CMOS-MEMS Based Optical Electrostatic Phase Shifter Array With Low Driving Voltage and High Fill Factor

Abstract

This work develops a novel 4 x 4 optical phase shifting micromirror array that achieves a lambda/4 vertical displacement and makes the mirror peak-to-valley deformation within lambda/10 (514 nm light source). Each individual micromirror pixel is controllable and driven by an electrostatic parallel plate actuator. The mirror reflective surface is an aluminum layer with a high optical reflectivity exceeding 90%. This device achieves a high fill factor of more than 90% without an additional flip-chip bonding process due to the parallel plate actuator and the hidden suspension beam structures. The phase shifter array is fabricated using the Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 mu m 2p4m CMOS process and post-CMOS process. An in-house post-process is utilized to reserve a 40 mu m thick bulk-silicon under the 200 mu m x 200 mu m mirror. This eliminates mirror deformation from residual stress after the device is released. The micromirror demonstrates a vertical displacement of lambda/4 at only 3 V and the resonant frequency is 3.6 kHz. Industry can use this phase-shifting micromirror array as a spatial light modulator in holographic data storage systems in the future.