EP2080214A1

EP2080214A1 - Device and method for producing high power microwave plasma

Info

Publication number: EP2080214A1
Application number: EP07818909A
Authority: EP
Inventors: Ralf Spitzl
Original assignee: iplas Innovative Plasma Systems GmbH
Current assignee: iplas Innovative Plasma Systems GmbH
Priority date: 2006-10-16
Filing date: 2007-10-11
Publication date: 2009-07-22
Also published as: US20100215541A1; DE102006048815B4; DE102006048815A1; CA2666117A1; AU2007312618A1; WO2008046551A1

Abstract

The invention relates to a device for producing high power microwave plasma. Said device comprises at least one microwave supply (1, 2, 4) that is surrounded by at least one dielectric tube. A dielectric fluid flows through the area between the microwave supply and the outer dielectric tube, said dielectric fluid having a small dielectric loss factor tan d in the region of between 10-2 to 10-7. At least the outer dielectric tube of the above mentioned device is cooled by a fluid.

Description

Apparatus and method for generating high power microwave plasmas

The invention relates to a method for producing high plasma density microwave plasma in a device having at least one microwave feed surrounded by at least one dielectric tube.

Devices for generating microwave plasmas are used in the plasma treatment of workpieces and gases. The plasma treatment is used for. As the coating, cleaning, modification and etching of workpieces, for the treatment of medical implants, for textile treatment, for sterilization, for light generation, preferably in the spectral range infrared to ultraviolet, for the implementation of gases or for gas synthesis and in the art for exhaust gas purification , In this case, the workpiece or gas to be treated is brought into contact with the plasma or the microwave radiation.

The geometry of the workpieces to be treated ranges from flat substrates, fibers and webs to moldings of any shape.

As a process gas, any known gas can be used. The most important process gases are noble gases, fluorine- and chlorine-containing gases, hydrocarbons, furans, dioxins, hydrogen sulfide, oxygen, hydrogen, nitrogen, tetrafluoromethane, sulfur hexafluoride, air, water and their mixtures. In the case of exhaust gas purification by microwave-induced plasma, the process gas consists of exhaust gases of all kinds, in particular carbon monoxide, hydrocarbons, nitrogen oxides, Aldehydes and sulfur oxides. However, these gases can readily be used as process gases for other applications.

Devices which produce microwave plasmas have been described in documents WO 98/59359 A1, DE 198 480 22 A1 and DE 195 032 05 C1.

The above-mentioned documents have in common that they describe a microwave antenna inside a dielectric tube. If microwaves are generated in the interior of such a tube, surface waves form along the outside thereof. These surface waves generate a linearly stretched plasma in a process gas which is under low pressure. Typical lower pressures are 0.1 mbar - 10 mbar. The volume inside the dielectric tube is typically at ambient pressure (generally normal pressure, about 1013 mbar). In some embodiments, a cooling gas flow through the tube is used to cool the dielectric tube.

For the supply of microwaves, inter alia, waveguide and coaxial, as coupling points in the wall of the plasma chamber, inter alia, antennas and slots are used. Such feed lines for microwaves and coupling points are described, for example, in DE 423 59 14 and WO 98/59359 A1.

The microwave frequencies used to generate the plasma are preferably in the range from 800 MHz to 2.5 GHz, more preferably in the ranges from 800 MHz to 950 MHz and 2.0 - 2.5 GHz, however, the microwave frequency can be in the entire range of 10 MHz to several 100 GHz.

DE 198 480 22 A1 and DE 195 032 05 C1 describe devices for generating plasma in a vacuum chamber by means of electromagnetic alternating fields, with a conductor which projects inside a tube of insulating material in the vacuum chamber, wherein the insulating tube at both ends by walls the vacuum chamber is held and sealed against the walls on its outer surface. The ends of the conductor are connected to a generator for generating the electromagnetic alternating fields.

With a device for the production of homogeneous microwave plasmas according to WO 98/59359 A1, particularly homogeneous plasmas can be produced over long distances, even at higher process pressures, due to the uniform coupling of the microwaves.

The possible uses of the abovementioned plasma sources are limited by a high energy output of the plasma on the dielectric tube. This release of energy can lead to excessive heating of the tube and ultimately to its destruction. Therefore, these sources are typically operated with microwave power of about 1 - 2 kW at a correspondingly low pressure (about 0.1 - 0.5 mbar). Although the process pressures can be 1 mbar - 100 mbar, but only under certain conditions and correspondingly lower power, so as not to destroy the pipe. With the above-mentioned devices, typical plasma lengths of 0.5 to 1.5 m can be achieved. Although larger lengths can be achieved with plasmas made from almost 100% argon, such plasmas are technically less relevant.

Another problem with such plasma sources lies in the process gas control, especially at higher process gas pressures (greater than 1 xnbar). The reason for this is due to the fact that the plasma density decreases sharply in increasing radial distance from the dielectric tube. This complicates the supply of new process gas to the areas of high carrier densities. Furthermore, at higher process pressures, the heat output delivered to the dielectric tube increases.

However, higher process gas pressures are preferred because they often result in significant process speed increases of 10 to 100 times.

The object of the present invention is to prevent or reduce the above-mentioned disadvantages of excessive heating of the dielectric tube and thus to enable an increase in the power of the plasma sources.

This is achieved according to the invention by a method according to claim 1. In a device for generating microwave plasmas, which has at least one microwave feed, which is surrounded by at least one dielectric tube, a dielectric fluid is passed through the space between the microwave feed and the dielectric tube. The dielectric fluid, which has a small dielectric electric loss factor tan δ in the range 10 ^"2 to 10 ^{~ 7} , flows through this space between microwave feed and dielectric tube.

The method advantageously enables the cooling of the dielectric tube by means of the passage of the fluid through the above-described tube arrangement. In the following, the device and the method will be described.

Suitable microwave feeds are known to the person skilled in the art. In general, a microwave feed consists of a structure that can radiate microwaves into the room. Structures that radiate microwaves are known to the person skilled in the art and can be implemented by all known microwave antennas and resonators with coupling points for coupling the microwave radiation into a room. Cavity resonators, rod antennas, slot antennas, helix antennas and omnidirectional antennas are preferred for the device described. Particularly preferred are coaxial resonators.

In operation, the microwave feed is connected via microwave feed lines (waveguide or coaxial conductor) to a microwave generator (eg klystron or magnetron). In order to control the properties of the microwaves and to protect the elements, it is also possible to introduce circulators, isolators, tuning elements (for example three-pin tuner or E / H tuner) and mode converters (for example rectangular to coaxial conductors) into the microwave feed. The dielectric tubes are preferably elongate. This means here that the ratio of pipe diameter: pipe length is between 1: 1 and 1: 1000, and preferably 1:10 to 1: 100. The two tubes can be the same length or have a different length. Furthermore, the tubes are preferably straight, but may also have a curved shape or corners along their longitudinal axis.

The cross-sectional area of the tubes is preferably circular, but generally any surface shapes are possible. Examples of other surface shapes are ellipses and polygons.

The elongated shape of the tubes requires an elongated plasma. Elongated plasmas have the advantage that, by moving the plasma apparatus relative to a flat workpiece, large areas can be treated in a short time.

The dielectric tubes should have a low dielectric loss factor tan δ for the microwave wavelength used at the given microwave frequency. Low dielectric loss factors tan δ ^' are in the range 10 ^{' 2} to 10 ^"7 .

Suitable dielectric materials for the dielectric tubes are metal oxides, semi-metal oxides, ceramics, plastics, and composites of these materials. Particular preference is given to dielectric tubes made of quartz glass or aluminum oxide with dielectric loss factors tan δ in the range 10 ^-3 . to 10 ^{~ 4} . In this case, the dielectric tubes may consist of the same material or different materials. According to a particular embodiment, the dielectric tubes are closed at the end faces with walls. A gas- or vacuum-tight connection between the pipes and the walls is advantageous. Connections between two workpieces are known to the person skilled in the art and can be, for example, adhesive, welding, clamping or screw connections. The tightness of the compound can range from gas-tight to vacuum-tight, being vacuum tight, depending on the working environment, tightness in a rough vacuum (300 - 1 hPa), fine vacuum (1 - 10 ^"3 hPa), high vacuum (10 ^'3 - 10 ^{" 7} hPa) or Ultra-high vacuum (10 ^{~ 7} - 10 ^"12 hPa) Generally, vacuum-tight means here a tightness in coarse or fine vacuum.

The walls may have passages through which a fluid can be passed. The size and shape of the passages is arbitrary. Depending on the application, each wall can contain at least one passage. In a preferred embodiment, there are no passages in the area covered by the end face of the inner dielectric tube.

Through these passages, the fluid can be conducted into the space between the outer dielectric tube and the inner dielectric tube and discharged again. Another possibility is the supply or discharge of the dielectric liquid via passages in the microwave feed on the one, and at least one of the passages in the walls on the other side. The pressure of the fluid may be greater than, less than or equal to the atmospheric pressure. The rate of flow and flow (laminar or turbulent) of the dielectric fluid through the dielectric tube should be selected so that the fluid has good contact with the edge of the dielectric tube and, in addition, liquid dielectric does not vaporize the dielectric fluid , The control of the flow rate and the Durchströmverhaltens means of the pressure and the shape and size of the passages is known in the art.

As the dielectric fluid, a dielectric fluid is preferably used. Since liquids generally have a much higher specific heat coefficient than gases, the cooling of the dielectric tube with a dielectric fluid is much more effective than with gas cooling, as described in DE 195 032 05 C1. However, a cooling of the dielectric tube by a liquid is not easy to realize, since the energy input of the microwaves to the liquid heats them. Each additional heating of the dielectric liquid reduces the cooling effect on the dielectric tube. This reduction in the cooling capacity can also lead to a negative cooling performance at high microwave absorption of the liquid, which corresponds to an additional heating of the dielectric tube by the cooling liquid.

In order to minimize heating of the dielectric liquid by the microwaves, the dielectric liquid must have a low dielectric loss factor tan δ in the range at the wavelength of the microwaves 10 ^"2 to 10 ^{" 7} . As a result, a microwave power input is avoided in the cooling medium or reduced to a tolerable level.

Such a dielectric fluid is, for example, an insulating oil having a low dielectric loss factor. Insulating oils are, for example, mineral oils, olefins (for example polyalphaolefin) or silicone oils (for example Coolanol® or dimethylpolysiloxanes). Preferred as a dielectric liquid is hexadimethylsiloxane.

By this fluid cooling of the outer dielectric tube, it is possible to reduce the heating of the outer dielectric tube. This allows higher microwave powers, which in turn lead to an increase in the concentration of the plasma on the outside of the outer dielectric tube. Furthermore, a higher process pressure is possible by the cooling than in uncooled plasma generators.

Another embodiment of the device is a double tube arrangement. In this case, a dielectric inner tube between the microwave feed and the dielectric tube is inserted.

The dielectric fluid can be guided between the two tubes in this embodiment (see Fig.2). In contrast to a gas cooling according to DE 195 032 05, in which the cooling gas has contact with the microwave feed, the contact between the fluid and the microwave feed is avoided here by the double tube arrangement, and thus precludes the possibility that the fluid with the microwave feed can react. Furthermore, maintenance of the microwave feed is considerably simplified by this separation of fluid and microwave feed.

In order to further reduce the microwave power requirement in the above-mentioned plasma sources, according to a further preferred embodiment, a metallic Uxnmante- ment can be attached to the outer dielectric tube, which covers this tube partially. This metallic sheath acts as a microwave shield and can be used e.g. consist of a metal tube, a bent metal sheet, a metal foil or even of a metallic layer and plugged, galvanized or applied in any other way. Such metallic microwave shields can arbitrarily limit the angular range in which the generation of the plasma takes place (for example to 90 °, 180 ° or 270 °) and thus reduce the power requirement accordingly.

In particular, in the embodiment with a metallic sheath of the devices for the production of microwave plasmas, it is possible to treat wide material webs with only a small power loss with a plasma. Through the jacket, the space region of the device, which is not facing the workpiece, shielded, and only produces a narrow strip of plasma between the workpiece and the device over the entire width of the workpiece.

All devices described above for producing plasmas form during operation on the outside of the dielectric tube from a plasma. Normally, the device is operated inside a room, a plasma chamber. Depending on the operating mode, this plasma chamber can have different shapes and openings and fulfill various functions. For example, the plasma chamber can contain the workpiece to be machined and the process gas (direct plasma process) or process gases and openings for the plasma exit have (remote plasma process, exhaust gas purification).

The invention will now be described by way of example with reference to the embodiments shown schematically in the drawings.

Figure 1 shows sectional drawings of the device described above.

FIG. 2 shows sectional drawings of the device described above with a double-tube arrangement. Figures 3 A and 3 B show two embodiments with metallic sheath.

Figure 4 shows a cross-sectional view of the device described above installed in a plasma chamber. Figures 5 A and 5 B show a possible embodiment for the treatment of large-scale workpieces.

Figure 1 shows the transverse and longitudinal section of a device for the production of microwave plasmas with a running as a coaxial resonator microwave feed. The microwave feed contains an inner conductor (1), an outer conductor (2) and coupling points (4). The microwave feed is surrounded by a dielectric tube (3), which separates the microwave supplying area from the plasma chamber (not shown) and on the outside of which the plasma is formed. The dielectric tube (3) is connected to the walls (5, 6) in a gas-tight or vacuum-tight manner. A dielectric fluid may be added or removed via the openings (8) and (9) in the walls. Another way of supplying or discharging the dielectric fluid is on the way (7) through the coaxial generator.

FIG. 2 shows, in front and side view, a further embodiment of the device with a microwave feed designed as a coaxial resonator, as described in FIG. 1, consisting of the inner conductor (1), the outer conductor (2) and the coupling points (4). The microwave feed is surrounded by a dielectric tube (3) which separates the microwave supplying area from the plasma chamber (not shown) and on the outside of which the plasma is formed. The dielectric tube (3) is connected to the walls (5, 6) in a gas-tight or vacuum-tight manner. Between the coaxial generator and the dielectric tube (3), a dielectric inner tube (10) is inserted, which is also connected to the walls (5, 6) gas-tight or vacuum-tight. Through the space between the dielectric tube (3) and the dielectric inner tube (10), the dielectric fluid via the openings (8) and (9) is supplied or removed. With this double tube arrangement, it is possible to separate the area through which the dielectric fluid flows from the microwave feed.

FIGS. 3 A and 3 B show cross sections of the embodiments illustrated in FIGS. 1 and 2, in which the dielectric tube (3) is surrounded by a metallic sheathing (11). Shown here is the case in which the metallic Uiranantelung the angular range in which the generation of the plasma takes place limited to 180 °.

Figure 4 shows a longitudinal section of a device (20), as described by Figure 1, when installed in a plasma chamber (21). The cooling liquid (22) flows in this example through passages in the two end faces. In the space (23) between the outer dielectric tube (3) and the wall of the plasma chamber, the plasma is formed during operation.

FIGS. 5 A and 5 B show, in a perspective illustration and in a cross section, an embodiment (20) in which the largest part of the outer surface of the outer dielectric tube is enclosed by a metal sleeve (11) and a plasma (31) which is located in the Drawing indicated by transparent arrows, can arise only in a narrow range. A workpiece (30) which moves relative to the device can be treated in this area with plasma over a large area.

All embodiments are powered by a microwave supply, not shown in the drawings, consisting of a microwave generator and possibly additional elements. These elements may include, for example, circulators, isolators, tuning elements (eg three-pin tuner or E / H tuner) as well as mode converters (eg, rectangular to coaxial). The fields of application of the apparatus and the method described above are manifold. The plasma treatment is used for. As the coating, cleaning, modification and etching of workpieces, for the treatment of medical implants, for textile treatment, for sterilization, for light generation, preferably in the spectral range infrared to ultraviolet, for the implementation of gases or gas synthesis, and in the art for exhaust gas purification. In this case, the workpiece or gas to be treated is brought into contact with the plasma or the microwave radiation. The geometry of the workpieces to be treated ranges from flat substrates, fibers and webs to moldings of any shape.

By increasing the plasma power thereby higher plasma densities and thus higher process speeds are possible than in devices and methods of the prior art.

Claims

claims

1. A method for generating microwave plasmas in a device having at least one microwave feed, which is surrounded by a dielectric tube (3), characterized in that the space between the microwave feed and the dielectric tube (3) is flowed through by a dielectric fluid , wherein the dielectric fluid has a small dielectric loss factor tan δ in the range 10 ^"2 to 10 ^{~ 7} .

2. The method according to claim 1, characterized in that between the microwave feed and the dielectric tube (3) a dielectric inner tube (10) is mounted, which surrounds the microwave feed, and that the space between the dielectric tube (3) and dielectric inner tube (10). flows through the dielectric fluid.

3. The method according to any one of claims 1 or 2, characterized in that each dielectric tube at its end faces with walls (5, 6), the passages (8, 9) are connected, and the dielectric fluid through passages (8, 9) in the walls or via a passage (7) in the microwave feed and at least one of the passages (8, 9) is supplied and discharged.

4. The method according to any one of claims 1 to 3, characterized in that the dielectric fluid is a dielectric fluid, preferably an insulating oil.

5. The method according to claim 4, characterized in that the dielectric liquid is or contains a mineral oil, silicone oil or a mixture of both oil groups.

6. The method according to claim 4, characterized in that the dielectric liquid is or contains a dimethylpolysiloxane, preferably hexadimethylsiloxane

7. The method according to any one of claims 1 to 3, characterized in that the fluid is or contains a gas.

8. The method according to any one of the preceding claims, characterized in that the pressure in the space between the inner dielectric tube (10) and the outer dielectric tube (3) is greater than the atmospheric pressure or equal to the atmospheric pressure.

A method according to any one of the preceding claims, characterized in that the pressure in the space between the inner dielectric tube (10) and the outer dielectric tube (3) is less than the atmospheric pressure.

10. An apparatus for carrying out the method for the production of microwave plasmas which has at least one micro-wave feed, which is surrounded by at least one dielectric tube (3), wherein each dielectric tube at its end faces with walls (5, 6) is completed, characterized in that both at least one of the walls and the microwave structure have at least one passage (7, 8, 9) or both walls (5, 6) each have at least one passage (8, 9). sen, and that the passages are adapted to pass a fluid.

11. The device according to claim 10, characterized in that the dielectric tubes are made of materials from the metal oxides, semi-metal oxides, ceramics, plastics and composite materials comprising these substances comprehensive group, preferably of quartz glass or alumina.

12. Device according to one of claims 10 or 11, characterized in that the outer dielectric tube (3) is partially surrounded by a metal casing (11).

13. The apparatus according to claim 12, characterized in that the metal casing (11) consists of a metal tube segment, a metal foil or a metal layer.

14. Device according to one of claims 12 or 13, characterized in that the metal casing (11) leaves free a region of the lateral surface of the outer dielectric tube (3), which extends over the entire length of the dielectric tube (3).

15. Device according to one of claims 10 to 14, characterized in that it comprises a process chamber outside the outer dielectric tube (3).

16. Device according to one of claims 10 to 15, characterized in that the microwave feed a Microwave antenna or a cavity resonator with coupling points, preferably a coaxial resonator is.

17. The device according to one of claims 10 to 16, characterized in that the microwave feed via microwave feed lines, preferably waveguide or coaxial, with a microwave generator, preferably a klystron or magnetron, is connected.

18. Use of a method according to one of claims 1 to 9 for the production of a plasma for coating, cleaning, modification and etching of workpieces, for the treatment of medical implants, for textile treatment, for sterilization, for light generation, preferably in the spectral range infrared to ultraviolet, for Implementation of gases or for gas synthesis as well as in the technology for exhaust gas purification.

19. Use of a device according to one of claims 10 to 17 for the production of a plasma for coating, cleaning, modification and etching of workpieces, for the treatment of medical implants, for textile treatment, for sterilization, for light generation, preferably in the spectral range infrared to ultraviolet, for Implementation of gases or for gas synthesis as well as in the technology for exhaust gas purification.