Correlated field evaporation as seen by atom probe tomography

doi:10.1016/j.susc.2006.10.019

Surface Science

Volume 601, Issue 2, 15 January 2007, Pages 536-543

https://doi.org/10.1016/j.susc.2006.10.019 Get rights and content

Abstract

We have studied the correlation between field evaporated atoms during an atom probe tomography experiment. The evaporated atoms have been shown to be highly correlated both in time and in space. This correlation can be explained by the dynamic distribution of the electric field at the surface of the sample. The evaporation sequence follows the distribution of zones where the electric field intensity is higher. The distance of correlation between successively evaporated atoms corresponds to the mean size of these zones and the time of correlation corresponds to the mean time necessary to evaporate the entire zone.

Introduction

The last 20 years have seen the emergence of a new class of analytical techniques in materials science. The Atom Probe Tomography enables the three dimensional visualization of chemical composition variations with a near atomic spatial resolution in small volumes of materials [1]. This technique is of great interest in the study of the early stages of phase transformations, surface and interface segregations, and more generally in nanotechnology applications [2], [3]. The basic principle of the technique is the pulsed field evaporation of specimen atoms and their detections by a position-sensitive detector with a high performance timing measurement accuracy. Chemical natures of detected ions from the probed volume are deduced by time-of-flight mass spectrometry.

The first generation of instrument, the position-sensitive atom probe (PoSAP) was developed at the end of the 80s by Cerezo and co-workers [4]. This instrument gave the first qualitative 3D images of various metallurgical cases. In this instrument, however, difficulties arises when more than one ion is evaporated from the probed area on a single evaporation pulse. Indeed, if two ions arrive simultaneously at the detector, their positions cannot be determined. This problem limits the accuracy of analyses performed on the resultant data. In order to avoid significant loss of data from the analysis, one solution is to decrease the mean rate of evaporation (down to 0.001 atom/pulse) to automatically reduce the statistical occurrence of multi-hits events. This is simply done by decreasing the electric field at the tip surface. However this decrease can result in undesirable effects such as field corrosion or preferential retention. Furthermore, it was shown recently that even at a low detection rate, the occurrence of multi-hit events is still important [5]. This prevents this type of instrument to obtain quantitative measurement for materials science applications.

To overcome these drawbacks, other instruments based on the use of detection system capable of multi-hit detection were developed. These instruments have proved to generally give reliable composition measurements [6] when the flight length of the instrument is not too short (>20 cm). The problem observed experimentally is that multi-hit events are shown to be highly correlated in time and in space. The ability to detect multi-hit events is thus related to the spatial and time resolving powers of the position-sensitive detector.

The correlation between hits on the detector is related to the process of field evaporation itself. It indicates that atoms close to each other on the tip surface are often evaporated on the same evaporation pulse, whatever the electric field at the tip surface. The correlated evaporation was studied in field ion microscopy [7], [3]. By studying the sequence of evaporation of surface atoms with this instrument, simultaneous evaporation is often observed. This phenomenon is particularly drastic at the centre of low index poles. Such types of burst evaporation are thought to be mainly due to the increase in electric field in proportion as the pole terrace diameter decreases. As field evaporation has a really critical dependance on the electric field, it is evident that the sequence of evaporation of the atoms can not be completely random, as it has been observed in field ion microscopy [8] or even in atom probe tomography [9], [10].

In this paper, the co-evaporation has been studied experimentally using atom probe tomography and the results have been compared with a simple analytical model and numerical simulation [11], [12]. Both models are based on basic principles of field evaporation.

Section snippets

Atom probe tomography (APT)

The atom probe tomography is based on the field evaporation of specimen surface atoms. The high electric field required is obtained by applying a high voltage to the sample prepared in the form of a sharply pointed needle with a end radius in the range 10–100 nm. The tip is biased at a high positive DC voltage in the range 1–20 kV. Surface atoms are field evaporated by means of electric pulses superimposed on the DC voltage V₀ with a pulse fraction (V_p/V₀) close to 20%. The electric field F at

Field evaporation process

Field evaporation [13], [14], refers to the process of removing an ion from a field-emitter surface with a high electric field. The required field intensity is in the range 10–60 V/nm for metal tip emitters. Theory of field evaporation is now fairly well developed. The basic concepts of field evaporation are best explained with reference to one-dimensional potential energy diagrams [15]. In the absence of electric field, neutral states are generally the stable states at the surface of materials.

Numerical modelisation of the evaporation process

A model was recently developed to reproduce either the electric field distribution and field evaporation process at the apex of a realistic specimen or to simulate the aberrations of ion trajectories in the atom probe. In this model, the tip was modelled as a 3D compact stack of polygons (Wigner–Seitz cells). These cells are initially confined inside a hemisphere. The atom centre is suppose to coincide with the centre of the cell. The tip is (0 0 1) orientated, and has a radius of curvature of

Experimental behaviour of the evaporation process

Experimental datasets were obtained on aluminium based alloys with the Rouen Tomographic Atom Probe (TAP) instrument. This instrument has a 45 cm flight length and the detector is a 10 × 10 cm² position-sensitive detector with multi-hits capability. The sample temperature was kept constant (80 K). The three different probability laws P_multi, P_pulses and P_d (Eqs. (7), (11), (13)) can be extracted from experimental datasets. An experimental raw dataset consists in a list of events that are filed

Modelling the co-evaporation process

As the last paragraph states, it is most likely that the correlation between the atoms are related to the dynamic distribution of the electric field on the surface of the sample. Considering Eq. (2), the evaporation process is a stochastic process governed by the temperature but also by the exact electric field that is applied to atoms to be evaporated. This electric field is dependent on the voltage applied to the tip, but also on the local curvature which is not homogeneous at the whole

Discussion

The atoms that are field evaporated during an atom probe tomography (or FIM) analysis have been shown to be highly correlated in time and in space. Our simulations show that this phenomenon can be explained by a non uniform distribution of the electric field at the surface of the tip. The atoms that are simultaneously evaporated are situated in regions of the surface where the electric field is similar. When the surface has reached its equilibrium shape, there are several atom positions on a

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Citation Excerpt :
Given these known limitations, there have been various attempts to improve the APT detector systems through hardware improvements, e.g., advanced delay line detectors and the analysis electronics [5,7–10], and via the software, i.e., algorithms applied to the signals generated from the hardware [7–9]. A reason given for multihit ion pile-up is co-evaporation, i.e., as one atom is field evaporated it exposes its neighboring atom(s), which then experience an enhanced field leading to an increased probability for a same pulse evaporation event [11]. These co-evaporated ions then arrive at the detector with an insufficient spatial separation to all be counted.
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Correlated field evaporation as seen by atom probe tomography

Abstract

Introduction

Section snippets

Atom probe tomography (APT)

Field evaporation process

Numerical modelisation of the evaporation process

Experimental behaviour of the evaporation process

Modelling the co-evaporation process

Discussion

Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

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Atom Probe Field Ion Microscopy

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