The microfabrication of capacitive ultrasonic transducers

The successful fabrication of surface micromachined ultrasonic transducers is reported. In a key step of the microfabrication, amorphous silicon is used as a sacrificial layer to form sealed nitride cavities. The process is fully CMOS compatible and allows for improved geometric control compared to previously reported work. Transmission experiments in both water and air are presented. A dynamic range in excess of 110dB is observed in air at 2.3MHz. In water, a single pair of transducers is able to operate from 2MHz to 20MHz. When tuned, a 3MHz tone burst results in a received signal with better than 80dB signal to noise ratio. The transducer behavior agrees with theoretical understanding of transducer dynamics.