Investigations towards nanoscale precise polishing of Nb3Sn thin films for SRF applications

Abstract

The superconducting radio frequency performance of Nb3Sn is significantly impacted by its near-surface composition and nanostructure. In this study, an innovative polishing technique for Nb3Sn thin films with nanoscale precision is proposed. Systematic angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis reveal that the mechanism of this polishing process involves the formation of a few nanometer-thick oxide layer on the film's surface, followed by the uniform removal of the oxide layers with HF rinse. It is approximated that the polishing depth achieved in a single cycle can reach several nanometers. Advanced material characterization demonstrates that optimized polishing process effectively eliminates compulsive tin droplets and potential tin-rich phases with poor superconductivity generated on the surface during the preparation of Nb3Sn thin films. Simultaneously, no additional impurity phase will be formed on the surface, and the surface roughness will remain undamaged. Meanwhile, the results of magnetic and electrical analysis show that the superconducting properties of the polished films are not affected. This polishing technique is anticipated to ease the stringent coating conditions for the preparation of high-performance Nb3Sn thin film superconducting cavity, and will be a valuable addition to the post-treatment process of Nb3Sn thin films.