EP2016809B1 - Cold plasma hand set for plasma treatment of surfaces - Google Patents

Description

This application claims the priority of the German patent application no. 10 2006 019 664.3 filed on April 27, 2006.

Technical area

The invention relates to a plasma tool for the plasma-assisted treatment, modification and coating of inner and outer surfaces of materials in air by means of a cold plasma jet according to the preamble of claim 1.

State of the art

The plasma technology, especially at high temperatures and high gas pressures, has long been known and described many times, eg in US Pat. No. 3,648,015 . US 4,626,648 . DE 41 08 499 A1 and DE 101 40 298 B4 ,

In WO 03/026365 A1 a device is described which allows to generate by means of microwaves a plasma, wherein the in WO 03/026365 described device allows, despite any pressure fluctuations in the process gas to produce a stable plasma flame.

Another plasma generator, which produces a plasma with high temperatures, is described in the German patent application 1 639 257 described. It is a high-frequency plasma jet generator with a cylindrical tube, on one end side of which to gas to be ionized and at the other end side of the generated plasma flows out, an induction coil whose one end is connected to ground and the other end is connected to a high-frequency generator. Between the two ends of the coil, a tap is arranged. The high-frequency voltage generated in the induction coil is higher than the excitation voltage. The tube in the area of the plasma outlet is made of metal and placed on the high-voltage end of the induction coil. The tube is concentrically and electrically isolated surrounded by a metallic housing. Due to the special arrangement, the gas discharge between the two adjacent ends of tube and housing takes place due to a capacitive coupling between these two components.

However, this generator is not suitable for the generation of a cold normal pressure plasma at least due to its electrode shape.

Low-temperature plasmas are also already known and have been used successfully in numerous surface treatment applications for the purpose of surface activation (changes in the adhesion properties, hydrophobization, hydrophilization) of etching, of polymerization, of layer deposition, of cleaning and of germ reduction. However, low-pressure plasmas have been used to date for these processes, in which the radicals, excited atoms, ions, electrons and UV radiation required for these applications can be generated to a defined extent by the selection of suitable process parameters. Low-pressure plasma processes, however, are not suitable for numerous industrial processes in which a corresponding surface modification is required, both for cost reasons and for procedural reasons.

A normal pressure plasma process that ionizes water vapor at a relatively low temperature can, is in EP 0 124 623 described. However, this method is hardly applicable in industrial production.

In order to make use of plasma technological surface treatment techniques for potential users of these industrial sectors, it is necessary to develop suitable non-thermal normal pressure plasma processes which are much less expensive and can be integrated into corresponding production lines. An essential requirement for the applicability of atmospheric pressure plasmas for this application is the generation of homogeneous plasmas. One way to achieve the required homogeneity is to generate a plasma jet outside the discharge space by a directed flow of the working gas (process gas).

All known types of discharge plasmas generated under normal pressure conditions, such as RF arc discharges, spark, corona, and barrier discharges, can be used to produce anisotropic normal pressure jet plasmas by implementing suitable process gas flows. Beamplasmas produced on this basis are the subject of various patents. For example, in the patent DE 3733492 a device for generating a jet plasma by corona discharge presented, which is suitable for the plasma treatment of surfaces. In this case, a gas stream is passed through a corona discharge gap between a rod-shaped inner and a tubular outer electrode. In the patent DE 19532412 A method of surface plasma treatment based on the generation of a plasma arc by non-transferred arc discharge is described. Subject of the patent documents US 6,194,036 . US 6,958,063 and US 6,262,523 are arrangements based on the RF excitation of normal pressure plasmas. In another patent document ( US 2002/122896 ) will be different Arrangements for producing normal pressure plasmas based on RF excited discharges in tubes of insulating material are described. In the field of medicine, plasmas of this type are used for argon plasma coagulation ( US 4,781,175 . US 4,060,088 . DE 19513338 ), for coatings on artificial implants to increase their biocompatibility, for the control of cell adhesion to surfaces, for the sterilization of medical instruments ( M. Laroussi: IEEE Trans. Plasma Sci. 30 4 (2002), 1409 ) and for the treatment of biological cells and tissues ( E. Stoffels et al .: Plasma Sources Sci. Technol., 11 (2002), 383 ) used.

The arrangements and methods for surface treatment by means of normal pressure plasma described hitherto in the technical or patent literature are solutions for limited tasks, which can not or only partially be adapted to the requirements of other applications due to their special construction and operation. Since the tasks and objectives of the plasma treatment of surfaces are very diverse, a solution is to be sought, which allows such an adaptation to different requirements with regard to the material or product to be treated or the desired effect on the surface to be treated. Arrangements for generating normal pressure plasmas based on RF-excited discharges have the advantage that they can be operated at fixed frequencies (13.56 MHz, 27.12 MHz, 40.68 MHz), which are released for industrial applications and on the other hand can be generated at lower voltages. However, they also have a significant disadvantage, which will be explained below.

High-frequency operated plasma reactors require a matching network (matchbox) for maximum power transmission from their RF generator. An often used circuit in the Matchbox is the n - circuit. It consists of two capacitors C1 and C2 and a coil (s. Fig. 1 ). In order to keep losses in the matchbox low, capacitors with air as a dielectric are used, which occupy a large volume. Since the current transport at these frequencies takes place mainly on the surface of an electrical conductor (skin effect), the coil and all other electrical leads consist of a relatively thick metal wire with high electrical conductivity on the surface (silver wire, silver-plated copper wire). As a result, such a matchbox is generally very voluminous. To ignite and maintain a gas discharge in the plasma reactor, high voltages are needed. These are achieved in the matchbox by the fact that the coil and the capacitor C2 form a series resonant circuit, which must be matched to the particular frequency of the RF generator used. To prevent losses, the supply line Z2 should consist of an unshielded cable and should be kept as short as possible. As a result, the matchbox and the plasma reactor actually form a relatively rigid, unwieldy unit. If you want to realize a handy plasma nozzle as a plasma reactor, which can be performed for example by a robot, such a bulky plasma reactor is useless.

The invention is therefore based on the object to realize a handy plasma nozzle, which can also be performed by hand and / or by robots.

Presentation of the invention

1. 1. Die Zuleitung Z1 (Koaxialkabel) vom Generator zur Plasmadüse kann wesentlich flexibler und länger gestaltet werden als dies für die Zuleitung Z2 gemäss Stand der Technik jemals möglich gewesen wäre.
2. 2. Änderungen in der Länge der Zuleitung Z1 sind mit Änderungen in der Kabelkapazität verbunden, die durch Änderung von C1 kompensiert werden können.
3. 3. Die Zuleitung Z2 wird durch das Spulenende zur Elektrode E1 gebildet und kann deshalb extrem kurz gestaltet werden.
4. 4. Die zwischen den Elektroden E1 und E2 gebildete Kapazität liegt parallel zu C2. Änderungen dieser Kapazität durch Toleranzen in der Herstellung der Plasmadüse oder bei Zündung des Plasmas können durch Veränderung von C2 kompensiert werden, so dass die Resonanzbedingung erhalten bleibt.
5. 5. Durch die sehr kurze Zuleitung Z2 wird automatisch die Gesamtkapazität, gebildet aus der Kapazität C2 und der Kapazität zwischen E1 und E2, klein gehalten, so dass die Induktivität L entsprechend der Festfrequenz maximal gewählt werden kann und somit eine hohe Güte des Reihenresonanzkreises (Erzeugung einer hohen Spannungsüberhöhung) erreicht werden kann.

In the context of the present invention, it has now been found that a very handy plasma nozzle can be obtained if an adaptation network in the form of a separate matchbox is dispensed with. Therefore, according to the invention, the coil and the capacitor C2 are integrated into the plasma nozzle. A possibly required capacitor C1 may be located anywhere between the generator and the plasma nozzle, but preferably the capacitor C1 is positioned directly on the generator outside (short lead) or directly inside. This achieves the following improvements:

1. 1. The supply line Z1 (coaxial cable) from the generator to the plasma nozzle can be made much more flexible and longer than would ever have been possible for the lead Z2 according to the prior art.
2. 2. Changes in the length of lead Z1 are associated with changes in the cable capacitance, which can be compensated by changing C1.
3. 3. The supply line Z2 is formed by the coil end to the electrode E1 and can therefore be made extremely short.
4. 4. The capacitance formed between the electrodes E1 and E2 is parallel to C2. Changes in this capacity due to tolerances in the production of the plasma nozzle or upon ignition of the plasma can be compensated for by changing C2, so that the resonance condition is maintained.
5. 5. Due to the very short supply line Z2, the total capacity, formed by the capacitance C2 and the capacitance between E1 and E2, is automatically kept small so that the inductance L can be maximally selected according to the fixed frequency and thus a high quality of the series resonant circuit (generation a high voltage overshoot) can be achieved.

The plasma tool according to the invention for the generation of a cold plasma jet comprises a plasma nozzle with a hollow body for the supply of a process gas or a process gas mixture, a Frequency generator and consisting of at least one coil and a capacitor C2 and optionally a capacitor C1 matching network for generating the required voltage and is characterized in that the matching network at least the coil and the capacitor C2 are integrated into the plasma nozzle.

In particular, in a plasma nozzle operated with a fixed frequency RF generator (13.56 MHz, 27.12 MHz, 40.68 MHz), the matching network integrates the coil L and the capacitor C2 into the plasma nozzle.

The capacitor C1 of the matching network can be arranged directly on or in the frequency generator and is advantageously arranged there.

In a specific embodiment, the plasma nozzle contains a capillary made of insulating material and the coil is arranged around this capillary.

In a particularly preferred embodiment in which the frequency generator is a high frequency generator, the matching network (matchbox) consists of a coil and two capacitors C1 and C2 with their connections. The coil and the capacitor C2 are integrated in the plasma nozzle and the capacitor C1 is arranged directly on or in the generator.

Although this description only mentions two capacitors C1 and C2, it is clearly stated here that the capacitors C1 and C2 can be made up of several partial capacitors and that capacitors constructed from partial capacitors are also referred to as C1 and C2 in the context of this invention.

Likewise provided by the present invention is a plasma nozzle into which at least one coil and a capacitor C2 are integrated. These can, as described above and in the embodiments resp. the figures shown to be installed.

Described but not the subject of the present invention is a frequency generator, in which either a capacitor suitable as capacitor C1 of a matching network capacitor integrated or mounted directly on the output of the generator.

As already described above, the embodiment with a capacitor C1 and a capacitor C2 relates in particular to commercially available RF generators having a fixed frequency, as described e.g. in Germany are released by the post office for technical concerns. A simplification, and therefore cheaper, of the combination RF generator - plasma nozzle results in the transition to lower frequencies (e.g., 3 MHz) and using a variable frequency generator. In such an embodiment, which is not the subject of the present invention, both capacitors C1 and C2 can be dispensed with, so that apart from a part of the lines of the matching network in the plasma nozzle only the coil remains, which together with that through the electrodes E1 and E2 formed capacitor forms a series resonant circuit. In this embodiment, the resonance state can be adjusted by varying the generator frequency.

A plasma nozzle according to the invention generally comprises a body side, ie on the plasma respectively. the nozzle facing away from the plasma nozzle, with a hollow body connected to a process gas supply. This hollow body is preferably made of insulating material. In a particularly space-saving variant, the coil forming part of the matching network is arranged around a part of this hollow body. The dimensions of the hollow body, or these dimensions together with another body, preferably an insulating body, are to be chosen such that the coil with the desired winding diameter can be arranged thereon. This coil must - if the hollow body or other body on which it is arranged, not made of insulating material, be self-insulated. This coil is connected to the nozzle side with an electrode E1 and optionally a variable capacitor C2. The electrode E1 may optionally be a ring electrode arranged around the insulating hollow body or a rod electrode arranged in the hollow body. The capacitor C2 and the coil are connected in series, so that it can adjust the voltage required at a given frequency. On the side facing away from the coil, the capacitor C2 is connected to the grounded housing. In a suitable for plasma generation distance from the first electrode E1 and arranged at the nozzle end of the hollow body thereon is a ring electrode E2, which is connected to the grounded housing. This housing has feeds for the electric current and feed openings for the process gas and a discharge opening for the plasma within the second electrode E2. Between the coil and the grounded housing there is a further insulating layer, which is important in particular with a small clearance between the coil and the housing. The connecting line between the electrode E1 and the capacitor C2 is usually on the coil side on the housing shielding insulation and in turn provided with an insulating layer.

For the generation of a cold plasma, it is important that the two electrodes E1 and E2 are well insulated from each other. As a result, the formation of an arc discharge is prevented, which would lead to an unwanted heating of the plasma.

Examples of suitable insulating materials are plastic, quartz glass, ceramics, etc., which may be used singly or in combination.

Since the current in the coil flows primarily over the surface, a material with high conductivity is preferred at least on the surface, such as silver-plated copper wire or pure silver wire.

Brief description of the drawings

• Figur 1 zeigt die generelle Beschaltung eines RF-betriebenen, kapazitiv gekoppelten Plasmawerkzeuges, wobei Figur 1a) den Plasmareaktor allgemein und Figur 1b) die Plasmadüse darstellen.
• Figur 2 zeigt eine erfindungsgemäße Ausführungsform, bei der die Spule L und der Kondensator C2 in den Generator resp. die Düse integriert sind.
• Figur 3 zeigt eine weitere nicht unter den Gegenstand der Erfindung fallende Ausführungsform mit einem Generator mit variabler Frequenz, bei der die Kondensatoren C1 und C2 entfallen können.
• Figur 4 zeigt eine erfindungsgemässe Plasmadüse mit RF-Ringelektrode.
• Figur 5 zeigt eine erfindungsgemässe Plasmadüse mit RF-Stabelektrode.
• Figur 6 zeigt eine erfindungsgemässe Plasma-Breitstrahldüse mit RF-Ringelektrode.

Further embodiments, advantages and applications of the invention will become apparent from the dependent claims and from the following description with reference to FIGS.

• FIG. 1 shows the general wiring of an RF-powered capacitively coupled plasma tool, wherein FIG. 1a ) the plasma reactor in general and FIG. 1b ) represent the plasma nozzle.
• FIG. 2 shows an embodiment of the invention, in which the coil L and the capacitor C2 in the generator resp. the nozzle are integrated.
• FIG. 3 shows another not falling under the subject invention embodiment with a generator with variable frequency, in which the capacitors C1 and C2 may be omitted.
• FIG. 4 shows a plasma nozzle according to the invention with RF ring electrode.
• FIG. 5 shows a plasma nozzle according to the invention with RF rod electrode.
• FIG. 6 shows a plasma broad-jet nozzle according to the invention with RF ring electrode.

Figure Legend

1

Kapillarentladung

2

Elektrode

3

RF-Elektrode

4

Hohlkörper (Kapillare), vorzugsweise aus Isoliermaterial

5

Isolierkörper

6

Spule (auch als L bezeichnet)

7

RF-Eingang

8

Gehäuse

9

Prozessgas

10

Strahlplasma / Plasmazone

11

RF Generator

12

Matchbox

13

Plasmareaktor

14

Plasmadüse (Plasmareaktor)

The reference numbers in the figures generally have the following meaning:

1

capillary discharge

2

electrode

3

RF electrode

4

Hollow body (capillary), preferably made of insulating material

5

insulator

6

Coil (also referred to as L)

7

RF input

8th

casing

9

process gas

10

Jet plasma / plasma zone

11

RF generator

12

Matchbox

13

plasma reactor

14

Plasma nozzle (plasma reactor)

Way (s) for carrying out the invention

In the FIG. 1 Illustrated embodiments of the prior art relate in particular to commercially available RF generators with a fixed frequency.

In the inventive embodiment, the in FIG. 2 2, the matching network, the matchbox, has been separated, with the capacitor C1 in the RF generator and capacitor C2 and the coil integrated into the plasma nozzle. A simplification and thus a more cost-effective variant of the combination RF generator - plasma nozzle results in the transition to lower frequencies (eg 3 MHz) and when using a generator with variable frequency. This variant, in which both capacitors C1 and C2 can be omitted, so that in the plasma nozzle only the coil is still, which forms a series resonant circuit together with the capacitor formed by the electrodes E1 and E2, and which is not the subject of the present invention, is in FIG. 3 shown. In this embodiment, the resonance state is adjusted by varying the generator frequency.

In Fig. 4 an embodiment for a plasma nozzle with a capacitively coupled capillary discharge 1 is shown. Two metallic ring electrodes 2, 3 are mounted at a suitable distance on a hollow body made of insulating material (dielectric) 4. On one, the hollow body 4 enclosing insulator 5, a coil 6 is wound, which is connected at one end to the RF electrode 3 and at the other end to the RF input 7 of the plasma nozzle. The RF electrode 3 is connected to the grounded case 8 via a rotary air capacitor C2. Via the hollow body 4, the process gas 9 (preferably noble gas) is fed to the discharge zone between the two electrodes 2 and 3. Both electrodes 2 and 3 and the dielectric 4 form a capacitance (a few pF), the parallel to C2. The coil 6 forms a series resonant circuit with these capacitances and can be adjusted via C2 to maximum voltage at the electrode 3. If a sufficiently high voltage at the electrode 3 has been reached via the calibration with C2, the electric field built up between the electrodes 3 and 2 leads to a capillary discharge whose plasma is driven outward by the gas flow 9 and forms a jet plasma 10. In order to keep the voltage drop across the capacitor, formed by the electrode 3, the dielectric 4 and the plasma inside the capillary, small, a dielectric with the highest possible dielectric constant should be selected.

Suitable dimensions and materials for the embodiment described in FIG. 4 are:

• Width of metallic ring electrodes: 5mm
• Distance between the metallic ring electrodes: 5 mm
• Material of metallic ring electrodes: stainless steel
• Dimensions of the hollow body made of insulating material (capillary): outer diameter 3mm, inner diameter 1mm
• Gas flow: 2 to 10 slm (standard liters per minute).
• Examples of process gases: noble gases, such as argon and helium
• Examples of additions to process gases: nitrogen, oxygen
• Dielectric with the highest possible dielectric constant, e.g. quartz glass

Values suitable for an RF generator with a fixed frequency of e.g. 27.12 MHz suitable / preferred are:

• The strength of the capacitance formed by both electrodes 2 and 3 as well as the dielectric 4 and lying parallel to C2: a few pF
• Inductance of the coil 1.9 μH
• Capacitor C2: Tunable in the range of 5 to 30 pF.
• Capacitor C1: 350 pF

In Fig. 5 a further embodiment of a plasma nozzle with a capillary discharge 1 is shown. In contrast to the variant described above, here the RF energy is coupled via a rod electrode 3 into the capillary discharge. The rod electrode should be made of low work function materials to minimize the voltage needed for capillary discharge. She should also be pointed forward, so as to achieve a high field strength. Between the tip and the grounded electrode 2, at sufficiently high voltages, a capillary discharge is formed, the plasma of which is in turn blown outward by the gas flow.

Essential dimensions / materials other than those described in the above and in FIG. 4 in this embodiment are:

• Hollow body made of insulating material (capillary) 4: outer diameter 6mm, inner diameter 2mm.
• Distance tip of the rod electrode to the end of the capillary 4: 1mm
• Diameter of the stick electrode: 1mm
• Material of the rod electrode: Tungsten.

In Fig. 6 a modified variant of the plasma nozzle is shown. The discharge is again generated between the electrodes 2 and 3 and enters the atmosphere through a slot. With a slot of 0.8 mm width and 4 cm length can be generated with this arrangement, a linearly expanded plasma of 4 cm width.

In all examples described, in a hollow body through which a process gas flows, insulating material, such as, for example, plastic, Quartz glass, ceramics, etc. (referred to in the above description as "plasma nozzle") by means of an RF discharge generated by a nozzle, directed normal pressure jet plasma with the desired properties (for example, non-thermal, floating, homogeneous and reactive) to which the The surface to be treated is exposed at a suitable distance from the nozzle in order to achieve its desired physicochemical change. The conditions in the jet plasma region can be changed by changing the geometrical arrangements and the dimensions inside the plasma nozzle, by using other process gases, their admixtures and flow velocities, by the arrangement and choice of the electrodes, by the kind of ignition and / or by variation of the electrical Parameters of the discharge are controlled.

The physical principles for the choice of dimensions within the nozzle as well as the definition of suitable operating conditions are known to those skilled in the field of plasma technology.

While preferred embodiments of the invention are described in the present application, it is to be understood that the invention is not limited thereto and may be embodied otherwise within the scope of the following claims.

Claims (10)

1. Plasma tool for generating a cold plasma beam with a plasma nozzle comprising a hollow body (4) for supplying process gas, a frequency generator and an adjustment circuit for generating the required voltage comprising a coil (6), a capacitor (C2) and if necessary a capacitor (C1), characterized in that the coil (6) and the condenser (C2) of the adjustment circuit are integrated into the plasma nozzle.
2. Plasma tool according to claim 1, with a plasma nozzle comprising two electrodes, E1 and E2, wherein the electrode E1 is optionally a ring electrode arranged around an isolating hollow body or a rod electrode arranged inside the hollow body and the electrode E2 is a ring electrode arranged at the nozzle-sided end of the hollow body (4) and on it at a distance which is appropriate for plasma generation, being connected to the grounded casing.
3. Plasma tool according to one of the preceding claims, characterized in that the adjustment circuit comprises a capacitor C1 and in that the capacitor C1 of the adjustment circuit is arranged directly on or inside the frequency generator.
4. Plasma tool according to one of the preceding claims, characterized in that the coil (6) is arranged around the hollow body (4) and preferably lies on this hollow body or on an isolating body (5) additionally surrounding this hollow body.
5. Plasma tool according to one of the preceding claims, characterized in that the generator is a fixed frequency RF generator, in that the adjustment circuit consists of a coil (6) and two capacitors C1 and
   C2 with their connections and in that the coil (6) and the capacitor C2 are integrated in the plasma nozzle and in that the capacitor C1 is arranged on or inside the generator.
6. Plasma tool according to one of the preceding claims, characterized in that the frequency generator is a generator which is adjustable with respect to frequency and the adjustment circuit consists of a coil (6) with connections, wherein the coil (6) is integrated into the plasma nozzle.
7. Plasma tool according to one of the preceding claims, characterized in that the adjustment circuit consists of a coil (6), the connections and either capacitor C1 or capacitor C2.
8. Plasma tool according to one of the preceding claims, characterized in that the plasma nozzle is dimensioned in such a way that it can be held in one hand during its use, particularly a plasma nozzle with the following dimensions:

   Diameter: 2 cm,

   Length: 17 cm,

   Length of the plasma zone: up to 1 cm.
9. Plasma tool according to one of the preceding claims, characterized in that the hollow body consists of isolating material.
10. Plasma nozzle, particularly a plasma nozzle for manual operation, characterized in that it comprises the coil (6) and the capacitor C2 of an adjustment circuit as described in one of the preceding claims.

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