FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

Abstract

Focused ion beam-scanning electron microscope (FIB-SEM) tomography is a powerful application in obtaining three-dimensional (3D) information. The FIB creates a cross section and subsequently removes thin slices. The SEM takes images using secondary or backscattered electrons, or maps every slice using X-rays and/or electron backscatter diffraction patterns. The objective of this study is to assess the possibilities of combining FIB-SEM tomography with cathodoluminescence (CL) imaging. The intensity of CL emission is related to variations in defect or impurity concentrations. A potential problem with FIB-SEM CL tomography is that ion milling may change the defect state of the material and the CL emission. In addition the conventional tilted sample geometry used in FIB-SEM tomography is not compatible with conventional CL detectors. Here we examine the influence of the FIB on CL emission in natural diamond and the feasibility of FIB-SEM CL tomography. A systematic investigation establishes that the ion beam influences CL emission of diamond, with a dependency on both the ion beam and electron beam acceleration voltage. CL emission in natural diamond is enhanced particularly at low ion beam and electron beam voltages. This enhancement of the CL emission can be partly explained by an increase in surface defects induced by ion milling. CL emission enhancement could be used to improve the CL image quality. To conduct FIB-SEM CL tomography, a recently developed novel specimen geometry is adopted to enable sequential ion milling and CL imaging on an untilted sample. We show that CL imaging can be manually combined with FIB-SEM tomography with a modified protocol for 3D microstructure reconstruction. In principle, automated FIB-SEM CL tomography should be feasible, provided that dedicated CL detectors are developed that allow subsequent milling and CL imaging without manual intervention, as the current CL detector needs to be manually retracted before a slice can be milled. Due to the required high electron beam acceleration voltage for CL emission, the resolution for FIB-SEM CL tomography is currently limited to several hundreds of nm in XY and up to 650 nm in Z for diamonds. Opaque materials are likely to have an improved Z resolution, as CL emission generated deeper in the material is not able to escape from it.

Scanning electron microscope cathodoluminescence imaging of subgrain boundaries, twins and planar deformation features in quartz

“…These estimates also provide an estimation for the resolution limit of SEM-CL images, because the CL emission at any point in the sample will be an average of the CL emission of the whole interaction volume of the electron beam with the sample. Interestingly, fast ion bombardment with gallium ions used to prepare the TEM sections did not destroy the CL signal, in agreement with CL studies on diamond (De Winter et al 2011).…”

D-CAT: Density and Clustering Annotation Tool for three dimensional electron microscopic volumes

“…These image stacks are often acquired at a 38°angle Hekking et al, 2009), and as a result, the imaged Y-resolution is a condensed representation of the sample (De Winter et al, 2011). Moreover, the pixel-size 1047-8477/$ -see front matter Ó 2011 Elsevier Inc. All rights reserved.…”

“…tual pixel-size of 3.17 nm/px in Y through the formula [1/cos /], where / is the acquisition-angle(De Winter et al, 2011)) with a Z-slice thickness of 40 nm. The FIB-SEM tomogram was obtained perpendicular to the vessel-wall.…”

Nano-Tomography of Porous Geological Materials Using Focused Ion Beam-Scanning Electron Microscopy

“…This is sufficient to avoid electron beam charging and damage, even for non-conductive materials. The interaction volume between Ga and the material is much smaller than that of electrons; the ions accelerated by a 30 kV voltage can penetrate tens of nanometres into the surface compared to microns for electrons with the same acceleration voltage [67,68]. Thus, for ion beam imaging and slice milling, a Pt-layer of about 1 µm deposited in situ was made to protect the ROI.…”