WO2025209832A1

WO2025209832A1 - Apparatus to trap debris

Info

Publication number: WO2025209832A1
Application number: PCT/EP2025/057372
Authority: WO
Inventors: Yue Ma; Andrey Nikipelov; Marcus Adrianus Van De Kerkhof; Hubertus Johannes VAN DE WIEL; Ruhou GAO; William Craig MILLAR; Jan Steven Christiaan Westerlaken
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-04-02
Filing date: 2025-03-18
Publication date: 2025-10-09
Anticipated expiration: 2026-10-02

Abstract

There is provided a debris trap structure for an EUV source or EUV utilization apparatus, the debris trap structure including at least one of: i) a heater configured to provide a surface of the debris trap structure at a temperature 100˚C or above; ii) a surface comprising a getter material for tin or lithium; and/or iii) a surface configured to suppress forward scattering of debris. Further provided is a lithographic system comprising an EUV source and an EUV lithographic apparatus comprising such a debris trap structure, sub-system, EUV source, or lithographic apparatus. Also described is a method of mitigating target material contamination in an EUV source or EUV utilization apparatus, the method including providing a debris trap structure and heating a surface of the debris trap structure to at least 100˚C.

Description

APPARATUS TO TRAP DEBRIS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/572,983 which was filed on 2 April 2024 which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to a debris trap structure for an EUV source or an EUV utilization apparatus. The present disclosure also relates to a sub-system of an EUV source or an EUV utilization apparatus comprising such a debris trap structure. The present disclosure also relates to an EUV source or EUV utilization apparatus comprising such a debris trap structure or sub-system. Also disclosed is a method of mitigating contamination in an EUV source or EUV utilization apparatus. The present disclosure has particular, but not exclusive, application to EUV lithography. The present disclosure has particular, but not exclusive, application to tin debris and lithium debris.

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 proj ect a pattern from a patterning device (e .g . a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

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

[0005] A lithographic apparatus includes a patterning device (e.g. a mask or reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. Methods for generating EUV light include, but are not limited to, altering the physical state of a source material, also known as a target material, to a plasma state. The source materials include 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, also referred to as an EUV source or radiation source. The radiation which is formed in such an EUV source passes through an intermediate focus disposed between the radiation source and a lithographic apparatus. Despite precautions in place which reduce the amount of target material, such as tin or lithium, inadvertently passing from the radiation source into the lithographic apparatus, some target material, such as tin or lithium, is still able to pass through. The target material which is able to pass through then can contaminate the sensitive optical elements of the apparatus, leading to, for example, lower performance of the optical elements by decreasing transmission of the radiation and also causing particle-derived defects on the reticle.

[0006] The present invention has been devised in an attempt to address at least some of the problems identified above.

SUMMARY

[0007] According to a first aspect of the present disclosure, there is provided a debris trap structure for an EUV source or EUV utilization apparatus, the debris trap structure including at least one of i) a heater configured to provide a surface of the debris trap structure at a temperature of 100°C or above; ii) a surface comprising a getter material for tin or lithium, and/or iii) a surface configured to suppress forward scattering of debris.

[0008] It will be appreciated that the structure may comprise one, two, or all three features as they may function either individually or in combination with one another.

[0009] The getter material may comprise or be a metal or metal alloy having an electronegativity higher than tin.

[00010] The surface may comprise a shape that supresses forward scattering/splashing of debris, such as tin droplets or lithium droplets, which propagate generally from the intermediate focus (IF) around an EUV cone or that supresses forward scattering of tin or lithium atoms, that propagate generally towards IF and/or orthogonal to an EUV cone axis. The surface may be configured to trap, block, and/or deflect droplets or atoms of target material. It will be appreciated that droplets of target material, such as tin or lithium, primarily travel in a direction similar to that of an EUV radiation beam since the direction of travel is restricted by the need to pass through the intermediate focus. As such, the debris trap structure may include one or more surfaces which are shaped to suppress forward scattering or splashing of such debris. It will also be appreciated that the droplets may be the source of atomic debris and that the atoms may propagate in a direction which is not the same as that of the particles. As such, the debris trap structure may include one or more surfaces which are shaped to suppress scattering of target material debris, such as tin atoms or lithium atoms, which propagate towards the intermediate focus and/or orthogonal to the EUV cone axis.

[00011] As such, the debris trap structure may include a geometry that supresses forward scattering and/or forward splashing of droplets, that propagate generally from intermediate focus outside of EUV cone.

[00012] Additionally or alternatively, the debris trap structure may include a geometry that supresses scattering of energetic tin atoms in the direction of illuminator optical elements, where energetic tin atoms originate generally within EUV cone near intermediate focus, and propagate generally in the direction orthogonal to EUV cone axis or in the direction towards the intermediate focus, in case or tin droplets that have escaped from the EUV source intro the EUV scanner.

[00013] Contamination from a radiation source into an illumination system, also referred to as an illuminator, may comprise target material debris, such as tin particles, tin vapour, lithium particles, and lithium vapour. Any debris in the illuminator can contaminate the sensitive optical elements which causes a transmission loss over time. In addition, debris particles can reach the reticle and settle on the reticle, thereby causing imaging defects. There are multiple strategies in place to reduce the amount of target material debris, whether as vapour (atoms) or as particles, which pass from the radiation source into the illumination system. A dynamic gas lock is provided between the radiation source and the illumination system. The dynamic gas lock provides a flow of gas directed towards a collector of the radiation source. This flow of gas is able to suppress gaseous debris or contamination, but is less able to stop particles as the particles travel very quickly. The flow of gas physically cannot be further increased enough to protect against particles as it is already at close to sonic speeds and, so attempting to increase the flow further is limited by the effect of choking; additionally the flow of gas is limited by pumping capacity, and so an additional strategy is required.

[00014] It has been found that a significant proportion of the target material which contaminates the illumination system is derived from particles (droplets) which pass through the intermediate focus. During transit, the droplets are illuminated by EUV radiation, which is strongly absorbed by the rear portion of the particles. Radiative cooling of droplets is ineffective due to the moderate black body coefficient of liquid target material, such as tin or lithium, and so the droplets dissipate heat via evaporation. Given the high fluence of EUV radiation near to the intermediate focus, around 1 to 10% of the mass of the droplet mass may be lost in a single EUV pulse, with larger particles losing a smaller mass portion and smaller particles losing a larger mass portion. In view of the high frequency of EUV pulses and the residence time of the droplets in an EUV cone near the intermediate focus, the particles may lose up to 90-99% of their mass in the first few centimetres after the intermediate focus. As a beam of EUV radiation passes through the intermediate focus, the EUV radiation beam diverges in the shape of a cone, which may be referred to as the EUV cone. It has been found that the tin vapour originating from the hot droplets, or lithium from hot lithium droplets, is directional, with the majority being emitted from the generally in the direction of the intermediate focus for the droplets that have left EUV source. Only a small proportion of the vapour is emitted within the EUV cone towards the illuminator optical elements and propagates towards the optical elements relatively unobstructed. Vapour can settle within the illumination system on surfaces close to the intermediate focus, where it is subject to reaction with hydrogen to form volatile stannane (SnEE), which is able to diffuse within the system or is picked up by the flow of hydrogen. SnEE is preferentially decomposed into metallic tin on the optical elements of the illuminator and causes EUV transmission loss. Alternatively, hot (about 2000 K) tin vapour can scatter in the direction of optical elements from the surfaces near the intermediate focus. Once it has been deposited on the optical elements, the interaction between the metallic tin or lithium and the material of the optical elements makes the deposited tin or lithium resistant to etching in hydrogen plasma and so it will remain on the optical elements indefinitely.

[00015] The present disclosure seeks to address these issues in a number of ways. Firstly, by providing a debris trap structure, such as a tin or lithium trap structure, which includes a heater which is configured to heat a surface of the structure to a temperature of 100°C or higher, and optionally below 500°C, the debris trap structure can intercept vapor (originating mainly from target material droplets that evaporate in an EUV cone) or it can intercept droplets, that propagate outside of the EUV cone generally from the intermediate focus in the direction of illuminator optics. Such temperatures may be utilised during nominal use of the EUV source or EUV utilization apparatus. By operating at a minimum temperature of 100°C, any stannane which is produced thermally decomposes and is redeposited as metallic tin locally instead of escaping towards the optical elements. This allows retention of collected tin and prevents its chemical sputtering. At higher temperatures, for example above 200 ° C stannane does not form at all, when the reactions like SnH_x => SnH_x.2 + EE (where x=2... 3) become dominant over chemical sputtering of tin: SnH_y + (H*, H+) => SnH_y+i (where y=0. . . 3). If the temperature of the surface is kept at below 232° C, which is the melting point of tin, the intercepted tin droplets remain solid, which in turn reduces the rate of explosive tin fragmentation (tin spitting). As such, maintaining the temperature of the debris trap structure between 100°C and 232°C is more beneficial for a trap structure that mainly intercepts droplets/particles, and maintaining the temperature between 200°C and 500°C is more beneficial for a debris trap structure that mainly intercepts vapor. At temperatures higher than 500°C even intermediate products of chemical sputtering SnH_z (z=1...3) become sufficiently volatile and debris trap efficiency for tin in the form of intermediate tin hydrides retention is reduced. At even higher temperatures, for example above 600 °C or 700 °C direct tin evaporation rate is so high that it compromises the tin retention of the debris trap structure.

[00016] The debris trap structure may include a surface comprising a getter material for target material, such as tin or lithium, such as noble metal, or a metal or metal alloy having an electronegativity higher than tin. A getter material is a material which is able to absorb, adsorb, or react with another material to retain the another material. Such metals catalytically dehydrogenate stannane and stabilise any metallic tin which is deposited thereon. As such, these metals are able to act as a getter material for tin vapor even at room temperature. The getter materials will, over time, saturate with the target material deposited on the surface. The target material may form a layer 2 to 3 monolayers in thickness, but any target material, particularly tin, in excess of this will be removed by hydrogen plasma via chemical sputtering. Even so, the getter material has a capacity to capture and retain tin or lithium, thereby preventing it from entering the sensitive areas containing optical elements and sensors and reducing the overall amount of tin or lithium available to contaminate the illumination system.

[00017] The debris trap structure may define a conical volume at least partially surrounding an EUV cone in nominal use. Preferably it is positioned in the vicinity of intermediate focus, for example within 0.1 to 10 cm from the intermediate focus. It will be understood that any EUV radiation which passes from an EUV radiation source through an intermediate focus expands in the shape of a cone, with the narrowest portion or tip of the cone being located at the intermediate focus. The debris trap structure may not block the EUV radiation as this would reduce the amount of EUV radiation available for imaging. By providing a debris trap structure which defines a conical volume, or a portion of a conical volume, the surface of the debris trap structure is close to the EUV cone and is therefore positioned to retain any tin vapour which is emitted from particles being evaporated within the EUV cone. In embodiments, the surface may partially impinge on the EUV cone. In this way, a small proportion of the EUV energy can be used to heat the surface of the debris trap structure. If the surface comprises a material with low emissivity, such as copper, molybdenum, silver, or ruthenium, which may be less than 0.05, then only a small amount of energy from the EUV radiation is needed to be intercepted to heat the surface. The surface may form a continuous wall around the EUV cone. The surface may form a discontinuous wall around the EUV cone. In other embodiments, the conical volume may be defined by a stacked series of plates having an aperture through which EUV radiation may pass. The aperture may increase in size to accommodate a widening EUV radiation beam. The increase in aperture size thereby defines a conical volume, albeit without continuous walls.

[00018] The surface of the debris trap structure may comprise a noble metal, or a metal or metal alloy having an electronegativity higher than tin. The getter material may be selected from one or more of ruthenium, iridium, molybdenum, rhodium, palladium, silver, tungsten, platinum, gold, copper, nickel, and iron. Silver, copper, nickel, and iron have electronegativity values which are lower than that of tin, but have electronegativities which are similar to that of tin, and, in embodiments where there is a heater which provides the surface of the debris trap structure at a temperature of above 100°C, they are able to decompose any stannane produced. The remaining specified elements have electronegativities which are higher than tin and so are able to dehydrogenate stannane without the need for heating, although heating to above 100°C can further enhance the effectiveness of surfaces comprising such elements.

[00019] The heater may be configured to provide the surface of the debris trap structure at a temperature above 100°C. At higher temperatures, the rate of decomposition of stannane is increased, and beneficially, even intermediate tin hydrides can decompose. In addition, a hotter surface enhances catalytic dehydrogenation, such as on ruthenium, iridium, or molybdenum surfaces, and also non- catalytic thermal decomposition, such as on steel or copper surfaces, of tin hydrides, such as stannane. As such, the heater may be configured to heat the surface to 100°C or higher during operation of an associated EUV utilization apparatus or EUV source.

[00020] In some embodiments, the maximum temperature of the surface is kept below the melting point of tin (232°C) so that any tin particles deposited on the surface remains solid. As such, the temperature is above that at which tin hydrides, such as stannane, spontaneously thermally decompose but below the melting point of metallic tin, so any tin hydride formed due to tin etching by hydrogen plasma is locally decomposed and redeposited, instead of escaping into the illumination system.

[00021] In some embodiments, it is preferably to heat the surface to a temperature higher than the melting point of tin. The hot surface temperature prevents the formation of tin hydride by EUV plasma, even when the thickness of any tin deposited on the surface already exceeds 2 or 3 monolayers. Even when the surface temperature is around 300°C within a low pressure environment, the rate of evaporation of the tin is very low, in the order of 2 x 10'⁴ monolayers per year.

[00022] The heater may be configured to provide the surface of the debris trap structure up to a temperature of around 1000°C. The temperatures of over 500°C may be utilised when the EUV utilization apparatus or source are undergoing a cleaning cycle which is intended to remove contamination therein. Whilst the surface may operate at a lower temperature in nominal use, for example, limited to 500 °C , it may be heated to higher temperatures during a cleaning operation in order to remove any tin which has been deposited. At a temperature of around 1000 K (around 727° C) and at the operational pressure of a lithographic apparatus, tin will sublimate. Under such conditions, the tin is estimated to sublimate at a rate of around 1 monolayer per 100 seconds, or even faster. As such, even for surfaces which strongly adhere tin, it is possible to remove the tin. The exact temperature used will depend on the desired rate of removal of tin, which may be selected experimentally or theoretically. In such a case, another cold tin getter can be placed near the temporary hot debris trap, in order to intercept tin vapour so it cannot reach sensitive optical elements, or a flow guides tin vapour away from the sensitive optical elements.

[00023] The debris trap structure may include a source of oxygen free and water vapour free gas configured to carry away sublimated tin or lithium. Preferably, the gas is dry noble gas, dry nitrogen, dry hydrogen, or a mixture thereof. Once the tin or lithium has sublimated, it is desirable to remove the tin or lithium vapour in order to prevent it from diffusing and depositing elsewhere. The source may be configured to direct the gas across the surface of the debris trap structure. In this way any sublimated tin or lithium is carried away in the flow of gas. The gas is oxygen free and water vapour free in order to avoid oxidation of the tin or lithium. This is because the vapour pressure of tin oxide or lithium oxide is significantly less than that of metallic tin or lithium at any temperature and so oxidation of the tin or lithium vapour could stall or stop sublimation.

[00024] The debris trap may include two or more plates. The surface of the debris trap may include two or more corrugations. The corrugations may be in the form of ridges, steps, zig-zags, or waves. The surface of the debris trap may include two or more plates and two or more corrugations. Since the surface may serve as a getter material and/or as a surface to thermally decompose tin hydrides, having an increased surface area provides an increase in capacity for holding tin or lithium. In addition, tin or lithium atoms originating from tin or lithium droplets evaporating within EUV cone may be energetic, since droplets temperature can exceed 2000 °C close to intermediate focus. Such energetic atoms may bounce off the surface . Thus it is beneficial to arrange the debris trap such, that an impact of an energetic atom is at grazing incidence and the most likely bounce direction (reflection) steers the atoms away from optical elements and towards other surfaces of the debris trap. This can be achieved by providing two or more plates and/or two or more corrugations, since it increases likelihood that energetic atoms will collide with the debris trap surface again or even multiple times after the first bounce and eventually will be captured by the debris trap.

[00025] The separation between the two or more plates and a length of the two or more plates may be at least 1:2. The ratio may be between 1:2 to 1: 10, such as, for example, 1:5. Similarly, the ratio of a separation of adjacent peaks of a corrugated surface to a depth of the corrugations may be at least 1:2. The ratio may be between 1:2 and 1: 10, such as, for example, 1:5. By providing plates or corrugations with such aspect ratios, tin atoms can be trapped more effectively.

[00026] The two or more plates may each define a respective plane. The plane of one or more of the two or more plates may be substantially perpendicular to a nominal direction of particles. Additionally or alternatively, the plane of at least one of the two or more plates may be angled relative to the direction of the particles in nominal use. As such, the plane of the plates may intersect the axis of the particles at 90° in the case where the two are exactly perpendicular. The plane of the plates may intersect the particles at any angle above 0° (where the plane and the direction of the particles are parallel meaning that they do not cross) and below 180° (where the plane and the axis are anti -parallel, again meaning that they do not cross). Preferably, the surface of the debris trap structure is angled relative to a nominal direction of particles such that the particles meet the surface at a grazing incidence in nominal use. By angling the surface, the particles can be deflected to cause them to lose energy and also to deflect them away from the illumination system and the sensitive optical elements therein.

[00027] The debris trap structure may include at least one trap cone or a number or trap cones, that encompass the EUV cone and connect to each other in the far field from the intermediate focus in an arrangement that intercepts tin droplets travelling outside of EUV cone. Such an arrangement prevents backward splashing of tin droplets due to first collision being grazing incidence. Such an arrangement prevents forward splashing of tin droplets towards illuminator optics. The trap cones can include sharp edges near intermediate focus to reduce the chance of back splashing.

[00028] The debris trap structure may be configured to engage with a structure having an opening which accommodates the intermediate focus of an EUV utilization apparatus or EUV source. As such, the debris trap structure may include at least one attachment feature, such as a connector. As such, the debris trap structure may include one or more connectors configured to reversibly install the structure in an EUV source or EUV utilization apparatus. Any suitable attachment feature may be provided. Preferably, the attachment feature is configured for reversible attachment. In this way, the debris trap structure can be swapped out for cleaning as required. The attachment feature may be thermally insulated.

[00029] The debris trap structure may have a proximal end and a distal end. The debris trap structure may be configured to locate the proximal end from 0 mm to 500 mm from an intermediate focus in an assembled condition. The distance may be less than 300 mm, less than 200 mm, or less than 150 mm. The debris trap structure may be configured to locate the proximal end from 0 mm to 30 mm from an intermediate focus in an assembled condition. The distance may be less than 25 mm, less than 20 mm, less than 15 mm, or less than 10 mm. Where the proximal end is at 0 mm, it is in contact with the intermediate focus. By separating the debris trap structure from the intermediate focus and/or dynamic gas lock supply system, thermal bridging is avoided and the additional heat provided to the debris trap structure does not directly pass into other components and the actively cooled components do not intervene with debris trap structure thermal setting.

[00030] The debris trap structure may include a controller configured to control a temperature of the surface of the debris trap structure. The amount of thermal power required to maintain the surface at a desired temperature may vary. The controller may be configured to ensure that the temperature of the surface is within a predetermined range. The controller may also be configured to selectively and temporarily increase the temperature of the surface of the debris trap structure to above a normal operating range in order to clean the surface.

[00031] The debris trap structure may be a ring or comprise multiple rings. The ring may surround the EUV cone. Since the tin may be evaporated in a hemispheric form and the tin vapour sources are located within the EUV cone, it is desirable to have the debris trap structure surround the source of tin or lithium contamination in order to intercept the tin or lithium.

[00032] The debris trap structure may have a cavity and/or a baffle to capture or trap forward scattered tin or lithium droplets. Where there is a gap between a proximal end of the debris trap structure and an intermediate focus, it is possible for tin or lithium droplets to be captured by the provision of a cavity and/or baffle. The cavity and/or baffle may be open in a direction facing opposite to the direction of a nominal EUV radiation beam or tin/lithium particles in nominal use.

[00033] The debris trap structure may have a v-shaped cross-section.

[00034] The heater may be a close loop controlled heating element.

[00035] According to a second aspect of the present disclosure, there is provided a sub-system of an EUV source or EUV utilization apparatus comprising an illumination system including the debris trap structure according to the first aspect of the present disclosure.

[00036] The illumination system includes a number of optical elements which reflect and shape an EUV radiation beam and directed it onto a reticle where it is patterned. The illumination system is susceptible to contamination by way of tin passing through an intermediate focus disposed between the illumination system and an EUV source. As such, the provision of the debris trap structure in the illumination system, preferably at an entrance of the illumination system where the EUV radiation and tin contamination enters, provides for capture of the tin, thereby reducing the likelihood of tin reaching the sensitive optical elements, such as mirrors or sensors.

[00037] The illumination system may include a source connection module and the debris trap structure may be connected to the source connection module. As described, the illumination system may be connected to a radiation source such that radiation generated in the radiation source can pass into the illumination system. The debris trap structure may be connected to the connection module which connects the illumination system to a radiation source.

[00038] According to a third aspect of the present disclosure, there is provided an EUV source or EUV utilization apparatus comprising a debris trap structure or sub-system according to the first or second aspect of the present disclosure. Preferably, the EUV utilization apparatus is an EUV lithographic apparatus. By providing a debris trap structure in accordance with the present disclosure, it is possible to reduce the amount of target material contamination which is able to reach sensitive optical elements or reticle.

[00039] According to a fourth aspect of the present disclosure, there is provided a lithographic system comprising an EUV source and an EUV lithographic apparatus comprising a debris trap structure, sub-system, EUV source, or lithographic apparatus according to any of the first to third aspects of the present disclosure.

[00040] According to a fifth aspect of the present disclosure, there is provided a method of controlling tin contamination in an EUV source or EUV utilization apparatus, the method including providing a debris trap structure and heating a surface of the debris trap structure to at least 100°C.

[00041] As described in respect of the first aspect, the features of which may be combined with the features of the fifth aspect of the present disclosure, by heating the surface of the debris trap structure, any tin hydrides are thermally decomposed which prevents them from diffusing into the EUV source or EUV utilization apparatus and causing damage.

[00042] The method may further include heating the surface of the debris trap structure to greater than around 700°C, preferably to around 1000°C, to sublimate any tin or lithium from the surface, and providing a flow of water-free and oxygen-free gas over the surface to remove any sublimated tin or lithium. This is preferably undertaken during a cleaning cycle in order to remove accumulate tin or lithium.

[00043] According to a sixth aspect of the present disclosure, there is provided the use of a debris trap structure according to the first aspect of the present disclosure, the sub-system according to the second aspect of the present disclosure, the EUV source or EUV utilization apparatus according to the third aspect of the present disclosure, the lithographic system according the fourth aspect of the present disclosure, or the method according to the fifth aspect of the present disclosure in an EUV utilization method or apparatus.

[00044] It will be appreciated that features described in respect of one embodiment may be combined with any features described in respect of another embodiment and all such combinations are expressly considered and disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS [00045] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing in which corresponding reference symbols indicate corresponding parts, and in which:

[00046] Figure 1 depicts a lithographic apparatus according to an embodiment of the disclosure;

[00047] Figures 2a to 2d depicts a cascade of events in which a particle or droplet of tin introduces tin vapour to an illuminator due to pulsed evaporation within EUV cone;

[00048] Figure 3 depicts the passage of a tin particle through an intermediate focus, how the particle is evaporated and tin vapour deposits on the surfaces near intermediate focus (excluding bouncing for tin atoms)

[00049] Figure 4 depicts the etching of tin from the surfaces as volatile hydride and deposition of tin on optical elements within an illumination system of an EUV utilization apparatus;

[00050] Figure 5 depicts a debris trap structure according to an embodiment of the present disclosure;

[00051] Figure 6 depicts a debris trap structure including a heater according to an embodiment of the present disclosure;

[00052] Figure 7 depicts a debris trap structure including a ridge or corrugated surface according to an embodiment of the present disclosure;

[00053] Figure 8 depicts a debris trap structure including a plurality of stacked plates according to an embodiment of the present disclosure;

[00054] Figure 9 depicts a debris trap structure including a plurality of stacked plates and a flow of dry, oxygen -free gas according to an embodiment of the present disclosure; and

[00055] Figures 10a to 10c depict an embodiment of a debris trap structure according to an embodiment of the present disclosure.

[00056] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[00057] Figure 1 shows a lithographic system according to the present invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The substrate table WT may include an electrostatic clamp (not shown) including an electrostatic sheet that is selectively polarised to clamp a substrate thereto. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B’ (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W. A debris trap structure 15 is depicted along the path of the radiation beam B and surrounding a portion of the EUV radiation cone 16. The patterning device MA may be referred to as the reticle. The support structure MT may be referred to as the reticle stage.

[00058] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[00059] The radiation source SO shown in figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Fuel emitter 3 may be connected to a droplet generator apparatus. Although tin is referred to in the following description, any suitable fuel may be used. The fuel is in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma. [00060] 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.

[00061] The laser may be separated from the radiation source SO. Where this is the case, the laser beam may be passed from the laser 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 and the radiation source SO may together be considered to be a radiation system.

[00062] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at a point to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus is located at or near to an opening in an enclosing structure of the radiation source. [00063] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.

[00064] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[00065] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[00066] Figure 2 depicts a cascade of events in which a particle or droplet of tin 17 introduces tin vapour into the illumination system IL of lithographic apparatus LA due to pulsed evaporation. In Figure 2a, a radiation beam B passing through an intermediate focus 6 is depicted with a particle or droplet of tin 17 within the EUV radiation cone 16. As shown in Figure 2b, the droplet of tin 17, having been illuminated at its rear hemisphere by the radiation B releases tin vapour 18, primarily rearwardly, namely towards the intermediate focus 6. Despite the presence of a low pressure (10 Pa or less) atmosphere of hydrogen, this is insufficient to stop the high energy tin atoms in vapour clouud 18, which is deposited on the wall 21 surrounding the intermediate focus 6, as shown in Figure 2c as tin contamination 20. Although the droplet of tin 17 is depicted as only slightly decreasing in size, in practice about 90 to 99% of the droplet is vapourized in the area around the intermediate focus 6 over many pulses of the EUV radiation. A dynamic gas lock (not shown) is provided to maintain the pressure differential between the radiation source SO and the illumination system IL and to contain most of tin particles and tin vapor in the EUV source. Primarily the gas flows towards the radiation source SO and the collector, but some gas also flows towards the illumination system IL as a dynamic gas lock flow 19. In operation, within the illumination system IL is a hydrogen plasma that generates tin hydride, such as stannane, via etching of the metallic tin contamination 20 that has been deposited through vapourization of tin droplets in the area around the intermediate focus 6. Hydrogen plasma etches the tin contamination 20 into volatile tin hydrides, which are introduced into the dynamic gas lock flow 19, where it is redistributed to different parts of the apparatus. Due to the dehydrogenation catalytic properties of metals, such as ruthenium, used in optical elements within the illumination system IL, tin hydrides are dehydrogenated upon contact to deposit metallic tin, which cannot be etched away by hydrogen plasma.

[00067] Figure 3 is similar to Figures 2a to 2d and depicts the evaporation of a tin droplet 132 in the EUV cone in the illuminator, that generates anisotropic tin vapor cloud 140. The radiation source SO, 100 includes an aperture cone 120 and a scanner/illuminator 200, also referred to as an illumination system IL. Tin particle 130, 131, 132 evaporates as it propagate in EUV beam 150 over multiple EUV pulses. A dynamic gas lock 110 maintains a pressure differential between the source and the scanner, and provides a stopping flow 111 and collateral outflow 112. Tin vapour expansion 140 after a pulse of EUV around the droplet 132 is not uniform as more vapour is generated towards the intermediate focus-facing rear hemisphere of the droplet 132. Energie tin atoms from the vapor cloud propagate through low pressure hydrogen without much deflection and deposit metallic tin (143, 142) on components of the source and components of the scanner 120, 210, alternatively, energetic tin atoms can scatter / bounce (not shown). Only a small amount of tin vapour propagates directly in the solid angle of the EUV radiation NA1, NA2 and can directly reach optical component 220, such as a field facet or pupil facet mirror. Most of the mass of the tin droplet is lost within the first 1 to 10 mm (H) of entering the illumination system IL, with additional but minor tin mass lost past 10 mm to 100 mm of entering the illumination system IL.

[00068] Figure 4 is also similar to Figure 3 and depicts how tin can contaminate optical elements. In particular, in the presence of EUV plasma, tin atoms of the tin contamination are converted to volatile tin hydrides 342, 343, such as stannane when they reside on room temperature steel or copper walls. This is even the case when there is sub-monolayer amount of tin contamination. Such tin hydrides are volatile and diffuse 300, 310 away from the surface on which they have been deposited and meet with the dynamic gas lock outflow 112 and are directed towards the optical element 310 within the EUV cone where it cannot be intercepted. As a result of convection and diffusion 320, the tin hydride can reach Ru-capped optics and deposit as well adsorbed metallic tin 350, which cannot be etched by EUV plasma unless it builds up to multiple monolayers, usually more than 3 monolayers.

[00069] Figure 5 depicts a debris trap structure 400 according to an embodiment of the present disclosure. The debris trap structure 400 includes a surface 401 of a getter material for tin. Such a getter material has a high affinity for tin, such as ruthenium, such that any tin deposited on the surface 401 is retained in monolayers of tin 410, 420, to a thickness of up to around 3 monolayers even at room temperature and despite presence of hydrogen EUV plasma. The height F of the debris trap structure is comparable to the length over which tin is evaporated, such as 10 mm to 100 mm. The surface 401 is provided around the EUV cone and is preferably substantially conformal to the EUV cone such that NA1 is approximately equal to NA3. The getter material is provided on surface 401 facing the EUV beam as this is where the majority of tin vapour is generated. Due to the getter material’s affinity for tin, tin will be adsorbed to the surface 401 and retained in place. Whilst monolayers greater than around 3 can be etched by the EUV plasma, the getter material is still able to hold onto the adsorbed tin.

[00070] Figure 6 is similar to Figure 5 and depicts a debris trap structure 500 including a heater 501. The surface 502 may be a getter material for tin, but could also be a material which does not adsorb tin strongly, such as steel, copper or molybdenum. As such, this embodiment may include a heater plus a surface comprising a getter material for tin or a noble metal, metal or metal alloy without a specific affinity for tin. Optionally, the heater is connected to a power source 504 via wires 503, 505. The heater is configured to heat the surface 502 to a temperature of above 100°C and below 500°C. In operation, the hot surface offsets any generation of SnH by thermal decomposition of the stannane as well as other tin hydrides, such as SnH, SnFE. and SnHs, and thereby stabilises multiple monolayers of tin 510, 520 on the surface 502, even in the presence of EUV hydrogen plasma. The surface can be any hydrogen plasma compatible material, which is a material which is not degraded to a damaging extent over the expected lifetime of the debris trap structure and which does not contaminate the illumination system, such as stainless steel or molybdenum, but is preferably ruthenium due to the additional stability provided for adsorbed tin. In operation, the temperature of the surface, which may be a getter material for tin, may be from around 100°C to around 232°C if it is desired to keep the tin as a solid, although the surface may be heater to higher temperatures, such as around 300°C, 400°C or 500°C since the rate of tin sublimation at such a temperature is still less than 0. 1 monolayers per year.

[00071] Figure 7 is similar to Figure 5, albeit the surface 601 is corrugated. It will be appreciated that a heater may be provided, similar to the embodiment of Figure 6, in order to heat the surface to thermally decompose tin hydrides. Additionally or alternatively, the surface 601 may comprise a getter material for tin. The corrugations are depicted as being triangular in shape, but they may be other shapes, such as steps or waves. The corrugations not only provide a larger surface area to adsorb tin contamination, but also assists in re-capturing high energy tin droplets or particles that may bounce off the surfaces before losing enough energy to allow them to be captured. The corrugations may have a depth K and a separation M. The aspect ratio (K/M) may be 2 or more, preferably 10 or more. Since the debris trap structure surrounds the EUV cone, the corrugations may be provided as concentric rings which extend around the inner surface of the debris trap structure.

[00072] Figure 8 depicts an embodiment of the present disclosure in which the debris trap structure includes a plurality of plates 700, 710. The plates 700, 710 may include integrated heaters and power supply 701, 702, 703 704 that stabilise deposited tin even in excess of a few monolayers 720. As depicted, the plates are substantially perpendicular to a central axis of the EUV cone, but can be angled upwardly or downwardly relative to the perpendicular position depicted. As with the other embodiments, the surface of the plates may comprise a tin getter material which stabilises adsorbed tin, or may comprise a material such as steel or copper, that had a lower affinity to tin, but which can be heated to above 100°C to thermally decompose tin hydrides, thereby reducing or preventing etching of the metallic tin. It will be appreciated that the plates are depicted in cross section and have a hole through which the EUV radiation can pass. The holes are of increasing size to accommodate the expanding EUV cone whilst keeping the plates close to the EUV cone so that the tin vapour can be effectively captured by the plates. It will also be appreciated that the plates may have a flat surface, or a corrugated surface such as a curved surface, a stepped surface, or a wavy surface. The corrugations may be provided in concentric rings.

[00073] Figure 9 is similar to Figure 8 and also includes a cross-flow of a dry, oxygen-free gas, such as nitrogen, hydrogen, noble gas or mixtures thereof. In cleaning mode, the plates 710 of the debris trap structure 700 are heated to a temperature sufficient to sublimate the tin contamination, such as greater than 700°C, or around 1000°C. A cross-flow of gas 750 is able to evacuate tin vapour contamination 740 from the plates where it is carried away in flow 735. The pressure of the gas may be 100 Pa or more and may have a density of lOg/mol or more. The gas is dry and oxygen-free to avoid oxidation of the tin to tin oxide, which is much less volatile than metallic tin at any temperature. The heating to a temperature to sublimate the tin and carry it away in a cross-flow is performed in a cleaning mode rather than during imaging.

[00074] Figures 10a to 10c depict a debris trap structure 800 according to an embodiment of the present disclosure. Figure 10a is a cross section of the debris trap structure 800 and it will be appreciated that the debris trap structure 800 is shaped to surround an EUV radiation cone in nominal use. The debris trap structure 800 is in the shape of a ring 801 with an aperture 802 through which an EUV radiation cone passes in nominal use. The aperture 802 includes an internal angled wall 803 which is shaped to substantially conform to the shape of the EUV radiation cone. Although the internal wall 803 is depicted as being smooth, it may include corrugations in order to increase surface area. The internal wall may comprise a getter material for tin, or may comprise a material which does not have a particular affinity for tin, such as copper, molybdenum or steel. The ring 801 includes circumferential slot 804 configured to receive a heater 805. An upper face 806 of the ring includes additional heaters 805’, 805”. It will be appreciated that the present invention is not particularly limited by the number and location of the heaters depicted in this figure and other numbers and locations of heaters may be used. The ring 801 includes an open cavity 807 facing away from the upper face 806. The cavity 807 has an inverted V-shaped cross section and includes concentric corrugations 808 configured to increase the surface area and to also deflect and trap tin particles or droplets. Also depicted is an optional mounting and thermal break attachment 809. The mounting and thermal break attachment 809 is configured to retain the debris trap structure in the desired location and serves to provide a gap between an exit of the intermediate focus and a proximal end of the ring 801. The mounting and thermal break attachment 809 also provides thermal insulation to avoid unwanted transfer of heat between components. By providing a gap, some of the tin particles which make it through the intermediate focus can be captured within the open cavity 807. In use, the particles which make it through the intermediate focus are vapourized by EUV radiation pulses and the vapour deposits on the debris trap structure, primarily but not exclusively on the upper face 806, internal angled wall 803, and within the open cavity 807. Since the tin is exposed to a hydrogen plasma in nominal operation, the hydrogen would normally etch the tin into volatile tin hydrides, which could diffuse away and redeposit tin elsewhere. Due to the presence of a heater, the debris trap structure is heated to a temperature of above 100°C to inhibit production of tin hydride and to thereby stabilise the metallic tin.

[00075] In summary, the present disclosure provides for a debris trap structure which includes one or both of a heater and a surface comprising a metal or metal alloy having an electronegativity higher than tin. Heating the debris trap structure to above the temperature at which volatile tin hydrides thermally decompose allows for the suppression of volatile tin diffusing and being redeposited on sensitive optical elements. In addition, providing a surface comprising a material having a higher electronegativity than tin similarly results in the breakdown of tin hydrides and the deposition and stabilisation of metallic tin. By combining a getter material and a heater, etching of tin is eliminated and it is possible to retain a thicker layer of metallic tin without it being etched away. By capturing tin vapour and preventing it from being able to form volatile compounds which can diffuse, the amount of contamination is heavily reduced and so the effectiveness and operational lifespan of an EUV utilization apparatus comprising such a debris trap structure is improved. Furthermore, the structure may be shaped to suppress forward scattering of debris, which additionally or alternatively, reduces the amount of contamination.

[00076] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[00077] 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

1. A debris trap structure for an EUV source or EUV utilization apparatus, the debris trap structure including at least one of: i) a heater configured to provide a surface of the debris trap structure at a temperature 100°C or above; ii) a surface comprising a getter material for tin or lithium; and/or iii) a surface configured to suppress forward scattering of debris.

2. The debris trap structure according to claim 1, wherein the debris trap structure defines a conical volume at least partially surrounding an EUV cone in nominal use.

3. The debris trap structure according to claim 1 or claim 2, wherein the surface of the debris trap structure comprises a noble metal, or a metal or metal alloy having an electronegativity higher than tin, optionally wherein the getter material is selected from one or more of ruthenium, iridium, molybdenum, rhodium, palladium, silver, tungsten, platinum, gold, copper, nickel, and iron.

4. The debris trap structure according to any preceding claim, wherein the heater is configured to provide the surface of the debris trap structure at a temperature above 100°C and optionally below 500°C.

5. The debris trap structure according to any preceding claim, wherein the heater is configured to provide the surface of the debris trap structure up to a temperature of between around 500°C and around 1000°C.

6. The debris trap structure according to any preceding claim, wherein the surface includes one or both of i) two or more plates, and ii) two or more corrugations.

7. The debris trap structure according to claim 6, wherein a ratio of a separation between the two or more plates and a length of the two or more plates is at least 1 :2, and/or wherein the ratio of a separation of adjacent peaks of a corrugated surface to a depth of the corrugations is at least 1:2.

8. The debris trap structure according to any preceding claim, wherein the surface of the debris trap structure is angled relative to a typical direction of tin atoms or tin particles, such that the atoms or particles meet the surface at a grazing incidence in nominal use.

9. The debris trap structure according to any preceding claim, wherein the debris trap structure includes a source of oxygen-free and waterfree gas configured to carry away sublimated tin, preferably wherein the gas is nitrogen, hydrogen, noble gas, or a mixture thereof.

10. The debris trap structure according to any preceding claim, wherein the debris trap structure is configured to engage with a structure including an opening which accommodates an intermediate focus of an EUV utilization apparatus or EUV source.

11. The debris trap structure according to claim 10, wherein the debris trap structure has a proximal end and a distal end, and wherein the debris trap structure is configured to locate the proximal end from 0 mm to 500 mm, preferably less than 300 mm, less than 200 mm, or less than 150 mm, from an intermediate focus in an assembled condition.

12. The debris trap structure according to any preceding claim, wherein the debris trap structure includes one or more connectors configured to reversibly install the structure in an EUV source or EUV utilization apparatus.

13. The debris trap structure according to any preceding claim, wherein the debris trap structure includes a controller configured to control a temperature of the surface of the debris trap structure.

14. The debris trap structure according to any preceding claim, wherein the debris trap structure is a ring.

15. A sub-system of an EUV source or EUV utilization apparatus comprising an illumination system including the debris trap structure according to any preceding claim.

16. The sub-system according to claim 15, wherein the illumination system includes a source connection module and wherein the debris trap structure is connected to the source connection module.

17. An EUV source or EUV utilization apparatus comprising a debris trap structure or sub-system according to any preceding claim, preferably wherein the EUV utilization apparatus is an EUV lithographic apparatus.

18. A lithographic system comprising an EUV source and an EUV lithographic apparatus comprising a debris trap structure, sub-system, EUV source, or lithographic apparatus according to any preceding claim.

19. A method of mitigating target material contamination in an EUV source or EUV utilization apparatus, the method including providing a debris trap structure and heating a surface of the debris trap structure to at least 100°C.

20. The method according to claim 19, the method further including heating the surface of the debris trap structure to greater than around 500°C, preferably to around 1000°C, to sublimate any tin from the surface, and providing a flow of water-free and oxygen-free gas over the surface to remove any sublimated tin.

21. The use of a debris trap structure according to any of claims 1 to 14, the sub-system according to claim 15 or 16, the EUV source or EUV utilization apparatus according to claim 17, the lithographic system according to claim 18, or the method according to claim 19 or 20 in an EUV utilization method or apparatus.