Microwave properties of barium strontium titanate thin films

Abstract

In recent years, barium strontium titanate (abbreviated as BST) thin films have attracted extensive research interest because their relative permittivity can be tuned by the application of an external bias field and can be used in agile microwave devices. In this work, Ba0.7Sr0.3Ti0.95O3 and Ba0.5Sr0.5TiO3 thin films prepared by the sol-gel and magnetron sputtering methods have been studied. X-ray diffraction (XRD) and atomic force microscopy (AFM) observations revealed that the films were well crystallized with uniform thickness. Coplanar electrodes were then deposited by magnetron sputtering and patterned by standard photolithography and wet chemical etching techniques. The processing-structure-property relationship of BST thin film-based interdigital capacitors was investigated and compared with the conventional parallel plate capacitors and consistent results have been obtained. The films annealed at higher temperature possessed larger relative permittivity and the relative permittivity was also found to be dependent on the film thickness. The films prepared under optimized conditions exhibited good dielectric properties at microwave frequencies. To find the properties of BST thin films with interdigital capacitors structure at high frequency, a simple measurement technique has been developed to remove the parasitic capacitance. Several technical issues in the characterization of interdigital capacitors are discussed. The dielectric tunability of BST thin films in interdigital capacitors was characterized. Under a 20 MV/m electric field, the tunability of BST on LAO substrate was found to be about 31%. Many processing factors were found to have influences on the tunability of the films. The relative permittivity and tunability of BST exhibited a strong dependence on film thickness. BST film-based microwave devices, such as resonators and phase shifters were prepared and tested. The performance of ring resonator with different structures (substrates and film thicknesses) were investigated and studied with the aid of a commercial simulation software IE3D. Also, a BST thin film-based phase shifter under dc bias has been developed and preliminary measurement results have been obtained.