Characteristics of dual mode AlN thin film bulk acoustic wave resonators

Thin film bulk acoustic wave resonators (FBAR) using piezoelectric AlN thin films have attracted extensive research activities in the past few years. Highly c-axis oriented AlN thin films are particularly investigated for resonators operating at the fundamental thickness longitudinal mode. Depending on the processing conditions, tilted polarization (c-axis off the normal direction to the substrate surface) is often found for the as-deposited AlN thin films, which may leads to the coexistence of thickness longitudinal mode and shear mode for the thin film resonators. Knowing that the material properties are strongly crystalline orientation dependent for AlN thin films, a theoretical study is conducted to reveal the effect of tilted polarization on the frequency characteristics of thin film resonators. The input electric impedance of a thin film resonator is derived that includes both thickness longitudinal and thickness shear modes in a uniform equation. Based on the theoretical model, the effective material properties corresponding to the longitudinal and shear modes are calculated through the properties transformation between the original and new coordinate systems. The electric impedance spectra of dual mode AlN thin film resonators are calculated using appropriate materials properties and compared with experimental results. The results indicate that the frequency characteristics of thin film resonators vary with the tilted polarization angles. The coexistence of thickness longitudinal and shear modes in the thin film resonators may provide some flexibility in the design and fabrication of the FBAR devices.