TABLE I. Sputtering conditions for BST deposition. Target Ba 0.5 Sr 0.5 TiO 3 Target diameter 3 in. Source to substrate distance 4 in. RF power 90 W Sputtering gas Ar/O 2 ͑sccm͒ Substrate temperature 450-650°C Gas pressure 50 mT Deposition rate 0.4 nm/min

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO 2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100-500 K determined that decreasing thermal expansion coefficient of the host substrate ͑i.e., larger tensile thermal strain͒ reduced the film dielectric permittivity. The experimentally determined Curie-Weiss temperature decreased with increasing tensile thermal strain and the Curie-Weiss constant increased with tensile strain as predicted by Pertsev et al. ͓J.

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A monolithic Ka-band phase shifter circuit that employs voltage tunable BaSrTiO 3 (BST) parallel plate capacitors is presented here. The circuit is capable of continuous 0-157 phase shift at 30 GHz with an insertion loss of only 5.8 dB and return loss better than 12 dB. In addition to promising loss performance (27.1 /dB) at 30 GHz, the circuit reported here has several advantages over previously reported BST phase shifters such as moderate control voltages (20 V), room temperature operation, and compatibility with monolithic fabrication techniques.

The dielectric permittivity, dielectric quality factor ͑inverse dielectric loss͒, and lattice parameter of 140 nm sputtered SrTiO 3 films were dependent on the oxygen partial pressure and total chamber pressure (O 2 ϩAr) during film growth. Films were grown at 25 and 75 mTorr ͑mT͒ in an oxygen rich and oxygen deficient sputtering gas environment concurrently on ͑100͒ SrTiO 3 and ͑111͒ Pt/͑0001͒ Al 2 O 3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate ͑200͒ peak in homoepitaxial SrTiO 3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO 3 films on platinized sapphire at 1 MHz for the 25 mT ͑50 sccm Ar/50 sccm O 2 ), 25 mT ͑90 sccm Ar/10 sccm O 2 ), 75 mT ͑50 sccm Ar/50 sccm O 2 ), and 75 mT ͑90 sccm Ar/10 sccm O 2 ) film growths were 320, 251, 209, and 102, respectively; likewise, the dielectric constants follow as 241, 230, 220, and 170, respectively. Improved film dielectric properties were observed for films closer to stoichiometric SrTiO 3 .