An SOI-MEMS Acoustic Sensor Based on Optical Grating Interferometer

Capacitive microelectromechanical system (MEMS) microphone in a small package encounters performance limits associated with damping in the air gap between diaphragm and backplate. In this work, we explored an optical-readout technique by fabrication of an optical interferometric acoustic sensor. The designed sensor chip forms a grating interferometer by a diffraction grating integrated backplate and a pressure-sensitive diaphragm. A scalar diffraction theory and a finite-element method have been applied to optimize the sensor parameters. The sensor device was fabricated on a silicon-on-insulator (SOI) wafer using complementary metal-oxide-semiconductor (CMOS) compatible processes. The performance of the sensor was studied using an acoustic testing system. The sensor demonstrates a good linear relationship with applied sound pressure. The frequency response is relatively consistent with a commercial reference microphone for audible range. The signal-to-noise ratio (SNR) of the sensor is about 43 dB at a frequency of 1 kHz. The proposed design shows good potential for high-performance acoustic sensors.