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 J_c uniformity across a 150-mm-diameter wafer. We outline the anodization process and describe the metrology techniques used to determine the NbO_x thickness grown. In the work described, we performed critical current I_c measurements on Josephson junctions distributed across a wafer. We then compared the J_c uniformity of pairs of wafers, fabricated together, differing only in the presence or absence of the anodization step. The cross-wafer standard deviation of J_c was typically ∼5% for anodized wafers but >15% for unanodized wafers. This difference in J_c 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 I_c uniformity - apparently the anodization improves only the J_c uniformity and not the variation in junction size. Control of J_c is important for all applications of superconductive electronics including quantum computation and rapid single-flux quantum (RSFQ) circuitry.