Thermal annealing of sputtered Nb3Sn and V3Si thin films for superconducting radio-frequency cavities

TL;DR

This study investigates how thermal annealing affects the structure and composition of sputtered Nb3Sn and V3Si thin films, aiming to improve their suitability for superconducting radio-frequency cavities.

Contribution

It provides new insights into optimizing annealing processes to achieve stoichiometric, strain-free, and stable superconducting thin films on different substrates.

Findings

Recrystallization of Nb3Sn at 950°C on Nb substrates.

Strain removal and grain growth in thick films with increased annealing temperature.

Phase stabilization and Sn/Si loss during high-temperature annealing.

Abstract

Nb3Sn and V3Si thin films are promising candidates as thin films for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues during deposition and post annealing. In this study, we explore the structural and chemical effects of thermal annealing, both in-situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films of varying thickness on Nb or Cu substrates, extending from our initial studies [1]. Through annealing at 950 {\deg}C, we successfully enabled recrystallization of 100 nm thin Nb3Sn films on Nb substrate with stoichiometric and strain-free grains. For 2 um thick films, we observed the removal of strain and a slight increase in grain size with increasing temperature. Annealing enabled a phase transformation from unstable to stable structure on V3Si films, while we observed significant Sn loss in 2 um thick Nb3Sn films after high temperature anneals. We observed similar Sn and Si loss on films atop Cu substrates during annealing, likely due to Cu-Sn and Cu-Si phase generation and subsequent Sn and Si evaporation. These results encourage us to refine our process to obtain high-quality sputtered films for SRF use.