Wide bandwidth silicon nitride membrane microphones

Small, low cost microphones with high sensitivity at frequencies greater than 20 KHz are desired for applications such as ultrasonic imaging and communication links. To minimize stray capacitance between the microphone and its amplifier circuit, process compatibility between the microphone and on-chip circuitry is also desired to facilitate integration. In this work, we have demonstrated micromachined microphones packaged with hybrid JFET amplifier circuitry with frequency response extending to 100 KHz, and voltage sensitivity of similar to 2.0 mV/Pa from 100Hz to 10 KHz, and 16.5 mV/Pa at 30 KHz with a bias voltage of 8.0 V. The microphones are fabricated with membranes and fixed backplates made of low temperature plasma-enhanced chemical vapor deposited (PECVD) silicon nitride. Because the maximum temperature of the fabrication process is 300 degrees C, microphones may be built on silicon wafers from any commercial CMOS foundry without affecting transistor characteristics, allowing integration with sophisticated amplifier circuitry. Low stress silicon nitride deposition was used to produce membranes up to 2.0 mm diameter and 0.5 mu m thickness with +/- 0.10 mu m flatness. The excellent planarity of both the diaphragm and the backplate, combined with a narrow sense gap (similar to 2 mu m) results in high output capacitance (up to 6.0 pF). The high output capacitance results in noise spectral density which is approximately 3x lower than silicon diaphragm microphones previously fabricated by the authors. Diaphragms with corrugations were fabricated to relieve tensile stress, to increase deflection per unit pressure, and to increase deflection linearity with pressure.