Fixed fingers Movable fingers Hidden springs Mirror Anchor z y x Fixed fingers Movable fingers Torsion springs Mirror (½) Lateral stopper LOW VOLTAGE MEMS ANALOG MICROMIRROR ARRAYS WITH HIDDEN VERTICAL COMB-DRIVE ACTUATORS Dooyoung Hah, Sophia Huang, Hung Nguyen, Hsin Chang, Jui-Che Tsai, and Ming C. Wu Department of Electrical Engineering, University of California, Los Angeles Los Angeles, CA 90095-1594 Hiroshi Toshiyoshi Institute of Industrial Science, University of Tokyo Tokyo, Japan 153-8505 ABSTRACT We report on a novel, polysilicon surface-micromachined one-dimensional (1-D) analog micromirror array. Large continuous DC scan angle (23.6° optical) and low operating voltage (6V) have been achieved using vertical comb-drive actuators. The actuators and torsion springs are placed underneath the mirror (137 × 120 μm 2 ) to achieve high fill-factor (91%). The measured resonant frequency of the mirror ranges from 3.4 kHz to 8.1 kHz. The static scanning characteristics show good uniformity (<±3.2%) for a 1 × 10 array with a mirror pitch of 150 μm. The measured DC scanning characteristics and resonant frequencies agree well with theoretical values. This micromirror array has applications in optical wavelength-division multiplexed (WDM) routers and wavelength-selective crossconnect (WSXC). INTRODUCTION The advance of photonic communication networks towards optical layer networking has created a great demand for many new functional optical network elements. Microelectromechanical- systems (MEMS) is a key enabling technology for many of these new devices. Optical MEMS devices for dynamic optical add-drop multiplexers (OADM) [1], 2-D [2-3] and 3-D optical crossconnects (OXC) [4] have been reported. Previously, we have reported a novel wavelength-division-multiplexed (WDM) router that employs a 1-D array of analog micromirrors [5]. Large continuous scan range and high fill factor are required to achieve high channel count and flat spectral response. In addition, low voltage, low power actuation is desired to reduce power consumption of drive electronics. Most of the micromirrors reported to date employ parallel- plate type electrostatic actuators. The pull-in phenomena in such actuators, however, limit the useful range of continuous scanning. The vertical comb-drive actuator reported recently [6, 7] offers several inherent advantages for actuating micromirrors. It can be designed to avoid pull-in effect and utilize the entire range of rotation. The large force density of vertical comb drives also lead to low voltage operation. However, the bulk-micromachined structures are not suitable for implementing high fill factor micromirror arrays where actuators are hidden underneath the mirrors. The surface-micromachining technique offers more flexibility for designing such multi-layer structures. In this paper, we report on a novel surface-micromachined analog micromirror array with hidden vertical comb drive actuators. High fill factor (91%), flat mirror surface (137x120 μm 2 ), low operating voltage (6 V), and large continuous scan angle (23.6° optical) are successfully achieved. This device is made possible by exploiting the chemical-mechanical planarization (CMP) processes in SUMMiT-V offered by Sandia National Laboratory. Figure 1. The schematic structure of the proposed micromirror. Figure 2. The SEM micrograph of the fabricated micromirror. The lower half of the micromirror was removed intentionally to reveal the underlying comb structures. THEORY The schematic of the analog micromirror is shown in Fig. 1. The vertical comb-drive actuators and the torsional springs were placed underneath the mirror to achieve high fill-factor, which is required for flat-top frequency response. Figure 2 shows the scanning electron microscope (SEM) image of the fabricated micromirror. Half of the mirror was intentionally removed to reveal the underlying structures. The height difference between the movable and the fixed fingers is illustrated by different contrast. 0-9640024-4-2/hh2002/$20©2002TRF DOI 10.31438/trf.hh2002.3 11 Solid-State Sensors, Actuators, and Microsystems Workshop Hilton Head Island, South Carolina, June 2-6, 2002