THE MICROFABRICATION OF CAPACITIVE ULTRASONIC TRANSDUCERS

Surface-micromachined capacitive ultrasonic transducers, which are sui table for operation in both air and water, have been fabricated and te sted. Amorphous silicon is used as a sacrificial layer because of its good etching selectivity versus a nitride membrane, and improved cell- size control is obtained by lithographic definition of cavity walls. I n addition, appropriate feature designs based on two-dimensional (2-D) process simulations make it possible to achieve device cavity sealing with g-line optical lithography, Transmission experiments in both wat er and air are presented. A dynamic range in excess of 110 dB is obser ved in air at 2.3 MHz, In water, a single pair of transducers is able to operate from 2 to 15 MHz. When tuned, a 3.5-MHz tone burst results in a received signal with better than 60-dB signal-to-noise ratio (SNR ), The transducer behavior agrees with a theoretical understanding of transducer dynamics, The dynamic ranges achieved in this paper are the best reported to date for surface-micromachined capacitive ultrasonic transducers.