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Title:
APPARATUS FOR USE IN A RADIATION SOURCE
Document Type and Number:
WIPO Patent Application WO/2021/073833
Kind Code:
A1
Abstract:
There is provided a container arranged to contain a waste product of a laser-produced plasma radiation source. The container comprises: a first portion which defines a chamber; and a second portion which at least partially defines an entrance to the chamber. In use, the waste product enters the chamber through the entrance. The second portion is formed from a material which comprises a ceramic material. The container may be particularly useful in a radiation source which is used in a lithographic system.

Inventors:
RIEPEN MICHEL (NL)
BROM PAUL (NL)
DE RUITER RIELLE (NL)
GEERDINK-GIELEN MARISE (NL)
MELISSOURGOS GEORGIOS (NL)
SCHOUT TWAN (NL)
Application Number:
PCT/EP2020/076092
Publication Date:
April 22, 2021
Filing Date:
September 18, 2020
Export Citation:
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Assignee:
ASML NETHERLANDS BV (NL)
International Classes:
H05G2/00
Domestic Patent References:
WO2019158492A12019-08-22
Foreign References:
US20180160518A12018-06-07
US20120248343A12012-10-04
US20030053594A12003-03-20
US20100243922A12010-09-30
US20080023657A12008-01-31
US20060193997A12006-08-31
US20160227637A12016-08-04
Attorney, Agent or Firm:
ASML NETHERLANDS B.V. (NL)
Download PDF:
Claims:
CLAIMS

1. A container arranged to contain a waste product of a laser-produced plasma radiation source, comprising: a first portion which defines a chamber; and a second portion which at least partially defines an entrance to the chamber; wherein, in use, the waste product enters the chamber through the entrance, and wherein the second portion is formed from a material which comprises a ceramic material.

2. The container according to claim 1, wherein the first portion is formed from a material which comprises molybdenum.

3. The container according to claim 1 or claim 2, wherein the ceramic material comprises boron and/or fluorine.

4. The container according to any of claim 1 to claim 3, wherein the ceramic material comprises: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and fluorine.

5. The container according to any of claim 1 to claim 3, wherein the ceramic material comprises a metal nitride.

6. The container according to claim 5, wherein the ceramic material comprises boron nitride.

7. The container according to claim 5, wherein the ceramic material comprises aluminium nitride.

8. A laser-produced plasma radiation source comprising the container according to any preceding claim.

9. A laser-produced plasma radiation source, comprising a component for use in a drainage system of the radiation source, wherein the component defines a chamber arranged to contain a waste product of the radiation source and wherein the component is formed from a material comprising a molybdenum.

10. A laser-produced plasma radiation source, comprising a component for use in a drainage system of the radiation source, wherein the component comprises a ceramic material comprising boron and/or fluorine.

11. A laser-produced plasma radiation source, comprising a component for use in a drainage system of the radiation source, wherein the component comprises a ceramic material comprising a metal nitride.

12. The laser-produced plasma radiation source of claim 10, wherein the ceramic material comprises: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and fluorine.

13. The laser-produced plasma radiation source of claim 10 or claim 11, wherein the component comprises a ceramic material comprising boron nitride.

14. The laser-produced plasma radiation source of claim 10, claim 11 or claim 13, wherein the component comprises a ceramic material comprising aluminium nitride.

15. The radiation source according to any of claim 8 to claim 14, wherein the radiation source further comprises a mechanism for providing hydrogen to one or more surfaces within the radiation source.

16. The radiation source according to any of claim 10 to claim 14, wherein the component at least partially defines an entrance to a chamber arranged to contain a waste product of the radiation source.

17. The radiation source according to any of claim 10 to claim 14, wherein the component is a pipe configured to transport a waste product of the radiation source.

Description:
APPARATUS FOR USE IN A RADIATION SOURCE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19203471.8 which was filed on October 16, 2019 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a radiation source. Particularly, the present invention relates to an apparatus suitable for transporting and/or containing a waste product of a radiation source, such as tin.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] EUV radiation may be generated using a laser-produced plasma (LPP) radiation source. An LPP radiation source may use a fuel such as liquid tin. After generating EUV radiation, tin may constitute a waste product of the radiation source. A drainage system may be used to remove a waste product (such as liquid tin) from the radiation source. In order to facilitate such removal of the waste product, parts of the drainage system may be maintained at a temperature above the melting point of the waste product so that the waste product can flow.

[0006] However, the waste product may accumulate in one or more components of the drainage system, particularly if components of the drainage system are not maintained above the melting point of the waste product. This may result in reduced flow and/or a blockage within the drainage system. Further, tin may be corrosive to components of the drainage system, and may therefore shorten their usable lifetime.

[0007] It may be desirable to overcome issues, such as those described above, associated with a waste product (such as tin) of a radiation source. Accordingly, embodiments of the present invention relate to new apparatuses suitable for transporting and containing a waste product of a radiation source. SUMMARY

[0008] According to a first aspect of the invention there is provided a container arranged to contain a waste product of a laser-produced plasma radiation source. The container may comprise a first portion. The first portion may define a chamber. The container may comprise a second portion. The second portion may at least partially define an entrance to the chamber. In use, the waste product may enter the chamber through the entrance. The second portion may be formed from a material which comprises a ceramic material.

[0009] It is the second portion, which at least partially defines an entrance to the chamber, which is formed from a material which comprises a ceramic material. In particular, one or more surfaces of the second portion which define the entrance to the chamber may be formed from a material which comprises a ceramic material. In some embodiments, the second portion may be formed from such a ceramic material. Alternatively, the second portion may be formed from another material, which may be coated by such a ceramic material.

[00010] The waste product of the radiation source may comprise tin.

[00011] The first portion may be referred to as a main body. The second portion may be referred to as a collar. The entrance to the chamber may be defined as an opening.

[00012] A laser-produced plasma (LPP) radiation source may generate radiation by providing droplets of liquid fuel, such as droplets of liquid tin, with energy via a laser beam. This liquid fuel may constitute a waste product of the radiation source. That is, the waste product may comprise liquid tin. It may be desirable to remove such a waste product from the radiation source. The radiation source may be provided with a drainage system. Any waste product within the radiation source may drain, via said drainage system, into a container. The container may also be referred to as a bucket. The container may be arranged so as to receive and store the waste product. In particular, the waste product may be received via the entrance to the chamber and may be stored within the chamber. [00013] The container advantageously comprises a first portion and a second portion. In particular, the first portion (which may constitute a main body of the container) may be formed from a first material and the second portion (which may constitute a collar of the container) may be formed from a second, different material. The material of the first portion and the material of the second portion may be selected to have different advantageous properties. For example, the material of the first portion may be advantageously suited for storing the waste product of the radiation source. The material of the second portion may be advantageously suited for being non-sticking and/or non wetting with the waste product.

[00014] The ceramic material from which the second portion is formed may be substantially non wetting to the waste product. If the waste product is incident on the second portion, it may be unlikely to wet a surface of the second portion. Further, the second portion may be substantially non-sticking with the waste product. The non-sticking properties of the ceramic material may facilitate the removal of the waste product from a surface of the second portion. Therefore, even though the second portion may be at a temperature which is lower than the melting point of the waste product, it is unlikely that any waste product will remain on a surface of the second portion. Advantageously, this may prevent a build-up of the waste product around the entrance to the chamber. This may prevent a blockage of the drainage system, of which the container forms part, from forming. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source and/or of a lithographic apparatus).

[00015] In particular, previous designs of container for use in a drainage system of a radiation source may not comprise a second portion formed from a ceramic material. These previous designs may be prone to developing a blockage due to a build-up of a waste product. In such previous designs, a waste product may wet a surface proximate to an opening in the container. In such previous designs, a waste product may stick to a surface proximate to an opening in the container. Therefore, over time, more and more of a waste product may be deposited proximate to an opening in the container. This may reduce efficiency of delivery of a waste product into such previous designs of container. This may eventually result in a blockage of a drainage system which makes use of such previous designs of container.

[00016] The new design of container, according to the first aspect of the present invention, substantially mitigates (and may even remove) the risk of such a blockage occurring. The new design of container, according to the first aspect of the present invention, therefore offers significant advantages over known containers.

[00017] Hydrogen gas may be provided within the radiation source. A mechanism for providing a flow of hydrogen gas within the radiation source may be provided. Provision of a flow of hydrogen gas across surfaces of components within the radiation source may assist in preventing a waste product from interacting with and/or building up on said surfaces.

[00018] Most materials containing metal may be generally non-wetting with the waste product in normal atmospheric conditions. This non-wetting property may be due to an oxide layer which forms on a surface of such materials. However, such an oxide layer may be removed in an environment containing hydrogen. In particular, in an environment in which hydrogen radicals are present, an oxide layer may be chemically reduced. This may result in increased wetting with the waste product. It may therefore be particularly challenging to prevent build-up of the waste product on a surface of the second portion due to the presence of hydrogen gas and/or hydrogen radicals.

[00019] A particular advantage of the container according to the first aspect of the present invention is that the ceramic material, even in the presence of hydrogen, may be generally non wetting with and generally non-sticking with the waste product. The second portion, formed from such a ceramic material, therefore provides the advantages described above even in the particularly challenging environment created in the presence of hydrogen. [00020] The first portion may be formed from a material which comprises molybdenum. The first portion may be formed from a material which comprises more than 90% molybdenum, for example more than 95% molybdenum. For example, the first portion may be formed from a material which comprises a titanium-zirconium-molybdenum alloy. Such a titanium-zirconium-molybdenum alloy may comprise 99.4% molybdenum, 0.5% titanium and 0.08% zirconium.

[00021] There may be negligible or no corrosion of the first portion, when formed from a material comprising molybdenum (such as, for example, a titanium-zirconium-molybdenum alloy), by the waste product. For example, there may be negligible or no corrosion of the first portion by liquid tin. In particular, there may be negligible or no corrosion of the first portion at a temperature at which the container may be held. This is particularly advantageous over known containers for storing a waste product of a radiation source (such as liquid tin), which may be formed from stainless steel. A waste product may react with stainless steel. Stainless steel may be corroded by a waste product. This may result in failure of such a container. Impurities, welding defects, and/or thermal stresses in the stainless steel may exacerbate this problem. Thermal gradients may exacerbate this problem.

[00022] TZM may be substantially resistant to hydrogen embrittlement. This is particularly advantageous due to the likelihood of hydrogen gas and or hydrogen radicals being present in a vicinity of the first portion.

[00023] The ceramic material may comprise boron and/or fluorine. Advantageously, ceramic materials comprising boron or fluorine have been found to be particularly non-wetting and non sticking for tin, even in a reducing environment in the presence of hydrogen gas and/or hydrogen radicals.

[00024] The ceramic material may comprise: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and fluorine.

[00025] Such a material may comprise the material marketed as MACOR™ by Corning Inc. which is incorporated in the USA. Such a material may enable the second portion to achieve the significant advantages described above by performing the first aspect of the invention. MACOR™ may be particularly non-sticking with the waste product (in particular, with liquid tin). MACOR™ may be particularly non-wetting with the waste product (in particular, with liquid tin). MACOR™, even in the presence of hydrogen, may be generally non-wetting to and generally non-sticking to the waste product. In particular, this material comprises boron and fluorine.

[00026] The ceramic material may comprise a metal nitride. Advantageously, metal nitrides (particularly boron nitride and aluminium nitride) have been found to be particularly non-wetting and non-sticking for tin, even in a reducing environment in the presence of hydrogen gas and or hydrogen radicals. Such metal nitrides may, for example be more suitable than may metal oxides.

[00027] The ceramic material may comprise boron nitride.

[00028] Such a material may enable the second portion to achieve the significant advantages described above by performing the first aspect of the invention. Boron nitride may be particularly non-sticking with the waste product (in particular, with liquid tin). Boron nitride may be particularly non- wetting with the waste product (in particular, with liquid tin). Boron nitride, even in the presence of hydrogen, may be generally non- wetting to and generally non-sticking to the waste product.

[00029] The boron nitride may be pyrolytic boron nitride.

[00030] That is, the ceramic material may comprise pyrolytic boron nitride. Such a material may enable the second portion to achieve the significant advantages described above by performing the first aspect of the invention. Pyrolytic boron nitride may be particularly non-sticking with the waste product (in particular, with liquid tin). Pyrolytic boron nitride may be particularly non-wetting with the waste product (in particular, with liquid tin). Pyrolytic boron nitride, even in the presence of hydrogen, may be generally non-wetting to and generally non-sticking to the waste product.

[00031] The ceramic material may comprise aluminium nitride.

[00032] The ceramic material may comprise boron nitride and aluminium nitride. Such a material may comprise the material marketed as SHAPAL™, SHAPAL™-M, and/or SHAPAL™ Hi-M Soft. Such a material may enable the second portion to achieve the significant advantages described above by performing the first aspect of the invention. Boron nitride and aluminium nitride may be particularly non-sticking with the waste product (in particular, with liquid tin). Boron nitride and aluminium nitride may be particularly non-wetting with the waste product (in particular, with liquid tin). Boron nitride and aluminium nitride, even in the presence of hydrogen, may be generally non wetting to and generally non-sticking to the waste product.

[00033] According to a second aspect of the invention there is provided a laser-produced plasma radiation source comprising the container according to the first aspect of the invention.

[00034] According to a third aspect of the invention there is provided a laser-produced plasma radiation source. The laser-produced plasma radiation source may comprise a component for use in a drainage system of the radiation source. The component may define a chamber arranged to contain a waste product of the radiation source. The component may be formed from a material comprising molybdenum.

[00035] The component may be formed from a material which comprises more than 90% molybdenum, for example more than 95% molybdenum. For example the component may be formed from a material which comprises a titanium-zirconium-molybdenum alloy. Such a titanium- zirconium-molybdenum alloy may comprise 99.4% molybdenum, 0.5% titanium and 0.08% zirconium.

[00036] The drainage system may be used to facilitate the removal of a waste product of the radiation source. The LPP radiation source according to the third aspect of the present invention is advantageous as there may be negligible or no corrosion of the component, when formed from a material comprising molybdenum (such as, for example, TZM), by the waste product. For example, there may be negligible or no corrosion of the component by liquid tin. The LPP radiation source according to the third aspect of the present invention is particularly advantageous over an LPP radiation source which comprises a component formed from, for example, stainless steel. For example, a waste product of a radiation source may react with a component formed from stainless steel. Stainless steel may be corroded by a waste product. This may result in failure of such a component. Impurities, welding defects, thermal gradients and/or thermal stresses in the stainless steel may exacerbate this problem.

[00037] TZM may be substantially resistant to hydrogen embrittlement. This is particularly advantageous due to the likelihood of hydrogen gas and or hydrogen radicals being present in a vicinity of the component within the radiation source.

[00038] The component may be referred to as a container. Advantageously, there may be negligible or no corrosion of the container by the waste product.

[00039] According to a fourth aspect of the invention there is provided a laser-produced plasma radiation source comprising a component for use in a drainage system of the radiation source, wherein the component comprises a ceramic material comprising boron and or fluorine.

[00040] The component of the radiation source according to the fourth aspect of the invention comprises a ceramic material. It will be appreciated that this is intended to include both: embodiments wherein the entire component is formed from such a ceramic material or, alternatively, embodiments wherein the component is formed from another material, which may be coated by such a ceramic material.

[00041] The LPP radiation source according to the fourth aspect of the present invention is advantageous as the ceramic material may be substantially non-wetting to the waste product. Advantageously, ceramic materials comprising boron or fluorine have been found to be particularly non-wetting and non-sticking for tin, even in a reducing environment in the presence of hydrogen gas and/or hydrogen radicals. If the waste product is incident on the component, it may be unlikely to wet a surface of the component. Further, the ceramic material may be substantially non-sticking with the waste product. The non-sticking properties of the ceramic material may facilitate the removal of the waste product from a surface of the component. Advantageously, this may prevent a build-up of the waste product on the component. This may prevent a blockage of the drainage system. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source and/or of a lithographic apparatus).

[00042] The ceramic material may comprise: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and fluorine. The drainage system may be used to facilitate the removal of a waste product of the radiation source. The ceramic material may comprise the material marketed as MACOR™ by Corning Inc. which is incorporated in the USA. Additionally or alternatively, the ceramic material may comprise boron nitride.

[00043] According to a fifth aspect of the invention there is provided a laser-produced plasma radiation source comprising a component for use in a drainage system of the radiation source, wherein the component comprises a ceramic material comprising a metal nitride. The ceramic material may comprise boron nitride and/or aluminium nitride.

[00044] The component of the radiation source according to the fifth aspect of the invention comprises a ceramic material. It will be appreciated that this is intended to include both: embodiments wherein the entire component is formed from such a ceramic material or, alternatively, embodiments wherein the component is formed from another material, which may be coated by such a ceramic material.

[00045] The drainage system may be used to facilitate the removal of a waste product of the radiation source. The LPP radiation source according to the fifth aspect of the present invention is advantageous as the ceramic material may be substantially non-wetting to the waste product. Advantageously, metal nitrides (particularly boron nitride and aluminium nitride) have been found to be particularly non-wetting and non-sticking for tin, even in a reducing environment in the presence of hydrogen gas and or hydrogen radicals. Such metal nitrides may, for example be more suitable than may metal oxides. If the waste product is incident on the component, it may be unlikely to wet a surface of the component. Further, the ceramic material may be substantially non-sticking with the waste product. The non-sticking properties of the ceramic material may facilitate the removal of the waste product from a surface of the component. Advantageously, this may prevent a build-up of the waste product on the component. This may prevent a blockage of the drainage system. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source and/or of a lithographic apparatus).

[00046] The ceramic material may comprise boron nitride and or aluminium nitride. The boron nitride may comprise pyrolytic boron nitride.

[00047] The ceramic material may comprise the material marketed as SHAPAL™, SHAPAL™- M, and or SHAPAL™ Hi-M Soft.

[00048] The radiation source according to the second, third, fourth or fifth aspect of the invention may further comprise a mechanism for providing hydrogen. The hydrogen may be provided to one or more surfaces within the radiation source.

[00049] Provision of a flow of hydrogen gas across one or more surfaces within the radiation source may assist in preventing a waste product of the radiation source from interacting with and or building up on said surfaces.

[00050] Most materials containing metal may be generally non-wetting with the waste product in normal atmospheric conditions. This non-wetting property may be due to an oxide layer which forms on a surface of such materials. However, such an oxide layer may be removed in an environment containing hydrogen. In particular, in an environment in which hydrogen radicals are present, an oxide layer may be chemically reduced. This may result in increased wetting with the waste product. It may therefore be particularly challenging to prevent build-up of the waste product on a surface of the second portion due to the presence of hydrogen gas and/or hydrogen radicals. [00051] A particular advantage of forming a component from the materials given in the second, third, fourth or fifth aspect of the invention is that these materials, even in the presence of hydrogen, may be generally non- wetting with and generally non-sticking with the waste product and/or may be generally resistant to corrosion from a waste product such as tin. The LPP radiation sources according to the second, third, fourth or fifth aspect of the invention therefore provide significant advantages even in the particularly challenging environment created in the presence of hydrogen.

[00052] The component of the radiation source according to the third, fourth or fifth aspect of the invention may at least partially define an entrance to a chamber. Said chamber may be arranged to contain a waste product of the radiation source.The component may correspond to the “second portion” according to the first aspect of the present invention. The component may be referred to as a collar. The collar may be substantially non-wetting to the waste product. If the waste product is incident on the collar, it may be unlikely to wet a surface of the collar. Further, the collar may be substantially non- sticking with the waste product. The non-sticking properties of the ceramic material may facilitate the removal of the waste product from a surface of the collar. Advantageously, this may prevent a build-up of the waste product on the collar. This may prevent a blockage of the drainage system. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source and or of a lithographic apparatus). [00053] The component of the radiation source according to the third, fourth or fifth aspect of the invention may comprise a pipe. The pipe may be configured to transport a waste product of the radiation source.

[00054] The pipe may be substantially non-wetting to the waste product. If the waste product is incident on the pipe, it may be unlikely to wet a surface of the pipe. Further, the pipe may be substantially non- sticking with the waste product. The non-sticking properties of the ceramic material may facilitate the removal of the waste product from a surface of the pipe. Advantageously, this may prevent a build-up of the waste product on the pipe. This may prevent a blockage of the drainage system. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source and or of a lithographic apparatus). [00055] The waste product of the radiation source according to the second, third, fourth or fifth aspect of the invention may comprise tin.

BRIEF DESCRIPTION OF THE DRAWINGS

[00056] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a schematic representation of a side view of a lithographic system comprising a lithographic apparatus and a radiation source;

Figure 2 depicts a three-dimensional rendering of a container for use in a radiation source; Figure 3 depicts a schematic representation of a side view of the container depicted in Figure

2; and

Figure 4 depicts a process by which the container of Figures 2 and 3 may be inserted into a drainage system of a radiation source.

DETAILED DESCRIPTION

[00057] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00058] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00059] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00060] A relative vacuum, i.e. a small amount of gas at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00061] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a carbon dioxide (CO2) laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma.

[00062] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00063] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[00064] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[00065] The process of converting tin droplets into tin plasma 7 is a highly energetic process. Any tin which is not converted into tin plasma 7 may be ejected from the plasma formation region 4 as a result of interacting with the laser beam 2. Tin which is not converted into tin plasma 7 may be incident on internal walls of the enclosing structure 9 of the radiation source SO and/or other components within the radiation source SO. Tin plasma 7 may also spread from the plasma formation region 4 (for example, as a result of interacting with the laser beam 2). As electrons and ions of the tin plasma 7 recombine (thereby generating radiation including EUV radiation), tin atoms are formed. These tin atoms may be incident on internal walls of the enclosing structure 9 of the radiation source SO and/or other components within the radiation source SO.

[00066] As described above, a small amount of gas at a pressure well below atmospheric pressure, may be provided in the radiation source SO. Hydrogen gas may be provided within the radiation source SO. A mechanism for providing a flow of hydrogen gas within the radiation source SO may be provided. Provision of a flow of hydrogen gas across surfaces of components within the radiation source SO may assist in preventing tin from interacting with and/or building up on said surfaces. [00067] The radiation source SO may be provided with a drainage system. Any tin within the enclosing structure 9 may drain, via said drainage system, into a container 20. The container 20 may also be referred to as a bucket. In particular, surfaces within the enclosing structure 9 may be so configured as to facilitate drainage of tin into the container 20. The container 20 may form part of the drainage system of the radiation source SO. A pipe 30 may form part of the drainage system of the radiation source SO. The pipe 30 may be used to transport tin to the container 20. Tin may drain from within the enclosing structure 9, through the pipe 30, into the container 20. The drainage system may comprise one or more pipes.

[00068] It may be desirable for tin which has drained into the container 20 to be in liquid form. This may be desirable as liquid tin, after being delivered to the container 20, may collect at one end of the container 20 (determined by gravity). The container 20 may be provided with a heating apparatus. The heating apparatus may heat the container 20 to a suitable operating temperature. The suitable operating temperature of the container 20 may be above the melting point of tin. It will be appreciated that a temperature of the radiation source SO (particularly within the enclosing structure 9) may be significantly higher than a temperature at which the container 20 operates.

[00069] In some embodiments, the container 20 may be located in, and or considered to be part of, the radiation source SO. In other embodiments, the container 20 may be at least partially located outside of, and/or considered to at least partially not form part of, the radiation source SO. In embodiments where the container 20 is located outside a vessel of the radiation source SO, there may be some differences from embodiments where the container is located inside the vessel of the radiation source SO, but the overall concept may remain the same.

[00070] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source (which, it will be appreciated, may also produce a waste product such as tin) may be used to generate EUV radiation. [00071] Figure 2 depicts a three-dimensional rendering of the container 20, according to an embodiment of the present invention.

[00072] The container 20 comprises: a main body 21; and a collar 25. The main body 21 may be referred to as a first portion. The collar 25 may be referred to as a second portion.

[00073] The container 20 also comprises: a plurality of connection points 22; a plurality of movement limiters 23; an opening 24; a recess 26; and a tray 27 having a splash cover 28.

[00074] The main body 21 is substantially cuboidal. Two edges of the main body 21 are chamfered, creating chamfered edges 21b, 21c. It will be appreciated that in other embodiments of container 20 the main body 21 may have a different shape. The main body 21 of the container 20 may be formed from multiple individual components. This may be advantageous for manufacturing considerations. The main body 21 is generally hollow. That is, the main body 21 generally defines a cavity. The cavity may be referred to as a reservoir. The opening 24 is an opening in a top surface 21a of the main body 21. It will be appreciated that the top surface 21a of the main body 21 is intended to refer to a surface of the main body 21 which, in use, faces a direction which is generally opposite to a direction in which gravity acts. That is, the top surface 21a, in use, may be described as generally being above other surfaces of the main body 21. The opening 24 may be described as an aperture or cutout. The opening 24 provides fluid communication between the cavity within the main body 21 and an environment in which the container 20 is disposed.

[00075] The plurality of connection points 22 may allow connection of the container 20 to an outer frame of the drainage system. Alternatively, the plurality of connection points may allow connection of the container 20 to any other component. It will be appreciated that each connection point 22 may comprise part of any standard mechanism for fastening one component to another, as is well known in the art. The plurality of movement limiters 23 may prevent unwanted movement of the container 20. In particular, the plurality of movement limiters 23 may prevent unwanted movement of the container 20 when the container 20 is used as part of the drainage system. The plurality of movement limiters 23 project from an outer dimension of the main body 21.

[00076] The collar 25 is a separate component to a component which forms part of the main body 21. The collar 25 is in contact with a top surface 21a of the main body 21. The collar 25 extends from the top surface 21a. The collar 25 extends from the top surface 21a in a direction which is generally perpendicular to a main plane of the top surface 21a. It will be appreciated that, in alternative embodiments, the collar 25 may extend in a different direction. The collar 25 extends outwards from the main body 21. That is, the collar 25 extends from the top surface 21a in a direction which is opposite the direction in which the cavity of the main body 21 is disposed.

[00077] The collar 25, in the cross-section perpendicular to the direction in which the collar 25 extends from the top surface 21a and in a portion of the collar which extends above the top surface 21a, is generally U-shaped. This portion of the collar 25, which extends above the top surface 21a of the main body 21, may be referred to as an upper portion 25e of the collar 25.

[00078] The collar 25, in the cross-section perpendicular to the direction in which the collar 25 extends from the top surface 21a and in a portion of the collar which generally does not extend above the top surface 21a, has a shape which is generally rectangular. This portion of the collar 25, which generally does not extend above the top surface 21a of the main body 21, may be referred to as a lower portion 25f of the collar 25. This lower portion 25f comprises material only at a perimeter of the lower portion 25f such that the lower portion 25f comprises a central aperture. The lower portion 25f forms a closed shape (which extends partially into the main body 21 so as to define the opening 24). [00079] By virtue of the collar 25 being generally U-shaped in the upper portion 25e of the collar 25, the collar 25 defines a notch 25 a between two generally mutually perpendicular extended portions 25b, 25c. The notch 25a may be described as an open portion of the collar 25. The collar 25 is disposed so as to partially surround the opening 24. In particular, the collar 25 defines an edge of the opening 24. There is fluid communication between the cavity within the main body 21 and an environment in which the container 21 is disposed, via a conduit defined by the opening 24 and the collar 25.

[00080] The container 20 is orientated at an angle with respect to the horizontal - i.e. with respect to the ground (which may be in the y-direction). This is because the radiation source SO is also angled with respect to the horizontal. In alternative embodiments, the container 20 may be horizontally orientated (i.e., a base of the container 20 may be orientated parallel with the y-direction, i.e. the ground).

[00081] The recess 26 is defined by a portion of the top surface 21a which is adjacent to the notch 25a of the collar 25. In particular, the recess 26 is defined by a portion of the top surface 21a which extends from the top surface 21a in the same direction in which the cavity of the main body 21 is disposed. The recess 26 may protrude only slightly from the top surface 21a. An edge of the recess 26 is proximate to the collar 25. In particular, the extended portions 25b, 25c of the collar 25 partially surround a portion of the recess 26 such that a base of the notch 25a is defined by said portion of the recess 26.

[00082] The recess 26 extends from the collar 25 to an edge of the top surface 21a of the main body 21. The tray 27 comprises a generally cuboidal component with an open face. The tray 27 may be substantially smaller than the main body 21 of the container 20. The tray 27 is disposed on a side surface 21d of the main body 21. The tray 27 is disposed such that the open face of the tray 27 is proximate to the edge of the top surface 21a of the main body 21 to which the recess 26 extends. The splash cover 28 comprises a generally rectangular component. The splash cover 28 may be described as a sheet. The splash cover 28 may be affixed to the tray 27. The splash cover 28 may be affixed to a face of the tray 27 which is opposite a face of the tray which is disposed on the side surface 2 Id of the main body 21. The splash cover 28 extends from the tray 27 so as to at least partially cover the edge of the top surface 21a of the main body 21 to which the recess 26 extends.

[00083] The main body 21 may be formed from a material which comprises molybdenum. Advantageously, molybdenum has a high resistance to corrosion from tin, which makes materials comprising molybdenum particularly suitable materials from which to form the main body 21. The main body 21 may further comprise one or more additional materials to increase a strength of the main body 21. The main body 21 may be formed from a material which comprises titanium. The main body 21 may be formed from a material which comprises zirconium. The main body 21 may be formed from a material which comprises a titanium-zirconium-molybdenum alloy (referred to as TZM). Such a titanium-zirconium-molybdenum alloy may comprise 99.4% molybdenum, 0.5% titanium and 0.08% zirconium. This may be formed by adding TiC and ZrC to molybdenum so as to improve the strength properties of the material (relative to pure molybdenum).

[00084] The collar 25 may be formed from a material which comprises a ceramic material. [00085] The collar 25 may be formed from a material which comprises any combination of: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and/or fluorine. In particular, the collar 25 may be formed from a material which comprises: silicon oxide; magnesium oxide; aluminium oxide; potassium oxide; boron oxide; and fluorine. The material may comprise the material marketed as MACOR™ by Corning Inc. which is incorporated in the USA.

[00086] The collar 25 may be formed from a material which comprises boron nitride (BN). The collar 25 may be formed from a material which comprises aluminium nitride (AIN). The collar 25 may be formed from a material which comprises boron nitride and aluminium nitride. The material may comprise the material marketed as SHAPAL™, SHAPAL™-M, and/or SHAPAL™ Hi-M Soft. The collar 25 may be formed from a material which comprises pyrolytic boron nitride, which may be referred to as PBN.

[00087] Several oxide compounds have been given herein. It will be appreciated that use of the word “oxide” in a compound may refer to any appropriate oxide compound (such as dioxide, trioxide, etc.). In particular, silicon oxide may refer to a silicon atom bonded with two oxygen atoms (S1O2), which may be referred to as silica. Magnesium oxide may refer to a magnesium atom bonded with an oxygen atom (MgO), which may be referred to as magnesia. Aluminium oxide may refer to two aluminium atoms bonded with three oxygen atoms (AI2O3), which may be referred to as alumina. Potassium oxide may refer to two potassium atoms bonded with one oxygen atom (K2O). Boron oxide may refer to two boron atoms bonded with three oxygen atoms (B2O3).

[00088] Figure 3 depicts a schematic representation of a side view of the container 20.

[00089] In use, the container 20 may be arranged so as to interface with a pipe of the drainage system (such as the pipe 30 shown in Figure 1). Figure 3 shows an outline of a section of the pipe 30 interfacing with the container 20. An end of the pipe 30 is proximate to the opening 24 of the container 20. In particular, the end of the pipe 30 is at least partially surrounded by the collar 25 (in Figure 3, the dashed line of the pipe 30 shows a section of the pipe which is partially surrounded by the collar 25). The end of the pipe 30 is at least partially surrounded by the upper portion 25e of the collar 25. The upper portion 25e of the collar 25 may be described as a partial section of a pipe. The lower portion 25 f of the collar 25 may be described as a full section of a pipe (as, unlike the upper portion 25e, the lower portion 25f does not comprise a notch and the lower portion 25f is therefore a closed shape). The collar 25 may interface with the pipe 30 of the drainage system. When the pipe 30 interfaces with the collar 25, this may be described as a formation of a composite pipe, comprising the pipe 30, the upper portion 25e of the collar 25e, and the lower portion 25f of the collar 25. The collar 25 further comprises a flange portion 25d. The flange potion 25d is arranged around an edge of the collar 25 such that, when the pipe 30 interfaces with the collar 25, the flange portion 25d is proximate to the pipe 30. The flange portion 25d increases an extent to which the upper portion 25e of the collar 25 partially surrounds the pipe 30. [00090] As described above, tin may drain from within the enclosing structure 9 of the radiation source SO, through one or more pipes (such as the pipe 30), into the container 20. A flow of tin 31 through the pipe 30 is depicted in Figure 3. The flow of tin 31 exits the end of the pipe 30. The flow of tin 31 then passes through the opening 24. The flow of tin 31 then enters the cavity of the main body 21 of the container 20. The container 20 may thereby receive and collect tin from the drainage system of the radiation source SO.

[00091] Known containers for storing liquid tin are formed from stainless steel. Liquid tin may react with stainless steel. Stainless steel may be corroded by liquid tin. For example, in known containers for storing liquid tin, approximately 100 um of stainless steel may be removed from a stainless steel surface for each year such a container is in operation. This may result in failure of such a container. Impurities in the stainless steel may exacerbate this problem. Welding defects (from the formation of the stainless steel container) may exacerbate this problem. Thermal stresses (from the formation of the stainless steel container) may exacerbate this problem. Temperature gradients within the container may also significantly exacerbate this problem.

[00092] As described above, according to an embodiment of the present invention, the main body 21 may be formed from a material which comprises molybdenum (for example TZM, as described above). Advantageously, there may be negligible or no corrosion of the main body 21 (formed from a material comprising molybdenum) by liquid tin. In particular, there may be negligible or no corrosion of the main body 21 at a temperature at which the container 20 is held.

[00093] TZM may have a thermal conductivity which is approximately 9 times greater than that of stainless steel. Therefore, advantageously, heating up and cooling down times of the main body 21 (formed from a material comprising TZM) may be shorter than those of a main body of a container formed from stainless steel. This may result in advantages for throughput (for example, of the radiation source SO and of the lithographic apparatus LA). It may also, advantageously, be possible to use a design of heating apparatus which is simpler than a design of heating apparatus which is necessary for a stainless steel container. Such an improved design of heating apparatus may comprise one or more heating elements which are separate from the container 20. Such an improved design of heating apparatus may comprise one or more heating elements which do not need to be removed from the radiation source SO when the cavity of the container 20 is generally filled with tin.

[00094] TZM may have a thermal expansion coefficient which is approximately 3 times lower than that of stainless steel. Therefore, advantageously, thermal stresses within the main body 21 (formed from a material comprising TZM) may be lower than those of a main body of a container formed from stainless steel. This may, advantageously, enable a simpler design of main body 21 to be formed (compared with a main body of a stainless steel container).

[00095] As described above, when the container 20 is used as part of the drainage system of the radiation source SO, the flow of tin 31 may exit the end of the pipe 30. When the container 20 is in use, the end of the pipe 30 is at least partially surrounded by the collar 25 (in particular, by the upper portion 25e of the collar 25). The collar 25may interface with the pipe 30 of the drainage system so as to form of a composite pipe. Advantageously, this may result in efficient delivery of tin to the container 20. In particular, this may result in efficient delivery of tin to the cavity of the main body 21 of the container 20 through the opening 24.

[00096] The flow of tin 31 may generally comprise liquid tin. Tin, after exiting the pipe 30, may be incident on the collar 25. Tin, after entering the cavity of the container 20, may splash back from a surface of the cavity towards the opening 24. The container 20 may be heated. In particular, a heating apparatus may heat the main body 21 to above the melting point of tin such that tin disposed within the cavity of the main body 21 is in a liquid state. However, a temperature of the collar 25 may be lower than the melting point of tin. This may be due to thermal properties of the collar 25 and/or proximity of the collar 25 to heating elements of the heating apparatus. For example, the collar 25 may be formed from a material having a relatively low heat conduction coefficient (for example a material having a heat conduction coefficient that is lower than that of a material from which the main body 21 of the container 20 is formed). Furthermore, there is a contact resistance between the collar 25 and the main body 21 of the container 20. Therefore, although the main body 21 of the container 20 may be maintained at a temperature of the order of 250 °C, the temperature of the collar 25 may be below the melting point of tin (232 °C). Even in situations whereby it is intended to heat the container 20 such that all parts of the container 20 are above the melting point of tin, in practice the temperature of the collar 25 may be below the melting point of tin.

[00097] As described above, according to an embodiment of the present invention, the collar 25 may be formed from a ceramic material which comprises MACOR™. MACOR™ is substantially non-wetting to liquid tin. If tin is incident on the collar 25 (formed from MACOR™), it may be unlikely to wet a surface of the collar 25. Further, MACOR™ is substantially non-sticking with liquid tin. The non-sticking properties of MACOR™ may facilitate the removal of tin from a surface of the collar 25. Therefore, even though the collar 25 may be at a temperature which is lower than the melting point of tin, it is unlikely that any tin will remain on a surface of the collar 25. Advantageously, this may prevent a build-up of tin around the opening 24. This may prevent a blockage of the drainage system from forming. This is advantageous as any blockage of the drainage system may result in a substantial reduction in throughput (for example, of the radiation source SO and of the lithographic apparatus LA).

[00098] In particular, previous designs of container for use in a drainage system of a radiation source do not comprise a collar (such as the collar 25) formed from a ceramic material (such as MACOR™). These previous designs are prone to developing a blockage due to a build-up of tin. In such previous designs, tin may wet a surface proximate to an opening in the container. In such previous designs, tin may stick to a surface proximate to an opening in the container. Therefore, over time, more and more tin may be deposited proximate to an opening in the container. This reduces efficiency of tin delivery into such previous designs of container. This eventually results in a blockage of a drainage system which makes use of such previous designs of container.

[00099] The new design of container 20, according to an embodiment of the present invention, substantially mitigates (and may even remove) the risk of such a blockage occurring. The new design of container 20, according to an embodiment of the present invention, therefore offers significant advantages over known containers.

[000100] As described above, the radiation source SO may comprise a mechanism for providing hydrogen to surfaces of components within the radiation source SO. Such hydrogen may be delivered in the form of a hydrogen gas. High energy radiation within the radiation source SO may generate hydrogen radicals from the hydrogen gas. Hydrogen gas and/or hydrogen radicals may propagate through the drainage system of the radiation source SO. Hydrogen gas and/or hydrogen radicals may therefore be present in a vicinity of the container 20.

[000101] Most materials containing metal may be generally non-wetting to liquid tin in normal atmospheric conditions. This non- wetting property may be due to an oxide layer which forms on a surface of such materials. However, such an oxide layer may be removed in an environment containing hydrogen. In particular, in an environment in which hydrogen radicals are present, an oxide layer may be chemically reduced. This may result in increased wetting with tin. It may therefore be particularly challenging to prevent build-up of tin on a surface of the collar 25 due to the presence of hydrogen gas and or hydrogen radicals.

[000102] A particular advantage of the container 20 according to an embodiment of the present invention is that MACOR™, even in the presence of hydrogen, is generally non-wetting to tin and generally non-sticking to tin. The collar 25, formed from a material comprising MACOR™, therefore provides the advantages described above even in the particularly challenging environment created in the presence of hydrogen.

[000103] It will be appreciated that advantages that may be achieved by using a collar 25 formed from a material comprising MACOR™, as described above, may be achieved by using a collar 25 formed from any other of the materials given. In particular, the same or similar advantages may be achieved by using a collar 25 formed from a material comprising: boron nitride; boron nitride and aluminium nitride (which may be marketed as SHAPAL™, SHAPAL™-M, and or SHAPAL™ Hi-M Soft); or pyrolytic boron nitride.

[000104] Another advantage of the container 20 according to an embodiment of the present invention is that TZM (from which the main body 21 may be formed, as described above) is substantially resistant to hydrogen embrittlement. This is particularly advantageous due to the likelihood of hydrogen gas and/or hydrogen radicals being present in a vicinity of the main body 21. Known designs of container may be formed from stainless steel, which is susceptible to corrosion from tin due to the corrosion of iron (the main component of stainless steel) by tin. Therefore, the container 20 according to an embodiment of the present invention is particularly advantageous over known designs of container.

[000105] The container 20 may be a replaceable component of the drainage system of the radiation source SO. The container 20 may be inserted into the drainage system. The container 20 may be removed from the drainage system. In particular, the container 20 may be removed from the drainage system when the cavity of the main body 21 contains a certain amount of tin. A new container (which may, for example, be equivalent to the container 20) may then be inserted into the drainage system. [000106] The container 20 may be reusable. The container 20 may be removed from the drainage system when the cavity of the main body 21 contains a certain amount of tin. Tin may be substantially removed from the container 20. Said tin may be in a liquid state. This may facilitate the removal of tin from the container 20. The container 20 may then be reinserted into the drainage system.

[000107] Figures 4a and 4b depict a process by which the container 20 may be inserted into the drainage system of the radiation source SO. Figure 4a shows a relative position of the pipe 30 and the container 20 before the container 20 is inserted into the drainage system. Figure 4b shows a relative position of the pipe 30 and the container 20 once the container 20 is inserted into the drainage system (this is also shown in Figure 3). The pipe 30 and the container 20 are shown against the same fixed background grid in Figures 4a and 4b.

[000108] When inserting the container 20 into the drainage system, the pipe 30 of the drainage system may remain fixed relative to the radiation source SO.. To insert the container 20 into the drainage system, the container 20 may be moved in the insertion direction 32.

[000109] Advantageously, the U-shaped cross-section of the upper portion 25e of the collar 25 allows the container 20 to be inserted into the drainage system such that the collar 25 at least partially surrounds the end of the pipe 30, thereby forming a composite pipe. In particular, the notch 25a provides a space through which the end of the pipe 30 may pass when the container 20 is being inserted into the drainage system. The notch 25a and the flared portion 25d (see Figure 2) therefore provide a mechanism through which an advantageous coupling between the container 20 and the pipe 30 may be achieved.

[000110] Tin may be incident on the top surface 21a of the main body 21. The drainage system may be configured such that any tin which is incident on the top surface 21a may generally be incident on the portion of the top surface 21a which constitutes the recess 26. The notch 25a of the collar 25 and the recess 26 may be arranged such that any tin which propagates through the notch 25a (e.g., tin which splashes out of the opening 24 from the cavity of the main body 21, tin which is incident on the collar 25 directly from the pipe 30) will be in the recess 26.

[000111] The container 20 may be orientated at an angle with respect to the horizontal - i.e., with respect to the ground - as described above and as depicted in Figures 3, 4a, and 4b. TZM (from which the main body 21 may be formed, as described above) may be non-wetting to tin. TZM may be non sticking to tin. Tin (which may generally be in a liquid state) which is disposed on the recess 26 may propagate, under the action of gravity, towards the edge of the top surface 21a proximate to which the tray 27 is disposed. Such propagation of tin may be advantageously facilitated by the non-wetting and non-wetting properties of TZM. One or more edges of the recess may guide the trajectory of tin such that the tin propagates towards the tray 27. Tin may enter the tray 27. The splash cover 28 may facilitate the receipt and/or retention of tin in the tray 27.

[000112] Advantageously, the recess 26, tray 27, and splash cover 28 may prevent the flow of tin 31 from contaminating the drainage system or other components. The recess 26, tray 27, and splash cover 28 may collect tin which exits from the pipe 30. The recess 26, tray 27, and splash cover 28 may collect tin which is incident on the container 20 when the container 20 is being inserted into or removed from the drainage system.

[000113] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid- crystal displays (LCDs), thin-film magnetic heads, etc.

[000114] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[000115] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[000116] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.