Effect of layer thickness on structural, morphological and superconducting properties of Nb$_3$Sn films fabricated by multilayer sequential sputtering

Superconducting Nb3Sn films can be synthesized by controlling the atomic concentration of Sn. Multilayer sequential sputtering of Nb and Sn thin films followed by high temperature annealing is considered as a method to fabricate Nb3Sn films, where the Sn composition of the deposited films can be controlled by the thickness of alternating Nb and Sn layers. We report on the structural, morphological and superconducting properties of Nb3Sn films fabricated by multilayer sequential sputtering of Nb and Sn films on sapphire substrates followed by annealing at 950 {\deg}C for 3 h. We have investigated the effect of Nb and Sn layer thickness and Nb:Sn ratio on the properties of the Nb3Sn films. The crystal structure, surface morphology, surface topography, and film composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray spectroscopy (EDS). The results showed Sn loss from the surface due to evaporation during annealing. Superconducting Nb3Sn films of critical temperature up to 17.93 K were fabricated.


Introduction
Nb3Sn is one of the type II superconductors that has wide applications from DC high-field magnets to radiofrequency cavities [1][2][3][4][5]. Magnetron sputtering is considered as one of the fabrication methods used to synthesize Nb3Sn films. Nb3Sn can be fabricated either from a stoichiometric Nb3Sn single target [6,7], or from sputtering of Nb and Sn followed by annealing [8]. Deposited films should maintain a controlled Sn composition range with atomic composition of 17-26% to obtain superconducting Nb3Sn films [3]. For multilayer growth, the atomic Sn concentration of the films can be controlled by varying the thickness of the Nb and Sn layers.
We report on the structural, morphological and electrical properties of Nb3Sn films fabricated by multilayer sequential sputtering of Nb and Sn films on sapphire substrates followed by annealing at 950 °C for 3 h.
The temperature and time were selected based on previously reported work on Nb3Sn fabrication [8]. The film properties were characterized for different layer thicknesses of Nb and Sn multilayers. The role of layer thickness was studied by varying the thickness of the Nb layers while keeping the Sn layer thickness constant, which adjusts the stoichiometry and by varying the thickness of both Nb and Sn layers while keeping the Nb:Sn thickness ratio constant.

Fabrication
Nb3Sn films were grown by annealing sequentially-sputtered multilayers of Nb and Sn. Some details on the deposition chamber, substrate, and target materials were described elsewhere [8]. Multiple layers of Nb and Sn with different thicknesses were deposited sequentially with the first layer Sn and the final layer Nb. The final Nb layer minimizes Sn loss during post-deposition annealing. Films were deposited in a high vacuum chamber at 4 × 10 -3 mbar Ar sputtering gas (99.999% purity) with a flow rate of 20 SCCM. The substrate holder was rotated at 30 rpm throughout the sputtering process to maintain a homogenous deposition.
Information about the film thickness is shown in Table 1. In the study of film stoichiometry, we chose a constant thickness of 10 nm of Sn and four thicknesses (10, 20, 30 and 40 nm) of Nb layers. These four conditions are referred to as conditions 1-4 in our results. The total thickness for all four conditions was kept constant (~900 nm) by varying the number of layers. In another set of films, we kept the Nb-to-Sn thickness ratio 2:1. For all films on these conditions, the total thickness was ~1.2 µm. These four conditions will be referred as conditions 5-8 in our results. All films were annealed in a separate vacuum furnace at 950 °C for 3 h with a temperature ramp rate of 12 °C/min.

Characterization
X-ray diffraction (XRD) patterns of the films were obtained from Rigaku Miniflex II X-ray diffractometer with Cu-Kα radiation. The crystallite sizes of the films were measured from the peak broadening using Scherrer's equation [9]. The film microstructure was observed by field-emission scanning electron microscopic (SEM) images (FESEM S-4700, Hitachi, Japan

Structural properties.
The Sn composition of as-deposited and annealed films are shown in Table 2. The Sn composition was reduced with increasing Nb layer thickness on as-deposited films. About 43.6% Sn was observed on films with condition 1, where both Nb and Sn layer thickness was maintained at 10 nm. All annealed films showed Sn loss after annealing. Large amount of Sn loss was observed on films with condition 1. The Sn composition changed from ~43.6% to ~23.8%. This large amount of Sn loss occurred due to sublimation of Sn during annealing. All annealed films showed Sn composition ~20-23%. The composition of the asdeposited films was ~16% in condition 4, however, the annealed film had ~20% Sn. Annealed film showed more Sn due to the uniform distribution of Sn throughout the film after annealing.  The crystallite size and lattice parameter corresponding to Nb3Sn (210) diffraction peak of the films as a function of Nb:Sn thickness ratio are shown in Figure 1 (b). The crystallite size decreased with increasing Nb layer thickness, which is in agreement with the grain size observed in SEM and AFM. Lattice parameters calculated from the d value of XRD peak are less than lattice parameter of bulk Nb3Sn (5.290 Å). This is because the Sn composition in the film is less than ideal condition of 25%.

Superconducting properties.
The resistivity versus temperature is shown in Figure 2 (c). The calculated Tc, and ∆Tc from the graph are shown in Table 2 and Figure 2 (d). All films exhibited good superconducting properties. The highest critical temperature is observed on the film fabricated with condition 1, where the Nb and Sn thicknesses were the same. Better Tc at this condition was obtained due to higher Sn composition on the annealed films. It has been reported that Tc of Nb3Sn is dependent on the Sn composition of the films [1]. The transition width also became wider with increasing Nb:Sn.

Structural properties.
All annealed films (conditions 4-8) had Sn composition in the range of 21-23%. Figure 3 shows

Superconducting properties.
The surface resistivity of the films for different Sn layer thicknesses is shown in Figure 4 (c) and the corresponding Tc and ∆Tc data are shown in Table 2 and Figure 4 (d), respectively. The Tc and ∆Tc of all films are close. Slight increased Tc is observed on the film for condition 8, however, the ∆Tc also increased.

Discussion
Sn composition plays an important role on the superconducting properties of Nb3Sn. In SRF cavities, Nb3Sn films coated by conventional Sn vapor diffusion method resulted to Sn deficient patchy regions which degraded the performance of SRF cavities [10,11]. Therefore, it is important to maintain the stoichiometry of the films after annealing. We have experienced Sn loss for almost all deposition conditions after annealing. This Sn loss resulted from the Sn diffusion and evaporation from the surface. The Sn composition for annealed films obtained from EDS in conditions 3, 4 and 8 are higher than that of asdeposited films. This is due to the limitation of the spatial resolution of EDS. For 15 keV electron, the Anderson-Hasler equation [12] gives the X-ray transmission fraction of 0.68 µm for Nb-Lα lines and 0.76 µm for Sn-Lα lines, which is shorter than the thicknesses of the deposited films. Because of the short Xray escape depth, layer ordering and the individual layer thickness affects the apparent EDS composition. The surface morphology of the films is affected by the coating parameters. It is possible to change the surface roughness varying the thickness of Nb and Sn layers. The grain sizes of the films were also varied when Nb:Sn ratio was varied. The cross-section image of these films could give more detailed idea about the grain structure near the surface as well as the interface. The technique has been used on vapor-diffused samples [15], but not yet on the present films.

Conclusion
Nb3Sn films were fabricated by sputtering multilayers of Nb and Sn with varied layer thicknesses followed by annealing. Throughout annealing, evaporation of Sn from the surface and growth of Nb3Sn by diffusion of Sn into Nb surface occur simultaneously. Sn loss is the outcome of the competition of these two processes. Sn loss depends on annealing parameters (annealing temperature and annealing time). For annealing at 950 °C for 3 h, Sn composition were 20-23% for all coating conditions. The surface morphology and the roughness of the films varied with the thickness of the layers. For different Nb:Sn thicknesses, the grain size and surface roughness decreased with increasing thickness ratio. For different layer thicknesses with constant thickness ratio, the surface roughness increased with increasing thickness of the layers, whereas the grain size did not vary significantly. All films showed good superconducting properties with a superconducting critical temperature Tc up to 17.93 K.