Effects of residual surface resistance on the microwave properties of YBCO thin films

The performance of a planar microwave device fabricated from a YBCO thin film is strongly dependent on the quality and uniformity of the film. In this paper we present a full set of results of the microwave surface impedance at 8 GHz of three films deposited on MgO substrates and patterned into coplanar resonators. We use the Modulated Optical Reflectance (MOR) technique as a complementary, noncontacting, room temperature method of assessing the uniformity of the films. The three films studied have highly uniform MOR signals across the whole of their surfaces, indicative of thin films of very good electrical quality. However, although the measured Q factors of all three resonators are high, they vary significantly from 15000 to 30000 at 15 K, a result that is uncorrelated with the observed uniformity of the MOR signals. We interpret this variability as being due to residual surface resistance associated with microstructural defects that are impossible to detect using MOR. On subtracting the residual resistance we find that the intrinsic, linear microwave properties (i.e. quasiparticle conductivity σ₁ and penetration depth λ) are very similar, consistent with the uniformity of the MOR signals.