Band-pass filters for 1.8 GHz frequency range using double-sided YBCO/Au films on CeO2-buffered sapphire

Microstrip band-pass filters for the frequency range of 1.8 GHz were manufa ctured using double-sided YBa2Cu3O7-delta (YBCO) films on CeO2-buffered 2-i n. sapphire wafers. Their S-parameters were measured. To protect YBCO films against degradation in air, they were coated with gold. Microwave surface resistance R-s(T) was measured by the copper endplate and microstrip resona tor techniques. The Au coating was proven to suppress e-factor of resonator s and to drastically increase the insertion losses of filters. A significan t peak of R-s(d(Au)) for 77 K at d(Au) approximate to 20 nm was experimenta lly observed. The peak is much higher and occurs at much lower d(Au) values than what is predicted by a simple electrodynamic calculation for the bila yer film. We suggest that this is due to a surface roughness, which was obs erved by atomic force microscopy. A dynamic proximity effect with an accoun t of Andreev reflection may be also important to understand the obtained re sults.