EP2198449B1 - Wide angle high resolution atom probe - Google Patents

Description

The present invention relates to improving the mass resolution of Large Angle Laser Tomographic Probes. It relates more particularly to the atomic probes called 3D atomic probes or "3D atom probe" according to the Anglo-Saxon name.

The atomic probe is an instrument well known to those skilled in the art which makes it possible to analyze samples on an atomic scale. Many instrumental configurations related to this analysis technique are described in the book " Atom probe field Ion microscopy, by Miller et al Published 1996by Clarendon Press / Oxford .

It is conventional to use, for this analysis, a peak-shaped sample, brought to a given potential with respect to the potential of the detector and to have, in the vicinity of this sample, an electrode brought to a potential intermediate between that of the sample and detector.

It is also conventional to have, in addition to this electrode another electrode grounded, or a grid also grounded. In this way the detector being grounded, the ions torn from the sample follow a trajectory that projects them on the detector without being influenced by any electric field that can modify this trajectory. Almost the entire path of the ions is thus performed in a so-called "no field" space.

It is also known that an essential parameter for obtaining a fine and precise measurement of the characteristics of the ions detected by an atomic probe is the measurement of the flight time of the ions detected, ie the time taken by the ion considered. to go through the space separating the sample from which they are torn from the detector. More specifically, the flight time is the time interval between an event triggering the pulling of the ion and its impact on the detector. The triggering event may be an electrical pulse carried on the electrode next to the sample or a pulse of a laser beam directed on the sample. Since measurement of the flight time is essential in the instrument to identify the ratio m / q of a detected ion, where m is the mass of the ion and q its electric charge, it is advantageous to increase the distance L between the sample and the detector to also increase the flight time. However, since the emitted ion beam is of divergent nature, a counterpart to this increase in the distance L is that a large part of the emitted beam can then escape the detector, the detector having meanwhile defined and necessarily limited dimensions. To overcome this drawback, it is known to interpose a convergent device such as an "Einzel" type lens between the sample and the detector to focus the ion beam on the detector. The "Einzel" lens is, moreover, a device well known in the optics of charged particles and whose principle is not detailed here. For more information on "Einzel" lenses, please refer to volume 2 of the book " Principles of electron optics, by PWHawkes and E. Kasper, published in 1989 by Academic Press .

Among the tomographic atomic probes, there are in particular the atomic probes known in the literature under the name of "3DAP" or " T ri D imensional A tom P gown" according to the Anglo-Saxon denomination or under the name of "PoSAP" or "Po sition ensitive S A P tom dress". These probes are advantageously characterized by the fact that with such a detector both the moment of the impact, which measures the flight time of an ion, the position in a plane of this impact on the detector. However, such a measure is only really possible if the position of the point of impact of a given ion is unequivocally linked to its position in the sample analyzed. This condition results in the fact that two distinct trajectories of ions must not lead to the same point of impact on the detector.

However, if it is easy to simply vary the emission angle captured by the detector with an Einzel lens, a strong focusing of the ion beam emitted with such a lens, leads to the appearance a spherical aberration on the lens, aberration which produces on the external trajectories very troublesome parasitic effects for the operation of the 3D probe. In practice, because of this aberration, distinct trajectories have the same end point of impact.

An object of the invention is to propose a solution for obtaining a tomographic probe, a pulsed 3D probe, a pulse probe laser in particular, simultaneously having a large angle of analysis (a large acceptance) and a large resolution in mass consecutive to a great length of flight.

• Un dispositif porte-échantillon pour recevoir un échantillon de matériau à analyser présentant une zone d'extraction de forme sensiblement pointue,
• Un détecteur sensible en position et en temps, de diamètre utile D, et espacé de l'échantillon d'une distance L;
• Une lentille électrostatique composée de trois électrodes, une première électrode ou extracteur, disposée à proximité de l'échantillon, une deuxième électrode, intermédiaire, et une troisième électrode, disposées entre l'électrode intermédiaire et le détecteur, les trois électrodes présentant une symétrie de révolution autour de l'axe Oz passant par la pointe de l'échantillon et perpendiculaire au plan P du détecteur;

et caractérisée en ce que la distance L étant supérieure à 2.75 D, les potentiels respectifs de l'échantillon, de la première électrode de la lentille et du détecteur sont tels que les ions issus de l'échantillon monté sur le porte-échantillon soient attirés vers la première électrode et vers le détecteur; le profil en coupe de l'électrode intermédiaire, dans un plan de coupe rOz, définissant trois points M₁, M₂ et M₃ de coordonnées respectives (r₁, z₁), (r₂, z₂) et (r₃, z₃) par rapport à une origine z₀ sur la pointe de l'échantillon, qui répondent aux conditions suivantes, étant entendu que le sens positif selon l'axe Oz va de l'échantillon au détecteur: $${\mathrm{z}}_1<{\mathrm{z}}_2<{\mathrm{z}}_3,$$

$$\left|{\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="imgf0005.png"\; state="\{\{state\}\}">z</figure-callout>}}_1-{\mathrm{z}}_0\right|<\mathrm{D}/\mathrm{3,}$$

$$\left|{\mathrm{z}}_2-{\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="imgf0005.png"\; state="\{\{state\}\}">z</figure-callout>}}_1\right|<{0.65}^{.}\mathrm{D},$$

$$\left|{\mathrm{z}}_3-{\mathrm{z}}_1\right|>{1.4}^{.}\mathrm{D}\mathrm{,}$$

$${\mathrm{r}}_2={\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0005.png"\; state="\{\{state\}\}">r</figure-callout>}}_1,$$

$${0.1}^{.}{\mathrm{D<<figure-callout\; id="1"\; label="r"\; filenames="imgf0005.png"\; state="\{\{state\}\}">r</figure-callout>}}_1<{0.65}^{.}\mathrm{D}\mathrm{,}$$

$$\mathrm{D}<{\mathrm{r}}_3<{1.6}^{.}\mathrm{D};$$

tous les points du profil en coupe de l'électrode étant situés en dehors de la zone du plan de coupe délimitée par le profil d'un cône à embout cylindrique limité par les points M₁, M₂ et M₃.For this purpose, the subject of the invention is a tomographic atomic probe comprising:

• A sample holder for receiving a sample of material to be analyzed having an extraction zone of substantially pointed shape,
• A position-and-time sensitive detector with a useful diameter D and spaced from the sample by a distance L;
• An electrostatic lens composed of three electrodes, a first electrode or extractor, disposed near the sample, a second electrode, intermediate, and a third electrode, disposed between the intermediate electrode and the detector, the three electrodes having a symmetry of revolution around the axis Oz passing through the tip of the sample and perpendicular to the plane P of the detector;

and characterized in that the distance L being greater than 2.75 D, the respective potentials of the sample, the first electrode of the lens and the detector are such that the ions from the sample mounted on the sample holder are attracted to the first electrode and to the detector; the sectional profile of the intermediate electrode, in a section plane rOz, defining three points M ₁ , M ₂ and M ₃ with respective coordinates (r ₁ , z ₁ ), (r ₂ , z ₂ ) and (r ₃ , z ₃ ) with respect to an origin z ₀ on the tip of the sample, which satisfy the following conditions, it being understood that the positive direction along the axis Oz is from the sample to the detector: $${\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="imgf0005.png"\; state="\{\{state\}\}">z</figure-callout>}}_1<{\mathrm{z}}_2<{\mathrm{z}}_3,$$

$$\left|{\mathrm{z}}_1-{\mathrm{z}}_0\right|<\mathrm{D}/3$$

$$\left|{\mathrm{z}}_2-{\mathrm{<figure-callout\; id="1"\; label="z"\; filenames="imgf0005.png"\; state="\{\{state\}\}">z</figure-callout>}}_1\right|<{0.65}^{.}\mathrm{D},$$

$$\left|{\mathrm{z}}_3-{\mathrm{z}}_1\right|>{1.4}^{.}\mathrm{D}\mathrm{,}$$

$${\mathrm{r}}_2={\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0005.png"\; state="\{\{state\}\}">r</figure-callout>}}_1,$$

$${0.1}^{.}{\mathrm{D\; <<figure-callout\; id="1"\; label="r"\; filenames="imgf0005.png"\; state="\{\{state\}\}">r</figure-callout>}}_1<{0.65}^{.}\mathrm{D}\mathrm{,}$$

$$\mathrm{D}<{\mathrm{r}}_3<{1.6}^{.}\mathrm{D};$$

all the points of the cross-sectional profile of the electrode being situated outside the region of the section plane delimited by the profile of a cone with a cylindrical tip limited by the points M ₁ , M ₂ and M ₃ .

According to an alternative embodiment of the tomographic atom probe according to the invention, the detector or a gate disposed near the detector is at a potential equal to that of the extractor.

According to an alternative embodiment of the tomographic atom probe according to the invention, the detector, or a gate disposed near the detector, is placed at an intermediate potential between that of the sample and that of the extractor electrode.

According to another variant embodiment of the tomographic atomic probe according to the invention, the diameter d of the opening of the extractor is adapted to intercept the peripheral portion of the emitted ion beam so as to block the ions having the most peripheral trajectories.

According to this alternative embodiment, the extractor comprises several diaphragms of different opening diameters, which can be alternately arranged at the central opening of the extractor.

According to this alternative embodiment, the different diaphragms are made on a mobile bar slidable in front of the opening of the extractor so as to place the desired diaphragm in front of the opening; the slide movement of the bar being automated.

According to a third variant embodiment of the tomographic atom probe according to the invention, the three electrodes are configured and arranged in such a way as to leave a free space in the flight chamber sufficient to house a removable device for adjusting the probe.

According to a fourth variant embodiment of the tomographic atom probe according to the invention, a second electrostatic lens is placed between the first electrostatic lens and the detector.

According to this alternative embodiment, the first electrostatic lens is configured to focus the least open paths near the median plane of the second electrostatic lens.

Advantageously, the various embodiments can be combined or associated.

The invention has the advantage of making it possible, for a given opening angle of the emitted ion beam and a given detector surface, to produce a tomographic atom probe, in particular a "3D" probe, having a length of analysis. significantly higher than existing probes.

• la figure 1, une illustration du principe général de fonctionnement d'une sonde tomographique classique;
• la figure 2, une illustration schématique, d'un échantillon en cours de mesure adapté à une sonde tomographique;
• la figure 3, une illustration du principe physique de la mesure réalisée au moyen d'une sonde tomographique;
• la figure 4, l'illustration du principe de fonctionnement d'une sonde tomographique intégrant une lentille "Einzel" dans la chambre de vol des ions;
• les figures 5 et 6, des illustrations du phénomène d'aberration qui apparaît dans le cas d'une forte focalisation;
• la figure 7, une illustration du dispositif de focalisation de la sonde atomique selon l'invention;
• la figure 8, une illustration d'un exemple de faisceau obtenu au moyen du dispositif de focalisation de la sonde atomique selon l'invention;
• la figure 9, l'illustration d'un autre exemple de faisceau focalisé obtenu au moyen du dispositif de focalisation de la sonde atomique selon l'invention;
• les figures 10, 11 et 12, des illustrations d'une variante de réalisation de la sonde atomique selon l'invention;
• la figure 13, l'illustration d'une autre variante de réalisation de la sonde atomique selon l'invention.

The characteristics and advantages of the invention will be better appreciated thanks to the description which follows, description which exposes the invention through a particular embodiment taken as a non-limiting example and which is based on the appended figures, figures which represent:

• the figure 1 , an illustration of the general principle of operation of a conventional tomographic probe;
• the figure 2 , a schematic illustration of a sample being measured adapted to a tomographic probe;
• the figure 3 , an illustration of the physical principle of measurement using a tomographic probe;
• the figure 4 , the illustration of the principle of operation of a tomographic probe incorporating an "Einzel" lens in the ion flight chamber;
• the Figures 5 and 6 , illustrations of the phenomenon of aberration that appears in the case of a strong focus;
• the figure 7 an illustration of the focusing device of the atomic probe according to the invention;
• the figure 8 an illustration of an example of a beam obtained by means of the focusing device of the atomic probe according to the invention;
• the figure 9 , illustration of another example of a focused beam obtained by means of the focusing device of the atomic probe according to the invention;
• the figures 10 , 11 and 12 illustrations of an alternative embodiment of the atomic probe according to the invention;
• the figure 13 , the illustration of another variant embodiment of the atomic probe according to the invention.

We are interested first Figures 1 to 3 , Figures which schematically show the basic structure of a tomographic atomic probe, including an atom probe called "3D" probe. This type of probe is well known to those skilled in the art, so it is not a question in this document to describe in detail such a device. The Figures 1 to 3 however, allow you to recall the following points.

A 3D tomographic atomic probe is intended to perform the analysis of a sample of material 11, atomic layer after atomic layer. For this purpose it basically comprises a sample holder on which is mounted the sample 11 of the material to be analyzed and a detector 12 located at a predetermined distance L of the sample. It also includes means (not shown on the figure 1 ) to evaporate (tear off), in ionic form, the atoms constituting the sample material analyzed and accelerate them so that the ions thus released follow a path that causes each ion 13 evaporated to strike the surface of the detector 12 in one given point 14 determined by the position of this ion on the surface of the sample before its tearing. Thus, by atom-by-atom erosion allowing a reconstruction of the composition of the atomic layer sample by atomic layer it is possible to determine the three-dimensional composition of the sample in question.

In order to isolate all the external disturbances, the probe also comprises a vacuum enclosure (not shown in FIG. figure 1 ), whose potential is, for example, that of the mass of the system in which the probe takes place.

This is how, as illustrated by figure 3 , a device comprising an ion source constituted by the sample 11, an analysis chamber, or flight chamber, of length L (analysis length) and a planar detector 12 whose dimensions cover a circular surface of diameter D. According to the type of probe, the electric field prevailing in the flight chamber takes variable values and may for example be zero. In the latter case, the ions propagate at constant speed inside the flight chamber.

At the arrival of an ion on the detector, it measures the position (x, y) on its surface of the point of incidence of the received ion. The detector also measures the "flight time", time counted from the moment corresponding to the tearing of the ion in question. A geometric correction is furthermore made to take into account the position of the point of impact in the calculation of the distance traveled between the tip and the detector. As a result, the position on the surface of the sample, occupied by the ion in question before it is torn off, is deduced in a known manner from the position of its point of impact on the surface of the detector, by application of a simple rule of projection.

In the case of a so-called "3D" tomographic probe, the detector 12 also determines the instant of arrival of the considered ion, with respect to a known time reference, generally corresponding to the time at which the analysis began. of the sample 11. The measurement of this instant advantageously makes it possible to know the depth at which the ion was situated with respect to the initial surface of the sample and thus to achieve a true three-dimensional positioning of the atom at the origin of the ion considered in the sample 11 of analyzed material.

As illustrated by Figures 1 to 3 the sample 11 is a piece of material having the shape of a substantially conical tip with an end forming a spherical cap of variable radius R over the analysis time. In fact, the tomographic analysis consisting in tearing off, evaporating one after the other, the atoms forming the layers of atoms constituting the material, the radius of this spherical cap 21, initially of given value R ₁ , a value R ₂ corresponding to the spherical cap 22, existing at the end of the analysis; the erosion of the tip leading at the same time an equivalent variation in the distance between the sample 11 and the detector 12.

As illustrated by figure 3 the length L of the analysis chamber and the diameter D of the detector define an angle θ such that tan (θ / 2) = (D / 2) / L. θ is called the "acceptance angle" of the probe. Only trajectories contained in the half-angle cone θ / 2 strike the detector. If D is too weak, some trajectories are not intercepted by the detector and the ions following these trajectories will have no impact on it. These undetected ions, which are not analyzed, will be evaporated at a loss.

Therefore, θ being thus defined, a tomographic atom probe can also be characterized, in known manner, by various parameters which are in particular its magnification G and by the potential difference V which must exist between the tip 11 constituting the sample and the input of the analysis chamber itself, potential difference responsible for the acceleration printed evaporated ions to cross the analysis chamber length L the electric field to be applied. This potential difference is conventionally defined by the relation E = V / R, where E represents the electric field of evaporation and R the radius of curvature of the tip, that is to say the radius of the spherical cap constituting its end.

The magnification is given by the relation G = L / bR, in which L substantially represents the length of the analysis chamber and bR the distance at the end 23 of the tip of a point P, or projection point, to from which the ion trajectories are all defined. The coefficient b which depends on the geometry of the instrumentation, tip, detector and vacuum chamber is typically between 1 and 2.

où M représente masse de l'ion, v sa vitesse, n le nombre de charges élémentaires portées par l'ion; e la charge élémentaire, c'est à dire la charge de l'électron et V la tension d'accélération appliquée.
Par suite, le temps de vol d'un ion étant donné par la relation: $$\mathrm{T}=\frac{\mathrm{L}}{\mathrm{v}},$$

la masse de l'ion sera déterminée d'après le temps de vol, selon la relation: $$\mathrm{M}=\frac{2\mathrm{neV}}{{\mathrm{L}}^2}{\mathrm{T}}^2$$

In such a device, the evaporated ions, by field effect, on the surface of the tip 11 are identified by time-of-flight mass spectrometry. Thus, the rate of displacement of the ions is determined by the acceleration voltage of the ions according to the formula: $$\frac{1}{2}{\mathrm{mv}}^2=\mathrm{Nev}$$

where M represents the mass of the ion, v its velocity, n the number of elementary charges borne by the ion; e the elementary charge, ie the charge of the electron and V the acceleration voltage applied.
As a result, the flight time of an ion is given by the relation: $$\mathrm{T}=\frac{\mathrm{The}}{\mathrm{v}},$$

the mass of the ion will be determined according to the flight time, according to the relation: $$\mathrm{M}=\frac{2\mathrm{Nev}}{{\mathrm{The}}^2}{\mathrm{T}}^2$$

Since the mass resolution δM / M is proportional to the accuracy on the flight time δT / T, it is advantageous to have the greatest possible flight time T, and consequently the greatest distance L possible. In other words, since the measurement of the flight time is essential in the instrument to identify the ratio m / q of a detected ion, m being the mass of the ion and q its electric charge, it is advantageous to increase the distance L between the sample and the detector to also increase the flight time. A counterpart to this increase in the distance L is a reduction in the acceptance angle θ = 2 arctan (D / 2L): A large part of the emitted beam can then escape the detector D dimension, some trajectories 15 being by intercepted by the detector 12.

Thus, to increase L, and therefore the mass resolution without reducing the acceptance angle θ, it is generally necessary to add to the arrangement illustrated by the Figures 1 to 3 , a device for focusing the ion beam emitted by the sample 11 on the detector 12 constituted. This device can for example be constituted as illustrated by figure 4 by an electrostatic lens 41 such as an "Einzel" lens, a device well known in charged particle optics, placed between the sample 11 and the detector 12. According to a well-known principle, the "Einzel" lens consists of three electrodes 42, 43 and 44, placed in the path of the ions and configured to make a portion of the trajectory of these ions on an electric field which acts directly on this path. In this way the initially diverging beam 45 is modified into a convergent beam 46, the convergence obtained being a function of the intensity of the electric field produced.

To create this electric field, the electrodes constituting the lens are placed at the appropriate potentials. Thus, for example, for a tomographic probe in which the detector is placed at the potential of the mass, the "Einzel" lens may comprise a first electrode 42 placed in the vicinity of the sample 11, itself to ground, and then a second electrode 43 brought to a positive potential, then finally a third electrode 44 also grounded, so that at the exit of the lens, the ions continue their trajectories in a space without an electric field. In this case, the first electrode 42 also plays the role of the extraction electrode, or counter electrode, or local electrode, which is generally implemented in the tomographic atomic probes to locate the electric field that produces the initial acceleration evaporated ions from the sample.

Such a focusing device advantageously makes it possible to limit the percentage of ions whose paths do not meet the detector. Nevertheless, its efficiency is generally limited by the fact that any electrostatic lens has what is called a spherical aberration which results in an overconversion of the outer region of the lens and a surfacing for the most eccentric trajectories which means that, as illustrated by Figures 5 and 6 (schematic sectional views) on two examples of lens configurations, the same point 51, 61, of the detector can intercept several distinct trajectories at once, resulting in a problem of indeterminacy of the original position of an ion having struck the detector at this point.

Regarding the configuration of the figure 5 this corresponds for example to an atomic probe in which the sample 11 sees its tip brought to a voltage of 15kV, while the first electrode 42 of the "Einzel" lens (closest to the sample), which serves as an extractor electrode, is grounded, that the second electrode 43 is brought to a voltage of 14kV and that the third electrode 44 (closest to the detector) is also grounded, just like the detector 12. In this configuration, the relative dimensions of the second and third electrodes are such that for the greater part of their path the ions remain under a focussing electric field.

Regarding the configuration of the figure 6 this corresponds for example to an atomic probe in which the sample 11 sees its tip brought to a voltage of 15kV, while the first electrode 42 of the "Einzel" lens (closest to the sample), which serves as an extracting electrode, is grounded, that the second electrode 43 is brought to a voltage of 12.5kV and that the third electrode 44 (closest to the detector) is also grounded, just like the detector 12. In this configuration, unlike the previous configuration, the relative dimensions of the second and third electrodes are such that for the greater part of their path the ions pass through. a space without a field, in which there is no focusing effect.

We are now interested in Figures 7 and 8 which make it possible to present the structure of the atomic probe according to the invention. This one has a well-known general structure, with a sample holder intended to receive the sample 11 of material to be analyzed, and a detector 12 sensitive to the impacts of ions evaporated from the sample and propelled against its sensitive surface. Conventionally, the probe according to the invention also comprises an accelerating electrode, or extractor, positioned near the sample and an electrostatic lens of the "Einzel" type for focusing the produced electron beam, consisting of three adjacent electrodes 71, 72 and 73, the first electrode of the "Einzel" lens being constituted by the accelerating electrode. Conventionally still, the electrodes of the electrostatic lens are polarized so that, taking into account the respective polarizations of the sample and the detector, the evaporated ions are initially accelerated towards the detector, to be then subjected for a part of their path, corresponding to the crossing of the lens, a focussing electric field.

The three electrodes are also preferably configured and arranged so as to provide the flight chamber with sufficient free space to house a removable device for adjusting the probe. The adjustment device may be for example a field emission ion microscope or "Field ion microscope" according to the English name.

The zone of the detector may furthermore, according to the embodiment considered, be brought to an intermediate potential between that of the sample and that of the extracting electrode 71. The setting of the potential considered is carried out directly or via a grid disposed near the detector. According to an alternative embodiment, this potential is that to which the extractor is carried.

In order to be able to have an analysis length L (flight length) substantially greater than that accessible with the existing probes, the The geometry and the arrangement of the electrodes 71, 72 and 73 constituting the electrostatic lens here correspond to the specific technical characteristics described in the following description.

According to the invention, the electrodes of the electrostatic lens consist of mechanical parts comprising a central opening and having a symmetry of revolution about a central axis, coinciding with the axis 74 joining the top of the tip forming the sample 11 of material to the detector 12 and perpendicular to the plane of the detector.

The first electrode 71, or extractor, located near the sample 11 and acting as an extracting electrode is preferably a thin piece having a hole 78 for passing ions, a circular hole for example.

Similarly, the third electrode 73 of the electrostatic lens is any electrode, preferably of relatively small thickness and having a central opening 79 with a diameter greater than or at least approximately equal to the diameter D of the detector 12, so as to allow the propagation up to to the detector of evaporated ions, whatever the path taken by these ions in the lens.

1. a) 0;1·D<r₁<0.65·D
2. b) r₂=r₁
3. c) D<r₂<1.6·D
4. d) |z₁-z₀|<D/3
5. e) |z₂-z₁|<0;65·D
6. f) |z₃-z₁|<1.4·D
7. g) en tout point M_i(r_i,z_i) de la zone M2M3 du profil 75, c'est-à-dire pour z_i>z₂ on a: $${\mathrm{r}}_{\mathrm{i}}\ge \mathrm{A}\cdot {\mathrm{z}}_{\mathrm{i}}+\mathrm{B}$$
   
   avec $$\mathrm{A}=\frac{{\mathrm{r}}_3-{\mathrm{r}}_2}{{\mathrm{z}}_3-{\mathrm{z}}_2}\mathrm{et\; B}=\frac{\mathrm{r}2\cdot {\mathrm{z}}_3-{\mathrm{r}}_3\cdot {\mathrm{z}}_2}{{\mathrm{z}}_3-{\mathrm{z}}_2}$$
   

Regarding the second electrode 72, the central electrode of the lens, the latter has a shape for defining an internal space whose dimensions advantageously vary over the length of the electrode. Thus, according to the invention the second electrode 72 comprises a first segment 711 adjacent to the first electrode 71 and having a cylindrical opening centered on the axis 74, of a radius r ₁ adapted to the passage of the evaporated ion beam. It also comprises a second segment 712, having a cylindrical opening centered on the axis 74 and radius r ₂ the radius r ₂ adapted to the width of the beam being greater than the radius r ₁ . It also comprises a third segment 713, having a conical opening connecting the opening of the first segment to that of the second segment. In this way, as illustrated by the sectional view of the figure 7 , the profile 75 of the inner surface of the second electrode describes a broken line passing through the three points M ₁ (z ₁ , r ₁ ), M ₂ (z ₂ , r ₂ ) M ₃ (z ₃ , r ₃ ). According to the invention, z ₁ , z ₂ , and z ₃ represent the abscissae on the axis 74 of the points M1, M2, and M3 with respect to an origin O of abscissa z ₀ situated at the level of the tip of the sample of material 11 and materialized in the figure by the intersection of axes 74 and 714. The parameters r ₁ , r ₂ and r ₃ represent the values of the radius of the opening at the point considered. These parameters are defined to meet the following conditions:

1. a) 0; 1 · D <r ₁ <0.65 · D
2. b) r ₂ = r ₁
3. c) D <r ₂ <1.6 · D
4. d) | z ₁ -z ₀ | <D / 3
5. e) | z ₂ -z ₁ | <0; 65 · D
6. f) | z ₃ -z ₁ | <1.4 · D
7. g) at any point M _i (r _i , z _i ) of the zone M2M3 of the profile 75, that is to say for z _i > z ₂ we have: $${\mathrm{r}}_{\mathrm{i}}\ge \mathrm{AT}\cdot {\mathrm{z}}_{\mathrm{i}}+\mathrm{B}$$
   
   with $$\mathrm{AT}=\frac{{\mathrm{r}}_3-{\mathrm{r}}_2}{{\mathrm{z}}_3-{\mathrm{z}}_2}\mathrm{and\; B}=\frac{\mathrm{r}2\cdot {\mathrm{z}}_3-{\mathrm{r}}_3\cdot {\mathrm{z}}_2}{{\mathrm{z}}_3-{\mathrm{z}}_2}$$
   

Condition g) amounts to stating that all the points of the sectional profile 75 of the electrode situated between M ₁ and M ₃ must be situated outside the zone of the section plane delimited by the profile of a cone limited by the points M ₂ and M ₃ .

Calculations carried out by the applicant, and not presented here, show that thanks to this particular configuration of the electrodes constituting the electrostatic lens, it is possible by applying the appropriate potentials on the different electrodes, as illustrated in FIG. to obtain a focus of the ion beam 81 sufficient to return to the detector a set of trajectories affected by small aberrations. The invention makes it possible in particular to use in an atomic probe a position detector 12 with a diameter D, placed at a distance L 'greater than 2L from the sample 11, L being such as the maximum analysis length without focusing, length defined in known manner by the relation tan (θ / 2) = (D / 2) / L, equivalent to L = 1.374 D when the half-opening θ / 2 is of 20 °. Thus, for an opening angle θ equal to ± 20 ° and for a detector diameter D equal to 80 mm, it is advantageously possible, thanks to the atomic probe according to the invention, to obtain an analysis length greater than twice the value L = 1.374 · D = 109 mm while intercepting with the detector all the trajectories of the emitted ions corresponding to this opening angle. By increasing the analysis distance by a factor of at least 2, it is possible to obtain, by applying to the ion beam a focussing electric field of appropriate intensity (ie applying to the electrodes the appropriate polarization voltages), a mass resolution improved by a factor greater than two. This resolution is advantageously obtained without undergoing at the detector the effects of confusion of the points of impact, or at least by undergoing these effects in a very weakened manner, effects consecutive to the spherical aberration of the electrostatic lens. The opening angle remains otherwise unchanged, the resolution increase is done here without causing additional limitation of the analyzed surface.

Thus, thanks to its structural characteristics, the probe according to the invention makes it possible to increase very considerably the analysis length that can be used. The intensity of the focus remains as for it defined by the value of the bias voltages applied to the different electrodes of the focussing lens produced. Depending on the polarizations applied, the ion beam will be more or less focused, the objective being however that the focused beam covers the largest possible area on the detector. The focused ion beam may then, for example, take the form of the beam 81 illustrated in FIG. figure 8 , or that of the beam 91 illustrated on the figure 9 . In the case of figure 8 the beam 81 is obtained by applying, for example, a voltage of 13.7 kV to the second electrode 72 and carrying the first and third electrode to ground, the detector itself being grounded and the sample being brought to a voltage of 15 kV. In the case of figure 9 , the beam 91 is obtained by applying, for example, a voltage of 15.1 kV to the second electrode 72 and carrying the first and third electrode to ground, the detector and the sample being otherwise, as in the previous case, carried respectively to the ground and to a voltage of 15 kV.

The architecture of the atomic probe according to the invention, as presented in the preceding paragraphs, corresponds to a basic common architecture, the probe according to the invention being able in practice to include certain variants corresponding to applications specific ones such as those presented in a nonlimiting manner in the following description.

We are now interested in Figures 10 to 12 , which illustrate a first embodiment variant of the probe according to the invention. In this variant, presented by way of non-limiting embodiment, the first electrode 71 constituting the focusing lens, the extracting electrode, comprises a central opening 78 equipped with a multiple aperture device. This device consists, as illustrated by figure 12 in a barrette of diaphragms 112 arranged to slide in front of the central opening 78 of the electrode 71. The diameters of the various diaphragms 111 of the bar 112, smaller than that of the central opening 78, are defined so as to to decrease more or less strongly the diameter of the orifice of passage of ions emitted by the sample 11. Thus, as illustrated by the figures 10 and 11 it is advantageously possible, according to the needs, to adapt the diameter of the opening 78 so as to let the entire emitted ion beam 81 pass or to eliminate from the beam the ions presenting the most peripheral trajectories, in particular for limit the width of the analyzed sample surface and therefore the opening angle of the corresponding ion beam that will be detected by the sensor. The diameter d of the opening of the extractor is thus adapted to intercept the peripheral portion of the emitted ion beam so as to block the ions having the most peripheral trajectories.

It should be noted that, as illustrated by figure 12 , the different diaphragms are arranged on the bar so that the distance between two contiguous diaphragms is sufficient so that, with the exception of the diaphragm used, all the others are perfectly masked by the electrode. The positioning in the two dimensions perpendicular to the axis of the beam can, moreover, be carried out by a suitable mechanism, possibly controlled by a computer and disposed outside the chamber of the probe.

We are then interested in figure 13 , which illustrates a second variant embodiment of the probe according to the invention. In this variant, also presented by way of non-limiting embodiment, the atomic probe according to the invention comprises a second focusing lens, of the "Einzel" lens type, for example, placed between the first lens and the detector. This particular configuration makes it possible to compensate for the residual spherical aberration that the first focusing lens exhibits, despite its particular configuration, this spherical aberration of the first lens can not always be avoided.

The atomic probe according to the invention, in this variant embodiment, comprises, besides the three electrodes 71, 72 and 73 constituting the first lens, two complementary electrodes 132 and 133, the electrode 132 being placed adjacent to the electrode 73 and the electrode 133 being placed adjacent to the electrode 132, between this electrode and the detector 12.

The electrode 133 is brought to a potential substantially equal to that of the electrode 73, while the electrode 132 is brought to a potential allowing all three electrodes 73, 132 and 133 to constitute a second lens. electrostatic inside which reigns an electric field.

• réaliser, à l'aide de la première lentille, la focalisation des trajectoires de faible ouverture sur le plan médian de la deuxième lentille, matérialisé par la ligne pointillée 134 sur la figure 13. De la sorte la deuxième lentille, constituée par les électrodes 73, 132 et 133, est sans effet sur les trajectoires de faible ouverture.
• opérer, à l'aide de la première lentille, une surfocalisation des trajectoires de plus fortes ouvertures. Les aberrations qui apparaissent alors sont corrigées en appliquant le potentiel approprié à l'électrode centrale 132 de la seconde lentille.

In this particular configuration with two lenses the second electrode 72 of the first lens and the second electrode 131 of the second lens are brought to defined potentials for:

• using the first lens, to focus the trajectories of small aperture on the median plane of the second lens, represented by the dotted line 134 on the figure 13 . In this way the second lens constituted by the electrodes 73, 132 and 133 has no effect on the trajectories of small aperture.
• operate, using the first lens, a surfocalisation of the trajectories of greater openings. The aberrations that appear then are corrected by applying the appropriate potential to the central electrode 132 of the second lens.

The electric field applied to the ion beam inside the second electrostatic lens may, depending on the case of use envisaged, be an accelerator or retarding field.

Such a device can for example be obtained from a structure such as that illustrated by FIG. figure 13 . The detector 12 being brought to the ground potential and the sample 11 to a potential of 15 kV, the extraction electrode 71 is then grounded as well as the electrodes 73 and 133, while the central electrode 72 of the first The lens is brought to a voltage of 15.3 kV and the central electrode 132 of the second lens is brought to a voltage of 14.5 kV.

Claims (15)

1. Tomographic atom probe comprising:

   - a sample-holding device for receiving a sample (11) of material to be analysed having an extraction area of substantially pointed shape,

   - a position-sensitive detector (12) of useful diameter D and spaced apart from the sample (11) by a distance L;

   - an electrostatic lens consisting of three electrodes, a first electrode (71) or extractor, arranged in proximity to the sample (11), an intermediate second electrode (72), and a distal third electrode (73) arranged between the intermediate electrode (72) and the detector (12), the three electrodes having a symmetry of revolution about the axis Oz passing through the point of the sample and perpendicular to the plane P of the detector;

   characterized in that, since the distance L is greater than 2.75 D, the respective potentials of the sample (11), of the first electrode (71) of the lens and of the detector (12) are such that the ions deriving from the sample (11) mounted on the sample-holder are attracted towards the first electrode (71) and towards the detector (12); the cross-sectional profile (74) of the intermediate electrode (72), in a cross-sectional plane rOz passing through the axis Oz, defining three points M₁, M₂ and M₃ of respective coordinates (r₁, z₁), (r₂, z₂) and (r₃, z₃) relative to an origin z₀ on the point of the sample, which satisfy the following conditions: $${\mathrm{z}}_{\mathrm{1}}<{\mathrm{z}}_2<{\mathrm{z}}_3,$$

   

   $$\left|{\mathrm{z}}_1-{\mathrm{z}}_0\right|<\mathrm{D}/\mathrm{3,}$$

   

   $$\left|{\mathrm{z}}_{\mathrm{2}}-{\mathrm{z}}_{\mathrm{1}}\right|<{0.65}^{.}\mathrm{D},$$

   

   $$\left|{\mathrm{z}}_{\mathrm{3}}-{\mathrm{z}}_{\mathrm{1}}\right|<{1.4}^{.}\mathrm{D},$$

   

   $${\mathrm{r}}_2={\mathrm{r}}_{\mathrm{1}},$$

   

   $${0.1}^{.}{\mathrm{D<r}}_1<{0.65}^{.}\mathrm{D},$$

   

   $$\mathrm{D}<{\mathrm{r}}_3<{1.6}^{.}\mathrm{D};$$

   

   all the points of the cross-sectional profile of the electrode being situated outside the area of the cross-sectional plane delimited by the profile (77) of a cone with cylindrical tip limited by the points M₁, M₂ and M₃.
2. Tomographic atom probe according to Claim 1, characterized in that the detector (12) or a grating arranged in proximity to the detector is at a potential equal to that of the extractor (71).
3. Atom probe according to Claim 1, characterized in that the detector or a grating arranged in proximity to the detector is at an intermediate potential between that of the sample (11) and that of the extractor electrode (71).
4. Tomographic atom probe according to one of Claims 1 to 3, characterized in that the diameter d of the aperture (77) of the extractor (72) is determined in such a way as to intercept the peripheral portion of the beam of emitted ions so as to block the ions that have the most peripheral trajectories.
5. Tomographic atom probe according to Claim 4, characterized in that the extractor comprises a number of diaphragms (111) of different aperture diameters, that can be alternately arranged at the level of the central aperture (77) of the extractor (72).
6. Tomographic atom probe according to Claim 5, characterized in that the different diaphragms (111) are produced on a moving bar (112) that can slide in front of the aperture (77) of the extractor (71) so as to place the desired diaphragm in front of the aperture; the sliding movement of the bar being automated.
7. Tomographic atom probe according to any one of the preceding claims, characterized in that the three electrodes (71, 72 and 73) are configured and arranged in such a way as to provide, inside the flight chamber, a free space that is sufficient to house a removable probe adjusting device.
8. Tomographic atom probe according to Claim 7, characterized in that the free space is sufficient to have a field ion microscope in the probe.
9. Tomographic atom probe according to any one of the preceding claims, characterized in that a second electrostatic lens (73, 131 and 132) is placed between the first electrostatic lens (71, 72 and 73) and the detector (12).
10. Tomographic atom probe according to Claim 9, characterized in that the first electrostatic lens (71, 72 and 73) is configured to focus the least open trajectories in proximity to the median plane of the second electrostatic lens (73, 131 and 132).
11. Topographic atom probe according to Claim 9, characterized in that the second electrostatic lens (73, 131 and 132) generates a delaying electrical field.
12. Topographic atom probe according to Claim 9, characterized in that the second electrostatic lens (73, 131 and 132) generates an accelerating electrical field.
13. Topographic atom probe according to Claim 1, characterized in that the extractor (71) is subjected to a pulsed potential.
14. Topographic atom probe according to Claim 1, characterized in that the ions of the material are separated from the sample (11) by means of a pulsed laser.
15. Topographic atom probe according to Claim 14, characterized in that the extractor (71) is subjected to a pulsed potential synchronized with the laser emission.

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