Finite element modeling and simulation of ultrasensitive film bulk acoustic resonator enabled by micropillar structure

Portable and ultra-sensitive film bulk acoustic resonator (FBAR) is a promising device to satisfy the requirement of detecting gas and biological molecule. In this work, a novel sensing device was developed to achieve ultrahigh sensitivity, by coupling polymer micropillars with a FBAR substrate to form a two-degrees-of-freedom resonance system (FBAR-micropillars). We systematically investigated the effects of micropillar structure on the characteristics of FBAR-micropillars device by finite element method (FEM). It was found that the resonant frequency shift increased with increasing the height of micropillars (h) within a certain range, and the FBAR-micropillars device displayed nonlinear frequency response, which was opposite to the linear response of conventional FBAR devices. In addition, a positive resonant frequency shift was captured near the “coupled resonant point” of the FBAR-micropillars device. The geometric parameters of micropillars, including micropillar diameter and micropillar spacing could also cause a change of Q-factor and mass sensitivity. The optimized design of the proposed device achieved a threefold improvement in sensitivity relative to conventional FBAR without pillars, suggesting a feasible method to improve the mass sensitivity of acoustic resonators.