GB2428599A

GB2428599A - Apparatus for treating a gas stream

Info

Publication number: GB2428599A
Application number: GB0515643A
Authority: GB
Inventors: Nigel Paul Schofield
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2005-07-29
Filing date: 2005-07-29
Publication date: 2007-02-07
Also published as: GB0515643D0

Abstract

Apparatus for treating a gas stream comprises a backing pump 14 which draws effluent gas from an abatment device 12. The apparatus for treating the gas stream further comprises a nozzle 100 for spraying liquid 102, usually water or aqueous solution, into a conduit 24 upstream from the backing pump 14. The backing pump is a rotary vane, scroll, Roots, Northey (or "claw") or screw pumping mechanism. The gas stream enters the pump 14 and is pulled into the running clearances between the stator and the rotor. Any soluble components of the gas stream, for example HF, become entrained in the liquid. The pump has an exhaust 106 for the liquid/gas mixture. The liquid gas mixture is collected in a fluid reservoir 108 which separates the components. The gas is exhausted via outlet 110 and the solution is conveyed back to nozzle 100 for reuse. The pressure difference between the liquid and the gas stream drives the flow of liquid towards the nozzle. The application is directed at the breakdown of liquid insoluble PFC or HFC gases which are converted into carbon dioxide and HF in the abatement device 12. The abatement device 12 may use incineration, plasma abatement, thermal decomposition or decomposition using gas additions and detail is provided with regard to the construction of a plasma abatement device, namely a plasma torch.

Description

GAS ABATEMENT

The present invention relates to gas abatement. The invention finds particular use in the abatement of gases exhaust from a process tool, for example a process tool used in the semiconductor or flat panel display manufacturing industry.

CF4, C2F6, NF3 and SF6 are commonly used in the semiconductor and flat panel display manufacturing industries, for example, in dielectric layer etching io and chamber cleaning. Following the manufacturing or cleaning process there is typically a residual PFC content in the effluent gas stream pumped from the process tool. PFCs are difficult to remove from the gas stream, and their release into the environment is undesirable because they are known to have relatively high greenhouse activity.

As illustrated in Figure 1, it is known to provide an abatement device for treating such gases exhaust from process tools. In the illustrated example, the abatement device 200 is located downstream from one or more evacuation systems each for evacuating a respective process chamber 202 of a process tool. In this example, each evacuation system comprises a Roots blower 204 or other secondary pump for drawing the gas stream from the process chamber 202, the Roots blower 204 being backed by a multi- stage dry pump 206 that exhausts the gas stream at or around atmospheric pressure to the abatement device 200. A suitable backing pump 206 comprises a combination of Roots and Northey ("claw") type pumping mechanisms.

The object of the abatement is to convert relatively harmful components of the gas stream into compounds that are less harmful to the environment and/or into compounds that can be more conveniently disposed of, for example, by a wet scrubber (not shown) located downstream from the abatement device 200. Conventional abatement devices include incineration, plasma abatement and thermal decomposition tools.

Semiconductor manufacturing processes typically produce as by-products particulates or powders that are drawn from the process chamber 202 by the evacuation system. As the pumping mechanisms of the backing pump 206 require close tolerances to be maintained between the rotor and stator components of the pumping stages during use, it is normal practice to inject an inert purge gas, such as nitrogen, into the pumping mechanisms. This purge gas serves to reduce the level of by-product contamination of the backing pump 206. However, in view of the relatively high flow rates of purge gas into each backing pump 206 (typically around 40 to 50 slm) in comparison to the flow rate of the gas stream exhaust from each process tool 202 (typically around 5 slm), the injection of purge gas into one or more of the backing pumps 206 significantly increases the power requirement of the abatement device 200, as abatement of an gas stream containing 70 to 90% purge gas requires significantly more power than the abatement of an gas stream containing no purge gas.

It is an aim of at least the preferred embodiment of the present invention to seek to provide a relatively simple, efticient and low cost technique for treating a gas stream.

In a first aspect, the present invention provides apparatus for treating a gas stream containing a liquid-soluble component, the apparatus comprising a pump and means for spraying a liquid into the gas stream upstream from the pump, the pump comprising an inlet for receiving the liquid-containing gas stream and an outlet for exhausting a liquid containing the liquid-soluble component of the gas stream.

As used herein, the term "liquid-insoluble component" means a component of the gas stream which is not soluble within the liquid sprayed into the gas stream, this liquid typically being water, and the term "liquid- soluble component" means a component of the gas stream which is soluble within that liquid, such as HF. Examples of such liquid-insoluble components are pertluorinated or hydrofluorocarbon compounds, such as CF4, C2F6, CHF3, CF8, and C4F8, which can be converted into CO2 and HF in a suitable abatement device upstream of the pump. The HF can be taken into solution in the pump. Other examples of liquid insoluble components include NF3, which can be converted into N2 and HF, and SF6, which can be converted into SO2 and HF.

As the gas stream is caused to come into contact with the liquid, any liquid- soluble components of the gas stream are washed into the liquid by means of the motion of the pumping mechanism within the pump, and thus removed from the gas stream before the gas stream is exhaust, at or around atmospheric pressure, from the pump with a solution of the liquid and the liquid-soluble components of the gas stream. A pump outlet allows for the discharge of the solution and any gas from the pump. By spraying the liquid into the gas stream upstream from the pump inlet, significant mixing of the liquid and the gas stream can be achieved, thereby enhancing the amount of the liquid-soluble component that is taken into solution with the pump.

The pump thus operates as both a wet scrubber and an atmospheric vacuum pumping stage for the gas stream. Where the pump is located downstream from the abatement device, a conventional wet scrubber is no longer required, thereby reducing costs. Furthermore, unlike a Roots or Northeytype pumping mechanism, any particulate or powder by-products exhaust from the tool do not have a detrimental effect on the pumping mechanism within the pump, and so there is no longer any requirement to provide any purge gas to the atmospheric pumping stage.

In one embodiment, the pump comprises a positive displacement pump.

Such pumps may comprise at least one of a rotary vane, scroll, Roots, Northey (or "claw") or screw pumping mechanism. In addition to removing the liquid-soluble components of the gas stream, the liquid can serve to the close the running clearances within the pump and to regulate the temperature within the pump.

Relatively cool running temperatures within the pump due to the liquid flow through the pump would allow significant use of polymeric material in the pump construction, and so at least part of the pump may be formed from plastics material. This can be particularly useful as one of the liquid-soluble io components of the gas stream is likely to be HF, which can result in hydrofluoric acid passing through the pump. This acid is particularly corrosive to standard engineering materials, such as iron and aluminium, and so forming the pump from plastics material may significantly increase the life of the pump.

Means may be provided for receiving the liquid and any gas exhaust from the pump with the liquid, and for separating the gas from the liquid. The separating means may comprise means for collecting the liquid exhaust from the pump. The collected liquid may be continuously or periodically drained from the collection device, for example for subsequent treatment.

Alternatively, or additionally, the gas spraying means may comprise means for returning the collected liquid to a nozzle for spraying the liquid into the gas stream. Depending on the amount of liquid-soluble component taken into solution within the pump, the liquid may be returned to the gas stream a number of times before it becomes saturated. The liquid returning means may comprise a conduit system extending between the collecting means and the nozzle, with atmospheric pressure serving to drive the flow of liquid to the nozzle. The liquid collected by the collecting means may be periodically replenished. Where the collected liquid is not returned to the nozzle, the liquid may be conveyed to the nozzle from a separate source.

The apparatus preferably comprises an abatement device for converting a component of the gas stream into a liquid-soluble component at a subatmospheric pressure, the pump being arranged to at least partially evacuate the abatement device. Therefore, in a second aspect the present invention provides apparatus for treating an effluent gas stream from a process tool, the apparatus comprising an abatement device for converting a component of the gas stream into a liquid-soluble component at a subatmospheric pressure, a pump for at least partially evacuating the abatement device, and means for spraying a liquid into the gas stream between the abatement device and the pump, the pump comprising an inlet for receiving the liquid-containing gas stream and an outlet for exhausting a liquid containing the liquid-soluble component of the gas stream.

The abatement device is preferably configured to convert a component of the gas stream into a different compound. For example, the abatement device may be configured to convert one or more components of the gas stream, such as SIH4 and/or NH3 into one or more compounds that are less reactive than said component with another component of the gas stream, such as F2.

Such gases may be present where the abatement device is configured to treat the gas streams exhaust from different process tools, or where different process gases are supplied to a process tool at different times. Pre-treating the SiH4 and NH3 gases can inhibit the formation of reactive gas mixtures within the gas stream. For example, the pre- treatment of SiH4 can form Si02.

In the preferred embodiment, the pump is located downstream of the abatement device such that, during use, the gas stream passes through the abatement device at a sub-atmospheric pressure. With this configuration, the abatement device can convert a component of the gas stream into a compound that is less reactive than said component with the liquid of the pump. For example, whilst F2 is soluble within water, it may react with water to form insoluble compounds, such as OF2. Conversion of F2 into HF can inhibit the formation of such compounds. Thus, with this configuration the abatement device can convert one or more components of the gas stream into components that are soluble within the liquid of the pump.

As mentioned above, the component of the gas stream may be initially liquid- soluble, or it may be liquid-insoluble. Examples of liquid-insoluble compounds are CF4, C2F6, CHF3, C3F8, C4F8, NF3 and SF6.

Any one of a range of equipment may be used to decompose the components of the gas stream. For example, a burner or such like may be provided to io thermally decompose these components. One suitable example is described in our European patent application no. 1,205,707, the contents of which are incorporated herein by reference. Alternatively, a plasma generator may be used to decompose these components. In one known plasma abatement technique, the gas stream is conveyed into a resonant cavity using microwave radiation to generate, from components such as PFCs, a microwave plasma.

Another known technique is to convey the gas stream into a dielectric tube, a high frequency surface-wave exciter being used to produce surface waves which generate a plasma within the tube to dissociate the PFCs. The plasma may be generated using radiation at a frequency of around 580 kHz, 13.56 MHz, 27 MHz, 915 MHz or 2.45 GHz. Alternatively, a glow discharge may be generated to decompose these components. As is well known, a glow discharge is a luminous, thermal plasma formed by applying to a gas a voltage that is greater than the breakdown voltage of that gas. The components may be decomposed by a discharge other than a glow discharge, for example by a corona discharge or an arc discharge. Such a discharge may be generated using a plasma gun, in which an electric arc is created between a water-cooled nozzle (anode) and a centrally located cathode. A stream of fluid passes through the electric arc and is dissociated thereby. The plasma of ionised fluid issuing from the nozzle resembles an open oxy-acetylene flame.

In an alternative abatement technique, the effluent waste stream is brought into contact with a stream containing a reactant for reacting with the components within the waste stream. For example, where these components are PFCs, a superheated stream of water vapour may be used to convert the PFCs into components, such as HF, which can be taken into solution in the pump. By providing a method in which reactive species are formed from a reactive fluid for subsequent reaction with such components of the gas stream, it has been found that the energy required to cause the destruction of the component in the gas stream, and the efficiency of that destruction, can be radically improved. For example, W and 0H ions formed from the dissociation of water are capable of reacting with, for example, a PFC contained in the gas stream at ambient temperature, and thus at a much lower temperature than would be required if the water had not been pre- ionised before being introduced into the gas stream. Further advantages are that a relatively cheap and readily available fluid, such as water vapour or a fuel, for example methane or an alcohol, can be used to generate W and/or 0H ions, as the reactive species, and that the reaction can take place at sub- atmospheric or atmospheric pressure.

Various techniques may be used to form the ions using a plasma gun. In a first technique, a plasma stream is formed and, prior to the injection of the plasma stream into the chamber, water (as an example of a suitable source of these ions) is conveyed to the stream so that a flame containing these ions is injected into the chamber to abate the gas stream within. The water may be conveyed to the plasma stream separately from the source gas, or within a fluid mixture comprising both water vapour and the source gas. In a second technique, both water and the gas stream are separately conveyed into the chamber. The water is dissociated by the flame to form heated ions within the chamber, which ions subsequently react with a PFC component of the waste stream. In a third technique, the gas stream is conveyed to the plasma stream prior to its injection into the reaction chamber, so that both the plasma stream and the gas stream, which may comprise the PFC and/or radicals generated from the PFC, are injected into the reaction chamber. Water may be conveyed to the plasma stream upstream from the aperture, that is, with one of the source gas or the gas stream, or separately therefrom, or may be conveyed to the plasma stream downstream from the nozzle, for example, directly to the reaction chamber. In this case, the water may impinge upon the plasma stream to form heated ions within the chamber for reacting with the PFC and/or the PFC radicals, and/or may react directly with the PFC radicals within the chamber for abatement thereof.

In the preferred embodiment, a single plasma gun is used to inject the plasma stream into the reaction chamber. However, a plurality of such guns may be provided to inject a plurality of plasma streams into the same chamber, each for abating a common or respective gas stream. Alternatively, a plurality of gas streams may be conveyed to a single chamber, into which a single plasma stream is injected. This can increase further the efficiency of the treatment of the waste stream. These guns may be connected to a common power source or to respective sources.

In a further aspect, the present invention provides a system for evacuating a process tool, the system comprising a vacuum pump for drawing an effluent gas stream from the tool and apparatus as aforementioned for receiving and treating the gas stream exhaust from the vacuum pump. Such a pump may comprise any convenient pump for exhausting the gas stream at a pressure in the range from 10 to 200 mbar. For example, the vacuum pump may comprise a turbomolecular pump, a molecular drag pump, or a multi-stage dry pump. Such a pump preferably comprises a plurality of Roots-type pumping stages, as such pumping mechanisms have larger tolerances than Northey- type mechanisms and so are less prone to seizure due to the accumulation of solid by-products within the running clearances of the pumping mechanism.

In a further aspect, the present invention provides a method of treating a gas stream containing a liquid-soluble component, the method comprising the -9- - - steps of spraying a liquid into the gas stream, conveying the liquid- containing gas stream to a pump, and exhausting from the pump a solution comprising the liquid and the liquid-soluble component of the gas stream.

In a yet further aspect the present invention provides a method of treating an effluent gas stream from a process tool, the method comprising the steps of conveying the gas stream to an abatement device for converting a component of the gas stream into a liquid-soluble component at a sub-atmospheric pressure, conveying the gas stream from the abatement device to a pump for at least partially evacuating the abatement device, spraying a liquid into the gas stream between the abatement device and the pump, and exhausting from the pump the liquid containing the liquid-soluble component of the gas stream.

Features described above in relation to apparatus aspects of the invention are equally applicable to method aspects, and vice versa.

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a system for evacuating a plurality of process chambers; Figure 2 illustrates a first embodiment of an apparatus for treating a gas stream; Figure 3 illustrates the fluid supply to an example of a plasma abatement device of the apparatus of Figure 2; Figure 4 illustrates in more detail the plasma abatement device of Figure 3; Figure 5 illustrates one embodiment of a plasma torch suitable for use in the device of Figure 4; Figure 6 illustrates the use of the torch of Figure 5 with a plurality of gas streams entering the abatement device; Figure 7 illustrates of a second embodiment of a plasma torch suitable for use in the device of Figure 4; Figure 8 illustrates schematically the spraying of liquid into the gas stream upstream from the pump; and I0 Figure 9 illustrates another example of a plasma abatement device suitable for use in the apparatus of Figure 2.

With reference to Figure 2, a first embodiment of an apparatus for treating a gas stream exhaust from one or more process chambers 10 of respective process tools comprises an abatement device 12 and a backing pump 14.

The abatement device 12 is located downstream from one or more high capacity secondary pumps 16 (three shown in Figure 2, although any suitable number may be provided). In the illustrated embodiment, each secondary pump 16 comprises a multi-stage dry pump, wherein each pumping stage is provided by a Roots-type pumping mechanism. Alternatively, one or more of the secondary pumps 16 may comprise a turbomolecular pump and/or a molecular drag mechanism, depending on the pumping requirements of the respective process chamber(s) 10.

The secondary pumps 16 draw gas streams from the process chambers 10 and exhaust the pumped gas streams at a sub-atmospheric pressure, typically in the range from 50 to 200mbar, and at a rate of around 5 slm, to the abatement device 12. The abatement device 12 receives the pumped gas streams and converts components of the gas streams, such as SiH4 and NH3, into substances which are less reactive with other components of the received gas streams, such as F2, and converts such components and others such as PFCs and F2 into substances that can be readily removed by liquid conveyed to the backing pump 14, as discussed in more detail below.

The abatement device 12 may utilise any technique which is suitable for the abatement of a sub-atmospheric gas stream, such as incineration, plasma abatement, thermal decomposition, decomposition using gas additions, or gas streams containing ions selected to cause the effluent gas to react and form the aforementioned substances. Examples of such an abatement device 12 will now be described with reference to Figures 3 to 7.

Figure 3 illustrates the gas supplies to the abatement device 12. The gas stream is conveyed to a first inlet 18 of the abatement device 12 by conduit 20, and is conveyed from the outlet 22 of the abatement device 12 by conduit 24. A source of 0H and/or W ions, in this example water, is supplied from a source 26 thereof to a second inlet 28 of the abatement device 12 by conduit 30, and an ionisable, plasma source gas, in this example nitrogen, is supplied from a source 32 thereof to a third inlet 34 of the abatement device by conduit 36.

With reference to Figure 4 the abatement device 12 comprises a reaction chamber 40 in which are formed the first inlet 18 for receiving the gas stream, the second inlet 28 for receiving the ion source, and the outlet 22 for exhausting from the chamber 40 a fluid stream containing byproducts from the abatement process and other, unabated gases contained within the gas stream entering the abatement device 12. The abatement device 12 further comprises a dc plasma torch 42 for receiving the nitrogen stream from the conduit 36 and generating a plasma stream that is injected into the chamber in the form of a flame emitted from an aperture or nozzle 44 of the plasma torch 42. As shown in Figure 4, the plasma torch 42 also receives a flow of water coolant that enters and leaves the torch via a conduit system indicated generally at 46 in Figure 4.

Figure 5 shows in more detail the configuration of one embodiment of the plasma torch 42. The plasma torch 42 comprises an elongate tubular cathode, or electron emitter, 48 having an end wall 50. Water coolant is conveyed through the bore 52 of the electron emitter 48 during use of the plasma torch 42. The bore 52 of the electron emitter 48 is aligned with the nozzle 44 formed in a start anode, or electrode 54 surrounding the end wall of the electron emitter 48. The start electrode 54 is mounted in an insulating block 56 surrounding the electron emitter 48. A bore formed in the insulating block 56 provides the third inlet 34 of the abatement device, and conveys a stream of plasma source gas into a cavity 58 located between the end wall 50 of the electron emitter 48 and the start electrode 54.

In operation of the plasma torch 42, a pilot arc is first generated between the electron emitter 48 and the start electrode 54. The arc is generated by a high frequency, high voltage signal typically provided by a generator associated with the power supply for the torch. This signal induces a spark discharge in the plasma source gas flowing in the cavity 58, and this discharge provides a current path. The pilot arc thus formed between the electrode emitter 48 and the start electrode 54 ionises the plasma source gas passing through the nozzle 44 to produce a high momentum plasma flame of ionised source gas from the tip of the nozzle 44. The flame passes from the nozzle towards a secondary anode 60 surrounding the nozzle 44 to define a plasma region 62.

The secondary anode 60 may be provided by part of the wall of the chamber 40, or may be a separate member inserted into the chamber 40, in which case the secondary anode 60 may be provided with apertures 64, 66 which align with the inlets 18, 28 of the chamber 40 to enable the ion source and the gas stream to be conveyed to the plasma region 62. The lower (as illustrated) portion of the secondary anode 60 may be profiled as shown in Figure 5 to enable the secondary anode to be used instead of the start electrode 54 to generate the plasma stream from the plasma source gas.

In use, the ion source, in this example water, is dissociated by the plasma flame emitted from the nozzle 44 of the torch plasma 42 to form H and 0H ions within the plasma region 62. These ions subsequently react within the chamber 40 with the PFC component(s) of the gas stream entering the chamber 40. The by-products from the reaction, and any unabated noble gases contained within the gas stream entering the chamber 40, are exhaust from the chamber 40 through outlet 22, and subsequently conveyed to the backing pump 14.

Some examples of reactions occurring within the chamber 40 will now be described.

Example 1

The reactive fluid is a source of W and 0H ions, for example, water vapour, and the gas stream contains a perfluorocompound, for example, CF4. The plasma flame dissociates the water vapour into W and 0H ions: H20 -H+ OH (1) which react with CF4 to form carbon dioxide and HF as by-products: CF4 + 20ft +2W -> CO2 + 4HF (2) A typical gas mixture for performing a dielectric etch in a process tool may contain differing proportions of the gases CHF3, C3F8, C4F8 or other perfluorinated or hydrofluorocarbon gas, but the chemical reactions of the W and 0H ions with these components of the gas stream will differ in detail but the general form will be as the scheme above.

Example 2

The reactive fluid is again a source of W and 0H ions, for example, water vapour, and the gas stream contains NF3. Process tool manufacturers are increasingly adopting NF3 as the chamber cleaning gas of choice for PECVD reactors. Whereas the utilisation of NF3 by the cleaning process is much higher than that of either CF4 or C2F6, the by-products produced are considerably more reactive and their uncontrolled release is potentially very dangerous. Within the plasma, NF3 dissociates to form N2, F2 and N2F4: 4NF3 -+ N+ 4F2 + N2F4 (3) II) with the N2F4 component of the gas stream subsequently reacting with the W and 0H ions generated from the impingement of the water vapour on the plasma flare: N2F4 + 2W + 20H -> N + 4HF + 02 (4) As illustrated by the above examples, the same ions may be used to remove various different components from a gas stream. Consequently, the abatement device is suitable to receive a plurality of gas streams, either from similar or different process tools, and convert similar or different components of those gas streams into liquid-soluble species. For example, as illustrated in Figure 6, the abatement device may be provided with an additional inlet for receiving an additional gas stream via conduit 20a, with an additional aperture 64a being provided in the secondary anode 60 to enable the additional gas stream to be conveyed to the plasma region 62.

In Example 1 above, the ions react with the CF4 component of the gas stream entering the chamber 40, and so it is not essential for the gas stream to pass through the plasma flare to decompose the CF4 prior to reaction with the ions.

In contrast, in Example 2 above, it is desirable to convey the gas stream through the plasma stream in order to dissociate the NF3 into species that are more reactive with the ions generated by the ion source. In the examples illustrated in Figures 4 to 6, the gas stream may be conveyed into the chamber 40 proximate the plasma region 62 so that the PFC passes through the plasma region. Figure 7 illustrates an example of a plasma torch 80 in which the contact of the gas stream with the plasma flare is maximised. In this example, the gas stream is conveyed directly to the plasma torch 80, rather than into the reaction chamber 40. As shown in Figure 7, the gas stream is conveyed from the first inlet 18 of the abatement device into the bore 52 of the electron emitter 48. The gas stream passes from the open end 82 of the electron emitter 48 into the cavity 58 between the electron emitter 48 and the start electrode 54 of the plasma torch 80. The cavity 58 also receives a stream of plasma source gas entering the abatement device through the third inlet 34 formed in the electrically insulting block 56 surrounding both the electron emitter 48 and the start electrode 54.

In use, similar to the example illustrated in Figure 5, a pilot arc is first generated between the electron emitter 48 and the start electrode 54 by supplying a high frequency, high voltage signal to a hafnium insert 84. The pilot arc thus formed between the electrode emitter 48 and the start electrode 54 ionises the plasma source gas entering the cavity 58 from the third inlet 34 to produce a high momentum plasma flame of ionised source gas from the tip of the nozzle 44. As the gas stream enters the cavity 58 from the open end 82 of the electron emitter 48, it mixes with the plasma source gas within the cavity 58 and is emitted from the nozzle 44 with the plasma stream into the plasma region 62. Water is supplied to the plasmaregion 62 from the second inlet 28, which in this example is also formed in the insulating block 56 of the torch 42. The water is decomposed by the plasma stream to form W and 0H ions, which react with the PFC, and/or with species formed from the dissociation of the PFC by the plasma stream, within the reaction chamber.

As mentioned above, the backing pump 14 draws the effluent gas from the abatement device 12. As illustrated in Figure 8, the apparatus for treating the gas stream further comprises a nozzle 100 for spraying a liquid 102, which is usually water or other aqueous solution, into the conduit 24 upstream from the backing pump 14. The backing pump 14 thus receives a liquid-containing gas stream. The backing pump 14 is preferably a positive displacement pump, and may comprise at least one of a rotary vane, scroll, Roots, Northey (or "claw") or screw pumping mechanism. The gas stream enters the pump 14 through gas inlet 104, and is pulled into the running clearances within the pump 14, for example, between rotor and stator elements of the pumping mechanism. Any liquid-soluble components of the gas stream, such as HF, become entrained within the liquid entering the pump 14 simultaneously with the gas stream. The pump 14 is provided with an exhaust 106 for exhausting from the pump 14 a liquid/gas mixture of a liquid solution comprising the liquid and the liquid-soluble components of the gas stream, and any gaseous species remaining from the gas stream.

Is The liquid/gas mixture exhaust from the backing pump is collected in a fluid reservoir 108, which serves to separate the liquid and gas components of the mixture exhaust from the pump. The gas is exhausted from the reservoir 108 through outlet 110 into the atmosphere, whilst the collected solution can be conveyed back to the nozzle 100 via a conduit 112 extending between the nozzle 100 and the reservoir 108 for re-use. The pressure difference between the collected liquid and the gas stream upstream from the backing pump 14 drives the flow of liquid towards the nozzle 100. A controllable valve may be provided within the conduit 112 for controlling the flow rate of liquid to the nozzle. Alternatively, the liquid can be periodically or continuously drained from the reservoir 108, with the liquid being conveyed to the nozzle 100 from a separate source.

Consequently, the backing pump 14 operates as both a wet scrubber and an atmospheric vacuum pumping stage for the gas stream. In addition to removing the liquid-soluble components of the gas stream, the liquid can serve to the close the running clearances within the pump and to regulate the temperature within the pump. Furthermore, the relatively cool running temperatures within the pump due to the liquid flow through the pump would allow significant use of polymeric material in the pump construction, and so the pump may be formed at least in part from plastics material.

As mentioned above, the abatement device 12 may utilise any technique that is suitable for the abatement of a sub-atmospheric gas stream, such as incineration, plasma abatement, thermal decomposition, decomposition using gas additions. A suitable combustion device is described in our European patent application no. 1,205,707, the contents of which are incorporated herein by reference. An alternative plasma abatement device 12 is illustrated in Figure 9. The device consists of a microwave resonant cavity 140. Within cavity 140 there is a dielectric insert 142 formed from, for example, PTFE, which is transparent to microwaves. The insert 142 has a circular internal section such that gases entering cavity 140 through the gas inlet 144 do so with a tangential component of velocity. The insert 142 may also act as a seal making the cavity 140 gas tight. Cavity 140 is coupled to a wave-guide (not shown) that is so dimensioned as to transmit microwave energy when connected to a 2.45 GHz microwave generator. Mounted within cavity 140 is a pair of.opposed field- enhancing electrodes 146, 148, which serve.to confine the plasma generated in cavity 140. The electrode 146 has a channel by means of which cooling water can be passed around the electrode 146. In use, the gas stream is water saturated prior to entering the abatement device.

The water-saturated gas stream exhaust enters the cavity 140 through the inlet 144 and spirals around electrodes 146,148 before entering the gap 150 between electrodes 146,148, wherein it is energised by the electric field created within the gap 150 by the microwaves entering the cavity 140 through the dielectric insert 142. By energising the gas stream within the gap 150, a plasma is formed, wherein reaction (2) above occurs for an gas stream containing CF4. The by-products from the reaction, namely CO2 and HF, leave the cavity 140 through the axial passageway 152 in electrode 148 prior to being received by the pump 14.

Claims

1. Apparatus for treating a gas stream comprising a liquid-soluble component, the apparatus comprising a pump and means for spraying a liquid into the gas stream upstream from the pump, the pump comprising an inlet for receiving the liquid-containing gas stream and an outlet for exhausting a liquid containing the liquid-soluble component of the gas stream.

2. Apparatus according to Claim 1, wherein the pump comprises a positive displacement pump.

3. Apparatus according to Claim 1 or Claim 2, wherein the pump comprises at least one of a rotary vane, scroll, Roots, Northey or screw pumping mechanism.

4. Apparatus according to any preceding claim, wherein the pump is formed at least in part from plastics material.

5. Apparatus according to any preceding claim, comprising means for receiving the liquid and any gas exhaust from the pump with the liquid, and for separating the gas from the liquid.

6. Apparatus according to Claim 5, wherein the separating means comprises means for collecting the liquid exhaust from the pump.

7. Apparatus according to Claim 6, wherein the gas spraying means comprises means for returning the collected liquid to a nozzle for spraying the liquid into the gas stream.

8. Apparatus according to Claim 7, wherein the liquid returning means comprises a conduit system extending between the collecting means and the nozzle.

9. Apparatus according to any of Claims 6 to 8, wherein the collecting means comprises a gas outlet for exhausting gas therefrom.

10. Apparatus according to any preceding claim, comprising an abatement device for converting a component of the gas stream into a liquid- soluble component at a sub-atmospheric pressure, the pump being arranged to at least partially evacuate the abatement device.

11. Apparatus for treating an effluent gas stream from a process tool, the apparatus comprising an abatement device for converting a component of the gas stream into a liquid-soluble component at a sub-atmospheric pressure, a pump for at least partially evacuating the abatement device, and means for spraying a liquid into the gas stream between the abatement device and the pump, the pump comprising an inlet for receiving the liquid-containing gas stream and an outlet for. exhausting a liquid containing the liquid-soluble component of the gas stream.

12. Apparatus according to Claim 10 or Claim 11, wherein the abatement device is configured to convert a component of the gas stream into a liquid-soluble component that is less reactive than said component with another component of the gas stream.

13. Apparatus according to any of Claims 10 to 12, wherein the abatement device is configured to convert a component of the gas stream into a liquid-soluble component that is less reactive than said component with the liquid of the pump.

- 20 -

14. Apparatus according to any of Claims 10 to 13, wherein the abatement device comprises means for decomposing a component of the gas stream.

15. Apparatus according to Claim 14, wherein the abatement device comprises a plasma abatement device for generating a plasma to decompose the component of the gas stream.

16. Apparatus according to any of Claims 10 to 15, wherein the abatement device comprises means for receiving a reactive fluid for reacting with a component of the gas stream.

17. Apparatus according to Claim 16, wherein the abatement device comprises means for generating an ionised fluid stream for impinging upon the reactive fluid to form reactive species for reacting with the component of the gas stream.

18. Apparatus according to any of Claims 10 to 17, wherein the abatement device comprises a reaction chamber and means for injecting into the reaction chamber an ionised fluid stream containing reactive species for reacting with a component of the gas stream.

19. Apparatus according to Claim 18, wherein the abatement device comprises means for thermally decomposing a reactive fluid to form the reactive species.

20. Apparatus according to any of Claims 10 to 19, wherein the abatement device comprises means for thermally decomposing a component of the gas stream. -21 -

21. Apparatus according to any of Claims 10 to 20, wherein the abatement device is configured to convert a liquid-insoluble component of the gas stream into a liquid-soluble component.

22. Apparatus according to Claim 21, wherein the abatement device is configured to convert a perfluorinated or hydrofluorocarbon compound into a substance that is soluble within the liquid of the pump.

23. Apparatus according to Claim 22, wherein the compound comprises one of CF4, C2F6, CHF3, C3F8, C4F8, NF3 and SF6.

24. Apparatus according to any preceding claim, wherein the liquid comprises water.

25. A system for evacuating a process chamber, the system comprising a vacuum pump for drawing an effluent fluid stream from the chamber and apparatus according to any preceding claim for receiving and treating the gas stream exhaust from the pump.

26. A system according to Claim 25, wherein the vacuum pump is configured to exhaust the gas stream at a pressure in the range from to 200 mbar.

27. A system according to Claim 25 or Claim 26, wherein the vacuum pump comprises a multi-stage dry pump.

28. A method of treating a gas stream comprising a liquid-soluble component, the method comprising the steps of spraying a liquid into the gas stream, conveying the liquid-containing gas stream to a pump, and exhausting from the pump a solution comprising the liquid and the liquidsoluble component of the gas stream.

29. A method of treating an effluent gas stream from a process tool, the method comprising the steps of conveying the gas stream to an abatement device for converting a component of the gas stream into a liquid-soluble component at a sub-atmospheric pressure, conveying the gas stream from the abatement device to a pump for at least partially evacuating the abatement device, spraying a liquid into the gas stream between the abatement device and the pump, and exhausting from the pump the liquid containing the liquid-soluble component of the gas stream.

30. A method according to Claim 29, wherein the liquid exhausted from the pump is returned to a nozzle for spraying the liquid into the gas stream.