Development of Nb3Sn on Cu by Pulsed magnetron sputtering for Superconducting RF applications.

Nb₃Sn in the form of thin film on copper is one of the most promising routes in the field of superconducting radio-frequency accelerating cavities for future colliders. At INFN-LNL, Nb₃Sn superconducting films has been developed using DC magnetron sputtering (DCMS), deposited at T ≤ 650 °C on copper substrates pre-coated with a 30 μm niobium buffer layer. The coatings exhibiting a critical temperature ≥ 17 K, furthermore, RF tests on a film deposited with the same recipe on a QPR sample in bulk niobium showed a surface resistance of 23 nΩ at 4.5 K, 20 mT, and 400 MHz. However, DCMS presents two main limitations: it requires low-power operation, as increasing the power can lead to preferential evaporation of the tin fraction in the target, and morphological analysis revealed the formation of Sn-rich clusters, which compromise the radiofrequency properties of the cavities. To overcome these limitations, this work focuses on the development of thin Nb₃Sn films using pulsed magnetron sputtering (PMS), deposited on pre-coated Nb buffer layers on copper. Sapphire and bulk niobium substrates were also employed to study the effect of substrate affinity on film growth and properties. Unlike DCMS, PMS allows the application of high peak power to the target while maintaining a comparable average power, which could reduce the risk of Sn evaporation and allows operation at higher powers. A preliminary analysis was carried out to maximize information on the dependence of Tc on process parameters. As a second step, a focused analysis was performed to assess the effect of each parameter on morphological responses and superconducting properties of the films.