GB2534890A

GB2534890A - Thermal plasma torch

Info

Publication number: GB2534890A
Application number: GB1501798.1A
Authority: GB
Inventors: Peter Knight Gary; Magni Simone
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2015-02-03
Filing date: 2015-02-03
Publication date: 2016-08-10
Also published as: WO2016124887A1; GB201501798D0; KR200493866Y1; KR20170003513U

Abstract

The plasma torch abatement apparatus is used for treatment of an effluent stream from a semiconductor processing tool comprises: an anode 14 and a cathode 16 arranged to generate a plasma stream 12 and a plasma discharge region 24; and an effluent stream conduit 60 arranged to convey the effluent stream to flow coaxially with the plasma stream. A conical end portion 18 of the torch causes the two gas streams to converge and mix in the torch end portion 24. In this way, rather than intersecting the effluent stream, the plasma stream instead flows with the effluent stream. A spiral channel 44 for the gas used to form the plasma generates a plasma vortex in the plasma gas generation region 24. The torch configuration helps to prolong the reaction time of the effluent stream in the plasma phase and reduces the risk of any of the effluent stream bypassing the plasma stream completely. This significantly improves the abatement effectiveness of the plasma torch.

Description

THERMAL PLASMA TORCH

FIELD OF THE INVENTION

The present invention relates to a thermal plasma torch. Embodiments relate to a thermal plasma torch abatement apparatus for treating an effluent stream from a processing tool.

BACKGROUND

Thermal plasma torches are known and are typically used for treating an effluent gas stream from a manufacturing process tool used in, for example, the semiconductor or flat panel display manufacturing industry. During such manufacturing, residual fluorinated or perfluorinated compounds (PFCs) and other compounds exist in the effluent gas stream pumped from the process tool. These compounds are difficult to remove from the effluent gas stream and their release into the environment is undesirable because they are known to have relatively high greenhouse activity.

One approach to remove the PFCs and other compounds from the effluent gas stream is to use a radiant burner as described, for example, in EP1773474.

However, when fuel gases normally used for abatement by combustion are undesirable or not readily available, it is also known to use a plasma torch abatement device.

Plasmas for abatement devices can be formed in a variety of ways. Microwave plasma abatement devices can be connected to the exhaust of several process chambers. Each device requires its own microwave generator, which can add considerable cost to a system. Plasma torch abatement devices are advantageous over microwave plasma abatement devices in terms of scalability and in dealing with powder (present in the effluent stream or generated by the 3o abatement reactions). In fact, with regard to microwave plasmas, if powder is present it can modify the dielectric characteristic of the reaction tube and render ineffective the microwave injection that sustains the discharge. The plasma generated by the plasma abatement device is used to destroy or abate unwanted compounds within the effluent gas stream. -2 -

Although these apparatus exist for processing the effluent gas stream, they each have their own shortcomings. Accordingly, it is desired to provide an improved technique for processing and effluent gas stream.

SUMMARY

According to a first aspect, there is provided a plasma torch abatement apparatus for treatment of an effluent stream from a processing tool, comprising: an anode and a cathode arranged to generate, in a plasma discharge region, a plasma stream from a plasma gas; and an effluent stream conduit arranged to convey the effluent stream to flow coaxially with the plasma stream.

The first aspect recognizes that the abatement efficiency of existing plasma torch devices is suboptimal. In particular, existing arrangements generate a plasma stream which is emitted by the plasma torch device and this plasma stream then intersects the effluent stream to be treated. For example, the effluent stream may flow through a conduit and the existing plasma torch device may be placed circumferentially on the conduit such that the plasma stream zo intersects the flow of the effluent stream by from the plasma torch device radially across the flow of the effluent stream. However, the first aspect recognizes that this provides for a relatively short reaction time as the effluent stream intersects the plasma stream and some of the effluent stream may even bypass the plasma stream altogether.

Accordingly, a plasma torch may be provided. The plasma torch may be for an abatement apparatus which treats an effluent stream from a processing tool. The plasma torch may comprise an anode and a cathode. The anode and cathode may generate a plasma stream from a plasma gas. The plasma 3o stream may be generated within or inside a plasma discharge region. The plasma torch may also comprise an effluent stream conduit. The effluent stream conduit may be arranged, configured or orientated to convey the effluent stream to flow coaxially together with the plasma stream. In this way, rather than intersecting the effluent stream, the plasma stream instead flows with the -3 -effluent stream. This helps to prolong the reaction time of the effluent stream in the plasma phase and reduces the risk of any of the effluent stream bypassing the plasma stream completely. This significantly improves the abatement effectiveness of the plasma torch.

In one embodiment, the effluent stream comprises abatement reagents introduced thereinto. Accordingly, appropriate abatement reagents, such as H20, Air, 02 Hz, and the like may be introduced into the effluent stream to assist the abatement reaction.

In one embodiment, the anode, the cathode and the effluent stream conduit are arranged to generate the plasma stream with a direction of flow and to convey the effluent stream to flow in the direction of flow with the plasma stream. Accordingly, both the plasma stream and the effluent stream may be conveyed so that they both flow in the same direction.

In one embodiment, the effluent stream conduit are arranged to convey the effluent stream to flow coaxially within the plasma stream. Accordingly, the effluent stream may be conveyed inside or within the plasma stream. Again, 20 this helps to avoid any of the effluent stream bypassing the plasma stream.

In one embodiment, the anode, the cathode and the effluent stream conduit are arranged to position the plasma stream as a boundary layer between the effluent stream and at least one of the anode and the cathode. By providing a boundary layer, degradation of the plasma torch by high activity compounds is reduced.

In one embodiment, the anode and the cathode are configured to generate the plasma stream as a cone and the effluent stream conduit is arranged to convey 3o the effluent stream to flow within the plasma stream along an axis of the cone.

In one embodiment, the anode and the cathode are configured to generate the plasma stream as a double cone and the effluent stream conduit is arranged to -4 -convey the effluent stream to flow within the plasma stream along an axis of the double cone.

In one embodiment the anode and the cathode are configured to generate the plasma stream as a double cone, each nappe of the double cone being coupled by a throat and the effluent stream conduit is arranged to convey the effluent stream to flow within the plasma stream along an axis of the double cone and the throat.

In one embodiment, the anode and the cathode are configured to impart a rotational component to the plasma gas flow to rotate the plasma about the direction of flow. In order to improve the stability of the plasma stream or flare, the plasma gas is stabilized by generating a spiral flow or vortex.

In one embodiment, one of the anode and the cathode comprises a swirl structure located in a gap between the anode and the cathode and configured to impart the rotational component to the plasma gas flow. The swirl structure may form part of the anode, the cathode or may be a separate item.

In one embodiment, the cathode comprises a generally cylindrical body received within the anode separated by a gap through which the plasma gas flows.

In one embodiment, the conduit comprises a bore within the cathode.

Accordingly, the effluent stream may be arranged to flow through a bore within the cathode around which the plasma gas flows in order to coaxially locate the effluent stream within the plasma stream.

In one embodiment, the conduit is coaxially aligned with an axis extending 3o along the cathode. It will be appreciated that the conduit need not extend along the whole length of the cathode, nor that the conduit is located at a centre of the cathode. -5 -

In one embodiment, the conduit is configured to impart a rotational component to the effluent stream to rotate the effluent stream about the direction of flow. Accordingly, the effluent stream may also be stabilized by generating a spiral flow or vortex. Typically, the effluent stream may be rotated in a direction which matches the rotation of the plasma stream.

In one embodiment, the conduit comprises a liner which terminates prior to the plasma discharge region of the cathode. Providing a liner helps to reduce degradation of the cathode by the effluent stream.

In one embodiment, the cathode comprises a high work function material proximate the plasma discharge region. Providing a high work function material helps to improve the plasma ignition and generation.

In one embodiment, the cathode comprises a frusto-conical annular ring proximate the plasma discharge region.

In one embodiment, the plasma torch abatement apparatus comprises a power supply unit operable to generate a breakdown and sustain a plasma discharge.

In one embodiment, the plasma torch abatement apparatus comprises a reaction area proximate the anode and operable to receive reagents (for example, H20, Air, 02 H2 and the like) to facilitate abatement. In one embodiment, the reaction area may be downstream of the anode.

According to a second aspect, there is provided a method of treating an effluent stream from a processing tool, comprising: generating, in a plasma discharge region, a plasma stream from a plasma gas using an anode and cathode; and conveying the effluent stream to flow coaxially with the plasma stream.

In one embodiment, the effluent stream comprises abatement reagents introduced thereinto. -6 -

In one embodiment, the method comprises generating the plasma stream with a direction of flow and conveying the effluent stream to flow in the direction of flow with the plasma stream.

In one embodiment, the method comprises conveying the effluent stream to flow coaxially within the plasma stream In one embodiment, the method comprises positioning the plasma stream as a boundary layer between the effluent stream and at least one of the anode and in the cathode.

In one embodiment, the method comprises generating the plasma stream as a cone and conveying the effluent stream to flow within the plasma stream along an axis of the cone.

In one embodiment, the method comprises generating the plasma stream as a double cone and conveying the effluent stream to flow within the plasma stream along an axis of the double cone.

zo In one embodiment, the method comprises generating the plasma stream as a double cone, each nappe of the double cone being coupled by a throat and conveying the effluent stream to flow within the plasma stream along an axis of the double cone and the throat.

In one embodiment, the method comprises imparting a rotational component to the plasma gas to rotate the plasma about the direction of flow.

In one embodiment, the method comprises locating a swirl structure in a gap between the anode and the cathode to impart the rotational component to the 3o plasma gas.

In one embodiment, the cathode comprises a generally cylindrical body received within the anode separated by a gap through which the plasma gas flows. -7 -

In one embodiment, the conduit comprises a bore within the cathode.

In one embodiment, the conduit is coaxially aligned with an axis extending along the cathode.

In one embodiment, the method comprises imparting a rotational component to the effluent stream to rotate the effluent stream about the direction of flow.

In one embodiment, the conduit comprises a liner which terminates prior to the plasma discharge region of the cathode.

In one embodiment, the cathode comprises a high work function material proximate the plasma discharge region.

In one embodiment, the generating comprises generating a breakdown and zo sustaining a plasma discharge with a power supply unit.

In one embodiment, the method comprises receiving reagents (for example, H2O, Air, 02 H2 and the like) to facilitate abatement at a reaction area proximate the anode. In one embodiment, the reaction area may be downstream of the anode.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in 3o combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. -8 -

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 illustrates a plasma torch according to one embodiment; and Figure 2 illustrates in more detail a swirl element shown in Figure 1.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments provide an arrangement whereby the effluent stream is mixed with the plasma stream by the plasma torch so that they flow together in the same direction, rather than causing the effluent stream to simply pass through the plasma stream in a downstream reaction chamber. In particular, the plasma torch generates the plasma stream and has a conduit which introduces the effluent stream to flow with the plasma stream. Making the effluent stream and plasma stream flow together, rather than having the effluent stream intersect the plasma stream in a reaction chamber, prolongs the reaction time and allows the plasma device to put more energy in the effluent stream hence improving abatement.

Typically, the effluent stream is introduced within the plasma stream so that the plasma stream surrounds the effluent stream to also reduce the likelihood of the effluent stream bypassing the plasma stream. The plasma gas used to generate the plasma stream also helps to cool the torch cathode and provides a boundary layer which stays close to the surface of the plasma torch components, thus reducing the extent to which reaction by-products corrode these components.

Plasma Torch 3o Figure 1 illustrates a plasma torch, generally 10, in cross-section. The plasma torch 10 comprises a generally tubular cathode 12 nested within an upstream opening of a generally tubular anode 14. The tubular anode 14 is generally kept cool by running a liquid coolant around its surface. An annular space 16 is -9 -provided between the cathode 12 and the anode 14 through which a plasma source gas such as argon or nitrogen can flow.

The cathode 12 and optionally the anode 14 is electrically connected to a power supply (not shown) which can be configured to apply a direct current between the cathode 12 and the anode 14 or an alternating current to either or both of the cathode 12 and the anode 14. The magnitude and frequency of the current required is generally determined and selected by reference to other process parameters, such as the effluent stream or plasma source gas species and flow rate, the cathode-anode spacing, gas temperature, etc. The voltage magnitude of the plasma discharge is directly influenced by these parameters. In any event, an appropriate, initial high-voltage regime is one that causes the plasma source gas to ionise and thereby form a plasma (in a process known as breakdown).

Dual Cone Configuration It can be seen that the interior geometry of the anode 14 comprises (going from the upstream end A to the downstream end B) a first inwardly-tapering frusto-conical portion 18 leading to a substantially parallel-sided throat portion 20, zo which leads to an outwardly-tapering frusto-conical portion 22. The effect of this geometry is to accelerate and compress the incoming gas to create a region 24 of relatively high-speed, relatively compressed gas in a region immediately downstream of the cathode 12.

The frusto-conical portion 22 can be contiguous to a reaction area which comprises (further) reagents needed to cope with the abatement reaction or process by-products; e.g. these reagents can be water fed by a water wall along a reaction tube or by means of spray nozzles.

3o Annular Rinq The cathode 12 comprises a generally cylindrical body portion leading to a chamfered free end portion whose external geometry substantially matches the internal geometry of the inwardly-tapering frusto-conical portion 18 of the anode 14. The cylindrical body portion of the cathode 12 is manufactured from a high-conductivity metal, such as copper. The chamfered free end portion of the cathode 12 formed by an annular ring 32. The annular ring 32 is coaxially co-located on the cathode 12. The annular ring 32 provides a preferential electrical discharge site. This is accomplished by selecting a different material for the annular ring 32 than the main body of the cathode, i.e. such that the cathode body is formed of a conducting material with a higher thermal conductivity than that of the therm ionic material of the annular ring 32. For example, it would be typical to use a copper cathode body and a hafnium or thoriated tungsten annular ring 32. The anode 14 can be formed of a similar material to the main body of the cathode 12; for example, copper.

The annular ring 32 is positioned in the region 24 of relative high-speed, relatively-compressed gas. The effect of such an arrangement is to create a region of preferential discharge for the plasma source gas and the arc is fed by a relatively compressed, high-speed state of the source gas. The plasma discharge is thus nucleated in the small region 24 immediately below the cathode 12, guided by the frusto-conical portion 18 of the anode 14 and exits as a jet via the throat 20 and expands and decelerates thereafter in the outwardly-tapering frusto-conical portion 22 of the anode 14.

In order to generate the plasma, the plasma source gas (typically a moderately inert ionisable gas such as nitrogen or argon) is conveyed to the annular space 16 via an inlet manifold (not shown). To initiate or start the plasma torch 10, a breakdown must first be generated between the annular ring 32 and the anode 14. This is typically achieved by a high-frequency, high voltage signal which may be provided by a generator associated with the power supply (not shown) for the plasma torch 10. The difference in thermal conductivity between the body of the cathode 12 and the annular ring 32 means that the cathode temperature will be higher and the electrons are preferentially emitted from the 3o annular ring 32. Therefore, when the signal is provided between the cathode 12 and the anode 14, an arc discharge is induced in the plasma source gas flowing into the region 24. The arc forms a current path between the anode 14 and the cathode 12, the plasma is then maintained by a controlled direct current between the anode 14 and the cathode 12. The plasma source gas passing through the throat 20 produces a high momentum plasma flare of ionized plasma source gas.

Swirl Element In most cases, the plasma flare will be unstable and cause anode erosion. It therefore needs to be stabilized by generating a spiral flow or vortex of the plasma source gas between the anode 14 and the cathode 12. This spiral flow or vortex causes the arc to rotate and change its attachment point avoiding anode erosion. One method for creating the vortex or gas swirl is by the use of a swirl element 40 on the surface of the cathode 12.

As illustrated in more detail in Figure 2, the swirl element 40 comprises a plurality of non-linear (for example, part-helical) grooves 44 or vanes 46 that form non-axial flow channels for sub streams of the plasma source gas. The effect of the veins 46 or grooves 44 is to cause discrete sub-streams of the plasma source gas to flow along spiralling trajectories, thereby creating a vortex in the region 24 where the individual sub streams of gas converge. The rotational component of the gas's momentum as it exits via the throat 20 causes the plasma jet to self-stabilize.

The swirl element 40 is formed of an electrically-conductive metal, or alloy, which can survive temperatures greater than 200°C, such as copper, stainless steel or tungsten. In this example, the swirl element 40 is integral and formed from the same material as the cathode 12. However, the swirl element 40 may be a separate element which is tightly engaged to and in electrical contact with the cathode body.

Insulating Liner In order for the torch 10 to function, the cathode 12 and the anode 40 must be 3o electrically isolated from one another. As such, any element interposed between and in contact with both the cathode 12 and the anode 14 must be electrically insulating.

Accordingly, the outer surface of the swirl element 40 is formed to cooperate with the inner surface of an annular ceramic liner 50 which is coaxially interposed between the outer surface of the swirl element 40 and the inner surface of the anode 14 to provide a ceramic electrical break. The downstream end of the ceramic liner 50 has an inwardly chamfered portion which matches the external geometry of the annular ring 32. The ceramic liner 50 has a radially outermost surface 56 that matches that of an annular recess 54 and a radially innermost surface 58 that is a continuation of, and which sits flush with, the tapering surface 18 of the anode 14. The ceramic liner 50 is located for cooperation with the swirl element 40 for forming a stabilizing plasma source gas vortex. The ceramic liner 50 may extend on each axial side of the swirl element 40, or at least on the downstream axial side thereof to ensure that arcing does not occur between the swirl element 40 and the anode 14.

When assembled, the cathode 12 is located within and concentric to the ceramic liner 50, which is located within and concentric to the copper anode 14. Thus, the anode 14 and the cathode 12 are spaced from one another to provide the conduit 16 therebetween. It will be appreciated that rather than forming the spiral veins or grooves in the swirl element 40, these may instead be formed in the ceramic liner 50.

The ceramic liner 50 is manufactured from a dielectric material which functions as an electrical insulator between the cathode 12 and the anode 14 and is also somewhat resistant to chemical attack by highly reactive plasma ions. The ceramic liner 50 is formed from a commercially available, inexpensive and easily machinable ceramic, such as a material formed by high-temperature resin, mica, glass and borosilicate (e.g. MACOR® made by Corning International or DOTEC® or DOTHERM® made by DOTHERM GmbH & Co. KG), boron nitride, silicon nitride or alumina which are all highly resistant to heat 3o and electrically insulating.

Effluent Conduit The cathode 12 is provided with a bore 60 which extends therethrough. In this example, the bore 60 is arranged coaxially and concentrically. The bore 60 is lined with a ceramic liner 62 through which the effluent stream passes. The effluent stream is introduced through an inlet (not shown) and flows in the direction A to B. Additional reagents needed for the abatement reaction (such as H20, Air, 02 H2, etc.) can be mixed with the effluent stream in the bore 60 made by the ceramic liner 62. The ceramic liner 62 has a tapering portion at the downstream end which reduces the thickness of the ceramic liner 62 in the vicinity of the annular ring 32. The ceramic liner 62 may be made of the similar material to the ceramic liner 50. Although the ceramic liner 62 in this embodiment is concentric and coaxially aligned within the cathode 12, it will be appreciated that this need not be the case and also that multiple conduits may also be provided.

In operation, once the plasma vortex has been generated, the effluent stream is conveyed through the ceramic liner 62 and introduced into the region 24. The effluent gas stream is surrounded by the vortex of plasma and abatement occurs for the entire period that the plasma vortex and effluent stream pass from the first inwardly tapering frusto-conical portion 18, through the throat 20 and the outwardly tapering frusto-conical portion 22. This helps to prolong the reaction time, thereby improving abatement. Also, because the effluent stream is introduced within the plasma vortex, the possibility of the effluent stream bypassing the plasma stream is minimized. This significantly improves the effectiveness of the abatement. Also, the provision of the rotating plasma source gas provides for a boundary layer which acts as a barrier to prevent the effluent stream and abatement by-products from contacting the anode 14 or the cathode 12.

As mentioned above, the key to efficient plasma abatement of an effluent stream such as a process gas flow is to ensure rapid and thorough mixing of the effluent stream with the thermal (hot) plasma, whilst preventing corrosion and 3o blockage of the apparatus. Embodiments provide for the effluent gas stream to be mixed with the plasma within the plasma torch, thereby providing fewer opportunities for the effluent stream to bypass the plasma. Embodiments utilize the plasma as a barrier to protect the components of the plasma torch from the reaction by-products. In particular, a cool boundary layer of the plasma source gas stays close to the surface of the components of the plasma torch, thus reducing the extent to which reaction by-products will corrode these.

Embodiments provide a vortex stabilised DC arc plasma torch having a tubular cathode 12 through which the effluent stream is injected. Using vortex stabilization generates a cone of nitrogen plasma which acts as a barrier preventing the effluent stream and effluent stream abatement by-products from contacting the anode 14 and the cathode 12, whilst also capturing the effluent stream flow so that it is forced to pass through the central core of the plasma rotating arc (which is the most hot and reactive part of the plasma). This can dramatically improve CF4 abatement, a compound requiring particularly high temperatures for its destruction.

In embodiments, the cathode 12 is cooled by nitrogen gas instead of water (which is used in existing techniques) which permits a higher running temperature of the cathode 12 which reduces deposition and prevents corrosion due to effluent stream by-products from poly and metal etch processes. Additionally, this stream of nitrogen, preheated by the cathode 12, is then used as the plasma supply, effectively transferring heat lost from the cathode 12 back into the plasma torch. This can typically help to save around 5% of the power loss. Due to the shape and characteristics of the anode 14 and the cathode 12, the running voltage will be higher and this also improves the efficiency by reducing the electrical current. Reducing the electrical current also helps to reduce the erosion rate of both the cathode 12 and the anode 14 since the arc erosion rate is mainly dependent on the arc current.

The thoriated tungsten annular ring 32 provides a substantial surface from which to seed the plasma arc during ignition and is exceptionally heat resistant. The size and shape of the swirl element 40 helps to cool the cathode 12 3o sufficiently for the materials of construction but maintains a temperature of around 200°C to prevent condensation of metal etch process gases and byproducts.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

CLAIMSA plasma torch abatement apparatus for treatment of an effluent stream from a processing tool, comprising: an anode and a cathode arranged to generate, in a plasma discharge region, a plasma stream from a plasma gas; and an effluent stream conduit arranged to convey said effluent stream to flow coaxially with said plasma stream.
The apparatus of claim 1, wherein said anode, said cathode and said effluent stream conduit are arranged to generate said plasma stream with a direction of flow and to convey said effluent stream to flow in said direction of flow with said plasma stream.
3. The apparatus of claim 1 or 2, wherein said effluent stream conduit is arranged to convey said effluent stream to flow coaxially within said plasma stream
4. The apparatus of any preceding claim, wherein said anode, said cathode and said effluent stream conduit are arranged to position said plasma stream as a boundary layer between said effluent stream and at least one of said anode and said cathode.
5. The apparatus of any preceding claim, wherein said anode and said cathode are configured to generate said plasma stream as a cone and said effluent stream conduit is arranged to convey said effluent stream to flow within said plasma stream along an axis of said cone.
6. The apparatus of any preceding claim, wherein said anode and said cathode are configured to generate said plasma stream as a double cone and said effluent stream conduit is arranged to convey said effluent stream to flow within said plasma stream along an axis of said double cone.The apparatus of any preceding claim, wherein said anode and said cathode are configured to generate said plasma stream as a double cone, each nappe of said double cone being coupled by a throat and said effluent stream conduit is arranged to convey said effluent stream to flow within said plasma stream along an axis of said double cone and said throat.8. The apparatus of any preceding claim, wherein said anode and said cathode are configured to impart a rotational component to said plasma gas to rotate said plasma about said direction of flow.9. The apparatus of claim 8, wherein one of said anode and said cathode comprises a swirl structure located in a gap between said anode and said cathode and configured to impart said rotational component to said plasma gas.10. The apparatus of any preceding claim, wherein said cathode comprises a generally cylindrical body received within said anode separated by a gap through which said plasma gas flows.11. The apparatus of claim 10, wherein said conduit comprises a bore within said cathode.12. The apparatus of any preceding claim, wherein said conduit is coaxially aligned with an axis extending along said cathode. 13. 14. 15. 16. 17.The apparatus of any preceding claim, wherein said conduit is configured to impart a rotational component to said effluent stream to rotate said effluent stream about said direction of flow.The apparatus of any preceding claim, wherein said conduit comprises a liner which terminates prior to said plasma discharge region of said cathode.The apparatus of any preceding claim, wherein said cathode comprises a high work function material proximate said plasma discharge region.The apparatus of any preceding claim, wherein said cathode comprises a frusto-conical annular ring proximate said plasma discharge region.A method of treating an effluent stream from a processing tool, comprising: generating, in a plasma discharge region, a plasma stream from a plasma gas using an anode and cathode; and conveying said effluent stream to flow coaxially with said plasma stream.18. A plasma torch abatement apparatus as hereinbefore described with reference to the accompanying drawings.19. A method of treating an effluent stream from a processing tool as hereinbefore described with reference to the accompanying drawings.