Implementation of an optical TAP: preliminary results

Invited review article: Atom probe tomography

The technique of atom probe tomography (APT) is reviewed with an emphasis on illustrating what is possible with the technique both now and in the future. APT delivers the highest spatial resolution (sub-0.3-nm) three-dimensional compositional information of any microscopy technique. Recently, APT has changed dramatically with new hardware configurations that greatly simplify the technique and improve the rate of data acquisition. In addition, new methods have been developed to fabricate suitable specimens from new classes of materials. Applications of APT have expanded from structural metals and alloys to thin multilayer films on planar substrates, dielectric films, semiconducting structures and devices, and ceramic materials. This trend toward a broader range of materials and applications is likely to continue.

Application of Fourier transform and autocorrelation to cluster identification in the three-dimensional atom probe

Because of the increasing number of collected atoms (up to millions) in the three-dimensional atom probe, derivation of chemical or structural information from the direct observation of three-dimensional images is becoming more and more difficult. New data analysis tools are thus required. Application of a discrete Fourier transform algorithm to three-dimensional atom probe datasets provides information that is not easily accessible in real space. Derivation of mean particle size from Fourier intensities or from three-dimensional autocorrelation is an example. These powerful methods can be used to detect and image nano-segregations. Using three-dimensional 'bright-field' imaging, single nano-segregations were isolated from the surrounding matrix of an iron-copper alloy. Measurement of the inner concentration within clusters is, therefore, straightforward. Theoretical aspects related to filtering in reciprocal space are developed.

Atom probe tomography: a technique for nanoscale characterization

Atom probe tomography is a technique for the nanoscale characterization of microstructural features. Analytical techniques have been developed to estimate the size, composition, and other parameters of features as small as 1 nm from the atom probe tomography data. These methods are outlined and illustrated with examples of yttrium-, titanium-, and oxygen-enriched particles in a mechanically alloyed, oxide-dispersion-strengthened steel.

Improvement of the mass resolution of the atom probe using a dual counter-electrode

As compared to other techniques, the mass resolution of the 3D atom probe is rather poor. This low mass resolution derives from the spread in energy of field-evaporated ions. In this work, the single counter-electrode used to remove atoms from the specimen was replaced with a dual counter-electrode. A positive standing voltage V(PA) is applied on the electrode facing the specimen while the second electrode is grounded. As a result, ions experience a post-acceleration between electrodes that lowers energy deficits of ions resulting in an improvement in the mass resolution. This paper reports the study of the resulting improvement in mass resolution as a function of the post-acceleration voltage. It is also shown that, because of the evaporation pulse, ions also undergo a dynamic post-deceleration in the between electrodes. This post-deceleration contributes to the mass resolution increase. Our results show that this very simple device makes it possible to significantly improve the mass resolution of the atom probe. For a low post-acceleration voltage, the mass resolution is 800 FWHM and 200 at full-width tenth-maximum.

A new step towards the lattice reconstruction in 3DAP

The spatial resolution of images produced by 3DAP is known not to be limited by the detector performance but by the physics of field ionisation and field evaporation. This resolution can be expressed by two parameters: the depth resolution and the lateral resolution, which have been shown to achieve, respectively, 0.06 and 0.2nm in an ideal case of a pure metal. By using this model and these values, the resolution along any crystallographic direction can be predicted. In addition, it is possible to predict the suitable sites of analysis where a given type of atomic planes can be resolved. Furthermore, it is shown that three types of planes with non-coplanar directions can be imaged in 3D reconstructed volumes. Thus, the reconstruction of the original tungsten lattice can be achieved with the use of Fourier Transform image analysis techniques. It is shown that the reliability of the method can be achieve the value of 90%.

Effects of incidence angles of ions on the mass resolution of an energy compensated 3D atom probe

We have used a first-order reflectron lens in an optical tomographic atom probe in order to improve the mass resolution. Calculations have been performed to determine the effect of second-order errors in ion energy and incidence angle on the performance of the lens. By applying a correction procedure based on the results of these calculations, we have been able to improve experimental mass resolution by 30%.