Electrostatic Combdrives Research Articles

Vacuum behavior and control of a MEMS stage with integrated thermal displacement sensor

We investigate the applicability of a MEMS stage in a vacuum environment. The stage is suspended by a flexure mechanism and is actuated by electrostatic comb-drives. The position of the stage is measured by an integrated sensor based on the conductance of heat through air. The vacuum behavior of the sensor and the stage is identified. A model for thermal conductivity and viscous damping as a function of the vacuum pressure is presented and validated by measuring the decreasing sensor response and the increasing Q-factor for decreasing pressure. We have identified the system in the frequency domain, which is used to compare the closed-loop behavior with three different controllers: an integral controller, an integral controller with low-pass filter, and an integral controller with notch filter. The integral controller can become unstable due to the high Q-factor in vacuum. Adding a low-pass or a notch filter improves the stability at low pressures. Since the integral controller with notch filter shows the lowest settling time, this is the preferred controller. Overall, we are able to control the position of the MEMS stage at a pressure of 1mbar with a reduced resolution of approximately 1.1μm, but with a good settling time of 2.2ms.

Flexures for large stroke electrostatic actuation in MEMS

The stroke of a microelectromechanical systems (MEMS) stage suspended by a flexure mechanism and actuated by electrostatic comb-drives is limited by pull-in. A method to analyze the electrostatic stability of a flexure mechanism and to optimize the stroke with respect to the footprint of flexure mechanisms is presented. Four flexure mechanisms for large stroke are investigated; the standard folded flexure, the slaved folded flexure, the tilted folded flexure and the Watt flexure. Given a certain stroke and load force, the flexures are optimized to have a minimum wafer footprint. From these optimizations it is concluded that the standard folded flexure mechanism is the best flexure mechanism for relatively small strokes (up to ±40 μm) and for larger strokes it is better to use the tilted folded flexure. Several optimized flexure mechanisms have been fabricated and experimentally tested to reach a stroke of ±100 μm. The displacement of the fabricated stages as a function of the actuation voltage could be predicted with 82% accuracy, limited by the fairly large tolerances of our fabrication process.

无交叠垂直梳齿平动微镜的驱动特性

To design a non-overlap vertical electrostatic combdriver for the optical translational micromirror of an optical phase modulator,a mathematic model based on the conformal mapping was built to research the driving performance of the combdriver in detail.According to the theory of complex variable function,an analytical model was established for the electrostatic field of comb actuator by conformal mapping.Then,the electrostatic force in a certain displacement range was deduced by the analytical model for researching the movable comb finger.The results were compared with the corresponding resolution by the Finite Element Method(FEM).Finally,a non-overlap vertical combdriver was successfully fabricated by Micro Electronic Mechanic System(MEMS) process,and an optical Michelson interference system was constructed to measure the static driving charateristics of the comb driver.The result shows that the displacement of proposed non-overlap vertical electrostatic combdriver is 325 nm(phase difference 2π) with a dc driving voltage of 28 V and the offset vertical comb actuator can be actuated to 2.07 μm under a dc driving voltage of 90 V.The measured results accord excellently with the simulated results using the conformal mapping and FEM method,which proves proposed analytical model to be correct.It can provide theoretical and pratical bases for design of non-overlap vertical electrostatic combdrivers.

Experimental Measurement of Resonance Frequencies of Asymmetric Micro-bridge Resonators

The experiment results on the resonance frequencies of micro-bridge resonators with attached electrostatic comb-drives are reported in this article. The resonators include a novel design of asymmetric resonators, in which the comb-drives are attached to off-midpoint of the beam. The experiment results confirm that by properly designing the location and mass of the comb-drives, the natural frequencies can exceed those of conventional symmetric resonators. Moreover, it has been shown that the second mode of vibration of asymmetric resonators, in contrast to that of symmetric resonators, can be excited, hence exploited for applications in which multiple working frequencies are desired. The experiment results are in good agreement with the analytical model. The deviations between these two are discussed in detail.

Multifunctional Tunable Optical Filter Using MEMS Spatial Light Modulator

In this paper, we introduce a multifunctional tunable microelectromechanical systems (MEMS) optical bandpass filter that can continuously and independently tune the center wavelength and the passband spectral width. The filter function is achieved by dispersing the input light on a MEMS spatial light modulator (SLM) that is implemented with gold-coated mirrors microassembled on a MEMS platform. The mirrors are actuated by electrostatic combdrives that are bidirectional and have a maximum stable displacement of 44 ¿m in both directions for a total range of 88 ¿m. By actuating the SLM, the 1-dB bandwidth of the filter can be continuously tuned from 0.1 to 1.3 nm, and the 3-dB bandwidth can be tuned from 0.3 to 1.5 nm. In addition, the center wavelength can be fine tuned by actuating the movable blocking mirrors and coarsely tuned over a large spectral range by rotating the grating.

Design and modeling of a six DOFs MEMS-based precision manipulator

In this paper a design is presented for a precision MEMS-based six degrees-of-freedom (DOFs) manipulator. The purpose of the manipulator is to position a small sample (10 μm × 20 μm × 0.2 μm) in a transmission electron microscope. A parallel kinematic mechanism with slanted leaf-springs is used to convert the motion of six in-plane electrostatic comb-drives into six DOFs at the end-effector. The manipulator design is based on the principles of exact constraint design, resulting in a high actuation compliance (flexibility) combined with a relatively high suspension stiffness. However, due to fabrication limitations overconstrained design has been applied to increase the stiffness in the out-of-plane direction. The result is a relatively large manipulator stroke of 20 μm in all directions combined with a high first vibration mode frequency of 3.8 kHz in relation to the used area of 4.9 mm × 5.2 mm. The motion of the manipulator is guided by elastic elements to avoid backlash, friction, hysteresis and wear, resulting in nanometer resolution position control. The fabrication of the slanted leaf-springs is based on the deposition of silicon nitride (SixNy) on a silicon pyramid, which in turn is obtained by selective crystal plane etching by potassium hydroxide (KOH). The design has been analyzed and optimized with a multibody program using flexible beam theory. A previously developed flexible beam element has been used for modeling the typical relatively large deflections and the resulting position-dependent behavior of compliant mechanisms in MEMS. The multibody modeling has been verified by FEM modeling. Presently only parts of the manipulator have been fabricated. Therefore, a scaled-up version of the manipulator has been fabricated to obtain experimental data and to verify the design and modeling.

High-Reflectivity Broadband Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant comb drive highly reflective broadband photonic crystal (PC) mirror microelectromechanical systems (MEMS) scanner with low dependence of reflectivity on incident angle and polarization. Two-dimensional PC mirrors have several advantages over metal coatings and dielectric stacks (1-D PCs) as mirrors. They can tolerate much higher processing temperatures and higher incident powers, as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, 2-D PC mirrors are compact and allow for simpler process integration, making them highly compatible with CMOS and MEMS processing. Our PC mirrors show broadband high reflectivity of > 90% in the wavelength range from 1345 to 1490 nm (> 95% for 1380 to 1450 nm) and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator wafers with the PC mirrors fabricated in polysilicon films on oxide. Although the scanner presented in this paper is single axis, the low angular and polarization dependence properties of the 2-D-PC mirror also make it useful for dual-axis scanners. The scanners are actuated by in-chip plane electrostatic combdrives on resonance. Dynamic deflection measurements show that the scanners can achieve 20deg total scan angle with an input square wave of 40 V and up to 120deg total scan angle with 190 V at a resonance frequency of 2.03 kHz. Although the achievable scan ranges are very large, the useful scan angles will be limited by the angular and polarization dependence properties of the PC reflector and its application. For the 2-D PC scanner presented in this paper, applications that can tolerate 10% decrease in reflectivity will have a scan range of 20deg.

Pull-In Analysis of Torsional Scanners Actuated by Electrostatic Vertical Combdrives

This paper presents pull-in analysis of torsional MEMS scanners actuated by electrostatic vertical combdrives with general comb gap arrangements and cross sections. The analysis is based on a 2-DOF actuator with a single voltage control. Three failure modes of the scanners are identified as in-plane twist, transversal motion, and out-of-plane twist. For each failure mode, analytical expressions of pull-in deflection are obtained by applying 2D analytical capacitance models to the derived pull-in equations. From these, the dominant pull-in mechanism is shown to be in-plane twist for scanners with high-aspect-ratio torsional springs. The analytical calculations for both symmetric and asymmetric capacitances are shown to be in good agreement with simulation results. The optimum scanner design is achieved when the pull-in deflection matches the capacitance maximum angle. The condition can be expressed in terms of the ratio of the comb thickness to the comb gap, which is smaller than the typical aspect ratio of deep reactive ion etching. The optimum tradeoff between the maximum deflection angle and the number of movable combs is achieved by adjusting the overlap of the movable and fixed combs and the distance of the comb sets from the axis of the rotation.

Two-Dimensional MEMS Scanner for Dual-Axes Confocal Microscopy

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

High Fill-Factor Two-Axis Gimbaled Tip-Tilt-Piston Micromirror Array Actuated by Self-Aligned Vertical Electrostatic Combdrives

In this paper, we present a high fill-factor micromirror array actuated by self-aligned vertical electrostatic combdrives. To meet the requirements of applications in free-space communication and imaging, each micromirror has three degrees of freedom of motion: rotation around two axes in the mirror plane and linear translation perpendicular to the mirror plane. Our approach is to integrate the high fill-factor reflectors into the fabrication process of the actuators on the wafer-scale. Multilevel silicon-on-insulator (SOI) bonding is utilized to form the high optical quality reflectors and high aspect-ratio vertical combdrive actuators. The wiring for electrical access to the multielectrode per pixel array is fabricated on separate wafers by thin film processing, and flip-chip bonded to the reflector/actuator chip. This architecture overcomes the fill-factor limitation of top-side accessed electrical addressing of mirrors made on SOI. Our 360/spl mu/m pixel size mirror array achieves a 99% fill-factor with optically flat reflectors.

A novel self-aligned vertical electrostatic combdrives actuator for scanning micromirrors

A novel self-aligned vertical electrostatic combdrives actuator has been developed and demonstrated, which enhances the capabilities and applications of high aspect ratio silicon-on-insulator microelectromechanical systems (SOI-MEMS) by enabling additional independent degrees of freedom of operation: both upward and downward vertical pistoning motion as well as bi-directional rotation. The present method utilizes four aligned masks greatly simplifying the existing SOI-MEMS fabrication methods for manufacturing high performance scanning micromirrors. Results from the micromirror device for the novel vertical combdrives show that a mechanical tilt angle of ±1° at 100 Vdc was achieved for a 450 µm diameter micromirror. The scanning micromirror can scan a large angle 62° at the resonance frequency of 10.46 kHz with a sinusoidal voltage input of 60 V in amplitude.

Single-crystalline silicon micromirrors actuated by self-aligned vertical electrostatic combdrives with piston-motion and rotation capability

We present micromirrors actuated by self-aligned, high aspect ratio, vertical electrostatic combdrives with multi-level electrical isolation that allows bi-directional and dual-mode (independent rotation and piston motion) operation. The fabrication process, which requires only front-side processing, is based on self-aligned deep-reactive ion etching (DRIE) of silicon-on-insulator (SOI) wafers with two device layers. The two oxide layers between the device layers provide electrode isolation and etch stops for thickness control. Self-alignment of electrostatic combs in the two device layers is accomplished by creating masking features in the upper layer that are transferred into the lower layer. The masking features are typically removed, but in some cases they remain as an integral part of the completed device. SEM pictures show perfectly aligned vertical combteeth both with and without removal of the masking features. We demonstrate micromirrors of this type with ±9° of optical scanning and 7.5 μm of piston motion under static actuation. Micromirrors designed for resonant scanning have optical scan angles up to ±25° at 13.5 kHz.

Self-aligned vertical electrostatic combdrives for micromirror actuation

In this paper, we analyze the effect of misalignment in electrostatic combdrives, and describe a fabrication technology that minimizes misalignment in vertical electrostatic combdrives by creating self-aligned, vertically staggered electrodes. Self-alignment of the interdigitated electrodes simplifies fabrication and minimizes failures due to electrostatic instability, thus enabling fabrication of narrow-gap, high-force actuators with high yield. The process is based on deep-reactive ion etching (DRIE) of buried-patterned silicon-on-insulator (SOI) wafers. Measurements on fabricated combdrives show relative misalignment of less than 0.05 /spl mu/m. This corresponds to less than 0.1% misalignment, which, according to our analysis, results in a travel range of 98% of that for perfectly aligned drives. The validity of the process is demonstrated by fabrication of scanning micromirrors measuring 300 /spl mu/m by 100 /spl mu/m. Optical angular deflections from 4/spl deg/ at low frequency to 40/spl deg/ at resonance were measured for an applied voltage of 75 Vpp. Resonant frequencies ranged from 5 kHz to 15 kHz for these devices, making them suitable for high-speed, high-resolution optical scanning and switching.