Development of Thin Film Multilayer Structures with Smooth Surfaces for HTS SFQ Circuits

We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x (La-YBCO) base electrode layers were prepared by a dc or rf magnetron sputtering method. The reproducibility of film quality for dc-sputtered La-YBCO films was better than that for rf-sputtered films, and the dc sputtered films exhibited the average surface roughness R a less than 1.0 nm and a T c zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO 3 /La-YBCO/ SrSnO 3 /La-YBCO on MgO substrate with R a below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x (La-YbBCO) counter electrodes have been fabricated by using this multilayer structure with dc-sputtered films. The fabricated junctions exhibited RSJ-type I - V characteristics with I c R n products of about 3 mV at 4.2 K. We also obtained a 1-σ I c spread of 8% for a 1000-junction series-array. The sheet inductance values at 4.2 K for the base and counter electrodes on La-YBCO ground planes were 0.8 pH and 0.7 pH per square, respectively. Operation of several types of elementary SFQ circuits has been successfully demonstrated hv using this multilayer structure.