Composition-control of magnetron-sputter-deposited (BaxSr1−x)Ti1+yO3+z thin films for voltage tunable devices

Precise control of composition and microstructure is critical for the production of $({\mathrm{Ba}}_x{\mathrm{Sr}}_{\text{1−x}}){\mathrm{Ti}}_{\text{1+y}}{\mathrm{O}}_{\text{3+z}}$ (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high-frequency devices. Here, we present results on composition-microstructure-electrical property relationships for polycrystalline BST films produced by magnetron-sputter deposition, that are appropriate for microwave and millimeter-wave applications such as varactors and frequency triplers. Films with controlled compositions were grown from a stoichiometric ${\mathrm{Ba}}_{\mathrm{0.5}}{\mathrm{Sr}}_{\mathrm{0.5}}{\mathrm{TiO}}_{\mathrm{3}}$ target by control of the background processing gas pressure. It was determined that the $\mathrm{(Ba+Sr)/Ti}$ ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total $({\mathrm{Ar+O}}_{\mathrm{2}})$ process pressure, while the ${\mathrm{O}}_{\mathrm{2}}\mathrm{/Ar}$ ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the $\mathrm{(Ba+Sr)/Ti}$ ratio are discussed. Optimized BST films yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabilities suitable for device applications.