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Doctoral Dissertation Announcement
Candidate: Chi-Jung Cheng
Doctor of Philosophy
Department: Electrical and Computer Engineering
Title: Design, Modeling and Fabrication of Shear Mode Bulk Acoustic Wave Sensor as a Potential Biosensor
Dr. Massood Z. Atashbar, Chair
Dr. Bradley Bazuin
Dr. Azim Houshyar
Date: Wednesday, May 16, 2012 4:00 p.m. to 6:00 p.m.
Parkview Campus, Room A-213
There has been an increasing interest in development of thin film bulk acoustic wave resonator (FBAR) devices for chemical and biological sensing applications in the environmental and biomedical industries. The zinc oxide (ZnO) thin film based FBAR devices provides attractive advantages, such as high resonant frequency, small size, and rapid response. Typically, the ZnO-based FBAR devices are operated in the longitudinal wave mode where the ZnO crystallites are perpendicular to the substrate. In gaseous environments, the longitudinal mode FBAR device provides high sensitivity for mass sensing. However, the longitudinal wave mode is adversely affected when used in liquid environments because the longitudinal wave is easily dissipated in a liquid media such as water, blood or serum. This phenomenon causes the decrease of quality factor (Q) and thus reduces the mass sensing resolution.
To overcome this limitation, the shear mode solidly mounted film bulk acoustic wave resonator (SMFBAR) device is presented in this dissertation. The shear acoustic wave propagation is an ideal acoustic wave mode for liquid sensing applications because it allows minor damping effects and thus reduces the energy dissipation in liquid media. Two proposed structures, the lateral field excitation structure and the c-axis inclined structure, for generating a shear acoustic wave are discussed in this study. The resonant frequency, electromechanical properties, and frequency response of the device are analyzed through the one-dimensional Mason’s equivalent circuit method. The two- and three-dimensional finite element structures were also built to study the wave propagation direction and the particle displacement. In addition, the effect of the thickness of the ZnO thin film and Bragg reflector layers on the resonance frequency was also discussed.
The shear mode SMFBAR device, with overall dimension of 12260 µm × 11900 μm was fabricated on a 4” silicon (Si) wafer. The devices were tested for parameters of interest such as resonant frequency quality factor, and electromechanical coupling as well as their ability to operate in liquids. The results show that the shear mode SMFBAR offers great potential as a biosensor. Lastly, the accomplishments of this study are summarized and future perspectives are provided.