HIGH-T(C) MULTILAYER STEP-EDGE JOSEPHSON-JUNCTIONS AND SQUIDS

We have developed a reliable process to fabricate high-quality YBa2Cu3O7-x (YBCO) superconductor-normal metal-superconductor (SNS) step-edge junctions and SQUIDs over YBCO ground planes. These multilevel circuits employ thin films of SrTiO3 and NdGaO3 as the insulating layer between the active device and the ground plane and use Ag as the normal metal in the Josephson junction. The reproducibility and uniformity of the junctions are better than our single-level devices grown directly on step edges cut into single-crystal substrates. Here the critical current variation among junctions on a single wafer is less than a factor of 2. Junctions grown on thin-film step edges of SrTiO3 have critical currents near 2 mA at 4 K, while those grown on NdGaO3 step edges have critical currents near 0.5 mA at 4 K.