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AbsaactFour-pole microsaip bandpass filters have been fabricated using both postannealed and in-situ-grown thin films of Y-Ba-Cu-0. The 4-GHz, 3-percent bandwidth filters exhibit, at 77 K, a passband insertion loss as low as 0.3 dB compared to the 2.8-dB loss of similar gold metallic filters at the same temperature. TntroductionThe newly discovered high-temperature superconductors (HTSs) offer great potential for passive microwave applications. The surface resistance of HTS materials, in particular Y-Ba-Cu-0, has been dramatically reduced recently as the quality of thin films of these materials has improved. Typical values of surface resistance at 4 GHz are better in state-of-the-art films than in copper by at least a factor of 10 at 77 K and a factor of 100 at 4.2 K1, offering the possibility of substantial improvement in microwave device performance by utilizing HTS material in place of normal metals such as gold. Filter DesiaTo demonstrate the potential performance benefits of HTS materials, we chose a four-pole Chebyshev bandpass design with 3-percent bandwidth, 0.05-dB passband ripple, and 4-GHz center frequency for implementation in Y-Ba-Cu-0. Fig. 1 shows the layout of the four-pole filter. The filter was designed.using an iterative technique described in Ref. 2 and was initially fabricated using gold signal lines on 500-pm-thick lanthanum aluminate (LaA103) substrates. This substrate material was selected because it has low-loss microwave properties and is chemically and structurally similar enough to Y-Ba-Cu-0 that high-quality thin films of Y-Ba-Cu-0 can be grown on its polished surface. The dielecmc constant of this perovskite material (E, -24) is larger than that which typical microwave circuit design routines can mat with accuracy, necessitating the iterative design technique using gold metalization. The design was optimized on 500-pm-thick LaAlO, and functioned with a slightly narrower passband on 425-pm-thick LaA109 This filter was fabricated in a microsaip configuration consisting of a signal line and ground plane on opposite sides of a single 1.3 x 2.3-cm substrate. After the filter design had been optimized, an all-niobium filter was fabricated as a reference for the performance of the HTS filters. Fabrication of the signal lines on the all-gold filters was accomplished by plating through a photoresist mask. For the all-niobium filters, niobium was deposited by sputtering and the signal line was pattemed by reactiveplasma etching. +.loo+Figure 1. Four-pole superconductive microstrip filter layout. The filters were fabricated on M O 3 substrates using gold, niobium and Y-Ba-Cu-0 signal lines.---Two types of growth techniques were used to deposit thin films of Y-Ba-Cu-0. The first technique is an ex-situ process in which amorphous Y-Ba-Cu-0 is deposited on a substrate by coevaporation of BaF2, Y, and Cu, followed by a postdepsition annealing, typically At 850 'C, in flowing 0 2 containing HzO vapor? The second technique for Y-Ba-Cu-0 film deposition is an in-situ-growth process using off-axis sing...

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