EP0232651A1 - Ion source operating at the electron cyclotron resonance - Google Patents

Abstract

1. Ion source with electronic cyclotronic resonance, comprising : - a closed enclosure (2) which has a lengthwise axis (4), a first (3) and a second (5) end which are oriented along this axis, this enclosure (2) containing a gas intended to form a plasma confined in the said enclosure, - a device (6, 8) for injecting a high-frequency electromagnetic field at the first end (3) of the enclosure, - means (12, 14) for generating in the enclosure a main magnetic field with axial symmetry defining a first polarity (N or S) at the first end of the enclosure and a second polarity (S or N) at the second end (3) of the enclosure, - a system (24, 26) for extracting ions from the enclosure, this system being situated at the second end, - a magnetic structure (16, 18, 36, 39) distributed around the axis (4) and made up of a number of layers stacked along the said axis, each layer generating radial local magnetic fields (20), which is characterized in that the end layer situated facing the first end (3) of the enclosure generating a local magnetic field of the poles of this first polarity and a magnetic performance inferior to that generated by the other layers.

Description

The present invention relates to an ion source with electronic cyclotron resonance allowing in particular the production of multicharged positive 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, a gaseous medium, consisting of one or more gases or metallic vapors, by means of 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.2π m / e, in which e represents the charge of electrons, m its mass, f the frequency of the electromagnetic field. This axial magnetic field is generally created by solenoids or coils surrounding the enclosure.

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

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 local radial fields are in particular generated by several magnetized bars, arranged symmetrically around the enclosure and each consisting of several elementary magnets, placed side by side. Such a magnetic configuration is described in particular in document 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 side of the ion extraction or on the side of the injection 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 FR-A-2 556 498.

On the other hand, these 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 parasitic energy results in annoying phenomena, linked to the transfer of electromagnetic energy to electrons located outside the confinement zone and in particular in an area with a strong decreasing magnetic gradient towards the walls of the enclosure. . This results in intense and localized bombardment 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.

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.

More specifically, the invention relates to an ion source with electronic cyclotron resonance, comprising:
a closed enclosure having a longitudinal axis, first and second ends oriented along this axis, this enclosure containing a gas intended to form a plasma confined in said enclosure,
- a device for injecting a high frequency electromagnetic field at the first end of the enclosure,
means for generating in the enclosure a magnetic field with axial symmetry, defining a first polarity at the first end of the enclosure and a second polarity at the second end of the enclosure,
a magnetic structure distributed around the axis and formed of several layers stacked along said axis, each layer generating local radial magnetic fields, the terminal layer located opposite the first end of the enclosure engen drant a local magnetic field lower than those generated by the other layers, and
- a system for extracting ions from the enclosure, this system being located at the second end.

By gas intended to form a plasma is meant a gaseous medium containing one or more gases or vapors of one or more solid materials. For this purpose, reference may be made to French patent application ^No. 83 10862 of 3 June 1983 filed on behalf of the applicant.

Furthermore, the stacked layers forming the magnetic structure can be fictitious or real.

The fact of creating a weaker local radial magnetic field on the side of the injection of the electromagnetic wave into the enclosure, makes it possible to prevent the resulting magnetic field (axial + radial) at this location having an amplitude satisfying the electronic cyclotronic resonance condition.

It should be noted that the outright removal of the terminal layer of the magnetic structure located opposite the high frequency injection device would only move the problem to the next layer.

In a first variant, the magnetic structure comprises a first and a second alternating series of magnetic bars, disjoint, distributed around the axis and formed of elementary magnets oriented radially, the poles of the bars of the first series oriented inwards of the enclosure having the first polarity and the poles of the bars of the second series oriented towards the inside of the enclosure having the second polarity, the terminal magnet of each bar of the first series, located opposite the first end of the enclosure, having magnetic performance lower than that of the other magnets of the corresponding bar.

The inventor has found that the annoying magnetic disturbances at the ends of the bars or edge effects only appear for the elementary magnets having the same polarity as that associated with the main magnetic field with axial symmetry.

Indeed, for a north polarity, associated with the main axial magnetic field, located on the injection side of the electromagnetic field, the terminal magnets located on this side, and having their north pole oriented towards the inside of the enclosure, create parasitic axial magnetic fields which are cut off from the main axial magnetic field resulting in a resulting magnetic field (axial + radial) whose amplitude is lower the stronger the radial magnetic field. As a result, the value B _r corresponding to the resonance can be easily achieved locally with all the drawbacks already mentioned. On the other hand, the parasitic axial magnetic fields created by the terminal magnets having their south pole oriented towards the interior of the enclosure are added to the main axial magnetic field generating a resulting magnetic field whose amplitude is much greater than B _r .

This therefore leads, for a north polarity located on the injection side of the electromagnetic field, to modify the magnetic performance of the terminal magnets located on this side and whose north pole is oriented towards the interior of the enclosure.

Conversely, if the polarity, associated with the main axial magnetic field located on the side of the electromagnetic injection is a south polarity, then these are the terminal magnets, located on the side of this injection and having their south poles oriented towards the inside of the 'enclosure which should have lower magnetic performance than other magnets of the corresponding bar.

The lower magnetic performances of the terminal magnet of each bar of the first series can be obtained by producing these terminal magnets in a material different from that constituting the other magnets of the corresponding bar.

In particular, the material of the terminal magnet of each bar of the first series may have a binary composition and that of the other magnets of the bar corresponding to a ternary composition. For example, the material of the terminal magnet can be based on cobalt and samarium and that of the other magnets based on iron, praseodymium and boron.

Another solution for reducing the magnetic performance of the terminal magnet of each bar of the first series lies in the reduction of the length of the terminal magnet compared to that of the other magnets of the corresponding bar. These terminal magnets of shorter length can be made of the same material as that of the other magnets of the corresponding bar or else of another material with lower magnetic performance, as described above.

Another solution for reducing the magnetic performance of the terminal magnets of the bars of the first series lies in the use of a wedge of non-magnetic material attached to each terminal magnet, this wedge, disposed towards the inside of the enclosure, has a thickness such that the length of the terminal wedge-magnet assembly is equal to the length of the other magnets of the corresponding bar. This solution is combined in particular with the use of terminal magnets of the bars of the first series made of a material different from those of the other magnets and in particular of lower magnetic performance.

According to another variant, the magnetic structure comprises first and second alternating series of magnetized and disjointed main bars, distributed around the axis and formed of elementary magnets oriented radially, the poles of the bars of the first series oriented towards the inside the enclosure having the first polarity and the poles of the bars of the second series oriented towards the inside of the enclosure, having the second polarity, as well as intermediate magnetic bars, inserted between the main bars and of length less than that of these main bars, the intermediate bars, set back relative to the main bars at the first end of the enclosure, having their poles in contact with the lateral faces of each main bar identical to the pole of the latter oriented towards the inside the enclosure.

If this terminal layer of the magnetic structure, devoid of intermediate magnets, does not generate a sufficiently weak local magnetic field, it is possible to play on the magnetic performances of the terminal magnets of the main magnetic bars of the first series, located next to the high frequency injection device. The magnetic modifications of these terminal magnets can be carried out by using a different material for these magnets, by reducing their length and / or by attaching thereto a shim made of non-magnetic material, as described above.

• - la figure 1 représente schématiquement, en coupe longitudinale, une source d'ions conformément à l'invention,
• - les figures 2a et 2b représentent schémati­quement des vues en bout selon la ligne II-II de la figure 1,
• - la figure 3 est une vue en perspective d'une structure magnétique particulière de la source d'ions de la figure 1, comportant des barreaux aimantés intermédiaires, et
• - la figure 4 est une vue schématique illus­trant l'influence magnétique des barreaux aimantés in­termédiaires de la structure magnétique de la figure 3.

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,
• FIGS. 2a and 2b schematically represent end views along the line II-II of FIG. 1,
• FIG. 3 is a perspective view of a particular magnetic structure of the ion source of FIG. 1, comprising intermediate magnetic bars, and
• FIG. 4 is a schematic view illustrating the magnetic influence of the intermediate magnetized bars of the magnetic structure of FIG. 3.

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 vacuum containment 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 2 has a longitudinal axis 4, passing through the center 7 of the enclosure, which in the case of a cylindrical enclosure represents the axis of revolution, and two ends 3 and 5 oriented along this axis.

The electromagnetic wave, produced by a source 6 such as a Klystron, is introduced into the resonant cavity, at the end 3 of the enclosure, by means of a waveguide 8 of circular or rectangular section.

A pipe 10 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. Enclosure 2 can be filled with hydrogen, neon, xenon, oxygen, carbon, nitrogen, tungsten, titanium, molybdenum, zirconium, etc. at a pressure of order of 10⁻⁵ mbar for a 10 GHz wave.

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

Coils 12 and 14 respectively surrounding the ends 3 and 5 of the enclosure 2 make it possible to create a magnetic field of axial symmetry symbolized by the arrows 15, in the form of a magnetic bottle. This magnetic field 15 has a minimum amplitude at the center 7 of the cavity and a maximum amplitude at the ends of the enclosure 2. This magnetic field which can be either continuous or pulsed is notably oriented from the end 3 to the end 5 of the enclosure. It then defines an axial north pole on the side of the end 3 of the enclosure 2 and an axial south pole on the side of the end 5 of said enclosure. An orientation of the axial magnetic field from the end 5 to the end 3 can of course be envisaged. The amplitude of the axial magnetic field 15 can vary from 3000 to 5000 Gauss for a wave of 10 GHz.

Magnetic bars 16 and 18 distributed all around the cavity and parallel to the longitudinal axis 4 of the enclosure make it possible to create local magnetic fields, symbolized by the arrows 20, located radially with respect to the axis 4. These fields Magnetic radials can go up to 5000 Gauss for a wave of 10 GHz.

The magnetic field 15 with axial symmetry and the local radial magnetic fields 20 allow the formation of at least one closed "equimagnetic" sheet 22 having no contact with the walls of the enclosure 2. This sheet 22, corresponding to the places of the points where the amplitude of the resulting magnetic fields has the same value, satisfies the condition of electronic cyclotron resonance. The resonance condition is notably satisfied for an amplitude B _r of 3600 Gauss and a frequency of the electromagnetic field of 10 GHz.

The existence of such a resonant sheet makes it possible to strongly ionize the gas or the vapors contained. in enclosure 2, thus giving rise to a plasma of very energetic electrons. This layer 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 thus formed can be extracted from the enclosure 2 by an extraction orifice 24 located on the side of the end 5. This orifice is located on the longitudinal axis 4 of the enclosure and in the extension of the waveguide 8 used for injecting the high frequency into the enclosure. An electrode 26 placed downstream of the extraction orifice 24, brought to a negative potential using a power source 28, ensures the extraction of the ions. The ions thus extracted 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.

The magnetic bars 16 and 18 can be four, six, eight, etc. The bars 16 are formed of several permanent elementary magnets joined 30 and 31 having their north poles oriented towards the inside of the enclosure 2 and their south poles facing outwards. Similarly, the magnetized bars 18 are formed of elementary magnets 32 joined, identical, having their south poles oriented towards the inside of the enclosure 2 and their north poles oriented towards the outside. These two types of bars, as shown in Figures 2a, 2b and 3, are arranged alternately, a bar 16 being interposed between two bars 18 and vice versa.

In order to avoid the formation of resonant caps, and therefore a loss of energy from the electromagnetic wave, the terminal magnets 31 of the bars 16, located on the side of the end 3 of the enclosure 2, that is to say on the side of the injection of the electromagnetic wave, exhibit magnetic performance lower than that of the magnets 30 of the corresponding bar 16. Indeed, this makes it possible, taking into account the direction of flow of the creepage line 33 of the terminal elementary magnets 31, shown in FIG. 1, to greatly reduce the antagonistic magnetic field created at the end 3 of the enclosure 2 by the component axial of the leakage field represented by 33.

The reduction of the magnetic performances of the terminal magnets 31 of the bars 16 can be obtained by using a binary material and in particular in SmCo₅ to constitute the magnets 31, the magnets 30 then being formed of a ternary material containing for example praseodymium, iron and boron.

Another solution consists, as shown in FIG. 2a, of using terminal magnets 31 for the bars 16 having a length l less than the length L of the magnets 30 of these same bars. For example the magnets 31 can measure 5 cm long and the magnets 30 can measure 7 cm long.

The terminal magnets 31 can be made of the same material as that constituting the other magnets 30 of the corresponding bars, but also of a different material having magnetic properties inferior to those of the magnets 30. In particular, the terminal magnets 31 can be made of SmCo₅ and the magnets 30 made of Fe-Pr-B.

In order to avoid any disturbance in the resulting magnetic field and consequently on the equimagnetic sheet 22 (FIG. 1) the terminal magnets 31 of the bars 16 will all have the same length and will all be made of the same material. Likewise the magnets 30 of the bars 16 and the magnets 32 of the bars reaux 18 will have the same length L and will be made of the same material.

The terminal magnets 31 of the bars 16 located on the side of the injection of the electromagnetic wave into the enclosure can moreover be each associated with a wedge 34 made of a non-magnetic material. Each wedge 34 is located between the corresponding terminal magnet 31 and the enclosure 2. Its thickness e is such that e + l ′ = L, if L represents the length of the magnets 30 of the corresponding magnetic bar and l ′ the length of l magnet 31 associated. The shims 34 may have a thickness ranging from 1 to 10 mm. These terminal magnets 31 associated with the shims 34 may possibly be made of a material different from those constituting the other magnets 30 and possibly 32.

To make the most of the performance of the elementary magnets constituting the bars 16 and 18, it is possible to use a special structure as shown in FIG. 3.

This structure comprises intermediate magnetic bars 36 and 39, inserted between the bars 16 and 18. The bars 36 and 39 are joined in pairs and are located respectively in contact with the lateral faces 38 and 40 of the bars 16 and 18.

The intermediate bars 36 have their north poles in contact with the lateral faces 38 of the bars 16 whose north is oriented towards the inside of the enclosure 2. Likewise, the intermediate bars 39 have their south poles in contact with the lateral faces 40 of the bars 18, the south of which is oriented towards the interior of the enclosure 2. This arrangement makes it possible, as shown in FIG. 4, to artificially increase the polar surfaces of the bars 16 and 18 and therefore to maximize the radial magnetic fields emitted by these bars near their inward-facing poles inside the enclosure. This structure makes it possible in particular to double the local radial fields emitted by the magnets constituting the magnetic bars 16 and 18.

In such a structure called "full magnet", it is possible to reduce according to the invention the local magnetic fields generated at the end 3 of the enclosure, where the high frequency injection takes place, by using bars intermediaries 36 and 39 shorter than the bars 16 and 18, the ends of the bars 16 and 18 projecting from the ends of the bars 36 and 39. Under these conditions, the local magnetic fields created near the injection of the high frequency are only due to the terminal magnets of the bars 16 and 18 projecting.

In the event that these terminal fields are not sufficiently weak to prevent the formation of resonant caps at the level of the injection of the high frequency, terminal magnets 31 of the bars 16 having magnetic performance lower than those of the bars 30 constituting can be used. these same bars 16 and possibly lower performance than the magnets 32 of the bars 18. This can be achieved by using terminal magnets 31 made of SmCo₅ and magnets 30 and 32 made of Pr-Fe-B and / or using magnets 31 shorter than magnets 30 and 32 and / or by inserting a non-magnetic shim between the magnets 31 and the enclosure 2, as described above.

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

In particular, the means making it possible to generate the local radial magnetic fields can be different provided that these are in the form of a magnetic structure, distributed around the longitudinal axis of the enclosure, generating at its end located opposite the high frequency injection of local magnetic fields lower than the rest of the magnetic structure. This magnetic structure could in particular be produced using cylindrical bars, arranged parallel to one another along the axis of the enclosure and being in a superconductive state (see US-A-4,417,178).

Furthermore, the magnetic field with axial symmetry could be produced by ferrites in place of the two coils surrounding the enclosure. This field could also be oriented from the extraction orifice to the high frequency injection device. In this case, it is the terminal magnets of the bars 18 which must have the lowest magnetic performance.

Likewise, the enclosure may have a shape other than a cylindrical shape, for example a rectangular or polygonal shape.

The ion source according to the invention makes it possible in particular to obtain ion beams highly charged with rare gases, such as for example ion beams of Ne⁺¹⁰, Ar⁺¹³, Xe⁺³³ as well as ions of C⁺⁶, N⁺⁷, etc ...

Claims (8)

1. Ion source with electronic cyclotron resonance, characterized in that it comprises:
- a closed enclosure (2) having a longitudinal axis (4), a first (3) and a second (5) ends oriented along this axis, this enclosure (2) containing a gas intended to form a plasma confined in said enclosure,
- a device (6, 8) for injecting a high frequency electromagnetic field at the first end (3) of the enclosure,
- Means (12, 14) for generating in the enclosure a magnetic field with axial symmetry defining a first polarity (N) at the first end of the enclosure and a second polarity (S) at the second end (3) of the enclosure,
- a magnetic structure (16, 18, 36, 39) distributed around the axis (4) and formed of several layers stacked along said axis, each layer generating local radial magnetic fields (20), the terminal layer located opposite the first end (3) of the enclosure generating a local magnetic field lower than those generated by the other layers, and
- a system (24, 26) for extracting the ions from the enclosure, this system being located at the second end. 2. ion source according to claim 1, characterized in that the magnetic structure comprises first and second alternating series of magnetic bars (16, 18), disjoint, distributed around the axis (4) and formed of elementary magnets (30, 31, 32) oriented radially, the poles of the bars (16) of the first series oriented towards the inside of the enclosure (2) having the first polarity (N) and the poles bars (18) of the second series oriented towards the inside of the enclosure (2) having the second polarity (S), the terminal magnet (31) of each bar (16) of the first series, located opposite of the first end (3) of the enclosure, having magnetic performance lower than that of the other magnets (30) of the corresponding bar (16). 3. Ion source according to claim 1, characterized in that the magnetic structure comprises first and second alternating series of main bars (16, 18) magnetized and disjoint, distributed around the axis (4) and formed d elementary magnets (30, 31, 32) oriented radially, the poles of the bars (16) of the first series oriented towards the inside of the enclosure (2) having the first polarity (N) and the poles of the bars (18 ) of the second series oriented towards the inside of the enclosure (2) having the second polarity (S), as well as intermediate magnetic bars (36, 39) interposed between the main bars (16, 18) and of shorter length to that of these main bars, the intermediate bars (36, 39), set back relative to the main bars (16, 18) at the first end (3) of the enclosure, having their poles in contact with the lateral faces (38, 40) of each main bar (16, 18) identical to the pole of the latter facing the interior of the enclosure. 4. Ion source according to claim 3, characterized in that each main magnetic bar of the first series comprises a terminal magnet (31), located opposite the first end (3) of the enclosure (2), having magnetic performance lower than that of the other magnets (30) of the corresponding bar (16). 5. Ion source according to claim 2 or 4, characterized in that the terminal magnet (31) of each bar (16) of the first series is made of a material different from that constituting the other magnets (30) of the corresponding bar (16). 6. Ion source according to claim 5, characterized in that the material of said terminal magnet (31) is based on cobalt and samarium and that of the other magnets (30) based on iron, praseodymium and boron. 7. ion source according to any one of claims 2, 4 to 6, characterized in that the terminal magnet (31) of each bar (16) of the first series has a length (l) less than that ( L) of the other magnets (30) of the corresponding bar (16). 8. An ion source according to claim 7, characterized in that a shim (34) made of non-magnetic material is attached to each terminal magnet (31), this shim (34) disposed towards the inside of the enclosure, having a thickness (e) such that the length of the terminal wedge-magnet assembly is equal to the length (L) of the other magnets (30) of the corresponding bar (16).

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