Evaluation of Piezoelectric Ta_2O_5 Thin Films Deposited on Sapphire Substrates

摘要

X-axis-oriented tantalum pentoxide (Ta_2O_5) piezoelectric thin films were deposited on sapphire (Al_2O_3) substrates using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave (R-SAW) propagation properties of the thin films were evaluated. For a Ta_2O_5 thin film deposited on the R-Al_2O_3 substrate plane, a similar preferential (200) orientation to that for a Ta_2O_5 thin film deposited on a SiO_2 glass substrate was obtained. For the first mode of the R-SAW on the R-plane Al_2O_3 sample, a coupling factor of 1.78% and a phase velocity of 4,895 m/s were obtained for a normalized film thickness of 0.225. For the c-Al_2O_3 substrate plane, the possibility of the epitaxial growth of a hexagonal Ta_2O_5 thin film with (203) orientation was shown. Furthermore, for the R-Al_2O_3 substrate plane, the possibility of the homoepitaxial growth of an orthorhombic Ta_2O_5 thin film with (201) orientation by using an oxide Ta thin film (TaO_x) as a buffer layer was also shown. However, no increase in coupling factor and no major improvement in propagation loss were observed upon these crystallizations because the hexagonal Ta_2O_5 has no piezoelectricity and the Ta_2O_5 thin film deposited on the TaO_x buffer layer had polycrystalline structure.