Scanning Micromirrors Fabricated by an SOI/SOI Wafer-Bonding Process

L. Zhou, J.M. Kahn, K.S.J. Pister
2006 Journal of microelectromechanical systems  
MEMS scanning micromirrors have been proposed to steer a modulated laser beam in order to establish secure optical links between rapidly moving platforms. An SOI/SOI wafer bonding process has been developed to fabricate scanning micromirrors using lateral actuation. The process is an extension of established SOI technology and can be used to fabricate stacked high-aspect-ratio structures with wellcontrolled thicknesses. Fabricated one-axis micromirrors scan up to 21.8° optically under a DC
more » ... tion voltage of 75.0 V, and have a resonant frequency of 3.6 kHz. Fabricated two-axis micromirrors scan up to 15.9° optically on the inner axis at 71.8 V and 13.2° on the outer axis at 71.2 V. The micromirrors are observed to be quite durable and resistant to shocks. Torsional beams with T-shaped cross sections are introduced to replace rectangular torsional beams in two-axis MEMS micromirrors, in order to reduce the cross-coupling between the two axial rotations. Fabricated bi-directional two-axis micromirrors scan up to ±7° on the outer-axis and from −3° to 7° on the inner-axis under DC actuation. I. Introduction The convergence of MEMS technology with communication and digital circuitry makes high-speed, low power, free-space communication links over distances up to several km possible. Free-space optical communication offers significant advantages over radio frequency (RF) communication, including secure links, wide bandwidth, small terminals, low power consumption, and freedom from frequency allocation issues. Thus, optical communication is an attractive option, provided that a line-of-sight propagation path is available. One of the key components in two-way free-space optical communication systems is a compact, reliable, and inexpensive laser beam steering device that provides a fast scanning capability for pointing, acquisition, tracking, and data communication. MEMS phased arrays composed of groups of relatively small micromirrors have been proposed to scan the laser beam [1][2]. They can be actuated through large deflection angles with substantially reduced response time. But they involve more complicated actuator design, i.e., requiring not only rotations around the two axes, but also vertical movements to compensate the phase differences between mirrors. Also active feedback controls over individual mirrors can be very complicated. Scanning micromirror based on MEMS technology have been introduced to steer modulated laser beams between moving unmanned aerial vehicles [3][4]. Scanning micromirrors have been developed for a wide range of other applications, such as optical crossbar switches [5], digital projectors [6], barcode readers [7], adaptive optics [8], and tunable lasers [9]. However, laser beam steering for freespace optical communication poses a somewhat unique set of requirements for micromirrors, such as large mirror sizes (~1 mm in diameter), rotation ability over two axes, large DC scan angles (±10° optical), fast switching ability (transition time between positions < 100 µs), low power consumption, and strong shock resistance (hundreds of g). While surface micromachining generally does not offer considerable scanning range for a large mirrors, MEMS micromirrors based on silicon-on-insulator (SOI) wafers and deep reactive ion etching (DRIE) technology overcome this problem by having an etched cavity under the micromirror. These fabrication technologies also offers attractive features such as excellent mirror flatness, high-aspect-ratio
doi:10.1109/jmems.2005.863736 fatcat:znhpbret4fe7zpp6ijbtlu7eru