292 Andreas Stoffers et al.
wafer extracted from the capping area of the ingot at 180mm height, where the concentration of impurities is known to be the highest. The samples were precharacterized by EBIC and EBSD to map the electrical activity of the present GBs toge- ther with the orientation relationships of the corresponding grains. In some cases, performing a detailed trace analysis based on the EBSD data can allow a global GB plane to be extracted (Stoffers, et al., 2015a). By the term “global GB plane” we refer to the fact that the actual interface planes of GBs can, in certain cases, decompose further into nanoscale facets, which cannot be detected by EBSD maps and so their exact topological analysis requires the use of HRSTEM.
METHODS
Aberration-corrected STEM allows the visualization of the local arrangement of single atomic columns, with the column intensities depending on the projected mean atomic number, which is an intuitive way to study the atomistic structure of small features and interfaces in two dimensions (2D). The optimum tool for a space-resolved analysis of the chemical composition in 3D at the nanoscale is APT. For a correlative approach, it is of highest interest, to analyze the structure and the chemistry at the same location or at least two locations that are very close together. This can be realized by two different approaches: (1) APT and HRSTEM are performed on the same specimen. As APT requires a tip-shaped specimen, this can only be implemented by conducting the HRSTEM directly on the APT tip. (2) APT and HRSTEM are performed on individual samples stemming from the same interface with a minimum lateral separation. The first approach has the advantage of enabling a true 1:1 correlation. However, the specimen has a tip shape with increasing thickness along the tip axis, quickly resulting in non-ideal conditions for HRSTEM. Furthermore, the examinable volume for structural analysis is drastically reduced compared with a conventional TEM lamella. Therefore, we mainly focus on the second approach here, but we also briefly report one example where the first approach has been applied toward the end of this manuscript. In order to extract samples for APT and TEM analysis
from closely spaced positions, we established a combination of the site-specific lift-out method for preparing APT specimens (Felfer et al., 2012) with the plan-view lift-out method for TEM lamellae (Langford et al., 2001), using a focused ion beam (FIB). A schematic view of the sample volume lifted out is sketched in Figure 1. In this example, the
samples are prepared from the area around a triple junction (TJ), obtaining a TEM foil containing the TJ along with two series of three APT specimens containing the two of the GBs on the right and left side of the lamella. Both theTEMsample and the APT specimens, are prepared parallel to the sample surface, enabling a direct correlation with the EBIC and EBSD information generated from the sample surface. Fur- thermore, the EBSD data were used to select appropriate regions, where all grains have a common zone axis direction close to the normal of the sample surface (<15°) and where the GBs can be viewed “edge-on”. This is essential for reaching atomic resolution across the GBs in HRSTEM. The size of the milled trench determines the maximum dimen- sions of the TEM lamella and in this study we milled trenches around a 30× 3 µm2 large area, resulting in a 3 µm wide TEM lamella. According to the adopted lift-out methods (Langford et al., 2001; Felfer et al., 2012), the volume is then attached to a micromanipulator by means of platinum deposition inside the FIB. Three small wedge- shaped portions of the right-hand GB are subsequently extracted and attached to the horizontally aligned supporting tips of a TEM half grid. Subsequently, the bigger portion for TEM lamella is glued to a conventional OmniProbe grid in the same way, followed by extraction of three wedges from the GB on the left-hand side. After mounting all these wedges, both grids are flipped by 90° into a vertical orienta- tion for the milling process. A custom-designed sample retainer was used (Herbig et al., 2015), which enables mounting and flipping of the grids in the different micro- scopes and thus minimizing the risk of damaging the samples during the preparation process. The APT specimens are then prepared in the conventional way by annular milling (Thompson et al., 2007b), with additional imaging of the GB position by transmission Kikuchi diffraction (TKD) in between the milling steps (Babinsky et al., 2014). Likewise, the TEM foil is prepared by FIB milling in the conventional way (Langford et al., 2001). Here, the milling is mainly performed on the thinner side of the wedge-shaped volume, to retain a large lamella surface and to speed up the milling process. In both cases, the energy of the ion beamis stepwise reduced during the milling process. Trenches are milled at 30 kV, and most of the milling is performed at 16 kV. Gentle milling at the later stages is performed at 8 kV, and final cleaning is performed at 2 kV to achieve a high surface quality (Schaffer et al., 2012). The success rate of the FIB-based sample preparation strongly depends on the user’s experience and was in the range of 75% in our case.
Figure 1. Schematic view of lift-out idea.
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