Analytical and experimental study on the second harmonic mode response of a bulk acoustic wave resonator

Abstract

A one-dimensional Mason model is employed to investigate the second harmonic mode response of a multilayer composite film bulk acoustic resonator (FBAR), particularly the dependence of the effective coupling coefficient (K2t, eff) on material properties and relative position in the acoustic stack. The simulation results for AlN-based FBAR with electrode layer having relatively low acoustic impedance and additional temperature compensation layer reveals that the maximum values of K2t, eff are obtained with a thickness ratio (between the non-piezoelectric layer and piezoelectric layer) that is close to its acoustic velocity ratio. The fundamental mode and second harmonic mode operation of an FBAR are compared. The maximum achievable K2t, eff is comparable (5.39% versus 5.16%) for the temperature compensated FBAR (with Mo as electrodes) operating at fundamental and second harmonic modes. However, the trimming-mass and crossover temperature sensitivities of the second harmonic mode are lower, indicating its potential advantage over the fundamental mode for high frequency applications above 2 GHz (such as filters, low phase noise temperature stable oscillator applications). Experimental results on a ZnO-based FBAR (Al/ZnO/Al/SixNy) operating around 5 GHz with various thicknesses of ZnO and SixNy show good agreement with numerical modeling.