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Fabrication of ultrasonic sensor using silicon membrane

Ultrasonic sensor was fabricated from silicon membrane of about 1.5 /spl mu/m thick by using ion implantation and anisotropic etching technique. This thin membrane is strong and the 4 mm square membrane is not broken by the atmospheric pressure (760 Torr). The sensor has a condenser-microphone structure with glass balls of about 10 /spl mu/m diameter as spacers. The sensitivity of the sensor having a 4 mm square membrane for the 10 kHz ultrasonic wave is 67 /spl mu/V/Pa when the applied voltage to the sensor is 18 V. The frequency dependence of the sensitivity is flat in the frequency region over 1 kHz. Angular dependence of the sensitivity for 40 kHz ultrasonic wave is close to that of the line antenna and the sensitivity becomes sharp as the membrane becomes large. The noise from the sensor can be effectively reduced by using MOSFET on the SOS substrate, which is used as an opposite electrode.