EP0252845B1 - Electron cyclotron resonance ion source - Google Patents

Description

The present invention relates to an ion source with electronic cyclotron resonance allowing in particular the production of positive multi-charged heavy ions. It finds many applications, depending on the different values of the kinetic energy of the extracted ions, in the field of ion implantation, microgravure, and more particularly in the equipment of particle accelerators, used both in the scientific than medical.

In sources with electronic cyclotron resonance, the ions are obtained by ionizing, in a closed enclosure of the microwave cavity type having an axis of symmetry, a gaseous medium, consisting of one or more gases or metallic vapors. This ionization results from an interaction between the gaseous medium and a plasma of electrons strongly accelerated by electronic cyclotron resonance.

This resonance is obtained thanks to the combined action of a high frequency electromagnetic field, injected at a first end of the enclosure, and a magnetic field with axial symmetry prevailing in this same enclosure. This axial magnetic field, which has an increasing amplitude from the center of the enclosure to the ends of the latter, has in particular an amplitude B _r which satisfies the condition of electronic cyclotron resonance B _r = f.2xnVe, in which e represents the charge of the electron, m its mass, f the frequency of the electromagnetic field.

The ions thus created are extracted from the enclosure by a second end.

Such a source is especially described in EP 0232651 (the French patent application ^No. 85 19252 filed on 26 December 1985 to the applicant).

The axial magnetic field is generally created by solenoids or coils, surrounding the enclosure, traversed by currents flowing in the same direction.

In this type of source, the quantity of ions that can be produced results from the competition between two processes, on the one hand the formation of ions by electronic impact on neutral atoms constituting the gas to be ionized, and on the other hand the destruction of these same ions by recombination due to a collision of these ions with a neutral atom; this neutral atom can come from the gas not yet ionized or else be produced on the walls of the enclosure by the impact of an ion on said walls.

The problem in this type of source is therefore to minimize the destruction of the ions formed by avoiding any collision of these with a neutral atom.

To remedy this drawback, provision has been made to confine the ions formed in the enclosure as well as the electrons used for their ionization. This is achieved by creating inside this enclosure a set of local, radial and axial magnetic fields, defining at least one closed sheet called "equimagnetic", having no contact with the walls of the enclosure and on which the electronic cyclotron resonance condition is satisfied. This tablecloth represents the place of the points where the amplitude of the local magnetic fields presents the same value.

Such a source has in particular been described in document US-A-4 417 178 filed in the name of the applicant.

The radial local fields are in particular generated by several magnetic bars, arranged symmetrically around the enclosure and going from one end to the other of the enclosure. They each consist of several elementary magnets, joined together. Such a magnetic configuration is notably described in document EP-A 0 145 586 (FR-A-2 556 498) filed in the name of the applicant.

This magnetic configuration has local magnetic disturbances (or edge effects) at these two ends generating parasitic axial magnetic components which add to or subtract, depending on the polarity of the magnets, from the main magnetic field with axial symmetry.

The modulation of these parasitic axial magnetic fields is all the stronger when the elementary magnets constituting the bars are more efficient.

However, it is currently appearing on the market of increasingly efficient magnets of ternary compositions, in particular magnets based on iron, praseodymium and boron. These rare earth magnets have the advantage of having a high magnetic rigidity, which, in addition to their high magnetic performance, allows the superposition of two opposing fields without risk of demagnetization of these magnets. A high magnetic rigidity allows in particular the making of composite multipole structures where the axial and radial fields are composed algebraically.

These high-performance magnets generate axial and radial magnetic fields at the ends of the magnetized bars which, when combined with the main axial field, lead to the condition of electronic cyclotron resonance. This resonance condition generates resonant caps at the two ends of the magnetic structure which, depending on whether they are on the ion extraction side or on the injection side of the electromagnetic wave, can be beneficial or harmful.

The production of resonant caps on the ion extraction side is generally beneficial as shown in document EP-A 0 145 586 (FR-A-2 556 498).

However, the extraction orifice being in an area with an unoptimized magnetic field where the plasma of electrons and multicharged ions is moderately dense, the extraction of ions along the axis of the enclosure is not optimal.

In addition, the resonant caps are always harmful on the side of the injection of the high frequency since they take energy in the electromagnetic wave which should normally be used for the ionization of the gaseous medium contained in the enclosure.

This absorption of stray energy results in annoying phenomena linked to the transfer electromagnetic energy to electrons located outside the confinement area and in particular in an area with a strong magnetic gradient decreasing towards the walls of the enclosure. This results in intense bombardment, located opposite the magnetic bars, of the wall of the enclosure by energetic electrons which can lead to its damage, a significant emission of X-rays and the creation of instabilities in the confined electron plasma.

In order to avoid damage to the internal wall of the enclosure, a heat shield must be provided on said wall.

In addition, magnetic structures in the form of a "cusped bottle" or cuspid have been known for more than a quarter of a century. This magnetic configuration is obtained using two coils, arranged symmetrically with respect to the center of the enclosure and supplied by currents flowing in opposite directions.

In such a configuration, the magnetic field has symmetry of axial revolution and symmetry with respect to the center of the source. The amplitude of the magnetic field increases in all directions of space from the center of the enclosure, until reaching a maximum near the coils, while passing by values defining closed equimagnetic surfaces on which the condition of electronic cyclotron resonance can be satisfied and whose dimensions increase from the center of the enclosure.

In particular, the magnetic field created by coils supplied in opposition has axial and radial components.

To obtain an optimum of the axial magnetic field, taking into account the supply in opposition of the coils, these must be as far apart as possible from each other. Conversely, to obtain an optimum of the radial magnetic field, in particular in the median plane of the enclosure, the two coils must be as close as possible to each other.

Between these two contradictory requirements, there is a compromise corresponding approximately to the equality of the axial magnetic fluxes in the coils and radial in the common lateral part. This is obtained for a distance separating the coils, equal to the mean radius of these coils.

Unfortunately in this case, there is a deficit of radial magnetic field in the median plane of the enclosure, compared to the axial magnetic field created, deficit due to the fact that the maximum of the radial magnetic field is made all against the coils (law of Biot and Savart). As a result, the equimagnetic surfaces are quickly limited.

The subject of the present invention is precisely an ion source with electronic cyclotron resonance which makes it possible to remedy the various drawbacks above.

• - une enceinte fermée ayant un axe de symétrie longitudinal, une première et une seconde extrémités orientées selon cet axe, cette enceinte contenant un gaz destiné à former par résonance cyclotronique un plasma confiné dans ladite enceinte,
• - un dispositif pour injecter à la première extrémité de l'enceinte un champ électromagnétique de haute fréquence,
• - un système pour extraire de l'enceinte les ions formés, situés à la seconde extrémité de l'enceinte,
• - une structure magnétique, disposée autour de l'enceinte et présentant un axe de symétrie confondu avec celui de l'enceinte, engendrant ds champs magnétiques locaux, axiaux et radiaux, définissant au moins une nappe équimagnétique sur laquelle la condition de résonance cyclotronique électronique est satisfaite, la structure magnétique comprenant pour créer une bouteille magnétique cuspée ou cuspidée:
• - une premiére et une seconde bobines disposées de part et d'autre et à égale distance d'un plan médian, perpendiculaire à l'axe longitudinal de l'enceinte et passant par le centre de la cavité, ces deux bobines étant parcourues par des courants de sens de circulation opposés, et
• - des moyens de concentration des lignes de forces magnétiques, entourant l'enceinte dans le plan médian et renforçant localement les champs magnétiques radiaux dans ce plan.

More specifically, the invention relates to an ion source with electronic cyclotron resonance comprising:

• a closed enclosure having a longitudinal axis of symmetry, first and second ends oriented along this axis, this enclosure containing a gas intended to form, by cyclotronic resonance, a plasma confined in said enclosure,
• - a device for injecting a high frequency electromagnetic field at the first end of the enclosure,
• - a system for extracting the ions formed from the enclosure, located at the second end of the enclosure,
• - a magnetic structure, arranged around the enclosure and having an axis of symmetry coincident with that of the enclosure, generating local, axial and radial magnetic fields, defining at least one equimagnetic sheet on which the condition of electronic cyclotronic resonance is satisfied, the magnetic structure comprising to create a cusped or cusped magnetic bottle:
• - a first and a second coil arranged on either side and at equal distance from a median plane, perpendicular to the longitudinal axis of the enclosure and passing through the center of the cavity, these two coils being traversed by opposite direction of flow currents, and
• means for concentrating the lines of magnetic forces, surrounding the enclosure in the median plane and locally reinforcing the radial magnetic fields in this plane.

By gas intended to form a plasma, it is necessary to understand a gaseous medium containing one or more gases or vapors of one or more solid materials. To this end, reference may be made to document EP-A 0 130 907 (FR-A-2 548 436) filed in the name of the applicant.

The use of two coils supplied in opposition makes it possible to obtain a cusped or cusped "magnetic bottle", having a symmetry of revolution along the longitudinal axis of the cavity and a symmetry with respect to the center of the cavity.

In this magnetic structure, there are values of the magnetic field for which the condition of electronic cyclotron resonance is satisfied.

The use of means for concentrating the magnetic force lines in the median plane of the enclosure constituting a unipolar structure makes it possible in particular to bring the elementary coils supplied in opposition closer together, the antagonistic influences of the coils being attenuated by the presence of these means of concentration. In addition, these means of concentration allow a homogeneous distribution of the layers or equimagnetic surfaces inside the enclosure.

As in any "magnetic bottle" there are radially oriented magnetic leaks, therefore causing leakage of energetic plasma along the corresponding magnetic force lines. However, in the source according to the invention, the radial leakage force lines all converge towards the median plane of the enclosure, which leads to a thermal impact of the leakage plasma in the entire median plane, on a circular band, and not on discrete generators of the enclosure as in the case of a multipolar system constituted by magnetic bars surrounding the enclosure according to the prior art. This considerably facilitates the thermal protection of the internal wall of the enclosure and therefore the local cooling of said wall.

Furthermore, the elimination of the magnetic bars compared to the prior art makes it possible to avoid magnetic disturbances at the ends of the bars which are most often harmful.

According to a first embodiment of the invention, the concentration means comprise a magnetic shield having the form of a ring surrounding the enclosure and arranged in the median plane.

In order to make the ion source according to the invention as compact as possible, in particular by bringing the coils as close as possible, the concentration means can be formed by a magnetic shield enclosing the two coils and having in section along a plane containing the longitudinal axis the shape of an E turned towards this longitudinal axis, the central leg of the E, located in the median plane, having a width twice that of the other legs of the E.

The central leg of the E plays the same role as the simple ring, provided above.

Preferably, the magnetic shielding, whether in the form of a simple ring or in the form of an E, seen in longitudinal section, is made of soft iron.

According to a second embodiment of the invention, the concentration means consist of several identical permanent magnets, arranged around the enclosure and in the median plane, the poles of these magnets oriented towards the longitudinal axis being placed side by side and located at the same distance from the longitudinal axis and having the same polarity.

Advantageously, these permanent magnets are made of an alloy of samarium and cobalt of formula SmCo ₅ .

If necessary, to increase the intensity of the magnetic field, it is possible to use simultaneously the magnetic shielding, having the shape of a simple ring or an E, in longitudinal section, and the permanent magnets.

• - la figure 1 représente schématiquement, en coupe longitudinale, une source d'ions conforme à l'invention,
• - la figure 2 est une vue schématique montrant la circulation des lignes de forces magnétiques créées par les bobines seules à l'intérieur de la source selon l'invention,
• - la figure 3 représente schématiquement une vue en coupe selon la ligne III-III de la figure 1,
• - la figure 4 est une vue schématique montrant la circulation des lignes de forces magnétiques créées à l'intérieur de la source de la figure 1,
• - la figure 5 représente schématiquement, en coupe longitudinale, une variante de la source d'ions de la figure 1,
• - la figure 6 représente schématiquement, en coupe transversale, selon le plan médian de l'enceinte d'une source de l'invention, une variante de réalisation des moyens de concentration des lignes de forces magnétiques situés dans ce plan,
• - la figure 7 est une vue schématique montrant la circulation des lignes de forces magnétiques créées à l'intérieur de la source de la figure 6,
• - la figure 8 représente schématiquement, en coupe transversale selon le plan médian de l'enceinte d'une source selon l'invention, une autre variante des moyens de concentration des lignes de forces magnétiques situés dans ce plan, et
• - la figure 9 est un schéma montrant la circulation des lignes de forces magnétiques créées à l'intérieur de la source de la figure 8.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration but not limitation, with reference to the appended figures, in which:

• FIG. 1 schematically represents, in longitudinal section, an ion source according to the invention,
• FIG. 2 is a schematic view showing the circulation of the lines of magnetic forces created by the coils alone inside the source according to the invention,
• FIG. 3 schematically represents a sectional view along line III-III of FIG. 1,
• FIG. 4 is a schematic view showing the circulation of the lines of magnetic forces created inside the source of FIG. 1,
• FIG. 5 schematically represents, in longitudinal section, a variant of the ion source of FIG. 1,
• FIG. 6 schematically represents, in cross section, along the median plane of the enclosure of a source of the invention, an alternative embodiment of the means for concentrating the lines of magnetic forces situated in this plane,
• FIG. 7 is a schematic view showing the circulation of the lines of magnetic forces created inside the source of FIG. 6,
• FIG. 8 schematically represents, in cross section along the median plane of the enclosure of a source according to the invention, another variant of the means for concentrating the lines of magnetic forces situated in this plane, and
• FIG. 9 is a diagram showing the circulation of the lines of magnetic forces created inside the source of FIG. 8.

In Figure 1, there is shown schematically in longitudinal section an ion source with electronic cyclotron resonance according to the invention.

This source comprises a containment vacuum enclosure 2 constituting a resonant cavity which can be excited by a microwave electromagnetic field, continuous or pulsed, having a frequency between 1 and 100 GHz.

This enclosure has a longitudinal axis 4 of symmetry, passing through the center 6 of the enclosure, which in the case of a cylindrical enclosure represents the axis of revolution; it further comprises two ends 8 and 10 situated in the extension of one another and oriented along the axis 6.

The electromagnetic wave, produced by a source 12 such as a Klystron, is introduced into the resonant cavity 2, at the end 10 of the enclosure, by means of a waveguide 14 of circular or rectangular section.

A pipe 16, equipped with a valve 18 makes it possible to introduce a gas or a vapor of a material, inside the cavity 2, intended to form a plasma in said cavity. This plasma can be a plasma of hydrogen, neon, xenon, oxygen, carbon, nitrogen, tungsten, titanium, molybdenum, zirconium, etc., at a pressure of the order of 1 m Pa (10-Smbar) for a 10 GHz electromagnetic wave.

Means not shown such as a vacuum pump can be mounted on the cavity 2.

Two coils 20 and 22 located respectively in the vicinity of the ends 8 and 10 of the enclosure make it possible mainly to create a magnetic field of axial symmetry.

According to the invention, these two coils 20 and 22 are traversed, continuously or pulsed, by currents of opposite direction of circulation. They generate a magnetic structure in which the magnetic field has the shape of a cusped magnetic bottle (called in English terminology cusp) whose lines of magnetic forces 24 are shown in FIG. 2.

The magnetic field created by these coils 20 and 22 has a symmetry of revolution along the axis 4 of the enclosure and a symmetry with respect to the center 6 of this enclosure. It is moreover zero at center 6, for reasons of symmetry; maximum and axial in the zones 25 of the enclosure situated inside each coil; maximum and radial in the zone 27 situated between the two coils and a certain distance from axis 4, and maximum but two components (axial and radial) in zones 29 of the enclosure near the turns of the coils.

The magnetic field notably defines an axial north pole at each end 8 and 10 of the enclosure.

In conclusion, the two coils supplied in opposition generate a magnetic structure in which the amplitude of the magnetic field increases in all directions of space from the center of the cavity until reaching a maximum near the coils, passing through values which can satisfy the condition of electronic cyclotron resonance.

One of the equimagnetic closed surfaces satisfying this condition is represented in FIG. 1 and bears the reference 28.

The resonance condition is notably satisfied for an amplitude B _r of 0.36T (3600 Gauss) and a frequency of the electromagnetic field of 10 GHz.

The existence of such a resonant surface or sheet 28 makes it possible to strongly ionize the gas or the vapors contained in the enclosure 2, thus giving rise to a plasma of very energetic electrons. This surface 28 also makes it possible to confine the ions and the electrons produced by ionization of the gas. Thanks to this confinement, the electrons created have time to bombard the same ion several times and ionize it strongly and even completely.

The highly charged ions formed can be extracted from the enclosure 2 by an extraction orifice 30 located on the side of the end 8 of the enclosure. This orifice is located on the longitudinal axis 4 and in the extension of the waveguide 14 used for the injection of the high frequency. An electrode 32, placed downstream of the extraction orifice 30 and brought to a negative potential using a power source 34, ensures the extraction of the ions.

The ions from the enclosure can then be selected according to their degree of ionization using any known means using a magnetic field and / or an electric field.

In order to obtain an optimum of the axial field and an optimum of the radial field, created by the coils supplied in opposition, a system for concentrating the lines of magnetic forces surrounding the enclosure 2 is provided in the median plane III-III of the enclosure 2, passing through the center 6 of the enclosure and perpendicular to the longitudinal axis 4 of the enclosure.

These means, as shown in Figures 1 and 3, may consist of a magnetic shield 36 of soft iron, having the shape of a continuous annular strip. This iron ring 36 is arranged in the median plane III-III of the enclosure and around it, and as close as possible to it.

The coils 20 and 22 are arranged on either side of this ring 36 and at equal distance from this ring.

The reluctance of the iron constituting the ring 36 then creates, as shown in FIG. 4, a concentration of the lines of magnetic forces 38 in the median plane III-III and in a radial direction thus allowing a homogeneous distribution of the equimagnetic surfaces 40 in the pregnant 2.

This soft iron ring 36 constitutes a southern unipolar magnetic structure for example.

To obtain the most compact ion source possible, the ring-shaped shield 36 can be replaced by a soft iron shield 42 as shown in FIG. 5.

This shielding 42 has in longitudinal section the shape of an E facing the longitudinal axis 4 of the enclosure 2. The central leg 44 of the E is located in the median plane III-III of the enclosure and plays the same role than the soft iron ring 36. This central leg 44 has a width L which is double the width 1 of the other legs respectively 46 and 48 of E.

The fact that the central leg 44 of E has a width double that of the legs 46 and 48 makes it possible to avoid the magnetic saturation of this leg 44. In fact, the magnetic flux passing through this leg is double given the supply of opposition of the coils 20 and 22.

A variant, represented in FIG. 6, consists in replacing the soft iron shielding 36 or 42 by permanent magnets 50 preferably made of samarium-cobalt (SmCos) distributed uniformly around the enclosure 2 and arranged in the median plane III-III of the enclosure. These magnets 50 are joined by their ends 52, oriented towards the longitudinal axis 4 of the enclosure; these ends 52 are located at an equal distance from the axis 4, as close as possible to the enclosure.

In order to form a unipolar structure, the ends 52 of the permanent magnets 50 all have the same polarity, south for example. This unipolar structure allows the strengthening of the radial field at the level of the median plane III-III of the enclosure 2 as shown in FIG. 7. In this figure, the lines of magnetic forces generated by the magnets 50 bear the reference 54.

In another variant shown in FIG. 8, it is possible to have in the median plane III-III of the enclosure 2 a magnetic shield in the form of a ring 36 and a crown of magnets 50, as described above, arranged concentrically, the ring of magnets 50 being located between the enclosure 2 and the ring 36.

It is also possible to replace in this particular case, the ring 36 by the shield 42 in the form of an E, along a longitudinal plane passing through the central axis 4 of the cavity.

In Figure 9, there is shown the magnetic force lines 56 generated by the magnet-ring assembly. These lines of magnetic forces are in no way different from those created by the ring 36 alone.

In the source according to the invention, the plasma of hot or energetic electrons and of multicharged ions is confined in the magnetic structure. Leaks of energetic plasma follow the lines of magnetic forces and preferentially escape where the magnetic field is weak (which corresponds to minimum work). Since the magnetic field has a minimum on the longitudinal axis 4 of the enclosure (it is maximum near the copper of the coils) (see Figures 4, 7 and 9) the densi The leakage plasma is therefore maximum on this axis where the ion extraction is located.

Consequently, the source of the invention allows the extraction, along the longitudinal axis of the enclosure, of multi-charged ions with maximum density unlike the structures of the prior art using a multipolar structure to form the radial field. .

The description given above has of course been given only by way of illustration, any modification, without however departing from the scope of the invention, can be envisaged.

In particular, the enclosure can have a shape other than a cylindrical shape, and for example a rectangular or polygonal shape.

Likewise, other means for concentrating the lines of magnetic forces in the median plane of the enclosure, by locally reinforcing the radial magnetic fields in this plane, can be used.

The ion source according to the invention is relatively compact. It makes it possible to obtain beams of ions of high density or intensity and in particular beams of ions several times charged with heavy materials up to uranium.

Claims (6)

1. Electron cyclotron resonance ion source comprising:

- a sealed enclosure (2) having a longitudinal axis of symmetry (4) a first end and a second end (8, 10) which are oriented along this axis, this enclosure (2) containing a gas intended to form by electron cyclotron resonance a plasma confined in the said enclosure,

- a device (12, 14) for injecting a high-frequency electromagnetic field at the first end (8) of the enclosure,

- a system (30, 32) for extracting the ions formed from the enclosure, which system is situated at the second end of the enclosure, and

- a magnetic structure, disposed about the enclosure (2) and exhibiting an axis of symmetry coincident with that of the enclosure, generating local, axial and radial magnetic fields defining at least one equimagnetic surface (28) on which the condition of electron cyclotron resonance is satisfield, the magnetic structure comprising, in order to create a cusped or crescent magnetic bottle:

- a first coil and a second coil (20, 22) which are disposed on either side of and at equal distance from a median plane (III-III), perpendicular to the longitudinal axis (4) of the enclosure and passing through the centre (6) of the cavity, currents of opposite directions of circulation flowing through these two coils (20, 22), and

- means (36, 42, 50) for concentrating the magnetic force lines (38), surrounding the enclosure in the median plane (III-III) and locally reinforcing the radial magnetic fields in this plane, at the location of the enclosure.

2. Ion source according to Claim 1, characterized in that the concentrating means comprise a magnetic shielding having the form of a ring (36) surrounding the enclosure (2) and disposed in the median plane (III-III).

3. Ion source according to Claim 1, characterized in that the concentrating means comprise a magnetic shielding (42) enclosing the two coils (20, 22) and exhibiting in cross-section, according to a plane (Figure 5) containing the longitudinal axis (4), the form of an E turned towards this longitudinal axis, the central limb (46) of the E, which limb is situated in the median plane (III-III), exhibiting a width (L) which is twice that (1) of the other limbs (46, 48) of the E.

4. Ion source according to Claim 2 or 3, characterized in that the shielding (36, 42) is constructed of soft iron.

5. Ion source according to any one of Claims 1 to 4, characterized in that the concentrating means comprise a plurality of identical permanent magnets (50), which are disposed about the enclosure (2) and in the median plane (III-III), the poles of these magnets oriented towards the longitudinal axis (4) being placed side by side and situated at a same distance from the longitudinal axis (4) being placed side by side and situated at a same distance from the longitudinal axis (4) as close as possible to the enclosure and exhibiting the same polarity (S).

6. Ion source according to Claim 5, characterized in that the permanent magnets (50) are constructed of SmCos.

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