FILM THICKNESS DEPENDENCE OF MICROWAVE SURFACE-RESISTANCE FOR YBA2CU3O7 THIN-FILMS

Microwave surface resistance close to 10 GHz has been measured as a function of temperature for epitaxial thin films of YBa2Cu3O7 (YBCO) on LaAlO3 in the film thickness range of 0.2-0.8 mum. The films were made by a reduced-temperature post-anneal technique. The surface resistance (R(s)) scaled to 10 GHz decreases with increasing film thickness as is expected due to the finite film thickness with respect to the magnetic penetration depth. Below about 70 K there is an increase in R(s) for the thickest films, attributed to a change in microstructure from c axis normal to the substrate plane, to c axis in the plane of the substrate; the resulting minimum in R(s) occurs at a film thickness of 0.6 mum. The critical current density (J(c)) at 77 K is highest for the thinner films, so that films with the highest J(c) do not have the lowest measured R(s). These results suggest that the optimum YBCO film thickness for microwave devices patterned from these films may be about 0.6 mum, depending on operating temperature.