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