EP0480518B1 - Electron source providing a particle retention device - Google Patents

Description

The present invention relates to a vacuum arc electron source comprising a plasma source having an anode and a cathode arranged opposite so as to form a plasma following the application of a potential difference. between the anode and the cathode, an electron extraction device and a material retention device located between the extraction device and the plasma source.

An electron source is known from EP-A-0 286 191, which describes a retention device in the form of a grid of baffles which are arranged downstream of an extraction device. The baffles define staggered openings, so that the plasma projections are collected.

A source of electrons is known from the article "Grid-controlled plasma cathodes" by S. HUMPRIES and collaborators in the "Journal of Applied Physics" vol. n ° 3 (February 1985) p. 700-713.

According to this prior art, the retention device is constituted by an ion control grid (ICG) which is arranged within the plasma and at the same electrical potential as the plasma source, and the extraction device comprises an extraction cathode K constituted by a grid polarized positively with respect to the plasma source as well as an electron collecting anode A. The function of the ICG ion control grid is to separate the ions from the electrons in the ICG grid - K cathode space, the electrons being extracted or not depending on the space charge in the extraction interval located between the cathode K and anode A.

Such a structure requires pulsed operation of the plasma source and in particular an operating condition is that the pulse length of the plasma must not be too large compared to the pulse length sought by the electrons to avoid charging. electrically grid and lead to breakdowns.

The basic idea of the invention is to decouple optically and electrically the plasma of the extraction zone in order to avoid the aforementioned drawbacks.

The electron source according to the invention is for this purpose characterized in that the material retention device comprises, in the direction of extraction of the electrons, an upstream baffle and an electrically conductive downstream baffle and having staggered openings, so that, when the baffles are brought to a given potential, the plasma does not extend downstream of the downstream baffle. This provides effective retention of materials, namely ions, neutralized or not, as well as neutrals and micro-particles emitted simultaneously.

At least one opening may be a transverse slit relative to the direction of extraction of the electrons.

At least one baffle may have around at least one opening, a folded edge on the side of the plasma source. This improves the retention of plasma ions, as well as neutrals and micro-particles emitted simultaneously by the vacuum arc. According to a preferred embodiment of the material retention device, the upstream baffle and the downstream baffle have said folded edges, aligned in the direction of extraction of the electrons.

The width of the openings can be greater than or equal to the interval between the openings. The distance between the baffles can be at least equal to the width of the openings and the interval between the openings. The quantity of electrons extracted indeed increases with the relative width of the openings compared to their interval, as well as with the distance between the baffles.

According to a particularly advantageous embodiment with regard to the homogeneity of the electron beam, there are provided, in the direction of extraction of the electrons, an upstream extraction electrode and a substantially parallel downstream extraction electrode, which are preferably spaced a distance at least equal to the pitch of said baffles.

At least one extraction electrode can advantageously be arranged in the passage located downstream of the openings of the downstream baffle in the direction of extraction of the electrons. This improves the extraction yield at equal potential.

The previous extraction structures lead to electron emissions at an energy (expressed in eV) close to the extraction voltage while remaining lower. To reduce this initial energy and obtain better control of the beam, it is advantageous to provide an electron energy reducing electrode disposed downstream of the extraction device in the direction of electron extraction, such reduction can then be obtained by bringing said electrode to an electrical potential lower than that of the extraction device.

• la figure 1, une source d'électrons selon l'art antérieur (état général de la technique), la figure 2 correspondant à l'article de HUMPRIES et al. précité
• la figure 3a à 3c, une source d'électrons selon un mode de réalisation de l'invention, la figure 3b étant un détail de la fig. 3a montrant les lignes de champ, et la fig. 3c un détail de la fig. 3a en vue de préciser les dimensions.
• la fig. 4 des diagrammes de courant et de tension en vue de l'extraction des électrons.
• la fig. 5, 6a, 6b et 6c des modes de réalisation des ouvertures des baffles selon l'invention, les figures 6b et 6c représentant respectivement en vue de dessus et en coupe XX′ un dispositif à symétrie cylindrique de révolution.
• la fig. 7 et 8 des modes de connexion de source de plasma en parallèle en vue d'obtenir une émission d'électrons de grande section.
• et les fig. 9 à 12 quatre variantes de l'invention présentant une extraction améliorée, la fig. 12 correspondant à un mode de réalisation préféré.

The invention will be better understood on reading the description which follows, given by way of nonlimiting example, in conjunction with the drawings which represent:

• FIG. 1, an electron source according to the prior art (general state of the art), FIG. 2 corresponding to the article by HUMPRIES et al. above
• FIG. 3a to 3c, an electron source according to an embodiment of the invention, FIG. 3b being a detail of FIG. 3a showing the field lines, and FIG. 3c a detail of fig. 3a in order to specify the dimensions.
• fig. 4 current and voltage diagrams for the extraction of electrons.
• fig. 5, 6a, 6b and 6c of the embodiments of the openings of the baffles according to the invention, FIGS. 6b and 6c respectively representing in top view and in section XX ′ a device with cylindrical symmetry of revolution.
• fig. 7 and 8 of the plasma source connection modes in parallel in order to obtain an emission of electrons of large section.
• and fig. 9 to 12 four variants of the invention presenting an improved extraction, FIG. 12 corresponding to a preferred embodiment.

According to Figure 1, an electron source has an ion source comprising at least a cathode 1 and an anode 2 (diode type) and possibly a trigger 3 (triode type) or else a secondary arc as in French patent 8708196 filed on June 12, 1987 by the Applicant and issued on November 24, 1989 under number FR 2616587 (tetrode type). For the diode type, the anode 2 and the cathode 1 are very close to one another and the initiation of the plasma arc P is simply obtained by application of a sufficient anode voltage. For the triode type, the trigger 3, whose position, shape and mode of supply allow the ignition of a cathode spot at the origin of the main arc P, is close to the cathode 1 while the anode 2 is far from it. For the tetrode type, the main plasma arc P is initiated by injection of a plasma coming from a secondary arc of short duration compared to the main arc P and dissipating a very low energy opposite the main arc P.

Likewise, these plasma sources can be produced in the form of thin layers deposited on insulators, generally allowing large instantaneous and more reproducible emissions, but with a reduced number of operating shots.

• l'énergie nécessaire pour l'obtention d'un arc stable (influence du courant électronique recherché)
• la dissipation thermique des électrodes, en particulier de la cathode, et les problèmes de refroidissement,
• le temps d'établissement de l'arc et la vitesse de projection du plasma (influence des températures de fusion des matériaux),
• l'aptitude à un traitement chimique ultérieur de nettoyage de la source,
• leur pureté, sous l'aspect désorption à chaud de gaz susceptible de perturber la qualité du vide.

Les électrons sont extraits du plasma P par un dispositif d'extraction d'électrons EE (par exemple une grille), le sens d'extraction (flèche F) étant perpendiculaire audit dispositif d'extraction EE. En tant que de besoin, un dispositif FA de focalisation et d'accélération dirige les électrons vers une cible A.The cathode materials used are in principle indifferent; their choice is a compromise between:

• the energy required to obtain a stable arc (influence of the electronic current sought)
• the heat dissipation of the electrodes, in particular of the cathode, and the cooling problems,
• the arc establishment time and the plasma projection speed (influence of material melting temperatures),
• suitability for further chemical cleaning of the source,
• their purity, under the hot desorption aspect of gas liable to disturb the quality of the vacuum.

The electrons are extracted from the plasma P by an electron extraction device EE (for example a grid), the direction extraction (arrow F) being perpendicular to said extraction device EE. When necessary, a focusing and acceleration device FA directs the electrons towards a target A.

Fig. 2 illustrates the device described in the publication by S. HUMPHRIES and collaborators, and according to which an ion control grid (ICG) is placed in the plasma P and at the same potential as the latter. An extraction cathode K acting as an extraction grid being positively polarized with respect to the grid (ICG), the potential difference thus created prevents the ions from entering the extraction space, that is to say say the space between cathode K and a target anode A. In the absence of an extraction potential, the electrons are prevented from crossing the extraction space AK. A condition on the density of extracted current is that the width of the space in which the separation between the ions and the electrons takes place is substantially equal to or greater than half the width of the openings of the extraction grid K. Another condition is that the length of the pulses producing the plasma cannot be much greater than the length of pulse designated for the electrons, this to avoid electrically charging the extraction grid K and to reduce the probability of breakdown. In other words, a plasma pulse can only correspond to one single electron extraction.

According to FIGS. 3a to 3c and 5, the cathode (or anode) plasma is optically isolated by two baffles 10 and 20, comprising, in the direction of electron extraction (arrow F) an upstream baffle 10, and a downstream baffle 20 , brought to ground or anode potential (for a cathode plasma), and provided with openings respectively 16 and 26 staggered with respect to each other. In practice, the two baffles 10 and 20 are constituted by a single member with two baffle functions (upstream and downstream) or by two mechanically separate members but electrically connected together so that they are always brought to the same potential. The plasma, in the absence of any extraction voltage, is intercepted by the baffles and cannot penetrate downstream of the downstream baffle 20. In the case of FIG. 2 (prior art), the ICG grid is arranged within the plasma P, which always extends downstream thereof until it comes close to the extraction cathode K. On the contrary, in the case of the invention, the plasma P is stopped by the baffles and cannot s 'extend downstream of these. The extraction electrode 30 is, whatever the operating conditions, free from any pollution by the plasma P which can therefore be maintained continuously for the entire time necessary for obtaining the desired number of electron extracts . In addition, such a baffle structure at at least two levels also allows the interception of the micro-projections emitted by the cathode 1. FIG. 4 shows, in a the profile of the current Iarc of the plasma source, in b the potential extraction (several pulses, for a single ignition of the plasma), and in c the Iext current of the extracted electrons. For an extraction voltage Vext (of a few kV) with a profile in flat plates, the current Iext. presents, in a conventional manner, trays with a negative slope.

FIG. 3b shows the equipotential lines between the separation surfaces 12 which delimit the contour of the plasma and according to which the electrons are extracted. These surfaces 12 are a function of the extraction voltage and the density, in electrical charges, of the plasma emitted. The surfaces 12 are located between the two baffles 10 and 20, in a general direction perpendicular to these and substantially from one edge to the other of the openings 16 and 26. The equipotentials (22 to 25) evolve between a shape ( 22) having a first part parallel to the baffle 20 and a second part clearly re-entering the inter-baffle space outside the plasma P, a shape (23) more downstream of the baffles and also with two parts, the second being less reentrant in the inter-cabinet space, a shape (24) even further downstream and which is practically planar, which makes it possible to direct the electrons essentially according to the direction of extraction F (they were indeed extracted essentially perpendicular to this direction), and finally a shape (25) substantially sinusoidal in the vicinity of the extraction grid 30.

FIG. 3c shows a practically ideal shape (14) of the separation surface 12, with a very marked digging which clearly increases the extraction surface, and therefore the extraction yield. It should be noted that the double baffle device makes it possible to easily design a geometry having an extraction surface greater than that of the prior art, that is to say on the surface of the extraction grid. On the other hand, the retention of plasma and material in the baffles, and especially in the downstream baffle (20) is favored by the presence of folded edges 21 (and / or 11), of a distance respectively d respectivement (and / or d₂) upstream.

The parameters influencing the extraction are the spacing h between the baffles 10 and 20, the width l₁ of the intervals between openings of the upstream baffle 10, the width l₂ of the openings 16 of the upstream baffle 10, it being understood that the downstream baffle 20 is the "negative" of the upstream cabinet 10.

• dans le même sens que h
• inversement à l'évolution de l₂ et de l₁ c'est'à-dire avec le nombre de cellules.

The quantity of electrons extracted increases:

• along the same lines as h
• conversely to the evolution of l₂ and l₁, that is to say with the number of cells.

In addition, the applied electric field is determining as for the quantity of electrons extracted. Two extreme positions (see Fig. 5) (30A: extraction electrodes not downstream of the edges of the openings; 30B: extraction electrodes in the center of the openings and the intervals between them) for identical polarizations correspond to the maximum extraction (30A) and minimum (30B), knowing that interception by the extraction electrode is maximum in (30A).

et à la forme 14 de ménisque de plasma de la figure 3c.The ideal maximum yield corresponds to: $$\text{l₁ ≦ l₂ <h}$$

and to the form 14 of the plasma meniscus of FIG. 3c.

• structures linéaires avec l₁ ≦ l₂ et h > l₂ avec les électrodes d'extraction constituées par des fils (ou barres) proches de l'alignement des bords relevés (11 et 21) des baffles 10 et 20, et légèrement masquées par le baffle 20 (figures 3a et 3b).
• structures à ouvertures circulaires (16′, 26′) (fig. 6a), pour des faisceaux cylindriques (de révolution ou non) et plus particulièrement lorsque l'homogénéïté doit présenter une symétrie axiale (figures 6b et 6c) : aux figures 6b et 6c, les baffles amont 10 et aval 20 deviennent des anneaux à bords relevés 10′ et 20′, reliés entre eux sur des rayons pour assurer le maintien mécanique (à la figure 6b, les anneaux 16′ sont représentés en pointillés).

The preferred configuration structures of baffles arise from these considerations:

• linear structures with l₁ ≦ l₂ and h> l₂ with the extraction electrodes constituted by wires (or bars) close to the alignment of the raised edges (11 and 21) of the baffles 10 and 20, and slightly masked by the baffle 20 (Figures 3a and 3b).
• structures with circular openings (16 ′, 26 ′) (fig. 6a), for cylindrical beams (of revolution or not) and more particularly when the homogeneity must have an axial symmetry (Figures 6b and 6c): in Figures 6b and 6c, the upstream 10 and downstream 20 baffles become rings with raised edges 10 ′ and 20 ′, interconnected on spokes to provide mechanical support (in FIG. 6b, the rings 16 ′ are shown in dotted lines).

$$\text{h > R₂₀′,i - R₂₀′,i-1}$$

For two rings (10 ′ and 20 ′ baffles) of rank i and i-1, we will have: $$\text{R₁₀ ′, i - R₁₀ ′, (i-1) ≦ R₂₀′i - R₂₀ ′, i-1}$$

$$\text{h> R₂₀ ′, i - R₂₀ ′, i-1}$$

According to Figures 7 and 8, a large source is obtained by paralleling n plasma sources (massive or layered) distributed so as to ensure a homogeneous plasma density on the baffles 10 and 20 (or 10 ′ and 20 ′). These sources are supplied either individually from a source (-HT) through a resistor R for each (fig. 7), or collectively through a single resistor R / n (fig. 8).

According to Figures 9 and 10, two extraction grids are used, referenced 30 and 31, located one behind the other. The second extraction grid 31, at the same potential as the first, screens the accelerating voltages of the electrons and allows free transit of these over a distance D greater than the pitch (l₁ + l₂) of the extraction baffles. This allows an overlap between the beams extracted from contiguous openings 26 of the downstream baffle 20 and attenuates the density distortions. According to FIG. 9, the upstream extraction grid is located near the downstream baffle 20, whereas according to Figure 10, it is removed.

According to FIGS. 11 and 12, a grid reducing the energy of the electrons is present (reference 40) downstream of the extraction grid (s) (30, 31). The grid 40 is brought to a potential lower than that of the extraction grid (s) (30, 31). In FIG. 11, only the extraction grid 30 is present. In FIG. 12, the grid 40 is associated with the two extraction grids 30 and 31, hence optimization of both the extraction and the energy of electrons. The potential of the gate 40 can be adjusted between the voltage of the extraction device (30, 31) and the bias voltage of the baffles (10, 20).

Claims (14)

1. A vacuum arc electron source comprising a plasma source (1) having an anode and a cathode facing each other such that they produce a plasma (P) after an appropriate voltage difference has been applied between the anode and the cathode, an electron extractor device (30) and a material-retaining device (10, 20) arranged between the extractor device and the plasma source, characterized in that the material-retaining device includes, arranged in the electron extraction direction (F), at least un upstream baffle (10) and a downstream baffle (20) which are both electrically conducting and have apertures (26, 26) arranged in quincunx, such that when the baffles (10) and (20) are brought to a given potential which is the same for both baffles, i.e. earth (for an anode plasma) or anode potential (for a cathode plasma), the plasma does not extend beyond the downstream baffle (20).
2. An electron source as claimed in Claim 1, characterized in that at least one aperture (16, 26) is a slot extending transversely of the direction of extraction of the electrons.
3. An electron source as claimed in one of the Claims 1 or 2, characterized in that at least on baffle (10, 20) has around at least one aperture (16, 26) an edge (11, 21) which is folded at the side facing the plasma source.
4. An electron source as claimed in Claim 3, characterized in that the upstream baffle (10) and the downstream baffle (20) include the said folded edges (11, 21), aligned in the electron extraction direction (F).
5. An electron source as claimed in anyone of the preceding Claims, characterized in that the width of the apertures (26) of the downstream baffle (20) exceeds or is equal to the gap between the apertures (26).
6. An electron source as claimed in anyone of the preceding Claims, characterized in that the distance (h) between the baffles is at least equal to the width of the apertures and the gap between the apertures.
7. An electron source as claimed in anyone of the preceding Claims, characterized in that the extractor device includes at least one extraction electrode (30).
8. An electron source as claimed in Claim 7, characterized in that at least one extraction electrode (30) comprises conductive wires which extend parallel to each other.
9. An electron source as claimed in Claim 7 or 8, characterized in that at least one extraction electrode (30) comprises a conductive grid.
10. An electron source as claimed in Claims 7 to 9, characterized in that it includes, arranged in the electron direction, an upstream extraction electrode (30) and a downstream extraction electrode (31), arranged substantially in parallel.
11. An electron source as claimed in Claim 10, characterized in that the interspace between the upstream extraction electrode (30) and the downstream extraction electrode (31) is at least equal to the pitch of the apertures (16, 26) of said baffles (10, 20).
12. An electron source as claimed in anyone of the Claims 7 to 11, characterized in that, at least one extraction electrode is provided in the path located downstream of the apertures (26) of the downstream baffles (20) in the extraction direction (F) of the electron.
13. An electron source as claimed in anyone of the preceding Claims, characterized in that it comprises an electron energy reducing electrode (40) which is arranged downstream of the electron extractor device (30, 31), in such a manner that the energy of the extracted electrons is reduced when the electron source is brought to an electric potential which is lower than that of the extractor device (30, 31).
14. An electron source as claimed in Claim 13, characterized in that it comprises means for adjusting the potential of the energy reducing grid (40) so that it lies between the voltage of the extractor device and the bias voltage of the baffles (10, 20).

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