Development of sputtered Nb₃Sn films on copper substrates for superconducting radiofrequency applications

Abstract

Superconducting radiofrequency (SRF) cavities that could provide a higher quality factor as well as a higher operational accelerating gradient at an affordable cost are in high demand for the future generation of particle accelerators. This study aims to demonstrate the potential of Nb3Sn as material of choice for such SRF applications. Due to its brittle nature, the only way to produce an Nb3Sn SFR cavity is to synthesise a thin layer inside a cavity made of niobium or copper. In this work, direct current magnetron sputtering using a stoichiometric target of Nb3Sn was employed to produce films on copper samples. Assessment of the morphology, microstructure and superconducting properties were performed in order to ensure that this approach is suitable for SRF applications. The potential of the method is proven by obtaining films, which exhibit a crack-free surface, dense morphology and critical temperatures (Tc) up to 16 K. The essential properties of the films have also been investigated with respect to the deposition and annealing conditions. The use of krypton as working gas during deposition increases the atomic percent of Sn in the film compared to argon. However, in contrast to argon, higher krypton pressures reduce the atomic percent of Sn. It was also found that long-lasting high-temperature annealing leads to higher superconducting critical temperatures due to an increased crystallographic order. Particular attention was given to the influence of the copper substrate on the film growth as well as the microstructural and superconducting characteristics. We discuss the main constraints introduced by the copper substrate, such as copper interdiffusion during annealing, lattice mismatch and difference in thermal expansion coefficients and methods to overcome them.

Properties of Nb3Sn films fabricated by magnetron sputtering from a single target

“…The experiments of Nb/Sn multilayers on oxidized Si and sapphire showed that, Nb 6 Sn 5 phase was present during the growth of A15 Nb 3 Sn. The Nb 6 Sn 5 phase disappeared above 800 • C. In this study, the highest T c of 17.45 K was observed for films annealed at 850 • C. Nb 3 Sn sputtered on copper [13,18,19] and Nb [20,21] substrates were studied for its application in SRF cavities. For Nb 3 Sn deposited on copper, the annealing temperature was limited to 830 • C due to a lower melting point of copper and the mismatch between the thermal expansion coefficients of copper and Nb 3 Sn [13].…”

“…The Nb 6 Sn 5 phase disappeared above 800 • C. In this study, the highest T c of 17.45 K was observed for films annealed at 850 • C. Nb 3 Sn sputtered on copper [13,18,19] and Nb [20,21] substrates were studied for its application in SRF cavities. For Nb 3 Sn deposited on copper, the annealing temperature was limited to 830 • C due to a lower melting point of copper and the mismatch between the thermal expansion coefficients of copper and Nb 3 Sn [13]. Niobium has a high melting point and a thermal expansion coefficient close to that of Nb 3 Sn [22].…”

“…Magnetron sputtering has been used to grow Nb 3 Sn films from a single stoichiometric Nb 3 Sn target [10][11][12][13], using separate Nb and Sn targets to deposit multilayers that are thermally interdiffused [14][15][16], or co-sputtering of Nb and Sn [17]. Early successful fabrication of Nb 3 Sn films by magnetron sputtering from a stoichiometric target was performed at Argonne National Lab [10,11].…”

Kinetically induced low-temperature synthesis of Nb3Sn thin films

“…Among the different intermetallic phases of the Nb-Sn system, only the A15-type Nb 3 Sn is superconducting above 10 K. However, thin film synthesis of high quality Nb 3 Sn is still a huge challenge. 2 Among the reported thin film deposition methods are tin evaporation, [3][4][5] chemical vapor deposition, 6 sputtering, [7][8][9][10][11] electron beam evaporation, 12 bronze processing, 13 and electrodeposition. 14 However, the tin diffusion process is the only process used so far for the production of cavities.…”

Nb3Sn multicell cavity coating system at JLAB

“…Various technique have been developed to grow Nb 3 Sn superconductor. Bronze routes, sputtering techniques, tin dip et cetera have been studied (see [7][8][9][10] and references therein), but require further development due to the exceptional quality of the RF surface necessary to sustain high RF fields. The best Nb 3 Sn-coated SRF cavities so far have been produced with the so-called vapor diffusion process [11].…”