A new type wide-frequency-range shear viscosity sensor using c-axis tilted ScAlN thin film on temperature stable AT-cut quartz thick plate

In order to study relaxation characteristics of viscoelasticity of the liquid, a wide frequency sweep in the range of MHz to GHz is desired. We, therefore, report a new type HBAR (High-overtone Bulk Acoustic Resonator: shear mode ScAlN film on AT-cut quartz plate) sensor which makes it possible to operate in a wide frequency range as shown in Fig. 1. When a thickness shear acoustic mode (TSM) resonator is in contact with a liquid sample, the shear wave penetrates into the liquid by the depth of δ (called penetration depth [1]). Thus, the liquid viscosity can be determined from the amount of the resonant frequency shift due to a δ thick mass loading layer mounted on the resonator. We choose AT-cut quartz as the substrate whose TCF is zero at room temperature to suppress frequency shifts due to the temperature change. TCF of the whole resonator stack greatly decreases considering the mass ratio between the thin piezoelectric film and the much thicker substrate. In this study, frequency shifts were measured for various concentration of glycerine solutions in order to demonstrate usefulness of the HBAR. [1] T. Nakamoto and T. Moriizumi, Jpn. J. Appl. Phys., 29 963 (1990).