Fabrication of high-temperature superconductor single-flux-quantum circuits using a multilayer structure with a smooth surface

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 Ra less than 2 nm, composed of a La0.2–Y0.9Ba1.9Cu3Ox ground plane and base electrode layer, and SrSnO3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La0.2–Yb0.9Ba1.9Cu3Ox counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in Ic 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.