Jonathan J. Bernstein
C.S. Draper Lab., 555 Technology Sq., Cambridge, MA 02139
A micromachined monolithic condenser hydrophone less than 1 mm in diameter has been designed, fabricated, and tested on a silicon chip. This device is a small variable capacitor with spring-supported silicon electrode and a fixed, perforated counter electrode. In use as a hydrophone, the device is filled with a dielectric fluid and sealed in a package that contains an acoustic cavity and an acoustic inlet port. Sensitivity with these devices has been measured at -223 dB re: 1 V/(mu)Pa. Tests of the sensor chips in air show they have potential as wideband microphones. Sensitivity was measured from 300 Hz to 100 kHz, with peak sensitivity measured at -60 dB re: 1 V/Pa. The chips were fabricated by standard micromachining technology. High-concentration boron diffusions are used to create silicon electrode and springs that remain after selective etching. Electroforming is used to create a closely spaced counter electrode with fine perforations. A wide variety of fill fluids have been investigated for use in filling these hydrophones, including silicone oils, halogenated hydrocarbons, esters, and liquid crystals. Device and fluid characterization by current-voltage and capacitance-voltage measurements allows the device capacitance and spring constant to be extracted. Details of these calculations will be given.