Study of fabrication problems with Nb3Sn films used for Josephson tunnel junctions

The fabrication of superconductive Nb3Sn thin films was done by two different techniques, composite target sputtering and the reaction of elemental films. Both techniques deposited films onto cooled substrates and then used annealing to form the high T/sub c/, A15 phase of the films. Transition temperature of 17.9 to 18.0K measured resistively were routinely achieved. Composite target films needed a surface-modification procedure before tunnel junctions showing large energy gap could be made. A range of fabrication parameters were tried. Reaction method films were mace by varying the Nb/Sn ratio, annealing temperature, sputter etch depth in sputter cleaning and cathode bias while sputter cleaning. Optimum values were 3:1; 8500C, 5-10 nm, and 250-500 V, respectively. All junctions made on these films exhibited leakage currents that showed a definite increase at the Pb energy gap indicating normal material in the film and x-ray diffraction showed very small amounts of Nb6/Sn5. An in-situ surface analysis system was built to study the surface of the Nb3Sn films after processing and to correlate these measurements with junctions fabricated directly on the characterized surfaces. Auger spectroscopy, ellipsometry, sputter cleaning and film annealing were some of the techniques and processes studied in this system. The Sn-poor surface created by sputter cleaning could be improved by an in-situ anneal