Improved Critical-Current-Density Uniformity by Using Anodization

We discuss an anodization technique for a Nb superconductive-electronics-fabrication process that results in an improvement in critical-current-density uniformity across a 150-mm-diameter wafer. We outline the anodization process and describe the metrology techniques used to determine the NbO thickness grown. In the work described, we performed critical current measurements on Josephson junctions distributed across a wafer. We then compared the uniformity of pairs of wafers, fabricated together, differing only in the presence or absence of the anodization step. The cross-wafer standard deviation of was typically 5% for anodized wafers but 15% for unanodized wafers. This difference in uniformity is suggestive of an in-process modification from an unknown cause that is blocked by the anodic oxide. It is interesting that small junctions do not see an improvement in uniformity-apparently the anodization improves only the uniformity and not the variation in junction size. Control of is important for all applications of superconductive electronics including quantum computation and rapid single-flux quantum (RSFQ) circuitry.