Broadband Dielectric Characterization of Sapphire/TiOx/Ba0.3Sr0.7TiO3 (111)-Oriented Thin Films for the Realization of a Tunable Interdigitated Capacitor

A complete microwave characterization up to 67 GHz using specific coplanar waveguides was performed to determine the dielectric properties (permittivity, losses, and tunability) of sapphire/TiO_x/Ba_0.3Sr_0.7TiO₃ (BST) (111)-oriented thin films. To that end, Ba_xSr_1-xTiO₃ thin films were deposited by RF magnetron sputtering on sapphire (0001) substrate. To control the preferred (111) orientation, a TiO_x buffer layer was deposited on sapphire. According to the detailed knowledge of the material properties, it has been possible to conceive, fabricate, and test interdigitated capacitors, the basic element for future microwave tunable applications. Retention of capacitive behavior up to 67 GHz and a tunability of 32% at 67 GHz at an applied voltage of 30 V (150 kV/cm) were observed. The Q-factor remains greater than 30 over the entire frequency band. The possibility of a complete characterization of the material for the realization of high-performance interdigitated capacitors opens the door to microwave device fabrication.