GB2399772A - A metallic filter and associated cooling means - Google Patents

Abstract

An apparatus for abating the back-flow of organic molecules from the inlet of a dry pump, comprises a porous mass of metallic material 2 and associated cooling means. Preferably the cooling means is associated with the inlet and/or the porous mass. The cooling means may be a hollow coil 1 through which is circulated water or a cryogenic coolant such as liquid hydrogen or nitrogen. Preferably the porous mass is a plurality of foamed metallic material such as aluminium, coated with activated charcoal or other material adsorptive of carbon based molecules. Also, the porous mass may be provided on the surfaces of at least one metallic baffle plate (2, Fig 1) providing part of the cooling means. Advantageously, the back-flow of volatile organic molecules such as hydrocarbons originating from lubricants in the drive mechanism of a dry pump used in lithographic systems in semi-conductor manufacture, are reduced.

Description

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ABATEMENT OF BACKFLOW CONTAMINANTS IN A DRY PUMP

The present invention relates to the abatement of the backflow of hydrocarbons and substituted hydrocarbons from a gaseous environment, in particular but not strictly limited to abatement of the backflow of such molecules which may arise from lubricant oil/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 semi-conductor manufacturing industry. Such pumps include an essentially dry (or oil free), pumping mechanism, but necessarily 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" operating (pump) mechanism is arranged to be physically separated from the lubricated components of the "wet" drive mechanism through a series of seals, some components will need to bridge the "wet" and "dry" sections of the pump.

Occasionally, contaminant molecules derived from lubricant oils can find their way, via such components, into the pumping mechanism and from there into the process environment.

In semi-conductor 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 light (EUVL), 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 À:e lee: cee. cee. À .e:e can be seriously impaired by contaminants forming deposits on their surfaces.

The use of EUVL necessitates a very low pressure (vacuum) operating environment and sometimes elevated temperatures. The vacuum is typically maintained by means of a primary pump backed by a dry pump. In such an environment, halogenated oils such as FOMBLIN_ (a perfluorinated polyether) typically used to lubricate the pump can, over time, infiltrate the optical system resulting in carbon deposits on the surfaces of lenses or mirrors used to focus and direct radiation in the lithographic system. Halogen substituted hydrocarbon molecules can also cause damage to other components such as a gas purifier (for example a "getters) which may be used in xenon recirculating systems to reduce, the levels of water vapour, whose presence can compromise the maintenance of a vacuum.

Previously used lithographic techniques have been performed at atmospheric pressure. The problematic, hydrocarbon and substitute hydrocarbon 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 EUVL. Consequently, these molecules can travel relatively easily through the process environment and form deposits on the surfaces of the optical components.

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 hydrocarbons in a dry pump (or 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 semi-conductor manufacture by abatement À c. À À C t À . . . . À À À À À À À À À of volatile organic molecules originating from lubricants oil in the drive mechanism of the dry pump.

In one aspect, the invention is an apparatus for abating the backflow of organic molecules in 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.

It has been observed that, in certain conditions and in the presence of metals, it is possible to electrically charge hydrocarbons and substituted hydrocarbons typically found in pump oil/grease (eg FOMBLIN_). This has been demonstrated using a simple- pumping circuit where a hydrocarbon/substituted hydrocarbon 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 a porous metallic mass forming part of the apparatus in the first aspect of the invention The means for cooling optionally comprises cryogenically or water cooled coil in thermal contact with and optionally encircling 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.

The cooling means serves to slow down volatile molecules thereby to increase the likelihood of coalescence to the surfaces of the porous mass.

The porous mass may comprise a plurality of layers each having selectively sized pores, for example, the layer adjacent the source of organic 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, etcetera. 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 hydrocarbon/substituted hydrocarbon molecules commonly found in dry pump lubricant oils.

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.

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 throughout with activated charcoal or other material adsorptive of carbon based molecules to provide an adsorptive surface for retaining molecules. Activated charcoal (and other such adsorptive materials) are electrically conducting and so tend to encourage electrical charging of hydrocarbon and In another aspect the 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 :.: ece Be; e:e:.

carbon based molecules and cooing means associated with the inlet and/or porous mass.

Preferably, the selectively adsorptive porous mass is provided on the surfaces of one or more metallic baffle plates which may comprise part of the cooling means. The porous mass is desirably activated carbon.

The activated carbon may be in the form of activated charcoal pellets. The activated carbon may be coated on or otherwise applied to surfaces of the baffles. Alternatively, the carbon may be glued to the surfaces.

The baffle plates desirably comprise a relatively inert metallic material, for example stainless steel. Where the- intended use of the apparatus of the inventions is in a semi-conductor manufacturing operation or any other vacuum based application, the glue is preferably vacuum compatible.

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.

In addition to its carbon based molecule adsorption properties, activated charcoal is desirable in that it also adsorbs water vapour which, :e aft.: e.e eee. .e:.

unchecked, may also contaminate the aforementioned optical components of lithographic systems.

As an alternative to activated carbon in the aforementioned embodiments, aluminas, silicas or zeolites may be used.

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

Cooling means optionally include, without limitation; a cryogenically or water cooled coil in thermal contact with and optionally encircling the porous mass, or, a thermally conductive foam or other porous body, coated throughout with the selectively adsorptive, porous mass.

Alternatively, or in addition, the means for cooling may be arranged adjacent or around the foreline of the pump near to the pump inlet.

The cooling means serves to reduce the kinetic energy and the vapour pressure of the volatile molecules thereby to reduce the likelihood of migration of molecules from the pump back to the foreline.

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 one aspect of the invention; Figure 2 illustrates a first variation of the embodiment of Figure 1; Figure 3 illustrates a second variation of the embodiment of Figure 2; Figure 4 illustrates an embodiment of another aspect of the invention.

c. :e 's. be: À À À. A:: ::: As can be seen from Figure 1 a dry pump 4 has adjacent its inlet and betwixt the inlet and pump foreline 5, an apparatus according to the invention comprising a first compartment 7 and a second compartment 1.

The first compartment has extending from its internal wall a plurality of cooling baffle plates 2. The cooling baffle plates 2 are cooled by means of a water cooling coil 3. Between the first compartment and the foreline 5 is a second compartment 1 having extending from its internal wall a plurality of interleaved baffles 6. These baffles are coated on their surfaces with activated charcoal pellets.

In use, the pump withdraws gas from the process- environment through the foreline 5 in the direction of the arrow shown in the foreline.

When the pump is at ultimate pressure, volatile contaminants from oil used to lubricate the driving mechanism of the pump travel in contra-flow to the withdrawn gas as illustrated by the remaining arrows. As the contaminant vapour passes through the first compartment, it is cooled and its vapour pressure reduced. Some gas condenses and/or coalesces to the metallic surfaces of the cooling baffles, however, depending upon the temperature and whether the pump is pumping a mass of gas or is at ultimate vacuum, (the latter being the worst case scenario where back migration of contaminants through the pump inlet is predominant) some molecules can continue into the second compartment. In the second compartment, the interleaved, coated baffles interfere with progress of the contaminant vapour through the compartment. Organic molecules which comprise a considerable proportion of carbon have a tendency to associate with the carbon coated on the baffle and become adsorbed thereto so that back migration of these organic molecules into the foreline is substantially reduced.

Figures 2 and 3 show arrangements broadly similar to that of Figure 1 but each with a different arrangement of coated baffles 6 in the second . :e Ale: eeese c:e c.e::: ....:: compartment 1. The mode of operation of the filters is essentially the same as for that shown in Figure 1.

As can be seen from Figure 4 an alternative filter in accordance with the invention comprises a metallic foam mass 2 consisting of 4 layers 4a, 4b, 4c, 4d having respectively progressively larger pores as they get closer to the inlet 3 of a dry pump (not shown). The foam mass 2 is surrounded by a hollow coil 1 through which coolant (for example water) can be circulated.

When the pump is in operation, materials are drawn through the filter to the inlet. Occasionally, volatile contaminants -such as fluorocarbons and hydrocarbons from oil grease used to lubricate the driving mechanism of the pump flow back against the main flow of materials exiting the pump, their direction of flow is illustrated by the arrows (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 aluminium foam 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 abatement device and into the processing environment.

Cooling 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.

Claims (27)

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1. An 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.

2. An apparatus as claimed in claim 1 wherein the means for cooling the porous mass includes a cooling coil through which a fluid coolant is circulated.

3. An apparatus as claimed in claim 1 or claim 2 wherein the means for cooling includes heat conducting baffle plates.

4. An apparatus as claimed in claim 2 or claim 3 wherein the coolant is water.

5. An apparatus as claimed in any preceding claim wherein the porous mass of material comprises a metallic foam.

6. An apparatus as claimed in claim 5 wherein the foamed metal comprises aluminium or an alloy thereof.

7. An apparatus as claimed in any preceding claim wherein the mass 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.

8. An apparatus as claimed in any preceding claim wherein the pores have a distribution in the range from about 4 pores per centimetre to about 16 pores per centimetre.

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9. An apparatus as claimed in any of claims 6 to 8 wherein the metallic foam is coated throughout with a material which is selectively adsorptive of carbon based molecules.

10. An apparatus as claimed in claim 9 wherein the selectively adsorptive material is activated charcoal.

11. An apparatus as claimed in any preceding claim wherein the cooling means is arranged to cool the pump foreline adjacent the inlet.

12. An apparatus substantially as described herein with reference to Figure 4.

13. An 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.

14. An apparatus as claimed in claim 13 wherein the porous mass comprises activated carbon.

15. An apparatus as claimed in claim 13 wherein the porous mass comprises materials selected from the groups; aluminas, silicas or zeolites.

16. An apparatus as claimed in any of claims 12 to 15 wherein the porous mass is provided in the form of a plurality of baffles.

17. An apparatus as claimed in claim 16 wherein the baffles comprise a metallic frame having the activated carbon coated or adhered thereon.

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18. An apparatus as claimed in claim 17 wherein the metallic frame comprises stainless steel.

19. An apparatus as claimed in any of claims 13 to 18 wherein the porous mass comprises activated carbon in the form of an agglomeration of pellets.

20. An apparatus as claimed in any of claims 16 to 19 wherein the baffles are interleaved.

21. An apparatus as claimed in any of claims 16 to 19 wherein the baffles are in a cone and plate configuration.

22. An apparatus as claimed in any of claims 13 to 21 wherein the means for cooling the porous mass includes a cooling coil through which a fluid coolant is circulated.

23. An apparatus as claimed in claims 22 wherein the coolant is water.

24. An apparatus as claimed in any one of claims 13 to 23 wherein the means for cooling includes heat conducting baffle plates.

25. An apparatus as claimed in any of claims 13 to 25 wherein the cooling means is arranged to cool the pump foreline adjacent the inlet.

26. An apparatus substantially as described herein and with reference to the Figures 1 to 3.

27. Use of the apparatus of any of the preceding claims in a dry pump inlet to abate the back flow of organic molecules originating from lubricant oil used to lubricate a drive mechanism of the dry pump.