Switchable and Tunable Ferroelectric Devices for Adaptive and Reconfigurable RF Circuits
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As wireless communication systems have become more prevalent, their role has broadened from simply a means of connecting individuals to one another to a means of connecting individuals to the vast
information and social network of the Internet. The resulting exponential increase in the utilization of wireless communication systems, the fundamental limitation of the finite wireless spectrum, and the use of conventional wireless communication systems that are designed to operate at fixed predetermined carrier frequencies pose a significant challenge. One method to address this problem is to use adaptive and reconfigurable wireless communication systems that can change their frequency and mode of operation based on the unused/available wireless spectrum in their environment as well as their surrounding environmental conditions. Unfortunately, currently available RF and microwave circuit components cannot meet the frequency agility specifications, performance requirements, and cost constraints necessary for the widespread commercialization of such systems.This thesis explores how the multifunctional properties of ferroelectrics such as barium strontium titanate (BST) can be used to design switchable and tunable RF circuits for use in adaptive and reconfigurable wireless communication systems. In particular, the electric field dependent permittivity, electrostriction, and electric field induced piezoelectricity of BST are utilized for the design of electroacoustic resonators and filters. The main contribution of this thesis is the demonstration of several different intrinsically switchable resonators and filters designs. First, BST film bulk acoustic wave resonators (FBARs), which exhibit electric resonances that are controlled by an applied dc bias voltage, are characterized. In addition, dual frequency two-element resonator arrays that utilizes intrinsically switchable and tunable BST FBARs are demonstrated for the first time. Second, intrinsically switchable and tunable ferroelectric FBAR filters with insertion losses as low as 4.1 dB at 1.6 GHz are presented. Furthermore, dual band BST FBAR filters that exhibit two different pass band responses in the low GHz range are demonstrated for the first time. Third, intrinsically switchable and tunable lateral (contour) mode resonators with frequencies as high as 1.67 GHz are demonstrated for the first time. Last of all, an RF magnetron sputtering system dedicated to BST thin film deposition is designed, assembled, and configured for continuing the improvements in ferroelectric thin film performance, developing novel ferroelectric based circuits, and designing larger and more complex circuits and systems. The performance of the deposited BST thin films have also been characterized.