Patent Application
20240222088
The invention concerns a device for plasma treatment of substrates. The device comprises a vacuum chamber with a confinement enclosure having a treatment zone and presenting a treatment direction along which substrates to be treated are to be moved through the treatment zone. This treatment zone is delimited in a direction transversal to the treatment direction by lateral walls of the confinement enclosure. At least one of these lateral walls has an opening to allow plasma to enter the treatment zone. The opening connects to a plasma source that is external to the treatment zone such that plasma generated by the plasma source can enter the treatment zone through this opening.
The invention relates, in general to a method and a device for plasma treatment of substrates, in particular cleaning, heating, or coating by plasma enhanced vapor deposition (PECVD). The substrates to be treated by the device or the method of the invention, are, for example, metal products in the form of strips, sheets, wires, girders, plates or profiles presenting all types of cross-sections, etc. The substrates or products can also comprise parts disposed on supports or carried by, for example, metal hooks or baskets. The substrates are transported through the treatment area by means of a transport system such as, for example, a bank of rollers, a conveyor or a monorail transport system.
According to the present invention one or several plasma sources are combined with a magnetic multipolar plasma confinement that is provided at lateral walls of the confinement enclosure. These lateral walls are preferably substantially parallel to the treatment direction and do not cross that direction.
Such a design provides two important advantages compared to other possible combinations: i) a better plasma uniformity in the treatment zone along said treatment direction allowing a more uniform treatment of products or substrates moving along the treatment direction, ii) a faster decay of the plasma density at the extremities of the plasma confinement area along the treatment direction, allowing the suppression of parasitic discharge outside the plasma confinement area. Parasitic discharges outside of the plasma confinement area are a serious issue in plasma processes where the running product is at the same electric potential as the internal walls of the vacuum chamber, more particularly at ground potential, when an anode is used in the plasma confinement area. This is specifically the case in product etching or PECVD coating with a bias polarization of the product against an anode.
The combination of multipolar magnetic cusps (MMC) and plasma sources, in particular inductive sources (see for example: U.S. Pat. No. 8,436,318 describing an ion source), is well known to those skilled in the art as well as the effect of the MMC to make the discharge profile more uniform (see for example: J. Hopwood, C. R. Guarnieri, S. J. Whitehair, J. J. Cuomo, “Langmuir probe measurements of a radio frequency induction plasma”, J. Vac. Sci. Technol. A11(1), January/February (1993)152). The MMC assembly always surrounds completely the aperture of the plasma source in order to obtain a uniform plasma density at the exit of the MMC confinement zone where the ion grid optic is secured in the case of ion sources or where lays the substrate in case of plasma sources. That configuration is generally used to treat a static substrate laying immobile under the apparatus.
It is also known to those skilled in the art to combine panels with MMC and electron sources as plasma generator (see for example: K. N. Leung, T. K. Samec, A. Lamm, “Optimization of permanent magnet plasma confinement”, Physics Letters, 51A (1975) 490).
The present invention is an improvement of methods and devices for plasma treatment, as disclosed by the prior art, in which the substrate moving through the treatment area is negatively polarized continuously or discontinuously, preferentially with a periodic signal, against one or several facing anodes arranged in this treatment area. For instance, U.S. Pat. No. 6,933,460 describes a plasma treatment method for a substrate crossing the treatment area in which the plasma is confined by magnetic mirrors. Document U.S. Pat. No. 8,835,797 discloses a plasma treatment method for a substrate crossing the treatment area in which the plasma is generated by inductive coupling through several antennas, or Faraday shielding, protecting the inductor from contamination and being the anode of the system.
These plasma treatment methods have in common that the substrate to be treated is moved in a given direction through a vacuum chamber presenting a treatment area in which an electrical discharge is created between one or several electrodes and the substrate, close to the surface of the latter.
Typical applications that are targeted by these plasma treatment methods of negatively polarized substrates with respect to one or several anodes are:
In general, the above methods can be applied to any substrate that is sufficiently conductive and, therefore, these can be applied to any substrate made from low alloy steel, stainless steel, aluminum, copper and other metals, but also to electrically conductive substrates coated with a thin electrically insulating layer. However, these methods have been developed essentially for industrial applications aimed to the pre-treatment of metal strips before coating by a vacuum deposition method or before PECVD treatment of the metal strips in a high vacuum coating plant.
Referring to the prior art cited above, if the substrate is kept at ground potential, which is always the case for the continuous treatment of metal products such as strips, sheets, wires or cords, the presence of an anode in a grounded vacuum chamber, the latter is always grounded for safety reasons, is generally subject to a parasitic plasma because the walls of the vacuum vessel constitute electrostatic reflectors for the discharge electrons, i.e. a hollow cathode allowing plasma formation.
The drawback of that is that the walls are subject to ion bombardment which means internal walls erosion and heating as well as important loss of electrical power and the contamination of the product by sputtered material from the vacuum vessel's walls.
The solution that is generally adopted to overcome this drawback is to electrically insulate or shield, with metallic panels secured onto electrical isolators, the internal walls of the vacuum chamber. However, this leads to a higher complexity in the vacuum chamber design because the feedthroughs must also cross the electrically insulated layers or shielding panels and the vacuum pumping ports must be electrically insulated or shielded as well, nevertheless allowing high vacuum pumping speed.
A second drawback of the prior art solved by the invention, in particular for the plasma treatment of continuously running products in the plasma treatment area, such as strips or wires for example, is the impossibility of removing the plasma treatment equipment for maintenance without prior removal of the product. That is clearly the case for the device of U.S. Pat. No. 6,933,460 for which the magnets used for the magnetic mirrors surround completely the product and for the apparatus of U.S. Pat. No. 8,835,797 for which the Faraday shielding and the inductor surround also the product during treatment.
A third drawback of the prior art, for which the invention brings a solution, is the problem of an uneven treatment along the transversal direction in the treatment area. This is particularly important when the product to be treated extends over a larger distance in a direction transversal to the moving direction than in the latter direction, for example when treating several products simultaneous like parallel wires, a wide strip or plates transported into the treatment area by a conveyor. That is particularly the case when, for reducing the possibility of arcing, a plasma treatment configuration such as described in U.S. Pat. No. 8,835,797 is used. Indeed, in that case, the plasma is generated by inductive coupling with an inductor surrounding the product and protected by a polarized Faraday shielding. It is well known that in such case, where the plasma is generated by volume ionization of the gas by primary electrons energized by inductive coupling and where plasma recombination arises essentially on surfaces, the plasma density is uneven (bell shaped with a maximum at the center of the discharge). This is, in particular, noticed along directions transversal to the running direction of the substrates, such as along the width and the height of the cross section of the treatment area. Hence, plasma treatment of the substrate, that is depending on the local plasma density, is uneven along the direction of the width of the plasma treatment area and presents a maximum in the middle of that area, i.e. at the position of the central axis following the direction travelled by the running product. Accordingly, the dose received by the product after plasma treatment varies along its width transversely to the running direction (or alternatively its height depending on the shapes of the product cross section and treatment area cross section).
The relative dose received by the product typically can vary from a ratio of one at the edges of the product to a ratio of two at its central axis along the direction travelled by the running product. It goes without saying that such a variation of the thickness of a coating across the product width is unacceptable, in particular in steel strip coating or in glass coating, because, in most cases, a maximum variation of thickness of ±5% is accepted.
Similarly, according to this prior art, in an etching or a heating process applied to a band or strip, the temperature increase in the middle of the product can be twice the temperature increase at its edge, which is unacceptable for mechanical and metallurgical reasons.
Another drawback of the prior art is that when the product width is much smaller than the width of the treatment area and the product is moving along that transverse direction when running through the treatment area, the plasma density profile varies in time because that profile is dependent on the temporal position and width of the product in the area. The same is true when the width of the strip changes along its length (frequent in treatment of steel strip). Indeed, the product area in the treatment area constitutes a significant area for charges recombination.
A further drawback of the apparatus disclosed by U.S. Pat. No. 8,835,797 is the contamination of the antennas that face directly the product treated when applied for etching or during PECVD. In particular, during low alloy steel etching, the contamination of the antennas, or Faraday shielding, by sputtered iron will decrease the efficiency of the process. As more energy is dissipated in heating the iron contamination layer the energy available for plasma generation decreases accordingly. Even more critical for the sustainability of the process: the contamination of the spacing between nearby antennas can lead to the process shutdown.
The invention aims to overcome the aforementioned drawbacks of the prior art by proposing a method and a device that permit to improve the uniformity of the plasma density along the treatment direction, i.e. along the moving direction of the substrates through the treatment zone. It is also the intention of the invention to extend the length of the treatment area along the treatment direction such that this length becomes larger than the diameter of plasma sources that are positioned in front of the moving substrate, i.e. product, in the treatment zone. Accordingly, a given dose (energy density or coating thickness for example) can be obtained in a longer process area than just the length defined by the plasma source aperture in front of the running substrate or product, whereby substantially the same working parameters are maintained at the plasma source(s).
Further, it is an object of the invention to control the plasma distribution in a direction that is transversal to the treatment direction.
Another object of the invention is to propose a method and equipment configuration for allowing plasma treatment of grounded substrates in a grounded vacuum vessel with unprotected internal walls in presence of an anode or multiple anodes, wherein any possibility of parasitic plasma in the vacuum vessel volume outside of the treatment area is prevented and wherein extremely high vacuum pumping speed of the treatment area is still allowed. This is particularly important when outgassing of the running product, which is to be treated, becomes important. This is, for example, the case for water vapor outgassing during plasma treatment of a product running at high speed, such as a strip or a foil.
Accordingly, the invention aims to provide:
The method and the device, according to the invention, can be applied on conductive substrates but also on electrically insulating substrates, such as glass plates and plastic webs, with some particular adaptations to the insulating materials.
In general, the device for plasma-treatment of substrates, according to the invention, concerns a device comprising a vacuum chamber with a plasma confinement enclosure having a treatment zone and presenting a treatment direction along which substrates to be treated are to be moved through the treatment zone. The treatment zone is delimited, in a direction transversal to the treatment direction, by lateral walls of the plasma confinement enclosure, wherein at least one of these lateral walls has an opening for allowing plasma to enter the treatment zone. This opening connects to a plasma source such that plasma generated by the plasma source can enter the treatment zone through this opening.
According to the invention, at least said wall presenting the opening for allowing plasma to enter, presents means for generating a multipolar magnetic cusp field extending along this wall around and adjacent to the opening for forming a plasma diffuser panel.
In an interesting manner, the means for generating said multipolar magnetic cusp field extend along said treatment direction over a distance substantially corresponding to the length of said treatment zone along the treatment direction.
Advantageously, the means for generating said multipolar magnetic cusp field comprise an assembly of permanent magnets or of electromagnets covering the exterior side of said wall with respect to the treatment zone.
According to a preferred embodiment of the device, said confinement enclosure has at least two opposite walls, preferably, extending parallel to said treatment direction on opposite sides of the treatment zone, each of these opposite walls forming a plasma diffuser panel provided with an assembly of permanent magnets or of electromagnets covering the exterior side of said wall with respect to the treatment zone. Each diffuser panel further presenting said opening for allowing plasma to enter the treatment zone, wherein adjacent to said opening are provided said assembly of permanent magnets or of electromagnets.
According to an interesting embodiment, of the device, according to the invention, at least one electron source is provided in the treatment zone for controlling the density of the plasma in the treatment zone.
According to a specific embodiment of the invention, at least one plasma current measurement probe is provided in the treatment zone cooperating with said at least one electron source to control plasma density along a direction transverse to said treatment direction.
According to a further embodiment of the invention, said confinement enclosure has an entry defined by an entry aperture and an exit defined by an exit aperture for said substrates, each of said apertures opening into a corresponding tubular passage extending along said treatment direction on opposite sides of the treatment zone, wherein said tubular passage is electrically isolated such as to be maintained at floating potential.
The invention also relates to a method for plasma treating substrates wherein the substrates are moved along a treatment direction through a treatment zone that is delimited in a direction transversal to said treatment direction by at least one wall forming a diffuser panel presenting an opening. Said plasma is introduced into the treatment zone trough said opening and the plasma is generated by means of a plasma source connecting to said opening. The method is characterized in that a multipolar magnetic cusp field extending along this wall around said opening and adjacent to said opening is generated such that said plasma entering the treatment zone trough said opening is distributed along said wall in the treatment zone.
Further details and advantages of the invention shall be given in the description below of some specific embodiments of the device and method, according to the invention. This description is only given as an example and does not restrict the scope of the protection claimed; the reference numerals used below pertain to the appended figures.
In the different figures, the same reference figures refer to identical or analogous elements.
The invention concerns in general a method and a device for generating a uniform plasma density profile across the transverse direction of a running product to be treated.
An interesting embodiment of the device, according to the invention, is shown in
The inside of the confined plasma treatment module 2 defines a treatment zone that is delimited by walls of the module 2. Accordingly, when plasma treating a product or a substrate, such as a metallic strip, the product enters the treatment zone through the entry opening 3 and, after having traversed the treatment zone, leaves this zone through the exit opening 4 while moving along the treatment or transport direction 5. The treatment zone is also called treatment area or plasma confinement enclosure, confinement area or confinement zone throughout this specification.
Opposite sides of the vacuum chamber 1 have an entry aperture 3′ and an exit aperture 4′ in front of the respective entry opening 3 or exit opening 4 of the plasma treatment module 2. The vacuum chamber 1 is connected at the entry aperture 3′ and the exit aperture 4′ to respective roller devices 6 via connecting bellows 7. Accordingly, a roller device 6 is provided upstream and downstream of the vacuum chamber 1. Both roller devices 6 are equipped with rollers 11 for the metallic strip that assure the electrical grounding of the strip and that support the latter.
The plasma treatment module 2 of the device represented in
Each subassembly 2a and 2b presents a corresponding plasma source 8. These plasma sources 8 are each secured to a corresponding cover flange 9. These cover flanges 9 of the subassemblies 2a and 2b constitute opposite walls of the vacuum chamber 1 (see
The inside of the confined plasma treatment module 2 communicates via pumping ports 10 with the interior of vacuum chamber 1. Vacuum pumping of the inside area of the confined plasma treatment module 2 is thus achieved through these ports 10. In the embodiment of the invention represented in the drawings, a pumping port is provided immediately above and beneath each entry opening 3 and exit opening 4.
The inside structure of a subassembly 2a or 2b of the confined plasma treatment module 2, adapted for plasma treatment of a running strip, is represented in
Each of the vacuum pumping ports 10 is formed by an aperture that is covered by an internal metallic mesh or plasma screen 10a and an external metallic mesh or plasma screen 10b. The internal and external metallic mesh or plasma screen 10a and 10b preferably extend parallel to each other, whereby the internal mesh or screen 10a extends at the side of the interior of the concerned subassembly 2a or 2b and the external mesh or screen 10b is directed to the exterior side of the subassembly 2a or 2b. The distance between the internal and external metallic mesh or plasma screen 10a and 10b is, preferably, larger than 5 mm. The pumping ports 10 are preferably, but not necessarily, situated close to or adjacent to the entry and exit openings 3 and 4 of the plasma confinement enclosure 15.
The subassemblies 2a and 2b have metallic internal walls 13 that are polarized against the substrate or product 21, in particular positively, such that the walls 13 form an anode with respect to the product 21. The walls 13 are electrically connected to the internal metallic mesh or plasma screen 10a of ports 10, whereas the external metallic mesh or plasma screens 10b are left at floating potential or are connected to the potential of the vacuum chamber 1, which is generally grounded.
The hot filament thermionic emitter 17 extends in front of each of the internal walls 13 and parallel to these walls 13. Each emitter 17 is supported by two electrical feedthroughs 18 that extend through the walls 13 and that are connected to respective connectors 19 fixed to the outside of the plasma treatment module 2. In the embodiment of the invention represented in the drawings, the emitters 17 are parallel to the transport direction 5 and extend at the lateral sides of the treatment zone close to the walls 13. However, for a person skilled in the art, it is clear that it is not required that the hot filament emitters 17 extend parallel to the transport direction 5.
The presence of hot filament thermionic emitters 17 allows to regulate the plasma density profile in a direction that is substantially transverse to the treatment direction 5. This regulation of the plasma density profile is realised by injecting electrons in the plasma treatment area at proximity of the multipolar magnetic cusps (MMC) at controlled current and controlled voltage following a feedback signal corresponding to plasma current measurements by probes 20a and 20b. Such probes 20a and 20b are secured at some specific locations in the plasma treatment area and are preferably formed by ion current probes.
Some advantages of combining the electron emitters 17 at proximity of the multipolar magnetic cusps (MMC) with the probes 20a and 20b are:
The internal walls 13, onto which condenses possibly process residues, are electrically insulated from the rest of the equipment by electrically insulating sheets 14. These sheets 14 extend along the external side of the walls 13.
In order to create a magnetic field inside the confined plasma treatment module 2 and adjacent to the walls 13, the latter are covered by multipolar magnetic cusps generated by magnets with alternating polarity directed towards the concerned wall 13. Said insulating sheets 14 are, preferably, provided between the walls 13 and the corresponding magnets. The side of the magnets, opposite to the walls 13, is supported by plates of ferromagnetic yoke or a magnetic induction shield. The walls 13 are made of non-magnetic material such as, for example, copper or aluminum, and are, preferably, water-cooled.
Process gas is introduced into the confined plasma treatment module 2 through the plasma source 8 via a gas nozzle 12 provided to this effect.
Thus, in general, the invention concerns a plasma treatment module 2 comprising one or more plasma sources combined with at least a plasma diffuser panel. Such a plasma diffuser panel presents a temperature controlled metal shield that faces the treatment zone and thus corresponds to said internal wall 13 of the module 2. The diffuser panel, and thus the wall 13, extends along said transport direction 5 of the products to be treated. Preferably, the panel is parallel to this transport direction 5, however, this is not required. It is clear that the panel must not cross the path of the product 21 moving through the treatment zone. Accordingly, the panel, or the corresponding wall 13, delimits the treatment zone along a direction transversal to transport direction 5.
The plasma diffuser panel is covered by a network of elements generating magnetic induction fields such that the surface of said metal shield of the panel, that is facing the treatment zone, is covered with multipolar magnetic cusps (MMC). The metal shield facing the plasma treatment area, i.e. wall 13, is electrically insulated, at the opposite side of the treatment area, from said network of elements generating magnetic induction fields. This electrical insulation is, for example, realised by said insulating sheet 14.
The plasma sources 8 connect to an aperture 23 through which the plasma generated by the concerned source 8 enters the treatment zone. According to the invention, said diffuser panel, in particular said multipolar magnetic cusps, extends adjacent to the aperture 23 of the plasma source 8. Preferably, the multipolar magnetic cusps extend around this aperture 23 such that this aperture 23 forms an opening in the concerned diffuser panel.
According to a preferred embodiment of the device of the invention, said network of elements comprises an assembly of permanent magnets. Accordingly, the multipolar magnetic cusps are generated by this assembly of permanent magnets.
Preferably, the treatment area is closed around the conveying path of the product 21 by said walls 13 such that the plasma treatment area is tubular. Accordingly, the embodiment of the device represented in
In the embodiment represented in
Further, in this embodiment of
These walls 13 are thus enclosing the tubular treatment area and are provided with said magnetic multipolar cusps (MMC) over substantially their entire surface as explained above.
At the entry opening 3 and at the exit opening 4 of the treatment zone, i.e. the plasma confinement enclosure 15, the extremities of the walls 13 connect to transverse walls extending transversal to the transport direction 5 of the product 21. Accordingly, a confined plasma enclosure 15, or plasma diffuser, is formed within the plasma treatment module 2 having only an aperture towards the vacuum chamber at said entry and at exit openings 3 and 4 wherein the edge of this aperture extends at a small distance from surface of the moving product 21.
When the device is adapted for treating only one side of a product 21, such as a represented in
A preferred embodiment of the device, according to the invention, is represented in
The entry opening 3 and the exit opening 4 connect to entry and exit tubular manifolds, in particular tubular passages 3a and 4a, enhancing plasma decay from the treatment area to the vacuum chamber volume. The internal walls of these tubular passages 3a and 4a are extending at a small distance from the product 21 that is moving through the treatment area along the transport direction 5. Accordingly, any electrical conduction by plasma between the treatment area and the volume of the vacuum chamber is prevented to avoid the formation of parasitic plasma in the volume of the vacuum chamber outside of the treatment area. The tubular passages 3a and 4, preferably, extend in the vacuum chamber 1 and outside the plasma confinement enclosure 15.
Preferably, said tubular passages 3a and 4a, or manifolds, are made of electrical insulating materials, or of a combination of metals and electrical insulating elements, such as to be electrically isolated from any electrode or from the ground. Accordingly, these tubular passages 3a and 4a are at floating potential.
According to an interesting embodiment of the invention, said tubular passages 3a and 4a present additional elements, also called closing means, for increasing the area of the passages 3a and 4a, in particular of the internal walls thereof, for enhancing plasma recombination and for decreasing the open area between the treatment area and the vacuum chamber volume. In general, such additional elements, also called closing means, are provided for decreasing the distance transversal to the transport direction 5 between the tubular passage, i.e the internal walls thereof, and the products moving through the passage. These elements are, for example, filling at least partially the passages 3a and 4a, and can possibly be in contact with the running products 21. Such elements, for example, comprise brushes, sliders, rolls, mobile shutters, etc.
According to an advantageous embodiment of the invention, said plasma diffuser panels are polarized continuously or discontinuously, in particular positively, against the substrate. Thus, the plasma diffuser panels, forming said walls 13, are on average positively polarised with respect to the substrate, for example.
As already indicated above, in an interesting way, the plasma diffuser panels, forming walls 13, are electrically connected to the internal mesh or plasma screen 10a of the pumping ports 10.
According to an interesting alternative embodiment of the device, according to the invention, said aperture 23 of the plasma source or sources 8 is covered by a set of parallel grids forming a so-called ion optic of a large beam ion source. In this way an ion source is obtained.
In an interesting way, such an ion source is combined with external electron sources directed towards the ion beam flowing between the source ion optic and the running substrate 21.
The one or more plasma sources 8 of the device, according the invention, are, in general, preferably inductively coupled.
Further, the plasma sources 8 may be protected by Faraday shielding. Such a Faraday shielding is preferably not directly facing to the plasma source aperture 23, i.e. the surface of the running product 21, in order to minimize Faraday shielding's contamination by condensation of process residues.
In general, the device, according to the invention is working at a gas pressure below 5·10−3 mbar. Thus, the gas pressure in the vacuum chamber 1, and consequently in the plasma confinement enclosure 15, is preferably maintained at a pressure below 5·10−3 mbar.
Thus, according to the invention in general, one or several plasma sources 8 for generating a plasma are combined with a magnetic multipolar plasma confinement system. This confinement system comprises multipolar magnetic cusps (MMC) that are provided along walls 13 that are extending substantially parallel to the transport direction 5 of the product 21 and wherein closing panels 22 extending substantially perpendicular to that direction do not present such MMC.
Such a configuration permits to improve the uniformity of the plasma along a direction transversal to the transport direction 5. The panels provided with MMC forming the walls 13, that extend along the transport direction 5, ensure that the plasma is extended along the transport direction 5, thereby improving the uniformity of the plasma density in a direction transverse to this transport direction 5. Thus, extending the areas covered with MMC along the transport direction 5 increases ionisation along this direction and reduces plasma diffusion in transverse direction, hence uniformity of the plasma density in this transverse direction is improved.
Another significant advantage of this configuration is that it permits to extend the length of the treatment area with respect to the length of the apertures 23 communicating with the plasma sources 8. Accordingly, a given dose (energy density or coating thickness for example) can be applied to the product 21 in a longer process area than just the length defined by the aperture 23 of the plasma source 8 in front of the running product while keeping substantially the same working parameters at the plasma source 8.
The advantage is a result from the fact that a distributed plasma in an extended area reduces the average plasma density such that the possibility of arcing is reduced. Further, this increases the surface of the equipment, in particular the walls, recovering the possible contamination generated by the process, for example, sputtered material from the substrate surface during a sputter etch process or deposit of a contamination layer during a PECVD process. This increase in length of the treatment area allows, for given conditions at the source(s) 8, more working time to get a given thickness of contamination onto the walls, i.e. the MMC panels, and the plasma source(s) 8 facing the running product. For example, if the contamination layer is formed by iron from plasma etching of a running low alloy steel product, that contamination layer shunt the magnetic induction field at the MMC increasingly with its thickness and decreases the efficiency of the MMC panels. There is a significant interest to distribute that contamination layer onto surfaces of equipment as widely as possible in order to increase the working time of the device which is generally linked to a maximum thickness allowable for a given contamination layer. The working time is the maximum time during which the device can be operated without interruption for removing contamination layers from the walls.
The absence of any MMC assembly on the closing panels 22 crossing the path of the product imparts two supplementary significant advantages:
Examples of different embodiments of the invention (non-limitative): Operating conditions and particular configurations of the invention for use in plasma treatment of steel strip
Prior to coating a steel strip by physical vapor deposition (PVD) or plasma enhanced chemical vapor deposition (PECVD), in order to enhance the adhesion of the coating, the steel strip surface has to be decontaminated from any contaminating adsorption layers such as water vapor, carbonaceous contaminants but also, in most cases, the native superficial oxide layer has to be removed. That is achieved by generating a plasma of argon maintained at a pressure of typically 10−3 mbar in the confined plasma treatment module 2 installed in the vacuum chamber 1 as shown in
The equipment just described above is specially adapted to etching but also to PECVD when the pressure is lower than about 5·10−3 mbar. That limit of pressure is actually determined by the confinement of electrons by the magnetic field at the MMC panels. At fixed power coupled to the plasma, a decrease in pressure improves the confinement of the electrons and decreases the rate of the process. A pressure of about 10−3 mbar is shown to be the optimum pressure for sufficient trapping of electrons by the magnetic field at the MMC panels in order to achieve enough ionization in the trapping zone, hence generate a uniform transversal plasma treatment, and also assure a maximum rate for that process. In a PECVD process where the goal is to produce a thin coating onto the substrate surface a mixture of reactive gases, instead of argon, is introduced in the confinement plasma treatment module through the gas nozzle 12. Examples of typical coatings that can be produced using the confined plasma treatment module, according to the invention, are:
Reactive etching can be used, for instance etching with a mixture of argon and hydrogen for low alloy steel strip etching prior to deposition. Prior art document EP 0780485 shows that the etching rate of low alloy steel can be increased at a fixed power by mixing the argon with hydrogen in a given proportion.
In order to treat only one face of the strip, only one subassembly equipped with one plasma source and MMC panels is needed, the plasma source facing the face of the strip to be treated as shown in
In order to prevent any parasitic plasma in the volume of the vacuum chamber, outside of the confined plasma treatment area, the subassembly 2b is closed by a closing panel 22. This closing panel 22 is made by the assembly of an internal metallic plate electrically connected to the potential of the metallic internal walls 13, electrically insulated from the electrical potential of the external shielding also metallic. The closing panel 22 is also provided with one half of the entry manifold 3a (not shown) and one half of the exit manifold 4a joining with the corresponding half manifolds secured onto the subassembly 2b. Because of the absence of any plasma source and magnetic confinement in the closing panel 22 and also because of the short distance between the internal surface of the closing panel 22 and the face of the strip 21 opposite to the treatment side, no plasma forms between the strip and the closing panel 22, hence this face of the strip 21 is not plasma treated. The subassembly 2b must generally be closed by a closing panel 22 when the product 21 is polarized (pulsed or DC) negatively against the walls 13.
Without polarization of the metallic internal walls 13, there is no risk of formation of parasitic plasma outside of the confined plasma treatment area. That is typically the case of PECVD onto electrically non-conducting substrates such as running glass plates. In that case the treatment is achieved by a simple subassembly such as shown in
Shaping the subassembly, for instance making it curved from entry to exit of the plasma confinement area, such as to hover at short distance above a bended foil supported onto a roll, allows to treat foil or web. In the case of a plastic web it could be a coating by PECVD, an activation treatment in an oxygen plasma etc.
Closing the subassembly, such as shown in
Prior to physical vapor deposition, in order to promote good adhesion of the coating, it is mandatory to outgas the adsorbed water vapor and sputter-etch the carbonaceous contamination and the oxide layer always present at the surface of the low alloy steel strip. That can be achieved advantageously using a confined plasma etching module as shown in
A strip can be coated by PECVD, either directly after etching, or after coating realized by PVD, using the same configuration of confined plasma treatment module as the one used for etching in example 1. For example, a passivating layer of 2.5 nm of SiO2 can be deposited onto both faces of the strip using one confined plasma treatment module of the same configuration as the one used for etching in example 1. Hot ion current probes 20a and 20b are used to prevent their contamination by SiO2. The typical configuration of the system is the following:
Textured TCO (transparent conducting oxide) surfaces are required in silicon thin film solar cells to improve light trapping efficiency. Ion beam treatment of the glass substrate prior to deposit ZnO:Al films by magnetron sputtering allows to achieve as-grown rough ZnO:Al films and suppress the need for ZnO:Al film etching in HCl after sputtering. Hence, that process offers the significant advantage to avoid to break vacuum between TCO and silicon deposition provided that appropriate interconnection process is available. The pretreatment of the glass surface is achieved using one subassembly of the confined plasma treatment module arranged in ion source as described in above section 5 (‘Large ion source for treatment of running products’). The ion source is equipped with a three grids ion optic closing totally the open aperture of the subassembly module, opposite to the aperture of the plasma source. The internal grid is at the same potential as the anode surface, the external, or accelerating, grid facing the running glass substrate is at ground potential. Both sides of the subassembly, at the extremities of the grounded shield of the two lateral MMC panels, are secured two hollow cathodes emitting electrons towards the transversal direction in the 50 mm of free space between the accelerating grid and the surface of the running substrate. These electrons neutralize the positive electric charge of the ion beam directed towards the glass substrate surface. The typical configuration of the system is the following:
The invention is of course not limited to the geometries of the device described hereabove. The device of the invention may present any possible geometry. For example, the plasma diffuser panels are not to be limited to one as represented in
The shape of the entry and exit openings 3 and 4, or of the tubular passages 3a and 4a, can be different from those represented on the figures, depending on the shape of the product cross-section or, for example, of the number of products treated in parallel. Accordingly, it is possible to provide tubular passages for wires, cords and long products. It is also possible to provide several passages in parallel, one for each product treated in parallel. The plasma diffuser MMC panels have just to be substantially parallel but do not need to be exactly parallel to the transport direction of the product, meaning that these panels may not cross the path of the product, but they may have different shapes than that of a surface parallel to the transport direction. These panels can, for example, simply make an angle with that direction, be curved, or “V shaped” or present a combination of these shapes. The geometry of the plasma source could be different etc.
The invention is not limited to MMC built with permanent magnets but the MMC can also be built by a magnetic field generated by passing a current in a structure of wires or so-called picket fence structure. The electron sources are not limited to hot filaments (HF) thermionic emitters but could be as well constituted by cold thermionic emitters, hollow cathodes or plasma bridges.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21171389.6 | Apr 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/060433 | 4/20/2022 | WO |
