Co-Sputtered Nb 3 Sn Thin Films for Superconducting RF Applications: Comparative Growth Study on Cu, SiO 2 and Al 2 O 3 Substrates

Bulk niobium is the standard material for superconducting RF cavities but requires operation at 2 K due to its critical temperature (Tc) of 9.3 K, leading to high cooling costs. Nb3Sn, with a higherTc(∼18.2 K), allows operation at 4.2 K, reducing cryogenic demands and improving efficiency. Although Nb3Sn theoretically offers a higher quality factor (Q), achieving this in practice remains difficult. Co-sputtering Nb3Sn films on copper cavities may improveQ, thermal stability, and delay quenching at high accelerating fields. In this study, Nb3Sn films were co-sputtered on different substrates to investigate theTcand lower critical field (Hc1). The films were deposited in a controlled atmosphere and analyzed using X-ray diffraction, resistivity measurements and superconducting quantum interference device magnetometry. The results confirm the successful formation of the Nb3Sn phase with well-defined superconducting transition. The measuredTcvalue for films grown on r-cut sapphire reached 17.8 K, close to the bulk Nb3Sn value. On the technically relevant substrate copper, aTcvalue of 15 K was obtained. Notably, theHc1value was found to be not only higher than that for bulk Nb3Sn but also higher than that of pure Nb. A high value ofHc1is critical for operating highQcavities at accelerating fields above 20 MV/m.