Mass sensitivity of thin film bulk acoustic resonator sensors based on polar c-axis tilted zinc oxide and aluminum nitride thin film

The mass sensitivity of thin film bulk acoustic wave resonator (FBAR) sensors using ZnO and AlN thin films with tilted polar c-axis has been theoretically investigated. The tilted c-axis orientation induces normal plane and in-plane polarizations, which leads to the coexistence of thickness longitudinal mode and thickness shear mode in the resonators. The equation for predicting electric impedance of FBARs with a mass loading layer was derived from the basic piezoelectric constitutive equations. The mass sensitivity of ZnO and AlN dual mode resonators was found by calculating the resonant frequency shifts of the thickness shear mode and thickness longitudinal mode due to thin film mass loading. In the calculation, ZnO and AlN thin film has a c-axis tilt angle from 0°–90°, 2 μm thickness, and 300×300 μm² electrode area; four different materials (Al, SiO₂, Au, and Pt) were considered as the thin film mass loading. It was found that both longitudinal resonance frequency and shear resonance frequency for different c-axis angle have a significant shift due to the mass loading; the mass sensitivities, defined as Δf/(fΔm), of the longitudinal and shear mode for this four mass loading materials are very close, and do not change much with c-axis tilt angle with a value rang around -900 cm²/g for ZnO FBARs and -1550 cm²/g for AlN FBARs. The results can be used in the design and applications of ultrahigh sensitive ZnO or AlN FBAR mass sensors.