Deposition of zinc oxide thin films for application in bulk acoustic wave resonator

High quality piezoelectric zinc oxide (ZnO) thin films were deposited on (1 0 0)-oriented silicon substrate by reactive rf magnetron sputtering for bulk acoustic wave resonator. In order to improve the ZnO thin films quality, structural and electrical characteristics have been compared before and after annealing in helium (He) by X-ray diffraction and reflection coefficient S11 measurements. Scanning electron microscopy (SEM) was used to study the crystallographic structure. A previous study has shown that a substrate temperature of 100 °C, a distance between the target and the substrate of 70 mm, and a pressure of 3.35 × 10−3 Torr in argon and oxygen mixed gas atmosphere, are the optimum conditions to sputter ZnO thin films with good homogeneity and a high degree of crystallinity. These films exhibit an electrical resistivity higher than 1010 Ω cm and an energy band gap of 3.3 eV at room temperature. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented with a full width at half maximum (FWHM) below 0.5°. The decrease of the FWHM after annealing treatment has shown the crystal quality improvement. A growth of c-axis (0 0 2)-oriented ZnO films allows predominant longitudinal wave propagation. Bulk acoustic wave (BAW) resonators have been fabricated by stacking different layers of Al/ZnO/Pt on a silicon substrate which could be used for the fabrication of humidity sensors based on the quartz microbalance principle.