Deposition and microwave performance of YBCO films on technical ceramics

YBCO films for microwave applications are usually deposited on expensive, single crystal substrates. Here, technical ceramics offer a variety of advantages with respect to rf-filters. They are available in arbitrary large size, the dielectric constant is isotropic, and can be selected from a range between 10 and 110 for ceramics with appropriate composition. The main shortcoming is the polycrystalline nature of such ceramic substrates. However, the ion beam assisted deposition (IBAD) technique allows YBCO growth with excellent crystalline orientation and yields superconducting properties comparable to epitaxial films on single crystal substrates. As an example we tested substrates made of SM 210, a standard low loss microwave ceramic from Kyocera, with a dielectric constant of 21 close to that of LaAlO₃. After proper polishing the 2″ substrates a 2 μm thick YSZ-IBAD buffer was applied by PLD. YBCO films of various thicknesses were deposited by thermal evaporation. These films exhibited critical current densities in excess of 1.5 MA/cm² at 77 K. We report evaluation of the surface resistance of the YBCO films and the substrate losses by a standard dielectric resonator technique in the frequency regime around 10 GHz and discuss the technological impact of these results on future filter development.