Towards a sub 15-dBA optical micromachined microphone.

Area of Science:

• Acoustics
• Microelectromechanical Systems (MEMS)
• Optical Metrology

Background:

• Optical microphones offer potential advantages for low-noise applications.
• Grating-based optical-interferometric readout is a key technology for these devices.
• Traditional microelectromechanical-system (MEMS) microphones face challenges in achieving ultra-low noise floors.

Purpose of the Study:

• To experimentally investigate a microphone diaphragm and backplate optimized for optical readout.
• To evaluate the thermal-mechanical noise performance of the designed optical microphone.
• To verify a device model for optical microphone systems and identify pathways for noise reduction.

Main Methods:

• Fabrication of a 1mm x 1mm backplate from a silicon-on-insulator wafer with a central diffraction grating.
• Integration of optoelectronic components within the microphone cavities.
• Measurement of thermal-mechanical noise, electrostatic frequency response, and noise spectra.

Main Results:

• The fabricated optical microphone system achieved a measured thermal-mechanical noise level of 22.6 dBA.
• A device model was verified using electrostatic frequency response and noise spectra measurements.
• The verified model suggests design improvements for achieving sub-15 dBA noise floors in MEMS microphones.

Conclusions:

• The developed optical microphone design demonstrates a viable approach to reducing thermal-mechanical noise.
• The experimental results validate the device model for optical microphone systems.
• Further optimization based on the model holds promise for next-generation ultra-low-noise MEMS microphones.