Fabrication of high-Tc Nb₃Sn films below 800 °C: A HiPIMS approach enabling copper-cavity applications

The synthesis of high-performance A15-phase Nb₃Sn films at temperatures compatible with copper substrates (< 800 °C) remains a significant challenge for next-generation superconducting radio-frequency (SRF) cavities. Conventional methods require high temperatures, which precludes direct deposition on Cu. This study demonstrates a low-temperature deposition strategy using high-power impulse magnetron sputtering (HiPIMS), which provides highly energetic ions to enhance adatom mobility and crystallization kinetics. By systematically optimizing the substrate temperature and bias voltage, we achieved the direct growth of high-quality Nb₃Sn films on sapphire substrates. Remarkably, the film deposited at 750 °C with a −50 V bias attained a high superconducting transition temperature (T_c) of 17.43 K without any post-annealing—a value that is among the highest reported for films prepared below 800 °C. Furthermore, this study investigated the influence of particles with different energy levels on the properties of Nb₃Sn films and elucidated the underlying mechanisms. This work establishes HiPIMS as a potent technique for low-temperature fabrication of Nb₃Sn coatings, paving the way for their integration into cost-effective and high-performance Cu-based SRF cavities.