Recipe Optimization and SRF Test of Cu-compatible Nb3Sn Films by DC Magnetron Sputtering from a Stoichiometric Target

The development of modern particle accelerators such as FCC-ee requires improved energy efficiency. On the SRF cavity side, the intermetallic compound Nb3Sn is a promising alternative to niobium: its higher critical temperature (18.3 K) results into a BCS surface resistance at 4.5 K comparable to the one of Nb at 2 K, potentially allowing improved performance and reduced cryogenic costs while maintaining operation at 4.5 K. However, its brittleness makes bulk machining impractical, restricting its application to thin-film coatings. This study presents Nb3Sn thin films deposited on copper substrates via DCMS using a single stoichiometric target. The optimization of the deposition parameters via the evaluation of the critical temperature, morphology, elemental composition and crystalline structure of the films is outlined. A niobium buffer layer is implemented to prevent copper-tin interdiffusion, and plays a key role in the film quality. The results demonstrate Nb3Sn films deposited at ≤ 650 °C on copper substrates pre-coated with a 30 μm niobium buffer layer which exhibit a critical temperature ≥ 17 K. The RF test of a film deposited via the same recipe on a bulk Nb QPR sample yielded an RF surface resistance of 23 nΩ at 4.5 K, 20 mT and 400 MHz. These findings open the way to a scalable approach to high-performance Nb3Sn/Cu cavities.