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

This work develops a novel 4 × 4 optical phase shifting micromirror array that achieves a λ/4 vertical displacement and makes the mirror peak-to-valley deformation within λ/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 μm 2p4m CMOS process and post-CMOS process. An in-house post-process is utilized to reserve a 40 μm thick bulk-silicon under the 200 μm X 200 μm mirror. This eliminates mirror deformation from residual stress after the device is released. The micromirror demonstrates a vertical displacement of λ/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.