WO2009010792A2

WO2009010792A2 - Plasma reactor

Info

Publication number: WO2009010792A2
Application number: PCT/GB2008/050569
Authority: WO
Inventors: Gary Peter Knight; Andrew Chambers
Original assignee: Edwards Limited
Priority date: 2007-07-19
Filing date: 2008-07-14
Publication date: 2009-01-22
Also published as: KR20100037609A; TW200914124A; WO2009010792A3; KR101490540B1; DE112008001790T5; GB0714025D0; CN103021779B; TWI433718B; CN101755322A; CN103021779A; CN101755322B

Abstract

A plasma reactor comprises a reaction chamber and an inlet head connected to the reaction chamber. The inlet head comprises an open end connected to the reaction chamber, a plasma inlet located opposite to the open end, an inner surface tapering from the open end towards the plasma inlet, and first and second gas inlets each located between the plasma inlet and the open end. A plasma torch injects a plasma stream into the reaction chamber through the plasma inlet, which is shaped to cause the plasma stream to spread outwardly towards the first and second gas inlets. This shaping of the inlet head and the plasma inlet can enable the plasma stream to impinge upon gas streams as they exit from the gas inlets and thereby cause a significant proportion of at least one component of the gas streams to be reacted before the gas streams begin to mix within the chamber.

Description

PLASMA REACTOR

The present invention relates to a plasma reactor. The apparatus finds particular use in a plasma abatement system for treating gas streams that have been exhausted from process chambers.

A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. Examples of gases supplied to the process chamber to form a thin film include, but are not restricted to:

• Silane and ammonia for the formation of a silicon nitride film; • Silane, ammonia and nitrous oxide for the formation of a SiON film;

• TEOS and one of oxygen and ozone for the formation of a silicon oxide film; and

• AI(CH₃J₃ and water vapour for the formation of an aluminium oxide film.

Plasma etching processes are typically also performed in the process chamber to etch circuit features. Etching gases are typically perfluorocompound gases such as CF₄, C₂F₆, CHF₃, NF₃ and SF₆.

The inside surface of the process chamber is also regularly cleaned to remove the unwanted deposition material from the chamber. One method of cleaning the chamber is to supply a perfluorocompound cleaning gas such as NF₃ or C₂F₆ to react with the unwanted deposition material.

A process tool typically has a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process. The composition of the gas stream exhausted from a process chamber typically includes a residual amount of the gas supplied to the process chamber, together with by-products from the process. Therefore, during processing a waste stream formed from a combination of the gases exhausted from the chambers may have various different compositions.

Process gases such as silane and TEOS, and cleaning gases such as perfluoro- compounds are highly dangerous if exhausted to the atmosphere, and so before the exhaust gases are vented to the atmosphere they are conveyed to abatement apparatus. The abatement apparatus converts the more hazardous components of the exhaust gases into species that can either be readily removed by conventional scrubbing, and/or can be safely exhausted to the atmosphere.

Current trends are to move towards fuel-free abatement techniques, and it is know that undesirable species within the exhaust gas from an etching process chamber can be removed with high efficiency and at a relatively low cost using a plasma abatement device. In the plasma abatement process, the gas stream is caused to flow into a high density plasma and under the intensive conditions within the plasma species within the gas stream are subjected to impact with energetic electrons causing dissociation into reactive species which can combine with oxygen or hydrogen to produce relatively stable by-products. For example, C₂F₆ can be converted into CO, CO₂ and HF, which can be removed in a further treatment step. It is therefore desirable to extend plasma abatement techniques to enable a single fuel-free abatement device to be used to treat exhaust gases from a range of process chambers.

However, depending on the processes taking place in the chambers the different exhaust gases may contain chemicals that are incompatible with one another. For example, the exhaust gas from a chamber in which a silicon oxide deposition process is taking place may contain TEOS, oxygen and SiO₂ particulates that have been generated within the chamber. On the other hand, the exhaust gas from a chamber in which an NF₃ cleaning process is taking place may contain fluorine (F₂). TEOS and fluorine will spontaneously combust on contact, potentially leading to fire or an explosion within the exhaust gas pipework. Whilst these gas streams may be treated separately using respective abatement apparatus, this increases the costs associated with the abatement system.

Furthermore, in order to optimise the destruction efficiency of microwave plasma abatement devices, the gas inlet is typically of the order of 1 mm². Consequently, the presence of particulates of only a few microns in diameter in the exhaust gas can lead to rapid clogging of the inlet of the plasma abatement device.

It is an aim of at least the preferred embodiments of the present invention to seek to solve these and other problems.

In a first aspect, the present invention provides a plasma reactor comprising: a reaction chamber; an inlet head having an open end connected to the reaction chamber, a plasma inlet located opposite to the open end, an inner surface tapering from the open end towards the plasma inlet, and first and second gas inlets each located between the plasma inlet and the open end; and a plasma torch for injecting a plasma stream into the reaction chamber through the plasma inlet; wherein the plasma inlet is shaped to cause the plasma stream to spread outwardly towards the gas inlets.

This shaping of the inlet head and the plasma inlet can enable the plasma stream to impinge upon gas streams as they exit from the gas inlets. The plasma stream can provide a source of energy that can cause a significant proportion of at least one component of the gas streams to be reacted before the gas streams begin to mix within the chamber.

For example, if one of the components of the gas stream is flammable and is conveyed into the reactor with a sufficient amount of oxidant, the plasma stream can provide ignition energy for causing substantially complete, controlled combustion of the flammable component before that gas stream mixes with the other gas stream within the reactor. This can inhibit an uncontrolled reaction occurring within the plasma reactor between the flammable component of one gas stream, for example TEOS, with a component of the other gas stream, for example fluorine. This component of the other gas stream can be reacted with a reactant, for example water vapour, supplied separately to the reaction chamber, or previously entrained within the gas stream, with the plasma stream providing a source of energy for promoting this reaction.

As a result, the plasma reactor may be used to treat simultaneously the gas streams exhaust from two process chambers with reduced power consumption and costs in comparison to an abatement system comprising a plasma reactor for each gas stream.

To create a high temperature reactor in which process powders are unable to collect, the reaction chamber preferably comprises an annular body and means for supplying gas to the inner surface of the annular body for inhibiting the build-up of deposits thereon. This feature may be provided in a plasma reactor which does not have the aforementioned inlet head, and so in a second aspect, the present invention provides a plasma reactor comprising: a reaction chamber; at least one gas inlet for supplying a gas to the reaction chamber plasma; and a plasma torch for injecting a plasma stream into the reaction chamber; wherein the reaction chamber comprises an annular body and means for supplying gas to the inner surface of the annular body for inhibiting the build-up of deposits thereon.

The annular body may comprise a porous annular member, with the gas supply means comprising a plenum chamber extending about the annular member for receiving the gas. The gas passes under pressure from the plenum chamber through the annular member and dislodges any deposits that may have accumulated on the inner surface of the annular member. Means may be provided for heating the gas supplied to the inner surface of the annular body. The means may be provided by electrical resistance heaters located in the plenum chamber, or alternatively by a heater surrounding the plenum chamber. Heating the gas supplied to the reactor chamber can enable a high temperature to be maintained along the length of the reactor chamber, thereby increasing the length of time during which the components of the gas stream are exposed to the high temperature conditions generated within the chamber and so enhancing the abatement performance of the reactor.

The gas may be an inert purge gas, for example nitrogen or argon, and may comprise a reactant, for example water vapour, oxygen, hydrogen or methane for reacting with a component of a gas stream entering the reactor through one of the gas inlets. This can provide a convenient mechanism for supplying the reactant to the reactor chamber, as no additional gas supply is required to supply the reactant to the reactor chamber.

A cooling column may be provided below and in fluid communication with the reaction chamber, along with means for maintaining a flow of water along the inner surface of the cooling column. This can enable the reaction product stream leaving the reaction chamber to be cooled whilst enabling acidic gases contained within the gas stream, such as HF, to be taken into solution by the water flow coating the inner surface of the column, and also enabling solid particulates to be captured by this water flow. The cooling column may also comprise a heat exchanger to reclaim heat to provide to other parts of the reactor.

The first gas inlet is preferably located diametrically opposite to the second gas inlet. The shape of the inner surface is preferably chosen so that it conforms closely to the shape of the plasma stream, thereby minimising the length of the gas path between a gas inlet and the plasma stream. For example, the inner surface of the inlet head may be substantially conical or frustro-conical, with the taper angle of the inner surface chosen to closely match the angle at which the plasma stream flares outwardly from the plasma inlet.

Each gas inlet comprises a nozzle for receiving a gas stream to be treated within the reaction chamber, and an annular passageway extending about the nozzle for receiving a purge gas. This purge gas can serve to cool the inlet head, with the additional advantage that the heat that is being drawn from the inlet head by the purge gas is re-introduced back into the reaction chamber by the purge gas. The purge gas may comprise a relatively inert gas, such as nitrogen or argon, and may also include a reactant, such as hydrogen, water vapour, oxygen or methane, for reacting with a component of the gas stream that is conveyed into the reactor by that gas inlet. The nozzle may terminate within the gas inlet to provide opportunity for the reactant to mix with the gas stream before it leaves the gas inlet. The inlet may also comprise a reactant gas inlet pipe extending into the nozzle concentrically surrounded by the nozzle and annular passageway. The reactant gas passed to the reactant gas inlet pipe may comprise, for example, hydrogen, water vapour, methane or oxygen. The reactant gas passed to the reactant gas inlet pipe may be in addition to, or instead of, any reactant passing to the annular passageway.

The inlet head may comprise means located between the plasma inlet and the reaction chamber for directing the plasma stream towards the first and second gas inlets, thereby reducing further the gas path extending from the gas inlet to the plasma stream. For example, a ceramic body may be located within the inlet head and shaped so as to direct the plasma stream into a conical or frustro-conical channel located between the body and the inner surface of the inlet head. Means may be provided for generating a gas layer on the outer surface of the body. For example, the ceramic body may be porous, with a stream of gas being supplied to the body to generate the gas layer on the outer surface of the body. This gas layer can provide a protective gas boundary for the ceramic body. The gas may also comprise a reactant for reacting with a component of a gas stream entering the reactor through one of the gas inlets. If cooling water is supplied to the body, this can provide a source of water vapour for reacting with the component of the gas stream.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a first embodiment of a plasma reactor;

Figure 2 is a cross-sectional view of a second embodiment of a plasma reactor; and

Figure 3 is a cross-sectional view of a third embodiment of a plasma reactor.

Figure 4 is a cross-sectional view of a fourth embodiment of a plasma reactor.

With reference first to Figure 1 , a first embodiment of a plasma reactor comprises a reactor chamber 12. The reactor chamber 12 is substantially cylindrical, and is bounded by the inner surface 14 of annular body 16. In this example, the annular body 16 is provided by a porous ceramic annular member, which is surrounded by a plenum volume 18 formed between the outer surface of the annular body and a cylindrical outer shell 20. As described in more detail below, a gas is introduced into the plenum volume 18 via one or more inlet nozzles 22 so that, during use, gas passes through the annular body 16 into the reactor chamber 12, as indicated at 24 in Figure 1.

The lower end (as shown) of the reactor chamber 12 is open to allow reaction products to be output from the reactor chamber 12. The upper end (as shown) of the reactor chamber 12 is connected to an inlet head 30 for supplying gas to be treated to the reactor chamber 12. The inlet head 30 comprises an open lower end 32 connected to the reaction chamber 12 and a plasma inlet 34 located opposite to the open end 32, with the inner surface 36 of the inlet head 30 tapering from the open end 32 towards the plasma inlet 34. A dc plasma torch 38 is located external from the inlet head 30 for injecting a plasma stream through the plasma inlet 34. The plasma stream may be generated from any ionisable plasma source gas, for example argon or nitrogen. The plasma inlet 34 is shaped to cause the plasma stream to spread outwardly as it leaves the plasma inlet 34. In this example, the plasma inlet 34 has an inner wall having a convergent section located adjacent the plasma torch 38, and a divergent section located adjacent the inner surface 36 of the inlet head 30, so that the plasma stream flares outwardly from the plasma inlet 34 with a plasma flare angle α. The inner surface 36 of the inlet head 30 is shaped to conform closely to the shape of the plasma stream. In this example the inner surface 36 is substantially frustro-conical, having a taper angle β which closely matches the plasma flare angle α.

The inlet head 30 also comprises a first gas inlet 40 and a second gas inlet 42, each located between the plasma inlet 34 and the open end 32 of the inlet head 30 and extending through the inlet head 30 in substantially parallel directions. Each gas inlet 40, 42 is connected to a respective gas supply conduit 44, 46 which supplies a gas to be treated within the plasma reactor to its respective gas inlet 40, 42. As shown in Figures 1 to 3 inlets 40, 42 may be arranged to supply the gas to be treated through the plasma reactor in a downward direction parallel to the inner surface 14 of the annular body 16. The inlets 40, 42 may also be arranged to supply the gas to be treated in a downward, spiral direction through the plasma reactor by directing the inlets 40, 42 at an angle into the inlet head 30 (not shown) and therefore increase the residence time of the gases in the reactor. A nozzle 48 is provided within each gas inlet 40, 42 for receiving gas from the gas supply conduit 44, 46 and injecting the gas into the reaction chamber 12. Each nozzle 48 is surrounded by an annular gas passageway 50 defined between the outer surface of the nozzle 48 and the inner surface of the gas inlet 40, 42, and to which a purge gas is supplied for cooling the inlet head 30 during use of the plasma reactor. The gas supplied to a gas inlet 40, 42 may comprise an exhaust gas from a semiconductor process chamber, with each gas inlet 40, 42 being arranged to receive gas from a different process chamber. For example, at a given time the gas conveyed to gas conduit 44 may be the exhaust gas from a process chamber in which a silicon oxide deposition process is taking place, and the gas conveyed to gas conduit 46 may be the exhaust gas from a different process chamber in which a cleaning process is taking place. Consequently, the gases being supplied to the reactor may be incompatible; in this example one gas may contain TEOS whilst the other gas may contain fluorine.

The design of the inlet head 30 can enable the plasma stream to impinge upon these gases as they enter the plasma reactor from the gas inlets 40, 42. The shape of the inner surface 36 means that there is only a relatively short gas path between each gas inlet 40, 42 and the plasma stream, and therefore little opportunity for the gases to mix before being struck by the plasma stream. The plasma stream can provide a source of energy that can cause a significant proportion of at least one component of the gases to be reacted before the gases begin to mix within the reaction chamber 12. For example, the plasma stream can provide an ignition source for the abatement of a flammable gas such as TEOS contained with the exhaust gas from a deposition process chamber. TEOS is generally exhausted from such a chamber with an amount of oxidant, such as oxygen or ozone, and so provided that there is sufficient oxidant within the gas substantially complete combustion of the flammable gas can take place within the reactor. In the event that there is insufficient oxidant contained within the exhaust gas for complete combustion of the flammable gas, additional oxidant may be supplied to the purge gas supplied to the annular passageway 50 surrounding the nozzle 48 of gas inlet 44.

As mentioned above, the gas conveyed to gas conduit 46 may be the exhaust gas from a process chamber in which a cleaning process is taking place, and so may contain cleaning gas, such as NF₃, together with fluorine (F₂) and SiF₄ generated during the cleaning process. The abatement of F₂ and SiF₄ is achieved by the heating of the gas by the plasma stream to a sufficient temperature that the reaction of these species with water vapour is rapid and complete. Again, the water vapour may be supplied to the reactor entrained within the purge gas which is supplied to the gas passageway 50 surrounding the nozzle of the gas inlet 46, so that the reactions can begin within the inlet head 30. Alternatively, as the flammable gas has been abated in close proximity to the gas inlet 44 there will be little, if any, flammable gas present within the reaction chamber 12, and so the water vapour may be supplied to the reaction chamber 12 entrained within the purge gas supplied to the plenum chamber 18 so that the reaction of fluorine and SiF₄ with water vapour take place wholly within the reaction chamber 12. Abatement of NF₃ and other perfluorocompounds requires an elevated temperature and a longer residence time, which is achieved by heating the (waterbearing) purge gas supplied to the plenum chamber 18. The purge gas may be heated using electrical resistance heaters located in the plenum chamber 18, or by a heating jacket surrounding the plenum chamber 18.

As one of the gases entering the plasma reactor may be the exhaust gas from a process chamber in which a silicon dioxide deposition is taking place, particulates of silicon dioxide may enter the reactor. This is because in the deposition process, the conditions immediate to the substrate are optimised to minimise gas-phase reactions and maximise surface reactions for the formation of a continuous film on the substrate. However, conditions elsewhere in the chamber and downstream from the chamber are not so optimised, and gas-phase nucleation can result in the formation of particulates. These particulates are generally formed with a range of sizes, from a few microns in diameter up to a few tens or hundreds of microns in diameter, and finer particulates can tend to agglomerate to form larger particulates. The supply of purge gas through the annular body 16 serves to dislodge any such particulates from the inner surface 14 of the annular body 16, thereby enabling the reaction chamber 12 to be maintained in a relatively clean condition during use of the reactor. The gas stream exhausted from the open bottom end of the reaction chamber 12 will therefore comprise by-products from the reactions taking place within the reactor, together with other gases that have passed through the reactor, such as purge gases and unconsumed reactants, and solid particulates. The open bottom end of the reaction chamber is connected to a cylindrical post-combustion chamber 60 comprising a water-cooling column 62 for receiving the gas stream flowing from the reaction chamber 12. Water is supplied to an annular trough 64 surrounding the cooling column 62 through a pipe (not shown) so that the water overflows from the top of the trough 64 and streams down the inner surface of the cooling column 62. The water serves to cool the gas stream and prevent solid particulates from being deposited on the surface of the cooling column 62. In addition, any acidic components of the gas stream may be taken into solution by the water. If any additional quenching is required, spray jets may be positioned at the lower end of the chamber 60 to introduce a water mist.

The gas stream and water which are discharged through the outlet of the chamber 60 may be conveyed to a separator (not shown) for separating the water, now containing solid particulates and acidic species, from the gas stream. The gas stream may then be conveyed through a wet scrubber to remove remaining acidic species from the gas stream before it is vented to the atmosphere.

A second embodiment of a plasma reactor is illustrated in Figure 2. The second embodiment includes all of the features of the first embodiment, and also includes a conical ceramic body 70 located opposite to the plasma inlet 34 for directing the plasma stream into a conical channel 72 located between the body 70 and the inner surface 36 of the inlet head 30. The body 70 may be connected to the inlet head 30, to the annular body 16 or to the bottom of the outer shell 20. The direction of the plasma stream into the conical channel 72 further reduces the gas path extending from each gas inlet 40, 42 to the plasma stream, thus making it more difficult for the gases to mix within the reactor before the gases have been treated to substantially remove at least one component from the gases. A third embodiment of a plasma reactor is illustrated in Figure 3. The third embodiment includes all of the features of the first embodiment, and also includes a second annular body 80 located between the inlet head 30 and the annular body 16. This second annular body is also preferably provided by a porous ceramic annular member, which is surrounded by a plenum volume 82 formed between the outer surface of the annular body 80 and a cylindrical outer shell 84. As for the annular body 16, a gas is introduced into the plenum volume 82 via one or more inlet nozzles 86 so that, during use, gas passes through the second annular body 80 into the reactor chamber 12, as indicated at 88 in Figure 3, to dislodge particulates from the inner surface 90 of the second annular body 80. As in the first embodiment, this gas is preferably heated before entering the reaction chamber 12.

In addition to increasing the length of the reaction chamber 12, and therefore the residence time for gases within the plasma reactor, the inclusion of this second annular body 80 and associated plenum chamber 82 can enable different purge gas flow rates, purge gas compositions and temperatures to be used along the length of the reaction chamber 12 so that the abatement chemistry can be optimised for the gases to be treated within the reactor. For example, the plenum chamber 82 may be fed with an oxidant-rich purge gas for abating hydrogen, whilst a water vapour-rich purge gas may be supplied to the plenum chamber 18 for abating an oxidant such as fluorine or NF₃.

A fourth embodiment of a plasma reactor is illustrated in Figure 4. The fourth embodiment includes all of the features of the third embodiment, and also includes reactive gas inlet pipes 100 provided inside of, and concentrically surrounded by, the nozzles 48 and annular gas passageways 50. Reactive gas is supplied to the gas to be treated via reactive gas inlet pipes 100 together with, or instead of, any reactive gas supplied to via gas passageway 50. It is to be understood reactant gas inlet pipes 100 may be provided with any of the embodiments described herein in Figures 1 to 3, and is not limited to the embodiment shown in Figure 4. Furthermore, the arrangement of the reactive gas inlet pipe with respect to the purge gas inlet is not limited to the arrangement shown in Figure 4. For instance the reactive gas inlet pipe 100 can be arranged such that all gases are mixed with one another prior to entry into chamber 12.

Claims

1. A plasma abatement reactor comprising: a reaction chamber; an inlet head having an open end connected to the reaction chamber, a plasma inlet located opposite to the open end, an inner surface tapering from the open end towards the plasma inlet, and first and second gas inlets each located between the plasma inlet and the open end; and a plasma torch for injecting a plasma stream into the reaction chamber through the plasma inlet; wherein the plasma inlet is shaped to cause the plasma stream to spread outwardly towards the gas inlets.

2. A plasma reactor according to Claim 1 , wherein the first gas inlet is located diametrically opposite to the second gas inlet.

3. A plasma reactor according to Claim 1 or Claim 2, wherein the inner surface of the inlet head is substantially frustro-conical.

4. A plasma reactor according to any preceding claim, wherein the gas inlets extend through the inlet head in substantially parallel directions.

5. A plasma reactor according to any preceding claim, wherein each gas inlet comprises a nozzle for receiving a gas stream to be treated within the reaction chamber, and an annular passageway extending about the nozzle for receiving a purge gas.

6. A plasma reactor according to any preceding claim, wherein the plasma inlet has an inner wall having a convergent section located adjacent the plasma torch, and a divergent section located adjacent the inner surface of the inlet head.

7. A plasma reactor according to any preceding claim, comprising means located opposite the plasma inlet for directing the plasma stream towards the first and second gas inlets.

8. A plasma reactor according to Claim 7, wherein the directing means comprises a body for directing the plasma stream into a conical channel located between the body and the inner surface of the inlet head.

9. A plasma reactor according to Claim 8, wherein the body is formed from ceramic material.

10. A plasma reactor according to Claim 8 or Claim 9, comprising means for generating a gas layer on the outer surface of the body.

11. A plasma reactor according to Claim 10, wherein the gas layer comprises a reactant for reacting with a component of a gas stream entering the reactor through one of the gas inlets.

12. A plasma reactor according to any of Claims 8 to 11 , comprising means for cooling the outer surface of the body.

13. A plasma reactor according to Claim 12, wherein the body cooling means is configured to supply water to the body.

14. A plasma reactor according to any preceding claim, wherein the reaction chamber comprises an annular body and means for supplying gas to the inner surface of the annular body for inhibiting the build-up of deposits thereon.

15. A plasma abatement reactor comprising: a reaction chamber; at least one gas inlet for supplying a gas to the reaction chamber plasma; and a plasma torch for injecting a plasma stream into the reaction chamber; wherein the reaction chamber comprises an annular body and means for supplying gas to the inner surface of the annular body for inhibiting the build-up of deposits thereon.

16. A plasma reactor according to Claim 14 or Claim 15, wherein the annular body comprises a porous annular member, and the gas supply means comprises a plenum chamber extending about the annular member for receiving said gas.

17. A plasma reactor according to any of Claims 14 to 16, comprising means for heating the gas supplied to the inner surface of the annular body.

18. A plasma reactor according to any of Claims 14 to 17, wherein the gas comprises a reactant for reacting with a component of a gas stream entering the reactor.

19. A plasma reactor according to any of Claims 14 to 17, wherein the reaction chamber comprises a second annular body located between the first-mentioned annular body and the plasma inlet, and second gas supplying means for supplying a second gas, different from the gas supplied to the first-mentioned annular body, to the inner surface of the second annular body for inhibiting the build-up of deposits thereon.

20. A plasma reactor according to Claim 19, wherein the second annular body comprises a porous annular member, and the second gas supply means comprises a plenum chamber extending about this annular member for receiving said second gas.

21. A plasma reactor according to Claim 19 or Claim 20, comprising means for heating the second gas.

22. A plasma reactor according to any of Claims 19 to 21 , wherein the second gas comprises a reactant for reacting with a component of a gas stream entering the reactor.

23. A plasma reactor according to any preceding claim, comprising a cooling column below and in fluid communication with the reaction chamber, and means for maintaining a flow of water along the inner surface of the cooling column.

24. A plasma reactor according to Claim 5 and Claim 15 further comprising a gas conduit to introduce a reactant gas to the gas to be treated prior to treatment by said plasma.