Micromachined microphones with diffraction-based optical interferometric readout

Abstract

A diffraction-based optical method for integrated interferometric detection of micromachined microphone diaphragm displacement is described. With multichip optoelectronics integration, this approach yields highly sensitive optical microphones in mm-cube volumes. Since the microphone sensitivity does not depend on capacitance, this method changes the paradigm for the backplate and gap structure design. As a result, one can use millimeter size diaphragms to achieve wide frequency response and low thermal mechanical noise levels characteristic of precision measurement microphones. Furthermore, the electrical port of the device, which is freed by optical detection, is used for electrostatic actuation of the microphone diaphragm to tune microphone sensitivity and to generate self-characterization signals. Prototype optical microphone structures have been fabricated using Sandia National Laboratories’ silicon based SwIFT-LiteTM process. Measurements on these diaphragms show an A-weighted diaphragm displacement noise of 2.4 pm and flat electrostatic response up to 20 kHz. These results indicate the feasibility of realizing measurement microphones with 1.5-mm-diam diaphragms, 15-dBA internal noise, and 40-kHz bandwidth. Application of the detection method in a bio-inspired directional microphone for hearing aids is also discussed. [Work partially supported by NIH Grant 5R01DC005762-03, Sensing and Processing for Hearing Aids.]