Optical element and optical assembly for reflecting radiation - Patents.com

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] [Reference to Related Applications] This application claims priority to German Patent Application No. 10 2021 201 715.0, filed on February 24, 2021, the entire disclosure of which is incorporated herein by reference.

[0002] The invention relates to an optical element for reflecting radiation, in particular for reflecting EUV radiation, comprising a substrate having a first and a second part that are joined at an interface, a reflective coating applied to a surface of the first part, a number 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 section formed in the substrate connecting a coolant inlet to the plurality of cooling channels, and a collection section formed in the substrate connecting the plurality of cooling channels to a coolant outlet. The invention also relates to an optical device, in particular an EUV lithography system, comprising at least one such optical element and a cooling device designed to direct a coolant flow through the plurality of cooling channels. [Background technology]

[0003] The thermal loads experienced by reflective optical elements for lithography, and in particular for EUV lithography, are increasing due to the increasingly high power of the radiation sources used in the operation of these elements. This applies in particular to the mirrors of projection systems for EUV lithography. In principle, for the substrates of this type of reflective optical element, also referred to below as mirrors for simplicity, attempts are made to use materials with a thermal expansion coefficient as close as possible to "zero". In practice, this requirement is best met at a certain temperature, also referred to as the zero crossing temperature.

[0004] The mirrors of such projection systems can only be operated near the zero cross temperature as a result of different degrees of heating depending on the setting or illumination conditions. This causes the mirror, or more precisely the surface with the reflective coating, to deform under thermal load during irradiation. With increasing thermal load, this "mirror heating" problem has a limiting effect on the performance of the optical device in which the mirror is located.

[0005] There are mechatronic approaches to solving this problem. Another relatively simple concept consists in cooling each mirror directly, i.e. by passing a cooling fluid through the mirror's substrate, 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, i.e. the mirror has a thermal reference.

[0006] Direct cooling of mirrors of optical devices, in particular EUV lithography devices, is subject to a number of boundary conditions for which an optimal balance must be found. It has proven to be advantageous if a number of substantially parallel cooling channels are formed in the substrate, which run below the surface to which the reflective coating is applied. In order to have sufficient play in the geometrical design, the channel geometries of these cooling channels are formed in two or more partial bodies of the substrate, which are interconnected at one or more interfaces by suitable bonding methods or possibly optical contact bonding. It is advantageous if the substrate has as few interfaces as possible.

[0007] Furthermore, to minimize direct connections at the mirror substrate, a distribution section connecting the substrate coolant inlet to the multiple cooling channels and a collection section connecting the substrate multiple cooling channels to the coolant outlet are required.

[0008] US 2007 / 0133963 A1 discloses an optical element in the form of a mirror having a first layer of a first material and a second layer of a second material that are joined along an interface. The optical element also has a cooling device extending in the region of the interface and configured to cool the optical element. The cooling device may have a number of cooling channels through which a coolant, for example cooling water, can flow. The cooling channels may extend parallel to one another and open laterally into side channels connected to a coolant inlet or a coolant outlet.

[0009] When a cooling fluid, in particular a cooling liquid, flows through the cooling channels, internal pressure can build up within the cooling channels, in particular within the distribution or collection sections, which can lead to undesirable deformations of the surface to which the reflective coating is applied. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] DE 10 2019 217 530 A1 Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the invention to provide an optical element and an optical device which make it possible to reduce deformations of the surface of an optical element provided with a reflective coating by direct cooling with a cooling fluid. [Means for solving the problem]

[0012] According to one aspect, this object is achieved by an optical element of the aforementioned type, in which the distribution portion and / or the recovery portion starts from the interface and extends further into the second part of the substrate than into the first part of the substrate.

[0013] The extension range of the distributor / collection part into the first and second partial bodies relates to the thickness direction of the substrate. The extension range of the distributor / collection part into the first partial body is generally very small. The maximum distance of the distributor / collection part from the interface to the first partial body may in particular be less than or equal to the maximum extension range of each cooling channel into the first partial body. In contrast, the (maximum) extension range of the distributor / collection part into the second partial body is generally (significantly) greater than the maximum extension range of each cooling channel into the second partial body. The extension range of the distributor / collection part into the second partial body may in particular correspond to 5 times or more the extension range of the distributor / collection part into the first partial body. The distributor and / or the collection part may possibly only extend from the interface to the second partial body and not to the first partial body.

[0014] Unlike the case of the side channels described in WO 2005 / 023363, which run along the interface between the two partial bodies, in this aspect of the invention the distributor and / or collector are mainly, possibly even completely, arranged in the second partial body, i.e. starting from the interface they extend further into the second partial body than into the first partial body. In this case the distributor and / or collector are connected to cooling channels which run along the interface. In this way it is possible to reduce the influence of deformations of the substrate on the shape of the surface of the first partial body in the area with the reflective coating, which may be caused by the internal pressure of the cooling fluid in the area of ​​the distributor / collection.

[0015] The cross section of each cooling channel can be divided between the two partial bodies. In this case, as described for example in the patent application WO 2005 / 023363, each groove-shaped recess can be formed in the first partial body and another groove-shaped recess can be formed in the second partial body, and when the two partial bodies are connected along the interface, the two groove-shaped recesses are joined to form a single cooling channel. In this case, each groove-shaped recess is milled both in the first partial body and in the second partial body. However, it is also possible that the groove-shaped recess is milled only in the first partial body or only in the second partial body, and the other partial body covers the recess like a lid to form the cross section of the cooling channel. In both cases, it is sufficient in principle to connect each cooling channel to a coolant inlet or coolant outlet if the distributor / collection part extends into the second partial body of the substrate up to the region of the interface, as a result of the interface extending into the cross section of each cooling channel or on the edge of the cross section. However, the distributor / collection part can also extend into the first partial body, for example to connect the cooling channels to each other in the cross section extending into the first partial body.

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

[0017] In yet another embodiment, the distributor and / or collector are aligned in the second section, possibly also in the first section, at an angle of 30° or less with respect to the thickness direction of the substrate, at least starting from the interface. In order to reduce the effect of the internal pressure of the distributor / collection on the surface with the reflective coating, it is advantageous to incline the distributor / collection with respect to the surface and / or interface of the first section, at least starting from the interface. In this way, the surface area of ​​the distributor / collection that may bulge due to the internal pressure of the cooling fluid is also inclined with respect to the surface, so that the effect of the bulge on the geometry of the surface is reduced. The distributor / collection can extend in particular 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 section, in the part connected to the interface, but this is not essential.

[0018] In yet another embodiment, the distributor and / or collector extends in the second part, possibly also in the first part, at least starting from the interface, below the partial area of ​​the surface that is not covered with the reflective coating. In order to avoid significant deformation of the area of ​​the surface applied with the reflective coating or of the optically utilized partial area of ​​the reflective coating, it is advantageous to position the distributor / collection as far away as possible from the optically utilized surface area. In this embodiment, it is usually necessary for the cooling channels to extend also to the partial area of ​​the surface that is not covered with the reflective coating. In order to reduce the effects of fluid pressure, it may also be possible for the distributor / collection to extend to the partial area of ​​the surface that is not covered with the reflective coating, but not to the optically utilized partial area of ​​the reflective coating. During irradiation of the optical element in an optical device, for example in an EUV lithography device, the optically utilized partial area is irradiated with the used radiation.

[0019] In yet another aspect of the present invention, particularly which can be combined with the above-mentioned aspects, the distribution section has a distribution chamber widening from the coolant inlet towards the interface, and / or the collection section has a collection chamber narrowing from the interface towards the coolant outlet.

[0020] In this aspect of the invention, the distributor / collection section, more precisely the distributor / collection chamber, can also extend along the interface between the first and second partial body without the distributor / collection section extending further into the second partial body than into the first partial body. In this case, it is advantageous for the distributor / collection chamber to have a form that is as flat as possible along the interface. A substantially triangular or funnel-shaped geometry of the distributor / collection section that is optimized for flow is achieved by widening or narrowing the flow cross section of each chamber. However, this geometry also results in a relatively large surface area of ​​the distributor / collection section. Due to this and the fact that the interface and thus the distributor / collection section extends a generally small distance from the surface to which the reflective coating is applied, the surface may also bulge due to the internal pressure of the distributor / collection chamber.

[0021] It has therefore proven advantageous if the distribution / collection chambers extend along the interface between the two halves, if these extend only under the partial area of ​​the surface which is not covered by the reflective coating (see above).

[0022] In a refinement of this embodiment, the distribution chamber extends from the coolant inlet to the interface and / or the collection chamber extends from the interface to the coolant outlet. In particular, in this case, it is advantageous if the distribution chamber / collection chamber are aligned substantially perpendicular to the thickness direction of the substrate starting from the interface as described above.

[0023] However, in this case the distribution / collection chamber still has a large surface area on which the internal pressure of the cooling fluid can act, which can then lead to deformation of the surface. It is therefore advantageous if the distribution / collection chamber does not extend to or is not directly connected to the interface, since it has a maximum lateral extent at the interface.

[0024] In an alternative embodiment, the distribution section and / or the collection section have a portion starting from the interface with a connecting channel connecting each at least one cooling channel to a coolant inlet or a coolant outlet, in this embodiment the cooling channel continues to a connecting channel connected to one or more of the cooling channels in the region of the interface.

[0025] The connecting channels may be aligned at an angle of not more than 30° to the thickness direction of the substrate starting from the interface. In this way, the cooling channels near the edge of the optically utilized part of the surface and / or near the part of the surface covered with a reflective coating are redirected substantially vertically. In this way, the cooling channels and / or the connecting channels may be distributed or merged in a further part of the distribution / collection part that is spaced apart from the surface of the substrate and / or from the interface in the thickness direction.

[0026] In principle, it is possible to assign a cooling channel to exactly one connecting channel, which in this case is the portion of the cooling channel that continues into the second part of the substrate. The connecting channel is usually drilled into the second part of the substrate, i.e. the connecting channel is a drilling.

[0027] Typically, a plurality of cooling channels, for example 10 or more, each having a relatively small cross-sectional area, are formed in the substrate. Therefore, in addition, when drilling connecting channels, which typically have a relatively large length or depth, there is a production-related risk that the second part of the substrate is damaged during the drilling operation.

[0028] In a refinement of the above embodiment, each connecting channel is connected to at least two, in particular exactly two, cooling channels. In this way, the cross-sectional area of ​​the connecting channel directly adjacent to a cooling channel is enlarged by at least twice its cross-sectional area, as a result of which the manufacturing risks during drilling of the connecting channels can be reduced.

[0029] In yet another refinement, the cross section of each connecting channel is tapered, in particular stepwise, from the interface. If the distance between adjacent connecting channels is relatively small and the surface area of ​​the connecting channels on which the fluid pressure is applied is relatively large due to the relatively large cross section of the connecting channels, it may be advantageous to vary the pore size of the connecting channels, in particular tapering from the interface. The cross section of each connecting channel can be tapered, in particular stepwise, i.e. the connecting channels have one or possibly several 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 part of the distribution part with a connecting channel of the distribution part and / or the collection chamber is connected to a part of the collection part with a connecting channel of the collection part. In this case, the connecting channel separates the distribution chamber / collection chamber in the thickness direction of the substrate from the surface with the reflective coating and / or from the interface between the two parts. The large distance from the surface ensures that the deformation of the substrate caused by the expansion of each chamber due to the pressure of the cooling fluid has a smaller effect on the geometry of the surface than in the above case where the distribution chamber / collection chamber is directly connected to the interface.

[0031] In a refinement of this embodiment, the distribution chamber and / or the collection chamber extend along another interface between the second part and a third part of the substrate, the third part being joined to the second part at said another interface. Said another interface can in particular extend substantially parallel to the interface where the first part is joined to the second part. In this way, the distribution chamber and / or the collection chamber are offset in the thickness direction of the substrate from the interface to the another interface. Said another interface is generally necessary if the distribution chamber and / or the collection chamber are to be offset in the thickness direction from the interface, since the chambers cannot be easily realised only in the second part due to the funnel-like geometry.

[0032] In yet another embodiment, the connecting channels of the distribution section lead to a common inlet channel connected to the coolant inlets and / or the connecting channels of the collection section lead to a common outlet channel connected to the coolant outlets. The inlet and / or outlet channels are usually in the form of perforations in the second part. The inlet and / or outlet channels may in particular run substantially parallel to the bottom surface of the second part and / or the substrate, but this is not essential. The inlet and / or outlet channels may form transverse holes in the second part leading to the connecting channels. The coolant inlets and / or outlets 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 the coolant outlet are formed in the second and / or third part of the substrate. The coolant inlet and / or the coolant outlet may for example form openings in the side of the second and / or third part, but it is also possible for the coolant inlet and / or the coolant outlet to be formed on the underside of the substrate, i.e. on the surface of the substrate opposite the interface and / or further interface. In the region of the coolant inlet and / or the coolant outlet, the substrate is usually shaped in such a way that coolant lines can be easily connected to the coolant inlet and / or the coolant outlet.

[0034] In yet another embodiment, the cross section of each cooling channel of the plurality of cooling channels is split between the first and second sub-bodies. As mentioned above, it is possible for the cross-sectional area of ​​the cooling channel or one or more of the cooling channels to be split between two sub-bodies. In this case, the cooling channel does not extend within or parallel to the substantially planar interface.

[0035] In yet another embodiment, the surface of the substrate applied with the reflective coating is curved and / or the cooling channels themselves are curved (in the thickness direction of the substrate), the curved cooling channels preferably having a constant distance from the curved surface. For substrates of this type, the above-mentioned division of the cross section of the cooling channels between the two sectional bodies is advantageous in particular when the interface itself does not follow the curvature of the surface but has, for example, a planar form. In this case, the division of the cross section between the two sectional bodies ensures that, despite the planar interface, the curved cooling channels follow the curved surface as a result of which the cooling channels extend at a constant distance from the curved surface. In this case, groove-like recesses with a curvature that follows the curvature of the surface are generally introduced not only in the first sectional body but also in the second sectional body. In this case, the cooling channels are formed by merging corresponding curved groove-like recesses of the first sectional body and groove-like recesses of the second sectional body along the interface. This makes it possible to ensure that the curved cooling channels have a constant channel cross section over their length.

[0036] Yet another aspect of the invention relates to an optical device, for example 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 number of cooling channels. The EUV lithography system may be an EUV lithography device for exposure of wafers or any other optical device using EUV radiation, for example an EUV inspection system for inspection of masks, wafers, etc. used in EUV lithography. The reflective optical element may in particular be a mirror of a projection system of an EUV lithography device. By way of example, the cooling device may be designed to flow a coolant in the form of a cooling fluid, for example a cooling liquid, for example in the form of cooling water, through the cooling channels. For this purpose, the cooling device may optionally comprise a pump and suitable supply and discharge lines. The optical device may also be a lithography system for another wavelength range, for example a DUV wavelength range, for example a DUV lithography system, or an inspection system for inspection of masks, wafers, etc.

[0037] Further features and advantages of the invention emerge from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing which show the essential details of the invention, and from the claims. Each of the individual features can be implemented alone or in any combination of several in a variant of the invention.

[0038] Exemplary embodiments are shown in the schematic drawings and explained in the following description. [Brief description of the drawings]

[0039] [Figure 1] 1 shows a schematic meridian section of a projection exposure apparatus for EUV projection lithography. [Diagram 2] FIG. 1 shows a schematic diagram of a mirror having multiple cooling channels extending along the interface between two halves of a substrate, as well as distribution and collection chambers. [Diagram 3] 3a, 3b show schematic diagrams of a mirror in which the distribution and collection chambers are formed only in the second part and extend through the thickness of the substrate. [Figure 4] 4a-b show schematic diagrams of a mirror having a distribution chamber and a collection chamber extending along another interface between a second and a third part of the substrate. [Diagram 5] 5a, 5b and 5c show schematic diagrams of a mirror with connecting channels running through the thickness to connect the cooling channels to the inlet channels of the distributor. [Figure 6] Figures 6a, 6b show schematic diagrams of a mirror similar to Figures 5a-5c with a curved surface, in which the cooling channels have a cross section extending into both the first and second halves. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] In the following description of the drawings, the same reference numbers are used for components that are the same or have the same function.

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

[0042] An embodiment of the illumination system 2 of the projection exposure apparatus 1 comprises, in addition to the light source or radiation source 3, an illumination optical unit 4 for illumination of an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the radiation source 3.

[0043] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable in particular in the scanning direction by a reticle displacement drive 9.

[0044] An embodiment of the illumination system 2 of the projection exposure apparatus 1 comprises, in addition to the light source or radiation source 3, an illumination optical unit 4 for illumination of an object field 5 in an object plane 6. In an alternative embodiment, the radiation source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not comprise the radiation source 3.

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

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

[0047] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, also referred to in the following as working radiation, illumination radiation or illumination light. In particular, the working radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 can be a plasma source, for example an LPP (Laser Produced Plasma) source or a GDPP (Gas Discharge Plasma) source. It can also be a synchrotron-based radiation source. Likewise, the radiation source 3 can be a Free Electron Laser (FEL).

[0048] The illumination radiation 16 leaving the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector mirror with one or more ellipsoidal and / or hyperbolic reflecting surfaces. The illumination radiation 16 may be incident on at least one reflecting surface of the collector mirror 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. The collector mirror 17 may be structured and / or coated, firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0049] The illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18 downstream of the collector mirror 17. The intermediate focal plane 18 may separate the source module comprising 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 arranged downstream thereof in the beam path. The deflection mirror 19 may be a plane deflection mirror or a mirror with a beam-influencing effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 may be in the form of a spectral filter which separates the used optical wavelength of the illumination radiation 16 from stray light of wavelengths deviating therefrom. The first facet mirror 20 comprises a number of individual first facets 21, also referred to as field facets in the following. FIG. 1 shows only some of said facets 21 by way of example. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optical unit 4. The second facet mirror 22 comprises a number of second facets 23.

[0051] The illumination optical unit 4 consequently forms a double-facet system. This basic principle is also called a fly's eye condenser (fly's eye integrator). The individual first facets 21 are imaged into the object field 5 by means of a second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

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

[0053] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1-M6. Four, eight, ten, twelve or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have an aperture for the passage of the illumination radiation 16. The projection system 10 is a double-shielded optical unit. The projection system 10 has an image-side numerical aperture which may be greater than 0.4 or 0.5, may be greater than 0.6, and may for example be 0.7 or 0.75.

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

[0055] 2 shows by way of example a mirror M4 of the projection system 10, which comprises a substrate 25 formed from a first partial body 26a and a second partial body 26b. The first partial body 26a, which in the illustrated example is plate-shaped, and the second partial body 26b, which forms the body of the substrate 25, are joined or connected to each other at a common interface 27, which in the illustrated example is planar, although this is not essential. The connection between the two partial bodies 26a, 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 partial body 26a and the second partial body 26b may be the same, but may also comprise different materials. In the illustrated example, both the material of the first partial body 26a and the material of the second partial body 26b are ultra-low expansion glass (ULE®). The substrate 25 or the two partial bodies 26a, 26b may also be made of another material with a thermal expansion coefficient as low as possible, for example a glass ceramic, for example Zerodur®.

[0056] A reflective coating 29 is applied to an exposed surface 28 of the first partial body 26a opposite the interface 27. Subregions 30 of the surface 28 located within the reflective coating 29 are struck by the EUV radiation 16 of the projection system 10, which subregions 30 form the optically useful subregions of the reflective coating 29. The reflective coating 29 can, for example, include a number of layer pairs of materials with different real parts of the refractive index, the layers being formed, for example, from Si and Mo for a wavelength of the EUV radiation 16 of 13.5 nm. The surface 28 of the first partial body 26a is represented as a plane in FIG. 2 but can also have a curvature.

[0057] In the example shown in FIG. 2, a number of cooling channels 31 extending below the surface 28 on which the reflective coating 29 is applied are formed in the substrate 25 in the region of the interface 27. In the example shown in FIG. 2, there are about 20 cooling channels 31, which extend below the surface 28 between a distribution section 32 and a recovery section 33 on either side of the optically utilized partial area 30 of the reflective coating 29. In the example shown in FIG. 2, the cooling channels 31 are aligned parallel to one another. In the example shown in FIG. 2, the distribution section 32 has a distribution chamber 32a that connects the cooling channels 31 to a common coolant inlet 34, which forms an opening in the second partial body 29b. Correspondingly, the recovery section 33 forms a recovery chamber that connects the cooling channels 31 to a common coolant outlet 35, which is also in the form of an opening in the second partial body 29b.

[0058] As can be seen in Fig. 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 collection 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 collection 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 Fig. 2, the distribution chamber 32a and the collection chamber 33a extend both in the first part 26a and in the second part 26b. The distribution chamber 32a and the collection chamber 33a have a substantially triangular geometry optimized with respect to the flow in order to ensure as even a distribution as possible between all the coolant channels 31 and to minimize the dynamic excitation due to the flow of the cooling water.

[0059] For supplying coolant to the coolant inlet 34 and for discharging the coolant through the coolant outlet 35, the projection exposure apparatus 1 comprises a cooling device 36, which is represented diagrammatically in Fig. 1. In the example shown, the cooling device 36 serves to supply coolant in the form of cooling water to the cooling channels 31 or to the mirror M4 and for this purpose comprises a supply line, not shown, which is liquid-tightly connected to the coolant inlet 34. The cooling device 36 also comprises a discharge line, not shown, for discharging the cooling water through the coolant outlet 35. For cooling purposes, the other mirrors M1-M3, M5, M6 of the projection system 10 can also be connected to the cooling device 36 or to further cooling devices, possibly provided for this purpose.

[0060] The pressure of the cooling water flowing through the distribution chamber 32a or the collection chamber 33a can cause the substrate 25 to expand, which can result in changes in the geometry of the surface 28. Due to the relative proximity of the distribution chamber 32a and / or the collection chamber 33a to the optically utilized sub-area 30 of the surface 28, undesirable deformations of the optically utilized sub-area 30 can thus occur.

[0061] In order to reduce the effect of the bulging of the distribution chamber 32a and / or the recovery chamber 33a on the optically utilized partial area 30 of the reflective coating 29, in the case of the mirror M4 shown in Fig. 3a, 3b the distribution part 32 or distribution chamber 32a and the recovery part 33 or recovery chamber 33a extend only from the interface 27 to the second partial body 26a of the substrate. In principle it is possible for the distribution chamber 32a and / or the recovery chamber 32b to extend from the interface 27 to the first partial body 26a in order to further connect the ends of the cooling channels 31 in the first partial body 26a as well. As can be seen in Fig. 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, not shown in Fig. 3a, also extends from the interface 27 to the coolant outlet 35, also 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 in this respect parallel to the thickness direction Z of the substrate 25. As can be seen in the partial cross section of FIG. 3a, the central plane M extends in the Z direction and in the X direction. The distribution chamber 32a is substantially mirror-symmetrical with respect to the central plane M. The central plane M also extends through the coolant inlet 34, which forms an opening on the underside of the second partial body 26b. In this case, the underside of the second partial body 26b extends perpendicularly 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 can swell due to a fluid pressure parallel to the surface 28 of the mirror M4 or the optically used partial area 30 of the surface 28. The inclination of the distribution part 32 and / or the recovery part 33 towards the second partial body 26b thus makes it possible to reduce deformations of the optically used partial area 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, but the distribution chamber 32a, more precisely its central plane M, can also be aligned at an angle α, which should generally be less than or equal to about 30°, relative to the thickness direction Z. The collection section 33 or collection chamber 33a, visible in the partial cross section of FIG. 3a, has in the illustrated example an identical structure to the distribution section 32 or distribution chamber 32a on the side of the optically active part area 30 of the surface 28 of the substrate 25 that is located opposite in the Y direction. However, a structurally identical design is not essential. For example, for flow reasons, it can be advantageous if the distribution section 32 and / or distribution chamber 32a and the collection section 33 and / or collection chamber 33a have different geometries.

[0064] As can be seen in particular in Fig. 3b, both the distribution chamber 32a and the recovery chamber 33a extend in the Z direction under a partial region 37 of the surface 28 that is not covered with the reflective coating 29, in particular also under the optically useful partial region 30 of the surface 28. This results in a large distance from the optically useful partial region 30 of the surface 28 of the triangular surface area that is under pressure and that can form and bulge in the distribution chamber 32a, which is visible in Fig. 3a. Such an arrangement is also basically possible in the case of the mirror M4 shown in Fig. 2, since the lateral construction space in the case of the mirror M4 shown in Fig. 2, in which the distribution chamber 32a and the recovery chamber 33a extend along the interface 27 between the two partial bodies 26a, 26b, is sufficient for this.

[0065] In the case of the mirror M4 shown in Fig. 4a, 4b, the substrate 25 has a third partial body 26c in addition to the first and second partial bodies 26a, 26b. The third partial body 26c is connected or joined to the second partial body 26b at another interface 38 and is also made of ULE®. The connection can be made like the above-mentioned connection at the interface 27 between the first and second partial bodies 26a, 26b. The collection part 32 shown in Fig. 4a, 4b has a part 39 connected to the interface 27 between the first and second partial bodies 26a, 26b and extending from the interface 27 to the second partial body 26b of the substrate 25. A connecting channel 40 extending in the thickness direction Z of the substrate 25 is made in the part 39 of the distribution part 32 connected to the interface 27.

[0066] As in the case of the example shown in Figures 3a, 3b, also in Figures 4a, 4b it is not essential to align the connection channels 40 in the thickness direction of the substrate 25, but it is possible to align the connection channels 40 at an angle α, which is typically less than or equal to 30°, to the thickness direction Z, as in Figures 3a, 3b. In principle it may be advantageous for the angle α at which the connection channels 40 are aligned to the thickness direction Z of the substrate 25 to vary across the substrate 25.

[0067] In the example shown in Figures 4a, 4b, each connecting channel 40 is connected to exactly one cooling channel 31 and continues downwards into the second partial body 26b. In other words, each cooling channel 31 is redirected from an arrangement parallel to the interface 27 into the second partial body 26b by means of the connecting channel 40 assigned to it. In the example shown in Figures 4a, 4b, the connecting channel 40 extends below a partial area of ​​the surface 28 that is not covered by the optically active partial area 30.

[0068] In the example shown in Fig. 4a, 4b, the coolant is distributed between the individual cooling channels 31 via a distribution chamber 32a connected to the cooling channels 40. The connection channel 40 leads to the distribution chamber 32a, which connects the connection channel 40 to the coolant inlet 34. In the example shown in Fig. 4a, 4b, the distribution chamber 32a extends along another interface 38 between the second and third halves 26b, 26c of the substrate 25. In the example shown, the another interface 38 extends in a plane parallel to the bottom surface of the third halves 26c, but such an arrangement is not required. The coolant inlet 34 forms an opening that extends through the third halves 26c and ends on the underside of the substrate 25. Alternatively, the coolant inlet 34 can be formed in the second halves 26c. In the case of the mirror M4 shown in Fig. 4a, 4b, the surface area of ​​the funnel-shaped distribution chamber 32a can be greater distanced from the surface 28 of the substrate 25 than in the case of the mirror M4 shown in Fig. 3a, 3b. The collection section 33 has a similar form to the distribution section 32 .

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

[0070] In the case of the mirror M4 shown in Fig. 5a-5c, the connecting channels 40 of the distribution parts 32 are connected to a common inlet channel 41. In the case of the mirror M4 shown in Fig. 5a-5c, the inlet channel 41 is in the form of a transverse or blind hole in the second partial body 26b. The connecting channels 40 branch off from the common inlet channel 41 upwards (Z direction) towards the surface 28 of the first partial body 26a. In the case of the example shown in Fig. 5a-5c, the coolant inlet 41 forms the opening of the inlet channel 41 formed in the side of the second partial body 26b of the substrate 25. The collection part 33 is structurally identical to the distribution part 32 and likewise has a connecting channel 40 leading to a common outlet channel 42, which is hidden by the substrate 25 in Fig. 5a-5c and is connected to the coolant outlet 35.

[0071] In both cases of the mirror M4 shown in Figures 4a, 4b and in Figures 5a-5c, the connecting channels 40 are in the form of drillings in the second partial body 26b of the substrate 25. If there are many connecting channels 40 extending relatively deep into the second partial body 26b, as in Figures 4a, 4b and 5a-5c, there is a not inconsiderable production risk during the drilling operation that the second partial body 26b may be damaged or, in the worst case, destroyed when making the connecting channels 40.

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

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

[0074] 6a, 6b show a cross section of the substrate 25 of a mirror M4, whose distribution section 32 and collection section 33 are designed as in FIG. 5a. In the case of the mirror M4 in FIGS. 6a, 6b, each of the connection channels 40 of the distribution sections is connected to a common inlet channel 41, which branches off from it towards the surface 28 of the first partial body 26a. The inlet channels 41 are connected to a coolant inlet, which is not shown in FIGS. 6a, 6b. The collection section is structurally identical and has a connection channel 40 for the cooling channels 31, which leads to a common outlet channel 42, which is connected to a coolant outlet, which is not shown in FIGS. 6a, 6b.

[0075] Unlike the mirror M4 shown in FIG. 5a, the cooling channel 31 in the example shown in FIG. 6a, 6b has a cross section that is divided between the two partial bodies 26a, 26b, i.e. the planar interface 27 between the two partial bodies 26a, 26b is smaller than the cross section or cross-sectional area A of the cooling channel 31. K 6a, 6b. The cooling channel 31 therefore consists of a first groove-like recess 43a formed in the first partial body 26a and a second groove-like recess 43b formed in the second partial body 26b. Such a division of the cross section of the cooling channel 31 between the two partial bodies 26a, 26b is particularly advantageous when the surface 28 of the substrate 25 is curved, as is the case in Figs. 6a, 6b.

[0076] Again, the distance D of the cooling channel 31 from the curved surface 28 should be substantially constant over the length of the cooling channel 31. This requires that the curvature of the cooling channel 31 be curved to follow or correspond to the curvature of the surface 28. Since the interface 27 between the two halves 26a, 26b is planar, the cross-sectional area A of the cooling channel 31 is constant over the length of the cooling channel 31. K can in this case only be realised if not only a first curved groove-like recess 43a is formed in the first partial body 26a but also a second curved groove-like recess 43b is formed in the second partial body 26b, as shown in Figures 6a, 6b. Needless to say, the cooling channels 31 of the mirror M4 described above in connection with Figures 2, 3a, 3b, 4a, 4b and 5b can also have a corresponding design, i.e. their cross section can be divided between the two partial bodies 26a, 26b.

[0077] Instead of a single distribution section 32 and / or a single collection section 33, it is optionally possible to form a plurality of distribution sections 32 and / or collection sections 33 in the substrate 25 in order to connect each of a plurality of cooling channels 31 extending below the surface 28 with the reflective coating 29 to a respective coolant inlet 34 and coolant outlet 35. However, in principle it is advantageous if only a single coolant inlet 34 and a single coolant outlet 35 are formed in the substrate 25.

[0078] Instead of the reflective coating 29 for EUV radiation 16, a reflective coating for radiation of a different wavelength range, for example for 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 less stringent, so that substrate materials other than those described above, for example conventional quartz glass, can be used.

Claims

1. an optical element (M4) for reflecting radiation, in particular EUV radiation (16), a substrate (25) having a first section (26a) and a second section (26b) joined at an interface (27); a reflective coating (29) applied to a surface (28) of the first section body (26a); a plurality of cooling channels (31) extending into the substrate (25) in the region of the interface (27) below the surface (28) having the reflective coating (29); a distribution portion (32) formed in said substrate (25) for connecting a coolant inlet (34) to said plurality of cooling channels (31); a collection section (33) formed in said substrate (25) for connecting said plurality of cooling channels (31) to a coolant outlet (35); In the optical element (M4), an optical element, characterized in that the distribution section (32) and / or the recovery section (33) extend, starting from the interface (27), farther into the second partial body (26b) of the substrate (25) than into the first partial body (26a) of the substrate (25), and a cross section of each cooling channel (31) is divided between the first partial body (26a) and the second partial body (26b).

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

3. 3. An optical element according to claim 1 or 2, wherein the distribution section (32) and / or the recovery section (33) in the second partial body (26b) extends below a partial region (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).

4. An optical element according to the preamble of claim 1, in particular according to any one of claims 1 to 3, wherein the distribution section (32) has a distribution chamber (32a) extending from the coolant inlet (34) and / or the recovery section (33) has a recovery chamber (33a) narrowing towards the coolant outlet (35).

5. 5. The optical element of claim 4, wherein the distribution chamber (32a) extends from the coolant inlet (33) to the interface (27) and / or the recovery chamber (33a) extends from the interface (27) to the coolant outlet (35).

6. An optical element according to any one of claims 1 to 4, wherein the distribution section (32) and / or the recovery section (33) have a portion (39) having a connecting channel (40) starting from the interface (27) and connecting at least one cooling channel (31) respectively to the coolant inlet (34) or the coolant outlet (35).

7. 7. An optical element according to claim 6, wherein each connecting channel (40) is connected to at least two, in particular exactly two, cooling channels (31).

8. 8. An optical element according to claim 6 or 7, wherein the cross section (A1, A2) of each connecting channel (40) narrows in particular stepwise from said interface (27).

9. An optical element according to any one of claims 6 to 8, wherein the distribution chamber (32a) is connected to a part of the distribution section (32) having the connecting channel (40) of the distribution section (32) and / or the recovery chamber (33a) is connected to a part of the recovery section (33) having the connecting channel (40) of the recovery section (33).

10. 10. The optical element of claim 9, wherein the distribution chamber (32a) and / or the recovery chamber (33a) extend along another interface (38) between the second partial body (26b) and a third partial body (26c) of the substrate (25), the third partial body (26c) being merged with the second partial body (26b) at the another interface (38).

11. An optical element according to any one of claims 6 to 8, wherein the connecting channels (40) of the distribution section (32) lead to a common inlet channel (41) connected to the coolant inlet (34) and / or the connecting channels (40) of the recovery section (33) lead to a common outlet channel (42) connected to the coolant outlet (35).

12. An optical element according to any one of claims 1 to 11, wherein the coolant inlet (34) and / or the coolant outlet (35) are formed in the second partial body (26b) and / or the third partial body (26c) of the substrate (25).

13. An optical element according to any one of claims 1 to 12, wherein the surface (28) on which the reflective coating (29) is applied is curved and / or the cooling channels (31) are curved, and the curved cooling channels (31) are preferably at a certain distance (D) from the curved surface (28).

14. An optical apparatus, in particular an EUV lithography system (1), comprising: At least one optical element (M1 to M6) according to any one of claims 1 to 13, A cooling device (36) designed to direct a coolant through the plurality of cooling channels (31); An optical device comprising:

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