True Atomic-Scale Imaging in Three Dimensions 211
variations. Simultaneously, the quality and the information retrieval ability in APT images overtook the FIM studies on the same specimen. FIM was downgraded to a complementary instrument to assess crystallographic information obtained in APT data sets. Currently, FIM images are, unfortunately, rarely used as
Figure 1. Isometric drawing showing the relationship of self- interstitial atoms in a crystalline lattice of tungsten to the positions of vacancies in a depleted zone (displacement cascade) detected in the (111) plane in a tungsten tip. Self-interstitial are marked by numbers. Image taken from Beavan et al. (1971) with permission.
(namely a sharply pointed needle or a tip). One can question why the 3D FIM is not more widely known and utilized in materials analyses? The most difficult aspect of 3D FIM studies is the retrieval of information contained within the small FIM specimens that are typically studied. An FIM image must be interpreted to obtain the ultimate elemental nature and positions of individual atoms in 3D direct space. Methods were developed to interpret the large number of frames of 35mm cine film recorded during the pulsed field- evaporation of hundreds of nanometers of different materi- als. In the 1970s, most of the work was performed manually, which was extremely tedious (Scanlan et al., 1974), involving techniques that had been developed by particle physicists in their quest for discovering fundamental particles. In addi- tion, special care must be taken to avoid misinterpretations and artifacts of the images produced by atomic surface
FIM as the standard field-ion-emission technique utilized to image and record 3D distributions of atoms in materials. APT was born at approximately the same time as the first powerful personal computers. Conversely to FIM images, APT data consists directly of a set of 3D coordinates with elemental identities, determined by time-of-flight mass spectrometry, so that 3D visualization is reasonably straightforward. By stacking FIM images into 3D data sets, volume rendering was proposed to transform FIM into a tomography technique (Cerezo et al., 1992). A strong degradation of the spatial resolution to downsize the data set was necessary at that time. Used for visualizing spinodal decomposition in an Fe–Cr alloy, the images were processed by a five-point median filter and histogram equalization technique, which reduced the contrast of individual atoms to leave only variations in brightness due to the composition
diffusion effects (Stiller & Andrén, 1982) or artifacts of field-evaporation (Schmid & Balluffi, 1971). Currently, atom probe tomography (APT) has replaced
part of APT investigations. The reason is mainly the absence of dedicated FIM, which yields high-quality FIM images, in modern APTmicroscopes.Wealso note that FIM studies are limited to pure materials or limited to materials containing atoms providing significant contrast differences between elements, which hampers its use as an analytical microscope. However, that FIM has several important advantages com- pared with APT. In most cases, the spatial resolutions (depth and lateral) are better than for APTs, and the visualization of the crystallography and the structure is more straightfor- ward, even though some crystallographic details can be extracted from APT data sets (Gault et al., 2012). The image of a single atom in FIM is the result of the ionization of thousands of image gas atoms per second. In addition, the theoretical imaging efficiency in selected regions of interest is 100%, compared with 80% for the best APTs (LEAP5000X S; Cameca, Madison, WI, USA), and about 50% in most commercial instruments (LEAP4000X Si, CAMECA), imply- ing that locatingmissing atoms in a latticemay be impossible using a LEAP5000X S.We also note that image distortions are less severe in FIM, because the image is formed from static atomic positions at a distance of a few angstroms from its surface. As a result, the development of 3D FIMapplications, aiming to capitalize on these advantages for atomic-scale analyses of materials, is undergoing a genuine revival. Recent research demonstrating the ability to fabricate tomograms from FIM images reveals full atomic positioning and the capacity to image crystalline defects (Vurpillot et al., 2007; Dagan et al., 2015). This article reviews these recent efforts, which may contribute to a renaissance in using FIM as an indispensable tool for a material’s 3D characterization at the atomic scale. Specifically, two approaches are being pursued in different research groups. The first one is the intensive use of workstations to stack 2Dmicrographs, which are corrected for artifacts induced by the imaging process. The method is the extension of the first stacking procedure proposed in the 1990s (Cerezo et al., 1992) and has been utilized to investigate several metallurgical problems. The second method proposed recently is an automated approach, similar to the earlier approach of Seidman et al. utilizing intensive image processing and pattern recognition algorithms.
METHODS AND RESULTS
Accurate Volume Rendering in an FIM Applied to Metallurgical Applications
Let us consider an FIM movie recorded during the con- tinuous pulsed field-evaporation of several atomic layers of a specimen. Each single FIM micrograph is a snapshot of the sample’s surface, with near atomic resolution at a given evaporation depth. Recall that the atomic contrast in an FIM
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