Annealing effect on the generation of dual mode acoustic waves in inclined ZnO films.

ZnO films with different inclined angles on steel substrates were sputter-deposited by changing the substrate tilt angle during deposition and then used to fabricate ZnO film ultrasonic transducers. The ultrasonic performance of those devices was characterized using a standard pulse-echo method. A dual mode wave with both longitudinal and shear wave components was detected from the ZnO device at 0° inclined angle. At a columnar inclined angle of 31°, longitudinal wave excitation was suppressed with a nearly pure shear wave detected. Post annealing of the ZnO film improved the crystallinity and decreased the film stress. The dispersion of the received echoes was observed when the grain sizes of ZnO films were increased after annealing. The frequency components of the waveforms were analyzed and identified using a short time Fourier transform. Post-annealing of the ZnO films changed the primary frequency and enhanced the propagation of the relative high-frequency acoustic wave.