Christian Oberdorfer

On the field evaporation behavior of dielectric materials in three-dimensional atom probe: a numeric simulation.

As a major improvement in three-dimensional (3D) atom probe, the range of applicable material classes has recently been broadened by the establishment of laser-assisted atom probes (LA-3DAP). Meanwhile, measurements of materials of low conductivity, such as dielectrics, ceramics, and semiconductors, have widely been demonstrated. However, besides different evaporation probabilities, heterogeneous dielectric properties are expected to give rise to additional artifacts in the 3D volume reconstruction on which the method is based. In this article, these conceivable artifacts are discussed based on a numeric simulation of the field evaporation. Sample tips of layer- or precipitate-type geometry are considered. It is demonstrated that dielectric materials tend to behave similarly to metals of reduced critical evaporation field.

Design of a laser-assisted tomographic atom probe at Münster University

To benefit from the latest technical improvements in atom probe analysis, a new tomographic atom probe has been built at the University of Münster, Germany. The instrument utilizes a femtosecond laser system with a high repetition rate combined with the ability of using a micrometer-sized extraction electrode and a wide angle configuration. Since field evaporation is triggered by laser pulses instead of high-voltage pulses, the instrument offers the ability to expand the range of analyzed materials to poorly conducting or insulating materials such as oxides, glasses, ceramics, and polymeric materials. The article describes the design of the instrument and presents characterizing measurements on metals, semiconductors, and oxide ceramic.

Laser-assisted atom probe tomography of oxide materials.

Atom probe tomography provides a chemical analysis of nanostructured materials with outstanding resolution. However, due to the process of field evaporation triggered by nanosecond high voltage pulses, the method is usually limited to conductive materials. As part of recent efforts to overcome this limitation, it is demonstrated that the analysis of thick NiO and WO3 oxide layers is possible by laser pulses of 500 ps duration. A careful analysis of the mass spectra demonstrates that the expected stoichiometries are well reproduced by the measurement. The reconstruction of lattice planes proves that surface diffusion is negligible also in the case of thermal pulses.

A full-scale simulation approach for atom probe tomography

A versatile approach for simulation of APT measurements is presented. The model is founded on a Voronoi cell partition of 3D space. The partition is used in dual role: First, the atomic structure of the field emitter is depicted in a one to one relationship by single Wigner-Seitz cells. Second, the construction of an adaptive tetrahedral mesh enables solving the Poisson equation on length scales covering seven orders of magnitude. Ion trajectories are computed in full-length comparable to experiments. Contrary to former simulation approaches the sequence of desorbing atoms is determined by field-induced polarization forces. Both results for cubic lattices in <001>, <011>, and <111> orientation are presented and the simulation of an APT measurement of a complex crystalline/amorphous layer structure is demonstrated. The example of a grain boundary addresses the new possibility of constructing models with structural defects. In this case, the simulation reveals strong artifacts in the reconstruction even if homogenous evaporation threshold is assumed.

Modeling Atom Probe Tomography: A review

Improving both the precision and the accuracy of Atom Probe Tomography reconstruction requires a correct understanding of the imaging process. In this aim, numerical modeling approaches have been developed for 15 years. The injected ingredients of these modeling tools are related to the basic physic of the field evaporation mechanism. The interplay between the sample nature and structure of the analyzed sample and the reconstructed image artefacts have pushed to gradually improve and make the model more and more sophisticated. This paper reviews the evolution of the modeling approach in Atom Probe Tomography and presents some future potential directions in order to improve the method.

Towards an accurate volume reconstruction in atom probe tomography

An alternative concept for the reconstruction of atom probe data is outlined. It is based on the calculation of realistic trajectories of the evaporated ions in a recursive refinement process. To this end, the electrostatic problem is solved on a Delaunay tessellation. To enable the trajectory calculation, the order of reconstruction is inverted with respect to previous reconstruction schemes: the last atom detected is reconstructed first. In this way, the emitter shape, which controls the trajectory, can be defined throughout the duration of the reconstruction. A proof of concept is presented for 3D model tips, containing spherical precipitates or embedded layers of strongly contrasting evaporation thresholds. While the traditional method following Bas et al. generates serious distortions in these cases, a reconstruction with the proposed electrostatically informed approach improves the geometry of layers and particles significantly.

Applications of a versatile modelling approach to 3D atom probe simulations

The article addresses application examples of a flexible simulation approach, which is based on an irregular mesh of Voronoi cells. The detailed atomic structure of APT field emitters is represented by Wigner-Seitz cells. In this way, arbitrary crystal structures can be modelled. The electric field results from the solution of the Poisson equation. The evaporation sequence of atoms from the emitter surface is enabled by calculation of the field-induced force, which acts on the surface cells. Presented examples show simulated field desorption maps of a cubic fcc <111> structure in comparison to the close-packed hcp <0001> structure. Additionally, the desorption maps of the cubic sc, bcc, and fcc lattices in <011> orientation are presented. The effect of inhomogeneous evaporation conditions on the emitter apex curvature is demonstrated. Reconstructions derived from the simulation of Σ5 GBs differently inclined with respect to the emitter axis are analyzed. Finally, the stress exerted on an embedded nano-particle during the simulated evaporation with inhomogeneous evaporation thresholds is estimated.

Controlled Field Evaporation of Fluorinated Self-Assembled Monolayers

Self-assembled monolayers of amino-undecanethiol and perfluoro-decanethiol are studied by atom probe tomography based on laser-assisted controlled field desorption. In the case of hydrogenated chains the identification of detected molecular species is difficult because of residual hydrocarbons. By contrast, fractions of the fluorinated chains can be unequivocally identified. Although chemically similar, the evaporation of both chains appears in significantly different molecular fractions. For the fluorinated chains, a well-ordered evaporation sequence is determined that allows conclusions to be drawn about the strength of bonds under field conditions and may lay the basis for the future numerical reconstruction of the chemical structure of such films.

Laser-assisted atom probe analysis of sol–gel silica layers

Semi-conducting nanocrystals embedded in a non-conducting matrix of silicate glass may be used as non-volatile data storage device. Structures of silicate glasses are conveniently produced by a sol-gel process, which offers the possibility to coat tip-shaped substrates with a silica layer. The study presents first results of their local chemical analysis by laser-assisted atom probe. Till date the exact mechanisms of laser pulsing are still controversial. But it is common sense that there is an at least considerable heating effect on the tip, which leads to a short temperature rise and a prolonged cooling period in materials of low heat conductivity. This effect alters the shape of mass peaks and is examined here using a one-dimensional model of heat transport.

Defect analysis by statistical fitting to 3D atomicmaps

In this article we present a statistical fitting method for evaluation of atomic reconstructions which does not require a coarse-graining step. The fitting compares different models of chemical structure in their capability to explain the measured data set by a least square type merit function. Only preliminary qualitative assumptions about the possible chemical structure are required, while accurate quantitative parameters of the chosen model are delivered by fitting. The technique is particularly useful for singular defect structures with very high composition gradients, for which iso-concentration surfaces determined by coarse-graining become questionable or impossible. We demonstrate that particularly detailed information can be gained from triple junctions and grain boundaries.