High-reliability Pb-alloy Josephson junctions for integrated circuits

The process developed and recently used at IBM for fabricating experimental Pb-alloy Josephson tunnel-junction devices, and the factors which influence the stability of such devices during repeated cycling between 300 and 4.2 K are reviewed. A new, fine-grained Pb 0.84 In 0.12 Au 0.04 alloy base electrode material has been developed that has excellent thermal cycling stability. In an experiment carried out to evaluate the cyclability of devices prepared with this material, excellent results were obtained: the cyclability of large-area junctions was improved by ∼100× compared to that of similar junctions prepared with the recently used, larger-grained Pb 0.84 In 0.12 Au 0.04 base electrodes. In the best cases, populations of 2600 large junctions and 2350 interferometers were found to withstand 400 and 700 thermal cycles to 4.2 K, respectively, before the first failures were observed. These results indicate that with the use of fine-grained electrodes, Pb-alloy Josephson devices have good potential for meeting the cycling requirements of computer systems.