Microstructural Characterization of Nb3Sn Thin Films Using FIB Tomography

Abstract

The accelerating gradient of Nb3Sn superconducting radiofrequency (SRF) cavities is currently limited, and the underlying cause remains an open question in the field. One leading hypothesis attributes this limitation to the presence of tin-deficient regions within the Nb3Sn coating, which can suppress the superheating field. Due to the relatively large coherence length of Nb3Sn, defects near the surface may significantly interact with the RF field. However, these subsurface defects have proven difficult to characterize. This research aims to investigate the structure and distribution of subsurface Sn deficient regions to better understand their influence on cavity performance. We employ focused ion beam (FIB) tomography to analyze the subsurface microstructure of Nb3Sn thin films. This technique enables three-dimensional reconstruction of both the tin distribution and the grain structure within the film. By correlating Sn content with grain structure, we find that Sn deficient regions are more prevalent that previously thought. However, the Sn deficient regions are consistently located below the surface of the film where RF fields are strongly attenuated by supercurrent screening and are likely not a limiting factor for cavity performance.

Figures (5)

• Figure 1: A diagram showing the voronoi-based reconstruction algorithm. First the grain orientation of a real sample is measured at discrete grid-like positions. Then the voronoi decomposition is calculated from the measurement positions to produce a nearest neighbor graph. The misorientation angle is calculated between nearest neighbor cells and GBs are determined using a minimum threshold angle. The grains are then reconstructed by traversing the graph and finding all regions that are enclosed by a GB.
• Figure 2: A 3D reconstruction of the Nb3Sn grains. Each grain is colored with a unique color. A horizontal plane is used to slice the sample. The plane is colored according to the Sn concentration at each point on the plane surface.
• Figure 3: In this figure we show the Sn concentration in a $Nb_3Sn$ film as a function of depth and distance from nearest grain boundary. Blue areas indicate Sn concentration below stoichiometric $Nb_3Sn$ and red areas above. The areas of highest Sn concentration are found most commonly near grain boundaries in the top half of the film and areas of low Sn concentration are found far from GBs below 500 nm from the surface. These Sn deficient regions are indicated by a red ellipse. The top 100 nm of the film also appear to have low Sn content however this is likely caused by the poor spatial resolution of the EDS measurement.
• Figure 4: Cross-sectional view of four different $Nb_3Sn$ grains. The cross section area is colored based on the Sn content. Grains consistently show Sn deficiency near the Nb substrate and the center of the grain far from GBs.
• Figure 5: A cross-sectional view of a $Nb_3Sn$ grain and the Nb substrate. The cross section is colored based on Sn concentration. The rest of the $Nb_3Sn$ grains are rendered translucent for reference.