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 SiO ₂/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 Å for the ZnO and 5000 Å 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 a substrate temperature of 430°C, a zinc acetate solution of 0.05 M doped with 20 atomic percent of LiCl, a 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" wafer, have c-axis orientation ratio over 99%, and have resistivity of the order of 10⁹ ohm·m