Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2007"

Nanoenergetic Materials for MEMS: A Review

Abstract: New energetic materials (EMs) are the key to great advances in microscale energy-demanding systems as actuation part, igniter, propulsion unit, and power. Nanoscale EMs (nEMs) particularly offer the promise of much higher energy densities, faster rate of energy release, greater stability, and more security (sensitivity to unwanted initiation). nEMs could therefore give response to microenergetics challenges. This paper provides a comprehensive review of current research activities in nEMs for microenergetics application. While thermodynamic calculations of flame temperature and reaction enthalpies are tools to choose desirable EMs, they are not sufficient for the choice of good material for microscale application where thermal losses are very penalizing. A strategy to select nEM is therefore proposed based on an analysis of the material diffusivity and heat of reaction. Finally, after a description of the different nEMs synthesis approaches, some guidelines for future investigations are provided.

Monolithically Fabricated Microgripper With Integrated Force Sensor for Manipulating Microobjects and Biological Cells Aligned in an Ultrasonic Field

Abstract: This paper reports an electrostatic microelectromechanical systems (MEMS) gripper with an integrated capacitive force sensor. The sensitivity is more than three orders of magnitude higher than other monolithically fabricated MEMS grippers with force feedback. This force sensing resolution provides feedback in the range of the forces that dominate the micromanipulation process. A MEMS ultrasonic device is described for aligning microobjects suspended in water using ultrasonic fields. The alignment of the particles is of a sufficient accuracy that the microgripper must only return to a fixed position in order to pick up particles less than 100 mum in diameter. The concept is also demonstrated with HeLa cells, thus providing a useful tool in biological research and cell assays

Design and Characterization of Artificial Haircell Sensor for Flow Sensing With Ultrahigh Velocity and Angular Sensitivity

Abstract: We report the development of an artificial hair cell (AHC) sensor with design inspired by biological hair cells. The sensor consists of a silicon cantilever beam with a high-aspect-ratio cilium attached at the distal end. Sensing is based on silicon piezoresistive strain gauge at the base of the cantilever. The cilium is made of photodefinable SU-8 epoxy and can be up to 700-mum tall. In this paper, we focus on flow-sensing applications. We have characterized the performance of the AHC sensor both in water and in air. For underwater applications, we have characterized the sensor under two flow conditions: steady-state laminar flow (dc flow) and oscillatory flow (ac flow). The detection limit of the sensor under ac flow in water is experimentally established to be below 1 mm/s. A best case angular resolution of 2.16deg is also achieved for the sensor's yaw response in air.

Single-Chip Multiple-Frequency ALN MEMS Filters Based on Contour-Mode Piezoelectric Resonators

Abstract: This paper reports experimental results on a new class of single-chip multiple-frequency (up to 236 MHz) filters that are based on low motional resistance contour-mode aluminum nitride piezoelectric micromechanical resonators. Rectangular plates and rings are made out of an aluminum nitride layer sandwiched between a bottom platinum electrode and a top aluminum electrode. For the first time, these devices have been electrically cascaded to yield high performance, low insertion loss (as low as 4 dB at 93MHz), and large rejection (27 dB at 236 MHz) micromechanical bandpass filters. This novel technology could revolutionize wireless communication systems by allowing cofabrication of multiple frequency filters on the same chip, potentially reducing form factors and manufacturing costs. In addition, these filters require terminations (1 kOmega termination is used at 236 MHz) that can be realized with on-chip inductors and capacitors, enabling their direct interface with standard 50-Omega systems

Design and Operation of a MEMS-Based Material Testing System for Nanomechanical Characterization

Abstract: In situ mechanical characterization of nanostructures, such as carbon nanotubes and metallic nanowires, in scanning and transmission electron microscopes is essential for the understanding of material behavior at the nanoscale. This paper describes the design, fabrication, and operation of a novel microelectromechanical-systems (MEMS)-based material testing system used for in situ tensile testing of nanostructures. The device consists of an actuator and a load sensor with a specimen in between. Two types of actuators, in-plane thermal and comb drive actuators, are used to pull the specimens in displacement control and force control modes, respectively. The load sensor works based on differential capacitive sensing, from which the sensor displacement is recorded. By determining sensor stiffness from mechanical resonance measurements, the load on the specimen is obtained. Load sensors with different stiffness were fabricated. The best resolutions were achieved with load sensors that are designed for testing nanotubes, reaching 0.05 fF in capacitance, 1 nm in displacement, and 12 nN in load. For the first time, this MEMS-based material testing scheme offers the possibility of continuous observation of the specimen deformation and fracture with subnanometer resolution, while simultaneously measuring the applied load electronically with nano-Newton resolution. The overall device performance is demonstrated by testing freestanding cofabricated polysilicon films and multiwalled carbon nanotubes.

A Polymer-Based Flexible Tactile Sensor for Both Normal and Shear Load Detections and Its Application for Robotics

Abstract: This paper proposes and demonstrates a novel flexible tactile sensor for both normal and shear load detections. For the realization of the sensor, polyimide and polydimethylsiloxane are used as a substrate, which makes it flexible. Thin metal strain gauges, which are incorporated into the polymer, are used for measuring normal and shear loads. The salient feature of this tactile sensor is that it has no diaphragm-like structures. The unit tactile cell characteristics are evaluated against normal and shear loads. The fabricated tactile sensor can measure normal loads of up to 4 N, and the sensor output signals are saturated against loads of more than 4 N. Shear loads can be detected by different voltage drops in strain gauges. The device has no fragile structures; therefore, it can be used as a ground reaction force (GRF) sensor for balance control in humanoid robots. Four tactile unit sensors are assembled and placed in the four corners of the robots sole. By increasing bump dimensions, the tactile unit sensor can measure loads of up to 2 kgf. When loads are exerted on the sole, the GRF can be measured by these four sensors. The measured forces can be used in the balance control of biped locomotion systems.

Penetration-Enhanced Ultrasharp Microneedles and Prediction on Skin Interaction for Efficient Transdermal Drug Delivery

Abstract: This paper presents penetration-enhanced hollow microneedles and an analysis on the biomechanical interaction between microneedles and skin tissue. The aim of this paper is to fabricate microneedles that reliably penetrate the skin tissue without using penetration enhancers or special insertion tools that were used in the previous studies. The microneedles are made of silicon and feature ultrasharp tips and side openings. The microneedle chips were experimentally tested in vivo by injection of dye markers. To further investigate the penetration, the insertion progression and the insertion force were monitored by measuring the electrical impedance between microneedles and a counter electrode on the skin. The microneedle design was also tested using a novel simulation approach and compared to other previously published microneedle designs. The purpose of this specific part of the paper was to investigate the interaction mechanisms between a microneedle and the skin tissue. This investigation is used to predict how the skin deforms upon insertion and how microneedles can be used to create a leak-free liquid delivery into the skin. The fabricated microneedles successfully penetrated dry living human skin at all the tested sites. The insertion characteristic of the microneedle was superior to an earlier presented type, and the insertion force of a single microneedle was estimated to be below 10 mN. This low insertion force represents a significant improvement to earlier reported results and potentially allows a microneedle array with hundreds of needles to be inserted into tissue by hand.

Dynamic Cell and Microparticle Control via Optoelectronic Tweezers

Abstract: This paper reports on cell and microparticle manipulation using optically induced dielectrophoresis. Our novel optoelectronic tweezers (OET) device enables optically controlled trapping, transportation, and sorting via dielectrophoretic forces. By integrating a spatial light modulator and using direct imaging, arbitrary dynamic manipulation patterns are obtained. Here, we demonstrate manipulation functions, including particle collectors, single-particle traps, individually addressable single-particle arrays, light-defined particle channels, and size-based particle sorting. OET-induced particle manipulation velocities are analyzed as a function of the applied voltage, optical pattern linewidth, and single-particle trap dimensions.

Miniaturized Flexible Temperature Sensor

Abstract: We realize a flexible temperature sensor based on a micropatterned thin-film platinum resistor. The sensor is situated at the end of a 100-mum-wide and 7-cm-long probe. The latter consists of a biocompatible high-temperature-resistant polyimide (U-Varnish-S, UBE Industries, Tokyo, Japan) in which the platinum resistor and the electrical contact leads are embedded. The sensor is operational from 0degC up to 400degC and has a time constant of less than 80 ms. Such a sensor can potentially be used to characterize the thermo-ablation process of tumors using hot steam injection via a microtube.

Thermoplastic Forming of Bulk Metallic Glass— A Technology for MEMS and Microstructure Fabrication

Abstract: A technology for microelectromechanical systems (MEMS) and microstructure fabrication is introduced where the bulk metallic glass (BMG) is formed at a temperature where the BMG exist as a viscous liquid under an applied pressure into a mold. This thermoplastic forming is carried out under comparable forming pressure and temperatures that are used for plastics. The range of possible sizes in all three dimensions of this technology allows the replication of high strength features ranging from about 30 nm to centimeters with aspect ratios of 20 to 1, which are homogeneous and isotropic and free of stresses and porosity. Our processing method includes a hot-cutting technique that enables a clean planar separation of the parts from the BMG reservoir. It also allows to net-shape three-dimensional parts on the micron scale. The technology can be implemented into conventional MEMS fabrication processes. The properties of BMG as well as the thermoplastic formability enable new applications and performance improvements of existing MEMS devices and nanostructures

Application of the Generalized Differential Quadrature Method to the Study of Pull-In Phenomena of MEMS Switches

Abstract: This paper reports on the pull-in behavior of nonlinear microelectromechanical coupled systems. The generalized differential quadrature method has been used as a high-order approximation to discretize the governing nonlinear integro-differential equation, yielding more accurate results with a considerably smaller number of grid points. Various electrostatically actuated microstructures such as cantilever beam-type and fixed-fixed beam-type microelectromechanical systems (MEMS) switches are studied. The proposed models capture the following effects: (1) the intrinsic residual stress from fabrication processes; (2) the fringing effects of the electrical field; and (3) the nonlinear stiffening or axial stress due to beam stretching. The effects of important parameters on the mechanical performance have been studied in detail. These results are expected to be useful in the optimum design of MEMS switches or other actuators. Further, the results obtained are summarized and compared with other existing empirical and analytical models.

Analytical Model of the DC Actuation of Electrostatic MEMS Devices With Distributed Dielectric Charging and Nonplanar Electrodes

Abstract: This paper gives a new insight into the problem of the irreversible stiction of RF microelectromechanical systems (MEMS) attributed to the dielectric charging. We present a model for the electrostatic actuation of MEMS devices taking into account the nonuniform distributions of the air gap and the charges in the dielectric layer. The major result of our study is the impossibility to invoke the sole uniform dielectric charging phenomenon to explain the irreversible stiction of electrostatic MEMS devices. In the absence of other forces, a nonzero variance of the charge distribution is required to explain the stiction of the device. Considering only uniform residual charge densities, previous reported works could only account for a drift of the actuation characteristics as a whole. In case of a uniform air-gap distribution, our analytical model can already account for an increase of the up-capacitance, a shift of the - , its narrowing, and the stiction by a closure of the pull-out window. We further show that the combined nonuniformities of air gaps and charges break the symmetry of the actuation characteristics. The asymmetry can be such that one of the pull-in points disappears, which is replaced by a continuous tuning range while the other pull-in point still exists.

Microparticle Concentration and Separation by Traveling-Wave Dielectrophoresis (twDEP) for Digital Microfluidics

Abstract: This paper describes highly efficient in-droplet particle concentration and separation where particles are concentrated and separated into droplets by traveling-wave dielectrophoresis (DEP) and subsequent electrowetting-on-dielectric droplet splitting. A successful concentration for 5-mum aldehyde sulfate (AS) latex particles was experimentally achieved using microfabricated devices, showing that 98% of the total particles were concentrated into a split daughter droplet. In addition, in-droplet particle separation was successfully achieved using the following two different cases of particle mixtures: case 1) a mixture of 5-mum AS latex beads and 8-mum glass beads; and case 2) a mixture of ground pine (GP) spores and 8-mum glass beads. In case 1), 97% of the total AS beads were separated into one split droplet and 77% of the total glass beads into the other split droplet. In case 2), over 92% of the GP spores were separated into a split daughter droplet, whereas 86% of the glass beads were separated into the other split daughter droplet. In all these concentration and separation experiments, the applied frequency and the conductivity medium are key parameters influencing the concentration and separation performance, which have been optimally determined by measuring the DEP and electrorotation spectra of the used particles prior to the concentration and separation experiments. This integrated in-droplet separation and concentration method may provide an additional functionality to digital microfluidics.

RF MEMS Sequentially Reconfigurable Sierpinski Antenna on a Flexible Organic Substrate With Novel DC-Biasing Technique

Abstract: Devices and systems that use RF microelectromechanical systems (RF MEMS) switching elements typically use one switch topology. The switch is designed to meet all of the performance criteria. However, this can be limiting for highly dynamic applications that require a great deal of reconfigurability. In this paper, three sets of RF MEMS switches with different actuation voltages are used to sequentially activate and deactivate parts of a multiband Sierpinski fractal antenna. The implementation of such a concept allows for direct actuation of the electrostatic MEMS switches through the RF signal feed, therefore eliminating the need for individual switch dc bias lines. This reconfigurable antenna was fabricated on liquid crystal polymer substrate and operates at several different frequencies between 2.4 and 18 GHz while maintaining its radiation characteristics. It is the first integrated RF MEMS reconfigurable antenna on a flexible organic polymer substrate for multiband antenna applications. Simulation and measurement results are presented in this paper to validate the proposed concept.[2007-0013]

Two-Dimensional MEMS Scanner for Dual-Axes Confocal Microscopy

Abstract: In this paper, we present a novel 2-D microelectromechanical systems (MEMS) scanner that enables dual-axes confocal microscopy. Dual-axes confocal microscopy provides high resolution and long working distance, while also being well suited for miniaturization and integration into endoscopes for in vivo imaging. The gimbaled MEMS scanner is fabricated on a double silicon-on-insulator (SOI) wafer (a silicon wafer bonded on a SOI wafer) and is actuated by self-aligned vertical electrostatic combdrives. Maximum optical deflections of plusmn4.8deg and plusmn5.5deg are achieved in static mode for the outer and inner axes, respectively. Torsional resonant frequencies are at 500 Hz and 2.9 kHz for the outer and inner axes, respectively. The imaging capability of the MEMS scanner is successfully demonstrated in a breadboard setup. Reflectance images with a field of view of are achieved at 8 frames/s. The transverse resolutions are 3.94 mum and 6.68 mum for the horizontal and vertical dimensions, respectively.

Sub-Micro-Gravity In-Plane Accelerometers With Reduced Capacitive Gaps and Extra Seismic Mass

Abstract: This paper presents a robust fabrication technique for manufacturing ultrasensitive micromechanical capacitive accelerometers in thick silicon-on-insulator substrates. The inertial mass of the sensor is significantly increased by keeping the full thickness of the handle layer attached to the top layer proof mass. High-aspect-ratio capacitive sense gaps are fabricated by depositing a layer of polysilicon on the sidewalls of low aspect- ratio trenches etched in silicon. Using this method, requirements on trench etching are relaxed, whereas the performance is preserved through the gap reduction technique. Therefore, this process flow can potentially enable accelerometers with capacitive gap aspect-ratio values of greater than 40:1, not easily realizable using conventional dry etching equipment. Also, no wet-etching step is involved in this process which in turn facilitates the fabrication of very sensitive motion sensors that utilize very compliant mechanical structures. Sub-micro-gravity in-plane accelerometers are fabricated and tested with measured sensitivity of 35 pF/g, bias instability of 8 mug, and footprint of <0.5 cm2.

Snap-Through and Pull-In Instabilities of an Arch-Shaped Beam Under an Electrostatic Loading

Abstract: The snap-through and pull-in instabilities of the micromachined arch-shaped beams under an electrostatic loading are studied both theoretically and experimentally. The pull-in instability that results in a system collision with an electrode substrate may lead to a system failure and, thus, limits the system maximum displacement. The beam/plate structure with a flat initial configuration under an electrostatic loading can only experience the pull-in instability. With the different arch configurations, the structure may experience either only the pull-in instability or the snap-through and pull-in instabilities together. As shown in our computation and experiment, those arch-shaped beams with the snap-through instability have the larger maximum displacement compared with the arch-shaped beams with only the pull-in stability and those with the flat initial configuration. The snap-through occurs by exerting a fixed load, and the structure experiences a discontinuous displacement jump without consuming power. Furthermore, after the snap-through jump, the structures are demonstrated to have the capacity to withstand further electrostatic loading without pull-in. Those properties of consuming no power and increasing the structure deflection range without pull-in is very useful in microelectromechanical systems design, which can offer better sensitivity and tuning range.

SU-8 Optical Accelerometers

Abstract: This paper presents the optimization and characterization of SU-8 quad beam optical accelerometers based on intensity modulation. An applied acceleration causes a misalignment between three waveguides, resulting in variation of losses. Mechanical simulations have focused on the evaluation of sensitivity and the design of a robust junction between the mechanical beams and the inertial mass. Results demonstrate that perfectly rounded structures show at least 4.4 times less stress than L-shaped counterparts. Optical simulation predicts that the optimal configuration in terms of sensitivity is obtained when the waveguides are not completely misaligned, since then losses are insensitive to variations in acceleration. Numerical sensitivities ranging between 11.12 and 32.14 dB/g have been obtained. Fabrication has been simplified, now requiring only two photolithographic steps and electroplating Cu as a sacrificial layer. Experimental results show a reproducible experimental sensitivity of at least 13.1 dB/g

Linear and Nonlinear Tuning of Parametrically Excited MEMS Oscillators

Abstract: Microelectromechanical oscillators utilizing noninterdigitated combdrive actuators have the ability to be parametrically excited, which leads to distinct advantages over harmonically driven oscillators. Theory predicts that this type of actuator, when dc voltage is applied, can also be used for tuning the effective linear and nonlinear stiffnesses of an oscillator. For instance, the parametric instability region can be rotated by using a previously developed linear tuning scheme. This can be accomplished by implementing two sets of noninterdigitated combdrives, choosing the correct geometry and alignment for each, and applying ac excitation voltages to one set and proportional dc tuning voltages to the other set. Such an oscillator can also be tuned to display a desired nonlinear behavior: softening, hardening, or mixed nonlinearity. Nonlinear tuning is attained by carefully designing combdrive geometry, flexure geometry, and applying the correct dc voltages to the second set of actuators. Here, two oscillators have been designed, fabricated, and tested to prove these tuning concepts experimentally

Glass Blowing on a Wafer Level

Abstract: A fabrication process for the simultaneous shaping of arrays of glass shells on a wafer level is introduced in this paper. The process is based on etching cavities in silicon, followed by anodic bonding of a thin glass wafer to the etched silicon wafer. The bonded wafers are then heated inside a furnace at a temperature above the softening point of the glass, and due to the expansion of the trapped gas in the silicon cavities the glass is blown into three-dimensional spherical shells. An analytical model which can be used to predict the shape of the glass shells is described and demonstrated to match the experimental data. The ability to blow glass on a wafer level may enable novel capabilities including mass-production of microscopic spherical gas confinement chambers, microlenses, and complex microfluidic networks

Bulk Micromachined Titanium Microneedles

Abstract: Microneedle-based drug delivery has shown considerable promise for enabling painless transdermal and hypodermal delivery of conventional and novel therapies. However, this promise has yet to be fully realized due in large part to the limitations imposed by the micromechanical properties of the material systems being used. In this paper, we demonstrate titanium-based microneedle devices developed to address these limitations. Microneedle arrays with in-plane orientation are fabricated using recently developed high-aspect-ratio titanium bulk micromachining and multilayer lamination techniques. These devices include embedded microfluidic networks for the active delivery and/or extraction of fluids. Data from quantitative and qualitative characterization of the fluidic and mechanical performance of the devices are presented and shown to be in good agreement with finite-element simulations. The results demonstrate the potential of titanium micromachining for the fabrication of robust, reliable, and low-cost microneedle devices for drug delivery

A Vibration Resistant Nanopositioner for Mobile Parallel-Probe Storage Applications

Abstract: We describe a planar microelectromechanical systems (MEMS)-based x/y nanopositioner designed for parallel-probe storage applications. The nanopositioner is actuated electromagnetically and has x/y motion capabilities of plusmn60 mum. The mechanical components are fabricated from a single-crystal silicon wafer using a deep-trench-etching process. To render the system robust against vibration, we utilize a mass-balancing concept that makes the system stiff against linear shock, but still compliant for actuation, and therefore results in low power consumption. We present details of the finite-element model used to design the device as well as experimental results for the frequency response, actuation, and vibration-rejection properties of the nanopositioner

Strength Distributions in Polycrystalline Silicon MEMS

Abstract: Safe and reliable design of MEMS components requires a statistical description of the material properties that are associated with failure. To this end, a series of microscale tensile tests was performed on polysilicon MEMS structures fabricated using Sandia National Laboratories' SUMMiT Vtrade process. Tensile bars were fabricated from each of the four freestanding polysilicon layers, with gage lengths ranging from 30 to 3750 mum. A two-parameter Weibull distribution appeared to adequately characterize the observed tensile strength distributions. The strength distribution was found to be dependent on the length of the tensile structures, as expected by the Weibull size effect, and unexpectedly strongly dependent on the layer from which the tensile bar was constructed. Specifically, the topmost polysilicon layer in the deposition process (poly4) was more than twice the strength of the bottom freestanding polysilicon layer (poly1). The mechanistic source of this layer-dependent strength appears to originate, at least in part, from process-dependent surface roughness, although other factors such as layer-dependent variations in microstructure, residual stress, and doping are also considered. A fracture mechanics analysis of the strength distributions suggests that the size of the critical flaws is in the vicinity from 50 to 150 nm. Fractography revealed crack origins along the sidewalls, corners, and top surfaces. Weibull strength distributions were also established at elevated temperatures: 200, 400, 600, and 800 degC in air and nitrogen environments. These results revealed the onset of ductility and reduction in strength at elevated temperatures: at 600 degC strength was less than 40% of the room temperature value. Most of the strength was regained if the material was tested at room temperature after a high-temperature exposure. In the discussion, we briefly review concepts for incorporating these observed strength distributions into probabilistic safe design of MEMS components

Chaos for a Microelectromechanical Oscillator Governed by the Nonlinear Mathieu Equation

Abstract: A variety of microelectromechanical (MEM) oscillators is governed by a version of the Mathieu equation that harbors both linear and cubic nonlinear time-varying stiffness terms. In this paper, chaotic behavior is predicted and shown to occur in this class of MEM device. Specifically, by using Melnikov's method, an inequality that describes the region of parameter space where chaos lives is derived. Numerical simulations are performed to show that chaos indeed occurs in this region of parameter space and to study the system's behavior for a variety of parameters. A MEM oscillator utilizing non interdigitated comb drives for actuation and stiffness tuning was designed and fabricated, which satisfies the inequality. Experimental results for this device that are consistent with results from numerical simulations are presented and convincingly show chaotic behavior.

Side-Wall Vertical Electrodes for Lateral Field Microfluidic Applications

Abstract: This paper describes the design, fabrication, and testing of microfluidic devices enabled by electrodes embedded vertically in the side walls of SU-8 microchannels. With vertical electrodes on the side walls, one can generate higher lateral electrical fields uniform along the vertical direction in the channel (perpendicular to the substrate). By designing the electrode shapes and configurations, uniform and nonuniform electrical fields in the lateral (planar) directions can be applied to manipulate flow or particles in microchannels for switching or sorting applications. The uniform field is demonstrated in a magnetohydrodynamic (MHD) microfluidic device for directing cells to different channel outlets while the nonuniform field is demonstrated in the generation of dielectrophoresis (DEP) forces for microbead focusing. Metal electrodes are fabricated by electroplating to form vertical electrodes aligned with the channel walls. The multilayer SU-8 lithography technique enables the four walls of the channel to be all SU-8. The thin precoated SU-8 layer on the substrate improves structure integrity of the SU-8 microchannels. The mechanical flexibility of PDMS compensates for the surface nonuniformity from the previous patterning steps to conformably cap the channel. The ability to integrate versatile electrodes design broadens the realm of electrical control and sensing for microfluidic applications

A Microassembled Low-Profile Three-Dimensional Microelectrode Array for Neural Prosthesis Applications

Abstract: This paper describes the design and micro- assembly process of a low-profile 3-D microelectrode array for mapping the functional organization of targeted areas of the central nervous system and for possible application in neural prostheses. The array consists of multiple planar complimentary metal-oxide-semiconductor stimulating probes and 3-D assembly components. Parylene-encapsulated gold beams supported by etch-stopped silicon braces allow the backends of the probes to be folded over to reduce the height of the array above the cortical surface. A process permitting parylene to be used at wafer level with bulk-silicon wet release has been reported. Spacers are used to fix the microassembled probes in position and are equipped with interlocking structures to facilitate the assembly process and increase yield. Four-probe 256-site 3-D arrays operate from plusmn5 V with an average per-channel power dissipation of 97 muW at full range stimulation with pulse widths of 100 mus at 500-Hz frequency. Thirty-two sites can be stimulated simultaneously with maximum currents of plusmn127 muA and a current resolution of plusmn1 muA. The microassembly techniques allow a variety of 3-D microstructures to be created from planar components.

Vapor-Phase Self-Assembled Monolayers for Anti-Stiction Applications in MEMS

Abstract: We have investigated the anti-stiction performance of self-assembled monolayers (SAMs) that were grown in vapor phase from six different organosilane precursors: CF3(CF2)5(CH2)2SiCl3 (FOTS), CF3(CF2)5(CH2)2Si(OC2H5)3 (FOTES), CF3(CF2)5(CH2)2Si(CH3)Cl2 (FOMDS), CF3(CF2)5(CH2)2Si(CH3)2Cl (FOMMS), CF3(CF2)7(CH2)2SiCl3 (FDTS), and CH3(CH2)17(CH2)2SiCl3 (OTS). The SAM coatings that were grown on silicon substrates were characterized with respect to static contact angle, surface energy, roughness, nanoscale adhesive force, nanoscale friction force, and thermal stability. The best overall anti-stiction performance was achieved using FDTS as precursor for the SAM growth, but all coatings show good potential for solving in-use stiction problems in microelectromechanical systems devices.

Fabrication of High-Aspect-Ratio Electrode Arrays for Three-Dimensional Microbatteries

Abstract: Silicon-micromachining techniques have been combined with conventional material-synthesis methods to develop microelectrodes for 3-D microbatteries. The resulting electrodes feature an organized array of high-aspect-ratio microscale posts fabricated on the current collector to increase their surface area and volume for a given footprint area of the device. The diameter of the posts ranges from a few micrometers to a few hundred micrometers, with aspect ratios as high as 50. The fabrication approach is based on micromolding of the electrode materials and subsequent etching of the mold to release the electrode structures. Deep reactive-ion-etching or photo-assisted anodic etching has been used to form an array of deep holes in the silicon mold. Electroplating or colloidal-processing method has been used to fill the mold with battery-electrode materials. Measurements on electrochemical half-cells indicated that the 3-D electrode arrays, which are composed of vanadium oxide nanorolls or carbon, exhibited much greater energy densities (per-footprint area) than that of the traditional 2-D electrode geometries. The use of electroplating enabled us to fabricate 3-D interdigitated arrays of nickel and zinc; and battery operation was demonstrated. [2006-0293].

A Micromachined Dual-Backplate Capacitive Microphone for Aeroacoustic Measurements

Abstract: This paper presents the development of a micro-machined dual-backplate capacitive microphone for aeroacoustic measurements. The device theory, fabrication, and characterization are discussed. The microphone is fabricated using the five-layer planarized-polysilicon SUMMiT V process at Sandia National Laboratories. The microphone consists of a 0.46-mm-diameter 2.25-mum-thick circular diaphragm and two circular backplates. The diaphragm is separated from each backplate by a 2-mum air gap. Experimental characterization of the microphone shows a sensitivity of 390 muV/Pa. The dynamic range of the microphone interfaced with a charge amplifier extends from the noise floor of 41 dB/ radicHz up to 164 dB and the resonant frequency is 178 kHz.

Resonant Pull-In Condition in Parallel-Plate Electrostatic Actuators

Abstract: Electrostatic parallel-plate actuators are a common way of actuating microelectromechanical systems, both statically and dynamically. In the static case, the stable actuation voltages are limited by the static pull-in condition, which indicates that the travel range is approximately limited to 1/3 of the initial actuation gap. Under dynamic actuation conditions, however, the stable voltages are reduced, whereas the travel range can be much extended. This is the case with the dynamic pull-in and the resonant pull-in conditions (RPCs). Using energy analysis, this paper extends the study of pull-in instability to the resonant case and derives the analytical RPC. This condition predicts snapping or pull-in of the structure for a given domain of dc and ac actuation voltages versus quality factor, taking into account the nonlinearities due to large amplitudes of oscillation. Experimental results are presented to validate the analytically derived RPC.