Clustering and pair correlation function in atom probe tomography

The measurement of the composition of small clusters from 3D maps as provided by atom probe tomography or Monte-Carlo simulations is a very tricky issue. A method based on pair correlation functions was developed. The analytical expression of the pair correlation function as a function of the phase composition, the number density and the size of spherical particles for a two-phase mono-dispersed system has been established. A best-fit procedure applied to experimental pair correlation function is shown to be a simple, fast and elegant way to determine the concentration of clusters and that of the parent phase as well as the radius and the number density of clusters. Application to carbon-doped silicon demonstrates the validity of this approach. Results were found very close to those derived by other means. This method was also applied to boron clustering in implanted silicon where clusters are not visible in 3D images. The advantage of this approach over other methods such as erosion or cluster identification is discussed.     

Analysis of clustering in Al-Mg-Si alloy by density spectrum analysis of atom probe data

Early stages of cluster formation in an Al-Si-Mg alloy were investigated by atom probe tomography and evaluated by a newly developed statistical method based on the nearest neighbour distributions. After solutionising and quenching, an alloy sample was naturally aged for one week. The atom probe data then measured was analysed for Mg, Si or Mg-Si clusters. For comparison specimen artificial aged with well developed precipitates was also investigated. A general approach for the analysis of density spectra was set up, which reduced the problem to the solution of an integral equation. Application of the method to the atom probe data set allowed us to detect clusters and to evaluate the atomic fractions within these clusters. This is also possible for an arbitrary number of nucleated phases. The higher-order next nearest neighbour distributions were used for the estimation of cluster sizes. Combining the density distribution method with a Monte Carlo simulation we found very small Si-Si and Mg-Mg clusters consisting of only a few atoms in the naturally aged state.     

Pragmatic reconstruction methods in atom probe tomography

Data collected in atom probe tomography have to be carefully analysed in order to give reliable composition data accurately and precisely positioned in the probed volume. Indeed, the large analysed surfaces of recent instruments require reconstruction methods taking into account not only the tip geometry but also accurate knowledge of geometrical projection parameters. This is particularly crucial in the analysis of multilayers materials or planar interfaces. The current work presents a simulation model that enables extraction of the two main projection features as a function of the tip and atom probe instrumentation geometries. Conversely to standard assumptions, the image compression factor and the field factor vary significantly during the analysis. An improved reconstruction method taking into account the intrinsic shape of a sample containing planar features is proposed to overcome this shortcoming.     

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.     

A sensitivity analysis of the maximum separation method for the characterisation of solute clusters

Variants of the maximum separation method have become the de-facto methodologies for the characterisation of nanometre scale clusters in atom probe tomography (APT) data obtained from dilute solid solutions. All variants rely on a number of parameters and it is well known that the precise values for these parameters strongly influence estimates of cluster size and number density. Quantitative analyses require an improved understanding of the inter-relationship between user-defined parameters, experimental parameters such as detection efficiency and the resultant parameterisation of the microstructure.A series of simulations has been performed to generate clusters with a range of compositions (50-100%) and diameters (1.5-2.5 nm) in a dilute solid solution. The data were degraded to simulate the effects of the finite detection efficiencies and positioning uncertainties associated with the ECOPoSAP and LEAP-3000X HR.An extensive analysis of each resultant dataset, using a range of values for the maximum separation parameters was then performed. Optimum values for each material condition were identified and it is shown that it is possible to characterise cluster size, number density and matrix chemistry. However, accurate estimates of cluster compositions are more difficult and absolute measurements must be treated with caution. Furthermore, it is shown that D_MAX must increase with decreasing detection efficiency and consequently clusters of a specific size will appear slightly larger in atom probes with a lower detection efficiency.     

Fabrication and characterization of APT specimens from high dose heavy ion irradiated materials

The next generations of advanced energy systems will require materials that can withstand high doses of irradiation at elevated temperatures. Therefore, a methodology has been developed for the fabrication of high-dose ion-irradiated atom probe tomography specimens at a specific dose with the use of a focused ion beam milling system. The method also enables the precise ion dose of the atom probe tomography specimen to be estimated from the local concentration of the implanted ions. The method has been successfully applied to the characterization of the distribution of nanoclusters in a radiation-tolerant 14YWT nanostructured ferritic steel under ion irradiation to doses up to 400 displacements per atom.     

Atom probe trajectory mapping using experimental tip shape measurements

Atom probe tomography is an accurate analytical and imaging technique which can reconstruct the complex structure and composition of a specimen in three dimensions. Despite providing locally high spatial resolution, atom probe tomography suffers from global distortions due to a complex projection function between the specimen and detector which is different for each experiment and can change during a single run. To aid characterization of this projection function, this work demonstrates a method for the reverse projection of ions from an arbitrary projection surface in 3D space back to an atom probe tomography specimen surface. Experimental data from transmission electron microscopy tilt tomography are combined with point cloud surface reconstruction algorithms and finite element modelling to generate a mapping back to the original tip surface in a physically and experimentally motivated manner. As a case study, aluminium tips are imaged using transmission electron microscopy before and after atom probe tomography, and the specimen profiles used as input in surface reconstruction methods. This reconstruction method is a general procedure that can be used to generate mappings between a selected surface and a known tip shape using numerical solutions to the electrostatic equation, with quantitative solutions to the projection problem readily achievable in tens of minutes on a contemporary workstation.     

Atom probe tomography characterization of solute segregation to dislocations

The extent and level of solute segregation to individual dislocations may be quantified by atom probe tomography. The technique is best applied to materials with high dislocation densities, such as cold worked, mechanically alloyed, or neutron-irradiated materials. Dislocations may be observed in field ion images by a change of the normal concentric atom terraces at crystallographic poles to spirals. Solute segregation is evident in field ion images by brightly imaging atoms near the core of the dislocation. Dislocations are evident in atom maps in the three-dimensional atom probe by linear regions of enhanced solute concentration. The maximum separation envelope and tracer methods may be used to quantify the levels of solute at the dislocation at the subnanometer scale. Examples of interstitial and substitutional element segregation in a mechanically alloyed, oxide dispersion strengthened ferrite steel and phosphorus segregation to dislocations in neutron-irradiated pressure vessel steels are presented.     

Atom probe tomography of reactor pressure vessel steels: an analysis of data integrity

In this work, the importance of optimising experimental conditions for the analysis of reactor pressure vessel (RPV) steels using atom probe tomography is explored. The quality of the resultant atom probe data is assessed in terms of detection efficiency, noise levels and mass resolution. It is demonstrated that artefacts can exist even when experimental conditions have been optimised. In particular, it is shown that surface diffusion of some minority species, including P and Si, to major poles prior to field evaporation can be an issue. The effects were most noticeable during laser pulsing.The impact of surface migration on the characterisation of dislocations and grain boundaries is assessed. The importance of selecting appropriate regions of the reconstructed data for subsequent re-analysis is emphasised.     

Image contrast of dielectric specimens in transmission mode near-field scanning optical microscopy: imaging properties and tip artefacts

Near-field scanning optical microscopy (NSOM) is a scanned probe technique utilizing a subwavelength-sized light source for high-resolution imaging of surfaces. Although NSOM has the potential to exploit and extend the experimental utility of the modern light microscope, the interpretation of image contrast is not straightforward. In near-field microscopy the illumination intensity of the source (probe) is not a constant value, rather it is a function of the probe–sample electronic environment. A number of dielectric specimens have been studied by NSOM to elucidate the contrast role of specimen type, topography and crystallinity; a summary of metallic specimen observations is presented for comparative purposes. Near-field image contrast is found to be a result of lateral changes in optical density and edge scattering for specimens with little sample topography. For surfaces with considerable topography the contributions of topographic (Z) axis contrast to lateral (X,Y) changes in optical density have been characterized. Selected near-field probes have also been shown to exhibit a variety of unusual contrast artefacts. Thorough study of polarization contrast, optical edge (scattering) contrast, as well as molecular orientation in crystalline specimens, can be used to distinguish lateral contrast from topographic components. In a few cases Fourier filtering can be successfully applied to separate the topographic and lateral contrast components.     

Characterization of dilute species within CVD‐grown silicon nanowires doped using trimethylboron: protected lift‐out specimen preparation for atom probe tomography

Three‐dimensional quantitative compositional analysis of nanowires is a challenge for standard techniques such as secondary ion mass spectrometry because of specimen size and geometry considerations; however, it is precisely the size and geometry of nanowires that makes them attractive candidates for analysis via atom probe tomography. The resulting boron composition of various trimethylboron vapour–liquid–solid grown silicon nanowires were measured both with time‐of‐flight secondary ion mass spectrometry and pulsed‐laser atom probe tomography. Both characterization techniques yielded similar results for relative composition. Specialized specimen preparation for pulsed‐laser atom probe tomography was utilized and is described in detail whereby individual silicon nanowires are first protected, then lifted out, trimmed, and finally wet etched to remove the protective layer for subsequent three‐dimensional analysis.     

3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography

The rapid advances in nanotechnology and the ever decreasing size of features in the microelectronics industry brings with it the need for advanced characterisation with high spatial resolution in two and three dimensions. Stereo microscopy allows some insight into the three-dimensional nature of an object but for true quantitative analysis, one has to turn to tomography as a way to reconstruct a three-dimensional object from a series of two-dimensional projections (images). X-ray tomography allow structures to be imaged at relatively large length scales, atom probe tomography at the atomic level. Electron tomography offers an intermediate resolution (of about 1nm) with a field of view of hundreds of nm making it ideal for the characterisation of many nanoscale devices. Whilst electron tomography has been used in the biological sciences for more than 30 years, it is only now being applied to the physical sciences. In this paper, we review the status of electron tomography, describe the basis behind the technique and some of the practicalities of recording and analysing data for tomographic reconstruction, particularly in regard to solving three-dimensional problems that are encountered in materials science at the nanometre level. We present examples of how STEM dark-field imaging and energy-filtered TEM can be used successfully to examine nearly all types of specimens likely to be encountered by the physical scientist.     

Anomalous distribution in atom map of solute carbon in steel

The distribution of carbon in atom probe tomography maps was investigated in various phases of steel. Carbon atoms in 3D atom maps of martensite and cementite phases showed an almost uniform distribution. On the other hand, carbon atoms in ferrite were consistently enriched along the zone line joining the (0 0 2) and the (2 2 2) poles, and in the depth direction of analysis, which was different from the actual distribution. The width and concentration of the enriched regions remained unchanged at a specimen temperature ranging from 90to 30 K. Moreover, the ratio of molecular carbon ions to total carbon ions decreased with decreasing temperature, but did not change between the enriched and diluted regions. Based on the results, the reason for the anomalous distribution of solute carbon atoms in atom maps is discussed.     

Identification of phases in gas-atomised droplets by combination of neutron and X-ray diffraction techniques with atom probe tomography

Powders of Al_68.5Ni_31.5 alloy have been produced by gas atomisation and sieved in different grain size families. The resulting families have been analysed by combined neutron and X-ray diffraction in order to investigate the structure and identify the existing phases at the surface and in the bulk of the grains. The weight fraction of the identified phases (Al₃Ni₂, Al₃Ni and Al) has been estimated from a profile refinement with the FULLPROF computer codes. An additional phase was observed but could not be identified in the diffraction patterns. Starting from grains less than 5 μm in diameter, samples have been shaped by annular focused ion beam into needles that were suitable for atom probe investigations. The structure and morphologies observed by different techniques are compared and discussed. It has also been possible to estimate the crystallite sizes and the strains corresponding to the different phases present in the powders from the refinement of the ND patterns. In addition to Al₃Ni₂ and Al₃Ni, a phase of composition close to the nominal one of the alloy was observed in the atom probe measurements. This phase could be one of the decagonal ones referred to in the literature. Small particles of composition close to Al₈₂Ni₁₈ are attributed to the metastable Al₉Ni₂ phase. The achieved conclusions demonstrate the complementarity of X-ray and neutron diffraction techniques and atom probe tomography to analyse complex structures.     

Determination of matrix composition based on solute‐solute nearest‐neighbor distances in atom probe tomography

In this study, we propose a fast automatic method providing the matrix concentration in an atom probe tomography (APT) data set containing two phases or more. The principle of this method relies on the calculation of the relative amount of isolated solute atoms (i.e., not surrounded by a similar solute atom) as a function of a distance d in the APT reconstruction. Simulated data sets have been generated to test the robustness of this new tool and demonstrate that rapid and reproducible results can be obtained without the need of any user input parameter. The method has then been successfully applied to a ternary Al‐Zn‐Mg alloy containing a fine dispersion of hardening precipitates. The relevance of this method for direct estimation of matrix concentration is discussed and compared with the existing methodologies. Microsc. Res. Tech., 2010. © 2010 Wiley‐Liss, Inc.     

Analytical TEM study of annealed nanocrystalline cobalt–phosphorous electrodeposits

Investigation of thermal stability of two nanocrystalline Co–P alloys shows that P atoms segregate to the grain boundaries upon annealing until precipitation of Co₂P and CoP precipitates takes place. The P-rich precipitates formed have been investigated by analytical transmission electron microscopy to obtain statistical results of precipitate size, volume fraction and spatial distribution. Electron spectroscopic imaging maps show that the P-rich precipitates are 33 ± 9 nm in Co–1.1at.%P and 33 ± 12 nm in Co–3.2at.%P. The main differences between the alloys are the precipitate size distribution (Co–3.2at.%P having broader distribution) and precipitate volume number density (Co–3.2at.%P has 1.8 times more precipitates than Co–1.1at.%P). The volume fraction of precipitates is 3.0% in Co–1.1at.%P and 4.4% in Co–1.1at.%P. Most of the precipitates are of nearly spherical or slightly elongated shape, and only a few have a platelet-like shape as expected from previous tomographic atom probe measurements. Due to the truncation and projection effects, the composition of the precipitates could not be determined.     

Atom probe tomography and transmission electron microscopy of a Mg-doped AlGaN/GaN superlattice

The electronic characteristics of semiconductor-based devices are greatly affected by the local dopant atom distribution. In Mg-doped GaN, the clustering of dopants at structural defects has been widely reported, and can significantly affect p-type conductivity. We have studied a Mg-doped AlGaN/GaN superlattice using transmission electron microscopy (TEM) and atom probe tomography (APT). Pyramidal inversion domains were observed in the TEM and the compositional variations of the dopant atoms associated with those defects have been studied using APT. Rarely has APT been used to assess the compositional variations present due to structural defects in semiconductors. Here, TEM and APT are used in a complementary fashion, and the strengths and weaknesses of the two techniques are compared.     

Atom-probe for FinFET dopant characterization

With the continuous shrinking of transistors and advent of new transistor architectures to keep in pace with Moore's law and ITRS goals, there is a rising interest in multigate 3D-devices like FinFETs where the channel is surrounded by gates on multiple surfaces. The performance of these devices depends on the dimensions and the spatial distribution of dopants in source/drain regions of the device. As a result there is a need for new metrology approach/technique to characterize quantitatively the dopant distribution in these devices with nanometer precision in 3D.In recent years, atom probe tomography (APT) has shown its ability to analyze semiconductor and thin insulator materials effectively with sub-nm resolution in 3D. In this paper we will discuss the methodology used to study FinFET-based structures using APT. Whereas challenges and solutions for sample preparation linked to the limited fin dimensions already have been reported before, we report here an approach to prepare fin structures for APT, which based on their processing history (trenches filled with Si) are in principle invisible in FIB and SEM. Hence alternative solutions in locating and positioning them on the APT-tip are presented. We also report on the use of the atom probe results on FinFETs to understand the role of different dopant implantation angles (10° and 45°) when attempting conformal doping of FinFETs and provide a quantitative comparison with alternative approaches such as 1D secondary ion mass spectrometry (SIMS) and theoretical model values.     

Comparison of radiation-induced segregation in ultrafine-grained and conventional 316 austenitic stainless steels

Due to a high number density of grain boundaries acting as point defect sinks, ultrafine-grained materials are expected to be more resistant to irradiation damage. In this context, ultrafine-grained 316 austenitic stainless steel samples have been fabricated by high pressure torsion. Their behavior under ion irradiation has been studied using atom probe tomography. Results are compared with those obtained in an ion irradiated conventional coarse-grained steel. The comparison shows that the effects of irradiation are limited and that intragranular and intergranular features are smaller in the ultrafine-grained alloy. Using cluster dynamic modeling, results are interpreted by a higher annihilation of point defects at grain boundaries in the ultrafine-grained steel.