Fabrication and frequency response of solidly mounted resonators with 1/4λ mode configuration

A solidly mounted resonator (SMR) consists of a multilayered structure and requires material interfaces that confine waves to resonate as standing waves in order to avoid wave energy loss. The selection of high or low acoustic impedance for the first layer beneath piezoelectric layer results in 1/4λ mode and 1/2λ mode resonance configurations.In this study, Mo/SiO2 is chosen to construct the Bragg reflector as the high/low acoustic impedance materials, respectively, and aluminum nitride (AlN) is used as the piezoelectric layer. For application at frequency of 2.5 GHz, the specific thicknesses of Mo, SiO2, and AlN are considered individually in the deposition processes. AlN of 1/4λ thickness is deposited on a seven-layer Bragg reflector. The 1/4λ mode SMR shows the distinct resonant characteristics at 1.3 GHz (shear mode) and 2.4 GHz (longitudinal mode). The coupling coefficient Keff2 of 6.9% is in agreement with the theoretical analysis.