Ferroelectric BaTiO₃/SrTiO₃ with optimized c-axis-oriented multilayered thin films were epitaxially fabricated on (001) MgO substrates. The microstructural studies indicate that the in-plane interface relationships between the films as well as the substrate are determined to be (001)_SrTiO3//(001)_BaTiO3//(001)_MgO and [100]_SrTiO3//[100]_BaTiO3//[100]_MgO. The microwave (5 to 18 GHz) dielectric measurements reveal that the multilayered thin films have excellent dielectric properties with large dielectric constant, low dielectric loss, and high dielectric tunability, which suggests that the as-grown ferroelectric multilayered thin films can be developed for room-temperature tunable microwave elements and related device applications.

A review of the temperature stability issues associated with Ba_xSr_1−xTiO₃ (BST) thin films and a summary of innovative materials designs, and novel process science solutions which serve to help mitigate this temperature sensitivity is presented. The experimental results from the literature and from our own work are reviewed; the correspondence between the theoretical results and the measured properties of tunable materials is critically analyzed; and reasons for discrepancy between literature results are discussed. Particular emphasis is concentrated on tunable phase shifters and filters to enable phased array antennas, radars and other advanced communications devices. This critical review provides the foundation to spawn new materials research solutions to further enable the development of BST films for microwave device applications.