Theory and Sensor Feasibility of Lateral Field Excited Acoustic Wave Devices

The lateral field excited (LFE) bulk acoustic wave (BAW) sensor has been under exploration as an alternative to the commonly used quartz crystal microbalance (QCM) for biosensing applications. For AT-cut quartz, both devices excite a desired resonant pure shear acoustic wave; however, the QCM only applies to mechanical property change detection. The thickness field excitation (TFE) of a QCM requires an electrode on the sensing surface, which severely blocks the detection of electrical property changes that would allow target analytes to be detected at far lower concentrations. An LFE sensor exhibits greater inherent sensitivity to mechanical and electrical property changes through its bare sensing surface by having both its electrodes on a single side of the piezoelectric plate. However, contrary to the established LFE theory, the measured LFE device response is far weaker than that of a TFE device for the same mode. Furthermore, the usual theory does not account for changes in electrical boundary conditions. This work aims to indicate how to improve LFE performance and to bridge the gap between theory and reality by expanding upon the existing theory through plate wave analysis that better captures the lateral wave nature of LFE operation. Alignment between the theoretical and experimental results is presented for AT-cut quartz and Z-cut lithium tantalate (LTO). This understanding of LFE operation is intended to facilitate LFE sensor applications through a better ability to predict the influences of measurands on the sensors.