Two-step deposition process of piezoelectric ZnO film and its application for film bulk acoustic resonators

In this article, a two-step deposition technique of piezoelectric zinc oxide (ZnO) film formation using radio-frequency (rf) sputtering and its successful applications for film bulk acoustic resonators (FBAR) are presented. Several critical sputtering process parameters such as deposition pressure, rf power, and O2 concentration were investigated to understand their impacts on the resulting crystal structures and surface morphologies of the ZnO films. The ZnO films formed by the two-step deposition have shown the growth characteristic of the strongly preferred orientation toward c axis. The FBAR with the ZnO films showed a large return loss of ∼50 dB at the center frequency of 1.49 GHz. It was also found that the impedance matching of the FBAR could be easily achieved by simply controlling the resonance area of the resonator.