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

Highlights

• Nb₃Sn superconducting films were fabricated by magnetron sputtering from a single target.

• Effects of annealing temperature and time on film structure and superconducting properties were examined.

• The highest critical temperature T_c = 17.83 K was observed for the Nb₃Sn films annealed at 800 °C for 24 h.

• Raman study of the films showed high sensitivity to the surface composition.

• RF surface resistance of Nb₃Sn on Nb shows the highest T_c = 17.44 K.

Abstract

Superconducting Nb₃Sn films were fabricated on sapphire and fine grain Nb substrates by magnetron sputtering from a single stoichiometric Nb₃Sn target. The structural, morphological and superconducting properties of the films annealed for 24 h at temperatures of 800–1000 °C were investigated. The effect of the annealing time at 1000 °C was examined for 1, 12, and 24 h. The film properties were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. The DC superconducting properties of the films were characterized by a four-point probe measurement down to cryogenic temperatures. The RF surface resistance of films was measured over a temperature range of 6–23 K using a 7.4 GHz sapphire-loaded Nb cavity. As-deposited Nb₃Sn films on sapphire had a superconducting critical temperature of 17.21 K, which improved to 17.83 K when the film was annealed at 800 °C for 24 h. For the films annealed at 1000 °C, the surface Sn content was reduced to ~11.3% for an annealing time of 12 h and to ~4.1% for an annealing time of 24 h. The Raman spectra of the films confirmed the microstructural evolution after annealing. The RF superconducting critical temperature of the as-deposited Nb₃Sn films on Nb was 16.02 K, which increased to 17.44 K when the film was annealed at 800 °C for 24 h.

Introduction

Nb₃Sn is of interest as a coating for superconducting radiofrequency (SRF) cavities due to its higher critical temperature T_c of ~18.3 K and superheating field H_sh of ~400 mT [1]. Nb₃Sn cavities have the potential to achieve high quality factor Q₀ when operated at 4 K and can replace the bulk Nb cavities that are operated at 2 K [2], [3]. The higher operating temperature of Nb₃Sn cavities compared to Nb can significantly reduce the operating cost. However, Nb₃Sn cannot be used as bulk material due to its fragile nature and poor thermal conductivity. Nb₃Sn thin films coated on niobium or copper are considered as potential alternative materials for SRF cavities [2], [3]. A 1.3 GHz single-cell Nb₃Sn/Nb cavity fabricated by Sn vapor diffusion at Jefferson Lab have demonstrated a Q₀ ≥ 2 × 10¹⁰ at 4 K before quenching at a field ≥15 MV/m [4], while Nb₃Sn/Nb CEBAF five-cell cavities had a low-field Q₀ of ~3 × 10¹⁰ [5] and maximum accelerating gradient up to 15 MV/m at 4 K [6]. An accelerating gradient of 22.5 MV/m at 4 K was achieved at Fermilab for a 1.3 GHz single-cell cavity [7]. At Cornell University, Nb₃Sn-coated 2.6 GHz cavities had a quality factor of 8 × 10⁹ at 4 K, which is 50 times higher than that of 2.6 GHz Nb cavities at 4 K, and preliminary data on Nb₃Sn-coated 3.9 GHz cavities showed a low field Q₀ of ~2 × 10⁹ at 4.2 K [8]. Nb₃Sn films on sapphire substrates also have potential applications for superconducting microwave devices [9].

Magnetron sputtering has been used to grow Nb₃Sn films from a single stoichiometric Nb₃Sn 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 Nb₃Sn films by magnetron sputtering from a stoichiometric target was performed at Argonne National Lab [10], [11]. There, Nb₃Sn films were grown on sapphire and the superconducting properties were examined for films grown at various sputter argon background pressures of 5–50 mTorr and substrate temperatures of 600–800 °C. The reported T_c and critical current density J_c of the grown films were up to 18.3 K and 15 × 10⁶ A/cm²_, respectively [11]. Nb₃Sn films sputtered from a single phase Nb₃Sn target on MgO substrates had a T_c of 15.3 K after annealing at 900 °C for 1 h [12]. The formation of Nb₃Sn by the reactive interdiffusion of Nb/Sn multilayers on oxidized Si and sapphire at high temperatures was investigated at AT&T Bell Laboratories [14]. The experiments of Nb/Sn multilayers on oxidized Si and sapphire showed that, Nb₆Sn₅ phase was present during the growth of A15 Nb₃Sn. The Nb₆Sn₅ phase disappeared above 800 °C. In this study, the highest T_c of 17.45 K was observed for films annealed at 850 °C. Nb₃Sn sputtered on copper [13], [18], [19] and Nb [20], [21] substrates were studied for its application in SRF cavities. For Nb₃Sn 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 Nb₃Sn [13]. Niobium has a high melting point and a thermal expansion coefficient close to that of Nb₃Sn [22]. Therefore, the deposition of Nb₃Sn on Nb and annealing of the grown film can be performed at temperatures above 830 °C. Nb₃Sn films grown by magnetron sputtering on different substrates have shown DC superconducting T_c of up to 15.7 K for copper [18], 17.7 K for niobium [21], and 17.93 K for sapphire substrates [23]. For SRF cavity applications, the RF properties of Nb₃Sn films are of interest. The RF properties of magnetron sputtered Nb₃Sn films on sapphire [24], [25] and copper substrates [26] were reported previously.

We present the impact of annealing temperatures over the temperature range of 800–1000 °C on the material, DC, and RF superconducting properties of Nb₃Sn films grown on sapphire and Nb by magnetron sputtering from a single stoichiometric Nb₃Sn target. The changes in film structure, morphology, and chemical composition due to different annealing conditions on different substrates is compared with the DC and RF superconducting properties of the films. We established the growth conditions to produce Nb₃Sn films with high T_c and very sharp superconducting transitions on sapphire substrates and applied the conditions to grow Nb₃Sn films on niobium substrates. The RF surface resistance of the Nb₃Sn films sputtered on Nb was then characterized with a 7.4 GHz sapphire-loaded Nb cavity.