EP0483004A1 - Electron cyclotron resonance ion source for highly charged ions with polarisable probe - Google Patents

Abstract

The invention relates to an ion source with polarisable probe (20) making it possible to produce, from neutral atoms, a quantity of highly charged ions. This source comprises a microwave cavity (2), means (14) of producing an axial magnetic field in the cavity, means (16) of producing a multipole radial magnetic field in this cavity, a high-frequency input (4), a gas inlet (6) into the cavity, means (10) of extracting ions, and a voltage-polarisable probe (20) for improving the ionisation of the gas. <IMAGE>

Description

The present invention relates to a source of highly charged positive ions with polarizable probe and electronic cyclotron resonance (ECR). It finds many applications, depending on the different values of the kinetic energy of the extracted ions, in the fields 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 gas, constituted for example of metallic vapors, by means of a plasma of electrons strongly accelerated by electronic cyclotron resonance. This resonance is obtained by the combined action of a high frequency electromagnetic field (HF) injected into the enclosure, containing the gas to be ionized, and a magnetic field, prevailing in this same enclosure, whose amplitude B satisfies the following condition of electronic cyclotron resonance: 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 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, single or multiple, during a collision of the latter 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 impact of an 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 sheet called "equimagnetic", having no contact with the walls of the enclosure and on which the condition of electronic cyclotron resonance is satisfied. This tablecloth has the shape of a rugby ball. The closer this equimagnetic sheet is to the walls of the enclosure, the greater its efficiency because it makes it possible to limit the volume of presence of neutral atoms and therefore the amount of ion-neutral atom collision. 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 the time to bombard the same ion several times and fully 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 an ion source of the prior art.

This source contains two stages.

The role of the first stage A is largely to provide a flow of electrons in the X axis of the source. This first stage A comprises a cavity 2a with symmetry of revolution of the solenoid coils 14a disposed at the two ends of the cavity 2a, creating an axial magnetic field, this field being augmented by a soft iron shield 18a located at the input of 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 capable of 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 passes into the second stage B. This stage B consists of a multimode cylindrical cavity 2 of high order, that is to say of large dimension with respect to the dimension of the wavelength of the electromagnetic field. Its axis of symmetry carries the reference X. This electromagnetic field is introduced radially by a second high frequency input 4.

The cavity 2 is joined at its end 5 to a vacuum pump 10b, by means of an extraction pipe in which are housed electrodes 10a. A power source 9 makes it possible to apply a potential difference to these electrodes.

This pump, pipe and electrode assembly constitutes the means for extracting the ions. The ions thus extracted from the cavity 2 can then be selected according to their degree of 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 radial magnetic field, generally of the hexapolar type. These axial and radial magnetic fields are superimposed on each other and distributed throughout the cavity; they thus form a resulting magnetic field which defines at least one equimagnetic surface inside the cavity 2.

The first problem in this type of source is the importance of the bulk; to this problem is added the difficulty of manufacture and therefore the cost of such a source.

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

The present invention relates to an ion source with cyclotron resonance which makes it possible to remedy these drawbacks by simplifying in particular 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 ;
• une sonde polarisable en tension 20 pour améliorer l'ionisation du gaz et augmenter ainsi le flux d'ions extrait, 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 ions essentially comprising the following elements:

• a microwave 2 cavity having 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;
• a voltage polarizable probe 20 for improving the ionization of the gas and thus increasing the flow of extracted ions, 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 metallic vapors.

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

In addition to the advantages described above, the invention allows a lower cost than that of existing sources such as Minimafios, the manufacture of the probe being easier than that of the first stage of Minimafios.

According to a preferred embodiment of the invention, the voltage supply means consist of a variable voltage source capable of supplying said probe with a negative voltage with respect to the potential of the cavity thus ensuring an increase in the ion current.

This negative voltage has an absolute value at least equal to about 100 volts for an increase in the charge of the ions.

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

According to the invention, the probe consists of an electron emitting metal.

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

In particular, the probe comprises a rod of this electron-emitting metal and a disc of this same metal fixed to one of the ends 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 gas is introduced along the axis of the cavity, parallel to the probe. This increases the ionization of neutral atoms. However, the gas can be introduced radially. Preferably, the introduction of the high frequency takes place along the axis of the cavity, on the probe side. It is however possible to introduce the high frequency radially. This source makes it possible in particular to obtain a current of argon ions seventeen times positively charged.

• 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 FIG. 4 give the variations of the ion current I _i expressed in microamperes, as a function of the charge of the krypton ion. The curve in FIG. 5 shows the variations in the approximate number N of pulses per second of the ion Ar¹⁷⁺ 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 comprises, 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 FIG. 1. On the other hand, the first stage A from the source in FIG. 1 has disappeared and has been replaced by a polarizable probe 20, supplied with voltage by a variable power source 8. Furthermore, this source comprises, at the input of the HF cavity, an element of soft iron 18 and another element of soft iron 12 at the outlet of the HF cavity, downstream of the electrodes 10a.

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

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

The probe 20 is supplied by the source 8 of variable voltage (0 - 200 V) whose electrical connection is such that the probe 20 receives a negative voltage with respect to the potential of the cavity which is higher by a few kilovolts (10 to 20 kV ) relative to mass. The probe 20 consists of a rod 20a of an electron emitting metal at the end of which is fixed a disk 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 of the electrons. This metal is in particular tantalum.

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

FIG. 3 schematically shows part of the source according to the invention and, in particular, the interior of the microwave cavity 2. The equimagnetic surface S in the shape of a rugby ball, created by the resulting magnetic field, is shown inside the 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 equimagnetic surface S in order to best avoid the possibilities of recombination of an ion with one or more electrons.

The curves in FIGS. 4 to 6 have been established for a frequency of 18 GHz and a voltage of 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 increase in the charge of the ions krypton which goes from fourteen times positively charged to seventeen times positively charged, for the maximum value of the current.

Conversely, the inventors have found that the probe, powered by a positive voltage with respect to the HF cavity, has the effect of reducing the ion current and increasing the low states of charge.

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

This curve was plotted by recording the intensity of the line K _α emitted by a beam of Ar¹⁷⁺ ions 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). It is here 104 ° for Q = 17. The line K _α (2.957 KeV) is observed with a hyper-pure germanium detector which looks at the target under a solid 4.10⁻⁵ steradian angle through a Kapton® window.

This measurement technique does not directly give the intensity of the ion current Ar¹⁷⁺ but allows its evolution to be followed without ambiguity. The intensity of the line being proportional to the current of Ar¹⁷⁺ 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 on the potential of the probe. The growth factor of the number of Ar¹⁷⁺ 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 above, in absolute value, 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 known two-stage sources and makes it possible to achieve at least the same performance as a two-stage source at a lower cost.

Claims (8)

Source of highly charged positive ions with polarizable probe and electronic cyclotron resonance essentially comprising the following elements: - a microwave cavity (2) comprising an axis of symmetry (X); - a high frequency input (4) opening into the cavity to create a high frequency electromagnetic field there; - introduction of gas (6) into the cavity; - Means for producing (14) 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 completely closed equimagnetic surface (S) inside the cavity; - means for extracting (10) the ions at the end (5) of the cavity; - A voltage polarizable probe (20) to improve the ionization of the gas and thus increase the flow of extracted ions, arranged upstream of the extraction means and having no contact with the equimagnetic surface; and - voltage supply means (8) of the probe. Source according to Claim 1, characterized in that the voltage supply means consist of a variable voltage source (8) capable of supplying said probe with a negative voltage with respect to the potential of the cavity ensuring an increase in the current. ions. Source according to claim 2, characterized in that this negative voltage has an absolute value at least equal to about 100 volts to increase the charge of the ions. Source according to any one of the preceding claims, characterized in that the probe is arranged along the axis (X) of the cavity and at one of its ends. Source according to any one of the preceding claims, characterized in that the probe consists of an electron-emitting metal. Source according to any one of the preceding claims, characterized in that the probe (20) comprises a rod (20a) of an electron-emitting metal and a disc (20b) of this same metal fixed to one end of said rod. Source according to any one of the preceding claims, characterized in that the introduction (6) of gas takes place along the axis of the cavity, parallel to the probe. Application of the source according to any one of the preceding claims, to obtaining a current of seventeen times positively charged argon ions.

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