Improvement in reproducibility of multilayer and junction process for HTS SFQ circuits

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 (R/sub a/) of typically less than 2 nm and a high T/sub c/ above 87 K. By employing La/sub 0.2/-Y/sub 0.9/Ba/sub 1.9/Cu/sub 3/O/sub x/ (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/sub 3//La-YBCO/SrSnO/sub 3//La-YBCO //MgO multilayer structures with R/sub a//spl les/2 nm and 100-junction arrays with 1-/spl sigma/I/sub c/ spread of 6-17% at 4.2 K depending on the average I/sub c/ level. The run-to-run reproducibility of average junction I/sub c/ and sheet inductance of both the base- and counter-electrodes for three samples was /spl plusmn/12%, /spl plusmn/3.0%, and /spl plusmn/3.4%, respectively, which are substantially smaller than those previously reported.