EP0295743B1 - Ion source with four electrodes - Google Patents

Description

The invention relates to a vacuum arc ion source comprising an anode and a cathode arranged opposite, polarized at different potentials and the main arc of which leads to the formation of a plasma directed perpendicular to the surface of the cathode is initiated by the projection of another plasma between said anode and said cathode for a short time relative to the arc pulse length.

When an arc is blown between two electrodes placed under vacuum, the material of the electrodes is locally vaporized under the effect of heating. This results in the formation of a plasma, that is to say of an ion-electron mixture with zero total charge.

The emission of this plasma projected with an average energy of a few tens of electron volts is made from very bright points and very small dimensions called cathode spots and the amount of ions in the plasma represents a few percent (5 to 10%) of the electric charge carried by the arc.

The projection has the shape of a cone. The ions can be extracted by means of an acceleration electrode brought to a negative voltage and of an extraction electrode, the latter being for example the anti-micro-projection system of particles.

Ion sources are used to create ions in isotopic separators, mass spectrometers, implanters, plasma machines, accelerators, neutron tubes etc ... They generally use the ionization of a gas injected into an almost closed volume.

• faibles dimensions pour la production du plasma,
• grand débit d'ions métalliques permettant l'utilisation de grandes surfaces d'extraction,
• fonctionnement sous vide et par conséquent ne nécessitant pas des systèmes de pompage différentiel de grand débit pour diminuer la pression de gaz dans la zone d'accélération des ions.

Compared to such gas discharge sources as the Penning source, vacuum arc sources have the following advantages:

• small dimensions for plasma production,
• high flow of metal ions allowing the use of large extraction surfaces,
• vacuum operation and therefore does not require high flow differential pumping systems to decrease the gas pressure in the ion acceleration zone.

Sources of vacuum arc ions of the kind mentioned in the preamble are of three electrode structure: anode, cathode and arc control trigger. An example of a structure commonly used is given in the article "Metal Vapor Vacuum Arc Ion Source" by T.G. Brown et al., Published in Review of Scientist Instruments, volume 57, No.6, June 1986, pages 1069-1084.

• la facilité de commande électrique par un générateur susceptible d'être indépendant sous l'aspect polarisation électrique,
• la simplicité de montage de la cathode de la source d'ions, montage rendu indépendant de la gâchette, et qui ne nécessite plus de tolérances mécaniques réduites comme dans le document référencé ci-dessus,
• la durée de fonctionnement et la fiabilité de la source d'ions en augmentant la partie active des gâchettes et en les éloignant de la cathode, dont la surface est fortement érodée et souvent déformée par les spots cathodiques (risque de court-circuits consécutifs à des fusions locales ou à des métallisations excessives).

The object of the invention is to increase:

• the ease of electrical control by a generator capable of being independent from the electrical polarization aspect,
• the simplicity of mounting the cathode of the ion source, mounting made independent of the trigger, and which no longer requires reduced mechanical tolerances as in the document referenced above,
• the duration of operation and the reliability of the ion source by increasing the active part of the triggers and by moving them away from the cathode, the surface of which is strongly eroded and often deformed by cathode spots (risk of short-circuits consecutive to local mergers or excessive metallization).

To this end, the ion source of the invention is remarkable in that the projection of the initial plasma is obtained by means of two autonomous triggers, one known as the cathode trigger which can be close to the anode, the other known as trigger anode may be close to the cathode and suitably polarized with respect to said anode and said cathode.

These triggers are formed, for example, by the superposition of two concentric circular rings, separated from each other, the anode and the cathode being arranged in the central zone of said rings and symmetrically with respect to their axis.

The following description with reference to the accompanying drawings, all given by way of example, will make it clear how the invention can be implemented.

Figure 1 shows in section the block diagram of an ion source according to the invention.

Figure 2 shows a particular embodiment of such a source.

Figure 3 shows the diagrams of some types of extraction electrodes.

In the sectional diagram of Figure 1 a cathode 1 of cylindrical shape is arranged opposite an anode.

This anode can be a metal disc 2 pierced with a circular opening in its center as shown in Figure 1a, or a metal grid 3 as shown in Figure 1b.

• soit de deux électrodes métalliques, massives, toriques, séparées par un espace de faible dimension,
• soit d'une couche métallique avec ou sans hydrure déposée sur un support isolant et sur laquelle on a tracé un sillon qui assure leur séparation,
• soit d'une couche semiconductrice avec émission de plasma par conduction (par exemple couche de carbone) et délimitée de même par le tracé d'un sillon.

Two independent triggers superimposed 4 and 5 in the form of concentric circular rings surround the active part of the anode and the cathode. These crowns are made up:

• either of two metallic, massive, toric electrodes, separated by a small space,
• either a metal layer with or without hydride deposited on an insulating support and on which a groove has been drawn which ensures their separation,
• or a semiconductor layer with emission of plasma by conduction (for example carbon layer) and similarly delimited by the drawing of a groove.

These various electrodes (anode, cathode and autonomous triggers) are suitably polarized by sources not shown. The initiation of an arc between the triggers 4 and 5 generates a plasma 6 called control plasma. The control plasma behaves like an electrical conductor of extensible form; during its passage between the cathode and the anode of the ion source, there is a short circuit between these two electrodes: the electrons of the control plasma are attracted by the anode and the ions by the cathode. In fact, physically, the process is as follows: the electrons in the plasma have a mobility much higher than the ions and the control plasma (out of respect for its overall electrical neutrality) will take the potential of anode 2 or 3. In these conditions, it appears between the plasma and the cathode 1 the potential difference applied to the ion source and the plasma ions are extracted by creating a cathode sheath whose height will depend on the ion density of the plasma. The resulting electric field on the cathode is very high and, depending on the trigger adjustment parameters (trigger current, duration of application: a few hundred nanoseconds to a few microseconds, cathode-anode trigger distance), there may be (or no) striking the arc. Under these conditions, if the electron current between the anode and the cathode is high enough to locally heat and vaporize the cathode, the metal vapor thus produced is ionized by the electrons and a cathode plasma 7 is formed from very bright points and very small dimensions (cathode spots). The arcing between the two triggers notably facilitates the control of the source in accordance with the invention.

Figure 2 shows an advantageous embodiment of the invention.

A metal part 8 pierced with a central opening 9 serves as a support for the entire device. On this part is mounted the support 10 of the anode 2 also serving for its polarization. An insulating ring 11 secured to the support 10 ensures the attachment of the triggers 4 and 5. The cathode 1 mounted on a metal rod 12 passing through the central opening 9 is adjustable longitudinally by means of the bellows 13; it is isolated from the triggers by the crown 14 also integral with the support 8. The output 15 for polarization of the triggers is made through another opening 16 likewise made in the support 8. The cathode plasma 7 is generated as indicated above.

• figure 3a : structure à simple orifice de type ponctuel 17 conduisant à des faisceaux extraits limités en débit et projetés sur une cible 19 à travers une électrode d'accélération 18,
• figure 3b : structure à grille simple de grande transparence 20 utilisée par exemple pour le bombardement d'une électrode 19,
• figure 3c : structure à un (ou plusieurs) orifice(s) d'extraction 21 de forme compatible avec les électrodes d'accélération 22 et conduisant à une définition du (ou des) faisceau(x) extrait(s) parfaitement maîtrisée,
• figure 3d : structure type "nid d'abeille" 23 permettant d'atténuer à l'extraction les variations de densité des flux de plasma cathodique.

FIG. 3 presents some examples of an extraction electrode, limiting the volume of expansion of the cathode plasma 7; the shape and structure of this electrode are a function of the ion acceleration mode selected, as shown in the diagrams below:

• FIG. 3a: structure with a single orifice of the punctual type 17 leading to extracted beams limited in flow and projected onto a target 19 through an acceleration electrode 18,
• FIG. 3b: simple, highly transparent grid structure 20 used for example for the bombardment of an electrode 19,
• FIG. 3c: structure with one (or more) extraction orifice (s) 21 of a shape compatible with the acceleration electrodes 22 and leading to a perfectly controlled definition of the extracted beam (s),
• FIG. 3d: a “honeycomb” type structure 23 making it possible to attenuate the variations in density of the cathode plasma streams during extraction.

These types of structure can be applied in particular to the embodiment of the ion source shown by way of example in FIG. 2.

Claims (5)

1. A vacuum arc ion source comprising an anode and a cathode which face each other, which are biased at different potentials and whose main arc which results in the formation of a plasma which consists of the material of the electrodes and which is directed perpendicularly to the cathode surface is triggered by the projection of a further plasma between said anode and said cathode during a period of time which is short relative to the length of the arc pulse, characterized in that the projection of this further plasma is obtained by means of two independent grids, one of which is denoted as the cathode grid and the other as the anode grid, and which are appropriately biased relative to said anode and said cathode.
2. An ion source as claimed in Claim 1, characterized in that said anode grid and said cathode grid are formed by the superpositioning of two concentric round rings which are separated one from the other.
3. An ion source as claimed in Claim 2, characterized in that said rings are constituted by two solid, annular metallic electrodes which are separated by a small gap.
4. An ion source as claimed in Claim 2, characterized in that said rings are constituted by a metallic layer, with or without hydride deposited on an insulating support, and bounded by a groove made in said layer.
5. An ion source as claimed in Claim 2, characterized in that said rings are constituted by a semiconductor layer, with plasma emission by conduction (for example a carbon layer), and bounded by a groove made in said layer.

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