IMPROVEMENT OF THE PERFORMANCE OF MICROPHONES WITH A SILICON-NITRIDE DIAPHRAGM AND BACKPLATE

The performance of a single-wafer fabricated silicon condenser microph one has been improved by increasing the stress and the acoustic hole d ensity of the backplate and by decreasing the diaphragm thickness. The best microphones show a sensitivity of 5.0 mV Pa-1, which corresponds to an open-circuit sensitivity of 10 mV Pa-1 for a microphone capacit ance of 6.6 pF. The measured frequency response is flat within +/-2 dB from 100 Hz to 14 kHz, which is better than the requirements for a he aring-aid microphone. The operating voltage of these microphones is on ly 5.0 V, which is about 60% of the collapse voltage. The measured noi se level of the microphones is 30 dBA SPL, which is approximately as l ow as required for a hearing-aid microphone (<29.5 dBA SPL).