[0001] This application claims priority of EP/US application 17198469.3 which was filed on 26 October 2017 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an apparatus for receiving a conductive fuel. In particular, it relates to an apparatus for receipt of a conductive fuel which is provided with a mechanism for applying a force to the received fuel so as to move it. The apparatus may be a fuel collector which may be suitable for receiving and at least partially containing conductive fuel which it receives. The apparatus may form part of a laser produced plasma radiation source.

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 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] 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. Such a radiation source is known as a laser produced plasma (LPP) radiation source. Within such an LPP radiation source at least a portion of the fuel may be incident on surfaces within the radiation source. It may be desirable to limit the amount of such fuel that is incident upon at the surfaces least of optical elements within the LPP source such as, for example, a radiation collector. The contamination of the surfaces of such optical elements in the radiation source with fuel may lead to a decrease in the performance of the radiation source, 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. [0006] It is an object of the present invention to obviate or mitigate at least one problem of prior art techniques.

SUMMARY

[0007] According to a first aspect of the invention there is provided an apparatus for receiving a conductive fuel, the apparatus comprising: a body defining a surface for receiving fuel; a current generating mechanism for generating an electric current in the body, the electric current having at least a component parallel to the surface; and a magnet generating mechanism arranged to generate a magnetic field with at least a component perpendicular to the surface.

[0008] The apparatus may be a fuel collector. The fuel collector may be suitable for receiving and at least partially containing conductive fuel receive which it receives.

[0009] A conductive fuel may be received by the surface defined by the body. At least a portion of the body proximate to the surface is formed from a conductive material so as to support the electric current. As the current generating mechanism generates an electric current in the body, at least a portion of the current may flow through any conductive fuel that is deposited on, and thereby in contact with, the surface. When a current flows through the fuel, it will exert a force on the fuel.

[00010] The force exerted is given by the Lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

[00011] The apparatus according to the first aspect of the invention is advantageous since it provides a surface for receipt of a conductive and the current generating mechanism and the magnet generating mechanism together provide a mechanism for applying a force to the fuel. This can allow, for example, the fuel to be directed away from an area or region of the surface upon which it is incident and towards, for example, a collection vessel or reservoir for the fuel.

[00012] A beneficial consequence of an apparatus according to the first aspect of the invention, which provides a mechanism for applying a force to fuel that is incident thereon, is that it can prevent build up of fuel on the surface. Generally, when fuel is incident on the surface, a first portion of the fuel may be deposited on the surface and a second portion of the fuel may scatter or rebound from the surface. It may be convenient, for example, to arrange the surface such that, in use, fuel is incident on it at a grazing incidence angle. With such an arrangement, the portion of fuel that is incident on the surface from an inlet aperture or the like and rebounds from the surface will tend to have a trajectory that is directed generally away from the inlet aperture. However, if a sufficiently large quantity of fuel accumulates on the surface then at least a portion of the fuel directed towards the surface may rebound back towards the inlet aperture.

[00013] The first aspect of the invention can therefore generally reduce the amount of fuel which back-scatters from the surface, i.e. rebounds in a direction generally anti-parallel to an initial trajectory of the incident fuel. [00014] The body may comprise a non-conductive support portion and a layer of conductive material, the conductive material defining the surface.

[00015] Such an arrangement is advantageous, as now discussed. The provision of a support portion allows a thickness of the layer of conductive material to be reduced relative to the thickness of material that would be required if the conductive portion of the body were self-supporting. Furthermore, by forming the support portion from a non-conductive material, the current generated in the body is constrained to flow within the, potentially relatively thin, layer of conductive material.

[00016] As explained above, as the current generating mechanism generates an electric current in the body, at least a portion of the current may flow through any conductive fuel that is deposited on the surface. The magnitude of the current flowing through the conductive fuel deposit is dependent on the relative electrical resistances of: the conductive fuel and the layer of conductive material which forms part of the body.

[00017] The electrical resistance of the layer of conductive material is dependent on its thickness (and on its specific resistance). In particular, by reducing the thickness of the layer of conductive material, the resistance of the layer of conductive material is increased. In turn, this increases the portion of current that flows through deposits of conductive fuel on the surface. This has the effect of increasing the force exerted on such deposits, improving the efficiency with which they are moved away from an area or region of the surface upon which it was incident, reducing the tendency of fuel to backscatter from the body.

[00018] The electrical resistance of a deposit of conductive fuel is also dependent on its thickness (and on its specific resistance). The larger the deposit of fuel, the lower the resistance of the deposit and, therefore, the greater the magnitude of current that flows through the deposit of conductive fuel will be. In turn, a greater current will result in a greater force being exerted on the deposit. In this way it can be seen that, advantageously, the apparatus according to the first aspect of the invention is self-regulating in that it applies a greater force to greater deposits (which may tend to cause more back-scattering).

[00019] At least a portion of the layer of conductive material may comprise a material that is easily wetted by tin.

[00020] It will be appreciated that a material that is easily wetted by tin may be defined as a material for which the contact angle at which the liquid-vapor interface of a tin droplet deposited on the material meets the interface between the tin and the material is less than 90°. Suitable materials include, for example, stainless steel ANSI 316L.

[00021] At least a portion of the layer of conductive material may comprise a layer of liquid tin. For example, the apparatus may be provided with a mechanism for wetting the surface with tin. [00022] The apparatus may further comprise a heater arranged to heat the surface. The heater may be arranged to heat the surface to a temperature greater than the melting point of a conductive fuel that, in use, is collected by the apparatus such as, for example, tin.

[00023] The apparatus may further comprise a receptacle for collecting at least a portion of fuel incident on the surface.

[00024] In use, conductive fuel may strike the surface and may then be directed towards the receptacle. The current generating mechanism may be arranged to generate an electric current and the magnet generating mechanism may be arranged to generate a magnetic field such that a force is exerted on the medium supporting the electric current, said force being directed generally towards the receptacle.

[00025] At least a portion of the electric current generated by the current generating mechanism may be supported by conductive fuel deposited on the surface. With an arrangement wherein the force exerted on the medium supporting the electric current is directed generally towards the receptacle, any conductive fuel deposited on the surface is directed towards the receptacle.

[00026] The current generating mechanism may comprise a power supply connected to the body via a physical link to enable the generation of the electric current in the body. It will be appreciated that such a power supply will be arranged to deliver current to, and receive current from, the body via the physical link so as to form an electrical circuit therewith. For example, the power supply may be operable to generate a voltage across at least a portion of the surface.

[00027] Alternatively, the current generating mechanism may be operable to generate the electric current in the body via electromagnetic induction. The current generating mechanism may comprise a magnet which is operable to generate a time varying magnetic field for generating eddy currents in the body.

[00028] For example, the magnet may comprise an electromagnet which may generate a time varying magnetic field by varying a current supplied to the magnet. Additionally or alternatively, the magnet may be movable relative to the surface such that the magnetic field generated by the magnet at a given point on the surface is time varying. The time varying magnetic field will induce eddy currents in the body (and any conductive fuel deposited thereon). For such embodiments, the same magnet may also form part of the magnetic field generating mechanism.

[00029] The current generating mechanism may be arranged to generate an electric current that flows across the surface generally in a first linear direction. With such an arrangement, the force exerted on the medium supporting the electric current by the magnetic field is generally perpendicular to the first linear direction. That is, the force exerted on the medium supporting the electric current by the magnetic field is generally in the same direction for all positions in a plane of the surface. For an arrangement wherein the magnetic field is perpendicular to the surface, the force exerted on the medium supporting the electric current is in a second linear direction which is generally parallel to the surface.

[00030] The magnet generating mechanism may comprise an electromagnet. Such an arrangement may be beneficial since such electromagnets may be operable to exert a magnetic field over a greater range of ambient temperatures. For example, the apparatus may form part of a laser produced plasma radiation source and may operate at temperatures in excess of the melting point of a fuel such as, for example, tin. That is, in use, the apparatus may operate at temperatures in excess of 232 °C. Another benefit of using an electromagnet is that this may be operable to also act as the current generating mechanism.

[00031] Alternatively, the magnet generating mechanism may comprise a permanent magnet. For such embodiments, the permanent magnet preferably has a Curie temperature which is above the melting point of a conductive fuel that it is desired to collect with the apparatus. For example, the apparatus may form part of a laser produced plasma radiation source and may operate at temperatures in excess of the melting point of a fuel such as, for example, tin. It may therefore be preferable for the permanent magnet to have a Curie temperature which is above the melting point of tin (232 °C).

[00032] The apparatus may further comprise a wall defining an inlet aperture. The wall may, for example, separate the body from a source of conductive fuel such as, for example, a fuel droplet generator within a laser produced plasma radiation source. The conductive fuel may pass through the inlet aperture and strike the surface. The wall may therefore provide a barrier between the body which receives the fuel and a region that it is desired to remain relatively free of such fuel (for example the vessel of a laser produced plasma radiation source).

[00033] The wall may form part of a housing that defines a cavity within which the body is disposed. Such a housing may better contain fuel received through the inlet aperture.

[00034] According to a second 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 first portion of the fuel into a radiation emitting plasma; and the apparatus of the first aspect of the invention configured to collect at least a second portion of the fuel.

[00035] The second portion of the fuel may comprise fuel that passes through the plasma formation region without being converted to radiation emitting plasma.

[00036] The surface of the body may be arranged such that the second portion of the fuel is incident on the surface at a grazing incidence angle. Such an arrangement reduces the amount of fuel that "back-scatters" from the surface. That is, less conductive fuel scatters or rebounds from the surface in a direction generally anti-parallel to the initial trajectory of the fuel.

[00037] It will be appreciated that here the term "grazing incidence angle" refers to the angle between the propagation direction of the conductive fuel and the surface that it is incident upon. This angle is complementary to the angle of incidence, i.e. the sum of the grazing incidence angle and the angle of incidence is a right angle. It will be further appreciated that fuel being incident on the surface at a grazing incidence angle is intended to mean that the grazing incidence angle is relatively small such that the path of the fuel is almost parallel to the surface. For example, the grazing incidence angle may be less than 30°. In some embodiments, the grazing incidence angle may be in the range of 2° to 12°, for example approximately 7°.

[00038] According to a third aspect of the invention 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 the first aspect of the invention arranged to provide at least some of said radiation to the lithographic apparatus.

[00039] The lithographic apparatus may comprise an illumination system configured to condition the at least some of said radiation so as to form a radiation beam. The lithographic apparatus may comprise a support structure constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam. The lithographic apparatus may comprise a substrate table constructed to hold a substrate. The lithographic apparatus may comprise a projection system configured to project the patterned radiation beam onto the substrate.

[00040] According to a fourth aspect of the invention there is provided a method for moving a deposit of conductive fuel on a surface of a body, the method comprising: generating a magnetic field with at least a component perpendicular to the surface; generating an electric current in the body, the electric current having at least a component parallel to the surface, such that at least a portion of the current flows through the deposit of conductive fuel.

[00041] When the electric current flows through the deposit of conductive fuel, it will exert a force on the fuel. The force exerted is given by the Lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

[00042] The method according to the fourth aspect of the invention is advantageous since it provides a simple arrangement for applying a force to the deposit of fuel. This can allow, for example, the fuel to be directed away from an area or region of the surface upon which it is incident and towards, for example, a collection vessel or reservoir for the fuel.

[00043] The electrical resistance of a deposit of conductive fuel is dependent on its thickness. The greater the deposit of fuel, the lower the resistance of the deposit and, therefore, the greater the magnitude of current that flows through the deposit of conductive fuel will be. In turn, a greater current will result in a greater force being exerted on the deposit by the magnetic field. In this way it can be seen that, advantageously, the method according to the fourth aspect of the invention is self- regulating in that it applies a greater force to greater deposits (which may tend to cause more back- scattering). [00044] The magnetic field and the electric current may be generated continuously. This may allow conductive fuel to be continuously moved on the surface, limiting the size of deposits of conductive fuel that can form on the surface. Alternatively, the magnetic field and the electric current may be generated intermittently or periodically. This may allow conductive fuel to build-up on the surface when the magnetic field and the electric current are not being generated and then for this build-up to be moved each time the magnetic field and the electric current are generated.

[00045] 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

[00046] 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 which incorporates a fuel collector according to an embodiment of the invention;

Figure 2 is a first view of a fuel collector according to an embodiment of the invention which may form part of the lithographic system shown in Figure 1 ;

Figure 3 is a second view of a fuel collector according to an embodiment of the invention which may form part of the lithographic system shown in Figure 1, showing a deposit of fuel on a surface of a body;

Figure 4 is a third view of a fuel collector according to an embodiment of the invention which may form part of the lithographic system shown in Figure 1, showing the magnetic field generated by a magnet and a Lorentz force acting on a deposit of fuel;

Figure 5 is a plan view of body of the a fuel collector shown in Figures 2, 3 and 4, showing the current generated by a power supply and a Lorentz force acting on a deposit of fuel; and

Figure 6 is cross sectional view of a portion of the body of the fuel collector shown in Figures 2, 3 and 4.

DETAILED DESCRIPTION

[00047] Figure 1 shows a lithographic system including a fuel collector 15 according to one embodiment of the invention. The lithographic system comprises a 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.

[00048] 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.

[00049] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source). A laser 1, which may for example be a CO2 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 16 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.

[00050] The EUV radiation is collected and focused by 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 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.

[00051] The radiation source SO further comprises a fuel collector 15. The fuel collector 15 may be arranged to collect at least part of the fuel that passes through the plasma formation region 4 of the radiation source SO (i.e. along trajectory 16) without being converted to radiation emitting plasma.

[00052] 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.

[00053] A supply of hydrogen may also be 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.

[00054] 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.

[00055] 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.

[00056] 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).

[00057] 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.

[00058] Figures 2 and 3 schematically show a fuel collector 20 according to an embodiment of the invention. The fuel collector 20 may be suitable for use as the fuel collector 15 shown in Figure 1. Therefore, the fuel collector 20 may be arranged 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. Such fuel may propagate generally along trajectory 21, which may coincide with trajectory 16 shown in Figure 1. [00059] 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 (see Figure 1).

[00060] The fuel collector 20 comprises a body 22 defining a surface 24 for receiving a conductive fuel (for example tin).

[00061] The fuel collector 20 further comprises a housing 26 that defines a cavity 28 within which the body 22 is disposed. A wall 30 of the housing defines an inlet aperture 32 of the fuel collector 20. In use, the fuel collector 20 is disposed such that fuel propagating generally along the trajectory 21 passes through the inlet aperture 32, into the cavity 28 and is incident on the surface 24 of the body 22.

[00062] The wall 30 of the housing 26 separates the body 22 from a source of conductive fuel such as, for example, the fuel emitter 3 within the LPP radiation source SO. The wall 30 therefore provides a barrier between the body 22 which receives the fuel and a region that it is desired to remain relatively free of such fuel (for example the vessel of the LPP radiation source SO). The housing 26 serves to at least partially contain fuel received through the inlet aperture 32.

[00063] In use, the fuel collector 20 is disposed such that any fuel incident on the surface 24 of the body 22 (which fuel propagates generally along trajectory 21) is incident on the surface 24 at a grazing incidence angle. Such an arrangement reduces the amount of fuel that "back-scatters" from the surface 24. That is, less conductive fuel scatters or rebounds from the surface 24 in a direction generally anti -parallel to the initial trajectory of the fuel (i.e. back along trajectory 21 and out of the inlet aperture 32). Rather, as indicated by arrows 34, fuel which scatters or rebounds from the surface 24 tends to propagate generally away from the inlet aperture 32.

[00064] It will be appreciated that here the term "grazing incidence angle" refers to the angle between the propagation direction (i.e. trajectory 21) of the conductive fuel and the surface 24 that it is incident upon. This angle is complementary to the angle of incidence, i.e. the sum of the grazing incidence angle and the angle of incidence is a right angle. It will be further appreciated that fuel being incident on the surface 24 at a grazing incidence angle is intended to mean that the grazing incidence angle is relatively small such that the path of the fuel is almost parallel to the surface 24. For example, the grazing incidence angle may be less than 30°. In some embodiments, the grazing incidence angle may be in the range of 2° to 12°, for example approximately 7°.

[00065] The fuel collector 20 further comprises a heater arranged to heat the surface 24. In particular, the heater may be arranged to heat the surface 24 to a temperature greater than the melting point of a conductive fuel that, in use, is collected by the apparatus such as, for example, tin. For example, the body 22 may be heated to a temperature between 250 to 350°C. [00066] It will be appreciated that such a heater may be implemented in various ways. For example, the heater may comprise a heating element configured to heat the body 22, which may, for example, be embedded in the body 22. Alternatively, the heating element may be provided separately from the body 22. Such a heating element may comprise tungsten or carbon, for example tungsten or carbon wires. Alternatively, the body 22 may be inductively heated. It will be appreciated that, in addition or alternatively, heating can also be achieved by an electrical current that is driven through the body 22.

[00067] As fuel is incident upon the surface 24 it tends to accumulate on the surface 24. As explained, arranging the fuel collector 20 such that fuel is incident on the surface 24 of the body 22 at a grazing incidence angle reduces the amount of fuel that "back-scatters" from the surface 24. However, if a sufficiently large quantity of fuel accumulates on the surface 24 then at least a portion of the fuel directed towards the surface may rebound back towards the inlet aperture. This is illustrated in Figure 3, which shows a deposit 36 of fuel on the surface 24. The deposit 36 may be of the form of a drop of fuel on the surface 24 which may, for example, have been formed from a plurality of droplets received from the fuel emitter 3. When such a deposit 36 forms on the surface, as indicated by arrows 38, a greater fraction of fuel which scatters or rebounds from the surface 24 tends to propagate generally towards the inlet aperture 32.

[00068] The fuel collector 20 further comprises a magnet 40. As shown in Figure 4, the magnet 40 is arranged to generate a magnetic field B (depicted by dashed lines) which is generally perpendicular to the surface 24, at least in a region of the surface 24 upon which fuel is incident. The region of the surface 24 upon which fuel is incident may be referred to as the landing zone and may have dimensions of the order of 2mm x 2mm. The magnet 40 may be arranged to generate a magnetic field over at least a region of the surface 24 with dimensions of the order of 5cm x 5cm and centered on the landing zone. The magnet 40 may be considered to be a magnetic field generating mechanism.

[00069] It will be appreciated that, at least with the arrangement shown in Figures 2 to 4, in order for the magent 40 to generate a magnetic field B in the landing zone on the surface 24, the body 22 may be made from a generally non-magnetizable material. That is, the body 22 may be formed form a material that has relatively low magnetic susceptibility so as to minimise any shielding of the magnetic field B by the body 22. Suitable materials for the body 22 include, for example, stainless steel.

[00070] In one embodiment, the magnet 40 comprises an electromagnet. Such an arrangement may be beneficial since such electromagnets may be operable to exert a magnetic field over a greater range of ambient temperatures. For example, in use, an LPP radiation source SO may operate at temperatures in excess of the melting point of a fuel such as, for example, tin. That is, in use, the fuel collector 20 may operate at temperatures in excess of 232 °C. Alternatively, the magnet 40 may comprise a permanent magnet. For such embodiments, the permanent magnet preferably has a Curie temperature which is above the melting point of a conductive fuel that it is desired to collect with the fuel collector 20 (for example tin).

[00071] As shown in Figure 5, the fuel collector further comprises a power supply 42 connected to the body 22 via a physical link, for example wires 44. The power supply 42 is operable to generate an output current and to deliver the output current to the body such that the output current I flows through the body 22 (depicted by dashed lines). The power supply 42 is therefore arranged to deliver current to, and receive current from, the body 22 via the wires 44 so as to form an electrical circuit therewith. The power supply 42 is operable to generate a voltage across at least a portion of the surface 24. As shown in Figure 5, the current I flows across and parallel to the surface 24. The power supply 42 and wires 44 may be considered to form a current generating mechanism that is operable to generate an electric current in the body 22.

[00072] The fuel collector 20 is suitable for receiving and at least partially containing conductive fuel (for example tin) which it receives.

[00073] In use, a conductive fuel may be received by the surface 24 defined by the body 22. At least a portion of the body 22 proximate to the surface 24 is formed from a conductive material so as to support an electric current. As the power supply 42 generates an electric current in the body 22, at least a portion of the current I may flow through any conductive fuel that is deposited on, and thereby in contact with, the surface 24. Due to the magnetic field B generated by the magnet 40, when a current flows through the fuel, a force will be exerted on the fuel. The force exerted is given by the Lorentz force formula. In particular, the force is perpendicular to both the magnetic field and the current.

[00074] The fuel collector 20 is advantageous since it provides a surface 24 for receipt of a conductive and the power supply 42 and the magnet 40 together provide a mechanism for applying a force to the fuel. This can allow, for example, the fuel to be directed away from an area or region of the surface 24 upon which it is incident and towards, for example, a collection vessel or reservoir for the fuel.

[00075] Since the fuel collector 20 has a mechanism for applying a force to fuel that is incident thereon the fuel collector 20 can prevent build up of fuel on the surface 24. In turn, this can generally reduce the amount of fuel which back-scatters from the surface 24 (i.e. rebounds in a direction generally anti-parallel to an initial trajectory 21 of the incident fuel) by reducing the build up of fuel which can cause such back-scattering (see Figure 3).

[00076] As mentioned above, an electrical current that is driven through the body 22 (such as, for example, the current I driven by power supply 42) can heat the body 22, for example to heat the surface 24 to a temperature greater than the melting point of a conductive fuel that, in use, is collected by the apparatus. Although, if in use the mechanism used to apply a Lorentz force to such fuel that is incident on the surface 24 is used intermittently (i.e. only at repeated time intervals) then a separate heat source or heater may also be provided.

[00077] The fuel collector 20 further comprises a receptacle 46 for collecting at least a portion of fuel incident on the surface 24. In use, conductive fuel may strike the surface 24 and may then be directed towards the receptacle 46.

[00078] The power supply 42 is arranged to generate an electric current I and the magnet 40 is arranged to generate a magnetic field such that a force is exerted on the medium supporting the electric current, said force being directed generally towards the receptacle 46, as now discussed.

[00079] The surface 24 defined by the body 22 may be generally planar. In the following, a direction generally perpendicular to the surface 24 may be referred to as the z direction. The directions in the plane of the surface 24 may be referred to as the x-y plane.

[00080] As shown in Figure 4, at least in the vicinity of the region of the surface upon which the conductive fuel is incident, the magnetic field B generated by the magnet 40 is perpendicular to the surface 24, i.e. the magnetic field B is generally in the z direction. As shown in Figure 5, the current I generated by the power supply 42 flows through the body 22 generally parallel to the surface 24 in the negative y-direction. At least a portion of the electric current I generated by the power supply 42 may be supported by conductive fuel deposited on the surface 24, also generally in the negative y- direction. With such an arrangement the force exerted on the medium supporting the electric current I is directed generally in the negative x-direction. As can be seen from Figure 4, this directs the fuel towards an edge 25 of the surface 24 which is disposed above the receptacle 46. The fuel can pass or drip from the edge 25 of the surface 24 and fall under gravity into the receptacle 46. In this way, any conductive fuel deposited on the surface 24 is directed towards the receptacle 46.

[00081] It will be appreciated that as the deposit 36 of fuel grows in size, its resistance will decrease and therefore the portion of current which flows through the deposit 36 of fuel increases. Therefore, although not shown in Figure 5 (which is rather schematic), in practice, the amount of current supported by the deposit 36 of fuel may be significantly greater than that supported by the surrounding surface 24 (or a layer of conductive fuel provided thereon).

[00082] A volume of the receptacle 46 may be selected such that the radiation source SO or one or more component(s) thereof may require replacing, before fuel collected in the receptacle 46 may need to be removed. In other words, the volume of the receptacle 46 may be selected such that an amount of fuel that is incident in a time period between maintenance operations of the radiation source SO can be collected and/or retained in the second receptacle. For example, the receptacle 46 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 receptacle 46, e.g. when the receptacle 46 is full or almost full. The suction device may be arranged to transfer the fuel from the receptacle 46 to an exterior of the radiation source SO. [00083] The receptacle 46 may be provided with a heater (not shown) with is configured to heat the receptacle 46 to a temperature that is equal or larger than a melting temperature of the fuel. By heating the receptacle 46 to a temperature that is equal or larger than a melting temperature of the fuel, the fuel in the receptacle 46 may be evenly distributed and/or the build-up of stalagmites may be prevented. The receptacle 46 may be frequently or infrequently heated to distribute the fuel therein and/or to prevent the build-up of stalagmites.

[00084] As shown in Figure 6, the body 22 comprises a non-conductive support portion 22a and a conductive portion 22b. In turn, the conductive portion 22b comprises a layer 48 of a material that is easily wetted by the conductive fuel (for example tin) and a layer 50 of the conductive fuel (for example tin). The conductive portion 22b may be referred to as the layer of conductive material 22b.

[00085] Note that, in general, the Lorentz force will be generated throughout the materials in the magnetic field B and within which the current I is flowing. Therefore, the Lorentz force is exerted on the entire conductive portion 22b which is in the magnetic field B.

[00086] It will be appreciated that a material that is easily wetted by tin may be defined as a material for which the contact angle at which the liquid-vapor interface of a tin droplet (i.e. the edge of the tin droplet) deposited on the material meets the interface between the tin and the material is in less than 90°. Such materials may be referred to as "tin-philic". Suitable materials include, for example, stainless steel ANSI 316L. Tungsten may be treated so as to make it tin-philic.

[00087] Stainless steel ANSI 316L has a resistivity (specific resistance) of 74 μΩΰΐη whereas tin has a resistivity (specific resistance) of 1100 μΩΰΐη. Therefore, for embodiments wherein the layer 48 of a material that is easily wetted by the tin comprises stainless steel ANSI 316L and the thickness of the layer 48 is approximately the same as that of the layer 50 of tin, the magnitude of the current passing through the tin will be approximately a factor of 14 greater than the magnitude of the current passing through the stainless steel.

[00088] It will be appreciated that the fuel collector 20 may be provided with a mechanism for wetting the surface of the layer 48 with tin so as to provide the layer 50 of liquid tin. The layer of conductive material 22b defines the surface 24.

[00089] The provision of a support portion 22a allows a thickness of the layer of conductive material 22b to be reduced relative to the thickness of material that would be required if the conductive portion 22b of the body were self-supporting. Furthermore, by forming the support portion 22a from a non-conductive material, the current generated in the body is constrained to flow within this, potentially relatively thin, layer of conductive material 22b.

[00090] As explained above, as the power supply 42 generates an electric current I in the body 22, at least a portion of the current may flow through any conductive fuel that is deposited on the surface 24. The magnitude of the current I flowing through the conductive fuel deposit is dependent on the relative electrical resistances of: the conductive fuel and the layer 22b of conductive material which forms part of the body 22.

[00091] The electrical resistance of the layer of conductive material 22b is dependent on its thickness. In particular, by reducing the thickness of the layer 22b of conductive material, the resistance of the layer 22b of conductive material is increased. In turn, this increases the portion of current that flows through deposits of conductive fuel on the surface 24. This has the effect of increasing the force exerted on such deposits, improving the efficiency with which they are moved away from an area or region of the surface 24 upon which it was incident, reducing the tendency of fuel to backscatter from the body 22.

[00092] The electrical resistance of a deposit of conductive fuel is also dependent on its thickness. The greater the deposit of fuel, the lower the resistance of the deposit and, therefore, the greater the magnitude of current that flows through the deposit of conductive fuel will be. In turn, a greater current will result in a greater force being exerted on the deposit. In this way it can be seen that, advantageously, the fuel collector 20 is self-regulating in that it applies a greater force to larger deposits (which may tend to cause more back- scattering).

[00093] The power supply is arranged to generate an electric current I that flows across the surface 24 of the body 22 generally in a first linear direction (the y direction in Figure 5). With such an arrangement, the force exerted on the medium supporting the electric current I by the magnetic field B is generally perpendicular to the first linear direction. That is, the force exerted on the medium supporting the electric current I by the magnetic field B is generally in the same direction for all positions in a plane of the surface 24. Since the magnetic field B is perpendicular to the surface 24, the force F exerted on the medium supporting the electric current I is in a second linear direction which is generally parallel to the surface (the negative x direction in Figures 4 and 5). Note that the directions of the current I and the magnetic field B are chosen such the force F exerted on the medium supporting the electric current I is directed towards the edge 25 of the surface 24 which is disposed above the receptacle 46.

[00094] For an embodiment wherein the magnet 40 exerts a generally constant magnetic field strength of 0.2 T over the landing zone and a current of 10 A is driven through the body 22 which has a length (in the y direction) of 0.05 m, the force experienced by the current is 0.1 N. If the bulk of this current is carried by a lump of tin with a mass of 1 g then the lump of tin will experience an acceleration of the order of 100 m/s ² .

[00095] According to an embodiment of the present invention, there is provided a method for moving a deposit of conductive fuel on a surface 24 of a body 22, as now discussed.

[00096] The method comprises: generating a magnetic field B with at least a component perpendicular to the surface 24; and generating an electric current I in the body 22, the electric current having at least a component parallel to the surface 24, such that at least a portion of the current I flows through the deposit of conductive fuel. When the electric current I flows through the deposit of conductive fuel, it will exert a force on the fuel. The force exerted is given by the Lorentz force formula. In particular, the force is perpendicular to both the magnetic field B and the current I.

[00097] This method is advantageous since it provides a simple arrangement for applying a force to the deposit of fuel. This can allow, for example, the fuel to be directed away from an area or region of the surface 24 upon which it is incident and towards, for example, a collection vessel 46 or reservoir for the fuel.

[00098] The electrical resistance of a deposit of conductive fuel is dependent on its thickness. The larger the deposit of fuel, the lower the resistance of the deposit and, therefore, the greater the magnitude of current I that flows through the deposit of conductive fuel will be. In turn, a greater current I will result in a greater force being exerted on the deposit by the magnetic field B. In this way it can be seen that, advantageously, the method is self -regulating in that it applies a greater force to greater deposits (which may tend to cause more back-scattering).

[00099] The magnetic field B and the electric current I may be generated continuously. This may allow conductive fuel to be continuously moved on the surface 24, limiting the size of deposits of conductive fuel that can form on the surface 24. Alternatively, the magnetic field B and the electric current I may be generated intermittently or periodically. This may allow conductive fuel to build-up on the surface when the magnetic field B and the electric current I are not being generated and then for this build-up to be moved each time the magnetic field B and the electric current I are generated.

[000100] In an alternative embodiment, the magnet 40 may be operable to generate both: (a) a magnetic field with at least a component perpendicular to the surface 24; and (b) an electric current in the body 22, as now discussed.

[000101] In this second embodiment, the magnet 40 is operable to generate a time varying magnetic field with at least a component perpendicular to the surface 24. For example, the magnet 40 may comprise an electromagnet which may generate a time varying magnetic field by varying a current supplied to the magnet 40. Additionally or alternatively, the magnet may be movable relative to the surface 24 such that the magnetic field at a given point on the surface 24 is time varying. The time varying magnetic field will induce eddy currents in the body 22 (and any conductive fuel deposited thereon). Therefore, in such embodiments, the magnet 40 may be considered to form a current generating mechanism that is operable to generate an electric current in the body 22 (in addition to a magnetic field generating mechanism). In turn, these eddy currents interact with the magnetic field, such that a Lorentz force will act on the medium supporting the eddy currents (for example a deposit of conductive fuel).

[000102] Note that in this second embodiment, the electric current I that flows through the body 22 will not be generally in a linear direction but will circulate in closed loops. Therefore, the force exerted on current carrying media in different positions on the surface 24 will, in general, experience forces in a range of different directions. It will be appreciated that the magent 40 is arranged such that the time varying magnetic field causes conductive fuel deposited on the surface in a landing zone (i.e. a region of the surface 24 upon which fuel is incident during use) to be transported away from the landing zone. For example, the time varying magnetic field may be arranged such that conductive fuel deposited on a landing zone is forced generally outwards from a center of the landing zone.

[000103] With such an embodiment the Lorentz force will not, in general, direct fuel towards the edge 25 of the surface 24 which is disposed above the receptacle 46. However, as shown in the Figures, the body 22 may be arranged such that the surface 24 is inclined relative to a horizontal direction. This may allow conductive fuel to flow, under gravity, generally towards the edge 25 to fall under gravity into the receptacle 46.

[000104] Such an embodiment does not comprise a power supply 42 connected to the body 22 via a physical link (although it may comprise a power supply for an electromagnet).

[000105] Although the specific embodiments described above comprise a fuel collector 20 which is provided with a mechanism for applying a force to conductive fuel which is incident on a surface, alternative embodiments may comprise other apparatus. Such other apparatus may, for example, comprise a component of a laser produced plasma radiation source SO. For example, such other apparatus may comprise a radiation collector. In general, embodiments of the invention may relate to any surface that it is desired to remove a conductive fuel (for example tin) from.

[000106] 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.

[000107] 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.

[000108] 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.

[000109] 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.

[000110] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[000111] 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.