Oxidation of multilayer HTS digital circuits

The issue of oxygen diffusion through insulating layers to buried supercond uctor films is common to any multilayer structure based on YBCO. Our earlie st technique for obtaining fully oxidized underlayers on a practical time s cale used reduced growth temperatures for strontium titanate insulating fil ms to introduce defects which enabled oxygen diffusion while maintaining th e integrity of electrical isolation. Since this approach did not work web w ith Sr-Al-Ta-O (SAT) and Sr-Al-Nb-O (SAN) insulators which have more desira ble dielectric properties, a plasma oxidation process was introduced. For d igital circuits based on HTS Josephson junctions where buried ground planes must be fully oxidized, plasma oxidation had profound effects on the prope rties of cobalt or calcium doped YBCO films used for N-layers in SNS edge j unctions, increasing junction critical currents by a factor of five. These experiments offer some insight into the role of oxygen in determining both individual junction properties and junction reproducibility, A third approa ch to oxidation of buried films relies on "oxygen vias" patterned in the in sulating layer to permit oxygen to diffuse in the a-b plane of YBCO films i nstead of diffusing through the insulating layer. We designed and measured test structures which set a practical limit of 20-30 micrometers for via sp acing.