EP1068778A1

EP1068778A1 - Plasma torch with a microwave transmitter

Info

Publication number: EP1068778A1
Application number: EP99920621A
Authority: EP
Inventors: Heinz-Jürgen BLÜM; Uwe Hofmann
Original assignee: Individual
Current assignee: MIKROWELLEN UMWELT TECHNOLOGIE GmbH
Priority date: 1998-04-02
Filing date: 1999-04-01
Publication date: 2001-01-17
Anticipated expiration: 2019-04-01
Also published as: US6388225B1; ES2192383T3; ATE232042T1; EP1068778B1; CA2327093A1; DE19814812C2; DE19814812A1; WO1999052332A1

Abstract

A plasma torch with a microwave transmitter which, for example, is used to coat surfaces and to produce radicals. The plasma torch exhibits a minimal energy loss during the transmission of microwaves to the produced plasma flame. The plasma torch includes a waveguide for transmitted microwaves and has a coaxial conductor. An electrode is provided with a duct, and a nozzle provided on the other end of the duct, the end facing away from the waveguide, are arranged in the coaxial conductor in an essentially axial manner. The plasma flame is produced at the nozzle. A coupling element is arranged between the waveguide and the coaxial conductor. The electrode is connected to the coupling element via a mounting plate and an electrically insulating intermediate element in a gas tight, thermally insulated manner such that microwaves are permitted to pass through.

Description

Plasma torch with a microwave transmitter

description

The invention relates to a plasma torch with a microwave transmitter according to the preamble of the claims, which is intended, for example, for coating surfaces and for generating radicals.

Known magnetron ion sources work with a magnetron to generate an alternating electric field, see DE 37 38 352 AI.

The disadvantage is that a quartz dome and external magnetic fields are required to generate the gas plasma. The intense

Magnetic field in the discharge space serves to adapt the

Cyclotron frequency to that of the microwave generator. An electrodeless microwave gas discharge is used. The device must be cooled when it is in operation. These plasma generators have a complex structure and are limited in their dimensions. The technical effort for microwave discharge systems is high. No large powers can be transmitted and it cannot be seen that high density plasmas are stable in the case of large powers.

Devices for generating a plasma by microwaves, as are known, for example, from DE 3905303 C2, DE 3915477 C2, US 5349154 A, work with quartz tubes. A magnetron

(Microwave transmitter unit) is attached to one end of a rectangular waveguide. The microwaves generated run through the waveguide and meet at its other end a quartz glass insert through which a special gas flows. The flow comes about through a negative pressure maintained in the recipient. In the quartz glass insert, a microwave is created by the microwave energy, which flows through the quartz glass insert into the recipient. The process is characterized in that it has no electrodes. These devices have the following disadvantages: The hottest point and the center of the plasma are in the part of the quartz glass insert which is arranged in the rectangular waveguide. As a result, the energy is not converted in the recipient, but in front of it, and if used appropriately, there are too few radicals available for the work process.

- There is a high proportion of wall effects in the quartz glass. - The mass throughput and the working pressures of 500 Pa to 3 kPa are too low.

- The quartz glass insert is not suitable for large-scale continuous operation. Because of the undesirably high temperatures, melting phenomena occur on it, or complex cooling devices have to be provided in addition.

- The efficiency of energy use is low.

- The vacuum tightness on the sealing surfaces is difficult to maintain.

- The glass can be destroyed during assembly or disassembly and due to the thermal expansion of the metal components.

Arrangements are also known in which there is a cross coupling of a rectangular waveguide with a coaxial conductor. In this case too, a microwave generating or transmitting device, the magnetron, is attached to one end of the waveguide. The microwaves generated run through the waveguide and hit a conductive elongated nozzle. The waveguide is closed with a short-circuit slide. The resulting electromagnetic wave can thus be tuned. Such a known arrangement can be carried out with a quartz tube (DE 195 11 915 C2) or without a quartz tube (US 4,611,108 A). Apart from the fact that the specific disadvantages mentioned above occur when using quartz tubes, this cross coupling has the following disadvantages:

- The use of microwave power is not very efficient. - There are energy losses at the cross coupling of rectangular waveguide and coaxial conductor.

- The whole structure is complicated.

- The maximum operating pressure and mass flow are too low.

From US 4,473,736 A a plasma generator is known in which a cavity and a coaxial conductor are capacitively coupled. Are there - 3 -

insulating, thin electrodes holding the electrode arranged over the entire cross section of the cavity and the coaxial conductor. Apart from the fact that this is not a hollow waveguide, this arrangement is not suitable for matching the impedance and for achieving a low-reflection hollow waveguide.

The invention is therefore based on the object of creating a plasma torch which generates a plasma in the region close to normal pressure with high densities. High performance should be able to be transferred. The plasma torch should be characterized by stable combustion and efficient use of microwave power. Avoid resulting quartz tubes or quartz domes for generating the plasma. A plasma torch which is overall simple in construction is to be created.

According to the invention, this is solved by the characterizing feature of the claim. It is initially irrelevant whether the coaxial conductor is directed transversely in a cross coupling or parallel to the waveguide in an axial coupling, that is to say whether its longitudinal axes preferably form a right angle with one another or whether their longitudinal axes essentially coincide with one another. The plasma torch (plasma generator) contains a vacuum chamber and a magnetron, which generates a field strength sufficient for plasma formation even within the val um chamber. A recipient adjoining the coaxial conductor is under a pressure of 100 Pa to 10 kPa, which is suitable for the formation of the plasma. A high degree of efficiency is achieved regardless of the type of coupling. Due to its simple axial construction using an antenna as an electrode, the plasma torch according to the invention manages without cooling and magnetic coils. The advantage of using a hollow waveguide instead of an AC waveguide is that the microwave power is coupled into the plasma not only in the vicinity of the nozzle, where the greatest field strengths occur, but via the hollow waves along the entire waveguide axis. This design enables a quasi-electrode-less coupling, which reduces the thermal load on the nozzle. The hollow electrode is advantageously designed as a truncated cone and fastened to a non-conductive intermediate piece which is connected to the coaxial conductor via a preferably disc-shaped holder. The nozzle is connected to a gas connection through the intermediate piece. The retaining washer is flanged to the coaxial conductor and the waveguide. The hollow electrode is advantageously designed as a truncated cone, the top surface of which faces the recipient. On this side, it has a nozzle, preferably screwed in and replaceable, which is inserted into its cavity and has four outlet openings for the process gas which are at regular intervals on a circle around the center of the outlet plane and in the outlet plane. This leads to an optimal conduction of the microwave to the outlet level (nozzle tip) and a more favorable energy input into the plasma flame. A nozzle suitable for high temperatures advantageously consists of a metallic-ceramic alloy. An electrically non-conductive insulator thermally insulates the space of the plasma flame from the coupling. A solution which is favorable for the operation of the plasma torch is obtained if the electrode is axially and possibly radially adjustable. In the cross coupling, a brass part and a second hissing piece advantageously connect the nozzle and the first intermediate piece to a gas connection. The brass part always guarantees the electromagnetic coupling of the waveguide and coaxial conductor. To tune the electromagnetic wave to the coupling, the waveguide, preferably a rectangular waveguide, of the cross coupling is provided with two screws. In the case of the waveguide, preferably the circular waveguide, of the axial coupling, the tuning is advantageously carried out by changing its length. For this purpose it consists, for example, of two parts which can be telescopically pushed into one another, even during the process. One of the tubes can be provided with longitudinal slots and resilient tabs remaining between them. A microwave seal is advantageously provided in an annular gap located between the tubes in the overlap area. For the transition from the coaxial conductor into the recipient, a passage of the electrode and the process gas is provided; this enables an efficient coupling of the electromagnetic wave. - 5 -

The invention is explained below with reference to the schematic drawing of two exemplary embodiments. Show it:

1 is a cross coupling of a rectangular waveguide with a coaxial conductor in longitudinal section,

Fig. 2 shows an axial coupling of a circular waveguide with a

Coaxial conductor in longitudinal section and Fig. 3 is an enlarged view of the front view of the nozzle.

In Fig. 1 is coupled to a rectangular waveguide 1 with a longitudinal axis XX, a cylindrical coaxial conductor 2 with a longitudinal axis YY via a coupling piece 3 near one of its ends so that the longitudinal axes XX and YY are directed at right angles to each other. The coupling piece 3 is bowl-shaped with a central opening 4 and a peripheral flange 5 and contains a receiving disk 6 for an intermediate piece 7 made of insulating material. The disc 6 is rigidly and tightly connected to the coupling piece 3 by means of a ring 8 screwed to the peripheral flange 5. The central opening 4 in the coupling piece 3 corresponds to an identical opening 9 in the rectangular waveguide 1, which is also surrounded by a flange 10 to which the coupling piece 3 is screwed. The ring 8 is the end part of a waveguide 20, which contains an insulator 11 and at the other end of which there is a recipient 12. Aufhahmescheibe 6, intermediate piece 7 and insulator 11 are made sufficiently solid and together form a gas-tight, thermally insulating, but microwave-permeable transition between the rectangular waveguide 1 and the waveguide 20. In addition, the intermediate piece 7 must have dielectric properties that a low-reflection waveguide on Ensure transition. A conical electrode 13 made of a metal-ceramic alloy is fastened to the intermediate piece 7 on its side facing the recipient 12 and, like the intermediate piece 7, has an axial passage 14 into which a nozzle 22 is fixed at the free end of the electrode 13 or used interchangeably, preferably screwed. The longitudinal axis of the electrode 13 coincides with the axis YY. On the other side of the intermediate piece 7, the bushing 14 is followed by a brass part provided with an axial bore 15 16 with an insulating, the axial bore 15 continuing connector 17, which leads to a gas connection 18. The connector 17 is held by a flat support 19 which is tightly screwed to the rectangular waveguide 1. The cylindrical waveguide 20 and the electrode 13 together form a coaxial conductor 2. The frustoconical electrode 13 is located in a corresponding recess 21 in the insulator 11 so that the nozzle 22 protrudes beyond the insulator 11 on the recipient side. The rectangular waveguide 1 is provided at the other end with a magnetron 23, from which microwaves are generated and transmitted through the conductor 1. Two screws (steps) 24 are used to influence the microwaves on the coupling.

The microwaves generated by the magnetron 23 pass through the conductor 1 and are matched to the coupling by the screws 24. Through the cross coupling, a longitudinal wave is coupled out into the coaxial conductor 2, and an axial electromagnetic field is created. The cross coupling consists of a coupling pin which is essentially identical to the electrode 13 with which it projects into the circular waveguide 20 and forms the coaxial line with it. The coupling pin 13 has the task of guiding the process gas and letting a plasma or a plasma flame 25 arise at the opening of the nozzle 22. The gas is fed into the coupling pin from the outer gas connection 18 via the bores 15 in the connecting piece 17 made of Teflon and in the brass part 16, and the passage 14 in the intermediate piece 7 likewise made of Teflon. The brass part 16 also ensures good coupling of the microwave. The electrode 13 is fastened in an insulated manner in the coaxial conductor 2 by the connecting piece 7. The geometry of the electrode 13 is optimally matched to the process requirements. It ensures maximum dielectric strength. What is important for the operation is its favorable length, which can be changed by the bushing 14 which can be adjusted by means of a thread in the electrode 13. Their cross-section is selected so that the coaxial conductor 2 ensures optimal conduction of the electromagnetic wave and the highest field strength occurs at the nozzle tip. This is very important because the plasma ignites at the point of greatest field strength. The nozzle 22 is made of a special material. she consists of a composite material that has ceramic components and is metallically conductive. The ceramic fulfills the task of thermal insulation of the plasma cloud from the electrode 13. The plasma can be operated up to a pressure of 35 kPa. A significantly larger mass throughput can thus be achieved. This is a great advantage in order to be able to generate many more reaction partners in a corresponding process. This makes it possible to greatly reduce process times due to the significantly increased mass throughput. Another advantage of this burner is that these parameters can also be achieved with air as the process gas. This eliminates all expensive additional gases, such as noble gases (argon).

In FIG. 2, an air-cooled magnetron 23 connected to a control unit 26 is attached to a base plate 30 with a fan 27, a temperature monitor 28 and a heating transformer 29. The magnetron 23 for generating the microwaves has a power of 2 kW and emits electromagnetic waves with a fixed frequency of 2.45 GHz and a wavelength of 12.24 cm. Its output can be regulated linearly between 10% and 100% of the maximum output by the control unit 26. The temperature monitor with the thermal switch is connected to the resonator of the magnetron 23. At a temperature of 120 ° C it switches off the magnetron for safety reasons. The base plate 30 is fastened to a round hollow conductor 31 which has an inner tube 32 with a diameter of 100 mm and a wall thickness of 2 mm and an outer tube 33 with a diameter of 104 mm and a wall thickness of 2 mm. The tubes 32, 33 are very well fitted into one another and are telescopically displaceable. They can be fixed to one another with a clamping screw 34. The outer tube 33 is provided to produce a certain pressure when moving with longitudinal slots 35, only one of which can be seen, so that resilient tabs are formed on the outer tube 33 between the slots 35, which press slightly against the inner tube and prevent unwanted displacement of both tubes 32 , 33 prevent each other as far as possible even when the clamping is released. At the same time, this improves the electrical contact between the tubes 32, 33, and it - 8th -

rollovers between the pipes are avoided. In order to ensure microwave tightness of the circular waveguide 31, a microwave seal 36, for example in the form of a metal gauze, can be inserted into the annular gap between the two tubes 32, 33. The outer tube 33 is provided at its end facing away from the magnetron 23 with a flange 37, via which the axial coupling takes place with an adjoining coaxial conductor 2, which has a common longitudinal axis X-Y with the circular waveguide 31. With this coupling, a longitudinal wave is coupled out into the coaxial conductor 2, and an axial electric field is created.

The coaxial conductor 2 and also an adjoining recipient 12 have the same diameter or cross-section as the outer tube 33. As a result, the recipient 12 simultaneously fulfills the task of a waveguide which prevents the waves from spreading laterally and in this way microwave power over a considerable distance couples into the plasma 25 behind the nozzle 22 along the axis XY (likewise the axis YY in FIG. 1). The coaxial conductor 2 also has at its end facing the circular waveguide 31 a flange 38 which is adapted to the flange 37, is screwed to it and essentially forms a coupling piece which corresponds to the coupling piece 3 of FIG. 1. Both flanges 37, 38 encompass the periphery of a receiving disc 6 made of any material (aluminum, quartz glass) and hold it vacuum-tight and firmly. In this disk 6, the inner conductor 39 of the coaxial line 2 is suspended in an electrically insulated manner via an intermediate piece 7 made of PTFE. The use of Teflon has the advantage that it is easy to work with and guarantees permanent vacuum tightness. This vacuum feedthrough also fulfills the task of carrying out the microwave waves in the recipient 12 and the thermal insulation of the waveguide 32 from the hot plasma 25. The inner conductor 39 serves to couple the circular waveguide and recipient, the gas supply and the expansion of the gas via an electrode

13 screwed nozzle 22 into the recipient 12. Its position in the coaxial conductor 2 and its length are adjustable to match the microwave. The electrode 13 is fastened to the intermediate piece 7 and, like this, has a passage 14 for the gas supply. This implementation

14 can be a compressed air hose 40 made of PE (polyethylene) - 9 -

Brass part (similar to Fig. 1) can be connected. Intermediate piece 7, electrode 13 and nozzle 22 form an antenna, the outer diameter of which is 20 mm. Its longitudinal axis coincides with the XY axis. The plasma 25 ignites at the nozzle 22 screwed in at the end of the antenna. A detachable connection between the electrode 13 and the nozzle 22 is important in order to be able to replace or replace the nozzle 22. Since the nozzle 22 is exposed to very high thermal loads, it is made of high-temperature resistant steel; For example, a metallic alloy with a maximum operating temperature of 1425 ° C is used. This material is characterized in that the nozzle 22 is metallically conductive and forms a ceramic surface under the influence of high temperatures, which can withstand the high temperatures. Since the frequency of the microwave used is below the plasma frequency, it cannot spread in the plasma 25. In order to achieve the best possible energy input into the plasma 25, the surface of the plasma cloud must assume a maximum. Therefore, the nozzle 22 ensures a strong swirling of the plasma 25. For this purpose, according to FIG. 3, four gas outlet openings 43 each having a diameter of 1 mm are provided in the outlet plane 41 of the nozzle 22, preferably in a regular arrangement on a circle 42. For thermal insulation of the plasma flame from the flanges 38, 39 or the disk-shaped receptacle 6, a thermal insulator 11 is arranged between the latter and the plasma flame 25, through which the electrode 13 with the nozzle 22 projects. The recipient 12, like the coaxial conductor 2, consists of a tube with a diameter of 104 mm, a wall thickness of 2 mm and a length of 300 mm. It can be provided with means, not shown, for temperature measurement, for pumping out and for monitoring the flame. Air can advantageously be used as the process gas. The operation of the plasma 25 is possible up to a pressure of 100 kPa. This enables an even greater mass throughput to be achieved. The axial coupling according to the invention is particularly well suited to generate the highest possible energy and many radicals in the recipient. Overall, the axial coupling according to the invention offers the following advantages: It enables efficient use of the microwave power. - It enables uncomplicated assembly. - 10 -

- It guarantees a high maximum operating pressure and mass throughput.

- It avoids the energy loss of the cross coupling.

Instead of using the clamping screw 34, the pipes 32, 33 can be mutually fixed with a clamp that encompasses both. A membrane bellows and interchangeable circular waveguide pieces can also be used to change the length of the circular waveguide 31. The quick, simple and precise adjustment of the length of the round waveguide is conducive to being able to adjust the diaphragm bellows in stages or continuously along a linear guide while the device according to the invention is in operation.

All features shown in the description, the following claims and the drawing can be essential to the invention both individually and in any combination with one another.

- 11 -

Reference list

1 rectangular waveguide

2 coaxial conductors

3 coupling piece

4, 9 openings

5, 10, 37, 38 flanges

6 mounting washer

7 intermediate piece

8 ring

11 isolator

12 recipient

13 electrode (coupling pin)

14 implementation

15 axial bores

16 brass part

17 connector

18 gas connection

19 carriers

20 waveguides

21 recess

22 nozzle

23 magnetron

24 screws

25 plasma

26 control unit

27 fans

28 temperature monitors

29 heating transformer

30 base plate

31 circular waveguide

32 inner tube (inner tube)

33 outer tube (outer tube)

34 clamping screw

35 (longitudinal) slots

36 Microwave seal

39 inner conductor

40 compressed air hose

41 Exit plane of the nozzle

42 circle

43 gas outlet openings XX, YY, XY (longitudinal) axes

Claims

- 12 patent claims

1. Plasma torch with a microwave transmitter, a waveguide for the transmitted microwaves and a coaxial conductor in which there is an electrode provided with a bushing and a nozzle provided on the end of the bushing facing away from the waveguide, essentially in an axial arrangement, with a plasma cloud on the nozzle is generated, characterized in that a coupling piece is arranged between the waveguide and the coaxial conductor and that the electrode is gas-tight and permeable to microwaves connected to the coupling piece via a disk-shaped receptacle and an electrically and thermally insulating intermediate piece held in this receptacle.

2. Plasma torch according to claim 1, characterized in that the electrode is designed as a truncated cone.

3. Plasma torch according to claim 1, characterized in that the nozzle is interchangeably fastened in the passage.

4. Plasma torch according to claim 2, characterized in that the nozzle with the electrode is adjustable parallel and at right angles to the longitudinal axis of the electrode.

5. Plasma torch according to at least one of the preceding claims, characterized in that the insulated connection of the electrode to the coupling piece has the shape of an intermediate piece located in a variable suspension.

6. Plasma torch according to claim 5, characterized in that the longitudinal axis of the electrode is directed transversely to the longitudinal axis of the waveguide. - 13 -

7. Plasma torch according to claim 5, characterized in that the longitudinal axis of the electrode is directed parallel to the common longitudinal axis of the waveguide and coaxial conductor.

8. Plasma torch according to claim 6 or 7, characterized in that on the waveguide side of the electrode and the intermediate piece is provided with a brass part provided with a bore, the bore being in the extension of the leadthroughs of the electrode and intermediate piece.

9. Plasma torch according to claims 6 and 8, characterized in that the bushings are connected to a gas connection via the holes in the brass piece and a connecting piece.

10. Plasma torch according to claim 1, characterized in that the nozzle consists of a ceramic-metallic compound.

11. Plasma torch according to claims 6 or 7, characterized in that a thermal insulator is provided between the plasma cloud and the disk-shaped receptacle.

12. Plasma torch according to claim 6, characterized in that at least one screw which is adjustable transversely to its longitudinal axis is arranged in the waveguide for tuning the microwave field.

13. Plasma torch according to claim 7, characterized in that the waveguide consists of two telescopically adjustable and tubes.

14. Plasma torch according to claim 13, characterized in that one, preferably the outer of the two tubes is provided with longitudinal slots over part of its length.

15. Plasma torch according to claim 13 or 14, characterized in that a gill means is provided for locking the tubes. - 14 -

16. Plasma torch according to one of claims 13 to 15, characterized in that a microwave seal is provided in an annular gap between the two tubes.

17. Plasma torch according to claim 1, 5 or 6, characterized in that the gas outlet openings of the nozzle are outside the nozzle axis.

18. Plasma torch according to claim 1, characterized in that the recipient has the same cross-section as the waveguide for the transmitted microwaves and protrudes considerably beyond the nozzle tip.