EP0483004B1 - Electron cyclotron resonance ion source for highly charged ions with polarisable probe - Google Patents

Description

The subject of the present invention is a source of highly charged positive ions with probe polarizable and electron cyclotron resonance (RCE). It finds many applications, in function of the different values of kinetic energy ions extracted, in the fields of implantation ionic, microgravure, and more particularly in the equipment of particle accelerators, used both in the scientific field than medical.

In cyclotron resonance sources electron, the ions are obtained by ionizing, in a closed cavity-like enclosure microwave, a gas, consisting for example of metallic vapors, using an electron plasma strongly accelerated by cyclotronic resonance electronic. This resonance is obtained thanks to the combined action of a high electromagnetic field frequency (HF) injected into the enclosure, containing the gas to be ionized, and a magnetic field, prevailing in this same enclosure, whose amplitude B satisfies under the condition of electronic cyclotron resonance following: B = f.2 πm / e in which e represents the charge of the electron, m its mass and f the frequency of the electromagnetic field.

In this type of source, the amount of ions that can be produced results from 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, single or multiple, in a collision of these last with a neutral atom; this neutral atom can come from gas not yet ionized or be produced on the walls of the enclosure by impact of a ion on said walls.

This drawback is avoided by confining, in the enclosure constituting the source, the ions formed as well as the electrons used for their ionization. This is achieved by creating inside the enclosure radial and axial magnetic fields, defining a closed tablecloth called "equimagnetic", having no contact with the walls of the enclosure and on which the electronic cyclotronic resonance condition is satisfied. This tablecloth is shaped like a balloon rugby. The closer this equimagnetic sheet walls of the enclosure, the more effective it is great because it limits the volume of attendance neutral atoms and therefore the amount of collision neutral atom-ions. This tablecloth also allows confine the ions and electrons produced by gas ionization. Thanks to this confinement, the created electrons have time to bomb several times the same ion and totally ionize it.

• "Minimafios - Surfaces magnétiques et parois" de Mrs GELLER et JACQUOT, publié à l'occasion du quatrième séminaire international sur les sources RCE et sujets annexes, en Janvier 1982, p. 14.1 à 14.14.
• "Source d'ions lourds multichargés triplemafios" de Mrs BRIAND, CHAN-TUNG, GELLER et JACQUOT, publié dans la revue de physique appliquée, en Août 1977, p.1 135 à 1 138.
• "Electron cyclotron resonance multiply charged ion sources" de Mr. GELLER, publié dans IEEE transactions on nuclear science, vol. NS 23, n° 2, en Avril 1976, p. 904 à 912.

The principle of such a source has been described in document FR-A-2 475 798, filed on behalf of the applicant and in the articles:

• "Minimafios - Magnetic surfaces and walls" by Mrs GELLER and JACQUOT, published on the occasion of the fourth international seminar on RCE sources and related subjects, in January 1982, p. 14.1 to 14.14.
• "Source of heavy ions multicharged triplemafios" of Mrs BRIAND, CHAN-TUNG, GELLER and JACQUOT, published in the review of applied physics, in August 1977, p.1 135 to 1 138.
• "Electron cyclotron resonance multiply charged ion sources" by Mr. GELLER, published in IEEE transactions on nuclear science, vol. NS 23, n ° 2, in April 1976, p. 904 to 912.

In Figure 1, there is shown schematically a source of ions of the prior art.

This source contains two stages.

The role of the first stage A is largely part of providing an electron flow in the axis X of the source. This first stage A has a cavity 2a with symmetry of revolution of the solenoidal coils 14a arranged at the two ends of the cavity 2a, creating an axial magnetic field, this field being plus 18a soft iron shielding located at the entrance from the source. The gas or vapor to be ionized is introduced through a conduit 6 inside the cavity 2a. When it comes to steam, it can be introduced into the cavity in the form of a rod suitable for vaporizing. An electromagnetic field is created inside the cavity 2a by a first high frequency input 4a.

The gas leaving the first stage A is pre-ionized and then goes into the second stage B. This stage B consists of a cylindrical cavity multimode 2 of high order, i.e. of dimension large compared to the dimension of the length of the electromagnetic field. Its axis of symmetry bears the reference X. This electromagnetic field is introduced radially by a second entry high frequency 4.

Cavity 2 is joined at its end 5 to a vacuum pump 10b, by means of a pipe in which electrodes are housed 10a. A power source 9 allows to apply a potential difference at these electrodes.

This pump, pipe and electrodes constitutes the extraction means 10 of ions. The ions thus extracted from cavity 2 can then be selected according to their degree ionization using any known means using a magnetic field and / or an electric field.

Around the cavity are arranged coils 14 creating an axial magnetic field and a set 16 of permanent magnets creating a field magnetic radial, generally of the hexapolar type. These axial and radial magnetic fields are superimposed one over the other and distributed throughout the cavity; they thus form a resulting magnetic field which defines at least one equimagnetic surface to inside the cavity 2.

The first problem in this type of source is the importance of clutter; to this problem there is also the manufacturing difficulty and therefore the cost of such a source.

Furthermore, the ion currents supplied by these sources are usually too weak.

The subject of the present invention is a ion source with cyclotron resonance allowing to remedy these drawbacks by simplifying including this type of source.

• une cavité hyperfréquence 2 comportant un axe de symétrie ;
• une entrée haute fréquence 4 débouchant dans la cavité pour y créer un champ électromagnétique de haute fréquence ;
• une introduction de gaz 6 dans la cavité ;
• des moyens de production 14 d'un champ magnétique selon l'axe dans ladite cavité ;
• des moyens de production 16 d'un champ magnétique radial multipolaire dans cette cavité, la superposition de ces champs magnétiques axial et radial formant un champ magnétique résultant réparti dans toute la cavité et définissant au moins une surface équimagnétique S complètement fermée à l'intérieur de la cavité ;
• des moyens d'extraction 10 des ions à l'extrémité 5 de la cavité ;

caractérisée en ce qu'elle comporte :

• une sonde polarisable en tension 20 pour améliorer l'ionisation du gaz et augmenter ainsi le flux d'ions extrait, réalisée en métal émetteur d'électrons, disposée en amont des moyens d'extraction et n'ayant aucun contact avec la surface magnétique ; et
• des moyens d'alimentation en tension 8 de la sonde.

More specifically, the invention relates to a source of highly charged positive ions with electronic cyclotron resonance comprising:

• a microwave cavity 2 comprising an axis of symmetry;
• a high frequency input 4 opening into the cavity to create a high frequency electromagnetic field there;
• an introduction of gas 6 into the cavity;
• means 14 for producing a magnetic field along the axis in said cavity;
• means 16 for producing a multipolar radial magnetic field in this cavity, the superposition of these axial and radial magnetic fields forming a resulting magnetic field distributed throughout the cavity and defining at least one equimagnetic surface S completely closed inside the cavity ;
• means for extracting ions 10 from the end 5 of the cavity;

characterized in that it comprises:

• a voltage polarizable probe 20 for improving the ionization of the gas and thus increasing the flow of extracted ions, made of electron emitting metal, disposed upstream of the extraction means and having no contact with the magnetic surface; and
• voltage supply means 8 for the probe.

By gas, we must also understand vapors metallic.

The main feature of the invention is to replace the first stage of the source Minimafios by a voltage polarizable probe.

In addition to the benefits described above, the invention allows a lower cost than that existing sources such as Minimafios, the manufacturing the probe being easier than that from the first floor of Minimafios.

According to a preferred embodiment of the invention, the voltage supply means consist of a variable voltage source suitable for supplying said probes a negative voltage compared to the potential of the cavity thus ensuring an increase in the current ions.

This negative voltage has an absolute value at least equal to about 100 volts for a increased charge of ions.

According to a preferred embodiment, the probe is arranged along the axis of the cavity, at a from its ends and from the side opposite to the means extraction. It is also possible to arrange it laterally.

In a preferred embodiment of the ion source, the probe is made of tantalum. Of course, any other electron-emitting metal can be considered and, in particular, tungsten and molybdenum.

In particular, the probe includes a rod of this electron emitting metal and a disc of this same metal attached to one end of this rod. It is possible to use a probe whose rod is made of a metal different from that of the disc.

Advantageously, the introduction of gas takes place along the axis of the cavity, parallel to the probe. This increases the ionization of neutral atoms. We can however introduce the gas radially. Preferably, the introduction of the high frequency is done along the axis of the cavity, on the probe side. However, it is possible to introduce the high frequency radially. This source allows in particular the obtaining of a current seventeen times positively charged argon ions.

• la figure 1, déjà décrite, représente schématiquement une source RCE de l'art antérieur ;
• la figure 2 représente schématiquement la source d'ions selon l'invention ;
• la figure 3 est une représentation schématique de la cavité de la source de la figure 2 ;
• les figures 4 à 6 sont des courbes de résultats obtenus lors d'expériences avec la source selon l'invention ; les courbes de la figure 4 donnent les variations du courant ionique I_i exprimé en microampères, en fonction de la charge de l'ion krypton. La courbe de la figure 5 montre les variations du nombre N approximatif d'impulsions par seconde de l'ion Ar¹⁷⁺ en fonction du potentiel U de la sonde exprimé en volts. La courbe de la figure 6 donne les variations du courant I_s de la sonde exprimé en milliampères en fonction du potentiel U de cette même sonde exprimé en volts.

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

• FIG. 1, already described, schematically represents an RCE source of the prior art;
• FIG. 2 schematically represents the source of ions according to the invention;
• Figure 3 is a schematic representation of the source cavity of Figure 2;
• Figures 4 to 6 are results curves obtained during experiments with the source according to the invention; the curves of Figure 4 show the variations of the ion current I _i expressed in microamperes, depending on the charge of the ion krypton. The curve in FIG. 5 shows the variations in the approximate number N of pulses per second of the Ar ¹⁷⁺ ion as a function of the potential U of the probe expressed in volts. The curve of FIG. 6 gives the variations of the current I _s of the probe expressed in milliamps as a function of the potential U of this same probe expressed in volts.

Figure 2 shows the ion source according to the invention. This source includes, like that of the prior art, a stage B equipped with the cavity 2 RCE almost identical to the second stage B of the source of figure 1. On the other hand, the first floor A from the source in Figure 1 has disappeared and has been replaced by a 20 polarizable probe, powered in voltage by a variable power source 8. Furthermore, this source includes, at the entry of the HF cavity, an element of soft iron 18 and a other element of soft iron 12 at the outlet of the cavity HF, downstream of the electrodes 10a.

The role of element 18 is identical to that of element 18a of the source of the figure 1. Element 12, meanwhile, allows the reduction of the axial magnetic field downstream of the electrodes output 10a.

In addition, the gas or steam supply 6 metallic is carried out along the X axis of the cavity HF and no longer radially in order to increase the quantity ions.

The probe 20 is supplied by the source 8 of variable voltage (0 - 200 V) including connection electric is such that the probe 20 receives a voltage negative compared to the potential of the cavity which is a few kilovolts higher (10 to 20 kV) relative to mass. The probe 20 is made up of a rod 20a of an electron emitting metal at end of which is fixed a disc 20b of the same metal. This disc has a diameter about ten times larger than that of the rod 20a in order to improve the emission electrons. This metal is in particular tantalum.

In the embodiment shown, the probe 20 is placed at the end 3 of the cavity 2, end opposite to that of the ion extraction means 10. In addition, it is placed along the X axis of the cavity 2, parallel to the high frequency input 4 and when the gas is introduced 6. The probe is fixed in this position using a shutter 22, insulating electric fitted with holes for passage, respectively, of the rod 20a, of the introduction gas 6 and high frequency input 4.

In Figure 3, there is shown schematically part of the source according to the invention and, in particular, the interior of the microwave cavity 2. The equimagnetic surface S shaped like a rugby ball, created by the field resulting magnetic, is shown inside of cavity 2, without contact with the walls of the cavity. The probe 20 located at the end 3 of the cavity 2 has no contact with this surface equimagnetic S in order to best avoid possibilities of recombination of an ion with one or several electrons.

The curves in Figures 4 to 6 have been established for a frequency of 18 GHz and a voltage 15 kV applied to the HF cavity.

FIG. 4 represents the ionization spectra of krypton, that is to say the variations of the ion current I _i of krypton, in microamperes, as a function of the state of charge Q of the krypton ion. The first spectrum a is obtained for a non-polarized probe and therefore for a zero probe voltage. The second spectrum b is obtained for a probe polarized by a negative voltage of -180 volts. We notice an increase in the current with the bias voltage; the maximum value is multiplied by a factor at least equal to two when the voltage of the probe goes from 0 to -180 volts.

Another consequence of this polarization of the probe is the increased charge of the ions krypton going fourteen times loaded positively to seventeen times positively charged, for the maximum current value.

Conversely, the inventors have found that the probe, powered by a positive voltage compared to the HF cavity, has the effect of decreasing the ion current and increase the weak states dump.

The curve in Figure 5 represents the variations in the quantity N of argon ions seventeen times positively charged, expressed as a number pulses per second, based on potential U of the probe, expressed in volts.

This curve was plotted by increasing the intensity of the K _α ray emitted by an Ar ion beam ¹⁷⁺ intercepted by a solid target. The ion beam is extracted from the source with a potential of 15 kV applied to the cavity relative to the mass and is deflected by a magnet, relative to the X axis, to be analyzed. The deflection angle is linked to the state of charge selected in M / Q (M is the mass of the ion and Q its charge). Here it is 104 ° for Q = 17. The line K _α (2.957 KeV) is observed with a hyper-pure germanium detector which looks at the target from a solid 4.10 ^-5 steradian angle through a Kapton® window.

This measurement technique does not directly give the intensity of the Ar ¹⁷⁺ ion current but makes it possible to follow its evolution without ambiguity. The intensity of the line being proportional to the current of Ar ¹⁷⁺ ions falling on the target. The energy of the X-rays being characteristic of this ion, one thus avoids any confusion with the states of charge having a close Q / M such as the nitrogen ion six times charged.

Note on this curve the strong dependence of the number of N ions with the potential of the probe. The growth factor for the number of Ar ¹⁷⁺ ions is approximately 100 for a potential of the probe passing from -5 volts to -150 volts.

The curve in FIG. 6 represents the variations in the current I _s of the probe in milliamps as a function of the potential U of this same probe in volts. Around U = 0, the tantalum probe current is negative, with an absolute value greater than 3 milliamps; this current corresponds to a capture of electrons. For voltages higher, in absolute value, to around 100 volts, the probe current is positive; it is then emission of electrons by the probe or else of an ion current of tantalum collected or else emission of electrons and of an ion current of tantalum collected.

The ion source according to the invention is simpler to manufacture than two sources known floors and makes it possible to reach at least the same performance as a two-stage source for lower cost.

Claims (7)

1. Source of highly charged, positive ions having electron cyclotron resonance comprising:

   a hyperfrequency cavity (2) having an axis of symmetry (X);

   a high frequency inlet (4) opening into the cavity so as to create there a high frequency electromagnetic field;

   a pipe (6) for admitting gas into the cavity;

   means (14) for producing a magnetic field along the axis in said cavity;

   means (16) for producing a multipolar radial magnetic field in this cavity, the superimposition of these axial and radial magnetic fields forming a resultant magnetic field distributed throughout the cavity and defining at least one equimagnetic surface (S) completely closed inside the cavity;

   means (10) for extracting ions at the extremity (5) of the cavity;

   characterized in that it comprises:

   a voltage-polarizable probe (20) so as to increase ionization of the gas and thus increase the flow of extracted ions made from an electron emitting metal, said probe being disposed upstream of the extraction means and having no contact with the equimagnetic surface; and

   means (8) for supplying the probe with voltage.
2. Source according to claim 1, characterized in that the voltage feeding means consist of a variable voltage source (8) able to provide said probe with a negative voltage with respect to the potential of the cavity ensuring an increase of the ionic current.
3. Source according to claim 2, characterized in that this negative voltage has an absolute value equal at least to about 100 volts so as to increase the charge of ions.
4. Source according to any one of the preceding claims, characterized in that the probe is disposed along the axis of the cavity and at one of its extremities.
5. Source according to any one of the preceding claims, characterized in that the probe has a rod (20a) of an electron emitting metal and a disk (20b) made of this same metal and fixed to one of the extremities of said rod.
6. Source according to any one of the preceding claims, characterized in that introduction of the gas takes place along the axis of the cavity, parallel to the probe.
7. Electron cyclotron resonance (ECR) source for obtaining an argon ion current positively charged seventeen times, characterized in that the source is in accordance with any one of the claims 1 to 6 and in that the gas introduced into the cavity contains argon.

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