Acoustical-Thermal Noise in a Capacitive MEMS Microphone

Since the size of silicon capacitive micro-electro-mechanical system (MEMS) microphones is getting smaller, the mechanical-thermal noise especially acoustical-thermal noise from the MEMS transducer becomes important taking a significant portion of the total noise. Acoustical-thermal noise caused by the flow resistance of microphone components, including the sound port on the package, perforation holes on the backplate, and the vent holes on the diaphragm, was investigated with lumped element models in this paper. The size of the ventilation holes on the backplate and the air-gap distance between the diaphragm and the backplate were designed to minimize the flow resistance. To estimate the acoustical-thermal noise sources, Johnson-Nyquist's relation was used for the flow resistance, and then the signal-to-noise ratio (SNR) was predicted only with acoustical-thermal noise. A capacitive MEMS microphone which has dual backplates to take advantage of the dual transductions has been compared with the single backplate microphone in terms of SNR. Although the advantage of dual transductions leads to SNR increases by $\sim 3$ dB, the prediction shows that the SNR of the double backplate microphones turns out to be merely 7% higher than the single backplate due to the viscous flow resistance and squeeze film damping caused by the additional backplate.