Scanning micro-interferometer array with sub-picometer resolution for MEMS inspection

Micro scanning grating interferometer (muSGI) is a micromachined optical sensor to inspect MEMS. This paper presents the design, fabrication, implementation and analysis of an array of such muSGIs. The muSGIs are capable of static and dynamic displacement measurements. The muSGIs have phase sensitive micromachined gratings which are actuated by electrostatic force. The muSGIs are fabricated on SOI wafers. The fabricated tunable gratings exhibit the first resonance mode at 48 kHz and a damping ratio of ~0.05. The gratings show sufficient (~400 nm) displacement range for tuning it to high sensitivity position, achieved with 30 V operating voltage range. To actively tune the gratings a novel control algorithm is implemented real-time in a field-programmable gate array (FPGA). It adjusts the position of each grating with respect to corresponding sample, to achieve high sensitivity. Active control scheme is modeled using MATLAB and the performance results show good agreement with the experiments. The control algorithm is successfully used to characterize the tunable gratings and to demonstrate parallel operation of the muSGIs. Experimental results demonstrate a vertical resolution of 0.2 pm over 1 Hz bandwidth.