Patent Application
20100290966
The present invention is concerned with the removal of fluorine from a gas stream, and finds particular use in the removal of fluorine from fluorine containing gases exhausting from a process tool used in the semiconductor or flat panel industry.
CF4, C2F6, NF3, SF6 and molecular fluorine (F2) are commonly used as etchant precursor gases in the semiconductor and flat panel industry to provide a source of excited fluorine species for processes such as dielectric layer etching and chamber cleaning.
In most cases excited fluorine species such atomic fluorine are formed in situ, or upstream of a chamber, to react with unwanted deposits and form volatile fluorinated compounds that are easily pumped away. However, gas use during these steps is often inefficient and so the effluent gas stream from the tool can have a residual amount of either the fluorinated pre-cursor gas or a range of other gaseous fluorinated reaction by-products.
Due to their toxicity and/or extremely high global warming potential any unused fluorinated gases and fluorinated reaction by-products must be removed (abated) from the chamber exhaust gas mixture prior to its release to the atmosphere. At present abatement is commonly carried out by combustive techniques, in devices such as inwardly fired burners, electrically heated burners, open flame burners or plasma abatement devices. In these devices the exhaust gases from the process chamber pass to a reactive zone where there is sufficient energy and reactive species, in the form of heat and fuel/oxidant gases respectively, to effect the conversion of the fluorinated gases to hydrofluoric acid (HF).
To meet stringent emission targets the hydrofluoric acid must then be removed from the gas stream, which is often achieved with wet scrubbers. These require large volumes of water to dissolve the HF, which is subsequently removed from the water by reaction with calcium salts as a calcium fluoride precipitate (CaF2). The precipitate is separated from the water by settling and “caking” allowing the water to be reused or passed to the sewerage system. The hydrofluoric acid may also be removed by passing the exhaust gases from the combustive destruction device through a dry reactive bed of Calcium salts, which are often inefficient due to the formation of a calcium fluoride layer on the surface of the bed, rendering it impervious to further reaction.
It is the intention of the invention described herein to address some of these problems.
In the first aspect the invention provides a method for the removal of a fluorinated gas from a gas stream comprising the steps of: conveying the gas stream containing the fluorinated gas to a combustion region, decomposing the fluorinated gas in the combustion region, entraining a calcium salt capable of reacting with the fluorinated gas decomposition products into the gas stream to react with the fluorinated gas decomposition products to form calcium fluoride, and then removing the calcium fluoride salt from the gas stream.
This provides a new method for the removal of fluorinated gases such as molecular fluorine (F2) and perfluorinated gases (for example nitrogen trifluoride (NF3), carbon tetrafluoride (CF4), hexafluoroethane (C2F6), silicon tetrafluoride (SiF4), and sulphur hexafluoride (SF6)) from a gas stream. The method advantageously reduces the requirement for expensive multi stage water treatment methods or inefficient dry bed reactors to remove the fluorinated decomposition (abatement) products from the gas stream on exiting the combustion region.
The combustion region is preferably, but not limited to, that of devices such as inwardly fired burners, electrically heated burners, open flame burners, DC plasmas and microwave plasma reactors. Such devices, already widely used for the destruction (abatement) of fluorinated gases, could advantageously be retro fitted with calcium fluoride entrainment devices, thus allowing the user to save energy without the need for large additional capital expenditure.
The calcium salts may be entrained into the gas stream containing the fluorinated gas prior to it entering the combustion region; when it is inside the combustion region; or when it has exited the combustion region. The calcium salts are preferably entrained in the gas stream either inside or close to the exit of the combustion region as the thermal conditions present enhance the reaction between the calcium salt and the fluorinated gas decomposition products.
The calcium salts may be entrained into the gas stream containing the fluorinated gas using many of the methods know to the skilled person such as air blown powder techniques, atomising spray techniques, simple spraying techniques and vaporisation techniques.
A carrier gases may be also used to entrain the calcium salt into gas stream including air, nitrogen, hydrogen, oxygen, methane, butane. The use of either a reducing or oxidising carrier gas can advantageously improve the reaction between the calcium salts and fluorinated decomposition products.
The calcium salt or its decomposition products must be able to react with the fluorinated decomposition products formed in the combustion region to form calcium fluoride, preferably without producing substantial quantities of undesirable additional gaseous by-products such as acids or NOx that would require further expensive scrubbing techniques. Examples of suitable calcium salts include, for example, CaO (Lime), CaCO3, Ca(OH)2 (Slaked Lime), and organo-calcium salts such as Ca(CH3CH2CO2)2.H2O. The calcium fluoride, which can then be removed from the gas stream by one or a combination of filter, cyclone and electrostatic precipitator devices. In apparatus where an atomising, or simple spraying technique are favoured it is preferable to use calcium acetate, Ca(CH3CH2CO2)2.H2O, as it readily forms a concentrated aqueous solution, thus maximising the amount of calcium and limiting the amount of water supplied to the apparatus and ensuring that the calcium fluoride salt, formed by reaction with the fluorinated gas decomposition products, remains substantially dry.
The removal of calcium fluoride from the gas stream can be carried out either in-situ by a device local to the combustion device or by a scrubber unit situated remotely from the combustion device, which allows the calcium fluoride from several combustion devices to be removed in a centralised area thus minimising the risk of particulate contamination of the ultra clean semiconductor processing areas.
Preferred features of the present invention will now be described by way of example with reference to the accompanying drawings, in which:
The drawings are not to scale
Like parts in different Figures are referred to below by the same reference numeral.
Path 6 conveys the calcium fluoride-rich gas stream exiting the combustion region 2 to calcium fluoride removal device or devices 10 in which the calcium fluoride is removed from the gas stream, with a calcium fluoride lean gas stream exiting the removal device 10 via path 12. Removal device 10 may be, for example, a cyclone, filter and electrostatic precipitator, or combinations thereof.
In order to both remove heat from the gas stream exiting the combustion region 2 and ensure that the calcium fluoride remains airborne, and is therefore prevented from collecting on the surfaces of the apparatus upstream of the removal device 10, an optional flow gas, such as air, oxygen or nitrogen, may be supplied to path 6 via inlet 8.
Each nozzle 16 is located within a respective bore 18 formed in a ceramic top plate 20, which defines the upper (as shown) surface of the combustion region 2. The combustion region 2 is within the bounds of the exit surface of a foraminous burner element 22 such as that described in EP-A-0 694 735. A plenum volume 24 is formed between the burner and a cylindrical outer shell 3. A mixture of fuel gas such as natural gas, or hydrocarbon, and air is introduced into the volume245 via one or more inlet nozzles (not shown) so that, during use, the mixture of fuel gas and air will burn without visible flame at the exit surface of burner element 22.
In order to optimize combustion conditions to achieve abatement of the particular fluorinated gas contained within the effluent gas stream it is preferable to supply additional fuel gas and/or oxygen to enrich the effluent stream prior to its entry into the combustion region 2. Therefore the apparatus includes means, by way of a lance 28 substantially concentrically positioned within the nozzle 16, for introducing oxygen into the effluent stream upstream of the combustion region 2, similar to that described in EP 0 802 370 A2. Additional fuel gas may be added by any suitable means (not shown) to the effluent gas stream at any convenient point upstream of the combustion region. However, for reasons of flammability, the oxygen and fuel gas should not be present in the effluent stream for any appreciable length of time prior to their injection to the combustion zone, and so the opening of the lance 28 into the nozzle 16 terminates between 0.7 and 3 pipe diameters prior to the point of injection of the effluent into the combustion region. The relative proportions of fuel gas and/or oxidant may be varied dependent on the type of fluorinated compounds present in the effluent stream.
Upon injection into the region 2 the enriched effluent gas mix ignites to form a flame extending from the nozzle 16 into the chamber 2 effecting the abatement of fluorinated gases by the general formula:
Fluorinated gas+Fuel+Oxygen→HF+CO/CO2 and H2O (1)
The apparatus also comprises one or more calcium fluoride entrainment devices 30, preferably an atomising spray device although other entrainment devices are also suitable, located within ceramic top plate 20. The calcium fluoride is entrained into the gas stream in the combustion region, by any of the aforementioned means, via inlet 4. The entrainment device/or devices 30 and their corresponding inlets 4 are positioned such that sufficient mixing of the entrained calcium fluoride and effluent gas stream occurs in the combustion region 2 without inhibiting the flame extending from the nozzle 16 into the region 2. Upon entrainment into the gas stream in region 2 the calcium salt, or its decomposition products, reacts with hydrofluoric acid from the abatement of the fluorinated gases to form calcium fluoride by the general formula:
HF+Calcium salt→CaF2 (2)
The gas stream and calcium fluoride then exit the abatement apparatus via exit 32 and are conveyed to one of the aforementioned device or devices 10 for the removal of the calcium fluoride from the gas stream.
As illustrated in
As also illustrated in
The first combustive gas mixture exiting apertures 46 is ignited by the pilot flame, formed from the second pilot gas mixture exiting apertures 52, to form a combustion flame extending into the combustion region 2 concentrically surrounding the path of the effluent gas stream exiting the nozzle 34. The effluent gas stream conveyed through the nozzle 34 enters the combustion region 2 via outlet 38 into the center of the combustion flame such that the fluorinated gases are abated according to the general formula (1) above.
The combustion chamber 2 is within the bounds of a substantially cylindrical conduit 54. The conduit 54 has a plurality of apertures 56 through which gas can pass from plenum chamber 58 formed between the conduit 54 and an outer shell 60. A suitable calcium fluoride entrainment device, preferably an air blow powder device or atomising spray device (not shown), although other entrainment devices are also suitable, is used to entrain a calcium salt into a gas stream, for example air, which is fed into the plenum chamber 58 via inlet 4. The calcium salt enriched gas enters combustion region 2 via apertures 56. The calcium salt, or its decomposition products, then reacts with the HF to form calcium fluoride according to the general formula 2 above. The effluent stream and calcium fluoride then exit the combustion region 2 via exit 62 and are conveyed from the abatement device to one of the aforementioned device or devices 10 for the removal of the calcium fluoride from the gas stream.
In use of the apparatus shown schematically in
A carrier gas, for example, air, oxygen and nitrogen may be optionally added via inlet 8 to the gas stream conveyed along path 6 to encourage the calcium fluoride to stay airborne until it reaches calcium fluoride removal device 10.
In use of the apparatus shown schematically in
A carrier gas, for example, air, oxygen and nitrogen may be optionally added to the gas stream conveyed along path 6 via inlet 8, downstream of inlet 4, to encourage the calcium fluoride to stay airborne until it reaches calcium fluoride removal device 10. The carrier gas may also be added with the calcium fluoride via inlet 4.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB07/50285 | 5/23/2007 | WO | 00 | 5/19/2010 |
