Reconfigurable Bulk Acoustic Wave Resonators and Filters Employing Electric-field-induced Piezoelectricity and Negative Piezoelectricity for 5G

Abstract

The ever-expanding wireless communications and sensing are influencing every aspect of human life. With the persistent demand for higher data capacity and recent advancements in wireless technologies, the design of current radio frequency front-end circuitry in communication devices calls for transformative changes. Frequency band proliferation is the biggest contributor to the added RF front-ends complexity in the design of future radios. To operate at various frequency bands, a complex combination of switches and filters is used in mobile devices, and the number of these frequency selective components in each device is expected to exceed 100 with the advent of 5th generation (5G) communication networks. Acoustic wave filters based on piezoelectric materials are the primary technologies employed in current communication systems, including mobile phones. Alternatively, the integration of multifunctional ferroelectric materials into reconfigurable frequency selective components promises reduced complexity, diminished size, and high performance for future radios, enabling them to support 5G wireless technologies and beyond. A promising reconfigurable bulk acoustic wave technology, employing electric-field-induced piezoelectricity and negative piezoelectricity in ferroelectrics, is presented in this dissertation. Successful implementation of ferroelectric filters would eliminate the need for external switcheplexers in the RF front-ends and reduce the number of required filters, leading to a significant reduction in size, cost, and complexity. Contributions of this work are categorized into three major parts. In the first part, an intrinsically switchable thin film bulk acoustic wave resonator (FBAR) based on ferroelectric BST with the highest figure of merit (i.e., Q_m×K_t^2) in the literature is presented. The BST FBARs are then employed to design intrinsically switchable filters with the lowest insertion loss to date. Such filters combine filtering and switching functionalities onto a single device, eliminating the need for external switches in RF front-ends. The second part of this work focuses on the development of frequency and bandwidth reconfigurable filters based on BST FBARs. The first switchless acoustic wave filter bank is presented in chapter 3, demonstrating the capability of BST FBARs in simplifying future agile radios. Next, a novel bandwidth reconfigurable filter based on BST FBARs is introduced in chapter 4, where the idea is experimentally validated with multiple design examples. Finally, through rigorous mathematical analysis and experimental validation, it has been demonstrated that a dynamic ‘non-uniform piezoelectric coefficient’ created within a composite structure made up of multi-layers of ferroelectrics allows the selective excitation of different mechanical Eigenmodes with a constant electromechanical coupling coefficient. Such technology overcomes the fundamental limitations associated with the electromechanical coupling coefficient of harmonic resonances in bulk acoustic wave resonators. To create ‘non-uniform piezoelectric coefficients’ in such structures, ferroelectrics’ electric-field-induced piezoelectricity and negative piezoelectricity has been exploited. This innovative technology provides a fundamentally new approach and a framework for synthesizing programmable frequency selective components, which leads to transformative advances in wireless systems’ front-end architecture. As part of the future direction, it is suggested that the multilayer structure presented in this section to be further studies as part of a new acoustic wave resonator design, which: (a) is capable of operation at a wide frequency range up to mm-wave frequencies designated for 5G (b). Such a structure has the potential to overcome the fundamental limitation of acoustic resonator’s ever-decreasing electromechanical coupling factors (Kt2) as their frequency of operation increases.