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Tunable RF phase shifters using strain engineered barium strontium titanate thin film capacitors
Dissertation

Tunable RF phase shifters using strain engineered barium strontium titanate thin film capacitors

John Carroll
Doctor of Philosophy (Ph.D.), Drexel University
Jun 2026
DOI:
https://doi.org/10.17918/00011409
pdf
Carroll_John_202617.25 MB
PDF Embargoed Access, Embargo ends: 30 Jun 2028

Abstract

This dissertation presents the development of tunable radio-frequency (RF) phase shifters based on barium strontium titanate (BST) thin films, with an emphasis on linking material properties to device-level performance. Rather than maximizing dielectric tunability alone, this work focuses on achieving a balance between tunability, dielectric loss, and impedance matching within a bridged-tee phase shifter architecture. BST thin films with varying compositions were grown using pulsed laser deposition and evaluated on multiple substrates to investigate the effects of epitaxial strain. Among the compositions studied, (Ba_[0.7]Sr_[0.3])TiO₃ (BST70) exhibited the most favorable combination of tunability and low loss. Devices fabricated on GdScO₃ substrates demonstrated improved dielectric and RF performance. Interdigitated capacitor structures were integrated into a high-pass bridged-tee phase shifter and experimentally characterized. Single-stage devices achieved phase shifts exceeding 200° under applied bias, while multi-stage configurations extended the phase range beyond 360°. The results further show that parasitic effects and device scaling strongly influence overall performance. This work demonstrates the viability of strain-engineered BST for tunable RF phase shifters and establishes a framework for co-designing materials and circuits to enable compact, low-loss, and highly tunable RF components.

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