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

Abstract

We have developed a multilayer structure with a smooth surface and fabrication processes forHTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughnessRa less than 2 nm,composed of a La0.2–Y0.9Ba1.9Cu3Ox ground plane and base electrode layer, andSrSnO3 insulating layers, was deposited by off-axis magnetron sputtering. We havefabricated interface-engineered junctions based on the multilayer structure using anLa0.2–Yb0.9Ba1.9Cu3Ox counter-electrode layer prepared by pulsed laser deposition. Thefabricated junctions exhibited excellent Josephson characteristics with a1-σ spread in Ic as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8–1.0 pHper square. Operation of several elementary SFQ circuits including a toggle flip-flop, aconfluence buffer, a set–reset flip-flop and other SFQ elements has been successfullydemonstrated at 30–60 K.