Mass sensitivity of thin film bulk acoustic wave resonator based on c-axis tilted ZnO film

In this paper, the mass sensitivity of dual mode thin film bulk acoustic wave resonator (FBAR) based on ZnO film 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 the longitudinal mode and shear mode in the resonator. Equation for predicting electric impedance of dual mode ZnO FBAR with a mass loading layer was derived from the basic piezoelectric constitutive equations. The mass sensitivity of dual mode ZnO resonator was achieved by calculation of resonant frequency shifts of thickness shear mode and longitudinal mode caused by a thin mass loading layer. In the simulation, ZnO thin film has a c-axis tilt angle from 0°-90°, 2μm thickness and 300μm by 300μm electrode area; Al, SiO₂, Au and Pt thin films were considered as the mass loading layers. It was found that both thickness longitudinal resonance frequency and shear resonance frequency for different c-axis tilt angle have a large shift due to the mass loading; the mass sensitivities of 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. The simulation results can be used for design and application of ZnO FBARs.