David A. Reinhard

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.

Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals

Abstract Atom-probe tomography (APT) and secondary ion mass spectrometry (SIMS) provide complementary in situ element and isotope data in minerals such as zircon. SIMS measures isotope ratios and trace elements from 1-20 μm spots with excellent accuracy and precision. APT identifies mass/ charge and three-dimensional position of individual atoms (±0.3 nm) in 100 nm-scale samples, volumes up to one million times smaller than SIMS. APT data provide unique information for understanding element and isotope distribution; crystallization and thermal history; and mechanisms of mineral reaction and exchange. This atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage, and can test for intracrystalline element mobility. Nano-geochronology is one application of APT in which Pb isotope ratios from sub-micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating. Based on SEM imaging and SIMS analysis, 11 needle-shaped specimens ~100 nm in diameter were sampled from one Archean and two Hadean zircons by focused ion-beam milling and analyzed with APT. The three-dimensional distribution of Pb and nominally incompatible elements (Y, REEs) differs at the atomic scale in each zircon. Zircon JH4.0 (4.007 Ga, Jack Hills, Western Australia) is homogeneous in Pb, Y, and REEs. In contrast, Pb and Y and REEs are clustered in sub-equant ~10 nm diameter domains, spaced 10-40 nm apart in zircons ARG2.5 (2.542 Ga, Grouse Creek Mountains, Utah) and JH4.4 (4.374 Ga, Jack Hills). Most clusters are flattened parallel to (100) or (010). U and Th are not collocated with Pb in clusters and appear to be homogeneously distributed in all three zircons. The analyzed domains experienced 4 to 8 × 1015 α-decay events/mg due to U and Th decay and yet all zircons yield U-Pb ages by SIMS that are better than 97% concordant, consistent with annealing of most radiation damage. The 207Pb/206Pb ratios for the 100 nm-scale specimens measured by APT average 0.17 for ARG2.5, 0.42 for the JH4.0, and 0.52 for JH4.4. These ratios are less precise (±10-18% 2σ) due to the ultra-small sample size, but in excellent agreement with values measured by SIMS (0.1684, 0.4269, and 0.5472, respectively) and the crystallization ages of the zircons. Thus Pb in these clusters is radiogenic, but unsupported, meaning that the Pb is not spatially associated with its parent isotopes of U and Th. For the domain outside of clusters in JH4.4, the 207Pb/206Pb ratio is 0.3, consistent with the SIMS value of 0.2867 for the zircon overgrowth rim and an age of 3.4 Ga. In ARG2.5, all Pb is concentrated in clusters and there is no detectable Pb remaining outside of the clusters. The Pb-YREE- rich clusters and lack of correlation with U in ARG2.5 and JH4.4 are best explained by diffusion of Pb and other elements into ~10 nm amorphous domains formed by α-recoil. Diffusion distances of ~20 nm for these elements in crystalline zircon are consistent with heating at temperatures of 800 °C for ~2 m.y. Such later reheating events are identified and dated by APT from 207Pb/206Pb model ages of clusters in JH4.4 and by the absence of detectable Pb outside of clusters in ARG2.5. SIMS datesfor the zircon rims independently confirm reheating of ARG2.5 and JH4.4, which were xenocrysts in younger magmas when rims formed. It is proposed that most domains damaged by α-recoil were annealed at ambient temperatures above 200-300 °C before reheating and only a small number of domains were amorphous and available to concentrate Pb at the time of reheating. The size, shapes, and orientations of clusters were altered by annealing after formation. The absence of enriched clusters in JH4.0 shows that this zircon was not similarly reheated. Thus APT data provide thermochronologic information about crustal reworking even for zircons where no overgrowth is recognized. The clusters in JH4.4 document Pb mobility at the sub-50 nm scale, but show that the much larger 20 μm-scale domains analyzed by SIMS were closed systems. The reliability of oxygen isotope ratios and other geochemical data from zircon can be evaluated by these means. These results verify the age of this zircon and support previous proposals that differentiated crust existed by 4.4 Ga and that the surface of Earth was relatively cool with habitable oceans before 4.3 Ga. These analytical techniques are of general applicability to minerals of all ages and open many new research opportunities.

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.

Toward atom probe tomography of microelectronic devices

Atom probe tomography and scanning transmission electron microscopy has been used to analyze a commercial microelectronics device prepared by depackaging and focused ion beam milling. Chemical and morphological data are presented from the source, drain and channel regions, and part of the gate oxide region of an Intel® i5-650 p-FET device demonstrating feasibility in using these techniques to investigate commercial chips.

A System for Simulation of Tip Evolution Under Field Evaporation

An improved method to numerically simulate the endform evolution of atom probe specimens has been developed in order to support reconstruction algorithm research. Similar to work described in [1], the system consists of a three-dimensional Poisson simulation and includes methods for choosing which atoms of the specimen to evaporate and a trajectory integrator to produce data that can be postevaporation processed, for example, by the IVAS system [2]. The current simulation uses a finite difference algorithm which specifies that atoms assigned to the tip are treated as part of the applied boundary conditions. The iteration used to solve for the potential is a standard multi-grid technique [3]. The simulation supports a wide range of tip geometries with included planar, spherical and cylindrical sub-structures. All components can be simulated with independently controllable compositions, evaporation fields and lattice structures.

Atom probe tomography (APT) of carbonate minerals.

Atom probe tomography (APT) combines the highest spatial resolution with chemical data at atomic scale for the analysis of materials. For geological specimens, the process of field evaporation and molecular ion formation and interpretation is not yet entirely understood. The objective of this study is to determine the best conditions for the preparation and analysis by APT of carbonate minerals, of great importance in the interpretation of geological processes, focusing on the bulk chemical composition. Results show that the complexity of the mass spectrum is different for calcite and dolomite and relates to dissimilarities in crystalochemical parameters. In addition, APT bulk chemistry of calcite closely matches the expected stoichiometry but fails to provide accurate atomic percentages for elements in dolomite under the experimental conditions evaluated in this work. For both calcite and dolomite, APT underestimates the amount of oxygen based on their chemical formula, whereas it is able to detect small percentages of elemental substitutions in crystal lattices. Overall, our results demonstrate that APT of carbonate minerals is possible, but further optimization of the experimental parameters are required to improve the use of atom probe tomography for the correct interpretation of mineral geochemistry.

Atomic-scale age resolution of planetary events

Resolving the timing of crustal processes and meteorite impact events is central to understanding the formation, evolution and habitability of planetary bodies. However, identifying multi-stage events from complex planetary materials is highly challenging at the length scales of current isotopic techniques. Here we show that accurate U-Pb isotopic analysis of nanoscale domains of baddeleyite can be achieved by atom probe tomography. Within individual crystals of highly shocked baddeleyite from the Sudbury impact structure, three discrete nanostructural domains have been isolated yielding average 206Pb/238U ages of 2,436±94 Ma (protolith crystallization) from homogenous-Fe domains, 1,852±45 Ma (impact) from clustered-Fe domains and 1,412±56 Ma (tectonic metamorphism) from planar and subgrain boundary structures. Baddeleyite is a common phase in terrestrial, Martian, Lunar and asteroidal materials, meaning this atomic-scale approach holds great potential in establishing a more accurate chronology of the formation and evolution of planetary crusts.

Approaches for Promoting Accurate Atom Probe Reconstruction

Comparison of atom probe tomography (APT) results from different instruments, different users, or different samples requires confidence in overall reconstruction accuracy. The conventional reconstruction algorithm utilizes a single point-projection onto a spherical surface [1,2]. Although there are a limited number of parameters used to prescribe how detector events are converted to a 3D set of reconstructed coordinates, some constraints are necessary to ensure minimal precision and accuracy. Key inputs in the reconstruction algorithm used in the commercially available IVAS software (CAMECA Instruments, Inc.) include ionic volume, detection efficiency, image compression factor (ICF), either tip radius (initial or final) or the product of geometrical field factor (k) and evaporation field (F), sphere-to-cone ratio, and radial evolution procedure (voltage, shank angle, or tip profile) [3]. When quality protocols are implemented, reliable comparisons can be made [4]. The available approaches for properly choosing reconstruction parameters will be the focus of this presentation.

Improving Data Quality in Atom Probe Tomography

Figure 1 schematically illustrates several methods to improve APT specimen yield through decreasing stress by decreasing the evaporation field required during data collection: 1) decrease ion detection rate, 2) increase specimen base temperature, 3) use laser rather than voltage pulsing, and 4) increase laser pulse energy. The current work explores the use of thin coatings to modify the thermal and/or optical properties of 302 stainless steel, SiN, or a Si/SiO2/Si/Ni test structure (Fig. 2) in order to improve yield [7]. The APT data collection parameters were nominally: specimen temperature 30 K, laser pulse energy 30 pJ at 625 kHz, and a detection rate of one ion every 333-1000 pulses.

Atom Probe Tomography of Zircon and Baddeleyite Geochronology Standards

Atom probe tomography (APT) makes it possible to study the compositional structure of geological materials at the nanoscale [1]. It is an analytical technique whereby single atoms (or small groups of atoms) are ionized under the presence of a large electric field and removed from the surface one-at-atime. Each removal (field evaporation event) is synchronized to a timing pulse and projected to a 2D position-sensitive detector providing ion identification through time-of-flight mass spectroscopy and, ultimately, a composition map of the surface [2]. Performed iteratively over many millions of ions, the evolving surface information is reconstructed as a 3D map of individual ions from which information can be extracted for a for a variety of real-space analyses.