Abstract
As Nb films are widely used as superconducting electrodes of Josephson junctions, it is important to investigate the properties of Nb films in order to fabricate high-quality Josephson junctions. In this work, we conducted a comprehensive analysis of the relationships among the properties of DC magnetron sputtered Nb films with a constant power fabricated at the National Institute of Metrology (China). The film properties, including superconductivity, stress, lattice constant, and surface roughness, were investigated. It was found that in the case of constant power and Ar pressure, the stress and other parameters of the Nb films can maintain a relatively stable state during the continuous consumption of the target material.
Highlights
DC magnetron sputtering is one of the thin-film deposition manufacturing processes which are applied in high-vacuum chambers filled with argon gas
Further studies were carried out on the relationships between the Nb film thickness, the Ar pressure, film stress, and superconducting transition temperature [3,4]; conclusions suggested that the transition temperature is hardly changed with the variation in the film thickness when the Nb film thickness is more than 200 nm [5], and low-stress Nb films can be obtained by different sputtering conditions
The main property parameters of Nb films used for Josephson junctions are stress, superconducting transition temperature, residual resistance ratio, lattice constant, and surface roughness
Summary
DC magnetron sputtering is one of the thin-film deposition manufacturing processes which are applied in high-vacuum chambers filled with argon gas. Since the 1960s, studies have been carried out to determine niobium (Nb) film deposition using DC magnetron sputtering in order to obtain high-quality superconducting electrodes: changing the impurity gas content in Nb films by ion bombarding the sample substrate [1] to improve the residual resistance ratio and superconducting transition temperature of Nb films, changing the distribution of Nb film stress by Ar ion bombardment [2], etc. The main property parameters of Nb films used for Josephson junctions are stress, superconducting transition temperature, residual resistance ratio, lattice constant, and surface roughness. A higher quality of Nb films exhibits a lower compressive stress with a residual resistance ratio of over 3, a smaller value of roughness, a higher superconducting transition temperature, and a lattice constant close to bulk Nb. This work mainly focused on the Nb film properties under a constant sputtering power and different Ar pressures, as well as the influence of the target-to-substrate distance. It was found that this relationship between stress and Ar pressure was stable and repeatable as long as the sputtering power was fixed at 500 W and target-tosubstrate distance was 110 mm; the Nb film stress can be controlled by adjusting the Ar pressure
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