Design of an Optical Microelectromechanical-System Microphone Toward Sub-15 dBA Noise Floor

date February 21, 2014
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Friday, February 21, 2014 4:00 p.m. ETC 4.150

Donghwan Kim
Department of Electrical and Computer Engineering
Microelectronics Research Center
The University of Texas at Austin

Microelectromechanical-system (MEMS) microphones with optical readout have been previously demonstrated. These microphones are similar to capacitive MEMS microphones, but the optical microphone can achieve potentially higher signal-to-noise ratio. The optical microphone measures sound pressure by detecting displacement of a compliant diaphragm. The displacement is measured using a diffraction-grating-based interferometer. Although the optical microphone requires a backplate to support the diffraction grating, the perforation density of the backplate can be much higher as compared to conventional capacitive microphones. Higher perforation density results in lower air damping and lower thermal-mechanical noise. A prototype sensor was fabricated at The Microelectronics Research Center of the University of Texas at Austin. The preliminary test demonstrates a 22.6 dBA noise floor, and shows that the flow resistance near the diffraction grating due to the squeeze-film damping effect between the diaphragm and the backplate is the dominant source of damping. The 22.6 dBA noise floor is approximately 6 dB better than commercially available capacitive MEMS microphones. System modeling suggests a better backplate design is possible which increases the SNR of the optical microphone further by an additional 8 dB, resulting in microphones with 10 dB higher SNR than the current state-of-the-art.