Patent Publication Number: US-2011073282-A1

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

Description:
The invention relates to a method for cooling a plasma treatment system for treating an especially gaseous fluid or fluid mixture, the system comprising means for coupling a microwave power source to an especially gaseous fluid mixture flowing in a dielectric tube past coupling means that transfer a part of the microwave energy to the fluid mixture in order to create therein a plasma that causes at least some of the chemical bonds of the fluid molecules to break, said dielectric tube being cooled by a flow of a coolant in thermal contact with the external wall of the tube to be cooled. 
     The invention also relates to a system for selectively destroying chemical molecules that uses this cooling method. 
     During the fabrication of integrated circuits, substances in the gaseous state are used in numerous steps for producing and interconnecting semiconductor elements—in ion implantation, etching and in physical or chemical deposition (PVD or CVD). Some of these substances may be what are called “greenhouse gases”, that is to say they contribute to global warming when present in the atmosphere, such as especially certain fluorine derivatives—in particular perfluorocarbon (PFC) or hydrofluorocarbon (HFC) gases—or they may be certain fluids and especially certain atmospheric pollution gases that present an immediate danger to life or health and that are more particularly toxic, corrosive, inflammable, pyrophoric and/or explosive. 
     Generally, in the fabrication of semiconductors, all the deposition precursor gases, all the etching gases, all the gases used for reactor cleaning, etc. are recovered downstream of the reactor in the form of mixtures and these effluents must be treated. 
     In other applications, like the fabrication of flat-screen plasma or LCD displays or even photovoltaic cells, again gases and a number of gaseous precursors, or precursors delivered in vapor form when they are initially solids or liquids, are used. 
     In other applications, like the separation of air gases, or the purification of gases such as krypton or xenon coming from the distillation residue of an argon distillation column in an air separation plant or extracted directly from an aquifer, the gas obtained comprises a small quantity of fluorinated gases as such, for example CF 4  or C 2 F 6 , which must be removed as best as possible from the gas to be purified. 
     To destroy greenhouse gases or deposition precursor gases from these reactors for fabricating integrated circuits, it is known, for example from EP-A-874537, to use atmospheric-pressure plasmas generated by coupling an ultrahigh frequency (UHF) or microwave (MW) frequency electromagnetic wave, transmitted in a waveguide to a wave applicator, into the gas mixture so as to create the gaseous plasma. Taking account of the fact that the use of electromagnetic frequencies is strictly 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 (ISM) use and in particular for the generation of these plasmas, in particular the frequencies 2.45 GHz, 915 MHz and 434 MHz. 
     Effluent gases such as, in particular, PFC or HFC effluents from etchers are systematically diluted with nitrogen in the rough vacuum pumps, because of their hazardousness. The gas mixture fed into a system for treating or destroying effluents of the above type therefore mainly consists of nitrogen. 
     Ionizing the gas and sustaining a nitrogen plasma, when using a carrier gas such as nitrogen at atmospheric pressure, require a lot of energy. 
     Moreover, employing especially ceramic tubes causes temperature withstand problems in the various materials used. Because of this, the discharge tube is cooled by a heat-transfer fluid that flows, from one end of the tube to the other, in a space between said tube and an outer, coaxial second tube that confines the liquid. When the plasma discharge is run for an extended period of time at high power in a gas consisting mainly of nitrogen or air, the excellent thermal conductivity of the ceramic may lead the temperature of the external surface in contact with the boundary layer of the dielectric coolant to exceed the temperature at which the physicochemical properties of the latter are stable. Thus, the onset of a solid-state polymerization may be observed on the wall of the tube, the deposit formed generally absorbing microwaves leading to a chain-reaction effect (because the absorption generally increases with temperature, the hotter the tube gets the more it is heated) and creating very highly overheated regions which tend to spread gradually. These very high thermal stresses, in a very small thickness of material, are likely to cause the tube to crack or break. The dielectric heat-transfer fluid may also undergo a volume transformation, becoming cloudy and malodorous due to the formation of decomposition products that are thought to be harmful. Without making any presumptions regarding the degradation of the functional properties of the fluid (i.e. dielectric and heat-transfer properties), the harmfulness of the waste product is unacceptable in an industrial plant. Thus, for example, the use of silicone fluids like polydimethylsiloxane (PDMS) was stopped due to the presumed harmfulness of its thermal decomposition products. 
     The invention aims to alleviate the various aforementioned drawbacks by providing a system for cooling the tube, especially a dielectric tube, in which the atmospheric-pressure plasma is generated, which is different from the systems of the prior art. 
     According to the invention, on the one hand, the coolant in thermal contact with the dielectric tube flows cocurrently to the fluid or fluid mixture in the dielectric tube and, on the other hand, the coolant comprises at least one oil chosen from among linear alpha-olefins having a carbon chain of at least ten carbon atoms and/or perfluorocarbon liquids having a dielectric constant ∈ lower than 2.5, a loss tangent tan δ of between 10 −2  and 10 −4  and a specific heat Cp&lt;0.6 g.cal/g.° C. 
     After many dielectric tubes had broken prematurely due to local overheating of the tube, the inventors successfully demonstrated a number of results that led them to the invention. In particular, when flowing the fluid mixture to be treated counter (downward injection) to the flow of the heat-transfer coolant (upward flow), it being generally recognized, by those skilled in the art, that countercurrent flow achieves the best heat exchange between fluids, the inventors demonstrated the existence of bubbles in the heat-transfer liquid next to 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 made of dissolved air gases and vaporized oils. These effects were confirmed by observing changes in the refractive index of the ceramic tube. Quite unexpectedly, reversing the flow direction of the oil (so as to flow cocurrently to the fluid mixture, that is to say, in the present example, from top to bottom), allows the ceramic/oil interface to be better cooled and stops the formation of a film of vaporized oil at this same interface. 
     It was also found that linear alpha-olefins, in particular C 14  linear alpha-olefins, give results that were already much better than conventional heat-transfer liquids (such as especially water). The use of perfluorocarbon (PFC) liquids again gave clearly improved results, in particular when these fluids had the following properties:
         dielectric constant ∈&lt;2.5, preferably ∈&lt;2.0;   10 −4 &lt;tan δ&lt;10 −2 , preferably &lt;10 −3 ; and   specific heat Cp such that: Cp≦0.6, preferably Cp≦0.3.       

     In addition, since these products have a very high density (almost three times greater than a C 14  alpha-olefin), the flow of liquid required to remove the same number of calories is clearly less, hence the flow rate of the heat-transfer fluid may be reduced by about 30%. 
     Furthermore, the thermal stability of these perfluoronated products is much higher, which makes the operation of the system of the invention safer. 
     Preferably, at least one linear alpha-olefin is used, preferably a C 14  linear alpha-olefin or 1-tetradecene and/or a perfluorocarbon (PFC) fluid having a dielectric constant ∈&lt;2 and/or a loss tangent tan δ&lt;10 −3  and/or a specific heat Cp≦0.3 g.cal/g.° C. 
     According to another preferable embodiment, the fluid mixture is injected into the tube at atmospheric pressure or at a pressure near atmospheric pressure. 
     According to another embodiment, the fluid mixture and/or a complementary inert gas are/is injected in the form of a vortex. 
     According to another preferable embodiment of the invention, the fluid to be treated and the coolant 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 that receives the fluid and/or the gas;   a microwave generator;   means for coupling microwaves to the fluid and/or a gas so as to create a plasma in the dielectric tube;   means for cooling the dielectric tube using a coolant, said means being placed outside the tube;   a source of linear alpha-olefin and/or of perfluorocarbon fluid connected to the means for cooling the tube; and       

     means for making the coolant flow cocurrently to the fluid or fluid mixture to be treated, preferably from top to bottom. 
    
    
     
       The invention will be better understood using the following embodiments, given by way of nonlimiting example, together with the figures which show: 
         FIG. 1 , a schematic overview of the system according to the invention; 
         FIG. 2   a , a vertical section view of a vortex-creating fluid injection head suitable for the system of  FIG. 1 ; 
         FIG. 2   b , a section view along the line A-A in  FIG. 1 ; 
         FIG. 2   c , a horizontal section view along the line B-B in  FIG. 2   a ; and 
         FIG. 3  is an embodiment of a vortex-creating injection head. 
     
    
    
     In  FIG. 1 , the plasma treating system A for treating gases comprises a surfaguide field applicator  1  as described in EP-A-874537, a heat exchanger B, wet scrubbing means C and then dry scrubbing means D (C and D can be placed in the reverse order if desired). 
     The system A is fed via the valve Vd with a gas used for striking the plasma and/or via the valve Vf with the gas to be treated. The gas to be treated is emitted from one of the reactors CVD 1 , CVD 2 , CVD 3 , . . . CVDn, via the valves V 1 , V 2 , V 3 , . . . Vn, respectively (these gases may be gasses emitted from reactors used in the fabrication of semiconductors or flat-screen displays or optical fibers of solar cells, etc.). 
     The system A also comprises a dielectric tube  16  surrounded by a cooling system comprising a heat-transfer  19  that absorbs the microwaves fluid sufficiently weakly for power to remain available to sustain the plasma and that flows in the space  18  defined by the silica outer tube  17  and the dielectric tube  16 . The inlet for the fluid  19  is located in the bottom part  13  of the system A and the outlet  20  for the fluid  19 , after having cooled the tube  16 , is located in the top part  24 . The dielectric tube  16  passes through the reduced central part  3  of the field applicator  1  (the height of the short side of the rectangular-section hollow waveguide decreases relative to the standard waveguide height), the silica tube  17  surrounding the space  18  in which the coolant flows. Electrically conducting jackets  7 ,  8 , which act as electromagnetic shields, are placed around the top and bottom parts of the aforementioned tubes, respectively. The bottom part of the jacket  7  and the dielectric tube are separated by an optimized radial distance so as to maximize the coupling between the waveguide and the tube, without the presence of the jacket interfering with the microwaves. 
     The top part of the jacket  8  and the tube next to the bottom part of the applicator  1  are separated by the same optimized radial distance. At their other ends, the jackets  7 ,  8  are adjacent the top part  24  and the bottom part  13 , respectively. The field applicator  1  formed from a hollow rectangular waveguide comprises a central part  3  having a reduced cross section relative to the standard cross section used at the input/output  2 ,  4  located on either side of this central part  3 . The microwave power, when the system is in operation, is transmitted from the lateral part  2  toward the central part  3 , in which central part the microwaves are concentrated and then launched along the tube  16 , from both sides of this central part  3  of the field applicator, so as to create a plasma in the tube by providing it with energy over the entire propagation length of the wave along the tube. This plasma is struck using the electrode  23  which is secured to the support  10  located above the top part  9  of the system A. The electrode  23  is kept substantially aligned with the axis of the dielectric tube  16 , said electrode being is connected to a high-voltage source or an ignition coil. 
     The system for striking the plasma is connected to a valve Vn and comprises essentially two branches: one connected to an argon (Ar) source via a mass flow controller and a valve V Ar , the other connected to a nitrogen source via a mass flow controller and a valve V N2 . 
     The heat exchanger B allows the hot gases emitted from the plasma of the system A to be cooled and then passed, at about 150° C. at most, to the wet scrubber C and the dry scrubber D (or vice-versa). 
       FIG. 2  shows a system for injecting gasses (whether gasses for striking the plasma or for treatment) in the form of a vortex. The gas and/or fluid injection ducts arrive tangentially at the vertical duct  54 , which prolongs the dielectric tube  16 , so as to create a swirling effect in the gasses and/or fluids injected. 
       FIG. 2   a  is a vertical section view of the top part  9 ,  24  of the plasma system A. Four gas-injection ducts ( 57 ,  51 ), ( 58 ,  62 ), ( 59 ,  53 ) and ( 60 , 64 ), all shown in  FIG. 2   b  (which is a section view along the line A-A in  FIG. 1 ) allow this vortex to be created in the duct  54 . The holder  10  for the electrode  23  is secured to the top part  9  ( 24 ). The four injection ducts, when viewed in the horizontal plane, are preferably at 90° to each other and, when viewed in the vertical plane, may be oriented either horizontally or downwardly. The ducts ( 70 ,  72 ) and ( 71 ,  73 ) (shown in  FIG. 2   c , which is a horizontal section along the line B-B in  FIG. 2   a ) are also connected tangentially to the central duct  54  and are at 180° to each other. They allow an additional gas (for example nitrogen) to be injected when the gas flow injected by the four injectors located in the plane A-A is insufficient to sustain a vortex. Such a vortex reduces heat exchange with the wall of the dielectric tube, prevents direct contact between the plasma and the same dielectric tube and thus prevents a too high a temperature, which could damage the tube, from being reached. 
       FIG. 3  shows a schematic view of an embodiment of an injection head  9  for injecting gas to be treated in the plasma, with which injection head an effective vortex is achieved. As in the other figures, the same elements fear the same references. This injection head  9  comprises an inlet ( 11 ) for introducing the gasses to be treated, which then flow via the channel  80  that is coaxial with the inlet  11  toward the peripheral channel, the successive portions  81 ,  82 ,  83  and  84  of which have been shown in cross section—this continuous channel winds around the solid central part (a structure similar to a spiral staircase around a central column  85 ). This solid central part  85  is preferably made of a conductor which has a conical bottom part  86  serving as electrode for striking the plasma created in the dielectric tube  16 . The solid parts  87 ,  88 ,  89 ,  90  and  91  that protrude relative to the axis  85  are solid parts that spiral around the axis  85  defining the gas channel. The top part  92  above the central part  85  is housed in a removable piece  93  that holds the central part stationary, an O-ring  94  being used as a vacuum seal. Preferably, as indicated in  FIG. 3 , the channel  81 ,  82 , . . . , through which the gas flows so as to create a vortex in the tube  16 , will have an axis inclined at an angle to the horizontal of between approximately 25° and 35°, more preferably about 30°. 
     EXAMPLE 
     Various cooling oils were used with the system described in  FIG. 1  (both with and without a vortex system as described in  FIGS. 2 and 3 . The cooling proved to be much better, particularly when the gasses to be destroyed were injected in the form of a vortex using the device of  FIG. 2  or  FIG. 3 ). The following oils were most satisfactory for cooling the dielectric tube. 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Type of oil 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Alpha- 
                   
                   
                   
               
               
                   
                 olefin C 14   
                 FC 40* 
                 FC 43* 
                 FC 70* 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Dielectric 
                 2.3 
                 1.87 
                 1.9 
                 1.98 
               
               
                 constant 
               
               
                 Tanδ 
                 5 × 10 −3   
                 7 × 10 −4   
                 7 × 10 −4   
                 7 × 10 −4   
               
               
                 Boiling 
                 250 
                 155 
                 174 
                 215 
               
               
                 point (° C.) 
               
               
                 Critical 
                   
                 270 
                 294 
                 335 
               
               
                 temperature 
               
               
                 (° C.) 
               
               
                 Density d 
                 0.771 
                 1.87 
                 1.88 
                 1.94 
               
               
                 (kg/m 3 ) 
               
               
                 Specific 
                 0.5 
                 0.26 
                 0.26 
                 0.26 
               
               
                 heat Cp 
               
               
                 (g · cal/g · ° C.) 
               
               
                   
               
               
                 *PFC oils from 3M. 
               
            
           
         
       
     
     If an FC 70 oil is used instead of a C 14  oil, the product d×Cp drops from 0.5 to 0.38 implying that the flow rate may be reduced by 30% for the same performance. 
     In applications in which a gas such as NF 3  is converted into a mixture of fluorine and nitrogen, the use of a PFC oil to cool the dielectric tube is much safer. However, one proviso is that fluoropolymers and fluoroelastomers cannot be used in conjunction to PFC oils.