Design and implementation of differential MEMS microphones using the two polysilicon processes for SNR enhancement

This study presents the design, fabrication and testing of the capacitive-type MEMS microphone with differential sensing electrodes to improve the sensitivity and signal-to-noise ratio (SNR). The microphone is designed and implemented based on an existing trench-refilled MOSBE process platform with two polysilicon structure layers, one Si3N4 electrical isolation layer and two sacrificial SiO2 layers. The microphone consists of top and bottom diaphragms and backplates. The top diaphragm and bottom backplate form a sensing electrode pair, and the bottom diaphragm and top backplate form the second sensing electrode pair. Moreover, the Si3N4 layer is exploited to partition the structures into three different electrical regions. Thus, the differential sensing MEMS microphone is achieved by using only two polysilicon structure layers. Various auxiliary components such as the central post and the U-shaped springs are also developed to realize the differential microphone. Measurements show that the typical fabricated microphone with a footprint of 800 μm diameter has the single-ended sensitivities of −45.8 and −47.2 dB (ref: 1 V/1 Pa), and the SNR is over 52 dB. Moreover, the ±3 dB bandwidth of the microphone ranges from 50 Hz-22 kHz. In summary, the presented microphone has the differential sensitivity of −40.5 dB (ref: 1 V/1 Pa) and the SNR of over 57.8 dB.