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We have developed an HTS multilayer structure including a superconducting ground plane and interface-engineered ramp-edge Josephson junctions for single flux quantum (SFQ) circuits, and tried to improve reproducibility in the multilayer and junction process. We found that one of the crucial factors to improve the reproducibility is use of a ground plane with small average surface roughness ( a ) of typically less than 2 nm and a high c above 87 K. By employing La 0 2 Y 0 9 Ba 1 9 Cu 3 O (La-YBCO) thin films deposited by dc off-axis magnetron sputtering, such high quality was maintained for 3-month successive use of a ceramic target. Use of such a high-quality ground plane reproducibly led to SrSnO 3 La YBCO SrSnO 3 La YBCO //MgO multilayer structures with a 2 nm and 100-junction arrays with 1 c spread of 6-17% at 4.2 K depending on the average c level. The run-to-run reproducibility of average junction c and sheet inductance of both the base-and counter-electrodes for three samples was 12%, 3.0%, and 3.4%, respectively, which are substantially smaller than those previously reported.

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We have developed a multilayer structure with a smooth surface and fabrication processes for HTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughness R a less than 2 nm, composed of a La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x ground plane and base electrode layer, and SrSnO 3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in I c as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8-1.0 pH per square. Operation of several elementary SFQ circuits including a toggle flip-flop, a confluence buffer, a set-reset flip-flop and other SFQ elements has been successfully demonstrated at 30-60 K.