Fig. 4 shows a photograph of a typical wafer after fabrication of the ultrasonic sensors. We were able to obtain these sensors with very high yield, with no cracking or peeling of the PZT films from the epitaxial SrRuO3/Pt/-Al2O3/Si layer structure. In contrast, many cracks appeared in the polycrystalline PZT thin films on the Pt/Ti/SiO2/Si, even though the sol–gel process was the same as that used for the epitaxial PZT film. Therefore,polycrystalline films could not be used for the sensing elements in this process. The wafers were diced to a few mm square and each die was mounted on a ceramic package. The top and bottom electrodes were connected to the package pads by Au wire bonding. The package output terminals were connected to a coaxial cable. In order to achieve electrical insulation, the
Au wires were coated in a silicon resin. The output terminals
of the package were also coated in the silicon resin and then
covered by a plastic film. The mechanical resonance frequency
of the thickness mode was estimated to be 491 MHz [14] by
using the density, elastic modulus and Poisson’s ratio values of
the PZT ceramic [15]. The resonance frequency is much higher
than that of commercially procurable ultrasonic transducers. In
the course of these measurements we did not attempt to achieve
impedance matching between the sensing element and the rest of
the system. Fig. 5 shows a photograph of the fabricated ultrasonic
sensor.