EP0184475A1 - Method and apparatus for igniting a hyperfrequency ion source - Google Patents

Abstract

The method of igniting a microwave ion source using in a known manner a resonant cavity (9) supplied by a gas or a vapor of a material intended to form a plasma, an injection system (8) in the microwave power cavity and a system for extracting (14) plasma ions from the cavity, is characterized in that the cavity being of the multimode type, electronic germs are created within the medium to be ionized the plasma is maintained after its ignition using only microwave power.

Description

The present invention applies to the field of microwave ion sources, which can also be used as an electron source or as a plasma generator.

It finds many applications, in the field of "sputtering" of thin layers, microgravure, ion implantation, electron irradiators, heating by beam of fast neutral particles from the plasma of fusion reactors, "tandem" accelerators, synchrocyclotrons and also cleaning and surface treatments.

Up to now, the realizations of microwave ion sources have been made according to the principle of ion sources with cyclotron resonance of electrons.

In such known sources, the ionization of a neutral gas and the appearance of a plasma are caused by combining, in an HF cavity, the synergistic effects of a microwave electromagnetic field of frequency F and a magnetic field. B constant so as to obtain the resonance between this latter frequency and the pulsation of the electrons in their circular trajectories around the lines of force of the field B. This condition is therefore written: F = a B 2πm formula in which e and m are the charge and mass of the electron, B the constant magnetic field present in the cavity, and which gives the relation linking F and B to obtain the electronic cyclotron resonance within the plasma. The electrons then describe trajectories in spiral around the lines of force of the field B by absorbing energy of the field B and thus acquiring a maximum kinetic energy to cause the ionization by shocks of the molecules of neutral gas present in the HF cavity from the source.

Such ion sources are described in particular by R. Geller, C. Jacquot and P. Sermet in the "Proceedings of the Symposium on ions sources and formation of ion beams", Berckeley (Oct. 1974) and by F. Bourg, R. Geller, B. Jacquot, T. Lamy, M. Pontonnier and JC Rocco in "Nuclear Instruments and Methods" North-Holland Publishing Company, 196 (1982) pp. 325-329. They are based on the establishment of a plasma confinement using a magnetic mirror configuration with maximum values of the magnetic field B greater than the value which ensures the cyclotronic resonance of the electrons.

The correct functioning of such known ion sources nevertheless requires certain special precautions and imposes a significant consumption of electrical energy, in particular for the creation of constant magnetic fields for the creation of the cyclotronic resonance of the electrons and for the extraction of the ions.

Thus, for example, in a known ion source of this nature, the maximum and minimum values of the magnetic induction are 0.42 and 0.32 Tesla respectively and the cyclotron resonance of the electrons is carried out at 0.36 Tesla, the frequency of the injected high frequency wave being fixed at around 10 GHz.

The ions created in the plasma are extracted by an extraction system, made up of electrodes brought to continuous potentials and which are downstream of the maximum of the magnetic field. Under these conditions, the ion current emitted by the source decreases in proportion to the value of the field at the place of extraction and, to obtain an intense ion current, it is necessary to extract the ions in a magnetic field at least of the same order. larger than the field of cyclotron resonance.

This need is unfortunately incompatible with good optical quality of the extracted ion beam if the magnetic field is canceled before the impact of the ions in the area of use. In this case, in fact, the ions take transverse energy, the diverging beam and its optical qualities are reduced, according to the effect described in the Bush theorem.

To maintain the optical qualities of the beam downstream of the ion source, the magnetic field must therefore be kept constant throughout the sliding space of the ion beam up to the point of its application or of the transformation of the ions in neutral particles. For the example described above, the field to be kept constant corresponds to an induction of approximately 0.36 Tesla, and the electric power consumed by the coils creating this magnetic field is of the order of 1 Megawatt.

When using low energy ions (less than 1 keV) the extraction system does not allow high densities to be extracted. To increase the latter, the ion beam can be compressed downstream of the ion source.

To compress the ion beam, the magnetic field must be increased proportionally.

The increase in the current density of the ions obtained is therefore limited by the technical problems which arise, concerning the production of magnetic fields of this order of magnitude.

• - d'abord la consommation d'énergie pour établir la configuration magnétique est très élevée,
• - ensuite, l'augmentation de la densité du courant d'ions à faible énergie cinétique est problématique à cause de la nécessité d'un champ magnétique élevé pour transporter ceux-ci en aval de l'extraction jusqu'au lieu d'utilisation.

In summary, the ion sources according to the known state of the art have the following main drawbacks:

• - first the energy consumption to establish the magnetic configuration is very high,
• - Then, increasing the density of the ion current with low kinetic energy is problematic because of the need for a high magnetic field to transport them downstream of the extraction to the place of use.

To overcome these drawbacks, various solutions have been proposed, such as that described in French patent application N 8308401 of May 20, 1983 in the name of the French Atomic Energy Commission.

In this unpublished French patent application, the configuration of the magnetic confinement was modified to allow the extraction of the ions in a magnetic field clearly lower than that of the ion sources of the prior art. This has already resulted in a considerable saving of the energy necessary to create the magnetic configuration in the source on the one hand and on the other hand downstream of the extraction, during a low-energy transport of the extracted beam.

However, the ion source described in this patent is always a source implementing the phenomenon of electron cyclotron resonance to create the plasma in the cavity, and it therefore always requires in this cavity the presence of a higher magnetic field or at least equal to that which creates the cyclotron resonance of the electrons.

The present invention specifically relates to a method of igniting a microwave ion source which operates without resorting to the phenomenon of cyclotron resonance of the electrons and, consequently, without the presence of a constant magnetic field prevailing for this purpose in the HF cavity.

This method of igniting a microwave ion source using in a known manner a resonant cavity supplied by a gas or a vapor of a material intended to form a plasma, a system for injecting into the cavity a microwave power and a system for extracting the ions from the plasma from the cavity, is characterized in that the cavity being of the multimode type, electronic germs are created within the medium to be ionized and the plasma is maintained after its ignition using only microwave power.

The essential novelty provided by the present invention resides in the fact that it has been possible to produce, contrary to all the ideas received and affirmed by those skilled in the art for more than twenty years, a source of microwave ions operating at using a resonant cavity without appealing to the phenomenon of electronic cyclotron resonance, that is to say without constant magnetic field inside this cavity. In other words, the applicant has demonstrated that it is possible, once the plasma has been ignited, to keep it in activity only using the only microwave power injected into the resonant cavity.

This unexpected result obviously makes it possible to produce microwave ion sources much simpler than those of the prior art and in particular much more economical due to the elimination of the significant electrical consumption that hitherto required the presence of a magnetic field to create the conditions for electronic cyclotron resonance.

According to the invention, the ignition of the ion source by creation of electronic germs within the medium to be ionized, can take place either uniquely during initial ignition, or repeatedly, that is to say -to say from time to time when the need arises, or even permanently, which is also rarely essential.

It should be noted that a single ignition allows the source to operate in pulsed mode, for a recurrence time of the order of ₁ 00 milliseconds.

Depending on the envisaged applications, an axial and / or multipolar magnetic configuration may nevertheless be necessary and used to confine and homogenize the plasma, but in this case, the values of the magnetic fields are much lower than those which were formerly necessary for the creation of the conditions of cyclotron resonance of electrons.

According to another characteristic of the ignition process which is the subject of the invention, the creation of electronic germs within the medium to be ionized is obtained by direct seeding of electrons.

According to another variant, the creation of these same electronic seeds is obtained by temporary and local application of a magnetic field of sufficient intensity to create, in a small volume of the cavity, the conditions for establishing an electronic cyclotron resonance. which in turn causes the creation of plasma.

According to another variant of the ignition method which is the subject of the invention, the creation of the same electronic germs within the medium to be ionized is obtained by temporary application of an overpressure in the cavity.

The present invention also relates to a device for igniting a microwave source implementing the above method in a particularly simple manner and using means per se known and easy to use.

In a first embodiment of the invention, this device for igniting a microwave ion source using in known manner a multimode resonant cavity supplied by a gas or a vapor of a material intended to form a plasma , a system for injecting microwave power into the cavity and a system for extracting plasma ions from the cavity, is characterized in that it consists of an electromagnet encircling the external wall of the cavity, a few centimeters downstream of the injection system and the magnetic carcass of which is applied to this cavity.

Thanks to this electromagnet located against the wall of the cavity, the ignition process is implemented which consists in creating in a small volume of the cavity, temporarily and locally, the conditions for establishing an electronic cyclotron resonance. which in turn causes the creation of plasma.

According to other embodiments of the ignition device of a microwave ion source object of the invention, one uses for the creation of electronic germs in the medium to be ionized one of the means chosen from the group comprising heated filaments, spikes with field emission, sources of sparks, ionization gauges, the chosen device being applied in the cavity or through its wall.

Finally, according to the invention, each of the three dimensions of the resonant cavity, length, width and height, must be greater than the short side or the diameter of the waveguide of the system for injecting the high frequency power of the cavity. This condition has proven to be effectively necessary in order to be able to obtain the ignition and self-maintenance of a plasma in a multimode resonant cavity having this particular form but in fact very frequently used.

Of course, the microwave ion sources implementing the ignition process, which is the subject of the invention, can be of any known nature and in particular include, like the other sources, the variants or improvements in detail recalled below. below ..

Thus, for example, such a source may have a magnetic configuration located downstream of the system for extracting ions from the plasma or electrons in order to carry out, under good conditions, the transport of the extracted beam and even to obtain its radial compression. .

In other variants, the ion or electron extraction system can be constituted by a single electrode brought to a determined potential.

Finally, according to other characteristics, which are used in a preferential but not compulsory manner, the device for igniting the microwave ion source is located at a distance of the order of a few centimeters downstream of the junction zone. between the microwave injector and the ion source cavity. This location has actually proven advantageous for obtaining a good ignition under conditions of maximum efficiency.

In the same way, it is advantageous to place the injection of the gas or of the vapor of the material which it is desired to ionize upstream from the ion or electron extraction system and in its vicinity, that is to say say at a relatively small distance from the latter.

• - la figure 1 représente en coupe selon l'axe une source d'ions à cavité résonnante hyperfréquence équipée du dispositif d'allumage selon l'invention,
• - la figure 2 représente une source identique à celle de la figure 1 sur laquelle on a placé en aval du faisceau une bobine supplémentaire de compression du plasma,
• - la figure 3 montre un exemple de source d'ions munie d'un dispositif d'allumage sous forme d'une pointe à émission de champ. Sur la figure 3a, on a représenté une coupe selon l'axe Z de la source d'ions et sur la figure 3b une coupe transversale selon aa du même dispositif,
• - la figure 4 montre un exemple de source d'ions munie de filament chauffé.

In any case, the present invention will be better understood by referring to the description which follows, given by way of explanation and in no way limiting, of an exemplary implementation of an ion source using the method according to the invention. This description will be made with reference to the appended drawings in which:

• FIG. 1 represents in section along the axis an ion source with a microwave resonant cavity equipped with the ignition device according to the invention,
• - Figure 2 shows a source identical to that of FIG. 1 on which an additional plasma compression coil has been placed downstream of the beam,
• - Figure 3 shows an example of ion source provided with an ignition device in the form of a field emission tip. In FIG. 3a, a section along the axis Z of the ion source has been shown and in FIG. 3b a cross section along aa of the same device,
• - Figure 4 shows an example of ion source provided with heated filament.

According to the invention, FIG. 1 schematically represents a simplified example of an embodiment of a source of ions, electrons or plasma, at microwave, in cross section comprising the central axis Z of the source .

In a vacuum cavity 9, of cylindrical shape of revolution for example, one end carries an injector 8 of microwave power through a window 13 and the other end is connected to the place of use ions, electrons or plasma. According to the invention, and as shown in FIGS. 1 and 2, the waveguide 15, which is of revolution, has a diameter smaller than that of the cavity 9.

It should be noted that the cavity 9 can have any shape depending on the nature of the use. In particular, the microwave power injection system 8 can be constituted by several microwave injectors in parallel.

According to the invention, the relative dimensions of a source such as that of FIGS. 1 and 2, with respect to the HF injector system are not arbitrary when the cavity 9 is parallelepiped. In this case, the dimensions of the three sides of the cavity 9 must be greater than the diameter or the short side of the waveguide which injects the HF power at 13, if one wants to be able to ignite and above all keep the plasma 10 in activity without resorting to cyclotron resonance of the electrons in this plasma.

A gas or a vapor intended to form a plasma is introduced at 17 at a low pressure of some 10 ' ³ to 10 _' ² Torr upstream of the system 14 for extracting the ions and in its vicinity.

In multimode operation of the cavity, there is indeed a pressure gradient in the source, this pressure increasing from window ₁ 3 to extraction, hence the choice to introduce the gas to be ionized in the vicinity of extraction.

In another embodiment, the plasma can be created at another location and then injected into the cavity 9.

The ignition system 7 is constituted in this example by a circular electromagnet surrounding the wall 9 and comprising an annular coil 11 and a soft iron carcass 12 applied against the wall 9. This electromagnet is capable of igniting the discharge by a pulse field by locally and temporarily creating in the cavity a magnetic field fulfilling the cyclotronic resonance conditions of the electrons, and the plasma ignites at 10. The system for extracting ions or electrons is represented here in the form of a single electrode 14.

If the microwave power per unit volume is increased, the ion current increases. We can then. extract larger ion currents or reduce the dimensions of the cavities, which allows the use of "mini-cavities" with low consumption of microwave power.

In any case, the absence in an ion source of this type of the high magnetic field of creation and maintenance of an electronic cyclotron resonance phenomenon leads to a very significant energy saving, major advantage of the present invention .

Finally, the beam extracted from the source can be compressed, downstream of the extraction electrodes by the application of an additional magnetic field, this is the case for example of the coil 15 in the example of FIG. 2.

• - oscillateur 2,45 GHz,
• - puissance HF fixe ₁ kW,
• - volume de la source 1 dm3,
• - gaz N, - P 5.10-4 mbar
• - densité électronique ne 2.1011/cm3,
• - température électronique T_e = 6eV ;

la densité de coupure est ne = 7.10¹⁰, valeur que l'on ne peut atteindre dans une source RCE monomode à cette fréquence et pour ce gaz.By way of example of implementation, an ion source according to the invention has operated, without a magnetic field of cyclotron resonance under the following conditions:

• - 2.45 GHz oscillator,
• - fixed HF power ₁ kW,
• - source volume 1 dm3,
• - N gas, - P 5.10-4 mbar
• - electronic density only 2.1011 / cm3,
• - electronic temperature T _e = 6eV;

the cutoff density is ne = 7.10 ¹⁰ , a value which cannot be reached in a single-mode RCE source at this frequency and for this gas.

In FIG. 3, an ion source according to the invention is seen, provided with a spark gap 18 of any shape, preferably pointed, isolated with respect to the ion source 9 by an isolator 19 and polarized by means of a food 20 compared to this same ion source 9.

This supply 20 can be alternative (simple transformer) or continuous in this case the tip 18 is brought to a negative potential with respect to the source 9.

• - P - 5.10-⁴ mbar,
• - d - 5 cm,
• - V alternatif - 3000 volts,
• - durée des impulsions ≤1 s.

The potential difference (alternative or continuous) is of the order of a few kilovolts but it depends on the pressure and the distance d between the tip and the wall farthest from the source. When the ion source 9 is rectangular, it is preferable to put the point on the short side of this source 9 (FIG. 3b). This igniter works on the principle of glow discharge. In an implementation variant, two tips can be used and the potential difference between the two tips can be applied. A source conforming to FIG. 3 works well for example with the following parameters:

• - P - 5.10- ⁴ mbar,
• - d - 5 cm,
• - alternating V - 3000 volts,
• - pulse duration ≤1 s.

In FIG. 4, the source 9 comprises, as the creator of the seed for plating the plasma, a filament 21 of refractory metal (W, Mo, Ta) emitting electrons. This filament 21 passes through the wall 9 of the source in an insulator 22; the filament heating supply 23 is continuous or alternative.

Common values of the operating parameters are: P - 5.10 - ⁴ mbar with a filament 21 having a few mm ² of surface brought to approximately 2000 ° C for approximately 1 second.

In conclusion, it can be said, on the one hand, that the reduction, or even the total elimination of the confining magnetic field at the level of the source leads to a very significant energy saving in the case of the use of coils or a significant gain. on investments in the case of the use of magnets; it follows, on the other hand, that, thanks to these gains in microwave and magnetic energies, the sources according to the invention find significant applications on high voltage platforms (Van de Graaf, synchrocyclotrons, etc.) .), which was not the case with traditional cyclotronic resonance sources of electrons.

Claims (9)

1. Method for igniting a microwave ion source using in a known manner a resonant cavity (9) supplied by a gas or a vapor of a material intended to form a plasma, an injection system (8) in the microwave power cavity and a system for extracting ( ₁ 4) plasma ions from the cavity, characterized in that the cavity being of the multimode type, electronic germs are created within the medium to be ionized and the plasma is maintained after its ignition using only microwave power. 2. An ignition method according to claim 1, characterized in that the creation of electronic germs within the medium to be ionized takes place uniquely during initial ignition. 3. An ignition method according to claim 1, characterized in that the creation of electronic germs within the medium to be ionized takes place repeatedly. 4. An ignition method according to any one of claims 1 to 3, characterized in that the creation of electronic germs within the medium to be ionized is obtained by direct seeding of electrons. 5. An ignition method according to any one of claims 1 to 3, characterized in that the creation of electronic germs within the medium to be ionized is obtained by temporary and local application of a magnetic field of sufficient intensity to create , in a small volume of the cavity, the conditions for establishing an electronic cyclotron resonance which in turn causes the creation of the plasma. 6. An ignition method according to any one of claims 1 to 3, characterized in that the creation of electronic germs within the medium to be ionized is obtained by temporary application of an overpressure in the cavity. 7. Device for igniting a microwave ion source using in a known manner a multimode resonant cavity (9) supplied by a gas or a vapor of a material intended to form a plasma, an injection system (8) in the microwave power cavity and a system (14) for extracting plasma ions from the cavity, characterized in that it consists of an electromagnet (11, 12) encircling the external wall of the cavity , a few centimeters downstream of the injection system (8) and whose magnetic carcass (12) is applied to the wall (9) of this cavity. 8. Device for igniting a microwave ion source using in a known manner a multimode resonant cavity (9) supplied by a gas or a vapor of a material intended to form a plasma, an injection system (8) in the microwave power cavity and a system for extracting (14) plasma ions from the cavity, characterized in that it is chosen from the group comprising heated filaments, field emission tips, sources of sparks, ionization gauges, the chosen device being applied in the cavity or through its wall. 9. Device for igniting a microwave ion source according to any one of claims 7 and 8, characterized in that the three dimensions of the multimode resonant cavity, length, width and height, are each greater than the short side or the diameter of the waveguide of the high frequency power injection system in the cavity.

Images