WO2004086610A2

WO2004086610A2 - Abatement of backflow contaminants in a dry pump

Info

Publication number: WO2004086610A2
Application number: PCT/GB2004/001198
Authority: WO
Inventors: Joanne Rachel Greenwood; Nathan Lee Kettlewell; Julian Richard Dean; John Spencer Kendall
Original assignee: The Boc Group Plc
Priority date: 2003-03-25
Filing date: 2004-03-19
Publication date: 2004-10-07
Also published as: WO2004086610A3; TW200427928A; GB2399772A; GB0306733D0

Abstract

A dry pump (10) has adjacent its inlet (12) and between the inlet and pump foreline (14) an apparatus for abating the back flow of organic molecules originating from a lubricant used to lubricate a drive mechanism of the pump. In one embodiment, the apparatus comprises a first compartment (18) having extending from its internal wall a plurality of cooling baffle plates (20). The cooling baffle plates are cooled by means of a water-cooling coil (22). Between the first compartment (18) and the foreline (14) is a second compartment 24 having extending from its internal wall a plurality of interleaved baffles (26). These baffles are coated on their surfaces with activated charcoal pellets for adsorbing halocarbon molecules back-streaming from the pump inlet.

Description

ABATEMENT OF BACKFLOW CONTAMINANTS IN A DRY PUMP

The present invention relates to the abatement of backflow contaminants such as halocarbons from a gaseous environment, in particular but not strictly limited to abatement of the backflow of organic molecules which may arise from lubricant oil or grease used in dry pumps and which, when the pump is in use, may migrate back through the pump inlet, contaminating the process environment.

Dry pumps are widely used in, for example, the semiconductor manufacturing industry. Such pumps include an essentially dry (or oil free) pumping mechanism, but typically also include some components, such as bearings and transmission gears, which require lubrication in order to be effective. Such components are used to drive the pumping mechanism. The presence of contaminants in the pumping mechanism can lead to contamination of the process environment which, in turn, can be severely detrimental to the quality of products manufactured in the process environment. In order to minimise the risk of such contamination, in such devices, the dry pumping mechanism is physically separated from the lubricated components of the drive mechanism by a series of seals. Nonetheless, ~^~ occasionally contaminant molecules derived from lubricant oils can find their way through the seals into the pumping mechanism and from there into the process environment.

In semiconductor manufacture, lithographic techniques are commonly used to pattern circuit features onto silicon wafers. Such techniques rely on high precision optics such as lenses. One recently developed lithographic technique utilises extreme ultra violet radiation (EUV), which is of such short wavelength it must be focussed by perfectly shaped mirrored surfaces, typically multi-layered with for example, molybdenum and silicon. The optical properties of these lenses and mirrored surfaces can be seriously impaired by contaminants forming deposits on their surfaces. The use of EUV necessitates a very low pressure (vacuum) process environment and sometimes elevated temperatures. The vacuum is typically maintained by means of a primary pump backed by a dry pump. The problematic organic molecules addressed by the invention have high vapour pressures, comparable to or higher than the ambient pressure in the vacuum or near vacuum environments required for EUV lithography. For example, the perfluorocarbon Fomblin™ Y- LVAC 25/6, chemical name propene, 1 ,1 ,2,3,3,3 hexa-fluoro, oxidized, polymerized, having the structure A-(C₃F₆0)_n-(CF₂0)m-CF₃, where A is one of CF₃0, C^F_δO, C₃F₇O, etc., and typically used to lubricate the drive mechanism of a dry pump, has a vapour pressure of 4x10^"8mbar at 20°C and 6x10^"5mbar at 100°C. Consequently, these molecules can travel relatively easily from the pump inlet to the process environment and form deposits on the surfaces of the optical components. The higher the temperature and the lower the vacuum, the more likely it is that there may be fluorocarbon contamination of the process environment. Furthermore, as well as forming deposits on the surface of optical components, fluorocarbon molecules can also cause damage to other components in the process environment, such as a gas purifier (for example a "getter") used in xenon recirculation systems to reduce the levels of water vapour, whose presence can compromise the maintenance of a vacuum.

As smaller wavelengths of radiation are used in lithographic techniques, the greater the need for optical precision becomes and increasingly smaller levels of contaminant can lead to problems.

The present invention aims to provide apparatus for abating the backflow of organic molecules, in particular halocarbon molecules, from the inlet of a dry pump (or any other machine with a "dry" operating mechanism). One application of the apparatus is to reduce deposits of contaminants on the aforementioned optical components of lithographic systems used in semiconductor manufacture by abatement of molecules originating from a lubricant in the drive mechanism of the pump. In one aspect, the invention provides apparatus for abating the backflow of organic, for example halocarbon, molecules from the inlet of a pump, the apparatus comprising means for reducing the partial pressure of the molecules flowing away from the pump, and a porous mass of material for capturing said molecules.

As used herein, the term "organic molecules" is intended to include any carbon containing species. Examples of organic molecules include halocarbons and perfluorocarbons.

Providing a porous mass of material increases the surface area to which the organic molecules can coalesce and/or adsorb. Reducing the partial pressure and the kinetic energy of the molecules reduces the likelihood of migration of molecules from the pump back to the foreline and increases the likelihood of coalescence and/or adsorbence of the molecules on the surfaces of the porous mass. Tests have shown that the back migration of a halocarbon pump oil such as Fomblin™ can be reduced by at least 50% with minimal effect on pumping efficiency.

The means for reducing the partial pressure of the organic molecules preferably comprises means for cooling the molecules flowing from the pump inlet. The means for cooling preferably comprises a cooling coil, for example a cryogenically or water-cooled coil, through which a fluid coolant is circulated. The coil is preferably in thermal contact with, and more preferably encircles, the porous mass. Other suitable coolants and cooling means will no doubt occur to the skilled addressee without departing from the true scope of the invention. Examples of suitable cryogenic coolants include a liquid hydrogen and liquid nitrogen. Alternatively, or in addition, the means for cooling may be arranged adjacent or around the pump foreline near to the pump inlet.

In one embodiment, the porous mass comprises a plurality of layers each having selectively sized pores, for example, the layer adjacent the source of molecules whose backflow is to be abated has relatively large pores, a second layer sandwiching the first with the source has smaller pores, a third layer sandwiching the second with the first layer has even smaller pores, and so on. Thus, the second and third layers provide an increasingly fine filter for smaller or more volatile contaminant molecules.

Pore densities of from about 4 pores per centimetre to about 16 pores per centimetre are suggested for molecules commonly found in dry pump lubricants. However, it is to be understood that specific pore sizes and densities are not essential, most suitable pore sizes and densities being variable and dictated by the nature and size of the molecules whose backflow is to be abated without causing any unnecessary conductance losses.

In one embodiment, the mass of porous material is formed from metallic material. It has been observed that, in certain conditions and in the presence of metals, it is possible to electrically charge organic molecules typically found in pump oil and grease (for example, Fomblin™). This has been demonstrated using a simple pumping circuit where a halocarbon is pumped through a mechanism using polymeric gears. Molecules exiting a stainless steel pipe were seen clearly to be electrically charged and attracted to the pipe, causing them to coalesce on the surface and flow backwards on the outer surface of the pipe. Whilst it is not intended to limit the scope of the invention as claimed herein, it is postulated that this effect could be beneficial in coalescing and capturing pump oil mist on the internal surfaces of the porous metallic mass.

Thus in another aspect the present invention provides apparatus for abating the backflow of organic molecules from the inlet of a dry pump, the apparatus comprising a porous mass of metallic material and cooling means associated with the inlet and/or porous mass. In this embodiment, the mass may be formed from a metallic material which has been foamed. A preferred metallic material comprises aluminium or an aluminium alloy.

The porous mass may further be coated with activated charcoal or other material, such as an alumina, silica or zeolite, adsorptive of carbon based molecules to provide an adsorptive surface for retaining halocarbon molecules. Activated charcoal is charcoal which has been treated with oxygen to open up millions of tiny pores between the carbon atoms. As a result, the large surface area of activated charcoal provides numerous bonding sites for the organic molecules to associate with the activated charcoal, the pores enabling other gases, such as nitrogen and oxygen which are not attracted to the carbon of the activated charcoal, to pass through.

Activated charcoal (and other such adsorptive materials) is also electrically conducting and so tends to encourage electrical charging of organic molecules. In addition to its adsorption of organic molecules, activated charcoal is desirable in that it also adsorbs water vapour which, unchecked, may also contaminate the aforementioned optical components of lithographic systems.

In another embodiment, the porous mass of material itself comprises activated charcoal or other material, such as an alumina, silica or zeolite, adsorptive of carbon based molecules. Thus in a further aspect the present invention provides apparatus for abating the backflow of organic molecules in the inlet of a dry pump, the apparatus comprising, a porous mass of material which is selectively adsorptive of carbon based molecules and cooling means associated with the inlet and/or porous mass.

This porous mass may be provided on the surfaces of one or more metallic baffle plates, which may comprise part of the cooling means. The activated carbon may be in the form of activated charcoal pellets, and may be coated on or otherwise applied to surfaces of the baffles. Alternatively, the activated carbon may be glued to the surfaces. Where the intended use of the apparatus of the inventions is in a semiconductor manufacturing operation or any other vacuum based application, the glue is preferably vacuum compatible.

The baffle plates preferably comprise a relatively inert metallic material, for example stainless steel. The baffle plates may be provided in a variety of configurations, for example they may be interleaved or overlapped.

In another embodiment the activated charcoal is provided in the form of a hollow cone placed adjacent the inlet and a baffle plate adjacent to but spatially separated from the cone. The baffle plate is desirably provided with a surface layer or coating of activated charcoal. The cone may comprise a metal framework provided with a surface layer or coating of activated charcoal.

In addition to the coated baffle plates, separate, metallic cooling baffles may also be provided between the pump inlet and the coated baffles to reduce the partial pressure of vapour flowing back through the pump inlet.

It is to be appreciated that other materials not already discussed may provide suitable selectively adsorptive materials. In particular it is envisaged that certain inert, vacuum compatible polymers may be used.

The present invention further provides a pump having apparatus as aforementioned at the inlet thereof.

The invention also provides a method of abating the backflow of organic molecules from the inlet of a pump, the method comprising the steps of cooling the molecules flowing from the pump to reduce the partial pressure thereof, and capturing the molecules in the pores of a porous mass of material. The invention further provides use of the apparatus as aforementioned at a pump inlet to abate the back flow of organic molecules originating from a lubricant used to lubricate a drive mechanism of the pump.

Features described above relating to apparatus aspects of the invention are equally applicable to method aspects.

For the purposes of exemplification some embodiments of the invention will now be further described with reference to the following figures in which:

Figure 1 illustrates a first embodiment of an apparatus for abating the backflow of halocarbon molecules;

Figure 2 illustrates a first variation of the embodiment of Figure 1 ;

Figure 3 illustrates a second variation of the embodiment of Figure 2; and

Figure 4 illustrates a second embodiment of an apparatus for abating the backflow of halocarbon molecules.

With reference first to Figure 1 a dry pump 10 has adjacent its inlet 12 for receiving fluid to be pumped and between the inlet 12 and pump foreline 14, an apparatus 16 for abating the backflow of organic molecules from the inlet 12 of the pump 10. The apparatus 16 comprises a first compartment 18. The first compartment 18 has extending from its internal wall a plurality of cooling baffle plates 20. In the embodiment shown in Figure 1 , the cooling baffle plates 20 are cooled by means of a water-cooling coil 22 extending about the cooling baffles plates 20.

Between the first compartment 18 and the foreline 14 is a second compartment 24 having extending from its internal wall a plurality of interleaved baffles 26. The surfaces of the interleaved baffles 26 are coated with activated charcoal pellets. , _{arrΛ Λ}

WO 2004/086610

- 8 -

ln use, the pump 10 withdraws gas from a process environment through the foreline 14 in the direction of the arrow 28 shown in the foreline 14. When the pump 10 is at ultimate pressure, volatile organic contaminants from oil, such as Fomblin™, used to lubricate the driving mechanism of the pump 10 travel in contra-flow to the withdrawn gas as illustrated by arrow 30. As the contaminant vapour passes through the first compartment 18, it is cooled and its vapour pressure reduced with the result that some of the contaminant vapour condenses and/or coalesces on to the metallic surfaces of the cooling baffles 20. However, depending upon the temperature and whether the pump 10 is pumping a mass of gas or is at ultimate pressure (the latter being the worst case scenario where back migration of contaminants through the pump inlet is predominant), some molecules of oil can continue into the second compartment 24. In the second compartment 24, the interleaved, coated baffles 26 interfere with progress of the contaminant vapour through the compartment. The organic molecules entering the second compartment 24 have a tendency to associate with the activated charcoal on the coated baffles 26 and become adsorbed thereto so that back migration of these organic molecules into the foreline 14 is substantially reduced. For example, where the oil is the perfluorocarbon Fomblin™ YLVAC25/6, having the structure A-(C₃F₆0)n-(CF₂0)_m-CF₃, where A is one of CF₃0, C₂F₅0, C₃F₇0, etc., tests have shown that the back migration of halocarbon molecules into the foreline 14 can be reduced by at least 50%. Similar results have been achieved where the oil is the perfluorocarbon Fomblin™ Y HVAC 25/9, perfluoropolyether vacuum-pump oil, chemical name propene, 1 ,1 ,2,3,3,3-hexafluoro, oxidized, polymerised, having a vapour pressure of 2 x 10^"9mbar at 20°C and of 2 x 10^"5 mbar at 100°C.

Figures 2 and 3 show arrangements broadly similar to that of Figure 1 but each with a different arrangement of coated baffles 26 in the second compartment 24. The mode of operation of each apparatus is essentially the same as for that shown in Figure 1. In Figure 3, the second compartment 24 contains a hollow cone 32 of activated charcoal placed adjacent the inlet 12 and a baffle plate 34 adjacent to , _{arrΛ Λ}

WO 2004/086610

but spatially separated from the cone 32. The baffle plate 34 is provided with a surface layer or coating of activated charcoal over a metal framework.

In the embodiment shown in Figure 4, the apparatus 40 for abating the backflow of organic molecules from the inlet 12 comprises a filter in the form of a metallic foam mass 42 consisting of four layers 44, 46, 48, 50 having respectively progressively larger pores as they get closer to the inlet 12 of the dry pump (not shown). The pores have a distribution in the range from about 4 pores per centimetre adjacent the inlet 12 to about 16 pores per centimetre adjacent the foreline 14.

The foam mass 42 is surrounded by a hollow coil 52 through which a coolant (for example water) can be circulated. The metallic foam may be conveniently formed from aluminium or an aluminium alloy.

When the pump is in operation, fluid for process equipment is drawn through the filter 40 to the inlet 12. Occasionally, volatile contaminants such as fluorocarbons from a Fomblin™ oil used to lubricate the driving mechanism of the pump flow back against the main flow of materials entering the pump, as illustrated by the arrows 54 (this occurs essentially when the pump is not moving a mass of gas but is pumping at ultimate vacuum). It is postulated that these contaminant molecules carry an electric charge causing them to be drawn towards the metallic surface of the foam mass 42 which acts as a capacitance plate across the vacuum, with the vacuum acting as a dielectric. The molecules coalesce onto the foam surface and are prevented from travelling through the filter and into the processing equipment. Cooling of the metallic foam further inhibits backflow of the contaminants from the inlet by reducing their partial pressure and encouraging condensation and coalescence to the surface of the foam mass 42. The foam mass 42 may also be coated with activated charcoal in order to improve the capture of the contaminants by the filter 40. Again, where the oil is the perfluorocarbon Fomblin™ YLVAC25/6 or Y HVAC 25/9, tests have shown that the back migration of organic molecules into the foreline 14 can be reduced by at least 50%.

Whilst in the embodiments described above the oil used in the drive mechanism of the pump 10 is a Fomblin™ oil, the apparatus is also suitable for use to abate the backflow of any other halocarbon or organic, carbon based molecules.

In summary, a dry pump 10 has adjacent its inlet 12 and between the inlet and pump foreline 14 an apparatus for abating the back flow of organic molecules originating from a lubricant used to lubricate a drive mechanism of the pump. In one embodiment, the apparatus comprises a first compartment 18 having extending from its internal wall a plurality of cooling baffle plates 20. The cooling baffle plates are cooled by means of a water-cooling coil 22. Between the first compartment 18 and the foreline 14 is a second compartment 24 having extending from its internal wall a plurality of interleaved baffles 26. These baffles are coated on their surfaces with activated charcoal pellets for adsorbing halocarbon molecules back-streaming from the pump inlet.

Claims

1. Apparatus for abating the back flow of organic molecules from the inlet of a dry pump, the apparatus comprising means for reducing the partial pressure of the molecules flowing away from the pump, and a porous mass of material for capturing said molecules.

2. Apparatus according to Claim 1 , wherein the means for reducing the partial pressure of the molecules flowing away from the pump comprises means for cooling the molecules.

3. Apparatus according to Claim 2, wherein the cooling means includes a cooling coil through which a fluid coolant is circulated.

4. Apparatus according to Claim 3, wherein the coolant is water.

5. Apparatus according to any of Claims 2 to 4, wherein the cooling means extends about the porous mass of material.

6. Apparatus according to Claim 2, wherein the cooling means comprises heat conducting baffle plates located between the pump inlet and the porous mass of material.

7. Apparatus according to Claim 6, wherein the cooling means comprises a cooling coil extending about the heat conducting baffle plates.

8. Apparatus according to any preceding claim, wherein the porous mass of material comprises a plurality of layers each having selectively sized pores, such that the average pore size for each layer differs from that of an adjacent layer.

9. Apparatus according to Claim 8, wherein the pores have a distribution in the range from about 4 pores per centimetre to about 16 pores per centimetre.

10. Apparatus according to any preceding claim, wherein the porous mass of material comprises metallic material.

11. Apparatus according to any preceding claim, wherein the porous mass of material comprises a metallic foam.

12. Apparatus according to Claim 11 , wherein the metallic foam comprises aluminium or an alloy thereof.

13. Apparatus according to Claim 11 or 12, wherein the metallic foam is coated with a material which is selectively adsorptive of carbon- based molecules.

14. Apparatus according to Claim 13, wherein the selectively adsorptive material is activated charcoal.

15. Apparatus according to any of Claims 1 to 7, wherein the porous mass of material comprises a material which is selectively adsorptive of carbon-based molecules.

16. Apparatus according to Claim 15, wherein the porous mass of material comprises activated carbon.

17. Apparatus according to Claim 15 or 16, wherein the porous mass of material comprises activated carbon in the form of an agglomeration of pellets.

18. Apparatus according to Claim 15, wherein the porous mass comprises an alumina, silica or zeolite.

19. Apparatus according to any of Claims 15 to 18, wherein the porous mass of material is provided in the form of a plurality of baffles.

20. Apparatus according to Claim 19, wherein the baffles comprise a metallic frame having the porous mass of material coated or adhered thereon.

21. Apparatus according to Claim 20, wherein the metallic frame comprises stainless steel.

22. Apparatus according to any of Claims 19 to 21 , wherein the baffles are interleaved.

23. Apparatus according to any of Claims 19 to 21 , wherein the baffles are in a cone and plate configuration.

24. Use of the apparatus according to any of the preceding claims at a pump inlet to abate the back flow of organic molecules originating from a lubricant used to lubricate a drive mechanism of the pump.

25. A pump comprising apparatus according to any of Claims 1 to 23 provided at the pump inlet.

26. A method of abating the back flow of organic molecules from the inlet of a pump, the method comprising the steps of cooling the molecules flowing from the pump to reduce the partial pressure thereof, and capturing the molecules in the pores of a porous mass of material.