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

Abstract

Nb 3 Sn and V3Si thin films are promising candidates for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues. This exploratory study investigates the structural and chemical effects of thermal annealing, both in−situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films with varying thicknesses, deposited on Nb or Cu substrates. Building upon our initial studies [Howard et al., Proceedings of the SRF’21, East Lansing, MI (JACoW, 2021), p. 82.], we provide fundamental insights into recrystallization, phase changes, and the issues of stoichiometry and strain. Through annealing at 950 °C, we have successfully enabled the recrystallization of 100 nm thin Nb3Sn films on Nb substrates, yielding stoichiometric and strain-free grains. For 2 μm thick films, elevated annealing temperatures led to the removal of internal strain and a slight increase in grain size. Moreover, annealing enabled a phase transformation from an unstable to a stable structure in V3Si films, while we observed significant Sn loss in 2 μm thick Nb3Sn films after high-temperature annealing. Similarly, annealing films atop Cu substrates resulted in notable Sn and Si loss due to the generation of Cu–Sn and Cu–Si phases, followed by evaporation. These results encourage us to refine our process to obtain high-quality sputtered films for SRF use.

Penetration depth and critical fields in superconducting NbTi thin films grown by co-sputtering at room temperature

“…Indeed, the estimated H c for NbTi is similar to the approximately 0.2 T of Nb [33] and to that expected for NbN [34]. The simplicity of the growth process of NbTi thin films developed in this study, along with the determined superconducting parameters, holds potential technological relevance by allowing its application of coatings or multilayers on both Nb and Cu cavities [32,35,36].…”

“…Our study provides a detailed characterization of the superconducting parameters, with a particular emphasis on determining λ values, which are often scarce in the literature. By utilizing these values along with ξ, we determine parameters such as the thermodynamic critical field that can be relevant for the application of NbTi films as coatings for SRF cavities [32,35,36]. Moreover, the development of ultra-thin films can be crucial for devices like superconducting nanowire single-photon detectors based on Nbbased systems [17,41,42].…”