Li₂CO₃-doped ZnO films prepared by RF magnetron sputtering technique for acoustic device application

It is necessary for zinc oxide film to have high resistivity for piezoelectric applications. The ZnO films have been deposited by RF sputtering deposition system using Li-doped ZnO ceramics as the target and high oxygen ratio (100% oxygen) for high film resistivity. The maximum resistivity of ZnO film measured was 10⁸ Ω cm in our experiments, and stronger intensity of c-axis orientation was grown at 50% oxygen ratio. Postdeposition annealing ZnO films in vacuum circumstance were found to relieve stress, avoid the electrode oxidation and increase resistivity one order. The preferred deposition conditions and annealing condition were obtained for piezoelectric application. Then, an over-mode resonator was made and showed a large return loss of 42 dB at the center frequency of about 2 GHz after annealing for 1 h in vacuum circumstance at 400 °C.