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 manufactured using double-sided YBa2Cu3O7−δ (YBCO) films on CeO2-buffered 2-in. sapphire wafers. Their S-parameters were measured. To protect YBCO films against degradation in air, they were coated with gold. Microwave surface resistance Rs(T) was measured by the copper end-plate and microstrip resonator techniques. The Au coating was proven to suppress Q-factor of resonators and to drastically increase the insertion losses of filters. A significant peak of Rs(dAu) for 77 K at dAu ≈ 20 nm was experimentally observed. The peak is much higher and occurs at much lower dAu values than what is predicted by a simple electrodynamic calculation for the bilayer film. We suggest that this is due to a surface roughness, which was observed by atomic force microscopy. A dynamic proximity effect with an account of Andreev reflection may be also important to understand the obtained results.