A broadband, capacitive, surface-micromachined, omnidirectional microphone with more than 200kHz bandwidth

A surface micromachined microphone is presented with 230 kHz bandwidth. The structure uses a 2.25 mu m thick, 315 mu m radius polysilicon diaphragm suspended above an 11 mu m gap to form a variable parallel-plate capacitance. The back cavity of the microphone consists of the 11 mu m thick air volume immediately behind the moving diaphragm and also an extended lateral cavity with a radius of 504 mu m. The dynamic frequency response of the sensor in response to electrostatic signals is presented using laser Doppler vibrometry and indicates a system compliance of 0.4 nm/Pa in the flat-band of the response. The sensor is configured for acoustic signal detection using a charge amplifier, and signal-to-noise ratio measurements and simulations are presented. A resolution of 0.80 mPa/ root Hz (32 dB sound pressure level in a 1 Hz bin) is achieved in the flat-band portion of the response extending from 10 kHz to 230 kHz. The proposed sensor design is motivated by defense and intelligence gathering applications that require broadband, airborne signal detection. (C) 2014 Acoustical Society of America.