Conventional metal materials affect the development of microwave integrated circuits, while the use of microwave electric materials can achieve the miniaturization and high-performance requirements. With the continuous exploration of modern microwave communication technology, microwave devices will face some opportunities and challenges like multi-frequency, and multi-mode capabilities. Tunable microwave devices are also an essential core of phased array radar and modern mobile technology, with the broad application prospects.

Ba_xSr_1-xTiO₃ (BST) film material is considered as one of the most materials for tunable microwave devices due to its high dielectric tunability and low dielectric loss with a high figure of merit (FOM). However, BST thin film material still has some problems in applications, such as the synergistic optimization of tunability and dielectric loss, as well as temperature stability. To optimize the performance of films, the dielectric properties are adjusted through the improved preparation processes, annealing modification, controlled composition ratio, and doping modification as well. These optimization measures provide some opportunities for the design and preparation of microwave devices based on the BST films.

To investigate the dielectric properties of BST thin film, high-annealing in the preparation process can repair defects and improve grain growth, while controlling growth atmosphere can optimize the surface roughness and grain boundary characteristics of the film. Selecting appropriate substrates or inserting buffer to control stress can optimize the film structure and improve the dielectric performance. In addition, the Ba/Sr/Ti molar ratio regulation has a significant impact on the ferro-dielectric properties (i.e., the Curie temperature) of the film. The use of oxide molecular beam epitaxy (MBE) can achieve a better stoichiometric control and a low defect density. Doping regulation can change the lattice parameters and ion interactions of BST films. Some methods such as acceptor doping donor doping, and multicomponent doping are used. For instance, the FOM of La/Fe co-doped Ba_0.65Sr_0.35TiO₃ films prepared by a solid-phase reaction is increase by 5 times, compared to pure BST. The super lattice structures prepared by alternate doping and the multilayer films prepared by different material combinations indicate better comprehensive dielectric properties.

The BST thin film materials have nonlinear characteristics, in which the dielectric constant changes with the applied electric field, and these dielectric properties play an important role as parameter indicators in microwaveable devices. It is indicated that the oxide bottom electrode is of great significance to improve the performance of BST thin film variable capacitors in high-frequency applications. The BST film capacitors with SrMoO₃ (SMO) oxide was designed as a bottom electrode instead of conventional precious metal electrode materials, which avoids an acoustic resonance at high frequencies. Using the BST tunable filters instead of conventional filters composed of multiple filters is more in line with the needs of miniaturized and integrated devices in radar and communication fields. However, current research is limited due to the high insertion losses in practical devices, and reducing the insertion losses remains an important issue for BST-based microwave devices. Some phase shifters made using BST films were reported, demonstrating a large phase shift range and a high figure-of-merit, and achieving continuous tunable phase shift capability at different frequency ranges. Film phase shifters are applied in antenna arrays and end-fire arrays, realizing radiation directionality steering and linear beam scanning, thus promoting the development of radar and antenna systems. The power oscillators using BST films and single-chip AlGaN/GaN high electron mobility transistors voltage controlled oscillator were reported, showing the potential of BST films in the field of microwave oscillators.