Oxidation of multilayer HTS digital circuits

Abstract

The issue of diffusion through insulating layers to buried superconductor films is common to any multilayer structure based on YBCO. Our earliest technique for obtaining fully oxidized underlayers on a practical time scale used reduced growth temperatures for strontium titanate insulating films to introduce defects which enabled diffusion while maintaining the integrity of electrical isolation. Since this approach did not work well with Sr-Al-Ta-O (SAT) and Sr-Al-Nb-O (SAN) insulators which have more desirable dielectric properties, a plasma oxidation process was introduced. For digital circuits based on HTS Josephson junctions where buried groundplanes must be fully oxidized, plasma oxidation had profound effects on the properties of cobalt or calcium-doped YBCO films used for N-layers in SNS edge junctions, increasing function critical currents by a factor of five. These experiments offer some insight into the role of in determining both individual junction properties and junction reproducibility. A third approach to oxidation of buried films relies on oxygen vias patterned in the insulating layer to permit to diffuse in the a-b plane of YBCO films instead of diffusing through the insulating layer. We designed and measured test structures which set a practical limit of 20-30 micrometers for via sparing.