[0001] This application claims priority of EP application 16198660.9 which was filed on November 14, 2016 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a fuel collector and to radiation system including such fuel collector. The present invention relates for example to a fuel collector and radiation system, for example a radiation source, for use with a lithographic system.

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

[0005] A lithographic system may comprise one or more radiation sources, a beam delivery system and one or more lithographic apparatuses. The beam delivery system may be arranged to deliver radiation from one or more of the radiation sources to each of the lithographic apparatuses.

[0006] The EUV radiation may be produced using a plasma. The plasma may be created, for example, by directing a laser beam at a fuel in the radiation source. The resulting plasma may emit the EUV radiation. Not all of the fuel passing through the radiation source may be converted into the EUV emitting plasma. For example, the laser may miss a portion of the fuel, for example, when the laser moves to a new substrate or the laser is intermittently operated. This portion may be captured by a fuel catch. However, as the amount of fuel in the fuel catch increases, some of the fuel entering the fuel catch may be scattered back into the radiation source. For example, in some systems about 1 % of the fuel entering the fuel catch may escape the fuel catch. This may result in contamination of the one or more component(s) of the radiation source, which may be difficult to clean. The contamination of one or more component(s) in the radiation source with fuel may lead to a decrease in the performance of the radiation source, e.g. the quality of the produced EUV radiation, which in turn can lead to degradation of performance of an associated lithographic apparatus. Ultimately, this can lead to significant down-time of the lithographic apparatus whilst components of the radiation source are cleaned or replaced

[0007] It is an object of the present invention to obviate or mitigate at least one problem of prior art techniques.

SUMMARY

[0008] In a first independent aspect of the invention, there is provided a radiation system comprising: a fuel emitter configured to provide fuel to a plasma formation region; an excitation device arranged to provide an excitation beam at the plasma formation region to convert at least a part of the fuel into a radiation emitting plasma; and a fuel collector configured to collect at least another part of the fuel, said at least other part of the fuel comprising fuel that passes through the plasma formation region without being converted to radiation emitting plasma, the fuel collector comprising a movable surface for receiving the fuel, the fuel collector being configured to move the surface at a velocity or speed that is selected in dependence on a velocity or speed of the fuel.

[0009] The fuel collector may be configured to move the surface at the selected velocity or speed in a direction that substantially corresponds to a direction of the fuel in the radiation system.

[0010] The velocity or speed of the surface may be selected such that collision forces between fuel incident on the surface and the surface are minimized.

[0011] The velocity or speed of the surface may be selected such that there is substantially no back- scattering from the surface of said at least other part of the fuel.

[0012] The velocity or speed of the surface may correspond or substantially corresponds to the velocity or speed of the fuel. For example, the velocity or speed of the surface may be the same or substantially the same as the velocity or speed of the fuel. An advantage of choosing the speed of the surface being the same or substantially the same as the velocity of the fuel droplets is that the kinetic energy of the incoming droplets is substantially reduced and thereby material ablation in the fuel collector in the droplet landing zone is also minimized. As a result, the lifetime of the moving surface may be increased, while also minimizing the risk of causing non-fuel contaminating elements ablated from the landing zone surface on a radiation collector for collecting and focusing EUV radiation (also referred herein as "collector"). The velocity or speed of the surface may be selected to be different from the velocity or speed of the fuel. For example, the velocity of speed of the surface may be larger or less than the velocity or speed of the fuel.

[0013] The velocity or speed of the surface may comprise a tangential velocity or speed of the surface.

[0014] The surface may be arranged or arrangeable to move relative to a path of the fuel such that the tangential velocity or speed of the surface in a direction along at least part of the path of the fuel corresponds or substantially corresponds to the velocity or speed of the fuel. For example, the surface may be arranged or arrangeable to move relative to a path of the fuel such that the tangential velocity or speed of the surface in a/the direction along at least part of the path of the fuel is the same or is substantially the same as the velocity or speed of the fuel.

[0015] The surface may be arranged or arrangeable to extend at an angle relative to the path of the fuel. For example, the surface may be arranged or arrangeable to extend at a grazing angle relative to the path of the fuel. Alternatively or additionally, the surface may be arranged or arrangeable to extend at an angle of at least 90 degrees. For example, the surface may be arranged or arrangeable to extend perpendicular or substantially perpendicular relative to the path of the fuel.

[0016] The fuel collector may be configured to rotate the surface relative to the fuel and/or fuel emitter

[0017] The fuel collector may comprise a first collecting portion for collecting fuel. The first collecting portion may comprise, form part of or be adjacent to the surface for receiving the fuel.

[0018] The first collecting portion may comprise or define the surface.

[0019] The fuel collector may comprise a plurality of support elements. The plurality of support elements may be arranged for supporting and/or mounting the first collecting portion.

[0020] The first collecting portion may be part of or comprised in the surface. The first collecting portion may be arranged or arrangeable to at least partially extend along a perimeter or around a circumference of the surface. For example, the first collecting portion may be substantially ring-shaped. [0021] The surface for receiving the fuel may form part of a rotatable device, for example a rotatable disk, and the first collecting portion may also form part of the rotatable device and/or in operation may rotate with the surface for receiving the fuel.

[0022] The first collecting portion may be arranged such that in operation centrifugal force causes at least some of the received fuel to pass from the surface for receiving the fuel to the first collecting portion. In operation at least some of the received fuel may remain on the surface, for example, to form a layer on the surface. The remaining fuel on the surface may reduce back-scattering of said at least other part of the fuel from the surface .e.g. due to cohesion forces imparted on the fuel incident on the surface by the remaining fuel.

[0023] The fuel collector may comprise a second collecting portion. The second collecting portion may be arranged or arrangeable for receiving at least a portion of the fuel from the surface.

[0024] The fuel collector may comprise a plurality of openings. The plurality of openings may be or comprise a plurality of through-holes. The plurality of openings may be arranged on the surface and/or the first collecting portion. The plurality of openings may be configured to allow passage of fuel from the surface and/or the first collecting portion, for example into the second collecting portion. It will be appreciated that the openings may be provided in any suitable form. For example, the fuel collector may comprise a porous material. The porous material may be comprised in or be part of the surface and/or the first collecting portion. The porous material may provide the plurality of openings.

[0025] The fuel collector may comprise a heating element. The heating element may be arranged for heating of the surface.

[0026] The heating element may be configured to heat the surface to a temperature that is equal to or greater than a melting temperature of the fuel.

[0027] The heating element may be configured to heat the surface to a temperature that is less than a melting temperature of the fuel.

[0028] The fuel collector may comprise an actuator. The actuator may be configured to move the surface at a velocity or speed that is selected in dependence on the velocity or speed of the fuel.

[0029] In a second aspect of the invention, which may be provided independently, there is provided a fuel collector for use with a radiation source, the fuel collector being configured to collect fuel that passes through a plasma formation region of the radiation source without being converted to radiation emitting plasma, the fuel collector comprising a movable surface for receiving the fuel, the fuel collector being configured to move the surface at a velocity or speed that is selected in dependence on a velocity or speed of the fuel. The fuel collector may comprise any of the features of the fuel collector defined in the first aspect.

[0030] In a third aspect of the invention, which may be provided independently, there is provided a radiation system comprising: a fuel emitter configured to provide fuel to a plasma formation region; an excitation device arranged to provide an excitation beam at the plasma formation region to convert at least a part of the fuel into a radiation emitting plasma; and a fuel collector configured to collect at least another part of the fuel, said at least other part of the fuel comprising fuel that passes through the plasma formation region without being converted to radiation emitting plasma, the fuel collector comprising a movable surface for receiving the fuel, the fuel collector being configured to move the surface at a velocity or speed that substantially corresponds to a velocity or speed of the fuel, optionally such that there is substantially no back-scattering from the surface of said at least other part of the fuel.

[0031] In a fourth aspect of the invention, which may be provided independently, there is provided a lithographic system comprising a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, and a radiation system according to first and/or third aspects arranged to provide at least some of said radiation to the lithographic apparatus.

[0032] In a fifth aspect of the invention, which may be provided independently, there is provided a method comprising: providing fuel to a plasma formation region; providing an excitation beam at the plasma formation region to convert at least a part of the fuel into a radiation emitting plasma; and receiving at least another part of the fuel, said at least other part of the fuel comprising fuel that passes through the plasma formation region without being converted to radiation emitting plasma, the receiving of the fuel comprising moving a surface for receiving the fuel at a velocity or speed that is selected in dependence on a velocity or speed of the fuel and receiving said at least other part of the fuel at the moving surface.

[0033] The method may comprise moving or rotating the surface at a velocity or speed of the surface that corresponds or substantially corresponds to the velocity or speed of the fuel.

[0034] The method may comprise removing received fuel from the surface.

[0035] The removing of the received fuel from the surface may comprise allowing cooling of the surface so that fuel collected on the surface solidifies.

[0036] The removing of the received fuel from the surface may comprise stopping or terminating movement or rotation of the surface. [0037] The removing of received fuel from the surface may comprise heating of the surface, e.g. to allow the collected fuel to pass from the surface to a collecting portion. The collector may be, or comprise any of the features of, the second collecting portion in the first aspect. For example, melted fuel may pass from the surface into the collecting portion via a plurality of openings. The plurality of openings may be part of or arranged in the surface.

[0038] The step of removing collected fuel from the surface may comprise heating of the collector.

[0039] The method may comprise gradually increasing the velocity or speed of the surface, for example, during or after the removing of the received fuel or when at least a portion of the received fuel is removed from the surface.

[0040] The method may comprise allowing the surface to cool to a temperature below a melting temperature of the fuel, for example, prior to the step of increasing the velocity or speed of the surface.

[0041] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] 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 lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment;

Figure 2a schematically depicts a side view of an exemplary fuel collector for use with the radiation source of Figure 1 ;

Figure 2b schematically depicts a top view of the fuel collector of Figure 2a;

Figure 3 a schematically depicts a side view of an exemplary fuel collector for use with the radiation source of Figure 1 ;

Figure 3b schematically depicts a top view of the fuel collector of Figure 3a;

Figure 4a schematically depicts a side view of an exemplary fuel collector for use with the radiation source of Figure 1 ;

Figure 4b schematically depicts a top view of the fuel collector of Figure 4a; and

Figure 5 schematically illustrates an exemplary method.

DETAILED DESCRIPTION [0043] Figure 1 shows a lithographic system including a radiation source according to one embodiment of the invention. The lithographic system comprises the radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[0044] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[0045] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source). An excitation device, which may be provided in the form of a laser 1, such as, for example, a C0 ₂ laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from 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 plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.

[0046] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to as a normal incidence radiation collector). The collector 5 may have a multilayer 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 elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[0047] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 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 1 and the radiation source SO may together be considered to be a radiation system.

[0048] In the embodiment of Figure 1 , a supply of hydrogen is also provided through the opening 1 generally along the same axis as the laser beam. Hydrogen may also be supplied around the perimeter of the collector 5, and/or optionally through supply ports. The hydrogen serves a number of purposes including maximising suppression of contamination of the collector 5 (and also optionally metrology modules, not shown), acting as a source of hydrogen radicals for decontamination, and conditioning the plasma to keep hot ionized gas away from the collector CO and metrology modules.

[0049] Returning to consideration of the radiation produced by the source, radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. 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.

[0050] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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.

[0051] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[0052] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0053] Figures 2a and 2b schematically depict an exemplary fuel collector 15 for use with a radiation source SO. The fuel collector 15 is configured to collect at least part of the fuel that passes through the plasma formation region 4 of the radiation source SO without being converted to radiation emitting plasma. The fuel collector 15 comprises a movable surface 16 for receiving the fuel. For example, the surface 16 may be moveable, e.g. rotatable or translatable, relative to the fuel. Figure 1 shows the exemplary fuel collector 15 arranged in the radiation source SO, for example, opposite the fuel emitter 3.

[0054] It will be appreciated that the term "fuel" may be considered as encompassing fuel droplets or drops. The at least part of the fuel that passes to the fuel collector may comprise drops or droplets that have not been converted to plasma and/or that have not been hit by the laser beam 2.

[0055] In the embodiment shown in Figures 2a and 2b the movable surface 16 is provided in the form of a rotatable disk 16. It will be appreciated that in other embodiments other suitable moveable surfaces may be used. For example, in other embodiments the moveable surface may be provided by moving band, such as a conveyor belt or the like.

[0056] The fuel collector 15 is configured to move the disk 16 at a velocity or speed that is selected in dependence on a velocity or speed of the fuel. For example, the fuel collector 15 may be configured to rotate the disk 16 a velocity or speed that is selected in dependence on the velocity or speed of the fuel.

[0057] The velocity or speed of the disk 16 may be selected such that collision forces between the fuel and the disk 16 are minimized. By minimizing collision forces between the fuel and the disk, back-scattering of the fuel into the radiation source may be prevented or reduced. For example, the velocity or speed of the disk 16 may be selected such that there is substantially no back-scattering from the diskl6 of the fuel. The terms "substantially no back-scattering" may be considered as encompassing no or almost no back-scattering or reflection of fuel from the disk 16. For example, the velocity or speed of the disk 16 may be selected such that substantially no fuel is deflected from the disk 16 into the radiation source SO and/or such that substantially no fuel exits the fuel collector 15. This may prevent or reduce contamination of one or more component(s) of the radiation source SO.

[0058] The velocity or speed of the disk 16 of the exemplary fuel collector 15 shown in Figures 2a and 2b may be selected to substantially correspond to the velocity or speed of the fuel. For example, the velocity or speed of the disk 16 may be selected to be the same (or substantially the same) as the velocity or speed of the fuel. The velocity or speed of the disk 16 may comprise a tangential velocity or speed of the disk 16. The disk 16 may be arranged to rotate relative to a path of the fuel in the radiation source SO such that the tangential velocity or speed of the disk 16 in a direction along at least part of the path of the fuel may substantially correspond to (e.g. may be substantially the same as) the velocity or speed of the fuel. This may result in the fuel that is incident on the disk 16 experiencing no or almost no difference in speed or velocity and/or may allow collision forces between the fuel and the disk to be minimized, for example, to prevent back-scattering of the fuel from the disk 16, e.g. into the radiation source. The term path of fuel may be considered as encompassing a path or stream of fuel in the radiation source SO, which passes through the plasma formation region 4 without being converted into radiation emitting plasma.

[0059] The disk 16 may define an area 16a, e.g. a landing area, on which fuel may be incident. The disk 16 may be arranged relative to the path of the fuel in the radiation source such that at least one point 16b, e.g. a landing point or soft landing point of the area 16a, may move or be considered as moving substantially in the same direction as the fuel. The point 16b may be a point on the area 16a on which the fuel is incident. On/at the point 16b of the area 16a the difference between the speed or velocity of the fuel and the moving disk 16 may be considered to be minimized and/or zero or almost zero. This may allow collision forces between the fuel and the disk 16 to be minimized and may prevent back-scattering of the fuel from the disk 16, e.g. into the radiation source SO. In the exemplary fuel collector 15 shown in Figures 2a and 2b, the area 16a may comprise a radius of approximately 10 cm. The fuel may, for example, have a speed or velocity of 100 m/s, which may result in selection of the speed or velocity of the disk 16 of 10000 rpm (approximately 1050 rad/s). This speed or velocity of the disk 16 may result in the difference between the speed or velocity of the fuel and the speed or velocity of the moving disk 16 to be minimized and/or to be zero or almost zero. [0060] As shown in Figure 2a, the disk 16 may be arranged to extend at an angle a relative to the path 17 of the fuel in the radiation source SO. In this example, the angle a is a grazing angle. In other word, the disk 16 may arranged relative to the path 17 of the fuel, so that the disk 16 extends in a direction that is almost parallel to the path 17 of the fuel. The grazing angle may be selected such that back-scattering of the fuel from the disk 16 is prevented or reduced. The grazing angle may be selected such as to compensate for deviation in the landing area on the disk 16, which may be due to deviation in the path of the fuel in radiation source. Deviation in the path of the fuel in the radiation source SO may, for example, be caused by the fuel emitter 3, e.g. prior to and/or subsequent to maintenance operation to the radiation source SO, which may require shut down or restart of the fuel emitter 3. The grazing angle may be selected such as to compensate for deviation in the landing area on the disk 16 without requiring an increase in dimensions, e.g. the diameter, of the disk 16. For example, the grazing angle may be in the range from 2 to 12 degrees, e.g. approximately 7 degrees. It will be appreciated that in other embodiments, the disk may be arranged at other suitable angles relative to the path of the fuel. Additionally or alternatively, it will be appreciated that the velocity or speed of the disk may be dependent on the angle of the disk relative to the path of the fuel.

[0061] The exemplary fuel collector 15 shown in Figure 2a and 2b includes a housing 18 in which the disk 16 is arranged. The housing 18 includes an opening 18a through which the fuel can pass towards the disk 16. The fuel collector 15 includes a collecting portion 19 for receiving fuel. The collecting portion 19 may be provided in the form of a gutter, channel or groove or the like. The collecting portion 19 may act as a first receptacle for the fuel on the disk 16. In the example shown in Figures 2a and 2b, the collecting portion 19 is provided adjacent the disk 16. The collecting portion 19 may be arranged to extend along a perimeter or around a circumference of the disk 16. For example, the collecting portion 19 may be connected to the disk 16 so as to extend around the circumference of the disk 16. Although the collecting portion 19 is shown in Figure 2a and 2b as extending along or around the entire circumference of the disk 16, it will be appreciated that in other embodiments the collecting portion may be arranged to extend at least partially along the perimeter or along the circumference of the disk. In the embodiment shown in Figures 2a and 2b, the collecting portion 19 may be considered as being comprised in or part of the disk 16. It will be appreciated that other embodiments the collector may be provided separately from the disk or instead of the disk 16. Although in Figures 2a and 2b, the collector portion 19 is shown as comprising a ring-shaped form, it will be appreciated that the exemplary collector portion disclosed herein is not limited to such shape and that in other embodiments other suitable shaped of the collector portion may be used.

[0062] The disk 16 and the collecting portion 19 may be considered as forming part of a rotatable device 15a. In operation, the collecting portion 19 rotates with the disk 16. The collecting portion 19 is arranged such that in operation centrifugal force causes at least some of the received fuel to pass from the disk 16 to the collecting portion 19. In operation at least some of the received fuel may remain on the disk, for example, to form a layer (not shown in Figure 2a) on the disk. The remaining fuel on the disk 16 may reduce back-scattering of the fuel from the disk 16 .e.g. due to cohesion forces imparted on the fuel incident on the disk 16 by the remaining fuel.

[0063] The exemplary fuel collector 15 includes a further collecting portion, which may comprise a second receptacle for receiving fuel. The second receptacle 20 may be arranged for receiving at least a portion of the fuel from the disk 16 and/or collecting portion 19. For example, the second receptacle 20 may be arranged below the disk 16 and/or the collecting portion 19. A diameter Dl, e.g. an outer diameter, of the second receptacle 20 may be larger than a diameter D2 of the disk 16 and the collecting portion 19. This may allow fuel from the disk 16 and/or collecting portion 19 to be collected in the second receptacle 20, as will be described below. The second receptacle 20 may be arranged to be ring-shaped or donut-shaped or the like. However, it should be understood that the exemplary second receptacle described herein may not be restricted to such shapes and that in other embodiments the second receptacle may have other suitable shapes for receiving fuel from the disk and/or the collecting portion. A volume of the second receptacle 20 may be selected such that the radiation source SO or one or more component(s) thereof may require replacing, before fuel collected in the second receptacle may need to be removed. In other words, the volume of the second receptacle may be selected such that an amount of fuel that is incident in a time period between maintenance operations of the radiation source can be collected and/or retained in the second receptacle. For example, the second receptacle may comprise a volume of 1 to 8 liters, e.g. 2 to 5 liters. Alternatively or additionally, a suction device (not shown), such as a pump or the like, may be arranged to extract fuel from the second receptacle, e.g. when the second receptacle is full or almost full. The suction device may be arranged to transfer the fuel from the second receptacle to an exterior of the radiation source SO.

[0064] The exemplary fuel collector 15 shown in Figures 2a and 2b comprises an actuator, which may be provided in the form of a motor 21 of the like. The motor 21 is configured to move the disk 16 and/or collecting portion 19 at the velocity or speed that is selected in dependence on the velocity or speed of the fuel, as described above. The motor 21 may be configured to gradually increase the velocity or speed of the disk 16 and/or collecting portion 19 from a first velocity or speed, which may be zero, to a second velocity or speed, which may be selected in dependence on the velocity or speed of the fuel, for example, during start-up operation of the radiation source SO or subsequent to maintenance operations to the radiation source SO. The motor 21 may be coupled to the disk 16 via an axle or shaft 22. The motor 21 may be coupled to the disk 16 to cause rotation of the disk 16 relative to the path 17 of fuel. By rotating the disk, fuel incident on the disk may be subjected to a centrifugal force, which may force the fuel radially outwards of the disk 16, for example, into the collecting portion 19. In Figure 2a, the motor 21 and/or axle or shaft 22 are shown as being arranged in the center of the second receptacle 20. It should be understood that the exemplary fuel collector disclosed herein is not restricted to such an arrangement of the motor and/or axle or shaft.

[0065] The exemplary fuel collector 15 shown in Figures 2a and 2b includes a heating element 23, which is arranged for heating of the disk 16 and/or collecting portion 19. The heating element 23 may be configured to heat the disk 16 and/or collecting portion 19 to a temperature that is equal to or greater than a melting temperature of the fuel. For example, the disk 16 and/or collecting portion may be heated to a temperature between 250 to 350°C. The heating element 23 may be configured to heat the disk 16 and/or collecting portion 19 such that fuel on the disk 16 and/or collecting portion 19 is melted. In operation, the disk 16 and/or collecting portion 19 may be heated such that melted fuel is be forced radially outwards of the disk 16, for example, into the collecting portion 19, due to the centrifugal forces imparted on the fuel by the rotating disk 16. The collecting portion 19 may be shaped to collect and/or retain the fuel. By heating the disk and/or collecting portion to a temperature that is equal to or greater than the melting temperature of the fuel, the fuel may be evenly distributed on the disk and/or in the collecting portion, which may allow the amount of fuel collected by the fuel collector may be increased. Additionally or alternatively, by heating the disk and/or collecting portion to a temperature that is equal to or greater than the melting temperature of the fuel, the formation of stalagmites on the disk and/or in the collecting portion, which may become detached from the disk and/or collecting portion during rotation of the disk, cause imbalance on the disk and/or collecting portion and/or may result in increased bearing loads on the disk and/or collecting portion, may be reduced or prevented. [0066] The collecting portion 19 may be configured to prevent passage or dripping of the fuel from the disk 16, for example, during rotation of the disk 16. A volume of the collecting portion

19 may be selected such that removal of the fuel in the collecting portion 19 may only be required while (other) maintenance operations are carried out on the radiation source SO. In other words, the volume of the collecting portion 19 may be selected such that an amount of fuel that is incident in a time period between maintenance operations of the radiation source can be collected and/or retained in the collecting portion 19. For example, the amount of fuel incident on the disk 16 may be about 146g/Gs, which corresponds to 20 cm ³ . The collecting portion 19 may have a radius of 15 cm and a cross-section of about 4 cm ² . These exemplary dimensions of the collecting portion 19 may allow the collecting portion 19 to collect around

20 Giga droplets of fuel (e.g. 20xl0 ⁹ droplets of the fuel) in the case of the illustrated embodiment. It should be understood that the exemplary collector is not restricted to these dimensions. For example, in other embodiments, the collector may comprise a different radius and/or cross-section, which may be selected such that the volume of the collector is sufficient to collect any fuel that is incident in a time period between other maintenance operations of the radiation source.

[0067] It will be appreciated that in other embodiments the heating element may be configured to heat the disk and/or collecting portion to a temperature that is less than a melting temperature of the fuel. Alternatively or additionally, the heating element may be configured to maintain the disk and/or collecting portion at a temperature that is below the melting temperature of the fuel. In this example, the fuel that is incident on the disk may solidify on the disk. The solidified fuel may be removed from the disk at a later stage, as will be described below. By allowing the fuel to solidify on the disk, the amount or layer of fuel on the disk may increase in use. However, by moving the disk at a speed or velocity that substantially corresponds to the speed or velocity of the fuel, back-scattering of the fuel in to the radiation source may be minimized and/or prevented. This may minimize or prevent contamination of the radiation source. It will be appreciate that in other embodiments the fuel may be incident on the collecting portion, where the fuel may solidify and/or from which the solidified fuel may be removed at a later stage, as will be described below.

[0068] The disk 16 and/or collecting portion 19 may be thermally conductive. The disk 16 and/or collecting portion 19 may comprise a ceramic material, such as Beryllium Nitride (Br3N ₂ ) or Aluminum Oxide (AI2O3). Alternatively or additionally, the disk 16 and/or collecting portion 19 may comprise a metal or metal alloy. The material of the disk 16 and/or collecting portion 19 may be selected to be a material that allows to be wetted by the fuel, e.g. comprises a high degree of wettability with the fuel. The material of the disk 16 and/or collecting portion 19 may be selected to be corrosion resistant, e.g. to be resistant against corrosion of the fuel in the environment in the radiation source SO, e.g. the hydrogen environment in the radiation source SO. For example, the material of the disk 16 and/or collecting portion 19 may be or comprise Molybdenum or Tungsten, or an alloy thereof.

[0069] The heating element may comprise tungsten or carbon, such as tungsten or carbon wires. The heating element may be embedded in the disk and/or collecting portion. It will be appreciated that in other embodiments, the heating element may be provided separately from the disk. In such embodiments, the heating element may be arranged to increase the temperature of the disk and/or collector. For example, in the embodiments shown in Figures 2a and 2b, the heating element 23 is provided in the form of a ring-shaped heating element 23, which is arranged below disk 16. It should be understood that the disk, collector and/or heating element disclosed herein are not restricted to the above described materials or shapes and that in other embodiments other suitable materials or shapes may be used for the disk, collector and heating element. It will be appreciated that in other embodiments, the disk and/or collecting portion may be inductively heated. For example, the heating element may be provided in the form of an electromagnetic element, e.g. a coil or the like. An electronic oscillator, e.g. an Radio Frequency generator, may be provided to generate electric currents in the electromagnetic element, which may result in heat being generated in the electromagnetic element.

[0070] The exemplary fuel collector 15 shown in Figures 2a and 2b comprises a plurality of openings 24. In the exemplary fuel collector shown in Figures 2a and 2b the plurality of openings is provided in the form of a plurality of through-holes 24. The plurality of through- holes may be arranged on the disk 16, e.g. on a circumferential region of the disk 16. Alternatively or additionally, the plurality of through-holes 24 may be arranged on/in the collecting portion 19. The plurality of through-holes 24 may be configured to allow passage of melted fuel from the disk 16 and/or collecting portion 19 into the second receptacle 20, for example, during removal of the fuel from the disk 16 and/or collecting portion 19, as will be described below. During rotation of the disk 16, the heating element 23 may heat the disk 16 and/or collecting potion 19 to a temperature equal to or above the melting temperature of the fuel, as described above. The temperature may be selected such that during rotation of the disk 16, the fuel on the disk 16 and/or collecting portion 19 is melted and/or the fuel can flow through the through-holes 24, e.g. to a lower side of the disk 16 and/or the collecting portion 19. For example, the collecting portion may comprise a first portion 19' and a second portion 19" . The first and second portions 19', 19" may be arranged to extend from the disk 16, e.g. in opposing directions from the disk 16. The first and second portions 19', 19" of the collecting portion 19 may be considered as extending at least partially in a direction substantially parallel to a rotational axis A, e.g. defined by the axle or shaft 22. The first portion 19' of the collecting portion 19 may define an upper side of the collecting portion 19. The second portion 19" of the collecting portion 19 may define the lower side of the collecting portion 19. The plurality of through-holes 24 may be configured to allow passage of melted fuel from the disk 16 and/or the first portion 19' of the collecting portion 19 to the second portion 19" of the collecting portion and/or the lower side of the disk 16. In operation, melted fuel in the collecting portion 19, e.g. the first portion 19' thereof, may flow through the through-holes 24 to the second portion 19" of the collecting portion 19 and/or the lower side of the disk 16, e.g. due to centrifugal forces acting of the fuel during rotation of the disk 16 and/or the collecting portion. The fuel collector 15 may comprise approximately 10 through-holes in some embodiments, which may be radially arranged on the disk 16 and/or collecting portion 19. In other embodiments, the fuel collector 15 may comprise approximately 5 to 20 through-holes or any other suitable number of through-holes. A size of each through-hole 24 may be selected such that melted fuel can flow through the through-holes 24, for example, from the disk 16 and/or collecting portion 19, e.g. the first portion 19' thereof, into the second portion 19" of the collecting portion and/or the second receptacle 20. For example, each through-hole 24 may comprise a diameter of about 8 μιη. In other embodiments, the diameter of each through-hole may be about 5 to 20 μιη or may have any other suitable value. Although in the exemplary fuel collector shown in Figures 2a and 2b the plurality of openings is provided in the form of a plurality of through-holes, it will be appreciated that in other embodiments the fuel collector may comprise a porous material. The porous material may be comprised in or be part of the disk and/or the collecting portion. The porous material may provide the plurality of openings.

[0071] The melted fuel may pass or drip from the disk 16 and/or collecting portion 19, e.g. the second portion 19" of the collecting portion 19, into the second receptacle 20 at one or more drip-off portion(s) 24a, which are indicated in Figure 2a. The drip-off portions 24a may be part of the disk 16 and/or collecting portion 19. For example, the second portion 19" of the collecting portion 19 may define at least one of the drip-off portions 24a. The drip-off portions 24a may be arranged on the lower side of the disk 16 and/or the collecting portion 19. The one or more drip-off portions may be shaped such that fuel can flows from the disk 16 and/or collecting portion 19, e.g. the second portion 19" of the collecting portion 19, into the second receptacle 20 at the one or more drip-off portions. For example, the melted fuel may flow from the disk 16 into the first portion 19' of the collecting portion 19 and through the through-hole 24 to the second portion 19' of the collecting portion 19 and/or the lower side of the disk 16 before passing into the second receptacle 20. The arrangement of the through-holes 24 on the disk 16 and/or collecting portion 19 may ensure that fuel flows into the second receptacle 20 at the drip-off portions 24a and/or may prevent fuel from flowing over the surface of the disk 16 and/or collecting portion 19. Additionally or alternatively, the arrangement of the through- holes 24 on the disk 16 and/or collecting portion 19 and/or the shape of the drip-off portions 24a may prevent fuel from contacting or passing on other parts of the fuel collector 15, e.g. the heating element 23, motor 21 and/or shaft 22.

[0072] The heating element 23 may be configured to heat the second receptacle to a temperature that is equal or larger than a melting temperature of the fuel. By heating the second receptacle 20 to temperature that is equal or larger than a melting temperature of the fuel, the fuel in the second receptacle may be evenly distributed and/or the build-up of stalagmites may be prevent. The second receptacle 20 may be frequently or infrequently heated to distribute the fuel therein and/or to prevent the build-up of stalagmites. In other embodiments, the fuel collector 15 may comprise a further heating element (not shown) for heating the second receptacle. The further heating element may be provided in addition to the heating element 23.

[0073] Figures 3a and 3b schematically depict another exemplary fuel collector 115. The fuel collector shown in Figures 3 a and 3b is similar to the fuel collector 15 shown in Figures 2a and 2b. However, in the fuel collector 15 shown in Figures 3a and 3b, the moveable surface is defined by the collecting portion 119 and the fuel is received directly by the collecting portion. The collecting portion 119 may comprise any of the features of the collecting portion 19 described above in relation to the embodiment shown in Figures 2a and 2b.

[0074] In the embodiment shown in Figures 3a and 3b, the fuel collector 115 is arranged such that the fuel that passes through the plasma formation region 4 without being converted to radiation emitting plasma is incident on the moveable collecting portion 119. The fuel collector 115 is configured to move the collecting portion 119 at a velocity or speed that is selected in dependence on a velocity or speed of the fuel. For example, the velocity or speed of the collection portion 119 comprises a tangential velocity or speed. The tangential velocity of speed of the collecting portion may be selected such as to substantially correspond to the velocity or speed of the fuel. The collecting portion 119 may be arranged to rotate relative to the path of the fuel 117 in the radiation source SO such that the tangential velocity or speed of the collecting portion 119 in a direction along at least part of the path of the fuel 117 may substantially correspond to the velocity or speed of the fuel. This may allow collision forces between the fuel and the collector to be minimized and may prevent back-scattering of the fuel from the collecting portion 119, e.g. into the radiation source.

[0075] The collecting portion 119 may define an area 119a, e.g. a landing area, on which fuel may be incident. The collecting portion 119 may be arranged relative to the path of the fuel 117 in the radiation source SO such that at least one point 119b, e.g. a landing point or soft landing point of the area 119a, may move or be considered as moving substantially in the same direction as the fuel. The point 119b may be a point on the area 119a on which the fuel is incident. On/at the point 119b of the area 119a the difference between the speed or velocity of the fuel and the moving collecting portion 119 may be minimized and/or zero or almost zero. This may allow collision forces between the fuel and the collector to be minimized and may prevent back-scattering of the fuel form the collecting portion 119, e.g. into the radiation source.

[0076] In the exemplary fuel collector 115 shown in Figures 3a and 3b, the collecting portion 119 may comprise a radius of approximately 10 cm. For example, the fuel may have a speed or velocity of 100 m/s, which may result in selection of a speed or velocity of the collecting portion 119 of 10000 rpm (approximately 1050 rad/s). This speed or velocity of the collecting portion 119 may result in a difference of the speed or velocity of the fuel and the moving collecting portion 119 to be minimized and/or may be zero or almost zero.

[0077] The exemplary fuel collector 115 shown in Figures 3a and 3b comprises a plurality of support elements 125, which may be provided in the form of plurality of support struts or spokes. The support elements 125 are arranged for supporting and/or mounting the collecting portion 119.

[0078] In the embodiment shown in Figures 3 a and 3b, the heating element 123 is arranged on a circumference of the collecting portion 19, for example, to surround the collecting portion 119. For example, the heating element 123 may be ring-shaped. This arrangement may allow heating of the collecting portion 119. The heating element 123 may be configured to heat the collecting portion 119 to a temperature that is equal to or greater than a melting temperature of the fuel. The heating element 123 may be configured to heat the collecting portion 119 such that fuel in the collecting portion 19 is partially melted. By heating the collecting portion, the fuel may be evenly distributed in the collecting portion, which may allow the amount of fuel collected by the fuel collector may be increased.

[0079] It will be appreciated that in other embodiments the heating element may be configured to heat the collecting portion to a temperature that is less than a melting temperature of the fuel. Alternatively or additionally, the heating element may be configured to maintain the collecting portion at a temperature that is below the melting temperature of the fuel. In this example, the fuel that is incident on the collecting portion may solidify in the collecting portion. The solidified fuel may be removed from the collecting portion at a later stage, as will be described below. By allowing the fuel to solidify in the collecting portion, the amount or layer of fuel in the collecting portion may increase in use. However, by moving the collecting portion at a speed or velocity that substantially corresponds to the speed or velocity of the fuel, back- scattering of the fuel from the collecting portion, e.g. into the radiation source, may be minimized and/or prevented. This may minimize or prevent contamination of the radiation source.

[0080] As described above, the second receptacle 120 (which is referred to as the second receptacle 20 in Figures 2a and 2b) may be ring-shaped or donut-shaped. An outer diameter Dl of the second receptacle 20 may be larger than an outer diameter D2 of the collecting portion 119. An inner diameter D3 of the second receptacle may be smaller than an inner diameter D4 of the collecting portion 119. This may allow melted fuel to be collected by the second receptacle 120, as will be described below.

[0081] Figures 4a and 4b schematically depict another exemplary fuel collector 215. The fuel collector 215 shown in Figures 4a and 4b is similar to that shown in Figures 3a and 3b. The fuel collector 215 may comprise any of the features of the fuel collector 115 described in relation to Figures 3a and 3b (e.g. second receptacle 220 is referred to as receptacle 20 in Figures 2a, b and receptacle 120 in Figures 3a, b).

[0082] The collecting portion 219 of the exemplary fuel collector 215 shown in Figures 4a and 4b is arranged to extend at an angle β of approximately 90 degrees relative to the path of the fuel 217 in the radiation source. For example, the collecting portion 219 may be arranged such that at least a part 219a of the collecting portion 219 on which the fuel is incident extends at an angle β of approximately 90 degrees relative to the path of the fuel 217 in the radiation source. By arranging the collecting portion (or the part thereof) to extend at an angle of approximately 90 degrees, the fuel collector 215 may comprise a reduced sensitivity to deviation in the path of the fuel emitted by the fuel source. It will be appreciated that in other embodiments the part of the collection portion on which the fuel is incident may extend at an angle of more than 90 degrees. The collecting portion 219 may be arranged so that in a transverse direction of the collecting portion 219, the collecting portion 219 extends at the grazing angle relative to the path of the fuel 217 in the radiation source SO, as shown in Figure 4a.

[0083] As described above, the collecting portion 219 may rotate relative to the path of fuel 217. Fuel that is incident on the collecting portion 219 (or the part 219a thereof) may be spread- out in the collecting portion 219, for example, over a surface of residue fuel on the collecting portion 219. The part 219a of the collecting portion 219 may define the area, e.g. the landing area 219a, on which fuel may be incident. A point 219b of the area 219a on which the fuel is incident may be considered as a landing point or hard landing point of the area 219a. The incident fuel may be subjected to centrifugal forces, which may be due to the moving or rotating collecting portion 219. Residue fuel that may be present on the collecting portion 219 may be at least partially melted and/or may impart cohesion forces on the incident fuel. This may prevent the incident fuel from being scattered and/or from plunging on to the collecting portion and forming splashes. The collecting portion 219 may be moved or rotated so as to spread the incident fuel in the collecting portion 219. The collecting portion in this embodiment may be referred to as an eraser.

[0084] In the exemplary fuel collector 215 shown in Figures 4a and 4b, the velocity of speed of the collecting portion 219 may be selected to be different to the velocity of speed of the fuel. For example, the velocity or speed of the collecting portion 219 may be selected to larger or less than the velocity or speed of the fuel. The velocity or speed of the collecting portion 219 may be selected such that back-scattering of fuel incident on the collecting portion 219 (or the part 219a thereof) is prevented. For example, the velocity of speed of the collecting portion 219 may be selected such fuel that is incident on the collecting portion 219 (or the part 219a thereof) may be spread-out in the collecting portion 219, for example, immediately on contact between the fuel and the collecting portion 219 (or the part 219a thereof).

[0085] The collecting portion may be configured for receiving and/or retaining fuel that is incident thereon. For example, a height of the collecting portion 219, e.g. a part of the collecting portion 219, such as the first portion 219' of the collecting portion 219, extending at least partially in a direction substantially parallel to the rotational axis A, e.g. defined by the axle or shaft 222, of the fuel collector 215, may be of 15 to 50 mm, e.g. approximately 30 mm.

[0086] Figure 5 shows steps of an exemplary method disclosed herein. The method will be described in relation to the fuel collector 15 shown in Figures 2a and 2b. However, it will be appreciated that the method may be applicable to other fuel collectors, such as the exemplary fuel collectors described in relation to Figures 3a, 3b, 4a and 4b. The method comprises providing fuel to a plasma formation region 4. The method comprises providing a laser beam 2 at the plasma formation region 4 to convert at least part of the fuel into a radiation emitting plasma. The method comprises collecting at least another part of the fuel by moving a surface for receiving the fuel at a velocity or speed that is selected in dependence on a velocity or speed of the fuel, as shown in Step A. The at least other part of the fuel comprises fuel that passes through the plasma formation region 4 without being converted to radiation emitting plasma. The surface may be provided in the form of disk 16 and/or collecting portion 19, as described above. The method may comprise rotating the disk 16 and/or collecting portion 19 relative to the fuel, e.g. around the rotational axis A defined by the shaft 22.

[0087] The method may comprise rotating or moving the disk 16 and/or collecting portion 19 at a velocity or speed of the surface that substantially corresponds to the velocity or speed of the fuel.

[0088] The method may comprise gradually increasing the velocity or speed of the surface from a first velocity or speed, which may be zero, to a second velocity or speed, which may be selected in dependence on the velocity or speed of the fuel, for example, during start-up operation of the radiation source SO or subsequent to maintenance operations to the radiation source SO.

[0089] As described above, the fuel collector 15 may comprise a heating element 23. The method may comprise heating of the disk 16 and/or collecting portion 19 to a temperature that is equal to or greater than a melting temperature of the fuel. For example, the disk 16 and/or collecting portion 19 may be heated while moving or rotating the disk 16 and/or collecting portion 19 relative to the fuel (Step B). By heating the disk and/or collector while rotating or moving the disk and/or collector relative to the fuel, fuel on the disk and/or collector may be subjected to centrifugal forces, which may move the fuel radially outwards of the disk 16 and/or collecting portion 19. Step B in Figure 5 shows the fuel 26 as being located in the collecting portion 19, e.g. due to the centrifugal forces imparted on the fuel during rotation of the disk 16. For example, the fuel 26 collected on the disk 16 and/or collecting portion 19 may be moved radially outwards into the first portion 19' of the collecting portion 19. In operation, the fuel 26 may flow through the through-holes 24 to the second portion 19" of the collecting portion 19. For example, the centrifugal forces acting on the fuel 26 during rotation of the disk 16 and/or collecting portion 19 may push or force the fuel 26 through the through-hole 24 into the second portion 19" of the collecting portion 19. This flow of fuel 26 from the first portion 19' to the second portion 19" of the collecting portion 19 may continue until the centrifugal forces acting on the fuel in each of the first and second portions 19', 19" compensate each other or cancel each other out. Some fuel may remain on the disk 16 and may, for example, form a layer of fuel 26a thereon. The remaining fuel 26a on the disk 16 may reduce reduced back-scattering of fuel from the disk 16, .e.g. due to cohesion forces imparted on the fuel incident on the disk 16 by the remaining fuel 26a. Alternatively or additionally, the method may comprise heating or maintaining of the disk and/or collecting portion to or at a temperature that is less a melting temperature of the fuel. In this example, the fuel that is incident on the disk and/or collecting portion may solidify on the disk. The solidified fuel may be removed from the disk and/or collecting portion at a later stage, as will be described below.

[0090] The method may comprise the step of removing collected fuel 26, 26a from the disk 16 and/or collecting portion 19 (e.g. the first and second portions 19' , 19" thereof). This step may be part of a drip-off sequence, which may be initiated when the provision of fuel to the plasma region 4 is stopped or interrupted, for example, during maintenance operation to the radiation source or one or more component(s) thereof.

[0091] The step of removing collected fuel 26, 26a from the disk 16 and/or collecting portion 19 may comprise allowing cooling of the disk 16 and/or collecting portion 19 so that the collected fuel 26, 26a solidifies, for example, by turning off the heating element 23. The disk 16 and/or collecting portion 19 may be allowed to cool down during movement or rotation of the disk 16 and/or collecting portion 19. The step of removing collected fuel 26, 26a from the disk 16 and/or collecting portion 19 may comprise stopping or terminating movement of the disk 16 and/or collecting portion 19 (Step C), for example, when the collected fuel on the disk 16 and/or collecting portion is solidified. The step of removing collected fuel 26, 26a from the disk 16 and/or collecting portion 19 may comprise heating of the disk 16 and/or collecting portion 19, for example, when the disk 16 and/or collecting portion 19 are stationary, to allow fuel 26, 26a collected on the disk 16 and/or collecting portion 19 to pass into the second receptacle 20 (Step D). Alternatively or additionally, the step of heating of the disk 16 may allow collected fuel 26, 26a to pass from the disk 16 into the collecting portion 19, e.g. into the first and second portions 19' , 19" thereof, as described above, before passing into the second receptacle 20. The disk 16 and/or collecting portion 19 may be heated to a temperature equal to or greater than the melting temperature of the fuel. The melted fuel 26, 26a may flow from the disk 16 and/or collecting portion 19 into the second receptacle 20 via the plurality of through-holes 24. For example, the melted fuel 26, 26a may flow from the disk 16 and/or first portion 19' of the collecting portion 19 to the second portion 19" of the collecting portion and/or the lower side of the disk 16 via the through-holes 24 and then flow from the second portion 19" of the collecting portion 19 and/or the lower side of the disk 16 into the second receptacle 20. The disk 16 and/or collecting portion 19 may be arranged to be inclined towards the second receptacle 20. This may allow the collected fuel 26, 26a to pass into the second receptacle 20, e.g. due to gravitational forces acting on the collected fuel 26, 26a. The arrangement of the through-holes 24 on the disk 16 and/or collecting portion 19, e.g. the second portion 19" of the collecting portion 19, may ensure that fuel flows into the second receptacle 20 at the one or more drip-off portions 24a and/or may prevent fuel from flowing over the surface of the disk 16 and/or collecting portion 19. Additionally or alternatively, the arrangement of the through-holes 24 on the disk 16 and/or collecting portion 19 and/or the shape of the drip-off portions 24a may prevent fuel from contacting or passing on other parts of the fuel collector 15, e.g. the heating element 23, motor 21 and/or shaft 22.

[0092] The second receptacle 20 may be heated, for example, by the heating element 23 or a further heating element (not shown). This may allow the fuel to be evenly distributed or spread in the second receptacle and/or may prevent the build-up of stalagmites in the second receptacle.

[0093] The method may comprise gradually increasing the velocity or speed of the disk and/or collecting portion 19, for example, when at least a portion 26b of the collected fuel is removed from the disk 16 and/or collecting portion 19. The velocity or speed of the disk 16 and/or collecting portion 19 may be increased, for example, while the disk 16 and/or collecting portion 19 have a temperature at which the fuel is at least partially melted. This may allow the fuel, e.g. residual fuel, on the disk 16 and/or collecting portion 19 to be distributed over the disk 16 and/or the collecting portion 19 and/or the disk 16 to remain balanced. Alternatively or additionally, the method may comprise allowing the disk 16 and/or collecting portion 19 to cool to a temperature below a melting temperature of the fuel, prior to the step of increasing the velocity of the disk 16 and/or collecting portion 19.

[0094] 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.

[0095] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4- 10 nm such as 6.7 nm or 6.8 nm.

[0096] Although Figure 1 depicts the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state. A radiation source of this type may be referred to as a discharge produced plasma (DPP) source. The electrical discharge may be generated by a power supply which may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.

[0097] 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.

[0098] 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.