Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2009"

$\hbox{Bi}_{2}\hbox{Te}_{3}$ -Based Flexible Micro Thermoelectric Generator With Optimized Design

Abstract: We demonstrate and discuss the performance of fully integrated and flexible micro thermoelectric generators (muTEGs). The devices are fabricated with a low-cost microfabrication process based on electrochemical deposition of a thermoelectric material into a polymer mold. Overall system optimization is demonstrated by means of NiCu and p- and n-type Bi2Te3-based muTEGs . Influences of design, material, fabrication, and performance parameters on device performance are explained by means of measurements and model calculations. The fabricated devices generate up to 2.6*10-3muWldrcm-2ldrK-2 for devices with NiCu thermocouples and up to 0.29 muWldrcm-2ldrK-2 for Bi2Te3-based generators in planar state. Mechanical testing on NiCu muTEGs demonstrated functionality of the generator when bent to curvatures down to 7.5 mm. This allows for enhanced thermal contact to nonplanar surfaces.

Temperature-Insensitive Composite Micromechanical Resonators

Abstract: Utilizing silicon and silicon dioxide's opposing temperature coefficients of Young's modulus, composite resonators with zero linear temperature coefficient of frequency are fabricated and characterized. The resulting resonators have a quadratic temperature coefficient of frequency of approximately -20 ppb/degC2 and a tunable turnover temperature in the -55degC to 125degC range. Reduction of the temperature dependence of frequency is shown in flexural-mode resonators (700 kHz-1.3 MHz) and extensional-mode ring resonators (20 MHz). The linear temperature coefficient of Young's modulus of silicon dioxide is extracted from measurements to be +179 ppm/degC. The composite resonators are fabricated and packaged in a CMOS-compatible wafer-scale hermetic encapsulation process. The long-term stability of the resonators is monitored for longer than six months. Although most devices exhibit less than 2 ppm frequency drift, there is evidence of dielectric charging in the silicon dioxide.

Permanent Magnets for MEMS

Abstract: This paper reviews the state of the art for the microfabrication of permanent magnets applicable to microelectromechanical systems (MEMS). Permanent magnets are a key building block for the realization of magnetically based MEMS sensors, actuators, and energy converters. In this paper, the basic theories and operational concepts for permanent magnets are first described. Then, different classes of permanent-magnet materials and associated performance tradeoffs are introduced. Challenges relating to the integration of permanent magnets into MEMS applications are then discussed. Last, a summary and review of previously reported fabrication strategies and material properties is provided.

A Six-Axis MEMS Force–Torque Sensor With Micro-Newton and Nano-Newtonmeter Resolution

Abstract: This paper describes the design of a six-axis microelectromechanical systems (MEMS) force-torque sensor. A movable body is suspended by flexures that allow deflections and rotations along the x-, y-, and z-axes. The orientation of this movable body is sensed by seven capacitors. Transverse sensing is used for all capacitors, resulting in a high sensitivity. A batch fabrication process is described as capable of fabricating these multiaxis sensors with a high yield. The force sensor is experimentally investigated, and a multiaxis calibration method is described. Measurements show that the resolution is on the order of a micro-Newton and nano-Newtonmeter. This is the first six-axis MEMS force sensor that has been successfully developed.

A Fast Liquid-Metal Droplet Microswitch Using EWOD-Driven Contact-Line Sliding

Abstract: Liquid-metal (LM) droplet-based MEMS switches have mostly been restricted to slow applications until now due to the following reasons: (1) a relatively large switching gap (distance) needed to accommodate imprecise volumes and locations of droplets on the device and (2) lack of high-speed actuation to move the droplets quickly across the switching gap. To combat these problems, we explore switching by sliding the solid-LM-gas triple contact line rather than the entire droplet. This new approach allows us to use a microframe, which not only consistently positions the LM droplet but also makes the switching gap less sensitive to the errors in the deposited-droplet volume, allowing us to design microswitches with very small switching gaps (e.g., 10 mum for 600 mum-diameter droplets). Furthermore, a study of electrowetting-on-dielectric identifies a regime of fast contact-line sliding at the onset of droplet spreading. By moving the contact line fast across a small switching distance, we demonstrate a low-latency LM switch with 60 mus switch-on latency ( ~ 20 times better than other LM-switch technologies) and better than 5 mus signal rise/fall time, while boasting no contact bounce, as expected from an LM switch. High power-handling capability and long-term reliability are also discussed.

An Electrothermal Tip–Tilt–Piston Micromirror Based on Folded Dual S-Shaped Bimorphs

Abstract: This paper presents the design, optimization, fabrication, and test results of an electrothermally actuated tip-tilt-piston micromirror with a large optical aperture of 1 mm. The fabrication of the device is a combination of thin-film surface micromachining and bulk silicon micromachining based on silicon-on-insulator wafers. The device has 3-DOF of actuations, including rotations around two axes in the mirror plane, and out-of-plane piston actuation. The micromirror shows an optical scan range of plusmn30deg about both x- and y-axes and displaces 480 mum in the z-axis, all at dc voltages that are less than 8 V. Dynamic testing of the micromirror shows that the thermal response time of each actuator is about 10 ms. Resonant frequencies of the piston and rotation motion are 336 and 488 Hz, respectively. The unique structural design of the device ensures that there is no lateral shift for the piston motion and no rotation-axis shift for the rotation scanning. With the large tip-tilt-piston scan ranges and low driving voltage, this type of device is very suitable for biomedical imaging and laser beam steering applications.

MEMS Vibration Energy Harvesting Devices With Passive Resonance Frequency Adaptation Capability

Abstract: Further advancement of ambient mechanical vibration energy harvesting depends on finding a simple yet efficient method of tuning the resonance frequency of the harvester to match the one dominant in the environment. We propose an innovative approach to achieve a completely passive, wideband adaptive system by employing mechanical nonlinear strain stiffening. We present analytical analysis of the underlying idea as well as experimental results obtained with custom fabricated MEMS devices. Nonlinear behavior is obtained through high built-in stresses between layers in these devices. We report experimentally verified frequency adaptability of over 36% for a clamped-clamped beam device at 2 g input acceleration. We believe that the proposed solution is perfectly suited for autonomous industrial machinery surveillance systems, where high amplitude vibrations that are necessary for enabling this solution, are abundant.

Widely Tunable MEMS-Based Fabry–Perot Filter

Abstract: This paper describes the use of strain stiffening in fixed-fixed beam actuators to extend the tuning range of microelectromechanical-systems-based Fabry-Perot filters. The measured wavelength tuning range of 1.615-2.425 mum is the largest reported for such a filter. Curvature in the movable mirror was corrected using a low-power oxygen plasma to controllably alter the stress gradient in the mirror. After curvature correction, the linewidth of a filter was 52 nm, close to the theoretical minimum for our mirror design. As a proof of concept, a filter was bonded to a broadband infrared detector, realizing a wavelength-tunable infrared detector. All measured data have been compared to theoretical models of the optics and mechanics of the filters, with excellent agreement between theory and measurement demonstrated in all cases. Finally, the Young's modulus and stress of the actuator materials were extracted directly from the measured voltage-displacement curves, demonstrating a novel technique for material property measurement.

Active Release of Microobjects Using a MEMS Microgripper to Overcome Adhesion Forces

Abstract: Due to force scaling laws, large adhesion forces at the microscale make rapid accurate release of microobjects a long-standing challenge in pick-place micromanipulation. This paper presents a new microelectromechanical systems (MEMS) microgripper integrated with a plunging mechanism to impact the microobject for it to gain sufficient momentum to overcome adhesion forces. The performance was experimentally quantified through the manipulation of 7.5-10.9-mum borosilicate glass spheres in an ambient environment under an optical microscope. Experimental results demonstrate that this microgripper, for the first time, achieves a 100% successful release rate (based on 200 trials) and a release accuracy of 0.70 plusmn0.46 mum. Experiments with conductive and nonconductive substrates also confirmed that the release process is not substrate dependent. Theoretical analyses were conducted to understand the release principle. Based on this paper, further scaling down the end structure of this microgripper will possibly provide an effective solution to the manipulation of submicrometer-sized objects.

Enhancing Parametric Sensitivity in Electrically Coupled MEMS Resonators

Abstract: In an array of identical resonators coupled through weak springs, a small perturbation in the structural properties of one of the resonators strongly impacts coupled oscillations causing the vibration modes to localize. Theoretical studies show that measuring the variation in eigenstates due to such vibration-mode localization can yield orders of magnitude enhancement in signal sensitivity over the technique of simply measuring induced resonant-frequency shifts. In this paper, we propose the application of mode localization for detecting small perturbations in stiffness in pairs of nearly identical weakly coupled microelectromechanical-system resonators and also examine the effect of initial mechanical asymmetry caused by fabrication tolerances in such sensors. For the first time, the variation in eigenstates is studied by coupling the resonators using electrostatic means that allow for significantly weaker coupling-spring constants and the possibility for stronger localization of vibration modes. Eigenstate variations that are nearly three orders of magnitude greater than the corresponding shifts in the resonant frequency for an induced perturbation in stiffness are experimentally demonstrated. Such high electrically tunable parametric sensitivities, together with the added advantage of intrinsic common-mode rejection, pave the way to a new paradigm of mechanical sensing.

PVDF-Based Microfabricated Tactile Sensor for Minimally Invasive Surgery

Abstract: This paper aimed to develop a miniaturized tactile sensor capable of measuring force and force position in minimally invasive surgery. The in situ measurement of tactile information is a step forward toward restoring the loss of the sense of touch that has occurred due to shift from traditional to minimally invasive surgeries. The sensor was designed such that it can sense low forces which could be comparable to those produced by pulsating delicate arteries, yet can withstand high forces comparable to grasping forces. The influence of some hidden anatomical features, such as lumps, voids, and arteries, on the stress distribution at the grasping surface was studied. In this paper, the capability of the sensor to determine and locate any point load was also investigated. The proposed sensor was designed and manufactured to be highly sensitive, using polyvinylidene fluoride (PVDF). The microfabrication procedure of the sensor, including corner compensation for toothlike projections and patterning of PVDF film, was discussed. The micromachined sensor was tested, and the experimental results were compared with the results of 3-D finite element modeling.

New On-Chip Nanomechanical Testing Laboratory - Applications to Aluminum and Polysilicon Thin Films

Abstract: The measurement of the mechanical properties of materials with submicrometer dimensions is extremely challenging, from the preparation and manipulation of specimens to the application of small loads and extraction of accurate stresses and strains. A new on-chip nanomechanical testing concept has been developed in order to measure the mechanical properties of submicrometer freestanding thin films allowing various loading configurations and specimen geometries. The basic idea is to use internal stress present in one film to provide the actuation for deforming another film attached to the first film on one side and to the substrate on the other side. The measurement of the displacement resulting from the release of both films gives access to the stress and the strain applied to the test specimen provided the Young's modulus and mismatch strain of the actuator film are known. Classical microelectromechanical-systems-based microfabrication procedures are used to pattern the test structures and release the films from the substrate. The design procedures, data reduction scheme, and a generic fabrication strategy are described in details and implemented in order to build a suite of test structures with various combinations of dimensions. These structures allow the characterization of different materials and mechanical properties and enable high throughputs of data while avoiding any electrical signal or external actuation. Results obtained on ductile aluminum and brittle polysilicon films demonstrate the potential of the method to determine the Young's modulus, yield stress or fracture stress, fracture strain, and strain hardening in ductile materials.

Large-Stroke Dielectric Elastomer Actuators With Ion-Implanted Electrodes

Abstract: In this paper, we present miniaturized polydimethylsiloxane (PDMS)-based diaphragm dielectric elastomer actuators capable of out-of-plane displacement up to 25% of their diameter. This very large percentage displacement is made possible by the use of compliant electrodes fabricated by low-energy gold ion implantation. This technique forms nanometer-scale metallic clusters up to 50 nm below the PDMS surface, creating an electrode that can sustain up to 175% strain while remaining conductive yet having only a minimal impact on the elastomer's mechanical properties. We present a vastly improved chip-scale process flow for fabricating suspended-membrane actuators with low-resistance contacts to implanted electrodes on both sides of the membrane. This process leads to a factor of two increase in breakdown voltage and to RC time constant shorter than mechanical time constants. For circular diaphragm actuator of 1.5-3-mm diameter, voltage-controlled static out-of-plane deflections of up to 25% of their diameter is observed, which is a factor of four higher than our previous published results. Dynamic characterization shows a mechanically limited behavior, with a resonance frequency near 1 kHz and a quality factor of 7.5 in air. Lifetime tests have shown no degradation after more than 4 million cycles at 1.5 kV. Conductive stretchable electrodes photolithographically defined on PDMS were demonstrated as a key step to further miniaturization, enabling large arrays of independent diaphragm actuators on a chip, for instance for tunable microlens arrays or arrays of micropumps and microvalves.

A Biaxial Stretchable Interconnect With Liquid-Alloy-Covered Joints on Elastomeric Substrate

Abstract: This paper reports a biaxial stretchable interconnect on an elastomeric substrate. To increase the stretchability of interconnects, a 2-D diamond-shaped geometry of gold on a polydimethylsiloxane substrate was adopted in which the potentially breakable points were covered with room temperature liquid alloy. Finite element model simulations were performed to identify the most vulnerable points subjected to stress concentration and optimize the design process. Simulations also indicated an optimum gold thickness and linewidth that result in a minimum stress when the substrate is stretched. Four different geometries were designed, fabricated, and characterized. These included: 1) 2-D diamond-shaped gold lines connected at circular junctions with an intersection angle of 90deg; 2) 2-D diamond-shaped gold lines connected at circular junctions with intersection angles of 120deg and 60deg; 3) 2-D diamond-shaped gold lines separated at circular junctions with an intersection angle of 90deg; and 4) 2-D diamond-shaped gold lines separated at circular junctions with intersection angles of 120 deg and 60deg. A maximum stretchability (DeltaL/L) of ~ 60% was achieved for the design in which the lines and circles were separated and had intersection angles of 120deg and 60deg. A resistance variation of (DeltaR/R) ~ 30% was measured for this configuration.

Waveform Design Methods for Piezo Inkjet Dispensers Based on Measured Meniscus Motion

Abstract: Waveform design methods for piezo inkjet dispensers based on measured meniscus motion are presented. The meniscus motion is measured from charge-coupled-device camera images wherein strobe lights from light-emitting diodes are synchronized with the jetting signal. Waveforms for the piezo dispenser are designed such that the number of experiments can be significantly reduced compared to conventional methods. Furthermore, the designed waveform can also be evaluated by the measured meniscus motion since the motion is directly related to jetting behavior.

A Microresonator Design Based on Nonlinear 1 : 2 Internal Resonance in Flexural Structural Modes

Abstract: A unique T-beam microresonator designed to operate on the principle of nonlinear modal interactions due to 1 : 2 internal resonance is introduced. Specifically, the T-structure is designed to have two flexural modes with natural frequencies in a 1 : 2 ratio, and the higher frequency mode autoparametrically excites the lower frequency mode through inertial quadratic nonlinearities. A Lagrangian formulation is used to model the electrostatically actuated T-beam resonator, and it includes inertial quadratic nonlinearities, cubic nonlinearities due to midplane stretching and curvature of the beam, electrostatic potential, and effects of thermal prestress. A nonlinear two-mode reduced-order model is derived using linear structural modes in desired internal resonance. The model is used to estimate static pull-in bias voltages and dynamic responses using asymptotic averaging. Nonlinear frequency responses are developed for the case of resonant actuation of a higher frequency mode. It is shown that the lower frequency flexural mode is excited for actuation levels above a certain threshold and generates response component at half the frequency of resonant actuation. The effects of damping, thermal prestress, and mass and geometric perturbations from nominal design are thoroughly discussed. Finally, experimental results for a macroscale T-beam structure are briefly described and qualitatively confirm the basic analytical predictions. The T-beam resonator shows a high sensitivity to mass perturbations and, thus, holds great potential as a radio frequency filter-mixer and mass sensor.

Post-CMOS-Compatible Aluminum Nitride Resonant MEMS Accelerometers

Abstract: This paper describes the development of aluminum nitride (AlN) resonant accelerometers that can be integrated directly over foundry CMOS circuitry. Acceleration is measured by a change in resonant frequency of AlN double-ended tuning-fork (DETF) resonators. The DETF resonators and an attached proof mass are composed of a 1-mum-thick piezoelectric AlN layer. Utilizing piezoelectric coupling for the resonator drive and sense, DETFs at 890 kHz have been realized with quality factors (Q) of 5090 and a maximum power handling of 1 muW. The linear drive of the piezoelectric coupling reduces upconversion of 1/f amplifier noise into 1/f 3 phase noise close to the oscillator carrier. This results in lower oscillator phase noise, -96 dBc/Hz at 100-Hz offset from the carrier, and improved sensor resolution when the DETF resonators are oscillated by the readout electronics. Attached to a 110-ng proof mass, the accelerometer microsystem has a measured sensitivity of 3.4 Hz/G and a resolution of 0.9 mG/radicHz from 10 to 200 Hz, where the accelerometer bandwidth is limited by the measurement setup. Theoretical calculations predict an upper limit on the accelerometer bandwidth of 1.4 kHz.

A Design Procedure for Wideband Micropower Generators

Abstract: We developed a design procedure for wideband electromagnetic micropower generators (WMPGs) based on piecewise-linear oscillators. We find that the dominant factors in the performance of this class of WMPGs are the stiffness ratio of the oscillator and the velocity of the moving structure at the point of impact with the stopper. We also find that designing these WMPGs requires additional steps beyond those required in the design of regular MPGs. The additional steps match the output power and bandwidth of the WMPG to the probability density function of environmental vibrations. While these steps add complexity to the design of WMPGs, they are shown to significantly increase harvested energy.

Design, Modeling, and Fabrication of MEMS-Based Multicapillary Gas Chromatographic Columns

Abstract: This paper describes different approaches to achieve high-performance microfabricated silicon-glass separation columns for microgas chromatography systems. The capillary width effect on the separation performance has been studied by characterization of 250-, 125-, 50-, and 25-?m -wide single-capillary columns (SCCs) fabricated on a 10 × 8 mm2 die. The highest plate number (12 500/m), reported to date for MEMS-based silicon-glass columns, has been achieved by 25-?m-wide columns coated by a thin layer of polydimethylsiloxane stationary phase using static coating technique. To address the low sample capacity of these narrow columns, this paper presents the first generation of MEMS-based ?multicapillary? columns (MCCs) consisting of a bundle of narrow-width rectangular capillaries working in parallel. The theoretical model for the height-equivalent-to-a-theoretical-plate (HETP) of rectangular MCCs has been developed, which relates the HETP to the discrepancies of the widths and depths of the capillaries in the bundle. Two-, four-, and eight-capillary MCCs have been designed and fabricated to justify the separation ability of these columns. These MCCs capable of multicomponent gas separation provide a sample capacity as large as 200 ng compared to 5.5 ng for 25-?m-wide SCCs.

Whole-Cell Impedance Analysis for Highly and Poorly Metastatic Cancer Cells

Abstract: A micro electrical impedance spectroscopy (muEIS) system has been developed and implemented to analyze highly and poorly metastatic head and neck cancer (HNC) cell lines with single-cell resolution. The microsystem has arrays of 16 impedance analysis sites, each of which is capable of capturing a single cell and analyzing its whole-cell electrical impedance spectrum. This muEIS system was used to obtain the electrical impedance spectra of the poorly metastatic HNC cell line 686 LN and the highly metastatic HNC cell line 686 LN-M4e over a frequency range of 40 Hz - 10 MHz. The 686 LN cells had higher impedance phase compared to that of 686 LN-M4e cells at frequencies between 50 kHz and 2 MHz. This result demonstrates that the metastatic state of HNC cells can be distinguished using the developed muEIS system. This system is expected to serve as a powerful tool for future detection and quantification of cancer cells from various tumor stages.

Viscoelastic Characterization and Modeling of Polymer Transducers for Biological Applications

Abstract: Polydimethylsiloxane (PDMS) is an important polymeric material widely used in bio-MEMS devices such as micropillar arrays for cellular mechanical force measurements. The accuracy of such a measurement relies on choosing an appropriate material constitutive model for converting the measured structural deformations into corresponding reaction forces. However, although PDMS is a well-known viscoelastic material, many researchers in the past have treated it as a linear elastic material, which could result in errors of cellular traction force interpretation. In this paper, the mechanical properties of PDMS were characterized by using uniaxial compression, dynamic mechanical analysis, and nanoindentation tests, as well as finite element analysis (FEA). A generalized Maxwell model with the use of two exponential terms was used to emulate the mechanical behavior of PDMS at room temperature. After we found the viscoelastic constitutive law of PDMS, we used it to develop a more accurate model for converting deflection data to cellular traction forces. Moreover, in situ cellular traction force evolutions of cardiac myocytes were demonstrated by using this new conversion model. The results presented by this paper are believed to be useful for biologists who are interpreting similar physiological processes.

Energetic Nanoporous Silicon Devices

Abstract: Nanoporous energetic silicon is a promising new material for on-chip integration of energetic materials. We demonstrate several advances in the integration of nanoporous energetic silicon, including monolithic integration of a hotwire initiator on nanoporous energetic silicon, with 2.8-V ignition. We also demonstrate lithographically patterned arrays of energetic devices that are independently addressable through integrated initiators with no sympathetic ignition. Monolithic integration of the energetic material with a surface micromachined microelectromechanical systems sensor is also shown. The performance and failure mechanisms of the hotwire initiator are examined, and preliminary measurements of the thrust and propagation velocity are reported.

Comparison of Au and Au–Ni Alloys as Contact Materials for MEMS Switches

Abstract: This paper reports on a comparison of gold and gold-nickel alloys as contact materials for microelectromechanical systems (MEMS) switches. Pure gold is commonly used as the contact material in low-force metal-contact MEMS switches. The top two failure mechanisms of these switches are wear and stiction, which may be related to the material softness and the relatively high surface adhesion, respectively. Alloying gold with another metal introduces new processing options to strengthen the material against wear and reduce surface adhesion. In this paper, the properties of Au-Ni alloys were investigated as the lower contact electrode was controlled by adjusting the nickel content and thermal processing conditions. A unique and efficient switching degradation test was conducted on the alloy samples, using pure gold upper microcontacts. Solid-solution Au-Ni samples showed reduced wear rate but increased contact resistance, while two-phase Au-Ni (20 at.% Ni) showed a substantial improvement of switching reliability with only a small increase of contact resistance. Discussion of the effects of phase separation, surface topography, hardness, and electrical resistivity on contact resistance and switch degradation is also included.

A Microfabricated Planar Electrospray Array Ionic Liquid Ion Source With Integrated Extractor

Abstract: This paper reports the design, fabrication, and experimental characterization of a fully microfabricated planar array of externally fed electrospray emitters that produces heavy molecular ions from the ionic liquids EMI-BF4 and EMI-Im. The microelectromechanical systems (MEMS) electrospray array is composed of the following two microfabricated parts: 1) an emitter die with as many as 502 emitters in 1.13 cm2 and 2) an extractor component that provides assembly alignment, electrical insulation, and a common bias voltage to the emitter array. The devices were created using Pyrex and silicon substrates, as well as microfabrication techniques such as deep reactive ion etching, low-temperature fusion bonding, and anodic bonding. The emitters are coated with black silicon, which acts as a wicking material for transporting the liquid to the emitter tips. The extractor electrode uses a 3-D MEMS packaging technology that allows hand assembly of the two components with micrometer-level precision. Experimental characterization of the MEMS electrospray array includes current-voltage characteristics, time-of-flight mass spectrometry, beam divergence, and imprints on a collector. The data show that with both ionic liquids and in both polarities, the electrospray array works in the pure ionic regime, emitting ions with as little as 500 V of bias voltage. The data suggest that the MEMS electrospray array ion source could be used in applications such as coating, printing, etching, and nanosatellite propulsion.

Flexible Tactile Sensor for Tissue Elasticity Measurements

Abstract: This paper presents a novel tactile sensing technique for tissue elasticity measurements. A prototype flexible tactile sensor has been successfully fabricated using polydimethylsiloxane as the structural material. The proposed sensor comprises an array of capacitors with no active elements used. By varying the sizes of sensing membranes within the capacitors, different stiffnesses of sensing diaphragms can be achieved. The elasticity of the targeted object can be thereafter measured based on the relative deflections of the sensing diaphragms. The fabricated sensor has been calibrated by an off-the-shelf polymer durometer hardness selector pack. The results show a sensing resolution of 0.1 MPa for elasticity measurement and a force sensing resolution as small as 5 mN. This flexible tactile sensor can be embedded on the distal portions of various endoscopic instruments for in vivo tissue elasticity measurements.

Double-Exposure Grayscale Photolithography

Abstract: A double-exposure grayscale photolithography technique is developed and demonstrated to produce three-dimensional (3-D) structures with a high vertical resolution. Pixelated grayscale masks often suffer from limited vertical resolution due to restrictions on the mask fabrication. The double-exposure technique uses two pixelated grayscale mask exposures before development and dramatically increases the vertical resolution without altering the mask fabrication process. An empirical calibration technique was employed for mask design and was also applied to study the effects of exposure time and mask misalignment on the photoresist profile. This technology has been demonstrated to improve the average step between photoresist levels from 0.19 to 0.02 mum and the maximum step from 0.43 to 0.2 mum compared to a single pixelated exposure using the same mask design.

A Novel Diamond Microprobe for Neuro-Chemical and -Electrical Recording in Neural Prosthesis

Abstract: This paper describes the design, microfabrication, and testing of a novel polycrystalline-diamond (poly-C)-based microprobe for possible applications in neural prosthesis. The probe utilizes undoped poly-C with a resistivity on the order of 105 Omega middot cm as a supporting material, which has a Young's modulus in the range of 400-1000 GPa and is biocompatible. Boron-doped poly-C with a resistivity on the order of 10-3 Omega middot cm is used as an electrode material, which provides a chemically stable surface for both chemical and electrical detections in neural studies. The probe has eight poly-C electrode sites with diameters ranging from 2 to 150 mum; the electrode capacitance is approximately 87 muF/cm2. The measured water potential window of the poly-C electrode spans across negative and positive electrode potentials and typically has a total value of 2.2 V in 1 M KCl. The smallest detectable concentration of norepinephrine (a neurotransmitter) was on the order of 10 nM. The poly-C probe has also been successfully implanted in the auditory cortex area of a guinea pig brain for in vivo neural studies. The recorded signal amplitude was 30-40 muV and had a duration of 1 ms.

Internal Dielectric Transduction in Bulk-Mode Resonators

Abstract: This paper investigates electrostatic transduction of a longitudinal-mode silicon acoustic resonator with internal dielectric films. Geometric optimization of internal dielectrically transduced resonators is derived analytically and shown experimentally. Analysis of internal dielectric transduction shows a maximum transduction efficiency with thin dielectric films at points of maximum strain of the desired resonant mode. With this design optimization, a silicon bar resonator is realized with a ninth harmonic resonance of 4.5 GHz and a quality factor of over 11 000, resulting in a record high f middotQ product in silicon of 5.1 times 1013. The novel dielectric transducer demonstrates improved resonator performance with increasing frequency, with optimal transduction efficiency when the acoustic wavelength is twice the dielectric thickness. Such frequency scaling behavior enables the realization of resonators up to the super-high-frequency domain.

Surfactant Adsorption on Single-Crystal Silicon Surfaces in TMAH Solution: Orientation-Dependent Adsorption Detected by In Situ Infrared Spectroscopy

Abstract: This paper focuses on two aspects, macroscopic and microscopic, of pure and surfactant-added tetramethylammonium hydroxide (TMAH) wet etching. The macroscopic aspects deal with the technological/engineering applications of pure and surfactant-added TMAH for the fabrication of microelectromechanical systems (MEMS). The microscopic view is focused on the in situ observation of the silicon surface during etching in pure and surfactant-added TMAH solutions using Fourier transform infrared (FT-IR) spectroscopy in the multiple internal reflection geometry. The latter is primarily aimed at investigating the causes behind the change in the orientation-dependent etching behavior of TMAH solution when the surfactant is added. Silicon prisms having two different orientations ({110} and {100}) were prepared for comparison of the amount of adsorbed surfactant using FT-IR. Stronger and weaker adsorptions were observed on {110} and {100}, respectively. Moreover, ellipsometric spectroscopy (ES) measurements of surfactant adsorption depending on the crystallographic orientation are also performed in order to gain further information about the differences in the silicon-surfactant interface for Si{100} and Si{110}. In this paper, we determine the differences in surfactant adsorption characteristics for Si{110} and Si{100} using FT-IR and ES measurements for the first time, focusing both on the mechanism and on the technological/engineering applications in MEMS.

MEMS Electrostatic Actuation in Conducting Biological Media

Abstract: We present design and experimental implementation of electrostatic comb-drive actuators in solutions of high conductivity relevant for biological cells. The actuators are operated in the frequency range 1-10 MHz in ionic and biological cell culture media, with ionic strengths up to 150 mmol/L. Typical displacement is 3.5 mum at an applied peak-to-peak signal of 5 V. Two different actuation schemes are presented and tested for performance at high frequency. A differential drive design is demonstrated to overcome the attenuation due to losses in parasitic impedances. The frequency dependence of the electrostatic force has been characterized in media of different ionic strengths. Circuit models for the electric double layer phenomena are used to understand and predict the actuator behavior. The actuator is integrated into a planar force sensing system to measure the stiffness of cells cultured on suspended structures.