Reliability evaluation of Nb 10 kA/cm2 fabrication process for large-scale SFQ circuits

Abstract

We developed a 10 kA/cm2 Nb fabrication process, whose device structure was composed of six planarized Nb-layers, an Nb/AlOx /Nb junction layer, an Mo resistor layer, and SiO2 insulator layers to make larger scale, higher speed single-flux-quantum (SFQ) circuits that have more than 100 k junctions.It is very important to evaluate the process reliability in relation to the uniformity of the device parameters and the defect rate of the fabrication process. To evaluate the process reliability, we designed and tested many kinds of process test circuits, including 100 k junctions and 60-k serially connected contacts. We obtained excellent insulation properties, even in the six-Nb-layer structure. The standard deviations of the critical currents for 2000 1.2-μm-square junctions were less than 1.9%. We obtained a critical current of more than 8 mA, even for the 60-k serially connected contacts. Based on these results, we obtained sufficient reliability to realize the SFQ circuits including more than 100 k junctions.