$Ba_{0.7}Sr_{0.3}TiO_3$ thin films were deposited by rf magnetron sputtering onto optimized Pt-base electrode $(Pt/SiO_2/Si)$. We investigated the effects of preparation conditions such as deposition temperature and $O_2$ content in plasma on electrical properties of BST films. The electrical properties of films with various thickness were also investigated. Considering electrical properties of films optimum deposition temperature were 590℃. Dielectric properties of films were improved as increasing $O_2$ content in plasma. An 33nm BST thin film deposited at 590℃ with 50% $O_2$ content in plasma had a dielectric constant of 275 (Tox=0.47nm) and a leakage current density of $4.4\times10^{-7}A/cm^2$ at 1.5V.
It was found that when BST films deposited on Pt bottom electrodes, low dielectric interfacial layer appeared to be formed between BST and Pt. We investigated the effects of interfacial layers on dielectric properties of BST films by increasing $O_2$ content in plasma only during deposition of interfacial layer, and calculated the thicknesses of interfacial layers with several assumptions. Dielectric properties of BST films were dominantly affected by those of interfacial layers. The thicknesses of interfacial layers on dielectric properties of BST films were calculated to be less than 7nm.
