Lateral field excitation of thickness shear mode waves in a thin film ZnO solidly mounted resonator

In recent years, interest in the development of highly sensitive acoustic wave devices as biosensor platforms has grown. A considerable amount of research has been conducted on AT-cut quartz resonators both in thickness excitation and in lateral excitation configurations. In this report, we demonstrate the fabrication of a ZnO solidly mounted resonator operated in thickness shear mode (TSM) using lateral field excitation of the piezoelectric film. Theoretical Christoffel equation calculations are provided to explore the conditions for excitation of a TSM wave in $c$-axis-oriented ZnO through lateral excitation. The existence of a TSM wave is verified through the comparison of theoretical and experimentally obtained acoustic velocity values from frequency versus thickness measurements and water loading of the resonators. A major strength of this design is that it incorporates a simple eight-layer, single-mask fabrication process compatible with existing integrated circuit fabrication processes and can be easily incorporated into multidevice arrays. With minimal electrode optimization, we have been able to fabricate resonators with nearly 100% yield that demonstrate $Q$ values of up to 550 and $K2$ values of 0.88% from testing of more than 30 devices.