A Thickness Shear Mode Zinc Oxide Liquid Sensor with Off-axis Excitation

A thickness shear mode acoustic sensor is developed based on a typical longitudinal mode electrode configuration through the use of a tuned acoustic mirror. The coupling of the electric field to a thickness shear mode occurs because of an off-axis electric field produced near the periphery of the top electrode. The existence of a lateral electric field component is verified through finite element simulations. The fabricated devices exhibit a Quality factor (Q) on average of 80 with an acoustic velocity of-3,086 m/s at 2.0 GHz. Sensor experiments are performed under liquid loading conditions, using glycerol-water solutions. The thickness shear mode resonance is monitored in real-time for a shift in frequency indicating sensitivity to the density-viscosity product. The highly sensitive mass sensor has the potential for use in biological sensing applications.