Radiation-reflecting optical elements and optical assemblies

Abstract

The invention relates to an optical element (M4) for reflecting radiation, in particular EUV radiation, comprising a substrate (25) having a first partial body (26a) and a second partial body (26b) joined at an interface (27), a reflective coating (29) applied to a surface (28) of the first partial body (26a), a number of cooling channels (31) extending into the substrate (25) in the region of the interface (27) below the surface (28) to which the reflective coating (29) is applied, a distribution portion (32) formed in the substrate (25) for connecting a coolant inlet (34) to the plurality of cooling channels (31), and a collection portion formed in the substrate (25) for connecting the plurality of cooling channels (31) to a coolant outlet (35). The distribution section (32) and / or the recovery section extend, starting from the interface (27), further into the second part (26b) of the substrate (25) than into the first part (26a) of the substrate (25).The invention also relates to an optical device, in particular an EUV lithography system, comprising at least one optical element (M4) as described above and a cooling device designed to direct a coolant through the cooling channels (31).

Description

Technical Field

[0001] [Citation of Related Applications] This application claims the priority of German Patent Application No. 10 2021 201 715.0 filed on February 24, 2021, and incorporates its entire disclosure herein by reference.

[0002] The present invention relates to an optical element that reflects radiation, particularly EUV radiation, and includes a substrate having a first part and a second part joined at an interface, a reflective coating applied to the surface of the first part, a plurality of cooling channels extending into the substrate in the region of the interface below the surface to which the reflective coating is applied, a distribution part formed on the substrate to connect a coolant inlet to the plurality of cooling channels, and a recovery part formed on the substrate to connect the plurality of cooling channels to a coolant outlet. The present invention also relates to an optical device, particularly an EUV lithography system, including at least one such optical element and a cooling device designed to flow a coolant through the plurality of cooling channels.

Background Art

[0003] The thermal load on reflective optical elements for lithography, particularly for EUV lithography, is increasing due to the higher power of the radiation sources used in the operation of these elements. This particularly applies to the mirrors of projection systems for EUV lithography. In principle, for the substrate of this type of reflective optical element, which is also referred to as a mirror for simplicity below, the use of materials with a coefficient of thermal expansion as close as possible to "zero" is attempted. In practice, it is best to meet this requirement at a specific temperature, also called the zero-crossing temperature.

[0004] Such mirrors of projection systems can only operate near the zero-crossing temperature as a result of different degrees of heating depending on the set or illumination state. This causes the mirror, more precisely the surface having the reflective coating, to deform due to the thermal load during irradiation. With the increase in the thermal load, this "mirror heating" problem has a limiting effect on the performance of the optical device in which the mirror is arranged.

[0005] A mechatronic approach can be used to solve this problem. Another, relatively simple concept is to directly cool each mirror, that is, to flow a cooling fluid through the mirror's substrate, or more precisely, through cooling channels formed in the substrate. The advantage of this concept is that the temperature of the mirror can be set relatively precisely by the temperature of the cooling fluid, meaning the mirror has a thermal reference.

[0006] Direct cooling of mirrors in optical devices, particularly EUV lithography systems, involves several boundary conditions that require finding an optimal balance. It has been found advantageous to form multiple substantially parallel cooling channels in the substrate extending beneath the surface coated with a reflective coating. To allow sufficient flexibility in the geometric design, the channel shapes of these cooling channels are formed in two or more substructures of the substrate, which are interconnected at one or more interfaces by an appropriate bonding method or, in some cases, optical contact bonding. Minimizing the number of interfaces in the substrate is advantageous.

[0007] Furthermore, in order to minimize direct connections between the mirrors and the substrate, a distribution unit is required that connects the coolant inlet of the substrate to multiple cooling channels, and a recovery unit that connects the multiple cooling channels of the substrate to the coolant outlet.

[0008] Patent Document 1 discloses an optical element in the form of a mirror having a first layer made of a first material and a second layer made of a second material bonded along an interface. The optical element also has a cooling device configured to extend into the interface region and cool the optical element. The cooling device may have a plurality of cooling channels through which a coolant, such as cooling water, can flow. The cooling channels extend parallel to each other and can laterally connect to side channels connected to a coolant inlet or coolant outlet.

[0009] When a cooling fluid, particularly a coolant, flows through a cooling channel, internal pressure is generated within the cooling channel, especially in the distribution or recovery section. This internal pressure can lead to undesirable deformation of the surface to which the reflective coating has been applied. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] German Patent Application Publication No. 10 2019 217 530 Specification [Overview of the project] [Problems that the invention aims to solve]

[0011] The object of the present invention is to provide an optical element and an optical device that enable the reduction of surface deformation of an optical element with a reflective coating by direct cooling with a cooling fluid. [Means for solving the problem]

[0012] According to one embodiment, this objective is achieved by an optical element of the type described above, wherein the distribution portion and / or collection portion starts from the interface and extends more significantly into the second portion of the substrate than into the first portion of the substrate.

[0013] The extent of the distribution / recovery sections to the first and second parts is related to the thickness direction of the substrate. The extent of the distribution / recovery sections to the first part is generally very small. The maximum distance of the distribution / recovery sections to the first part from the interface may be less than or equal to the maximum extent of each cooling channel to the first part. In contrast, the (maximum) extent of the distribution / recovery sections to the second part is generally (significantly) greater than the maximum extent of each cooling channel to the second part. The extent of the distribution / recovery sections to the second part may be more than five times the extent of the distribution / recovery sections to the first part. The distribution and / or recovery sections may, in some cases, start from the interface and extend only to the second part and not to the first part.

[0014] Unlike the side channel described in Patent Document 1, which extends along the interface between two subparts, in this embodiment of the present invention, the distribution and / or recovery section is located mainly, and possibly entirely, in the second subpart, i.e., it starts from the interface and extends more into the second subpart than into the first subpart. In this case, the distribution and / or recovery section is connected to a cooling channel that extends along the interface. In this way, it is possible to reduce the effect of substrate deformation on the surface shape of the first subpart in the region having a reflective coating, which may occur due to the internal pressure of the cooling fluid in the distribution / recovery section region.

[0015] The cross-section of each cooling channel may be divided between two parts. In this case, as described in Patent Document 1, for example, each groove-like recess may be formed in the first part and another groove-like recess may be formed in the second part, and when the two parts are connected along the interface, the two groove-like recesses are joined to form a single cooling channel. In this case, each groove-like recess is milled in both the first and second parts. However, it is also possible that the groove-like recess is milled in only the first part or only the second part, with the other part covering the recess like a lid to form the cross-section of the cooling channel. In either case, as a result of the interface extending within or on the edge of the cross-section of each cooling channel, it is in principle sufficient to connect each cooling channel to the coolant inlet or coolant outlet if the distribution / recovery section extends into the second part of the substrate to the interface region. However, the distribution / recovery section may also extend into the first part to connect the cooling channels to each other in a cross-sectional portion that extends into the first part, for example.

[0016] The distribution and recovery sections can, in principle, have the same configuration. In this case, the distribution section can only be distinguished from the recovery section in an optical element when the cooling medium flows through the optical element and / or the cooling channel. However, in order to optimize the flow of the cooling medium, the distribution and recovery sections may have different designs, i.e., different geometric shapes.

[0017] In yet another embodiment, the distribution and / or recovery section is positioned at an angle of 30° or less with respect to the thickness direction of the substrate, at least in the portion beginning at the interface, in the second substructure, and possibly in the first substructure as well. To reduce the effect of the internal pressure of the distribution / recovery section on the surface having a reflective coating, it is advantageous to incline the distribution / recovery section with respect to the surface of the first substructure and / or the interface, at least in the portion beginning at the interface. In this way, the surface area of ​​the distribution / recovery section that may bulge due to the internal pressure of the cooling fluid is also inclined with respect to the surface, and as a result the effect of bulging on the geometric shape of the surface is reduced. The distribution / recovery section may, in the portion connected to the interface, extend particularly in the thickness direction of the substrate and / or parallel to the thickness direction of the substrate, i.e., perpendicular to the substantially planar bottom surface of the second substructure, but this is not mandatory.

[0018] In yet another embodiment, the distribution and / or recovery section extends, in the second sub-part, and possibly in the first sub-part, at least from the interface, beneath a portion of the surface not covered by the reflective coating. To avoid significant deformation of the surface area coated with the reflective coating, or the optically utilized portion of the reflective coating, it is advantageous to position the distribution / recovery section as far away from the optically utilized surface area as possible. In this embodiment, the cooling channel typically needs to extend into the portion of the surface not covered by the reflective coating. To reduce the effects of fluid pressure, it may also be possible for the distribution / recovery section to extend into the portion of the surface not covered by the reflective coating, but not into the optically utilized portion of the reflective coating. When irradiating optical elements in an optical device, such as in an EUV lithography device, the radiation used irradiates the optically utilized portion.

[0019] In yet another embodiment of the present invention, which can be combined with the embodiments described above, the distribution unit has a distribution chamber that widens from the coolant inlet toward the interface, and / or the recovery unit has a recovery chamber that narrows from the interface toward the coolant outlet.

[0020] In this aspect of the invention, the dispensing / recovery part, more precisely the dispensing chamber / recovery chamber, can also extend along the interface between the first sub-body and the second sub-body without the dispensing / recovery part extending more greatly towards the second sub-body than the first sub-body. In this case, it is expedient for the dispensing chamber / recovery chamber to have as flat a form as possible along the interface. By the flow cross-section of each chamber widening or narrowing, a substantially triangular or funnel-shaped geometry of the dispensing / recovery part optimized with respect to the flow is realized. However, this geometry also makes the surface area of the dispensing / recovery part relatively large. This, and the fact that the interface and thus the dispensing / recovery part extends generally at a small distance from the surface coated with the reflective coating, means that the surface may bulge due to the internal pressure in the dispensing chamber / recovery chamber.

[0021] Therefore, it has been found to be expedient if, when the dispensing chamber / recovery chamber extends along the interface between the two sub-bodies, they only extend under a partial area of the surface not covered by the reflective coating (see above).

[0022] In an improvement of this embodiment, the dispensing chamber extends from the coolant inlet to the interface and / or the recovery chamber extends from the interface to the coolant outlet. In particular, in this case, it is expedient if the dispensing chamber / recovery chamber is aligned substantially perpendicular to the thickness direction of the substrate starting from the interface as described above.

[0023] However, in this case, the dispensing chamber / recovery chamber still has a large surface area on which the internal pressure of the cooling fluid acts and which can thus lead to deformation of the surface. Therefore, it is expedient for the dispensing chamber / recovery chamber not to extend to the interface or to be directly connected to the interface, since it has the largest lateral size at the interface. <00000​​In an alternative embodiment, the distribution part and / or the collection part have a portion starting from the interface, having connection channels that connect at least one cooling channel of each to a coolant inlet or a coolant outlet. In this embodiment, the cooling channels are followed by connection channels connected to one or more of the cooling channels in the region of the interface.

[0025] The connection channels can be aligned at an angle of 30° or less, particularly in the thickness direction of the substrate, in the portion starting from the interface. In this way, the cooling channels near the edge of the optically utilized portion region of the surface and / or near the partial region of the surface covered by the reflective coating are substantially redirected in the vertical direction. In this way, the cooling channels and / or the connection channels can distribute or merge at another part of the distribution part / collection part spaced apart in the thickness direction from the surface of the substrate and / or from the interface.

[0026] In principle, it is possible to assign exactly one connection channel to a cooling channel. In this case, the connection channel becomes the portion where the cooling channel continues to the second part of the substrate. The connection channel is usually drilled in the second part of the substrate, that is, the connection channel is a drilled hole.

[0027] Generally, a plurality of cooling channels, each having a relatively small cross-sectional area, for example, 10 or more, are formed in the substrate. Therefore, further, when drilling a connection channel having a generally relatively large length or depth, there is a manufacturing-related risk that the second part of the substrate is damaged during the drilling operation.

[0028] In an improvement of the above embodiment, each connection channel is connected to at least two, particularly exactly two, cooling channels. In this way, the cross-sectional area of the connection channel directly adjacent to the cooling channel is enlarged to at least twice its cross-sectional area, and as a result, the manufacturing risk during the drilling of the connection channel can be reduced.

[0029] Another improvement involves the cross-section of each connecting channel being reduced in a stepwise manner from the interface. When the distance between adjacent connecting channels is relatively small and the surface area of ​​the connecting channels subjected to fluid pressure is relatively large due to the relatively large cross-section of the connecting channels, it may be advantageous to change the diameter of the connecting channels, especially by reducing it from the interface. The cross-section of each connecting channel can be reduced in a stepwise manner, i.e., the connecting channel has one or possibly more steps in which the cross-section of the connecting channel is gradually reduced. In principle, it is also possible to continuously change or reduce the cross-section of each connecting channel.

[0030] In yet another embodiment, the distribution chamber is connected to a portion of the distribution section having a connecting channel, and / or the recovery chamber is connected to a portion of the recovery section having a connecting channel. In this case, the connecting channel causes the distribution chamber / recovery chamber to be spaced in the thickness direction of the substrate from the surface having the reflective coating and / or from the interface between the two portions. The greater distance from the surface reduces the effect on the surface geometry of the substrate deformation caused by the bulging of each chamber due to the pressure of the cooling fluid compared to the above case where the distribution chamber / recovery chamber is directly connected to the interface.

[0031] In this improved embodiment, the distribution chamber and / or recovery chamber extends along another interface between the second portion and the third portion of the substrate, where the third portion is joined with the second portion. This other interface can extend substantially parallel to the interface where the first portion is joined with the second portion. In this way, the distribution chamber and / or recovery chamber are offset in the thickness direction of the substrate from one interface to the other. When attempting to offset the distribution chamber and / or recovery chamber in the thickness direction from an interface, the other interface is generally necessary because the chamber cannot be easily realized on the second portion alone due to its funnel-shaped geometric form.

[0032] In yet another embodiment, the connection channel of the distribution section leads to a common inlet channel connected to the coolant inlet, and / or the connection channel of the recovery section leads to a common outlet channel connected to the coolant outlet. The inlet and / or outlet channels are typically in the form of perforations in the second substructure. The inlet and / or outlet channels may, but are not required, extend substantially parallel to the bottom surface of the second substructure and / or substrate. The inlet and / or outlet channels may form transverse holes in the second substructure leading to the connection channels. The coolant inlet and / or coolant outlet may be in the form of openings at the free ends of the inlet and outlet channels, respectively.

[0033] In yet another embodiment, the coolant inlet and / or coolant outlet are formed in the second and / or third portion of the substrate. The coolant inlet and / or coolant outlet may be formed by openings on the sides of the second and / or third portion, for example, but it is also possible to form the coolant inlet and / or coolant outlet on the underside of the substrate, i.e., on the surface of the substrate opposite to the interface and / or other interface. In the region of the coolant inlet and / or coolant outlet, the substrate is usually shaped in such a way that coolant lines can be easily connected to the coolant inlet and / or coolant outlet.

[0034] In yet another embodiment, the cross-section of each cooling channel in a plurality of cooling channels is divided between a first sub-part and a second sub-part. As described above, it is possible to divide the cross-sectional area of ​​a cooling channel, or one or more cooling channels, between two sub-parts. In this case, the cooling channel does not extend within or parallel to a substantially planar interface.

[0035] In yet another embodiment, the surface of the substrate coated with a reflective coating is curved and / or the cooling channel itself is curved (in the thickness direction of the substrate), and it is preferable that the curved cooling channel is at a certain distance from the curved surface. In the case of this type of substrate, the above division of the cross section of the cooling channel between the two parts is particularly advantageous when the interface itself has a planar shape, for example, rather than following the curvature of the surface. In this case, the division of the cross section between the two parts ensures that, despite the planar interface, the curved cooling channel follows the curved surface, resulting in the cooling channel extending at a certain distance from the curved surface. In this case, groove-like recesses having a curvature that follows the curvature of the surface are generally introduced not only in the first part but also in the second part. In this case, the cooling channel is formed by joining the corresponding curved groove-like recess of the first part and the groove-like recess of the second part along the interface. This makes it possible for the curved cooling channel to have a constant channel cross section over its length.

[0036] A further aspect of the present invention relates to an optical device, such as an EUV lithography system, comprising at least one optical element formed as described above and a cooling device designed to flow a coolant through a plurality of cooling channels. The EUV lithography system may be an EUV lithography apparatus for wafer exposure, or any other optical device using EUV radiation, such as an EUV inspection system for inspecting masks, wafers, etc., used in EUV lithography. The reflective optical element may be a mirror in the projection system of the EUV lithography apparatus. As an example, the cooling device may be designed to flow a coolant in the form of a cooling fluid, such as a coolant in the form of cooling water, through the cooling channels. For this purpose, the cooling device may optionally have a pump and appropriate supply and discharge lines. The optical device may be a lithography system for a different wavelength range, such as a DUV wavelength range, such as a DUV lithography system, or an inspection system for inspecting masks, wafers, etc.

[0037] Further features and advantages of the present invention will become apparent from the following description of exemplary embodiments of the invention with reference to the drawings illustrating essential details of the invention, and from the claims. Each of the individual features can be implemented individually or in any combination of several in one variant of the invention.

[0038] An exemplary embodiment is shown in the schematic diagram and described below. [Brief explanation of the drawing]

[0039] [Figure 1] This shows a schematic meridian cross-section of a projection exposure apparatus for EUV projection lithography. [Figure 2] A schematic diagram of a mirror having multiple cooling channels extending along the interface between two parts of a substrate, as well as a distribution chamber and a recovery chamber, is shown. [Figure 3] Figures 3a and 3b show schematic diagrams of a mirror in which the distribution chamber and recovery chamber are formed only in the second part and extend in the thickness direction of the substrate. [Figure 4] Figures 4a and 4b show schematic diagrams of a mirror having a distribution chamber and a collection chamber extending along another interface between the second and third parts of the substrate. [Figure 5] Figures 5a, 5b, and 5c show schematic diagrams of a mirror having connecting channels that extend in the thickness direction to connect the cooling channels to the inlet channels of the distribution section. [Figure 6] Figures 6a and 6b show schematic diagrams of mirrors similar to those in Figures 5a-5c, having a curved surface with cooling channels extending to both the first and second parts of the mirror. [Modes for carrying out the invention]

[0040] In the following drawings, the same reference numerals shall be used for components that are the same or have the same function.

[0041] The essential components of the optical apparatus for EUV lithography in the form of microlithography projection exposure apparatus 1 are described below as an example with reference to Figure 1. The description of the basic configuration of projection exposure apparatus 1 and its components should not be understood as having a limiting effect in this case.

[0042] One embodiment of the illumination system 2 of the projection exposure apparatus 1 includes, in addition to the light source or radiation source 3, an illumination optical unit 4 for illuminating the object field of view 5 on the object surface 6. In an alternative embodiment, the light source 3 may be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the radiation source 3.

[0043] A reticle 7 positioned in the object field of view 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable, particularly in the scanning direction, by a reticle displacement drive 9.

[0044] One embodiment of the illumination system 2 of the projection exposure apparatus 1 includes, in addition to the light source or radiation source 3, an illumination optical unit 4 for illuminating the object field of view 5 on the object surface 6. In an alternative embodiment, the radiation source 3 may be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the radiation source 3.

[0045] For illustrative purposes, a Cartesian xyz coordinate system is shown in Figure 1. The x-direction extends perpendicular to the plane of the figure. The y-direction extends horizontally, and the z-direction extends vertically. In Figure 1, the scanning direction extends in the y-direction. The z-direction extends perpendicular to the object plane 6.

[0046] The projection exposure apparatus 1 includes a projection system 10. The projection system 10 functions to image the object field of view 5 onto the image field of view 11 of the image plane 12. The structure on the reticle 7 is imaged onto the photosensitive layer of the wafer 13, which is positioned in the region of the image field of view 11 of the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by a wafer displacement drive 15, particularly along the y-direction. On the one hand, the displacement of the reticle 7 by the reticle displacement drive 9 and on the other hand, the displacement of the wafer 13 by the wafer displacement drive 15 can be synchronized with each other.

[0047] Radiation source 3 is an EUV radiation source. Radiation source 3 emits EUV radiation 16, which is also referred to below as the radiation used, illumination radiation, or illumination light. In particular, the radiation used has a wavelength in the range of 5 nm to 30 nm. Radiation source 3 may be a plasma source, such as an LPP (laser-generated plasma) source or a GDPP (gas discharge plasma) source. It may also be a synchrotron-based radiation source. Similarly, radiation source 3 may be a free electron laser (FEL).

[0048] Illumination radiation 16 emitted from radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector mirror having one or more elliptical and / or hyperbolic reflecting surfaces. Illumination radiation 16 may be incident on at least one reflecting surface of the collector mirror 17 at an oblique incidence (GI), i.e., at an incidence angle greater than 45°, or at a perpendicular incidence (NI), i.e., at an incidence angle less than 45°. The collector mirror 17 may be structured and / or coated to first optimize its reflectivity for the radiation used, and second to suppress external light.

[0049] The illumination radiation 16 propagates downstream of the collector mirror 17 through the intermediate focal point of the intermediate focal plane 18. The intermediate focal plane 18 can separate the radiation source module, which has the radiation source 3 and the collector mirror 17, from the illumination optical unit 4.

[0050] The illumination optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 located downstream of it in the beam path. The deflection mirror 19 may be a planar deflection mirror or a mirror having a beam influence effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 may be in the form of a spectral filter that separates the wavelength of light used by the illumination radiation 16 from stray light of wavelengths deviating from it. The first facet mirror 20 includes a plurality of individual first facets 21, also referred to below as field facets. Figure 1 shows only some of these facets 21 as examples. A second facet mirror 22 is located downstream of the first facet mirror 20 in the beam path of the illumination optical unit 4. The second facet mirror 22 includes a plurality of second facets 23.

[0051] The illumination optical unit 4 consequently forms a dual-facet system. This basic principle is also referred to as a fly-eye condenser (fly-eye integrator). Each first facet 21 is imaged into the object field of view 5 using a second facet mirror 22. The second facet mirror 22 is the last beam shaping mirror or, in fact, the final mirror for the illumination radiation 16 in the upstream beam path of the object field of view 5.

[0052] The projection system 10 includes a plurality of mirrors Mi, which are numbered sequentially according to their arrangement in the beam path of the projection exposure apparatus 1.

[0053] In the example shown in Figure 1, the projection system 10 includes six mirrors M1 to M6. Substitution with four, eight, ten, twelve, or any other number of mirrors Mi is equally possible. The second-to-last mirror M5 and the final mirror M6 each have apertures through which illumination radiation 16 passes. The projection system 10 is a double-shielded optical unit. The projection system 10 has an image-side numerical aperture greater than 0.4 or 0.5, and may be greater than 0.6, for example, 0.7 or 0.75.

[0054] Similar to the mirrors of the illumination optical unit 4, mirror Mi can have a highly reflective coating for illumination radiation 16.

[0055] Figure 2 shows an example of a mirror M4 of a projection system 10, which includes a substrate 25 formed from a first sub-part 26a and a second sub-part 26b. In the illustrated example, the first sub-part 26a, which is plate-shaped, and the second sub-part 26b, which forms the main body of the substrate 25, are joined or connected to each other at a common interface 27, which is planar in the illustrated example, but this is not mandatory. The connection between the two sub-parts 26a and 26b is provided by conventional bonding or joining processes, for example, by high-temperature or low-temperature bonding, or by optical contact bonding. The materials of the first sub-part 26a and the second sub-part 26b may be the same, but may include different materials. In the illustrated example, both the material of the first sub-part 26a and the material of the second sub-part 26b are ultra-low expansion glass (ULE®). The substrate 25 or the two sub-parts 26a and 26b may be made of another material with the smallest possible coefficient of thermal expansion, such as glass ceramic, for example, Zerodur®.

[0056] A reflective coating 29 is applied to the exposed surface 28 of the first sub-part 26a opposite to the interface 27. EUV radiation 16 from the projection system 10 strikes a sub-region 30 of the surface 28 located within the reflective coating 29, and the sub-region 30 forms an optically utilized sub-region of the reflective coating 29. The reflective coating 29 may include, for example, multiple layers made of materials with different real parts of refractive indices, and the layers may be formed from, for example, Si and Mo when the wavelength of the EUV radiation 16 is 13.5 nm. The surface 28 of the first sub-part 26a is represented as a plane in Figure 2, but may also have curvature.

[0057] In the example shown in Figure 2, multiple cooling channels 31 are formed in the substrate 25 in the region of the interface 27, extending beneath the surface 28 to which the reflective coating 29 is applied. In the example shown in Figure 2, there are approximately 20 cooling channels 31, which extend beneath the surface 28 between the distribution section 32 and the recovery section 33 on either side of the optically utilized portion 30 of the reflective coating 29. In the example shown in Figure 2, the cooling channels 31 are aligned parallel to each other. In the example of Figure 2, the distribution section 32 has a distribution chamber 32a that connects the multiple cooling channels 31 to a common coolant inlet 34, which forms an opening in the second portion 29b. Correspondingly, the recovery section 33 forms a recovery chamber that connects the multiple cooling channels 31 to a common coolant outlet 35, which is similarly in the form of an opening in the second portion 29b.

[0058] As shown in Figure 2, the distribution chamber 32a widens in a funnel shape from the coolant inlet 34 to the end of the cooling channel 31 leading to the distribution chamber 32a. Correspondingly, the recovery chamber 33a narrows in a funnel shape from the end of the cooling channel 33 to the coolant inlet 35. The distribution chamber 32a and the recovery chamber 33a extend along the interface 27 and are formed as flat as possible in the thickness direction of the substrate 25. In the example shown in Figure 2, the distribution chamber 32a and the recovery chamber 33a extend to both the first portion 26a and the second portion 26b. The distribution chamber 32a and the recovery chamber 33a have a substantially triangular geometric shape optimized with respect to flow in order to distribute the coolant as evenly as possible among all the coolant channels 31 and to minimize dynamic excitation due to the flow of the coolant.

[0059] To supply coolant to the coolant inlet 34 and discharge the coolant from the coolant outlet 35, the projection exposure apparatus 1 includes a cooling device 36 schematically shown in Figure 1. In the illustrated example, the cooling device 36 serves to supply coolant in the form of cooling water to the cooling channel 31 or mirror M4, and for this purpose includes a supply line (not shown) that is liquid-tightly connected to the coolant inlet 34. The cooling device 36 also includes a discharge line (not shown) for discharging the cooling water from the coolant outlet 35. For cooling purposes, the other mirrors M1-M3, M5, and M6 of the projection system 10 may also be connected to the cooling device 36, or optionally to further cooling devices provided for this purpose.

[0060] The pressure of the cooling water flowing through the distribution chamber 32a or the recovery chamber 33a may cause the substrate 25 to bulge, potentially altering the geometric shape of the surface 28. Because the distribution chamber 32a and / or the recovery chamber 33a are relatively close to the optically usable portion 30 of the surface 28, undesirable deformation of the optically usable portion 30 may occur in this manner.

[0061] To reduce the effect of the bulge of the distribution chamber 32a and / or recovery chamber 33a on the optically utilized portion 30 of the reflective coating 29, in the case of the mirror M4 shown in Figures 3a and 3b, the distribution section 32 or distribution chamber 32a and the recovery section 33 or recovery chamber 33a extend only from the interface 27 to the second portion 26a of the substrate. In principle, the distribution chamber 32a and / or recovery chamber 32b can also extend from the interface 27 to the first portion 26a in order to further connect the ends of the cooling channels 31 in the first portion 26a. As seen in Figure 3a, the distribution chamber 32a extends from the coolant inlet 34 formed on the underside of the substrate 25 to the interface 27. Correspondingly, the recovery chamber 33a, which is not shown in Figures 3a and 3b, also extends from the interface 27 to the coolant outlet 35 formed on the underside of the substrate 25.

[0062] The distribution chamber 32a, more precisely, the central plane M of the distribution chamber 32a, is aligned at this point parallel to the thickness direction Z of the substrate 25. As can be seen in the partial cross section of Figure 3a, the central plane M extends in the Z and X directions. The distribution chamber 32a is substantially mirror-symmetric with respect to the central plane M. The central plane M also extends through the coolant inlet 34, which forms the lower opening of the second sub-body 26b. In this case, the lower side of the second sub-body 26b extends perpendicular to the thickness direction in the XY plane of the XYZ coordinate system. This significantly reduces the surface area of ​​the distribution chamber 32a that could bulge due to fluid pressure parallel to the surface 28 of the mirror M4 or the optically usable portion 30 of the surface 28. Therefore, by inclining the distribution section 32 and / or the recovery section 33 over the second sub-body 26b, it is possible to reduce the deformation of the optically usable portion 30 of the surface 28 of the mirror M4.

[0063] It is not essential that the distribution chamber 32a extends in the thickness direction Z of the substrate 25. The distribution chamber 32a, or more precisely, its central plane M, can be aligned at an angle α with respect to the thickness direction Z, which should generally be about 30° or less. The recovery section 33 or recovery chamber 33a, which can be seen in the partial cross-section of Figure 3a, in the illustrated example, is located on the side opposite in the Y direction of the optically active partial region 30 of the surface 28 of the substrate 25, and has the same structure as the distribution section 32 or distribution chamber 32a. However, structurally identical designs are not essential. For example, for flow reasons, it may be advantageous for the distribution section 32 and / or distribution chamber 32a and the recovery section 33 and / or recovery chamber 33a to have different geometric shapes.

[0064] In particular, as seen in Figure 3b, both the distribution chamber 32a and the retrieval chamber 33a extend in the Z direction beneath a portion 37 of the surface 28 not covered by the reflective coating 29, and not particularly beneath the optically usable portion 30 of the surface 28. This increases the distance of the pressure-sensitive, potentially bulging triangular surface region formed within the distribution chamber 32a, as seen in Figure 3a, from the optically usable portion 30 of the surface 28. Such an arrangement is also possible in the case of the mirror M4 shown in Figure 2, as the lateral structural space is sufficient for this, since the distribution chamber 32a and the retrieval chamber 33a extend along the interface 27 between the two portions 26a and 26b.

[0065] In the case of the mirror M4 shown in Figures 4a and 4b, the substrate 25 has a third portion 26c in addition to the first and second portions 26a and 26b. The third portion 26c is connected to or joined with the second portion 26b at another interface 38 and is similarly made of ULE®. The connection can be formed as described above at the interface 27 between the first and second portions 26a and 26b. The collection portion 32 shown in Figures 4a and 4b has a portion 39 connected to the interface 27 between the first and second portions 26a and 26b and extending from the interface 27 to the second portion 26b of the substrate 25. A connecting channel 40 extending in the thickness direction Z of the substrate 25 is formed in the portion 39 of the distribution portion 32 connected to the interface 27.

[0066] As in the examples shown in Figures 3a and 3b, and also in Figures 4a and 4b, it is not essential to align the connection channel 40 in the thickness direction of the substrate 25. As shown in Figures 3a and 3b, it is usually possible to align the connection channel 40 at an angle α of 30° or less with respect to the thickness direction Z. In principle, it may be advantageous for the angle α at which the connection channel 40 is aligned with respect to the thickness direction Z of the substrate 25 to vary in the substrate 25.

[0067] In the example shown in Figures 4a and 4b, each connection channel 40 is connected to exactly one cooling channel 31, which extends downward to the second substructure 26b. In other words, each cooling channel 31 is redirected from an arrangement parallel to the interface 27 to the second substructure 26b by the connection channel 40 assigned to it. In the example shown in Figures 4a and 4b, the connection channels 40 extend below a subregion of the surface 28 that is not covered by the optically active subregion 30.

[0068] In the example shown in Figures 4a and 4b, the coolant is distributed between the individual cooling channels 31 via a distribution chamber 32a connected to the cooling channel 40. The connecting channel 40 leads to the distribution chamber 32a, which connects the connecting channel 40 to the coolant inlet 34. In the example shown in Figures 4a and 4b, the distribution chamber 32a extends along another interface 38 between the second and third portions 26b, 26c of the substrate 25. In the illustrated example, the other interface 38 extends in a plane parallel to the bottom surface of the third portion 26c, but such an arrangement is not mandatory. The coolant inlet 34 extends through the third portion 26c and forms an opening that ends on the underside of the substrate 25. Alternatively, the coolant inlet 34 may be formed in the second portion 26c. In the case of the mirror M4 shown in Figures 4a and 4b, the surface area of ​​the funnel-shaped distribution chamber 32a can be further away from the surface 28 of the substrate 25 than in the case of the mirror M4 shown in Figures 3a and 3b. The collection unit 33 has the same configuration as the distribution unit 32.

[0069] In the case of the mirror M4 shown in Figures 4a and 4b, another interface 38 is required to connect the connection channel 40 extending in the Z direction to the coolant inlet 34.

[0070] In the case of the mirror M4 shown in Figures 5a to 5c, the connection channel 40 of the distribution unit 32 is connected to a common inlet channel 41. In the case of the mirror M4 shown in Figures 5a to 5c, the inlet channel 41 is in the form of a lateral hole or blind hole in the second part 26b. The connection channel 40 branches upward (in the Z direction) from the common inlet channel 41 toward the surface 28 of the first part 26a. In the example shown in Figures 5a to 5c, the coolant inlet 41 forms the opening of the inlet channel 41 formed on the side surface of the second part 26b of the substrate 25. The recovery unit 33 is structurally identical to the distribution unit 32 and similarly has a connection channel 40 leading to a common outlet channel 42, but the common outlet channel 42 is hidden by the substrate 25 in Figures 5a to 5c and is connected to the coolant outlet 35.

[0071] In both the case of mirror M4 shown in Figures 4a and 4b and the case of mirror M4 shown in Figures 5a to 5c, the connection channel 40 is in the form of a perforation in the second portion 26b of the substrate 25. When there are many connection channels 40 that extend relatively deeply into the second portion 26b, as in Figures 4a, 4b and 5a to 5c, there is a considerable manufacturing risk that the second portion 26b may be damaged or, in the worst case, destroyed during the perforation process when creating the connection channels 40.

[0072] To mitigate this risk, in the example shown in Figure 5b, each connection channel 40 is connected to two adjacent cooling channels 31 instead of just one. In this way, the connection channels 40 can be manufactured with a larger cross-section than in the example shown in Figure 5a. In some cases, to further reduce manufacturing risk, it is also possible to connect three or more generally adjacent cooling channels 31 to the same connection channel 40.

[0073] If the cross-sectional area of ​​the pressure-bearing connection channel 40 is too large and / or the ribs between the connection channels 40 on the substrate 25 are too small, it is advantageous to make the connection channels 40 in the form of stepped holes, as shown in Figure 5c. In this case, the connection channels 40 are directly adjacent to the interface 27 and have a first cross-sectional area A1 sufficient to connect each connection channel 40 to each of the two cooling channels 31. In one step, the first cross-sectional area A1 is reduced to a smaller second cross-sectional area A2, and as a result the distance between each of the two adjacent connection channels 40 increases. Each connection channel 40 may also have two or more steps in order to reduce the cross-sectional areas A1, A2, etc. from the interface 27 to the inlet channel 41. Reducing the cross-sectional areas A1, A2, etc. of each connection channel 40 from the interface 27 to the distribution chamber 32a is also possible in the case of the mirror M4 shown in Figures 4a and 4b.

[0074] Figures 6a and 6b show cross-sections of the substrate 25 of the mirror M4, in which the distribution section 32 and the recovery section 33 are designed as shown in Figure 5a. In the case of the mirror M4 in Figures 6a and 6b, each connection channel 40 of the distribution section is connected to a common inlet channel 41, from which it branches out toward the surface 28 of the first sub-part 26a. The inlet channel 41 is connected to a coolant inlet, which is not shown in Figures 6a and 6b. The recovery section is structurally identical and has connection channels 40 to the cooling channels 31, and the connection channels 40 lead to a common outlet channel 42 connected to a coolant outlet, which is not shown in Figures 6a and 6b.

[0075] Unlike the mirror M4 shown in Figure 5a, the cooling channel 31 in the example shown in Figures 6a and 6b has a cross-section divided between two parts 26a and 26b, that is, the planar interface 27 between the two parts 26a and 26b is the cross-section or cross-sectional area A of the cooling channel 31. K It extends through the first part 26a. Thus, the cooling channel 31 consists of a first groove-shaped recess 43a formed in the first part 26a and a second groove-shaped recess 43b formed in the second part 26b. Such division of the cross-section of the cooling channel 31 between the two parts 26a and 26b is particularly advantageous when the surface 28 of the substrate 25 is curved, as in the cases of Figures 6a and 6b.

[0076] In this case as well, the distance D from the curved surface 28 to the cooling channel 31 should be substantially constant over the length of the cooling channel 31. This requires curving the cooling channel 31 so that its curvature follows or corresponds to the curvature of the surface 28. Since the interface 27 between the two sub-parts 26a and 26b is planar, the cross-sectional area A is constant over the length of the cooling channel 31. K The cooling channel 31 having the curved first groove-like recess 43a can only be realized in this case, as shown in Figures 6a and 6b, if not only is the curved first groove-like recess 43a formed in the first part 26a but the curved second groove-like recess 43b is also formed in the second part 26b. Needless to say, the cooling channels 31 of the mirror M4 described above in relation to Figures 2, 3a, 3b, 4a, 4b, and 5b can also have a corresponding design, that is, their cross-section can be divided between two parts 26a and 26b.

[0077] Instead of a single distribution unit 32 and / or a single recovery unit 33, it is also possible to optionally form multiple distribution units 32 and / or recovery units 33 on the substrate 25 to connect each of the multiple cooling channels 31 extending beneath the surface 28 having a reflective coating 29 to the coolant inlet 34 and coolant outlet 35, respectively. However, in principle, it is convenient if only a single coolant inlet 34 and a single coolant outlet 35 are formed on the substrate 25.

[0078] Instead of the reflective coating 29 for EUV radiation 16, a reflective coating for radiation in a different wavelength range, such as the EUV wavelength range, may be applied to the optical element described above. Generally, the requirements regarding the thermal expansion of the substrate 25 for such reflective optical elements are not very strict, so a substrate material other than the one described above, such as conventional quartz glass, can be used.

Claims

1. An optical element (M4) that reflects EUV radiation (16), A substrate (25) having a first sub-part (26a) and a second sub-part (26b) joined at the interface (27), A reflective coating (29) applied to the surface (28) of the first portion (26a), A plurality of cooling channels (31) extending into the substrate (25) in the region of the interface (27) beneath the surface (28) to which the reflective coating (29) is applied, A distribution section (32) is formed on the substrate (25) to connect the coolant inlet (34) to the plurality of cooling channels (31), A recovery section (33) is formed on the substrate (25) to connect the plurality of cooling channels (31) to the coolant outlet (35) and In the optical element (M4) provided, The distribution section (32) and / or the recovery section (33) start from the interface (27) and extend more to the second section (26b) of the substrate (25) than to the first section (26a) of the substrate (25), and the cross-section of each cooling channel (31) is divided between the first section (26a) and the second section (26b), The distribution unit (32) and / or the recovery unit (33) has a portion (39) having a connecting channel (40) that starts from the interface (27) and connects each of the cooling channels (31) to the coolant inlet (34) or the coolant outlet (35), An optical element characterized in that the connection channel (40) of the distribution unit (32) is connected to a common inlet channel (41) connected to the coolant inlet (34), and / or the connection channel (40) of the recovery unit (33) is connected to a common outlet channel (42) connected to the coolant outlet (35).

2. The optical element according to claim 1, wherein the distribution portion (32) and / or the recovery portion (33) are optical elements in the second sub-part (26b) that are aligned at an angle (α) of 30° or less with respect to the thickness direction (Z) of the substrate (25) in at least the portion (39) starting from the interface (27).

3. The optical element according to claim 1 or 2, wherein the distribution portion (32) and / or the recovery portion (33) are optical elements in the second portion (26b) that extend below a portion (37) of the surface (28) that is not covered by the reflective coating (29), at least in the portion (39) starting from the interface (27).

4. The optical element according to claim 1 or 2, wherein the distribution unit (32) has a distribution chamber (32a) that widens from the coolant inlet (34), and / or the recovery unit (33) has a recovery chamber (33a) that narrows toward the coolant outlet (35).

5. An optical element according to claim 1 or 2, wherein each connection channel (40) is connected to at least two, and more precisely, two cooling channels (31).

6. An optical element according to claim 1 or 2, wherein the cross-sections (A1, A2) of each connection channel (40) are particularly steppedly reduced from the interface (27).

7. An optical element according to any one of claims 1 to 6, wherein the coolant inlet (34) and / or the coolant outlet (35) is an optical element formed on the second portion (26b) of the substrate (25).

8. An optical element according to any one of claims 1 to 7, wherein the surface (28) to which the reflective coating (29) is applied is curved, and / or the cooling channel (31) is curved.

9. The optical element according to claim 8, wherein the curved cooling channel (31) is located at a certain distance (D) from the curved surface (28).

10. At least one optical element (M1 to M6) according to any one of claims 1 to 9, A cooling device (36) designed to circulate a coolant through multiple cooling channels (31) and An EUV lithography system (1) equipped with [a specific feature].

11. An optical element (M4) that reflects EUV radiation (16), A substrate (25) having a first sub-part (26a) and a second sub-part (26b) joined at the interface (27), A reflective coating (29) applied to the surface (28) of the first portion (26a), A plurality of cooling channels (31) extending into the substrate (25) in the region of the interface (27) beneath the surface (28) to which the reflective coating (29) is applied, A distribution section (32) is formed on the substrate (25) to connect the coolant inlet (34) to the plurality of cooling channels (31), A recovery section (33) is formed on the substrate (25) to connect the plurality of cooling channels (31) to the coolant outlet (35) and In the optical element (M4) provided, The distribution section (32) and / or the recovery section (33) start from the interface (27) and extend more to the second section (26b) of the substrate (25) than to the first section (26a) of the substrate (25), and the cross-section of each cooling channel (31) is divided between the first section (26a) and the second section (26b), The optical element is characterized in that the distribution portion (32) and / or the recovery portion (33) in the second portion (26b) extends below a portion (37) of the surface (28) that is not covered by the reflective coating (29), at least in a portion (39) starting from the interface (27).

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