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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.