Effects of mode couplings on the vibration characteristics of partially electroded thin-film bulk acoustic wave resonators

Mode coupling vibrations in thin-film bulk acoustic wave resonators (FBARs) were systematically analyzed in the series of our work. Previously, the two-dimensional theory of multi-layered FBAR plate has been established and used for the analysis of both free and forced vibrations of a fully electroded plate. In this paper, coupling vibrations in a partially electroded FBAR were taken into account, where the top electrode only covers the central part of the film. The two-dimensional equations were written into matrix form based on the state-vector approach. Transfer matrix functions of the global structure were obtained when continuous and traction-free boundary conditions were satisfied. Aspect ratios of both electroded and unelectroded regions were varied to research the influence of them on the coupling effects. The admittance versus frequency response was also obtained through forced vibration analysis to investigate the coupling effects. Results in this paper show the existence of energy-trapping behavior and the importance of structure size on mode coupling effects, which is important to the design of real FBAR devices.