Quartz crystal resonator sensors under lateral field excitation—a theoretical and experimental analysis

AT-cut quartz crystals have been driven with the so-called lateral field excitation. When applying a voltage to the two electrodes placed on one crystal surface and separated by a small gap an electric field is generated, which is mainly confined in the lateral direction. Extraordinary changes can be observed in the conductance spectrum when applying a liquid to the other (bare) surface of the crystal. In contrast to traditional quartz crystal sensors, these changes must be attributed to electrical properties of the adjacent medium. It is suggested that a redistribution of the exciting electric field from the lateral toward the thickness direction occurs. The assumption is supported by simulations which allow insights into the piezoelectric excitation and transduction mechanism of the acoustic device under varied electrical boundary conditions at the bare surface as well as the shear displacement patterns involved. Results from 10 MHz plano–plano and 6 MHz plano–convex sensors which have been exposed to liquids of varying permittivity show the strong dependence of the sensor response on liquid permittivity which overlies the known dependence on density–viscosity.