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Sensitivity improvement of no-back-plate MEMS microphone using polysilicon trench-refilled process

This study demonstrates the structural design and fabrication process of the no-back-plate MEMS condenser microphone for sensitivity improvement. The no-back-plate microphone, as in Fig.1, consists of the planar interdigitated sensing electrodes, deformable diaphragm, and back chamber. Thus, the in-use pull-in and in-process stiction between diaphragm and back-plate are prevented. Merits of this study are: (1) the HAR (high-aspect-ratio, 13 in this study) sensing electrodes are implemented to provide the required sensing capacitance, (2) the flat diaphragm with smaller thickness is achieved to obtain the larger out-of-plane deformation, and (3) the polysilicon trench-refilled process is utilized to define the different thickness of the diaphragm and sensing electrodes. The proposed microphone with 760μm diameter diaphragm (ROC=91mm) and 565fF sensing capacitance is realized and tested. The acoustic sensitivity is −45.5dB (ref. 1V/1Pa) at 1 kHz, the +3dB bandwidth is form 562∼2,200 Hz, and the SNR is 45dB.