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Fundamental issues in the design of capacitive microelectremechanical systems (MEMS) microphones, and the MEMS microphones built by Bell Labs are described. It is found that self thermal noise is a serious design challenge for MEMS microphones and the most significant way to increase the MEMS signal- to-noise-ratio, is to increase the area of membrane.

In this paper, we have fabricated two kinds of silicon microoptical mirrors to make optical axes of two high-power laser diodes close at an interval of 100 [micro]m, utilizing anisotropic etching of silicon with the self-alignment mask. One of them is a microrectangular prism mirror (MRPM). It has two orthogonal reflection mirrors, composed of polished (110) and anisotropically etched (111) planes of silicon. The other is a micro two-reflection mirror (MTRM). It has two pairs of two reflectors facing each other in parallel. The use of MTRM made the mounting process easier than that of MRPM. The use of the self-alignment mask made mirror surface smoother and it has been confirmed that full widths at half maximum (FWHM) are almost the same with and without reflections by MRPM or MTRM, respectively. It has also been shown that the etched (111) plane has the etching-condition dependence of surface roughness. [1049] Index Terms--Anisotropic etching, etching condition, micromirror, parallel beams, surface roughness.

In this paper, we have successfully developed an integrated micromechanical system for controlling tunneling current. A pair of nanoscale tunneling tips have been integrated with a silicon micromachined electrostatic actuator of high-aspect ratio. The tip sharpness has been observed to be as sharp as commercial tips by scanning over surface of carbon graphite as an atom scale. We have also succeeded to observe the tunneling current in the air and in the vacuum condition (in TEM). [0941] Index Terms--Deep-reactive ion etching (DRIE), ICP-RIE, microactuators, scanning tunneling microscopy (STM), tunneling current.

A methodology for robust design analysis of microelectromechanical systems is presented by considering the example of comb microresonators and uncertainty in parameters governing the resonant frequency and the transconductance values. Analytical models for the variability in the resonant frequency and transconductance are developed as function of the parameter uncertainty, and used as objective functions sought to be minimized in a robust design endeavor. An enumeration search over the design space is utilized to determine the optimal design of microresonators that minimize the variability subject to constraints on performance requirements. The results are presented over a wide interval of operating resonant frequency, and can be used in the robust design of microresonators. [0960] Index Terms--Comb microresonator, microelectromechanical systems (MEMS), parameter uncertainty, reliability, robust design.

This paper presents an interface circuit for low-frequency dithering measurements of resistor-based transducers. It is demonstrated in the context of a polyimide-shank scanning thermal microscopy probe which provides high thermal sensitivity and spatial resolution, but has a low bandwidth from both mechanical and thermal perspectives. These pose challenges in temperature dithering and control, as well as noise immunity. The circuit includes a proportional-integral controller and a demodulator, along with appropriate amplifier and filter blocks. It keeps the average temperature of the probe tip constant while synchronously detecting variations in the second harmonic of the modulated signal as the tip is scanned across the sample surface. Strategic choices in the circuit architecture and topology are evaluated, and the overall system including the sensor and the circuit is simulated. Measurements of the implemented system show that a signal-to-noise ratio (SNR) of 15.7 is achieved while scanning a photoresist sample of 218 nm thickness on a silicon substrate, and that the detection limit for variations in thermal conductance is < 3 pW/K. [1125] Index Terms--Polyimide probe, scanning probe, thermal imaging.

A silicon-based micromachined, floating-element sensor for low-magnitude wall shear-stress measurement has been developed. Sensors over a range of element sizes and sensitivities have been fabricated by thin-wafer bonding and deep-reactive ion-etching techniques. Detailed design, fabrication, and testing issues are described in this paper. Detection of the floating-element motion is accomplished using either direct or differential capacitance measurement. The design objective is to measure the shear-stress distribution at levels of O(0.10 Pa) with a spatial resolution of approximately O(100 [micro]m). It is assumed that the flow direction is known, permitting one to align the sensor appropriately so that a single component shear measurement is a good estimate of the prevalent shear. Using a differential capacitance detection scheme these goals have been achieved. We tested the sensor at shear levels ranging from 0 to 0.20 Pa and found that the lowest detectable shear-stress level that the sensor can measure is 0.04 Pa with an 8% uncertainty on a 200 [micro]m x 500 [micro]m floating element plate. [0918] Index Terms--Capacitive sensing, microfabrication, microsensor, shear-stress measurement.

This paper presents a semi-empirical procedure for determining the through-the-thickness variation of the eigenstrain (eigenstrain is a generic term ['or any inelastic strain, including plastic strain, free thermal expansion, phase transformation, etc.) that develops during the growth of thin polysilicon films formed using low-pressure chemical vapor deposition (LPCVD). This variation is assumed to depend on the polysilicon microstructure and deposition conditions, but not on the characteristics of the (single crystal silicon) substrate. The procedure involves the use of an elastic laminated plate model to determine the eigenstrain distribution that predicts the experimentally measured substrate curvatures. In comparison to the 'shaving method' presented by Ni et al., which relies on incremental etching of a single specimen, an alternative experimental procedure is followed to measure the substrate curvatures of a series of different thickness films. While being significantly more time-consuming, the alternative procedure was expected to lead to improved predictions of the eigenstrain distribution, as it avoids the nonuniform film thicknesses produced by the etching procedure. However, a comparison of the curvature histories measured using the two approaches demonstrates that, as long as sufficiently small increments are used in the shaving method, then the improvement is insignificant. This suggests that the plasma etching dues not alter the polysilicon's intrinsic growth strain, and that the etch rate nonuniformities across the substrate are small. The eigenstrain distributions could be used, in conjunction with structural mechanics models, to design multilayered polysilicon devices with prescribed curvatures. [1175] Index Terms--Growth strain, microelectromechanical systems (MEMS), polysilicon, residual stress, stress gradient.

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels apprnaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic .journal and thrust bearings. This paper presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbumachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling. [1161] Index Terms--Bearings, lubrication, microengines, microturbine, power MEMS, turbomachinery.

A submicron thick and detect-free palladium-silver (Pd-Ag) alloy membrane is fabricated on a supporting microsieve by using microfabrication techniques. The microfabrication process also creates a robust wafer-scale membrane module, which can easily be inserted into a membrane holder to have gas-tight connections to the outer world. The microfabricated membrane demonstrated high separation fluxes of up to 4 mol [H.sub.2]/[m.sup.2] x s with a minimal selectivity of 1500 for hydrogen over helium ([H.sub.2]/He) at 450 [degrees]C and 83 kPa [H.sub.2] retentate pressure. The present membrane has great potential for hydrogen purification and in applications like dehydrogenation chemistry. In addition, the presented technology can be used to fabricate other kinds of ultrathin but strong and defect-free membranes to set up new applications. [1235] Index Terms--Hydrogen separation, membrane module, membrane reactor, microfabrication technology, Pd-Ag membrane, SiN microsieves, {110} wafers.

Microfabricated interdigitated electrode array is a convenient form of electrode geometry for dielectrophoretic trapping of particles and biological entities such as cells and bacteria within microfluidic biochips. We present experimental results and finite element modeling of the holding forces for both positive and negative dielectrophoretic traps on microfabricated interdigitated electrodes within a microfluidic biochip fabricated in silicon with a 12-[micro]m-deep chamber. Anodic bonding was used to close the channels with a glass cover. An Experimental protocol was then used to measure the voltages necessary to capture different particles (polystyrene beads, yeast cells, spores and bacteria) against destabilizing fluid flows at a given frequency. The experimental results and those from modeling are found to be in close agreement, validating our ability to model the dielectrophoretic filter for bacteria, spores, yeast cells, and polystyrene beads. This knowledge can be very useful in designing and operating a dielectrophoretic barrier or filter to sort and select particles entering the microfluidic devices for further analysis. [1181] Index Terms--Dielectrophoretic filter, interdigitated electrode, microfluidic biochip.

We present the realization and characterization of a new type of plastic micropump based on the magnetic actuation of a magnetic liquid. The pump consists of two serial check-valves that convert the periodic motion of a ferrofluidic plug into a pulsed quasi-continuous flow. The ferrofluid is actuated by the mechanical motion of an external NdFeB permanent magnet. The water-based ferrofluid is synthesized in-house using a coprecipitation method and has a saturation magnetization of 32 mT. The micropump consists of various layers of polymethylmethacrylate (PMMA), which are microstructured by powder blasting or by standard mechanical micromachining techniques, and are assembled in a single plastic structure using a monomer gluing solution. Two soft silicone membranes are integrated in the microfluidic structure to form two check-valves. Water has been successfully pumped at flow rates of up to 30 [micro]L/min and pumping is achieved at backpressures of up to 25 mbar. [1068] Index Terms--Ferrofluid, lab-on-a-chip, micropump, polymethylmethacrylate (PMMA), powder blasting, silicone check-valve.

This paper reports a MEMS microforce sensor with a novel configuration of bulk micromachined differential triplate comb drives that overcomes the difficulty of electrically isolating the two stationary capacitor comb sets in bulk micromachining. A high-yield fabrication process using deep-reactive ion etching (DRIE) on silicon-on-insulator (SOI) wafers and only three lithographic masks was utilized to construct the high aspect ratio devices. The process features dry release of both suspended structures and the entire device in order to protect fragile components. The sensor has a high sensitivity (1.35 mV/[micro]N), good linearity (< 4%), and a large bandwidth (7.8 kHz), and is therefore well suited for characterizing flight behavior of fruit flies (Drosophila melanogaster). The technique allows for the instantaneous measurement of flight forces, which result from a combination of aerodynamic forces and inertial forces generated by the wings, and demonstrates a novel experimental paradigm for exploring flight biomechanics in the fruit fly. The average lift force is determined to be 9.3 [micro]N ([+ or -] 2.3 [micro]N), which is in the range of typical body weights of fruit flies. The potential impact of this research extends beyond gathering flight data on Drosophila melanogaster by demonstrating how MEMS technology can be used to provide valuable tools for biomechanical investigations. [1208] Index Terms--Capacitive sensor, deep-reactive ion etching (DRIE) on silicon-on-insulator (SOI), dice free dry release, differential triplate comb drive, Drosophila melanogaster, fruit flies, high aspect ratio, microforce sensing.

In this paper, we present a micromachined differential viscosity detector suitable for integration into an on-chip hydrodynamic chromatography system. The general design, however, is applicable to any liquid chromatography system that is used for separation of polymers. The micromachined part of the detector consists of a fluidic Wheatstone bridge and a low hydraulic capacitance pressure sensor of which the pressure sensing is based on optical detection of a membrane deflection. The stand-alone sensor shows a resolution in specific viscosity of 3 x [10.sup.-3], in which specific viscosity is defined as the increase in viscosity by a sample, relative to the baseline viscosity of a solvent. [0947] Index Terms--Microfluidics, viscometer, viscosity detection.

This paper reports on the design, fabrication, and testing of an axial-flux permanent magnet electromagnetic generator. The generator comprises a polymer rotor with embedded permanent magnets sandwiched between two silicon stators with electroplated planar coils. Finite element simulations have been carried out using ANSYS to determine the effects on performance of design parameters such as the number of layers in the stator coils, and the rotor-stator gap. The effect of including soft magnetic pole pieces on the stators has also been studied. A prototype device with a diameter of 7.5 mm has been tested, and shown to deliver an output power of 1.1 mW per stator at a rotation speed of 30 000 rpm. The generator has been integrated with a microfabricated axial-flow microturbine to produce a compact power conversion device for power generation and flow sensing applications. [1265] Index Terms--Axial-flow microturbine, energy harvesting, laser micromachining, permanent magnet machine, power generator, power MEMS.

This paper presents a microsystem that utilizes inductive power and data transfer through a backscatter-modulated carrier and a transducer interface that monitors its environment through embedded capacitive transducers. Formed on a single chip, transducers for temperature, pressure, and relative humidity are realized using a silicon-on-glass process that combines anodic bonding and a silicon-gold eutectic to realize vacuum-sealed cavities with low-impedance (6 [ohm]) electrical feedthroughs. Temperature is sensed capacitively using a row of Si/Au bimorph beams that produce a sensitivity of 15 fF/ [degrees]C from 20 to 100[degrees]C. The absolute pressure sensors have a sensitivity of 15 fF/torr and a range from 500 to 1200 torr, while the relative humidity sensor responds with 39 fF/%RH from 20 to 95% RH. A relaxation oscillator implements low-power capacitance-to-frequency conversion on a second chip with a sensitivity of 750 Hz/pF at 10 kHz, forming a 341 [micro]W transducer interface. The system is remotely powered by a 3-MHz carrier and has a volume of 32 [mm.sup.3], including the hybrid antenna wound around the perimeter of the system. [1241] Index Terms--Anodic bonding, backscatter modulation, bimorph sensor, capacitive interface, electrical feedthrough, eutectic bonding, passive telemetry, relaxation oscillator, wireless sensor.

We have fabricated sub-millimeter-sized quad corner-cube retroreflectors (CCRs) for free-space optical communication. Each quad CCR structure comprises three mirrors micromachined from silicon-on-insulator wafers, and is designed to facilitate manual assembly with accurate angular alignment. Assembled CCRs exhibit mirror nonflatness less than 50 nm, mirror roughness less than 2 nm, and mirror misalignment less than 1 mrad, leading to near-ideal optical performance. The quad CCR incorporates a gap-closing actuator to deflect a base mirror common to the four CCRs, allowing their reflectivity to be modulated up to 7 kb/s by a drive voltage less than 5 V. We have demonstrated a 180-m free-space optical communication link using a CCR as a passive optical transmitter. Quad CCRs have been integrated into miniature, autonomous nodes that constitute a distributed wireless sensor network. We present an analysis of the signal-to-noise ratio of CCR-based links, considering the impact of CCR dimensions, ambient light noise, and other factors. [907] Index Terms--Assembly, corner-cube retroreflector, free-space communication, microelectromechanical systems (MEMS).

A new class of fully compliant bistable mechanisms with the added benefit of integrated self-retraction has been developed (hereafter identified as Self-Retracting Fully compliant Bistable Mechanism or SRFBM). A technique using tensural pivots to manage compressive loading in compliant mechanisms is introduced and implemented in the SRFBM. The elimination of traditional kinematic joints and their associated clearance allows a total displacement between stable positions of 8.5 [micro]m, and the mechanism size is less than 300 [micro]m square when using 2.0 [micro]m minimum line widths. Maximum actuation force is approximately 500 [micro]N. The SRFBM's small linear displacement and reasonable actuation force facilitate integration with efficient thermal actuators. Furthermore, fully compliant mechanisms allow greater freedom in fabrication as only one mechanical layer is needed. Systems with on-chip actuation have been fabricated and tested, demonstrating bistability and on-chip actuation, which requires approximately 150 mW. A single fatigue test has been completed, during which the SRFBM endured approximately 2 million duty cycles without failure. [746] Index Terms--Fully compliant bistable micromechanisms, microelectromechanical systems (MEMS), switching.

A MEMS electromagnetic optical scanner for horizontal scanning of a commercial confocal laser scanning microscope has been developed. The purpose is to replace currently used commercially available scanner with our new MEMS scanner in an existing microscope product, and therefore, the scanner specifications have to be compatible with those for the current one. Electromagnetic actuation is selected because of the millimeter-sized mirror, and single crystal silicon hinge is used for realizing high-speed scanning with sufficient scan angle. In order to maintain a mirror flatness for high quality optics requirement, the whole wafer thickness (300 [micro]m) is used as the mirror, resulting in a large moment of inertia, and it has been taken into consideration in the actuator design. Although few MEMS actuators have been commercialized to date, it has successfully satisfied all the specifications including not only the fundamentals such as resonant frequency and scan angle but also the ones for the commercial product such as scanning stability and reliability. It has been commercialized as a part of our product, Olympus OLS1100 (remodeled as OLS1200 in August 2002). [837] Index Terms--Closed-loop control, electromagnetic, laser scanning microscope, optical scanner, reliability.

New applications continue to emerge for Microelectromechanical systems (MEMS) including the areas of aerospace propulsion and engine control. In addition, engineers are looking to take entirely unorthodox and new approaches to the development of future flight and engine controls through the use of MEMS.