Development of ZnO thin films for SAW devices by the ultrasonic spray pyrolysis technique

ZnO thin films for use in SAW filters have been prepared on SiO₂/Si substrates by the ultrasonic spray pyrolysis technique. Thickness of the films has been determined through a SAW propagation simulation on the substrate as 8000 angstroms for the ZnO and 5000 angstroms for the SiO₂ at which the coupling factor and the Rayleigh wave velocity are 0.9% and 2900 m/sec, respectively. For use in SAW filters, the ZnO thin film should have high resistivity and strong piezoelectric activity, i.e., good c-axis orientation. For simultaneous achievement of the two desired properties, we have rigorously investigated the effects of various deposition parameters through experiments. The optimal deposition conditions have been determined as the substrate temperature of 430 °C, the zinc acetate solution of 0.05 M doped with 20 atomic percent of LiCl, the flow rate of 2.5 ml/min., reactor pressure of 3 Torr, and annealing and cooling down in oxygen environment. Deposited ZnO thin films show good thickness uniformity over a 4 inches wafer, have c-axis orientation ratio over 99%, and have resistivity in the order of 10⁹ ohm·m.