A reproducible method for damage-free site-specific preparation of atom probe tips from interfaces

Atom probe tomography (APT) is a mass spectrometry method with atomic-scale spatial resolution that can be used for the investigation of a wide range of materials. The main limiting factor with respect to the type of problems that can be addressed is the small volume investigated and the randomness of common sample preparation methods. With existing site-specific specimen preparation methods it is still challenging to rapidly and reproducibly produce large numbers of successful samples from specifically selected grain boundaries or interfaces for systematic studies. A new method utilizing both focused ion beam (FIB) and transmission electron microscopy (TEM) is presented that can be used to reproducibly produce damage-free atom probe samples with features of interest at any desired orientation with an accuracy of better than 50 nm from samples that require very little prior preparation.     

Development of atom probe specimen preparation techniques for specific regions in steel materials

More elaborated specimen preparation techniques for atom probe analysis were developed using a focused ion beam with a sample lift-out system so as to expand the application field in steel materials. The techniques enable atom probe analysis of sample steel at site-specific regions of interest. The preferable form of the needle specimen was provided by electrostatic field calculation using a finite element method. The new techniques were applied to the observation of a bainite-ferrite interface in a low carbon steel, and atomic-scale partitioning and segregation of alloying elements at the phase interface were directly observed in three dimensions.     

Dynamic reconstruction for atom probe tomography

Progress in the reconstruction for atom probe tomography has been limited since the first implementation of the protocol proposed by Bas et al. in 1995. This approach and those subsequently developed assume that the geometric parameters used to build the three-dimensional atom map are constant over the course of an analysis. Here, we test this assumption within the analyses of low-alloyed materials. By building upon methods recently proposed to measure the tomographic reconstruction parameters, we demonstrate that this assumption can introduce significant limitations in the accuracy of the analysis. Moreover, we propose a strategy to alleviate this problem through the implementation of a new reconstruction algorithm that dynamically accommodates variations in the tomographic reconstruction parameters.     

Some aspects of atom probe specimen preparation and analysis of thin film materials

Some of the factors in the preparation of atom probe specimens of metallic multilayer thin films have been investigated. A series of Ti/Nb multilayer films were sputtered deposited on n-doped Si [001] substrates with either 5 or 0.05Omega cm resistivity. Each wafer was pre-fabricated into a series of 5 microm x 5 microm x approximately 80 microm island posts by photolithography and reactive ion etching. Once the film was grown on the wafer, a Si post was mounted to either a tungsten or stainless steel fine tip needle that was mechanically crimped to a Cu tube for handling. The specimen was then loaded into a Focus Ion Beam instrument where a sacrificial Pt cap was in situ deposited onto the surface of the film and subsequently annularly ion milled into the appropriate geometry. The Pt cap was found to be an effective method in reducing Ga ion damage and implantation into the film during milling. The multilayers deposited on the high resistivity Si exhibited uncontrolled field evaporation which lead to high mass tails in the mass spectra, a reduction in the mass resolution, high background noise, propensity for "flash-failure", and a variation in the apparent layer thickness as the experiment elapsed in time. The multilayers deposited on lower resistivity Si did not suffer from these artifacts.     

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.     

Contingency table techniques for three dimensional atom probe tomography

A contingency table analysis procedure is developed and applied to three dimensional atom probe data sets for the investigation of fine-scale solute co-/anti-segregation effects in multicomponent alloys. Potential sources of error and inaccuracy are identified and eliminated from the technique. The conventional P value testing techniques associated with chi(2) are shown to be unsatisfactory and can become ambiguous in cases of large block numbers or high solute concentrations. The coefficient of contingency is demonstrated to be an acceptable and useful basis of comparison for contingency table analyses of differently-conditioned materials. However, care must be taken in choice of block size and to maintain a consistent overall composition between experiments. The coefficient is dependent upon block size and solute composition, and cannot be used to compare analyses with significantly different solute compositions or to assess the extent of clustering without reference to that of the randomly ordered case. It is shown that as clustering evolves into larger precipitates and phases, contingency table analysis becomes inappropriate. Random labeling techniques are introduced to infer further meaning from the coefficient of contingency. We propose the comparison of experimental result, mu(exp), to the randomized value, micro(rand), as a new method by which to interpret the quantity of solute clustering present in a material. It is demonstrated that how this method may be utilized to identify an appropriate size of contingency table analysis blocks into which the data set is partitioned to optimize the significance of the results.     

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.     

Using atom probe tomography to analyse MAX-phase materials

Ti₂AlC belongs to an interesting group of materials with both metallic and ceramic properties. This material is highly attractive as a candidate for corrosion resistant coatings. The process of fabrication of such coatings is in the investigation stage only and the detailed knowledge of the structure and chemistry of the produced coatings is important for optimisation of their properties. In this work the applicability of atom probe tomography for investigation of both Ti₂AlC bulk materials and coatings was tested. We show that for the first time, Ti₂AlC has successfully been analysed using laser pulsing mode in a local electrode atom probe and the results from analysis of both bulk Ti₂AlC and Ti₂AlC based spray deposited coatings are presented. It appears that, in this particular material system, it is difficult to obtain the accurate stoichiometry. This is due to the loss of detection because of unavoidable multiple events and due to the peak overlaps present. Methods of how to approach these problems are discussed.     

Multivariate statistical analysis of atom probe tomography data

The application of spectrum imaging multivariate statistical analysis methods, specifically principal component analysis (PCA), to atom probe tomography (APT) data has been investigated. The mathematical method of analysis is described and the results for two example datasets are analyzed and presented. The first dataset is from the analysis of a PM 2000 Fe–Cr–Al–Ti steel containing two different ultrafine precipitate populations. PCA properly describes the matrix and precipitate phases in a simple and intuitive manner. A second APT example is from the analysis of an irradiated reactor pressure vessel steel. Fine, nm-scale Cu-enriched precipitates having a core-shell structure were identified and qualitatively described by PCA. Advantages, disadvantages, and future prospects for implementing these data analysis methodologies for APT datasets, particularly with regard to quantitative analysis, are also discussed.     

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.     

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.     

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.     

Dopant distributions in n-MOSFET structure observed by atom probe tomography

The dopant distributions in an n-type metal-oxide-semiconductor field effect transistor (MOSFET) structure were analyzed by atom probe tomography. The dopant distributions of As, P, and B atoms in a MOSFET structure (gate, gate oxide, channel, source/drain extension, and halo) were obtained. P atoms were segregated at the interface between the poly-Si gate and the gate oxide, and on the grain boundaries of the poly-Si gate, which had an elongated grain structure along the gate height direction. The concentration of B atoms was enriched near the edge of the source/drain extension where the As atoms were implanted.     

Evaporation mechanisms of MgO in laser assisted atom probe tomography

In this paper the field evaporation properties of bulk MgO and sandwiched MgO layers in Fe are compared using laser assisted Atom Probe Tomography. The comparison of flight time spectra gives an estimate of the evaporation times as a function of the wavelength and the laser energy. It is shown that the evaporation takes place in two steps on two different time scales in MgO. It is also shown that as long as the MgO layer is buried in Fe, the evaporation is dominated by the photon absorption in Fe layer at the tip apex. Eventually the evaporation process of MgO is discussed based on the difference between the bulk materials and the multilayer samples.     

The influence of voxel size on atom probe tomography data

A methodology for determining the optimal voxel size for phase thresholding in nanostructured materials was developed using an atom simulator and a model system of a fixed two-phase composition and volume fraction. The voxel size range was banded by the atom count within each voxel. Some voxel edge lengths were found to be too large, resulting in an averaging of compositional fluctuations; others were too small with concomitant decreases in the signal-to-noise ratio for phase identification. The simulated methodology was then applied to the more complex experimentally determined data set collected from a (Co_0.95Fe_0.05)₈₈Zr₆Hf₁B₄Cu₁ two-phase nanocomposite alloy to validate the approach. In this alloy, Zr and Hf segregated to an intergranular amorphous phase while Fe preferentially segregated to a crystalline phase during the isothermal annealing step that promoted primary crystallization. The atom probe data analysis of the volume fraction was compared to transmission electron microscopy (TEM) dark-field imaging analysis and a lever rule analysis of the volume fraction within the amorphous and crystalline phases of the ribbon.     

Effect of laser power and specimen temperature on atom probe analyses of magnesium alloys

The influence of laser power, wave length, and specimen temperature on laser assisted atom probe analyses for Mg alloys was investigated. Higher laser power and lower specimen temperature led to improved mass and spatial resolutions. Background noise and mass resolutions were degraded with lower laser power and higher specimen temperature. By adjusting the conditions for laser assisted atom probe analyses, atom probe results with atomic layer resolutions were obtained from all the Mg alloys so far investigated. Laser assisted atom probe investigations revealed detailed chemical information on Guinier-Preston zones in Mg alloys.     

Quantitative comparison of energy-filtering transmission electron microscopy and atom probe tomography

Whereas transmission electron microscopy (TEM) is a well established method for the analysis of thin film structures down to the sub-nanometer scale, atom probe tomography (APT) is less known in the microscopy community. In the present work, local chemical analysis of sputtered Fe/Cr multilayer structures was performed with energy-filtering transmission electron microscopy (EFTEM) and APT. The single-layer thickness was varied from 1 to 6 nm in order to quantify spatial resolution and chemical sensitivity. While both the methods are able to resolve the layer structure, even at 2 nm thickness, it is demonstrated that the spatial resolution of the APT is about a factor of two, higher in comparison with the unprocessed EFTEM data. By calculating the influence of the instrumental parameters on EFTEM images of model structures, remaining interface roughness is indicated to be the most important factor that limits the practical resolution of analytical TEM.     

Application of Delaunay tessellation for the characterization of solute-rich clusters in atom probe tomography

This work presents an original method for cluster selection in Atom Probe Tomography designed to be applied to large datasets. It is based on the calculation of the Delaunay tessellation generated by the distribution of atoms of a selected element. It requires a single input parameter from the user. Furthermore, no prior knowledge of the material is needed. The sensitivity of the proposed Delaunay cluster selection is demonstrated by its application on simulated APT datasets. A strong advantage of the proposed methodology is that it is reinforced by the availability of an analytical model for the distribution of Delaunay cells circumspheres, which is used to control the accuracy of the cluster selection procedure. Another advantage of the Delaunay cluster selection is the direct calculation of a sharp envelope for each identified cluster or precipitate, which leads to the more appropriate morphology of the objects as they are reconstructed in the APT dataset.     

Quantitative binomial distribution analyses of nanoscale like-solute atom clustering and segregation in atom probe tomography data

The applicability of the binomial frequency distribution is outlined for the analysis of the evolution nanoscale atomic clustering of dilute solute in an alloy subject to thermal ageing in 3D atom probe data. The conventional chi(2) statistics and significance testing are demonstrated to be inappropriate for comparison of quantity of solute segregation present in two or more different sized system. Pearson coefficient, mu, is shown to normalize chi(2) with respect to sample size over an order of magnitude. A simple computer simulation is implemented to investigate the binomial analysis and infer meaning in the measured value of mu over a series of systems at different solute concentrations and degree of clustering. The simulations replicate the form of experimental data and demonstrate the effect of detector efficiency to significantly underestimate the measured segregation. The binomial analysis is applied to experimental atom probe data sets and complementary simulations are used to interpret the results.     

Atom probe specimen preparation with a dual beam SEM/FIB miller

Dual beam scanning electron microscope/focused ion beam (SEM/FIB) methods complement electropolishing methods and enable specimens to be made from a wider range of materials. Several methods have been developed to fabricate specimens from different forms of materials, including thin ribbons, mechanically ground sheet and fine powders. In addition, FIB-based methods can be used in conjunction with electropolishing methods to improve the shape, surface finish and taper angle of specimens. Several lift-out (LO) methods have been developed for selecting specific microstructural features or other regions of interest such as phases, interfaces, grain boundaries, subsurface or implanted regions and interdendritic regions. These LO methods make use of an in situ nanomanipulator and platinum deposition to transfer and attach the lifted out volume to a post for final annular milling into a needle-shaped specimen. In order to improve the efficiency and to facilitate the LO procedure, some special specimen mounts that hold both the specimen and the support post at the appropriate working distance have been developed.