Micromachined microphones with integrated optical nanoscale displacement sensors

Micromachined microphones with diffraction-based optical displacement detection are presented. A compliant membrane is made part of a phase-sensitive diffraction grating, and the deflection resulting from external acoustic pressure alters the intensities of the diffracted orders which are monitored with integrated photodiodes. The scheme provides the displacement sensitivity of a Michelson interferometer and can be integrated without beam splitters or critical alignment problems into volumes on the order of 1 mm3. Preliminary characterization with ultrasonic sensors shows a displacement resolution of 1×10−4 Å/Hz1/2 near 100 kHz with 60 μW of laser power incident on the photodetector. Current research is aimed at achieving similar displacement resolution in the audio frequency range while demonstrating the potential for high-fidelity miniature microphone arrays for hearing and measurement applications. The approach is implemented and characterized using microphone membranes with integrated diffraction grating bottom electrodes fabricated on silicon using Sandia National Laboratories’ dedicated processing platform. Preliminary results on nonoptimized implementations show a flat frequency response to 15 kHz with internal noise levels below 40 dBA. [The authors would like to thank NIH and DARPA for supporting this research.]