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Microelectromechanical Systems, Journal of

Issue 2 • Date Jun 1998

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Displaying Results 1 - 17 of 17
  • Electrothermal frequency tuning of folded and coupled vibrating micromechanical resonators

    Page(s): 164 - 171
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    The use of constrained thermal expansion to tune the resonant frequency of vibrating micromechanical resonators is explored. A simple model is developed to predict the power sensitivity obtained with folded and unfolded geometries, including the effects of electrothermal heating, and conduction and convection cooling. It is shown that the sensitivity of folded structures can change sign as the ambient gas pressure is lowered in contrast to the behavior of unfolded structures. Tuning is then by tensile axial stress rather than compressive stress. Using folded laterally resonant bulk-micromachined comb-drive electrostatic actuators, tuning ranges of -25% and +50% are obtained (at atmospheric pressure and at 10 mTorr, respectively, with powers of 10 and 1.5 mW, respectively). A nested coupled resonator is then presented in which the frequency of one resonator may be tuned without effecting that of the other, thus allowing frequency matching to be obtained View full abstract»

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  • MEMS milliactuator for hard-disk-drive tracking servo

    Page(s): 149 - 155
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (236 KB)  

    This paper describes the design, fabrication, and operational characteristics of a MEMS milliactuator designed for servo tracking in a hard-disk drive (HDD). The actuator is designed to increase the bandwidth of an HDD tracking servo and pack more recording tracks on a disk. An Invar (low thermal expansion metal) electrode position process was developed to meet the thermal stability requirement. The electroplated Invar's thermal coefficient of expansion is as low as 6.3×10-6/K, which is almost half of that of pure nickel. For the plating mold pattern definition, a high-aspect-ratio polymer etching technique was developed. A high-aspect-ratio structure line-and-gap definition is required to achieve both a high directional stiffness ratio and electrode efficiency for the actuator. The etching technique described can etch through a thick (<40 μm) polymer layer with an aspect ratio of 16:1 at an etch rate of <2 μm/min. Low-cost/high-volume manufacturing is achievable by this batch fabrication technique. A milliactuator was fabricated and assembled with a suspension and a slider weighted at around 2 mg. The slider was successfully driven by the milliactuator while the slider was flying on a spinning disk. The operational characteristics (frequency response) of the in-flight milliactuator were measured, and the results indicate that the actuator is suitable for high-bandwidth HDD servo-tracking applications View full abstract»

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  • Fabrication of thick Si resonators with a frontside-release etch-diffusion process

    Page(s): 201 - 206
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (148 KB)  

    A frontside-release etch-diffusion process has been developed to create released single-crystalline Si microstructures without the need for wafer bonding. This frontside-release process is simple and requires only a single mask. A deep dry etch in an electron cyclotron resonance source is used to define the structures, followed by a short boron diffusion to convert them to p++ Si. A short etch in ethylenediamine pyrocatechol (EDP) is then used to undercut and release the structures from the frontside of the Si wafer. The structures are isolated from the substrate using a reverse-biased p++/n junction. Since the structures have a high aspect ratio, beams longer than 1 mm can be released without sticking to the substrate, and thick resonators are flat with no bending due to stresses. Resonant microstructures with thicknesses ranging from 10 to 55 μm thick have been fabricated using this process and their resonant frequency has been measured. For typical clamped-clamped beam resonators that were 24 μm thick, 5 μm wide, and 400 μm long, with 2-μm comb gaps, a resonant frequency of 90.6 kHz and a quality factor of 362 were measured in air View full abstract»

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  • Fabrication of three-dimensional microstructures by electrochemically welding structures formed by microcontact printing on planar and curved substrates

    Page(s): 261 - 266
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    This paper describes two convenient techniques for the fabrication of three-dimensional (3-D) structures with micron-sized features. The methods use microcontact printing (μCP) to define patterns with feature sizes as small as 20 μm both on planar substrates and on cylinders (diameter ~2 mm). Electrodeposition serves as a micron-scale tool for metal deposition and welding that transforms these patterned surfaces and cylinders into metallic complex 3-D microstructures (e.g., tetrahedra). Final tetrahedra (~2-cm sides) have feature sizes as small as 50 μm View full abstract»

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  • Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

    Page(s): 252 - 260
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    We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH3 (CH2)17SiCl3, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF3(CF2)7(CH2)2 SiCl3, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 μm thick, 2 μm above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 μJ/m2 for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m2 for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450°C in an N2 ambient. In air, the OTS film begins to degrade at 150°C while the fluorinated coating remains intact up to 400°C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment View full abstract»

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  • A novel integrated silicon capacitive microphone-floating electrode “electret” microphone (FEEM)

    Page(s): 224 - 234
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    A novel principle “electret” microphone, i.e., floating electrode electret microphone, is proposed and implemented in this study. Single-chip fabrication and corrugation technique are used in the design and fabrication of the microphone. The floating electrode is encapsulated by highly insulated materials to ensure that there is no electric-leakage passage between the floating electrode and the electrodes of the microphone. Net-free electronic charges (not “bonded” charges as in traditional electret) in the floating electrode can excite the electric field, which is similar to that of the traditional electret. The floating electrode can be easily charged by use of the “hot” electron technique, available using the avalanche breakdown of the p+-n junction. Therefore, the electret microphone is rechargeable, which can greatly increase the lifetime of the device. The preamplifier has been on-chip integrated in a junction-field-effect transistor (JFET) source-follower type with resistors by use of ion implantation. Electret charges are banded in a deep potential trap, thus, this microphone can operate at a high temperature (as high as 300°C) and has high stability and reliability. Experiments show that the prototype has a 3-mV/Pa sensitivity and a larger than 21-kHz frequency bandwidth in a 1 mm ×1-mm diaphragm area. Microphone performance can be further improved by optimized process and design. The fabrication is completely integrated-circuit (IC) compatible, hence, the microphone shows promise in integrated acoustic systems View full abstract»

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  • Fully integrated magnetically actuated micromachined relays

    Page(s): 181 - 191
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (380 KB)  

    A fully integrated magnetically actuated micromachined relay has been successfully fabricated and tested. This particular device uses a single-layer coil to actuate a movable upper magnetically responsive platform. The minimum current for actuation was 180 mA, resulting in an actuation power of 33 mW. Devices have been tested which can make and break 1.2 A of current through the relay contacts when the relay is electromagnetically switched. Operational lifetimes in excess of 850000 operations have been observed. Contact resistances as low as 22.4 mΩ have been observed under electromagnetic actuation. Magnetic and structural finite-element (FE) simulations have been performed using ANSYS to calculate both the actuation and contact forces View full abstract»

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  • Design and fabrication of an angular microactuator for magnetic disk drives

    Page(s): 141 - 148
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (344 KB)  

    Angular electrostatic microactuators suitable for use in a two-stage servo system for magnetic disk drives have been fabricated from molded chemical-vapor-deposited (CVD) polysilicon using the HexSil process. A 2.6-mm-diameter device has been shown to be capable of positioning the read/write elements of a 30% picoslider over a ±1-μm range, with a predicted bandwidth of 2 kHz. The structures are formed by depositing polysilicon via CVD into deep trenches etched into a silicon mold wafer. Upon release, the actuators are assembled onto a target wafer using a solder bond. The solder-bonding process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be reused for subsequent polysilicon depositions, amortizing the cost of the deep-trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Furthermore, 100-μm-high structures may be made from a 3-μm deposition of polysilicon, increasing overall fabrication speed View full abstract»

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  • Characterization of a high-sensitivity micromachined tunneling accelerometer with micro-g resolution

    Page(s): 235 - 244
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    A new high-sensitivity bulk-silicon-micromachined tunneling accelerometer with micro-g resolution has been successfully fabricated and tested at Stanford University. This accelerometer is a prototype intended for underwater acoustics applications and is required to feature micro-g resolution at frequencies between 5 Hz and 1 kHz and can be packaged with circuitry in an 8-cm3 volume with a total mass of 8 g. This paper briefly describes the mechanical design of this tunneling accelerometer and focuses on the experiments carried out in our laboratory to test the tunneling transducer as well as on the experimental determination of accelerometer resolution. The exponential relationship between tunneling gap and tunneling current is verified and results in an effective tunneling barrier height of about 0.2 eV. The goal of this paper is to outline the measurements which are necessary to verify that the sensor is actually tunneling and to confirm that the accelerometer performance is consistent with what should be expected from a tunneling accelerometer View full abstract»

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  • Parametric modeling of a microaccelerometer: comparing I- and D-optimal design of experiments for finite-element analysis

    Page(s): 274 - 282
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    Parametric modeling of a microaccelerometer is used to compare two optimization criteria in the design of experiments for finite-element analysis (FEA). I- and D-optimality criteria are used to specify a near-minimal set of 38 input points in multidimensional input space at which FEA is performed. The accelerometer is comprised of a rigid proof mass and four suspension beams. Its response is determined as a function of acceleration along the input and cross axes, temperature, and two structural variables, and the related performance compromises are identified. It is shown that both I- and D-optimality criteria provide good results, with displacement residuals spread over ranges of magnitude 1.6 and 2.3 μm, respectively, in a large input domain over which the range of displacement has a magnitude exceeding 13 μm. Additionally, the results suggest that for the particular device that was considered, the temperature coefficients of offset and sensitivity have smaller magnitudes when the suspension beams are attached near the center of each side of the proof mass View full abstract»

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  • On the calibration of multicomponent microforce sensors

    Page(s): 156 - 163
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    A calibration method for multicomponent microforce sensors using Lorentz forces is proposed. The method permits generation of six force and moment components in three orthogonal directions on two-dimensional (2-D) structures that can be fabricated through existing silicon micromachining technology. Two multicomponent microforce sensor and calibration devices were developed and tested to verify the method View full abstract»

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  • A low-temperature IC-compatible process for fabricating surface-micromachined metallic microchannels

    Page(s): 267 - 273
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    In this paper, a low-temperature integrated-circuit (IC)-compatible process for fabricating metallic microchannels is described. Arrays of 1-100 metallic microchannels have been fabricated on silicon and glass substrates. The process can be extended to many planar substrate materials including polymers and ceramics. The microchannels are formed using microelectro-formed metals. The microchannels demonstrated in this paper use nickel as the structural material and gold as the surface coating on the inside walls of the microchannels. The inner dimensions of the individual microchannels fabricated to date range from 30 μm to 1.5 mm in width, 0.5 mm to several centimeters in length, and 5-100 μm in thickness. The wall thickness ranges from 5 to 50 μm. The microchannel fabrication technology enables the fabrication of surface microchannels with a relatively large cross-sectional area. The metallic microchannels can be fabricated to extend from the substrate edge. Interfacing schemes are given for attaching external pressure feeds View full abstract»

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  • A high-sensitivity z-axis capacitive silicon microaccelerometer with a torsional suspension

    Page(s): 192 - 200
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (316 KB)  

    This paper presents a new z-axis high-sensitivity silicon-micromachined capacitive accelerometer fabricated using a three-mask dissolved-wafer process (DWP). It employs capacitive sensing using overlap-area variations between comb electrodes and a torsional suspension system to provide high sensitivity without compromising bandwidth, full-scale range, or the pull-in voltage ceiling. Excellent electrical sensitivity is obtained by using high-aspect-ratio comb fingers with narrow air gaps of 2 μm and a large overlap area of 12 μm ×300 μm. Torsional suspension beams 150 μm long with a cross-sectional area of 12 μm ×3 μm are used to improve the mechanical gain. Simulations of the capacitance between sense fingers show a highly linear region over a wide 14-μm tip deflection range. Accelerometers were fabricated and yielded sensitivities of 263-300 mV/g, a nonlinearity less than 0.2% over a range of -4 to +3 g, a full-scale range of -4 to +6 g, and pull-in voltages greater than 8 V. A 3 dB cutoff frequency of 35 Hz was measured in air. The calculated thermomechanical noise in the sensor is 0.28 mg over this bandwidth View full abstract»

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  • Multistator LIGA-fabricated electrostatic wobble motors with integrated synchronous control

    Page(s): 214 - 223
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    A novel design of an electrostatic wobble motor is presented, which incorporates multiple concentric rotor-stator surface pairs to increase the space-usage efficiency and a segmented bearing for the implementation of synchronous control. The device has been successfully fabricated using the Lithographie, Galvanoformung, Abformung (LIGA) technique. The finite-element method is used to predict the available output torque, and the potential for extracting the useful output from the device is assessed View full abstract»

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  • Independent tuning of linear and nonlinear stiffness coefficients [actuators]

    Page(s): 172 - 180
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    Using a combination of electrostatic actuators, we present a method to independently tune the linear and nonlinear stiffness coefficients of a uniaxial micromechanical device. To demonstrate the method's capability, we investigated the tuning of an oscillator with linear and cubic restoring forces. We successfully tuned the cubic stiffness from 0.31×1011 to -5.1×1011 N/m3 without affecting the resonant frequency or the linear stiffness. Numerical results are presented which characterize the actuators and indicate important design parameters. Finally, issues such as actuator design, quadratic stiffness, and stability are discussed View full abstract»

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  • High-aspect-ratio Si vertical micromirror arrays for optical switching

    Page(s): 207 - 213
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    Arrays of bulk-micromachined high-aspect-ratio vertical Si mirrors were designed, fabricated, and characterized for optical switching applications. These 50-μm-tall vertical mirrors were fabricated by the deep-etch shallow-diffusion process. The released Si mirrors have smooth sidewalls with 5-nm surface roughness. An electrostatic comb drive applied to actuate the mirrors which were supported by folded or serpentine beams. For 800-μm-long, 3-μm-wide, and 50-μm-thick folded suspension beams, a lateral mirror movement of 34 μm was achieved by a driving voltage of 30 V. Resonant frequency of 987 Hz was obtained for similar devices at atmospheric pressure. There are tradeoffs between mirror displacement and resonant frequency. The mirror displacement increases with beam length (BL) and decreases with beam width (BW). However, the resonant frequency increases with BW, but decreases with BL, which is in agreement with the theoretical calculations. These vertical Si micromirrors were coated with Au to increase the reflectivity. Magnetron sputtering of Au at low pressure was applied to achieve uniform and smooth sidewall coverage. Optical measurement on 50-μm-tall Au-coated Si mirrors showed a reflectivity of 85% (-0.71 dB). Dynamic response of the Si micromirror switches was measured, and the switching function was successfully demonstrated at a frequency up to 1.2 KHz View full abstract»

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  • Thin-film shape-memory alloy actuated micropumps

    Page(s): 245 - 251
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    Micropumps capable of precise handling of low-fluid volumes have the potential to revolutionize applications in fields such as drug delivery, fuel injection, and micrototal chemical analysis systems (μTAS). Traditional microactuators used in micropumps suffer from low strokes and, as a result, are unsuitable for achieving large fluid displacement. They also suffer low-actuation work densities, which translate to low forces. We investigate the use of the shape-memory effect (SMA) in sputter-deposited thin-film shape-memory alloy (SMA) titanium nickel (TiNi) as an actuator for microelectromechanical systems (MEMS)-based microfluidic devices, as it is capable of both high force and high strains. The resistivity of the SMA thin film is suitable for Joule heating, which allows direct electrical control of the actuator. Two micropump designs were fabricated-one with a novel complementary actuator and the other with a polyimide-biased actuator-which provided thermal isolation between the heated microactuator and the fluid being pumped. A maximum water flow rate of 50 μl/min was achieved View full abstract»

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Aims & Scope

The topics of interest include, but are not limited to: devices ranging in size from microns to millimeters, IC-compatible fabrication techniques, other fabrication techniques, measurement of micro phenomena, theoretical results, new materials and designs, micro actuators, micro robots, micro batteries, bearings, wear, reliability, electrical interconnections, micro telemanipulation, and standards appropriate to MEMS. Application examples and application oriented devices in fluidics, optics, bio-medical engineering, etc., are also of central interest.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Christofer Hierold
ETH Zürich, Micro and Nanosystems