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.     

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.     

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.     

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.     

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.     

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.     

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti₂AlC grains in different materials as well as on large M₂₃C₆ precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the ¹³C isotope, and calculating the concentration of ¹²C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti₂AlC an additional difficulty arises from the overlap between C₂⁺, C₄²⁺ and Ti²⁺ at the mass-to-charge 24 Da.     

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.     

Broadening the applications of the atom probe technique by ultraviolet femtosecond laser

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.     

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.     

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±0.10 nm 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.     

Atomic scale investigation on the distribution of boron in medium carbon steels by atom probe tomography and EELS

The medium carbon (0.5 wt% C) steels containing various boron contents were studied to observe the distribution of boron using atom probe tomography and electron energy loss spectroscopy. APT revealed the segregation of boron atoms at retained austenite for 100 ppm boron added steels and the trapped carbon atoms at micro-twins for 50 ppm boron treated steels. Moreover, it was also found that boron was randomly distributed for 20 ppm boron added steels regardless of the interactions between carbon and boron.     

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.     

Gallium-enhanced phase contrast in atom probe tomography of nanocrystalline and amorphous Al-Mn alloys

Over a narrow range of composition, electrodeposited Al-Mn alloys transition from a nanocrystalline structure to an amorphous one, passing through an intermediate dual-phase nanocrystal/amorphous structure. Although the structural change is significant, the chemical difference between the phases is subtle. In this study, the solute distribution in these alloys is revealed by developing a method to enhance phase contrast in atom probe tomography (APT). Standard APT data analysis techniques show that Mn distributes uniformly in single phase (nanocrystalline or amorphous) specimens, and despite some slight deviations from randomness, standard methods reveal no convincing evidence of Mn segregation in dual-phase samples either. However, implanted Ga ions deposited during sample preparation by focused ion-beam milling are found to act as chemical markers that preferentially occupy the amorphous phase. This additional information permits more robust identification of the phases and measurement of their compositions. As a result, a weak partitioning tendency of Mn into the amorphous phase (about 2 at%) is discerned in these alloys.     

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.     

Quantitative analysis of carbon content in cementite in steel by atom probe tomography

Atom probe performance in the quantitative analysis of carbon atoms in steel was investigated through analysis of stoichiometric spherical cementite (Fe₃C) in steel. The carbon concentration was estimated by determining the mean carbon number of molecular ions having a mass-to-charge ratio of 24. The apparent carbon concentration of cementite increased as the specimen temperature decreased, and it was several at% higher than the stoichiometric value (25 at%) under the preferable condition of low specimen temperature. On the other hand, the apparent carbon concentration was not changed by pulse fraction. These results indicate that the large deviation from the stoichiometric value did not arise from the preferential retention and evaporation between carbon and iron. The other mechanisms explaining the phenomenon have been discussed.     

Qualification of the tomographic reconstruction in atom probe by advanced spatial distribution map techniques

New and improved spatial distribution map (SDM) methods are developed to identify and extract crystallographic information within atom probe tomography three-dimensional (3D) reconstructions. Detailed structural information is retrieved by combining z-SDM offset distribution analyses computed in multiple crystallographic directions, accurately determining inter-planar spacings and crystallographic angles. The advantages of this technique in comparison to applying the complete z-SDM and complementary xy-SDM analysis to a single crystallographic direction are investigated. Further, in determining these multidirectional z-SDM and xy-SDM profiles, background noise reduction and automatic peak identification algorithms are adapted to attain increased accuracy and is shown to be particularly effective in cases where crystal structure is present but poorly resolved. These techniques may be used to calibrate the reconstruction parameters and investigate their dependence on the design of individual atom probe experiments.     

Correlated ion analysis and the interpretation of atom probe mass spectra

Several techniques are presented for extracting information from atom probe mass spectra by investigating correlations within multiple-ion detector events. Analyses of this kind can provide insights into the origins of noise, the shape of mass peaks, or unexpected anomalies within the spectrum. Data can often be recovered from within the spectrum noise by considering the time-of-flight differences between ions within a multiple event. Correlated ion detection, particularly when associated with shifts in ion energies, may be used to probe the phenomenon of molecular ion dissociation, including the questions of data loss due to ion pile-up or the generation of neutrals in the dissociation process.     

Ordering and site occupancy of D03 ordered Fe3Al-5 at%Cr evaluated by means of atom probe tomography

Addition of ternary elements to the D0₃ ordered Fe₃Al intermetallic phase is a general approach to optimise its mechanical properties. To understand the physical influences of such additions the determination of the probability of site occupancies of these additions on the lattice site and ordering parameters is of high interest. Some common experimental techniques such as X-ray diffraction or Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) are usually applied to explore this interplay.Unfortunately, certain published results are partly inconsistent, imprecise or even contradictory. In this study, these aspects are evaluated systematically by atom probe tomography (APT) and a special data analysis method. Additionally, to account for possible field evaporation effects that can falsify the estimation of site occupancy and induce misinterpretations, APT evaporation sequences were also simulated. As a result, chromium occupies most frequently the next nearest neighbour sites of Al atoms and local ordering parameters could be achieved.