Patent Application
20100301012
This application is a National Stage application of International Application No. PCT/EP2007/008839, filed on Oct. 11, 2007, which claims priority of German application number 10 2006 048 814.8, filed on Oct. 16, 2006, both of which are incorporated herein by reference in their entireties.
The present invention relates to a device for producing microwave plasmas with a high plasma density, comprising at least one microwave feed that is surrounded by at least one dielectric tube. Furthermore, the present invention relates to a method for producing microwave plasmas with a high plasma density by using the device.
Devices for generating microwave plasmas are being used in the plasma treatment of workpieces and gases. Plasma treatment is used, for example, for coating, cleaning, modifying and etching workpieces, for treating medical implants, for treating textiles, for sterilisation, for light generation, preferably in the infrared to ultraviolet spectral range, for converting gases or for gas synthesis, as well as in waste gas purification technology. To this end, 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, fibres and webs, to any configuration of shaped articles.
The most important process gases are inert gases, fluorine-containing and chlorine-containing gases, hydrocarbons, furans, dioxins, hydrogen sulfides, oxygen, hydrogen, nitrogen, tetrafluoromethane, sulfur hexafluoride, air, water, and mixtures thereof In the purification of waste gases by means of microwave-induced plasma, the process gas consists of all kinds of waste gases, especially carbon monoxide, hydrocarbons, nitrogen oxides, aldehydes and sulfur oxides. However, these gases can be used as process gases for other applications as well.
Devices that generate microwave plasmas have been described in the documents WO 98/59359 A1, DE 198 480 22 A1 and DE 195 032 05 C1.
The above-listed documents have in common that they describe a microwave antenna in the interior of a dielectric tube. If microwaves are generated in the interior of such a tube, surface waves will form along the external side of that tube. In a process gas which is under low pressure, these surface waves produce a linear elongate plasma. Typical low pressures are 0.1 mbar-10 mbar. The volume in the interior of the dielectric tube is typically under ambient pressure (generally normal pressure; approx. 1013 mbar). In some embodiments a cooling gas flow passing through the tube is used to cool the dielectric tube.
To feed the microwaves, hollow waveguides and coaxial conductors are used, inter alia, while antennas and slots, among others, are used as the coupling points in the wall of the plasma chamber. Feeds of this kind for microwaves and coupling points are described, for example, in DE 423 59 14 and WO 98/59359 A1.
The microwave frequencies employed for generating the plasma are preferably in the range from 800 MHz to 2.5 GHz, more preferably in the range from 800 MHz to 950 MHz and 2.0-2.5 GHz, but the microwave frequency may lie in the entire range from 10 MHz up to several 100 GHz.
DE 198 480 22 A1 and DE 195 032 05 C1 describe devices for the production of plasma in a vacuum chamber by means of electromagnetic alternating fields, comprising a conductor that extends, within a tube of insulating material, into the vacuum chamber, with the insulating tube being held at both ends by walls of the vacuum chamber and being sealed with respect to the walls at its outer surface. The ends of the conductor are connected to a generator for generating the electromagnetic alternating fields.
A device for producing homogenous microwave plasmas according to WO 98/59359 A1 enables the generation of particularly homogeneous plasmas of great length, even at higher process pressures, as a result of the homogeneous input coupling.
The possible applications of the above-mentioned plasma sources are limited by the high energy release of the plasma to the dielectric tube. This energy release may result in an excessive heating of the tube and ultimately lead to the destruction thereof. For that reason, these sources are typically operated at microwave powers of about 1-2 kW at a correspondingly low pressure (approximately 0.1-0.5 mbar). The process pressures can also be 1 mbar-100 mbar, but only under certain conditions and at a correspondingly low power, in order not to destroy the tube.
With the above-mentioned devices, typical plasma lengths of 0.5-1.5 m can be achieved. With plasmas of almost 100% argon it is possible to achieve greater lengths, but such plasmas are of little technical importance.
Another problem with such plasma sources lies in the channelling of the process gas, especially at higher process gas pressures (above 1 mbar). The reason for this is that with increasing radial distance from the dielectric tube, the plasma density decreases strongly. This makes it more difficult to supply new process gas to the areas of high charge carrier density. In addition, at higher process pressures, the thermal power dissipated to the dielectric tube increases.
However, higher process gases are preferred since they frequently result in a clear, tenfold to hundredfold, increase in the process velocity.
It is the object of the present invention to overcome the above-mentioned disadvantages and thereby to achieve an increase in plasma concentration and in the process gas pressure.
In accordance with the invention, this object is achieved by a device for generating microwave plasmas in accordance with the present invention. This device comprises at least one microwave feed that is surrounded by an inner dielectric tube. Said inner dielectric tube is in turn surrounded by at least one outer dielectric tube. A space is thereby formed which is suitable for receiving and conducting a fluid.
By means of the device it is possible to conduct a fluid through the above-described double-tube arrangement in an advantageous manner, which fluid can be used for cooling or for being supplied to the process gas.
Suitable microwave feeds are known to those skilled in the art. Generally, a microwave feed consists of a structure which is able to emit microwaves into the environment. Structures that emit microwaves are known to those skilled in the art and can be realised by means of all known microwave antennae and resonators comprising coupling points for coupling the microwave radiation into a space. For the above-described device, cavity resonators, bar antennas, slot antennas, helix antennas and omnidirectional antennas are preferred. Coaxial resonators are especially preferred.
In service, the microwave feed is connected via microwave feed lines (hollow waveguides or coaxial conductors) to a microwave generator (e.g. klystron or magnetron). To control the properties of the microwaves and to protect the elements, it is furthermore possible to introduce circulators, insulators, tuning elements (e.g. 3-pin tuners or E/H tuners) as well as mode converters (e.g. rectangular and coaxial conductors) in the microwave supply.
The dielectric tubes are preferably elongate. This means that the tube diameter: tube length ratio is between 1:1 and 1:1000, and preferably 1:10 to 1:100. The two tubes may be equally long or be of a different length. Furthermore, the tubes are preferably straight, but they may also be of a curved shape or have angles along their longitudinal axis.
The cross-sectional surface of the tubes is preferably circular, but generally any desired surface shapes are possible. Examples of other surface shapes are ellipses and polygons.
The elongate shape of the tubes produces an elongate plasma. An advantage of elongate plasmas is that by moving the plasma device relative to a flat workpiece it is possible to treat large surfaces within a short time.
The dielectric tubes should, at the given microwave frequency, have a low dielectric loss factor tan δ for the microwave wavelength used. Low dielectric loss factors tan δ are in the range from 10-2 to 10-7.
Suitable dielectric materials for the dielectric tubes are metal oxides, semimetal oxides, ceramics, plastics, and composite materials of these substances. Particularly preferred are dielectric tubes made of silica glass or aluminium oxide with dielectric loss factors tan δ in the range from 10-3 to 10-4. The dielectric tubes here may be made of the same material or of different materials.
According to one particular embodiment, the dielectric tubes are closed at their end faces by walls.
A gas-tight or vacuum-tight connection between the tubes and the walls is advantageous. Connections between two workpieces are known to those skilled in the art and may, for example, be glued, welded, clamped or screwed connections. The tightness of the connection may range from gas-tight to vacuum-tight, with vacuum-tight meaning, 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 ultrahigh vacuum (10-7-10-12 hPa). Generally, the term “vacuum-tight” here refers to tightness in a rough or fine vacuum.
The walls may be provided with passages, through which a fluid can be conducted. The size and shape of the passages can be chosen at will. Depending on the application, each wall may contain at least one passage. In a preferred embodiment, there are no passages in the region that is covered by the face end of the inner dielectric tubes.
The fluid is conducted through the space between the outer dielectric tube and the inner dielectric tube, and is fed and discharged, respectively, via the apertures in the walls at the face ends of the dielectric tubes.
The flow velocity and the flow behaviour (laminar or turbulent) of the dielectric fluid flowing through the dielectric tube is to be chosen such that the fluid, in particular if it is a liquid, has good contact with the boundary of the dielectric tube and that, in addition, where a liquid fluid is used, there does not occur any evaporation of the dielectric liquid. How the flow velocity and flow behaviour can be controlled by means of pressure and by means of the shape and size of the passages is known to those skilled in the art.
Suitable for use as the dielectric fluid are both a gas and a dielectric fluid.
However, cooling of the dielectric tube by means of a fluid cannot be realised in an easy fashion since the energy input of the microwaves to the fluid leads to the heating of the latter.
Any additional heating of the fluid will decrease the cooling effect on the dielectric tube. This decrease in the cooling performance can also, at a high microwave absorption by the fluid, lead to a negative cooling performance. This corresponds to an additional heating of the dielectric tube.
To keep the heating of the fluid by the microwaves as low as possible, the fluid must, at the wavelength of the microwaves, have a low dielectric loss factor tan δ in the range of 10-2 to 10-7. This prevents a microwave power input into the fluid or reduces said input to an acceptable degree.
Because of their higher thermal coefficient, liquid fluids absorb more thermal power than gaseous fluids.
An example of such a dielectric liquid is an insulating oil that has a low dielectric loss factor. Insulating oils are, for instance, mineral oils, olefins (e.g. poly-alpha-olefin) or silicone oils (e.g. COOLANOL® or dimethylpolysiloxane). Hexadimethylsiloxane is preferred as the dielectric liquid.
By means of this fluid cooling of the outer dielectric tube, it is possible to reduce the heating of the outer dielectric tube. This enables higher microwave powers which, in turn, lead to an increase in the concentration of the plasma at the outside of the outer dielectric tube. In addition, the cooling enables a higher process pressure than in uncooled plasma generators.
By contrast to the gas cooling according to DE 195 032 05, where the cooling gas is in contact with the microwave feed, in the device described herein the contact between the fluid and the microwave feed is prevented by the double-tube arrangement, thereby excluding any possibility of the fluid reacting with the microwave feed. Furthermore, this separation of fluid and microwave feed greatly facilitates the maintenance of the microwave feed.
In a preferred embodiment according to the invention, the material of the outer dielectric tube is replaced by a porous dielectric material. Suitable porous dielectric materials are ceramics or sintered dielectrics, preferably aluminium oxide. However, it also possible to provide tube walls made of silica glass or metal oxides that have small holes.
When a gas flows between the dielectric tubes, part of the gas escapes through said pores. Since the highest microwave field strengths are present at the surface of the outer dielectric tube, the gas molecules, upon passing through the outer dielectric tube, travel through the zone of the highest ion density.
Furthermore, after passing through the pores, the gas has a resultant movement direction radially away from the tube.
If the same gas is used for cooling as is used as the process gas, the portion of the excited particles is increased by the passage of the process gas through the region of the highest microwave intensity. In this way, an efficient transport of excited particles to the workpiece is ensured. This increases both the concentration and the flow of the excited particles.
Furthermore, such an arrangement is also particularly suitable for carrying out pure gas conversion processes such as waste gas purification or gas synthesis processes. Further process gases can, if required, be fed through further porous tubes of the processing chamber.
Due to the porosity of the outer dielectric tube and the gas pressure, the flow (molecules per area per time) of the process gas or process gas mixture is governed by the outer dielectric tube.
Furthermore, in this waste gas purification method, all gas molecules have to pass through the tube wall, and thus through the region of highest ion density. This constitutes an advantage over established methods, wherein the furnace chamber is located in the interior of a volume and the microwaves are irradiated from outside. With an established method, the portion of the purified waste gas is smaller than in the method presented herein since in such a conventional method those portions of the gas which are in the vicinity of the volume are not ionised due to the low field strengths which are present there.
Any known gas may be used as the process gas. The most important process gases are inert gases, fluorine-containing and chlorine-containing gases, hydrocarbons, furans, dioxins, hydrogen sulfides, oxygen, hydrogen, nitrogen, tetrafluoromethane, sulfur hexafluoride, air, water, and mixtures thereof. In the purification of waste gases by means of microwave-induced plasmas, the process gas consists of all kinds of waste gases, especially carbon monoxide, hydrocarbons, nitrogen oxides, aldehydes and sulfur oxides. However, these gases can be used as process gases for other applications as well.
According to a further embodiment, a further dielectric tube may be installed within the outer dielectric tube, said further dielectric tube surrounding the inner dielectric tube and likewise being connected with the walls at its end faces in a gas-tight or vacuum-tight manner. In this embodiment, the space between the outer dielectric tube and the inner dielectric tube is divided into an outer and an inner space.
If the process gas is guided through the outer space and a fluid is guided through the inner space, it is possible to cool the inner dielectric tube and the microwave structure. This, in turn, enables a better process performance. The fluid should not absorb the microwaves. Especially where a liquid is used as the fluid, the liquid should have a low dielectric loss factor tan δ in the range of 10-2 to 10-7 for the microwave wavelength used.
In order to further reduce the microwave power requirement for the above-mentioned plasma sources, according to another preferred embodiment it is possible for a metallic jacket to be applied around the outer dielectric tube, said jacket partially covering the tube. This metallic jacket here acts as a microwave shield and may be made, for example, of a metallic tube, a bent sheet metal, a metal foil, or even a metallic layer, and may be plugged or electroplated thereon, or applied thereon in another way. Such metallic microwave shields are able to limit the angular range in which the generation of the plasma takes place as desired (e.g. 90° , 180° or 270°) and thereby reduce the power requirement accordingly.
Especially in the case of the embodiment comprising a metallic jacket of the devices for generating microwave plasmas, it is possible to treat broad material webs with a plasma at a low power loss. The jacket shields that region of the space present in the device which does not face the workpiece, and there is generated only a narrow plasma strip between the workpiece and the device, over the entire width of the workpiece.
All of the above-described devices for plasma generation, during operation, form a plasma at the outside of the dielectric tube. In a normal case, the device will be operated in the interior of a chamber (plasma chamber). This plasma chamber may have various shapes and apertures and serve various functions, depending on the operating mode. For example, the plasma chamber may contain the workpiece to be processed and the process gas (direct plasma process), or process gases and openings for plasma discharge (remote plasma process, waste gas purification).
In one method for producing microwave plasmas in an above-described device, a fluid is guided through the space between the inner dielectric tube and the outer dielectric tube, preferably through passages provided in the walls. In this case, the fluid may be a gas or a liquid.
The pressure of the fluid may be above, below or equal to the atmospheric pressure.
In an advantageous embodiment, a gaseous fluid, preferably a process gas, more preferably a waste gas, is conducted through the porous tube of the above-described device, comprising a porous external tube, and is thereby fed to the plasma process. The fluid here preferably has a low dielectric loss factor tan δ in the range of 10-2 to 10-7.
According to another advantageous embodiment, in the above-described device, comprising an inner middle and an outer dielectric tube, a gas, preferably a process gas, flows in the space between the outer dielectric tube and the middle dielectric tube, and a fluid flows in the space between the inner dielectric tube and the middle dielectric tube, said fluid preferably having a low dielectric loss factor tan δ. The outer dielectric tube here preferably has a porous wall.
In the following, the invention will be explained, by way of example, by means of the embodiments which are schematically represented in the drawings.
In a preferred embodiment, wherein the outer dielectric tube (3) has a porous tube wall, as outlined in
All of the embodiments are fed by a microwave supply, not shown in the drawings, consisting of a microwave generator and, optionally, additional elements. These elements may comprise, for example, circulators, insulators, tuning elements (e.g. three-pin tuner or E/H tuner) as well as mode converters (e.g. rectangular or coaxial conductors).
There are numerous fields of application for the above described device and the above described method. Plasma treatment is employed, for example, for coating, purification, modification and etching of workpieces, for the treatment of medical implants, for the treatment of textiles, for sterilisation, for light generation, preferably in the infrared to ultraviolet spectral region, for conversion of gases or for the synthesis of gases, as well as in gas purification technology. The workpiece or gas to be treated is brought into contact with the plasma or microwave radiation. The geometry of the workpieces to be treated ranges from flat substrates, fibres and webs to shaped articles of any shape.
Due to the increased plasma density and the increased plasma power, it is possible to achieve higher process velocities than with devices and methods according to the prior art.
What has been described above are preferred aspects of the present invention. It is of course not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, combinations, modifications, and variations that fall within the spirit and scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 048 814.8 | Oct 2006 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/008839 | 10/11/2007 | WO | 00 | 7/20/2009 |
