Multifunctional Ferroelectrics based Devices for Next Generation RF Front-ends

Abstract

This dissertation presents designs of acoustic wave resonators based on two different multifunctional ferroelectric materials: Barium Strontium Titanate (BaxSr1-xTiO3, BST) and Scandium Aluminum Nitride (ScxAl1-xN, ScAlN), in order to address different aspects of acoustic filter limitations. The first, BST, is employed in the design of ferroelectric based reconfigurable filters that require no external switches to transition between different bands of interest, specifically in the sub-6 GHz. As modules sizes continues to shrink, utilizing filters that are capable of multiple functions provide significant reduction in the overall complexity, cost, and size of the RF front-end. The second material, ScAlN, is employed to address frequency scaling limitations of current bulk acoustic wave (BAW) technology. Employing MBE grown film in a multilayer structure, a novel mm-Wave acoustic resonator, where polarization switching of the ScAlN layer is demonstrates for the first time, is presented. Contribution of this work are categorized into three major parts. First, BST thin film bulk acoustic wave resonators (FBARs) are employed to design intrinsically switchable filters based on electrostrictive transduction. As a proof-of-concept, a systematically designed switchless, quad band acoustic wave filter bank with the lowest insertion loss to date in the literature is fabricated and measured. The filter bank is capable of switching between frequency bands of interest solely through a direct current (DC) bias applied across the associated BST FBARs and demonstrates its capability to streamline future radio systems. Beyond switching on and off the filter response, the second contribution employs BST’s electrostriction effect in a hybrid-based approach to develop filters with the capability to alter their transfer function. The use of reconfigurable frequency-selective components along the RF front-end is highly attractive, as the same component potentially supports functionality across multiple different bands. For the first time, two designs of reconfigurable RF circuits on the BST-on-Si platform (bandstop to an all-pass response and bandpass to all-reject response) are experimentally validated at significantly higher operational frequencies than previously published results at the time. Moreover, this hybrid approach offers a method to attain fractional bandwidth (FBW), not limited to the inherent coupling of the associated resonators. The final part of this work incorporates thin film ferroelectric ScAlN in the design of a novel mm-Wave higher order mode FBAR. Selective excitation of multiple eigen modes within a multilayer FBAR structure through alternating configurations of positive and negative piezoelectricity (AlN and ScAlN, respectively) is highly advantageous compared to single layer BAW devices, which suffer from increased losses and reduced performance (quality factor, Q, and electromechanical coupling coefficient, kt2) with increased operating frequency. For the first time, a composite trilayer piezoelectric-ferroelectric-piezoelectric AlN-ScAlN-AlN FBAR exciting a third order thickness-extensional mode is demonstrated. To the author’s knowledge, this is the first published work that demonstrates complete ScAlN polarization switching within a composite structure. The reported kt2 does not follow the same roll-off as typical BAW devices when transitioning between fundamental and third mode, and such resonators can be incorporated in the design of mm-Wave filters with greater FBW requirements.