Four-layer polarity inverted ZnO piezoelectric thin film for bulk acoustic wave resonator

Bulk acoustic wave (BAW) resonators are attractive for application in molecular sensors. We propose a new sensor principle using acoustic energy confinement in a polarization inverted structure. For example, the fourth overtone mode operation in the four-polarization inverted resonator apparently acts as the resonator apparently has four times large acoustic impedance. This allows sensors to operate in a liquid environment. In this study, BAW resonators with four-layer polarity-inverted ZnO thin films were fabricated using RF magnetron sputtering by controlling the substrate position and substrate temperature. We have found that Zn-polar ZnO film is grown in the case that a substrate is set inside of the discharge with substrate heating (240°C), whereas O-polar ZnO film is grown in the case that a substrate is set out side of the discharge with substrate heating (250°C). The presence or absence of the energic negative ion bombardment at substrate probably has an effect on the change of polarization. In the oxide sputtering process, a large flux of negative ions generates on the oxide sputtering target. The sign of polarity the film was determined from the sign of the piezoelectric response using an oscilloscope when compressive stress was applied. The fourth-order mode thickness extensional mode resonance at the 1.6 GHz was clearly observed using a network analyzer.