Bias-sputtered Nb for reliable wirings in Josephson circuits

The authors applied bias-sputtered Nb films 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 voltages less than -150 V. The authors studied the characteristics of contacts formed between two Nb wirings. By using bias-sputtered Nb as the upper wiring, the lc of the contacts increased 2--10 times. Reduction in contact Ic after annealing was markedly suppressed. This paper verifies that the bias-sputtered Nb was feasible in Josephson circuits as reliable wirings, with no compromise of junction characteristics