Seal assembly

Abstract

A seal assembly which has a housing that defines a channel. A conduit first end, of a conduit which has a central axis, extends into the channel. The conduit first end defines a tip portion of the conduit. The seal assembly further has a radial protrusion that is disposed adjacent the tip portion of the conduit first end. The seal assembly further has a seal element, which extends about the tip portion of the conduit first end. The seal element is fully enclosed by the housing, the tip portion of the conduit, and the radial protrusion. The seal assembly further has a biasing assembly, which is configured to exert an axial compressive load on the seal element via the radial protrusion and the housing. The seal element is configured to urge against the housing, the tip portion of the conduit, and the radial protrusion by the axial compressive load.

Description

SEAL ASSEMBLYCROSS REFERENCE TO EARLIER APPLICATIONS

[0001] The present application claims priority benefits of European application No. 24215703.0, filed 27 November 2024.FIELD OF INVENTION

[0002] The present invention relates to a seal assembly associated with an EUV radiation source. The present invention also relates to a target material preparation assembly, a droplet generation assembly, an EUV radiation source, and an EUV exposure system.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] 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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. 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 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] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses a reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patternedmask inspection and can be carried out for the quality control of EUV patterned masks. For example, the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourth, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] One type of radiation source for producing EUV radiation is a laser produced plasma (LPP) radiation source. In an LPP radiation source, fuel droplets, which are produced using a fuel droplet nozzle assembly, are irradiated with a laser in order to form a plasma which will emit EUV radiation. The fuel droplet nozzle assembly is supplied with fuel from a fuel preparation assembly. The fuel is provided to the fuel droplet nozzle assembly in fluid form. The fuel preparation assembly includes a number of seal assemblies. It may be desirable to provide a seal assembly which at least partially addresses one or more problems in known seal assemblies that are used in such fuel preparation assemblies, whether identified herein or otherwise.SUMMARY

[0007] In a first aspect of the invention there is provided a seal assembly for an EUV radiation source. The seal assembly comprises a housing. The housing defines a channel. The seal assembly further comprises a conduit. The conduit defines a central axis, and a conduit first end. The conduit first end extends into the channel. The conduit first end defines a tip portion of the conduit. The conduit is configured to transfer a target material, typically in the form of a target fluid. The seal assembly further comprises a radial protrusion that is disposed adjacent the tip portion of the conduit first end. The seal assembly further comprises a seal element. The seal element extends about the tip portion of the conduit first end. The seal element is fully enclosed by the housing, the tip portion of the conduit, and the radial protrusion. The seal assembly further comprises a biasing assembly. The biasing assembly is configured to exert an axial compressive load on the seal element via the radial protrusion and the housing. Upon exertion of the axial compressive load, the seal element is configured to urge against the housing, the tip portion of the conduit, and the radial protrusion.

[0008] The first end and second end of the conduit may be understood to refer to a first end region and a second end region of the conduit. The seal element extending about the first end of the conduit may be understood to mean that the seal element extends about the first end region of the conduit.

[0009] The conduit may completely encircle the central axis.

[0010] The target material may be tin.

[0011] Since the seal element is fully enclosed and urges against the housing, the tip portion of the conduit, and the radial protrusion upon exertion of the axial load, the fluid pressure that the seal elementis able to withstand without fluid leakage is advantageously improved (compared with a conventional seal arrangement). This is because there is limited space available for the seal element to deform into when compressed via the axial load. Were part of the seal element exposed, upon compression of the seal element, the seal element would deform into the region adjacent the exposed region of the seal element, which reduces the sealing pressure of the seal element. Since the seal element of the present invention is fully enclosed, the sealing pressure of the seal element is advantageously increased, which allows the seal element to withstand higher fluid pressure as compared to known seal assemblies.

[0012] The seal element may define a first end that tapers radially inwardly and a second end. The channel of the housing may define a tapered surface. The tapered surface may be engaged by the first end of the seal element.

[0013] Where the seal element defines a first end that tapers radially inwardly and a second end, and the channel of the housing defines a tapered surface that is engaged by the first end of the seal element, assembly of the seal assembly is more efficient. This is because the user is able to insert the seal element into the housing more readily. Furthermore, the tapered surface of the housing advantageously makes maintenance of the seal assembly more efficient. This is because the tapered surface is more accessible to a user during cleaning, as compared to, for example, a 90 degree comer.

[0014] A circumferential groove may be arranged in the tip portion of the conduit.

[0015] The tip portion may define the circumferential groove.

[0016] Where the tip portion of the conduit defines a circumferential groove, the seal element is better retained on the tip portion of the conduit first end. This is as compared to where the groove is not provided. Better retaining the seal element advantageously reduces the likelihood of the seal element being inadvertently removed from the tip portion of the conduit during removal of the conduit, and seal element, from the housing, such as during maintenance of the seal assembly and / or tin handling system. This advantageously makes maintenance of the seal assembly and / or tin handling system more efficient.

[0017] The biasing assembly may comprise a first load transfer member. The first load transfer member may extend radially outwards from, and be secured to, the conduit adjacent the first end of the conduit. The biasing assembly may further comprise a second load transfer member. The second load transfer member may extend radially outwards with respect to the central axis. The second load transfer member may be axially spaced apart from the first load transfer member. The second load transfer member may be connected to the housing. The biasing assembly may further comprise at least one biasing member. The at least one biasing member may be disposed between the first load transfer member and the second load transfer member such that the at least one biasing member urges against the first load transfer member and the second load transfer member to provide the axial load.

[0018] The at least one biasing member may comprise a spring. The spring may be a disc spring.

[0019] Where the biasing assembly comprises a first load transfer member, a second load transfer member, and at least one biasing member, a more consistent sealing pressure can be provided. This isbecause in this arrangement, the number of tolerances to be accounted for is reduced, which allows the seal assembly to be assembled with improved accuracy.

[0020] The seal assembly may further comprise an annular member. The annular member may extend from the second load transfer member to the housing.

[0021] Where an annular member is provided that contacts the housing, the annular member advantageously functions as a heat sink, which removes heat from the housing. This advantageously reduces the likelihood of the housing overheating in use. Overheating of the housing is undesirable because it can lead to the pressure of the fluid in the conduit increasing, which increases the likelihood of a fluid leak. Therefore, the annular member advantageously reduces the likelihood of a fluid leak.

[0022] The first load transfer member may comprise a sleeve portion. The sleeve portion may be engaged around the conduit.

[0023] The radial protrusion may be integrally formed with the conduit.

[0024] Where the radial protrusion is integrally formed with the conduit, assembly of the seal assembly is advantageously more efficient. This is because the seal assembly includes fewer components, and so the time required to assemble the seal assembly is reduced.

[0025] The radial protrusion may be integrally formed with the first load transfer member.

[0026] The radial protrusion may fully encircle the central axis.

[0027] Where the radial protrusion fully encircles the central axis, the axial load applied to the seal element is advantageously distributed in a uniform manner. This further increases the fluid pressure that the seal element is able to withstand.

[0028] The channel may comprise a first uniform portion. The first uniform portion may define a first cross sectional area.

[0029] The channel may comprise a second uniform portion. The second uniform portion may adjoin the first uniform portion. A second cross-sectional area of the second uniform portion may be less than the first cross-sectional area of the first uniform portion.

[0030] The second uniform portion may adjoin the first uniform portion via a transition portion. The seal element may engage the transition portion.

[0031] At least part of the tip portion of the conduit first end may be axially slidable into the second uniform portion.

[0032] At least part of the radial protrusion may be axially slidable into the first uniform portion.

[0033] The channel may further comprise atapered portion. The tapered portion may adjoin an opening of the channel. The tapered portion may adjoin the first uniform portion. The opening of the channel defines a larger cross sectional area than the first cross-sectional area.

[0034] Where the channel comprises a tapered portion that adj oins an opening of the channel, assembly of the seal assembly is advantageously more efficient. This is because a user is able to more readily insert the conduit into the channel.

[0035] The biasing assembly may be dedicated to the seal element such that, in use, an entirety of the axial compressive load provided by the biasing assembly may be exerted on the seal element.

[0036] Where the biasing assembly is dedicated to the seal element such that, in use, an entirety of the axial load provided by the biasing assembly is exerted on the seal element, the axial load exerted on the seal element can be better controlled. Better control of the axial load provides a more robust seal assembly because the likelihood of applying an axial load that is too great or too small is reduced.

[0037] In a second aspect of the invention there is provided a target material preparation assembly for an EUV radiation source. The target material preparation assembly comprising a seal assembly according to the first aspect of the invention.

[0038] The target material preparation assembly may comprise a plurality of seal assemblies, each seal assembly of the plurality of seal assemblies being in accordance with the first aspect of the invention.

[0039] In a third aspect of the invention there is provided a droplet generation assembly for an EUV radiation source. The droplet generation assembly comprises a target material droplet nozzle assembly; and a target material preparation assembly according to the second aspect of the invention. In use, the target material droplet nozzle assembly receives target material from the target material preparation assembly;

[0040] In a fourth aspect of the invention there is provided a EUV radiation source comprising the droplet generation assembly of the third aspect of the invention.

[0041] In a fifth aspect of the invention there is provided a EUV exposure system comprising the EUV radiation source of the fourth aspect of the invention and an EUV exposure apparatus.

[0042] Features disclosed in relation to one aspect of the invention may be combined with other aspects of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 shows a system for (actinic) mask inspection;Figure 3 schematically depicts a target material preparation assembly associated with the lithographic system of Figure 1;Figure 4 is a perspective view of a seal assembly that may be used with the target material preparation assembly of Figure 3 according to a first embodiment of the present invention; Figure 5 is a cross-sectional view of the seal assembly of Figure 4;Figure 6 is a close up view of region A of Figure 5;Figure 7 is a cross-sectional view of a seal assembly that may be used with the target material preparation assembly of Figure 3 according to a second embodiment of the present invention; Figure 8 is a cross-sectional perspective view of the seal assembly of Figure 7; and Figure 9 is a close up view of region B of Figure 7.DETAILED DESCRIPTION

[0044] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The lithographic apparatus LA is an example of an exposure apparatus. The radiation source SO is configured to generate an 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.

[0045] 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.

[0046] 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).

[0047] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0048] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) 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.

[0049] Figure 2 depicts a system for (actinic) mask inspection. The mask inspection system is an example of an exposure apparatus. A mask inspection system can be used to identify or inspect defectsin a mask to be used in a lithographic process by means of a lithographic system such as for example the one described in Figure 1. The mask inspection system comprises an radiation source SO and an illumination system IL and a detection system DS. A patterning device (i.c. mask) MA is placed on a support structure MT, which may be a mask stage, and illuminated by the illumination system IL reflecting radiation incident from the radiation source SO. The radiation coming from the illuminated patterning device MA is reflected by the detection system. In this way an image is formed on a detector DE. The radiation source SO of Figure 2 may be modelled after the radiation source system of Figure 1.

[0050] 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 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. The fuel may be referred to as a target fluid or target material. 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 may be used to generate EUV radiation.

[0055] Figure 3 shows a target material preparation assembly 16. The target material preparation assembly 16 comprises a target material priming tank 18. The target material, which may be in solid form, is inserted into the target material priming tank 18 where it is melted. The target material may be heated to at least 260 degrees Celsius. It will be appreciated that the temperature to which the target material is heated is determined, at least in part, by the target material that is used. The target material preparation assembly 16 further comprises a refdl tank 20. Molten target material is held in the refdl tank 20. The refdl tank 20 is downstream of the target material priming tankl8. The target material preparation assembly 16 further comprises valve system 22. The valve system 22 is downstream of the refdl tank 20. A target material emitter 25 is disposed downstream of the valve system 22.

[0056] A first pressure tank 24 and a second pressure tank 26 are in fluid communication with the valve system 22. The first pressure tank 24 and the second pressure tank 26 are configured to increase a pressure of the target material. In some embodiments, only one of the first pressure tank 24 and the second pressure tank 26 are provided. In some embodiments, three or more pressure tanks may be provided. The first pressure tank 24 comprises variable volume portion 27 and a pressurisation portion 29. The pressurisation portion 29 surrounds the variable volume portion. Put another way, the variable volume portion 27 is disposed in the pressurisation portion 29. In use, target material is disposed in the variable volume portion 27. In use, a pressurised fluid, such as a hydraulic fluid or gas, is introduced into the pressurisation portion 29. This allows the target material to be discharged from the variable volume portion 27 at a constant, or near-constant, pressure. The pressure at which the target material is discharged may be at least 300 bara, at least 500 bara, or at least 800 bara. Additionally or alternatively, the pressure at which the target material is discharged may be up to 3000 bara, up to 2000 bara, up to 1500 bara, up to 1000 bara. The construction and operation of the second pressure tank 26 are identical to the first pressure tank 24, and so the construction and operation of the second pressure tank 26 will not be discussed separately. The valve system 22 is operable to control which of the first pressure tank 24 and the second pressure tank 26 is in fluid communication with the refill tank 20, and which of the first pressure tank 24 and the second pressure tank 26 is in fluid communication with the target material emitter 25.

[0057] Each of the components of the target material preparation assembly 16 are fluidly connected via a plurality of fuel transfer lines 28 (only one of which is labelled in Figure 3). The fuel transfer lines 28 may also be referred to as target material transfer lines. The fuel transfer lines 28 may also be referred to as conduits. A plurality of seal assemblies 30 are also provided. Seal assemblies of the plurality of seal assemblies 30 are provided at a respective interface between each fuel transfer line 28 and the components to which the fuel transfer line is connected. Seal assemblies of the plurality of seal assemblies 30 are also provided at interfaces between portions of the fuel transfer lines 28.

[0058] Embodiments of the present invention relate to novel seal assemblies that may be used in a fuel (or target material) transfer line in or to the target material emitter 25. Figure 4 shows a perspective view of a seal assembly 30 according to an embodiment of the present invention. The seal assembly 30 may be a seal assembly of the plurality of seal assemblies shown in Figure 3. The seal assembly 30 comprises a housing 32. The housing 32 comprises a first housing portion 34 and a second housing portion 36. The second housing portion 36 may be a part of a component of the target material preparation assembly 16. For example, the second housing portion 36 may be a part of the target material priming tank 18, the refill tank 20, the valve system 22, the first pressure tank 24, or the second pressure tank 26. Alternatively, the second housing portion 26 may be a standalone component, to allow multiple sections of fuel transfer lines to be sealingly connected to one another. The first housing portion 34 is secured to the second housing portion by virtue of a plurality of fasteners 38 (only one of which is labelled in Figure 4). An end cap 40 is secured to the first housing portion 34. A fuel transfer line 28 extends through an opening 41 that extends through the end cap 40.

[0059] Figure 5 shows a cross-section view of the seal assembly 30. The fuel transfer line 28 defines a first end 42. The term “end” in this context may be understood to refer to an end region, or an absolute end, of the fuel transfer line 28. The first end 42 of the fuel transfer line 28 is disposed in a channel 44 that is defined by the second housing portion 36. The fuel transfer line 28 comprises a through bore 46. The fuel transfer line 28 defines a central axis 48. The first end 42 of the fuel transfer line is axially slideable into the channel 44. As noted above, the fuel transfer line 28 may be referred to as a conduit.

[0060] Figure 6 shows a close up view of region ‘A’ of Figure 5. The channel 44 comprises a first uniform portion 58 which may adjoin an opening 56 of the channel 44. The first uniform portion defines a first cross sectional area which is constant over an axial interval of the channel 44. In some embodiments, the first uniform portion 58 may be replaced by a non-uniform portion. Alternatively, a first tapered portion 54 may optionally be provided adjoining the opening 56 of the channel 44 and the first uniform portion 58. The first tapered portion 54 advantageously simplifies assembly of the seal assembly 30. This is because the first tapered portion allows for easier insertion of the fuel transfer line 28 into the channel 44. The channel 44 may further comprise a second optional tapered portion 60. The second tapered portion adjoins the first uniform portion 58. A first end 66 of a seal element 64 engages the second tapered portion 60 of the channel 44. The size and shape of the first end 66 of the seal element 64 may be complementary to the size and shape of the second tapered portion 60. The second tapered portion 60 may be referred to as a transition portion. In some embodiments, the transition portion may be in the form of a shoulder (e.g. extending perpendicular to the axis 48). The channel 44 further comprises a second uniform portion 62. The second uniform portion 62 adjoins the second tapered portion 60. The cross-sectional area of the second uniform portion 62 is less than the cross- sectional area of the first uniform portion 58. It will be appreciated that the channel 44 may take any other suitable shape. For example, the tapered portions 54, 60 need not be provided such that channel44 comprises only the first uniform portion 58 and the second uniform portion 62, separated by a shoulder.

[0061] The first end 42 of the fuel transfer line 28 defines a tip portion 50. A radial protrusion 52 is disposed adjacent the tip portion 50. In the present embodiment, the radial protrusion 52 is integrally formed with the fuel transfer line 28. However, as will be discussed in more detail below, this need not be the case. The radial protrusion 52 fully encircles the central axis 48. However, in some, non -depicted embodiments, this need not be the case.

[0062] The tip portion 50 of the fuel transfer line 28 is axially slidable into the second uniform portion 62 of the channel 44. When assembled, the tip portion 50 of the fuel transfer line 28 is at least partially disposed in the second uniform portion 62 of the channel 44. The radial protrusion 52 is axially slidable into the first uniform portion 58 of channel 44.

[0063] The seal element 64 extends about the tip portion 50 of the fuel transfer line 28. The seal element 64 is fully enclosed by the housing 32 and the fuel transfer line 28. In particular, the seal element 64 is enclosed by the second housing portion 36, by the tip portion 50 of the fuel transfer line 28, and by the radial protrusion 52. Being fully enclosed means that the (annular) space defined between the tip portion 50, the second housing portion 36, the radial protrusion 52 and a first point of contact between the tip portion 50 and the second uniform portion 62, is fully occupied by the seal element 64. The seal element 64 being fully enclosed cannot escape and this advantageously allows the seal element 64 to withstand higher fluid pressures during use (compared with a conventional seal arrangement). The seal element 64 defines a first end 66 and an opposed second end 68. The first end 66 of the seal element optionally tapers radially inwardly. In some embodiments, the first end 66 of the seal element 64 does not taper radially inwardly. The seal element 64 is axisymmetric about the central axis 48. The size and shape of the seal element 64 may be complementary to the (annular) space defined between the tip portion 50, the second housing portion 36, the radial protrusion 52 and a first point of contact between the tip portion 50 and the second uniform portion 62.

[0064] Referring back to Figure 5, the seal assembly 30 further comprises a biasing assembly 70. The biasing assembly 70 comprises a first load transfer member 72, a second load transfer member 74, and a plurality of biasing members 76 (only one of which is labelled in Figure 5). In the present embodiment, the plurality of biasing members 76 comprise disc springs. However, in other, nondepicted, embodiments, the plurality of biasing members 76 may take any other suitable form. For example, the plurality of biasing members 76 may be replaced by one or more helical springs. In the present embodiment, the plurality of biasing members comprises ten biasing members. In some, nondepicted embodiments, the plurality of biasing members may comprise any suitable number of biasing members. In some embodiments, the plurality of biasing members 76 may be replaced by a single biasing member. The plurality of biasing members 76 are disposed between the first load transfer member 72 and the second load transfer member 74. The plurality of biasing members 76 urge against the first load transfer member 72 and the second load transfer member 74 to provide an axial load whichis exerted on the seal element 64 to compress the seal element 64. The seal element 64 is axially compressed between the radial protrusion 52 and the housing 32, in particular the second housing portion 36. That is to say, the biasing assembly 70, in particular the plurality of biasing members 76, exert an axial compressive load on the seal element 30, via the radial protrusion 52 and the housing 32.

[0065] The first load transfer member 72 comprises a sleeve portion 78 and a flange portion 80. The first load transfer member 72 defines a central bore 82. The central bore 82 extends along the central axis 48. The central bore 82 is threaded. An outer periphery of the fuel transfer line 28 is also threaded such that the first load transfer member 72 can be screwed onto the fuel transfer line 28 to secure the first load transfer member 72 to the fuel transfer line 28. However, it will be appreciated that other methods of securing the load transfer member 72 to the fuel transfer line 28 may be used.

[0066] The second load transfer member 74 is generally annular. The second load transfer member 74 may be non-axisymmetric about the central axis 48. The second load transfer member 74 comprises at least one lobe (not visible in Figure 5 - it is out of the plane of the cross-section). The at least one lobe engages a corresponding portion of the first housing portion 34. This allows the second load transfer member 74 to function as an anti-rotation feature. The second load transfer member 74 may comprise any suitable circumferential discontinuity that functions as an anti -rotation feature. In some embodiments, the second load transfer member 74 may be axisymmetric. The second load transfer member 74 is not secured to the fuel transfer line 28. The second load transfer member 74 is not secured in the axial direction with respect to the sleeve portion 78 of the first load transfer member 72. Axial movement of the second load transfer member 74, in a direction away from the second housing portion 36, is restricted by the end cap 40.

[0067] The flange portion 80 of the first load transfer member 72 defines a first side 84. The first side 84 faces away from the second housing portion 36. The flange portion 80 of the first load transfer member defines a second side 86. The second side 86 is spaced apart from the first side 84 along the central axis 48. The second side 86 is opposed to the first side 84. The second load transfer member 74 defines a first side 88. The first side 88 faces away from the second housing portion 36. The second load transfer member 74 defines a second side 90. The second side 90 is spaced apart from the first side 88 along the central axis 48. The second side 90 is opposed to the first side 88. The first side 84 of the flange portion 80 of the first load transfer member 72 is engaged by a biasing member of the plurality of biasing members 76. The second side 90 of the second load transfer member is engaged by a biasing member of the plurality of biasing members 76. The first side 88 of the second load transfer member 74 is engaged by the end cap 40. The end cap 40 restricts axial movement of the second load transfer member 74 in a direction that is axially away from the second housing portion 36. The second load transfer member 74 extends about the sleeve portion 78 of the first load transfer member 72. The second load transfer member 74 is axially spaced apart from the first load transfer member 72. The second load transfer member 74 extends radially with respect to the central axis 48.

[0068] When assembled, the tip portion 50 of the fuel transfer line 28 is at least partially disposed in the second uniform portion 62 of the channel 44 of the first housing portion 36, and in axially sliding engagement therewith. Therefore, the tip portion 50 of the fuel transfer line 28 is axially slidable into the second uniform portion 62. The radial protrusion 52 is axially slidable into the first uniform portion 58 of the channel 44.

[0069] Assembly of the seal assembly 30 will now be discussed. First, the first housing portion 34 is secured to the second housing portion 36 using the plurality of fasteners 38. Next, the fuel transfer line 28 is passed through the opening 41 of the end cap 40. The plurality of biasing members 76 are then positioned about the first load transfer member 72, in particular about the sleeve portion 78 of the first load transfer member 72, such that the flange portion 80 of the first load transfer member 72 is engaged by a biasing member of the plurality of biasing members 76. The second load transfer member 74 is then positioned about the sleeve portion 78 of the first load transfer member 72. Next, the first load transfer member 72 is threaded on to the fuel transfer line 28. Next, the seal element 64 is positioned about the tip portion 50 of the fuel transfer line 28. The seal element 64 is positioned such that it engages the radial protrusion 52. Next, the tip portion 50, along with the seal element 64, is inserted into the channel 44. Doing this also positions the biasing assembly 70 in the housing 32, in particular in the first housing portion 34. The end cap 40 is then secured to the first housing portion 34. The end cap 40 urges against the first side 88 of the second load transfer member 74, which urges the second side 90 of the second load transfer member 74 against a biasing member of the plurality of biasing members 76. This exerts an axial load on the plurality of biasing members 76 which, in turn, is applied to the seal element 64 via the first load transfer member 72, the fuel transfer line 28, and the radial protrusion 52. Upon exertion of the axial load the seal element 64 urges against the second housing portion 36, the tip portion 50 of the fuel transfer line 28, and the radial protrusion 52.

[0070] Application of an axial load to the seal element 64 via a biasing assembly, as discussed above, advantageously allows the seal element 64 to provide a robust seal over a large range of pressures. For example, the seal element 64 is able to prevent leakage of target material at pressures from a vacuum state to several thousand bar. Furthermore, since the axial load is applied via the plurality of biasing members 76, the impact of strains that may result from variation in the coefficients of thermal expansion of the components of the seal assembly 30 is minimised. This is because the compression of the biasing members 76 accounts for this.

[0071] Figure 7 shows a cross-sectional view of a seal assembly 130 in accordance with a second embodiment of the present invention. Like numerals, prefixed with a ‘ 1 ’ , will be used to refer to features that are also present in the first embodiment. Unless noted otherwise, the description above in relation to the first embodiment applies to the second embodiment unless noted otherwise.

[0072] In this embodiment, the radial protrusion 152 is formed as part of (i.e., is integrally formed with) the first load transfer member 172. In some embodiments, the radial protrusion 152 may be separately formed from the first load transfer member 172 and subsequently attached via any suitablemeans. The radial protrusion 152 extends axially from the flange portion 180 of the first load transfer member 172. The radial protrusion 152 defines a radial width. In this embodiment, an outer diameter of the fuel transfer line 128 maybe constant along the central axis 148. However, this need not be the case. Since the outer periphery of the fuel transfer line 128 is threaded, an outer diameter of the fuel transfer line 128 being constant may be understood to refer to the major diameter, a pitch diameter, or a minor diameter of the threading applied to the outer periphery of the fuel transfer line 128 being constant along the axial length of the fuel transfer line.

[0073] In this embodiment, the flange portion 180 of the first load transfer member 172 is separately formed from the sleeve portion 178 of the first load transfer member 172. However, this need not be the case and, as in the previous embodiment, the sleeve portion 178 and the flange portion 180 may be formed as a single piece. The flange portion 180 defines a through bore 194. At least part of the through bore 194 is threaded. An external surface 196 of the sleeve portion 178 is threaded. At least part of the thread of the through bore 194 of the flange portion 180 is engagable with at least part of the thread of the external surface 196 of the sleeve portion 178. This allows the flange portion 180 to be secured to the sleeve portion 178. This is achieved by screwing the flange portion 180 onto the sleeve portion 178.

[0074] As in the previous embodiment, at least part of the outer periphery of the fuel transfer line 128 is threaded. The sleeve portion 178 comprises athrough bore 191. One or both of the through bore 191 of the sleeve portion 178 and the through bore 194 of the flange portion 180 is threaded. The thread of one or both of the through bore 191 of the sleeve portion 178 and the through bore 194 of the flange portion is engagable with the thread of the outer periphery of the fuel transfer line 128. This secures the axial position of the first load transfer member 172 with respect to the fuel transfer line 128.

[0075] The seal assembly 130 comprises an annular member 192. When assembled, the annular member 192 extends from the second load transfer member 174, in particular from the second side 190 of the second load transfer member 174, to the second housing portion 136. When assembled, the annular member 192 engages both the second load transfer member 174 and the second housing portion 136. The annular member 192 advantageously facilitates heat transfer away from the second housing portion. In addition, since the annular member 192 extends from the second side 190 of the second load transfer member 174 to the second housing portion 136, the distance from the second side 190 of the second load transfer member 174 to the second housing portion 136 is known with a greater degree of accuracy, as compared to if the annular member 192 were not present. This advantageously allows the force exerted by the plurality of biasing members 176 to be applied with greater accuracy. This advantageously reduces the likelihood that too great or too small a load is exerted on the sealing element 164. The load being too great or too small may increase the likelihood of a leak occurring in use.

[0076] A channel 187 is defined between the first housing portion 134 and the second housing portion 136. A circumferential portion of an axial end 189 of the annular member 192 is spaced apart from the second housing portion 136 to define a groove 191. The remainder of the circumference of the annular member 192 engages the second housing portion. The axial and circumferential position of the groove191 corresponds with the axial and circumferential position of the channel 187. This allows a load applied to the seal element 164 to be checked. To do so, a user inserts a gauge through the channel 187 and through the groove 191. This allows a spacing between the second side 186 of the flange portion 180 and the second housing portion 136 to be checked. The gauge being loose in the space between the second side 186 of the flange portion 180 and the second housing portion 136 indicates that the load is too small. If the gauge cannot fit into the space between the second side 186 of the flange portion 180 and the second housing portion 136, the load is too great. This may also indicate that the seal element 164 is not present.

[0077] A pin 195 extends through an opening 197 provided in the first housing portion 134. The pin 195 extends into a slot 199 that is provided in the annular member 192. The pin 197 is provided to restrict rotation of the annular member 192. This advantageously maintains alignment of the channel 187 and the groove 191. In addition, the pin 197 restricts axial movement of the annular member 192, which retains the annular member 192 within the first housing portion 134 during assembly and disassembly.

[0078] Figure 8 shows a cross-sectional perspective view of the seal assembly 130. In Figure 8, the end cap is not shown for clarity. As can be seen, the second load transfer member 174 comprises lobes 133. In the embodiment depicted, the second load transfer member 174 comprises two lobes 133. However, the second load transfer member 174 may comprise any suitable number of lobes, including only a single lobe. As discussed above in relation to the previous embodiment, the lobes function as an anti-rotation feature. In some, non-depicted, embodiments, the anti-rotation feature need not be in the form of one or more lobes, but may take any other suitable form. As can also be seen, the first housing portion 134 comprises a central opening 135. The central opening 135 extends along the full axial length of the first housing portion 134. The central opening 135 comprises a threaded portion 137. The threaded portion 137 comprises threads that are engaged by threads of the end cap. The central opening 135 comprises two groove portions 139. The number of groove portions 139 may correspond to the number of lobes 133 of the second load transfer member 174. The groove portions 139 receive the lobes 133 of the second load transfer member 174. This restricts rotation of the second load transfer member 174. The lobes 133 are axially slidable within the grooves 139. The lobes 133 engage the annular member 192. This restricts axial movement of the annular member 192. The lobes 133 may be the only portion of the second load transfer member 174 that engages the annular member 192.

[0079] Figure 9 shows a close up view of region ‘B’ of Figure 7. The tip portion 150 comprises an annular groove 198. The annular groove 198 is disposed axially adjacent the radial protrusion 152. However, in some, non-depicted, embodiments, the annular groove 198 may be axially spaced apart from the radial protrusion 152. The presence of the annular groove 198 better retains the seal element 132 on the tip portion 150, for example, during assembly and disassembly of the seal assembly 130. The tip portion 50 of the fuel transfer line 28 of the previous embodiment may comprise an annulargroove in accordance with the present embodiment. In either embodiment, the annular groove 198 need not be provided.

[0080] Referring back to Figure 7, to assemble the seal assembly 130, first, the first housing portion 134 is secured to the second housing portion 136 using the plurality of fasteners (not visible in Figure 7). Next, the sleeve portion 178 of the first load transfer member 172 is threaded onto the fuel transfer line 128. The fuel transfer line 128 and the sleeve portion 178 of the first load transfer member 172 are then passed through the opening 141 of the end cap 140. Next, the second load transfer member 174 is positioned about the sleeve portion 178 of the first load transfer member 172. Next, the plurality of biasing members 176 are positioned about the sleeve portion 178 of the first load transfer member 172. Next, the flange portion 180 of the first load transfer member 172 is threaded on to the sleeve portion 178 of the first load transfer member 172. Next, the annular member 192 is positioned about the plurality of biasing members 176, and about the flange portion 180 of the first load transfer member 172. Next, the seal element 164 is positioned on the tip portion 150 of the fuel transfer line 128. The tip portion 150 and seal element 164 are then inserted into the channel 144. Next, the pin 195 is inserted into the opening 197 of the first housing portion 134 and into the slot 199 of the annular member 192. Next, the end cap 140 is secured to the first housing portion 134. The end cap 140 urges against the first side 188 of the second load transfer member 174, which urges the second side 190 of the second load transfer member 174 against the plurality of biasing members 176. This exerts an axial load on the plurality of biasing members 176 which, in turn, is applied to the seal element 164 via the first load transfer member 72. Upon exertion of the axial load the seal element 64 urges against the second housing portion 36, the tip portion 50 of the fuel transfer line 28, and the radial protrusion 52.

[0081] The seal assembly described herein is particularly suitable in association with an EUV radiation source. Specifically, it may be applied in a target material preparation assembly for an EUV radiation source. More specifically, it may be applied in droplet generator assembly for an EUV radiation source, in particular where the droplet generator assembly requires target material at high pressure, for instance 800 bara or higher up to 3000 bara. In the context of the present specification, such a target material preparation assembly and / or droplet generation assembly may be considered to be part of the EUV radiation source.

[0082] 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.

[0083] 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 (orother 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.

[0084] 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.

[0085] 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.

Claims

CLAIMS1. A seal assembly for an EUV radiation source, the seal assembly comprising: a housing, the housing defining a channel; a conduit that defines a central axis, and a conduit first end, wherein the conduit first end extends into the channel, wherein the conduit first end defines a tip portion of the conduit, and wherein the conduit is configured to transfer a target material; a radial protrusion that is disposed adjacent the tip portion of the conduit first end; a seal element that extends about the tip portion of the conduit first end, the seal element being fully enclosed by the housing, the tip portion of the conduit, and the radial protrusion; and a biasing assembly that is configured to exert an axial compressive load on the seal element via the radial protrusion and the housing; wherein, upon exertion of the axial compressive load, the seal element is configured to urge against the housing, the tip portion of the conduit, and the radial protrusion.

2. The seal assembly of claim 1, wherein the seal element defines a first end that tapers radially inwardly and a second end, and wherein the channel of the housing defines a tapered surface that is engaged by the first end of the seal element.

3. The seal assembly of claim 1 or claim 2, wherein a circumferential groove is arranged in the tip portion of the conduit.

4. The seal assembly of any preceding claim, wherein the biasing assembly comprises: a first load transfer member that extends radially outwards from, and is secured to, the conduit adjacent the first end of the conduit; a second load transfer member that extends radially outwards with respect to the central axis and is axially spaced apart from the first load transfer member, wherein the second load transfer member is connected to the housing; at least one biasing member, the at least one biasing member being disposed between the first load transfer member and the second load transfer member such that the at least one biasing member urges against the first load transfer member and the second load transfer member to provide the axial load.

5. The seal assembly of claim 4, further comprising an annular member that extends from the second load transfer member to the housing.

6. The seal assembly of claim 4 or claim 5, wherein the first load transfer member comprises a sleeve portion that is engaged around the conduit.

7. The seal assembly of any of claims 1 to 6. wherein the radial protrusion is integrally formed with the conduit or wherein the radial protrusion is integrally formed with the first load transfer member.

8. The seal assembly of any preceding claim, wherein the radial protrusion fully encircles the central axis.

9. The seal assembly of any preceding claim, wherein the channel comprises a first uniform portion defining a first cross sectional area.

10. The seal assembly of claim 9, wherein the channel comprises a second uniform portion that adjoins the first uniform portion, and wherein a second cross-sectional area of the second uniform portion is less than the first cross-sectional area of the first uniform portion.

11. The seal assembly of claim 10, wherein the second uniform portion adjoins the first uniform portion via a transition portion, and wherein the seal element engages the transition portion.

12. The seal assembly of claim 10 or 11, wherein at least part of the tip portion of the conduit first end is axially slidable into the second uniform portion.

13. The seal assembly of any of claims 9 to 12, wherein at least part of the radial protrusion is axially slidable into the first uniform portion.

14. The seal assembly of any of claims 9 to 13, wherein the channel further comprises a tapered portion that adj oins an opening of the channel and adj oins the first uniform portion, whereby the opening of the channel defines a larger cross sectional area than the first cross-sectional area.

15. The seal assembly of any preceding claim, wherein the biasing assembly is dedicated to the seal element such that, in use, an entirety of the axial compressive load provided by the biasing assembly is exerted on the seal element.

16. A target material preparation assembly for an EUV radiation source, the target material preparation assembly comprising a seal assembly according to any preceding claim.

17. A droplet generation assembly for an EUV radiation source, the droplet generation assembly comprising: a target material droplet nozzle assembly; and a target material preparation assembly according to claim 16; wherein, in use, the target material droplet nozzle assembly receives target material from the target material preparation assembly;18. A EUV radiation source comprising the droplet generation assembly of claim 17.

19. An EUV exposure system comprising the EUV radiation source of claim 18 and an EUV exposure apparatus.

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