EP0514255B1 - Electron cyclotron resonance ion source - Google Patents

Description

The present invention relates to an improvement of an ion source with electronic cyclotron resonance (ECR) allowing, in particular, the production of multicharged ions.

It finds numerous applications as a function of the different values of the kinetic energy of the ions produced, in the field of ion implantation, of microgravure, and more particularly in the equipment of the particle accelerators used both in the scientific field. than medical.

dans laquelle e représente la charge de l'électron, m sa masse et f la fréquence du champ électromagnétique.In ion sources with electronic cyclotron resonance, the ions are obtained by ionization, in a closed enclosure, such as a microwave cavity, of a gaseous medium consisting of one or more gases or metallic vapors, by means of electrons strongly accelerated by electronic cyclotron resonance. This resonance is obtained thanks to the combined action of a high frequency electromagnetic field (HF) injected into the enclosure, containing the gas to be ionized, and of a magnetic field, prevailing in this same enclosure, whose amplitude B satisfies the following electronic cyclotronic resonance condition: $$\text{B = f.2 π m / e,}$$

in which e represents the charge of the electron, m its mass and f the frequency of the electromagnetic field.

In these sources, 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 so-called "equimagnetic" surface, having no contact with the walls of the enclosure and on which the condition of electronic cyclotron resonance is satisfied. This surface has the shape of a rugby ball. The closer this equimagnetic surface 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 collisions between ions and neutral atoms. This surface 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.

Such an ion source was described in the document filed on March 13, 1986, in the name of the applicant and published under the number FR-A-2 595 868.

In Figure 1, there is shown schematically an ion source according to the prior art. This source includes an enclosure 1 constituting a resonant cavity which can be excited by a high frequency electromagnetic field (HF). This electromagnetic field is produced by a generator 3 of electromagnetic waves; it is introduced inside the enclosure 1 via a waveguide 5 and a transition cavity 20.

This source also includes an externally shielded magnetic structure (7, 9, 11), the shielding 11 of which makes it possible to magnetize only the volume useful for electronic cyclotron resonance in the enclosure 1.

This magnetic structure comprises, in addition to the shielding 11, permanent magnets 7 and solenoids 9, arranged around the enclosure 1 and respectively creating a radial magnetic field and an axial magnetic field. These two magnetic fields are superimposed and distributed throughout the enclosure; they thus form a resulting magnetic field which defines a resonant equimagnetic surface 13 inside the enclosure 1.

A magnetic axis 15, which is also the longitudinal axis of the source, crosses the shield 11 through two openings 17 and 19, arranged in said shield 11 to allow respectively the extraction of the ions from the enclosure 1, as well as the introduction of electromagnetic waves and gas or solid samples.

First and second dielectric lines 23 connect the opening 19 of the shield 11 to respective openings 25 and 27 of the transition cavity 20, these openings being located on the lateral faces of the cavity 20 which has the shape of a cube .

est transverse à la direction de propagation des ondes et perpendiculaire à la surface des conducteurs, c'est-à-dire des canalisations 21, 23.The ratio of the diameters of these two pipes 21, 23 is such that it is possible to assimilate the latter to a coaxial line of characteristic impedance of the order of 85 Ω. Such a coaxial line preferably propagates an electromagnetic Transverse Electro-Magnetic (TEM) mode in which the electromagnetic field $\overrightarrow{\text{E}}$

is transverse to the direction of propagation of the waves and perpendicular to the surface of the conductors, that is to say pipes 21, 23.

To ionize a gas, said gas is introduced into the enclosure 1 by means of a gas pipeline 30 connected to the opening 27 of the transition cavity 20. The gas and the electromagnetic waves introduced into the cavity 20 are transmitted to enclosure 1 by the first and second pipes 21 and 23, the role of which is to enable said waves to be transmitted to said enclosure and to inject them there along the longitudinal axis 15.

In enclosure 1, the combination of the axial magnetic field and the electromagnetic field makes it possible to strongly ionize the gas introduced. The electrons produced are then strongly accelerated by electronic cyclotron resonance, which leads to the formation of a plasma of hot electrons confined in the volume limited by the equimagnetic surface 13.

The ions then formed in enclosure 1 are extracted therefrom by an electric extraction field generated by a potential difference applied between an electrode 31 and the enclosure 1. The electrode 31 and the enclosure are both connected to a source 33 of electrical power, the electrode 31 being positioned outside the opening 17 of the enclosure 1.

To control the intensity of the ion current, it is possible to control the average power of the electromagnetic field by acting on a pulse generator 35, itself located upstream of a power source 37 connected to the generator d 'electromagnetic waves. Said pulse generator 35 controls said power source 37 by adjusting the useful cycle, namely the ratio between the duration of a pulse and the period of the pulses.

In addition, means 39 for measuring total pressure are connected to an input of a comparator 41, the output of which is itself connected to a valve 43 of the gas pipe 30. On a second input of the comparator 41, a reference voltage R is applied and compared with the measured value of the ion current to give, at the output of the comparator, the value to be transmitted to the valve 43. This valve 43 makes it possible to act on the quantity of gas to be introduced into enclosure 1, so as to automatically regulate the ion current.

In addition, an adaptation piston 45, connected to a third lateral opening 29 of the cavity 20, makes it possible to adjust the internal volume of said cavity 20. The adjustment of said piston 45 is used to tune all of the internal volumes of the cavity 20 on the frequency of the electromagnetic waves in order to obtain a minimum of reflected waves, that is to say waves which return to the wave generator 3. When these internal volumes are tuned to the frequency of the electromagnetic waves , the waves injected into the cavity 20 by the generator 3 are almost completely transmitted, via the pipes 21 and 23, to the enclosure 1 containing the plasma, then absorbed by the equimagnetic surface 13.

In this ion source of the prior art, the second pipe 23 is transparent to electromagnetic waves at its end 23a, end close to the opening 19 of the enclosure 1, located opposite the shielding 11.

In the interior volume of this transparent part 23a, there is an axial magnetic field from the solenoids, an electromagnetic field and a high gas pressure. The electromagnetic field comes from electromagnetic waves transmitted between the first pipe 21 and a non-transparent part 23b of the second pipe 23, and which pass through the transparent part 23a of the second pipe 23. Therefore, an electronic cyclotron resonance can take place at inside the end 23a of the second pipe 23 in a volume where there is a high gas pressure. The denser the plasma produced by electronic cyclotron resonance inside the end 23a, the better the transmission of electromagnetic waves, this dense plasma cord itself becoming conductive. In addition, this plasma cord has the same outside diameter as the part 23b of the second pipe. The characteristic impedance of the coaxial line is therefore not modified, which avoids the reflection of electromagnetic waves.

This end transparent to electromagnetic waves therefore constitutes a self-regulated pre-ionization stage, where the excess incident power of the electromagnetic waves is transmitted without reflection to the zone of electronic cyclotron resonance constituted by the equimagnetic surface 13.

Thus, to optimize an ion source, as described in the prior art, it is necessary on the one hand, to adjust the volume of the transition cavity 20 by acting on the adaptation piston 45 and, on the other hand , adjust the intensity of the current in the solenoids 9. These adjustments, even made by an experienced experimenter, can be very long: they can last for hours or even days without necessarily leading to the optimum performance of the source . Indeed, these settings do not obey any known rule used to optimize the ion source.

The article by C. C. TSAI et al. entitled "Potential applications of an electron cyclotron resonance multicusp plasma source" and extract from Journal of Vacuum Science and Technology, part A, vol. 8, no. 3, June 1990, New York, US, describes a source of plasma production in which the pressure can be modified, by variation of the excitation current in the solenoid, to allow the ionization of different gases.

The object of the present invention is precisely a source of RCE ions comprising a device making it possible to rationally optimize said source.

• une enceinte contenant un plasma d'ions et d'électrons formés par résonance cyclotronique électronique ;
• une structure magnétique comportant un blindage extérieur, ladite structure entourant l'enceinte et créant à l'intérieur de celle-ci deux champs magnétiques radial et axial assurant un confinement du plasma dans l'enceinte ;
• une cavité de transition reliée à un générateur d'ondes électromagnétiques ; et
• une première et une seconde canalisations diélectriques reliant l'enceinte et la cavité, la seconde canalisation comportant une partie transparente en regard du blindage dans laquelle se produit une résonance en un point déterminé C,

ce dispositif se caractérise par le fait qu'il comporte une pièce tubulaire placée autour de la seconde canalisation au niveau de la partie transparente et pouvant être translatée parallèlement aux canalisations, afin de régler de façon optimale la position dudit point de résonance dans la seconde canalisation diélectrique.More specifically, the invention relates to a device for optimizing an ion source with electronic cyclotron resonance comprising:

• an enclosure containing a plasma of ions and electrons formed by electronic cyclotron resonance;
• a magnetic structure comprising an external shielding, said structure surrounding the enclosure and creating inside it two radial and axial magnetic fields ensuring confinement of the plasma in the enclosure;
• a transition cavity connected to an electromagnetic wave generator; and
• first and second dielectric pipes connecting the enclosure and the cavity, the second pipe comprising a transparent part facing the shielding in which resonance occurs at a determined point C,

this device is characterized by the fact that it comprises a tubular piece placed around the second pipe at the level of the transparent part and which can be translated parallel to the pipes, in order to optimally adjust the position of said resonance point in the second pipe dielectric.

According to a preferred embodiment of the invention, the tubular part comprises, on its external peripheral part, a thread so as to form, with the shielding, a screw / nut system.

• la figure 1, déjà décrite, représente schématiquement une source d'ions RCE selon l'art antérieur ;
• la figure 2 représente un schéma électrique créé à l'intérieur de la source d'ions lorsque ladite source est optimisée ;
• la figure 3 représente schématiquement une source d'ions RCE optimisée selon l'invention.
• la figure 4 représente schématiquement le dispositif inventé pour optimiser la source RCE de la figure 1.

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 drawings in which:

• FIG. 1, already described, schematically represents a source of RCE ions according to the prior art;
• FIG. 2 represents an electrical diagram created inside the ion source when said source is optimized;
• FIG. 3 schematically represents a source of RCE ions optimized according to the invention.
• FIG. 4 schematically represents the device invented for optimizing the RCE source of FIG. 1.

de l'onde électromagnétique, les ondes stationnaires sont des ondes de tension. Il existe alors une succession de noeuds et de ventres de tension entre les deux canalisations 21 et 23, la distance entre deux noeuds ou deux ventres étant égale à la demi-longueur d'onde λ des ondes électromagnétiques injectées dans la source d'ions.On a coaxial line, as described above, on which the electromagnetic mode TEM propagates, there are generally standing waves due to the reflection of the wave. propagated. For the electric field $\overrightarrow{\text{E}}$

of the electromagnetic wave, standing waves are voltage waves. There is then a succession of knots and bellies of tension between the two pipes 21 and 23, the distance between two nodes or two bellies being equal to the half-wavelength λ of the electromagnetic waves injected into the ion source.

c'est-à-dire que la pulsation des ondes électromagnétiques HF a la même valeur que la pulsation giromagnétique des électrons, à savoir la pulsation "cyclotron électronique" qui a pour expression : $${\text{(E2)\hspace{1em}\hspace{1em}\hspace{1em} ω}}_{\text{ce}}\text{=}\frac{\text{e}}{\text{m}}\text{Br,}$$

dans laquelle e est la charge de l'électron, m sa masse et Br la valeur de l'induction résonante.In such an ion source, there are three remarkable points A, B and C on the magnetic axis 15. At these three points A, B, C, the condition of electronic cyclotron resonance (ECR) is verified, namely: $${\text{(E1) ω}}_{\text{HF}}{\text{= ω}}_{\text{this}}\text{,}$$

that is to say that the pulsation of the electromagnetic waves HF has the same value as the gyromagnetic pulsation of the electrons, namely the pulsation "electronic cyclotron" which has for expression: $${\text{(E2) ω}}_{\text{this}}\text{=}\frac{\text{e}}{\text{m}}\text{Br,}$$

in which e is the charge of the electron, m its mass and Br the value of the resonant induction.

Thus, at these remarkable points A, B, C, the following expression, deduced from (E1) and (E2) is verified: $$\text{|}\overrightarrow{\text{Br}}\text{| =}\frac{\text{m}}{\text{e}}{\text{ω}}_{\text{HF}}\text{.}$$

For an ion source, such as that described above, the points A and B represent the ends of the equimagnetic surface 13, also called closed resonant surface, located in the confinement plasma. The point C is located in the second dielectric line 23, in the preionization plasma, that is to say at the level of the shielding 11 magnetic, said shielding 11 causing the sudden drop in magnetic induction. The part of the pipes 21, 23 located at the level of the shield 11 is an area of strong magnetic gradient, that is to say an area where the magnetic induction varies greatly.

sont alors optimum aux points de résonance A, B et C.In Figure 2, there is shown the electrical diagram which is created inside the RCE ion source when said source is optimized. Electric fields $\overrightarrow{\text{E}}$

are then optimum at the resonance points A, B and C.

atteint sa valeur maximale, qu'il est perpendiculaire au champ d'induction résonante et qu'il est sur un cylindre de faible rayon, c'est-à-dire sur la seconde canalisation 23 de faible rayon.Indeed, the RCE resonance is optimized at point C, when the electric field $\overrightarrow{\text{E}}$

reaches its maximum value, that it is perpendicular to the resonant induction field and that it is on a cylinder of small radius, that is to say on the second pipe 23 of small radius.

In addition, when this optimized RCE resonance exists, the preionization plasma created in the dielectric lines 21, 23 is so dense that it becomes practically conductive, flourishing up to the equimagnetic surface 13, thus reaching point B. This equimagnetic surface 13 also contains a dense plasma which is capable of absorbing and reflecting electromagnetic waves, thus making said surface 13 semi-conductive, from point B to point A.

Thus, from an electromagnetic point of view, the RCE ion source behaves like a coaxial line up to point A of the magnetic axis 15. This open line is then the seat of standing waves between point A and piston 45.

From a more practical point of view, the diameters d and D of the respective conductors 23 and 21 are fixed optimally by respecting the following law: $$\frac{\text{D - d}}{\text{2}}\text{=}\frac{\text{<figure-callout id="3" label="λ" filenames="imgf0001.png,imgf0003.png" state="{{state}}">λ</figure-callout>}}{\text{3}}\text{.}$$

Likewise, the diameters D ′ and d ′ of the enclosure 1 and of the equimagnetic surface 13 respectively are chosen in an optimal manner when this same law mentioned above is verified, namely: $$\frac{\text{D '- d'}}{\text{2}}\text{=}\frac{\text{<figure-callout id="3" label="λ" filenames="imgf0001.png,imgf0003.png" state="{{state}}">λ</figure-callout>}}{\text{3}}\text{.}$$

soient également optimum en A et B, une condition importante concernant la distance entre ces points A, B, C doit être vérifiée. Les distances entre deux points A et B, B et C, ou C et A sont égales à un nombre entier (n ou m) de fois la demi-longueur d'onde λ des ondes électromagnétiques introduites dans la source.To obtain such a source of ions, i.e. so that the electric fields $\overrightarrow{\text{E}}$

are also optimum in A and B, an important condition concerning the distance between these points A, B, C must be verified. The distances between two points A and B, B and C, or C and A are equal to an integer (n or m) of times the half wavelength λ of the electromagnetic waves introduced into the source.

et $$\text{AC = m}\frac{\text{λ}}{\text{2}}\text{,}$$

expressions pour lesquelles la longueur d'onde λ est une valeur connue dès l'instant où l'on connaît la fréquence f des ondes électromagnétiques injectées par le générateur 3, la longueur d'onde équivalant au rapport célérité c de la lumière sur fréquence f des ondes introduites.So : $$\text{AB = n}\frac{\text{λ}}{\text{2}}\text{,}$$

and $$\text{AC = m}\frac{\text{λ}}{\text{2}}\text{,}$$

expressions for which the wavelength λ is a known value from the moment one knows the frequency f of the electromagnetic waves injected by the generator 3, the wavelength equivalent to the speed ratio c of light on frequency f waves introduced.

Figure 3 is a schematic representation of the ion source comprising the device according to the invention for optimizing the position of points A, B and C.

The RCE source represented in FIG. 3 is the same as the RCE source of the prior art, to which the optimization device of the invention has been added, said RCE source having been described at the beginning of the description. All the elements, cited during the description of Figure 1, retain the same references in Figure 3, which will be described.

The device for optimizing an RCE source is shown in FIG. 4. It consists of a tubular part 47, also called magnetic screw, this screw 47 being placed around the first pipe 21, with a comfortable clearance of approximately 0.5 mm , in order to avoid any friction with said pipe 21, when the latter is moved in translation relative to the shield 11. This tubular part 47, of the same thickness as the shield 11, comprises, on its periphery, a thread 47a capable of be screwed onto the threaded part (11a) of the shield 11. The screwing / unscrewing of the magnetic screw 47 on the shield 11 moves said magnetic screw 47.

This magnetic screw 47 is made of iron. Therefore, there is a strong magnetic gradient at the level of the shielding which makes it possible to act on the position of the point C of resonance. In fact, point C almost follows the movement of said tubular part 47 relative to the shield 11.

The displacement of the tubular part 47 is carried out using a special tool provided with two pins which engage in two of the four holes 47b included in the tubular part 47. These four holes 47b are regularly distributed over the external surface of the magnetic screw 47 and are each on an axis parallel to the magnetic axis 15. The special tool provided with its two pins comes s '' Engage in two diametrically opposite holes, thus making it possible to turn the screw 47.

The translation of the magnetic screw 47 is carried out in the absence of a magnetic field, that is to say when the RCE source is stopped. In the presence of the magnetic field created by the solenoids 9, an interaction is established between the magnetic screw 47 and the shielding 11. In fact, a large magnetic force then opposes the translation of the screw 47, the thread 47a of the screw 47 then pressing strongly on the thread 11a of the shield 11 thus ensuring magnetic continuity in the shield of the RCE source.

However, to achieve full optimization of the ion source, this adjustment of the positioning of point C by action on the magnetic screw 47 must be supplemented by two adjustments depending on said adjustment of the screw 47. These adjustments allow optimization of the source by successive approaches.

au point C est obtenu, à forte pression de gaz, de façon à optimiser la source sur les faibles états de charge ionique. Cet optimum s'apprécie en réglant d'une part la vis 47 et d'autre part la position du piston 45. On a alors une première position du point C. Selon la connaissance préalable du profil magnétique axial de la source RCE, les points A et B sont positionnés par réglage de l'intensité du courant dans les deux solénoïdes 9, cette intensité étant contrôlée par des alimentations extérieures qui fournissent, par exemple, un courant variant de 0 à 1 000 ampères.The optimum electric field $\overrightarrow{\text{E}}$

at point C is obtained, at high gas pressure, so as to optimize the source on low states of ionic charge. This optimum is assessed by adjusting on the one hand the screw 47 and on the other hand the position of the piston 45. There is then a first position of the point C. According to prior knowledge of the axial magnetic profile of the RCE source, the points A and B are positioned by adjusting the intensity of the current in the two solenoids 9, this intensity being controlled by external power supplies that provide, for example, a current varying from 0 to 1000 amps.

All of these three adjustments are repeated several times, for increasingly low gas pressures, until optimization of the source is obtained on the strong states of ionic charge.

According to an exemplary embodiment of an RCE source in accordance with the invention, the diameter of the enclosure 1 is approximately 6 centimeters and the wavelength λ of the waves introduced is three centimeters, ie a frequency f of 10 GHz. For such a source, all of the settings must be made for an electromagnetic wave power of less than 100 Watts. A knowledgeable experimenter is able to optimize this source in five or six operations, that is to say in a few minutes.

Claims (2)

1. Device to optimize a source of ions with electron cyclotron resonance including:

   - a chamber (1) containing a plasma of ions and electrons formed by electron cyclotron resonance;

   - a magnetic structure (7,9,11) comprising an outer armouring (11), said structure surrounding the chamber and creating inside the latter two fields, namely one radial and one axial magnetic field ensuring a confinement of the plasma inside the chamber;

   - a transition cavity (20) connected to an electromagnetic wave generator (3); and

   - a first and a second dielectric pipe (21,23) connecting the chamber and the cavity, the second pipe (23) comprising a transparent portion (23a) opposite the armouring in which resonance is produced at a specific point (C),

   characterized in that said device has a tubular piece (47) placed around the second pipe at the transparent portion and which can be translated parallel to the pipes, in order to regulate in optimum manner the position of said resonance point in the second dielectric pipe.
2. Device according to claim 1, characterized in that the tubular piece comprises, on the outer peripheral portion, a thread (47a) so as to form, with the armouring, a screw/nut system.

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