Assessment of the acoustic shear velocity in SiO2 and Mo layers for acoustic reflectors

We present a method to assess the shear acoustic velocity in SiO₂ and Mo sputtered films, which are materials commonly used as low and high acoustic impedance layers in the acoustic reflectors of solidly mounted resonators. The method consists in the analysis of the frequency response of AlN-based resonators operating in the shear mode using acoustic mirrors specifically designed to obtain a half-wavelength resonance in the layer under study, the thickness of which is thickened to achieve a significant signal. After measuring the thickness and mass densities of all the layers composing the resonator, the fitting by Mason´s model of the electrical response in a wide frequency range provides accurate values of both the longitudinal and the shear mode velocities. The shear acoustic velocities of porous SiO₂ and Mo are 3150 m/s and 3450 m/s respectively. This method allows assessing the shear mode acoustic velocity of any material by adequately designing the test structure.