MEMS Capacitive Microphones: Acoustical, Electrical, and Hidden Thermal-Related Issues

This paper reports on the design and implementation of high performance MEMS microphones. The main parameters defining microphone performance, i.e., sensitivity, acoustic overload level, and signal-to-noise ratio, have been extracted from an acoustical-mechanical-electrical model of the MEMS sensor and designing a proper architecture of the readout circuit. Moreover, some subtle problems linked to the integration of the MEMS sensor with its readout circuit in the same package have been highlighted. These hidden issues are the degradation of the power supply rejection at low frequencies, the contamination from radio frequency interference, and the presence of a spurious tone in the audio band when digital microphones are operated in stereo configuration. They can seriously impact the performance of advanced analog and digital MEMS microphones if not properly tackled and solved. The MEMS are made by a polysilicon membrane with serpentine spring structures placed at the corners of the diaphragm, whereas the readout application-specified integrated circuits are realized in a 0.5-mu m single-poly, four metal layers, N-well CMOS process with MIM capacitors.