Abstract
An optical element for reflecting radiation comprises: a substrate having first and second partial bodies put together at an interface; a reflective coating applied to a surface of the first partial body; a plurality of cooling channels running in the substrate in the region of the interface below the surface to which the reflective coating; a distributor in the substrate for connecting a coolant inlet to the plurality of cooling channels; and a collector in the substrate for connecting the plurality of cooling channels to a coolant outlet. The distributor and/or the collector extend, proceeding from the interface, further into the second partial body of the substrate than into the first partial body of the substrate. An optical arrangement, for example in an EUV lithography system, comprises: at least one such optical element; and a cooling device designed for flowing a coolant through the plurality of cooling channels.
Claims
1. An optical element configured to reflect radiation, the optical element comprising: a substrate comprising first and second partial bodies joined at an interface; a reflective coating supported by a surface of the first partial bodies; a plurality of cooling channels in the substrate in a region of the interface below the surface supporting the reflective coating; a distributor in the substrate and configured to connect a coolant inlet to the plurality of cooling channels; and a collector in the substrate and configured to connect the plurality of cooling channels to a coolant outlet, wherein: at least one member selected from the group consisting of the distributor and the collector extends, proceeding from the interface, further into the second partial body of the substrate than into the first partial body of the substrate; and a cross section of a respective cooling channel is divided between the first partial body and the second partial body.
2. The optical element of claim 1, wherein the at least one member, in the second partial body at least in a portion proceeding from the interface, is aligned at an angle of at most 30° relative to a thickness direction of the substrate.
3. The optical element of claim 1, wherein the at least one member, in the second partial body at least in a portion proceeding from the interface, is below a partial region of the surface that is not covered by the reflective coating.
4. The optical element of claim 1, wherein the distributor comprises a distributor chamber which widens from the coolant inlet, and/or wherein the collector comprises a collecting chamber which tapers toward the coolant outlet.
5. The optical element of claim 4, wherein the distributor chamber extends from the coolant inlet to the interface, and/or wherein the collecting chamber extends from the interface to the coolant outlet.
6. The optical element of claim 1, wherein the at least one member comprises a portion which proceeds from the interface and comprises connecting channels configured to connect at least one cooling channel to the coolant inlet or to the coolant outlet.
7. The optical element of claim 6, wherein a connecting channel is connected to at least two cooling channels.
8. The optical element of claim 6, wherein a cross section of a connecting channel decreases from the interface.
9. The optical element of claim 6, wherein the distributor chamber is connected to the portion of the distributor having the connecting channels of the distributor, and/or wherein the collecting chamber is connected to the portion of the collector that has the connecting channels of the collector.
10. The optical element of claim 9, wherein at least one chamber selected from the group consisting of the distributor chamber and the collecting chamber extends along a further interface between the second partial body and a third partial body of the substrate that is joined with the second partial body at the further interface.
11. The optical element of claim 6, wherein the connecting channels of the distributor open into a common inlet channel which is connected to the coolant inlet, and/or wherein the connecting channels of the collector open into a common outlet channel which is connected to the coolant outlet.
12. The optical element of claim 1, wherein at least one inlet selected from the group consisting of the coolant inlet and the coolant outlet is in at least one partial body selected from the group consisting of the second partial body and a third partial body of the substrate.
13. The optical element of claim 1, wherein the surface supporting the reflective coating is curved, and/or wherein the cooling channel is curved.
14. The optical element of claim 13, wherein the cooling channel is a constant spacing from the surface supporting the reflective coating.
15. The optical element of claim 13, wherein the at least one member comprises a portion which proceeds from the interface and comprises connecting channels configured to connect at least one cooling channel to the coolant inlet or to the coolant outlet.
16. The optical element of claim 1, wherein the radiation comprises EUV radiation.
17. An optical arrangement, comprising: an optical element according to claim 1; and a cooling device configured to provide a coolant to the plurality of cooling channels.
18. The optical arrangement of claim 17, wherein the radiation comprises EUV radiation.
19. An apparatus, comprising: an optical arrangement, comprising: an optical element according to claim 1; and a cooling device configured to provide a coolant to the plurality of cooling channels, wherein the apparatus is a lithography projection exposure apparatus.
20. The apparatus of claim 19, wherein the radiation comprises EUV radiation.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] Exemplary embodiments are depicted in the schematic drawings and are explained in the following description. In the figures:
[0038] FIG. 1 shows a schematic meridional section through a projection exposure apparatus for EUV projection lithography;
[0039] FIG. 2 shows a schematic illustration of a mirror having a plurality of cooling channels and a distributor chamber and a collecting chamber which run along an interface between two partial bodies of a substrate;
[0040] FIGS. 3A-3B show schematic illustrations of a mirror, in the case of which the distributor chamber and the collecting chamber are formed only in the second partial body and extend in the thickness direction of the substrate;
[0041] FIGS. 4A-4B show schematic illustrations of a mirror having a distributor chamber and a collecting chamber which run along a further interface between the second partial body and a third partial body of the substrate;
[0042] FIGS. 5A-5C show schematic illustrations of a mirror having connecting channels running in the thickness direction, in order to connect the cooling channels to an inlet channel of the distributor; and
[0043] FIGS. 6A-6B show schematic illustrations of a mirror, similar to FIGS. 5A-5C, which has a curved surface and in the case of which the cooling channels have a cross section which runs both in the first partial body and in the second partial body.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0044] In the following description of the drawings, identical reference signs are used for components that are the same or have the same function.
[0045] Certain components of an optical arrangement for EUV lithography in the form of a microlithographic projection exposure apparatus 1 are described by way of example below with reference to FIG. 1. The description of the basic construction of the projection exposure apparatus 1 and its components should not be understood as having a limiting effect in this case.
[0046] An embodiment of an illumination system 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.
[0047] 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 for example in a scanning direction by way of a reticle displacement drive 9.
[0048] An embodiment of an illumination system 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.
[0049] For explanation purposes, a Cartesian xyz coordinate system is depicted in FIG. 1. The x direction runs perpendicularly to the plane of the drawing. The y direction runs horizontally, and the z direction runs vertically. The scanning direction runs in the y direction in FIG. 1. The z direction runs perpendicularly to the object plane 6.
[0050] The projection exposure apparatus 1 comprises a projection system 10. The projection system 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable for example along the y direction by way of a wafer displacement drive 15. The displacement on the one hand of the reticle 7 by way of the reticle displacement drive 9 and on the other hand of the wafer 13 by way of the wafer displacement drive 15 may be synchronized with one another.
[0051] The radiation source 3 is an EUV radiation source. The radiation source 3 emits, for example, EUV radiation 16, which is also referred to below as used radiation, illumination radiation or illumination light. For example, the used radiation has a wavelength in the range between 5 nm and 30 nm. The radiation source 3 may be a plasma source, for example an LPP source (Laser Produced Plasma) or a GDPP source (Gas Discharge Produced Plasma). It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).
[0052] The illumination radiation 16 emanating from 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 hyperboloidal reflection surfaces. The illumination radiation 16 may be incident on at least one reflection surface of the collector mirror 17 with grazing incidence (GI), i.e. at angles of incidence of greater than 45°, or with normal incidence (NI), i.e. at angles of incidence of less than 45°. The collector mirror 17 may be structured and/or coated, firstly to optimize its reflectivity for the used radiation and secondly to suppress extraneous light.
[0053] 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 constitute a separation between a radiation source module, having the radiation source 3 and the collector mirror 17, and the illumination optical unit 4.
[0054] The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20. The deflection mirror 19 may be a plane deflection mirror or, alternatively, a mirror with a beam-influencing effect going beyond a pure deflection effect. As an alternative or in addition, the deflection mirror 19 may be in the form of a spectral filter that separates a used light wavelength of the illumination radiation 16 from stray light of a wavelength deviating therefrom. The first facet mirror 20 comprises a multiplicity of individual first facets 21, which are also referred to as field facets below. FIG. 1 depicts only some of the facets 21 by way of example. In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.
[0055] The illumination optical unit 4 consequently forms a doubly faceted system. This is also referred to as a fly's eye condenser (fly's eye integrator). The individual first facets 21 are imaged into the object field 5 with the aid of the second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror or indeed the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5.
[0056] The projection system 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.
[0057] In the example depicted in FIG. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are similarly possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical unit. The projection system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, and can be for example 0.7 or 0.75.
[0058] Just like the mirrors of the illumination optical unit 4, the mirrors Mi can have a highly reflective coating for the illumination radiation 16.
[0059] FIG. 2 shows, by way of example, a mirror M4 of the projection system 10, the mirror comprising a substrate 25 which is formed from a first partial body 26a and a second partial body 26b. The first partial body 26a, which is plate-shaped in the example shown, and the second partial body 26b, which forms a main body of the substrate 25, are put together or connected to one another at a common interface 27, which in the example shown is a planar surface, although this is not mandatory. The connection between the two partial bodies 26a, b is established by a conventional joining or bonding process, for example by high-temperature or low-temperature bonding or by optical contact bonding. The material of the first partial body 26a and of the second partial body 26b may be identical, but different materials may also be involved. In the example shown, 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, b, may also be made from another material which has as low as possible a coefficient of thermal expansion, for example a glass ceramic, for example Zerodur®.
[0060] A reflective coating 29 is applied to an exposed surface 28 of the first partial body 26a that faces away from the interface 27. A partial region 30 of the surface 28, which is located within the reflective coating 29, is struck by the EUV radiation 16 of the projection system 10 and forms an optically utilized partial region of the reflective coating 29. The reflective coating 29 may comprise, for example, a plurality of layer pairs made of materials with different real parts of the refractive index, the layers possibly being formed from Si and Mo, for example, in the case of a wavelength of the EUV radiation 16 of 13.5 nm. The surface 28 of the first partial body 26a is represented as a planar surface area in FIG. 2, although it may also have a curvature.
[0061] In the example shown in FIG. 2, a plurality of cooling channels 31, which run below the surface 28 to which the reflective coating 29 is applied, is formed in the substrate 25 in the region of the interface 27. In the example shown in FIG. 2, there are approximately twenty cooling channels 31, which extend below the surface 28 between a distributor 32 and a collector 33, which are on opposite sides of the optically utilizable partial region 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 of FIG. 2, the distributor 32 has a distributor chamber 32a, which connects the plurality of cooling channels 31 to a common coolant inlet 34, which forms an opening in the second partial body 29b. Correspondingly, the collector 33 forms a collecting chamber, which connects the plurality of cooling channels 31 to a common coolant outlet 35, which is likewise in the form of an opening in the second partial body 29b.
[0062] As can be seen in FIG. 2, the distributor chamber 32a widens in funnel-shaped fashion from the coolant inlet 34 to the ends of the cooling channels 31, which open into the distributor chamber 32a. Correspondingly, the collecting chamber 33a narrows in funnel-shaped fashion from the ends of the cooling channels 33 to the coolant outlet 35. The distributor chamber 32a and the collecting chamber 33a extend along the interface 27 and are formed as flatly as possible in the thickness direction of the substrate 25. In the example shown in FIG. 2, the distributor chamber 32a and the collecting chamber 33a extend both into the first partial body 26a and into the second partial body 26b. The distributor chamber 32a and the collecting chamber 33a have a substantially triangular geometry that is optimized in terms of flow, in order as far as possible to achieve a uniform distribution of the coolant among all the coolant channels 31 and as low as possible a dynamic excitation owing to the flow of the cooling water.
[0063] To feed the coolant to the coolant inlet 34 and to discharge the coolant from the coolant outlet 35, the projection exposure apparatus 1 comprises a cooling device 36, which is represented schematically in FIG. 1. In the example shown, the cooling device 36 serves to feed a coolant in the form of cooling water to the cooling channels 31 or to the mirror M4, and to this end comprises a feed line, not depicted here, which is connected to the coolant inlet 34 in fluid-tight fashion. The cooling device 36 also comprises a discharge line, not depicted here, in order to discharge the cooling water from the coolant outlet 35. For cooling purposes, the other mirrors M1-M3, M5, M6 of the projection system 10 may also be connected to the cooling device 36 or optionally to further cooling devices provided to this end.
[0064] The pressure of the cooling water flowing through the distributor chamber 32a or through the collecting chamber 33a may lead to bulging of the substrate 25, the result of which is a change in the geometry of the surface 28. Owing to the relative proximity of the distributor chamber 32a and/or the collecting chamber 33a to the optically utilized partial region 30 of the surface 28, in this way undesired deformation of the optically utilized partial region 30 can occur.
[0065] In order to reduce the effects of the bulging of the distributor chamber 32a and/or the collecting chamber 33a on the optically utilized partial region 30 of the reflective coating 29, in the case of the mirror M4 shown in FIGS. 3A-3B the distributor 32, or the distributor chamber 32a, and the collector 33, or the collecting chamber 33a, extend from the interface 27 only into the second partial body 26a of the substrate 25. It is possible for the distributor chamber 32a and/or the collecting chamber 32b to extend from the interface 27 into the first partial body 26a a little, in order to connect the cooling channels 31 to one another at their ends additionally also in the first partial body 26a. As can be seen in FIG. 3A, the distributor chamber 32a extends from the coolant inlet 34, which is formed on the underside of the substrate 25, to the interface 27. Correspondingly, the collecting chamber 33a, not depicted in FIGS. 3A-3B, also extends from the interface 27 to the coolant outlet 35, which is likewise formed on the underside of the substrate 25.
[0066] The distributor chamber 32a, more precisely a center plane M of the distributor chamber 32a, in this respect is aligned parallel to the thickness direction Z of the substrate 25. As can be seen in the partial section of FIG. 3A, the center plane M runs in the Z direction and in the X direction. The distributor chamber 32a is substantially mirror-symmetrical in relation to the center plane M. The center plane M also runs through the coolant inlet 34, which forms an opening in 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 an XY plane of an XYZ coordinate system. This considerably reduces the surface area of the distributor chamber 32a that can bulge owing to the fluid pressure parallel to the surface 28 or to the optically utilized partial region 30 of the surface 28 of the mirror M4. Therefore, tilting the distributor 32 and/or the collector 33 into the second partial body 26b makes it possible to reduce deformations of the optically utilized partial region 30 on the surface 28 of the mirror M4.
[0067] It is not mandatory for the distributor chamber 32a to run in the thickness direction Z of the substrate 25; rather, the distributor chamber 32a, more precisely its center plane M, may also be aligned at an angle α to the thickness direction Z which generally should be no more than approximately 30°. The collector 33 which can be seen in the partial section of FIG. 3A, or the collecting chamber 33a, in the example shown has an identical structure to the distributor 32, or the distributor chamber 33a, on that side of the optically active partial region 30 of the surface 28 of the substrate 25 that is situated opposite in the Y direction. However, a structurally identical design is not mandatory. It may be advantageous, for example for flow-related reasons, if the distributor 32 and/or the distributor chamber 32a and the collector 33 and/or the collecting chamber 33a to have a different geometry.
[0068] As can be seen for example in FIG. 3B, both the distributor chamber 32a and the collecting chamber 33a run below, in the Z direction, a partial region 37 of the surface 28 that is not covered by the reflective coating 29, for example also not below the optically utilized partial region 30 of the surface 28. This enlarges the spacing of the triangular surface area, visible in FIG. 3A, that is subjected to pressure, is formed within the distributor chamber 32a and can bulge, from the optically effective partial region 30 of the surface 28. Such an arrangement is fundamentally also possible in the case of the mirror M4 shown in FIG. 2, for which the distributor chamber 32a and the collecting chamber 33a extend along the interface 27 between the two partial bodies 26a, b, since the structural space in the lateral direction in the case of the mirror M4 shown in FIG. 2 is sufficient for this.
[0069] In the case of the mirror M4 illustrated in FIGS. 4A-4B, the substrate 25 has a third partial body 26c in addition to the first and the second partial body 26a, b. The third partial body 26c is connected to or put together with the second partial body 26b at a further interface 38 and is likewise made of ULE®. The connection can be formed like the connection described above at the interface 27 between the first and the second partial body 26a, b. The collector 32 shown in FIGS. 4A-4B has a portion 39 which is connected to the interface 27 between the first and the second partial body 26a, b and extends from the interface 27 into the second partial body 26b of the substrate 25. Connecting channels 40, which extend in the thickness direction Z of the substrate 25, are formed in that portion 39 of the distributor 32 that is connected to the interface 27.
[0070] As in the case of the example described in FIGS. 3A-3B, it is also not mandatory in FIGS. 4A-4B for the connecting channels 40 to be aligned in the thickness direction Z of the substrate 25; rather, as in FIGS. 3A-3B, an alignment of the connecting channels 40 at an angle α of typically no more than 30° to the thickness direction Z is possible. It can also be advantageous if the angle α, at which the connecting channels 40 are aligned in relation to the thickness direction Z of the substrate 25, varies in the substrate 25.
[0071] In the example shown in FIGS. 4A-4B, a respective connecting channel 40 is connected to exactly one cooling channel 31 and downwardly continues the latter into the second partial body 26b. In other words, a respective cooling channel 31 is deflected from an alignment parallel to the interface 27 into the second partial body 26b by a connecting channel 40 which is assigned to it. In the example shown in FIGS. 4A-4B, the connecting channels 40 run below a partial region of the surface 28 that is not covered by the optically active partial region 30.
[0072] In the example shown in FIGS. 4A-4B, the coolant is distributed among the individual cooling channels 31 via a distributor chamber 32a, which is connected to the connecting channels 40. The connecting channels 40 open into the distributor chamber 32a, which connects the connecting channels 40 to the coolant inlet 34. In the example shown in FIGS. 4A-4B, the distributor chamber 32a extends along the further interface 38 between the second and the third partial body 26b, c of the substrate 25. In the example shown, the further interface 38 extends in a plane parallel to the base area of the third partial body 26c, although such an alignment is not mandatory. The coolant inlet 34 forms an opening which runs through the third partial body 26c and ends on the underside of the substrate 25. As an alternative, the coolant inlet 34 may be formed in the second partial body 26b. In the case of the mirror M4 shown in FIGS. 4A-4B, the surface area of the funnel-shaped distributor chamber 32a may be spaced apart from the surface 28 of the substrate 25 to a greater extent than is the case for the mirror M4 shown in FIGS. 3A-3B. The collector 33 has a similar form to the distributor 32.
[0073] In the case of the mirror M4 shown in FIGS. 4A-4B, a further interface 38 is used to connect the connecting channels 40, which run in the Z direction, to the coolant inlet 34.
[0074] In the case of the mirror M4 shown in FIGS. 5A-5C, the connecting channels 40 of the distributor 32 are connected to a common inlet channel 41. In the case of the mirror M4 shown in FIGS. 5A-5C, the inlet channel 41 is in the form of a transverse bore, or blind bore, in the second partial body 26b. The connecting channels 40 branch off from the common inlet channel 41 upward (in the Z direction), toward the surface 28 of the first partial body 26a. In the case of the example shown in FIGS. 5A-5C, the coolant inlet 41 forms an opening of the inlet channel 41 which is formed on a side face of the second partial body 26b of the substrate 25. The collector 33 is structurally identical to the distributor 32 and likewise has connecting channels 40 which open into a common outlet channel 42, which in FIGS. 5A-5C is concealed by the substrate 25 and is connected to the coolant outlet 35.
[0075] In the case of both the mirror M4 shown in FIGS. 4A-4B and that shown in FIGS. 5A-5C, the connecting channels 40 are in the form of bores in the second partial body 26b of the substrate 25. For the case in which, as in FIGS. 4A-4B and FIGS. 5A-5C, there are many connecting channels 40 which extend relatively deeply into the second partial body 26b, there is a considerable manufacturing risk that, during the boring operation, the second partial body 26b is possibly damaged or in the worst case destroyed when the connecting channels 40 are being produced.
[0076] In order to reduce this risk, in the example shown in FIG. 5B a respective connecting channel 40 is connected not to one but to two respective adjacent cooling channels 31. In this way, the connecting channels 40 can be manufactured with a larger cross section than is the case in the example shown in FIG. 5A. If appropriate, it is also possible for more than two, generally adjacent cooling channels 31 to be connected to one and the same connecting channel 40, in order to further reduce the manufacturing risk.
[0077] For the case in which the cross-sectional areas of the connecting channels 40 that are subjected to pressure are too large and/or the ribs between the connecting channels 40 in the substrate 25 are too small, it is favorable for the connecting channels 40 to be in the form of stepped bores, as illustrated in FIG. 5C. In this case, the connecting channels 40 have, directly adjacent to the interface 27, a first cross-sectional area A1 which is enough to connect a respective connecting channel 40 to two respective cooling channels 31. At a step, the first cross-sectional area A1 is reduced to a second, smaller cross-sectional area A2, as a result of which the spacing between two respective adjacent connecting channels increases. A respective connecting channel 40 may possibly also 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. A reduction in the cross-sectional area A1, A2, etc. of a respective connecting channel 40 from the interface 27 to the distributor chamber 32a is also possible in the case of the mirror M4 shown in FIGS. 4A-4B.
[0078] FIGS. 6A-6B show a section through a substrate 25 of a mirror M4, in the case of which the distributor 32 and the collector 33 are designed as in FIG. 5A. In the case of the mirror M4 of FIGS. 6a, b, a respective connecting channel 40 of the distributor is connected to a common inlet channel 41 and branches off from the latter toward the +surface 28 of the first partial body 26a. The inlet channel 41 is connected to a coolant inlet, which is not depicted in FIGS. 6A-6B. The collector is structurally identical and has connecting channels 40 to the cooling channels 31 which open into a common outlet channel 42 connected to a coolant outlet, which is not depicted in FIGS. 6A-6B.
[0079] By contrast to the mirror M4 shown in FIG. 5A, the cooling channel 31 in the case of the example shown in FIGS. 6a, b has a cross section which is divided between the two partial bodies 26a, b, that is to say the planar interface 27 between the two partial bodies 26a, b runs through the cross section, or the cross-sectional area AK, of the cooling channel 31. The cooling channel 31 is thus composed of a first groove-shaped depression 43a, formed in the first partial body 26a, and a second groove-shaped depression 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, b is favorable for example when the surface 28 of the substrate 25 is curved, as is the case in FIGS. 6A-6B.
[0080] In this case as well, the spacing D of the cooling channel 31 from the curved surface 28 should be substantially constant over the length of the cooling channel 31. This involves the cooling channel 31 being curved, with the curvature of the cooling channel 31 following or corresponding to the curvature of the surface 28. Since the interface 27 between the two partial bodies 26a, b is planar, a cooling channel 31 with a cross-sectional area AK that is constant over the length of the cooling channel 31 can only be realized in this case if not only a first, curved groove-shaped depression 43a is formed in the first partial body 26a but also a second, curved groove-shaped depression 43b is formed in the second partial body 26b, as shown in FIGS. 6A-6B. It goes without saying that the cooling channels 31 of the mirror M4 that was described above in connection with FIG. 2, FIGS. 3A-3B, FIGS. 4A-4B and FIGS. 5B-5C may also have corresponding designs, that is to say that their cross section can be divided between the two partial bodies 26a, b.
[0081] Instead of a single distributor 32 and/or a single collector 33, it is optionally possible for multiple distributors 32 and/or collectors 33 to also be formed in the substrate 25 in order to connect a respective plurality of cooling channels 31, which run below the surface 28 with the reflective coating 29, to a respective coolant inlet 34 and to a respective coolant outlet 35, respectively. However, it can be favorable if only a single coolant inlet 34 and only a single coolant outlet 35 are formed on the substrate 25.
[0082] Instead of a reflective coating 29 for EUV radiation 16, a reflective coating for radiation in a different wavelength range, for example for the DUV wavelength range, may also be applied to the optical element described above. As a rule, there are less stringent desired thermal expansion properties for the substrate 25 for such a reflective optical element, and so use can be made of different substrate materials to those described above, for example conventional fused silica.
