EP0285020A2 - Arrangement for coupling microwave energy to a leaky microwave line - Google Patents

Abstract

Device for introducing microwave energy into a container which is at least partially permeable to microwave energy. At least one microwave transmitter (21, 22) and an open microwave conductor (1) connected to the microwave transmitter are located in the immediate vicinity of the container. In order to increase the linear expansion of the plasma generated by the microwave energy and to even out the energy distribution, the open microwave guide (1) is provided with a microwave feed line (17, 18; 24, 25) at least at both ends.

Description

The invention relates to a device for introducing microwave energy into an at least partially permeable container for microwave energy with at least one microwave transmitter and an open microwave conductor connected to the microwave transmitter, which is located in the immediate vicinity of the container.

Such a container can either consist entirely of a material which is permeable to microwave energy, that is to say can be formed by a quartz tube, for example, or have a microwave-permeable window made of such a material.

A device of the type described above with a window is known for example from DE-PS 31 47 986. The "open microwave conductor" consists of a metallic, ladder-shaped structure with two parallel bars and numerous rungs, which are at right angles to the bars and connect the bars in an electrically conductive manner, and which are equidistant from one another. The rungs alternate with each other With such structures it is possible to emit the microwave power supplied at one end approximately uniformly over the length of the structure into the interior of the container. In the English language area such structures are also referred to as "slow wave structures", which can also be translated with "delay structures".

"In principle, delay structures are microwave conductors, over which a bound electromagnetic wave travels in a defined direction. The electrical power is in the form of the electrical field, that surrounds the structure with the so-called group velocity v _g . Regardless of this, the phase of the wave moves with the phase velocity v _Ph . The two speeds are also independent in terms of their sign, so that it can easily happen that group and phase speeds run in opposite directions. The term "slow" in connection with the waveguide structure can refer to both the phase velocity and the group velocity. In general, the group speed can be between 0 and the speed of light in a vacuum. The value "0" applies to a structure that consists of resonance resonant circuits that are not coupled to each other and over which no wave runs. In contrast, the speed of light is achieved in a smooth conductor. The phase speed, on the other hand, can also assume values that are greater than the speed of light.

The group speed is reduced by the load on a straight conductor with periodically arranged obstacles, in the case of the above-mentioned conductor structure by the "rungs" mentioned. Depending on the arrangement of these obstacles, speeds between 1 and 100% of the speed of light can be set in practice. A detailed one A discussion of the properties of periodic structures at microwave frequencies can be found in AS Harvey, IRE Transactions on Microwave Theory and Techniques, January 1960, 30.

A whole series of slow-wave structures for feeding microwave energy into containers and / or matter is described in the literature:

It is of particular interest that the electrical field associated with a certain power flow along the structure must obviously become larger with decreasing group speed, ie the relationship applies to a constant power flow:
E²∼1 / v _g .

This field can become so large for small group speeds and extend so far into space from the leading structure that it is suitable for generating non-thermal low pressure plasmas. Such an application is discussed, for example, by the group of authors ES Hotston, JM Weaver, DJH Wort, in "UKAEA Research Report", CLM-R78, 1968. This is a theoretical paper dealing with the coupling of microwave energy into a plasma. The energy is coupled in from a slow-wave structure located outside the plasma. The Plasma itself is limited by a magnetic field.

From US Pat. No. 3,663,858, it is known to generate a low-pressure plasma in a plasma container using a slow-wave structure which is of helical design.

In order to even out the energy input over the length of the structure, the structure or structures at an acute angle of attack is proposed both in the aforementioned DE-PS 31 47 986 as well as in US Pat. No. 3,472,200 and in US Pat. No. 3,814,983 to be placed opposite the neighboring matter. When using a container, a uniform plasma is to be achieved over the entire length of this container. The effectiveness and limits of this measure are described in DE-PS 31 47 986, so that the use of two structures intersecting at an acute angle with opposing energy supply is proposed to eliminate the disturbing influences. The known solution led to a uniformity sufficient for many applications over a length of the excited plasma of approx. 450 mm, which varies between ± 5% and ± 10% depending on the monomers used for the plasma process. At the same time, practical operation shows a number of limitations of the known solution:
- Due to the complexity, it is a relatively complex and expensive device.
- Since the substrate successively traverses two plasma zones with generally different thicknesses, this also results in a change in the structure of the layer.
- The structure length cannot be increased arbitrarily. This is because the difficulty arises that as the structure length increases, the required angle of attack relative to the microwave window becomes increasingly acute, which leads not only to mechanical but also to electrical instabilities.
- When the plasma length is divided into two or three structures arranged in series for large substrate widths, the phenomenon of so-called "plasma bellies" occurs in the optically visible plasma, which is also reflected in an uneven layer thickness distribution. The cause of this phenomenon has not yet been clarified.

The invention is therefore based on the object of improving a device of the type described at the outset in such a way that a great length expansion of the excited plasma with an extremely uniform energy distribution in the plasma is achieved with little structural outlay.

In the device described at the outset, the object is achieved in that the open microwave guide is provided with a microwave feed line at least at both ends thereof.

This design is in complete departure from the previously practiced design regulations to provide only a single microwave feed at the end of each open microwave conductor. At most, a so-called reactive load was then connected to the opposite end of the open microwave conductor via a microwave discharge line. This measure is also described in DE-PS 31 47 986, but it does not lead to the solution of the existing problems.

When using the measure according to the invention, each microwave conductor can first be arranged completely parallel to the container surface or to the window.

The substrate no longer has to pass through two plasma zones one after the other, possibly with different thicknesses, so that a much more homogeneous layer structure can be achieved.

Since the open microwave conductor is practically as often as desired along its length with other microwaves supply lines, there are no practical limits to the length change. By dispensing with an acute angle of attack, the arrangement is not only mechanically but also electrically stabilized.

The phenomenon of "plasma bellies" does not occur.

If the number of microwave inlets is an even number, e.g. two microwave feed lines can be connected to a single microwave transmitter via a T-shaped divider.

It is particularly advantageous if at least two microwave feed lines with a fixed phase relationship are connected to the at least one microwave transmitter. This is preferably possible by coupling one transmitter to two microwave feed lines through the line branch described above. However, several transmitters with a fixed phase relationship can also be used, so that in this case the microwave feed lines are each connected to their own microwave transmitter.

entspricht, wobei "λ" die Wellenlänge der Mikrowelle und "n" der Brechungsindex des Fenstermaterials für die Mikrowelle ist.According to a development of the invention, it is particularly advantageous in connection with a container in which there is a window made of a microwave-permeable material in a manner known per se, if its thickness of the relationship

corresponds, where "λ" is the wavelength of the microwave and "n" is the refractive index of the window material for the microwave.

The prior art does not deal with this dimensioning rule, which may be primarily due to the fact that the entire state of the art deals only with very small-sized devices in which the wall thickness or the thickness of the window is very small compared to the wavelength, which is about 12 cm (f = 2.5 GHz) at the commonly used frequencies.

In large-scale plants, however, with regard to the pressure difference to be expected that the window has to withstand, window thicknesses of approximately 15 to 20 mm can be expected purely by calculation. Here, however, partial reflection of the microwave energy occurs both on the front and on the back of the window, so that a corresponding interference can be observed. In addition, under the influence of the electric field around the delay structure, a corresponding electromagnetic wave is formed in the dielectric of the window.

It has been found according to the invention that this problem does not occur when the dimensioning rule described above is applied. In the case of quartz glass with an n = 1.94, the calculated window thickness is d = 30 mm, which has proven to be optimal.

Experiments carried out with windows dimensioned in this way resulted in a significant improvement in the uniformity of the plasma zone with, at the same time, a noticeably reduced weakening of the microwave power coupled into the plasma. It could be observed that the coupled power drops significantly if the thickness of the window is reduced from 30 mm to 15 mm. The result was that the plasma did not ignite at all or only with difficulty.

It is again particularly advantageous in the context of the further embodiment of the invention if the surface of the window facing the microwave guide has a covering made of a material with a smaller refractive index than that of the window material.

This makes it possible to reduce the reflection of the microwave on the atmosphere-side surface of the microwave window. Practical tests have shown that optimal conditions occur when a polytetrafluoroethylene plate with a thickness of about 5 mm is used as the covering. Two opposing effects had to be weighed up: On the one hand, the Ver uniformity of the plasma with increasing layer thickness of the topping, on the other hand the coupling of the microwave energy became noticeably less.

Finally, it is again particularly advantageous if the microwave guide is V-shaped in a symmetrical arrangement and has a smaller distance from the window at the apex of the “V” than at its feed points.

Exemplary embodiments of the subject matter of the invention and their mode of operation are explained in more detail below with reference to FIGS. 1 to 5.

• Figur 1 Teilausschnitte aus einem Fenster und einem darüber angebrachten Mikrowellenleiter,
• Figur 2 eine perspektivische Ansicht einer Vakuum­kammer mit einem Fenster, einen Mikrowellen­leiter und zwei Einspeisungsstellen, jeweils in Form von Teilausschnitten,
• Figur 3 eine Seitenansicht eines Mikrowellenleiters mit vier Einspeisungsstellen und zwei Mikrowellensendern,
• Figur 4 ein Diagramm mit Kurven über den Schicht­dickenverlauf und
• Figur 5 ein weiteres Diagramm über die Reflexionsver­hältnisse bei Fenstern ohne und mit einem Belag aus Polytetrafluoräthylen.

Show it:

• FIG. 1 partial sections from a window and a microwave guide attached above it,
• FIG. 2 shows a perspective view of a vacuum chamber with a window, a microwave guide and two feed points, each in the form of partial cutouts,
• FIG. 3 shows a side view of a microwave conductor with four feed points and two microwave transmitters,
• 4 shows a diagram with curves over the course of the layer thickness
• Figure 5 shows a further diagram of the reflection conditions in windows without and with a covering made of polytetrafluoroethylene.

FIG. 1 shows a partial length of an open microwave conductor 1 of the type described above above a microwave-permeable window 7. The microwave conductor 1 is made of metal, namely from two parallel bars 2 and 3, which are bridged at equidistant intervals by rungs 4 extending at right angles to the bars. The rungs are alternately connected to one of two center conductors 5 and 6.

The microwave-permeable window 7 is also only shown in a partial section. It consists of a quartz glass pane 8 facing the plasma with a thickness of 30 mm and a covering 9 made of polytetrafluroethylene with a thickness of 5 mm.

Such a window 7 is also shown in FIG. 2, it closes the upper wall 10 of a vacuum chamber 11, which is bounded at the bottom by a further wall 12. A substrate holder 13 with numerous substrates 14 becomes through the space between the walls 10 and 12 emotional. The direction of movement is indicated by an arrow 15; it runs at right angles to the longest axis of window 7.

Above the window 7, the open simplified microwave guide 1 is arranged in parallel. Only the rear part of a shielding housing 16 is shown.

The microwave conductor 1 is provided at intervals with microwave feed lines 17 and 18 which are connected to a microwave transmitter 21 via a T-shaped distributor piece 19 and a connecting piece 20. In the present case, the microwave feed line 17 is connected to one end of the microwave conductor 1. The microwave feed line 18 opens into the microwave guide 1 at a location distant therefrom. A mirror-symmetrical arrangement is located at the other end of the microwave guide 1, which is not shown further here.

FIG. 3 shows an arrangement which can be thought of as a result of doubling the relationships according to FIG. 2. In this case, a further microwave transmitter 22 is provided, which merges via a T-shaped distributor piece 23 into two further microwave feed lines 24 and 25, which open into the microwave guide 1 in a mirror-symmetrical arrangement to the feed lines 17 and 18. The microwave transmitters 21 and 22 are operated in a fixed phase relationship to one another, and the same fixed phase position is also ensured with respect to the microwave conductor 1 by the line branches or distributors 19 and 23.

FIG. 4 shows a graphic representation of the test results for two differently designed microwave conductors, each 900 mm long. This microwave guide was provided at both ends with the microwave feed lines 17 and 18. This resulted in a layer thickness distribution as given by the upper curve 26. The weak formation of a minimum in the middle of the microwave guide was observed. Through a weakly V-shaped configuration of a microwave conductor 1 a, which is indicated by the dashed lines, this minimum could be eliminated in the middle, as is evident from the lower curve 27. It should be noted that a length of the microwave guide of 900 mm already doubles the length of the previously available microwave guide. The deviations in the layer thickness uniformity were between approximately ± 5% and ± 10%. The ordinate shows the layer thickness belonging to curves 26 and 27 for coatings made from hexamethylene disilazane and from acetylene in nanometers (nm).

In FIG. 3, 28 denotes an adapter which is designed as a 3-bar tuner. This has a measuring point for the forward power P _V. Upstream of each is a circulator 29 for decoupling the reverse power P _R , which is fed via a microwave line ^{30 to} a reactive load, not shown. This protects the transmitters 21 and 22 from reverse power.

FIG. 5 shows the reflection of a microwave with a frequency of 2.5 GHz from the atmosphere-side surface of the microwave window in both polarization directions for all angles of incidence. The angles of incidence from 0 to 90 degrees are plotted on the abscissa. The solid lines represent a quartz glass surface; the dashed lines for a window with a covering of polytetrafluoroethylene. Integrated across all angles of incidence, there is a significant reduction in reflection to about half if, instead of a pure quartz glass window, one is used which is provided with a 5 mm thick covering made of polytetrafluoroethylene.

Claims (7)

1. Device for introducing microwave energy into an at least partially permeable to microwave energy container with at least one microwave transmitter and an open microwave conductor connected to the microwave transmitter, which is located in the immediate vicinity of the container, characterized in that the open microwave conductor (1) at least at its both ends are provided with a microwave feed line (17, 18; 24, 25), 2. Apparatus according to claim 1, characterized in that the open microwave conductor (1) is provided in addition to its two ends and at least one point between the ends, each with a microwave feed line (18, 24). 3. Apparatus according to claim 1, characterized in that the at least two microwave feed lines (17, 18; 25, 25) are connected to the at least one microwave transmitter (21, 22) with a fixed phase relationship. 4.Device according to claim 1, characterized in that the microwave feed lines (17, 18; 24, 25) are each connected to its own microwave transmitter (21, 22). 5. The device according to claim 1, characterized in that the container (11) in a manner known per se has a window (7) made of a microwave-permeable material, the thickness of the relationship

corresponds, where "λ" is the wavelength of the microwave and "n" is the refractive index of the window material for the microwave. 6. The device according to claim 5, characterized in that the microwave conductor (1) facing surface of the window (7) has a covering (9) made of a material with a smaller refractive index than that of the window material. 7. The device according to claim 1, characterized in that the microwave conductor (1a) is symmetrically V-shaped and has a smaller distance from the window (7) in the apex of the "V" than at its feed points (17, 18).

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