Uma Krishnamoorthy

A raster-scanning full-motion video display using polysilicon micromachined mirrors

Abstract As portable computers become more widespread, there is increasing need for lightweight, low-power, inexpensive video displays with high information content. Many established display technologies are useful for large-format displays, but do not satisfy the weight and power requirements of demanding portable display applications. Micromachined raster-scanning displays, however, look attractive for portable computing applications. We describe the operation of a raster-scanning full-motion video display constructed using surface micromachined mirrors. The 41×52 pixel display is interfaced directly to a computer video card. Display resolution is limited by dynamic deformation of the mirror surface. Mirror-scan irregularity is shown to be negligible compared to diffraction from the mirror aperture and dynamic deformation of the mirror surface. The line-scan micromirror has been operated for more than 45 billion cycles with less than 1% change in the mirror resonant frequency.

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.

Scalable optical cross-connect switch using micromachined mirrors

This letter describes a free-space optical fiber cross-connect that uses a pair of micromirror arrays to redirect optical beams from an input-fiber array to an output array. This confocal switch architecture is well suited for simultaneous switching of multiple wavelength channels. We show that confocal switches with low insertion loss, low crosstalk, and large port counts can be implemented with surface-micromachined mirror arrays, and we demonstrate a 2/spl times/2 single-mode (1550 nm) switch configuration with insertion loss of -4.2 dB and crosstalk of -50.5 dB. Our micromirror design has sufficient size and angular deflection for scaling to 32/spl times/32 ports.

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.

Dual-mode micromirrors for optical phased array applications

We present design, analysis and characterization of dual-mode vertical comb-drive actuated micromirrors. The micromirrors are actuated by vertical comb drive actuators that enable large deflections at relatively low voltages with continuous stable control over the full range of motion. The electrode layout of the dual-mode actuators allows us to independently control the piston motion and tilt of the micromirrors. This enables precise control of the phase of optical beams reflected from arrays of micromirrors. Our experimental results show that dual-mode actuation significantly increases the angular range of motion of the mirrors, and we demonstrate phase-front control by spatially scanning an optical beam using a phased array consisting of four dual-mode micromirrors.

Vertical Mirror Fabrication Combining KOH Etch and DRIE of (110) Silicon

This paper presents fabrication of MEMS-actuated optical-quality vertical mirrors as the key active optical components in a silicon optical bench (SOB) technology. The fabrication process is based on a combination of potassium hydroxide (KOH) etch and deep reactive ion etching (DRIE) of (110) SOI wafers. The process starts by creating optical-quality vertical surfaces by KOH etch, followed by an oxidation step to protect them. The patterned wafer is then etched by DRIE to define actuators. The process is designed to allow the KOH etch and DRIE to be independently optimized without compromising either while at the same time meeting the challenge of lithography on high-aspect-ratio structures. Three variations of the fabrication process are demonstrated, two that use double masking layers and one that uses a silicon masking layer. We demonstrate in-plane scanners and fast translational vertical mirrors fabricated using these processes. In addition, we propose extensions of the fabrication process to account for DRIE aspect-ratio limitations. Mask layouts of key SOB building blocks, including vertical mirrors, beam splitters, and parallel-plate actuators, are also presented. [2008-0146]

High-resolution, high-speed microscanner in single-crystalline silicon actuated by self-aligned dual-mode vertical electrostatic combdrive with capability for phased array operation

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 is based on self-aligned DRIE of SOI wafers with two device layers. The two oxide layers provide electrode isolation and etch stops for thickness control. Only front-side processing is required. We demonstrate micromirrors of this type with /spl plusmn/9 degrees of optical scanning and 7.5 /spl mu/m of piston motion under static actuation. Micromirrors designed for resonant scanning have optical scan angles up to /spl plusmn/25 degrees at 13.5 kHz.

Coherent micromirror arrays

A one-dimensional, spatial light phase modulator consists of a coherent array of programmable micromirrors. Each micromirror has rotation as well as elevation control to permit high-fidelity, piecewise linear approximations to a desired spatial phase profile. We demonstrate programmable, coherent spatial modulation by configuring a four-micromirror array to act as a blazed diffraction grating, showing five diffraction orders. In sharp contrast, adjustment of each mirror elevation converts the grating into the equivalent of a single, large tilted mirror that reflects only in a new specular direction. Higher-order diffraction is effectively suppressed.

Process for High Speed Micro Electro Mechanical Systems (MEMS) Scanning Mirrors with Vertical Comb Drives

We present a three-level fabrication process for fast scanning micromirrors actuated by vertical comb drives. The process uses only front side lithography to create combdrives with selfaligned comb teeth. Selfalignment of the movable and stationary comb teeth allows the use of high voltages that are necessary for fast scanning. The fabrication process is based on two wafer-bonding steps to create a three-level structure that is well suited for array applications.

Microoptical phased arrays for spatial and spectral switching

This article describes two optical devices based on linear arrays of micromirrors. The first is a phased array of micromirrors that can be rotated as well as translated vertically to maintain coherence across the array. We demonstrate experimentally that such micromirrors are capable of high-diffraction-efficiency phased-array scanning of laser beams. The second device is a Gires-Tournois (1969) interferometer with a micromirror array that provides tunable phase modulation for the multitude of partially reflected beams within the interferometer. We demonstrate experimentally that the MEMS-GT interferometer can operate as a tunable deinterleaver for dense wavelength-division multiplexed fiber optic communication.