A monolithic three-axis micro-g micromachined silicon capacitive accelerometer

A monolithic three-axis micro-g resolution silicon capacitive accelerometer system utilizing a combined surface and bulk micromachining technology is demonstrated. The accelerometer system consists of three individual single-axis accelerometers fabricated in a single substrate using a common fabrication process. All three devices have 475-/spl mu/m-thick silicon proof-mass, large area polysilicon sense/drive electrodes, and small sensing gap (<1.5 /spl mu/m) formed by a2004 sacrificial oxide layer. The fabricated accelerometer is 7/spl times/9 mm/sup 2/ in size, has 100 Hz bandwidth, >/spl sim/5 pF/g measured sensitivity and calculated sub-/spl mu/g//spl radic/Hz mechanical noise floor for all three axes. The total measured noise floor of the hybrid accelerometer assembled with a CMOS interface circuit is 1.60 /spl mu/g//spl radic/Hz (>1.5 kHz) and 1.08 /spl mu/g//spl radic/Hz (>600 Hz) for in-plane and out-of-plane devices, respectively.     

Using dynamic voltage drive in a parallel-plate electrostatic actuator for full-gap travel range and positioning

The nonlinear dynamics of the parallel-plate electrostatically driven microstructure have been investigated with the objective of finding a dynamic voltage drive suitable for full-gap operation. Nonlinear dynamic modeling with phase-portrait presentation of both position and velocity of a realistic microstructure demonstrate that instability is avoided by a timely and sufficient reduction of the drive voltage. The simulation results are confirmed by experiments on devices fabricated in an epi-poly process. A 5.5-V peak harmonic drive voltage with frequency higher than 300 Hz allows repetitive microstructure motion up to 70% of gap without position feedback. The results of the analysis have been applied to the design of a new concept for positioning beyond the static pull-in limitation that does include position feedback. The measured instantaneous actuator displacement is compared with the desired displacement setting and, unlike traditional feedback, the voltage applied to the actuator is changed according to the comparison result between two values. The "low" level is below the static pull-in voltage and opposes the motion, thus bringing the structure back into a stable regime, while the "high" level is larger than the static pull-in voltage and will push the structure beyond the static pull-in displacement. Operation is limited only by the position jitter due to the time delay introduced by the readout circuits. Measurements confirm flexible operation up to a mechanical stopper positioned at 2 /spl mu/m of the 2.25 /spl mu/m wide gap with a 30 nm ripple.     

Self-balanced navigation-grade capacitive microaccelerometers using branched finger electrodes and their performance for varying sense voltage and pressure

Presents a navigation-grade capacitive microaccelerometer, whose low-noise high-resolution detection capability is achieved by a new electrode design based on a high-amplitude anti-phase sense voltage. We reduce the mechanical noise of the microaccelerometer to the level of 5.5 /spl mu/g//spl radic/Hz by increasing the proof-mass based on deep RIE process of an SOI wafer. We reduce the electrical noise as low as 0.6 /spl mu/g//spl radic/Hz by using an anti-phase high-amplitude square-wave sense voltage of 19 V. The nonlinearity problem caused by the high-amplitude sense voltage is solved by a new electrode design of branched finger type. Combined use of the branched finger electrode and high-amplitude sense voltage generates self force-balancing effects, resulting in an 140% increase of the bandwidth from 726Hz to 1734 Hz. For a fixed sense voltage of 10 V, the total noise is measured as 2.6 /spl mu/g//spl radic/Hz at the air pressure of 3.9torr, which is the 51% of the total noise of 5.1 /spl mu/g//spl radic/Hz at the atmospheric pressure.     

A high-sensitivity silicon accelerometer with a folded-electrode structure

A high-sensitivity capacitive silicon accelerometer with a new device structure is presented in this paper. The structure uses a fixed rigid electrode suspended between a proof mass and a stiff moving electrode to provide differential capacitance measurement and force rebalancing. High sensitivity is achieved by forming a thick silicon proof mass and a narrow uniform air gap over a large area. The mechanical noise floor is reduced by incorporating damping holes in the electrodes. The accelerometer structure is all silicon and is fabricated on a single silicon wafer. The measured sensitivity for a device with 2.6 mm /spl times/ 1 mm proof mass and 1.4 /spl mu/m air gap is /spl ap/11 pF/g per electrode. The calculated mechanical noise floor for the same device is 0.18 /spl mu/g//spl radic/Hz at atmosphere.     

An in-plane high-sensitivity, low-noise micro-g silicon accelerometer with CMOS readout circuitry

A high-sensitivity, low-noise in-plane (lateral) capacitive silicon microaccelerometer utilizing a combined surface and bulk micromachining technology is reported. The accelerometer utilizes a 0.5-mm-thick, 2.4/spl times/1.0 mm/sup 2/ proof-mass and high aspect-ratio vertical polysilicon sensing electrodes fabricated using a trench refill process. The electrodes are separated from the proof-mass by a 1.1-/spl mu/m sensing gap formed using a sacrificial oxide layer. The measured device sensitivity is 5.6 pF/g. A CMOS readout circuit utilizing a switched-capacitor front-end /spl Sigma/-/spl Delta/ modulator operating at 1 MHz with chopper stabilization and correlated double sampling technique, can resolve a capacitance of 10 aF over a dynamic range of 120 dB in a 1 Hz BW. The measured input referred noise floor of the accelerometer-CMOS interface circuit is 1.6/spl mu/g//spl radic/Hz in atmosphere.     

Encapsulated submillimeter piezoresistive accelerometers

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (相似文献   

InP-based optical waveguide MEMS switches with evanescent coupling mechanism

An optical waveguide MEMS switch fabricated on an indium phosphide (InP) substrate for operation at 1550 nm wavelength is presented. Compared to other MEMS optical switches, which typically use relatively large mirrors or long end-coupled waveguides, our device uses a parallel switching mechanism. The device utilizes evanescent coupling between two closely-spaced waveguides fabricated side by side. Coupling is controlled by changing the gap and the coupling length between the two waveguides via electrostatic pull-in. This enables both optical switching and variable optical coupling at voltages below 10 V. Channel isolation as high as -47 dB and coupling efficiencies of up to 66% were obtained with switching losses of less than 0.5 dB. We also demonstrate voltage-controlled variable optical coupling over a 17.4 dB dynamic range. The devices are compact with 2 /spl mu/m/spl times/2 /spl mu/m core cross section and active area as small as 500 /spl mu/m/spl times/5 /spl mu/m. Due to the small travel range of the waveguides, fast operation is obtained with switching times as short as 4 /spl mu/s. Future devices can be scaled down to less than 1 /spl mu/m/spl times/1 /spl mu/m waveguide cross-sectional area and device length less than 100 /spl mu/m without significant change in device design.     

Polymer-based wide-bandwidth and high-sensitivity micromachined electron tunneling accelerometers using hot embossing

The first PMMA-based membrane tunneling accelerometers were fabricated by hot embossing replication with silicon molds. The silicon molds were prepared by a combinative etching technique involving anisotropic bulk etching and modified plasma dry etching. The constructed molds hold both pyramid pits and positive profile sidewalls with smooth surfaces and steep angles, which were necessary for the hot embossing demolding. After electrodes patterned on embossed PMMA structures, the accelerometers, 8 mm /spl times/8 mm /spl times/1 mm, were packaged and assembled on a measurement circuit board. The exponential relationship between tip currents and applied deflection voltages presented a tunneling barrier height of 0.17 eV. The natural frequency of sensors was about 128 Hz. The bandwidth of the feedback system was 6.3 kHz. The sensitivity of voltage over acceleration was 20.6 V/g, and the resolution was 0.2485 /spl mu/g//spl radic/Hz (g=9.8 m/s/sup 2/).     

Vertical-actuated electrostatic comb drive with in situ capacitive position correction for application in phase shifting diffraction interferometry

This research utilizes the levitation effect of electrostatic comb fingers to design vertical-to-the-substrate actuation for optical phase shifting interferometry applications. For typical polysilicon comb drives with 2 /spl mu/m gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 /spl mu/m above the stationary comb fingers. This distance is ideal for most phase shifting interferometric applications. A parallel plate capacitor between the suspended mass and the substrate provides in situ position sensing to control the vertical movement, providing a total feedback-controlled system. The travel range of the designed vertical microactuator is 1.2 /spl mu/m. Since the levitation force is not linear to the input voltage, a lock-in amplifier capacitive sensing circuit combined with a digital signal processor enables a linearized travel trajectory with 1.5 nm position control accuracy. A completely packaged micro phase shifter is described in this paper. One application for this microactuator is to provide linear phase shifting in the phase shifting diffraction interferometer (PSDI) developed at LLNL which can perform optical metrology down to 2 /spl Aring/ accuracy.     

Resonant Pull-In Condition in Parallel-Plate Electrostatic Actuators

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.     

Uncooled multimirror broad-band infrared microbolometers

A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earth's Radiant Energy System) instrument to measure the radiation flux at the Earth's surface and the radiant energy now within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 /spl mu/m, a wide-band channel of 0.3 to 100 /spl mu/m and a window channel from 8 to 12 /spl mu/m each housing a 1.5 mm x 1.5 mm microbolometers or alternatively 400 /spl mu/m x 400 mm microbolometers in a 1 /spl times/ 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 /spl mu/m wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si/sub 3/N/sub 4/ bridge layer. The temperature coefficient of resistance was measured to be -2.8%/K. The voltage responsivities were over 10/sup 3/ V/W, detectivities above 10/sup 8/ cm Hz/sup 1/2//W, noise equivalent power less than 4 /spl times/ 10/sup -10/W/Hz/sup 1/2/ and thermal time constant less than 15 ms.     

Flatness-Based Control of Electrostatically Actuated MEMS With Application to Adaptive Optics: A Simulation Study

Typical adaptive optics (AO) applications require continual measurement and correction of aberrated light and form closed-loop control systems. One of the key components in microelectromechanical system (MEMS) based AO systems is the parallel-plate microactuator. Being electrostatically actuated, this type of devices is inherently instable beyond the pull-in position when they are controlled by a constant voltage. Therefore extending the stable travelling range of such devices forms one of the central topics in the control of MEMS. In addition, though certain control schemes, such as charge control and capacitive feedback, can extend the travelling range to the full gap, the transient behavior of actuators is dominated by their mechanical dynamics. Thus, the performance may be poor if the natural damping of the devices is too low or too high. This paper presents an alternative for the control of parallel-plate electrostatic actuators, which is based on an essential property of nonlinear systems, namely differential flatness, and combines the techniques of trajectory planning and robust nonlinear control. It is, therefore, capable of stabilizing the system at any point in the gap while ensuring desired performances. The proposed control scheme is applied to an AO system and simulation results demonstrate its advantage over constant voltage control.1613     

Extending the travel range of analog-tuned electrostatic actuators

The pull-in instability limits the travel distance of elastically suspended parallel-plate electrostatic microactuators to about 1/3 of the undeflected gap distance. In this paper, we examine the “leveraged bending” and “strain-stiffening” methods for extending the travel range of electrostatic actuators. The leveraged bending effect can be used to achieve full gap travel at the cost of increased actuation voltage. The strain-stiffening effect can be used to minimize actuation voltage for a given travel range. An analytical approximation shows that the strain-stiffening effect can be used to achieve a stable travel distance up to about 3/5 of the gap. A tunable reflective diffraction grating known as the polychromator has been designed using these actuation techniques, and selected designs have been fabricated and tested for actuation behavior. Gratings with 1024 flat, closely packed grating-element actuators have been fabricated with over 1-cm-long mirrors, achieving stable vertical travel distances of more than 1.75 μm out of a 2-μm gap     

Pull-in-based μg-resolution accelerometer: Characterization and noise analysis

The pull-in time (t_pi) of electrostatically actuated parallel-plate microstructures enables the realization of a high-sensitivity accelerometer that uses time measurement as the transduction mechanism. The key feature is the existence of a metastable region that dominates pull-in behavior, thus making pull-in time very sensitive to external accelerations. Parallel-plate MEMS structures have been designed and fabricated using a SOI micromachining process (SOIMUMPS) for the implementation of the accelerometer. This paper presents the experimental characterization of the microdevices, validating the concept and the analytical models used. The accelerometer has a measured sensitivity of 0.25 μs/μg and a bandwidth that is directly related to the pull-in time, BW = 1/2t_pi ≈ 50 Hz. These specifications place this sensor between the state of the art accelerometers found both in the literature and commercially. More importantly, the resolution of the measurement method used is very high, making the mechanical-thermal noise the only factor limiting the resolution. The in-depth noise analysis to the system supports these conclusions. The total measured noise floor of 400 μg (100 μs) is mainly due to the contribution of the environmental noise, due to lack of isolation of the experimental setup from the building vibrations (estimated mechanical thermal noise of 2.8 μg/√Hz). The low requirements of the electronic readout circuit makes this an interesting approach for high-resolution accelerometers.     

Deep-reactive ion-etched compliant starting zone electrostatic zipping actuators

This paper presents the modeling, design, fabrication and testing of monolithic electrostatic curved-electrode zipping actuators fabricated by deep reactive ion etching (DRIE). In contrast to traditional curved-electrode zipping actuators, the design of the actuators presented here utilizes a compliant starting cantilever to significantly reduce the initial pull-in voltage by closing the gap (kerf) generated by DRIE. Thus, the actuators achieve high actuation force at a relatively low voltage. For example, two actuators each with dimensions of 4.5 mm*100 /spl mu/m*300 /spl mu/m are used to drive a bistable MEMS relay. Together, the two actuators provide up to 10 mN of force over their 80 /spl mu/m stroke at 140 V. Measurements of the force-displacement relation of these actuators confirm theoretical expectations based both on numerical and analytical methods. Finite element analysis is employed to predict the behavior of the complete bistable relay system. [1231].     

Grating-assisted optical microprobing of in-plane and out-of-plane displacements of microelectromechanical devices

In this paper, we present an optical sensing method that is capable of detection of both in-plane and out-of-plane translational motions of a micromachined structure that incorporates a diffraction grating. In the proposed method, the out-of-plane displacement sensing is based on optical intensity modulation of a phase-sensitive diffraction grating, while the in-plane displacement sensing is based on a modified grating interferometry. Preliminary experimental results on a surface micromachined grating structure fabricated within the shuttle of a comb-drive resonator demonstrate an in-plane resolution of 0.23 nm//spl radic/Hz and an out-of-plane resolution of 0.03 nm//spl radic/Hz in a 1 Hz bandwidth centered at 950 Hz. The proposed method can be configured for many promising applications, including optically interrogated high sensitive single/dual-axis microaccelerometers or gyroscopes.     

Performance improvement of an electrothermal microactuator fabricated using Ni-diamond nanocomposite

In this paper, a low-temperature stress-free electrolytic nickel (EL) deposition process with added dispersed diamond nanoparticles (diameter <0.5 /spl mu/m) is developed to synthesize Ni-diamond nanocomposite for fabricating electrothermal microactuators. Device characterization reveals dramatic performance improvements in the electrothermal microactuator that is made of the nanocomposite, including a reduction in the input power requirement and enhanced operation reliability. In comparison with the microactuator made of pure nickel, the nanocomposite one can save about 73% the power for a 3 /spl mu/m output displacement and have a longer reversible displacement range, which is prolonged from 1.8 /spl mu/m to more than 3 /spl mu/m. Furthermore, the nanocomposite device exhibits no performance degradation after more than 100 testing cycles in the reversible regime. The enhancements increase with the incorporation of the nanodiamond in a nickel matrix, so the Ni-diamond nanocomposite has potential for application in MEMS fabrication.     

Fuzzy H/sub /spl infin// output feedback control design for singularly perturbed systems with pole placement constraints: an LMI approach

This paper examines the problem of designing an H/sub /spl infin// output feedback controller with pole placement constraints for singular perturbed Takagi-Sugeno (TS) fuzzy models. We propose a fuzzy H/sub /spl infin// output feedback controller that not only guarantees the /spl Lscr//sub 2/-gain of the mapping from the exogenous input noise to the regulated output to be less than some prescribed value, but also ensures closed-loop poles of each subsystem are in a prespecified linear matrix inequality (LMI) region. In order to alleviate the numerical stiffness caused by the singular perturbation /spl epsiv/, the design technique is formulated in terms of a family of /spl epsiv/-independent linear matrix inequalities. The proposed approach can be applied both standard and nonstandard singularly perturbed nonlinear systems. A numerical example is provided to illustrate the design developed in this paper.     

Inherently robust micromachined gyroscopes with 2-DOF sense-mode oscillator

Commercialization of reliable vibratory micromachined gyroscopes for high-volume applications has proven to be extremely challenging, primarily due to the high sensitivity of the dynamical system response to fabrication and environmental variations. This paper reports a novel micromachined gyroscope with two degrees-of-freedom (DOF) sense-mode oscillator that provides inherent robustness against structural parameter variations. The 2-DOF sense-mode oscillator provides a frequency response with two resonant peaks and a flat region between the peaks, instead of a single resonance peak as in conventional gyroscopes. The device is nominally operated in the flat region of the sense-mode response curve, where the amplitude and phase of the response are insensitive to parameter fluctuations. Furthermore, the sensitivity is improved by utilizing dynamical amplification of oscillations in the 2-DOF sense-mode oscillator. Thus, improved robustness to variations in temperature, damping, and structural parameters is achieved, solely by the mechanical system design. Prototype gyroscopes were fabricated using a bulk-micromachining process, and the performance and robustness of the devices have been experimentally evaluated. With a 25 V dc bias and 3 V ac drive signal resulting in 5.8 /spl mu/m drive-mode amplitude, the gyroscope exhibited a measured noise-floor of 0.64/spl deg//s//spl radic/Hz over 50 Hz bandwidth in atmospheric pressure. The sense-mode response in the flat operating region was also experimentally demonstrated to be inherently insensitive to pressure, temperature, and dc bias variations.     

Piezoelectric unimorph microactuator arrays for single-crystal silicon continuous-membrane deformable mirror

Micromachined deformable mirror technology can boost the imaging performance of an otherwise nonrigid, lower-quality telescope structure. This paper describes the optimization of lead zirconium titanate (PZT) unimorph membrane microactuators for deformable mirrors. PZT unimorph actuators consisting of a variety of electrode designs, silicon-membrane thickness, and membrane sizes were fabricated and characterized. A mathematical model was developed to accurately simulate the membrane microactuator performance and to aid in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. Using the above approach, we have successfully demonstrated a 2.5-mm-diameter PZT unimorph actuator. A measured deflection of 5 /spl mu/m was obtained for 50 V applied voltage. Complete deformable mirror structures consisting of 10-/spl mu/m-thick single-crystal silicon mirror membranes mounted over the aforementioned 4/spl times/4 4 PZT unimorph membrane microactuator arrays were designed, fabricated, assembled, and optically characterized. The fully assembled deformable mirror showed an individual pixel stroke of 2.5 /spl mu/m at 50 V actuation voltage. The deformable mirror has a resonance frequency of 42 kHz and an influence function of approximately 25%.