Strontium titanate thin films were deposited by electron‐cyclotron‐resonance plasma‐assisted radio‐frequency magnetron sputtering. Electron‐cyclotron‐resonance plasma assistance was employed because of its ability to be used as a source of low‐energy bombardment by a high density of species that are highly activated. It was found that both the structure and composition improve with the application of microwave plasma during the deposition. Analysis of the capacitance‐voltage characteristics of metal‐insulator‐semiconductor devices revealed that the quality of the film/substrate interface is dependent on the pressure, atmosphere, and temperature of the deposition. Interfacial traps which give rise to charged surface states and silicon oxide formation have detrimental effects on films deposited on bare silicon substrates. Films on platinum‐coated silicon substrates show good dielectric properties. The small‐signal dielectric constant and dissipation factor at a frequency of 100 kHz were 170 and 0.033, respectively. For a 0.37‐&mgr;m‐thick film a charge storage density of 28 fC/&mgr;m2and a unit area capacitance of 3.7 fF/&mgr;m2were obtained at an applied electric field of 200 kV/cm.