c-Axis Zig-Zag ZnO film ultrasonic transducers for designing longitudinal and shear wave resonant frequencies and modes

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO₂ low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason´s equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.