Micromachined ultrasonic transducers using silicon nitride membrane fabricated in PECVD technology

Capacitive ultrasonic transducers, consisting of thin membranes scratched over a conducting backplate, offer many advantages compared to piezoelectric transducers, such as low impedance mismatch, low energy density and low cost. Recent developments in microfabrication technology have spurred novel design for transducers in the ultrasonic range both for air and water applications. In this paper we report the fabrication process of transducers using PECVD deposition technology. With this process it is possible to change the stress from compressive to tensile, varying the temperature and time parameters. The resulting film is of very good quality and experiences irreversible modification after annealing process which prevents changes at lower temperature. Using this technology we succeeded in fabricating transducers with 3.8 MHz resonant frequency in air.