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The effect of photomask pattern shape for a counter-electrode on critical current (I(sub c)) uniformity and controllability in Nb/AlO(sub x)/Nb junctions is evaluated. Although no difference is found in the (I(sub c)) uniformity between square and OPC junctions, the OPC junctions exhibited smaller shrinkage in junction size than the square junction.

Abstract: We developed a fabrication process for planarized multi-layer Nb SFQ circuits. This new process has several new steps for achieving a planarized multi-layer structure with a high critical current density. We have so far fabricated about 30 wafers, and we have measured the characteristics of Josephson junctions in multi-layer structures. We confirmed small spreads and high reproducibility in the critical current, and we also achieved a high junction yield. In contrast, we detected a minor degradation of the gap voltage, which had no serious influence on circuit operation. Based on the measured results, we believe that planarization was the most probable cause for this degradation. [Copyright &y& Elsevier]

Abstract: We developed a 10 kA/cm2 Nb fabrication process, whose device structure was composed of six planarized Nb-layers, an Nb/AlO x /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. [Copyright &y& Elsevier]

The authors evaluated the effect of photomask pattern shape for a counter-electrode on critical current Ic uniformity and controllability in Nb/AlOx/Nb junctions. Circular, square, and 2 kinds of optical proximity correction (OPC) square patterns were used as the mask pattern shape. Although there was no difference in the Ic uniformity between square and OPC junctions, the OPC junctions exhibited smaller shrinkage in junction size than the square junction. In addition, the OPC junctions improved the chip-to-chip variation in the shrinkage. The circular junction exhibited the smallest variation in the shrinkage, and had an ad- vantage in 4 uniformity for smaller than 1.0 μm² junctions in comparison with the other junctions. The shrinkage of the circular junction was the largest of all the junctions. This paper describes the recommended choice of the photomask pattern shape for several Nb LSI technologies. [ABSTRACT FROM AUTHOR]