High-aspect-ratio Si vertical micromirror arrays for optical switching

Arrays of bulk-micromachined high-aspect-ratio vertical Si mirrors were designed, fabricated, and characterized for optical switching applications. These 50-μm-tall vertical mirrors were fabricated by the deep-etch shallow-diffusion process. The released Si mirrors have smooth sidewalls with 5-nm surface roughness. An electrostatic comb drive applied to actuate the mirrors which were supported by folded or serpentine beams. For 800-μm-long, 3-μm-wide, and 50-μm-thick folded suspension beams, a lateral mirror movement of 34 μm was achieved by a driving voltage of 30 V. Resonant frequency of 987 Hz was obtained for similar devices at atmospheric pressure. There are tradeoffs between mirror displacement and resonant frequency. The mirror displacement increases with beam length (BL) and decreases with beam width (BW). However, the resonant frequency increases with BW, but decreases with BL, which is in agreement with the theoretical calculations. These vertical Si micromirrors were coated with Au to increase the reflectivity. Magnetron sputtering of Au at low pressure was applied to achieve uniform and smooth sidewall coverage. Optical measurement on 50-μm-tall Au-coated Si mirrors showed a reflectivity of 85% (-0.71 dB). Dynamic response of the Si micromirror switches was measured, and the switching function was successfully demonstrated at a frequency up to 1.2 KHz