Fabrication and characterization of hybrid Nb-YBCO DC SQUIDs

Hybrid low T_c/high T_c DC SQUIDs of two types were fabricated. The first utilizes niobium tunnel junctions and a YBCO (Y₁Ba₂Cu₃O_7-x ) film strip as the most inductive portion of the SQUID loop. This configuration allows a direct measurement of the inductance of the YBCO microstrip from which the effective penetration depth can be calculated. The successful fabrication of these SQUIDs has required superconducting Nb-to-YBCO contacts, deposition and patterning of an SiO₂ insulation layer over YBCO, and selective patterning of niobium and SiO ₂ relative to YBCO. All These process steps are pertinent to the eventual use of YBCO thin films in electronic devices. The large effective YBCO film penetration depth inferred (~1.2 μm) is consistent with the polycrystalline structure of these films. The second squid utilizes grain boundary YBCO junctions engineered by the use of bare sapphire substrate. A single-turn niobium drive coil is then deposited on-chip, separated from the YBCO SQUID by an SiO₂ insulation layer. Good isolation was obtained, and the observed periodicity of the SQUID response corresponds to the calculated flux from the niobium drive coil penetrating the YBCO SQUID loop. These results and the circuit design and fabrication are presented and discussed