EP2286641A1

EP2286641A1 - Method for cooling microwave plasma and system for the selective destruction of chemical molecules using said method

Info

Publication number: EP2286641A1
Application number: EP09753765A
Authority: EP
Inventors: Daniel Guerin; Christian Larquet; Jean-Christophe Rostaing; Michel Moisan; Pascal Moine; Bruno Depert; Valère Laurent
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: 2008-05-28
Filing date: 2009-04-30
Publication date: 2011-02-23
Also published as: KR20110021816A; JP2011522691A; US20110073282A1; WO2009144110A1; TW200952568A; EP2131633A1

Abstract

The invention relates to a method for cooling a plasma processing system for processing a mixture of particularly gaseous fluids, which comprises a means for coupling a source of microwave power to a mixture of particularly gaseous fluids flowing in a dielectric tube at the level of the coupling means adapted for transferring a portion of the microwave energy to the fluid mixture in order to generate therein a plasma for initiating the breaking of at least some chemical bonds of the fluid molecules, wherein said dielectric tube is at least partially cooled down by a coolant flow in thermal contact with the outer wall of the tube to be cooled. According to the invention, on the one hand, the coolant flow in thermal contact with the dielectric tube flows in a direction opposite to that of the fluid or fluid mixture in the dielectric tube and, on the other hand, the coolant includes at least one oil selected from linear alpha-polyolefins having a carbonated chain of at least ten carbon atoms and/or perfluorocarbonated liquids having a dielectric constant ε lower than 2.5, a microwave absorbance tan δ of between 10^-2 et 10^-4, and a specific heat Cp < 0.6 g. cal/g.°C.

Description

METHOD FOR COOLING MICROWAVE PLASMA AND SYSTEM FOR

SELECTIVE DESTRUCTION OF CHEMICAL MOLECULES

USING THIS METHOD

The invention relates to a method for cooling a plasma treatment system of a fluid or a mixture of fluids, especially gaseous fluids, comprising means for coupling between a microwave power source and a mixture of particularly gaseous fluids circulating in a tube. dielectric at the level of the coupling means for transferring a portion of the microwave energy to the fluid mixture to create a plasma therein to cause the breaking of at least some of the chemical bonds of the fluid molecules, said tube dielectric being cooled by a circulation of a cooling fluid in thermal contact with the outer wall of the tube to be cooled.

The invention also relates to a system for the selective destruction of chemical molecules using this cooling method.

During the fabrication of integrated circuits, the multiple steps of making the semiconductor elements and their interconnections use gaseous substances used in ionic implants or etching and physical or chemical deposition reactors ( "PVD" or "CVD"). Some of these substances may be so-called greenhouse gases, that is to say, contributing to the global warming of the climate when they are present in the atmosphere, such as in particular certain fluorinated derivatives, in particular gases. known as "PFC" (perfluorinated gas) or "HFC" (hydrofluorocarbon gas) or certain fluids and in particular certain atmospheric pollutants immediately dangerous for life or health, and more particularly those that are toxic, corrosive, flammable, pyrophoric and / or explosive. Generally, in the manufacture of semi conductors ^¬ all said gases "precursors" of deposit and the etching gas, cleaning reactors, etc .. are recovered at the outlet of the reactors as mixtures and these effluents must be treated.

In ^λ of other applications such as the manufacture of flat screens of plasma or LCD-type or that of photovoltaic cells, are also used gas and a number of gaseous precursors or delivered as vapors when initially to liquid or solid state.

In other applications, such as the separation of gases from the air or the purification of gases such as krypton or xenon from the distillation residue of an argon column in an air separation plant or directly extracts of a mixture from a groundwater, the gas obtained comprises a small amount of fluorinated gases such as for example CF4 or C2F6 that it is necessary to best remove the gas to be purified.

In order to destroy the greenhouse gases or the precursor precursor gases originating from these integrated circuit manufacturing reactors, it is known, for example, from EP-A-874537, to use atmospheric pressure plasmas which are generated by coupling of an ultra-high frequency (UHF) or microwave (microwave or microwave) electromagnetic wave transported in a waveguide to an "applicator" system of the wave in the gaseous mixture which makes it possible to create the gaseous plasma. Given the fact that the use of electromagnetic waves is highly regulated (because of potential interference with civil and military telecommunications), only a few UHF or microwave bands are available and authorized for industrial, scientific and medical uses (ISM ) and in particular for the realization of these plasmas, and in particular the frequencies 2.45 GHz, 915 MHz, 434 MHz. The gaseous effluents such as in particular the PFC or HFC type effluents emanating from the etching chambers are systematically diluted in nitrogen at the level of the primary vacuum pumps because of their dangerousness. The mixture of gases entering a treatment system or destruction of effluents of the type named above is therefore mainly constituted by nitrogen.

The use at atmospheric pressure of a carrier gas such as nitrogen requires a large amount of energy to ionize the gas and to maintain a nitrogen plasma. Furthermore, the use of tubes including ceramic, causes problems of temperature resistance of the different materials used. The discharge tube is in fact cooled by a coolant circulating from one of its ends to the other, in a determined space between said tube and a second outer coaxial tube for confinement to the liquid. When the plasma source is operated for a long time at high power in a gas consisting mainly of nitrogen or air, because of the excellent thermal conductivity of the ceramic, the temperature of the external surface in contact with the boundary layer of the cooling dielectric fluid may exceed the limit of physicochemical stability of the latter. One can thus note a solid polymerization beginning on the tube wall, the deposit formed generally absorbing the microwaves from which an effect of self ^¬ runaway (because absorption generally increases with temperature, so that the more tube is hot, the more it tends to heat up even more) and the creation of very highly superheated areas tending to spread gradually. These very high thermal stresses in a very small thickness are likely to lead to cracking or rupture of the tube. The dielectric fluid heat transfer can also undergo a transformation in volume and become cloudy and smelly, corresponding to the formation of decomposition products suspected of being harmful. Without prejudging the degradation of the functional properties of the fluid (character dielectric and property of heat transfer), the harmfulness of the used product is unacceptable in an industrial environment. For example, the use of silicone fluids such as dimethylpolysiloxane (DMPS) has been abandoned given the presumed harmfulness of the products of decomposition to heat.

The invention aims to overcome the various disadvantages mentioned above by using a cooling system of the tube, in particular dielectric, in which is generated the plasma at atmospheric pressure, different from the systems used in the prior art.

According to the invention, on the one hand the circulation of the cooling fluid in thermal contact with the dielectric tube is carried out co-currently with the circulation of the fluid or mixture of fluids in the dielectric tube and on the other hand the fluid of cooling comprises at least one oil chosen from linear alpha olefins having a carbon chain of at least ten carbon atoms and / or perfluorocarbon liquids having a dielectric constant ε of less than 2.5, absorbance micro waves ^¬ tan δ ranging between 10 ^{~ 2} and 10 ^{~ 4} and a specific heat Cp <0.6 g. cal / g. ⁰ C.

After having noted many premature failures of dielectric tubes due to local overheating of the tube, the inventors have succeeded in highlighting a certain number of results which allowed them to realize the invention. In particular, during the counter-current circulation of the mixture of fluids to be treated (injection downwards) and coolant coolant (upward circulation), countercurrent circulation which is usually recognized by the skilled person as allowing the best heat exchange between the fluids, the inventors have demonstrated the existence of bubbles in the coolant at the ceramic tube. Thus, the film of cooling oil in contact with the wall of the tube is not continuous because of these bubbles which are composed of dissolved gas gas and vaporized oils. These phenomena have been confirmed by observing changes in the refractive index of the ceramic tube. Quite unexpectedly, the inversion of the direction of circulation of the oil (co-current with the circulation of the mixture of fluids, that is to say in the present example, from top to bottom) allows a better cooling at the ceramic / oil junction and avoids the formation of a vaporized oil film at the same junction.

It has also been found that while linear alpha-olefins, especially those of the Cl 4 type, already give much better results than normal heat transfer liquids (such as water, in particular), the use of perfluorocarbon liquids (PFCs) still gives results. significantly improved, especially when these fluids had the following properties:

- Dielectric constant ε <2.5, preferably ε <2.0; - 10 ^"4 <tan δ <10 ^{" 2} , preferably <10 ^"3 ;

Specific heat Cp such that: Cp ≤ 0.6, preferably Cp <0.3.

In addition, these products having a very high density (almost three times higher than a C-14 alpha-olefin) the amount of liquid to circulate to evacuate the same number of calories is significantly lower, which translates by reducing the flow rate of the heat transfer fluid of the order of 30%. In addition these perfluorinated products are much more thermally stable, which increases the operational safety of the system of the invention.

Preferably, at least one linear alpha olefin, preferably a linear olefin C-14 or tetradecene-1 and / or a perfluorocarbon fluid (PFC) having a dielectric constant ε <2 and / or an absorbance tan δ <10, will be used. ^{~ 3} and / or a specific heat Cp <0.3 g. cal / g. ⁰ C.

According to a preferred embodiment, the injection of the fluid mixture into the tube is carried out at atmospheric pressure or at a pressure close to atmospheric pressure. According to another variant, the injection of the fluid mixture and / or an inert complementary gas in the form of a vortex into the dielectric tube will be carried out.

According to another preferred embodiment of the invention, the fluid to be treated and the cooling fluid flow from top to bottom.

The invention also relates to a plasma treatment system comprising: means for injecting a fluid and / or a gas; a dielectric tube receiving the fluid and / or the gas; a microwave generator; means for coupling the microwaves and the fluid and / or a gas to create a plasma in the dielectric tube; means for cooling the dielectric tube placed outside the tube, using a cooling fluid; a source of linear olefin alfa and / or perfluorocarbon fluid connected to the cooling means of the tube; means for circulating the cooling fluid cocurrently of the fluid or mixture of fluids to be treated, preferably from top to bottom.

The invention will be better understood with the aid of the following exemplary embodiments, given in a nonlimiting manner, together with the figures which represent: FIG. 1, an overall schematic view of the system according to the invention; Figure 2a, a vertical sectional view of a fluid injection head using a vortex and adaptable to the system of Figure 1; - Figure 2b, a sectional view along AA of the view of Figure 1; Figure 2c, a horizontal sectional view along BB of the view of Figure 2a; Figure 3 is an alternative embodiment of an injection head creating a vortex.

In FIG. 1, the plasma gas treatment system A comprises a surgeide type field applicator 1 as described in EP-A-874537, a heat exchanger B and washing means C, and cleaning means at dry D (or arranged in reverse order if desired). The system A is supplied via the plasma start gas valve Vd and / or via the valve Vf to the gas to be treated and comes from one of the reactors CVD1, CVD2, CVD3,. .. CVDn, via respective valves V1, V2, V3, ..Vn (these gases can be gases from semiconductor manufacturing reactors or flat screens or optical fibers or solar cells, etc.).

The system A also comprises a dielectric tube 16 surrounded by a cooling system comprising a coolant 19 sufficiently weakly absorbing the microwaves in order to keep the power available to maintain the plasma, circulating in the space 18 delimited by the outer tube. silica 17 and the dielectric tube 16. The fluid inlet 19 is located in the lower part 13 of the system A and the outlet 20 of the fluid 19 after cooling the tube 16 is located in the upper part 24. The field applicator 1 in its central reduced portion 3 (reduction of the small side of the hollow rectangular waveguide section relative to the standard) is traversed by the dielectric tube 16, the silica tube 17 surrounding the space 18 for circulation of the cooling fluid . Sleeves of electrically conductive material 7, 8 acting as electromagnetic screens are arranged respectively around the top and bottom of the aforementioned tubes. Between the lower part of the sleeve 7 and the dielectric tube, an optimal radial distance is provided in order to obtain the maximum coupling between the guide wave and tube, without disturbance of microwaves by the presence of the sleeve.

The same optimized radial distance is provided between the upper part of the sleeve 8 and the tube at the lower part of the applicator 1. At their other ends, the sleeves 7, 8 are respectively adjacent to the upper part 24 and the lower part 13. The field applicator 1 hollow rectangular waveguide comprises a central portion 3 of reduced section relative to the standard section used at the inlet / outlet 2, 4 located on either side of this central portion 3. The microwave power, when the system is in operation, flows from the side part 2 to the central part 3, at which the microwaves are concentrated to be thrown along the tube 16 of the part and the another of this central portion 3 of the field applicator, so as to create a plasma in the tube 16 by yielding energy throughout the propagation of the wave along the tube. This plasma is started using the electrode 23 which is integral with the support 10 situated above the upper part 9 of the system A. The electrode 23 is maintained substantially along the axis of the dielectric tube 16 and is connected to a high voltage source or starter coil. The plasma starting system is connected to the valve Vn and essentially comprises two branches: one connected to an Ar argon source via a mass flow regulator and a VAr valve, the other to a nitrogen source via a mass flow controller and a valve VN ₂ . The heat exchanger B makes it possible to cool the hot gases coming from the plasma of the system A and to send them at around 150 ^° C. at most to the washer C and the dry cleaner D (or vice versa). In Figure 2 is shown a gas injection system (starting or process) in the form of a vortex. The gas injection ducts and / or fluid arrive tangentially in the vertical duct 54, located in the extension of the tube dielectric 16, to create a rotational effect of the gases and / or fluids injected.

FIG. 2a is a vertical sectional view of the upper part 9, 24 of the plasma system A. Four gas injection pipes (57, 51), (58, 62), (59, 53), and (60, 64) all visible in Figure 2b (which is a sectional view along AA of Figure 1) to create the vortex in the conduit 54. The support 10 of the electrode 23 is secured to the upper portion 9 (24) . The four injection lines are preferably oriented (in the horizontal plane) at 90 ° from each other and can be oriented (in the vertical plane) either horizontally or from top to bottom. The ducts (70, 72) and (71, 73) (visible in FIG. 2c which is a horizontal section BB in FIG. 2a) are also connected tangentially to the central duct 54 and oriented at 180 ° relative to one another. 'other. They allow the injection of an additional gas (for example nitrogen) when the flow of gas injected into the four injectors located in the plane A-A is insufficient to maintain a vortex. (Such a vortex makes it possible to reduce the thermal exchanges with the wall of the dielectric tube, to avoid the direct contact of the plasma with this same dielectric tube and thus to avoid a too high temperature detrimental to the dielectric tube).

FIG. 3 represents a schematic view of an alternative embodiment of an injection head 9 of gas to be treated in the plasma, with which an effective vortex is produced. As in the other figures, the same elements have the same references. This injection head 9 has an inlet (11) for introducing the gases to be treated which are then conducted via the channel 80 which is coaxial with the inlet 11 to the peripheral channel, the successive portions of which are shown in section 81, 82, 83 and 84, this continuous channel, surrounding the solid central portion 85 (similar structure to that of a spiral staircase around a central column 85). This solid central portion 85 is preferably made of conductive material and ends with a portion lower conical 86 serving as ignition electrode of the plasma which is created in the dielectric tube 16. The solid parts 87, 88, 89, 90 and 91 projecting from the axis 85 are the solid parts arranged in a spiral around of the axis 85 delimiting the passage of the gas. The upper portion 92 above the central portion 85 is housed in a movable part 93 ensuring the attachment of this central portion and the gas tightness by the O-ring 94. Preferably, as shown in Figure 3, the channel 81, 82, ... leading the gas to give it a vortex effect in the tube 16, will have an axis inclined relative to the horizontal between about 25 ° and 35 °, more preferably of the order of 30 °.

EXAMPLE: Different cooling oils were used with the system described in FIG. 1 (with and without a vortex system as described in FIGS. 2 and 3). The cooling proved to be much better, especially with the injection of the gases at destroy in the form of a vortex using the device of Figure 2 or Figure 3). The following oils were very satisfactory for cooling the dielectric tube.

• *: PFC oils from the company "3M".

If an "FC 70" oil is used instead of a "C-14" oil, the product dx Cp goes from a value of 0.5 to a value of 0.38, which results in a reduction of 30% throughput at equal performance.

In applications in which a gas such as NF ₃ is converted to generate a mixture of fluorine and nitrogen, the use of a PFC type oil for cooling the dielectric tube will be much safer. A constraint is however not to use fluorinated polymers or elastomers in connection with these PFC oils.

Claims

1 - Process for cooling a system (A) for plasma treatment of a fluid or a mixture of fluids, especially gaseous fluids, comprising coupling means (7, 8) between a source (1) of microwave power and a mixture particularly gaseous fluids circulating in a dielectric tube (16) at the coupling means which transfer a portion of the microwave energy to the fluid mixture to create a plasma therein so as to cause the rupture of at least some of the chemical bonds of the fluid molecules, said dielectric tube (16) being at least partially cooled by circulation of a cooling fluid in thermal contact with the outer wall of the tube to be cooled, characterized in that, on the one hand, the circulation of the cooling fluid in thermal contact with the dielectric tube is co-current with the circulation of the fluid or mixture of fluids in the dielectric tube and secondly, the fluid method of cooling comprises at least one oil selected from linear alpha olefins having a carbon chain of at least ten carbon atoms and / or perfluorocarbon liquids having a dielectric constant ε of less than 2.5, a microwave absorbance tan δ between 10 ^{~ 2} and 10 ^{~ 4} and a specific heat Cp <0.6 g. cal / g. ⁰ C.

2 - Process according to claim 1, characterized in that at least one linear alpha olefin, preferably a linear olefin C-14 or tetradecene-1 and / or a perfluorocarbon fluid (PFC) having a dielectric constant ε < 2 and / or an absorbance tan δ <10 ^{~ 3} and / or a specific heat Cp <0.3 g. cal / g. ⁰ C. 3 - Process according to one of claims 1 or 2, characterized in that the injection of the fluid or mixture of fluids to be treated in the tube is carried out at atmospheric pressure or at a pressure close to atmospheric pressure.

4 - Process according to one of claims 1 to 3, characterized in that one carries out the injection of the mixture of fluid and / or an inert complementary gas in the form of a vortex.

5 - Process according to one of claims 1 to 4, characterized in that the fluid to be treated and the cooling fluid flow from top to bottom.

6 - System (A) for plasma treatment comprising: a) means for injecting a fluid (19) and / or a gas; b) a dielectric tube (16) receiving the fluid and / or the gas; c) a generator (1) of microwaves; d) coupling means (7, 8) of the microwaves and the fluid and / or a gas to create a plasma in the dielectric tube (16); e) cooling means of the dielectric tube placed outside the tube (18), with the aid of a cooling fluid (19); f) a source of linear alpha olefin and / or perfluorocarbon fluid connected to the cooling means of the tube; g) means for circulating the cooling fluid cocurrently of the fluid or mixture of fluids to be treated, preferably from top to bottom.