Micromachined piezoelectric ultrasonic transducers on dome-shaped-diaphragm in silicon substrate

We have successfully fabricated piezoelectric ultrasonic airborne transducers built on 1.5 μm thick dome-shaped silicon-nitride diaphragm (2,000 μm in radius, with a circular clamped boundary on a silicon substrate) with electrodes and a piezoelectric ZnO film. The key idea of fabricating such a large 3-D diaphragm ultrasonic transducer is in producing a large spherical etch front (2 mm in radius) in a 2 mm thick silicon substrate by an isotropic silicon etching with a 75 μm thick tape as an etch mask. Also, we use a shadow mask during Al thermal evaporation in order to deposit (with a good step coverage) patterned electrodes over the sharp edge in the dome diaphragm. Flat-diaphragm transducers are also fabricated to demonstrate the effectiveness of a dome-shaped diaphragm for an ultrasonic transducer. Compared to the flat-diaphragm transducer, the dome transducer generates much higher sound output in 15-200 kHz range and this without distortion (unlike the flat-wrinkled-diaphragm transducer). The sound-pressure level at 50 mm away from the dome transducer is measured to be around 0.5 Pa at 145 kHz. The linearity of the dome transducer's response is also measured to be very good, as expected