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_a) of typically less than 2 nm and a high T_c above 87 K. By employing La_0.2-Y_0.9Ba_1.9Cu₃O_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 SrSnO₃/La-YBCO/SrSnO₃/La-YBCO //MgO multilayer structures with R_a≤2 nm and 100-junction arrays with 1-σI_c spread of 6-17% at 4.2 K depending on the average I_c level. The run-to-run reproducibility of average junction I_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.