WO1994021096A1

WO1994021096A1 - Plasma production device

Info

Publication number: WO1994021096A1
Application number: PCT/FR1994/000243
Authority: WO
Inventors: Christian Maxime Dupret; Odile Madeleine Marie-Thérèse DESSAUX; Pierre Wolf Numa Goudmand; Christophe Hoyez
Original assignee: Universite Des Sciences Et Technologies De Lille
Priority date: 1993-03-05
Filing date: 1994-03-04
Publication date: 1994-09-15
Also published as: FR2702328A1; FR2702328B1

Abstract

Device comprising a cavity (1) delimited by an outer metal wall (10) and crossed by a discharge tube (2) containing a plasma gas, a microwave generator and means (4, 5) for conveying the microwaves between the generator outlet and the cavity interior. A waveguide (4) and an antenna (5) in the shape of a metal rod (50) extend from the waveguide interior to the cavity interior. According to the invention, the discharge tube (2) has a large diameter and the antenna (5) is provided with a capacitive bar (6) to increase its effective height. A filter (7) prevents fundamental mode (TE) from developing within the cavity (1) and the lateral filters (8, 8') prevent leakage of evanescent waves. The invention is for use in high power plasma applications.

Description

"Plasma production device"

The present invention relates to a device for producing a plasma of the type comprising a cavity delimited by a metallic external wall and traversed by a discharge tube in which a plasma gas (or plasma tube) circulates, a microwave generator and means of transporting microwaves between the generator outlet and the interior of the cavity.

This device constitutes a microwave / plasma coupler for producing a plasma by a method using the resonance of electromagnetic waves in the microwave range. Couplers of the above type are well known in the art of plasma production. The resonant cavity most often has a cylindrical or parallelepiped shape and, in a first type of embodiment, the waves produced by the generator are transported towards the cavity by a waveguide sized according to the wavelength of the waves to transmit. Between the waveguide and the cavity is disposed an iris, that is to say an orifice equipped with a number of adjustment screws (STUB). The aim of this adjustment is to "tune" the coupler, that is to say to maximize the ratio of the energy transmitted to the plasma to the energy delivered by the generator. The cavity is also dimensioned as a function of the wavelength, so that inside waves form standing waves. In general, the geometry of the system is studied so that the plasma gas circulates in the cavity, in a place where the electric field is maximum because the higher this field, the higher the energy communicated to the plasma gas. These couplers of the prior art suffer from a limitation of the output power of the plasma. In the example of a nitrogen plasma, it seems impossible to exceed a transmitted power of 800 W, even with a 6 kW microwave generator. Consequently, for many applications requiring very energetic plasmas, it is essential to provide several couplers and several generators, which greatly complicates this application. In addition, in certain industrial processes requiring an energetic plasma, it is not possible to have several couplers for reasons of space. Finally, the iris and the adjustment device require numerous manipulations, so that they are not very suitable for use in an industrial process.

In a second type of embodiment, known for example from French patent application FR-A-2,074,715, the microwaves are transported between the generator and the cavity by means of a coaxial cable. However, in the current state of technology of these coaxial cables, they do not make it possible to convey significant energy densities due to the insufficient resistance of the interlayer dielectric when it is subjected to an intense electric field. In practice, the couplers described in this patent application are currently limited to an incident power of the order of 800 W. These limitations make couplers with coaxial cables not very suitable for the production of very energetic plasmas (typically more than 1 kW) .

To remedy the drawbacks of known couplers, the Applicant has proposed in the French patent application FR-A-2 665 323, a simpler device and above all making it possible to generate a very energetic plasma by optimizing the ratio of the power transmitted to plasma at generator output power. This device for producing a plasma comprises a cavity delimited by a metallic external wall and traversed by a discharge tube in which circulates in service a plasma gas, a microwave generator and microwave transport means between the generator outlet and the interior of the cavity. In this device, the microwave transport means comprise a waveguide and a metal rod-shaped antenna extending between the interior of the waveguide and the interior of the cavity.

Microwave transport is therefore made possible by a waveguide / antenna arrangement, which has the advantage of great simplicity that the waveguide couplers of the prior art did not obtain because of the obligation to use an iris and an associated adjustment device. In addition, this arrangement minimizes the power losses at the transition to the cavity, which makes it possible to achieve very high values of power transmitted to the plasma gas. This transition does not impose the presence of dielectric elements which, in known couplers with coaxial cable, considerably limit the possible incident powers.

Finally, in a particularly advantageous embodiment, the gas circulating in the plasma tube is exposed to the waves transmitted inside the cavity over a reduced length compared to the dimension of the cavity parallel to the direction of the plasma tube.

Thanks to such a device, the Applicant has been able to observe powers transmitted to the plasma much higher than those encountered with conventional couplers. In the example of a nitrogen plasma, we thus arrive at transmitted powers of several kilowatts whereas a conventional coupler is limited to approximately 800 W. However, experience has shown, precisely because of the large powers which allow reach this device, that local overheating of the plasma tube could occur. In very unfavorable cases, for example in continuous plasma production, it has happened that this tube, made of quartz, melts. We cannot therefore fully exploit the potential offered.

However, the preferred applications targeted by the invention are such that it is sought to obtain a large transmitted power.

It is therefore one of the main aims of the invention, namely to transmit the maximum possible energy to the gas circulating in the plasma tube.

Another aim, corollary of the first, is to increase the flow rate of the plasma generated.

One might think that, to solve the problem of local overheating, one could use a double jacket tube and circulate between the two envelopes a coolant. However, this automatically leads to a reduction in efficiency because the inner tube, in which the gas circulates, is too far from the region where the field is maximum. This provision therefore goes against the aim sought.

According to the invention, a larger plasma tube diameter is used than those used in the known art. By this arrangement, the plasma flow generated can be increased. However, the frequency of the microwaves used in the preferred fields of application targeted by the invention is generally in a high range, typically, without this being naturally limiting, at 2450 MHz, which constitutes a de facto standard. .

Increasing the diameter of the tube in which the gas circulates is therefore incompatible with the frequency choices which have just been recalled, at least so that the device according to the invention fully achieves the set goals.

To do this, according to the invention, use is made of provisions which simultaneously authorize a strong flow, which allows a large diameter tube, and optimal coupling, that is to say a TOS (Stationary Wave Rate) little different from the unit.

A first arrangement consists in providing the antenna with a capacitive bar so that the latter sees its effective length increased, the large diameter of the plasma tube limiting the penetration of the antenna into the cavity.

A second arrangement consists in placing a mode filter in the cavity. The purpose of this filter is to avoid the development of the fundamental mode TE -,., Of the cavity, mode not desired in this application.

In a preferred embodiment of the invention, evanescent wave "traps" are further used to prevent microwave leakage.

The subject of the invention is therefore a device for producing a plasma comprising a resonant cavity delimited by a metallic external wall and traversed by a discharge tube in which circulates in service a plasma gas, a microwave generator and means microwave transport means, between the generator outlet and the interior of the cavity, these microwave transport means comprising a waveguide and a metal rod antenna extending in a determined direction between the inside of the waveguide and the inside of the cavity, characterized in that said discharge tube has an inside diameter greater than or equal to the half-wavelength of said microwaves, wavelength such that measured under isotropic propagation conditions, in that said metal rod-shaped antenna comprises means for increasing the effective length thereof and in that it comprises means arranged inside the resonant cavity to prevent the establishment within it of the fundamental TE _{1 L} resonance mode specific to this cavity. By the provisions adopted, the invention has many advantages, some of which have already been mentioned.

The large diameter of the plasma tube allows a high flow.

These arrangements also allow a large volume of plasma generation, giving a high contact time [species-electromagnetic field].

The increase in the effective length of the antenna allows good coupling, the large diameter of the plasma tube limiting the penetration of the antenna into the cavity.

In addition, the device of the invention fully retains the advantages offered by the device according to the French patent application FR-A-2 665 323 cited above. In particular the adjustment of the T.O.S. is obtained using a single organ, in this case thanks to a threaded wheel.

The invention will be better understood and other characteristics and advantages will appear on reading the description which follows with reference to the appended figures and among which:

- Figure 1 is a sectional view, in a direction x-x ', of a device according to a preferred embodiment of the invention;

- Figure 2 is a partial sectional view, in a direction orthogonal to the direction x-x ', of the same device;

- Figure 3 schematically illustrates a filter used in the production of the device according to

The invention;

- Figures 4 and 5 are diagrams relating to certain plasmagenic gases which can be used in the context of the invention. In what follows, to fix the ideas, it will be assumed, firstly, that the plasma gas is nitrogen and that the frequency of the microwaves used is 2450 MHz. It goes without saying that other gases can be used and that other frequencies can be selected. Examples will be given later. As previously indicated, the device according to the invention, in addition to the provisions which are specific to it, incorporates the main characteristics taught by the aforementioned French patent application FR-A-2,665,323 and retains the advantages thereof. We can profitably refer to this patent application for a detailed description of this device.

The device of the invention will now be described with reference to Figures 1 and 2 which illustrate an exemplary embodiment according to a preferred embodiment. The device according to the present invention comprises a resonant cavity 1 delimited by a metallic external wall 10. A cylindrical quartz tube 2, called discharge tube, crosses the cavity 1 along an axis x-x '. The plasma gas flows in the tube 2 in the direction indicated by the arrows F. This direction could moreover be reversed without losing the effects sought by the invention.

The outer wall 10 of the cavity 1 has a cylindrical shape in the example illustrated. It defines, inside the cavity 1, a peripheral surface 11 of revolution around the axis xx 'of the discharge tube 2 and two transverse surfaces 12, comprising two orifices 14, 15, provided to let the tube pass. discharge 2. Conduits 18 opening into the peripheral surface 11 of the cavity are connected to a device, not shown, intended to circulate compressed air in the cavity 1 in order to cool the cavity 1 and the discharge tube 2 by operation. A microwave generator 3 delivers microwaves of predetermined frequency, for example from frequency f = 2450 MHz. The generator 3 can be a conventional magnetron generator.

A waveguide 4 and an antenna 5 comprising a rod 50 transport the microwaves between the output of the generator 3 and the interior of the cavity 1. The waveguide 4 extends in a direction zz ' shown in phantom in Figure 2 and has a rectangular section defined by four surfaces. It is fixed to the generator 3 by screws (not shown) passing through holes 44 provided on a base 45. Of course, the waveguide 4 is dimensioned, conventionally, as a function of the wavelength of the micro¬ waves to transmit.

These constructional arrangements are, in large part, similar to those taught by the aforementioned patent application.

The arrangements specific to the invention will now be described.

One of the essential characteristics of the invention is to use a large diameter plasma tube 2. This tube will allow a significant flow. By way of nonlimiting example, the outside diameter of the tube can be chosen equal to approximately 60 mm. More generally, the diameter of the tube must be of the order or greater than the half wavelength of the excitation microwaves. However, it should be clearly understood that this wavelength is not that which can be measured inside the cavity 1 but it will be defined as that measured under isotropic propagation conditions.

As mentioned before, one of the usual frequencies used in microwave generators is the 2450 MHz frequency, which is a quasi-standard. This frequency is therefore relatively high. However, even if lower frequencies are used, as will be indicated later, the large diameter selected for the plasma tube then prevents optimum coupling because this arrangement limits the penetration of the antenna into the cavity 1.

According to another important characteristic, the antenna 5 is provided with means making it possible to increase the effective length thereof. In a preferred embodiment, there is available on the end of the rod 50 of the antenna 5, close to the cavity 1, a capacitive metal bar 6. This metal bar 6 is arranged at right angles to the rod 50 and s 'extends parallel to the x-x axis'. Under the conditions previously indicated (frequency equal to 2450 MHz in particular), the strip has typical dimensions of 10 mm wide, 3 mm thick and a length of approximately / 2. It is made of a metal which can be chosen from the following: copper, brass, etc.

This arrangement artificially increases the height of the antenna 5. It therefore no longer needs to penetrate deep into the cavity to ensure good energy coupling.

The waveguide 4 and the cavity 1 are provided with orifices, respectively 46 and 16 to provide a coupling between these two members, via an intermediate tube 47, allowing the rod 50 of the antenna 5 to pass through.

One must also take care that does not develop within cavity 1 the fundamental mode of resonance TE *** ^ aimed. To do this, according to another arrangement adopted by the invention, an element is inserted into the cavity avoiding the establishment of this mode. In a preferred embodiment, there is a "trap" or mode 7 filter. This can be constituted by a metal ring 70, for example brass. This in turn is surrounded, in the example illustrated, by a ring 71 made of insulating material, for example Teflon. To maintain this assembly in the center of the resonant cavity 1, relative to the side walls 12 and 13, wedging tabs 72, also made of Teflon, can be used. In the example illustrated, three legs, regularly distributed, were used at 120 °.

These arrangements make it possible, in combination, to ensure both a large volume of generation of the plasma, giving a high contact time [species-electromagnetic field]. They also ensure good homogenization. The non-confinement of the plasma makes it possible to avoid the problems of fusion of the plasma tube under high power.

Tests have shown that the coupler according to the invention can be operated for powers equal to or greater than 6 KW.

One can further improve the coupling and perfect the operation of the device according to the invention by adopting an additional arrangement. We place, on both sides of the central "trap" 7 eliminating the fundamental mode, side "traps" 8 and 8 'with evanescent waves; thus avoiding the escape of microwaves.

These "traps" can consist of metal cylinders surrounding the plasma tube. In the example illustrated by FIG. 1, the cavity 1 is extended along the axis x-x ', by two lateral sleeves surrounding the plasma tube 2, starting from the orifices 14 and 15.

In reality, still in the example illustrated, the walls 12 and 13 ending the cavity 1 are an integral part of the parts constituting the lateral traps. But this is only a purely technological choice which in no way limits the scope of the invention. The resonant cavity 1 will be constituted in this case by a simple tube 10 at the ends of which will be fixed, by any suitable means (screws, etc.), the walls 12 and 13 of the two lateral traps. These walls close the cavity 1.

Additional improvements can be made to the device of the invention. The first possible improvement consists in browning all or part of the pieces carrying high frequency currents to limit losses by skin effect. These are in particular the internal surfaces of the cavity 1: 11 to 13; waveguide 4: 40 to 43; from tube 47: 470; the internal surface 48 of the antenna well 5 and the surface of the rod 50 thereof: 51.

The second possible improvement is to cool the body of the resonant cavity. Various methods are commonly used in the known art, in particular it can be provided with external cooling fins, circulating coolant on the external wall 17 of the cavity 1, to evacuate the calories, etc., in particular if one targets high powers continuously.

Finally, the device of the invention, in its preferred embodiment, also has the advantage of being able to adjust the TOS using a single means, namely a threaded button 9. This member is similar to that described in the aforementioned patent application and there is no need to detail it again. To avoid rotation of the bar 6 when adjusting the tuning by the screw 9, there is advantageously provided a system for transmitting the rotary movement in translational movement.

The adjustment of the antenna 5 by sliding along its axis y-y 'makes it possible to tune the resonant cavity. This agreement consists in minimizing the TOS standing wave rate, defined by the formula:

P _R [TOS - 1] ² (1)

P _j [TOS + l] ² formula in which P _R denotes the power reflected by the plasma and P _j the incident power output from the generator. The reflected power is the power which is not transmitted to the plasma. In the present invention, the sliding of the antenna 5 modifies the geometry of the field lines inside the cavity 1, because it modifies the penetration length of the antenna 5 in this cavity. An optimal position exists to minimize the TOS; the agreement is to seek that position.

The tuning of the coupler by the thumb wheel 9 is of course optional in the context of the present invention. In particular, if the coupler is intended for a single application, with a single plasma gas, it may be advantageous to remove the adjustment button 9. The coupler is, in this case, tuned once and for all and the adjustment cannot be lost.

To fix the ideas, we will now indicate, without limiting the scope of the invention, some of the important typical numerical values.

Of course, the waveguide 1 is dimensioned, in a conventional manner, as a function of the wavelength of the microwaves to be transmitted. As indicated, the wavelength „of the microwaves in the waveguide 1 differs from the wavelength _v of the same microwaves in vacuum.

The length of the waveguide 1, between walls 12 and 13 is equal to. The mode filter 7 is arranged at

/ ₂ and the axis yy 'of the antenna 5 (perpendicular to x- g x') is located _ / 4 of the wall 13 (in the example illustrated). Of course, it could also be the opposite wall 12. It is assumed that the quartz tube 2 has an outside diameter of 60 mm and a wall thickness of 1.5 mm. The metal ring 70 has a thickness of 5 mm. It is substantially the same for the ring 71 and the retaining tabs 72. The lateral "traps" have a length at least equal to _σ / 4, between the respective side walls 12 and 13 of the resonant cavity 1 and their ends. Finally, as in the aforementioned French patent application, the end of the antenna 5, close to the adjustment wheel 9, is arranged in a well of height _v / 4, to obtain an infinite impedance and therefore create a wall fictitious at the outlet of the antenna 5 in the guide 4.

So far, it has been accepted that the plasma gas is nitrogen. However, various plasmagenic gases can be used: among these, in addition to nitrogen, it is possible to use, without being limiting, air, helium, argon, oxygen, hydrogen, ammonia , sulfur oxide or any gaseous mixture.

The Applicant has carried out experiments with a device in accordance with the invention.

The results obtained with nitrogen, helium, argon and air are indicated in the following, under various conditions of pressures and incident powers.

1. For nitrogen and air: a) At 10 hPa, whatever the incident power between 1 and 6 kW, the reflected power is constant and substantially equal to 50 W. b) At atmospheric pressure (# 1013 hPa), whatever the incident power between 1 and 6 kW, the reflected power does not exceed 650 W. 2. For helium and argon, the results are more complex and are given by the curves of the Figures 4 and 5, respectively.

It is noted, for incident powers greater than 1 to 2 kW, that the reflected power increases very quickly, all the more so when the pressure is low. In addition, this effect is more marked for helium. In conclusion, this cavity is particularly suitable for nitrogen or air for any pressure less than or equal to atmospheric pressure, and for mono-atomic gases for pressures greater than lOhPa. The present invention is not limited to the examples described above with reference to the drawings.

Reference was made to the frequency value of 2450 MHz for the microwaves used. This frequency corresponds to a standard imposed in a certain number of countries. It goes without saying that the invention can be implemented for other normative values (such as for example 915 MHz, 896 MHz or 433 MHz) or for any value in the microwave range. The dimensioning of the components of the device is proportional to the wavelength of the microwave radiation and constitutes a usual execution operation for a person skilled in the art of the technique concerned.

It is understood that the discharge produced by the coupler according to the invention can be used to form a plasma of any chemical composition. The preferred applications of the invention are those requiring very energetic plasmas, such as those in which the properties of post-discharge plasmas are exploited with a large useful volume.

By way of nonlimiting example, as a particularly advantageous application of the device of the invention, it is possible to cite the generation of plasma for surface treatment, the plasma generated being continuously pumped into a large enclosure or reactor, this- ci containing parts to be treated. The device of the invention allowing high speed and the production of a stable and homogeneous plasma, is therefore particularly well suited for this application. Mention may also be made of plasma-assisted deposits or polymerization by the addition of any gas downstream from the discharge: the so-called P.D.P.E.C.V.D method (from the Anglo-Saxon "Poεt Discharge Plasma Enhanced Chemical Vapor Deposition").

Claims

1. Device for producing a plasma comprising a resonant cavity (1) delimited by a metal wall (11, 12, 13) and traversed by a discharge tube (2) in which circulates in service a plasma gas, a generator of microwave (3) and microwave transport means (4, 5), between the generator outlet (3) and the interior of the cavity (1), these microwave transport means comprising a waveguide (4) and an antenna (5) in the form of a metal rod (50) extending in a determined direction (yy ¹ ) between the interior of the waveguide (4) and the interior of the cavity (l), characterized in that said discharge tube (2) has an internal diameter greater than or equal to the half-wavelength of said microwaves, wavelength as measured under isotropic propagation conditions, in that said antenna (5) in the form of a metal rod (50) comprises means (6) for increasing the effective length thereof and in that that it comprises means (7) arranged inside the resonant cavity (1) to prevent the establishment, within it, of the fundamental mode of resonance TE _{1; L} specific to this cavity (1).

2. Device according to claim 1, characterized in that said resonant cavity (1) has a cylindrical shape, limited by two side walls

(12, 13) distant by a wavelength () as measured inside this cavity (1), of axis of symmetry (x-x ') orthogonal to said determined direction (yy ¹ ) and in that this determined direction is a quarter of a wavelength apart

() of one of the walls (13) of the cavity (1), as measured therein.

3. Device according to claim 1, characterized in that the said means for increasing the effective height of the antenna (5) comprise a capacitive bar (6) arranged in a direction (x- x ') orthogonal to said determined direction (yy ¹ ) of the rod (50) of the antenna.

4. Device according to claim 3, characterized in that said bar (6) is a parallelepiped metal bar whose large dimension is orthogonal to said determined direction (y-y ') of the rod (50) of the antenna (5 ).

5. Device according to claim 1, characterized in that said means (7) for preventing the establishment of the fundamental mode of resonance (TE _{1; L} ) comprise a metal ring (70) surrounding the discharge tube (2) and disposed in the center of the resonant cavity (1).

6. Device according to any one of claims 1 to 5, characterized in that it further comprises means (8, 8 ') arranged on either side of the resonant cavity (1), to prevent leakage evanescent waves.

7. Device according to claim 6, characterized in that said means for preventing the escape of evanescent waves comprise two metal sleeves (8, 8 '), each sleeve being threaded on the discharge tube (2), on the side and d other of the resonant cavity (1) and extending it by a distance at least equal to a quarter of the wavelength () as measured inside the cavity (1).

8. Device according to any one of claims 1 to 7, characterized in that said antenna (5) is slidably mounted in said determined direction (y-y ') to allow its penetration length to be adjusted inside the cavity (1).

9. Device according to claim 8, characterized in that said sliding is obtained using single adjustment means comprising a threaded wheel (9).

10. Device according to one of claims 1 to 9, characterized in that at least one surfaces (11, 12, 13, 40 to 43, 470, 51, 48) in contact with the microwaves, when the device is in operation, includes gilding.

11. Device according to any one of claims 1 to 10, characterized in that it comprises means (16) for cooling the cavity (1) and / or the discharge tube (2) in operation.

12. Device according to claim 11, characterized in that the means for cooling the cavity (1) comprise a cooling circuit intended for the circulation of a refrigerant fluid.

13. Device according to claim 12, characterized in that the cooling circuit is in contact with the outer wall (17) of the cavity (1).

14. Device according to any one of claims 1 to 13, characterized in that the plasma gas is chosen from the following: mono, di or polyatomic.

15. Device according to any one of claims 1 to 14, characterized in that the frequency of the microwaves is chosen equal to one of the industrial microwave frequencies 2450 MHz, 915 MHz and 896 MHz.

16. Device according to claim 15, characterized in that the external diameter of the discharge tube (2) is chosen to be substantially equal to 60 mm.