Non-Contact 3D Surface Metrology of Large Grain High Purity Niobium by SLCM and FESEM Z.H. Sung, P.J. Lee and D.C, Larbalestier Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University, FL, 32310. A large number (~20,000) of high quality superconducting rf (SRF) cavities will be required for the proposed International Linear Accelerator, imposing stringent requirements on production quality, uniformity and yield. One source of variability is from surface topology introduced by the chemical removal (either by chemical or electro-polishing) of damaged and contaminated layers from the interior of the cavity after mechanical fabrication from high purity Nb sheet.[1] Making cavities from large grain (10-50 mm) Nb ingots is a recent approach that both reduces the effect of differential surface etching and the number of fabrication steps [2]. Such large-grain slices also provide excellent material for the study of the superconducting properties of individual Nb grains and grain boundaries in a form that allow us to improve our understanding of the relationship between materials and processing, and cavity performance.[3] We have developed a sample preparation technique that allows us to measure the local superconducting properties on bi-crystal samples by magneto-optical imaging and resistivity as well as local chemistry by EELS [4]. In order to correctly interpret these results we must also have an accurate measurement of the surface topology at the grain boundary. Because the soft high purity Nb is easily damaged, and each sample must undergo multiple characterization tests, a non-contact measurement technique must be used to quantify the local topology. In this study we compare surface height measurements from 1. a scanning (blue) laser confocal microscope, SLCM (Olympus LEXT OLS3100), that provides spatial resolution in the z-direction by eliminating out-of-focus information in object planes of the optical system, and 2. a commercial software package [4] that extracts height information from sets of 2 or 3 tilted eucentric images, in this case obtained using a high resolution FESEM (Zeiss 1540EsB). The surface topological data produced by the SLCM confocal microscope is illustrated in Fig. 1: as a 2D contour map (a) and a 3D image (b). Together they show the characteristic topology in the vicinity of the GB created by the “optimized” cavity surface treatment. The topology is automatically obtained with a height resolution of ~ 10nm as part of the standard multi-plane SLCM image acquisition technique. Digital stage control allows us to make volumetric analyses of our entire specimens which is useful for our electrical measurements. However for detailed lateral analysis beyond the ~0.12 µm x-y plane resolution of the SLCM we need to use scanning electron microscopy. Because of the high depth of field of SEM we need to use software to interpret height information from the geometric movement of features when the sample is tilted. For our samples the optimum tilt angles and working distance were found to be ±7° and 5 mm respectively. Fig. 2 shows (a) the 2D contour image determined from 3D image. processing by combining three captured SE images along with eucentric tilts from -7°, 0°, to +7° and (b) the height profiles obtained along the red line on 3D height image (SCLM) or 2D contour image (FESEM). Both results provide comprehensive depth information for the GB. However, compared to Fig.1, the results from the SEM imaging indicate additional sub-micron height variations that do correspond to visible surface features observed in the

secondary electron image. The magnitude of the grain boundary height change is in good agreement with that from the same area from the SLCM. References: [1] J. Knobloch, et al 9th SRF workshop (1990). [2] P. Kneisel et al., Proc. PAC’05, IEEE Cat. No. 05CH37623, Paper TPPT076 (2005). [3] P.J. Lee et al., Physica C 441(1), 126–129 (2006). [4] Z.H. Sung et al., ibid [5] Mex 5.0; 3D software (Alicona Imaging GmbH, www.alicona.com). [6] This work support in part by US DOE under grants DE-FG02-05ER41392 and DE-FG02-07ER41451 and by the State of Florida.

FIG. 1.(a) 2D contour image (b) 3D height map (Olympus LEXT OSL3100).

FIG. 2 (a) Eucentric tiled SE2 images & 2D contour image obtained from the Mex5.0 digital imaging process in Carl Zeiss 1540 EsB-FESEM. (b) Comparison of Height profiles from the red bar on each 2D height images