Towards a sub 15-dBA optical micromachined microphone

Abstract

Micromachined microphones with grating-based optical-interferometric readout have been demonstrated previously. These microphones are similar in construction to bottom-inlet capacitive microelectromechanical-system (MEMS) microphones, with the exception that optoelectronic emitters and detectors are placed inside the microphone's front or back cavity. A potential advantage of optical microphones in designing for low noise level is the use of highly-perforated microphone backplates to enable low-damping and low thermal-mechanical noise levels. This work presents an experimental study of a microphone diaphragm and backplate designed for optical readout and low thermal-mechanical noise. The backplate is 1 mm  1 mm and is fabricated in a 2-lm-thick epitaxial silicon layer of a silicon-on-insulator wafer and contains a diffraction grating with 4-lm pitch etched at the center. The presented system has a measured thermal-mechanical noise level equal to 22.6 dBA. Through measurement of the electrostatic frequency response and measured noise spectra, a device model for the microphone system is verified. The model is in-turn used to identify design paths towards MEMS microphones with sub 15-dBA noise floors.

A Grating Interferometric Acoustic Sensor Based on a Flexible Polymer Diaphragm

“…The cavity gap usually has a machining error during the fabrication of the sensor [ 14 , 19 , 32 ]. Thus, the initial operating point is uncertain.…”

“…Therefore, a thinner metal reflector should be used while ensuring that the diaphragm has a certain optical reflectivity. The cavity gap usually has a machining error during the fabrication of the sensor [14,19,32]. Thus, the initial operating point is uncertain.…”

“…Considering the position and optical utilization, the optical intensity of the first diffraction spot was chosen as the detection signal. The optical interference curve of the first intensity analyzed based on Equation ( 2 The cavity gap usually has a machining error during the fabrication of the sensor [14,19,32]. Thus, the initial operating point is uncertain.…”

“…In contrast, optical interferometric acoustic pressure detection methods are immune to the scaling laws as electrical methods [ 7 ]. Optical interferometric acoustic sensors are mainly based on Sagnac interferometer (SI) [ 8 ], Fabry-Perot interferometer (FPI) [ 9 , 10 , 11 ], laser self-mixing interferometer (LSMI) [ 12 ], and grating interferometer (GI) [ 13 , 14 ]. The common implementation of the SI and the FPI requires optical fibers, which makes it difficult to integrate [ 15 ].…”

On the theoretical maximum achievable signal-to-noise ratio (SNR) of piezoelectric microphones

“…A complete description of all packaging-induced noise sources is presented in several references, including [17–19]. The A-weighted vent noise contribution is completely determined by C ad , C ab , and f 0 .…”