Resonance Effects of Bilayered Piezoelectric Films Used for Bulk Acoustic Wave Sensors

The resonance vibrations of acoustic sensors with two layers of (1120) textured hexagonal piezoelectric films are studied. When the acoustic and electric fields satisfy a special match condition, i.e. the phase variation of thickness shear mode (TSM) at each film equals π, both piezoelectric layers with opposite polarization directions reduce the first TSM and generate the second TSM with higher frequency and a higher quality factor. The excited second TSM can increase the product of the operating frequency and the quality factor, which is useful for improving the mass sensitivity and resolution of acoustic sensors. Additionally, both of the piezoelectric films have larger thickness and decrease the risk of mechanical damage in device production processes.