4D-5 Lateral Field Excited High Frequency Bulk Acoustic Wave Sensors

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM) which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM the exciting electric field is primarily perpendicular to the crystal surface resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation in which both electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that could result in high frequency operation and hence increased sensitivity. In this work a theoretical search and experimental measurements are performed to identify the existence of high frequency temperature compensated TSMs in alpha-quartz and LiTaO₃ (LTO). Prototype LFE sensors were fabricated and tested and found to be capable of operating at frequencies in excess of 1 GHz and displayed significant increases in sensitivity to both mechanical and electrical property changes in liquids.