Performance of a narrow band microwave filter implemented in thin-film YBa2Cu3O7−δ with ohmic contacts

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Abstract
Using in situ deposited thin‐film Y1Ba2Cu3O7−δ on LaAlO3 substrates, we have developed a pseudo‐interdigital three‐pole stripline filter with a minimum passband loss under 3.5 dB at 9.5 GHz and 77 K. The passband is very narrow, having a bandwidth of only 50 MHz (0.5%). The filter occupies a volume of less than 0.1 cm3. An equivalent copper filter produced a loss of 18.5 and 28 dB at 77 and 300 K, respectively. The input and output coupling is achieved by ex situ sputtered Au thin‐film contacts defined on the Y1Ba2Cu3O7−δ by a lift‐off technology and ultrasonically bonded to hermetic K connectors. Some of the major concerns in the realization of narrow band filters such as air gaps between high dielectric constant substrates and uniformity of the substrate’s dielectric constant are addressed.