Patent Publication Number: US-2016230989-A1

Title: Process gas abatement

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process gas abatement apparatus and method. 
     BACKGROUND OF THE INVENTION 
     Apparatus for treating an effluent gas stream from a manufacturing process tool operating at a sub-atmospheric pressure used in, for example, the semiconductor or flat panel display manufacturing industry are known. During such manufacturing, residual perfluorinated compounds (PFCs) and other compounds exist in the effluent gas stream pumped from the process tool. PFCs are difficult to abate or remove from the effluent gas and their release into the environment is undesirable because they are known to have relatively high greenhouse activity. 
     One way of performing effluent gas abatement is to pump the effluent gas from the process tool to a higher sub-atmospheric pressure before being fed to a radiant burner. The radiant burner uses combustion to remove the PFCs and other compounds from the process gas stream. Typically, the effluent gas stream is a nitrogen stream containing PFCs and other compounds. A fuel gas is mixed with the effluent gas stream and that gas stream mixture is conveyed into a combustion chamber that is laterally surrounded by the exit surface of a foraminous gas burner. Fuel gas and air are simultaneously supplied to the foraminous burner to affect flameless combustion at the exit surface, with the amount of air passing through the foraminous burner being sufficient to consume not only the fuel gas supplied to the burner, but also ail the combustibles in the gas stream mixture injected into the combustion chamber. The resultant treated gas stream is exhausted from the radiant burner. Thereafter, the treated gas stream is pumped to atmospheric pressure before being vented. 
     Although techniques exist for processing the effluent gas stream, they each have their own shortcomings. Accordingly, it is desired to provide an improved technique for processing an effluent gas stream. 
     SUMMARY 
     According to a first aspect, there is provided a process gas abatement apparatus, comprising: a burner comprising: a combustion chamber operable to receive an effluent gas stream from a manufacturing process tool to be treated within the combustion chamber at a sub-atmospheric pressure, the combustion chamber being further operable to receive a fuel, oxidant and diluent, the fuel, oxidant and diluent controlling combustion within the combustion chamber to treat the effluent gas stream to produce a treated exhaust stream, the diluent being condensable in the treated exhaust stream. 
     The first aspect recognises that with existing approaches, as mentioned above, the burner will be operated at a pressure between that of the process tool, but below atmospheric pressure. For example, the burner is typically operated at approximately 200 mbar, with process gases being brought up to this pressure by means of a multi-stage dry pumping mechanism, with the combustion by-products being brought up to a second pressure, for example atmospheric pressure by means of second pump such as, for example, a liquid ring pump. 
     Typically, a hydrocarbon fuel provides the energy source for the combustive abatement within the combustion chamber and often this fuel is methane. This burns with the process gas ‘P’ to produce a treated process gas P′ according to reaction (1) below: 
       10P+CH 4 +2O 2 =CO 2 +2H 2 O+10P′  (1)
 
     If it is assumed that atmospheric pressure combustion properties also occur for sub-atmospheric combustion, then each standard litre per minute (slm) of methane can abate around 10 standard litres per minute of process exhaust. So with CH 4  and pure oxygen, the volumetric gain between the volume of gas being input to the combustion chamber and the volume of gas being output by the combustion chamber is only 10% (i.e. 10 slm of process exhaust is input into the combustion chamber and 11 slm is output from the combustion chamber). 
     However, ordinarily, the O 2  would be delivered as 20.9% by volume in air and hence would be accompanied by a substantial volume of N 2 . Using air is both a convenient source of O 2  and also is helpful within the combustion chamber as the N 2  helps to moderate the flame speed and temperature within the combustion chamber. 
     With air, burning as equation (2) below: 
       10P+CH 4 +2O 2 +8N 2 =CO 2 +8N 2 +2H 2 O+10P′  (2)
 
     However, the first aspect recognises that the volumetric gain between the volume of gas being input to the combustion chamber and the volume of gas being output by the combustion chamber almost doubles (i.e. 10 slm of process gas input into the combustion chamber and 19 slm output from the combustion chamber). 
     Accordingly a process gas abatement apparatus may be provided. The apparatus may comprise a burner. The burner may comprise a combustion chamber which receives a process or effluent gas stream from a manufacturing process tool. The effluent gas stream may be treated within the combustion chamber at a sub-atmospheric pressure. The combustion chamber may receive a fuel, oxidant and diluent. The fuel, oxidant and diluent may control combustion within the combustion chamber to treat the effluent gas stream and produce a treated exhaust stream. The diluent may be condensable in the treated exhaust stream. 
     The first aspect recognises that since the purpose of the N 2  provided in existing approaches is to moderate the flame speed and temperature within the combustion chamber, it is only by convenience that N 2  is used as it is ordinarily present in air. If this N 2  could be replaced with a diluent in the form of, for example, an inert condensable, the volume gain within the combustion chamber will be reduced, which reduces the volume of the exhaust stream and reduces the volumetric load on the second pump. The volume gain reduces because the diluent shifts phase in the exhaust stream, thereby effectively removing the contribution of the diluent to the volume of the exhaust stream. This leads to considerable power saving since a lower volume of gas is output from the combustion chamber which would need to be brought up to the second pressure, for example atmospheric pressure, by means of the second pump. 
     In one embodiment, the diluent, when introduced to the combustion chamber, comprises a vapour. Accordingly, the diluent may be mixed in vapour form with the fuel and oxidant to effect combustion with the required characteristics in order to treat the effluent gas stream. The transition from, for example, an inert condensable vapour to a liquid within the exhaust stream enables the diluent to both contribute to the characteristics of the combustion whilst also reducing the volume gain because the diluent shifts phase in the exhaust stream, thereby effectively removing the contribution of the diluent to the volume of the exhaust stream. 
     In one embodiment, the diluent comprises a liquid prior to being vaporised for introduction to the combustion chamber. It will be appreciated that this significantly simplifies storage of the diluent. 
     In one embodiment, the diluent condenses to a liquid in the treated exhaust stream. It will be appreciated that the phase from a vapour to a liquid causes a significant reduction in volume. 
     In one embodiment, the diluent is introduced into the combustion chamber with a first volumetric rate and occupies the treated exhaust stream with a second volumetric rate, the second volumetric rate being lower than the first volumetric rate. 
     In one embodiment, the diluent is provided at specified volumetric rate to control combustion conditions within the combustion chamber to treat the effluent gas stream. 
     In one embodiment, the diluent is combined with at least one of the fuel and oxidant prior to being introduced into the combustion chamber. It will be appreciated that this significantly simplifies storage of the diluent and/or the fuel and oxidant. 
     In one embodiment, at least one of the fuel and the oxidant is dissolved by the diluent prior to being introduced into the combustion chamber. 
     In one embodiment, both the fuel and the oxidant are dissolved by the diluent prior to being introduced into the combustion chamber. 
     In one embodiment, at least one of the fuel and the oxidant dissolved by the diluent is vaporised prior to being introduced into the combustion chamber. 
     In one embodiment, at least one of the fuel and the oxidant dissolved by the diluent and the diluent are co-vaporised prior to being introduced into the combustion chamber. 
     In one embodiment, the diluent comprises at least one of water, a perfluorocarbon and a hydrocarbon. 
     In one embodiment, the burner comprises a radiant burner and the combustion chamber has a porous sleeve through which the fuel, oxidant and diluent pass for combustion proximate to a combustion surface of the porous sleeve. 
     In one embodiment, the treated exhaust stream is provided to a liquid ring pump for compression to atmospheric pressure. 
     In one embodiment, the diluent condenses in the liquid ring pump. Hence, the liquid ring pump may also act as an efficient condenser. 
     In one embodiment, the liquid ring pump is operable to scrub the treated exhaust stream. Hence, the liquid ring pump may also act as an efficient scrubber. 
     According to a second aspect, there is provided a process gas abatement method, comprising: receiving an effluent gas stream to be treated from a manufacturing process tool within a combustion chamber at a sub-atmospheric pressure, receiving a fuel, oxidant and diluent within the combustion chamber, the fuel, oxidant and diluent controlling combustion within the combustion chamber to treat the effluent gas stream to produce a treated exhaust stream; and condensing the diluent in the treated exhaust stream. 
     In one embodiment, the step of receiving comprises introducing the diluent to the combustion chamber as a vapour. 
     In one embodiment, the diluent comprises a liquid prior to being vaporised for introduction to the combustion chamber. 
     In one embodiment, step of condensing comprises condensing the diluent to a liquid in the treated exhaust stream. 
     In one embodiment, the step of receiving comprises introducing the diluent into the combustion chamber with a first volumetric rate and the step of condensing comprises occupying the treated exhaust stream with a second volumetric rate, the second volumetric rate being lower than the first volumetric rate. 
     In one embodiment, the step of receiving comprises providing the diluent at specified volumetric rate to control combustion conditions within the combustion chamber to treat the effluent gas stream. 
     In one embodiment, the method comprises the step of combining the diluent with at least one of the fuel and oxidant prior to being introduced into the combustion chamber. 
     In one embodiment, the method comprises the step of dissolving at least one of the fuel and the oxidant by the diluent prior to being introduced into the combustion. 
     In one embodiment, the method comprises the step of dissolving both the fuel and the oxidant by the diluent prior to being introduced into the combustion. 
     In one embodiment, the step of receiving comprises vaporising at least one of the fuel and the oxidant dissolved by the diluent prior to being introduced into the combustion chamber. 
     In one embodiment, the step of receiving comprises co-vaporising at least one of the fuel and the oxidant dissolved by the diluent and the diluent prior to being introduced into the combusion chamber. 
     In one embodiment, the diluent comprises at least one of water, a perfluorocarbon and a hydrocarbon. 
     In one embodiment, the burner comprises a radiant burner and the combustion chamber has a porous sleeve through which the fuel, oxidant and diluent pass for combustion proximate to a combustion surface of the porous sleeve. 
     In one embodiment, the method comprises providing the treated exhaust stream to a liquid ring pump for compression to atmospheric pressure. 
     In one embodiment, the step of condensing comprises condensing the diluent in the liquid ring pump. 
     In one embodiment, the method comprises the step of scrubbing the treated exhaust stream using the liquid ring pump. 
     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 combinations other than those explicity 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a process gas abatement apparatus according to one embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Overview 
     Before discussing the embodiments in any more detail, first an overview will be provided. In embodiments, a sub-atmospheric combustion system is operated with a diluent which condenses in its exhaust stream in order to reduce the volume of exhaust emitted. This reduces the volume of exhaust which needs to be compressed to atmospheric pressure prior to be vented to atmosphere. 
     Process Gas Abatement 
       FIG. 1  illustrates a process gas abatement apparatus, generally  100 , according to one embodiment. A first pump stage  10  evacuates a process chamber, such as a semiconductor process chamber, and takes a process or effluent gas stream P provided at a first pressure, such as 1 mbar and compresses the effluent gas stream P to an intermediate pressure, such as 100-200 mbar. The first pump stage  10  typically comprises a dry pump. 
     A radiant burner  20  or other combustion apparatus receives the effluent gas stream P at the intermediate pressure. In addition, the radiant burner  20  receives a fuel/oxidant mixture, in addition to a diluent D. The effluent gas stream P is provided into a combustion chamber that is laterally surrounded by the exit surface of a foraminous gas burner. The fuel/oxidant mixture is simultaneously supplied with the diluent D to the foraminous burner to affect flameless combustion at the exit surface. The amount of oxidant passing through the foraminous burner is sufficient to consume not only the fuel supplied to the burner, but also all the combustibles in the effluent gas stream injected into the combustion chamber. The diluent D is provided with an amount sufficient to control the flame speed at the exit surface of the foraminous burner and to control the temperature and other combustion characteristics within the combustion chamber. The treated effluent gas stream P′ is exhausted from the radiant burner, together with the other by-products of the combustion within the combustion chamber. The diluent D condenses within the treated effluent gas stream. 
     The treated effluent gas stream P′ is provided to a secondary pump stage  30 , such as a liquid ring pump, which compresses the treated effluent gas stream P′, together with the other by-products of the combustion within the combustion chamber to a second pressure, such as atmospheric pressure, prior to being vented to atmosphere. 
     EXAMPLE OPERATION 
     In this example, the effluent gas stream P is provided at a rate of 10 slm (standard litres per minute) from the first pump stage  10  to the radiant burner  20 . In order to treat the effluent gas stream P, the fuel/oxidant mixture is provided at a rate of 3 slm, together with the diluent D at a rate of 8 slm in order to adequately control the flame speed, temperature and other combustion characteristics within the combustion chamber in accordance with the reaction (3) to correctly treat the effluent gas stream P: 
       10P+CH 4 +2O 2 +8D( g )=CO 2 +8D(I)+2H 2 O+10P′  (3)
 
     For example, such a radiant burner  20 , operating at approximately 200 mbar is fuelled with a hydrocarbon, for example methane, and oxygen. This is diluted to suitable concentration of diluent D. 
     Since the diluent D condenses in the effluent gas stream P′, it is typically a liquid under ambient conditions and so is heated in order to be vaporised prior to being provided to the combustion chamber. The liquid diluent D can therefore be mixed with the fuel and/or with the oxidant in order to store these in a convenient manner prior to being introduced into the combustion chamber. 
     Diluent 
     In one example, the diluent D is conveniently water. An added advantage of the provision of substantial quantities of water vapour at flame temperature in the combustion chamber is that this provides additional reagent for F 2  abatement in the effluent gas stream P according to equation (4) below: 
       F 2 +H 2 O=2HF+½O 2   (4)
 
     The excess O 2  generated also helps since it reacts with deposition gases such as, for example, SiH 4 . 
     The fuel may be dissolved within the water for convenient storage. For example, an alcohol may be dissolved within the water to provide an aqueous solution, which is then vaporised prior to being introduced into the combustion chamber. Likewise, the oxidant may be dissolved within the water for convenient storage. For example, hydrogen peroxide may be dissolved within the water to provide an aqueous solution, which is then vaporised prior to being introduced into the combustion chamber. Similarly, both the fuel and oxidant may be dissolved within the water for convenient storage, if this is derived from a 70° C./300 mbar source, this would require approximately 2600 J/g to produce. The power to do this would be around: 
       (8/22.4)×18×2600/60=280 Watts.
 
     This power may be derived from waste heat generated in the vacuum pump. In an integrated system, the water (and the pump) may be pre-heated electrically and the temperature maintained by the balance between evaporation and heat generation. 
     Considering the example above where there is around 10 slm of process gas P to be treated, typically, around 1 slm of CH 4  and 2 slm of O 2  will be required. To dilute this and provide similar combustion characteristics as would be achieved with air, around 8 slm of H 2 O as the diluent D will also be needed. 
     The water condenses within the treated effluent gas stream and so around 10 slm of processed effluent gas stream P′, together with 1 slm of CO 2  is provided. This means that rather than 19 slm being provided to the secondary pump stage  30 , only around 11 slm is provided, which considerably reduces the amount to be compressed and reduces the power consumption to achieve this. 
     The secondary pump stage  30  may be a liquid ring pump. Providing a liquid ring pump is particularly advantageous as this assists both the condensation of the diluent and can be used to scrub the gas stream provided. 
     Although the above example utilised water as the diluent, it will be appreciated that the diluent may be any suitable compound which condenses in the effluent gas stream such as, for example, a perfluorocarbon or a hydrocarbon. 
     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.