Fabrication of a silicon micromachined capacitive microphone using a dry-etch process

A silicon micromachined capacitive microphone has been fabricated on (100) silicon with PECVD silicon nitride as the diaphragm and backplate material. Amorphous silicon is used as the sacrificial layer and a dry-etch release method is used to free the diaphragm from the backplate. The dry-etch release method eliminates problems associated with the diaphragm and backplate sticking together, which often occurs due to capillary force of the rinse liquid with a wet-etch process. The fabricated microphone has a measured capacitance of 9.5 pF and an open-circuit sensitivity of 7 mV Pa-1 at a bias voltage of 6 V. This bias voltage is about 60% of the maximum allowed voltage that will electrostatically pull the diaphragm and backplate together. The measured frequency response of the microphone is flat within 4 dB from 100 Hz to 10 kHz and shows a gradual increase at higher frequencies.