Title,text
A Versatile MEMS Quadrupole Platform for Portable Mass Spectrometry Using the First and Second Stability Regions,"The Micro Gas Analyzer Program aims to develop portable, low-power, fast and low-false-alarm-rate gas analyzer technology for a wide range of applications. One of the subsystems of the gas analyzer is a mass filter. An array of micro-fabricated quadrupole mass filters is being developed for this purpose. The quadrupoles will sort out the ions based on their specific charges. Both high sensitivity and high resolution are needed over a wide range of ion masses, from 15 to 650 amu. In order to achieve this perfor-mance, multiple micro-fabricated quadrupoles, each operating at a specific stability region and mass range, are operated in paral-lel.The proof-of-concept device is a single, linear quadrupole that has a micro-fabricated mounting head with meso-scaled DRIE-pat-terned springs. The mounting head allows micron-precision hand assembly of the quadrupole rods [1] –critical for good resolution and ion transmission. The micro-fabricated mounting head can implement quadrupoles with a wide range of aspect ratios for a given electrode diameter. The springs can be individually actu-ated using spring tip handlers. The current version of the spring-head is able to interact with rods with diameters from 1588 µm down to 250 µm. The quadrupoles that have been implemented thus far span the aspect ratio range from 30 to 60. The choice of electrode diameter takes into account the dimensional uncer-tainties and alignment capabilities with respect to the expected resolution and transmission goals. Figure 1 shows an assembled MEMS quadrupole with 250-micrometer diameter rods. Figure 2 shows the experimental data of one of these quadrupoles us-ing FC-43 as a calibration compound, where a mass resolution of 2 amu and a full mass range of 650 amu are demonstrated, while using a 1.44 MHz RF power supply to drive the quadrupole with a constant-width circuit made by the Extrel company (Pitts-burgh, PA). To obtain better resolution, the MEMS quadrupoles have been driven with up to 4 MHz RF sources, resulting in 0.7 amu peak width. Also, the devices have been driven in the second stability regions to obtain 0.4 amu of peak width and smoother peaks. Current research efforts concentrate on developing RF power supplies of higher frequency and further exploration of the second stability region to obtain better performance."
First Principles Optimization of Mass-producible Microscaled Linear Quadrupoles for Operation in Higher Stability Regions,"In recent years, there has been a desire to scale down linear quad-rupoles. The key advantages of this miniaturization are the por-tability it enables and the reduction of pump-power needed due to the relaxation on operational pressure. Various attempts at making microscaled linear quadrupoles met with varying degrees of success [1-2]. Producing these devices involved some com-bination of precision machining or microfabrication and down-stream assembly. For miniature quadrupole mass filters to be mass-produced cheaply and efficiently, manual assembly should be removed from the process.A purely microfabricated quadrupole mass filter comprising a planar design and a rectangular electrode geometry is proposed. Quadrupole resolution is inversely-proportional to the square of the electrode length, thus favoring a planar design since electrodes can be made quite long. Rectangular rods are considered since that is the most amenable geometric shape for planar microfabri-cation. This deviation from the conventional round rod geometry calls for optimization and analysis. Electrode designs were pa-rameterized, and the potential fields were solved using Maxwell 3D (Figure 1). The fields were decomposed using a multipole ex-pansion to examine the higher-order coefficients (Figure 2). This process was used to minimize the significant high-order terms, thus optimizing the design and determining the ultimate limita-tions of the device.Higher-order field contributions arising from geometric non-ide-alities lead to non-linear resonances. These resonances manifest as peak splitting that is typically observed in quadrupole mass spectra. Reported work involving linear quadrupoles operated in the second stability region show improved peak shape without these splits [3]. It is believed that operating the device in the second stability region will provide a means to overcome the non-linear resonances introduced by the square electrode geometry. This study was conducted to justify a fully microfabricated, mass-producible, MEMS linear quadrupole mass filter. Successful im-plementation of such devices will lead into arrayed configurations for parallel analysis and aligned quadrupoles operated in tandem for enhanced resolution."
A Single-gated Open Architecture carbon nanotube Array for Efficient Field Ionization,"Mass spectrometers require a suitable ionizer to be able to discern the chemical composition of the sample that they are analyzing. Traditional ionizers for gases use either chemical ionization (CI) or electron impact ionization (EI). In the latter case, electrons from thermionic sources produce ions by colliding with neutral molecules. More efficient carbon nanotube-based field emitted electron impact ionizers have been developed [1]. However, one of the drawbacks of electron impact ionization is that the sam-ple is transformed into fragmentation products. Several samples could have similar fragmentation spectra but be quite different compounds, with radically different properties (for example, one substance can be a poisonous agent while another is a harmless material). Therefore, an approach to reduce the fragmentation products would improve the informational power of the mass spectrometer.Field ionization soft-ionizes molecules, thus reducing the frag-mentation products. In the field ionization scheme, ions are cre-ated by directly tunneling electrons from the outer shell of neutral molecules by virtue of a very high electric field [2]. The electric field is produced by high aspect ratio field enhancers and the ap-plication of a large (up to 1 kV) bias voltage. Carbon nanotubes are ideal field enhancers because of their high aspect ratio and their reduced tip radius. A good field ionizer should work in the field-limited regime instead of the molecular flux-limited regime, where all the molecules that approach the high field region are thus ionized. In the case of the electron impact ionizers, a closed architecture is implemented because it is intended to protect the field enhancers from back streaming ions [3]. Therefore, an open architecture, where the field enhancers surround a through-hole, is a more suitable approach to produce field ionization. We plan to implement a single-gated field ionizer array with an open ar-chitecture. Figure 1 shows a schematic of the open architecture concept. Figure 2 shows a cross section of the device. Current research effort focuses on device characterization."
A Fully Micro-fabricated Planar Array of Electrospray Emitters for Space-propulsion Applications,"Electrospray thrusters work by extracting ions or charged droplets di-rectly from a liquid surface using an electrostatic field and accelerating them in that field to produce thrust [1]. This method could lead to more efficient and precise thrusters for space propulsion applications. Emission occurs from sharp emitter tips, which enhance the electric field and constrain the emission location. The electrospray process limits the thrust from a single tip. To get into the millinewton range will require an array with tens of thousands of emitters. Batch micro-fabrication is well suited to making this array.We have designed, built, and tested a thruster made in silicon using deep reactive ion etching (DRIE) and wafer-bonding technology (see Figure 1). This thruster comprises two components. The emitter die has up to 517 emitters in a 0.75 cm2 area, formed using DRIE and SF6 etching, and is plasma treated so that liquid can be transported to the tips in a porous black silicon surface layer. The extractor die incor-porates the extractor electrode, a Pyrex layer for insulation, and the springs, which are used to reversibly clamp the emitter die [2]. This versatile assembly method allows the extractor die to be reused with multiple emitter dies and potentially with emitter concepts radically different from the one we have experimented with.Figure 2 shows data collected when firing the thruster with the ionic liquid EMI-BF4. Measurable emissions occurred for extraction volt-ages down to 700 V. The current collected on the extractor electrode was less than 3% of the emitted current over a wide operating range and often less than 0.1 %. Beam-divergence half-angles were between 15 and 30 degrees, depending on the operating conditions. Emitted currents of 500 nA/emitter were observed in stable operation, for ex-pected thrusts of 25 nN/emitter. Time-of-flight measurements prove operation in the ion emission regime, which is most efficient for pro-pulsion."
Carbon Nanotube Electron Sources for Space Propulsion Applications,"Low-power, low-voltage, efficient field emission neutralizers for FEEPs [1], colloid thrusters [2], and other micro-propul-sion engines are attractive for nanosatellites because they do not use mass flowrate to operate, unlike more conventional neutralizing solutions such as hollow cathodes [3]. Electrons are field-emitted from the surface of metals and semicon-ductors by the application of a high electrostatic field. Field emitters use high aspect ratio structures to generate very high fields even when low voltages are applied. The ideal field en-hancing structure is a rounded whisker [4]. Micro-engineered field emission neutralizers would have smaller starting volt-ages, better area usage, and more uniform I–V characteristics, compared to macro/meso fabricated field emitter versions. Plasma-Enhanced Chemical Vapor Deposited (PECVD) Car-bon Nanotubes (CNTs) are rounded whiskers with 100 nm or less of tip radius and 13 µm or more tall. The adoption of CNTs as electron-emitting substrate has recently being shown to have advantages compared to Spindt emitters because of the higher aspect ratio of CNTs and their superior resistance to harsh environments. This research focuses on the devel-opment of a batch-fabricated MEMS neutralizer that uses PECVD CNTs as field enhancers (Figure 1). As a reference, a previously made Busek-MIT MEMS CNT device that uses a randomly oriented CNT matrix produced by Busek Co. (Natick MA) with a proprietary arc-based process yielded de-vices with Fowler-Nordheim emission, startup voltage as low as 100 V, and electron currents as large as 3.2 mA/cm2 with about 20% of gate current interception."
Carbon Nanotubes for Electrospray Nanofluidic Applications,"Electrospray is the technique to soft-ionize liquids by applying a high electric potential to a liquid meniscus. The liquid menis-cus is deformed into a cone [1], and charged species are emit-ted from its apex. The emission can be solvated ions, charged droplets, or a mix of the two. This low-divergence charged species source can be used in diverse applications such as mass spectrometry, propulsion, printing, and etching. Our research group has successfully developed several multiplexed MEMS electrospray sources, mainly intended for space propulsion ap-plications. These devices include internal pressure-fed spouts that emit charged droplets [2] and externally surface tension fed spouts that emit solvated ions [3]. In all cases, the emit-ter field enhancers and the hydraulic impedance are provided using silicon-based structures. Furthermore, the devices use a 3D packaging technology that allows decoupling the process flows of the subsystems without loss in emitter density [4]. Consequently, it is possible to use radically different fabrica-tion techniques and materials to implement MEMS electro-spray arrays. This project intends to investigate the application of Plasma Enhanced Chemical Vapor Deposition Carbon Nanotubes (PECVD CNTs) in multiplexed electrospray sources. Two research directions are currently pursued: the use of CNTs as hydraulic impedance to ballast the emitter array (both in internal and external architectures) and the use of CNTs as emitter field enhancers. On the one hand, PECVD CNT for-ests can be custom tailored to match a desired morphology. On the other hand, PECVD CNTs have remarkable field en-hancing properties. Figure 1 shows a silicon-based externally fed electrospray linear emitter array that uses PECVD CNTs as hydraulic impedance, while Figure 2 shows the PECVD CNT forest grown on top of the silicon structures, using our group’s reactor. Current research is focused on exploring the wettability of CNT forests using different liquids, catalysts, and growth conditions. These results will be used to choose the proper nanostructure to be used in an externally fed MEMS electrospray head that will eventually include CNT-based field enhancers."
A High-density Electron Source that Uses Un-gated Transistors for Ballasting,"Electrons are field emitted from the surface of metals and semiconductors when the potential barrier (work function) that holds electrons within the metal or semiconductor is deformed by the application of a high electrostatic field. Field emitters use high aspect ratio structures with tips that have nanometer dimensions to produce a high electrostatic field with a low ap-plied voltage. We are implementing two types of field enhanc-ers: carbon nanofibers (CNFs) and silicon conical tips (Figure 1). Spatial variation of tip radius results in the spatial variation of the emission currents and non-uniform turn-on voltages. Small changes in the tip radius result in huge changes in the current density because of the exponential dependence of the emitted current on the bias voltage, as described by the Fowler-Nordheim theory. If the emitters are ballasted, the spatial non-uniformity can then be substantially decreased. Furthermore, ballasting individual emitters prevents destructive emission from the sharper tips allowing higher overall current emission because of the inclusion of duller tips. Ballasting also results in more reliable operation. The use of large resistors in series with the field emitters is an unattractive ballasting approach because of the resulting low emission currents and power dis-sipation in the resistors. A better approach for ballasting field emitters is the use of un-gated field effect transistors that ef-fectively provide high dynamic resistance with large saturation currents. In the past our research group demonstrated the use of a MOSFET to ballast the emission of electrons from silicon tips [1]. We plan to implement vertical un-gated transistors in series to the field emitters to obtain spatial uniformity in the current emission and I-V characteristics of the array [2]. The ballast structure is an n-doped, single-crystal silicon column, patterned using Deep Reactive Etching, and thinned using wet oxidation. Figure 2 shows a cross section of the un-gated transistors consisting of a 1-million elements in 1 cm2. The field emitters are formed on top of the columns. Current ef-forts focus on device testing."
Nanoelectromechanical Switches and Memories,"The ability to change shape is a compelling attraction of molecu-lar semiconductors. Compared to rigid inorganic materials, mol-ecules are soft and malleable, and their conformational changes are essential to the functionality of biological systems. Applica-tions of nano-electro-mechanical (NEM) molecular devices in-clude memories and transistors: Information can be stored in the conformation of molecules, potentially leading to very high den-sity memories, and molecular transistors that change shape under bias could exhibit sub-threshold slopes of << 60 mV/decade [1]. Indeed, as an example of the potential of NEMs, voltage-gated ion channels possess sub-threshold slopes of approximately 15 mV/decade [2].Although many materials are available for NEM applications, carbon nanotubes exhibit low resistance and good mechanical properties. In this project, we are constructing an NEM testbed. The proposed design for our relay is shown in Figure 1. Nano-tubes are directly grown at the bottom of an electron-beam de-fined trench etched in Si. Leaving tube growth to the final step gives us better control of the nanotube and removes the need for additional steps that are required for the removal of surfac-tants and organics from the surface of the nanotubes. Because the nanotubes are vertically oriented, we are able to take advantage of the smallest size feature of the carbon nanotube, its diameter, which enables us to create dense arrays of relays for applications such as memory or logic devices. The vertical orientation allows NEM structures with very large aspect ratios. Theoretical results [3] have shown that increasing the aspect ratio of a carbon nano-tube reduces the voltage needed to pull in the nanotube, thereby reducing the power consumption. Furthermore, because of the ability to easily functionalize the surface of nanotubes, we can functionalize the tube with multiple charges to lower the pull-in voltage even further."
Exciton Coupled Surface Plasmon Resonance Biosensor,"The development of portable and cost-effective biological sen-sors promises benefits to medical care, pharmaceutical testing, and the detection of biological warfare agents. Electronic devices are compact and readily integrated into microfluidic substrates, making them a promising alternative to today’s bio-detection requirements. Our sensor design aims to exploit the sensitivity of surface plasmon resonance (SPR). Unlike conventional SPR sensors, the plasmon is detected in the near field using a thin film organic photovoltaic (PV). High absorption coefficients make organic semiconductors ideal candidates for the detection of surface plasmons. Organic materials are also easily deposited on microfluidics, enabling use of these devices outside the laboratory environment in a convenient portable package.In the initial demonstration, plasmon modes are excited in the top surface of the gold cathode by a p-polarized laser beam, when the horizontal component of the light wavevector matches the plasmon wavevector [1]. The plasmon is absorbed by the organic semiconductor and split into holes and electrons at the interface between the donor and acceptor layers composing the PV cell (Figure 1). The plasmon resonance measured indirectly as reflect-ed power and photocurrent (Figure 2) has a strong angular loca-tion dependence on the adjacent layer’s dielectric constant that is altered upon binding of bio-molecular species. The steep slope of the resonance enables sensitive detection as well as measure-ment of kinetic parameters of the binding event."
Micromechanical Substrates for Reconfigurable cell culture,"We have previously demonstrated the use of microfabricated cell culture substrates (Figure 1) to implement reconfigurable cell cul-ture (Figure 2) [1]. Specifically, we studied interactions between liver hepatocytes and supportive stromal cells. We found that preservation of liver-specific function depended on signaling from the stroma. Specifically, signaling both through direct contact and through diffusible secreted factors was important. However, while the secreted factors needed to be maintained for the entire dura-tion of culture (2 weeks), direct contact was required only for an 18-hour period early in culture. In addition, the secreted factors were found to have a limited effective range of less than 400 µm. Through FEM diffusion modeling, we showed that a half-life on the order of hours would result in such short-range signaling.Currently, we are exploring the use of this platform in a variety of applications including identification of the signaling factors in hepatocyte co-culture, stabilizing liver endothelial cells in culture, toxicity models for drug testing, preconditioning of hepatocytes prior to encapsulation in a 3D gel, and patterning cells directly on the combs to study contact signaling mechanisms."
Aligned Multimask Patterning of Biomolecules and cells,"Surface engineering of cell culture substrates has developed into a powerful tool for controlling multicellular organization at the micrometer scale. This new capability has brought valuable in-sight into the biological mechanisms by which the cellular micro-environment determines cell fate and function. However, studies requiring more complex tissue structures have been hindered by limitations in surface patterning. Typically, molecules that medi-ate cell attachment are patterned against a non-adhesive back-ground, allowing arrays of a single cell type to be formed with control of cell positioning and relative spacing. Alternatively, patterns composed of two different adhesive regions can be em-ployed to form patterned co-cultures of two different cell types, as long as one cell type selectively attaches to a specific region. How-ever, there have been a few examples where multiple attachment chemistries have been successfully combined with non-adhesive surfaces in a multicomponent pattern. This has prevented the re-alization of configurations in which cell-cell contact and spacing between different cell types are controlled.The use of photolithography with multiple aligned masks is well established for generalized multicomponent patterning, but it is often too harsh for biomolecules. We report a two-mask photo-lithographic process that is tuned to preserve bioactivity in pat-terns composed of covalently coupled polyethylene glycol (PEG), adsorbed extracellular matrix protein (e.g., collagen I), and ad-sorbed serum proteins (e.g., vitronectin). Thereby, we pattern two cell types—primary hepatocytes and 3T3 fibroblasts—demon-strating control over contact and spacing (20-200 µm) between the two cell types for over one week. This method is applicable to the study of intercellular communication in cell biology and tissue engineering."
Collective Hydrodynamics and Kinetics of Sickle Cell Vaso-occlusion and Rescue in a Microfluidic Device,"The pathophysiology of sickle cell disease, the first to be implicat-ed with a genetic origin, is complicated by the multi-scale nature of the processes that link the molecular genotype to the organis-mal phenotype. Here, we show that it is possible to evoke, control and inhibit the vaso-occlusive crisis event in sickle cell disease us-ing an artificial microfluidic environment. We use a combination of geometric, physical, chemical and biological means to quantify the phase space for the onset of a jamming crisis, as well as its dis-solution, as shown in Figure 1.The microfluidic chip designed to independently vary the various parameters that control the onset of vaso-occlusion in a sickle cell crisis is shown in Figure 2. This device allows us to dissect and probe the hierarchical dynamics of this multi-scale process by manipulating the geometrical, physical, chemical and biologi-cal determinants of the process. The chip consists of a series of bifurcating channels of varying diameters that grossly mimics the geometry of vasculature. By controlling the physical pressure gradient across the chip, we can vary the kinetic time scale for transit of red blood cells. The channels are separated from a gas reservoir by a thin gas-permeable polydimethylsiloxane (PDMS) membrane. As the geometries are microscopic, gas diffusion is rapid and the oxygen concentration in the microchannels is gov-erned by the concentration in the gas reservoir. By changing the mixture of this reservoir, we control oxygen concentrations in the channels and hence the onset of microscopic hemoglobin polym-erization. By using blood with varying concentrations of HbS and different hematocrits, we can mimic the variability among individuals. This device was used to study the phase space of jamming governed by pressure, channel dimensions and oxygen concentration as shown in Figure 1. Our experimental study integrates the dynamics of collective processes at the molecu-lar, polymer, cellular and multi-cellular level; lays the foundation for a quantitative understanding of the rate limiting processes; provides a potential tool for optimizing and individualizing treat-ment; and serves as a bench test for dynamical drugs."
Plasma-activated Inter-layer Bonding of Thermoplastics for Micro- and nano-fluidic Manufacturing,"Plasma-activated polymer–polymer bonding is a promising way of encapsulating micro- and nano-fluidic channels across large substrate areas, without the substantial distortion of channel ge-ometries that can plague thermally- and solvent-assisted bond-ing. The process involves treating the surfaces to be bonded with an oxygen or air plasma, and then pressing the surfaces together to allow an irreversible chemical bond to form [1]. A conve-nient method is desired for measuring the toughness of such a bonded interface. Simple crack-opening tests (whereby a blade prizes apart the two bonded layers and the length of the inter-layer crack determines the bond toughness [2]) are clumsy and hard to automate. We propose that built-in microscopic crack-opening test sites be distributed across manufactured substrates [3]. At each test site, a polymeric film bonded over a step in the substrate would peel back from the step after bonding, by a dis-tance depending on the toughness of the bond. The presence of a wedge-shaped air gap between the covering film and the substrate leads to visible interference fringes, the spacing of which can be used to extract the bond strength (Figure 1). Arrays of these in situ cracks might be imaged without removing the substrate from a production line and would allow us to monitor both substrate-to-substrate and cross-substrate bond toughness variation. Bond toughness and polymer layers’ surface energies are of par-ticular relevance in planning the fabrication of very shallow flu-idic channels whose widths, w, are much larger than their depths, h. The risk of channels’ collapsing during fabrication must be controlled. For channels with h ~ 1 µm or less that are fabricated with thermoplastics, we expect collapsing to occur through lo-cal deformation of the surrounding material rather than through plate-like bending of the cover plate [4]. Our analysis suggests that the pressure applied during bonding, together with the polymer–polymer interface energies that exist before and after plasma-activated bonding, will delineate, on a w/h against h plot, regions in which collapsing will and will not occur. We have dem-onstrated nanochannels fabricated from polymethylmethacrylate (PMMA) that are 80 nm deep and 10 µm wide and other chan-nels that are 110 nm deep and 20 µm wide (Figure 2)."
Environmentally Benign Manufacturing of Three-dimensional Integrated circuits,"Along with scaling down in size, novel materials have been in-troduced into the semiconductor industry to enable continued improvements in performance and cost as predicted by Moore’s law. It has become important now more than ever to include an environmental impact evaluation of future technologies, before they are introduced into manufacturing, in order to identify po-tentially environmentally harmful materials or processes and un-derstand their implications, costs, and mitigation requirements. In this project we introduce a methodology to compare alterna-tive options on the environmental axis, along with the cost and performance axes, in order to create environmentally aware and benign technologies. This methodology also helps to identify po-tential performance and cost issues in novel technologies by tak-ing a transparent and bottoms-up assessment approach. This methodology is applied to the evaluation of the MIT 3D IC technology in comparison to a standard CMOS 2D IC approach. Both options are compared on all three axes–performance, cost, and environmental impact. The “handle wafer” unit process in the existing 3D IC technology, which is a crucial process for back-to-face integration, is found to have a large environmental impact because of its use of thick metal sacrificial layers and high-en-ergy consumption. We explore three different handle wafer op-tions: between-die channel, oxide release layer, and alternative low-temperature permanent bonding. The first two approaches use a chemical handle wafer-release mechanism while the third explores solid liquid inter-diffusion (SLID) bonding using cop-per-indium at 200°C. Preliminary results for copper-indium bonding indicate that a sub-micron thick multi-layer copper-in-dium stack, when bonded to a 300-nm-thick copper film, results in large voids in the bonding interface primarily due to rough as-deposited films. Finally, we conduct an overall assessment of these and other proposed handle wafer technologies. The overall assessment shows that none but the oxide release layer approach appears promising; however, each process option has its strengths and weaknesses, which need to be understood and pursued ac-cordingly."
An Implantable MEMS Drug-delivery Device,"A novel drug-delivery system based on MEMS technology is be-ing developed. This implantable microchip is capable of deliv-ering vasopressin, a known vasoconstrictor that can prevent or delay death by hemorrhagic shock [1]. The device is specially tailored to treat hemorrhagic shock in ambulatory settings and is intended for in vivo use as a micro-implant in the peritoneum for people in high-risk situations.The device has a modular design and is composed of three layers (shown schematically in Figure 1): a large reservoir layer, where the drug solution is stored; a membrane layer from where the drug is ejected; and a bubble-generating layer, where bubbles are formed. The reservoir layer is defined by drilling through a Pyrex 7740 wafer with a diamond bit. Wafer thickness and hole diameter can be modified to change reservoir capacity. The membrane layer is composed of silicon nitride membranes cover-ing through-holes etched by DRIE into a silicon substrate. Thin gold fuses can be patterned on the membranes to detect ruptures, which then shows as an open circuit. The bubble-generating layer is defined by micro-resistors, which can quickly and locally heat the contained fluid to generate bubbles. The pressure exerted by these bubbles causes rupture of the silicon nitride membranes and forces the contained solution out of the device.In vitro operation of the device has been demonstrated, as shown in Figure 2. Further developments of this device include reduc-tion of power consumption during activation, wireless activation, and adaptation of the device for a pen-size, transdermal delivery system. We believe that the ramifications of this MEMS-based drug delivery system can be useful for a vast number of medical applications."
High Speed Three-dimensional Scanner for in vivo non-invasive Optical Biopsy using Two-photon Microscopy,"We have recently demonstrated the modeling, design, and micro-fabrication process of a millimeter-scale, high-speed endoscopic scanner that is to be integrated at the distal end of an endomicro-scope [1]. The scanner system consists of (1) an active Silicon op-tical bench (SOB), as shown in Figure 1, which constrains, aligns, and thermally actuates (1) mm-size optics (GRIN lens and prism) at 5 Hz and (2) a slim fiber resonator that excites the double-clad photonic bandgap fiber at ~1 kHz. The scanner system has a 7-millimeter device envelope with a range of 100 micrometers in X, Y and Z. The design of a two-photon endoscope requires scanning of focused light to create tissue images, and scanning actuator technology still proves to be a bottleneck for practical endoscope design. The performance (force-speed-stroke) criteria for the prototype endomicroscope design are generated based on clinical needs. The strict force, speed, and stroke requirements (~10 mN, 1 kHz, and 100 µm) call for a new method for ac-tuation. The low voltage requirement for future in vivo examina-tion/operation makes a new class of thermomechanical actua-tors (TMAs) a suitable candidate among other micro-actuation technologies.The two-photon imaging technique requires scanning of focused light to create tissue images. The endoscopic scanner may en-able the design and construction of a miniaturized two-photon microscopic system to image the surface and sub-surface cells (up to 200 microns depth) of internal tissues with sub-cellular reso-lution. The two-photon endomicroscope is designed to perform non-invasive, in vivo, optical biopsy, which has numerous benefits over excisional biopsy. For example, non-invasive optical screen-ing may decrease the number of excision biopsies required, and optical biopsy can provide more informed selection of excisional biopsy sites, minimizing incorrect diagnosis due to random sam-pling. This is useful for detecting cancer at an early stage among other diseases. The chevron TMAs on the SOB are optimized through the geometric contouring method [2] to provide enhanced force, displacement and reduced power consumption compared to common chevron actuators. This also allows the TMAs to be operated at lower temperature and thus makes the TMAs more suitable for precision actuation. Figure 2 presents an example of a contoured chevron TMA. Early models and experiments of the contour shaping method have confirmed that the maximum achievable thermal strain of a driving beam may be increased by 29%, the actuator stroke may be increased by a factor of 3 or more, and identical force or displacement characteristics may be achieved with 90% reduction in power. A new high-speed pulsing technique has also been investigated recently; it enhances the dynamic performance of the contoured TMAs [3]. Prelimi-nary simulation results indicate a 12% bandwidth increase, 30% stroke enhancement, and 70% power reduction. This technique, together with the geometric contouring method for TMAs, may potentially increase the bandwidth of the endoscopic scanner by a factor of 10 and therefore meet the functional requirements for a two-photon scanning endomicroscope."
Electromagnetically-driven Meso-scale nanopositioners for nano-scale Manufacturing and Measurement,"Nanopositioners – be they nano-, micro-, or macro-scale in physi-cal size – enable us to move large or small parts with nanome-ter-level or better precision. They therefore set the limits on our ability to measure, understand, manipulate, and affect physical systems. Six-axis small-scale nanopositioners enable a combina-tion of faster speed and better resolution. They are therefore important in scientific and commercial applications where speed and small-dimensions are important: biological sciences, data storage and nanomanufacturing equipment and instruments [1-4]. Emerging applications in these fields will benefit from por-table, multi-axis nanopositioners that are capable of nanometer-level positioning over tens-of-microns at speeds of hundreds to thousands of Hertz. Toward this end, our work aims to create a meso-scale, high-speed, six-axis nanopositioner.The nanopositioner is designed to operate with a range-of-mo-tion of larger than 10 micrometers in the X-, Y- and Z- direc-tions, possess a natural frequency of 1 kHz, and exhibit better-than-10-nm resolution. The nanopositioning system, shown in Figure 1, is composed of three sets of micro-actuators. Within each set, micro-coils are suspended above a linear array of 1 mm3 permanent magnets via a silicon flexure system [5]. Each actua-tor is composed of two independent coils that apply in-plane and out-of-plane forces to the flexure. The actuator inputs are com-bined to control the stage position in six axes. Figure 2, which shows the actuator’s force output capability vs. coil footprint, was generated using a numerical model. The micro-coils consist of two stacked copper micro-coils that are electrically isolated by a layer of silicon dioxide. They are created by electroplating cop-per within silicon and photoresist molds. The flexures are etched using deep reactive-ion etching. The system will be applied to the high-speed and precise positioning of small parts such as probes and thin-films. The system is scheduled to be integrated into a bench-top scanning-probe microscope and within a nano-electro-discharge machining station [4]."
Barcoded Microparticles for Multiplexed Detection,"The detection of multiple targets in a single sample is important for many applications, including medical diagnostics, genotyp-ing, and drug discovery. The current approaches to multiplexing, such as planar arrays (such as DNA microarrays) and suspension (particle-based) arrays, require expensive or cumbersome means of encoding, decoding, or functionalizing substrates. Currently, commercially available approaches for multiplexed analysis are cost-prohibitive for high sample throughput, low-cost applica-tions such as bedside diagnostics. We have developed a method [1], based on multifunctional bar-coded particles, for the sensitive and accurate multiplexed detec-tion of biomolecules. Our method is unique in that (1) we can fabricate, encode, and functionalize particles in a single step, (2) the particles are composed of poly(ethylene glycol) hydrogel to increase both sensitivity and specificity, and (3) only a single fluo-rescent wavelength is required to decode the particles and quan-tify the corresponding targets. Using an efficient one-step method based on continuous-flow lithography, we synthesize micropar-ticles with multiple functional regions (Figure 1). Each particle bears a fluorescent dot-pattern barcode (capable of providing over a million unique codes) to identify the target(s) it is looking for and one or more spatially separated regions containing a probe where those targets can bind and be detected via fluorescence. In this way, particles from a library can be mixed and incubated in a single sample to simultaneously detect many targets, such as DNA oligomers (Figure 2). The detection of targets is not only sensitive but also extremely specific due to the porous and bio-inert nature of the hydrogel structure that allows target molecules to diffuse and bind deep into the transparent particle surfaces."
Single-molecule DnA Mapping in a Fluidic Device,"The ability to controllably and continuously stretch large DNA molecules in a microfluidic format is important for gene-mapping technologies such as Direct Linear Analysis (DLA). We have recently shown that electric field gradients can be readily generated in a mi-crofluidic device and the resulting field is purely elongational. We have performed a single-molecule fluorescence microscopy analysis of T4 DNA (169 kbp), stretching in the electric field gradients in a hyperbolic contraction microchannel. In addition, we are able to se-lectively pattern a crosslinked gel anywhere inside the microchannel. With an applied electric field, DNA molecules are forced to reptate through the gel and they stretch moderately as they exit the gel. By placing a gel immediately in front of the hyperbolic contraction, we bypass “molecular individualism” and achieve highly uniform and complete stretching of T4 DNA. This device offers a new method to efficiently stretch DNA for single-molecule mapping studies."
DnA Dynamics in nanofluidic Devices,"In dilute polymer solutions, the shape, motion, dynamic response, and solvent-interaction (HI) of single polymer molecules change when geometric constraints reach the length scales of the equi-librium polymer conformation. Our study seeks to understand these changes using double-stranded DNA as a model polymer and to utilize these confinement effects to tune the dynamic re-sponse of single molecules. This ability is useful in processes that rely on controlling the conformation of a biomolecule for analysis [1] or in the manipulation of molecules for separations and/or reactionsOur experiments [2-3] use thermally-bonded pyrex channels with heights ranging from 75 to 500 nm and widths of 150 µm. The Brownian motion of stained DNA molecules is observed us-ing epi-fluorescence microscopy. By following the time evolution of the center-of-mass and orientation of single molecules, we can obtain the diffusion coefficient (D) and longest relaxation time (τ1) of the polymer independently. We find that scalings with molecu-lar weight of both D and τ1 agree with a free-draining polymer model, indicating that, in contrast to bulk solution, HI is not im-portant in slit confinement at length scales comparable to the size of the molecule. We find that the relaxation time of the polymer increases with confinement, which promises easier manipulation of DNA conformations. Our results in well-defined nanofluidic devices may also provide insight into polymer behavior in the less-controlled confinement that occurs in concentrated polymer solu-tions. We are currently working to stretch DNA in confinement and to study the effects of confinement far from the equilibrium conformation of the polymer"
Microfluidic Bubble Logic,"Large-scale microfluidic integration promises to revolutionize the fields of biology and analytical chemistry. The “Lab-on-a-Chip” community has long sought the ability to precisely control very small volumes (nanoliters) of fluid packets . Current mechanisms for fluid routing depend on external control elements with no feedback, limiting scalability and integration. In [1] we describe Bubble logic, an all-fluidic universal logic family implemented in a two-phase microfluidic system. The presence or absence of a drop or a bubble represents a bit of information. Non-linear hydrodynamic interactions of these elements in microfluidic ge-ometries are exploited to build logic gates (AND, NOT), bistable memory (toggle flip-flop), ring oscillators, ripple counters, and synchronizers. This provides an on-chip internal flow control mechanism with all the properties of a digital logic family includ-ing gain, bistability, cascadability, feedback and synchronization. Since no external control elements are required, bubble logic can also find applications in diagnostic instrumentation in resource-poor settings, controlled drug delivery or computation in harsh settings. Previous attempts at an all-fluidic computation mechanism used inertial effects (significant only at high Reynolds numbers) or non-Newtonian fluids (like polymer blends). Bubble logic operates at both low Reynolds and capillary numbers, allowing us to reduce length scales further and thus operate in nanoliter or smaller re-gimes. Figure 1 depicts device geometries for universal AND-NOT logic gate and a toggle flip-flop. The devices are fabricated using soft-lithography in PDMS bonded to glass. Propagation time for the logic gate and toggle flip-flop is ~10ms. Figure 2 de-picts a ring oscillator consisting of three AND gates and a delay line with photomicrographs of the device in operation (recorded by a high speed video camera). We are currently working on in-tegrating bubble logic elements to build high-density, random-ac-cess chemical memories."
Perfused Multiwell Tissue culture Plates for Development of Drug and Disease Models,"A new platform for three-dimensional hepatic tissue engineering has been developed. It is based on the conventional multiwell tis-sue culture plate format but it allows the tissue to be continuously perfused with cell culture medium [1]. The new capability is achieved by a microfluidic perfusion system that re-circulates cell culture medium between reactors and reservoir (Figure 1). It fea-tures a network of microfluidic valves and pumps integrated into the plate [2]. Flow pulsatility is controlled by fluidic capacitors. In order to measure performance of fluidic capacitors, fluid was pumped through a capillary and a high-speed video camera was used to track the end position of the fluid. Figure 2 compares the performance of measured and modeled capacitors. As predicted by the model, the 10-mm capacitor effectively filters fluid pulses and generates a nearly constant flow. Flow with this characteristic is critical during the initial cell attachment time-period.Phase contrast and fluorescent imaging, measurement of oxygen consumption, accumulation of taurocholic acid, gene expression profiling, and drug metabolism assays are used to characterize the performance of the 3D perfused cultures. Because the new system features a standard multiwell tissue culture plate footprint, it is readily amenable to numerous high-throughput assays com-patible with automated technologies commonly used in pharma-ceutical development. The system provides a means to conduct assays for toxicology and metabolism and can be used as a model for human diseases such as hepatic disorders, exposure-related pathologies, and cancer."
A Patterned Anisotropic nanofilter Array for continuous-flow Separation of DnA and Proteins,"Microfabricated regular sieving structures hold great promise as an alternative to gels to improve biomolecule separation speed and resolution. In contrast to the disordered gel porous network, these regular structures also provide well-defined environments ideal for study of molecular dynamics in confining spaces. How-ever, previous regular sieving structures have been limited for sep-aration of long DNA molecules, and separation of smaller, physi-ologically-relevant macromolecules, such as proteins, still remains as a challenge. Here we report a microfabricated anisotropic siev-ing structure consisting of a two-dimensional periodic nanoflu-idic filter array (an Anisotropic Nanofilter Array, or ANA). The designed structural anisotropy in the ANA causes differently-sized molecules to follow different trajectories, leading to efficient sepa-ration. Continuous-flow Ogston sieving-based separation of short DNA and proteins as well as entropic trapping-based separation of long DNA were achieved, thus demonstrating the potential of the ANA as a generic sieving structure for an integrated biomol-ecule sample preparation and analysis system."
"Cell Stimulation, Lysis, and Separation in Microdevices","Quantitative data on the dynamics of cell signaling induced by different stimuli requires large sets of self-consistent and dynamic measures of protein activities, concentrations, and states of modi-fication. A typical process flow in these experiments starts with the addition of stimuli to cells (cytokines or growth factors) under controlled conditions of concentration, time, and temperature, followed at various intervals by cell lysis and the preparation of extracts. Microfluidic systems offer the potential to do laborious assays in a reproducible and automated fashion [1].Figure 1 shows quantification of the stimulation of a T-cell line with antibodies performed in a micro-fluidic device with integrat-ed cell lysis. The device is capable of resolving the very fast kinet-ics of the cell pathways, with protein activation levels changing 4-fold in less than 15 seconds [2]. The quantification of the lysate is currently performed off-chip using electrophoretic separation. To effectively extract meaningful data from cellular preparations, many current biological assays require similar labor-intensive sample purification steps.Micro-electrophoretic separators have several important advan-tages over their conventional counterparts, including shorter separation times, enhanced heat transfer, and the potential to be integrated into other devices on-chip. However, the high voltages required for these separations prohibit using metal electrodes in-side the microfluidic channel. A PDMS isoelectric focusing device with polyacrylamide gel walls [3] has been developed to perform rapid separations by using electric fields orthogonal to fluid flow. This device and its variants have been shown to focus organelles, low-molecular-weight dyes, proteins, and protein complexes (Fig-ure 2a) in seconds. Simulations have driven the development of improved device configurations, such as tandem IEF stages (Fig-ure 2b)."
Microreactors for Synthesis of Quantums Dots,"We have fabricated gas-liquid, segmented-flow reactors with multiple temperature zones for the synthesis and the overcoating of quantum dots (QDs). In contrast to single-phase flow reactors, the segmented flow approach enables rapid mixing and narrow residence time distribution, factors which strongly influence the ultimate QD size distribution. The silicon-glass reactors accom-modate a 1-m-long reaction channel (hydraulic diameter ≈ 400 µm) and swallow side channels for multiple additional injections of precursors inside the main channel (Figure 1). Pressure-drop channels were added in order to avoid backflow into the side chan-nels. Two temperature zones are maintained, a heated region (> 260 °C) and a cooled quenching region (< 70 °C). Measurements of the flow distribution (Figure 2a) show that this side manifold de-sign results in very uniform distribution even at very low nominal flow rates. As a model system, monodispersed CdSe and CdSe/ZnS QDs were prepared using this reactor. For the preparation of CdSe QDs, cadmium and selenium precursor solutions were de-livered separately in the cooled region and were thereafter mixed in the heated region. An inert gas stream is introduced further downstream to form a segmented gas-liquid flow, thereby rapidly mixing the precursors and initiating the reaction, as was shown in a previous work [1]. In the case of the synthesis of CdSe/ZnS QDs, CdSe cores are introduced directly inside the main channel, while Zn and S precursors are added through the side swallow channels, allowing the overcoating. The reaction is stopped when the fluids enter the cooled outlet region of the device. When we vary the process parameters (temperature, precursors flow rates). the size of the cores material can be tuned without sacrificing the monodispersity. In addition, the overcoating of CdSe cores allows shifting the absorbance spectrum (5 nm), due to the presence of the ZnS layering (Figure 2b)."
Microfluidic Synthesis and Surface Engineering of Colloidal nanoparticles,"There has been considerable research interest over the last de-cade in fabricating core-shell materials with tailored optical and surface properties. For example, core-shell particles of silica and titania have drawn attention due their potential for trapping light at specific frequencies. This optical property depends on the for-mation of nanolayers on nano- or micro-cores. To obtain useful particles, these layers need to be uniform and even. These lay-ered particles also need to be distinct and monodispersed. While nanolayer formation is successful in batch reactions, nonunifor-mity, agglomeration, and secondary nucleation often occur. We have developed microfluidic routes for synthesis and surface-coat-ing of colloidal silica and titania particles. The chief advantages of a microfluidic platform are precise con-trol over reactant addition and mixing and continuous operation. Microfluidic chemical reactors for the synthesis and overcoating of colloidal particles are shown in Figures 1a and 1b, respectively [1-2]. Figure 2a is an SEM micrograph of silica particles synthe-sized in a microreactor (Figure 1a) operated in segmented gas-liq-uid flow mode. Figure 2b shows a silica nanoparticle coated with a thick shell of titania. We have also fabricated integrated devices combining synthesis and overcoating to enable continuous multi-step synthesis of core-shell particles."
Organic Synthesis in Microreactor Systems,"Enhanced heat and mass transfer, reduced reaction volume, and the ability to run several experiments in parallel render mi-croreactors powerful instruments for scanning and optimizing chemical reaction conditions. Furthermore, the high mechanical strength and thermal conductivity of silicon microreactors permit the exploration of organic syntheses at higher temperatures and pressures than can be achieved with conventional bench-scale equipment. An example of these benefits is demonstrated in the aminocarbonylation reaction study [1]. Traditionally, these reac-tions are performed at atmospheric conditions and with tempera-tures at or below the boiling point of the solvent (toluene, 110ºC). However, in silicon microreactors (Figure 1), it is possible to reach pressures exceeding 100 bar [2] and temperatures above 800ºC [3]. Exploration of the aminocarbonylation reaction offers in-formation that can be useful for the optimizing selectivity of the synthesis; higher CO pressures enhance α-ketoamide formation and increased temperatures favor amide formation.Once the chemical reaction is complete, it is desirable to separate the toxic gas from the liquid phase. Although negligible on the macro-scale, surface forces play a dominant role in microfluidics. Creating a capillary-based system (Figure 2) [4] makes it possible to take advantage of these forces. The liquid phase wets the cap-illaries and prevents the gas from penetrating the capillary ma-trix through the proper adjustments of pressure drops across the separator. Similarly, this concept can be applied to heterogeneous reactions that involve two immiscible liquids. Due to this micro-technology, microreactor systems can be assembled for multi-step synthesis and separation that could not easily be achieved in tra-ditional laboratory environments. As a result, high throughput experiments can be performed and entire chemical processes can be optimized efficiently with microreactor systems."
Autothermal catalytic Micromembrane Devices for Portable High-purity Hydrogen Generation,"The high efficiency and energy density of miniaturized fuel cells provide an attractive alternative to batteries in the portable-power-generation market for consumer and military electronic devices [1-3]. The best fuel cell efficiency is typically achieved with hydrogen, but safety and reliability issues remain with cur-rent storage options. Consequently, there is continued interest in reforming of liquid fuels to hydrogen. The process typically in-volves high-temperature reforming of fuel to hydrogen combined with a low-temperature PEM fuel cell, which implies significant thermal loss. Owing to its high hydrogen content (66%) and ease of storage and handling, methanol is an attractive fuel. However, partial oxidation of methanol also generates CO, which can poi-son the fuel cell catalyst [1]. Previously [4] we have successfully demonstrated hydrogen pu-rification using thin (~200 nm) Pd-Ag membranes using electri-cal heating. Further, integration of these devices with LaNiCoO3 catalyst allowed methanol reforming at 475oC with 47% fuel con-version [5]. In the current work, we fabricate a novel autother-mal reformer for hydrogen generation and purification using bulk micromachining techniques. This device combines the reforming unit with a catalyst loaded microreactor for combustion of hy-drogen not recovered through the Pd- Ag membrane, generated CO, and unreacted methanol. The energy from the combustion heats the reformer to the operating temperature (~450C). High thermal conductivity of silicon ensures efficient heat transfer from combustor to reformer. In the first phase, Pd-Ag membrane stability post-fabrication was tested; results indicated a pin-hole- and crack-free layer. Further, we successfully demonstrate high-pressure operation (up to 1.6 atm) of the device for enhanced hydrogen flux. The microburner has also been characterized with hydrogen oxidation over platinum catalyst. Work on reforming methanol for hydrogen generation and characterization of ther-mal responses is currently under progress."
Thermal Management in Devices for Portable Hydrogen Generation,"The development of portable-power systems employing hydro-gen-driven solid oxide fuel cells continues to garner significant interest among applied science researchers. The technology can be applied in fields ranging from the automobile to personal elec-tronics industries. This work focuses on developing microreaction technology that minimizes thermal losses during the conversion of fuels – such as light-end hydrocarbons, their alcohols, and am-monia – to hydrogen. Critical issues in realizing high-efficiency devices capable of operating at high temperatures have been addressed: specifically, thermal management, the integration of materials with different thermophysical properties, and the devel-opment of improved packaging and fabrication techniques.A new fabrication scheme for a thermally insulated, high-temper-ature, suspended-tube microreactor has been developed. The new design improves upon a monolithic design proposed by Arana et al. [1]. In the new modular design (Figure 1), a high-temperature reaction zone is connected to a low-temperature (~50°C) package via the brazing of pre-fabricated, thin-walled glass tubes. The design also replaces traditional deep reactive ion-etching (DRIE) with wet potassium hydroxide (KOH) etching, an economical and time-saving alternative. A brazing formulation that effectively ac-commodates the difference in thermal expansion between the silicon reactor and the glass tubes has been developed. Autother-mal combustion of hydrogen, propane (Figure 2), and butane has been demonstrated in ambient atmosphere and in a vacuum."
Microfluidic Systems for the Study of Vascular networks,"Mechanical forces are important regulators of cell biology in health and disease. Cells in the vascular system are subjected to fluid shear stress, cyclic stretch, and differential pressure [1-3],[1-3],, and at the same time they receive multiple biochemical cues. All these factor into the integrated response of the tissue. A micro-fluidic bioreactor has been constructed to facilitate studies into the roles of both biophysical and biochemical factors on capil-lary morphogenesis. The device is made of PDMS, cured on The device is made of PDMS, cured on The device is made of PDMS, cured onThe device is made of PDMS, cured on an SU-8 patterned wafer. Then a scaffold material, collagen, is induced into a specific region of devices that was designed to keep its shape and properties. Cells are seeded via one flowCells are seeded via one flow channel on the surface of the scaffold and then subjected tothe scaffold and then subjected to scaffold and then subjected to and then subjected toand then subjected to controlled mechanical factors like surface shear and trans-endo-mechanical factors like surface shear and trans-endo-surface shear and trans-endo-thelial pressure, or biochemical angiogenic factors, inducing theor biochemical angiogenic factors, inducing theinducing the formation of vascular sprouts that extend across the scaffold to a second flow channel. With the bioreactor, cells on the scaffold. With the bioreactor, cells on the scaffoldcells on the scaffold form a confluent monolayer and generate sprouts. They show. They show different responses and interactions with the scaffold, followingfollowing the angiogenic factors, fluidic factors, surface characteristics and scaffold properties. Experiments are now under way to find theExperiments are now under way to find thefind the relations between cell responses and controlled factors. The de- The de- The de-The de-veloped system is the first system that can control biochemical andis the first system that can control biochemical andthat can control biochemical andcan control biochemical andcontrol biochemical and biochemical and mechanical factors together, and it can be used for comparing thecan be used for comparing thecomparing the effects of angiogenic factors under controlled environment withenvironment with with enhanced view. It can also be applied to study the process of an-to study the process of an-giogenesis that entails the growth of vascular sprouts emanating from one endothelial surface and connecting with the other."
Patterning and Processing of Thermosensitive Hydrogels for,"Hydrogels have been an active area of research for a variety of applications due to their ability to retain large volumes of water within their polymer gel networks. Stimuli-responsive hydrogels provide the added advantage of the ability to control the water retention by means of external stimuli. For example, N-isopro-pylacrylamide (NIPAAm) is a thermosensitive hydrogel that ex-hibits a Lower Critical Saturation Temperature (LCST) around 32°C, above which the gel becomes hydrophobic and expels the water molecules, resulting in a drastic swelling/shrinking ratio. The goal of this project is to utilize this pseudo-binary transition in the fields of microfluidics and drug delivery.By imbedding magnetic nanoparticles into the gel networks, the Hamad-Schifferli group [1] could control the temperature of the gels by inducing eddy currents by means of an oscillating magnet-ic field. We are developing the concept further into micro-scale devices that can be monolithically integrated into many micro-fluidic systems. We have demonstrated the ability to photopattern the hydrogels and have shown control of the swelling behavior by controlling the amount of cross-linking in the network. This al-lowed for the creation of hydrogel valves for microfluidic devices. Unlike pressure controlled valves, these valves do not require any physical interconnects to macro-scale devices. This advantage could prove extremely useful in the commercialization of micro-fluidic analysis systems where users might not have equipment such as syringe pumps or air compressors available. In addition to valves, applications of the swelling behavior to micropumps are also being examined."
"A Large-strain, Arrayable Piezoelectric Microcellular Actuator by Folding Assembly","A low-power, piezoelectric, contracting cellular MEMS actuator has been developed that demonstrates a peak strain of 3% under a 10 V stimulus. Since the motion of the end effecter is linear and in-plane, the actuator can be arrayed in series to amplify the total stroke or in parallel to amplify the total force, as needed. Loca-tion of the piezoelectric member through the structural center of stiffness reduces the potential for parasitic out of plane bending present in previous designs [1].Cellular actuators arrays can be assembled into a larger array of actuators. We demonstrated that sets of cellular microactuators can be assembled out of plane by folding them over thin gold hinges. To our knowledge, this study is the first effort in this field. The gold hinges serve dually as mechanical assembly guides and electrical interconnects. Long chains of devices may be assem-bled by rolling out of plane. Figure 2 shows a smaller collection, assembled by folding three actuator triplets onto one another. Actuation of the collection is contingent on the manufacturing of functional thin-film PZT."
Thermal Ink Jet Printing of Lead Zirconate Titanate Thin Films,"The ferromagnetic and piezoelectric properties of ceramic lead zirconate titanate (PZT) thin films have made PZT an appealing choice for micro-sensors and actuators. Significant work has been done integrating PZT with standard MEMS processes, including the development of PZT sol-gels for spin coating [1-2]. Crack-ing is often a problem with PZT spin coating due to the brittle nature of the films coupled with the thermal strain experienced during annealing. This propensity for cracking limits the overall thickness deposited and the size out of plane features over which PZT can be reliably coated. Furthermore, spin coating requires a large volume of the expensive PZT precursor solution. We pro-pose thermal ink-jet printing of a modified PZT sol-gel as a new method of depositing PZT films for MEMS applications. Pre-liminary work has shown ink jetting to be a reliable method for depositing PZT films of the correct thickness for MEMS applica-tions and that annealed films can crystallize into the piezoelectric perovskite phase using the same thermal process developed for spin-coated PZT (see Figure 1) [3]. The goal of this research is to develop a deposition process that will enable reliable manufactur-ing of high-quality PZT films with greater deposition flexibility and lower material costs than spin coating. Thermal ink jetting technology supports a wide range of ink viscosities and solid particle contents. The ink composition can therefore be adjusted to control both the contact angle of solution with the substrate (1000Å Pt/ 200Å Ti) and the as-deposited film thickness. This flexibility allows for the deposition of films with thickness and uniformity that are acceptable for the fabrication of piezoelectric devices (see Figure 2). Multiple layers can be depos-ited to attain the thickness as needed. Currently, annealed films have been prepared as thick as 0.5 µm, corresponding to an as deposited thickness of approximately 1 µm. This is comparable to the current limit of standard spin-coated PZT sol-gel processed; printing of thicker films is under investigation."
Piezoelectric Micro-power-penerator: MEMS Energy-harvesting Device for Self-powered Wireless corrosion-monitoring System,"A novel thin-film, lead zirconate titanate Pb(Zr,Ti)O3 (PZT), energy-harvesting MEMS device is being developed for autono-mous wireless monitoring systems. It is designed to harvest en-ergy from parasitic vibrational energy sources and convert it to electrical energy via the piezoelectric effect. We envision that harvesting parasitic energy from the vortex-induced vibration of the oil pipelines will deploy a massive number of microsensors along the hundreds of miles of pipeline in very cold and remote areas. The proposed system consists of a corrosion sensor, a ra-dio transceiver, a microcontroller, a power management module, and a piezoelectric micro power generator (PMPG) to supply the needed power of the system without replacing batteries. The new pie-shaped design for the harvester (about a size of a nickel) has a radical departure from previous design concepts. This energy harvester design can be regarded as revolutionary as the first self-rectifying piezoelectric power generator. The new design avoids the high Q resonance, which is also a big change from previous designs. This will enable more robust power gen-eration even if the frequency spectrum of the source vibration varies unexpectedly. Furthermore, the beam shape is optimized to achieve uniform allowable strain throughout the PZT layer. Cur-rently, the first prototype, which is shown schematically, is being fabricated at MTL."
Lateral-line-inspired MEMS-array Pressure Sensing for Passive Underwater navigation,"A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the lateral line sensory organ in fish, which enable some species to form three-dimensional maps of their surroundings [1-2]. The canal subsystem of the organ can be described as an array of pressure sensors [3]. Interpreting the spatial pressure gradients allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby objects [2]. It also aids schooling [5]. Similarly, by measuring pressure variations on a vehicle surface, an engineered dense pressure sensor array allows the identifica-tion and location of obstacles for navigation (Figure 1). We are demonstrating proof-of-concept by fabricating such MEMS pressure sensors by using KOH etching techniques on SOI wafers to construct strain-gauge diaphragms.The system consists of arrays of hundreds of pressure sensors spaced about 2 mm apart on etched silicon and Pyrex wafers. The sensors are arranged over a surface in various configurations (Fig-ure 2). The target pressure resolution for a sensor is 1 Pa, which corresponds to the noiseless disturbance created by the presence of a 0.1-m-radius cylinder in a flow of 0.5 m/s at a distance of 1.5 m. A key feature of a sensor is the flexible diaphragm, which is a thin (20 µm) layer of silicon attached at the edges to a silicon cavity. The strain on the diaphragm due to pressure differences across the diaphragm is measured. At this stage, the individual MEMS pressure sensors are being constructed and tested.In parallel to the construction of a sensor array, techniques are being developed to interpret the signals from a dense pressure ar-ray by detecting and characterizing wake structures such as vorti-ces and building a library of pressure distributions corresponding to basic flow obstructions. In order to develop these algorithms, experiments are being performed on coarse arrays of commercial pressure sensors"
Fabrication of a Fully-integrated Multiwatt µTurboGenerator,"There is a need for compact, high-performance power sources that can outperform the energy density of modern batteries for use in portable electronics, autonomous sensors, robotics, andsensors, robotics, and, robotics, and other applications. Building upon the results presented in [1], the current research is aimed at fabricating a fully-integrated, multiwatt micro turbogenerator on silicon that can produce 10 micro turbogenerator on silicon that can produce 10 W DC output power (Figure 1). One of the main challenges in-in-volves the seamless integration between silicon and the magnetic the seamless integration between silicon and the magnetic components required to generate power. The generator requires a NiFe soft magnetic back iron and laminated stator for flux redirection as well as NdFeB permanent magnet pieces to serve as flux sources (Figure 2). In addition, copper windings must be fabricated above the laminated stator to couple to the alternating flux in order to extract electrical power from the machine.Great strides have been made in the past year to quantify the requirements on the magnet pieces that will go into the rotor housing. Manufacturing accuracy of the pieces is critical because variations in the magnet geometries create an overall rotor imbalance, which can cause the rotor to crash during transcritical operation. A procedure in which the gaps around in which the gaps around the gaps around the magnet pieces are filled with solder and then polished back using chemical-mechanical planarization has been developed; this process can reduce the effective imbalance of the rotor by anan order of magnitude.The assembly and packaging procedure for the turbogenerator is also critical because the embedded permanent magnets cannot withstand temperatures much above 150 oC. This temperature restriction rules out the use of fusion bonding for the final die-level assembly after rotor insertion. Based on results presented by Choe, et al. [2], an eutectic In-Sn bonding scheme that requires only 140 oC has been researched. In this scheme, Cr/Au is de-posited on one bonding surface and Cr/Sn/In/Au is deposited on the other surface; both depositions are done using an e-beam evaporator without breaking vacuum. By painting no-clean flux on both surfaces and compressing the dies together on a hot plate, we form the bond."
A Portable Power Source Based on MEMS and carbon nanotubes,"There is a growing need for small, lightweight, reliable, highly efficient and fully rechargeable portable power sources. The focus of this project is the design and modeling of a system in which energy is stored in the elastic deformation of carbon nanotube (CNT)-based springs. The CNTs are coupled to a MEMS electric generator. When the CNT deformation is released, the stored energy actuates the generator, which then converts the energy into electricity. The MEMS generator may be operated in reverse, as a motor, in order to wind the CNT springs and recharge the system. Alternatively, the stored elastic energy may be used to supply a mechanical load directly. This project is motivated by recent research into the mechanical properties of CNTs. The CNTs have a high stiffness, low defect density, and a consequently high yield strain that enables them to store elastic energy with significantly greater energy density than typical spring materials such as high-carbon steel. Models suggest that CNTs can be reversibly stretched by up to 15% [1]; lower strains of up to 6% have been demonstrated experimen-tally to date [2-3].This type of system offers several important potential advantages. First, due to CNTs’ high strength, high flexibility, and low defect density, they can store energy at very high energy density. Con-sidering just the CNT-based spring itself, the energy density of an array of CNTs stretched to a reversible 15% strain is about 1500 W-hr/kg, about ten times the energy density of Li-ion batteries. The energy density of the final system will be lower because of the finite conversion efficiency of the generator and the weight of both the supporting structure and the generator hardware. In addition, because energy storage in the CNT system is based on stretching chemical bonds rather than breaking and reforming chemical bonds as in batteries, the CNT-MEMS generator sys-tem has the potential to operate at higher power densities, un-der harsher conditions, to deeper discharge levels, and through a greater number of charge-discharge cycles than a chemical bat-tery.The system architecture consists of a CNT-based energy storage element, an energy release rate mechanism, and a MEMS gen-erator. This project is examining and modeling different varia-tions on this system architecture that incorporate different modes of deformation of the CNT-based energy storage element, vari-ous types of generators, different types of coupling between the storage element and the generator, and different size scales for the various components. One conceptual example is illustrated below, in which the axial relaxation of an axially-stretched CNT-based storage element is converted to rotational motion of a wheel. The wheel is coupled to a piezoelectric generator through a mechanism that regulates the rate of energy release, much as in a mechanical watch."
A MEMS Steam Generator,"Previous work [1] has shown that MEMS technology has signifi-cant potential to create more compact, higher- performing hard-ware for chemical oxygen iodine lasers (COIL). In COILs, the laser medium is a flowing gas that must be pumped through the system at high mass flow rates to ensure proper system opera-tion. As a result, compact pumps with high pumping rates are a key element of the COIL system. One promising component of a MEMS COIL system would be a compact MEMS pump system in which the pump action is provided in part by micro steam ejectors and the micro steam generators that supply their driving fluid. This work describes the design and modeling of a microscale hydrogen peroxide (H2O2)-based steam generator to supply such a MEMS pump system. Hydrogen peroxide is a read-ily available, inexpensive, nontoxic, and environmentally friendly fluid that may be catalytically decomposed to form steam. Steam generation by the catalytic decomposition of H2O2 also finds oth-er important applications in the MEMS field beyond pumping, particularly in the area of thrust generation. Compared to their macroscale counterparts, MEMS H2O2-based steam generators offer better performance, notably improved mixing, and higher uniformity due to the absence of moving parts [2-3].A complete MEMS steam generator consists of a peroxide res-ervoir, an injector, a reactor, and a converging-diverging nozzle to accelerate the exiting flow, as shown in Figure 1. Initial work focuses on the design of the reactor and nozzle. Liquid H2O2 in aqueous solution is injected into the reactor, where it decom-poses into steam and oxygen gas upon contact with the catalyst. A continuous supply of homogeneous liquid catalyst is used, as it avoids the aging problem typically exhibited by heterogeneous catalysts [4]. The gaseous products of the reaction are then accel-erated to supersonic velocities through the converging-diverging nozzle. The work to date indicates that a MEMS steam generator designed to minimize heat transfer to the environment can pro-vide complete, compact, uniform decomposition of peroxide into steam suitable to drive a MEMS pumping system."
Microscale Singlet Oxygen Generator for MEMS-based cOIL Lasers,"Conventional chemical oxygen iodine lasers (COIL) offer several important advantages for materials processing, including short wavelength (1.3 µm) and high power. However, COIL lasers typi-cally employ large hardware and use reactants relatively ineffi-ciently. This project is creating an alternative approach called microCOIL. In microCOIL, most conventional components are replaced by a set of silicon MEMS devices that offer smaller hardware and improved performance. A complete microCOIL system includes microchemical reactors, microscale supersonic nozzles, and micropumps. System models incorporating all of these elements predict significant performance advantages in the microCOIL approach [1].Initial work is focused on the design, microfabrication, and demonstration of a chip-scale singlet oxygen generator (SOG), a microchemical reactor that generates singlet delta oxygen gas to power the laser. Given the extensive experience with micro-chemical reactors over the last decade [2], it is not surprising that a microSOG would offer a significant performance gain over large-scale systems. The gain stems from basic physical scaling; sur-face-to-volume ratio increases as the size scale is reduced, which enables improved mixing and heat transfer. The SOG chip being demonstrated in this project employs an array of microstructured packed-bed reaction channels interspersed with microscale cool-ing channels for efficient heat removal [3]. To date the device has produced oxygen concentrations of 1017 cm-3, yields approaching 80% and molar flowrates in excess of 600x10-4 moles/L/sec [4]. The yield and molar flowrates indicate a significant improvement over the macroscale SOG designs."
An Integrated Microelectronic Device for Label-free nucleic Acid Amplification and Detection,"While there have been extensive advances in miniaturized poly-merase chain reaction (PCR) systems, progress on integrated mi-crofabricated readout mechanisms has been rather limited, and most systems rely on off-chip optical detection modules to mea-sure the final product. Existing optical detection platforms typi-cally include CCD cameras, photodiodes, and photomultiplier tubes. While such hardware has adequate sensitivity for detecting PCR products in sample volumes significantly lower than that of bench-top systems, most are difficult to miniaturize and integrate into a compact analytical system. For example, some portable systems incorporating external LEDs and photodetectors can weigh between 1 kg and 4 kg each. To address these limitations, several groups have successfully embedded photodetectors within integrated PCR platforms. However, these devices still rely on external excitation sources.To address this limitation, we have developed an integrated mi-croelectronic device for amplification and label-free detection of nucleic acids (Figure 1) [1]. Amplification by PCR is achieved with on-chip metal resistive heaters, temperature sensors, and microfluidic valves. We demonstrate a rapid thermocycling with rates of up to 50°C/s and a PCR product yield equivalent to that of a bench-top system. Amplicons within the PCR product are detected by their intrinsic charge with a silicon field-effect sensor. Similar to existing optical approaches with intercalators such as SYBR Green, our sensing approach can directly detect standard double-stranded PCR products while in contrast our sensor oc-cupies a micron-scale footprint, dissipates only nano-watt power during operation, and does not require labeling reagents. By com-bining amplification and detection on the same device, we show that the presence or absence of a particular DNA sequence can be determined by converting the analog surface potential output of the field-effect sensor to a simple digital true/false readout."
Monitoring of Heparin and its Low Molecular Weight Analogs by Silicon Field Effect,"Heparin is a highly sulfated glycosaminoglycan that is used as an important clinical anticoagulant. Monitoring and control of the heparin level in a patient’s blood during and after surgery is essen-tial, but current clinical methods are limited to indirect and off-line assays. We have developed a silicon field-effect sensor for di-rect detection of heparin by its intrinsic negative charge [1]. The sensor consists of a simple microfabricated electrolyte-insulator-silicon (EIS) structure encapsulated within microfluidic channels (Figure 1). As heparin-specific surface probes, we used the clinical heparin antagonist protamine or the physiological partner an-tithrombin III. The dose-response curves in 10% PBS revealed a detection limit of 0.001 U/ml, which is orders of magnitude lower than clinically relevant concentrations. We also detected heparin-based drugs, such as the low-molecular-weight heparin enoxaparin (Lovenox®) and the synthetic pentasaccharide hepa-rin analog fondaparinux (Arixtra®) (Figure 2), which cannot be monitored by the existing near-patient clinical methods. We de-monstrated the specificity of the antithrombin III functionalized sensor for the physiologically active pentasaccharide sequence. As a validation, we showed correlation of our measurements to those from a colorimetric assay for heparin-mediated anti-Xa activity. These results demonstrate that silicon field-effect sensors could be used in the clinic for routine monitoring and maintenance of therapeutic levels of heparin and heparin-based drugs and in the laboratory for quantitation of total amount and specific epitopes of heparin and other glycosaminoglycans."
"Weighing of Biomolecules, Single cells and Single nanoparticles in Fluid","Nanomechanical resonators enable the measurement of mass with extraordinary sensitivity. Previously, samples as light as 7 zepto-grams (1 zg = 10-21 g) have been weighed in vacuum, and proton-level resolution seems to be within reach. Resolving small mass changes requires the resonator to be light and to ring at a very pure tone—that is, with a high quality factor. In solution, viscosity severely degrades both of these characteristics, thus preventing many applications in nanotechnology and the life sciences where fluid is required. Although the resonant structure can be designed to minimize viscous loss, resolution is still substantially degraded when compared to measurements made in air or vacuum. An entirely different approach eliminates viscous damping by placing the solution inside a hollow resonator that is surrounded by vac-uum (Figure 1). We have recently demonstrated that suspended microchannel resonators can weigh single nanoparticles (Figures 2), single bacterial cells. and sub-monolayers of adsorbed proteins in water with sub-femtogram resolution (1 Hz bandwidth). Cen-tral to these results is our observation that viscous loss due to the fluid is negligible compared to the intrinsic damping of our sili-con crystal resonator. The combination of the low resonator mass (100 ng) and high quality factor (15,000) enables an improvement in mass resolution of six orders of magnitude over a high-end commercial quartz crystal microbalance [1]. This gives access to intriguing applications, such as mass-based flow cytometry, the direct detection of pathogens, or the non-optical sizing and mass density measurement of colloidal particles."
Integrated System for cancer Biomarker Detection,"There is evidence to suggest that the next generation of cancer-screening tests may employ not just one, but a small panel of less than ten biomarkers that together add statistical power to the de-tection of specific cancers. While immunoassays such as ELISA are well established for detection of antigen-based biomarkers, the fidelity of the assay is governed by the disassociation constant, Kd, of the antibody-antigen complex. If the antigen concentra-tion is significantly below Kd, then the binding kinetics are slow and readout precision of the antigen-antibody complex can be degraded by noise.We propose a general approach for improving the performance of ligand-receptor assays. The approach is based on a nano-fluidic device that controllably concentrates a dilute sample and an ultra-sensitive suspended microchannel resonant mass sensor that detects specific biomarkers within the concentrate. Since the amplification (or gain) of the concentrator is adjustable, the dynamic range and detection limit of the immunoassay can be governed by the properties of the concentrator and not Kd. Since the integrated concentration/detection system is batch-fabricated by conventional foundry-level processing techniques, the cost per device could potentially be less than ten dollars.Over the past year, we have fabricated the first generation of in-tegrated systems (Figure 1). The devices appear to be functional based on initial visual inspections. We are currently validating the performance of the system by using quantum dots for a calibration assay. We are also in the process of validating the performance of the concentrator and mass sensor (as individual components) with prostate-specific antigen so that we can make comparisons to existing methods in terms of sensitivity and selectivity."
"Passive Microwave Transponders for Passive, Real-time, and High-sample-rate Localization","Passive surface acoustic wave (SAW) transponders have been used for RFID applications because of their zero-power and long-range (100m) capabilities. This work presents current research into utilizing SAW transponders for localizing objects. The SAW transponders have many advantages over existing localization solutions, including small size (mm x mm), zero-power, high-ac-curacy, longer range (100m), and kilohertz update rates. Unhin-dered localization of objects is desirable for many applications, from human computer interaction to product tracking to security. The SAW transponders offer improvements to existing solutions for asset tracking, location of lost articles, ubiquitous computing, tracking of people with special needs or prisoners, workers in hazardous situations, human machine interfaces, virtual training environments, security, location of short-range mobile sensors, and biomedical research. The goal of this project is to prototype this tracking system and evaluate the feasibility of a commercial system.Multiple RADAR measurement stations use phase-encoded chirps to selectively track individual SAW transponders by tri-angulation of range and/or angle measurements. Update rates on the order of 10kHz with accuracies better than 10cm3 are conceivable. A 300-nm deep-uv contact-mask lithography nano-fabrication process to create these SAW devices is under develop-ment. Figure 1 shows the block diagram of the electronic test setup that is being used to characterize devices. Figure 2 shows a micrograph of a few of our fabricated devices. Recent results from this investigation will be presented, including the character-ization of our first devices."
Macroscopic Interfaces to Parallel Integrated Bioreactor Arrays,"Macroscopic fluidic interfaces are important for improving the usability of microfluidic devices. For example, in our previously developed parallel integrated bioreactor arrays [1], two needle punctures were required to fill each fluidic reservoir, one for fluid injection using a syringe and another needle to vent the air dis-placed by the injected fluid. While suitable for internal labora-tory use, such an inconvenient fluid injection procedure impedes the adoption of this new bioreactor technology. We have developed a fluid injection port that automatically vents the displaced air and is compatible with standard laboratory pi-pette tips. The principle of operation is shown in Figure 1. On opposite sides of each fluid reservoir (Figure 1a), there is a fluid injection channel and a vent channel. Both of these channels ter-minate in vias that connect the channels to the seat of the pipette interface nipple. The pipette-interface-nipple (Figure 1e) is an elastomer structure, which, when compressed by its housing (Fig-ure 1c), seals closed both vias such that fluid can neither escape nor enter the fluid reservoir. When a pipette tip is inserted into the interface-nipple, it is deformed and allows air to escape from the vent port (Figure 1f). Meanwhile, the inserted pipette tip seals to the via connected to the fluid injection channel and fluid can be injected into the reservoir. The particular bow-tie shape of the pipette-interface-nipple was chosen such that when it is inserted into a rectangular housing, sufficient compressive force would seal the central slit closed while also allowing space for the nipple to expand upon insertion of the pipette tip. Fabricated devices exhibited good operating characteristics and provided a seal against a greater-than-10psi back pressure."
MIT-OSU-HP Focus center on non-lithographic Technologies for MEMS and nEMS,"This newly formed center is part of an overall set of centers on MEMS/NEMS fundamentals supported by DARPA. The MIT-OSU-HP Focus Center aims to develop new methods for fabrica-tion of MEMS and NEMS that do not use conventional litho-graphic methods. The Center leverages the leading expertise of MIT and OSU in MEMS and printed devices, with the printing expertise of HP. The focus center is organized into four primary areas: tools, materials and devices, circuits, and demonstration systems.In the area of tools, we are leveraging the existing thermal inkjet (TIJ) technology of HP and augmenting it with specific additional features, which expand the palette of available materials for print-ing. We are developing materials and devices over a broad spec-trum from active materials, photonic and electronic materials, to mechanical materials. In the circuits area, we are studying the behavior of the devices that can be realized in this technology with the goal of developing novel circuit architectures. Lastly, we intend to build several “demonstration” systems that effectively communicate the power of the new technologies that will emerge from this center."
A Micromachined Printhead for the Evaporative Printing of Organic Materials at Ambient Pressure,"Organic optoelectronic devices are promising for many commer-cial applications if methods for fabricating them on large-area, low-cost substrates become available. Our project investigates the use of MEMS in the direct patterning of materials needed for such devices. By depositing the materials directly from the gas phase, without the liquid phase coming in contact with the sub-strate, we aim at avoiding the limitations due to inkjet printing of such materials.We developed a MEMS-enabled technique for evaporative print-ing of organic materials. This technique does not require a vac-uum ambient, has a fast printing rate (1 kHz), and can be scaled up to an array of individually addressable nozzles. The MEMS printhead comports a microporous layer with integrated heaters for local evaporation of the materials. Figure 1 shows the micro-fabricated device: an array of 2 micron pores and an integrated thin film platinum heater sit in the center of a silicon membrane. The material to be printed is delivered to the porous region in liquid or gas phase and deposits inside the pores (see Figure 1, top left). The integrated heater then heats up the porous area (see Fig-ure 2, top) and the material is re-evaporated from the pores onto the substrate. The main limitation of this printhead is the failure of the thin-film platinum heater at temperatures above 800°C (see Figure 2 bottom).This printhead was used, together with inkjet technology for the delivery of material to the pores, to print molecular organic semi-conductors (see other abstract in this volume). Our technique enables printing of organic optoelectronics over large areas and can be used to print on a variety of substrates, does not require a vacuum ambient, and thus could enable low-cost printing of optoelectronics."
Surface Micromachining Processes using non-lithographic Technologies,"Conventional MEMS fabrication relies heavily on planar lithog-raphy and IC technology. While these techniques are well-suited for relatively flat devices such as the semiconductors, they are drastically limited in the design and fabrication of three-dimen-sional devices such as MEMS. From a commercial viewpoint, the semiconductor paradigm is also a poor fit for MEMS because the lower volume and demands make it more difficult to offset the high production costs. Ridding MEMS fabrication of its reliance on such techniques may introduce several advantages, namely a wider base of substrate materials and decreased costs.Our project investigates severing MEMS fabrication from the semiconductor paradigm via non-lithographic technologies. We have previously shown how MEMS can be used for the direct patterning of small molecular organics [1]. Using similar con-cepts, we intend to show that surface micromachining can also be achieved.The first stage of the project is to directly pattern a structural layer over a spacer and successfully release a cantilever. We have successfully patterned metal silver over various spacer materials, including polyethylene glycol (PEG), polyvinyl acetate (PVA), and UNITY™ sacrificial polymer, and we are currently working on the release process. This technique will ultimately be used to con-struct simple structures, such as cantilevers and bridges, to test the structural material’s mechanical properties. The next stage of this project will consist of using this process to fabricate cantile-vers and integrate them with other non-lithographic techniques to fabricate an accelerometer. Subsequent stages will consist of creating a library of non-lithographic processes so that entire MEMS devices can be fabricated without the use lithography."
Micromechanical Actuators for Insect Flight Mechanics,"This project aims to develop MEMS actuators to aid in the study of insect flight mechanics. Specifically, we are developing actua-tors that can stimulate the antennae of the crepuscular hawk moth Manduca Sexta. The possible mechanosensory function of antennae as airflow sensors has been suggested [1], and recent discoveries of our collaborators reveal that mechanosensory in-put from the antennae of flying moths serves a similar role to that of the hind wings of two-winged insects, detecting Coriolis forces and thereby mediating flight stability during maneuvers [2]. Early evidence suggests that mechanical stimulus of the antennae may enable flight control. In addition, the crepuscular hawk moth Manduca Sexta has a wide wingspan (~110 mm) and is capable of carrying at least one quarter of its own weight. Thus, studying the flight of M. Sexta by attachment of microsystems seems plausible. The goal of our project is to design and fabricate micromechani-cal actuators, which will be mounted onto the moth antennae (Figure 1). Our collaborators will study the flight control mecha-nism by mechanical stimulation. Our first step is to fabricate “dummy” silicon rings for our biolo-gist collaborators for implant experiments. The diameters along the antenna vary from tip to base, being thickest in the middle. As a result, in order to prevent the ring’s being thrown off, the mounting of the silicon ring onto the base cannot be as simple as pushing it from the tip with a large inner hole. On the other hand, the sizes of the antennae vary from moth to moth. Two-piece construction was designed and fabricated to be like a “zip strip” to meet the mounting requirements (Figure 2). Future work will focus on refining the design and fabrication of the mounting kit and integrating actuators into it. To generate adequate displace-ment, strain amplification will be needed, such as reported by Conway, et al. [3]."
MEMS Micro-vacuum Pump for Portable Gas Analyzers,"There are many advantages to miniaturizing systems for chemi-cal and biological analysis. Recent interest in this area has led to the creation of several research programs, including a Micro Gas Analyzer (MGA) project at MIT. The goal of this project is to develop an inexpensive, portable, real-time, and low-power approach for detecting chemical and biological agents. Elements entering the MGA are first ionized, then filtered by a quadrupole array, and sensed using an electrometer. A key component en-abling the entire process is a MEMS vacuum pump, responsible for routing the gas through the MGA and increasing the mean free path of the ionized particles so that they can be accurately detected.There has been a great deal of research done over the past 30 years in the area of micro pumping devices [1, 2]. We are cur-rently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven active-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We have constructed an accurate compressible mass flow model for the air flow [4] as well as a nonlinear plate deformation model for the stresses experienced by the pump parts [5]. Using these models, we have defined a process flow and fabricated three generations of the MEMS vacuum pump over the past year and are currently working on the fourth. A schematic of the pump is shown in Figure 1. For ease in testing we have initially fabricated only Layers 1-3 and have constructed a testing platform that, under full computer control, drives the pistons and monitors the mass flows and pressures at the ports of the device. The lessons learned from the first three generations of the pump have led to numerous improvements. Every step from the modeling to the etching and bonding to the testing has been modified and improved along the way. The most recent third generation pump test data is shown in Figure 2. Figure 2a shows the pressure versus flow rate characteristics of the pump; note that the data compares very well with models. Figure 2b shows the output flow rate versus actuation characteristics of the pump. Notice that the flow goes to zero each time the piston is actuated upwards (red bar). All three pistons demonstrated similar perfor-mance illustrating a pump with fully functioning pistons and teth-ers. Next, we hope to characterize the pumping characteristics of this and the upcoming fourth-generation pumps."
Microfabricated Electrodes for Solid Oxide Fuel cells,"The solid oxide fuel cell (SOFC) is an energy-conversion device that produces electricity directly through the electrochemical re-action of hydrogen (H2) and oxygen (O2). The SOFC also allows utilization of hydrocarbons, such as methane (CH4), via internal reforming or direct electrochemical oxidation, giving these sys-tems the flexibility of using a variety of commercially available fuels. The high energy-density of hydrocarbon fuels makes the SOFC attractive for large-scale stationary systems and as a re-placement to batteries for powering portable electronic devices. Standard SOFCs operate in the temperature range of 800°C-1000°C and have an open circuit voltage of approximately 1-volt. The most common SOFC materials are yttria-stabilized zirconia (YSZ) for the electrolyte, strontium-doped lanthanum manganite (LSM) for the cathode and nickel (Ni)-YSZ for the anode. Al-though elevated temperature operation allows the use of non-noble metal catalysts and excellent high-grade heat exhaust to be used for additional power generating cycles, there are numer-ous advantages to lowering the SOFC operating temperature to around 600°C. Benefits such as lower thermal stress, reduced cell degradation, utilization of metallic components, and shorter startup times are a few. However, at these lower temperatures the poor electrochemical activity of the electrodes, in particular the LSM cathode, leads to unacceptable voltage losses that lower the efficiency and performance of the SOFC. Oxygen reduction mechanisms on the perovskite material LaxSr1-xMnO3-d (LSM) has been widely studied; however, no final con-clusion on the molecular level mechanisms for oxygen reduction has been made. To probe the oxygen reduction reaction, we fabricate electrodes with precise geometries (50-200 µm) using thin-film deposition techniques (sputtering and laser ablation) and subsequent photolithography to investigate the fundamen-tal electrode mechanisms and rate-determining reactions. The electrochemical impedance spectroscopy (EIS) response of a La1-xSrxMnO3-d (LSM) microelectrode on 8YSZ is then analyzed as a function of geometry and temperature using a microprobe sta-tion equipped with a high temperature stage, as Figure 1 shows. Our preliminary EIS results1 shown in Figure 2a show at least four distinct reaction processes for oxygen reduction on LSM/8YSZ: (i) ion transport in 8YSZ with average activation energy (Ea) of 1.16±0.02eV, (ii) surface diffusion on LSM with Ea rang-ing from 1.34±0.05eV to 1.65±0.03eV, (iii) at least one surface chemical process on LSM with Ea ranging from 1.71±0.02eV to 1.88±0.02eV and an average capacitance 3.4x10-4 F/cm2, and (iv) a mixed bulk/TPB charge transfer process with Ea ranging from 2.42±0.02eV to 3.05±0.03eV and an average capacitance of 3.2x10-3 F/cm2. The overall oxygen reduction process is il-lustrated in Figure 2b, with the rate-limiting reaction for ORR found to be from mixed bulk/TPB charge transfer processes be-low 700°C and shifts to surface chemical reactions above 700°C."
A MEMS-relay for Power Applications,"Contact travel and heat dissipation are important requirements of electrical power switching devices such as MEMS-relays and MEMS-switches. Whereas low-power MEMS-based RF switches have been vigorously studied, few studies have been reported on high-power MEMS-relays. This paper presents a MEMS-relay for power applications. The device is capable of make-break switching; has large contact travel, on the order of 10’s of µm; and has low contact resistance, on the order of 120 mΩ. Testing has demonstrated current carrying capacity on the order of sev-eral amperes and hot-switching of inductive loads, on the order of 10mH, without performance degradation.The MEMS-relay, shown in Figure 1a, is bulk micromachined in (100) silicon and bonded to a glass substrate. Anisotropic etching is used to fabricate the oblique and parallel (111) contact surfaces, having nanometer-scale surface roughness [1]. Figure 1b shows a cross section of the open fabricated contacts. An offset between the wafer-top and the wafer-bottom KOH masks produces the contact geometry shown. The silicon contact metal surfaces are created by evaporation and electroplating with a conductive film, shown in Figure 1c. A thermal oxide layer provides insulation between the actuators and the contacts. Deep reactive ion-etch-ing (DRIE) is used to pattern a parallelogram-flexure compliant mechanism and a pair of rolling-point “zipper” electrostatic ac-tuators [2]. Nested masks are used to pattern both wafer-through etches. Figure 2 illustrates the process used to fabricate the de-vice."
"A Silicon-etched, Electrical-contact Tester","We are developing a bulk micromachined contact tester to inves-tigate the electro-tribological performance of micro- and nano- structured planar electrical contacts [1]. The test device features parallel, planar, nanometer-scale surface roughness contacts etched in silicon coated with thin conductive films. Contacts used in microsystems, probes and interconnects are subject to heat dis-sipation and to electro-mechanical tribological effects. With an understanding of how nanoscale surface and subsurface material structure affect electrical contact resistance and mechanical con-tact wear, a deterministic manufacturing process could be devel-oped to design electrical contacts from crystalline plane surfaces as potential high performance contacts for MEMS devices and related applications. The microfabricated contact tester, shown in Figure 1 and in Figure 2, consists of a pair of parallel planar contact surfaces with nanometer roughness patterned onto two (100) Si substrates. Anisotropic etching is used on one of the substrates to create a membrane that serves as a compliant mechanism for the con-tact tester. A thin conductive film, i.e., Au, is patterned onto the contacts in a Kelvin configuration. The two-piece tester archi-tecture allows for inspection of the contacts before, during, or after testing without destruction of the test device. In one em-bodiment of the tester, a quasi-kinematic coupling enables the alignment between the substrates while providing the initial gap between the contacts. Similar quasi-kinematic designs fabricated in silicon substrates have reported repeatability on the order of 1 micrometer [2]. In a second embodiment of the MEMS-tester a patterned oxide film is used to provide the initial space between the contacts. The tester will be loaded using a commercial na-noindenter to bring the surfaces into contact as contact resistance is measured as a function of the force."
Microfluidic Studies of Biological cell Deformability and Rheology,"It is well known that many hereditary or infectious diseases, as well as certain types of cancer, produce alterations in mechanical properties of human cells. In certain diseases, such as malaria, infected cells exhibit reduced deformability and increased cy-toadherance. Such changes alter the circulatory response of red blood cells (RBCs) and limit physiological responses to such dis-eases, such as splenic clearance of parasitized RBCs, and restrict circulation of RBCs in the microvasculature. In other diseases, such as pancreatic cancer under certain conditions, cancerous cells may exhibit enhanced deformability, which may contribute to an increased probability of metastasis.Several projects in the Suresh research group aim to utilize micro-fabricated structures to experimentally evaluate the circulatory re-sponse of diseased human cells. In these studies, microfabricated channels of polydimethylsiloxane (PDMS) are used in conjunc-tion with a fluidic system and a high-speed camera to quantify the biorheological behavior of cells under different conditions. In the case of diseases involving RBCs, cells are made to pass through a narrow (~3 micrometer square) channel under a known pressure differential. During this process, shown in Figure 1, the average velocity and characteristic entrance and exit and shape recovery times are indicative of the overall biorheological response of the cell during microcirculation. Current results indicate large dif-ferences in rheological behavior of cells of different ages and in-cubation times. In addition, initial results from Plasmodium fal-ciparum parasitized RBCs indicate a possible effect of exported proteins from the malaria-inducing parasite to the surface of the RBC membrane on the rheological behavior of RBCs. Similar experiments are also being conducted on pancreatic cancer cells (Panc-1), as seen in Figure 2. In these cells, the biorheology of cells is assessed in environments similar to those experienced dur-ing metastasis. Future work with these systems will aim to further identify particular proteins or biochemical interactions that affect the biorheolgical and circulatory behavior of diseased cells with the aim of developing enhanced understanding of the mecha-nisms of disease progression and possible avenues for treatment."
Phase change Materials for Actuation in MEMS,"Phase change materials (Sb and Te alloys) are used for optical data storage in commercial phase change memories, such as rewrit-able compact discs (CD±RW) and rewritable digital video disks (DVD±RW, DVD-RAM) [1]. Recently, they have also shown high potential for the development of phase change random ac-cess memories (PC-RAMs or PRAMs), which might replace flash memories in the future [2]. In this project, thin films of phase change materials are systematically analyzed with regard to their transformation behavior under laser-induced amorphization and crystallization. The goal of this project is to gain a better under-standing of the relationship among the laser parameters, the ma-terial-specific transformation kinetics, and the involved volume changes (and associated mechanical stresses) over a wide range of alloy compositions.The approach to pursuing this goal is to use microfabricated SiN cantilevers as substrates for thin film deposition: The SiN cantile-vers are manufactured by chemical vapor deposition of low-stress SiN on Si wafers, patterning the SiN film using optical lithogra-phy and revealing the cantilevers using dry etching and wet etch-ing. Thin films of phase change materials are subsequently sput-ter-deposited on these SiN cantilevers and are locally switched by laser heating from the amorphous to the crystalline phase (and vice versa). The associated stresses induce a cantilever bending, which is measured by optical microscopy and non-contact in-terferometry as a function of laser annealing parameters, laser quench rate and alloy composition (Figures 1 and 2). Additionally, amorphous films are hot-stage crystallized, which allows the study of the kinetics associated with the crystallization process as well as the force associated with the cantilever bending.The results of this project will help to increase the number of write-erase cycles and the data transfer rate in phase change memories and may lead to other applications of phase change materials in MEMS actuation."
Microfabricated Thin-film Electrolytes and Electrodes for Solid Oxide Fuel cells,"There is growing interest in the microfabrication of electrodes for solid oxide fuel cells (SOFCs) in microionic devices [1]. Recently, we reported the fabrication of Pt/(Zr,Y)O2 (YSZ) nanocomposite electrodes by reactive magnetron co-sputtering [2]. Use of X-ray diffraction and X-ray photoelectron spectroscopy (XPS) charac-terization show these composites to be a two-phase system with no change of oxidation state from the constituent compounds. Electrical characterization via impedance spectroscopy demon-strated promising electrochemical properties at low temperatures; an area-specific resistance of 500 Ω cm2 was achieved at 400ºC. To test whether microfabricated thin-film electrolytes may suffer from degradation due to grain boundaries acting as short-circuit-ing diffusion pathways, sputtered NiO diffusion source films were in-diffused along grain boundaries into nanocrystalline CeO2 thin films grown by pulsed laser deposition (PLD), at tempera-tures from 700-800oC. The diffusion profiles were measured by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) at the Institute for Physical Chemistry at RWTH Aachen Univer-sity, Germany. These SIMS spectra, shown in Figure 2, point to a single diffusion mechanism, believed to be grain boundary diffusion, at these relatively low temperatures. Further work to systematically determine the unique opportunities and challenges associated with microstructured SOFCs is currently underway."
Nanowire- and Microsphere-templated Gas Sensors,"Novel materials synthesis techniques were used to fabricate nano-structured and macroporous semiconducting metal oxide (SMO) films exhibiting exceptionally high sensitivity to reducing and oxi-dizing gases, as compared to conventionally prepared specimens. Increased sensitivity resulted from an elevated surface area and reduced specimen cross section. Several processing routes were pursued including electronspinning of semiconducting metal ox-ide (SMO) nanowires into a highly porous mat structure and mi-crosphere templating followed by pulsed laser deposition (PLD) of macroporous SMO material onto the microsphere templates.The TiO2/poly(vinyl acetate) composite nanofiber mats were electrospun onto interdigitated Pt electrode arrays, producing a mesh of 200-500 nm sheaths filled with ~10 nm thick single-crys-tal anatase fibrils. Testing in the presence of NO2 gas at 300°C demonstrated a minimum detection limit (MDL) of below 1 ppb1. Chemical and physical synthesis routes were combined to prepare macroporous CaCu3Ti4O12 and TiO2 thin films by PLD onto PMMA microsphere-templated substrates. Stable quasi-or-dered hollow hemispheres with diameter and wall thicknesses of 800 nm and 100 nm, respectively, were obtained (Figure 1). Cur-rent-voltage and impedance spectroscopy measurements point to the crucial role played by grain boundary barriers in controlling the electrical properties of these films. The macroporous CaCu-3Ti4O12 films exhibited a much superior H2 gas sensitivity (55ppm MDL) to non-templated films2 (Figure 2), while macroporous TiO2 films exhibit excellent NOx sensitivity. Studies are continu-ing to more carefully correlate sensor response with SMO micro-structure, morphology, and chemistry."
BioMEMS for control of the Stem cell Microenvironment,"The stem cell microenvironment is influenced by several factors including cell-media, cell-cell, and cell-matrix interactions. Al-though conventional cell-culture techniques have been success-ful, they offer poor control of the cellular microenvironment. To enhance traditional techniques, we have designed a microscale system to perform parallel cell culture on a chip while controlling the microenvironment in novel ways. To control cell-matrix and cell-cell interactions, we use cell pat-terning. We have developed a simple cell-patterning technique (Figure 1 upper) that can pattern single cells onto arbitrary sub-strates [1]. Using this technique, we patterned clusters of mouse embryonic stem cells (mESCs) with different numbers of cells in each cluster (Figure 1 lower). We have also developed methods for single-cell patterning using dielectrophoresis (DEP), which uses non-uniform AC electric fields to position cells on or be-tween electrodes [2].To control cell-media interactions, we have developed a microflu-idic device for culturing adherent cells over a logarithmic range of flow rates [3]. The device (Figure 2, left) controls flow rates via a network of geometrically-set fluidic resistances connected to a syringe-pump drive. We use microfluidic perfusion to explore the effects of continuous flow on the soluble microenvironment. We cultured mESCs in standard serum-containing media across a 2000× range of flow rates. On day 1, colony areas were roughly constant along the axis of perfusion, implying negligible nutrient depletion. However, by day 3, we observed a significant decrease in colony size along the axis of perfusion at mid-range flow rates (Figure 2, right). At higher flow rates, colonies were uniformly large along the axis of perfusion, implying that nutrient depletion was not significant above certain flow rates. This microfabricated system will serve as an enabling technology that can be used to control the cellular microenvironment in pre-cise and unique ways, allowing us to perform novel cell biology experiments at the microscale."
Microfabricated Devices for Sorting cells Using complex Phenotypes,"This research involves the development of numerous microfab-ricated sorting cytometer architectures for genetic screening of complex phenotypes in biological cells. Our various approaches combine the ability to observe and isolate individual mutant cells within surveyed populations. In this work we merge benefits of both microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities in a single platform. Biologists will leverage these new affordances to isolate cells on the basis of observed dynamic and/or intracellular responses, enabling novel avenues for population screening. Our most recent approach to image-based sorting, which comple-ments our earlier work, utilizes a microfabricated array of PDMS microwell structures positioned in the floor of a microfluidic flow chamber (Figure 1) [1-2]. These microwells capture and hold cells in place for microscopy-based imaging, and can be optimized to trap single cells. After inspecting the array using microscopy to determine cells of interest, we apply radiation pressure from an infrared (IR) laser diode to levitate target cells out of the wells and into a flow stream. Released cells can be collected downstream for further analysis. The interconnect-free architecture scales eas-ily; we have presently implemented trap arrays containing more than 10,000 sites.Manipulating live cells, irrespective of the technique, will certain-ly have some effect on cellular behavior and physiology. It is im-perative that we understand the effects of our sorting techniques (both optical and electrical) on cellular physiology over a range of operating conditions for two main reasons: (1) to determine whether there are any gross effects (such as viability and changes in proliferation), and (2) to determine whether there are more subtle effects that alter complex phenotypes of interest. To this end we are designing a microfabricated device to perform electri-cal and optical “dose responses” to determine optimal regions of operation and using fluorescence-based stress reporter cell lines as sensors of physiological state (Figure 2).Figure 1: Microwell-based optical sorting. (A) Schematic sort based on fluorescence localization (nucleus vs. cytoplasm). Laser levitates target cells into the flow stream for downstream collection. (B) Section of well array. We remove a membrane-stained cell from a population of purely nuclear-stained cells. We used an argon laser here; we now use IR-beams to mitigate cell-damage concerns.p Figure 2: Sensing physiological state. (A) Green fluorescent protein (GFP) based stress reporter cell line which shows significant increase in fluorescence intensity, compared to control (B) after a 30-minute heat shock at 44ºC and a 14-hour recovery at 37ºC. Such a live cell sensor will allow us to perform fluorescence-based cell health assays on thousands of cells. Scale bars 20 µm."
Combined Microfluidic/Dielectrophoretic Microorganism Concentrators,"This project focuses on the development of microfabricated mi-crofluidic/dielectrophoretic devices capable of concentrating micron-size particles from complex liquids. The concentrated particles of interest, such as pathogenic bacteria and spores, can then be delivered in small aliquots to the appropriate sensor for identification. Our micro-concentrator exploits the phenomenon of dielectrophoresis (DEP)–the force on polarizable particles in spatially non-uniform electric field [1]–to trap particles from the flow stream in order to subsequently concentrate them by release into a smaller volume of liquid. We create the non-uniform elec-tric field using interdigitated electrodes (IDE) at the bottom of the flow channel (Figure 1). To maximize the exposure of particles to the DEP field, we em-ploy a passive microfluidic mixer to circulate the liquid (Figure 1). One question that arises is how to determine the optimal mixer geometry for circulating the liquid, which may differ from the ide-al geometry for mixing two liquids. To answer this question we developed modeling tools and an experimental methodology to quantitatively predict the trapping behavior of particles in these systems. As Figure 2 shows, our modeling is able to predict the efficiency of different mixer configurations, without any fitting parameters. Among the four mixers tested (herringbone mixer (HM) slanted groove mixer (SGM), staggered herringbone mixer (SHM), and smooth channel (SMOOTH)), the HM and SHM perform similarly. This result is unexpected, as the HM is known to be a poor mixer of two liquids, while here we show that it is fine for circulating one liquid [2]."
DEP cell-patterning for controlling cellular Organization,"The ability to place cells at specific locations on a substrate is a useful tool to study and engineer interactions between cells [1], perform image-based cell selection [2], and create cell-based bio-sensors [3]. The ability to pattern with single-cell resolution is necessary in order to perform studies of single-cell physiology in which these cells are interacting with other cells. We have pre-viously created nDEP-based traps that were used to hold single micron-size beads at chosen locations on a substrate [4]. We have recently extended this work by modifying the design to allow us to manipulate and pattern single cells. We accomplished this modification by adding interdigitated electrodes to minimize non-specific cell adhesion and determining operating parameters that minimized heating and electric field exposure. The resulting structures are termed nDEP microwells to reflect that fact that they present an electrical microwell to incoming cells, allowing only cell-substrate attachment inside the DEP trap. With these nDEP microwells we have been able to place non-adherent cells and pattern adherent cells (Figure 1). Additionally, we have dem-onstrated that our cell-patterning technique does not affect gross cell phenotype as measured by morphology and proliferation. Fi-nally, we have developed a method that combines pressure-driven and convective flows to manipulate cells in two dimensions (Fig-ure 2)."
Iso-dielectric Cell Separation,"Increased throughput in the techniques used to engineer new metabolic pathways in unicellular organisms demands similarly high throughput tools for measuring the effects of these pathways on phenotype. For example, the metabolic engineer is often faced with the challenge of selecting the one genomic perturbation that produces a desired result out of tens of thousands of possibilities [1]. We propose a separation method – iso-dielectric separation, or IDS – which separates microorganisms continuously based on their intrinsic dielectric properties [2-3]. Because IDS is an equilibrium method, sorting cells according to their unique equi-librium positions in an energy landscape, it offers enhanced speci-ficity over other label-free separation methods [4]. This technol-ogy would enable high throughput screening of cells based upon electrically distinguishable phenotypes. Iso-dielectric separation uses dielectrophoresis (DEP) and media with spatially varying conductivity to create the energy landscape in which cells are separated according to their effective conduc-tivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis, and is thus applicable to uncharged particles, such as cells [5]. The IDS leverages many of the advantages of microfluidics and equilibrium gradient separation methods to create a device that is continuous-flow, capable of parallel separations of multiple (>2) subpopulations from a heterogeneous background, and label-free. We demon-strate the simultaneous separation of three types of polystyrene beads based upon surface conductance as well as sorting non-vi-able from viable cells of the budding yeast Saccharomyces cerevisiae (Figure 2). Current efforts are focused on the separation of Esch-erichia coli based upon the amount of the intracellular polymer poly(hydroxybutyrate) each cell contains."
MEMS Vibration Harvesting for Wireless Sensors,"The recent development of “low power” (10’s-100’s of µW) sens-ing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of indi-vidual wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients [1]. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of structural health monitoring, par-ticularly for aircraft.This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam structures have been developed to pre-dict the electrical and mechanical performance obtainable from ambient vibration sources. The optimized models have been vali-dated by comparison to prior published results [2] and verified by comparison to tests on a macro-scale device [3]. A non-opti-mized, uni-morph beam prototype (Figure 1) has been designed and modeled [4-5]. Dual optimal frequencies with equal peak powers and unequal voltages and currents are characteristic of the response of such coupled devices when operated at optimal load resistances (Figure 2). Design tools to allow device optimi-zation for a given vibration environment have been developed for both geometries. Future work will focus on fabrication and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. System integration and development, including modeling the power electronics, will be included."
"Design, Fabrication, and Testing of Multilayered Microfabricated Solid Oxide Fuel cells (SOFcs)","Microfabricated solid oxide fuel cells were investigated for por-table power applications requiring high energy densities [1]. The thickness of the electrolyte, the travel length of oxygen ions, was reduced down to ~150nm. The tri-layers (yttria-stabilized zir-conia (YSZ) as an electrolyte and platinum-YSZ cermet as cath-ode/anode) were sputter-deposited on a silicon wafer, and then they were released as square plates by KOH etching the silicon through patterned silicon nitride masks on the back side. High intrinsic and extrinsic (thermal) stresses due to fabrication and operation (25-600°C) [2], respectively, require careful thermome-chanically stable design of µSOFCs.First, material properties of the ultra-thin YSZ were character-ized experimentally and found to be significantly different than those of bulk YSZ [3]. Second, based on the obtained proper-ties, maximum stresses in the plates at 625ºC were analyzed us-ing non-linear von Karman plate theory [4]. The stresses showed three regions with sidelength variation: an un-buckled regime, a buckled regime with high stresses, and post-buckling regime with lower stresses (see Figure 1). The µSOFCs were fabricated in the post-buckling regimes with ~80-~180µm sidelength and total ~450nm thickness. With the plates buckled as shown in Figure 2, the µSOFCs produced power output of 0.008mW/cm, lower than the expected power from their electrochemical test. Given the high-performance predicted for the underlying nano-struc-tured ultra-thin electrolyte, anode, and cathode layers, additional studies are needed to improve specimens and test setup and to assess µSOFCs’ long-term operational stability."
A Compact Flash X-Ray Source Based Upon Silicon Field Emitter Arrays,"X-rays are used for non-destructive imaging in virtually every industry today. They enable doctors to make diagnostic decisions to quality assurance for electronics and industrial components. The machines that do these tasks are large, bulky, and occasionally slow due to the design of the X-ray system. Every X-ray imaging system is generally limited to up to 2 X-ray sources due to the cost but primarily the size. The majority of commercial X-ray sources are based upon thermionic emission or a heated filament similar to that of an incandescent light bulb. Because of the high temperature of the thermionic source, it is difficult to shrink the size of the X-ray tube. Field emission is a solution that has been touted for decades, but it has always had reliability problems. We resolve these problems by demonstrating that a high-performance and potentially compact flash X-ray source can be realized. The current X-ray setup, completed in collaboration with Massachusetts General Hospital (MGH) and shown in Figure 1, has been shown to be reliable enough to take hundreds of images for computed tomography to reconstruct a 3D image. These X-rays were taken in pulsed mode, where short bursts of a few hundred nanoseconds are used to turn the field emitter on, the first demonstration of its kind."
A Silicon Field Emitter Array as an Electron Source for Phase-controlled Magnetrons,"Magnetrons are a highly efficient (>90%), high-power vacuum-based microwave source. In a magnetron, free electrons in vacuum are subject to a magnetic field while moving past open metal cavities, resulting in the emission of resonant microwave radiation. Current state-of-the-art magnetrons use a heated metal filament to thermionically emit electrons into vacuum continuously and are not addressable. This work seeks to replace the heated metal filament as a source of electrons with Si field emitter arrays to improve efficiency and increase power, especially when several sources are combined. Si field emitter arrays, schematically shown in Figure 1, are devices that are normally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of many sharp Si tips sitting on long Si nanowires that limit the current of the electron emission. Electrons from the Si tips tunnel into a vacuum as a result of the high electric field of the applied bias on the polysilicon gate. Pulsing the electric field applied on the gate can turn the arrays on and off. The proposed use of Si field emitter arrays in a magnetron will allow injection locking and hence phase control of magnetrons. Phase-controlled magnetrons have multiple applications in areas where high-power microwave sources are desired. Currently, Si field emitter arrays have been designed for the magnetron; our collaborators at Boise State University are testing them."
Acoustically-active Surface for Automobile Interiors Based on Piezoelectric Dome Arrays,"The surfaces of automobile interiors can be rendered acoustically active by mounting on them thin, wide-ar-ea membranes with arrays of small acoustic transduc-ers. Each small, individually addressable transducer functions as a speaker or a microphone, and an en-tire pixelated acoustic membrane enables directional sound generation and sensing. The frequency response of the wide-area acoustically-active surface is deter-mined by those of the small isolated acoustic transduc-ers, which thereby yields better tunability of the band-width through designing pixel dimensions. As a result, the acoustically-active surface can work either in the audio frequency range for noise cancellation, personal entertainment, and communication with the vehicle, or - in the ultrasonic frequency range - for gesture de-tection, alertness monitoring, etc., which collectively improve the comfort and safety of the automobiles.This project seeks to develop and demonstrate a thin, wide-area acoustic “wallpaper” based on an array of dome-shaped piezoelectric transducers, which exhib-its outstanding performance and is deemed the most suitable option for miniaturization and scalable fabri-cation. Dependencies of device performance for both speaker and microphone applications on the material properties, dome dimensions, and back cavity struc-ture have been studied through theoretical modeling and numerical simulation. For speaker applications, a 12-μm thick, 8-inch wafer-size acoustic wallpaper con-sisting of an array of PVDF domes that are sub-1-mm in diameter is capable of generating over 60 dBSPL a half meter away with a 1-kHz, 10-V driving voltage, which can be further enhanced by scaling up the area and reducing the thickness of the membrane. On the other hand, reducing the radius of PVDF domes will lead to an extended bandwidth into the ultrasound range at a small cost of sound pressure level in the audio frequen-cy range. We have developed a scalable process to fabri-cate such acoustic wallpapers. A 1×1 cm2 sample (Figure 1) has been fabricated for demonstration and will be scaled up to a 10×10 cm2 wallpaper to explore prospec-tive applications of acoustically active surfaces."
The Scanning Anode Field Emission Microscope: A Tool for Mapping Emission Characteristics of Field Emitter Array Devices and Structures,"We developed a scanning anode field emission micro-scope (SAFEM) for characterizing field emission array (FEA) devices and structures (see Figure 1). The SAFEM is designed to accurately and precisely scan and posi-tion a probing anode tip over emitter tips of an FEA and acquire maps of emitter tip current IE(x,y) and an-ode-to-emitter voltage VAE(x,y,) from which the map of spatial variation of the field factor β(x,y) and the distri-bution f(β) are extracted. The scanning and positioning movement is achieved by using two positioning stages. The first stage holds the device so a sample can move in the xyz direction with a travel range of 26 mm and resolution of 6 nm. The second stage holds the probing anode and can also move in the xyz direction with a travel range of 5 mm and a resolution of 0.03 nm. Its probing and imaging of emission currents from FEA devices and structures enable use of the SAFEM to ob-serve the aging, i.e., temporal evolution of FEA devices in vacuum and gas ambient, and to measure accurately the turn-on and operating voltage of FEA devices. The SAFEM is operational and has been used to obtain cur-rent maps of various FEA devices, as in Figure 2. Images obtained by the SAFEM have been used to optimize the fabrication of FEAs for high-current and long-lifetime operations. The resolution of the SAFEM is determined by scan step size, relative sizes of emitter tip radii (rE), an-ode tip radius (rA), and emitter array pitch (TP): rE << rA << TP . To obtain a well-resolved current map of FEA tips, the SAFEM is operated in the pulsed mode. The scanning motion, voltage (or current) sourcing, and current (or voltage) measuring functions of the SAFEM can be independently operated in either a pulsed or continuous mode. In the completely pulsed mode, the scanning anode moves a discrete scan step and enters a wait state long enough for the movement response to reach steady state. Once the steady state is reached, the source measure unit turns on the anode voltage and waits for the emission current response to also reach a steady value before the command to measure the cur-rent is issued. The duty cycles of the pulsing operation are critical to obtaining a well resolved current map. Operation in the pulsed mode significantly reduces the described noise current from the FEA substrate, nearby edges, nearby tips, and stage movements compared to other modes of operation."
Highly Uniform Silicon Field-Emitter Arrays,"Cold cathodes based on silicon field-emitter arrays (FEAs) have shown promise in a variety of applications requiring high-current-density electron sources. However, FEAs face a number of challenges that have prevented them from achieving widespread use in commercial and military applications. One problem limiting the reliability of FEAs is emitter tip burnout due to Joule heating. The current fabrication process for FEAs results in a non-uniform distribution of emitter tip radii. At a fixed voltage, emitters with a small radius emit a higher current while emitters with a large radius emit a lower current. Therefore, emitters with a small radius reach their thermal limit due to Joule heating at lower voltages and consequently burn out. Previous solutions to mitigating tip burnout have focused on limiting the emitter current with resistors, transistors, or nanowires in order to obtain a more uniform emission current.In this project, we focused on increasing the uniformity of emitter tip radii as a means to reduce tip burnout. Figure 1 shows a typical distribution of emitter tip radii for FEAs. The non-uniform distribution of emitter tip radii first forms during the photolithography step that defines the array of “dots” that become the etching mask for the silicon tips. In our FEA fabrication process, we used a trilevel resist process that nearly eliminated the light wave reflected at the photoresist/silicon interface and hence improved the uniformity of the dot diameter. Furthermore, we integrated the emitter tips with silicon nanowires to improve their reliability. Figure 2 shows a diagram of the fabricated structure. Our fabrication process resulted in FEAs with a more uniform emission current and a potentially longer lifetime."
CMOS Opto-nanofluidics,"CMOS (complementary metal-oxide-semiconductor) foundries offers designers access to nanometer scale patterns, a suite of readout and interface circuits, and, more importantly, the capability to mass-manufacture their designs. These features of microelectronic CMOS foundries have been extensively utilized for photonic applications. This abstract introduces their application to opto-nanofluidics. In MicroTAS 2017, we first reported the process of fabricating nanofluidic channels inside CMOS chips by defining the channels using the polysilicon gate layer and releasing the channels by sacrificial etching. Since then, we developed a packaging approach to accommodate mm-sized dies, which are the norm in multi-project wafer runs. The CMOS opto-nanofluidic chip in Figure 1 was fabricated in a 65-nm SOI CMOS process (10LP+) with integrated photodetectors.The packaging approach employs a low-cost epoxy material to extend the die area and a back-side machining step to planarize and thin the wafer down for subsequent lithography and etch steps. The I-V curves in Figure 2 indicate that the photodetectors are fully operational after the substrate extension and nanochannel release. Future chips will include an integrated amplifier with 0.6 μV/√Hz simulated input referred noise for improved sensitivity by lock-in detection. We plan to apply this CMOS opto-nanofluidic platform for single-molecule manipulation and sensing applications."
Increasing the Yield of Atmospheric Pressure Microsputtering for Fabrication of Agile Electronics,"Additive manufacturing (AM) promises new, flexible production; however, while AM excels at creating structural parts, it cannot make functional objects well, e.g. multi-material structures such as electronic components and circuits. Sputtering, which removes material from a target atom-by-atom by using a plasma, is used in IC fabrication finely layered, multi-material fabrication. By miniaturizing the dimensions of the plasma reactor down to sub-millimeter scale, the sputterer can operate at atmospheric pressure, obviating the need for a vacuum. However, at atmospheric pressure, collisions with gas molecules scatter most of the sputtered material, preventing it from reaching the substrate. We develop plasma microsputterer technology that allows for high-resolution, high-quality deposition of arbitrary patterns, without any templates, pre-, or post-processing; recent results with a gold target include creating imprints with electrical conductivity within an order of magnitude to that of bulk metal. We explore two methods to minimize sputtered ma-terial scattering and to increase the deposition rate (yield). The first method minimizes the gap between the sputtering target and the substrate (Fig. 1): the sputtering target is placed 150 µm above the substrate. Dielectric barriers confine the plasma, forcing the plasma to connect the target wire and anode without damaging the substrate. This approach yields 0.2 nm/s (40 pg/s)—twice previous results. However, significant substrate heating occurs, which is incompatible with temperature-sensitive substrates. The second method harnesses convection to drive the sputtered material towards the substrate (Fig. 2). We surround the microsputter target (100 µm diameter) with a strong jet of air (100 m/s, 0.5 mm thick coaxial flow) to force air molecules to transport the sputtered material. This method greatly increases the yield (1 nm/s, 20 ng/s)— 30% of the sputtered material reaches the substrate. Current work focuses on further increasing the deposition rate by increasing the rate at which atoms are sputtered."
Silicon MEMS Compatible Bipropellant Micro Rocket Engine Using Steam Injector,"Rocket engines miniaturized and fabricated using MEMS or other techniques have been an active area of research for two decades. At these scales, miniaturized steam injectors like those used in Victorian-era steam locomotives are viable as a pumping mechanism and offer an alternative to pressure feed and high-speed turbo-pumps. Storing propellants at low pressure reduces tank mass, and this improves the vehicle empty-to-gross mass ratio; if one propellant is responsible for most of the propellant mass (e.g., oxidizer), injecting it while leaving the others solid or pressure-fed can still achieve much of the potential gain. Previously, the principal investigator and his group built and tested ultraminiature-machined micro jet injectors that pumped ethanol and also explored liquid and, more recently, hybrid engine designs. Recent work has focused on designing and implementing a whole-engine test article that simultaneously integrates a steam injector, boiler, decomposition chamber, fuel injector and thrust chamber, that is practical to build, and that is compatible with MEMS fabrication. An axisymmetric engineering mockup in brass was built to demonstrate the feasibility of the design concept (see Figure 1). Configurations that combine electrically-driven pumps with steam injectors by, for example, using electric pumps to pump fuel or coolant and a steam injector motivated by boiled coolant to pump oxidizer are also being explored. These would allow pressurized tanks to be avoided altogether while still being compatible with miniaturization via MEMS."
Gated Silicon Field Ionization Arrays for Compact Neutron Sources,"Neutron radiation is widely used in various applications, ranging from the analysis of the composition and structure of materials and cancer therapy to neutron imaging for security. However, most applications require a large neutron flux that is often achieved only in large infrastructures such as nuclear reactors and accelerators. Neutrons are generated by ionizing deuterium (D2) to produce deuterium ions (D+) that can be accelerated towards a target loaded with either D or tritium (T). The reaction generates neutrons and isotopes of He, with the D-T reaction producing the higher neutron yield. Classic ion sources require extremely high positive electric fields, on the order of 108 volts per centimeter (10 V/nm). Such a field is achievable only in the vicinity of sharp electrodes under a large bias; consequently, ion sources for neutron generation are bulky. This work explores, as an alternative, highly scalable and compact Si field ionization arrays (FIAs) with a unique device architecture that uses self-aligned gates and a high-aspect-ratio (~40:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array (Figure 1). The tip radius has a log-normal distribution with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm in diameter and is within 200 nm of the tip. Field factors, β, > 1 × 106 cm-1 can be achieved with these Si FIAs, implying that gate-emitter voltages of 250-300 V (if not less) can produce D+ based on the tip field of 25-30 V/nm. In this work, our devices achieve an ionization current of up to 5 nA at ~140 V for D2 at pressures of 10 mTorr. Gases such as He and Ar can also be ionized at voltages (<100 V) with these compact Si FIAs (Figure 2)."
Silicon Field Emitter Arrays (FEAs) with Focusing Gate and Integrated Nanowire Current Limiter,"The advent of microfabrication has enabled scalable and high-density Si field emitter arrays (FEAs). These are advantageous due to compatibility with complementary metal-oxide semiconductor (CMOS) processes, the maturity of the technology, and the ease in fabricating sharp tips using oxidation. The use of a current limiter is necessary to avoid burn-out of the sharper tips. Active methods using integrated MOS field-effect transistors and passive methods using a nano-pillar (~200-nm wide, 8-µm tall) in conjunction with the tip have been demonstrated. Si FEAs with single gates reported in our previous works have current densities of >100 A/cm2 and operate with lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essential to control the focal spot size of the electron beam as electrons leaving the tip have an emission angle of  12.5. The focus electrode provides a radial electric field that reduces the lateral velocity of stray electrons and narrows the cone angle of the beam reaching the anode. Varying the voltage on the focus gate reduces the focal spot size or achieves an electron beam modulator for radio frequency applications. In this work, we fabricate dense (1-μm pitch) double-gated Si with an integrated nanowire current limiter (Figure 2). The apertures are ~350 nm and ~550 nm for the extractor and focus gates, respectively, with a 350-nm-thick oxide insulator separating the two gates. Electrical characterization of the fabricated devices shows that the focus-to-gate ratio (VFE/VGE) can be used to control the anode current (Figure 2). When the focus voltage exceeds the gate voltage, the field superposition increases the extracted current, and vice versa. These devices can potentially find applications as high-current focused electron sources in flat panel displays, nano-focused X-ray generation, and microwave tubes."
Electron Transparent Anodes for Field Emission Cathodes in Poor Vacuum,"Nanoscale Vacuum Channel Transistors (NVCTs) using field emission sources could potentially have superior performance compared to solid state devices of similar channel length. This is due to ballistic transport of electrons, shorter transit time and higher breakdown voltage in vacuum. Furthermore, there is no opportunity for ionization or avalanche carrier multiplication imbuing NVCTs with very high Johnson figure of merit (~1014 V/s). However, field emitters need ultra-high vacuum (UHV) for reliable operation as the field emission process is sensitive to barrier height variations induced by adsorption/desorption of gas molecules. Small changes in the barrier height cause exponential variations in current. Poor vacuum also leads to generation of energetic ions that bombard the emitters, altering the work function and degrading electrical performance. To overcome the UHV requirement, graphene can be used to nano-encapsulate the field emitter in UHV or a gas (e.g. He) with high ionization energy. Separation of the electron tunneling region from the electron acceleration region enables emission of electrons in UHV and electron transport in poor vacuum, if not atmospheric conditions. For mechanical strength, a multi-layer graphene structure that is transparent to electrons while being impervious to gas molecules/ions is necessary. In this work experimentally characterize the electron transparency of graphene membranes using arrays of gated Si field emitters with 1 µm pitch (Figure 1) that exhibit transistor-like characteristics. Using an energized multi-layer graphene/grid structure (Figure 2) in combination with emitter arrays, we measured extremely high electron yield perhaps due to secondary emission from electrons impinging on the graphene layer. Adopting this architecture for NVCTs will allow the realization of empty state electronics capable of functioning at higher frequencies (THz regime) higher power and harsher conditions (high radiation and high temperature) compared to solid state electronics."
GaN Vertical Nanowires with Self-Aligned Gates for Field Emission Applications,"Field emitters (FE), or namely vacuum transistors, are promising for harsh-environments and high-frequency electronics thanks to their radiation hardness and scattering-free electron transport. However, the stability and operating voltage still need improvement to enable circuit applications. To overcome these issues, III-Nitrides are excellent candidates due to their strong bonding energies and tunable electron affinities. Though the material properties of III-Nitrides are promising, so far, there are few works demonstrating sub-100 V turn on as most III-N FEs are still two-terminal structures.In this work, we demonstrate a novel GaN nanowire (NW) FEs based on self-aligned gates to reduce the gate-emitter turn-on voltage (VGE, ON) below 30 V. The GaN on Si wafer was grown by Enkris Semiconductor, Inc. Thanks to a new GaN processing technology, we successfully fabricate GaN NWs with width of 60 nm and aspect-ratio of 5 (Figure 1 (a)). The gate stack is then conformally deposited. We then finish the device fabrication by dry etching to open FEs’ tips (Figure 1 (b)). We measure the transfer characteristics with a suspended 0.5-mm-diameter tungsten ball biased at +500 V as an anode (Fig. 2(a)). Device turns on at 27 V. This device demonstrates the lowest turn-on voltage among GaN field emitters in literature, as well as excellent current density (Fig. 2 (b)) and shows great potential for integrated circuit applications."
"3D-Printed, Miniature, Multi-material, Valve-less, Magnetically Actuated Liquid Pumps","Miniaturized pumps can be used to supply precise flow rates of liquid in compact systems. Numerous micro-fabricated positive displacement pumps for liquids with chamber volumes that are cycled using valves have been proposed. Pumps made via standard (i.e., cleanroom) micro-fabrication typically cannot deliver large flow rates without integrating hydraulic amplifi-cation or operating at high frequency due to their small pump chambers.3D-Printing has recently been explored as a pro-cessing arena for microsystems; in particular, research-ers have reported 3D printed pumps for liquids and gas-es with performance on par with or better than coun-terparts made with standard microfabrication. Building on earlier work on printed magnetically actuated liquid pumps, we 3D-printed multi-material, magnetically driven, valve-less miniature liquid pumps. We used the fused filament fabrication (FFF) method: a thermoplastic filament is extruded from a hot nozzle to create, layer by layer, a solid object. The body of the pump is printed in Nylon 12, while the actuation mag-net is printed in Nylon 12 containing NdFeB micro-par-ticles. The devices are driven by a non-contact rotating magnet and employ valve-less diffusers to greatly sim-plify operation. Our low-cost, leak-tight, miniature devices are microfabricated using 150- and 225-µm layers with a multi-step, multi-material printing process (Figure 1) that monolithically creates all key features with <13-µm in-plane misalignment. Each pump has a frame, a 225-µm-thick membrane connected to a piston with an embedded magnet, a chamber, two diffusers, and two fluidic connectors (Figure 2). Fabrication of the pump requires under 75 minutes and costs less than $3.89. Fi-nite element analysis of the actuator predicts a maxi-mum stress of 15.7 MPa @ 100 μm deflection, i.e., below the fatigue limit of Nylon 12 for infinite life (i.e., 19 MPa). Water flow rate up to 1.68 ml/min at an actuation fre-quency of 204 Hz was measured."
"Measurement of the Condensation Coefficient of Water Using an Ultrathin, Nanoporous Membrane","In applications ranging from electronics cooling and power generation cycles to distillation, liquid-vapor phase change phenomena play a critical role. At their fundamental (kinetic) limits, evaporation and con-densation are dictated by the resistance to molecules crossing the liquid-vapor interface, which is quantified by the condensation coefficient. Despite its fundamen-tal importance and widespread use in heat transfer models such as the Schrage equation, the condensation coefficient of water has been difficult to characterize, with experimental results and theoretical calculations spanning three orders of magnitude. Experimental measurement has been challenging because three con-ditions must be satisfied: sensitivity to the condensa-tion coefficient is high, temperature of the liquid-vapor interface is precisely yet noninvasively measured, and the concentration of contaminants at the liquid-vapor interface is low. To achieve a precise measurement of the condensation coefficient of water, we have fabricat-ed an ultrathin (~200 nm), nanoporous (~150 nm diame-ter), hydrophobic membrane for forward-osmosis (FO) driven transport (Figure 1). Due to the ultrathin, low-as-pect ratio dimensions of the membrane, we achieve high sensitivity to the condensation coefficient and avoid undesired contaminant buildup at the interface because the membrane is freestanding. Since transport is driven by osmotic pressure, the system can be main-tained at isothermal conditions such that the tempera-ture can be precisely measured in the bulk water with-out interfering with the liquid-vapor interface. These experimental measurements of the condensation co-efficient of water are crucial for modeling liquid-vapor phase change in nanoscale systems and advanced ther-mal management devices."
In-plane Gated Field Emission Electron Sources via Multi-material Extrusion,"Field emission is the quantum tunneling of electrons to vacuum due to local high electrostatic fields; such high fields can be generated at a moderate voltage using nanosharp, high-aspect-ratio tips. Compared to thermionic counterparts, field emission cathodes consume less energy, respond faster, and can operate in poorer vacuum, making them attractive in compact applications such as nanosatellite electric propulsion, portable mass spectrometry, and handheld X-ray generation. A wide variety of materials has been explored as field emitters; the research in field emission electron sources has focused on carbon nanotubes (CNTs) due to their nanosized tip diameter, high aspect-ratio, high electrical conductivity, and excellent chemical stability. However, most manufacturing methods for CNT field emission electron sources have associated large cost, long processing time, need of static masks for defining in specific locations the nanostructured emitting material and/or the electrode(s), and large gate interception (or the need for advanced assembly methods to attain high transmission). In this project, we are developing low-cost field emission cathodes via multi-material extrusion. The devices are flat plates with two concentric imprints (Figure 1): an imprint made of CNTs (emitting electrode), symmetrically surrounded on both sides by an imprint made of Ag microparticles (extractor gate). Unlike the great majority of field emission cathodes reported that have an out-of-plane gate electrode, our devices have an in-plane gate that significantly reduces the cost and manufacturing complexity of the device and also facilitates high gate transmission. Our devices can emit electrons in vacuum with as little as 62 V applied between the CNT imprint and the Ag imprint and achieve over 97% gate transmission (Fig. 2). Current work focuses on increasing the imprint density to attain larger current density emission and on developing ballasting structures for attaining large and uniform array emission."
"Additively Manufactured, Miniature Electrohydrodynamic Gas Pumps","A corona discharge is a self-sustained physical phenomenon induced around the sharper electrode of a diode due to sharply nonuniform electric fields within the interelectrode space. Ion propagation across such a space is accompanied by collisions with neutral particles, resulting in bulk fluid movement known as ionic wind. In contrast to traditional counterparts, ionic wind pumps have no moving parts, respond faster, and produce significantly less noise, drawing great interest in applications such as air propulsion and electronics cooling. Currently, ionic wind pump technology is far from practical in applications that require large flow velocity, flow rate, and power efficiency; another concern is the stability of the pump, given that ion accumulation in the interelectrode space can cause an electric short during sustained operation. Researchers have proposed using active electrodes with a plurality of field enhancers arranged in parallel (multiplexing) to maximize throughput; however, the reported multi-needle devices are serially assembled, and their performance is inferior to that of single-needle counterparts. This project uses metal additive manufacturing and electropolishing to create miniature, multi-needle ionic wind pumps. Our devices are needle-ring corona diodes composed of a monolithic inkjet binder-printed active electrode (Figure 1), made in stainless steel 316L, with a plurality of sharp, conical needles and a thin plate copper counter-electrode, with electro-chemically etched apertures aligned to the needle array. Five-needle ionic wind pumps eject air at 2.9 m/s and at a volumetric flow rate of 343 cm3/s, three times larger than the flow rate of a single-tip device with comparable efficiency (Figure 2). Current work systematically studies the relevant parameters to optimize the design of the electrohydrodynamic pump."
3D-Printed Silver Catalytic Microreactors for Efficient Decomposition of Hydrogen Peroxide,"Microreactors increase the surface-to-volume ratio of their reactants and by-products, resulting in faster, more efficient reactions and better heat transfer than in their non-miniaturized counterparts, leading to higher throughput per unit of reactor active volume and to better selectivity in the species produced by the reactor. The great majority of microreactors are made of polydimethylsiloxane (PDMS)—a material that cannot operate at elevated pressures or temperatures. Other reported microreactors are made in silicon, ceramics, or metals; although these materials are compatible with high-pressure and high-temperature operation, they have associated a very high production cost because they are made in a semiconductor cleanroom or with specialized, low-throughput tooling, e.g., electro discharge machining. Hydrogen peroxide (H202), a water-soluble oxidant, spontaneously decomposes in the presence of heat or a catalyst. Applications of a H2O2 catalytic reactor include monopropellant rocket propulsion, steam generators, and pumping; miniaturized versions of such catalytic reactors are of great interest to PowerMEMS. Here, we developed a novel additive manufacturing technique based on silver clay extrusion to create high-pressure compatible and high-temperature compatible, monolithic microfluidics; silver is also a very efficient and effective catalyst for the decomposition of H2O2. Our microreactors are composed of a water-tight microchannel connected to the exterior via two fluidic ports (Figure 1). The experimental performance of the microreactor as a catalytic decomposer of H2O2 matches well our reduced-order modeling estimates (Figure 2), attaining a decomposition efficiency of 87% for a flow rate of 5 μL/min of H2O2 with an initial concentration of 30% w/w. Current research focuses on exploring other applications, e.g., heat exchangers."
Management of Brine Effluent from the Desalination Plant,"While large-scale desalination has been a mainstay in a country with a severe water shortage for many decades, management of high concentration brine effluent (> 50,000 TDS) has posed technological, economic, and environmental challenges. There are two research directions to treat the brine effluent effectively: (1) reduce the total volume of effluent onshore and (2) discharge the effluent offshore to minimize environmental impact. Interestingly, the production of effluent is tons of liters, but both studies implement a diffusion process of molecule in the effluent, which appears on a microscopic scale. Here, two technologies are briefly introduced: ion concentration polarization (ICP) to reduce effluent volume and offshore discharge of effluent using plunging liquid jets to minimize environmental impact.ICP process is a novel electrochemical desalination technology, which emerged within the last decade as a viable option for effluent treatment (Figure 1). ICP employs only a cation exchange membrane (CEM) to utilize a higher diffusivity of chloride (t^(β-)), which is the majority ion in the effluent. Compared with conventional electrochemical desalination such as ED (electrodialysis), it is more energy-efficient, less susceptible to various fouling, and can be implemented with a much smaller footprint. Our group has developed and matured the technology over the years to realize the first-ever lab-scale ICP desalination prototype (~0.1L/min), demonstrating its technical and economic feasibility, and secured several key intellectual properties on this technology. For the offshore discharge of brine, we are investigating the use of plunging liquid jets (Figure 2) through laboratory experiments. Similar to the widely used offshore discharge outfalls such as submerged or surface jets, plunging jets also utilize the high momentum and negative buoyancy of brine to induce mixing with the surrounding ocean water and reduce the concentration of contaminants such as salt, anti-fouling agents, and anti-scalants that, in turn, reduce the environmental impact. However, unlike these outfalls, plunging jets also introduce air into the water column which, when dissolved, can reduce the environmental impact associated with the creation of hypoxic (low dissolved oxygen) zones."
Reduced-order Modeling of Oil Transport in Internal Combustion Engines Based on Autoencoder,"Reducing emissions of internal combustion engines is the major focus in the modern automotive industry. Lubrication oil leakage from the piston ring pack is critical to oil consumption and emission. The oil transport mechanism is not well understood due to the computational complexity of the oil motion and ring pack dynamics and experimental difficulty. This raises our interest to build a reduced-order model predicting the oil movement inside the ring pack with acceptable accuracy and efficiency.In this work, we proposed a neural-network-based method to perform model order-reduction (MOR) on the computational fluid dynamics (CFD). First, we use a variational autoencoder (VAE) to address the reduce basis of the fluid field and encode the original space into the reduced space. Second, we apply a recurrent neural network (RNN) to learn the dynamics of the reduced space. To guarantee the stability of system dynamics, certain physics-based conservation law and stability regularization are included in the loss function. This method can reduce the fluid dynamics model calculation time by orders of magnitude with acceptable accuracy for analysis. With the reduced-order oil transport model coupled with the piston ring dynamics model, we can quantitatively analyze the mechanisms for oil leakage and inspire design optimization in automotive industry. The methodology developed in the work is not limited to fluids. The same procedures are applicable to other physical system modeling. Further, the methodology can interpret the neural network behavior from the perspective of model order-reduction."
A Four-terminal Nanoelectromechanical Switch Based on Compressible Self-assembled Molecules,"Nanoelectromechanical (NEM) switches are under investigation as complements to, or substitutes for, CMOS switches owing to their intrinsic quasi-zero static leakage, large ON-OFF conductance ratio, and high robustness in harsh environments. For most NEM contact switches, a trade-off between high actuation voltage and the risk of stiction failure seems inevita-ble due to the strong van der Waals attraction between contacts at the nanoscale. This attraction leads to un-favorable dynamic power consumption and decreased reliability. Through this research, we have developed a novel tunneling NEM switch, termed a “squitch”, based on a metal-molecule-metal junction whose tunneling gap can be modulated by compressing the molecule lay-er with electrostatic force created by a voltage applied between the metal electrodes. In contrast to conven-tional NEM contact switches, direct contact of squitch electrodes in the ON state is avoided by assembling a molecular spacer between the electrodes; the molecu-lar spacer acts to hold the squitch together and helps reduce hysteresis and the possibility of stiction failure.A multi-terminal squitch has been demonstrated using a chemically-synthesized Au nanorod as a floating top electrode, and bottom Au electrodes patterned with electron beam lithography. With the help of a peeling technique that we have developed, Au electrodes are created with sub-nanometer roughness. The electrodes include two actuation gates recessed by several nanometers via a graphene sacrificial layer. By choosing molecules with appropriate chain lengths, we are able to define nanometer-wide electrode-to-nanorod gaps, which can be subsequently adjusted by a bias voltage applied between the gate electrodes. With a proper bias voltage, we can exponentially modulate the conduction current through a small variation of the gating voltage. Our squitch has been experimentally demonstrated to exhibit low actuation voltage and hysteresis, which supports its prospects in ultra-low power logic applications."
Silicon Field Ionization Arrays Operating > 200 V for Deuterium Ionizers,"Devices that can field-ionize gas molecules at low bias voltages are essential for many applications such as ion mobility spectrometry and highly selective porta-ble gas sensing. Field ionization consists of a valence electron of a gas atom or molecule tunneling through a potential barrier, commonly into a vacant energy state of the conduction band of a metal at the anode. Clas-sic ion sources require extremely high positive electric fields, of the order of 108 volts per centimeter. Such fields are only achievable in the vicinity of very sharp electrodes under a large bias. Ion sources based on mi-crowave plasma generation have demonstrated high currents and high current densities. Yet, they are bulky and require large magnetic fields. Alternatively, single or arrays of gated tip structures have been used as field ionizers, but they emit low currents (~10 nA). Early tip burn-out due to non-uniform tip distribution and low voltage breakdown are the two main causes of such low currents. In this work, Si field ionization arrays (FIAs) with a unique device architecture that uses a high-aspect-ratio (~50:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array is proposed. The nanowires are 10 μm in height, 1 µm apart and 100-200 nm in diameter. A dielectric matrix of (Si3N4/SiO2) supports a poly-Si gate while a 3 μm thick dielectric holds the contacts. Current densities >100 A/cm2 and lifetime > 100 hours have already been reported. The tip radius has a log-normal distribution varying from 2 to 8 nm with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm. Field factors, β, > 1 × 106 cm-1 can be achieved with these devices implying that voltages of 250-300 V (if not less) can produce D+ ions based on the tip field of 25-30 V/nm. Completed chips on a package are shown in Figure 1 together with a schematic of the test set-up for field ionization.Breakdown at the mesa edge at voltages ~70 V was the reported by Guerrera, et al. However this has now been overcome by etching a vertical sidewall profile (Figure 2) with a combination of both SF6 and C4F8 flowing simultaneously (Figure 2). I-V characterization in air demonstrates breakdown occurs within the active region (Figure 2) possibly due to the narrow gate apertures and the short oxide thickness from the tip to the poly-Si gate. Initial results show that further etching this oxide to expose the nanowire increases the oxide separation to the gate, which in turns increases the breakdown voltage (Figure 2), thus enabling the Si FIAs to be operated at voltages exceeding 200 V."
Highly Uniform Silicon Field Emitter Arrays,"Cold cathodes based on silicon field emitter arrays (FEAs) have shown promising potential in a variety of applications requiring high current density electron sources. However, FEAs face a number of challenges that have prevented them from achieving widespread use in commercial and military applications. One problem limiting the reliability of FEAs is emitter tip burnout due to Joule heating. The current fabrication process for FEAs results in a non-uniform distribution of emitter tip radii. At a fixed voltage, emitters with a small radius emit a higher current while emitters with a large radius emit a lower current. Therefore, emitters with a small radius reach their thermal limit due to Joule heating at lower voltages and consequently burnout. Previous solutions to mitigating tip burnout have focused on limiting the emitter current with resistors, transistors, or nanowires in order to obtain more uniform emission current.In this project, we focus on increasing the uniformity of emitter tip radii as a means to reduce tip burnout. Figure 1 shows a typical distribution of emitter tip radii for FEAs. The non-uniform distribution of emitter tip radii first forms during the photolithography step that defines the array of “dots” which become the etching mask for the silicon tips. In our FEA fabrication process, we use a tri-level resist process that nearly eliminates the light wave reflected at the photoresist/silicon interface, and hence improves the uniformity of the dot diameter. Furthermore, we integrate the emitter tips with silicon nanowires to improve their reliability. Figure 2 shows a diagram of the fabricated structure. We expect our fabrication process to result in FEAs with more uniform emission current and potentially longer lifetime."
High Current Density Silicon Field Emitter Arrays (FEAs) with Integrated Extractor and Focus Gates,"Cold electron sources have been identified as alter-natives to thermionic emitters due to their lower op-erating temperature, instant response to the applied electric field, and their exponential current-voltage characteristics. With the advent of microfabrication, the generation of high electric fields around sharp emitters to tunnel electrons was made possible. Scal-able and high-density field emitter arrays (FEAs) based on Si are advantageous due to compatibility with CMOS processes, maturity of technology, and the ease to fabricate sharp tips using oxidation. The use of a cur-rent limiter is necessary to avoid burning of the sharp-er tips; active method using an integrated MOSFET, or passive methods using a nanopillar (~200 nm wide, 10 µm tall) in conjunction with the tip has been demon-strated. Si FEAs reported by Guerrera, et al., exhibited high current densities exceeding 100 A/cm2 and having lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essential to control the focal spot size of the beam as the tips become blunt with time and as a consequence, the turn-on voltage also increases (Figure 2). With the focus electrode, stray electrons extracted by the gate closest to the tip will be captured and only electrons emitted within a certain cone angle will reach the anode, thus achieving a narrower focal spot size compared to a single gated Si FEA. The voltage on the focus gate can be varied with time to maintain a fixed focal spot size or even as an electron control switch. Indeed having a high positive focus voltage pulls all the extracted electrons and can be used to prevent electrons reaching the anode. This also offers the opportunity for fast switching of these Si FEAs, which has been a limitation thus far of these devices. In this work we are optimizing the process steps to fabricate Si FEAs with the two integrated gates and current limiter, to characterize the effects of the focus gate on electron emission. These devices will find applications in flat panel displays, nanofocused X-ray sources, microwave tubes, and triodes."
A Silicon Field Emitter Array as an Electron Source for Phase Controlled Magnetrons,"Magnetrons are a highly efficient (>90%), high-pow-er vacuum-based microwave source. In a magnetron, free-electrons in vacuum are subject to a magnetic field while moving past open metal cavities, resulting in resonant microwave radiation to be emitted. Current state-of-art magnetrons use a heated metal filament to thermionically emit electrons into vacuum continu-ously and are not addressable. This work seeks to re-place the heated metal filament as a source of electrons with silicon field emitter arrays in order to improve the efficiency and increase the power, especially when sev-eral sources are combined. Silicon field emitter arrays, schematically shown in Figure 1, are devices that are normally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of many sharp tips made of silicon sitting on long silicon nanowires that limit the current of the electron emission. Electrons from the silicon tip tun-nel into a vacuum as a result of the high electric field of the applied bias on the polysilicon gate. Pulsing the electric field applied on the gate can turn the arrays on and off. The proposed use of silicon field emitter arrays in a magnetron will allow injection locking and hence phase control of magnetrons. Phase-controlled magnetrons have multiple applications in areas where high- power microwave sources are desired. Currently, Si field emitter arrays have been designed for the mag-netron and are undergoing testing with collaborators at Boise State University."
"Development of a Tabletop Fabrication Platform for MEMS Research, Development, and Production","A general rule of thumb for new semiconductor fabrica-tion facilities (fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for research and development and for groups seeking to commercialize new semiconductor devic-es aimed at smaller market segments and requiring a dedicated process. To eliminate this cost barrier, we are working to create a suite of tools that will process small (~1”) substrates and cumulatively cost less than $1 million. This suite of tools, known colloquially as the 1” Fab, offers many advantages over traditional fabs. By shrinking the size of the substrate, we trade high die throughputs for significant capital cost savings, as well as substantial savings in material usage and energy consumption. This substantial reduction in the capital cost will drastically increase the availability of semi-conductor fabrication technology and enable experi-mentation, prototyping, and small-scale production to occur locally and economically. Our research in the last few years has been primarily focused on developing and characterizing tools for the 1” Fab. In previous years, we demonstrated a deep reactive ion etching (DRIE) tool and a corresponding modular vacuum tool architecture, and we are now working to develop a reactive magnetron sputtering tool and an inductively coupled plasma-based PECVD tool (ICP-CVD) for depositing a wide variety of materials. The reactive magnetron sputtering tools operates using a 2” target and a direct sputtering configuration and is fully integrable with the modular tool architecture of the 1” Fab. We have demonstrated the functionality of the tool with the depositions of copper, aluminum, and via reactive sputtering, aluminum nitride. The system has been characterized using a response surface methodology and consistent, uniform depositions with <6% variation across the wafer have been shown. The ICP-CVD tool has also been built within the modular tool architecture and is being tested with depositions of SiO2, SiNx, and a-Si. The use of an ICP source allows depositions to occur at temperatures as low as 25°C, with low hydrogen incorporation, and quality approaching that of LPCVD depositions. Film stress and index of refraction are also controllable."
Is the Surface Wickability the Single Descriptor of Critical Heat Flux during Pool Boiling?,"Enhancement and estimation of critical heat flux (CHF) are two of the most important research areas of pool boiling. It is well-known that microstructured surfaces can extend the limit of CHF up to ~250% higher than that of a flat surface. The mechanism for this enhance-ment has generally been accepted as the wickability of structured surfaces originating from liquid propaga-tion within the surface structures driven by capillary pressure. We investigated the applicability of this the-ory based on the accumulated data of previous studies and our experimental data. We first calculated capil-lary pressure and permeability of structured surfaces to characterize liquid propagation rate analytically. We then performed pool boiling experiments on silicon mi-cropillar surfaces to measure CHF values. We found that there is no distinct relationship between the CHF and wickability contrary to a general notion. Our results suggest that although liquid wicking has been found to be important, the parameter wickability defined by previous works alone is not sufficient to describe CHF. In addition to the wickability, we propose that there may be other important parameters that also change along with the surface structures, e.g., the diameter of vapor columns and bubble departure size, among others, which need further investigation."
Carbon Nanotubes Based-field Emitters by 3-D Printing,"In field emission, electrons are ejected from a solid sur-face via quantum tunneling due to the presence of a high local electrostatic field. Compared to state-of-the-art thermionic electron sources, field emission cathodes consume significantly less power, are faster to switch, and could operate at higher pressure. Field emission cathodes have a wide range of applications such as X-ray sources, flat-panel displays, and electron microscopy. Several materials, e.g., Si, ZnO, and graphene, have been explored as field emission sources; however, carbon nanotubes (CNTs) are very promising to implement field emission cathodes due to their high aspect ratio, high electrical conductivity, excellent mechanical, and chemical stability, and high current emission density. Reported approaches for fabricating CNT field emitters include screen printing and direct growth of nanostructures (e.g., plasma-enhanced chemical vapor deposition) where a static stencil, i.e., mask, is involved to produce patterned structures in specific locations. These masks increase the time and cost needed to iterate the pattern, affecting the prototype optimization of the cathode. Ink direct writing (IDW), i.e., the creation of imprints by extrusion of liquid suspensions through a small nozzle, has emerged as an attractive maskless patterning technique that can accommodate a great variety of materials to create freeform imprints at low-cost (Figure 1). An imprint with CNTs protruding from the surface of the imprint (Figure 2), strongly adhered to the substrate can achieve stably high-current emission when an electric field is applied. We are currently working on the design and optimization of the formulation of a CNT-based ink, to eventually demonstrate low-cost field emission sources."
Electron Impact Gas Ionizer with 3-D Printed Housing and NEMS Si Field Emission Cathode for Compact Mass Spectrometry,"Mass spectrometry is widely used to quantitatively determine the composition of samples. However, the bulky size and high-power consumption of conven-tional mass spectrometry instruments limit their por-tability and deployability. One of the key components of a mass spectrometer (MS) is the ionizer. State-of-the-art electron impact gas ionizers use a stream of electrons produced by a thermionic cathode to create ions by fragmentation. Field emission cathodes, based on quantum tunneling of electrons triggered by high electrostatic fields, are a better alternative for portable mass spectrometry of gases compared to mainstream thermionic cathodes because they consume signifi-cantly less power, are faster to switch, and could oper-ate at higher pressure.In this project, we are developing a compact electron impact gas ionizer based on a cleanroom-microfabricated cathode and a 3-D printed ionization housing (Figure 1). The cathode is an array of nano-sharp silicon field emitters with proximal, self-aligned extractor gate, while the ionization housing is composed of an ionization region surrounded by an ionization cage, an anode electrode, a repeller electrode, and a dielectric structure that holds together the electrodes. To produce ions (i) a high enough bias voltage is applied between the extractor gate and the silicon tips, shooting electrons into the ionization region, (ii) the anode electrode attracts the emitted electrons, forcing them to interact with the neutral gas molecules within the ionization region, (iii) the bias voltage of the ionization cage maximizes the ionization yield of the interaction between the electrons and the neutral gas molecules, and (iv) the repeller electrode pushes ions out of the ionization cage. Figure 2 shows an assembled ionizer. Current work is focused on characterization of the field emission cathode and gas ionizer at various conditions."
Printed Piezoelectric Thin Films via Electrohydrodynamic Deposition,"Piezoelectric components have found applications in a variety of fields including energy harvesting, biological and chemical sensing, and telecommunications. The creation of piezoelectric thin films has made possible the implementation of exciting devices that operate at higher frequency (a consequence of the reduction of the thickness of the piezoelectric material) including highly sensitive gravimetric biosensors and acousto-fluidic actuators. However, traditional manufactur-ing methods for piezoelectrics require a high vacuum, show low deposition rates, involve expensive and com-plex equipment, and require additional microfabrica-tion processes to achieve the required geometries via patterning and lithography.Electrohydrodynamic deposition harnesses the electrospray phenomenon to create ultrathin imprints from liquid feedstock (Figure 1). When the electrospray emitter operates in the cone-jet mode, stable jetting of the liquid feedstock allows for the direct writing of structures, thus, eliminating the need for steps for material removal, e.g., mask transfer and etching (Figure 2). In addition, electrohydrodynamic deposition can operate at room temperature without the need for a vacuum and can be scaled-up via electrospray emitter multiplexing.This project aims to produce piezoelectric thin films suitable for acoustic resonators and actuators via electrospray jetting of nanoparticle-doped liquid feedstock. Initial work revolved around the optimization of the deposition parameters and formulation of the liquid feedstock for the reduction of the printed line’s width and thickness, elimination of the “coffee ring” effect, and analysis of the crystallographic orientation of the films. Current work focuses on improving the film’s homogeneity, increasing the crystal orientation towards a highly oriented film, and its piezoelectric characterization and application as a sensor."
Gravitationally-driven Wicking Condensation,"Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Film-wise condensation, where the condensate completely covers the condenser surface, is prevalent in typical in-dustrial-scale systems. Dropwise condensation, where the condensate forms discrete liquid droplets, can im-prove heat transfer performance by an order of mag-nitude compared to filmwise condensation; however, current state-of-the-art dropwise technology relies on functional hydrophobic coatings, which are often not robust and therefore undesirable in industrial condi-tions. Furthermore, low surface tension condensates, like hydrocarbons, pose a unique challenge since coat-ings used to shed water often do not repel these fluids. We demonstrated a method to enhance condensation heat transfer using gravitationally-driven flow through a porous metal wick, which takes advantage of the condensate’s affinity to wet the surface and also eliminates the need for condensate-phobic coatings. The condensate-filled wick has a lower thermal resistance than the fluid film observed during filmwise condensation, resulting in an improved heat transfer coefficient of up to an order of magnitude and comparable to that observed during dropwise condensation. The improved heat transfer realized by this design presents the opportunity for significant energy savings in natural gas processing, thermal management, heating and cooling, and power generation."
Boron Arsenide Crystals with High Thermal Conductivity and Carrier Mobility,"Overheating presents a major challenge in modern electronics industry due to the increasingly higher power density. High temperatures not only limit de-vice performance, but also greatly reduce reliability and lifetime. To effectively dissipate heat from an elec-tronic chip, materials with high thermal conductivity (k) are crucial. Common electronic materials such as copper and silicon exhibit a room temperature (RT) k of 401 Wm-1K-1 and 148 Wm-1K-1, respectively. In compar-ison, diamond holds the current k record of about 2000 Wm-1K-1 at RT. However, natural diamond is scarce, and synthetic diamond still suffers from slow growth, low quality, and high cost. In addition, significant thermal stresses can arise from the large mismatch in the co-efficient of thermal expansion between diamond and common semiconductors.Recently, first-principles calculations predicted a very high RT k of about 1400 Wm-1K-1 for cubic boron arsenide (BAs), rendering it a close competitor for diamond. Our materials collaborators from the University of Houston and UCLA have grown samples of different sizes and qualities. We carried out thermal transport measurement of these sub-millimeter to millimeter-sized samples using time-domain thermoreflectance (TDTR) (Figure 1), among other methods. In some samples, we have reached thermal conductivity as high as 1200 Wm-1K-1.We have also carried out the first-principles calculation of electron and hole mobility in boron-based III-V materials. We predict that BAs has both high electron (1400 cm2V-1s-1 at RT) and hole (2110 cm2V-1s-1 at RT) mobility (Figure 2). These characteristics, together with the high thermal conductivity, make BAs attractive for microelectronics applications both as device materials and as heat sink materials."
A Simple Fabrication Method for Doubly Reentrant Omniphobic Surfaces  via Stress Induced Bending,"We developed omniphobic, doubly reentrant surfac-es fabricated with a simple method suitable for use with traditional microfabrication processes. Intrinsic stresses in deposited layers of silicon nitride induced bending of a singly reentrant microstructure, creat-ing the doubly reentrant geometry. Due to the use of standard microfabrication processes, this approach may be extended to a variety of materials and feature sizes, increasing the viability of applying omniphobic doubly reentrant structures for use in areas such as superomniphobicity, anti-corrosion, heat transfer en-hancement, and drag reduction.Figure 1 shows the fabrication process, in which standard photolithography and etches used to create singly reentrant microstructures are adopted. However, due to the stresses in deposited layers of silicon nitride, the singly reentrant structure is bent into a doubly reentrant geometry that renders the surface omniphobic. Figure 2 shows the contact angle of water and FC 40 on the surface. FC 40 has a much lower surface tension than water, which typically makes it difficult to repel. However, due to the double reentrant geometry of this surface, it is repelled."
Micro-engineered Pillar Structures for Pool Boiling Critical Heat Flux Enhancement,"Increasing the performance of phase-change heat trans-fer phenomena is key to the development of next-gen-eration electronics, as well as power generation systems and chemical processing components. Surface-engineer-ing techniques could be successfully deployed to achieve this goal. For instance, by engineering micro/nano-scale features, such as pillars, on the boiling surface, it is pos-sible to attain 100% enhancement in pool boiling criti-cal heat flux (CHF). Researchers have been working on several CHF enhancing micro- and nano-structured sur-faces for years. However, due to the complexity of CHF phenomena, there is still no general agreement on the enhancement mechanism. An investigation of the effect of micropillar height on surface capillary wicking and the associated pool boiling CHF enhancement has been conducted. Several 1 cm × 1 cm silicon micropillar surfac-es with different micro-pillar heights have been fabricat-ed using MTL's photolithography and DRIE facilities.The surfaces were characterized using MTL's Scanning Electron Microscope (SEM), as shown in Figure 1a. The surfaces were then characterized by measuring the capillary wicking rate using high-speed imaging and a custom-built capillary tube approach as presented in Figure 1b. The capillary wicking experimental results are presented in Figure 1c is demonstrating the increase in liquid transport capability by increasing the micro-pillar heights.Finally, the performances of such structures were characterized through traditional pool boiling experiments and compared with a flat silicon heater (Figure 1d). The surfaces were tested at atmospheric pressure and saturation temperature using DI water as the working fluid. The results demonstrate the benefits of wicking promoted by these structures in terms of CHF enhancement."
Superhydrophilic and Superhydrophobic Nanostructured Surfaces for Microfluidics and Thermal Management,"Nanostructured features can be used to magnify the intrinsic hydro-phobicity or hydrophilicity of a material to create superhydropho-bic and superhydrophilic surfaces [1, 2]. There has been particular interest in these surfaces for a variety of applications including self-cleaning and drag reduction with superhydrophobic surfaces [3-5]. Superhydrophilic surfaces are of interest in anti-fogging and thermal management [6-8]. Past work has demonstrated significant changes in contact angle with minimal hysteresis with the introduc-tion of nanostructured surface features [9]. Current efforts, how-ever, focus on the dynamic robustness and spreading of liquids on such surfaces We have fabricated silicon pillar arrays with cross sections of 500 nm× 500 nm, spacings between pillars of 800 nm, and heights of 5 µm (Figure 1). The pillar arrays are naturally oxidized in air to make them hydrophilic. The interaction of the spreading liquid with the fabricated pillars was studied using diffraction limited microscopy and with an environmental scanning electron microscope (Figure 2). The preliminary data (Figure 2) shows that the liquid-air inter-face is pinned diagonally. Using an energy minimization approach, theory is currently being developed to understand the effect of pil-lar spacing, height, and diameter on spreading dynamics. We have also concurrently coated the silicon pillars with a silane chemistry to create superhydrophobic surfaces. The effect of shape and size of the nanostructures on hydrophobic robustness is currently being investigated."
Design of a Micro-breather for Venting Vapor Slugs in Two-phase Microchannels,"Boiling is currently used in a variety of industries as an efficient method of cooling. Boiling, and phase-change in general, are attrac-tive because the latent heat of vaporization can be used to carry and dissipate large heat fluxes. Two-phase microchannels have been of recent interest because they promise compact and efficient solu-tions [1].However, phase-change in microchannels leads to challenges that are not present in macroscale counterparts because the governing forces are different. Surface tension forces become dominant at the microscale whereas buoyancy forces can be neglected. As a result, flow instabilities, large pressure fluctuations, and local liquid dry-out occur in microchannels, which severely limits the overall ther-mal performance.To address these problems, a few solutions have been proposed [2, 3], including the use of porous membranes or hydrophobic ports that allow vapor bubbles to escape from the microchannels as they form. The proposed solutions have drawbacks, including the inabil-ity to sufficiently remove vapor bubbles effectively and eliminate dry-out within the channels.We propose a design for a microscale breathing device that uses the combination of surface chemistry and geometry to separate vapor from a liquid flow. To better understand the physics and governing parameters for the microscale breather, we designed a test device that allows for cross-sectional visualization of a breathing micro-channel (Figure 1). We have conducted various experiments and col-lected image data to help direct our vapor breather design to achieve high vapor removal efficiencies with minimal fabrication effort and control requirements (Figure 2).The successful implementation of a microchannel with an efficient breather will allow for new cooling technologies with higher heat removal capacities that can be effectively used by the semiconduc-tor industry. The breathers also have significant promise as liquid vapor separators for use in micro-fuel cells and other applications that require phase separation at the microscale."
Microfluidic Patterning of P-Selectin for Cell Separation through Rolling,"Cell separation based on markers present on the cell surface has ex-tensive biological applications. However, current separation meth-ods involve labeling cells and label removal steps that are often slow and intrusive. Recently, we discovered that it is possible to steer cells interacting transiently with the surface through patterning of recep-tors on the surface [1]. In this paper we report microfluidic pattern-ing of P-selectin receptors to control cell rolling for label-free sepa-ration of cells. We envision a microfluidic device that would perform label-free separation of cells by rolling them on receptor patterned surfaces (Figure 1). The present work is the first step towards real-izing these devices.A microchannel defining the pattern was fabricated in PDMS and reversibly bonded onto a polystyrene substrate. Human P-Selectin was filled inside the microchannel and left overnight for physisorb-tion to complete. Later the PDMS mask was removed, and the surface was washed with PBS and finally incubated in Fetal Bovine Serum to block non-specific interactions. HL-60 myeloid cell suspension was flowed over the surface to verify patterning of P-selectin. We observed that cells interacted selectively with the P-selectin region, showing that the patterning technique was successful (Figure 2). Rolling was clearly observed on the selectin-coated bands and some deflection of cells at the edge was also observed. A few cells were also seen to detach from one band and reattach at another selectin band downstream. This work demonstrates microfluidic patterning of P-selectin that is essential for a device for cell separation based on cell rolling. In the future, these patterns will be incorporated in a smaller microfluidic flow chamber with multiple inlets and outlets for label-free, con-tinuous-flow cell separation."
Electrical Detection of Fast Reaction Kinetics in Nanochannels with an Induced Flow,"Nanofluidic channels can be used to enhance surface binding reac-tions, since the target molecules are closely confined to the surfaces that are coated with specific binding partners. Moreover, diffusion-limited binding can be significantly enhanced if the molecules are steered into the nanochannels via either pressure-driven or elec-trokinetic flow. Monitoring the nanochannel impedance, which is sensitive to surface binding, has led to electrical detection of low analyte concentrations in nanofluidic channels within response times of 1-2 h [1]. This finding represents a ~54 fold reduction in the response time using convective flow compared to diffusion-limited binding [2]. At high flow velocities, the presented method of reac-tion kinetics enhancement is potentially limited by force-induced dissociations of the receptor-ligand bonds [3]. Optimization of this scheme could be useful for label-free, electrical detection of bio-molecule binding reactions within nanochannels on a chip."
Integration of Actuated Membranes in Thermoplastic Microfluidic Devices,"PolyDiMethylSiloxane (PDMS) is a common material for fabrication of microfluidic devices. Elasticity provided by PDMS enables the creation of active devices that utilize pressurized membranes such as pumps and mixers. However, for structures requiring dimen-sional stability, rigidity, or disposability, plastics have the required properties [1]. Plastics can be manufactured using mass fabrication technologies such as injection molding and hot embossing with es-tablished bonding processes [2], but at the cost of sacrificing active device functionality. A new fabrication process combining plastic substrates with PDMS membranes enables the creation of active microfluidic devices inside dimensionally stable systems, merging the functionality of PDMS with established plastic fabrication tech-nologies.Irreversible bonding between PDMS and plastics for fluidics re-quires interfaces that can handle high pressure and harsh chemical environments. Hydrolytic stability under acidic or basic conditions is particularly important. Direct bonding between PMMA and PDMS has been explored [3], but interfaces withstood only 2.5 psi before failure. Surface modification of polycarbonate and PMMA surfaces with AminoPropylTriEthoxySilane (APTES) [4] has also been shown to enable PDMS plasma bonding [5], but no data on hydrolytic sta-bility was shown. To improve hydrolytic stability, two additional silanes were explored, BisTriEthoxySilylEthane (BTESE) and Bis(TriMethoxySilylPropyl)Amine (BTMSPA), for thin and thick primer coatings, respectively. Devices with PDMS membranes suspended over 25-µL fluid reser-voirs were fabricated in PC and PMMA to test interface robustness. For all devices, membrane ruptures occurred instead of delamina-tion at 60 psi, making the devices suitable for active valves. Blisters were then subjected to NaOH and HCl solutions from the PDMS side at 70 C for 2 hours, followed by pressure testing. Figure 1 shows that hydrolytic stability improves over APTES with addition of BTMSPA to the primer solution for thick coatings or BTESE for monolayer coatings. A test chip containing peristaltic pumps and mixers was then fabricated, and pump rate versus frequency was measured as shown in Figure 2."
Teflon Films for Chemically-inert Microfluidic Valves and Pumps,"Like transistors in electronic microprocessors, microfluidic valves and pumps are the fundamental elements of logic and control in many lab-on-a-chip devices. Flexible elastomers make good can-didates for the moving parts in valves and pumps, and elastomers like polydimethylsiloxane (PDMS) have found widespread use in a variety of normally-open and normally-closed microfluidic valves. Unfortunately, the limited chemical compatibility of PDMS has complicated its use in many microfluidic applications. Many chemi-cals commonly used in organic synthesis readily swell PDMS devices or dissolve PDMS oligomers from the elastomer. Small hydrophobic molecules readily partition into and out of bulk PDMS, complicating the determination of their on-chip concentration. Some reusable glass microfluidic devices must be equipped with removable, dis-posable valves because the PDMS valves would be destroyed by the harsh acid used to clean the device before reuse. For these reasons, a large variety of interesting and useful chemistries may be unsuitable for use in native PDMS devices.We have developed a simple alternative method for fabricating Teflon monolithic membrane valves and pumps in glass micro-fluidic devices [1]. We have found that inexpensive, commercially-available fluorinated ethylene-propylene (FEP) Teflon films can be bonded between etched glass wafers to form chemically-inert monolithic membrane valves and pumps. Both FEP and polytetra-fluoroethylene (PTFE) are comprised entirely of carbon and fluorine and are similarly inert. But while PTFE is opaque and must be cut or skived to make rough thin sheets, FEP is transparent and available as a smooth, uniform thin film. Chemical compatibility data from nearly 50 years of use as a commercial product show that FEP is re-sistant to virtually all chemicals except “molten alkali metals, gas-eous fluorine, and certain complex halogenated compounds such as chlorine trifluoride at elevated temperatures and pressures.”[2] The resulting glass-FEP-glass devices are optically transparent and suit-able for imaging or fluorescence applications (Figures 1, 2). The FEP Teflon valves permit unimpeded (0.9 µL/s) flow while open and neg-ligible (< 250 pL/s) leakage while closed against 14 kPa fluid pressure. The FEP pumps can precisely meter nanoliter-scale volumes at up to microliter/second rates. The pumps also show excellent long-term durability with < 4% change in pumping rate after 13 days of con-tinuous operation. By combining ease of fabrication with extreme chemical inertness, these Teflon monolithic membrane valves and pumps enable research involving a vast array of chemistries that are incompatible with native PDMS microfluidic devices."
Nanofluidic System for Single-particle Manipulation and Analysis,"Nanopores are versatile sensors for detection of single molecules and particles in solution. When a molecule passes through a nano-pore with a voltage bias applied across it, the resulting transient blockage of the nanopore yields a detectable current change that enables single-molecule sensing [1, 2]. Different molecules may[1, 2]. Different molecules may, 2]. Different molecules may2]. Different molecules may. Different molecules may Different molecules maymay exhibit different current blockage and duration profiles, which mayfferent current blockage and duration profiles, which may current blockage and duration profiles, which may, which may be used to characterize the molecules. However, the sensing ability of nanopores is often limited by the quick transit times of molecules through the nanopore that result in poor signal-to-noise ratio. To ToTo address this issue, we are developing a nanofluidic system to manip-ulate single particles and molecules that will enable multiple mea-surements on the same molecule (Figure 1). Nanofluidic channels will function as traps to localize the molecule in the system after its translocation (transit) through the nanopore. When the electric When the electricWhen the electric field across the nanopore is reversed, the same molecule will travel through the nanopore again. Feedback control will be used to reverse the applied voltage bias and thus ensure multiple translocations of a molecule through the nanopore. This technique will enable inte- This technique will enable inte-This technique will enable inte-gration of a signal over multiple translocations, thereby improving the signal-to-noise ratio. For proof-of-concept, translocation signals For proof-of-concept, translocation signalsFor proof-of-concept, translocation signals of DNA molecules through a PDMS (polydimethylsiloxane) nano-(polydimethylsiloxane) nano- nano-pore will be measured using a patch-clamp amplifier. Techniques of E-beam lithography and UV lithography are adopted to create aE-beam lithography and UV lithography are adopted to create aa master mold of the device consisting of nanopores and reservoirs.mold of the device consisting of nanopores and reservoirs.of the device consisting of nanopores and reservoirs.the device consisting of nanopores and reservoirs.nanopores and reservoirs. Soft lithography with PDMS will be used for rapid and reproducible PDMS will be used for rapid and reproducible fabrication of nanopores connecting two reservoirs (Figure 2) [3].nanopores connecting two reservoirs (Figure 2) [3]. [3].[3]. This approach will demonstrate a new paradigm in sensing by using nanopores, and it may enable an unprecedented level of character-ization of nanoparticles and biomolecules."
Microfluidic Systems for Continuous Crystallization,"Microfluidic systems offer a unique toolset for discovering new crys-tal polymorphs and for studying the growth kinetics of crystal sys-tems because of well-defined laminar flow profiles and online opti-cal access for measurements. Traditionally, crystallization has been achieved in batch processes that suffer from non-uniform process conditions across the reactors and chaotic, poorly controlled mix-ing of the reactants, resulting in polydisperse crystal size distribu-tions (CSD) and impure polymorphs. This reduces reproducibility, increases difficulty in obtaining accurate kinetics data, and manu-factures products with inhomogeneous properties. The short length scale in microfluidic devices allows for better control over the pro-cess parameters, such as the temperature and the contact mode of the reactants, creating uniform process conditions across the reac-tor channel. Thus, these devices have the potential to generate more accurate kinetics data and produce crystals with a single morphol-ogy and a more uniform size distribution. In addition, microfluidic systems decrease waste, provide safety advantages, and require only minute amounts of reactants, which is most important when deal-ing with expensive materials such as pharmaceutical drugs. Figure 1 shows a microfluidic device used for crystallization and Figure 2 shows optical images of different polymorphs of glycine crystals grown in reactor channels. A key issue for achieving contin-uous crystallization in microsystems is to eliminate heterogeneous crystallization – irregular and uncontrolled formation and growth of crystals at the channel surface-- and aggregation of crystals, which ultimately clogs the reactor channel. We have developed a micro-crystallizer using soft lithography techniques that introduces the reagents to the reactor channel in a controlled manner, preventing heterogeneous crystallization and aggregation. We have also inte-grated an online spectroscopy tool for in situ polymorph detection. Our ultimate goal is to develop an integrated microfluidic system for continuous crystallization with the ability to control and detect the crystal morphology, as well as obtain kinetics of crystallization through online detection."
Massively-parallel Ultra-high-aspect-ratio Nanochannels for High-throughput Biomolecule Separation,"Many bottom-up approaches have been used to build nano/meso-porous materials/filters with a good size control, but the integration of these systems into a microsystem format has been a challenge. Top-down nanofilter fabrications, on the other hand, suffered from small open volume and low throughput. For this paper, we devel-oped a top-down fabrication strategy for massively-parallel, regular vertical nanochannel membranes with a uniform, well-controlled gap size of ~50 nm and a depth up to ~40 µm, by using only stan-dard semiconductor fabrication techniques [1]. The vertical nanofil-ter membranes were fabricated into an anisotropic nanofilter array, which demonstrates the ability to integrate nanofilters and micron-sized channels/pores seamlessly. We demonstrated efficient con-tinuous-flow separation of large DNAs in a two-dimensional verti-cal nanochannel array device as shown in Figure 2. Compared with planar nanofilter systems [2], an important feature of our device is a sample processing rate as high as ~ 1 µL hour-1, and further im-provement of throughput can be achieved simply by upscaling the channel depths. These ultrahigh-aspect-ratio nanochannels have the advantage of large open volume, enabling high-throughput ap-plications."
Microfluidic Control of Cell Pairing and Fusion,"Currently, several different methods have been used to reprogram somatic cells to an embryonic stem cell-like state. Nuclear transfer and fusion methods [1] use either oocytes or embryonic stem cells (ESCs) as a source of reprogramming factors. Recently, defined fac-tors have been identified that are capable of inducing pluripotency in somatic cells2 [2]. While all three approaches can be used success-fully for reprogramming, cell lines generated are not yet suitable for potential therapeutic applications in humans and many questions remain about the process of nuclear reprogramming.We have developed a microfluidic system in which thousands of ESCs and somatic cells (SCs) are properly paired and immobilized, resulting in a high number of one-to-one fusions that can be clearly identified for further studies [3]. The device consists of thousands of cell traps in a millimeter-sized area, accessed by microfluidic chan-nels (Figure 1). The traps consist of larger frontside and smaller back-side capture cups made from a transparent biocompatible polymer. Cells are loaded sequentially in a 3-step loading protocol enabling capture and pairing of two different cell types. The geometry of the capture comb precisely positions the two cells, and flow through the capture area keeps the cells in tight contact in preparation for fu-sion. Pairing efficiencies of ~70% are possible over the entire device (Figure 2).The device is compatible with both chemical and electrical fusion. The PEG-mediated fusion is initiated by flowing PEG past the cells for 3 minutes and then rinsing with warm media. With 4 doses of PEG, we have observed that 15 % of the traps contain cells that have exchanged fluorescent proteins, and 25 % of the traps contain cells whose membranes have reorganized. A control protocol done in a standard conical tube yielded only 6 ± 4 % fusion of the same fluo-rescent cells. Electrofusion is made possible by bonding the PDMS device to a glass slide with pre-patterned metal electrodes that are then connected to a standard fusion power supply. We have ob-served membrane fusion efficiencies up to 90% and can achieve > 50% properly paired and fused cells, based on exchange of fluores-cence, over the entire device. Control fusion protocols, performed using the same power supply with a commercial electrofusion chamber, obtained only 11 +/- 9 % fusion. We have demonstrated pairing and fusion of mESCs and mEFs and are currently using the device to explore fusion-based reprogramming."
BioMEMS for Modulating Stem Cell Signaling,"The stem cell microenvironment is influenced by several factors in-cluding cell-cell, cell-matrix, and cell-media interactions. Although conventional cell-culture techniques have been successful, they provide incomplete control of the cellular microenvironment. To en-hance traditional techniques, we have developed several microscale systems for adherent cell culture of mouse embryonic stem cells (mESCs) while controlling the microenvironment in novel ways [1]. We are using stencil cell patterning and microscopic analytical tools to investigate cell-cell interactions, in particular the role of colony-colony interactions in self-renewal of mESCs. Since autocrine sig-naling in mESCs has not been thoroughly characterized, we validate our platform using a model autocrine cell line, A431 epidermoid carcinoma cells. By precisely controlling the colony size, spacing, and the medium replenishing frequency, we modulate the degree of colony-colony interactions (Figure 1). We are also using the Bio Flip Chip to investigate cell-cell signaling in mESCs. The chip is made from PDMS using replica molding, and it contains hundreds-to-thousands of microwells, each sized to hold either a single cell or small numbers of cells (Figure 1) [2]. A microscale cellular manipula-tion technique for cell-matrix interactions involves the patterning of specific protein signals around live mESC colonies in order to study the local effects of signal presentation. A photopolymerizable polymer (PEG-diacrylate) with attached proteins has been used to pattern structures around growing cell colonies in vitro, thereby exposing them to a very controlled microenvironment. Using these methods, we have patterned a known regulator of pluripotency, LIF, around mESC colonies and analyzed how far this signal propagated through the colony (Figure 1).To control cell-media interactions we have developed a two-layer PDMS microfluidic device that contains two sets of triplicate cham-bers, allowing implementation of different culture conditions on the same chip. The device incorporates a valve architecture modeled af-ter Irimia et al. [3], which enables different parts of the device to be fluidically isolated during different stages of the experiment. Using our system, we demonstrated that microfluidic perfusion can affect the soluble microenvironment. We showed that defined serum-free media (N2B27), sufficient for differentiating cells into neuronal pre-cursors in static culture [4], did not allow cells to proliferate or dif-ferentiate on-chip. On the same chip we cultured cells in N2B27 that had been supplemented with media containing cell-secreted factors from a static culture. In this media, we were able to restore growth and differentiation (Figure 2)."
Microfabricated Slits in Series: A Simple Platform to Probe Differences in Cell Deformability,"Change in cell stiffness is a characteristic of blood cell diseases, such as sickle cell anemia [1], malaria [2], and leukemia [3]. Often, increas-es in blood cell stiffness lead to loss of the cells’ ability to squeeze through capillaries, resulting in organ failure, coma, and ultimately death [4]. The goal of this project is to create a microfluidic device that can quickly and accurately screen, diagnose, and treat disorders involving cell deformability. We report the creation of a microfabri-cated device consisting of a series of 1-2 µm wide polymeric slits, as Figures 1 and 2 show. This device can potentially be used to screen and diagnose disorders involving cell deformability.The device fabrication process is depicted in Figure 1 and follows approaches similar to those in [5]. First, a 2-level negative PDMS stamp was made using soft lithography techniques from a silicon template, Figure 1a. A droplet of UV-sensitive prepolymer NOA 81 was stamped on a glass slide and exposed to UV, Figure 1b. Similarly, a droplet of NOA was stamped using a flat PDMS slab and exposed to UV on a PDMS cover sheet treated with oxygen plasma to improve the adhesion. After the stamps were peeled off , the two pieces were brought in contact and bonded by completing the crosslinking with a second exposure to UV, as in Figure 1C. Figure 1 details the device’s operation and results of fabrication."
Microfluidic System for Screening Stem Cell Microenvironments,"Embryonic stem cell (ESC) differentiation is a potentially powerful approach for generating a renewable source of cells for regenerative medicine. It is known that the microenvironment greatly influences ESC differentiation and self-renewal. Most biological studies have aimed at identifying individual molecules and signals. However, it is becoming increasingly accepted that the many kinds of signals inmany kinds of signals in of signals in the ESC microenvironment interact in a synergistic and antagonistic manner based on their temporal and spatial expression, dosage, and specific combinations. This interplay of microenvironmental factors regulates the ESC fate decisions to proliferate, self-renew, differen-tiate, and migrate. Despite this complexity, the systematic study of stem cell cues is technologically challenging, expensive, slow, and labor-intensive. Here we propose to develop a high-throughput mi-crofluidic based system that overcomes many of these challenges. We will subsequently analyze the resulting high-throughput system in elucidating specific aspects of mesodermal and endodermal dif-ferentiation in a systematic manner. A simple microfluidic screening device consisted of fluidic chan-nels, control channels, and poly(ethylene glycol) (PEG) microw-ells has been developed (Figure 1). A microfluidic screening device was fabricated by multi-layer soft lithography technique [1]. The fluidic channel made by positive photoresist (AZ 4620) is 10-µm--thick pattern with a round shape and the pneumatically actuatedpattern with a round shape and the pneumatically actuatedwith a round shape and the pneumatically actuated control channel fabricated by negative photoresist (SU-8 2150) is a 40-µm-thick pattern. To obtain a round profile of a fluidic channel, pattern. To obtain a round profile of a fluidic channel,. To obtain a round profile of a fluidic channel, the positive photoresist (AZ 4620) was reflowed at 200°C for 120 sec after development. A replica of the fluidic channel was obtained by spin-coating poly(dimethylsiloxane) (PDMS) at 1700 rpm for 1 min followed by baking at 70 °C for 1.5 hours. This process resulted in a 20-µm-thick PDMS membrane containing the fluidic channel. The crossing of the control channel over the fluidic channel formed the on-chip barrier valve. We used ES-green fluorescent protein (GFP) cells that can express Octamer-4 (Oct4), a homodomain transcrip-tion factor. The ES-GFP cells were seeded into a fluidic channel and localized within PEG microwells in a flow-based microfluidic screening device (Figure 2). The ES cells were well docked and pat-terned within a microwell, while cells that were not localized within a microwell were flowed into a reservoir. The ES cells expressed by Oct4 (green) maintained self-renewal during media perfusion (0.3 µl/min). The ES cells docked within a microwell showed high cell viability (> 90%)."
Self-assembly of Cell-laden Microgels with Defined 3D Architectures on Micro-patterned Substrate,"Most living tissues are composed of repeating units on the scale of hundreds of microns; these units are ensembles of different cell types with well-defined three-dimensional (3D) microarchitectures and tissue-specific functional properties (i.e., islet, nephron, or si-nusoid) [1]. To generate engineered tissues, the recreation of these repeating structural features is of great importance in enabling the resulting tissue function. Here, we tried to self-assemble cell-laden microscale hydrogel (microgel) units as 3D tissue constructs with defined architecture by using hydrophobic/hydrophilic interactions. By micro-contact printing [2], we patterned the glass slides with spe-cific hydrophobic and hydrophilic regions [3]. We hypothesized that the hydrophilic microgels tend to stick to the hydrophilic patterns, while not on the hydrophobic patterns. Therefore, we could control the architecture of the microgel assembly by creating different hy-drophilic patterns.To achieve microcontact printing, we first created different SU 8 pat-terns on the silicon wafer based on the photomask by using stan-dard photolithography. The SU 8 patterns were transferred to the PDMS mold, which was soaked with the hydrophobic ink. The ink was printed with specific patterns on the glass slide by microcon-tact printing. Afterwards, the slides were covered in DPBS (~600 µL of DPBS) containing microgels (approximately 1500 gel units each patterned slide). After a few minutes, the slides were tilted over, to allow the liquid to drain off the slide. Microgels remained on only the hydrophilic glass surfaces, as predicted. Below are images of mi-crogel assembly on a 1600-um square pattern on the glass slide."
High-throughput Study of Cell-ECM Interactions in 3D Environment Using Microwell Arrays,"The extracellular matrix (ECM) is critical in developing an integrat-ed picture of the role of the microenvironment in the fate of many cells. A two-dimensional (2D) microarray method was reported for cell-ECM interaction study [1]. These 2D approaches can be comple-mented by three-dimensional (3D) approaches such as embedding cells within ECM gels [2]. However, 3D microarray methods are diffi-cult to develop due to difficulties such as ECM array fabrication and nanolitre liquid handling. To overcome these difficulties, microwell array and robotic spotting may be useful.In this study, we develop an approach using a microarrayer (Piezorray) and microwell arrays for cell-ECM interaction study with high throughput. The microwell array was fabricated with soft li-thography (Figure 1). The diameter of a microwell was 400 µm with a pitch of 600 µm. In total, 2100 microwells were fabricated on a single slide with numbers and alphabets in between for identifica-tion (Figure 2A). As a proof-of-concept experiment, it was shown that dye solutions can be printed accurately into these microwells preloaded with collagen solution (Figure 2B). For future study, we will print the ECM component in a combinatorial manner into the microwell array preloaded with cells in prepolymer solutions. Then, the mixtures will be UV-crosslinked to immobilize the ECM mixture inside each isolated microwell for cell-ECM interaction study."
Amplified Electrokinetic Response by Concentration Polarization near Nanofluidic Channel,"Due to a strong electrokinetic response inside an ion-depletion re-gion created by concentration polarization, the velocity of non-equi-, the velocity of non-equi- the velocity of non-equi-librium electroosmotic flows (EOF) inside the ion-depletion zone can be 10 times faster than any equilibrium EOFs. [1, 2] Fast fluid. [1, 2] Fast fluid, 2] Fast fluid] Fast fluid Fast fluid vortices were generated at the anodic side of the nanochannel due to the non-equilibrium EOF. The vortex flow speed was estimated to be usually about 1000 µm/sec, which is about ~10X higher than that of primary EOF under the same electrical potential, and was propor-tional to the square of applied voltage, as shown in Figure 1(a). At the steady state, we can clearly observe the two counter-rotating vorti-ces beside the nanochannel, as Figure 1(b) shows. In the dual-sided nanochannel device, since the ions were depleted through both walls, the four independent vortices were formed in the four divided regions, as shown in Figure 1(c). One can independently suppress the convective part of the phenomena by decreasing the microchan-nel thickness. As Figure 1(d) shows, the size of the vortex in the dot-ted circle was approximately 2µm, which corresponded to the depth of the microchannels. We also observed that, once the particles pass the depletion zone and entered the downstream low concentration zone, they travel 25 times faster than in the buffer zones, as Figure 2 shows. These results indicate that the concentration polarization (depletion) can be utilized to make efficient and novel electrokinetic pumps and fluid switching devices, at an efficiency that has never been demonstrated."
Micropipette Interfaces for Lab-on-a-Chip Systems,"We have developed a simple to use, pipette-compatible, integrated fluid injection port to interface closed microfluidic chambers for ap-plications such as cell culture or microchamber PCR that are sensi-tive to external contamination. In contrast to open systems where fluid can be easily loaded into wells or flow-through microfluidic systems where interfacing involves bridging millimeter scale tubing with micrometer scale channels [1], filling closed chambers requires either first applying vacuum or venting the chamber. We have fab-ricated a pipette interface that automatically vents and seals upon insertion and removal of a pipette tip that can be directly integrated into fluidic devices.The injection port is composed of a deformable elastomer nipple, compression housing, and flow and vent channels that interface with the fluid chamber. A schematic of the components is shown in Figure 1a and photographs are shown in Figures 1b through 1e. When the elastomer nipple (Figure 1c) is inserted in the compression hous-ing (Figure 1d), the slit of the elastomer nipple is sealed closed, iso-lating the fluidic chamber from the external environment. Insertion of the pipette tip into the slit (Figure 1e) causes the nipple to deform, which opens the venting channel to the air while the pipette tip seals against the fluid flow channel. Actuation of the pipette plunger forces fluid into the chamber while air is vented around the pipette tip. Removing the tip reseals the port to prevent external contamina-tion. The seal can withstand at least 15psi of backpressure. The integrity of the injection port seal against bacterial contamina-tion was tested using the device shown in Figure 2, which comprised eight closed chambers of 150µL in volume interfaced with an inte-grated injection port. By visual inspection and plating, the sealed ports prevented contamination while the negative controls were clearly contaminated."
Multiplexed Proteomic Sample Preconcentration Chip Using Surface-patterned Ion-selectiveChip Using Surface-patterned Ion-selective Using Surface-patterned Ion-selective Membrane,"We report a new method of fabricating a high-throughput proteine report a new method of fabricating a high-throughput protein preconcentrator in poly(dimethylsiloxane) (PDMS) microfluidic chip format. We print a submicron-thick ion-selective membrane on the glass substrate by using standard patterning techniques. By simply plasma-bonding a PDMS microfluidic device on top of the printed glass substrate, we can integrate the ion-selective mem-brane into the device and rapidly prototype a PDMS preconcentrator without complicated microfabrication and cumbersome integration processes. The PDMS preconcentrator showed a high preconcen-tration efficiency with a factor as high as ~104 in just 5 min., which was 12x higher than our previous PDMS preconcentrator fabricated by junction gap breakdown [1]. Moreover, we have demonstrated a [1]. Moreover, we have demonstrated a. Moreover, we have demonstrated a fabrication of 10 single preconcentrators in an array format which increased the preconcentrated volume by 3 orders of magnitude compared to our previous result obtained with the silicon nanoflu-idic preconcentrator [2]. The ability to build a massively parallel ar- [2]. The ability to build a massively parallel ar-. The ability to build a massively parallel ar-ray using this technique is significant in terms of the integration of our preconcentrator to an external sensing unit such as mass spec-trometer. In addition to a shorter preconcentration time, the array can offer a sufficient amount of the concentrated sample volume to transfer it to an external sensing unit. Due to this capability, we ex-pect a high potential of our PDMS preconcentrator chip as a signal enhancement tool for a mass spectrometer to detect low-abundance proteins and peptides. Furthermore, the PDMS microfluidic format of this device would allow the integration of preconcentrator into many different BioMEMS platforms, including cellular BioMEMS devices."
Improving the Sensitivity and Binding Kinetics of Surface-based Immunoassays,"Immunoassays are currently among the most widely used diagnos-tics tools in the healthcare industry. The usage of current immuno-assays is limited by the availability of good antigen-antibody pairs, time-consuming incubation, and sensitivity limits. In particular, the sensitivity and binding kinetics are limited by the usually low concentration of molecules that we are trying to detect. One of the most common methods to overcome the limitations of sensitivity is by adding a post-binding amplification step, meaning that signals get enhanced after molecules are bound to capture antibodies. This method helps improve the sensitivity of the assay, but it fails to re-duce the time required for the sensor to reach an equilibrium value because the low concentration of molecules still takes the same time to saturate the sensor. An alternative to post-binding amplification is pre-binding amplification. By increasing concentration of mol-ecules of interest prior to their capture by antibodies, pre-binding amplification improves both the sensitivity of the sensor and kinet-ics of binding. Our lab has developed and integrated a nanofluidics-based con-centrator and has successfully integrated the device with an immu-noassay [1]. The principle behind the concentrator, electrokinetic trapping, is a space-charge induced phenomenon that can be fine-tuned using external voltage controls. After application of appropri-ate voltages, a charge-depletion zone forms near the nanochannels and excludes all charged species. If the analyte-containing fluid is continuously moved into this zone, the analytes would accumulate and their concentration increase (Figure 1). The concentrator can be combined with an on-chip assay for improved assay sensitivity. In our lab, a 1,000-fold increase in sensitivity of assays has been demonstrated with fluorescent proteins in simple buffers (Figure 2). Currently, efforts are underway to adapt the system for use with non-natively fluorescent proteins in a more complex background such as serum. Development of a surface-coating method and a pre-concentration scheme for non-natively fluorescent proteins is cur-rently the main focus of this project."
Mass-based Readout for Agglutination Assays,"Agglutination assays based on nanometer- and micrometer-sized particles were originally inspired by natural agglutination of cells [1] and provide a simple, rapid means for diagnostic testing. There are several commercial examples of agglutination assays used for clini-cal diagnostics applications. These assays are typically straightfor-ward to administer and provide fast response times. Techniques for measuring agglutination include turbidity, dynamic light scattering, and UV – Vis spectroscopy. In some cases, particle-counting tech-niques such as flow cytometry and image analysis can improve sen-sitivity by quantifying small aggregates that are produced during the initial stages of aggregation, allowing a reduction of the required in-cubation times. Additionally, particle-counting enables gathering of more specific information about the agglutination distribution in a population, rather than reliance on the average agglutination infor-mation typically obtained by ensemble measurement techniques. Furthermore, microfluidic approaches for particle-counting can re-duce the required sample volume from milliliters to microliters and enable integration with sample treatment steps. We have developed a non-optical alternative for particle counting in which early-stage aggregation is quantified by measuring mass with the suspended microchannel resonator (SMR) [1]. In SMR detec-tion, each aggregate is weighed in real-time by measuring transient changes in resonant frequency as it flows through the vibrating mi-crochannel (Figure 1). Using a model system of streptavidin-func-tionalized microspheres and biotinylated antibody as the analyte, we obtain a dose-response curve showing particle agglutination over a concentration range of 630 pM to 630 nM (Figure 2). We show that the results are comparable to what has been previously achieved by image analysis and conventional flow cytometry."
"Measuring the Mass, Density, and Size of Particles and Cells Using a Suspended Microchannel Resonator","Nano- and micro-scale particles and colloidal solutions are central to numerous applications in industrial manufacturing, nanotech-nology, and the life sciences. We demonstrate the measurement of mass, density, and size of cells and nanoparticles using suspended microchannel resonators (SMRs) [1]. The masses of individual par-ticles are quantified as transient frequency shifts while the particles transit a microfluidic channel embedded in the resonating cantile-ver. Mass histograms resulting from these data reveal the distribu-tion of a population of heterogenously sized particles. Particle den-sity is inferred from measurements made in different carrier fluids, since the frequency shift for a particle is proportional to the mass difference relative to the displaced solution (Figure 1). We have char-acterized the density of polystyrene particles, Escherichia coli and human red blood cells with a resolution down to 10-4 g/cm3.The SMR’s particle measurement capabilities are a valuable comple-ment to light scattering and other particle sizing methods currently used in numerous industrial and research applications. Of particular note is the SMR’s ability to directly measure the mass/density of in-dividual particles with high precision and accuracy. These capabili-ties provide a counterpoint to optical “ensemble” techniques such as laser diffraction, which are sensitive to the shape and optical properties of the target particles, and which for some samples are prone to artifacts and irreproducibility. In its current incarnation, the SMR excels for particles from ~ 50 nm to ~ 10 µm. Future im-provements in mass resolution may allow measurement of particles down to the ~10-nm scale. The SMR’s ability to measure particle density is unique among particle size analyzers and may be applied to applications such as the measurement of porosity and capacity of drug-loaded microspheres; the characterization of engineered porous silica used in coatings, slurries, and optoelectronics; and ex-amination of the structure of submicron-sized particles."
"Making it Stick: Convection, Reaction and Diffusion in Surface-based Biosensors","The past decade has seen researchers from a diverse range of disci-plines develop and apply novel technologies for biomolecular de-tection, at times approaching hard limits imposed by physics and chemistry. In nearly all types of biomolecular sensors, the diffusive and convective transport of target molecules to the sensor can play as critical a role as the chemical reaction itself in governing binding kinetics and, ultimately, performance. This is particularly true as ever-smaller sensors are developed to interrogate ever-more-dilute solutions. Yet rarely does an analysis of the interplay between diffu-sion, convection and reaction motivate experimental design or data interpretation.We have developed a physically intuitive and practical understand-ing of analyte transport for researchers who develop and employ bi-osensors based on surface capture [1]. Using a model sensor embed-ded within a microfluidic channel (Figure 1), we explore the quali-tatively distinct behaviors that can result (Figure 2), develop rules of thumb to quickly determine how a given system will behave, and derive scaling relations that give order-of-magnitude estimates for fundamental quantities of interest, such as fluxes, collection rates, and equilibration times. We pay particular attention to collection limits for micro- and nano-sensors and highlight unexplained dis-crepancies between reported values and theoretical limits."
Iso-dielectric Separation of Cells and Particles,"The electrical properties of cells and particles offer insight into their composition and structure as well as provide an intrinsic handle upon which separations can be based. Over the past several decades, dielectrophoresis (DEP) [1], electrorotation [2], and impedance spec-troscopy [3] have been used to characterize the electrical properties of cells. Not surprisingly, these techniques – in particular, DEP - have also proven effective for cell sorting [1]. One significant barrier in de-veloping effective electrical sorts of cells, however, is our relatively poor understanding of cells’ electrical properties and how they vary under different environmental conditions. Better understanding of how phenotype and genotype manifest themselves through the electrical properties of a cell under different environmental condi-tions is crucial for developing new screens. Towards this end, we have created a separation method – iso-dielectric separation, or IDS – that separates continuous streams of cells and particles according to their intrinsic dielectric properties [4, 5]. Iso-dielectric separation uses dielectrophoresis (DEP) and a medium with spatially varying conductivity to sort cells according to their ef-fective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [6]. The IDS leverages many of the advantages of microfluidics and equilibrium gradient separation methods to create a device that is continuous-flow, capable of par-allel separations of multiple (>2) subpopulations from a heteroge-neous background, and label-free. Additionally, because IDS offers analog separation of cells and particles according to their intrinsic properties, it can be also be used as a platform to characterize par-ticles. We have demonstrated the separation and characterization of particles ranging from polystyrene beads, to the budding yeast Saccharomyces cerevisiae, to mouse pro B cells (Figure 2), represent-ing three orders of magnitude in particle volume (~1-1000 μm3) and conductivity (~0.001–1 S/m)."
"Sub-cellular, Precision, On-chip Immobilization, Imaging, Manipulation, and Sorting of Small Animals","Today, pharmacological drug and genetic screens require use of in-vitro cell cultures due to the absence of high-throughput technolo-gies for studying whole animals. However, isolated cells do not rep-resent truly the physiology of live animals, and many multi-cellular processes cannot be screened using cell cultures alone. Although small-animal studies have significantly impacted cellular biology and continue to do so, the lack of techniques for rapid and high-throughput observation and manipulation of sub-cellular features in live animals has significantly limited the use of small-animal as-says for drug/genetic discoveries. We have recently invented and developed the first technologies to conduct critical high-through-put drug/genetic studies on whole animals at cellular resolution at unprecedented speeds [1, 2]. These technologies can greatly acceler-ate drug discovery using small animals for target identification and validation as well as compound mode-of-action screens.Using microfluidic large-scale-integration techniques, we enable sub-cellular precision high-throughput screening of C. elegans, a small semi-transparent nematode that is a powerful model organ-ism for studying a wide variety of biological phenomena. We have created a whole-animal sorter (Figure 1a), which makes use of single and multiple suction channels and an additional control layer to isolate and immobilize a single animal from a group. A microfluidic valve is opened at the input, and the circulating animals enter the sorter. A single small suction channel held at a low pressure is used to capture a single worm, and the remaining animals are washed away. The single worm is then partially immobilized in a straight configuration using multiple aspiration channels on the opposite site of the sorter. The aspiration immobilizes animals only partially, and it is not sufficient to completely restrict their motion. In order to fully immobilize the animals, we create a seal around them that re-stricts their motion completely. This is done by using a flexible seal-ing membrane that separates a press-down channel from the flow channel underneath. The press-down channel can be rapidly pres-surized to expand the thin membrane downwards. The membrane flexes on top of the captured animals, wrapping around them and forming a tight seal that completely constrains their motion in a lin-ear orientation (Figure 1b). Although the animals are constrained by the PDMS membrane from the top and bottom, they still have access to liquid media via the multiple aspiration channels on the side. The stability of the immobilization is comparable to that achieved using anesthesia, which allows imaging using high-magnification optics (Figure 1c) as well as the use of advanced techniques including fem-tosecond microsurgery and multi-photon imaging, both of which we have demonstrated on-chip [2]. The ability to flow worms at a high density, combined with the high actuation speed of the valves, means that animals can be isolated and immobilized for analysis very quickly and sorted based on highly complex phenotypes."
High-throughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Massthroughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Masshroughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Massbased Fractionation of Biological Samples in Microfluidic Chip for Massased Fractionation of Biological Samples in Microfluidic Chip for Mass Spectrometry,"We have developed a microfluidic chip for pI (isoelectric point)-e have developed a microfluidic chip for pI (isoelectric point)-have developed a microfluidic chip for pI (isoelectric point)- microfluidic chip for pI (isoelectric point)-microfluidic chip for pI (isoelectric point)-based fractionation of peptides and proteins as a sample preparationfractionation of peptides and proteins as a sample preparation of peptides and proteins as a sample preparation step for mass spectrometry (MS). The sorting chip with its multiplesorting chip with its multiple chip with its multiple with its multiple outlets allows continuous-flow binary sorting of the proteomic sam-s continuous-flow binary sorting of the proteomic sam- continuous-flow binary sorting of the proteomic sam-continuous-flow binary sorting of the proteomic sam- binary sorting of the proteomic sam-ples into positively and negatively charged molecules without usings into positively and negatively charged molecules without using into positively and negatively charged molecules without using without using any carrier ampholytes (Figure 1a). When coupled with pH titration,. When coupled with pH titration, When coupled with pH titration,with pH titration, pH titration, two fractionation steps enable us to isolate molecules within a pre-fractionation steps enable us to isolate molecules within a pre-steps enable us to isolate molecules within a pre-enable us to isolate molecules within a pre-us to isolate molecules within a pre-to isolate molecules within a pre-isolate molecules within a pre-molecules within a pre-pre-determined pI range from proteomic sample mixtures [1]. The pIpI range from proteomic sample mixtures [1]. The pIfrom proteomic sample mixtures [1]. The pI proteomic sample mixtures [1]. The pIsample mixtures [1]. The pI [1]. The pI. The pIpI information of the isolated molecules that is not provided by theof the isolated molecules that is not provided by thehat is not provided by theat is not provided by the is not provided by thenot provided by thethe standard ion-exchange chromatography can lead to a substantialcan lead to a substantial lead to a substantial reduction of peptide sequencing time in shotgun proteomics [2].peptide sequencing time in shotgun proteomics [2].sequencing time in shotgun proteomics [2]. in shotgun proteomics [2]. [2].The electrical junction inside the sorting chip was created simplyas created simply created simplycreated simply by patterning multiple submicron-thin hydrophobic layers on glasspatterning multiple submicron-thin hydrophobic layers on glassmultiple submicron-thin hydrophobic layers on glass-thin hydrophobic layers on glassthin hydrophobic layers on glasslayers on glass on glassglass substrate prior to plasma bonding with the PDMS chip (Figure 1b).to plasma bonding with the PDMS chip (Figure 1b).plasma bonding with the PDMS chip (Figure 1b). bonding with the PDMS chip (Figure 1b).bonding with the PDMS chip (Figure 1b). To demonstrate the sorting capability, we used three pI markers, 10.3, 8.7 and 6.6, in 20mM phosphate buffer solution with pH 8.4. As Figure 1c) shows, three bands were clearly visible in presence of1c) shows, three bands were clearly visible in presence of) shows, three bands were clearly visible in presence of an electric field of 200 V/cm. We could also separate two different of 200 V/cm. We could also separate two different200 V/cm. We could also separate two different00 V/cm. We could also separate two different. We could also separate two differentWe could also separate two differente could also separate two different proteins, GFP and R-Phycoerythrin, differing by only 0.5 pI units, into two streams (Figure 1d). In addition, we demonstrated the high-1d). In addition, we demonstrated the high-). In addition, we demonstrated the high-In addition, we demonstrated the high-addition, we demonstrated the high-, we demonstrated the high-we demonstrated the high- the high-he high-throughput capability of the device by processing raw samples at 1at 1 µL/min, which is sufficient for downstream, standard biomolecule, which is sufficient for downstream, standard biomolecule assays such as MS.We validated the two-step sorting result of a peptide mixture, pI 9.7,e validated the two-step sorting result of a peptide mixture, pI 9.7, pI 7.2 and pI 5.1, into three different fractions with the MALDI-MS. into three different fractions with the MALDI-MS.with the MALDI-MS. As Figure 2 shows, pI 7.2 (falling between pI 6-8) could be isolated shows, pI 7.2 (falling between pI 6-8) could be isolated, pI 7.2 (falling between pI 6-8) could be isolated from the mixture. The test of the device with more complex samplesThe test of the device with more complex samples such as human serum will ultimately demonstrate its potential in sample preparation for mass spectrometry.. Its successful develop-ment will have a significant impact on MS-based bioanalysis."
Microfabricated Devices for Sorting Cells Using Complex Phenotypes,"This research involves the development of sorting cytometer archi-tectures for genetic screening of complex phenotypes in biological cells. Our approaches combine the ability to observe and isolate individual mutant cells within surveyed populations. In this work we merge the benefits of microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities in a single platform. Biologists will leverage this flexibility to isolate cells based upon imaged dy-namic or intracellular responsesOur most recent electrical approach to image-based sorting [1] com-bines microfabricated weir structures and their efficient single-cell capture mechanics with negative dielectrophoretic (n-DEP) actua-tion (Figure 1). In these designs, we “pin” individual cells in desig-nated on-chip locations using “capture cups” formed from a pho-topatterned silicone polymer [2]. Negative DEP forces then operate as a switch to unload targeted subgroups of the weirs and prevent site-specific loading altogether in arrayed weir grouping. This func-tionality enables the placement of multiple cell types in organized single-cell patterns on a common substrate, permitting new screen-ing and response assays for cell-cell signaling dynamics. With this platform, manipulations prove feasible in standard cell-culture me-dia, thus avoiding cell health concerns associated with comparative p-DEP approaches. We have also continued developing our optical approach to image-based cell sorting. In this approach, cells are captured in a 10,000-site silicone microwell array. Following imaging, we use an infrared laser to levitate and thus sort cells out of microwells. Over the past year we have demonstrated the ability to purify cell populations up to >150× as well as sort cells based upon a localization-based pheno-type [3].Additionally, we are investigating the effects of DEP manipulation on cell physiology using a microfabricated, high-content screening (HCS) platform that applies electrical stimuli to cells and monitors the resulting subcellular molecular responses via automated fluo-rescence microscopy. The platform consists of a chip with individu-ally addressable arrayed electrodes and peripheral support electron-ics (Figure 2). We seed cells onto the chip and then expose them to a variety of electrical stresses. By monitoring the response of the cells via a fluorescent reporter cell line, we can assess how cells respond to the electric fields."
Inkjet Stimulation of Neurons,"Electrical excitation is the standard method for stimulating neural tissue [1]. Although widely used, it is not the most efficient method. We have been investigating the use of potassium ions as a method of stimulating neural tissue. The use of ionic stimulation allows for a more biocompatible and low-power method of stimulation. Initial in-vitro experiments on rabbit retina show that a modest increase (~10mM) of extracellular potassium ion concentration elicits neural responses.Our initial experiments were performed by pressure ejection of KCl using a multi-barrel glass pipette and performed on the epi-retinal side of the retina [2], as in Figure 1. However, the final envisioned device will be situated in the sub-retinal space. Furthermore, a dif-ferent in-vitro experimental platform needs to be designed to over-come the limitations of the existing setup. Furthermore, a device that allows for ejection of very small volumes (pL compared to nl) and also allows for accurate estimation of volumes ejected would greatly enhance the development of a prosthetic device using this concept. An additional advantage of ionic stimulation over electri-cal stimulation, as an investigation tool for neuroscience, is that ionic stimulation does not induce a stimulus artifact that allows for simultaneous recording from multiple neurons. Thus, it would be advantageous to build an ionic stimulation plat-form that has the capability of array stimulation. Inkjet printing technology naturally lends itself to this endeavor and is the platform of choice for our device. Figure 2 illustrates the scheme . However, our initial experiments using thermal inkjet technology met with failure for reasons including inkjet head construction and cham-ber size. Simple experiments performed using a piezoelectric inkjet head showed more promise and we are currently building a custom in-vitro stimulation platform using a piezoelectric inkjet head con-trolled by custom electronics and using a software platform based on LabView."
Flexible Multi-site Electrodes for Moth Flight Control,"Significant interest exists in creating insect-based Micro-Air-Vehicles (MAVs) that would combine advantageous features of in-sects—small size, relatively large payload capacity, navigation abil-ity—with the benefits of MEMS and electronics—sensing, actua-tion and information processing. In this work, we have developed a flexible electrode array that provides multi-site stimulation in the moth’s abdominal nerve cord. These flexible multi-site electrodes (FMEs) are implanted into moth (Manduca Sexta) pupae and direct-ly interface with the central nervous system (CNS) of the moths for flight control.The FMEs are composed of two layers of polyimide with gold sand-wiched in between and have 4 – 8 stimulation sites (Figure 1). The FMEs have a split-ring design that allows the FME to encircle the nerve cord, and the electrodes on the FME are on flexible tabs that protrude into the split ring and can bend back to make good con-tact with the nerve cord. The split-ring and tab design makes the FME adaptable to a wide range of nerve cord diameters, maintain-ing good contact as animals undergo metamorphosis and the nerve cord diameter increases. These FMEs were inserted into pupae as early as 7 days before the adult moth emerges and could stimulate pupae and adult moths. In pupae, we observed abdominal flexion using square wave pulses of ≥4 volts at various pairs of the stimulation sites, and similar behav-ior was observed in tethered adult moths. The electrode implanta-tions and stimulation experiments were performed by our collabo-rators at the University of Arizona and University of Washington, respectively. Finally, in loosely tethered flight, we have used this ab-dominal ruddering to cause the normally hovering moth to change its abdominal angle, leading to a change in flight direction (Figure 2). This demonstrates our ability to create MEMS-based electrodes that can be implanted in pupae, directly interface with the CNS, and enable control of insect flight."
Protein Separation by Free-flow Isoelectric Focusing,"Disposable, inexpensive microfluidic devices have the potential to become a robust new tool for proteomic research involving difficult proteins and protein complexes. In this work, a preparative scale free-flow IEF isoelectric focusing (FF-IEF) device was designed, in-vestigated, and optimized. Prior work on micro FF-IEF has described devices with volumes in the range of 1-2 μL [1] and a flow rate of sub-microliters per minute. A larger FF-IEF device was developed to address the needs of molecular biologists working with samples of milligrams in mass and milliliters in volume. Earlier work [1] with IEF simulations has confirmed the advantages of using non-rectilinear channel geometries. Here we present a triangular-shaped prepara-tive IEF device fabricated by soft lithography in PDMS and having 24 outlets. The triangular design facilitates the development of the pH gradient with a corresponding increase in separation efficiency and decrease in focusing time. The unique design of a triangular separation channel required the electric fields across the central channel to be optimized. After the shaping of the PDMS prior to the device binding, a functionalized polyacrylamide gel region at the bottom of the device was selec-tively controlled to adjust the ratio of the applied potential across the separation channel (Figure 1). At the device depth of 160 mm, the electric fields of as high as over 300 V/cm could be achieved. To further investigate the separation of the protein complex mixture on the microdevice, whole cell lysate of U20S was applied and separat-ed under denaturing conditions. To validate the performance of the free-flow IEF separation, selective fractions representing the acidic, neutral, or basic region were run on a traditional 2D gel. As Figure 2 shows, effective isolation of acidic (blue), neutral (green), and basic (orange) proteins from the whole cell lysate was achieved. High-mo-lecular-weight proteins were retained by FF-IEF (shown in the blue box), but they are mostly missing from the 2D gel separation. Thus using the IEF device is an advantage for biologists interested in high-molecular-weight proteins, which presently are difficult to iso-late with conventional IEF-strip 2D gel techniques. The devices can process complex biological samples and fractionate whole cell lysate at rates between 10-30 uL/min while providing greater separation of traditionally difficult proteins. These findings show the promise of inexpensive, disposable microfluidic FF-IEF devices in proteomics research."
Multiplexed Comet Assay for DNA Damage and Repair,"The use of DNA damage as a biomarker with predictive value for cancer and other diseases requires the development of a robust as-say that enables routine assessment of DNA damage levels in human samples. Many applications, such as toxicity testing and epidemio-logical studies, require an assay that is capable of testing many con-ditions or many samples in parallel. To this end, we are developing a high-throughput version of the comet assay, a well-known assay for DNA damage. The basic principle of the assay is that undamaged DNA is supercoiled and highly compact, whereas damaged DNA is composed of relaxed loops and fragments and is more mobile when electrophoresed in an agarose gel. Our assay offers many distinct advantages over other DNA damage assays, including a high level of sensitivity and the ability to detect multiple damage types. The assay can also be implemented as a measure of DNA repair kinet-ics. Despite the assay’s apparent benefits, it has been underutilized because of poor reproducibility, both from laboratory to laboratory and from user to user, and the time- and labor-intensive process of performing the assay. The major goal of this project is to overcome this assay’s limitations, such as its low throughput and poor repro-ducibility, to create a multiplexed assay for DNA damage and repair. The goal is a new tool that will be useful in a broad range of clinical, epidemiological, and experimental settings.Cell micropatterning enables spatial encoding of assay conditions, and it vastly improves spatial utilization of chips over the case with randomly placed cells. Both of these features are critical to a truly multiplexed assay platform. We have implemented single-cell mi-cropatterning using microwells1-2 fabricated directly into agarose gel (Figure 1). A negative relief mold of the microwells is fabricated us-ing photolithography of SU-8 on a silicon substrate. Molten agarose is applied to the mold and allowed to solidify, resulting in an agarose gel with patterned microwells. A cell suspension with 1− 2 ×106 cells/ml is applied to the gel, and cells are allowed to settle into the wells by gravity. Afterwards excess cells are rinsed, leaving only cells contained in the microwells. Finally, another agarose layer is applied to encapsulate the cells and to contain the DNA during the comet assay. Examples of microarrayed comets appear in Figure 2. The size of the well is a tunable parameter, which allows us to control the number of cells trapped in a single well. Additionally, this method places the cells in the same focal plane, which facilitates automated imaging, and it gives control over the cell microenvironment. We are currently developing a method for applying multiple chemical dam-aging agents to a single comet chip. With 100% filling efficiency, 200-µm cell spacing requires only 4 mm2 for 100 cells/condition, which would allow 200 conditions on a single comet chip (20 × 50 mm2 imaged area). Combining a platform for applying multiple con-ditions with the existing comet chip would provide the first truly multiplexed assay for DNA damage."
Microfluidic Devices for Studying Early Response of Cytokine Signaling,"This study presents the design, fabrication, and characterization of a microfluidic device (as shown in Figure 1) integrated with cell cul-ture, cell stimulation, and protein analysis as a single device towards efficient and productive cell-based assay development. In particu-lar, it demonstrates the feasibility of culturing human cancer cells in microliter-volume reactors in batch and fed-batch operations, stimulating the cell under well-controlled and reproducible condi-tions at early stages, and detecting the protein signals with an im-munocytochemical (In-Cell Western) assay. These microfluidic devices take advantage of microfabrication tech-niques to create an environment suitable for cell culture, biome-chanical and biochemical stimulation of cells, and protein detection and analysis. The microfluidic approach greatly reduces the amounts of samples and reagents necessary for these procedures and the re-quired process time compared with their macroscopic counterparts. Moreover, the technique integrates unit operations, such as cell cul-ture, stimulation, and protein analysis, in a single microchip. The microfluidic technique presented in this study correlates the space in the microchannels with the biological process time (cell stimulation time). Thus, a single experiment in one microfluidic device is capable of generating a multiple experimental complete temporal cell response curve, which otherwise would have required multiple experiments and manual assays by standard microwells and pipetting techniques. The developed method also provides high time-resolution and reproducible data for studies of cell signaling events, especially at early stages. These cell signaling events are dif-ficult to investigate by conventional techniques. This study reports the development not only of a cell population analysis method, but also of a single-cell detection and analysis technique to explore cell-to-cell variations. In this study, a new mi-croscope stage holder was designed and machined, and an auto cell counting algorithm was developed for single-cell analysis. This sin-gle-cell method provided data on cell-to-cell variations and showed that the average cell signaling profiles were consistent with those by population-based analysis. The integration of single-cell imaging and microfluidic-enabled measurements shows promise as a tech-nique for exploring cell signaling with single-cell resolution."
Micromechanical Actuators for Insect Flight Mechanics,"This project aims to develop MEMS actuators to aid in the study of insect flight mechanics. Specifically, we are developing actua-tors that can stimulate the antennae of the crepuscular hawk moth Manduca Sexta. The possible mechanosensory function of anten-nae as airflow sensors has been suggested [1], and recent discoveries of our collaborators reveal that mechanosensory input from the an-tennae of flying moths serves a similar role to that of the hind wings of two-winged insects, detecting Coriolis forces and thereby medi-ating flight stability during maneuvers [2]. Early evidence suggests that mechanical stimulus of the antennae may enable flight control. In addition, the crepuscular hawk moth Manduca Sexta has a wide wingspan (~110 mm) and is capable of carrying at least one quarter of its own weight. Thus, studying the flight of Manduca Sexta by at-tachment of microsystems seems plausible. The goal of our project is to design and fabricate micromechanical actuators, which will be mounted onto the moth antennae. Our collaborators will study the flight control mechanism by mechanical stimulation.Our first step was to fabricate “dummy” silicon rings for our biolo-gist collaborators for implant experimentation. Due to the nature of the moth antennae, ring-beam-ring construction was designed and fabricated, like a “shackle,” to meet the mounting requirements [3]. Our current work focuses on integrating actuators onto the mount-ing kit. A piezoelectric-bender and piezoelectric-stack are consid-ered the actuator (Figure 1). Live testing is also done while the moth is resting or flipping its wings (Figure 2). The moth apparently re-sponds to the mechanical stimulus under both circumstances by swinging its wings and abdomen. Future work will refine the actua-tor design and quantitatively analyze the moth’s reaction to the me-chanical stimulation, which might lead to successful flight control of the moth."
Biomimetically Inspired MEMS Pressure Sensor Assays for Passive Underwater Navigation,"A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the later-al line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1, 2]. The canal sub-system of the organ can be described as an array of pressure sensors [3]. Interpreting the spatial pressure gradients allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby ob-jects [2]. It also aids schooling [5]. Similarly, by measuring pressure variations on a vehicle surface, an engineered dense pressure sensor array allows the identification and location of obstacles for naviga-tion (Figure 1). We are demonstrating proof-of-concept by fabricat-ing such MEMS pressure sensors by using KOH etching techniques on SOI wafers to construct strain-gauge diaphragms.The system consists of arrays of hundreds of pressure sensors spaced about 2 mm apart on etched silicon and Pyrex wafers. The sensors are arranged over a surface in various configurations (Figure 2). The target pressure resolution for a sensor is 1 Pa, which corresponds to the noiseless disturbance created by the presence of a 0.1-m-radius cylinder in a flow of 0.5 m/s at a distance of 1.5 m. A key feature of a sensor is the flexible diaphragm, which is a thin (20 μm) layer of silicon attached at the edges to a silicon cavity. The strain on the diaphragm due to pressure differences across the diaphragm is mea-sured. At this stage, the individual MEMS pressure sensors are being constructed and tested.In parallel to the construction of a sensor array, techniques are be-ing developed to interpret the signals from a dense pressure array by detecting and characterizing wake structures such as vortices and building a library of pressure distributions corresponding to basic flow obstructions. In order to develop these algorithms, experi-ments are being performed on coarse arrays of commercial pressure sensors."
Piezoelectric Micro-power-generator: MEMS Energy-arvesting Device for Self-powered Wireless Monitoring Systems,"A novel thin-film lead zirconate titanate Pb(Zr,Ti)O3 (PZT) MEMS energy-harvesting device is designed and developed for powering autonomous wireless sensors. It is designed to harvest energy from parasitic vibrational energy sources and convert it to electrical en-ergy via the piezoelectric effect [1-4]. The new pie-shaped design always generates positive tension on the PZT layer and then positive charge output throughout vibration cycles. It produces mono-polar-ity output charge without using any additional bridge rectifier cir-cuitry, which will be a huge cost saving for commercial production of scaled-up products. Contrary to the high-Q cantilever designs, the new design has a low-Q, doubly anchored beam design, which provides a wide bandwidth of operational frequency. This will en-able more robust power generation even if the frequency spectrum of the source vibration varies unexpectedly. Furthermore, the beam shape is optimized to achieve uniform strain throughout the PZT layer [5]. In this new design, the whole thickness of the silicon wafer is used as the proof mass to increase the power of the generator. The fab-rication includes CVD of 10-micron-thick oxide, followed by spin-coating, patterning, wet-etching, and annealing a thin ZrO2 layer as the diffusion barrier layer, followed by three layers of PZT. The top interdigitated electrodes are patterned by the lift-off method out of gold. A long BOE etching through the oxide followed by a DRIE of silicon from the wafer’s back finalizes the device structure and re-leases the beams and proof mass (Figure 1). The SEM images of the released multi-beam cantilever beam design with a common heavy proof mass (an improved version of type-I PMPG) and a pie-shaped device with a center proof mass (type-II device) are imaged using scanning electron microscope (SEM) as shown in Figure 2."
MEMS Vibration Harvesting for Wireless Sensors,"The recent development of “low-power” (10’s-100’s of μW) sensing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of individu-al wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients [1]. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of aircraft structural health monitoring (SHM).This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy, compared to other transduction principles. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam and plate structures have been developed to predict the electrical and me-chanical performance obtainable from ambient vibration sources. The optimized models have been validated by comparison to prioroptimized models have been validated by comparison to prior models have been validated by comparison to prior published results [2] and verified by comparison to tests on a mac-ro-scale device [3]. A non-optimized, uni-morph beam prototype3]. A non-optimized, uni-morph beam prototype]. A non-optimized, uni-morph beam prototype (Figure 1) has been designed and modeled [4-5]. Dual optimal fre-4-5]. Dual optimal fre-]. Dual optimal fre-quencies with equal peak power and unequal voltages and currents are characteristic of the response of such coupled devices when op-erated at optimal load resistances (Figure 2). Design tools to allow device optimization for any given vibration environment have been developed for both geometries. Future work will focus on fabrica-tion and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. This work will include system integration and development, including modeling the power electronics."
A Muscle-inspired Cellular Piezo Actuator,"A muscle-inspired linear actuator that combines many piezoelectric micro-actuator “cells” into a single functional collection is designed and fabricated via a folding assembly technique. A triplet of individ-ually contractive MEMS actuator cells is designed and fabricated in series and three triplets are assembled by folding them out-of-plane around gold ribbon hinges. A triplet demonstrates peak unblocked displacement of 15.24µm, which is about 30 times amplification of the PZT strain at 10V stimulus. The loaded displacement measure- at 10V stimulus. The loaded displacement measure-at 10V stimulus. The loaded displacement measure-ments predict the 9.21µN blocking forces for the single triplet. Since the motion of the end effecter is linear and in plane, the device is arrayable in series. The use of flexible gold ribbon hinges and the folding method out-of-plane allows assembly of strings of actuators around the hinges [1].The final goal of this study is to array these actuators massively in se-ries and in parallel to make a linear actuator like an artificial muscle bundle. An improved folding assembly method such as a stacking assembly will be used to assemble hundreds of discrete piezoelec-tric MEMS actuators. This muscle-like actuator can be attached di-rectly to the skeletal structure without tendon wires and additional transmission mechanisms, simplifying micro-robot systems."
A System for Measuring Micro-scale Contact Resistance,"Designing devices utilizing micro-scale electrical contacts requires a precise knowledge of the relationship between contact force and contact resistance [1]. This relationship must be obtained experimen-tally because traditional contact theory does not always hold at the micro-scale, particularly at very low contact forces [2]. Additionally, this relationship has been shown to change with repeated cycling. The changes in this relationship are linked to physical changes of the contacts [3]. A system has been developed that measures the relationship between contact force and contact resistance for flat-on-flat micro-scale electrical contacts and also permits the contacts to be observed intermittently during testing [4]. This system is com-posed of two separate coupons, each containing a metal trace of the contact material and three KOH-etched pits. The coupons are as-sembled by placing stainless steel ball bearings into the KOH-etched pits of the bottom coupon and then placing the pits of the top cou-pon over the balls. An integrated flexure on the top coupon allows the metal traces to be brought into and out of contact, as shown in Figure 1. This kinematic coupling configuration allows the coupons to be assembled and reassembled with a repeatability on the order of a few microns [5]. During testing the metal traces are brought into contact while a load cell measures force and an integrated Kelvin structure measures contact resistance, as Figure 2 shows. This type of measurement has previously been used to measure the contact resistance of carbon nanotubes [6]. The contact surfaces are then separated and observed with an SEM. The cycle of repeated mea-surements and observation of the contact surface can be used to quantify the relationship between contact resistance and contact force and describe how this relationship and the physical attributes of the contact surface change with cycling."
A MEMS-Relay for Make-Break Power-Switching Applications,"We present a horizontal-displacement, electrostatically-actuated, MEMS relay for make-break power switching applications. The relay features {111}-plane silicon-etched electrical contacts. Experimental relays exhibit a minimum total on-state contact resistance of 130 mΩ, a response time of 750 µs, a theoretical electrical isolation in excess of 1 kV (tested to 450 V with available equipment), and a cur-rent-carrying capacity of 800 mA. The MEMS relay has been hot-switched in excess of 105 cycles without signs of performance deg-radation [1].The relay, shown in Figure 1, is composed of four double-parallelo-gram flexures (1) that serve as bearings, eight pairs of engaging and disengaging electrostatic “zipper” actuators (2), one moving {111} contact (3), and a pair of static {111} contacts (4a, 4b). The {111}-plane contacts [2] offer several advantages over traditional MEMS-relay metal contacts: they provide large travel, on the order of 70 µm, which exceeds the 30 µm required to withstand contact erosion and the 10 µm required to prevent arcing while operating in air at atmo-spheric pressure; the oblique contact geometry introduces contact wipe, which is known to enhance the contact reliability; and the contact geometry allows for an enhanced metallization process that provides low on-state contact resistance. The relay is etched in (100) Si through a combination of KOH etching and DRIE using nested masks. After evaporation of a gold seed-layer on the contacts using a shadow mask, the silicon is bonded to a glass substrate. Next, the contacts are plated with a 10-µm-thick copper and a 2-µm-thick pal-ladium-cobalt film. The device is released by dicing and packaged in a pin grid array for testing.During testing, voltages and currents were continuously monitored as the relay cycled, and the instantaneous total contact resistance was computed, as shown in Figure 2. The load current and voltage were increased until the relay showed any signs of temporary con-tact-sticking during any actuation cycle throughout the test. The maximum hot-switched current achieved without any signs of con-tact sticking was 800 mA with a resistive load and 350 mA with a 1 mH inductive load. While operating at or below these currents, the MEMS relay was hot-switched in excess of 105 cycles without signs of performance degradation. While the device operated at higher currents than the threshold, the sticking phenomenon was found to occur sporadically and to be reversible. Once stuck, the contacts recovered after the relay was cycled with the load disconnected."
Fabrication and Testing of a Fully-Integrated Multiwatt TurboGenerator,"There is a need for compact, high-performance power sources that can outperform the energy density of modern batteries for use in portable electronics, autonomous sensors, robotics, and othersensors, robotics, and other, robotics, and other applications. The current research aims to produce a fully-integrat- The current research aims to produce a fully-integrat-ed, synchronous permanent magnet microturbogenerator capable of generating 10 W DC output power using compressed air as its energy source. Past conference abstracts by Yen, et al. [1, 2] focused on the theoretical design as well as core fabrication procedures and techniques. Presently, all the silicon die fabrication is complete, andis complete, and complete, and the magnetic components are being integrated onto the die in prep-aration for power generation testing.While the magnetic integration is in progress, efforts are underway to separately test and qualify the gas-lubricated bearings that will support the magnetic rotor to very high speeds. To make the tests relevant, they are conducted on silicon dies similar to the final gen-erator dies, with the only differences being the lack of surface wind-ings and a laminated magnetic stator. Figure 1 shows a bearing rig die enclosed in an acrylic package, as well as the metal tubulations and o-rings used to bring nitrogen into the die.Three sets of bearing rig tests are currently planned. A light rotor made purely of silicon and shown in Figure 2 will be used to assess the nominal imbalance, defined as the distance between the geo-metric and mass center of the rotor, introduced by the fabrication process. This rotor has approximately half the mass of the magnetic rotor, so a solder-filled rotor twice as heavy will be tested next to determine whether the bearings perform well with a massive rotor. After these two sets of experiments are complete, the magnetic ro-tor, which has permanent magnets and a soft magnetic back iron embedded in it, will be characterized. Because the silicon die can be easily opened along its eutectic interface [2], it is anticipated that the magnetic rotor can be removed from the die after testing and reused for the generator die."
MEMS Micro-vacuum Pump for Portable Gas Analyzers,"There are many advantages to miniaturizing systems for chemical and biological analysis. Recent interest in this area has led to the cre-ation of several research programs, including a Micro Gas Analyzer (MGA) project at MIT. The goal of this project is to develop an in-expensive, portable, real-time, and low-power approach for detect-ing chemical and biological agents. Elements entering the MGA are first ionized, then filtered by a quadrupole array, and sensed using an electrometer. A key component enabling the entire process is a MEMS vacuum pump, responsible for routing the gas through the MGA and increasing the mean free path of the ionized particles so that they can be accurately detected.A great deal of research has been done over the past 30 years in the area of micro pumping devices [1, 2]. We are currently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven ac-tive-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We have constructed an accurate compress-ible mass flow model for the air flow [4] as well as a nonlinear plate deformation model for the stresses experienced by the pump parts [5]. Using these models, we have defined a process flow and fabricat-ed five generations of the MEMS vacuum pump over the past years and are currently working on improving the overall design. Figure 1 shows a schematic of the pump. For ease in testing we have initially fabricated only layers 1-3 and have constructed a testing platform which, under full computer control, drives the pistons and monitors the mass flows and pressures at the ports of the device. The lessons learned from the first four generations of the pump have led to numerous improvements. Every step from the modeling, to the etching and bonding, to the testing has been modified and improved along the way. The most recent fifth generation pump test data ap-pears in Figure 2. Figure 2a shows the measurements of the vacuum being generated in an external volume (5.6cm3) by the micropump operating at 2Hz. The pump was able to reduce the external volume pressure by 163 Torr. Figure 2b shows the micropump-generated flow rate as a function of pumping frequency (driven in a 6-stage cycle by a controlling microprocessor to move the gas from the input to the output). The performance of this pump compares very well with that of other similar scaled micropumps in the literature. Next, we plan to fabricate and test an improved overall design and develop a final set of models to fabricate any future micropumps to the de-sired specifications."
Batch-fabricated Linear Quadrupole Mass Filters,"In recent years, there has been a desire to scale down linear quad-rupoles. The key advantages of this miniaturization are the por-tability it enables and the reduction of pump power needed due to the relaxation on operational pressure. Attempts at making mi-croelectromechanical systems-based linear quadrupoles met with varying degrees of success [1-3]. Producing these devices involved some combination of precision machining or microfabrication and downstream assembly. For miniature quadrupole mass filters to be mass-produced cheaply and efficiently, manual assembly should be removed from the process.A purely microfabricated quadrupole mass filter consisting of a pla-nar design and a rectangular electrode geometry has been made. Rectangular rods were utilized since they are most amenably shaped for planar microfabrication. This deviation from the conventional round rod geometry required optimization and analysis. After we minimized unwanted effects through various simulations, we pro-posed a design (Figure 1), conceived a process flow, and fabricated the Micro-Square Electrode Quadrupole Mass Filter (MuSE-QMF) (Figure 2). The process requires the bonding of five silicon wafers and the use of deep reactive ion etching to pattern the features. It is a relatively simple process, furthering the case for mass-production of these devices.This non-conventional design will introduce non-linear resonances that manifest as peak splitting in the mass spectrum. Reported work involving linear quadrupoles operated in the second stability region shows improved peak shape without these splits [3]. It is believed that operating this device in the second stability region will provide a means to overcome the nonlinear resonances introduced by the square electrode geometry. Successful implementation of this de-vice will lead into arrayed configurations for parallel analysis and aligned quadrupoles operated in tandem for enhanced resolution."
MEMS Ejector Pumps Driven by MEMS Steam Generators,"Vacuum pumping of gases at the MEMS scale is an ongoing chal-lenge; MEMS pumps typically have pressures far above and pumping rates far below those of their macroscale counterparts. To meet this challenge, we are creating high-mass-flow-rate MEMS steam-ejec-tor pumps that are driven by MEMS-based steam generators. Ejector pumps scale favorably to the MEMS scale because the entrainment of the flow to be pumped by the driving fluid takes place over a much shorter distance in a narrow, millimeter-scale channel than in a wide macroscale channel [1]. However, delivering driving fluid from a compact source remains a significant challenge. Our solution to this challenge is MEMS steam generators that decompose hydro-gen peroxide with a liquid catalyst in order to produce the ejectors’ driving fluid. The creation of MEMS steam generators enables the creation of systems of high-performance MEMS pumps. One important objective of this work is the design, fabrication, and demonstration of the MEMS steam generator to drive the pumps. The generator decomposes hydrogen peroxide using a homoge-neous (liquid) catalyst to produce steam, which is then accelerated through a nozzle to the high velocities required for effective pump-ing. Hydrogen peroxide is selected for its availability and environ-mental friendliness. The use of a liquid catalyst eliminates common problems of catalyst poisoning and aging, and the system is sized and designed to minimize thermal losses and enable complete de-composition of the peroxide.Our work to date has primarily focused on the design and modeling of the steam generator and its interface with the pumps. Conceptually, the MEMS steam generator consists of a microscale mixer, a reactor, and a converging-diverging nozzle to accelerate the exiting flow, as shown in Figure 1. One or more steam generators would be coupled to a MEMS ejector as shown in Figure 2. Liquid H2O2 is mixed with the catalyst in the generator’s “engulfment” mixer [2] and then in-jected into the reactor, where the peroxide decomposes into steam and oxygen gas. The mixing timescale is much less than the reac-tion timescale, so that the reaction and vaporization take place in uniformly-mixed fluid inside the reaction chamber. The gaseous products are then accelerated to supersonic velocities through the converging-diverging nozzle. Models predict adequate thermal management and high performance for the generator-driven MEMS pumping system. The research now focuses on the realization and experimental demonstration of the MEMS steam generator to drive the MEMS pumping system."
Micro-Reaction Technology for Energy Conversion,"The development of portable-power systems remains an important goal, with applications ranging from the automobile industry to the portable electronics industry. The focus of this work is to develop microreaction technology that converts fuels – such as light hydro-carbons and their alcohols-- to hydrogen for use in solid oxide fuel cells or directly into electricity. Developing high-efficiency devices requires addressing difficulties in high temperature operation: spe-cifically, thermal management, material integration, and improved packaging techniques. In addition, recent work has included efforts to harness energy rejected to the environment as heat.The microreactor designed for combustion has been improved, resulting in longer residence times within the reactor. This longer residence time ensures full combustion of propane fuel over a plati-num catalyst. The channels within the reactor are etched using wet potassium hydroxide, which is the most economical etch technique available. The reactor remains suspended via thin-walled glass tubes, reducing conductive heat losses and allowing the reactor to operate at high temperatures. The tubes are brazed to the micro-reactor using a thermally-matched glass braze technique that was developed in-house. The coupling of two reactors has allowed for combustion to occur in one and ammonia cracking in the other, re-sulting in autothermal hydrogen generation.A combined reforming/separation device has been developed and demonstrated. Specifically, the hydrogen generation unit combines a 200-nm-thick palladium-silver film with a methanol reforming catalyst, e.g., LaNiCoO3. The catalytic combustion unit employs a platinum catalyst. Both units are formed in a silicon wafer by bulk silicon micromachining techniques. The energy generated in the combustion unit is efficiently transferred to the hydrogen produc-tion unit in the thermal conduction of silicon support. With a modi-fied brazing technique, the reactor is thermally insulated from its environment. The system has been demonstrated to purify hydro-gen at elevated pressures (up to 2 atm). Joint combustion/purifica-tion of the system has also been demonstrated, in which combus-tion and reforming occur simultaneously with the purification of the resulting hydrogen.Recent work has also included efforts to harness waste heat in the form of electrical energy. Thermophotovolatics (TPV) cells are be-ing integrated to harness radiation energy. Work is also ongoing to integrate thermoelectric (TE) devices to harness waste heat through intimate contact of the TE device with the microreactor."
Microfabricated Thin-film Electrolytes and Electrodes for Solid Oxide Fuel-cell Electrodes,"Micro-solid oxide fuel cells (SOFCs) are currently under intense in-vestigation for portable power applications, such as notebook com-puters and mobile phones [1, 2]. While thin-film nanostructured solid electrolytes result in lower cell losses due to ohmic resistance, grain boundaries may serve as fast diffusion pathways for cations, resulting in poorer long-term stability. The effects of grain bound-ary chemistry and interdiffusion on ionic transport have yet to be systematically investigated. To explore the relationship between performance and stability, CeO2 thin films were grown by pulsed laser deposition (PLD), as shown in a transmission electron micrograph (TEM) in Figure 1 [3]. Thin dif-fusion sources of NiO and Gd2O3 were deposited, and samples were annealed in the temperature range of 700-800˚C to in-diffuse the Ni cations heterogeneously along the grain boundaries. Confirmation of diffusion along the grain boundaries was achieved via time-of-flight secondary ion mass spectrometry (ToF-SIMS). After modifi-cation, the diffusion source was removed by a wet etching process, and Pt microelectrodes were prepared via a photolithographic lift-off process. The electrical conductivity was measured by impedance spectroscopy and two-point DC techniques, and it decreased 10x following grain boundary in-diffusion. These results are being mod-eled by examination of changes in charge-carrier profiles induced by the in-diffusion in the space charge region adjacent to the bound-ary."
Chemical Synthesis with Online Optimization in Microreactor Systems,"Microreactors are powerful instruments for scanning and optimiz-ing chemical reactions due to their enhanced heat and mass trans-fer, reduced reaction volume, and the ability to run several experi-ments in parallel. Applying fabrication principles that have been developed for integrated circuits, such as lithography, deep reactive ion etching (DRIE), oxidation, anodic bonding, and electron beam metal deposition, microreactors can be designed to accommodate a comprehensive set of chemistries. In addition to the study of chemi-cal reactions under these enhanced conditions, such as high tem-perature and high pressure, use of other process components such as mixers, heat exchangers, and phase separators can be incorpo-rated on a chip to provide a multifunctional microreactor (Figure 1). Previous work in our group has focused on exploiting these benefits in order to determine optimal reaction conditions (e.g., tempera-ture, pH, etc.) quickly, as well as to accurately evaluate the reaction kinetics for chemical syntheses related to pharmaceutical and fine chemistry sectors.[1, 2]Microreactors can also be integrated with physical sensors to pro-vide online measurement of process variables. Pressure sensors can be used to determine liquid flow rates, and temperature sensors are readily integrated by using thin film resistors or by incorporating a thin thermocouple. The progress of the chemical reaction can be monitored on-chip through UV/Vis, infrared, or Raman spectros-copy. Incorporating these measurements with traditional feedback control and optimization algorithms enables the optimization pro-cedure to be completely automated. Such an ‘intelligent’ microreac-tor system was applied experimentally for a multi-step reaction, the oxidation of benzyl alcohol by chromium trioxide to benzaldehyde with further oxidation to benzoic acid, to determine the conditions that maximize the yield of the intermediate, benzaldehyde. In a multi-parameter (e.g., reaction temperature and reagent flow rates) optimization approach, the system performed approximately 30 ex-periments in a completely automated fashion to determine the opti-mal yield of 82 – 84% (Figure 2)."
Novel Synthesis of Polymeric Nanoparticles for Drug Delivery Applications Using Microfluidic Rapid Mixing,"The development of smart targeted nanoparticles (NPs) that can deliver drugs at a sustained rate directly to specific cells may pro-vide better efficacy and lower toxicity for treating many diseases. For these applications, control of the NP properties such as size and polydispersity is of utmost importance for the particles’ end thera-peutic effects. Here we report the use of rapid microfluidic mixing using hydrodynamic flow focusing to control self-assembly of poly-meric NPs. Self-assembly occurs through nanoprecipitation, a pro-cess that involves dilution of a block copolymer from a solvent to an anti-solvent resulting in the precipitation of NPs [1]. We demon-strated that through the rapid mixing of precursors with anti-solvent (i.e., water), the particle size could be tuned and more homogeneous NPs could be synthesized. This work is the first implementation of nanoprecipitation on a microfluidic platform. The PDMS microfluidic devices were used to synthesize PLGA-PEG NPs by mixing PLGA7400-PEG3500 in acetonitrile (50 mg/ml) with water (anti-solvent). Hydrodynamic focusing was achieved by con-trolling flow rates with syringe pumps. Figure 1 shows the polymer stream being focused by two water streams as well as a TEM image of the resulting NPs. Figure 2 shows the change in PLGA-PEG particle size as mixing time is varied. Mixing time (~ 1-10 ms) can be tuned by changing the flow ratio of the solvent to anti-solvent. These re-sults agree with the idea that self-assembly of block copolymers into NPs by nanoprecipitation yields smaller particles as mixing time is decreased [2].This work demonstrates that microfluidic synthesis of polymeric nanoparticles with rapid mixing allows for tuning of NP size and dis-tribution through control of flow rates. These results lay the founda-tions of a microfluidic platform for controlled synthesis of NPs that may result in improved performance in drug delivery applications."
"Design, Fabrication, and Testing of Multilayered, Microfabricated Solid Oxide Fuel Cells (SOFCs)","Microfabricated solid oxide fuel cells were investigated for portable power applications requiring high energy densities [1]. The thick-ness of the electrolyte, the travel length of oxygen ions, was reduced down to ~150nm. The tri-layers (yttria-stabilized zirconia (YSZ) as an electrolyte and platinum-YSZ cermet as cathode/anode) were sputter-deposited on a silicon wafer, and then they were released as square plates by KOH-etching the silicon through patterned silicon nitride masks on the back side. High intrinsic and extrinsic (thermal) stresses due to fabrication and operation (25-600oC) [2], respective-ly, require careful thermomechanically stable design of µSOFCs. First, material properties of the ultra-thin YSZ were characterized experimentally and found to be significantly different than those of bulk YSZ [3]. Second, based on the obtained properties, maximum stresses in the plates at 625°C were analyzed using non-linear von Karman plate theory [4]. The stresses showed three regions with sidelength variation: un-buckled regime, buckled regime with high stresses, and post-buckling regime with lower stresses (see Figure 1). The µSOFCs were fabricated in the post-buckling regimes with ~80-~180-µm sidelength and total ~450-nm thickness. With the plates buckled as shown in Figure 2, the µSOFCs produced power output of 0.008mW/cm, lower than the expected power from their electrochemical test. Given the high-performance predicted for the underlying nano-structured ultra-thin electrolyte, anode, and cath-ode layers, additional studies are needed to improve specimens and test setup and to assess µSOFCs’ long-term operational stability."
Microscale Singlet Oxygen Generator for MEMS-based COIL Lasers,"Conventional chemical oxygen iodine lasers (COIL) offer several important advantages for materials processing, including short wavelength (1.3 µm) and high power. However, COIL lasers typically employ large hardware and use reactants relatively inefficiently. This project is creating an alternative approach called microCOIL. In microCOIL, most conventional components are replaced by a set of silicon MEMS devices that offer smaller hardware and improved per-formance. A complete microCOIL system includes microchemical reactors, microscale supersonic nozzles, and micropumps. System models incorporating all of these elements predict significant per-formance advantages in the microCOIL approach [1].Initial work is focused on the design, microfabrication, and dem-onstration of a chip-scale singlet oxygen generator (SOG), a micro-chemical reactor that generates singlet delta oxygen gas to power the laser. Given the extensive experience with micro-chemical reactors over the last decade [2], it is not surprising that a microSOG would offer a significant performance gain over large-scale systems. The gain stems from basic physical scaling; surface-to-volume ratio in-creases as the size scale is reduced, which enables improved mixing and heat transfer. The SOG chip being demonstrated in this project employs an array of microstructured packed-bed reaction channels interspersed with microscale cooling channels for efficient heat re-moval [3]. To date the device has produced oxygen concentrations of 1017 cm-3, yields approaching 80%, and molar flowrates in excess of 600x10-4 moles/L/sec [4]. The yield and molar flowrates indicate a significant improvement over the macroscale SOG designs."
Templated Assembly by Selective Removal,"Templated assembly by selective removal (TASR) is an effective technique for site-selective multi-component assembly at the nano- and micro-scales. In this project, the TASR approach has been cre-ated, demonstrated and quantitatively modeled; work to expand the technology and exhibit practical applications is now underway. The TASR approach offers great promise for assembling arbitrary (not necessarily periodic) systems of multiple different types of na-noscale components, such as electronics and biological or chemical sensing devices. It also offers a path to a new kind of shape- and size-selective chromatography.TASR employs a combination of chemistry, surface topography and controllable ultrasonically-induced fluid forces to assemble diverse sets of objects selectively from fluid into designated sites on a 2D surface [1]. Figure 1 shows a schematic layout of the process set-up. The components and the substrate, after undergoing chemical sur-face modification by a coating of an adhesion promoter, are placed in a fluid environment for the assembly process and megahertz fre-quency ultrasound is applied to the fluidic bath. Competition be-tween the chemical adhesive effects and fluidic removal forces takes place in which adhesive forces emerge as stronger for components in a well-matched site. The selectivity is based on the degree to which the component to be assembled matches the shape and dimensions of the surface topography at that location. Figure 2 is an optical mi-crograph showing the successful assembly of 600 nm and 2 mm di-ameter silica microspheres using TASR. Experiments are now being conducted to extend the technique to a variety of different materials such as biological cells, polymers and nanorods which vary mark-edly not only in their physical configuration and properties but also in their chemical interaction with the substrate onto which they are to be assembled. Thus, TASR can be used as a low-cost nanofabrication method with the ability to create complex, arbitrary patterns. We are also investigating the extension of TASR to a shape- and size-sensitive separation mechanism enabling the fabrication of a filtering device with chromatography applications. Present work focuses on the ex-tension of TASR to smaller size scales, a diverse set of component shapes and materials, and improved template fabrication tech-niques with the goal of demonstrating numerous practical applica-tions enabled by this approach."
Transplanting Assembly of Individual Carbon Nanotubes to MEMS Devices,"The biggest challenge in integrating nanostructures to MEMS is how to handle and assemble individual nanostructures. We demonstrate a novel assembly method for fabricating CNT-tipped atomic force microscopy (AFM) probes at a high rate and controllable quality via integrating the CNT into MEMS. Its key idea is to grow individual CNTs on a separate substrate and to transplant a well-grown CNT to the target location on a MEMS cantilever (Figure 1). This assem-bly concept transforms the scale of CNT assembly from nano-scale to micro-scale, which enables even manual assemble of individual CNTs in a deterministic way. An array of CNTs is grown from the nickel (Ni) nano-dots defined on Si substrates using electron-beam lithography followed by metal de-position and lift-off processes. Each CNT is embedded into a MEMS scale polymer block that serves as a CNT carrier. A double polymeric layer encapsulation process with SU8 (top) and PMGI (bottom) en-ables an easy release from the substrate and a deterministic length control of the CNT tip. Manual assembly of a polymer block to the end of a tipless AFM cantilever forms a CNT-tipped AFM probe. No laborious weeding, trimming, or welding process was required, and the transplanting assembly technique enables reliable assembly of CNT tips on various AFM cantilevers. The exposed CNT tip normal to the sample surface is 1.5 µm long, which corresponds to the thick-ness of the bottom layer (Figure 2, top). The scanning results over a grating with 3-µm pitch and 100-nm-deep vertical trenches shows that our CNT-tipped AFM probe scans the vertical trenches close to their vertical walls. The scanning on a biological sample (filament actins) demonstrates the potential of a CNT-tipped AFM probe for use with soft biological samples (Figure 2, bottom). This technology makes readily feasible massive parallel assembly, which will be pursued in the future."
Surface Micromachining via Digital Patterning,"Conventional microelectromechanical systems (MEMS) fabrication relies heavily on the semiconductor manufacturing paradigm. While this model is well-suited for planar devices such as integrated cir-cuits, it is drastically limited in the design and fabrication of three-dimensional devices such as MEMS. From a commercial viewpoint, this paradigm also poorly fits MEMS because the lower market de-mand makes it harder to offset the high production costs. Ridding MEMS fabrication of its reliance on such techniques may introduce several advantages, namely a wider base of substrate materials as well as decreased manufacturing costs.Our project investigates severing MEMS fabrication from the tradi-tional paradigm via digital patterning technologies. We have previ-ously shown how MEMS can be used for the direct patterning of small molecular organics [1]. Using similar concepts, we have shown that surface micromachining can also be achieved.In 2007-2008, we identified a viable material set for our surface mi-cromachining process’ sacrificial and structural layers: poly-meth-ylmethacrylate (PMMA) and silver nanoparticles. To account for surface non-uniformity of the deposited PMMA, we employed sol-vent vapors to effectively lower the polymer’s glass transition tem-perature and cause reflow at room temperatures [2]. To limit surface wetting and increase material loading of the silver nanoparticles, we deposited a PMMA reservoir to contain the silver nanoparticle solu-tion (Figure 1). Free-standing cantilevers were fabricated (Figure 2), confirming that these techniques can be used for a surface micro-machining process.The next stage will be to fabricate additional MEMS structures and test the silver nanoparticle’s mechanical properties. These proper-ties will be used to design and fabricate a demonstration system based on our surface micromachining process. Subsequent stages will consist of creating a library of digital fabrication processes so that entire MEMS devices can be fabricated without the use of semi-conductor manufacturing techniques."
Vertical Growth of Individual CNTs/CNFs as Building Blocks for Functional Nano-devices,"We grow the vertically aligned single-strand CNT/CNF with thegrow the vertically aligned single-strand CNT/CNF with the the vertically aligned single-strand CNT/CNF with thewith the plasma-enhanced chemical vapor deposition (PECVD) machine-enhanced chemical vapor deposition (PECVD) machineenhanced chemical vapor deposition (PECVD) machine machine we developed [1]. We found that ammonia (NHdeveloped [1]. We found that ammonia (NHWe found that ammonia (NHe found that ammonia (NHammonia (NH (NH3) gas etching is one etching is one of the key process parameters in growing vertically aligned CNTs/ in growing vertically aligned CNTs/vertically aligned CNTs/ aligned CNTs/s//CNFs. The NHs. The NH. The NH3 gas etches Ni catalyst layers to form nanoscale islands while the NH3 plasma etches deposited amorphous carbon.A 30-nm-thick Ni layer is deposited on top of a 25-nm-thick titaniumNi layer is deposited on top of a 25-nm-thick titaniumis deposited on top of a 25-nm-thick titaniums deposited on top of a 25-nm-thick titaniumtop of a 25-nm-thick titanium25-nm-thick titanium-nm-thick titaniumnm-thick titanium-thick titaniumthick titanium layer where CNT/CNF forest can grow vertically. For individual where CNT/CNF forest can grow vertically. For individual. For individual CNT/CNF growth at deterministic locations, 100~200-nm-sized growth at deterministic locations, 100~200-nm-sized, 100~200-nm-sized-nm-sizednm-sized-sizedsized nano dots were made by the E-beam lithography process (Raith,ere made by the E-beam lithography process (Raith, made by the E-beam lithography process (Raith,the E-beam lithography process (Raith,E-beam lithography process (Raith, process (Raith, (Raith, SEBL). The individual CNT/CNF growth requires shorter NHThe individual CNT/CNF growth requires shorter NHindividual CNT/CNF growth requires shorter NH growth requires shorter NH shorter NHshorter NH3 etch-ing time than is needed for a large-area forest growth. We obtained. We obtainedWe obtainede obtained a well-grown array of vertically aligned individual CNTs/CNFs withwell-grown array of vertically aligned individual CNTs/CNFs with-grown array of vertically aligned individual CNTs/CNFs withgrown array of vertically aligned individual CNTs/CNFs witharray of vertically aligned individual CNTs/CNFs withvertically aligned individual CNTs/CNFs withindividual CNTs/CNFs with CNTs/CNFs withs/CNFs with/CNFs withs with with 5~10 µm in length (Figure 1). High-resolution transmission electronµm in length (Figure 1). High-resolution transmission electronm in length (Figure 1). High-resolution transmission electronin length (Figure 1). High-resolution transmission electronlength (Figure 1). High-resolution transmission electronHigh-resolution transmission electron microscopy (HRTEM) images show fishbone structures with mul-tiple layers parallel to the etched surface of a Ni dot and the spac-and the spac-the spac-ing between the layers is measured as 0.34 nm, which confirms that they are stacked graphene layers (Figure 2). (Figure 2).In this research, we obtained vertically aligned individual CNTs/n this research, we obtained vertically aligned individual CNTs/s//CNFs on predefined location. We found that NH3 time in gas states on predefined location. We found that NH3 time in gas state on predefined location. We found that NH3 time in gas statepredefined location. We found that NH3 time in gas state location. We found that NH3 time in gas state is one of important parameters which affect in growing CNTs by PECVD. These individual CNTs/CNFs will be excellent candidatess as building blocks for functional nano-devices such as an AFM tip,building blocks for functional nano-devices such as an AFM tip,s for functional nano-devices such as an AFM tip, for functional nano-devices such as an AFM tip,an AFM tip,AFM tip, photovoltaic cell, super capacitor, and so on."
"High-pressure, High-temperature, Continuous Micro-flow Synthesis of Narrow Size-distribution Quantum Dots","We have developed a high-pressure, high-temperature continuous-flow Silicon-Pyrex microreactor for the synthesis of CdSe quantum dots (QDs). The microreactor consists of a 400-µm-wide and 250-µm-deep channel with a 0.1- m-long mixing zone maintained at room temperature and a 1-m-long reaction zone heated up to 350°C. The two zones are separated by a thermally isolating halo etch that allowed for a temperature gradient of over 250°C. High-pressure modular compression fluidic connections are realized by compress-ing the microreactor between two stainless steel parts using silicone O-rings. In this configuration, the set-up allows reaching high pres-sure (up to 15 MPa) and temperature (up to 350 °C in the heated sec-tion). The entire set-up (Figure 1) is first pressurized from the inlet to the outlet using a pressurized nitrogen gas cylinder. Thereafter, the nitrogen valve is closed and the two precursor solutions are de-livered independently using a high pressure syringe pump, insuring good control of the flow rate. Applying pressure allows the use of more conventional solvents like hexane, instead of high-boiling-point solvents (squalane) used previously [1]. One can even reach the low-viscosity supercritical fluid phase of hexane (Tc = 234.7 °C and pc = 3.03 MPa, 20 < η < 70 µPa.s). In contrast to viscous single-phase flow reactors, the supercritical fluid flow approach enables narrow distribution of residence time, factors which have strong influence on the ultimate QD size distribution, as well as higher nucleation rates. Cadmium and selenium precursor solutions are delivered sep-arately in the cooled mixing region and are thereafter allowed to re-act in the heated region. The use of supercritical hexane has a strong effect on the size distribution of the QDs and consequently the Full Width at Half Maximum (FWHM) of the emission peak (Figure 2). The size distribution for QDs synthesized in hexane, 4 - 5% (FWHM: 25 - 26 nm), is much smaller than for those synthesized in liquid squalane, 9 - 11% (FWHM: 45 - 49 nm)."
Modeling of Pattern Dependencies in Hot Embossing Processes,"The embossing of thermoplastic polymeric layers has proved to be both a lithographic technique with exceptional lateral resolu-tion and a promising approach to high-volume microfabrication. Understanding of the mechanics of hot embossing is well developed thanks to experimentation and meticulous finite-element model-ing [1], but it is not practicable to extend such approaches to the feature-rich embossed patterns of real devices. What is needed is a computationally efficient simulation technique that can predict the fidelity of an embossed topography, given an arbitrary stamp layout and a chosen embossing temperature, pressure, and loading dura-tion. Previous attempts to develop efficient embossing simulators have modeled the polymer as a Newtonian fluid [2], an assumption that neglects the elasto-plastic and rubbery behavior that is present between the glass-transition and melting temperatures of popular embossing materials such as polymethylmethacrylate (PMMA). We present a highly computationally efficient way of simulating the deformation of a polymeric layer when embossed with an arbitrarily patterned stamp [3]. Our approach takes a discretized stamp design and iteratively finds the distribution of stamp–polymer contact pressure that is consistent with the stamp’s remaining rigid while the polymer deforms. We model the polymer in its rubbery regime as a perfectly elastic layer with a temperature-sensitive Young’s modulus; we find the overall embossed topography by convolving the pressure distribution with the response of the polymeric sur-face to unit pressure applied in one cell of the discretized region. This topography is assumed to be “frozen” in place by cooling be-fore unloading. The simulation is implemented in Matlab and the convolution uses Fast Fourier Transforms so that we can complete simulations containing ~106 elements within a few minutes using a standard desktop computer. We can additionally represent plastic flow of the polymer during embossing by scaling the point-load-re-sponse function and performing a time-stepped simulation."
Inexpensive Metrology Approaches for Process Variation in Polymeric MEMS,"Polymeric materials, often inexpensive, tough and transparent, are attractive for manufacturing micro- and nano-fluidic devices. Here we describe three projects to develop tools for monitoring polymer-ic microstructure production. The first uses diffraction to identify dimensional defects in embossed thermoplastic components. A col-limated laser beam is shone through a component whose micro-embossed structure includes a specially designed holographic test pattern that spatially modulates the phase of the transmitted light (Figure 1). A far-field diffraction pattern is formed, yielding infor-mation about the embossed topography without requiring precise alignment of, or contact with, the manufactured part [1].The second project uses moiré interference to study distortions of hot-embossed polymeric substrates. The only apparatus required is a desktop image scanner and a precisely printed reference grid. The reference grid and a substrate embossed with a grid of the same tar-get pitch are placed on the scanner and rotated by hand until moiré fringes are seen. At least two scans are made, each with a different relative reference–part rotation. These rotations are extracted from the images and, together with the moiré fringes’ orientations and spacings, reveal the part’s distortions.Thirdly, we are designing a way of testing the toughness of bonds between polymeric layers. The UV/ozone- and plasma-activated bonding of polymeric layers is appealing because, unlike with ther-mal fusion bonding, microstructures at the interface remain intact [2]. However, the lack of a simple bond test method has impeded the development of these processes. Our approach is to pattern one of the layers with one or more steps to ~1 µm deep. The bonding pro-cess is then performed and, immediately after bonding, the layers peel apart locally around each step (Figure 2). The lengths of the re-sulting cracks are measured with optical microscopy, revealing the bond’s toughness. Interfacial cracks are usually shorter than a mil-limeter, meaning that these test structures can be interspersed with functional devices."
Relationship between Pad Properties and CMP Planarization,"Chemical mechanical polishing (CMP) is a key technology in semi-conductor and micromachining processes. In previous work, our group proposed semi-empirical and physically-based die-level CMP models to understand and optimize the dielectric planarization process [1]. In this work, we seek to understand how planarization model parameters relate to specific pad properties. In particular, we are interested in how pad bulk stiffness and pad surface properties affect both within-chip planarization and step-height reduction or planarization efficiency. Our recent work has investigated pad hardness effects on polish-ing performance by fitting experimental data from the polishing of patterned wafers and by extracting model parameters related to effective pad Young’s modulus and height distribution of surface as-perities. We polished wafers with the same pattern-density arrays and the same initial oxide thickness using four pads with different hardness (standard, high, low and very low), and measured the ox-ide thickness and step-height evolution during the process.From the data fitting and model prediction, we conclude that the standard hardness pad achieves faster planarization and has better linear step-height removal for a longer time than the other pads, as shown in Figure 1. All pads have a pattern-density dependency ef-fect; however, the stiffer pads show less oxide thickness variation across the chip. Figure 2 shows the evolution of step height at the test point on a 50% pattern density array boundary next to a 10% layout pattern density area. This edge planarizes faster than the ar-ray center point because of the lower effective pattern density. We also see that the very low hardness pad has substantially different step-height removal behavior than the other pads, indicating that the pad surface asperity height distribution may be substantially dif-ferent for this pad. Current work is seeking to make direct pad physi-cal measurements in order to verify the relationship between both pad surface (asperities) and bulk effective modulus and the resulting planarization performance."
Cascaded Mechanical Alignment for 3D MEMS Assembly,"The fabrication of MEMS devices relies, for the most part, on tools and processes developed originally for the fabrication of electronic chips in the IC industry. However, in contrast to electrical circuits, functional micro-electro-mechanical systems need features that are three-dimensional (3D) in structure. To capitalize on the well-de-veloped techniques and equipment of 2D-patterning technologies, we have developed a method to create 3D MEMS devices by folding, aligning and latching 2D micro-fabricated films.The folding process is relatively well developed. Various methods for bending films out-of-plane have been demonstrated, including thermal contraction [1], stress gradients [2], surface tension [3], and external magnetic forces [4]. However, aligning the folded segments and latching them, while maintaining the structural integrity of the MEMS devices, remain challenges.We have designed, fabricated, and tested a mechanical alignment mechanism that enables the precise angular positioning of 2D mem-branes to form 3D structures. The alignment system is based on a cascaded set of triangular protrusions on the target segment and rhombic holes on its corresponding aligning segment. Upon fold-ing, the protrusion-hole pairs start to engage sequentially, starting with the pair closest to the fold. The alignment progresses in a zip-per-like manner, allowing a large range of correction as well as high alignment accuracy (Figure 2). We have demonstrated our align-ment mechanism by assembling a corner-cube retro reflector. The alignment system’s accuracy was within 16 mrad and the measured range of correction was 0.38 rad [5]. We have also demonstrated the ability to simultaneously align multiple segments at different angles (Figure 2)."
Direct Printing of PZT Thin Films for MEMS,"In 2007-2008, we reported a new method for depositing piezoelec-tric thin films via thermal ink jet (TIJ) printing of a modified PZT sol-gel [1]. Direct printing of lead zirconate titanate (PZT) thin films eliminates the need for photolithographic patterning and etching, allows for controlled deposition over non-planar topographies, and enables the deposition of films with varied thickness. We developed conditions of deposition and crystallization for high-quality PZT thin films via thermal inkjet printing, including solution chemistry, printing conditions, and thermal processing parameters. The inks developed for this work were based on a commercially available PZT sol gel. Dilution of the sol is required to control the evaporation rate and characteristic dimensionless numbers of the ink, and our work included a jetability study of various solution chemistries. This study resulted in an ink that can be jetted reliably and is made up of 50% isopropanol, 30% 2-methoxyethanol, %15 A6 sol-gel, and 5% ethylhexanoic acid. This work also investigated factors that control the droplet size and the contact angle of the PZT ink deposited on a Pt substrate. The edge roughness of deposited lines was controlled to +/- 10µm. We further investigated the effect of droplet size, spac-ing, ink boiling point and substrate temperature on the deposited film uniformity. Figure 1 demonstrates the effect of substrate tem-perature on the film topography. Films between 100-500nm in thickness with a variation of less than 40nm were produced (Figure 1b). A capacitor test device was fabricated with approximately 400 nm of printed PZT between two platinum electrodes. The bottom electrode was 200 nm Pt/20nm Ti/200 nm SiO2/Si. The capacitor area was 6.25·10-4 cm2. Finally, it was determined that the modified ink requires a prolonged drying step to remove added solvents, and pre-dryed films showed a drastically improved polarization perfor-mance (Figure 2)."
Printable Microfluidic Valves Composed of Thermosensitive Hydrogels,"A method for fabricating compact microfluidic valves using ther-mal inkjet printing is presented. Poly(N-isopropylacrylamide) (poly(NIPAAm)) is a temperature-sensitive hydrogel that shrinks when heated above a Lower Critical Solution Temperature (LCST) (~32°C). With the swelling behavior of poly(NIPAAm) as a flow con-trol mechanism, a compact microfluidic valve has been designed and fabricated. The proposed valve provides a series of benefits over conventional microfluidic valves such as the “Quake” valves [1], in that they allow for the use of single-layer PDMS microchannels. Additionally, the need for a bulky external pump is eliminated by localized electromagnetic heating if the hydrogel valve.The design of the proposed valve is composed of an SU-8 microwell into which the prepolymer NIPAAm solution is printed (Figure 1). The well contains micro-anchors to ensure that the hydrogel always shrinks downward in order to prevent any unintended blocking of the flow channel. After the prepolymer solution with photoinitia-tors has been printed into the wells, it is polymerized using ultra-violet light. Finally, the PDMS channel is placed above the well. This channel contains discontinuities at the location of the valves, which block the flow when the hydrogel is in a swollen state. When the poly(NIPAAm) valves are heated above the LCST, the hydrogel plug shrinks and allows flow (Figure 2). The amount by which the gel plug shrinks depends on monomer concentration and UV expo-sure energy. By fabricating the wells on a separate substrate from the channels, users can use the same valve substrate with a variety of different fluidic circuit designs. Device geometry was chosen using CFD to minimize pressure drops across the valve."
Integration of Printed Devices and MEMS,"As part of an overall effort on Non-Lithographic Technologies for MEMS and NEMS, we are de-veloping processes for the integration of printed MEMS and devices. The goal of this project is to demonstrate the power of a printed technology for microsystems. We have already developed a surface micromachined cantilever technology that utilizes silver as a structural material and a novel organic spacer. Further, we have developed a family of both inorganic and organic devices that can ulti-mately be printed. As an initial demonstration, we are building a MEMS capacitive accelerometer that integrates the silver surface micromachined proof mass and spring with a capacitive sense circuit fab-ricated using organic FETs."
The MIT-OSU-HP Focus Center on Non-lithographic Technologies for MEMS and NEMS,"This center is part of a set of centers on MEMS/NEMS fundamen-tals supported by DARPA. The MIT-OSU-HP Focus Center aims to develop new methods for fabrication of MEMS and NEMS that do not use conventional lithographic techniques. The Center leverages the leading expertise of MIT and OSU in MEMS and printed devices, with the printing expertise of HP. The Focus Center is organized into four primary areas: tools, materials and devices, circuits, and dem-onstration systems.In the area of tools, we are leveraging the existing thermal inkjet (TIJ) technology of HP and augmenting it with specific additional features, which expand the palette of available materials for print-ing. We are developing materials and devices over a broad spectrum from active materials and photonic and electronic materials to me-chanical materials. In the circuits area, we are studying the behavior of the devices that can be realized in this technology with the goal of developing novel circuit architectures. Lastly, we intend to build several “demonstration” systems that effectively communicate the power of the new technologies that will emerge from this center. In the past year, the center has succeeded in demonstrating a number of the key “building blocks” for a fully printed system. Specifically, we have created printed transistors, printed optical elements (light emitters and photodetectors), printed active materials (piezoelec-trics), and a printed MEMS structure (micro-cantilever). Looking forward, we will begin efforts to integrate some of these building blocks."
Inkjet-printed Quantum Dot and Polymer Composites for AC-driven Electroluminescent Devices,"We introduce a technique for the reliable deposition of intricate, multicolored patterns using a quantum dot (QD) and polymer com-posite and demonstrate its application for robust AC-driven displays with high brightness and saturated colors. The AC electrolumines-cent (AC EL) devices are a well-established technology [1]. Their rel-atively simple fabrication and long operating lifetimes make them desirable for large-area displays; however, a major challenge with AC EL remains finding efficient and stable red phosphors for mul-ticolored displays. Colloidally synthesized QDs are robust, solution-processable lumophores offering tunable and narrowband photolu-minescence across the visible spectrum [2]. By integrating QDs into an AC EL device, we demonstrate patterning of saturated red, green, and blue pixels that operate at video brightness.The concept behind the device operation is optical downconver-sion: red and green QDs absorb blue electroluminescence from phosphor grains and then emit at longer wavelengths. The device, pictured schematically in Figure 1, is fabricated with a layer-by-lay-er approach that is compatible with flexible substrates. A QD and polyisobutylene (PIB) solution is printed on conductive indium tin oxide (ITO) using a Hewlett Packard Thermal Inkjet Pico-fluidic dis-pensing system (TIPs). Figure 2a shows examples of the intricate and multicolored patterns possible. The electroluminescent phosphor paste (ZnS:Cu powder in a transparent binder from Osram-Sylvania) is deposited uniformly over the sample using a disposable mask and doctor-blading to define the device area. Top contacts are made with conductive tape from 3M. This basic device structure is assembled and tested entirely under atmospheric conditions. When an AC voltage waveform is applied across the device, we measure spectrally pure QD emission in the red and green and ~100 Cd/m2 brightness. Photographs of the red, green, and blue pixels of a working, AC-driven device appear in Figure 2b. The Commission International d’Eclairage (CIE) coordinates of the pixels device define a color triangle that is comparable to the International Telecommunication Union HDTV standard."
Milli-watt Energy-harvesting from Low-frequency Vibrations,"This project is part of the Hybrid Insect Microelectromechanical System MEMS (HI-MEMS) program sponsored by the Defense Advanced Research Projects Agency (DARPA). The main objective of this program is to establish the interface between adult neural systems and appropriate computational and MEMS systems. Here, insects are the first test bed, and they will be directed to fly to spe-cific locations in real time via remote control. In order to support the flight-control systems, a local energy-harvesting power system is re-quired on the moth. The energy-harvesting system has two ports: the mechanical port and the electrical port. Mechanical power is in-put from moth motion at the mechanical port, and electrical power is output for general consumption at the electrical port. Internal to the harvester between the two ports are an electromechanical en-ergy converter (generator) and the power electronics. In the past 12 months, a 0.1-mW bench-top electromechanical energy converter, which extracts power from low-frequency vibration, was designed and fabricated. Figure 1 shows the harvester.The electromechanical energy converter has two major components: a resonator with moving magnets and a coil. The magnets serve as a proof mass, and as the resonator vibrates, the magnets sweep past coils through which power will be harvested. In collaboration with the Daniel Group at the University of Washington, we determined the resonating frequency to be 25 Hz by tracking the three-dimen-sional inertial motion of a moth and taking the Fourier transform of the moth’s motion. Figure 2 shows a snapshot of the moth carrying a resonator during flight. The coils are wound on a plexiglass form, such as that shown in Figure 1; future flight-qualified windings will be made with flexible printed-circuit technology. The electrome-chanical energy converter was tested on a shaker table, which simu-lates the vibration of a moth, and 0.1 mW of time average power was extracted from the output of the series coil connection.We are now optimizing a more compact advanced energy-harvester that has flexible printed-circuit windings, neodymium iron bo-ron magnets. Simulations indicate that 1-mW energyharvesting is achievable at a cost of 0.27g. Harvester components including the magnets and windings have been designed and are under fabrica-tion. Currently, we are beginning the analysis and design of the power electronic circuit. The first pass will focus on switched-ca-pacitor power electronics, and the next milestone will be testing the advanced energy-harvester on a shaker table and developing power electronics compatible with radio micro- fabrication."
"Development of a Tabletop Fabrication Platform for MEMS Research, Development, and Production","A general rule of thumb for new semiconductor fabrica-tion facilities (Fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for research and development and for groups seeking to commercialize new semiconductor devic-es aimed at smaller market segments and requiring a dedicated process. To eliminate this cost barrier, we are working to create a suite of tools that will process small (~1”) substrates and cumulatively cost less than $1 million. This suite of tools, known colloquially as the 1” Fab, offers many advantages over traditional fabs. By shrinking the size of the substrate, we trade high die throughputs for significant capital cost savings, as well as substantial savings in material usage and energy con-sumption. This substantial reduction in the capital cost will drastically increase the availability of semiconduc-tor fabrication technology and enable experimentation, prototyping, and small-scale production to occur locally and economically. To implement this suite of 1” Fab tools, our current research has primarily been focused on developing a 1” Fab deep reactive ion etcher (DRIE). DRIE tools are used to create highly anisotropic, high aspect-ratio trenches in silicon—a crucial element in many MEMS processes that will benefit from a 1” Fab platform. In 2015-2016 we completed the development of the tool, and in this past year, our focus has been on optimizing its design for manufacturability. We ultimately demonstrated the manufacturability of the tool by setting up a satellite laboratory in Beijing, China with our research collaborators at the General Research Institute for Nonferrous Metals (GRINM). (See Figure 1 for a photo of the system set up in China). Our GRINM colleagues are helping develop etch recipes and providing feedback on the operation of the tool. We have also been working with the Perreault group at MIT to develop a low-cost, resistance-compression-based impedance matching network for use with this DRIE system and other plasma-based processing toolsIn addition to the optimization of the DRIE tool, we are currently developing novel PECVD and magnetron sputtering tools. In the PECVD research, we are exploring the use of inductively coupled plasma sources and non-pyrophoric mixtures of silane gas for Si-based film depositions. For sputtering, we are looking at novel techniques for creating low cost multi-layer film stacks. These two new systems will leverage the pre-existing 1” Fab modular infrastructure and will be fully compatible with the common base assembly that was developed for the 1” Fab DRIE system, as shown in Figure 2."
Development of in-situ Depth Profiler for Real-Time Control in a Deep Reactive Ion Etcher,"Standard process development for micro and nanofab-rication etching cycles rely on open-loop trial and error testing of recipes to achieve optimal etch depths and uniformities. This strategy is non-optimal for research and fabrication of novel devices where one-of-a-kind experiments can not justify lengthy process develop-ment times. As an alternative, we are developing an in-situ depth measurement device for real-time feed-back of etch depth and uniformity. This will facilitate far shorter process development times, ideally enabling the desired etch to be achieved on the first process run.Many system constraints make this very difficult and preclude the use of existing technology. We are pursuing an optical measurement approach based on a parallelized confocal design. The measurement must be done at a distance of around 8” through an aperture of around 2” in diameter, significantly limiting the numerical aperture. We are currently investigating the fundamental resolution limits of a confocal depth measurement under these conditions. We expect the dominant noise source to be laser speckle which will result from coherent illumination of the rough surface left by the plasma etching process. Calculations and simulations indicate that the confocal depth measurement is significantly corrupted by this speckle noise, severely limiting the depth resolution to around 100 μm. The desired depth resolution is around 1 μm which should be achievable if the specke noise could be removed. By measuring and characterizing the statistical properties of the rough surface’s height distribution, we hope to remove the speckle noise and significantly improve the achievable depth resolution."
Resonant Body Transistor with MIT Virtual Source (RBT-MVS) Compact Model,"High-Q mechanical resonators are crucial components for filters and oscillators that are essential for radio frequency and analog circuits. It is highly desirable for resonators to scale to GHz-frequencies and beyond to meet today’s challenging requirements in terms of speed and data rates. Furthermore, aggressive scal-ing requirements call for monolithic integration with complementary metal-oxide semiconductor (CMOS) circuits to allow for a smaller footprint and reduced parasitics and power consumption. Micro-electro- mechanical (MEM) resonators represent a potential solution for frequency and footprint scaling, along with monolithic integration in CMOS.A resonant body transistor (RBT) is a MEM resonator with a field-effect transistor (FET) incorporated into the resonator structure. The FET is intended for active sensing of the mechanical vibrations through piezoresistive modulation of the channel mobility. RBTs also rely on electrostatic internal dielectric transduction for actuation, by means of metal–oxide–semiconductor capacitors (MOSCAPs). Such sensing and actuation enable these devices to easily scale to multi-GHz frequencies while being compatible with CMOS manufacturing technologies.Compact modeling for these devices is essential to gain a deeper insight into the tightly coupled physics of the RBT while emphasizing the effect of the different parameters on the device performance. It also grants circuit designers and system architects the ability to quickly assess the performance of prospective RBTs while minimizing the need for computationally intensive coupled-multiphysics finite element method (FEM) simulations.The RBT compact model is developed as a set of modules, each representing a physical phenomenon. Mechanical resonance, FET sensing, MOSCAP driving, and thermal modules are the most notable. The modules are interconnected through a set of nodes (namely, mechanical nodes and a thermal node) to represent the coupling between the different physics. This modular approach enables the seamless expansion of the RBT model either by incorporating new physics, adding driving or thermal sources, or mechanically coupling multiple RBTs together. A modified version of the MIT Virtual Source (MVS) model is used to implement both the electrostatic driving (as a MOSCAP) and the piezoresistive active FET sensing. The full model is developed in Verilog-A and available on nanohub.org."
"Shielded, Flexible, and Stretchable Tactile Pressure and Shear Sensors Based on Deformable Microwave Transmission Lines","Tactile sensors and skins aimed at replicating the human sense of touch are an active topic of research with numerous potential applications in areas includ-ing robotics, healthcare, and prosthetics. Current skin technology is limited by mechanical fragility, complex fabrication, and the need for large numbers of connec-tions to external electronics. We have developed a new sensing technology based on microwave transmission lines that address these challenges.The pressure sensor (Figure 1) consists of a shielded flexible and stretchable 3-mm-thick transmission line constructed with conductors made of stretchable conductive cloth and a dielectric made of silicone rubber. Where pressure is applied, the dielectric deforms causing a change in the local characteristic impedance of the line. We have developed an algorithm that can reconstruct the deformation of the line as a function of position, based on the terminal impedance of the line measured across a wide frequency range (30 MHz to 6 GHz). This algorithm can also correct for resistive loss in the transmission line. To demonstrate this sensor, three different pressure deformations were applied at each of three locations, and the responses were combined to create Figure 1. Due to the shielding, the sensor performs correctly even when tied in a knot (with updated baseline subtraction).We have also developed a shear sensor (Figure 2) capable of measuring deformation due to applied pressure, and separately, deformation due to the force applied parallel to the surface of the sensor. This device consists of two independent transmission lines, which are constructed so that pressure causes equal impedance change but shear causes unequal change, allowing pressure to be differentiated from shear. Shear sensors are rare in the field of tactile skins; this technique, requiring only two connections, has promise for inexpensive and simple wide-area flexible and stretchable pressure and shear sensors."
Micro-Engineered Pillar Structures for Pool-Boiling CHF Enhancement,"Increasing the performance of phase-change heat transfer phenomena is key to the development of next-generation electronics as well as power gener-ation systems and chemical processing components. Surface-engineering techniques could be successfully deployed to achieve this goal. For instance, by engineer-ing micro/nano-scale features, such as pillars, on the boiling surface, it is possible to attain 100% enhance-ment in pool boiling critical heat flux (CHF). Research-ers have been working on several CHF enhancing micro- and nano-structured surfaces for years. However, due to the complexity of CHF phenomena, there is still no general agreement on the enhancement mechanism. An investigation of the effect of micropillar height on surface capillary wicking and the associated pool- boiling CHF enhancement has been conducted. Several silicon micropillar structures have been fabricated using MTL photolithography and DRIE facilities.The surfaces were characterized using MTL’s scan-ning electron microscope (SEM), as shown in Figure 1a. The surfaces were then characterized by measuring the capillary wicking rate as presented in Figure 1b. A mechanistic capillary wicking estimation has been provided and compared with experimental wicking results (Figure 1c). Finally, the performance of such structures was characterized through traditional pool boiling experiments (Figure 1d). The results demonstrate the benefits of wicking promoted by these structures in terms of CHF enhancement. The microstructured surfaces fabricated at MTL have also been tested in pool-boiling with an electric field applied to replace for low gravity in space applications. A further increase in CHF has been observed due to the application of the electric field, on both flat and micro-structured silicon heaters. Notably, the combined use of passive (micro-structured surfaces) and active (electric field) CHF enhancement techniques has produced the maximum CHF enhancement."
An Ultra-Thin Nanoporous Membrane Evaporator,"Evaporation is a ubiquitous phenomenon found in nature and widely used in industry. Fundamental un-derstanding of the interfacial transport during evap-oration remains limited to date as it is generally chal-lenging to characterize the heat/mass transfer at the interface level, especially when the heat flux is high (> 100 W/cm2). In this work, we were able to accurate-ly monitor the temperature of the liquid-vapor inter-face, reduce the thermal-fluidic transport resistance, and mitigate the clogging risk due to contamination. This was done with an ultra-thin (≈ 200-nm thickness) nanoporous (≈ 130-nm pore diameter) membrane evap-orator, Figure 1 a, b, and c. At a steady state, we demon-strated high heat fluxes across the interface (≈ 500 W/cm2) with pure evaporation into an air ambient over a total evaporation area of 0.20 mm2. In the high flux regime, we showed the breakdown of Fick’s first law of diffusion and the importance of convective trans-port caused by evaporation itself (Figure 2). The pres-ent work improves the fundamental understanding of evaporation and paves the way for applications of high flux phase change devices."
Thin-Film Evaporation from Nanoporous Membranes for Thermal Management,"Performance and lifetime of emerging electronics are often dictated by the ability to dissipate heat generated in the device. In fact, a number of advanced electronics can generate heat fluxes exceeding 1000 W/cm2, such as gallium nitride high electron mobility transistors, and pump lasers. To put that in context, the heat flux of a typical electric stovetop is more than 100x less. The large heat fluxes generated in these devices, coupled with the negative impact on the device’s performance, has created the need for new thermal management techniques. Thin-film evaporation from nanopores has emerged as a promising candidate by reducing the thermal transport resistance across the liquid film while simultaneously providing capillary pumping. The combination of low resistance and large capillary pumping allows large heat fluxes to be dissipated with minimal temperature rise in the device.In this work, we study the dependence of evaporation from nanopores on a variety of geometric parameters, including pore diameter, membrane porosity, and the location of the meniscus within the pore. Anodic aluminum oxide membranes were used as an experimental template. A bi-philic treatment was used to create a hydrophobic section of the pore to control meniscus location. This membrane was sealed in a text fixture shown in Figure 1. Heat was supplied to the membrane, and the resulting temperature was monitored. We demonstrated different heat transfer regimes and observed more than an order-of-magnitude increase in dissipated heat flux by confining fluid within the nanopore, as seen in Figure 2. Similar tests were run systematically varying pore diameter, porosity, and meniscus location within the pore. We were able to show that pore diameter had little effect on evaporation performance at these pore diameters due to the negligible conduction resistance from the pore wall to the evaporating interface. The dissipated heat flux scaled linearly with porosity as the evaporative area increased. Furthermore, it was demonstrated that moving the meniscus as little as 1 μm into the pore could decrease performance significantly. The results of this study provide a better understanding of evaporation from nanopores and provide guidance in future high heat flux thermal management device design."
Suppressing High-Frequency Temperature Oscillations in Microchannel Heat Sinks with Surface Structures,"Thermal management of high performance electron-ic devices such as gallium nitride (GaN) power ampli-fiers and solid-state lasers is critical for their efficient and reliable operation. Two-phase microchannel heat sinks are attractive for thermal management of high heat flux electronic devices, yet flow instability, which can lead to thermal and mechanical fatigue, remains a significant challenge. Much work has focused on long-timescale (~seconds) flow oscillations, which are usually related to the compressible volume in the loop. However, the rapid growth of vapor bubbles, which can also cause flow reversal, occurs on a much shorter timescale (~tens of milliseconds). While this high-frequency oscillation has often been visualized with high-speed imaging, its effect on the instantaneous temperature has not been fully investigated due to the typical low sampling rates of the sensors. We propose to suppress this high-frequency temperature oscillation using surface microstructures that promote capillary wicking during flow boiling. We fabricated microchannels with micropillar arrays on the bottom heated surface (Figure 1). The geometries of the micropillars were optimized based our previously developed numerical model that maximizes the capillary flow. We then investigate the temperature response as a result of the high-frequency flow oscillation in microchannel heat sinks with smooth and microstructured surfaces with a measurement data acquisition rate of 1000 Hz. For smooth surface microchannels, the fluid flow oscillated between a complete dry-out and a rewetting annular flow due to the short-timescale flow instability, which caused high-frequency and large amplitude temperature oscillations (10 °C in 25 ms, Figure 2a). In comparison, hydrophilic surface structures on the microchannel promoted capillary flow, which delayed and suppressed dry-out in each oscillation cycle, and thus significantly reduced the temperature oscillation (Figure 2b) at high heat fluxes. This work suggests that promoting capillary wicking via surface structures is a promising technique to reduce thermal fatigue in high heat flux, two-phase, microchannel thermal management devices."
EWOD Actuation of a Vertical Translation and Angular Manipulation Stage,"Adhesion and friction during physical contact of sol-id components in microelectromechanical systems (MEMS) often lead to device failure. Translational stag-es that are fabricated with traditional silicon MEMS typically face these tribological concerns. Meanwhile, electrowetting, a phenomenon whereby the contact angle of a fluid can be changed with an applied volt-age, allowing control of droplet shape, has had a limited role in MEMS applications. We show through modeling and experimental demonstration that the electrowet-ting-on-dielectric (EWOD) technique has the potential to eliminate solid-solid contact during MEMS stage operation by actuating via deformable liquid droplets placed between the stage and base to achieve stage dis-placement as a function of applied voltage (Figure 1).Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators (Figure 2). We demonstrate with our model that a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices."
Additively Manufactured Miniature Diaphragm Vacuum Pumps,"Miniaturized pumps supply fluids at precise flow rates and pressure levels in a wide variety of microfluidic sys-tems. In particular, microfabricated positive displace-ment pumps that exploit gas compressibility to create vacuum have been reported as a first pumping stage in non-zero flow, reduced-pressure miniaturized systems, such as mass spectrometers. Compared to standard microfabrication, additive manufacturing offers the advantages of rapid prototyping, larger displacements for better vacuum generation and larger flow rate, freeform geometries, and a broader material selection while attaining minimum feature sizes on par with mi-crofluidic systems (out-of-plane features in the 10-300-μm range and in-plane features in the 25-500-μm range). In addition, a number of 3-D printing techniques make possible the definition of leak-tight, closed channels or cavities, sometimes involving a second sacrificial material that is removed after printing. Using polyjet 3-D printing technology with 42-μm XY pixelation and 25-μm layer height, a single-stage vacuum pump design with active valves and a total pumping volume of 1 cm3 with 5% dead volume was implemented (Figure 1a). Devices were printed in the acrylate based, UV curable photopolymer TangoBlack Plus® (Shore 27A) in one piece (Figure 1b) or in two halves for ease in removing the sacrificial material. The pumps were pneumatically actuated and consistently pumped down a 1 cm3 volume from atmosphere to 330 Torr in under 50 seconds operating at 3.27 Hz (Figure 2); from the data, the effective flow rate of the device is estimated at 8.7 cm3/min.The compression chamber diaphragms exhibited lifetimes approaching 20,000 cycles, while the valves’ membranes have not leaked after >1-million cycles. Current work focuses on increasing the diaphragm lifetime, reducing the ultimate pressure, and improving the mass flow rate vs. pressure pump characteristics."
Evaluation of Lost-Wax Micromolding for Additive Manufacturing of Miniaturized Metallic Vacuum Components,"In contrast to traditional subtractive methods, additive manufacturing (AM) is a process of joining materials layer by layer to generate solid structures from comput-er-aided design (CAD) data. Benefits of AM include the reduction of the raw materials required to make the part, fast manufacturing speed, versatility, and adaptabili-ty. Furthermore, AM has the potential to enable novel designs that could not be fabricated with conventional machining practices and to enhance the capability of true 3-D micromanufacturing. Standard 3-D printing of metallic parts is done via selective laser sintering, where a coherent photon beam is used to create a solid from the melting of metal powders. However, the printed struc-tures are coarse and porous with profusely outgassing surfaces and have electrical conductivity and mechanical strength less than those of the bulk material. Therefore, there is a need for better AM technologies to fabricate vacuum-compatible miniaturized metallic structures. In this project, we are exploring lost-wax micromolding as an alternative AM technology for metal parts. Wax masters printed via stereolithography were duplicated in sterling silver by encasing the master in a ceramic mold, removing the wax by melting it, and filling-in with metal the cavities left within the mold after wax removal; finally, the parts are extracted from the mold and polished. An array of pillars (Figure 1) with diameter varying from 350 μm to 500 μm and height from 400 μm to 950 μm was created to characterize feature size repeatability (Figure 2). We found close agreement between the intended and cast heights for cylinders 400 μm to 750 μm tall; however for taller cylinders, the measured values are smaller than expected, and the standard deviation is also larger. This might be related to the way high aspect-ratio pillars with a small diameter solidify during casting. Further work will focus on completing the exploration of this technology to print solid, pore-free metal parts including characterization of physical properties such as roughness, thermal diffusivity, and vacuum outgassing."
3-D Printed Multiplexed Electrospinning Sources for Large Production of Nanofibers,"Electrospinning is a versatile process that creates ul-trathin nanofibers via electro-hydrodynamical jetting. Electrospun nanofibers are used in a wide variety of bio-medical (i.e., tissue healing/scaffolding, drug delivery), energy (i.e., electrodes, solar cells), and microsystem applications (i.e., sensors, batteries). Even though elec-trospinning is the only technique capable of generating nanofibers of arbitrarily length using a wide variety of feedstock, the throughput of an electrospinning emit-ter is very low, making difficult the use of these fibers in commercial products. Multiplexing the emitters, i.e., implementing arrays of emitters that work in parallel, is an attractive approach to increase the throughput of electrospinning sources without sacrificing the quality of the fibers generated. Microfabricated multiplexed electrospinning sources that achieve uniform opera-tion at low voltage and large emitter density have been reported. However, these devices do not really solve the problem well as they are made with standard microfab-rication, which is expensive and time-consuming. In this project, we are exploring stereolithography (SLA) to create disposable electrospinning sources capable of high-throughput generation of fibers. In SLA, UV light is focused on a photopolymer while 3-D layers are created through crosslinking, making it possible to print complex three-dimensional structures. The SLA process has several advantages over competing approaches such as a higher resolution, higher quality surface, higher customization, and the creation of watertight imprints. Devices with emitters with 300-µm internal diameter have been created (Figure 1). Measured per-emitter vs. flow rate characteristics using a PEO solution demonstrates that the arrays operate uniformly. Current research focuses on maximizing the throughput of the sources by emitter multiplexing, exploring approaches for charging up the emitted jets to produce thinner fibers, and in collecting and characterizing aligned PEO nanofibers using a drum as a collector system for tissue engineering applications (Figure 2)."
Atmospheric Microplasma-Based 3-D Printing of Metallic Microstructures,"State-of-the-art additive manufacturing techniques for metallic microstructures cannot yet deliver the feature resolution, electrical conductivity, and material choice flexibility needed for high-performance micro-circuits. Further, many current and proposed additive manufacturing approaches for fine-geometry metal features require high-temperature post-processing and restrict the substrate material. We aim to develop a mi-croplasma-based sputtering system able to direct write a wide range of materials onto any substrate. We have modeled, designed, and constructed a first-generation system that sputters gold onto a substrate. By manip-ulating the metal at the atomic level, we retain the re-sistivity of bulk metal, and by sputtering the metal, we eliminate the need for post-processing or lithographic patterning. We use a microplasma to sputter metal at atmospheric pressure, obviating the need for a vacuum. Our microplasma generator uses electrostatic fields to focus the imprints. With a suitable electrode arrangement, we can shape electrostatic fields that will guide the ionized fraction of the working gas towards a localized spot on the substrate. The directed ions will collide with other gas atoms and, crucially, with sputtered metal atoms from the sputtering target. The net force due to these collisions will indirectly guide the metal atoms towards the desired part of the substrate. This indirect electrostatic focusing not only mitigates the inherent spread of the sputtered material caused by collisions at atmospheric pressure, but also enables feature definition. In the absence of collisions, the printed line will be wider than the sacrificial cathode. By focusing the sputtered material, we achieve imprints significantly narrower than the cathode. This precludes the need to machine sacrificial electrodes as small as our desired printed lines.Our microplasma head has a central target wire acting as the cathode, surrounded by four electrodes (Figure 1), two biased at a positive voltage (relative to the grounded target) to form the plasma, and the other two biased at a negative voltage to focus the plasma. By both pulling and pushing the plasma, COMSOL simulations predict imprints orders of magnitude narrower than the cross section of the target wire (Figure 2)."
MEMS Electrohydrodynamic High-Throughput Core-Shell Droplet Sources,"Coaxial electrospraying is a microencapsulation tech-nology based on electrohydrodynamic jetting of two immiscible liquids that allows precise control with low size variation of the geometry of the core-shell particles it generates. Coaxial electrospraying is a very promising microencapsulation technique because (i) it is easy to implement, (ii) it can operate at room tem-perature and at atmospheric pressure, (iii) it does not require a series of steps in the encapsulation process, (iv) it can generate compound droplets with narrow size distribution, and (v) it can be used to encapsulate a great variety of materials of interest to biomedical and engineering applications. State-of-the-art coaxial elec-trospray sources have very low throughput because they have only one emitter. Consequently, coaxial elec-trosprayed compound particles are compatible with only high-end applications and research. An approach to increasing the throughput of a coaxial electrospray source without affecting the size variation of the emitted compound microparticles is to implement arrays of coaxial emitters that operate in parallel. However, no miniaturized coaxial array sources have been reported, probably due to the inherent three-dimensionality of the emitter geometry and the hydraulic network required for uniform array operation, which is at odds with the planar nature of traditional microfabrication. In this project, we demonstrated the first MEMS multiplexed coaxial electrospray sources in the literature. Miniaturized core-shell particle generators with up to 25 coaxial electrospray emitters (25 emitters·cm-2) were fabricated via digital light projection/stereolithography (DLP/SLA, Figure 1), which is an additive manufacturing process based on photopolymerization of a resin that can create complex microfluidics. The characterization of emitter arrays with the same emitter structure but different array size demonstrates uniform array operation. The core/shell particles produced by these additively manufactured sources are very uniform (Figure 2); the size distribution of these compound microparticles can be modulated by controlling the flow rates fed to the emitters."
High Current Density Si-field Emission Arrays (FEAs),"Silicon field emitter arrays (FEAs) are excellent cold cathodes that have not been fully exploited due to the nonzero tip radius distribution causing lower utilization of the arrays. This discrepancy in emitter tips causes sharper tips to burn out (by Joule heating) before duller tips, and therefore the maximum current achievable is small. In this work, we focus on achieving high current density Si FEAs, by integrating high-aspect ratio Si nanowires as to limit the supply of electrons and hence saturate the maximum current to avoid the burn-out of the sharper tips. Si nanowires of height ~10 µm and 100-200-nm diameter limit the current and improve reliability through velocity saturation and the pinch-off of majority carriers. To prevent charge injection and minimize the gate-substrate capacitance, a 2-µm-thick SiO2 insulator is added, and the Si nanowires are embedded in a conformal dielectric matrix consisting of Si3N4 and SiO2. High current densities are achieved as the nanowires (current limiter) are integrated with each field emitter, thereby preserving a high density of operational emitters (~108 emitters/cm2) without burning out. These Si FEAs have also been shown to provide consistent current scaling of array sizes from a single emitter to 25,000 emitters, low voltage (VGE < 60 V ), high current density ( J > 100 A/cm2), and long lifetime ( > 100 hours at 100 A/cm2, > 100 hours at 10 A/cm2, and > 300 hours at 100 mA/cm2). Compared to conventional Si FEAs operating without a current limiter, the device architecture shown here demonstrate a current density improvement of > 10 folds and low turn-on voltage (8.5 V). Cold cathodes based on Si-FEAs incorporating a current limiter have high potential in applications ranging from X-ray imaging, RF amplifiers, and THz sources to deep UV sources, ion sources, and neutron sources."
Field Emission from Silicon Tips Embedded in a Dielectric Matrix,"Field emitter arrays (FEAs) are a class of cold cathodes with promising potential in a variety of applications requiring high current density electron sources. How-ever, FEAs have not yet achieved widespread usage because of fundamental challenges that limit their reliability in systems. Field emission from conduct-ing surfaces requires high fields and pristine surfaces; these surfaces are vulnerable to adsorption-desorption processes by residual gas molecules, leading to emis-sion current fluctuations and tip erosion. Moreover, electron transport through insulators often leads to im-pact ionization and dielectric breakdown. This project explores electron emission from field emitter tips that are embedded in a dielectric matrix, specifically silicon dioxide, as a potential approach to address reliability problems in classical field emitters.In the project, arrays of silicon emitter tips that are individually regulated by silicon nanowires are being fabricated. The silicon nanowires have diameters between 100-200 nm and heights of 10 µm, resulting in an aspect ratio of 50-100:1. The emitter tips typically have radii of 5 nm with a log-normal distribution and a density of 108 tips/cm2. Further, the silicon nanowires function as current limiters that improve reliability by preventing premature tip burn-out due to Joule heating, thermal runaway, and cathodic arcs. Chemical mechanical polishing (CMP) was used to form the self-aligned gates. The silicon tips formed by oxidation sharpening are embedded in a dielectric matrix and are not released. A diagram of the structure is shown in Figure 1."
A Silicon Field Emitter Array as an Electron Source for Phase Controlled Magnetrons,"Magnetrons are a highly efficient (>90%), high-pow-er vacuum-based microwave source. In a magnetron, free-electrons in vacuum are subject to a magnetic field while moving past open metal cavities, resulting in resonant microwave radiation to be emitted. Current state-of-art magnetrons use a heated metal filament to thermionically emit electrons into vacuum continu-ously and are not addressable. This work seeks to re-place the heated metal filament as a source of electrons with silicon field emitter arrays in order to improve the efficiency and increase the power, especially when sev-eral sources are combined. Silicon field emitter arrays, schematic shown in Figure 1, are devices that are nor-mally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of a many sharp tips made of silicon sitting on long sil-icon nanowires that limit the current of the electron emission. Electrons from the silicon tip tunnel into a vacuum as a result of the high electric field of the ap-plied bias on the polysilicon gate. Pulsing the electric field applied on the gate can turn the arrays on and off. The proposed use of silicon field emitter arrays in a magnetron will allow injection locking and hence phase control of magnetrons. Phase-controlled mag-netrons have multiple applications in areas where high- power microwave sources are desired."
Ion Electrospray Thrusters for Microsatellite Propulsion,"Ion electrospray propulsion systems (iEPS) are high specific impulse, low thrust, and extremely scalable devices; these characteristics make them excellent candidates for propulsion systems on microsatellites, which require some small amount of maneuverability primarily for station-keeping. Like other ion engines, they utilize an electrostatic potential to accelerate charged particles across a gap to relatively high veloci-ties to generate thrust. Utilizing ionic liquids – a special class of molten salts that do not evaporate in vacuum, thanks to their negligible vapor pressure – drastically increases the propellant density and obviates the need for a stage in which the propellant is first ionized, thus further reducing mass and volume requirements. The thrusters themselves are extremely simple in that they are passively fed through capillary action and require no moving parts. However, high electric fields, on the order of 109 V m-1, are required to extract ions from the liquid. This entails careful fabrication of the porous emitter substrates, which feature an array of roughly five hundred tips, patterned into the surface via laser ablation. By providing a sharp tip, the electric field is effectively intensified to the point that ions can be ex-tracted, through a sharpening effect similar to coronal discharge. The thrusters are constructed from several component parts. The frames are made via microelectromechanical (MEMS) processing: a silicon base layer, an insulating glass layer, and finally a top silicon layer with alignment features to correctly locate the tip array is etched and then anodically bonded. To those frames a porous substrate is affixed after being shaped and polished. A tip array is patterned into the substrate via laser ablation. Next, a silicon electrode grid, also fabricated with MEMS processing, is bonded to the frame so that the grid holes are aligned to the tip array, completing the emitter. The emitters are then bonded onto tanks that passively transport propellant to the emitter. The tanks are mounted on electronic power supply/control boards, creating a finished engine that may be integrated into a spacecraft. Four small satellites (CubeSats) equipped with these thrusters have already been launched into space. Our team is currently working on a new project, set to launch during Q1 of 2018."
Enhanced Water Desalination in Electromembrane Systems,"Currently, reverse osmosis (RO) is considered the leading technology for desalination, and the operational efficien-cy of RO has been significantly improved over the last two decades with a thorough energy analysis. On the other hand, electrical desalination can be more advan-tageous in certain applications due to the diversity of allowed feed conditions, operational flexibility, and the relatively low capital cost needed (the size of a system is generally small). Yet, electromembrane desalination techniques such as electrodialysis (ED) have not been modeled in full detail, partially due to scientific challeng-es involving the multiphysics nature of the process.In addition, while current ED relies on bipolar ion conduction (Figure 1b), removing one pair of a cation and an anion simultaneously, one final but most important point is that desalination achieved by means of an anion exchange membrane (AEM) and a cation exchange membrane (CEM) should be considered separately and independently (Figure 1a). Based on the intrinsically different ion transport near AEM and CEM, our group previously presented a novel process of ion concentration polarization (ICP) desalination (Figure 1b), which can basically enhance the amount of salt reduction, by examining unipolar ion conduction through both experiments and numerical modeling. In our studies, we investigate the effects of embedded microstructures on mass transport enhancement; these microstructures affect the electrical energy efficiency of an ED system for its current application of brackish (low salinity) water desalination (Figure 1c); we also explore the technical and economic feasibility of the ICP desalination for potential applications in the emerging field of high-salinity brine desalination (Figure 1d)."
Digital Optical Neural Networks for Large-scale Machine Learning,"Artificial intelligence is becoming ubiquitous in our society; specifically, artificial deep neural networks (DNNs) have enabled breakthroughs in image classification, translation and prediction. The recent adoption of DNNs in a wide variety of fields is largely due to algorithms with improved accuracy that leverage more compute power and larger datasets. However, throughput and energy efficiency are currently limiting the further expansion and adoption of DNNs. We have proposed optical neural networks (ONNs), which we have theoretically shown to achieve low-energy, high-throughput DNN processing. Our latest results include a proof-of-concept demonstration of a digital ONN with little drop in classification accuracy on the MNIST dataset (-0.6% on a custom, fully-connected, 3-layer network, due to optical crosstalk). In this scheme, we use optics for passive digital data fan-out and routing. Owing to the length-independence of energy and latency in optical data transmission, we find that the digital ONN may enable more efficient DNN hardware. This work showcases the promise of ONNs as a new computing paradigm, which is required to unlock the full potential of DNNs."
Charge-carrier Recombination in Halide Perovskites,"The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique among semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge-carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba splitting), and (4) photon recycling (Figure 1). We scrutinize the evidence for each case and the implications of each process for carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that shallow trap states and the possible indirect nature of the bandgap (e.g., Rashba splitting), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties."
In-situ Gamma Radiation Damage on SiC Photonic Devices,"In this report, we demonstrate real-time, in-situ anal-ysis of radiation damage in integrated photonic devic-es. The devices, integrated with an optical fiber array package and a baseline-correction temperature sensor, can be remotely interrogated while exposed to ionizing radiation over a long period without compromising their structural and optical integrity. We also introduce a method to deconvolve the radiation damage respons-es from different constituent materials in a device. The approach was implemented to quantify gamma radia-tion damage and post-radiation relaxation behavior of SiO2-cladded SiC photonic devices. Our findings sug-gest that densification induced by Compton scattering displacement defects is the primary mechanism for the observed index change in SiC. Additionally, post-radia-tion relaxation in amorphous SiC does not restore the original pre-irradiated structural state of the material. Our results further point to the potential of realizing radiation-hard photonic device designs taking advan-tage of the opposite signs of radiation-induced index changes in SiC and SiO2. The devices fabricated following CMOS-compat-ible protocols are symmetrically cladded with PECVD SiO2. In device packaging, the as-fabricated devices were packaged with optical fiber arrays (SQS Vlaknova Optika) using ultraviolet-curable epoxy (Masterbond UV15TK) as the bonding agent. Fibers with an incident angle of 15° were first active aligned to the on-chip grating couplers to maximize the transmitted power. Epoxy was applied onto the chip to securely bond the fibers to the chip. The active alignment was repeated after epoxy application to ensure optimal coupling. The epoxy was then cured through flood UV exposure. We monitored the device resonance peak position and Q-factor as gamma radiation progressed. The refractive index and absorption coefficient change of a-SiC core and a-SiO2 cladding were extract and plotted in Figure 1. As indicated in the graph, we clearly observe an op-posite in signs of index change in these two materials, suggesting the potential of realizing radiation-hard photonic devices."
Variation-aware Compact Models for Yield Prediction of Coupled-resonator Optical Wave-guides,"Silicon photonics is a growing design platform due to all the potential applications and enhancements it can offer. Among these attractive applications are the sig-nificant computing system performance gains that can be achieved by transferring information using optical rather than electrical signals. Achieving this optical transmission requires on-chip optical buffers. Coupled resonator optical waveguides (CROWs), which chain a number of ring waveguides together as in Figure 1a, can be used as buffers. However, CROWs are challenged by the spatial variations within die or across the wafer, as CROWs are large structures extending hundreds of microns to mil-limeters in length depending on the number of constit-uent rings. These variations can change the passband or, more importantly, may cause the CROW to fail if the spatial variations cause the resonances of the coupled rings to lose their alignment. Moreover, varying the ring (constituting a CROW) design requires regenerat-ing the S-parameters, which is computationally expen-sive and time-consuming, if many variants need to be considered for Monte-Carlo statistical simulations or during design optimization. This highlights the need for a variation-aware compact model. We develop a method and variation-aware com-pact models that can be used to simulate and predict the CROW behavior (S-parameters) against spatially correlated process variations in thickness and width. Figure 1b compares the simulated performance of a 28-ring CROW using S-parameters generated directly from FDTD simulation and using S-parameters gener-ated using the developed compact model. This parame-trized compact-model-generated S-parameters can be used to facilitate and speed up design optimization, run Monte-Carlo simulations, and predict yield. Figure 2 shows the yield prediction of CROWs satisfying a suf-ficient amplitude pass band (above -20dB), in response to width variation as a function of spatial correlation length ( ) and amplitude (σ). This compact model can serve as a building block for a variation-aware process design kit (PDK) for photonics."
Graphene-loaded Slot Antennas for Multispectral Thermal Imaging,"Color cameras are ubiquitous in everyday life. However, most color imagers rely on color filter arrays (CFAs), resulting in most incoming light being filtered out instead of detected. More generally, for a filter-based imaging array with N different colors, only 1/N of the incoming light is actually used. While lossless spectral imagers are available, they rely on bulky optics such as diffraction gratings or interferometers to achieve spectral resolution, which is often undesirable. In the thermal IR wavelength range, the problem of filter loss is exacerbated by reduced sensor detectivity compared to visible light sensors. We propose an efficient and compact thermal IR spectral imager based on a metasurface consisting of sub-wavelength-spaced, differently-tuned antennas with photosensitive loads. The different antenna resonances combine to yield broadband optical energy transfer to the loads exceeding the 1/N efficiency limit of CFAs. In particular, we investigate slot antennas due to their unidirectionality and high efficiency compared to typical dipole antennas. We use graphene as our photosensitive load because its 2D nature makes it easily adaptable to this imager architecture. To aid in the design of these slot antenna metasurfaces, we establish a circuit model for the optical properties of the antennas and demonstrate consistency between this model and full-wave electromagnetic simulations. We also show simulations results demonstrating broadband ~36% free space to graphene coupling efficiency for a six-spectral-band metasurface. Finally, we demonstrate a fabrication process which yields slot antennas with smooth surfaces suitable for graphene transfer on top. This research represents the first steps towards compact, monolithic, and potentially CMOS-integratable mid-IR spectral imagers whose low bulk and low energy consumption suit them for deployment on small drones for remote sensing and free-space communication purposes."
Room-temperature Strong Light-matter Interactions in Hybrid Perovskites,"State-of-the-art perovskite materials demonstrate photoluminescence quantum efficiencies (PLQE) above 90% due to low non-radiative recombination rates and unparalleled defect tolerance. The optoelectronic properties that have allowed perovskites to emerge as a leading active layer material in high-efficiency thin-film photovoltaics (PVs)–high absorption coefficient, small Stokes shift, high PLQE, solution processability, and chemical tunability – simultaneously situate perovskites to function superbly as a coherent quantum material. In this work, we explore perovskites as a platform for strong light-matter coupling to sustain all-optical operations. Although light is weakly interacting, it is possible to form interacting quasi-particles, called exciton-polaritons, that have characteristics of both light and matter. Traditionally, polaritons have been studied at cryogenic temperatures in all-inorganic semiconducting materials (e.g., GaAs heterostructures). Here, we study the room-temperature formation of exciton-polaritons with large Rabi splittings in semiconductor microcavities, using solution-processed 2D perovskites as self-assembled quasi-quantum well structures (Figure 1). Polariton formation is probed by angle resolved reflectivity and photoluminescence measurements through a k-space imaging setup. Enhanced polariton propagation is explored by microstructuring the microcavity to funnel polaritons generated in smaller cavity length regions to lower confined photon energy regions of longer cavity length. The realization of stable, facilely-fabricated room-temperature exciton-polaritons has the potential to revolutionize a wide range of devices, from PVs to low-threshold lasers to all-optical switches."
Amorphous Silicon Carbide for Nonlinear Integrated Photonics,"Silicon carbide (SiC) has been actively researched in recent years as a platform for linear and nonlinear photonics due to its large bandgap, large refractive index, low thermo-optic coefficient, excellent mechanical and chemical stability, and large Kerr nonlinearity. We have demonstrated amorphous SiC waveguides with propagation losses as low as 3 dB/cm, which enable their application in integrated photonics. We have demonstrated amorphous SiC ring-resonators on SiO2 insulator substrate with an intrinsic quality factor as high as 1.6×105. The Kerr nonlinearity obtained at 1550-nm wavelength was 4.8 ×10-14cm2/W, which was the highest value reported in both crystalline and amorphous SiC material, making it a promising platform for CMOS-compatible nonlinear photonics.The amorphous SiC photonic devices were fabricated in the MIT.nano cleanroom facilities. A plasma-enhanced chemical vapor deposition (PECVD) system using a silane and methane reactive gas mixture was used to deposit an amorphous SiC thin film on a 6-inch Si wafer that had a 3-µm thermal oxide insulating layer. Electron beam lithography was used to pattern the SiC-on-insulator ring resonators. Fluorine chemistry was used to dry etch SiC using reactive ion etching. We characterized the optical properties of the amorphous SiC photonic devices in collaboration with Dr. Peng Xing and Professor Dawn Tan at the Singapore University of Technology and Design (SUTD) and achieved the largest quality factor among all crystalline and amorphous SiC materials tested to date. The Kerr coefficient of the amorphous SiC film was extracted by fitting the nonlinear Schrödinger equation. The Kerr nonlinearity measured in our amorphous SiC is almost one order of magnitude higher than that reported in the literature for crystalline and amorphous SiC. Nonlinear behavior was observed for the first time for a-SiC at the wavelength of 1550 nm, with a high incident pulse peak power."
High Sensitivity Mid-Infrared/Thermal Detectors,"Infrared detectors that are fast, high-detectivity, and room-temperature-operable are needed to enable next-generation hyperspectral arrays. While photo conductor (pc) detectors can achieve high detectivity and ~100 ps time constants, pc detectors suffer from a narrow spectral range and must be cooled to cryogenic temperatures for efficient detection beyond ~ 4 µm wavelength. Thermal detectors, meanwhile, exhibit a flat detectivity response with respect to wavelength and can, ideally, reach a detectivity of 1.98*1010 cmHz1/2W-1 at room temperature. In this work we focus on two sensitive novel thermal detector architectures: (1) a nanogap based thermomechanical bolometer and (2) a pyroelectric gated field-effect transistor (FET ) biased in the subthreshold regime. The thermomechanical (thm) bolometer achieves high sensitivity by closing a ~1.3-nm gap as the surrounding materials expand due to infrared light absorption, resulting in an exponential increase in current. The suspended thm bolometer is made of two metal cantilever arms connected by a 5-nm-thick platinum wire (see Figure 1). The nanogap detectors are mechanically stabilized via a self-assembled monolayer (SAM). Early experimental results show temperature coefficient of resistance (TCR) values as high as 0.16 K-1, which is higher than the state-of-the-art ~ 0.1 K-1. Studies to characterize the noise of these devices, measure their response to laser illumination, and determine their detectivity are in progress.We are also exploring an additional low-power and sensitive bolometer design using subthreshold, pyroelectric gated thin-film transistors. When infrared light is absorbed, dipole charges in the pyroelectric material align and gate the transistor channel. We estimate that these devices can achieve TCR values of 0.6275/I0 K-1, where I0 is the bias current. The proposed device structure can be found in Figure 2. We are currently exploring the design space of Hf0.5Zr0.5O2 ferroelectric/pyroelectric FETs and optimizing them for 5 µm – 10 µm wavelength detection."
3D Integrated Photonics Platform with Deterministic Geometry Control,"3D photonics promises to expand the reach of photonics by enabling both the extension of traditional applications to non-planar geometries and adding novel functionalities that cannot be attained with planar devices. However, current fabrication methods limit the range of available materials options (e.g. to low index contrast polymers for 3D printing) or device geometries (e.g. to curvilinear geometries that are inherently 2D in topology). As an application example, the much-needed ability to monitor stress in biological samples such as cell cultures and tissue models requires a platform that provides precise measurements at multiple, pre-defined locations in 3D, which none of the current fabrication methods for 3D integrated photonics can offer.In this work, we report a fully-packaged 3D integrated photonics platform with devices placed at arbitrary pre-defined locations in 3D using a fabrication process that capitalizes on the buckling of a 2D pattern. The final structure consists in several buckled strips joining two planar edge platforms, as shown on Figure 1a. Each strip may contain waveguides and waveguide-coupled components such as resonators. We show that our fabricated devices (see Figure 1b) precisely match theoretical shapes. Finally, we demonstrate the amenability of this platform for mechanical strain sensing, e.g. in 3D cell cultures, by calibrating its stress-sensing response. Our results indicate a strain measurement accuracy of 0.01%, for materials with a Young's modulus down to 300 Pa. A key benefit of our fabrication approach for 3D integrated photonics lies in the wide range of physical and chemical sensing applications of optical resonators, as well as the possibility to multiplex resonators spec-trally and spatially. Our platform is thus amenable to monitoring a variety of parameters at a large number of locations in a distributed sensor array, potentially enabling multifunctional sensing, mapping, and light delivery in the 3D space."
"Single-element, Aberration-free Fisheye Metalens","Wide-angle optical functionality is crucial for implementation of advanced imaging and image projection devices. Conventionally, wide-angle operation is attained with complicated assembly of multiple optical elements. Recent advances in nanophotonics have led to metasurface lenses or metalenses, a new class of ultra-thin planar lenses utilizing subwavelength nanoantennas to gain full control of the phase, amplitude, and/or polarization of light. Here we present a novel metalens design capable of performing diffraction-limited focusing and imaging over an unprecedented > 170° angular field of view (FOV). Similar to a Chevalier landscape lens, our metalens design concept spatially decouples the metasurface and aperture stop, but positions them on a common, planar substrate (Fig. 1a). This optical architecture allows input beams incident at various angles (indicated with colored arrows in Fig. 1a) to be captured on distinct yet overlapping areas of the metasurface. The metasurface further forms the pencil-beams, in such a way, that all of the focal spots are positioned in the same image plane. We fabricated the metasurface using PbTe meta-atoms of rectangular and H-shaped blocks, which induce distinct phase shifts arising from the electric and magnetic resonant multipole modes (Figure 1b). The meta-atom library consisted of eight elements covering the 360° phase space with a discrete step of 45° for linearly polarized light at the mid-infrared wavelength of 5.2 σm. The implemented metalens produced diffraction limited focal spots when illuminated with a laser beam at the incident angles ranging from 0σ to 85σ. We further demonstrated that the metalens can perform aberration-free imaging of the USAF resolution charts over the entire FOV. Our metalens design concept is generic and can be readily adapted to other meta-atom geometries and wavelength ranges to meet diverse application demands. In the scope of this project, we also explored machine learning approaches to generate free-form metalens designs with improved performance."
Ultra-sensitive All-optical Membrane Transducers for Photoacoustics,"Photoacoustic imaging (PAI) has attracted much attention over the past two decades for various biomedical imaging applications. However, it is surprising to note that this unique imaging modality has not yet spun out much in commercial applications. One of the key obstacles in this direction is the limited sensitivity of the currently available ultrasound transducers. Existing acoustic transducer technologies based on bulk PZT, piezoelectric, and capacitive micromachined ultrasonic transducers have a significantly low sensitivity in orders of 0.2 – 2.0 mPa/sqrt(Hz). This feature limits the imaging depth, reliability, and molecular sensitivity of the current PAI systems.Our research work explores on-chip CMOS-compatible all-photonic architecture to develop PAI systems with significantly improved sensitivities, improved detection limits, and reduced power consumption. Spiral-shaped silicon nitride waveguides realized on suspended silicon-oxide membranes designed to have a center frequency between 5 MHz to 10 MHz are used as Mach-Zender arms for highly sensitive ultrasound reception with <1 mPa/sqrt(Hz) noise equivalent pressure. Hence, this approach allows fast intensity-based acquisition as opposed to interferometric acquisition, thus allowing on-chip optical interrogation. A few previously reported attempts in this direction have been limited to a single sensor element. Here, we attempt to leverage the benefits of existing photonic-based signal conditioning schemes and adopt them to multiplex the ultrasound reception from multiple sensor elements. The presented transducer technology has a multitude of advantages. Ultra-high sensitivity combined with an all-optical implementation will allow easy scaling-up of the technology and miniaturization for wearable applications."
Large-scale Integration of Diamond Qubits With Photonic Circuits,"Quantum technologies can potentially offer dramatic speed-up and enhanced security in information pro-cessing, communication, and sensing. Such tasks would require the scalable construction and control of a large number of quantum bits (qubits). Here, we report the fabrication and characterization of the largest integrat-ed artificial atom-photonics chip. Defects such as color centers in diamond behave like “artificial atom” (AA) spin qubits in that they can be controlled via light and microwaves and can maintain long coherence times. Scaling such systems requires (1) high-yield qubit fabrication, (2) efficient photonic wires to route and manipulate single photons, and (3) post-tuning capability to compensate for inhomogene-ities between different qubit modules. Rather than fabricating a low-yield monolithic system with these necessary requirements, we intro-duced the heterogeneous integration of “quantum mi-cro-chiplets” (QMCs) into an integrated photonics pro-cess. The QMC (Figure 1A) consists of AA qubits in a di-amond waveguide array, while the photonic integrated circuit (PIC) is an aluminum nitride (AlN)-on-sapphire platform. We used a pick-and-place process to transfer the QMCs in diamond to the AlN photonics chip with success probability over 90% (Figure 1B). As Figure 1C shows, the diamond and AlN modules meet at tapered waveguide interfaces for efficient photon routing from the diamond layer to the integrated photonics layer. Room temperature and cryogenic measurements reveal single-photon emission in all 128 integrated waveguide channels (Figure 2). Additionally, the emit-ters exhibit near-lifetime-limited linewidths, indicating high optical coherence of emitters in nanostructures. Finally, we demonstrated on-chip tuning of the qubit optical transitions via strain fields in the waveguide. Our platform paves the way for on-chip generation and manipulation of large entangled quantum states and demonstrates the scalability of optically active spin qu-bits in solids for quantum information processing."
Transmittance Enhancement at Graphene/Al Interfaces,"When two metal films stack together forming “hete-ro-film,” it has been generally accepted that the effective transparency is lower than in the respective metal film as a result of the absorption accumulation. In this work, we investigated the counterintuitive transmittance en-hancement of graphene/aluminum hetero-films. Single layer graphene was first grown by chemical vapor depo-sition and transferred on SiO2 substrate. Subsequently, an aluminum coating with a thickness of 4-20 nm was produced by an e-beam evaporator with a target of 99.99% pure aluminum. We acquired the transmittance spectra of graphene/aluminum hetero-films using a UV-vis-NIR spectrophotometer. One interesting obser-vation is that transmittance increased in samples with graphene, indicating a novel physical or chemical inter-action between graphene and aluminum. For 4-nm Al film, graphene induced transparency enhancement at UV range of 200 to 300 nm. As film thickness increas-es to 8 nm, the transparency enhancement extends to a wider UV range of 10-660 nm. In a 12-nm sample, we observe an averaged 12% increase in transmittance for the wavelength range of 500-2500 nm in the sample with graphene, compared with a pure Al coating on the substrate. More surprisingly, similar transparency en-hancement is captured when Al film was deposited on the graphene film. Due to the counterintuitive observa-tion, we anticipate this work will benefit the communi-ty in fundamental understanding and reliable utiliza-tion of graphene and Al interactions."
Decomposed Representation of S-Parameters for Analysis of Silicon Photonic Variation,"Silicon photonics offers great potential for monolithic integrated photonic and electronic components using existing integrated circuit (IC) fabrication infrastructure. However, methods to analyze the impact of IC process variations on performance of photonic components remain limited. Statistical models based on either simulations or experiments that quantify the effect of these variations are necessary to achieve high-yield manufacturing. To cope with the non-linearity in the S-parameters of photonic device components and circuits, non-linear parameter fitting is often used prior to statistical modeling, e.g., rational polynomial fitting of ring resonator responses. The traditional approach treats the amplitude and phase of the S-parameters separately in the fitting process; however, this method can be problematic when the behavior of the S-parameters becomes complicated under the variations since it neglects the strong correlation between amplitude and phase. For example, the seemingly complicated spike in group delay shown in Figure 1 is actually where a smooth S-parameter accidentally crosses the origin point.We present a novel representation of S-parameters that decomposes the complex-numbered S-parameters into several components, each having a simple response that does not require non-linear parameter fitting and that supports subsequent statistical analysis. We apply the proposed S-parameter decomposition method to Y-splitters with imposed line edge roughness (LER) variations. In contrast to the difficulty of the traditional amplitude-phase representation, the decomposed representation shows improvement in statistical modeling of variation ensembles, e.g., using principle component analysis (PCA) (Figure 2).The method can be extended to other photonic components and circuits with other process variations to help quantify the effect of process variations for statistical analysis and to help designers predict and optimize photonic component performance and yield."
DC-DC Converter Implementations Based on Piezoelectric Resonators,"Power electronics play a vital role in the technologi-cal advancement of transportation, energy systems, manufacturing, healthcare, information technology, and many other major industries. Demand for power electronics with smaller volume, lighter weight, and lower cost often motivates designs that better utilize a converter's energy storage components, particularly magnetics. However, the achievable power densities of magnetic components inherently reduce as volume decreases, so further progress in converter miniaturiza-tion will eventually require new energy storage mech-anisms with fundamentally higher energy density and efficiency capabilities. This prompts investigation into piezoelectric en-ergy storage for power conversion; piezoelectrics have comparatively superior volume scaling properties. While piezoelectrics have been used extensively for sensing, actuation, transduction, and energy harvest-ing applications, their adoption in power conversion has been more limited. Converter designs based on single-port piezoelectric resonators (PRs) report limit-ed power and/or performance capability, but without investigation into the full realm of possible converter implementations.In this work, we conduct a systematic enumeration and downselection of practical dc-dc converter switch-ing sequences and topologies that best leverage PRs as their only energy storage components. In particular, we focus on switching sequences that facilitate high-effi-ciency behaviors (e.g., low-loss resonant charging/dis-charging of the PR’s input capacitance and all-positive instantaneous power transfer) with voltage regulation capability. To analyze and compare implementations, we demonstrate methods for mapping PR state trajec-tories across a switching cycle, imposing practical con-straints on PR behavior, evaluating PR utilization, and estimating PR efficiency. Effective use of the PR's resonant cycle enables these converter implementations to achieve strong ex-perimental performance with peak efficiencies >99%, even with presently commercially-available PRs. This suggests that these PR-based converters are promis-ing alternatives to those based on traditional energy storage. With further development, PR-based convert-ers may pave the way for high-performance converter miniaturization in applications spanning consumer electronics, biomedical implants, and flight."
High Capacity CMOS-compatible Thin Film Batteries,"The miniaturization of sensors through advancements in low-powered MEMS devices in integrated circuits has opened up new opportunities for thin film microbatteries. However, many of the available thin film battery materials require a high-temperature process that necessitates additional packaging volume, which reduces the overall energy density of these batteries. Previous research with collaborators in Singapore demonstrated an all-solid-state materials system with high volumetric capacity that exclusively utilizes CMOS-compatible (i.e., room temperature) processes. This process allows integration of these batteries directly onto CMOS circuits, thereby achieving energy densities comparable to bulk batteries for applications in distributed power supplies and integrated autonomous microsystems (Figure 1). Additionally, the ability to deposit all components of the battery at room temperature makes it possible to fabricate these batteries on thin, flexible substrates that can be densely stacked to achieve a wide range of capacities without sacrificing their high energy density.We have successfully demonstrated a full thin film microbattery using Ge and RuO2 as anode and cathode materials, respectively, with LiPON as the solid-state electrolyte (Figure 2b). Although RuO2 has traditionally been used as an anode material, it has significantly higher volumetric capacity than typical cathode materials and sufficiently high electrochemical potential versus Ge to provide an output voltage of ~ 0.5V at a capacity of ~40 Ah/cm3 (Figure 2a)."
"State Estimation, Parameter Inference, and Observability Analysis of Electrical Distribution Networks","In modern electrical power systems, distribution networks facilitate the final step of power delivery to homes and businesses. Distributed energy resources (DERs) such as Tesla powerwalls and rooftop PV systems, automated sensing devices equipped with telemetry capabilities such as micro-Phasor Measurement Units (μ-PMUs) and smart meters, and active loads, which are capable of responding to real-time pricing signals, all significantly disrupt the standard operating procedures of distribution networks. One of the primary roadblocks to successful operation and control of these systems is the lack of network observability. Due to the significant cost and effort associated with sensor deployment in ultra-large distribution networks, system operators must alternatively leverage the physical model of the network and various measurement sets to reconstruct the so-called “state” (i.e., voltage equilibrium) of the network. State estimation, therefore, is a vitally important tool for distribution system operators. Because network parameter values span many orders of magnitude and sensors are critically under-deployed, the traditional state estimation problem is severely ill-conditioned and is seldom deployed in the field.Standard DSSE techniques rely on strong, yet potentially unjustified, regularization to combat the ill-conditioning of the problem. In this project, we represent the operation of a distribution system as a sequence of nonlinear maps that relate measurements, states, controller decisions, and operational performance. Using advanced uncertainty quantification techniques, we quantify the subspace of input perturbations whose response is practically “unobservable” at the output of each nonlinear map. These sensitivity results (which must be regathered each time state estimation is employed) guide the selection of appropriate regularization methods whose application can be probabilistically justified. We therefore carefully apply varying degrees of statistical regularization, such as Bayesian priors, and physics-based regularization to solve the state estimation problem. Further uncertainty quantification not only gauges the quality of the result, but also suggests optimal field testing and optimal placement of future deployed sensors to system operators. Despite regularization, the Hessian used to iteratively solve the state estimation problem can still exhibit severe numerical ill-conditioning. To overcome this numerical ill-conditioning, we are developing a set of computationally efficient and numerically robust methods to invert the Gauss-Network “gain” matrix. This solution utilizes a semi-explicit LU decomposition in conjunction with a matrix series expansion (i.e., Neuman expansion) and sequential applications of the so-called Woodbury matrix identity. Homotopy methods are used to scale the measurement variances and line lengths to decrease the number of iterations needed to converge on a final solution."
Maximizing the External Radiative Efficiency of Hybrid Perovskite Solar Cells,"Despite rapid advancements in power conversion efficiency (PCE)in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite’s external radiative efficiency (ERE) and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. If defects are strategically targeted, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved."
Blade Coating of Perovskite Solar Cells Toward Roll-to-roll Manufacturing,"High efficiency combined with transformative roll-to-roll (R2R) printability makes metal halide perovskite-based solar cells the most promising solar technology to address the terawatt challenge of the future energy demand. However, translation from lab-scale deposition solution processing techniques, such as spin coating, to large-scale R2R compatible methods has been a significant challenge due to fundamental differences in coating fluid dynamics and resulting drying and crystallization processes with the different coating methods. Here we address this challenge by developing processes and device architectures with high-speed (> m min-1) blade-coating (Figure 1A), which is R2R manufacturing compatible. We constructed solar cells with structure of Glass/FTO/SnO2/FA0.8MA0.16Cs0.04PbBr0.16I0.84/Spiro-MeOTAD/MoOX/Ag (Figure 1B), where the SnO2 is blade-coated at an environment of 49% relative humidity and with overall device thickness of less than 1 μm, excluding the glass substrate. We demonstrated a light-to-electricity conversion efficiency up to 17%, with open-circuit voltage of 1.112 V, short-circuit current of 22.12 mA cm-2, and fill factor of 69.1% (Figure 1C). The application of blade-coating of SnO2 has been a first step to show the potential of scaling highly efficient perovskite solar cells with transformative R2R compatible manufacturing techniques."
Solid State Batteries: Interfacial Degradation Between Solid Electrolyte and Oxide Cathodes,"All-solid-state batteries (SSBs) promise safer and higher performance energy storage than the present liquid-electrolyte Li-ion batteries. Li7La3Zr2O7 and Li1+xAlxTi2−x(PO4)3 are promising solid electrolytes for Li-ion SSBs. The wide electrochemical window of Li7La3Zr2O7 enables usage of a Li metal anode and high-voltage oxide cathodes. This combination makes Li7La3Zr2O7 a promising candidate for a high-capacity battery cell. Li1.4Al0.4Ti1.6(PO4)3 has a high stability win-dow and excellent chemical stability against moisture, enabling large-scale production with minimal cost. However, the development of SSBs in both systems is hindered mainly due to the high cathode |electrolyte interfacial resistance, which impedes the Li-ion trans-fer and ultimately the durability and power density. Sintering, which is necessary to get good contact be-tween a cathode and an electrolyte, leads to the forma-tion of detrimental phases that are insulting for Li-ion transfer. Despite the importance of the issue, only lim-ited understanding of the interfacial chemistry exists so far. The lack of research comes from the challenges in investigating buried interfaces.We aim to advance the understanding and control over the stability of the cathode|electrolyte interfaces. We use model systems made of thin-film cathode layers on dense electrolyte pellets (Figure 1). This approach enables us to use surface-sensitive and non-destruc-tive techniques such as X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorp-tion fine structure (EXAFS) to study buried interfaces. Our findings show that interfacial degradation is high-ly dependent on the gas environment used in the sin-tering process (Figure 2). Annealing in O2 environment does not lead to formation of a detrimental phase at the interface. In contrast, annealing in air or in CO2 led to severe degradation. We attribute this to the forma-tion of Li2CO3 and delithiated phases at the interface. The findings from this project will lead to identifying suitable process parameters to develop a stable cath-ode-electrolyte interface with good electrochemical properties."
Techno-economic Assessment and Deployment Strategies for Vertically-mounted Photovoltaic Panels,"Conventional schemes of panel mounting require horizontal space, on the order of 20,000 to 40,000 m2 per megawatt peak (MWp), prompting us to investigate new strategies for deploying solar panels. Mounting solar photovoltaic (PV) panels vertically to the sides of existing structures, such as facades of buildings, offers one such strategy. Vertically-mounted PVs take advantage of otherwise unused vertical real estate in the built environment, with minimal additional structural reinforcement costs and no need for additional land area use. Uniquely, the peak electricity generation time of west-facing vertically-mounted PV panels occurs closer to the hour of maximum consumer power demand, allowing increased electricity generation when the same PV panels, if conventionally mounted, would generate lower amounts of power.Keeping these advantages in mind, we identified a set of potential profitable markets in the United States (U.S.) and enumerated the technical challenges to expanding PV usage into these markets. We calculated the levelized cost of electricity (LCOE) for vertically-mounted PVs as a function of the azimuth panel; then using county-level resolution we estimated economic viability for these installations in the contiguous U.S. The LCOE calculations allow us to identify target specifications for vertical PV panels to be economically competitive when compared to the commercial grid electricity. We show that lightweight, flexible and bifacial form factors attainable with the next-generation PV technology can lead to installation cost reductions. We are developing roof-of-concept prototypes to validate our hypothesized deployment strategies."
Hafnia-filled Photonic Crystal Emitters for Mesoscale Thermophotovoltaic Energy Converters,"Thermophotovoltaic (TPV) systems are promising as small scale, portable generators for power sensors, small robotic platforms, and portable computational and communication equipment. In TPV systems, an emitter at high temperature emits radiation that is then converted to electricity by a low bandgap photovoltaic cell. Our group’s approach to increase both TPV power and efficiency is to use two-dimensional, hafnia-filled tantalum photonic crystals (PhCs) as emitters. These emitters consist of a 2D array of cylindrical cavities etched in tantalum and filled with hafnia (HfO2). They work by enabling efficient spectral tailoring of thermal radiation for a wide range of incidence angles; they can increase the fuel-to-electricity efficiency of our group’s propane-based TPV system from 4.3% to above 12%. However, fabricating these PhCs is difficult: while the deep cavities of the PhC must be filled as completely as possible, using atomic layer deposition to fill the cavities layer by layer leads to an uneven and thick top hafnia surface that adversely impacts the emittance. Because the PhC optical performance improves with a flatter top hafnia surface, we explore methods to planarize the top surface, in particular by depositing a sacrificial oxide layer and etching it back. With a single iteration, the average height difference between the hafnia crest and trough is reduced from about 200nm to 90nm in silicon PhC samples, suggesting a method to fabricate PhCs with improved geometry and emittance. Precise fabrication of PhC emitters can enable high TPV performance and pave the way toward portable micro-generators for off the grid applications."
Fabric Integration of Organic Photovoltaics,"In recent years, wearable technologies have emerged as a platform beyond basic functionality, such as a watch or a headphone, into highly integrated tools capable of communications, biosensing, navigation guidance, and performing financial transactions. Yet these technologies remain localized on the body in a bulky form-factor such as a smartwatch, AR glasses, or earbuds. Seamless integration of electronics over large areas into the most indispensable wearable, clothing, remains a distant goal. Forgoing conventional discrete/bulky electronics in place of emerging thin-film alternatives promises to bridge this gap. In this project, we report integration of organic photovoltaics (OPV) into ultra-lightweight composite fabrics (Dyneema) as a first step towards realizing elec-tronically active fabrics. The devices are fabricated on CVD-deposited ultra-thin dielectric substrates, which lend themselves for use on fabrics through transfer lamination. Employing standard thermal evaporation and RF sputtering processes, we have demonstrat-ed fabric-integrated OPV devices with over 1% power conversion efficiencies. In an effort to realize photo-voltaics with higher efficiencies that can power larger electronic devices, we are currently exploring the use of electronic polymer inks, which can be coated/print-ed through scalable roll-to-roll processes. Techniques developed in this project can also enable integration of other devices including displays, sensors, speakers, and actuators."
Balancing Actuation and Computing Energy in Low-power Motion Planning,"We study a novel class of motion planning problems, in-spired by emerging low-power robotic vehicles, such as insect-size flyers, high-endurance autonomous blimps, and chip-size satellites for which the energy consumed by computing hardware while planning a path can be as large as the energy consumed by actuation hard-ware during the execution of the same path. For these new applications, we must consider the total energy of executing and computing a candidate solution to eval-uate a motion plan. Figure 1 shows average actuation energy and computing energy curves for a selected robotic platform and computing platform. Here, min-imizing only the actuation energy does not minimize the total energy. Instead, stopping computing earlier and accepting a higher actuation energy cost for a low-er computing energy cost minimizes the total energy.We propose a new algorithm, called Computing Energy Included Motion Planning (CEIMP). CEIMP op-erates similarly to other anytime planning algorithms, except it stops when it estimates further computing will require more computing energy than potential savings in actuation energy. The algorithm relies on Bayesian inference to estimate future energy savings to evaluate the trade-off between the computing ener-gy required to continue sampling and the potential fu-ture actuation energy savings after such computation. CEIMP outperforms the average baseline of using max-imum computing resources in realistic computational experiments involving 10 MIT building floor plans. On the ARM Cortex-A15, for a simulated vehicle that uses 1 Watt to travel 1 m/s, CEIMP saves 2.1-8.9x the total ener-gy on average across floor plans compared to the base-line, translating to missions that can last 2.1-8.9x longer on the same battery. Figure 2 shows CEIMP in action; while the path returned by CEIMP is longer than the path returned by the baseline, CEIMP’s total energy is much closer to the true minimum of total energy than the baseline."
Enabling Low-cost Electrodes in PbS Solar Cells through a Nickel Oxide Buffer Layer,"The versatile characteristics of lead sulfide quantum dots (PbS QD) make them an attractive material to de-velop high-efficiency, low-cost, and flexible photovol-taics (PVs). Hole transport layers (HTLs) and electron transport layers are essential building blocks in these solar cell architectures. PbS QDs with an EDT ligand are widely used as an HTL in high-efficiency QDPVs. However, the limited compatibility of the EDT with different electrode materials prevents the continued development of QDPVs into manufacturing capable de-vice architectures. Specifically, the dependence on gold electrodes is cost-prohibitive for depositing QDPVs on a large scale.While gold cannot be used on a commercial scale, less expensive but more chemically reactive materials can be used. Replacing gold with aluminum or copper would cut material costs by a factor of at least 1,200. Through the use of a nickel oxide (NiOx) buffer layer, these devices become compatible with lower-cost elec-trodes. As a p-type metal oxide, NiOx is a favorable HTL material with a high work function, large band gap, and film stability.In fact, through the use of a NiOx buffer layer, power conversion efficiencies for devices with low-er-cost electrodes are equivalent to their gold electrode counterparts. However, even though NiOx buffer layer devices show improved performance and stability com-pared to devices without NiOx buffer layers, the power conversion efficiency drops after a couple of months due to a new barrier within the device stack. Current research focuses on improving the stability of QDPVs with low cost electrodes through identifying and miti-gating the barrier formation."
Architecture-level Energy Estimation of Accelerator Designs,"With Moore's law slowing down and Dennard scaling ending, energy-efficient domain-specific accelerators have become a promising direction for hardware de-signers to continue bringing energy efficiency improve-ments to data and computation intensive applications. To ensure fast exploration of accelerator design space, architecture-level energy estimators, which perform energy estimations without requiring complete hard-ware description of the designs, are critical to design-ers. However, it is hard to use existing architecture-lev-el energy estimators to obtain accurate estimates for accelerator designs, as accelerator designs are diverse and sensitive to data patterns.To solve this problem, we present Accelergy (Fig-ure 1), an architecture-level energy estimation method-ology. Accelergy allows the users to define their own components in their designs to allow descriptions of the diverse design space. At the same time, to reflect the energy differences brought by special data patterns, e.g., sparsity in data, Accelergy also allows the users to define special actions types related to the components. To enhance flexibility, Accelergy defines an interface to communicate with other estimators that focus on en-ergy estimations of specific types of components in the designs (e.g., memory storage components). To illustrate the usage of Accelergy methodology, we implemented an example framework for energy estimations of deep neural network (DNN) accelerator designs. We further integrate Accelergy with Timeloop, a DNN mapping space exploration tool, to enable accurate estimation of processing-in-memory (PIM) based DNN accelera-tor designs. We validated the Accelergy framework on a conventional digital design Eyeriss as well as a PIM-based design, both achieving a total energy estimation accuracy of 95% and accurate energy breakdowns of various components in the designs (Figure 2)."
Low-frequency Buckled Beam MEMS Energy Harvester,"Vibrational energy harvesting at the MEMS scale is a unique challenge for low-frequency sources which are ubiquitous but do not operate at resonant frequencies of structures on the micro scale. It is nature’s law that resonant frequency is inversely proportional to mass, which is a great challenge for micro-scale energy har-vesting devices operating at low frequencies (less than 100Hz). A bi-stable buckled beam design is presented that does not rely on resonance of a MEMS structure but rather operates by snapping between buckled states at low frequencies. A fully functional piezoelectric MEMS energy harvester is designed, monolithically fabricated, and tested. An electromechanical lumped parameter mod-el is developed to analyze the nonlinear dynamics and to guide the design of the nonlinear oscillator-based energy harvester. Multi-layer beam structure with re-sidual stress induced buckling is achieved through the progressive residual stress control of the deposition processes along the fabrication steps. Dynamic test-ing, however, demonstrated that optimizing the beam stiffness to proof mass ratio was challenging given the presence of undesired modes of vibration. A new iter-ation of the design was fabricated with changes to the proof mass geometry which stabilize the oscillations by reducing rotational inertia, a key variable in enhanc-ing dynamic performance of the device."
A CMOS-based Energy Harvesting Approach for Laterally-arrayed Multi-bandgap Concentrated Photovoltaic Systems,"When high solar conversion efficiency is desired, peo-ple often adopt concentrated photovoltaic systems with multi-junction cells. However, traditional tandem structures widely used in such systems can suffer from current-mismatch effects with spectrum variations, whereas the Laterally-Arrayed Multi-Bandgap (LAMB) cell structure is a potentially higher-efficiency and lower-cost alternative.Here we show an energy harvesting approach de-signed to take full advantage of the LAMB cell struc-ture. Individual cells within a sub-module block are connected for approximate voltage-matching, and a Multi-Input Single-Output (MISO) buck converter combines the energy and performs Maximum Pow-er Point Tracking locally. A miniaturized MISO dc-dc converter prototype is developed in a 130nm CMOS process. For 45-160mW power levels, >95% peak effi-ciency is achieved in a small form factor designed to fit within available space in a LAMB cell block. The results demonstrate the potential of the LAMB CPV system for enhanced solar energy capture."
Engineering a 2D Hole Layer in Hydrogen-terminated Diamond Using Transition Metal Oxides,"The quest for a suitable wide-bandgap semiconductor for high-power and high-frequency applications is well motivated; wide-bandgap semiconductors generally ex-hibit a high breakdown field and can therefore support a high voltage over short distances. Diamond (5.5 eV) in particular is an attractive prospect since its thermal conductivity and radiation hardness far surpass that of other wide-bandgap semiconductors. However, practi-cal transistors require the ability for the charge density to be engineered through substitutional doping, which has proven to be difficult considering the strong cova-lent bonds that make up bulk diamond.We use an alternative doping mechanism, surface transfer doping; it takes advantage of the unformed bonds at the diamond surface and generates a highly conductive 2D hole sheet at the surface with carrier densities up to 1014 cm-2. Surface transfer doping using stable high electron affinity transition-metal oxides (TMO) such as WO3 along and the novel contact-first process explored in this work shows great promise to advance process stability while maintaining the high current densities desired for future power diamond transistors. We are exploring various methods to reproducibly achieve high values of sheet hole concentration and hole mobility on the diamond surface that can be in-corporated into a transistor fabrication process. Our proposed design for characterizing mobility and sur-face conductivity combines a transmission line and Van der Pauw test structures simultaneously, as shown in Figure 1. We chose tungsten as the ohmic contact for its thermal stability and attractive process character-istics. We are examining different H-plasma processes for diamond surface bond passivation and the use of the hydrogen isotope deuterium. Preliminary results show increased carrier concentration and mobility with Al2O3 as the surface dopant, as in Figure 2. The methods explored in this work show promise towards the enhancement of diamond conductivity and repro-ducibility."
Dynamic Approach to Quantifying Strain Effects on Ionic and Electronic Defects in Functional Oxides,"The search for novel electronic and magnetic properties in functional oxides has generated a growing interest in understanding the mobility and stability of ionic and electronic defects in these materials. Instead of altering material content, most research views mechanical strain as a lever for modulating defect concentration and mobility more finely and continuously in both semi-conductors and functional oxides. Previous studies also proposed that strain may increase ionic mobility by orders of magnitude, which is crucial for lowering the operation temperature of solid oxide fuel cells.However, experimental and computational results from research groups differ significantly due to the convoluted effect of mechanical strain and film/substrate interface on defect content and mobility. Such reliance on substrate selection to induce strain in the oxide thin film also limits the range of strain accessibility, with limited data available to dateWe have developed an experimental technique that facilitates application of in-plane strain to functional oxide thin films continuously on the same substrate. First, we combine photolithography and metal sputtering in MIT Nano to deposit an interdigitated Pt electrode down to a 2-micrometer finger distance on our sample (Figure 1). Next, we conduct 3- or 4-point bending and concurrent conductivity measurement of the thin film-on-substrate device (Figure 2). This approach is accessible to a wide temperature range and has precise gas control relevant to mixed ionic-electronic conducting oxides with extremely high reproducibility (error < 3%) over a long period of time. We can strain and measure the transport properties of the same functional oxide thin film at high temperature in situ, over a range of strains applied to a single system. Combining these experiments with our ab initio computational simulations and predictions of carrier dominance over a range of strains and temperatures, we also aim to measure the carrier mobility in Nb-doped SrTiO3 as a function of applied strain, to observe the sudden change of carrier mobility and temperature dependency. We believe this will also be a powerful technique for studying the strain effect on surface reactions like exsolution or catalytic reaction."
First Demonstration of GaN CMOS Logic on Si Substrate Operating at 300 Degrees C,"The power density (and form-factor) of power electronic circuits is mostly dominated by the size of the passive energy storage components like inductors and capacitors, which depend on the switching frequency. Increasing the switching frequency of power electronic circuits can significantly reduce the energy storage requirement of these components to allow for smaller components. However, the maximum operating frequency of state-of-the-art GaN transistors, promising candidates for high-voltage compact switches, is usually limited by the gate inductance between the gate electrode and the driver circuit. Monolithically integrating the GaN-based driver circuit with that of the GaN power transistor on the same chip can significantly reduce this inductance.To increase the efficiency of such GaN-based integrated circuits, a CMOS-like circuit technology is needed. Major benefits of such a technology include zero/negligible static power dissipation, higher noise immunity, and linearity. However, the lack of high-performance GaN p-FETs and the challenges of their monolithic integration with E-mode n-FET devices are major roadblocks towards achieving such a technology. This work demonstrates a new GaN-based complementary circuit platform on 6-inch Si substrate. Figure 1(a) shows the voltage transfer characteristics (VTC) of the inverter for a VDD of 5 V along with output current. The inverter shows a record voltage gain of 27 V/V for a voltage switching of 0-to-5 V. Figure 1(b) shows the VTC of the same inverter for VDD=3 V, exhibiting excellent inverting behavior with Vswing=2.91 V and maximum gain of ~15 V/V. The dynamic switching of the inverter was characterized by connecting the inverter input to a pulse generator and the output to the high impedance port of an oscilloscope. The VDD was kept at 3 V because of the high gate leakage in the p-GaN gated n-FET above that voltage. The voltage of the input pulses varied from −0.2 V to 3 V with a ramp time of 100 ns. Figure 1(c)-(d) presents measured waveforms of the input and output signals. The output signal showed a voltage swing close to 0~3 V. The fall time was 1 µs; the rise time was 20 µs. It should be noted that these times represent an upper bound on the fall and rise times, as the very high input capacitance of the oscilloscope port (~ 350 pF) limits the measurements.High-temperature measurement of the inverter shows a reduction in the voltage gain, as shown in Figure 2. The maximum available voltage swing at the output is also reduced due to the rise of low-level Vout, which can be attributed to the reduction in ON-OFF current ratio of the p-FET at high temperature. At high temperature, because of the higher activation of Mg dopants, the threshold voltage of p-FET moves towards the positive zone, making it D-mode, which in turn reduces the ON-OFF current ratio. While room exists for significant performance improvement, this demonstration opens a number of application domains for GaN such as integrated CMOS driver circuits, CMOS logic, logic, and signal conditioning under harsh environment operation, among many others."
100-nm Channel Length E-mode GaN p-FET on Si Substrate,"GaN-CMOS-technology could be instrumental towards realizing high-power-density, high-speed, low-form-factor, and highly-efficient power electronic circuits, which sparked many efforts to develop a high performance GaN p-FET. However, most of these demonstrations show normally-ON operation with ON-resistance over 1 kΩ∙mm. GaN/AlInGaN heterostructure-based p-FET shows low ON-resistance because of higher 2-DHG density and hole mobility but with D-mode operation. A GaN/AlN heterostructure based p-FET shows E-mode operation with RON of 640 Ω∙mm. However, n-FET integration with this p-FET requires regrowth. In this work, we demonstrate a self-aligned p-FET with a GaN/Al0.2Ga0.8N (20 nm)/GaN heterostructure grown by metal-organic-chemical vapor deposition (MOCVD) on Si substrate. The utilization of GaN-on-Si platform offers lower cost, availability of 200-mm-diameter substrates, and potential to integrate with high performance logic and analog functionality. While most of the GaN p-FET demonstrations so far in the literature mainly focused on recessed gate MISFET structure, we choose to develop a self-aligned structure (see Figure 1 for the device structure) as it offers the following advantages over a recessed gate MIS p-FET: (1) the shortest possible source to the drain distance, cutting down the access region; (2) low ON-resistance because of negligible access resistance: and (3) easier gate alignment.Our 100-nm channel length self-aligned device with recess depth of 70 nm exhibits a record ON-resistance of 400 Ω∙mm and ON-current over 5 mA/mm with ON-OFF ratio of 6×105 when compared with other p-FET demonstrations based on a GaN/AlGaN heterostructure (see Figure 2 for benchmarking of our device with other p-FET demonstrated in the literature). The device shows E-mode operation with a threshold voltage of −1 V, making it a promising candidate for a GaN-based complementary circuit that can be integrated on a Si platform. A monolithically integrated n-channel transistor with p-GaN gate is also demonstrated."
Characterizing and Optimizing Qubit Coherence Based on SQUID Geometry,"Superconducting qubits are leading candidates to implement quantum hardware capable of performing certain computational tasks more efficiently than their classical counterparts. A prerequisite for scalable quantum computation is a sufficiently low noise level in the participating qubits. The dominant source of decoherence in frequency tunable superconducting qubits is 1/f flux noise, presumably originating from magnetic defects located at the interfaces of their SQUID loops. Here, we measure the flux noise amplitudes of more than 50 capacitively shunted flux qubits and study their dependence on geometric parameters of their SQUID loops. Each of six chips (Figure 1) holds ten capacitively shunted flux qubits, featuring two copies of five different SQUID loop geometries, respectively. Dispersive readout of each qubit is performed using a common transmission line and individual readout resonators. We perform a series of spin-echo measurements in the vicinity of the flux sweet spot of the qubits, showing that the pure dephasing rate is proportional to the slope of the qubit spectrum, which is in turn related to the flux noise amplitude for each qubit. Our data (Figure 2) show good agreement with a previously presented microscopic model for independent spin impurities, which has so far eluded experimental verification. Due to a limited applicability of the proposed model for superconducting films of finite thickness, we provide numerical simulations of the current distribution in our SQUIDs, which extend and refine the considered model. Our improved model is in excellent quantitative agreement with our data both in terms of absolute numbers and geometry dependence (Figure 2b). Our results demonstrate that flux noise is suppressed in SQUIDs with small perimeters, fat wires, and thick superconducting films therefore serve as a guide for minimizing the flux noise susceptibility in future circuits."
Control of Conducting Filaments Properties in TiO2 by Structural and Chemical Disorder for Neuromorphic Computing,"Resistive switching (RS) random access memories are considered as possible artificial synapses in next-generation neuromorphic networks, mostly due to their predicted high memory density, energy efficiency and scalability. Integration of these devices in a neuromorphic computing system could allow solving intensive computing tasks actually only handled by the human brains such as speech and character recognition as well as grammar and noise modeling. Within their architecture, redox-based RS memory devices store binary code information using the electric field-induced resistance change of an oxide layer by conductive filament (CF) formation and rupture (Figure 1a). Nevertheless, a lack of control on the properties of CFs, which mainly forms at chemical and structural defects, causes detrimental cycle-to-cycle and device-to-device variations. We are therefore studying the effect of strain on the microstructure, chemistry and RS properties of TiO2 thin films to get insights into defects formation with the objective of selectively doping along these defects (Figure 1b). We found that the microstructural properties of pulsed laser deposited epitaxial TiO2 films depend on both the film thickness and the nature of the bottom electrode, suggesting a potential method to better control defects properties and improve consistency in RS."
Manipulation of Coupling and Magnon Transport in Magnetic Metal-insulator Hybrid Structures,"Ferromagnetic metals and insulators are widely used for generation, control, and detection of magnon spin signals. Most magnonic structures are based primarily on either magnetic insulators or ferromagnetic metals, while heterostructures integrating both of them are less explored. Here, by introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure grown on Si substrate (Figure 1(a)), we studied the magnetic cou-pling and magnon transmission across the interface of the two magnetic layers. After the film growth by mag-netron sputtering, atomic force microscopy (AFM) mea-surements were performed (Figure 1(b)) to characterize the film quality, which indicates a surface roughness of approximately 1 nm. Moreover, we found that with-in this structure, Py and YIG exhibit an antiferromag-netic coupling field as strong as 150 mT, as evidenced by both the vibrating-sample magnetometry (VSM) (Figure 1(c)) and polarized neutron reflectometry mea-surements. By controlling individual layer thicknesses and external fields, we realize parallel and antiparallel magnetization configurations, which are further uti-lized to control the magnon current transmission. We show that a magnon spin-valve with an ON/OFF ratio of ~130% can be realized out of this multilayer structure at room temperature through both spin pumping and spin Seebeck effect experiments. Owing to the efficient control of magnon current and the compatibility with Si technology, the Pt/YIG/Py hybrid structure could po-tentially find applications in magnon-based logic and memory devices."
Nonvolatile Control of Long-distance Spin Transport in an Easy-plane Antiferromagnetic Insulator,"How an antiferromagnet transmits spin angular momentum by the quanta of spin-wave excitations, viz. magnons is one of the core topics of antiferromagnetic magnon spintronics. It is generally believed that only easy-axis antiferromagnets can support spin transmis-sion, a natural inference of the fact that the circularly polarized magnons there have finite spin angular momentum. In contrast, easy-plane antiferromagnets would destroy spin transport due to the vanished angular momentum carried by their linearly polarized magnons.In this work we show that contrary to this traditional picture, spin transmission over micrometer distance indeed happens in an easy-plane insulating antiferromagnet, α-Fe2O3 thin film. A model involving superposition of linearly polarized propagating magnons is proposed to account for the observations. Enabled by this physical insight, our work opens up additional possibilities for nonvolatile, low magnetic field control of spin transmission, where a spin-current switch with a 100% on/off ratio is realized."
Gigahertz Frequency Antiferromagnetic Resonance and Strong Magnon-magnon Coupling in the Layered Crystal CrCl3,"Antiferromagnetic spintronics is an emerging field with potential to realize high-speed memory devices. Compared to ferromagnetic materials, antiferromagnetic dynamics are less well understood, partly due to their high intrinsic frequencies that require terahertz techniques to probe. Here, we introduce the layered antiferromagnetic insulator CrCl3 as a tunable platform for studying antiferromagnetic dynamics. Because of weak interlayer coupling, the antiferromagnetic resonance (AFMR) frequencies are within the range of typical microwave electronics (<20 GHz). This allows us to excite different modes of AFMR and to induce a symmetry-protected mode crossing with an external magnetic field. We further show that a tunable coupling between the optical and acoustic magnon modes can be realized by breaking rotational symmetry. Recently, strong magnon-magnon coupling between two adjacent magnetic layers has been achieved, with potential applications in hybrid quantum systems. Our results demonstrate strong magnon-magnon coupling within a single material and therefore provide a versatile system for microwave control of antiferromagnetic dynamics. Furthermore, CrCl3 crystals can be exfoliated down to the monolayer limit, allowing device integration for antiferromagnetic spintronics. We transferred layered bulk CrCl3 onto a coplanar waveguide (CPW) and secured it with Kapton tape. The crystal c-axis is normal to the CPW plane. We measure microwave transmission in a cryostat by fixing the excitation frequency and sweeping the applied magnetic field. When the field is applied in-plane and parallel to the in-plane radio frequency field, both acoustic and optical modes of AFMR are observed. The mode frequency evolutions are well-described by theoretical formulas. When the field is canted out-of-plane, two magnon modes hybridize because of rotational symmetry breaking, and the coupling strength is tunable by rotation angle. Our results demonstrate that CrCl3 serves as a convenient platform for studying AFMRs in microwave frequencies and shows the possibility to realize magnon-magnon coupling utilizing van der Waals assembly."
High-density Microwave Packaging for Superconducting Quantum Information Processors,"Quantum information processors hold the promise to solve specific computational problems much faster than classical computers. Superconducting qubits are among the leading candidates for realizing near-term quantum processors. Beyond lithographic scalability, superconducting qubits offer long computational operation windows–coherence times–relative to short operational gate times that have enabled the demonstration of the first practical quan-tum algorithms. Despite this progress, engineering challenges must be met to further scale these devices. In particular, qubits require a precisely engineered microwave environment to suppress energy decay and corresponding information loss. For instance, the corruption of information can occur due to lossy package modes interacting with the qubit electric field. As the number of qubits increases, qubit packages must be adapted to support an increasing number of input/output ports without adding additional loss channels.Our qubit package, shown in Figure 1 (a), provides a well-defined electromagnetic (EM) environment. It consists of an aluminum-coated copper cavity and a microwave interposer with 32 waveguides. We performed full-wave simulations of the signal launches, the package cavity, and superconducting wirebonds to establish principles needed to construct larger packages. We evaluated the presence and absence of lossy package modes using high-coherence qubits as sensors, illustrated in panel (b). A weakly driven package mode causes EM-field enhancement with increased microwave photon number fluctuations, which, when coupled to the qubit, shifts its energy levels. The resulting qubit energy fluctuations result in qubit dephasing inferable via Ramsey interferometry. Sweeping a probe tone in frequency while monitoring the coherence time reveals the presence of parasitic package modes. Panel (c) exhibits a mode-free operating environment up to 11 GHz. Our EM model can reproduce the observed package modes, shown in panel (d). Current work focuses on the design of packages to support more complex qubit chips and modular interconnects to facilitate fast chip exchange."
Scanning Transmission Electron Microscopy Imaging of Materials,"Properties of materials are controlled by the arrangement and type of atoms in the structure. Many characterization techniques can provide information about the crystal structure and micro scale features, but atomic scale information is critical for fully understanding a material system. Through advanced scanning transmission electron microscopy (STEM) techniques, atomic column intensity and positions can be extracted to provide useful information about ordering, local distortions, and defects.Materials such as strontium titanate, SrTiO3, demonstrate the capabilities of this powerful imaging technique. Annular dark field (ADF) STEM imaging shows atom column contrast from Sr and Ti cations, as expected from the crystal structure, but no contrast from the oxygen anion atom columns due to the low atomic number of oxygen (Figure 1 a). Integrated differential phase contrast (iDPC) imaging in the STEM mode makes the lighter oxygen atoms visible (Figure 1 b). Additionally, the electric field vector map in projection can be found from the differential phase contrast data acquired from a four-segment detector (Figure 1 c). Projected charge density maps obtained from differential phase contrast imaging clearly show symmetrical charge contours revealing non-polar behavior in the SrTiO3 sample (Figure 1 d). Such a projected charge density imaging technique is useful in studying polar functional material.The positions and intensity of each atom column can be extracted from the STEM images using image analysis techniques. Detailed, quantitative analysis of bond lengths, bond angles, and atomic contrast can be used to find regions of order, local distortions, and defects. Structural nanoscale features such as ferroelectric/ferromagnetic domains or chemical/distortion-ordered regions can be correlated with the electrical, mechanical, ferroelectric, magnetic, and other properties of the material to elucidate the nanoscale origin of macroscale properties."
Degradation Under Forward Bias Stress of Normally-off GaN High Electron Mobility Transistors,"Energy-efficient electronics have been gaining much attention as a necessary path to meet the growing demand for energy and sustainability. GaN field-effect transistors (FETs) show great promise as high-voltage power transistors due to their ability to withstand a large voltage and carry a high current with minimum losses. For best circuit reliability and performance, a normally-off transistor is highly desirable. An attractive design is the p-doped gate AlGaN/GaN high electron mobility transistor (p-GaN HEMT).Our research aims to better understand the reliability issues impeding widespread adoption of p-GaN power HEMTs for power management applications. One key issue is device degradation under electrical stress, where key device performance figures such as the threshold voltage and the gate leakage current change with electrical stress.Understanding reliability issues of p-GaN power HEMTs is obstructed by the complex gate stack of the devices. First, both holes and electrons are present in the gate stack: holes in the p-doped GaN region and electrons in the 2-dimensional electron gas at the AlGaN/GaN interface. Furthermore, holes and electrons encounter several barriers (shown in the energy band diagram of Figure 1), obfuscating understanding of the electrostatics and transport physics under forward-bias stress. Coupled with the often time-dependent nature of degradation, p-GaN power HEMT reliability remains difficult to fully understand. For instance, Figure 2 shows the time evolution of the gate leakage with different constant gate voltage stress. As can be easily seen, the gate leakage current decreases with time at lower biases and high biases but increases with time at intermediate biases, showing a complex multi-regime behavior. Nevertheless, an on-going reliability analysis such as breakdown voltage indicates that p-GaN HEMTs show great promise as robust and efficient next-generation power transistors."
Vertical Leakage Characteristics of GaN Power Transistor,"The great promise of Gallium Nitride Metal-Insula-tor-Semiconductor High Electron Mobility Transistors (GaN MIS-HEMTs) in the growing power electronic market has rapidly positioned these devices at the forefront of a new technology wave. This has triggered a vast amount of worldwide research and yielded con-tinuous improvements in device performance and electrical reliability. Regarding reliability, a key consid-eration in any new device technology, the maximum breakdown voltage is ultimately limited by the vertical breakdown of the drain-body junction. This is particu-larly a concern for devices with conductive substrates. A way to mitigate premature drain-body break-down under high positive drain voltage consists of ap-plying a positive voltage to the body with respect to the source so that the drain-body voltage can be reduced. A potentially problematic consequence of this is exces-sive source-body leakage current under off conditions. This is undesirable. In this work, we study the source-body leakage in commercially prototype devices for negative voltage at the source with respect to the body. Figure 1 shows the body current as the source is swept negative and then positive at 26. The different paths that are followed and the “eye” that appears could be due to trapping or a floating-body effect. Figure 2 shows the temperature dependence of the negative sweep. The sharp corner in the characteristics that coincides with the maximum widening of the “eye” opening ap-pears to have a negative temperature coefficient of -0.13 eV. These and other interesting features are critical to understanding the origin of the reverse bias source-body current so that it can be suppressed."
Quantum Landscape Engineering of Superconducting Circuit Ground States for Higher-order Coupler Design,"Superconducting circuits provide a versatile engi-neering platform for the study of quantum systems and their use as a computational resource. Their ap-plication ranges from studying fundamental princi-ples such as the physics of quantum entanglement to the demonstration of large-scale control of quantum bits simulating spin models in solid state physics. Many-body interactions of multiple spins simultane-ously are one aspect of spin models that has not been demonstrated to date.In this work, we exploit that the response of the quantum ground state energy of a superconducting cir-cuit to external magnetic flux can be shaped by design to engineer artificial spin couplers. We propose a meth-odology for adding higher-order polynomial terms into the ground state energy versus flux by strongly cou-pling a series of rf SQUIDs. The fundamental instance of two rf SQUIDs generating a ground state with 4th-or-der terms is implemented experimentally. Probing this circuit with a sensor flux qubit, the qubit’s transition frequency maps the derivative of the quartic ground state in accordance with simulation. Modest levels of qubit coherence are maintained despite the relatively strong inductive coupling. These results demonstrate the viability of this design for use as a 4-local coupler and show promise for extending it to higher polynomi-al order."
Vertical Gallium Nitride FinFETs for RF Applications,"From wireless communication systems like the 4G and 5G cellular services that enable 4K video streaming, to the high-resolution radars that are vital to national defense, radio frequency (RF) systems have become a ubiquitous part of modern life. A fundamental building block within these systems is the RF power amplifier. As amplifier technology progresses, the relentless de-mand for improved performance necessitates develop-ment of new transistor technologies that can operate at higher power levels and over larger bandwidths. While traditional planar processing techniques have led to countless successful RF amplifiers, the fact that all conduction takes place very near the wafer’s surface fundamentally limits their performance. If instead we utilize a compact vertical transistor design, the bulk material can be used to withstand large voltages in the vertical direction as opposed to lateral designs, which need large device areas. Additionally, bulk conduction improves thermal spreading, thereby reducing cooling needs, and vertical gate patterning techniques trade ex-pensive high-resolution lithography for relatively easy control of etch depth. This work presents novel vertical GaN RF tran-sistors. As the cross-sectional diagram in Figure 1 shows, the vertical GaN RF finFET consists of narrow fins to confine the current and has sidewall gates to modulate the conductivity within the fins. To enable high-frequency system integration, these devices were fabricated on sapphire, a highly insulating substrate, with a top-side drain contact to remove the need for through-wafer vias. To reduce costs and allow easier integration with existing technology, the same devices can be fabricated on GaN on Si as well. Figure 2 shows a scanning electron microscope (SEM) cross section of a fabricated device. These devices achieve a current den-sity of over 7 kA-cm-2 and a power gain cut-off frequen-cy, fmax, of 5.9 GHz, demonstrating a promising first step toward vertical GaN transistors in RF applications."
Towards Sub-10-nm-Diameter Vertical Nanowire III-V Tunnel FETs,"Recently, III-V compound semiconductors have emerged as a promising family of materials for future complementary metal-oxide semiconductor (CMOS) technology, thanks to their superior electron trans-port properties. To enable continued scaling, a high aspect-ratio vertical nanowire (VNW) transistor geom-etry with a gate-all-around (GAA) structure is highly fa-vorable due to effective charge control and robustness to short-channel effects. Another big advantage of the vertical nanowire geometry is that it allows engineer-ing of the energy band structure along the transport di-rection, enlarging the device design space. In particular, device structures that potentially break the thermal limit of the subthreshold behavior become possible. In our research, we are pursuing the demonstra-tion of broken-band GaSb/InAs vertical nanowire tun-nel field-effect-transistors (TFETs) with sub-10-nm di-ameter for ultra-low power logic applications. We aim to exploit the recent demonstration of high-quality III-V MOS interface characteristics using in-situ ther-mal atomic-layer etching in combination with atomic layer deposition of the gate stack.In our work, we have developed a top-down ap-proach for sub-10-nm VNW fabrication, as shown in Figure 1. Hydrogen silsesquioxane (HSQ) hardmask is patterned by electron beam lithography (EBL), fol-lowed by Cl-based reactive-ion-etching (RIE) and al-cohol-based digital etch (DE). Planarization is anoth-er critical step, in which insulating layers are formed around the VNWs with good vertical location control. We have developed a method to accurately control the thickness of an HSQ film using EBL with different elec-tron doses. Figure 2 shows the final height of HSQ as a function of e-beam dose. The insets in Figure 2 show an example of a 60- nm-thick planarized HSQ spacer formed around a 230-nm-high InAs VNW."
W Contacts to H-terminated Diamond,"Diamond is considered a leading candidate for harsh environment high-power electronics due to their ex-traordinary thermal and electrical properties. One of the many challenges facing diamond electronics is creating reliable and stable ohmic contacts to hydro-gen-terminated diamond (D:H). In this work we ex-plored a novel approach for scalable and self-aligned ohmic contacts to D:H. Our results show that using this approach stable ohmic contacts can be obtain with state-of-art contact resistance. The diamond surface conductivity is governed by its surface termination. H-termination leads to a con-ductive surface, while O-termination (D:O) results in in-sulating diamond. The different terminations are typi-cally obtained by exposing the diamond surface to H or O plasma (fig. 1). Since in D:H all the dangling bonds are practically passivated, it is typically hydrophobic and suffers from poor adhesion to most materials which are only weakly attached by Van der Waals forces. D:O however, is hydrophilic and can provide good adhesion. This create a problem for ohmic contacts which usually need to be laid over a conductive surface. To overcome this issue in our approach we first pattern W contact on D:O providing good adhesion. After this, the dia-mond surface is exposed to the H plasma. We use W in this approach since it is one of the few metals that can withstand prolongued exposure to H plasma at elevat-ed temperature without being damaged or go through embrittlement.To test our approach, we fabricated four terminal TLM test structures with nano contacts. From the anal-ysis of the data (Fig. 2), we extract the contact resistance (black markers), as well as the D:H (blue markers) as a function of contact length Lc (Fig. 1 right). Since this is a ‘side contact’, it does not follow the classical transfer length behavior obtained when Ohmic contacts are overlapping a conductive surface (Fig. 2, full line). Rath-er, the contact resistance is insensitive to the contact length. Notably, the sheet and contact resistance are in par with other approaches to obtain ohmic contacts to D:H."
Cryogenic GaN HEMT Technology for Application in Quantum Computing Electronics,"High performance and scalable cryogenic electronics is an essential component of future quantum informa-tion systems, which typically operate below 4K. Cur-rent electronics rely on technology like CMOS (Si), or heterojunction bipolar transistor (e.g. SiGe, InP). This work explores the use of wide band gap het-erostructure electronics, specifically the AlGaN/GaN high electron mobility transistor (HEMT), for cryo-genic low-noise applications. These structures take advantage of the polarization-induced two-dimen-sional electron gas to create a high mobility channel, hence eliminating the use of heavy doping as in the other semiconductor technologies. Epitaxially-grown GaN-on-Silicon wafers are available in large (8 inch / 200 mm) substrates, therefore making the technology an excellent candidate for scalable RF electronics in quantum computing systems.Furthermore, the use of electrodes using supercon-ducting materials is proposed to significantly reduce the parasitic components and therefore push the RF performance of cryogenic devices. Short-channel tran-sistors with NbN gates of length 100 nm have been demonstrated with promising performance.In the next step, the effect of the superconduct-ing gate on RF characteristics of the transistors will be studied, with the eventual goal of pushing the fre-quency performance of these transistors to new limits. These transistors will be integrated into low noise am-plifier circuits for applications in readout and control electronics at cryogenic temperature. Furthermore, the developed cryogenic GaN HEMT technology would bring us one step closer to an all-nitride integrated elec-tronics-quantum device platform."
Quantitative Study on Current-induced Effects in an Antiferromagnetic Insulator/Pt Bilayer Film,"Electrical control and detection of magnetic ordering inside antiferromagnets have attracted considerable interests, for making next generation of magnetic random access memory with advantages in speed and density. However, a full understanding of the recent prototypical spin-orbit torque antiferromagnetic mem-ory devices requires more quantitative and systematic study. Here we study the current-induced switching in a canted antiferromagnetic insulator α-Fe2O3, similar to previous demonstrations of antiferromagnetic memo-ries, but make good use of its uniquely small spin flop field. We compare the current-induced Hall resistance to the field-induced one, and look into the nature of the switching. We raise the concern that the signal in these memory devices can be complicated by two neglected sources that are unrelated to spin-orbit torques, while the contributions from spin-orbit torques are much smaller than expected. This work provides a pathway towards the clear realization of a spin-orbit torque antiferromagnetic insulator memory device.We epitaxially grew the α-Fe2O3 (0001) film on α-Al2O3 substrate. In Pt/α-Fe2O3 bilayer, we found a typical antiferromagnetic spin Hall magnetoresistance (SMR). We performed the conventional current-in-duced switching in the Hall cross devices and obtained a sawtooth-like behavior. However, it remained almost unchanged under magnetic field, which means a purely resistive switching. To exclude that, we measured the angle-dependent SMR curve subject to an in-plane ro-tating field when applying different sensing currents. The current always tilts the Néel vector towards itself, which is quantified by two effective magnetic energy changes, with 180° and 360° angle period, respectively. Macrospin simulation based on the conventional damp-ing-like torque cannot reproduce the results, while a newly-proposed thermo-magnetoelastic effect well ex-plains the data. The 360° period energy change, instead, can be explained by a field-like spin-orbit torque."
High-performance 2D Material Devices for Large-scale Integrated Circuits and Power Electronic Applications,"Among all the possible back-end-of-line (BEOL) solu-tions to improve the integration density and func-tionality of conventional silicon circuits, 2D material devices are believed to be very promising, due to their high mobility, relatively large band gaps, and atom-lev-el thickness. These devices are beneficial for both logic integrated circuits and power electronic applications. However, the large area growth of high quality 2D ma-terial thin films and 2D material devices and achieving low contact resistance have always been challenging and hinder the development of 2D material devices and circuits.Recently, by using Au contacts and MOCVD tech-nique, we have fabricated back-gated MoS2 transistors on 4-inch MoS2 wafer with 200-nm channel length and have obtained excellent device performance, i.e., high on-state current of around 220 µA/µm (Figure 1) and low contact resistance of around 9.9 kΩ·µm (Figure 2). In order to have larger scale 2D materials with better quality, we are currently building a MOCVD system in EML labs to grow 2D materials, e.g., MoS2 and WSe2, on 6-inch wafers. We are also using Li-induced phase tran-sition in the source/drain regions to further reduce the contact resistance of 2D material transistors. Moreover, top-gated MoS2 transistors with a multilayer hBN gate dielectric are also being investigated to improve the gate controllability and the mobility of the channel ma-terials. In the very near future, we hope to demonstrate 2D material circuits, such as multiplexers, and DC-DC converters with high performance 2D material devices."
Polarization Switching in Highly Scaled Ferroelectric MOS Capacitor,"Ferroelectric FETs (FeFET) are promising candidates for low-power, scalable, and non-volatile memory-enabling applications such as in-memory computing, artificial intelligence (e.g., analog synapses, coupled oscillator networks, spiking neurons) and quantum computing (i.e., cryogenic memory). Ultra-thin doped HfO2 based thin-films have emerged as an attractive option for FeFETs due to precise thickness control through atom-ic layer deposition (ALD) and Complemnetary Metal Oxide Semiconductor(CMOS) compatibility. However, the design space of a FeFET-based memory that op-erates with a low supply voltage, a sufficient memory window, and high endurance is not well understood. In this work, we systematically investigate ferroelectric Hf0.5Zr0.5O2 MOS capacitors to study the electrostatics of the device, which solidifies the design criteria for low voltage FeFETs. In this study, MOS capacitors (Figure 1) are fabri-cated on p-Si wafers using standard CMOS processing with different ferroelectric thicknesses. The dielectric constant, k, of the annealed Hf0.5Zr0.5O2 film is higher than those of HfO2 and ZrO2(k = 25) for all thicknesses, as observed in the small signal capacitance-voltage (C-V) characteristics (Figure 2) due to the film’s orthorhom-bic phase. At low gate biases, the HZO film is hystere-sis-free (Figure 2 inset) and shows negligible frequency dispersion, indicating a high-quality interface. At high gate bias, the thinner films show rapid increase of the capacitance, resembling the peak of butterfly-like be-havior of standard ferroelectric capacitors as the net charge exceeds the critical charge required to achieve the coercive field. However, this behavior is absent in the thick HZO film, where the coercive field is higher than the breakdown field. The high dielectric constant and relatively low effective charge of the ferroelectric thin film, in combination with the ultrathin SiO2 inter-layer, enables the polarization switching of the thinner dielectrics. This is the first observation of polarization switching ferroelectric MOS capacitors using small-sig-nal measurement.These results indicate that our technology can enable FeFETs operating at 2.5 V with highly scaled dielectrics (tFE =5 nm) that are required for a future transistor topology. This is a significant improvement compared to state-of-the-art flash memory. However, to enable lower switching voltage FeFET, additional materials and device engineering would be required as the switching voltage weakly scales with ferroelectric thickness."
First Demonstration of GaN Vertical Power FinFETs on Engineered Substrates,"GaN vertical power Fin Field Effect Transistors (Fin-FET) are promising high-voltage switches for the next generation of high-frequency power electronics ap-plications. Thanks to a vertical fin channel, the device offers excellent electrostatic and threshold voltage control, eliminating the need for epitaxial regrowth or p-type doping, unlike other vertical GaN power tran-sistors. Vertical GaN FinFETs with 1200 V breakdown voltage (BV), 5 A current rating and excellent switching figures of merit have been demonstrated recently on free-standing GaN substrates. Despite this promising performance, the commercialization of these devices has been limited by the high cost ($50-$100/cm2) and small (~ 2 inch) diameter of free-standing GaN sub-strates. The use of GaN-on-Si wafers could reduce the substrate cost by 1000; however, the growth of the thick (~10 μm or thicker) drift layers required for kV class applications is extremely challenging on Si. Alter-natively, GaN grown on engineered substrates (QST®) with a matched thermal expansion coefficient could enable low-cost vertical GaN FinFETs with thick (>10 μm) drift layers and large wafer diameters (8-12 inch). In this work, we have demonstrated a quasi-vertical GaN FinFET on engineered QST® substrates for the first time.A conformal oxide-based planarization and etch-back technology was used for gate etching and source-to-gate spacer etching. The device demonstrates a cur-rent density of JDS=3.8 kA/cm2 at VGS= 1.5 V and VDS= 4 V (Figure 2), and a maximum gm = 2 kS/cm2 at VDS= 4 V when normalized with respect to the total device area (fin width and spacing between fins), a record for vertical and quasi-vertical MOSFETs on non-GaN sub-strates. The current density in each fin is higher than 30 kA/cm2 at the same bias condition. The on-resis-tance is currently limited by non-ideal source contacts, as is evident in the Schottky-like behavior of the drain current at low VDS. The source contact resistance can be improved by either higher doping density or rapid thermal annealing of the metal stack after contact for-mation. The results are very promising for large wafer scale manufacturing and commercialization of vertical GaN power FinFETs."
Tuning Plant Cell Culture Parameters for Improved Model Physiologies,"In vitro plant culture models provide valuable insights into factors governing plant growth and development. Improved understanding of genetic and biochemical pathways in plants has facilitated advancements in a variety of industries —from guiding the development of more robust crops, to enabling increased biofuel yields by tuning biomass genetics. Despite the utility of plant culture models, translation of cellular findings to the plant-scale is hindered in current culture systems. These limitations are, in part, because culture systems fail to recapitulate physical aspects of the natural cellular environment. This work investigates the role of extra-cellular mechanical and chemical influences such as scaffold stiffness, hormone concentrations, media pH, and cell density on cell development and growth patterns. Early results indicate that tuning of biomechanical and biochemical cues leads to cell growth which deviates from typical culture morphologies and better resembles natural plant tissue structures. New analytical methods and measurement metrics were developed to monitor cell enlargement, elongation, and differentiation in response to altered culture conditions. Through factorial design of experiments, optimal conditions for maintenance of long-term cell viability or elevated differentiation rates have been identified. Maps of cell response over a range of extracellular conditions allows for tuning of plant cell models to allow for the exhibition of de-sired physiological compositions. With the aid of these new data maps, plant tissues which are traditionally difficult to access or study in real-time can be better replicated for study in the laboratory setting."
Conformable Ultrasound Patch with Energy-efficient In-memory Computation for Bladder Volume Monitoring,"Continuous monitoring of urinary bladder volume aids management of common conditions such as post-oper-ative urinary retention. Urinary retention is prevented by catheterization, an invasive procedure that greatly increases urinary tract infection. Ultrasound imaging has been used to estimate bladder volume as it is porta-ble, non-ionizing, and low-cost. Despite this, ultrasound technology faces fundamental challenges limiting its usability for next generation wearable technologies. (1) Current ultrasound probes cannot cover curved hu-man body parts or perform whole-organ imaging with high spatiotemporal resolution. (2) Current systems require skilled manual scanning with attendant mea-surement variability. (3) Current systems are insuffi-ciently energy-efficient to permit ubiquitous wearable device deployment.We are developing an energy-efficient body con-tour conformal ultrasound patch capable of real-time bladder volume monitoring. This system will incorpo-rate (1) deep neural network- (DNN) based segmenta-tion algorithms to generate spatiotemporally accurate bladder volume estimates and (2) energy-efficient stat-ic random-access memory (SRAM) with in-memory dot-product computation for low-power segmentation network implementation. We aim to develop platform technology embodiments deployable across a wide range of health-monitoring wearable device applica-tions requiring accurate, real-time, and autonomous tissue monitoring. We are training a low-precision (pruned and quan-tized weights) DNN for accurate bladder segmentation. DNNs are computation-intensive and require large amounts of storage due to high dimensionality data structures with millions of model parameters. This shifts the design emphasis towards data movement between memory and compute blocks. Matrix vector multiplications (MVM) are a dominant kernel in DNNs, and In-Memory computation can use the structural alignment of a 2D SRAM array and the data flow in ma-trix vector multiplications to reduce energy consump-tion and increase system throughput."
Arterial Blood Pressure Estimation Using Ultrasound Technology,"Hypertension, or high blood pressure (BP), is a major risk factor for cardiovascular diseases. Doctors prefer monitoring BP waveforms of ICU patients as the mor-phology and absolute values of these signals help to assert the cardiovascular fitness of the patient. At pres-ent, doctors use invasive radial catheters to record these waveforms. Invasive transducers are inconvenient and can be painful and risky to the patient. Hence, we are developing an algorithm to estimate BP waveforms us-ing non-invasive ultrasound measurements at the bra-chial and carotid arteries. Ultrasound probes are a commonly used sensing modality for non-invasive cardiovascular imaging. For instance, doctors use a linear array transducer to image superficial blood vessels like the brachial or the carot-id artery. These multifunctional probes can record the lumen area waveform of these arteries and measure the velocity of the blood. In this project, we will record the aforementioned signals with a commercial ultra-sound probe and a custom-designed probe (see Figure 1) and use the physics of the arterial pulse wave trans-mission to estimate the shape and absolute values of the pressure waveform. The pressure waves originat-ing from the heart traverse the arterial wall with a velocity commonly referred to as pulse wave velocity (PWV). According to the physics of the arterial pulse wave transmission, we can calculate PWV from the ul-trasound signals. Compliance and pulse pressure of the pressure waves in the artery may be obtained using the Bramwell-Hill equation. Finally, absolute values of the pressure will be derived using a combination of a trans-mission line model of the artery and machine learning algorithms."
Superficial Blood Vessel Lumen Pressure Measurement with Force-coupled Ultrasound Image Segmentation and Finite-element Modeling,"Blood pressures of arteries and veins are valuable indicators of cardiovascular health. Systolic and diastolic arterial pressure can be obtained in vivo noninvasively and accurately with a blood pressure cuff on one of the limbs. However, no noninvasive means to evaluate lumen pressure in veins exists other than visual assessment of the internal jugular vein, which often requires ample skill to execute despite its inaccuracy. What is more, venous pressure is constantly evaluated in the context of congestive heart failure in determining diuretic treatment. Heart failure cardiologists face the difficult decision between ordering an invasive test with plenty of inherent risk or noninvasively but inaccurately evaluating jugular venous pressure.Our group has developed a force-coupled ultrasound probe attachment, providing the ability to measure the force applied by an ultrasound probe for each image obtained. We can segment a superficial blood vessel of fewer than 5 cm of depth and without bone between it and the skin to track its deformation in response to external force applied by the ultrasound probe. Furthermore, we can create a forward finite-element model of a blood vessel cross section to predict vessel deformation in response to the known force applied. We can nest this forward model into a combined iterative inverse model with the observed force and vessel deformation to optimize over the lumen pressure by comparing predicted deformation to observed deformation. This method has the potential to noninvasively and accurately derive sampled arterial and venous pressure waves."
Development of Fully-automated and Field-deployable Sample Preparation Platform Using a Spiral Inertial Microfluidic Device,"Sample separation is a key step in sample preparation to isolate target analytes from interferents in the biofluid sample for a particular analysis. As the current standard, centrifugation and affinity-based (labeling) methods or their combination are used for sample separation. Although those methods themselves are straightforward, they are labor-, energy-, and time-intensive and require large volumes of sample (on the order of 1 mL) and well-trained operators; expensive labeling reagents should be employed for the labeling methods. More importantly, the centrifugation process and cell labeling can cause damage of sample (e.g., ex vivo cell activation), which leads the challenges in assessing the host’s immune response or leukocyte functions correctly.To overcome these limitations, we propose a new type of spiral cell-sorting process using a multi-dimensional double spiral (MDDS) device, where particles are concentrated through a first smaller-dimensional spiral channel and subsequently separated through a second, larger-dimensional spiral channel (Figure 1a). Because of the initial focusing in the first spiral channel, particle dispersion can be significantly decreased, and smaller particles can be effectively extracted into the outer-wall side of the channel, resulting in increase of separation resolution (Figure 1b). To obtain a more purified and concentrated output, we also developed a new recirculation platform based on a check-valve that allows only one-way flow. In the platform, the separated output can be extracted back into the input syringe by the withdrawal motion of a syringe pump and processed again through the MDDS device by the infusion motion of a syringe pump, resulting in higher purity and concentration (Figure 1c). The developed platform can be operated in a fully-automated or even hand-powered manner with a great separation performance. Therefore, we expect that the developed platform could provide an innovative sample preparation solution for point-of-care analyses and diagnostics."
Nanofluidic Monitoring of the Quality of Protein Drugs During Biomanufacturing,"Biologics are drugs produced from any biological source (e.g., mammalian cells, bacteria, yeast). Biologics include recombinant therapeutic proteins, vaccines, monoclonal antibodies, and other living cells. Because of their high ef-fectiveness and reduced complications, biologics can be used to treat many complex conditions, such as cancers and autoimmune disorders, and are transforming mod-ern medicine. Biologics are typically produced through a biomanufacturing process including large-scale bioreac-tor cultivation, purification, and quality checks. Quality checking is critical during this process; quality deviation can significantly compromise drug efficacy and safety.To ensure the quality of biologics, quality control laboratories at manufacturing sites routinely use con-ventional analytical technologies, such as liquid chro-matography and mass spectroscopy. Most analytical technologies require (1) labor intensive manual sample preparation, (2) large sample volume, and (3) technical expertise from scientists/technicians. In addition, these techniques have limited data throughput due to offline and discontinuous analysis. To overcome these limita-tions, micro/nanofluidics can be used to monitor critical quality attributes during biomanufacturing. With the ad-vantages of easy automation, continuous-flow, and small sample volume, micro/nanofluidic technologies can pro-duce a large amount of quality data for improved quality control and understanding of biologics. Previously, our group introduced a new nanofluidic device for contin-uous-flow multi-parameter quality analytics. Recently, this nanofluidic device was integrated with continuous biomanufacturing to monitor protein size in a fully auto-mated, continuous, online manner (Figure 1).We are expanding the capability of our nanofluidic device to monitor other critical quality attributes such as binding affinity and glycosylation of monoclonal an-tibodies during biomanufacturing. With optimization of the monitoring system, we aim to achieve “real-time” and “multi-modal” quality analytics. This nanofluidic an-alytics is expected to improve the safety and efficiency of biomanufacturing in the future."
Measuring Eye Movement Features Using Mobile Devices to Track Neurodegenerative Diseases,"Current clinical assessment of neurodegenerative dis-eases (e.g., Alzheimer’s disease) requires trained special-ists, is mostly qualitative and is commonly done only intermittently. Therefore, these assessments are affect-ed by an individual physician’s clinical acumen and by a host of confounding factors, such a patient’s level of attention. Quantitative, objective and more frequent measurements are needed to mitigate the influence of these factors. A promising candidate for a quantitative and ac-cessible diseases progression monitor is eye movement. In the clinical literature, an eye movement is often mea-sured through a pro/anti-saccade task, where a subject is asked to look towards/away from a visual stimulus. Two features are observed to be significantly different between healthy subjects and patients: reaction time (time difference between a stimulus presentation and the initiation of the corresponding eye movement) and error rate (the proportion of eye movements towards the wrong direction). However, these features are com-monly measured with high-speed, IR-illuminated cam-eras, which limits the accessibility. Our goal is to devel-op a novel system that measures these features outside of the clinical environment.Previously, we showed we can accurately measure reaction time using iPhone cameras, by combining a deep convolutional neural network (CNN) for gaze es-timation with a model-based approach for saccade on-set determination. We showed that there is significant intra- and inter-subject variability in reaction time, which highlights the importance of individualized tracking. We have since developed an app to facilitate data collection and include error rate measurement. With a large amount of data, we can validate the effect of age on these features and identify confounding fac-tors, leading to a better understanding of relationship between eye movement features and disease progres-sion. By facilitating repeat measurements, our frame-work opens the possibility of quantifying patient state on a finer timescale in a broader population than pre-viously possible."
Noninvasive Monitoring of Single-cell Mechanics by Acoustic Scattering,"The monitoring of mechanics in a single cell throughout the cell cycle has been hampered by the invasiveness of mechanical measurements. Here we quantify mechanical properties via acoustic scattering of waves from a cell inside a fluid-filled vibrating cantilever with a temporal resolution of < 1 min. Through simulations, experiments with hydrogels, and the use of chemically perturbed cells, we show that our readout, the size-normalized acoustic scattering (SNACS), measures stiffness. To demonstrate the noninvasiveness of SNACS over successive cell cycles, we used measurements that resulted in deformations of < 15 nm. The cells maintained constant SNACS throughout interphase but showed dynamic changes during mitosis. Our work provides a basis for understanding how growing cells maintain mechanical integrity and demonstrates that acoustic scattering can be used to noninvasively probe subtle and transient dynamics."
"Modular Optoelectronic System for Wireless, Programmable Neuromodulation","Optogenetics is a technique that uses visible light stimulation to activate or inhibit neurons genetically modified to express light-sensitive proteins from the microbial rhodopsin family. It offers light-sensitive opsin proteins to the region of interest and provide advantages such as cell type specificity, millisecond temporal precision, and rapid reversibility. Furthermore, compared to the electrical stimulation, it causes negligible electrical perturbation to the environment, which enables simultaneous electrical recording while stimulating a region of interest. The stimulation of the targeted neurons can be achieved using lasers, light-emitting diode (LED)-coupled optical fibers, or wireless μLEDs. This work presents a modular, light-weight head-borne neuromodulation platform that achieves low-power wireless neuromodulation and allows real-time programmability of the stimulation parameters such as the frequency, duty cycle, and intensity. This platform is composed of two parts: the main device and the optional intensity module (Figure 1). The main device is functional independently; however, the intensity control module can be introduced on demand (Figure 2). The stimulation is achieved through the use of LEDs directly integrated in the custom-drawn fiber-based probes. Our platform can control up to 4 devices simultaneously, and each device can control multiple LEDs in a given subject. Our hardware uses off-the-shelf components and has a plug-and-play structure, which allows for fast turnover time and eliminates the need for complex surgeries. The rechargeable, battery-powered wireless platform uses Bluetooth Low Energy (BLE) and is capable of providing stable power and communication regardless of orientation. This platform presents a potential advantage over the battery-free, fully implantable systems that rely on wireless power transfer, which is typically direction-dependent, requires sophisticated implantation surgeries, and demands complex experimental apparatuses. Although the battery life is limited to several hours, this is sufficient to complete the majority of behavioral neuroscience experiments. Our platform consumes 0.5 mW and has a battery life of 12 hours."
Nanoparticle for Drug Delivery Using TERCOM,"Targeted drug delivery has been an area of active investigation for many decades. Some approaches target cell-borne receptors; others use external stimuli such as heat or radio waves to drive spatially-localized release. In this work, particles estimate their own location within the body by correlating their sensed fluid environment (e.g. temp., press., salinity, sugar, pH, etc.) against an embodied map and release on the basis of this estimate; the approach is related to terrain contour matching (TERCOM), a technique used in air navigation. Preliminarily explored particle concepts have included liposomes and proteins (bottom-up fab) and thin films (top-down fab). As envisioned, a mixture of drug-laden and empty permeable vessels, each with a different environmental response, interconnect through a capacitive volume separated from the surroundings by a permeable film. In another envisioned approach, the monomer sequence of polypeptides or other polymers is selected to provide the greatest activity in preferred capillaries, the sequence of experienced environments affecting the conformation. In both, using item response theory, the mixture's or particle's composition is tailored to deliver a larger dose or greater activity to preferred capillaries. A chip concept that implements a microarray with a half-toned chemical library and material data drawn from conventional surgical analogs has also been considered as a means of screening candidate compositions for the desired spatial sensitivity. Overall, the work builds on a past effort by the PI and his group to develop nanoparticles which record their experience in DNA. Current efforts focus on the theory of estimating location within the body from vectors of sensed variables and on the development of concepts for particles and chips. The ultimate objective is to demonstrate a nanoparticle that implements TERCOM- or DSMAC-like navigation in the body and a chip that can evaluate its selectivity. The concept is outlined in Figure 1."
Multiplexed Graphene Sensors for Detection of Ions in Electrolyte,"Nowadays wearable electronics such as sweat sensors targeting key biomarkers have been heavily investigated. However, these electronics typically contain only one sensor for each type of analyte and the performance is evaluated and optimized separately. When applied to real-world application with complex environment, the reproducibility and the reliability of such device is questionable. Here we present a platform technology for multiplexed, large-area sensing array for more reliable measurement. Graphene is used as signal transducer because of its high surface-to-volume ratio and excellent electrical properties. By utilizing a material jetting 3D printer, we can deposit different types of functionalization on specific regions of the array to achieve multiplexed sensing. Here we demonstrate a fully integrated sensing array with three types of ion-selective membranes (ISMs) to achieve detection of sodium, potassium and calcium(see Figure 1). Each types of functionalization covers over 70 working devices and in total more than 200 devices are functional in one array. The sensor array is first tested with various concentration of solutions contain pure K, Na or Ca ions. All three types of sensors show excellent Nernstian sensitivity towards their target ion and moderate level of sensitivity towards other two types of ions. Using Principle Component Analysis, we can cluster and identify the type of ion as shown in Figure 2. The sensor array is also tested with a set of mixture solutions that are prepared by fixing the concentration of interfering ions while varying concentration of a specific type of ions. Similar clusters are observed indicating the sensor array’s ability for identifying which type of ion concentration is changing within a complex mixture solution. This work demonstrates the possibility of achieving highly reliable multiplexed sensing array that can be deployed in complex environments. By collecting data from a statistically significant sample size, we would be able to apply more sophisticated statistical methods or machine learning models to further associate complex mixtures for real-world applications."
Analytical and Numerical Modeling of Microphones for Fully Implantable Assistive Hearing Devices,"Fully implantable cochlear implants (CIs) could take advantage of the natural enhancement of pressure and binaural cues afforded by the outer ear. They would also allow for hearing 24/7 and mitigate the limitations and inconvenience of an external device. To enable a fully implantable CI, we are developing two piezoelectric implantable microphones to be embedded inside a cochlear implant electrode array or the middle ear cavity as shown in Figure 1. The first type senses pressure along the CI array and has a form factor similar to conventional CI arrays. It will not sense at the base of the cochlea where unwanted noise can originate and scarring and bony growth occurs. The latter sits adjacent to the eardrum and senses any umbo displacement. We have built prototypes of such piezoelectric microphones made with polyvinylidene fluoride (PVDF), a piezoelectric film. We have inserted these prototype microphones inside the scala tympani through the round window and in the middle ear cavity. Preliminary tests show promise for achieving good sensitivity, low noise, and wide bandwidth with this structure. Our approach combines analytical models for design guidance, numerical models for design verification, and bench-test experiments for validation. Analytical modeling is driven by the differential equations of solid mechanics and piezoelectricity. Numerical modeling is enabled by the COMSOL Multiphysics software where we have created simulations of the piezoelectric sensor and use ear mechanics measurements to choose the appropriate boundary conditions.Progress has been made to advance both prototypes into a practical implantable microphone. We have created a platform for system optimization and started the iterative design process. In the near future we will begin sensing circuit design which will modify the system’s overall sensitivity. We will verify numerical model parameters, conduct bench testing imitating cochlear conditions, develop surgical implantation methods, and generate device manufacturing processes"
Spontaneous Relaxation towards Dislocation-free Heteroepitaxy,"Epitaxy laid a foundation for conventional electronic systems as it produces high-quality single crystalline materials. To grow various materials through epitaxy, heteroepitaxy is required as a limited set of available substrates exists. However, a lattice-mismatched issue in heteroepitaxy leads to degradation in the materials’ quality by introducing dislocations to release accumulated strain energy due to the lattice-mismatch. Here, we report a unique approach to release the accumulated strain energy in heteroepitaxy by coating graphene on substrates. As graphene provides a slippery nature on substrates, deposited particles are easily moved around to have energetically favorable atomic lattice. Thus, inserted graphene allows us to grow strain-free single-crystalline materials, a process named spontaneous relaxation. We expect this spontaneous relaxation will be useful to realize the monolithic integration of various lattice-mismatched systems. Figure 1 shows a mechanism of strain relaxation in conventional epitaxy. GaP was grown on GaAs substrate that has 3.7 % misfit strain. Because of the lattice-mismatch, a substantial number of dislocations was introduced to release the accumulated strain energy above a critical strain level. This energy is known as a source to degrade the material’s properties. On the other hand, Figure 2 shows a scenario of strain relaxation through spontaneous relaxation. GaP was grown on a graphene-coated GaAs substrate. As graphene has lattice transparency and provides a slippery surface on top of the substrates, strain-released GaP was obtained. These results demonstrate the feasibility of another strain relaxation pathway on graphene-coated substrates, which will broaden the materials set available for heteroepitaxy."
Graphene-based Tunneling Nanoelectromechanical Switch,"Nanoelectromechanical (NEM) switches are considered to be a promising complementary technology for conventional logic switches because of their zero static power consumption and potential for low-voltage operation. However, they can suffer from stiction caused by significant van der Waals forces acting on their nanoscale structures. Such stiction can easily lead to the permanent failure of a conventional NEM switch and generally prevents miniaturization, leading to a high actuation voltage. Therefore, for NEM switches to be competitive, it is necessary to develop a NEM switch with high switching reliability, low-voltage operation, and ultra-low power consumption. The fabrication of such a switch should also be scalable to enable its popularization within the digital integrated-circuit industry. This work advances the development of a novel squeezable NEM switch, called a squitch. To fabricate the squitch as shown in Figure 1, a pair of nanometer-smooth gold electrodes are fabricated via electron beam lithography and transferred to a glass substrate. A monolayer of polyethylene glycol (PEG)-thiol is then deposited on electrodes via a self-assembly process. Finally, a single layer of graphene is patterned and transferred onto the bottom part of the squitch. Varying the voltage applied between the gold electrodes can electrostatically modulate the thickness of the compressible PEG-thiol monolayer, enabling an exponential change of the current tunneling through it. At this point, as shown in Figure 2, an on/off current ratio of 100:1 with sub-1 V actuation has been achieved. The devices can also survive 10 to 100 cycles of operations, showing observable durability. The fabrication yield is up to ~ 40% and can be further improved by modifying the methods of transferring graphene and exploring new molecules with the appropriate mechanical properties. In the future, we plan to design a squitch based completely on graphene while keeping the current structure to avoid the potential effect of electromigration."
Low-temperature Ferroelectric Hf0.5Zr0.5O2 for InGaAs-channel Negative Capacitance Field-effect-transistors,"Negative capacitance (NC) MOSFETs by integrating ferroelectric (FE) hafnium zirconium oxide (HZO) film in the MOS gate stack have generated enormous interest due to its performance-boosting and CMOS process compatibility. Stable ferroelectricity in the HZO film is usually obtained after a rapid thermal annealing (RTA) step at 500℃. This is because film crystallization under the right conditions is crucial for the formation of the FE orthorhombic phase. However, in order to achieve NC InGaAs-channel MOSFETs, as is our goal, a low-temperature process is essential to preserve the integrity of the gate oxide/InGaAs channel interface. This is also needed for precise capacitance matching. In our work, we have focused our attention towards enabling a low thermal budget process for FE formation of HZO film.After optimization of the HZO atomic layer deposition (ALD) process, Metal – FE – Metal (MFM) capacitors were fabricated to characterize the FE properties, as shown in Figure 1. To provide higher tensile stress and promote the formation of the orthorhombic phase in the HZO film during RTA, 100 nm-thick TiN as electrode was introduced. Figure 2 shows the polarization – voltage characteristics of MFM capacitors annealed at 500℃ and 400℃. The result demonstrates that the HZO film attains FE properties with a 400℃ thermal process. This is also confirmed by the strong FE switching current peaks observed in Figure 3.We are in addition exploring the further decrease of process temperature of HZO crystallization through the introduction of a ZrO2 seed layer under the HZO film (Figures 2 and 3). This has been shown to boost orthorhombic phase formation. Going beyond, we have observed that HZO film deposited by plasma-enhanced ALD (PE-ALD) yields ferroelectric behavior with a 350℃ thermal process. Our research will continue by integrating the optimized gate stack with our established InGaAs MOSFET platform for developing InGaAs NCFETs."
Artificial Heterostructuring of Single-crystalline Complex-oxide Membranes,"Epitaxial heterostructures are the backbone of many important electrical and photonic devices used today. Although many dissimilar crystals can be utilized, epitaxy is limited by the choice of substrates. In other words, the epitaxial film must be similar to the crystal structure of the host wafer. Such limitations impede the advancement of heterostructure engineering and prevent many novel physical phenomena from being discovered because they prevent epitaxial growth of dissimilar materials on a single substrate.To overcome this limitation, we have developed a method to easily remove the epitaxial layer and transfer it onto any arbitrary substrate by using graphene as a release platform in a method called remote epitaxy. By extending this method to complex-oxide material systems, we have, for the first time, artificially created a complex-oxide membrane heterostructure by stacking piezoelectric PMN-PT and magnetostrictive CoFe2O4 (CFO) and hybridizing their properties. Both membranes were released from the substrates and were manually stacked by hand, with the PMN-PT membrane having a thickness of 500 nm and CFO having a thickness of 300 nm. The multiferroic heterostructure was fabricated into a device that allowed measurement of the voltage generated across the PMN-PT membrane (Figure 1).The device was measured by applying a magnetic field across the entire heterostructure and measuring the resulting voltage generation across the PMN-PT membrane. In this device, the magnetic field strains the CFO membrane, and that strain is transferred to the PMN-PT, generating voltage (i.e., magnetoelectric coupling). We noticed that completely freestanding devices generated higher voltages by several factors than devices still clamped to the substrate (Figure 2). These results demonstrate the feasibility of creating novel heterostructures that have never been possible before using remote epitaxy and show the advantages of utilizing freestanding membranes as opposed to those still stuck on their substrates."
Morphological Stability of Nanometer-scale Single-crystal Metallic Interconnects,"Continued IC scaling requires interconnects with cross-sectional dimensions in the <10nm range. At these dimensions the resistance of interconnects increases dramatically due to surface and grain boundary electron scattering. The reliability of interconnects with nanoscale dimensions is also expected to be compromised by reduced morphological stability. As a part of a collaborative program focused on ballistic conduction and morphological stability of single-crystal nanometer-scale interconnects, we are investigating the crystallographic dependence of the morphological stability of Ru wires.Thin single-crystal films agglomerate into small particles via capillary driven surface diffusion in a process known as solid-state ""dewetting."" With decreasing film thickness, the temperature at which dewetting occurs is well below the constituent materials melting temperature. However, previous work on single-crystal (FCC) Ni films has demonstrated that crystalline anisotropy gives rise to special crystallographic orientations along which single-crystal wires are kinetically stable (Figure 1). Interconnects composed of such wires should have decreased vulnerability to morphological instabilities during processing and circuit operation. These wires will have strongly faceted surfaces which are predicted to reduce electron scattering and decrease interconnect resistance. Ru is a candidate material for future interconnects, and exploratory work with single-crystal (0001) films suggests that wires oriented along directions will be particularly stable (Figure 2). Work on patterning and testing of such wires is currently underway. In addition to this experimental work, we are working toward accurate simulations of anisotropic solid-state dewetting. These simulations reproduce the dramatic effect that stable surfaces can have on wire stability and provide an opportunity to systematically probe the effects of individual material properties. Combining the results of these experiments and simulations with those of past work on Ni will provide insights that will enable optimization of interconnect structural and crystallographic factors for design of morphologically stable nanowires with cross-sectional dimensions significantly below 10nm."
Modern Microprocessor Built from Complementary Carbon Nanotube Transistors,"Electronics is approaching a major paradigm shift because silicon transistor scaling no longer yields historical energy-efficiency benefits, spurring research towards beyond-silicon nano-technologies. In particular, carbon nanotube field-effect transistor (CNFET)-based digital circuits promise substantial energy-efficiency benefits, but the inability to perfectly control intrinsic nanoscale defects and variability in carbon nanotubes has precluded the realization of very-large-scale integrated systems. Here we overcome these challenges to demonstrate a beyond-silicon microprocessor built entirely from CNFETs: RV16X-NANO. This 16-bit micro-processor is based on the RISC-V instruction set, runs standard 32-bit instructions on 16-bit data and addresses, comprises more than 14,000 complementary metal–oxide–semiconductor CNFETs and is designed and fabricated using industry-standard design flows and processes. We propose a manufacturing methodology (MMC) for carbon nanotubes, a set of combined processing and design techniques for overcoming nanoscale imperfections at macroscopic scales across full wafer substrates. The key elements of MMC are:(1) RINSE (removal of incubated nanotubes through selective exfoliation). We propose a method of removing CNT aggregate defects through a selective mechanical exfoliation process. RINSE reduces CNT aggregate defect density by >250× without affecting non-aggregated CNTs or degrading CNFET performance.(2) MIXED (metal interface engineering crossed with electrostatic doping). Our combined CNT doping process leverages both metal contact work function engineering as well as electrostatic doping to realize a robust wafer-scale CNFET CMOS process. We experimentally yield entire dies with >10,000 CNFET CMOS digital logic gates (2-input ‘not-or’ gates with functional yield 14,400/14,400, comprising 57,600 total CNFETs), and present a wafer-scale CNFET CMOS uni-formity characterization across 150-mm wafers.(3) DREAM (designing resiliency against metallic CNTs). This technique overcomes the presence of metallic CNTs entirely through circuit design. DREAM relaxes the requirement on metallic CNT purity by about 10,000× (relaxed from a semiconducting CNT purity requirement of 99.999999% to 99.99%),"
"Nanostructured, Additively Manufactured, Miniature Ionic Liquid Ion Sources","Electrospraying is a high-electric field physical phenomenon that transforms electrically conductive liquids into fine, uniform streams of micro/nanoparticles; the applications of electrospraying include mass spectrometry, nanosatellite propulsion, combustors, and agile manufacturing. Unfortunately, electrospray emitters have very low throughput; consequently, several research groups have investigated, for about two decades, greatly increasing the electrospray source’s throughput via emitter multiplexing, using micro- and nanotechnology to attain lower startup voltage and denser emitter arrays. Although successful, the reported implementations harness cleanroom microfabrication, which has an associated high cost that is incompatible with many applications of electrospraying. In this project, we explore the use of additive manufacturing to create, at a very low cost, monolithic arrays of electrospray emitters capable of ion emission. We have succeeded at demonstrating the first additively manufactured ionic liquid electrospray sources in the literature; our devices produce per-emitter current comparable to that produced by silicon microfabricated counterparts, at a small fraction of their fabrication cost. The devices are diodes composed of an emitting electrode and an extractor electrode: the emitting electrode is a monolithic array of digital light projection (DLP)-printed solid, conical, polymeric needles covered by a conformal layer of hydrothermally grown zinc oxide (ZnO) nanowires as a wicking material (Figure 1), while the extractor electrode is a laser-cut SS 316L plate with an array of apertures that matches the pattern of the array of needles. Characterization of the devices in vacuum using the ionic liquid EMI-BF4 demonstrates bipolar, uniform array emission of solvated ions—in agreement with the literature on ionic liquid ion sources. Current research efforts focus on increasing the number of emitters per unit of area and on exploring other materials and designs for implementing the devices."
Soldiers’ Hearing Health Protection and Auditory Augmentation Using Electrostatic NEMS,"Our work on acoustic nanomembrane electromechan-ical transducers (NEMS) that can safely fit and operate inside an ear is motivated by the desire to improve U.S. soldiers’ auditory health. From the time soldiers set foot on training grounds to their deployment in war zones, they are consistently exposed to deleterious noise from rapid gunshots, friendly fire, explosions, jet engines, and armored personnel carriers. Noise levels from these sources often exceed safe hearing thresh-olds and can inflict irreversible ear trauma and hear-ing loss. Existing hearing protection solutions that at-tempt to mitigate tactical noise are often insufficient and compromise communication, combatant response time, and response accuracy. Indeed, it is recognized that hearing loss resulting from military service is a massive financial and clinical burden, which needs a technical solution that can protect soldiers and assist veteran civilians.To simultaneously provide hearing protection, ef-fective tactical communication, and situational aware-ness, an in-the-ear acoustic system is needed, one that can operate at low power with distortion-free acoustic output over the entire human auditory range. Scalable versions of such systems do not yet exist, which moti-vates us to suggest that the design of our electrostatic NEMS enables them to operate as ultra-low-power mi-crospeakers. The low weight, the material composition, and the geometry of our NEMS membranes ensure linear and near-uniform acoustic output in the human auditory range. Hence, if integrated with earmolds, our NEMS could be used as high-fidelity microspeakers for speech enhancement in communication and noise at-tenuation. The same transducers can also act as acous-tic dosimeters and as ambient-facing microphones for sensing acoustic signals. This composite reversible device is designed to attenuate harmful sounds while enhancing verbal communication and maintaining acoustic transparency with the surroundings. Leverag-ing our nanomembrane transducer technology, we aim to reach superior size, weight, and power consumption specifications, with no compromise in performance."
Proton-based Resistive Memory for Analog Computing Applications,"The size of state-of-the-art deep neural networks (DNNs) and consequent computation load have been increasing ever since the beginning of their outbreak. Since bigger and deeper neural networks trained with larger data sets generally provide better performance, this trend is expected to accelerate in the future. However, this poses as a significant problem for conventional digital architectures as the energy and time consumption for training DNNs have become unmanageable. The idea of using analog resistive crossbars to do parallel vector-matrix multiplications based on Ohm's and Kirchhoff's Laws have been known since 1960s. It was recently discovered that rank-one outer product-based updates can also be performed in parallel using pulse-coincidence for multiplication and incremental conductance change of devices for accumulation. These advancements have fueled the investigation of various non-volatile analog resistive memory technologies to realize fast, energy-efficient and versatile platforms for deep learning. In this work we implement a three-terminal electrochemical (i.e. battery-like) resistive memory device, employing the smallest ion, the proton. The conductance of the device is determined by the proton concentration intercalated inside the channel material. Electrical pulses applied to the gate enable protons to drift between the reservoir and the channel through the solid electrolyte as electrons move through the external circuit in the same direction (Fig. 1). When the gate is electrically open, proton movement through the electrolyte is forbidden since electrons cannot flow through the outside circuit, enabling non-volatile memory. We have demonstrated this concept on devices composed of a WO3 channel, Nafion electrolyte and Pd as hydrogen reservoir (Fig. 2) Devices have very low energy consumption (18 aJ/(μm2 x nS), nearly symmetric modulation characteristics and long cycling lifetime. Future work will involve optimizing the material stack, scaling and integration of these devices ultimately realizing a full-scale DNN training accelerator"
High-throughput Vapor Transport Deposition of Organic-inorganic Perovskite Films,"Vapor transport deposition (VTD) is a promising technique for enabling high-throughput large-area deposition of next-generation perovskite solar cells. VTD uses a carrier gas to transfer sublimed salts from source to substrate, where they react to form perovskite films. Unlike vapor thermal evaporation, VTD decouples the material deposition rate from material temperature, allowing for high-throughput deposition. VTD allows independent control of chamber pressure and deposition rate, parameters which can be tuned to change the film crystallization kinetics. Like thermal evaporation, VTD permits precise control of thickness and eliminates hazardous solvents from device fabrication, allowing facile growth of complex multi-layer device structures such as tandem solar cells. The high throughput deposition coupled with low vacuum operation reduces the capex requirement for VTD deposition tools and has led to commercialization of the technique for CdTe and organic semiconductor materials manufacturing.Through the use of a custom home-built VTD setup, we study perovskite solar cell active-layer film formation via co-evaporation of lead iodide and methylammonium iodide (MAI). We determined parameters and deposition conditions necessary to form high-quality methylammonium lead iodide films. We found that control of MAI degradation and its deposition rate during VTD is a critical challenge that must be overcome. Last, we developed numerical simulations of material diffusion and gas flow necessary to narrow the VTD design parameter space. We are assembling the next-generation VTD reactor to study the impact of critical parameters such as substrate temperature, carrier gas flow rate, chamber pressure, and deposition rate on film formation kinetics by examining metrics such as photoluminescence, x-ray diffraction, morphology, and device efficiency. We aim to demonstrate VTD to be a viable new deposition method for large-area high-throughput manufacturing of perovskite solar cells."
Imaging Moiré Periodicities at the 2D/3D Interface Using 4D STEM,"Structure and defects at the interface between 2D materials and their 3D bulk adjuncts greatly influence nanoscale device properties, such as contact resistance, photo-response, and high-frequency performance. Knowledge of fundamental charge transfer at this interface is critical for the continued and rapid development of devices that utilize 2D materials. Recent advances in scanning transmission electron microscopy (STEM), such as aberration correction and 4D STEM, allow analysis of interface growth, structure, and ordering. In this work, we use 4D STEM to directly image hidden moiré periodicities arising from epitaxial growth of nanoislands on 2D materials in ultra-high vacuum. Epitaxial growth creates moiré patterns arising from rotation and lattice mismatch between the nanoisland and underlying 2D material substrate. We use 4D STEM to directly image these periodic superlattices, which are not visible in conventional TEM or STEM and often can result in strong electron correlations. DFT calculations show that this moiré is directly responsible for a periodic modulation of electronic structure in the 2D material. Our work illustrates the essential role of emerging microscopy techniques to unveil the mechanisms of moiré superlattices, and we explore the implications of these on physical properties at the 2D/3D interface, such as enhanced charge transfer and moiré-modulated local interactions."
Templated Solid-state Dewetting of Single-crystal Thin Films,"Solid-state dewetting of single-crystal thin films leads to an ordered array of particles that align along a few specific crystallographic orientations due to crystalline anisotropy. When single-crystal thin films are pre-patterned and then subjected to dewetting, which is known as templated dewetting, a regular array of complex features can be fabricated. The features that result from templated dewetting are affected by various instabilities that develop at the retracting edges of pre-patterns and the characteristics they produce. One instability that retracting edges can develop is pinch-off (see Figure 1a), which leads a wire parallel to the retracting edge; this process can occur repeatedly to form multiple wires . Alternatively, retracting edges can be subject to fingering instability (see Figure 1b), which leads to an array of wires aligned along the finger propagation direction. Understanding and controlling whether pinch-off or fingering occurs is important for controlled pattern formation. In the past year we have demonstrated that the initial roughness of a film edge determines whether pinch-off or fingering occurs, with rough edges leading to fingering (Figure 1). To further understand this phenomenon and to control it, we patterned large rectangular patches, with edges having controlled patterned roughness to template the fingering instability (Figure 2a). The edges of the patches were also aligned along various in-plane crystallographic orientations to study the effects of crystalline anisotropy on the templated fingering instability. We have found that templating of edge roughness can cause a fingering instability, with a very narrow distribution of the width and period of the fingers and wire, and that the wavelength of patterned roughness can control the steady-state finger period (Figure 2b). We further found that t the period of fingers affects the steady-state finger propagation velocity, so we developed a kinetic model that predicts steady-state finger propagation velocity as a function of the finger period. Strong effects of crystalline anisotropy on the templated fingering instability were observed. We found that edges that were aligned along a kinetically stable orientation resisted development of a fingering instability, even with the templating, and the patterned roughness disappeared as they retracted and became straight. Edges with orientations other than a kinetically stable orientation all developed fingering instabilities, but the finger propagation orientation was affected by the initial edge orientation; as a result, the steady-state finger period and propagation velocity were also affected. Furthermore, we studied effects of initial film thickness on the templated fingering instability using the same range of wavelengths of patterned roughness and the same range of in-plane orientations of the edge. During this study we found that the steady-state finger propagation velocity increases as film thickness decreases and that width of the wires that form due to propagation of the fingers decreases with film thickness, while the wavelength of patterned roughness still controls the finger period. Through these studies, we are developing techniques through which templated dewetting can be used as a patterning method."
Nucleation and Growth of Metal Thin Films and Nanocrystals on Two-dimensional Materials,"The interface between metals and 2D materials (2DMs) influences device properties such as contact resistance, photoresponse, and high-frequency performance. In this project, we study the nucleation and growth of a variety of metals, including Ag, Au, Ti, Cr and Nb, on 2DMs (graphene, hexagonal boron nitride (hBN), WSe2, MoS2). We use transmission electron microscopy (TEM) to provide direct insight into crystal size, shape and orientation, epitaxy, and diffusivity. Besides the basic parameters that affect growth mode and epitaxy such as diffusivity, binding, and cohesive energies, we also explore the effects of 2DM thickness, temperature during deposition, and substrate (SiO2, hBN, or suspended). Combining the knowledge of deposition conditions, templating, and nucleation control greatly enhances routes towards tailored interface design for emerging 2DM device applications.Temperature during deposition greatly affects the diffusivity of metal atoms on the 2D surface. As expected from crystal growth models, temperature determines whether the crystal morphology is dendritic or compact and faceted on 2DMs (Figure 1). The effect of the substrate on the epitaxy and crystal morphology is relatively unexplored, and we find suspended 2DMs exhibit the largest epitaxial alignment (Figure 2). This is seen even for relative thick 2DMs, suggesting that substrate effects such as surface charges play little role in the crystal growth. Rather, 2DM roughness may be a determining parameter, which is decidedly lower for suspended 2DMs. In suspended layers, we also find that diffusion distance depends on 2DM thickness, with longest diffusion distances (>2 μm) on suspended Gr >8 monolayers thick. This project aims to contribute to the emerging field of 2D material devices through atomic scale characterization of metal nanocrystal growth on 2DMs, facilitating the design of contacts, heterostructures, and coupled materials for future 2DM dimensional devices."
An Energy-efficient Configurable Accelerator for Post-quantum Lattice-based Cryptography,"Public key cryptography protocols, such as RSA and elliptic curve cryptography, will be rendered insecure by Shor’s algorithm when large-scale quantum computers are built. Cryptographers are working on quantum-resistant algorithms, and lattice-based cryptography has emerged as a prime candidate. However, the high computational complexity of these algorithms makes it challenging to implement lattice-based protocols on low-power embedded devices. To address this challenge, we present an energy-efficient lattice cryptography processor with configurable parameters. Efficient sampling, with a SHA-3-based PRNG, provides two orders-of-magnitude energy savings; a single-port RAM-based number theoretic transform memory architecture is proposed, which provides 124k-gate area savings, while a low-power modular arithmetic unit accelerates polynomial computations. This is the first ASIC implementation to demonstrate multiple lattice-based protocols proposed for post-quantum standardi-zation by NIST.Figure 1 shows the architecture of our lattice cryptography processor along with the chip micrograph. Our test chip was fabricated in TSMC 40-nm low-power CMOS process and supports voltage scaling from 1.1V down to 0.68V. The cryptographic core occupies 0.28 mm2 area consisting of 106k logic gates and 40.25 KB SRAM. It can be programmed with custom instructions for polynomial arithmetic and sampling and it coupled with a low-power RISC-V micro-processor to demonstrate NIST Round 2 lat-tice-based key encapsulation and digital signature protocols Frodo, NewHope, qTESLA, CRYSTALS-Kyber and CRYSTALS-Dilithium, achieving up to an order-of-magnitude improvement in performance and energy-efficiency compared to state-of-the-art hardware implementations. All key building blocks are constant-time and secure against timing and simple power analysis side-channel attacks. The cryptographic core can also be programmed to implement masking-based differential power analysis side-channel countermeasures, with additional computation cost, with no change to the hardware."
Conformable Ultrasound Patch with Energy-efficient In-memory Computation for Bladder Volume Monitoring,"Continuous monitoring of urinary bladder volume aids management of common conditions such as post-oper-ative urinary retention. Urinary retention is prevented by catheterization, an invasive procedure that greatly increases urinary tract infection. Ultrasound imaging has been used to estimate bladder volume as it is porta-ble, non-ionizing, and low-cost. Despite this, ultrasound technology faces fundamental challenges limiting its usability for next generation wearable technologies. (1) Current ultrasound probes cannot cover curved hu-man body parts or perform whole-organ imaging with high spatiotemporal resolution. (2) Current systems require skilled manual scanning with attendant mea-surement variability. (3) Current systems are insuffi-ciently energy-efficient to permit ubiquitous wearable device deployment. We are developing an energy-efficient body con-tour conformal ultrasound patch capable of real-time bladder volume monitoring. This system will incorpo-rate (1) deep neural network (DNN) based segmenta-tion algorithms to generate spatiotemporally accurate bladder volume estimates and (2) energy-efficient stat-ic random-access memory (SRAM) with in-memory dot-product computation for low-power segmentation network implementation. We aim to develop platform technology embodiments deployable across a wide range of health-monitoring wearable device applica-tions requiring accurate, real-time, and autonomous tissue monitoring. We are training a low-precision (pruned and quan-tized weights) DNN for accurate bladder segmentation. DNNs are computation-intensive and require large amounts of storage due to high dimensionality data structures with millions of model parameters. This shifts the design emphasis towards data movement between memory and compute blocks. Matrix vector multiplications (MVM) are a dominant kernel in DNNs, and In-Memory computation can use the structural alignment of a 2D SRAM array and the data flow in ma-trix vector multiplications to reduce energy consump-tion and increase system throughput."
Bandwidth Scalable Current Sensing with Integrated Fluxgate Magnetometers,"Contactless current sensing finds use in many industrial applications including power line monitoring, motor control, and electric vehicle battery management, as it provides inherent galvanic isolation over direct shunt-sensing. Magnetometers indirectly sense current through a wire by measuring the magnetic fields around it. For stray magnetic field rejection, magnetic sensors need to be placed in the air gap of a magnetic core around each wire. This solution is costly, bulky, and inconvenient to install. [1] proposes a plug-in solution with an array of integrated fluxgate (IFG) magnetometers for contactless current sensing in industrial internet of things applications. IFG offers a better alternative than Hall sensors in terms of dynamic range (~10^5), sensitivity (200 V/T), linearity (0.1%), and low temperature drift. IFG sensors work by driving magnetic cores in and out of saturation and sensing the resulting voltage difference. They achieve high linearity by balancing external magnetic fields within the core using compensation current, which can be quite power hungry, requiring up to 1W power for a three-phase measurement. Previous IFG sensors are designed for continuous operation at high sampling rates and cannot be duty cycled efficiently due to the long convergence time needed per measurement. The primary goal of this work is to reduce the energy needs of IFG sensors so they can be used in an array in energy constrained environments. Secondary goals are to increase the bandwidth to >100 kHz for fault detection and increase the measurement range to +/- 60 A at 0.5 cm away from the wire for a compact solution. We achieve these goals through a mixed signal front-end design to enable energy-efficient duty cycling in a bandwidth scalable fluxgate magnetic-to-digital converter. This work achieves higher measurement range, > 100 kHz bandwidth, and considerable energy savings with duty cycling from >100 kHz bandwidths for machine health monitoring to <1 kHz for power quality management."
SynCells - Electronic Microparticles for Sensing Applications,"Autonomous electronic systems smaller than the diam-eter of a human hair (<100 µm) presents a great oppor-tunity for sensing applications because they allow us to interact with the environment at a much smaller scale. These microsystems could be used for example to de-tect chemicals in very confined spaces like the human body or microfluidic channels. Alternatively, they are small enough to be sprayed on surfaces to form distrib-uted sensor networks or even be incorporated into fi-bers to make smart clothing. However, fabricating and designing such microsystems is difficult due to integra-tion challenges and a limited power budget.In this work, we present a 60x60x2 µm3 electronic platform, called Synthetic Cell or SynCell, that over-comes these issues by leveraging the unique integra-tion capabilities of 2D material on an SU-8 substrate and the use of functional materials to reduce power consumption. We integrated several components on this platform including molybdenum disulfide-based transistors and chemical sensors, analog timers based on eroding germanium films, and magnetic iron pads (see Figure 1). These building blocks represent a broad set of capabilities and enable functions like computa-tion, sensing, time tracking and remote actuation, re-spectively. Over the past years, we have optimized the SynCell fabrication and lift-off process and recently demonstrated a yield close to a hundred percent of ful-ly working SynCells. To show the potential of SynCells in confined spaces, we magnetically positioned several SynCells in a microfluidic channel to detect putrescine in a proof-of-concept experiment, see Figure 2. After we extracted them from the channel, we successfully read out the timer and sensor signal, the latter of which was am-plified by an onboard transistor circuit. In the future, SynCells may be useful in a wide variety of fields, from clinical research to printable/sprayable sensor coatings."
Wideband Sub-THz Components for Ultra-efficient Meter-class Interconnect,"With the growing interest in millimeter wave and terahertz (THz) electronics, there has been an associated interest in the various components that are required to realize these systems. In one such application, guided and modulated sub-THz (approximately 220-330 GHz) waves are used to transport high-rate data over backplane-scale distances. Such a scheme is attractive for a number of reasons, including broad available fractional bandwidth, compact system size (driven by smaller wavelengths compared to lower-frequency operations), relative robustness to misalignment during assembly versus optical systems, and lower transmission losses than those exhibited by copper lines for high-speed data transmission. One of the challenges associated with the development of the above link system is the realization of compact, low-loss channelizers over the wide operating bandwidth afforded by these types of lines. While waveguide-based channelizers have been demonstrated at lower bands and waveguide components are available at higher operating frequencies, they are relatively large and require more expensive packaging and interface schemes. This type of scheme would require a planar integration approach to be economically feasible.We have demonstrated the best-in-class channel-ization performance on a new Intel organic packaging process over 40% fractional bandwidth and occupying up to 200x less area than competing approaches. The de-sign makes use of a very fast circuit-EM co-design tech-nique to overcomes computational hurdles associated with large-scale, full-wave dimensional optimization to rapidly optimize the design. The work utilizes a ridged-SIW resonator design, enabled by the Intel packaging technology, provides superior performance, enables the wide operating band, and reduces the device size by 40%. This design methodology, the selected channel-izer topology, and the packaging technology provide a feasible path toward ubiquitous, highly-integrated, and low-cost THz-communication systems-in-package at the board/back-plane level."
A CMOS-based Dense 240-GHz Scalable Heterodyne Receiving Array with Globally-accessible Phase-locked Local Oscillation Signals,"Driven by the thrust of sensor miniaturization, there is a growing interest in forming steerable beams on the chip scale, which calls for pushing the operation frequency of beam-steering systems towards the terahertz (THz) range. However, this requires disruptive changes to traditional THz receiver architectures, e.g. square-law direct detector arrays (low sensitivity and no phase information preserved) and small heterodyne mixer arrays (bulky and not scalable). The major issue that prevents the latter case from being scalable is the need of large-scale power distribution network for local oscillation signals (LO), which can be very lossy at such high frequency. Here, we report a highly scalable 240-GHz 4×8 heterodyne array achieved by replacing the LO power distributor with a network that couples LOs generated locally at each unit. Now the major challenge for this specific architecture is that each unit should fit into a tight λ/2×λ/2 area to suppress side lobes from beam forming - it makes integrating mixer, local oscillator, and antenna into a unit difficult. Our design addresses this challenge well: the highly compact units ultimately enable the integration of two interleaved 4×4 phase-locked sub-arrays in 1.2-mm2.The architecture of the entire array is shown in Figure 1(a). Its core component is a self-oscillating harmonic mixer (SOHM), which can simultaneously (1) generate high-power LO signal and (2) down-mix the radio frequency (RF) signal. Since coupling is designed to be global, LOs generated in all units are all locked to an external reference signal by phase-locking two units only. The die (Figure 1(b)) photo shows the placement of the array and the PLL. The measured sensitivity (required incident RF power to achieve SNR=1 at baseband) over 1-kHz detection bandwidth is 58fW, which is more than 4000× improvement over state-of-the-art large-scale square-law detector arrays. Figure 2 shows that this work has pushed the boundary of THz receiver arrays in terms of scale and sensitivity."
Method and Countermeasure for SAR ADC Power Side-channel Attack,"Analog-to-digital converters (ADCs) are essential building blocks of most electronic systems as they convert analog signals into digital bits. Since the demand for digital signal processing keeps growing, researchers have focused on enhancing the ADC performance to keep up with the demand of digital processors. However, recent studies have raised a hardware security concern regarding the ADC-related security loophole, warning that private signal information can be leaked through power supply current waveforms of an ADC.Figure 1 illustrates an example ADC power side-channel attack (PSA) scenario in sensing hardware that is acquiring a private signal (e.g., healthcare, smart home devices, industrial monitoring). By employing an encryption engine equipped with a PSA-countermeasure, an attacker is prevented from performing eavesdropping and extracting the secret key of the encryption algorithm by tapping into the power supply of the encryption engine. Also, a tamper-proof package can be used to prevent an attacker from directly tapping into the sensor output signal. However, for practical reasons such as a provision for battery replacement and a limitation on physical dimensions, the tamper-proof package may not extend to the ADC power supplies, allowing an attacker to tap into the power supply waveforms of the ADC. Due to the strong correlation between the ADC power supply current waveforms and the ADC digital outputs, an at-tacker can perform an ADC PSA to obtain the private signal data of the sensing hardware.This work explores both aspects of ADC PSA: method and countermeasure with an emphasis on SAR ADCs. In this work, neural networks are used as a mapping function that converts a SAR ADC power supply current waveform into the corresponding ADC digital output. To protect a SAR ADC from the proposed PSA method, switched-capacitor circuits called current equalizers are used to decorrelate the on-chip ADC activity and the ADC power supply current waveforms. Figure 2 shows the experimental PSA results on a custom-designed SAR ADC (Figure 2c) that demonstrate the effectiveness of the proposed SAR ADC PSA method and countermeasure schemes."
Reconfigurable CNN Processor for Compressed Networks,"Convolutional neural networks (CNNs) have become the standard for performing complex tasks such as image classification due to their high accuracy. However, they typically involve substantial computation (~109 multiplies and adds) to process a single image and require a large amount of storage (~10 to 100 MB) for the fixed weight parameters and intermediate output activations. This makes it challenging to process CNNs locally on edge devices with low power and low latency. To address this, we need custom hardware accelerators to exploit the high parallelism present in the computations. At the same time, they should be flexible enough to support various networks, especially as new and better networks are continuously being developed. Because of the memory constraints on edge devices, we focus on networks compressed by techniques such as Deep Compression and Trained Ternary Quantization, which quantize the weights to a small number of unique values (usually 16 or fewer).We propose a scalable architecture for efficiently processing compressed networks by reordering the multiplications and additions. Instead of performing each multiply-and-add separately, we accumulate all the activations multiplied by the same weight together and perform the multiplication at the end. With a small number of unique weights, the number of multiplications is greatly reduced, and consequently decreasing the average energy per operation. To enable the tradeoff between accuracy and efficiency, we added reconfigurability for different weight and activation bit widths. This allows us to use shorter bit widths in applications where energy must be minimized and a drop in accuracy can be tolerated. With added support for residual connections and depthwise convolutions, our accelerator can run modern networks such as ResNet and MobileNet, enabling CNN processing for a wide range of applications on energy-constrained devices including cell phones and IoT nodes."
Simulation and Analysis of GaN CMOS Logic,"There is an increasing demand for electronics that can operate in high-temperature conditions, such as spacecraft and sensors for industrial environments. A promising solution exists in electronics based on wide-bandgap materials, among which gallium nitride (GaN) stands out as a strong candidate due to its excellent material properties and potential for monolithic integration. Most current demonstrations of GaN logic are based on nMOS technology, which has a high static power consumption. GaN CMOS technology, which has lower static power consumption, is desired.This work studies the effect of p-channel transistor performance and circuit parameters on the performance of CMOS digital logic circuits. The industry-standard MIT virtual source GaN-FET model (MVSG) was used to accurately model the behavior of the n-channel and p-channel transistors, which were fabricated on the developed GaN-complementary circuit platform. Furthermore, excellent matching was achieved between the experimental data of a fabricated CMOS logic inverter and the simulated compact models. Several building blocks of digital logic, namely, the logic inverter, multi-stage ring oscillator, and static random-access memory (SRAM) cell, were studied using the developed computer-aided design (CAD) framework. Device-circuit co-design was conducted to optimize circuit performance, using a variety of design parameters including transistor sizing and supply voltage scaling. The high temperature performance of the circuits, simulated based on experimentally observed trends of the devices, was projected. The results indicate that GaN CMOS technology based on our monolithically integrated platform has potential for a variety of use cases, including harsh-environment digital computation. This technique will be scaled up for more complex combinational and sequential logic building blocks, with the eventual goal of realizing a GaN CMOS microprocessor."
Energy-efficient SAR ADC with Background Calibration and Resolution Enhancement,"Many signals, for example, medical signals, do not change much from sample to sample most of the time. Conventional switching schemes for SAR ADCs do not exploit this signal characteristic and test each bit start-ing with the MSB. Previous work called least-signifi-cant-bit (LSB)-first saves energy and bit-cycles by start-ing with a previous sample code and searching for the remainder by testing bits from the LSB end. However, certain code transitions consume unnecessary energy, even when the code change over the previous code is small.This work addresses this problem with a new algo-rithm called Recode then LSB-first (RLSB-first) that re-duces the switching energy and bit-cycles required for all cases of small code change across the full range of possible previous sample codes. RLSB-first uses split-DAC to systematically encode the previous code before LSB-first. RLSB-first lowers switching energy by up to 2.5 times and uses up to 3 times fewer bit-cycles than LSB-first. In addition to creating an energy-efficient SAR ADC, this work aims to use the savings for back-ground calibration and resolution enhancement."
Rethinking Empirical Evaluation of Adversarial Robustness Using First-order Attack Methods,"Deep neural networks (DNNs) are known to be vulnerable to adversarial perturbations, which are often imperceptible to humans but can alter predictions of machine learning systems; robustness against those perturbations is becoming an important design factor. A practical approach to measuring adversarial robustness of DNNs is to use the accuracy of those models on examples generated by adversarial attack methods as a proxy for adversarial robustness. However, the failure of those attack methods to find adversarial perturbations cannot be equated with being robust. In our work, we identify three phenomena that inflate accuracy against popular bounded first-order attack methods: 1) a loss function numerically becoming zero when using standard floating point representation, resulting in non-useful gradients; 2) innate non-differentiable functions in DNNs, such as ReLU activation and Max Pooling, incurring “gradient masking”; and 3) certain regularization methods used during training to induce the models to be less amenable to first-order approximation. For each case, we propose compensation methods to improve the evaluation metric for adversarial robustness. The impact of these three sources of overestimated adversarial robustness can be significant when comparing different model capacities for adversarial robustness. For example, Figure 1 shows the adversarial robustness of deep models with the same architecture but different number of neurons per layer. Compensating for these three phenomena can change the relative benefit of using larger models in terms of adversarial accuracy. Similarly, Figure 2 shows adversarial robustness when we iteratively prune weights of an over-parameterized deep model. Adversarial accuracy against the baseline attack method significantly drops as we prune the model; however, actually there is little difference between the original dense model and the sparser models in their adversarial robustness when we properly compensate for these phenomena. Therefore, it is important to rethink the metric we use before we draw conclusions on model capacities or other design factors for their adversarial robustness."
Efficient Video Understanding with Temporal Shift Module,"Hardware-efficient video understanding is an important step towards real-world deployment, both in the cloud and on the edge. For example, there are over 10^5 hours of videos uploaded to YouTube every day to be processed for recommendation and ad ranking; similarly, terabytes of sensitive videos in hospitals need to be processed locally on edge devices to protect privacy. All these industry applications require both accurate and efficient video understanding.Traditionally, a 2D convolutional neural network (CNN) is more efficient but cannot model temporal information; 3D CNN can perform spatial-temporal feature learning, but at the cost of high computation. In this paper, we propose a novel temporal shift module (TSM), which achieves 3D CNN performance at 2D cost. By shifting some of the channels bi-directionally along the temporal dimension, we can facilitate temporal reasoning in 2D CNN at the cost of zero FLOPs and zero parameters. We also propose a uni-directional TSM for online video understanding, supporting online classification and detection from a streaming video.TSM is efficient and accurate: on temporal related datasets, we can improve the performance by double digits at almost no overhead compared to a 2D network. TSM ranks first on Something-Something leaderboard upon submission. TSM is highly scalable: it can be scaled up to 1,536 GPUs and finish the training on Kinetics in 15 minutes; it can also be scaled down to edge deployment, achieving 77 FPS on Jetson Nano and 29 FPS on Galaxy Note 8."
Secure System for Implantable Drug Delivery,"Recent advances in microelectronics and medical technology have enabled Internet-connected IMDs that the patients/users can control through external handheld or wearable devices. However, several proof-of-concept attacks have been demonstrated on such devices by exploiting weaknesses in authentication protocols or their implementations. While such connected implantable devices have the potential to enable many emerging medical applications such as on-command implantable drug delivery, security concerns pose a threat to their widespread deployment. To address this challenge, we present a secure low-power integrated circuit (IC) with sub-nW sleep-state power, energy-efficient cryptographic acceleration, and a novel dual-factor authentication mechanism that ensures that the ultimate security of the IMD lies in the hands of the user. As a solution, we propose a dual-factor authentication scheme in which cryptographic authentication is supplemented with a voluntary response from the user. The voluntary response serves as a guarded action from the user; that is, it represents consent from the user for executing the desired action without causing them much inconvenience. In our protocol, we have selected a touch-based voluntary response where the user taps on their skin near the IMD. Since most implants are subcutaneous, they can easily detect the tap-pattern and authenticate using this second-factor response. Clearly, for an adversary to provide correct second-factor response to the IMD without alerting the user is difficult, which provides higher security guarantees. In addition to second-factor authentication, the human voluntary factor (human touch) is also used for waking up the system. This provides dual benefits of achieving extremely low-power wake-up and protecting against energy-drainage attacks. Through circuit-level optimizations, energy-efficient architecture and a novel dual-factor authentication mechanism, this work demonstrates a low-power IC for securing connected biomedical devices of the near future."
A Sampling Jitter Tolerant Continuous-time Pipelined ADC in 16-nm FinFET,"Almost all real-world signals are analog. Yet most data is stored and processed digitally due to advances in the integrated circuit technology. Therefore, analog-to-digital converters (ADCs) are an essential part of any electronic system. The advances in modern communication systems including 5G mobile networks and baseband processors require the ADCs to have large dynamic range and bandwidth. Although there have been steady improvements in the performance of ADCs, the improvements in conversion speed have been less significant because the sampling clock jitter limits the speed-resolution product (Figure 1). The effect of sampling clock jitter has been considered fundamental. However, it has been shown that continuous-time delta-sigma modulators may reduce the effect of sampling jitter. But since delta-sigma modulators rely on relatively high oversampling, they are unsuitable for high frequency applications. Therefore, ADCs with low oversampling ratio are desirable for high-speed data conversion. In conventional Nyuistrate ADCs, the input is sampled upfront (Figure 2). Any jitter in the sampling clock directly affects the sampled input and degrades the signal-to-noise ratio (SNR). It is well known that for a known rms sampling jitter σt the maximum achievable SNR is limited to 1/(2πfinσt,) where fin is the input signal frequency. In an SoC environment, it is difficult to reduce the rms jitter below 100 fs. This limits the maximum SNR to just 44 dB for a 10 GHz input signal. Therefore, unless the effect of sampling jitter is reduced, the performance of an ADC would be greatly limited for high frequency input signals.In this project, we propose a continuous-time pipelined ADC having reduced sensitivity to sampling jitter. We are designing this ADC in 16-nm FinFET technology to give a proof-of-concept for improved sensitivity to the sampling clock jitter."
Bandgap-less Temperature Sensors for High Untrimmed Accuracy,"Temperature sensors are extensively used in measurement, instrumentation, and control systems. A sensor that integrates the sensing element, analog-to-digital converter, and other interface electronics on the same chip is referred to as a smart sensor. CMOS- based smart temperature sensors offer the benefits of low cost and direct digital outputs over conventional sensors. However, they are limited in their absolute ac-curacy due to the non-ideal behavior of the devices used to design them. Therefore, these sensors require either calibration or gain/offset adjustments in the analog domain to achieve desired accuracies (Figure 1). The latter process, also called trimming, needs additional expensive test equipment and valuable production time and is a major contributor to the cost of the sensors. To enable high volume production of CMOS- based temperature sensors at low cost, it is imperative to achieve high accuracies without trimming.This work proposes the design of a CMOS temperature sensor that uses fundamental physical quantities resilient to process variations, package stress, and manufacturing tolerances, in order to achieve high accuracies without trimming. Simulation results prove that 3σ inaccuracy of less than 1o C can be obtained with the proposed method."
Low Power Time-of-flight Imaging for Dynamic Scenes,"Depth sensing is useful for many emerging applications, which include mobile augmented reality and robotics. Time-of-flight (ToF) cameras are appealing depth sensors that obtain dense depth measurements, or depth maps, with minimal latency. However, because these sensors obtain depth by emitting light, they can be power-hungry and limit the battery-life of mobile devices. To address this limitation, we present two approaches, shown in Figure 1, that reduce the power for depth sensing by leveraging the other available data: (1) when RGB images are concurrently collected, our technique reduces the usage of the ToF camera and estimates new depth maps using a previous depth map and the consecutive images; (2) when only the data from a ToF camera is available, we adaptively vary the amount of light that the ToF camera emits to infrequently obtain high-power depth maps and to use them to denoise subsequent low power ones. In the second scenario, the ToF camera is always on, but we reduce the overall amount of emitted light while still obtaining accurate depth maps.In contrast to our previous approaches that dealt with rigid environments, our techniques here can be used for applications that operate in dynamic environments, where the ToF camera and objects in the scenes can have independent, rigid, and non-rigid motions. For dynamic scenes, we show two benefits: (1) when RGB images are concurrently collected, our algorithm can reduce the usage of the ToF camera by over 90%, while still estimating new depth maps with a mean relative error (MRE) of 2.5% when compared to depth maps obtained using a ToF camera; and (2) when only the data from a ToF camera is available, our algorithm can reduce the overall amount of emitted light by up to 81% and the MRE of the low power depth maps by up to 64%. For these techniques, our algorithms use sparse operations and linear least squares to efficiently estimate or denoise depth maps at up to real-time (e.g., 30 fps) using the CPUs of a standard laptop computer and an embedded processor. Our work taken together enables energy-efficient, low latency, and accurate depth sensing for a variety of emerging applications."
CMOS Molecular Clock Using High-order Rotational Transition Probing and Slot-array Couplers,"Recently, chip-scale molecular clock (CSMC) referenced to sub-THz transitions of carbonyl sulfide (OCS) gas has emerged as a low-cost solution to achieve high stability with a small size. However, the long-term stability of the first CSMC is limited by the non-flat transmission baseline, which is susceptible to environmental disturbance. In order to enhance the long-term stability, we presented a CSMC chip that enables high-order dispersion curve locking. Since Nth-order dispersion curve can be comprehended as Nth-order derivative of the OCS line profile, the baseline tilting becomes negligible with high-order dispersion curve. Also, our chip adopts a pair of slot array couplers (SAC) for low loss chip-to-waveguide connection. Figure 1 shows the clock architecture which consists of a spectroscopic transmitter (TX), referenced to a 60 MHz voltage-controlled crystal oscillator (VCXO), and a spectroscopic receiver (RX). In order to generate the TX probing signal which is wave-length-modulated at a rate of fm=100kHz, high-accuracy, differential sine signal at fm is generated by a pair of 8bit DACs and then fed to varactors in the 57.77 GHz VCO in TX PLL2. The harmonic-rejection lock-in detector (HRLKD) is referenced to fLKREF=3fm, since the 3rd-order dispersion curve is used in this work. Figure 2 shows the structure and simulated S parameter of the SAC. The simulated loss and 3dB fractional bandwidth of the SAC are 5.2dB and 22%, respectively. The chip was fabricated in a 65nm bulk CMOS process and its DC power consumption was 70mW. The measured Allan Deviation are 3.2×10-10@τ=1s and 4.3×10-11@τ=103s, respectively, and the measured magnetic sensitivity of the unshielded clock is ±2.9×10-12/Gauss. With an on-chip temperature sensor and a 2nd-order polynomial compensation, the frequency drift over temperature range of 27~65°C is ±3.0×10-9. This work based on very compact size and low cost demonstrates stability performance that is comparable with chip-scale atomic clocks. Its applications include 5G cellular basestations, portable navigation systems, communication and sensing under GPS-denied conditions."
FastDepth: Fast Monocular Depth Estimation on Embedded Systems,"Depth sensing is a critical function for many robotic tasks such as localization, mapping and obstacle detection. There has been a significant and growing interest in performing depth estimation from a single RGB image, due to the relatively low cost and size of monocular cameras. However, state-of-the-art single-view depth estimation algorithms are based on fairly large deep neural networks that have high computational complexity and slow runtimes on embedded platforms. This poses a significant chal-lenge when performing real-time depth estimation on an embedded platform, for instance, mounted on a Micro Aerial Vehicle (MAV). Our work addresses this problem of fast depth estimation on embedded systems. We investigate efficient and lightweight encoder-decoder network architectures. To further improve their computational efficiency in terms of real metrics (e.g., latency), we apply resource-aware network adaptation (NetAdapt) to automatically simplify proposed architectures. In addition to reducing encoder complexity, our proposed optimizations significantly reduce the cost of the decoder network (Figure 1). We perform hardware-specific compilation targeting deployment on the NVIDIA Jetson TX2 platform. Our methodology demonstrates that it is possible to achieve similar accuracy as prior work on depth estimation, but at inference speeds that are an order of magnitude faster (Figure 2). Our network, FastDepth, runs at 178 fps on a TX2 GPU and at 27 fps when using only the TX2 CPU, with active power consumption under 10 W."
Design Considerations for Efficient Deep Neural Networks in Processing-in-memory Accelerators,"Deep neural networks (DNNs) deliver state-of-the-art accuracy on a wide range of artificial intelligence tasks at the cost of high computational complexity. Since data movement tends to dominate energy consumption and can limit throughput for memory-bound workloads, processing in memory (PIM) has emerged as a promising way for processing DNNs. Unfortunately, the design of efficient DNNs specifically for PIM accelerators has not been widely explored. In this work, we highlight the key differences between PIM and digital accelerators and summarize how these differences need to be accounted for when designing DNNs for PIM accelerators. The key design considerations include (1) resilience to circuit and device non-idealities, which affect accuracy; (2) data movement of feature map activations, which affects energy consumption and latency; and (3) utilization of the memory array, which affects energy consumption and latency. We examine the use of PIM accelerators on 18 DNNs published since 2012 for image classification on the ImageNet dataset to highlight the importance of the various design considerations. Our experiment results show that the common principles used to design efficient DNNs for digital accelerators (e.g., making a DNN deeper with smaller layers) may not suit PIM accelerators. Therefore, we need to rethink how to design efficient DNNs for PIM accelerators."
A Terahertz FMCW Comb Radar in 65-nm CMOS with 100GHz Bandwidth,"The increasing demands for low-cost, compact, and high-resolution radar systems have driven the op-eration frequency to terahertz due to the shorter wavelength and larger bandwidth. However, conven-tional single-transceiver frequency-modulated contin-uous-wave (FMCW) radar chips provide only limited signal bandwidth, especially when implemented using Complementary metal–oxide–semiconductor (CMOS) technologies with low fT and fmax. Therefore, prior THz integrated radars are based on compound semicon-ductors and have severely degraded performance near the band edges. That not only limits their applications in high-accuracy scenarios but also creates tradeoffs between bandwidth and detection range. To avoid such limitations, we adopt a frequency-comb-based scalable architecture using a paralleled transceiver array as shown in Figure 1. The concept of the FMCW comb radar is illustrated as a wideband chirp signal is divided into N identical segments that sweep simultaneously using an array of transceivers with equally-spaced carrier frequencies. Each transceiver has its own on-chip antenna, and the received echo sig-nal is mixed with the transmitted signal to generate an IF output. The presented high-parallelism scheme of-fers several advantages over single-transceiver radars. Firstly, it achieves scalable bandwidth extension and enables implementations in less advanced technologies as well as flatter frequency responses across the entire operation band. Secondly, the flat frequency response also leads to higher linearity of the equivalent chirp signal. Thirdly, the SNR of comb radar is improved by N for a given total detection time. Implemented in a 65-nm bulk CMOS process, a five-transceiver radar chip is prototyped with seamless coverage of the entire 220-to-320GHz band as shown in Figure 2. Across the total chirp bandwidth of 100GHz, 0.6dBm/20dBm (with/without lens) multi-channel-ag-gregated EIRP with 8.8dB output power fluctuation, and 22.8dB minimum RX noise figure are achieved. With all five channels stitched together, 2.5-mm separa-tion of two objects is clearly detected. This chip has an area of 5mm2 and consumes 840mW of power. This is the first demonstration of THz radar in CMOS process, and a record FMCW bandwidth is achieved."
GaN Electronics for High-temperature Applications,"Gallium nitride is a promising candidate for high-tem-perature applications. However, despite the excellent performance shown by early high-temperature proto-types, several issues in traditional lateral AlGaN/GaN HEMTs could cause early degradation and failure un-der high-temperature operation (over 300°C). These in-clude ohmic degradation, gate leakage, buffer leakage, and poor passivation. Additionally, enhancement-mode HEMTs are preferred from the application point of view because they reduce the circuit complexity and cost. At the same time, the two-dimensional electron gas induced by AlGaN/GaN heterostructures makes HEMTs naturally depletion-mode devices. Devices capable of high-temperature operation were demonstrated by combing gate injections tran-sistors (GITs) with ion-implanted refractory metal con-tacts. A self-aligned gate-first process, together with an etch-stop process, was developed and optimized to improve fabrication efficiency and device uniformity for large-scale integration. Basic logic building blocks including inverters, a NAND gate, a NOR gate, SRAM, and a ring oscillator have been demonstrated and char-acterized at both room temperature and high tempera-ture (300°C)."
Hybrid Intelligence in Design,"One of the greatest challenges facing society is addressing the complexities of big picture, system-level, interdisciplinary problems in a holistic way. Human designers, architects, and engineers have come to rely on steadily improving computational tools to design, model, and analyze their systems of interest. At this stage one might ask several questions: “How could we teach junior engineers, architects, and scientists to design complex systems successfully without spending years on job training? Could we also assist human experts to minimize the probability of failure by leveraging recent developments in artificial intelligence (AI) and big data?” While the resurgence of AI and machine learning suggest ways to even more fully automate downstream tasks in the design process, we propose to go up-stream of design, where all the key concepts are determined. Could machine intelligence help this early stage of designing beyond routine design and the optimization of pre-specified goals toward the generation of good, novel designs? To capture the benefit of machine learning for design, the information and knowledge embodied in design must be represented in a method that machines can understand, memorize, and retrieve, with the goal of enhancing the practice of design. Preliminary investigation has shown how Natural Language Processing (NLP) models can be applied to accurately estimate design metrics such as functional independence based solely on descriptions of different design cases, as shown in Figure 1. With a framework for representing design knowledge, machines can effectively augment the work of human designers at the early stages of the design process."
Partition WaveNet for Deep Modeling of Automated Material Handling System Traffic,"The throughput of a modern semiconductor fabrication plant depends greatly on the performance of its automated material handling system. Spatiotemporal modeling of the dynamics of a material handling system can lead to a multi-purpose model capable of generalizing to many tasks, including dynamic route optimization, traffic prediction, and anomaly detection. Graph-based deep learning methods have enjoyed considerable success in other traffic modeling domains, but semiconductor fabrication plants are out of reach because of their prohibitively large transport graphs. In this report, we consider a novel neural network architecture, Partition WaveNet, for spatiotemporal modeling on large graphs. Partition WaveNet uses a learned graph partition as an encoder to reduce the input size combined with a WaveNet-based stacked dilated 1D convolution component. The adjacency structure from the original graph is propagated to the induced partition graph. We discuss the motivation for framing our problem as a supervised learning task instead of a reinforcement learning task, as well as the benefits of Partition WaveNet over alternative neural network architectures. We evaluate Partition WaveNet on data from a simulated and a real semiconductor fabrication plant. We find that Partition WaveNet outperforms other spatiotemporal networks using network embeddings or graph partitions for dimensionality reduction."
Efficient AutoML with Once-for-all Network,"We address the challenging problem of efficient inference across many devices and resource constraints, especially on edge devices. Conventional approaches either manually design or use neural architecture search (NAS) to find a specialized neural network and train it from scratch for each case, which is computationally prohibitive (causing CO2 emission as much as 5 cars' lifetime) and thus unscalable. In this work, we propose to train a once-for-all (OFA) net-work that supports diverse architectural settings by decoupling training and search, to reduce the cost. We can quickly get a specialized sub-network by selecting from the OFA network without additional training. To efficiently train OFA networks, we also propose a novel progressive shrinking algorithm, a generalized pruning method that reduces the model size across many more dimensions than pruning (depth, width, kernel size, and resolution). It can obtain a surprisingly large number of sub-networks that can fit different hardware platforms and latency constraints while maintaining the same level of accuracy as training independently. On diverse edge devices, OFA consistently outperforms state-of-the-art (SOTA) NAS methods (up to 4.0% ImageNet top1 accuracy improvement over MobileNetV3, or same accuracy but 1.5x faster than MobileNetV3, and 2.6x faster than EfficientNet w.r.t measured latency) while reducing GPU hours and CO2 emission by many orders of magnitude. In particular, OFA achieves a new SOTA 80.0% ImageNet top1 accuracy under the mobile setting (<600M MACs).OFA is the winning solution for the 3rd Low Power Computer Vision Challenge (LPCVC, classification DSP track) and the 4th LPCVC (both classification track and detection track)."
Robustness Verification and Defense for Tree-based Machine Learning Models,"Although adversarial examples and model robustness have been extensively studied in the context of linear models and neural networks, research on this issue in tree-based models is still limited, despite the prevalence of tree-based models in manufacturing and other domains. In this work, we develop a novel algorithm to learn robust trees, as well as an efficient algorithm to evaluate the robustness of a tree-based model. Our first algorithm aims to optimize the performance under the worst-case perturbation of input features, which leads to a max-min saddle point problem. Incorporating this saddle point objective into the decision tree building procedure is nontrivial due to the discrete nature of trees—a naive approach to finding the best split according to this saddle point objective will take exponential time. To make our approach practical and scalable, we approximate the inner minimizer in this saddle point problem and present implementations for classical information gain-based trees as well as state-of-the-art tree boosting models such as XGBoost. As demonstrated in Figure 1, experimental results on real world datasets demonstrate that the proposed algorithms can substantially improve the robustness of tree-based models against adversarial examples. Formal robustness verification of decision tree ensembles involves finding the exact minimal adversarial perturbation or a guaranteed lower bound, which is NP-complete in general. We show that for tree ensembles, the verification problem can be cast as a max-clique problem on a multipartite graph with bounded boxicity. For low dimensional problems when boxicity can be viewed as constant, this reformulation leads to a polynomial time algorithm. For general problems, by exploiting the boxicity of the graph, we develop an efficient multi-level verification algorithm that can give tight lower bounds on the robustness of decision tree ensembles while allowing iterative improvement and anytime termination. As in Figure 2, our algorithm is much faster than a previous approach that requires solving mixed integer linear programming (MILP) and can give tight robustness verification bounds on large models with one thousand deep trees."
An Efficient and Continuous Approach to Information-theoretic Exploration,"Exploration of unknown environments is embedded in many robotics applications: search and rescue, crop survey, space exploration, etc. The central problem an exploring robot must answer is “where should I move next?” The answer should balance travel cost with the amount of information expected to be gained about the environment. Traditionally, this question has been an-swered by a variety of heuristics that provide no guar-antees on their exploration efficiency. Information-the-oretic methods can produce an optimal solution, but until now they were thought to be computationally intractable.In our recent work we describe the Fast Continu-ous Mutual Information (FCMI) algorithm, which com-putes the information-theoretic exploration metric ef-ficiently. FCMI takes as input an incomplete occupancy map like the one shown in Figure 1, where white pix-els indicate free space, black pixels indicate occupied space, and gray pixels indicate unknown space. It then returns an information surface as shown in Figure 2, where the brightness of each pixel indicates how much information is expected to be gained by exploring at that location. The algorithm also works on multi-reso-lution or 3-dimensional maps. FCMI has a lower asymp-totic complexity than existing methods and our exper-iments demonstrate that it is hundreds of times faster than the state-of-the-art for practical inputs.The key insight that enables FCMI is to consid-er the occupancy map as a continuous random field rather than a discrete collection of cells. This reveals a nested information structure that makes it possible to recursively reuse computation from one map location in adjacent locations. The continuous structure also provides more general insights that are relevant to any occupancy mapping system.For practical map sizes, FCMI runs in seconds on a single threaded laptop CPU which is well within the timing constraints for most robotic applications. It provides considerable savings to energy constrained systems by reducing both the exploration travel cost and the computation cost. FCMI is also highly paral-lelizable and suited for a rapid, low energy, embedded implementation."
On the Use of Deep Learning for Retrieving Phase from Noisy Inputs in the Coherent Modulation Imaging Scheme,"Low-dose light imaging is of significance in many cases when minimal radiation exposure of samples is desired. In biological imaging, high-dose light may induce phototoxic effects at the cost of larger signal-to-noise ratio (SNR). In particle imaging, for instance, imaging integrated circuits (IC) with high-power beam leads to destructive side-effects, e.g., heat-induced deformation. However, quantum nature of photon detection influences and degrades the quality of intensity measurements, and on top of the Poisson statistics, other types of noise sources, e.g., thermal or readout noise, add up.Deep neural networks (DNNs) have been used for retrieving phase information from noisy intensity measurements. Nonetheless, the ill-posedness of the inversion problem, governed by a physical system design, could not be sufficiently addressed when the DNN alone was used. Due to the ill-posedness of the system, residual artifacts remained in reconstructions, thus a decrease in image fidelity. Therefore, we suggest the application of random phase modulation on an optical field, also known as a coherent modulation imaging (CMI) scheme, along with the DNNs as a method of reconstruction.In this work, we provide both quantitative and qualitative results that unwanted artifacts in reconstructions are largely removed in the coherent modulation imaging scheme under low-light conditions in conjunction with the DNNs. Here, phase extraction neural network (PhENN), which is an encoder-decoder DNN architecture based on ResNet specifically optimized for phase retrieval tasks, was used as a design of the DNN."
Rapid Uniformity Tuning in Ion Implantation Systems Using Bayesian Optimization,"As the size of integrated circuits continue to shrink, variations in their fabrication processes become more significant, hindering their electrical performances and yields. One such wafer-scale variation occurs in ion implantation processes, where an ion beam implants charged particles into a substrate. As the beam is scanned across the wafer, its shape and intensity often change, resulting in a non-uniform implantation. This effect can be compensated for by adjusting the speed of the ion beam as it moves across the wafer; however, in order to do so, the dynamics of the ion beam shape must be known.Our work focuses on using Bayesian optimization, a form of reinforcement learning, to rapidly learn how the beam shape changes, and to optimize the beam speeds in order to reduce non-uniformities. Here, we capture our knowledge of the beam shapes by treating its intensities as multivariate, normally distributed, random variables. After observing new implantations, we then use this framework to update our belief of the beam shapes, then solve for a new set of scan speeds which result in our desired profile under this updated model. We then continue this process until we converge to our desired profile. After this initial tuning, the same tuning algorithm continues to run during normal operation. Implantation measurements are periodically made, the model is updated using these measurements, and any corrections to the scan speeds are made in order to maximize uniformity. This process allows us to both quickly tune a new implantation recipe, while also allowing us to learn and compensate for any changing conditions in the tool."
A Mutual Information Accelerator for Autonomous Robot Exploration,"Robotic exploration problems arise in various contexts, ranging from search and rescue missions to underwater and space exploration. In these domains, exploration algorithms that allow the robot to rapidly create the map of the unknown environment can reduce the time and energy for the robot to complete its mission. Shannon mutual information (MI) at a given location is a measure of how much new information of the unknown environment the robot will obtain given what the robot already know from its incomplete understanding of the environment. In a typical exploration pipeline, robot starts with an incomplete map of the environment. At every step, the robot computes the MI across the entire map. Then, the robot can select the location with the highest mutual information for exploration in order to gain the most information about the unknown environment.However, on the CPUs and GPUs typically found on mobile robotic platforms, computing MI using the state-of-the-art Fast Shannon Mutual Information (FSMI) algorithm across the entire map takes more than one second, which is too slow for enabling fast autonomous exploration. As a result, the emerging literature considers approximation techniques, and many practitioners rely on heuristics that often fail to provide any theoretical guarantees. To eliminate the bottleneck associated with the computation of MI across the entire map, we propose a novel multicore hardware architecture (Figure 1) with a memory subsystem that efficiently organizes the storage of the occupancy grid map and an arbiter that effectively resolves memory access conflicts among MI cores so that the entire system achieves high throughput. In addition, we provide rigorous analysis of memory subsystem and arbiter in order to justify our design decisions and provide provable performance guarantees. Finally, we thoroughly validated the entire hardware architecture by implementing it using a commercial 65nm ASIC technology (Figure 2)."
Ionic Analog Synapses for Deep Learning Accelerators,"The recent progress in novel hardware/software co-optimizations for machine learning has led to tremendous improvement of the efficiency of neural networks. Nevertheless, the energy efficiency is still orders of magnitude lower than biological counterpart – the brain. Digital CMOS architecture has inherent limitations for deep learning applications due to their volatile memory, spatially separated memory and computation, and the lack of connectivity between nodes. Crossbar arrays of non-volatile memory devices, able to perform simple operations (e.g. bit multiplication), can potentially achieve a 30000× improvement in energy efficiency. State-of-the-art analog “synaptic” devices based on resistive memories suffer from stochastic, asymmetric, and non-linear weight updates, detrimental to training accuracy. Electrochemical ionic devices have been shown to be fast, energy efficient, and exhibit symmetric, linear weight updates. However, electrolytes used for the electrochemical reaction are often CMOS incompatible and suffers from scalability. Here we propose a new transistor-based analog synapse, consisting of a proton-doped SiO2 gate oxide which electrostatically modifies the threshold voltage of the semiconductor channel, tuning the channel conductance (Figure 1). Non-volatility is maintained by trapping of protons in the oxide. Due to electrostatics, we expect to observe a symmetric and linear shift in threshold voltage, leading to linear weight updates. We study the proton diffusion and electrostatic effects through device simulation via Silvaco Atlas and analytical modeling. Simulations show a threshold voltage shift of the MOS gate stack due to the presence of ions in the gate oxide (Figure 2). We fabricate n-Si/ALD SiO2/Al MOS capacitor and to demonstrate the feasibility of our ionic device. We observe that the MOS gate stack exhibits hysteretic behavior below 2V, indicating non-volatility and low-voltage operation. The results of this work will shed light on the feasibility of simple CMOS-compatible ionic devices for the next generation of neural network hardware accelerators."
Efficient 3D Deep Learning with Point-voxel CNN,"3D deep learning has received increased attention thanks to its wide applications: e.g., AR/VR and autonomous driving. These applications need to interact with people in real time and therefore require low latency. However, edge devices (such as AR/VR headsets and self-driving cars) are tightly constrained by hardware resources and battery. Previous work processes 3D data using either voxel-based or point-based NN models. However, both approaches are computationally inefficient. The computation cost and memory footprints of the voxel-based models grow cubically with the input resolution, making it memory-prohibitive to scale up the resolution. As for point-based networks, up to 80% of the time is wasted on structuring the sparse data which have rather poor memory locality, not on the actual feature extraction.To this end, we propose Point-Voxel CNN (PVCNN) that represents the 3D input data as point clouds to take advantage of the sparsity to reduce the memory footprint, and leverages the voxel-based convolution to obtain the contiguous memory access pattern (Figure 1). Evaluated on semantic and part segmentation datasets, it achieves a much higher accuracy than the voxel-based baseline with 10× GPU memory reduction; it also outperforms the state-of-the-art point-based models with 7× measured speedup on average (Figure 2). We validate its general effectiveness on 3D object detection: Frustrum PVCNN outperforms Frustrum PointNet++ by up to 2.4% mAP with 1.8× measured speedup and 1.4× GPU memory reduction."
Learning Human-environment Interactions Using Scalable Functional Textiles,"Living organisms extract information and learn from the surroundings through constant physical interactions. For example, humans are particularly receptive to tactile cues (on hands, limbs, and torso), which enable the performing of complex tasks like dexterous grasp and locomotion. Observing and modeling interactions between humans and the physical world are fundamental for the study of human behavior, healthcare, robotics, and human-computer interactions. However, many studies of human-environment interactions rely on more easily observable visual or audible datasets because it is challenging to obtain tactile data in a scalable manner. Recently, Sundaram et al. coupled tactile-sensing gloves and machine learning to uncover signatures of the human grasp. However, the recording and analysis of whole-body interactions remain elusive, as this would require large-scale wearable sensors with low cost, dense coverage, conformal fit, and minimal presence to permit natural human activities. We present a textile-based tactile learning platform that enables researchers to record, monitor, and learn human activities and the associated interactions. Realized with inexpensive piezoresistive fibers (0.2 USD/m) and automated machine knitting, our functional textiles offer dense coverage (> 1000 sensors) over large complex surfaces (> 2000 cm2). Further, we leverage machine learning for sensing correction, ensuring that our system is robust against potential variations from individual receptors. To validate the capability of our sensor, we capture diverse human-environment interactions (> 1,000,000 tactile frames) and demonstrate that machine learning techniques can be used with our data to classify human activities, predict whole-body poses, and discover novel motion signatures. This work opens new possibilities in wearable electronics, healthcare, manufacturing, and robotics."
Automated Fault Detection in Manufacturing Equipment Using Semi-supervised Deep Learning,"Our project investigates the use of semi-supervised deep learning systems for automated fault detection and predictive maintenance of manufacturing equipment. Unexpected equipment faults can be highly costly to manufacturing lines, but data-driven fault detection systems often require a high level of domain-specific expertise to implement as well as continued human oversight. To this end, we are developing and testing general-purpose fault detection systems that require minimal labeled data. Our system trains deep autoencoders to function as a non-linear compression algorithm for sensor readings from manufacturing equipment. The compressed sensor signals are used as a proxy for the equipment’s hidden state, and the reconstruction error is used to detect unexpected behavior. The compressed representation and reconstruction error are combined to provide a robust anomaly score. Instances in time with the highest anomaly score are then flagged to be labeled by a human operator as faulty or nominal. With sparsely labeled faults, the system then uses Gaussian mixture models to classify different types of errors and predicts future faults by monitoring parameter drift towards known fault states. Our system is currently being trained to detect failed runs on a plasma etcher (used for integrated circuit fabrication) using internal sensors that take voltage, current, pressure, and temperature readings. In preliminary tests, the system was able to correctly detect 88% of failed etching runs and identify specific markers in different signals indicative of faults. For example, a failure mode of the plasma etcher involves an abnormally high temperature (Figure 1). Without any labeled errors, the system flagged the higher temperatures as possibly indicative of faults (Figure 2).We are currently testing the system on a wider range of applications, including estimating wear of milling machine cutting tools and predicting the risk of breakage. We are also developing prototypes of contactless voltage/current sensors that can easily be retrofitted onto older machinery to test the efficacy of fault detection systems using only external power draw."
Control of Conductive Filaments in Resistive Switching Oxides,"There is a growing interest in using specialized neuro-morphic hardware for artificial neural network appli-cations such as image and speech processing, which require significant computational resources. These neuromorphic devices show promise for reducing the demands of such applications by increasing speed and decreasing power consumption compared to current software-based methods. One approach to achieving this goal is through oxide thin film resistive switching devices arranged in a crossbar array configuration. Re-sistive switching can mimic several aspects of neural networks, such as short- and long-term plasticity, via the dynamics of switching between multiple analog conductance states-dominated by the creation, annihi-lation, and movement of defects within the film (such as oxygen vacancies). These processes can be stochas-tic in nature and contribute significantly to device vari-ability, both within and between individual devices. Our research focuses on reducing the variability of the set/reset voltages and enhancing control of the con-ductance state with voltage pulsing using model sys-tems of HfO2 grown on Nb:SrTiO3 substrates through the control of film growth and processing parameters. We show that depending on the growth temperature, substrate orientation, and substrate surface treatment, devices can exhibit forming-free switching or forming voltages ranging from 4 to 7 V. Forming-free devices show lower variability in the high and low conduc-tance states but have a lower on/off conductance ratio. We rationalize these results using film microstructure information obtained from 2D X-ray diffraction and cross-sectional transmission electron microscopy. This work provides a significant step towards controlling the mechanisms behind device variability and achiev-ing devices that meet the strict requirements of neuro-morphic computing."
Variational Inference for Model-free Simulation of Dynamic Systems with Unknown Parameters,"Complex physical, biological, and engineering processes can be modelled using dynamic systems with few parameters. However, in real-world applications including manufacturing, it is possible to encounter systems for which the dynamics are not well understood and identifying the parameters is challenging.“Model-free” approaches aim to learn the dynamics of the system from data. Classical statistical models assume the dynamics are linear to make the inference analytically tractable. Extension to nonlinearity usually requires partial knowledge about the system. Our goal is to achieve modeling of nonlinear dynamic systems purely by using data with the strength of deep learning.In this work, we formulate the learning task as variational inference by considering the unknown parameters as random variables. Then, we use two recurrent neural networks and a feedforward network as the variational autoencoder to learn an approximate posterior distribution. The first recurrent neural network is a pre-trained encoder that encodes the input into a dense representation. Then, the feedforward network transforms the representation into the posterior distribution. Finally, the second recurrent neural network receives samples from the posterior distribution to predict the mean and variance of the output. Loss functions include pretraining loss, reconstruction loss, and KL divergence loss with regard to the prior. Figure 1 gives an overview of our model.The numerical experiments show that the proposed model produces a more accurate simulation than the standard recurrent neural networks, especially when the Monte Carlo method is applied to perform multiple-step simulations. In addition, by analyzing the learned posterior distribution, we show that our approach can correctly identify the number of underlying parameters."
SpArch: Efficient Architecture for Sparse Matrix Multiplication,"Generalized sparse matrix-matrix multiplication (SpGEMM) is the key computing kernel for many algorithms such as compressed deep neural networks. However, the performance of SpGEMM is memory-bounded on the traditional general-purpose computing platforms (CPU, GPU) because of the irregular memory access pattern and poor locality brought by the ex-tremely sparse matrices. For instance, the density of Twitter's adjacency matrix is as low as 0.000214%. Previous accelerator OuterSPACE proposed an outer product method that has perfect input reuse but poor output reuse due to enormous partial matrices, thus achieving only 10.4% of the theoretical peak.Therefore, we propose SpArch (HPCA'2020) to jointly optimize input and output data reuse. We obtain input reuse by using the outer product and output reuse by on-chip partial matrix merging (Figure 1).We first design a highly parallelized merger to pipeline the two computing stages, Multiply and Merge. However, the number of partial matrices can easily exceed the on-chip merger's parallelism and incurs even larger DRAM access. We thus propose a condensed matrix representation for the left input matrix, where all non-zero elements are pushed to the left, forming much denser columns and fewer partial matrices. Unfortunately, the condensed rep-resentation can still produce more partial matrices than the merger's parallelism. Since the merge order impacts DRAM access, we should merge matrices with fewer non-zeros first. To this end, we design a Huffman tree scheduler to decide the near-optimal merge order of the partial matrices. Finally, we propose a row prefetcher to prefetch rows of the right matrix and store to a row buffer, thus improving the input reuse.We evaluate SpArch on real-world datasets from SuiteSparse, SNAP, and rMAT, achieving 4×, 19×, 18×, 17×, and 1285× speedup and 6×, 164×, 435×, 307×, and 62× energy saving over OuterSPACE, MKL, cuSPARSE, CUSP and ARM Armadillo, respectively. Figure 2 shows the speedup breakdown of SpArch over OuterSPACE."
Efficient Natural Language Processing with Hardware-aware Transformers (HAT),"Transformers have been widely used in Natural Language Processing (NLP) tasks, providing a significant performance improvement over previous convolutional and recurrent models. Nevertheless, transformers cannot be easily deployed in mobile/edge devices due to their extremely high cost of computation. For instance, to translate a sentence with only 30 words, a Transformer-Big model executes 13G Mult-Adds and takes 20 seconds on Raspberry Pi 4, making real-time NLP impossible.We found two critical phenomena that impact the transformer’s efficiency: (1) FLOPs cannot reflect real latency and (2) efficient model architecture varies for different hardware. The reason is that for different hardware, the latency influencing factors differ a lot. For example, the embedding size has a large impact on Raspberry Pi but can hardly influence GPU latency.Inspired by the success of neural architecture search (NAS), we propose to search for hardware-aware transformers (HAT, ACL'2020) by directly involving hardware latency feedback in the design loop (Figure 1). Hence, we do not need FLOPs as a latency proxy and can search hardware-specific models. We first construct a large search space with two features: (1) arbitrary encoder-decoder attention to allow all decoder layers to attend to multiple and different encoder layers and (2) heterogeneous layers to let different layers have different architectures. To conduct a low-cost search, we first train a SuperTransformer, which contains many Sub-Transformers with weight-sharing. Then we perform an evolutionary search in the Super-Transformer to find the best SubTransformers under hardware latency constraints.We evaluate our HAT with three translation tasks on Raspberry pi ARM CPU, Intel CPU, and Nvidia GPU. HAT achieves up to 3× speedup and 3.7× smaller size over the conventional Transformer-Big model (Figure 2). With 10000× less search cost, HAT outperforms the Evolved Transformer with 2.7× speedup and 3.6× smaller size. Therefore, HAT enables efficient NLP on mobile devices."
Flexible Low Power CNN Accelerator for Edge Computing with Weight Tuning,"Smart edge devices that support efficient neural network (NN) processing have recently gained public attention. With algorithm development, previous work has proposed small-footprint NNs achieving high performance in various medium complexity tasks, e.g. speech keyword spotting (KWS), human activity recognition (HAR), etc. Among them, convolutional NNs (CNNs) perform well, which gives rise to the deployment of CNNs on edge devices. A hardware platform for edge devices should be (1) flexible to support various NN structures optimized for different applications; (2) energy efficient to operate within the power budget; (3) achieving high accuracy to minimize spurious triggering of power-hungry downstream processing, since it is often part of a large system. This work proposes a weight tuning algorithm to improve the energy efficiency by lowering the switching activity of weight-related components, e.g. weight buses and multipliers. To achieve that, the algorithm reduces the Hamming distance between successive weights as shown in Figure 1. A flexible and runtime-reconfigurable CNN accelerator is co-designed with the algorithm. The system is fully self-contained for small CNNs. Speech keyword spotting is shown as an example with an integrated feature extraction frontend. As shown in Figure 2, a fully integrated custom ASIC is fabricated for this system. Based on post place-and-route simulation of the ASIC, the weight tuning algorithm reduces the energy consumption of weight delivery and computation by 1.70x and 1.20x respectively with little loss in accuracy."
Protonic Solid-state Electrochemical Synapse for Physical Neural Networks,"Physical neural networks made of analog resistive switching processors are promising platforms for analog computing and for emulating biological synapses. State-of-the-art resistive switches rely on either conductive filament formation or phase change, processes that suffer from poor reproducibility or high energy consumption, respectively. To avoid such shortcomings, we establish an alternative synapse design (Figure 1a) that relies on a deterministic charge-controlled mechanism, modulated electrochemically in solid state, that consists of shuffling the smallest cation, the proton. This proof-of-concept, protonic solid-state electrochemical synapse is a three-terminal configuration and has a channel of active material (A), here taken as WO3. By protonation/deprotonation, we modulate the electronic conductivity of the channel over seven orders of magnitude, obtaining a continuum of resistance states (Figure 1b). A solid proton reservoir layer (R), PdHx, serves as the gate terminal. A proton conducting solid electrolyte (E), Nafion, separates the channel and the reservoir. By probing the atomic, electronic, and crystal structures (Figure 1c-d) involved during proton intercalating, we reveal an increase in the electronic conductivity of WO3 resulting from the increase of both the carrier density and the mobility. This switching mechanism has several key advantages over other switching mechanisms, in-cluding low energy dissipation and good reversibility and symmetry in programming.We are also working to improve device properties and integrability of this protonic synapse by exploring alternative materials for both the active channel and the solid-state electrolyte. On one hand, promising host materials for the intercalation of protons and multivalent ions, such as vanadium pentaoxide, graphene oxide, and tantalum pentaoxide, are being investigated as potential active materials. On the other hand, nanocrystalline yttrium-doped barium zirconate and gadolinium-doped cerium oxide are being studied as possible room-temperature fast proton conductor ceramics."
WSe2 Thin Film Solar Cells,"Our group is interested in exploring the ultimate limits of microsystem scaling and functionality. The amount of energy available to the system is one of the key constraints, and solar cells based on transition metal dichalcogenides (TMDs) could be a key component of future highly-integrated microsystems.Single atomic layer TMDs have been explored extensively for ultrathin optoelectronic applications due to their direct bandgap and strong light-matter interactions. However, optoelectronic applications of multi-layer TMD thin-films have not been as extensively studied despite their strong absorption characteristics and wide absorption frequency. Nevertheless, published work has shown that a p-n junction made with chemically doped multilayer MoS2 can achieve an efficiency of 2.8%, and a vertical Schottky junction WSe2 solar cell can achieve efficiencies as high as 6.7%. Most intriguingly, it has been shown that with careful design, a 15nm WSe2 solar cell can absorb 90% of 633nm incident light, demonstrating that TMDs can push the limit of thin film photovoltaics.In this work, we study the electronic transport and photovoltaic characteristics of multilayer (~100 nm) WSe2 devices that can later be integrated as the energy harvester in a micro-scale sensing system. We have demonstrated a Schottky junction WSe2 solar cell using dissimilar metal contacts. The proof-of-concept dual-metal device showed an open-circuit voltage of ~ 0.2 V, short circuit current density ~ 4 mA/cm2 and power conversion efficiency ~ 2% under white light illumination with input power of 300 W/m2. This study is extended to explore methods to better optimize the WSe2 based solar cell using experimental and modeling techniques. We are currently developing hole and electron transport layers to improve the device efficiency."
Critical Design Parameters for Omnidirectional 2-D Filled Photonic Crystal Selective Emitter for Thermophotovoltaics,"Thermophotovoltaic (TPV) systems are promising as small-scale, portable generators to power sensors, small robotic platforms, and portable computational and communication equipment. In TPV systems, an emitter at high-temperature emits radiation that is then converted to electricity by a low bandgap pho-tovoltaic cell. One approach to improve the efficiency is to use hafnia-filled two-dimensional (2-D) tantalum (Ta) photonic crystals (PhCs). These emitters enable ef-ficient spectral tailoring of thermal radiation for a wide range of incidence angles. However, fabricating these PhCs is difficult. We use focused ion beam (FIB) imaging and simulations to investigate the effects of fabrication imperfections on the emit-tance of a fabricated hafnia-filled PhC and to iden-tify design parameters critical to the overall PhC performance. We demonstrate that, more so than uniform cavity filling, the PhC performance relies on the precise cavity period and radius values and thickness of the top hafnia layer."
Low-power Management IC for Vibrational Energy Harvesting Applications,"Vibration-based machine health monitoring provides an efficient real-time method for tracking the health of industrial motors, thereby achieving predictive main-tenance and avoiding machine downtime. Vibration sensors are attached to the vibrating motors, and peri-odically transmit data indicative of machine health. To power such monitors, we demonstrate a vibration-based energy harvesting system whose schematic is shown in Figure 1. It extracts power from 50Hz industrial motors and comprises a co-designed MEMS-based transducer and associated low-power management circuit.The MEMS-based energy harvester shown in Figure 1 can generate about 1 mW output power under matched load at resonance. However, its high quality-factor results in significant reduction in output power and voltage at off-resonance conditions. The system is made resilient to manufacturing variations which cause a mismatch between the harvester’s natural resonance and the motor frequency by using the interface power electronics. A Meissner oscillator circuit shown in Figure 2 is used to achieve battery-less cold-start from low harvester-voltages at off-resonance. A regular operation circuit is designed to operate once the cold-start circuit generates above-1V output voltage (Vout). This circuit employs an H-bridge to interface the harvester whose FETs are switched based on current-feedback. The load-storage element is toggled between the two ports of the harvester to synthesize the desired load-current at any frequency. The circuit thus accomplishes conjugate-impedance matching for efficient power extraction from the harvester. Further, it can tune the harvester’s source reactance to electrically shift its resonance to achieve increased bandwidth of operation.The IC implemented in the Taiwan Semiconductor Manufacturing Company (TSMC) 180nm process (shown in Figure 1) is co-designed with the harvester achieves cold-start from 150mV-peak AC-voltage from the harvester at 5% off-resonance (10x state-of-the-art). The H-bridge circuit is able to deliver 800 μW to the load at 71% efficiency at resonance as shown in Figure 2. It is also able to perform frequency tuning to account for manufacturing tolerances (A first low-power IC demonstration for this application)."
Electromagnetic MEMS Harvester for Vibrational Energy Harvesting Applications,"Powering machine health monitoring sensors with the motions from the machinery allows install-and-forget implementation of the machine health monitoring net-work. Electromagnetic MEMS based-transducer provides an efficient interface between industrial machines and the rest of the vibration-based energy harvesting system. Implementing the mechanical harvester’s spring system on silicon, allows the mechanical system and the circuit to be manufactured through the same process, cutting down on both assembly time and complexity.The transducer design uses a modified version of the classic 4 bar linkage spring design. The long beams are tapered such that the end connecting to the guide rod is wider than the connecting region to the shuttle, which houses the magnet (see Figure 1). This alleviates the stress experienced at the joints of the beam, which is the typical weak point of the structure. With the tapered beam, the current design achieves a full stroke of 1.6mm and is more robust with regard to handling during the assembly process. The design also offers good modal separation, with the modal frequency of the first undesirable mode several hundred Hz above the desired, horizontal translational mode. The coils are manually wound using 42 AWG enamel coated copper wires with two coils placed at 150mm above and below the magnet’s plane of motion. The coils and the spring system are each fixed in a plastic package. When attached to the source of vibration, the harvester’s magnet vibrates in between the coils, inducing an EMF in the coils in accordance with Lenz’s Law. The coils are connected in series, and the induced voltages add to produce an output voltage, which is interfaced with custom designed circuitry for energy harvesting. The assembled mechanical harvester can deliver 1mW of output power at resonance with a matched load."
Micro-buckled Beam Based Ultra-low Frequency Vibration Energy Harvester,"MEMS energy harvesting has been keenly pursued to provide perpetual power for many wireless applica-tions including distributed sensor networks and up-coming IoT systems. However, scavenging a sufficient amount of power for wireless communication from environmentally available vibrations, typically at low frequency (<70Hz) and low acceleration (0.5g), has nei-ther been successful nor reported at the MEMS scale. Here we present a bi-stable buckled beam MEMS en-ergy harvester which could meet those requirements in terms of low operating frequency, wide bandwidth, and power, all packaged in the size of a coin. This new design does not rely on conventional linear or non-lin-ear resonance of the MEMS structure, but instead oper-ates with large snapping motions of buckled beams at very low frequencies. A fully functional piezoelectric device has been designed, monolithically fabricated, and tested to induce bi-stable buckling of ~200µm. The first batch device generated peak power of 85 nW with 50% half-power bandwidth under 70Hz at 0.5g.Our bi-stable nonlinear oscillator-based MEMS energy harvester has a clamped-clamped beam structure with a stack of thin-films having 28 pairs of beams 0.4mm wide in a silicon frame of 15mm×12mm. Each beam has approximately 500 interdigitated Au fingers over 0.2 µm thick PZT. A proof mass is located in the middle, connecting the beams to synchronize their out-of-plane motion and minimize undesirable torsion. Thin-film layers of various stresses have an effective total compression and balanced stress with respect to the neutral axis to achieve bi-stable buckling. The residual stress and the thickness of the thin films are monitored for each deposition step, and progressive feedback control of subsequent deposition is employed to minimize deviation from the design target. The final released device (Figure 1) shows desired bi-stable buckling of about 200µm Figure 2) which is within 5% of the designed value. The dynamic testing with a laser vibrometer validates the design concept that the buckled beam device could have large-amplitude oscillations with low-frequency and low-amplitude inputs (<70Hz and 0.5g)."
RuO2 as Cathode Material of Thin Film Lithium-ion Batteries (LIB),"Technologies for the Internet of Things (IoT) are be-ing developed for a vast number of networking appli-cations. Thin film batteries are important for IoT sys-tems as they are better integrated within an integrated circuit (IC) and can store energy that is harvested by green generators (e.g., solar cells) and provide it to sen-sors. RuO2 had been found to have a larger specific ca-pacity compared to other cathode materials of lithium ion batteries (LIB), and thus, is a good candidate as a cathode material of thin film LIB. We are currently studying the reaction mechanism of RuO2 and lithium in parallel with the fabrication of full battery devices. To analyze the mechanism of lithium storage in thin film RuO2, we performed cyclic voltammetry (CV) tests with varying lower limits, as shown in Figure 1. Sur-prisingly, the lithiation process consists of 3 peaks while the delithiation process consists of 4 peaks. Moreover, the 3rd delithiation peak does not appear in sequential order relative to the other delithiation peaks. To reveal the correspondence between the peaks and specific re-actions, ex situ cross-sectional TEM, electron diffraction, Raman spectroscopy, and XPS are currently being used. In addition to characterizing the lithiation of RuO2, we have also built full battery devices that include a lithiated Si anode, a lithium phosphorous oxynitride (LiPON) electrolyte, and RuO2 cathode. Figure 2 shows the cycle performance of the microbattery at a rate of C/10. It could deliver a highly reversible capacity of approximately 150 µAh cm-2 µm-1 after 100 cycles, which is still 2.5 times higher than commercial CYMBET microbatteries. Ongoing work is focused on improving the cyclability of the RuO2 and silicon anodes through stress engineering, as well as improving the volumetric capacity through process improvements. These initial results suggest a promising route towards IC integratable batteries for on-chip power delivery."
Kinetic Study of the Reversible Lithiation in Si Thin Film Anodes,"Among all the known anode materials for Li-ion batter-ies, Si is a promising candidate for applications in CMOS- compatible microbatteries. It has extraordinarily high capacities (8375 Ah/cm3, 3579 Ah/kg), which is a result of the unique alloying mechanism during lithiation that involves bond breakage and a series of formation of new short-range structures. The reversible lithiation of Si anodes (Figure 1, highlighted) has not been exten-sively studied, and there have also been debates over whether it is a diffusion process or a phase-transition process. Here we adopt the potentiostatic technique to study the reversible phase transitions that occur in the second and subsequent lithiation cycles.It was found that there is always a peak in the current vs. time curve under desirable potentiostatic test conditions in the reversible lithiation regime (Figure 2). The existence of the peak suggests there is phase transition in the reversible lithiation, rather than pure diffusion where current should decrease monotonically with time. The time at which the peak occurs (tpeak) increases with the applied potential, which indicates slower kinetics for the phase transition. Kinetic parameters could be extrapolated from the current vs. time curves upon modeling and fitting."
Modeling Discharge Pathways in Li-O2 Batteries to Optimize Capacity,"Li-O2 batteries offer the possibility of storing twice the gravimetric energy density of Li-ion batteries. Li-O2 batteries operate by reacting oxygen with lithium ions in a non-aqueous solvent to form Li2O2 on a conductive cathode material. However, Li2O2 has poor electronic conductivity and passivates the electrode area. Achiev-ing high capacity requires careful attention to Li-O2 dis-charge mechanisms in order to optimize cathode void space filling by Li2O2.Li-O2 discharge occurs by two competing mechanistic pathways which are responsible for two possible morphologies of Li2O2 discharge product. The surface pathway involves two consecutive electron transfers to form a ~10 nm thin film of Li2O2. The solvent pathway involves the solvation of the reaction intermediate Li+-O2-, which then reacts in solution to form ~100 nm in diameter toroids of Li2O2. Since toroids allow for greater volumes of Li2O2 to form with less electrode area coverage, toroids are preferable to maximize capacity. However, the exact dependence of each pathway on different discharge conditions and solvent properties to promote toroid formation is not fully understood.Rotating ring-disk electrode (RRDE) experiments were performed to understand these pathway trends. A rotating rod creates convection currents that sweep reactants to the central disk electrode (Figure 1). Li2O2 film and soluble Li+-O2- are formed at the disk. Soluble Li+-O2- is swept to the ring electrode and oxidized, providing a measure of the relative size of the solvent pathway. By comparing ring and disk currents, the separate contribution of each discharge pathway can be determined.We then developed a model based on nucleation and growth of the Li2O2 film to explain potentiostatic discharge curves collected from RRDE experiments under different discharge conditions, such as varying solvent water content (Figure 2). The model demonstrates that high Li+-O2- solvent solubility inhibits the surface pathway and that this effect is primarily responsible for toroid promotion."
Multi-cell Thermogalvanic Systems for Harvesting Energy from Cyclic Temperature Changes,"Technologies for the Internet of Things (IoT) are be-ing developed. An IoT network consists of large quan-tities of networked sensors that are often in remote or difficult to access locations, which drives the need for self-powered systems. Here, we come up with two types of multi-cell thermogalvanic systems that gener-ate electrical power through temperature cycles. The dual-temperature, dual-stack, self-powered electrochemical system is depicted in Figure 1. This dual-temperature system uses two identical electrochemical stacks, which can be a single battery or multiple batteries connected in series; however, each electrochemical stack is held at a different temperature. On the other hand, a single-temperature system works similarly, with the electrochemical stacks having similar operating potentials but oppositely signed temperature coefficients. Its operation is illustrated in Figure 2. Both systems can harvest energy from temperature cycles.We have tested both dual-temperature systems and single-temperature systems with different cathode/anode materials, load resistances, and frequencies of temperature cycles. The largest energy conversion efficiency was obtained from the dual-temperature experiment with two homemade LiCoO2/Li coin cells in which the cathodes with composition Li0.85CoO2 were cycled between 20 °C and 50 °C. The loads were two 100Ω resistors. The current is shown in Figure 3, and the efficiency was calculated to be 0.22%. This value is comparable to the efficiency obtained using charging-free thermally regenerative electrochemical cycles (TRECs), thermocapacitive cycles and ionic thermoelectric supercapacitors, but with more flexibility of material selection. In the meantime, we have also tested two single-temperature systems with four LiV2O5/Li-Al and three LiCoO2/Li cells, and one LiMnO2/Li-Al and one LiV2O5/Li-Al cell, respectively. Although the efficiency and power were still limited, they confirmed the feasibility of this concept. These systems can be further optimized by using materials with higher temperature coefficients and decreasing internal resistance at the same time."
Ising-Model-Based Computation by using Block Copolymer Self-assembly,"Directed self-assembly of block copolymers can gener-ate complex and well-ordered nanoscale patterns for lithography. Previously, self-consistent field theory has been commonly used to model and predict the block co-polymer morphology resulting from a given template. In this work, we map block copolymer self-assembly onto an Ising model using two-dimensional post lattice template. We describe a simple and fast Ising-model-based simulation method for block copolymer self-as-sembly. With the Ising lattice setup, we demonstrate Ising-model-based logic gates.Figure 1 shows a diagram of the Ising lattice setup. To define the Ising lattice, we used a post lattice template with horizontal and vertical pitch equal to the equilibrium block copolymer periodicity, L0. After block copolymer processing, we defined a binary state, +1 or −1, between each adjacent pair of posts. We assigned +1 to a state when two adjacent posts were connected by a block copolymer structure, and −1 otherwise. The Ising Hamiltonian is given bywhere J’s and h’s were assumed to be independent of lat-tice location. We calculated the minimum Hamiltonian configuration using simulated annealing and compared the simulation results with previously reported results.To perform Ising-model-based computation, we encoded Boolean operations into the ground states of Ising lattices by designing specific Hamiltonians. Figure 2 shows a template design for a buffer where a boundary was defined by incommensurate double posts. Inside the boundary, an input state and an output state were defined. Prior to block copolymer processing, the input state was determined by the orientation of double posts while the output state was undetermined. After block self-assembly, the output state was set equal to the input state, performing the buffer operation."
Initiated Chemical Vapor Deposition (iCVD) of Cross-linked Polymer Films for the Directed Self-assembly of Block Copolymers,"Directed self-assembly (DSA) of block copolymer (BCP) thin films is a promising approach to enable next-genera-tion patterning at increasingly smaller length scales. DSA uses a combination of physical and chemical constraints to force the BCP domains to self-assemble with the de-sired orientation with respect to the substrate. Physical constraints, such as holes and trenches, are formed using conventional lithographic techniques. Chemical con-straints, or wetting layers, are thin films that are either sandwiched between the BCP film/substrate interface or coated on top of the BCP film. These wetting layers ensure the pattern formed upon self-assembly has the appropriate orientation with respect to the substrate. Controllable chemistry combined with facile processing is key to the integration of these wetting layers. Here, we demonstrate that initiated chemical vapor deposited (iCVD) polydivinylbenzene (pDVB) ultra-thin films can direct the self-assembly of poly(styrene-block-methylmethacrylate) (PS-b-PMMA). iCVD allows for the simultaneous synthesis and formation of polymer thin films via a surface free radical polymerization. Additionally, methyl radicals formed at increased filament temperatures can change the chemical structure of the growing pDVB film in situ. By tuning the degree of backbone methylation, we systematically changed the wetting properties of iCVD pDVB from weakly PMMA preferential to complete PS preference. Conformal coatings of weakly preferential iCVD pDVB films on topographical line and space patterns produced self-assembled BCP films with both perpendicular orientation and long-range alignment (Figure 1a and 1b). Current research efforts aim to use iCVD pDVB films to enable contact hole shrinkage. To minimize the diameter of the template hole, the BCP should assemble with a central, cylindrical PMMA domain. Preliminary experiments examined the effects of strongly PS-preferential, conformal iCVD pDVB films on a contact hole shrink template. The dark spot within each hole in Figure 1c corresponds to the PMMA domain. These results indicate that iCVD pDVB films are a viable method to enable contact-hole shrinkage."
Geometry-dependent Properties of Synthetic Monolayer MoS2,"Two-dimensional transition metal dichalcogenides (2-D TMDs) have shown great promise to be an ideal candi-date for post-silicon technology. Their atomic thickness-es and large carrier effective masses can offer excellent electrostatic gate control, a reduced source-to-drain leakage current, and a higher on-current in the ballistic regime, potentially enabling ultra-scaled devices, tunnel field-effect transistors, and ballistic transistors. Howev-er, the intricacy and diversity of the structural defects in 2-D TMDs significantly affect their electrical and optical properties, in either beneficial or detrimental ways. In the case of monolayer MoS2, several challenging issues including Femi level pinning at metal/MoS2 interface, unintentional n-type doping, and carrier scatterings, non-radiative excitonic recombinations, etc., have been attributed to a considerable amount of sulfur vacancy in monolayer MoS2. On the other hand, specific types of defects, if controlled carefully, also offers the access to engineer the nature of monolayer MoS2, such as channel polarity modification for realization of low-power MoS2–based CMOS integrated circuits, exciton reservoirs to prolong the excitonic lifetime for high-performance optoelectronic and photonic devices.This work explores the correlation between the domain geometries and the presence of different types of defects in monolayer MoS2 synthesized by chemical vapor deposition through transport and spectroscopy measurements. We show that the shapes of MoS2 domain can modulate the photoluminescence intensity and work function of MoS2 monolayers and the threshold voltage in the MoS2 field-effect transistors. Based on a two-defect-state model, the geometry-modulated behavior can be explained. This work not only offers a strategy to engineer the nature of MoS2 from the synthesis perspective, but also pave a path to realize low-power MoS2 CMOS integrated circuits."
Atmospheric Microplasma Sputter Deposition of Interconnects,"We have preliminarily developed an apparatus that allows for the continuous, direct writing of interconnect-quality conductive lines. An atmospheric-pressure microplasma obviates the need for a vacuum while allowing for fine resolution imprints. We tested and characterized a novel focusing mechanism in which collisions with the working gas are harnessed to transfer electrostatic force to neutral sputtered atoms. This method compresses the deposit’s width in one dimension while expanding its length in the perpendicular dimension. We find that for an ideal set of parameters, the imprint is narrower than the sacrificial sputtering target (i.e., 9 µm wide imprint from a 50 µm diameter target). Other sets of parameters lead to other results, as computer simulation predicted, ranging from an unfocused spot 400 µm in diameter to a narrow line with 20:1 compression in the direction of focus, i.e., width, and 20:1 expansion in length (Figure 1), as compared to the unfocused spot.The microstructure of the deposit is of particular interest. As is typical of sputterers, the deposit could be smooth (55 nm roughness), and the resistivity can be as low as 1.1 µΩ·m (with no annealing). However, the resistivity greatly depends on the microstructure, which in turn depends on the deposition conditions. It is well known that sputtering at high-pressure results in a grain structure, as the early deposits shadow parts of the bare substrate, keeping sputtered material from fully coating the substrate. Traditionally, vacuum sputtering prevents this problem by allowing the sputtered material to impact the substrate normal to the surface; however, we sputter at atmospheric pressure, and thus, the sputtered material is redirected by random collisions. In our case, we use a combination of directed gas flow and electrostatic forces to prevent this shadowing effect (Figure 2)."
Influence of TMAH Development on Niobium Nitride Thin Films,"Patterning of superconducting thin films at the na-noscale has enabled numerous technologies used in signal detection and digital circuits. For instance, super-conducting nanowire single photon detectors (SNSPDs) and, more recently, the nanocryotron (nTron) both make use of the ability to pattern niobium nitride films at dimensions < 100 nm. Electron beam lithography of these devices often employs the negative tone resist hy-drogen silsesquioxane (HSQ) due to its high resolution and superior line edge roughness. Development of HSQ and adhesion promotion of HSQ to the substrate sur-face are both facilitated by tetramethylammonium hy-droxide (TMAH), making it an integral chemical in the fabrication process. However, despite the prevalent use of TMAH in patterning superconducting films, its influ-ence on the film itself has yet to be fully studied.Here we have investigated the effects of exposing NbN thin films to 25% TMAH. We show that TMAH modifies the surface chemistry of the film by reacting with the NbN to form niobium-based clusters, which are visible by scanning electron micrograph inspection (Figure 1). In addition to thinning the overall NbN film and reducing its critical temperature, the formation of niobium clusters creates a barrier to reactive ion etching in CF4, threatening the lithographic pattern transfer (Figure 2). While characterization such as FTIR has been employed to identify the compounds created by this reaction, future work is needed to study the mechanism through which the hexaniobate species interfere with the reactive ion etch chemistry."
Roll-to-Roll Transfer of Conductive Graphene Sheets,"Graphene technology has been widely explored to pro-duce large sheets of conductive film to facilitate the manufacturing of flexible transparent photovoltaics. Monolayer-thick graphene has 97% transmittance in the visible regime and outstanding mechanical and electrical properties: that makes graphene suitable for transparent electrodes in order to replace the current state-of-the-art ITO electrodes, which are less flexible and are limited by the low indium supply on earth. However, scaling up the graphene manufacturing is tricky since it is typically grown on copper foils by chemical vapor deposition (CVD), and therefore, an ad-ditional transfer step is required to insert the graphene sheet into practical devices. The success of the transfer process is critical for the performances and the scal-ability of the graphene film. Given the compatibility with the manufacturing processes in organic and flexible electronics, we explore roll-to-roll (R2R) to enable the deployment of large area graphene on plastic substrates. We investigate how to avoid defects and fractures in the graphene film upon transfer. We scan over several options in order to figure out how the interplay of adhesion forces between the graphene and the host substrate works out. These investigations will advance the progress of the application of graphene in future flexible electronics."
Additive Manufacturing of High-temperature Compatible Magnetic Actuators,"Various MEMS devices require large displacement and large force actuation to be efficient, such as miniature pumps. Magnetic actuation delivers large displacement and large force in a compact form factor. Additive man-ufacturing has recently been explored as a processing toolbox for MEMS; researchers have reported additive-ly manufactured microsystems with performance on par or better than counterparts made with standard microfabrication. In this work, miniature actuators are printed in pure Nylon 12 using the fused filament fab-rication method where a thermoplastic filament is ex-truded from a hot nozzle to create layer by layer a solid object. The actuators have embedded magnets that are not demagnetized by the heated nozzle (@ 250 °C) while being sealed in place midstream in the printing process.We have demonstrated the first miniature, additively manufactured, monolithic magnetic actuators compatible with high temperature (>200 °C) operation (Figure 1). The displacement of a 150 μm-thick, single-layer membrane actuator is characterized by various DC coil bias voltages, resulting in a maximum membrane displacement of 302 μm with 20V DC applied to the driving coil; in addition, the magnetic force is proportional to the square of the current drawn by the coil as expected from theory (Figure 2)."
“Soft” Epitaxy in DNA-Nanoparticle Thin Films,"The programmability of DNA makes it an attractive structure-directing ligand for the assembly of nanoparticle superlattices that mimic atomic crystallization. However, synthesizing multilayer single-crystals of defined size remains a challenge. This work studies growth temperature and interfacial energetics to achieve epitaxial growth of single crystalline nanoparticle thin films over arbitrarily shaped 500 × 500 μm2 areas on lithographically patterned templates. Both surface morphology and internal structure are examined to provide an understanding of particle attachment and reorganization (Figure 1).Importantly, these superlattices utilize a “soft,” elastically malleable building block, resulting in significant strain tolerance when subjected to lattice mismatch. Calculations of interaction potentials, small-angle X-ray scattering data, and electron microscopy images show that the oligomer corona surrounding a particle core can deform to store seven times more elastic strain than atomic films. DNA-nanoparticles dissipate strain both elastically through coherent relaxation of mismatched lattice parameter and plastically (irreversibly) through formation of dislocations or vacancies (Figure 2). Additionally, the DNA cannot be extended as readily as compressed, and thus, the thin films exhibit distinctly different relaxation behavior in the positive and negative mismatch regimes. These observations provide a more general understanding of utilizing rigid building blocks coated with soft compressible polymeric materials to control nano- and microstructure through “soft heteroepitaxy.”"
Aligned CNT-based Microstructures and Nanoengineered Composite Macrostructures,"Materials comprising carbon nanotubes (CNTs), such as hierarchical nanoengineered advanced composites for aerospace applications, are promising new materials thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and hierarchical nanoengineered advanced composites macrostructures hybridized with aligned CNTs.Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests. Here the polymer is typically an unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm2 Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus, the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), transmission electron (TEM) microscopy, 3-D TEM, and X-ray computed tomography (CT) to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive mechanical property testing and modeling are underway, including 3-D constitutive relations and fracture toughness.Nanoengineered hierarchical composites hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies (“nanostitching”), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup. Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Extensions of the CNT-based architectures to ceramic-matrix nanocomposites and towards multifunctional capabilities are being developed, including structural health monitoring and deicing."
Electrospray-printed Physical Sensor,"Electrospray deposition (ESD) has recently gained at-tention as a manufacturing technology to develop novel nanostructured composites to produce low-cost micro- and nano-devices. ESD is also a remarkably versatile printing technique due to its capability to create ultra-thin films made from a great variety of liquid feedstock (e.g., suspensions of polymeric, dielectric, metallic par-ticles) that can be doped with organic nanostructures to modulate the physical properties of the imprint. No-tably, the resulting nanoreinforced composites might show enhanced transduction, which, in combination with printing on flexible substrates, might be relevant for exciting applications such as wearable biomedical devices.This project aims to develop an additively manufactured, low-cost, flexible physical sensor based on an ultrathin nanocomposite film doped with functionalized carbon nanostructures. The Taylor cone on an electrospray emitter fed with nanocomposite feedstock is shown in Figure 1a, while an electrospray-deposited imprint on a substrate is shown in Figure 1b. Essentially, this project is divided in (i) down-selecting and optimizing the formulation of the liquid feedstock, (ii) optimizing the fabrication of the ultrathin (~100 nm) nanostructured composite, and (iii) demonstrating a flexible physical sensor with transducing component made of the optimized nanostructured composite (see Figure 2)."
Empirical Modeling of Copper Semi-additive Electro-chemical Plating,"Semi-additive electro-chemical plating (ECP) is a common process for fabricating copper interconnects in many advanced packaging technologies, such as Wafer Level Integrated Fan Out (InFO) packaging. While cost efficient, this process suffers from thickness variations in the height of the plated copper. The most significant of these variations are layout dependent, where areas with dense interconnects plate slower than sparse areas (Figure 1). If left unchecked, these variations can lead to significant complications in later stages of the fabrication process, and ultimately to decreased electrical performance of the final packaged device. Previously, there were limited methods for predicting these variations, and foundries had to rely on experimentally determining which layouts would perform acceptably. Recently, we have developed a model that predicts these variations and allows errors to be predicted and corrected without the need to first fabricate the layout in question. While a model based on fundamental physics could in principle be developed to predict these variations, we instead develop an empirical model based on experimental data. This approach is well suited for many industrial applications, as empirical models can often be developed more quickly, without a significant loss of accuracy, and can be rapidly tuned or adapted to accommodate effects whose causes are uncertain. Our ECP model is divided into four stages as summarized in Figure 2. First, the effective pattern density of each point on the layout is determined with a learned spatial filter. These pattern densities are then mapped to effective conductances using a ratio-of-polynomials approximation. Next, these conductances are masked with the original layout, as photoresist prevents copper from growing in unwanted areas. Finally, the current flowing through each point of the wafer is solved for, and these currents are then converted to the plating height at each point in the layout."
150 °C Copper Bonding Technology with Graphene Interlayer,"Bonding technology plays a significant role in elec-tronic packaging as it provides physical and electrical connections between semiconductor chips. Reliability of bonding joints affects the energy consumption and speed of an electronic system. Hence, it is important to have a reliable bonding technology. Copper (Cu) bonding technology is one of the most frequently-used bonding technologies nowadays. However, two critical issues have been limiting the reliability of lead-free Cu bond-ing technology: high bonding temperature (~260 °C) and aging degradation.We have devised a graphene-based Cu bonding technology that is of low bonding temperature and high reliability. By integrating nanoscale graphene/Cu composite on the Cu substrate prior to thermocompression bonding, Sn-Cu joints can be fabricated at a bonding temperature as low as 150 °C, which is the lowest reported value to date for Cu bonding technology. Specifically, we electrochemically deposit a layer of Cu nanocone array on the Cu substrate and cover it with a graphene sheet, prior to the bonding process. When subjected to heat, microscale Sn solder deforms and replicates the Cu nanocone array morphology, and hence transforming into nanoscale Sn. Compared to microscale Sn, nanoscale Sn has much lower melting points and facile surface diffusion. This phenomenon effectively contributes to the low bonding temperature observed in our bonding technology. The presence of the graphene layer prevents the formation of Cu-Sn intermetallic compounds thus significantly slows down the aging degradation. With the advancement in graphene synthesis and transfer technology, we anticipate the graphene-based Cu bonding technology presented in this work can be integrated into the existing commercial Cu bonding technology for industrial applications in the near foreseeable future."
Chemical Vapor Deposition of Multiple Transition Metal Disulfides in One Synthesis Step,"Recently, transition metal disulfides (TMD) have received tremendous attention due to their exceptional optical and electrical properties. Many techniques have been explored to obtain monolayer TMD and chemical vapor deposition (CVD) synthesis using transition metal oxide, and chalcogenide solid precursors is the most common method used in laboratories now. However, the quantity of solid precursors used is usually surplus giving rise to chemical reactions between precursors in each of their crucibles, as a result of precursors’ diffusion at growth temperature. Hence, a CVD setup is normally dedicated for the growth of only one type of TMD to avoid cross-contamination (except for hetero-structures synthesis), and it is impossible to grow multiple monolayer TMD in one synthesis step. Here, we report a new technique to synthesize MoS2 and WS2 monolayer films in one CVD process. We first disperse a minuscule amount of metal oxide precursor on targeted substrates, which were then loaded to the furnace in slanting position, rather than horizontal, followed by a sulfur annealing to concurrently grow monolayer MoS2 and WS2 on separate substrates. The synthesized TMD films exhibit good properties as confirmed by Raman, PL, XPS, STEM analyses, and electrical measurements."
"A Nanofabricated, Path-separated, Grating Electron Interferometer","Recent progress in focused-ion-beam (FIB) technol-ogy has enabled the fabrication of electron optical elements such as zone-area plates, phase plates, and beamsplitters. These nanofabricated elements can be used to perform Zernike phase-contrast imaging, ho-lography and beam aberration correction in a conven-tional transmission electron microscope (TEM). We have fabricated a grating-Mach-Zehnder-electron-in-terferometer, using FIB milling of a single-crystalline silicon workpiece. As shown schematically in figure 1(a), the interferometer uses two thin layers of silicon as dif-fraction gratings; the first to split the incident electron beam, and the second to recombine two of the diffract-ed beams. The gap between the gratings in our interfer-ometer was 20 µm. Fabrication of the gratings from a single crystalline silicon workpiece ensures alignment and precise positioning. We obtained a rotational align-ment of ~100 µrad and a grating positioning accuracy of 100 nm. Figure 1(b) is a scanning electron micro-graph of this interferometer. We inserted this interferometer in the sample holder of a 200 kV TEM (JEOL 2010F). We used an electron beam with a diameter of 240 nm on the first grating and convergence semi-angle of 4 mrad in our experiment. As shown in figure 2(b), when imaging the second grating (figure 2(a), sample z-height z1) at high-resolution (Ψ0), we obtained a lattice-resolved image of silicon. As we raised our sample holder z-height to move the imaging plane below the second grating (z2-z5), the first-order diffracted beam from this grating (Ψ0g) moved closer to the first-order diffracted beam from the first grating (Ψgg), and the two beams overlapped 20 µm below the second grating (z6). Figure 2(c) is a high-resolution image of the overlapping beams, showing interference fringes of period 0.32 nm, which was expected from the interference of first-order silicon diffracted beams. This interferometer could be used to perform electron holography in any TEM, as well as interaction-free imaging using the Elitzur-Vaidman scheme."
A Scheme for Low-dose Imaging via Conditional Sample Re-illumination,"Recently, several electron-beam-based low-damage im-aging schemes for radiation-sensitive samples (such as proteins and biomolecules) have been investigated. It is now possible to incorporate a Mach-Zehnder inter-ferometer (MZI) in a standard transmission electron microscope (TEM) to perform Elitzur-Vaidman Inter-action-free imaging (IFI). We are theoretically inves-tigating the performance of an MZI-based IFI with a Poisson source. We combined IFI with a conditional re-illumination scheme that reduced the probability of imaging errors at low illumination doses.As a first step, we considered imaging of purely black-and-white pixels. As shown in figure 1, we considered two schemes: classical and IFI, with various imaging detectors. We quantified error as the probability of incorrectly inferring the transparency of a pixel (Perr), and damage as the mean number of electrons that scatter off a black pixel (ndamage), respectively. At the start of our calculations, we assumed a prior probability q of a given pixel being black. Then, we found expressions to update q based on the electron detection statistics, assuming a Poisson beam with mean λt. If the value of q was within a pre-defined minimum acceptable error threshold ∈, we made an inference on whether the pixel was black or white. If this condition was not met, we re-updated q using a second round of detection statistics. This process was repeated a maximum of Nmax times. Figure 2 shows the results of imaging utilizing conditional re-illumination, for both classical and IFI. These results were calculated with Nmax=1 (circles with dotted line) and Nmax=50 (crosses with dashed lines) illuminations. For both schemes, conditional re-illumination offered a reduction in ndamage at 50 illuminations as compared to single-stage illumination. For classical imaging, Ndamage was reduced to 1, and for IFI, ndamage saturated to 0.67, at Nmax=50.We are now working on extending these calculations to semi-transparent samples, as well as implementing this illumination scheme in a scanning TEM."
Towards Dislocation-free GaN,"The performance of advanced GaN-based electronics and optoelectronics can rely heavily on the structural quality of the epilayer used in its fabrication. The lay-er’s characteristics, such as dislocation density or sur-face roughness, are largely inherited from the initial GaN growth. Due to the limited availability and the cost of high-quality bulk GaN substrates, heteroepitaxy of GaN on foreign substrates such as Al2O3, SiC, and Si is conventionally used. The lattice and thermal-expan-sion-coefficient mismatch of these substrates to GaN unavoidably lead to the formation of dislocations, as well as potential cracks and wafer bow. In addition, the majority of the substrate material is usually removed from state-of-the-art devices to lower the thermal resistance of the packaged devices and improve performance. The removal of GaN devices from bulk/foreign substrates is very challenging and is an ongoing subject of research. Existing removal processes involving photoelectrochemical etching, mechanical spalling, and laser interface decomposition suffer from slow processing speed and/or significant surface roughening and cracking, limiting the process yield and practicality of substrate reusing.Recently, we discovered that the epitaxial registry of adatoms could be determined by the underlying substrate remotely without direct contact with the substrate, but through a narrow gap defined by monolayer graphene. Therefore, homoepitaxial growth can be performed remotely through the single-atom-thickness gap, with the dislocation density of the epitaxial thin film at the same level as the high-quality substrate. In addition, because of the van der Waals interaction at the graphene interface, the epitaxial thin film can be precisely and rapidly exfoliated from the substrate, demonstrating the atomic flatness at the released surface mimicking the morphology of graphene surface. We performed the remote epitaxy of GaN on GaN/sapphire substrate with monolayer graphene as an interlayer to demonstrate high-quality, low dislocation density GaN thin films. We obtained GaN epilayer with material quality identical to the GaN/sapphire substrate in terms of surface morphology and dislocation density. We further exfoliated the GaN epitaxial thin film from the substrate achieving free-standing GaN of 300nm thick.Ultimately, we will develop the process of GaN remote epitaxy on bulk GaN substrate with minimal defects, enabling the GaN-based electrical and optoelectronic devices approaching intrinsic performance without the limitation from material quality. On the other hand, the cost of such high-performance devices will be significantly reduced since expensive substrates will be reused."
Fabrication of Small-pitch Gratings for Smith-Purcell Radiation from Low-energy Electrons,"Swift-moving electrons carry evanescent near-field, which can be coupled to far-field radiation when the electrons move closer to a periodic structure and in parallel to the periodic structure plane. This effect was named after Smith and Purcell, following their first experimental demonstration of the effect. The wavelength of Smith-Purcell radiation depends on the grating pitch and the electron energy. Here, we demon-strate Smith-Purcell radiation in the optical regime by using gratings with 50-60 nm pitch and electrons with 1.5-6 keV kinetic energy. These results have potential applications in tunable nanoscale light sources.Our gratings were fabricated on gold-coated silicon substrates. The 200-nm-thick gold coating layer was used to suppress cathodoluminescence from silicon. The grating patterns were defined using electron beam lithography in PMMA resist, followed by 0 °C cold development in 3:1 IPA:MIBK. 20 nm gold was then deposited via electron-beam evaporation and lifted-off in hot NMP. Figure 1 shows an SEM image of a 50-nm-pitch grating.To measure Smith-Purcell radiation, the grating samples were mounted inside a modified SEM with an optical attachment to collect the radiated light and measure its spectrum. Electrons with 1.5-6 keV kinetic energy were used to induce the Smith-Purcell radiation. Figure 2 shows the measured Smith-Purcell radiation spectra from a 50-nm-pitch grating using electron beams with different kinetic energies. The peaks of the radiation spectra match well with the theoretical predictions (vertical dashed lines). We demonstrate the Smith-Purcell radiation wavelength decreases as we increase the electron kinetic energy or decrease the grating pitch."
Remote Epitaxy through Graphene for Two-dimensional Material Based Layer Transfer,"Van der Waals epitaxy (vdWE) has gained great inter-est for crystalline growth as it substantially relaxes the strict lattice matching requirements in conventional heteroepitaxy and allows for facile layer release from the vdWE surface. In recent studies, vdWE was inves-tigated on two-dimensional (2-D) materials grown or transferred on arbitrary substrates, with the primary notion that the 2-D material is the sole epitaxial seed layer in vdWE. However, the underlying substrate may still play a role in determining the orientation of the overlayers since the weak vdW potential field from 2-D materials may barely screen the stronger potential field from the substrates. Here, we reveal that the epitaxial registry of adatoms during epitaxy can be assigned by the underlying substrate remotely through 2-D materials by modulating the interaction gap between the substrate and the epilayer. Our study shows that remote epitaxial growth can be performed through a single-atom-thick gap defined by monolayer graphene at the substrate-epilayer interface. Simulations using density functional theory (DFT) prove that remote epitaxy can occur within a ~9 Å substrate-epilayer gap. We experimentally demonstrate successful remote homoepitaxy of GaAs(001) on GaAs(001) substrates through monolayer graphene (Figure 1). Characterization by high-resolution scanning transmission electron microscopy (HRSTEM) confirms single crystalline growth of GaAs film through graphene with an interaction gap of 5 Å below the critical limit outlined by the simulation. The concept of remote homoepitaxial growth is further extended to other compound semiconductors such as InP, GaP, GaN, as well as functional oxides, SrTiO3, and fluoride material systems, LiF (Figure 2). Following the growth, the single-crystalline films are rapidly released from the vdW surface of graphene to provide large-scale, single-crystalline films. This concept, here termed 2-D material based layer transfer (2-DLT), suggests a universal method to copy/paste epitaxial films of any material systems based on the underlying substrates through 2-D materials then rapidly release and transfer to substrates of interest. The potential to reuse graphene-coated substrates suggests 2-DLT will greatly advance non-Si electronics and photonics by displacing the high cost of non-Si substrates."
Controlling Concentration and Nature of Oxygen Defects in Layered Cuprate-based Materials by Electrical Bias,"Both the nature and concentration of oxygen defects in oxide materials can have a significant impact on their physical and chemical properties, as well as key interfa-cial reaction kinetics such as oxygen exchange with the atmosphere. Most commonly, the desired oxygen defect concentration, or equivalently oxygen nonstoichiom-etry, is attained in a given material by controlling the oxygen partial pressure and temperature in which it is equilibrated or annealed. This approach, however, is lim-ited by the range of oxygen partial pressures readily ex-perimentally achievable and requires knowledge of the applicable defect chemical model. In this study, we fine-tune oxygen defect concentrations in promising rare earth cuprate (RE2CuO4: RE = rare earth) solid oxide fuel cell (SOFC) cathode materials by application of electrical potentials across a yttria-stabilized zirconia (YSZ) supporting electrolyte. These layered perovskites can incorporate both oxygen interstitials and vacancies, thereby broadening the range of investigations. Here, we show a strong correlation between oxygen nonstoichiometry values (which are determined by in situ measurement of chemical capacitance) and oxygen surface exchange kinetics (which is inversely proportional to the area-specific-resistance). Both types of oxygen defects – interstitials and vacancies – dramatically enhance surface kinetics. These studies are expected to provide further insight into the defect and transport mechanisms that support enhanced SOFC cathode performance."
Coherent Soft X-ray Imaging of Magnetic Nanotextures,"The ability to image the nanoscale structure of materi-als with tunable magnetic textures is pivotal for the de-velopment of low-power and nonvolatile data storage technologies. Soft X-ray imaging has emerged in the last decade as a powerful and accurate methodology to resolve the bulk domain structure of several magnetic materials — magnetic multilayers, buried interfaces, or skyrmion lattices — as well as nanoelectronic devices under operating conditions. Soft X-ray imaging relies on two main requirements: (i) the ability to focus a collimated X-ray beam on a spot the size of a few tens of nm and (ii) the ability to scan the focused X-ray beam with nm precision. We have commissioned a new soft X-ray nanofocusing setup installed at beamline CSX-1 of the National Synchrotron Light Source II. The schematics of this setup are shown in Figure 1. A key element is the Fresnel zone plate (inset D), which acts as a diffractive phase mask to focus X-rays to a 70-nm spot at the sample, and is fabricated using e-beam lithographic tools. The beam spot can be moved with the aid of piezo-based nanopositioners (inset C), which translate the X-ray optics while keeping the sample in a fixed position. Diffracted X-rays are collected with a CCD camera in the far field (~30 cm from the sample). The resulting magnetic scattering intensity encodes local antiferromagnetic strength and can be acquired in less than a second. By scanning the X-ray beam across the sample, we are able to probe the spatial distribution of antiferromagnetic order.We applied this new method to the study of antiferromagnetic rare earth NdNiO3. In particular, and for the first time, we unveil the inhomogeneous nature of the spin-ordered ground state (inset B). Furthermore, we identify the spatial distribution of antiferromagnetic domains and show that it follows a scale-free distribution. Our future focus is to extend our studies to the imaging of nanoscale magnetic textures in antiferromagnetic spintronic materials and devices, including in operando studies as a function of applied current."
Electro-Chemo-Mechanical Studies of Perovskite-Structured Mixed Ionic-electronic Conducting SrSn1-xFexO3-x/2+δ,"High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive for conversion of fuels to electricity. Reduced operating temperatures, desirable for reduced costs and extended operation, however, result in significant losses in efficiency. This loss has been traced primarily to slow cathode surface reaction kinetics. In this work, we extend previous studies on the promising mixed ionic and electronic conducting perovskite-structured SrTi1-xFexO3-x/2+δ (STF) materials system whose exchange kinetics were correlated with the minority electron charge density by replacing Ti with Sn, due to its distinct band structure and higher electron mobility. Oxygen nonstoichiometry and the defect chemistry of the SrSn1-xFexO3-x/2+δ (SSF) system were examined by thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in the STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure, associated with the substitution of Ti by Sn, impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure, while surface kinetics were examined using AC impedance spectroscopy. The electrochemical properties of SSF were found not to differ significantly from the corresponding composition in STF. Though slightly shifted by the larger size of Sn, the defect equilibria and the cathode area specific resistance differed only in a limited way from that in STF. This was attributed to properties being largely dominated by Fe and not by the substitution of Ti with Sn. However, due to asymmetry in the crystal structure caused by the larger size of Sn, both thermal and chemical expansion coefficients of SSF35 were found to be around 20% and 10% lower than those of STF35, thus making SSF35 much more chemo-mechanically stable in SOFC operating conditions."
Experimental Characterization and Modeling of Templated Solid-state Dewetting of Thin Single-crystal Films,"Solid-state dewetting is a physical phenomenon that dis-integrates a continuous film into islands when the film is heated above a characteristic dewetting temperature but kept well below its melting temperature. It is driven by surface energy minimization and mediated via sur-face diffusion of atoms. Solid-state dewetting has been thought of as an issue in microelectronics, however, it has also demonstrated its potential as a simple patterning method that can be used to generate a complex and reg-ular array of micro- and nano-sized structures in a highly reproducible way [Figure 1a, reference 1]. It starts either from edges of the film or in the continuous flat region by forming a natural hole. Various instabilities that develop at retracting edges have been understood via modeling and experimenting over the past years, including “pinch-off,” “corner instability [reference 2],” and “Rayleigh-like in-stability’[reference 3].” The fingering instability as shown in Figure 1b, which is another instability that creates wire-like structures at retracting edges, is our current focus. Through experiments, we have found conditions that lead to the fingering instability and have learned that spacing between fingers can be controlled via templating of film edge. We have also found that controlling the period of the fingering process affects the kinetics of the fingering, and we have developed an analytical model that predicts a relationship between the retraction rate and finger period. This model agrees well with experimental results. Our increased understanding of the various instabilities at retracting edges can be used to design templates that will lead to specific complex structures during solid-state dewetting.However, before we can fully exploit our understanding of templated solid-state dewetting to make designed structures, we must understand natural hole formation in thin films. In polycrystalline films, grain boundary triple junctions facilitate hole formation in a well-understood way, but the formation of holes in single-crystal films (Figure 2) is not well understood. Studying this phenomenon is critical because holes create new edges from which the film retracts. Furthermore, thinner single-crystal films develop more natural holes per unit area, and the growth of these holes can come to dominate the overall reduction of film surface area. Unsuppressed natural hole formation interrupts edge retraction modes that were intentionally patterned to create a specific structure. If controlled, however, the formation of holes could be used to pattern periodic nanostructures that span large length scales, up to several centimeters. In parallel with studying the fingering instability, we are currently working with both Ni films on MgO substrates and Ru films on sapphire substrates to identify and understand the causes of natural hole formation in single- crystal films. By understanding these mechanisms, we aim to develop templated solid-state dewetting into a powerful and cost-effective method for producing nanostructures."
Optofluidic Real-time Cell Sorter for Longitudinal CTC Studies in Mouse Models of Cancer,"Circulating tumor cells (CTCs) play a fundamental role in cancer progression. However, in mice, limited blood volume and the rarity of CTCs in the bloodstream preclude longitudinal, in-depth studies of these cells using existing liquid biopsy techniques. We developed an optofluidic system (Figures 1, 2) that continuously collects fluorescently labeled CTCs from a genetically engineered mouse model (GEMM) for several hours per day over multiple days or weeks. The system is based on a microfluidic cell-sorting chip connected serially to an unanesthetized mouse via an implanted arteriovenous shunt. Pneumatically controlled microfluidic valves capture CTCs as they flow through the device, and CTC-depleted blood is returned back to the mouse via the shunt. To demonstrate the utility of our system, we profile CTCs isolated longitudinally from animals over four days of treatment with the BET inhibitor JQ1 using single-cell RNA sequencing (scRNA-Seq) and show that our approach eliminates potential biases driven by inter-mouse heterogeneity that can occur when CTCs are collected across different mice. The CTC isolation and sorting technology presented here provides a research tool to help reveal details of how CTCs evolve over time, allowing studies to credential changes in CTCs as biomarkers of drug response and facilitating future studies to understand the role of CTCs in metastasis."
Continuous Online Monitoring of Biologics Quality during Continuous Biomanufacturing using Micro/Nanofluidic System,"The growing trend in the biopharmaceutical industry is to adopt continuous biomanufacturing to reduce manufacturing cost and improve product quality. How-ever, several challenges must be solved. First, a reliable and efficient cell retention device is required. Currently, using a hollow fiber membrane is a widely adopted cell retention method in industry to maintain suspended cells in the bioreactor and remove biologics from the bioreactor. However, it suffers from membrane fouling/clogging due to cells and cell debris. Moreover, product recovery efficiency becomes significantly low over cul-tivation time, resulting in low manufacturing efficien-cy.Second, there is no robust online sensor for critical quality attributes, such as purity and binding affinity, during manufacturing to understand the real-time relationship between the critical quality attributes and bioprocesses. For example, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), size exclusion chromatography (SEC), and capillary gel electrophoresis (CGE or CE-SDS) are commonly used to check protein purity, but they offer only at-line/offline discontinuous analysis. In this context, we developed a novel micro/nanofluidic system to demonstrate continuous online monitoring of protein size distribution of cell culture supernatant during perfusion culture (Figure 1). The system consists of perfusion culture, online sample preparation, and detection of protein size distribution. To enable long-term perfusion culture, we used a membraneless microfluidic cell retention device. The cell retention is based on size-based cell sorting. Its high cell-concentration-capacity (>40E+6 cells/mL), scalability, long-term biocompatibility, and high product recovery efficiency have already been demonstrated. The online sample preparation consists of buffer-exchange, cell clarification, protein labeling, and denaturation. At the end of the system, the nanofluidic device continuously monitors protein size distribution. It has nanofilter array and supports continuous-flow size-based protein separation and concentration."
Electrokinetic Purification for DNA Analysis,"Rapid detection of ultralow-abundance pathogenic DNAs in complex clinical samples, which are often as low as <100 copies/ml (~0.1 aM), is critical for early diagnosis of infectious diseases. Despite the unprece-dented amplification capacity of the polymerase chain reaction (PCR), its detection sensitivity and specificity are limited by the ability to purify DNAs from clinical samples with minimal loss. Currently, DNA extraction relies on slow and labor-intensive spin column-based solid phase extraction, which introduces significant losses of DNAs during capture, elution, and final sam-pling, especially for ultralow-abundance samples.Ion-concentration-polarization (ICP)-based electro-kinetic trapping (ET) has attracted much attention in the past decade as a viable approach for the rapid concentration of DNAs and other biomolecules, with enrichment speeds of ten-thousand-fold in ∼10 minutes. Based on this technique, we report the direct enrichment and purification of DNAs in complex biological samples by pressure-modulated selective electrokinetic trapping (PM-SET). We showcase the utility of PM-SET in human serum that contains 60–80 mg/mL total serum protein and perhaps represents one of the most complex backgrounds for molecular detection. Through modulating the hydrostatic pressure applied to the ICP-based ET device, we demonstrate the selective trapping of DNAs (of high electrophoretic mobility) while the majority of background proteins (of low electrophoretic mobility) are simultaneously removed (Figure 1), achieving an enrichment factor of >4800 in 15 minutes for DNAs.With these advantages, we believe that PM-SET could potentially play an enabling role in developing lab-on-a-chip devices toward point-of-care diagnostics, on-site food and environment monitoring, and a variety of other applications in resource-limited settings."
Multi-parameter Cell-tracking Intrinsic Cytometry for Characterization of Single Cells,"Cells possess biochemical properties that require ex-trinsic tags, e.g., fluorescent dyes, for detection and biophysical properties, e.g., morphological, mechanical, electrical, and optical properties, which are intrinsic and do not require any labels. While extrinsic labeling techniques are highly specific to cell states, analyses of label-free biophysical properties are more suitable for applications, which require quick turnaround, and for analysis where biochemical labels for targeting cell states are not known. Development of single-cell biochemical analysis techniques with high sensitivity, throughput, and multiplexing capability has advanced understanding of complex biological systems and has established their presence in biological research labs and clinical practice. In contrast, development of sin-gle-cell biophysical analysis techniques is often limited to proof-of-concept due to the low-specificity nature of the intrinsic markers and the lack of approaches to combine multiple biophysical assays for high-dimen-sional biophysical phenotyping of single cells.To address this challenge, we propose a general approach to combine multiple biophysical measurements of single cells via optical tracking. To show specific implementation of this approach, we developed a microfluidic platform that measures up to five intrinsic markers of single cells, including size, deformability, and polarizability at three frequencies (Figure 1). We chose these intrinsic markers because each has been associated with important biological functions and proven useful for cell characterization, and they are rarely studied together. Cell tracking was demonstrated on the fully integrated platform, and multiple intrinsic markers of single cells were measured from cell samples treated with varying concentrations of actin polymerization inhibitor. An unsupervised dimensionality reduction technique, viSNE, was implemented to visualize the five-dimensional intrinsic marker measurements in two-dimensional visualization (Figure 2). Our analysis showed that an increase in number of intrinsic markers measured by our intrinsic cytometry platform resulted in an increase in classification accuracy of cell states induced by drug treatment."
Point-of-Care Biomarker Detection through Electronic Microfluidics,"Identification of protein biomarkers is a vital step for numerous biomedical applications including clinical diagnostics, monitoring, and treatment. However, tra-ditional blood analysis techniques require large sam-ple volumes and centralized laboratories with trained technicians to perform tests. This translates to long wait times (~days) for patients and healthcare provid-ers to receive testing results. Point-of-Care (PoC) de-vices have emerged as promising alternatives to these traditional blood assays as they are capable of rapid analysis (~mins) in non-laboratory settings. Thus, we are developing an integrated and electronically operat-ed PoC platform for the rapid identification of protein biomarkers. As shown in Figure 1, in principle, the PoC system is a blood-to-result platform that incorporates an interface for automated sample withdrawal from blood collection devices, an electrochemical assay, and an electronic readout. The electrochemical assay was developed as a bead-based electronic enzyme-linked immunosorbent assay (ELISA) to reduce assay time, lower required sample volume (~µL), and enable platform automation. The workflow of this bead-based electronic assay is shown in Figure 2. Following sample infusion, magnetic microbeads conjugated with antibodies and enzymes are introduced to the sample. Target biomarkers bind to antibodies on the surface of the microbeads and are then sent to electrodes for detection. Biomarker-bound beads then attach to capture antibodies coated on the electrodes. Following an incubation period, enzymes on the attached microbeads catalyze chemical reactions to generate current, which is measured electronically. Electrical readouts are then correlated with the target biomarker concentrations on the transducer. Results have indicated that this sensor has the sensitivity range required for clinically relevant concentrations of various biomarkers for a variety of biomedical applications. Furthermore, initial testing has shown that the platform produces rapid results (within 30 mins) using small volumes (~µL) of blood."
A Microfluidic System for Modeling Human Atherosclerosis and Pathophysiology,"Hemodynamic flows and consequent fluid shear stress-es (FSS) directly regulate endothelial function (EF), which in turn regulates atherosclerotic disease pro-gression (atherogenesis). Laminar, helical flows with a high-magnitude pulsatile FSS waveform enhance EF (i.e., are atheroprotective), while multidirectional flows with a low-oscillatory FSS waveform impede EF (i.e., are atheroprone). Understanding atherogenesis re-quires a microenvironment with representative flows regulating EF (Figure 1). Current systems usually pro-vide a single aspect of atheroprotective or atheroprone flows: high/low shear, oscillatory/unidirectional flow, uniform/pulsatile flow, etc., but do not recreate all the spatiotemporal flow features to mimic the complexity of physiological flows.We have developed a microfluidic system that, for the first time, fully recapitulates in vivo-like spatiotemporal atheroprotective flow simultaneously with atheroprone flows, both with complex but programmable features. Applying these flows upon primary human endothelial cells (hECs), we can concurrently monitor maintenance of EF and the emergence of endothelial dysfunction in precise locations within a single cellular monolayer, as it occurs in vivo. We utilize on-chip valves to dynamically modulate flows—and hence FSS applied on cells—mimicking in vivo waveform dynamics and magnitude. Additionally, we utilize patterned grooves within the device to impart specific spatial profiles of flow, enabling us to recapitulate the complete spatiotemporal flow signatures found in vivo.Our platform allows us to monitor hECs cultured under spatiotemporal flows and execute relevant biological assays for assessing EF. As an example, we observe cell alignment exclusively under atheroprotective flows compared to atheroprone flows, matching known in vivo morphology of functional hECs (Figure 2). Overall, with this highly relevant platform, we can, for the first time, systematically and simultaneously control unexplored hemodynamic flow parameters that condition hECs to regulate human disease susceptibility."
Biochip for Drug Delivery using TERCOM,"Targeted drug delivery has been an area of active investigation for several decades. Most approach target cell-borne receptors chemically or genetically. Some use external stimuli such as heat or radio waves to drive spatially-localized release. In one approach, particles estimate their own location within the body by correlating their sensed environment (e.g., temperature, pressure, salinity, sugar levels, pH, etc.) or its time history against a carried map and releases a charge of a drug based on this estimate. This eliminates external aids and is closely related to terrain contour matching (TERCOM) and scene correlation (DSMAC), techniques used in aircraft navigation. Previous work by the PI and his group focused on the development of nanoparticles capable of sensing and retaining a memory of their environment with noisy DNA. Current efforts focus on the theory of estimating location within the body from vectors of sensed variables and on development of a SiO2 MEMS biochip (microarray) that can test or screen particles and molecules for such sensitivity. Preliminarily explored particle concepts have included liposomes and proteins (bottom-up fab) and thin films (top-down fab). A chip concept that implements a microarray with a half-toned chemical library and material data drawn from conventional surgical analogs has also been considered. The objective is to demonstrate a targeted nanoparticle that implements TERCOM- or DSMAC-like navigation in the body and a biochip that can evaluate its selectivity. The concept is outlined in Figure 1."
Nanocone-arrayed SERS Substrate for Rapid Detection of Bacterial Sepsis,"Rapid detection of bacteria is a very critical part of treating infectious disease. Sepsis kills more than 25 percent of its victims, resulting in as many as half of all deaths in hospitals before identification of the patho-gen for patients to get the right treatment. Raman spectroscopy is a promising candidate in pathogen di-agnosis, given its fast and label-free nature if the con-centration of the pathogen is high enough to provide reasonable sensitivity. This work develops a new kind of surface-enhanced Raman spectroscopy (SERS) sub-strate that will provide high enough sensitivity and fast and close contact of the target structure to the hot spots for an immunomagnetic-based, bacteria-concen-trating and -capturing technique. The substrate uses an inverted cone structure array made of transparent PDMS to funnel the light to the top of the cones, where plasmonic nanorods are located. A high-reflective and low-loss layer is deposited on the outer surface of the cone. Given the geometry of the cone, photons are multi-reflected by the outer layer and thus the number density of photon increases by at least an order. After the pattern and geometric shape of the cones are optimized, the hot spots of the proposed SERS substrate could have an enhancement factor of 108 or higher, which could be high enough to detect immunomagnetically densified bacteria."
Arterial Blood Pressure Estimation using Ultrasound Technology and a Transmission Line Arterial Model,"This work describes the application of a transmission line model to arterial measurements in order to derive useful cardiovascular parameters. Non-invasive ultrasound tech-niques are used to make these measurements, which has several benefits over invasive methods such as arterial catheterization. However, invasive methods are seen as the “gold standard” measurements and therefore the most ac-curate. Having accurate measurements performed non-in-vasively is very desirable for cardiologists to determine their patients’ risk of developing cardiovascular disease.This work details how to obtain the “blood” flow and pulse pressure waveforms with ultrasound transducers using a flow phantom with blood mimicking fluid (BMF) shown in Figure 1. Two transducers, one for imaging and one for Doppler, are used together to derive these pulse pressure waveforms from distension and “blood” flow velocity measurements. Unfortunately, the pulse pressure waveform does not contain diastolic pressure information. By decomposing the backward and forward pulse and flow waves and using the transmission line model, the diastolic pressure can be determined, yielding a complete arterial blood pressure waveform."
Human Subject Studies of Ultrasound for Continuous and Non-invasive Arterial Blood Pressure Waveform Monitoring,"Arterial blood pressure (ABP) is a key physiological parameter for evaluating the circulatory system of pa-tients. The ABP reflects the pathophysiologic states of the cardiovascular system. Currently, the ABP wave-form is usually obtained via an arterial line (A-line) in intensive care settings; while considered the gold stan-dard, the A-line is invasive. Thus, our goal is to develop a reliable, continuous, and non-invasive ABP waveform estimation system. Ultrasound is an ideal imaging mo-dality to achieve this goal due to its low cost and por-tability. Two human subject studies are in progress us-ing prototype ultrasound devices to develop this ABP waveform estimation system.The first human subject study is being done in collaboration with the Boston Medical Center to compare the measured ABP waveform on patients with A-lines with the pulse pressure waveform measured with the Flow Method we developed at the carotid artery. In the Flow Method, the blood flow is measured with pulsed Doppler using a single ultrasound transducer while the arterial area and distention are measured by using M-mode imaging with a second single ultrasound transducerA drawback of the Flow Method is that it provides only the pulse pressure waveform rather than the absolute ABP waveform. Thus, a transmission line model of the arterial blood flow system is being developed to make an estimation of the diastolic pressure, which provides the baseline for the absolute ABP waveform. The pulse pressure waveform on its own gives no information on diastolic blood pressure. However, the transmission line model suggests that the waveform may contain information regarding the patient’s vascular resistance. By decomposing the waveform into the forward and backward traveling waves, we can derive the reflection coefficient. The reflection coefficient provides an estimate of the vascular resistance, which is multiplied with the measured diastolic blood flow to yield the diastolic pressure. The second human subject study is underway in collaboration with Massachusetts General Hospital to compare the measured ABP waveform on patients with A-lines with mean arterial pressure (MAP) calculated by the transmission line model at the brachial arteries."
Measuring Saccade Latency using Smartphone Cameras,"With current clinical techniques, it is difficult to ac-curately determine the condition of a patient with a neurodegenerative disease (e.g., Alzheimer’s disease). The most widely used metrics are qualitative and vari-able, exposing the need for a quantitative, accurate, and non-obtrusive metric to track disease progression. Clinical studies have shown that saccade latency--an eye movement measure of reaction time--can signifi-cantly differ between healthy subjects and patients. We propose a novel system that measures saccade latency outside the clinical environment using videos recorded with a smartphone camera. This is challenging, given the absence of infrared illumination and high-speed cameras, adverse lighting conditions, and the instabil-ity of the tracking device. To overcome these challenges and therefore enable tracking of saccade latency in large cohorts of subjects, we combined a deep convolutional neural network (CNN) for gaze estimation with a model-based approach for saccade onset determination that provides automated signal-quality quantification and artifact rejection (Figure 1). A variant of the iTracker gaze estimation CNN and a hyperbolic tangent model resulted in mean saccade latencies and associated standard deviations on iPhone recordings that were essentially the same as those obtained from recordings using a high-end, high-speed camera. With our system, we recorded over 19,000 latencies in 29 self-reported healthy subjects and observed significant intra- and inter-subject variability, which highlights the importance of individualized disease tracking (Figure 2). Our framework shows that unobtrusive, individualized tracking of neurodegenerative disease progression is possible."
A Simplified Design for Modeling Coronary Capillary Fluid Transport in a PDMS Model,"Myocardial injury is the leading cause of adult mortal-ity in the United States. Despite the tremendous scien-tific interest in modeling the cardiac capillary damage that is characteristic of this event, few platforms exist to model in-vivo fluid dynamics, especially capillary interactions, accurately. Tissue-interface-mimicking microfluidic devices are the few in-vitro models for studying the critical behavior of capillaries, but fre-quently used models require single micrometer resolu-tion photolithography tools. This study examines and evaluates an accessible alternative design that employs centimeter-resolution photolithography to achieve similar flow properties. Although fundamental fluid dynamics properties of the new design are in accor-dance with expectations, some suggestions are made to improve the applicability of the new design for model-ing cross-membrane diffusion in capillaries."
A Bidirectional LLC Converter using Common Mode and Differential Mode Current Injection,"Power converters are ubiquitous in today’s world of electronics, and the push for higher-power-density converters has opened new realms of applications for them. One popular converter topology for high-perfor-mance, high-power-density converters is the LLC res-onant converter, which relies on the frequency-depen-dent gain of an LLC network for voltage conversion. This LLC network consists of a capacitor, inductor, and transformer in series, with the transformer’s magnetiz-ing inductance serving as the LLC’s second inductance. This LLC network’s gain characteristic is advantageous because it allows the converter to achieve a wide range of input/output voltage gain with only a narrow range of switching frequencies. However, with a traditional LLC converter, this valuable gain characteristic is pres-ent for power conversion only in the forward direction. This trait is inopportune for bidirectional converters.In this work, we have demonstrated a converter topology that achieves the LLC gain characteristic during both forward and backward operation. This topology splits the traditional LLC topology into two equal halves, as Figure 1 illustrates. Then, we add an auxiliary inductor Lmb between the two inverter switch nodes to serve the magnetizing inductance role during reverse operation. Both halves are driven identically in parallel (the voltages at points A and B are always equal) for forward operation, resulting in common-mode current injection into the LLC resonant tank and no current through the auxiliary inductor. During reverse operation, the two halves are driven 180 degrees apart, resulting in differential-mode current injection that passes through the auxiliary inductor. As a result, the resonant tank exhibits a gain characteristic resembling that of an LLC network in both directions. This topology brings the high-performance of LLC resonant converters to a variety of new applications requiring bidirectional power flow, such as consumer electronics, electric vehicles, and grid energy storage."
Electro-Chemo-Mechanical Studies of Perovskite-structured Mixed Ionic-electronic Conducting SrSn1-xFexO3-x/2+δ,"Solid oxide fuel cells (SOFCs) convert chemical ener-gy directly to electricity and thus have high potential conversion efficiency. Thermo-mechanical stability and high cathode surface reaction kinetics are two ma-jor criteria for good SOFC cathodes. In this work, we extend previous studies on the promising mixed ionic and electronic conducting perovskite-structured Sr-Ti1-xFexO3-x/2+δ (STF) materials system whose exchange kinetics were correlated with the minority electron charge density by replacing Ti with Sn, due to its dis-tinct band structure and higher electron mobility. Oxygen nonstoichiometry and the defect chemistry of the SrSn1-xFexO3-x/2+δ (SSF) system were examined by thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in the STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure, associated with the substitution of Ti by Sn, impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure while surface kinetics were examined using AC impedance spectroscopy. The electrochemical properties of SSF were found not to differ significantly from the corresponding composition in STF. Though slightly shifted by the larger size of Sn, the defect equilibria and the cathode area specific resistance differed only in a limited way from that in STF. This small difference was attributed to properties being largely dominated by Fe and not by the substitution of Ti with Sn. However, due to asymmetry in the crystal structure caused by the larger size of Sn, both thermal and chemical expansion coefficients of SSF35 were found to be around 20% and 10% lower, respectively, than those of STF35, thus making SSF35 much more chemo-mechanically stable in SOFC operating conditions."
High Capacity CMOS-compatible Thin Film Batteries on Flexible Substrates,"The miniaturization of sensors through advancements in low-powered MEMS devices in integrated circuits has opened up new opportunities for thin film micro-batteries. However, many of the available thin film battery materials require high-temperature process-es that necessitate additional packaging materials, which reduce the overall energy density of these bat-teries. Previous research with collaborators in Singa-pore demonstrated an all-solid-state materials system with high volumetric capacity that exclusively utilizes CMOS-compatible (i.e., room temperature) processes. This process allows integration of these batteries with CMOS circuits as distributed power supplies or for in-tegrated autonomous microsystems. Additionally, the ability to deposit all components of the battery at room temperature makes it possible to fabricate these bat-teries on thin, flexible substrates that can be densely stacked to achieve energy densities comparable to bulk batteries, which has been the focus of this project.We have successfully demonstrated a full thin film microbattery using germanium (Ge) and ruthenium dioxide (RuO2) as anode and cathode materials, respectively, with lithium phosphorous oxynitride (LiPON) as the solid-state electrolyte (Figure 1b). Although RuO2 has traditionally been used as an anode material, it has significantly higher volumetric capacity than typical cathode materials and sufficiently high electrochemical potential versus Ge to provide an output voltage of about 0.5V at a capacity of about 40 Ah/cm3 (Figure 1a). These materials are deposited onto a thin (~5 μm), flexible polyimide substrate with integrated interconnects and peeled off the handle substrate (Figure 2). These battery films can be stacked for higher power and energy densities and folded to fit any volume."
Kinetic Study of Lithiation-induced Crystallization in Amorphous Germanium Anodes in Thin Film Batteries,"Germanium (Ge) is one of the most promising anode materials for complementary metal-oxide semiconduc-tor (CMOS)-compatible lithium-ion microbatteries. An intercalation or allowing anode is needed to avoid the presence of metallic Li for this application. Ge has a vol-umetric capacity of 7366 mAh/cm3, which is ten times as large as the graphite anodes used in commercial bulk batteries. When Ge is discharged below a threshold voltage, a crystalline phase Li15Ge4 forms. This phase is expected to affect the performance of Ge anodes. The degree of crystallinity is hugely affected by the cutoff voltage during lithiation (Figure 1), as well as other fac-tors including cycle number and initial film thickness. In addition to structural analyses and cyclic voltam-metry techniques, we have developed a potentiostatic technique to study the kinetics of crystallization at low voltage in amorphous Ge anodes.We found double peaks in the current vs. time curves under specific potentiostatic test conditions (Figure 2). The existence of double peaks indicates that two phase transitions occur under the given conditions. The appearance of peak 1 in Figure 2 exhibits clear correlations with the applied voltage, cycle number, and initial film thickness, which all indicate the formation of the c-Li15Ge4 phase. Combining kinetic studies with previously reported spectroscopic studies, we can attribute peak 1 to the amorphous-to-crystalline transition, while peak 2 corresponds to an amorphous-to-amorphous transition. The extent to which the crystalline phase forms has a dramatic effect on the delithiation behavior (Figure 1)."
Mechanisms of Li Storage in RuO2 Electrodes for Thin Film Batteries,"It has been demonstrated that RuO2 films can serve as high-performance electrodes for thin film lithium-ion batteries due to their large volumetric charge capacity, excellent cyclability, and rate capability. Unlike oth-er electrode materials, RuO2 films also do not require high-temperature processing, making them suitable for integration with low-power CMOS circuits and fabrica-tion on flexible membranes. However, lithiation and delithiation mechanisms for RuO2 are poorly under-stood, and an improved understanding is required for optimization of battery performance and yield.Lithium is stored in RuO2 films through a complex sequence of phase transformations. We have carried out detailed electrochemical studies coupled with the physical characterization of sputtered RuO2 thin films. The sequence of phase transformations during lithiation and delithiation was electrochemically characterized using galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) measurements (Figure 1). These characterizations were correlated with ex-situ selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and in-situ electrochemical impedance spectroscopy (EIS) results. This allows identification of phase transformations α, β, γ, and δ as reactions of Li storing between the grain boundaries between nanosized grains, formation of a reversible SEI layer, main conversion reaction, and formation of alloy LixRuO2, respectively, as Figure 2 shows. Current studies are focused on application of these insights to optimization of the performance of RuO2 electrodes in full thin film batteries. The methodology developed in this study can also be applied to other candidate thin film electrode materials. In addition, lessons from studying thin films can be applied to more complex powder-based electrodes used in bulk batteries."
Crystal Engineering of Mixed Cation Perovskite for Fabrication of Highly Efficient Solar Cells,"Inorganic-organic perovskite solar cells (PSCs) have caught tremendous interest from many research groups in the field of photovoltaic devices due to their low cost, ease of fabrication, and excellent optical and electrical properties, which resulted in a record certi-fied personal consumption expenditure (PCE) of 23.3%. The presence of surface and grain boundary defects in organic–inorganic halide perovskite films is detrimen-tal to both the performance and operational stability of PSCs. Here, we study the effect of chloride (Cl) addi-tives on the bulk and surface defects of mixed-cation and halide PSCs. We found that using an anti-solvent technique divides the perovskite film into two separate layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. Moreover, we demonstrate that the addition of formamidinium chloride (FACl) into the precursor solution removes the small grain perovskite capping layer and suppresses the formation of bulk and surface defects (Figure 1). This modification by FACl provides the perovskite film with remarkably improved orientation, crystallinity, and large grain size up to over 1 μm (Figure 2a). Time-resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1-adamantylamine hydrochloride (ADAHCl) than for the reference sample. Based on these treatments, we improve the quality of perovskite film and increase the power conversion efficien cy (PCE) from 19.43% for a reference sample to 21.2% for the modified device by Cl additives. This efficiency is among the highest reported values for a planar perovskite solar cell. This PCE enhancement is mostly due to the improvement of open circuit voltage (Voc) from 1110 mV to 1152 mV (Figure 2b). Moreover, the device modified by Cl additives shows a lower hysteresis effect than the reference sample. Importantly, the molecular engineering created by applying Cl additives greatly enhances the stability of the PSCs, which show only 5% degradation after aging for 90 days, which is higher than the 16% PCE loss of the reference device (Figure 2c). Additionally, we found that the modified device with Cl additives shows a smaller ideality factor of 1.8 than 2.1 for the reference device, due to the lower recombination. Our proposed approach opens up a new direction for the commercialization of efficient and stable solar cell devices."
Buckled MEMS Beams for Energy Harvesting from Low-frequency Vibrations,"Vibration energy harvesters based on the resonance of the beam structure work effectively only when the operating frequency window of the beam resonance matches that of the available vibration source. None of the resonating micro-electro-mechanical system (MEMS) structures can operate with low-frequency, low-amplitude, and unpredictable ambient vibrations since the resonant frequency rises as the structure gets smaller. A bi-stable buckled beam energy harvester is has been developed to lower the operating frequency window below 100Hz for the first time at the MEMS scale. This design does not rely on the resonance of the MEMS structure but operates with the large snapping motion of the beam at very low frequencies when in-put energy overcomes an energy threshold. A fully functional piezoelectric MEMS energy harvester was designed, monolithically fabricated, and tested. An electromechanical lumped parameter model was developed to analyze the nonlinear dynamics and to guide the design of the nonlinear oscillator-based energy harvester. Multi-layer beam structure with residual stress-induced buckling was achieved through the progressive residual stress control of the deposition processes along with fabrication steps. The surface profile of the released device shows bi-stable buckling of 200𝜇𝑚, which matches well with the amount of buckling designed. Dynamic testing demonstrates that the energy harvester operates with 50% bandwidth under 70Hz at 0.5g input, operating conditions that have not been demonstrated by MEMS vibration energy harvesters before."
A Robust Electromagnetic MEMS Vibration Energy Harvester,"Modern production plants lack an effective way to autonomously monitor equipment health. It is uneco-nomical to engage personnel solely to monitor ma-chines that function normally most of the time and impractical to wire plant-wide arrays of sensors for power and communication. As an alternative, vibration energy harvesters could power autonomous sensor networks that communicate wirelessly. Further, vibra-tion-based machine health monitoring could be an ef-fective method of assessing real-time machine perfor-mance. Such monitoring could become preventive by prompting maintenance prior to unrecoverable plant failures. To this end, this project seeks to advance the state of vibration energy harvesting.Our previous work yielded silicon-micro-electro-mechanical systems (MEMS) electromagnetic vibration energy harvesters suitable for powering machine health sensors. To further improve robustness and increase electrical power output, a new harvester is designed, fabricated, and demonstrated using the MP35N alloy. Its design and optimization follow that developed for earlier silicon harvesters. The new material has a mechanical modulus close to that of the silicon while not being brittle. Thus, with similar material thickness, we maintain the harvester footprint while improving robustness . The MP35N alloy allows for less stressful full stroke operation, enabling improved output power while being much more tolerant of external shock.Fabrication of the new harvester combines electric discharge machining and water-jet cutting for prototype production. The Lorentz-force harvester, with its folded-spring- suspended magnets, is packaged between two coupling coils using 3D-printed plastic package parts. The new harvester can survive large transient accelerations, common in an industrial setting; such accelerations are unsustainable by a comparable silicon harvester. This added durability brings the harvester much closer to practical application. The improved robustness enables the installation of back-irons, further improving the output power. The power output and power density (1.47 mW/cm3) are comparable to that of the previous record-setting silicon device."
Foulant-agnostic Coatings for Extreme Environments,"Fouling is ubiquitous in large-scale energy production, decreasing efficiency and increasing cost due to foulant buildup. Fouling degrades systems that rely on fluid flow and heat transfer by increasing system pressure drops, impeding heat transfer, and accelerating corro-sion by fostering oxidation or concentrating chemical species within the foulant itself. This leads directly to system derating and early failure. To restore these functions, the deposits must be removed by techniques such as ultrasonic cleaning or manual removal, or the affected part must be replaced. However, these actions are often impractical, prolonging system outages and incurring significant costs due to downtime and com-ponent replacement. Therefore, it is crucial to prevent foulant deposition in the first place. The adhesion of foulant particles is due to their interaction with materi-al surfaces, which can comprise many different types of surface forces. This attraction is dominated by van der Waals (vdW) forces in extreme environments of interest to large-scale energy production, where temperatures and pressures are too high to support electrochemical double layers, and in the absence of other forces like magnetism, static charge, or steric bonding. Therefore, minimizing vdW forces should create an atomistically slick surface, preventing foulant deposition.Here, we hypothesize and experimentally demonstrate a design principle for anti-fouling coatings that exploits the relation between vdW forces and the refractive index of the coating, when vdW forces are dominant. These coatings can be made foulant-agnostic. Both experimental results and first-principles calculations support our hypothesis. As can be seen in Figure 1, the findings show that the closer the refractive index spectrum of a coating to the surrounding fluid, the better it resists the deposition of all foulants. Immediate implications include improving the efficiency of both geothermal reservoirs and nuclear power plants, which are two of the largest sources of carbon-free electricity."
All-solid-state Glucose Fuel Cell for Energy Harvesting in the Human Body,"Efficiently powering sensors, pacemakers, and bio-elec-tronic devices for the human body defines a new era of medicine to track, support, and operate bodily func-tions. Glucose fuel cells have seen a renaissance in re-cent years as an implantable power source harvesting energy from readily available fuels in the human body. Compared to existing implantable batteries, glucose fuel cells require much less frequent replacement sur-gery. However, state-of-the-art glucose fuel cells are based primarily on relatively bulky polymer electro-lytes , suffer from long-term stability issues, and exhib-it low power densities. Here, we innovate a miniaturized glucose fuel cell that is fully composed of solid-state materials based on thin film processing. This all-solid-state glucose fuel cell can be scaled down to the sub-micrometer range for unprecedented miniaturization and is built on a Si chip using semiconductor fabrication methods suitable for integrated and direct powering of bio-electronic implants. Through the use of abiotic catalysts instead of conventional biological catalysts such as enzymes and microbes, long-term stability and increased power density are in perspective. Free-standing fuel cell membranes based on a proton conducting oxide on Si chips were assembled using a microfabrication route with standard semiconductor processing techniques. Oxide thin films were prepared via pulsed laser deposition. The anode is in contact with glucose in phosphate-buffered saline solution to mimic blood, whereas the cathode is in contact with oxygen. Performance characterizations were carried out via electrochemical impedance spectroscopy and galvanostatic polarization curve measurements. We report that the proposed cell is electrochemically active and shows promise in functioning as the first all-solid-state glucose fuel cell with a roughly 100-fold lowered thickness of the device (only 250 nm) compared to polymer-based glucose fuel cells."
Hybrid Intelligence in Design,"One of the greatest challenges facing society is address-ing the complexities of the big-picture, system-level, interdisciplinary problems in a holistic way. Human designers, architects, and engineers have come to rely on steadily improving computational tools to design, model, and analyze their systems of interest. The de-sign of real-world systems (engineering, architecture, software, industrial, financial, and social systems) is, however, often a tumultuous endeavor fraught with great triumphs and, at times, significant regrets. Many believe that only human experts can conceptualize and orchestrate big projects upstream of designing sys-tems. There are two challenging issues in the current practice of a heuristic way of systems design. Firstly, it takes too long (decades) to become area experts through accumulating experience in many successes and some failures. Secondly, human experts also fail sometimes, especially at critical times. The questions one might ask at this stage are, “How could we teach junior engineers, architects, and scientists to design complex systems successfully without spending years of effort training on the job? Could we also assist human experts to minimize the probability of failure by leveraging recent developments in AI and big data?” While the resurgence of artificial intelligence and machine learning suggests ways to even more fully automate downstream tasks in the design process, we propose to go upstream of design, where all the key concepts are determined. Could machine intelligence help this early stage of designing beyond routine design and the optimization of pre-specified goals toward the generation of good, novel designs? Our solution to the question above will be the use of Hybrid Intelligence: combining human intelligence, which grows through experience, and machine intelligence, which can learn from all the past successes and failures and does not forget them at all. Early-stage design across disciplines requires high-level intelligence based on one’s intuition and experiential perceptions to understand challenges, constraints, and requirements in achieving the goals set. Instead of replacing humans with computational systems such as machine intelligence, we see humans and computers as working together within an ecosystem where each must bring their strengths to bear. We propose in the long run a fundamentally broad investigation of this likely convergence across the disciplines of Architecture, Structural Engineering, System Engineering, Mechanical Engineering, and Product Design. We call this approach Hybrid Intelligence because our concern is not with the intelligence of artifice, or the constraining of human designers, but rather with the effectiveness of their hybridized combination. Hybrid Intelligence for design is an umbrella term in which humans and computers collaborate from their strengths to find new processes for thinking, working, and designing."
HAQ: Hardware-aware Automated Quantization,"Model quantization is a widely used technique to compress and accelerate deep neural network (DNN) inference. Emergent DNN hardware accelerators be-gin to support mixed precision (1-8 bits) to improve the computation efficiency further. This goal raises a great challenge to find the optimal bitwidth for each layer: it requires domain experts to explore the vast design space, trading off among accuracy, latency, ener-gy, and model size, which is both time-consuming and sub-optimal. The conventional quantization algorithm ignores the different hardware architectures and quan-tizes all the layers uniformly. In this paper, we introduce the Hardware-aware Automated Quantization (HAQ) framework, which leverages the reinforcement learning to determine the quantization policy automatically, and we take the hardware accelerator’s feedback in the design loop. Rather than relying on proxy signals such as FLOPs and model size, we employ a hardware simulator to generate direct feedback signals (latency and energy) to the RL agent. Compared with conventional methods, our framework is fully automated and can specialize the quantization policy for different neural network architectures and hardware architectures. Our framework effectively reduced the latency by 1.4-1.95x and the energy consumption by 1.9x with negligible loss of accuracy compared with the fixed bit width (8 bits) quantization. Our framework reveals that the optimal policies on different hardware architectures (i.e., edge and cloud architectures) under different resource constraints (i.e., latency, energy, and model size) are drastically different. We interpreted the implications of different quantization policies, which offer insights for both neural network architecture design and hardware architecture design."
AMC: AutoML for Model Compression and Acceleration on Mobile Devices,"Model compression is a critical technique to efficiently deploy neural network models on mobile devices, which have limited computation resources. Conventional model compression techniques rely on hand-crafted heuristics and rule-based policies that require domain experts to explore the large design space, which is usu-ally sub-optimal and time-consuming. In this paper, we propose AutoML for Model Compression (AMC), which leverages reinforcement learning to provide the model compression policy. This learning-based policy outperforms conventional rule-based policy by having a higher compression ratio, better preserving the accu-racy, and freeing human labor. Under 4x floating point operations per second (FLOPs) reduction, we achieved 2.7% better accuracy than the hand-crafted compres-sion policy for VGG-16 on ImageNet. We applied this automated compression pipeline to MobileNet and achieved a 1.81x speedup of measured inference latency on an Android phone and 1.43x speedup on the Titan XP GPU, with only 0.1% loss of ImageNet accuracy."
Transferable Automatic Transistor Sizing with Graph Neural Networks and Reinforcement Learning,"Automatic transistor sizing is challenging due to the large design space, complex performance trade-offs, and fast technology advancement. Although much work has focused on transistor sizing targeting one circuit, limited research has explored transferring knowledge from one circuit to another to reduce re-de-sign overhead. We propose leveraging a Reinforcement Learning (RL) algorithm to conduct knowledge trans-fer between different technology nodes and schemat-ics. Inspired by the fact that circuits are graphs, we also propose to learn on the schematic graph with Graph Convolutional Neural Networks (GCN). The GCN-RL agent extracts features on the schematic graph, whose vertices are transistors and edges are wires. By learning the schematic information, our method consistently achieves higher Figures of Merit (FoMs) on four differ-ent circuits than conventional black box optimization methods (Bayesian Optimization, Evolutionary Algo-rithms). Experiments on transfer learning between five technology nodes and two circuit schematics demon-strate that with the same number of simulations, RL with transfer learning can achieve much higher FoMs than agents without knowledge transfer.To the best of our knowledge, we are the first to leverage RL to transfer knowledge between technology nodes and schematics and to leverage GCN to learn on the schematic graph. Our work makes three main contributions. First, we leverage the schematic graph information in the optimization loop (open-box optimization) to build a GCN based on the circuits schematic graph to open the optimization black box effectively and embed the domain knowledge of circuits to improve performance. We use RL as an optimization algorithm; it consistently achieves better performance than a human expert, random search, Evolution Strategy, Bayesian Optimization, and MACE. Third, we use knowledge transfer with GCN-RL between technology nodes and circuit schematics to reduce the required number of simulations and shorten the design cycle."
ProxylessNAS: Direct Neural Architecture Search on Target Task and Hardware,"Neural architecture search (NAS) has a great impact by automatically designing effective neural network architectures. However, the prohibitive computational demand of conventional NAS algorithms (e.g., 104 GPU hours) makes it difficult to directly search the archi-tectures on large-scale tasks (e.g., ImageNet). Differ-entiable NAS can reduce the cost of GPU hours via a continuous representation of network architecture but suffers from the high GPU memory consumption issue (grow linearly w.r.t. candidate set size). As a result, they need to utilize proxy tasks, such as training on a smaller dataset, or learning with only a few blocks, or training just for a few epochs. These architectures optimized on proxy tasks are not guaranteed to be optimal on the target task. In this paper, we present ProxylessNAS that can directly learn the architectures for large-scale target tasks and target hardware platforms. We address the high memory consumption issue of differentiable NAS and reduce the computational cost (GPU hours and GPU memory) to the same level of regular training while still allowing a large candidate set. Experiments on CIFAR-10 and ImageNet demonstrate the effectiveness of directness and specialization. On CIFAR-10, our model achieves 2.08% test error with only 5.7M parameters, better than the previous state-of-the-art architecture AmoebaNet-B, while using 6X fewer parameters. On ImageNet, our model achieves 3.1% better top-1 accuracy than MobileNetV2, while being 1.2X faster with measured GPU latency. We also apply ProxylessNAS to specialize neural architectures for hardware with direct hardware metrics (e.g., latency) and provide insights for efficient CNN architecture design."
Defensive Quantization: When Efficiency Meets Robustness,"Neural network quantization is becoming an industry standard to efficiently deploy deep learning models on hardware platforms such as CPU, GPU, TPU, and FPGAs. However, we observe that the conventional quantization approaches are vulnerable to adversarial attacks. This paper aims to raise awareness about the security of the quantized models, and we designed a novel quantization methodology to optimize the effi-ciency and robustness of deep learning models jointly. We first conduct an empirical study to show that vanilla quantization suffers more from adversarial attacks. We observe that the inferior robustness comes from the error amplification effect, where the quantization operation further enlarges the distance caused by amplified noise. Then we propose a novel defensive quantization (DQ) method by controlling the Lipschitz constant of the network during quantization, such that the magnitude of the adversarial noise remains non-expansive during inference. Extensive experiments on CIFAR-10 and Street View House Number datasets demonstrate that our new quantization method can defend neural networks against adversarial examples and even achieves superior robustness to their full- precision counterparts while maintaining the same hardware efficiency as vanilla quantization approaches. As a by-product, DQ can also improve the accuracy of quantized models without adversarial attack."
Scalable Free-space Optical Neural Networks,"The transformative impact of deep neural networks (DNNs) in many fields has motivated the develop-ment of hardware accelerators to improve speed and power consumption. We present a novel photonic ap-proach based on homodyne detection where inputs and weights are encoded optically and can be repro-grammed and trained on the fly. This architecture is naturally adapted to free-space optics where both ful-ly-connected and convolutional networks can be im-plemented and scaled to millions of neurons. By utiliz-ing passive optical fan-out and performing arithmetic coherently with optical interference, this scheme cir-cumvents fundamental limits of irreversible electronic processing. We study the effect of detector shot noise on neural-network accuracy to establish a “standard quantum limit” for this system. This bound, which can be as low as 50 zJ/FLOP, suggests performance below the Landauer (thermodynamic) limit is theoretically possible with photonics."
DeeperLab: Single-shot Image Parser,"Image parsing is the process of partitioning an image into multiple semantically meaningful regions (called semantic segmentation), such as car and road, and tell-ing different countable instances apart (called instance segmentation), such as car A and car B. It is a long-last-ing unsolved problem in computer vision and a basic component of many applications, such as autonomous driving. Recent approaches to image parsing typically employ separate standalone neural networks for the semantic and instance segmentation tasks and require multiple passes of inference.Instead, the proposed DeeperLab image parser performs image parsing with a significantly simpler, more fully convolutional approach that jointly addresses the semantic and instance segmentation tasks and requires only one pass of inference (i.e., one-shot), resulting in a streamlined system that better lends itself to fast processing. For quantitative evaluation, we use both the instance-based panoptic quality (PQ) metric and the proposed region-based parsing covering (PC) metric, which better captures the image parsing quality on non-countable classes and larger object instances. We report experimental results on the challenging Mapillary Vistas dataset, in which our single model achieves 31.95% (val) / 31.6% PQ (test) and 55.26% PC (val) with 3 frames per second (fps) on a graphics processing unit (GPU) or near real-time speed (22.6 fps on GPU) with reduced accuracy."
Architecture-level Energy Estimation of Accelerator Designs,"With Moore’s law slowing down and Dennard scaling ending, energy-efficient domain-specific accelerators, such as deep neural network (DNN) processors for machine learning and programmable network switch-es for cloud applications, have become a promising direction for hardware designers to continue bringing energy-efficiency improvements to data and computa-tion intensive applications. To ensure fast exploration of accelerator design space, architecture-level energy estimators, which perform energy estimations without requiring complete hardware description of the de-signs, are critical to designers. However, using existing architecture-level energy estimators to obtain accurate estimates for accelerator designs is hard, as accelerator designs are diverse and sensitive to data patterns (e.g., sparsity in DNNs).To solve this problem, we present Accelergy (Figure 1), an architecture-level energy estimation methodology for accelerator designs. Accelergy interprets a design in terms of its components (e.g., an arithmetic logic unit (ALU) design consists of multipliers and adders). Since accelerator design space is very diverse, Accelergy allows users to define their own components to describe the designs. At the same time, to reflect the energy differences brought by special data processing (e.g., zero-gating in DNN accelerators), Accelergy allows users to define special actions types related to the components (e.g., read and gated read actions for SRAM). To illustrate the usage of Accelergy methodology, we implemented a sample framework for energy estimations of DNN accelerators. The framework provides a set of primitive components for users to describe the design or construct their new components. To further enhance flexibility, Accelergy provides an interface to communicate with different primitive component estimators for system-level estimations of designs that involve emerging technologies (e.g., optical DNN). Accelergy achieves 95% accuracy on total energy estimation with a well-known accelerator design – Eyeriss. Accelergy can also produce accurate energy breakdown across components estimations comparing to other estimation methodologies (Figure 2)."
FastDepth: Fast Monocular Depth Estimation on Embedded Systems,"Depth sensing is a critical function for many robotic tasks such as localization, mapping, and obstacle de-tection. There has been significant and growing inter-est in performing depth estimation from a single red-green-blue image, due to the relatively low cost and size of monocular cameras. However, state-of-the-art single-view depth estimation algorithms are based on fairly large deep neural networks that have high com-putational complexity and slow runtimes on embed-ded platforms. This poses a significant challenge when cameras perform real-time depth estimation on an embedded platform, for instance, mounted on a micro aerial vehicle. Our work addresses this problem of fast depth estimation on embedded systems. We investigate efficient and lightweight encoder-decoder network architectures. To further improve their computational efficiency in terms of real metrics (e.g., latency), we apply resource-aware network adaptation (NetAdapt) to automatically simplify proposed architectures. In addition to reducing encoder complexity, our proposed optimizations significantly reduce the cost of the decoder network (Figure 1). We perform hardware-specific compilation targeting deployment on the NVIDIA Jetson TX2 platform. Our methodology demonstrates that it is possible to achieve accuracy similar to that of prior work on depth estimation, but at inference speeds that are an order of magnitude faster (Figure 2). Our proposed network, FastDepth, runs at 178 fps on a TX2 GPU and at 27 fps when using only the TX2 CPU, with active power consumption under 10 W."
Low-power Adaptive Time-of-Flight Imaging for Multiple Rigid Objects,"Time-of-Flight (ToF) cameras are becoming increasing-ly popular for many mobile applications. To obtain ac-curate depth maps, ToF cameras must emit many puls-es of light, which consumes a lot of power and lowers the battery life of mobile devices. However, lowering the number of emitted pulses results in noisy depth maps. To obtain accurate depth maps while reducing the overall number of emitted pulses, we propose an al-gorithm that adaptively varies the number of pulses to infrequently obtain high-power depth maps and uses them to help estimate subsequent low- power ones as shown in Figure 1. To estimate these depth maps, our technique uses the previous frame by accounting for the 3D motion in the scene. We assume that the scene contains independently moving rigid objects and show that we can efficiently estimate the motions. In contrast to our previous work, this approach uses only the data from the ToF camera and does not need RGB images to estimate the 3D motion in the scene. The resulting algorithm estimates 640 × 480 depth maps at 30 frames per second on an embedded processor. We evaluate our approach on data collected with a pulsed ToF camera and show that we can reduce the mean relative error of the low-power depth maps by up to 65% (see Figure 2) and the number of emitted pulses by up to 80%."
Fast Shannon Mutual Information Accelerator for Autonomous Robotics Exploration,"Robotic exploration problems arise in various contexts, ranging from search and rescue missions to underwater and space exploration. In these domains and beyond, exploration algorithms that can rapidly reduce uncer-tainty can provide significant benefits, for instance, by shortening time and reducing resources required for exploration. Unfortunately, principled algorithms based on rigorous information-theoretic metrics, such as maximizing Shannon mutual information (MI) along the exploration path, are computationally extremely demanding.We propose a novel computing hardware architecture to efficiently compute Shannon MI on an occupancy grid map, which is the standard probabilistic representation for a 2D environment. The proposed architecture consists of multiple MI computation cores, each evaluating the MI between a single sensor beam and the occupancy grid map. We find that parallelization alone is not sufficient for high-throughput computation due to the limited bandwidth of the memory. In fact, it is critical to consider 1) memory management of the occupancy grid map storage and 2) data delivery from the occupancy grid map to MI cores. Thus, our key contributions consist of 1) a novel memory architecture that diagonally partitions the occupancy grid map into multiple banks to minimize the memory access conflicts among multiple cores (Figure 1); 2) a fast and fair memory request arbiter that ensures effective utilization of all MI computation cores; and 3) an energy-efficient, high-throughput MI computation core.This architecture (Figure 2) was optimized for 16 MI computation cores and was implemented on a field-programmable gate array. We show that it computes the MI metric for an entire map of 20m × 20m at 0.1m resolution in near real time, at 2 frames per second, which is approximately two orders of magnitude faster, while consuming an order of magnitude less power than an equivalent implementation on a Xeon CPU."
An Energy-efficient Configurable Lattice Cryptography Processor for the Quantum-secure Internet of Things,"Modern public-key cryptography protocols, such as Rivest-Shamir-Adleman and elliptic-curve cryptog-raphy (ECC) will be rendered insecure by Shor’s algo-rithm when large-scale quantum computers are built. Therefore, cryptographers are working on quantum-re-sistant algorithms, and lattice-based cryptography has emerged as a prime candidate. However, the high computational complexity of these algorithms makes it challenging to implement lattice-based protocols on resource-constrained Internet of things (IoT) devices, which need to secure data against both present and future adversaries. To address this challenge, we pres-ent a lattice cryptography processor with configurable parameters that enables energy savings of up to two orders of magnitude and 124k-gate reduction in system area through architectural optimizations. This is also the first ASIC implementation that demonstrates mul-tiple lattice-based protocols proposed in the National Institute of Standards and Technology’s post-quantum standardization process.Figure 1 shows a block diagram of our system along with the chip micrograph. The chip was fabricated in a 40-nm low-power CMOS process and supported voltage scaling from 1.1V down to 0.68V. Our lattice cryptography processor occupies 106k NAND Gate Equivalents and uses 40.25KB of SRAM. When executing the Kyber-768 and NewHope-1024 key exchange schemes, our design is 28x and 37x more energy-efficient, respectively, than Cortex-M4 software, after accounting for voltage scaling. Moreover, post-quantum key exchange using our processor is 30x more energy-efficient than state-of-the-art pre-quantum ECC-based key exchange at the same pre-quantum security level. Through architectural and algorithmic optimizations, this work demonstrates practical hardware-accelerated quantum-resistant lattice-based cryptographic protocols that can be used to secure resource-constrained IoT devices of the near future."
Power Side-channel Attack on Successive Approximation Register Analog-to-digital Converters,"When sensing hardware is used to acquire a private signal, there must be no information leakage through-out the entire signal chain. Applications that require such security include biomedical and military sensor platforms. Industrial and infrastructure monitoring sensing hardware must also be secure to prevent po-tentially harmful activities of adversaries. By using well-established cryptographic primitives, communica-tion links for sensing hardware can be protected from hackers. However, once hackers physically access the sensing hardware, the sensor-interface circuit can leak critical information via its power side-channel.Both analog and digital circuit blocks of the sensor-interface circuit can leak through a power side-channel as their operations depend on the sensor output value. Since the first discovery of the encryption engine’s power side-channel leakage, countermeasures against digital circuit’s power side-channel attacks have been researched in the cryptographic hardware community. However, unlike digital circuit blocks that can be protected by countermeasures, analog/mixed-signal circuit blocks are now vulnerable to side-channel attacks as their exploitations have not been recognized yet. In this work, we have developed practical power side-channel attack scenarios that make analog/mixed-signal circuit blocks become the security loophole of the entire system. We chose analog-to-digital converters (ADCs) as our target block of study. We focused our research on successive approximation register (SAR) ADCs because they are more power-efficient than other ADC types in the performance range (resolution, sampling rate) that is suitable for most sensor platforms. To experiment with power side-channel attack on SAR ADCs, we devised an attack method and mounted it on two SAR ADC products from different manufacturers. The experimental results show that SAR ADCs’ input waveforms could be faithfully reconstructed from their current traces."
Energy-efficient SAR ADC with Background Calibration and Resolution Enhancement,"Many signals, for example, medical signals, do not change much from sample to sample most of the time. Conventional switching schemes for SAR ADCs do not exploit this signal characteristic and test each bit start-ing with the MSB. Previous work called least-signifi-cant-bit (LSB)-first saves energy and bit-cycles by start-ing with a previous sample code and searching for the remainder by testing bits from the LSB end. However, certain code transitions consume unnecessary energy, even when the code change over the previous code is small.This work addresses it with a new algorithm called Recode then LSB-first (RLSB-first) that reduces the switching energy and bit-cycles required for all cases of small code change across the full range of possible previous sample codes. RLSB-first uses split-DAC to systematically encode the previous code before LSB-first. RLSB-first lowers switching energy by up to 2.5 times and uses up to 3 times fewer bit-cycles than LSB-first. In addition to an energy-efficient SAR ADC, this work aims to use the savings for background calibration and resolution enhancement."
An 8-bit Multi-GHz Flash ADC with Time-based Techniques,"High-speed and medium-to-low-resolution flash an-alog-to-digital converters (ADCs) are widely used in applications such as 60-GHz receivers, serial links, and high-density disk drive systems. In this project, we propose an 8-bit, multi-gigahertz flash ADC with two major innovations: the time-based comparator offset calibration and the time-based 4x interpolation.A high-speed, low-power comparator with low noise and offset requirements is a key building block. Figure 1 shows the two-stage dynamic comparator used in our design. With the scaling of CMOS technology, the offset voltage of the comparator keeps increasing due to greater transistor mismatches, making offset calibration a necessity. Traditional offset calibration methods that use digitally-controlled capacitor banks or extra input transistor pairs add extra parasitics to the comparators and slow down the operation. In this work, the proposed time-based comparator offset calibration put no additional load on the comparators and avoids the speed penalty of traditional methods.The number of comparators in a flash ADC grows exponentially with resolution. This is a major drawback of flash ADCs. Time-domain interpolation is a popular technique that utilizes the timing information from adjacent comparators to resolve extra bits of resolution without adding comparators. Figure 2 shows the pro-posed flash ADC. Sixty-five comparators are used to achieve the six most significant bits (MSBs). Sixty-four interpolators are inserted between the comparators to obtain two extra bits by comparing the delay from neighboring comparators. The input capacitance of this design is ¼ of the conventional 8-bit flash ADC. Therefore, a higher operating speed can be achieved. We introduce gating logic so that only one interpolator is enabled during operation, which reduces power consumption significantly.The prototype ADC is realized in 65-nm CMOS technology. At 2.8 GS/s, the prototype measures an SNDR of 43.3 dB at Nyquist input frequency and achieves a state-of-the-art figure-of-merit."
A Sampling Jitter-tolerant Continuous-time Pipelined ADC in 16-nm FinFET,"Analog-to-digital converters (ADCs) interface re-al-world analog signals with digital systems, and hence they are an essential part of any electronic system. Al-though there have been steady improvements in the performance of ADCs, the improvements in conversion speed have been less significant because the speed-res-olution product is limited by the sampling clock jitter. The effect of sampling clock jitter has been considered fundamental. However, it has been shown that con-tinuous-time delta-sigma modulators may reduce the effect of sampling jitter. Since delta-sigma modulators rely on relatively high oversampling, they are unsuit-able for high-frequency applications such as 5G base-band processors. Therefore, ADCs with low oversam-pling ratio are desirable for high-speed data conversion.In conventional Nyquist-rate ADCs, the input is sampled upfront (Figure 1). Any jitter in the sampling clock directly affects the sampled input and degrades the signal-to-noise ratio (SNR). For fast varying input signals, the sampling jitter severely limits the maximum attainable SNR. It is well known that for a known rms sampling jitter σt, the maximum achievable SNR is limited to 1/(2πfinσt), where fin is the input signal frequency. Typically, reducing the rms jitter below 100 fs is difficult. This challenge limits the maximum SNR to just 44 dB (which is equivalent to 7 bits) for a 10-GHz input signal. Therefore, unless the effect of sampling jitter is reduced, the performance of an ADC would be greatly limited for high-frequency input signals.In this project, we propose a hybrid ADC with reduced sensitivity to sampling jitter. We are designing this ADC in 16-nm FinFET technology to give a proof-of-concept for improved sensitivity to the sampling clock jitter."
Studies on Long-term Frequency Stability of OCS Molecular Clock,"Miniature clocks with high long-term stability are critical to navigation, sensing, and communication networks. Crystal/micro-electro-mechanical sys-tems (MEMS) oscillators with typical stability of 10-4 to 10-8 are not well suited for high-precision systems. Small-volume atomic clocks improved the stability to 10-11 to 10-12 by probing hyperfine transitions of Cs and Rb atoms at microwave frequencies, but their compli-cated electro-optical implementation leads to exceed-ingly high cost. Recently, complementary metal-oxide semiconductor (CMOS) molecular clocks emerged as a promising alternative to miniature clocks with high long-term stability. By probing the rotational lines of gaseous carbonyl sulfide (OCS) molecules at 267.530 GHz and then calibrating the clock’s 10 MHz output fre-quency according to the measured terahertz transition frequency of OCS, the molecular clock achieved Allan deviation of 1×10−11 with fully electronic operations. To verify the clock’s robustness to external environmental variations, two critical metrics related to the long-term stability of THz OCS clocks were studied: temperature and magnetic field. The intrinsic frequency OCS transition line is very robust to the temperature change having the temperature coefficient of a few parts per trillion per kelvin. However, the clock’s sensitivity to temperature is increased by the baseline tilting, which is mainly caused by the reflection of the THz wave at the waveguide vacuum sealing window. Also, with the presence of a magnetic field, the rotational energy levels associated with different magnetic quantum numbers deviate from their degenerate value at zero field due to the Zeeman effects. While first-order Zeeman effects of all transition sub-levels maintain the symmetry of the transition line and introduce no shift, the clock shift caused by the second-order Zeeman effects is, by theory, 4×10−13. In our preliminary testing, the temperature coefficient of the clock is ∼1.3×10−10/◦C without ovenized temperature stabilization and temperature compensation, and the upper limit of the magnetic-induced shift in response to a 75-Gauss external magnetic field is 4×10−11. This study verifies the molecular clock’s high robustness under temperature variations and strong-magnetic conditions."
"Miniaturized, Ultra-stable Chip-scale Molecular Clock","Mobile electronic devices require stable, portable, and energy-efficient frequency references (or clocks). However, current approaches using quartz-crystal and micro-electro-mechanical systems (MEMS) oscil-lators suffer from frequency drift. Recent advances in chip-scale atomic clocks, which probe the hyperfine transitions of evaporated alkali atoms, have led to de-vices that can overcome this issue, but their complex construction, cost, and power consumption limit their broader deployment. Here, we show that sub-terahertz rotational transitions of polar gaseous molecules can be used as frequency bases to create low-cost, low-pow-er miniaturized clocks. A molecular clock probing 231.061 GHz (J=19←18) spectral line of carbonyl sulfide (16O12C32S) is shown in Figure 1. Based on complementary metal–oxide semiconductor (CMOS) technology, a terahertz phase-locked loop with built-in frequency-shifting-keying (FSK), referenced to an 80-MHz crystal oscillator, and generates the probing signal. The OCS molecules are accessed within a compact WR4.3 waveguide gas cell. The relative frequency error through comparing the probing frequency and selected spectral line center is detected by envelope rectification and phase-sensitive detection in a CMOS receiver. Finally, a type-I frequency locking feedback loop is established to stabilize the crystal frequency. Figure 2 shows the photograph of the CMOS molecular clock chipset. Figure 3 shows that with an averaging time of 103 s, the clock stability (defined by Allan deviation) achieves 3.8×10-10. Compared with chip-scale atomic clocks, our approach is less sensitive to external influences (temperature variation, electromagnetic field fluctuation, and mechanical vibration); offers faster frequency error compensation; and, by eliminating the need for alkali metal evaporation, offers faster start-up time and lower power consumption. Our work demonstrates the feasibility of monolithic integration of atomic-clock-grade frequency references in mainstream silicon-chip systems."
A Dense 240-GHz 4×8 Heterodyne Receiving Array on 65-nm CMOS Featuring Decentralized Generation of Coherent Local Oscillation Signals,"There is a growing interest in pushing the frequency of beam-steering systems towards the terahertz range, in which case narrow beams can be formed at chip scale. However, this calls for disruptive changes to traditional terahertz receiver architectures, e.g., square-law direct detector arrays (with low sensitivity and no phase in-formation preserved) and small heterodyne mixer ar-rays (bulky and not scalable). Specifically, for the latter case, corporate feed (for generating and distributing the local oscillation (LO) signals), typically a necessary component, can be very lossy at large scale. Here, we report a highly scalable 240-GHz 4×8 heterodyne array achieved by replacing the LO corporate feed with a net-work that couples LOs generated locally at each unit. A major challenge for this architecture is that each unit should fit into a tight λ/2×λ/2 area to suppress side lobes in beamforming--it makes the integration of mixer, lo-cal oscillator, and antenna in a unit extremely difficult. This challenge is well addressed in our design, where highly compact units enable the implementation of two interleaved 4×4 phase-locked sub-arrays in an area of 1.2 mm2.The architecture of the entire array is shown in Figure 1(a). Its core component is a self-oscillating harmonic mixer (SOHM), which simultaneously (1) generates high-power LO signal and (2) down-mixes the radio frequency (RF) signal. Owing to the coupling, LOs generated in each unit are all locked to an external reference signal, so that the array is coherent. Die photo showing the placement of the array and the phase-locked loop (PLL) is given in Figure 1(b). A measured spectrum at 475-MHz (beyond the noise corner frequency) baseband signal is shown in Figure 2. The measured sensitivity (required incident RF power to achieve SNR=1 at baseband) over 1-kHz detection bandwidth is 58fW–a more than 4000× improvement over prior state-of-the-art large-scale square-law detector arrays in silicon."
A PLL-free Molecular Clock based on Second-order Dispersion Curve Interrogation of a Carbonyl Sulfide Transition at 231 GHz,"Miniature clocks with high long-term stability are critical to navigation, sensing, and communication networks. Crystal/MEMS oscillators with a typical stability of 10-4 to 10-8 are not well suited for high-pre-cision systems. Small-volume atomic clocks improved the stability to 10-11 to 10-12 by probing hyperfine tran-sitions of Cs and Rb atoms at microwave frequencies, but their complicated electro-optical implementation leads to exceedingly high cost. Recently, CMOS molec-ular clocks that use a sub-THz spectrometer to probe the absorption lines of carbonyl sulfide molecules have emerged to achieve a low-cost miniature clock with high long-term stability.To generate the sub-THz probing signal within the lock-range, molecular clocks require a voltage-controlled crystal oscillator (VCXO) and a fractional-N phase-locked loop (PLL) as a frequency multiplier. However, eliminating the VCXO and PLL is necessary to further reduce the power consumption and form factor. In addition, using PLL leads to degraded in-band noise because of the high-frequency multiplication factor of the PLL. This work proposes a molecular clock without a VCXO and a PLL. A sub-THz voltage-controlled oscillator (VCO) is directly controlled by a negative feedback loop and then locked to the center of the absorption line. For frequency initialization and coarse frequency tuning, the second-harmonic dispersion curve of the absorption line profile was utilized instead of a PLL. Since the polarity of the second-harmonic dispersion curve is positive only when the frequency of the probing signal is very close to the absorption line, detection of the absorption line does not depend on the signal strength. Also, the second harmonic signal is robust against spectral baseline variations. By eliminating the VCXO and PLL from the loop and using the proposed coarse frequency tuning method, the noise performance of the proposed molecular clock is expected to improve, and further miniaturization of an ultra-stable clock can be achieved."
Chip-scale Scalable Ambient Quantum Vector Magnetometer in 65-nm CMOS,"Room-temperature coherent spin state control and de-tection of nitrogen-vacancy (NV) centers in diamond have enabled magnetic field sensing with high sensitiv-ity and spatial resolution. However, current NV sens-ing apparatuses use bulky off-the-shelf discrete com-ponents, which increases the system scale and limits practical applications. To address this challenge, we de-veloped a hybrid complementary metal-oxide semicon-ductor (CMOS)-NV platform to shrink this spin-based magnetometer to chip scale. In this work, we present a fully integrated CMOS-NV quantum sensor fabricated using a 65-nm CMOS process. Magnetic field sensing is accomplished by the excitation and detection of the spin states of the NV. The frequency of the spin states is determined through optically detected magnetic resonance (ODMR). The magnetic field is proportional to the frequency splitting of the spin states (2.8 MHz/Gauss). Our CMOS-NV magnetometer system is composed of (i) a microwave generation and delivery system to control the NV’s spin states and (ii) an optical system for the readout of spin states. We implement a highly scalable microwave delivery structure, which consists of an array of current-carrying conductors. We control the current flowing in each conductor to achieve a uniform magnetic-field profile. This uniform field enables coherent driving of the NV centers, which enhances the sensitivity. The on-chip optical readout follows the microwave manipulation of the NV spin ensembles. We implemented a CMOS-compatible, three-layer grating structure to filter out the green excitation. The filter reduces the shot noise of the photo-detector caused by the input green laser. The Talbot effect is used in the filter, where we place layers of gratings with positions aligned with the maxima and minima of the green and the red diffraction patterns generated from the preceding grating layer. We detect the spin-dependent red fluorescence of the NV centers using on-chip N-Well/P-sub photodiode. This work presents a hybrid NV-CMOS platform that can perform coherent spin control and readout of the NV ensemble’s spin state: a highly advanced, scalable, and compact platform for quantum sensing."
Broadband Inter-chip Link using a Terahertz Wave on a Dielectric Waveguide,"The development of data links between different mi-crochips of an onboard system has encountered a speed bottleneck due to the excessive transmission loss and dispersion of the traditional inter-chip elec-trical interconnects. Although high-order modulation schemes and sophisticated equalization techniques are normally used to enhance the speed, they also lead to significant power consumption. Silicon photonics pro-vides an alternative path to solve the problem, thanks to the excellent transmission properties of optical fi-bers; however, the existing solutions are still not fully integrated (e.g., off-chip laser source) and normally re-quire process modification to the mainstream comple-mentary metal-oxide semiconductor (CMOS) technolo-gies. Here, we aim to utilize a modulated THz wave to transmit broadband data. Similar to the optical link, the wave is confined in dielectric waveguides, with suf-ficiently low loss (~0.1dB/cm) and bandwidth (>100GHz) for board-level signal transmission (Figure 1). In com-mercial CMOS/BiCMOS platforms, we have previously demonstrated high-power THz generation with modu-lation, frequency conversion, and phase-locking capa-bilities. In addition, a room-temperature Schottky-barrier diode detector (in 130-nm CMOS) with <10pW/Hz1/2 sensitivity (antenna loss excluded) is also reported. The prototype data link will leverage these techniques to achieve a ~100Gbps/channel transmission rate with <1pJ/bit energy efficiency. As the first step of this project, we have designed a new broadband chip-to-fiber THz wave coupler, passive channelizers, broadband THz modulators, and sub-harmonic carrier generation. In contrast to previous couplers using off-chip antennas, our THz coupler is entirely implemented using the metal backend of a CMOS process and requires no post-processing (e.g., wafer thinning). The structure is also fully shielded, which prevents THz power leakage into the silicon substrate. Conventional on-chip radiators using ground shield work are the resonance type (e.g., patch antenna) and have only <5% bandwidth. In comparison, our design is based on a traveling-wave, tapered structure, which supports broadband transmission. A proof-of-concept is shown in Figure 1: two on-chip couplers are connected with a 2-cm waveguide using Rogers 3006 dielectric material. The entire back-to-back setup exhibits only ~11dB insertion loss across over 60-GHz bandwidth (Figure 2). Additionally, our on-chip and on-interposer channelizers provide a compact and efficient means of reducing ISI while combining incoherent parallel data streams."
Low-energy Current Sensing with Integrated Fluxgate Magnetometers,"The ability to sense current is crucial to many industri-al applications including power line monitoring, motor controllers, battery fuel gauges, etc. We are developing smart connectors with current sensing abilities for use in the industrial Internet of things (IoT). These connec-tors can be used for 1) power quality management to measure real power, reactive power, and distortion and 2) machine health monitoring applications for continu-ous monitoring, control, prevention, and diagnosis. At the system level, the smart connectors need to 1) measure AC, DC, and multiphase currents; 2) reject stray magnetic fields; and 3) detect impending connector failure. On the sensor level, they need high accuracy and performance and a small area to fit inside the outer plastic encasing of the connectors. Therefore, the sensors must not use large external magnetic cores as field concentrators.A good system solution is to use an array of integrated fluxgate (FG) sensors (Figure 1), which offer a better alternative than Hall/magneto-resistive sensors and shunt-sensing in terms of dynamic range (~10^5), sensitivity (200 V/T), linearity (0.1%), low temperature drift, and inherent isolation. But high power consumption is a drawback for FG sensors. FG sensors work by driving magnetic cores in and out of saturation and sensing the resulting voltage difference (Figure 2). They achieve high linearity by balancing the external magnetic fields within the core with an equivalent compensation current, which can be quite power-hungry. We need to reduce the energy needs of the FG sensors so they can be used in an array, especially in energy-constrained environments. We propose a low-energy front-end design with bandwidth scalability and lower energy per measurement for FG sensors. We use a mixed-signal architecture with quick convergence techniques to enable duty cycling from >50 kHz bandwidth for machine health monitoring to <1 kHz for power quality management."
Contactless Current and Voltage Detection using Signal Processing and Machine Learning,"Measuring current and voltage in electrical systems is a critical task in industrial environments and can be used to monitor power quality and machine and pro-cess performance. Easily retrofitted contactless mea-surements are preferred, but they can require difficult installations and bulky hardware. In contrast, we are developing a contactless clip-on sensor that will esti-mate voltage and current in three-phase power cables. Our goal is to create a measurement system that uses less hardware than present state-of-the-art solutions while maintaining a high level of accuracy. Current is estimated using an array of magnetic field sensors embedded in a yoke that fits around the cables, as shown in Figure 1. The measurements are filtered to remove magnetic fields from external sources, such as adjacent cables or eddy currents. This filtering employs a Best Linear Unbiased Estimate of cable currents that is based on a covariance matrix calculated from a probabilistic model of external magnetic fields detected by the sensor array. Additionally, we are using collected data to train neural networks and explore whether machine learning can generate a better estimate. To estimate voltage, we employ guarded electrodes in the yoke that fit snugly against the cables. We then sense cable voltage capacitively coupled to the electrodes and use a physical model of the electrode system to estimate the voltage differences between cables. A voltage estimate example is shown in Figure 2.At present, our system can estimate voltage with an error of less than 1% and current with an error of less than 2%, even in the presence of electric and magnetic field interference. This performance is comparable to currently used contactless detection systems but uses significantly less hardware and should thus be less costly to manufacture. Furthermore, since our estimates produce full current and voltage waveforms, we can calculate quantities such as instantaneous power and power quality."
SHARC: Self-healing Analog Circuits with RRAM and CNFETs,"Next-generation applications require processing of a massive amount of data in real time, exceeding the ca-pabilities of electronic systems today. This has spurred research in a wide range of areas: from new devices to replace silicon field-effect transistors (FETs) to im-proved circuit implementations to new system archi-tectures with dense integration of logic and memory. However, isolated improvements in any one area are insufficient. Rather, enabling these next-generation applications will require combining benefits across all levels of the computing stack: leveraging new devices to realize new circuits and architectures. For instance, carbon nanotube (CNT) field-effect transistors (CNFETs) for logic and resistive random-access memory (RRAM) for memory are two promising emerging nanotechnologies for energy-efficient electronics. However, CNFETs suffer from inherent imperfections (such as of metallic CNTs (m-CNTs)), which have prohibited realizing large-scale CNFET circuits in the past. M-CNTs create shorts between the CNFET source and drain, which translates into (1) a 100x intrinsic gain reduction for analog circuits causing the failure of the whole system and (2) high power consumption and degraded noise margin for digital circuits. This work proposes a circuit design technique (called self-healing analog circuitry with RRAM correction (SHARC)) that integrates and combines the benefits of both CNFETs and RRAM to realize three-dimensional circuits that are immune to m-CNTs. Non-volatile RRAMs are 3D-integrated with CNFETs, whereas each CNFET is split into multiple minimum-width FETs (i.e., “sub-CNFETs”), with a RRAM cell in series fabricated directly under (or over) the source or drain contact of each sub-CNFET.SHARC is a non-volatile technique that self-reconfigures the circuit by programming RRAMs. The sub-FETs including m-CNTs become connected in series to reset high-resistance RRAM that effectively removes those sub-CNFETs from the circuit, while CNFETs containing only semiconducting CNTs are connected in series with set low-resistance RRAM. Leveraging this technique, we experimentally demonstrate the first and largest CMOS CNFET mixed-signal systems robust to m-CNTs (by implementing SHARC in amplifiers and switches) such as a 4b-DAC and 4b-SAR ADC. SHARC can also be combined with additional existing circuit techniques to further improve performance for very-large-scale integrated circuits."
DISC-FETs: Dual Independent Stacked Channel Field-effect Transistors,"We experimentally demonstrate a three-dimensional (3D) field-effect transistor (FET) architecture leverag-ing emerging nanomaterials: dual independent stacked channel FET (DISC-FET) (Fig. 1). DISC-FET is composed of two FET channels vertically integrated on separate circuit layers separated by a shared gate. This gate mod-ulates the conductance of both FET channels simulta-neously. This 3D FET architecture enables new oppor-tunities for area-efficient 3D circuit layouts. The key to enabling DISC-FET is low-temperature processing to avoid damaging lower-layer circuits. As a case study, we use carbon nanotube (CNT) FETs (CNFETs) since they can be fabricated at low temperature (e.g., <250 ºC). We demonstrate wafer-scale CMOS CNFET-based digital logic circuits: 2-input “not-or” (NOR2) logic gates designed using DISC-FETs with independent NMOS CNT channels below and PMOS CNT channels above a shared gate (Fig. 2 and 3). This work highlights the potential of 3D integration for enabling not only new 3D system architectures, but also new 3D FET architectures and 3D circuit layouts."
Modeling and Optimizing Process Uniformity using Gaussian Process Methods,"Modeling process uniformity is critical for achieving the required specifications in many advanced process technologies. For example, sputter deposition systems are prone to significant wafer-scale deposition rate variations due to the complex dynamics of the cham-ber plasma. Our work focuses on developing and apply-ing machine learning methods for modeling and mini-mizing these non-uniformities. Traditionally, modeling this process using first physics principles has been par-ticularly difficult due to the chaotic nature of plasma physics. Instead, we model this process using a Gauss-ian process (GP) framework, which uses historical data to model the deposition rate across the wafer as a func-tion of both process parameters, such as power and chamber pressure, and as a function of the equipment configuration (Figure 1). Recent work focused on creating a method for optimizing the process parameters and the equipment configuration once a predictive GP model has been fit. As the input space to our process is extremely high-dimensional, many sets of process parameters and equipment configurations may lead to our desired response. For this reason, it is neither possible to explore and model the whole space, nor required to find a configuration that meets our specifications. Therefore, when optimizing a specific process, we search not only for process inputs that lead to our desired response, but also ones that lead to tight confidence intervals (Figure 2), allowing us to accurately model only a portion of the input space and converge on a solution that meets our requirements with relatively few deposition runs. Future work will focus on additional data collection and comparing the convergence rates of our Bayesian optimization method to standard process optimization methods."
Magic-angle Graphene Superlattices: A New Platform for Strongly Correlated Physics,"Understanding strongly correlated quantum matter has challenged physicists for decades. Such difficul-ties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum ma-terials. Here, we present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, when two graphene sheets are twisted by an angle close to the theoretically pre-dicted “magic angle,” the resulting flat band structure near the Dirac point gives rise to a strongly correlated electronic system. These flat bands exhibit half-fill-ing insulating phases at zero magnetic field, which we show to be a correlated insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping this system, we find electrically tunable superconductivity in it, with many characteristics similar to the superconductivity of high-temperature cuprates. These unique properties of magic-angle twisted bilayer graphene open a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without a magnetic field. We also present data demonstrating nematicity in the superconducting state, strange metal behavior at correlated fillings with near Planckian dissipation, and correlated states in other types of graphene superlattices. This novel platform may pave the way towards more exotic correlated systems."
Giant Enhancement of Interlayer Exchange in an Ultrathin 2D Magnet,"A primary question in the emerging field of two-di-mensional van der Waals magnetic materials is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a dif-ferent magnetic ground state for ultrathin exfoliated films, but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order (Figure 1), finding a ten-fold enhance-ment in the antiferromagnetic interlayer exchange compared to bulk crystals.Moreover, polarization-dependent Raman spectroscopy (Figure 2) reveals that exfoliated thin films of CrCl3 possess a different low-temperature stacking order than bulk crystals. Temperature-dependent Raman spectra further attribute this difference in stacking to the absence of a stacking phase transition in these thin films, even though it is well established in bulk CrCl3. We hypothesize that this difference in stacking is the origin of the unexpected magnetic ground states in the ultrathin chromium trihalides. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3."
Splitting of 2D Materials with Monolayer Precision,"Traditionally, two-dimensional (2D) heterostructures at the micrometer-scale level are formed by using adhesive tape, which requires isolating 2D flakes in monolayers from bulk material. However, this is a very time-consuming and random process. Moreover, al-though flakes have been isolated into a nominal mono-layer, the lateral dimensions (hundreds of micrometers) are not sufficient to guarantee the fabrication of large-scale 2D heterostructures.We introduce a layer-resolved splitting (LRS) technique that can be applied universally to harvest multiple 2D material monolayers at the wafer scale (5-centimeter diameter) by splitting single stacks of thick 2D materials grown on a single wafer. Figure 1 shows a schematic of the LRS process. The LRS process is initiated by depositing a Ni film and exfoliating the entire WS2 stack from the sapphire wafer. A Ni layer is deposited on the bottom of the WS2 film while retaining the top tape/Ni/WS2 stack as-exfoliated to harvest the a continuous WS2 monolayer. The Ni/WS2 stack is separated upon peeling while the bottom Ni strongly adheres to the WS2 monolayer, leaving a monolayer of WS2 on the bottom Ni layer. We transferred this monolayer film onto an 8-inch (20.3 cm) Si wafer coated with 90 nm of SiO2 (Figure 2A). Figure 2B shows the wafer-scale photoluminescence mapping image, which indicates that the 2D monolayer isolation was uniform across the entire 2-inch wafer area.We then fabricated arrays of 2D heterostructure devices at the wafer scale (10x10 arrays of MoS2 transistors on a 1-cm2 wafer) (Figure 2C). The transistors without h-BN exhibited very large hysteresis in their drain current-gate voltage sweep, which is detrimental to a transistor’s operation. However, substantial suppression of hysteresis has been observed in transistors with h-BN (Figure 2D)."
Investigation of Atomic Interaction through Graphene via Remote Epitaxy,"Remote epitaxy opens the possibility of growing epi-taxial films that “copy” the substrate crystal structure through a 2D material interlayer, enabling the produc-tion of ultrathin components for device integration. We report advances in understanding the physics of the in-teraction between the substrate and the epitaxial film. Remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. The ionicity of the substrate material determines the distance at which its potential field is still effective for epitaxy (Figure 1). However, such field penetration can be substantially attenuated by hexagonal boron nitride (hBN), which itself has polarization in its atomic bonds. A transition from remote epitaxy to van der Waals epitaxy can be seen with an increasing number of hBN layers (Figure 2). Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes (Figure 3), which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications."
Experimental Characterization and Modeling of Templated Solid-state Dewetting of Thin Single Crystal Films,"Templated solid-state dewetting of thin single crystal films has shown potential for use as a self-assembly method for fabrication of regular, complex structures with sub-lithographic length scales (Figure 1 and first Reading below). This potential can be realized by un-derstanding and controlling dewetting instabilities and mechanisms that lead to different dewetting morphol-ogies. Since dewetting instabilities, and hence the re-sulting morphologies , depend on a number of param-eters, including crystal structure of the film (fcc, hcp, etc.), texture of the film, initial film thickness, annealing ambient, temperature, and geometry of the initial tem-plate for the film before subject to dewetting, there is a great opportunity/challenge that we are addressing through both experiments and computationally.During the past several years, we have used pre-patterned single-crystal Ni(110) or (100) epitaxial films grown on MgO as a model system and have identified and studied individual dewetting instabilities, including corner-induced instability and Rayleigh-like instability. We are currently focusing on a fingering instability that can occur during edge retraction and results in the formation of a parallel array of wire-like features. An observation motivated our current study that rough edges produced by poor lithographic edge definition led to fingering instabilities. To better understand the effect of edge roughness on the fingering instability and to control the instability, we patterned edges of large Ni(110) lithographically defined patches with a wide range of periodic perturbations. The edges of patches with the same periodic perturbation were also aligned along different crystallographic in-plane orientations to studying anisotropic effects on a templated fingering instability. We have found that fingering can be induced from those film edges with periodic perturbations (Figure 2), demonstrating that development of the fingering instability has a strong correlation with edge roughness. Furthermore, the template not only induced fingering instabilities but also provided control of the period of the fingers and the corresponding parallel wire-like structures. We have developed a kinetic model that predicts the relationship between the retraction rate of fingers and the templated finger period and are testing this model through additional experiments."
"Toward Robust, Condensation-resistant, Omniphobic Surfaces","Surfaces that are repellent to liquids have broad appli-cations in anti-fouling, chemical shielding, heat trans-fer enhancement, drag reduction, self-cleaning, water purification, and icephobic surfaces. State-of-the-art omniphobic surfaces based on reentrant surface struc-tures repel all liquids, regardless of the surface mate-rial, without requiring low-surface-energy coatings. While omniphobic surfaces have been designed and demonstrated, they fail catastrophically during con-densation, a phenomenon ubiquitous in both nature and industrial applications. Specifically, as condensate nucleates within the reentrant geometry, omniphobic-ity is destroyed. Here, we show a nanostructured surface that can repel liquids even during condensation (Figure 1). This surface consists of isolated reentrant cavities with a pitch on the order of 100 nanometers to prevent droplets from nucleating and spreading within all structures. We developed a model to guide surface design and subsequently fabricated and tested these surfaces with various liquids. We demonstrated repellency to various liquids up to 10 °C below the dew point and showed durability over three weeks. Furthermore, the design is robust to defects or damage to the surface. This work provides important insights for achieving robust, omniphobic surfaces."
Observation of Second Sound in Graphite above 100K,"Second sound is an unusual phenomenon in which heat transports in a wave-like manner, rather than by the more usual diffusive motion. This wave-like motion is a result of the dominance of normal phonon-phonon scattering, which conserves the total phonon momen-tum over any other phonon resistive scatterings. Simi-lar to a gas system, where particles scatter without los-ing total momentum, phonons gain an average velocity under a temperature gradient when normal scattering dominates, and their transport is said to be in the hy-drodynamic regime. In this regime, a heat pulse prop-agates as a wave similar to the way a pressure impulse generates sound waves, and this wave is called second sound. Previously, second sound has been observed in only a few materials at very low temperature (<20 K). Recently, we successfully predicted and observed second sound in graphite up to 150 K using the transient thermal grating (TTG) technique. In TTG, two transient pump laser beams create interference at the sample surface and generate a thermal grating. A probe beam detects the transient decay of the thermal grating. When the phonon system is diffusive, the thermal grating will decay diffusively with fixed peak and valley positions, corresponding to an exponential decay signal, as shown by the curves at 200 K and 300 K in Figure 1A. However, between 85-150K, the heat wave motion leads to an oscillating exponential signal (hallmark of second sound in TTG), as shown by the curves at 85-150 K. The experimental result is well supported by an ab initio simulation (Figure 1B)."
Ultrahigh Thermal Conductivity and Mobility in c-BAs,"As the transistor density gets larger and larger in to-day’s central processing unit, thermal management becomes necessary to improve reliability and prevent overheating failure. Utilizing ultrahigh thermal con-ductivity materials that can help efficiently dissipate the generated heat from the chips is one of the passive cooling strategies in electronics. In this way, diamond, as the highest thermally conducting material, has been used as the heat spreader. However, diamond is limited by its high cost and interface issues like poor thermal and mechanical coupling to common semiconductors. Therefore, finding other ultrahigh thermal conductiv-ity materials that can totally or partially overcome the limitation of diamond can be significantly beneficial. Recently, our group with collaborators has predicted, synthesized, and measured ultrahigh thermal conductivity in cubic barium arsenides (c-BAs). First, c-BAs samples of mm-size were successfully synthesized by the chemical vapor transport technique at the University of Houston. With a metal layer coated on top of the sample surface as the transducer, we carried out thermal transport measurements on the samples using time-domain thermoreflectance and frequency-domain thermoreflectance, and the measured thermal conductivity is as high as ~1200 W/mK at room temperature (Figure 1). This places c-BAs as the second most heat-conducting cubic material. In addition, c-BAs is a semiconductor with an indirect bandgap around 1.7 eV. We predict that they have comparably high mobility for both electrons and holes (Figure 2). The high thermal conductivity and high mobility of c-BN promise interesting applications in microelectronics."
Morphological Stability of Single Crystal Co and Ru Nanowires,"High-performance integrated circuits contain tens of kilometers of metal interconnects, the cross-sectional area of which must shrink in lockstep with shrinking transistors. The reliability of integrated circuits is con-tingent upon morphologically stable interconnects. At the tiny length scales of next-generation intercon-nects, the electrical resistance of Ru and Co nanowires is expected to be lower than that of nanowires based on current copper technology; thus the morphological stability of these two materials is of particular practical interest. Solid-state dewetting by surface self-diffusion is often the dominant mechanism by which the mor-phology of micro- and nano-scale features evolve at el-evated temperatures. As feature dimensions decrease, the temperature at which dewetting also occurs drops, which can lead to significant morphological degrada-tion at surprisingly low homologous temperatures. Al-though solid-state dewetting is fairly well understood in isotropic systems, the dewetting behaviors of aniso-tropic, crystalline solids are far more complicated and more experimental, and modeling work is required to identify crystallographic characteristics that will opti-mize morphological stability.Previous work on single-crystal Ni films has demonstrated that crystalline anisotropy gives rise to special crystallographic orientations along which single-crystal wires are kinetically resistant to morphological instabilities. The strongly faceted surfaces of these wires are also predicted to reduce electron scattering and decrease interconnect resistance. For Ru nanowires, exploratory work with single-crystal (0001) films suggests that wires oriented along <1-210> directions will be particularly stable. Work on patterning and testing of such wires is currently underway. We have also begun similar experiments on single-crystal Co films and will compare our results across the Co, Ni, and Ru systems to construct a more fundamental understanding of dewetting behavior in crystalline nano-scale structures such as interconnects."
Field Controlled Defects in Layered Cuprate-based Materials,"Both the nature and concentration of oxygen defects in oxide materials can have a significant impact on their physical and chemical properties, as well as on key interfacial reaction kinetics such as oxygen exchange with the atmosphere. Most commonly, the desired oxy-gen defect concentration, or equivalently oxygen non-stoichiometry, is attained by doping with aliovalent cations and/or controlling the oxygen partial pressure and temperature in which the materials are equilibrat-ed or annealed. These approaches, however, are limit-ed by dopant solubility limits and the range of oxygen partial pressures readily experimentally achievable, and they require knowledge of the applicable defect chemical model. In this study, we fine-tune oxygen defect concentrations in rare earth cuprate (RE2CuO4: RE = rare earth) solid oxide fuel cell (SOFC) cathode materials by application of electrical potentials across an yttria-stabilized zirconia (YSZ) supporting electrolyte. These layered perovskites can incorporate both oxygen interstitials, and vacancies, thereby broadening the range of investigations. Here, we show a strong correlation between oxygen nonstoichiometry values (which are determined by in-situ measurement of chemical capacitance) and oxygen surface exchange kinetics (which are inversely proportional to the area-specific-resistance) without changing cation chemistry. Both types of oxygen defects, interstitials and vacancies, dramatically enhance surface kinetics. These studies are expected to provide further insights into the defect and transport mechanisms that support enhanced SOFC cathode performance."
Mixed Electron-proton Conductor Membrane Mediates H2 Oxidation,"Electrochemical transformations are key to the inter-conversion of electrical and chemical energy and ubiq-uitous in the formation of commodity chemicals. Elec-trocatalysts which enable these transformations must serve to both activate chemical bonds and facilitate electron-proton transfer. In conventional electrocatal-ysis, these two functions occur at a singular catalyst electrolyte interface that prevents independent opti-mization of either process; changes to the interface will inherently affect both functions. Critically, the optimal interface for one function often does not coincide with the optimal structure for the other. We have shown that for hydrogen oxidation reaction (HOR), these two functions can be segregated by interposing a mixed electron-proton conductor (MEPC) membrane between the catalyst and electrolyte.We have designed a device that enables concurrent electrochemical proton-electron extraction at an MEPC electrolyte interface and H2 activation at a gas catalyst interface. A reduced WO3 (WOx) membrane supported on a porous support is decorated with a platinum catalyst on one side (Figure 1). At the gas Pt interface, H2 is dissociatively activated at Pt surfaces to generate H-atoms. The resulting H-atoms migrate across the Pt WOx boundary to intercalate into the WOx via H-spillover and diffuse through the WOx membrane. At the MEPC electrolyte interface, the applied electrochemical potential drives the separation of protons and electrons with protons entering the solution and electrons passing current through the external circuit. This work represents the first demonstration of employing an MEPC membrane to segregate the bond activation and charge transfer functions in electrocatalysis.These devices exhibit respectable current densities that exceed 20 mA cm–2 at 0.5 V vs. RHE. We found that the thickness of the membrane does not limit the rate of H2 oxidation catalysis, suggesting that H-diffusion within the WOx membrane is relatively rapid. Instead, the condensed MEPC membrane serves as a barrier to prevent impurities and poisoning species dissolved in the electrolyte to degrade HOR catalysis. On the other hand, the rate of HOR depends on the Pt sputtering time (Figure 2). An increasing rate was found, up to 35 s of Pt deposition, which decreased upon continued sputtering. This suggests that H-spillover across the Pt WOx boundary limits the overall rate of HOR and a 35 s deposition of Pt maximizes the Pt WOx boundary line density. Future work focuses on a selection of materials for these devices to enable a library of diverse reactivity."
Dynamic Approach of Quantifying Strain Effects on Ionic and Electronic Defects in Functional Oxides,"The search for novel electronic and magnetic proper-ties in functional oxides has generated a growing in-terest in understanding the mobility and stability of ionic and electronic defects in these materials. Instead of altering material content, most research views me-chanical strain as a lever for modulating defect concen-tration and mobility more finely and continuously in both semiconductors and function-al oxides. Previous studies also proposed that strain may increase ionic mobility by orders of magni-tude, which is crucial for lowering the operation temperature of solid oxide fuel cells.However, experimental and computational results differ significantly among research groups due to the convoluted effect of mechanical strain and film/substrate interface on defect content and mobility. Such reliance on substrate selection to induce strain in the oxide thin film also limits the range of strain accessible, with limited data available to date.We have developed an experimental technique that facilitates application of in-plane strain to functional oxide thin films continuously on the same substrate. First, we combine photolithography and metal sputtering to deposit an interdigitated Pt electrode on our sample (Figure 1). Next, we conduct 3- or 4-point bending and concurrent conductivity measurement of the thin film-on-substrate device (Figure 2). This approach is accessible to a wide temperature range and precise gas control relevant to mixed ionic-electronic conducting oxides. We can strain and measure the transport properties of the same functional oxide thin film at high temperature in situ, over a range of strains applied to a single system. Combining these experiments with our ab initio computational simulations and predic-tions of carrier dominance over a range of strains and temperatures, we also aim to measure the carri-er mobility in Nb-doped SrTiO3 as a function of applied strain, to observe the sudden change of carrier mobility and temperature dependency. We believe this will also be a powerful technique for studying the strain effect on surface reactions like exsolution or catalytic reaction."
3-D Printed Microarchitected Ceramics for Low-heat Capacity Reactors,"Efficient heat and mass transfer for catalytic reactors are desirable for a broad array of biological and envi-ronmental applications and are of great import to the automotive and power plant industry. The conversion efficiency of catalytic reactors relies on the tempera-ture of the constituent substrate and its thermal re-sponse. Although porous substrates with thinner cell/pore walls and higher cell/pore density enable faster catalyst activation due to low thermal mass and, larger surface area, the manufacturing of well-engineered structures with thin walls and higher cell/pore density remains a challenge. For example, there is a practical limit to the maximum cell density and the minimum wall thickness of the honeycomb substrate caused by difficulties in the extrusion-based process, such as larger flow drag force and inhomogeneity. Another promising candidate, the open-cell foams, also suffer manufacturing and assembly difficulties due to their low mechanical strength, high flow resistance, and high heat capacity caused by random-pore architectures. To overcome these limitations, we proposed manufacturing-friendly structural design and additive manufacturing for microarchitected ceramic substrates having both a large catalytic surface area and low thermal mass. Our idea for achieving efficient catalytic substrates is leveraging 3D micro-lattices of thin-walled tubular networks instead of conventional honeycomb monoliths (Figure 1). We measured surface temperature by thermal IR camera (Figure 1) and investigated the thermal response of each architecture (Figure 2). The proposed 3D hollow micro-lattice was heated up and cooled down faster than the monoliths’ structure. This result verifies the low thermal mass of the proposed 3D micro-lattice ceramic to enable a faster thermal response for the faster catalytic activation. Therefore, we expect that the proposed 3D ceramic micro-lattice structure will have a high catalytic conversion efficiency and accelerate the development of an efficient gas purification system for automotive and environmental applications."
Additively Manufactured Externally-fed Electrospray Sources,"Additive manufacturing (AM) is a layer-by-layer fabri-cation technique that creates solid objects by putting material where needed, instead of removing material from stock. Recent advances in AM have made possi-ble the implementation of microsystems that surpass the performance of state-of-the-art counterparts made in a clean room, as well as the demonstration of devic-es that are challenging or unfeasible to create using standard microfabrication–particularly in the area of microfluidics. In addition, AM is inherently compatible with implementing, with great precision, hierarchical structures with features spanning orders of magnitude in size to accomplish multiple tasks efficiently.In this project, we are exploring AM to develop, at a low-cost, massively multiplexed externally-fed electrohydrodynamic liquid ionizers (Figure 1) for a wide range of applications such as mass spectrometry, nanosatellite propulsion, species transport, and agile manufacturing. These devices are mesoscaled arrays of high-aspect-ratio, hundreds-of-microns tall, micron-sharp tips that are conformally covered with a nanostructured layer that transports and regulates the flow of liquid from the reservoir to the emission sites. Manufacturing issues such as inter-process compatibility and tip array uniformity need to be addressed to implement devices that operate efficiently successfully. Current work focuses on exploring and optimizing various manufacturing techniques to monolithically create the electrospray source out of different structures made of different materials; future work includes assessment of device performance, e.g., emission characteristics and uniformity."
Additive Manufacturing of Microfluidics via Extrusion of Metal Clay,"Most microfluidics uses closed microchannels to effi-ciently accomplish tasks such as species mixing, heat transfer, and particle sorting by increasing the sur-face-to-volume ratio of the fluid(s) involved in the pro-cess. However, the current manufacturing techniques for microfluidics present disadvantages such as high-cost, long production time, no device customization, elaborated design iteration, restriction in the kinds of structures that can be made, and low fabrication yield.Recent research results demonstrate that additive manufacturing can readily address the shortcomings outlined, often yielding devices that surpass the state of the art or for which traditional microfabrication creates no counterpart. However, most 3D-printed microfluidics are made of polymeric feedstock, which is not compatible with high-pressure and/or high-temperature applications. Mainstream 3D printing methods for metal include lost-wax micro molding, inkjet binder, and direct metal laser sintering; these processes are unideal to produce monolithic closed-channel microfluidics because they either require internal dummy structures or create internal voids filled in with unprocessed printable material, both of which are challenging to remove from the printed part.In this project, we are exploring the use of extrusion of metal clay to implement closed-channel microfluidics; the technique is arguably similar to fused filament fabrication and can readily create voids without spurious infill or post-processing required. Via the extrusion of metal clay, leak-tight metal microchannel with monolithic, working ports have been created (Figure 1). A cross-section of the microfluidic shows an unclogged microchannel, evidencing the feasibility of the technique to create closed channels with hydraulic diameters of relevance to microfluidics (Figure 2). Current work focuses on exploring the design space of the technology and demonstrating an application of relevance."
"3D-Printed, Low-cost, Miniature Liquid Pump","Many compact systems use pumps to precisely set flow rates of liquid or, in general, to manipulate small liquid volumes for effective mass transport, cooling, or momentum transfer. Numerous microfabricated posi-tive displacement pumps for liquids with chamber vol-umes that are cycled using valves have been proposed. Pumps made via standard (i.e., cleanroom) microfabri-cation typically cannot deliver large flow rates with-out integrating hydraulic amplification or operating at high frequency due to their small pump chambers.Additive manufacturing, i.e., the layer-by-layer fabrication of objects using as template a computer-aided design model, has recently been explored as a processing arena for microsystems. In particular, researchers have reported 3-D printed pumps for liquids and gases with performance on par or better than counterparts made with standard microfabrication. Building upon earlier work on printed MEMS magnetic actuators, we recently developed miniature liquid pumps printed in pure nylon 12 via fused filament fabrication (FFF) whereby a thermoplastic filament is extruded from a hot nozzle to create a solid object layer by layer.Our low-cost, leak-tight, miniature devices are microfabricated using 150- to 300-µm layers with a multi-step printing process (Figure 1) that monolithically creates all key features with <13- µm in-plane misalignment. Each pump has a rigid frame, a 21-mm-diameter, 150-µm-thick membrane connected at its center to a piston with an embedded magnet, chamber, passive ball valves, and two barbed fluidic connectors (Figure 2). Pump fabrication under 2 hours and costs less than $4.65 are achieved. Finite element analysis of the actuator predicts a maximum stress of 18.7 MPa @ 2-mm deflection, about the fatigue limit of nylon 12 (i.e., 19 MPa). A maximum water flow rate of 1.37 ml/min at 15.1 Hz actuation frequency is calculated, comparable to reported values of miniature liquid pumps with up to 200X higher actuation frequency."
3D-Printed Microfluidics to Evaluate Immunotherapy Efficacy,"Microfluidic devices are conceptually an ideal platform for the provision of personalized medical evaluations as they require small analyte volumes and facilitate rapid and sensitive investigations. However, inher-ent challenges in device fabrication have impeded the widespread adoption of microfluidic technologies in the clinical setting. Additive manufacturing could ad-dress the constraints associated with traditional mi-crofabrication, enabling greater microfluidic design complexity, fabrication simplification (e.g., removal of alignment and bonding process steps), manufacturing scalability, and rapid and inexpensive design iterations. We have developed an entirely 3D-printed microfluidic platform that enables modeling of interactions between tumors and immune cells, providing a microenvironment for testing the efficacy of immunotherapy treatment. The monolithic platform allows for real-time analysis of interactions between a resected tumor fragment and resident or circulating lymphocytes in the presence of immunotherapy agents. Our high-resolution, non-cytotoxic, transparent device monolithically integrates a variety of microfluidic components into a single chip, greatly simplifying device operation vs. traditionally-fabricated microfluidic systems. The 3D-printed device sustains viability of biopsied tissue fragments under dynamic perfusion for at least 72 hours while enabling simultaneous administration of drug treatments, illustrating a useful tool for drug development and precision medicine for immunotherapy. Confocal microscopy of the tumor tissue and resident lymphocytes in the presence of fluorescent tracers provides real-time monitoring of tumor response to various immunotherapy. The platform and accompanying analysis methods distinguish between a positive immune response and a lack of tissue response in the presence of immunotherapeutic agents.This platform introduces novel methodologies in modeling and analyzing tumor response to improve prediction of patient-specific immunotherapy efficacy. To the best of our knowledge, this is the first report of human tumor fragments cultured in a dynamic perfusion system capable of testing the effect of circulating immune checkpoint inhibitors on resident tumor-infiltrating lymphocytes."
Electrohydrodynamic Printing of Ceramic Piezoelectric Films for High-frequency Applications,"The high operating frequencies that ceramic piezoelec-tric ultra-thin films attain have made possible exciting applications such as energy harvesting, telecommuni-cations’ filters, high sensitivity biosensors, and acous-tofluidic devices; however, the inherently high cost and complexity of current manufacturing methods limit, in general, their widespread use. Additive manufacturing (AM), which has proven successful in creating complex devices and components of relevance to micro and nanosystems, could overcome these disadvantages; nevertheless, AM of piezoelectrics has been achieved only with polymer-based materials–unsuitable for said applications.We report the first additively manufactured ceramic ultra-thin piezoelectric films compatible with high-frequency applications using electrohydrodynamic deposition (EHD) at room temperature. The films were made by electrospraying a zinc oxide (ZnO) nanoparticle liquid feedstock, directly writing line imprints as thin as 213 nm and as narrow as 198 µm. We harness a previously unreported effect to align the polar axis of the imprint and obtain overall piezoelectricity. As Figure 1a shows, the (100) orientation monotonically increases as the linear density of the deposition is reduced by increasing the raster speed or reducing the feedstock flow rate (Q)–provided two conditions are met: the feedstock is ionized (via EHD), and a small separation between emitter and substrate is used. Notably, the orienting effect directly acts on the direction of the polar axis by means of the rastering direction (Figure 2a), allowing for vibration modes and resonator configurations that were previously unfeasible. The macroscopic piezoelectric behavior is shown through piezoforce response microscopy (PFM) (Figure 2b) and the suitability for high-frequency applications was demonstrated by testing thin-film bulk acoustic resonators (FBAR) on a flexible polymer substrate, where the resonant frequency of ~5 GHz was used to calculate the acoustic speed of the films (~2,000 m/s), which is close to the transversal wave speed of ZnO."
"3D-Printed, Monolithic, Multi-tip MEMS Corona Discharge Ionizers","A corona discharge is a high-electric field ionization phenomenon caused by the development of a self-sus-tained electron avalanche between a sharp electrode (i.e., corona electrode) and a blunt electrode; the ions create a plasma region around the corona electrode and in their travel to the opposite electrode transfer momentum to the surrounding fluid. In this project, we are harnessing advanced metal inkjet printing technol-ogy to demonstrate massively multiplexed MEMS coro-na discharge ionizers (Figure 1), with the aim to increase greatly their ionization throughput and optimize their transduction mechanism to be able to implement ex-citing applications such as no-moving-parts pumps for gases and compact ion mobility spectrometers.A 1D electrohydrodynamic coaxial cylinder model was implemented in COMSOL Multiphysics to study the ionization and collision processes in air at atmospheric pressure and room temperature of a 1-tip device, predicting a 400-µm-thick corona region surrounding the corona tip. The onset voltage estimated from the simulation is 5.849 kV, which is close to the theoretical value from Peek’s formula of 6.416 kV. In addition, current over voltage (I/V) versus bias voltage minus the onset voltage (V-V0) characteristics were collected for different ionizer array designs while varying the separation between the corona electrode and the collector electrode; the data follow the Townsend current-voltage model (Figure 2). Moreover, the data show that the corona current decreases with increased spacing of the corona electrode-to-collector electrode due to the decrease of the electric field on the tips; however, a smaller separation between the corona electrode and the collector electrode results in larger fluctuations in the corona discharge current. Devices with different numbers of tips tend to generate the same total corona current at the same bias voltage although more tips are set to discharge as the number of tips increases; this increase can be ascribed to the stronger interference between adjacent tips when the tip-to-tip spacing decreases. Current research efforts focus on optimizing the array design to minimize electric field shadowing and sharpening the tips to achieve operation at a lower bias voltage."
3D-Printed Gas Ionizer with CNT Cathode for Compact Mass Spectrometry,"Mass spectrometers are powerful chemical analytical instruments used to quantitatively characterize the composition of unknown samples via ionization and mass-to-charge ratio species sorting. However, main-stream mass spectrometers are large, heavy, power hungry, and expensive, limiting their applicability in real-time and in-situ applications. Gas molecules can be ionized via electron impact ionization (EII), for which a source of electrons, i.e., a cathode, is required. Cold cathodes emit electrons into a vacuum via quantum tunneling due to high surface electric fields that low-er and narrow the barrier that traps electrons within the material; typically, high-aspect-ratio, nano-sharp tips are used to produce such fields with moderate bias voltages. Compared to thermionic cathodes, field emission electron sources have faster response and less power consumption. Compared to other field emitters, carbon nanotubes (CNTs) are less affected by back-ion bombardment and chemical degradation. There are nu-merous reports of gas ionizers with CNT cathodes EIIs; however, these devices are microfabricated using clean-room technology and/or use ion-generating structures machined with standard technologies, affecting their cost and size.In this project, we are harnessing additive manufacturing (AM) to develop novel electron impact ionizers that circumvent these challenges. AM has unique advantages over traditional manufacturing methods including compatibility with creating complex 3D geometries, print customization, and waste reduction. Our design (Figure 1) uses inkjet binder printing of SS 316L to create electrodes to efficiently generate ions and steer charged species, stereolithography of polymer resin to define the dielectric structures that electrically isolate the different electrodes, and an additively manufactured CNT electron source. We have successfully characterized the ionizers at pressures as high as 5 mTorr while achieving ionization efficiencies as high as 8.5% (Figure 2)."
Printed CNT Field Emission Sources with Integrated Extractor Electrode,"Field emission cathodes are promising electron sourc-es for exciting applications such as flat-panel displays, free-electron lasers, and portable mass spectrometry where fast switching, low-pressure operation, and low power consumption are favored metrics. A field emitter quantum tunnels electrons to a vacuum due to the high electrostatic fields at its surface; this tunneling is typi-cally done at low voltage using a whisker-like structure. Carbon nanotubes (CNTs) are attractive structures to produce electron field emission due to their ultrasharp tip diameter, high aspect ratio, high electrical conduc-tivity, and excellent mechanical and chemical stability. Although CNT-based cold cathodes have been widely reported in the literature, their manufacture could be quite expensive (e.g., devices partially or fully made in a semiconductor cleanroom), or the extractor electrode of the cathode is an external mesh, causing high-beam interception (e.g., in screen-printed devices) or requir-ing an advanced method of assembly to the emitting component to achieve high transmission.In this project, we are developing novel field emission sources that are fully additively manufactured to circumvent the aforementioned challenges. The devices are made via direct ink write (DIW) printing, which is one of the least expensive and most versatile additive manufacturing methods as is capable of monolithic multi-material printing. Compared to screen printing, DIW does not involve static masks to transfer patterns and produces significantly less waste. The fully-printed field emission electron source is composed of two continuous imprints: a spiral trace made of a CNT compound, acting as an emitting electrode, symmetrically surrounded on both sides by a spiral trace made of silver nanoparticles, acting as in-plane extractor electrode (Figure 1). After printing, the CNT spiral receives a mechanical treatment that releases the CNT tips from the bulk of the imprint (Figure 2), enabling field emission from the CNT imprint. Characterization of the printed CNT field emission sources in triode configuration (i.e., using an external anode) shows low turn-on voltage and low interception of the emitted current by the extractor electrode. Current work focuses on design optimization and experimental characterization of the devices."
Controlling the Nanostructure in Room-temperature-microsputtered Metal,"Sputter deposition involves the ejection of atoms from a target and the atoms’ subsequent deposition on a nearby substrate. Because the deposition is done on the atomic level, the nanostructure of the deposit can vary significantly. This variance is of concern, as it can great-ly affect material strength and conductivity. Tradition-al sputtering relies on vacuum and thermal annealing to ensure dense, highly conductive deposits. However, agile manufacturing on temperature-sensitive sub-strates is not compatible with these two solutions.To enable high-quality material without heating the material or requiring a vacuum, we performed a statistically-motivated set of experiments to determine what deposition parameters improve the material quality. We developed an empirical model and found that an appropriate electrical bias voltage, applied either to the substrate or to a conductive plate under the substrate, has the greatest impact on the material quality. This is due to the presence of charged nanoparticles, formed by collisions between sputtered atoms in the dense plasma around the sputter target. The applied electric field attracts positively charged nanoparticles, allowing the nanoparticles to strike the substrate with more energy than their temperature alone would dictate. This extra energy enhances the mobility of the deposited metal, allowing it to form denser, more energetically favorable coatings (Figure 1) without significant substrate heating. With this technique, we have improved the conductivity of the sputter coating to 5x bulk metal (15 µΩ·cm) at room temperature.Applied electric fields also improve the coating’s thickness. In the absence of electric fields, the sputtering process is self-limiting. As the positively charged sputtered material reaches the substrate, charge builds upon the substrate, repelling charged sputtered material and preventing the deposit from thickening. However, biasing the substrate with a negative voltage prevents this charge from accumulating, allowing for thicker (> 200 nm) films."
Gated Silicon Field Ionization Arrays for Compact Neutron Sources,"Neutron radiation is widely used in various applications, ranging from the analysis of the composition and structure of materials and cancer therapy to neutron imaging for security. However, most applications require a large neutron flux that is often achieved only in large infrastructures such as nuclear reactors and accelerators. Neutrons are generated by ionizing deuterium (D2) to produce deuterium ions (D+) that can be accelerated towards a target loaded with either D or tritium (T). The reaction generates neutrons and isotopes of He, with the D-T reaction producing the higher neutron yield. Classic ion sources require extremely high positive electric fields, on the order of 108 volts per centimeter (10 V/nm). Such a field is achievable only in the vicinity of sharp electrodes under a large bias, and consequently, ion sources for neutron generation are bulky. This work explores, as an alternative, highly scalable and compact Si field ionization arrays (FIAs) with a unique device architecture that uses self-aligned gates and a high-aspect-ratio (~40:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array (Figure 1). The tip radius has a log-normal distribution with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm in diameter and is within 200 nm of the tip. Field factors, β, > 1 × 106 cm-1 can be achieved with these Si FIAs, implying that gate-emitter voltages of 250-300 V (if not less) can produce D+ based on the tip field of 25-30 V/nm. In this work, our devices achieve ionization current of up to 5 nA at ~140 V for D2 at pressures of 10 mTorr. Gases such as He and Ar can also be ionized at voltages (<100 V) with these compact Si FIAs (Figure 2)."
Silicon Field Emitter Arrays (FEAs) with Focusing Gate and Integrated Nanowire Current Limiter,"The advent of microfabrication has enabled scalable and high-density Si field emitter arrays (FEAs). These are advantageous due to compatibility with comple-mentary metal-oxide-semiconductor (CMOS) process-es, the maturity of the technology, and the ease in fabri-cating sharp tips using oxidation. The use of a current limiter is necessary to avoid burn-out of the sharper tips. Active methods using integrated MOS field-effect transistors and passive methods using a nano-pillar (~200 nm wide, 8 µm tall) in conjunction with the tip have been demonstrated. Si FEAs with single gates re-ported in our previous works have current densities >100 A/cm2 and operate with lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essential to control the focal spot size of the electron beam as electrons leaving the tip have an emission angle of ≈ 12.5°. The focus electrode provides a radial electric field that reduces the lateral velocity of stray electrons and narrows the cone angle of the beam reaching the anode. Varying the voltage on the focus gate reduces the focal spot size or achieves an electron beam modulator for radio frequency applications. In this work, we fabricate dense (1-μm pitch) double-gated Si with an integrated nanowire current limiter (Figure 2). The apertures are ~350 nm and ~550 nm for the extractor and focus gates, respectively, with a 350-nm-thick oxide insulator separating the two gates. Electrical characterization of the fabricated devices shows that the focus-to-gate ratio (VFE/VGE) can be used to control the anode current (Figure 2). When the focus voltage exceeds the gate voltage, the field superposition increases the extracted current, and vice versa. These devices can potentially find applications as high-current focused electron sources in flat panel displays, nano-focused X-ray generation, and microwave tubes."
Highly Uniform Silicon Field Emitter Arrays,"Cold cathodes based on silicon field emitter arrays (FEAs) have shown promise in a variety of applica-tions requiring high-current-density electron sources. However, FEAs face a number of challenges that have prevented them from achieving widespread use in commercial and military applications. One problem limiting the reliability of FEAs is emitter tip burnout due to Joule heating. The current fabrication process for FEAs results in a non-uniform distribution of emit-ter tip radii. At a fixed voltage, emitters with a small ra-dius emit a higher current while emitters with a large radius emit a lower current. Therefore, emitters with a small radius reach their thermal limit due to Joule heating at lower voltages and consequently burn out. Previous solutions to tip burnout have focused on lim-iting the emitter current with resistors, transistors, or nanowires to obtain more uniform emission current.In this project, we focused on increasing the uniformity of emitter tip radii as a means to reduce tip burnout. Figure 1 shows a typical distribution of emitter tip radii for FEAs. The non-uniform distribution of emitter tip radii first forms during the photolithography step that defines the array of “dots” that become the etching mask for the silicon tips. In our FEA fabrication process, we used a trilevel resist process that nearly eliminated the light wave reflected at the photoresist/silicon interface and hence improved the uniformity of the dot diameter. Furthermore, we integrated the emitter tips with silicon nanowires to improve their reliability. Figure 2 shows a diagram of the fabricated structure. Our fabrication process resulted in FEAs with more uniform emission current and potentially a longer lifetime."
Development of a Subnanometer-Precision Scanning Anode Field Emission Microscope,"Field emitter arrays (FEA) have not found widespread adoption in demanding applications such as THz, RF, Deep UV, X-ray, electron, ion, and neutron sources, where high current (>1 mA) and long operating lifetime (>10,000 hrs.) are required. These limitations arise as a result of the sensitivity of emitted electron and ion cur-rent to the spatial non-uniformity of emitter tip radi-us and the temporal non-uniformity of work function due to adsorption and desorption of gas molecules at the surface of the emitter tips. These non-uniformi-ties result in the variation of the field factor (β), a key performance parameter for field emitters and ioniz-ers. Variations in β result in severe underutilization of emitter tips as only few tips with large β contributes to emission or ionization current. These tips, if not pro-tected, burn out, leading to low emission current and very short operation lifetime. Emission current and operational lifetime of a FEA could thus be improved by making emitters with more uniform characteristics. We are developing a subnanometer-precision scanning anode field-emission microscope (SAFEM) that could be used to probe the fundamental processes in the operation of emitter tips of FEA. The SAFEM is designed to precisely and accurately position, with subnanometer resolution, a probing anode over the tips of an FEA and in scanning mode directly acquire the spatial map of emission tip current. From the map, other characteristics of the emitter tip such as anode voltage, tip radius, density, and field factor can be extracted. The map of extracted parameters could yield insight into the operation of the FEAs. Also, a statistical distribution of the field factor will enable study of the dependence of tip characteristics on the fabrication process and thereby enable exploration of novel process for engineering high-performance FEAs with high current densities and long operational lifetimes."
Nanoscale Vacuum Channel Transistors Operation in Poor Vacuum,"Nanoscale vacuum channel transistors (NVCTs) using field emission sources could potentially have supe-rior performance compared to solid-state devices of similar channel length. This superior performance is due to ballistic transport of electrons, shorter transit time, and higher breakdown voltage in vacuum. Fur-thermore, there is no opportunity for ionization or avalanche carrier multiplication imbuing NVCTs with very high Johnson figure of merit (~1014 V/s). However, field emitters need ultra-high vacuum (UHV) for reli-able operation as the field emission process is sensitive to barrier height variations induced by adsorption/de-sorption of gas molecules. Small changes in the barrier height cause exponential variations in current. Poor vacuum also leads to the generation of energetic ions that bombard the emitters, altering their work func-tion and degrading electrical performance. Graphene can be used to nano-encapsulate the field emitter in UHV or a gas (e.g., He) with high ionization energy to overcome the UHV requirement. Separation of the electron tunneling region from the electron acceleration region enables emission of electrons in UHV and electron transport in poor vacuum, if not atmospheric conditions. For mechanical strength, a multi-layer graphene structure that is transparent to electrons while being impervious to gas molecules/ions is necessary. In this work, we studied the electron transparency and robustness of multi-layer graphene. We fabricated arrays of gated Si field emitters with 1-µm pitch and integrated a nanowire current limiter (Figure 1) that exhibits transistor-like characteristics. Using an energized multi-layer graphene/grid structure (Figure 2) in combination with emitter arrays, we achieved an electron transparency of ≈ 20%. We envision electron transparencies close to ~100% with an optimized design. Adopting this architecture for NVCTs will allow the realization of empty state electronics capable of functioning at higher frequencies (THz regime), higher power, and harsher conditions (high radiation and high temperature) than solid-state electronics."
Micro Rocket Engine using Steam Injector and Peroxide Decomposition,"Rocket engines miniaturized and fabricated using silicon MEMS have been an active area of research for two decades. At these scales, miniaturized steam injectors like those used in Victorian-era steam locomotives are viable as a pumping mechanism and offer an alternative to pressure feed and high-speed turbo-pumps. Storing propellants at low pressure reduces tank mass, and this improves the vehicle empty-to-gross mass ratio; if one propellant is responsible for most of the propellant mass (e.g., oxidizer), injecting it while leaving the others solid or pressure-fed can still achieve much of the potential gain. Previously, the PI and his group demonstrated the feasibility of this pumping concept by designing and testing two ultraminiature-machined stainless steel micro jet injectors that pumped ethanol and by exploring liquid bi-propellant engine designs. Current efforts focus on designing a test article and fabrication process that integrates a jet injector, a decomposition chamber, and a thrust chamber with a solid or liquid fuel to form an injector-pumped partially-pressure-fed or hybrid micro rocket (see Figures 1 and 2). The proposed demonstration launch vehicle integrates these with suitably-sized propellant tanks and structures, derived from or like those found in hobby rocketry. Other applications have also been explored."
Ptychography Development for Soft X-ray Imaging at the Nanoscale,"Ptychography, a powerful imaging modality, has been applied successfully to many experimental systems. Ptychography visualizes an extended object via re-trieval of the far-field phases of a wave scattered from it, from which a complex Fourier transform is extract-ed and a real space image formed by simple Fourier in-version. Removing the need for a high-quality imaging optic, ptychography improves the resolution of micros-copy experiments using high-energy probes, e.g., x-ray and electron microscopy, where lens quality limits ac-curacy. Ptychography also enables quantitative phase contrast imaging. X-ray ptychography in the Bragg geometry enables high-spatial-resolution quantitative studies of nanoscale structures in most electronic and magnetic materials: bulk crystals, thin films, hetero-interfaces, or composite devices. However, inevitable contamination by experimental sources of error limits progress in this area.X-ray ptychography requires high source coherence, monochromaticity, and stability plus precision motion of the source-forming optics. Studying electronic phases with soft x-rays in the Bragg geometry requires high vacuum, a stable cryogenic sample environment, and a full diffractometer; experiments inevitably face multiple serious sources of error. This challenge has deterred serious effort in this direction, despite enormous scientific potential. Intimate knowledge of these issues enabled us to develop computational tools to handle multiple qualitatively different sources of error simultaneously. We define experiment-specific forward models that incorporate parameters describing the dominant sources of error. Applying automatic differentiation and gradient descent-based optimization algorithms such as Adam allows reconstruction of contributions to the error during reconstruction of sample features.We validated our automatic differentiation-based approach with a series of proof-of-concept experiments at the CSX beamline of the National Synchrotron Light Source II (Brookhaven Natl Lab) in transmission and Bragg geometries. Experimental issues included extreme probe positioning errors, large fluctuations in the probe fluence, and reduced probe coherence. We successfully reconstructed test samples with known structures and produced consistent high-quality reconstructions from more realistic samples (see Figure 1). Future directions include studies of electronic symmetry breaking that are enabled by this novel capability, improvements to various error sources, and the ultimate resolution of the reconstructed images."
"Control of the Density, Location, and Properties of Conducting Filaments in TiO2 by Chemical Disorder for Energy-efficient Neuromorphic Computing","Inspired by the efficiency of the brain, redox-based resistive switching (RS) random access memories are considered the next-generation devices to mimic neuromorphic core architectures for pattern recogni-tion and machine learning due to their predicted high memory density, energy efficiency, and speed. Within their metal−insulator−metal architecture, these de-vices store binary code information using the electric field-induced resistance change of the insulating layer by conductive filament (CF) formation and rupture. However, a lack of control on the location and spacing of CF formation, which occurs at chemical and struc-tural defects, and or their properties cause detrimental variation in the devices. We recently initiated a study on the effect of strain on the microstructure, chemistry, and RS properties of TiO2 thin films to get insights into defect formation in view of selectively doping along these defects to eliminate stochasticity in CF formation as schematically depicted in Figure 1."
Control of Conductive Filaments in Resistive Switching Oxides,"There has been a growing interest in using specialized neuromorphic hardware for artificial neural network applications such as image and speech processing. These neuromorphic devices show promise for meet-ing the significant computational demands of such applications with higher speed and lower power con-sumption than software-based implementations. One approach to achieving this goal is through oxide thin film resistive switching devices arranged in a crossbar array configuration. Resistive switching can mimic sev-eral aspects of neural networks, such as short and long-term plasticity, via the dynamics of switching between multiple analog conductance states--dominated by the creation, annihilation, and movement of defects with-in the film (such as oxygen vacancies). These processes can be stochastic in nature and contribute significantly to device variability, both within and between individ-ual devices. This study focuses on reducing the variability of the set/reset voltages and enhancing control of the conductance state with voltage pulsing using model systems of HfO2 and SrTiO3 grown on Nb:SrTiO3 and Si/TiN substrates, by control of film synthesis parameters and composition. By comparing the electrical characteristics of a large number of devices (~100) from each processing condition, film growth conditions may be optimized for maximum resistive switching repeatability. Because the device requirements for practical resistive switching arrays are significant, controlling the variability of individual devices will likely be a consideration for every fabrication and processing step. This work provides a significant step towards understanding the mechanisms behind device variability and achieving devices that meet the strict requirements of neuromorphic computing."
Design and Characterization of Superconducting Nanowire-based Processors for Accelerating  Deep Neural Network Training,"Training of deep neural networks (DNNs) is a compu-tationally intensive task and requires massive volumes of data transfer. Performing these operations with the conventional von Neumann architectures creates un-manageable time and power costs. Recent studies have shown that mixed-signal designs involving crossbar architectures can achieve acceleration factors as high as 30,000× over the state-of-the-art digital processors. These approaches involve the use of non-volatile mem-ory elements as local processors. However, no technolo-gy has been developed to date that can satisfy the strict device requirements for the unit cell. This work presents the superconducting nanowire-based processing element as a cross-point device. The unit cell has many programmable non-volatile states that can be used to perform analog multiplication. Importantly, these states are intrinsically discrete due to the quantization of flux, which provides symmetric switching characteristics. The operation of these devices in a crossbar is described and verified with electro-thermal circuit simulations. Finally, validation of the concept in an actual DNN training task is shown using an emulator."
Metal Oxide Thin Films as the Basis of Memristive Nonvolatile Memory Devices,"The design of silicon-based memory devices over the past 50+ years has driven the development of increas-ingly powerful and miniaturized computers with de-mand for increased computational power and data storage capacity continuing unabated. However, fun-damental physical limits are now complicating further downscaling. The oxide-based memristor, a simple M/I/M structure, in which the resistive state can be re-versibly switched by application of appropriate voltag-es, can replace classic transistors in the future. It has the potential to achieve operating power that is an or-der of magnitude lower than existing RAM technology and paves the way for neuromorphic memory devices relying on non-binary coding. Our studies focus on un-derstanding the mechanisms that lead to memristance in a variety of insulating and mixed ionic electronic conductors, thereby providing guidelines for material selection and for achieving improved device perfor-mance and robustness."
Ion-implantation and Multilayer Oxides with Conductive Spacers for Highly Consistent Resistive Switching Devices,"Resistive switching devices are actively being pursued for use as the fundamental units in next-generation hardware deep-learning or neuromorphic systems. However, these devices are still tricky both to fabri-cate and to operate. Circuits that deploy these resistive switching devices in large arrays start off with a defi-cient capacity, operate erratically, and further degrade throughout their operational lifespan.We identified that simultaneous use of noble metal atom doping and of multilayer oxides will guarantee that devices have high yields after fabrication and high device-to-device and cycle-to-cycle switching consistency (Figure 1). Resistances in the low and high resistance states (LRS/HRS) span just 0.3 decades across devices and 0.05 decades across cycles on the logarithm scale, in comparison to a more typical span of 0.5 to 1.5 decades.Implanted Au atoms in Al2O3 act as bridging atoms for mobile oxygen vacancies in the formation and dissipation of conductive filaments of a hybrid composition. Density functional theory studies found that Au atoms stabilize neighboring oxygen vacancies, can act as a reservoir of vacancies, and enable the ease of switching between LRS and HRS. The DFT studies have also guided further experimental verifications that Pt and Pd are also highly suitable dopants to achieve high-consistency switching. Multi-bit switching could then be easily demonstrated without setting current compliances or using pulsing schemes.The strategy we used to achieve high yield and highly consistent resistive switching devices is broadly applicable to almost all material systems, which means that existing optimized devices can perhaps be further optimized without having to overhaul the existing material stack. The improvement in switching consistency will not just lead to more functional devices, but also simplify the study of future devices in uncovering more of the physics that governs the resistive switching mechanism."
Lithium Neuromorphic Computing and Memories,"Advances over the last years on the understanding and implementation of memristor technology have posi-tioned memristors as a major candidate to overcome the current bottleneck in current electronic-based tran-sistors in terms of downscaling capabilities and energy consumption. In particular, current challenges pre-venting a widespread implementation of oxygen-based memristors in today’s integrated circuits include the need to address cycle-to-cycle and device-to-device variabilities while circumventing electroforming; these inherent issues are associated with the filamentary nature of the switching mechanism. An alternative strategy to tackle challenges might arise by looking at other mobile ions. It remains surprising that despite their fast diffusivity and stability, Li solid-state oxide conductors have been almost neglected as switching materials. On the other hand, the field of Li solid-state batteries has already shown that high Li conductivities are reachable and that the internal capacity to accu-mulate or deplete Li at oxide interfaces can vary over a huge range for electrode materials, enabling a perfect playground for performance-switching engineering.However, the defect chemistry leading to the switching behavior of Li-based materials and the impact of lithiation degree on their performance remain unclear. Our group is researching the problems for Li-based thin films. In particular, we report for the first time the non-volatile, non-filamentary bipolar resistive switching characteristics of lithium titanates compounds, Li4+3xTi5O12, as a function of the lithiation degree. We have employed a recently proposed strategy to overcome Li loss during thin film deposition and finely control the final lithiation degree of the films to create delithiated Li4Ti5O12 and overlithiated Li7Ti5O12 memristive devices. Changing the Li content from a delithiated to an overlithiated phase results in the capability to tune the performance in a wide range in terms of accessible resistance window (from ratios of 102 to 106 at low voltage operation), symmetry (from highly asymmetric to symmetric behavior, respectively) and retention (from a few minutes up to 105 s at room temperature, respectively), among others. In other words, controlling the lithiation degree might offer a suitable path to reduce the stochasticity from which current filamentary memristive devices inherently suffer, mainly due to the difficulties in controlling the number of vacancies generated, and paves the way to further control of ionic migration for novel nanoelectronic devices."
Sub-5-nm Fin-width InGaAs FinFETs by Thermal Atomic Layer Etching,"As complementary metal-oxide semiconductor (CMOS) technology continues to scale down and transistor structures become more three-dimensional, semicon-ductor manufacturing is increasingly more challeng-ing. In recent years, 3D transistors with sub-10-nm physical dimensions have been demonstrated. Pushing forward requires breakthroughs in device fabrication technologies with sub-nm-scale precision and fidelity. In this research, we demonstrated the first III-V 3D transistors with sub-5-nm fin width. This size is made possible by the development of a novel fabrication technology called thermal atomic layer etching (ALE). Thermal ALE can be thought of as the reverse of atomic layer deposition (ALD). Thermal ALE is a plasma-free and benign chemical process that can be integrated with ALD in an in-situ approach in the same reactor. In this work, we have demonstrated the first thermal ALE on III-V compound semiconductors. We achieved a highly controllable etching rate of InGaAs of merely 0.2 Å/cycle. Figure 1 shows a fully suspended InGaAs fin with minimum fin width of 3 nm, covered by an Al2O3/W gate stack, fabricated by the in-situ thermal ALE-ALD technique. Moreover, we illustrated the device worthiness of the thermal ALE technique by fabricating the most aggressively scaled InGaAs fin field-effect transistors (FinFETs) to date, with record fin width down to 2.5 nm. We demonstrated working FinFETs with 2.5-nm fin width and 60-nm gate length, as shown in the subthreshold characteristics in Figure 2. Record ON- and OFF-state transistor characteristics highlight the extraordinary device potential of the in-situ ALE-ALD process."
Impact of Fin Width on Performance in Nanometer-scale InGaAs FinFETs,"InGaAs is a promising n-channel material candidate for future CMOS technology due to its superior electron transport properties and low-voltage operation. In-GaAs fin field-effect transistors (FinFETs) have drawn much interest as they provide both superb transport advantages and great scaling potential. Recently, our group has demonstrated impressive InGaAs FinFETs with fin width down to 5 nm and record channel-aspect ratio. Figure 1 shows cross-sectional schematics of a device across the fin and along the channel directions. The channel is 50-nm-thick InGaAs, and the gate oxide consists of 1 monolayer of Al2O3 and 3-nm HfO2 deposited by atomic layer deposition. These are some of the most aggressively scaled and highest-performing InGaAs FinFETs in the world. Our results show a rapid degradation in performance as the fin width scales down to single-nanometer dimensions. Figure 2 shows that as fin width narrows below about 10 nm, the DC peak transconductance (gm,max) sharply decreases. Our study focuses on understanding the underlying reason behind such degradation.One of the most critical challenges facing III-V semiconductors is the lack of a good native oxide. Severe electron trapping in the oxide has been reported, resulting in hysteresis, threshold voltage instability, and frequency dispersion in InGaAs metal-oxide semiconductor FETs (MOSFETs). In this work, the same problematic issues are observed in our FinFETs. To gain deeper understanding, we use high-frequency measurement techniques to isolate the intrinsic characteristics in InGaAs FinFETs free from the influence of oxide trapping. We find that at 1 GHz, where most oxide traps are unresponsive, gm,max extracted from S-parameters is much higher than DC and degrades more slowly (Figure 2). This suggests significantly higher intrinsic performance potential even in narrow InGaAs FinFETs than what is observed under DC conditions. Our results also highlight the importance of minimizing oxide trapping in future scaled InGaAs FinFETs."
Excess Off-state Current in InGaAs FinFETs,"InGaAs is a promising channel material candidate for CMOS technologies beyond the 7-nm node. In these di-mensions, only high-aspect-ratio 3D transistors with a fin or nanowire configuration can deliver the necessary performance while suppressing short-channel effects. Recently, impressive InGaAs FinFET (Figure 1) proto-types have been demonstrated.However, InGaAs FinFETs are challenged by relatively high leakage of current in the OFF state (Figure 2). This leakage originates from band-to-band tunneling at the drain end of the channel that is amplified by a parasitic bipolar effect as a result of its floating body. In this work, we present a simple model of the parasitic bipolar effect in InGaAs FinFETs that captures the key gate length and fin width dependences. Our model accounts for surface recombination at the sidewalls of the fin as well as bulk recombination at the heavily doped source. When compared with experimental results, our model suggests that fin sidewall recombination dominates in long gate length transistors and leads to an exponential gate length dependence of the current gain of the parasitic bipolar junction transistor. The model enables the extraction of the carrier diffusion length, which exhibits the predicted dependence on fin width. For short gate length transistors, source recombination is shown to dominate, and the parasitic bipolar gain scales with the inverse of the gate length."
Digital-etch Effect on Transport Properties of III-V Fins,"One of the key process technologies to improve the interface quality of modern III-V transistors is digital etching (DE). DE is a self-limiting etching process that consists of dry oxidation of the semiconductor surface and wet etching of the oxide. DE is also the last process step before the gate oxide is deposited over the fins in FinFETs . DE, therefore, plays a crucial role in surface preparation and holds the key to further improve-ments to device transport and electrostatics.In this work, we compare the electrical performance of two sets of InGaAs FinFETs (Figure 1a) processed side by side that differ only in the type of DE that is applied. In one case, the oxide removal step was accomplished using H2SO4; in the other, HCl was used. While the etching property is similar for both processes, the surface termination is different (Figure 1b). Consequently, each treatment results in a different interface trap density (Dit) profile. To study the impact of surface treatments, we compare the electrical performance of the devices, as summarized in Figure 2. There are a few notable differences. In the OFF state, the HCl sample shows a larger subthreshold swing than the H2SO4 sample (Figure 2a). This suggests that HCl treatment results in a higher interface state density (Dit) toward the valence band. In the ON state, however, the intrinsic transconductance, gm,i, exhibits a peculiar trend. For wide fins, the HCl sample shows higher performance, but in very narrow fins (Wf<20 nm), H2SO4 performs better (Figure 2b). This implies that HCl yields higher mobility but lower carrier concentration at comparable overdrive. For aggressively scaled fins, the carrier concentration in the fin becomes comparable to Dit, and, as a result, the intrinsic gm of H2SO4 sample prevails."
InGaAs MOSFET with Integrated Hf0.5Zr0.5O2 in the Gate Stack for Investigating the Dynamic Operation of Negative Capacitance,"Achieving negative capacitance (NC) by incorporating a ferroelectric (FE) material in the gate stack of a met-al-oxide semiconductor field-effect transistor (MOS-FET) has recently attracted considerable interest. This interest is because of its potential for achieving a steep subthreshold swing in ultra-scaled semiconductor de-vices. Among the FE materials, HfxZr1-xO (HZO) thin film is the most promising and readily available option, because it is scalable and fully compatible with the current complementary metal-oxide semiconductor process. For these reasons, various experiments and simulations have been demonstrated so far. However, the dynamic response of the NC effect remains con-tentious and unclear. In this work, we present InGaAs MOSFETs with integrated Hf0.5Zr0.5O2 as a first step in the investigation of the dynamic operation of NCFETs. Figure 1 shows the schematic of a typical self-aligned device fabricated through a gate-last process. The gate stack was formed with 4 nm of Al2O3 as an interlayer and 10 nm of HZO, followed by a TiN layer deposited by an in-situ atomic layer deposition process. Rapid thermal anneal is performed at 500 °C for 1 min to activate the FE property of the HZO film, as confirmed by the hysteresis loop in Figure 2a. The control device has an identical structure except for the absence of the HZO layer in the gate stack. Integrating the HZO into the gate stack shows a plausible FE characteristic, which is ΔVth<0 during a full cycle sweep of the subthreshold characteristics, as shown in Figure 2b. However, the devices do not manifest the NC effect, for example, sub-60-mV/decade subthreshold swing and ON-current boost, even though the capacitance matching process was performed. In conclusion, we have observed characteristics consistent with the FE effect in InGaAs MOSFETs that incorporate HZO in the gate stack. Furthermore, we realize the device should be fabricated with a thinner interlayer to scrutinize the NC effect in the high-frequency regime."
High-temperature Electronics Based on GaN Technology,"Compared to conventional Si or GaAs based devices, wide-bandgap GaN has fundamental advantages for high-temperature applications thanks to its very low thermal carrier generation below 1000 °C. However, in spite of the excellent performance shown by early high-temperature prototypes, several issues in traditional lateral AlGaN/GaN high-electron mobility transistors (HEMTs) could cause early degradation and failure under high-temperature operation (over 300 °C). These include ohmic degradation, gate leakage, buffer leakage, and poor passivation. To enable digital circuit processing, it is critical to have enhancement-mode HEMTs, while two-dimensional electron gas induced by AlGaN/GaN heterostructure makes HEMTs into natural depletion-mode devices. Gate injection transistors (GIT) are being considered to overcome this problem at high temperatures.Our previously reported tungsten Si-implanted ohmic contact shows great thermal stability over 300 °C by combining a refractory metal such as tungsten (W) with Si-ion implantation, which locally dopes the material n-type and reduces the contact resistance. However, ion implantation technology in GaN is still challenging due to activation and damage recovery. The implanted contact performance is limited by high access resistance. High-temperature activation annealing over 1200 °C would cause irreversible lattice relaxation and degrade the AlGaN/GaN heterostructure quality, as shown in Figure 1. The implanted contact performance at different activation annealing condition is also shown in Figure 2."
Novel GaN Transistor Design for High Linearity Applications,"Enhancing the linearity of gallium nitride (GaN) high-electron-mobility transistors (HEMTs) is essen-tial for future radio frequency (RF) applications that require extremely low intermodulation distortion and gain compression. Existing power amplifiers with high linearity specifications make use of gallium arse-nide (GaAs)-based heterostructure bipolar transistors (HBTs) or digital pre-distortion, but these solutions are insufficient to fulfill the needs of next-generation power amplifiers operating at the Ka band and beyond. Therefore, device-level solutions are required to im-prove the linearity of power amplifiers. This study fo-cuses on the origins of device non-linearities in GaN-based transconductance amplifiers (Classes A through C) and proposes device-level solutions to enhance the linearity at the amplifier level.First, the drop in transconductance (gm) at high current levels observed in GaN transistors can be miti-gated with either self-aligned or fin field-effect transis-tor- (FinFET-) like structures. This is due to the higher current-driving capability of the source access region on these devices.Second, the large second derivative of the transcon-ductance with respect to gate-source voltage (Vgs) (gm"" or gm3) results in gain compression in the RF amplifier. This can be overcome by using a new generation of en-gineered FinFET transistors where the width of each fin is optimized for minimizing gm"".Third, the non-linear behavior of the device capac-itances (Cgs, Cgd) with gate bias voltage plays a signifi-cant role in limiting the maximum achievable linearity of the amplifier, especially at large signal swings. Nano-structures could be used to improve the capacitance behavior and hence linearity.Last, but not least, memory effects due to surface traps and buffer traps/defects contribute to non-linear-ity in amplifiers. They, too, could be overcome through the use of nanostructures."
Vertical GaN Fin Transistors for RF Applications,"The demand for improved wireless connectivity and data speeds has continuously increased and outpaces hardware’s abilities. Transistors for radio frequency (RF) amplifiers must be developed to satisfy this grow-ing market. Recently, lateral GaN-based high electron mobility transistors (HEMTs) have succeeded in the RF power market. However, the strong confinement of current near the surface plagues HEMTs with current collapse and self-heating. To circumvent these limita-tions, we use a vertical transistor design where current conducts through the bulk of the material, minimizing surface effects. This vertical design offers reduced cur-rent collapse, area independent breakdown, increased power density, and improved heat dissipation, which enable unmatched RF performance. Figure 1 shows a schematic of our vertical transistors. A fin-based structure adopted from devices we developed for vertical GaN power transistors confines current. Us-ing fins has the added benefit of improving linearity through threshold voltage engineering, where varying the width of each fin optimizes the transconductance curve. With 100-nm gate lengths and optimized drift re-gions, these devices are designed for 30 GHz operation with 200 V breakdown.Figure 2 shows the first GaN RF fin transistors fabricated at MTL. To fabricate them, an array of 200-nm fins is patterned with electron beam lithography and etched using a combined dry and wet etching technique that produces highly vertical, smooth sidewalls. Forming the gate uses a sputter and etch-back process to allow gate lengths unconstrained by lithographic resolution limits. A conformal silicon dioxide coating fills spaces between fins and is subsequently etched back to expose the tops of the fins. Ohmic metal for the source and drain is finally deposited. Keeping all contacts on the top surface and utilizing an insulating, high thermal conductivity substrate like silicon carbide enables integration with existing microwave integrated circuits to meet the demands of the next generation of wireless communication systems."
GaN Power Transistor Reliability,"Gallium nitride (GaN) metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT) tech-nology is the most recent development in the power semiconductor market. Owing to its large bandgap and other unique material properties, GaN exhibits a breakdown field up to ten-fold higher than Si. The MIS field-effect transistor (FET) architecture was adopted to optimize the breakdown voltage and demonstrate reliable and highly-efficient operation at and over 650 V. Combined with low on-resistance and fast switching capability, the GaN MISFET is a promising platform for numerous applications in the power electronics mar-ket.A successful commercial technology must meet strict reliability requirements. We are interested in gate oxide breakdown through a process known as time-dependent dielectric breakdown (TDDB) in the OFF state of transistor operation. This occurs with a large drain-source voltage and the channel turned off. For estimation of transistor lifetime under operating conditions in an effective manner, suitable acceleration of the degradation rate needs to be introduced. This is often done through voltage or temperature acceleration. However, since the GaN MISFET architecture employs a conductive substrate, a concern arises about substrate leakage that, under accelerated conditions, can trigger a potential vertical breakdown path through the buffer layer. Our work seeks to develop a test procedure for isolating and evaluating transistor time-to-failure due to TDDB by suitable temperature and voltage acceleration and to distinguish this from other failure mechanisms.Commercial prototype devices are tested at the Microsystems Technology Laboratories with only one acceleration factor changed at a time. The experimental design accounts for higher stress voltages or temperatures at which devices break in minutes. Weibull distributions are then fitted to the data as a function of different conditions; from these results, the acceleration factors and lifetime estimations are derived. This method is effective at giving intrinsic failure modes a physical interpretation and at predicting mean-time-to-failure under use conditions. Figure 1 shows gate current for five devices measured under the same stress condition. Figure 2 shows the Weibull distribution plot for these devices with a line fit and a shape parameter β estimate of 2.8."
Time-dependent Dielectric Breakdown under AC Stress in GaN MIS-HEMTs,"GaN has emerged as a promising next-generation can-didate for high-performance energy-efficient electron-ics. In particular, the GaN metal-insulator-semicon-ductor high-electron-mobility transistor (MIS-HEMT) has been identified recently as a promising candidate for high-voltage and high-power applications due to its high current drive while minimizing gate leakage. However, reliability concerns with this device type are hampering its widespread commercial deployment. A key reliability issue is time-dependent dielectric break-down (TDDB), in which prolonged electrical stress leads to catastrophic breakdown of the gate dielectric. There has recently been great progress in understand-ing TDDB in GaN FETs. However, much of the work to date has been done under constant voltage stress con-ditions, mostly due to ease of instrumentation. Here, we investigate time-dependent dielectric breakdown (TDDB) in AlGaN/GaN MIS-HEMTs under forward bias AC stress, which better emulates real-world operational conditions. To this end, we have performed TDDB experiments across a wide range of frequencies, temperatures, and recovery voltage levels. We find that TDDB under AC stress shows longer breakdown times than under DC stress and that this increase is more prominent with higher frequency, lower-temperature, and more negative recovery voltage. We hypothesize that this is due to the dynamics of the gate stack in GaN MIS-HEMTs biased with a high positive gate voltage. Under these conditions, a second electron channel forms at the dielectric/AlGaN interface. This process is relatively slow as these electrons come from the 2DEG at the AlGaN/GaN interface and must overcome the energy barrier presented by the AlGaN. At the same gate voltage, then, the electric field across the gate oxide is lower in magnitude under AC stress at high enough frequency than under DC stress, explaining the obtained results."
Reliability of GaN High-electron-mobility Transistors,"Gallium nitride-based high-electron-mobility tran-sistors (GaN HEMTs) are particularly attractive for high-power and high-frequency applications. While there have been some successful commercialization of these devices, large-scale market adoption has not yet occurred. This lack is partially due to an unclear under-standing of the origin of low device fabrication yield and reliability.Using research devices made by collaborators, we have systematically studied the origin of high gate-leakage currents in AlGaN/GaN HEMTs. Devices that initially had a low gate-leakage current (good devices) were compared with ones that had a high gate-leakage current (bad devices). The apparent zero-bias Schottky barrier height of bad devices (0.4 < ϕB0 < 0.62 eV) was found to be lower than that of the good devices (ϕB0=0.79 eV). From transmission electron microscopy and electron energy loss spectroscopy analysis, we found that this difference is due to the presence of carbon impurities in the nickel layer in the gate region, as shown in Figure 1. The carbon is likely the residue from a lift-off process.In ongoing research, we are also characterizing the reliability of commercial GaN HEMTs. Different failure modes have been identified for both on-state and off-state testing, as shown in Figure 2. Statistical reliability models will be developed and compared with research devices."
MIT Virtual Source Ferroelectric FET (MVSFE) Model: Application to Scaled-Lg FeFET Analog Synapses,"Conventional multi-purpose hardware based on von-Neumann architecture does not satisfy the energy-efficiency requirements of large-scale implementations of deep neural networks (DNN). Hardware accelerators are, therefore, key to improve the power efficiency of many big data applications based on deep learning, such as image classification and speech recognition. Emerging non-volatile memory devices such as resistive random-access memory, phase change memory, floating gate memory, and ferroelectric field-effect transistors (FeFETs) are potential candidates for these DNN accelerators due to their synaptic functionality, i.e., analog conductance modulation. FeFET analog synapses are 3-terminal devices and one of the most promising non-volatile memory devices that can improve the classification accuracy and yield low latency in neuromorphic accelerators. This is due to their high conductance ratio, operation capability with sub-100-ns pulse, and seamless integration with CMOS process flow. The initial proof-of-concept FeFET synapses have been demonstrated in a custom-built Si platform with large device footprint (Lg = 0.6 µm, W=20 µm). This work presents a comprehensive physics-based compact modeling platform, MIT Virtual Source Ferroelectric FET (MVSFET), that is used to study the scaled (Lg =45 nm) three-terminal FeFETs calibrated against a state-of-the-art highly scaled MOSFET. The MVSFE model captures FeFET characteristics by combining the MVS-model that describes underlying Si-MOSFET ballistic transport together with Preisach (static) and VDST (dynamic) models that govern the full-dynamics of Fe-oxide. The robustness of the model verified for synaptic operation with different pulse schemes was used to predict the advantage of technology scaling in reduced latency and improved energy efficiency while maintaining a high classification accuracy in a system-level multilayer perceptron network. The current work reveals the potential of FeFET analog synapses for system-level applications used in advanced technological node platforms."
"X3D: Heterogeneous Monolithic 3D Integration of “X” (Arbitrary) Nanowires: Silicon, III-V, and Carbon Nanotubes","We experimentally demonstrate a new paradigm for monolithic three-dimensional (3D) integration: X3D, which enables a wide range of semiconductors includ-ing silicon (Si), III-V, and nanotechnologies such as car-bon nanotubes (CNTs) to be heterogeneously integrat-ed together in monolithic 3D integrated systems (Fig. 1). Such flexible heterogeneous integration has the po-tential for a wide range of applications, as each layer of monolithic X3D integrated circuits (ICs) can be custom-ized for specific functionality (e.g., wide-bandgap III-V-based circuits for power management, CNT field-effect transistors (CNFETs) for energy-efficient computing, and tailored materials for custom sensors or imagers). As a case study, we experimentally demonstrate monolithic X3D ICs with 5 vertical circuit layers heterogeneously integrating 3 different semiconductors: Si junctionless nanowire field-effect transistors (JNFETs), III-V JNFETs, and CNFETs (also junctionless). The layers of monolithic X3D IC are, from bottom to top: Si p-JNFETs, n-CNFETs, Si n-JNFETs, p-CNFETs, and III-V n-JNFETs (Fig. 2). Each layer is fabricated using an identical process flow for ease of integration. Importantly, we show that circuits fabricated on each vertical layer are agnostic to subsequent monolithic X3D processing, experimentally demonstrating the ability to interleave these “X” (arbitrary) semiconductors in arbitrary vertical ordering (Fig. 3). As a final demonstration, we fabricate complementary digital logic circuits comprising different technologies that span multiple vertical circuit layers. This work demonstrates a new paradigm for ICs, allowing for flexible and customizable electronic systems."
Strong Coupling between Cavity Photons and Nano-magnet Magnons,"Coupled microwave photon-magnon hybrid systems offer promising applications by harnessing various magnon physics. At present, to realize high coupling strength between the two subsystems, bulky ferro-magnets with large spin numbers N are utilized, which limits their potential applications for scalable quantum information processing. By enhancing single-spin cou-pling strength using lithographically defined super-conducting resonators, we report high cooperativities between a resonator mode and a Kittel mode in nano-meter-thick Permalloy wires. The on-chip, lithographically scalable, and super-conducting quantum-circuit-compatible design provides a direct route toward realizing hybrid quantum systems with nanomagnets, whose coupling strength can be precisely engineered and whose various mechanisms derived from spintronic studies can control dynamic properties. We pattern superconducting niobium films into coplanar waveguide (CPW) resonators and deposit nanometer-thick Py wires on top of them. An in-plane magnetic field is applied to adjust the resonance frequency of the Kittel mode in Py, which interacts with the resonator mode to create mode-splitting near resonance. By fitting the resonance mode’s evolution, we confirmed the scaling of g with N by varying the Py sizes. To further lower N, we employ low-impedance resonators that greatly enhance the magnetic field near the Py wires. A g/2π of 74.5 MHz is obtained for 40um × 2um × 10nm Py, corresponding to 4x1010 spins. Compared with previous works, our experiment shows a more than six orders-of-magnitude reduction in spin number. This highly engineerable device design and the large coupling strength with nanomagnets provide a direct avenue towards scalable hybrid quantum systems that can benefit from various magnon physics, including nonlinearity, synchronized coupling, non-Hermitian physics, and current- or voltage-controlled magnetic dynamics."
Magnon Spin Generation and Transport in Heavy Metal-magnetic Insulator-ferromagnet Hybrid Structure,"Magnons (or spin waves) are collective excitations of electrons’ spin angular momenta in magnetic or non-magnetic materials. Magnons can be used to trans-port spin current and enable information transmission with much higher energy efficiency than conducting electron spin current. The excitation and tunabili-ty of magnons in low-damping magnetic materials are particularly interesting because they could offer much longer magnon propagation length and poten-tial broad spintronic applications. However, the exci-tation of magnons in ferromagnetic metals is usually accompanied by a rectification effect that can hinder the effective detection of magnon spin current. The goal of our project is to utilize a heavy metal/magnetic insulator/ferromagnet hybrid structure for definitive and efficient magnon spin generation, transport, and manipulation. In our work, a platinum (Pt)/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure is studied, as depicted in Figure 1(a), where YIG is a low-damping magnetic insulator, Py is a low-damping ferromagnetic metal, and the whole structure is grown on the gadolinium gallium garnet (GGG) substrate by magnetron sputtering. Through external microwave excitation, the YIG layer and the Py layer can be excited to reach the ferromagnetic resonance (FMR) modes individually, as shown in Figure 1(b). Spin current generated by the Py spin pumping process can transmit through the YIG layer and be converted to voltage signal in the platinum (Pt) layer through the inverse spin Hall effect, where the rectification effect from the Py layer can be completely ruled out. More importantly, the perpendicular standing spin waves (PSSWs) have been detected in the YIG layer, as shown in Figure 1(b). At specific frequency (~7 GHz), the PSSWs in YIG can be coupled with the magnon mode in Py, as indicated in Figure 1(c), and facilitate the magnon spin transport from the Py layer to the bottom Pt layer, as demonstrated by Figure 1(d). This result indicates that the PSSWs in the YIG layer could offer additional tunability to control the magnon spin transmission from Py to the bottom Pt layer, which is promising for building magnon spin switches or amplifiers for magnonic device applications."
Tunable Spin-charge Conversion across the Metal-insulator Transition in Vanadium Dioxide,"The charge-to-spin conversion efficiency is a crucial parameter in determining the performance of many useful spintronic materials. Usually, this conversion efficiency is an intrinsic material property, which can-not be easily modified without invoking chemical or structural changes in the underlying system. Here we demonstrate successful tuning of charge-spin conver-sion efficiency via the metal-insulator transition in a prototypical metal-insulator transition material. Vanadium dioxide (VO2), a quintessential strongly correlated electron compound, undergoes a temperature-driven structural phase transition near room temperature. This abrupt change in its electrical, optical, thermal, and magnetic properties at transition has generated great interest from both technological and fundamental research perspectives. By employing ferromagnetic-reso- nance-driven spin pumping and the inverse spin Hall effect measurement, we find that the pumped spin signal and charge-spin conversion efficiency undergo a swift, dramatic enhancement upon transition. The large enhancement (80%) in the spin pumping signal across the metal-insulator transition provides the first evidence of variable spin-charge conversions of this material. In combination with the recently observed electric-field, irradiation, or strain mediated phase transitions in VO2, this tunable spin-charge conversion can be used to make practical spintronic devices. The abrupt, dramatic change in the structural and electrical properties of this material, therefore, provides additional knobs to modulate the spin-charge conversion efficiency, leading to extra flexibilities in spintronic device design as well as providing new functionalities for spintronic devices, such as tunable spin-based memory and energy-harvesting devices."
Mutual Control of Coherent Spin Waves and Magnetic Domain Walls in a Magnonic Device,"Spin waves, the collective excitation of electronic spins inside magnetic materials, offer new opportunities for wave-based computing. Here we experimental-ly demonstrate interactions between spin waves and magnetic domain walls, where the magnetic domain walls manipulate the phase and magnitude of spin waves and a strong spin wave, in turn, moves the posi-tion of magnetic domain walls. The discovery of mutu-al control between a spin wave and a magnetic domain wall can lead to efficient mechanisms for modulating spin wave propagation, which opens the possibility of realizing all-magnon-based reading/writing devices. In the first part of this work, we experimentally demonstrate that nanometer-wide magnetic domain walls can be used to manipulate the phase and magnitude of coherent spin waves in a non-volatile manner. A coherent spin wave is excited and detected in Co/Ni multilayers, the perpendicular magnetic anisotropy, and a relatively low damping factor, which allows the coexistence of domain walls and zero-field coherent spin wave excitation. By comparing the transmitted signals of the spin wave in a device with and without a domain wall, we observe a more than 10-dB change in magnitude and a nearly 180° shift in phase when the spin wave passes through the domain wall. In the second part of this work, we observe that the domain wall moves opposite the direction of the spin wave propagation and reaches maximum efficiency at the spin wave resonance frequency, which is consistent with the picture of spin transfer torque from the magnon spin current. The combination of these two effects can potentially provide a platform for realizing efficient spin-wave-based memory, computing, and information processing that lie in the domain of single spin waves."
Research on CMOS-compatible High-k Dielectrics for Magneto-ionic Memory,"High-k dielectrics play a key role in modern microelec-tronic circuitry, given their ability to provide reduced leakage currents while providing adequate capacitance in ever-smaller nano-dimensioned metal-oxide semi-conductor field-effect transistor (MOSFET) devices. Re-cently, the Beach group at MIT demonstrated the abil-ity to modulate the magnetic properties of transition metal thin films by electrical bias across thin films of Gd2O31. The reversible switching was demonstrated to be assisted by the electro-migration of ions to and away from the transition metal/Gd2O3 interface. This novel process, now called “magneto-ionic control,” creates new opportunities for nonvolatile information storage. To better understand the mechanisms of ionic transport in these devices, we are examining the defect, electrical, and transport properties of Gd2O3 via impedance spectra as a function of temperature and oxygen partial pressure considering Gd2O3 as a model oxide for ionic migration-controlled devices. In this research, we find that Gd2O3 can be an electronic or mixed ionic-electronic conductor at high-temperature depending on dopant type, concentration, and phase. This research is being extended to the lower-temperature regime to understand the correlations between the behavior of such devices and their defect chemistry."
Sub-10 nm Diameter InGaAs Vertical Nanowire MOSFETs,"In future logic technology for the Internet of Things and mobile applications, reducing transistor power consump-tion is of paramount importance. Transistor technolo-gies based on III-V materials are widely considered as a leading solution to lower power dissipation by enabling dramatic reductions in the transistor supply voltage. Ver-tical nanowire (VNW) transistor technology holds prom-ise as the ultimately scalable device architecture.In this work, we present the smallest vertical nanowire transistors of any kind in any semiconductor system. These devices are sub-10 nm diameter InGaAs VNW metal–oxide–semiconductor field-effect transistors (MOSFETs). They are fabricated by a top-down approach, using reactive ion etching, alcohol-based digital etch, and Ni alloyed contacts. A record ON current of 350 μA/μm at OFF current of 100 nA/μm and supply voltage of 0.5 V is obtained in a 7 nm diameter device. The same device exhibits a peak transconductance of 1.7 mS/μm and minimal subthreshold swing of 90 mV/dec at a drain voltage of 0.5 V. This yields the highest quality factor (defined as the ratio between transconductance and subthreshold swing) of 19 reported in vertical nanowire transistors. Excellent scaling behavior is observed with peak transconductance and ON current increasing as the diameter is shrunk down to 7 nm. The performance of our devices exceeds that of the best Si/Ge transistor by a factor of two at half the supply voltage."
10-nm Fin-Width InGaSb p-Channel FinFETs,"Recently, III-V multi-gate MOSFETs have attracted great interest to replace silicon in future CMOS tech-nology. This is due to III-V semiconductor’s outstand-ing carrier transport properties. Although impressive n-type transistors have been demonstrated on materi-als such as InAs and InGaAs, research in III-V p-chan-nel devices is lagging. The antimonide system, such as InGaSb, has the highest hole mobility among all III-V compound semiconductors, and its hole mobility can be further improved by applying compressive strain. Therefore, InGaSb is regarded as one of the most prom-ising semiconductors to replace p channel Si MOSFETs.FinFET is a nonplanar transistor in which the conducting channel sticks out of the wafer top in a similar way as the fin of a shark above the ocean surface. In a FinFET, the gate wraps around the fin helping to reduce leakage current when the device is OFF and mitigating short-channel effects. FinFET is the state of the art transistor architecture in current Si CMOS technology, and demonstration of III-V FinFETs is imperative.In this work, we greatly advance the state-of-the-art of antimonide-based electronics by demonstrating deeply-scaled InGaSb p-channel FinFETs through a fully CMOS-compatible fabrication process. To achieve this, we have developed a novel antimonide-compatible digital etch technology, which has a consistent etch rate of 2 nm/cycle on InGaSb. It is the first demonstration of digital etch on InGaSb-based transistors of any kind. The new technologies enabled the first fabricated InGaSb FinFETs featuring fin widths down to 10 nm and gate lengths of 20 nm. Single fin transistors with fin width of 10 nm and channel height of 23 nm (aspect ratio of 2.3) have achieved a record transconductance of 160 μS/μm at VDS = 0.5 V. When normalized to device footprint, we achieve a record transconductance of 704 μS/μm. Digital etch has been shown to effectively improve the turn-off characteristics of the devices. This work not only highlights the potential of InGaSb p-channel multigate MOSFETs, but also pushes the state-of-the-art of antimonide fabrication technology significantly for general applications in which the antimonide-based compounds can shine."
Digital-etch Effect on Transport Properties of III-V Fins,"InGaAs is a promising candidate as channel material for CMOS technologies beyond the 7 nm node. In this dimensional range, only high aspect-ratio (AR) 3-D tran-sistors with a fin or nanowire configuration can deliver the necessary performance. Impressive InGaAs FinFET prototypes have been demonstrated recently. However, as the fin width is scaled down to 10 nm, severe ON-cur-rent degradation is observed. The origin of this perfor-mance degradation is largely related to the quality of the high-K/semiconductor interface at the fin sidewalls. One of the key process technologies to improve the interface quality is digital etch (DE). DE is a self-limiting etching process that consists of dry oxidation of the semiconductor surface and wet etch of the oxide. This process allows for the accurately scaling down of the fin width and smoothing the sidewalls. Digital etch is also the last process step before the gate oxide is deposited over the fins. It. Therefore, plays a crucial role in surface preparation and holds the key for further improvements to device transport and electrostatics.In this work, we compare the electrical performance of two identical sets of InGaAs FinFETs processed side-by-side that differ only in the type of digital etch that is applied. In one case, the oxide removal step was accomplished using H2SO4, in the other, HCl was used. The starting material consists of 50 nm thick (HC) moderately-doped InGaAs channel layer on top of InAlAs buffer (both lattices matched to InP), as shown in Figure 1(a). Fins are first patterned using E-beam lithography and RIE etched. After this, four cycles of digital etch are applied. Then, the gate dielectric composed of 3 nm HfO2 is deposited by Atomic Layer Deposition. and Mo is sputtered as gate metal and patterned by RIE. In this process, the HSQ that defines the fin etch is kept in place. This makes our FinFETs double-gate transistors with carrier modulation only on the fin sidewalls. The device is finished by via opening and ohmic contact and pad deposition. Transmission Electron Microscopy (Figure 1(b)) is used to verify that the fin shape and dimensions are similar in both samples.Well-behaved characteristics and good sidewall control are obtained in both types of devices. There are a few notable differences. In the OFF state, the HCl sample shows lower gate leakage but larger subthreshold swing compared to the H2SO4 sample (Figure 2(a)). This suggests that HCl treatment results in a higher interface state density (Dit) toward the valence band. In the ON state, however, the intrinsic transconductance, gm,i, exhibits a peculiar trend. For wide fins, the HCl sample shows higher performance but in very narrow fins (Wf<20 nm), H2SO4 performs better (Figure 2(b)). This implies that HCl yields a higher mobility but lower carrier concentration at comparable overdrive. For aggressively scaled fins, the carrier concentration in the fin becomes comparable to Dit, and, as a result, the intrinsic gm of H2SO4 sample (with a lower Dit toward the conduction band) prevails."
Transconductance Dispersion in InGaAs MOSFETs,"InGaAs is a promising n-channel material candidate for future CMOS technology due to its superior electron transport properties and low voltage operation. Due to the lack of good native oxide, it has been challenging to achieve a high-quality gate stack, which includes the gate oxide as well as the oxide/semiconductor interface. Many have observed hysteresis and threshold voltage instability in InGaAs MOSFETs that are attributed to interface and oxide defects. In this work, we study the frequency dispersion of InGaAs MOSFETs, an import-ant electrical parameter that is also affected by gate stack defects. The InGaAs MOSFETs used in this study are fabricated in a contact-first, gate-last self-aligned manner. Figure 1 shows the device schematic. The intrinsic channel consists of 8 nm-thick In0.7Ga0.3As. The gate insulator is a 2.5 nm-thick HfO2, deposited by Atomic Layer Deposition (ALD) at 250oC. The gate metal Mo is 35 nm thick, deposited by evaporation. These devices show state-of-the-art performance. We have carried out frequency-dependent electrical characterization from DC to 10 GHz. For the frequency range between 100 kHz and 10 MHz, we employ a lock-in setup and measure the AC drain current induced by AC gate voltage. For frequency range from 100 MHz to 10 GHz, the device S-parameters are measured using a vector network analyzer. From these measurements, we extract the intrinsic transconductance, gm,i. Figure 2 (a) shows the frequency dispersion of the intrinsic transconductance (gm,i) from DC to 10 GHz. As AC frequency increases, deep-level trap states can no longer respond, and device performance improves. gm,i increases from 775 mS/mm to 2200 mS/mm from DC to 10 GHz. The dispersion throughout the entire frequency range also indicates defect states with different time constants. It is remarkable how much unrealized intrinsic performance is left at DC. Figure 2 (b) shows peak gm,i at 10 GHz as a function of gate voltage. Here it is clear that the higher the gate voltage, the larger the gap between DC and 10 GHz gm,i. At the highest gm,i, the ratio is about a factor of 3.In conclusion, we have found large frequency dispersion of intrinsic transconductance in InGaAs MOSFETs, leading to a compromised device performance at DC. Thus, it is important to mitigate the oxide and interface defects in order to unveil the intrinsic outstanding transport properties of InGaAs."
Vertical Gallium Nitride Power Transistors,"Lateral and vertical gallium nitride (GaN)-based devic-es are excellent candidates for next-generation power electronics. They are expected to significantly reduce the losses in power conversion circuits and enhance the power density. Vertical GaN devices can achieve high-er breakdown voltage (BV) and handle higher current/power than lateral GaN devices and are therefore prom-ising for high-voltage and high-power applications. The development of vertical GaN power transistors has been hindered by the need to perform epitaxial regrowth or dope the layer p-type. The epitaxial regrowth greatly increases the complexity and cost of device fabrication. p-type GaN has low ratio for the acceptor activation, memory effects, and much lower carrier mobility compared to that in n-GaN.We demonstrate a novel normally-off vertical GaN power transistor with submicron fin-shaped channels. This vertical fin transistor only needs n-GaN layers, with no requirement for epitaxial regrowth or p-GaN layers (Figure 1). A specific on-resistance of 0.2 mΩ·cm2 and a BV over 1200 V have been demonstrated, with a threshold voltage of 1 V rendering normally-off operation (Figure 2). These results set a new record performance for 1200-V class power transistors and demonstrate the great potential of vertical GaN fin power transistors for high-power applications."
Vertical Gallium Nitride Power Diodes on Silicon Substrates,"Vertical gallium nitride (GaN) devices are excellent can-didates for next-generation power electronics. How-ever, their commercialization has been hindered so far by the high cost and small diameter of GaN substrates. GaN vertical devices on low-cost silicon (Si) substrates are therefore highly desired, as they could allow for at least 50-to-100-fold lower wafer and epitaxy costs as well as the possibility of processing on 8-inch Si substrates. However, the insulating buffer layers typically found on GaN-on-Si wafers make it challenging to realize vertical current conduction.Since 2014, we have developed three generations of vertical GaN-on-Si power diodes. The first generation utilized a quasi-vertical structure, where the anode and cathode are placed on a mesa step on the same wafer side (Figure 1(a)). We then demonstrated fully-vertical diodes by flip-chip-bonding the GaN-on-Si wafer to another Si wafer followed by the removal of insulating buffer layers. Recently, a novel technology was developed for making fully-vertical diodes (Figure 1(b)). Si substrate and buffer layers were selectively removed, and the bottom cathode was formed in the backside trenches. A specific differential on-resistance of 0.35 mΩ·cm2 and a breakdown voltage of 720 V were both demonstrated (Figure 2), setting a new record performance in all vertical GaN power diodes on foreign substrates."
Vertical GaN Transistors for RF Applications,"Stemming from their high breakdown voltages, large power densities, and high efficiency, GaN devices have quickly grown in popularity over the last two decades. With uses in millimeter wave applications like radar, satellite communication, and electronic warfare, the ev-er-increasing demand for high power devices that oper-ate over large bandwidths requires that new transistor technology is created. Since vertical device dimensions and doping can be carefully controlled during wafer growth, a vertical design is ideal for RF devices which need short gate lengths. Moreover, by utilizing the ver-tical dimension, we can achieve excellent power density at millimeter-wave frequencies with minimal die area, and since most transport occurs through the bulk of the material, we also expect thermal management and reliability improvements when compared to the tradi-tional GaN high electron mobility transistor (HEMTs). In this project, we adopt the design of recently devel-oped vertical GaN transistors, which were initially op-timized for high power applications, and modify them for improved RF performance. Another important benefit of a vertical fin design is the ability for threshold voltage engineering. In RF devices, an important metric to non-linearity is g m’’ (the second derivative of device transconductance), which is ideally flat. One method for correcting this is through threshold voltage engineering where devices of varying VT are connected in parallel. Since shifting V T also shifts the peaks of gm’’, with careful design, the peaks of one transistor’s gm’’ can effectively cancel those of another when superimposed. The resultant device will then have a flatter transconductance response with improved RF performance. Through the fin-based design of the transistors in this project, the transconductance can be adjusted by simply altering the width of each fin, thus allowing for optimized large signal response for RF applications. At MTL, we are fabricating the first vertical GaN fin RF transistors. For this, we are using electron beam lithography paired with a combination of dry and wet etching to achieve 100-300 nm tall fins with very smooth and vertical sidewalls. A molybdenum gate allows for a well-controlled etch-back process which coats only the sidewalls in metal. Further dry/wet etching can then be used to access the highly doped drain layer, which was defined during wafer growth. With the gate, source, and drain all on the top surface, this design will be compatible with GaN on Si technology, capable of significantly reducing material costs."
High-temperature GaN Technology,"Gallium nitride (GaN)-based transistors are very prom-ising candidates for high power applications due to their high electron mobility and high electric break-down field. Compared to conventional Si or GaAs based devices, wide bandgap GaN also has fundamental ad-vantages for high-temperature applications thanks to their very low thermal carrier generation below 1000°C. However, in spite of the excellent performance shown by early high-temperature prototypes, several issues in traditional lateral AlGaN/GaN HEMTs could cause ear-ly degradation and failure under high-temperature op-eration (over 300°C). These include ohmic degradation, gate leakage, buffer leakage and poor passivation. In addition, to enable digital circuits, it is critical to have enhancement-mode HEMTs, while two-dimensional electron gas induced by AlGaN/GaN heterostructure makes HEMTs be natural depletion-mode devices. In this work, we are developing a new GaN technology for high-temperature applications (>300°C). For this, we are first increasing the temperature stability of the ohmic contacts in GaN HEMTs, by combining a refractory metal such as tungsten (W) with Si-ion implantation, which locally dopes the material n-type and reduces the contact resistance. The schematic cross section is shown in Figure 1. An R c of 0.8 Ω mm, I max of 700 mA/mm were obtained with the W ohmic contacts in a transistor with a gate length of 4 µm. The W ohmic contacts were stable at least up to 300°C in air for at least 30 min, as seen in Figure 2, while conventional alloyed Ti/Al/Ni/Au ohmic contacts showed a strong temperature dependence and their contact resistance increased from 0.47 Ω mm (RT) to 2.15 Ω mm (300°C).Gate injection transistors (GIT) have also been studied for enhancement-mode HEMTs. The structure used in this work had a 110nm extra p-GaN layer on 15nm Al0.2Ga0.8N barrier layer to fully deplete 2DEG under gate area. As shown in Ids-Vgs in Figure 3, a positive VT around 3V was achieved, and their high-temperature stability is currently under investigation."
Novel GaN Transistor Design for High Linearity Applications,"Enhancing the linearity of Gallium Nitride (GaN) high-electron-mobility transistors (HEMTs) is essen-tial for future RF applications that require extremely low intermodulation distortion and gain compres-sion. In this project, we have studied the origins of non-linearities in GaN-based amplifiers and propose device-level solutions to improve linearity. First, the drop in transconductance (gm) at high current levels observed in GaN transistors can be mitigated with ei-ther self-aligned or finFET-like structures. This is due to the higher current-driving capability of the source access region on these devices. The second cause of de-vice non-linearity has been linked to the large second derivative of the transconductance with respect to gate-source voltage (Vgs) (gm”). This can be overcome by using a new generation of engineered finFET tran-sistors where the width of each fin is optimized for minimizing gm” [3]. In addition, the non-linear behavior of the device capacitances with operating voltage also plays a very important role in device non-linearities. In this case, too, nanostructures can be used to improve device performance. Finally, memory effects due to sur-face and buffer trap also contribute to non-linearities in amplifiers, and they can also be overcome through the use of nanostructures."
Sub-micron p-Channel GaN Tri-gate MISFET,"Remarkable attributes of GaN has led to the develop-ment of transistor technology for both power electron-ics and RF applications. Even though much attention is given to n-channel GaN transistor technology, p-chan-nel GaN transistors still lack attention. Development of p-channel GaN transistors is a must to harness the full potential that GaN technology has to offer in achieving high-efficiency power conversion. In this work, we have demonstrated for the first time sub-micron p-channel tri-gate MISFET with fin width of 200 nm. Figure 1(a) shows the schematic of fabricated device structure along with device dimensions. Figure 1(b) and 1(c) show the SEM image of the final device and the fins respectively. Because of the relatively thin AlGaN layer, the measurement results show significant electron contribution to the total drain current. However, if we deduct the current due to 2-DEG at the interface of AlGaN/GaN, we can extract the hole current. Figure 2 shows the IDS-VDS characteristics of the hole current. To prove that the current in Figure 2 predominantly is not because of the holes in the top p-GaN layer rather than the 2-DHG present at the GaN/AlGaN interface, we performed a low-temperature measurement. Because of relatively higher activation energy of Mg (~240 meV) in GaN, the p GaN layer is expected to be frozen out at around 77K leaving only the 2-DHG channel for the hole current. Figure 3 shows the hole current at 80K."
Reliability of GaN High Electron Mobility Transistors,"High electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures have been studied in lit-erature for a variety of high-frequency and high-pow-er applications. To minimize lattice mismatch and suppress defects generation, HEMTs, under study, are mostly fabricated on sapphire or SiC substrates. Cur-rently, there is strong interest to fabricate GaN HEMTs on silicon substrates due to its low cost and compatibil-ity with complementary metal–oxide–semiconductor (CMOS) integration technology. However, market adop-tion of this technology is still limited by the HEMT de-vice reliability.We have investigated the effects of SixN1-x passivation density on the reliability of AlGaN/GaN-on-Si HEMTs. Upon stressing, devices degrade in two stages: fast-mode degradation, followed by slow-mode degradation (Figure 1). Both degradations can be explained by different stages of pit formation at the gate edge. Fast-mode degradation is caused by pre-existing oxygen at SixN1-x /AlGaN interface. It is not significantly affected by the SixN1-x density. On the other hand, slow-mode degradation is associated with SixN1-x degradation caused by electric-field-induced oxidation. By using high-density SixN1-x, the slow-mode degradation can be minimized.Devices for research purposes are usually designed and fabricated in a way that certain failure can be magnified to study the failure mechanism better. However, commercial devices focus more on reliability and performance maximization. In ongoing research, we are also interested in characterizing the reliability of commercial GaN HEMTs produced by CREE Inc. A statistical reliability model will be developed, and comparison with devices produced by SMART-LEES will be made. Figure 2 shows the initial characterization of GaN HEMTs produced by CREE, Inc. Reliability testing of these devices is underway."
Dielectric Breakdown in a Novel GaN Power Field-effect Transistor,"Gallium Nitride (GaN) transistors are increasing in pop-ularity for high voltage power electronics applications. The most promising device structure is the metal-insu-lator-semiconductor high electron mobility transistor (MIS-HEMT). MIS-HEMTs are of interest because of their high breakdown voltage, low gate leakage cur-rent, and high channel conductivity. However, before commercial deployment, more work is required to improve the reliability and to reduce the instability of GaN MIS-HEMTs (Figure 1). Our work is focused on the characterization, ON-state time-dependent dielectric breakdown (TDDB), OFF-state TDDB, and Weibull sta-tistical analysis of a novel GaN transistor. Our goal is to study and understand the physics behind gate dielec-tric breakdown in this device in order to assess device robustness to prolonged operation. We have complet-ed many studies on these devices to determine break-down location along the channel, chip to chip variation, temperature dependence, voltage dependence, thresh-old voltage shift, and projected lifetime.During sustained ON-state bias at a high voltage, these devices exhibit trapping effects, stress-induced leakage current (SILC), progressive breakdown and eventually, hard dielectric breakdown (Figure 2). This is comparable to past MIS-HEMT studies in our group. As expected, hard breakdown time decreases as both temperature and drain voltage (VDS) are increased.OFF-state TDDB proved difficult because of parasitics, test implementation, and a high variability of over three orders of magnitude in hard breakdown time. An alternative methodology was used, increasing VDS in a linear ramp until hard breakdown occurred. This allows us to characterize the instantaneous breakdown voltage of the devices. Analyzing these results using a Weibull distribution shows a two-slope distribution. This can mean that two breakdown mechanisms are present or that there are multiple layers in the gate stack with different rates of defect generation.Our present research focuses on determining a methodology to accurately evaluate device lifetime during the application of a large drain bias while the device is in the OFF state."
Gate Dielectric Reliability under Mechanical Stress in High-voltage GaN Field-effect Transistors,"Energy-efficient electronics have been gaining atten-tion as a solution to meet the growing demand for ener-gy and sustainability. GaN field-effect transistors (FET) show great promise as high-voltage power transistors due to their ability to withstand a large voltage and car-ry large current. However, at the present time, the GaN metal-insulator-semiconductor high-electron-mobili-ty-transistor (MIS-HEMT), the device of choice for elec-tric power management, is limited from commercializa-tion due to many challenges, including gate dielectric reliability. Under continued gate bias, the dielectric ultimately experiences a catastrophic breakdown that renders the transistor useless, a phenomenon called time-dependent dielectric breakdown (TDDB).One key issue is the impact of mechanical strain on TDDB. In particular, when studying OFF-state stress conditions where the drain-source bias is very positive and gate-source bias is negative, the presence of unknown traps at both the interfaces and the bulk of the heterolayers can detrimentally impact dielectric reliability. Mechanical strain introduced during fabrication steps may be causing further reliability problems by amplifying the presence of traps.To understand the impact of mechanical strain on TDDB, we apply external strain by physically bending the devices. We compare the TDDB distributions which follow the Weibull statistical distribution at different external strain.Figure 1 shows TDDB under ON-state stress conditions. Under this situation, the gate is held at a positive bias while the drain and the source are grounded. Since the channel is not depleted, the electric field across the dielectric is distributed throughout the entire gate length and therefore traps make minimal impact on TDDB. Indeed, the breakdown statistics show that for two different mechanical strain, there is little change.On the other hand, figure 2 shows that TDDB under OFF-state stress condition changes with external strain. Under this stress condition, the majority of the electric field through the dielectric is focused at the gate/drain edge. As more of the electric field is focused in a small area, traps can play a significant role in TDDB.Understanding the role of mechanical stress in amplifying trap effects will help the efforts to understand the physics behind TDDB."
High-performance Graphene-on-GaN Hot Electron Transistor,"Hot electron transistors (HETs) are promising devices for high-frequency operation and probing the fun-damental physics of hot electron transport. In a HET, carrier transport is out of plane (Figure 1) due to the injection of hot electrons from an emitter to a collec-tor which is modulated by a base electrode. HETs have been used to probe scattering events, band nonparabo-licity, size-quantization effects, and intervalley trans-fer in different material systems. Monolayer graphene, being the thinnest available conductive membrane in nature, provides us with the opportunity to study the HET transport properties at the ultimate scaling limit. Previously, we have demonstrated graphene-base HET with GaN/AlN emitter and a graphene/WSe2 van der Waals heterostructure collector base-collector stack that can overcome the performance limitation of the graphene-base HETs with oxide barriers. In this work, we studied the effect of material parameters on the transport properties of the heterojunction diodes (i.e., Emitter-Base and Base-Collector) of HETs, and their impact on the HET performance. Temperature dependent transport measurements identify quantum mechanical tunneling as the major carrier transport mechanism in HETs. We demonstrate a new generation of graphene-base HET with record current density above kA/cm2 (Figure 2) by scaling the tunneling barrier thickness and device geometry optimization. Preliminary simulations show that with further optimization graphene-on-GaN HET can outperform the bulk HETs towards ultra-high frequency operation."
Circuit-performance Evaluation of Negative Capacitance FETs using MIT Virtual Source Negative Capacitance FET (MVSNC) Model,"Negative Capacitance Field Effect Transistors (NCFETs) have emerged as promising candidates for CMOS tech-nology scaling due to their potential for sub-60-mV/de-cade operation by utilizing negative capacitance effects in ferroelectric materials. A ferroelectric oxide (FE-ox-ide) capacitor in series with the normal gate-stack ca-pacitor of a conventional MOSFET forms the NCFET as shown in Figure 1. A physics-based compact model, MVSNC, is proposed to capture the device–behavior under static and dynamic operating conditions using the MVS-framework for the underlying MOSFET and the Landau-Khalatnikov (L-K) equation to model the FE-oxide as shown in sub-circuit of Figure 1.The baseline MOSFET is characterized against Intel-45nm data and while PZT oxide of tFE=5 nm is chosen for NCFETs. The model is implemented in Verilog-A, and transient simulations are performed using a commercial simulator (ADS®). The simulated device-level IV- and CV-characteristics of NCFET and baseline FET are shown in Figure 1. With same off-currents, NCFETs exhibit steep subthreshold-swing (SS) due to stabilization of negative capacitance (NC)-state in FE-oxide and VG,int-amplification compared to VG. Higher on-current (at same VG) with reduced or negative DIBL at certain VD regimes can also be seen. The CV-characteristics show capacitance-amplification in sub-threshold regime. Leakage in FE-oxide that can potentially remove the SS-steepness advantage in NCFETs is studied along with work-function engineering (WFE) that is proposed to mitigate the impact of FE-leakage. By shifting the FE-oxide’s Q-V curves along voltage-axis, WFE allows NC-state to be reached at low-VDD. The energy-delay (E-td) figure-of-merit of the NCFETs can be compared against baseline CMOS using loaded ring-oscillator (RO)-simulations. 21-stage ROs loaded with a constant capacitance CL whose value is equal to total on-capacitance (CGG at VD=0 and VG=1V) of the constituent baseline FETs of inverter are shown in Figure 2. Here, VDD is swept to get the energy-delay plot. The figure shows reduced E-td in NCFETs even under leakage because of lower switching loss in CL (0.5CLV2DDf). The benefit of lower E-td with NCFETs is significant at scaled VDD nodes and can be preserved even under DE-leakage scenarios by adopting WFE."
Negative Capacitance Carbon Nanotube Field-effect Transistors,"As continued scaling of silicon field-effect transistors (FETs) grows increasingly challenging, alternative paths for improving digital system energy efficiency are actively being pursued. These paths include replacing the transistor channel with emerging nanomaterials (such as carbon nanotubes: CNTs), as well as utilizing negative capacitance effects in ferroelectric materials in FET gate stacks, e.g., to improve sub-threshold slope beyond the 60 mV/decade limit (at temperature = 300 °K) for conventional FETs (which in itself is difficult to achieve due to short-channel effects). However, which path provides the largest energy efficiency benefits, and whether these multiple paths can be combined to achieve additional energy efficiency benefits, is still un-clear. Here, we experimentally demonstrate the first negative capacitance carbon nanotube FETs (CNFETs: Figure 1), combining the benefits of both carbon nanotube channels (which offer superior electrostatic control vs. silicon-based FETs, simultaneously with superior carrier transport) and negative capacitance effects. We experimentally demonstrate negative capacitance CNFETs (NC-CNFETs) that achieve sub-60 mV/decade sub-threshold slope. Across 50 NC-CNFETs, our experimental results show an average sub-threshold slope of 55 mV/decade at room temperature, compared to 70 mV/decade for baseline CNFETs, i.e., without negative capacitance (Figure 2). The average on-state drive current (ION) of these NC-CNFETs improves by 2.1× vs. baseline CNFETs, for the same off-state leakage current (IOFF). This work demonstrates a promising path forward for future generations of energy-efficient electronic systems."
MoS2 FETs with Doped HfO2 Ferroelectric/Dielectric Gate Stack,"Atomically thin layered two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS2) have been proposed to enable aggressive minia-turization of FETs. We previously reported ultra-short channel MoS2 FETs with channel length down to 15 nm and 7.5 nm using graphene and directed self-assembly pattern technique, respectively. However, the power scaling in such devices suffers from the same issues as in CMOS technology. Obtaining a subthreshold swing (SS) below the thermi onic limit of 60 mV/dec by exploiting the negative-ca pacitance (NC) effect in fer-roelectric (FE) materials is a novel effective technique to allow for the reduction of the supply voltage and power consumption in field-effect transistors (FETs). Conventional ferroelectric materials, i.e., lead zirconate titanate, bismuth ferrite, and polymer ferroelectric di-electrics such as P(VDF)-TRFE are not technologically compatible with standard CMOS fabrication process-es. On the other hand, fluorite-type doped HfO2 ferro-electric thin-films deposited by ALD offers the CMOS compatibility and scalability required for advanced electronic applications. In this work, we demonstrate NC-MoS2 FETs by incorporating a ferroelectric doped HfO2 (Al:HfO2 or Si: HfO2 ) in the FET gate stack. Standard HfO2 has monoclinic crystal structure which can be transformed into orthorhombic phase by temperature, pressure, or doping. The electrical properties of the doped HfO2 thin-films can be tuned from dielectric to ferroelectric and even antiferroelectric by changing dopant type (Zr, Al, Si, Gd, Y, etc.), dopant fraction and/or capping layer. The ferroelectric nature of typical doped HfO2 thin film can be confirmed by the polarization measurement (Figure 1). Here, Si:Hf composition is kept fixed by controlling the 3DMAS/TEMAH pulses during the ALD. We observe steep SS in FETs when used these FE in the gate stack with carefully matched FE/DE bilayer. The NC-MoS2 FET built on a typical FE/DE bilayer showed a significant enhancement of the SS to 57 mV/dec at room temperature, compared with SSmin = 67 mV/dec for the MoS2 FET with only HfO2 as a gate dielectric."
Graphene-based Ion-sensitive Field-effect Transistor Sensors for Detection of Ionized Calcium,"Ion-sensitive field-effect transistors (ISFET) are used for measuring ion concentration in solution. Typical ISFET is silicon-based and suffers stability problems. Graphene is an atomically thin material with excellent electrical, mechanical, optical, and chemical properties. It can be used to replace silicon for biological and chem-ical sensing with the potential of being light weighted, flexible, and transparent. This work develops a sensing platform (Figure 1A) with 152 individual ISFETs and an automatic data acquisition system. The array is functionalized with an ion-selective membrane and acts as a calcium sensor with excellent selectivity, sensitivity and response time. In particular, only calcium ion can be transported from the solution phase into the membrane via a charge neutral ionophore. At equilibrium, a stable Nernstian interface potential is achieved. With higher calcium concentration, the interface potential increases causing an effective shift in the sensor I-V characteristic. Hence, the sensor can detect and quantify changes in ionized calcium concentration through the shift in sensors I-V characteristic.The shift in I-V characteristic is quantified by the location of minimum conduction point in graphene’s V-shaped curve, Dirac point. The theoretical rate of change in potential versus calcium concentration at room temperature is approximately 30mV/decade for bivalent ions such as calcium. Our data shows an average slope of 30.1 mV/decade with a standard deviation of 1.9 mV/decade, which agrees very well with the theory, therefore, indicates excellent sensitivity. By matching data from transient response with data from I-V characteristic, we can calculate the concentration of calcium with a single calibration reference. As depicted in Figure 1C, sensors are capable of quantifying ionized calcium concentrations spanning over five orders of magnitude. This proof-of-concept work represents a milestone in the development of graphene-based sensors for solution-phase chemical detection of analytes such as ionized calcium."
High Breakdown Voltage in Solution-processed High-voltage Organic Thin Film Transistors,"Organic thin film transistors (OTFT) are excellent can-didates for large area electronics on arbitrary and flex-ible substrates, enabling novel flexible displays as well as wearable electronics such as artificial skin. However, enabling truly-ubiquitous electronics through OTFTs demands not only high performance and high degree of flexibility, but also a wide range of operating voltag-es. Applications such as electrophoretic displays, digi-tal X-ray imaging, photovoltaic systems-on-glass, and TFT-MEMS integration for large actuation are but a few that can enable high driving voltages on an OTFT technology platform.We are currently developing a solution-processed 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) high-voltage, organic, thin film transistor (HVOTFT) with self-assembled monolayer (SAM) treatments that is capable of driving voltages beyond -450 V while operating with threshold voltages below -10 V. The ability to modulate such high-voltages with a relatively low gate voltage is highly appealing for future MEMS integration. The HVOTFT is defined by a dual channel architecture comprised of a gated and offset region, enabling FET and high-voltage capabilities, respectively. Furthermore, a high-k cubic pyrochlore dielectric Bi1.5 Zn1Nb1.5O7 (BZN) is employed to achieve low gate leakage currents and low threshold voltages.A combination of organosilane self-assembled monolayers and a self-shearing drop cast method is used to grow thin (< 100 nm) crystal bands of TIPS-pentacene on the HVOTFT structures. Controlling the thickness of the organic semiconductor layer is critical in achieving high breakdown voltages of -450 V as well as high ION/IOFF current ratios of 104 A/A. Recent efforts in developing a self-aligned solution-process using surface energy engineering to enhance control of the crystal growth as well as to have transistor-to-transistor isolation have proven promising."
Characterization of Room-temperature Processed Thin Film Capacitors under Curvature,"Organic thin film transistors (TFTs) have been of great interest lately because of their potential applications in flexible systems, enabling devices such as electronic skins or implantable medical devices. With the ability to bend these new systems comes the question of how bending affects device perform. Consequently, thicker oxide layers are desirable because they are less likely to be stretched thin when flexed, preventing tunneling processes and high leakage currents. High-k dielectrics, such as the cubic pyrochlore Bi1.5Zn1Nb1.5O7 (BZN), have the potential to improve the reliability of this technol-ogy because they allow for a thicker film without de-creasing capacitive coupling.In this work, we investigated how the operating characteristics, like capacitance, change when devices are flexed. When the BZN is bent, strain is introduced into the crystal structure which can affect the dielectric constant. To explore this, we fabricated MIM capacitors and measured capacitance at different degrees of curvature to extract the dielectric constant. The capacitors, shown in Figure 1, were fabricated with a reactive sputtered BZN. Frequently, BZN is annealed at temperatures of 500-700°C; however, many flexible substrates, such as the Kapton polyimide films used here, are not compatible with such high-temperatures. Without annealing, the BZN was amorphous with a dielectric constant of around 30 as compared to values up to 200 found in crystalline BZN.We found that when the devices were bent to the radii of curvature shown in Figure 2, the capacitance dropped to 85-95% of the original capacitance when flat. As there was no apparent change in thickness or area of the devices, we’ve attributed this to a change in dielectric constant caused by strain in the crystal structure altering the alignment of electric dipoles in the material. When the devices were again laid flat, the capacitance returned to 95-99% of the original value. The information found in the MIM capacitor could be used to infer how device bending would affect behavior of a BZN-based OTFT for flexible applications."
Room Temperature Spin-orbit Torque Switching Induced by a Topological Insulator,"Recent studies on the topological insulators (TI) have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic de-vices. In particular, the strongly spin-moment coupled electronic states have been extensively pursued to re-alize efficient spin-orbit torque (SOT) switching. How-ever, so far current-induced magnetic switching with TI has been observed only at cryogenic temperatures. Whether the topologically protected electronic states in TI could benefit from spintronic applications at room temperature remains a controversial issue.In this work, we report SOT switching in a TI/ferromagnet heterostructure with perpendicular magnetic anisotropy (PMA) at room temperature. Ferrimagnetic cobalt-terbium (CoTb) alloy with robust bulk PMA is directly grown on a classical TI material, Bi2Se3. The low switching current density provides definitive proof of the high SOT efficiency from TI and suggests the topological spin-momentum locking in TI even if it is neighbored by a strong ferromagnet. Furthermore, the effective spin Hall angle of TI is determined to be several times larger than commonly used heavy metals. Our results demonstrate the robustness of TI as an SOT switching material and provide an avenue towards applicable TI-based spintronic devices."
Current-induced Domain Wall Motion in Compensated Ferrimagnets,"Antiferromagnetic materials show promises compared to ferromagnetic materials for spintronic devices due to their immunity to external magnetic fields and their ultra-fast dynamics. However, difficulties in controlling and determining their magnetic state are limiting their technological applications. At the compensation point, the two antiparallel sub-lattices in a ferrimag-net have the same magnetic moment, and the materi-al is an antiferromagnet. Compensated ferrimagnets are expected to exhibit fast magnetic dynamics like an antiferromagnet, and yet their magnetic state can be manipulated and detected like a ferromagnet, and therefore, have been pursued as a candidate system for ultrafast spintronic applications. Previously, it was demonstrated that current-induced spin-orbit torque could provide an efficient switching mechanism for a compensated ferrimagnet. However, limited by the quasi-static measurement technique, the nature of the switching dynamics in these experiments is yet to be revealed. In this work, we provide the first experimen-tal proof of current-induced fast domain wall (DW) mo-tion in a compensated ferrimagnet.Using a magneto-optic Kerr effect microscope, we determine the spin-orbit torque-induced DW motion in Pt/Co1-xTbx microwires with perpendicular mag-netic anisotropy. The DW velocity is determined as a function of applied current amplitude. A large en-hancement of the DW velocity is observed in angular momentum compensated Pt/Co0.74Tb0.26 microwires compared to single layer or multi-layer ferromagnetic wires (Figure 1). Using analytical model, we also find that near angular momentum compensation point, the domain walls do not show any velocity saturation unlike ferromagnets or uncompensated ferrimagnets since both the effective gyromagnetic ratio and effec-tive damping diverge at this composition (Figure 2). Moreover, by studying the dependence of the domain wall velocity with the longitudinal in-plane field, we identify the structures of ferrimagnetic domain walls across the compensation points. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnets open new opportunities for spintronic logic and memory devices."
Research on CMOS-compatible High-k Dielectrics for Magneto-ionic Memory,"High-k dielectrics play a key role in modern microelec-tronic circuitry, given their ability to provide reduced leakage currents while providing adequate capacitance in ever smaller nano-dimensioned metal-oxide semi-conductor field-effect transistor (MOSFET) devices. Re-cently, the Beach group at MIT demonstrated the ability to modulate the magnetic properties of transition met-al thin films by electrical bias across thin films of Gd2O3. The reversible switching was demonstrated to be assist-ed by the electro-migration of oxygen ions to and away from the transition metal/Gd2O3 interface. This novel process, now called “magneto-ionic control” creates new opportunities for nonvolatile information storage. Like magneto-ionic device, there is another important emerging device called “memristor” which applies field driven ionic transport-controlled property toggling. Though this device has been researched widely for a decade and defect chemistry of dielectrics is critical to the device operation, understanding of defect chemistry of dielectrics used for memristors are still limited. Here, we have examined electrical and transport properties of Gd2O3 via impedance spectra as a function of temperature and oxygen partial pressure considering Gd2O3 as a model oxide for ionic migration-controlled devices. In this research, we found that Gd2O3 can be electronic or mixed ionic-electronic conductor at high-temperature via controlling doping and phase. This research will be continued to the lower temperature regime to understand the correlation between the behavior of such devices and defect chemistry of dielectrics.In addition, we have begun an investigation of the mechanism of magneto-ionic devices in a viewpoint of considering magneto-ionic device as an electrochemical cell. Previous research indicated that this device behaves in a manner similar to high-temperature electrochemical devices. We are preparing model devices that reflect features of both magneto-ionic and electrochemical devices and are examining their properties in situ."
Probing 2-D Magnetism in van der Waals Crystalline Insulators via Electron Tunneling,"In this work, we introduce tunneling through layered insulators as a versatile probe of nanoscale magnetism. We fabricate van der Waals heterostructures of two graphite sheets separated by a magnetic CrI3 tunnel barrier (Figure 1). For magnetic tunnel junctions, the barrier height is lowered for electrons aligned with the magnetic layer, resulting in a direct dependence of the conductance across the junction on the magnetic or-dering in the CrI3 barrier.Layers of CrI3 align their spins perpendicular to the crystal, either up or down. By sweeping an applied magnetic field, we detect discrete steps in the junction conductance (Figure 2) corresponding to individual layers in the CrI3 barrier flipping out-of-plane magnetization. For example, when the magnetic field is swept up past 1 T in the bilayer device, the spins in the two layers of CrI3 both align with the field, resulting in a 95% magnetoresistance.Moreover, we can control the spin polarization of the output current with applied magnetic field, generating currents with up to 99% polarization. Thus, in addition to studying 2-D magnetic crystals using electrical readout of the magnetization, this result could also be applied to develop novel magnetic memory devices incorporating spin-orbit torques and other spintronic techniques."
Microwave Modulation of Relaxation Oscillations in Superconducting Nanowires,"Superconductors are ideal platforms for studying non-linear behavior due to their nonlinear switching dy-namics and phase relationships. Josephson junctions (JJs), the most common superconducting devices, have a nonlinear current-phase relationship that allows them to phase lock to weak external periodic drives. This phenomenon, known as the AC Josephson effect, produces distinct DC steps in the time-averaged cur-rent-voltage characteristics at voltage intervals of Vn = nhf/2e, where n is an integer, h is Planck’s constant, f is the frequency of the external radiation, and e is the electronic charge. Such a relationship has enabled tech-nology such as the Josephson voltage standard and an-alog-to-digital converters.Unlike JJs, superconducting nanowires are governed by a thermal nonlinearity that controls the switching into and out of the resistive state. In this work, we have studied fast oscillations in superconducting nanowires based on the electrothermal feedback between the nanowire hotspot and an external shunt resistor with a series inductance. In addition to studying how circuit parameters influence the frequency of the oscillations, we show that the oscillations can mix with an external microwave drive and eventually phase lock (Figure 1). This process produces a nanowire analog to the AC Josephson effect, with steps occurring at intervals of Vn = nfIcL, here n is an integer, f is the frequency of the drive, Ic is the critical current of the nanowire, and L is the series inductance (Figure 2). In addition to offering a potential avenue for measuring inductance through the appearance of phase-locked steps, the ability of these oscillations to mix with an external drive is promising for applications such as parametric amplification and frequency multiplexing."
A Superconducting Nanowire Based Memory Cell,"The development of a practical supercomputer relies on having a scalable memory cell, energy efficient con-trol circuitry, and the ability to read and write a state without sacrificing density. Typical superconducting memories relying on Josephson junctions (JJs) have demonstrated extremely low power dissipation (10-19 J) and rapid access times (< 10 ps), but suffer from large device dimensions and complex readout circuitry, mak-ing scalability a considerable challenge.As an alternative to JJ-based superconducting memories, we have made a memory based solely on lithographic niobium nitride nanowires. The state of the memory is dictated by persistent current stored in a superconducting loop, while the write and read operations are facilitated by nanowire cryotron devices patterned alongside the memory loop in a single lithographic process. In addition to ease of fabrication, superconducting nanowires offer the advantage of relying on kinetic rather than geometric inductance, allowing the memory cell to be scaled down for high device density without sacrificing performance. Additionally, since persistent current is stored without Ohmic loss, the memory cell has minimal power dissipation in the static state.We have demonstrated a 3 µm x 7 µm proof-of-concept device with an energy dissipation of ~ 10 fJ and a bit error rate < 10-7. Current work focuses on developing a multilayer fabrication process to expand the single memory element into an array and to reduce device dimensions for further density optimization."
"Novel Device (Resistive Switching Device, Memristor) Structure for Neuromorphic Computing Array","Although several types of architectures combining memories and transistors have been used to demon-strate artificial synaptic arrays, they usually present limited scalability and high-power consumption. Ana-log-switching devices may overcome these limitations, yet the typical switching process they rely on, forma-tion of filaments in an amorphous medium, is not eas-ily controlled and hence hampers the spatial and tem-poral reproducibility of the performance.Here we demonstrate single-crystalline SiGe epiRAM with minimal spatial/temporal variations with long retention/great endurance, and high analog current on/off ratio with tunable linearity in conductance update, thus justifying epiRAM’s suitability for transistor-free neuromorphic computing arrays. This is achieved through one-dimensional confinement of conductive Ag filaments into dislocations in SiGe and enhanced ion transport in the confined paths via defect selective etch to open up the dislocation pipes. In SiGe epiRAM, the threading dislocation density can be maximized by increasing Ge contents in SiGe or controlling degree of relaxation23, and we discovered that 60 nm-thick Si0.9Ge0.1 epiRAM contains enough dislocations to switch at tens of nanometer scale devices. Our simulation-based on all those characteristics of epiRAM shows 95.1% accurate supervised learning with the MNIST handwritten recognition dataset. Thus, this is an important step towards developing large-scale and fully-functioning neuromorphic-hardware."
Metal Oxide Thin Films as Basis of Memristive Nonvolatile Memory Devices,"The design of silicon-based memory devices over the past 50+ years has driven the development of increas-ingly powerful and miniaturized computers with de-mand for increased computational power and data storage capacity continuing unabated. However, fun-damental physical limits are now complicating further downscaling. The oxide-based memristor, a simple M/I/M structure, in which the resistive state can be reversibly switched by application of appropriate volt-ages, offers to replace classic transistors in the future. It has the potential to achieve an order of magnitude lower operation power compared to existing RAM technology and paves the way for neuromorphic mem-ory devices relying on non-binary coding. Our studies focus on understanding the mechanisms that lead to memristance in a variety of insulating and mixed Ionic electronic conductors; thereby providing guidelines for material selection and for achieving improved device performance and robustness."
Lithium Neuromorphic Computing and Memories,"Ionically-controlled memristors could allow for the realization of highly functional, low-energy circuit elements operating on multiple resistance states and to encode information beyond binary. The application of a sufficiently high electric field induces a non-volatile resistance change linked to locally induced redox processes in the oxide. State-of-the-art devices operate mainly on O2−, Ag+ or Cu2+ ions hopping over vacancies. Surprisingly, despite their fast diffusivity and stability towards high voltages, lithium solid-state oxide conductors have almost been neglected as switching materials. Our work investigates lithium ionic carrier and defect kinetics in oxides to design material architectures and interfaces for novel Li-operated memristors as alternative memory material. Extensive efforts were devoted to understand the growth of the chosen Li-oxides conductor thin films by Pulsed Laser Deposition (PLD) and to microfabricate model thin film architecture devices. In-house overlithiated pellets of the selected oxides were synthesized and used as PLD targets. Dense, crack-free thin film oxides have been successfully grown on Pt/Si3N4/Si substrates, including multilayer heterostructures of two selected Li-oxide materials. Remarkably, Pt/Li-oxide/Pt structures (Figure 1a and b) show a significant bipolar resistive switching effect with a resistance ratio Roff/Ron~104-105 at beneficial low operation voltages to reduce the footprint at operation (~3V for a non-device lab optimized architecture) (Figure 1a). In addition, sweep rate, thickness, and area dependence studies suggest that the bulk oxide plays a major role in the diffusion of the ionic species for achieving a large and tunable resistance ratio. This phenomenon makes the new investigated Li-oxides novel candidate material as new neuromorphic computing element. In situ Raman Spectroscopy and TEM experiments will shed light on the microstructure and its defects and will allow a better understanding of the underlying physical mechanism of the switching behavior. Also, new routes are explored to modify the lithiation degree of the thin films and would add an extra parameter to tune and alter switching kinetics and resistance retention."
Effects of Line Edge Roughness on Photonic Device Performance through Virtual Fabrication,"Silicon photonics has garnered a large amount of in-terest in recent years due to its potential for high data transfer rates and for other, more novel applications. One attractive feature of silicon photonics is its rela-tively seamless integration with existing CMOS fab-rication technologies. That means, however, that it is subject to similar random and systematic variations as are known to exist in CMOS manufacturing processes.One common source of process variation is Line Edge Roughness (LER), which occurs during lithography. Since LER produces random perturbations to the component geometry, it is likely to influence the light-guiding abilities of photonic components and devices subject to LER.We study the effect of LER on the performance of a fundamental component, the Y-branch, through virtual fabrication simulations (Figure 1). Ideally, the Y-branch transmits the input power equal to its two output ports. However, imbalanced transmission between the two output ports is observed when LER is imposed on the Y-branch (Figure 2) depending on the statistical nature (amplitude and correlation length) of the LER. The imbalance can be as low as 1% for small LER amplitudes, and reach up to 15% for large LER amplitudes (Figure 3). These results can be captured as worst-case corner models and included in variation-aware photonic compact models."
Reprogrammable Electro-Chemo-Optical Devices,"Photonic devices with programmable properties allow more flexibility in manipulation of light. Recently, sev-eral examples of reconfigurable photonic devices were demonstrated by controlling the local/overall index of refraction in thin films, either by thermally induced phase change in chalcogenides or by intercalation of lithium into oxides. We propose a novel approach for design of reprogrammable photonic devices based on electrochemical modification of ceria-based elec-tro-chemo-optical devices. Previously, it was shown that the refractive index of PrxCe1-xO2-δ (PCO) is a function of oxygen nonstoichiometry, δ that can be controlled electrochemically via closely spaced electrodes in a lateral device configuration. For modified transverse configurations, a PCO thin film on yttrium stabilized zirconia (YSZ) substrate with Transparent Conducting Oxide (TCO) top electrode allows for voltage controlled oxygen exchange. Enhanced spatial resolution can be further achieved with the aid of lithographically patterned nano-dimensioned oxide layers."
On-chip Infrared Chemical Sensor Leveraging Supercontinuum Generation in GeSbSe Chalcogenide Glass Waveguide,"In this report, we demonstrate the first on-chip spec-troscopic chemical sensor with a monolithically inte-grated supercontinuum (SC) light source. Unlike tradi-tional broadband, blackbody sources used in benchtop Infrared Radiation (IR) spectrophotometers waveguide SC sources feature high spatial coherency essential for efficient light coupling and manipulation on a photon-ic chip. Compared to tunable lasers, SC offers superior bandwidth coverage. The broadband nature of SC facil-itates access to wavelengths that are difficult to cover using semiconductor lasers, and thereby, significantly expands the identifiable molecule repertoire of spec-troscopic sensors. In our experiment, we use chalco-genide glass (ChG) as the waveguide material for both SC generation and evanescent wave sensing. ChGs are known for its broadband infrared transparency, large Kerr nonlinearity, and low two-photon absorption (TPA), ideal characteristics for our application. 400 nm thick Ge22Sb18Se60 (GeSbSe) films were thermally evaporated onto 4” silicon wafers with 3 µm thermal oxide as an under cladding from GeSbSe glass powders. GeSbSe waveguides with varying length were fabricated using our previously established protocols. In the process, a 350-nm-thick ZEP resist layer was spun onto the substrate followed by exposure on an Elionix ELS-F125 tool at a beam current of 10 nA. The resist pattern was then developed by immersing in ZED-N50 developer for one minute. Reactive ion etching was performed in a PlasmaTherm etcher to transfer the resist pattern to the glass layer. The etching process used a gas mixture of CHF3 and CF4 at 3:1 ratio and 5 mTorr total pressure. The incident Radio Frequency (RF) power was fixed at 200 W.Finally, the device was immersed in N-Methyl-2-pyrrolidone (NMP) overnight to remove the ZEP resist and complete device fabrication. The waveguides assume a zigzag geometry with lengths up to 21 mm. Figure 1a plots the SC spectra in GeSbSe waveguides with the different lengths and the optimal dimensions (W = 0.95 µm, H = 0.4 µm). As indicated in the figures below, the SC bandwidth extends to over half an octave, albeit with decreased total output power when the waveguide length increases to 21 mm. In the sensing experiment, the GeSbSe waveguide was immersed in carbon tetrachloride (CCl4) solutions containing varying concentrations of chloroform (CHCl3). The CCl4 solvent is optically transparent across the near-IR, whereas the C-H bond in chloroform leads to an overtone absorption peak centering at 1695 nm, a wavelength outside the standard telecommunication bands. SC spectra near the chloroform absorption peak obtained with GeSbSe waveguides of different lengths are presented in Figure 1b. The data were normalized to the background (collected in pure CCl4)."
Sensing Chemicals in the mid-Infrared using Chalcogenide Glass Waveguides and PbTe Detectors Monolithically Integrated On-chip,"Chemical sensors are important for many applications, from sensing explosive residues for homeland security and defense to sensing contaminants in air and water for environmental monitoring. However, the sensors currently used for these purposes are either bulky, not very sensitive, or not able to identify a chemical specifi-cally. Integrated photonic sensors, which include a light source, photonic sensing element, and photonic detec-tor integrated directly on-chip, that can operate in the mid-infrared (MIR) chemical fingerprint region, prom-ise to be small, sensitive, and specific chemical sensors. They achieve this by confining light within waveguides packed into a small area and using the evanescent field that exists outside the waveguides to sense the pres-ence of a chemical through absorption spectroscopy, identifying chemicals by their unique absorption spec-tra. This work focuses on designing and fabricating the first ever MIR integrated sensing element combined with a detector, operating at room temperature.A spiral waveguide design was chosen for the sensing element due to its long interaction length, which improves sensitivity, while still maintaining a small area footprint. Fabrication was done using a double layer electron beam lithography and liftoff technique to reduce the waveguide sidewall roughness, and therefore loss, of the thermally evaporated chalcogenide glass waveguides. The thermally evaporated polycrystalline PbTe detector was deposited directly underneath the waveguide using photolithography and liftoff. This direct integration of the detector with the waveguide improves coupling of light into the detector while also reducing the size, and therefore noise, level of the detector, allowing it to function at room temperature when most MIR detectors need cooling. Figure 1 shows the spiral sensing element and waveguide integrated PbTe detector. The results from sensing methane gas using 3.3 μm light are shown in Figure 2, demonstrating that this integrated sensing element and detector can effectively sense the presence of chemicals using their MIR absorption spectra."
Broadband Low-loss Nonvolatile Photonic Switches Based on Optical Phase Change Materials (O-PCMs),"Optical switching is an essential function in photon-ic integrated circuits. Recently, a new class of devices based on O-PCMs have emerged for on-chip switch-ing. Unlike electro-optic or thermo-optic effects which are minuscule, phase transition in O-PCMs generates huge optical property modulation conducive to ul-tra-compact device architectures. In addition, such phase changes can be non-volatile, exemplified by the transition between amorphous (a-) and crystalline (c-) states in chalcogenide alloys. Despite these attractive features, the performances of existing PCM-based pho-tonic switches are severely compromised by the high optical absorption in traditional O-PCMs.Here we report the design and modeling of a new kind of photonic switches combining low-loss phase change alloys and a “nonperturbative” design to boost the switching performance. On the one hand, we use a low-loss O-PCM for this application: Ge2Sb2Se4Te1 (GSS4T1). Fig 1a and 1b show the optical constants of GSS4T1 compared with traditional PCM Ge2Sb2Te5 (GST225), as measured by ellipsometry. At telecommunication wavelength, the material figure-of-merit, which is defined as index change over extinction coefficient, is 6 times higher. Moreover, the loss of amorphous state GSS4T1 is 0.00017 measured by waveguide cutback method, which is two orders of magnitude lower. On the other hand, the switch design is based on the huge index change of O-PCMs. The basic element is a directional coupler comprised of a bare waveguide (WG1) and a waveguide covered with a PCM strip (WG2). At (a-) state, their indices are matched, and light will be coupler from WG1 to WG2. At (c-) state, due to the large index change of O-PCM, the modal profile will be completely different, and effective index of WG2 will increase a lot so that coupling will not happen. This helps to keep the loss at a low level since light will not travel in WG2 when GSS4T1 is in its (c-) state. Fig 2 and 3 show the state-of-the-art performance of the 1 by 2 and 2 by 2 switches designed by this method."
Chalcogenide Glass Waveguide-integrated Black Phosphorus mid-Infrared Photodetectors,"Black phosphorus (BP) is a promising 2-D material that has unique in-plane anisotropy and a 0.3 eV direct bandgap in the mid-IR. However, waveguide integrated black phosphorus photodetectors have been limited to the near-IR on top of Si waveguides that are unable to account for BP’s crystalline orientation. In this work, we employ mid-IR transparent chalcogenide glass (ChG) both as a broadband mid-IR transparent wave-guiding material to enable waveguide-integration of BP detectors and as a passivation layer to prevent BP deg-radation during device processing as well as in ambient atmosphere.Our ChG-on-BP approach not only leads to the first demonstration of mid-IR waveguide-integrated BP detectors, but also allows us to fabricate devices along different crystalline axes of black phosphorus to investigate, for the first time, the impact of in-plane anisotropy on photoresponse of waveguide-integrated devices. The best device exhibits responsivity up to 40 mA/W and noise equivalent power as low as 30 pW/Hz1/2 at 2185 nm wavelength. We also found that photodetector responsivities changed by an order of magnitude with different black phosphorus orientations. This work validates black phosphorus as an effective photodetector material in the mid-IR and demonstrates the power of the glass-on-2-D-material platform for prototyping of 2-D material photonic devices."
An Ultrasensitive Graphene-polymer Thermo-mechanical Bolometer,"Uncooled mid-infrared (Mid-IR) detection and imaging technologies are highly desired for night vision, secu-rity surveillance, remote sensing, industrial inspection, medical, and environmental chemical sensing. Tradi-tional mid-IR detection technologies operating at room temperature are all associated with thermal related phenomena that transfer the optical signals into elec-trical signals through changes of temperature on the device. Here we propose and implement a new signal transducing scheme where the energy transfer path is optical-thermal-mechanical-electrical. By combining highly sensitive strain sensors made with percolative graphene nano-flake films synthesized by Marangoni self-assembly method, and the highly efficient polymer opto-thermo-actuators, we were able to demonstrate the proof-of-concept bolometric type mid-IR detectors (Figure 1) that could be more sensitive than state-of-the-art technologies. Two types of photoresponse behaviors were observed in our devices: a gradual change in resistance in terms of temperature (Figure 2(a)), which may be associated with the average overlap area decrease of adjacent nano-flakes; and an abrupt “switch” like response (Figure 2(b)) that is presumably due to the decrease of the number of conduction paths of the percolative film. Microscopic characterizations and theoretical modeling were carried on to understand such behaviors. Theoretical analysis showed that our new technology could be at least one order of magnitude more sensitive than the fundamental limit of existing uncooled mid-IR technologies (Figure 2(c))."
Nanocavity Design for Reduced Spectral Diffusion of Solid-state Defects,"The negatively charged nitrogen-vacancy (NV) center in diamond has an electronic spin state that can be optical-ly initialized, manipulated, and measured. Entanglement generation between two spatially separated quantum memories can be generated by coupling them to optical modes. Coupling NV centers to nanophotonic devices such as waveguides and cavities will boost the NV-NV entanglement rate by increasing the emission and collec-tion rate of photons entangled with the spin resonators.We can fabricate 1D photonic crystal nanobeam cavities in diamond with quality factors larger than 16,000. Unfortunately, an optimally coupled NV center in such a cavity will be only 30 nm from surfaces, and the linewidths of NV centers in such cavities is increased to 10s of GHz (1000x the natural lifetime limited linewidth) due to spectral diffusion.To obtain NV centers with GHz linewidths in a cavity with a high-quality factor, we design and fabricate novel “Alligator” cavities. A bandgap is created via a sinusoidal width modulation. A high-Q mode is trapped in a defect created by reducing the amplitude of the modulation. The optimized mode (seen in Figure (a)) has a Q > 100,000 in simulation. We fabricate these cavities from single crystal bulk diamond. A scanning electron micrograph of one is seen in Figure (b). In experiment, we measure cavities with a mean Q value of ~7000 (Figure (d)). Figure (c) shows the spectrum of such a cavity. These structures should allow coupling between single NV centers with limited spectral diffusion and high-quality factor cavity modes."
Two-dimensional Photonic Crystal Cavities in Bulk Single-crystal Diamond,"Color centers in diamond are leading candidates for quantum information processing. Recent demonstra-tions of entanglement between separated spins of the nitrogen-vacancy (NV) color center constitute a major milestone in generating and distributing quantum in-formation with solid-state quantum bits. However, the generation of entanglement in local quantum nodes containing NV centers is an inefficient process due to the largely incoherent NV optical transitions, as the zero-phonon-line (ZPL) constitutes only 4% of the NV’s spontaneous emission. This fraction can be modified if the NV center is placed in a photonic cavity, which modifies the electromagnetic environment, and thus, the NV’s emission properties via the Purcell effect. Pho-tonic crystal (PhC) slab nanocavities offer high-quality factors (Q) and small mode volumes (V), which consid-erably increase the fraction of emission into the ZPL. Figure 1(A) shows the electric field profile in such a nanocavity, where the lattice constant is a = 214 nm and the thickness of the slab is H = a. The fabrication of such structures, however, typically requires laborious reactive-ion etching (RIE) thinning of a bulk diamond down to a thickness of H. This need arises because high-quality single-crystal diamond thin films are not available and the chemically inert nature of diamond precludes wet undercutting techniques. In this work, we fabricate PhC nanocavities in diamond directly from bulk diamond. Electron beam lithography and reactive ion etching (RIE) first defines the PhC structures, after which alumina deposited using atomic layer deposition conformally coats and protects the diamond sidewalls. Then, anisotropic oxygen plasma undercuts the diamond slabs and, finally, hydrofluoric acid removes the hard mask and alumina to reveal suspended diamond structures (Figure 1(B)). We find high Q resonances near the NV ZPL wavelength of 637 nm, as shown in the photoluminescence spectra in Figure 1(C). The fabrication details and cavity measurements are in the last reference.In conclusion, we report the first fabrication of photonic crystal slab nanocavities in bulk diamond. Immediate steps include the coherent coupling of a single NV center to the nanocavity, which will serve as a node in a quantum repeater and for solid-state cavity quantum electrodynamics investigations. This 2-D platform considerably expands the toolkit for classical and quantum nanophotonics in diamond."
Quasi-Bessel-Beam Generation using Integrated Optical Phased Arrays,"Due to their unique diffractive properties, Bessel beams have contributed to a variety of important ad-vances and applications, including multiplane optical trapping, reduced scattering and increased depth of field microscopy, improved laser corneal surgery, and adaptive free-space communications. Recent work has turned toward generation of Bessel beams using com-pact form factors, including spatial light modulators, Dammann gratings, and metasurfaces. However, these demonstrations do not provide full on-chip integration, and most are fundamentally limited to static beam for-mation.In this work, integrated optical phased arrays, which manipulate and dynamically steer light, are proposed and demonstrated for the first time as a method for generating quasi-Bessel beams in a fully integrated, compact-form-factor system (Figure 1). First, the phase and amplitude distributions necessary for generating phased-array-based Bessel-Gauss beams are derived analogously to bulk-optics Bessel implementations. Next, a splitter-tree-based CMOS-compatible phased array architecture (Figure 2) is developed to passively encode arbitrary phase and amplitude feeding of the array – necessary for Bessel-Gauss-beam generation. Finally, the developed theory and system architecture are utilized to demonstrate a 0.64 mm × 0.65 mm aperture integrated phased array that generates a quasi-one-dimensional Bessel-Gauss beam with a ~14 mm Bessel length and ~30 μm power FWHM (Figure 3)."
See-through Light Modulators for Holographic Video Displays,"In this research (a collaboration with Dr. Daniel Smalley of Brigham Young University), we design and fabricate acousto-optic, guided-wave modulators in lithium nio-bate for use in holographic and other high-bandwidth displays. Guided-wave techniques make possible the fabrication of modulators that are higher in bandwidth and lower in cost than analogous bulk-wave acous-to-optic devices or other spatial light modulators used for diffractive displays; these techniques enable simul-taneous modulation of red, green, and blue light. In particular, we are investigating multichannel variants of these devices with an emphasis on maximizing the number of modulating channels to achieve large total bandwidths. To date, we have demonstrated multi-channel full-color modulators capable of displaying ho-lographic light fields at standard-definition television resolution and at video frame rates. Our current work explores a device architecture suitable for wearable augmented reality displays and other see-through applications, in which the light outcouples toward the viewer (Figure 1), fabricated using femtosecond laser micromachining (Figure 2)."
A Scalable Single-photon Detector Array Based on Superconducting Nanowires,"Detecting single photons over large numbers of spa-tial modes is crucial for photonic quantum informa-tion processing. This measurement usually requires an array of time-resolved single-photon detectors. The superconducting nanowire single-photon detec-tors (SNSPDs) are currently the leading single-photon counting technology in the infrared wavelength and have the highest performance in timing jitter, detection efficiency, and counting rate. In a conventional readout scheme, each SNSPD requires one coaxial cable in the cryostat, a low-noise RF amplifier, and a high-resolu-tion time-to-digital converter. Implementing a system of a few SNSPD channels with the conventional read-out is possible, but scaling them to tens or hundreds of channels requires formidable resources and remains an outstanding challenge.Here, we report a scalable two-terminal SNSPD array that only requires one pair of RF cables for the readout. Figure 1 shows the architecture of the array, where a chain of detectors was connected using superconducting nanowire delay lines. The nanowire delay lines were designed to be slow-wave transmission lines with a phase velocity of only 0.016c, where c is the speed of light in vacuum. When a detector absorbs a photon and fires, it generates a pair of counter-propagating pulses towards the two terminals. By registering the pulses on the two terminals, and performing simple timing logic, one can resolve the arrival locations of up to two incident photons (see Figure 2). By analyzing the electrical pulse shapes, we also showed photon-number-resolving capability in a 4-element device. This device architecture will be useful for multi-photon coincidence detection in photonic integrated circuits."
Utilization of BaSnO3 and Related Materials Systems for Transparent Conducting Electrodes,"Efficient, transparent electrode materials are vital for applications in smart window, LED display, and solar cell technologies. These materials must possess a wide band gap for minimal optical absorption in the visible spectrum while maintaining high electrical conductivi-ty. Tin-doped indium oxide (ITO) has been the industry standard for transparent electrodes, but the use of the rare element indium has led to a search for better mate-rial alternatives. BaSnO3 represents a promising alter-native due to its high electron mobility and resistance to property degradation under oxidizing conditions, but the mechanisms by which processing conditions and de-fect chemistry affect the final material properties are not well understood.This work seeks to better understand the relationships between processing, defect chemistry, and material properties of BaSnO3, to better establish the consistent and controllable use of BaSnO3 as a transparent electrode. To accomplish these goals, methods such as in situ resistance and impedance monitoring during annealing will be applied. In addition, a variety of novel methods such as the in situ monitoring of optical transmission (shown in Figure 1) during annealing and the in situ monitoring of resistance during physical vapor deposition will be utilized to investigate BaSnO3. Direct measurements of the key constants for the thermodynamics and kinetics of oxidation in donor-doped BaSnO3 will be experimentally determined for the first time. This increase in understanding will provide a predictive model for determining optical properties, carrier concentrations, and electron mobilities in BaSnO3, which may be become increasingly important due to its high electron mobility, high-temperature stability, and favorable crystal structure."
A Sampling Jitter-tolerant Pipelined ADC,"In a conventional pipelined ADC, the input signal is sampled upfront as shown in Figure 1. Any jitter in the sampling clock directly affects the sampled input and degrades the signal-to-noise ratio (SNR). Therefore, for fast varying input signals, the sampling jitter severe-ly limits the SNR. The error in sampled voltage due to clock jitter isΔv = (dv/dt) · Δtwhere dv/dt is the time-derivative of the input signal at the sampling instant and ∆t is the jitter in the sampling clock. Since the sampling clock jitter is random, it introduces a random noise in the sampled input signal. Also, the error voltage is proportional to dv/dt and hence to the amplitude and frequency of the input signal. Thus, as the frequency of the input signal increases, the effect of sampling clock jitter becomes more pronounced. In fact, it can be shown that for a known rms sampling jitter st the maximum SNR is limited toSNRmax = 1/(2πfinst )where fin is the input signal frequency. Typically, it is difficult to reduce the rms jitter below 100 fs. This limits the maximum SNR to just 44 dB (which is equivalent to 7 bits) for a 10 GHz signal. Therefore, unless the effect of sampling jitter is reduced, the performance of an ADC would be greatly limited for high frequency input signals. It has been shown that continuous-time delta-sigma modulators (CTDSM) reduce the effect of sampling jitter. But since CTDSMs rely on oversampling, they are not suitable for high frequency signals. Therefore it is imperative to develop sampling jitter-tolerant architectures for Nyquist-rate data converters. In this project, we propose a new topology that provides increased tolerance to sampling jitter. At present, we are designing the pipelined ADC in 16-nm CMOS technology to give a proof-of-concept for tolerance to sampling jitter."
A Pipelined ADC with Relaxed Op-amp Performance Requirements,"Among various analog to digital converter (ADC) ar-chitectures, pipelined ADCs are well suited for appli-cations that need medium to high resolution above hundreds-of-megahertz sampling rate. To obtain good linearity, conventional pipelined ADCs must mini-mize multiplying digital to analog converter (MDAC) charge-transfer error by employing high-gain, fast-set-tling op-amps. However, such an op-amp design has become increasingly difficult due to the reduced in-trinsic gain and voltage headroom in a fine-line CMOS technology. With low intrinsic gain devices, either a gain-boosting technique or a multi-stage topology is necessary to make the op-amp meet the gain require-ment. Decreased power supply demands a larger capac-itance to maintain the same level of SNR. As a result, the power consumption of these op-amps becomes prohibitively large. Op-amp non-idealities have been removed or relaxed in digital domain by taking advantage of digital computation to address this issue. In this project, we propose a digital calibration scheme for op-amp-based pipelined ADCs. The ADC relaxes first stage op-amp performance requirements by using a shadow ADC and a simple digital domain calibration algorithm. To validate the functionality of the proposed calibration technique, a proof-of-concept ADC has been designed in 28nm CMOS technology and is currently being tested."
Data-dependent Successive-Approximation-Register Analog-to-Digital Converter,"This work on successive-approximation-register (SAR) analog-to-digital converters (ADCs) (Figure 1) aims at improving data-dependent savings in energy in key components of a SAR ADC by leveraging the informa-tion available from signal’s immediate past samples and the signal type. The dominant energy consuming components are the digital-to-analog converter (DAC) and the comparator.Energy expenditure in the DAC per sample conversion depends on the DAC topology and sequence of steps taken during successive approximation. Energy in the comparator is directly proportional to the number of comparisons done per sample conversion. A design with data-dependent savings takes advantage of the correlation between successive samples in completing the conversion in fewer bit-cycles and also operates the DAC more energy-efficiently.Previous work presented data-dependent savings by doing least-significant-bit (LSB)-first successive approximation to convert an input sample. By starting with a previous sample and using LSB-first, the algorithm converges in a fewer number of cycles than conventional most-significant-bit (MSB)-first SAR conversion when the present signal is close to the previous signal. Fewer cycles translate into energy savings in the comparator and the DAC. Another work developed successive approximation algorithms to find a sub-range from the full range in a few cycles before carrying on a binary search in this small range. In this work, we investigate a SAR ADC with a search algorithm based on the statistical characteristics of the signal for optimum energy expenditure."
GaN HEMT Track-and-Hold Sampling Circuits with Digital Post-correction on Dynamic Nonlinearity for High-Performance ADCs,"Analog-to-digital converters (ADCs) often limit the performance of integrated systems for emerging ap-plications such as next-generation communication systems, data centers, and quantum computing. The ADC performance is, in turn, limited at least partly by a track-and-hold sampling circuit (THSC). The low sup-ply voltage of deeply scaled complementary metal-ox-ide-semiconductor (CMOS) transistors determines the THSC input signal range, therefore becoming a fun-damental upper bound to the effective number of bits (ENOBs) of CMOS ADCs. This research work envisions to realize THSCs in GaN-on-Si technology, which monolithically integrates GaN high-electron-mobility transistors (HEMTs) with Si-CMOS transistors, for future ultrahigh-performance ADCs. Operating GaN HEMTs at a high voltage (>30 V) allows a very large input swing (>16 V), providing signal-to-noise ratio (SNR) performance orders of magnitudes beyond the limit of CMOS THSCs. We designed and implemented two GaN HEMT THSCs. The first THSC was fabricated in a commercial GaN foundry technology on SiC substrate, providing 98-dB SNR at 200 MS/s. The second THSC design was fabricated in a GaN technology that was developed at MTL on Si substrate, which operates at 1 GS/s thanks to a higher current-gain cutoff frequency fT and external gate-bootstrapping clock (Figure 1). While these GaN HEMT THSCs achieved an unprecedentedly high SNR at a given input frequency, they suffer from dynamic nonlinearity from the GaN HEMT source-follower buffers for gate-bootstrapping sampling clock generation. Although dynamic nonlinearity correction techniques are mature with RF power amplifiers (PAs), these conventional pre-distortion techniques have high sensitivity to DC offsets, and thus, cannot be directly applied to GaN HEMT THSCs.To overcome this challenge, we are developing a digital post-correction (DPC) technique, which will demonstrate improved linearity of GaN HEMT THSCs without using a dedicated reference ADC. By applying a DPC technique based on modified Volterra series (Figure 2), we have recently demonstrated that THSC linearity can be improved by more than 20 dB. We are presently working to enhance the linearization performance by applying advanced DPC techniques."
GaN Circuit-device Interaction in Fully Integrated RF Power Amplifiers,"Highly integrated GaN RF power amplifiers (PAs) have been developed for mobile devices and connected cars ap-plications using the physics-based RF transistor compact model, MIT Virtual Source GANFET (MVSG). RF power amplifiers are required to operate in a linear region to prevent signal distortion and resultant data loss, which is mainly affected by inherent device-level nonlinear be-havior. Since the second derivative of transconductance, g3, is an intrinsic source of intermodulation distortion, many studies aimed to cancel it, especially in CMOS tech-nology. However, the high mobility and thermal effect of GaN devices make the device nonlinearity compensation harder than in CMOS devices. Thus, we have looked into the large signal linearization considering both power gain and third-order harmonics rather than g3 alter-ation techniques that cannot be properly functional in a high-power amplifier with large signal input.In our previous design, the Class-AB + Class-C configuration was proposed for a fully integrated GaN RF amplifier, demonstrating improved linearity and efficiency. Recently, we designed another GaN RF power amplifier with the Common-Source & Common Gate (CS-CG) configuration to further improve the intermodulation distortion by optimizing the third order harmonics performance from the viewpoint of compensating for the large signal distortion. The CS-CG outperforms the Class-AB + Class-C in terms of the third order harmonics and intermodulation distortion, which means that the average time-varying composite g3 of the CS–CG is lower than that of the Class-AB + Class-C.To study the impact of the device and technology parameters on the circuit performance, we used both the MVSG model and the CS-CG amplifier and isolated some device parameters which affect the DC and RF performance at both device and circuit levels. Figure 1 shows the circuit implementation using 0.25μm GaN technology and its gain and third- order harmonics with varying DIBL, δ. Intermodulation distortion is further investigated with varying δ, short channel effects such as moderate punch-through, nd, and parasitics, i.e., Cds, and Cdg, as depicted in Figure 2."
Cryptographically Secure Ultra-fast Bit-level Frequency Hopping for Next-generation Wireless Communications,"Current Internet-of-Things devices communicate via Bluetooth Low Energy (BLE). Unfortunately, BLE-con-nected devices are vulnerable to a wide range of attacks; this work specifically addresses selective jamming denial of service where the adversary corrupts transmitted mes-sages targeting a single victim. Selective jamming is par-ticularly challenging as it conceals the attacker’s identity contrary to broadband-wireless jamming. To illustrate this type of attack, we demonstrate selective jamming against a commercial fitness BLE-device as shown in Figure 1. This form of attack can cause serious harm such as in the case of insulin pump medical devices.The primary vulnerability of BLE is founded in the communication protocol which uses frequency hopping to send a message, which is decomposed into data packets, over rapidly changing sub-frequencies. The carrier frequency hops among these sub-frequencies at a relatively slow rate of 612µs per data packet (Figure 2). Conversely, an attacker needs only 1µs to identify the carrier frequency, then block the remainder of the data packet sent on that sub-frequency. To counter this attack, we developed physical-layer security through an ultra-fast bit-level frequency hopping scheme which sends every data bit on a unique carrier frequency while achieving a 1µs hop period (Figure 2). In addition, a challenging issue is that traditional modulation schemes, such as the BLE Gaussian frequency shift keying (GFSK) modulation with fixed carrier offset of ± 250kHz for Bit 1 and Bit 0, permit the attacker to selectively overwrite individual bits in a packet once the carrier frequency is localized. The attacker gains control over the packet that will be received by the victim. We protect against this attack by implementing a cryptographically secure data-driven dynamic channel selection scheme that enables 80-way pseudorandom FSK modulation and provides data encryption in the physical layer.In this work, we demonstrated the first integrated bit-level frequency-hopping transmitter that hops at 1μs period and uses data-driven random dynamic channel selection to enable secure wireless communications with data encryption in the physical layer."
A Dense 240-GHz 4×8 Heterodyne Receiving Array on 65-nm CMOS Featuring Decentralized Generation of Coherent Local Oscillation Signal,"There is a growing interest in pushing the frequency of beam-steering systems towards terahertz range, in which case narrow-beam response can be realized at chip scale. However, this calls for disruptive chang-es to traditional terahertz receiver architectures, e.g., square-law direct detector arrays (low sensitivity and no phase information preserved) and small heterodyne mixer arrays (bulky and not scalable). In the latter case, corporate feed for generating and distributing the local oscillation signals (LO)— typically a necessary component—can be very lossy at large scale. Here, we report a highly scalable 240-GHz 4×8 heterodyne array achieved by replacing the LO corporate feed with a net-work that couples LOs generated locally at each unit. A major challenge for this architecture is that each unit should fit into a tight λ/2×λ/2 area to suppress side lobes in beamforming, making the integration of the mixer, local oscillator, and antenna into a unit ex-tremely difficult. This challenge is well-addressed in our design. We have built highly-compact units, which ultimately enables the integration of two interleaved 4×4 phase-locked sub-arrays in 1.2-mm2.The schematic of the circuit of one unit is shown in Figure 1(a). Its core component is a self-oscillating harmonic mixer (SOHM), which can simultaneously (1) generate high-power LO signal and (2) down-mix the radio frequency (RF) signal. The SOHM is connected to both an intra-unit slot antenna (TL4 and TL4’) for RF receiving and a co-planar waveguide (CPW)/slotline mesh (TL3) for strong LO coupling with neighboring SOHMs. Owing to the coupling, LOs generated in each unit can be all locked to an external reference signal so that the array is coherent. Die photo showing the placement of the array and the PLL is given in Figure 1(b). Measured spectrum of 4.6-MHz (below the noise corner frequency) baseband signal is shown in Figure 2, from which we obtain a sensitivity (required incident RF power to achieve SNR=1 at baseband) over 1-kHz detection bandwidth of 38.8pW – more than 6× improvement over state-of-the-art large-scale homodyne arrays."
THz-Comb-Based Radar for Ultra-Broadband 3-D Imaging,"Low-cost 3-D imaging recently becomes increasingly attractive because of its enormous potential in security applications. In particular, waves in the low terahertz (THz) range provide powerful capabilities for 3-D imag-ing due to the large available bandwidth and improved angular resolution (compared with radio frequency and mm-wave signals), and good transmission (<0.01 dB/m) through extreme weather conditions (compared with infrared and visible light).We propose a comb radar architecture to increase the bandwidth to more than 0.1 THz without using ultra-wideband components. Shown in Figure 1, it utilizes equally-spaced signal tones with frequency modulation; the generated IF signals are then combined in the digital domain. The proposed comb radar architecture has many advantages compared with conventional Frequency-Modulated Continuous-Wave (FMCW) radar in silicon: peak performance is maintained across a large bandwidth, finer Doppler frequency resolution, larger intermediate frequency (thus smaller flicker noise) and higher linearity. Similar to our previous frequency-comb-based THz spectrometer, in this radar, all components including antennas can be integrated on a single chip, our solution has merits of low cost, small volume, and lightweight.Figure 2 shows the architecture of the proposed comb radar. It consists of multiple channels with a suitable bandwidth in each channel, leading to an aggregated bandwidth that is larger than 0.1 THz. Note that the number of channels is not limited by the architecture, so the aggregated bandwidth is only limited by the bandwidth of a single channel. The FMCW signal is fed into the first channel directly and up-converted through single sideband mixers to the subsequence channels step by step. The transmitter and the receiver share one on-chip antenna to save the area and power. The mixer first receiver utilizes the transmit power as local oscillator signal and down-converts the received echo signal to IF for further image processing. In addition, since backside radiation has asymmetric radiation pattern and multiple reflections in the attached silicon lens, front-side radiation is desired. To this end, we adopt a substrate-integrated-waveguide antenna utilizing its multiple high-order resonance modes in orthogonal directions. Compared with patch antenna, the new on-chip antenna design has much wider bandwidth (>10% fractional bandwidth)."
CMOS Chip-scale Vector Ambient Magnetic Field Sensing Based on Nitrogen-vacancy (NV) Centers in Diamond,"Nitrogen-vacancy (NV) centers in diamond have attract-ed attention for spin-based quantum sensing in ambi-ent conditions. They have demonstrated outstanding nanoscale sensing and imaging capabilities for magnet-ic-fields. However, these sensing systems require many discrete devices to operate. This limits their scalability. In this work, we demonstrate a chip-scale CMOS and NV in-tegrated platform for magnetic field sensing. The CMOS chip performs the required spin manipulation and read-out functions for NV sensing protocols.Magnetic field sensing is accomplished by determining the spin states of the NV. The frequency of the spin states is determined by through optically detected magnetic resonance (ODMR). The magnetic field is proportional to the frequency splitting of the spin states (2.8 MHz/Gauss). Our system has an on-chip microwave (MW) signal generator, operating from 2.6 GHz to 3 GHz. In addition, an on-chip coil with parasitic loops radiates the AC magnetic field with an amplitude up to 10 Gauss with 95% uniformity over 50 µm x 50 µm. This MW radiation efficiently manipulates the NV spin ensembles. This is followed by on-chip optical readout of the spin state. A CMOS-compatible metal-dielectric structure filters out the optical pump (532 nm) with an isolation of 10 dB. An on-chip patterned P+/N-Well photodiode, beneath the MW coil and the filter, detects the NV red fluorescence. This photodiode is patterned to reduce the unwanted coupling to the MW coil. The measured photodiode responsivity is 230mA/W. The proposed system opens the door for a highly integrated quantum system with applications in the life sciences, tracking, and advanced metrology."
An Energy-efficient Reconfigurable DTLS Cryptographic Engine for End-to-End Security in IoT Applications,"End-to-end security protocols, like Datagram Trans-port Layer Security (DTLS), enable the establishment of mutually authenticated confidential channels be-tween edge nodes and the cloud, even in the presence of untrusted and potentially malicious network infra-structure. While this makes DTLS an ideal solution for IoT, the associated computational cost makes soft-ware-only implementations prohibitively expensive for resource-constrained embedded devices. We ad-dress this challenge through the design of energy-effi-cient hardware to accelerate the DTLS protocol along with associated cryptographic computations.Figure 1 shows a block diagram of our system, which consists of a 3-stage RISC-V processor, and a memory-mapped DTLS engine supporting the AES-128 GCM, SHA-256, and prime curve elliptic curve cryptography (ECC) primitives. We demonstrate hardware-accelerated DTLS which is 438x more energy-efficient and 518x faster than software implementations. The use of dedicated hardware for DTLS also reduces code size by 78KB and data memory usage by 20KB, thus increasing processor resources available to the application stack.The test chip, shown in Figure 2, was fabricated in a 65nm LP CMOS process, and it supports voltage scaling from 1.2V down to 0.8V. The RISC-V processor achieves 0.96DMIPS/MHz, consuming 40.36μW/MHz at 0.8V. The DTLS engine consumes 44.08μJ per DTLS handshake, and 0.89nJ per byte of application data, both at 0.8V. Therefore, through the design of reconfigurable energy-efficient cryptographic accelerators and a dedicated protocol controller, this work makes DTLS a practical solution for implementing end-to-end security on resource-constrained IoT devices."
"Ultra-Low-Power, High-sensitivity Secure Wake-up Transceiver for the Internet of Things","The Internet of Things (IoT) connects together an ex-ponentially growing number of devices with an esti-mate of more than 70 billion devices in less than ten years from now. Such devices revolutionize the per-sonal heart monitoring, home automation, as well as the industrial monitoring systems. Unfortunately, the wireless IoT nodes consume a huge portion of their en-ergy on communicating with other devices. On the oth-er hand, a longer battery lifetime or even a batteryless energy-harvesting operation requires a sub-microwatt consumption without significant performance deg-radation. In this work, we propose protocol optimiza-tions as well as circuit-level techniques in the design of a -80dBm sensitivity ultra-low power wake-up receiver for on-demand communication with IoT nodes.Wireless protocols such as Bluetooth low-energy (BLE) are optimized for short-length packets with small preambles and reduced header sizes. However, the power consumption of a low duty-cycled node in the default connected-mode is limited by the periodic beacons dictated by the protocol. Commercial BLE chips are then limited to tens of microwatts even though their standby power is in the nanowatt range. This wake-up receiver exploits the lower limit of the standby power to achieve significant power reduction through a wake-up scheme wrapped around the BLE advertising protocol. The receiver, shown in Figure 1, employs such duty-cycled wake-up scheme to mitigate the power/sensitivity trade-off achieving sub-microwatt average power at the required BLE sensitivity. When the receiver decodes its wake-up pattern inside the BLE advertising packet, depicted in Figure 2, it generates a wake-up signal then reconfigures its correlator with a new pattern. Figure 3 illustrates the power/latency trade-off where a user with a commercial app can use a cellphone to wake any sleeping IoT node up using the BLE standard according to the application at hand."
Contactless Current Sensing for Industrial IoT,"The ability to sense current is crucial to many industri-al applications including power line monitoring, motor controllers, battery fuel gauges, etc. We are developing smart connectors with current sensing abilities for use in the industrial internet of things (IoT). These connec-tors can be used for 1) power quality management: to measure real power, reactive power, and distortion, and 2) machine health monitoring applications for continu-ous monitoring, control, prevention, and diagnosis. At the system level, the smart connectors need to 1) mea-sure AC, DC, and multiphase currents, 2) reject stray magnetic fields, and 3) detect impending connector fail-ure. On the sensor level, they need to provide high mea-surement bandwidth (BW) and low power operation. Current can be sensed directly by using a shunt resistor, but it leads to large power dissipation for measuring high current levels (10-100 A). Indirect/contactless sensing, which senses the magnetic field strength, is a better option as it offers galvanic isolation and the ability to operate safely in high voltage applications. Examples of contactless current sensors include Hall, magneto-resistive (MR), and fluxgate (FG) sensors. FG sensors with integrated magnetics offer higher sensitivity than Hall sensors (nT vs. µT) and higher linearity and lower offset hysteresis than MR sensors, making them a good choice for industrial current sensing. The proposed system consists of a central processor and multiple low-power, high-BW FG sensors to make synchronous measurements (Figure 1). The measured data from all sensors is stored on the central processor, which runs preliminary analytics on the data before sending it to the cloud. Figure 2 shows the workings of a basic fluxgate sensor design. The proposed sensor makes use of various power saving techniques to reduce the energy per measurement, as well as digitally assisted analog circuits to push for high BW and BW scalability with duty cycling, from >100 kHz BW for machine health monitoring to <1 kHz for power quality management."
Navion: An Energy-efficient Accelerator for NanoDrones Autonomous Navigation in GPS-denied Environments,"Drones are getting increasingly popular nowadays. Nanodrones specifically are easily portable and can fit in your pocket. Equipped with multiple sensors; the drone functionality is getting more powerful and smart (e.g., track objects, build 3-D maps, etc.). These ca-pabilities can be enabled by powerful computing plat-forms (CPUs and GPUs), which consume a lot of energy. The size and battery limitations of Nanodrones make it prohibitive to deploy. This work presents Navion, an energy-efficient accelerator for visual-inertial odometry (VIO) that enables autonomous navigation of miniaturized robots, and augmented reality on portable devices. The chip fuses inertial measurements and mono/stereo images to estimate the camera’s trajectory and a sparse 3-D map. VIO implementation requires large irregularly structured memories and heterogeneous computation flow. The entire VIO system is fully integrated on-chip to eliminate costly off-chip processing and storage. This work uses compression and exploits structured and unstructured sparsity to reduce on-chip memory size by 4.1x. Navion is fabricated in 65nm CMOS. It can process 752x480 stereo images at 171 fps and inertial measurements at 52 kHz, consuming an average 24mW. It is configurable for maximizing accuracy, throughput, and energy-efficiency across different environments. This is the first fully integrated VIO in an ASIC."
Fast Frontier-exploration for Unmanned Autonomous Vehicles with Resource Constraints,"Unmanned Autonomous Vehicles (UAV) have received wide attention. Their capability to autonomously navigate around the environment enables many ap-plications including search-and-rescue, surveillance, wildlife protection and environment mapping. The key technique to empower such capabilities is the frontier-exploration algorithm, which periodically makes decisions on where the vehicle should explore next in an unknown environment based on previous-ly acquired knowledge. However, such algorithms are computationally expensive. In a practical system, the computation is usually offloaded to a powerful com-puter, causing a significant delay in the response time. This also makes the system strongly dependent on the presence of a stable wireless connection. These factors prohibit the application of the frontier-exploration al-gorithm to resource-constrained miniature UAVs with limited battery and computation power.In this work, we present an algorithm to reduce the computation cost of the state-of-the-art mutual information based frontier-exploration algorithm. The key idea behind the algorithm is to use the same computations between different parts of the mutual information computation and reduce redundant computations. Additionally, our approach seeks a more compact representation of the environment, which minimizes the number of operations required to run the algorithm.In practice, our algorithm enables the complicated frontier-exploration algorithm to be deployed to a battery-powered miniature UAVs with limited computation power. The algorithm makes it possible for the UAV to explore a closed unknown environment with no stable wireless connections. Thanks to the capability of local computation, the latency of running the algorithm is reduced, enabling the UAVs to explore faster and quickly react to the changes in the environment. The saved computation power can be allocated to the actuators of the UAV, enabling the UAVs to stay in the air longer and therefore explore a larger area given fixed power budget."
Efficient Processing for Deep Neural Networks,"Artificial Intelligence powered by deep neural networks (DNNs) has shown great potential to be applied to a wide range of industry sectors. Due to DNNs’ high computa-tional complexity, energy efficiency has ever-increasing importance in the design of future DNN processing sys-tems. However, there is currently no standard to follow for DNN processing; the fast-moving pace in new DNN algorithm and application development also requires the hardware to stay highly flexible for different con-figurations. These factors open up a large design space of potential solutions with optimized efficiency, and a systematic approach becomes crucial.To solve this problem, we address the co-optimization among the three most important pillars in the design of DNN processing systems: architecture, algorithm, and implementation. First, we present Eyeriss, a fabricated chip that implements a novel data flow architecture targeting energy-efficient data movement in the processing of DNNs (Figure 1). Second, we develop Energy-Aware Pruning (EAP), a new strategy of removing weights in the network to reduce computation so that it becomes more hardware-friendly and yields higher energy efficiency (Figure 2). Finally, we present a tool to realize fast exploration of the architecture design space under different implementation and algorithmic constraints."
Energy-efficient Deep Neural Network for Depth Prediction,"Depth sensing and estimation is a key aspect of posi-tional and navigational systems in autonomous vehi-cles and robots. The ability to accurately reconstruct a dense depth map of a surrounding environment from RGB imagery is necessary for successful obstacle de-tection and motion planning. Since deep convolutional neural networks (DNNs) have proven to be successful at achieving high accuracy rates in image classification and regression, recent work in the deep learning space has focused on designing neural networks for depth prediction applications. However, the high accuracy of DNN processing comes at the cost of high computa-tional complexity and energy consumption, and most current DNN designs are unsuitable for low-power applications in miniaturized robots. In this project, we aim to address this gap by applying recently developed methodologies for estimating and improving the ener-gy-efficiency of DNNs to an existing depth-prediction DNN. We envision an outcome in which the depth-pre-diction DNN is modified to be better suited for a spe-cialized hardware implementation that could be in-tegrated with a low-power visual-inertial odometry system to result in a combined navigational system for miniaturized robots."
Depth Estimation of Non-Rigid Objects for Time-of-Flight Imaging,"Depth sensing is used in a variety of applications that range from augmented reality to robotics. Time-of-flight (TOF) cameras, which measure depth by emitting and measuring the roundtrip time of light, are appeal-ing because they obtain dense depth measurements with minimal latency. However, as these sensors be-come prevalent, one disadvantage is that many TOF cameras in close proximity will interfere with one an-other, and techniques to mitigate this can lower the frame rate at which depth can be acquired. Previously, we proposed an algorithm that uses concurrently col-lected optical images to estimate the depth of rigid ob-jects. Here, we consider the case of objects undergoing non-rigid deformations. We model these objects as lo-cally rigid and use previous depth measurements along with the pixel-wise motion across the collected optical images to estimate the underlying 3-D scene motion, from which depth can then be obtained. In contrast to conventional techniques, our approach exploits previ-ous depth measurements directly to estimate the pose, or the rotation and translation, of each point by find-ing the solution to a sparse linear system. We evaluate our technique on a RGB-D dataset where we estimate depth with a mean relative error of 0.58%, which out-performs other adapted techniques."
Small-footprint Automatic Speech Recognition Circuit,"With the advanced technology of speech and natural language processing, spoken language has become a feasible way for human-machine interaction. Due to the high complexity of articulated speech signal, au-tomatic speech recognition (ASR) generally requires intensive computation and memory size to achieve good performance. However, due to its widespread ap-plications on robots, wearables, and mobile devices, it’s desirable to design circuit to implement ASR locally in a resource-limited environment, particularly in which power consumption is a critical concern.In this work, we first scrutinize software speech recognition procedure; evaluate the memory and computational resource needed when transferring to hardware, and take advantage of circuit design to minimize size and power usage. We design small-footprint ASR system (Figure 1) with cutting-edge neural network that can best perform acoustic modeling with memory restrictions, along with weight truncation and quantization. Dedicated arithmetic unit design, parallelization, and resource dispatching further reduce latency. We implement weighted finite-state transducer (WFST) to incorporate the phonetic probability with language model to select the best word transcription. Model compression, caching, and lattice truncation are adopted to adapt the ASR to circuit and optimize the design. Our ASR design leveraging powerfulness and robustness of neural network in hybrid ASR model outperforms conventional model in recognition accuracy, whereas conducting ASR tasks on-chip sees great reduction in power compared to CPU. We show a 2.4X reduction in neural network weight size compared to previous hardware design. Our work demonstrates the feasibility to operate an ASR in a small-footprint environment in applications with small vocabulary size and optimized model."
In-Memory Computation for Low Power Machine Learning Applications,"Convolutional Neural Networks (CNN) have emerged to provide the best results in a wide variety of ma-chine learning (ML) applications, ranging from image classification to speech recognition. However, they require huge amounts of computation and storage. When implemented in the conventional von-Neumann computing architecture, there is a lot of data move-ment per computation between the memory and the processing elements. This leads to a huge power con-sumption and long computation time, making CNNs unsuitable for many energy-constrained applications, e.g., smartphones, wearable devices, etc. To address these challenges, we propose embedding computation capability inside the memory (Figure 1). By doing that, we can significantly reduce data transfer to/from the memory and also access multiple memory addresses in parallel, to increase processing speed. The basic convo-lution operation in a CNN layer can be simplified to a dot-product between the layer inputs (X) and the filter weights (w), to generate the outputs (Y) for that layer. In this work, CNNs are trained to use binary filter weights (w = +/- 1), which are stored as a digital ‘0’ or ‘1’ in bit-cells of the memory array. The digital inputs (X) are converted to analog voltages and sent to the array, where the dot-products are performed in the analog domain. Finally, the analog dot-product voltages are converted back into the digital domain outputs (Y) for further processing.To demonstrate functionality for a real CNN architecture, the Modified National Institute of Standards and Technology (MNIST) handwritten digit recognition dataset is used with the LeNet-5 CNN (Figure 2). We demonstrated a classification accuracy of 98.35%, which is within 1% of what can be achieved with an ideal digital implementation. We achieved more than 16x improvement in the energy-efficiency in processing the dot-products vs. full-digital implementations. Thus our approach has the potential to enable low-power ubiquitous ML applications for smart devices in the Internet-of-Everything."
Reconfigurable Neural Network Accelerator using 3-D Stacked Memory Supporting Compressed Weights,"The recent success of machine learning, with the help of emerging techniques, such as convolutional neural networks, have been rapidly changing the way many traditional signal processing problems are being solved, including vision processing, speech recognition, and other prediction and optimization problems. Howev-er, neural networks require a large number of weight parameters and processing power that are difficult to accommodate efficiently using a normal CPU architec-ture. This necessitates dedicated on-chip solutions. A major challenge in recent on-chip neural network processors is reducing the energy consumed by memory accesses, as the cost for data operations becomes relatively cheaper than the cost for data movement in recently advanced processes. One approach is to simply reduce the amount of data movement by using compression schemes (i.e., reducing the bit-width of weights and activations). Han, et al. develop a deep compression technique to non-uniformly quantize floating point weights to 4-bit values, without any loss of accuracy. This was further extended to quantizing to only 2-bit ternary weights. Another approach is to increase the memory capacity, for example with 3-D stacked memory, to reduce the required number of costly external DRAM accesses.Our proposed design takes full advantage of these compression schemes by directly integrating the decompression within the processing element. In addition, the design can be reconfigured to perform more general fixed-point computations with variable bit-widths. Combining this with a closely integrated memory chip through 3-D stacking makes it possible to run large networks with less data movement to and from the external DRAM, resulting in improved energy efficiency compared to other implementations."
Bandwidth-efficient Deep Learning: Algorithm and Hardware co-Design,"In the post-ImageNet era, computer vision and ma-chine learning researchers are solving more complicat-ed Artificial Intelligence (AI) problems using larger data sets driving the demand for more computation. How-ever, we are in the post-Moore’s Law world where the amount of computation per unit cost and power is no longer increasing at its historic rate. This mismatch be-tween supply and demand for computation highlights the need for co-designing efficient machine learning al-gorithms and domain-specific hardware architectures. By performing optimizations across the full stack from application through hardware, we improved the efficiency of deep learning through smaller model size, higher prediction accuracy, faster prediction speed, and lower power consumption. Our approach starts by changing the algorithm, using “Deep Compression” that significantly reduces the number of parameters and computation requirements of deep learning models by pruning, trained quantization, and variable length coding. “Deep Compression” can reduce the model size by 18× to 49× without hurting the prediction accuracy. We also discovered that pruning and the sparsity constraint not only applies to model compression but also applies to regularization, and we proposed dense-sparse-dense training (DSD), which can improve the prediction accuracy for a wide range of machine learning tasks. To efficiently implement “Deep Compression” in hardware, we developed EIE, the “Efficient Inference Engine,” a domain-specific hardware accelerator that performs inference directly on the compressed model which significantly saves memory bandwidth. Taking advantage of the compressed model, and being able to deal with the irregular computation pattern efficiently, EIE improves the speed by 13× and energy efficiency by 3,400× over GPU."
"Ultra-thin, Reconfigurable, High-efficiency Meta-optical Devices in Mid-infrared","The mid-infrared (MIR) is a frequency band strategi-cally important for numerous biomedical, military, and industrial applications. Further development of MIR devices is hindered by the lack of inexpensive and efficient basic optical elements such as lenses, wave plates, filters, etc. Furthermore, the available compo-nents are typically bulky and passive. Our research ad-dresses these challenges by leveraging novel low-loss optical phase-change materials (Ge-Sb-Se-Te) and their sub-wavelength patterning to achieve ultra-thin (thick-ness < 0/5), high-efficiency (>25%), and multi-function-al MIR components. As a proof-of-principle, we demonstrated a reconfigurable bifocal meta-lens with a switchable focus. Our metalens principle is based on collective Mie scattering of incident plane waves by subwavelength dielectric structures, which sustain both electric and magnetic dipolar resonances. Each of the scatterers, also known as Huygens’ meta-atoms, contributes to the phase and amplitude of the incident beam. The amount of phase shift was controlled by the meta-atom geometry and its refractive index. Proper spatial arrangement of meta-atoms can reconstruct a desired phase profile. For instance, lens functionality can be achieved by introducing a hyperboloid phase distribution. In amorphous state (A-state) the lens focuses the incident light at a focal length of 1 mm; after the heating-induced material state transition, the focal length changes to 1.5 mm (C-state). The switching of the focal length was attained by changing the hyperboloidal phase profiles. For simplicity, we performed binary discretization of original continuous phase distributions: 0° and 180° phase shifts. Then, we formed a library of four distinct meta-atoms that can realize the binary transitions. The metalens was fabricated by depositing a 1-m-thick Ge2Sb2Se4Te1 film onto CaF2 substrate followed by patterning processes involving electron-beam lithography patterning and reactive ion etching with a mixture of fluoromethane gases. We believe that our findings will enable a new range of compact, multi-functional spectroscopic, and thermal imaging devices."
Reprogrammable Electro-Chemo-Optical Devices,"Photonic devices with programmable properties allow more flexibility in the manipulation of light. Recently, several examples of reconfigurable photonic devices were demonstrated by controlling the local/overall in-dex of refraction in thin films, either by a thermally induced phase change in chalcogenides or by interca-lation of lithium into oxides. We propose a novel ap-proach for design of reprogrammable photonic devices based on electrochemical modification of ceria-based electro-chemo-optical devices. Previously, it was shown that the refractive index of PrxCe1-xO2-δ (PCO) is a function of oxygen nonstoichiometry δ , that can be controlled electrochemically via closely spaced electrodes in a lateral device configuration. For transverse modified configurations, a PCO thin film on yttrium-stabilized zirconia (YSZ) substrate with transparent conducting oxide (TCO) top electrode allows for voltage-controlled oxygen exchange. Enhanced spatial resolution can be further achieved with the aid of lithographically patterned nano-dimensioned oxide layers."
Y-Branch Compact Model Including the Line Edge Roughness Effect,"Silicon photonics is a booming design platform due to its ability to support high data rates and enable novel applications. Since the CMOS fabrication infrastruc-ture is leveraged in silicon photonics, it becomes crucial to provide process-variation-aware compact models as optical components inherit the process variations found in CMOS. These models would help designers, enhance yield, and serve as a building block in the sili-con photonics process design kit (PDK). We develop a compact model for a basic photonic component, a Y-branch, that specifies the variations in the transfer characteristics against line-edge roughness (LER). LER is a common statistical random process variation that causes imbalanced transmission between the two output ports of the Y-branch, which is supposed to be balanced. As a random process variation, LER affects the Y-branch transmission in a random behavior, so the transmission can be described by its mean (µ) and variance (σ2). This model provides the transmission mean and variance as a function of LER parameters, amplitude (A) and correlation length (LC), across the operating wavelength range of interest (λ). The flow of modeling, shown in Figure 1, starts by simulating different A and LC combinations with multiple instantiations for each to get a statistical sense of the variations. Afterward, the optical behavior of the Y-branch with the imposed LER is extracted and used to develop the compact model. The model is developed using the Gaussian process regression method where the R2 score for both mean and variance predictions is 0.99. Figure 2 shows the model’s performance on test data for predicting mean and variance. This model can be used in photonic integrated circuit simulators to predict the performance across process variations and worst corner cases as the models we rely upon in CMOS design."
Particle Defect Yield Modeling for Silicon Photonics,"Silicon photonics, where photons instead of electrons are manipulated, shows promise for higher data rates, lower energy communication and information process-ing, biomedical sensing, and novel optically based func-tionality applications such as wavefront engineering and beam-steering of light. In silicon photonics, both electrical and optical components can be integrated on the same chip, using a shared silicon integrated circuit (IC) technology base. However, silicon photonics does not yet have a mature process, device, and circuit vari-ation models for the existing IC and photonic process steps; this lack presents a key challenge for design in this emerging industry.Our goal is to develop key elements of a robust design for manufacturability methodology for silicon photonics. As one part of the goal, here we focus on the impact of particle defects in silicon photonics, which can arise in photolithography, deposition, etching, and other processes. The model and result will be used to help generate layout design rules and critical area extraction methods, predicting and optimizing the yield of complex silicon photonic devices and circuits for tomorrow’s silicon photonics designers, just as IC designers do today.We model the impact of different types of particle defects (Figure 1) on different device components, e.g., straight waveguides (Figure 2) and y-splitters (Figure 3). We modify and apply the adjoint method, which is widely used in optimization, to accelerate the speed of simulation and reduce numerical error. The result from the adjoint method shows good consistency with direct simulation over different types of particles, different device components, and wavelengths ranging from 1500 to 1600 nm. The same methodology can be used on the circuit level and thus predict the yield of the chip. Present research also focuses on generating layout design rules and critical area extraction based on results from the adjoint method."
See-through Light Modulators for Holographic Video Displays,"In this research, we design and fabricate acousto-optic, guided-wave modulators in lithium niobate for use in holographic and other high-bandwidth displays. Guid-ed-wave techniques make possible the fabrication of modulators that are higher in bandwidth and lower in cost than analogous bulk-wave acousto-optic devices or other spatial light modulators used for diffractive displays; these techniques enable simultaneous modu-lation of red, green, and blue light. We are investigating multichannel variants of these devices with an emphasis on maximizing the number of modulating channels to achieve large total bandwidths. To date, we have demonstrated multichannel full-color modulators capable of displaying holographic light fields at standard-definition television resolution and video frame rates. Our current work explores a device architecture suitable for wearable augmented reality displays and other see-through applications, in which the light outcouples toward the viewer (Figure 1), fabricated using femtosecond laser micromachining (Figure 2)."
Bio-inspired Photonic Materials: Producing Structurally Colored Surfaces,"Advances in science and engineering are bringing us closer and closer to systems that respond to human stimuli in real time. Scientists often look to biology for examples of efficient, spatially tailored multifunctional systems, drawing inspiration from photonic structures like multilayer stacks similar to those in the morpho butterfly. In this project, we develop an understand-ing of the landscape of responsive, bio-inspired, and active materials, drawing on principles of photonics and bio-inspired material systems. We are exploring material processing techniques (starting with electron beam lithography and moving to direct laser writing) to produce and replicate structurally colored surfaces while developing simulation and modeling tools (such as inverse design processes) to generate new structures and colors. Such complex biological systems require advanced fabrication techniques. Our designs are re-alizable through fabrication using direct laser writing techniques such as two-photon polymerization. We aim to compare our model system and simulations to fabricated structures using optical microscopy, scan-ning electron microscopy, and angular spectrometry. This process provides a toolkit with which to examine and build other bio-inspired, tunable, and responsive photonic systems and expand the range of achievable structural colors.Unlike with natural structures, producing biomimetic surfaces allows researchers to test beyond tunability that occurs naturally and explore new theory and models to design structures with optimized functions. The benefits of such biomimetic nanostructures are plentiful: they provide brilliant, iridescent color with mechanical stability and light-steering capabilities. By producing biomimetic nanostructures, designers and engineers can capitalize on unique properties of optical structural color and examine these structures based on human perception and response."
Reversible Electrothermal Switching of Nonvolatile Metasurfaces Based on Optical Phase Change Materials,"Chalcogenide phase change materials (PCMs) are high-ly attractive for active metasurface applications due to their nonvolatile switching capability. So far, revers-ible switching of PCM-based metasurfaces is realized via either laser pulsing or electrical-current-induced phase transition. Both methods require raster-scanned writing and bulky off-chip instruments (lasers or AFM setups), making them incompatible with large-scale on-chip integration. A robust and scalable, on-chip, PCM-based metasurface switching method is therefore high-ly desired. Here we report an electrothermal switching method employing on-chip metal heaters, enabling large-area reversible switching for PCM-based meta-surfaces.Figure 1a shows the optical constants of the low-loss optical PCM (O-PCM) we choose for this application: Ge2Sb2Se4Te1 (GSST), which exhibits low-loss at both its amorphous and crystalline phases over a broad spectral range. Moreover, its improved amorphous phase stability gives rise to a larger critical switching thickness than that of traditional PCMs (e.g., GST-225). These two factors make GSST a preferred material for metasurface applications. Figure 1b illustrates the design of the switching platform. Ti/Pt are used as a metal heater for its excellent conductivity. After an atomic layer deposition of Al2O3, GSST is subsequently deposited and patterned via electron beam lithography. The thickness of the GSST meta-atoms is designed to be 220 nm. Finally, a SiO2 capping layer is deposited to prevent oxidation and evaporation of the PCM. The devices are wire-bonded onto a custom printed circuit board carrier to enable in-situ Raman and Fourier transform infrared (FTIR) characterizations. Figure 1c shows the SEM and optical microscope images of a fabricated device. The boundary of the heat is optimized for uniform heating in the PCM area. Figure 1d confirms the complete reversible switching of the PCM utilizing the distinct Raman peaks of amorphous and crystalline states. Figure 1e shows that more than 40% reflection contrast is achieved using this platform. Figure 1f, on the other hand, demonstrates that applying different voltages can achieve any arbitrary levels of crystallization, therefore providing possibilities for quasi-continuous tuning using this platform."
Graphene Microheaters for Controlled Switching of Optical Phase Change Materials,"The integration of optical phase change materials (O-PCMs) into photonic devices enables a long-sought functionality: nonvolatile reconfiguration, the ability to switch between at least two distinct configurations with no power consumption to retain either one. Ener-gy-efficient, highly cyclable integrated optical devices such as switches, memories, metasurfaces, color pixels, and brain-inspired computing elements are success-ful examples of O-PCMs applications. However, these results use optical switching mechanisms that are challenging to scale up for architectures comprising hundreds of large-area active cells. To tackle this chal-lenge, we present a hybrid electro-optical framework in which we use graphene microheaters for thermal switching of Ge2Sb2Se4Te1 (GSST). We choose GSST be-cause of its broadband transparency in the infrared be-yond 18.5-µm wavelengths in both the amorphous and the crystalline states. Similarly, we choose graphene for our integrated approach because of its minimal optical loss (~ 0.1–1.2 dB/mm), high thermal conductivity, and stability. Such a device benefits from scalable electrical control, while having a reconfigurable optical response.We demonstrate large-area switching of 50-nm thick, 4×3-µm2 GSST using a 5×10-µm2 graphene heater (Figures 1A and 1B). The chip was wire-bonded onto a printed circuit board to enable in-situ Raman probing while electrically testing each integrated device (Figures 1C and D). To switch the as-deposited GSST to the crystalline state (heat up over ~280°C), we used 6V pulses with varying lengths between 10-20 ms. To reamorphize (melt over 650°C and quench), we triggered 13-µs electrical pulses with a peak voltage of 7.5V. We demonstrate repeatable electrical switching by in-situ Raman spectroscopy of GSST after each pulse excitation (Figure 1E), done by tracking the amorphous and crystalline signature peaks at 159 cm-1 and 120cm-1, respectively (Figure 1F). Furthermore, the change in color observed in the inset microscope images of Figure 1F demonstrates the nonvolatile modulation of the optical properties upon GSST switching."
Highly Sensitive Nanogap-based Mechanical Sensors for Infrared Detection,"Many new physical phenomena show up only on na-noscale structures; with these phenomena, we can design novel devices with unprecedented functional-ity. Nanoengineering makes it possible to fabrication nanometer-sized quantum tunneling barriers that can be tuned mechanically. Such a tremendous mechanical tunability can be harnessed for mechanical sensors and many other types of sensors with extremely high sen-sitivity. Here we demonstrate two nanostructures that implement such a mechanically tunable tunneling barrier and use them for either a mechanical/strain sensor or a mid-infrared bolometric detector. The first nanostructure is the self-assembled graphene nanoflake network (Figure 1 (a)). It is composed of a resistance network of sub-micron graphene flakes that connect with <100 nm overlap. The second nanostructure is a metal nanogap with the gap defined by self-assembled monolayers (SAMs) (Figure 1 (b)). The proposed structures show high gauge factors and/or improved linear dynamic range as strain sensors (Figure 1 (c)). Such mechanical sensors can also be integrated with a thermal actuator to realize a highly sensitive, uncooled bolometer-type mid-infrared detector (Figure 2(a) and (b)). The measured temperature coefficient of resistance (TCR) can be as high as 5 K-1, which is more than one order of magnitude better than the state of the art (Figure 2(c))."
Oxide Passivation on MoS2-based Field-effect Transistors for Sensing Applications,"Two-dimensional materials have attracted much at-tention as candidates for next-generation sensing platforms because of their unique electrical, optical, mechanical, and chemical properties. Due to its natural bandgap, MoS2 is one of the most popular two-dimen-sional materials for sensing. The sensing signal can be amplified as charges transfer onto the MoS2 channel and result in strong modulations in current with the present of the analyte. The large surface-to-volume ra-tio also contributes to the high sensitivity of a MoS2-based sensor. However, high sensitivity also results in much noise as vapor molecules and other interfering molecules absorb on the exposed MoS2 surface. Also, unprotected MoS2 can degrade in an ambient envi-ronment due to oxidation and surface contaminants. Therefore, a suitable passivation layer is needed to pro-tect the channel surface but still preserve the sensitiv-ity of MoS2.In this work, back-gated MoS2 field-effect transistors (FETs) were fabricated, and a thin layer of Al2O3 was deposited to passivate the channel surface. Prior to atomic layer deposition of Al2O3 as a seed layer, 2 nm of aluminum was deposited. Approximately 13 nm of Al2O3was added to the final device. With the oxide passivation, the hysteresis of both output and transfer characteristics was greatly reduced, indicating effective protection from fast absorbent-type trapping site. The sensing ability of oxide-passivated MoS2 FETs was also tested with a series of the electrolyte solution of pH ranging from 5 to 10. As shown in Figure 2, a near-linear relationship between relative change in resistance and change in pH was achieved. This work proves that Al2O3is a great passivation layer for MoS2-based sensor devices. With oxide being the outmost layer, other oxide-compatible surface functionalization can also be used to improve the selectivity of such sensors while still benefiting from MoS2’s natural sensitivity."
SynCells - Electronic Microparticles for Sensing Applications,"Although transistors have dramatically decreased in size over the past decades, thanks to Moore’s law, the overall size of electronics has roughly stayed constant. However, shrinking electronics systems to the size of biological cells presents a big opportunity for sensing applications because it allows us to interact with the environment at a much smaller scale. These microsys-tems could be used, for example, to detect chemicals in very confined spaces like the human body or micro-fluidic channels. Alternatively, they are small enough to be sprayed on surfaces to form distributed sensor networks or even be incorporated into fibers to make smart clothing.To realize this vision, we have developed a microscopic sensor platform built a on 3-µm- thick SU-8 polymer substrate that we call synthetic cells or SynCells. The SynCells contain a variety of electric components, including molybdenum disulfide-based transistors and chemical sensors, analog timers based on eroding germanium films, and magnetic iron pads (see Figure 1). Over the past years, we have optimized the SynCell fabrication and lift-off process, and we recently demonstrated a yield close to a hundred percent of fully working SynCells. Additionally, we have shown high sensitivities of the MoS 2 sensors to amines such as putrescine in both water and air. Using rare-earth magnets, we are also able to move and pivot the SynCells in solution from over 50 cm away.As the next step, we want to use SynCells in a complex task, where we move them to a specific location in a microfluidic channel using a magnet to measure the chemical concentration (see Figure 2). Additionally, the germanium timer will measure the time spent in water, while the transistors will be used to amplify the chemical sensor signal. If successful, SynCells could enable microscale smart sensors for healthcare, environmental monitoring, or smart material composites."
Piezoresistive Sensor Arrays and Touch-sensitive Textile for Robot Manipulation and Control,"Humans rely on tactile feedback for object manipula-tion as well as many other dexterous tasks. In contrast, modern robots are tactile-blind; therefore, tactile sen-sors have been widely applied in robotic manipulation, policies control, and human-computer interaction. Large-scale electronic skins and touch-sensitive tex-tiles with high densities, durability, and flexibility will be important tools to understand human behavior as well as to monitor and improve robot manipulation and control. In this work, a high density flexible piezoresistive pressure sensor array with high robustness is fabricated. Commercial piezoresistive films are sandwiched between two layers of stainless-steel threads to assembly a 32×10 sensor array, which is then attached to the surface of a robot gripper (Figure 1). The shape and densities can be customized for different applications. Pressure maps will be recorded during the operation of gripper by a printed circuit board with a buffered reading circuit. Data retrieved from the sensor array will be further analyzed to monitor or improve robot manipulation. Moreover, smart garments with tactile sensors are fabricated by incorporating electronic textiles into a fully knitted garment, which will have huge opportunities in human-computer interaction. Piezoresistive fibers are fabricated by coating graphite/polydimethyl-siloxane mixture over stainless conductive thread (Figure 2). We are presently working to improve the compatibility between piezoresistive fibers and the fully automated knitting machine."
High-throughput Measurement of Single-cell Growth Rates using Serial Microfluidic Mass Sensor Arrays,"Methods to rapidly assess cell growth would be useful for many applications, including drug susceptibility testing, but current technologies have limited sensi-tivity or throughput. Here we present an approach to precisely and rapidly measure growth rates of many individual cells simultaneously.We flow cells in suspension through a microfluidic channel with 10–12 resonant mass sensors distributed along its length, weighing each cell repeatedly over the 4–20 min it spends in the channel (Figures 1, 2). Because multiple cells traverse the channel at the same time, we obtain growth rates for >60 cells/h with a resolution of 0.2 pg/h for mammalian cells and 0.02 pg/h for bacteria. We measure the growth of single lymphocytic cells, mouse and human T cells, primary human leukemia cells, yeast, Escherichia coli and Enterococcus faecalis. Our system reveals subpopulations of cells with divergent growth kinetics and enables assessment of cellular responses to antibiotics and antimicrobial peptides within minutes."
Iso-dielectric Separation of Cells and Particles,"The development of new techniques to separate and char-acterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. We are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This meth-od, iso-dielectric separation (IDS), uses dielectrophoresis (DEP, the force on a polarizable object) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductiv-ity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [Figure 1].Sepsis is an uncontrolled activation of the immune system that causes an excessive inflammatory response. There is an unmet need to develop tools to monitor sepsis progression, which occurs quickly and provides few clues to indicate if treatment is effective. Previously, we have found the electrical profile of leukocytes changes with activation state, and we have applied IDS to characterize the electrical profile of leukocytes for monitor sepsis. After working with neutrophils, we also found that IDS can be used to distinguish different types of leukocytes having different dielectric properties. As Figure 2 suggests, once cell properties such as size, permittivity and conductivity of each part change, Clausius-Mossotti (CM) factor changes and it explains the reason why we can distinguish different types of cells in IDS. We could distinguish neutrophils and T-cells (the majority of lymphocytes) at the frequency of 5 MHz and the area under ROC curve was 0.8473. To advance the automation of the system and reduction sample preparation for clinical deployment, we could integrate the upstream separator such as inertial microfluidic sorter for removal of red blood cells (RBC) from the patient’s blood samples. It might be possible to monitor sepsis from patients in pseudo-real time."
Microfluidic Electronic Detection of Protein Biomarkers,"Traditional blood tests are performed in centralized laboratories by trained technicians and need days to deliver results. The need of ~mL blood sample also makes it challenging to apply the traditional tests to premies or even newborns. We are developing a min-iaturized microfluidic electronic biosensor, which gives immediate results (within 30 minutes) and needs ~μL blood, for diagnosis of neonate sepsis. To achieve this goal, we developed portable PCB-based multiplexed amperometry circuitry and a bead-based electronic enzyme-linked immunosorbent assay. Combining the circuitry and bead-based assay, we have demonstrated measurement of human interleukin-6, a potential neo-natal sepsis biomarkers, in serum with clinically rele-vant limit of detection (e.g., < 40pg/ml)."
Continuous Biomanufacturing Using Micro/nanofluidics,"Continuous biomanufacturing is a growing trend in the biopharmaceutical industry because it can reduce manufacturing cost and increase product quality. Ideas from micro/nanofluidics can be employed in all aspects of continuous biomanufacturing to enhance the over-all productivity as well as the efficacy and safety of the final products.First, we introduce a novel cell retention device based on inertial sorting for perfusion culture (Figure 1). The cell retention device maintains cells in the bioreactor and removes biologics and metabolites. Hollow fiber membrane is commonly used in the biopharmaceutical industry. However, it has challenges, such as membrane clogging/fouling, low product recovery, and inability to remove dead cells. In this context, we developed a membrane-less microfluidic cell retention device and demonstrated perfusion culture of high-concentration mammalian cells producing monoclonal antibodies for >3 weeks with high product recovery (>99%). Second, we present a nanofluidic system for continuous-flow, multi-variate (purity, bioactivity, and protein folding) protein analysis for real-time critical quality assessments (Figure 2). This size-based nanofluidic system can complement the existing bench-type conventional analytical tools, such size exclusion chromatography and gel electrophoresis, to meet quality assurance requirements of current and future biomanufacturing systems. We demonstrated rapid purity and bioactivity monitoring of protein drugs, such as hGH, IFN-alfa-2b, and G-CSF, using the nanofluidic system."
Ion Concentration Polarization Desalination using Return Flow System,"While the conventional electrodialysis (ED) relies on bipolar ion conduction employing two ion exchange membranes, anion exchange membrane (AEM) and cation exchange membrane (CEM), our group has pro-posed unipolar ion conduction, so-called ion concen-tration polarization (ICP) desalination, employing only CEM to enhance energy efficiency. Because chloride ion, the majority salt in nature, has faster diffusivity than sodium ion, ICP desalination theoretically has a cur-rent utilization (CU) of 1.2, but the ED has only that of 1. To facilitate the ICP desalination, our group has devel-oped series of technology from Bifurcate ICP system to Trifurcate ICP (Tri-ICP) system. Here, we have developed a return flow (RF-ICP) desalination system with a newly designed flow path for improving energy efficiency.Figure 1 shows a schematic of RF-ICP desalination system, which has three channels separated by two nano-porous membranes. The three channels consist of a concentrate channel on the anodic side, a diluate channel on the cathodic side, and an intermediate channel in between. A feed solution flows through the inlet of intermediate channel with the highest pressure and flows through the outlet of both side channels with the lowest pressure. As the feed solution flows through the channels, a portion of the feed solution flows through the porous membrane (Por-flow) due to the pressure difference. The Por-flows facilitate two types of flow barriers, a suppressor for a chaotic electroconvection in the diluate stream and a preventer for a salt leakage from the concentrate stream. The remaining solution returns at the end of channel (RF-flow) and induces the effect of sweeping a mass on the CEM surfaces by shear stress.We demonstrate that the developed RF-ICP system reduces a power consumption compared to the previously developed Tri-ICP system. Also, the RF-ICP system showed symmetrical product concentrations between diluate and concentration (data not shown), and the recovery rate increased to 50% compared to the Tri-ICP system, which was 25%. To improve the performance of RF-ICP system, more optimized system would be developed by various operating controls for recovery rate increase or spacer designs for energy efficiency increase."
A Printed Microfluidic Device for the Evaluation of Immunotherapy Efficacy,"Inherent challenges in device fabrication have impeded the widespread adoption of microfluidic technologies in the clinical setting. Additive manufacturing could address the constraints associated with traditional microfabrication, enabling greater microfluidic design complexity, fabrication simplification (e.g., removal of alignment and bonding process steps), manufacturing scalability, and rapid and inexpensive design iterations. We have fabricated an entirely 3-D-printed microfluidic platform enabling the modeling of interactions between tumors and immune cells, providing a microenvironment for testing immunotherapy treatment efficacy. The monolithic platform allows for real-time analysis of interactions between a resected tumor fragment and resident or circulating lymphocytes in the presence of immunotherapy agents. Our high-resolution, non-cytotoxic, transparent device monolithically integrates a variety of microfluidic components into a single chip, greatly simplifying device operation when compared to traditionally-fabricated microfluidic systems. Human tumor fragments can be kept alive within the device. In addition, the tumor fragment within the device can be imaged with single-cell resolution using confocal fluorescence microscopy."
Biocompatible Dielectric-conductive Microsystems Monolithically 3-D Printed via Polymer Extrusion,"Additive manufacturing (AM), i.e., the layer-by-layer construction of devices using a computer-aided design (CAD) file, has been recently explored as a manufactur-ing toolbox for MEMS. The demonstration of mono-lithic multi-material devices in 3-D printed MEMS has the potential to implement better, more complex, and more capable microsystems at a small fraction of the time and cost typically associated with semiconductor cleanroom microfabrication. Fused filament fabrica-tion (FFF) is an AM technique based on extrusion of thermoplastic polymers that is arguably the simplest and cheapest commercial 3-D printing technology available.Here, we report additively manufactured monolithic microsystems composed of conductive and dielectric layers using an FFF dual extruder 3-D printer. The base material is a biocompatible polymer, polylactic acid (PLA), which can be doped with micro/nanoparticles to become electrically conductive. Characterization of the printing technology demonstrates close resemblance between CAD files and printed objects, generation of watertight microchannels, high-vacuum compatibility, and non-cytotoxicity. A large (~23) piezoresistive gauge factor was measured for a certain graphite-doped conductive PLA, suggesting its utility to implement 3-D printed strain transducers via FFF. Multiplexed electrohydrodynamic liquid ionizers (Figure 1) with integrated extractor electrode and threaded microfluidic port were also demonstrated. The per-emitter current vs. per-emitter flowrate characteristic shows a power dependence with 0.6 coefficient (Figure 2), close to the square-root dependence predicted by de la Mora’s law for the cone-jet emission mode."
Mini Continuous Stirred Tank Reactors (mini-CSTR) for Cell and Tissue Culture Applications,"An ideal cell culture system will provide a well-con-trolled, homogeneous, and steady environment for cells and tissues. For instance, well-controlled steady states would greatly benefit organ-on-chip experiments, stem cell culture, and tissue propagation (among other rele-vant biomedical applications). At present, no continu-ously stirred mini-reactors are commercially available for lab-scale culture applications.We are developing simple, low-cost, and user-friendly miniaturized continuously stirred tank reactors (CSTRs) for biomedical and biotechnological agitations. These well-mixed mini-CSTRs will enable cost-efficient continuous culture at small scales. We cast Polydimethylsiloxane (PDMS) casting on poly(methyl-methacrylate) molds, or directly use high resolution 3-D-printing, to fabricate these CSTRs and an Arduino platform to measure and control key parameters, such as agitation, temperature, and pressure, in small portable incubators (Figure 1). Nutrients are fed by syringe pumps, and well-controlled low-speed (benign) agitation is provided by a custom-made magnetic system. Since the reactor behaves as a well-mixed reservoir, all bulk-liquid concentrations can be measured at the outlet stream, thereby greatly reducing the need for intrusive instrumentation. We are currently validating the use of this culture platform in two model applications: (a) the extended culture of breast cancer spheroids, and (b) the culture of Chinese Hamster Ovary Cells (the warhorse for biopharmaceutical production) for continuous production of biopharmaceutical compounds (Figure 2)."
Chaotic Flows as Micro- and Nanofabrication Tools,"Nature generates densely packed micro- and nano-structures that enable key functionalities in cells, tis-sues, and other materials. Current fabrication tech-niques are far less effective at creating microstructure, due to limitations in resolution and speed. Chaos is one of the many mechanisms that nature exploits to create complexity with simple “protocols.” For example, chaotic flows have the extraordinary capacity to create microstructure at an exponential rate. We are currently developing a set of microfabrication strategies that we term chaotic printing—the use of chaotic flows for rap-id generation of complex, high-resolution microstruc-tures.In our experiments, we use two classic mixing systems as models—Journal Bearing (JB) flow and the Kenics mixer—to demonstrate the usefulness of chaotic printing. In a miniaturized JB flow (miniJB), we induced deterministic chaotic flows in viscous liquids (i.e., methacryloyl-gelatin and poly-dimethylsiloxane), and deformed an “ink” (i.e., a drop of a miscible liquid, fluorescent beads, or cells) at an exponential rate to render a densely packed lamellar microstructure that is then preserved by curing or photocrosslinking. In a continuous version of chaotic printing, we created chaotic flows by co-extruding two streams of alginate (two inks) through a printing head that contains an on-line miniaturized Kenics mixer. The result was a continuous 3-D-printing of multi-material lamellar structures with different degrees of surface area and full spatial control of the internal microstructure (Figure 1). The combined outlet stream was then submerged in an aqueous calcium chloride solution to crosslink the emerging alginate fibers containing the microstructure.The exponentially rapid creation of fine microstructure achievable through chaotic printing exceeds the limits of resolution and speed of the currently available 3-D printing techniques. Moreover, the architecture of the microstructure created with chaotic printing can be predicted using computational fluid dynamic (CFD) techniques. We envision diverse applications for this technology, including the development of densely packed catalytic surfaces and highly complex multi-lamellar and multi-component tissue-like structures for biomedical and electronics applications (Figure 2)."
On-chip Photonic Aerosol Spectrometer for Detection of Toxic Inhalable Materials,"Aerosol particles are distributed in the atmosphere and can constitute serious health threats depending on their chemistry, size, and concentration. For instance, particles of different sizes are deposited in different parts of a lung airway and can lead to specific respirato-ry complications; and aerosols with certain functional groups can be more harmful than others. So, the com-prehensive sensing of aerosol particles is critical for human health, particularly with timely monitoring of environmental pollution, industrial pollution, and de-fense threats. Most existing aerosol sensors are based on free-space detection methods using optical scatter-ing, IR spectroscopy, and electrical property determi-nation. These sensors can suffer from poor sensitivity and be expensive and bulky.We have developed an on-chip photonic aerosol spectrometer that can perform in situ particle sizing, counting, shape, and chemical characterization. The device is based on an integrated array of waveguide and microresonator structures built on a silicon nitride-on-insulator platform using simple UV photolithography. We have demonstrated that the sensors can estimate the size of particles ranging from 100 nm to 5 microns with particle concentrations over ~500 to 105 particles/cm3. An aerosol particle falling on the microresonator sensor interacts with the evanescent field of the resonators and acts as a scatterer causing energy loss. The interaction of these particles with the evanescent mode of the microresonators depends on the particle size, shape and count. Coupled with theoretical scattering models of Mie and Rayleigh, we use the measured data to extract physical properties of the airborne particles. The Q-factor of these resonators is as high as 105 enabling sensing resolution to that of an individual aerosol particle. Similarly, by selecting a combination of the resonant wavelengths in microresonators to develop infra-red spectrum sensitive to the distinctive bands of organic and inorganic functional groups inherent in molecularly structures aerosol particles, the spectrometer can be used to do chemical characterization of aerosol particles. This multi resonator platform is tailorable to single or multi-species detection that can be deployed for a variety of aerosol chemistry sensing applications. The technology offers various advantages in particle sensing modalities by offering improved sensitivity, response time and reduced cost and size of the device."
Close-packed Silicon Microelectrodes for Scalable Spatially Oversampled Neural Recording,"A major goal of neuroscience is to understand how the activity of individual neurons yields network dynam-ics, and how network dynamics yields behavior (and causes disease states). Innovative neuro-technologies with orders-of-magnitude improvements over tradi-tional methods are required to reach this goal. Nano-fabrication can provide the scalable technology plat-form necessary to record with single-spike resolution the electrical activity from a large number of individ-ual neurons, in parallel and across different regions of the brain. By combining innovations in fabrication, design, and system integration, we can scale the num-ber of neural recording sites: from traditionally a small number of sparse sites, to currently over 1000 high-den-sity sites, and in the future beyond many thousands of sites distributed through many brain regions.We designed and implemented close-packed silicon microelectrodes (Figure 1), to enable the spatially oversampled recording of neural activity (Figure 2) in a scalable fashion, using a tight continuum of recording sites along the length of the recording shank, rather than discrete arrangements of tetrode-style pads or widely spaced sites. This arrangement, thus, enables spatial oversampling continuously running down the shank so that sorting of spikes recorded by the densely packed electrodes can be facilitated for all the sites of the probe simultaneously.We use MEMS microfabrication techniques to create thin recording shanks and a hybrid lithography process that allows a dense array of recording sites which we connect to with submicron dimension wiring. We have performed neural recordings with our probes in the live mammalian brain, and illustrate the spatial oversampling potential of closely packed electrode sites in Figure 2."
Building Synthetic Cells for Sensing Applications,"Miniaturized sensors, less than 100 μm in diameter, equipped with communication capabilities could en-able a new paradigm of sensing in areas such as health care and environmental monitoring. For example, in-stead of measuring a patient’s blood sugar by pricking their finger and analyzing a drop of blood externally, a microscopic sensor in the bloodstream could sense the glucose concentration internally and communicate data to the outside world non-invasively.In this project, we work towards this vision by integrating chemical sensors and transistors on 100-µm-wide flexible polymer disks that we call synthetic cells or “SynCells” (see Figure 1). The transistor channels and sensors are made of molybdenum disulfide (MoS2), which it is an excellent material to build digital electronics and highly sensitive sensors. To use the SynCells, they are mixed into a target solution. Upon exposure to a specific substance, the chemical sensors permanently change their electrical resistance. Afterward, the SynCells are retrieved and analyzed externally.During the last year, we improved our SynCell fabrication process and increased our transistor yield significantly. Furthermore, we successfully demonstrated chemical detection of triethylamine (see Figure 2). As next steps, we want to explore the behavior of our SynCells in microfluidic channels and investigate ways to include time-awareness in these systems."
The AutoScope: An Automated Point-of-Care Urinalysis System,"Urinalysis is one of the most common diagnostic tech-niques in medicine. Over 200 million urine tests are ordered each year in the US, costing between $800 to $2,000 million in direct costs. 46% of all urinalysis tests include microscopic analysis, which involves identify-ing and counting each particle found in the urine. Mi-croscopic urinalysis is a costly and complex process often done in medical laboratories. An inexpensive and automated cell-counting system would (1) increase access to microscopic urinalysis and (2) shorten the turn-around time for physicians to make diagnostic de-cisions by permitting the test to be done at the point-of-care.The AutoScope is an automated, low-cost microscopic urinalysis system that can accurately detect red blood cells (RBCs), white blood cells (WBCs), and other particles in urine. We use a low-cost image acquisition system combined with two neural networks to identify these particles. By not using any optical magnification, we achieve costs three orders of magnitude less than the only commercially available semi-automated urinalysis system and a device size of 8.3 x 6.0 x 8.8cm. To validate the system, we calculated the accuracy, sensitivity, and specificity of the Autoscope. The specificity and sensitivity were determined by generating 209 digital urine specimens modeled after urine received in medical labs. The Autoscope had a sensitivity of 88% and 91% and a specificity of 89% and 97% for RBCs and WBCs, respectively. Next, we determined the Autoscope’s accuracy by fabricating 8 synthetic urine samples with RBCs, WBCs, and microbeads. The reference results were confirmed through a medical laboratory. The AutoScope’s counts and the reference counts were linearly correlated to each other (r2= 0.980) across all particles. The sensitivity, specificity, and R-squared values for the AutoScope are comparable (and mostly better) than the same metrics for the iQ-200, a $100,000-$150,000 state-of-the-art semi-automated urinalysis system."
Cardiac Output Measurement using Ballistocardiography and Electrocardiography,"Cardiac output (CO) is one of several parameters used by cardiologists to stratify risk of patients with cardio-vascular disease and has significant clinical relevance. CO is currently obtained in the ICU setting through right heart catheterization, an invasive method. This kind of procedure brings with it increased financial cost and risk to the patient. Consequently, non-inva-sive methods, such as ballistocardiography (BCG), have been gaining more traction and are seen as potential candidates for measuring cardiovascular parameters such as CO.BCG utilizes detection of the body’s recoil from the ejection of blood into the arterial system. Due to its nature, BCG is prone to noise and ensemble averaging of multiple cardiac cycles is used to obtain a waveform with higher signal-to-noise ratio. An electrocardiogram (ECG) handlebar is used to generate the ECG waveform that sets the timing of the cardiac cycles for this technique. The most notable features of the BCG waveform (I, J, and K waves) are driven by the difference in blood pressure between the inlet and outlet of the ascending aorta during a cardiac cycle. Several parameters derived from these features in the waveform, such as I-J amplitude, IJK width, and the R-J interval, can be used to determine a patient’s stroke volume. Once the stroke volume is known, it can be used alongside the heart rate to calculate the cardiac output. This kind of device can be used for continuous monitoring of a patient in the home setting, removing many of the limitations seen with invasive methods."
Continuous and Non-invasive Arterial Pressure Waveform Monitoring using Ultrasound,"An arterial blood pressure (ABP) waveform provides valuable information for understanding cardiovascu-lar diseases. The ABP waveform is usually obtained through an arterial line (A-line) in intensive care settings. Although considered the gold standard, the disadvan-tage of this method is its invasive nature. Non-invasive methods such as vascular unloading and tonometry are not suitable for prolonged monitoring. Therefore, reli-able non-invasive ABP waveform estimation has long been desired by medical communities. Medical ultra-sound is an attractive imaging modality because it is inexpensive, cuff-less, and suitable for portable system implementation.The proposed ultrasonic ABP waveform monitoring is achieved by ultrasonography to observe the pulsatile change of the cross-sectional area and identify the vessel elasticity, represented by the pulse wave velocity (PWV); the propagation speed of a pressure wave along an arterial tree) with a diastolic pressure measurement. The local PWV can be estimated from the flow-area plot during a reflection-free period (e.g., the early systolic stage). A prototype ultrasound device was designed to conduct application-specific ultrasonography in a portable form factor, shown in Figure 1. The first human subject validation shows the agreement between this method on the common carotid artery and the ABP waveform obtained at a middle finger using the vascular unloading method. Motion-tolerant ultrasonography is explored to improve the measurement stability from the first design for long term monitoring. The second human subject study in a transient stress situation demonstrates the proof-of-concept of this method for the stress testing. Currently, the human subject study to compare the A-line with this method in collaboration with Boston Medical Center is in progress."
Breathable Electronic Skin Sensor Array through All-in-One Device Transfer,"Skin electronics, which can laminate on human skin, have emerged as essential tools for human/Internet of things (IoTs) interfaces such as real-time health moni-toring and instantaneous medical treatment. Amid this sweeping trend, human skin has been treated as merely a flexible, stretchable, and soft space for mounting of skin electronic devices. The skin is the outmost and the largest organ covering the external body surface and plays a vital role to maintain human life. Thus, homeo-stasis of the skin should be maintained even beneath the electronics. However, conventional thin-film device design, neglecting the skin, can induce problems (e.g., inflammation).Here we propose a breathable skin electronics, not blocking physiological activity of the skin. Sweat pore-inspired micro-hole pattern in a skin patch secure ~100% breathability and an elastic modulus of the skin patch has comparable value of the skin, which can replicate mechanical deformation of the skin with strong adhesion.Furthermore, we develop all-in-one device transfer process that high-temperature processed (~500 °C), photo-patterned inorganic device array is directly transferred onto the skin patch (Figure 1). High-quality inorganic semiconductors on skin-like patch lead to highly sensitive electromechanical devices such as strain sensors (Figure 2)."
Secure System for Implantable Drug Delivery,"Recent years have witnessed a growing increase in the use of implantable and wearable medical devices for monitoring, diagnosing, and treating our medical conditions. Advancements in electronics have opened up new avenues for deploying these devices towards applications previously overlooked, such as implant-ing an entire repository of a medical drugs within the human body for effective time-released delivery. The advantages of a time-released implant offer over some conventional oral dosage forms are site-specific drug administration for targeted action, minimal side-ef-fects, and sustained release of therapeutic agent. Pa-tient compliance is more positive with the treatment regimen associated with an implantable device as it is considerably less burdensome than pills or injections. The prominent application for implantable drug deliv-ery includes diabetes management, contraception, HIV/AIDS prevention, and chronic pain management. In many of these applications, the control of the command to these devices lies with the patient, who can program the device as needed. For example, a woman can program her monthly schedule of contraception for her family planning and allow the device to release regular doses of contraception, alleviating daily doses. However, an alarming concern that is associated with it is the generic security concerns with regular IoT devices, and potentially, with much more catastrophic effects. Any compromise of the controller device/cell phone would render the system ineffective. The fact that there is no direct feedback from the implantable to the patient makes it even more difficult. A simple example is a malicious cell-phone continuously commanding the device to release drug without the knowledge of patient. Our work focusses on solving this problem with a combination of energy-efficient cryptography with relevant physiological properties of the user. This makes it very difficult for any attacker, even with significant control over the controller, to break the system, while providing legitimate feedback to the user."
Enabling Saccade Latency Measurements with Consumer-grade Cameras for,"Quantitative and accurate tracking of neurodegenera-tive disease remains an ongoing challenge. Diagnosis re-quires patients to undergo time-consuming neuropsy-chological tests that suffer from high-retest variability, making it difficult to assess the progression of the dis-ease or a patient’s response to experimental treatments.We tackle the lack of an objective measurement to track the progression of neurodegenerative diseases by designing algorithms that can quantify subtle changes across time in eye movement patterns that correlate with disease progression. One such feature is saccade latency – the time delay between the appearance of a visual stimulus and when the eye starts to move towards said stimulus. As a result, an unobtrusive tool that measures saccade latency (or other metrics of eye movement) consistently across time can enable the quantification of disease progression and the assessment of a patient’s response to treatment. We propose a pipeline (Figure 1) to modify and evaluate a set of candidate eye-tracking algorithms to operate on video sequences obtained from an iPhone 6, for accurate and robust determination of saccade latency. A variant of the iTracker algorithm performed most robustly and resulted in mean saccade latencies and associated standard deviations on iPhone recordings that were essentially the same as those obtained from recordings using a high-end, high-speed camera (Figure 2). Our results suggest that accurate and robust saccade latency determination is feasible using consumer-grade cameras and might, therefore, enable unobtrusive tracking of neurodegenerative disease progression."
Comparing Piezoelectric Materials and Vibration Modes for Power Conversion,"Major industries such as transportation, energy sys-tems, manufacturing, healthcare, consumer electronics, and information technology vitally depend on power electronics for processing electrical energy. Power elec-tronics are often the bulkiest components in the sys-tems they serve, and smaller converter designs are typ-ically limited by magnetic energy storage components (i.e., inductors and transformers). The power density and efficiency capabilities of magnetics fundamentally decrease at low volumes, which motivates exploration of other energy storage mechanisms that are more con-ducive to miniaturization. One promising alternative is piezoelectric energy storage; piezoelectrics store energy in the mechanical compliance and inertia of a piezoelectric material, and they offer several potential advantages to power conversion. In previous work, we have demonstrated a converter implementation capable of >99% peak efficiency using a commercially available piezoelectric resonator (PR). However, criteria for selecting piezoelectric materials and/or designing PRs themselves remain murky in the context of power conversion.In this work, we derive figures of merit (FOMs) for piezoelectric materials and vibration modes specifically for use in power electronics. In particular, we focus on maximum efficiency and maximum power density FOMs for PRs in realistic converter switching sequences. These FOMs are shown to depend on only material properties for each of seven vibration modes, and they correspond to specific PR geometry conditions for realizing both maximum efficiency and maximum power density in PR designs.We validate these FOMs and their geometry condi-tions using a numerical solver for converter operation as well as experimental results for six commercially available PRs (shown in Figures 1-2). The proposed FOMs are demonstrated to be highly representative metrics for PR efficiency and power density capabili-ties, and these properties are likewise shown to scale favorably for converter miniaturization. Thus, by en-abling smaller-volume converters, piezoelectrics are positioned to both reduce system costs and open new application spaces for power conversion."
Acoustically Active Surface for Automobile Interiors Based on Piezoelectric Dome Arrays,"The surfaces of automobile interiors can be rendered acoustically active by mounting on them flexible, wide-area thin-films with arrays of small acoustic transducers. Each small, individually addressable transducer functions as a speaker, a microphone, or an ultrasonic transceiver. Engineering the structures and dimensions of individual transducers on the acoustic surface offers widely tunable performance. Coordinat-ing the phased transducer array based on adaptive con-trol could enable unique functionalities of the acoustic surface such as directional sound generation and de-tection. As a result, the acoustically active surface can work either in the audio frequency range for noise can-cellation, personal entertainment, and communication with the vehicle or in the ultrasound frequency range for gesture detection, alertness monitoring, etc., which collectively improve the comfort and safety of the au-tomobile.This project seeks to develop and demonstrate a wide-area, paper-thin, robust, and even transparent acoustic surface based on an array of dome-shaped piezoelectric transducers. Dependencies of the acoustic performance on the design variables of the piezoelectric domes are studied through theoretical modeling, simulation, and experimental characterization of dome vibration and sound radiation by the acoustic surface. A 12-μm-thick, 10×10 cm2 acoustic surface consisting of an array of polyvinylidene difluoride (PVDF) can be further enhanced by scaling up the area, utilizing superior piezoelectric materials, enlarging the dome size, and/or reducing the film thickness. A scalable micro-embossing process has been developed to fabricate the small domes with high precision and at low cost. 10×10 cm2 samples (Figure 1) were prepared with different dome dimensions and tested in an anechoic chamber. The results confirm outstanding performance of the acoustic surface, owing to the existence of active microstructures in an array, and thereby show great promise for broad application scenarios."
Advanced Microfluidic Heat Exchangers via 3D Printing and Genetic Algorithms,"Power electronics are fundamental in many high-tech applications, e.g., electric cars. Adequate heat dissipa-tion of these electronic components is essential for them to operate properly and attain long lifespans. Cooling high-power electronics typically employs heat exchangers that put a liquid in contact with hot sur-faces to extract heat. Using microfluidics can greatly in-crease the surface-to-volume ratio of the liquid, boost-ing heat transfer. However, classically designed heat exchangers do not properly address the non-uniformi-ty of the heat field, e.g., localized hot spots. In addition, better power microelectromechanical system microflu-idics can be created via additive manufacturing, involv-ing better materials and implementing more effective geometries than in mainstream cleanroom microfabri-cation. In particular, metal 3D printing can monolithi-cally create complex microfluidic devices while greatly simplifying the manufacturing process and requiring significantly less time than subtractive manufacturing.Genetic algorithms (GAs) can be used to implement an iterative design process inspired in natural selection that can potentially create better engineering solutions by generating unexpected implementations. In a nutshell, GAs are used to create multiple generations of randomized mutations of the parent designs (called subjects), looking to optimize the solution’s performance by minimizing/maximizing a particular fitness function.In this project, we are exploring metal 3D printing and GAs to implement better microfluidic heat exchangers. The fitness function employed ponders trade-offs between temperature and pressure drop in the cold plate to minimize the maximum temperature. We use a finite element solver with a computational fluid dynamics module to obtain solutions of the flow and temperature fields of each subject of each generation and then we used software to compare their performance across each generation and down-select the best designs. The software creates and analyzes new generations until it attains a certain threshold value in the fitness function (Figure 1). The resultant devices are complex, often counter-intuitive, and unlikely to be synthesized by a human using first principles (Figure 2), surpassing the performance of traditional designs."
3D-Printed Miniature Vacuum Pumps,"Compact pumps that create and sustain vacuum en-vironments while supplying precise gas flow rates are essential to implement a variety of microsystems. Positive displacement vacuum pumps, e.g., diaphragm pumps, create and maintain vacuum by cycling pockets of gas that are compressed from rarified conditions to atmospheric pressure. Miniaturized positive displace-ment vacuum pumps typically have dead volumes very similar to the maximum displacement of their com-pression chambers, resulting in the creation of modest vacuums. Magnetic, long-stroke actuators could be used to implement pump chambers with large compression ratios; an exciting possibility to implement such actuators at a low cost is additive manufacturing. In this project, we demonstrated the first miniaturized, additively manufactured, magnetic diaphragm pumps for liquids in the literature where all constitutive parts, including the magnets, are monolithically 3D-printed. The devices were created in nylon-based feedstock via fused filament fabrication, in which thermoplastic filament was extruded from a hot nozzle to create a solid object layer by layer. The miniature pumps use 150-μm- or 225-μm- thick membranes connected to a piston with an embedded magnet, a chamber, two diffusers, and two fluidic connectors (Figure 1). We also experimentally observed that the same pumps for liquids can be used as vacuum pumps if they are first moistened with a small amount of water to enable the pump diffusers to seal during actuation. The miniature 3D-printed pumps can attain an ultimate pressure of 540 Torr at an operating frequency of 230 Hz, i.e., the pumps achieve a pressure of 220 Torr below atmospheric pressure (Figure 2). The ultimate pressure achieved by our pumps is close to values reported from commercially available, non-microfabricated, miniature diaphragm pumps with comparable diaphragm diameters. We speculate that changing the design of the pump chamber to increase its compression ratio and printing a more flexible and compliant material could attain lower ultimate pressure."
"3D-Printed, Miniaturized Retarding Potential Analyzers for Cubesat Ionospheric Studies","The ionosphere is an upper region of the atmosphere that is made of plasma created and sustained by solar UV radiation. Little is known about some of the layers of the ionosphere, e.g., the thermosphere. Comprehend-ing the processes taking place in the thermosphere is essential to understand local and global weather and global warming. There is evidence that global warming is cooling down the thermosphere, causing serious is-sues, e.g., variation in satellites’ drag and less recycling of water. In-situ data would provide more and better information.Plasma sensors are used to characterize plasmas, measuring one or more properties that can be derived from the position and velocity distributions of the particles that make up the plasma. A retarding potential analyzer (RPA) is a multi-gridded sensor that measures the ion energy distribution of a plasma. In an RPA, the diameter of the apertures of the outermost grid (the floating grid) measures up to two Debye lengths to trap the plasma outside the sensor while the inter-grid spacing measure up to four Debye lengths to avoid space charge effects that would smear the measurements. The Debye length in the ionosphere is about 1 mm. Sending hardware to space is quite expensive because, among other reasons, of the physics of rocket propulsion, e.g., requiring ejecting propellant many times the mass of the spacecraft. Therefore, technologies that yield smaller, lighter, and cheaper space hardware without sacrificing performance are of great interest. Consequently, there is great interest in developing mission-focused miniaturized satellites, i.e., cubesats (1-10 Kg, a few L in volume).In this project we are harnessing additive manufacturing to demonstrate better and cheaper cubesat plasma sensors. Our RPA design uses laser-micromachined stainless steel grids integrated to a 3D-printed ceramic housing made via vat polymerization using 60-µm by 60-µm by 100-µm XYZ voxels (Figure 1). Each grid is assembled to the housing using a set of engineered springs that provide active alignment. Experiments show that the per-level assembly precision is better than 100 µm (Figure 2). Inter-grid alignment results in larger current signals. Current work focuses on completing, fabricating, and characterizing the RPA design."
Multi-Dimensional Double Spiral Device for Fully Automated Sample Preparation,"Sample preparation is the process of extracting tar-get analytes from interferents for the sensitive and successful downstream analysis of samples. To over-come the limitations of the current standard (centrif-ugation), which entails many energy-consuming steps, various microfluidic devices have been developed. Among them, the inertial spiral microfluidic device has been extensively utilized due to its inherent advantag-es including label-free, high-throughput, and reliable operation without any external force field. However, improvement of separation efficiency and usability is required for field-deployable applications.In response to this critical need, we developed a new type of spiral device, the multi-dimensional double spiral (MDDS) device. The MDDS device is composed of two sequentially connected spiral channels having different dimensions. Particles can be concentrated through the first, smaller-dimensional spiral channel and subsequently separated through the second, larg-er-dimensional spiral channel (Figure 1a). The initial focusing in the first spiral channel can significantly de-crease particle dispersion and effectively extract small-er particles into the outer-wall side of the channel, in-creasing separation resolution and efficiency (Figure 1b).To achieve more purified and concentrated output, we also developed a new recirculation platform based on a check-valve which allows only one-way flow. In the platform, an output from the MDDS separation can be extracted back into the input syringe and processed again repeatedly via programmed back-and-forth mo-tions of a syringe pump, resulting in higher purity and concentration (Figure 1c). The developed platform can be operated in a fully automated or even hand-powered manner. Using the platform, we successfully demon-strated the isolation of white blood cells from a dilut-ed blood sample by removing abundant red blood cells (up to 99.99%). We expect that the developed platform could provide an innovative field-deployable sample preparation solution to point-of-care sample analyses (not limited to blood) and diagnostics."
"Internally Fed, Additively Manufactured Electrospray Thruster","Electrospray engines produce thrust by electrohydro-dynamically ejecting high-speed ions or droplets. Elec-trospray emitters work better if miniaturized because their start-up voltage decreases with the square root of the emitter diameter. A single emitter has very low thrust; multiplexing the emitters, so they uniformly operate in parallel, makes it possible to increase the thrust delivered. Electrospray thrusters are typically created via precision subtractive manufacturing tech-niques, which is time-consuming and expensive. For New Space, i.e., the development of a commercial space industry, additive manufacturing is an attractive possi-bility to create complex hardware that is inexpensive and exquisitely iterated and optimized.Our group recently demonstrated the first additively manufactured ionic liquid electrospray thrusters in the literature; these devices attain pure ion emission in both polarities, maximizing their specific impulse. However, the propellant flow rate, which has an upper bound for pure ionic emission, limits the thrust per emitter that can be attained for a given bias voltage. An engine that can deliver larger per-emitter thrust, at the expense of using less efficiently the propellant, is of interest for impulsive maneuvers.Consequently, we are also interested in developing additively manufactured, low-specific impulse, high per-emitter thrust electrospray engines. Unlike the externally fed, nanoporous fluidic structure used in the ionic thrusters previously described, an internally fed emitter architecture is a better fit to produce droplets (Figure 1), which are heavier than ions, resulting in higher per-emitter thrust. We use the vat polymerization method called digital light processing to make emitters with narrow channels that provide high hydraulic resistance. Using resolution matrices drawn in ~25 µm voxels and a resin chemically resistant to an ionic liquid, we verified the high fidelity of the printed parts to the computer-aided design (CAD) models (Figure 2). Current research efforts focus on exploring the resolution limits of the printable feedstock for solid and negative features and developing device designs with hydraulic networks that provide a high and uniform hydraulic impedance to each emitter."
Planar Field-Emission Electron Sources via Direct Ink Writing,"Vacuum electron sources appear in numerous technologies, from microscopy to displays to mass spectrometry. The two main forms vacuum electron sources can take are thermionic and field emission. Thermionic sources emit electrons by raising the temperature of a conductor so that many of its electrons have an energy greater than the potential barrier trapping them, allowing them to escape. Field-emission sources use an applied electric field to lower the potential barrier, allowing electrons to quantum tunnel out of the conductor. Field-emission sources can therefore operate at lower temperatures, in a poorer vacuum, faster, and using less energy, all of which increase the usability of the electron source.Field-emission sources’ emitting electrodes have been made from many materials, but research has focused on carbon nanotubes (CNTs). CNTs’ nanosized tips and high aspect ratio lead to high electric fields at modest voltages, which is useful since the emitted current increases with electric field; in addition, CNTs have excellent chemical resistance, e.g., resisting oxidation by the trace gases in the vacuum. Manufacturing CNT field-emission sources is often a costly and time-intensive effort, particularly when the CNT growth locations are restricted by desired device geometry.To affordably implement CNT field emission cathodes, this project explores direct ink writing to create in-plane, gated field-emission sources. A spiral CNT ink trace is printed on an insulating substrate, along with a symmetric, co-planar trace (see Figure 1) of a different conducting (e.g., silver nanoparticle) ink. A voltage applied between the traces induces an electric field, causing electron tunneling from the CNT tips. The planar design reduces manufacturing complexity and increases electron transmission. Current work includes printable feedstock material selection, exploration of geometric modifications to increase device longevity, and increasing imprint density to allow for greater emission current density."
Micro Rocket Engine Using Steam Injector and Electric Fuel Pump,"Micro-fabricated miniature chemical rocket engines have been an active area of research at MIT and else-where for two decades; they are a compelling propul-sion option for small launch vehicles and spacecraft. At these scales, miniaturized steam injectors like those used in Victorian-era steam locomotives are viable as a pumping mechanism and offer an alternative to pres-sure feed and high-speed turbo-pumps. Storing pro-pellants at low pressure reduces tank mass, and this improves the vehicle empty-to-gross mass ratio; if one propellant is responsible for most of the propellant mass (e.g., oxidizer), injecting it while leaving the others solid or pressure-fed can still achieve much of the po-tential gain. Previously, the principal investigator and his group built and tested ultraminiature-machined micro jet injectors that pumped ethanol and explored pressure-fed liquid and hybrid engine designs. Current work has focused on configurations that use a battery and electric pump to replace the pressure-feed portion of past designs; electric pumps pump fuel and/or cool-ant while a steam injector motivated by boiled coolant pumps the oxidizer. This replacement allows pressur-ized tanks to be avoided altogether, greatly simplify-ing implementation and the sourcing of components while still being compatible with miniaturization via a micro-electromechanical system (MEMS). Current work has focused on designing and implementing an axisymmetric whole-engine mock-up or test article that simultaneously integrates a steam injector, boiler, decomposition chamber, fuel injector, thrust chamber, and electric fuel pump while being practical to build and also retaining compatibility with 2D MEMS fabri-cation (see Figure 1)."
Nonvolatile Electrically Reconfigurable Photonic Circuits Based on Low-Loss Phase-Change Materials,"Low-power active components are crucial to achiev-ing programmable photonic integrated circuits (PICs). Reaching this goal drives the development of active components with outstanding performance in the gigahertz-frequency operation required in telecom-munication applications but also on slower scales for active reconfiguration of PICs. However, these compo-nents are all volatile, which is not ideal for applications where the configurations are performed sporadically or just once. In the latter case, nonvolatile reconfigu-ration capable of retaining any configuration with ze-ro-power consumption is the desired functionality. To fill this gap, we employ Ge2Sb2Se4Te1 (GSST), a low-loss broadband optical phase-change material. GSST allows refractive index modulation by using a heat stimulus to switch between the amorphous and the crystalline states, which results in an outstanding modulation of optical properties (∆n ~ 1.7). We patterned ~1018 cm-3 n-doped silicon microheaters to provide the heat stimuli and electro-thermally configure the state of GSST, which was evanescently coupled to the propagating mode of a half-etched rib waveguide (Figure 1). We evaporated 30 nm of GSST, which theoretically introduced a π phase-shift with a 5-µm-long cell. We demonstrated 50 cycles of reversible and repeatable switching between the amorphous and two partially crystallized states of a 3-µm-long GSST and the subsequent phase shift on a ring resonator (Figure 1c). We used 3.5V×20 ms and 5V×50 µs pulses to crystallize and amorphize, respectively. Our analysis reveals that doped Si contributes only to 0.03 dB/µm absorption, amorphous GSST shows zero loss, and crystalline GSST shows 0.57 dB/µm. Furthermore, we demonstrate GSST-based phase-shifters on a balanced 2×2 MZI switch structure (Figure 1d, Figure 1e). We measured the variations on the two output channels as a function of the state of a 10-µm-long GSST in each arm. Using the same pulse sequence as above, we achieved π/2 phase-shift upon amorphization followed by full recrystallization with a 30-dB extinction ratio."
Extremely Dense Arrays of Si Emitters with Self-Aligned Extractor and Focusing Gates,"The advent of microfabrication has enabled scalable and high-density Si field-emitter arrays (FEAs). These are advantageous due to compatibility with comple-mentary metal-oxide-semiconductor (CMOS) process-es, the maturity of the technology, and the ease in fabri-cating sharp tips using oxidation. The use of a current limiter is necessary to avoid burn-out of the sharper tips. Active methods using integrated MOS field-effect transistors and passive methods using a nano-pillar (~200-nm wide, 8-µm tall) in conjunction with the tip have been demonstrated. Si FEAs with single gates re-ported in our previous works have current densities of >100 A/cm2 and operate with lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essen-tial to control the focal spot size of the electron beam as electrons leaving the tip have an emission angle of  12.5. The focus electrode provides a radial electric field that reduces the lateral velocity of stray electrons and narrows the cone angle of the beam reaching the anode. Varying the voltage on the focus gate reduces the focal spot size or achieves an electron beam modu-lator for radio-frequency applications. In this work, we fabricate the densest (1-μm pitch) double-gated Si with an integrated nanowire current limiter (Figure 2). The apertures are ~350 nm and ~550 nm for the extractor and focus gates, respectively, with a 350-nm-thick oxide insulator separating the two gates. Electrical character-ization of the fabricated devices shows that the focus-to-gate ratio (VFE/VGE) can be used to control the anode current (Figure 2). When the focus voltage exceeds the gate voltage, the field superposition increases the ex-tracted current, and vice versa. These devices can pot-entially find applications as high-current focused elec-tron sources in flat panel displays, nano-focused X-ray generation, and microwave tubes."
Gated Silicon Field-Ionization Arrays for Compact Neutron Sources,"Neutron radiation is widely used in various applications, ranging from the analysis of the composition and structure of materials and cancer therapy to neutron imaging for security. However, most applications require a large neutron flux, which is often achieved only in large infrastructures such as nuclear reactors and accelerators. Neutrons are generated by ionizing deuterium (D2) to produce deuterium ions (D+) that can be accelerated towards a target loaded with either D or tritium (T). The reaction generates neutrons and isotopes of He, with the D-T reaction producing the higher neutron yield. Classic ion sources require extremely high positive electric fields, on the order of 108 volts per centimeter (10 V/nm). Such a field is achievable only in the vicinity of sharp electrodes under a large bias; consequently, ion sources for neutron generation are bulky. This work explores, as an alternative, highly scalable and compact Si field-ionization arrays (FIAs) with a unique device architecture that uses self-aligned gates and a high-aspect-ratio (~40:1) Si nanowire current limiter to regulate electron flow to each field emitter tip in the array (Figure 1). The tip radius has a log-normal distribution with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm in diameter and is within 200 nm of the tip. Field factors, β, of > 1 × 106 cm-1 can be achieved with these Si FIAs, implying that gate-emitter voltages of 250-300 V (if not less) can produce D+ based on the tip field of 25-30 V/nm. In this work, our devices achieve an ionization current of up to 5 nA at ~140 V for D2 at pressures of 10 mTorr. Gases such as He and Ar can also be ionized at voltages (<100 V) with these compact Si FIAs (Figure 2)."
Field Emission from a Single Nanotip in Controlled Poor Vacuum,"For reliable field emission performance, nano-emitters require ultra-high vacuum, which is bulky and costly. In poor vacuum, the adsorption/desorption of gas mol-ecules on the surface causes work function variations, which results in exponential changes in the emitted current. In this work, we assess the dependence of the Fowler-Nordheim slope, bFN, in different gases using a single un-gated Si emitter. These measurements are enabled by using a scanning anode field emission microscope that has a W tip (radius <1 μm) as the anode and with the Si emitter placed on a nano-positioning stage. We first characterized the devices in ultra-high vacuum and in the following gases: Ar, He, N2, O2, and H2. I-V characteristics are recorded by varying the distance, d, between the anode and the emitter (Figure 1). Using the measurement in ultra-high vacuum (UHV, 10-9 Torr) as a reference, we extract the geometrical field-factor, β from bFN. In poor-vacuum measurements (10-8 Torr – 10-5 Torr), we use this β to extract the “modified” work-function of the surface for each gas and each pressure investigated. We find that as pressure increases, the performance in Ar changes very little at the distances scanned (Figure 2). As expected (Figure 3), operation in O2 resulted in substantial increase in bFN and hence the work-function; however, in H2, we measured a decrease in the slope, suggesting a reduction in the work function. This work provides the premise in assessing which gases and pressures are responsible for performance degradation in Si field-emitter arrays, to achieve more stable field-emission current in poor vacuum."
GaN Vertical Nanostructures Sharpened by A New Digital Etching Process for Field Emission Applications,"Field emitters (FE), namely vacuum transistors, are promising for harsh-environments and high-frequency electronics. III-nitrides are excellent candidates as FEs due to their tunable electron affinities. However, so far, few works demonstrate sub-100 V turn on in III-nitride field emission devices because of relatively large tip siz-es and the lack of self-aligned gate structures.In this work, we develop a novel wet-based-only digital etching (DE) process for GaN nanopyramid field emission arrays (FEAs). Conventional oxygen-plasma-based DEs on III-nitrides are anisotropic, and they do not sharpen vertical tips. Furthermore, the use of a biased plasma could potentially damage tips. Therefore, a new digital etching process is developed. By using this new technology, tip width can be sharpened from 40 nm down to sub-20 nm with a reasonably controlled etching rate per cycle of DE (Figure 1 (a)).Combining the sharpened GaN nanopyramids with a self-aligned-gate structure (Figure 1 (b)) we developed before, we demonstrate the world’s-best GaN vertical field emission devices with the lowest gate-emitter turn-on voltage (VGE, ON) of 20 V and the highest max current density of 150 mA/cm2 at VGE = 50 V (Figure 2). The turn-on voltage and field factor of this device are also already comparable with the-state-of-art Si FEAs. Furthermore, the gate leakage is still only about 0.5 % of the anode current, indicating a space to have future improvement for more drive current. Further performance improvements are expected when applying the developed technologies to N-polar III-nitrides and AlGaN-alloys."
Integrating Object Form and Electronic Function in Rapid Prototyping and Personal Fabrication,"Rapid prototyping is a key technique that enables us-ers to quickly realize their digital designs and therefore has been widely used in early-stage prototyping and small-scale customized fabrication. A long-term vision in human-computer interaction is to create interactive objects for which all functions are directly integrat-ed with the form and fabricated at once. So far, rapid prototyping has focused mainly on fabricating passive objects for which the form of an object can be freely designed; recently we have also moved towards digital specification and fabrication of object functions for in-teractive design. These advances offer the promise that eventually in rapid function prototyping, the interac-tive object form and function would be under the same design consideration; therefore, the object form could follow its designated function, and the function could adapt to its physical form."
MEMS Energy Harvesting and AI-based Design Processing,"Vibrational energy-harvesting devices seek to deliver useable electric power in remote or mobile applications by drawing energy from ambient sources of vibration. Due to the spectrum of such ambient vibrations oc-curring at a very low frequency (below 100Hz), major design challenges must be overcome when developing a piezoelectric energy harvesting device to function in these conditions, namely generating strain at the mi-cro-scale and operating over a wide bandwidth of low input frequencies. The culmination of three genera-tions of this microelectromechanical systems (MEMS) design effort by our research group is a bi-stable buck-led beam energy harvester that relies on non-linear oscillations to translate input vibrations to the axial strain of piezoelectric elements to produce sizable elec-tric energy at the MEMS-scale devices.Various long-term research efforts such as this at MIT produce documentation detailing novel devices and corresponding process designs that could benefit future micro and nano systems designers if the knowledge and design concepts explored for them could be computationally retrievable. To benefit from past designs, their functional requirements must be identified and structured in a searchable and trainable knowledge base which future designers may navigate. Currently, we are developing an AI-based Natural Language Processing (NLP) model which can automate the reading of vast MEMS documentation produced by MTL and MIT.nano. By automatically processing and representing decades of research knowledge at MIT and elsewhere, faster and successful design and innovation in MEMS and Nano-scale systems can be achieved."
Chip-Scale Quadrupole Mass Filters for a Micro Gas Analyzer,"In recent years, there has been a desire to scale down linear quadrupoles. The key advantages of this miniaturization are the portability it enables and the reduction of pump-power needed due to the relaxation on operational pressure. Various attempts at making MEMS-based linear quadrupoles have met with varying degrees of success [1]-[3]. Producing these devices involved some combination of precision machining or microfabrication followed by electrode assembly. For miniature quadrupole mass filters to be mass-produced cheaply and efficiently, the electrode assembly should be removed from the process.A chip-scaled quadrupole mass filter comprising a planar design and square electrodes was conceived, fabricated, and tested. Rectangular electrodes were utilized since this is the most amenable geometric shape for planar microfabrication. This deviation from the conventional round rod geometry required optimization and analysis, which was conducted with Maxwell 2D and MATLAB [4]. The fabrication process consists of thermal oxidation, the use of DRIE to define the features, and the fusion bonding of five patterned silicon wafers. This relatively simple process flow furthers the case for mass-production of these devices. A completed device measures 33 x 15 x 4 mm3 and contains integrated ion optics as shown in Figure 1.This non-conventional design introduces non-linear resonances that degrade the peak shape in the mass spectrum. Reported work with linear quadrupoles shows improved peak shape by operation in the second stability region [3]. Characterization of the device was conducted using FC-43, a standard calibration compound, and air as the analytes. The MuSE-QMF demonstrated a mass range of 250 amu using the first stability region and a minimum peak width of 0.7 amu in the second stability region. The main peaks for air (nitrogen, oxygen, argon, carbon dioxide) can be clearly distinguished in Figure 2.In future work, we plan on modifying the processing and the mask layout to improve device performance. The design and fabrication concepts of this device can be expanded into arrayed configurations for parallel analysis and aligned quadrupoles operated in tandem for enhanced resolution."
Tactile Sensors and Actuators for Smart Surface Applications,"Novel tactile sensor and actuator devices using zinc oxide nanowires have been developed to enhance the interaction between people and their environment for smart surface applications. Both the sensor and actuator device use the piezoelectric effect of zinc oxide (ZnO) nanowires. The devices are based on a cross-bar network comprising a top and bottom array of electrodes around a composite of vertically grown nanowires and an insulating polymer. This cross-bar network allows for individually addressable locations for both sensing and actuation. The results for the tactile pressure sensor show a clear spike in current when an insulating tip is placed on and removed from the surface (Figure 1). This result is compared to controls including a touch on the adjacent cross electrodes and testing another device without wires. Both tests show at least an order of magnitude difference in current between the control and the pressure sensor. The actuator device utilizes a thin membrane of thermally grown silicon dioxide that is oscillated at resonance to induce tactile sensation. The oxide membrane is fabricated by using a deep back-side etch of a silicon wafer and utilizing the thermally grown oxide as an etch stop. The rest of the device is very similar to the pressure sensor with an electrode cross bar network and a zinc oxide nanowire polymer composite. The nanowires are grown in a furnace by chemical vapour deposition or by a low temperature hydrothermal method, producing wires of length of 1–12µm [1], [2]. The system is actuated by applying an alternating current through the top and bottom electrodes. The piezoelectric nanowires expand and contract according to the AC signal [3]. The results show a first resonance peak at 139kHz, followed by a slightly lower peak at 191kHz. The amplitude of oscillation is still not known precisely, but it is estimated to be approximately 15nm at 33V. Currently, haptic feedback for portable electronic devices such as mobile phones is limited to vibration over a large area or the whole phone [4], [5]. This project addresses these issues by making the tactile actuators and sensors smaller than the pixel size that the finger can sense. This small pixel size leads to virtual buttons and textured surfaces that are software-controlled and infinitely variable. The long term goal of the project is to have a transparent and flexible device so that it can be incorporated into a variety of different displays and surfaces."
MEMS-based Plasma Probes for Spacecraft Re-entry Monitoring,"NASA’s strategic plan calls for a focus on advanced sensing that would assure continued safe operations. We propose a set of three cost-effective and reliable MEMS-based sensors to diagnose in real time the conditions of the plasma surrounding the spacecraft during reentry. The proposed sensors are (i) arrays of Langmuir probes, (ii) arrays of retarded potential analyzers, and (iii) arrays of GPS antennas. Each sensor is targeted to gather specific information of the plasma, and it is operated in such a way that allows fast data collection. There are reports of MEMS-based devices for plasma diagnostics such as Langmuir probes [1]. Although these sensors work, their shield is made of polyimide. Therefore, these sensors are not compatible with the high-temperature or high-density plasmas that the spacecraft encounters at re-entry. Silicon Carbide (SiC) is a semiconductor material that is very resistant to hostile environments [2]. There are current research efforts to develop SiC-based MEMS intended for harsh environments, including pressure, acceleration, temperature, and strain transducers, as well as transistors [3]-[5]. The SiC is a promising material to implement low-cost and reliable plasma diagnostics. We are exploring SiC both as a coating and as fabrication substrate. The work has focused on the Langmuir probe development. Langmuir probe densities as large as 106/cm2 have been demonstrated (Figure 1). Also, fabrication experiments using a plasma-enhanced chemical vapor deposited (PECVD) SiC coatings have been conducted (Figure 2). Future research includes the development of an RPA based on an ionizer we recently developed [6] and experimental validation of the sensors."
Investigating Stem Cell Dynamics Utilizing Microfluidic-based Time-lapse imaging,"An understanding of the mechanisms underlying stem cell fate and function has recently been augmented by the application of microfabricated systems, designed to systematically probe important environmental stimuli and intrinsic genetic programs [1]. In particular, current approaches leveraging these systems aim to enhance both the spatial and temporal resolution of stem cell analysis, providing a more complete picture of dynamic stem cell processes. Microfluidics represents a promising technology for the parallel analysis of cellular responses to numerous perturbations simultaneously within a single device [2], although it can be difficult to implement in traditional biology laboratory settings. To examine the dynamics of embryonic stem (ES) cell self-renewal and differentiation, we have employed a simple microfluidics platform, without valves or specialized equipment, coupled with near-simultaneous time-lapse imaging. This integrated system incorporates a miniaturized 96-well, ~6 x 4 mm2 imaging area with a variable input/output channel design and enables the interrogation of ES cell kinetics within multiple environments. We have tested the platform with both feeder-independent mouse ES cell lines as well as co-cultures of mouse ES cells with supportive mouse embryonic fibroblast (MEF) feeder layers and demonstrated self-renewal over 3-4 days of analysis. The examination of ES cells containing fluorescent protein fusions was utilized to monitor chromosome dynamics during self-renewal and to evaluate proliferation kinetics; furthermore, perturbation with an anti-mitotic agent demonstrated the dynamic response to exogenous factors within the device. Overall, these studies illustrate the capacity to dynamically assess and manipulate stem cell processes through the integration of a simple, but modular, microfluidics-based imaging platform."
Direct Patterning of Metallic MEMS through Microcontact Printing,"Standard photolithography-based methods for fabricating microelectromechanical systems (MEMS) present several drawbacks including expense, incompatibility with flexible substrates, and limitations to wafer-sized device arrays. We have developed a new fabrication method for rapid fabrication of large-area MEMS that breaks the paradigm of lithographic processing using a scalable, large area microcontact printing method to define three-dimensional electromechanical structures. Our PDMS Lift-Off Transfer (PLOT) involves the rapid removal of a pick-up stamp from a transfer pad to transfer a continuous metal film from the pad to the stamp. A stamp that forms the membrane suspension supports is fabricated by molding a thin layer of PDMS against a silicon master with a predefined relief. The metal membranes are deposited by thermal evaporation onto a transfer pad which has been prepared with an organic molecular release layer. To achieve transfer of the metal membrane over the supports of the device, the stamp is brought into conformal contact with the transfer pad and then released by rapidly peeling away. MEMS bridge structures, such as the ones shown in Figure 1, have been fabricated using PLOT, and their performance as variable capacitors has been characterized. In Figure 2, the capacitance of these devices increases with applied voltage, indicating mechanical deflection of the bridges due to the electrostatic force. PLOT forms MEMS structures without requiring elevated temperature processing, high pressure, or wet chemical or aggressive plasma release etches, providing compatibility with sensitive material sets for the fabrication of integrated micro- or opto-electronic/MEMS circuits. Flexible, paper-thin device arrays produced by this method may enable such applications as pressure sensing skins for aerodynamics, phased array detectors for acoustic imaging, and novel adaptive-texture display applications."
Design of Micro-scale Multi-axis Force Sensors for Precision Applications,"Multi-axis force-sensing at the micro-scale is necessary for a wide range of applications in biology, materials science, and nanomanufacturing. A three-degree-of-freedom force sensor (Figure 1) was designed that is capable of accurately and precisely measuring the adhesion forces (nanoNewtons) between biologically active surfaces. This force sensor is positioned and actuated using a Hexflex nanopositioner and Lorenz force actuators as seen in Figure 2.In order to design high-accuracy, high-precision, multi-axis MEMS force sensors, a closed form model was developed to optimize the strain sensitivity of the MEMS force sensor. This model first sets constraints on the system due to package size, fabrication techniques, desired degrees of freedom, and force range. The layout of the flexure system is optimized to meet the kinematic and manufacturing constraints of the MEMS force sensor. The geometry of the flexures is set to maximize the strain at the sensor locations.This model was incorporated into a thermal/electric model to fully characterize all of the inputs to the system. The resolution of the force sensor is a function of the noise from the strain sensors, the noise in the electronics, the thermomechanical noise, and the sensitivity of the strain sensors to a force input. Based on this model, the dominant noise sources are identified and the sensor system is optimized to reduce these noise sources. The thermal/electric model is also used to determine the major factors limiting the accuracy of the force sensor. In most cases, the drifts in both the electronics and sensors caused by fluctuations in room temperature were the major sources of accuracy errors. Therefore, an environmental enclosure with closed-loop control over temperature was designed and implemented. Overall, the final design of the force sensor is capable of producing sub-nanoNewton-resolution force measurements with nanoNewton-level accuracy."
Design of a Six Degree of Freedom Nanopositioner for  Use in Massively Parallel Probe-based Nanomanufacturing,"In probe-based nanomanufacturing a micro-scale probe tip is used to create or measure nm-scale features. The serial nature of probe-based manufacturing dictates that practical throughput rates will require the use of two-dimensional tip arrays. These arrays must be controlled in six degrees-of-freedom to maintain parallelism with respect to the work surface. Meso-scale, 6-axis nanopositioners [1] will be needed because they (1) are lower cost ($100s US versus $10,000s), (2) possess higher bandwidth, and (3) are more thermally stable than macro-scale nanopositioners. Furthermore, their small size enables arraying many nanopositioners in a small footprint. Sensing is important as this enables closed loop position control and therefore control in a nanomanufacturing process. We have designed and microfabricated low-cost nanopositioners with nm-level accuracy and resolution that are equipped for closed-loop operation throughout a 50x50x50 µm3 work volume. Figure 1 shows the nanopositioner (less actuators [2] and electronics) that contains an integrated 6-axis piezoresistive sensing system [3]. The figure inset shows the piezoresistor arrangement, wherein a first sensor is placed along the beam’s neutral axis and the second sensor is placed at the beam’s edge. Both sensors are placed near the root of the cantilever where maximum device strain occurs. The neutral axis sensor experiences strain primarily from out-of-plane bending while the sensor on the edge of the beam experiences strain from in- and out-of-plane bending. Biasing these signals makes it possible to obtain in-plane and out-of-plane measurements from the sensors while keeping them located on the same face of the flexible beam. The structure of the nanopositioner was microfabricated from a 400 µm thick silicon wafer with 500 nm polysilicon piezoresistors fabricated onto the flexural beams. Each nanopositioner costs approximately $250 US and initial tests indicate the nanopositioner will have 2 nm out-of-plane resolution and 20 nm in-plane resolution."
"Magnetically-assisted Assembly, Alignment, and orientation of Micro-scale Components","The use of magnetic forces to improve fluidic self-assembly of micro-components has been investigated using Maxwell 3D to model the forces between Ni thin films on semiconductor device micro-pills and Sm-Co thin films patterned on target substrates [1]. Orienting and restraining forces on pills far in excess of gravity are predicted, and it is found that the fall-off of these forces with pill-to-substrate separation can be engineered through the proper design of the Sm-Co patterns to retain only properly oriented pills [1], [2]. Micro-scale hybrid assembly is a potentially important way of doing heterogeneous integration, i.e., of integrating new materials on silicon integrated circuits to obtain functionality not readily available from silicon device structures alone, and fluidic self-assembly is an attractive way to automate micro-scale assembly. A serious limitation of fluidic self-assembly, however, is the lack of a good method for holding properly assembled components in place and accurately positioned until all of the components have been assembled and they have been permanently bonded in place. We have shown, based on our modeling, that suitably patterned magnetic films can be used to provide the forces necessary to retain, and to accurately orient and position, assembled micro-components.Our motivation for pursuing micro-scale hybrid assembly is our general interest in doing optoelectronic integration, specifically of vertical cavity surface emitting lasers (VCSELS), edge-emitting lasers (EELs), and light emitting diodes (LEDs), with state-of-the-art, commercially processed Si-CMOS integrated circuits. Our ongoing research integrating these devices on silicon described elsewhere in this report provides the context for this work and illustrates the types of applications we envision for magnetically assisted self-assembly using the results of this study.Assembly experiments to verify and demonstrate the theoretical predictions are currently in progress using two sizes of 6-µm-thick pills (50 µm by 50 µm and 50 µm by 100 µm) and a variety of magnetic thin film patterns. Recesses with different dimensions are also being studied [2]."
Microfabricated Slits in Series: A Simple Platform to Probe Differences in Cell Deformability,"Change in cell stiffness is a characteristic of blood cell diseases such as sickle cell anemia, malaria1, and leukemia2. Often, increases in blood cell stiffness lead to loss of the cells’ ability to squeeze through capillaries, resulting in organ failure, coma, and ultimately death. The spleen is the organ in the human body that is responsible for removing these less deformable cells. It functions by forcing cells in blood to squeeze between endothelial cells arranged like the staves of a wooden barrel. The goal of this project is to create a microfluidic device that can quickly and accurately screen, diagnose, and treat disorders involving cell deformability. We report the creation of a microfabricated device consisting of a series of 1-2 µm-wide polymeric slits, modeled on those of the spleen, Figure 1. Using this device, we demonstrate unambiguous mobility differences between cells differing solely in stiffness. Figure 2 shows mobility differences for red blood cells (RBCs) treated with different concentrations of GA in a 2-µm slit device. The GA acts as an amine-crosslinker, making the cell membrane and cytosol stiffer. The RBCs are slightly larger than this slit size and must deform to traverse the slit. Velocities of 0.001% GA-treated cells were within experimental error to untreated cells. The cells treated with 0.01% GA exhibited a velocity of 0 µm/s, as they were too rigid to pass through the slits. At a concentration of 0.003 %, the cells were semi-rigid and showed decreased mobility compared to the untreated cells. Cell size was observed to be the same throughout the range of GA concentrations.These results demonstrate that increased membrane stiffness can cause statistically significant mobility differences through a series of slits. Additionally, the low-cost aspect of this device makes it ideal for on-site disease (e.g., malaria) screening in resource-poor settings."
Microfabricated Devices for Portable Power Generation,"The development of portable power-generation systems remains an important goal, with applications ranging from the automobile industry to the portable electronics industry. The focus of this work is to develop microreaction technology that converts the chemical energy stored in fuels–such as light hydrocarbons and their alcohols—directly into electricity or into a different energy vector such as hydrogen. Developing devices with high energy-conversion efficiency requires addressing difficulties in high temperature operation: specifically, thermal management, material integration, and improved packaging techniques.A catalytic combustion-based device intended for the direct conversion of thermal energy to electricity has been developed. The combustor has been designed to achieve attractive energy and power densities while addressing system challenges such as mechanically robust fluidic connections and minimal parasitic power losses related to pressurization of air. The channels of the combustor are etched using wet potassium hydroxide, which is the most economical etch technique available. Straight channels (1mm by 1mm in cross-section) are arranged in parallel and separated by 100-µm–thick silicon walls, in order to achieve low pressure drop (< 300 Pa at 10 SLPM gas flow) with significant surface area (~1 cm2 per channel) for catalyst deposition. Two identical reactors are stacked using metal thermocompression bonding to increase reactor volume without a significant increase in exposed surface area. External gas distribution manifolds are compression-sealed to the reactor, eliminating the need for glass brazing of tubes, increasing the mechanical robustness of the device, and avoiding large pressure losses associated with flow constrictions. Platinum-on-alumina catalyst has been washcoated on the channel surfaces for the catalytic combustion of butane with air.A combined reforming/separation device has been developed and demonstrated. The hydrogen generation unit combines a 200-nm-thick palladium-silver film with a methanol reforming catalyst (supported palladium). The catalytic combustion unit employs a supported platinum catalyst. Both units are formed in a silicon wafer by bulk silicon micromachining techniques. The energy generated in the combustion unit is efficiently transferred to the hydrogen production unit by the thermal conduction of silicon support. The system has been demonstrated to purify hydrogen at elevated pressures (up to 2 atm). Joint combustion/purification of the system has also been demonstrated, in which combustion and reforming occur simultaneously with the purification of the resulting hydrogen."
Microfluidic Systems for Continuous Crystallization,"Microfluidic systems offer a unique toolset for discovering new crystal polymorphs and for studying the growth kinetics of crystal systems because of well-defined laminar flow profiles and online optical access for measurements. Traditionally, crystallization has been achieved in batch processes that suffer from non-uniform process conditions across the reactors and chaotic, poorly controlled mixing of the reactants, resulting in polydisperse crystal size distributions (CSD) and impure polymorphs. Consequently, batch crystallization suffers from reproducibility issues, increases difficulty in obtaining accurate kinetics data, and manufactures products with inhomogeneous properties. The small length scale in microfluidic devices allows for better control over the process parameters, such as the temperature and the contact mode of the reactants, creating uniform process conditions across the reactor channel. Thus, these devices have the potential to generate more accurate kinetics data and produce crystals with a controlled morphology and a more uniform size distribution. In addition, microfluidic systems decrease waste, provide safety advantages, and require only minute amounts of reactants, which is most important when dealing with expensive materials such as pharmaceutical drugs. Figure 1 shows a microfluidic device used for crystallization; Figure 2 shows optical images of different polymorphs of glycine crystals grown inside reactor channels. A key issue for achieving continuous crystallization in microsystems is to eliminate heterogeneous crystallization–irregular and uncontrolled formation and growth of crystals at the channel surface–and aggregation of crystals, which ultimately clogs the reactor channel. We have developed a microcrystallizer using soft lithography techniques that introduce the reagents to the reactor channel in a controlled manner, preventing heterogeneous crystallization and aggregation. We have used optical microscopy in situ to obtain high-resolution images of crystals grown in continuous microreactors and use image analysis to derive growth kinetics of crystals of different morphologies and shapes. In addition, we have integrated an online spectroscopy tool for in situ polymorph detection. In summary, we have developed a microfluidic system for continuous crystallization of small organic molecules and integrated it with in situ detection tools for size and morphology characterization."
Multistep Microfluidic Systems for Synthetic Chemistry,"Microchemical systems have recently gained prominence for use in reaction screening and augmentation. However, most chemical syntheses combine several reaction and work-up steps, and independently studying each step limits understanding of how they are coupled in a process. To that end, microfluidic systems have been integrated to realize multistep reaction and liquid-liquid extraction steps [1], [2]. However, other separation techniques are needed in traditional batch synthetic transformations such as filtration, evaporation, and distillation. Consequently, developing a fundamental understanding of microfluidic distillation has been undertaken.Distillation is a ubiquitous method of separating liquid mixtures based on differences in volatility. This unit operation is fundamental to a number of industrial processes, and performing such separations in microfluidic systems is difficult because interfacial forces dominate over gravitational forces. The concept of distillation has been engineered on a silicon-based microfluidic chip as shown by the device shown in Figure 1 [3]. Microfluidic distillation is realized by establishing vapor-liquid equilibrium during segmented flow. Enriched vapor in equilibrium with liquid is then separated using capillary forces, thus enabling a single-stage distillation operation. As shown in Figure 2, separation of binary liquid mixtures (e.g., methanol (MeOH) and toluene) is made possible by carrying out microfluidic distillation. These experimental results were consistent with phase equilibrium predictions."
Direct Printing of PZT Thin Films for MEMS,"In 2008-2009, we continued our work on thermal ink-jet printing of PZT [1], further optimizing the deposition process and thermal post-processing. Early work showed that modified sol-gel inks often have reduced performance due to porosity, pin holes, and void formation. Multi-layer deposition was investigated as a means to seal voids. Multiple ferroelectric capacitors were fabricated, all with approximately 400nm of printed PZT. Multi-layer films showed consistently improved dielectric properties over single-layer films, with less leakage current and higher resistivity. The continued refinement of the thermal processing profile developed in 2007-2008 lead to a 3hr pyrolysis at 400C followed by a 650C anneal in an O2 environment. These small adjustments improved organic removal, increased film densification, and provided improved piezoelectric response (Figure 2). The remanent polarization of each capacitor was measured as metric for piezoelectric performance. Finally, printing of devices with different thicknesses on a single wafer was demonstrated, something that cannot be accomplished with conventional coating techniques. Future work includes further development of a thermal treatment for multi-layer films. The samples in figure 2 were annealed between each layer, potentially affecting the alignment of the ferroelectric domains between layers. Work on devices in which the entire stack is annealed together is ongoing. Once this annealing is accomplished, thermal ink-jet printing of PZT of the highest dielectric and piezoelectric quality will have been realized."
Nonlinear Pie-shaped MEMS-scale Energy-harvester,"A novel nonlinear pie-shaped thin-film lead zirconate titanate Pb(Zr,Ti)O (PZT) MEMS energy-harvester has been developed. It harvests energy from parasitic ambient vibration via piezoelectric effect and converts it to electrical energy. The new nonlinear pie-shaped design tries to exploit the maximum theoretical power density of PZT for small levels of vibration and wide range of frequencies in a robust way. Contrary to the traditional designs based on cantilever high-Q oscillators which use bending strain, the new design heuristically utilizes the stretching strain in doubly-anchored beams in order to maximize the strain and power. It also provides a wide-bandwidth of operational frequency due to the system’s nonlinearity and enables a robust power generation amid the unexpected change in the vibration spectrum. The device is microfabricated by a combination of surface and bulk micromachining processes in order to use the whole thickness of wafer to form a heavy proof mass. For the structural layers of the beams, 2-µm-thick, high-quality, low-stress silicon nitride is used; it is deposited using low-pressure chemical vapor deposition (LPCVD). Layers of thin-film PZT and ZrO2 as the diffusion barrier are deposited by sol-gel spin-coating, wet-etched, and annealed to form the active area of the device. E-beam deposition and lift-off is used to place interdigitated (IDT) electrodes that extract the generated charge, exploiting the d piezoelectric mode of PZT. Deep reactive-ion-etching (DRIE) from top and back of the wafer patterns the nitride beams and silicon proof mass and finally a XeF2 etching of silicon fully releases the device. Released devices are super-glued on Pin Grid Array (PGA) packages in such a way that the proof mass is located on top of the cavity to give it enough space for motion in response to the base vibration. The pads on the device are wire-bonded to the package’s pads. Devices are heated to 100C and poled at 180kV/cm for 30 minutes using the setup shown in Figure 1. The piezoelectric properties of each device are electrically verified by Polarity/Voltage measurement (Figure 2). Currently, the poled devices are under electromechanical testing to verify their energy-harvesting characteristics."
Templated inkjet Printing for MEMS,"Drop-on-demand (DoD) printing has shown great promise as a low-volume production method for MEMS. A new method for depositing lead zirconate titanate (PZT) piezoelectric thin films via thermal inkjet (TIJ) printing was recently reported by authors [1]. We demonstrated that well optimized printing conditions could provide thickness uniformity with less than 100-nm variation. However, the printed pattern showed more than +/- 10-µm edge (line) roughness, which is far bigger than the necessary minimum feature size for most MEMS devices. In general, the minimum possible line width created by most droplet-based deposition processes has been bigger than ~25 mm due to the possible spot resolution, and 3-5 mm roughness was demonstrated only in a research environment [2]. A pre-fabricated dam or trench can be a solution for defining fine edges by printing, which requires additional dam patterning with lithography or laser trimming and additional post-processing steps for dam structure removal [1], [3].We show that an imprinted self-assembled mono-layer (SAM) template behaves as a wetting/non-wetting barrier for water-based inkjetted droplets and confines water-based inks within the hydrophilic region. The SAM imprinting is done by micro-contact printing with fluorinated thiol ink. The smallest droplet size tested in this work was 3pL, which could define 20-µm line roughness at best. The inkjet droplets were printed between the imprinted square patterns as shown in Figure 1. The pitch between each droplet and the dropping interval were controlled as shown in Figure 2. The left figures show the patterns without imprint guided inkjet printing and the right figures show the printing with template assistance. The pattern with imprint assisted printing shows a line roughness of less than +/- 1µm, which could not be achieved with the current inkjet printing methods."
"A 1-mW, 25-Hz Vibration-energy-harvesting System","This project is part of the Hybrid Insect MEMS (HI-MEMS) program sponsored by the Defense Advanced Research Projects Agency (DARPA). The main objective of this program is to establish the interface between adult neural systems and external electronics. Here, insects are the first test bed, and they will be directed to fly to specific locations in real time via wireless remote control through the external electronics. In order to provide sustainable energy for the controlling on-moth electronics, a local energy-harvesting system is required. The energy-harvesting system has two major parts: the vibration-energy-harvester [1] and the DC-DC boost converter [2]. In the past 12 months, a 1-mW vibration-energy-harvester was designed, fabricated, and tested. Figure 1 shows the harvester. A DC-DC 10-mV to 1-V boost converter has also been designed and is ready for tape out. Figure 2 shows the topology of the boost converter.The vibration-energy-harvester consists of a resonator with moving magnets and a coil. As the resonator vibrates, neodymium iron boron magnets sweep past coils through which power will be harvested. The coils are made with flexible printed-circuit technology to maximize the flux linkage and minimize the coil mass. The harvester was tested on a shaker table, which simulates the vibration of a moth. After testing, 1-mW of time average power was extracted at a mass cost of 1.067g. Work is now underway to significantly reduce the mass of the harvester.The boost converter takes in the AC output voltage of the harvester, rectifies it to a DC voltage and boosts the voltage to 1V. The converter is a two-stage boost converter with off-chip inductors to increase the quality factor and overall efficiency. Due to the low input voltage of the harvester, synchronous rectification using low-power discontinuous comparators is employed. Spice simulation indicates that the converter can achieve 80% efficiency. The power processing switches have been laid out and are currently in the queue to be fabricated in 0.18-um CMOS process."
Development and Application of Distributed MEMS Pressure Sensor Array for AUV object Avoidance,"A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the lateral line sensory organ in fish, which enables some species to form three dimensional maps of their surroundings. The lateral line is a sensory system which measures the flow velocity and pressure distribution over the fish’s surface, enabling behaviors such as collision avoidance [1] and object recognition [2]. These behaviors are related to a particular subset of the lateral line organ, which measures only the pressure gradient [3]. We report progress in fabricating a sensor array capable of measuring similar quantities as the lateral line organ.The system consists of arrays of hundreds of pressure sensors spaced about 2 mm apart on etched silicon and Pyrex wafers. The sensors are arranged over a surface in various configurations. The target pressure resolution for a sensor is 1 Pa, which corresponds to the noiseless disturbance created by the presence of a 0.1-m-radius cylinder in a flow of 0.5 m/s at a distance of 1.5 m. A key feature of a sensor is the flexible diaphragm, which is a thin (20 µm) layer of silicon attached at the edges to a silicon cavity. The strain on the diaphragm due to pressure differences across the diaphragm is measured. At this stage, the individual MEMS pressure sensors are being constructed and tested.The output voltage was measured and the relative change in resistance ∆R/R for the resistors as functions of pressure were calculated (Figure 2). For a diaphragm with a width of 2.82 mm, we obtained the experimental values of (∆R/R)/P are –2.94 × 10-7 Pa, –2.78 ×10-7 Pa, 2.52 × 10-7 Pa and 2.65 × 10-7 Pa. The theoretical value is ±1.07 × 10-7 Pa. There are several explanations for the discrepancy between theory and experiment. Regardless, the sensitivity of the sensor is better than the original expectations."
Integrated Measurement of the Mass and Surface Charge of  Discrete Microparticles Using a Suspended Microchannel Resonator,"Measurements of the mass and surface charge of microparticles are employed in the characterization of many types of colloidal dispersions. The suspended microchannel resonator (SMR) is capable of measuring individual particle masses with femtogram resolution. Here we employ the high sensitivity of the SMR resonance frequency to changes in particle position relative to the cantilever tip to determine the electrophoretic mobility of discrete particles in an applied electric field [1]. When a sinusoidal electric field is applied to the suspended microchannel, the transient resonance frequency shift corresponding to a particle transit can be analyzed by digital signal processing to extract both the buoyant mass and electrophoretic mobility of each particle (Figure 1). These parameters, together with the mean particle density, can be used to compute the size, absolute mass, and surface charge of discrete microspheres, leading to a true representation of the mean and polydispersity of these quantities for a population. We have applied this technique to an aqueous suspension of two types of polystyrene microspheres in order to differentiate them on the basis of their absolute mass and their surface charge (Figure 2). The integrated measurement of electrophoretic mobility using the SMR is found to be quantitative based on comparison with commercial instruments and exhibits favorable scaling properties that will ultimately enable measurements from mammalian cells."
Surface Micromachining via Digital Patterning,"Conventional microelectromechanical systems (MEMS) fabrication relies heavily on the semiconductor manufacturing paradigm. While this model is well-suited for planar devices such as integrated circuits, it is drastically limited in the design and fabrication of three-dimensional devices such as MEMS. From a commercial viewpoint, this paradigm also poorly fits MEMS because the lower market demand makes it harder to offset the high production costs. Ridding MEMS fabrication of its reliance on such techniques may introduce several advantages, namely a wider base of substrate materials as well as decreased manufacturing costs.Our project investigates severing MEMS fabrication from the traditional paradigm via digital patterning technologies. We have previously shown how MEMS can be used for the direct patterning of small molecular organics [1]. Using similar concepts, we have shown that surface micromachining can also be achieved.In 2007-2008, we identified a viable material set for our surface micromachining process’ sacrificial and structural layers: poly-methylmethacrylate (PMMA) and silver nanoparticles. To account for surface non-uniformity of the deposited PMMA, we employed solvent vapors to effectively lower the polymer’s glass transition temperature and cause reflow at room temperatures [2]. To limit surface wetting and increase material loading of the silver nanoparticles, we deposited a PMMA reservoir to contain the silver nanoparticle solution (Figure 1). Free-standing cantilevers were fabricated (Figure 2), confirming that these techniques can be used for a surface micromachining process.The next stage will be to fabricate additional MEMS structures and test the silver nanoparticle’s mechanical properties. These properties will be used to design and fabricate a demonstration system based on our surface micromachining process. Subsequent stages will include creating a library of digital fabrication processes so that entire MEMS devices can be fabricated without the use of semiconductor manufacturing techniques."
Integration of Printed Devices and MEMS,"As part of an overall effort on Non-Lithographic Technologies for MEMS and NEMS, we are de veloping processes for the integration of printed MEMS and devices. The goal of this project is to demonstrate the power of a printed technology for microsystems. We have already developed a surface micromachined cantilever technology that utilizes silver as a structural material and a novel organic spacer. Further, we have developed a family of both inorganic and organic devices that can ulti mately be printed. As an initial demonstration, we are building a MEMS capacitive accelerometer that integrates the silver surface micromachined proof mass and spring with a capacitive sense circuit fab ricated using organic FETs."
The MIT-OSU-HP Focus Center on Non-lithographic Technologies for MEMS and NEMS,"This center is part of a set of centers on MEMS/NEMS fundamentals supported by DARPA. The MIT-OSU-HP Focus Center aims to develop new methods for fabrication of MEMS and NEMS that do not use conventional lithographic techniques. The Center leverages the leading expertise of MIT and OSU in MEMS and printed devices, with the printing expertise of HP. The Focus Center is organized into four primary areas: tools, materials and devices, circuits, and demonstration systems.In the area of tools, we are leveraging the existing thermal inkjet (TIJ) technology of HP and augmenting it with specific additional features, which expand the palette of available materials for printing. We are developing materials and devices over a broad spectrum from active materials and photonic and electronic materials to mechanical materials. In the circuits area, we are studying the behavior of the devices that can be realized in this technology with the goal of developing novel circuit architectures. Lastly, we intend to build several “demonstration” systems that effectively communicate the power of the new technologies that will emerge from this center. In the past year, the center has succeeded in demonstrating a number of the key “building blocks” for a fully printed system. Specifically, we have created printed transistors, printed optical elements (light emitters and photodetectors), printed active materials (piezoelectrics), and a printed MEMS structure (micro-cantilever). Looking forward, we will begin efforts to integrate some of these building blocks."
MEMS Micro-vacuum Pump for Portable Gas Analyzers,"There are many advantages to miniaturizing systems for chemical and biological analysis. Recent interest in this area has led to the cre ation of several research programs, including a Micro Gas Analyzer (MGA) project at MIT. The goal of this project is to develop an in-expensive, portable, real-time, and low-power approach for detect ing chemical and biological agents. Elements entering the MGA are first ionized, then filtered by a quadrupole array, and sensed using an electrometer. A key component enabling the entire process is a MEMS vacuum pump, responsible for routing the gas through the MGA and increasing the mean free path of the ionized particles so that they can be accurately detected.A great deal of research has been done over the past 30 years in the area of micro pumping devices [1, 2]. We are currently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven ac tive-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We have constructed an accurate compress ible mass flow model for the air flow [4] as well as a nonlinear plate deformation model for the stresses experienced by the pump parts [5]. Using these models, we have defined a process flow and fabricat ed five generations of the MEMS vacuum pump over the past years and are currently working on improving the overall design. Figure 1 shows a schematic of the pump. For ease in testing we have initially fabricated only layers 1-3 and have constructed a testing platform which, under full computer control, drives the pistons and monitors the mass flows and pressures at the ports of the device. The lessons learned from the first four generations of the pump have led to numerous improvements. Every step from the modeling, to the etching and bonding, to the testing has been modified and improved along the way. The most recent fifth generation pump test data ap pears in Figure 2. Figure 2a shows the measurements of the vacuum being generated in an external volume (5.6cm3) by the micropump operating at 2Hz. The pump was able to reduce the external volume pressure by 163 Torr. Figure 2b shows the micropump-generated flow rate as a function of pumping frequency (driven in a 6-stage cycle by a controlling microprocessor to move the gas from the input to the output). The performance of this pump compares very well with that of other similar scaled micropumps in the literature. Next, we plan to fabricate and test an improved overall design and develop a final set of models to fabricate any future micropumps to the de sired specifications."
Phase-change Materials for Actuation,"Phase-change materials (chalcogenide alloys) are used for optical data storage in commercial phase-change memories, such as rewritable compact discs (CD±RW) and rewritable digital video disks (DVD±RW, DVD-RAM). Recently, they have also shown high potential for the development of phase-change random access memories (PC-RAMs or PRAMs), which might replace flash memories in the future. In this project, we suggest a different application of phase-change materials in optically triggered micro actuators [1]. The suggested device consists of a thin film of a phase-change material deposited on a micro-fabricated low-stress SiN cantilever. The SiN cantilevers are manufactured by chemical vapor deposition of low-stress SiN on Si wafers, patterning the SiN film using optical lithography and revealing the cantilevers using dry etching and wet etching. Amorphous thin films of phase-change materials are subsequently sputter-deposited on these cantilevers. A laser-induced crystallization in the film initiates a cantilever deflection since this transformation is accompanied by a large density change at the order of 6-9%. Then we will re-amorphize the crystalline part of the film by short laser pulses, and the cantilever tip should return to its initial position. Both the amorphous and crystalline states of phase-change materials are stable at room temperature, and the resulting device can serve as a bi-stable micro actuator.We have also used a similar technique to investigate the stress change as a function of film thickness and capping layer [2]. This approach can be used in optimization of chalcogenides for use in PRAMS.In addition to chalcogenides, the cantilevers used with combinatorial deposition have been used to investigate the crystallization-induced stress for a metallic amorphous alloy system (Cu-Zr). It was discovered that the magnitude of the stress change scaled with the ease of glass formation, yielding fundamental new insight into the materials requirements for amorphization [3]."
Origin and Control of intrinsic Stresses in  Metallic Thin Films for N/MEMS Applications,"Because mechanical properties strongly influence the reliability and performance of films in N/MEMS applications, understanding and controlling of the intrinsic stresses in as-deposited films is of great importance. For high-atomic-mobility metals (e.g., Au, Ag, Al, Cu) deposited on amorphous substrates, much of the observed tensile stress can be attributed to grain structure evolution during which individual islands grow, impinge, and coalesce to form a continuous film. The stress state shifts from tensile during island coalescence to compressive as the film grows past continuity (see Figure 1).The origin of post-coalescence compressive stress has been debated extensively over the past decade. Models associated with adatom-surface [1], [2] and adatom-grain boundary [3] interactions have been proposed to explain the compressive stress generation during deposition and its relaxation during interruptions of growth. Using an in-situ stress measurement system and ex-situ TEM characterization, we have experimentally shown that, for films with the same thickness, grain size has an impact on stress behavior during a growth interruption. The relationship between the inverse of grain size and the corresponding reversible stress rise was found to be linear, with zero stress for heteroepitaxial film (interpreted as films with “infinite” grain size) (see Figure 2) [4]. This experimental result strongly indicates that the microstructure of the as-deposited film, especially the grain boundary, is critical to the origin and control of intrinsic compressive stress in these films.Current investigations are focused on analysis of the effects of processing conditions, e.g., substrate temperature and deposition rate, on the magnitude of the residual stresses in polycrystalline films We are also investigating the use of substrate topography to control island formation and stress evolution."
Microfluidic Perfusion for Modulating Stem Cell Diffusible Signaling,"Stem cell phenotype and function are influenced by microenvironmental cues comprised of cell-cell, cell-extracellular matrix (ECM), cell-media interactions, and mechanical forces. Although conventional cell-culture techniques have been successful, they provide incomplete control of the cellular microenvironment. Our research focuses on developing microscale systems for controlling the cellular microenvironment of mouse embryonic stem cells (mESCs) to control their function. To modulate cell-media interactions, we have developed a two-layer PDMS microfluidic device that incorporates a valve architecture, debubblers, and cell culture chambers, allowing for a rich set of culture conditions on the same chip [1-3]. We are using our microfluidic system to determine the minimal media sufficient for mESCs to maintain their self-renewal characteristics under constant flow. Upon growing mESCs in defined, serum-free media conditions under perfusion, we have observed a change in the preponderance and the heterogeneity of stem cell markers. Using a combination of assays, we have observed similar or upregulated levels of the stem cell marker Nanog, as well as a more stem cell-like morphology of cells under perfusion (Figure 1). The use of ESCs for clinical therapeutic applications requires expansion of the pluripotent cells. This usually necessitates the use of a bioreactor where the cells are subjected to mechanical forces: fluid shear stresses [4]. We are quantitatively investigating the effect of fluid shear stress on ESC self-renewal by using a 1x6 logarithmic flow rate microfluidic device. By specifying the dimensions of the flow rate-setting resistor channels, we were able to apply shear stress varying by a factor of 4 across chambers, enabling us to simultaneously study shear stress effects on mESC self-renewal over a range of 1024× (Figure 2a). Initial results show that mESC proliferation is negatively correlated to shear stress over a range of 0.016 to 16 dynes/cm2 (Figure 2a).ESCs dynamically interact with their extracellular matrix (ECM) and culture substrate. In particular, different substrates adsorb ECM differently, which in turn affects cell attachment and function. Standard culture techniques typically utilize tissue culture polystyrene (TCPS), a treated polystyrene substrate that promotes ESCs attachment. We developed a process that integrates micro-patterned polystyrene onto glass substrates, combining the cell culture compatibility of polystyrene with the fabrication compatibility of glass (Figure 2b). This process integrates cell culture surfaces directly within a device and preserves the standard microfluidic assembly process of plasma bonding. We have demonstrated a simple technique for realizing multi-functional polystyrene patterns for the fabrication of complex, highly integrated microfluidic cell culture platforms."
Microfluidic Control of Cell Pairing and Fusion,"Currently, several different methods have been used to reprogram somatic cells to an embryonic stem-cell-like state, including somatic cell nuclear transfer, forced expression of transcription factors, and cell fusion. Cell fusion is an appealing method by which to study reprogramming as the delivery of cells is easily visualized. However, conventional methods to fuse cells en masse do not control the pairing between the cell populations, resulting in heterogeneous output populations that must be further purified.We have developed a microfluidic system in which thousands of ESCs and somatic cells (SCs) are properly paired and immobilized, resulting in a high number of one-to-one fusions that can be clearly identified for further studies [1]. The device consists of thousands of microscale cell traps in a millimeter-sized area. The traps consist of larger frontside and smaller backside capture cups made from a transparent biocompatible polymer. The key to pairing cells efficiently is to load them sequentially in a 3-step loading protocol enabling capture and pairing of two different cell types (Figure 1). The geometry of the capture comb precisely positions the two cells, and flow through the capture area keeps the cells in tight contact in preparation for fusion. With this approach we have obtained pairing efficiencies of ~70%. The device is compatible with both chemical and electrical fusion, and, in agreement with the literature, we have obtained higher performance with electrofusion. When we compared fusion performance in our device to commercial approaches, we obtained significant improvements in overall performance for both PEG-mediated fusion and electrofusion. Specifically, we have measured fusion efficiencies of ~80% in our device using electrofusion, about 5× greater than that obtained in commercial systems. We are also able to remove fused cells from the device and culture them, demonstrating that the device creates viable fused cells (Figure 2a-b). Finally, by fusing mouse embryonic stem cells (mESCs) with mouse embryonic fibroblasts (mEFs, a somatic cell type), we have demonstrated the ability to reprogram the somatic cells to a pluripotent state as evidenced by morphology, alkaline phosphatase staining (Figure 2c), and activation of an oct4-GFP reporter present in the somatic cell genome (Figure 2d)."
Flexible Multi-site Electrodes for Moth Flight,"Significant interest exists in creating insect-based Micro-Air-Vehicles (MAVs) that would combine advantageous features of insects—small size, effective energy storage, navigation ability—with the benefits of MEMS and electronics—sensing, actuation and information processing. The key part of the insect-based MAVs is the stimulation system, which interfaces with the nervous system of the insect to bias the insect’s flight path. In this work, we have developed a flexible split-ring electrode (FSE) for insect flight control; the FSE uses a set of electrodes arranged around a split ring to provide circumferential stimulation around an insect’s nerve cord (Figure 1). The FSE is made of two layers of polyimide with gold sandwiched in between in a split-ring geometry using standard MEMS processing. The stimulation sites are located at the each end of protruding tips that are circularly distributed inside the split-ring structure. These protruding tips penetrate through the cuticle tissues of the nerve cord and enable stimulation on the axon-rich region of the nerve cord. We have been able to insert the electrode into pupae of Manduca sexta as early as 7 days before the adult moth emerges, and we are able to stimulate multi-directional graded abdominal motions in both pupae and adult moths. The direction of the abdominal movements depends on the particular pair of stimulation sites excited. The pupal implantation allows for tissue growth around the FSE before the adult moth emerges, which enhances the attachment of the FSE. Also, as compared to the adult moth, the body of the pupae is relatively immobile, easing the difficulty of insertion surgery. Finally, we have demonstrated that the FSE is able to stimulate abdominal motion that can in turn cause ruddering to alter adult moth flight path (Figure 2) [1]."
Measuring the Effects of Electric Fields on Cell Phenotype,"One overarching goal of our research group involves using electric fields to manipulate, position, and ultimately sort living biological entities [1], [2]. To enable such exquisite control over living organisms, we leverage a technique called dielectrophoresis (DEP), which uses spatially non-uniform electric fields to “push” or “pull” cells towards or away from electrodes. The processing of biological samples is more readily achieved using systems on the length-scale of the samples themselves. Such biological microelectromechanical systems, or BioMEMS, enable integrated sample preparation and analysis; they leverage techniques such as DEP to enable cell manipulation. Hence, it is imperative that we understand the effects of DEP manipulation on cell physiology to determine whether DEP manipulation itself can alter particular phenotypes of interest and confound downstream biological assays. To this end, we have developed a microfabricated, high-content screening (HCS) platform that can apply a large number of different electrical stimuli to cells and then monitor the molecular effects of those stimuli using automated fluorescence microscopy. The platform consists of a chip with individually addressable arrayed electrodes and support electronics to generate the desired waveforms (Figure 1). Mammalian cells are seeded on the chip and then the entire assembly is clamped and placed in a standard cell culture incubator, where a computer-controlled custom-designed switch box automatically and autonomously applies arbitrary stimulation waveforms (varying voltage, frequency, and duration) to individual electrode sites. Since this platform uses transparent electrode structures, it can equally be used with both inverted and fluorescent microscopy techniques. Using this HCS platform, we have been able to elucidate the response of cells to electric fields using a custom-designed live-cell stress sensor. This stress sensor was designed using transfection and cloning techniques, and it forms the basis for the read-out of our biological assay. Stressful events in the environment around the cells, such as temperature elevation (due to Joule heating) and the generation of oxygen radicals are sensed by our stress sensor and reported as a distinct fluorescence level. These fluorescent signals are collected for individual cells using automated microscopy and quantified using image-processing algorithms. The results obtained from one such set of experiments are displayed in Figure 2 (adapted from [3]). This HCS platform enables the molecular-level biological assays across a very wide range of electric field conditions, a feat challenging to accomplish with previously developed systems or assay platforms."
Image-based Sorting of Cells,"This research involves the development of architectures for screening complex phenotypes in biological cells. We augment microscopy with the ability to retrieve cells of interest. This capability will permit cell isolation on the basis of dynamic and/or intracellular responses, enabling new avenues for screening. Currently, such sorts require expensive, specialized equipment, widely prohibiting such sorts.We have explored microfabricated/microfluidic approaches to cell sorting. These approaches employed purely dielectrophoretic (DEP) trap arrays [1], passive hydrodynamic trap arrays with active DEP-based cell release [2], and passive microwell arrays with optical cell release to permit sorting of non-adhered cells [3]. We recently developed a photolithography-inspired method that allows sorting of adherent cells without the use of microfluidics [4], illustrated in Figure 1. Here we plate adherent cells in a dish and assay them, identifying the locations of cells of interest. We then use a computer and standard office printer to automatically generate a transparency mask. After alignment of the transparency mask to the back of the cell culture dish, opaque mask features reside beneath desired cells. We then add a prepolymer to the dish, containing cell culture media, a UV-photoinitiator, and poly(ethylene glycol) diacrylate (PEGDA) monomer. Next we use a standard fluorescence lamp to shine UV light through the mask, crosslinking a hydrogel over all unmasked locations and encapsulating all undesired cells. Desired cells can be enzymatically released (Figure 2) and re-captured. Our sorting process requires standard equipment found in biology labs and inexpensive reagents (<$10 per experiment), simplifying widespread adoption. We have demonstrated cell release from 500-µm-diameter wells, as well as the isolation of perfectly pure, viable target cells from a background population of undesired cells. Further efforts will reduce well size, enabling the sorting of denser cell populations. The simplicity and inexpensiveness of our method will allow for widespread dissemination and new cell sorting paradigms."
Cell Micropatterns for Studying Autocrine Signaling,"Autocrine signaling plays a key role in tumorigenesis and in the maintenance of various physiologic states. Due to its intrinsic, closed-loop nature, autocrine signaling is, however, difficult to investigate experimentally. Our research involves the use of cell- patterning techniques to investigate the role of autocrine signaling during in vitro maintenance of embryonic stem cells, stem cell differentiation, and uncontrolled expansion of cancer cells.First we use stencil cell patterning to examine the spatial distribution of autocrine systems. Typical techniques to quantify autocrine signaling rely on bulk measurement of autocrine pathway activation using randomly plated cells. Such random cell positioning usually masks the effects of local ligand concentration gradients, reducing the chance to observe spatially varying cell responses. We fabricated regular arrays of cell patches with varying colony size and spacing and generated graded levels of autocrine ligands in space while maintaining the same global ligand concentration (Figure 1A). Using the TGFα/EGFR paradigm in A431 cells as our model, we have determined the effective length scale where autocrine signaling contributed to promote growth of adjacent cell patterns (Figure 1B) [1]. We are applying the developed platform to determine the contribution of autocrine signaling in preserving a homogeneous population of mouse embryonic stem cells (mESCs) in vitro.Expanding on our previous work on Bio Flip Chips, we have used them to create patterns of single cells at varying densities [2]. We then studied the effects of plating density on the colony-forming efficiency of mESCs and found that the colony-forming efficiency increases with density (Figure 1C). We have confirmed this result by performing growth assays in a traditional well-plate format and in a defined medium. In this second set of assays, we found that the growth of mESCs increases with density (for a certain range), both in the first 24 hours and in the next 24 hours after plating of cells (Figure 1D). Finally, we checked that medium that has been conditioned by cells enhances the growth of mESCs. Together, these results prove that mESCs produce at least one diffusible factor that aids survival.In addition to localization of a single cell type on the substrate, we have also developed a novel technique to fabricate complex heterotypic patterns-within-patterns [3]. Stencil-delineated electroactive patterning (S-DEP) combines dielectrophoresis (DEP) and stencil patterning to create cell clusters with customizable shapes, positions, and internal cell organization (Figure 2). Stencils define overarching tissue-like construct geometries, and negative-dielectrophoretic forcing guides subgroupings of cells to desired positions within constructs. The S-DEP enables correlation of cells’ cluster location to phenotype and provides avenues for creating mosaic tissue-like constructs of phenotypically or genetically distinct cells. Such diversified chimeric cell clusters help us evaluate the impact of diffusive signaling on stem-cell differentiation."
Iso-dielectric Separation of Cells and Particles,"The development of new techniques to separate and characterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. Continuing or improving upon this trend – for example, by developing new avenues for performing genetic and phenotypic screens – requires continued advancements in cell sorting technologies. Towards this end, we are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object [1]) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [2],[3]. While dielectrophoresis has been widely used in cell separation [4], iso-dielectric separation offers a unique combination of features that could be potentially enabling for new genetic screens. It is continuous-flow, capable of parallel separations of multiple (>2) subpopulations from a heterogeneous background, and label-free. Additionally, in contrast to many other separation techniques, IDS leverages physical interactions between particles as they are separated to achieve better performance, and it is thus ideally suited to operation at high particle concentrations with correspondingly high throughput (Figure 2A). Finally, using IDS as a tool for cell characterization could identify electrical phenotypes and map them to specific genes. This improved understanding of the relationship between a cell’s genotype and its physical properties is critical for developing new screens. We have demonstrated the separation and characterization of particles ranging from polystyrene beads, to the budding yeast Saccharomyces cerevisiae, to mouse pro B cells (Figure 2B), representing three orders of magnitude in particle volume (~1-1000 µm3) and conductivity (~0.001–1 S/m) [5]."
Fully integrated Air Pumped Heat Exchanger (PHUMP),"The ever-increasing computational power of modern electronics entails an associated increase in heat generation in the chip; microprocessors without a thermal management system are easily capable of melting themselves. Exotic thermal management systems such as liquid cooling allow high thermal power densities but require large volumes and complex implementations. The Fully Integrated Air-Pumped Heat-Exchanger (PHUMP) heat sink allows this cost-effective technology to keep pace with the cooling demands of the advancing electronics industry.The PHUMP will provide reduced thermal resistance and reduced power demand in a compact volume. It will be designed to operate in a range of thermal and mechanical shock environments, for an extended period of time. These goals will be achieved by incorporating heat pipes into the extended surface of the heat sink as well as incorporating fan rotors along each wall of the extended surface to maximize heat transfer. Heat pipes are enclosed systems that have a very high effective thermal conductivity by generating a two-phase flow in a working fluid contained within them [1], [2]. The improved heat transfer to the extended surface allows the PHUMP to operate at lower speeds and generate less mass flow than traditional air-cooled heat sinks. This improved heat transfer reduces the power required to turn the fan and allows the PHUMP to achieve high coefficients of performance."
Model-based Design of MEMS Vibration-energy-harvesters for Wireless Sensors,"The recent development of “low power” (10s-100s of µW) sensing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of individual wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of structural health monitoring (SHM), particularly for aircraft. This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam structures have been developed to predict the electrical and mechanical performance obtainable from ambient vibration sources. The optimized models have been verified by comparison to tests on a macro-scale device both without [1] and with a proof mass at the end of the structure (Figure 1) [2]. A non-optimized, uni-morph beam prototype (Figure 2) has been designed and fabricated [3], [4]. Design tools to allow device optimization for a given vibration environment have been under detailed investigation considering various geometries of the device structures and fabrication constraints, especially in microfabrication. Future work will focus on fabrication and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. System integration and development, including modeling the power electronics, will be included."
A Tabletop Deep Reactive Ion Etching System for MEMS Development and Production,"A general rule of thumb for new semiconductor fabrica-tion facilities (fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for groups seeking to commercialize new semi-conductor devices aimed at smaller market segments that require a dedicated process. To eliminate this cost barrier, we are working to create a suite of tools that will process small (~1”) substrates and cost less than $1 million. This suite of tools, known colloquially as the 1” Fab, offers many advantages over traditional fabs. By shrinking the size of the substrate, we can realize sub-stantial savings in material usage, energy consumption, and, most importantly, capital costs. This substantial reduction in capital costs will drastically increase the availability of semiconductor fabrication technology and enable experimentation, prototyping, and small-scale production to occur locally and economically. The first 1” Fab tool we have developed is a deep reactive ion etcher (DRIE). DRIE tools are used to create highly anisotropic, high aspect-ratio trenches in silicon—a crucial element in many MEMS processes that will benefit from a 1” Fab platform. A labeled image and rendering of the 1” Fab DRIE is shown in Figure 1. The modularized design of our DRIE system can be easily adapted to produce other plasma-based etching and deposition tools (like PECVD and RIE). Using the switched-mode Bosch Process, the 1” Fab DRIE system currently can achieve silicon etch rates up to 6 µm/min with vertical sidewall profiles, an estimated photoresist selectivity greater than 50:1, and etch depth non-uniformity to less than 2% across the substrate. Several examples of anisotropic etches performed with our system are included in Figure 2. Presently, we are working to refine the mechanical design of the system and optimizing recipes for high-aspect ratio etching."
A Miniature MEMS Vacuum Pump with Curved Electrostatic Actuation,"Portable sensing devices such as microscale mass spec-trometers need vacuum pumping to lower samples at atmospheric pressure to the desired measurement pressure range. Further improvements for MEMS accelerometers, gyros, and other resonant sensors require internal pressures as low as a few microtorr, which is possible only with active vacuum pumping. While these pressures are easily achieved using mac-roscale vacuum pumps, the larger pumps are not por-table, negating the benefits gained from making small, low-power sensors in the first place. To realize the full potential of portable sensors, a chip-scale vacuum pump needs to be developed.We are developing what is to our knowledge the first two-stage MEMS displacement pump with integrated electrostatic actuation. Two pump stages, along with an efficient layout that minimizes dead volume and a new actuation scheme, should enable it to reach pressures below 30 Torr. Actuation is achieved by electrostatically zipping a thin flexible membrane down onto a stiff curved electrode. This actuator topology allows for large displacements and large forces at relatively low voltages (< 100 V). An image of a fabricated two-stage micropump is shown in Figure 1 below.We have developed two methods for producing curved electrodes in MEMS devices: 1) hot air trapped during wafer bonding expands with enough pressure to plastically deform a thin silicon membrane and 2) strain induced when epoxy cures can pull a membrane into a curved shape. We have demonstrated that we can reliably and repeatably zip a thin membrane using these curved electrodes at low voltages and we have mapped out how the critical voltage depends on the deformation magnitude and the oxide thickness. Finally, we have developed models to predict the extent of plastic deformation and the onset of pullin for these curved electrostatic electrodes. A comparison of the model and experimental data is shown in Figure 2 below."
Additive Manufacturing of Three-Dimensional Microfluidics,"In many cases, microfluidics are manufactured in clean-rooms using semiconductor industry processes and materials, making them fairly expensive to produce. In addition, the device architecture is often a compro-mise between what should be made based on model-ing and what can be made based on the planarity and thickness/depth limitation of most microfabrication processes. Moreover, a change of any of the in-plane features of the design typically requires the fabrication of one or more new lithography masks, incurring sub-stantial costs and time delays. A manufacturing tech-nology that can circumvent these difficulties without sacrificing device performance would greatly extend the kind of devices that can be made and the kind of commercial applications beyond research, high-end products, and large-volume products that can satisfied by microfluidic chips.Additive manufacturing is a group of layer-by-layer fabrication methods that use a computer file to generate solid objects. Additive manufacturing started as a visualization tool of passive, mesoscaled parts; however, given the recent improvements in the resolution capabilities and cost of commercial 3D printers, additive manufacturing has recently been explored as a fabrication technology that could address the complexity of certain microsystems, e.g., microfluidics.We are exploring the use of stereolithography to manufacture freeform microfluidics with three-dimensional hydraulic networks with features (range of dimensions, aspect ratio, morphology) that would be very hard to make using standard microfabrication processing. Stereolithography is an additive fabrication process that uses a computer file (Figure 1) to manufacture structures based on spatially controlled solidification of a liquid resin by photo-polymerization. For example, we have developed fabrication process flows for the creation of three-dimensional structures that can be used as multiplexed, externally fed electrospray emitter arrays (Figure 2); these structures have a minimum feature size and emitter density comparable to reported single-crystal silicon multiplexed electrospray devices. Current work focuses on exploring the resolution limits and capabilities of the 3D printing process, as well as in demonstrating working microfluidic chips."
Piezoelectric Nonlinearity in GaN Lamb Mode Resonators,"This paper reports on the measurement of nonlinear-ity in GaN Lamb mode resonators subjected to power levels between 10 and +10 dBm. In these devices, non-linearity manifests itself as both frequency shift (Δf/f of 60-128 ppm) and change in motional impedance (ΔRm/Rm of 13-33%). In this work, we decouple the contributions from self-heating and strain-induced piezoelectric nonlinearity to ΔR/R , and conclude that strain-induced change in piezoelectric coefficients Δe31 and Δe33 is the dominant cause of ΔR/R , accounting for 31% of the total 33% observed shift. The result is consistent with 2nd order nonlinear coefficients previ-ously derived analytically.Whether for use in radio filters or in frequency references, the MEMS resonator’s capability to handle large RF power is crucial for system performance. It is therefore important to understand any nonlinearity in piezoelectric MEMS resonators. Studies have shown that self-heating is a primary contributor to frequency shift in AlN Lamb mode resonators. In this paper, we show that GaN Lamb mode resonators (Figure 1) are subject not only to frequency shift (Δf) from self-heating, but also to an increase in motional impedance (ΔRm) with increasing power levels due to a significant nonlinearity in piezoelectric coefficients (Figure 2). After ruling out these two factors, we conclude that the amplitude-induced Δe31 and Δe31 are the dominant contribution to ΔRm, consisting about 31% of the total 33% change.The paper also concludes that self-heating is the main cause of frequency shift and nonlinearity in piezoelectric coefficient will dominate IIP3 ( the input power at the third-order intercept point ), an importance specification for weakly nonlinear devices in RF communication"
Controlled Fabrication of Nanoscale Gaps using Stiction,"As dimensions are continuously scaled down to achieve devices with higher performance and novel principles, developing methods for the controlled fabrication of nanogaps is important for enabling functional devices. Nanogaps are particularly critical for advancements in nanoelectromechanical systems (NEMS) and molec-ular electronics. Various methods of fabricating such gaps have been reported in the literature. However, these approaches are developed mainly for two-termi-nal devices, involve multiple processing steps, and com-monly lack robustness, thus limiting their applications. In this work we present an approach to controlled fabrication of nanoscale gaps through use of stiction, i.e., permanent adhesion between device components, an otherwise common mode of failure in electromechanical systems. In this scheme, laterally actuated cantilevers are patterned through electron beam lithography in polymethyl-methacrylate (PMMA). During the wet-developing process, the cantilever (labeled Electrode 1 in Figure 1) undergoes deflection due to the capillary forces, permanently adhering (stiction) to the opposing structure (Electrode 2). The deflection and stiction promote formation of nanogaps, smaller than originally patterned, between the cantilever and opposing electrode. Lastly, gold (Au) is evaporated onto the substrate defining the metallic electrodes onto the PMMA structures. The Au evaporation further reduces the gap size depending on the thickness of the film. The extent of deflection and its profile can be controlled through balancing the surface adhesive forces by altering the device geometry such that desired widths are achieved. The tunability of the gap size through device design is shown in Figure 2, where relative placement of the electrode with respect to the point of stiction defines the widths of the gap achieved. Furthermore, through modifications of device design, the nanogaps can be optimized to be electromechanically tunable or filled with molecular layers making them suitable for applications in tunneling electromechanical switches, nanoelectromechanical systems, and molecular electronics."
Printed MEMS Membrane Electrostatic Microspeakers,"This work reports the fabrication and operation of elec-trostatic microspeakers formed by contact-trans fer of 125-nm-thick gold membranes over cavities pat terned in a micron-thick silicon dioxide (SiO2) layer on a conducting substrate. Upon electrostatic actuation, the membranes deflect and produce sound. Addition ally, membrane de-flection upon pneumatic actuation can be used to monitor pressure. The microspeaker fabrication process reported enables fabrication of MEMS diaphragms without wet or deep reactive-ion etching, thus obviating the need for etch-stops and wafer-bonding. This process enables mono-lithic fab rication of multiple completely enclosed drum-like structures with non-perforated membranes to dis place air, in both individual-transducer and phased-array geometries. We characterized the mechanical deflection of the gold membranes using optical interferometry. The membranes show a repeatable peak center deflection of 121±13 nm across gaps of ~25 microns at 1 kHz sinusoidal actuation with 60 V peak-to-peak amplitude and a 30 V DC bias (Figure 1). The acoustic performance of the microspeakers is characterized in the free field. Sound pressure level of the microspeaker increases with frequency at 40 dB/decade (Figure 2), indicating that its sound pressure output is proportional to the acceleration of its diaphragm, as expected in the spring-controlled regime for free field radiation. The microspeaker consumes 262 μW of real electric power under broadband actuation in the free field and outputs 34 dB(SPL/Volt) of acoustic pressure at 10 kHz drive. The silicon wafer substrate (~500 μm thick) dominates the total thickness of the microspeakers; the active device thickness is less than 2 μm. These thin microspeakers have potential applications in hearing aids, headphones, and large-area phased arrays for directional sound sources."
Electromagnetic Imaging of Nanostructures,"This objective of this project is to develop a system to perform high bandwidth, subsurface, electromagnet-ic imaging of microfabricated devices. The intent is to simultaneously detect surface topologies, buried con-ductors/insulators, and doped regions. The proposed system promises to offer very high measurement band-width, enabling rapid measurement of large areas with high resolution which is critical to the time-efficient scanning of complex semiconductor wafers.Our imaging approach is based on high-frequency impedance measurements through an array of electrodes capacitively coupled to a microfabricated device. As the electrode array is scanned over the device surface, the resulting impedance variations will be measured and transformed into a 3D tomographic map of the near-surface spatial distributions of the sample permittivity and conductivity. Also, nonlinearities in the current/voltage relationship of P-N junctions allow detection of the dopant boundaries by measuring the harmonic distortion. We plan to drive the electrodes with GHz excitation frequencies, and maintain the electrode array at a submicron flying height above the semiconductor surface. High excitation frequencies are necessary for the electric field generated from the sensor array to penetrate the silicon substrate in sufficient depth, thereby being coupled to the sub-surface features. The imaging system will consist of a MEMS probe head, precision mechatronics, and RF electronics. The probe head will be fabricated from an array of gold electrodes that will be sandwiched between guard electrodes to prevent stray fields from interfering with the capacitance measurement; see Figure 1 for details. These probes will then fan out back to a vector network analyzer (VNA) which measures the impedance of each probe tip at high frequencies (0.5 GHz – 6 GHz). Different excitation patterns may applied from the VNA to the gold probes to control the depth of penetration of the electric fields into the nanostructure to be imaged. RF electronics will be used to mitigate losses at high frequencies while guarding against unwanted stray electric fields. Finally, an inversing imaging algorithm will be developed to compute a final image from the measured impedance data.For the experimental setup, the test sample is mounted onto an air bearing spindle and the probe will be placed perpendicularly to the sample, as in Figure 2. Next, the spindle is rotated at a predefined angular velocity and the change in impedance, as the probe tip passes over the test sample, is measured by the RF equipment. After the data is collected, it is processed using the inverse imaging algorithm to output a map of the material composition of the test structure."
Purification of High Salinity Brine by Multi-Stage Desalination via Ion Concentration Polarization (ICP),"There is an increased need for the desalination of high concentration brine (> TDS 35,000ppm) efficiently and economically, either for the treatment of produced wa-ter from shale gas/oil development, or minimizing the environmental impact of brine from existing desalina-tion plants. Although electro-membrane desalination (e.g., electrodialysis) has been underestimated and considered as a limited technology for brackish water treatment, we have found its multiple advantages for brine treatment. Based on our earlier works (Figure 1) showing better salt removal and energy efficiency than conventional electrodialysis (ED), we demonstrates technical and economic viability of ion concentration polarization (ICP) electrical desalination for the high saline water treatment by adopting a novel multi-stage operation. According to our analysis with a miniatur-ized microfluidic platform (Figure 2a), one can achieve competitive water cost (~$1/bbl) of highly concentrat-ed brine desalination by optimizing the energy use by adopting the strategy of incremental, multi-stage salt removal in electrical desalination (Figure 2b). We also demonstrate that ICP desalination has the advantage of removing both salts and diverse suspended solids simultaneously, and of less susceptibility to membrane fouling/scaling, which is a significant challenge in any membrane processes."
Enhanced Flow Boiling in Microchannels via Incorporated Surface Structures,"The increasing power densities in various electronic de-vices including concentrated photovoltaics, power elec-tronics, and laser diodes pose significant thermal man-agement challenges for the electronics industry. The use of two-phase microchannel heat sinks to cool high-per-formance electronic devices is attractive because they harness the latent heat of vaporization to dissipate high heat fluxes in a compact form factor. However, the chal-lenges with such a scheme are associated with flow in-stability and the need to increase the critical heat flux (CHF), which is the highest heat flux the device is capa-ble of dissipating before heat transfer failure. Recently, incorporating micro/nanostructures onto the surfaces of the microchannels has opened up new opportunities for performance enhancement. Here we investigate the role of surface microstructures on flow boiling heat transfer in microchannels. We designed and fabricated microchannels with well-defined silicon micropillar arrays (heights of ~25 μm, diameters of 5-10 μm and pitches of 10-30 μm) on the bottom heated channel wall. The design decouples thin film evaporation and nucleation by promoting capillary flow on the bottom heated surface while facilitating nucleation from the sidewalls. The structured surface microchannels showed significantly reduced temperature and pressure drop fluctuation. Visualization of the flow indicates that the micropillar surface can promote capillary flow and enhance flow stability and heat transfer by maintaining a stable annular flow, which resulted in high-performance thin film evaporation and an enhanced critical heat flux. The fabricated devices achieved significantly enhanced heat transfer coefficient (40%) compared to that without micropillars, and a maximum CHF value of 720 W/cm2 was achieved on a structured surface microchannel (diameters of 5 μm and pitches of 15 μm). The experimental results suggest that capillary flow can be maximized without introducing large viscous resistance when the microstructure geometry is optimized. This work is a first step towards guiding the design of stable, high-performance two-phase microchannel heat sinks."
Experimental Characterization of Thin-Film Evaporation from Silicon Micropillar Wicks,"To the credit of Moore's Law, the exponential rise in the number of transistors in a single chip, the increase in clock speed and functionality, and the continual overall size reduction in device architecture of electronic devices have generated concentrated heat loads in excess of 100 W/cm2. Furthermore, this heat flux is projected to exceed 300 W/cm2 in a few years [1] creating a thermal management challenge. While enhanced air convection cooling strategies have done the job in the past, direct extension of the state-of-the-art air cooling technology is inadequate to remove heat loads in excess of 100 W/cm2. As a result, novel thermal management solutions such as thin-film evaporation [2] that utilize the latent heat of vaporization as the working fluid changes phase from liquid to vapor are required to mitigate this thermal management challenge.In this work, we have experimentally characterized thin-film evaporation from silicon micropillar wicks. The micropillars were created using contact photolithography and deep-reactive ion etching. For integrated testing and measurement, a thin-film heater and microsensors were incorporated using e-beam evaporation and acetone lift-off. The microsensors measure local temperature while the heater emulates the heat generated in electronic devices. The experiment was conducted in a vacuum chamber and de-ionized water was passively transported to the evaporator surface via capillary-wicking (Figure 1). The water was syphoned into the microstructured surface from the surrounding reservoir in response to the input heat flux. Steady state thin-film evaporation in the absence of nucleate boiling was demonstrated. The liquid meniscus recedes and the microstructured surface dries out when the capillary wicking mechanism cannot deliver sufficient liquid to sustain the evaporation by overcoming viscous losses. Dryout heat fluxes of ≈46 W/cm2 were dissipated at 19°C superheat (Figure 2) over a 1cm×1cm microstructured area and the effects of micropillar wick geometry were captured through systematic study. Experimental results show that the dryout heat flux scales with micropillar wick thickness. Furthermore, for a given micropillar wick thickness, an optimum pillar diameter and spacing is identified which maximizes the capillary-limited evaporation dryout heat flux. Our study provides mechanistic understanding of the liquid transport and heat transfer processes of thin-film evaporation from well-defined micropillar wicks."
Elementary Framework for Cold Field Emission: Emission from Quantum-Confined Emitters,"Cold field emission is the emission of electrons from a metal at T=0K, induced by an electrostatic field. Field emitted current density (ECD) is traditionally predict-ed with the Fowler-Nordheim (FN) equation, which assumes a bulk, planar, metal emitter. Due to the en-hancement of a static electric field at highly curved sur-faces (lightning rod effect), the conventional strategy for increasing the ECD is to fabricate ever smaller and more highly-curved emitter tips. However, for suitably small field emitters, the effects of quantum confine-ment (QC) at the emitter tip may play a significant role in determining the total ECD since the specific shape of a quantum system determines the its electronic wave functions and distribution of energy levels. In order to study the competing effects of a reduced electron supply due to QC and increased electron transmission probability from local field enhancement, our previ-ously developed elementary framework for cold field emission has been reformulated to treat emission from non-planar surfaces of QC metal emitters.The framework was employed to derive ECD equations for emission from the planar surface of a normally unconfined (NU) 1D cylindrical nanowire (CNW) and the curved side of a normally confined (NC) 1D CNW, which are illustrated in Figure 1. The energy level spacing, energy level degeneracy, and transverse zero-point energy unique to each emitter geometry led to certain geometries producing larger ECDs than others under equivalent conditions. The close energy level spacing and lack of a transverse zero-point energy in the NC CNW geometry led to exceptionally large ECD peaks, an average ECD that exceeded the FN limit at typical values of EF, and an increasing trend in the ECD with decreasing emitter dimensions in the presence of field enhancement, which is shown in Figure 2. These results suggest that highly curved emitter geometries may be ideal for emission from the standpoint of not only tip electrostatics, but also the electron supply. Current work includes the application of the framework to more realistic emitter tip geometries, such as paraboloids, and the development of an analogous framework for emission from non-planar, quantum-confined semiconductor emitters."
High-Throughput Manufacturing of Nanofibers using Planar Arrays of Microfabricated Externally Fed Emitters,"Electrohydrodynamic jetting occurs when a strong elec-tric field is applied to the free surface of a conductive liquid; the process can uniformly produce ion plumes, fine aerosol droplets, or continuous fibers with submi-cron diameters, i.e., nanofibers, depending on the prop-erties of the liquid used and the ionization conditions. Nanofabrication via electrohydrodynamic jetting has received attention as a promising candidate for produc-tion of nanostructures because of its ability to create nano-thick films of high quality at lower temperature than standard solid-state processing. A key advantage of electrospinning, i.e., electrohydrodynamic jetting of nanofibers, over other fiber generation methods is its versatility in producing fibers of arbitrary length from a range of materials including polymers, metals, ceramics, and semiconductors. The applications of elec-trospun nanofibers include dye-sensitized solar cells, scaffolds for tissue engineering, electrodes for ultraca-pacitors, and separation membranes. We created a technology for high-throughput generation of polymer nanofibers using planar arrays of microfabricated externally fed electrospinning emitters. Devices with emitter density as high as 25 emitters/cm2 (Figure 1) deposit uniform imprints comprising fibers with diameters on the order of a few hundred nanometers using solutions of dissolved polyethylene oxide in water and ethanol as working fluid (Figure 2). We measured mass flux rates as high as 417 g/hr/m2, i.e., 4x the reported production rate of leading commercial free-surface electrospinning sources. Throughput increases with increasing array size at constant emitter density, showing that the design can be scaled up with no loss of productivity. The largest measured mass flux resulted from arrays with larger emitter separation operating at larger bias voltages, indicating the strong influence of electrical field enhancement on the performance of the devices. Inclusion of a ground electrode surrounding the array tips helps control the spread of the imprints over large distances."
"Optimization of the Morphology of Arrays of Nano-Sharp, Photon-Triggered Silicon Field Emitters to Maximize their Total Current Emission","Femtosecond ultrabright cathodes with spatially structured emission are a critical technology for ap-plications such as free-electron lasers, tabletop coher-ent x-ray sources, and ultrafast imaging. State-of-the-art UV photocathodes have several disadvantages: (i) they need to be fabricated, stored, and operated in ul-tra-high vacuum and (ii) producing high current puls-es reduces their lifetime due to the rapid degradation of the low workfunction material. Cathodes based on photon-triggered field emission, i.e., tunneling of elec-trons due to the interaction of high-intensity optical pulses with field enhancing structures, are a promising technology to bypass these shortcomings. We recent-ly reported batch-fabricated photon-triggered field emission cathodes composed of massively multiplexed arrays of nano-sharp high-aspect-ratio silicon pillars; the devices are made using standard complementary metal-oxide semiconductor batch fabrication process-es, are stored at atmospheric conditions, and can be operated at lower vacuum levels than standard photo-cathodes with no degradation. The devices are capable of pC-level emission with multi-kHz repetition, greatly increasing the total emitted charge per pulse compared to single-emitter sources. Through experiment and simulations, this work explores the optimization of the total electron yield of ultrafast photon-triggered field emission cathodes composed of arrays of nanosharp, high-aspect-ratio, single-crystal silicon pillars by vary-ing the emitter pitch and height.Arrays of 6-nm-tip-radius silicon emitters with emitter densities between 1.2 and 73.9 million tips.cm-2 and emitter height between 2.0 μm and 8.5 μm were characterized using 35-fs 800-nm laser pulses (Figure 1). Of the devices tested, the arrays with emitter pitch equal to 2.5 μm produced the highest total electron yield; arrays with larger emitter pitch suffer area sub-utilization; and in devices with smaller emitter pitch, the larger emitter density does not compensate for the smaller per-emitter current due to the electric field shadowing that results from the proximity of the adjacent tips (Figure 2). Experimental data and simulations suggest that 2-μm-tall emitters achieve practical optimal performance as shorter emitters have visibly smaller field factors due to the proximity of the emitter tip to the substrate, and taller emitters show marginal improvement in the electron yield at the expense of greater fabrication difficulty."
Advanced X-Ray Sources for Absorption Imaging of Low-Z Materials,"X-rays are widely used in applications such as healthcare, airport security, crystallography, spectroscopy, and micro-fabrication. The development of miniaturized X-ray sourc-es could satisfy applications where the target areas are small or where the smaller dimensions and lighter weight of the X-ray source enable desirable capabilities such as portability. For example, compact X-ray sources can revolu-tionize computerized tomography (CT) by making possible the implementation of a system with multiple X-ray sourc-es that provides a wide range of information without the need to implement a rotating gantry.A field emission cathode is an attractive alternative to a conventional thermionic cathode as an electron source in a portable X-ray source because of the lower vacuum it requires to operate, its faster response, and its resilience to traces of reactive gases. Field emission cathodes use high-surface electric fields on the emitter tip surface to narrow the potential barrier that traps electrons in the material, allowing electrons to quantum tunnel into vacuum. Miniaturization and multiplexing of field emitters result in nanostructured field-emitter arrays capable of high-current emission at a low (< 150 V) voltage. The field emitters used in our X-ray source are capable of generating mA-level dc currents even when operated continuously for many hours. High-current cathodes make it possible to capture images in a short time, which helps to reduce any blurriness of the image due to movement of the sample.X-rays generated from a target anode can be catego-rized as either bremsstrahlung or fluorescent. On the one hand, bremsstrahlung X-rays span the entire ener-gy range of the bombarding electrons with the maxi-mum energy being determined by the voltage applied to the anode. On the other hand, fluorescent X-rays are characteristic of the target material and appear as spe-cific sharp peaks in the X-ray spectrum. While brems-strahlung X-rays give rise to low-contrast polychromat-ic images, fluorescent X-rays could be used to produce quasi-monochromatic, high-contrast images.For over four years our group has developed advanced field-emission-enabled, near-monochromatic X-ray sources capable of imaging soft tissue structures. Our latest development is a portable X-ray source (200 cm3 chamber size) with a reflection anode composed of a copper rod coated with a molybdenum thin film and a field emission cathode (Figure 1). A 25 l/s portable ion pump keeps the chamber base pressure at approximately 10-8 Torr. At an anode bias voltage of 35 kV, the X-ray source maximizes the percentage of photons with 17.8 keV, which corresponds to the Kα peak of Mo; these X-rays are energetic enough to go through air without significant attenuation (~95% transmission) but are of low-enough energy to generate high-contrast absorption images when interacting with soft tissue. Using the X-ray source, we obtained absorption images of ex-vivo samples captured on a CsI scintillator operated in fluoroscopic mode (Figure 2). Features as low as 160 µm were visible in the images."
A Field Emission-Based Ultra-High Vacuum Pump for Cold-Atom Interferometry Systems,"The discovery of magneto-optical trapping of alkali metal vapors in the late 1980s generated a strong in-terest in developing miniaturized atomic clocks and sensors based on cold alkali atom interferometry. Chip-scale, high-precision atomic sensors can be used in a great variety of exciting applications including funda-mental scientific discovery (e.g., general relativity and geophysics), inertial navigation (e.g., gyroscopes and accelerometers), and geological survey (e.g., magne-tometers and gravimeters). Cold-atom interferometry needs ultra-high vacuum (UHV, pressure < 10-9 Torr) to operate; therefore, portable cold-atom sensors require miniaturized UHV pump technology compatible with alkali vapor that operates at low power. Standard UHV ion pumps, which use high magnetic fields to increase the ionization probability, are not ideal to maintain vac-uum in a chip-scale atomic sensor because the intensi-ty of the magnetic field increases with the reduction in size of the pump and because the magnetic field of the pump can alter the quantum states of the laser-cooled atoms, leading to incorrect measurements. A better al-ternative is to use an electron source to provide a sur-plus of electrons to increase the ionization probability, eliminating the need for a magnetic field. A field emis-sion electron source is a good choice for that because, unlike state-of-the-art thermionic cathodes, they do not require high temperature to operate, which makes them compatible with the reactive alkali environment inside atomic vapor cells.We preliminarily demonstrated a magnetic-less ion pump design (Figure 1) that uses field electron emission to create a self-sustained plasma within a 200 cm3 vacuum chamber. A silicon-based, nanostructured, self-aligned, gated field emitter array (FEA) is used as electron source. Two electrodes, both consisting of structural rings wrapped with titanium wire, are placed above the FEA and biased at voltages that enable collection of either electrons or ions. The ion collector is the getter of the pump, capturing the ions both physically (bombardment) and chemically (chemisorption). The apparatus has a rubidium dispenser for releasing the alkali metal vapor inside the chamber, and the chamber is connected to an external pump system capable of maintaining a base pressure of ~10-8 Torr within the chamber. The performance of the field emission cathode did not deteriorate due to the presence of Rb at pressures as high as 7×10-6 Torr. The pump performance is shown in Figure 2. An initial rise in pressure (due to electron scrubbing) was followed by a 25% drop in pressure (from 4.0×10-7 Torr to 3.0×10-7 Torr) when the ion current was increased from 0 to 0.5 nA (by increasing the bias on the negatively charged ion collector). Current work focuses on the optimization of the electron impact ionization process to improve pumping performance."
100-nm Channel Length E-mode GaN p-Channel Field Effect Transistor (p-FET) on Si Substrate,"GaN-complementary circuit technology could be in-strumental towards realizing high-power-density, high-speed, low-form-factor, and highly efficient power electronic circuits, which has sparked many efforts to develop a high performance GaN p-channel field-effect transistor (p-FET). However, most of these demonstra-tions show normally-ON operation with ON-resis-tance over 1 kΩ∙mm. The GaN/AlInGaN heterostruc-ture-based p-FET shows low ON-resistance because of higher 2-DHG density and hole mobility but with D-mode operation. A GaN/AlN heterostructure-based p-FET shows enhanced-mode (E-mode) operation with RON of 640 Ω∙mm. However, n-FET integration with this p-FET requires regrowth. In this work, we demonstrate a self-aligned p-FET with a GaN/Al0.2Ga0.8N (20 nm)/GaN heterostructure grown by metal-organic-chemical vapor deposition (MOCVD) on Si substrate. The utilization of a GaN-on-Si platform offers lower cost, availability of 200-mm-diameter substrates, and potential to integrate with high performance logic and analog functionality. While most of the GaN p-FET demonstrations so far in the literature focus mainly on recessed gate metal-insulator-semiconductor FET (MISFET) structure, we choose to develop a self-aligned structure (see Figure 1 for the device structure) as it offers the following advantages over a recessed gate MIS p-FET: (1) the shortest possible source to the drain distance, cutting down the access region; (2) low ON-resistance because of negligible access resistance; and (3) easier gate alignment.Our 100-nm-channel-length self-aligned device with recess depth of 70 nm exhibits a record ON-resistance of 400 Ω∙mm and ON-current over 5 mA/mm with ON-OFF ratio of 6×105 when compared with other p-FET demonstrations based on a GaN/AlGaN heterostructure (see Figure 2 for benchmarking of our device with other p-FETs demonstrated in the literature). The device shows E-mode operation with a threshold voltage of −1 V, making it a promising candidate for a GaN-based complementary circuit that can be integrated on a Si platform. A monolithically integrated n-channel transistor with p-GaN gate is also demonstrated."
GaN CMOS Gate Driver for GaN Power Transistor,"In combination with its excellent transport properties, the high critical electric field of GaN, allows for GaN power transistors with much shorter drift regions and narrower gate widths than their Si or SiC counterparts. This allows for significantly lower gate capacitances and faster switching frequencies than traditional pow-er switches at the same operating voltages. To take full advantage of the reduced gate capacitance and high switching speed of GaN power transistors, it is neces-sary to minimize parasitic inductances between the power switches/transistors and the gate driver circuit. For this, the GaN community has traditionally lever-aged enhancement-mode/depletion-mode logic also known as direct coupled logic (DCL) to integrate rela-tively simple gate-driver circuits on the same chip as the GaN power devices; however, this technology suf-fers from significant power consumption and limited circuit design flexibility. To overcome these issues, re-cently there has been much research on a new all-GaN complementary technology that allows integration of high-performance n-channel and p-channel GaN en-hancement-mode transistors on the same chip without the need for epitaxial regrowth. The epitaxial structure used for the demonstration of the all-GaN complemen-tary technology consists of a GaN/AlGaN/GaN dou-ble heterostructure. This structure was grown by the company Enkris Semiconductor on 6” silicon wafers. Enhancement-mode p-type transistors were fabricated by contacting the two-dimensional hole gas at the top GaN/AlGaN interface while n-type devices are obtained by recessing the structure to allow direct ohmic con-tact connection to the two-dimensional electron gas at the bottom AlGaN/GaN interface. A simple gate driver with a 350-pF load is switched at 100-kHz frequency."
Self-Aligned p-FET with ION~100 mA/mm,"GaN-complementary circuit technology could be in-strumental towards realizing high-power-density, high-speed, low-form-factor, and highly efficient power electronic circuits, which has sparked many efforts to develop a high performance GaN p-channel field-effect transistor (p-FET). However, most of these demonstra-tions show normally-ON operation with ON-resistance over 1 kΩ∙m. In this work, we demonstrate a self-aligned p-FET with a GaN/Al0.2Ga0.8N (20 nm)/GaN heterostruc-ture grown by metal-organic-chemical vapor deposi-tion on Si substrate. The utilization of a GaN-on-Si plat-form offers lower cost, availability of 200-mm-diameter substrates, and potential to integrate with high perfor-mance logic and analog functionality. While most of the GaN p-FET demonstrations so far in the literature focus mainly on a recessed gate metal-insulator-semi-conductor FET (MISFET) structure, we choose to devel-op a self-aligned structure as it offers the following ad-vantages over a recessed gate MIS p-FET: (1) the shortest possible source to the drain distance, cutting down the access region; (2) low ON-resistance because of negligi-ble access resistance: and (3) easier gate alignment. The device with LSD=200 nm shows a record combination of ION~50 mA/mm and ON-OFF ratio of 104 when com-pared with other p-channel transistor demonstrations. The device also exhibits enhancement mode operation with threshold voltage of -0.5 V. The best device shows a record current density of 100 mA/mm but at the ex-pense of a lower ON-OFF ratio of 102. A monolithically integrated n-channel transistor with p-GaN gate is also demonstrated."
Reliability of AlGaN/GaN-on-Si High-Electron-Mobility Transistors,"AlGaN/GaN high-electron-mobility transistors (HEMTs) are of interest for high-frequency and high-power applications such as in 5G networks and autonomous vehicles. Fabrication of GaN-based devic-es on silicon substrates can lead to reduced costs as well as enable monolithic integration of Si-complementary metal-oxide-semiconductor devices with GaN HEMTs. However, due to the large mismatches in lattice con-stant and coefficient thermal expansion between GaN and Si, GaN-on-Si HEMTs face challenges in terms of long-term reliability. Our previous work has focused on degradation of the maximum drain current observed after off-state and on-state stressing in reverse or zero gate bias conditions. Decreases in the drain current, ID, and increases in the gate leakage current, IG, were found to be associated with electrochemical oxidation and pit formation at the gate edges. This degradation can be suppressed by using high-density passivation layers, reducing the threading dislocation density and reducing oxygen impurities at the GaN-cap/passivation layer interface. Given that HEMTs also operate in forward gate bias, we have more recently focused on forward-bias stressing of research-grade HEMTs by setting the gate bias VG-stress at 2, 2.5, 3, or 4 V with VD = VS = 0 V. ID vs. VG measurements were made at time increments during testing. As shown in Figure 1(a), the drain saturation current and threshold voltage do not change significantly over time while the leakage current increases dramatically. Moreover, increasing the stressing voltage highly accelerates this degradation (Figure 1(b)). This increase in the leakage current was accompanied by a decrease of the Schottky barrier height and an increase in the ideality factor, suggesting the degradation likely occurred at the Schottky gate contact. Further analysis using photon emission microscopy (PEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) revealed that carbon impurities in the gate metal layer (nickel) were responsible for this degradation (Figure 2(a) and 2(b)). The carbon impurities likely originated from photoresist residues from the gate lift-off process."
CMOS-Compatible Vanadium Pentaoxide-Based Programmable Protonic Resistor for Analog Deep Learning,"Deep learning proficiency in classification and cluster-ing of data representations has fundamentally changed how information is processed. However, state-of-the-art digital processing units based on complementary metal–oxide–semiconductor (CMOS) circuits require large memory space and high power consumption to train deep neural networks. Improvements in com-puting performance therefore require designing novel scalable, fast, and energy-efficient hardware structures with both processing and storage capabilities using an-alog crossbar arrays.The building block of these arrays is a programmable, non-volatile resistor, which should display multiple conductance states that are modulated reversibly, symmetrically, and reproducibly. Several device technologies, based mainly on filamentary conduction and phase-change mechanisms, have been proposed for analog deep learning; none of these however yet meets all device performance requirements. A recent concept, ion intercalation in transition-metal oxides, can potentially circumvent issues faced by other mechanisms.We are therefore investigating a CMOS-compatible proton intercalation resistor that relies on a deterministic charge-controlled mechanism. The use of protons, the smallest cations, as the doping ion presents several advantages including high operation speed, good compatibility with current patterning processes, and long lifetime. Our initial design, shown in Figure 1, with a PdHx solid hydrogen reservoir and a WO3 active channel, demonstrated promising device characteristics but needs to be improved as : 1) it relied on Nafion, a non-CMOS-compatible electrolyte that strongly reacts with some other promising channel materials, and 2) the conductance of WO3 and device energy consumption (conductance reading) increases through protonation. Herein, we present our progress towards device integrability using an inert CMOS-com-patible electrolyte and on the reduction of the device energy consumption using a vanadium pentaoxide (V2O5) channel, which conductance decreases through protonation and can be modulated in a non-volatile, symmetric, reversible, and reproducible way, as shown in Figure 2."
Waveguide Quantum Electrodynamics with Superconducting Artificial Giant Atoms,"Models of light-matter interactions typically invoke the dipole approximation, within which atoms are treated as point-like objects when compared to the wavelength of the electromagnetic modes that they interact with. However, when the ratio between the size of the atom and the mode wavelength is increased, the dipole ap-proximation no longer holds, and the atom is referred to as a “giant atom.” Thus far, experimental studies with solid-state devices in the giant-atom regime have been limited to superconducting qubits that couple to short-wavelength surface acoustic waves, probing the properties of the atom at only a single frequency.Figure 1 shows an alternative architecture that realizes a giant atom by coupling small atoms to a waveguide at multiple, but well separated, discrete locations. We also show how multiple giant atoms can be coupled to the same waveguide in a braided fashion to enable interactions between the qubits that are mediated by the waveguide. Figure 2 shows how our realization of giant atoms enables tunable atom-waveguide couplings with large on-off ratios and a coupling spectrum that can be engineered by device design. We also demonstrate decoherence-free interactions between multiple giant atoms that are mediated by the quasi-continuous spectrum of modes in the waveguide-- an effect that is not possible to achieve with small atoms. These features allow qubits in this architecture to switch between protected and emissive configurations in situ while retaining qubit-qubit interactions, opening new possibilities for high-fidelity quantum simulations and non-classical itinerant photon generation."
Dynamics of Hf0.5Zr0.5O2 Ferroelectric Structures: Experiments and Models,"Due to its complementary metal-oxide-semiconduc-tor compatibility, ferroelectric HfZrO2 (FE-HZO) has attracted enormous interest in various semiconduc-tor device areas, such as analog computing, logic, and memory. Despite intense research, controversy re-mains about the ferroelectric switching dynamics and the existence of negative capacitance (NC). To develop fundamental understanding, we have carried out de-tailed experimental studies of the FE-HZO switching dynamics of metal-ferroelectric-metal (MFM) and met-al-ferroelectric-insulator-metal (MFIM) structures. To extract the intrinsic dynamic response of the structures, our experimental methodology has paid close attention to minimizing and calibrating all circuit and sample parasitics. Based on the measured MFM dynamics, we have proposed a new nucleation-limited switching (NLS) model that captures the incubation and growth of polarization domain nuclei within each grain of a polycrystalline ferroelectric film, as described by a Weibull distribution. Figure 1 shows that the model describes well all observed behavior including major and minor charge-voltage loops under a broad range of conditions. Further, our work reveals that in R-MFM circuit configurations with an external resistor, the MFM dynamics show no evidence of NC-like behavior in contrast with other reports. Our study suggests that erroneous consideration of parasitic capacitance could explain earlier claims of NC effects in the MFM dynamics. We observed clear NC behavior in MFIM structures. We confirm the transient quasi-static S-like FE behavior described in the literature and observe a dynamic response that displays hysteretic behavior in the NC region. A model based on the Landau-Khalatnikov equation that incorporates FE dynamics via a phenomenological frictional resistance adequately describes the observed results when that resistance is made dependent on the direction of the voltage drive vs. time, as in Figure 2. Mitigation of this hysteretic NC behavior will be crucial to harness NC in practical metal-oxide-semiconductor field-effect transistors."
Fault Detection for Semiconductor Processes Using One-Class Parzen Window Classifiers,"Faults in fabrication processes are extremely costly. When undetected and unaddressed, they will contin-ue to ruin wafer lots until the underlying problem is corrected, leading to massive yield losses. Our work uses one-class Parzen window classifiers to raise alerts when faults are suspected by monitoring process sen-sor information, reducing future yield loss. These mod-els are kernel-based density estimation methods that determine the similarity of incoming data to known good process data. The method uses only nominal pro-cess data, which is desirable as faults are often unique, and examples will not be available before they occur. Using historical examples of a wide variety of faults in plasma etching and ion implantation (Figure 1), our fault- detection methodology captures more than 90% of faults, with a false positive rate of less than 0.5%. This method can be applied to a wide variety of pro-cesses without significant adjustment, making it ideal for generalized fault detection."
Bias Temperature Instability under Forward Bias Stress of Normally-Off GaN High-Electron-Mobility Transistors,"Energy-efficient electronics have been gaining much attention as a necessary path to meet the growing demand for electrical energy and sustainability. GaN field-effect transistors (FETs) show great promise as high-voltage power switches due to their ability to withstand a large voltage and carry high current. For best circuit reliability, safety, and performance, a nor-mally-off transistor is highly desirable. An attractive design is the p-doped GaN-gate high-electron-mobility transistor (p-GaN HEMT).Our research aims to better understand the reliability issues impeding widespread adoption of p-GaN power HEMTs. One key issue is device degradation under prolonged operation, where key device performance metrics such as threshold voltage and gate leakage current change with electrical stress.We show that device degradation under forward-bias electrical stress, i.e., when the transistor is turned on, shows multiple regimes that are voltage and time dependent. Due to the complex gate stack that includes a p-doped GaN layer, the device exhibits bias temperature instability degradation with signature characteristics of electron and hole trapping. Furthermore, we show that some of the degradation is recoverable. Altogether, our research reveals the presence of rich and dynamic degradation physics for the p-GaN HEMTs that must be well understood before the commercial success of this technology."
Morphological Stability of Nanometer-Scale Single-Crystal Metallic Interconnects,"Continued integrated-circuit scaling requires intercon-nects with cross-sectional dimensions in the <10-nm range. At these dimensions, the resistance of intercon-nects increases dramatically due to surface and grain boundary electron scattering. The reliability of inter-connects with nanoscale dimensions is also expected to be compromised by reduced morphological stability. As a part of a collaborative program focused on ballis-tic conduction and stability of single-crystal nanome-ter-scale interconnects, we are investigating the crys-tallographic dependence of the morphological stability of Ru wires.Thin single-crystal films agglomerate into small particles via capillary-driven surface diffusion in a process known as solid-state “dewetting.” With decreasing film thickness, the temperature at which dewetting occurs is well below the constituent material’s melting temperature. However, previous work on single-crystal Ni films has demonstrated that crystalline anisotropy gives rise to special crystallographic orientations along which single-crystal wires exhibit greatly enhanced morphological stability. Ru is a candidate material for future interconnects, and we have studied the morphological stability of arrays of Ru nanowires lithographically patterned from single-crystal (0001) films, such that the individual wires have axes lying in different crystallographic directions. After annealing, we find nanowires oriented along directions that are particularly stable; see Figures 1 and 2. Interconnects composed of such wires should have decreased vulnerability to morphological instabilities during processing and circuit operation. These wires also have strongly faceted surfaces, with facets parallel to the wire axis (Figure 3), which are predicted to reduce electron scattering and decrease interconnect resistance. This high degree of morphological stability and faceting also suggests that wires with these orientations will be particularly resistant to electromigration. Combining new data from this material system with results from past work on Ni, which has weaker surface energy anisotropy, will provide insights that will enable optimization of interconnect structural and crystallographic factors for design of morphologically stable nanowires with cross-sectional dimensions significantly below 10 nm."
Switching Reliability of GaN Power High-Electron-Mobility Transistors,"GaN electronics constitutes a new technology with su-perior power-handling capabilities compared to those of Si and other semiconductors in many applications. Power management applications typically involve op-erating the GaN transistors under rapid switching con-ditions between a high-voltage off-state and a high-cur-rent on-state. Depending on the system topology and specifications, two switching modes apply to power applications: soft switching and hard switching. The reliability and robustness of GaN transistors under re-peated switching is a concern, particularly when they operate under hard-switching conditions. The Double-Pulse Test is the most effective test for emulating high-power switching close to the mode of operation of the devices in electrical power management applications. In our work we have constructed a unique experimental setup to implement the Double-Pulse Testing technique. Figure 1a illustrates a physical implementation of the experimental setup. The system can conduct testing under severe stress conditions and monitor the induced degradation of device parameters up to the point of catastrophic device failure. Figure 1b shows a typical waveform of the Double-Pulse Test. The system allows users to repeat the test multiple times and measure device parameters at fixed intervals. Figure 1c shows an example of catastrophic degradation of dynamic RDS,ON when the transistor is subjected to hours of repeated switching operation. Degradation and hard-fail data will be used to verify failure modes and develop life-time models in order to project device survivability under various conditions."
Automated Design of Superconducting Circuits and Its Application to 4-Local Couplers,"Quantum processors are well-controlled quantum sys-tems capable of performing complex computational tasks. They have been shown to hold promise for the simulation of fundamental physical effects, as well as for solving computationally expensive yet practical problems. Superconducting circuits have emerged as a promising platform to build such quantum processors. These are microscale electrical circuit structures that are fabricated on an on-chip device. In a cryogenic envi-ronment, the chip behaves quantum mechanically and can be controlled using microwave pulses.The challenge of designing a circuit is to compromise between realizing a set of performance metrics and reducing circuit complexity and noise sensitivity. At the same time, one needs to explore a large design space, and computational approaches often yield long simulation times. Here, we automate the circuit design task using superconducting circuit closed-loop automated design (SCILLA). The software SCILLA performs a parallelized, closed-loop optimization to design superconducting circuit diagrams that match predefined properties, such as spectral features and noise sensitivities. We employ SCILLA to design 4-local couplers for superconducting flux qubits and identify a circuit that outperforms an existing proposal with a similar circuit structure in terms of coupling strength and noise resilience for experimentally accessible parameters. Our results are important for the future development of quantum processors in two ways. First, the coupler circuit that we have found is expected to boost the capabilities of quantum processors. Second, our method demonstrates how automated design can facilitate the development of complex circuit architectures for quantum information processing."
CMOS-Compatible Protonic Programmable Resistor Based on Phosphosilicate Glass Electrolyte for Analog Deep Learning,"The success of deep learning in classifying and clus-tering representations of data at multiple levels of abstraction has fundamentally changed how infor-mation is processed. However, conventional digital ar-chitectures face increasing difficulties in supporting the heavy computational workloads required to train state-of-the-art deep neural networks. The pressing need for faster and more energy-efficient deep learning processors has therefore led to an intensive investiga-tion of in-memory computation schemes using analog crossbar arrays.The building block of analog crossbar arrays is the crosspoint element, which can be described as a programmable, non-volatile resistor. Ion intercalation-based programmable resistors have emerged as a potential next-generation technology for analog deep learning applications. Protons, being the smallest ions, are the most promising candidate to enable devices with high modulation speed, low energy consumption, and enhanced endurance. The main bottleneck with developing protonic programmable resistors has been the absence of a suitable solid-state electrolyte that conducts protons but blocks electrons. All designs so far have relied on approaches that either cannot be integrated and scaled down, such as using organic materials; use chemically and thermally sensitive polymers; or suffer from energy inefficiency such as high electric field-induced water hydrolysis. In this work, we report on the first back-end complementary metal-oxide-semiconductor- (CMOS) compatible protonic programmable resistor enabled by the integration of phosphosilicate glass (PSG) as the proton electrolyte layer. PSG is an outstanding electrolyte material that displays both excellent protonic conduction and electronic insulation characteristics. Moreover, it is a well-known material within conventional Si fabrication that enables high deposition control and scalability. Our scaled three-terminal devices show desirable modulation characteristics in terms of symmetry, retention, endurance, and energy efficiency. Protonic programmable resistors based on PSG, therefore, represent promising candidates to realize nanoscale analog crossbar processors with monolithic CMOS-integration."
III-V Broken-Band Vertical Nanowire Esaki Diodes,"Further reducing transistor power consumption of metal-oxide-semiconductor field-effect transistors (MOSFETs) in logic applications requires transport mechanisms other than thermionic emission over an energy barrier. Among all possible mechanisms, quan-tum tunneling emerges as one of the most promising. Therefore, the design and demonstration of tunnel field-effect transistors (TFETs) have received much at-tention recently. In spite of intense research, the results to date have been disappointing.In our research, we aim to utilize the unique broken-band alignment and the superior carrier transport properties in the GaSb/InAs material system to obtain high drive current with tunneling. In order to quantitatively evaluate the quality of the tunneling junction, GaSb/InAs(Sb) vertical nanowire (VNW) Esaki diodes have been fabricated and electrically characterized."
Mysterious Layer on a Hydrogen-Terminated Diamond Surface,"The surface conductivity of H-terminated diamond (D:H) is usually explained by the transfer doping mod-el. The model assumes a surface dopant layer is formed on the D:H surface that generates a two-dimensional hole gas (2DHG) in the diamond. The dopant layer is typically assumed to be of atomic dimensions. Howev-er, since the D:H surface is almost perfectly passivated, there are no chemical bonds out of the surface, and the dopants are weakly held by van der Waals forces. Consequently, analysis of the capacitance of MOSFETs built on D:H show it to be much smaller than expect-ed. To study the nature of this interfacial layer, we have analyzed the scaling properties of the gate capacitance of Al/Al2O3/D:H MOS structures. A comparison of the obtained results against Poisson-Schrodinger simula-tions suggests the existence of an “air gap” of 0.5-1 nm in thickness at the Al2O3/D:H interface. If confirmed, this gap will have important implications for the current drivability of diamond MOSFETs."
Impact of Ionizing Radiation on Superconducting Qubit Coherence,"The practical viability of technologies that rely on qu-bits requires long coherence times and high-fidelity operations. Superconducting qubits are a promising platform for achieving these objectives. However, their coherence is affected by broken Cooper pairs, referred to as quasiparticles. The experimentally observed den-sity of quasi-particles is orders of magnitude higher than the value predicted at equilibrium by the Bar-deen-Cooper-Schrieffer theory of superconductivity. Our results suggest that ionizing radiation from cos-mic rays and from environmental radioactive materials contribute to the observed difference. In this work, we use a radioactive 64Cu source to measure the impact of ionizing radiation on superconducting qubits under controlled levels of radiation. While the activity of the source decayed over time, we observed an increase in the coherence of the qubits, see Figure 1. From independently measured level of naturally occurring background radiation, we can extrapolate the impact of ionizing radiation on quasi-particle generation and the qubit coherence. We predict that the ionizing radiation would limit the coherence times of superconducting qubits of the type we measured to the millisecond regime.Next, we demonstrate that shielding the qubits with lead can mitigate the impact of radiation on the qubits, see Figure 2. We continuously raised and lowered the shield and measured the corresponding change in the qubit energy-relaxation rate. Albeit a small effect in today’s qubits, the change in the relaxation time positively correlated with the increased shielding, confirming our hypothesis that naturally occurring ionizing radiation affects the qubit coherence."
NbN-Gated GaN Transistor Technology for Applications in Quantum Computing Systems,"High-performance and scalable cryogenic electronics is an essential component of future quantum infor-mation systems, which typically operate below 4K. Su-perconducting qubits need advanced radio-frequency (RF) and pulse-shaping electronics, which typically oc-cupies large instrumentation racks operating at room temperature. This approach is not scalable to the mil-lions of qubits needed in future quantum systems.This work explores the use of wide band gap heterostructure electronics, specifically the AlGaN/GaN high electron mobility transistor (HEMT), for cryogenic low-noise applications. These structures take advantage of the polarization-induced two-dimensional electron gas to create a high mobility channel, hence eliminating the heavy doping needed in the other semiconductor technologies. Epitaxially-grown GaN-on-Silicon wafers have been demonstrated in large (12 inch / 300 mm) substrates, therefore making the technology an excellent candidate for scalable RF electronics in quantum computing systems.Furthermore, the use of electrodes of superconducting materials is proposed to significantly reduce the parasitic components and therefore push the RF performance of cryogenic devices. Short-channel transistors with NbN gates of length 250 nm have been demonstrated with promising performance.The next step will study the effect of the superconducting gate on RF characteristics of the transistors, with the eventual goal of pushing the frequency performance of these transistors to new limits. These transistors will be integrated into low-noise amplifier circuits for applications in readout and control electronics at cryogenic temperature. Furthermore, the demonstrated NbN-gated GaN transistor paves the way for the application of high-frequency GaN technology in cryogenic electronics, notably in scalable quantum computing systems. When combined with other highlights in GaN electronics, e.g., a GaN complementary metal-oxide-semiconductor (CMOS) platform, the reported technology brings usone step closer to an all-nitride integrated electronics-quantum device platform."
Metal Alloy Enables Reliable Silicon Memristor Synapses,"In the age of artificial intelligence (AI), memristors have emerged as an artificial synapse for neuromorphic computing, overcoming the limitations of convention-al silicon (Si)-based digital synapses. Interestingly, Si has also been utilized to develop memristor synapses via combination with a silver (Ag) electrode. An electri-cal conductance of Si medium is reversibly modulated by Ag injection, corresponding to the synaptic weight changes. Owing to the thermodynamic instability of Ag in Si medium (Ag is immiscible in Si), injected Ag exhib-its high mobility, resulting in a high-weight modulation ratio and high switching endurance. Unfortunately, large switching variations and poor weight stability occur at the devices and are also induced by the ther-modynamic instability. Thus, to mitigate such dilem-mas in performance, the regulation of thermodynamic stability of Ag in Si medium would be the fundamental strategy. Here we have developed Ag alloy for precision tuning of thermodynamic interactions of Ag and Si, thereby achieving highly balanced synaptic performance. We selected copper (Cu) as an alloying element due to its (1) high diffusivity into Si and (2) favorable thermodynamic interactions with both Ag and Si. Our hypothesis was that Cu would migrate into Si simultaneously with Ag and enhance thermodynamic stability of Ag in Si (i.e., be a stabilizer). The device’s performance results clearly confirm our metallurgical strategy that switching uniformity and weight retention are significantly enhanced by a Ag-Cu alloyed electrode (Figure 1). It should be noted that other alloying elements such as Ni cannot improve the synaptic performance due to their repulsive interactions with Ag.With promising device performance test results, we have demonstrated 32 × 32 Si memristor array and successfully performed image storage (Figure 2) and image processing (Figure 3), which are only enabled by Ag-Cu active electrode (Figure 3). We believe our alloying strategy can be expanded to other memristive synapses to resolve performance issues in neuromorphic computing applications."
Highly Tunable Junctions in Magic Angle Twisted Bilayer Graphene Tunneling Devices,"The recent observation of superconductivity and cor-related insulating states in “magic-angle” twisted bi-layer graphene (MATBG) featuring nearly flat bands at twist angles close to 1.1 degrees presents a highly tunable two-dimensional material platform capable of behaving as a metal, an insulator, or a superconduc-tor. Local electrostatic control over these phases may enable the creation of versatile quantum devices that were previously not achievable in other single materi-al platforms. Our research shows how we can exploit the electrical tunability of MATBG to engineer Joseph-son junctions and tunneling transistors all within one material, defined solely by electrostatic gates. Our multi-gated device geometry offers complete control over the Josephson junction, with the ability to inde-pendently tune the weak link, barriers, and tunneling electrodes. Utilizing the intrinsic bandgaps of MATBG, we also demonstrate monolithic edge tunneling spec-troscopy within the same MATBG devices and mea-sure the energy spectrum of MATBG in the supercon-ducting phase. Furthermore, inducing a double barrier geometry permits the devices to be operated as a sin-gle-electron transistor, exhibiting a Coulomb blockade. These MATBG tunneling devices, with versatile func-tionality encompassed within a single material, may find applications in graphene-based tunable supercon-ducting qubits, on-chip superconducting circuits, and electromagnetic sensing in next-generation quantum nanoelectroni"
Electronic Cells: Autonomous Micromachines from 2D Materials,"Electronic cells are micromachines encompassing au-tonomous on-board functions such as sensing, com-putation, communication, locomotion, and power management. Akin to their biological counterparts, electronic cells bring specialized capabilities to previ-ously inaccessible locations. Here, we present the de-sign and fabrication of the first-of-its-kind electronic cell composed of the nanoelectronic circuit on top of an SU-8 particle. Powered by a 2D material-based pho-todiode, the on-board circuit connects a chemiresistor element and a memristor element, enabling on-board detection and storage capabilities. We demonstrate how our cells sense and record information about the presence of ammonia and dispersed soot when aerosol-ized in the enclosed tubes, dispersed in a hydrodynamic flow of pipelines, or sprayed over large surfaces. Elec-tronic cells may find widespread application as probes in confined environments, such as the human digestive tract, oil and gas conduits, chemical and biosynthetic reactors, and autonomous environmental sensors."
Decoding Complexities in Relaxor Ferroelectrics Using Electron Microscopy,"Relaxor ferroelectrics show slim hysteresis loops, low remanent polarization, high saturation polarization, and exceptional electromechanical coupling, finding applications in ultrasound imaging and energy storage devices. Developing a structure-property relationship in relaxors has been a seemingly intractable prob-lem due to the presence of nanoscale chemical and structural heterogeneities. We have employed aberra-tion-corrected scanning transmission electron micros-copy (STEM) to quantify the various contributions of nanoscale heterogeneity to relaxor ferroelectric prop-erties in a PMN-PT system. Specifically, we found three main contributions-- chemical ordering, oxygen octa-hedral tilting and oxygen octahedral distortion--that are difficult to otherwise differentiate. STEM reveals the elusive connection between chemical and struc-tural heterogeneity and local polarization variation. Further, the effects of strain and thickness on PMN-PT thin films has been examined. These measurements elucidate the design principles for next-generation re-laxor material."
Printed MEMS Membrane Electrostatic Microspeakers,"This work reports the fabrication and operation of electrostatic microspeakers formed by contact-trans-fer of 125-nm-thick gold membranes over cavities pat-terned in a micron-thick silicon dioxide (SiO2) layer on a conducting substrate. Upon electrostatic actuation, the membranes deflect and produce sound. Addition-ally, membrane deflection upon pneumatic actuation can be used to monitor pressure. The microspeaker fabrication process reported enables fabrication of MEMS diaphragms without wet or deep reactive-ion etching, thus obviating the need for etch-stops and wafer-bonding. This process enables monolithic fab-rication of multiple completely-enclosed drum-like structures with non-perforated membranes to dis-place air efficiently, in both individual-transducer and phased-array geometries. We characterized the mechanical deflection of the gold membranes using optical interferometry. The membranes show a repeatable peak center deflection of 121±13 nm across gaps of ~25 microns at 1 kHz sinusoidal actuation with 60 V peak-to-peak amplitude and a 30 V DC bias (Figure 1). The acoustic performance of the microspeakers is characterized in the free field. Microspeaker sound pressure level increases with frequency at 40 dB/decade (Figure 2), indicating that its sound pressure output is proportional to the acceleration of its diaphragm, as expected in the spring-controlled regime for free field radiation. The microspeaker consumes 262 µW of real electric power under broadband actuation in the free field, and outputs 34 dB(SPL/Volt) of acoustic pressure at 10 kHz drive. The silicon wafer substrate (~500 μm thick) dominates the total thickness of the microspeakers; the active device thickness is less than 2 µm. These thin microspeakers have potential applications in hearing aids, headphones, and large-area phased arrays for directional sound sources."
Stretchable Pressure and Shear Sensitive Skin,"In the fields of robotics and prosthesis design, there is need for inexpensive, wide-area pressure- and shear-sens-ing arrays that can be integrated into a flexible and stretchable skin analog. This project seeks to meet this need by building combined pressure and shear sensors based on the well-documented piezoresistive (strain de-pendent resistance) property of composites made from polydimethylsiloxane (PDMS) and carbon black (CB).The sensor skins are fabricated of three materials which are all PDMS-based: A CB/PDMS mixture is used as the active sensing material, a CB/PDMS and ~1 µm silver particle mixture is used to form strain-insensitive conductors into the skin, and pure PDMS is used to form the base of the skin. These materials are mixed, vacuum degassed, and then molded in custom-machined acytal and aluminum molds to fabricate the sensor arrays. Each sensor consists of a roughly hemispherical piece of CB/PDMS molded on top of a line of three conductors, thus allowing the resistance of each half of the CB/PDMS sensor to be measured independently. A schematic representation of a single sensor is shown in Figure 1. Our own characterization experiments performed on bulk (1 cm3) CB/PDMS samples have shown that the resistance of CB/PDMS increases under tensile, compressive, and shear strain but is much more sensitive to tensile strain then compressive strain. This symmetry allows the device to sense both pressure and shear. Under pressure, each half of the sensor has roughly equal compressive strain, and thus the resistance of each half of the device increases roughly equally. However, under shear, one half of the device is under tensile strain while the other half is under compressive strain. Due to the asymmetric response of the CB/PDMS, the resistance of the half of the sensor under tension increases much more than the half under compression, allowing a differentiation between pressure and shear; see Figure 2."
Tunneling Nanoelectromechanical Switches Based on Molecular Layers,"Nanoelectromechanical (NEM) switches have emerged as a promising competing technology to the conventional complementary metal-oxide semicon-ductor (CMOS) transistors. NEM switches can exhibit abrupt switching behavior with large on-off current ratios and near-zero off-state leakage currents. How-ever, they typically require large operating voltages exceeding 1 V and suffer from failure due to stiction. To address these challenges, this work presents NEM switches utilizing metal-molecule-metal switching gaps. These switches operate by electromechanical modulation of the tunneling current through electro-statically-induced compression of the molecular film (Figure 1). The molecular layer helps define few-nano-meter-thick switching gaps to achieve low-voltage operation. In addition, the compressed molecules prevent direct contact between the electrodes while providing a restoring force to turn off the device once the applied voltage is removed, thereby preventing permanent adhesion between the electrodes and eliminating stiction. A prototype two-terminal tunneling NEM switch is fab-ricated as a laterally actuated cantilever using electron beam-lithography (Figure 2). A fluorinated decanethiol layer is deposited over the device area and into the gap between Electrodes 1 and 2 using self-assembly through thiol-chemistry. During the assembly process, Elec-trode 1 collapses onto the opposing electrode to form a metal-molecule-metal junction with a nanometer thickness. Experimental results based on a device with a self-assembled fluorinated decanethiol layer demon-strate repeatable switching, indicating the importance of the molecular film in alleviating stiction. Compari-son of these results to the theoretically expected device behavior suggests the compression of the molecular layer during the switching process, confirming the elec-tromechanical modulation of tunneling current as the switching mechanism. Our current research focuses on engineering the molecular layer and the device design to optimize the NEM switch performance to achieve stiction-free sub-1-V actuation with more than 6 orders of magnitude on-off current ratio."
Electrically-Tunable Organic Microcavities,"The availability of a compact, single-system tunable visible light source would benefit a wide range of fields such as remote sensing, spectroscopy, and optical switches. Organic-based materials are attractive for visible light emission over a broad tunable spectrum. However, previously demonstrated frequency-tunable lasing devices required either complex fabrication techniques, external micro-actuated mirror stages, or manual switching between gain media. Tunable air-gap MEMS microcavity structures offer a scalable, inte-grated solution but their typical fabrication processes are incompatible with solvent- and temperature-sensi-tive organic gain materials. We have demonstrated a method for fabricating integrated organic optical microcavities that can be mechanically or electrostatically actuated to dynamically tune their output emission spectra. Fabrication of the micro-opto-electro-mechanical system (MOEMS) structures (as in Figure 1) is enabled by a solvent-free additive transfer-printing method for composite membranes that we have developed. The suspended membrane incorporates an organic laser gain medium, Alq3:DCM, into the microcavity, and the completed capacitive structure can be electrostatically actuated for dynamic tuning of the optical spectra (see Figure 2). Electrical actuation and optical characterization of a completed cavity structure show reversible resonance tuning greater than 20 nm for net membrane deflections of over 200 nm at 50 V. The device structure and transfer technique are easily scalable for large area fabrication with applications in tunable lasers as well as remote all-optical pressure sensing and low-power optical switches."
MEMS Tactile Displays,"Providing information to people who are blind or have low vision is critical for enhancing their mobility and situational awareness. Although refreshable 2D graph-ical interfaces are preferred, it is challenging to create actuators that are compact enough to be arrayed into an unlimited number of rows and columns while still being robust, easy to sense, and rapidly switchable. Electroactive polymer actuators are small enough to be arrayed with a few- millimeter pitch and to provide quasistatic millimeter-scale actuations, but they typ-ically have actuation times on the order of seconds. An alternative integrates piezoelectric bending beam actuators perpendicular to the tactile sensing plane, enabling large bending beam actuators to be tightly packed for fully 2D displays. Ideally, the display’s resolution should be about one tactel (i.e., tactile element) per mm2 , which is the density of mechanoreceptors in human finger pads. It should be refreshable in real time (hundreds of Hz, i.e., the frequency response of human touch), allowing the contents of the display to keep up with rapidly changing inputs. Since humans are much more sensitive to motions and changing stimuli than to static patterns, the display should code information not only as static patterns, but also as simulated motion against the user’s finger pads. Finally, the power consumption of the display should be compatible with portable use. Although existing displays meet various subsets of these requirements, no existing display can meet all requirements simultaneously. We are developing tactile displays based on a new type of MEMS tactile actuator created to target these requirements. This new actuator concept uses an extensional piezoelectric actuator that operates a scissor amplifier that transforms the in-plane movement of the piezo into amplified out-of-plane movement (see Figure 1). We have shown these tactile elements to be effective at the milliscale. Their measured performance agrees with the models, with maximum deflections of greater than 10 µm and maximum forces above 45 mN (as in Figure 2) that place the devices well above the sensing threshold. Our analytical model based on ideal pinned hinges is shown to be useful for predicting the behavior of tactels with flexural hinges, especially when coupled with FEA to predict hinge failure. The analytical model validation provides support for further downscaling of the tactile elements to achieve 100 tactels/cm2. The measured performance confirms sensing thresholds of less than 4 µm and 2 mN for the most effective tactile devices."
Real-time Manipulation with Magnetically Tunable Structures,"Responsive actuating surfaces have attracted signifi-cant attention as promising materials for liquid trans-port in microfluidics, cell manipulation in biological systems, and light tuning in optical applications via their dynamic regulation capability. Significant efforts have focused on fabricating static micro and nano-structured surfaces, even with asymmetric features to realize passive functionalities such as directional wet-tability and adhesion. Recent advances in utilizing ma-terials that mechanically respond to thermal, chemical or magnetic stimuli have enabled dynamic regulation. However, the challenges with these surface designs are associated with the tuning range, accuracy, response time, and multi-functionality for advanced systems. Here we report dynamically tunable micropillar arrays with uniform, reversible, continuous, and extreme tilt angles with precise control for real-time fluid and optical manipulation. Inspired by hair and motile cilia on animal skin and plant leaves for locomotion, liquid transportation, and thermal-optical regulation, our flexible uniform responsive microstructures (µFUR) consist of a passive thin elastic skin and active ferromagnetic microhair whose orientation is controlled by a magnetic field. We experimentally show uniform tilt angles ranging from 0° to 57° and developed a model to accurately capture the tilting behavior. Furthermore, we demonstrate that the µFUR can control and change liquid spreading direction on demand, manipulate fluid drag, and tune optical transmittance over a large range. The versatile surface developed in this work enables new opportunities for real-time fluid control, cell manipulation, drag reduction, and optical tuning in a variety of important engineering systems, including applications that require manipulation of both fluid and optical functions."
GaN MEMS Resonator Using a Folded Phononic Crystal Structure,"We present a gallium nitride (GaN) Lamb-wave resona-tor using a phononic crystal (PnC) to selectively confine elastic vibrations with wide-band spurious mode sup-pression. A unique feature of the design demonstrated here is a folded PnC structure to relax energy confine-ment in the non-resonant dimension and to enable routing access of piezoelectric transducers inside the resonant cavity. This feature provides a clean spectrum over a wide frequency range and improves series resis-tance relative to transmission line or tethered resona-tors by allowing a low-impedance path for drive and sense electrodes. We demonstrate GaN resonators with wide-band suppression of spurious modes, f.Q product up to 3.06×1012, and resonator coupling coefficient keff2 up to 0.23% (filter BW up to 0.46%). Furthermore, these PnC GaN resonators exhibit record-breaking power han-dling, with IIP3 of +27.2dBm demonstrated at 993MHz.This work focuses on developing MEMS resonators for channel-select filtering in RF receiver front ends. For a MEMS band pass filter, the presence of spurious modes in the constituent resonators strongly impacts filter performance. Resonators with a clean frequency spectrum help reduce ripples in the pass-band and prevent interference from unwanted signals outside the pass-band. Conventional MEMS resonator designs with free mechanical boundaries are inherently prone to spurious modes, since free boundaries act as acoustic reflectors over all frequencies. To resolve this issue, the resonator boundary needs to be frequency selective. One way is by using PnCs, which involve periodic scatters to achieve highly reflective boundary conditions only for frequencies in a specific range. This acoustic band gap can be engineered based on the unit cell size and material configuration. While the acoustic band gap of these PnCs helps reduce resonance outside the band gap, these structures provide no spurious mode suppression inside the band gap. Further, transducers must be routed through the PnC in these configurations, leading to resistive loading of Q. In this work, we demonstrate a new resonant structure leveraging both PnC acoustic confinement and the electromechanical benefits of GaN. The proposed GaN folded PnC structure provides several important benefits:wide-band spurious mode suppression, both outside and inside the PnC band gap, through relaxed confinement in the non-resonant dimension,low-loss electrical routing to the resonant cavity, improved heat dissipation relative to other PnC or tethered resonators, androbust design that is immune to residual stress and handling.The folded PnC design achieves these improvements while maintaining quality factor and transducer cou-pling comparable to traditional tethered resonators."
GaN RF MEMS Resonators in MMIC Technology,"As a wide bandgap semiconductor with large break-down fields and saturation velocities, gallium ni-tride (GaN) has been increasingly used in high-power, high-frequency electronics and monolithic microwave integrated circuits (MMICs). At the same time, GaN also has excellent electromechanical properties, such as high acoustic velocities and low acoustic losses. To-gether with a strong piezoelectric effect, these make GaN an ideal material for RF MEMS resonators. This work focuses on the optimization of L-band (1-2 GHz) GaN resonators in standard MMIC technology.For monolithically integrated resonators, various constraints of the technology must be considered, such as the thickness of the GaN MMIC heterostructure, residual stresses in the GaN film, and the lack of bottom electrodes. Residual stress due to high temperature growth can affect the mechanical properties of the resonators and even lead to cracking and breaking. To achieve high performance resonators with multiple frequencies on the same chip within this technology, we designed 5th-order extensional resonators driven piezoelectrically with a top metal interdigitated transducer (IDT) as shown in Figure 1. These resonators have achieved mechanical quality factors >5500 at 1GHz, with f·Q products >5.5×1012, the highest demonstrated in GaN to date. Enhanced signal-to-noise ratios (SNR)at high frequencies can be obtained by using active transistor sensing. We demonstrate the first mechanically-coupled Resonant Body Transistor, in which the drive transducer and sensing high electron mobility transistor (HEMT) are embedded in two separate cavities, as shown in Figure 1. This additional electrical isolation between drive and sense allows for an improvement in the SNR of >50× compared to previous designs. The large SNR, together with high Q (Figure 2), makes these resonators ideal for monolithically integrated low-phase noise oscillators, with applications in clocking and wireless communications."
Ion Energy Measurements of Dense Plasmas with a Microfabricated RPA,"The energy of ions determines the efficiency of plasma propulsion systems and governs surface chemical reac-tions in plasma etching chambers. In plasma diagnostics, the instrument used to measure the ion energy distri-bution is the Retarding Potential Analyzer (RPA). How-ever, high-density plasmas of interest require tens- to hundreds-of-microns scale dimensions. Through MEMS processing techniques, our RPA achieves the small aper-ture sizes necessary to measure dense plasmas. Precise alignment between successive microfabricated grids is achieved through compliant support structures in the housing (as Figure 1 shows). The silicon spring tips mate with corresponding notches in the electrodes to provide robust alignment on the order of 1 µm and to increase the overall sensor’s ion transmission.Our previously reported RPA, deemed “hybrid” on account of incorporating microfabricated electrodes in a conventionally machined sensor, demonstrated improved performance over conventional RPAs. By reducing the aperture size while enforcing some degree of aperture alignment, we achieved a better resolution with no loss in signal strength compared to conventional mesh RPAs. Measurements of the ion energy distribution in a helicon plasma were obtained at MIT’s Plasma Science and Fusion Center using our sensors with microfabricated electrodes having 100 µm apertures. However, as a consequence of its larger apertures, the conventional RPA design was unable to effectively trap the plasma, and therefore no ion distribution could be extracted with this traditional device.Figure 2 shows ion energy distributions obtained with an ion source comparing the performance of a conventional RPA (with 152 µm apertures), the hybrid RPA (with 100 µm apertures), and MEMS RPA (with 150 µm apertures). The MEMS RPA design utilizes a fully microfabricated housing to improve upon the inter-grid aperture alignment over the hybrid sensor. Additionally, various aperture diameters are utilized in the electrode stack to mitigate current interception within the sensor. These RPA improvements result in an order of magnitude increase in signal strength over the conventional device and a threefold increase in energy distribution resolution."
High Throughput Electrospinning of Nanofibers from,"Nanofibers promise to be a key engineering material in the near future due to their unique, nanoscale mor-phological properties. In particular, the large specific surface area of the porous webs they form make them highly desirable as scaffolds for tissue engineering; layers in multifunctional filters/membranes; and com-ponents in devices such as fuel cells, solar cells, and ul-tra-capacitors. However, their integration into almost all of these technologies is unfeasible as a result of the low throughput, high cost, and poor control of current production methods. The most common process for producing nanofibers involves applying strong elec-tric fields to polar, high-molecular-weight polymeric liquids pumped through a syringe in what is known as electrospinning. Electrospinning is the only known technique that can generate nanofibers of arbitrary length; it has tremendous versatility as it can create non-woven or aligned mats of polymer, ceramic, semi-conducting, and/or metallic fibers.We implement high throughput arrays of externally-fed, batch-microfabricated electrospinning emitters that are precise, simple, and scalable. We fabricate monolithic, linear emitter arrays that consist of pointed structures etched out of silicon using DRIE and assemble these into a slotted base to form a two-dimensional array. By alter-ing the surface chemistry and roughness of the emitters, we can modify their wetting properties to enable wicking of fluid through the micro-texture (as in Figure 1). The in-terplay between electric, viscoelastic, and surface tension forces governs the fluid transport and fiber formation. We achieve over 30 seconds of stable electrospinning of polyethylene oxide (2-4% w/v in 60/40 water/ethanol solu-tion) from 9 emitters in a two-dimensional array with a density of 11 emitters/cm2 using bias voltages around 10kV (see Figure 2). This density is 7 times greater than the emitter density achieved in similar array-based ap-proaches. Current work focuses on characterization of larger, denser arrays to demonstrate uniform emission."
Near-Monochromatic X-ray Sources Using a Nanostructured Field Emission Cathode and a Transmission Anode for Markerless Soft Tissue Imaging,"A conventional X-ray generator consists of a thermion-ic cathode and a reflection anode inside of a vacuum chamber that has an X-ray transmission window. The cathode generates a beam of electrons that is acceler-ated towards the anode, which is biased at tens of kilo-volts above the cathode voltage. Some of the electrons collide with the anode and convert their kinetic energy into radiation, a fraction of which escapes the vacuum chamber through a transmission window made of a suitable material, such as beryllium. The X-ray emis-sion is a mix of bremsstrahlung radiation (broad, con-tinuous spectrum) and fluorescence (emission at spe-cific peaks corresponding to atomic shell transitions). Conventional X-ray technology requires high vacuum to operate, does not efficiently produce X-rays, and has overall low power efficiency. Conventional X-ray gen-erators cannot image well soft tissue unless contrast media, i.e., markers, are employed.We are developing efficient X-ray generators capable of soft tissue imaging using batch-microfabricated field emission cathodes composed of arrays of self-aligned, gated, and nanometer-sharp n-silicon tips, and a microstructured transmission anode (Figure 1). The nanostructured silicon cathode operates at low voltage and reliably achieves high-current emission with high transmission. The transmission anode efficiently generates X-rays while reducing the background radiation, resulting in emission of X-rays with narrow spectral linewidth for sharp imaging of biological tissue.Using our first-generation X-ray source (a tabletop apparatus), we have obtained absorption images of ex-vivo samples that clearly show soft tissue and fine bone structures (Figure 2). Current work focuses in miniaturizing the X-ray source into a portable system, and in improving the cathode and anode components to achieve generation of coherent X-rays to make possible phase contrast imaging at a low cost."
Multiplexed MEMS Electrospray Emitter Arrays with Integrated Extractor Grid and CNT Flow Control Structures for High-Throughput Generation of Ions,"Electrospray is a process to ionize electrically conduc-tive liquids that relies on strong electric fields. Charged particles are emitted from sharp tips that serve as field enhancers to increase the electrostatic pressure on the surface of the liquid, overcome the effects of sur-face tension, and facilitate the localization of emission sites. Ions can be emitted from the liquid surface if the liquid is highly conductive and the emitter flowrate is low. Previous research has demonstrated successful operation of massive arrays of monolithic batch-mi-crofabricated planar electrospray arrays with an inte-grated extractor electrode using ionic liquids EMI-BF4 and EMI-Im—liquids of great importance for efficient nanosatellite propulsion and nanomanufacturing. The current design builds upon a previous electrospray array designs from our group by increasing the area density of the emitter tips and increasing the output current by custom-engineering nanofluidic structures for flow control.Our MEMS multiplexed electrospray source consists of an emitter die and an extractor grid die (Figure 1), both made of silicon and fabricated using deep reactive ion etching. The two dies are held together using a MEMS high-voltage packaging technology based on microfabricated springs that allows precision packaging of the two components with low beam interception. The emitter die contains dense arrays of sharp emitter tips with over 1,900 emitters in 1 cm2. A voltage applied between the emitter die and the extractor grid die creates the electric field necessary to ionize the ionic liquid. A carbon nanotube forest grown on the surface of the emitters transports the liquid from the base of the emitters to the emitter tips. Our electrospray arrays operate uniformly (Figure 2), and mass spectrometry of the emission demonstrates that our devices only produce ions."
"Exploration of the Packing Limits of Ultrafast, Optically-triggered Silicon Field-emitter Arrays Using the Finite Element Method","Ultrafast optically-triggered field emission cathodes bypass several disadvantages demonstrated by current state-of-the-art ultrafast cathodes, such as requiring ultra-high vacuum to operate and short lifetime, and are a promising technology for implementing spatial-ly-structured electron sources for applications such as free-electron lasers, compact coherent X-ray sourc-es, and attosecond imaging. Ultrafast optically-trig-gered cathodes composed of massive arrays of high aspect-ratio silicon pillars capped by nano sharp tips and 5 µm pitch were fabricated at MIT MTL. The effect of the geometry and the morphology of the Si pillar ar-rays on the ultra-fast emission characteristics of such cathodes is now explored using the finite element mod-eling in 2D and 3D. Since the field-emitted current depends exponentially on the surface electric field, we are interested in studying how the electric field is enhanced by the geometry and the morphology of the Si pillar arrays. We selected COMSOL Multiphysics to simulate the electric field of the devices. The 3D model (see Figure 1) consists of a single tapered pillar 2.0 µm tall and 0.7 µm wide at the base with a 6-nm radius hemispherical cap. Perfectly matched layers (PMLs) are added on the top and bottom to absorb the excited and higher order modes. Floquet periodicity is applied on the four sides of the unit cell to simulate the infinite 2D array. The port boundary condition is applied on the interior boundary of the PML as the excitation port to simulate the 800-nm incident wave at a glancing angle of 84° from normal (the same experimental setup described in the third reading below). This model is validated by verifying the Fresnel equations between Si and vacuum before inserting the Si pillar. The 2D slice contour plot (see Figure 2) shows the simulated electric field from a 1 GV/m incident field on an emitter with 1-µm pitch using frequency domain analysis. The maximum electric field at the tip is about 4.2 GV/m, i.e., the emitter tip has an field enhancement factor of ~4.2. Both 2D and 3D models are utilized to explore the effect of the geometry and the morphology of the Si pillar arrays on the field enhancement."
High-Current Field Emission Cold Cathodes with Temporal and Spatial Emission Uniformity,"Field emission arrays (FEAs) are an attractive alterna-tive to mainstream thermionic cathodes, which require high vacuum and high temperature to operate. Field emission of electrons consists of the following two processes: first, the transmission of electrons (tunnel-ing) through the potential barrier that holds electrons within the material (workfunction φ) when the barrier is deformed by a high electrostatic field and second, the supply of electrons from the bulk of the material to the emitting surface. Either the transmission process or the supply process could be the limiting step that de-termines the emission current of the field emitter. Due to the exponential dependence on the field factor, the emission current from the tips is extremely sensitive to tip radii variation. We have a process to achieve uni-form emission from nanosharp FEAs by both fabricat-ing highly uniform tip arrays and controlling the sup-ply of electrons to the emitting surface (see Figure 1). We have designed and fabricated FEAs in which each field emitter is individually ballasted using a vertical ungated field effect transistor (FET) made from a high aspect ratio (40:1) n-type silicon pillar. Each emitter has a proximal extractor gate that is self-aligned for maximum electron transmission to the anode (col-lector). Our modeling suggests that these cathodes can emit as much as 30 A.cm-2 uniformly with no deg-radation of the emitters due to Joule heating; also, these cathodes can be switched at microsecond-level speeds. The design process flow, mask set, and pil-lar arrays have been completed (as Figure 2 shows) with the self-aligned extractor gate. An ultra-high vacuum chamber has been built to test the devic-es. The chamber can test full 150mm wafers with six high voltage feed through and a step-down anode at 2x10-10 torr pressure while also imaging the electron emission on a phosphorus screen."
Photoactuated Ultrafast Silicon Nanostructured Electron Sources for Coherent X-ray Generation,"Nanostructured cathodes that can be switched at an ultrafast time scale (<50 ps) have applications in free-electron lasers and coherent X-ray sources. This project is creating the theory, modeling, and exper-imental results for a compact coherent xray source for phase contrast medical imaging based on inverse Compton scattering of relativistic electron bunches. The X-ray system requires a low-emittance electron source that can be switched at timescales in the low femtosecond range. The focus of our work has been the design, fabrication, and characterization of massive arrays of a nanostructured high aspect-ratio silicon structures to implement low-emittance and high-brightness cathodes that are triggered using ultrafast laser pulses to produce spatially uniform electron bunches. Laser pulses at 35 fs, 800 nm and a 3 kHz repetition rate from a titanium sapphire laser at an 84º glancing angle, inside a vacuum chamber at ~10-8 torr bathe a highly uniform array of ~2200 silicon pillars with a 5-μm pitch. The cathode chip is connected to ground through a picoammeter while the anode, a 0.25-inch plate 3mm above the cathode, connects to a voltage supply (see Figure 1). The cathodes show stable emission and emit over 1.2 pC average charge for over 8-million pulses when excited with 9.5-μJ laser energy with no degradation of the emission characteristic of the cathode. This result shows that silicon-based photon-triggered cathodes processed with standard CMOS processes and operated at high vacuum can function for extended periods without performance degradation. The cathodes are fabricated from single-crystal <100> n-Si 1-10 Ω-cm wafers. The result is massive arrays of pillars (over half a million elements with 5-µm hexagonal packing) capped by tips with under-5-nm average tip radius and less than 1-nm standard deviation (see Figure 2). Through simulation and experiment we have demonstrated that the emitters operate in two distinctive regimens, i.e., the low-electric field multi-photon regime (similar to a typical photocathode), and the high-field quantum tunneling regime (similar to a field emission cathode). Actuation of the devices with laser pulses of 10 µJ or lower results in electron emission with no device degradation."
Field Emission Neutralizers for Electric Propulsion of Small Spacecraft in Low Earth Orbit,"Electric propulsion (EP) systems are excellent candi-dates for small spacecraft since EP systems consume less propellant than chemical rockets. In EP systems such as field emission electric propulsion thrusters (FEEPs), ion engines, and hall thrusters, a beam of posi-tive ions is ejected at high speed to produce thrust. If the ejecting charge is not compensated, the operation of the EP system will negatively charge the spacecraft, reduc-ing the propulsion efficiency and eventually stopping the thruster. Hence, development of robust, low-pow-er, and high-current neutralizers that do not consume propellant is necessary to advance the state of the art of EP systems for small spacecraft. Field emission neutral-izers (FENs) are promising candidates because of their low power consumption, high specific current, small size, and lack of propellant consumption. For operation in LEO, neutralizers must withstand long-term opera-tion in environments with oxygen partial pressures of ~5×10-7 Torr. Carbon nanotube-based FENs could satisfy these requirements; however, they require biases higher than 600 V for 1 mA emission current.This work develops arrays of Pt-coated, self-aligned, gated tips as low-voltage FENs for electric propulsion of small spacecraft in low Earth orbit. The neutralizers consist of 320,000 tips with 10 µm pitch and 5-10 nm tip radii; they have an integrated self-aligned gate electrode with 3 µm apertures. The devices emit currents higher than 1 mA at bias voltages as low as 120 V, i.e., similar currents at five-fold less bias voltage and emission area than state-of-the-art CNT neutralizers. The devices have a 2.5-µm-thick gate dielectric to prevent device failure due to dielectric breakdown; the tips are coated with a 10-nm-thick Pt film to improve the tip resistance against ion bombardment and reactive gasses. Continuous emission for 3 hours at pressures of 5×10-6 Torr in air was demonstrated. Less than 60 V increase in the gate-emitter voltage was sufficient to maintain the emission current at 1 mA."
Field Emission Arrays with Integrated Vertical Current Limiters and Self-aligned Gate Apertures,"Field emission cold cathodes are some of the bright-est electron sources ever reported, making them an ideal source in a variety of applications, including mi-croscopy, lithography, imaging, and the generation of terahertz and X-ray radiation. Field emission arrays (FEAs) suffer from emitter tip radius variation across the array and sensitivity to the state of the emitting surface, resulting in spatial and temporal variations of emission current. To address these issues, we previous-ly demonstrated that a high-aspect-ratio silicon ver-tical current limiter (VCL) that is connected in series with each field emitter in a field emission array could regulate the supply of electrons to each emitter and result in uniform emission; however, due to the lack of an integrated extractor gate, these devices operate at high extraction voltages and 99% of the total emitted current is intercepted by the extraction gate. Large ex-traction gate voltages are required due to the low field factor, β (cm-1), and result in a high Fowler-Nordheim (FN) slope bFN (V), arising from the large extractor gate−tip distance. To reduce the extractor voltage and enable low-voltage operation, we report Si FEAs with 1 million individual field emitters that have a 1-micron pitch with integrated VCLs poly-silicon extractor gates. These VCLs are Si pillars that have diameter less than 100 nm and are 10 microns tall, with tip radius under 20 nm. A schematic diagram and circuit diagram are shown in Figure 1 (a),(b). To fill in the gaps between the pillars and to support the self-aligned gate, a novel gap-filling process consisting of silicon dioxide and silicon-rich nitride deposition and chemical-mechanical planarization was employed, resulting in the structure shown in Figure 1 (c). The diameter of the extractor gate aperture is under 200 nm. As shown in Figure 1(d), these devices exhibit turn-on voltages less than 20 V and saturation currents of approximately 1 pA / emitter."
Low-Voltage High-Pressure Gas Field Ionizers,"Low power consumption, soft-ionization capability, and the potential for operation at high pressures are characteristics desired in gas ionizers for application to portable analytical instruments. Unlike impact ion-ization techniques, field ionization provides an efficient method for producing stable molecular ions—even from complex organic compounds. Consequently, field ion sources can generate nonfragmented ions for exact measurement of the mass-to-charge ratio of an analyte. These devices are used in various analytical instru-ments such as field ion mass spectrometers (FIMS) and atom beam microscopes. Other applications include gas chromatography FIMS for analysis of petroleum prod-ucts and neutron generators for detection of shielded nuclear material and oil-well logging. Despite the attrac-tive features offered by field ion sources, long-term, reli-able, and high pressure operation has not been reported due to high voltages (> 500 V) needed for field ionization using the current state-of-the-art devices. We have developed low-voltage Torr-level gas field ion-izers with operating voltages as low as 150 V even for He, which has the highest ionization potential among mol-ecules. The ionizer consists of a large array of Pt-coat-ed self-aligned gated Si tips with radii <10 nm and gate apertures of 3 µm. The tips were designed to generate fields above 20 V/nm at gate-to-tip voltages lower than 200 V while the field at the edge of the gate remains be-low 0.2 V/nm. A 2.5-µm-thick stack of silicon oxide/sili-con nitride was employed as the gate dielectric to limit the field intensity inside the gate dielectric to less than 100 V/µm, allowing prolonged operation of the device. Continuous field ionization of He and N2 for 104 s was achieved at pressures as high as 10 Torr. A slow decay in ion current was observed over time, which can be explained by adsorption of particles at the tip surface. Nevertheless, the original device characteristics can be recovered by operating the device as field emitter in a high vacuum (<10-7 Torr)."
Large-Area Field Emission Arrays for High-Current Applications,"Gyrotrons, free electron lasers (FELs), and THz vac-uum electronic devices require intense high-current electron beams. High-current, high-current-density electron beams are also needed for X-ray generation, pumping of gaseous lasers, and surface treatment of materials. Field emission sources show great promise for these applications as they can produce current densities higher than 10 A/cm2 at voltages below 100 V. Despite these promising attributes, the state-of-the art devices have produced currents less than 300 mA due to limitated array size (1–10 mm2) because of fabrication issues that result in failure or severe sub-utilization of the array. The major challenges include low yield of fabrication, large variation in gate and tip dimensions across the array, and point defects in the gate dielectric.We have developed a high-yield process for fabrication of large-area, self-aligned, gated tip arrays with low sensitivity to processing conditions. The fabricated field emission arrays (FEAs) demonstrate average field factor >106 cm-1 using nanometer-scale tips (radii < 10 nm) surrounded by individual gates with 1.5 µm radius of aperture. This ensures low-voltage operation of the device and a turn-on voltage below 50 V. For reliability a thin Pt layer was deposited over the FEA and a SiOx/SiNx dielectric stack thicker than 2.5 µm was used as the gate insulator. The Pt coating ensures chemical resistivity of the tips against corrosive gasses/ions, and the thick insulator stack limits the field inside the gate dielectric to < 150 V/µm at Gate-Emitter voltages of < 300 V. Our FEAs consisting of 320,000 tips in 0.32 cm2 are capable of emitting currents as high as 350 mA at densities of ~1.1 A/cm2. The device operation at higher emission currents was prevented due to plasma ignition because of the excessive outgassing of the anode. At low pressures, long-term (~3 hrs) operation not only was possible but also lowered emission voltage and gate current."
Evaporation through Nanoporous Membranes for High Heat Flux,"The development of ever more compact electronic cir-cuits has brought the demands for thermal management to unprecedented levels. Although there has been exten-sive research on single phase and multi-phase cooling in microchannels, evaporative cooling in the thin film regime has the potential to reach an even higher heat flux. We report the design and fabrication of a novel sil-icon-based evaporation device for direct integration into high power density electronics. We designed a micro-scale device, relying on the evaporation of a very thin liquid film to dissipate over 1000 W/cm2 with an overall temperature difference of less than 30 K. Evaporation occurs in a 200 nm thick silicon membrane patterned with 100 nm pores using interference lithography. The nanopores create a large thin-film evaporation area and generate a large capillary pumping pressure to supply fluid to the membrane. The membrane is thermally bonded to an arrayed supply network of 4 µm x 4 µm microchannels whose walls provide mechanical support and a thermal conduction pathway from the substrate. The substrate is resistively heated, and the temperature is measured with RTDs fabricated with a lift-off pattern. A finite element model is developed to optimize the microchannel and membrane geometry. The convective heat transfer coefficient is modeled by numerically solving governing equations of heat, mass, and momentum conservation at the pore level. Evaporation through nanoporous membranes has the potential for achieving ultra-high heat flux dissipation (5 kW/cm2) for high-performance electronic devices."
Experimental Investigation of Thin-film Evaporation from Microstructured Surfaces for Thermal Management,"Thermal management is a primary design concern for numerous high power density devices such as in-tegrated circuits, electric vehicles, military avionics, photonic devices, and solar energy convertors. This is especially true in the microelectronics industry where the increase in the number of integrated circuits and operating speed has increased the waste heat that is generated at the device footprint from 30 W/cm2 in the 1970’s to 100 W/cm2. Moreover, this heat flux is projected to reach 300 W/cm2 in the next few years introducing new challenges in thermal management that has forced the industry to seek advanced cooling solutions. Unfortunately, the widely used convention-al single-phase cooling systems are inferior in perfor-mance and cannot be used for applications that require removal of high heat fluxes in excess of 100 W/cm2. As a result, state-of-the-art single-phase cooling systems are limited to low heat flux devices and the proposed solution is to use liquid-vapor phase change systems such as thin-film evaporation [1, 2] to make use of the high latent heat of vaporization that can be harnessed during the phase change process.In this experimental study, we investigated the com-plex fluidic and thermal transport processes when a thin-liquid film is evaporating from a microstructured surface. We fabricated well-defined microstructured surfaces using contact photolithography and deep reac-tive ion etching. In addition to offering rich opportuni-ties to manipulate the fluid dynamics, microstructured surfaces in combination with chemical functionaliza-tion have long been recognized for enhancing thermal performance in phase-change process. The induced roughness generates capillary pressure for passive liquid transport [3]. The liquid transport was further assisted by incorporating microchannels which reduce the overall flow resistance of the porous media. For in-tegrated heating and temperature measurement, we used electron-beam evaporation and acetone lift-off to create a thin-film heater and sensors. This work eluci-dates new and innovative techniques to utilize micro-structured surfaces for thermal management."
Scalable and Direct Water Purification Technology by Ion Concentration Polarization,"We demonstrate a scalable and direct water purification technology using ion concentration polarization (ICP). Although nonlinear ICP was shown to generate a strong depletion zone near the ion exchange membrane (IEM), several challenges (power consumption, expandability, etc.) must be overcome for ICP to be a competitive technology in desalination. To resolve and improve them, we propose a modified ICP platform for water desalination by involving two identical cation exchange membranes (CEMs); it demonstrates better salt removal and energy efficiency than conventional electrodialysis (ED), as Figure 1 shows. Between two parallel CEMs, ion depletion/enrichment zones are generated near the CEMs under an electric field. As cations selectively transfer through the CEMs, anions relocate to achieve electro-neutrality, resulting in a decreased/increased concentration in the ion depletion/enrichment zone. Given that the desalted and brine flow streams form on the cathodic and anodic CEM in the main channel, respectively, we can separate and collect each desalted and brine flow by bifurcating the channel at the end. Our technique offers a significant advantage for reducing the number of water purification stages over other conventional technologies since we can obtain desalted flow delivering any charged particles (contaminants) to the brine channel simultaneously. To visualize the electrokinetic phenomena between the membranes, we fabricated a PDMS-based microfluidic chip with thin channel depth (~0.2mm) and injected a sodium chloride solution mixed with fluorescent dye, as in Figure 2(a). Additionally, to increase system throughput, we built a plastic-based desalination prototype (~1ml/min) by expanding the channel depth and successfully operated it over ten hours, as shown in Figures 2(b) and 2(c). Therefore, we expect our ICP desalination to be a practical technology for water purification, providing both lower energy cost and high throughput."
Electrostatic Precursor Films,"When a liquid spontaneously spreads over a solid sur-face, a progressive microscopic structure—convention-ally known as van der Waals driven precursor film—de-velops ahead of the moving contact line. Here, we report a new class of electrostatically assisted precursors con-taining microscopic charged particles. This precursor manifests itself as the late stage of forced-spreading of a macroscopic dielectric film subjected to a unipolar ionic discharge in a gas containing particulates. We put a model forward to predict dynamic behavior of this electrostatic precursor dynamics. The spreading of the precursor film is predicted to be proportional to the square root of exposure time, which is consistent with the ellipsometric measurements."
Continuous-flow Microcalorimetry Using Silicon Microreactors and Off-the-shelf Components,"Calorimetry is an important method for studying the kinetics and energy requirements of chemical and/or biological reactions. In particular, calorimetry can characterize the heats of reaction (ΔH) to determine the necessary heat transfer requirements when scaling up production, for example whether the system has the appropriate amount of heating/cooling elements to sustain its optimal reaction conditions. There are many products and devices capable of characterizing ΔH, such as differential scanning calorimeters, thermal activity monitors, and isothermal nanocalorimeters; however, these systems utilize fixed volumes of reac-tants and are inherently incapable of being run in-line with continuous flow without complex modifications. Unlike traditional calorimeters, this microcalorimeter is designed for continuous flow and to run in-line with an automated microfluidic reaction optimization system with little-to-no modifications. Previously, a similar microcalorimeter was proposed; however, the design had a high heat flux threshold (>50mW), limiting its usefulness to high-energy and/or high-concentration reactions (>1M for reactions where ΔH ≥ 50kJ/mol). This previous design had several other drawbacks including long thermal time constants due to its large thermal mass and requiring a control (baseline) reaction to be run sequentially with the sample reaction. Our design utilizes a parallel-reactor setup, enabling the baseline and sample reactions to run concurrently and allows for direct measurement between the parallel reactors. This parallel setup reduces the thermal mass and experimental time and results in a predicted 5x increase in thermal sensitivity. As such, the microcalorimeter is capable of characterizing ΔH’s faster than the previously mentioned design while at lower (<1M) concentrations. Currently, the continuous-flow microcalorimeter consists of two parallel silicon microreactors, one running the chemical reactions and the other running a baseline reaction. The microreactors are sandwiched in between a series of thermoelectric modules and a machined aluminum jig, and the ΔH is measured by heat flux between the microreactors. The microcalorimeter was used to characterize a Paal-Knorr reaction, resulting in the thermoelectric modules measuring a voltage of 3.70±0.27mV, corresponding to a heat flux of 170.7±12.5mW. When running the baseline reaction in both reactors, the system had a noise floor of 0.19mV. Extrapolating the signal to the noise floor, we predict that the microcalorimeter will be capable of measuring heat fluxes as low as 8.6mW.Our next step for the microcalorimeter is to continue refining the thermal control mechanisms to further improve the heat flux sensitivity. Additionally, we will designed and fabricate a specialized silicon microreactor for the ΔH characterization of solar thermal fuels, molecules designed by our collaborators that are capable of storing solar energy and subsequently releasing the solar energy as heat at a later date. Finally, the system will be inserted into an automated cycling setup to monitor to analyze the stability and cycling longevity of the solar thermal fuels."
MEMS Two-stage Diaphragm Vacuum Pump,"Portable sensing devices such as microscale mass spec-trometers need vacuum pumping to lower samples at atmospheric pressure to the desired measurement pressure range. Further improvements for MEMS accelerometers, gyros, and other resonant sensors require internal pressures as low as a few microtorr, which is possible only with active vacuum pumping. While these pressures are easily achieved using mac-roscale vacuum pumps, the larger pumps are not por-table, negating the benefits gained from making small, low-power sensors in the first place. To realize the full potential of portable sensors, a chip-scale vacuum pump needs to be developed.We have developed what is to our knowledge the first two-stage MEMS displacement pump with integrated electrostatic actuation. Two pump stages, along with an efficient layout that minimizes dead volume and a new actuation scheme should enable it to reach pressures below 30 Torr. Actuation is achieved by electrostatically zipping a thin flexible membrane down onto a stiff curved electrode. This actuator topology allows for large displacements and large forces at relatively low voltages (< 100 V). An image of a fabricated two stage micropump is shown in Figure 1 below.We have developed two methods for producing curved electrodes in MEMS devices: 1) hot air trapped during wafer bonding expands with enough pressure to plastically deform thin silicon membrane and 2) strain induced when epoxy cures can pull a membrane into a curved shape. Using the plastically deformed electrodes, we have demonstrated that we can reliably and repeatably zip a thin membrane at low voltages and we have mapped out how the critical voltage depends on the deformation magnitude and the oxide thickness. This is shown in Figure 2 below. These accomplishments have helped us understand the fabrication process and physics of device operation. We plan in the next year to further examine the reliability of plastic deformation in our process, testing the actuators at low pressures, and we hope to fabricate a working micropump that is capable of achieving pressures as low as 30 Torr."
A Tabletop Fabrication System for MEMS Development and Production,"A general rule of thumb for new semiconductor fabrica-tion facilities (fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for small entities seeking to develop or commercialize new semiconductor devices. The barrier is especially for-midable for those groups whose devices target smaller market segments or those which require exotic materials or nontraditional process sequences. The foundry fab model has arisen partially to overcome this inefficiency, but to remain profitable, these foundries typically offer only a few standardized processes that limit customer customization. The limited diversity afforded by these foundries can make some devices with smaller market sizes economically viable, but many devices (particularly in the MEMS sector) require process customization be-yond the level currently offered by commercial foundries.To address these problems, we are working to create a suite of tools that processes 1-2” substrates. This suite of tools (known colloquially as the 1” Fab) takes advantage of modern processing techniques, but at a fraction of the normal cost. We anticipate a full set of tools for product development and small-scale production to cost ~$1 million and require <50 ft2 of space (roughly a large conference table, see Figure 1 for a rendering), compared to >$1 billion and >50,000 ft2 for a typical 8” fab. In addition to the reductions in equipment cost and required space, a 1” Fab also uses significantly less total materials and reagents, requires far less energy to operate, and lessens the environmental impact of fabrication. The total throughput possible in a 1” Fab certainly cannot match a typical 8” fab, but the vast majority of devices that are unsuitable for traditional foundries simply do not require this advantage in production rate. For these devices, the cost savings of the 1” Fab platform and its ability to quickly prototype designs far outweigh any expansion in production schedules. We are currently developing a deep reactive ion etcher (DRIE) tool for the 1” Fab. DRIE tools are used to create highly anisotropic, high aspect-ratio trenches in silicon—a crucial element in many MEMS processes. The modularized design of our DRIE system can be easily adapted to produce other plasma-based etching and deposition tools (like PECVD and RIE). Our DRIE, shown in Figure 2, is the about the size of a large microwave oven and costs just a small fraction of a commercial system. We have demonstrated etch rates of >6 microns per minute and anticipate achieving etch rates of 10µm/min with further process tuning. In the coming year we will continue to optimize our DRIE design and begin developing PECVD and high-temperature process (e.g. oxidation and LPCVD) tools for the 1” fab."
A Double-gated CNF Tip Array for Electron-impact Ionization and Field Ionization,"Carbon nanofibers have been investigated for a wide range of applications today. In particular, due to their remarkable conductivity, carbon nanofibers have generated a lot of interest for applications in vacuum microelectronic devices [1-2]. For example, the ionization sensor for gases is one of the most important applications since the conventional ionization sensors are bulky, require high-voltages, and consume high power. The purpose of this project is to fabricate carbon nanofiber field emission and field ionization arrays, which can be utilized in a micro-gas sensor. This device can help reduce the size of the sensor and operating voltages required for gas analysis.In this project, the PECVD method is used to grow vertically oriented carbon nanofibers. The number of carbon nanofibers per site is controlled by the Ni catalyst dot size. It has been demonstrated that the diameter of the Ni catalysts disk must be 300 nm or less to ensure the growth of only a single carbon nanofiber [3]. The 250-nm Ni dots used in this work were defined by ebeam lithography. Figure 1 shows a close-up SEM picture of vertically aligned single carbon nanofiber array grown by PECVD. Later, these vertically-aligned single carbon nanofibers will be integrated into a double-gated field emission/ionization structure developed by L. Dvorson [4]. Figure 2 shows the schematic drawing of the final device.Using the device shown in Figure 2, two approaches for ionizing gas molecules will be investigated for micro-gas sensors. One approach is electron impact ionization, which uses strong electric fields to emit electrons followed by collision between the energetic electrons and neutral gas molecules resulting in ionization. The second approach is field ionization, which is a gentler process in comparison to electron impact ionization. It results in molecular ionization and a simpler mass spectrum due to lower fragmentation of molecules."
Hand-assembly of an Electrospray Thruster Electrode Using Microfabricated Clips,"This work [1] explores a method to precisely assemble two planar MEMS components. Our intended application is the assembly of the extractor electrode of an electrospray thruster, in which holes in the extractor must be aligned precisely with emitter needles or ridges (see [2] in this volume). In this method, the components can be accurately assembled by hand. Moreover, the assembly is made using a system of flexures, allowing considerable flexibility in the choice of materials and coatings for the components.Figure 1 shows a diagram of our device. The extractor electrode (red) needs to be assembled in a recess on the base of the electrospray thruster (blue). To do this, the extractor is placed by hand in its recess. This step is easy as there are a few hundred micrometers of slack. The extractor is then rotated. As it rotates, features around its periphery force it to align its center to within 50 micrometers of its final position. As it continues the rotation, flexible fingers on the base part get flexed by the extractor, until the fingertips fall into notches in the sidewalls of the extractor.Our devices, shown in Figure 2, were initially made out of Silicon using deep reactive-ion etching (DRIE). To show the flexibility of the method, we have also produced laser-cut polyimide extractors. The polyimide extractors have allowed us to achieve electrical insulation between the extractor and the rest of the device, which is vital for our intended application.We have measured front-to-back alignment on all our silicon devices and found that they are within 9 micrometers RMS of their intended location. However, multiple assembly/disassembly cycles on a specific device show that the position is repeatable to within 1.5 micrometers of standard deviation. This measurement suggests that much of the misalignment we are observing occurs due to misalignments during the various photolithography and bonding steps."
A Fully Microfabricated Planar Array of Electrospray Ridge Emitters for Space Propulsion Applications,"Electrospray thrusters work by extracting ions or charged droplets directly from a liquid surface using an electrostatic field and accelerating them in that field to produce thrust. This method could lead to more efficient and precise thrusters for space propulsion applications. The propellant liquid is generally placed at the tip of a needle to enhance the electrostatic field. The electrospray process limits the thrust from a single emitter needle. To get into the millinewton range will require an array with thousands of emitters. Batch microfabrication is well suited to making such an array.We have designed and built a prototype thruster that consists of two silicon parts (Figure 1) made using deep reactive ion etching (DRIE) and SF6 plasma etching. The thruster base holds the electrospray emitters. Its surface is treated to control the areas where propellant can go. The extractor produces the electric field, which generates the electrospray. It is equipped with slits to allow the accelerated particles through. The two parts are positioned relative to each other using a kinematic mount, in which alumina balls rest in holes on the silicon dies (Figure 1). Alumina screws hold the assembly together.In this design, we have replaced the needles that are typically used in electrospray thrusters by ridge emitters: vertical slabs with sharp tips spaced along their length (Figure 2). We have shown that our process for needles [1] can be extended to ridge shapes, and a modeling effort is underway to better control the shapes of the emitters.Our thruster has been fired with the ionic liquid EMI-BF4. This experiment shows successful electrical insulation, even in the presence of the liquid. Challenges we now face are reducing the amount of emission that is intercepted by the extractor and determining where on the ridges the emission is coming from."
"A Double-gated Silicon Tip, Electron-Impact Ionization Array","A device with the ability to ionize gases is needed for a variety of applications, of which the mass spectrometer (MS) [1-2] is one of the most important. The ionization method in the majority of gas analyzers in MS is electron-impact ionization, which uses a beam of electrons that collides with gas molecules. Through this collision process, energy is transferred from the electrons to the gas molecules, which causes electrons on the gas molecules to be stripped off (i.e., ionization of the gas molecules). Traditionally, thermionic emission, which consists of a filament that produces electrons when heated, is the most common way of generating electrons for MS using electron impact ionization. However, thermionic emission has several disadvantages: slow switch-on time, large power consumption, and lack of robustness. These disadvantages, however, are eliminated when field emission is used instead.In this project, a double-gated silicon field emission device is used to generate the electron source for electron impact ionization. Figure 1 shows a SEM picture of a double-gated silicon field emission device used here. Using this device, we have demonstrated the linear relationship between the emission current (IE) and the ion current (II) at a fixed pressure (10-4 torr) as shown in Figure 2."
A MEMS Electrometer for Gas Sensing,"The DARPA-funded micro gas analyzer program aims to develop portable, low-power, fast, and reliable gas analyzer technology for a wide range of applications. The system architecture of the gas analyzer contemplates a MEMS electrometer at the end of the system. The electrometer characterizes the ionized species that are filtered by the quadrupole. The sensitive element of the electrometer is a MEMS structure embedded in a feedback loop of a precise oscillator circuit. The electrometer has a comb drive that sets the electrometer to oscillate. Shifts in the oscillation frequency are related to changes in the capacitance of the electrometer due to ion interception. The resolution of the device is estimated at 100 e/√Hz in vacuum [1]. Figure 1 shows a fabricated MEMS electrometer. Figure 2 shows the experimental data of one of these MEMS electrometers, in air. The experimental resonant frequency is 6.2 kHz, and the conversion gain was estimated at 2 × 109 V/C (theoretical value is 7 × 109 V/C). Current research focuses on implement lock-in detection, which will remove the noise from the drive signal because the output has twice the frequency of the input signal."
A Single-Gated CNT Field-Ionizer Array with Open Architecture,"The micro gas analyzer project aims to develop the technology for portable, real-time sensors intended for chemical warfare and civilian air purity control. The device is composed of four micro-fabricated subsystems: an ionizer, a mass filter based on a quadrupole array [1], an electrometer [2], and a positive displacement pump [3]. We are developing a single-gated fieldionizer array based on gated carbon nanotubes (CNTs). The devices achieve species ionization by tunneling of outer shell electrons due to the presence of high electric fields that the device sets. We use CNTs as field enhancers because of their small radii and high aspect ratio while the gate proximity ensures high fields at low voltage. State-of-the-art ionizers use electron-impact ionization (thermionic cathodes), incurring in excessive power consumption, low current, current density, ionization efficiency, and short lifetime. The field-ionizer arrays (Figure 1) are able to soft-ionize species, thus achieving molecule ionization. The reliability and lifespan of the field-ionizer arrays are larger than the corresponding values for electron-impact ionizer arrays because the CNTs are biased at the highest potential in the circuit, thus making it unlikely for ionized molecules to back-stream. Figure 2 shows two SEM pictures of a single-gated CNT array that implements a selective CNT-growth process. This process reduces the fabrication complexity of the device because it grows CNTs from an un-patterned catalyst (Ni). Current research efforts concentrate on improving the device and data acquisition, including benchmarking the performance of the ionizer in low-pressure oxidizing environments."
A MEMS Quadrupole that Uses a Meso-scaled DRIE-patterned Spring Assembly System,"The DARPA-funded micro gas analyzer program aims to develop portable, low-power, fast, and reliable gas analyzer technology for a wide range of applications. One of the subsystems of the gas analyzer is a mass filter. An array of micro-fabricated quadrupole mass filters is being developed for this purpose. The quadrupoles will sort out the ions based on their specific charge. Both high sensitivity and high resolution are needed over a wide range of ion masses, from 20 to 200 atomic mass units. In order to achieve this performance, multiple micro-fabricated quadrupoles, each operating at a specific stability region and mass range, are operated in parallel. The proof-of-concept device is a single, linear quadrupole that has a micro-fabricated mounting head with meso-scaled DRIE-patterned springs. The mounting head allows micron-precision hand assembly of the quadrupole rods [1]–critical for good resolution and ion transmission. The micro-fabricated mounting head can implement quadrupoles with a wide range of aspect ratios for a given electrode diameter. There are currently two versions of the mounting head, able to interact with rods of diameters equal to 1588 and 559 micrometers. The choice of electrode diameter results from pondering the dimensional uncertainties and alignment capabilities with respect to the expected resolution and transmission goals. Figure 1 shows an assembled MEMS quadrupole, including some detail of the spring structure near the quadrupole transmission region. The quadrupoles that have been implemented so far span the aspect ratio range from 30 to 60. Figure 2 shows the experimental data of one of these quadrupoles on a FC-43 sample, where a mass resolution of 2 amu and a full mass range of 200 amu are demonstrated, while using a 1.2-MHz RF power supply to drive the quadrupole. Current research efforts concentrate on developing RF power supplies of higher frequency to obtain better performance from the same device."
Digital Holographic Imaging of Micro-structured and Biological Objects,"The need for understanding the trophodynamics of the ocean has led to the development of several instruments for monitoring plankton communities, critical indicators of the ocean’s health and the base of the aquatic food chain. The three competing methods for plankton observation utilize direct, acoustic, and optical sampling techniques; however none of the current systems can provide the complete data set required for predictive modeling capabilities. The goal of this project is to develop a small, low-power, digital holographic imaging (DHI) system that allows for in situ monitoring of plankton and other aquatic communities. This system allows microbiologists to collect high-resolution, spatio-temporal data on species-specific population structures. In addition to biological studies, the DHI camera can be utilized in diverse areas such as medical analysis, quality control inspection, and MEMS device characterization.DHI uses a digital sensor to record holograms, formed by the interference pattern between a reference wave and a field produced by scattered light from an illuminated object. The illumination source is coherent and typically provided by a laser. The recorded images are processed on a computer to reconstruct the original object field at a given axial location [1]. From the reconstructed images, information about the object such as morphology, topology, and 3D coordinates can be computed throughout a large sample volume. In addition, velocity and 3D trajectories are available under slightly modified methods.Experiments have focused on biological applications, including marine and microbial organisms ranging from 5 to 2000 microns. In addition to the inline configuration (Figure 1), several setups have been implemented to explore smaller scales, including the use of spherical reference waves, 4f telescopes, and microscope objectives. Figure 1 shows our compact benchtop prototype DHI camera, currently being developed to be used as a sea-going instrument for deep-sea microbiology. Using a lens-free spherical configuration with a working distance of 50 mm, all lines on a 1951 USAF resolution target can be resolved, down to 2.2 microns in width. A 4f system was used to track the trajectories of 7 micron algae over several seconds. Small plankton, 50 to 500 microns long, have been imaged using all three setups with excellent clarity. Figure 2 shows a reconstruction from an inline configuration of an adult copepod. Future work includes incorporating the DHI camera into an underwater vehicle. Additional work will focus on tracking small particles under turbulent flow conditions."
Aligning and Latching Nano-structured Membranes in 3D Micro-Structures,"The 3D micro-electro-mechanical systems (MEMS) manufacturing is an emerging technology that promises to solve many of the problems in the microfabrication industry. In microelectronics, as the feature sizes of the components approach their physical limits, packing more transistors on a microprocessor or on a memory chip requires expanding the circuitry into the third dimension. In optical switches, the traditional 2D MEMS-based switches do not scale easily beyond 32 ports; to increase the number of ports, companies have been developing 3D micro-mirror arrays that can reflect light in multiple directions. The Nanostructured Origami™ 3D fabrication process is a two-step method for fabricating 3D MEMS; it involves patterning films on a surface and then folding the patterned films to create three-dimensional structures [1]. This method is advantageous because it uses state-of-the-art 2D patterning methods and it involves patterning all the parts of the structure in one step, eliminating problems of feature misalignment. However, in creating the 3D structures, two major challenges arise; the first is to accurately place the folded membranes in their desired positions and the second is to fix the membranes in those positions to maintain the final 3D configuration. Current positioning solutions involve the use of mechanical motion-limiters that prevent folded membranes from moving beyond a certain point [2]. We propose two methods for aligning and latching folded nano-patterned membranes in 3D microstructures. The first method uses photoresist pads to glue together two mating surfaces of the structure (Figure 1). What distinguishes this method from previous polymer gluing attempts is that we use dense gold patterns as a local heater to melt the photoresist pads. This allows us to control the membranes we latch and the time when we latch them. We use patterned gold wires to form the hinges that hold the membranes together. Thin dense gold patterns also serve as local heaters to melt the photoresist gluing pads. The surface tension in the molten pads aligns the surfaces and solidification of the photoresist latches them together. The second method uses mechanical alignment and latching features that allow edges-to-surface latching (Figure 2). One major advantage of this method is that the structural components and the alignment features are patterned in the same lithographic step, which lowers costs and minimizes misalignment errors. Another interesting aspect is the cascaded alignment; the alignment features are designed so that they function sequentially, starting from the features closest to the hinge. With proper design of those features, the alignment system can achieve accurate positioning using the features away from the hinge while tolerating a large initial positioning error range by virtue of the short radius sustaining the features closest to the hinge."
A Microfabricated Platform for Investigating Multicellular Organization in 3-D Microenvironments,"Understanding how complex intrinsic and external cues are integrated to regulate cell behavior is crucial to the success of cell-based therapies in the treatment of human disease. Systematic and quantitative investigation of these microenvironment signals was first enabled by precise cell positioning using 2-D micropatterning tools [1]. However, cellular signaling is often altered in adherent tissue culture where structural cues are lacking (including tumor, stem, and differentiated cells), in contrast to 3-D culture systems that more closely resemble in vivo cell behavior [2]. Our goal was to develop new micropatterning tools capable of micron-scale cell patterning and organization within a 3-D hydrogel with tissue-like properties. We developed a technique for the rapid formation of reproducible, high-resolution, 3-D cellular structures within a photo-crosslinkable hydrogel using dielectrophoretic forces (Figure 1) [3]. We demonstrate parallel formation of ~20,000 cell clusters of precise size and shape within a 1 x 2 cm2 slab of tissue (Figure 2a), with high cell viability and differentiated cell function maintained over 2 weeks in culture. By modulating cell-cell interactions in clusters of various size (independent of hydrogel geometry, chemistry, or volumetric seeding density; Figure 2b), we present the first evidence that 3-D microscale tissue organization regulates chondrocyte behavior (Figure 2c) [3]. This dielectrophoretic cell patterning (DCP) technology enables further investigation of the role of tissue architecture in many other multicellular processes from embryogenesis to regeneration to tumorigenesis."
Microfluidic Hepatocyte Bioreactor,"This project utilizes microfluidic systems to study how groups of liver cells acquire emergent tissue properties. Hepatocytes (the parenchymal cells of the liver) respond to many cues in their microenvironment: neighboring cells, growth factors, extracellular matrix, dissolved oxygen, and their interactions. One tissue property of interest is the compartmentalization of gene expression in multicellular domains along the liver sinusoid. This process, often described as “zonation,” underlies much of liver physiology and regional susceptibility to toxins. We have previously shown oxygen gradients can be used to compartmentalize mixed populations of hepatocytes in a large-scale reactor [1]. Here, we present a microdevice that enables one to explore the crosstalk between two inputs (oxygen gradients and soluble growth factors) in a systematic fashion. The device consists of a two-layer PDMS microfluidic network with an on-chip dilution tree bound to a glass slide with an array of microreactors. Hepatocyte zonation is induced in each microreactor through local oxygen concentration, which is modulated through gas channels separated from the bioreactor by a 100-µm PDMS layer as shown in Figure 1. The local oxygen concentration in the microchannels is quantified in Figure 2. Primary rat hepatocytes are seeded into microreactors together with 3T3 fibroblasts, which act to stabilize the hepatocyte phenotype as described previously. This device will be useful to further explore liver tissue biology in vitro including the dynamics of zonation, mechanisms of oxygen sensing, and the role of growth factors in zonal response."
Micromechanical Control of Cell-Cell Interaction,"Cellular behavior within tissues is driven by environmental cues that vary temporally and spatially with granularity on the order of individual cells. Local cell-cell interactions via secreted and contact-mediated signals play a critical role in these pathways. In order to study these dynamic small-scale processes, we have developed a micromechanical platform to control microscale cell organization so that cell patterns can be reconfigured dynamically. This tool has been employed to deconstruct the mechanisms by which liver-specific function is maintained in hepatocytes upon co-cultivation with stromal support cells. Specifically, we examine the relative roles of cell contact and short-range soluble signals, duration of contact, and the possibility of bi-directional signaling.The device consists of two silicon parts that can be locked together either to allow cell-cell contact across the two parts or to separate the cells by a uniform gap of approximately 80 µm (Figs. 1 and 2). Switching between these two states is actuated simply by pushing the parts manually using tweezers; no micromanipulation machinery is necessary. Micron-scale precision is possible due to a 10:1 mechanical transmission ratio and microfabricated snap locks, both of which are monolithically incorporated into the silicon structure. The entire device is fabricated in a simple single-mask process using through-wafer deep reactive ion etching. To provide a surface compatible with cell culture, the surface is coated with a layer of polystyrene and plasma-treated, providing a standard tissue-culture surface."
Characterization and Modeling of Non-uniformities in DRIE,"Our previous work on spatial non-uniformities in deep-reac-tive ion etch (DRIE) has provided a method by which an etch-ing tool and associated “recipes” of operating parameters may be pre-characterized [1]. That work allowed the wafer-average pattern opening density (or “loading”) to be related to wafer-scale etch rate variations. Such variations have been attributed to loading-dependent interactions of the flux densities of SxFy ions and F neutrals and to shifts in the gross flows of fluorine across the wafer [2]. Unlike some other approaches [3–5], our method captures asymmetries in the fluxes within the chamber. Our approach is now supplemented by an understanding of how uniformity depends on the localization of etched patterns within the wafer (Figure 1). A semi-physical model represents the diffusion of monatomic fluorine etchant parallel to the wa-fer’s surface, giving a two-dimensional filter which translates a discretized map of pattern density into a prediction of how etch rate will vary within and between dies [6]. This die-level mod-el is readily combined with the existing wafer-level model. To tune this combined model for a new recipe, a set of about five test wafers is etched, and fitting algorithms are run with etched-depth data. Collaborative experiments with Surface Technology Systems Ltd have demonstrated our approach in use with a prototype etch tool. Further experiments have compared the characteristics of different manufacturers’ tools. We have also quantified a memory effect whereby the aver-age pattern density of one etched wafer can affect the average rate and non-uniformity with which a subsequent wafer etches (Figure 2). In the future we aim to incorporate well-known fea-ture size or aspect ratio effects into our model [7]. We envisage our approach being integrated into computer-aided design systems for MEMS and believe that it will be of particular use when one is keen to preserve a fast-average etch rate and is thus loath to win uniformity by reducing the chamber pres-sure."
Understanding Uniformity and Manufacturability in MEMS Embossing,"The hot embossing of thermoplastic materials, such as polymethylmethacrylate (PMMA) or cyclo- olefin copolymer (COC), is a promising way to manufacture microfluidic channels and networks [1]. Hot embossing potentially offers lower per-area cost than the micromachining of quartz or silicon and easier scaling-up of production than soft lithography using polydimethylsiloxane [2]. In hot embossing, a microfabricated mold (typically of silicon or nickel) is pressed into a flat sample of polymeric material that has been softened by heating it above its glass-transition temperature. We are particularly interested in how the spatial distribution of mold features—their diameters, shapes, and areal densities—may influence the quality of embossed patterns. We are developing a simulation approach whose building-block is a simple model in which, for given embossing conditions, a feature-sized disk of viscous polymer is compressed at a rate inversely proportional to the square of the radius of the disk [3] (Figure 1). Such a model implies that the mold will sink into the substrate at a spatially uniform rate when the product of the areal density of mold features and the square of their average radius remains constant across the mold. We aim to construct a reliable model that is computationally efficient and that can predict the combination of embossing pressure and duration required by any mold design. We are investigating the measurement of birefringence of embossed samples [4] as a way of monitoring the embossing process (Figure 2). We are also pursuing a technique for the bonding of polymer surfaces that promises minimal deformation of pre-embossed features: the polymer surfaces are exposed to an oxygen plasma for ~1 minute and then pressed together [5]."
A MEMS Drug Delivery Device for the Prevention of Hemorrhagic Shock,"Hemorrhagic shock is the number one cause of preventable death on today’s battlefield [1]. It is a hypotensive state of deficient organ perfusion caused by blood loss from wounds of the extremities or internal injuries. Hemorrhagic shock is normally treated by hemorrhage control, fluid replacement, and the injection of vasoconstrictors. Battlefield conditions, however, can prevent the timely administration of these measures. Hemostatic dressings developed for battlefield application are useful in controlling open wound hemorrhage but cannot stop internal bleeding or avert shock if too much blood has been lost [1]. Arginine vasopressin is a vasoconstrictor that causes peripheral and abdominal arteries to constrict, shunting blood to the vital organs in case of hemorrhage [2]. It improved survival by restoring blood pressure in pre-clinical experiments and clinical case studies of hemorrhagic shock when treatment was not immediately available [3-7]. This property makes it a perfect candidate for battlefield injection to keep wounded soldiers alive until they can be properly treated. Self-injection may not always be possible, however, due to the nature of these traumas.We are currently developing an implantable drug delivery microelectromechanical system (MEMS) to deliver vasopressin to wounded soldiers on the battlefield. This device consists of a silicon substrate in which pyramidal wells are etched using common MEMS processing techniques. The wells are capped by metallic membranes and the chip is hermetically bonded to a Pyrex macroreservoir (Figure 1). The macroreservoir can be injected with 25 µL of a vasopressin solution to be released on demand. Applying an electric pulse through a metallic membrane melts it by resistive heating, exposing the macroreservoir to the environment. We also observed the formation of multiple thermal bubbles inside the macroreservoir, which enabled rapid delivery of the solution. We are redesigning the device to better control this mechanism. Future challenges include insuring long-term hermeticity and wireless activation of the device."
Application of Input Shaping® and HyperBit™ Control to Improve the Dynamic Performance of a Six-axis MEMS Nano-positioner,"We have recently demonstrated how Input Shaping® and HyperBitTM control may be used to obtain fine-resolution motion and minimize vibration errors in all six axes of a six-axis, MEMS nano-positioner [1]. The dynamic problems in MEMS positioners, e.g., ringing/overshoot, that are conventionally addressed by damping must be resolved using control techniques since it is difficult to incorporate damping into micro-scale devices. Secondly, a positioner’s range to resolution ratios has to be 1 million or larger and also its “on-chip” digital-to-analog converters would need to be minimized on the expensive silicon real estate. These issues will be resolved by the applying the Input Shaping and HyperBit control.We first study the dynamic characterization of the nanopositioner, the microHexFlex [2], including the natural frequencies and their corresponding mode shapes. We then demonstrate the effect of Input Shaping and HyperBit on the nano-positioner’s resolution and settling time. Using these techniques, it is possible to obtain ms settling times with sub-nanometer resolution. The practical implications of this work are that future small-scale precision devices will be able to use these techniques to provide low-cost, multi-axis positioning at high-speeds speed and with fine resolution.The micro-HexFlex nanopositioner possess a 2.5-mm footprint and consists of two layers of single crystalline silicon with one layer of silicon dioxide in between. The stage of the micro-HexFlex is supported by axi-symmetric micro-scale flexures. Thermomechanical actuators are used to drive the Micro-HexFlex. In our tests, the thermomechanical actuators were driven via a voltage that was preconditioned using an Input-shaping controller. The controller [3] is an implementation of a feed-forward technique that acts to remove ringing and overshoot by modifying the input signal to the actuators so as to obtain the best possible performance from the positioner.We also add HyperBit DAC technology, a recently developed technique [4] for extending the resolution of digital-to-analog converters (DACs), for instance using a 4- bit DAC to obtain 12-bit functionality. Since DAC update rate capabilities are significantly faster than the bandwidths of the devices being driven, this technique allows the idle time-domain capacity of “low-bit” DACs to emulate that of “high-bit” DACs. The improvement in resolution is therefore obtained with simpler DAC equipment/circuitry that is more easily fabricated and integrated with micro- and meso-scale devices. Experimental results indicate reduction in dynamic errors by two orders of magnitude when the positioner was given 100-Hz square wave commands."
Multi-Axis Electromagnetic Moving-Coil Microactuator,"Electromagnetic (EM) micro-actuators are becoming increasingly important in micro-systems requiring moderate forces operating over a large range of motion. The applications that benefit from the performance advantages of EM micro-actuators include micro-scanning systems, micro-fluidic pumps, and positioning systems. Advantages of electromagnetic actuation over other classes of micro-actuators include low-voltage operation, moderate power density, large operating distances, linear response, multi-axis capability, and high bandwidth [1]. This work leverages the advantages of EM interactions to design a moving-coil micro-actuator that enables two-axes actuation with moderate forces (10+ mN) over large operating distances (10+ micrometers) at moderate mechanical frequencies (1+ kHz) using assembled permanent magnet field sources.The two-axes electromagnetic actuator consists of moving coils suspended on compliant silicon flexure springs above an array of 3 rectangular permanent magnets, as shown in Figure 1. The phase of the stacked coils results in Lorentz forces that are independently controllable in-the-plane and out-of-the-plane. The coil-spring fabrication scheme includes electroplating of copper coils, followed by a deep reactive-ion etch (DRIE) to pattern and release the compliant springs. Millimeter-sized permanent magnets are then aligned to the spring layer using an alignment chip. Successfully fabricated micro-coil structures have been shown to sustain current densities over 1000 Amps per square millimeter.A quasi-analytic electromagnetic force model for the device has been developed and experimentally validated against a centimeter-size bench-level prototype actuator. Figure 2 shows the predicted lateral-actuator force per coil-footprint versus current input for a typical actuator with 900-µm2 coil cross section. The actuator will be implemented in a high-speed meso-scale nano-positioner with applications in nano-fabrication and scanning-probe microscopy. When equipped with this micro-actuator, the nano-positioner is expected to be able to position millimeter-sized samples in six axes of motion (x, y, z, tip, tilt, yaw) with repeatability better than 10 nanometers at frequencies greater than 1 kHz."
Multiwell Cell Culture Plate Format with Integrated Microfluidic Perfusion System,"Recent reports indicate that it takes nearly $800 million dollars and 10-15 years of development to bring a drug to market. Nearly 90% of the lead candidates identified by current in vitro screens fail to become marketable drugs. One of the reasons for the high failure rate of drug candidates is the lack of adequate models. To address the problem, we have developed a new cell culture analog amenable to routine use in drug development. It is based on the standard multiwell cell culture plate format but it provides perfused three-dimensional cell culture capability.The multiwell plate microbioreactor array [1, 2] consists of a fluidic and a pneumatic manifold with a diaphragm sandwiched in between them. The fluidic manifold contains an array of microbioreactor and reservoir pairs (Figure 1). Each microbioreactor/reservoir pair is fluidically isolated from all other microbioreactors on the plate. A key component of a microbioreactor is a scaffold for tissue morphogenesis (Figure 2). The scaffold is a thin wafer containing an array of channels in which cells self-assemble into 3D pieces of tissue. It is backed by a filter and a support scaffold. Tissue in the scaffold is perfused by cell culture medium. The medium is re-circulated between the reactor and reservoir by a diaphragm pump. The diaphragms of all pumps and rectifying valves are actuated in parallel via three pneumatic lines distributed by the pneumatic manifold. Fluidic capacitors control flow pulsatility.The system provides a means to conduct high throughput assays for target validation and predictive toxicology in the drug discovery and development process. It can be also used for evaluation of long-term exposure to drugs or environmental agents and as a model to study viral hepatitis, cancer metastasis, and other diseases and pathological conditions."
Characterization of Nanofilter Arrays for Biomolecule Separation,"In the past decade, microfabricated devices have been developed that can separate, detect, and analyze various biomolecules [1]. In contrast to the sieving gels that are historically used in these studies, microfabricated devices are precisely designed and constructed. The deterministic structure of these devices facilitates experiment design and testing of theory. Periodic nanofilter arrays have been shown to separate DNA from 100 bp to 10 kbp [2]. These nanofilters consist of a regular sequence of free and constricted regions, with 50-100 nm being the characteristic dimension of the constricted region. In this context, the DNA is smaller than the constriction size, suggesting applicability of the Ogston sieving mechanism. Movement is characterized by the partitioning between the free and constricted regions due to steric constraints [3-4]. DNA has a persistence length of 50 nm (150 bp) and can be approximated as semi-rigid rods in this size range, facilitating theoretical analysis.We investigated the effects on separation efficiency and resolution of changing various device and experiment parameters. These parameters include the strength of the electric field; depth of the deep region; depth of the thin and deep regions, while maintaining their ratio; silicon substrate bias; buffer strength; and period of the nanofilter array."
Patterned Periodic Potential-energy Landscape for Fast Continuous-flow Biomolecule Separation,"Manipulation of charged biomolecules through confining environments has broad applications in life science. Recent progress in fabricating well-defined spatial constraints allows direct observation of novel molecular dynamic behavior in molecular-sized confining structures. Further, it shows exceptional promise for providing regular sieving media with superior separation performance. Here we demonstrate a continuous-flow, biomolecule-separation device that makes use of a patterned anisotropic sieving matrix consisting of a two-dimensional periodic array of nanofilters. The electrophoretic drift of biomolecules in the sieving medium involves a differential bidirectional motion through two-dimensional, periodically modulated, free-energy landscapes that results in a vectorial apparent electrophoretic mobility that directs molecules of different sizes to follow radically different paths. This method provides a novel basis for dispersing small fluid-borne biomolecules into distinct fractions. A fluorescently labeled dsDNA mixture (50-766 bp) used to characterize the device was separated in 1 minute with a resolution of about 10%. The patterned anisotropic sieve was also used for size-fractionation of SDS-protein complexes of size ranging from 11 to 200 kDa in 1 minute. By virtue of its gel-free and continuous-flow operation, this device suggests itself as a key component of an integrated microsystem that prepares and analyzes biomolecule samples."
Fabrication of Massively-parallel Vertical Nanofluidic Membranes for High-throughput Applications,"Nanofluidics has gained tremendous successes in the last few years because they provide unique capability in biomolecular manipulation and control. For nanofluidic applications, one critical issue is the availability of reliable, reproducible fabrication strategies for nanometer-sized structures. A simple technique, without nanolithography or special tools, has been developed to generate planar nanochannels with precise control of depths to the nanometer scale for many applications including separation [1] and preconcentration [2]. However, one big issue with these planar nanofluidic channels is the limited fluidic conductance that results in low throughput.Here we describe a novel fabrication approach to generate massively-parallel vertical nanochannels with the well-controlled gap size down to 100 nm [3]. We use anisotropic wet etching (KOH) to make deep, vertical trenches on Si (110) substrate (Figure 1A). Alternatively, conventional deep reactive ion etching (DRIE) can be performed to produce very deep trenches, and then the sidewalls can be smoothed by a short KOH etching. Then the width of the trench channel is further decreased to a desired thickness even below 50 nm (Figure 1B), by growing thermal oxide. Also, backside etching of the Si wafer can yield thin membranes over a wide area (~ 6-inch wafer) with well-defined membrane thickness, if needed. Our method requires neither expensive nanolithography expertise nor other tools and allows the integration of a large number of narrow, vertical nanofluidic filters with fluidic conductance 10~100 times higher than planar nanochannels. Furthermore, we have demonstrated efficient, high-throughput separation of large DNA molecules in our vertical nanofilter array device based on the entropic trapping mechanism (Figure 2). We believe that these membrane devices could be a key to the high-throughput nanofluidic sample-preparation microsystems."
Continuous-flow pI-based Sorting of Proteins and Peptides in a Microfluidic Chip Using Diffusion Potential,"In this work, we have developed a simple microfluidic chip that can sort biomolecules based on their isoelectric point (pI) values in a simple buffer system. The new method differs from previous approaches such as transverse isoelectric focusing [1] or free-flow electrophoresis [2] in that this process involves no external power supply and no special ampholyte. Instead, we utilize the diffusion potential generated by the diffusion of different buffer ionic species in situ at the laminar flow junction. The use of diffusion potential in microfluidics was previously demonstrated with the mass transport of dye molecules between the two streams in [3]. However, they did not explicitly demonstrate a separation of two species. In our device, we establish a laminar flow junction between two buffers with different pH and concentrations. A potential gradient is developed across the liquid junction, generating a high-enough electric field to mobilize and to collect biomolecules at the boundary when their pI values fall between the two buffer pH values. The computational modeling shows a decreasing potential gradient from 17.1 V/cm to 6.9 V/cm along the 2-mm-long microchannel (20 µm deep, 100 µm wide), as the concentration gradient becomes shallower toward the end of the channel due to mixing (Figure 1). In our initial experiment, two pI-markers (Figure 2) as well as two proteins were successfully sorted in this device, with a flow rate of 5~10 µL/min. To characterize the accuracy of this pI-based sorting process, we tested sorting behavior of the device by changing the pH value of the sample buffer in 0.1 pH step. It was shown that a peptide can be sorted into a different output stream with a ~0.1pH unit resolution. We are currently working on the development of new buffer systems as well as on the hybrid approach with a superimposed external electric field to increase the sorting efficiency and resolution. Once fully developed, it can potentially be a pI-based sample fractionation tool for proteomic analysis of complex biomolecule samples."
"Cell Stimulation, Lysis, and Separation in Microdevices","Quantitative data on the dynamics of cell signaling induced by different stimuli require large sets of self-consistent and dynamic measures of protein activities, concentrations, and states of modification. A typical process flow in these experiments starts with the addition of stimuli (cytokines or growth factors) to cells under controlled conditions of concentration, time, and temperature, followed at various intervals by cell lysis and the preparation of extracts. Microfluidic systems offer the potential to do this in a reproducible and automated fashion. Figure 1 shows quantification of the stimulation of a T-cell line with antibodies performed in a microfluidic device with integrated cell lysis. The device is capable of resolving the very fast kinetics of the cell pathways, with protein activation levels changing 4-fold in less than 15 seconds. The quantification of the lysate is currently performed off-chip using electrophoretic separation. To extract meaningful data from cellular preparations, many current biological assays require similar labor-intensive sample purification steps to be effective. Micro-electrophoretic separators have several important advantages over their conventional counterparts including shorter separation times, enhanced heat transfer, and the potential to be integrated into other devices on-chip. However, the high voltages required for these separations prohibit metal electrodes inside the microfluidic channel. A PDMS isoelectric focusing device with polyacrylamide gel walls has been developed to perform rapid separations by using electric fields orthogonal to fluid flow (Figure 2). This device has been shown to focus low molecular weight dyes, proteins, and organelles in seconds."
Thermal Management in Devices for Portable Hydrogen Generation,"The development of portable-power systems employing hydrogen-driven solid oxide fuel cells continues to garner significant interest among applied science researchers. The technology can be applied in fields ranging from the automobile to personal electronics industries. This work focuses on developing microreaction technology that minimizes thermal losses during the conversion of fuels – such as light-end hydrocarbons, their alcohols, and ammonia – to hydrogen. Critical issues in realizing high-efficiency devices capable of operating at high temperatures have been addressed, specifically, thermal management, the integration of materials with different thermophysical properties, and the development of improved packaging and fabrication techniques.A new fabrication scheme for a thermally insulated, high temperature, suspended-tube microreactor has been developed. The new design improves upon a monolithic design proposed by Arana et al.[1]. In the new modular design (Figure 1), a high-temperature reaction zone is connected to a low-temperature (~50°C) package via the brazing of pre-fabricated, thin-walled glass tubes. The design also replaces traditional deep reactive ion etching (DRIE) with wet potassium hydroxide (KOH) etching, an economical and time-saving alternative. A brazing formulation that effectively accommodates the difference in thermal expansion between the silicon reactor and the glass tubes has been developed."
Autothermal Catalytic Micromembrane Devices for Portable High-Purity Hydrogen Generation,"The high efficiency and energy density of miniaturized fuel cells provide an attractive alternative to batteries in the portable power generation market for consumer and military electronic devices [1-3]. The best fuel cell efficiency is typically achieved with hydrogen, but safety and reliability issues remain with current storage options. Consequently, there is continued interest in reforming liquid fuels to hydrogen. The process typically involves high temperature reforming of fuel to hydrogen combined with a low temperature PEM fuel cell, which implies significant thermal loss. Owing to its high hydrogen content (66%) and ease of storage and handling, methanol is an attractive fuel. However, partial oxidation of methanol also generates some CO, which may poison the fuel cell catalyst. Previously [4] we successfully demonstrated hydrogen purification using thin (~200 nm) Pd-Ag membranes using electrical heating. Further, integration of these devices with LaNiCoO3 catalyst allowed methanol reforming at 475oC with 47% fuel conversion [5]. Since microreactors possess high surface area to volume ratio, minimizing heat loss is important. Hydrogen flux across the Pd membranes is an equilibrium controlled process. Thus to achieve thermal management, the unextracted hydrogen, generated CO, and unreacted methanol can be completely oxidized in a separate reactor. In the current work, we explore the realization of autothermal hydrogen generation by fabricating silicon- based reactors using bulk micromachining techniques. The hydrogen generation unit comprises a 200-nm palladium-silver membrane coated with a reformer catalyst while the combustor is loaded with platinum catalyst. High thermal conductivity of silicon ensures autothermal operation. Upon thermal isolation using vacuum packaging [6], we characterize the performance of this integrated, autothermal hydrogen generation system in terms of energy efficiency and hydrogen production."
Multiphase Transport Phenomena in Microfluidic Systems,"Fluid interfaces provide unique opportunities for microfluidic and nanofluidic systems. Applications range from microscale heat exchangers and miniature fuel cells to microreactors for materials synthesis. Multiphase flow in such devices can be challenging, as the interfacial forces naturally favor axisym-metric geometries that are difficult to microfabricate. The ad-vantages of surface tension dominated microfluidics include a much richer dynamic flow behavior and enhancement of heat and mass transfer by creating secondary flows. These advantages offer many uses beyond enabling gas-liquid and fluid-solid reactions [1]. In particular, we are interested in segmented flow of gas and liquid in hydrophilic channels. Figure 1 shows several key features of this flow for reaction purposes. The presence of bubbles reduces the amount of dispersion of liquid flowing through the channels, ensuring that reactants and products spend a uniform amount of time in the system. For nanopar-ticle synthesis in microfluidic networks, a uniform residence time distribution translates into narrowly distributed particle sizes [2-3]. Liquid segments are efficiently mixed by circula-tion motion and gas bubbles are separated from microchan-nel walls by only a thin film (thickness < 1 µm). Thin films re-duce mass transfer resistance to components immobilized on the walls, such as catalysts [4] or analytical reagents and anti-bodies. We are also interested in the dynamics of multiphase flow through microchannels that are populated with a forest of micropillars (diameters: 50 µm – 100 µm). The observed flow patterns (Figure 2) connect to fundamental studies of flow in porous media and to catalysis. Gas-liquid and liquid-liquid flow patterns and their dynamics are determined in pulse-laser fluorescent micrographs and with microscale par-ticle image velocimetry (PIV) measurements. Characteristics of such three-phase systems, such as persistent static fractions, axial dispersion and mixing, are compared with multiphase flow in macroscopic unstructured beds and porous media."
Microfluidic Synthesis and Surface Engineering of Colloidal Nanoparticles,"Metal oxide colloidal particles such as silica (SiO2) and titania (TiO2) have many diverse applications ranging from catalysis, pigments and photonic band-gap materials to health care products. There has also been considerable research interest over the last decade in fabricating core-shell materials with tailored optical and surface properties. Core-shell particles such as titania-coated silica often exhibit improved physical and chemical properties over their single-component counterparts and hence are potentially useful over a broader range of applications. Newer methods of engineering such materials with controlled precision are required to overcome the difficulties with conventional production techniques, which are limited to multi-step batch processes. We have developed microfluidic routes for synthesis and surface-coating of colloidal silica and titania particles. The chief advantages of a microfluidic platform are precise control over reactant addition and mixing and continuous operation. Microfluidic chemical reactors for the synthesis and overcoating of colloidal particles are shown in Figure 1a and Figure 1b, respectively [1-2]. Figure 2a is an SEM micrograph of silica particles synthesized in a microreactor (Figure 1a) operated in segmented gas-liquid flow mode. Figure 2b shows a silica nanoparticle coated with a thick shell of titania. We have also fabricated integrated devices combining synthesis and overcoating to enable continuous multi-step synthesis of core-shell particles."
Microreactor Enabled Multistep Chemical Synthesis,"As a demonstration of how microsystems can enable quantitative study and improved production of chemistries that have been too hazardous to pursue via traditional means, the kinetics of direct sodium nitrotetrazolate (NaNT) synthesis were characterized and a microsystem for its commercial production has been constructed (Figure 1). A PDMS modular microreactor system capable of both multi-step synthesis and rapid scale-out was constructed. This system minimized the necessary volume of the unstable diazonium intermediate, enabling the study of NaNT, an energetic material used in the construction of fire suppression systems that was too dangerous to test with traditional techniques. In the direct synthesis of NaNT, 5-aminotetrazole (5-AT) reacts with nitrous acid to produce the diazonium intermediate that, in a second reaction, undergoes a Sandmeyer type reaction that displaces the diazonium group by the nitrite ion. The rapid mixing and safety advantages of microsystems were incorporated into a flexible architecture, presenting an improved ability to safely probe the conditions of the reaction. The modular design of this system also enabled the same set of modules to be rearranged as parallel reactor chains for small-scale production. A second generation microsystem was constructed from silicon micromixer modules (Figure 2); this micro-system is not only more robust than the PDMS design but also capable of accommodating higher flow rates (>2 mL/min) and higher temperatures. This system allows higher throughput and longer operational lifetimes and is currently being optimized for use as a full-scale production platform."
Integrated Microreactor System,"The realization of integrated microchemical systems will revolutionize chemical research by providing flexible tools for rapid screening of reaction pathways, catalysts, and materials synthesis procedures, as well as faster routes to new products and optimal operating conditions. Moreover, such microsystems for chemical production will require less space, use fewer resources, produce less waste, and offer safety advantages. The need for synthesizing sufficient quantities of chemicals for subsequent evaluation dictates that microchemical systems are operated as continuous systems. Such systems require fluid controls for adjusting reagent volumes and isolating defective units. The integration of sensors enables optimization of reaction conditions as well as the extraction of mechanistic and kinetic information.We are developing integrated microchemical systems that have reactors, sensors, and detectors with optical fibers integrated on one platform. We are exploring new approaches for connecting modular microfluidic components into flexible fluidic networks. Real-time control of reaction parameters using online sensing of flowrate, temperature, and concentration allows for precise attainment of reaction conditions and optimization over a wide range of temperatures and flow-rates. The multiple microreactors on the system can be used together to give higher throughputs or they can be used independently to carry out different reactions at the same time. Figure 1 shows a schematic of an integrated microreactor platform along with an early stage microreactor “circuit board” [1]."
Crystallization in Microfluidic Systems,"Microfluidic systems offer a unique toolset for discovering new crystal polymorphs and for studying the growth kinetics of crystal systems because of well-defined laminar flow profiles and online optical access for measurements. Traditionally, crystallization has been achieved in batch processes that suffer from non-uniform process conditions across the reactors and chaotic, poorly controlled mixing of the reactants, resulting in polydisperse crystal size distributions (CSD) and impure polymorphs. This reduces reproducibility and manufactures products with inhomogeneous properties. The short length scale in microfluidic devices allows for better control over the process parameters, such as the temperature and the contact mode of the reactants, creating uniform process conditions. Thus, these devices have the potential to produce crystals with a single morphology and a more uniform size distribution. In addition, microfluidic systems decrease waste, provide safety advantages, and require only minute amounts of reactants, which is most important when dealing with expensive materials such as pharmaceutical drugs. Figure 1 shows a microfluidic device used for crystallization and Figure 2 shows optical images of different shapes and sizes of glycine crystals produced in reactor channels. A key issue for achieving continuous crystallization in microsystems is to eliminate heterogeneous crystallization – irregular and uncontrolled formation and growth of crystals at the channel surface, which ultimately clogs the reactor channel. We have developed a sheath flow microcrystallizer using microfabrication and hot embossing of poly(dimethylsiloxane) (PDMS) and cyclic olefin copolymer (COC) to prevent heterogeneous crystallization. We are currently working on integrating an online spectroscopy tool for in situ polymorph detection. Our ultimate goal is to develop an integrated microfluidic system for continuous crystallization with the ability to control polymorphism and online detection."
Microreactors for Synthesis of Quantum Dots,"We have fabricated a gas-liquid segmented flow reactor with multiple temperature zones for the synthesis of quantum dots (QDs). In contrast to single-phase flow reactors, the segmented flow approach enables rapid mixing and narrow residence-time distributions, factors which have a strong influence on the ultimate QD size distribution. The silicon-glass reactor accommodates a 1-m long reaction channel (hydraulic diameter ~400-µm) and two shallow side channels for collecting reaction aliquots (Figure 1). Two temperature zones are maintained, a heated reaction region (>260°C) and a cooled quenching region (<70°C). As a model system, monodisperse CdSe QDs with excellent optical properties were prepared using the reactor. Cadmium and selenium precursor solutions are delivered separately into the heated section. An inert gas stream is introduced further downstream to form a segmented gas-liquid flow, thereby rapidly mixing the precursors and initiating the reaction. The reaction is stopped when the fluids enter the cooled outlet region of the device. Under conditions for a typical synthesis, the gas and liquid segments are very uniform (Figure 2a-b), and the QDs produced in the reactor possess narrow spectral features, indicative of monodisperse samples. The narrow particle size distributions arise directly from the enhanced mixing and narrow residence-time distribution realized by the segmented flow approach. Furthermore, the QD size can be tuned without sacrificing monodispersity by varying the Cd and Se precursor flow rates. In Figure 2d, the Se/Cd molar ratio was varied while keeping the total liquid and gas flow rates constant. Decreasing Se/Cd results in a substantial red shift of the QD effective band-gap (first absorption feature and photoluminescence peak), corresponding to larger QD diameters."
Polymer-based Microbioreactors for High Throughput Bioprocessing,"This project aims at developing high-throughput platforms for bioprocess developments. Based on the membrane-aerated microbioreactor [1], we have realized a microliter-volume, actively-mixed, and polymer-based microbioreactor by microfabrication and precision machining of PDMS and PMMA for batch [2] and continuous cultures [3] of microbial cells. Biological applications of microbioreactors, such as global gene expression of yeast cells [4], were demonstrated, and the parallel operation of multiple batch fermentations was realized by a multiplexed system [5].As a very important operation for bioprocess developments, fed-batch process allows extensive control over environmental conditions in fermentations. Fed-batch fermentations in the microbioreactor were made possible by applying water evaporation through the PDMS membrane as a fluidic exit, and by combining passive feeding of water and active feeding of base, acid, and glucose solutions. Commercial microvalves were used to control pressure-driven liquid feeds to realize closed-loop pH control in the microbioreactor. For Escherichia coli fermentations, the pH value was successfully maintained within a certain range (Figure 1). Cells were physiologically healthier and remained active for longer periods of time (as shown by the dissolved oxygen curve), which in return yielded significantly higher biomass concentration at the end of experiments.The microbioreactor was also integrated with the plug-n-pump microfluidic connectors [6], as well as incorporation of fabricated polymer micro-optical lenses and connectors for biological measurements to realize “cassettes” of microbioreactors (Figure 2). The fabrication process included precision machining and thermal bonding of PMMA devices. These integrations greatly simplified the setup and operation procedure and increased the signal-to-noise ratio for optical measurements for the cassettes, thus made the microbioreactors more compatible with high-throughput bioprocessing in multiplexed systems."
Micro-fluidic Bioreactors for Studying Cell-Matrix Interactions,"Mechanical forces are important regulators of cell biology in health and disease. Cells in the vascular system are subjected to fluid shear stress, cyclic stretch, and differential pressure [1]. Numerous investigations have revealed the vast pathological and physical responses of endothelial cells to fluid shear stress by culturing the cells on the rigid surfaces of a flow chamber. This approach, however, fails to mimic the true environment of cells in vivo that grow on flexible, porous basement membranes with a defined microstructure [1-4]. In order to create an improved model for this in vivo condition, we developed a new microfluidic bioreactor system that enables us to study cell-matrix interactions on soft substrates made of gel under conditions of controlled shear stress and pressure difference. A gel cage consisting of three thin layers (Figure 1) is constructed from PDMS using a silicon master made by the deep RIE process. Flow chambers, also made of PDMS, are cured on an SU-8 patterned master. Separate channels are included that allow for filling this central chamber with a gel that mimics the extracellular matrix and also allows for independent control over the flows in the upper and lower channels. The assembled bioreactor is shown in Figure 2. To conduct experiments, we introduce a peptide solution into the gel cage, allow it to gel, and then seed cells on the gel surface exposed through the holes of gel cage. After cell adhesion, the flow chambers are sealed by the application of a vacuum to the top and bottom sides of the gel cage. Flows are then applied to each chamber with controlled pressures and flow rates. With this system, we will apply controlled shear stress and pressure on the cell layers. We plan to study the process of angiogenesis that entails the growth of vascular sprouts emanating from one endothelial surface and connecting with the other."
A Nanoscanning Platform for Biological Assays,"An in-plane nanoscanning platform with switchable stiffness being developed at the Micro & Nano Systems Laboratory (MNSL) [1] can be an alternative to the existing atomic force microscope (AFM) system. The nanoscanning platform has a carbon nanotube (CNT) tip, which is known as one of the ideal candidates for AFM tips because of their superior mechanical and chemical properties. Raman Spectroscopy has gained a lot of interest as a tool for single molecule detection since it has easy and fast sample preparation and measurement compared to the existing technologies, such as X-ray crystallography and nuclear magnetic resonance. Among the several approaches attempted in order to enhance the weak Raman signals is tip enhanced raman spectroscopy (TERS). The enhancement of the electric field due to the plasmon resonance on the coated metal surface was predicted qualitatively [2]. The metal-coated CNT or CNT filled with Ag, Au, or Cu with a small diameter tip and high aspect ratio is ideal for TERS. The switchable stiffness AFM can work as a tool for imaging and placing the tip at the sub-nanometer proximity to a soft, molecular-scale biological sample, which would enhance the Raman signals."
"A Large Strain, Arrayable Piezoelectric Microcellular Actuator","To provide a competitive actuating solution, micro-electromechanical-systems (MEMS)-based actuators need low operating power and form factors. Piezoelectrics provide substantially higher work-output/volume for a given voltage, when compared to other actuating solutions. A bow amplifier constructed of SU-8 beams and short length flexural pivots has been designed [1] and has demonstrated an amplification ratio of greater than 10:1. Current research focuses on increasing this amplification ratio and achieving the goal of 10% axial strain, while reducing parasitic out-of-plane bending inherent in the current fabrication process.The overall goal of this project is to array one such actuator massively in series and in parallel in order to create a macro-scale, muscle-like actuator. Such a device would have widespread applications in mobile robotics, medicine, and aero/astronautics, where low power, high efficiency, and small form factors might be required."
Self-powered Wireless Monitoring System Using MEMS Piezoelectric Micro Power Generator,"A thin-film lead zirconate titanate Pb(Zr,Ti)O3 (PZT), MEMS Piezoelectric Micro Power Generator has been integrated with a commercial wireless sensor, Telos, to simulate a self-powered RF temperature monitoring system (Figure 1). Such a system has many important applications, ranging from structure to rotary system monitoring. Telos consumes 2270 µJ for 221 ms per measurement. The PMPG and power management module are designed to satisfy such power requirementsThe first prototype of PMPG provides an average 1 µW, with a natural frequency of 13.9 kHz (Figure 2). It has an energy density of 0.74 mW-h/cm2, which compares favorably to lithium ion batteries [1]. The second generation PMPG is designed to provide 0.173 mW of power at 3 V with a natural frequency of 150 Hz and maximum strain of 0.12% [2]. We increased the effective mass of the PMPG by adding a Si substrate with thickness of 525 µm to the beam structure. The increase in the effective mass increases the energy store in the device and its power output. The beam length is also increased to achieve a low resonant frequency. The third generation PMPG will use a serpentine structure, which can achieve a low frequency with minimum volume. Since PMPG offers limited power, a storage capacitor and a power management module are implemented to power the sensor node at discrete time intervals [3]. The PMPG is first connected to a rectifier that converts AC to DC voltage. Each cycle consists of a charging interval, in which PMPG charges the capacitor, and operation intervals, in which Telos uses the energy from capacitor. We developed a test bed, which mimics that of a liquid gas pipe used in the Alaska where the PMPG device will be used to generate power for temperature sensors. Scaling/dimension factors as well as cost and robustness are considered in the design."
MEMS Pressure-sensor Arrays for Passive Underwater Navigation,"MEMS pressure sensors have had broad applications in fields such as mining, medicine, automobiles, and manufacturing. Another application to be explored is in underwater vehicular navigation. Objects within a flow generate pressure variations that characterize the objects’ shape and size. Sensing these pressure variations allows the unique identification and location of obstacles for navigation (Figure 1). This concept is inspired by existing biological systems. Fish have such a sensory lateral line, which they use to monitor all aspects of their hydrodynamic environment, including obstacles [2,5].We propose to develop low-power sensors that passively measure dynamic and static pressure fields with sufficient resolution to detect objects generating the disturbance. We will also develop processing schemes that use the information from the sensors to identify objects in the flow environment. These sensors and processing software emulate the capabilities of the lateral line in fish. While active acoustic means can be used for object detection, the process is power-intensive, and depends strongly on the acoustic environment. A simpler alternative is to use a passive system that can resolve the pressure signature of obstacles. The system consist of arrays of hundreds or thousands of piezoresistive pressure sensors fabricated on etched silicon and Pyrex wafers [1,3,4,6] with diameters around 1 mm; the sensors are arranged over a flat or curved surface in various configurations, such as a single line, a patch consisting of several parallel lines (Figure 2), or specialized forms to fit the hull shape of a vehicle or its fins. The sensors will be packaged close together at distances of a few millimeters apart in order to resolve pressure and flow features near the array spacing, which in turn can be used to identify the overall features of the flow."
An Integrated Multiwatt Permanent Magnet Turbine Generator,"There is a need for compact, high-performance power sources that can outperform the energy density of modern batteries for use in portable electronics, autonomous sensors, robotics, and other applications. Previous research efforts on a micro-scale, axial-flux, permanent-magnet turbine generator [1-2] culminated in a spinning rotor test stand that delivered 8 W DC output power through a diode bridge rectifier with an overall generator system efficiency of 26.6%. In these experiments, the generator rotor was mounted via a steel shaft to an air-driven, ball-bearing supported spindle and spun to the desired operational speed.Current research efforts aim to fully integrate the permanent-magnet (PM) generator design into the silicon micro-turbine engine fabrication process and create devices that can deliver 10 W DC output power when driven by compressed air. The integrated generator will couple energy from the compressed air to the rotor through microfabricated turbine blades attached to the backside of the rotor. One important challenge in this integration process is the structural integrity of the magnetic rotor spinning at a tip speed near 300 m/s, or equivalently 450 krpm.Based on power requirements, a 300-µm thick circular NdFeB PM with an inner radius of 2.5 mm and an outer radius of 5 mm must be embedded into the silicon rotor on top of a 150 µm FeCoV back iron. FEA analysis shows that the maximum principle stress at 450 krpm in the silicon rotor, 900-µm thick and 12 mm in diameter, with bonded annular PM and back iron pieces, will be approximately 180 MPa through the entire structure. This stress is well below the tensile strength of silicon and FeCoV. However, because the PM is brittle and has a typical tensile strength around 83 MPa, it is unclear whether the material will fracture. Tests are currently underway to characterize the reference strength and Weibull modulus of the PM, and from these results, a working rotor design will be proposed."
Micro-scale Singlet Oxygen Generator for MEMS-based COIL Lasers,"Conventional chemical oxygen iodine lasers (COIL) offer several important advantages for materials processing, including short wavelength (1.3 µm) and high power. However, COIL lasers typically employ large hardware and use reactants relatively inefficiently. This project is creating an alternative approach called microCOIL. In microCOIL, most conventional components are replaced by a set of silicon MEMS devices that offer smaller hardware and improved performance. A complete microCOIL system includes micro-chemical reactors, micro-scale supersonic nozzles, and micro-pumps. System models incorporating all of these elements predict significant performance advantages in the microCOIL approach [1]. Initial work focuses on the design, microfabrication, and demonstration of a chip-scale singlet oxygen generator (SOG), a micro-chemical reactor that generates singlet delta oxygen gas to power the laser. Given the extensive experience with micro-chemical reactors over the last decade [2-4], it is not surprising that a micro-SOG would offer a significant performance gain over large-scale systems. The gain stems from basic physical scaling; surface-to-volume ratio increases as the size scale is reduced, which enables improved mixing and heat transfer. The SOG chip demonstrated in this project, shown in Figure 1, employs an array of micro-structured packed-bed reaction channels interspersed with micro-scale cooling channels for efficient heat removal. Production of singlet oxygen has been confirmed via spontaneous emission (as shown in Figure 2) and mass spectrometry techniques. The yield (or fraction of singlet oxygen produced) is estimated at 70%, making the micro-SOG competitive with macro-scale alternatives."
Label-free Microelectronic PCR Quantification,"The introduction of real-time monitoring of the polymerase chain reaction (PCR) represents a major breakthrough in specific nucleic acid quantification. This technique employs fluorescent intercalating agents or sequence-specific reporter probes to measure the concentration of amplified products after each PCR cycle. However, the need for optical components can limit the scalability and robustness of the measurement for miniaturization and field-uses. Moreover, the addition of external fluorescent reagents can induce inhibitory effects [1] and require extensive optimization [2].We have developed a robust and simple method for direct label-free PCR product quantification using an integrated microelectronic sensor (Figure 1) [3]. The field-effect sensor can sequentially detect the intrinsic charge of multiple unprocessed PCR products and does not require sample processing or additional reagents in the PCR mixture. The sensor measures nucleic acid concentration in the PCR relevant range and specifically detects the PCR products over reagents such as Taq polymerase and nucleotide monomers. The sensor can monitor the product concentration at various stages of PCR and can generate a readout that resembles that of a real-time fluorescent measurement using an intercalating dye but without its potential inhibition artifacts (Figure 2). The device is mass-produced using standard semiconductor processes, can be reused for months, and integrates all sensing components directly on-chip. As such, our approach establishes a foundation for the direct integration of PCR-based in vitro biotechnologies with microelectronics."
Atomic Force Microscopy with Inherent Disturbance Suppression for Nanostructure Imaging,"Scanning probe imaging is often limited by disturbances, or mechanical noise, from the environment that couple into the microscope. We demonstrate on a modified commercial atomic force microscope that adding an interferometer as a secondary sensor to measure the separation between the base of the cantilever and the sample during conventional feedback scanning can result in real-time images with inherently suppressed out-of-plane disturbances (Figure 1) [1]. The modified microscope has the ability to resolve nanometer-scale features in situations where out-of-plane disturbances are comparable to or even several orders of magnitude greater than the scale of the topography. We present images of DNA in air from this microscope in tapping mode without vibration isolation, and show improved clarity using the interferometer as the imaging signal (Figure 2). The inherent disturbance suppression approach is applicable to all scanning probe imaging techniques.We do not claim that image improvement will be comparable to these results on all SPMs and in all imaging environments. At present, this technique will be most effective in very noisy environments, such as a microfabrication facility, where Z disturbances overwhelm sample topography. However, there are two significant implications of this work: 1) vibration isolation, which is costly and consumes space, can be rendered unnecessary for noisy environments; and, 2) this technique can potentially outperform vibration isolation in any environment with further reduction of the interferometer noise floor."
Vacuum-Packaged Suspended Microchannel Resonant Mass Sensor for Biomolecular Detection,"Microfabricated transducers enable the detection of biomolecules in microfluidic systems with nanoliter size sample volumes. Their integration with microfluidic sample preparation into lab-on-a-chip devices can greatly leverage experimental efforts in systems biology and pharmaceutical research by increasing analysis throughput while dramatically reducing reagent cost. Microdevices can also lead to robust and miniaturized detection systems with real-time monitoring capabilities for point-of-use applications.We have recently fabricated, packaged, and tested a resonant mass sensor for the detection of biomolecules in a microfluidic format [1]. The transducer employs a suspended microchannel as the resonating element, thereby avoiding the problems of damping and viscous drag that normally degrade the sensitivity of resonant sensors in liquid (Figure 1). Our device differs from a vibrating tube densitometer in that the channel is very thin, which enables the detection of molecules that bind to the channel walls; this provides a path to specificity via molecular recognition by immobilized receptors. The fabrication is based on a sacrificial polysilicon process with low-stress LPCVD silicon nitride as the structural material, and the resonator is vacuum packaged on the wafer scale using glass frit bonding (Figure 2). Packaged resonators exhibit a sensitivity of 0.8 ppm/(ng•cm2) and a mechanical quality factor of up to 700. To the best of our knowledge, this quality factor is among the highest so far reported for resonant sensors with comparable surface mass sensitivity in liquid."
Microbial Growth in Parallel Integrated Bioreactor Arrays,"Bioprocesses with microbial cells play an important role in producing biopharmaceuticals such as human insulin and human growth hormone and other products such as amino acids and biopolymers. Because bioprocesses involve the complicated interaction between the genetics of the microorganisms and their chemical and environmental conditions, hundreds or thousands of microbial growth experiments are necessary to develop and optimize them. In addition, efforts to develop models for bioprocesses require numerous growth experiments to study phenotypes of microorganism. We have designed and developed integrated arrays of microbioreactors that can provide the oxygen transfer and control capabilities of a stirred tank bioreactor in a high-throughput format. The devices comprise a novel peristaltic oxygenating mixer and microfluidic injectors (Figure 1), which are fabricated using a process that allows the combination of multiple scale (100 µm-1 cm) and multiple depth (100 µm-2 mm) structures in a single mold. The microbioreactors have a 100 µL working volume, a high oxygen-transfer rate (kLa ≈ 0.1s-1), and closed loop control over dissolved oxygen and pH (±0.1). Overall, the system supports eight simultaneous batch cultures in two parallel arrays with two dissolved oxygen thresholds, individual pH set points, and automated near real-time monitoring of optical density, dissolved oxygen concentration, and pH. These capabilities allowed the demonstration of multiple Escherichia coli aerobic fermentations with growth to high cell densities (>12g-dcw/L, Figure 2), and individual bioreactor performance on par with bench scale stirred tank bioreactors. The successful integration of diverse microfluidic devices and optical sensors in a scalable architecture opens a new pathway for continued development of parallel bioreactor systems."
Vacuum-Sealing Technologies for Micro-chemical Reactors,"Current portable power sources may soon fail to meet the demand for increasingly larger power densities. To address this concern, our group has been developing MEMS power generation schemes that are focused around fuel cells and thermophotovoltaics. At the core of these systems is a suspended tube micro-reactor that has been designed to process chemical fuels [1]. Proper thermal management is critical for high reactor efficiency, but substantial heat loss is attributed to conduction through air. A straightforward solution is to eliminate the heat-loss pathways associated with air by means of a vacuum package. This work explores a glass-frit bonding method for vacuum sealing.Optimization of pre-sintering and bonding parameters of the glass frit produced a repeatable and robust hermetic seal. Encounters with outgassing issues prompted an alternate two-step packaging process illustrated in Figure 1. New capping dies were fabricated, test devices were packaged, and the final seal-off was attempted with various materials [2]. Several experimental results appear in Figure 2. The glass frits are undesirable since they produce holes from material breakdown when heated in a vacuum. The gold-indium solder appears promising but holes formed due to internal outgassing. Extended heating to assist outgassing resulted in the delamination of the solder from the wetting metal. Recent work has been conducted to evaluate oxidized caps and lead-tin solder as solutions to these problems. Enhancements through the incorporation of non-evaporable getters will be assessed once a vacuum package is achieved."
Direct Patterning of Organic Materials and Metals Using Micromachined Printheads,"Organic optoelectronic devices are promising for many commercial applications if methods for fabricating them on large-area, low-cost substrates become available. Our project investigates the use of MEMS in the direct patterning of materials needed for such devices. By depositing the materials directly from the gas phase, without liquid phase coming in contact with the substrate, we aim to avoid the limitations of inkjet printing such materials.In our first demonstration, we used an electrostatically actuated micromachined shutter integrated with an x-y-z manipulator to modulate the flux of evaporated organic semiconductors and metals and to generate patterns of the deposited materials. We printed arbitrary patterns of organic semiconductor Alq3 (tris(8-hydroxyqunolinato) aluminum) and metal silver on glass substrates. We also printed pentacene/silver organic field effect transistor (OFET) and arrays of organic light emitting devices (OLED), as shown in Figure 1. This printing technique can pattern small-molecule organic light-emitting devices at high resolution (800 dpi).The next stage of this project investigates the use of a microporous layer with integrated heaters for local evaporation of the materials. The microfabricated device is shown in Figure 2. The material to be printed is delivered to the porous region in liquid or gas phase and deposits inside the pores. An integrated heater then heats up the porous area and the material is re-evaporated from the pores onto the substrate. Compared to the first generation of printheads, the problems of crashing and stiction are avoided, since there is no moving part. Clogging is also limited since most of the material is removed during each printing cycle. Other advantages include the smaller quantity of organic material used, and the reduced substrate heating. Such a printhead would ultimately be integrated with an ink-jet printer for the delivery of liquid phase material into the porous region."
A Thermophotovoltaic (TPV) MEMS Power Generator,"For a number of years, batteries have not kept up with the fast development of microelectronic devices. The low energy densities of even the most advanced batteries are a major hindrance to lengthy use of portable consumer electronics such as laptops and of military equipment that most soldiers carry today. Furthermore, battery disposal constitutes an environmental problem. Hydrocarbon fuels exhibit very high energy densities in comparison, and micro-generators converting the stored chemical energy into electrical power at even modest levels are therefore interesting alternatives in many applications. This project focuses on building thermophotovoltaic (TPV) micro-generators, in which photocells convert radiation from a combustion-heated emitter into electrical power. TPV is an indirect conversion scheme that goes through the thermal domain and therefore does not exhibit very high efficiencies (10-15% max). However, because of its simple structure and because the combustor and photocell fabrication processes do not need to be integrated, the system is simpler to micro-fabricate than other generator types, e.g., thermoelectric systems and fuel cells. It is also a mechanically passive device that is virtually noiseless and less subject to wear than engines and turbines. In this TPV generator, a catalytic combustor, the suspended micro-reactor (SµRE) (Figure 1), is heated by combustion of propane and air, and the radiation emitted is converted into electrical energy by low-bandgap (GaSb) photocells. Net power production of up to 1 mW has been achieved [1], constituting a promising proof of concept. A new version of the SµRE is currently under fabrication. This new design (Figure 2) aims to address several problems existing in the earlier version, including fabrication difficulties, low burst pressure of the tubes, and low emitter surface area."
MEMS Vacuum Pump,"There are many advantages to miniaturizing systems for chemical and biological analysis. Recent interest in this area has led to the creation of several research programs, including a micro gas analyzer (MGA) project at MIT. The goal of this project is to develop an inexpensive, portable, real-time, and low-power approach for detecting chemical and biological agents. Elements entering the MGA are first ionized, then filtered by a quadrupole array, and sensed using an electrometer. A key component enabling the entire process is a MEMS vacuum pump, responsible for routing the gas through the MGA and increasing the mean free path of the ionized particles so that they can be accurately detected. There has been a great deal of research done over the past 30 years in the area of micro pumping devices [1, 2]. We are currently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven active-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We constructed accurate computer models for all aspects of the pump’s operation: a compressible mass flow model of the flow rates, the pressure, the density, and the Mach number in the different parts of the pump in both the sonic and subsonic regimes [4], and a nonlinear plate deformation model of the stresses experienced by the pistons, tethers, and walls of the pump during operation [5], for any chosen dimensions and material properties. Using these models we have defined a process flow for our first-generation MEMS vacuum pump designed to meet our first-term goals. A schematic of this pump that we started fabricating is shown in Figure 1 below. For ease in testing we have decided to fabricate only Layers 1-3 and constructed a testing platform that will drive the pistons pneumatically. This will allow for rapid characterization of pumping performance as well as chamber and valve designs for several dies at once without having to incorporate piezos in each case. The final device will be driven using low-voltage, low-loss, piezoelectric-stacks incorporated into Layer 4 and will include Layer 5 for structural support."
Rapid and Shape-Controlled Growth of Aligned Carbon Nanotube Structures,"We present approaches for growth of aligned carbon nanotube (CNT) structures on silicon substrates, based on atmospheric pressure chemical vapor deposition (CVD) using a Fe/Al2O3 catalyst film in C2H4/H2. First, vertically-aligned films of small-diameter (5-10 nm) multi-walled CNTs (MWNTs) are grown to 0.9 mm thickness in 15 minutes and 1.8 mm in 60 minutes, using a conventional 1-inch-diameter tube furnace [1]. The catalyst is patterned by photolithography, and the growth rate of CNT microstructures depends on the local areal density of catalyst, which is analogous to loading effects in plasma etching process. Further, using a novel apparatus where the silicon substrate is resistively heated, we achieve CNT film thickness of 3 mm in just 20 minutes along with rapid (100oC/s) control of the substrate temperature and optically image the film during growth (Figure 1).By placing a weight on the catalyst-coated substrate, we measure the force which can be exerted by a growing CNT film and demonstrate that the film thickness after a fixed growth time and the alignment of CNTs within the film decrease concomitantly with increasing applied force [2]. We utilize this principle to fabricate three-dimensional structures of CNTs (Figure 2) that conform to the shape of a microfabricated template. This technique is a catalytic analogue to micromolding of polymer and metal microstructures; it enables growth of nanostructures in arbitrarily-shaped forms and does not require patterning of the catalyst.Finally, we perform combinatorial flow studies of CNT growth using an array of parallel microchannels fabricated by KOH etching of silicon [3]. We observe transitions in CNT yield and quality along the microchannels, grow CNT structures that are aligned by gas flows in the microchannels, and fabricate CNT-filled microchannels for applications such as microfluidic filters."
A Low Contact Resistance MEMS-Relay,"A low contact resistance MEMS-relay featuring highly parallel and planar oblique contacts has been fabricated and is currently being tested. The contacts are etched in silicon using a potassium hydroxide (KOH) solution. An offset between the wafer-top and the wafer-bottom KOH masks produces the oblique contact geometry schematically shown in Figure 1A. In contrast, many prior art MEMS devices [1-3] have rough, non complementary contacts. As these surfaces touch, they do so in a small number of high points, as shown in Figure 1B, which significantly reduces the effective contact area and leads to a high contact resistance and a low current carrying capacity. Additionally, vertical contacts are prone to poor metallization, which further affects the device’s contact resistance. Our MEMS-relay, shown in Figure 2, is composed of a compliant mechanism (B), a pair each of engaging (C) and disengaging (D) rolling-point “Zipper” actuators [4-5], and a pair of planar and parallel contacts (E).The relay is fabricated by a combination of deep reactive ion etching (DRIE) and KOH etching. Nested masks are used to pattern both wafer-through etches. Low stress silicon nitride (Si3N4), which will later be used as a KOH mask, is patterned initially on both sides of the device wafer. A silicon oxide film is deposited on the KOH mask. The compliant mechanism and actuators are then etched through DRIE and a second Si3N4 film is deposited. The second Si3N4 film is patterned using a “shadow” (through-etched) wafer as a mask. The oxide is selectively etched to reveal the buried nitride mask. The contacts are etched in KOH solution. Both Si3N4 and oxide films are stripped and a thermal oxide, which insulates both the electrostatic actuators and the relay contacts from the rest of the device, is grown. Gold is evaporated over both sides of the insulated contacts and the device wafer is anodically bonded to a Pyrex handle wafer. Experimental pull-in and drop-out voltages of 70 V and 40 V, respectively, agree with the model. Contact travel of 50 µm prevents arcing as the load circuit is switched on and off. A contact resistance of 50 mΩ was demonstrated by our group using an externally actuated structure as a proof of concept for the contact design [4]. Our group continues to develop these MEMS relays for power applications."
Fast Three-Dimensional Electrokinetic Pumps for Microfluidics,"Electrokinetic pumps are attractive for portable and flexible microfluidic analysis systems, since they operate without moving parts using low (battery-powered) alternating potentials. Since the discovery of AC electro-osmosis (ACEO) in the late 1990s, there has been much work in designing planar, periodic pumps, which exploit broken symmetry in electrode spacing and width to produce a streaming flow over a surface. Although surface-height modulation has been suggested as another means of breaking symmetry[1], it has never been numerically or experimentally pursued. Recently, Bazant and Squires described more general flows due to induced charge electro-osmosis (ICEO) around three-dimensional metal structures[2], which has since been realized experimentally in microfluidic systems[3]. Motivated by ICEO around raised electrodes, we are developing a variety of new three-dimensional AC electrokinetic pumps capable of much faster directional flows than planar ACEO pumps (for the same applied voltage and minimum feature size) by an order of magnitude according to the usual low-voltage model. This phenomena and an example microfabricated device are illustrated in Figure 1. We test and improve our theoretical designs experimentally in a microfluidic loop[4], as shown in Figure 2. Our pumps involve interdigitated planar electrodes with raised metal structures from a simple electroplating step, which leads to greatly enhanced pumping."
BioMEMS for Control of the Stem-cell Microenvironment,"The stem-cell microenvironment is influenced by several factors including cell-media, cell-cell, and cell-matrix interactions. Although conventional cell-culture techniques have been successful, they offer poor control of the cellular microenvironment. To enhance traditional techniques, we have designed a microscale system to perform massively parallel cell culture on a chip.To control cell-matrix and cell-cell interactions, we use dielectrophoresis (DEP), which uses non-uniform AC electric fields to position cells on or between electrodes [1]. We present a novel microfabricated DEP trap designed to pattern large arrays of single cells (Figure 1, left). We have experimentally validated the trap using polystyrene beads and cells, showing excellent agreement with our model predictions [2]. In addition, by placing interdigitated electrodes between the traps, we can prevent cells from sticking to the substrate outside the traps (Figure 1, right).To control cell-media interactions, we have developed a microfluidic device for culturing adherent cells over a logarithmic range of flow rates (Figure 2, left) [3]. The device controls flow rates via a network of geometrically-set fluidic resistances connected to a syringe-pump drive. We use microfluidic perfusion to explore the effects of continuous flow on the soluble microenvironment. We have demonstrated logarithmically-scaled perfusion culture of mouse embryonic stem cells over 4 days, with flow rates varying > 300x across the array. Cells cultured at the slowest flow rate did not proliferate while colonies at higher flow rates demonstrated healthy round morphology (Figure 2, upper and lower right) and expressed the stem-cell marker Oct-4. These microfabricated platforms will enable precise and unique control over the cellular microenvironment, allowing novel cell biology experiments at the microscale."
Microfluidic/Dielectrophoretic Approaches to Selective Microorganism Concentration,"This project focuses on the development of microfabricated microfluidic/dielectrophoretic devices capable of concentrating micron-size particles from complex liquids, for example water containing contaminants such as dust, sand, protein or soot. The concentrated particles of interest, such as pathogenic bacteria and spores, can then be delivered in small aliquots to the appropriate sensor for identification.The micro-concentrator exploits the phenomenon of dielectrophoresis–the force on polarizable particles in spatially non-uniform electric field [1]–to trap the particles from the flow stream in order to subsequently concentrate them by release into a smaller volume of liquid. Dielectrophoresis does not negatively affect the liquid or the particles on which it operates. In our device the non-uniform electric field is created by interdigitated electrodes (IDE) at the bottom of the channel through which the contaminated solution is passed (Figure 1). To maximize the exposure of particles to the DEP field, we mix the liquid using passive micro-fluidic mixers (Figure 1). Preliminary results with different fabricated micro-fluidic mixers exhibit up to 70% improvement in trapping efficiency as compared to devices without mixers (Figure 2). Although both the herringbone mixer (HM) and slanted groove mixer (SGM) show notable improvements over smooth channel configurations, the staggered herringbone mixer (SHM) provides the greatest enhancement in trapping efficiency. We believe that the chaotic mixing associated solely with the SHM exposes more particles to the concentrator’s bank of IDEs, thus resulting in higher trapping efficiency when compared to other mixer types.The magnitude and direction of the dielectrophoretic (DEP) force depends on the particle’s dielectric properties (i.e., conductivity and permittivity); therefore, when the operating frequency of the field and the conductivity of the medium are chosen, the DEP force can be selectively applied to trap and concentrate some particles (bacterial spores of interest) and not others (dust, soot, sand or protein). In our device, initial banks of interdigitated electrodes are driven to maximize interferent trapping, while final stages capture spores from a purified solution. Using this mode of operation, we demonstrated selective trapping of B. subtilis spores while rejecting interferents such as pollen, chitin, sand and depleting interferents such as soot and dust. Future work will focus on improving purity and efficiency of trapping."
Microfabricated Approaches for Sorting Cells Using Complex Phenotypes,"We are developing microfabricated approaches to create sorting cytometers for genetic screening of complex phenotypes in biological cells. Our goal is to create technologies that combine the ability to observe with the ability to isolate individual mutant cells from a population under study. Such cytometry merges benefits of microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities on a single platform. Biologists will be able to use these technologies to isolate cells based upon dynamic and/or intracellular responses, permitting creation of new types of genetic screens.We currently are developing optical and electrical approaches to enable image-based sorting. One of our current approaches uses an array of switchable traps (Figure 1) that rely upon the phenomena known as dielectrophoresis (DEP) [1]. The DEP-enabled traps allow for capturing and holding cells in defined spatial locations and then subsequently releasing a desired subpopulation for further study. The traps in our device are controlled using a series of row and column electrical connections. This setup avoids any need for separate connections to each of the traps in our arrays. Our chip-to-world interconnect needs thus scale only as 2√n for any n × n trap footprint. This condition enables site-specific addressing within arrays sized appropriately for bio-relevant assays (10,000 sites) using a minimal number of electrical ties (200 wires). To date, we have captured, held, and sorted small populations of individual HL60 human leukemia cells using a demonstrative 4 × 4 trap array [2]. Figure 2 shows a proof-of-concept assay where orange- and green-stained HL60 cells are first held in the 16-site array and then we sorted each of the green cells from the grid. Developing and scaling such a platform for screening applications requires performance characteristics that are easily met only by using quantitative modeling [3]. Using such an approach, we have developed updated trap geometries and system configurations for use in larger 20 × 20 array structures. Currently we are fabricating these enhanced devices, their affiliated control and automation systems, and specific RFP-tagged cell lines for planned complex phenotype-based sorting assays. In tandem with this design cycle, we are investigating the effects of DEP trapping on cell health and the impact that it may have on our ability to assess specific phenotypic behaviors. Complementary and alternative approaches for implementing these sorting functionalities are similarly under study in an attempt to lower the threshold for acceptance and use in biological laboratories."
"A Continuous, Conductivity-Specific Micro-organism Separator","Increased throughput in the techniques used to engineer new metabolic pathways in unicellular organisms demands similarly high throughput tools for measuring the effects of these pathways on phenotype. For example, the metabolic engineer is often faced with the challenge of selecting the one genomic perturbation that produces a desired result out of tens of thousands of possibilities [1]. We propose a separation method–iso-dielectric separation, or IDS–which separates microorganisms continuously based on their dielectric properties. This technology would enable high-throughput screening of cells based upon electrically distinguishable phenotypes. Iso-dielectric separation uses dielectrophoresis (DEP) and media with spatially-varying conductivity to separate cells by their effective conductivity. It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis, and is thus applicable to uncharged particles, such as cells [2]. We apply this method to the separation of polystyrene beads (based on surface conductance), vesicles (based on the conductivity of the internal fluid), and cells (based on viability). Current efforts are focused on the separation of Escherichia coli based upon the amount of the intracellular polymer poly(hydroxybutyrate) that each cell contains."
MEMS Vibration Harvesting for Wireless Sensors,"The recent development of “low power” (10’s-100’s of µW) sensing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of individual wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients [1]. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of structural health monitoring, particularly for aircraft.This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam structures have been developed to predict the electrical and mechanical performance obtainable from ambient vibration sources. The uniform models have been validated by comparison to prior published results [2] and verified by comparison to tests on a macro-scale device [5]. Models of a uniform harvester with proof mass are currently undergoing macro-scale testing and validation. A non-optimized, uni-morph beam prototype (Figure 1) has been designed and modeled to produce 30 µW/cm3 [3]. A MEMS fabrication process for a prototype device is presented based on past work at MIT [4]. Dual optimal frequencies with equal peak powers and unequal voltages and currents are characteristic of the response of such coupled devices when operated at optimal load resistances (Figure 2). Design tools to allow device optimization for a given vibration environment have been developed for both geometries.Future work will focus on fabrication and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. System integration and development, including modeling the power electronics, will be included."
Fabrication and Structural Design of Ultra-thin MEMS Solid Oxide Fuel Cells,"Microfabricated solid oxide fuel cells are being investigated for portable power applications requiring high energy densities [1-2]. Reducing the thickness of the fuel cell stack (anode, electrolyte, and cathode) improves the electrochemical performance over that of traditional devices. This motivation for thinner structures, combined with significant temperature excursions during processing and operation (~600-1000 °C), leads to a major challenge of thermomechanical stability of such membranes. Figure 1 shows a buckled electrolyte/SiN thin film. To predict and control structural stability and failure, the structural characterization of thin films is being investigated. Our group has characterized the residual stress and microstructure of the electrolyte layer. Complete studies were done on residual stress in sputter-deposited yttria-stabilized zirconia (YSZ) thin films (5 nm-1000 nm thickness) as a function of substrate temperature [3]. The results indicate variations in intrinsic stress from ~-0.5GPa to ~50 MPa as in Figure 2. Changes in microstructure are characterized using x-ray diffraction of as-deposited and annealed films and correlated with relevant mechanisms/models of residual stress evolution. Based on the design frameworks using the data above, a large-area full fuel cell stack (anode, electrolyte, and cathode) has been fabricated and tested to be thermomechanically stable at high operating temperatures. Tri-layers (Pt-YSZ/YSZ/Pt-YSZ, 50-200-µm wide, each 250-nm-thick) were sputter-deposited at high temperature (500-600C). Devices are being tested for electrochemical performance and power generation. In addition, proton-conducting electrolytes, typically capable of significant power generation at temperatures lower than YSZ are also being investigated in ultra-thin film form. Crack-free barium cerium-yttrium-oxide (BaCeYO) films with uniform thickness (300-500-nm thick) have been successfully sputter-deposited. Electrochemical and residual stress characterization for this material is currently underway. Additional ongoing work includes bulge-testing to determine the electrolyte’s elastic/thermal/fracture properties in ultra-thin membrane form, investigation of the mechanical and chemical properties of anode cathode materials, and nonlinear modeling of film postbuckling and failure."
Nanoscale Manipulation of Biological Entities Using Magnetic Particles and Fields,"An increasing number of “lab-on-a-chip” technologies and therapeutic treatments rely on the rapid isolation of clinically or scientifically relevant proteins, cells, and nucleic acids. Magnetic fields and forces provide a useful means of sorting and manipulating such biological entities. Researchers have successfully used magnetic particles, often decorated with target-specific antibodies, for applications in human leukocyte antigen (HLA) diagnostics, cell enrichment or depletion, protein isolation, biomechanics measurements, and the electrophoresis of nucleic acids. The goal of our research is to use uniform and non-uniform magnetic fields in MEMS devices to manipulate magnetic particles or bound entities for the purpose of developing tools that can more rapidly and efficiently sort DNA, blood cells, and cellular organelles.We have previously demonstrated the electrophoresis of DNA in a microchannel using an array of self-assembled posts of magnetic particles [1]. We intend to investigate the effect of column spacing on separation efficiency and also the use of “blinking” magnetic fields (Figure 1) as a more rapid means to separate long-chain DNA, which tends to migrate very slowly in a static matrix. In addition, we have demonstrated, experimentally and through simulation, the ability to direct columns of magnetic beads laterally across a microfluidic channel, using patterned materials and a uniform magnetic field (Figure 2). This mechanism is the first step toward our development of a continuous, incubation-free cell-sorting device. Furthermore, we have utilized “saw-tooth” magnetic fields with aqueous ferrofluids to sort submicrometer (510 and 840nm) non-magnetic particles [2]. We believe this magnetophoresis will be useful in sorting subcellular, like-sized biological bodies, such as organelles and viruses."
Suspended Microchannel Resonators for Biomolecular Detection,"We have demonstrated a new approach for detecting biomolecular mass in the aqueous environment. Known as the suspended microchannel resonator (SMR), target molecules flow through a suspended microchannel and are captured by receptor molecules attached to the interior channel walls [1]. As with other resonant mass sensors, the SMR detects the amount of captured target molecules via the change in resonance frequency of the channel during the adsorption (Figures 1,2). However, what separates the SMR from the myriad of existing resonant mass sensors is that the receptors, targets, and their aqueous environment are confined inside the resonator, while the resonator itself can oscillate at high Q in an external vacuum environment, thus, yielding extraordinarily high mass resolution. Figure 1: a) Suspended microchannel resonator (SMR); b) Cross-section of vibrating SMR; c) Targets bind to immobilized receptors (not shown), and the high surface concentration lowers the resonant frequency. Since biomolecules are more dense than solution (~1.4 g/cm3), the resonant frequency is reduce by ∆ω.Figure 2: a) Electron micrograph of three suspended microchannel resonators; b) Relative frequency shift for a 40 kHz resonant microchannel after injection of the following solutions: buffer (black), avidin (blue), bBSA (red), and avidin (blue). The adsorption of the biomolecules to the interior channel walls increases the overall mass and lowers the resonant frequency."
A Combined Microfluidic/Dielectrophoretic Microorganism Concentrator,"This project focuses on the development of a microorganism concentrator for pathogen detection applications. A common problem in microfluidic systems is the mismatch between the volume of a sample and the volume that a device, such as a detector, can process in a reasonable amount of time. Concentrators can, therefore, be used in pathogen detection and other microfluidics applications to reduce sample sizes to the micro-scale without losing particles of interest.The concentrator, illustrated in Figure 1, is an active filter that uses dielectrophoresis to concentrate bacterial spores in low-conductivity solution. Dielectrophoresis uses spatially nonuniform, alternating electric fields to move particles by polarizing them and then acting on the induced dipole [1]. This concentrator uses positive dielectrophoresis, pulling particles toward electric field maxima. In operation, we set up the electric fields by lining the bottom of the channel with interdigitated electrodes. We combine a passive mixer [2] with these electrodes to enable trapping at high flowrates: the mixer circulates the liquid, bringing particles to the bottom of the channel where they are trapped by the electrodes. When enough particles have been collected, they are all released at once in a small volume, thereby producing a concentrated sample. Figure 2 shows a plot of output concentration over time as a sample of beads is released. The plot was produced by sampling discrete droplets at the output of the device and measuring their bead concentration using a spectrophotometer. This result shows a concentration enhancement of 25x between the input (C0) and output (Drop #5) concentrations."
single Molecule Analysis of DNA in Electric Fields,"Recent advances in gene therapy and crime investigation have spurred a demand for rapid “gene mapping” of large (kbp-Mbp) DNA molecules. Because current electrophoresis technologies are inadequate for large DNA, several promising MEMS designs for DNA mapping have been recently proposed that require either: 1) a DNA molecule negotiating an obstacle course in a microchannel or 2) stretching a DNA coil for linear analysis. The goal of our research is to experimentally probe the fundamental physics that underlie these DNA mapping designs. In general, the governing physics is complex due to the confinement of the microchannel, the coiled-nature of long DNA molecules, and the induced electric field gradients from obstacles and changes in channel dimensions. With single molecule microscopy, we have demonstrated many of the governing physical mechanisms at play in these gene mapping microfluidic devices [1-3]. For example, we have shown the experimental scaling for the diffusion coefficient of DNA in a confined channel (Figure 1a) and the probability distribution for the “collision time” of a DNA molecule unhooking from a small obstacle (Figure 1c). In addition, we have thoroughly investigated DNA stretching in electric field gradients created by a contraction and an obstacle (Figure 2). Just as a flow gradient stretches a polymer, an electric field gradient can stretch a charged polymer like DNA. Because electric field gradients have no local rotational components, a charged polymer will experience purely extensional deformation. These findings will aid the design of DNA separation devices that contain many obstacles and contractions, and they also offer an attractive way to completely stretch DNA for linear analysis."
Microfabricated Mechanical Biosensor with Inherently Differential Readout,"Intermolecular forces that result from adsorption of biomolecules can bend a micromachined cantilever and enable the detection of nucleic acids and proteins without any prior labeling of target molecules. Often, the cantilever deflection is detected using the optical lever method, i.e., by focusing a laser beam at the tip of the cantilever and measuring the changes in position of the reflected beam. Researchers have also shown that, by using the optical lever method to separately measure the bending of two identical cantilevers, the reliability of the signal resulting from the molecular binding reaction is improved by monitoring the relative or differential bending. [1]We developed an interferometric sensor that inherently measures the differential bending between two adjacent cantilevers, thereby eliminating the need for two separate optical setups and alignment steps. The two cantilevers constitute a sensor-reference pair, whereby only the sensing surface is functionalized with receptors that are specific to the ligand to be detected (Figure 1). The two cantilevers have closely matched responses to background disturbances. Hence, disturbance-induced nonspecific deflections are suppressed upstream, i.e., before the optical signal is measured. We have previously shown that in air, the resolution of the interferometric cantilever-based sensor at high frequencies (40-1000 Hz) is limited by its sub-angstrom thermomechanical noise (~0.2 ÅRMS). However, at lower frequencies, the sensor exhibits a flicker or 1/f-type behavior, which yields noise levels that are much higher (~10 ÅRMS) than the thermomechanical noise. For biological applications of cantilever-based sensors, it is the low-frequency behavior in liquid that governs the detection limit. We have measured the low-frequency behavior of the sensor in liquid and demonstrated that it can be improved by differential detection (Figure 2) [2]."
Micromechanical Detection of Proteins Using Aptamer-Based Receptor Molecules,"Numerous studies have been conducted on using antibodies as receptors for detecting proteins. Although antibodies can be used to detect proteins with high sensitivity and specificity, they are generally produced in vivo, which introduces difficulties in engineering their properties. In contrast, aptamers (nucleic-acid binding species) can be selected in vitro and have been produced against a wide range of targets, from small molecules, to proteins, to whole cells. Aptamers are DNA or RNA molecules, which can form tertiary structures that recognize and bind to their respective targets.We have investigated the capability of an aptamer-protein binding event to generate changes in surface stress that bend a flexible micromachined cantilever (Figure 1) [1]. We used a receptor-ligand system that was previously investigated and characterized in solution. The ligand, i.e. the target molecule, was Thermus aquaticus (Taq) DNA polymerase, an enzyme that is frequently used in polymerase chain reaction (PCR). The recognition element (receptor) of the sensor was an anti-Taq aptamer modified with a thiol group at one end to enable covalent linking onto a gold surface. The sensor cantilever was functionalized with aptamer molecules, and the reference cantilever was functionalized with oligonucleotides of nonspecific sequence. The differential bending between the two cantilevers was determined directly by using interferometry. We characterized the system in terms of its response to variation in ligand concentration, as well as, its ability to recognize a particular ligand in a complex mixture and to discriminate against nonspecific binding (Figure 2). Our results indicate that aptamers can be used with cantilever-based sensors for sensitive, specific, and repeatable protein detection."
Plasmon Microscopy on Gold and Gold/Oxide surfaces,"Surface plasmon resonance has primarily been used as a technique for measuring the thicknesses of very thin organic and polymer films on metallic surfaces with low lateral resolution. Its ability to sense unlabeled molecules and its speed of measurement are advantageous when observing real-time adsorption, desorption, or reactions, of biological molecules.In this study, we will use the surface plasmon technique to create an imaging microscope to study planar lipid bilayers. We develop imaging optics that collect the plasmon reflectivity in a CCD (charged-coupled device) camera to provide real images of the optical thickness of absorbates as shown in Figure 1. To improve the lateral resolution, we will utilize protein barriers to restrict the motion of the lipids and to uniformly divide the observational field. We print these with a PDMS (polydimethylsiloxane) stamp made from photoresist masters created in the MTL Technology Research Laboratory. To provide a surface commensurate with other experimentation on the lipids, we coat the metallic interface with a 10 nm layer of silicon dioxide, which has a minimal effect on sensitivity. The metallic surface and the silicon dioxide coating are evaporated in the MTL Exploratory Materials Laboratory. In Figure 2, we show a static corral pattern with 50x50 µm2 areas of 40% 1,2-dioleoyl-sn-glyceri-3-phosphocholine (DOPC)/30% egg-sphingomyelin/30% cholesterol surrounded by 10 micrometer wide BSA (Bovine Serum Albumin) protein spacers. The width is foreshortened by the experimental setup.After improving the lateral resolution, this technique will be able to image the domain dynamics caused by enzyme reactions in a high throughput way."
Use of stamped Protein corrals in High Throughput studies of Lipid Membrane Model systems,"Supported lipid bilayers are useful in vitro mimics for natural biological membranes, and various biotechnological applications are facilitated by their planar geometry. In this study, variable compositions or conditions will be created on supported planar lipid bilayers in order to study the coupled effects of enzyme, membrane, and solution composition on the sphingomyelinase enzymatic reaction. We combine gradients produced by microfluidic flows with membranes confined to surface patterned corrals in order to achieve a high throughput experimental system in which the preparation and measurement times can be greatly reduced. We employ poly(dimethylsiloxane) (PDMS) stamps, which are made from photoresist masters created in the MTL Technology Research Laboratory, to print proteins [1] onto glass surfaces to create barriers capable of restricting the motion of lipids to specific regions of the surface called corrals, as shown in Figure 1. The various membrane conditions in the corrals can be created by incorporating the patterned surface within a microfluidic device. The laminar flow in the micofluidic channel causes fluid elements to follow streamlines, mixing across the streamlines only by diffusion. To create varying lipid bilayer compositions, vesicles are deposited from solution and irreversibly stick to form a continuous bilayer within each corral. As a consequence, a particular vesicle composition in the microfluidic channel is captured by the surface and is restricted in each corral, as shown in Figure 2. Likewise, we can create gradients in the bulk solutions (e.g. enzyme concentration or buffer conditions) by varying the composition in neighboring laminar streams. The desired corralled lipid composition gradient or desired solution condition gradient upon corralled lipid can be adjusted by flow parameters and scale of corral size."
Use of Microfluidic Device to study Protein-Polymer Interactions,"In recent years, the importance of polymer architecture on their physical properties has been recognized. We are studying the effect of a polymer’s macromolecular architecture on its ability to interact with other molecules, in particular with proteins. In order to study a variety of protein-polymer interactions we developed a microfluidic platform. We monitor polymer-protein interactions by means of fluorescence resonance energy transfer (FRET), where the polymer molecules are unlabeled and two populations of protein molecules are fluorescently labeled with a FRET donor and an acceptor pair. Because a FRET signal is highly distance-dependent, without interaction we observe little FRET, and upon complexation, we observe a strong FRET signal (Figure 1). We are interested in the effects of polymer branching on protein aggregation and have chosen a model system of different generations of Poly(amidoamine) PAMAM dendrimers and fluorescently labeled Streptavidin. We can manipulate the overall charge of PAMAM dendrimers either by selecting dendrimers generation (G0, G2, G4, etc) or by adjusting the solution pH.We create a microfluidic device from polydimethylsiloxane (PDMS). The laminar flow in these channels allows us to directly compare polymer or control solutions interacting with the protein solution by interdiffusion. Our initial results show qualitative differences between Streptavidin/PAMAM (G2) and Streptavidin/PAMAM (G4) interactions (Figure 2). As molecules move along the channel, they start interacting. We observe both a shift in peak position, as well as, changes in intensity profiles as the molecules move away from the junction point. The peak position shift indicates that, indeed, both polymers interact with Streptavidin and that changes in intensity profiles are not solely caused by diffusion. Differences between the intensity profiles of Streptavidin/PAMAM G2 and Streptavidin/PAMAM G4 show that indeed both polymers interact differently with Streptavidin molecules; we are currently analyzing these FRET profiles to provide a quantitative measure of protein-polymer interaction."
Super-Hydrophobic Surfaces for Hemocompatibility,"It is well known that in fluid systems, as geometric scale decreases, the effect of surface forces increases relative to body forces. This property has been exploited to modify the wetting behavior of fluids on a surface by structuring the surface. By reducing feature size, surfaces have been developed that have a contact angle with water that approaches 180˚ when the flat-surface contact angle of the material is closer to 100˚ [1]. Our project focuses on making these so-called super-hydrophobic surfaces practical to manufacture. We have manufactured surfaces with water contact angles above 160˚ by casting poly-dimethylsiloxane (PDMS), a material with a flat-surface water contact angle of approximately 100˚. Our methods are limited by the size of a low temperature oven, not by wafer size. Thus, we can scale production size up beyond the limits of typical microfabrication techniques. Additionally, we are interested in the application of super-hydrophobic surfaces in bio-medical systems to improve hemocompatability. A material is hemocompatable if it does not react unfavorably in the presence of blood. Hemocompatible surfaces are crucial to the performance of many biomedical devices. One of the requirements for such surfaces is the ability to resist the coagulation of proteins from blood. The increase in contact angle for super-hydrophobic surfaces is driven by a reduction in the interaction between the fluid and the surface. We are investigating the hypothesis that reducing the fluid-surface interaction between blood and a surface by microstructuring will decrease protein deposition on the surface."
Electrical Properties of the Tectorial Membrane Measured with a Microfabricated Planar Patch clamp,"The tectorial membrane (TM) is a mechanical structure in the cochlea that plays a critical role in hearing. Although its composition suggests that it contains an abundance of charged molecules–charges that may contribute to its mechanical properties–measuring the concentration of this fixed charge has been difficult. Since the TM lacks an insulating cell membrane, traditional micropipette techniques have not yielded stable measurements of the electrical potential of the TM. We have developed a microfabricated chamber that overcomes this problem by placing the TM as an electrochemical barrier separating two fluid baths. The chamber consists of a small aperture into a microfluidic channel (Figure 1), similar to previous planar patch clamp designs [1]. The aperture diameter was chosen to be small enough to be covered by the TM, while large enough to contribute little electrical resistance. The microfluidic channel allows perfusion of the fluid below the TM, so the ionic composition of fluids in both baths can be rapidly changed. Varying the ionic concentration of the baths changes the electrical potential between baths in a manner that depends on the fixed charge of the TM. The microfabricated chamber has enabled the first stable, repeatable measurements of this electrical potential (Figure 2). The results suggest that the TM contains sufficient charge to completely account for its mechanical rigidity."
Microfabricated shearing Probes for Measuring Material Properties of the Tectorial Membrane at Audio Frequencies,"The tectorial membrane (TM) is ideally located to exert shearing forces on sensory hair cells in the cochlea in response to sound. Consequently, measuring the shear impedance of the TM is important for understanding the mechanical basis of hearing. However, few direct measurements of TM shear impedance exist, because the small size of the TM and the need to measure its properties at audio frequencies render traditional impedance measurement methods infeasible. We have overcome these limitations by designing and microfabricating shearing probes that are comparable in size to the TM and that can exert forces at audio frequencies. The probes consist of systems of cantilevers designed to apply forces in two dimensions (Figure 1). Forces applied to the base of the probe are coupled through the cantilevers to a shearing plate, which is brought into contact with the TM. By measuring the relative deflection of the base and plate and knowing the probe stiffness, we can determine the shear impedance of the TM. A variety of probes with different stiffnesses and geometries allow measurement of impedance over many orders of magnitude. Figure 2 shows a probe whose shearing plate is in contact with the TM. To determine TM impedance at audio frequencies, we have coupled these probes to a computer microvision system that allows measurements of nanometer-scale motions at high frequencies [1]. The probes were calibrated, and could exert forces with amplitudes in the range 3-300 nN at frequencies from 10-9000 Hz, a large fraction of the hearing range. Measurements of TM shear impedance, using these microfabricated probes, have helped to characterize this enigmatic component of the cochlea."
Implantable MEMs for Drug Delivery,"We have developed an implantable silicon microelectrome-chanical system (MEMS) device for biomedical applications [1]. This device contains an array of wells that hermetically store its contents. Activation of the device electrochemically dissolves gold membranes covering the wells, by application of an anodic voltage through a wire-bonded connector (Figure 1). The well contents are then exposed to the surrounding en-vironment. This system allows temporal control of several acti-vations and the ability to store a variety of contents separately. Targeted application for this device is local drug delivery.We have focused our drug delivery efforts on carmustine (BCNU), a potent brain cancer drug. Local delivery of BCNU from an implanted device results in efficacious concentrations at the tumor site, coupled with reduced systemic toxicity, which is a major drawback of the systemic delivery of BCNU [2]. We have achieved successful in vitro and in vivo release of BCNU, and have shown it to significantly impede tumor growth in rats as a result of co-formulation with polyethylene glycol (PEG) to improve release kinetics, and of the development of a new, Pyrex-based package that increases the capacity of the device [3]. Combination therapy of BCNU with Interleukin-2 (IL-2), however, has been shown to be more effective than either alone against tumors [4]. We, therefore, plan to use our device to achieve combination releases, to fully utilize the advantages of our MEMS, i.e., temporal control and multi-drug releases."
A Peristaltic Oxygenating Mixer for Miniature Integrated Bioreactors,"We have developed a mixer and corresponding fabrication process to address problems involved in the development of a miniaturized parallel integrated bioreactor array system, whose functional objectives include: (1) the ability to support cell growth of aerobic micro-organisms without oxygen limitation, (2) scalability to a large number of reactors, (3) online sensing of culture parameters, and (4) individual control over pH. In order to achieve these design objectives, we have developed a flat form factor, all PDMS (silicone elastomer), peristaltic oxygenating mixer (Figure 1), using a fabrication process that allows integrating multiple scale (100µm-1cm) and multiple depth (100µm-2mm) structures in a simple molding process. The flat form factor ensures a high surface area to volume ratio for high oxygen transfer rates, and the peristaltic action achieves in-plane homogeneous mixing within 5-20 seconds, depending on the depth of the well and actuation parameters, which is three orders of magnitude faster than lateral mixing from diffusion alone. The peristaltic action also contributes to mixing in the vertical direction, which further improves the oxygen transfer rate. The volumetric oxygen transfer coefficient (kLa) was measured by a gassing-in method [1], using an integrated platinum-octaethylporphyrine based dissolved oxygen sensor [2]. Calibrated measurements of the oxygen transfer coefficient (Figure 2) in devices of various well depths agree with theoretically expected oxygen transfer coefficients for unmixed devices. For devices mixed with various actuation frequencies, the measured oxygen transfer coefficient falls short of the theoretical values due to non-instantaneous vertical mixing. Even with non-optimized devices, preliminary results from eight simultaneous bacteria growth experiments, using four different medium compositions with online measured optical density and dissolved oxygen concentration, indicate that the oxygen transport is sufficient to maintain a greater than 55% dissolved oxygen concentration for the duration of the bioreaction."
Microfluidic Platform for High-Density Multiplexed Biological Assays,"We have developed a microfluidics-based technology that will support the ongoing need to reduce the cost and increase the capabilities of genetic testing in areas such as: population studies for the identification of inherited disease genes, more effective evaluation of drug candidates, and rapid determination of gene expression in tissues for disease management. This technology will also reduce the cost of the clinical testing of novel genetic targets related to disease risk and drug response.Specific improvements promised by this technology are the following:Provides a flexible microfluidic enabling platform for genomic, proteomic and cellular array-based assays;Can be used with current diagnostic protocols and instrumentation;Tests many samples in parallel on the same microarray; Reduces the time it takes to perform genetic tests on microarrays from hours to minutes.The elastomeric microfluidic device can print high-density DNA microarrays with dimensions as small as 10 µm. The device (Figure 1), which hermetically seals to a glass slide, patterns hundreds of DNA targets in parallel as lines on the glass surface. DNA samples are introduced into the sample entry ports and drawn along the channels, where they are exposed to and bind to the slide. After patterning, subsequent probe-target hybridization is simply achieved by running fluorescently labeled samples orthogonally over the target DNA-patterned glass slide, using a second microfluidic chip. Hybridization is achieved in less than 5 minutes; orders of magnitude faster than conventional DNA microarrays that require 16 hours for the same process. Using 10 µm wide microchannels, the hybridization spot density can be increased to over 400,000 assays per cm2."
Polymer-Based Microbioreactors for High Throughput Bioprocessing,"This project aims to develop high-throughput platforms for bioprocess discovery and developments, specifically automated microbioreactors; each with integrated bioanalytical devices, and operating in parallel. By microfabrication and precision machining of polymer material such as poly(dimethylsiloxane) (PDMS) [1] and poly(methylmethacrylate) (PMMA) [2, 3], we realize microliter (5~150 µl) microbioreactors (Figure 1) with integrated active magnetic mixing and dissolved oxygen, optical density, and pH optical measurements (Figure. 2) for monitoring nutrients and products. Reproducible batch and fed-batch [2] fermentation of Escherichia coli and Saccharomyces cerevisiae have been demonstrated in the microbioreactor. With the integration of local temperature control, cell-resistance surface modification, and pressure-driven flow at ~µL/min rates, the microbioreactor was also proven to be capable for chemostat continuous cell culture [3], which is a unique and powerful tool for biological and physiological research. As examples of bioanalysis, HPLC [1] and gene expression analysis [4] using microbioreactors have demonstrated potential applications in bioprocess developments. Parallel microbial fermentations were undertaken in a multiplexed system demonstrating the utility of microbioreactors in high-throughput experimentation [5]. A key issue for high-throughput bioprocessing is to have inexpansive and disposable microbioreactors to save operation time and labor. Current works include the integration of plug-n-pump microfluidic connections [6] in the microbioreactor system, as well as, incorporation of fabricated polymer micro-optical lenses and connectors for biological measurements to produce “cassettes” of microbioreactors."
"Cell stimulation, Lysis, and separation in Microdevices","Quantitative data on the dynamics of cell signaling induced by different stimuli requires large sets of self-consistent and dynamic measures of protein activities, concentrations, and states of modification. A typical process flow in these experiments starts with the addition of stimuli to cells (cytokines or growth factors) under controlled conditions of concentration, time, and temperature, followed at various intervals by cell lysis and the preparation of extracts (Figure 1). Microfluidic systems offer the potential to do these experiments in a reproducible and automated fashion. Figure 1 shows a schematic of a microfluidic device for rapid stimulus and lysis of cells. The fluidic systems with stimulus and lysis zones are defined using soft lithography in a poly(dimethylsiloxane) (PDMS) layer, which is then bonded to a glass slide. Temperature regulation for the two zones is achieved by using a thermo electric (TE) heater at 37oC to mimic physiological conditions during stimulation and a TE cooler at 4oC to inhibit further stimulus during lysis. Mixing in the device is enhanced by the use of segmented gas-liquid flow.To extract meaningful data from cellular preparations, current biological assays require labor-intensive sample purification to be effective. Micro-electrophoretic separators have several important advantages over their conventional counterparts, including shorter separation times, enhanced heat transfer, and the potential to be integrated into other devices on-chip. A PDMS isoelectric focusing device has been developed to perform rapid separations by using electric fields orthogonal to fluid flow (Figure 2). This device has been shown to separate low molecular weight dyes, proteins, and organelles [1]."
Microfluidic Devices for Biological cell capture,"Over the past century, cellular biology and biomechanical engineering blazed ahead in areas, such as: genome sequencing, optical probes, and high-throughput biochemical testing. For example, an increasing variety of optical imaging probes now are available for chemical and biological analyses of molecular events, physiological processes, and pathologic conditions. In contrast, cell culture techniques have remained virtually stagnant [1]. Advances in MEMS, including microfluidics and soft lithography, are providing a toolset from which to develop biological MEMS devices. In addition to miniaturizing macro biological analysis tools, techniques, and assays, microfluidic devices can utilize microscale phenomena and systems to probe single- and multi-cellular levels yielding complimentary static and dynamic data sets [1,2]. Combining these advances with more traditional microtechnology provides groundwork for developing a new generation of cell culture and analysis. Assay protocols can be run in parallel, and dynamic single-cell event information can be collected on a small or large population of cells. Cells can be probed rapidly and inexpensively in large or small quantities with small sample sizes in custom, portable microenvironments developed to more physiologically resemble in vivo conditions [2]. Modular microfluidic devices are expanding possibilities, enabling snap-in modifications for different or second-pass assays. Biological cell capture and analysis devices are shown in Figure 1 and Figure 2. Designed to capture and maintain a specific number of cells in predetermined locations, the devices yield a mechanism by which to study isolated cells or cell-to-cell interaction. Once captured, the cells can be probed and static and dynamic data extracted on the single- and multi-cellular levels."
Manipulating solid Particles In Microfluidic systems,"Microfluidic systems offer a unique toolset for the separation of microparticles and for the study of the growth kinetics of crystal systems because of laminar flow profiles and good optical access for measurements. Conventional separation techniques for particles, such as sieving, are limited to sizes larger than ~ 50 microns with large dispersion. Sorting microparticles (e.g. small crystals, single cells), requires different techniques. Dielectrophoresis is particularly attractive for microfluidic systems because large electric field gradients that drive the force are easily generated at low voltages using microfabricated electrode structures, and fixed charges are not required as in electrophoresis. It is possible to continuously separate particles of 1-10 microns with ~ 1 micron resolution (Figure 1) using dielectrophoresis with asymmetric electric fields and laminar flow (Figure 2).Microfluidic devices can also be used to study crystallization and extract kinetic parameters of nucleation and growth, and to study different polymorphs of a system. Crystallization has been achieved in some batch processes that do not have uniform process conditions or mixing of the reactants, resulting in polydisperse crystal size distributions (CSD) and impure polymorphs. Microsystems allow for better control over the process parameters, such as the temperature and the contact mode of the reactants, creating uniform process conditions. Thus, they have the potential to produce crystals with a single morphology and uniform size distribution."
BioMEMs for control of the stem cell Microenvironment,"The stem cell microenvironment is influenced by several factors, including: cell-media, cell-cell, and cell-matrix interactions. Although conventional cell-culture techniques have been successful, they offer poor control of the cellular microenvironment. To enhance traditional techniques, we have designed a microscale system to perform massively parallel cell culture on a chip. To control cell-matrix and cell-cell interactions, we use dielectrophoresis (DEP), which uses non-uniform AC electric fields to position cells on or between electrodes [1]. We present a novel microfabricated DEP trap, designed to pattern large arrays of single cells. We have experimentally validated the trap using polystyrene beads, and have shown excellent agreement with our model predictions without the use of fitting parameters (Figure1A) [2]. In addition, we have demonstrated trapping with cells by using our traps to position murine fibroblasts in a 3x3 array (Figure 1B).To control cell-media interactions, a 4x4 microfluidic parallel cell culture array has been designed and fabricated (Figure 2A). Each of the 16 culture chambers has microfluidic inlets and outlets that geometrically control the flow rate and type of media in each cell culture chamber. Reagent concentration is varied along one axis of the array, while the flow rates are varied along the other axis. The system is fabricated out of multilayer polydimethylsiloxane (PDMS) on glass and includes an on-chip diluter to generate a range of concentrations. We have cultured murine fibroblasts in a similar PDMS-on-glass environment at comparable flow rates (Figure 2B). This microfabricated system will serve as an enabling technology that can be used to control the cellular microenvironment in precise and unique ways, allowing us to do novel cell biology experiments at the microscale."
Development of Microfluidic channels for Endothelial cell chemotaxis,"Many cells have the ability to sense the direction of external chemical signals and respond by polarizing and migrating toward chemoattractants or away from chemorepellants. This phenomenon, called chemotaxis, has been shown to play an important role in embryogenesis, neuronal growth and regeneration, immune system response, angiogenesis, and other biological phenomena.[1] In addition, cell migration is also important for emerging technologies, such as tissue engineering and biochemical implants.[2] This simple behavior is apparently mediated by complex underlying diffusion and migration mechanisms that have been the focus of many studies and models. These mechanisms may be studied by various chemotactic assays. There have been several chemotaxis assay chambers developed in the past. The most widely used is the Dunn chamber.[3] The drawback of this chamber is that the cells that are squeezed between the cover glass and the chamber walls might release toxic enzymes and organells, and their effect, if any, on the viable neighborhood cells can not be easily quantified. Additionally, the linear gradient of chemokines lasts only 1 to 2 hours. The Whitesides group at Harvard designed a chemotaxis assay chamber using soft lithography.[4] They incorporated several serial mixers to generate multi-profile chemical gradient. This chamber can generate gradient with a simple linear or complex profile without limit in time, but it needs continuous flow to maintain gradient in the direction normal to gradient direction, which is not physiological. A novel chemotaxis chamber using diffusion characteristics to develop a chemotactic gradient has been developed.[5] This chamber generates a stable and linear gradient along a narrow channel without limitation in time and unnecessary physical stresses. The chamber has 2 inlet ports for 2 kinds of solutions and 1 outlet. One of the input solutions is mixed with a growth factor, and the other solution is mixed with a fluorescent dye or microspheres to verify that there is no bypass flow through the cross channel that supports diffusion. There are two main channels through which the input solutions flow and one narrow cross channel that connects the two main channels, into which a growth factor diffuses from one main channel by diffusion."
A Microfabricated sorting cytometer,"This research involves the development of a microfabricated sorting cytometer for genetic screening of complex phenotypes in biological cells (Figure 1). Our technology combines the ability to observe and isolate individual mutant cells from a population under study. The cytometer merges the benefits of both microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities on a single technology platform. Biologists will be able to use this platform to isolate cells based upon dynamic and/or intracellular responses, enabling new generations of genetic screens. We are implementing this technology by developing an array of switchable traps that rely upon the phenomena known as dielectrophoresis (DEP) [1,2]. The DEP-enabled traps allow for capturing and holding cells in defined spatial locations, and subsequently, releasing (through row-column addressing) a desired subpopulation for further study. Using DEP, non-uniform electric fields induce dipoles in cells that, in turn, enable cellular manipulations. At present, no scalable DEP-based trap configuration exists that can robustly capture single cells and is also amenable to high-throughput microscopy. Such a platform requires performance characteristics that can only be met through quantitative modeling. We have undertaken much of the front-end work necessary for such a system and are continuing our efforts to realize this desired functionality.To date, we have developed second-generation trap geometries implemented in 4x4 trap arrays (Figure 2) to compare our front-end simulation-based modeling with the performance of actual devices. We have designed, fabricated, and tested both n-DEP (cells held at electric-field minima) and p-DEP (cells positioned at electric-field maxima) based configurations [3]. Our first design and test iteration demonstrated partial functionality and first-order proof of concept, while offering insight for future design improvements. We are also investigating the effects of DEP trapping on cell health and the impact that it may have on our ability to assess specific complex phenotypic behaviors."
Vacuum sealing Technologies for Microchemical Reactors,"Current portable power sources may soon fail to meet the increasing demand for larger and larger power densities. To address this concern, our group has been developing MEMS power generation schemes that are focused around fuel cells and thermophotovoltaics. At the core of these systems is a suspended tube microreactor that has been designed to process chemical fuels [1]. Proper thermal management is critical for high reactor efficiency, but substantial heat loss is attributed to conduction and convection through air, as shown in Figure 1. A straightforward solution is to eliminate the heat loss pathways associated with air by utilizing a vacuum package. We are exploring a glass frit bonding method for vacuum sealing.The leading cause of failure for a glass frit hermetic seal is large voids that are formed in the frit while bonding [2]. Progress has been made toward the optimization of presintering and bonding parameters to reduce or eliminate void formation. A vacuum package of 150 mTorr was obtained after optimization, but became leaky shortly after. An alternate packaging method using a two-step bond process, inspired by [3], was devised and developed. Recent experiments of the process, depicted in Figure 2, show that the initial box bond is capable of producing a hermetic seal. Enhancements through the incorporation of non-evaporable getters will be assessed once a vacuum package is achieved."
Scaled-out Multilayer Microreactors with Integrated Velocimetry sensors,"Microreactors are a new class of continuous reactors, with feature sizes in the submillimeter range, which have emerged over the last decade and, for a number of applications, present capabilities exceeding those of their macroscale counterparts. Unlike conventional reactors, the throughput of microreactors is increased by “scale-out,” i.e., operating a large number of identical reaction channels in parallel under equal reaction conditions. We have developed a scaled-out gas-liquid microreactor, built by silicon processing, which consists of three vertically stacked reaction layers, each containing twenty reaction channels. The reaction channels are operated in parallel from single gas and liquid feeds with a liquid volumetric throughput of 80 mL/h. Gas and liquid are introduced to the device through single inlet ports, flow vertically to each reaction layer, and are distributed horizontally to the reaction channels via individual auxiliary channels that provide a significantly larger pressure drop than that across a single reaction channel. These auxiliary channels eliminate cross-talk between reaction channels and ensure uniform flow distribution. The product mixture flows out of the device through a single outlet port. The design rationale of the scaled-out microreactor is illustrated in Figure 1. It is based on flow visualization studies and pressure drop measurements, obtained in a single channel, with the same channel geometry as the reaction channels of the scaled-out device (triangular cross section, channel width = 435 µm, channel length ~ 20 mm) [1]. A photograph of the scaled-out unit is presented in Figure 2. Flow visualization by pulsed-fluorescence microscopy across the top reaction layer reveals that the same flow regime is present in all channels. To further validate the reactor design and monitor flows during continuous operation, pairs of integrated multiphase flow regime sensors are integrated into the device [2]. Comparable slug velocities are measured across the reaction layers."
Multiphase Transport Phenomena in Microfluidic systems,"Microscale multiphase flows (gas-liquid and liquid-liquid) possess a number of unique properties and have applications ranging from use in microchemical synthesis systems to heat exchangers for IC chips and miniature fuel cells. Our work is focused on gas-liquid flows in microfabricated channels of rectangular or triangular cross section. We characterize the phase distribution and pressure drop of such flows and apply such information to a systematic design of gas-liquid microchemical reactors. The inherently transient nature of such multiphase flows provides a rich variety of flow regimes and dynamic flow properties. Characterization is done using pulsed-laser fluorescence microscopy and confocal microscopy (spinning disk and scanning), as well as by integrated flow regime sensors. Superficial gas and liquid velocities were varied between 0.01-100 m/s and 0.001-10 m/s, respectively.Particular attention is given to segmented (slug or bubbly) flows in hydrophilic channels. Figure 1a illustrates the distribution of gas and liquid in the channel. Gas bubbles are surrounded by thin liquid films (thickness ~ 1µm) at channel walls and liquid menisci in the corners. Such flows create a recirculation in the liquid segments (Figure 1b) and can, therefore, be used to efficiently mix two miscible liquids on the microscale within a length of only a few tens of the microchannel width [1,2]. We demonstrate that the transient nature of gas-liquid flows can be used to significantly improve mixing of miscible liquids compared with existing methods. After mixing is accomplished–Figure 2 (bottom) provides an illustration for mixing of two differently colored streams–the gas can be removed from the mixed liquid phase in a capillary phase separator for arbitrary velocities and flow patterns [1]. In addition to providing mixing enhancement, segmented flows narrow the distribution of residence times of fluid elements in the liquid phase, as compared to single-phase flows [1]. A narrower residence time distribution is particularly essential for particle synthesis on a chip."
Integrated Microreactor system,"Individual microreactors have been fabricated for many different chemical reactions, but the development of microreaction technology will require combining separation with microreactors to enable multi-step synthesis. The realization of integrated microchemical systems will revolutionize chemical research by providing flexible tools for rapid screening of reaction pathways, catalysts, and materials synthesis procedures, as well as, faster routes to new products and optimal operating conditions. Moreover, such microsystems for chemical production will require less space, use fewer resources, produce less waste, and offer safety advantages. The need for synthesizing sufficient quantities of chemicals for subsequent evaluation dictates that microchemical systems are operated as continuous systems. Such systems require fluid controls for adjusting reagent volumes and isolating defective units. The integration of sensors enables optimization of reaction conditions, as well as, the extraction of mechanistic and kinetic information.We are developing integrated microchemical systems that have reactors, sensors, and detectors with optical fibers integrated on one platform. New approaches for connecting modular microfluidic components into flexible fluidic networks are being explored. Real-time control of reaction parameters, using online sensing of flowrate, temperature, and concentration, allows for precise attainment of reaction conditions and optimization over a wide range of temperatures and flow-rates. The multiple microreactors on the system can be used together to give higher throughputs or they can be used independently to carry out different reactions at the same time. Figure 1 shows a schematic of an integrated microreactor platform along with an early stage microreactor “circuit board” [1]."
Micro Gas Analyzer,"The US Department of Defense is currently interested in developing the technology to sense, in real time, deployable agents used in chemical warfare. The Micro Gas Analyzer Project (MGA) is the result of this interest, and aims to develop a portable sensor of wide rage and robustness. Current state-of-the-art technology involves bulky equipment (not portable), high power consumption due to the use of thermionic sources and impact ionization mechanisms, high voltage (in the kilovolt range), and long processing times. Thus, the project has a number of key technological challenges, such as the enhancement of the state-of-the-art sensitivity and specificity capabilities, power consumption reduction, and portability, while keeping the processing time below two seconds. The MGA is composed of an ionizer (a CNT field ionization array / CNT field emission array), a mass filter (a micro quadrupole mass spectrometer -µQMS), an ion counter/multiplier, an electrometer/mass detector, and a pumping system (passive – absorption pump/active – piezoelectric pump). A schematic of the MGA system is shown in Figure 1. The goal is to make low vacuum (in the millitor range), ionize the species inside the gas using the CNT arrays, filter them with the quadrupole, and then, sense them with the electrometer. The project team is composed of MIT (Ionizer, µQMS, µPump, Valves), University of Texas (Ionization, µPump), Cambridge University (Ion Counter), and Raytheon/CET (System Integration)."
Micro Quadrupole Mass spectrometer,"One of the subsystems of the Micro Gas Analyzer Project is a mass filter. The purpose of this filter is to select the kind of species that will be sensed downstream by the electrometer. A microfabricated quadrupole mass filter array is being developed for this purpose where a confining potential sorts the unwanted species (Figure 1). Both high sensitivity and high resolution are needed over a wide range of ion mass-to-charge ratios, from 20 to 200 atomic mass units, to achieve the versatility and resolution that are intended for the program. In order to achieve the high resolution and sensitivity, multiple micro-fabricated quadrupoles, each with specific geometrical parameters, are operated in conjunction with each other. From a theoretical point of view, the Mathieu equations describe the dynamics of a particle inside the quadrupole. These equations predict a series of stability regions (Figure 2). Each stability region has its own strengths, such as: less power consumption, less operational voltage, or more sensitivity. For example, lower stability zones are used to improve ion transmission, whereas, higher stability zones are used to improve the selectivity of the filter. Therefore, we plan to explore the stability regions of the Mathieu equations to optimize our design. Two sets of variable voltage sources are needed for the mass filter to operate properly, with voltages ranging between 20 and 200 V, at frequencies of 250 and 500 MHz. We plan to try three different approaches to build the device: LIGA (a german acronym for the process that generates high aspect ratio metallic structures), rods assembled using micro-fabricated deflection springs [1], and rod mounts made with KOH [2]. The device has a cross-sectional area of 20 mm2. The aperture of the individual quadrupoles ranges from 10 to 100 microns."
Design Tools for Bio-Micromachined Device Design,"Using micromachining for biological applications requires complicated structures such as mixers, separators, preconcentrators, filters, and pumps; and these elements are used to process biomolecules or biological cells. To accelerate the design of these complicated devices, new tools are needed that can efficiently simulate mixing and particle or cell motion in complicated three-dimensional flows. In addition, for microfluidic devices intended for use in molecular separation, the length scales are such that noncontinuum fluid effects must be considered, and therefore, hybrid approaches that combine molecular and continuum models must be developed. Finally, the wide variety of structures being developed implies that generating models for system-level simulation will require efficient simulation combined with automated model extraction [3]. Our recent work in addressing these problems includes: the development of efficient time integration techniques for cells in flow [1], techniques for accurately extracting diffusion constants from measurements [2], and efficient techniques for extracting models from detailed simulations [4]."
Microfabricated solid-Oxide Fuel cell systems,"Solid-Oxide Fuel Cells (SOFCs), employing ceramic electrolytes, are a promising alternative to low-temperature PEM (proton-exchange membrane) fuel cells for portable power applications. The use of an oxygen-ion conducting electrolyte, operating at high temperatures, offers the potential for internal reforming of a variety of fuels, with improved tolerance to competitively adsorbing species at the anode (e.g. CO); thus, removing the need for pretreatment stages for conversion of hydrocarbon fuel to high-purity hydrogen. However, the appropriate thermal management of this high-temperature fuel cell system is required to achieve an energy-efficient device. A chip-scale micromembrane architecture has been developed for thermally efficient thin-film applications1 and has been successfully demonstrated for hydrogen separation via ultra-thin palladium films. Resistive heaters placed directly upon a thermally-isolated membrane allow for rapid heating and cooling of the supported thin film at a minimum expenditure of energy. In addition, the mechanical strength provided by the micromembrane support allows the use of sub-micron films for significant improvement in ion permeability. For these reasons, the micromembrane architecture has been investigated for SOFC development. The extension of this technology is achieved, utilizing a silicon-nitride girder-grid support system to mechanically reinforce the solid-oxide thin films (Figures 1 and 2).Efforts include: the determination of optimal free-standing fuel cell stack dimensions, integration of individual stacks into a reinforced membrane structure, design of current collectors, and electrical performance tests of fabricated devices. Stability tests of free-standing membranes of varying length scales and aspect ratios are performed for a variety of fuel cell stacks and individual stack layers, with results compared to mechanical models of layered free-standing films. The resulting information is incorporated into the design of a silicon-nitride reinforced free-standing membrane architecture. Lastly, microdevice testing stations allow for performance studies of prototype microdevices."
Catalytic Micromembrane Devices for Portable High-Purity Hydrogen Generation,"The development of portable-power systems employing hydrogen-driven fuel cells continues to garner significant interest in the scientific community, with applications ranging from the automotive industry to personal electronics. While progress has been made in the development of efficient hydrogen-storage devices, it is still preferable for portable-power systems to operate from a liquid fuel with a high energy density (e.g., methanol, ammonia). This necessitates the integration of a hydrogen generator capable of converting stored fuels to hydrogen to drive the fuel cell. Previous research has focused upon the development of novel catalysts and autothermal microreactor designs for efficient conversion of liquid fuels (e.g. methanol, ammonia) into hydrogen for use by a polymer-electrolyte fuel cell [1]. Additionally, micromembrane devices (Figure 1) have been developed for purification of the resulting hydrogen stream to remove impurities (e.g. CO) that adversely affect fuel cell performance [2]. Our current research aims to integrate (i) catalyst design, (ii) autothermal microreformer design, and (iii) micromembrane technology to realize microscale chemical systems capable of producing high-purity hydrogen for fuel cell operation. By combining microfabrication techniques for generation of micromembrane devices with wet-chemical deposition methods for a variety of catalysts, multiple membrane reactor applications for hydrogen generation can be realized, taking full advantage of superior mass transport and film permeabilities achievable at the microscale. Results obtained for LaNi0.95Co0.05O3 perovskite catalysts integrated with 23 wt% Ag-Pd membranes (Figure 2) demonstrate promising high-purity hydrogen yields at low methanol feed compositions, and demonstrate the applicability of catalytic membrane reactors effected at the microscale for efficient production of high-purity hydrogen. Resulting microdevices are directly applicable as part of an integrated portable-power system."
Thermal Management in Devices for Portable Hydrogen Generation,"As power requirements of portable electronic devices continue to increase, the development of an efficient portable power generation scheme has remained an active research area. Specifically, hydrogen-driven fuel cells have received significant attention. This work focuses on microreaction technology for the conversion of fuel to electrical power. Emphasis has been placed on developing microreactors for high-purity hydrogen production. Critical issues in realizing high-efficiency devices capable of operating at high temperatures have been addressed: specifically, thermal management, the integration of materials with different thermophysical properties, and the development of improved packaging and fabrication techniques.A microfabricated suspended-tube reactor (Figures 1, 2) has been developed for efficient combustion and reforming of chemical fuels.[1] The reactor, designed specifically to thermally isolate the high-temperature reaction zone from the ambient, consists of thin-walled U-shaped silicon nitride tubes formed by deep reactive ion etching (DRIE) and subsequent nitride deposition via chemical vapor deposition (CVD). Thin-film platinum resistors are integrated into the reactor for heating and temperature sensing. Detailed thermal characterization demonstrates reactor operation up to 900ºC and quantifies heat losses. Additionally, this high-temperature microcombustor is applicable for thermophotovoltaic generation. A new fabrication scheme for the suspended-tube reactor incorporates wet potassium hydroxide (KOH) etching, an economical and time-saving alternative to DRIE]. In this design, pre-fabricated thin-walled glass tubes replace the silicon nitride tubing to provide inlet and outlet conduits. The thermal conductivity of the resulting tubes is 50% lower than that of silicon nitride. Hence, this technique allows for the incorporation of robust tubing, while maintaining thermal efficiency."
Materials and structures for a MEMs solid Oxide Fuel cell,"Microfabricated solid oxide fuel cells are currently being investigated for portable power applications requiring high energy densities [1, 2]. Reducing the thickness of fuel cell stack materials improves the electrochemical performance versus traditional devices. This motivation for thinner structures, combined with significant temperature excursions during processing and operation (~600 – 1000 °C), presents the thermomechanical stability of such membranes as a major challenge. A buckled electrolyte/SiN thin film is shown in Figure 1. The prediction and management of structural stability (buckling) and failure require accurate knowledge of many parameters including: thermomechanical properties, residual stress, and fracture strength.Our group has characterized the residual stress and microstructure of the electrolyte layer of the fuel cell stack. Residual stress in sputter-deposited yttria stabilized zirconia (YSZ) thin films (5nm – 1000nm thickness), as a function of deposition pressure and substrate temperature, has been completed [3]. The results indicate variations in intrinsic stress from ~0.5GPa compressive to mildly tensile (~50 MPa) (Figure 2). Changes in microstructure are subsequently characterized using X-ray diffraction of as-deposited and annealed films and correlated with relevant mechanisms/models of residual stress evolution. Frameworks for using such residual stress data to design mechanically stable membranes for µSOFC devices have also been developed.Current research areas include: continued microstructural and residual stress characterization under thermal cycling, elastic/fracture properties characterization, design and fabrication of thermomechanically stable fuel cell stacks, exploration of proton conducting solid oxide thin films for lower-temperature operation, investigation of the mechanical properties of anode and cathode materials, and nonlinear modeling of film postbuckling and failure."
Microfabricated Proton-conducting solid Oxide Fuel cell system,"Owing to their high efficiency and energy density, miniaturized fuel cells are an attractive alternative to batteries in the mW-W power generation market for portable consumer and military electronic devices [cf. 1-3]. Hydrogen is being actively considered as a fuel for power generation. It can be supplied either by storage devices or its in-situ generation using reformers. However, safety and reliability issues persist with current storage choices, such as zeolites and carbon nanotubes [4]. For these reasons, fuel cells based on direct fuel reforming are advantageous. The processes typically involve either high temperature reforming of fuel to hydrogen combined with a low temperature Proton exchange membrane (PEM) fuel cell, which implies significant thermal loss. Alternatively, fuel reforming can be combined with solid oxide fuel cells capable of operating at high temperatures. Typical components of a solid oxide fuel cell include electrodes and an electrolyte. Typically ZrO2, CeO2, and LaGaO3, which are oxide ion conductors are used as separator materials [5]. However, one of the disadvantages of these materials is the need for operation at high temperatures (~700oC). These operating temperatures, in turn, lead to associated problems of materials compatibility and low tolerance with respect to variations in operating conditions. As an alternative, proton conducting solid oxide membranes, typically alkaline earth metal substituted perovskites, such as BaCeO3, SrCeO3, and BaZrO3, exhibit high protonic conductivity even at 400oC [6]. In the current research, we explore the possibility of fabricating a fuel cell using these low temperature electrolytes. Previous work on Pd-based membranes on MEMS-supported membranes indicates that hydrogen yields up to 93% can be achieved for methanol using LaNiCoO3 anode catalyst at 475oC. We plan to extend this concept further to prepare a complete fuel cell assembly and test its performance."
Thermophotovoltaic (TPV) MEMs Power Generators,"Batteries have, for a number of years, not kept up with the fast development of microelectronic devices. The low energy densities of even the most advanced batteries are a major hindrance to lengthy use of portable consumer electronics, such as laptops, and of military equipment that most soldiers carry with them today. Furthermore, disposing of batteries constitutes an environmental problem. Hydrocarbon fuels exhibit very high energy densities in comparison, and micro-generators converting the stored chemical energy into electrical power at even modest levels, are, therefore, interesting alternatives in many applications. This project focuses on building thermophotovoltaic (TPV) micro-generators, in which photocells convert radiation from a combustion-heated emitter, into electrical power. TPV is an indirect conversion scheme that goes through the thermal domain and therefore, does not exhibit very high efficiencies (10-15% max). However, because of its simple structure and because the combustor and photocell fabrication processes do not need to be integrated, the system is simpler to micro-fabricate than other generator types (e.g. thermoelectric systems and fuel cells). It is also a mechanically passive device that is virtually noiseless and less subject to wear than engines and turbines. In this TPV generator, a catalytic combustor, the suspended micro-reactor (Figure 1) is heated by combustion of propane and air, and the radiation emitted is converted into electrical energy by low-bandgap (GaSb) photocells (Figure 2). Net power production of up to 1 mW has been achieved [1], constituting a promising proof of concept. Work is underway to build a new micro-reactor more suited for the needs of TPV than the original design."
Thermoelectric Energy conversion: Materials and Devices,"Thermoelectric devices based on Peltier effect and Seebeck effect use electrons as a working fluid for energy conversion. These solid-state energy conversion devices have important applications in refrigeration and electrical power generation. Our work follows two directions: nanostructured materials and microdevices. The efficiency of thermoelectric devices is characterized by the nondimensional thermoelectric figure of merit 2ZT S T / k= σ, where S is the Seebeck coefficient, σ the electrical conductivity, and k the thermal conductivity of their constituent materials, and T is the average device temperature. Identifying materials with a large ZT has been challenging because of the interdependency of those three properties. With both quantum size effects on electrons and classical size effects on phonons, nanostructures provide an alternative way to engineer thermoelectric properties.1,2 Our current effort is focused on designing, synthesizing, and characterizing nanostructures in bulk form that can be produced for mass applications. Figure 1 illustrates ballistic phonon transport in a unit cell of a nanocomposite, which leads to low thermal conductivity.3 We are also working on fabricating micro thermoelectric devices, first using thin film devices such as SiGe alloy and Si-Ge superlattices,4 and more recently on thick films to reduce parasitic heat losses.5 In addition, we are also exploring novel microdevice configurations that can improve energy conversion efficiency, by utilizing the hot electron concepts.6,7"
Far-Field spectral control and Near-Field Enhancement of Thermal Radiation Transfer for Energy conversion Applications,"The performance of thermophotovoltaic (TPV) energy conversion systems is greatly affected by the radiation characteristics of the thermal emitter. Ideally, one would want a selective emitter with high emissivity above the band gap and low emissivity below the band gap. Various approaches have been proposed to fabricate effective selective emitters with 2D or 3D photonic crystals, which involve considerable intricate microfabrication. Instead, we have proposed a simpler-to-fabricate 1D structure that exhibits many of the features of its 2D and 3D counterparts [1]. The key has been to use ultra thin metallic films arranged as a periodic multilayer stack with a suitable non-absorbing dielectric material in-between. Figure 1 shows the numerical computation of the total hemispherical emissivity of two such structures as a function of wavelength. In addition to improving the selective emission of thermal radiators, we are also exploring near field effects to improve the energy density and efficiency of thermal-to-electric energy conversion devices. Electromagnetic surface waves, like surface phonon polaritons or surface plasmon polaritons, can increase the energy transfer by two or three orders of magnitude compared to the near-field enhancement between materials that do not support such surface waves. Our work has shown that such enhancements in thermal radiative transfer can not only increase the power density and efficiency of TPV devices [2] but can also contribute to the improvement of thermoelectric devices [3]. We are also exploring a new TPV device structure involving interdigitized hot-and-cold fingers with increased surface area, built-in photon recycling, and potentially built-in spectral control [4]. Experimental work involving microfabrication and device testing is in progress."
Development of a High Power Density Microscale Turbocharger,"A microscale turbocharger has been fabricated as part of a program to develop a microfabricated gas turbine generator to serve as a battery replacement with seven times the energy density of today’s best batteries. The turbocharger will evolve into the gas turbine generator with minimal fabrication process changes. The turbocharger lacks an electric generator, and its turbine and compressor flow paths are independent; otherwise, the two devices are virtually identical. The turbocharger is a test vehicle for developing fabrication processes and turbomachinery/bearing technology. The turbocharger is formed by fusion bonding six silicon wafers. The hatched structure in Figure 1 is the rotor, which is free to spin within the device on hydrostatic gas bearings. The turbocharger has a design rotation rate of 1.2 million rpm and a design compressor pressure ratio of 2.2.Journal bearing dimensional control is a key challenge: 15 +/- 0.75 µm in width and 330 +/- 5 µm in depth. The bearing width tolerance, which is half that of previous devices in this program, is achieved through refinements in the etch recipe as well as modifications to the masking material profile. The masking material must be carefully controlled because of its finite etch rate and the effects of sidewall-passivation-layer erosion from ions deflected by the resist slope. The journal bearing specification is met on device wafers with a yield of more than 60%. Another challenge for this device is obtaining a rotor blade height uniformity of about 1%, which is critical for low levels of imbalance in the rotor.A turbocharger has been operated to a rotation rate of 480,000 rpm, which is equivalent to a tip speed of 200 m/s (450 miles per hour). Figure 2 shows the measured compressor pressure ratio for two runs of the same device with different throttle settings. The compressor achieved a pressure ratio of 1.21 with a flow rate of 0.14 g/s at its top speed. The measured pressure and flow characteristics are consistent with the design models for this device."
A MEMS Electroquasistatic Induction Turbine-Generator,"Presented here is a microfabricated electroquasistatic (EQS) induction turbine-generator that has generated net electric power. A maximum power output of 192 µW was achieved under driven excitation. We believe that this is the first report of electric power generation by an EQS induction machine of any scale in the open literature. This work forms part of a program at MIT to fabricate a MEMS-scale gas turbine-generator system. Such a system converts the enthalpy of combustion of a hydrocarbon fuel into electric power. For even modest efficiency levels of the gas turbine engine cycle (10-15%), a small gas turbine would be a portable energy source with higher energy density than the best batteries available [1]. In MIT's device, this small engine provides the shaft power needed to drive a small electric generator. Although magnetic machines are preferred at large scales, EQS machines become attractive at small scales, primarily because very small airgaps between the rotor and stator allow higher breakdown electric fields of approximately 108 V/m. The generator comprises five silicon layers (Figure 1) fusion bonded together at 700oC. The stator is a platinum electrode structure formed on a thick 20 µm recessed oxide island. The rotor is a thin film of lightly doped polysilicon also residing on an oxide island, which is 10 µm thick. We also present a generalized state-space model for an EQS induction machine that takes into account the machine and its external electronics and parasitics. This model correlates well with measured performance, and was used to find the optimal drive conditions for all driven experiments. Figure 2 shows the results of an experiment under driven excitation. In this particular experiment, 108 µW was generated at 245krpm. Good correlation with the models is observed. In other experiments, self-excited operation was attained. In this case, the generator self-resonates and generates power without the use of any external drive electronics [3]."
Multi-Watt Electric Power from a Microfabricated Permanent-Magnet Generator,"Presented here are the design, fabrication, and characterization of three-phase permanent magnet (PM) machines that convert 2.3 W of mechanical power and deliver 1.1 W of DC electrical power to a resistive load at a rotational speed of 120,000 rpm. Such microgenerators are an important system-level component of compact MEMS-based power sources, such as combustion-driven or air-driven microengines [1].The generators are three-phase, eight-pole, synchronous machines, each consisting of a surface-wound stator (Figure 1) and a multi-poled PM rotor (Figure 2(a)). The stator uses three Cu windings that are dielectrically isolated from a 1-mm thick NiFeMo (Supermalloy) substrate by a 3 µm spin-on-glass layer and/or 5 µm polyimide layer. The coils were fabricated using a two-layer electroplating process [2]. They were measured to be 80-120 µm thick and 50-550 µm in width. The microfabricated coils, with their small inter-conductor gaps and variable width geometry, are the key for enabling high power output. The rotor contains an annular SmCo PM and a ferromagnetic FeCoV (Hiperco50) backiron, each 9.525 mm OD, 3.175 mm ID, and 500 µm thick. The SmCo PM and FeCoV backirons were, then, assembled and glued into a pre-formed PMMA cup, which was fit onto a 1.6 mm shaft (Figure 2(b)).For characterization, a high-speed spinning rotor test stand, incorporating an air-turbine driven spindle, was constructed. The stator was positioned under the rotor using an xyz-micropositioner, which permitted precise (± 5 µm) adjustment of the air gap. A three-phase step-up transformer (1:6 turn ratio) and Schottky diode bridge were used to rectify the output voltage for DC power generation across a load resistor. The power data for the 2-turn/pole machine shows a quadratic dependence on speed for a fixed load (Figure 2(c)) and typical power transfer dependence for varying loads (Figure 2(d)), with a maximum demonstrated power of 1.1 W (2.9 MW/m3 power density)."
High-speed Micro-scale Gas Bearings for Power MEMs,"The high-speed micro hydrostatic gas journal bearings used in the high-power density MIT micro-engines are of very low aspect ratio, with a bearing length-to-diameter ratio of less than 0.1, and are running at surface speeds of order 500 m/s. These ultra-short high-speed bearings exhibit whirl instability limits and dynamic behavior very different from conventional hydrostatic gas bearings. The design space for stable high-speed operation is confined to a narrow region and involves singular behavior [1]. The narrow design space together with the limits on achievable fabrication tolerance that can be achieved in the silicon chip manufacturing technology severely affects journal bearing operability and limits the maximum achievable speed of micro turbomachinery. The hydrostatic gas thrust bearings are located near the center of the rotor, and play a vital role in providing axial support for the rotor. The thrust bearing geometry is designed to provide the required axial and tilting stiffness, and ensures stable thrust bearing operation at high-speed [2].Our technical approach involves the combination of numerical simulations, experiment, and simple, first principles based on modeling of the gas journal and gas thrust bearing flow fields and the rotordynamics. A novel variation of the axial-flow hydrostatic micro-gas journal bearing concept is introduced that yields anisotropy in bearing stiffness [3]. By departing from axial symmetry and introducing biaxial symmetry in hydrostatic stiffness (Figure 1), the bearing's top speed is increased and fabrication tolerance requirements are substantially relieved, making more feasible extended stable high-speed bearing operation. An existing analytical hydrostatic gas journal bearing model [4] is extended and modified to guide the journal bearing design with stiffness anisotropy. In addition, a novel micro gas thrust bearing model is established. High-speed experimental spin tests were conducted in several micro-bearing test devices, and all 11 test devices were spun to high-speed, achieving an average rotor speed of 720,000 rpm. Figure 2 depicts a typical test run, and shows good agreement between the newly established bearing theory and the measurements."
Piezoelectric Micro Power Generator (PMPG): A MEMS-based Portable Power Device,"A thin-film lead zirconate titanate Pb(Zr,Ti)O3 (PZT), MEMS energy-harvesting device is developed to enable autonomous sensors for in-service integrity monitoring of large scale infrastructures. It is designed to resonate at specific frequencies from external vibrational energy sources, thereby creating electrical energy via the piezoelectric effect. The corresponding energy density of the 1st prototype is 0.74 mW-h/cm2, which compares favorably to lithium ion batteries. [1] Current efforts are focused on improving the harvest efficiency of the device. A geometric optimization of the cantilever design is made to suppress damping contributions from air and structural dissipation. Additionally, a serpentine cantilever has been designed to achieve a low resonant frequency structure. The dominant contributors to low Q factor at the MEMS scale are air damping and internal structure damping. For 2nd generation PMPG [3], we have optimized the cantilever shape to minimize the damping effect. Analytical modeling of PMPG predicts a 77% decrease of the damping coefficient of a new PMPG device.[4] This reduced damping coefficient enables 4.3 times larger resonance amplitude of the cantilever structure and 10.2 times larger maximum strain of the PZT layer. As a result, power density increases up to 1850% of the old PMPG device at the same footprint. We also designed a serpentine cantilever to achieve a low resonant frequency structure, as well as, a low damping effect, when it resonates. (Figure 2)PMPG has been integrated with a commercial wireless sensor, Telos, to simulate a self-powered RF temperature monitoring system. Such devices will play an important role in remote sensing network applications. Telos on average consumes 350µJ for 38 ms per measurement. Since PMPG offers limited power, a storage capacitor and a power management module are implemented to power the node at discrete time intervals."
MEMs Piezoelectric Ambient Vibration Energy Harvesting for Wireless sensors,"Recently, numerous investigations have focused on the development of distributed wireless sensor node networks. Power for such devices can be supplied through harvesting ambient environmental energy, available as: mechanical vibrations, fluid motion, radiation, or temperature gradients [1]. Envisioned applications include: building climate control and warehouse inventory control, identification and personalization (RFID tags), structural health monitoring (aerospace and automotive sectors), agricultural automation, and homeland security.Advances in “low-power” DSP’s (Digital Signal Processors) and trends in VLSI (Very Large Scale Integration) system design have reduced power requirements to 10’s-100’s of µW. These power levels are obtainable through piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures. Coupled electromechanical models have been developed to predict the electrical and mechanical performance obtainable from known low-level ambient vibration sources. These models have been validated by comparison to prior published results [2] and tests on a MEMS device. A non-optimized, uni-morph beam prototype (Figure 1) has been designed and modeled to produce 30 µW/cm3 [3]. A MEMS fabrication process for a prototype device is presented based on past work at MIT [4]. Dual optimal frequencies with equal peak powers and unequal voltages and currents are characteristic of the response of such coupled devices when operated at optimal load resistances (Figure 2).Future work will explore active sources, such as: aircraft skin for harvestable power, fabrication and testing of the uni-morph prototype beam, and optimization of device configurations for aerospace structural health monitoring applications. System integration and development, including modeling the power electronics, will be included."
Micro chemical Oxygen Iodine Lasers (MicrocOIL),"Conventional Chemical Oxygen Iodine Lasers (COIL) offer several important advantages for materials processing, including short wavelength (1.3 µm) and high power. However, COIL lasers typically employ large hardware and use reactants relatively inefficiently. This project is creating an alternative approach called microCOIL. In microCOIL, most conventional components are replaced by a set of silicon MEMS devices that offer smaller hardware and improved performance. A complete microCOIL system includes: microchemical reactors, microscale supersonic nozzles, and micropumps. System models incorporating all of these elements predict significant performance advantages in the microCOIL approach [1]. Initial work is focused on the design, microfabrication, and demonstration of a chip-scale Singlet Oxygen Generator (SOG): a microchemical reactor that generates singlet delta oxygen gas to power the laser. Given the extensive experience with microchemical reactors over the last decade [2-4], it is not surprising that a microSOG would offer a significant performance gain over large scale systems. The gain stems from basic physical scaling; surface to volume ratio increases as the size scale is reduced, which enables improved mixing and heat transfer. The SOG chip being demonstrated in this project employs an array of microstructured packed-bed reaction channels interspersed with microscale cooling channels for efficient heat removal. Figure 1 shows a schematic top view of the microSOG chip, including inlets and outlets for the reactant and product flows, and packed-bed reaction channels. Figure 2 shows a schematic diagram of stacked microSOG chips, micronozzles, and micropumps forming a complete microCOIL system."
Linear Array of Electrospray Micro Thrusters,"Electrospray thrusters are electrostatic accelerators of charged particles that use the electrohydrodynamic effect known as Taylor cone as propulsive effect [1]. These particles could be charged droplets, solvated ions, or a mix of the two. Since the new advances in electrospray technology that occurred in the late 1980s [2], the field of electrospray propulsion has experienced a renaissance, specifically aiming to provide efficient high-tunable precision low-thrust engines for micro-satellites and high accuracy astrophysics missions [3]. The MIT Space Propulsion Laboratory and the Microsystems Technology Laboratories are currently pursuing the development of a micro-fabricated electrospray emitter array for space propulsion. The project is developing in parallel two radically different concepts, a pressure-fed engine, and a surface tension-fed engine. This abstract reports the design, fabrication, and experimental characterization of a micro-fabricated, internally-fed linear array of electrospray emitters (Figure 1). This work demonstrates the feasibility of high clustering of electrospray emitters. The linear array is composed of 1 plenum, 12 manifolds, and 240 emitters. The emitters are sharpened to reduce the startup voltage. The electrodes are micro-fabricated with conductive paths made of tungsten and electrical insulation provided by vacuum gaps 350 µm wide and 10 µm thick PECVD silicon oxide. The electrodes are hand-assembled to the engine using a novel technique that relies on clusters of micro-fabricated springs [4]. This assembly scheme allows us to have two independent process flows for the electrodes and the engine hydraulics. The emitter-to-emitter separation is 130 µm, and the hydraulic diameter is 12 µm. The length of each channel is 15 mm. The engine uses highly doped formamide as propellant, with electrical conductivity in the 0.3 – 3.0 S/m range. The electrospray array operates in the single Taylor cone droplet emission regime, and it requires about 2000 V to become activated. The engine implements the concept of hydraulic and electrodynamic flow rate matching to achieve electrical control. Current versus flowrate characteristics of the engine are in agreement with a well-established reduced order model (Figure 2). Experimental data, demonstrating the low divergence of electrospray emitter arrays operated in the single Taylor cone, is in qualitative agreement with a reduced order mode that assumes the absence of a thermalized tail in the plume."
Planar Array of Electrospray Micro Thrusters,"Electrospray thrusters are electrostatic accelerators of charged particles using the electrohydrodynamic effect known as Taylor cone to generate thrust [1]. These particles could be charged droplets, solvated ions, or a mix of the two. Since the new advances in electrospray technology that occurred in the late 1980s [2], the field of electrospray propulsion has experienced a renaissance, specifically aiming to provide efficient high-tunable precision low-thrust engines for micro-satellites and high accuracy astrophysics missions [3]. The MIT’s Space Propulsion Laboratory and the Microsystems Technology Laboratories are currently pursuing the development of a micro-fabricated electrospray emitter array for space propulsion applications. The project is developing, in parallel, two radically different concepts, a pressure-fed engine and a surface tension-fed engine. This abstract reports the design, fabrication, and experimental characterization of a hybrid macro-fabricated/micro-fabricated, externally fed planar array of micro-fabricated electrospray emitters with macro-fabricated electrodes (Figure 1). An externally-fed engine has a number of advantages compared to the other implementations reported in the literature. For example, the engine lacks a static pressure difference between the plenum and the emitters; therefore, there cannot be propellant emission unless it is electrically activated. In this sense, the planar array is less vulnerable to unplanned propellant emission compared to pressure fed schemes. Additionally, clogging is not an issue in this engine because the propellant is not doped, and the flow channels are open. The planar array uses the ionic liquid EMI-BF4 as a propellant. The ionic liquid EMI-BF4 has a very low vapor pressure, making it suitable to be used in an open architecture engine. The array is composed of a set of spikes, i.e., emitters, coming out from a propellant pool. There are two configurations for the emitters: fully sharpened slender emitters, i.e., pencils, and truncated pyramidal emitters, i.e., volcanoes. The arrays have between 4 and 1024 emitters in an active area of 0.64 cm2. The surface of the engine (tank and emitters) is covered with “black silicon” that acts as wicking material. The hydraulic system has been experimentally characterized, including: start-up tests (Figure 2), wettability tests, current-per-emitter versus voltage characteristics, imprints of the exit stream on a collector, and a thrust test in agreement with the current-per-emitter versus voltage characteristics and the time-of-flight measurements that we have independently obtained at the Space Propulsion Laboratory. Preliminary results demonstrating the feasibility of obtaining substantially larger emission currents at the same extraction voltage by controlling the temperature have also been obtained. The emission from the array seems to be described by a Schottky emission mechanism."
Numerical Techniques for Integral Equations,"Finding computationally efficient numerical techniques for simulation of three-dimensional structures has been an important research topic in almost every engineering domain. Surprisingly, the most numerically intractable problem across these various disciplines can be reduced to the problem of solving a three-dimensional potential problem with a problem-specific Greens function. Application examples include: electrostatic analysis of sensors and actuators, electromagnetic analyses of integrated circuit interconnect and packaging, detailed analysis of frequency response and loss in photonic devices, drag force analysis of micromachined structures, and potential flow based aircraft analysis. Over the last fifteen years, we have been developing fast methods for solving these problems, and have developed widely used programs such as FastCap (capacitance), FastHenry (magnetoquasistatics), FastLap (general potential problems), FastImp (full wave impedence extraction),and FastStokes (fast fluid analysis). Our most recent work is in developing higher order methods[1], methods that efficiently discretize curved geometries[2], methods that are more efficient for substrate problems [3], and methods for analyzing rough surfaces [4]."
Characterization and Modeling of Nonuniformities in DRIE,"We contribute a quantitative and systematic model to capture etch nonuniformity in the deep reactive ion etching (DRIE) of microelectromechanical systems (MEMS) devices [1]. DRIE is commonly used in MEMS fabrication where high-aspect ratio features are to be produced in silicon. It is typical for many devices, of diameters on the order of 10 mm, to be etched simultaneously into a silicon wafer of diameter 150 mm. Devices containing a range of feature diameters exhibit aspect ratio-dependent etching rates, a phenomenon that is well understood [3]. In addition, equivalent features within supposedly identical devices are observed to etch at varying rates. These spatial variations have been explained in terms of uneven distributions of SxFy ions and fluorine neutrals at the wafer scale, and of competition for those species at the device, or die, level. An ion–neutral synergism model [7] is constructed from data obtained by etching several layouts of differing pattern opening densities (Figure 2). Such a model is used to predict wafer-level variation with an r.m.s. error below 3% (Figure 1). This model is combined with a die-level model, which we have reported previously [2,8], on a MEMS layout. The two-level model is shown to enable prediction of both within-die and wafer-scale etch rate variation for arbitrary wafer loadings."
Measuring the Mechanical Properties of Thin Films Using MEMS structures,"Simple micromechanical devices are being developed to measure the mechanical properties of thin films in localized areas after processing. The simplest devices to fabricate are cantilevers overhanging a pit formed using an anisotropic etch. Cantilevers formed from a material of interest can be used to measure the through-thickness stress-gradient and the elastic modulus of that material. Measuring the elastic modulus requires applying a known force to the tip of the cantilever and measuring the subsequent deflection or curvature. We have developed a technique for high accuracy modulus measurement by application of a force with a beam having known properties, with deflection measurements made in an optical profilometer.Membrane devices, as shown in Figure 1, can be used to measure the stress in a thin film without further processing. The membranes are fabricated using an SOI wafer as the starting material. An anisotropic etch from the backside is used to form the membrane, which consists of two layers: buried silicon dioxide under the device single crystal silicon. The membrane buckles because the buried silicon dioxide is under compressive stress relative to the silicon. The amount of buckling is determined by the mechanical properties and the geometry of the membrane, and is measured using optical profilometry. Depositing a film on either side of the membrane changes the buckling, and therefore, the stress of the new material can be determined. Films deposited on both sides of the membrane contribute to the change in deflection; consequently, the stress in CVD films can be measured. Buckling of doubly-supported beams can be used to charac-terize compressive stresses. To characterize tensile stresses, we have recently developed a new type of device, a V-shaped beam, as shown in Figure 2(a). The V-beam is made from a material of interest. A tensile stress causes out-of-plane bending that can be measured using an optical profilometer. The measured deflections are then compared to finite element analyses. Two modes of bending have been seen in V-beams produced from silicon nitride thin films. Finite element models of the 2 modes showing vertical deflection contours can be seen in Figures 2(b) and 2(c). Mode 1 bending is symmetric and produces very large deflections that are often too large to measure in an optical profilometer. Most beams tend to bend into Mode 2, which is asymmetric, but easily measured us-ing an optical profilometer. Mode 2 deflections also have the advantage that the through-thickness stress gradient does not change the deflection. Because all the devices described above are small, they can be placed in many locations on the wafer."
Scanning Probe Microscopy with Inherent Disturbance Suppression Using Micromechanical Devices,"Scanning probe microscopes are notoriously susceptible to disturbances, or mechanical noise, from the surrounding environment that couple to the probe–sample interaction. These disturbances include vibrations of mechanical components, piezo drift, and thermal expansion. Disturbance effects can be substantially reduced by designing a rigid microscope, incorporating effective vibration isolation, and selecting an appropriate measurement bandwidth and image filter. However, it is not always possible to satisfy these requirements sufficiently, and as a result, critical features in an image can be obscured. The cause of this problem is that the actuator (control) signal is used both to readout topography and correct for disturbances. We have introduced a general approach for inherently suppressing out-of-plane disturbances in scanning probe microscopy [1]. In this approach, two distinct, coherent sensors simultaneously measure the probe-sample separation. One sensor measures a spatial average distributed over a large sample area, while the other responds locally to topography underneath the nanometer-scale probe. When the localized sensor is used to control the probe-sample separation in feedback, the distributed sensor signal reveals only topography. This configuration suppresses disturbances normal to the sample. We have applied this approach to scanning tunneling microscopy (STM) with a microcantilever that integrates a tunneling tip and an interferometer (Figure 1) and have shown that it enables Angstrom resolution imaging of nanometer-sized gold grains in a noisy environment (Figure 2). For disturbances applied normal to the sample, we measured disturbance suppression of -50 dB at 1 Hz, compared to 0 dB with conventional imaging."
In-Plane AFM Probe with Tunable stiffness,"We developed an in-plane Atomic Force Microscope (AFM) probe that is specifically tailored to the needs of biological applications. It features a variable stiffness, which makes the stiffness of the probe adjustable to the surface hardness of the sample [1]. The inherent capability of the in-plane AFM probe for building a massively parallel array is also an important feature that greatly affects the speed of the AFM scanning process. Concept and FunctionalityThe switchable stiffness probe allows the scanning of biological samples with varying surface hardness without changing probes during scanning and therefore, prevents a loss of positional information, as is unavoidable with conventional devices. For the integration of the components into a MEMS device, the conventional cantilever-type design of AFM probes has been abandoned in favor of an in-plane design. The new design has an advantage in that it facilitates a high-density array of AFM probes and allows for easy surface micromachining of the integrated device. It also enables the integration of micro-fluidic channels for reagent delivery and nanopipetting. For scanning nano-scale trenches and grooves, a multi-walled carbon nanotube, embedded in a nanopellet [2], is mechanically assembled to the AFM probe as a high-aspect-ratio tip. Design and FabricationThe variable stiffness is accomplished in a mechanical way by engaging or disengaging auxiliary beams to the compliant beam structure by the means of electrostatically actuated clutches (Figure 1). Figure 2 shows the integrated AFM probe system. For actuation, an electrostatic combdrive is considered to move the probe tip up and down. The vertical displacement of the tip can be measured by a capacitive sensor, which can easily be integrated into the system."
Direct Patterning of Organic Materials and Metals Using a Micromachined Printhead,"Organic optoelectronic devices are promising for many commercial applications, if methods for fabricating them on large area low-cost substrates become available. Our project investigates the use of MEMS in the direct patterning of materials needed for such devices.In our first demonstration, we used an electrostatically actuated micromachined shutter integrated with an x-y-z manipulator to modulate the flux of evaporated organic semiconductors and metals and to generate patterns of the deposited materials. The micromachined printhead consists of a free-standing silicon microshutter actuated over a 25 micron square aperture by a comb-drive actuator. Figure 1 shows the microshutter and aperture. The device is fabricated, starting with a SOI (silicon on insulator) wafer, and using deep reactive ion etching to pattern both the through-wafer aperture and the free-standing structure and actuation mechanism. An operating voltage of 30 V is needed to obstruct the aperture with the microshutter. The simulated first mechanical resonant frequency of the device is 6 kHz.We tested the printing method in a vacuum chamber by depositing an organic semiconductor, Alq3 (tris (8-hydroxyqunolinato) aluminum), and silver on glass substrates. We also printed arrays of organic light emitting devices (OLED). Figure 2 shows patterns obtained using this method: photoluminescence image of 40 micron pixels of Alq3, optical microscope image of 30 microns wide line patterns of silver, and electroluminescence of 30 micron pixels arrays of TPD:10%DCM/Alq3/TAZ at 20 V (with blue filter), and of TPD/Alq3/TAZ at 10V (no filter). The results show that this printing technique is capable of patterning small molecule organic light emitting devices at high resolution (800 dpi in our case).The next stage of this project will involve investigating the use of a microporous layer with integrated heaters for local evaporation of the materials."
Nanometer-Level Positioning in MEMs without Feedback control,"Traditional macro-scale nanopositioners rely on sensors and feedback control to achieve nanometer-level accuracy and repeatability. The need for low-cost, high-speed precision positioning devices has led to a trend in miniaturization of these machines. Miniaturization of precision positioning devices is problematic as precision positioners require feedback control, and feedback control is not readily adapted to small-scale machines. The difficulty in adaptation is due mainly to the challenges encountered during the integration of small-scale sensors, mechanisms, and actuators. In this work, we are designing multi-axis MEMS that are capable of nanometer-level positioning without sensing/feedback control. The approach has grown from binary actuation technologies used in macro-scale robotics [1,2].In our approach, Digital Nanoactuation Technology (DNAT), a positioner is equipped with actuator-flexure building blocks. The blocks consist of a pair of binary actuators that work together to generate discrete, repeatable positions. The actuators are attached to a positioning stage via flexures such that the actuator-flexure sets are diametrically opposed. An actuator set is shown on the left side of Figure 1. The opposed flexures differ in stiffness, one compliant, KC, and one stiff, KS. When both actuators are activated (four possible on-off combinations), four repeatable positions may be obtained. DNAT building blocks may be superimposed to provide many position states. For example, the 64 states shown on the right side of Figure 1 are obtained by superimposing the output of three blocks. The number of states scales with the number of actuator pairs, N, as 22N. A positioner with N = 6 is capable of over 4000 discrete positions. If these points are encompassed within a space of a few microns, simple on and off actuator commands may be used to obtain nanometer-level repeatability without sensing/feedback. A macro-scale analogy of a small-scale device has been constructed and tested [3] to demonstrate that nanometer-level positioning is possible. The small-scale prototype shown in Figure 2 is being tested to characterize a 64 state prototype before we progress to a smaller, 4000 state device."
"An Electrostatic, circular Zipping Actuator for the Application of a Tunable capacitor","A tunable capacitor is devised using a circular zipping actuator, based on its ability to potentially control a gap between two large surfaces with nanometer resolution [1]. The device consists of three wafers; a SOI (Silicon-On-Insulator) wafer sandwiched by two Pyrex glass wafers that are anodically bonded together, as shown in Figure 1. In the center of the device is a circular membrane that is supported by tethers that are connected to the outer walls. A cylindrical fulcrum, fabricated by the deep reactive ion etching technique, acts as the pivot for the membrane and divides the membrane into the outer actuator region and the center capacitor region. The top of the fulcrum is bonded to the top glass wafer for structural rigidity. The SOI layer is used as the membrane-actuator because of its uniform thickness and the low stress of single-crystal silicon. Thermally grown silicon dioxide is used as dielectric insulation. The bottom wafer contains the bottom electrodes for the actuator and the capacitor. The actuator electrode is etched into the glass to form the gap of the actuator. Gold is deposited on top of the glass wafer as both actuator and capacitor electrodes. Voltage is applied between the top and the bottom actuator electrodes. At a certain threshold, the outer membrane snaps down. With increasing actuation voltages, the membrane zips along the radial direction, as shown in Figure 2, and results in the separation of the two capacitor surfaces. Because of the poor adhesion of gold to oxide, the membrane will not be bonded to the gold surface, although the two are in close contact during operation. Thus, the design makes it possible to have two initially closed-contacted surfaces that can be pried apart. By changing the gap between the two plates of the capacitor, the capacitance can be tuned.The device is modeled using both numerical methods with Matlab and FEM with ANSYS. Tests are done using a laser interferometer to measure the center displacement and a network analyzer to measure the capacitance change."
A Low contact Resistance MEMs Relay,"An electrostaticaly driven, bulk micromachined, low contact resistance MEMS cross bar relay has been designed, and is currently under fabrication. This relay will be used to study and optimize the behavior of micro-scale contacts for power applications.Many MEMS relays have been reported in the literature [1,2,3]; most, however, are not suited for practical power applications due to their high contact resistance. A contact resistance of 50 mΩ [4] has been achieved by our group using a bulk micromachined, externally actuated structure as a proof of concept for this design [4].The electrostatic “zipper” actuators [4,5] are designed for low pull-in voltage (~100 V) and large contact travel (~40 µm) to prevent arcing as the load circuit (up to 600V) is switched on and off. Figure 1 shows the MEMS relay. Figure 2 shows a detailed view of the actuator. The two arms of the parallelogram flexure are used as the traveling electrodes of the electrostatic actuators. Each traveling electrode, or arm of the parallelogram flexure, is adjacent to a pair of stationary electrodes: an engaging and a disengaging stationary electrode. The relay is engaged by electrostatic attraction between the traveling electrodes and the engaging stationary electrodes. Similarly, the MEMS relay is disengaged through electrostatic attraction between the traveling electrodes and the disengaging stationary electrodes. Each stationary electrode is comprised of a stiff component and a compliant, cantilevered component. The cantilevered component reduces the pull-in voltage by reducing the distance between the electrodes. As the actuator is energized, the compliant end of the stationary electrode, having the lower stiffness, is attracted by and deflected toward the moving electrode, making initial contact at the loose end of the cantilever. As the actuation voltage is increased, the contact point between the electrodes is displaced along the stationary electrode over the stiff component of the electrode in a “zipping” motion. Our group continues to develop these MEMS relays for power applications."
A Variable capacitor Made from single crystal silicon Fracture surfaces,"A process for the fracture fabrication of single crystal silicon surface pairs with nanoscale roughness has been developed, and a prototype variable capacitor, featuring fracture surfaces as the moveable parallel plates, has been fabricated. The surfaces are fabricated by notching a portion of a compliant structure with either potassium hydroxide (KOH) or Focused Ion Beam (FIB) milling to produce a stress concentration. The device is fractured by pulling on the compliant structure with a probe. Post-fracture, the compliant structure acts as a bearing so the two surfaces can be brought back into intimate contact without misalignment. Proper alignment ensures that nanometer scale gaps can be maintained with surfaces that are perfectly smooth or complementary. Complementary surfaces have been closed to gaps less than 20 nm. For a successful fracture, the notch must be very sharp and properly aligned to the crystal structure, and the compliant structure (typically etched into the device layer of a Silicon On Insulator (SOI) wafer) must attenuate stray forces and moments and withstand the trauma of fracture. Experiments with different specimens have shown 10 µm to be the optimal thickness (Figure 1).An updated version of the device used for the surface fabrication experiments has been fabricated, assembled, and sealed (Figure 2). This device includes an integrated zipper actuator [1] for controlling the separation of the surfaces, as well as, provision for wirebonding the device into its hermetically sealed package. Testing has confirmed that the actuator functions properly and that the specimens survived the fabrication process. The device also validated the electrical model used to design the capacitance measurement circuitry. Unfortunately, fracturing of these new devices has been problematic: growing the actuator’s thermal oxide has likely blunted the notches. The fabrication process has been debugged, and a new round of fabrication (with an improved design) is nearing fruition."
A High-Q Widely Tunable Gigahertz Electromagnetic cavity Resonator,"RF systems need high-frequency widely tunable high-Q bandpass filters for channel selection filters and local oscillators. Our work describes the design, fabrication, and testing of an electromagnetic cavity resonator designed for such applications. Alternative technologies provide wide tuning or high Q, but not both, and are generally not tunable. This resonator is distinguished by its simultaneous high Q near 200 and its wide high-frequency tuning range of 2.5 GHz to 4.0 GHz, which have been experimentally demonstrated. The resonator is fabricated using standard MEMS technologies and consists of a gold-lined capacitor and toroidal inductor cavity formed by etching silicon in potassium hydroxide (Figure 1). Frequency tuning is performed by compressing the cavity to close the capacitor gap. Testing was done with a piezoelectric actuator for this task. The match between the modeled and measured impedance is extremely good up to and beyond 5 GHz, with less than a 1% error in magnitude and phase."
"Lateral, Direct contact RF MEMs switch with PZT Actuation","A novel direct contact MEMS switch is developed with compliant lateral metal contacts to address the need for low contact resistance and long life cycles. The device is unique in its self-alignment of the contact surfaces, self-cleaning of particles generated at each contact cycle, and mechanical anchoring method of the contact metal to the side of the Su-8 beam structures. The fabricated device maintains less than 0.1Ω contact resistance for up to 10 billions of cycles of contact. A fabricated device is shown in Figure 1 (a). Each switching member consists of two parallel beams with angled contact surfaces. One side of the contacting surfaces is undulated with micro grooves, as shown in Figure 1 (b). When the movable member is actuated to meet the fixed one, the gold on each side of the contact creates a short circuit. When the movable member is on the other side, enough gap is maintained to open the circuit with high isolation. The angled contact orientation makes the undulated surface slide over the static surface, which pushes entrapped particles or generated micro-weldments into the micro-grooves. By cleaning the surface at every cycle of switching, the micro-undulated surface ensures a low contact resistance over long cycles of switching operation. The grooved contact surfaces show successfully that the self-cleaning concept works and that a low contact resistance below 0.1Ω has been maintained over 10 billion cycles. (Figure 2) Applications of the self-cleaning MEMS switch, such as tunable antennas, are being investigated to assess the commercial potential of our switch."
Design and Fabrication of Nano-Tweezers,"Since the invention of atomic force microscopes (AFM) that provided researchers with a convenient tool to observe objects at nanoscale, manipulation tools at nanoscale have been in high demand. There have been several attempts to create nanomanipulation devices, such as nano-tweezers, to address this challenge. Most such attempts have amounted to single proofs of concepts rather than a practical, readily producible manipulation tool. The goal of this project was to further the current state of nanomanipulators, by producing nano-tweezers that are consistently producible, using batch microfabrication processes. In addition, given the regularity and practicality of the AFM as a nano-scale research tool, the nano-tweezers were intended to also serve as a scanning probe for the AFM. This way, the same tool can to be used to both image and manipulate samples, and the utility of the devices is increased.A two-fold approach was used to tackle the problem. First, using complete batch fabrication methods, a process was created to generate nano-scale tweezer tips separated by a nano-scale gap. This process uses standard micron scale batch lithography to define pyramidal walls in silicon. It then produces an extremely thin cut that self-aligns to the apex of the pyramid. Thus far, tip separations of 358nm and tip widths of 50nm have been repeatably produced. The alignment of the process is within 35nm and is much smaller than that of the lithography tool. The second phase was to create free standing, protruding structures that can serve as the tweezing arms and move with nano-scale resolution. Cantilevered flexural members, coupled with electro-static actuation, were successfully fabricated. These slender cantilevered flexural components measure only 1-2 um in width. A novel process was developed that overcomes problems due to surface tension, and protects the released devices all the way through die separation.The devices have shown actuation behavior that is consistent with theory and design intent. Resolution of motion of 40nm has been verified using SEM through the entire working range of the device. Resolution of less than 10nm is expected based on data but has not been verified due to the limits of this SEM."
Induced-charge Electro-Osmotic Pumps and Mixers for Portable or Implantable Microfluidics,"Microfluidic technology offers great promise in diverse fields such as bioinformatics, drug delivery, and analytical chemistry. In spite of involving microchannels, however, current lab-on-chip technologies are mostly limited to bench-top analysis due to various bulky external elements. For example, peristaltic pumping in soft-polymer channels requires complicated tubing and flow meters, and capillary electro-osmosis requires a high-voltage power supply. Miniaturizing and integrating the power source is a crucial next step toward portable or implantable devices for medical diagnostics, localized drug delivery, artificial organs, or pressure control to treat diseases such as glaucoma.We are developing new kinds of pumps and mixers exploiting “induced-charge electro-osmosis” (ICEO) [1], as a potential platform for portable microfluidics. ICEO refers to the slip of a liquid electrolyte at a polarizable (metal or dielectric) solid surface, driven by an electric field acting on its own induced surface (double-layer) charge. Unlike classical (fixed-charge) electro-osmosis, which requires large DC voltages (>100V) applied down a channel, ICEO can be driven locally by small AC voltages (<10V). It is sensitive to the geometry, ionic strength, and driving frequency and scales with the square of the applied voltage. The effect generalizes “AC electro-osmosis” at planar electrode arrays [2] and offers some more flexibility. We originally demonstrated ICEO flow in dilute KCl around a platinum wire by comparing flow profiles from micro-particle-image velocimetry (µPIV) to our theory [3]. We have also fabricated many devices involving electroplated gold structures on glass in PDMS microchannels, which exhibit mm/sec flow rates in 100 V/cm fields at kHz AC, and further optimization is underway. As a first application, we are developing a portable ICEO-powered biochip to detect blood exposure to toxic warfare agents by lysing cells and amplifying and detecting target genes."
Resonant Body Transistor with MIT Virtual Source (RBT-MVS) Compact Model,"High-Q mechanical resonators are crucial components for filters and oscillators that are essential for RF and ana-log circuits. It is highly desirable for resonators to scale to GHz-frequencies and beyond to meet today’s challenging requirements in terms of speed and data rates. Further-more, aggressive scaling requirements call for monolith-ic integration with CMOS circuits to allow for a smaller footprint and reduced parasitics and power consumption. Micro-electromechanical (MEM) resonators represent a potential solution for frequency and footprint scaling, along with monolithic integration in CMOS.A resonant body transistor (RBT) is a MEM resonator with a field-effect transistor (FET) incorporated into the resonator structure. The FET is intended for active sensing of the mechanical vibrations through piezoresistive modulation of the channel mobility. RBTs also rely on electrostatic internal dielectric transduction for actuation, by means of MOS capacitors (MOSCAPs). Such sensing and actuation enable these devices to easily scale to multi-GHz frequencies, while being compatible with CMOS manufacturing technologies.Compact modeling for these devices is essential to gain a deeper insight into the tightly coupled physics of the RBT while emphasizing the effect of the different parameters on the device performance. It also grants circuit designers and system architects the ability to quickly assess the performance of prospective RBTs, while minimizing the need for computationally intensive coupled-multi-physics finite element method (FEM) simulations.The RBT compact model is developed as a set of modules, each representing a physical phenomenon. Mechanical resonance, FET sensing, MOSCAP driving, and thermal modules are the most notable. The modules are interconnected through a set of nodes (namely, mechanical nodes and a thermal node) to represent the coupling between the different physics. This modular approach enables the seamless expansion of the RBT model either by incorporating new physics, adding driving or thermal sources, or mechanically coupling multiple RBTs together. A modified version of the MIT Virtual Source (MVS) model is used to implement both the electrostatic driving (as a MOSCAP) as well as the piezoresistive active FET sensing. The full model is developed in Verilog-A and available on nanohub.org."
Piezoelectric Micro-Machined Ultrasonic Transducer Array for Medical Imaging,"Diagnostic medical ultrasound imaging is becoming in-creasingly widespread because it is relatively inexpen-sive, portable, compact, and non-invasive compared to other diagnostic scanning techniques. However, com-mercial realization of advanced imaging trends will re-quire cost-effective, large-scale arrays of miniaturized elements, which are expensive to fabricate with the current bulk piezoelectric transducers. At high volume, micro-fabricated transducers based on micro-electro-mechanical (MEMS) technology are an array-compati-ble and low-cost option. The piezoelectric micro-machined ultrasonic transducer (pMUT) is a promising alternative to previously proposed capacitive MUT devices since it does not suffer from electrostatic transduction limitations, including potentially unsafe high bias voltage, and non-linearity. With more effective transformation via the piezoelectric effect, pMUTs have already demonstrated viability for deep penetration imaging via high acoustic pressure output. However, insufficient modeling has produced pMUT devices that often fall short of predictions, resulting in low electromechanical coupling and reduced bandwidth. With an improved modelling framework and optimization, pMUT based arrays have the potential for efficient, low-power, and high-pressure operation necessary for wearable applications.Based on a high force-output figure of merit, a 31-mode, lead zirconate titanate (PZT)-based pMUT plate cell design is selected. Our previous work developed and validated an analytical, electro-acoustic model of the single cell through experiment and finite element simulation. By leveraging and building on the validated single-cell model, we further optimized parallelized multi-cell elements to achieve high acoustic power and power efficiency. These elements are incorporated into 1D arrays (Figure 1) to demonstrate basic beamforming and image collection capabilities of a pMUT-based ultrasound system.Current work focuses on fabrication of the pMUT arrays (Figure 2) using common micro-fabrication techniques including a PZT sol-gel deposition process. Beyond fabrication, the project aims to generate proof-of-concept images to demonstrate the commercial viability of pMUT-based array systems."
Development of a Tabletop Deep Reactive-Ion Etching System for MEMS Development and Production,"A general rule of thumb for new semiconductor fabrica-tion facilities (fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for research and development and groups seeking to commercialize new semiconductor devices aimed at smaller market segments that require a dedicated pro-cess. To eliminate this cost barrier, we are working to create a suite of tools that will process small (~1”) sub-strates and cost less than $1 million. This suite of tools, known colloquially as the 1” Fab, offers many advan-tages over traditional fabs. By shrinking the size of the substrate, we trade off high throughputs for significant capital cost savings while incurring substantial savings in material usage and energy consumption. This sub-stantial reduction in the capital cost will drastically increase the availability of semiconductor fabrication technology and enable experimentation, prototyping, and small-scale production to occur locally and econom-ically. To implement this suite of 1” Fab tools, our cur-rent research has been focused primarily on developing a deep reactive-ion etching (DRIE) system. DRIE tools are used to create highly anisotropic, high aspect-ratio trenches in silicon—a crucial element in many MEMS processes that will benefit from a 1” Fab platform. A la-beled image of the 1” Fab DRIE system is shown in Fig-ure 1. The load lock and wafer lift assembly allow up to 2” wafers and pieces to be easily loaded and processed, and the modularized design of the processing chamber means that the (currently DRIE) system can be easily adapted to produce other plasma-based etching and deposition tools (such as PECVD and RIE). Using the switched-mode Bosch Process, the 1” Fab DRIE system currently can achieve silicon etch rates up to 6 µm/min with vertical sidewall profiles, an estimated photoresist selectivity greater than 75:1, and etch depth non-unifor-mity to less than 2% across the substrate. Several exam-ples of anisotropic etches performed with our system are included in Figure 2. Presently, we are working to refine the thermal design of the system and optimize recipes for high-aspect ratio etching."
MEMS Energy Harvesting from Low-Frequency and Low-Amplitude Vibrations,"Vibration energy harvesting at the microelectrome-chanical system (MEMS) scale will promisingly advance exciting applications such as wireless sensor networks and the Internet of Things by eliminating troublesome battery-changing or power wiring. On-site energy gen-eration could be an ideal solution to powering a large number of distributed devices usually employed in these systems. To enable the envisioned battery-less systems, a fully assembled energy harvester at the size of a quar-ter-dollar coin should generate robustly 101~102 µW of continuous power from ambient vibrations (mostly less than 100 Hz and 0.5 g acceleration) with wide bandwidth. We are inching close to this goal in terms of power densi-ty and bandwidth, but not in terms of low-frequency and low-amplitude operations.Most reported vibration energy harvesters use a linear cantilever resonator to amplify the energy absorption from weak ambient vibrations. While such structures are easy to model, design, and build, they typically have unusably narrow bandwidths. In contrast, nonlinear resonators have a different dynamic response and greatly increase the bandwidth by hardening or softening the resonance characteristic. Our previous research with nonlinear resonating bridge-structure-based energy harvesters achieved 2.0 mW/mm3 power density with >20% power bandwidth. However, they were operated with input vibrations of >1 kHz at 4 g, which practically limits the use of this technology for harvesting energy from real environmentally available vibrations. Many believed this is an inherent limitation imposed on the MEMS-scale structures.We approached this problem with a buckled-beam-based bi-stable nonlinear oscillator. Compared to mono-stable nonlinear oscillations, we found bi-stable oscillations could bring more dynamics phenomena to help reduce the operating frequency. An electromechanical lumped model has been built to simulate the dynamics of the buckled clamped-clamped beam-based piezoelectric energy harvesters. The two oscillation modes, intra-well and inter-well with respect to the double-well energy potential of the bi-stable system, have been predicted. The characteristic spring softening and spring stiffening responses corresponding to the simulations were observed by testing a meso-scale prototype. The testing results also verify the theoretical prediction on low-frequency operation, showing a shifted response of bi-stable configuration, which generates more power than the mono-stable configuration at lower frequencies. A MEMS mechanical bi-stable oscillator has also been fabricated to verify the operating frequency and amplitude of the new design (Figure 1). The multi-layer bridge structure has employed compressive residual stress in the micro-fabricated thin films to induce buckling and lower the operation frequencies. The dynamic responses were measured by a laser Doppler vibrometer (Figure 2). The wide-band nonlinear response shows a one-order-of-magnitude lower frequency range at low g’s. The fully functional piezoelectric devices are under fabrication."
Close-Packed Silicon Microelectrodes for Scalable Spatially Oversampled Neural Recording,"The extracellular recording of brain activity in the mammalian brain provides an important tool for un-derstanding neural codes and brain dynamics. Extra-cellular electrodes with recording sites that are closely packed can enable spatial oversampling of neural activ-ity, which facilitates data analysis; such oversampling becomes important when we aim to scale up the num-ber of neurons used for recording.We designed and implemented close-packed silicon microelectrodes (Figure 1) to enable the spatially oversampled recording of neural activity (Figure 2) in a scalable fashion, using a tight continuum of recording sites along the length of the recording shank, rather than discrete arrangements of tetrode-style pads or widely spaced sites. This arrangement, thus, enables spatial oversampling continuously running down the shank, so that sorting of spikes recorded by the densely packed electrodes can be facilitated for all the sites of the probe simultaneously.We use MEMS microfabrication techniques to create thin recording shanks, and a hybrid lithography process that allows a dense array of recording sites, which we connect with submicron dimension wiring. We have performed neural recordings with our probes in the live mammalian brain. Figure 2 illustrates the spatial oversampling potential of close packed electrode sites."
Wearable Energy Harvesters Based on Aligned Mats of Electrospun Piezoelectric Nanofibers,"Battery recharging and replacement are still chal-lenging after several decades of developing energy sources for portable and wireless devices. For this reason, new power sources have become essential for current and future stand-alone devices. Energy harvesters are an attractive alternative for supplying power in these systems. We are developing wearable energy harvesters based on electrospun piezoelectric nanofibers as transducing elements. The proposed harvesting device consists of a set of flexible interdigitated electrodes on a flexible substrate; the electrodes are coated with aligned piezoelectric nanofibers. Each time the substrate is stretched or bent, the piezoelectric nanofibers produce voltage and charge that can be used to feed low-power devices. Our energy harvesters could be integrated into garments, allowing people to carry less weight and volume in batteries, which is particularly advantageous on long journeys and when located far from the electrical grid. The piezoelectric nanofibers of our energy harvester are made of poly(vinylidene difluoride), i.e., PVDF, using the electrospinning technique. In electrospinning, a solution rich in long-chain polymers that is subject to a high electrostatic field ejects a jet that is thinned to a submicron diameter due to the interaction of the electric field and surface tension effects on the fiber (Figure 1). Highly aligned fiber deposition on the interdigitated electrodes of the energy harvester is necessary to achieve high efficiency. With this goal in mind, we developed a custom rotating collector system that allows control of the alignment and diameter of the deposited nanofibers. The collected fibers tend to be more aligned and exhibit smaller fiber diameters when the collector drum rotates at thousands of revolutions per minute (Figure 2). Current work focuses on controlling the morphology of the PVDF fibers and nanofiber mats, as well as on testing nanofiber harvester prototypes using a custom apparatus and benchtop electronics."
Prediction and Characterization of Dry-Out Heat Flux in Micropillar Wick Structures for Thermal Management Applications,"Thin-film evaporation in wick structures for cooling high-performance electronic devices is attractive be-cause it harnesses the latent heat of vaporization and does not require external pumping. However, optimiz-ing the wick structures to increase the dry-out heat flux is challenging due to the complexities in modeling the liquid–vapor interface and the flow through the wick structures. In this work, we developed a model for thin-film evaporation from micropillar array wick structures (Figure 1) and validated the model with ex-periments. The model numerically simulates liquid velocity, pressure, and meniscus curvature along the wicking direction by conservation of mass, momen-tum, and energy based on a finite volume approach. Specifically, the three-dimensional meniscus shape, which varies along the wicking direction with the lo-cal liquid pressure, is accurately captured by a force balance using the Young–Laplace equation. The dry-out condition is determined when the minimum con-tact angle on the pillar surface reaches the receding contact angle as the applied heat flux increases. With this model, we predict the dry-out heat flux on various micropillar structure geometries (diameter, pitch, and height) in the length scale range of 1–100 μm and discuss the optimal geometries to maximize the dry-out heat flux (seen in Figure 2). We also performed detailed experiments to validate the model predictions, which all show good agreement. This work provides many insights into the role of surface structures in thin-film evaporation and also offers important design guidelines for enhanced thermal management of high-performance electronic devices."
Fabrication of Core-Shell Microparticles Using 3-D Printed Microfluidics,"Coaxial electrospraying is an electrohydrodynamic process that creates core-shell microparticles by at-omization of a coaxial electrified jet composed of two immiscible liquids. Coaxial electrospraying has several advantages over other microencapsulation technol-ogies including higher encapsulation efficiency and more uniform size distribution. Coaxial electrosprayed compound microparticles can be used in exciting ap-plications such as feedstock microencapsulation, con-trolled drug release, and self-healing composites.Unlike traditional, i.e., uniaxial, electrospraying that has been investigated for over 100 years and of which many MEMS implementations exist, coaxial electrospraying was first described in 2002 and no microfabricated coaxial electrospray source had been reported due to the inherent three-dimensionality and complexity of its hydraulic system.Stereolithography (SLA) is a layer-by-layer additive manufacturing process that creates solid objects via photopolymerization of a resin using ultraviolet light. Additive manufacturing started as a visualization tool for mesoscaled objects, but recent developments in the resolution and capabilities of 3-D printing suggest that these manufacturing processes could address the complexity, three-dimensionality, and material requirements of many microsystems. In particular, high-resolution SLA can be used to manufacture freeform microfluidics at a small fraction of the cost per device, infrastructure cost, and fabrication time of a typical silicon-based microfluidic system.We developed SLA 3-D printed coaxial electrospray sources with one or two emitters that are fed by two helical channels (Figure 1). Each emitter spout is designed to produce a coaxial flow and to enhance the electric field on the liquid meniscus. Using these devices, we produced uniform core-shell microparticles using deionized water as the inner liquid and sesame oil as the outer liquid (Figure 2). The size of the droplets can be modulated by controlling the flow rates fed to the emitters. Electrical characterization of the devices demonstrates that the emitters operate uniformly. Current research efforts focus on demonstrating massively multiplexed sources with uniform array operation."
"Thin, Flexible, and Stretchable Tactile Sensor Based on a Deformable Microwave Transmission Line","Over the past decade, there have been numerous pub-lications on tactile sensors and skins aimed at repli-cating the human sense of touch in applications such as robotics, healthcare, and prosthetics. A variety of technologies are used, with the dominant ones being piezoresistive and capacitive, but both of these tech-nologies have limitations due to mechanical fragility, complex fabrication, and the need for large numbers of connections to external electronics. We have devel-oped another sensing technology that is mechanically robust, simple to fabricate, and requires only one con-nection to external electronics.The new sensor, shown in Figure 1, consists of a flexible and stretchable 1.6-mm-thick microstrip transmission line with conductors made of stretchable silver-based conductive cloth and a dielectric made of soft silicone rubber (PDMS). When pressure is applied to the line, the dielectric is deformed, causing a local impedance discontinuity in the line. We have developed an algorithm that can reconstruct the deformation of the line as a function of position, based on the measured impedance of the line across a wide frequency range (30 MHz to 6 GHz).To characterize the sensor and algorithm, the sensor was precisely deformed using a custom-designed jig based on a micrometer head while its impedance was measured in real time with a vector network analyzer. The analyzer was connected to a computer. in which the output was processed to display a plot of the reconstructed deformation, also in real time. To correct for imperfections in fabrication, any deformation present with the sensor at rest was subtracted from the responses with pressure applied. Three different pressures were applied at each of three locations, and the responses were combined to create Figure 2. Note that the reconstruction algorithm is derived entirely from physical theory and was calibrated to the measured velocity factor of the line but was not otherwise tuned to match the individual device."
Extreme Heat Flux Thermal Management via Thin-film Evaporation,"Thermal management is a primary design concern for nu-merous power-dense equipment such as power amplifiers, solar energy convertors, and advanced military avionics. During operation, these devices generate large amounts of waste heat (>1 kW/cm2) from sub-millimeter areas. These concentrated heat loads are spatially and tempo-rally non-uniform and cause hotspots which are localized regions with extreme heat flux and exceedingly high tem-perature that can adversely impact device performance and reliability. In this study, we demonstrate an extreme heat flux thermal management solution targeted towards cooling hotspots. Our test devices utilize well-defined silicon micropillar arrays which were fabricated via contact photolithography and deep-reactive-ion-etching for passive fluidic transport (i.e., capillary-wicking). Resistive thin-film heaters were integrated on the back side of our test device via electron-beam evaporation and acetone lift-off to emulate the heat generated by actual electronic chips during operation. The heaters which were used to measure temperature in addition to providing heating were calibrated prior to experiments in a convection oven. The hotspots (640×620 μm2) were spatially distributed over the microstructured surface (1×1 cm2). Uniform background heating was provided by heating the entire microstructured surface using a 1×1 cm2 thin-film heater. Experiments were conducted in a temperature-controlled stainless steel environmental chamber which was maintained at saturated temperature and the corresponding pressure. We dissipated ≈6 kW/cm2 from a single hotspot without background heating before the microstructured surface dried out (Figure 1a). Dryout occurs due to liquid starvation when the viscous losses exceed the capillary pressure generated owing to the meniscus shape. We activated concurrent hotspots on our test devices over the 1×1 cm2 microstructured surface and examined the hotspot dryout heat flux. Our experiments show that this hotspot dryout heat flux decreased monotonically when concurrent hotspots were present on the microstructured surface (left ordinate, Figure 1b). The dryout heat flux, which was ≈6 kW/cm2 when a single hotspot (H2) was present, decreased to ≈4 kW/cm2 per heater when two hotspots (H1/H3) were present (left ordinate, Figure 1b). This dryout heat flux decreased further to ≈3 kW/cm2 per heater when three hotspots (H1/H2/H3) were present (left ordinate, Figure 1b). When a 10 W/cm2 and 20 W/cm2 uniform background heating was superposed with a hotspot, the hotspot dryout heat flux, which was ≈6 kW/cm2 without background heating, decreased to ≈4 kW/cm2 and ≈3 kW/cm2, respectively (left ordinate, Figure 1c). Despite the decrease in the hotspot dryout heat flux, the total heating power increased when concurrent hotspots were created (right ordinate, Figure 1b) or when uniform background heating was superposed with a hotspot (right ordinate, Figure 1c). Our experiments show that thin-film evaporation is a promising thermal management solution for the next generation of power amplifiers and radio-frequency devices which generate extreme heat fluxes in excess of 1 kW/cm2. The insights gained from this study can be used to improve the design of wicking structures which are commonly used in phase-change-based thermal management devices such as heat pipes and vapor chambers."
Enhanced Water Desalination in Electrochemical System,"Currently, reverse osmosis (RO) is considered the lead-ing technology for desalination, and the operational efficiency of RO has been significantly improved over the last two decades with a thorough energy analysis. On the other hand, electrical desalination can be more advantageous in certain applications due to the diver-sity of allowed feed conditions, operational flexibility, and the relative low capital cost needed (the size of a system is generally small). Yet electrical desalination techniques such as electrodialysis (ED) have not been modeled in full detail, partially due to scientific chal-lenges involving the multiphysics nature of the pro-cess. In addition, while current ED relies on bipolar ion conduction, removing one pair of a cation and an anion simultaneously, one final but most important point is that desalination achieved by means of an an-ion exchange membrane (AEM) and a cation exchange membrane (CEM) should be considered separately and independently (Figure 1a). Based on the intrinsically different ion transport near AEM and CEM, our group previously presented a novel process of ion concentra-tion polarization (ICP) desalination (Figure 1b), which can basically enhance the amount of salt reduction, by examining unipolar ion conduction through both experiments and numerical modeling (Figure 1b). Since our experimental works are done in a model system for scalable electrochemical systems, the microfluidic device (Figure 1c) enables more scientific knowledge about ion transport phenomena through visualization. Meanwhile, the high-throughput module (stacked layer system, Figure 1d) enables us to realize a practical op-eration and evaluate the system’s performance. Along with the ICP desalination, we also employed an ED system as a model to investigate the mass transport effects of embedded microstructures between the ion exchange membranes. In this work, therefore, we aim to perform a high-level analysis of ion transport near IEMs in order to enhance water desalination in electro-chemical system."
3-D Printed Massively Multiplexed Electrospray Sources,"Electrospray is a electrohydrodynamic phenomenon that produces from a meniscus a stream of micro/nanoparticles that, depending on the properties of the liquid and the process conditions, can be droplets, ions, or fibers. The low spread in size and specific charge of the emitted particles makes the use of electrospray at-tractive in applications such as combustors, maskless micro/nanomanufacturing, and nanosatellite propul-sion. However, the throughput of an electrospray emit-ter is very low, limiting the applicability of single-emitter electrospray sources to a few practical cases, e.g., mass spectrometry of biomolecules. An approach to increase the throughput of an electrospray source without increasing the size variation of the emission is implementing arrays of electrospray emitters that operate in parallel. Miniaturization of the electrospray emitters results in less power consumption and lower onset voltage; in addition, using micro-fabrication, monolithic arrays of miniaturized emitters with large array size and emitter density can be made. Researchers have demonstrated a variety of MEMS multiplexed electrospray sources that operate uniformly. Although these devices work satisfactorily, they present a number of issues: (i) the device archi-tecture is often a compromise between what should be made based on the modeling and what can be made given the limitations of traditional microfabrication, sacrificing device performance; (ii) a change in any of the in-plane features of the design requires the redesign and fabrication of one or more lithography masks while causing added costs and time delays; (iii) these devices are fairly expensive because they are made in a multi-million semiconductor-grade cleanroom with advanced tools that are operated by highly trained staff, which restricts their application to high-end applications and research.We recently demonstrated the first 3-D printed multiplexed electrospray sources in the literature (Figure 1). The devices were fabricated with stereolithography and have associated two orders of magnitude less fabrication cost per device, fabrication time, and manufacturing infrastructure cost compared to a silicon MEMS multiplexed electrospray source. The 3-D printed devices include features not easily attainable with other microfabrication methods, e.g., tapered channels and threaded holes. Through the optimization of the fabrication process, arrays with as many as 236 internally fed electrospray emitters (236 emitters in 1 cm2) were made, i.e., a twofold increase in emitter density and a sixfold increase in array size compared with the best reported values from multiplexed, internally fed, electrospray sources made of polymer. The characterization of devices with a different array size suggests a uniform emitter operation (Figure 2)."
Optimization of Capillary Flow through Open Microstructured Arrays,"Liquid propagation through porous microstructures has received significant attention due to the importance of precisely controlling flow in microfluidic systems. Peri-odic surface structures, e.g., arrays of open micropillars or open microchannels, sometimes can be used to con-trol the flow in a microsystem, introducing benefits such as direct access to the porous structure, device reusabil-ity, and resilience against clogging. In an open fluidic structure, the liquid is not actively pumped, e.g., using an upstream pressure signal; instead, the microstructured surface passively drives the liquid via capillary action. However, the same surfaces driving the flow via sur-face tension’s pull simultaneously impede it by way of viscous resistance. Therefore, optimization of the geom-etry of the microstructured surface is required to maxi-mize the flow rate it transports.We developed semi-analytical models that describe the dynamics of capillary flow against gravity in (i) vertical arrays of open microchannels with rectangular cross-section and (ii) arrays of open micropillars with square packing and square cross section. We also extended our analysis to capture the shear-thinning behavior typical of many non-Newtonian fluids. Our models indicate the existence of multiple flow rate maxima with respect to pore size. One maximum, which occurs only in micropillar arrays, arises from the trade-off between capillary pressure and viscous resistance. The two other maxima, which occur for both micropillar and microchannel arrays, are related to meniscus and gravitational effects and only appear at low aspect-ratio (i.e., in channels/gaps between adjacent pillars that are about as wide as they are deep) and high Bond number, respectively. Experimental capillary rise data demonstrate that incorporating first-order gravitational effects and the impact of meniscus curvature improved flow rate predictions relative to models that neglect these factors (Figures 1 and 2; in both figures the working liquid is 1% PEO in 40/60 ethanol/water). Experimental capillary rise data also confirm the existence and location of a flow maximum with respect to the width of an open-microchannel; operating at any of the maxima decreases the sensitivity of flow rate to geometric variation, allowing for more robust microfluidic systems. Finally, we demonstrated electrospray emission from the edge of a microstructured surface as an example of an application of the porosity geometries we in-vestigated in this study; the supply-limited regime of the current-voltage characteristics of these devices are in agreement with the literature on electrospray droplet emission, opening the possibility to implement arrays of externally-fed electrohydrodynamic jetting emitters that can operate continuously while producing droplets or nanofibers using suitable working liquids."
Chip-Scale Electrostatic Vacuum Ion Pump with Nanostructured Field Emission Electron Source,"Cold-atom interferometry of alkali atoms can be used in a variety of high-precision sensors and timing devices such as atomic clocks, gyroscopes, accelerometers, magnetom-eters, and gravimeters. These devices require ultra-high vacuum (UHV, pressure < 10-9 Torr) to operate; therefore, chip-scale versions require miniaturized UHV pumps re-silient to alkali metal vapors that consume power at lev-els compatible with device portability. In a macro-sized chamber, UHV-level vacuum can be maintained using a conventional magnetic ion pump, where electrons that swirl around the magnetic lines of a magnet create ions by impact ionization of neutral molecules, which in turn sputter a Ti getter. While scaled-down versions of mag-netic ion pumps have been reported, these are incompat-ible with miniaturized cold-atom interferometry systems because (i) a reduction in the pump size increases the required threshold magnetic field for electron trapping, and (ii) the larger magnetic field associated with a minia-turized ion pump can interfere with the operation of the cold-atom sensor, yielding flawed readings. Non-evapo-rable getter (NEG) pumps are used in some cold-atom in-terferometry systems, e.g., commercial chip-scale atomic clocks; however, NEG pumps are unable to pump noble gases such as He and N2 that are present in the chamber, and they inefficiently pump alkali vapors.We are developing vacuum ion pumps compatible with chip-scale cold-atom interferometry devices. The proposed field emitter array (FEA)-based magnet-free ion pump architecture is shown in Figure 1. In this pump design, a helical electron collector pulls the electrons toward itself, forcing them to first travel beyond the height of the electron collector, to then get pushed back due to the electrostatic mirror effect of the annular-shaped ion collector. Therefore, the trajectory of the electrons is significantly increased compared to a pump design with a parallel-capacitor electrode configuration, augmenting the probability of impact ionization. The FEA consists of arrays nano-sharp silicon tips, each surrounded by a self-aligned gate electrode; we have shown that these FEAs do not degrade in the presence of Rb vapor. Figure 2 shows the semi-log plot of the minus time derivative of the pressure versus time during pump-down, with the horizontal axis denoting the time since the beginning of each pump-down cycle; in these experiments, the pressure inside the chamber reached values as low as ~7×10-7 Torr. Each data point in the plot represents an average of the minus time derivative of the pressure considering all pump-down cycles. The R2 of the linear fit of the data evidences that our reduced-order model accurately explains the dynamics of the pump. The slope of the linear fit of the data estimates the experimental pumping time constant at about 161 seconds."
Hardware Trojan Detection using Unsupervised Deep Learning on High Spatial Resolution Magnetic Field Measurements,"One major vulnerability of integrated circuits (ICs) is the difficulty of ensuring that an IC fabricated in a third-party foundry is not a maliciously modified ver-sion of the original design. Such modifications by at-tackers, called hardware Trojans (example shown in Figure 1a), can leak private data from an IC, change its functionality, or have other effects. Attackers can de-sign Trojans so that their effects are not visible during simple functional tests, making detection difficult. However, side channel methods (Figure 1b) can mea-sure differences in circuit activity resulting from the modified logic to detect Trojans prior to the presence of functional changes.In this work, we achieve a method of detecting small footprint hardware Trojans in a field-programmable gate array by performing high spatial resolution and wide field-of-view imaging of the circuit magnetic fields using a quantum diamond microscope. These images are then separated into Trojan-free and Trojan-inserted measurements in an automated framework by using an unsupervised convolutional neural network and clustering. With this framework, we show detection ability comparable to previous literature without requiring any knowledge of the Trojan at test time."
A Low-Power BLS12-381 Elliptic Curve Pairing Crypto-Processor,"Pairing-based cryptography (PBC), a variant of elliptic curve cryptography (ECC), uses bilinear maps between elliptic curves and finite fields to enable novel applica-tions beyond traditional key exchange and signatures, e.g., signature aggregation and functional encryption. These protocols require special pairing-friendly ellip-tic curves; recent cryptanalysis has compromised the security of commonly used 254b BN curves. There-fore, the new BLS12-381 curve, based on a 381b prime field, is being standardized for PBC applications. How-ever, with strong security, the new curve has higher computational complexity, making implementating low-power embedded devices challenging. To address this challenge, we present the first BLS12-381 elliptic curve pairing crypto-processor, which enables two or-ders-of-magnitude energy savings through efficient hardware acceleration, implements countermeasures against timing and power side-channel attacks, and provides the flexibility to implement ECC and PBC pro-tocols for securing Internet of Things applications.Figure 1 shows the architecture of a pairing cryptoprocessor with the chip micrograph. Our test chip was fabricated in TSMC 40-nm low-power complementary metal-oxide-semiconductor process and supports voltage scaling from 1.1V down to 0.66V. The cryptographic core occupies a 0.2-mm2 area consisting of 112k logic gates and 16 KB SRAM. Programming with custom instructions for modular arithmetic, elliptic curve point and line arithmetic, pairing operations, control, and branching is possible. Key building blocks are constant-time and secure against timing and simple power analysis side-channel attacks. For high-security use, our chip can be configured to protect against stronger differential power analysis side-channel attacks at the cost of increased energy consumption. We have evaluated pairing-based public key cryptography protocols on our chip, including signature aggregation, identity-based signatures, identity-based encryption, inner product functional encryption, and multi-party key exchange. Our hardware-accelerated implementations are 130-140× more energy-efficient than software. The programmability of our pairing crypto-processor allows new protocols, algorithm optimizations, and side-channel countermeasures to be easily implemented using one chip."
Direct Hybrid Encoding for Signed Expressions SAR ADC for Analog Neural Networks,"Artificial intelligence (AI) has proven itself to be one of the most powerful techniques for computer vision, nat-ural language processing, and the automobile industry. Current AI algorithms that are based on deep neural networks (DNNs) are facing a crucial challenge from ef-ficient computing. State-of-the-art DNNs need millions of weights and plenty of computation. The huge ener-gy consumption is neither environmentally friendly nor practical in the battery-constraint edge devices. Conventional DNN hardware is based on fully digital implementation, where data movement is becoming the bottleneck. Data movement typically takes orders of magnitude more energy than the actual computa-tion. Analog neural networks (ANNs) are a promising solution for energy-efficient AI inference. The ANNs perform the in-memory-computing to reduce the ener-gy of data movement. Thus, the analog/digital interface circuits are a critical part of the ANNs and are often the key bottleneck of the performance, power consump-tion, and area of the resulting system.The hybrid encoding for signed expression (HESE) scheme is based on the booth encoding but with additional rules to provide the minimum-length signed-digit-representations (SDR) for efficient encoding for both DNNs including ANNs. This work focuses on a successive approximation register (SAR) analog-to-digital converter (ADC) that produces HESE encoded output on the fly. This ADC has two thresholds for 2-bits look ahead (LA). The proposed SAR ADC can directly encode the analog input to HESE instead of binary encoding. Preliminary results show that in a typical DNN, over 95% of weights can be represented by 5 terms of HESE for a 12-bits resolution."
Efficient Computation of Map-scale Continuous Mutual Information on Chip in Real Time,"Exploration tasks are essential to many emerging ro-botics applications, ranging from search and rescue to space exploration. The planning problem for explora-tion requires determining the best locations for future measurements that will enhance the fidelity of the map, for example, by reducing its total entropy. A wide-ly studied technique involves computing the mutual information (MI) between the current map and future measurements and utilizing this MI metric to decide on the locations for future measurements.However, computing MI for reasonably sized maps is slow and power-hungry, which has been the bottleneck in fast and efficient robotic exploration. In this paper, we introduce a new hardware accelerator architecture for MI computation that features 16 high-efficiency MI compute cores and an optimized memory subsystem that provides sufficient bandwidth to keep the cores fully utilized. Each core employs interleaving to counter the recursive algorithm and workload balancing and numerical approximations to reduce latency and energy consumption.We demonstrate an optimized architecture on a field-programmable gate array (FPGA) implementation, which can compute MI for all cells in an entire 201-by-201 grid (e.g., representing a 20.1-m-by-20.1-m map at 0.1-m resolution) in 1.55 ms while consuming 1.7 mJ of energy, thus finally rendering MI computation for the whole map in real time and at a fraction of the energy cost of traditional compute platforms. For comparison, this particular FPGA implementation running on the Xilinx Zynq-7000 platform is two orders of magnitude faster and consumes three orders of magnitude less energy per MI map compute than a baseline GPU implementation running on an NVIDIA GeForce GTX 980 platform. The improvements are more pronounced when compared to CPU implementations of equivalent algorithms."
Simulation and Analysis of GaN CMOS Logic,"There is an increasing demand for electronics that can operate in high-temperature conditions, such as space-craft applications and sensors for industrial environ-ments. Electronics based on wide-bandgap materials offer a promising solution, among which gallium ni-tride (GaN) stands out as a strong candidate due to its excellent material properties and potential for mono-lithic integration. Most current demonstrations of GaN logic are based on nanometal-oxide-semiconductor (nMOS) technology, which has a high static power con-sumption. Therefore, we are developing GaN comple-mentary metal-oxide-semiconductor (CMOS) technolo-gy, which has lower static power consumption.This work studies the effect of a p-channel transistor and circuit parameters on the performance of CMOS digital logic circuits. We used the MIT Virtual Source GaN-field-effect transistor (MVSGFET) model to accurately model the behavior of the n-channel and p-channel transistors, which were fabricated on the developed GaN complementary circuit platform. We simulated and studied several building blocks for digital logic, namely, the logic inverter, multi-stage ring oscillator, and static random-access memory (SRAM) cell, using the developed computer-aided design (CAD) framework. We conducted device-circuit co-design to optimize circuit performance, using a variety of design parameters, including transistor sizing and supply voltage scaling. We projected the high-temperature performance of the circuits through simulations based on experimentally observed device behaviors. The results indicate that GaN CMOS technology based on our monolithically integrated platform has potential for a variety of use cases, including harsh-environment digital computation. We will apply this technique for more complex combinational and sequential logic building blocks, with the eventual goal of realizing a GaN CMOS microprocessor."
A 0.31 THz CMOS Uniform Circular Antenna Array Enabling Generation/Detection of Waves with Orbital-Angular Momentum,"Multiplexing of electromagnetic (EM) waves with dif-ferent frequencies, polarizations, and coding has been extensively exploited in wireless systems. Recently, an-other dimension of EM waves-the orbital angular mo-mentum (OAM), is attracting increasing attention. An OAM-based wave possesses a wavefront with a helical phase distribution around the central axis of the beam. Different OAM modes, determined by the handedness and the total phase change () of the wavefront twist, are orthogonal. Wireless communication uses multi-OAM mode transmission to enhance spectral efficiency and physical-layer security. Conventional OAM-generation approaches incorporate dielectric spiral-phase plates, passive uniform circular antenna arrays, or metasur-faces in conjunction with separate signal drivers. These discrete solutions, however, lead to very bulky and cost-ly systems.In this project, we demonstrate the first chip-based (at any frequency) CMOS front-end that generates and receives electromagnetic waves with OAM, shown in Figure 1. The chip, based on a uniform circularly placed patch antenna array at 0.31THz, transmits reconfigurable OAM modes, which are digitally switched among the (plane wave), (left-handed), (right-handed), and superposition states. The chip is also reconfigurable into a receiver mode that identifies different OAM modes with >10dB rejection of unintended modes. The array, driven by only one active path, has a measured EIRP of -4.8dBm and consumes 154mW of DC power in the OAM source mode. In the receiver mode, it has a measured conversion loss of 30dB and consumes 166mW of DC power. The OAM chip output mapped from a repeated Keccak-generated data sequence was verified, and the time-domain outputs of the Rx with different SPP configurations are shown in Figure 2, which shows good correlation with matched modes, partial correlation of multiplexed mode, and rejection of unmatched modes."
Stability Improvement of CMOS Molecular Clocks Using an Auxiliary Loop Based on High-Order Detection and Digital Integration,"Recently, chip-scaled molecular clocks (CSMC) have achieved high-frequency stability with low power and compact size by using a rotational-mode transition of carbonyl sulfide (OCS) centered around 231.061 GHz as a frequency reference (f0). In the molecular clock, the probing signal generated from the transmitter is fre-quency-modulated at fm around the center frequency (fc). Since fc is locked to f0 in a feedback loop, the output frequency inherits the excellent stability of the OCS transition frequency.Due to its fully-electronic implementation, CSMC has reduced the cost of high-stability miniaturized clocks. However, the frequency stability is still limited by a finite loop gain of the frequency locked loop and detection non-idealities, which are susceptible to environmental disturbance even though an invariant physical constant is used as the frequency reference. In this work, we propose a new dual-loop CSMC architecture based on both fundamental and high-order transition probing as well as digital integration.In order to achieve a high long-term stability without compromising the signal-to-noise ratio, the fundamental harmonic detection forms the main loop while the higher-order probing is used in an auxiliary loop. The loop fine-tunes the phase-locked loop’s frequency multiplication ratio according to the sign of the high-order detection output. With a proper selection of gain and bandwidth in each loop, the main loop enables the fast correction of frequency, and the auxiliary loop responds against long-term frequency variation. Also, the frequency offset between the clock output and the OCS reference can be eliminated when the clock is locked because the auxiliary loop includes a digital integrator to obtain an infinite DC gain. As a result, the proposed CSMC implemented in 65-nm CMOS process achieved Allan deviation of 5.4×10-10 and 2×10-11 at 1 s and 104 s averaging times, respectively, with 71 mW power consumption."
A Sampling Jitter Tolerant Continuous-Time Pipelined ADC in 16-nm FinFET,"Almost all real-world signals are analog. Yet most of the data is stored and processed digitally due to advances in the integrated circuit technology. Therefore, ana-log-to-digital converters (ADCs) are an essential part of any electronic system. The advances in modern com-munication systems including 5G mobile networks and baseband processors require the ADCs to have large dy-namic range and bandwidth. Although there have been steady improvements in the performance of ADCs, the improvements in conversion speed have been less sig-nificant because the speed-resolution product is limit-ed by the sampling clock jitter (Figure 1). The effect of sampling clock jitter has been considered fundamen-tal. However, continuous-time delta-sigma modulators may reduce the effect of sampling jitter. But since del-ta-sigma modulators rely on relatively high oversam-pling, they are unsuitable for high-frequency applica-tions. Therefore, ADCs with low oversampling ratio are desirable for high-speed data conversion. In conventional Nyquist-rate ADCs, the input is sampled upfront (Figure 2). Any jitter in the sampling clock directly affects the sampled input and degrades the signal-to-noise ratio (SNR). It is well known that for a given root mean square (rms) sampling jitter σt the maximum achievable SNR is limited to 1/(2πfinσt), where fin is the input signal frequency. In an SoC environment, it is difficult to reduce the rms jitter below 100 fs. This limits the maximum SNR to just 44 dB for a 10 GHz input signal. Therefore, unless the effect of sampling jitter is reduced, the performance of an ADC would be greatly limited for high-frequency input signals.In this project, we propose a continuous-time pipelined ADC having reduced sensitivity to sampling jitter. We are designing this ADC in 16-nm FinFET technology to give a proof-of-concept for improved sensitivity to the sampling clock jitter."
Bandgap-Less Temperature Sensors for High Untrimmed Accuracy,"Temperature sensors are extensively used in measure-ment, instrumentation, and control systems. A sensor that integrates the sensing element, analog-to-digital converter, and other interface electronics on the same chip is referred to as a smart sensor. Complementa-ry metal-oxide-semiconductor- (CMOS) based smart temperature sensors offer the benefits of low cost and direct digital outputs over conventional sensors. How-ever, they are limited in their absolute accuracy due to the non-ideal behavior of the devices used to design them. Therefore, these sensors require either calibra-tion or gain/offset adjustments in the analog domain to achieve desired accuracies (Figure 1). The latter pro-cess, also called trimming, needs additional expensive test equipment and valuable production time and is a major contributor to the cost of the sensors. In order to enable high volume production of CMOS-based tem-perature sensors at low cost, achieving high accuracies without trimming is imperative.This work proposes the design of a CMOS temperature sensor that uses fundamental physical quantities resilient to process variations, package stress, and manufacturing tolerances to achieve high accuracies without trimming. Simulation results prove that 3σ inaccuracy of less than 1o C can be obtained with the proposed method."
High Angular Resolution THz Beam Steering Antenna Arrays in 22-nm FinFET Technology,"THz phased arrays are a promising emerging technolo-gy for many applications, including THz imaging, radar, communications, and other sensing applications. This is largely a result of the smaller wavelength at THz fre-quencies and accordingly smaller array size and weight. However, challenges exist in their design, particularly the design of THz phase shifters, which are often lossy, power hungry, and physically large, precluding their use in dense arrays. These losses often arise from the high-resolution nature of the phase shifters. In ad-dition, lossy on-chip transmission lines significantly degrade system performance. In this work, we apply phased array principles to yield dense THz antenna ar-rays with only one bit of phase resolution, yielding per-formance benefits in terms of DC power, THz loss, size, bandwidth, and simplicity. In addition, by distributing radio frequency (RF) power spatially, we mitigate many of the losses with RF signal distribution. This approach is termed reflectarray (reflector array). We demonstrate our approach on complementary metal-oxide semicon-ductor silicon in the form of a 4x4 mm2 chip containing 7x7 antenna elements, operating at 260 GHz. The chip is designed in Intel 22-nm FinFET process so that mul-tiple chips can be tiled to create large arrays that can be scaled in size based on performance requirements. The use of one-bit phase shifters comes at a cost in sys-tem-level performance by introducing sidelobes in the radiation pattern. Our work introduces a number of ap-proaches to mitigate this, allowing the one-bit phased array design to approach the performance of a phased array with a continuous, analog phase shifter. While still in progress, this work pushes towards practical large-scale THz phased arrays."
DC-DC Converter Implementations Based on Piezoelectric Transformers,"Power converters play major roles in many applica-tions ranging from power generation and distribution in electric grids to everyday devices such as mobile phones and computers. As many applications require small form factors, there has been a significant demand to miniaturize power converters while maintaining high performance. Typical converters rely on magnetic energy storage components, but the achievable power densities of magnetics fundamentally decrease at low volumes and therefore limit converter miniaturization. Piezoelectrics, which have more favorable power density scaling properties than magnetics, are a promising energy storage alternative to meet the demands for low-volume power electronics. Furthermore, multi-port piezoelectric transformers (PTs) offer the additional benefits of galvanic isolation and inherent voltage conversion ratios. Despite their potential, PTs have seen little use in converters without magnetics, and such design attempts have unreported or limited efficiencies.In this work, we systematically enumerate isolated and non-isolated converter switching sequences and topologies that best utilize PTs as their only energy storage components. We constrain this search for (1) high-efficiency behaviors such as zero voltage switching (ZVS) and all-positive instantaneous power transfer and (2) practical characteristics such as voltage regulation capability and control simplicity. To evaluate the selected switching sequences, we also develop a model for estimating the PT’s efficiency.Initial experimental results of these converter designs demonstrate promising high-efficiency behaviors and peak whole-converter efficiencies higher than reported for most magnetic-less PT-based converters in the literature. The prototype displayed in Figure 1 is based on a commercially available PT and achieves a peak efficiency of 89.3%, which is close to our estimation model’s predictions. These results suggest that PT-based converters can offer high efficiencies in addition to the low-volume scaling benefits of piezoelectrics. Such characteristics can be advantageous to high-voltage, low-power applications such as portable electronics and biomedical devices, particularly those requiring galvanic isolation."
Closed Loop Control for a Piezoelectric-Resonator-Based DC-DC Power Converter,"Electronics such as computers, mobile phones, house-hold appliances, and even electric vehicles can vary greatly in terms of supply requirements; power elec-tronics are necessary to power these devices from stan-dard sources. Reducing the sizes of power converters allows them to be more cost-effective and useful to a wider range of applications. Traditional DC-DC power converters make use of magnetics for energy storage, but these are less efficient and power-dense when scaled down to small sizes. Our prior work has explored the use of piezoelectric resonators (PRs) as alternative energy storage mechanisms for DC-DC converters, and we successfully demonstrated a magnetics-less PR-based converter with >99% efficiency. However, our initial prototypes depended on open-loop switching times that were manually tuned, meaning the convert-er could not dynamically handle transients or adjust operation when the load or temperature changed. This work presents a closed-loop control scheme for the PR-based DC-DC converter. For high efficiency, the converter is designed to cycle through a specific 6-stage “switching sequence” during each PR resonant cycle. In this sequence, the PR is switched between fixed-voltage energy transfer stages and resonant transition stages (shown in Figure 1), which is challenging to implement in a simple manner. The converter is controlled by two active switches, as shown in Figure 2. Both switches are triggered to turn on purely by voltage measurements of the PR node voltages. Switch 1’s on-time is modulated to control power output, and switch 2’s on-time is modulated to reach the specific high-efficiency point. Simulation results have shown that this control scheme is effective, and we are currently validating it on hardware. The successful implementation of this closed-loop control scheme will allow the PR-based converter to operate on its own, paving the way for use of these small and efficient DC-DC converters in commercial applications."
Leveraging Multi-Phase and Fractional-Turn Planar Transformers for Power Supply Miniaturization in Data Centers,"Data centers are the backbone of the Internet. Their servers represent an important and growing electrical load, and there is strong interest in miniaturizing the supplies that power them. Miniaturization is challeng-ing as it requires both a reduction in volume and an in-crease in efficiency and is bottlenecked in this applica-tion by the need for a high-current transformer. A common approach toward improving the current carrying capability of the transformer is to increase its phase count by employing multiple identical transformers in parallel. Every phase that is added proportionally decreases the “copper loss” (ohmic loss) of the transformer while proportionally increasing its core loss (i.e., loss in the magnetic material). We call this “linear rebalancing.”In this work, we fundamentally re-think the nature of the transformer to maximally leverage the connecting electronics. In particular, by careful placement of the active switching devices required in a converter around and the passive copper and magnetic material comprising the transformer, we can create a “fractional turn” transformer. Employing a half-turn fractional transformer reduces copper loss by a factor of four while increasing core loss by a factor of 2β, where β is between 2 or 3 depending on the core material. Thus, fractional turn transformers yield an “exponential rebalancing” of core and copper loss.We show that the fractional turn concept can also be combined with the common approach of adding transformer phases, enabling multi-phase fractional-turn transformers. For example, a split-phase half-turn transformer (SPHTT) combines the linear and exponential rebalancing of each of those transformers and allows a designer to get closer to the true optimum loss trade-off for a given application. We show that a SPHTT is optimal for a data center application, yielding 3.1x lower loss than a single-phase transformer in the same volume and demonstrating its clear miniaturization benefit."
Soft-Actuated Micro Aerial Vehicles with High Agility,"Developing agile and robust micro-aerial-vehicles (MAVs) that can demonstrate insect-like flight capa-bilities poses significant scientific and engineering challenges. Previously, we chose dielectric elastomer actuators (DEAs) to substitute for rigid actuators and achieved the first take-off and controlled flight of a soft-actuated MAV. In this work, we substantially im-prove the robot’s flight capability through redesigning the actuator, robot wings, and transmission. The new MAV weighs approximately 665 mg and can complete a somersault within 0.16 s. Furthermore, its vertical as-cending speed exceeds 70 cm/s, which makes it among the fastest soft mobile robots, and it outperforms rig-id-powered subgram MAVs. A major contribution to this excellent performance is that we switch to a less viscoelastic elastomer, Elastosil P7670. Compared to our previously used elastomer (5:4 mixture of Ecoflex 0030 and Sylgard 184), this new elastomer has a higher resonance peak, which implies a larger displacement at the resonant frequency. In addition, it has higher a di-electric strength and a shorter pot time. Based on our measurement, the new MAV achieves a high lift-to-weight ratio of 2.2:1, which is 83% better than our previous work. The large lift force enables us to demonstrate hovering flight, ascending flight, in-flight collision recovery, and--more impressively—a somersault. As shown in Figure 2, the MAV takes off and hovers, accelerates upward, flips along its body pitch axis, recovers attitude, and finally returns to hover. The somersault is completed in 0.16 s; during the body flip, the motion capture system loses tracking for approximately 0.1 s. This loss results in the MAV’s hitting the ground before recovering its attitude. Despite experiencing disturbance caused by the collision, the MAV quickly stabilizes its attitude and returns to hover. This is the first time that a soft-driven MAV performs agile tasks that rigid-driven MAVs have not yet demonstrated."
Adjusting for Autocorrelated Errors in Neural Networks for Time Series Regression,"Time series data are ubiquitous. Researchers in many fields, including the social sciences, operations re-search, and engineering often collect time series data to create models for systems without prior or precise knowledge of the model structure and, in turn, provide insight for such systems. During this process of collec-tion and creation, errors inevitably occur. Usually, the assumption is that the errors are uncorrelated at dif-ferent time steps. However, in practice, errors can be autocorrelated when (1) the function space of the mod-el and the true underlying system do not intersect, (2) some key explanatory variables are not collected, or (3) a measurement error at a current time step carries over to future time steps. To solve this issue, previous literature, such as the Cochrane–Orcutt estimation, focuses only on cases where the model is linear or contains only predefined nonlinearity. This focus greatly limits usage, as many systems today (such as in semiconductor manufacturing) are almost certainly nonlinear while the underlying nonlinearity is unknown.Here, we propose to use neural networks (NNs) to approximate the unknown nonlinearity and treat the autocorrelation coefficient ρ as a trainable parameter. The input to our model is a vector of features (i.e., regressors) at time t, and the output is the target scalar (i.e., regressand) also at time t. During training, we jointly optimize model parameters with the autocorrelation coefficient to adjust for the autocorrelated errors. This optimization enables us to train a NN that can fit the nonlinearity and adjust correspondingly to its autocorrelated noise. Compared to previous methods, this one has the advantages of (1) fitting unknown nonlinearity with autocorrelated noise and (2) better optimization via joint training of model parameters and autocorrelation coefficient. Our experimental results show that we obtain a better estimate of the autocorrelation coefficient and improve the model performance especially when the autocorrelated errors are substantial."
Terahertz Wireless Link for Quantum Computing in 22-nm FinFET,"Quantum computing can provide exponential speed-up in solving many of today’s intractable problems such as quantum chemistry, RSA encryption, DNA analysis, etc. In order to implement an error-protected quantum computer (QC), we will require approximately a million or thousands of qubits. State-of-the-art QCs have only around 100 qubits but still demand large-form-factor room- temperature electronics with many radio-fre-quency (RF) cables to realize the control and readout of quantum processors. These RF cables routed from room temperature to cryogenic temperature consume a non-negligible power due to the heat load, limiting the scalability and practical implementations of QCs.We propose a terahertz (THz) wireless link to efficiently deliver the control signals to the cryogenic environment, reducing the heat loss due to the physical conductive links (Figure 1). We implement a cryogenic THz receiver to send multi-Gb/s control signals modulated on a THz carrier (e.g., 260-GHz). The THz operation allows for a small antenna aperture size, high data rate, and minimal interference with the operation of the qubits, working around a few GHz. For the de-modulation of the sub-THz downlink control signal, a THz square-law detector, operating with zero drain bias, is used first to rectify the input to baseband, and then a low-power transimpedance amplifier followed by a post-amplifier are used to boost the baseband signal so that the subsequent digital circuits can operate reliably. Figure 2 shows the chip photo of this prototype. This system opens the door for scalable and practical realization of cryogenic quantum systems."
Energy-Efficient System Design for Video Understanding on the Edge,"With the rise of various applications including auton-omous driving, object tracking for unmanned aerial vehicles, etc., the need increases for accurate and ener-gy-efficient video understanding on the edge. Although plenty of deep learning chips designed for images ex-ist, little work has been done for videos. Video under-standing on the edge has three major challenges. First, video understanding requires temporal modeling. For example, it identifies the difference between opening and closing a box, which is distinguishable only with temporal information. Second, many applications are delay-critical, such as self-driving cars. Third, high en-ergy efficiency matters for edge devices with a tight power budget. Due to temporal continuity, consecu-tive frames might share much information, providing a potential to improve processing efficiency. However, an image-based processing system, which processes frames individually, cannot utilize that. In this project, we co-designed algorithms and hardware for energy-efficient video processing on delay-critical applications (Figure 1). We applied temporal shift module (TSM) on the backbone built on 2D convolutional neural network (Figure 2). To the best of our knowledge, our work is the first chip with temporal modeling support. Moreover, we propose a Real-Time DiffFrame method to reduce on-chip energy and DRAM traffic. It is based on the linearity of convolution, which has Conv(ft) = Conv(ft - ft-1) + Conv(ft-1), where ft and ft-1 are the successive frames. Due to temporal continuity, ft - ft-1 is usually sparse. Instead of the ordinary sparsity-aware convolution in previous work, our method utilizes SparseConv, which does not dilate the input pattern and further improves energy efficiency. The load and store of Conv(ft-1) are the overhead of the DiffFrame method. We propose a scheme to reduce memory traffic for real-time processing. The preliminary results show that our method achieves 1.6x reduction in DRAM traffic over previous work and 1.8x estimated reduction in computation and memory access over the baseline."
"Sparseloop: An Analytical, Energy-Focused Design Space Exploration Methodology for Sparse Tensor Accelerators","Many popular applications (e.g., deep neural networks) involve tensor computations (e.g., cross products) whose operand and result tensors can have high spar-sity. Due to the nature of multiplication, zero multipli-cands always result in zero products. Such computa-tions (which are called ineffectual) can be exploited by hardware sparse optimization features to improve ener-gy efficiency and throughput. We classify these sparse optimization features into three categories: zero-gat-ing, zero-skipping, and zero-compression. Zero-gating improves energy efficiency by keeping the associated hardware components idle for ineffectual computa-tions. Zero-skipping further improves throughput by skipping cycles where ineffectual computations would have taken place. Zero-compression reduces required storage by storing only nonzero values. In recent years, a variety of sparse tensor accelerators have been proposed. Based on the designer’s intuitions, each design applies variations of the aforementioned sparse optimization features differently to the storage and compute levels of the architecture. However, these specific designs are just points in a large and diverse space of sparse tensor accelerators. A fast, flexible, and accurate modeling framework would enable architects to perform early design space exploration in the complete space instead of picking specific points based on intuition.Existing tensor accelerator models are either very detailed and design-specific, leading to slow and limited design space exploration, or fast and flexible but unable to systematically evaluate the impact of sparse optimization features, resulting in inaccurate modeling. In this work, we propose Sparseloop, an analytical modeling infrastructure for performing fast design space exploration of sparse accelerators that vary in both (1) properties associated with sparsity (e.g., compression formats, ineffectual operations’ gating/skipping, and workload attributes) and (2) architecture properties (e.g., organization of the storage hierarchy). To the authors’ knowledge, Sparseloop is the first analytical model that allows systematic evaluation of sparse tensor accelerators."
"Multi-Inverter Discrete-Backoff: A High-Efficiency, Very-Wide-Range RF Power Generation Architecture","Radio-frequency (RF) power amplifiers (PAs) for indus-trial applications, e.g., plasma generation for semicon-ductor processing equipment, operate into variable load impedances at high frequency (e.g., tens of MHz) and power levels (e.g., peak power in kWs), and often with wide overall power ranges and high peak-to-aver-age-power ratios. To meet the evolving needs for semi-conductor processing, goals for RF PAs in these applica-tions include (1) operation over a wide load impedance (as determined by the plasma load); (2) operation across a very wide range of output power (e.g., 100:1 or 20 dB); (3) very fast dynamic response to output commands (e.g., at μs scale); and (4) high peak and average effi-ciency (to reduce cooling requirements and electricity costs). Unfortunately, meeting all these goals has not been possible to date, and efficiency is often sacrificed in order to meet the other performance metrics.This work introduces a scalable power amplifier architecture and control approach suitable for such applications. The architecture consists of modular PAs organized in groups and employs (1) a technique which we call Multi-Inverter Discrete Backoff (MIDB), which losslessly combines the outputs of parallel-grouped switched-mode PAs and modulates the number of active PAs within the same group to provide discrete steps in RF output voltage, and (2) outphasing among the voltage outputs of PA-groups, for fast-response and continuous output power control over a wide range. To further expand the high-efficiency output power range of the system, discrete drain modulation may be optionally employed. In doing so, the MIDB-based architecture can maintain high efficiency and fast RF power control across a very wide range of power backoff."
Programming a Quantum Computer with Quantum Instructions,"The use of quantum bits to construct quantum com-puters opens the door to dramatic computational speedups for certain problems. The maturity of mod-ern quantum computers has moved the field from be-ing predominantly a quantum-device-focused research area to also include practical quantum-computing-ap-plication-focused research. Our research explores a new experimental result on a foundational aspect of how to program quantum computers. A central prin-ciple of classical computer programming is the equiv-alence between data and instructions about what to do with that data. In quantum computers, this equiv-alence is broken: classical hardware is used to generate the sequence of operations to be executed on the quan-tum data stored in the quantum computer. Our experi-ment shows for the first time how the instruction-data symmetry can be restored to quantum computers. We use superconducting qubits as a platform to imple-ment high-fidelity quantum operations enabling the so-called density matrix exponentiation algorithm to generate these quantum instructions. This algorithm provides large quantum speedups for a family of other quantum algorithms."
Silicate-Based Composite as Heterogeneous Integration Packaging Material for Extreme Environments,"Electronic microsystems are foundational to today’s computational, sensing, communication, and informa-tion processing capabilities, therefore impacting indus-tries such as microelectronics, aerospace, healthcare, and many more. Cell phones are an example of what is possible when a variety of systems can be tightly integrated into a highly portable and capable system. However, as we aim to improve our ability to interact and operate (e.g., sense, communicate, record, compute, move, etc.) in extreme environments (such as outer space or the human body), new methods and materials must be developed to manufacture such integrated sys-tems that will endure post-processing, environmental, and operational challenges.Typical organic-based packaging materials (e.g., polymer adhesives, coatings, and molding materials) often suffer from outgassing and leaching that can lead to system contamination, as well as coefficient of thermal expansion (CTE) mismatches that can lead to warpage and breakage with fluctuations in system temperature during operation. This work demonstrates an alternative, by using a silicate-based inorganic glass composite as an electronics packaging material for stability in extreme environments. Combining liquid alkali sodium silicate (water glass) and nanoparticle fillers, composites can be synthesized and cured at low temperatures into chemically, mechanically, and thermally (up to 400oC) stable structures using high-throughput processing methods such as spin and spray coating. Further, this material can be processed into thick layers (10s to 100s of microns), fill high aspect ratio gaps (13:1), withstand common microfabrication processes, and have its CTE tailored to match various subs"
Rethinking Plant-Based Materials Production: Selective Growth of Tunable Materials via Cell Culture,"Current systems for plant-based materials production are inefficient and place unsustainable demands on en-vironmental resources. Traditionally cultivated crops present low yields of industrially useful components and require extensive post-harvest processing to re-move extraneous portions of the plants. Large-scale monoculture remains the unchallenged standard for biomass production despite the negative impacts of the practice to the surrounding biome as well as a suscepti-bility to season, climate, and local resource availability. This work proposes a novel solution to these shortcom-ings based on the selective cultivation of useful, tun-able plant tissues using scalable, land-free techniques. By limiting biomass cultivation to only desirable plant tissues, ex planta farming promises to improve yields while reducing plant waste and competition for arable land. Employing a Zinnia elegans model system, we provide the first proof-of-concept demonstration of isolated, tissue-like plant material production by way of gel-mediated cell culture. Parameters governing cell development and morphology including hormone concentrations, medium pH, and initial cell density are optimized and implemented to demonstrate the tunability of cultured biomaterials at cellular and macroscopic scales. Targeted deposition of cell-doped, nutrient-rich gel scaffolds via injection molding and 3D bioprinting enable biomaterial growth in near-final form (Figure 1), reducing downstream processing requirements. These investigations demonstrate the implementation of plant cell culture in a new application space, propose novel methods for quantification and evaluation of cell development, and characterize morphological developments in response to critical culture parameters—illustrating the feasibility and potential of the proposed techniques.The proposed concept of selectively grown, tunable plant materials via gel-mediated cell culture is believed to be the first of its kind. This work uniquely quantifies and modulates cell development of cultured primary plant products to optimize and direct growth of plant materials."
Absolute Blood Pressure Waveform Monitoring using Philips Ultrasound Probe,"In an Intensive Care Unit (ICU), physicians use an inva-sive radial catheter to measure blood pressure (BP) to track the hemodynamic status of the subject, and these measurements are neither easy nor feasible to perform outside an ICU environment. In such non-ICU settings as a step-down clinical ward or an outpatient clinic, clinicians prefer to use a non-invasive arm-cuff device to measure BP. Even though these measurements are convenient, these devices cannot record the absolute BP (ABP) waveform. Hence, strong interest exists in developing a non-invasive device to monitor the ABP waveform as a quantitative option to perform rapid he-modynamic profiling of patients who cannot undergo invasive BP measurements. This project uses a Philips ultrasound-based transducer (XL-143) to measure BP from superficial arteries (carotid and brachial) proximal to the heart. We measure the arterial diameter and blood flow velocity waveforms from these arteries; an algorithm computes BP from this data in three stages, as illustrated in Figure 1. The algorithm uses the arterial area (A) calculated from arterial diameter and the blood flow velocity (F) waveforms to estimate the height of the ABP waveform, known as pulse pressure (PP), via standard fluid dynamics principles. Further, the algorithm uses a transmission line model of the human vasculature to estimate the mean arterial pressure (MAP)."
Electrochemical Neuromodulation Using Electrodes Coated with Ion-Selective Membranes,"Developing precise and effective means of modulating the nervous system is a major challenge in neural pros-theses. While modalities such as deep brain stimula-tion (DBS), vagus nerve stimulation, and electric acous-tic stimulation (EAS) for cochlear implants are finally being realized on the clinical level, there still remains work to be done with respect to our ultimate goal. In the Micro/Nanofluidic BioMEMS research group, we are developing a type of electrode modified with an ion-selective material that can change the concentra-tion of chemicals around a nerve, which will enhance the level of control compared to traditional electrical stimulation.A type of material called the ion-selective membrane (ISM) has been used in the field of analytical chemistry for decades to measure ion concentrations. These membranes are composed of a polymer matrix modified with a chemical called an ionophore, which makes them selective to a particular ion species. In work published by our group, the functionality of these electrodes was inverted, using them for electrochemical stimulation in ex vivo studies of a frog sciatic nerve (see Figure 1 from Song et al.). As a continuation of this work, we are: (1) developing computational models that describe and predict physical behavior of chemical transport from galvanostatic operation of polymeric neutral-carrier based ion-selective membrane electrodes, (2) fabricating and characterizing practical devicesfor implementing ISM-based neuromodulation(see Figure 2 from Flavin et al.), and (3) employingprototype devices in in vitro and in vivo animal models. A successful implementation of this work will pavethe way for more advanced operations such as centralnervous system (CNS) intervention."
Ultrasound-Based Cerebral Arterial Blood Flow Measurement,"Ultrasound-based cerebral blood flow (CBF) monitor-ing is vital in the diagnosis and treatment of a variety of acute neurologic conditions. While flow velocity can be measured using Doppler ultrasound, accurate CBF measurement is difficult as vessel diameters cannot be determined reliably due to acoustic aberrations in-troduced by the skull and because cranial attenuation necessitates low frequency (1-2 MHz) insonation with poor spatial resolution.We have developed a CBF estimation technique that achieves the spatial resolution required for CBF determination by estimating the point spread function of the imaging system. The received data are then deconvolved to increase spatial resolution, and a correction is applied to account for cranial aberrations. Doppler data were collected from phantom blood vessels with diameters between 2 and 6 mm over a 150-mL/min range using a clinical ultrasound device.Our method achieved an RMSE of 26 mL/min, withinacceptable range for cerebral perfusion monitoring atthe bedside."
Force-Coupled Ultrasound for Noninvasive Venous Pressure Assessment,"Congestive heart failure is a clinical syndrome that affects about 6 million people and accounts for about 1 in 9 deaths in the United States. In this condition, the pumping ability of the heart decreases, causing a buildup of blood volume and pressure in the venous system as it returns blood to the heart. This buildup further decreases the pumping ability of the heart by over-stretching its ventricles. Additionally, increased venous pressure can lead to fluid migrating from the veins to the interstitial space, which is called edema. Left unchecked, edema can lead to death. Proper ad-ministration of diuretic drugs can allow venous pres-sure to drop back down by lowering intravascular vol-ume, which will improve a patient’s condition. However, thus far, only invasive catheterization can produce an accurate and reliable venous pressure measurement. Our goal is to produce an accurate, noninvasive means of assessing venous pressure by means of force-coupled ultrasound. By positioning our force-coupled ultrasound probe at the base of the neck, we can observe the compression of the internal jugular vein, which returns blood from the cerebral vasculature to the heart. Unlike in the case of an artery, we can safely observe compression from zero force all the way to complete occlusion of the vein. We can also observe compression of the internal jugular vein while increasing its pressure with the Valsalva maneuver, exhalation against a closed airway, and while decreasing its pressure by elevating it above the supine position. We expect these observations to give us excellent insight for our computational models to accurately assess venous pressure."
An Electrokinetic-Based Concentrator for Ultra-Low Abundant Target Detection,"The recent COVID-19 outbreak has sparked urgent in-terest in rapid and reliable viral identification. In fact, this is a recurring challenge in many other pathogen detection and diagnostics, where only a few target vi-ruses or bacterial cells are present in milliliters or even liters of volume, necessitating that a large volume of the sample must be concentrated for the targets to be introduced into the downstream detection system. Un-fortunately, due to the size of the virus or biomolecule, concentrating or retrieving the virus or biomolecule with a filter, ultracentrifuge, or any kind of method is extremely hard. Figure 1 (a) shows the purpose of this work and the overall concept. This technology is based on microfluidic devices that couple microchannel and cation exchange membrane (CEM) to play an electrophoretic force off a hydraulic drag force to enable charge-based concentration, without any physical filter. Under the electric field, the virus experiences the electrophoretic force and hydraulic drag force at the same time. The electrophoretic force is driven by the intensive electric field focused near the CEM while drag force is driven simply by the hydraulic flow. Efforts are being made to build electrokinetic concentrators using materials and processes that are more robust and scalable than those of traditional microfluidics. Instead of using polydimethylsiloxane (PDMS) that is patterned using photolithography, one can laser etch channels and ports into acrylic polymethyl methacrylate (PMMA). Thin adhesive films can have custom patterns cut into them using a digital die cutter and then be used to bond PMMA layers and seal channels. Designing the device in manner seen in Figure 1b also allows the use of ion- exchange membranes that are commonly used in electrodialysis and fuel cell systems, meaning these materials are robust and relatively inexpensive."
Micro/Nanofluidic Technologies for Next-Generation Biomanufacturing,"Biomanufacturing of therapeutic proteins and vaccines is crucial for modern medicine. Recently, the biophar-maceutical industry started to focus more on process intensification through continuous biomanufacturing. New therapeutic modalities such as cell and gene thera-pies are rapidly emerging as well. Accordingly, it has be-come increasingly important for biomanufacturers to improve manufacturing efficiency, quality, and safety. Compared to conventional biomanufacturing technol-ogies, micro/nanofluidic technologies can contribute to the improvement with their unique advantages. Here, we introduce our new micro/nanofluidic technologies for efficient, high-quality, and safe biomanufacturing. First, we developed spiral microfluidic devices for reliable and efficient perfusion culture and adventitious agent (AA) clearance. The devices enable size-based cell sorting without any physical barriers, so that mammalian cells can be continuously separated from cell culture. Using this feature, the spiral device was used for 1) cell retention for perfusion culture and 2) rapid AA clearance (Figure 1). This microfluidic technology could overcome the limitations (biofouling, cell damage) of conventional cell separation techniques (e.g., membrane-based filtration, centrifugation).Second, we introduce a new nanofluidic device for monitoring critical quality attributes (purity, binding affinity, glycosylation, etc.) of antibody therapeutics during biomanufacturing. The device has a nanofilter array and enables continuous-flow size or charge-based protein separation. Using this device, we demonstrated a fully automated continuous online protein-size monitoring during continuous perfusion culture. We are currently expanding the capability of the nanofluidic device to monitor binding affinity and glycosylation of antibodies at real-time speed (Figure 2). The technology could complement conventional protein-quality-monitoring equipment while producing a large amount of information about biologics quality."
Measuring Eye Movement Features using Mobile Devices to Track Neurodegenerative Diseases,"Current clinical assessment of neurodegenerative dis-eases (e.g., Alzheimer’s disease) requires trained special-ists, is mostly qualitative, and is commonly done only intermittently. Therefore, these assessments are affect-ed by an individual physician’s clinical acumen and by a host of confounding factors, such a patient’s level of attention. Quantitative, objective, and more frequent measurements are needed to mitigate the influence of these factors. A promising candidate for a quantitative and accessible diseases progression monitor is eye movement. In the clinical literature, an eye movement is often measured through a pro/anti-saccade task, where a subject is asked to look towards/away from a visual stimulus. Two features are observed to differ significantly between healthy subjects and patients: reaction time (time difference between a stimulus presentation and the initiation of the corresponding eye movement) and error rate (the proportion of eye movements towards the wrong direction). However, these features are commonly measured with high-speed, IR-illuminated cameras, which limits accessibility. A portable measurement system is required to track them longitudinally. Previously, we enabled ubiquitous tracking of eye-movement features by enabling app-based measurements of visual reaction time and error rates. In this work, we further show how we learn potential trends in these eye-movement features using Gaussian process modeling. Such modeling has allowed us to discover subjects’ task-performing strategies such as trading off between speed and accuracy. We hope that once we have collected data from patients, we can use the model to a) compare the trends of the features with the clinical assessments, b) distinguish the effect of strategies from the effect of disease progression, and c) evaluate the potential to use our system to track disease progression more frequently and widely than previously possible."
A Comparison of Microfluidic Methods for High-Throughput Cell Deformability Measurements,"The mechanical phenotype of a cell is an inherent bio-physical marker of its state and function, with many applications in basic and applied biological research. Microfluidics-based methods have enabled single-cell mechanophenotyping at throughputs comparable to those of flow cytometry. As shown in Figure 1, we present a standardized cross-laboratory study com-paring three microfluidics-based approaches for mea-suring cell mechanical phenotype: constriction-based deformability cytometry (cDC), shear flow deforma-bility cytometry (sDC), and extensional flow deform-ability cytometry (xDC). All three methods detect cell deformability changes induced by exposure to altered osmolarity. However, a dose-dependent deformability increase upon latrunculin B-induced actin disassembly was detected only with cDC and sDC, which suggests that when cells are exposed to the higher strain rate imposed by xDC, cellular components other than the actin cytoskeleton dominate the response. The direct comparison presented here furthers our understand-ing of the applicability of the different deformability cytometry methods and provides context for the inter-pretation of deformability measurements performed using different platforms."
"Electronics for Transparent, Long-Lasting Respirators","The use of personal protective equipment (PPE), includ-ing the N95 respirators and surgical masks, is essential in reducing airborne disease transmission, particularly during the COVID-19 pandemic. Unfortunately, there has been a shortage of PPE since the beginning of the pandemic. Also, the available N95 masks have major limitations, including masking facial features, waste, and lack of integrity after decontamination, forcing re-searchers to find alternatives.This work presents a transparent, elastomeric, adaptable, long-lasting respirator with an integrated biometric interface. The mask is made mostly of silicon rubber and comes with two replaceable filter cartridges. The electronic interface uses one of the filter insert locations to measure temperature, humidity, pressure, and air quality. The system uses Bluetooth Low Energy and sends real-time sensor data to a phone or a computer. The data can be used to inform the user regarding mask fit, fatigue, mask condition, and potential diagnostic information."
Self-Editing or “Lamarckian” Genomes Using the Bio/Nano TERCOM Approach,"Gene editing has been an area of active investigation for many decades. Some approaches introduce per-manent edits; others modify expression. In this work, conceptually, cells or cell-free reactions estimate their location by correlating the evolution of their sensed fluid environment (e.g., temp., salinity, sugar, pH, ion concentration, etc.) against an embodied map and then self-edit the content of their genomes in a way that depends on said estimate; editing the genome shifts the expressed phenotype and the heritable genotype. This approach is related to terrain contour matching (TERCOM), a technique used in air navigation. Current efforts focus on a reaction mixture containing a plas-mid that experiences path-dependent self-edits while en route to a target site. As envisioned (see Figure 1), a read-only so-called “junk DNA” segment of a plasmid transcribes into mRNA strands having coding heads and consumable tails; the tails are attacked by an ex-onuclease, the activity of which depends jointly on re-moved monomer species and local ion concentration (or another environmental variable), causing the tails to function as path-sensitive fuses and the mix of sur-viving mRNA to depend on the path. The surviving mRNA is reverse-transcribed into DNA and integrated as expressible genes in a read-write portion of the plas-mid; concurrent random erasures keep overall length roughly constant. In this process, the genetic composi-tion of the read-write region evolves with the changing environmental path. A related heritage effort explores drug delivery using particles that exhibit path-depen-dent doses or conformation. The current and heritage efforts build on prior study by the PI and his group of nanoparticles that record the trajectory of their environment. An experimental apparatus has been designed to test these various TERCOM-like reaction mixtures. Progress on the present effort may allow the engineering of organisms that exhibit Lamarckian evo-lution or gene therapies that confer this ability."
Balancing Actuation Energy and Computing Energy in Motion Planning,"Inspired by emerging low-power robotic vehicles such as insect-size flyers, high-endurance autonomous blimps, and chip-size satellites, we identify a new class of motion-planning problems in which the energy consumed by the computer while planning a path can be as large as the energy consumed by the actuators during the execution of the path. Figure 1 shows how the energy to move one meter on various low-powered robotic platforms is of a similar magnitude to the ener-gy to compute one second on various embedded com-puters. As a result, minimizing energy requires mini-mizing both actuation energy and computing energy since computing energy is no longer negligible. Figure 2 shows average actuation energy and computing energy curves for a selected robotic platform and a computing platform. Here, minimizing only actuation energy, as is conventionally done, does not minimize total ener-gy. Instead, stopping computing earlier and accepting a higher actuation energy cost for a lower computing energy cost minimizes total energy.We propose the first algorithm to address this new class of motion planning problems, called Computing Energy Included Motion Planning (CEIMP). CEIMP operates similarly to other anytime planning algorithms, except that it stops when it estimates that while further computing may save actuation energy by finding a shorter path, the additional computing energy spent to find that path will negate those savings. We evaluate CEIMP on realistic computational experiments involving 10 MIT building floor plans, and CEIMP outperforms the average baseline of using maximum computing resources. In one representative experiment on an embedded CPU (ARM Cortex A-15), for a simulated vehicle that uses one Watt to travel one meter per second, CEIMP saves 2.1-8.9x of the total energy on average across the 10 floor plans over the baseline, which translates to missions that can last equivalently longer on the same battery."
Absolute Blood Pressure Measurement using Machine Learning Algorithms on Ultrasound-based Signals,"Hypertension, or high blood pressure (BP), is a major cardiovascular risk factor. Therefore, measuring BP is of significant clinical value. At present, there are a few disadvantages for devices that measure a patient’s BP. For instance, in an Intensive Care Unit (ICU), physicians use an invasive radial catheter to measure BP, which is not feasible outside an ICU. In non-ICU settings, clini-cians use a non-invasive arm-cuff device to measure BP. This is convenient but can provide only a systolic and a diastolic pressure value and does not output the abso-lute BP (ABP) waveform. These devices also neglect the dynamic nature of the arterial system as they do not measure the morphology of the BP waveform, which may contain information on the underlying patho-physiology.In this work, we propose a non-invasive way to get BP waveform with blood flow velocity and arterial area obtained from non-invasive ultrasound signals. One key drawback of the ultrasound-based device is that the output BP waveform has an arbitrary reference, so we have to estimate the mean arterial pressure (MAP). We propose to use a machine learning model containing 1D convolution and Transformer encoder layers to regress the MAP accurately. The input features are arterial area, flow velocity, and several other scalar features such as pulse wave velocity and pulse pressure. They are first embedded into a 512-dimension vector. Then, the convolution layers perform feature extractions, and a transformer models the relationship between time steps. We perform the training on the Pulse Wave Database (PWDB) synthetic dataset and test on seven real patients. The model provides accurate results, with mean absolute error 2.6 mmHg and std 2.1 mmHg. This algorithm has large potential to make affordable BP waveform measurements accessible to everyone."
Analytical and Numerical Modeling of an Intracochlear Hydrophone for Fully Implantable Assistive Hearing Devices,"Cochlear implants with fully implantable microphones would allow directional and focused hearing by taking advantage of ear mechanics. They would be usable in almost all environmental conditions throughout the day and night. Current implantable microphones suf-fer from unstable mechanics, poor signal-to-noise ratio (SNR), and low bandwidth.In this work, we used analytical modeling, a finite element model, and experiments to design a polyvinylidene (PVDF) intracochlear hydrophone for high-bandwidth sensitivity, surgical viability, and improved SNR by electrical shielding and circuit design. Our analysis shows that the copolymer PVDF-TrFE should be used due to its higher hydrostatic sensitivity, the area of the sensor should be maximized to maximize gain, and the length should not exceed a maximal value determined by the bandwidth requirement. A short-circuit topology charge amplifier maximizes the SNR of the sensor by minimizing noise and attenuating electromagnetic interference by shielding. These advances in sensor performance bring fully implantable systems closer to reality."
Fluorescent Janus Droplet and Its Application in Biosensing of Listeria Monocytogenes,"Dynamic complex droplets afford versatile platforms for biosensing. The biosensing methods based on drop-lets enable a combination of advantages including speed, cost-effectiveness, and portability. This research explores a sensing method based on the agglutination of Janus emulsions for Listeria monocytogenes, which is a gram-positive bacterium and is responsible for a potentially lethal foodborne bacterial illness. We create a bio-recognition interface between the Janus emul-sions that comprises equal volumes of hydrocarbon and fluorocarbon oils in Janus morphology by attach-ing antibodies to a functional surfactant polymer with a tetrazine/trans-cyclooctene (TCO) click reaction. The Listeria antibodies would be on the surface of the hy-drocarbon hemisphere since the surfactant will stay at the interface of the hydrocarbon and water phase. Ag-glutinations of Janus droplets are formed when Liste-ria is added because of the strong binding between Listeria and the Listeria antibody located at the hydro-carbon surface of the emulsions. By incorporating one emissive dye in the fluorocarbon phase and a blocking dye in the hydrocarbon phase of Janus droplets, we conduct a two-dye assay, which enables the rapid detec-tion of trace Listeria in two hours via an emissive sig-nal produced in response to Listeria binding. To clarify, the Janus structures are tilted from their equilibrium position as a result of the formation of agglutinations and produce emissions that would ordinarily be ob-scured by a blocking dye. Overall, this method not only provides rapid and inexpensive Listeria detection with high sensitivity but also can be used to create a new class of biosensors by connecting with other related recognition elements."
Dance-Inspired Investigation of Human Movement,"This research focuses on efforts to formalize a dancer’s approach to movement. The overarching hypothesis is that dancers stabilize their joints through stretches – which are observed during common activities such as walking and running. However, most untrained indi-viduals are able to apply this form of stabilization only during activities such as walking that seemingly “just happen,” much as we “see.” In contrast, the best dancers and athletes are able to generalize this stretch-based joint stabilization beyond walking to their art form. To understand how dancers organize movement through stretches, the researchers use motion tracking and elec-tromyography. This work will potentially benefit sever-al fields, including soft robotics, neuroscience, and AI."
Nanoscale Insights into the Mechanisms of Cellular Growth and Proliferation,"The growth and proliferation of human cells are con-trolled by the large molecular machine called mTORC1 that acts as a molecular equivalent of an AND logic gate. mTORC1 integrates multiple environmental sig-nals, such as nutrients and growth factors, and orders the cell to either grow and divide in times of plenty or stand by and recycle when nutrients are scarce. Using electron cryomicroscopy, we revealed how mTORC1 recognizes nutrient signals, which provided a na-noscale-precision blueprint for the design of therapies aimed at deregulated mTORC1 in diseases of cellular growth, such as cancer."
A Polarization-Encoded Photon-to-Spin Interface,"The central goal of quantum communication is to de-liver quantum information in a way that is resilient against eavesdropping. One notable approach is the measurement-device-independent quantum key dis-tribution (MDI-QKD) protocol, in which a secret key is shared between two parties connected by quantum and classical channels. Essential to this architecture, however, is the ability to faithfully transfer quantum states between two distant qubits. Here, we propose an integrated photonics device for mapping qubits encod-ed in the polarization of a photon onto the spin state of a cavity-coupled artificial atom: a “polarization-encod-ed photon-to-spin interface” (PEPSI). We perform theo-retical analysis of the state fidelity’s dependence on the device’s polarization extinction ratio and atom-cavity cooperativity. Furthermore, we explore the rate-fidelity trade-off through analytical and numerical models. In simulation, we show that our design enables efficient, high-fidelity photon-to-spin mapping."
Reliability of AlInGaP-on-Si Light-Emitting Diodes,"Micro-sized light-emitting diodes (μLEDs) are emerg-ing candidates for next-generation microdisplays. To achieve high resolution, it is preferable to integrate red, green, and blue self-emissive LEDs with a Si-com-plementary metal-oxide-semiconductor (CMOS) driv-er within a single die using a monolithic CMOS-com-patible process. Therefore, fabrication of AlInGaP (for red emission) and InGaN (for blue and green emission) LEDs directly on Si substrates is of great interest. We have reported on the reliability of InGaN on Si LEDs in previous years. Similar to InGaN on Si, the mismatch in lattice constant between AlInGaP and Si is large, so it is very challenging to grow high-quality AlInGaP on Si. AlInGaP layers on germanium-on-insulator (GOI) on Si substrates with a threading dislocation density of ~1.2x10-6 cm2 have recently been made using wafer bonding and layer transfer techniques. We have conducted constant current stressing of AlInGaP-on-Si LEDs made using this process by measuring the light intensity over time. Four stages of degradation of the light emission were observed (Figure 1(a)), and the degradation was seen to be non-uniform across the devices (Figure 2). The rate and degree of degradation are seen to be strongly dependent on the stressing current. The initial increase of light emission in stage I is due to the carbonization of organic hydrocarbon residues. These carbonized residues enhance current spreading and therefore increase the light emission. The stage II and III degradation is caused by the oxidation of the top C-doped p-GaAs layer by organic residues. No structural degradation is observed in the multiple quantum well layers. Finally, as the oxidation increases the contact resistance, the applied voltage also increases to keep the stressing current constant, leading to the avalanche breakdown of the contact, which is indicated as stage IV in Figure 1."
Enhancing SiN Waveguide Optical Nonlinearity via Hybrid Gallium Sulfide Integration,"Silicon nitride (SiN) has become an increasingly preva-lent material platform for integrated photonic circuits. SiN enables low-loss waveguides, and it is transparent in both visible and near-infrared (0.25 – 4.3 ). In spite of its small nonlinear index, SiN has been a par-ticularly popular platform for chip-based nonlinear photonic applications, such as supercontinuum gener-ation and frequency combs. However, if SiN’s nonlin-ear index could be increased, the intensity threshold for these useful nonlinear processes could be further reduced. The group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te) are a class of layered van der Waals materials with strong second- and third-order optical nonlinearities. Gallium sulfide (GaS) in particular has a bulk bandgap of 2.5 eV, large enough to make multi-photon absorption at telecom wavelengths negligible. In this work, we create hybrid waveguides that benefit from the low-loss processing of SiN and the large non-linear index of the group-III monochalcogenides.Mechanically exfoliated GaS crystals are transferred onto planarized SiN microring resonators. Figure 1a shows an optical microscope image of a uniform GaS flake fully covering a microring resonator and its coupling region. As the simulated optical TE mode profile in Figure 1b shows, due to the large refractive index (2.6) of GaS, the mode from the SiN core is drawn into GaS. To characterize the nonlinear properties of our hybrid waveguide structures, we measure all-optical cross-wavelength modulation in the microring resonator. We measure enhanced all-optical modulation from the GaS up to 10 MHz (limited by equipment) and measure its nonlinear index to be 10 times larger than that of SiN. This work shows the potential for future incorporation of the group-III monochalcogenides in hybrid waveguides for enhanced optical nonlinearities."
Waveguide-Integrated Mid-Infrared Photodetection Using Graphene on a Scalable Chalcogenide Glass Platform,"The development of compact and fieldable mid-infra-red (IR) spectroscopy devices represents a critical chal-lenge for distributed sensing with applications from gas leak detection to environmental monitoring. Green-house gases in particular represent an opportunity for IR gas sensing technology, as many of them are rela-tively inert and cannot be detected by chemical means. Recent work has focused on mid-IR photonic integrat-ed circuit (PIC) sensing platforms and waveguide-in-tegrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus, and tellurene. However, material bandgaps and reliance on SiO2 substrates limit operation to wavelengths λ < 4 μm, whereas the main absorption peaks of the most potent greenhouse gases occur at longer wavelengths. Here we overcome these challenges with a chalcogenide glass-on-CaF2 PIC architecture incorporating split-gate photothermoelectric graphene photodetectors, shown in Figure 1. Figure 2 plots the photovoltage map of our device, with a maximum responsivity of 1.5 V/W. Our design extends operation to λ = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz1/2 with no fall-off in photoresponse up to f3dB = 1 MHz and a predicted 3-dB bandwidth of f3dB > 1GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications."
Imaging Transparent Objects through Dynamic Scattering Media Using Recurrent Neural Networks,"Transparent objects in biological imaging and X-ray im-aging are imaged by solving inverse problems based on their diffraction intensity patterns. However, the scat-tering process induced by their complex interiors com-plicates inverse problems with a severity depending on the statistics of the refractive index gradient and contrast profiles. Recently, static neural networks were used to retrieve original information from the scatter-ing. Here, we propose a novel dynamical machine learn-ing approach to image phase objects through dynamic diffusers. The motivation of this study is to accommo-date the input with spatiotemporal dynamics, such as a temporal recording of time-varying scattering pro-files. This dynamical machine learning architecture is adopted to strengthen and exploit the correlation among adjacent scattering patterns during the train-ing and testing processes. To impart dynamics, we pro-pose a simplified dynamical model as follows. We use the on-axis rotation of a diffuser and utilize multiple speckle measurements from different angles to form a sequence of images for training. Our recurrent neu-ral network (RNN) architecture effectively discards any redundancies and enhances/filters out the static pattern, that is, the quantitative phase information of transparent objects. This method is also applicable to other imaging applications that involve any other spa-tiotemporal dynamics."
Field-Based Design of a Resonant Dielectric Antenna for Coherent Spin-Photon Interfaces,"We propose a field-based design for dielectric antennas to interface diamond color centers in dielectric mem-branes with a Gaussian propagating far field. This an-tenna design enables an efficient spin-photon interface with a Purcell factor exceeding 400 and a 93% mode overlap to a 0.4 numerical aperture far-field Gaussian mode. The antenna design with the back reflector is ro-bust to fabrication imperfections, such as variations in the dimensions of the dielectric perturbations and the emitter dipole’s location. The field-based dielectric an-tenna design provides an efficient free-space interface to closely packed arrays of quantum memories for mul-tiplexed quantum repeaters, arrayed quantum sensors, and modular quantum computers."
Strategies for High-Performance Solid-State Photon Upconversion Based on Triplet Exciton Annihilation,"Photon upconversion, a non-linear optical process to convert low-energy photons into higher energies, has various applications such as photovoltaics, infrared sensing, and bio-imaging. In particular, upconversion based on triplet exciton annihilation is one of the most promising approaches to achieve high efficiency at low excitation intensity for practical applications. Howev-er, the reported performance in solid-state is limited due to energy back transfer, materials aggregation, and weak optical absorption, which complicates the inte-gration with solid-state applications (Figure 1).Here, we propose strategies to improve the performance in solid-state via device structure engineering. In a green-to-blue upconverter consisting of a bilayer of an absorbing and an upconverting material, we reduce energy back transfer by inserting a blocking layer in between and mitigate aggregation by doping the absorber into a host material. The upconversion efficiency had a 7-fold enhancement, with the excitation intensity reduced by 9 times (Figure 2a). To improve optical absorption, we investigate an infrared- to-visible upconverter and integrate the upconverting layers into a Fabry-Pérot microcavity. At the resonant wavelength, infrared absorption increases 74-fold, and the threshold excitation intensity for upconversion is reduced by two orders of magnitude to a sub-solar flux (Figure 2b). Our work demonstrates the importance of device structure engineering to improve the performance of solid-state photon upconversion and offers a path towards practical applications."
Magnet Field-Switchable Laser via Optical Pumping of Rubrene,"Optical imaging of magnetic fields is used in spintron-ics, magnetic resonance imaging, and radiology. Most conventional approaches to magnetic field imaging rely on expensive crystalline materials or garnets, but the cost of these materials makes them poorly suited to high-area imaging. Magnetic sensing applications may benefit from cheaper magnetically active dyes. We demonstrate that the well-studied organic mole-cule rubrene can be used to spatially resolve magnetic fields. Furthermore, we report a 460% enhancement in rubrene brightness under a 0.4-T magnetic field in a first-of-its-kind magnetic field-switchable laser. We attribute the high magnetic sensitivity of rubrene to the magnetic field dependence of singlet fission, a pro-cess whereby one spin-singlet excitation splits into two spin-triplet excitations. These results suggest that rubrene—and other organic molecules that exhibit singlet fission—are promising candidates for low-cost, high-sensitivity magnetic imaging."
Multiplexed Raman Sensors Using Swept-Source Excitation,"Spontaneous Raman spectroscopy is routinely used in pharmaceutical production, chemical analysis, and the semiconductor industry for characterization of struc-tural features, strain, and doping. Standard Raman systems require dispersive spectrometers and often specialized cooled charge-coupled device (CCD) detec-tors to compensate for the low signals, making them prohibitively expensive and bulky. In this work we in-troduce and demonstrate a novel Raman system archi-tecture using a swept-source laser excitation, replac-ing the spectrometer. The laser is delivered through optical fibers to custom-made Raman probes, which are designed to be compatible with either single-mode or telecom-standard multimode optical fibers. Each probe delivers the excitation light onto a sample and collects the Raman signal, which is then detected using a narrow optical filter in front of a room-temperature high-gain Si photodiode. With a standard telecom op-tical switch, we can multiplex up to 16 channels and deploy remote probes using an optical fiber network. As an initial proof-of-concept, we present the spectra collected with our probes for both solid polystyrene and liquid urea solutions and further show that the ac-quired spectra have signal-to-noise ratios comparable to that collected with our lab-built bench top Raman system. We believe this new system, in which a single tunable laser can serve a distributed sensor network, significantly reduces the space and cost of current spectrometer-based Raman systems and promotes the use of Raman for online process control."
Hafnia-Filled Photonic Crystal Emitters for Mesoscale Thermophotovoltaic Generators,"Thermophotovoltaic (TPV) systems are promising as small-scale, portable generators for power sensors, small robotic platforms, and portable computational and communication equipment. In TPV systems, an emitter at high temperature emits radiation that is then converted to electricity by a low- bandgap photo-voltaic cell. One way to increase both TPV power and efficiency is to use two-dimensional, hafnia-filled-and-capped tantalum photonic crystals (PhCs); they enable spectral tailoring of thermal radiation for a wide range of angles. However, two key features are hard to real-ize simultaneously: a uniformly filled cavity and a thin capping film. Cavity-filling leads to a capping film that is both thick and uneven, so that trying to thin the film removes hafnia from within the cavity. Here, we pres-ent a method to reduce the film roughness and better control the thickness. Improved PhCs can pave the way toward high-performance TPV micro-generators for off-the-grid applications."
High-Performance Non-Mechanically-Tunable Meta-Lens,"Optical metasurfaces, i.e., ultra-thin arrays of sub-wave-length antennae, have enabled a new range of photonic devices with unprecedented functionalities in sculpt-ing wavefronts and a substantially reduced form-fac-tor. Recently special interest has been drawn to a class of so-called “active metasurfaces,” whose optical prop-erties can be modulated post-fabrication by non-me-chanical effects. A variety of tuning mechanisms have been harnessed; however, demonstrated meta-optical devices often incur narrow tuning ranges and low op-tical efficiencies. Here, we implemented an active var-ifocal meta-lens based on phase-change materials that offers 1) aberration-free performance across arbitrary optical states; 2) extremely low crosstalk of nearly 30 dB; and 3) considerably enhanced focusing efficiency exceeding 20% in both states with a clear pathway for further improvement. This advancement will further unveil a new cohort of exciting applications of active metasurfaces in imaging, sensing, display, and optical ranging."
GaN µLEDs for Microsystem Optical Communications,"Electronic systems smaller than 50 µm are promising for ubiquitous sensing; however, wireless communica-tion with such systems is challenging since radio-fre-quency communication is inefficient at the micron scale. Small length scales motivate the use of optical communications for micro-devices, which must be low-power due to the size constraint on solar cell sur-face area. Here we present an analysis of blue GaN microLEDs (µLEDs) for optical communications with 50 x 50 µm2 sensor microsystems called SynCells. We analyzed µLEDs with sizes from 5 x 10 µm2 up to 150 x 150 µm2, developing a test setup that can detect an LED driven by only 1 nW (Figure 1). We found higher external quantum efficiency (EQE) for larger µLEDs; also, EQE increased with current density up to a peak value, after which we observed an efficiency droop resulting from Auger recombination. GaN µLEDs operating at maxi-mally efficient current density will be able to produce detectable optical signals at sufficiently low power for practical use in SynCells."
Large-area Optical Metasurface Fabrication using Nanostencil Lithography,"Optical metasurfaces promise optical components with on-demand control of light and reduced size, weight, and power (SWaP) compared to their bulk counter-parts. However, fabrication of metasurfaces in the opti-cal spectral range often relies on electron beam lithog-raphy due to the high resolution requirements, which makes fabrication scale poorly with the device dimen-sions. Recently, deep ultraviolet (DUV) lithography has been validated as a scalable manufacturing route for optical metasurfaces. However, DUV lithographic fab-rication requires significant capital investment and is also limited to standard materials and processes avail-able in foundries.We are developing nanostencil lithography as an alternative technique for scalable, versatile, and rapid prototyping of metasurface devices. Nanostencils are nano-scale shadow masks, which allow repeated fabrication of a pattern via any anisotropic deposition process once the nanostencil is made. Previous research that used nanostencil lithography required only deposition of very thin layers (100 nm or less) through the nanostencil, while transmissive dielectric metasurfaces require significantly thicker layers, especially as the wavelength of light increases. Previous nanostencils were also limited to 1 mm by 1 mm in size. We improved previous processes for fabricating and using nanostencils, increasing the yield of nanostencil fabrication by not fully etching through the nanostencil membrane before the KOH etch and improving the consistency of metasurface fabrication by developing a resist-based spacer layer. Figure 1 outlines the resulting processes.To show the effectiveness of nanostencil lithography for large-area optical metasurface fabrication, we used 2 mm by 2 mm nanostencils to fabricate 1.5-mm-diameter PbTe metalenses on CaF2. The lenses showed diffraction-limited focusing, with a representative focal spot shown in Figure 2, and focusing efficiencies comparable to efficiencies reported in state-of-the-art large-area dielectric metalenses."
The Effect of O:N Ratio in a HfOxNy Interlayer on Triplet Energy Transfer In Singlet-Fission-Sensitized Silicon,"With the climate changing, the Sun is a promising source of renewable energy. However, silicon photovol-taics, the current industry standard, are approaching their theoretical efficiency limit. A proposed method to exceed this limit is to sensitize silicon with a ma-terial that undergoes singlet exciton fission, a carrier multiplication process with the potential to reduce thermalization losses by creating two excited electrons from a single photon. Successful transfer of these two electrons to silicon could result in increased photo-current and improved device efficiency. Recently, our group demonstrated the first proof-of-concept solar cell incorporating tetracene as a singlet fission material to produce additional carriers that are transported to silicon via a thin layer of hafnium oxynitride (HfOxNy). With the aim of improving singlet-fission-sensitized silicon solar cells, our research focuses on understand-ing what properties are necessary for the transport layer between singlet fission materials and silicon and the mechanism for the energy transfer process. In this work, the defect density of a HfOxNy interlayer is varied by changing the oxygen-to-nitrogen ratio, and different interlayer thicknesses are studied to examine the role of defect states on energy transfer from tetracene to silicon. The transfer efficiency is inferred via magnetic field modulation of the silicon photoluminescence. The results form a preliminary basis for unravelling the ex-citon transfer mechanism, which will subsequently be studied using time-resolved spectroscopy. Ultimately, knowledge of interlayer material properties and in-sights into the mechanism of energy transfer to silicon may inform the design of sensitizing layers for silicon and pave the way to commercializing the use of singlet fission to boost silicon photovoltaic efficiencies."
LION: Learning to Invert 3D Objects by Neural Networks,"Non-destructive three-dimensional imaging is import-ant to establish the internal structure of a 3D object non-invasively. In our work, objects of interest are in-tegrated circuits (ICs). This operation requires suffi-cient measurements for computational reconstruction, such as measurements from different angles or depths. Meanwhile, if the required acquisition time is too long, the operation may become impractical, not to mention the risk of instabilities or even damage to the samples from X-ray radiation. Reducing the number of angular views and the radiation dosage per view can be used to limit beam exposure, but low-dose data acquisition schemes yield noisy measurements that significantly reduce the quality of the reconstructed image.Our goal in this project is to reduce the acquisition time by a factor of 10 100 through augmenting image reconstruction with machine learning. In the LION approach, we embed the physics of X-ray propagation and interaction with matter into the learning process. This improves both the fidelity and the efficiency of learning. We study two types of information-starved 3D imaging: limited-angle tomography and low-dose tomography. A limited-angle tomography combined with an advanced ptychography technique, achieves high-resolution (10 nm) reconstruction with the advanced technique of recurrent neural network and generative adversarial network (see Figure 1a, b). On the other hand, low-dose tomography suffers from shot noise as the photon budget reduces (<50 photons per ray). Regardless of 20 of reduction in photon budget for 50 photons case, the physics-assisted machine learning still reconstructs ICs with high fidelity (Test PCC: 0.80) (see Figure 1c)."
Light Sources and Single-Photon Detectors in Bulk CMOS,"Silicon photonics realized in complementary metal-ox-ide semiconductor (CMOS) processes has transformed computing, communications, sensing, and imaging. Here, we demonstrate a chip-to-chip fiber optic link that implements both the light source and detector in bulk CMOS. A high-brightness infrared light emission in forward bias for a silicon p-n junction is implement-ed in an open-foundry CMOS process – 55 BCDLite. Emission intensity of 50 mW/cm2 light at a wavelength of 1020 nm is realized at room temperature by using a deep vertical junction with lightly doped rings.An infrared-enhanced single photon avalanche di-ode (SPAD) was designed in the same process and used to collect the 1020-nm light emission. We find that a de-tector using the p-substrate as part of the p-n junction achieves good detection (6% quantum efficiency for a 20-µm diameter device) even at a wavelength of 1020 nm. This device has a breakdown voltage of -24V and can be operated in Geiger mode to achieve photon de-tection at low light levels. The capabilities of the two devices combine to demonstrate a complete chip-to-chip optical interconnect utilizing only silicon CMOS devices."
Inference of Process Variations in Silicon Photonics from Characterization Measurements,"Silicon photonics, which manipulates photons instead of electrons, shows promise for higher data rates, low-er-energy communication and information processing, biomedical sensing, and novel optically based function-ality applications such as wavefront engineering and beam steering of light. In silicon photonics, both elec-trical and optical components can be integrated on the same chip, using a shared silicon integrated circuit (IC) technology base. However, silicon photonics does not yet have mature process, device, and circuit variation models for the existing IC and photonic process steps; this lack presents a key challenge for design in this emerging industry.Our goal is to develop key elements of a robust design for manufacturability (DFM) methodology for silicon photonics. One of the key steps for the goal is to find the distribution map of process variation in the ac-tual fabrication, which is usually inferred from well-de-signed test structure measurements.In this work, we develop a Bayesian-based meth-od to infer the distribution of systematic geometric variations in silicon photonics that aims to reduce the extraction error caused by measurement noise. We ap-ply this method to characterization data from multiple silicon nitride ring resonators with different design parameters and produce the estimated spatial map of device geometric variations (e.g., waveguide width, Si3N4 on SiO2 thickness, partial etch depth), as shown in Figure 1. Our results show that this characterization scheme can serve as a good test structure for process variation inference. Since characterization measure-ments are commonly used for device optimization de-sign, our method provides an efficient alternative ap-proach to study process variation in silicon photonics without requiring separate or replicated test structure design and thus facilitates the design of high-yield sili-con photonic circuits in the future."
Integrated Photonics and Electronics for Chip-Scale Control of Trapped Ions,"Trapped atomic ions are promising candidates for quan-tum information processing and quantum sensing. Current state-of-the-art trapped-ion systems require many lasers and electronics to achieve precise timing and control over quantum states. Usually, electronic signals are sent into vacuum chambers via wire feed-throughs, and laser light is focused down to a trapped ion’s location with external lenses mounted outside viewports on the chamber. These requirements lead to dense and complex setups that may be prone to drift and limit the amount of control that can be achieved.We have made recent progress toward integrating control technology into the substrate of the ion trap itself. By using a planar trap design, which is compat-ible with lithographic fabrication, we may implement other well-developed processes in order to enhance the function of the ion trap. In one experiment, we demon-strate an ion trap with integrated, complementary met-al-oxide-semiconductor-based high-voltage sources that can be used to control the motional frequency and position of a trapped ion. In another demonstration, we use photonic waveguides and diffractive grating couplers to route light around a chip and focus it onto ions trapped above the surface.Integrating controls into ion traps has the potential to increase the density of independently controllable ions on a chip in next-generation systems, but there are also many immediate practical benefits. Reducing the number of required feedthroughs allows chambers to be made more compact, which may be useful for ion-based clocks or sensors. We also show that integrated-photonic platforms help to reduce vibration-induced noise seen when using external optics, which may enable portable systems based on trapped-ion quantum information processing."
Lithiation Mechanisms of Si and Ge Thin Film Battery Electrodes,"Thin film batteries (TFBs) made using complementa-ry metal-oxide-semiconductor- (CMOS) compatible materials and processes can be integrated with CMOS circuits and energy harvesting and sensing devices to produce low-cost autonomous sensors with small form factors. As part of our research on CMOS-compatible Li-ion TFBs, we are studying Si and Ge films to be used as anode electrodes. While these materials have the highest known charge capacity (8300 mA/cm3 for Si and 7300 mA/cm3 for Ge), they tend to have poor reli-ability (low cyclability) due to mechanical failures as-sociated with large volume changes. The mechanisms through which lithium is stored in these materials are also poorly understood but are known to be related to poor cyclability. We have carried out mechanistic stud-ies of reversible lithium storage in Si and Ge films us-ing both electrochemical and physical characterization techniques.Figure 1 shows current-voltage measurements made during the first three lithiation/delithiation cycles of a 315-nm-thick amorphous Si film (a cyclic voltammogram, CV). The current corresponds to Li being stored in the electrode (lithiation) or removed (delithiation). Peaks in these curves indicate accelerated charging or discharging associated with phase transitions, all of which are between amorphous phases with different stoichiometries (increasingly Li-rich for lithiation). An irreversible transition is seen in the first cycle (peak 1 in Figure 1), and two reversible transitions are seen in all three and all subsequent cycles (peaks 2-2’ and 3-3’). Through new potentiostatic and transmission electron microscopy techniques, we have established that the irreversible transition occurs through propagation of a reaction through the thickness of the film (Figure 2b) and that the reversible transitions occur through an amorphous-to-amorphous nucleation and growth process (Figure 2b), sometimes referred to as a polyamorphous phase transition. In similar experiments on Ge, we have focused on the reversible transition of a Li-rich amorphous phase to a crystalline phase, which also occurs through a nucleation and growth process. These studies have been correlated with the reliability of TFBs."
Gated Nonreciprocal Magnon Transmission from Direction-Dependent Magnetic Damping,"An important application of magnetic materials in information technology is to provide nonreciprocity, which allows unidirectional signal transmission. A rep-resentative device is the two-terminal microwave isola-tor. A ferromagnet inside naturally breaks the time-re-versal symmetry and allows microwave transmission only from port 1 to port 2, while signals from port 2 to port 1 are suppressed. Despite wide applications, these conventional nonreciprocal devices suffer from their bulk volume and the difficulty of being integrated into high-density circuits. Nowadays, new mechanisms that can provide passive and directional isolation of signals are being pursued at sub-micrometer scale. Among var-ious proposals, magnons, the quanta of the collective excitation of magnetic moments, show unique poten-tial due to the tunability and the possibility for on-chip integration. So far, nonreciprocal magnon transmission has been achieved only at resonant conditions with gigahertz frequency. It is unclear if nonreciprocity can still be observed for magnons with a broad spectrum up to terahertz frequency.Here we show that using a magnetic gate, one can realize tunable nonreciprocal propagation in spin Hall effect-excited incoherent magnons, whose frequency covers the spectrum from a few gigahertz up to terahertz. We further identify the direction-dependent magnetic damping as the dominant mechanism for the nonreciprocity, which originates from the interlayer dipolar coupling and works both in the ballistic and diffusive regions of magnons. As a natural result of the chiral magnon-magnon coupling, our findings provide a general mechanism for introducing directional magnon transmission and lead to a design of passively gated magnon transistors for applications of information transmission and processing."
Gigahertz Frequency Antiferromagnetic Resonance and Strong Magnon-Magnon Coupling in Layered Crystal CrCl3,"Magnon-magnon hybrid systems have recently been realized between two adjacent magnetic layers, with potential applications to hybrid quantum systems and coherent information processing. Realizing mag-non-magnon coupling within a single material requires antiferromagnetic (AFM) or ferrimagnetic materials with magnetic sublattice structures. However, con-ventional AFM resonance lies in terahertz frequencies, which require specialized techniques to probe. In this work, we realize strong magnon-magnon coupling within a single material, CrCl3. CrCl3 is a layered van der Waals AFM material, with parallel intralayer alignment and antiparallel interlayer alignment of magnetic moments (Figure 1). Because of weak anisotropy and interlayer magnetic coupling, we observe both optical and acoustic modes of AFM resonances within the range of typical microwave electronics (<20GHz), in contrast to conventional AFM resonances. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be realized between the two resonant modes (Figure 2). Our results demonstrate strong magnon-magnon coupling within a single material and establish CrCl3 as a convenient platform for studying AFM dynamics in microwave frequencies. Because CrCl3 is a van del Waals material that can be cleaved to produce air-stable monolayer thin films, these results open up the possibility to realize magnon-magnon coupling in magnetic van der Waals heterostructures by symmetry engineering."
"Nanoparticle-Enhanced Microsputtered Gold Thin Films for Low-Cost, Agile Manufacturing of Interconnects","Silicon and gold are ubiquitous in the microelectronics industry—silicon as the cornerstone of semiconduc-tor devices, and gold as a material with unmatched electro-optical properties. However, gold films do not adhere well to silicon or silicon dioxide, necessitating the need for an adhesion layer made of a third material. This need increases complexity and cost. Also, rework-ing interconnects via traditional (cleanroom) technolo-gy poses challenges, e.g., thermal budget, vacuum com-patibility. In this project, we explore microplasma sputtering to implement at low-cost interconnects for agile electronics. We have shown that under proper operational conditions, a microplasma sputterer creates at room temperature and atmospheric pressure dense, highly conductive gold films with a fivefold better adhesion than the state of the art, without using an adhesion layer, annealing, or any other pre/post printing steps. If the gold film is sputtered in an atmospheric-pressure microsputterer in the presence of a fast-moving jet of air, gold nanoparticles form. The high collisionality of the atmospheric-pressure gas and high energy of the plasma facilitate nanoparticle formation, while the jet carries the nanoparticles to the substrate. The speed of the jet of air determines the size of the nanoparticles. These nanoparticles then act as an adhesion layer to allow a gold film, made of these nanoparticles and individual atoms, to adhere well to a silicon or silicon dioxide substrate. By rastering the printhead over the desired deposition area, we can interweave large nanoparticles and smaller atoms, creating a dense film (Figure 1). This process allows us to optimize adhesion, density, and conductivity simultaneously. Conductivity of the resultant films is also near-bulk (120% of bulk gold—the highest value reported for a room-temperature additive manufacturing method), allowing for their use in microelectronics."
Large-Scale 2D Perovskite/Transition Metal Dichalcogenide Heterostructure for Photodetector,"Monolayer transition metal dichalcogenides (TMDCs) have been attractive nanomaterials for optoelectron-ics due to their extremely high quantum efficiency, but their atomically thin thickness prevents them from ab-sorbing sufficient light for optoelectrical applications.To improve the optoelectrical performance of TMDCs, during the past years, 2D Ruddlesden-Popper perovskites (PVSKs)/TMDC heterostructures have been demonstrated. Thanks to their high absorption coefficient, long diffusion length of charge carrier, sharp exciton emission, and high power conversion efficiency, 2D PVSKs have been used as an absorption layer for TMDCs. However, 2D PVSK/TMDC heterostructures are limited in the micrometer scale, since 2D PVSKs have been fabricated only by the tape-exfoliation method. We reported a layer resolved splitting (LRS) technique to isolate multilayer 2D materials into a monolayer in wafer scale in 2018. To improve the scalability of 2D PVSKs for large-scale application, in this work, we successfully split micrometer-thick 2D PVSKs into nanometer-thick and millimeter-width scale with the LRS technique. We also obtained 90% of photoluminescence quantum yield, which is the world’s best record to the best our knowledge. Then we plan to demonstrate large-scale 2D PVSK/TMDC heterostructures for photodetectors, which only has been previously demonstrated up to micrometer scale."
Grayscale Stencil Lithography,"In this work, we demonstrate a new eBeam evaporation method with fixed single stencil shadow mask to gener-ate 2D patterns with spatial thickness variation across wafer-scale substrate. This method outperforms con-ventional approaches like the iterative photo-lithogra-phy-and-lift-off method or grayscale photolithography, due to their limitations of efficiency, material choices, and manufacturing complexity. We applied the method to create a multi-spectral reflective color filter arrays with two layers of variable thickness. It offers a broad-er design space to achieve a wide color spectrum with simple and efficient fabrication procedures. This meth-od shows potential for scaling up and high-resolution patterning, which could be widely applied in manufac-turing for optical imaging, sensing, and computing. The method takes inspiration from the “pin-hole imaging” in optics to generate the convoluted pattern of the material source and the stencil shadow mask, as shown in Figure 1. The ejection of materials in the eBeam pockets is analogous to the “light source,” which passes thorough the pin holes on the shadow masks to finally cast the “image” in terms of deposition thickness. By controlling the deposition dose (T) and the tilting angle (θ) of the substrate, we could create the “point spreading function” (PSF) of the material target passing through the shadow mask, which can create smooth deposition with less than 5-nm surface roughness. As show in Figure 2, we applied this method to create a multi-spectral color filter array and a 2D pattern of the MIT “Dome” to demonstrate the capability of this method for customizable patterning. Higher resolution deposition is also possible by combining the available micro/nano stencil lithography techniques and our grayscale stencil lithography method."
Unraveling the Correlation between Raman and Photoluminescence in Monolayer Molybdenum Disulfide through Machine Learning Models,"Two-dimensional (2D) transition metal dichalcogenides (TMDs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelec-tronic and photonic applications such as light-emitting diodes, photodetectors, and single-photon emitters. Among the standard characterization tools for 2D ma-terials, Raman spectroscopy stands out as a fast and non-destructive technique capable of probing mate-rials’ crystallinity and perturbations such as doping and strain. However, the correlation between photo-luminescence and Raman spectra in monolayer MoS2 remains elusive due to its highly nonlinear correlation. Here, we systematically explore the correlation be-tween PL signatures and Raman modes through ma-chine learning models. First, we adopt a convolution neural network, DenseNet, to predict PL by spatial Ra-man maps with relatively small pixel dimensions but deep channels. Moreover, we apply a gradient boosted trees model (XGBoost) with the Shapley additive expla-nation (SHAP) to evaluate the impact of individual Ra-man features in PL behavior, which allows us to further link the strain and doping of monolayer MoS2 with its PL behavior. Our analytical method unravels the non-linear correlations of physical or chemical properties for 2D materials and provides the knowledge for tun-ing and synthesizing 2D semiconductors for high-yield photoluminescence."
Additively Manufactured Electrospray Ion Thrusters for Cubesats,"Putting satellites in orbit is very expensive: typical rocket launches cost up to hundreds of millions of US dollars, and typical per-kilogram of payload costs are up to tens of thousands of US dollars). Therefore, great interest exists to develop smaller, lighter, and cheaper space satellites with adequate performance. In partic-ular, since the 1990s, research groups across the world have been developing and launching cubesats, i.e., 1-10 Kg, a few L in volume, miniaturized, mission-focused satellites. Multi-material additive manufacturing is of great interest for fabricating cubesats, as it can mono-lithically create complex, multi-functional objects composed of freeform components made of materials matched to performance.Electrospray engines produce thrust by electrohydrodynamically ejecting charged particles from liquid propellant. Electrospray thrusters are an attractive choice for propelling cubesats because their physics favors miniaturization, e.g., their start-up voltage scales with the square root of the emitter diameter. The thrust of an electrospray emitter is very low; thus , electrospray engines have large arrays of emitters to greatly boost the thrust they can deliver.We recently demonstrated the first additively manufactured electrospray engines. Our devices are composed of large arrays of conical emitters coated by a conformal forest of zinc oxide nanowires (ZnONWs) that transport the propellant to the emitter tips (Figure 1). The ZnONWs provide a large hydraulic impedance that regulates and uniformizes the flow across the emitter array, restricting the flow rate per emitter to attain ionic emission. Our devices are also remarkable because, unlikely all the other electrospray ionic liquid engines reported in the literature, they emit only ions using the ionic liquid EMI-BF4 as propellant (Figure 2), which maximizes their specific impulse for a given bias voltage, i.e., they produce more thrust per unit of propellant flow rate. Current work focuses on optimizing device design and fabrication and on developing a multi-electrode stack to control the plume."
Low-Temperature Growth of High Quality MoS2 by Metal-Organic Chemical Vapor Deposition,"The large-area and high-quality synthesis of molybde-num disulfide (MoS2) plays an important role in real-izing industrial applications of flexible, wearable, and ultimately scaled devices due to its atomically thin thickness, sizable bandgap, and dangling-bond-free interface. However, currently used synthesis of MoS2 by chemical vapor deposition (CVD) require high tem-perature and a transfer process, which limits its utili-zation in device fabrications. In this work, we achieved the direct synthesis of high-quality monolayer MoS2 by metal-organic chemical vapor deposition (MOCVD) at a low temperature of 320oC by designing the exper-imental setup for better controlling the flow rate of the organic precursors. Large single-crystal monolayer MoS2 with a domain size up to 120 μm can be obtained on SiO2/Si substrate (Figure 1). Owing to the low sub-strate temperature, the MOCVD-grown MoS2 exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced elec-tronic performance with high electron mobility of 68.3 cm2 V-1s-1 at room temperature. In addition, we propose a model to quantitatively analyze the shape change of the MoS2 flakes grown under different conditions, which provides an insight into the growth mechanism for optimizing growth conditions."
Self-Assembly via Defect-Mediated Metal Nanoisland Nucleation on 2D Materials,"Patterning point defects, nanopores, and nanoribbons can enhance (opto-)electronic properties of two-di-mensional (2D) materials. Moreover, metal adatoms and small clusters can nucleate on point defects in 2D materials. This nucleation suggests that defect pattern-ing may be used for templated self-assembly of metal nanoislands on 2D materials, enabling applications in plasmonics and single-photon emission. Focused ion beams (FIBs) are well-suited for patterning 2D materi-als with nanometer precision and can be used for the controlled creation of point defects and sub-10-nm fea-tures. For applications that require control of the loca-tions of metal islands, the optimization of FIB irradia-tion parameters for metal nucleation is crucial. In this work, we study the structural changes that arise from FIB patterning of suspended 2D materials and the influence of patterning on metal nucleation and growth. We calibrate the irradiation parameters to achieve patterning with minimal damage to the 2D material, and the features are characterized by scanning transmission electron microscopy (STEM) (Figure 1a). Using these patterned 2D materials, we study the extent to which the defects, ion species, dose rate, and sample thickness affect the nucleation and growth of metals. Figure 1 shows representative results after the deposition of Au. At high deposition amounts, Au forms small islands around graphene nanopores, indicative of defect-mediated nucleation (Figure 1b). The templating and nucleation control presented here can be generalized to anchor other materials on 2D materials, such as Si and Ge via chemical vapor deposition or other metals via thermal and e-beam evaporation. This strategy opens routes towards the directed self-assembly of semiconducting and metallic nanoislands on 2D materials with optimized charge transfer and strong light-matter interactions."
Strain Control of Nanocatalyst Synthesis,"A central theme in renewable energy technologies to-day is designing nanostructured catalysts for desired reactions. Exsolution generates stable and catalytical-ly active metal nanoparticles via phase precipitation out of a host oxide. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This strong metal-oxide interaction makes the exsolved nanoparticles more resistant against particle agglom-eration than the infiltrated ones. While exsolution is an exciting and promising pathway for generating stable oxide supported nanoparticles, rational control over the exsolved particles has yet to be achieved. In par-ticular, controlling the size and density of the exsolved nanoparticles remains a big challenge.In this work, we propose point defect formation in the oxide lattice to be the fundamental knob to control exsolution and demonstrate this approach in epitaxial La0.6Sr0.4FeO3 (LSF) thin films. By combining in-situ surface characterization and ab-initio defect modeling, we show oxygen vacancy and Schottky defects to be the primary points defects formed upon Fe0 exsolution. Lattice strain tunes the formation energy, and thus the abundance of these defects, and alters the amount and size of the resulting exsolution particles. As a result, the tensile strained LSF with a facile formation of these critical point defects results in a higher Fe0 metal concentration, a larger density of nanoparticles, and reduced particle size at its surfaces. These observations highlight the critical role of point defects in controlling the size and density of the exsolved nanoparticles on the perovskite surface. The strain-controlled synthesis of nanocatalysts can benefit a wide range of applications in clean energy conversion and fuels generation such as solid oxide cells (SOCs), chemical looping (CL), and ceramic membrane reactors (CMRs)."
Controlled Cracking to Improve Mechanical Stability of RuO2 Thin-Film Li-ion Electrodes,"Thin film Li-ion batteries are of interest for low-cost autonomous sensors. We have investigated high-per-formance electrode materials, such as Si and Ge for an-odes and RuO2 for cathodes, that can reversibly store high concentrations of Li. RuO2 is of particular interest as a cathode material because of its ability to revers-ibly store high concentrations of Li without requir-ing high-temperature processing, unlike conventional cathode materials. While high Li capacities are benefi-cial for high energy density, high Li concentrations lead to large volume changes, which can lead to mechanical degradation during battery cycling. In particular, re-moval of Li (delithiation) leads to tensile stresses that can cause cracking and delamination of electrodes, which can severely limit the number of times that bat-teries can be charged and discharged. Motivated by the finding that patterned small patches of Si demonstrated higher mechanical stabil-ity compared to continuous films, patterned arrays of holes with stress-raising corners were fabricated with-in sputtered RuO2 thin films (Figures 1a and b). After lithiation and delithiation, channel cracks form along the directions defined by the hole array (Figure 1c). We found that this method for controlled crack formation led to increased mechanical stability, as no delamina-tion occurred within the patterned area (Figure 2, right side), while severe delamination occurred in the unpat-terned areas (Figure 2, left side). These results may oc-cur because the formation of the controlled crack array dissipates the strain energy that would otherwise drive delamination.It was further discovered that the formation of cracks was reversible. After re-lithiation, the RuO2 patches expanded, and the channel cracks closed again (Figure 1d). The sizes of the channel cracks were con-trolled by the state of charge of the film. In addition to use for mechanical stabilization of thin film electrodes, this process has potential application for creation of channel networks with electrochemically modulated channel sizes, which might be of use in microfluidic de-vices."
Seeing Superlattices: Imaging Moiré Periods at the Nanoisland-2D Material Interface Using Scanning Transmission Electron Microscopy,"Opportunities are emerging to combine van der Waals (2D) materials with (3D) metals /semiconductors to explore fundamental charge-transport phenomena at their interfaces and exploit them for devices. Recent advances in scanning transmission electron microsco-py (STEM) allow detailed analysis of atomic structure, properties, and ordering at these interfaces. We use 4D STEM and integrated differential phase contrast (iDPC) to directly image moiré periodicities arising from epitaxial growth of nanoislands on 2D materials in ultra-high vacuum. Our research explores the role of emerging microscopy techniques in unveiling the alignment and ordering of moiré superlattices and the implications of moiré periodicities for the properties of 2D/3D junctions."
Small-Molecule Assemblies Inspired by Kevlar: Aramid Amphiphile Nanoribbons,"Small-molecule self-assembly offers a powerful bot-tom-up approach to producing nanostructures with high surface areas, tunable surfaces, and defined in-ternal order. Historically, the dynamic nature of these systems has limited their use to specific cases, especial-ly biomedical applications, in solvated environments. Here, we present a self-assembling small-molecule platform, the aramid amphiphile (AA), which over-comes these dynamic limitations. AAs incorporate a Kevlar-inspired domain within each molecule to pro-duce strong interactions between molecules. We ob-serve that AAs spontaneously form nanoribbons when added to water with aspect-ratios exceeding 4000:1. Ro-bust internal interactions suppress the ability of AAs to move between assemblies and result in nanoribbons with mechanical properties rivaling silk. We harness this stability to extend small-molecule assemblies to the solid-state for the first time, forming macroscopic threads that are easily handled and support 200 times their weight when dried. The AA platform offers a nov-el route to use small-molecule self-assembly to achieve aligned nanoscale materials in the solid-state"
"Magnetically-assisted Assembly, Alignment, and Orientation of Micro-scale Components","The use of magnetic forces to improve fluidic self-assembly of micro-components has been investigated using Maxwell 3D to model the forces between Ni thin films on semiconductor device micro-pills and Sm-Co thin films patterned on target substrates [1]. Orienting and restraining forces on pills far in excess of gravity are predicted, and it is found that the fall-off of these forces with pill-to-substrate separation can be engineered through the proper design of the Sm-Co patterns to retain only properly oriented pills[1],[2].Micro-scale hybrid assembly is a potentially important way of doing heterogeneous integration, i.e., of integrating new materials on silicon integrated circuits to obtain functionality not readily available from silicon device structures alone, and fluidic self-assembly is an attractive way to automate micro-scale assembly. A serious limitation of fluidic self-assembly, however, is the lack of a good method for holding properly assembled components in place and accurately positioned until all of the components have been assembled and permanently bonded in place. We have shown, based on our modeling, that suitably patterned magnetic films can be used to provide the forces necessary to retain, and to accurately orient and position, assembled micro-components.Our motivation for pursuing micro-scale hybrid assembly is our general interest in doing optoelectronic integration, specifically of vertical cavity surface emitting lasers (VCSELS), edge-emitting lasers (EELs), and light emitting diodes (LEDs), with state-of-the-art, commercially processed Si-CMOS integrated circuits. Our ongoing research integrating these devices on silicon described elsewhere in this report provides the context for this work and illustrates the types of applications we envision for magnetically assisted self-assembly using the results of this study.Assembly experiments to verify and demonstrate the theoretical predictions are currently in progress using two sizes of 6-µm-thick pills (50 µm by 50 µm and 50 µm by 100 µm) and a variety of magnetic thin film patterns. Recesses with different dimensions are also being studied[2]."
Batch-Microfabricated RPA for Ion Energy Measurements,"Plasma diagnostic tools are required in numerous fields, from experimental physics to aerospace and beyond. Various sensors exist to capture plasma saturation current and plasma potential, and will infer other properties through theory. The retarding potential analyzer (RPA) is device that will directly measure ion energy, a property of interest monitoring exterior craft conditions at hypersonic speeds. Through microfabrication, our device expands the present state-of-the-art to achieve improved mechanically enforced grid alignment, while maintaining the required micron-scale features.Advantages of enforcing alignment across successive RPA electrodes was already demonstrated in a hybrid device yielding a threefold increase in signal amplitude[1]. In utilizing microelectromechanical system (MEMS) batch-fabrication techniques, alignment precision is refined to the order of 1μm[2]. This MEMS RPA exemplifies the same modularity as the hybrid device, such that grids may be interchanged within the same housing, and otherwise incompatible fabrication techniques might be used. Figure 1 shows the schematic of a complete sensor and the corresponding six-wafer housing stack. Alignment in the assembled device is enforced by curved silicon springs in the housing (Figure 2) which allow for a slight mismatch in nominal and actual grid and housing dimensions, and can accommodate changing dimensions due to thermal expansion. Another advantage of microfabrication is the batch-processing of devices in an effort to drive down costs. MEMS RPA housings are manufactured 30 devices at a time.Measurements with our MEMS RPA have shown an additional jump in peak signal strength resulting in an order of magnitude increase over its conventional counterpart[3]. In using thicker silicon electrodes over mesh grids, the device is expected to be more robust when exposed to harsher environments. Finally, if batch-fabrication can lead to a more widely available ion energy sensor, this device could find application in monitoring micromachining processes in-situ."
Micron- and Submicron-thick Parylene Substrates for Transfer Printing and Solar Applications,"Transfer printing of thin metal films enables the fabrication of both planar and suspended membrane electrodes for microelectromechanical (MEMS) sensors and actuators in an additive process. In addition, transfer-printed metal films can be used to form abrupt junctions between organic and metal layers. In contrast, conventional deposition processes such as evaporation or sputtering can cause the metal atoms to penetrate into underlying organic layers.We have developed a solvent-free transfer printing method using micron- or submicron-thick films of a robust, flexible polymer, parylene-C, as a carrier membrane. The transparent parylene films are initially deposited by chemical vapor deposition onto a rigid or semi-rigid handle substrate for ease of handling during fabrication. Metal and/or organic layers are subsequently deposited and patterned on top. The entire stack is then peeled away from the handle in a continuous sheet and transferred to the receiving substrate. After transfer, the ultrathin parylene carrier may be left in place or removed by oxygen plasma.Using this method, we have demonstrated electrostatically-actuated gold membrane-covered cavity arrays for microspeakers with larger areas (>1cm2) than previously possible using a solvent-assisted contact transfer printing method[1]. The solvent-free method has also been employed to deposit metal electrodes on top of solvent-sensitive organic layers in metal-molecule-metal structures for tunneling nanoelectromechanical switches[2].Parylene films can also serve as ultrathin, lightweight substrates for organic photovoltaics (OPVs). Their chemically inert and insoluble nature enables the use of common vapor- and solution-phase methods for thin-film deposition, including thermal evaporation and layer-by-layer spin-casting. Since the full active layer stack in an OPV is itself less than a micron thick, moving to thinner, lighter substrates could significantly reduce total weight and cost[3]. Although our current devices absorb strongly in the visible wavelength range, further materials and architecture engineering could yield a fully transparent solar cell on parylene with exceptional flexibility and versatility for optoelectronic applications."
Tunneling Nanoelectromechanical Switches Based on Organic Thin Films,"With the silicon-based electronics reaching physical limits that inhibit continued improvements in device performance, much research has been directed towards nanoelectromechnical (NEM) switches as a promising alternative. NEM switches exhibit abrupt switching behavior and near-zero leakage current but suffer from large actuation voltages and failure due to stiction[1][2][3]. To overcome these challenges, this work presents a NEM switch, squitch, that operates based on electromechanical modulation of tunneling current through a nanometer-thick organic thin film (Figure 1). The switching process is initiated by the electrostatic compression of the organic film sandwiched between conductive contacts. As the organic layer is compressed, the tunneling distance is reduced, leading to an exponential increase in the tunneling current. The presence of the organic layer prevents direct contact between the electrodes. Furthermore, when the electrostatic force is removed, the deformed organic layer provides the restoring force necessary to displace the top electrode to the off-state position, helping to mitigate stiction.Theoretical analysis of two- and three-terminal devices suggests the possibility of achieving large on-off current ratio exceeding 6 orders of magnitude, sub-1-V actuation voltage, and nanosecond switching time. Various fabrication techniques have been explored to form the metal-organic-metal junction with the organic layer created mainly through use of a self-assembled monolayer of thiolated molecules. While the bottom metallic contact is commonly formed through thermal evaporation, the top electrode can be fabricated through evaporation, nanotransfer printing of a metallic thin film, or assembly of graphene sheets. An example of a metal-organic-metal junction fabricated through nanotransfer printing of a gold (Au) top electrode is shown in Figure 2. In addition to device fabrication, our current work is focused on investigating the electromechanical response of organic-based tunneling junctions through simultaneous use of electrical and optical measurements."
Hydrophobic Rare-earth Oxide Ceramics with Applications to Sustained Dropwise Condensation,"Hydrophobic materials that are robust to harsh environments are needed in a broad range of applications[1][2][3]. Although durable materials such as metals and ceramics, which are generally hydrophilic, can be rendered hydrophobic by polymeric modifiers[4], these materials deteriorate in harsh environments. Here we show that a class of ceramics comprising the entire lanthanide oxide series, ranging from ceria to lutecia, is intrinsically hydrophobic (Figure 1)[5]. We attribute their hydrophobicity to their unique electronic structure, which inhibits hydrogen bonding with interfacial water molecules. We also show with surface-energy measurements that polar interactions are minimized at these surfaces and with FTIR/GATR that interfacial water molecules are oriented in the hydrophobic hydration structure. Moreover, we demonstrate that these ceramic materials promote dropwise condensation, repel impinging water droplets, and sustain hydrophobicity even after exposure to harsh environments[5]. These ceramics can also be used to fabricate superhydrophobic surfaces using various techniques. As an example, we fabricated superhydrophobic surfaces by sputtering a thin layer (~200-350 nm) of ceria onto nanograss-covered silicon microposts (Figure 2a). Water droplets display high contact angles (Figure 2b) and exhibit extreme mobility due to low contact angle hysteresis (<10°) on these surfaces. In addition, impinging water droplets completely bounce off the surface, leaving it dry (Figure 2c). Hence, we envision that this class of robust hydrophobic materials will have far-reaching technological potential in various industrial applications, where water repellency and dropwise condensation are desirable."
Electro-chemical Stimulation of Neuromuscular Systems Using Ion-selective Membranes,"Spinal cord injury (SCI) leads to paralysis, decrease in quality of life, and high lifetime medical costs. Direct nerve functional electrical stimulation (FES) induces muscles to contract by electrically stimulating nerves, and it shows promise for clinical applications in restoring muscle function in SCI. FES is limited by the lack of graded response in muscle contraction and by high fatigability due to the reversal of the order in which motor units are recruited. Previous work showed that ion-selective membranes can be used to modulate Ca2+ ions in-situ, decreasing the current threshold for nerve stimulation and eliciting a more graded muscle contraction response1. This work has developed polyimide-based cuff ion-selective electrodes to enable the future application of this technique in-vivo. The developed electrodes are flexible, elastic, and conductive. In-vitro tests of the electrodes by stimulation of frog sciatic nerve reproduced the decrease in the stimulation current threshold, which had been observed in planar glass-based electrodes, in the flexible polyimide-based electrodes. Additionally, cuffing the stimulated nerves with ion-selective electrodes was more effective at decreasing current threshold than planar stimulation. This work also analyzed data on twitch width, contraction time, and relaxation time to infer effects of ion-selective electrodes on recruitment order. Stimulation with the ion-selective electrodes had higher twitch width, contraction time, and relaxation time than traditional electrical stimulation at all force levels. The difference was particularly high at low force levels, indicating an effect of calcium ion depletion on recruitment order[1]."
Designing Complex Digital Systems with Scaled Nano-electro-mechanical Relays,"Silicon CMOS circuits have a well-defined lower limit on their achievable energy efficiency due to sub-threshold leakage. Once this limit is reached, power constrained applications will face a cap on their maximum throughput independent of their level of parallelism. Avoiding this roadblock requires an alternative device with a steeper sub-threshold slope, i.e., lower VDD/Ion for the same Ion/Ioff. One promising class of such devices is electrostatically actuated nano-electro-mechanical (NEM) switches with nearly ideal Ion/Ioff characteristics. Although mechanical movement makes NEM switches significantly slower than CMOS, they can be useful for a wide range of VLSI applications if we reexamine traditional system- and circuit-level design techniques to take advantage of the electrical properties of the device.Basic circuit design techniques and functionality of some main building blocks of VLSI systems, such as logic, memory, and clocking structures, have been demonstrated in our previous works[1][2][3]. Recently, by employing pass-transistor logic design, we have designed and demonstrated complex relay-based arithmetic units such as multipliers (Figure 1b-c)[4]. Simulation results of an optimized 16-bit relay multiplier built in a 90-nm equivalent relay process model predict ~10x improvement in energy-efficiency over optimized CMOS designs in the 10-100 MOPS performance range. The relative performance of the multiplier enhancements are in line with what was previously predicted by a NEM relay 32-bit adder[3], suggesting that complete VLSI systems, such as a microprocessor, would expect to see similar energy/performance improvements from adopting NEM relay technology[3],[4].Since scaling is crucial for performance, energy, and total area improvement, we have developed a scaled version of the original relay, a 6-terminal relay, which is 25x smaller and offers enhanced functionality. The operation of the main building block of the NEM-relay based multiplier, the (7:3) compressor, built with these scaled devices is experimentally demonstrated. This circuit, consisting of 46 scaled relays, is the largest scaled relay circuit successfully tested to date (Figure 2)."
Microfluidic Electronic Detection of Protein Biomarkers,"Immunoassays use antibodies to detect protein biomarkers, with a substantial global market and significant importance for clinical practice. However, traditional immunoassays are performed in centralized laboratories using optical detection methods, which means that results take days and cannot be highly multiplexed, in turn increasing patient visits, healthcare costs, and decreasing healthcare outcomes. In our project, we are developing an all-electronic immunoassay. All-electronic immunoassays have three major benefits: 1) we can achieve high-throughput immunoassays, allowing us to detect over 100 proteins in a single sample and potentially even to measure all approved protein biomarkers; 2) we can reduce cost by taking advantage of decreasing costs of silicon electronics; and 3) patients can get results before meeting with their physicians, since impedance detection is much faster than optical readout.The basic idea of an all-electronic biosensor is that by looking into the impedance change caused by association of antigens to antibodies, we can determine if certain antigens are present in the blood. Our biosensor is illustrated in Figure 1. To facilitate the sensor design, we determined three parameters (detection sensitivity, response time and sensor dimension) to characterize the performance of the sensor. We built two models to gain insight into the relationship between the sensor design and performance. The transport model of molecular binding indicated that the response time is determined mainly by the height of the channel, and sensors with higher channels need more time to reach equilibrium but can have better signal-to-noise ratio (Figure 2a). The electric model of impedance change shows that the interfacial impedance determines the electric readout and that sensors with wide electrode can offer good signal-to-noise ratio but end up with large size (Figure 2b). Therefore, the three key performance parameters cannot be achieved simultaneously, and the sensor design needs to be optimized.Another critical issue is chemical modification of the electrode’s surface; the goal is to eventually immobilize probe proteins on the electrodes. To this end, we built alkanethiol self-assembled monolayers (SAMs)as the links between gold electrodes and probe proteins. Since creating SAMs is the first step of surface modification, it is critically important to have successful SAMs. We used different surface characterization techniques, including Fourier transform infrared spectroscopy and impedance spectroscopy. Both methods showed that the self-assembled monolayers have been successfully built (Figures 2c and 2d)."
Stretchable Pressure- and Shear-sensing Skin Printed from PDMS,"In the fields of robotics and prosthesis design, there is need for inexpensive, wide-area pressure- and shear-sensing arrays that can be integrated into a flexible and stretchable skin analog[1],[2]. This project seeks to meet this need by building combined pressure and shear sensors based on the well documented piezoresistive property of composites made from PDMS and carbon black (CB)[3],[4]. These sensors can be printed by extruding the uncured composite and can be molded directly into a solid PDMS membrane to form a sensing skin that is water- and heat-resistant, flexible, and stretchable. The sensor concept consists of three electrical contacts, arranged linearly, with the PDMS/CB composite deposited on top of them. When this arrangement is exposed to pressure, the resistance between the center contact and each of the edge contacts changes equally. However, when sheared, the resistance changes differently between the center contact and the two edge contacts. In this way the effects of pressure and shear can be separated; see Figure 1. In fabrication, a silver/PDMS composite is used to form the contacts, a PDMS/CB composite is used to form the sensors, and pure PDMS is used to form the bulk of the skin. Each of these materials is extruded onto a plastic membrane, which is removed after heat-curing. Due to the early point in development and fabrication, the sensors have excessive drift that makes quantitative measurements difficult. However, subtracting the resting voltages (representing the resistances of each half of the sensor) measured before shear or pressure is applied from those measured after their application and plotting these differences enables observation of the separation of pressure and shear; see Figure 2. At present this method works for a carefully selected area of the sensor and degrees of pressure and shear."
"Wide-bandwidth, Low-frequency, Low-g Piezoelectric MEMS Energy Harvesters","Our group proposes the piezoelectric energy harvester based on a nonlinear resonator to address the narrow bandwidth issue of conventional energy harvesters. The wide bandwidth and significantly higher power than previously reported devices have been proved by our experiments (Fig. 1), which showed a ~20% bandwidth and a 2W/cm3 power density. Increasing number of researchers have been exploring different nonlinear resonance based designs, including beam stiffening, asymmetry structure, buckled structure, and magnetic restoring force, to name a few. The Duffing mode resonance has been observed in diverse designs, and the wide bandwidth is their shared property. However, the theoretical proof of a wide bandwidth and high-power density energy harvester is not available. By building the electromechanical model for the nonlinear system, we have found the analytical solution of the output power of a nonlinear resonance based piezoelectric energy harvester and have proved a much wider power bandwidth can be obtained by varying electrical load, and the maximum power can be achieved by matching the electrical damping to mechanical damping”. By understanding the advantages of the nonlinear design, we are making new designs to bring it closer to real applications. The two gap between the nonlinear piezoelectric energy harvesters at MEMS scale now are, the operating frequency (>1000Hz) is usually much higher than the frequencies of ambient vibrations (~100Hz), and the excitation level when testing in laboratory (4~5g) is also higher than the common vibration’s amplitude (<0.5g). By optimizing the beam composition, tuning the proof mass and redesigning the beam geometry, we anticipate that our new design will work in the frequency range between 100Hz to 200Hz at the excitation of 0.5g. The new design will be fabricated and tested soon."
Design and Fabrication of Magnetically Tunable Microstructured Surfaces,"Micro- and nanostructured surfaces have broad applications ranging from liquid transport in microfluidics and cell manipulation in biological systems to light tuning in optical applications[1][2]. While significant efforts have focused on fabricating static micro/nanostructured arrays[3], uniform arrays that can be dynamically tuned have not yet been demonstrated. We present a novel fabrication process for magnetically tunable microstructured surfaces, where the tilting angle can be controlled upon application of an external magnetic field. We also demonstrate this platform for droplet and bubble manipulation in heat transfer applications.The tunable surfaces consist of ferromagnetic (Ni) pillars on a soft PDMS substrate. The pillars have a diameter of 23-35 µm, pitch of 60-70 µm, and height of 70-80 µm. We used vibrating sample magnetometry to obtain hysteresis loops of the Ni pillar arrays, which match well the properties of bulk Ni. With a field strength of 0.5±0.1 Tesla and a field angle of 60±15°, a uniform 10.5±0.5° tilting angle of the pillar arrays was observed. Meanwhile, simulations using Abaqus to determine the equilibrium positions of the pillars under different applied fields show good agreement with the experiments. We also investigated how these tunable pillars changed the contact angle of water droplets on the surface. An external magnetic field of 0.3 Tesla changed the water droplet contact angle by ~15°. Future work will focus on using these surfaces to actively transport water droplets and spread the liquid film via pillar movement. These tunable surfaces promise new fluid manipulation capability for applications in condensation, evaporation, and boiling."
Scalable 3-D Microelectrode Recording Architectures for Charac-terization of Optogenetically Modulated Neural Dynamics,"Optogenetics is commonly used for precision modulation of the activity of specific neurons within neural circuits[1], but assessing the impact of optogenetic neuromodulation on the neural activity of local and global circuits remains difficult. Our collaborative team recently initiated a project (Scholvin et al., SFN 2011) to design and implement 3-D silicon-micromachined electrode arrays with customizable electrode locations, targetable to defined neural substrates distributed in a 3-D pattern throughout a neural network in the mammalian brain and compatible with simultaneous use of a variety of existing light delivery devices.We have developed a series of innovations aimed at facilitating the scalability aspect of these probes – that is, aspects of probe design that should enable them to scale to 1000 channels of neural recording or more. First, we have developed streamlined electrode fabrication methodologies that enable micromachined probes to be first fabricated using conventional silicon micromachining and then rapidly assembled into custom 3-D arrays, with semi-automated formation of the necessary electrical connections and mechanical constraints. Second, we have developed a set of surgical and insertion technologies to enable the insertion of electrode arrays with a high number of electrode shanks into the brain, while minimizing probe insertion damage. Finally, to facilitate scaling of the channel count beyond what is feasible with external amplifiers, we are exploring new approaches for integrating amplifier circuits directly on the probe arrays themselves, to remove bottlenecks associated with connecting probes to the outside world."
Aligned CNT-based Microstructures and Nano-engineered Composite Macrostructures,"Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs.Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests[1]. Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest[2],[3], the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is typically an unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm2 Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive mechanical property testing is underway, including 3D constitutive relations and fracture toughness.Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies[4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in-situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup[5][6][7]. Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Extension of the CNT-based architectures to ceramic-matrix composites and towards multifunctional capabilities including structural health monitoring and deicing is underway."
Nanoporous Elements with Layer-by-layer Assembly in MEMS with a Focus on Microfluidic Bioparticle Separation,"We have integrated ultra-porous (99% porous) elements (nanoporous forests of vertically aligned carbon nanotubes (VACNTs)) in MEMS, showing their use in microfluidic applications for bioparticle isolation and health diagnostics. Distinct from works where the effects of fluids on VACNT elements resulted in either structural deformation or catastrophic forest collapse[1], our approach enables creation of high aspect ratio (~1-mm) nanoporous elements and preserves their shape under flow-through conditions. Figure 1 shows a device consisting of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enable flow around and through the VACNT elements, enhancing physical interaction between the particles in the flow and the functional elements. The large surface-to-volume ratio of nanoporous materials yields a significant increase in the functional surface area (~250-500X for the layouts analyzed in our works[2] ), with permeability comparable to that of macro-scale porous materials[3], thus further promoting bioparticle capture[4]. To utilize these attributes, assembly of polymer films on individual carbon nanotubes via layer-by-layer (LbL) techniques was explored. Conformal coating surrounding the VACNTs provides the opportunity to control intra-CNT spacing as well as surface functionality.Initial work on VACNT-LbL assemblies has been performed on various geometries at the same flow conditions. FITC-PAH-SPS at appropriate pH levels was used to perform LbL. Preliminary results indicate conformal coating on both the inside and outside surfaces of the VACNT wall elements. Effects of flow conditions, other polymer systems, and surface functionalization are topics of ongoing work. Optimization of functionalizing similar polymer films on flat surfaces has been performed, with the goal of applying the same surface chemistry to CNT microfluidic elements for bioparticle capture and manipulation."
Waveguide Micro-probes for Optical Control of Excitable Cells,"Optogenetics is the safe, effective delivery of light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively[1]. This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health and for studies of brain function and interconnectivity.We have developed multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired[2]. Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion (Figure 1a). Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage. We have recently created 2-D arrays of such probes (Figure 1b) so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at a resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics[3].The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns [2]. Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe (Figure 1c) functions as designed[2]. The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable. The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule. This allows for increased modularity and control in initial probe testing.We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping. Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and ultimately to enable a new generation of neural control prosthetics."
Development of Porous Piezoresistive Materials and Its Applications for Underwater Pressure Sensors and Tactile Sensors,"Microelectromechanical system (MEMS) pressure sensor arrays are gaining attention in the field of underwater navigation because they are seen as alternatives to current sonar and vision-based systems that fail to navigate unmanned undersea vehicles (UUVs) in dark, unsteady, and cluttered environments. Other advantages of MEMS pressure sensor arrays include lower power consumption and the fact that their passive nature makes them covert. This work focuses on the development of a flexible pressure sensor array for UUVs, where the sensor array is inspired by the ability of fish to form three-dimensional maps of their surroundings[1],[2]. Fish are able to decipher various pressure waves from their surroundings using the array of pressure sensors in their lateral line sensory organs that can detect minute pressure differences. Similarly, by measuring pressure variations using an engineered pressure-sensor array on the surface of an UUV, this project aims to aid UUVs in the identification and location of obstacles for navigation. The active material of the pressure sensor array is a porous polydimethylsiloxane (PDMS)-carbon black composite made out of a sugar sacrificial scaffold that shows great promise for satisfying the proposed applications. The proposed device structure is flexible, easily fabricated, cost efficient, and capable of being implemented on a large-area and curved UUV surface. Although hysteresis occurs during the electromechanical test, the piezoresistivity of this porous PDMS-carbon black composite is reversible and reproducible. Compared to its non-porous counterpart[3], this porous composite shows a six-times increase in piezoresistivity and a greatly reduced Young’s Modulus. When tested underwater, this porous composite was able to differentiate water waves that had a frequency of 1 Hz and 2 Hz, which is promising for its underwater application. This porous composite was also extended to the application of tactile sensors using a different device architecture, which showed excellent response under mechanical testing."
Continuous RBC Removal Using Spiral Channel with Trapezoidal Cross-section,"Red blood cells (RBC) are the most abundant cell component in many biological fluids, including blood, bone marrow aspirate, and peritoneal aspirate. Depletion of contaminating RBCs from those samples is often an indispensable sample preparation step before the application of any clinical and diagnostic tests[1], while avoiding artificial alteration on the phenotypes of sorted cells is an important criterion for all studies. The achievement of minimal artifact is especially important in the case of removing RBCs from human blood to isolate white blood cells (WBCs), which play a key role in carrying out and mediating the immune response to various pathogens. The information extracted from the isolated WBCs would be meaningful to facilitate disease prognosis only when the key features of WBCs’ original state are not masked by the sample preparation artifacts. However, several cases have been reported that the conventional methodologies for blood cell separation on the macroscale, including differential centrifugation and selective erythrocyte lysis, could result in altered imuno-phenotype[2] or impaired viability[3] of the isolated WBCs. Meanwhile, passive continuous microfluidic separation techniques utilizing the size-dependent hydrodynamic effects[4][5][6] have been considered as an alternative approach to bypass the issues associated with macroscale blood cell separation methods.In this work, we improved the separation resolution of curvilinear microchannel while maintaining the high-throughput feature by modifying the channel cross-section to be trapezoidal rather than rectangular and demonstrated its ability for efficient RBC depletion from human blood sample with negligible effect on polymorphonuclear leukocyte (PMN) immune-phenotype as compared to selective erythrocyte lysis method[7]. To our knowledge, this is the first experimental demonstration where the asymmetry velocity field within a trapezoid spiral channel affects the inertial focusing phenomenon, indicating the feasibility of using channel cross-sectional geometry (other than width and depth) as a parameter for optimization of a curvilinear inertial microfluidic sorter."
Mechanisms for Intrinsic Stress Evolution during Deposition of Polycrystalline Films,"Complex kinetic processes are involved during the growth of polycrystalline films, which is typically carried out far from equilibrium. Minor changes in processing conditions can lead to tremendous differences in surface morphology, grain structure, and residual stress in the films. This result strongly influences their performance and reliability in nano- and micro-electromechanical devices and systems (N/MEMS)[1]. Control of the residual stress is especially important in the devices based on micro-beam structures, such as electrically actuated switches and accelerometers. For example, for doubly-supported beams, an average compressive stress as small as 10 MPa can cause buckling, while a tensile stress can cause changes in the beam’s stiffness. Figure 1 shows examples of problems caused by residual stresses in released micro-structures and devices.Through in situ real-time measurements we have studied the intrinsic stress evolution of a number of materials at different homologous temperatures (deposition temperature divided by the melting temperature) for several materials (Figure 2). One general trend can be found: the stress becomes more compressive when the homologous temperature is higher. In particular, at intermediate homologous temperatures, the instantaneous stress changes from compressive to tensile during film thickening. Previous models[2][3][4] are inadequate to explain this transition behavior. By characterizing the film microstructure as a function of the film thickness, we conclude that this transition originates from the increase of grain size during film thickening, which has two consequences. First, it changes the bulk stress of the film during deposition and causes a tensile component of the instantaneous stress. Second, it changes the grain size dependence of the compressive component, the magnitude of which is controlled by the competition between adatom-2D island incorporation and adatom-GB incorporation."
MEMS Pull-in and Lift-off Simulation Using Continuation Methods,"The voltages at which MEMS actuators and sensors become unstable, known as pull-in and lift-off voltages, are critical parameters for almost any design. However, current general-purpose simulators compute these critical parameters by directly sweeping the voltage, leading to either excessively large computational cost or convergence failure near the instabilities. This work applies two kinds of continuation methods to simulate the pull-in and lift-off effects.The first algorithm uses arc-length continuation algorithm to compute the multiple static solutions of a given MEMS design. Using a tangent predictor and a correction scheme, a next solution point can be calculated based on the previous solution point. This method can efficiently avoid the convergence failures of Newton iterations when a direct sweeping method is applied to solve for the static solutions[1].The second algorithm uses a different idea to generate a single-solution curve. The basic idea is that we first apply arc-length continuation until a pull-in or lift-off point is approaching. After that, a homotopy method is applied to directly solve for the next point after a sharp transition of the solution curve.Both algorithms have been implemented in a commercial MEMS+IC co-design software package, and they have been tested by various industrial MEMS design cases. Figures 1 and 2 show that the simulation results from both algorithms are the same. These results are consistent with that from a commercial simulator, CoventorWare, which utilizes detailed but time-consuming finite-element and boundary-element analysis."
Deep Trench Capacitor Drive of Unreleased Si MEMS Resonator,"With frequency-quality factor products (f•Q) often exceeding 1013, MEMS resonators offer a high-Q, small footprint alternative to conventional LC tanks and off-chip crystals for clocking and wireless communication. Over the past three decades, much progress has been made in the key figures of merit of MEMS resonators including small footprint, high Q, low motional impedance, and efficient energy coupling kT2. In parallel, efforts have focused on system-level metrics including high yield, low cost, robustness, easy packaging, and integration with circuits. A key challenge in MEMS resonator design is to achieve high performance yet manufacturable devices. The unreleased deep trench (DT) resonators in this work address this challenge.Beyond the performance goals of high Q and low loss, these devices target two key features desired for monolithically integrated MEMS resonators. First, lithographic definition of resonance frequency enables a broad range of frequencies to be fabricated on a single chip. Second, unreleased bulk-acoustic resonators do not require any low-yield, complex steps to freely suspend the moving structure and are robust in harsh environments without packaging. Unreleased resonators such as the HBAR[1] have been demonstrated but have thickness-defined frequency. Lateral bulk acoustic resonators with lithographically defined frequency such as the LoBAR[2] have achieved high Q but require low d31 coupling to drive and sense resonance. Meanwhile, sidewall AlN resonators[3] excite lateral resonance with d33 coupling but still require a release step. This work provides the benefits of all of these devices with high Q, efficient dielectric transduction, lateral resonance, and no release step. The DT resonator implements deep trench capacitors as both electrostatic transducers and Acoustic Bragg Reflectors (ABRs), defined in a single mask and self-aligned (Figure 1). While ABRs provide acoustic isolation in a solid medium, the DT capacitors function as internal dielectric transducers[4], which have achieved the highest frequencies in Si to date[5]. A 3.3-GHz unreleased Si resonator is demonstrated with Q of 2057 and motional impedance RX of 1.2 kΩ (Figure 2). This realization of high-Q unreleased resonators in a bulk Si process provides a high yield, low cost, no packaging solution for on-chip clocking, wireless communication, and electromechanical signal processing."
Preventing Catastrophic Failures: Nano-engineered Multi-physics Materials for Structural Applications,"Catastrophic structural failures cause many physical and personal losses, with prevention estimated at billions of dollars in savings each year. Non-destructive evaluation (NDE) techniques have been pursued and employed for damage detection of such structures to detect cracks and other damage at pre-critical levels for remediation[1],[2]. Here, a novel multi-physics approach is reported that addresses drawbacks in existing techniques by taking advantage of the effects that damage, such as a crack, has on the electric and thermal transport in a material containing a CNT network distributed in the bulk material. When a potential is applied to a nano-engineered structure (see Figure 1), electric field lines concentrate in the vicinity of cracks as electrons flow around damage, causing field concentrations and “hot spots” via Joule heating, an effect which is amplified because the heat flow is also impeded in areas of damage (e.g., across a crack face). These changes of temperature can be localized through a conventional infrared thermal camera. Low power operation (a 9V standard battery is exemplary, providing a 15C rise at 1 Watt as in Figure 2) and high spatial resolution are demonstrated that are beyond state-of-the-art levels in non-destructive evaluation. Multiple applications have been identified using this technique such as crack detection in composite components that are joined by metallic fasteners, structures having internal nonvisible damage due to impact, and in situ progressive damage monitoring during a tensile strength test. The thermal nano-engineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring the next generations of safer infrastructure[3][4][5]. Further expansion on this work has yielded significant technologies in ice protection systems (IPS) for vehicle structures such as unmanned aerial systems (UAS)[6] with application to infrastructure needs such as wind turbines and bridges."
MEMS-enabled Tactile Displays for the Blind and Visually Impaired,"According to the World Health Organization, more than 285 million people have visual impairments worldwide, and 39 million of those are blind. About 20% of visual impairments cannot be prevented or cured; in these cases, assistive technologies are critical to enable independent integration into professional and social settings. There is a pressing need for technologies that enable the blind and visually impaired to acquire graphical information or navigate in unstructured environments. The purpose of this project is to enable compact, rapidly refreshable tactile displays that provide information in an intuitive format as part of a broader system for situational awareness, navigation, and perception of graphical information. The overall system is a collaborative effort among MIT’s Computer Science and Artificial Intelligence and Microsystems Technology Laboratories, and researchers from Northeastern University; the tactile display is the focus of this abstract.We are developing the scientific and engineering knowledge for high-resolution displays of rapidly-updatable, vibrating tactile elements, i.e., tactels (Figure 1), using a combination of macro- and microscale batch manufacturing techniques. The proposed architecture is entirely distinct from the conventional piezoelectric bending beams of refreshable Braille readers or the Optacon[1], as well as from the actuators of electroactive polymer displays[2]. Our tactels use structures that receive in-plane displacement from piezo beams to produce amplified out-of-plane displacements that can be sensed by human hands. Current work focuses on parametric multiphysics modeling of the tactels and development of the manufacturing process and assembly approach for the MEMS displays."
Erythrocyte Deformability Correlates to Intracellular Calcium Level,"Elevated intracellular calcium level ([Ca++]i) and reduced deformability in red blood cells (RBCs) are commonly associated with blood-related diseases[1][2][3] as well as in-vivo ageing[4]. The correlation between RBC deformability and [Ca++]i has been established at the bulk level, typically accompanied with changes in ATP level and RBC size[4]. It is, however, unclear whether changes in RBC deformability would correspond to [Ca++]i at the single-RBC level.In this project, we attempt to establish the connection between single RBC deformability and [Ca++]i using a microfluidic device as described[5]. Calcium ionophore A23187 was used for calcium loading, creating different levels of [Ca++]i. RBC deformability is assessed by measuring the microcirculatory velocity of RBCs in a microfluidic device with narrow gaps[5]. Simultaneous measurement of calcium intensity and transit velocity was performed while an individual RBC traversed the microchannels."
Thin Film Piezoelectric Micro-machined Ultrasonic Transducer for Medical Imaging,"Ultrasound is an attractive 3D medical imaging technique because it is relatively inexpensive, portable, compact, and non-invasive. However, for 3D real time imaging to be commercially realizable, scans must be consistent and high resolution and occur at a fast acquisition rate–all factors that are inhibited by the current bulk piezoelectric transducer technology[1]. Highly manual manufacturing limits the size of current transducers to millimeter length scales and the high acoustic impedance of the bulk piezoelectric limits resolution reducing bandwidth and sensitivity[2]. At high volume, micro-fabrication is high yield and less expensive, and it would enable element miniaturization for high resolution, small form factor ultrasound probes.Our group has designed a piezoelectric micro-machined ultrasonic transducer (pMUT) that transmits acoustic signals via high frequency vibrations of a thin diaphragm. These vibrations are actuated by applying a voltage across a thin film piezoelectric lead zirconate titanate (PZT) film deposited via a sol-gel technique. For sensing, acoustic waves reflected from an imaging target strain the diaphragm generating a current signal.Device fabrication begins with growth of thermal oxide on a silicon-on-insulator wafer. The bottom electrode is then deposited via a lift-off process, and PZT is deposited and patterned with a wet etch. The lift-off process is then repeated to create the top electrode. Finally, diaphragms are released and the substrate is divided into chips (Figure 2) with a back-side deep reactive ion etch. A schematic of the fabricated device is shown in Figure 1.Through electrode size optimization, our pMUT is designed to maximize deflection, which is ideal for generating the high acoustic pressure necessary to overcome signal attenuation in deep penetration imaging[3]. In the future, we hope to incorporate the optimized pMUT transducer design in pMUT arrays with a small form factor to enable 3D real time medical imaging."
Thermal Ink Jet Printing of CNT films,"Ink jet printing allows for rapid, scalable, and low-cost patterning process. It does not need a vacuum environment or toxic etching process, which facilitates the integration of ink jet printing into other micro-machining process. And ink jet printing can pattern on curved surface or 3D structure. The characteristics of this technique are advantageous in industry. Previously, we demonstrated highly repeatable and uniform PZT thin film with ink jet printing[1],[2]. With this know-how in PZT film printing, we apply carbon nanotubes to ink jet printing. Due to their mechanical, thermal conductive and electrical properties, carbon nanotubes find diverse applications. The percolation network of carbon nanotubes is a transparent and conductive material, and strong bonding among carbon nanotubes enables fabrication of a bendable, foldable, and stretchable electrode. Previous research from other groups fabricated an ink jet-printed carbon nanotubes conductor[3], but the performance was not good enough to replace the current ITO transparent electrode. In this research, we controlled the amount of carbon nanotube deposition by changing the pitch of the printed line, which determines conductivity and transparency. Electrodes of 30% transparency can be fabricated using 10-um pitch, which has 36.6 S/cm conductivity and 247 ohm/sq sheet resistance. Increasing the pitch size up to 20 um gives the electrode properties of 74% transparency, 30.8 S/cm conductivity and 1.1 Kohm/sq sheet resistance. This result shows better performance compared with previous research. Figure 1 shows the SEM image of fabricated electrodes depending on the pitch size. Printed film shows uniformly deposited carbon nanotubes percolation network within 30- um pitch size. In the future, we plan to produce devices that utilize the full capabilities of this process to achieve better performance in both conductance and transparency, which will enable applications such as touch panels, organic light-emitting diodes, and solar cells."
Switchable Piezoelectric Transduction in AlGaN/GaN MEMS Resonators,"High-Q MEMS resonators, with small footprint and monolithic integration, are excellent building blocks for configurable RF systems. While these resonators provide narrow bandwidth selectivity, broad-band operation typically requires a large bank of switchable devices. This bank introduces a large capacitive load at the input due to the drive transducers. Typically, piezoelectric resonators have strong electromechanical coupling coefficients enabling low loss filters. However, they must be switched in line of the RF signal, resulting in insertion loss and reduced power handling.This work presents a new implementation of piezoelectric transduction in an AlGaN/GaN heterostructure that enables on/off switching of transduction with DC voltage applied out-of-line of the RF signal and reduces the capacitive load of the resonator by >10× when in the off state. This transducer is formed in the AlGaN, between a top Schottky electrode and a 2D electron gas (2DEG) as a second electrode[1],[2] (Figure 1). When a negative bias of -7 volts is applied to the Schottky electrode, the 2DEG is depleted. The removal of this bottom electrode suppresses electromechanical transduction and serves to reduce the drive capacitance by >10×.Mechanical resonances can be detected with a passive transducer equivalent to the drive, or with a high electron mobility transistor (HEMT) embedded in the resonator, which has been previously shown to enable sensing at higher frequencies[3],[4]. The HEMT-sensed device is illustrated in Figure 1c. The DC behavior of the embedded HEMT is shown in Figure 2a, while the measured frequency response of the resonator is illustrated in Figure 2b. Applying a negative bias to the drive transducer depletes the 2DEG and suppresses the resonance signal while reducing the drive capacitance by 13×. The resonance at 2.67 GHz has Q of 650 in air with f·Q of 1.7×1012, the highest in GaN resonators to date."
Iso-dielectric Separation of Cells and Particles,"The development of new techniques to separate and characterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. We are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object[1] and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes[2][3][4].Sepsis is a clinical condition caused by infection and, despite state-of-the-art facilities and treatments, it has a mortality rate of ~30%. Sepsis induces inflammation and organ failure; a possible treatment would require removing inflammatory agents, such as activated neutrophils, from whole blood. We used a CLP mouse model of sepsis (Figure 2a) and PMA-activated human granulocytes (Figure 2b) to monitor electrical differences between septic blood and leukocytes. With human granulocytes we saw a shift in their average isodielectric position (IDP) at high frequencies. Based on these results we did an IDP profile of leukocytes in healthy mouse blood and established a gate for the activated leukocytes. Applying the same gate under the same conditions with blood from CLP mice (n=4), we saw an increase in the number of activated leukocytes (Figure 2c). Finally we took aliquots of the same samples from healthy and CLP mice and measured common activation biomarkers with flow cytometry. Comparing both results, we see good correlation between our estimation of activated cells and the number of activated granulocytes measured in flow cytometry."
Quantifying Particle Coatings Using High-precision Mass Measurements,"Microparticles are currently used in a variety of industrial and biomedical applications and are often coated with different materials to impart functionality for applications such as drug delivery, cell extraction, and biomolecular detection. In many cases, the amount of coating affects the functionality of the particle. Label-based methods such as fluorescence are commonly used for biomolecular detection applications. However, labeling is not always practical and may not be an option in cases where a material layer is added. Although there is a wide range of label-free approaches for measuring the amount of coating on a flat surface, there are very few analogous approaches for particles.We have previously demonstrated that the suspended microchannel resonator (SMR) can weigh individual microparticles with femtogram precision. Although this level of precision is sufficient to resolve meaningful differences in coating thicknesses between populations of microparticles, such measurements have remained challenging for two reasons: (i) since the weight of the microparticle is generally many orders of magnitude larger than its coating, variation in particle mass across even the most monodisperse population can obscure the mass of the coating and (ii) sample-to-sample variations in the density of the carrier solution and density drift during the measurement of an individual sample give rise to significant differences in buoyant mass. Here we address these limitations by adjusting the density of the carrier solution to diminish the buoyant mass of the particle with respect to its coating and by monitoring solution density throughout the measurement using rapid fluid exchanges with a reference solution in an adjacent bypass[1]. This method is appropriate for polymer-based microparticles coated with materials of a different density. For a protein coating on a 3-μm polystyrene microsphere, we can resolve approximately 10% of a full layer (Figure 1)."
Contact-printed MEMS Membranes,"It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates[1][2].A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm2 in area are fabricated, and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28-μm-diameter cavities, as shown in Figure 1[3]. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of these devices."
Automated Parameterized Dynamical Modeling of RF MEMS Resonators,"Design and optimization of novel RF micro-electro-mechanical system (MEMS) resonators such as resonant body transistors (RBT) require modeling across multiple domains including mechanical (distributed stress and elastic wave models), electrical (semiconductor devices and RF small signal models), and thermal. These domains are all cross-coupled in nonlinear ways and require lengthy finite element multi-physics analyses to solve. Due to the complexity of these structures embedded in the CMOS stack and sensed using active FETs, the finite element multi-physics simulation prevents quick, intuitive parameterization of device design. A reduced model parameterized across all three domains is therefore necessary both for rapid prototyping and for device optimization.In this work, we are currently developing an algorithm to automatically generate compact and accurate models for RF MEMS resonators from input/output measurements. In a future stage of the project, we will develop physics based models from first principles and from finite element solvers. In both cases, our compact models will be suitable for AC, DC, and RF operation of the device and allow the circuit designers to run circuit-level time-domain simulations using any commercial circuit simulator[1]. The compact models are parameterized, so that the circuit designer will be able to instantiate instantaneously models within the circuit simulator for different values of the key device parameters[2]. Key parameters included in the compact parameterized models for RF MEMS resonators are resonant frequency, quality factor, presence of spurious modes, and operating temperature. Values for the model coefficients are calibrated using measurements from MEMS resonator devices. A critically important feature of our models is to guarantee that when circuit designers change arbitrarily values for the device parameters, the compact models will always preserve the physical properties of the original device and will never cause numerical instabilities and convergence issues when connected to other blocks within the circuit simulator[1]. We can use these models for sensitivity analysis and automated design space exploration. Numerical results are presented for a hybrid MEMS-CMOS Si based resonator[3] shown in Figure 1. For these devices, we model temperature dependent transconductance[4]. Figure 2 demonstrates an excellent match between the output of our identified models and the given measured data."
A Microfluidic ‘‘Baby Machine’’ for Cell Synchronization,"Common techniques used to synchronize eukaryotic cells in the cell cycle often impose metabolic stress on the cells or physically select for size rather than age. To address these deficiencies, a minimally perturbing method known as the ‘‘baby machine’’ was developed previously. In the technique, suspension cells are attached to a membrane, and as the cells divide, the newborn cells are eluted to produce a synchronous population of cells in the G1 phase of the cell cycle. However, the existing ‘‘baby machine’’ is suitable only for cells that can be chemically attached to a surface. Here, we present a microfluidic ‘‘baby machine’’ in which cells are held onto a surface by pressure differences rather than chemical attachment (Figures 1 and 2). As a result, our method can in principle be used to synchronize a variety of cell types, including cells that may have weak or unknown surface attachment chemistries. We validate our microfluidic ‘‘baby machine’’ by using it to produce a synchronous population of newborn L1210 mouse lymphocytic leukemia cells in G1 phase[1]."
High-throughput Electrospinning of Nanofibers from Batch-microfabricated Arrays,"Nanofibers’ unique morphological properties promise to make them a key engineering material across many disciplines. In particular, the large specific surface area of the porous webs they form make them highly desirable as multifunctional layers in protective soldier clothing; scaffolds in tissue engineering; and components in devices such as fuel cells, solar cells, and ultra-capacitors[1]. However, their integration into almost all of these technologies is unfeasible as a result of the low throughput and high cost of current production methods. The most common process for producing nanofibers involves applying strong electric fields to polar, high molecular weight polymeric liquids pumped through a syringe in what is known as electrospinning. Electrospinning is the only known technique that can generate nanofibers of arbitrary length and has tremendous versatility as it can create non-woven or aligned mats of polymer, ceramic, semiconducting, and/or metallic fibers.We implement high-throughput arrays of externally fed, batch-microfabricated electrospinning emitters that are precise, simple, and scalable. We fabricate monolithic, linear emitter arrays that consist of pointed structures etched out of silicon using DRIE and assemble these into a slotted base to form a two-dimensional array. By altering the surface chemistry and roughness of the emitters, we can modify their wetting properties to enable “hemi-wicking”[2] of fluid through the micro-texture (Figure 1). The interplay between electric, surface tension, and viscoelastic forces governs the fluid transport and fiber formation. We achieve more than 30 seconds of continuous, stable electrospinning simultaneously from 9 emitters in a two-dimensional array less than 1 cm2 using bias voltages under 15kV (Figure 2). This represents a 4-fold increase in run time compared to similar externally fed approaches[3] and a 7-fold increase in emitter density compared to state-of-the-art MEMS electrospinning sources[4]. Future work should explore denser arrays and integration of a proximal extractor electrode."
"Impact of SiNx Passivation on IDS,max of AlGaN/GaN HEMTs","Dielectric films such as SiNx, SiO2, and Al2O3 play key roles in AlGaN/GaN heterostructure field-effect transistors (HFETs) both as surface passivation and as gate-insulating layers[1]. Several groups, including ours, have observed that thin SiNx (<50 nm) deposition on AlGaN/ GaN HFETs by plasma-enhanced chemical vapor deposition (PECVD) can significantly change the two-dimensional electron gas (2DEG) density (ns). The origin of this change in ns has not been carefully analyzed until now. The study of the effect of a SiNx layer on AlGaN/GaN high-electron-mobility transistors (HEMTs) has been challenging since it is difficult to decouple the effects of the SiNx-induced strain from changes in surface potential.In this work, we have investigated the impact of SiNx passivation in the transport properties of AlGaN/GaN heterostructures grown on Si(111) substrates. After the fabrication of standard AlGaN/GaN membrane HEMTs, the Si substrate underneath several devices was selectively etched away using a deep reactive ion etching system with SF6 chemistry[2]. Then, the transistors were passivated with compressive (LFSiN) and tensile (HFSiN) stress SiNx layers deposited by PECVD.A comparative study of compressive and tensile SiNx dielectrics on AlGaN/GaN grown on Si (111) shows a decrease by ~ 40 % and an increase by ~13% in current, respectively (Figure 1). At the same time, a threshold voltage (VT) shift towards 0V is observed after the LFSiN deposition, unlike with the HFSiN passivation cap layer (Figure 2). It was found that surface strain induced by the passivation layer is the main contributor to the change in ns and current density in the GaN-based devices when tensile stress SiNx is deposited, unlike the compressive stress SiNx cap layer. These results pave the way to a new degree of freedom in the design of GaN electronic devices and local strain engineering."
Cell Pairing for Studying Immunity,"Cell-cell interactions are crucial for proper functioning of the immune system because direct cell-cell contacts largely govern the successful progression of adaptive immune responses. The heterogeneity inherent in these interactions plays a critical role in the functional outcome produced. Assessing the heterogeneity in the initial activation and connecting it with the endpoint function would, therefore, clarify the signaling cascades involved in the observed outcomes. Current methods to study this heterogeneity are mainly limited by the control over pairing, and thus by initiation of activation and low throughput. To remedy this situation, we developed a high-throughput microfluidic cell-pairing platform for studying the activation kinetics of immune cells. We adapted the microfluidic device from a previously developed chip for studying cell reprogramming[1] and altered device design and geometries to accommodate much smaller immune cells. The device comprises a dense array of weir-based hydrodynamic cell traps that contain a back-side single-cell trap and front-side two-cell trap (Figure 1a). Using a 3-step loading protocol (Figure 1b), we achieved 67 ± 12 % (n=18) pairing efficiencies with a range of 40-86 %, the highest ever reported for such smaller cells (Figure 1c). We used our microfluidic platform to dissect the activation kinetics of T cells from two lines of trans-nuclear mice, Trp1-hi and Trp1-lo, which recognize the identical peptide-MHC complex with markedly different affinities yet are equivalent in their ability to curtail the growth of B16 melanoma in vivo. We paired Trp1-hi/lo T cells with antigen-loaded B cells in a highly parallel and synchronous manner and measured the activation profiles through Ca2+ imaging. Our results show inherent heterogeneity within the clonal population of each cell type and indicate significant differences in the activation dynamics and activation percentages between the two lines (Figure 1d). These findings emphasize how immune cells dissociate the affinity of interaction and heterogeneity in the initial activation from a robust in vivo protective ability. Study of activation cascades in this well-defined system provides substantial insight into how variation leads to robust functional outcomes in immunity."
Electrokinetic Control of Axonal Growth,"Dynamic control of axonal outgrowth holds the potential to establishing a range of in-vivo and in-vitro applications including perpiheral nerve section injury recovery, neural computers, and neural interfaces. Although many axon guidance clues like surface topography[1], biochemical[2] or external forces[3] have been investigated, those methods do not provide downscaling and dynamic control of axonal growth.We have introduced the use of AC electrokinetics to dynamically control axonal growth in cultured rat hippocampal neurons. We find that the application of modest voltages at frequencies on the order of 105 Hz can cause developing axons to be stopped adjacent to the electrodes while axons away from the electric fields exhibit uninhibited growth. By switching electrodes on or off, we can reversibly inhibit or permit axon passage across the electrodes. We make use of our dynamic control over axon elongation to create an axon-diode via an axon-lock system that consists of a pair of electrode “gates” that either permit or prevent axons from passing through as shown in Figure 1a-c. Finally, we developed a neural circuit consisting of three populations of neurons, separated by three axon-locks to demonstrate the assembly of a functional, engineered neural network as shown in Figure 1b. Action potential recordings demonstrate that the AC electrokinetic effect does not harm axons, and Ca2+ imaging demonstrated the unidirectional nature of the synaptic connections. AC electrokinetic confinement of axonal growth has potential for creating functional, configurable, and directional neural networks[4].We have extended this work to demonstrate the control of axonal growth in collagen scaffold. The scaffold is confined in a microfluidic channel of three different heights where axons can develop over electrodes. When the channel height was limited to the size range of the growth cone (~ 3 mm), axon repulsion in a 2D plane was observed. We find that developing axons in the microchannel are repelled from the electrodes and follow the field lines until lower field strength (Figure 2a). Axons that grow in the same channel but further away from the electrodes show uninhibited growth. When the channel height is in the range of the growth cone (~10 mm), axonal growth is slowed down by the AC field. Finally, when the channel height is significantly bigger than the growth cone (~ 50 mm), axons develop in 3D into the scaffold and are repelled from the electrodes in the depth of the scaffold itself, where the minimum plane height is linked to the magnitude of the electric field as presented in Figure 2a-b.This new technology provides a powerful tool to confine axonal growth and leads the way to dynamic configurable neuronal networks."
A Novel Microfluidic “Cell-based” Blood Dialysis Platform for Septic Murine Model,"Sepsis is an adverse systemic inflammatory response caused by microbial infection in blood. We have reported a microfluidic approach for removal of microbes and inflammatory cellular components from whole blood, inspired by the in vivo phenomenon of leukocyte margination[1],[2]. We also developed a multiplexed blood filtration platform to demonstrate the bacteria removal capability in vivo using a septic mouse model (Figure 1). As blood flows through the margination channel, deformable red blood cells migrate to the axial center (Fahreaus effect), resulting in margination of other cell types towards the sides. Bacteria-depleted blood is collected at the center outlet and returned directly to the animal, as in a complete dialysis circuit. In vitro experiments using human blood spiked with FITC-conjugated Escherichia coli (E. coli) indicated a bacteria removal efficiency of ~70%; inflammatory cellular components (platelets and leukocytes) were also depleted by >70% (Figure 2). To mimic in vivo mouse filtration, a blood sample (~1mL, similar to mouse blood volume) spiked with fluorescent E. coli was subjected to continuous filtration in a closed loop circuit using a peristaltic pump. Experimental data obtained were in good agreement with the Monod kinetics model, achieving ~40% decrease in bacteria concentration after 30 minutes of filtration. The developed technique offers significant advantages: high throughput (~2mL/hr) and label-free separation for non-specific removal of blood-borne pathogens. The device is ideal for the mouse model: the filtration flow rate (90 mL/kg/hr) is comparable to high-volume hemofiltration (45-60 mL/kg/hr) used for humans in clinical settings. Unlike current extracorporeal blood purifications which mostly focus on cytokines removal, we hypothesize that a broad spectrum removal of bacteria and inflammatory cellular components (platelets and leukocytes) could help modulate the host inflammatory response as a blood cleansing method for sepsis treatment."
Preventing Catastrophic Failures: Nano-engineered Multi-physics Structural Damage Detection,"Catastrophic structural failures cause many physical and personal losses, with prevention estimated at billions of dollars in savings each year. Non-destructive evaluation (NDE) techniques have been pursued and employed for damage detection of such structures to detect cracks and other damage at pre-critical levels for remediation [1] [2] . Here, a novel multi-physics approach is reported that addresses drawbacks in existing techniques by taking advantage of the effects that damage, such as a crack, has on the electric and thermal transport in a material containing a CNT network distributed in the bulk material. When a potential is applied to a nano-engineered structure(see Figure 1), electric field lines concentrate in the vicinity of cracks as electrons flow around damage, causing field concentrations and “hot spots” via Joule heating, an effect which is amplified because the heat flow is also impeded in areas of damage (e.g., across a crackface). These changes of temperature can be localized through a conventional infrared thermal camera. Low power operation (a 9V standard battery is exemplary, providing a 15C rise at 1 Watt as in Figure 2) and high spatial resolution are demonstrated that are beyond state-of-the-art levels in non-destructive evaluation.Multiple applications have been identified using this technique such as crack detection in composite components that are joined by metallic fasteners, structures having internal nonvisible damage due to impact, and in situprogressive damage monitoring during a tensile strength test. The thermal nano-engineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring the nextgenerations of safer infrastructure [3] [4] ."
Aligned CNT-based Microstructures and Nano-engineered Composite Macrostructures,"Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs.Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests [1] . Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest [2] [3] , the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm2 Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive physical property testing is underway.Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies [4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in-situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup [5] [6] [7] . Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Interestingly, toughness improvement has demonstrated a favorable nano-scale size effect [7] . Analysis of the multifunctional properties and nanoscale interactions between the constituents in both the nanocomposites and hybrid macrostructures is underway. A new route to fabricate these materials in a continuous way has been developed."
RF MEMS Resonators in 32-nm SOI CMOS Technology,"This work presents the first hybrid RF MEMS-CMOS resonators demonstrated in silicon at the transistor level of IBM’s 32-nm SOI CMOS process, without the need for any post-processing or packaging. The unreleased, Si bulk acoustic resonators are driven capacitively and sensed using a field effect transistor (FET). MEMS-CMOS Si resonators with acoustic Bragg reflectors (ABRs) are demonstrated at 11.1 GHz with Q~18 and a footprint of 5µm × 3µm.The majority of electromechanical devices require a release step to freely suspend moving structures, which necessitate costly complex encapsulation methods and back-end-of-line (BEOL) processing of large-scale devices [1] . Development of unreleased Si-based MEMS resonators in CMOS allows integration into front-end-of-line (FEOL) processing with no post-processing or packaging. We have previously demonstrated the Resonant Body Transistor (RBT), which employs active FET sensing of acoustic vibrations [2] [3] , which amplifies the mechanical signal before parasitics. Realization of the RBT in CMOS technology leverages high fT, high-performance transistors, enabling RF-MEMS resonators at frequencies orders of magnitude higher than possible with passive devices.The hybrid MEMS-CMOS RBT presented in this work is a Si bulk-acoustic resonator with electrostatic drive formed using the gate dielectric and a body-contacted nFET sense transducer (see Figure 1). Acoustic vibrations in the unreleased resonator are confined using 7 pairs of 1D ABRs surrounding the device, which are patterned using shallow trench isolation (STI). The DC characteristics of the sense transistor are similar to standard body-contacted nFETs of the 32-nm SOI process and show no direct effect of the capacitor drive voltage on the FET behavior. The frequency response of an 11.1-GHz resonator is shown in Figure 2 for multiple bias conditions, verifying the mechanical nature of the resonance.This first demonstration of an unreleased hybrid MEMS-CMOS resonator paves the way for monolithically integrated RF MEMS frequency sources and signal processors."
Electrospray Nanoprinting on Electrospun Nanofiber Mats for Low-cost Biochemical Detection,"An electrospray emitter ionizes polar liquids using high electrostatic fields. The electric field produces suction on the free surface (meniscus) of an electrically conductive liquid, and the surface tension of the liquid tends to counteract the effect of the electrostatic suction. If the electric field is larger than a certain threshold, the meniscus snaps into a conic shape called a Taylor cone [1] (see Figure 1). A Taylor cone emits charged particles from its apex due to the high electrostatic fields present there; these particles can be ions, droplets, fibers, etc., depending on the working liquid and the emitter flowrate [2] . In particular, electrospray in cone-jet mode [3] creates near-monodispersed charged droplets that can be used for many applications including mass spectrometry [4] , etching [5] , and nanosatellite propulsion [6] . In this project we are exploring electrospray in cone-jet mode as a technology to create controlled nanoimprints on electrospun nanofiber mats with liquids such as fluorescent dye and nanoparticles solutions, as an alternative technology to nano-pipetting or ink jet printing. Using a shadow mask, we have shown imprints in close agreement with the dimensions of the mask aperture (see Figure 2). The long-term goal of the project is to investigate the design space of the technology to make low-cost and low false-positive biochemical detectors by exploring the multiplexing and scaling-down limits of cone-jet mode electrospray sources using batch micro- and nanofabrication [7] ."
Cell Sorting and Identification for Immunology,"A major challenge in immunobiology is to better understand how the immune cells dynamically interact with each other and with their environment. Any insights in that direction can help provide a clearer picture of the immune response’s evolution and reaction to infectious diseases; to this end, the use of clinical samples is extremely valuable [1] . However, most of the current analytical techniques used to characterize cells in a sample (e.g., ELISA, flow cytometry, PCR, DNA/RNA sequencing) do not preserve the cells after the analysis, and they typically fail to give multiple parameters of interest from the same sample. In addition, those analyses return only a snapshot of the sample state at a given time, which in most cases cannot be realistically compared with the in vivo conditions. Therefore, there is a need to develop new approaches for high-throughput and multiparameter analyses on clinical samples in a time-dependent manner and with single-cell resolution, to obtain the maximum information from a clinical sample.Recently developed single-cell, multiparameter analytical platforms [2] [3] are oriented to this purpose and can be complemented with DNA/RNA analysis and sequencing to provide a complete picture of the immune cells from a sample, as illustrated in Figure 1. Nevertheless, linking the results of those different approaches requires keeping the identity of the cells all along the multiple analyses. The proposed solution consists of four steps (i) a stochastic bead-based labeling of the cells of interest in the multiparameter analytical platform, (ii) imaging of the labeled cells, (iii) an optical cell release to allow sorting of the selected cells [4] and finally, (iv) a read-out of the labels on the cells in the final platform to re-assign the cell identity."
Flexible Multi-functional Electrodes for Neural Interfacing,"Interfaces with the nervous system are important for understanding basic neurobiology and for neuromedicine. We are part of a multi-university NSF Engineering Research Center (ERC) focused on sensorimotor neural engineering. One of the challenges that our team is addressing is making multi-functional interfaces with the nervous system. This work builds upon a previous collaboration developing flexible multi-site electrodes (FME) for insect flight control that directly interfaced with the animal’s central nervous system (Figure 1). The FMEs are made of two layers of polyimide with gold sandwiched in between in a split-ring geometry using standard MEMS processing (Figure 2) [1] . The FMEs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord of the moth Manduca Sexta. Additionally, we integrated carbon nanotube (CNT)-Au nanocomposites into the FMEs to enhance the charge injection capability of the electrode.As part of the NSF ERC, we are working with collaborators to extend this work by integrating the latest knowledge on electrode design into the probes. We are also investigating addition of multi-functionality to the probes, for example integrating both sensing and actuation modalities onto the same device. This integration would allow closed-loop operation of the probes, which we believe will have applicability both to uncover the basic mechanisms behind neurological disorders as well as to serve as eventual “smart” therapeutic devices."
Cell-based Sensors for Measuring Impact of Microsystems on Cell Physiology,"The use of microsystems to manipulate and study cells in microenvironments is continually increasing. However, along with such increase in usage comes a growing concern regarding the impact of these microsystems on cell physiology. In this project, we are developing a set of cell-based fluorescent sensors to measure the impact of common stresses experienced in microsystems on cell physiology. We are including stress agents commonly found in microsystems (e.g., UV exposure, heat shock, fluid flow, etc.). Each sensor is designed to respond to one particular stress agent but can also be combined for multiplexed analysis of multiple stresses at once, as might be experienced in a typical microsystem. Each sensor will use different colors to both indicate the type of sensor and the strength of the signal, to ease multiplexed analysis.We are currently developing a sensor that responds to activation of the p53 protein pathway, for generalized DNA damage analysis. Similar to the heat shock sensor we previously reported [1] [2] , which coupled fluorescent protein expression to activation of heat shock factor 1, the DNA damage sensor will couple fluorescent protein expression to activation of p53. The new sensor will have cyan (cerulean) as the constitutive color and red (RFP) as the activation color. Figure 1 shows the DNA damage sensor response after 30 min of UV exposure. One of the more relevant sources for physiological stress on cells cultured in microfluidic devices is shear stress. Construction of a shear stress sensor cell line requires an understanding and characterization of the gene expression mechanisms and mechanotransduction pathways, especially since the pathways are known to have a varying correlation towards cell types, magnitudes, and dynamics of applied stresses. Therefore, we are using a multi-flow microfluidic device (see Figure 2) that can simultaneously apply different flows to cells across a 1000× range to first understand the behavior of NIH3T3 mouse fibroblast cells under flow [3] . Specifically, these cells are seeded in 6 chambers concurrently, exposed to flow for 1-6 hours, and assayed for gene expression changes. Once they are characterized, we will construct a transfected NIH3T3 cell line with RFP expression correlating to shear stress."
Iso-dielectric Separation of Cells and Particles,"The development of new techniques to separate and characterize cells with high throughput has been essential to many advances in biology and biotechnology. We are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object [1] ) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [2] [3] [4] .Sepsis is a clinical condition caused by infection; despite state-of-the-art facilities and treatments, sepsis has a mortality rate of ~30%. Sepsis induces inflammation and organ failure and a possible treatment would require removing inflammatory agents from whole blood such as activated neutrophils. Using an automated IDS system (see Figure 2a) we could see electrical differences between white and red blood cells (Figure 2b). Furthermore, we measured the electrical properties of activated vs. non-activated neutrophils (see Figure 2c). The populations show differences that indicate that the populations are amenable to efficient separation. Using the position as a classifier to determine if a neutrophil is activated or non-activated yields receiver operating characteristic (ROC) curves with high area-under-curve (AUC), which would result in good specificity (see Figure 2d)."
Microfluidic Perfusion for Modulating Stem Cell Diffusible Signaling,"Stem cell phenotype and function are influenced by microenvironmental cues that include cell-cell, cell-extracellular matrix (ECM), and cell-media interactions (i.e., diffusible signaling), which we can control using microscale systems. Our research focuses on cell-ECM and cell-media control of mouse embryonic stem cells (mESCs). Cells are constantly secreting and responding to soluble signals, the removal of which can be mediated by modulating flow properties at the microscale [1] . To assess the contribution of cell-secreted factors to mESC differentiation and self-renewal, we utilized a two-layer microfluidic perfusion device allowing for parallel comparison of different cell culture conditions (see Figure 1A).Our results demonstrate that mESCs do not grow in differentiation conditions with minimal autocrine signaling, even with supplementation by Fgf4, a signal that has been shown to be a crucial factor in differentiation toward a neuronal stem cell fate, while they do grow when supplemented with media saturated with soluble signals (conditioned media, CM) (see Figure 1B). Consistent with this effect, inhibiting the Fgf4 receptor does not affect growth of mESCs in differentiation conditions (as Figure 1C shows), but it does affect differentiation toward a neuronal stem cell fate (as in Figure 1D) [2] .ESCs grown under self-renewal conditions are able to proliferate without conditioned media, but they lose expression of the self-renewal marker Nanog (see Figure 2A), results that, together with signaling and downstream differentiation assays, indicate differentiation towards an epiblast-like state under conditions that had previously been shown to be sufficient for self-renewal. This differentiation can be reversed by disrupting the ECM using sodium chlorate, which affects the ability of growth factors to bind to the ECM (as Figure 2B shows). This effect is evident based on colony morphology and can be duplicated by disrupting the matrix using collagenase (see Figure 2C) [3] . Together, these results indicate the importance of diffusible cell-secreted signals for mESC growth and ECM-based signals for mESC self-renewal."
Image-based Sorting of Cells,"Microfluidic approaches to cell sorting include purely dielectrophoretic (DEP) trap arrays [1] , passive hydrodynamic trap arrays with active DEP-based cell release [2] , and passive microwell arrays with optical cell release to permit sorting of non-adhered cells [3] . As in the preceding technologies, we developed an image-based single-cell sorting method that enables parallel cell sorting using a dual-photopolymerization scheme. Our approach enables simultaneously sorting multiple cells of interest following high-resolution imaging with high purity using a method that requires only common equipment at modest cost. Our overall approach was to spatially segregate cells using a microwell array, image them, and then remove desired cells from the array by encapsulating all the undesired cells in a photopolymer (see Figure 1). To demonstrate the sorting of minority populations (e.g., rare cell isolation), we mixed the GFP- and mCherry-expressing cells at a ratio of 1:100 and targeted to sort the GFP-expressing cells, while RFP-expressing cells were undesired. First, the desired GFP-expressing cells were targeted via microscopy (as Figure 2a shows). The desired GFP-expressing cell in the center was isolated from its surrounding mCherry-expressing cells by the photopolymerized PEGDA sorting well, while the undesired mCherry-expressing cells were encapsulated in the cross-linked gel (in Figure 2b, the gel is autofluorescent in the green channel). Finally, the desired GFP-expressing cells were removed by simply washing the array, leaving the undesired mCherry-expressing cell (see Figure 2c). Figure 2d shows a 1 mm × 1.4 mm region of the array after the desired cells were sorted. The two layers of microwells are evident: the trapping wells made from the photopolymerized optical adhesive and the sorting wells made from the photopolymerized PEGDA hydrogel. The overall technique requires standard equipment found in biological labs and inexpensive reagents (<$10 per experiment), encouraging widespread adoption."
Nanoporous Elements in MEMS with a Focus on Microfluidic Bioparticle Separation,"We integrated ultra-porous (99% porous) elements (nanoporous forests of vertically aligned carbon nanotubes (VACNTs)) in MEMS, showing their use in microfluidic applications for bioparticle isolation and health diagnostics. Distinct from works where the effects of fluids on VACNT elements resulted in either structural deformation or catastrophic forest collapse [1] , our approach enables creation of high aspect ratio (~ 1-mm) nanoporous elements and preserves their shape under flow-through conditions. Figure 1 shows a device, consisting of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enable flow around and through the VACNT elements, enhancing physical interaction between the particles in the flow and the functional elements. Multiple device layouts demonstrated a ~7X increase in specific bioparticle capture when transitioning to VACNT porous designs [1] . The large surface-to-volume ratio of nanoporous materials yields a significant increase in the functional surface area (~250-500X for the layouts analyzed in our works [2] [3] ), with permeability comparable to that of macro-scale porous materials [4] , thus further promoting bioparticle capture.Specific isolation of bioparticles ranging over 4 orders of magnitude in size (from cells to viruses) was experimentally demonstrated (Figure 1), including the ability to perform simultaneous multiphysics, multiscale isolation on a single chip [4] . Particles smaller than the average distance between single nanotubes in the VACNT elements (~80 nm) can penetrate the elements and can be isolated using chemical affinity; simultaneously, particles larger than 80 nm cannot enter the nanoporous elements and can be isolated on the elements’ outer surfaces using both mechanical filtration and biomolecular recognition. The nanoporous elements are versatile and could provide access to underexplored sub-micron particles (e.g., proteins, exosomes)."
An Analytical Approach for Characterizing the Complete Stress State in Thin-film CMOS Layered Materials and 3D MEMS Design via Postbuckling,"Characterization of thin film layered materials is critical for many MEMS devices. Residual stresses from production determine both final shape and performance of microdevices and should therefore be accurately determined. Stresses are typically extracted using simple test structures (clamped beams and cantilevers, see Figures 1a-b) that allow for mean and gradient residual stress estimation [1]. However, current approaches to material characterization have two major limitations. First, their accuracy is directly proportional to their cost. This is especially true for mean compressive stress, where more accurate estimates require a larger number of different test structures. Second, they oversimplify test-structure boundary conditions by considering them to be ideal (e.g., perfectly clamped in the case of buckled beams for mean compressive stress determination [1] ). To overcome these issues, we have developed a new methodology for characterizing the complete stress state (effective mean and gradient stresses) in CMOS layered materials that also assesses non-ideality of clamped boundaries [2] [3] . The approach uses a closed-form solution of the postbuckling problem of micromachined beams including non-ideal boundaries (Figure 1). In Table 1 we show the results relative to the characterization of four different CMOS material combinations. The outcomes show mean compressive stresses ranging between -15 and -105MPa, thus demonstrating the method’s ability to characterize structures subjected to both large and small compressive stresses. This ability contrasts with traditional critical length methods that encounter difficulties in quantifying small compressive stresses due to their inability to distinguish between mean stress and gradient stress effects [2] . For the CMOS materials examined here, the accuracy was ± 2MPa for mean stresses and ±3MPa/µm for gradients. Boundary non-ideality is found to be 90% of perfectly clamped for the CMOS-released films, having such a significant effect on the extracted stresses that it must be considered. The analytical tool can also be extended to 3D MEMS design, where buckling is used to controllably place structural elements outside the wafer plane. Using this approach, we have demonstrated out-of-place architectures for applications from three-axis thermal sensing to 3D flow measurement [4] )."
Growth of Vertically Aligned Carbon Nanotubes on a Continuously Moving Substrate,"Vertically-aligned carbon nanotube (CNT) arrays are grown on a moving substrate, demonstrating continuous growth of nanoscale materials with long-range order. A cold-wall chamber with an oscillating moving platform (see Figure 1) is used to locally heat a silicon growth substrate coated with a Fe/Al2O3 catalyst film for CNT growth via chemical vapor deposition. The reactant gases are introduced over the substrate through a directed nozzle to attain high-yield CNT growth [1] . Aligned multi-wall carbon nanotube (MWNT) arrays (or “forests”) with heights of ≈1 mm are achieved at substrate speeds up to 2.4 mm/s. Arrays grown on moving substrates at different velocities are studied to identify potential physical limitations of repeatable and fast growth on a continuous basis. No significant differences are noted between static and moving growth as characterized by SEM (as in Figure 2) and Raman spectroscopy, although overall growth height is marginally reduced at the highest substrate velocity. CNT arrays produced on moving substrates are also found to be comparable to those produced through well-characterized batch processes consistent with a base-growth mechanism. Growth parameters required for the moving furnace are found to differ only slightly from those used in a comparable batch process; thermal uniformity appears to be the critical parameter for achieving large-area uniform array growth.Once the parameters have been optimized, a desktop continuous will growth apparatus has been designed and implemented to grow VACNTs on silicon wafers (Figure 2), flexible sheets, and alumina fibers continuously. We have demonstrated and reported the ability to manufacture VACNT arrays in a continuous manner, significantly reducing the time spent, energy consumed, and reaction products created as compared to batch processing of these technologicallyvaluable assemblies of nanoscale materials [2] ."
Flush-mounted MEMS Langmuir Probe Arrays for HF-S Band Plasma-sensing,"Arrays of MEMS Langmuir probes that are flush-mountable (Figure 1) can serve as a sensorial skin on a spacecraft for fine spatial and temporal resolution of plasma phenomena. The technology can also provide diagnostics for other applications such as tokamaks and nanosatellite scientific payloads [1] . The benefits are innumerable for deeper understanding of plasma physics, which is in great need of these microprobes [2] . For instance, multiplexed microprobes that are flush-mounted on all the faces of a 3-D “tip” can allow for simultaneous capture of a detailed “whole picture” of plasma behavior in different axes at a given timescale. In addition, two or more different sensory configurations, e.g., single-, double-, triple-probe methods, etc., can be adapted into the same flat die, profiting at the same time from their individual data acquisition strengths. Protruded probes cannot offer these advantages. Another area of deployment is in the observation of electron phase-space holes, self-consistent nonlinear plasma structures that are formed from strong current- or beam-driven turbulence and found in magnetic reconnection regions, which are magnetic field topology modifiers responsible for the explosive release of magnetic energy in magnetospheric storms, solar flares, and laboratory plasmas [3] . Fast micro-Langmuir probes that work at high frequencies are indispensable for studying these plasma fluctuations. We developed a system of flush-mounted MEMS Langmuir probes and apparatus with fast timescale; i.e., shorter time compared to the timescale of reconnection events in the Versatile Toroidal Facility at MIT (Figure 2); and wide bandwidth extending across regions of magnetosphere-photosphere, i.e., considering both electron and ion plasma frequencies associated with these regions."
Silicon Field Emitter Arrays for Chip-scale Vacuum Pumping,"Development of miniature vacuum pumps that can be integrated with electronic or MEMS components is necessary for producing advanced equipment such as portable analytical instruments [1] and high performance sensors [2] . The proposed approach graphically illustrated in Figure 1 is based on electron impact ionization (EEI) or field ionization (FI) of the gas molecules using nano-scale sharp silicon tips. The ionized gas molecules are then evacuated from the chamber using a strong electric field to accelerate the ions and implant them permanently into a getter medium made of Ti or Al. In the EEI mode of the operation, a positive voltage is applied between the gate and the emitter to extract electrons that are used to ionize the background gas. In the FI regime, the Si sharp tips are biased at a positive voltage with respect to the gate to extract electrons from the outer shell of the gas molecules in a quantum tunneling process. The former process occurs at electric fields in the range of 3 – 6 ×107 V/cm while the later process initiates at electric fields above 108 V/cm [3] . Despite the larger required voltage, the operation in the FI regime is mandatory since the back-streaming of the positive ions during EEI mode of operation will damage the field emitter (FE) tips at mTorr-pressure range. Although state-of-the-art field emitters have been reported [4] [5] [6] , the focus of this work is to improve the reliability of the FE or FI devices for extended operation times and large currents necessary for pumping application. Since these devices demand application of large voltages between the gate and the tip of the FE/FI, wear or breakdown of the insulating dielectric is a major issue. Finite element modeling (shown in Figure 2) has been conducted to optimize the design of the device for pumping application. A new fabrication process is also being developed for high-yield fabrication of an array with more than 300K Si FEs/FIs."
Measuring Ion Energy Distribution Using Batch-microfabricated RPAs,"The need to measure particle energies arises in many applications, from calibrating electron sources for electron guns in precision microscopes to determining the efficiency of space-based ion beam thrusters. Retarding potential analyzers (RPAs) are capable of filtering particles based on their energy and have been used as early as the late 1950s and early 1960s for such purposes [1] . However, these devices maintain limited application due to stringent dimensional constraints driven by plasma Debye length. Cold dense plasmas require minute apertures and tight spacing tolerances between biasing grids that are difficult to enforce using conventional means. We suggest microelectromechanical system (MEMS) batch-fabrication techniques in order to achieve unprecedented alignment accuracy of successive electrodes while incorporating the necessary micron-scale features. Assembly to a precision of a few tens of microns has been demonstrated with a hybrid RPA (see Figure 1a) [2] . Figure 1b shows the fully MEMS-fabricated sensor inspired by in-plane assembly of high-voltage devices, which will have tolerances on the order of 1μm [3] .Augmenting the optical transparency of RPAs provides a more direct path for particles to the collector plate. Signal strength is thus improved as the effective collection area is increased. Preliminary results and comparisons between MEMS-fabricated electrodes and conventional stainless steel mesh have revealed an ameliorated signal quality. Figure 2 shows a greater than two-fold improvement in peak signal strength with the micromachined grids over the conventional RPA [2] . Currents captured by the various grids and simulations suggest the possibility of ion beam focusing and interception of ions prior to reaching the collector. Alteration of the internal dynamics of the sensor provides a cleaner signal that may lead to a better interpretation of the measurements than with models that incorporated the stochastic behavior of charged species through randomly oriented electrode apertures."
Cathode for X-ray Generation with Arrays of Individually Addressable Field Emitters Controlled by Vertical Ungated FETs,"This work focuses on the design and fabrication of a cathode for a portable x-ray source. The cathode is made of an array of individually addressable electron guns, each containing double-gated field emitters. Compared to thermionic cathodes, field emission arrays operate at lower vacuum and lower temperatures, use less power and are more portable. The electron beam from each gun is extracted by a proximal gate and collimated using a distal gate before it hits an anode in a micron-sized spot that generates Bremsstrahlung x-rays. The architecture of the cathode is shown in Figure 1. Each field emitter is fabricated on top of a vertical ungated field-effect transistor (FET) [1] [2] that acts as a current source due to the velocity saturation of electrons in silicon when the voltage across the FET is above a saturation voltage. Current source-like behavior provides spatial and temporal uniformity of the output current across the emitter array; it also protects against emitter burnout and current surges. Individual addressability is achieved by fabricating the structure on SOI wafers to create electrically isolate strips of silicon. The extractor and focus gates are monolithically integrated with the cathode chip. They are patterned in strips that are orthogonal to the silicon strips, so that a single electron gun can be turned on at once. Each vertical ungated FET is a 25-μm-tall column with a 0.5-μm diameter, and emitter tip radius is in the range of 20 nm. The saturation current and saturation voltage of the silicon columns are plotted as a function of doping density in Figure 2. Wafer doping of 10-20 Ω cm provides a saturation current of 0.5 μA and an output impedance of 2×109 Ω. With 100 emitters per chip, the total output current per chip is 50 μA, corresponding to a current density of 139 μA/cm2."
Batch-Microfabricated Electrospray Arrays with Integrated Electrode Stack for Ionic Liquids,"Electrospray is a process to ionize electrically conductive liquids that relies on strong electric fields; charged particles are emitted from sharp tips that serve as field enhancers to increase the electrostatic pressure on the surface of the liquid, overcome the effects of surface tension, and facilitate the localization of emission sites. Ions can be emitted from the liquid surface if the liquid is highly conductive and the emitter flowrate is low. Previous research demonstrated successful operation of massive arrays of monolithic batch-microfabricated planar electrospray arrays with an integrated extractor electrode using ionic liquids EMI-BF4and EMI-Im [1] [2] – liquids of great importance for efficient nanosatellite propulsion. The current work aims to build upon the previous electrospray array designs by increasing the density of the emitter tips, increasing the output current by custom-engineering suitable nanofluidic structures for flow control, and improving the ion optics to gain control of the plume divergence and exit velocity.The basic version of the MEMS electrospray array consists of an emitter die and an extractor die (shown in Figure 1), both made of silicon and fabricated using deep reactive ion etching. The two dies are held together using a MEMS high-voltage packaging technology based on microfabricated springs that allows precision packaging of the two components with less than 1% beam interception [3] [4] . The emitter die contains dense arrays of sharp emitter tips with as many as 1,900 emitters in 1 cm2. A voltage applied between the emitter die and the extractor electrode creates the electric field necessary to ionize the ionic liquid (see Figure 2). A nanostructured material transports the liquid from the base of the emitters to the emitter tips. The present research focuses on engineering the nanofluidic structure to attain higher emitter current while maintaining good array emission uniformity and on developing batch microfabricated advanced ion optics to control the electrospray plume."
"Externally-fed, Microfabricated Electrospinning Device for Increased Throughput of Polymer Nanofibers","Electrospinning is a process in which a membrane-like web of thin fibers can be produced using high electrostatic fields and polar liquids with high viscosity. It is the only known technique that can generate continuous fibers with controlled morphology in the 10-500 nm diameter range and has tremendous versatility as it can create non-woven or well-aligned mats of polymer, ceramic, semiconductor, and/or metallic fibers using the same hardware. Electrospinning is also capable of conformally coating 3D complex shapes with ultrathin layers that have complex multi-layered structure and thickness variation across the surface. In particular, polymer electrospun fibers have been proposed to develop multi-stack functional fiber mats for protective gear, because they show high breathability, elasticity, and filtration efficiency. In addition, electrospun fibers made of the appropriate materials could also be used in flexible electronics (graphene) and in structural reinforcement against mechanical trauma. However, the production of electrospun nanofibers has very low throughput due to the small fiber diameter, which limits their applications to high-end products. In this project we are investigating the development of high-throughput electrospun nanofibers using batch-microfabricated arrays of externally fed electrospinning emitters. Externally-fed emitters are attractive, because they do not require high pressure drops as internally-fed emitters do. Also, they do not clog and can process liquids that bubble.An aspect of this project is looking into the physics of wicking to optimize the fluidic micro/nanostructures that control the emitter flow rate. For solids with intrinsic contact angles below some critical value determined by roughness geometry, it becomes energetically favorable for a droplet to completely impregnate the roughness and spread through it [1] . This process of hemi-wicking has been described in pillar arrays of varying shapes and sizes [2] [3] . For externally-fed electrospinning, we must ensure a sufficient and steady flow rate of polymer solution to avoid broken or irregular fibers. We are theoretically and experimentally investigating optimal morphologies of both the micro/nano fluid control structures and the emitter geometry to attain good array emission uniformity."
Evolution of Intrinsic Stress and Grain Structure in Polycrystalline Films for Nano/Micro-electromechanical System Applications,"Controlling the intrinsic stress in polycrystalline thin films is of great importance in a wide variety of applications, especially those in which mechanical properties and reliability issues are critical, e.g., Nano-/microelectromechanical systems (N/MEMS). Using capacitance techniques, intrinsic stress can be monitored in situ and in real time during deposition processes. We do this in a UHV e-beam evaporator in which we grow FCC metal films at a range of homologous temperatures, in a range of deposition rates, and with variable vacuum conditions. These studies show an evolution to a high tensile stress during film formation (Type I behavior), or an evolution first to a tensile stress and then to a compressive stress at higher thicknesses (Type II behavior). The origin of this behavior, especially Type II behavior, is not well understood. In recent studies we have found that Pd and Ni deposited at intermediate homologous temperatures undergo a behavior intermediate to that of Type I and Type II (Figure 1), where the stress evolves from tensile to compressive and back to tensile. Transmission electron microscopy (TEM) reveals that the grain size increases during deposition at low or intermediate homologous temperatures. The grain size in Ni films deposited from 300K to 473K forms a linear relation with film thickness. Figure 2 shows representative bright field TEM images of Ni films deposited at 473K. It is known that grain growth in a constrained film leads to tensile stress. We believe that while the first tensile rise is associated with a coalescence stress, the second is associated with grain growth. Grain growth itself leads to a tensile stress and also to a lower rate at which ad-atom are trapped at boundaries to cause compressive stresses. We find that changes in the deposition conditions can modify this behavior."
Designing Complex Digital Systems with Nano-electro-mechanical Relays,"Silicon CMOS circuits have a well-defined lower limit on their achievable energy efficiency due to sub-threshold leakage. Once this limit is reached, power constrained applications will face a cap on their maximum throughput independent of their level of parallelism. Avoiding this roadblock requires an alternative device with steeper sub-threshold slope – i.e., lower VDD/Ion for the same Ion/Ioff. One promising class of such devices is electro-statically actuated nano-electro-mechanical (NEM) switches with nearly ideal Ion/Ioff characteristics. Although mechanical movement makes NEM switches significantly slower than CMOS, they can be useful for a wide range of VLSI applications by reexamining traditional system- and circuit-level design techniques to take advantage of the electrical properties of the device. NEM relay circuits with pass-transistor logic design combine as many propagating electrical delays into as few mechanical delays as possible, parallelizing the tasks to do more operations in less time.Basic circuit design techniques and functionality of some main building blocks of VLSI systems, such as logic, memory, and clocking structures, have been successfully demonstrated in our previous works [1] [2] [3] .Recently, complex arithmetic units such as relay-based multipliers have been developed (Figure 1b-c) [4] . Simulation results of an optimized 16-bit relay multiplier built in a scaled relay process predicts ~10x improvement in energy-efficiency over optimized CMOS designs in the 10-100 MOPS performance range. The relative performance of the multiplier enhancements are in line with what was previously predicted by a NEM relay 32-bit adder [3] , suggesting that complete VLSI systems (e.g., a microprocessor or an ASIC) would expect to see similar energy/performance improvements from adopting NEM relay technology [3] [4] . The operation of the main building block of the MEM-relay based multiplier, the (7:3) compressor, is experimentally demonstrated. This circuit, consisting of 98 MEM-relays, is the largest MEM-relay based circuit successfully tested to date (Figure 2) [4] ."
Measuring Single-cell Density,"We have used a microfluidic mass sensor to measure the density of single living cells. By weighing each cell in two fluids of different densities (see Figure 1), our technique measures the single-cell mass, volume, and density of approximately 500 cells per hour with a density precision of 0.001 g mL−1. We observe that the intrinsic cell-to-cell variation in density is nearly 100-fold smaller than the mass or volume variation. As a result, we can measure changes in cell density indicative of cellular processes that would be otherwise undetectable by mass or volume measurements. Here, we demonstrate this with four examples: identifying erythrocytes infected with Plasmodium falciparum malaria in a culture, distinguishing transfused blood cells from a patient’s own blood (as in Figure 2), identifying irreversibly sickled cells in a sickle cell patient, and identifying leukemia cells in the early stages of responding to a drug treatment. These demonstrations suggest that the ability to measure single-cell density will provide valuable insights into cell state for a wide range of biological processes."
Design and Modeling of a PZT Thin-film-based Piezoelectric Micromachined Ultrasonic Transducer,"Although new software techniques enable higher-resolution medical ultrasound imaging, commercial ultrasonic transducer technology has remained largely unchanged for a few decades. Current transducers are fabricated from bulk PZT using assembly steps that are labor-intensive and limit individual transducers to millimeter-sized features. With micro-fabrication technology, micro-scale transducers can be easily manufactured at very low cost, but their acoustic power and efficiency may be compromised. We revisit a piezoelectric micro-machined ultrasonic transducer (PMUT) based on a lead zirconate titanate (PZT) thin film with a view to improve acoustic performance. Our initial findings show that the inherently high piezoelectric coupling of thin-film PZT produces the deflection necessary for high acoustic pressure applications without significant power requirements or application of a DC bias voltage if the design can be optimized. With its high acoustic pressure output and small size, a PMUT could be used for deep penetration and non-invasive medical imaging, e.g., intracranial monitoring of head injuries.Our group has derived the equivalent circuit for a bimorph PMUT [1] . This configuration sandwiches a PZT between top and bottom electrodes and actuates it with an applied voltage across the electrodes. Adding a structural support layer, such as silicon, creating a multimorph device increases the model’s complexity. With separate definition of mechanical and electrical neutral axes, the equivalent circuit derivation extends to include the multimorph design [2] . With this advance, transduction behavior of the PMUT can be more accurately predicted, designs more easily optimized, and results validated with a complete model. An analytical solution for deflection based on electrode coverage has been derived and the optimum electrode coverage for maximum deflection has been determined. Based on the modeling results, fabrication of an optimized PMUT design is now underway. Our eventual goal is to incorporate PMUT elements into 1D and 2D arrays with a small form factor to enable high resolution medical imaging."
Applications of Piezoresistive Nanocomposites in Electronics,"Polymer materials doped with conductive particles exhibit piezoresistive properties. These materials are fabricated such that their conductivity changes with an applied compressive force. When compressed, the formation of percolation pathways allows increased electrical conduction through tunneling between the particles. This work explores and utilizes this property of composites to fabricate various devices with the ultimate goal of developing integrated flexible systems resembling sensory skins.As a first generation of piezoresistive devices, a squeezable switch (squitch) is fabricated with a three-terminal configuration shown in Figure 1 [1] . In this study, the squitch is fabricated from a composite of polydimethylsiloxane doped with 60 wt% Ni microparticles that shows more than 5 orders of magnitude change in conductivity over a 20% strain (Figure 2). In the absence of an applied gate bias, the composite is a poor conductor. An applied gate voltage generates an electrostatic force between the source and the gate that compresses the composite, causing the squitch to conduct. To allow fabrication of reliable and reproducible devices, the composite needs to be engineered such that its mechanical properties are more stable. To achieve this goal, current research explores the effects of the type of polymer and conductive particles and the method of fabrication on the properties of the nanocomposite and performance of the squitch. The surfaces of the metal particles are chemically treated to allow better distribution in the polymer matrix while also chemically binding the particles to the polymer preventing particle migration over repeated use of the device. After the composite is optimized, future work will involve extending the squitch design to fabricate devices such as analog amplifiers, digital inverters, and various sensors and developing processes to allow large-area fabrication. The devices will then be integrated to develop artificial skins."
Nano-electromechanical System Digital Switches,"Nano-electromechanical systems (NEMS) are an emerging area of research with potential applications as low-power switches for electronic circuits. The proliferation of electronics in both stationary and portable applications demands the development of more energy-efficient devices than are currently available. While solid-state silicon MOS-based transistor circuits, the dominant technology in today’s electronics, have greatly reduced their power requirements by aggressive scaling, the concurrent increase in off-state leakage current limits their energy efficiency. In contrast, microelectromechanical relays have been demonstrated with zero off-state currents and abrupt switching characteristics [1] [2] . As these and other electromechanical devices are shrunk to the nanoscale, their actuation voltages, and hence power requirements, are expected to be reduced significantly.Our group recently presented a three-terminal electromechanical switch based on a piezoresistive polymer nanocomposite as the active material [3] . The metal-polymer composite consisted of a polydimethylsiloxane polymer matrix doped with 60 wt% nickel particles. A schematic diagram of this squeezable switch, or “squitch,” is shown in Figure 1. In its initial state, the conductive metal particles are separated by the insulating polymer matrix. Thus, the active material is highly resistive, and little current flows through the device (in the “off” state). When compressed, the metal-metal distances decrease until the onset of tunneling allows current to flow from source to drain (“on” state). The first-generation squitch demonstrated transistor-like behavior with drain-source conduction modulation over 4 orders of magnitude when electromechanical force was applied. However, the large mechanical dimensions of this concept demonstration necessitated higher supply voltages than desired. Our current work focuses on incorporating the squitch concept into nanoscale devices by (a) developing improved device structures and fabrication methods and (b) exploring new materials such as ligand-coated nanoparticles and self-assembled monolayers as active materials."
MEMS Pressure-Sensor Arrays for Passive Underwater Navigation,"A pressure sensor array is under development for unmanned undersea vehicles (UUV). This project is inspired by the lateral line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1] [2] . The canal subsystem of the organ can be described as an array of pressure-sensors [3] . The lateral line allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby objects [2] [5] . Similarly, by measuring pressure variations on the surface of an UUV, an engineered pressure-sensor array supports the identification and location of obstacles for navigation.To be compatible with the doubly-curved surface of a typical UUV hull, the pressure sensor array must be flexible. Further, it is desirable that the array be amenable to wide-area fabrication. Correspondingly, the design pursued here is fabricated primarily from a PDMS polymer, some parts of which are doped with conducting nanoparticles so as to become piezoresistive. As shown in Figure 1 below, a pressure sensor array consists of piezoresistive strain-gauges patterned onto PDMS membranes suspended over cavities formed in a PDMS substrate [6] . The resistance of each strain gauge is measured using a four-point probe array with a common current source shared by all sensors. The strain-gauge resistance can be related to the deflection of its corresponding membrane, and hence the pressure difference across the membrane. All cavities are connected together so that all pressure sensors have a common reference.During the past year, flexible pressure sensor arrays were mounted on the side of a kayak for open-water tests, as shown in Figure 2a below. The pressure measurement from one sensor is shown in Figure 2b together with measurements from nearby commercial reference sensors. The similarity of the measurements demonstrates the functionality of the PDMS pressure sensors in an uncontrolled environment."
Microfluidic Device for Characterization of Dynamic Red Blood Cell Deformability,"The average diameter of human red blood cells (RBCs) is around 8µm. As RBCs circulate in the body and transport oxygen, they have to deform repeatedly in small blood capillaries. RBC deformability is therefore an important mechanical attribute for efficient oxygen delivery. Several blood related diseases such as malaria, sickle cell anemia, and sepsis are marked with significant alterations in RBC deformability [1] [2] [3] .This project studies RBC dynamic deformability using a simple, portable microfluidic device [4] . The deformability of individual RBCs can be assessed by the average velocity of RBCs passing through narrow microfluidic channels. The repeated deformations to be experienced by RBCs simulate in vivo blood capillary system. Several blood-related diseases are included in our studies."
Diffusive Transport of Acid through Mucus Hydrogels inside a Microfabricated Device,"In the stomach, the biological hydrogel known as mucus protects the stomach wall from the damaging effects of strongly acidic digestive juices inside the stomach lumen. Altered mucus function is linked to gastric diseases including ulcers and cancers. The biophysical mechanisms underlying the barrier are not well understood, due partly to a lack of suitable in vitro tools.In this work, we developed an in vitro microfluidic system designed to mimic mucus secretion in the stomach (see Figure 1) [1] . In our system, mucus components are pumped continuously on-chip into an acidic flow, mimicking in vivo mucus secretion into an acidic stomach lumen. A fluorescent pH indicator added to the samples allows optical tracking of acid diffusion. Our microfluidic system is superior to in vitro macroscale techniques currently used to assay mucus function [2] . Advantages of our system include study of barrier function under secretion rather than static conditions, ability to optically measure the pH profile inside the mucus layer, and low sample volume requirement enabling experiments using difficult-to-purify mucus components.With this system, we demonstrate that continuous secretion of mucin glycoprotein, the dominant protein component of mucus, hinders the diffusion of acid (Figure 2) due to the ability of mucins to directly bind and sequester H+ (see [1] for more details). We further estimate that the barrier function resulting from direct binding of H+ to mucin constitutes a significant portion of the in vivo mucus barrier. This “mucus-secretion-on-a-chip” platform may be used to systematically study the barrier function of each mucus layer component, perform diagnostics of mucus function using small amounts of clinical sample, and test mucus-targeted drugs."
Particle Behavior inside Planar Straight and Spiral Microchannels,"Although inertial force-induced lateral migration has been extensively studied for almost 50 years and has been utilized in various microchannels to perform size-based separation for cell research, the mechanism of inertial focusing is generally described as the interplay between inertial lift force and dean drag force and lacks information on particle behavior in depth direction, leaving several missing pieces from the physical understanding of inertial focusing parameter space [1] [2] [3] . Here we present an exploratory study of inertial focusing in planar straight microchannels and spiral microchannels with varying geometry to identify the regimes of particle behavior in response to flow rate and channel dimension. To gather accurate information on the depth direction of a straight channel, we fabricated a pair of straight channels with the same cross-sectional dimensions but different orientations and recorded the focusing positions of particles in the top-down images under the same conditions using these two devices, respectively. Combining the data from these two channels provides unambiguous information on the cross-sectional particle focusing positions. We also developed a polymer-casting technique to fabricate PDMS devices with smooth sidewalls through which one can observe the particle positions at the outermost loop of the planar spiral in the channel depth direction. The data gathered for the same spiral channel but from different directions allowed us to map the distribution of particles in cross-section with a simulated velocity field. With accurate information on particle positions in the cross-sections of straight and spiral channels, we would be able to relate the effect of channel dimension on the force field with the related particle- focusing behavior and identify the key parameters for the optimal design of a size-based separation device targeted at specific size range."
Removal of Pathogen and Inflammatory Components from Blood using Cell Margination,"Sepsis is an adverse systemic inflammatory response caused by microbial infection in blood. In this work, we report a simple microfluidic approach for intrinsic, non-specific removal of both microbes and inflammatory cellular components (platelets and leukocytes) from whole blood, inspired by the in vivo phenomenon of leukocyte margination [1] . As blood flows through a narrow microchannel (20 × 20 µm), deformable red blood cells (RBCs) migrate axially to the channel center, resulting in margination of other cell types (bacteria, platelets and leukocytes) towards the channel sides (see Figure 1) [2] . With the use of a simple cascaded channel design, the blood samples undergo a 2-stage bacteria removal in a single pass through the device, thereby allowing higher bacterial removal efficiency. As an application for sepsis treatment, we demonstrated separation of Escherichia coli and Saccharomyces cerevisiae spiked into whole blood, achieving high removal efficiencies of ~80% and ~90%, respectively (Figure 2A). Inflammatory cellular components were also depleted by >80% in the filtered blood samples, which could help to modulate the host inflammatory response and potentially serve as a blood-cleansing method for sepsis treatment. The developed technique offers significant advantages including high throughput (~1mL/hr per channel) and label-free separation that allows non-specific removal of any blood-borne pathogens (bacteria and fungi). The continuous processing and collection mode potentially enables the return of filtered blood to the patient directly, similar to a simple and complete dialysis circuit setup. Due to design simplicity, further multiplexing is possible by increasing channel parallelization or device stacking to achieve higher throughput comparable to convectional blood dialysis systems used in clinical settings."
Waveguide Micro-probes for Optical Control of Excitable Cells,"Professor Ed Boyden uses light to precisely control neural activity. His lab has invented safe, effective ways to deliver light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively [1] . This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health, and his lab has developed animal models of epilepsy and Parkinson’s disease to explore the use of optical control to develop new therapies.We have recently developed mass-fabricatable multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired [2] . Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion. Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage. We are currently developing 2-D arrays of such probes so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at the resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics. Such devices will allow the substrates that causally contribute to neurological and psychiatric disorders to be systematically analyzed via causal neural control tools. Given recent efforts to test such reagents in nonhuman primates, these devices may also enable a new generation of optical neural control prosthetics, contributing directly to the alleviation of intractable brain disorders.The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide, and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns [2] . Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe functions as designed [2] . The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable (see Figure 2). The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule. This allows for increased modularity and control in initial probe testing.We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping. Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and to subserve a new generation of neural control prosthetics."
Compact Parameterized Modeling of RF Nano-Electro-Mechanical (NEM) Resonators,"Design and optimization of novel RF Nano-Electro-Mechanical (NEM) resonators such as Resonant Body Transistors (RBT) require modeling across multiple domains, including mechanical (distributed stress and elastic wave models), electrical (semiconductor devices and RF small signal models), and thermal. These domains are all cross-coupled in nonlinear ways and require lengthy finite element multi-physics analyses to solve. Due to the complexity of these structures embedded in the CMOS stack and sensed using active FETs, the day-long time scale of each finite element simulation prevents quick, intuitive parameterization of device design. A reduced model parameterized across all three domains is therefore necessary both for rapid prototyping and for device optimization.In this work, we are developing an algorithm to automatically generate compact models for NEM resonators. Our compact models are suitable for AC, DC and RF operation of the device and allow the circuit designers to run circuit-level time-domain simulations using any commercial circuit simulator [1] [2] . The compact models are “parameterized,” so that the circuit designer will be able to instantiate instantaneously models within the circuit simulator for different values of the key device parameters. Key resonator parameters included in the compact parameterized model are resonant frequency, quality factor, signal strength, isolation, presence of spurious modes, and operating temperature. Values for the model coefficients are calibrated using measurements from NEMS resonator devices. A critically important feature of our models is to guarantee that when circuit designers change arbitrarily values for the device parameters, the compact models will always preserve the physical properties of the original device and will never cause numerical instabilities and convergence issues when connected to other device models and circuits within the circuit simulator [3] . Figure 1 shows the layout of a Si-based NEMS-CMOS resonator. Numerical results show a great promise for our technique. We have achieved high quality fit to the measured data, as Figure 2 shows, which offered modeling challenges including the presence of noise and spurious resonant peaks."
Recombination Dynamics of Charge Carriers in Nanostructured Solar Cells,"Nanostructured solar cells are attracting increasing attention as a promising photovoltaic (PV) technology [1] . Generation of free charge carriers in nanostructured PV devices occurs at the electron donor-acceptor interface, analogous to the pn-junction interface in traditional crystalline silicon solar cells. However, recombination at this interface constitutes one of the major charge carrier loss pathways. Thus characterizing and controlling recombination dynamics is critical for informing the design of novel device architectures. Recombination parameters also enable comparisons between different device architectures.In this work, we employ the transient photovoltage (TPV) technique [2] to probe recombination mechanisms under standard operating conditions in three different solar cells, as shown in Figure 1: a poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction; a chloroaluminium phthalocyanine and fullerene (ClAlPc:C60) planar mixed heterojunction; and a lead sulfide quantum dot and zinc oxide (QD PbS:ZnO) pn-heterojunction. The normalized TPV data acquired at 0.5-sun illumination intensity are shown in Figure 2a, which compares the recombination lifetimes of charge carriers in these devices. The observed differences in carrier lifetimes may arise from variations in the respective interface morphologies: for example, the slower recombination transients observed in the ClAlPc:C60 device may be attributed to the intrinsic planarity of this particular architecture. We can also measure the charge carrier lifetime as a function of the light intensity, as shown in Figure 2b; this result confirms that recombination dynamics are faster in P3HT:PCBM and QD PbS:ZnO than in ClAlPc:C60 PV devices."
Contact-printed MEMS Membranes,"It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates.A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm2 in area are fabricated, and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28-μm-diameter cavities, as shown in Figure 1. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of these devices."
An On-Chip Test Circuit for Characterization of MEMS Resonator,"Electromechanical resonators such as quartz crystals, surface acoustic wave (SAW) resonators, and ceramic resonators have become essential components in electronic systems. However, due to their large footprint and difficulty in integrating with CMOS processes, there has been much development in realizing microelectromechanical system (MEMS) resonators that achieve comparable performance yet have smaller footprint and are compatible with CMOS. As with other semiconductor devices, with increasing frequency and with decreasing device size into the submicron scale, variability has started to become a critical issue in MEMS resonators. However, one of the critical challenges is the lack of a characterization method that is accurate but efficient enough to be used for testing the large number of devices necessary to acquire accurate statistical distribution of the parameters of interest. This project proposes an on-chip test circuit that can accurately characterize a large number of resonators for variation analysis and that is general enough that it can be used with a wide range of resonators, not limited to specific frequencies or other properties. The proposed test circuit is based on a transient step response method using a voltage step that can accurately measure the resonant frequencies and the quality factor of devices [1] . The circuit employs a sub-sampling method to capture the high-frequency decay signal [2] and a simple analog-to-digital converter (ADC) [3] allowing complete digital interface, an important feature for test automation. SPICE level simulation combined with a behavioral simulation tool that was developed showed acceptable extraction errors of <1% for RS, <0.1% for Lx, <0.1% for Cx, <100 ppm for fs, and <1% for Qs. A test chip implementing the proposed test circuit has been designed and fabricated in NSC 0.18-um CMOS process."
DNA-templated Assembly of Droplet-derived Microtissues,"Paracrine and autocrine cell signaling are critical factors guiding tissue development and maintenance, and dysregulation of these cues contributes to the pathogenesis of diseased states such as cancer. Patterning multiple cell types is thus a critical step for engineering functional tissue [1] , but current “top-down” approaches such as dielectrophoresis and photopatterning are challenging to scale-up for the construction of mesoscale tissues. On the other hand, “bottom-up” methods wherein small tissue building blocks are assembled into larger structures have potential for constructing multicellular tissues in a facile, scalable fashion. Synthetic microtissues composed of cell-laden hydrogels in this size range represent appropriate fundamental building blocks of such bottom-up methods [2] .To specify the placement of many different microtissues relative to one another, we have developed a “bottom-up” approach for fabricating multicellular tissue constructs that utilizes DNA-templated assembly of 3D cell-laden hydrogel microtissues (Figure 1a). A microfluidic flow focusing-generated emulsion of photopolymerizable prepolymer is used to produce 100-µm monodisperse microtissues at a rate of 100 Hz (105/hr) (Figure 1b-d). Multiple cell types, including suspension and adherently cultured cells, can be encapsulated into the microtissues with high viability (~97%) (Figure 1e). We then use a DNA coding scheme to self-assemble microtissues “bottom-up” from a template that is defined using “top-down” techniques. The microtissues are derivatized with single-stranded DNA using a biotin-streptavidin linkage to the polymer network and are assembled by sequence-specific hybridization onto spotted DNA microarrays. Using orthogonal DNA codes, we have achieved multiplexed patterning of multiple microtissue types with high binding efficiency and >90% patterning specificity (Figure 2a). We have also demonstrated the ability to organize multicomponent constructs composed of epithelial and mesenchymal microtissues while preserving each cell type in a 3D microenvironment (Figure 2b)."
AC Variability Characterization of MOSFETs,"The high-frequency variability characterization of MOSFETs is becoming more necessary due to new process developments such as high-K metal gates, elevated source-drain junctions, strained silicon, and others. Some of the effects of these variability sources can be seen at low frequencies by characterizing MOSFET parameters such as threshold voltage or saturation current. However, the nature of some of these sources of variability may manifest itself only at high frequencies. Two circuits have been designed and implemented to assess the potential manifestations of these short time-scale, or AC, variation sources.The first circuit is a simple array-based test structure consisting of 128 devices under test (DUTs) whose relative delays are characterized using a logic gate-based delay detector circuit, as shown in Figure 1 [1] . The delay measurement technique only requires a single off-chip DC voltage measurement for each DUT. A design-time optimization is performed on each DUT array to ensure that the measured delays of each DUT primarily reflect its AC, or short time-scale, characteristics rather than previously well-studied DC characteristics such as saturation current, threshold voltage, and channel length.The second circuit, shown in Figure 2, is a ring oscillator (RO)-based test structure which transforms small delay variations into easily measurable digital and DC quantities [2] . This enables the calculation of a parameter that primarily reflects the AC, or short time-scale, characteristics of the DUT. An array of such ROs is designed in order to obtain statistics on the DUTs. The array-based circuit and RO-based circuit occupy areas of 400 um x 20 um and 1600 um x 20 um, respectively, and are both implemented in an advanced CMOS PD-SOI technology. Simulations show that both circuits exhibit good sensitivity towards potential AC variation sources in transistors."
An On-Chip Test Circuit for Characterization of MEMS Resonators,"Electromechanical resonators such as quartz crystals, surface acoustic wave (SAW) resonators, and ceramic resonators have become essential components in electronic systems. However, due to their large footprint and difficulty in integrating with CMOS processes, there has been much interest in developing microelectromechanical systems (MEMS) resonators that achieve comparable performance yet have smaller footprint and are compatible with CMOS. Recently, MEMS resonators have been proposed that overcome physical limitations in traditional resonators to reach frequencies in the GHz range. In addition, they have the potential for compatibility with CMOS, opening up possibilities for new circuits and systems [1] . As with other semiconductor devices, with increasing frequency and with decreasing device size into the submicron scale, variability has started to become a critical issue in MEMS resonators. Thus vigorous characterization of important device parameters such as resonant frequencies, quality factors, and variations associated with them has become necessary. However, one of the critical challenges is the lack of a characterization method that is accurate but efficient enough to be used for testing of the large number of devices necessary to acquire accurate statistical distribution of the parameters of interest. This project proposes an on-chip test circuit that can accurately characterize a large number of resonators for variation analysis. The desired test circuit is general enough that it can be used with a wide range of resonators, not limited to specific frequencies or other properties. Previous works have attempted to achieve similar goals, but most of them were restricted to characterization of a single device or a narrow range of properties. The proposed test circuit is based on a transient step response method using a voltage step that can accurately measure the resonant frequencies and the quality factor of devices [2] . The circuit employs a sub-sampling method to capture the high-frequency decay signal [3] and a simple analog-to-digital converter (ADC) [4] allowing complete digital interface, an important feature for test automation."
Micro-contact Printed MEMS,"It is desirable to extend the functionality of MEMS to different form factors including large area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates. We present a new method for fabricating thin (140-nm-thick) suspended metal films in MEMS using micro-contact printing. These films can be utilized in pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of MEMS sensors.Our approach to MEMS fabrication involves the use of a stamp and a donor viscoelastic transfer pad that is coated with an organic release layer and a thin film of metal. The stamp consists of a layer of patterned polydimethylsiloxane (PDMS) atop a glass slide that is coated with a layer of electrically conducting indium tin oxide (ITO). The surface of this patterned PDMS stamp is placed in contact with the thin metal film on the donor transfer pad, and then the stamp is rapidly peeled away, picking up the metal film. The metal film ends up bridging the gaps in the patterns of the PDMS stamp, forming a capacitive MEMS structure. A continuous film of metal is lifted onto the stamp only if the stamp is peeled off the transfer pad rapidly.This process avoids the use of solvents and etchants, eliminating the need for deep reactive-ion etching and other harsh chemical treatments. Solvent absence during fabrication also avoids the detrimental effects of MEMS stiction that can result during wet processing. MEMS fabrication on flexible polymeric substrates is also possible due to the absence of elevated temperature processing.Thin films up to 0.78 mm2 in area have been fabricated using the aforementioned process, as shown in Figure 1. These MEMS devices are actuated electrostatically to demonstrate the deflection of 25-μm-diameter films (see Figure 2)."
Malaria-diagnostic System Based on Electric Impedance Spectroscopy,"Malaria prevails mainly in the countries that lack proper medical facilities, and it kills about a million people worldwide a year. This parasitic disease invades human red blood cells (RBCs), and it is life-threatening unless treated immediately [1] .This work focuses on utilizing a single cell analysis technique to develop a rapid malaria diagnostic test system among various approaches to diagnose the disease in its early stage. Single cell analysis based on electronics enables high throughput tests of biological cells. The specific analysis method used in this research is electric impedance spectroscopy (EIS), which measures the electric impedance of biological cells flowing continuously over a pair of electrodes, so that it can differentiatecells whose impedance is highly correlated to cell size and cytoplasm permittivity [2] [3] [4] [5] [6] .The system consists of two parts: a MEMS probe and a reader circuit. To investigate one cell at a time and to achieve enough sensitivity to tiny (<10 µm) human red blood cells, a MEMS device consisting of a microfluidic channel and micro-electrodes is fabricated. The probe MEMS device is made of transparent materials except the electrodes for convenience of monitoring. In addition, a printed-circuit-board using a commercial impedance-to-digital chip is made to continuously measure electric impedance in high-speed manner. The circuit board generates a sinusoidal voltage signal, measures the DFT of the resulting current, and calculates the impedance from DFT. We are seeing this system as a possible solution for developing a low-power, highly sensitive, and cost-effective malaria diagnostic device."
Automated Passive Dynamical Model Extraction of Thin Film Bulk Acoustic Resonators (FBAR) for Time Domain Simulations,"Thin Film Bulk Acoustic Resonators (FBARs) are widely used in the design of modern radio frequency components including duplexers, filters, and oscillators. The overall goal of this project is to incorporate the performance parameters of these resonators into the design flow of the overall system. As a first step, the frequency response of the fabricated devices is measured. Traditionally, an equivalent circuit is then built based on least squares fitting of the frequency response of a simple RLC network to the measured data [1] . Such a technique is fairly simple, and the resulting equivalent model does capture important performance parameters, such as quality factor and resonant frequency. However, this technique cannot capture spurious resonances and other second order effects, which quite often play a significant role in the overall performance of the device.In this work, we are developing tools that will automatically generate accurate, compact, and passive dynamical models for FBARs. Given measured transfer function samples, we identify a rational transfer function model that minimizes the mismatch at the given frequencies. These dynamical models can be interfaced with commercial circuit simulators for time domain simulations of a larger interconnected system. To guarantee the stability of the overall simulation, we ensure the passivity of our generated models by enforcing semidefinite constraints during the fitting process as proposed in [2] . Figure 1 shows the 3D layout of an FBAR. Numerical results are presented for resonators configured to constitute a bandpass frequency response. Figure 2 compares the output of our identified models with the given measured data."
Single-cell Trapping and DNA Damage Analysis Using Microwell Arrays,"DNA damage has been found to play critical roles in cancer, aging, and heritable disease. There is rising interest in studying DNA damage and repair kinetics in cells, but the lack of a robust, inexpensive, and high-throughput device for quantitative DNA damage analysis makes such investigations far from routine. The single cell gel electrophoresis or “comet” assay is one of the best-established methods for detection of DNA lesions and strand breaks. Based on the principle that relaxed loops and fragments of damaged DNA migrate farther under the influence of an electrical field in agarose gel than undamaged DNA, the level of DNA damage can be assessed by measuring the relative amount of DNA migration. Although extremely versatile and inexpensive, the comet assay is restrained from wider acceptance due to its low throughput, poor reproducibility and laborious and potentially biased analysis methods. Through incorporation of microfabrication techniques, we have developed a microarray platform to perform high throughput single cell electrophoresis with improved consistency. Different from randomly dispersed cells in the traditional comet assay, cells on our platform are patterned into spatially registered microscopic wells. These microwells are formed by direct stamping of hydrated gels with molds that contain patterned microposts fabricated through Su-8 photolithography (Figure 1). Cells are arrayed through passive settling into the microwells and can then undergo treatment and analysis. We have also developed software that utilizes the unique patterning feature to automatically analyze images with high accuracy and reproducibility (Figure 1). By sandwiching our patterned agarose gel between a bottomless 96-well plate and glass plate, we have transformed our assay into a multiwell version, referred to as “the CometChip.” This 96-well format enables simultaneous investigation of different chemical conditions among different cells samples, as well as analysis of repair kinetics. Importantly, the CometChip is compatible with standard automated liquid handling and imaging. The efficacy and increased throughput of the CometChip for DNA damage analysis is demonstrated by comparing an irradiation dose response to the traditional comet assay (Figure 2). All doses and replicates were assayed on a single CometChip in significantly shorter time and with less labor. Our research has demonstrated a significant technological advance to traditional methodology and opened countless possibilities in epidemiology and drug development applications."
Continuous Signal Enhancement for Sensitive Aptamer Mobility Shift Assay Using Electrokinetic Concentration,"Aptamers are emerging as popular alternatives to antibodies as affinity probes in immunoassays. From a point-of-care diagnostics standpoint, aptamers have an advantage over antibodies since they are stable over a wide range of conditions and can be chemically synthesized at low cost. Affinity probe capillary electrophoresis (CE) [1] [2] is a promising platform with which to perform aptamer-based biomarker detection as it features fast homogeneous reaction kinetics and requires only one affinity probe species, although sensitivity is still limited due to band dispersion, complex dissociation and lack of amplification reaction.We have previously demonstrated microfabricated nanofluidic preconcentration devices that can continuously accumulate a charged biomolecule species at a specified location [3] [4] [5] . In this work, we showed that these devices can also efficiently separate biomolecules with different mobilities by focusing them at different locations. This phenomenon lends itself well to aptamer affinity probe CE, where aptamers undergo a significant mobility shift upon binding to larger target proteins. The important advantage of this scheme compared to conventional CE is that aptamer-protein dissociation and band broadening effects are counteracted by electrokinetic focusing. By simultaneously focusing and separating free aptamers from aptamer-protein complex in this device, we can obtain highly sensitive and quantitative measurement of target biomarkers using aptamers.With this scheme, we showed enhanced detection sensitivity for IgE and HIV-1 RT in simple buffer solution. The limits of detection obtained (4.5 pM for IgE and 9 pM for HIV-1 RT) are among the lowest reported in the literature. The limit of detection for IgE in 10% serum was 10-fold higher due to nonspecific interactions between aptamers and serum proteins. Due to the simple readout for this assay, multiple samples can be assayed in parallel. As the assay is driven by gravitational flow, uses low voltages (30 V), and does not require multiple processing steps, it is well-suited towards low-cost point-of-care analysis."
Dynamic Cell Deformability Study in Microfluidic Devices,"The mechanical properties of tissues and cells have important implications on their differentiated state, functions and responses to injury. Altered cell deformability is both a cause of and biomarker for potentially severe diseases, such as cancer, sickle cell anemia and malaria. In the past, several techniques have been developed to measure single-cell deformability including micropipette aspiration, atomic force microscopy, and optical tweezers. However, many of these measurements assess only static cell deformations which often fail to reflect in vivo situation when cells are in microcirculation. Additionally, the low throughput of the techniques limits sampling size per experiment, which may potentially lead to misrepresentation of population-wide trait. Therefore, we aim to develop a microfluidic device which measures cell dynamic deformability with high sensitivity and high throughput.In this project, the relation between cell dynamic deformability and disease state is aimed to be established for several representative cell lines including human erythrocytes, breast cancer cells, and mesenchymal stem cells. The impact of microenvironmental controls such as temperature fluctuation and drug treatment on the deformability of malaria infected cells is also investigated."
An Integrated Microfluidic Probe for Concentration-enhanced Selective Single Cell Kinase Activity Measurement,"We present an integrated microfluidic probe that captures the contents of selected single adherent cells from standard tissue culture platforms and directly measures specific protein kinase activities in the captured lysate using either a fluorimetric assay in a small isolated chamber or a concentration-enhanced mobility-shift assay in an integrated nanofluidic concentrator. We demonstrate the use of the probe by measuring kinase activity in a single human hepatocellular carcinoma (HepG2) cell.Traditional cellular assays measure average properties of 103-106 of cells, missing differences (e.g., drug responses) between individual cells in supposedly homogenous populations that have consequences for treatment of diseases [1] . Recent microfluidic or traditional tools [2] have studied genetic differences between single cells using nucleic acid amplification. These tools fail to capture important non-genetic sources of heterogeneity that create unique proteomes in different cells. Direct measurement of protein activities from single cells remains difficult due to limited assay sensitivity. In addition, difficulties in interfacing with adherent cells in a standard culture have led to the use of cell suspensions in microfluidic single cell assays [2] .The integrated device (Figure 1) reported here interfaces with standard tissue culture plates using a microfluidic probe [3] that creates a limited, tunable lysis zone at its tip by simultaneously dispensing and collecting lysis agents and lyses and collects contents of selected single cells from adherent cell populations. The captured cytosol is mixed with assay reagents and flowed into a small reaction chamber, which is isolated for observation, using pneumatic micro-valves. The integrated ion-selective hydrogel-based nanofluidic concentrator [4] is then used to trap/concentrate the proteins/reaction products in the mixture to yield very high kinase assay sensitivity [5] , sufficient to probe proteins from single cells. This single cell detection platform is agnostic to specific sensing chemistry, so other biochemical assays can also be implemented with minimal modification."
Microfluidic Platforms for Studying the Role of the Biophysical and Cellular Microenvironment in Tumor Invasion,"Tumor invasion has received considerable attention as a critical step in cancer metastasis, and is a promising target for developing new cancer drugs. Current understanding of the role of the biophysical and cellular microenvironment in tumor invasion is limited, because of the lack of appropriate in vitro and in vivo models. We have adapted our previous microfluidic platforms [1] for studying the role of the endothelium on tumor intravasation (entry into the vascular system) and the effects of interstitial flow on tumor cell migration, along with the development of new hard plastic devices for commercial transition.Recent results from the tumor-endothelial interaction assay demonstrated the capability to form a confluent endothelial monolayer on collagen type I matrices, in the presence of invading tumor cells in 3D (Figure 1). Stimulating the layer with inflammatory cytokines, we demonstrated an increase in diffusive permeability to fluorescent dextrans, in agreement with a measured increase in the number of intravasation events. These results demonstrate the utility of this assay for studying the role of the endothelial barrier function in tumor cell intravasation.We also developed a microfluidic system for investigating the role of interstitial flow in tumor cell migration (Figure 2). Tumor cells exposed to interstitial flow preferentially migrated along streamlines, and the relative percentage of cells migrating upstream and downstream was found to be a function of chemokine receptor activity and cell density. Interstitial flow stimulated downstream tumor cell migration through CCR7-mediated autocrine signaling. However, flow also stimulated upstream cell migration through a competing, mechanically mediated pathway, as evidenced by significant increases in FAK activation in devices with flow. Relative strengths of the autocrine and mechanical stimuli determine whether cells migrate upstream or downstream.We applied known commercially-viable manufacturing methods to a cyclic olefin copolymer (COC) to fabricate a microfluidic device with controlled surface properties and improved potential for high-volume applications. Culture of cells in the new COC device indicated no adverse effects. Therefore, this transition of platform demonstrates a capability of using microfludic devices for 3D cell culture across the range from the scientific research to applications with broad clinical impact."
Thermal Ink Jet Printing of PZT Thin Films,"We recently demonstrated a process of thermal ink jet printing of PZT thin films for MEMS applications [1] [2] . Previous methods for deposition of solution-based PZT were painstaking and low-yield; they also imposed significant processing and design constraints. Thermal ink jet printing allows for rapid, low-cost deposition of patterned PZT films over a wide range of geometries and provides for greater flexibility in process sequencing. With this technique, PZT may be easily integrated into devices with large out-of-plane features after the micro-machining process, which enables the formation of more complex device structures. In 2010-2011 the printing process was modeled in detail, including the dynamics of droplet formation as well as the internal flows that occurring during film drying. Models proposed by others were extended to include printed PZT films [3] . Experiments were carried out to confirm the modeling. Specifically, high-speed camera images (Figure 1) were taken to visualize the droplet formation, and the effects of surface tension and temperature were investigated through droplet drying tests. As a result the conditions required for highly repeatable and uniform printed films were determined. Further development work focused on the integration of printed PZT into a range of micro-machined structures including cantilevers and bridges with energy harvester applications as well as resonators for ultrasonic transduction (Figure 2). These devices provide a proof of concept for a fully integrated PZT device fabrication process. In the future we plan to produce devices that utilize the full capabilities of this process to reach energy densities and acoustic coupling greater than those of devices based on current deposition techniques."
Piezoelectric Transducers for Advanced Ultrasound Imagining Systems and Energy Harvesting,"In this project, a piezoelectric 2-D array of ultrasound transducers will be developed for compact, portable 3-D ultrasound imaging systems. Piezoelectric materials have been used for macro-scale ultrasound systems due to their high polarization density. However, making tiny 2-D array of transducers with conventional piezoelectric materials (all ceramic or polymeric composite) has been extremely difficult. Dicing and bonding of crystallized piezoelectric ceramic bulk and subsequent delicate assembly operations require a lot of manual effort, which limits production yield, rate, and quality. In addition, piezo-ceramics inherently have high acoustic impedance, which is difficult to match in liquid or air medium. Capacitive Micromachined Ultrasonic Transducers (CMUTs) have been developed to leverage the MEMS fabrication techniques for small form factor transducer fabrication and to mitigate the acoustic impedance mismatch [1] . A CMUT consists of metallized silicon nitride membranes suspended above highly doped silicon bulk. These membranes vibrate when an electrostatic charge is generated under each membrane. Each membrane can also detect the reflected sound wave by measuring the capacitance change at the gap under each membrane. CMUTs offer greater bandwidth than piezoelectrics and are tunable [2] . Moreover, many of the available MEMS processing technologies could be used to make micro-scale arrays of CMUT elements effectively. However, CMUTs still have some technical issues such as high voltage requirement, which makes them not suitable for in vivo operations, result in insulator breakdown, and cause static charge accumulation at the membrane surface.This research project will focus on developing PZT micromachined ultrasound transducers (PMUTs) and designing novel 2-D array PMUTs with a reliable PZT process technique of PZT. The initial goal of this project is to study the PZT structure appropriate for a 64×64 array and actuation voltage less than 10 volts. A prototype PZT structure will be fabricated and characterized to demonstrate the feasibility of the technology. In addition, the low voltage limits, potential efficiency, and sensitivity will be determined and optimized. Fabricating an array of PZT pillars with size less than 50 mm is one of the major challenges of this project. A new and flexible on-demand deposition process for high quality PZT thin films developed by Bathurst et al. will be used to solve this problem [3] .In addition to the advanced medical applications, the core technology developed in this project will be applied to further improve the ultra-wide bandwidth of energy harvesters. This will lead energy harvesters to be deployable in real world applications including sensors for energy efficient buildings, structural monitoring devices of crude oil pipelines, and leak detectors in water supply networks."
"Design of Low-frequency, Low-g, Nonlinear Resonating Piezoelectric Energy Harvesters","To overcome the limitations of piezoelectric energy harvesters such as narrow bandwidth and low power density, our group has recently demonstrated a broadband harvester, which is based on amplitude-stiffened Duffing mode resonance. This nonlinear resonance greatly increases the bandwidth by keeping the harvester resonant until jumping down to a low energy state. Furthermore, the stretching strain of the nonlinear beam produces much higher maximum extractable electrical energy than that of a linear bending-based harvester. This design has been fabricated into a compact MEMS device, which is about the size of a US quarter coin. The test results show more than one order of magnitude improvements in both bandwidth (~20% of the peak frequency) and power density (up to 2W/cm3) in comparison to the devices previously reported. To make the energy harvester better scavenge energy from ambient vibrations, which typically have low frequency spectra and low-g excitation, we are exploring new designs based on the nonlinear resonance. We have found that it is possible to bring the working frequency down to the range of 100 Hz to several hundred Hz, and lower the excitation level to ~0.5 g, by tuning the design parameters such as the dimensions of the resonator and external proof mass. The new low frequency, low-g designs will be implemented and tested soon. We anticipate that the broadband, low frequency, low-g piezoelectric energy harvesters will be used to power a wide range of devices including portable electronic devices and self-powered wireless sensors."
MEMS Pressure-sensor Arrays for Passive Underwater Navigation,"A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the lateral line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1] [2] . The canal subsystem of the organ can be described as an array of pressure-sensors [3] . The lateral line allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby objects [2] [5] . Similarly, by measuring pressure variations on a vehicle surface, an engineered pressure-sensor array allows the identification and location of obstacles for navigation. Several strain-gauge-based approaches to the sensing element are being tested. The two types presented here are silicon- and polymer-based technologies.Both sensor designs share the following features. The array consists of thin diaphragms. Each sensor has an empty cavity behind the membrane connected via a common backplane to the others. A set of strain gauge resistors on the diaphragms responds to pressure changes. When the sensor is placed in a Wheatstone bridge, the resulting output voltage can be used to determine the change in resistance in the strain gauges and thus the pressure difference between the two sides of the diaphragm. The two technologies differ chiefly in how strain is measured. In the silicon-based approach, the shape of the resistor is altered slightly by strain. In the polymer-based approach, the distances between conducting particles embedded in the material adjusts as strain is applied.The amplified voltage output bridges with strain-gauge resistors on diaphragms of various sizes as was measured as a function of applied pressure. Generally, larger diaphragms are more stable and more sensitive, whereas small diaphragms maintain linearity over a wider range and are more physically robust. The deflections of the centers of silicon diaphragms are measured as functions of applied pressure. Although larger diaphragms exhibit non-linear behavior, there are no hysteretic effects, thus enabling their usage for static and dynamic pressure sensing.For the conductive polymer strain-gauge patterned onto a PDMS membrane, the resistances of the strain gauges are measured against the segment length. The resulting linear fit demonstrates consistency of resistivity across the patterned structure. Finally, observing output voltage in response to dynamic pressure applied with a syringe connected to the sensor indicates a bandwidth fast enough for underwater sensing."
MEMS Space Thrusters: The ion Electrospray Propulsion System (iEPS),"Electric Propulsion (EP) brings benefits for space missions requiring relatively large changes in satellite velocity, for example by reducing the propellant mass compared to traditional, less fuel-efficient chemical engines. Introducing EP in small satellites would enable them to perform interesting missions, such as long term attitude control/drag cancellation, orbital modification and, perhaps, deep space travel [1] . However, most EP technologies are challenging to miniaturize to the required levels, especially for nano/pico-satellites. Our group has developed an ion Electrospray Propulsion System (iEPS) as a candidate of an EP technology amenable for efficient miniaturization. The thruster core is based on a porous metal structure, which is bonded to an oxidized silicon package frame, followed by masking of the metal with a pattern of circles. The metal is then electrochemically etched in a regime that prevents material removal inside the pores, thus forming an array of porous tips [2] , as shown in Figure 1. To finalize the device, an extractor silicon grid with a matching array of holes and coated with a gold film is aligned and bonded to the frame holding the porous metal. Electrical isolation is provided by the bonding material and grown silicon oxide layers. A zero vapor pressure ionic liquid (the propellant) is then injected to the device from the back through a port in the silicon frame. The liquid wicks through the porous structure reaching the tips. Ion emission is the produced when applying a voltage of about 1kV between the metal and extractor grid. Figure 2 shows a typical I-V curve and a picture finished devices on a CubeSat [3] . A thruster pair should be able to produce 60-70 micro-N, enough to raise the orbit of a 1 kg CubeSat by 400 km in about 25 days of operation consuming 6-7 grams of propellant with 1W of power."
Suspended Microchannel Resonators with Piezoresistive Sensors,"Precision frequency detection has enabled the suspended microchannel resonator (SMR) to weigh single living cells, single nanoparticles, and adsorbed protein layers in fluid. To date, the SMR resonance frequency has been determined optically, which requires the use of an external laser and photodiode and cannot be easily arrayed for multiplexed measurements. Here we demonstrate the first electronic detection of SMR resonance frequency by fabricating piezoresistive sensors using ion implantation into single crystal silicon resonators [1] . To validate the piezoresistive SMR, buoyant mass histograms of budding yeast cells and a mixture of 1.6-, 2.0-, 2.5-, and 3.0-mm-diameter polystyrene beads are measured. Figure 1 shows our experimental setup. For piezoresistive detection, a Wheatstone bridge is built with the piezoresistor and three external resistors. The bias voltage (5 V) is selected to maximize the signal while limiting the temperature increase in the piezoresistor due to resistive heating. Figure 6 shows mass resolution derived from mass sensitivity and Allan variance. In summary, the mass resolution achieved with piezoresistive detection is comparable to what can be achieved by the conventional optical-lever detector in 1 kHz bandwidth. Eliminating the need for expensive and delicate optical components will enable new uses for the SMR in both multiplexed and field deployable applications."
Continuous Microbioreactors,"For systems biology, the models are more often limited by the absence of reliable experimental data than by available computational resources. Unfortunately, there is still great difficulty in making the leap from genetic and biochemical analysis to accurate verification with conventional culture growth experiments due to variations in culture conditions. Measurements of metabolic activity through substrate and product interactions or cellular activity through fluorescent interactions are highly dependent on environmental conditions and cellular metabolic state. For such experiments to be feasible, continuous cultures [1] [2] utilizing control strategies must be developed to measure chemical concentrations, introduce chemical inputs, and remove waste. An integrated microreactor system with built-in fluid metering will enable environmental control and programmable experiments capable of generating reproducible data.The chip shown in Figure 1 is fabricated out of a rigid plastic polycarbonate, utilizing PDMS membranes for actuation and pumping [3] . The fabrication process for bonding plastic-PDMS hybrid devices has been described previously [4] . Mixing and oxygen delivery are performed through membranes between the fluidic and actuation layers of the growth chamber sections. A growth volume of 1 mL ensures the ability to couple sampled volume to offline chemical analysis. Culture experiments are performed using E. coli strain FB21591 grown on defined media. The glucose input is separated to provide input control. As shown in Figure 2, various metabolic states are observable through continuous flow control. Cell density is directly dependent on glucose input, and acid production is proportional to cell density in chemostat mode. In turbidostat mode, cell density can be kept constant and glucose utilization can be observed, demonstrating the direct observation of overflow metabolism."
Controlling the Intrinsic Stresses in Polycrystalline Metallic Films for N/MEMS Applications,"Polycrystalline metallic thin films are vital in a wide variety of applications, including microelectronics, plasmonics, magnetic storage, N/MEMS and catalysis. Because mechanical properties strongly influence their reliability and performance, understanding and controlling the intrinsic stresses in as-deposited films are of great importance. When the capacitance or multi-beam laser technique is used, real-time stress measurements can be performed during thin film deposition. The measurement results do not only provide a useful tool to define the stress evolution history but also an insightful picture for the study of structure evolution processes. When combining the in situ stress measurement with other characterization techniques and theoretical modeling, we are able to move towards a comprehensive understanding of the underlying atomic processes during Volmer-Weber growth.We have experimentally studied the intrinsic stress evolution at different homologous temperatures. Figure 1 shows the general trend—compressive stress is favored at higher temperatures and tensile stress is favored at lower temperatures. This trend indicates that the compressive stress generation processes are thermally activated. Furthermore, we found the incremental stress changes from compressive to tensile during growth at intermediate homologous temperatures, e.g., Ni at 398K. The origin of the compressive-tensile transition is not known exactly. None of the previous models [1] [2] [3] are able to explain the transition behavior. We have also studied the stress behaviors during long interruptions of gold films. Particularly, we studied the long interruptions of gold films with different thicknesses. Figure 2 demonstrates that the total released stress is dependent on the film thickness during long interruptions. This result strongly suggests that the stress relaxation during long interruptions is a bulk process. Meanwhile, we found abnormal grain growth occurs in as-deposited gold film at room temperature. By calculating the densification stress due to grain growth and comparing its value with the measured bulk relaxation stress, we conclude that grain growth is the main process of bulk stress relaxation."
MEMS Langmuir Probes for Atmospheric Reentry Plasma Diagnostics,"One of the most fundamental technical problems concerning spacecraft design is preparing the vehicle to survive the extreme conditions encountered during reentry into the Earth’s atmosphere [1] . When a hypersonic vehicle travels through the atmosphere, a high-density, low-temperature plasma sheath forms around it [2] . The reentry plasma sheath affects heat transfer to the spacecraft, aerodynamics, and perhaps most notably, communications. A communications blackout is a major threat, bringing about a complete loss of RF signal strength between the reentry vehicle and the ground. A thorough knowledge of reentry plasma sheath properties is needed to effectively develop systems capable of maintaining communications during reentry. However, the reentry plasma sheath occurs due to processes that are not well understood. Furthermore, the conditions of the plasma sheath rapidly change throughout reentry, which introduces additional complications. Analytical approaches alone are not sufficient to gain a complete understanding of the plasma sheath. Therefore, instrumentation must be developed to measure properties of the plasma sheath during reentry [3] .We propose a novel approach to reentry plasma diagnostics, utilizing planar arrays of MEMS Langmuir probes to perform real-time measurements of the electron temperature and number density of the reentry plasma sheath. The MEMS Langmuir probes, shown in Figure 1, consist of an array metallic vias in a high temperature-resistant dielectric substrate, which can be blended onto the outer surface of a reentry vehicle (i.e., as a sensorial skin). Figure 2 shows one of the early prototypes we made as proof of concept of the device process flow. The MEMS Langmuir probes are made using electroplated gold and an ultrasonic drilled Pyrex substrate. The performance of the MEMS probes will be validated experimentally in laboratory plasmas similar to those encountered by spacecraft during reentry."
Scaling of High Aspect Ratio Current Limiters for the Individual Ballasting of Large Arrays of Field Emitters,"Field Emitter Arrays (FEAs) are excellent cold cathodes, but they have not found widespread adoption in demanding device applications because of several major challenges, including spatial/temporal current variations emanating from emitter tip radius distribution and the work function fluctuation. A consequence of tip radius variation is that the sharper emitters burn out from Joule heating before duller emitters turn on, reducing the current attainable from FEAs.Addressing these challenges, groups have incorporated current limiting (ballasting) elements including large resistors [1] , diodes [2] , and MOSFETs [3] into FEAs, but none of these simultaneously provide high current, high emitter density, and high current density. Velasquez-Garcia et al. demonstrated silicon vertical ungated FETs integrated with FEAs, resulting in a Si tip on Si pillar structure [4] . The ungated FET has a current-source-like I-V characteristic, providing effective individual ballasting of emitters while allowing uniform and high current emission without thermal runaway [4] . To limit emission current, the device uses pinch-off and velocity saturation of carriers in a Si high aspect ratio channel. Their pillars have a diameter of 1 µm, height of 100 µm, and 10-µm pitch, resulting in a density of 106 emitters/cm2. However, a consequence of tip radius variation and ballasting is that the energy distribution of emitted electrons is larger when compared to un-ballasted FEAs.To obtain FEAs with higher current densities, lower operating voltages, and reduced energy spread while retaining current uniformity, we expanded on previous work by scaling their tip on Si pillar structure. We developed vertical ungated FET current limiters 100 nm in diameter, 8 µm tall, and with 1-µm pitch, increasing the density to 108 emitters/cm2 (Figure 1). These devices demonstrate excellent current saturation of 15 pA / pillar with a linear conductance of 2.6×10-10 S/pillar and an output conductance under 10-13 S/pillar. The current saturates at a drain to source voltage under 0.2 V. These are the highest density, smallest diameter, and lowest operating voltage Si vertical ungated FETs ever reported."
Batch-micromachined RPAs for Plasma and Ion Measurements,"Retarding potential analyzers (RPAs) were first developed in the 1960’s. RPAs find widespread application including characterization of near-spacecraft environments and assessment of the propulsion efficiency of plasma-based space thrusters. In this project we are exploring the multiplexing and scaling-down limits of RPAs using micro and nanotechnology. Miniaturized RPAs will weigh visibly less, which will reduce the cost of a nanosatellite-based mission. Also, miniaturized RPAs will provide better diagnostics of spacecraft plasma plumes as smaller projected area will be less disruptive to plasma under observation. In addition, batch-fabricated miniaturized RPAs can be used as part of a spacecraft “sensorial skin” that provides detailed local information of the plasma surrounding the spacecraft, particularly during re-entry, when monitoring exterior conditions is essential to ensuring safety during the mission.An improvement of our work from the state-of-the-art RPAs is the introduction of enforced aperture alignment. When the apertures of each successive grid are aligned, the optical transparency of the sensor increases, which should result in improved signal strength. We recently developed a first-generation prototype of a hybrid microRPA (Figure 1). The hybrid microRPA has micromachined electrodes and a stainless steel housing. Internal dynamics of this type of energy analyzer, however, are more complex than simple transmission or reflection of the various ion species [1] [2] . This fact is made evident by the experimental characterization of the microRPA using a commercial thermionic ion source for mass spectrometry. Figure 2 shows that the measured data reveal a peak in the energy distribution function around 5.4 V of retarding potential when the ionization region is at 10 V. Therefore, the observed ion energy distribution (dotted) deviates from the expected (continuous line) by approximately 4.6 V, a shift that is constant for a wide range of ionization region potentials. We speculate that changes in the internal dynamics due to enforced aperture alignment, sources of error in the applied voltages due to the materials selected, or a combination thereof are cause for this anomaly. Exploration of these potential sources of error continues, as well as the manufacturing of a fully batch-microfabricated RPA sensor with housing based on 3D HV packaging technology [3] [4] ."
Electron-impact-ionization Pump Using Double-gated Isolated Vertically Aligned Carbon Nanotube Arrays,"There is a need for microscale vacuum pumps that can be readily integrated with other MEMS and electronic components at the chip-scale level. Vacuum pumps exhibit favorable scaling and are promising for a variety of applications such as portable mass spectrometers [1] and vacuum amplifiers. This project aims to develop the technology for a micro-fabricated electron-impact-ionizer pump. The micropump consists of a field-emission electron source that is an array of double-gated isolated vertically aligned carbon nanotubes (VA-CNTs), an electron-impact-ionization region, and an ion implantation getter, as shown in Figure 1. The pump works as follows: first, electrons are field-emitted from the VA-CNT array; then, the electrons are accelerated at a bias voltage that maximizes the probability of collision with neutral gas molecules, this way achieving ionization by fragmentation of the molecules; finally, ions are implanted into the getter.In a double-gated field-emitter array, the first gate (extractor) is used to modulate the tunneling of electrons out of the tip, while the second gate (focus) is biased at a lower voltage than the first gate to focus the emitted electrons and to collect the back-streaming ions, thus protecting the tip [2] . As part of this work, we designed and fabricated single-gated isolated VA-CNT field-emission arrays, shown in Figure 2(a), to quantify the effectiveness of the field emitter-extractor diode to enhance the electric field on the emitter tip (i.e., estimate the extractor field factor), through experiments and simulations using the commercial software COMSOL. Figure 2(b) shows the solution of electric field using the same geometry of the device we fabricated. Each emitter has a 15-nm tip radius and 2-µm height with a 1-µm aperture from a single gate. From the simulation results we obtain an extractor field factor of 7.35×105V/cm. Figure 2(c) is the experimental FN plot of an array of ~10,000 single-gated emitters. From the slope of the plot we estimate a field factor of 7.8×105V/cm, which is in good agreement with the prediction of the extractor field factor from the COMSOL simulation."
Near-ultraviolet Sensor Based on Horizontal Low-Temperature Solution-Grown Zinc Oxide Nanowires,"A near-ultraviolet (UV) sensor based on zinc oxide (ZnO) nanowires (NWs) that is sensitive to photo excitation at or below 400-nm wavelength has been fabricated and characterized. The device uses a single optical lithography step, and the NWs are grown at a low temperature from solution. ZnO is a wide direct band gap (3.37 eV) semiconductor whose absorption edge is in the near-UV range, making it an ideal near-UV photodetector. This is the first reported ZnO NW near-UV sensor that is insensitive to visible light (visible blind) and fabricated using a low temperature solution process [1] . At a voltage bias of 1V across the device, a 29-fold increase in current is observed in comparison to dark current when the NWs are photo excited by 400-nm light-emitting diode (LED), 8.91 µA (photo excitation current) vs. 311 nA (dark current).The fabrication of the near-UV sensor device is based on a single optical lithography step with no processing steps that exceed 100°C. The devices are compressed of a thin ZnO film with a metal cap. The sidewall of the ZnO film within the material stack acts as a seed for lateral growth of ZnO NWs. The metal cap restricts vertical growth of the NWs and doubles as the device electrodes. The symmetric devices have multiple electrode shapes and gaps between the electrodes ranging from 1-20 µm. The horizontally grown ZnO NWs bridge the gap between the two electrodes. The wires vary in length from 0.8 to 8.4 µm and diameter from 80 to 300 nm, depending on growth time. The result is a self-aligned ZnO NW ‘visible blind’ near-UV sensor that utilizes a low temperature process and a simple one-mask optical lithography step that can be integrated on a flexible substrate."
Massively Parallel Microfluidic Cell-pairing Platform for the Statistical Study of Immunological Cell-cell Interactions,"Many immune responses are mediated by cell-cell interac­tions. In particular, cytotoxic T cells form conjugates with pathogenic and cancer cells in order to fight disease. More­over, T cell maturation and activation is governed by direct cell interactions with antigen-presenting cells (APCs). Er­rors in these processes can lead to the progression of severe diseases, such as multiple sclerosis (MS) and type 1 diabe­tes. The study of these intricate cell-cell interactions at the molecular scale is therefore crucial for understand­ing the dynamics and specificity of the immune response. One important feature of these interactions is the variability of response across populations. Cell-to-cell variability in pre­sumably homogeneous populations exposed to the same environmental conditions is ubiquitous, yet has long been neglected in immunology due to the limitations of conven­tional assay methods [1] [2] . Traditional methods to study cell-cell interactions, such as bulk measurements [3] or im­mobilization of cell pairs on a dish [4] [5] , suffer from both the inability to control cell-pairing at the single cell lev­el and the inability to study dynamic cell-cell interaction processes with high spatial and temporal resolution. We have overcome these limitations by developing a platform that can control cell pairing across thousands of individual immune cell pairs simultaneously while allowing visual­ization of the resulting responses. This approach enables us to quantify and understand variations in cell-cell inter­actions within large cell populations at the resolution of individual cell pairs. Previously, we developed a microflu­idic device with the capability to create thousands of such single cell pairs for the study of stem cell reprogramming (Figure 1, [6] ). To adapt the approach to work with smaller primary immune cells, we performed hydrodynamic mod­eling to guide redesign of the trap geometry (Figure 2). The modeling was used to determine how to adjust the trap ge­ometry to maximize flow through the center of the cups, which is crucial to the loading process. We determined that altering the cup-to-cup spacing transverse to the flow had the greatest impact on flow through the cups. We fabricated redesigned traps and are in the process of test­ing their pairing efficiency with primary immune cells."
Cell-based Sensors for Measuring Impact of Microsystems on Cell Physiology,"The use of microsystems to manipulate and study cells in microenvironments is continually increasing. However, along with such increase in usage is also a growing concern regarding the impact of these microsystems on cell physiology. In this project, we are developing a set of cell-based fluorescent sensors to measure the impact of common stresses experienced in microsystems on cell physiology. We are including stress agents commonly found in microsystems (e.g., UV exposure, heat shock, fluid flow, etc.). Each sensor is designed to respond to one particular stress agent but can also be combined for multiplexed analysis of multiple stresses at once, as might be experienced in a typical microsystem. Designed to ease multiplexed analysis, each sensor will use different colors to both indicate the type of sensor and the strength of the signal.One sensor in the system will be a heat shock sensor that responds to activation of the heat shock pathway, which is a generalized stress pathway in cells. We are adapting a version of this sensor that we previously reported [1] [2] , which coupled fluorescent protein expression to activation of heat shock factor 1, from green fluorescent protein (EGFP) to a red fluorescent protein (RFP) and from red (DsRed) to yellow (YPet) for the constitutive color. Figure 1 shows the heat shock sensor response to 15 min heating at 42 ºC. Alongside this effort, we are using a multi-flow microfluidic device that can simultaneously apply different flows to cells across a 1000× range to understand the behavior of cells in flow [3] . Figure 2 is an image of the multi-flow device used to test NIH3T3 mouse fibroblast cells. Cells are seeded in 6 chambers concurrently and exposed to flow for 24 hrs, after which we can extract PCR from each chamber to study the cell response."
Microfluidic Perfusion for Modulating Stem Cell Diffusible Signaling,"Stem cell phenotype and function are influenced by microenvironmental cues that include cell-cell, cell-extracellular matrix (ECM), and cell-media interactions, as well as mechanical forces. Our research focuses on developing microscale systems for controlling the cellular microenvironment of mouse embryonic stem cells (mESCs), in particular mechanical forces (i.e., shear stress) and cell-media interactions (i.e., diffusible signaling).Many emerging technologies used for ESC expansion or differentiation require perfusion culture, an example being pluripotent stem cell expansion in bioreactors for clinical applications [1] . We employ a multiplex microfluidic perfusion array to study the effects of shear stress on mESCs across a wide range of flow rates in a graded, quantitative manner. Using this device, we are able to show that perfusion elicits phenotypic changes and that the specific shear-responsive phenotype is due to mechanosensing by heparan sulfate proteoglycans (HSPGs, Figure 1A-C). This is the first study describing the ESC machinery capable of responding to shear stress, thus providing a foundation for further shear mechanotransduction studies [2] .Cells are constantly secreting and responding to soluble signals, the removal of which can be mediated by modulating flow properties at the microscale. To assess the contribution of cell-secreted factors to mESC differentiation and self-renewal, we utilized a two-layer microfluidic perfusion device allowing for parallel comparison of different cell culture conditions (Figure 2A) [3] . Our results demonstrate that mESCs do not grow in differentiation conditions with minimal autocrine signaling, even with supplementation by Fgf4, a signal that has been shown to be a crucial factor in differentiation toward a neuronal stem cell fate (Figure 2B). Conversely, under self-renewal conditions, mESCs proliferate but lose self-renewal markers and upregulate differentiation markers (Figure 2C). These results, together with signaling and downstream differentiation assays, indicate that a differentiation towards an epiblast-like early differentiation state under conditions that had previously been shown as sufficient for self-renewal. Together, these results indicate the importance of cell-secreted signals for mESC growth and self-renewal."
Micropatterning of Cells to Study Autocrine Signaling,"Autocrine signaling is a mode of chemical signaling that occurs when cells can capture self-secreted diffusive factors. Apart from its major role in sustaining cancer growth, autocrine signaling is also involved in the positive-feedback regulation of various physiological processes. Due to the closed-loop nature and complex interplay of this signaling with other signaling cues, it is difficult to validate the presence and function of autocrine loops. Studying these loops typically requires the use of specific inhibitors to perturb the underlying ligand/receptor pairing, limiting investigation of poorly characterized autocrine loops.To promote the examination of autocrine signaling in broader biological systems, we have developed a general method for modulating autocrine activity using cell patterning. In addition to capturing self-secreted ligands, cells with autocrine loops also acquire ligands from their neighbors. By modulating the relative positioning between cells, we are able to modulate capture of autocrine ligands without needing specific inhibitors. In particular, we use stencil cell patterning to organize cells as square-latticed arrays of circular patches of varying array spacing (Figures 1A & B). We found that the cell-patterning platform can maintain uniform local cell density at all array spacings, in contrast to randomly plated cells, which exhibit increasing local cell density (Figure 1C). By reducing the influence of these other environmental cues, we are able to more explicitly study the effect of autocrine signaling on cell phenotype.In addition to studying the direct role of autocrine signaling, the cell-patterning platform can also be used to investigate the interplay of autocrine signaling with other signaling cues and to evaluate its contribution towards cell-to-cell variability. To determine the concurrent role of cell-cell contacts, we can compare cell responses between single patches and multiple patches where the cell number of both designs is equal (Figures 2A & B). To evaluate the contribution of autocrine loops in causing cell-to-cell variability, we can determine how the inclusion of a large cell patch will perturb the response of an array of small patches (Figure 2C). These innovative cell-patterning designs provide us novel tools for characterizing the impact of autocrine signaling without prior knowledge of the underlying ligand/receptor interactions."
Iso-dielectric Separation of Cells and Particles,"The development of new techniques to separate and characterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. Continuing or improving upon this trend – for example, by developing new avenues for performing genetic and phenotypic screens – requires continued advancements in cell sorting technologies. Towards this end, we are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object [1] ) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [2] [3] [4] .Previously, we have demonstrated the ability to perform continuous-flow, label-free, non-binary separations using IDS on a wide variety of cells and particles, while simultaneously extracting quantitative information from these samples as they are sorted [4] . In order to make IDS discovery more unknown cell types, dynamically changing the conductivity gradient is crucial for increasing the efficiency of finding the optimal separation condition. Therefore, we are developing a tri-syringe pump system to dynamically control conductivity gradients. We have verified the stability of the tri-syringe pump system via quantitative fluorescence imaging. Combining this gradient control system with a computer-controlled function generator and automated microscope, we plan to fully automate IDS separation and electrical profile screening (Figure 2)."
Image-based Sorting of Cells,"Microfabricated/microfluidic approaches to cell sorting, include purely dielectrophoretic (DEP) trap arrays [1] , passive hydrodynamic trap arrays with active DEP-based cell release [2] , and passive microwell arrays with optical cell release to permit sorting of non-adhered cells [3] . As these proceeding technologies were best suited to operate with non-adherent cells, we are developing a solution for adherent cells. Our approach to sorting adherent cells based on the morphological features uses a photolithography-inspired method, illustrated in Figure 1 (a). We first plated adherent cells into a dish and imaged cells using a microscope. Machine learning algorithm-based software CellProfiler [4] was used to quantitatively characterize the morphological features of the imaged cells, covering the cell area, shape, fluorescent intensity, and texture. As shown in Figure 1 (b), four classes were defined according to the fluorescent intensity difference in cell cellular compartments. A set of judging rules was generated by iteratively training the classifier based on hundreds of quantitative cell feature measurements to cluster the cells of similar phenotypes into a particular class. Desired cells were identified according to the classification. An alignment mark image was generated, with black features corresponding to locations of desired cells. Aligning the transparency mask to the back of the cell culture dish showed that opaque mask features resided beneath desired cells. We then mixed a prepolymer solutizon consisting of cell culture media, a UV-photoinitiator, and poly(ethylene glycol) diacrylate (PEGDA) monomer. We added the prepolymer to the cell culture dish and shined ultraviolet (UV) light from a standard fluorescence microscope fluorescence source through the transparency and into the dish. The prepolymer then crosslinked into a hydrogel in all unmasked locations, encapsulating undesired cells. The desired cells, which were not encapsulated, can be enzymatically released from the substrate and recovered, as shown in Figure 2. The overall technique requires standard equipment found in biological labs and inexpensive reagents (<$10 per experiment), encouraging widespread adoption."
Flexible Multi-site Electrodes for Moth Flight Control,"Significant interest exists in creating insect-based Micro-Air-Vehicles (MAVs) [1] [2] [3] that would combine advantageous features of insects—small size, effective energy storage, navigation ability—with the benefits of MEMS and electronics—sensing, actuation and information processing. The key part of the insect-based MAVs is the multi-site electrode, which interfaces with the nervous system of the insect to bias the insect’s flight path by controlling insect’s abdominal motions.In this work, we have developed a flexible multi-site electrode (FME) for insect flight control that directly interfaces with the animal’s central nervous system as shown in Figure 1b. The FMEs are made of two layers of polyimide with gold sandwiched in-between in a split-ring geometry using standard MEMS processing [3] . The FMEs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord of the Moth Manduca Sexta (Figure 1b) and allows for an efficient surgical process for implantation (Figure 1d). Additionally, we have integrated carbon nanotube (CNT)-Au nanocomposites into the FMEs to enhance the charge injection capability of the electrode.To quantify the performance of the FME, we have developed a custom stimulation and measurement system that allows computer-controlled stimulation and automated image analysis of the resulting abdominal motion (Figure 2a). We measured the magnitude (r) and direction (q) of the abdominal movement by the position of the red dot on the abdomen tip of the moths (Figure 2 b) versus the stimulations signal delivered at various magnitude and across various site pair. Moreover, we measured the voltage and current across pairs of stimulation sites during stimulation signal delivery (Figure 2c); hence, we could estimate the power consumption and injection charge density for the FME stimulations. Finally, we have integrated the FMEs into a wireless system (Figure 1a). In the flight control experiment, we can force a freely flying animal to perform turning motions via the FME stimulations."
High-flux Pool Boiling with Micro-engineered Surfaces,"The mechanism of critical heat flux (CHF) is commonly attributed to two limits during boiling behavior: 1) the hydrodynamic limit due to Helmholtz instability and 2) the capillary limit determined by surface wettability [1] . In recent years, a significant amount of research has been focused on CHF enhancement by utilizing micro/nanostructured surfaces to improve wettability [2] [3] [4] , with CHF of ~200 W/cm2 being demonstrated [4] . While most works are focused on making small structure sizes to improve surface wettability, the effect of this roughness-augmented wettability on CHF is poorly understood. The limit of CHF enhancement with roughness-augmented wettability, where hydrodynamic instability becomes the dominant mechanism for CHF, has not been investigated. In addition, boiling on nanostructured surfaces suffers from the requirement of high superheat due to bubble geometries closer to the homogeneous nucleation limit. As a result, the heat transfer coefficient (HTC) on nanostructured surfaces is sacrificed [3] , which impairs the heat removal capability especially for applications demanding small temperature difference.In this study, micro/nanopillar arrays are fabricated with a series of pitch and diameter size, as shown in Figure 1. The sizes of pillar are designed to ensure that bubbles in the Cassie state, where vapor bubbles are suspended on the pillars, are energetically favorable such that bubble detachment is enhanced. The series of sizes of the structured arrays generate various capillary forces, which allow the study on the mechanism for CHF and the limit of CHF enhancement with roughness-augmented wettability. Furthermore, the investigation on surface roughness, where hydrodynamic instability dominants, gives the optimal size of structures for CHF enhancement and explores the feasibility of heterogeneous bubble nucleation on surfaces with proper structure geometry to reduce superheat."
Acoustic Bragg Reflectors for Q-enhancement of Unreleased MEMS Resonators,"Two of the greatest challenges in MEMS are those of packaging and integration with CMOS technology. Development of unreleased MEMS resonators at the transistor level of the CMOS stack will enable direct integration into front-end-of-line (FEOL) processing and minimal or no packaging, making these devices an attractive choice for on-chip signal generation.Toward this goal, the authors have previously demonstrated the first fully unreleased MEMS resonator operating at 39 GHz with a quality factor (Q) of 129 [1] . The Si bulk acoustic resonator, surrounded on all sides by SiO2, demonstrates the feasibility of unreleased resonators, providing a Q that is only 4x lower than its released counterpart [2] . At mm-wave frequency in the Landau-Rumer regime, resonator Q is limited primarily by anchor loss [3] . In the case of fully-clad resonators, the quality factor can be significantly improved by localization of acoustic energy using acoustic Bragg reflectors [4] .The HybridMEMS lab has performed a study of fully unreleased resonator surrounded by lithographically defined ABRs, embedded in a homogeneous cladding layer (Figure 1). This one-mask design enables resonator banks of various frequencies on the same chip, providing multiple degrees of freedom in ABR design. With the goal of direct integration into FEOL CMOS processing, resonator performance is investigated for materials commonly found in the CMOS stack. The characteristics of these unreleased structures are compared with freely suspended resonators, released resonators isolated with lithographically defined ABRs [5] , and phononic crystal [6] based unreleased resonators (Figure 2)."
MEMS RESONATOR OSCILLATOR DESIGN AND VARIATION STUDY,"Electromechanical resonators such as quartz crystals, surface acoustic wave (SAW) resonators, and ceramic resonators have become essential components in electronic systems. However, due to their large footprint and difficulty in integrating with CMOS process, there has been much interest in developing MEMS resonators that achieve comparable performance yet have a smaller footprint and are compatible with CMOS. Recently, MEMS resonators have been proposed that overcome physical limitations in traditional resonators to reach frequencies in the GHz range. In addition, they have the potential for compatibility with CMOS, opening up possibilities for new circuits and systems [1].As with other semiconductor devices, with increasing frequency into the SHF and EHF range and with decreasing device size into the submicron scale, variability has started to become a critical issue in electromechanical resonators. Also, integration with CMOS process makes it more difficult or impossible to use the conventional frequency trimming methods employed for traditional resonators. Thus, in order for wider use of these resonators, more accurate characterization of important parameters and variations associated with these parameters is necessary. Some of these parameters include resonant frequency, the quality factor, and series resistance.This work aims to characterize and model the variation in resonant frequency of Si-based MEMS bulk acoustic resonators. The test structure consists of an array of resonators, multiplexing structures, and an oscillator loop. During measurement, each resonator is connected into a self-sustained oscillation loop through the multiplexing structure and the oscillation frequency is measured through a digital counter. On-chip measurement circuits such as this make it possible to measure a large number of devices, otherwise difficult to do with the traditional test methods of individual device probing, thus allowing more accurate characterization and modeling of critical device parameters and variations associated with them."
"A COMPUTATIONALLY SIMPLE, WAFER-TO-FEATURE-SCALE MODEL OF ETCH-RATE VARIATION IN DEEP REACTIVE ION ETCHING","Modeling etch-rate variation in Deep Reactive Ion Etching (DRIE) helps to identify possible defects in MEMS and IC devices arising from sub-optimal etching depths and times. Besides tool-specific properties, such as the chamber design, another cause for the observed non-uniformity effects is the particular wafer pattern employed, which causes distinctive effects at the wafer-, die-, and feature-scales [1]. We present a model to capture these pattern-dependencies by tracking the spatial and temporal distribution of the ion and radical species within the DRIE chamber. The model implementation uses a time-stepped algorithm with three levels – corresponding to the three different length scales – and a coarse-grained approach whereby multiple features in a given region are characterized by a particular shape, size, and density. The local radical species concentration distribution above the wafer is determined at each time step using current feature geometries to compute their Knudsen transport coefficient, which is linked to the radical transport mechanisms within other areas in the chamber. At the end of each time step, etch-rate estimates based on this radical concentration distribution and current feature geometries are used to update feature depth information for the next time interval. At the wafer scale, our modeling results, shown in part in Figure 1, achieve a success comparable to that of previously-developed wafer-level models with an etch-rate RMS error percentage between 2.1% and 8.2%. The results also show that feature-level etch evolution substantially impacts the wafer-level fluorine concentration and thereby modifies the wafer and die etch-rate uniformity. We expect a similar model could be incorporated into CAD software tools to evaluate masks and correct potential design issues before they are made. Our results also shed light on possible tool and process modifications to allow users the capability of altering across-wafer etch-rate variability."
SIZE-SELECTIVE SORTING OF CELLS USING TEMPLATED ASSEMBLY BY SELECTIVE REMOVAL,"This work presents the size-selective sorting of single biological cells using Templated Assembly by Selective Removal (TASR). We have demonstrated the selective self-assembly of single SF9 cells (clonal isolate derived from Spodoptera frugiperda IPLB-Sf21-AE cells) into patterned hemispherical sites on rigid assembly templates using TASR. Experimental success with SF9 cells, which are nearly spherical and resistant to shear, suggests that self-assembly using TASR can also be extended to other cells and biological materials that are spherical. Examples include white blood cells and, in general, cells that maintain a well-defined morphology for short durations when dispersed in culture media, agitated mildly using megasonic excitation, and allowed to settle on a patterned substrate. Therefore, TASR-based biological self-assembly holds potential for several applications, such as cell-sorting for medical research or diagnostics, or isolation of single cells for studying their biological and mechanical behavior.In TASR, the system’s free energy is minimized when objects assemble in holes that match their shapes and sizes on the template’s surface (Figure 1). A combination of chemical and mechanical effects selectively removes objects from poorly matched holes. Previous work on TASR has shown that microcomponents made from relatively rigid materials such as silica [1] [2] and deformable materials like polystyrene [3] can be assembled effectively on similarly rigid patterned templates using this technique. In an extension of the application of TASR to biological systems, SF9 cells (which come in a range of sizes with a mean diameter of 15 microns) were successfully assembled using TASR onto patterned silicon templates. The assembly sites comprised holes with nearly hemispherical profiles etched in a silicon substrate using DRIE. Figure 2 shows optical micrographs of the assembly and demonstrates the size-selectivity of the process as well as the high yield of cell assembly using this technique."
MEMS TUNABLE ORGANIC LASER CAVITIES,"Standard photolithography-based methods for fabricating micro-electromechanical systems (MEMS) present several drawbacks including incompatibility with flexible substrates and limitations to wafer-sized device arrays. Recently our group has demonstrated a new method for rapid fabrication of metallic MEMS that breaks the paradigm of lithographic processing using an economically and dimensionally scalable, large-area microcontact printing method to define 3D electromechanical structures. Here we utilize this MEMS printing method to create tunable optical cavities, with the ultimate goal of creating an electrically color-tunable organic laser. The device concept is shown in Figure 1. The bottom half of the optical cavity is fabricated by forming ridges of polydimethylsiloxane (PDMS) on a dielectric mirror using a silicon master. The top half of the cavity is independently fabricated by thermally evaporating an organic release layer on a planar PDMS substrate followed by layers of Au and Ag, which act both as the top mirror of the cavity and as the flexible MEMS element. The organic lasing medium (DCM) is evaporated on the metallic layers. This layer structure is stamped onto the PDMS ridges followed by a rapid peel-off, forming the structure shown in Figure 1b.The photoluminescence from the DCM gain layer under 532-nm excitation shows clear evidence of a vertical optical cavity formed between the dielectric mirror and the Ag flexible membrane, demonstrating an optical microcavity formed by a scalable MEMS printing method. The optical resonances of the device can be varied by applying a bias voltage to the top flexible membrane, thus changing the distance between the mirrors (Figure 1a). This structure paves the way to developing color-tunable organic lasers on flexible substrates over large areas using an economical MEMS fabrication technique."
MICROFLUIDIC PERFUSION FOR MODULATING STEM CELL DIFFUSIBLE SIGNALING,"Stem cell phenotype and function are influenced by microenvironmental cues comprised of the cell-cell, cell-extracellular matrix (ECM), cell-media interactions, and mechanical forces. Our research focuses on developing microscale systems for controlling the cellular microenvironment of mouse embryonic stem cells (mESCs), in particular mechanical forces (i.e., shear stress) and cell-media interactions (i.e., diffusible signaling).The use of embryonic stem cells (ESCs) for clinical therapeutic applications requires expansion of the pluripotent cells in bioreactors, where the cells are subjected to fluid shear stresses [1]. In comparison to macroscale bioreactors, microfluidic perfusion systems allow for study of shear stress effects on mESCs across a wide range of flow-rates. We demonstrate the utilization of a multiplex microfluidic perfusion device as a powerful experimental tool to probe shear stress effects in a graded, quantitative manner, allowing us to identify a shear-modulated marker, Fgf5 (Figure 1A-C). Such approach facilitates further targeting of molecular players implicated in shear stress mechanotransduction.Flow properties at the microscale enable fine-tuning of soluble factors transport, which plays a crucial role in determining ESC fate. To modulate cell-media interactions in mESCs self-renewal and differentiation, we utilized a two-layer microfluidic perfusion device allowing for parallel comparison of different cell culture conditions (Figure 2A) [2]. Our results demonstrate that mESCs under conditions of reduced autocrine signaling in self-renewing culture conditions express decreased levels of self-renewal markers and at the same time upregulated levels of early differentiation markers. In contrast, an interruption of cell-ECM interactions yields opposite results (Figure 2B). Similarly, we demonstrated that mESCs failed to proliferate under conditions of minimized autocrine signaling during neuronal differentiation. In addition, we found that autocrine Fgf4 signaling is implicated as a crucial factor in acquiring neuronal identity of mESCs, while alone it cannot restore the growth of mESCs undergoing differentiation (Figure 2C)."
CONTINUOUS-FLOW DEFORMABILITY-BASED SORTING OF P. FALCIPARUM-INFECTED RED BLOOD CELLS USING A MICRO-FILTER ARRAY,"Change in cell stiffness is a characteristic of several blood cell diseases, such as sickle cell anemia [1], malaria [2], and leukemia [3]. In humans, the spleen acts like a filter to remove these more rigid cells by pushing blood through slits between endothelial cells and removing cells that cannot pass. In this work, we create a microfluidic device that mimics the architecture of the spleen to achieve continuous-flow fractionation of cells based on their rigidity. We demonstrate successful operation of this device by separating malaria-infected from normal, uninfected red blood cells (RBCs). Applications include disease diagnosis and sample preparation for downstream analysis. Figure 1 illustrates device operation and presents details regarding device design.The device fabrication process involves the use of photo-curable polyurethane, NOA 81. The high Young’s modulus of this material enables the creation of high aspect ratio slits in an inexpensive and simple manner, as contrasted with PDMS and silicon-glass. Fabrication details have been presented elsewhere [4].We separated schizont-stage malaria-infected RBCs from uninfected RBCs. Figure 2A presents a snapshot from a video showing an infected cell sliding along the edge of a slit and uninfected cells following the direction of the fluid flow. Figure 2B presents a histogram showing the separation sensitivity of the device. As shown, this device was able to separate 4-infected cells from 164 uninfected cells.Applications of this device include sample preparation for biochemical and cell culture assays. Deformability-based cell sorting can also be useful for situations when cell surface markers are not clearly identified for FACS. Lastly, the low-cost aspect of this device makes it ideal for on-site disease (e.g., malaria) screening in resource-poor settings."
INCREASE OF SENSITIVITY OF ELISA USING A MULTIPLEXED ELECTROKINETIC PRECONCENTRATOR,"Enzyme-Linked Immunosobent Assay (ELISA) is the gold standard in biodetection due to its high reproducibility and sensitivity. Still, the ultimate detection sensitivity of ELISA is not good enough to tackle the challenges of biomarker detection. This led to the development of many novel ultra-high-sensitivity immunoassay platforms. However, additional sensitivity comes with the cost of added complexity in amplification chemistry or complicated devices. Being able to measure analytes at concentrations below the current detection limits, without modifying the basic workflow of ELISA, would be very useful.We utilized a nanofluidic preconcentration device to continuously accumulate enzymatic product molecules from ELISA into a much smaller volume and increase its local concentration significantly, therefore achieving a lower limit of detection. An important advantage of this scheme is that the enzymatic product being concentrated is the same, and thus the same device setup is applicable regardless of the antigen. We also demonstrated multiplexing capability by assaying five samples simultaneously in a single device.We showed enhanced detection sensitivity in two cases: 1) when the enzymatic reaction is on-chip and 2) when the enzymatic reaction is off-chip. In the first case, immunobinding was performed using beads as a solid phase, and the beads were physically trapped in a microfluidic channel. When fresh substrate solution was flown through the bead pack, it was enzymatically converted into fluorescence product and accumulated downstream. In the second case, the complete ELISA workflow, including substrate conversion, was performed in a 96-well plate. After that, the converted molecules were injected into the device and electrokinetically concentrated. Using the first method, we demonstrated a 65-fold lower limit of detection for CA 19-9, a human pancreatic cancer marker, in control serum. The detection limit for human prostate specific antigen (PSA) in donkey serum was 100 times lower using the second method."
IMPROVEMENTS TO A CHIP-SCALE QUADRUPOLE MASS FILTER FOR PORTABLE MASS SPECTROMETRY,"Mass spectrometers are powerful analytical instruments that are known to be the gold standard for chemical analysis. The Micro-Gas Analyzer Project attempts to leverage the cost reduction, performance enhancement, and increased portability associated with MEMS to create a microfabricated mass spectrometer for chemical species detection. One of the key components of the mass spectrometer is the mass analyzer, which functions to separate ionized species by their mass-to-charge ratios (m/z). There are various types of mass analyzer with respective advantages and disadvantages, but our group decided to utilize the quadrupole mass filter due to its inherent robustness.Various attempts at making MEMS-based linear quadrupoles have met with varying degrees of success [1] [2] [3] [4]. Our group devised a new class of chip-scaled quadrupole mass filter that utilizes square electrodes to address the issue of mass producibility, a property not readily achieved with the other technologies. The proof-of-concept device demonstrated a mass range of 250 amu when operated in the first stability region and a maximum resolution of ~40 when operated in the second stability region [5].A new improved version of the device demonstrated an increased mass range of up to 650 amu in the first stability region and a resolution of ~90 when operated in the second stability region [6]. Additionally, we demonstrated the functionality of integrated ion optics. The major improvement in the new version was a different process flow that raised the yield, electrical breakdown voltage, and structural robustness of the device. In future work, we plan on modifying the mask layout and device dimensions to further improve the performance through using a smaller device radius, minimizing the parasitic capacitances, and including prefilters for enhanced transmission."
COMPONENT INTEGRATION OF A MICRO-GAS ANALYZER,"The Micro-Gas Analyzer Project attempts to leverage the cost reduction, performance enhancement, and increased portability associated with MEMS to create a microfabricated mass spectrometer for chemical species detection. Mass spectrometers are powerful analytical instruments that are mainly comprised of an ionizer, a mass analyzer, a detector, and a vacuum pump. Our group and collaborators have made substantial progress on these various components, spanning carbon-nanotube-based electron impact ionizers [1], chip-scaled quadrupole mass filters with square electrodes [2], and time-modulated capacitance electrometers [3]. Each component functions and performs adequately on its own, but a complete system requires the integration of these three parts, as demonstrated by other researchers [4] [5].An integration plan was conceived to sequentially combine the various components in a logical manner. A testing jig was designed and machined so the electron impact ionizers would have electrical connections to the power supplies and compatibility with our in-house characterization system. After validating the functionality of the ionizer with a macro-scaled quadrupole and a Channeltron electron multiplier, we plan to use our chip-scaled quadrupole instead of the macro-version. Once these two vital components are well characterized, we will check the functionality of the electrometer with a commercial ion source and a macro-scaled quadrupole. Finally, we will put all three components together to be tested and characterized in a vacuum chamber."
MANIPULATION OF LIQUID SPREADING ON ASYMMETRIC NANOSTRUCTURED SURFACES,"The manipulation of liquid spreading is important for a broad range of microfluidic, biological, and thermal management applications [1] [2] [3]. In this work, we investigated the ability to manipulate droplet spreading to a single direction on-demand on asymmetric nanostructured surfaces using electrowetting. Asymmetric nanopillar arrays were fabricated with diameters of 500 to 750 nm and deflection angles of 3 to 52 degrees. Figure 1 shows scanning electron micrographs (SEMs) of three representative asymmetric nanopillar arrays with deflection angles, φ, (as defined in the Figure 1 inset) ranging from 7°-25°. A Cartesian coordinate system is defined for convenience, as shown in Figure 1, where the pillars deflect in the positive X (+X) direction. In the presence of asymmetric nanostructures, the spreading behavior can be dynamically controlled with electrowetting, which utilizes an electric potential, V, across the droplet and nanostructured surface to change the surface energy (Figure 2a). With this approach, different droplet-spreading directionalities can be achieved based on the magnitude of the electric potential. If we apply V= 1.5 V to an initially static symmetric droplet, the liquid pins in the –X direction and spreads in +X, i.e. uni-directionally (Figure 2b). In the case of an applied V= 2.1 V, the liquid unpins in –X and spreads bi-directionally. The spreading, however, is asymmetric: the rate is three times faster in +X as compared to –X (Figure 2c). Moreover, with increasing applied V, the asymmetry decreases. In the case of an applied V> 2.5V, the liquid spreading is nearly symmetric, i.e. the rates in +X and –X are approximately equal (Figure 2d). The study provides design guidelines to tune the droplet’s behavior from uni-directional to asymmetric or symmetric bi-directional spreading using both nanostructure design and applied electric fields for a variety of microfluidic applications."
MEMS STEAM GENERATORS FOR EJECTOR PUMPS,"Microscale ejector pumps offer the potential for high-flow-rate pumping of gases, a functionality that is greatly needed in MEMS technology [1]. These pumps have many additional characteristics, such as their simplicity of design and their lack of moving parts, which favor them over other MEMS gas pumps. One of the challenges associated with driving ejector pumps, however, is providing a compact source of motive fluid. This fluid is the high-speed gas that drives the pumping action. The current work delivers a MEMS device capable of generating steam at speeds suitable for driving an ejector pump in a compact fashion. To that end, the device utilizes the homogeneous catalytic decomposition of hydrogen peroxide. Analysis shows that a MEMS ejector pump driven by this device is capable of achieving mass flow rates per unit pump volume on the order of 10-2 g/s/cm3, which is two orders of magnitude higher than the rates of state-of-the-art MEMS gas pumps. In addition to pumping, the steam generator may also be used for microrocket thrust generation in micropropulsion applications.This work involves the design, fabrication, and testing of the MEMS steam generator. To our knowledge, this device is the first of its kind in the literature that works successfully, and it achieves results that have been sought by other groups for over a decade. The device consists of a mixing section for the peroxide and catalyst streams, a reactor section where the peroxide decomposes, and finally a nozzle section where the gaseous products of the decomposition are accelerated to the required velocities. A schematic is shown in Figure 1. To design the device, multidomain (chemical, thermal, and fluidic) numerically-implemented modeling is used to study the underlying physics and arrive at an optimized, microfabricatable design. The modeling takes into account the key challenges of thermal management, achieving fast mixing [2], and boundary layer compensation. The device is then fabricated from a stack of four silicon wafers and one Pyrex wafer using deep-reactive-ion etching and wafer bonding. A photograph of a microfabricated device is shown in Figure 2. The modeling also guides the design of a mica-based ceramic package which provides both thermal insulation and piping ports. The system is then experimentally tested using 90% high-test hydrogen peroxide and ferrous chloride tetrahydrate solution as the catalyst. The device is characterized using temperature measurements, refractive index analysis, and visual inspection during operation. Successful performance is demonstrated via the full decomposition of the peroxide and the complete vaporization of the water produced. The experimental results are also compared with those from the simulation. Good agreement is observed between experiment and theory, providing comprehensive model verification."
DESIGN AND DEMONSTRATION OF INTEGRATED MICRO-ELECTRO-MECHANICAL (MEM) RELAY CIRCUITS FOR VLSI APPLICATIONS,"Silicon CMOS circuits have a well-defined lower limit on their achievable energy efficiency due to subthreshold leakage. Once this limit is reached, power constrained applications will face a cap on their maximum throughput independent of their level of parallelism. Avoiding this roadblock requires an alternate device with steeper sub-threshold slope – i.e., lower VDD/Ion for the same Ion/Ioff ((H. Kam, et al., “Circuit Level Requirements for MOSFET Replacement Devices,” in IEDM Tech. Dig., 2008, pp. 427.)). One promising class of such devices is electro-statically actuated micro-electro-mechanical (MEM) relays with nearly ideal Ion/Ioff characteristics. Although mechanical movement makes MEM relays significantly slower than CMOS, they can be useful for a wide range of VLSI applications by reexamining traditional system- and circuit-level design techniques to take advantage of the electrical properties of the device. Unlike in CMOS circuit design, logic functions in MEMS circuit design should be implemented as a single complex gate with minimum-sized relays, resulting in significantly reduced logic complexity. We have recently shown that with optimized circuit topologies MEM relays may potentially enable ~10x lower energy over CMOS at up to ~0.1-1GHz frequencies [1]. This work takes initial steps towards experimental validation of these principles by leveraging recently developed relay technology and reliability enhancements [2] [3] to demonstrate several monolithically integrated MEM relay-based building blocks. Specifically, our chip includes logic, memory, I/O, and clocking structures, and we demonstrate successful basic functionality and circuit composition [4]. These relay circuits illustrate a range of important functions necessary for the implementation of integrated VLSI systems, and give insight into circuit design techniques that leverage the physical properties of these devices."
MEMS SIC LANGMUIR PROBES FOR PLASMA DIAGNOSTICS OF SPACECRAFT DURING REENTRY,"During reentry, the formation of a high-density, low-temperature plasma sheath around a spacecraft results in a communications blackout [1]. Advanced plasma sensors onboard a reentry-bound spacecraft will enhance this spacecraft’s ability to maintain communications through the plasma sheath. We propose cost-effective and reliable batch fabricated MEMS-based silicon carbide (SiC) Langmuir probe arrays to provide real-time diagnosis of plasma conditions surrounding a spacecraft during reentry. The Langmuir probe array provides data associated with specific parameters of the plasma, including plasma temperature and plasma density. The Langmuir probe array operates so that each probe in the array is individually addressable. Our MEMS-based Langmuir probe arrays are fabricated from SiC, a semiconductor material that is relatively inexpensive and very resistant to hostile environments such as spacecraft reentry [2]. Current research efforts to develop SiC-based MEMS intended for harsh environments include transistors, and transducers to measure pressure, acceleration, temperature, and strain [3] [4] [5]. Langmuir probe densities as large as 106/cm2 have been demonstrated (Figure 1). Also, fabrication experiments using plasma-enhanced chemical-vapor-deposited (PECVD) SiC coatings have been conducted (Figure 2). Future research includes the development of MEMS-based Langmuir probe arrays fabricated from SiC substrate materials and micromolded SiC. Langmuir probe performance will be validated experimentally in laboratory plasma sources that generate plasma densities similar to those encountered by spacecraft during reentry."
MONITORING CELL PHYSIOLOGY IN MICROBIOREACTORS,"Mammalian cell culture dominates the biopharmaceutical industry for biotherapeutics including monoclonal antibodies, vaccines, and growth factors. However, mammalian cells are also the most sensitive to changes in the culture environment, e.g., mechanical agitation, nutrient depletion, and waste byproduct accumulation. Hence, maintaining cell viability in mammalian cell cultures for an extended period of time is an important limiting factor for mammalian cell cultures [1]. Currently, state-of-the-art micro-bioreactors estimate cell density by measuring the turbidity of the culture using an Optical Density (OD) sensor [2]. Unfortunately, OD sensors measure light scattered by cells which may not be accurate due to cell aggregation and the influence of cell shape. Additionally, biomass measurements do not discriminate between live and dead cells. An online sensor that explicitly measures viable cells in a micro-bioreactor is necessary.Dielectric spectroscopy is a promising online sensor for cell viability in micro-bioreactors [3]. The difference between the low-frequency and high-frequency capacitance measurements (ΔC = CLF – CHF) in the radio frequency regime gives the capacitance contributed by the cells to the total capacitance of the suspension. Due to the fact that most dead cells no longer have an intact membrane, defined as a membrane that is selectively impermeable to ions in the solution, they do not contribute to the capacitance reading (ΔC). By calibrating the measured capacitance with cell suspensions of known cell densities, the number of live cells in the culture can be determined, shown in Figure 1. When dielectric spectroscopy is combined with OD measurements, the percent cell viability can be utilized to optimize the yield of a mammalian cell culture in a micro-bioreactor. The first part of our project involves designing and calibrating a dielectric spectroscopy sensor for a micro-scale bioreactor, shown in Figure 2."
A MEMS SINGLET OXYGEN GENERATOR FOR POWERING CHEMICAL LASERS,"Singlet delta oxygen (O2(a)) may be synthesized through the highly exothermic multiphase reaction of gaseous Cl2 with an aqueous mixture of concentrated H2O2 and KOH. Among other applications, O2(a) may be used to drive a chemical oxygen iodine laser. The laser application of O2(a) generation requires a high yield (i.e., a high fraction of product oxygen in the O2(a) state) and conversion of Cl2 to O2(a) to sustain laser emission; the high yield is achieved in part by effective mixing of the gas and liquid reagents. Modeling suggests that the MEMS singlet-oxygen generator (SOG) has key advantages as compared with fully macroscale implementations [1]. These advantages include smaller hardware size for the same power level, higher yield, more efficient reactant utilization, gravity independence, and feasible batch manufacturing.Previously, the MIT microSOG team has demonstrated working microSOG devices [2] [3]. These devices produced O2(a) with yields of up to 78% and O2(a) flows per unit volume of up to 0.067mol/s/L; this yield was a significant improvement and the flows per unit volume were comparable to macroscale counterparts. Building on recent successes in this area of innovation [2] [3], we are creating new microSOG chips to improve the Cl2 utilization and O2(a) flow rate by optimizing the mixing, flow distribution systems, and device packaging. Mixing is promoted by an array of microstructured posts that fill the reaction channels; the size of these posts is reduced as compared with earlier systems to enable more compact microSOG chips and a higher contact area per unit volume, which enables high Cl2 utilization (more than 95%) and O2(a) yield (more than 70%). The new device is predicted to operate at a maximum O2(a) flow rate of 1000sccm, for a 7x increase in output flow per unit volume from 0.067mol/L/s [3] to 0.50 mol/L/s. The chip was fabricated from a stack of two silicon wafers and one Pyrex wafer using bulk micromachining (primarily DRIE and bonding technology). An SEM image of the generator and the finished chip are shown in Figures 1 and 2, respectively. Testing is underway."
METALLIC AND OTHER GLASSES FOR MEMS,"Chalcogenide glasses are widely used as “phase change materials” for optical data storage media in rewrit­able compact discs (CD±RW) and rewritable digital video disks (DVD±RW, DVD-RAM). Recently, they have also shown high potential for use in phase-change random ac­cess memories (PC-RAMs or PRAMs), which might replace flash memories in the future. In these applications, information storage is accomplished by reversible amorphization and crystallization. Metallic glasses have also recently generated interest because of their having a combination of high elastic moduli and fracture strengths. They have been used, for example, as torsional springs in micro-mirror arrays. In both types of applications, the mechanical properties of films in the glassy state are important in determining their functionality and reliability. Of particular interest is the stability in the glassy state and stress changes associated with crystallization, both of which can be studied through measurements of cantilever displacements associated with crystallization.In prior work we have shown that the stress change associated with crystallization of chalcogenides depends strongly on the thickness of films, and on whether or not the films are capped with materials with high stiffness (Figure 1). This large dependence is the result of inelastic accommodation of a component of the large volume change (~6%). The yield stress of the glasses is strongly thickness- and encapsulation-dependent [1] [2].In more recent studies we have focused on metallic glasses, both for applications as memory media and as mechanical components in MEMS devices. We have used combinatorial deposition on cantilever arrays to characterize the volume change on crystallization in the binary Cu-Zr [3] and ternary Cu-Zr-Al systems [4]. We found significant and complex dependences of the volume/stress change on composition (Figure 2). These variations were also found to correlate with the ease with which these alloys of specific compositions could be deposited or quenched into the glassy state. We are now measuring the compositional dependence of the elastic modulus, thermal expansion coefficient, and hardness of as-deposited and annealed metallic glasses."
ULTRA-WIDE-BANDWIDTH MICRO ENERGY-HARVESTER,"A novel ultra-wide-band resonating thin-film PZT MEMS energy-harvester has been developed. It harvests energy from parasitic ambient vibration at a wide range of amplitude and frequency via the piezoelectric effect. Up to this point, the designs of most piezoelectric energy devices have been based on high-Q linear cantilever beams that use the bending strain to generate electrical charge via the piezoelectric effect [1] [2]. They suffer from very small bandwidth and low power density, which prohibit practical use. Contrary to the traditional designs, our new design utilizes the tensile stretching strain in doubly-anchored beams [3]. The resultant stiffness nonlinearity due to the stretching provides a passive feedback and consequently an ultra-wideband resonance [4]. This wide bandwidth of resonance enables a robust power generation amid the uncertainty of the input vibration spectrum. This work includes the design, microfabrication, and testing of a MEMS-scale prototype that aims to harvest up to 0.1mW electrical power in a wide range of excitation frequencies. Mechanical testing has shown 10-fold improvements in the displacement bandwidth. Our simulation predicts 100-fold improvement in the electrical power bandwidth compared to the conventional linear designs. Currently, a new generation of the device is under fabrication and testing at MTL and MNSL facilities."
SCALE-DOWN CELL CULTURE FOR BIOPHARMACUETICALS,"Developing highly productive cell-culture processes is an essential step in the production of biopharmaceutical products. Process development involves experimentally determining the combination of cell line, culture medium composition, and process parameters that will produce the highest quality and quantity of product. Scale-down models for large-scale bioreactors are essential for cell-culture process development in order to achieve the throughput necessary to conduct a sufficient number of experiments for a reasonable cost. Bioreactor systems based on 24 well plates [1] and a highly automated robotic system with passive microreactors [2] have recently been developed in order to overcome the shortcomings of traditional shake-flask and bench-scale stirred-tank-based scale-down models. However, in these systems mixing and fluid-handling require external machinery/robotics or manual intervention. We are using an alternative approach that integrates fluid-handling and mixing capabilities into the bioreactor device utilizing previously developed fluid injectors and mixing devices [3]. Figure 1 shows a schematic view and photographs of the mammalian-cell-culture reactor. It is fabricated with injection-molded and machined biocompatible polycarbonate layers and an actuated silicone membrane. Initial CHO cell cultures show comparable growth and viability to shake flask cultures."
DEVELOPMENT OF A MICROMACHINED RETARDING POTENTIAL ANALYZER FOR SENSING PLASMA CONDITIONS AT SPACECRAFT RE-ENTRY,"To ensure safe operation during space missions, NASA is sponsoring research on advanced sensing systems. In this endeavor, one particular facet is the development of a sensing skin that would monitor the spacecraft re-entry conditions. A retarding potential analyzer (RPA) is a device that would provide useful information about the ion energy distribution of the plasma that forms around the spacecraft while it re-enters the atmosphere. As a consequence of the harsh conditions during re-entry, the sensor must be made of suitable materials such as silicon carbide or tungsten. We are currently developing a hybrid MEMS-macro RPA that uses grids made of either bulk silicon coated with SiC or W. Design constraints of the RPA require closely packed holes with large aspects ratios. Therefore, the grids are etched using deep-reactive-ion-etching (DRIE). We plan to investigate the effects that various probe parameters have on the RPA’s performance, which is driven mainly by the Debye length of the plasma [1]. To this extent, the hybrid sensor has been designed to allow variations in grid hole diameter, pitch, and transparency. Limitations in grid-to-grid spacing will also be examined. The modular assembly of the device is shown in Figure 1. Based on this experimental exploration, a fully micromachined RPA will be built using a MEMS 3D packaging technology that we pioneered for multiplexed high voltage MEMS [2] [3]. An example of what this MEMS RPA may eventually look like is given in Figure 2, where the first grid is shown on the left and the collector plate is displayed on the right. The proposed MEMS RPA has five electrodes to insure proper data collection. Future work will further reduce the footprint of this final design as well as explore the possibility of machining the grids using bulk SiC."
MICROFLUIDIC CONTROL OF CELL PAIRING AND FUSION,"Cell fusion has been used for many purposes, including generating hybridomas and reprogramming somatic cells. The fusion step is the key event in initiating these procedures. Standard fusion techniques, however, provide poor and random cell contact, leading to low yields. While different approaches can be used successfully for reprogramming, the cell lines generated are not yet suitable for potential therapeutic applications in humans, and many questions remain about the process of nuclear reprogramming. A more efficient cell-pairing and fusion method could answer these questions. Skelley et al. have therefore developed a microfluidic device to trap and properly pair thousands of cells [1] (Figures 1 and 2). Using this device, the pairing efficiency for different cell types, including fibroblasts, mouse embryonic stem cells and myeloma cells, is as high as 70%. The device is compatible with both chemical and electrical fusion protocols, with membrane reorganization efficiencies of up to 89%. These properties render the device particularly suitable for our current study of the underlying mechanism of fusion-based reprogramming.Having established the basis for high-yield cell-pairing measurements using stem cells, we are developing a similar device for the statistical, kinetic study of immune cell populations. Immune responses are largely mediated by cell-cell interactions. In particular, natural killer cells and cytotoxic T cells form conjugates with pathogenic and cancer cells in order to fight disease Errors in these and other immune cell-cell interactions can lead to fatal immune diseases. The study of these intricate cell-cell interactions at the molecular scale is crucial for understanding the dynamics and specificity of the immune response. Conventional techniques, such as bulk measurements or immobilization of cell pairs on a dish, preclude gathering of sufficient data on single cell pairs for meaningful statistics of cell-cell interactions. We propose to overcome the limitations of traditional methods by visualizing and controlling the pairing of thousands of individual immune cell pairs simultaneously. Primary mouse lymphocytes (~6 μm diameter), a common model system in immunology, are significantly smaller than stem cells (~18 μm diameter), making a device redesign necessary. We have employed a numerical, finite- element fluid- dynamic model as a guide to optimize pairing efficiencies by altering geometric properties (Figure 3). Our device is furthermore compatible with standard staining methods, such as antibody staining and ratiometric calcium flux measurements. The next step will be to fabricate the new devices and evaluate their pairing efficiency."
MICROFLUIDIC STUDIES OF CANCER INVASION: NEW HIGH-THROUGHPUT ASSAYS TO STUDY MICROENVIRONMENTAL EFFECTS,"Microfluidics offer a unique platform for screening the effects of the biochemical and biophysical microenvironment on cellular phenotype, while allowing for increasing the experimental throughput compared to traditional assays. In this work we study the effect of interstitial flow on tumor cell migration and the interactions between tumor and endothelial cells during tumor cell entry into a blood vessel (intravasation) in a 3D extracellular matrix. Based upon previous work over the past years in our lab [1] [2], we recently developed a new design with a 10-fold increase in the number of matrix regions allowing for increased data collection capabilities. The design consists of independently addressable microfluidic channels interconnected through 3D matrices, wherein tumor and endothelial cells can be seeded, while establishing interstitial flow and/or chemoattractant gradients through the matrix. We have also developed a protocol for applying fluid-flow induced shear stress on the endothelial monolayer on the channel for studying its effects on tumor cell intravasation. After applying a shear stress of 3 dynes/cm2 for 24 hours, we observed alignment of the endothelial cells in the flow direction consistent with previous studies. Using live cell imaging and in the absence of any directional microenvironmental cues, we observed both a quiescent and sprouting endothelium, while some tumor cells invaded in 3D randomly and others reached towards the endothelial monolayer.Tumor cells exposed to interstitial flow preferentially migrated along streamlines, and the relative percentage of cells migrating upstream and downstream was a function of chemokine receptor activity and intercellular distance. At low seeding densities, cells preferentially migrated upstream. However, at high intercellular distances, cells preferentially migrated downstream but reverted to upstream migration with blocking of a single cell receptor, CCR7. These data provide supporting evidence to the autologous chemotaxis model and suggest that a competing paracrine pathway provides a stimulus for upstream migration."
SINTERED METAL WICKS FOR LOOP HEAT PIPES,"Loop heat pipes (LHPs) are a widely-used component in the thermal management of high-power electronics. LHPs transfer heat by utilizing the latent heat of a working fluid, which is circulated by the capillary pumping of a porous wick. A typical design consists of an evaporator and condenser connected in a closed loop. As the capillary wick, located in the evaporator, influences the operating characteristics and limits of the LHP, much work has been devoted to optimizing the wick’s capillarity and permeability to extend the range of LHP operation [1] [2] [3].This work investigates the wick characteristics necessary to operate a novel, multi-condenser LHP. To ensure controlled condensation and full utilization of all condensers, an additional wick must be integrated into the condensers. Condensation occurs on the wick’s surface, and the wick is used to separate the vapor and liquid phases. The wick must therefore have high capillary pressure at the interface to separate the phases and high bulk permeability for ease of liquid flow.To achieve high capillary pressure at the wick surface and high bulk permeability, a bi-layer sintered wick structure was fabricated in two steps (Figure 1). The bulk wick was first made by sintering coarse (120-140 µm) copper powder at 850 °C for 30 minutes. The sintering was performed in a tube furnace under a hydrogen-nitrogen atmosphere. A thin layer of high-capillarity wick was then fabricated on top of the bulk wick by layering fine (5-15 µm) copper powder and sintering at 650 °C for 30 minutes. A controlled fabrication procedure resulted in a repeatable thickness of the fine layer of approximately 100 µm. The bi-layer wick showed improvement over the single-layer coarse wick, matching the permeability (10-11 m2) of the coarse wick in the bulk while increasing the surface capillary pressure from 100 to 680 Pa for the advancing meniscus."
DIRECT SEAWATER DESALINATION BY ION-CONCENTRATION POLARIZATION,"The shortage of fresh water is a serious global problem, and an energy-efficient desalination strategy can provide a substantial solution for the water-crisis [1]. Current desalination methods utilizing reverse-osmosis and electrodialysis mechanisms require high power consumption or large-scale infrastructures, which are not suitable for resource-limited settings. This work elucidates a novel desalination process utilizing ion-concentration polarization (ICP) [2].Often called ion depletion or enrichment, ICP occurs due to the mismatch of the charge carriers at the nanoporous membrane. Once ICP is triggered, the concentrations of both cations and anions decrease on the anodic side of the membrane (ion depletion) and increase on the cathodic side (ion enrichment) [3]. With a strong ICP, both anions and cations are depleted near the nanojunctions. If ICP is combined with a pressure-driven flow, a steady-state depletion zone is obtained using the device depicted in Figure 1(a). In the experiments done with seawater obtained from Crane Beach, MA in Figure 1(b), the depletion zone was formed to divert charged ions (represented by dyes) into the “salted” stream. It was also shown the ICP layer acts as a virtual barrier for all charged particles, including most solid particles and biomolecules found in water. Therefore, both small salt ions and large microorganisms can be removed from the “desalted” stream, significantly reducing membrane fouling. Figure 1(c) shows in situ conductivity measurement of “desalted” stream. Measurements of the desalted stream indicate the seawater’s conductivity dropped from ~50mS/cm to ~0.5mS/cm, which corresponds to ~3mM salinity. This level of salinity is below the 10mM threshold required for potable water.The steady-state current required was found to be ~30µA, equating power consumption of ~3.5Wh/L. This technology could be applied to small-scale desalination systems, possibly with the option of battery/solar cell-powered operation. The footprint of the device is small, potentially allowing ~3X104 parallelization on the wafer scale for a total throughput of ~300mL/min."
IMAGE-BASED CELL SORTING,"This research involves the development of architectures for screening complex phenotypes in biological cells. We augment microscopy with the ability to retrieve cells of interest. This capability will permit cell isolation on the basis of dynamic and/or intracellular responses, enabling new avenues for screening. Currently, such sorts require expensive specialized equipment, widely prohibiting such sorts.We previously reported a technique for selective hydrogel-based photoencapsulation of undesired cells in cell cultures to enable sorting [1]. We have increased the resolution of this technique, improving purification and enrichment performance. We have also adapted this approach into an even simpler technique that permits sorting via photopatterned free-radical toxicity termed radical-activated cell sorting, or RACS [2] (Figure 1). Here we plate adherent cells into a dish, assay them using microscopy, and note the positions of cells of interest. We then use an inexpensive inkjet printer to print a transparency mask with opaque features corresponding to locations of cells of interest. After we align the mask to the dish, features reside beneath target cells. We then add a solution to the dish containing a UV-photoinitiator. We expose the dish through the mask with UV light, which causes the photoinitiator to split into toxic radicals that attack and kill unmasked, undesired cells, leaving behind live, desired cells (Figure 2). The method permits culture on arbitrary substrates and requires standard hardware found in biology labs and an inexpensive photoinitiator, facilitating dissemination.We have demonstrated the ability to pattern viability with resolution < 500 μm as well as the functional sorting of mixed MCF7 cell cultures predicated on microscopy-based selection. Further development will increase the range of cell types that can be sorted using this technique and resolution. The straightforward operation and low cost of RACS will especially appeal to biologists, bringing straightforward image-based cell-sorting technology to individual labs."
MICROSCALE CONTROLLED CONTINUOUS CELL CULTURE,"For systems biology, the models are more often limited by the absence of reliable experimental data than by available computational resources. Unfortunately, there is still great difficulty in making the leap from genetic and biochemical analysis to accurate verification with conventional culture growth experiments due to variations in culture conditions. Measurements of metabolic activity through substrate and product interactions or cellular activity through fluorescent interactions are highly dependent on environmental conditions and cellular metabolic state. For such experiments to be feasible, continuous cultures [1] [2] utilizing control strategies must be developed to measure chemical concentrations, introduce chemical inputs, and remove waste. An integrated microreactor system with built-in fluid metering will enable environmental control and programmable experiments capable of generating reproducible data.The chip shown in Figure 1 is fabricated out of a rigid plastic, polycarbonate, utilizing PDMS membranes for actuation and pumping [3] The fabrication process for bonding plastic-PDMS hybrid devices has been described previously [4]. Mixing and oxygen delivery is performed through membranes between the fluidic and actuation layers of the growth chamber sections. Chip reliability is demonstrated over a 2-week culture where multiple steady states are reached. Culture experiments are performed with E. coli ATCC31883 in 5 g/L glucose minimum-salts defined medium supplemented with 100 ug/ml ampicillin. Cell density is measured through forward-scattering with an optical sensor at 585 nm in a path length of 0.8 mm. Multiple operation modes are shown in Figure 2 including batch, fed batch, oxystat, chemostat, and turbidostat. Full control is demonstrated under turbidostatic steady state, where the cell density is closed-loop-controlled at a cell density of 2, by dynamically varying the flow rate. Turbidostatic steady state also allows the extraction of the cell maximum growth rate of 0.79 h-1 in minimum salts medium."
USING BUOYANT MASS TO MEASURE THE GROWTH OF SINGLE CELLS,"Understanding how the rate of cell growth changes during the cell cycle and in response to growth factors and other stimuli is of fundamental interest. Over the decades, various approaches have been developed for describing cellular growth patterns, but different studies have often reached irreconcilable conclusions, even for the same cell types. Several factors may contribute to the discrepancies between different growth models: i) cells are minute, irregularly shaped objects; ii) proliferating cells increase their size only by a factor of two, so distinguishing between different cell growth models with mathematical rigor requires highly precise measurements; iii) a wide variety of methods have been used to measure growth, including approaches that average across populations as well as those that monitor individual cells; and iv) a cell’s size includes both volume and mass, which can change at different rates. An ideal method for measuring cell growth rates would directly and continuously monitor the mass and volume accumulation of single unperturbed cells with high precision. In recent years, optical microscopy has been the closest match to this ideal, but volume determination by microscopy has lacked the precision to conclusively distinguish between cell-growth models. Potential alternatives include using fluorescent protein reporters that are correlated with cell size5 or using phase microscopy to measure dry mass during cell growth. We have developed a system that can precisely monitor the growth of single cells in terms of buoyant mass and show that bacteria, yeast, and mammalian lymphoblast cells grow at a rate that is proportional to their buoyant mass (Figures 1 and 2). Buoyant mass is dependent on the amount of biomass in the cell, most of which is denser than water, and so is analogous to the dry mass of the cell."
NONMONOTONIC ENERGY DISSIPATION IN MICROFLUIDIC RESONATORS,"Micro- and nanomechanical cantilevers are widely used as sensitive probes for physical measurements in materials science, engineering, and biology. In vacuums and air, detecting shifts in the resonance frequency enables exquisitely sensitive measurements of mass and detection of single DNA molecules, single viral particles, and single bacterial cells. However, numerous applications in nanotechnology and the life sciences require samples to be contained in liquid. Recently, measurements have demonstrated that viscous damping is substantially reduced by confining the (liquid) sample to a microfluidic channel embedded inside a cantilever beam surrounded by vacuum (Figure 1). Such devices enable mass measurements of nanoparticles, single bacterial cells, and submonolayers of adsorbed proteins with femtogram sensitivity in liquid. A key outstanding question is how energy dissipation, and hence sensitivity, scales with the size of the resonator and the density and viscosity of the fluid. This is of particular interest since two of the most intriguing size regimes for these devices remain to be explored: (i) where the resonators are small enough to acquire mass spectra of viruses, protein complexes, and ultimately single molecules directly in solution and (ii) where the channel is large enough to measure the growth of mammalian cells by monitoring their mass with high precision. We have addressed this question through theory and through measurements; our results reveal surprising connections between the fluid properties, energy dissipation, and the device dimensions in liquid-filled microcantilevers. While the quality factor of conventional cantilever sensors submersed in fluid always degrades with increasing viscosity, we show that damping in liquid-filled cantilevers can increase or decrease as viscosity increases (Figure 2)."
RF MEMS RESONATORS FOR BODY-AREA NETWORK TRANSCEIVERS,"Traditionally, RF circuits for wireless communications have used large-sized and low-quality-factor (Q) electrical RF components such as oscillators and filter banks, which create a bottleneck to miniaturization. Silicon-based micromechanical resonators can complement or even replace their electrical counterparts in existing wireless technology by providing RF building blocks with small size, low power, high-Q and multi-GHz frequency (high speed) functionality.In this project, we are developing bulk acoustic resonators for use in low-power transceivers in Body-area Networks (BAN), as part of the Healthy Radios project sponsored by MARCO IFC/MSD. These networks will transmit data from multiple sensors for parallel monitoring of medical or environmental variables. An integrated solution for transceiver design employing multiple channels separated by 1 MHz in the 2.36 to 2.4 GHz Medical BAN band requires a bank of RF resonators with quality factors greater than those achievable using conventional LC tanks. We explore high-Q micromechanical resonators using lateral dielectric transduction [1] [2] at multi-GHz frequencies to achieve channel-select filtering and narrow-bandwidth frequency sources in the BAN transceivers. These devices are compared to piezoelectrically transduced devices, successfully demonstrated in the multi-GHz domain [3].The coupling coefficients of piezoelectric transducers are in general greater than those of their electrostatic counterparts. However, piezoelectric devices have limited quality factor due to mechanical losses in the piezoelectric material. Unlike piezoelectrics, dielectric transducers provide high Q and compatibility with CMOS for monolithic transceiver design. To this end, we scale Si-based lateral dielectrically transduced resonators to the BAN band at 2.4 GHz (Figure 1). The nth harmonic of longitudinal vibration in the bar is driven and sensed on alternate fingers of an interdigitated electrode design. Figure 2 provides the analytical frequency-dependence of motional impedance (Rx) and nominal capacitance (C0) for various harmonics with 20 nm of HfO2 transduction film on a 340-nm Si device layer. A motional impedance of ~10 kΩ is obtained for a 48-um device operating at the 15th harmonic but it is necessary to optimize for the nominal capacitance for these specifications. Optimization of Rx and C0 allows for integration of the resonators into the transceiver circuitry by making a feedback loop possible for an oscillator and providing low insertion loss for a bandpass filter."
MICROFABRICATED DEVICES FOR PORTABLE POWER-GENERATION,"The development of portable power-generation systems remains an important goal, with applications ranging from the automobile industry to the portable electronics industry. The focus of this work is to develop microreaction technology that converts the chemical energy stored in fuels– such as light hydrocarbons and their alcohols— directly into electricity or into a different energy vector such as hydrogen. Developing devices with high energy-conversion efficiency requires addressing difficulties in high-temperature operation: specifically, thermal management, material integration, and improved packaging techniques.We have developed a catalytic combustion-based device intended for the direct conversion of thermal energy to electricity. The combustor has been designed to achieve attractive energy and power densities while addressing system challenges such as mechanically robust fluidic connections and minimal parasitic power losses related to pressurization of air. The channels of the combustor are etched using wet potassium hydroxide, which is the most economical etch technique available. Straight channels (1mm by 1mm in cross-section) are arranged in parallel and separated by 100-μm –thick silicon walls, in order to achieve low pressure drop (< 300 Pa at 10 SLPM gas flow) with significant surface area (~1 cm2 per channel) for catalyst deposition. Two identical reactors are stacked to increase reactor volume without a significant increase in exposed surface area. External gas distribution manifolds are compression-sealed to the reactor, eliminating the need for glass brazing of tubes, increasing the mechanical robustness of the device, and avoiding large pressure losses associated with flow constrictions. Platinum-on-alumina catalyst has been washcoated on the channel surfaces for the catalytic combustion of butane with air.The catalytic combustor has been shown to transfer up to 360 W of heat through surfaces intended for thermoelectric power generation, at a maximum surface temperature of 465°C and a thermal transfer efficiency in the range of 73 – 78% (based on fuel lower heating value). The experiments have been performed using conductive heat sinks designed to have a thermal resistance similar to that of a thermoelectric module. As designed, the reactor could also be used for heat integration of multiple reactions, such as catalytic combustion and steam reforming of alcohol for hydrogen production."
MODEL-BASED DESIGN OF MEMS VIBRATION-ENERGY-HARVESTERS FOR WIRELESS SENSORS,"The recent development of “low power” (10s-100s of μW) sensing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of individual wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of structural health monitoring (SHM), particularly for aircraft. This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam structures have been developed to predict the electrical and mechanical performance obtainable from ambient vibration sources. The optimized models have been verified by comparison to tests on a macro-scale device both without [1] and with a proof mass at the end of the structure (Figure 1) [2]. A non-optimized, uni-morph beam prototype (Figure 2) has been designed and fabricated [3] [4]. Design tools to allow device optimization for a given vibration environment have been under detailed investigation considering various geometries of the device structures and fabrication constraints, especially in microfabrication. Future work will focus on fabrication and testing of optimized unimorph beams for not only the {3-1} mode but also the {3-3} mode of operation using an interdigitated electrode configuration. System integration and development, including modeling the power electronics, will be included."
STUDYING AUTOCRINE SIGNALING FOR GROWTH IN TUMOR AND STEM CELLS,"Autocrine signaling plays a key role in tumorigenesis and in the maintenance of various physiologic states. Our research involves the use of cell-patterning techniques to investigate the role of autocrine signaling during in vitro expansion of embryonic stem cells and cancer cells.Expanding on our previous experimental work, recently we have also developed numerical models of autocrine signaling. Cells act as sources for autocrine factors. However, cells also possess receptors that the factors can bind to. Finally, in addition to protein transport, uptake of nutrients and production of metabolites are also important processes to account for, especially for longer culture periods. We have previously found an optimal density for 2-day culture of mouse embryonic stem cells. Our models suggest that the positive feedback on growth provided by autocrine signaling combined with the negative feedback provided by nutrient depletion can account for the presence of such an optimal density (Figure 1).For our study of autocrine signaling in tumor cells, we continued to investigate the role of autocrine signaling on heterogeneity in tumor growth. Using A431 epidermoid carcinoma cells as our model, we used stencil-cell patterning to position cells as square-latticed arrays of circular cell patches of varying size and spacing (Figure 2). Unlike randomly-plated cell culture where cells experience differing amounts of cell-cell contacts and irregular intercellular spacing, our platform ensures the direct modulation of autocrine signaling while keeping other contributing signals intact. We are investigating the use of the developed platform as a novel tool to quantify the spatial operation of autocrine signaling. Existing methods require prior knowledge of the underlying autocrine loops and therefore cannot be applied to less characterized biological systems. Our technique will be useful for in vitro investigation of cancer therapeutics and enables the systematic modulation of autocrine-promoted growth."
ISO-DIELECTRIC SEPARATION OF CELLS AND PARTICLES,"The development of new techniques to separate and characterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. Continuing or improving upon this trend – for example, by developing new avenues for performing genetic and phenotypic screens – requires continued advancements in cell-sorting technologies. Towards this end, we are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (DEP, the force on a polarizable object [1]) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity. It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [2] [3].Previously, we have demonstrated the ability to perform continuous-flow, label-free, non-binary separations using IDS on a wide variety of cells and particles, while simultaneously extracting quantitative information from these samples as they are sorted [4]. We are currently focusing on extending these capabilities to perform genome-wide characterizations of electrical properties in the budding yeast Saccharomyces cerevisiae. The most recent implementation of the device uses a valve scheme that enables real-time control of the conductivity gradient, along with the ability to rapidly switch samples for sorting and characterization (Figure 1). These developments increase the throughput of the device, making the systematic characterization of both pooled and unpooled cell libraries feasible. To date, we have applied IDS to a pooled screen of the yeast deletion collection, identifying several genes associated with distinct electrical properties (Figure 2). The improved understanding of the relationship between a cell’s genotype and its physical properties enabled by IDS suggests its potential as a new high-content screening platform."
CHARACTERIZING IMMOBILIZED CATALYSTS USING PACKED-BED MICROREACTORS,"Catalyst immobilization on heterogeneous supports affords several advantages over homogeneous catalysts for chemical synthesis in continuous flow processes. The use of packed beds in flow systems offers built-in catalyst separation from the effluent while allowing for high catalyst loadings. However, heterogeneous catalysts typically suffer from two problems: 1) reduced activity compared to the homogeneous analogue and 2) loss of activity over time due to deactivation or leaching. A requirement for using immobilized catalysts in continuous processes is an understanding of the activity and stability of the catalyst over long periods.Towards this end, we have developed a platform for characterizing immobilized catalysts using silicon microreactors. These devices have void volumes of 28-140 µL, which allow complete characterization using milligram quantities of material. The devices are fabricated using deep reactive-ion-etching (DRIE), coated with silicon nitride to enhance chemical compatibility, and capped with pyrex to allow visual access. The microfabricated weir has 25 µm wide channels (Figure 1); thus, particles larger than 25 µm can be retained. Fluidic connections are made using a compression packaging scheme that was recently developed in our group [1] (Figure 2). Both polymer beads and silica gel have been loaded and retained, though polymer beads offer challenges due to their tendency to swell in organic solvents. Application of this system to studying covalently bound catalysts and physisorbed catalysts [2] is ongoing."
DESIGN OF A THREE-AXIS MEMS FORCE-SENSOR,"Multi-axis force-sensing at the micro-scale is necessary for a wide range of applications in biology, materials science, and nanomanufacturing. A three-degree-of-freedom force-sensor (Figure 1) was designed that is capable of accurately and precisely measuring the adhesion forces (nanoNewtons) between biologically active surfaces. This force sensor is positioned and actuated using a Hexflex nanopositioner and Lorenz force actuators. The fabricated device is shown in Figure 2.In order to design high-accuracy, high-precision, multi-axis MEMS force sensors, a closed-form model was developed to optimize the strain-sensitivity of the MEMS force-sensor. This model first sets constraints on the system due to package size, fabrication techniques, desired degrees of freedom, and force range. The layout of the flexure system is optimized to meet the kinematic and manufacturing constraints of the MEMS force-sensor. The geometry of the flexures is set to maximize the strain at the sensor locations.This model was incorporated into a thermal/electric model to fully characterize all of the inputs to the system. The resolution of the force-sensor is a function of the noise from the strain-sensors, the noise in the electronics, the thermomechanical noise, and the sensitivity of the strain-sensors to a force input. Based on this model, the dominant noise sources are identified and the sensor system is optimized to reduce these noise sources. The thermal/electric model is also used to determine the major factors limiting accuracy of the force-sensor. In most cases, the drifts in both the electronics and sensors caused by fluctuations in room temperature were the major sources of accuracy errors. Therefore, an environmental enclosure with closed-loop control over temperature was designed to reduce the thermal variation. Overall, the final design of the force sensor is capable of producing sub-nanoNewton-resolution force measurements with nanoNewton-level accuracy."
LIQUID PROPAGATION IN MICROPILLAR ARRAYS,"Prediction and optimization of liquid propagation rates in micropillar arrays are important for various lab-on-chip [1], biomedical [2], and thermal management applications [3]. We developed a semi-analytical model based on the balance between capillary pressure and viscous resistance to predict liquid propagation rates in micropillar arrays with height-to-period ratios greater than 1 and diameter-to-period ratios less than 0.57. These geometries represent the most useful regimes for practical applications requiring large propagation rates. The capillary pressure was obtained using an energy approach in which the meniscus shape was predicted using Surface Evolver simulations and verified by interference microscopy. The interference microcopy image of the liquid meniscus is shown in Figure 1. The viscous resistance was determined using Brinkman’s equation [4] with a numerically obtained permeability [5] and corroborated with finite element simulations. The model shows excellent agreement with one-dimensional propagation experiments of de-ionized water in silicon micropillar arrays, which highlights the importance of capturing the details of the meniscus shape and the viscous losses. Furthermore, an effective propagation coefficient was obtained through dimensionless analysis that is functionally dependent only on the micropillar geometry. The relationship is plotted in Figure 2. The work offers design guidelines to obtain optimal liquid propagation rates on micropillar surfaces.The current model obtained an average driving pressure during the propagation process. More specifically, two distinct time scales were observed as the liquid front advanced on the bottom surface between pillars or wetting the sides of the pillars. When the height of the pillars is smaller than the period of the pillar array, the former time scales dominate and our model overestimates the propagation rate. Future work will focus on the detailed dynamics of the liquid front."
NANOFABRICATED REFLECTION AND TRANSMISSION GRATINGS,"Diffraction gratings and other periodic patterns have long been important tools in research and manufacturing. Diffraction occurs due to the coherent superposition of waves and is a phenomenon with many useful properties and applications. Waves of many types can be diffracted, including visible and ultraviolet light, x-rays, electrons, and even atom beams. Periodic patterns have many useful applications in fields such as optics and spectroscopy; filtering of beams and media; metrology; high-power lasers; optical communications; semiconductor manufacturing; and nanotechnology research in nanophotonics, nanomagnetics, and nanobiology.A long-standing problem with transmission gratings in the extreme ultraviolet (EUV) and soft x-ray bands has been the strong absorption of photons upon transmission and thus a low diffraction efficiency in this important wavelength band. We have recently solved this problem with the invention and fabrication of critical-angle transmission (CAT) gratings. This new design for the first time combines the high broadband efficiency of blazed grazing-incidence reflection gratings with the superior alignment and figure tolerances and the low weight of transmission gratings [1] [2]. The CAT gratings consist of ultrahigh-aspect-ratio, nm-thin freestanding grating bars with sub-nm smooth sidewalls that serve as efficient mirrors for photons incident at graze angles below the angle for total external reflection (see Figures 1 and 2). Blazing can concentrate diffracted power into a single or a few desired diffraction orders and has been confirmed through x-ray tests. Blazing also enables the use of higher diffraction orders and leads to manifold increases in spectral [3] and spatial resolution in spectrometer or focusing applications, respectively. We have achieved grating bar aspect ratios of ~ 150 in 6-micron-deep, 200-nm-period CAT gratings and are currently focusing on the minimization of internal support structures.Work on high-resolution (R ~ 10,000 – 100,000) applications is also ongoing in the areas of high-precision patterning of silicon-immersion echelle gratings in infrared telescopes for astronomy [4] and blazed reflection gratings for high-resolution EUV and soft x-ray synchrotron applications [5]."
FREE-FLOW ZONE ELECTROPHORESIS OF PEPTIDES AND PROTEINS IN PDMS MICROCHIP FOR NARROW ISOELECTRIC-POINT (PI) RANGE SAMPLE PREFRACTIONATION COUPLED WITH MASS SPECTROMETRY,"Isoelectric point (pI)-based fractionation is ideally suited for the first-dimensional separation because the pI value of any peptide or protein can be simply estimated from the sequence information. Therefore, the pI-based fractionation techniques can be highly specific to target peptides and proteins. Due to this benefit, there have been previous efforts to integrate isoelectric focusing (IEF) into mass spectrometry (MS)-based proteome analysis processes [1] [2] [3] [4]. While the physical coupling between capillary IEF and ESI-MS is straightforward, the buffer systems for IEF separation (carrier ampholytes, a complex mixture of amphoteric small molecules) have low compatibility with electrospray (ESI)-MS interfaces. In view of this current deficit, we have developed an ampholyte-free, two-step cascaded microfluidic sorting technique based on free-flow zone electrophoresis that isolates the molecules of interest from a small sample volume of 100 mL within a narrow and freely adjustable pI range (£ 1 pH units), even below pH 3 and beyond pH 10 [5]. To create a salt bridge for free-flow electrophoresis in PDMS chips, we printed a submicron-thick hydrophobic layer on a glass substrate and created an electrical junction gap for free-flow zone electrophoresis. With this sorting device, as shown in Figure 1, we demonstrated binary sorting of peptides and proteins in standard buffer systems and validated the sorting result with liquid chromatography (LC)/MS. In Figure 2, the sorting result of the acidic peptides < pH 7 is shown as an example. Furthermore, we coupled two sorting steps via off-chip titration and isolated peptides within specific pI ranges from sample mixtures, where the pI range was simply set by the pH values of the buffer solutions. This pI-based binary sorting device, with its simplicity of fabrication and a sorting resolution of 0.5 pH unit, can potentially be a high-throughput sample fractionation tool for targeted proteomics using LC/MS."
STENCIL-AND-FLIP CELL PATTERNING FOR GENERATION OF DUAL STEM CELL MICROENVIRONMENTS,"Embryonic development is a complex dynamic process, whereby spatial organization of molecular signals instructs stem cells to adopt various differentiation fates at different locations [1]. Recapitulating this process in vitro will greatly facilitate the mechanistic understanding of embryonic development. However, current in vitro models are unable to reproduce the developmental environment adequately. To realize the aim of building a complex developmental model, we developed the Stencil-and-Flip Cell Patterning (SAF-CP) technique to present multiple spatially organized microenvironments to a single population of stem cells [2]. SAF-CP combines two established patterning technologies i.e., stencil and Bio Flip Chip (BFC) [3] but uses them in a sequentially aligned format to create spatially organized stem cell microenvironments in vitro (Figure 1). To validate that spatial organization of microenvironments translates to differential instruction of stem cell fates, we used SAF-CP to selectively present self-renewing (STO fibroblast feeder + N2B27 medium) (Figure 2A) and neuronal differentiating (gelatin + N2B27 medium) (Figure 2B) microenvironments to a single mouse embryonic stem cell (mESC) colony. (Figure 2C) After one week of culture, we observed that self-renewal (Sox2) and neuronal precursor (Nestin) markers had a spatial distribution coinciding with the patterned dual microenvironments (Figure 2D). The ratio of self-renewal-to-neuronal phenotypes within a colony was observed to be dependent on the relative extent of the two microenvironments presented to the colony (quantifiable by the percentage colony area on STO and gelatin, respectively) (Figure 2E). Our results demonstrate the utility of the SAF-CP in building in vitro models that recapitulate the organizational-instructive traits of stem cell niches, allowing us to emulate stem cell development more realistically."
FLEXIBLE MULTI-SITE ELECTRODES FOR MOTH FLIGHT CONTROL,"Significant interest exists in creating insect-based Micro-Air-Vehicles (MAVs) [1] [2] [3] that would combine advantageous features of insects—small size, effective energy storage, navigation ability—with the benefits of MEMS and electronics—sensing, actuation and information processing. The key part of the insect-based MAVs is the stimulation system which interfaces with the nervous system of the insect to bias the insect’s flight path.In this work, we have developed a flexible multi-site electrode (FME) for insect flight control that directly interfaces with the animal’s central nervous system. The FMEs are made of two layers of polyimide with gold sandwiched in-between in a split-ring geometry using standard MEMS processing [3]. The FMEs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord of the Moth Manduca Sexta and allows for an efficient surgical process for implantation (Figure 1). Additionally, we have integrated carbon nanotube (CNT)-Au nanocomposites into the FMEs to enhance the charge injection capability of the electrode.We are able to elicit graded and multi-direction abdominal movements in both the pupae and adult moths using FME stimulation.Moreover, the CNT coated FMEs are able to elicit abdominal motion of the moths with a stimulation voltage significantly less (1.0 V vs. 2.0 V, p < 0.001, n=10 moths) than that of uncoated FMEs. Finally, we have integrated the FMEs into a wireless system and in the flight control experiment, we are able to force a freely flying animal to perform turning motions (Figure 2a) using the abdominal ruddering with these elicited abdomen motions. These turning motions are well repeatable and the changes in the yaw angle of the moth with 4 successive stimulations are shown in Figure 2b."
ORIGIN AND CONTROL OF INTRINSIC STRESSES IN METALLIC THIN FILMS FOR N/MEMS APPLICATIONS,"The mechanical properties of thin films, especially residual stresses in as-deposited films, strongly influence the reliability and performance of microelectromechanical devices and systems. Residual stresses can be as high as 1GPa and can be tensile or compressive, depending on the material, deposition technique, and, very sensitively, deposition conditions. When evaporative deposition is used, the two broad categories of behavior occur (Figure 1). Type I is characterized by development of a high tensile stress that is retained during and after continued deposition. This behavior is common when materials are deposited at low temperatures relative to their melting temperature, e.g. among refractory metals and semiconductors. In Type II behavior, a tensile stress develops as the film first forms, but compressive stresses (as high as 200MPa) develop during continued deposition. This behavior is characteristic of materials with relatively low melting temperatures such as Au, Ag, and Al.The tensile rise seen in both behaviors is thought to arise as islands coalesce to form a film. The origin of post-coalescence compressive stress has been debated extensively over the past decade. Models associated with adatom-surface [1] [2] and adatom-grain boundary [3] interactions have been proposed to explain the compressive-stress generation during deposition and its reversible relaxation during growth interruptions.We use in-situ stress measurements to follow stress evolution as films are deposited. The measurements can be done with sensitivity to variations associated with sub-monolayer deposition and can be carried out at high sampling frequencies. Through correlation of stress and microstructural evolution, we have shown that reversible stress relaxations that occur during interruptions of growth of Au films vary with the grain size, in general agreement with the model given in reference [3] [4]. However, we also find that the temperature dependence of the reversible stress change is too weak to be consistent with any proposed models. To further characterize stress evolution phenomenology, especially with respect to deposition temperature, we have begun studies of a range of materials. Figure 2 shows results for Ni films deposited at room temperature and 200C. Two observations are noteworthy: the observed behavior is intermediate to Type I and II, and the behavior depends strongly on the deposition temperature. Those results show that the processes leading to the development of compressive stresses are temperature-dependent, even if the process involved in reversible stress relaxations is not.Through these and other studies, we aim for a comprehensive understanding of factors that affect the residual stress in thin films, so that this stress can be better controlled."
POROUS IONIC-LIQUID-ION-SOURCE EMITTER ARRAYS FOR SPACECRAFT PROPULSION,"Ionic Liquid Ion Sources (ILIS) are a subset of electrospray emitters characterized by pure ion emission from room-temperature ionic liquids. Previously these sources have been shown to be a simple and efficient source of both positive and negative ions that could be used, amongst other applications, for spacecraft propulsion [1]. However, the low (<1µN) thrust levels per emitter require practical thrusters to employ arrays of emitters. At modest packing densities, a few tips per square millimeter, the thrust per unit area approaches that of more traditional plasma based ion thrusters [2]. Initial efforts focused on creating arrays of externally wetted emitters from silicon [3]. These studies were plagued by poor and/or inconsistent wetting. As an alternative, we are developing arrays fabricated from bulk porous materials. Here capillarity alone provides passive and consistent wetting of the emission sites. We have observed that this feed mechanism can yield a larger current range than externally wetted emitters while continuing to operate in the purely ionic regime [2]. Using porous emitters, a thruster configuration as in Figure 1 is envisioned.Fabricating arrays using bulk porous material has resulted in a number of inherent challenges. Electrochemical etching has been a useful tool for fabricating ILIS in the past [1], and our recent findings suggest it may be well suited for etching the surface of a porous material. Specifically, when etching is used with an appropriate tool, a transport limited etch rate can be achieved that promotes smooth, near-isotropic etching of the surface of a porous material with minimal penetration into the pores. Figure 2 demonstrates the results for both porous and solid nickel samples. Additionally, we are beginning to investigate the feasibility of high-pressure molding of powder metals to fabricate the emitters and substrate with molds fabricated using more traditional techniques. To date, basic extraction and acceleration grids have been fabricated from silicon with die-level alignment; however, we plan to move towards a more complete and optimized package within the near future."
PECVD CNT-ENABLED ELECTRON-IMPACT GAS IONIZERS FOR PORTABLE MASS SPECTROMETRY,"Research efforts on MEMS-based analytical instrumentation have focused on the development of rugged gas chromatography and mass spectrometry (GC/MS) systems that are smaller, lighter, cheaper, faster, and more power-efficient [1]. The power consumption, size, and weight of these systems are driven by the pump requirements. Therefore, relaxation of the pressure level at which the system components can operate would enable the systems’ portability. Portable GC/MS systems, either as individual units or as parts of massive networks, can be used in a wide range of applications including in-situ geological surveys, law enforcement, environmental monitoring, and space exploration [2].The ionizer is one of the core components of an MS system. We have developed a carbon nanotube (CNT)-based MEMS/NEMS electron-impact gas ionizer with integrated extractor gate for portable mass spectrometry. The ionizer achieves low-voltage ionization using sparse forests of plasma-enhanced chemical-vapor-deposited (PECVD) CNTs as field-emitters and a proximal extractor grid with apertures aligned to the CNT forests to facilitate electron transmission. The extractor gate is integrated into the ionizer by using a high-voltage MEMS packaging technology based on Si springs defined by deep-reactive-ion etching (DRIE) [3]. The ionizer also includes a high aspect-ratio silicon structure (μfoam) that facilitates sparse CNT growth and also enables uniform current emission. The experimental data show that the MEMS extractor gate transmits up to 66% of the emitted current, and that the ionizers are able to produce up to 0.139 mA of ion current with up to 19% ionization efficiency at 22 mtorr while consuming 0.39 W [4]. Figure 1 shows a cross-section schematic and a picture of a fabricated ionizer; Figure 2 shows experimental data that demonstrate that the ionizers work as described by the electron-impact-ionization model."
MEMS PRESSURE-SENSOR ARRAYS FOR PASSIVE UNDERWATER NAVIGATION,"A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the lateral line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1] [2]. The canal subsystem of the organ can be described as an array of pressure-sensors [3]. Interpreting the spatial pressure gradients allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby objects [2]. It also aids schooling [5]. Similarly, by measuring pressure variations on a vehicle surface, an engineered dense pressure-sensor array allows the identification and location of obstacles for navigation (Figure 1). We are demonstrating proof-of-concept by fabricating such MEMS pressure sensors by using KOH etching techniques on SOI wafers to construct strain-gauge diaphragms.The system consists of arrays of hundreds of pressure-sensors spaced about 2 mm apart on etched silicon and Pyrex wafers. The sensors are arranged over a surface in various configurations (Figure 2). The target pressure resolution for a sensor is 1 Pa, which corresponds to the noiseless disturbance created by the presence of a 0.1-m radius cylinder in a flow of 0.5 m/s at a distance of 1.5 m. A key feature of a sensor is the flexible diaphragm, which is a thin (20-μm) layer of silicon attached at the edges to a silicon cavity. The strain on the diaphragm due to pressure differences across the diaphragm is measured. At this stage, the individual MEMS pressure sensors are being constructed and tested.In parallel to the construction of a sensor array, techniques are being developed to interpret the signals from a dense pressure array by detecting and characterizing wake structures such as vortices and building a library of pressure distributions corresponding to basic flow obstructions. In order to develop these algorithms, experiments are being performed on coarse arrays of commercial pressure-sensors."
PROTEIN DYNAMICS INVOLVED IN THE CYTOKINESIS OF FISSION YEAST,"Cytokinesis is the final stage of cell division when eukaryotes assemble a contractile acto-myosin ring to physically divide their cytoplasm and genetic material to create two daughter cells. Global and local concentrations of protein components involved in the contractile ring assembly of fission yeast have been using quantitative fluorescence microscopy [1]. However, the dynamics of the ring protein remains unknown due to the limitations of conventional imaging and image analysis approaches. In this project, we use highly integrated computational-experimental research approaches that consist of high-resolution imaging with the assistance of micro-well arrays manufactured in MTL, computational image analysis (using an image-correlation spectroscopy algorithm) [2] [3], and data-driven computational modeling. Our goal is to establish a molecular-level model that describes mechanistically how the core set of ring proteins in fission yeast is organized prior to, as well as during, its constriction in cytokinesis. Specifically, the summer project includes an experimental imaging part and a computational modeling part."