Lithographic apparatus components and methods

Abstract

A reticle stage for a lithographic apparatus is provided. The reticle stage includes at least one optical element having an area to be cleaned, a radiation beam target, and a control system configured to direct the radiation beam toward the radiation beam target. Interaction of the radiation beam and a gas generates an optical element cleaning plasma. The radiation beam target is positioned to provide the optical element cleaning plasma to the at least one optical element for cleaning the optical element. The at least one optical element is a different element from the radiation beam target, and is preferably a thermally sensitive element. A fiducial for a lithographic apparatus with at least one curved edge, a reticle stage or a lithographic apparatus including the fiducial, and a method for cleaning an optical element of the reticle stage are also described.

Description

[Technical Field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Application No. 20212524.1 filed December 8, 2020, and European Application No. 20214417.6 filed December 16, 2020, which are incorporated herein by reference in their entireties.

[0002] [Technical field] The present invention relates to a reticle stage for a lithographic apparatus, a datum for a lithographic apparatus, a lithographic apparatus comprising such a reticle stage and / or datum, a method for cleaning at least one optical element of a reticle stage of a lithographic apparatus, and the use of a reticle stage, a datum, or such a method in a lithographic apparatus or method, and a computer program and computer readable medium configured to perform the steps of the methods described herein. [Background technology]

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate. Lithographic apparatus are used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus projects a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) that has been formed on the substrate.

[0004] The wavelength of the radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on the substrate. Lithographic apparatus that use EUV radiation, which is electromagnetic radiation having a wavelength in the range of 4-20 nm, may be used to form smaller features on a substrate than conventional lithographic apparatus (which may, for example, use electromagnetic radiation having a wavelength of 193 nm). Summary of the Invention [Problem to be solved by the invention]

[0005] During operation, contaminants such as carbon may be deposited on surfaces within a lithographic apparatus. Some of these surfaces are located on optical elements of the apparatus, and are therefore elements that interact with a radiation beam during operation. The performance of such optical elements is reduced due to the deposition of contaminants such as carbon on their surfaces. It is therefore desirable to clean contaminated surfaces within the lithographic apparatus, particularly surfaces that interact with a radiation beam.

[0006] The present invention has been devised to provide an improved or alternative system for cleaning one or more optical elements that are heat sensitive elements of a lithographic apparatus. [Means for solving the problem]

[0007] According to a first aspect of the present invention, there is provided a reticle stage for a lithographic apparatus, the reticle stage comprising at least one optical element having an area to be cleaned, a radiation beam target, and a control system configured to direct a radiation beam towards the radiation beam target. Interaction of the radiation beam and a gas generates an optical element cleaning plasma. The radiation beam target is positioned to provide the optical element cleaning plasma to the at least one optical element for cleaning the optical element. The at least one optical element is a different element from the radiation beam target. The reticle stage is configured to receive an object. In one embodiment, the object is a reticle.

[0008] Conventionally, carbon cleaning of fiducials in lithography apparatuses using EUV-induced plasma leads to very high temperatures on the fiducials, which are optical elements, and this quickly leads to damage to the fiducials and deterioration of their reflectivity. This necessitates replacement of the fiducials, which takes a long time and results in extended downtime of the apparatus, which is obviously undesirable. A fiducial may be understood as an optical element. An optical element may be used to reflect a radiation beam and / or to interact with a radiation beam during operation and / or to obtain information from the reflected radiation beam. Fiducials are used in the reticle stage of a lithography apparatus, and if they are cleaned directly with EUV radiation, the fiducials are likely to deteriorate and may no longer be able to function properly. This necessitates replacement of the entire reticle stage, resulting in extended downtime of the apparatus. The present invention aims to solve this problem. Fiducials and other heat-sensitive elements are also used elsewhere in the lithography apparatus, and if they are damaged, they similarly require replacement or repair.

[0009] It has been discovered that heat-sensitive elements, such as optical elements in a lithography apparatus, can be indirectly cleaned by directing a radiation beam, such as an EUV radiation beam, onto a radiation beam target that is different from the optical element itself. The radiation beam interacts with a gas (typically hydrogen) to generate a plasma within the lithography apparatus. The plasma is generated at a location where it can reach the optical element to be cleaned and remove contaminants therefrom. It has not previously been recognized that optical elements in a lithography apparatus can be cleaned in this manner. Typically, the contaminants include carbon, but in other cases may be other materials such as tin, and the plasma can remove the contaminants from the heat-sensitive element. An advantage of the present invention is that the high thermal load on the heat-sensitive optical element can be reduced because the plasma can be generated by the radiation beam striking a radiation beam target that is different from the heat-sensitive optical element to be cleaned, rather than striking the heat-sensitive optical element itself. Because direct radiant energy, such as from an EUV radiation beam, is not absorbed by the heat-sensitive optical element, significant improvements are seen in thermal effects on the heat-sensitive optical element. A further advantage of the present invention is that because the radiation beam does not directly illuminate the heat-sensitive optical element, hydrocarbons generated by the interaction between carbon contaminants and the hydrogen plasma are not immediately re-decomposed into carbon. In systems where direct illumination is performed, this mechanism can partially reverse the cleaning process. The present invention enables cleaning of any heat-sensitive optical element affected by contaminants. The heat-sensitive element is preferably a fiducial mark. The heat-sensitive element may be physically connected to, attached to, and / or included in the reticle stage. In this manner, the present invention advantageously extends the life of the reticle stage. Previously, non-optical elements might be cleaned by generating a cleaning plasma, but such elements are not as sensitive as optical elements, and it was not discovered that the plasma generated as described herein could clean optical elements without damaging them.

[0010] The radiation beam target may be an illumination shutter for limiting the area of ​​the heat-sensitive element to be illuminated, a reticle clamp, a beam dump, or an absorbing element configured to absorb energy from the radiation beam and generate a plasma. The illumination shutter may also be referred to as a reticle masking shutter, or a ReMa blade or unit, or simply ReMa. As such, the illumination shutter may be for limiting the area of ​​the substrate to be illuminated and / or the area of ​​the optical element to be illuminated. An important feature of the beam target is that it can withstand direct exposure to a radiation beam, such as an EUV radiation beam, to generate a plasma from an ambient gas. Because the beam target is not the heat-sensitive element to be cleaned, the radiation beam target can be fabricated from a material that can withstand direct exposure to the radiation beam (but not necessarily the heat-sensitive element).

[0011] The control system may be configured to move the radiation beam target relative to the optical element having the area to be cleaned. Alternatively or additionally, the control system may be configured to move the optical element having the area to be cleaned to a position where it can interact with the optical element cleaning plasma. To provide optimal cleaning performance, it is desirable to generate plasma near the area to be cleaned. Because the plasma recombines over time, cleaning will not occur if the optical element is too far from where the plasma is generated. Therefore, several modes are possible. One is a stationary mode in which the at least one optical element and the radiation beam target are stationary, and the radiation beam is directed onto the radiation beam target to generate plasma for cleaning the at least one optical element. Another mode may involve scanning the radiation beam over the radiation beam target, optionally partially exposing a portion of the optical element having the area to be cleaned that is different from the absorbing area of ​​the optical element. For example, the thermally sensitive optical element may be a fiducial mark used in a lithography apparatus to measure at least one parameter, such as position or alignment. The scanning mode may provide more uniform cleaning of the thermally sensitive element being cleaned.

[0012] The apparatus may include a gas source configured to supply a gas. The gas may be hydrogen. The gas source may be configured to maintain a gas pressure in the range of about 1 Pa to about 20 Pa. In order to generate a plasma, a gas must be present within the lithographic apparatus. Typically, the interior of a lithographic apparatus is a low-pressure hydrogen environment. The gas source is configured to supply such a gas. The pressure of the gas (preferably hydrogen) may be increased system-wide or locally to increase the rate at which the optical elements are cleaned.

[0013] The radiation beam target may be connected to the at least one optical element. The radiation beam target may be located within about 25 mm of the at least one optical element. Because the plasma is generated by the interaction of the radiation beam, the radiation beam target and the surrounding gas, it is desirable to locate the radiation beam target close to the at least one optical element in order to generate the plasma close to the area of ​​the optical element to be cleaned.

[0014] The radiation beam target may be provided with a conditioning system: As the radiation beam target is directly illuminated and heated by the radiation beam, a conditioning system may be provided to cope with this heat load.

[0015] The radiation beam target may be a beam dump, optionally comprising chrome. The beam dump, like any radiation beam target described herein, may include a conditioning system. The beam dump may be positioned on a reticle stage.

[0016] The radiation beam target may be positioned close to the optical element, thus minimizing the distance between the plasma generation region and the area to be cleaned, leading to less plasma loss and higher cleaning rates.

[0017] The at least one optical element may include a fiducial mark having a mark area. The mark area may be rectangular, with at least one side further comprising a hemispherical or semi-elliptical area. Alternatively or additionally, the mark area may include at least one curved edge. Existing fiducials have a rectangular shape, with a large portion of the radiation beam illuminating an absorbing portion of the fiducial. This leads to very high temperatures (e.g., above 300°C) on the fiducial, causing rapid deterioration of reflectivity. This results in the need for repair or replacement of the fiducial, resulting in significant downtime for the lithography apparatus. It has been found that a curved fiducial can reflect more of the incoming radiation and reduce the fiducial's heat absorption. In particular, the curved fiducial can reduce the temperature from 350°C to approximately 145°C and increase the time the fiducial can be stationary and exposed to the radiation beam from less than 10 seconds to approximately 100 seconds. To further minimize the radiation-absorbing area, the reflective area of ​​the fiducial may be enlarged or extended to the edge of the fiducial. This can provide a reduced thermal load and reduced temperature gain reference without changing the in-production scanning behavior of existing reticle stages.

[0018] The at least one illumination shutter comprises two blades, and the control system is configured to window the at least one optical element with the two blades, which may be referred to as reticle masking blades, also known as ReMa blades.

[0019] The blades may be arranged parallel to one another. At least one optical element may be arranged between the blades. At least one optical element may be arranged in a plane different from the plane of the blades. In an embodiment, the optical element and the radiation beam target are in the same plane or in parallel planes.

[0020] The reticle stage may include a conditioning circuit and a spacer. The spacer may be connected to the at least one optical element. The conditioning circuit may be configured to condition the at least one optical element and the spacer. The conditioning circuit may be referred to as a thermal conditioning circuit.

[0021] The gas may be hydrogen, which is typically used in lithographic tools, and hydrogen plasma can remove carbon deposits in the tool.

[0022] The radiation beam may be an EUV beam.

[0023] A reticle stage may be a system or subsystem of a lithographic apparatus in which the reticle stage supports a reticle that is used in the lithographic apparatus to provide a pattern in an incoming radiation beam.

[0024] At least one optical element may be a fiducial mark. A fiducial or fiducial marker may be provided to assist in alignment measurements to determine the precise alignment of elements of the apparatus. The fiducial may be patterned into a portion of the radiation beam for measurement purposes. For example, the fiducial may comprise a reflective diffraction grating which may produce multiple diffraction orders when illuminated. In either case, the fiducial interacts with the radiation to provide useful information. If the fiducial is contaminated, for example with carbon, it may undesirably no longer reflect radiation effectively.

[0025] According to a second aspect of the present invention, there is provided a fiducial for a lithographic apparatus, the fiducial comprising at least one curved edge. As mentioned above, it has been surprisingly found that the curved edge increases reflectivity and thereby reduces the temperature rise of the fiducial upon exposure to a radiation beam. Existing fiducials are rectangular and do not comprise at least one curved edge.

[0026] The fiducial may comprise two opposing linear edges. It has been found advantageous for the fiducial to include at least one curved edge, although not all of the edges need to be curved to exhibit an advantageous effect. Thus, the fiducial may comprise two opposing linear edges. The opposing linear edges may be parallel or divergent. The fiducial may comprise one substantially linear edge and one curved edge opposite. Thus, in embodiments, the fiducial comprises three linear edges and one curved edge. The fiducial according to the second aspect may be an optical element according to the first aspect of the invention.

[0027] According to a third aspect of the present invention, there is provided a lithographic apparatus comprising a reticle stage or a fiducial according to the first or second aspect of the present invention.

[0028] According to a fourth aspect of the present invention, there is provided a method for cleaning at least one optical element of a reticle stage of a lithographic apparatus, the method comprising providing a reticle stage according to any aspect of the present invention, and directing a radiation beam towards a radiation beam target to generate an optical element cleaning plasma.

[0029] According to a fifth aspect of the invention, there is provided the use of a reticle stage, a datum or a method according to any aspect of the invention in a lithographic apparatus or method.

[0030] According to a sixth aspect of the present invention, there is provided a computer program product comprising instructions for causing an apparatus according to any aspect of the present invention to perform the steps of the method according to the fourth aspect of the present invention. There is also provided a computer readable medium having stored thereon a computer program according to the sixth aspect of the present invention.

[0031] It is understood that features described with respect to one aspect or embodiment may be combined with any feature described with respect to any other aspect or embodiment, and that all such combinations are expressly contemplated and disclosed herein. [Brief explanation of the drawings]

[0032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0033] FIG. 1 shows a lithographic apparatus that may include a reticle stage according to an embodiment of the invention.

[0034] FIG. 2 is a schematic diagram of one embodiment of a reticle stage according to the present invention.

[0035] FIG. 3 is a schematic diagram of an embodiment of a reticle stage according to the present invention.

[0036] FIG. 4 is a schematic diagram of one embodiment of a reticle stage according to the present invention that includes a beam dump.

[0037] FIG. 5 illustrates a cleaning process in which the heat-sensitive element and the radiation beam target are in relative motion.

[0038] FIG. 6a shows a fiducial shape of the prior art, and FIGS. 6b and 6c show fiducial shapes according to an embodiment of the present invention, having at least one curved edge.

[0039] Features and advantages of the present invention will become more apparent from the detailed description set forth below when considered in conjunction with the drawings in which like reference characters identify corresponding elements. In the drawings, like reference numerals generally indicate identical, functionally similar, and / or structurally similar elements. The following detailed description describes the invention with respect to an optical element having an area to be cleaned, but it will be understood that the invention is also applicable to cleaning other heat-sensitive elements. DETAILED DESCRIPTION OF THE INVENTION

[0040] Figure 1 shows a lithographic apparatus. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate a beam of extreme ultraviolet (EUV) radiation B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident on the patterning device MA. The projection system is configured to project the radiation beam B (patterned by the mask MA) onto the substrate W. The substrate W may include a previously formed pattern. In this case, the lithographic apparatus aligns the patterned radiation beam B to the previously formed pattern on the substrate W.

[0041] The 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 (e.g. hydrogen) at a pressure below atmospheric pressure may be provided in the source SO. A vacuum may be provided in the illumination system IL and / or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure below atmospheric pressure may be provided in the illumination system IL and / or the projection system PS.

[0042] The radiation source SO shown in FIG. 1 is of a type that may be referred to as a laser-produced plasma (LPP) source. A laser, which may be, for example, a CO laser, is configured to inject energy via a laser beam into a fuel, such as tin (Sn), provided from a fuel emitter. While the following description refers to tin, any suitable fuel may be used. The fuel may be, for example, in liquid form or may be a metal or alloy. The fuel emitter may include a nozzle configured to direct the tin, for example, in the form of droplets, along a trajectory toward the plasma formation region. The laser beam is incident on the tin in the plasma formation region. The injection of laser energy into the tin generates a plasma in the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of the ions of the plasma.

[0043] The EUV radiation is collected by a near-normal incidence radiation collector (more commonly referred to as a normal incidence radiation collector). The collector may have a multi-layer structure configured to reflect EUV radiation (e.g., EUV radiation having a desired wavelength, such as 13.5 nm). The collector may have an elliptical configuration with two elliptical foci. The first focus may be at the plasma formation region, and the second focus may be at an intermediate focus, as described below.

[0044] The laser may be separate from the radiation source SO, in which case the laser beam may be passed from the laser to the radiation source SO by a beam delivery system (not shown), for example comprising appropriate directing mirrors and / or beam expanders and / or other optical elements. The laser and radiation source SO may together be considered a radiation system.

[0045] The radiation reflected by the collector forms a radiation beam B. The radiation beam B is focused to a point to form an image of the plasma formation region and 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 within or near an aperture in an enclosure of the radiation source.

[0046] The radiation beam B travels from a radiation source SO into an illumination system IL that 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. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B from the illumination system IL is incident on a patterning device MA that is held by a 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 facetted field mirror device 10 and the facetted pupil mirror device 11.

[0047] Following reflection from the patterning device MA, the patterned radiation beam B enters a projection system PS. The projection system comprises a number of mirrors 13, 14 configured to project the radiation beam B onto a substrate W held by a substrate table WT, which may also be referred to as a substrate stage. The projection system PS may apply a demagnification factor to the radiation beam to form an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 may be applied. Although in Figure 1 the projection system PS comprises two mirrors 13, 14, the projection system may include any number of mirrors (e.g. six mirrors).

[0048] The radiation source SO shown in Figure 1 may include components that are not shown. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transparent to EUV radiation but may substantially block other wavelengths of radiation, such as infrared radiation.

[0049] FIG. 2 is a schematic diagram of one embodiment of the present invention. In this exemplary embodiment, a reticle stage comprises a support structure MT for a patterning device MA. It is understood that the reticle stage may thus be a system or subsystem of a lithographic apparatus. The reticle stage is indicated by the numeral 15. The reticle stage 15 comprises one or more optical elements 16. The illustrated example includes two optical elements 16, but in some embodiments there may be only one or more than two optical elements 16. In the illustrated example, the optical element 16 is a fiducial. The fiducial is used to calibrate the lithographic apparatus and also to provide information about its positioning relative to the radiation beam. The optical element 16 is contaminated with a contaminant 17, which may be carbon. The radiation beam target 18 in the illustrated example is an illumination shutter for limiting the area of ​​the substrate to be illuminated. In other embodiments, the radiation beam target may be provided near the optical element 16 or may be a beam dump or a reticle clamp. During use, a radiation beam 19 is directed towards the radiation beam target 18. The interaction of the radiation beam 19, the radiation beam target 18 and the surrounding gas (preferably hydrogen) creates a plasma 20. The plasma 20 can react with the contaminants 17 and clean the optical element 16. Because the optical element 16 is not the radiation beam target 18, the optical element 16 is protected from direct exposure to the radiation beam 19 and is less susceptible to damage from overheating.

[0050] FIG. 3 is similar to FIG. 2 except that it shows a gas source 21. The gas source 21 supplies a gas (typically hydrogen) to the area around the reticle. This creates what is commonly referred to as a reticle mini-environment. As will be described, the present invention utilizes the generation of a plasma to clean thermally sensitive elements of the reticle stage, such as fiducials on the reticle stage. The gas source 21 is configured to generate a gas curtain that functions to reduce contaminants on the surface of the reticle.

[0051] FIG. 4 shows an embodiment in which the beam dump 22 is positioned adjacent to the optical element 16. In this manner, when the radiation beam illuminates the beam dump 22, a plasma is generated adjacent to the optical element 16, allowing for cleaning of contaminants 17. The beam dump 22 may be separate from the optical element 16, in contact with the optical element 16, or connected to the optical element 16. There may be a conditioning system 23 configured to condition the beam dump 22 and / or the optical element 16. It is understood that a conditioning system may be provided in embodiments in which the radiation beam target is a different element from the beam dump 22. The invention is not particularly limited by the specific nature of the conditioning system selected. In an embodiment, the conditioning system 23 may include a water cooling circuit.

[0052] 5 illustrates one embodiment of the present invention in which a heat-sensitive optical element and a radiation beam target are moved relative to one another to clean contaminants 17 from the heat-sensitive optical element. In a first stage, a radiation beam target 18 and a radiation beam 19 are positioned to generate a plasma 20 in a first region of contaminants 17 present on a heat-sensitive element 16, which is an optical element. As the cleaning process progresses, the optical element 16 and the radiation beam target 18 move relative to one another such that the radiation beam target 18 and the radiation beam 19 are positioned to generate a plasma 20 in a second region of contaminants 17 present on the optical element 16 to clean the second region. The cleaning process may further progress such that the optical element 16 and the radiation beam target 18 move relative to one another such that the radiation beam target 18 and the radiation beam 19 are positioned to generate a plasma 20 in a third region of contaminants 17 present on the optical element 16 to clean the third region. It will be appreciated that this may continue until the optical element 16 is sufficiently cleaned. If necessary, the process may be repeated one or more times. The relative movement between the optical element and the radiation beam target may be continuous or stepped as required. In other embodiments, the optical element and the radiation beam target may remain stationary relative to each other, and the radiation beam may move over the radiation beam target to generate plasma at different locations and clean the optical element. It will be appreciated that the optical element and the radiation beam target may initially move relative to each other in order to be positioned at the required position for a subsequent scan of the radiation beam.

[0053] FIG. 6a shows a prior art fiducial that is rectangular with four linear edges. In contrast, the fiducials according to the present invention illustrated in FIGS. 6b and 6c have at least one curved edge. In particular, the fiducial in FIG. 6b has two opposing curved edges, while the fiducial in FIG. 6c has a single curved edge. The fiducial has a reflective portion and an absorbing portion. Therefore, for a fiducial of the conventional shape shown in FIG. 6a, a large portion of the incoming radiation beam strikes the absorbing portion. This leads to very high temperatures on the fiducial (e.g., above 300°C) and rapid deterioration of reflectivity. This results in the need for repair or replacement of the fiducial. By providing a fiducial with at least one curved edge, as shown in FIGS. 6b and 6c, the temperature of the fiducial is significantly reduced (e.g., from above 300°C to below 150°C). Additionally, the maximum time the fiducial may remain stationary and be exposed to the radiation beam is increased from less than 10 seconds to more than 1 minute (e.g., up to 100 seconds).

[0054] Another way in which the temperature of the fiducial can be managed is by increasing the amount of reflective area (reducing the amount of absorbing parts) in the fiducial so that it absorbs less energy from the incoming radiation beam.

[0055] In summary, the present invention provides for cleaning of optical elements of a lithographic apparatus without directly exposing such elements to the radiation beam. This prevents damage to the elements and reduces downtime of the lithographic apparatus. The radiation beam target may be configured to withstand direct exposure to the radiation beam and to be unaffected by direct illumination by the radiation beam. A gas (typically hydrogen) may be converted into a plasma which can clean contaminants (typically carbon) from the optical elements. A further advantage of the present invention is that the problem of carbon being redeposited on heat-sensitive elements due to decomposition of hydrocarbons is reduced or eliminated.

[0056] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced otherwise than as described.

[0057] The foregoing description is intended to be illustrative, not limiting, and 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. at least one optical element having an area to be cleaned; a radiation beam target; a control system configured to direct a radiation beam at said radiation beam target; Equipped with interaction of the radiation beam and the gas to generate an optical element cleaning plasma; the radiation beam target is positioned to provide the optical element cleaning plasma to the at least one optical element for cleaning the optical element; the at least one optical element is a different element from the radiation beam target; A reticle stage for a lithography apparatus.

2. 2. A reticle stage for a lithographic apparatus according to claim 1, wherein the radiation beam target is an illumination shutter for limiting an area of ​​the substrate to be illuminated, a reticle clamp, a beam dump, or an absorbing element configured to absorb energy from the radiation beam and generate a plasma.

3. 3. A reticle stage for a lithographic apparatus according to claim 2, wherein the control system is configured to move the radiation beam target relative to the optical element having an area to be cleaned, and / or the control system is configured to move the optical element having an area to be cleaned to a position where it can interact with the optical element cleaning plasma.

4. 4. A reticle stage for a lithographic apparatus according to claim 1, further comprising a gas supply configured to supply the gas, optionally the gas supply configured to maintain a gas pressure in the range of 1 Pa to 20 Pa.

5. 5. A reticle stage for a lithographic apparatus according to claim 1, wherein the radiation beam target is connected to the at least one optical element or is located within 25 mm of the at least one optical element.

6. The reticle stage for a lithographic apparatus according to any of claims 2 to 5, wherein the radiation beam target further comprises a conditioning system.

7. A reticle stage for a lithographic apparatus according to any of claims 2 to 6, wherein the radiation beam target is a beam dump, optionally the beam dump comprises Cr.

8. 8. A reticle stage for a lithographic apparatus according to claim 1, wherein the at least one optical element having the area to be cleaned comprises a reference mark having a mark area, and optionally the mark area is rectangular in shape, one side of which further comprises a hemispherical area or a semi-elliptical area, or has at least one curved edge.

9. the illumination shutter comprises two blades; the control system is configured to window the at least one optical element with the two blades; A reticle stage for a lithographic apparatus according to claim 2 or 3.

10. The blades are arranged parallel to one another, the at least one optical element is disposed between the blades; 10. A reticle stage for a lithographic apparatus according to claim 9.

11. a conditioning circuit and a spacer; the spacer is connected to the at least one optical element; the conditioning circuit is configured to condition the at least one optical element and the spacer. A reticle stage for a lithographic apparatus according to any one of claims 1 to 10.

12. A reticle stage for a lithographic apparatus according to any of claims 1 to 11, wherein the gas is hydrogen.

13. 13. A reticle stage for a lithographic apparatus according to any of the preceding claims, wherein the radiation beam is an EUV beam.

14. Providing a reticle stage for a lithographic apparatus according to any one of claims 1 to 13; directing a radiation beam at the radiation beam target to generate an optical element cleaning plasma; 1. A method for cleaning at least one optical element of a reticle stage, comprising:

15. A computer program product comprising instructions for causing a reticle stage for a lithographic apparatus according to any one of claims 1 to 13 to perform the steps of the method according to claim 14.

16. 16. A computer readable medium storing the computer program of claim 15.

17. A lithographic apparatus comprising a reticle stage for the lithographic apparatus according to any one of claims 1 to 13.

18. A reticle stage for a lithographic apparatus according to any one of claims 1 to 13 or use of the method according to claim 14 in a lithographic method or apparatus.

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