Fabrication of NbN Films by dc Bias Sputtering and Their Application to Superconducting Bridges

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Abstract
NbN thin films have been prepared by double cathode dc reactive sputtering method. The effect of the substrate bias and the nitrogen partial pressure on the films' resistivity and superconducting transition temperature were investigated. A high critical temperature of around 16 K was obtained for 300 nm films made at about -80 V substrate bias voltage and suitable nitrogen partial pressure. The thickness dependence of the films' critical temperature was determined for film thickness down to 5 nm. NbN variable thickness bridges with length of about 0.2 µm were fabricated and shown to be feasible for use as Josephson devices. Microwave induced steps were observed over the bridges' I-V characteristics over rather narrow temperature ranges.

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