Three-Dimensional Reconstruction of Nb3Sn Films by Focused Ion Beam Cross Sectional Microscopy

Niobium has been the material of choice for SRF cavities for several decades due to its formability and superconducting properties. The accelerating gradient of niobium cavities is, however, rapidly approaching a theoretical limit. To achieve higher accelerating gradients a new material is needed that can sustain high fields. Nb3Sn is a promising competitor with a higher superconducting transition temperature and a higher critical field than pure niobium. However, Nb3Sn is very brittle and cannot be formed readily into a cavity. The main method for creating Nb3Sn cavities is to form a Nb3Sn film into a niobium surface using a tin vapor-diffusion method. This technique creates a microcrystalline Nb3Sn thin film on the inner surface of the cavity. Tin depleted regions are known to form in the film during this process. Previous studies have analyzed these regions using transmission electron microscopy on cross-sectional lamellae prepared by focused ion beam/scanning electron microscope (FIB/SEM). This method does not provide any three-dimensional (3-D) information about the distribution of tin-deficient regions. In this study we employ a focused ion beam tomographic technique to analyze the 3-D structure of the film. Electron dispersive X-ray spectroscopy is used to image the tin concentration of the film in 3-D. Tin-deficient regions are discovered close to the surface of the Nb3Sn film.