Mechanism Of Piezoelectric Bulk Acoustic Wave Sensors Based On Lateral Field Excitation

Piezoelectric bulk acoustic wave (BAW) devices based on thickness field excitation (TFE), e.g. quartz crystal microbalance (QCM) sensors, have been investigated and widely used in biochemical sensors for a long time. However, traditional BAW sensors have been found obvious disadvantages in many practical applications. The lateral field excitation (LFE) sensors, on the other hand, employ two electrodes located on the same major surface of the substrate to launch the thickness shear mode (TSM) and are superior in sensing performance compared to TFE devices. It is believed that there is a great potential for LFE sensors when used in liquid biochemical sensing applications.The dissertation is devoted to the fundamental theory of LFE shear wave propagating in solids, LFE materials and design method of LFE devices, through which the sensing mechanism and operating modes of LFE BAW sensors were investigated in depth. The major research work includes the following.The wave propagating characteristics excited by LFE in solid mediums was studied using the Christoffel-Bechmann method. The LFE piezoelectric coupling factor and acoustic wave phase velocity on different crystal orientations were calculated for quartz, langasite, lithium niobate and lithium tantalite. It is presented for the first time that BAW devices with a LFE configuration in fact work at three different modes, namely the pure-LFE, quasi-LFE and pseudo-LFE.The energy trapping of the thickness-shear vibration in LFE devices was investigated using the elastic wave propagation theory and the cutoff frequency. A novel method for energy-trapping LFE device design was proposed and four electrode configurations were fabricated and tested. The result agrees well with the theoretical analysis for energy-trapping mode LFE devices.The temperature characteristics of LFE devices fabricated on above materials were analyzed and compared to those of TFE devices. The Mason model and Butterworth-Van-Dyke equivalent circuit were derived for LFE devices using the one-dimensional wave propagation theory.Using the static capacitance compensation technique, a phase-locked detector and the liquid sensor system were developed. LFE devices working at pure-LFE operating mode with four novel electrode configurations on LiNbO3 were tested in air, water and NaCl solutions, and the sensing mechanism was also verified by experiments.The significance of the current research lies in the fact that the working principle of LFE sensors is identified as different operation modes. The research is not only important for academic merit, but also for designing novel biochemical sensors for various applications.