The resonance characteristics of solidly mounted resonators with 1/2 λ and 1/4 λ configurations

The solidly mounted resonator (SMR) is composed of a piezoelectric AlN thin film sandwiched between two electrodes and a Bragg reflector consisting of alternating high and low acoustic impedance layers of quarter-wavelength thickness. According to the different arrangements of high and low acoustic impedance layers, SMR devices form two resonance modes, i.e. 1/2 lambda and 1/4 lambda mode configurations. The purpose of this study is to compare the resonance characteristics of these two configurations. The thickness of the piezoelectric layer for 1/4 lambda mode configuration is set equal to a half of that for 1/2 lambda mode configuration. Therefore, the piezoelectric thin film for 1/2 lambda mode configuration exhibits better crystallization owing to its film thickness. It leads to a high coupling coefficient, k², and a wide bandwidth if piezoelectric thin film is fabricated with good crystalline characteristics. As to 1/4 lambda mode configuration with moderate film thickness, it has a better surface smoothness and a lower stress resulting in a slight scattering of the acoustic wave. As a result, the SMR device with 1/4 lambda mode configuration leads to a high quality factor. The return loss outside passband is obviously close to zero for the 1/4 lambda mode configuration.