WO2004088711A2

WO2004088711A2 - Method and system for generating a plasma

Info

Publication number: WO2004088711A2
Application number: PCT/NL2004/000220
Authority: WO
Inventors: Marius Pieter De Goeje; Gerardus Titus Van Heck; Johannes Petrus Zijp; Yves Lodewijk Maria Creijghton; Antonius Maria Bernardus Van Mol
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 2003-04-01
Filing date: 2004-04-01
Publication date: 2004-10-14
Anticipated expiration: 2005-10-01
Also published as: WO2004088711A3; NL1023072C2

Abstract

A method and system respectively for generating plasma (8) between electrodes (1, 2) separated by a gaseous or vaporous dielectric, to which a voltage source has been connected. In order to prevent flashover, the electrodes may be provided with a solid dielectric (3, 4). Pollution and/or ageing of the electrode protective layer can successfully be prevented by regularly 'changing' this protective layer by means of a movable protective layer (5, 9) which is moved over or along the respective electrode, through the area in which the plasma is generated. When the plasma is used for treating a film-shaped base material (coating, etching, etc.) this film can at the same time be used as movable electrode shielding.

Description

Title: Method and system for generating a plasma

Field of the invention

The invention relates to respectively a method and system for generating plasma for the purpose of, for instance, processing materials. Herein, plasma is understood to mean: "An electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a phase of matter distinct from solids, liquids, and normal gases".

Background of the invention Generating plasma under atmospheric (air) pressure ("atmospheric plasma") is currently a hot topic because of the potential possibilities. Plasma technique may for instance be used for coating a base material ("plasma deposition") or modifying or otherwise treating the microstructure and/or composition of the base material ("etching"). Plasma treatment and plasma deposition often take place under low pressure. If plasma treatment can take place under atmospheric pressure, this will be very much easier and less expensive because of the absence of vacuum systems. However, a stable plasma is not easy to obtain at atmospheric pressure; various research groups all over the world are conducting research into improvement thereof.

Plasma can be generated between electrodes separated by a gaseous or vaporous dielectric, to which a DC (for instance RF or HF) or AC source has been connected. In order to prevent disruptive discharge of the gaseous dielectric between the electrodes, one or more electrodes can be provided with a covering or protective layer from a solid (non-gaseous) material having a relatively high disruptive voltage. As a result of this covering from solid insulating material, after applying a suitable electric voltage over the electrodes, a plasma will be created between the electrodes without disruptive discharge of the gas taking place between the electrodes.

However, a problem occurring in practice is that, with the passage of time, the solid dielectric by which the electrodes are covered ages and/or becomes polluted by inter alia the generated plasma and the substances present therein. As a result of this degradation and/or pollution of the covering material, at a certain moment, disruptive discharge of the gaseous dielectric between the electrodes will yet again take place, for instance initiated by disruptive discharge of the degraded solid dielectric or by flashover along the polluted surface of this solid dielectric.

Summary of the invention

The invention is based on the insight that the effects of pollution and/or ageing of the electrode protective layer can successfully be prevented by regularly "changing" this protective layer by moving this protective layer, such that the protective layer is never exposed to the influences of the plasma and/or the electric field between the electrodes and/or the gaseous or vaporous dielectric for so long that a disastrous pollution and/or degradation of the protective layer is the result.

Preferably, use is made of a protective film having good dielectric properties which is moved over or along the respective electrode, through the area in which the plasma is being generated.

The protective layer may be moved either continuously or discontinuously .

When the plasma is used to treat a film-shaped base material (coating, etching, etc.), this film itself can at the same time be used as (movable) electrode shield. In other cases, a separate (inexpensive) shielding film can be used from, for instance, polyethylene or polypropylene. The movable film can serve to protect the solid electrode protective layer (covering) but may also wholly or partly replace this protective layer, if desired. In that case, depending on the configuration, certain requirements need to be imposed on the dielectric properties of the movable layer with regard to layer thickness and dielectric material properties, particularly the disruptive voltage.

Systems in which — in accordance with the method according to the invention — a protective layer is moved over or along the respective electrode can be implemented indifferent manners, which will be discussed in more detail in the following sections.

Figures

Figures 1-6 show different exemplary embodiments of a system according to the invention.

Description of the Figures

Figure 1 shows electrodes 1 and 2 to which a voltage source (not shown) can be connected, so that an electric field is created between the electrodes 1 and 2. The two electrodes 1 and 2 are - in any case on the sides facing each other - provided with protective layers 3 and 4 respectively from good dielectric material, for instance polyethylene, polypropylene, polyethylene terephtalate or polyimide. So, each electrode configuration comprises electrodes 1 and 2 respectively and solid dielectric high-quality protective layers 3 and 4 respectively. When, hereinafter, the term "electrode" is used, this can be understood to mean "electrode configuration", comprising the actual electrode and the solid electrode protective layer. Incidentally, to prevent flashover between the electrodes 1 and 2, it is sufficient in itself to cover only one of these electrodes with an insulating protection, provided that this insulating layer has sufficient disruptive strength. Over the surface of the solid protective layer 4 of electrode 2, a movable protective layer - for instance in the form of a protective film 5 - is led, which is moved via a set of guides 6 and 7, driven by a drive 6a. In the area between the electrodes 1 and 2, a plasma is created as a result of ionization of the gas (for instance air) or vapor (hereinafter to be referred to as "gas" for the. sake of convenience) between the electrodes. Disruptive discharge of the gas is prevented in that the protective layers 3 and 4 are located against the surfaces of the electrodes 1 and 2. Since the disruptive voltage of the protective layers 3 and 4 is many times greater than that of the gas, these protective layers will not break down and thus, flashover (disruptive discharge of the gaseous dielectric) between the electrodes 1 and 2 is prevented.

However, pollution of the surfaces of the protective layers 3 and 4 may, with the passage of time, cause flashover between the electrodes 1 and 2 to occur yet again, for instance as a result of creeping discharge along the (polluted) surfaces of the protective layers 3 and 4, so that the plasma generation of the system stops and the system can also become severely damaged. Besides by pollution of the surfaces of the protective layers 3 and 4, the material of these layers may also chemically and/or physically degrade, so that, at a certain moment, disruptive discharge of the protective layers 3 and 4 occurs all the same, resulting in disruptive discharge of the gas.

In order to prevent the pollution and degradation problem, in Fig. 1, electrode 2 is provided with a protective film 5 sliding over the protective layer 4 of electrode 2. This film 5 is led over the protective layer 4 either in a continuous or in a discontinuous — for instance intermittent — movement, in the direction of the arrow. The speed at which film 5 is moved may, if desired, be adjusted to the intensity of the plasma, for instance by driving the film at a speed which is a function of the magnitude of the electric current through the electrodes 1 and 2. Although, in certain cases, it may be sufficient to use the configuration of Fig. 1, a configuration which is shown in Fig. 2 will, however, be preferred in many cases. In Fig. 2, movable protective films 5 and 9 respectively run along the two electrodes 1 and 2, that is to say over the surfaces of their respective protective layers 3 and 4, via sets of guide rollers 6/7 and 10/11. Each of these has the same function, namely shielding the surfaces of the electrode protective layers 3 and 4 from the influences of particularly dirt, the generated plasma and the gas. The films are driven by the drives 6a and 10a. Since the plasma can be used (see hereinabove) for treatment of, for instance, a film which - like the protective films 5 and 9 — is led through the plasma 8 and which is, for instance, etched or coated by this plasnla, waste products may be formed in this treatment which may also pollute and/or affect the electrode protective layers 3 and 4. It is noted that it is not absolutely necessary that the protective films 5 and 9 respectively have good dielectric properties, as long the underlying electrodes 1 and 2 respectively are covered with protective layers 3 and 4 respectively which do have good dielectric properties (dielectric constant, disruptive strength, creep resistance, etc.). The resistance of the movable films 5- and 9 respectively to pollution, action of the plasma 8 and/or the (high) electric field intensity and/or waste products etc. need not be extremely great, since the films 5 and 9 respectively are — by moving thereof — only relatively briefly exposed to these influences.

The film which is led through the plasma for processing (etching or coating) may serve as protective film at the same time, instead of the films 5 and 9. This can be represented by having, for instance in Fig. 2, film 5 be a film which is, in this case, being etched or coated. Film 9 could also be processed simultaneously, in the same manner. So, the films 5 and 9 respectively processed by the plasma 8 then simultaneously provide protection of the dielectric protective layers 4 and 3 respectively or can even replace them, as the following Figures will show.

As Fig. 3 shows, the films 5 and 9 may also take over the role of dielectric electrode protective layer from the protective layers 4 and 3 shown in Figs. 1 and 2. Obviously, more stringent requirements are then imposed on the dielectric properties and/or the thickness of the film. However, the resistance to pollution and/or degradation does not need to be very great since the films 5 and 9 respectively are regularly "changed" by means of the moving thereof along the electrodes 1 and 2. Fig. 4 shows an embodiment in which the thickness of the films 5 and 9 is unequal. This may be desired if, for instance, one of the films exclusively has the purpose of preventing initiation of flashover between the electrodes — the role of the solid protective layers 3 and 4 in Fig. 2 — and the other film (also) needs to be processed or treated (etched, coated) by the generated plasma. Also, unequal thicknesses may be the result of different dielectric constants of the film materials of the films 5 and 9 respectively and/or of applying a non-symmetric voltage over the electrode configurations 1 and 2.

Fig. 5 shows an exemplary embodiment of an apparatus according to the invention in which use is made of one continuous film 5, for the protection of both the one electrode 1 and the other electrode 2. The film is supplied via guide roller 11, led over electrode 1 and discharged via a guide roller 12 along electrode 2 and the guide roller 7 driven by a drive 7a. This exemplary embodiment appears, to be very suitable for processing (for instance etching) the film 5 moved via guide roller 11, reversing roller 12 and guide roller 7 along the electrodes 1 and 2. Here, the film 5 also serves as movable dielectric protective layer for the electrodes 1 and 2. The process (for instance etching) takes place in two stages, namely during the moving of the film along electrode 1 and then — after being reversed by guide roller — during the moving along electrode 2. This embodiment enables a double processing speed compared to the "single film transport" embodiments in the previous Figures.

It is noted that, in the exemplary embodiment shown, the shape of the electrodes 1 and 2 transverse to the direction in which the film 5 is led is slightly spherical. This may, incidentally, also be the case in the other exemplary embodiments. This spherical shape allows the film to be led somewhat more tightly and more reliably over the contacts 1 and 2. The contacts 1 and 2, which are thus preferably slightly bent in the plane of the drawing, as Fig. 5 shows, are preferably straight (parallel to each other) in the plane perpendicular to the plane of the drawing (both in Fig. 5 and in the other Figures), so that the plasma generated between the contacts is homogeneous over the whole width of the film 5, transverse to the plane of the drawing.

Finally, Fig. 6 shows an alternative embodiment, in which the static electrodes have been replaced by rotating electrodes 13 and 14. Also in this case, the film 5 is led over/along the electrodes 1 and 2, during which passage, the film 5, on the one hand, "shields" the electrode and thus prevents flashover of the gaseous medium between the electrodes, and, on the other hand, is processed by the plasma generated between the ^■ electrodes.

The advantage of rotating electrodes is that the film 5 passes the electrodes 13 and 14 virtually frictionlessly, so that the film does not sustain any frictional wear. This apparatus is therefore very suitable for processing of very delicate films. Incidentally, the embodiments of Figs. 5 and 6 - as can simply be understood from the Figures - can also be used for electrodes which - like those in Figs. 1 and 2 - have been covered with a solid dielectric. Covering of the electrodes may also be desired with a view to preventing damage of the film to be treated resulting from friction along the - usually metal — electrodes. Also, for this reason, a solid protective layer can be chosen which does not directly form a very good dielectric, but in particular has a very low friction coefficient. An example of a material having a very low friction coefficient as well as excellent insulating properties is polytetrafluorethylene . Finally, it is noted that, in order to achieve that the movable layers 5 and 9 respectively properly abut against the surfaces of the electrodes 1 and 2 and of the solid dielectric layers 3 and 4 respectively, this can (additionally) be brought about by ensuring that the gas pressure in the area between the electrodes 1 and 2 is higher than the ambient pressure. In Figs. 5 and 6, this is indicated by means of arrows P, representing that the gaseous dielectric is blown in from the indicated side via a blow nozzle 15.

An alternative for obtaining a higher pressure on the side of the arrows P is, for instance, to exhaust, via — in Fig. 5 through exhaust openings 16 — or in the immediate vicinity of the electrodes 1 and 2 respectively, the gaseous or vaporous dielectric and/or ambient medium (for instance ambient .air), represented by arrows P in Fig. 5.

Blowing in and exhausting can also be used simultaneously. The result is that the movable solid dielectric, the. film 5, is respectively pressed and pulled against both electrode 1 and electrode 2, which prevents the creation of a thin gas layer between the movable film and the respective electrode, in which gas layer, under the influence of the electric voltage applied, plasma could, undesirably, also be created.

Claims

1. A method for generating a plasma (8) between electrode configurations (1,2,13,14) connected to a voltage source, separated from each other by a gaseous or vaporous medium, wherein with at least one of the electrode configurations, a layer of solid material (5,9) is led along this electrode configuration.

2. A method according to claim 1, wherein the layer (5,9) led along the electrode configuration has good dielectric properties.

3. A method according to claim 1, wherein the layer (5,9) led along the electrode configuration is provided with a continuous movement.

4. A method according to claim 1, wherein the layer (5,9) led along the electrode configuration is provided with a discontinuous movement.

5. A method according to claim 1, wherein the layer (5,9) led along the ^' electrode configuration is provided with an intermittent movement.

6. A method according to claim 1, wherein the layer (5,9) led along the electrode configuration is provided with a movement which is a function of the intensity of the generated plasma (8).

7. A method according to claim 6, wherein the layer (5,9) led along the electrode configuration is provided with a movement which is a function of the electric current intensity through the plasma (8).

8. A method according to claim 1, wherein the layer (5) led along the electrode configuration is also led along at least one of the other electrode configurations.

9. A method according to claim 1, wherein at least one of the electrode configurations comprises an electrode (1) and a material layer (3) having good dielectric properties which is not movable with respect to this electrode.

10. An apparatus for generating a plasma (8), comprising electrode configurations (1,2) connectable to a voltage source, separated from each other by a gaseous or vaporous medium, and means (6a,10a,7a) for leading, with at least one of the electrode configurations, a layer of solid material (5,9) along this electrode configuration.

11. An apparatus according to claim 10, wherein the layer (5,9) led along the electrode configuration has good dielectric properties.

12. An apparatus according to claim 10, wherein the means are suitable to lead the layer (5,9) led along the electrode configuration along the electrode configuration in a continuous movement.

13. An apparatus according to claim 10, wherein the means are suitable to lead the layer (5,9) led along the electrode configuration along the electrode configuration in a discontinuous movement.

14. An apparatus according to claim 10, wherein the means are suitable to lead the layer (5,9) led along the electrode configuration along the electrode configuration in an intermittent movement.

15. An- apparatus according to claim 10, wherein the means are suitable to lead the layer (5,9) led along the electrode configuration along the electrode configuration in ^;a movement which is a function of the intensity of the generated plasma (8).

16. An apparatus according to claim 15, wherein the means are suitable to lead the layer (5,9) led along the electrode configuration along the electrode configuration in a movement which is a function of the electric current intensity through the plasma (8).

17. An apparatus according to claim 10, wherein the means are suitable to also lead the layer (5) led along the electrode configuration along at least one of the other electrode configurations.

18. An apparatus according to claim 10, wherein at least one of the electrode configurations comprises an electrode (1) and a material layer (3) having good dielectric properties which is not movable with respect to this electrode.

19. An apparatus according to claim 10, wherein the electrode configurations comprise stationary electrodes.

20. An apparatus according to claim 10, wherein the electrode configurations comprise rotatable electrodes.

21. An apparatus according to claim 10, comprising means (15,16) for achieving that the pressure of the gaseous or vaporous medium between the electrode configurations is such that the layer (5,9) led along the respective electrode configuration is pressed against this electrode configuration.

22. An apparatus according to claim 21, comprising means (15,16) for achieving that the pressure of the gaseous or vaporous medium between the electrode configurations is higher than the ambient pressure.

23. An apparatus according to claim 22, comprising means (15) for blowing in the gaseous or vaporous medium between the electrode configurations.

24. An apparatus according to claim 21, 22 or 23, comprising means (16) for exhausting the gaseous or vaporous medium and/or ambient medium on the side of the layer (5,9) led along the respective electrode configuration (1,2) facing the respective electrode configuration (1,2).