EP2193694A1

EP2193694A1 - Microwave plasma generating devices and plasma torches

Info

Publication number: EP2193694A1
Application number: EP08837221A
Authority: EP
Inventors: Zenon Zakrzewski; Michel Moisan; Daniel Guerin; Jean-Christophe Rostaing
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2007-09-20
Filing date: 2008-09-16
Publication date: 2010-06-09
Also published as: CN101803471A; FR2921538A1; US20120018410A1; JP2010539669A; CN101803471B; WO2009047441A1; FR2921538B1; US20140138361A1

Abstract

The invention relates to a plasma generating device that comprises at least one very high frequency source (> 100 MHz) connected via an impedance adaptation device to an elongated conductor attached on a dielectric substrate, at least one means for cooling said conductor, and at least one gas supply in the vicinity of the dielectric substrate on a side opposite to that bearing the conductor. The invention also relates to plasma torches using said device.

Description

PLASMA GENERATING DEVICES, MICROWAVE AND TORCHES

PLASMA.

The invention relates to devices for generating plasmas by coupling electromagnetic power to a gas. Such devices are also called "plasma sources". The terms "plasma generating device" or "plasma source" will be used interchangeably in the present description. The popularization of cold plasma surface treatment technologies requires an improvement of the devices whose function is to generate these plasmas by coupling an electromagnetic power to a gas. These devices or "plasma sources" must be: - simple and inexpensive, suitable for scale extension, and possibly non-planar geometries, capable of operating in a wide range of pressure levels between a significant vacuum, of the order of 10 ^" mbar and atmospheric pressure, or above the latter.

In addition, the efficiency of plasma transmission of the electromagnetic power from the generator must be as high as possible, that is to say: - that the operation must generate only a minimum of losses by heating of the structure of the device for coupling the electromagnetic power to the plasma, that the residual radiation to the outside must be negligible (safety and impossibility of interference with devices operating in the vicinity at the same authorized industrial frequencies), that only a small fraction of the incident power must be reflected back to the generator, ie a good impedance match must be made between the power supply line and the plasma source using the same power.

This latter condition should as far as possible remain true for a wide range of operating regimes, without the need to retouch settings in real time.

Plasmas excited by very high frequencies

(well above 100 megahertz) including microwave (microwave), for example 434 MHz, 915 MHz, 2450 MHz and 5850 MHz (frequencies authorized by the international regulation for the industrial band,

Scientific and Medical), have a particular interest because of their high electron density. This involves a more intense activation of physico-chemical processes in the landfill, including a higher rate of formation of the active species involved in a surface treatment process. This treatment is then more complete and / or faster: for example the deposition rate of thin-film materials is higher and the production yield is more favorable.

Beyond a limit of a few tens of MHz, electromagnetic waves, because of their propagation properties, can not be applied to a gas to create a plasma by means of electrodes connected to a power supply circuit. as is the case continuously or radiofrequency. The microwaves are conveyed from the generator by a hollow rectangular waveguide or a coaxial cable, and then guided by a conductive structure of a specific architecture, internal or contiguous to the treatment chamber. This must allow distribution and distributed absorption of microwaves to create a plasma with the required characteristics, and sufficiently uniform. Microwave plasma generating devices have been developed, among which mention may be made of the system

"Duo-plasmaline" (E. Rauschle et al., J. of Physics IV (8),

PR7, 99 (1998)), two-dimensional slot antenna applicators (H. Sugai, Plasma Fusion Research 72, 621 (1996) and H.

Sugai et al., Plasma Sources Science and Technology 7, 192

(1998)), microstrip field applicator sources for analytical applications (A.M. Bilgic et al., Plasma

Sources Science and Technology 9, 1-4 (2000)), electron cyclotron resonance systems and multidipolar magnetron systems.

However, all these devices are complex architecture and expensive to achieve. They are on the other hand too dependent on a configuration and a given dimension for the plasma treatment reactor.

In particular, we can focus on the work of the Bilgic et al. mentioned above (reference may also be made to documents DE-198 51 628 and US2003 / 008068) relating to the use of microstrip systems. It should be noted, however, that the source in question is very small and intended to maintain low power plasmas (about 10 W) at atmospheric pressure in argon in a capillary channel (section 1). approximately 1 mm ² ) bored axially in a silica rod of rectangular section. The very small section plasma channel is fully contained within a micro-ribbons transmission line. There is no mention of the possibility of extending the system in two or three dimensions, and it is therefore difficult to imagine the possibility of using such a structure for large and continuous surface treatment.

As will be described in more detail in the following through comparative figures, the systems described by Bilgic et al. implement a continuous conducting plane, maintained in the ground, on the opposite face of the dielectric, which solution has drawbacks, among which:

The coupling with the plasma is rather resonant type, which is best avoided because the impedance agreement is then sharp to achieve, which is a constraint often unbearable for real and practical applications.

In such a configuration, it is not clear where to position the plasma: It is possible to dig a channel in the dielectric, or to place the ground plane away from the lower face of the dielectric, but in any case this distance is limited to a few mm

(Since the line and the ground plane must "see" electrically), which in practice will considerably limit the applications of such a configuration.

We will explain how we realize according to the invention progressive wave propagation and not resonant couplings, and how we eliminate when necessary the depth constraint. We will see in particular that it is the merit of the present invention to have thought to consider that the plasma is a conductor with its own potential and can therefore perfectly serve as a mass reference, thus supporting by itself the propagation of the progressive wave that creates it.

The inventors have surprisingly and unexpectedly found that planar sources based on microstrip field applicators and more generally using an elongated conductor of small section in front of its length (whether it is micro-ribbon type or hollow line type, for example cylindrical) constitute Plasma sources very simple and easy to implement and which have all the required qualities.

Thus, the plasma generating device according to the invention comprises at least one very high frequency source connected to an elongated conductor of small section in front of its length (for example of microstrip or hollow line type) fixed on a support dielectric device, at least one impedance matching means between the very high frequency source and the connection to the conductor, at least one means for cooling said conductor, and at least one gas supply near the dielectric support on the opposite side to the supporting the driver.

As will be understood from reading the above, the expression "very high frequencies" according to the invention means frequencies greater than 100 MHz, and in particular the "discrete" frequencies at 434 MHz, 915 MHz, 2450 MHz and 5850 MHz which are authorized by the international regulations for the Industrial, Scientific and Medical (ISM) band. Similarly, for the supply of gas "near" or "near" the dielectric support means an inlet typically emerging less than 15 mm from the support, and preferably less than 10 mm of the support.

The plasma is generated below the surface of the dielectric opposite to the surface supporting the driver and facing the latter. Thus, the device according to the invention can be moved with respect to the surface to be treated so that the plasma is in contact with this surface to be treated or the surface to be treated can move under the plasma generating zone, the device according to the invention then remaining fixed. According to the orientation of the conductor relative to the surface to be treated and according to the distance separating the dielectric and the surface to be treated, the treatment will be done directly by plasma or by post-discharge. By "post-discharge" the person skilled in the art hears the region immediately contiguous to the actual plasma zone characterized by its intense luminescence. In the post-discharge the charged species have practically disappeared but there are still neutral active and / or excited species. Thus, when the conductor is perpendicular to the surface to be treated, the latter does not meet the plasma area and the treatment will be by post-discharge whereas when the conductor is parallel to the surface to be treated (the most common case), treatment will be by direct contact with the plasma.

In the present invention, the term "microstrip" means an electrically conductive element of elongated shape and thin, typically of the order of one millimeter or less than one millimeter. The length and the width of the microstrip are not arbitrary and will be dimensioned so as to optimize the propagation properties of the power along the transmission line constituted by the microstrip. As a variant, as already mentioned above, the microstrip may be replaced by a hollow elongated element, in particular of round, rectangular or square section, the thickness of the wall of the hollow tube being sufficient for good mechanical strength. and without effect on electrical behavior. The microstrip / conductor is not constrained to flat, straight geometry, but may also adopt a curved shape in the plane or a shape that is left in the direction of its length with concave or convex curvatures. As will be understood, we will discuss indifferently in the following conductor or microstrip, without at any time that the present invention can be restricted to only one of these types of line. In view of the fact that the currents in high frequency circulate by obeying the effect of skin or "skin effect" and that this one depends on the frequency and the conductivity of the material constituting the conductor, the practical thickness in which goes circulating the current will be much less than 0.1 mm. However, given the fact that the powers transported are high, of the order of a few hundred watts, and the conductivity of the metal decreases with the increase in temperature, the thickness of the micro-ribbon will be much greater than the Theoretical thickness defined by the skin effect and it will be necessary to cool the micro-ribbon so that it retains its physical integrity. Thus, the microstrip will have a thickness of the order of a millimeter and be made of a good electrical and thermal conductor material selected from those having good mechanical strength, which may be copper alloys such as brass or preferably beryllium copper. To maintain the good conductivity of the microstrip, it is advantageous to coat the surface of the latter with a deposit of metal at least as good electrical conductor and insensitive to oxidation (for example gold). Thus it ensures the maintenance over time of the good electrical characteristics in a normal operating environment where the cuprous alloys tend to slightly oxidize or pollute the surface.

Advantageously, the conductive micro-ribbon is mechanically plated on the dielectric. It can also be screen printed on the dielectric, if the powers involved are sufficiently low. The dielectric used must have not only good dielectric properties, that is to say a small value of the ratio of the imaginary part to the real part of its dielectric function (delta tangent), typically comprised between 10 ^{~ 4} and 10 ^{~ 2} , reflecting low dielectric losses at the frequency of use concerned, but also excellent resistance to thermal shock (the thermal gradient connected to the plasma in contact with the wall opposite the microstrip can be very Student) .

Thus silica can be chosen as dielectric for its excellent resistance to thermal shocks or preferably ceramics and in particular boron nitride or aluminum nitride. Different cooling means of the microstrip can be used. According to a first embodiment, in an insulating casing placed on the dielectric and above the micro-ribbon, a cooling fluid is circulated which is electrically insulating and has a dielectric constant ε less than that of the solid dielectric of the substrate. The coolant must have good heat transfer capacity. It must also be a good dielectric so as not to disturb the propagation of electromagnetic waves along the line nor to dissipate a large fraction of the power by absorption. The heat transfer dielectric fluid may for example advantageously be an alpha-olefin such as tetradecene (C14). Thus the device according to the invention comprises a housing disposed on the dielectric and above the microstrip confining the circulation of the cooling fluid.

According to a second embodiment, the cooling is carried out indirectly by disposing on the entire free face of the microstrip a radiator made of a dielectric material which may be a ceramic, preferably a good thermal conductor (eg alumina, nitride, etc.). aluminum), in which a cooling fluid is circulated. In this case, as the circulation is not made in direct contact with the microstrip but at a certain distance from it, the cooling fluid does not circulate in a region of high electromagnetic power density and is not constrained to low wave absorption, and may therefore be water. According to a third embodiment, in the case where the microstrip is replaced by a hollow elongated conductive element, a cooling fluid is circulated in the hollow part thereof. The cooling fluid may be water since the electromagnetic field is zero on the inner wall of the hollow element. Indeed, the wall thickness of said element is much greater than the skin thickness. This solution allows a much better cooling than the cooling systems described above and allows the passage of higher very high frequency currents and therefore a higher transmitted power without increasing electrical losses. The line thus formed with a hollow conductor of rectangular, square or circular cross section, is similar to a hybrid structure from the electrical point of view in comparison with a plane microstrip line. Experimentally it has been verified that this type of line has a characteristic impedance relatively close to that of a microstrip structure. The fact of no longer having an intermediate radiator significantly simplifies the assembly, and the contact of the electrode on the dielectric is provided by a plating device identical to the mounting of a microstrip flat structure.

According to another embodiment, the device according to the invention may also be provided with at least one dielectric cooling means. Cooling means may consist of channels in the dielectric in which a cooling fluid circulates. Another way may be to place the dielectric on a support having channels in which a cooling fluid circulates.

In order not to emit microwaves in the external environment, which would be a waste of power and would create problems for operator safety or electromagnetic compatibility, it is advantageous for the device for coupling the microwave power to wave formed by the micro-ribbons line is enclosed in a conductive housing acting as a Faraday cage.

Depending on the frequency used, the power supply of the devices according to the invention can be directly transposed from the power semiconductor industry applied to telecommunications. Power generators based on this "solid state" technology are more compact and reliable than generators based on vacuum tubes such as magnetrons driven by a switching power supply. Unlike the latter, they do not require any maintenance, in particular the periodic replacement of the magnetron is eliminated. In addition, the price of these generators drops rapidly with the production effect in medium and large series.

The supply of micro-ribbon lines can be carried out in several ways: in progressive wave mode, by connecting the very high frequency wave generator at one end of the microstrip and by connecting an impedance load adapted to the opposite end of the micro-ribbon; in progressive wave mode, by connecting a very high frequency wave generator at each end of the microstrip to, on the one hand, increase the total power, on the other hand, compensate the attenuation of the wave by absorption along its spread to sustain the plasma. In this case, it is necessary to use a different generator at each end to not have a phase correlation between the two signals, otherwise we would maintain a stationary wave mode; in stationary wave mode, by connecting a very high frequency wave generator at one end of the microstrip and providing an adjustable short circuit at the opposite end to achieve the impedance matching; in stationary wave mode, by connecting a very high frequency wave generator to a divider device, each branch of which is connected to one end of the microstrip.

Connections and connections are provided by standard components of the trade (for example by a coaxial cable having a characteristic impedance of 50Ω).

The device according to the invention has the additional advantage over waveguide systems that the impedance matching is also more convenient to achieve. For example, the transformation and impedance matching components can be realized in the form of an adaptation network in conventional circuitry (networks consisting of inductances and capacitors), but also directly in the structure of the micro-ribbon lines themselves. by making a quarter-wave impedance transformer (the principle of which is known to those skilled in the art), or by relating microstrip sections of appropriate dimensions (referred to in this industry as "stubs") , outgrowth of the propagation lines with as a corollary a simplicity in the integration, the impossibility of maladjustment (the values being fixed by the geometry and the nature of the dielectric used) and an optimization of the transfer of the very high frequency power (losses reduced in connectivity and connections). Thus the impedance matching between the very high frequency generator and the microstrip applicator can be done by a T, IT or L grating, or by means of a "stub" perpendicular to the microstrip. ribbon. The adaptation of the impedance and thus the dimensioning of the stub and the microstrip are within the reach of the person skilled in the art and can be made according to a quasi-static analysis in which it is assumed that the mode of propagation is exclusively TEM (see the work of Gupta et al: "Microstrip Unes and slot Unes", KC Gupta, R.Garg, IJ Bahl (Hartech House, Norwood, MA 1979). impedance of the devices in which the microstrip is immersed in a dielectric constant cooling fluid greater than 1, or in which a dielectric radiator having a dielectric constant greater than 1 is plated on the microstrip.

In order to simultaneously and uniformly treat a larger area, it is advantageous to combine several devices according to the invention. The juxtaposition of a plurality of plasma generating devices will indeed make it possible to generate a plasma layer over large areas. This at least applies to the continuous parade process.

As many elements as necessary can be combined to achieve a continuous surface treatment with the desired production yield. Each of the plasma generator devices thus associated comprises at least one very high frequency source connected through an impedance matching to a conductive micro-ribbon attached to a dielectric support, at least one cooling means of said micro-ribbon, and at least one a gas supply close to the dielectric support on the opposite side to the side supporting the microstrip.

For surface treatment applications operating at atmospheric pressure with the need for scrolling the substrate under the active area we can imagine different arrangements of plasma modules for easy integration while enjoying the inherent simplicity of this type of sources. The plasma generating devices can be placed end to end to cover the width of the substrate or can be placed with an offset in the direction of travel so as to recover the area to be treated. It is also possible to add the plasma generating devices in the direction of travel, if necessary to increase the contact time of the active zone as a function of the running speed, notably in order to increase the productivity.

The assembly of the different devices between them can be achieved through a common base or mechanical structure which provides the functions of gas distribution, cooling and the connection of electromagnetic power.

The connectivity can advantageously be very limited by connecting directly to the micro-ribbon amplifier module of the very high frequency power generator, with its integrated impedance matching device.

The assembly of different plasma generating devices with each other thanks to a base or mechanical structure which provides the functions of gas distribution, cooling and electromagnetic connection has the following advantages: its realization and its integration are simple, which makes possible mass production and limits manufacturing costs, and makes maintenance easy; by reducing the electrical connection to a simple connector (no coaxial cable), the losses in the transport of power to the plasma module are reduced which has a significant impact on the dimensioning and therefore the cost of the very high frequency part.

In addition, with the devices according to the invention, it is possible to use excitation frequencies of the plasma modules slightly lower than the microwave domain, for example 434 MHz (ISM band), which makes it possible to benefit from all-semiconductor technology with good performance.

Another object of the invention relates to plasma torches of medium power and small and modular size also enjoying the same advantages as those described above. These plasma torches take the provisions and shapes (micro-ribbon / flat conductor or hollow) of the previous applicators. More particularly, the dielectric on which the conductor is placed is traversed right through by a longitudinal channel. Gas is introduced through one end and the plasma is formed in the channel and extends throughout the channel. By adjusting the gas flow rate and the very high frequency power, it will be possible to extract the plasma at the end of the torch or to use the post-discharge by moving the substrate to be treated. The section of the channel can of course be optimized to confine the plasma.

Thus, a plasma torch in accordance with the invention comprises at least one source of very high frequency with its integrated impedance matching device connected to a conductor (for example of the microstrip type or hollow conductor type) fixed on a dielectric support, at least one cooling means of said conductor, said dielectric support being traversed longitudinally by a channel at one end of which the gas is introduced and in which the plasma is formed.

Due to their simple design it is possible to use this type of plasma torch on a robot arm to apply the plasma treatment by scanning a surface to be treated.

According to one aspect of the invention, the device according to the invention, and contrary to what was advocated in the prior art (ie the presence of a ground plane extending at least in relation to the entire surface of the conductive transmission line, on the opposite surface of the dielectric), the device according to the invention therefore comprises a ground plane but which is in no way continuous, only a minority surface of the transmission line (micro- conductive ribbon) is next to a ground plane.

This aspect of the invention will be described in conjunction with the accompanying Figures 14, 15, and 16 which illustrate the case of using an elongated microstrip-type conductor.

Figure 14 illustrates the case of the prior art including the work of the team Bilgic et al. The structure consists of the micro-ribbon and a continuous and total mass plane, separated by the dielectric substrate. In this case, as stated above, it is implicitly not possible to maintain the plasma beyond the geometrical limit constituted by this continuous and total mass plane, for example to treat a substrate which would be arranged in an extended room below. In fact, another useful configuration could be used in finding that a microwaves field of edges extends in space from the lateral slots determined between the edges of the ribbon and the ground plane. If there is no confinement of the field in the near area above the conductive strip and the dielectric, then it is possible alternatively to create an extended plasma in this area (optionally providing a dielectric superstrate sandwiched the conductive ribbon line with the substrate, said substrate being then constitute the window of a treatment chamber). However this provision would not be advantageous because on the one hand it is more complex, on the other hand the plasma can be maintained only by the edge fields fleeing through the slots determined between conductor ribbon (s) and plane mass, so spatially discontinuous. At atmospheric pressure in particular, it is a very serious drawback because because of the low average free path, the plasma will have great difficulty in homogenizing to be practically usable. The width of the conductive ribbon as well as the thickness of the substrate are small relative to the wavelength in the free space. The propagation mode along such a line is in first approximation the TEM mode. However, one could also imagine embodiments where the active conductive parts take instead the form of rectangles. This provision is not a priori more advantageous than the previous one.

In the end, a configuration (of the Bilgic et al. Type or other) where there is a continuous and total mass plan appears to be particularly crippling.

As was said above also, it is the merit of the present invention to have thought to consider that the plasma layer is a conductor with a potential of its own that can therefore serve as a reference mass. The arrangement of FIG. 15 is then obtained. In this case, the wave field also extends into the plasma. To "throw" such a wave, an appropriate distribution of the field in the right section of the propagation line must be imposed at the origin of the line.

It is then proposed according to the present invention a partial metal ground plane at the origin of the line (where the microwaves arrive) which will be sufficient to launch and maintain the propagation of the progressive wave and the maintenance of a continuous plasma all along the line, opposite the latter and under the dielectric.

More generally, according to one of the embodiments of the invention, a ground plane fraction is implemented, but its normal projection on the propagation line intercepts a minority surface of the section of the line.

Figures 16-a) and 16-b) illustrate two embodiments of the invention.

The launch zone of the wave, at the input of the transmission line, has a conventional structure with the microstrip, a metal ground plane and the dielectric wall of the treatment chamber serving as a substrate. The metal ground plane stops at a short distance from the entrance and is replaced by the plasma extending with the micro-ribbon over the rest of the length of the conductor line (Fig. 16-a)) .

But it is also possible, because the interface between a dielectric wall and a plasma layer may form a guide structure for an electromagnetic wave, alternatively, to dispense with extending the microstrip substantially beyond the limit of the metallic mass plane (fig 16-b)). In this case, the analog of a device and a surface wave plasma mode is obtained, but in planar geometry.

The partial surface of the microstrip facing a fraction of the ground plane may be not only at the origin of the line (end edge) but also take the form of an overlapping of the lateral edges micro-ribbon with a boundary line of the ground plane. For example, a window substantially matching the shape of the micro- ribbon but slightly smaller can be opened in the surface of the ground plane.

Other features and advantages of the invention will be detailed with the aid of the appended drawings in which: FIGS. 1a-Ib show front and sectional views of one embodiment of the device according to the invention according to which the microstrip is planar but of curved shape, and allowing plasma post-discharge treatment of a non-planar surface; FIGS. 2a-2b show front and sectional views of an embodiment of the device according to the invention in which the microstrip is of left-hand shape and allows a direct treatment in the plasma of a non-planar surface of a substrate; FIGS. 3a-3d show schematically different connections of the conductive microstrip to the very high frequency generator; - Figures 4a-4c schematically show the possibility of adapting the impedance of the device; FIG. 5 represents in cross-section a device according to the invention with a plane micro-ribbon provided with a first embodiment of the cooling means; FIG. 6 represents in cross-section a device according to the invention with a plane micro-ribbon provided with a second embodiment of the cooling means; Figures 7 and 8 show in cross section a device according to a second embodiment of the invention with a hollow section propagation line element which is an alternative to the microstrip; Figures 9a and 9b are longitudinal and transverse sectional representations of a device according to the invention with a planar microstrip; FIGS. 10a and 10b are longitudinal and transverse sectional representations of a device according to the invention provided with a hollow section propagation line element which is an alternative to the microstrip; Figure 11 shows in cross section an assembly of devices according to the invention; - Figure 12 shows in section another assembly of devices according to the invention; Figures 13a and 13b show longitudinal and transverse sections of a plasma torch implementing a device according to the invention. In Figures la and Ib is schematically illustrated a device 1 according to the invention according to which the microstrip 2 which has a flat but curved shape is connected to a very high frequency generator. This micro-ribbon 2 is fixed on the surface of a dielectric support 3, an edge of which coincides with one of the curved edges of the ribbon. A slot 4 is provided in the dielectric in which the gas is introduced and generated the plasma 5. A substrate 6 to be treated, perpendicular on average to the plane of the microstrip and having a left shape matching the curvature of the dielectric and the microstrip is driven below the device in the direction indicated by the arrow. According to this embodiment, the substrate being perpendicular to the microstrip, the treatment is done by plasma post-discharge.

FIGS. 2a and 2b are schematically illustrated a device 7 according to the invention according to which the micro-ribbon

8 which has a left shape is connected to a very high frequency generator. This micro-ribbon 8 is fixed on the surface itself left of a dielectric 9. The gas is brought to near the face 9a of the dielectric and the plasma 10 is generated under the face 9a facing the micro-ribbon 8. A substrate 11 to be treated, having a left shape matching that of the dielectric 9 and the micro-ribbon 8 is driven below the device 7 in the direction indicated by the arrow. According to this embodiment, the substrate 11 being perpendicular to the microstrip, the treatment is done directly by the plasma.

In FIGS. 3a to 3d are diagrammatically shown the different ways of connecting the conductive microstrip to the very high frequency power supply. Thus, according to a first embodiment (FIG. 3a), the microstrip 12 is powered so as to propagate a progressive wave along the microstrip. The very high frequency wave generator is connected via a coaxial line, for example a characteristic impedance of 50Ω (this value generally corresponding to the industrial standard) at one end 12a of the micro-ribbon 12, the other end 12b being connected. to a suitable impedance load 14, that is to say that there is no reflection of the waves at said end opposite the connection to the generator and therefore no standing wave along the microstrip . In this embodiment the intensity of the wave decreases very significantly along the microstrip, due to the gradual absorption of power to maintain the plasma. The latter is therefore not uniform along the microstrip.

According to a second embodiment, illustrated in FIG. 3b, the microstrip 15 is powered so as to propagate two opposite progressive waves from each of its ends, so that their intensities add up. For this, one end 15a of the microstrip is connected via a coaxial line 17 to a first very high frequency wave generator 16 and the opposite end 15b of the microstrip is connected via a coaxial line 18 to a second very high frequency wave generator 19. Since the signal phases of two different generators are decorrelated, the intensities of the two waves propagating in the opposite direction are added, and not their amplitudes (which would lead to the appearance by interference of a standing wave), partially offsetting the observed gradient with a single source at one end.

According to a third embodiment illustrated in FIG. 3c, the microstrip 20 is powered so as to create a stationary wave mode along the microstrip. An end 20a of the microstrip 20 is connected via a coaxial line 21 to a very high frequency generator. At the other end 20b is connected a device 22 of short circuit. This short-circuit device 22 is adjustable to vary the complex reflection coefficient and adapt the impedance so as to optimize the characteristics of the standing wave.

According to a fourth embodiment illustrated in FIG. 3d, the microstrip 23 is powered so as to create a stationary wave mode along the microstrip. A very high frequency generator is connected via a coaxial line 24 to a power divider device 25 (standard industrial equipment known to those skilled in the art) each branch 26a and 26b is connected to one end 23a and 23b of the micro-ribbon 23. The phases of the waves coming from the same generator being correlated, it is the amplitudes of the waves which are added and not their intensities, giving rise by interference to a standing wave. As a power divider, it is possible for example to use a "Wilkinson" type device known in the literature.

Figures 4a to 4c are schematically shown three modes of adaptation of the impedance. Thus, in FIG. 4a the very high frequency generator is connected to the microstrip 27 by an impedance matching network which is in this particular case a T-shaped network 28. In FIG. 4b, the very high frequency generator is connected directly to the microstrip 29 on the side where it is provided with a "stub" 30 micro-ribbon length L and W width, perpendicular to the microstrip 29. The choice of geometric parameters L and W allows to modify the electrical effect of the stub and thus to provide the desired correction to the resulting impedance of the system. In FIG. 4c, the very high frequency generator is connected to the microstrip 31 via a quarter wave impedance transformer made in microstrip 32 arranged in the longitudinal extension of the main microstrip and having an effective electrical length of λ / 4. λ being the propagation wavelength along the micro-ribbon line reported on the given dielectric constant substrate, at the considered value of the very high frequency. The function of the quarter - wave impedance transformer is to cause the incident power from the generator to "channel" an effective impedance equal to the characteristic impedance of the main microstrip line forming the field applicator, the plasma being lit (the micro-ribbon + plasma assembly constituting a complex charge). The general rule of sizing a quarter-wave impedance transformer on a transmission line is well known. If Z ₀ is the output impedance of the generator and Z _L the characteristic impedance of the microstrip line (with the plasma on), the impedance Z _t of the quarter-wave transformer will be Z _t = (Z _c x Z _L ) ^1/2 .

FIG. 5 is a cross sectional view of a device 33 according to the invention comprising a microstrip 34 attached to a dielectric which is an element parallelepipedic having an elongate recess forming a channel 36 and placed on a support of conductive material 37 forming electrical reference plane, traversed over its entire height by a slot 38 and on either side of said slot by slots 39a and 39b longitudinal and symmetrical with respect to the slot 38 and allowing the supply of gas. The conductive support 37 acts as a partial mass plane as defined above, the slot 38 being narrower and shorter than the microstrip 34 so that there is a fraction of conductive ground plane facing the ends of the ribbon , and facing the lateral edges of said ribbon along its entire length. On the upper face of the dielectric 35a supporting the microstrip 34 is fixed a housing made of dielectric material 40 in which circulates a dielectric cooling fluid 41, the entire microstrip 34 being in contact with the cooling fluid 41. A Faraday cage 42 encloses the dielectric 35 and the confining housing of the cooling fluid 40. The plasma 43 is generated in the channel 36 and the active species escape through the slot 38 in the direction of the arrow due to the entrainment by the flow gas .

FIG. 6 shows in cross-section a device 44 according to the invention which differs from the embodiment shown in FIG. 5 in that the insulating housing containing a cooling liquid in contact with the microstrip is replaced by a radiator 45 which is a parallelepiped of dielectric material plated on the upper free surface (opposite the substrate and the plasma) of the microstrip 34 and traversed by a channel 47 in which circulates a cooling liquid 48 which is no longer necessarily a very good dielectric at the very high frequency considered, but can be for example water. FIG. 7 shows in cross-section a device 49 according to the invention which differs from the embodiment shown in FIG. 6 in that the micro-ribbon 34 and the dielectric radiator 45 have been replaced by a line element. transmission member 50 which is a hollow conductive element of circular section in which circulates a cooling liquid 51. The surface 35a of the dielectric 35 has of course been modified to adapt to the shape of the conductive element 50. In FIG. 8 is shown in cross section a device 52 according to the invention which differs from the embodiment shown in Figure 7 in that the transmission line element 53 is a hollow conductor of rectangular section in which circulates a coolant 51. The surface 35a of the dielectric 35 is then flat as for the embodiments of FIGS. 5 and 6.

A plasma generating device 54 having a cooling system such as that of FIG. 6 is fully represented in FIGS. 9a and 9b.

This device 54 consists of the following different elements stacked on each other: a base 55 traversed by two symmetrical longitudinal channels 56a and 56b in which water circulates, and by two symmetrical channels 57a and 57b of distribution of the gas entering the discharge with in the center an exit slot 58 allowing the extraction of the active species from the plasma 59; the cooling of the base is necessary because of the heat generated by the plasma which is in contact with the dielectric substrate; a dielectric 60 forming, above said slot 58, a channel 61 of the same width as the microstrip 62 and of the same length; said microstrip 62 consisting of a conductive metal strip and connected to the connector for transmitting the very high frequency power from the generator, and being fixed above said dielectric 60; a dielectric radiator 63 made of ceramic, having a longitudinal channel 64 in which water circulates, and plated over the entire surface of the microstrip 62.

A clamping system 65 of the stack allows the plating and holding of the elements on the base 55. A not shown O-ring located in the lower part ensures the tightness of the volume in which the discharge develops.

The entire device is confined in a conductive casing 66 acting as a Faraday cage in order to prevent radiation leakage to the external environment, with the associated problems of security and electromagnetic compatibility.

A plasma generating device 67 having a cooling system such as that of FIG. 7 is fully represented in FIGS. 10a and 10b.

This device 67 differs from that of FIGS. 9a and 9b in that the micro-ribbon assembly 62 + insulating radiator

63 is replaced by a longitudinal transmission line element of hollow circular section in which water circulates. The transmission line element is held by a dielectric wedge inserted into the remainder of the stack and immobilized by clamping means 70.

FIG. 11 shows an assembly 71 of three devices (by way of example, this number being able to be increased without any particular limitation) plasma generators each comprising a very high frequency power supply module 72 enabling very high frequency power supply. a conductive micro-ribbon 73. The micro-ribbon is cooled thanks to a dielectric radiator 74 in the inner channel 75 which circulates water. The micro-ribbon is fixed on a dielectric substrate 76. The various micro-ribbon / dielectric / very high frequency power supply / dielectric radiator units are held together by a distribution block incorporating gas supply ramps 79, ramps supply of cooling water 80. The plasma 81 is generated at the lower face of the dielectric substrate facing the microstrip. The substrate 82 to be treated runs under each of the plasma sources. In the case where the substrate 82 is conductive, for example if it involves treating a sheet of steel or aluminum, said substrate can act as a ground plane. In the case where the substrate is a dielectric, a ground plane fraction (not shown) must be provided under the dielectric block 76, for example a plane conductive element extending a limited distance from the end of the microphone -Ruban powered in the direction perpendicular to the plane of the figure (generic arrangement of Figure 16). FIG. 12 shows another type of assembly 83 comprising two units (in a nonlimiting manner) dielectric 84 / microstrip 85 allowing the formation of a plasma 86 in the slot 87 supplied with gas by the gas inlet 88. The gas is then led to the gas outlet 89. The cooling of the microstrip is ensured by a circulation of dielectric cooling fluid in the channel 90 surrounding the microstrip. The cooling of the distribution block 91 is provided by channels 92 in which water circulates. In the general principle of the invention, in order to keep the plasma as potential reference and avoid a resonant system, the mass blocks delimiting the slits 87 facing the micro-ribbons 85 will be in conductive material only over a limited length from of the end of the microstrip power, the rest of the total length of the block (in the direction perpendicular to the plane of the figure) may consist of a dielectric bar. FIG. 13 shows a plasma torch 93 comprising a base 94 incorporating a coaxial longitudinal channel 95 closed at one end and in which circulates water with inlet and outlet at the other end. Above this base 94 is placed a dielectric 96 traversed right through by a longitudinal channel 97 into which the gas is introduced and in which the plasma 98 is generated. Above the dielectric is fixed the connected micro-ribbon 99 at the very high frequency generator. On the free face of the micro-ribbon 99 is placed a dielectric radiator in which water 101 flows. The assembly is inserted into a Faraday cage 102.

Claims

1. Plasma generator device (1) which comprises at least one power source with a frequency greater than 100 MHz, connected via an impedance matching system to an elongated conductor (2) fixed in intimate contact over its entire lower surface on a dielectric support (3), at least one means for cooling said conductor, and at least one gas supply close to the dielectric support on the side opposite the side supporting the conductor.

2. Device according to claim 1, characterized in that the conductor has a thickness of the order of a millimeter.

3. Device according to claim 1 or claim 2, characterized in that the conductor is a micro-ribbon.

4. Device according to claim 1 or claim 2, characterized in that the conductor is an elongated hollow element, in particular of round, rectangular or square section.

5. Device according to one of the preceding claims, characterized in that it comprises a partial electrical ground plane extending opposite the face of the dielectric opposite the side supporting the conductor, the partial nature of the ground plane being expressing by the fact that only a minority surface of the conductor line is facing a ground plane.

6. Device according to claim 5, characterized in that the partial ground plane is located at the origin of the conductor line, where the microwaves arrive in the device.

7. Device according to claim 6, characterized in that the wave launching zone, at the entrance to the conductive line has a classic structure assembling the elongated conductor, the dielectric and the partial ground plane, the plane of mass interrupting a short distance from the entrance to the conductive line and then being replaced by the plasma extending with the conductor over the entire remainder of the length of the conductive line.

8. Device according to claim 6, characterized in that the wave launching zone, at the entrance to the conductive line, has a classic structure assembling the elongated conductor, the dielectric and the partial ground plane, the plane mass interrupting a short distance from the entrance to the conductive line and then being replaced by the plasma, the conductor not extending appreciably beyond the limit of the ground plane.

9. Device according to one of the preceding claims, characterized in that the conductor is made of a copper alloy chosen from the group comprising brass or preferably beryllium copper.

10. Device according to any one of claims 1 to 9, characterized in that the conductor is mechanically fixed to the dielectric.

11. Device according to any one of claims 1 to 9, characterized in that the conductor is screen printed on the dielectric.

12. Device according to any one of claims 1 to 11, characterized in that the dielectric has a tangent of dielectric losses tg delta between 10 ^~4 and 10 ^"2

13. Device according to any one of the preceding claims, characterized in that the dielectric is silica or a ceramic, preferably aluminum nitride or boron nitride.

14. Device according to any one of claims 1 to 13, characterized in that the device is placed in a conductive housing playing the role of a Faraday cage.

15. Device according to any one of the preceding claims, characterized in that a dielectric housing is arranged on the dielectric substrate of the conductive line and above the conductor, and that a cooling fluid having low dielectric losses circulates in said housing.

16. Device according to any one of claims 1 to 14, characterized in that on the entire free face of the conductor is placed a radiator made of dielectric material through which a cooling fluid passes.

17. Device according to any one of claims 1 to 14, characterized in that the elongated conductor is a hollow longitudinal conductor provided at each of its ends with an opening allowing the circulation of a cooling fluid.

18. Device according to any one of claims 1 to

16. characterized by the fact that it comprises means for cooling the dielectric substrate.

19. Device according to claim 18, characterized in that the dielectric has channels in which a cooling liquid circulates or in that the dielectric is arranged on a support comprising channels in which a cooling liquid circulates.

20. Device according to one of the preceding claims, characterized in that the surface of the conductor is coated with a deposit of metal that is a good electrical conductor and insensitive to oxidation, such as gold.

21. Device according to one of the preceding claims, characterized in that said impedance matching system is made from impedance tuning components made in the structure of the conductor itself.

22. Plasma generator device, characterized in that it consists of an assembly of at least two, preferably from 2 to 15, and even more preferably from 3 to 8, devices as described in one any of claims 1 to 21.

23. Plasma torch comprising at least one very high frequency source connected via an impedance matching device to an elongated conductor (2) fixed on a dielectric support, at least one means for cooling said conductor, said dielectric support being crossed longitudinally by a channel through one end of which the gas is introduced and in which the plasma is formed, the active species of which are extracted by the gas flow through the opposite end.