EP1307607A1

EP1307607A1 - Method and device for plasma treatment of moving metal substrates

Info

Publication number: EP1307607A1
Application number: EP01962488A
Authority: EP
Inventors: Pierre Vanden Brande; Alain Weymeersch
Original assignee: Cold Plasma Applications Cpa
Current assignee: Cold Plasma Applications Cpa
Priority date: 2000-08-04
Filing date: 2001-08-06
Publication date: 2003-05-07
Also published as: BR0113141A; JP2004506096A; US6933460B2; AU2001283717A1; WO2002012591A1; ZA200301533B; US20040026412A1; EP1178134A1; CA2423138A1

Abstract

The invention concerns a method for treatment, in particular for cleaning and/or heating, a metal substrate (1) moving substantially continuously in a vacuum chamber (3) having a treatment zone wherein an electric discharge (10), that is a plasma, and a magnetic field are produced in a gas maintained at a pressure lower than atmospheric pressure between at least the substrate (1) forming an electrode, and at least a counter electrode (9) so that the substrate (1) is bombarded with ions produced in the electric discharge (10). Said method is characterised in that a magnetic confinement induction field is generated all around the substrate (1) in the treatment zone so that the electric discharge (10) is likewise confined all around the substrate (1) in said treatment zone by the confinement of the electrons released in the electric discharge (10).

Description

PROCESS AND DEVICE FOR PROCESSING METAL SUBSTRATES ON A PLASMA RUN

The present invention relates to a method of treatment, in particular of cleaning and / or heating by an electric discharge, i.e. by plasma, metallic substrates such as products in the form of wires, tubes, beams, strips or and sheets. In this method, the substrate to be treated is moved in a determined direction in an enclosure having a treatment zone in which an electric discharge is created between a counter-electrode and the substrate, near the surface of the latter.

This process makes it possible to remove a contamination layer from the substrate, such as for example surface metal oxide and surface carbon, so as to promote the adhesion of a coating subsequently applied to this product by a vacuum deposition technique. . This process also makes it possible to efficiently heat the substrate and can therefore be used for annealing metal products. It can be applied to mild steel, stainless steel, aluminum, copper and other metals substrates.

The known methods for cleaning or heating a metallic product by plasma have several disadvantages. According to the state of the art, it is for example not possible to treat, in a single operation, all the external surfaces of the product.

In particular, to treat a flat metal strip with an upper face and a lower face, the two faces are treated successively by producing a plasma successively near the two faces. This has the disadvantage

25 important, a possible recontamination of the face which was treated in the first step during the treatment of the second face in the second step.

To be able to treat the metal strip effectively, we realize generally on the surface of the latter a magnetron discharge by arranging at the rear of the surface to be treated networks of magnets.

Another major drawback of the known technique is the adaptation of the size of the magnet networks to the bandwidth and the impossibility of treating wires and tubes by this technique.

One of the essential aims of the present invention is to propose a method which makes it possible to remedy this type of drawback.

To this end, in the method according to the invention, a magnetic induction induction field is produced all around the substrate in the treatment zone so that the electrical discharge is confined all around the substrate by the confinement of the released electrons. in the discharge, so as to allow the formation of a high density plasma all around the substrate.

Advantageously, a magnetic induction field is produced in the aforementioned treatment zone substantially parallel to the axis of travel of the substrate in this treatment zone in such a way as to thus allow the formation of a magnetron discharge around the substrate by a flow of electrons from the discharge along paths extending around the substrate.

According to a particularly advantageous embodiment of the method, according to the invention, at least one magnetic mirror is produced at least partially around the substrate, so that the electrical discharge is confined mainly by this magnetic mirror around the substrate in the treatment zone .

Interestingly, a magnetic induction field is created which extends substantially transversely to the direction of movement of the substrate, and which has a minimum value near the surface of the substrate where the electric discharge is confined. In particular, the intensity of this magnetic field increases by at least a factor of two from the substrate to the aforementioned magnetic mirror. The electrons moving away from the substrate, are, therefore, reflected and returned towards the substrate having the effect of confining the electrical discharge around the substrate.

According to another advantageous embodiment, at least two magnetic mirrors are produced which are crossed by the substrate and which define the entry and exit of the treatment zone. These magnetic mirrors make it possible to confine the electrical discharge in a direction substantially parallel to the axis of travel of the substrate in the treatment area.

A magnetic induction field is formed by these magnetic mirrors which is substantially parallel to the direction of travel of the substrate. The intensity of this induction field decreases as it moves away from each mirror towards the center of the treatment area.

The invention also relates to a device for treating a metallic substrate, in particular for cleaning and / or heating a metallic substrate, comprising a vacuum chamber provided with an inlet opening and a outlet opening for the substrate. In this vacuum chamber, which has a treatment zone in which the electrical discharge is created, can pass the substrate in a substantially continuous manner. The device has magnetic means for confining the electrons produced in the electric discharge and comprises at least one counter-electrode disposed opposite the substrate forming an electrode. The device according to the invention is characterized in that the magnetic confinement means are arranged in such a way with respect to the substrate so that the ions produced in the electric discharge can be maintained all around the substrate and thus bombard the surface of the last. More particularly, the magnetic confinement means are provided at least partially around the treatment zone.

Advantageously, at least one magnetic mirror is arranged around the treatment zone in which the substrate passes, in such a way as to reflect and return towards the substrate the electrons moving away from the latter and, consequently, to confine the discharge. around the substrate in this area. The magnetic confinement means preferably comprise at least one solenoid around the treatment zone, with an axis substantially parallel to the direction of travel of the substrate. The cross section of the solenoid, perpendicular to its axis, can be of generally any shape, for example circular for a cylindrical solenoid or rectangular for a parallelepipedic solenoid.

In an interesting embodiment of the invention, the device comprises at least two magnetic mirrors between the inlet opening and the outlet opening, delimiting the processing area in the direction of travel of the substrate.

Other details and particularities of the invention will emerge from the description given below, by way of nonlimiting example, of some particular embodiments of the invention with reference to the appended drawings.

Figure 1 is a schematic cross-sectional view parallel to the axis of travel of the product of a first embodiment of the device according to the invention.

FIG. 2 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 1.

Figure 3 is a schematic view in cross section parallel to the axis of travel of the product of a second embodiment of the device according to the invention. FIG. 4 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 3.

Figure 5 is a schematic cross-sectional view parallel to the axis of travel of the product of a third embodiment of the device according to the invention.

FIG. 6 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 5.

Figure 7 is a schematic cross-sectional view parallel to the axis of travel of the product of a fourth embodiment of the device according to the invention.

FIG. 8 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 7. Figure 9 is a schematic cross-sectional view parallel to the axis of travel of the product of a fifth embodiment of the device according to the invention.

FIG. 10 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 9.

Figure 11 is a schematic cross-sectional view parallel to the axis of travel of the product of a sixth embodiment of the device according to the invention. FIG. 12 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 11.

Figure 13 is a schematic cross-sectional view parallel to the axis of travel of the product of a seventh embodiment of the device according to the invention.

FIG. 14 is a graph which represents the value of the component of the magnetic induction field in the vicinity of the substrate along the direction of movement of the substrate in the device of FIG. 13.

FIG. 15 is a detail in cross section parallel to the axis of travel of the product of a schematic representation of the discharge indicating the behavior of the electrons between two magnetic mirrors.

Figure 16 is a schematic cross-sectional view perpendicular to the direction of product movement, illustrating the formation of a magnetron discharge around the product. In the different figures, the same reference numbers relate to identical or analogous elements.

The method according to the invention consists in creating near the surface of a substrate to be cleaned or in heating a plasma in a gas mixture comprising for example one or more of the following elements: Ar, He, H ₂ , O ₂ or N ₂ , or hydrocarbon compounds. The gas is maintained at a pressure P lower than atmospheric pressure, so as to generate radicals and ions allowing cleaning and the increase in temperature of this substrate. The pressure P can for example be between 10 ^"2 Pa and 1000 Pa.

This process allows cleaning or heating of substrates with high kinetics since it is based on the creation of a high density plasma n in the gas maintained at low pressure. This plasma density n generally corresponds to an electronic density between 10 cm ^"3 and 10 ¹² cm ^{" 3} . Since the plasma is of high density, high powers can be dissipated on the surface of the substrate, mainly in the form of an ion bombardment.

In particular, the electric discharge is produced under a maximum potential difference maintained between the substrate and a counter electrode less than or equal to 1000 V for average power densities per unit area of substrate between 1 Wcm ^" 2 and 200 Wcm ^" 2.

The discharge, for the formation of a plasma, is obtained by polarizing the substrate negatively with respect to a counter electrode, either continuously or cyclically. In the first case, it is a continuous electrical discharge and the counter-electrode is an anode. In the second case, it is an alternative discharge during which the substrate is bombarded by ions only when it is negatively polarized with respect to the counter-electrode.

The form of the signal of the voltage used for an alternating discharge can be sinusoidal, simply rectified, square in the case of pulsed currents or generally any. The duration of the positive part of the voltage signal is not necessarily equal to that of the negative part over a period. The excitation frequency is typically between 1 kHz and 1 MHz and is preferably between 10 kHz and 100 kHz. Advantageously, the substrate is kept grounded.

Preferably, the time during which the counter electrode is negatively polarized with respect to the surface of the substrate is shorter than the time during which it is positively polarized.

The confinement of the discharge electrons between two or more successive magnetic mirrors crossed by the metal substrate makes it possible to increase the density of a plasma produced around the latter. By magnetic mirror, it is meant, within the framework of the present invention, a magnetic field of a relatively high value compared to a neighboring magnetic field so that electrons present in this neighboring field and moving towards the high value magnetic field can be reflected in the reign area the lower value fields.

These magnetic mirrors can for example be produced by means of two solenoids surrounding the metal substrate or also by means of permanent magnets.

The method according to the invention is particularly advantageous for treating long or continuous substrates. The substrate is then moved in its longitudinal direction through an enclosure in which the plasma is created.

In a preferred configuration, the magnetic mirrors are placed in such a way that they generate an induction field substantially parallel to the axis of displacement of the substrate, the field increasing in the direction of each mirror. The magnetic induction field produced in the middle of a magnetic mirror is preferably at least equal to 5.10 ^" T or 50 Gauss.

As shown schematically in Figure 15, the secondary electrons generated during the bombardment of the surface of the substrate 1 by the ions of the discharge and accelerated in a sheath 16 surrounding the surface of the substrate, are reflected by the magnetic mirrors as long as they n have not lost most of their kinetic energy in the ionization processes.

Indeed, above a certain critical value of the perpendicular component v _perp of the velocity vector of an electron with respect to the direction of the induction field, which is substantially parallel to the direction of movement of the substrate 1, the electron remains confined between two neighboring magnetic mirrors.

By sheath is understood the space charge zone, the thickness of which generally does not exceed more than a few millimeters, which is established and naturally separates any surface in contact with a plasma. When this surface is that of an electrode and this electrode is brought to a negative potential compared to that of a counter-electrode or anode, the sheath which is formed therein bears the name cathode sheath. Most of the potential difference applied between this cathode and the counter electrode or anode is then found at this cathode sheath. For this reason, the cathode cladding is the seat of the acceleration of positive ions towards the cathode and the seat of the acceleration of the secondary electrons emitted towards the plasma following the impact of the ions at the cathode. If B _mln represents the minimum magnetic induction field between two successive mirrors and B _max corresponds to the magnetic induction field parallel to the central axis, between these two mirrors, generated at each mirror, we show that, for a axial symmetry with B _min parallel to the direction of movement of the substrate 1, an electron remains confined between these two mirrors when the ratio of the peφendicular component v _perp of the speed of the electron to its module total speed v is greater than a critical value equal to sinθ _cr = (B _min / B _max ) 1/2 with θ the angle between the direction of the induction field and the total speed v. See for example: [J. . Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2, pp 75-90]. Refer to Fig. 15. We express that an electron escapes through the mirrors when the escape angle θ is less than the critical escape angle θ _cr , which is expressed mathematically in the form:

The electric field E of the sheath 16 being always perpendicular to the magnetic induction field B since the latter is substantially oriented along the axis of movement of the substrate 1 to be treated and the sheath tension 16 being high, the peφendicular component of the speed vector of the electron v is always much higher than its component parallel to the magnetic induction field v _para since the latter corresponds substantially to the average thermal speed of the electrons in the plasma corresponding to a much lower kinetic energy. The average thermal energy of the electron is typically less than 10 eV in this type of plasma. This explains why the magnetic confinement of secondary electrons remains very effective until they lose in inelastic collisions. almost all of the energy gained during their acceleration in the cathode cladding 16. These electrons thus bounce between the magnetic mirrors until the energy gained in the cladding 16 is exhausted.

The voltage of the sheath 16 is typically of the order of several hundred volts.

Besides this longitudinal movement, the electrons also describe a movement in a direction peφendicular to the induction field and in a direction peφendicular to the electric field, around the substrate 1 (see Fig. 16). These electrons thus finally describe a spiral path 17 around the substrate 1 to be cleaned by bouncing continuously between the magnetic mirrors until the energy gained during the acceleration in the sheath 16 is exhausted. in fact a magnetron discharge confined between two successive mirrors around the substrate 1 to be cleaned or heated. Only the electrons whose escape angle θ is less than the critical angle θ _cr can escape from the confinement zone in a proportion once again dependent on the ratio of the induction fields B _min between the mirrors and B _raax at the level of the mirrors. Indeed, only electrons "thermalized" during numerous collisions can escape through magnetic mirrors. By "thermalised" electrons is understood electrons whose kinetic energy is reduced and has reached the average value of the energy of the energy distribution of the electrons in the electric discharge. It is therefore possible to place the counter-electrodes or the anodes at the level of the magnetic mirrors without risk of capturing secondary electrons before thermalization. We can show that the fraction F _th of the fully thermalized electrons that are trapped between the magnetic mirrors is worth:

(JR Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2). Different specific configurations are possible for the confinement of the electrons, and consequently for the confinement of the plasma, towards the external surface of the substrate:

A first method consists in confining the discharge in the direction peφendicular to the axis of movement of the substrate between two neighboring mirrors, by the presence of an axial magnetic field, as represented in FIGS. 1, 5 and 7. The latter makes it possible to reduce the ambipolar diffusion coefficient D _{perp (a)} of the plasma in the direction peφendicular to the axis of travel at the value of the diffusion coefficient D _{perp (e)} of the electrons in this same direction, this value being much lower than the value the ambipolar diffusion coefficient of a non-magnetized discharge D _a . See [MA Lieberman, AJ Lichtenberg in Principles of Plasma Discharges and Material Processing, Wiley Interscience, ISBN 0471005770, NY (1994), pp 129-145].

The presence of the magnetic induction field along the direction of movement of the substrate, in fact limits the diffusion of the plasma in the direction transverse to the direction of movement of the substrate perpendicular to the lines of magnetic induction field by forcing the electrons to rotate around of the same field line until a collision with another particle, such as an atom or a gas molecule, has not taken place. This configuration also has the advantage of generating a magnetron discharge around the treated substrate and therefore also has the effect of improving the confinement of the plasma around the substrate. Two magnetic mirrors generating a field in the same direction make this configuration possible.

To generate such a magnetron discharge, the magnetic induction field parallel to the axis of travel of the substrate is advantageously at least equal to l0 ^"3 T (l T = l tesla) and is preferably between 10 ^{" 3} T and 0.25 T in the treatment area.

In a preferred embodiment of the invention, the magnetic induction field is reinforced in the direction of movement of the substrate by other means such as for example by adding a third solenoid between the two magnetic mirrors or by the presence of permanent magnets between the mirrors. It is also possible to provide the treatment area only with a magnetic induction field parallel to the direction of movement of the substrate without that it is necessary to install magnetic mirrors if the treatment zone is long enough so that the plasma losses at the entry and at the exit of this zone are low compared to the production of plasma in this same zone.

A second method consists in confining the secondary electrons in the space delimited by two mirrors by making use of a particular configuration of these latter, known under the name of magnetic cusp point or "magnetic cusp" as represented in FIGS. 3 and 9. In this case, the fields parallel to the scrolling axis produced by two neighboring mirrors are inverted at a cusp, which has the effect of producing a confinement zone for the electrons as well in the direction of displacement of the substrate only in the direction peφendicular to this direction.

Indeed, this configuration has the advantage of generating a magnetic mirror around the substrate in the treatment zone in addition to the magnetic mirrors crossed by the product. The component parallel to the running axis of the magnetic induction field is canceled out at the cusp point See [J. R. Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2, p 83].

A third method uses multipolar magnetic confinement on a surface of revolution surrounding the substrate to be treated between the two magnetic mirrors crossed by the latter. This multipolar confinement can be achieved, for example, by juxtaposition of permanent magnets as shown in FIG. 11.

Another known method consists in passing in tight solenoids, surrounding the product, and spaced from each other, currents in opposite directions (Fig. 13). These solenoids can be reduced to wires surrounding the product, traversed by currents successively in opposite directions. See [JR Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2, p. 88-90] and [MA Lieberman, AJ Lichtenberg in Principles of Plasma Discharges and Material Processing, Wiley Interscience, ISBN 0471005770, NY (1994), pp 146-150]. A great advantage of the process according to the invention is that, unlike conventional magnetron pickling or "etching" processes, it allows realize a magnetron discharge around substrates of cross sections of different shapes. The method is suitable, for example, for cleaning substrates which consist of beams in the form of T, I or U, wires or multitudes of wires, strips, sheets, etc. It also makes it possible to treat the edges of sheet metal. The method according to the invention does not in fact require the presence of permanent magnets placed at the rear of the substrate, as well as complicated mechanical devices for making the adaptations to the different bandwidths. See for example: EP535568, EP780485, EP879897, D136047, EP878565, EP908535, [S. Schiller, U. Heisig, K. Steinfelder and K. Gehn, Thin Solid Films, 51 (1978) 191]. The method can be used to clean the exterior surface of a product by bombardment of ions from the plasma. In this case, for the cleaning kinetics to be high, it is preferable to work at a sufficiently low pressure, preferably at a pressure below 1 Pa, so that the ions accelerated in the cathode sheath are not subjected to no collision before impact with the surface of the substrate.

To clean the substrate, an electric discharge is generated in a gas advantageously maintained at a pressure of less than 1 Pa and preferably between 1 Pa and 0.01 Pa in the treatment zone.

When the process is used to heat a metallic product and one wants to avoid the erosion of the exterior surface by ion bombardment, it is preferable to work at a pressure such that the ions undergo numerous collisions with the atoms or molecules of the gas during their acceleration in the cathode cladding. In this case, the energy being brought to the surface of the substrate by a larger number of particles, the latter is much less eroded. Indeed, the particles, which arrive on the external surface of the metallic product, do not have sufficient kinetic energy to atomize the surface atoms. In general, in a heating process one works at a pressure above 1 Pa, and preferably at a pressure of the order of 10 Pa.

To heat the substrate, an electric discharge is produced in a gas advantageously maintained between 10 Pa and 1000 Pa.

FIG. 1 represents a first embodiment of the device, according to the invention, in which a substrate 1 is displaced along a displacement axis 2 in the direction of the arrow 12 through a tank 3, constituting a vacuum chamber, connected to a pumping group 4. The pumping group 4 keeps a gas at low pressure in the tank 3. A tube 5 passes through the wall of the tank 3 in order to be able to inject a gas or a gaseous mixture therein, necessary for the cleaning or heating process.

In this particular case, the device is provided with six coaxial solenoids 6 (A, B, C, D, E, and F) which are inside the tank 3 and which make it possible to create a magnetic induction field. B at the center of the solenoids 6, substantially parallel to the axis of displacement 2.

The general profile of the magnetic induction field lines created by the solenoids 6 is indicated by line 8.

Inside each of the solenoids 6 is arranged a counter-electrode 9 of rectangular or cylindrical section so that it surrounds the entire surface of the substrate 1.

In the space between the solenoids 6 and the counter-electrodes 9 is mounted an electrostatic confinement enclosure 11 allowing electrical insulation with respect to the walls of the tank 3. This electrostatic confinement enclosure 11 is electrically isolated from the substrate 1 and from the tank 3 and is therefore kept at floating potential with respect to the potential of the substrate 1 and with respect to the potential of the tank 3. Thus a formation of a discharge on the walls of the tank 3 is avoided and the solenoids 6 are protected against with respect to the material removed from the substrate 1 during cleaning. This enclosure 11 also allows the recovery of the metal torn from the substrate by the bombardment of ions. It is preferably of tubular appearance. A discharge 10 is produced between the substrate 1 and the counter-electrodes

9. The magnetic induction field B, generated by means of the solenoids 6 traversed by electric currents of the same direction, as indicated by the orientation of the arrow 7 which is peφendicular to the plane of Figure 1, is substantially parallel to l 'axis of displacement 2. It follows that the orientation of the induction field B is substantially constant. The intensity of the latter varies along the axis of displacement 2. The arrow 7 represents the current vector seen by one of its ends. The induction field is maximum and has a value B _max at the level of each solenoid and it presents a minimum (B _min ) between two successive solenoids 6 as clearly shown in FIG. 2.

Consequently, the electrons are generally confined between two successive solenoids 6 as long as the escape angle θ remains greater than the critical escape angle θ _cr . The solenoids are close enough to allow the presence of a minimum induction field B _rain , parallel to the axis of displacement 2, not zero. As a result, a magnetron discharge is established around the product to be treated between two successive solenoids. The embodiment of the device, according to the invention, shown in FIG. 3 is different from the previous embodiment in that a discharge is produced between three solenoids 6 traversed by a current flowing successively in the opposite direction. In this case, the magnetic induction field 8 remains substantially parallel to the axis of displacement 2 only at the level of the solenoids 6. Between two successive solenoids, it is transverse to the axis of displacement 2. The field of magnetic induction 8 is therefore maximum, in absolute value, at the center of the solenoids A, B and C. On the other hand, the field parallel to the axis of displacement 2 vanishes quickly between two solenoids in the vicinity of the product as shown in FIG. 4 .

This configuration allows the formation of a magnetic mirror around the product 1 between two successive solenoids as clearly shown in Figure 3 by the concentration of field lines 8 around the product between two successive solenoids.

The plasma 10 is, therefore, also confined in a direction peφendicular to the axis 2 because the intensity of the magnetic induction field in this direction increases due to the inversion of the field produced by 2 successive solenoids. The plasma is therefore confined in an area delimited by two successive solenoids 6 which form, on the one hand, magnetic mirrors peφendicular to the axis of movement, and on the other hand, a magnetic mirror surrounding the substrate 1, the magnetic mirror surrounding the substrate 1 of generally arbitrary section and adapted to the shape of the product, ie rectangular for a strip, cylindrical for a beam or wire, being located between two successive solenoids.

Figure 4 clearly shows the presence of a point for which the magnetic induction field B parallel to the axis of displacement is canceled out to reverse on axis 2. The embodiment of the device, according to invention, shown in Figure 5 corresponds to the preferred case where three successive solenoids A, B and C are traversed by a current in the same direction and generate a magnetic field in the same direction.

A magnetron discharge around the substrate 1 is produced between two successive solenoids 6. The mirrors A and C, and the third solenoid B make it possible to produce a magnetic induction field parallel to the axis of displacement 2. This configuration is particularly advantageous because it makes it possible to carry out a magnetron discharge around the substrate 1 over a length generally any one under relatively uniform conditions with high plasma density. The use of solenoids A, B and C of rectangular sections makes it possible to treat metal strips, while the use of solenoids of cylindrical sections makes it possible to treat long substrates such as for example wires, beams or tubes.

The electrons of the discharge are confined inside the solenoid B by the magnetic mirrors constituted by the solenoids A and C as long as the exit angle θ is greater than the critical angle θ _cr (see above). The intensity of the component of the magnetic induction field parallel to the axis of displacement 2 is maximum in the middle of the solenoids A and C and minimum in the middle of the solenoid B as shown in FIG. 6.

Successive assemblies of this type can be arranged one behind the other by making each mirror (solenoid A or C) follow a solenoid of type B allowing the magnetron discharge according to a sequence of the type: ABABABA.

The configuration of the device, according to the invention, represented in FIG. 7 is a configuration of the magnetic bottle type no longer produced by means of solenoids through which an electric current flowing in the same direction as shown in FIG. 'permanent magnets 13 magnetized in a direction parallel to the direction of movement 2 of the product. Three permanent magnets 13 are mounted in the direction of movement 2. The north pole of one of the magnets is arranged opposite the south pole of the neighboring magnet. This has the consequence that between two neighboring magnets, the magnetic field is oriented in the same direction. It is substantially parallel to the axis of displacement 2 and has a minimum value in the middle of the zone between two successive magnets as shown in FIG. 8. The secondary electrons of the discharge 10 are reflected by the mirrors that constitute the magnetic poles of the magnets or the magnetic field is maximum and equal to B _max . The discharge is confined between each pair of successive magnets. A magnetron discharge forms around the substrate in each zone located between two successive magnets. Of course, the return of the magnetic field requires an inversion of the direction of the induction field in the free passage of the permanent magnets allowing the product to pass through. This configuration is particularly advantageous in the case of the treatment of metallic wires.

Another embodiment of the device, according to the invention, shown in Figure 9, differs from that of Figure 7 in that the three permanent magnets 13 are mounted with the north pole of one of the magnets oriented facing the north pole of the neighboring magnet and vice versa the south pole opposite the south pole of the neighboring magnet. In this way, a cusp of the parallel component of the induction field in the direction of travel 2 can be obtained, as in the device in FIG. 3. We can clearly see the presence of a point of inversion of the sign of the component parallel to axis 2 of the induction field between two successive magnets as shown in Figure 10.

The plasma 10 is confined along the axis 2 between the two magnetic surfaces of the same polarity facing each other. Radially, the plasma 10 is confined due to the formation of a magnetic mirror on a closed surface surrounding the product by the concentration of the field lines B away from the axis 2 between two successive magnets.

In the embodiment of the device, according to the invention, of FIG. 11, the discharge is carried out between two magnetic mirrors A and C which allow the axial confinement of the electrons. The plasma is confined around the product by means of a series of permanent magnets 14 arranged around the enclosure electrostatic 11. These magnets allow the electrons and therefore the plasma of this enclosure 11 to be magnetically repelled towards product 1.

Indeed, these permanent magnets 14 constitute a series of magnetic mirrors which force the electrons to return in the direction of the substrate 1. It goes without saying that different arrangements of the magnets are possible, such as those described in the literature [J. R. Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2]. This configuration is interesting if we want to avoid the presence of any magnetic field in the plasma confinement zone 10. As shown in Figure 12, the magnetic field is negligible at the level of the axis of movement 2 between the mirrors magnetic A and C. This configuration also makes it possible to produce plasmas 10 in areas of large volumes.

The configuration of the device shown in Figure 13 is identical to the previous one except for the means for generating the magnetic field on the surface of the enclosure 11. In fact, in this case, the permanent magnets are replaced by a series of solenoids 15 traversed by currents successively in opposite directions.

FIG. 14 indicates that the variation of the magnetic induction field of the device of FIG. 13 is similar to that of FIG. 12.

FIG. 15 represents the discharge and the behavior of the electrons in the treatment zone located between two magnetic mirrors where the magnetic induction field B is equal to B _mjn . The magnetic mirrors are arranged in planes peφendicular to the axis of movement 2 of the substrate 1. The latter is negatively polarized with respect to the counter-electrode 9.

We distinguish the formation of a cathode sheath 16 on the surface of the substrate 1. In this sheath 16, the electric field E makes it possible to accelerate the ions towards the surface of the substrate. During the impact of the latter, the secondary electrons emitted are accelerated towards the plasma by the same field and it follows that the peφendicular component v _perp , relative to the direction of the B field, their speed is on average much more higher than the parallel component v _para of this same speed. The confinement angle θ is therefore greater than the critical confinement angle θ _cr as long as a significant part of their energy has not been lost in inelastic collisions. As a result, these electrons are reflected in the discharge along trajectories 17, as long as this condition is satisfied. The counter-electrode 9 allows the return of electrons which have lost their kinetic energy, and which are therefore thermalized, towards the generator not shown in the figure.

The counter electrode can function as an anode in direct current, or successively as a cathode and as an anode in alternating current. To avoid, in this case, that the latter is not attacked by the ions when the polarization is negative relative to the product, several means can be implemented. In a first means, the anodes are placed only at the level of the magnetic mirrors, that is to say in an area where the plasma density is normally lower than between two mirrors because the electrons are repelled there.

A second means consists in reducing the time during which the counter-electrode is negatively polarized compared to the time during which it is positively polarized. Indeed, the negative polarization time of the substrate can be much longer than the positive polarization time because the electrons move much faster than the ions due to their much lower mass. Thus, it is possible to use generators with pulsed currents or alternatively generators with simply rectified currents (not smoothed). The advantage of using an alternating current discharge is the marked reduction in the probability of the appearance of "unipolar" arcs on the surface treated by electrical neutralization of the charges forming on the surface of dielectric impurities such as greases and oxides always present on the surface of the product to be treated. These charges form naturally during the bombardment of ions and are neutralized by the flow of electrons when the product is positively polarized with respect to the counter-electrode.

Typically an alternating current is used with a frequency between 10 kHz and 100 kHz.

In direct current the position of the anode is generally indifferent. Preferably, the substrate is maintained at the potential of the mass of the installation. FIG. 16 represents a sectional view in a transverse direction peφendicular to the direction of displacement 2. The magnetic field B is therefore peφendicular with respect to the plane of the sheet. This figure illustrates how the electrons move around the substrate 1 along a trajectory 17 under the joint influence of the magnetic field and the cladding electric field 16. A magnetron discharge therefore forms around the substrate as illustrated in FIG. 16. It As a result, the process naturally adapts to any variation in thickness (th) and width (w) of the substrate 1.

The electrons rotate around the field lines while the ions bombard the substrate 1. The trajectories 17 of the electrons are due to the peculiarity of the direction of the magnetic induction field with the direction of the cladding electric field which is always oriented peφendicular to the surface of the substrate.

All the magnetic confinement devices can indifferently be located inside (as illustrated in the different figures) or outside the tank 3. In the latter case, the tank 3 must be made of a non-ferromagnetic material such than stainless steel or aluminum to allow penetration of the magnetic induction field inside the tank 3.

For the same reason, the confinement enclosure 11 must also be made of a non-ferromagnetic material.

Examples of practical applications

1. Cleaning of hot-rolled mild steel sheets

A device for cleaning hot-rolled mild steel sheets has been produced according to the configuration presented in FIG. 5.

In this device, the three solenoids A, B and C of cross sections peφendicular to the axis of movement 2, of rectangular shapes are of the same dimensions. The length of each solenoid is 400 mm, while the average dimension of the rectangular section is 400 mm by 1000 mm. Each solenoid is formed by winding copper wire.

The two external solenoids constituting the magnetic mirrors A and C consume an electrical power of 2,240 kW each. The central solenoid is supplied by a direct current of 0.7 A at 800 V, or an electrical power consumed of 560 W. The total electrical power consumed to generate the induction field B is therefore approximately 5 kW. The three solenoids are placed in sealed carcasses permeable to the magnetic field surrounding the product inside the tank 3.

This configuration provides a maximum field B _max of 5.10 ^"

9 9

T and a minimum induction field B _min equal to 2.5 10 ^" T in the central zone where a magnetron discharge is produced around the substrate in argon maintained at a pressure of 0.5 Pa (5 mbar).

Under these conditions, the voltage between anode and cathode is of the order of 500 V for an electrical current of 304 A, or an electrical power consumed of 152 kW in the discharge.

This results in an average power dissipated on the surface of the substrate, constituting the cathode of the system, by bombardment of argon ions of the order of 19

9 1 "3

W / cm, which corresponds to an electronic density in the plasma of 1.14x10 cm ^" .

The critical angle of escape of the electrons θ _{cr is} worth 45 ° under these conditions because sinθ _cr = (250/500) ¹⁷² .

The secondary electrons accelerated in the cladding are therefore largely confined by the magnetic mirrors before inelastic collisions, the escape angle θ just after acceleration being worth 84 °, which is greater than 45 °.

We can also show that under these conditions 71% of the "thermalized" electrons are confined between the two magnetic mirrors A and C.

The return of current to the counter-electrodes placed at the level of mirrors A and C is therefore due to electrons whose escape angle is less than 45 ° which corresponds to 29% of the population of "thermalized" electrons. The radius of gyration of the secondary electrons accelerated under 500 V is 3 mm and therefore exceeds the value of the thickness of the cathode cladding which is of the order of 0.5 mm at the surface of the substrate. The ions bombard the surface of the substrate after acceleration in the sheath, and without undergoing a collision during this acceleration, almost at normal incidence.

Four devices of the previous type allowed sufficient cleaning of the sheet at a speed of 100 m / min to ensure excellent adhesion to a deposit of thickness of 7 μm of zinc produced by "ion plating" on the two faces of a sheet. a millimeter thick, the temperature of which did not exceed one hundred degrees centigrade after treatment.

The sheet was kept grounded while the counter-electrodes were supplied with pulsed current at 40 kHz. These formed the anodes of the system during most of the voltage cycle. During this anode phase of the counter-electrodes, the surface of the sheet was bombarded with argon ions, while the short cathode phase of the counter-electrodes made it possible to neutralize the positive charges on the surface of the sheet and thus ensured a discharge. without electric arc. The same device made it possible to treat sheets of smaller widths and different thicknesses without any modification of the device, except for the adaptation of the electrical powers consumed. A series of parallel wires or strips could moreover be treated without any other modification of the cleaning process than an adaptation of the electric power according to the product treated.

2. Heating of a cold rolled steel sheet in a vacuum annealing process with cold plasma.

The same device as that described in the previous point comprising four units of three successive solenoids, each provided with anodes in the middle of the magnetic mirrors, made it possible to heat a sheet metal 1 m wide and 0.18 mm thick to a temperature of 600 ° C. at a speed of 100 m / min, before cooling at low pressure on metal cooling rollers, themselves cooled with water. To avoid excessive erosion of the sheet by ion bombardment, the argon pressure was maintained at 10 Pa. Each of the four heating units consumed a power of 180 kW, which corresponded at a total power of 720 kW. The power density dissipated in the treatment area was 22.5 W / cm ² . The four heating units being distributed over 8 m, the average temperature rise rate is estimated at 125 ° C / s. By modifying the number of heating units and the power consumed by the heating units, it was possible to modify both the value of the temperature to be reached and the rate of temperature rise. The same device made it possible to process sheets of smaller widths and different thicknesses without any modification to the device except the adaptation of the electrical powers consumed.

3. Cleaning of a mild steel wire before galvanizing

A mild steel wire was cleaned using a device such as that shown in Figure 7. The wire passed through eleven processing units each bounded by a pair of permanent NeFeB magnets. The useful length of each treatment area was 10 cm, or 1.1 m in total useful treatment length. For a steel wire with a diameter of 5 mm, traveling with a speed of 300 m / min, effective cleaning before zinc plating by "ion plating" was done at an argon pressure of 0.5 Pa using a electrical power of the order of 15 kW. The average power density in this case was 90 W / cm ² and the temperature rise of the wire remained below 40 K.

As illustrated in the description, the present invention has in all cases the main advantage of allowing the obtaining of a magnetically confined discharge around the outer surface of a metallic substrate in the process thus allowing continuous treatment of its entire outer surface. , whatever the shape or the shape variation of this substrate during the process. Only the power adaptation must be carried out.

In a particularly advantageous configuration of the method according to the invention, the presence of an induction field parallel to the product allows, in addition to confining the discharge around the product, the realization of a high density magnetron discharge around this latest. In this case it is clear that the use of magnetic mirrors is useful to limit plasma losses entering and leaving the treatment area but not essential.

It is understood that the invention is not limited to the various embodiments of the method and the device described above and shown in the appended figures, but that many variants can be envisaged with regard in particular to the construction and the shape of the tank 3, the position and shape of the solenoids or permanent magnets, and the number of magnetic mirrors without departing from the scope of this invention.

Claims

1. A method of treatment, in particular of cleaning and / or heating, of a metal substrate (1) traveling in a substantially continuous manner in a vacuum chamber (3) having a treatment zone in which an electric discharge (10 ), i.e. a plasma and a magnetic field are produced in a gas maintained at a pressure below atmospheric pressure between at least the substrate (1), forming an electrode, and at least a counter-electrode (9) so that the substrate (1 ) can be bombarded by ions produced in the electrical discharge (10), characterized in that a magnetic confinement induction field is formed all around the substrate (1) in the treatment zone so that the electrical discharge ( 10) is also confined all around the substrate (1) in this treatment zone by the confinement of electrons released in the electric discharge (10).

2. Method according to claim 1, characterized in that one carries out in the aforementioned treatment zone a magnetic induction field substantially parallel to the axis of travel (2) of the substrate (1) in this treatment zone d in such a way as to thus allow the formation of a magnetron discharge around the substrate (1) by a circulation of electrons from the discharge along paths (17) extending around the substrate (1).

3. Method according to either of Claims 1 and 2, characterized in that a magnetic induction field is produced, the component of which is parallel to the traveling axis (2) is at least equal to 10 ^"3 T (10 Gauss) and preferably between 10 ^{" 3} T (10 Gauss) and 0.25 T (2500 Gauss) in the aforementioned treatment zone.

4. Method according to claim 1, characterized in that one realizes, in the treatment zone, at least one magnetic mirror all around the substrate.

5. Method according to claim 4, characterized in that a magnetic induction field substantially transverse to the axis of travel (2) of the substrate (1) is generated whose intensity increases by at least a factor of two since the substrate (1) to the magnetic mirror.

6. Method according to any one of claims 1 to 5, characterized in that at least two magnetic mirrors are produced which are traversed by the substrate (1) and which define the entry and exit of the treatment zone , in order to confine the electrical discharge in a direction substantially parallel to the axis of travel of the substrate (1) in the treatment zone.

7. Method according to claim 6, characterized in that a magnetic induction field is formed by said magnetic mirrors which is substantially parallel to the axis of travel of the substrate (1) whose intensity decreases away from each mirror toward the center of the treatment area.

8. Method according to any one of claims 6 and 7, characterized in that a magnetic induction field is produced in the middle of the magnetic mirrors greater than any value of the magnetic induction field in a direction substantially parallel to the the scroll axis in the treatment zone, and preferably at least equal to twice the value of the minimum magnetic induction field in the treatment zone in the vicinity of the substrate (1).

9. Method according to any one of claims 6 to 8, characterized in that the maximum induction field produced by a magnetic mirror is at least equal to 5.10 ^"3 T (50 Gauss).

10. Method according to any one of claims 1 to 9, characterized in that a direct voltage is applied between the substrate (1) forming an electrode and the counter-electrode (9) so that the substrate ( 1) is negatively polarized with respect to this counter-electrode (9).

11. Method according to any one of claims 1 to 9, characterized in that an alternating voltage is applied between the substrate (1) forming an electrode and the counter-electrode (9).

12. Method according to claim 11, characterized in that an alternating voltage is applied at a frequency between 1 kHz and 1 MHz and preferably between 10 kHz and 100 kHz between the substrate (1) and the counter electrode ( 9).

13. Method according to either of claims 11 and 12, characterized in that the counter-electrode (9) is polarized negatively with respect to the substrate (1) for a shorter time than the time during which it is positively polarized.

14. Method according to any one of claims 1 to 13, characterized in that, to heat the substrate (1), an electrical discharge is produced in a gas maintained at a pressure greater than or equal to 1 Pa (0.01 mbar ) and preferably between 10 Pa (0.1 mbar) and 1000 Pa (10 mbar).

15. Method according to any one of claims 1 to 13, characterized in that, to clean the substrate (1) by a bombardment of positive ions, an electric discharge is generated in a gas maintained at a pressure below 1 Pa (0.01 mbar) and preferably between 1 Pa (0.01 mbar) and 0.01 Pa (10 ^" ⁴ mbar) in the aforementioned treatment zone.

16. Method according to any one of claims 1 to 15, characterized in that one carries out the electrical discharge in a rare gas, such as argon or helium and / or a molecular gas such as of hydrogen, nitrogen, oxygen and / or hydrocarbon compounds.

17. Method according to any one of claims 1 to 16, characterized in that an electric discharge is produced between the counter electrode (9) and the substrate (1) under a maximum potential difference maintained between the counter electrode (9) and the substrate (1) less than or equal to 1000 V for average power densities per unit of external surface area of substrate (1) between 1 Wcm ^"2 and 200 Wcm ^{" 2} .

18. Device for processing a metal substrate, in particular for implementing the method according to any one of claims 1 to 17, comprising a vacuum chamber (3) in which a metal substrate (1) can pass in a substantially continuous manner, a treatment zone, in which an electrical discharge is created, magnetic means for confining the electrons produced in the electrical discharge, and at least one counter-electrode (9) arranged opposite the substrate (1 ) forming an electrode for creating this electrical discharge, characterized in that the magnetic confinement means are arranged in such a manner with respect to the substrate (1) so that the ions produced in the discharge can be maintained all around the substrate (1) and thus bombard the surface of the latter.

19. Device according to claim 18, characterized in that at least one magnetic mirror is arranged around the treatment zone in which the substrate (1) runs, in such a way as to reflect and return towards the substrate (1) the electrons moving away from the latter and, consequently, to confine the electric discharge around the substrate (1) in this zone.

20. Device according to claim 19, characterized in that said magnetic mirror is produced by a juxtaposition of at least two successive solenoids (6) which can be crossed by the substrate (1) and arranged along the axis of travel of the substrate ( 1), traversed by electric currents successively in opposite directions.

21. Device according to claim 19, characterized in that said magnetic mirror is produced by at least two successive magnets magnetized in an opposite direction in the direction of travel of the substrate (1) and having an opening which can be traversed by the substrate (1 ).

22. Device according to claim 19, characterized in that said magnetic mirror is produced by a series of magnets arranged around the treatment zone one next to the other in such a way as to be able to reflect the electrons towards the substrate, and magnetized in a direction transverse to the direction of travel of the substrate (1).

23. Device according to either of claims 21 and 22, characterized in that the magnets consist of an assembly of magnets magnetized in the same direction and in the same direction.

24. Device according to any one of claims 18 to 23, characterized in that it comprises, around the treatment zone, at least one solenoid (6) with an axis substantially parallel to the axis of travel of the substrate ( 1).

25. Device according to any one of claims 18 to 24, characterized in that it comprises at least two magnetic mirrors delimiting the treatment zone in the direction of travel of the substrate (1).

26. Device according to claim 25, characterized in that it comprises at least two successive magnets magnetized in the same direction in the direction of travel of the substrate (1) and having an opening which can be traversed by this substrate (1).

27. Device according to claim 25, characterized in that it comprises at least two successive magnets magnetized in an opposite direction in the direction of travel of the substrate (1) and having an opening which can be traversed by the substrate (1).

28. Device according to claim 25, characterized in that it comprises, at least two solenoids (6) capable of creating a magnetic induction field in the direction of travel of the substrate (1), and being arranged around the axis scrolling of this substrate (1) so that it can be crossed by the last.

29. Device according to any one of claims 18 to 28, characterized in that said counter-electrode (9) is disposed substantially in the vicinity of the magnetic mirrors which can be traversed by the substrate (1).

30. Device according to any one of claims 18 to 29, characterized in that the vacuum chamber (3) comprises a confinement enclosure (11) delimited by walls of non-ferromagnetic material which is electrically isolated from the substrate (1 ) and the counter electrode (9).

31. Device according to claim 30, characterized in that the magnetic confinement means are arranged outside the confinement enclosure (11) between the outer wall of the latter and the inner wall of the vacuum chamber (3 ).

32. Device according to any one of claims 18 to 30, characterized in that the magnetic confinement means are located outside the vacuum chamber, the walls of which are then made of non-ferromagnetic materials.