A silicon condenser microphone with structured back plate and silicon nitride membrane

The fabrication process of a silicon condenser microphone and experimental results of the acoustic measurements are described. The microphone consists of two chips. One chip carries the 150 nm thick silicon nitride membrane, which has an area of 0.8 mm × 0.8 mm. The second chip contains the back electrode, the spacer and the contact pads of the microphone. In order to reduce the streaming resistances in the air gap, the back-electrode area is either structured with grooves by a plasma etching technique or with holes by an anisotropic etching technique. A frequency-independent sensitivity of 10 mV/Pa (open circuit, 1.8 mV/Pa measured) up to 30 kHz is obtained as a result of this structuring of the back-electrode area. Since the air-gap height is only 2 μm, the capacitance of the transducers ranges from 1 to 1.3 pF. The total size of the silicon microphone is 1.6 mm × 2 mm × 0.56 mm.