Broadband self-calibrating micromachined microphones with integrated optical displacement detection

Abstract

An optical displacement detection method for micromachined microphones is described and experimental results are presented. The microphone membrane is fabricated on a transparent substrate and the back electrode is patterned in the form of diffraction fingers. This structure forms a phase sensitive diffraction grating, providing the displacement sensitivity of an optical interferometer. The diffraction fingers are also used for electrostatic actuation, providing sensitivity adjustment and self-calibration capabilities. Optically semitransparent coatings are also employed to create Fabry–Perot resonant cavities that enhance the optical detection sensitivity by an order of 10 dB. The high electrical sensitivity provided by optical displacement detection relaxes requirements on mechanical sensitivity, and small microphone membranes on the order of 200 μm with vacuum-sealed and air-sealed cavities are used to fabricate microphones with a flat response from dc to over 200 kHz. The optical detection and electrostatic actuation capabilities are demonstrated on fully integrated devices with aluminum microphone membranes micromachined on quartz substrates and bonded to microfabricated silicon photodiodes. [Work supported by DARPA.]