Effects of two-step deposition and thermal treatment on the frequency response characteristics of ZnO SAW devices

Polycrystalline ZnO thin films are deposited on SiO₂/Si(100) substrate using RF magnetron sputtering. The film deposition performed in this research is composed of the following two procedures; 1^st-deposition for 30 min without oxygen at 100 W and 2^nd-deposition with oxygen in the range O₂/(Ar+O₂)=10∼50%. Deposited ZnO films reveal a strongly c-axis preferred-orientation (the corresponding texture coefficient ∼100%) as well as a high resistivity (>10⁷Ωcm). It is also observed that the crystallite size of ZnO is noticeably increased by thermal-annealing. In addition, surface acoustic wave (SAW) devices are also fabricated by using a lift-off method, with the configuration of IDT/ZnO/SiO₂/Si(100). Frequency response characteristics (including S₂₁) of fabricated SAW devices are measured and device parameters including insertion losses and side-lobe rejection level estimated from frequency response are compared.