G. Da Costa

Design of a delay-line position-sensitive detector with improved performance

A delay-line position-sensitive detector with improved performance is presented. In this device, timing is carried out by means of fast digitizer boards. The use of dedicated signal processing procedures leads to a timing accuracy of 70 ps and a dead-time below 1.5 ns. As a result, the spatial resolving power of this detector is close to 1 mm leading to a high multihit capability. A temporary detector has been designed in which the delay-line anode is combined with a phosphor screen allowing additional positioning to be made via a charge-coupled device camera. This additional positioning is used to unambiguously quantify performances in terms of spatial resolution and multihit capabilities. A three-dimensional atom probe analysis of a material containing low evaporation field phases is used to demonstrate the capabilities of this detector.

Structural analyses in three-dimensional atom probe: a Fourier transform approach

The three‐dimensional atom probe (3DAP) technique gives the elemental identities and the position of atoms within the small volume analysed (on the order of 10u2003×u200310u2003×u2003100 nm3). The large number of atoms collected (up to two million) and the excellent spatial resolution of this instrument allows the observation of some crystallographic features of phases chemically identified. This paper shows that the application of a discrete Fourier transform algorithm to a 3DAP dataset provides information that is not easily accessible in real space. The derivation of the mean size of particles from Fourier intensities is an example. Using 3D ‘dark‐field’ imaging, single ordered grains were isolated from the disordered matrix of a ternary alloy. Moreover, the intrinsic spatial resolution of the instrument was evaluated by this method for pure metal; the resolution reaches 0.2u2003nm laterally and 0.06u2003nm in depth. This excellent resolution is shown to be sufficient to give access to the crystalline lattice. The use of image filtering in the reciprocal space enables for atomic columns to be imaged the first time from 3DAP data.

Clustering and nearest neighbour distances in atom-probe tomography

The measurement of chemical composition of tiny clusters is a tricky problem in both atom-probe tomography experiments and atomic simulations. A new approach relying on the distribution of the first nearest neighbour (1NN) distances between solute atoms in the 3D space composed of A and B atoms was developed. This new approach, the 1NN method, is shown to be an elegant way to get the composition of tiny B-enriched clusters embedded in a random AB solid solution. The theoretical statistical distributions of first neighbour distances P(r) for both random solid solution and solute-enriched clusters finely dispersed in a depleted matrix are established. It is shown that the most probable distance of P(r) gives directly the phase composition. Applications of this model to both one-phase SiGe alloy and boron-doped silicon containing small clusters indicate that this new approach is quite reliable.

Implementation of an optical TAP: preliminary results

Abstract The FIM Group of the University of Rouen is developing a new type of detector that can be used in atom probe and 3D atom probe applications. This detector consists of an array of conductive and transparent strips covered with a phosphorescent material. After having produced light, electrons generated by ionic impacts onto microchannel plates produce signals on strips that are used for timing. At the same time, light produced by ions on the phosphor screen is recorded by means of a CCD image sensor. Then, the comparison between positions and the distribution of time of flight on the strip array makes it possible to correlate positions and times for each event. In this contribution, preliminary results obtained with a 5×5xa0cm 2 detector having eight strips will be shown. Potential capabilities and performance will be discussed.

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.

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.

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%.

Advance in multi-hit detection and quantization in atom probe tomography

The preferential retention of high evaporation field chemical species at the sample surface in atom-probe tomography (e.g., boron in silicon or in metallic alloys) leads to correlated field evaporation and pronounced pile-up effects on the detector. The latter severely affects the reliability of concentration measurements of current 3D atom probes leading to an under-estimation of the concentrations of the high-field species. The multi-hit capabilities of the position-sensitive time-resolved detector is shown to play a key role. An innovative method based on Fourier space signal processing of signals supplied by an advance delay-line position-sensitive detector is shown to drastically improve the time resolving power of the detector and consequently its capability to detect multiple events. Results show that up to 30 ions on the same evaporation pulse can be detected and properly positioned. The major impact of this new method on the quantization of chemical composition in materials, particularly in highly-doped Si(B) samples is highlighted.

Field evaporation mechanism of bulk oxides under ultra fast laser illumination

The controlled field evaporation of single atoms from an oxide surface assisted by ultra fast laser pulses has recently been demonstrated. When UV light is used, a photoionization mechanism was proposed. However, experimental results observed when the laser intensity and wavelength are changed cannot be explained by this mechanism. Instead, a thermal assisted evaporation mechanism characterized by two evaporation times is proposed. The fast and slow evaporation rates are associated to two cooling processes inside the tip sample. Experiments are carried out on TiO2 and MgO field emitter tips to check the dependence of the evaporation process on structural properties of the oxide. A good agreement between the predictions of our model and the experimental data is found.

A model to predict image formation in Atom probeTomography

A model devoted to the modelling of the field evaporation of a tip is presented in this paper. The influence of length scales from the atomic scale to the macroscopic scale is taken into account in this approach. The evolution of the tip shape is modelled at the atomic scale in a three dimensional geometry with cylindrical symmetry. The projection law of ions is determined using a realistic representation of the tip geometry including the presence of electrodes in the surrounding area of the specimen. This realistic modelling gives a direct access to the voltage required to field evaporate, to the evolving magnification in the microscope and to the understanding of reconstruction artefacts when the presence of phases with different evaporation fields and/or different dielectric permittivity constants are modelled. This model has been applied to understand the field evaporation behaviour in bulk dielectric materials. In particular the role of the residual conductivity of dielectric materials is addressed.