256 Michal Dagan et al.
The 3Dreconstruction of the analyzed volumes was therefore heavily restricted by the time-consuming manual analysis. More recently, 3D FIM was further advanced by the
development of procedures for the digital-stacking of time- ordered images (Vurpillot et al., 2007), applied, for example, to the investigation of Guinier-Preston (GP) zones in steels (Danoix et al., 2012), nitrides in Fe–Cr alloys (Jessner et al., 2009) and fine-scale precipitates in CuFeNi (Cazottes et al., 2012). Theoretically these images evolve, atom-by-atom and layer-by-layer, where each slice approximately represents a constant increment in the depth of the analysis. The stacking parameters can also be calibrated to match prominent crystallographic features in the specimen (i.e., spacing between atomic planes). Bending of the stack can be performed to account for the highly curved specimen shape. This procedure proved highly successful to uncover features that did not display clear contrast on the 2D FIM images but could be directly observed in a cross-section of the 3D FIM reconstruction, such as dislocations extending along the tip axis (Vurpillot et al., 2007). However, this image stacking approach can become limiting for the precise identification of individual point defects in the matrix. In recent years, FIM has been somewhat overshadowed
by the emergence of wide-field-of-view atom probe tomo- graphy (APT). APT is similarly based on the concept of field evaporation of specimen atoms from the surface (Gault et al., 2012). The evaporated ions strike a position-sensitive detector and their time-of-flight is measured, informing as to their chemical identity. Ultimately, using the digitally recorded detector coordinates and a reverse-projection model, a 3D atomistic reconstruction of the tip is obtained (Vurpillot et al., 2013). The automated 3D atom-by-atom imaging combined
with highly resolved elemental discrimination offered by APT provides significant advantages in comparison with FIM. However, these strengths are achieved at the expense of detection efficiency, that is, due to the incorporation of the microchannel plates in current APT position-sensitive detectors, a significant number of ions striking the detector are not registered by the experiment. Although the maxi- mum achievable detection efficiency in the latest generation of commercial atom probe instruments is reported to reach 80%, in the case of many existing reflectron-fitted instru- ments up to 65% of the atoms evaporated from the specimen are stochastically omitted from the final analysis (Kelly & Miller, 2007; Miller et al., 2012). The incomplete nature of APT information makes it impossible to directly image every atomic site on the lattice and requires the application of advanced methods to characterize individual sites (Moody et al., 2014). Reliable characterization of atomic-sized defects such as vacancies and interstitials is therefore not possible with APT. In contrast, FIM is not subject to the same limitations of
detection efficiency. The imaging of each surface atom in the analysis is effectively formed by the combined contributions of thousands of imaging gas ions per second. The data therefore contains full representation of all atoms in the
analyzed volume, and enables the detection of defects as small as vacancies that could not be characterized with APT (Dagan et al., 2015). In order to fully utilize the unique imaging capabilities
of FIM and explore larger volumes accurately and con- sistently, this study introduces the first automated “atom-by- atom” approach to 3D FIM data reconstruction. The proce- dure relies on minimal underlying assumptions, is sensitive to atomic information and subangstrom displacements in the imaged positions of the atoms, and results in an atom- ically resolved 3D reconstruction of the crystalline lattice. The proposed reconstruction procedure is demonstrated here for the analysis of tungsten, currently a leading candi- date for plasma-facing components of nuclear fusion reac- tors (Davis et al., 1998; Neu et al., 2005; Philipps, 2011). However, the method is applicable to a wide range of mate- rials which can be imaged by FIM such as steels and catalyst metals.
The approach developed here for reconstructing FIM
data maintains the truly atomistic nature of the information in FIM images. We demonstrate the procedure for FIM analysis of the atomically resolved [2,2,2] pole of a tungsten needle, characterized by 7771 images, describing the eva- poration of 69 crystallographic layers in-depth into the material and comprising 2450 tungsten atoms. We demon- strate how each atom can be automatically identified across the sequence of images, its position tracked across all images, from the time it is first revealed on the surface of specimen to the instant when it is field evaporated. Ultimately, all of this information is brought together to reconstruct original position of each atom on the crystal lattice and create a complete atomistic 3D image of the analyzed volume. The main stages of the reconstruction algorithm are demon- strated, and significant challenges in the reconstruction procedure discussed. Optimal implementation of the algo- rithm requires high-quality data underpinned by a high level of control in the evaporation process, atomic resolution, and an ordered evaporation structure. However, it can also be implemented on FIM data sets that do not possess all of these properties. To explore the implementation of the algorithm to more complex material systems and data sets, a second reconstruction case study of carbides in a M50 bearing steel is discussed, demonstrating how simple adjustments can be
applied to the method to reconstruct larger portions of the field of view, as well as more complex alloy systems.
EXPERIMENT
A tungsten FIM sample was electropolished in a 5% NaOH solution with the application of 5–8V of AC voltage from a tungsten wire oriented along the [011] direction, in a one- step electropolishing procedure. The final specimen apex radius was estimated to be ~20 nm. The sample was subject to self-ion implantation to a dose consistent with primary stages of radiation damage, where damage cascades caused by the implantation of individual ions are not interacting.
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