Characterization of ZnO polycrystalline films on silica glass by the LFB ultrasonic material characterization system

We evaluated eleven ZnO polycrystalline films with different thicknesses fabricated on silica glass substrates by DC sputtering and RF sputtering methods using a line-focus-beam ultrasonic material characterization (LFB-UMC) system. We measured fH (product of frequency f and film thickness H) dependences of leaky surface acoustic wave (LSAW) velocities for each ZnO-film specimen. The calculated LSAW velocities decreased from 3424.3 m/s for silica glass to 2671.6 m/s for Z-cut ZnO single crystal as fH increased. The measured LSAW velocities became lower than the calculated ones: decreases of 42 m/s for the DC-ZnO film and 27 m/s for the RF-ZnO film from a calculated value of 2672.1 m/s at fH = 1680 Hz·m. These velocity decreases were related to the FWHM in c-axis orientation, resulting in decreases in elastic constant c₄₄ ^E associated with ZnO polycrystalline film structure: about 6% for the DC-ZnO film and about 3% for the RF-ZnO film. We also demonstrated the capability of the system for evaluating film thickness distributions through LSAW velocity distributions. This ultrasonic method is useful for characterization of polycrystalline and epitaxial films.