Date of Award

Summer 8-16-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

Advisor

John F. Vetelino

Second Committee Member

Nuri Emanetoglu

Third Committee Member

Mauricio Pereira da Cunha

Abstract

The most common bulk acoustic wave (BAW) sensing platform is the AT-cut quartz crystal microbalance (QCM), which exhibits a pure transverse shear mode (TSM) that is temperature compensated near room temperature. Due to the inability of shear waves to propagate in liquids, a pure TSM is desirable for applications such as biosensing, where the target is typically detected from within a fluid. Resonant BAW modes that are thickness field excited (TFE) by electrodes on both major faces of the crystal substrate, prevent electrical property detection at the sensing surface. In contrast, lateral field excitation (LFE) only requires electrodes on a single face of the substrate, leaving the sensing surface bare. This difference results in sensitivity to changes in both mechanical and electrical properties. Theory and preliminary experimentation have indicated that, via LFE, a temperature compensated pure TSM also exists near the Z-cut of lithium tantalate (LiTaO3). With electromechanical coupling several times greater than that of quartz, an LFE LiTaO3 sensor may provide superior sensitivity for detecting minute variations in measurands, such as the biomarkers associated with certain cancers. This work involves the theoretical and experimental search for a LiTaO3 orientation which has an LFE pure TSM that is temperature compensated at room temperature. A set of orientations ranging from (YXwl) 0°/-85° to 0°/-90° was chosen for experimental verification and a method for temperature characterization of LFE LiTaO3 BAW resonances was developed. The resonant frequency temperature response of each sample was shown to be parabolic, with a linear relationship between crystal orientation angle and temperature inflection point, i.e. turnaround temperature. Specifically, the (YXwl) 0°/-87° cut with a turnaround temperature at 26.4°C demonstrates that a pure TSM in LiTaO3, that is temperature compensated near room temperature, can be excited via LFE. For this LiTaO3 cut, the temperature coefficient of frequency (TCF), defined as the derivative of resonant frequency shift with respect to temperature, is a linear function of temperature that is zero-valued at 26.4°C. In contrast, the cubic resonant frequency temperature response of AT-cut quartz results in a parabolic TCF curve with an inflection point at room temperature and less resonant frequency shift per °C in the vicinity of room temperature. Although the parabolic temperature characteristic of LiTaO3 does not grant the degree of temperature stability provided by the cubic temperature characteristic of AT-cut quartz, the LFE TSM in LiTaO3 has a piezoelectric coupling that is nearly seven times higher than that of the room temperature compensated LFE mode in AT-cut quartz [1]. This higher coupling may contribute to a sensor with increased dynamic range and possibly increased sensitivity. Future work focusing on the development of an LiTaO3 sensing platform could profoundly impact sensor systems in agriculture, homeland security, global warming, and medical applications.

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