Brian P. Geiser

Local Electrode Atom Probe Tomography

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Advances in the reconstruction of atom probe tomography data

Key to the integrity of atom probe microanalysis, the tomographic reconstruction is built atom by atom following a simplistic protocol established for previous generations of instruments. In this paper, after a short review of the main reconstruction protocols, we describe recent improvements originating from the use of exact formulae enabling significant reduction of spatial distortions, especially near the edges of the reconstruction. We also show how predictive values for the reconstruction parameters can be derived from electrostatic simulations, and finally introduce parameters varying throughout the analysis.

Spatial Distribution Maps for Atom Probe Tomography

A real-space technique for finding structural information in atom probe tomographs, spatial distribution maps (SDM), is described. The mechanics of the technique are explained, and it is then applied to some test cases. Many applications of SDM in atom probe tomography are illustrated with examples including finding crystal lattices, correcting lattice strains in reconstructed images, quantifying trajectory aberrations, quantifying spatial resolution, quantifying chemical ordering, dark-field imaging, determining orientation relationships, extracting radial distribution functions, and measuring ion detection efficiency.

Evolution of tip shape during field evaporation of complex multilayer structures.

Atom‐probe tomography analysis of complex multilayer structures is a promising avenue for studying interfacial properties. However, significant artefacts in the three‐dimensional reconstructed data arise due to the field evaporation process. To clarify the origin and impact of these artefacts for a FeCoB/FeCo/MgO/FeCo/IrMn multilayer, tip shapes were observed by transmission electron microscopy and compared to those obtained by finite difference modelling of electric fields and evaporation processes. It was found that the emitter shape is not spherical and its surface morphology evolves during successive evaporation of the different layers. This evolving morphology contributes to the artefacts generally observed in the reconstructed atom‐probe data for multilayer structures because algorithms for three‐dimensional reconstruction are based on the assumption that the shape of the emitter during field evaporation is spherical. Some proposed improvements to data reconstruction are proposed.

Wide-Field-of-View Atom Probe Reconstruction

In atom probe tomography, it is usually desirable to obtain the largest possible field of view (FOV) in the analysis and recent advances in instrumentation [1] have made significant increases in FOV. However, the most commonly used data reconstruction techniques were designed for much smaller FOV instruments and as such, the small-angle approximations employed are less accurate for the current generation of instruments. Prior to the advent of wide FOV instruments, the geometric assumptions described by Blavette [2], and later applied by Bas [3], were widely considered the standard global reconstruction technique [4]. This model incorporates a simple point projection to account for lateral magnification and uses geometric models of the global tip shape to reconstruct depth information. It also assumes that the original shape of acquired volumes is small enough in lateral extent to be considered cylindrical and the radius of the tip is determined atom-by-atom by the specimen voltage. In the early reconstructions [3], the actual shank angle is ignored and it is assumed to be zero in the calculation of the volume increment. In using the voltage as a proxy for the tip radius it will often be the case that the reconstructed geometry is not conical, and indeed may be extremely irregular. This can happen for instance in a multi-layer system where evaporation fields are rapidly changing. In this case the assumption of a fixed evaporation field is clearly erroneous and induces rapidly changing model geometry.

Reconstructing atom probe data: A review

Atom probe tomography stands out from other materials characterisation techniques mostly due to its capacity to map individual atoms in three-dimensions with high spatial resolution. The methods used to transform raw detector data into a three-dimensional reconstruction have, comparatively to other aspects of the technique, evolved relatively little since their inception more than 15 years ago. However, due to the importance of the fidelity of the data, this topic is currently attracting a lot of interest within the atom probe community. In this review we cover: (1) the main aspects of the image projection, (2) the methods used to build tomographic reconstructions, (3) the intrinsic limitations of these methods, and (4) future potential directions to improve the integrity of atom probe tomograms.

Atom Probe Tomography Spatial Reconstruction: Status and Directions

Abstract In this review we present an overview of the current atom probe tomography spatial data reconstruction paradigm, and explore some potential routes to improve the current methodology in order to yield a more accurate representation of nanoscale microstructure. Many of these potential improvement methods are directly tied to extensive application of advanced numerical methods, which are also very briefly reviewed. We have described effects resulting from the application of the standard model and then introduced several potential improvements, first in the far field, and, second, in the near field. The issues encountered in both cases are quite different but ultimately they combine to determine the spatial resolution of the technique.

Improvements in planar feature reconstructions in atom probe tomography

Standard atom probe tomography spatial reconstruction techniques have been reasonably successful in reproducing single crystal datasets. However, artefacts persist in the reconstructions that can be attributed to the incorrect assumption of a spherical evaporation surface. Using simulated and experimental field evaporation, we examine the expected shape of the evaporating surface and propose the use of a variable point projection position to mitigate to some degree these reconstruction artefacts. We show initial results from an implementation of a variable projection position, illustrating the effect on simulated and experimental data, while still maintaining a spherical projection surface. Specimen shapes during evaporation of model structures with interfaces between regions of low‐ and high‐evaporation‐field material are presented. Use of two‐and three‐dimensional projection‐point maps in the reconstruction of more complicated datasets is discussed.

Electrostatic simulations of a local electrode atom probe: The dependence of tomographic reconstruction parameters on specimen and microscope geometry

We electrostatically model a local electrode atom probe microscope using the commercial software IES LORENTZ 2D v9.0 to investigate factors affecting the reconstruction parameters. We find strong dependences on the specimen geometry and voltage, and moderate dependences on the tip-aperture separation, which confirm that the current approach to atom probe reconstruction overlooks too many factors. Based on our data, which are in excellent agreement with known trends and experimental results, we derive a set of empirical relations which predict the values of the reconstruction parameters. These may be used to advance current reconstruction protocols by enabling the parameters to be adjusted as the specimen geometry changes.

Effect of analysis direction on the measurement of interfacial mixing in thin metal layers with atom probe tomography.

The accuracy and precision of thin-film interfacial mixing as measured with atom probe tomography (APT) are assessed by considering experimental and simulated field-evaporation of a Co/Cu/Co multilayer structure. Reconstructions were performed using constant shank angle and Z-scale reordering algorithms. Reconstruction of simulated data (zero intermixing) results in a 10-90% intermixing width of ~0.2 nm while experiential intermixing (measured from multiple runs) was 0.47 ± 0.19 and 0.49 ± 0.10nm for Co-on-Cu and Cu-on-Co interfaces, respectively. The experimental data were collected in analysis orientations both parallel and anti-parallel to film growth direction and the impact of this on the interfacial mixing measurements is discussed. It is proposed that the resolution of such APT measurements is limited by the combination of specimen shape and reconstruction algorithms rather than by an inherent instrumentation limit.