EP0473488A2 - High transmission stigmatic mass spectrometer - Google Patents

Abstract

The mass spectrometer according to the invention includes, arranged between an entrance slit (W1y, W1z) and an exit slit (W4y, W4z) through both of which pass particles emitted by a sample, an optical coupling system (1) placed between two sectors, electrostatic (2) and magnetic (3). Its optical coupling system comprises at least two lenses (Ly) and (Lz) with slit, which are oriented respectively in a first direction (Y) along which the trajectory of the ions is bent by the electrostatic (2) and magnetic (3) sectors, and in a direction (Z) perpendicular to the plane of the trajectory. The positions of the two lenses (Ly) and (Lz) on the optical axis of the spectrometer are determined so as to obtain a compensation for the chromatic dispersions throughout the axis downstream of the spectrometer, a stigmatic image of the entrance slit in the exit plane of the spectrometer and a stigmatic image downstream of the spectrometer of the entrance slit, of a plane which is not conjugate with the entrance slit, in a plane different to the exit plane. <??>Application: high transmission stigmatic mass spectrometer. <IMAGE>

Description

The present invention relates to a high transmission stigmatic mass spectrometer of the type known by the English abbreviation SIMS and whose descriptions can be found in the book Chemical Analysis vol. 86 entitled "Secondary ion mass spectrometry" by A. Benninghoven et al and published by John Whiley and sons in section 4 pages 329 to 664.

In a SIMS device, the part of the device which is located downstream of the sample and the secondary ion extraction devices forms a mass spectrometer whose spectrometry differs greatly from that of thermionization spectrometers in that the ions secondary emitters generally have a much more significant energy dispersion which can typically range up to 20 eV. Under these conditions, it is advantageous in these devices not to filter the beam of particles into energy so as to conserve all of the available ionic signal, one of the performances expected from a spectrometer being to have good transmission for a determined mass resolution. , the term transmission designating the part of the secondary beam which is accepted by the spectrometer and the term mass resolution designating the smallest mass difference between two masses which are measured separately. However as in any spectrometer it is necessary to diaphragm the particle beam to obtain a fixed mass resolution, it is intuitive to think that the more the mass resolution required will be important, the more the transmission of the beam will be limited and the more the signal available for the measurement will be weak.

suivant laquelle B désigne le champ magnétique, m la masse de la particule, V la tension d'accélération de la particule, q sa charge électrique et v sa vitesse. Cependant la relation (1) met en évidence un phénomène de dispersion chromatique du secteur magnétique qui peut limiter sérieusement la résolution en masse par le fait que, le rayon de courbure R_m dépend aussi bien de la tension d'accélération V que de la masse m et que, les dispersions énergiques dans les analyses SIMS sont relativement importantes.Mass dispersion is obtained by causing the particle beam to pass through a magnetic field created by the magnet of a magnetic sector "SM". Each non-relativistic particle that crosses the magnet is then deviated along a trajectory circular with radius of curvature R _m defined by a relation of the form: $${\text{R}}_{\text{m}}\text{= mv / qB = (1 / B). (2mV / q) ½}$$

according to which B denotes the magnetic field, m the mass of the particle, V the acceleration voltage of the particle, q its electric charge and v its speed. However the relation (1) highlights a phenomenon of chromatic dispersion of the magnetic sector which can seriously limit the resolution in mass by the fact that, the radius of curvature R _m depends as well on the acceleration voltage V as on the mass m and that the energetic dispersions in the SIMS analyzes are relatively large.

dans laquelle V représente la tension d'accélération de la particule, E le champ électrique régnant entre les deux électrodes et q la charge électrique de la particule. La relation (2) montre que le rayon R_e dépend de la tension d'accélération mais pas de la masse. Cela permet de dire qu'un champ électrique disperse chromatiquement, mais qu'il ne disperse pas en masse. De ce fait pour réaliser un dispositif à la sortie duquel les trajectoires présentent une déviation dépendant de la masse mais non de l'énergie de la particule, il est habituel d'associer un secteur électrostatique SE qui crée un champ électrique sur le parcours de la particule à un secteur magnétique SM qui crée un champ magnétique. Bien entendu dans cette association, les caractéristiques des secteurs électriques et magnétiques et leur agencement doivent être tels que les dispersions chromatiques se compensent exactement pour que le spectromètre ainsi obtenu soit achromatique. Toujours suivant l'art connu deux types de spectromètre peuvent habituellement être considérés suivant que l'achromatisme a lieu en un point unique de l'axe de sortie du spectromètre ou qu'il a lieu sur tout l'axe de sortie du spectromètre. Dans ces spectromètres le plan achromatique de sortie d'un secteur électrique ou magnétique est le plan où est situé le point d'où semblent issues les trajectoires dispersées en énergie. Le plan achromatique d'entrée est symétrique de celui de sortie si le secteur est symétrique. Egalement dans ces spectromètres toute trajectoire de particules présentant un écart en énergie Δ E et convergeant vers le plan achromatique sort toujours sur l'axe, et l'achromatisme sur l'axe implique que le plan achromatique de sortie du secteur électrostatique SE et le plan achromatique d'entrée du secteur magnétique SM soient conjugués.As further explained in Mr. Benninghoven's book, this problem is usually solved by compensating for the chromatic dispersion of the magnetic sector by that of an electric field created by a voltage applied between two electrodes between which the particle passes. Under these conditions the particle is still deflected along another circular trajectory whose radius of curvature R _e verifies a relation of the form: $${\text{R}}_{\text{e}}\text{= 2V / qE}$$

in which V represents the acceleration voltage of the particle, E the electric field prevailing between the two electrodes and q the electric charge of the particle. The relation (2) shows that the radius R _e depends on the acceleration voltage but not on the mass. This makes it possible to say that an electric field disperses chromatically, but that it does not disperse in mass. Therefore to achieve a device at the outlet of which the trajectories have a deviation depending on the mass but not on the energy of the particle, it is usual to associate an electrostatic sector SE which creates an electric field on the path of the particle with a magnetic sector SM which creates a magnetic field. Of course in this association, the characteristics of the electrical and magnetic sectors and their arrangement must be such that the chromatic dispersions are compensate exactly so that the spectrometer thus obtained is achromatic. Still according to the prior art, two types of spectrometer can usually be considered depending on whether the achromatism takes place at a single point on the exit axis of the spectrometer or whether it takes place on the entire exit axis of the spectrometer. In these spectrometers the achromatic plane of exit from an electric or magnetic sector is the plane where the point from which the trajectories dispersed in energy seem to originate. The input achromatic plane is symmetrical to the output plane if the sector is symmetrical. Also in these spectrometers any trajectory of particles having a difference in energy Δ E and converging towards the achromatic plane always leaves on the axis, and the achromatism on the axis implies that the achromatic plane of exit of the electrostatic sector SE and the plane input of the magnetic sector SM are combined.

An arrangement for performing chromatography on the entire axis of a spectrometer is for example known from the IMS3F device marketed and manufactured by the Applicant Company CAMECA, a corresponding description of which can be found in an article by M. Lepareur entitled "the second generation micro-analyzer CAMECA module 3F "published in the journal THOMSON CSF Vol. 12 no.1, March 1980.

The advantage of this arrangement is that it makes it possible, in addition to performing achromatism over the entire axis, to project onto an image intensifier, the ionic image of the sample filtered in mass.

However, like the electrostatic or magnetic sectors, have, in addition to their deflection properties, focusing properties which depend on the shape which is given to the electrodes of the SE or to the pole pieces of the SM, these generally do not have symmetry of revolution, and the convergence in the two directions Oy and Oz, normal to the optical axis is not identical. However in the IMS3F device, the sphere-shaped electrodes of the sector electrostatic SE, have a focal fe equal in the directions Oy and Oz and the planes located on either side of the electrostatic sector SE, at a distance from the input faces equal to the radius of the electrostatic sector SE are conjugate. In addition, the shape given to the input faces of the magnetic sector SM gives it an optical diagram equivalent to that formed by a pair of lenses. In this way the lenses have equal focal lengths fm for the two directions Oy and Oz, and the space which separates them is not identical in the two directions. An entry slot is arranged at a distance fe upstream from the entry face of the electrostatic sector SE; an outlet slot is arranged at a distance f _m downstream of the magnetic sector SM. These two slots are optically combined and have an approximately equivalent role. It is the setting of the input slit that determines the mass resolution. To select according to a chromatic criterion the particles to be taken into account by the analysis, an "energy" slot is arranged at a distance fe from the exit face of the electrostatic sector SE. In normal operation, the pupil of illumination of the secondary emission also called "cross-over" is imaged on the entry slit and therefore on the exit slit. The plane of the sample is imaged on a diaphragm located just at the entrance to the electrostatic sector SE. Because of the disparity in the directions Oy and Oz of the spaces separating the lenses in doublet from the equivalent diagram of the magnetic sector SM, a stigmator is necessary in a plane close to the exit slit to allow to project a stigmatic image of the plane of the 'sample.

In this device, a coupling optic is placed between the electrostatic sector SE and the magnetic sector SM to compensate, on the one hand, for the chromatic dispersion of the electrostatic sector by that of the magnetic sector and, on the other hand, to combine the planes of the inlet slot and the outlet slot. As also described in the article by M. Lepareur cited above, the coupling optics can be achieved by means of a single lens. Once the optical characteristics of the two electrostatic and magnetic sectors SE and SM, there is only one configuration of the following parameters, (distance between the two sectors, position of the lens, and excitation of the lens) which makes it possible both to compensate for chromatic dispersions and to conjugate the exit slit with the entry slit.

In this configuration, the exit achromatic plane of the SE and the achromatic plane of the SM are combined.

où - Δ M/M est la résolution en masse.

• Wys est la largeur de l'image gaussienne de la fente d'entrée au niveau du plan de sortie du spectromètre
• K_M est le coefficient de dispersion en masse de l'aimant défini par dy = K_M dM/M
• Ky et Kz sont les coefficients d'aberration du second ordre du spectromètre.
• ϑys et ϑzs sont les ouvertures angulaires du faisceau au plan de sortie.

However, as in any spectrometer, the mass resolution ΔM / M with respect to the width of the entry slit and the characteristics of the sectors is determined by a relation of the form: $$\frac{\text{ΔM}}{\text{M}}\text{=}\frac{{\text{W}}_{\text{ys}}}{{\text{K}}_{\text{M}}}\text{+}\frac{{\text{K}}_{\text{y}}}{{\text{K}}_{\text{M}}}{\text{ϑ²}}_{\text{ys}}\text{+}\frac{{\text{K}}_{\text{z}}}{{\text{K}}_{\text{M}}}{\text{ϑ²}}_{\text{zs}}$$

where - Δ M / M is the mass resolution.

• Wys is the width of the Gaussian image of the entry slit at the exit plane of the spectrometer
• K _M is the mass dispersion coefficient of the magnet defined by dy = K _M dM / M
• Ky and Kz are the second order aberration coefficients of the spectrometer.
• ϑys and ϑzs are the angular openings of the beam at the exit plane.

The relation (3) expresses that the angular openings in the orthogonal directions 0Y and OZ in the plane of the slits produce second order aberrations in the direction 0Y which is the direction of the mass dispersion.

As the quantity of ions taken into account by the analysis is proportional to the product Wys x ϑys, there must certainly exist an optimum of the couple (Wys, ϑys) which minimizes the resolution in mass. However, there is every benefit in reducing the aperture ϑzs by optical means, it being understood that the Wzs image is enlarged, but this latter effect has no effect on the mass resolution.

An attempt to reduce the opening of the Z beam was made on an Australian device called SHRIMP where a quadrupole lens placed before the SE electrostatic sector crushes the Z beam. A description of this device is described in an article by S. Clément , W. Compston, G. New-stead entitled "Design of a large, high resolution ion micro-probe" in Proceeding of International Conference on SIMS and ion microprobes. A. Benninghoven editor, Springer Verlag 1977. But in this instrument, there is no possibility of projecting the image of the analyzed sample after the spectrometer.

The object of the invention is to overcome the aforementioned drawbacks.

To this end, the subject of the invention is a double focusing high transmission stigmatic mass spectrometer of the type comprising, arranged between an entry slit and an exit slit crossed by particles emitted by a sample, an optical system for coupling placed between two electrostatic and magnetic sectors respectively, characterized in that the optical coupling system comprises at least two slit lenses oriented respectively in a first direction in which the ion trajectory is curved by the electrostatic and magnetic sectors and in a direction perpendicular to the plane of the trajectory, the positions of the two lenses on the optical axis of the spectrometer being determined to obtain compensation for the chromatic dispersions on the entire axis downstream of the spectrometer, a stigmatic image of the entry slit in the plane from the spectrometer and a stigmatic image downstream of the spectrometer, of a plane not conjugated with the entry slit, in a plane distinct from the exit plane.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the appended drawings which represent:

Figure 1 the construction of the images of the entry slit in an IMS3F type device.

FIG. 2 the positions of the images of the plane of the sample in an apparatus of the IMS3F type.

Figure 3 the positions of the achromatic planes of the electrostatic and magnetic sectors of an IMS3F device.

FIGS. 4A and 4B an embodiment of an optical coupling according to the invention by means of two lenses L _y and L _z arranged between the two sectors SM and SE.

FIGS. 5A and 5B an embodiment of an optical coupling according to the invention by means of a lens Ly and two lenses L _1z and L _2z .

FIG. 6A an embodiment of an optical coupling by means of only a single lens Ly.

FIG. 6B an embodiment of an optical coupling by means of two lenses L _1y and L _2y .

The invention takes advantage of the fact that the opening aberrations in the direction of the axis OZ are much greater in a magnetic sector SM than in an electrostatic sector SE. It allows the realization of an optical coupling between the two sectors allowing to keep the advantages of the IMS3F device. The spectrometer shown in FIG. 1 comprises, arranged on either side of an optical system 1 for coupling L _Y in a direction Y situated in the plane of FIG. 1 perpendicular to the direction X of the optical axis of the spectrometer , an electrostatic sector 2 and a magnetic sector 3. An entry slit W _1y arranged in an entry plane P1 has for image through the electrostatic sector 2 a slit W _2y arranged in the plane P2 image of the plane P1.

In FIG. 2, S _2y is the virtual image of the sample provided by the electrostatic sector in the plane P′2.

In FIG. 1 the optical system L _y transforms the slit W _2y into an image W _3y arranged, FIG. 1, in a plane P3 and in FIG. 2 it transforms the virtual image of the sample S _2y into an image S _3y in a plane P′3.

In this way, the pairs of planes (P2, P3) and (P′2, P′3) appear to be conjugate with respect to the optical system 1. The arrangements of the achromatic planes of the electrostatic and magnetic sectors are shown in FIG. 3.

soit vérifiée.By designating by K _M and K _E the energy dispersion coefficients of sectors 2 and 3, achromatism on the X axis requires that, in addition to the conjugation of the achromatic planes, a condition on the magnification such as the relation : $${\text{W}}_{\text{3Y}}{\text{/ W}}_{\text{2Y}}{\text{= K}}_{\text{M}}{\text{/ K}}_{\text{E}}$$

be verified.

soit vérifiée où λ est un coefficient proche de 5.In the direction normal to the plane of Figures 1 and 2, the same optical coupling system materialized by a lens L _Z , not shown, is used to conjugate the same plane couples (P2, P3) and (P′2, P ′ 3). Similarly to the operation of the lens L _y the lens L _Z transforms the slit W _2Z into a slit W _3Z situated in the plane P3 and transforms the image S _2Z , not shown, into an image S _3Z , not shown, at the plane P′3 on the condition that the following relation $${\text{W}}_{\text{3Z}}{\text{/ W}}_{\text{2Z}}{\text{= λ (K}}_{\text{M}}{\text{/ K}}_{\text{E}}\text{)}$$

be verified where λ is a coefficient close to 5.

Under these conditions the multiplication coefficients applied on the magnifications makes it possible to obtain a reduction of 1 / λ on the openings.

et sa focale f est donnée par $$\frac{\text{1}}{\mathit{\text{f}}}\text{=}\frac{\text{1}}{\text{X-x₂}}\text{-}\frac{\text{1}}{\text{X-x′₂}}$$

However, it is not absolutely essential to produce an isotropic image of the entry slit in the plane of the exit slit because, to operate in mass discrimination, it is sufficient to ensure only this condition in the Y direction. But experience shows that the settings are simplified in a completely interesting way when an isotropic image of the entry slit is available at the exit plane. Given the two pairs of planes (P2, P′2), (P3, P′3) positioned on the optical axis X at the coordinate points (X₂, X′₂) and (X₃, X′₃), the lens allows the two pairs of planes to be conjugated simultaneously if its abscissa X satisfies the equation:

and its focal length f is given by $$\frac{\text{1}}{\mathit{\text{f}}}\text{=}\frac{\text{1}}{\text{X-x₂}}\text{-}\frac{\text{1}}{\text{Xx′₂}}$$

Relations (4), (6) and (7) suggest that the two roots of the equation can represent the respective positions of a lens L _y and a lens Lz each conjugating the two pairs of planes (P2, P ′ 2) and (P3, P′3). L _y denotes a slotted lens or a rectangular lens which is active in the Y direction and practically neutral in the Z direction. L _z denotes a slit lens or a rectangular lens which is active in the Z direction and practically neutral in the Z direction direction Y.

Figures 4A and 4B where elements homologous to those of Figures 1, 2 and 3 are shown with the same references illustrate this arrangement. More particularly, this arrangement corresponds to an embodiment in which the electrostatic sector 2 is a spherical sector with a radius of one meter; therefore K _E = 2 meters and P2 is one meter from the outlet of the electrostatic sector 2 SE. In FIG. 4A, as in FIG. 4B, the origin of the abscissae is on the achromatic plane of the electrostatic sector 2. The achromatic plane P′2 is one meter upstream from the exit of the electrostatic sector. The magnetic sector 3 is flanked by two spaces such that the entry plane is conjugated with the exit plane, its chromatic dispersion K _M = 1.2 m at the exit plane P4 and its achromatic entry plane is distant from 1 , 6 m from the entry plane P3.

The relation (5) imposes that L _y realizes a magnification W _3y / W _2y = 0, 6. This relation ¡anointed with the condition of conjugation between the achromatic planes of the sectors determines the position of the lens X _y = 1.449 m, its focal length f _y = 0.827 m and consequently the abscissa of the image plane P3 (X₃ = 1.780m) and of the achromatic plane entering the magnetic sector SM is 3.380m.

In FIG. 4A, P′2 is taken at the origin, as in the case of the IMS3F. P′2 is consequently the achromatic plane of exit of the electrostatic sector 2 and consequently, P′3, conjugate plane of P′2, is the achromatic plane of entry of the magnetic sector SM.

To find the position and the focal length of the lens L _{z in} Figure 4B, it is necessary to solve equation (6) with x₂ = 2m, x′₂ = Om, x₃ = 1,780 m, x′₃ = 3.380 m. The two roots of the equation are on the one hand, the abscissa X _y which is already known and on the other hand the abscissa of the lens Lz, ie X _z = 2.306 m. The focal length f _Z is in these conditions equal to 0.732m.

The magnification in Z, W _3z / W _2x is then 1.716 or 2.86 times more than the magnification in Y.

This arrangement makes it possible to put an energy discrimination slot on the plane P3 where there is a real image of the input slot. However, it is limited in its practical applications in the sense that to obtain an appreciable magnification ratio in the directions Y and Z, it is necessary that the magnification according to Y is smaller than 1 what, according to the relation ( 4) necessarily corresponds to a K _E / K _M ratio greater than 1 and therefore, since the chromatic dispersions are very strongly dependent on the rays, to a radius of SE of the electrostatic sector 2 greater than that of the magnetic sector 3 and this all the more so as one seeks to create a significant magnification ratio between the directions Z and Y. Thus in the case of FIGS. 4A and 4B the ratio of 2.86 can only be obtained if the radius of the electrostatic sector SE is one meter, the radius of the magnetic sector SM being imposed at 0.585 m. Such a dimension can be entirely excessive, by the bulk to which it leads and by the technological difficulty of producing the electrostatic sector.

According to a second alternative embodiment of the invention, the arrangement shown in FIGS. 5A and 5B overcomes this constraint: it comprises a lens Ly (FIG. 5A) located downstream of the plane P2 and two active lenses at Z, L _1z and L _2z (Figure 5B). The lens L _1z images the slit W _2z with a magnification close to 1 and the lens L _2z acts as a magnifying glass to produce an image with a magnification of the order of 5 while respecting relation (6). With an electrostatic sector having the form of a spherical sector with a radius of 0.585 meters, K _E = 1.17 meters and the plane P2 is at 0.585 meters from the exit of the electrostatic sector 2. The achromatic plane is at 0.585 meters upstream from the exit of the electrostatic sector. The magnetic sector 3 is identical to that of the previous example.

As before, the origin of the abscissae is located on the achromatic plane of the electrostatic sector 2. The relation (4) requires that the lens L _there realizes a magnification W _3y / W _2y = 1, 03. This relation joined to the condition of conjugation between the achromatic planes of the sectors, determines the position of the lens X _y at 1.190 m and its focal length f _y is equal to 0.679 m. As a result, the abscissae of the image plane P3 and of the achromatic plane of the magnetic sector 3 are arranged respectively at 1.169 m and 2.769 m,

Positions de L_1z =

1, 336 m

Focale de L_1z =

0,1 m

Position de L_2z =

1, 760 m

Focale de L_1z =

0,241 m

avec cette configurat¡on le grandissement intermédiaire W_21z/W_2z est de -1, 538. Cet arrangement permet de mettre une fente de discrimination en énergie au plan P2 où l'on trouve une image réelle de la fente d'entrée.From the moment when L _1z is fixed in position and in convergence, the position of the lens L _2z and its focal length are determined by reporting in the equations (6) and (7) the values X₂ = 1, 17 m, X ′ ₂ = Om, X₃ = 1, 169 m and X′₃ = 2, 769 m. As an example, a possible arrangement to obtain a magnification W _3z / W _2x 5, 3 times stronger than the magnification W _3y / W _2y = 1.03 can be obtained with the following characteristics =

Positions of L _1z =

1.336 m

Focal length of L _1z =

0.1m

Position of L _2z =

1,760 m

Focal length of L _1z =

0.241 m

with this configuration the intermediate magnification W _21z / W _2z is -1, 538. This arrangement makes it possible to put an energy discrimination slit on the plane P2 where there is a real image of the input slit.

Finally, according to a third embodiment of the invention, it may be advantageous to replace the lens L _Y by two lenses L _1y and L _2y located on either side of the plane P2. As already shown in FIGS. 5A and 5B, the position and the convergence of the lens L _Y are imposed very strictly by the optical characteristics of the electrostatic 2 and magnetic sectors 3. However, these are not necessarily known with precision at the time of mounting. of the camera and if it is obviously easy to adjust the focal length of the lens L _y , it is not the same for its position. Under these conditions the replacement of the lens Ly by two lenses L _1y and L _2y achieves a zoom effect which gives the operator a latitude of adjustment which he does not have with a single lens. This arrangement makes it possible to put an energy discrimination slot in the intermediate plane P21 where there is a real image of the entry slot. This has an advantage, if the magnification W _3y / W _2y must be close to 1.

Position de L_1y =

0,900 m

Focale de L_1y =

2,250 m

Position de L_2y =

1,250 m

avec une distance focale de L_1y =

0, 820 m

FIGS. 6A and 6B illustrate how the lens L _y of the example of FIGS. 5A and 5B can be replaced by two lenses L _1y and L _2y positioned as follows

Position of L _1y =

0.900 m

Focal length of L _1y =

2,250 m

Position of L _2y =

1,250 m

with a focal length of L _1y =

0.820 m

The magnetic sector 3 being a device which generally does not have the same focusing properties in Y and in Z it is necessary to compensate for its astigmatism. As indicated above, the shape given to the input faces of the magnetic sector 3 gives the latter an equivalent optical diagram consisting of a pair of lenses. The lenses then have an equal focal length for the two directions Oy and Oz, that is to say f _m , but the space which separates them is not identical in the two directions.

To make it possible to remove a stigmator placed just upstream of the exit slit so as to be able to project a stigmatic image of the sample onto an intensifier, it suffices in equation (6) to modify the parameter X′₃ so as to take into account the disparity of the spaces along the directions Y and Z in the equivalent diagram of the magnet so that downstream of the magnet, both the image of the entrance slit and that of the sample are stigma.

The advantage of eliminating the stigmator located before the entry slot is to completely free up the space at this level and therefore to allow, for example the collection in parallel of several different masses.

Claims (14)

Double focus high transmission stigmatic mass spectrometer of the type comprising arranged between an entry slit (W _1y , W _1z ) and an exit slit (W _4y , W _4z ) traversed by particles emitted by a sample, a system optical coupling (1) placed between two sectors respectively electrostatic (2) and magnetic (3), characterized in that the optical coupling system comprises at least two lenses (L _y ) and (L _z ) with slit oriented respectively according to a first direction (Y) in which the trajectory of the ions is curved by the electrostatic (2) and magnetic (3) sectors and in a direction perpendicular (Z) to the plane of the trajectory, the positions of the two lenses (L _y ) and ( L _z ) being determined on the optical axis of the spectrometer to obtain compensation for chromatic dispersions over the entire axis downstream of the spectrometer, a stigmatic image of the entry slit in the exit plane of the spectrometer and a stigmatic image, downstream of the spectrometer, of a plane not conjugated with the entry slit in a plane distinct from the exit plane. Spectrometer according to claim 1, characterized in that the plane of the entrance slit is conjugated or close to it with the opening angle of the ion beam emitted by the sample and in that the plane of the sample is the other plane, the assembly of which creates the stigmatic image. Spectrometer according to claim 1, characterized in that the first lens (L _y ) and the second lens (L _z ) are located on either side of a plane (P2) conjugated with the entry slit by the sector electrostatic (2) and in that the position of the lens (L _y ) and its focal length are determined on the one hand, to conjugate the image (W _2y ) of the slit (W _1y ) obtained in the plane (P2) by an image in a plane (P3) with a magnification (W _3y / W _2y ) equal to the ratio of the mass dispersion coefficients (K _M ) and (K _E ) respectively of the sector magnetic (3) and the electrostatic sector (2), and secondly, to combine the virtual image of the sample (S _2y ) given by the electrostatic sector (2) in a plane (P′2) in one image (S _3y ) in an achromatic plane (P′3) of the magnetic sector (3). Spectrometer according to claim 3, characterized in that the position and the focal length of the lens (L _z ) are determined on the one hand, to conjugate the planes (P2, P3) or the planes (P′2 and P′3) to transform the image (W _2z ) of the slit (W _1Z ) by the electrostatic sector (2) into an image (W _3z ) in the plane P3 with a magnification (W _3z / W _2z ) proportional to the ratio of the coefficients of mass dispersion (K _M ) and (K _E ) of the magnetic sector (3) and the electrostatic sector (2) and on the other hand, to combine the virtual image of the sample (S _2z ) given by the electrostatic sector (2) in the plane (P′2) in an image (S _3z ) in the achromatic plane (P′3) of the magnetic sector (3). Spectrometer according to either of Claims 3 and 4, characterized in that the position of the lens (L _z ) is determined by the roots of an equation of the form:

in which (x₂, x′₂) and (x₃, x′₃) denote the positions of the plane couples (P₂, P′₂) and (P₃, P′₃) and its focal length f is determined by the relation $$\frac{\text{1}}{\mathit{\text{f}}}\text{=}\frac{\text{1}}{\text{X - x₂}}\text{-}\frac{\text{1}}{\text{X - x′₂}}$$

Spectrometer according to claims 1 and 3, characterized in that it comprises an energy slit disposed between the lenses (L _y ) and (L _z ) in a plane conjugated with the entry slit for the lens (L _y ). Spectrometer according to claim 3, characterized in that it comprises yn active optical system in the z direction formed by the assembly of two lenses, a first lens (L _1z ) and a second lens (L _2z ), in that the first lens (L _1z ) reverses the image of the entry slit produced by the electrostatic sector (2) with a ratio close to -1 and in that the second lens (L _2z ) acts as a magnifying glass on the inverted image of the slot provided by the first lens (L _1z ). Spectrometer according to claim 7, characterized in that the position of the lens (L _2z ) is determined by the roots of an equation of the form

in which (x₂, x′₂) and (x₃, x′₃) denote the positions of the plane couples (P₂, P′₂) and (P₃, P′₃) and in that its focal length f is determined for a form relation $$\frac{\text{1}}{\mathit{\text{f}}}\text{=}\frac{\text{1}}{\text{X - x₂}}\text{-}\frac{\text{1}}{\text{X}}$$

Spectrometer according to claim 1 or 5, characterized in that the first lens (L _y ) is unique. Spectrometer according to any one of claims 1, 8 and 9, characterized in that an energy slit is arranged between the output of the electrostatic sector (2) and the first lens (L _y ) in the plane (P₂) conjugate of the entry slit (W _1y ) by the electrostatic sector (2). Spectrometer according to either of Claims 1 and 7, characterized in that the two lenses (L _1y ) and (L _2y ) are situated on either side of a plane (P₂) conjugated with the slot (W _1y ) of entry by the electrostatic sector (2). Spectrometer according to claim 11, characterized in that an energy slit is arranged between the two lenses (L _1y ) and (L _2y ) in the conjugate plane of the slit (W _1y ) by the lens (L _1y ). Spectrometer according to any one of Claims 1 to 12, characterized in that the adjustment of the first and second lenses (L _1z ) and (L _2z ) is used to compensate for the stigmatism defects of the magnetic sector (3). Use of the spectrometer according to any one of claims 1 to 13 in a SIMS device.

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