Synthesis and characterization of (BxSr1-x)Ti1+yO3+z thin films and integration into microwave varactors and phase shifters

Precise control of composition and microstructure is critical for the produ ction of (BaxSr1-x)Ti1+yO3+z (BST) dielectric thin films with the large dep endence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into t unable high frequency devices. Here we review recent results on composition -microstructure-electrical property relationships of polycrystalline BST fi lms produced by magnetron sputter deposition, that are appropriate for micr owave devices such as phase shifters. Films with controlled compositions we re grown from a stoichiometric Ba0.5Sr0.5TiO3 target by control of the back ground processing gas pressure. It was determined that the (Ba+Sr)/Ti ratio s of these BST films could be adjusted from 0.73 to 0.98 by changing the to tal (Ar+O-2) process pressure, while the O-2/Ar ratio did not strongly affe ct the metal ion composition. Film crystalline structure and dielectric pro perties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST layers yielded capacitors with low dielectric losses (0.0047), among the be st reported for sputtered BST, while still maintaining tunabilities suitabl e for device applications. These BST films were used to produce distributed -circuit phase-shifters, using a discrete periodic loading of a coplanar wa veguide with integrated BST varactors on high-resistivity silicon. Phase sh ifters yielding 30 degrees of phase shift per dB of insertion loss were dem onstrated at 20GHz.