Double-chip condenser microphone for rigid backplate using DRIE and wafer bonding technology

A novel silicon condenser microphone with a rigid backplate has been proposed and fabricated. The backplate is fabricated on a separated wafer from the membrane to enhance acoustic characteristics, and they are combined together by gold–tin (Au/Sn) eutectic solder bonding. On a 2.5 mm × 2.5 mm, 0.5 μm thick low stress silicon nitride membrane, 2 mm × 2 mm Au/Ni/Cr membrane electrode is deposited including 3 μm thick Au/Sn layer. A 2 mm × 2 mm, 150 μm thick single crystal silicon rigid backplate has 1.8 mm × 1.8 mm backplate electrode, and an air gap, which is fabricated by bulk micromachining, and a silicon deep reactive ion etching. Slots and 50–60 μm radius circular acoustic holes to reduce air damping are also formed in the backplate chip. The fabricated microphone sensitivity is 39.8 μV Pa−1 (−88 dB re. 1 V Pa−1) at 1 kHz using 28 V polarization voltage. The microphone shows flat frequency response within 1 dB from 20 Hz at least up to 5 kHz.