Influence mechanism of RF bias on microstructure and superconducting properties of sputtered niobium thin films

Abstract

In this study, radio frequency (RF) biased direct current (DC) magnetron sputtering was used to simultaneously improve the surface roughness and superconducting properties of Nb thin films for Josephson circuits application. The surface roughness of the Nb films decreased with an increase in RF powers, whereas the transition temperature, transition width, and residual resistance ratio of the Nb film were optimum at an RF power of 20 W but will deteriorate at higher RF powers. The film resistivities at 10 K obviously increased when the RF power was increased to 40 W and 60 W, which indicates that electron-impurity scattering and scattering on lattice defects are severer in these Nb films. The microstructures, impurities (mainly Ar) and dislocation states were correlated with the electrical and superconducting properties of Nb films. In summary, an optimal process for depositing Nb films with simultaneously improved surface roughness and superconducting properties was found, the Nb film deposited at a DC power of 500 W and an RF bias power of 20 W with a substrate bias voltage of approximately −100 V had the best overall quality. The method is prospective to be applicated in multilayer superconducting electronic devices fabrication based on Nb and other superconductors.