Tungsten Oxide Layers of High Acoustic Impedance for Fully Insulating Acoustic Reflectors

Gravimetric sensors based on solidly mounted resonators require fully insulating acoustic reflectors to avoid parasitics when operating in liquid media. In this work, we propose a new high-acoustic impedance material, tungsten oxide (WO₃), for acoustic reflectors. We have optimized the sputtering conditions of WO₃ to obtain nonconductive layers with mass density around 6700 kg/m³ and acoustic velocities for the shear and the longitudinal modes up to 2700 and 4500 m/s, respectively. Compared to other conventionally used high impedance layers, WO₃ films display several manufacture advantages, such as high deposition rates, great reproducibility, and good adhesion to underlying substrates. We have demonstrated the applicability of WO₃ in practical shear mode bulk acoustic wave resonators that display good performance in liquid environments.