Fuel system, method of fueling, and lithographic apparatus

Abstract

Disclosed herein is a fuel system for a source of EUV radiation, the fuel system comprising: a container having an internal region for receiving solid fuel; at least one holding pin arranged to hold solid fuel in the internal region spaced apart from walls of the container; a first heating system arranged to heat the walls of the container; and a second heating system arranged to heat each holding pin.

Description

FUEL SYSTEM, METHOD OF FUELING, AND LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23190072.1 which was filed on 07 August 2023 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a system for loading fuel into a radiation source of a lithographic apparatus. In particular, the invention relates to a system for loading fuel into a radiation source that uses plasma to generate EUV radiation for a lithographic apparatus.BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of around 50 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, is used in photolithography processes to produce extremely small features in and on substrates, for example, silicon wafers, that are used to produce integrated circuits and various other microelectronic devices.

[0004] Methods for generating EUV light include, but are not limited to, altering the physical state of a source material to a plasma state. The source material includes a compound or an element, for example, xenon, lithium, or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma is produced by irradiating a source material, for example, in the form of a droplet, stream, or cluster of source material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment. The source material, such as xenon, lithium, or tin, which emit in the EUV range when in the plasma state, are commonly referred to as target material since they are targeted and irradiated by the drive laser.SUMMARY

[0005] According to a first aspect of the invention, there is provided a fuel system for a source of EUV radiation, the fuel system comprising: a container having an internal region for receiving solid fuel; at least one holding pin arranged to hold solid fuel in the internal region spaced apart from walls of the container; a first heating system arranged to heat the walls of the container; and a second heating system arranged to heat each holding pin.

[0006] According to a second aspect of the invention, there is provided a laser powered plasma based EUV source comprising the fuel system according to the first aspect.

[0007] According to a third aspect of the invention, there is provided an EUV lithographic apparatus comprising the fuel system according to the first aspect.

[0008] According to a fourth aspect of the invention, there is provided a method of generating a supply of molten fuel for an EUV radiation source, the method comprising: loading solid fuel into an internal region of a fuel system; holding the solid fuel in the internal region with the solid fuel spaced apart from the walls of the fuel system; heating the walls to a first temperature that is substantially at, or above, the melting point of the solid fuel; extracting, from the fuel system, any fluid released by the walls of the fuel system when the walls are heated to the first temperature; reducing the temperature of the walls to a second temperature that is lower than the first temperature; and generating a supply of molten fuel by heating the solid fuel to a temperature that is at, or above, the melting point of the solid fuel.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0010] Figure 1 is a block diagram of an apparatus including a first reservoir system, a second reservoir system, a priming system, and a fluid control system, and being configured to supply a nozzle supply system with a fluid target material during continuous operation of the nozzle supply system;

[0011] Figure 2 schematically shows a known priming system;

[0012] Figure 3 schematically shows a priming system according to a first embodiment;

[0013] Figure 4 schematically shows a cross-section through a holding pin according to embodiments;

[0014] Figure 5 schematically shows a priming system according to a second embodiment; and

[0015] Figure 6 is a flowchart of a method according to embodiments.DETAIEED DESCRIPTION

[0016] Referring to Figure 1 , an apparatus 100 is configured to supply a nozzle supply system 140 with a fluid target material 120 during continuous operation of the nozzle supply system 140.

[0017] The nozzle supply system 140 can be configured to supply the fluid target material 120 in the form of a stream of targets 121 for use by a system 124. The fluid target material 120 is a target material that is in a fluid state (such as a liquid state).

[0018] The apparatus 100 includes a first reservoir system 102, a second reservoir system 103, a priming system 104, and a fluid control system 190. The fluid control system 190 is fluidly connected to the priming system 104, the first reservoir system 102, the second reservoir system 103, and the nozzle supply system 140. In some implementations, the fluid control system 190 includes a fluid communication apparatus 116 and a fluid controller 106 operable to adjust one or more aspects of the fluid communication apparatus 116.

[0019] The priming system 104 is configured to receive a solid matter 122 that includes the target material. The fluid communication apparatus 116 is an adjustable fluid flow path that is able to be in fluid communication with the first reservoir system 102, the second reservoir system 103, the priming system 104, and the nozzle supply system 140. For example, the fluid communication apparatus 116 includes fluid transmission lines 115 and one or more regulation devices 118 and 119 configured to regulate, direct, or control the flow of a fluid through the fluid transmission lines by, for example, opening, closing, or partially obstructing various passageways within the fluid transmission lines. The fluid communication apparatus 116 can also include a regulation device 117 along the fluid flow path toward the nozzle supply system 140 to control fluid flow to the nozzle supply system 140.

[0020] Because the apparatus 100 includes two reservoir systems 102, 103 in addition to the priming system 104 (which enables refill of the target material into the entire apparatus 100), the fluid target material 120 can be transferred between the first and second reservoir system 102, 103 while the solid matter 122 of the target material 120 is being added to the priming system 104. In particular, during operation of the nozzle supply system 140 to supply the stream of targets 121, the fluid control system 190 controls the fluid communication between and among the first reservoir system 102, the second reservoir system 103, and the priming system 104 to maintain a continuous supply of the fluid target material 120 to the nozzle supply system 140 so that the stream of targets 121 supplied to the system 124 is not interrupted.

[0021] Additionally, the actions that take place within the apparatus 100 do not adversely impact performance of the nozzle supply system 140 that could otherwise arise due to disturbances in fluid pressure. Moreover, in some implementations, while fluid is being supplied to the nozzle supply system 140, the first reservoir system 102 is in fluid communication with the nozzle supply system 140 and is also maintained at a high pressure, and the first reservoir system 102 can provide a primary source of fluid target material 120 to the nozzle supply system 140.

[0022] Fluid control between and among each of the first reservoir system 102, the second reservoir system 103, the priming system 104, and the nozzle supply system 140 can be independently controlled by the fluid control system 190, and in this way, at least one of the reservoir systems 102 or 103, which is in fluid communication with the nozzle supply system 140, can provide a source of fluid target material 120 to the nozzle supply system 140 at all times during operation of the nozzle supply system 140.

[0023] As discussed above, the fluid communication apparatus 116 includes the fluid transmission lines 115 and the one or more regulation devices 117, 118, 119 configured to regulate, direct, or control the flow of a fluid through the fluid transmission lines 115 by, for example, opening, closing, or partially obstructing various passageways within the fluid transmission lines 115. The fluid transmission lines 115 can include, for example, one or more interconnected tubings formed of tantalum tungsten (TaW) or other suitable materials that can contain the fluid target material 120 at varying and high pressures. The tubings can be flexible. The fluid communication apparatus 116 can include other various fluidcontrol devices not shown that are configured to provide a controllable fluid flow path between the various parts of an apparatus 100. The fluid communication apparatus 116 can include (in addition to the noted fluid transmission lines 115 and one or more regulation devices 117, 118, 119) one or more valves, tubes, fluid flow regulation apparatuses, and tanks.

[0024] Each of the regulation devices 117, 118, 119 can include a valve control apparatus. In this way, fluid flow through a particular regulation device 117, 118, 119 can be adjusted by opening or closing the valve within its valve control apparatus. Each valve control apparatus can include a fluid valve, which can be, for example, a thermally-controlled valve, a manual valve, and / or a motor. In a thermally- controlled valve (which is also referred to as a freeze valve), the pathway is heated to maintain the fluid within the flow path in a liquid state, and the pathway is permitted to cool down or actively cooled to thereby convert the fluid into a solid state. Thus, in some implementations discussed herein, the valve control apparatus within regulation devices 118, 119 includes a freeze valve. Each valve control apparatus of the regulation device 117, 118, 119 can have any suitable shape, such as, for example, a 90° bent tubing, a restricted passage tubing, or a cylindrical tubing.

[0025] The first reservoir system 102 includes a first fluid reservoir 112 that is a vessel configured to contain the fluid target material 120 and be in continuous fluid communication with the nozzle supply system 140 during operation of the nozzle supply system 140. The first fluid reservoir 112 is a volume that is defined by a structure that can be formed, lined, or reinforced with molybdenum (Mo), forged Mo, or any material that remains stable and solid above the melting point of the fluid target material 120 and is also not chemically reactive with the fluid target material 120. The first fluid reservoir 112 is in fluid communication with the nozzle supply system 140 via the fluid communication apparatus 116 that is fluidly coupled and controlled by the fluid controller 106.

[0026] The second reservoir system 103 includes a second fluid reservoir 113 that is a vessel configured to contain the fluid target material 120 and be in fluid communication with the first reservoir system 102 for at least part of the time during operation of the nozzle supply system 140. The second fluid reservoir 113 is a volume that is defined by a structure that can be formed, lined, or reinforced with molybdenum (Mo), forged Mo, or any material that remains stable and solid above the melting point of the fluid target material 120 and is also not chemically reactive with the fluid target material 120. The second fluid reservoir 113 is in fluid communication with the first reservoir system 102 via the fluid communication apparatus 116 and under control of the fluid controller 106.

[0027] The priming system 104 includes a priming tank 114 that is a vessel configured to contain the fluid target material 120 (that is produced from the solid matter 122). The priming tank 114 is in fluid communication with one or more of the first reservoir system 102 and the second reservoir system 103 for at least part of the time during operation of the nozzle supply system 140. The priming system 104 is configured to produce the fluid target material 120 from the solid matter 122 and can fluidly communicate with one or more of the second reservoir system 103 to refill the second reservoir system 103 with the fluid target material 120 and the first reservoir system 102 to refill the first reservoir system102 with fluid target material 120. The priming system 104 can also include a priming chamber 130 configured to receive the solid matter 122 that contains the target material. The priming chamber 130 can include, for example, an openable and / or removable cover such that the solid matter 122 can be replaced inside the priming chamber 130.

[0028] Figure 2 schematically shows a known priming system 200 that may be the same as, or comprised by, the priming system 104 shown in Figure 1.

[0029] The main body of the priming system 200 comprises a container 203. The priming system 200 has a cover 206 that may be opened, and / or removed, so that solid matter 201 may be loaded into the container 203. The solid matter 201 may be received by a heater 202 within the container 203. The heater is arranged to heat the solid matter 201 so that it melts. The priming system 200 has a gas outlet 204. A gas extraction system (that is not shown in Figure 2) may extract gas through the gas outlet 204 to thereby generate a vacuum inside the priming system 200. The priming system 200 has a liquid outlet 205 for allowing a flow of the molten solid matter 201 out of the priming system 200.

[0030] The solid matter 201 may contain the target material and be the same as the solid matter 122 described with reference to Figure 1. Accordingly, the solid matter 201 may be, for example, tin, lithium, or any other suitable target material for use in the generation of EUV radiation.

[0031] To fuel / re-fuel the priming system 200, the cover 206 is opened, in a cleanroom, and solid matter 201 is loaded into the priming system 200. The cover 206 is then closed. The heater 202 then melts the solid matter 201. The molten solid matter is pressurized and supplied to the nozzle supply system 140.

[0032] The solid matter 201 is typically ultra-pure and clean tin that is supplied in the form of tin ingots. This is expensive high quality tin that has gone through a specially designed purification process in order to prevent, or substantially reduce, the presence of contamination and unwanted particles. The tin ingots are handled carefully so as to avoid any contamination and saturation by moisture (i.e. water).

[0033] A problem that may occur in known nozzle supply systems 140, and related parts of an EUV source, is conduits, such as the fluid transmission lines 115, and nozzles becoming clogged due to the presence of tin oxide. Clearing clogged conduits and nozzles typically requires an extremely long downtime of the source.

[0034] The generation of tin oxide may result from the exposure of pure molten tin to either oxygen or water. Tin oxide may also result from the exposure of solid pure tin to either oxygen or water. However, the formation of tin oxide will stop after a layer of tin oxide has formed on the surfaces of the solid tin.

[0035] The walls of container 203 may be made from molybdenum (Mo) and / or stainless steel. These materials absorb moisture, i.e. water, when exposed to air. The inner surfaces of the walls of the container 203 will be exposed to air when the cover 206 of the container 203 is opened. It is necessary to open the cover 206 whenever the priming system 200 is fueled / re-fueled by loading a new tin ingot into the container 203. When a vacuum is generated inside the container 203 following the loading of a tin ingot, some of the moisture absorbed by the walls is released. However, the vacuum may not resultin water from some of the deeper layers within the walls being released. To release more water, a bake out process may be performed in which the walls of the container are heated.

[0036] With the known priming system 200 shown in Figure 2, the same heater is used to both melt the tin and also heat the walls of the container 203. The release of water from the walls of the container 203 is dependent on the temperature of the walls of the container 203. Accordingly, when the temperature of the walls is below the melting point of the tin, there is still water retained within the walls. When the temperature is increased so as to melt the tin, some of this retained water within the walls is released. The bake out process therefore cannot be performed in a way that avoids water coming into contact with molten tin. The known priming system 200 shown in Figure 2 thereby experiences the problem of a relatively high level of tin oxide formation occurring.

[0037] A further problem with the known priming system 200 shown in Figure 2 is that the molten tin may not be fully drained from the container 203 and this may leave residual tin in the container 203. The residual tin, that cannot be removed by the bake out processes, is exposed to air when the cover 206 is opened and therefore mostly becomes tin oxide.

[0038] Embodiments solve one or more of the above problems to thereby improve the fueling / re- fueling process of a priming system.

[0039] Figure 3 schematically shows a priming system 300 according to a first embodiment.

[0040] The priming system 300 comprises a container 303. The container 303 is the main body of the priming system 300. The priming system 300 has a gas outlet 304. A gas extraction system (that is not shown in Figure 3) may extract gas through the gas outlet 304 to thereby generate a vacuum inside the priming system 300. The priming system 300 has a cover 306 that may be opened, and / or removed, so that solid matter 301 may be positioned within an internal region of the container 303. The solid matter301 may be the same as the solid matter 201 for the known priming system 200 described earlier with reference to Figure 2. Accordingly, the solid matter 301 may be an ingot of clean and ultra-pure tin, or any other material suitable for use in the generation of EUV radiation.

[0041] The priming system 300 comprises holding pins 302 that extend into the internal region of the container 303. The holding pins 302 are arranged to receive and hold the solid matter 301 when the solid matter 301 is loaded into the internal region of the container 303. The holding pins 302 may pass through the walls of the container 303 and / or the cover 306. The substantial part of each holding pin302 is not in direct physical contact with the walls of the container 303 or the cover 306. The part of each holding pin 302 that is in direct physical contact with the walls of the container 303 or cover 306 may be covered with thermal insulation so as to reduce the rate of any heat transfer between each holding pin 302 and the walls of the container 303 or cover 306.

[0042] Embodiments include the use of any number of holding pins 302. For example, the number of holding pins may be in the range 1 to 10.

[0043] The solid matter 301 is held within the internal region of the container 303, by the holding pins 302, so that the solid matter 301 is not in direct physical contact with the walls of the container 303, orany other part of the priming system 300 than the holding pins 302. Accordingly, the solid matter 301 is separated from the walls of the container 303 and this separation reduces the rate of heat transfer from the walls of the container 303 to the solid matter 301.

[0044] The priming system 300 comprises a first heating system 308 arranged to heat the walls of the container 303. The first heating system 308 may comprise heaters, such as electrical heating elements. The first heating system 308 may comprise one or more heating elements that are located within the walls of the container 303. Alternatively, or additionally, the first heating system may be provided by one or more heaters located around the outer walls of the container 303. Although not shown in Figure 3, one or more thermometers may be arranged on, or in, the walls of the container 303 for measuring the temperature of the walls. The thermometers may be thermocouples.

[0045] In addition to heating the walls of the container 303, the first heating system 308 will heat the cover 306, and other parts of the priming system 300, due to the direct physical contact between the different parts of the priming system 300. However, the heating pins 302 will be heated slower than the walls due to the thermal insulation on the parts of the heating pins 302 that contact the walls or the cover 306. As described in more detail later, a consequence of this is that the solid matter 301 is not melted during the bake out process of the rest of the priming system 300.

[0046] The priming system 300 also comprises a second heating system arranged to heat the solid matter 301 so that it melts. The second heating system is provided within the holding pins 302.

[0047] Figure 4 schematically shows a cross-section through a holding pin 302. The holding pin 302 has an elongate main body 404. The holding pin 302 comprises a heater 401 that may be an electrical heater. The holding pin 302 may also comprise a thermometer 402 that may be a thermocouple. The main body 404 of the holding pin may comprise channels so that the heater 401 and the thermometer 402 may be inserted into the holding pin 302. A distal end 403 of the holding pin 302 may be shaped with a lip that the solid matter 301 may sit on. The heater 401 within the holding pin 302 is arranged to heat the parts of the heating pin that contact the solid matter 301 when the solid matter 301 is loaded within the priming system 300.

[0048] The priming system 300 comprises a liquid outlet 305 for a flow of the molten solid matter 122 out of the priming system 300. The walls of the lower part of the container 303 may be angled relative to horizontal so as to direct a flow of the molten solid matter 122 to the liquid outlet 305. In particular, the lower part of the container 303 may be shaped as a funnel 307 and the liquid outlet 305 located at the end of the funnel 307. The walls of the funnel 307 may be at any angle that is not horizontal and arranged so that molten solid matter 122 flows along the walls to the liquid outlet 305. For example, the walls of the funnel 307 may be angled in the range 10° to 80° relative to horizontal.

[0049] The process of fueling / re-fueling the priming system 300 according to the first embodiment is described below.

[0050] To fuel / re-fuel the priming system 300, the cover 306 is opened, or removed, in a clean room. Solid matter 301 is then loaded into an internal region of the container 303 of the priming system 300.The solid matter 301 may be an ingot of clean and ultra-pure tin. During this loading process, the solid matter 301 and the walls of the container 303 are exposed to oxygen and moisture from the ambient environment within the clean room. The cover 306 is then closed, or replaced, so that the priming system 300 is sealed. A substantial vacuum is then generated in the internal region of the priming system 300 by a gas extraction system extracting gas through the gas outlet 304.

[0051] A bake out process may then be performed to release any water within the walls of the container 303. The bake out process may comprise using the first heating system 308 to heat the walls of the container 303 so that water within the walls is released. The first heating system 308 may be arranged to heat the walls to a temperature that is substantially at, or above, the melting point of the solid matter 301 in a vacuum. For example, the walls of the container 303 may be heated to about 280°C, whereas the melting of tin in a vacuum may start at about 232°C. The heating may cause fluid, such as water / gas, to be released from the walls of the container 303 and this released fluid may be extracted through the gas outlet 304.

[0052] The heating by the first heating system 308 may be performed without any heating by the heaters 401 of the second heating system. Due to the substantial thermal decoupling between the solid matter 301 and the walls of the container 303, the first heating system 308 may heat the solid matter 301 a lot less than the walls of the container 303. For example, when the walls of the container are heated to a temperature of about 280°C, the solid matter 301 may be heated to a temperature of about 220°C. Since the solid matter 301 is not heated to its melting point, the heating by the first heating system 308 may not cause any melting of the solid matter 301.

[0053] The heating by the first heating system 308 may then be reduced, or stopped, so that the temperature of the walls of the container 303 reduces. This reduction of temperature may stop any further fluid release, such as water / gas release, from the walls of the container 303. The temperature of the walls of the container 303 may be reduced to about 220°C.

[0054] The second heating system may then heat the solid matter 301 to above its melting point so that the solid matter 301 starts to melt. For example, the solid matter 301 may be heated to about 260°C. The molten solid matter 301 may drop into the funnel 307 and flow out of the priming system 300 through the liquid outlet 305. The sloped surfaces of the funnel may result in substantially all of the molten solid matter 301 flowing out of the priming system 300 through the liquid outlet 305.

[0055] Advantageously, the above processes according to the first embodiment release fluid, such as water, from within the walls of the container 303, and remove the released fluid from the inside of the priming system 300, prior to the solid matter 301 being melted. This prevents, or greatly reduces, any contact between the molten solid matter 301 and water and thereby reduces the formation of any oxides of the solid matter 301. In particular, when the solid matter 301 is tin, the generation of tin oxide may be prevented, or substantially reduced. The use of a funnel 307 also prevents, or substantially reduces, residual amounts of the solid matter 301 forming on the inner surfaces of the walls of the container 303.

[0056] Figure 5 schematically shows a priming system 500 according to a second embodiment. The priming system 500 may differ from the priming system 300 of the first embodiment by the container 501 being shaped, and the holding pins 302 arranged, so that the solid matter 301 is loaded at an angle relative to horizontal. The longitudinal axis of the solid matter 301 may be at any angle that is not horizontal. For example, the longitudinal axis of the solid matter 301 may be at about 10° to 80°, and preferably at about 30° to 60°, relative to the horizontal when the solid matter 301 is loaded in the priming system 500.

[0057] The priming system 500 according to a second embodiment may otherwise be substantially the same as that of the first embodiment. The priming system 500 according to a second embodiment may be operated in the substantially the same way as that of the first embodiment.

[0058] Figure 5 shows a reservoir 502 of the molten solid matter 301 that may build up in the container 303 before flowing out of the priming system 500.

[0059] In the priming system 500 of the second embodiment, angled orientation of the solid matter 301 in the container 303 may define a flow path of the molten solid matter 301 to, and falling from, the lowest edge of the solid matter 301. This may result in a more controlled flow of the molten solid matter 301 than with the priming system 300 of the first embodiment.

[0060] Embodiments include a number of modifications and variations to the above described techniques.

[0061] The priming system 300 of the first embodiment, or the priming system 500 of the second embodiment, may be used in the apparatus 100 shown in Figure 1 in place of the known priming system 104. Embodiments also include the priming system 300 of the first embodiment, or the priming system 500 of the second embodiment, being used in other types and / or designs of apparatus for generating molten matter that may be used in the generation of EUV radiation. The priming system 300, 500 of the first and second embodiments may both alternatively be referred to as fuel systems or fuel reload systems.

[0062] The process of fueling, or re-fueling, the priming system 300, 500 may alternatively be referred to a reload process. In particular, when the solid matter 301 is tin, the fueling, or re-fueling, of the priming system 300, 500 may be referred to as tin reload.

[0063] In the priming systems 300, 500 of the first and second embodiments the liquid outlet 305 is not restricted to being centrally located in a plan view of the priming system 300, 500. The funnel 307 may alternatively be asymmetrically shaped so that the liquid outlet 305 is close to a side of the priming system 300, 500.

[0064] In a preferred implementation of both the first and second embodiments, all of the holding pins are the same. However, embodiments also include the use of different holding pins. For example, some, but not all, of the holding pins may comprise a heater 401 and / or a thermometer 402.

[0065] Figure 6 is a flowchart of a process of generating a supply of molten fuel for an EUV radiation source according to embodiments.

[0066] In step 601, the process starts.

[0067] In step 602, solid fuel is loaded into an internal region of a fuel system.

[0068] In step 603, the solid fuel is held in the internal region with the solid fuel spaced apart from the walls of the fuel system.

[0069] In step 604, the walls are heated to a first temperature that is substantially at, or above, the melting point of the solid fuel.

[0070] In step 605, any fluid released by the walls of the fuel system when the walls are heated to the first temperature is extracted from the fuel system.

[0071] In step 606, the temperature of the walls is reduced to a second temperature that is lower than the first temperature.

[0072] In step 607, a supply of molten fuel is generated by heating the solid fuel to a temperature that is at, or above, the melting point of the solid fuel.

[0073] In step 608, the process ends.

[0074] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin film magnetic heads, etc.

[0075] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0076] 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 ambient (non- vacuum) conditions.

[0077] 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, where the context allows, is not limited to optical lithography.

[0078] Embodiments include the following numbered clauses:1. A fuel system for a source of EUV radiation, the fuel system comprising: a container having an internal region for receiving solid fuel; at least one holding pin arranged to hold solid fuel in the internal region spaced apart from walls of the container; a first heating system arranged to heat the walls of the container; and a second heating system arranged to heat each holding pin.2. The fuel system according to clause 1 , further comprising a liquid outlet for providing a flow of molten fuel out of the system; wherein a lower part of the container comprises walls that are angled relative to horizontal such that molten fuel in the lower part flows to the liquid outlet; and preferably the walls of the lower part are angled in the range lOo to 80o relative to horizontal.3. The fuel system according to clause 2, wherein the lower part comprises a funnel.4. The fuel system according to any preceding clause, further comprising a gas outlet for the extraction of fluid from within the fuel system so as to generate substantial vacuum conditions within the fuel system.5. The fuel system according to any preceding clause, further comprising an openable and / or removeable cover such that solid fuel is loadable into the internal region.6. The fuel system according to any preceding clause, wherein, when solid fuel is held in the internal region, the solid fuel is tin.7. The fuel system according to any preceding clause, wherein, when solid fuel is held in the internal region, the solid fuel is an ingot.8. The fuel system according to clause 7, wherein the container and at least one holding pin are arranged such that, when the ingot is held in the internal region, a longitudinal axis of the ingot is angled relative to horizontal; and preferably the longitudinal axis of the ingot is angled in the range lOo to 80o relative to horizontal.9. The fuel system according to any preceding clause, wherein the first heating system is arranged to heat the walls of the container to a temperature that is substantially at, or above, the melting point of the solid fuel.10. The fuel system according to any preceding clause, wherein the second heating system is arranged to heat the solid fuel to a temperature that is substantially at, or above, the melting point of the solid fuel.11. The fuel system according to any preceding clause, wherein the first heating system and the second heating system are operable independently of each other.12. The fuel system according to any preceding clause, wherein there are a plurality of holding pins.13. The fuel system according to any preceding clause, wherein each holding pin comprises a heater; and the second heating system comprises the heater in each holding pin.14. A laser powered plasma based EUV source comprising the fuel system according to any preceding clause.15. An EUV lithographic apparatus comprising the fuel system according to any of clauses 1 to 13.16. A method of generating a supply of molten fuel for an EUV radiation source, the method comprising: loading solid fuel into an internal region of a fuel system; holding the solid fuel in the internal region with the solid fuel spaced apart from the walls of the fuel system; heating the walls to a first temperature that is substantially at, or above, the melting point of the solid fuel; extracting, from the fuel system, any fluid released by the walls of the fuel system when the walls are heated to the first temperature; reducing the temperature of the walls to a second temperature that is lower than the first temperature; and generating a supply of molten fuel by heating the solid fuel to a temperature that is at, or above, the melting point of the solid fuel.17. The method according to clause 16, wherein the heating of the walls to a first temperature is performed with a different and independently operable heating system from that used to heat the solid fuel.18. The method according to clause 16 or 17, wherein the method is performed with: a fuel system according to any of clauses 1 to 13; a laser powered plasma based EUV source according to clause 14; and / or an EUV lithographic apparatus according to clause 15.

[0079] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A fuel system for a source of EUV radiation, the fuel system comprising: a container having an internal region for receiving solid fuel; at least one holding pin arranged to hold solid fuel in the internal region spaced apart from walls of the container; a first heating system arranged to heat the walls of the container; and a second heating system arranged to heat each holding pin.

2. The fuel system according to claim 1 , further comprising a liquid outlet for providing a flow of molten fuel out of the system; wherein a lower part of the container comprises walls that are angled relative to horizontal such that molten fuel in the lower part flows to the liquid outlet; and preferably the walls of the lower part are angled in the range 10° to 80° relative to horizontal.

3. The fuel system according to any preceding claim, further comprising a gas outlet for the extraction of fluid from within the fuel system so as to generate substantial vacuum conditions within the fuel system.

4. The fuel system according to any preceding claim, further comprising an openable and / or removeable cover such that solid fuel is loadable into the internal region.

5. The fuel system according to any preceding claim, wherein, when solid fuel is held in the internal region, the solid fuel is tin, or wherein the solid fuel is an ingot.

6. The fuel system according to claim 5, wherein when the solid fuel is an ingot wherein the container and at least one holding pin are arranged such that, when the ingot is held in the internal region, a longitudinal axis of the ingot is angled relative to horizontal; and preferably the longitudinal axis of the ingot is angled in the range 10° to 80° relative to horizontal.

7. The fuel system according to any preceding claim, wherein the first heating system is arranged to heat the walls of the container to a temperature that is substantially at, or above, the melting point of the solid fuel.

8. The fuel system according to any preceding claim, wherein the second heating system is arranged to heat the solid fuel to a temperature that is substantially at, or above, the melting point of the solid fuel.

9. The fuel system according to any preceding claim, wherein the first heating system and the second heating system are operable independently of each other.

10. The fuel system according to any preceding claim, wherein there are a plurality of holding pins, and wherein each holding pin comprises a heater; and the second heating system comprises the heater in each holding pin.

11. A laser powered plasma based EUV source comprising the fuel system according to any preceding claim.

12. An EUV lithographic apparatus comprising the fuel system according to any of claims 1 to 10.

13. A method of generating a supply of molten fuel for an EUV radiation source, the method comprising: loading solid fuel into an internal region of a fuel system; holding the solid fuel in the internal region with the solid fuel spaced apart from the walls of the fuel system; heating the walls to a first temperature that is substantially at, or above, the melting point of the solid fuel; extracting, from the fuel system, any fluid released by the walls of the fuel system when the walls are heated to the first temperature; reducing the temperature of the walls to a second temperature that is lower than the first temperature; and generating a supply of molten fuel by heating the solid fuel to a temperature that is at, or above, the melting point of the solid fuel.

14. The method according to claim 13, wherein the heating of the walls to a first temperature is performed with a different and independently operable heating system from that used to heat the solid fuel.

15. The method according to claim 13 or 14, wherein the method is performed with: a fuel system according to any of claims 1 to 10; a laser powered plasma based EUV source according to claim 11 ; and / or an EUV lithographic apparatus according to claim 12.

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