Y-Ba-Cu-O thin films on 3" sapphire wafers for microwave devices

Epitaxially grown large area YBCO films are essential for superconducting high frequency devices, e.g. microstrip filters for telecommunications. Therefore, c-axis oriented, 250 to 500 nm thick YBCO films were simultaneously deposited by sputtering from hollow cylindrical targets on both sides of CeO₂ buffered 3" sapphire wafers. The lateral homogeneity was analyzed by mapping the surface resistance, R_s, at 145 GHz and 75 K. Values of R_s below 50 mΩ were reached over the whole area. The HF power handling capability was demonstrated by a break down field higher than 10 mT at 8.5 GHz and 77 K. In order to demonstrate the quality of our sputtered films with respect to large area high frequency applications, simple microstrip resonators were patterned by UV photo lithography and wet chemical etching. At a circulating power of up to 1 W, very high unloaded quality factors, Q₀, of up to 32000 at 4 GHz and 77 K were achieved. Above 5 W, Q₀ decreased due to the field dependence of R_s. The operation of complex structures was demonstrated by 3 pole and 7 pole bandpass filters operated at 77 K and 2 GHz and 6 GHz, respectively. The filters, tuned by brass tuning elements, showed transmission losses of less than -0.3 dB and reflection losses of more than -17 dB