Assessment of the shear acoustic velocities in the different materials composing a high frequency solidly mounted resonator

Thin film acoustic resonators operating in the shear mode are being increasingly used for in-liquid sensing applications. A good design of such sensors requires accurate knowledge of the acoustic properties of the materials composing the whole device, which specifically includes their shear velocities. Here we present a method to assess the shear acoustic velocity of high and low acoustic impedance films commonly used in AlN-based solidly mounted resonators (SMRs), using test devices specifically designed to induce a half-wavelength resonance in the layer under study. Provided that the thickness and mass densities of all the layers are known, fitting the electrical response by Mason's model over a wide frequency range gives accurate values of both longitudinal and shear mode velocities. The assessment of porous and dense SiO2, Mo, W and Ta2O5 sputtered films yields shear velocities of 3150 m/s, 3950 m/s, 3450 m/s, 3350 m/s and 2900 m/s, respectively. In addition, the resonances stimulated in the Ir and Au top electrodes enable deriving their shear modes velocities, with values of 3950 m/s and 2350 m/s, respectively.