In this paper, we present the design, fabrication, and measurements of a two-dimensional (2-D) optical scanner with electrostatic angular vertical comb (AVC) actuators. The scanner is realized by combining a foundry-based surface-micromachining process (Multi-User MEMS Processes-MUMPs) with a three-mask deep-reactive ion-etching (DRIE) postfabrication process. The surface-micromachining provides versatile mechanical design and electrical interconnect while the bulk micromachining offers high-aspect ratio structures leading to flat mirrors and high-force, large-displacement actuators. The scanner achieves dc mechanical scanning ranges of /spl plusmn/6.2/spl deg/ (at 55 Vdc) and /spl plusmn/4.1/spl deg/ (at 50 Vdc) for the inner and outer gimbals, respectively. The resonant frequencies are 315 and 144 Hz for the inner and the outer axes, respectively. The 1-mm-diameter mirror has a radius of curvature of over 50 cm. [1454].

/pdf/surface-and-bulk-micromachined-two-dimensional-scanner-2zhim8ttcw.pdf

Surface- and bulk- micromachined two-dimensional scanner driven by angular vertical comb actuators

In this paper, we demonstrate full closed-loop control of electrostatically actuated double-gimbaled MEMS mirrors and use them in an optical cross-connect. We show switching times of less than 10 ms and optical power stability of better than 0.2 dB. The mirrors, made from 10-/spl mu/m-thick single-crystal silicon and with a radius of 400-450 /spl mu/m, are able to tilt to 8/spl deg/ corresponding to 80% of touchdown angle. This is achieved using a nonlinear closed-loop control algorithm, which also results in a maximum actuation voltage of 85 V, and a pointing accuracy of less than 150 /spl mu/rad. This paper will describe the MEMS mirror and actuator design, modeling, servo design, and measurement results.

http://www.jaywalkertc.com/publications/JAW_live_papers/JMEMSservo.pdf

Design and nonlinear servo control of MEMS mirrors and their performance in a large port-count optical switch

The aim of this paper is to provide a review of micromirror array (MMA) technologies (2631 MMA research papers and patents were reviewed for this effort). The performance capabilities of 277 MMA designs from 49 companies and 23 academic research groups are categorized and compared. The designs are categorized according to (i) their array’s dimension (e.g., 2D arrays consisting of mirrors that cover a surface, 1D arrays consisting of mirrors in a row, and 0D arrays consisting of only a single mirror), (ii) the nature of the surface of their mirrors (e.g., continuous or discrete), (iii) what combination of tip, tilt, and/or piston degrees of freedom (DOFs) they achieve, and (iv) how they are actuated. Standardized performance metrics that can be systematically applied to every MMA design (e.g., mirror area, fill factor, pitch, range of motion, maximum acceleration, actuator energy density, and number of uncontrolled DOFs) are defined and plotted for existing designs to enable their fair comparison. Theoretical bounds on what is physically possible for MMAs to achieve are also derived and depicted in these plots to highlight the amount of performance improvement that remains to be achieved by future designs and guidelines are provided to aid in the development of these future designs.

A review of micromirror arrays

This paper presents high-resolution control of torsional electrostatic micromirrors beyond their inherent pull-in instability using robust sliding-mode control (SMC). SMC enables compact realization of a robust controller tolerant of device characteristic variations, non-linearities, and many types of inherent instabilities. Robustness of the control loop is demonstrated through extensive simulations and measurements on MEMS with a wide range in their characteristics. Control of two-axis gimbaled micromirrors beyond their pull-in instability with overall 14 milli-degrees pointing accuracy is confirmed experimentally.

Robust sliding-mode control of electrostatic torsional micromirrors beyond the pull-in limit

This paper presents a 2-axis optical scanner fabricated by a MEMS process for use in optical coherence tomography (OCT) imaging. It comprises planar coils placed above a single permanent magnet; it is driven by an electromagnetic force in the static or low frequency (~10 Hz) mode, which is required for OCT systems. Two coils are fabricated on a movable plate for X-axis rotation with a mirror, and four coils are fabricated on a movable frame for Y-axis rotation; each coil has a double-layer structure. Two types of torsion beams are designed and tested. The maximum optical scanning angle of the optical scanner with meandering torsion beams was ±8° for a drive current of ±4.6 mA and ±10.3 mA in the X- and Y-axes, respectively. The resonant frequencies of the optical scanner were 106 Hz and 80.5 Hz in the X- and Y-axes, respectively; the frequency characteristics indicated that the scanner can be driven from dc to several Hz. The fabricated optical scanner was applied to an OCT system and cross-sectional images of an onion were obtained by scanning along both axes.

A 2-axis optical scanner driven nonresonantly by electromagnetic force for OCT imaging

In this paper we introduce a new electrostatic actuator that provides a large increase in neutral position (no rotation) torque and suppression of pull-in compared to a conventional parallel-plate structure. The actuator geometry incorporates raised electrodes on the side of the rotating element that generate torque versus angle behavior much different than convention parallel plate actuators. Using this electrode configuration we have demonstrated 17-degree stable mechanical rotation on large (950 /spl mu/m diameter) mirrors with 100 V single ended drive-a rotation of 60% of the gap. Driving the mirrors with a differential drive scheme we have demonstrated stable 8-degree rotation with as little as /spl plusmn/18 V.

Pull-in suppression and torque magnification in parallel plate electrostatic actuators with side electrodes

High-density micromirror arrays have been fabricated with a process that combines the benefits of both bulk and surface micromachining. Arrays fabricated with this technique are characterized by flat mirrors, high spring constant uniformity, and high yield. Each pixel consists of a mirror-in-gimbal structure that allows the mirror to rotate about two orthogonal axes when a bias is placed on electrodes residing beneath the mirror. A new dry release process is introduced as a simple, effective alternative to critical point drying for releasing the structures.

C.C. Wang

Papers

Pull-in suppression and torque magnification in parallel plate electrostatic actuators with side electrodes

Electrostatic micromirror arrays fabricated with bulk and surface micromachining techniques