WO2025108636A1

WO2025108636A1 - Scrubber system, euv radiation source and euv utilization system comprising the same, and method of removing contamination

Info

Publication number: WO2025108636A1
Application number: PCT/EP2024/079525
Authority: WO
Inventors: William Peter VAN DRENT; Yue Ma
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-11-23
Filing date: 2024-10-18
Publication date: 2025-05-30
Anticipated expiration: 2026-05-23
Also published as: TW202525406A

Abstract

A scrubber system for an EUV radiation source, wherein the scrubber system comprises a conduit having an entrance configured to receive gas and contamination and an exit configured to output gas from which contamination has been removed, the scrubber system further comprising scrubber vanes and a cooling circuit configured to carry cooling fluid, the scrubber vanes and the cooling circuit being located within the conduit.

Description

SCRUBBER SYSTEM. EUV RADIATION SOURCE AND EUV UTILIZATION SYSTEM

COMPRISING THE SAME. AND METHOD OF REMOVING CONTAMINATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/602,425 which was filed on 23 November 2023 and which is incorporated herein in its entirety by reference.

FIELD

[0002] In one aspect, the present invention relates to a scrubber system. The scrubber system may form part of an EUV radiation source, such as a laser produced plasma (LPP) radiation source, a discharge produced plasma (DPP) source, a laser assisted DPP source or any variations thereof. The EUV radiation source may produce extreme ultraviolet (EUV) radiation and may form part of an EUV utilization system, such as a lithographic, an inspection or a metrology system. In another aspect, the invention relates to a method of removing contamination from a gas of an EUV 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 of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

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

[0005] An inspection system may be configured to inspect a mask for potential defects. This could be achieved by illuminating the mask by EUV light generated by means of the EUV radiation source. The inspection system may include an illumination system and an optical detection system. The EUV light may be reflected by means of the optical detection system to the mask to be inspected. In this way an image may be formed on a detector. This detector could for example be a time delay integration camera.

[0006] EUV radiation for a lithographic apparatus may be produced by a laser produced plasma (LPP) radiation source. Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation.

[0007] The radiation source may comprise a chamber within which the fuel droplets are irradiated. Gas (e.g. Hydrogen) is pumped through the chamber in order to carry contamination out of the chamber and thereby reduce the extent to which contamination builds up within the chamber. A scrubber system is used to remove contamination from the gas, following which the gas may be recirculated to the chamber.

[0008] It may be desirable to provide a scrubber system which overcomes a problem associated with the prior art, in a manner which is not disclosed or suggested by the prior art.

SUMMARY

[0009] According to a first aspect of the invention there is provided a scrubber system for an EUV radiation source, wherein the scrubber system comprises a conduit having an entrance configured to receive gas and contamination and an exit configured to output gas from which contamination has been removed, the scrubber system further comprising scrubber vanes and a cooling circuit configured to carry cooling fluid, the scrubber vanes and the cooling circuit being located within the conduit.

[00010] Advantageously, the cooling circuit lowers the temperature of the gas and contamination, thereby reducing spitting of contamination from the scrubber vanes.

[00011] The cooling circuit may be located upstream of the scrubber vanes.

[00012] The cooling circuit may comprise a plurality of pipes which extend into the conduit.

[00013] A plurality of rows of pipes may be provided.

[00014] The plurality of pipes may extend fully across the conduit.

[00015] The plurality of pipes may extend from one side of the conduit and return to that same side of the conduit.

[00016] The cooling circuit may be configured to deliver cooling fluid to upstream ends of the pipes and to remove cooling fluid from downstream ends of the pipes.

[00017] The cooling circuit may be located upstream of an area of the scrubber vanes which is hottest during use.

[00018] The cooling circuit may be not equally distributed across the conduit.

[00019] The cooling circuit may comprise the scrubber vanes, the scrubber vanes having hollow portions configured to receive cooling fluid.

[00020] The cooling circuit may be configured to deliver cooling fluid to upstream ends of the hollow portions and to remove cooling fluid from downstream ends of the hollow portions.

[00021] The scrubber vanes may have hollow portions formed using additive manufacturing.

[00022] The cooling circuit may comprise a valve configured to stop a flow of cooling fluid.

[00023] The cooling circuit may comprise a valve configured to direct the cooling fluid via a chiller or via a heater.

[00024] The cooling circuit may be configured to carry cooling gas. The cooling circuit may be configured to carry cooling liquid.

[00025] The contamination may be tin. [00026] The scrubber system may further comprise a controller configured to control the cooling provided by the cooling circuit in order to hold the temperature of the scrubber vanes below a predetermined temperature.

[00027] The controller may be configured to control the cooling provided by the cooling circuit to hold the temperature of the scrubber vanes below a melting point of the contamination, and to then intermittently raise the temperature of the scrubber vanes above the melting point of the contamination. [00028] According to a second aspect of the invention, there is provided an EUV radiation source comprising the scrubber system of any preceding aspect.

[00029] According to a third aspect of the invention, there is provided an EUV utilization system comprising the EUV radiation source of the second aspect. Examples of EUV utilization systems include a lithographic system, an inspection system, and a metrology system.

[00030] According to a further aspect of the invention, there is provided a lithographic system comprising the EUV radiation source of the second aspect, and further comprising a lithographic apparatus.

[00031] According to a further aspect of the invention, there is provided an inspection system or metrology system comprising the EUV radiation source of the second aspect.

[00032] According to a fourth aspect of the invention, there is provided a method of removing contamination from a gas of an EUV radiation source, the method comprising directing the gas and contamination into a conduit, using a cooling circuit in the conduit to cool the gas and contamination, and using scrubber vanes in the conduit to remove contamination from the gas.

[00033] Features of different aspects of the invention may be combined together.

BRIEF DESCRIPTION OF THE DRAWINGS

[00034] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figure 2 schematically depicts an example of a scrubber system;

Figure 3 schematically depicts another example of a scrubber system;

Figure 4 schematically depicts a cooling fluid circuit which may form part of a scrubber system; and

Figure 5 schematically depicts in cross-section a scrubber vane which may form part of a scrubber system.

DETAILED DESCRIPTION

[00035] Figure 1 shows a lithographic system. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00036] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00037] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00038] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00039] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a pulsed laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser pulsed beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma.

[00040] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal -incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[00041] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the main laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00042] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[00043] A relative vacuum, i.e. a small amount of gas (e.g. Hydrogen) at a pressure below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00044] The enclosing structure 9 of the radiation source SO defines a chamber 20. Generation of EUV radiation using tin (or other fuel) does not fully convert all of the fuel into radiation. As a result, some tin (or other fuel), which is generally referred to herein as contamination, is present in the chamber 20. This contamination may accumulate upon the collector 5, internal walls of the enclosing structure 9, and other locations an undesirable manner. To prevent or reduce this accumulation of contamination, a flow of gas (e.g. Hydrogen) through the chamber 20 is provided.

[00045] A gas source 22 is depicted in Figure 1. Although a single gas source 22 is depicted, in practice, many sources of gas may be provided. The gas provided from the gas source(s) may for example be Hydrogen. A pump 24 is connected to the chamber 20 via a scrubber system 26. The scrubber system 26 is described further below. The pump 24 draws gas and contamination out of the chamber 20 and through the scrubber system 26. The scrubber system 26 removes the contamination from the gas before the gas enters the pump 24. If the contamination was not removed from the gas before the gas entered the pump 24, then contamination would accumulate in the pump 24 and would stop the pump from working.

[00046] The scrubber system 26 is connected to the chamber 20 by a funnel 28. The funnel 28 is configured to direct the gas and contamination into the scrubber system 26. The funnel 28 may include rounded surfaces configured to redirect the gas and contamination into the scrubber system 26.

[00047] A scrubber system 26 and pump 24 are schematically depicted in Figure 2. The scrubber system 26 comprises the funnel 28, a conduit 30, cooling pipes 32, scrubber vanes 34, and a contamination receptacle 36. The funnel 28 is connected to the conduit upstream of the cooling pipes 32 and scrubber vanes 34. Contamination receptacle 36 is located downstream of the scrubber vanes 34. A pump 24 is connected to the conduit 30 downstream of the scrubber vanes 34. A heater 38 is provided around a wall 40 of the conduit 30.

[00048] Three rows of cooling pipes 32 are depicted in Figure 2. However, two rows of cooling pipes, or more than three rows may be provided. A single row of cooling pipes may be provided. A plurality of cooling pipes are provided. For example, 5 or more cooling pipes may be provided, e.g. 10 or more cooling pipes. The number of cooling pipes may be selected based upon a flow rate of gas and contamination through the conduit 30. Other considerations may also influence the number of cooling pipes.

[00049] The cooling pipes 32 may have a cylindrical outer surface in cross-section. The cooling pipes may have an ellipsoid or other curved shaped outer surface in cross-section. The cooling pipes may have a cross-sectional area of at least 5mm if the fluid is liquid. The cooling pipes may have a cross-sectional area of at least 3mm if the cooling fluid is gas.

[00050] The cooling pipes 32 extend into the conduit 30. The cooling pipes 32 may extend fully across the conduit 30 (e.g. as straight lines). Alternatively the cooling pipes 32 may be configured such that at least one cooling pipe extends from one side of the conduit and returns to that same side of the conduit. In such a case, the cooling pipe 32 may have the general form of a U-shape, with ends of the U being received in the wall 40 of the conduit 30. Cooling fluid may be delivered to an upstream end of the cooling pipe 32, and removed from a downstream end of the cooling pipe.

[00051] In use, gas and contamination enters the conduit 30 via the funnel 28. A cooling fluid, for example cooling gas, is pumped through the cooling pipes 32. The cooling pipes 32 remove heat from the gas and contamination, thereby reducing the temperature of the gas and contamination.

[00052] The gas and contamination are then incident upon the scrubber vanes 34. In one example the contamination comprises tin, and the gas is Hydrogen gas which is super saturated with tin. That is, the tin is in a vapor form carried by the Hydrogen gas. Because the Hydrogen gas is super saturated with tin, when the Hydrogen gas comes into contact with the scrubber vanes 34, the tin is released from the vapor form and condenses onto the scrubber vanes. In this way, the tin is removed from the Hydrogen gas. The tin may drip from the scrubber vanes 34 into the contamination receptacle 36. Alternatively, the tin may flow from the scrubber vanes 34 to the wall 40 of the conduit 30, and down the walls to the contamination receptacle 36. The scrubber vanes 34 may include grooves or other shapes configured to a direct a flow of tin in a desired direction. The Hydrogen gas, from which the tin has been removed, is pumped away by the pump 24. The Hydrogen gas referred to in this example may include (e.g. small amounts of) Hydrogen radicals, Stannane, and Oxygen.

[00053] The melting temperature of tin is 232°C. The heater 38 is configured to maintain an inner surface of the wall 40 of the conduit 30 at a temperature of 232°C or more. This is desirable because it prevents tin from solidifying on the walls 40 of the conduit 30 and causing undesirable contamination of the wall. Instead, the tin remains in liquid form and flows down the walls 40 into the contamination receptacle 36.

[00054] The gas and contamination which enters the scrubber system 26 may have a high temperature, for example a temperature of more than 300°C, for example more than 375°C. If the cooling pipes 32 were not present, then this high temperature gas would be directly incident upon the scrubber vanes 34 and would cause the scrubber vanes to heat up to a high temperature (e.g. greater than 300°C, e.g. greater than 375°C). Having the scrubber vanes 34 at such high temperatures is not desirable because this will cause violent spitting of liquid tin, as explained below.

[00055] Liquid tin on the surface of a scrubber vane 34 receives Hydrogen radicals (the gas carried into the scrubber system 26 comprises Hydrogen radicals and contamination). The Hydrogen radicals dissolve into the tin and stop at the surface of a scrubber vane 34. Each Hydrogen radical recombines with another Hydrogen radical to form a Hydrogen atom. The Hydrogen atoms remain at the surface of the scrubber vane 34 and coalesce with other Hydrogen atoms. In this way a bubble of Hydrogen is formed. The Hydrogen bubble grows, and then when it reaches a critical size it pops. This causes spitting of the tin. The amount of spitting which occurs increases with temperature, and thus it is desirable to control the temperature of the scrubber vane 34. For example, it may be desirable to prevent the scrubber vane from exceeding a temperature of 350° C.

[00056] Spitting of tin is undesirable because the tin may travel back into the chamber 20 of the EUV radiation source SO, and may travel into the lithographic apparatus LA, causing undesirable contamination.

[00057] Advantageously, the cooling pipes 32 lower the temperature of the gas and contamination before the gas and contamination are incident at the scrubber vanes 34. Consequently, the temperature of the scrubber vanes 34 is lower than it would be if the cooling pipes 32 were not present. As a result, the amount of spitting which occurs is reduced.

[00058] The cooling pipes 32 may be configured to remove sufficient heat from the gas and contamination that the temperature of the scrubber vanes 34 remains for example below 300°C. Cooling pipes 32 may remove sufficient heat from the gas and contamination that the scrubber vanes remain below a temperature of, for example 250°C. In general, the lower the temperature of the scrubber vanes 34, the less spitting occurs and the less contamination is caused by spitting.

[00059] The cooling pipes 32 may be configured such that the removal of heat from the gas and contamination does not reduce the temperature of the scrubber vanes 34 to below 232°C. This is so that tin which is received at the scrubber vanes does not solidify but instead remains in liquid form. The tin then flows down the scrubber vanes 34 and falls into the contamination receptacle 36.

[00060] In an alternative arrangement, the scrubber vanes 34 may be held at a temperature below 232°C, as a result of which the tin solidifies at the scrubber vanes. The scrubber vanes may then be heated (e.g. periodically) to a temperature of 232°C or above so that the tin melts and then flows down to the contamination receptacle 36. The tin scrubber vanes 34 may be heated by reducing the cooling provided by the cooling pipes 32. This may be achieved for example by stopping or reducing a flow of cooling fluid through the cooling pipes 32. Alternatively, a heater may be used to heat the cooling fluid, which is circulated to heat the cooling pipes 32.

[00061] The intermittent increase of the temperature of the scrubber vanes 34 by reducing cooling provided by the cooling pipes 32 may be referred to as thermal cycling. Thermal cycling may be performed on a periodic basis. For example, thermal cycling may be performed on a daily basis (e.g. with the scrubber vanes 34 being heated above 232° C for an hour). Periodic thermal cycling may have a duty cycle of less than 10% (i.e. with the scrubber vanes being above 232°C for less than 10% of the time).

[00062] Alternatively and/or in addition, thermal cycling may be performed when a level of buildup of contamination (e.g. tin) has occurred. This may be determined for example based upon a predetermined number of pulses of EUV radiation having been emitted by the source SO, or based upon other parameters. It may be determined via a measurement of the tin build up. It may be determined by monitoring a pressure drop in the conduit between a postion upstream of the scrubber vanes and a position downstream of the scrubber vanes (the pressure drop will increase as the amount of tin builds up on the scrubber vanes, obstructing the gas flow).

[00063] Figure 3 schematically depicts another scrubber system 126. In the embodiment of Figure 3, many of the elements of the scrubber system 126 correspond with those depicted in Figure 2. Where this is the case, those elements are not described again and are provided with the same reference numerals. As with the embodiment in Figure 2, a pump 24 is used to pump gas from the conduit 30 of the scrubber system 126.

[00064] In Figure 3, the cooling pipes depicted in Figure 2 are not present. The scrubber vanes 134 are hollow and are configured to allow cooling fluid to be pumped through them. This is schematically depicted in Figure 3 by the scrubber vanes 134 being hollow rectangles. The scrubber vanes 134 may have any suitable shape, provided that they include a hollow portion through which cooling fluid may be pumped.

[00065] The hollow portion in a scrubber vane 134 may have any suitable shape. The hollow portion may comprise an entrance through which cooling fluid is received, and an exit out of which cooling fluid is passed. The hollow portion, and the scrubber vane, may extend fully across the conduit from one side to the other. Alternatively, the hollow portion may extend from one side of the conduit and return to that same side of the conduit. In such a case, the hollow portion may have the general form of a U-shape, with ends of the U being received in the wall 40 of the conduit 30. In this arrangement, cooling fluid may be delivered to an upstream end of the hollow portion, and removed from a downstream end of the hollow portion.

[00066] An advantage of the embodiment of Figure 3 is that it is simplified compared with the embodiment of Figure 2. [00067] The scrubber vanes 134 may be connected by manifolds (not depicted). The scrubber vanes may extend fully across the conduit 30 such that the manifolds are located outside of the conduit and thus are not in the flow of gas and contamination.

[00068] The scrubber vanes 134 may be fabricated using additive manufacturing. Advantageously, this allows more complex scrubber vane shapes and hollow interiors to be fabricated compared with conventional manufacturing. Similarly, manifolds connected to the scrubber vanes 134 may be formed using additive manufacturing.

[00069] The cooling fluid provided to the scrubber vanes may remove sufficient heat from the scrubber vanes 134 that the temperature of the scrubber vanes 134 remains for example below 300°C. The cooling fluid may remove sufficient heat from the scrubber vanes 134 that the temperature of the scrubber vanes remains for example below 250°C. In general, the lower the temperature of the scrubber vanes 134, the less spitting occurs and the less contamination is caused by spitting.

[00070] The cooling fluid may hold the scrubber vanes 134 at a temperature of 232°C or more, so that tin which is received at the scrubber vanes does not solidify but instead remains in liquid form. The tin then flows down the scrubber vanes 134 and falls into the contamination receptacle 36.

[00071] In an alternative arrangement, the cooling fluid may cool the scrubber vanes 134 such that they are at a temperature below 232°C, as a result of which the tin solidifies at the scrubber vanes. The scrubber vanes may then be heated (e.g. periodically) to a temperature of 232°C or above so that the tin melts and then flows down to the contamination receptacle 36. The scrubber vanes 134 may be heated by reducing the cooling provided by the cooling fluid. This may be achieved for example by stopping or reducing a flow of cooling fluid through the scrubber vanes 134. Alternatively, a heater may be used to heat the cooling fluid which is circulated, to heat the scrubber vanes 134.

[00072] The intermittent increase of the temperature of the scrubber vanes 134 may be referred to as thermal cycling. Thermal cycling may be performed on a periodic basis. Instead or inaddition, thermal cycling may be performed when a level of build-up of contamination (e.g. tin) has occurred. This may be determined for example based upon a pre -determined number of pulses of EUV radiation having been emitted by the source SO, or based upon other parameters.

[00073] Scrubber vanes which are cooled by cooling fluid may be provided in addition to cooling pipes located upstream of the scrubber vanes.

[00074] Scrubber vanes maybe attached to cooling pipes. An example of this is schematically depicted in Figure 5. The cooling pipe 232 is generally cylindrical (although it may have a different shape). The scrubber vane 234 is attached to the cooling pipe 232 and is downstream of the cooling pipe (the direction of gas flow is indicated by arrows). The scrubber vane 234 extends at an angle relative to the flow direction (this may apply for any type of scrubber vane). The scrubber vane 234 includes a bend such that a distal portion extends at an angle which his opposite in direction to the first angle (this may apply for any type of scrubber vane). The scrubber vane 234 is thinner than the cooling pipe 232. Because the cooling pipe 232 is at the leading edge and the scrubber vane (which his thinner) is downstream, this provides less of a pressure drop compared with for example providing the cooling pipe at the trailing edge.

[00075] The cooling fluid provided to the cooling pipes or the scrubber vanes may be gas. An advantage of using cooling gas (compared with liquid) is that the gas can have a wide range of different temperatures. This means that the cooling gas can be used to provide cooling to the cooling pipes or cooled scrubber vanes when the pipes or scrubber vanes are being used to cool the Hydrogen flowing through the scrubber system, and can remain in situ in the pipes or scrubber vanes when the cooling gas is not being cooled (or is being heated). That is, the cooling gas can be allowed to become hot (e.g. hotter than 300°C) without this causing any damage to the scrubber system 26. If the cooling fluid is a liquid, then it may be desirable to ensure that the liquid does not get too hot, such that the liquid changes to gas (if this were to happen then it could burst the cooling pipes or cooled scrubber vanes) . A specialist cooling fluid such as silicon oil, which is capable of being heated to temperatures above 300°C, may be used. The cooling fluid may for example be Galden HT 270, available from Kurt J. Lesker Company of Sussex, UK, or for example Solvay Fomblin Y LVAC 25/6, available from Solvay S.A. of Brussels, Belgium.

[00076] In cases in which the cooling fluid is liquid, the cooling fluid may be removed from the cooling pipes, or hollow scrubber vanes, if it is desired to allow the pipes or scrubber vanes to heat up in order to melt tin.

[00077] The cooling pipes 32 may be formed for example from molybdenum, tungsten or tantalum, or other refractory metal. It may be preferable not to form the cooling pipes from steel because the steel may react with the Hydrogen radicals. Steel may be used if the temperature of the cooling pipes is kept below 300°C because in such cases reaction with the Hydrogen radicals may be avoided. The steel may for example be 300 series stainless steel. The same considerations apply to the material used to form the scrubber vanes 34, 134.

[00078] The rate of flow of cooling fluid through the cooling pipes 32 may be adjusted as needed in order to keep the cooling pipes at a desired temperature. For example, the rate of flow of cooling fluid may be adjusted to keep the temperature of the cooling pipes below 300°C. The temperature of the cooling pipes may be monitored using a thermal sensor such as a thermocouple. The thermocouple may for example be provided within the cooling fluid flow adjacent to an exit of a cooling pipe. Alternatively the thermocouple may be provided in thermal contact with an exterior of a cooling pipe. [00079] The temperature of the cooling fluid on entering a cooling pipe and on leaving that cooling pipe may both be measured. These two measured temperatures may be used to determine how much heat is being taken up by the cooling fluid. This measurement, along with a known rate of flow and a known thermal capacity of the cooling fluid, may be used to determine the temperature of the Hydrogen gas being cooled by the cooling pipe. [00080] A temperature sensor extending into the gas flow may be provided downstream of the cooling pipes, thereby measuring the temperature of the gas after it has been cooled by the cooling pipes.

[00081] A control loop may be used to control a rate of flow of the cooling fluid and/or a level of chilling applied to the cooling fluid, in order to keep the temperature of the Hydrogen gas below a desired temperature, or to keep the temperature of the cooling pipes below a desired temperature (e.g. below 300°C, e.g. below 250°C). The control loop may use any of the measurements described above. [00082] A control loop may be used to control a rate of flow of the cooling fluid and/or a level of chilling applied to the cooling fluid, in order to keep the temperature of the Hydrogen above a desired temperature, or to keep the temperature of the cooling pipes above a desired temperature (e.g. above 232°C). The control loop may use any of the measurements described above.

[00083] The above may be applied for scrubber vanes having a hollow portion that receives cooling fluid. That is, a rate of flow of cooling fluid through the scrubber vanes may be adjusted as needed in order to keep the scrubber vanes at a desired temperature. For example, the rate of flow of cooling fluid may be adjusted to keep the temperature of the scrubber vanes below 300°C (e.g. below 250°C). The temperature of the scrubber vanes may be monitored using a thermal sensor such as a thermocouple. The thermocouple may for example be provided within the cooling fluid flow adjacent to an exit of a scrubber vane. Alternatively the thermocouple may be provided in thermal contact with an exterior of a scrubber vane.

[00084] The temperature of the cooling fluid on entering a scrubber vane and on leaving that scrubber vane may both be measured. These two measured temperatures may be used to determine how much heat is being taken up by the cooling fluid. This measurement, along with a known rate of flow and a known thermal capacity of the cooling fluid, may be used to determine the temperature of the Hydrogen gas being cooled by the scrubber vanes.

[00085] A control loop may be used to control a rate of flow of the cooling fluid and/or a level of chilling applied to the cooling fluid, in order to keep the temperature of the Hydrogen gas below a desired temperature, or to keep the temperature of the scrubber vanes below a desired temperature. The control loop may use any of the measurements described above.

[00086] It may be desirable for the scrubber vanes to be thin. Thin scrubber vanes provide a greater total surface area for interaction with the gas and contamination, and thereby provide more effective removal of the contamination from the gas. In examples in which cooling fluid is not provided in the scrubber vanes, the scrubber vanes may for example have a thickness of around 2mm or more. In examples in which cooling fluid is provided through scrubber vanes, the scrubber vanes may be thicker in order to accommodate channels for the cooling fluid. The scrubber vanes may for example have a thickness of at least 3mm if the cooling fluid is gas, and may for example have a thickness of at least 5mm if the cooling fluid is liquid. The minimum thickness of a cooled scrubber vane may apply to a cooled portion of the scrubber vane. Other parts of the scrubber vane may be thinner. [00087] The scrubber vanes may be made for example from molybdenum, tungsten, tantalum or another refractory metal. The scrubber vanes may be made for example from steel (e.g. series 300 stainless steel). Steel may be used if the temperature of the scrubber vanes is kept below 300°C because in such cases reaction with the Hydrogen radicals may be avoided.

[00088] Additive manufacturing may be used to construct hollow scrubber vanes, for example using molybdenum, tungsten, tantalum or another refractory metal. Additive manufacturing may be used to construct hollow scrubber vanes using steel. A thickness of at least XXmm for each scrubber vane wall may be desirable when additive manufacturing is used, in order to allow formation of crystals of the metal which forms the scrubber vanes.

[00089] A cooling fluid circuit which may form part of a scrubber system is schematically depicted in Figure 4. Three cooled scrubber vanes 134 which extend across the conduit 30 are depicted (in practice more scrubber vanes will be provided). The cooling scrubber vanes 134 and conduit 30 are depicted in cross section viewed from above. Figure 4 is schematic, and for ease of illustration the conduit 30 is depicted as having a rectangular cross section whereas in practice it may have a cylindrical or other smooth cross sectional shape without comers.

[00090] A pump 140 is configured to pump the cooling fluid around a cooling fluid circuit 142. A manifold 144 provides the cooling fluid to the scrubber vanes 134. Another manifold 146 receives the cooling fluid from the scrubber vanes 134. A three-way valve 148 is used to direct the cooling fluid either to a chiller 150 or to a heater 152. When cooling of the scrubber vanes 134 is desired, the three- way valve 148 directs the cooling fluid via the chiller 150. When heating of the scrubber vanes 134 desired (e.g. to melt accumulated contamination such as tin) then the cooling fluid is directed by the valve 148 via the heater 152. In an alternative arrangement, if heat provided by the Hydrogen gas and contamination is sufficient to raise the temperature of the scrubber vanes 134 above 230°C, then the heater 152 may be omitted. Where this is the case, the three-way valve may be used to bypass the chiller 150.

[00091] Although the cooling fluid circuit comprises scrubber vanes 134, an equivalent cooling fluid circuit comprising cooling pipes may be provided.

[00092] In alternative configuration (not depicted), a valve which stops and starts flow may be provided. The valve may be open when cooling is desired, and may be closed when cooling is not desired (i.e. allowing heating to occur).

[00093] Although a closed fluid circuit is depicted in Figure 4, an open fluid circuit may be used instead. That is, a cooling gas or liquid may be delivered into the circuit, passed through the cooling pipes or cooling scrubber vanes, and then exhausted from the circuit. The cooling circuit may thus be a closed cooling circuit or an open cooling circuit. The term “cooling circuit” may be interpreted as meaning a hollow conduit which carries cooling fluid. [00094] The cooling provided by the cooling circuit may be controlled by a controller (not depicted). The control provided by the controller may be automatic, taking into account inputs such as measured temperature, pressure drop, etc. Such inputs are explained further below

[00095] In cases in which the cooling fluid is gas, the gas may be Nitrogen. Nitrogen gas advantageously will not interact with the material (e.g. Molybdenum) used to form the cooling pipes or scrubber vanes. In general, it may be desirable not to use Oxygen as the cooling gas, in order to avoid oxidation with the material (e.g. Molybdenum) used to form the cooling pipes or cooling scrubber vanes.

[00096] Cooling pipes may be located upstream of an area of the scrubber vanes which is hottest during use. For example, a central portion of the scrubber vanes may become hottest during use. In such a situation, cooling pipes may be configured to extend across a central portion of the conduit. Cooling pipes may be omitted from some side portions of the conduit. In other words, the cooling pipes are not configured to provide an equal amount of cooling across the conduit. In other words, cooling is not equally distributed across the conduit.

[00097] The above description relates to examples in which tin is the fuel used to generate EUV radiation (and thus the contamination referred to is tin). A different fuel may be used to generate EUV radiation. Where this is the case, then the melting temperature of the fuel will be different (i.e. will not be 232°C). Where this is the case, the temperatures referred to above may be modified accordingly. For example, if the melting temperature of the fuel is 242° C then 10°C may be added to the various temperature values set out above. A corresponding adjustment may be applied for other fuel melting temperatures.

[00098] Cooling pipes (not depicted) may be located downstream of the scrubber vanes.

[00099] Although embodiments have been described in connection with an LPP EUV radiation source, the invention may be used with other EUV radiation sources. For example, the invention may be used with a discharge produced plasma (DPP) source, a laser assisted DPP source or any variations thereof. A DPP radiation source applies high voltage between electrodes. A target material (e.g. tin) is supplied or provided in liquid state to the electrodes. A high density high temperature plasma is generated by the discharge so as to extract and use the extreme ultra violet light radiated therefrom. Contamination (e.g. tin) may be generated by a DPP radiation source.

[000100] A laser assisted DPP radiation source is a variation of a DPP radiation source. In the laser assisted DPP source a target material (e.g. tin) in liquid state can be supplied to surfaces of the electrodes at which the discharge is to be generated. The raw material is evaporated by irradiating the raw material with an energy beam such as a laser beam. Then the high temperature plasma is generated by the discharge. Contamination (e.g. tin) may be generated by a laser assisted DPP radiation source.

[000101] Further variations of EUV radiation sources exist comprising one or more rotatable elements to which target material is provided. Also in these radiation sources a laser can be used for in the EUV light generation process. The invention may be used in connection with such EUV radiation sources.

[000102] Although specific references in the present disclosure may have been made in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments may form part of a mask inspection system, a metrology system, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These systems or apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non -vacuum) conditions. The invention may be particularly advantageous in lithographic tools which generate EUV radiation using an LPP source, DPP source, laser assisted DPP source or any variations thereof.

[000103] An inspection system may be configured to inspect a mask for potential defects. This could be achieved by illuminating the mask by EUV light generated by means of the EUV radiation source. The inspection system may include an illumination system and an optical detection system. The EUV light may be reflected by means of the optical detection system to the mask to be inspected. In this way an image may be formed on a detector. This detector could for example be a time delay integration camera.

[000104] While tin mitigation is of importance in a lithographic process, tin mitigation may be of even higher importance in an inspection or metrology process.

[000105] CLAUSES

1. A scrubber system for an EUV radiation source, wherein the scrubber system comprises a conduit having an entrance configured to receive gas and contamination and an exit configured to output gas from which contamination has been removed, the scrubber system further comprising scrubber vanes and a cooling circuit configured to carry cooling fluid, the scrubber vanes and the cooling circuit being located within the conduit.

2. The scrubber system of clause 1, wherein the cooling circuit is located upstream of the scrubber vanes.

3. The scrubber system of clause 2, wherein the cooling circuit comprises a plurality of pipes which extend into the conduit.

4. The scrubber system of clause 3, wherein a plurality of rows of pipes are provided.

5. The scrubber system of clause 3 or claim 4, wherein the plurality of pipes extend fully across the conduit.

6. The scrubber system of clause 3 or clause 4, wherein the plurality of pipes extend from one side of the conduit and return to that same side of the conduit.

7. The scrubber system of clause 6, wherein the cooling circuit is configured to deliver cooling fluid to upstream ends of the pipes and to remove cooling fluid from downstream ends of the pipes.

8. The scrubber system of any of clauses 2 to 7, wherein the cooling circuit is located upstream of an area of the scrubber vanes which is hottest during use. 9. The scrubber system of any of clauses 2 to 8, wherein the cooling circuit is not equally distributed across the conduit.

10. The scrubber system of any preceding clause, wherein the cooling circuit comprises the scrubber vanes, the scrubber vanes having hollow portions configured to receive cooling fluid.

11. The scrubber system of clause 10, wherein the cooling circuit is configured to deliver cooling fluid to upstream ends of the hollow portions and to remove cooling fluid from downstream ends of the hollow portions.

12. The scrubber system of clause 10 or clause 11, wherein the scrubber vanes having hollow portions are formed using additive manufacturing.

13. The scrubber system of any preceding clause, wherein the cooling circuit comprises a valve configured to stop a flow of cooling fluid.

14. The scrubber system of any of clauses 1 to 12, wherein the cooling circuit comprises a valve configured to direct the cooling fluid via a chiller or via a heater.

15. The scrubber system of any preceding clause, wherein the cooling circuit is configured to carry cooling gas.

16. The scrubber system of any preceding clause, wherein the contamination is tin.

17. The scrubber system of any preceding clause, further comprising a controller configured to control the cooling provided by the cooling circuit in order to hold the temperature of the scrubber vanes below a predetermined temperature.

18. The scrubber system of clause 17, wherein the controller is configured to control the cooling provided by the cooling circuit to hold the temperature of the scrubber vanes below a melting point of the contamination, and to then intermittently raise the temperature of the scrubber vanes above the melting point of the contamination.

19. An EUV radiation source comprising the scrubber system of any preceding clause.

20. A lithographic system comprising the EUV radiation source of clause 19, and further comprising a lithographic apparatus.

21. An inspection system or metrology system comprising the EUV radiation source of clause 19.

22. An EUV utilization system comprising the EUV radiation source of clause 19.

23. A method of removing contamination from a gas of an EUV radiation source, the method comprising directing the gas and contamination into a conduit, using a cooling circuit in the conduit to cool the gas and contamination, and using scrubber vanes in the conduit to remove contamination from the gas.

[000106] 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

2. The scrubber system of claim 1, wherein the cooling circuit is located upstream of the scrubber vanes.

3. The scrubber system of claim 2, wherein the cooling circuit comprises a plurality of pipes, preferably a plurality of rows of pipes, which extend into the conduit.

4. The scrubber system of claim 3, wherein the plurality of pipes extend fully across the conduit.

5. The scrubber system of claim 3, wherein the plurality of pipes extend from one side of the conduit and return to that same side of the conduit.

6. The scrubber system of claim 5, wherein the cooling circuit is configured to deliver cooling fluid to upstream ends of the pipes and to remove cooling fluid from downstream ends of the pipes.

7. The scrubber system of any of claims 2 to 6, wherein the cooling circuit is located upstream of an area of the scrubber vanes which is hottest during use.

8. The scrubber system of any of claims 2 to 7, wherein the cooling circuit is not equally distributed across the conduit.

9. The scrubber system of any preceding claim, wherein the cooling circuit comprises the scrubber vanes, the scrubber vanes having hollow portions configured to receive cooling fluid.

10. The scrubber system of claim 9, wherein the cooling circuit is configured to deliver cooling fluid to upstream ends of the hollow portions and to remove cooling fluid from downstream ends of the hollow portions.

11. The scrubber system of claim 9 or claim 10, wherein the scrubber vanes having hollow portions are formed using additive manufacturing.

12. The scrubber system of any preceding claim, wherein the cooling circuit comprises a valve configured to stop a flow of cooling fluid.

13. The scrubber system of any of claims 1 to 11, wherein the cooling circuit comprises a valve configured to direct the cooling fluid via a chiller or via a heater.

14. The scrubber system of any preceding claim, wherein the cooling circuit is configured to carry cooling gas.

15. The scrubber system of any preceding claim, wherein the contamination comprises tin.

16. The scrubber system of any preceding claim, further comprising a controller configured to control the cooling provided by the cooling circuit in order to hold the temperature of the scrubber vanes below a predetermined temperature.

17. The scrubber system of claim 16, wherein the controller is configured to control the cooling provided by the cooling circuit to hold the temperature of the scrubber vanes below a melting point of the contamination, and to then intermittently raise the temperature of the scrubber vanes above the melting point of the contamination.

18. An EUV radiation source comprising the scrubber system of any preceding claim.

19. An EUV utilization system comprising the EUV radiation source of claim 18.

20. A method of removing contamination from a gas of an EUV radiation source, the method comprising directing the gas and contamination into a conduit, using a cooling circuit in the conduit to cool the gas and the contamination, and using scrubber vanes in the conduit to remove the contamination from the gas.