Design and Modeling of Ferroelectric BST FBARs for Switchable RF Bulk Acoustic Wave Filters.

Abstract

Multi-standard smartphones have become ubiquitous in everyday life. Such systems operate under different communication standards (2G, 3G, 4G-LTE, WLAN, GPS, Bluetooth, etc.) at different frequencies. Compact and high-performance filters are indispensable for RF front-ends in mobile phones, and RF bulk acoustic wave (BAW) filters, based on piezoelectric film bulk acoustic resonators (FBARs), have become prevalent. Moreover, due to the upcoming Internet of Things (IoT) and 5G, the demand for new technologies that can be employed to design switchable/tunable filters has increased. This dissertation presents one of the new promising technologies, known as intrinsically-switchable BAW filters employing newly-investigated electrostrictive effect in BST thin films. Successful implementation of switchable filters would eliminate/minimize external switches in the design of filter banks, thus leading to significant reduction in their size, cost, and complexity. Contributions of this work are categorized into three major parts. First, the nonlinear circuit modeling procedure for BST FBARs is presented. The nonlinear circuit model, essential for the material characterization and device characterization including linearity analysis, is developed based on the physics of electrostriction-based intrinsically switchable FBARs. Modeling results are in close agreement with dc-bias-voltage and RF-power-level dependent measurement results for BST FBARs. Second, the design methods for BST-on-Si composite FBARs are presented. The designed composite FBAR shows a record Q of 970 at 2.5 GHz among switchable BST resonators. Temperature-dependent characteristics of BST-on-Si composite FBAR devices are also presented with the measured TCF of -35 ppm/K. Furthermore, a raised-frame technique, which has been used to eliminate lateral-wave spurious-modes in piezoelectric BAW resonators, is first employed for switchable ferroelectric FBARs, demonstrating the effectiveness of the frame technique. Finally, the design method for intrinsically switchable BST FBAR filters is presented. The filter design method for ladder-type BAW filters is developed based on image parameters. Closed-form equations are derived for the first time enabling one to accurately design BAW filters. A systematically-designed pi-type BST FBAR filter is fabricated and measured, exhibiting a 1.22% bandwidth at 1.97 GHz with an isolation of greater than 22 dB, having a very small device size of 0.021 mm2.