Bias-sputtered Nb for reliable wirings in Josephson circuits

Bias-sputtered Nb films were applied to wirings in Josephson circuits. Applying bias voltage during sputtering improved Nb step coverage and stability against annealing. Deteriorated superconducting characteristics in films were negligible for bias voltage less than -150 V. The characteristics of contacts formed between two Nb wirings were studied. By using bias-sputtered Nb as the upper wiring, the I _c of the contacts increased 2-10 times. Reduction in contact I_c after annealing was markedly suppressed. The authors verified that the bias-sputtered Nb was feasible in Josephson circuits as reliable wirings, with no compromise of junction characteristics