Optical element with channels and lithography system

Abstract

The invention relates to an optical element comprising: a preferably monolithic substrate (26) with an optical surface (27) onto which a reflective coating for reflecting radiation, in particular for reflecting EUV radiation, is applied, wherein a plurality of fluid-flowable channels (29) extend in the substrate (26) below the optical surface (27), the channels being preferably aligned in parallel. In the optical element, the cross-sectional area of ​​at least one channel (29) varies in the longitudinal direction (x) of the channel (29) and / or the lateral distance between any two adjacent channels (29) varies. The invention also relates to a lithography system, in particular an EUV lithography system, comprising at least one such optical element.

Description

Background of the invention

[0001] The invention relates to an optical element comprising: a preferably monolithic substrate with an optical surface onto which a reflective coating for reflecting radiation, in particular for reflecting EUV radiation, is applied, wherein a plurality of channels through which a fluid flows, preferably oriented parallel, extend in the substrate below the surface. The invention also relates to a lithography system comprising at least one such optical element.

[0002] The lithography system can be a lithography unit for exposing a wafer or another optical arrangement for lithography. In particular, it can be an EUV lithography system, i.e., a lithography system that uses EUV radiation. For example, it can be an EUV inspection system, e.g., for inspecting masks, wafers, or the like used in EUV lithography. The optical element with the reflective coating can be, in particular, a mirror, which is why the optical surface is subsequently also referred to as the mirror surface.

[0003] As the power of the radiation sources in lithography systems increases, thermally induced deformations of mirror surfaces and associated aberrations also increase. One way to counteract these effects is direct temperature control of the mirrors. In direct temperature control, channels or hollow structures are introduced into the volume of the substrate, through which a fluid, typically a liquid, can flow for temperature control. The liquid is usually water. Typically, the mirror is cooled during temperature control, meaning the fluid has a lower temperature at the inlet than the surrounding substrate. However, it is also possible, in principle, to heat the optical element using the channels.

[0004] The channels typically run at a short distance below the optical surface to effectively dissipate the thermal energy released when radiation strikes the surface, to precisely control the absolute temperature level at the surface, and to reduce local temperature gradients. The design and arrangement of the channels have a significant impact on the performance of the optical element.

[0005] Even with directly tempered or cooled optical elements, high temperature differences between the optical surface and the channels through which the radiation flows, as well as high temperature gradients along the optical surface, can occur, depending on the (local) power density of the radiation incident on the optical surface. Spatially concentrated high power peaks at the optical surface are particularly problematic, as they lead to locally high temperatures and temperature gradients. Object of the invention

[0006] One object of the invention is to provide an optical element and a lithography system in which the geometry and / or the arrangement of the channels is selected in such a way as to reduce temperature gradients on the optical surface. Subject matter of the invention

[0007] This task is solved by an optical element of the type mentioned above, in which a cross-sectional area of ​​at least one channel varies in the longitudinal direction of the channel and / or in which a lateral distance between any two adjacent channels varies.

[0008] Typically, channels used to cool the optical surface of a mirror, particularly an EUV mirror, have a constant cross-sectional area in the longitudinal direction, and the lateral spacing between any two adjacent channels is constant. With constant lateral spacing and a constant, identical cross-sectional area of ​​the usually parallel channels, the cooling, or rather the cooling capacity, across the optical surface can be considered essentially constant, although the cooling capacity generally decreases towards the edges of the hollow structure formed by the channels. The longitudinal direction of the channels corresponds to the flow direction of the fluid.

[0009] In cases where the thermal load on the optical surface is not uniformly distributed during operation of the optical element in a lithography system, for example, because the power of the useful radiation incident on the surface varies depending on location, the cooling capacity of the fluid flowing through the channel can be adjusted by the locally varying cross-sectional area of ​​the channel and / or by different lateral distances between the channels. In particular, the locally varying cooling capacity can be adjusted so that the resulting relative temperature at the optical surface exhibits a comparable pattern or gradient to that observed when the optical element is not subject to any thermal load.

[0010] In particular, in areas of the optical surface with high thermal stress, increased cooling capacity can be achieved by appropriately varying the cross-sectional area of ​​each channel and / or by appropriately varying the lateral distance between adjacent channels, in order to create the most homogeneous temperature distribution possible or the lowest possible temperature gradients on the optical surface. This can also compensate for the effect that, even with homogeneous heat input and homogeneous cooling capacity, the optical surface is generally colder at the edges than in the center.

[0011] The lateral distance between two adjacent channels is defined as the distance between the centers of their cross-sectional areas. If the lateral distance varies along the longitudinal direction of the channels, it is defined as the arithmetic mean of the longitudinal lateral distances of two adjacent channels. It is understood that for the lateral distance between any two adjacent channels to vary, at least three channels must be present below the optical surface.

[0012] The substrate material can be, for example, quartz glass, particularly titanium-doped quartz glass, or a glass-ceramic. The substrate can be monolithic and have no interface in the channel area. This is possible because the aforementioned materials are transparent to radiation at wavelengths between approximately 500 nm and 2000 nm. Therefore, laser radiation in the form of ultrashort laser pulses can be focused within the volume of such substrates. In a given focus area, the laser radiation power is typically absorbed by multiphoton processes, leading to a modification of the material in the irradiated volume. In so-called selective laser etching, the modified material in the irradiated volume is removed by wet chemical etching.Therefore, in a monolithic substrate made of such a material, the channels can be created by selective laser etching, as described, for example, in WO2021115643A1.

[0013] The channels can also be created in a monolithic substrate using other methods, such as laser ablation, for example, backside laser ablation, in which the material in the irradiated volume is directly removed, forming an ablation front that migrates through the material. To remove the ablated material and for cooling, the ablation front is flushed with a fluid, typically a liquid. When creating long channels, it is usually necessary to feed the fluid along the ablation front, for example, by inserting a flexible tube into the already created part of the channel. Channels can also be created using other methods, such as electrical discharge machining (EDM). They can also be produced in a monolithic substrate manufactured by additive manufacturing (3D printing).

[0014] A channel manufactured in the manner described above can have virtually any cross-sectional geometry. For example, the channel can have a polygonal geometry (e.g., rectangular or square), a rounded rectangle, a rounded shape, an oval, an elliptical shape, etc. It is also possible, in principle, to manufacture the channels by joining two substrate components at a joint surface using a suitable bonding process, as described, for example, in WO 2022 / 008155 A1.

[0015] In one embodiment, at least one channel has at least one divergence section with an increasing cross-sectional area and / or at least one convergence section with a decreasing cross-sectional area, wherein the divergence section and / or the convergence section preferably runs between channel sections with a constant cross-sectional area. The increase or decrease in cross-sectional area relates to a common direction corresponding to the flow direction of the fluid through the channel.

[0016] The transition between a divergence section or a convergence section and a channel section with a constant cross-sectional area is typically gradual or continuous, meaning there is no abrupt change in cross-sectional area. Typically, the cross-sectional area in the divergence section increases by the same amount by which the cross-sectional area in the convergence section decreases. The channel may, in particular, have a channel section with a constant cross-sectional area at both of its longitudinal ends and a channel section with a larger, constant cross-sectional area between the divergence and convergence sections to accommodate thermal stress on the optical surface. Typical diameters of the channel sections with a constant cross-sectional area are on the order of millimeters.It is understood that one and the same channel can have several divergence sections and convergence sections, whereby neither the period, i.e. the distance between successive divergence and convergence sections, nor the amplitude of the cross-sectional change nor the strength of the divergence or convergence need be constant.

[0017] During further development, the cross-sectional area of ​​the channel increases in both spatial directions during the divergence section and / or decreases in both spatial directions during the convergence section. In this case, the cross-sectional area of ​​the channel is varied in two mutually perpendicular spatial directions during the divergence and / or convergence sections. The variation or scaling can be the same in both spatial directions, so that a circular or square cross-sectional area remains circular or square in the divergence or convergence section, respectively. However, it is also possible for the variation to differ in the two spatial directions, so that, for example, a circular cross-sectional area becomes an elliptical cross-sectional area or a square cross-sectional area becomes a rectangular cross-sectional area.

[0018] In an alternative flow path, the cross-sectional area of ​​the channel in the divergence section increases only in one spatial direction and / or the cross-sectional area of ​​the channel decreases only in one spatial direction in the convergence section. In this case, the cross-sectional area in the other spatial direction remains constant or is not scaled.

[0019] The diameter of the channel, or its extent in a given spatial direction where the cross-sectional area varies, can increase or decrease linearly in the divergence and convergence sections, for example. In this case, the convergence or divergence section has a constant opening angle in the respective spatial direction. The value of the opening angle depends on various parameters, including the cross-sectional geometry of the channel.

[0020] In a further development, the spatial direction in which the cross-sectional area increases or decreases is aligned parallel to or perpendicular to the normal direction of the optical surface. The extent of the cross-sectional area does not change in the spatial direction perpendicular to this. If the cross-sectional area varies perpendicular to the normal direction of the optical surface, the cross-section cooling the surface increases laterally, but the extent of the channel in the normal direction of the surface remains constant. If the channel has a constant, circular cross-section outside the divergence or convergence zone, this changes to an elliptical cross-section. It is understood that the cross-sectional area can alternatively vary in the vertical direction, i.e.,parallel to the normal direction of the optical surface at the respective position on the optical surface under which the channel runs.

[0021] In another embodiment, the distance of the channel from the optical surface or the distance of the channel from a surface of the substrate facing away from the optical surface is constant in the divergence section and / or in the convergence section.

[0022] For the purposes of this application, the distance to the optical surface is understood to be the minimum distance that the respective channel, or more precisely its circumferential surface, has to the optical surface. Similarly, the distance to the surface of the substrate facing away from the optical surface is understood to be the minimum distance that the respective channel, or more precisely its circumferential surface, has to the surface of the substrate facing away from the optical surface.

[0023] A constant distance to the optical surface can be maintained, for example, if the increase or decrease in the cross-sectional area of ​​the channel occurs only in the lateral direction, i.e., parallel to the surface. If the increase or decrease in the cross-section occurs in the vertical direction, i.e., perpendicular to the optical surface, the cross-sectional area perpendicular to the optical surface increases or decreases only on the side of the channel's surface facing away from the optical surface, while the side of the channel's surface facing the optical surface maintains its constant distance from the optical surface. If the distance to the surface facing away from the optical surface is to remain constant, the cross-sectional area decreases accordingly.the cross-section on the side of the channel's lateral surface facing the optical surface increases or decreases, while the distance between the opposite side of the lateral surface and the surface of the substrate facing away from the optical surface remains constant.

[0024] In a further development, in the divergence region of a first channel, an adjacent second channel exhibits a convergence region, and vice versa. It can be advantageous for the performance of the optical element or the lithography system if the designs of adjacent channels are matched with respect to their cross-sectional areas. For example, in a longitudinal region of a first channel where a divergence region is formed, a second, adjacent channel can exhibit a convergence region, and vice versa. In this way, channels or channel structures can be formed in the manner of a string of beads or a honeycomb structure.

[0025] In principle, it is possible to connect or mesh adjacent channels one or more times, for example, to create an array of channels below the optical surface. Preferably, a divergence section of the first channel is connected to a convergence section of the second channel via a connecting segment, and vice versa. In principle, all convergence sections can be used as an additional inlet and all divergence sections as an additional outlet for the respective "honeycomb." Furthermore, all possible partial combinations of connections can be used; for example, only pairs of adjacent channels can be connected.

[0026] In another embodiment, the distance of at least one channel, and in particular all channels, from the optical surface is 10 mm or less, preferably 5 mm or less. The smaller the distance of the channels from the optical surface, the smaller the temperature difference typically is between the optical surface and the fluid flowing through the channels to cool the optical element. In principle, the temperature gradients at the optical surface are thus reduced as the distance of the channels from the optical surface decreases, regardless of the power distribution of the radiation incident on the surface. However, the distance of the channels to the optical surface cannot be arbitrarily small due to manufacturing limitations.

[0027] In a further aspect of the invention, which can in particular be combined with the aspect of the invention described above, the channels are connected via distribution channels to a fluid distributor which is designed as an inlet channel, wherein the distribution channels branch off from the inlet channel at a respective convergence section with decreasing cross-sectional area, and / or the channels are connected via collector channels to a fluid collector which is designed as an outlet channel, wherein the collector channels branch off from the outlet channel at a respective convergence section with decreasing cross-sectional area.

[0028] As described above, the channels typically run in a near-surface region below the optical surface, i.e., at a distance of 10 mm or less. The fluid distributor, in the form of the inlet channel, and the fluid collector, in the form of the outlet channel, each typically have a larger cross-sectional area than a single cooling channel. This allows for the establishment of favorable flow conditions. The fluid distributor and / or the fluid collector are preferably located at a greater distance from the optical surface than the channels. The fluid distributor is typically connected to a fluid inlet, and the fluid collector is typically connected to a fluid outlet. Each channel running below the surface can be connected to exactly one distributor channel and exactly one collector channel, but it is also possible, in principle, for a group of two or possibly more channels to be connected.more than two channels are connected to a common distribution channel and a common collector channel.

[0029] The fluid collector, in the form of the inlet channel, and the fluid distributor, in the form of the outlet channel, generally run essentially transversely to the longitudinal direction of the distributor channels and the collector channels, respectively. In this case, the fluid distributor and the fluid collector can, for example, be designed as cylindrical channels extending into the substrate from an inlet opening or an outlet opening on the outer surface. The inlet and outlet channels can be formed, for example, as bores, but it is also possible for the inlet and outlet channels to be produced by the selective laser etching described above or by an ablation process.

[0030] The distribution and collector channels typically branch off from the inlet and outlet channels at essentially right angles and have a constant cross-sectional area. This presents challenges for fluid flow during operation of the optical element, as a sharp transition between the inlet and outlet channels is suboptimal for fluid flow, negatively impacting cooling performance below the optical surface. The convergence section, with its decreasing cross-sectional area, ensures increased fluid flow and reduced turbulence due to a smoother transition from the inlet to the distribution channel or from the collector to the outlet channel, thereby reducing flow-induced vibrations. Furthermore, a narrow transition between the inlet channel and a respective distribution or collector channel is not recommended.The situation is unfavorable for the outlet channel and its respective collector channel if, as described above, a flexible hose is guided along the ablation front during laser ablation. The hose can be inserted into the inlet or outlet channel in a lance-like guide, for example, and must be inserted or threaded into the respective distribution or collector channel. Threading the hose can lead to wear, and in the worst case, kinking or damage that permanently impairs its function. A possible consequence of inadequate flushing of the ablation front during channel fabrication is material damage that cannot be sufficiently repaired by subsequent processes, such as etching.

[0031] The convergence section, whose cross-sectional area decreases from the inlet or outlet channel, acts as a funnel. This allows the maximum cross-sectional area of ​​the convergence section to be adapted to the target geometry of the connecting channel and ensures the safe insertion of the hose for flushing the removal front. The maximum cross-sectional area of ​​the convergence section at its opening into the inlet or outlet channel is adapted to the specific requirements. The opening angle at which the longitudinal direction of the convergence section is aligned with a longitudinal direction of the inlet or outlet channel is also adapted to the requirements and depends, among other things, on the cross-sectional geometry. In principle, the convergence section can have a cross-sectional area of ​​any geometry; for example, it can be a polygonal funnel.whether it is a polygonal cross-sectional area or a round cross-sectional area.

[0032] In this embodiment, the fluid collector and the fluid distributor generally run essentially transversely to the longitudinal direction of the distributor channels and the collector channels, respectively. The distributor channels and the collector channels typically branch off from the inlet channel and the outlet channel, respectively, at essentially right angles. In this case, the fluid distributor and the fluid collector can, for example, be designed as cylindrical channels extending into the substrate from an inlet opening and an outlet opening on the outside of the substrate. The inlet channel and the outlet channel can, for example, be designed as bores.

[0033] In another embodiment, the convergence section has a constant opening angle or is rounded. As described above, possible geometries for the convergence section include straight lines or rounded geometries or radii. In the first case, the convergence section is a linearly tapered funnel; in the second case, it is a radially tapered funnel where the opening angle decreases continuously from the opening at the inlet or outlet channel.

[0034] The cross-sectional geometry of the convergence section can be rotationally symmetric with respect to the longitudinal direction of the distribution channel or collector channel, meaning that the lateral surface of the convergence section is a surface of revolution. However, it is also possible for the convergence section to have an asymmetric geometry with respect to the longitudinal direction of the distribution channel or collector channel.

[0035] In a further development, the convergence section has an edge oriented perpendicular to either the inlet or outlet channel. As described above in connection with the channels, the reduction in the cross-sectional area of ​​the convergence section occurs only on one side of its lateral surface, while the reduction on the other side, i.e., at the edge, is not observed. The edge is typically formed on the side of the convergence section facing away from the fluid inlet or outlet, as this side is generally less critical for fluid flow and flow-induced vibrations. If the hose described above is fed from the opposite side of the inlet or outlet channel, the convergence section can, for example,The convergence section should be radially symmetrical. The reduction in cross-sectional area described above should be implemented at least on the side of the convergence section from which the fluid enters or exits during operation, or on the side from which the hose is inserted.

[0036] In another embodiment, the angle between the inlet channel and a respective distribution channel is between 60° and 120°, preferably between 80° and 100°, and / or the angle between the outlet channel and a respective collector channel is between 60° and 120°, preferably between 80° and 100°. The angle at which the distribution channel branches off from the inlet channel, or the angle at which the collector channel branches off from the outlet channel, need not necessarily be 90°. An oblique angle, in particular an obtuse angle, can offer additional advantages in fluid dynamics and reduce the formation of turbulence.

[0037] In another embodiment, the convergence section has a non-rotationally symmetrical geometry with respect to a longitudinal direction of the distributor channel or collector channel. This is typically the case when the distributor channel or collector channel is oriented at an angle other than 90° to the intake or exhaust channel, respectively. However, it can also be advantageous for the convergence section to have a non-rotationally symmetrical geometry when the distributor channel or collector channel is oriented at an angle of 90° to the intake or exhaust channel, respectively.

[0038] Another aspect of the invention relates to a lithography system, in particular an EUV lithography system, comprising: at least one optical element as described above. The lithography system may be a lithography system, in particular an EUV lithography system, for exposing a wafer, or another optical arrangement that uses EUV radiation, for example, an EUV inspection system, e.g., for inspecting masks, wafers, or the like used in EUV lithography. The variation of the cross-sectional area of ​​the channel in the longitudinal direction of the channel and / or the variation of the lateral distance between any two adjacent channels of the optical element may, for example, be determined depending on a local power density at the optical surface during operation of the lithography system.

[0039] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention. drawing

[0040] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows Fig. 1. Schematic representation in meridional section of a projection exposure system for EUV projection lithography, Fig. 2a-c schematic sectional views of an EUV mirror with a substrate having a plurality of channels through which a fluid can flow, Fig. 3a-d schematic sectional views of a channel having a longitudinally varying cross-sectional area with divergence sections and convergence sections, Fig. 4a-c schematic cross-sectional views of adjacent channels, in which the divergence sections and convergence sections are aligned or connected to each other, Fig. 5a-c schematic representations of a substrate in which the channels running below the optical surface are connected to an inlet channel via distribution channels and to an outlet channel via collector channels, as well as Fig. 6a-c schematic sectional views of an inlet channel from which distribution channels branch off via a convergence section.

[0041] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0042] The following will refer to Fig. 1. The essential components of an optical arrangement for EUV lithography, in the form of a projection exposure system 1 for microlithography, are described as an example. The description of the basic structure of the projection exposure system 1 and its components is not to be understood as restrictive.

[0043] One embodiment of a lighting system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, a lighting optic 4 for illuminating 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 lighting system. In this case, the lighting system does not include the light source 3.

[0044] A reticule 7 located in the object field 5 is illuminated. The reticule 7 is held by a reticule holder 8. The reticule holder 8 can be moved, particularly in one scanning direction, via a reticule displacement drive 9.

[0045] In Fig. Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. The scan direction runs in the Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane. 6.

[0046] The projection exposure system 1 comprises a projection system 10. The projection system 10 serves to image the object field 5 onto an image field 11 in an image plane 12. A structure on the reticulum 7 is imaged onto a light-sensitive layer of a wafer 13 located in the image plane 12 within the area of ​​the image field 11. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, particularly along the y-direction, via a wafer transfer drive 15. The displacement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized with each other.

[0047] Radiation source 3 is an EUV radiation source. Specifically, radiation source 3 emits EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP source (laser-produced plasma) or a DPP source (gas-discharged produced plasma). It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0048] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloid reflective surfaces. The at least one reflective surface of the collector mirror 17 can be illuminated by the illumination radiation 16 at grazing incidence (GI), i.e., with angles of incidence greater than 45°, or at normal incidence (NI), i.e., with angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated to optimize its reflectivity for the useful radiation and to suppress stray light.

[0049] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0050] The illumination optics 4 comprise a deflecting mirror 19 and, downstream in the beam path, a first faceted mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with a beam-shaping effect in addition to its deflecting function. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 16 from stray light of a different wavelength. The first faceted mirror 20 comprises a plurality of individual first facets 21, which are hereinafter also referred to as field facets. Of these facets 21, the Fig. 1 Only a few examples are shown. In the beam path of the lighting optics 4, a second faceted mirror 22 is arranged downstream of the first faceted mirror 20. The second faceted mirror 22 comprises a plurality of second facets 23.

[0051] The illumination optics 4 thus form a double-faceted system. This basic principle is also known as a honeycomb condenser (fly's eye integrator). With the aid of the second faceted mirror 22, the individual first facets 21 are imaged into the object field 5. The second faceted mirror 22 is the last beam-shaping, or indeed the last, mirror for the illumination radiation 16 in the beam path before the object field 5.

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

[0053] In the Fig. In the example shown, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection system 10 is a double-obscured optic. The projection optic 10 has an image-side numerical aperture greater than 0.4 or 0.5, and which can also be greater than 0.6, for example, 0.7 or 0.75.

[0054] The mirrors Mi, just like the mirrors of the lighting optics 4, can have a highly reflective coating for the lighting radiation 16.

[0055] Fig. Figures 2a-c show a cross-sectional view of a mirror 25, which could, for example, be one of the mirrors Mi of the projection optics 10 or another reflective optical element of the EUV lithography system 1. The mirror 25 has a substrate 26 made of a material with a low coefficient of thermal expansion, such as titanium-doped quartz glass or a glass-ceramic. The substrate 26 has an optical surface 27, which in the example shown is planar, but can also deviate from a planar geometry and be, for example, concave or convex. A reflective coating 28 is applied to the optical surface 27.In the example shown, the reflective coating 28 has a plurality of layer pairs of materials with different real parts of the refractive index, which, for example, can be made of Si and Mo at a wavelength of 13.5 nm of the EUV radiation 16 incident on the optical surface 27.

[0056] As in Fig. As can be seen in Figure 2a, the substrate 26 has a plurality of channels 29 through which a fluid F flows. In the example shown, the fluid F is water, but it could also be another liquid or possibly a gas. In the example shown, the channels 29 run in a straight line along a longitudinal direction corresponding to the x-direction of an xyz coordinate system. All channels 29 are arranged at a constant distance d from the optical surface 27 of the mirror 25, which in the example shown is approximately 5 mm. The channels 29 have a circular cross-section with a diameter D that is constant along the longitudinal direction of the channels 29, which can, for example, be on the order of a few millimeters. The lateral distance p between the centers of the cross-sectional areas A of each pair of adjacent channels 29 is the same for all channels 29 and is approximately 5 mm in the example shown, as shown in Figure 2a. Fig. 2b based on a cross-sectional view along the in Fig. 2a is represented by dashed lines.

[0057] The cooling capacity achieved with the aid of the cooling channels 29 is in the Fig. The temperature of the optical surface 27 is essentially constant, as shown in Figure 2b. However, with an inhomogeneous heat input from the EUV radiation 16 onto the optical surface 27, an inhomogeneous temperature distribution T(x,y) can occur at the optical surface 27, despite the essentially equal cooling power. In this distribution, the temperature in the center of the optical surface 27 is significantly higher than at the edge of the optical surface 27. Fig. 2b the temperature distribution T(x,y) is represented by dashed ellipses which represent the isotherms of the temperature distribution T(x, y).

[0058] To homogenize the temperature distribution T(x, y), it is advantageous if the cooling power generated by the fluid F flowing in the channels 29 is greater in the center of the optical surface 27 than at the edge. This can be achieved by making the lateral distance p between any two adjacent channels 29 smaller in the center of the optical surface 27 than at the edge of the optical surface 27, as shown in Fig. Figure 2c shows the temperature distribution T(x, y) shown there, which is more homogeneous and therefore has fewer isotherms than the one in Figure 2c. Fig. 2b shows the temperature distribution T(x, y).

[0059] The variable lateral distance p between the parallel channels 29 allows the temperature distribution T(x, y) to be homogenized transversely to the channels 29, i.e., in the Y-direction, but not in the longitudinal direction x of the channels 29. Such homogenization can be achieved if the cross-sectional area A of the channels 29 varies in the longitudinal direction x of the channels 29, as shown below. Fig. 3a-d is described.

[0060] In Fig. Figure 3a above shows the temperature distribution T(x) along the x-direction at the optical surface 27 when it is cooled by an underlying channel 29 with constant cross-sectional area A. As in Fig. As can be seen in Figure 3a, the temperature T(x) is at its maximum approximately in the middle of the length of channel 29. To increase the cooling capacity in this area, channel 29 has a divergence section 30 and a convergence section 31. In the divergence section 30, the cross-sectional area A of channel 29 increases continuously in the longitudinal direction x, while in the convergence section 31, the cross-sectional area A of channel 29 decreases continuously in the longitudinal direction x. Between the divergence section 30 and the convergence section 31, channel 29 has a central channel section 32 with a constant cross-sectional area A, which is larger than the cross-sectional area A of channel 29 in the respective end-channel sections 33a and 33b, which border the divergence section 30 and the convergence section 31, respectively, to the left and right.Due to the larger cross-sectional area A of the channel 29 in the middle channel section 32, the cooling power increases there and the temperature distribution T(x) on the optical surface 27 in the longitudinal direction x of the channels 29 is homogenized.

[0061] At the in Fig. In the example shown in Figure 3a, the cross-sectional area A of channel 29 increases in both spatial directions z and y in the divergence section 30, and the cross-sectional area A of channel 29 decreases in both spatial directions z and y in the convergence section 31, with the increase and decrease in both spatial directions z and y being equal. The round or circular geometry of channel 29 in channel sections 33a, 32, and 33b with a constant cross-sectional area A is therefore maintained in both the divergence section 30 and the convergence section 31.

[0062] As in Fig. As can be seen in Figure 3a, when the channel 29 widens in the Z-direction, i.e., perpendicular to the optical surface 27, the distance d of the channel 29 to the optical surface 27 also changes. If this is not desired, the cross-sectional area A can vary only on one side, i.e., only on one side of the lateral surface of the channel 29, in the divergence section 30 and in the convergence section 31, as shown in Figure 3a. Fig. 3b is shown. In the Fig. In the example shown in 3b, the distance d between the channel 29 and the optical surface 27 remains constant. Fig. Figure 3c shows a channel 29 in which a distance d' to a surface 27' facing away from the optical surface 27 on the underside of the substrate 26 is constant. It is understood that in the Fig. In the examples shown in 3b,c, the cross-sectional geometry or the shape of the cross-sectional area A is changed in the divergence section 30 and in the convergence section 31.

[0063] It is not absolutely necessary that the cross-sectional area A of the channel 29 in a respective divergence section 30 or in a respective convergence section 31 changes in both spatial directions z, y; rather, the cross-sectional area A of the channel 29 in the divergence section 30 can only increase in one spatial direction z or y, and the cross-sectional area A of the channel 29 in the convergence section 31 can only decrease in one spatial direction z or y. Fig. Figure 3d shows an example of such a channel 29, where the cross-sectional area A of the divergence section 30 and the convergence section 31 varies only in the y-direction, i.e., perpendicular to the normal direction z of the optical surface 27, while the cross-sectional area A parallel to the normal direction z of the optical surface 27 remains constant. The shape of the cross-sectional area A changes from a circular geometry in the channel sections 33a, b, 32 with a constant cross-sectional area A to an elliptical geometry in the divergence section 30 and in the convergence section 31. As shown in Fig. As can also be seen in 3D, one and the same channel 29 can exhibit several divergence sections 30, 30', 30" and several convergence sections 31, 31', 31". As shown in Fig. As can be seen in 3d, the distance between adjacent divergence sections 30, 30', 30" and adjacent convergence sections 31, 31', 31", the amplitude of the increase or decrease of the cross-sectional area A and the opening angle or the degree of increase and decrease of the cross-sectional area A can vary.

[0064] The variation in the cross-sectional areas A of adjacent channels 29 can be coordinated, as exemplified below by means of Fig. 4a-c is described. Fig. As can be seen in Figure 4a, in the region of a divergence section 30 of a first channel 29, an adjacent second channel 29' can have a convergence section 31', and vice versa. The same applies to the second channel 29' and an adjacent third channel 29", resulting in a structure similar to a honeycomb structure.

[0065] It is also possible to connect or mesh adjacent channels 29, 29', 29" at the divergence sections 30 and the convergence sections 31 to create an array of channels 29 below the optical surface 27. Fig. Figure 4b shows such a network using the example of a first and second channel 29, 29', in which a respective divergence section 30 of the first channel 29 is connected to a respective convergence section 31' of the second channel 29' via a connecting section 34, and vice versa. Fig. 4c shows the case where not only two, but all adjacent channels 29, 29', 29", ... are connected to the channels associated with Fig. are interconnected or networked in the manner described in 4b. Fig. The dashed lines shown in 4b,c serve to illustrate the transition between the divergence section 30 or convergence section 31 and the respective connecting section 34.

[0066] Fig. 5a shows mirror 25 of Fig. 2a in a three-dimensional representation, whereby the reflective coating has been omitted for the sake of clarity. The substrate 26 has channels 29 that run below the optical surface 27 and through which the fluid F can flow, as shown in Fig. 5a is indicated by an arrow.

[0067] As in Fig. As can be seen in Figure 5a, the fluid F enters the substrate 26 through an inlet opening 35 into a cylindrical inlet channel 36, which forms a fluid distributor. A plurality of distributor channels 37 branch off from the inlet channel 36, each of which is connected to one of the plurality of channels 29 that run below the optical surface 27. From the channels 29, the fluid F flows through a plurality of collector channels 38 to a cylindrical outlet channel 39, which forms a fluid collector. The fluid F exits the substrate 26 through an outlet opening 40 at an end face of the cylindrical outlet channel 39.

[0068] Fig. 5b and Fig. Figure 5c shows a detail of the inlet channel 36 of Fig. 5a with one of the distribution channels 37 in longitudinal section along the longitudinal direction y of the inlet channel 36 or in cross-section in a plane xz perpendicular to the longitudinal direction y. As in Fig. As can be seen in Figures 5b and 5c, the distribution channel 37 branches off from the inlet channel 36 at an angle of approximately 90° and has a constant cross-sectional area. The transition between the inlet channel 36 and the distribution channel 37 is correspondingly sharply defined. Such a sharp or angular transition is not optimal for the flow of fluid F during operation of the mirror 25 and can also lead to turbulence, which can result in undesirable flow-induced vibrations. Furthermore, during the production of the channels 29 or the hollow structure in the substrate 26 by laser ablation, a flexible hose used to guide a flushing fluid cannot be easily inserted into the distribution channel 37. This can lead to damage to the hose or, if necessary, to the substrate 26 material if the hose cannot be guided as desired.

[0069] Fig. Figures 6a-c show examples of the inlet channel 36, from which the distribution channels 37, 37', ... branch off at a respective convergence section 41, 41', ... with decreasing cross-sectional area A. It is understood that the collector channels 38 can also be connected to the outlet channel 39 via corresponding convergence sections 41, 41', ... The convergence section 41, 41', ... essentially fulfills a funnel function, with the cross-sectional area A of the convergence section 41 generally converging towards the target geometry of the distribution channel 37. The cross-sectional shape of the convergence section 41, 41', ... does not necessarily have to match the cross-sectional shape of the distribution channel 37; for example, the convergence section 41, 41', ... can have a polygonal geometry that opens into a distribution channel 37 with a round cross-sectional geometry.

[0070] There are various possibilities for shaping the geometry of the convergence section 41, 41', ... The one in Fig. The convergence section 41 shown on the left in Figure 6a has a constant opening angle α and a linearly tapered funnel shape, which in Fig. The convergence section 41' shown in Figure 6b on the right is rounded and has an opening angle that varies along the length of the convergence section 41', or a radially tapered funnel shape. In the sectional view of Fig. 6a the lateral surface of the convergence section 41' shown on the right runs in the form of a quarter circle, but it is understood that roundings with different geometries are also possible.

[0071] In Fig. Figure 6b on the left shows a convergence section 41, which in the sectional view is rotationally symmetric to the longitudinal direction of the distribution channel 37 and has a circular cross-section. Such a rotationally or mirror-symmetrical convergence section 41 is advantageous if the flow direction of the fluid F, indicated by an arrow, is opposite to the direction in which a flexible hose 42 is to be inserted into the distribution channel 37. The in Fig. 6b The central convergence section 41' has a right edge 43 that is oriented perpendicular to the inlet channel 36 or to its longitudinal direction y. This is possible if both the fluid F and the hose 42 are fed into the distribution channel 37' from the left. Similarly, if both the fluid F and the hose 42 are fed into the distribution channel 37' from the right, the left edge 43 of the convergence section 41' can be oriented perpendicular to the inlet channel 36, as shown in Fig. 6b is shown on the right.

[0072] Fig. Figure 6c shows the case where the angle γ between the inlet channel 36 and a respective distributor channel 37, 37' deviates from 90°. In the example shown, the angle γ is approximately 100°, i.e., it is an obtuse angle measured to the left side of the distributor channel 37, from which the fluid F flows into the distributor channel 37. It is understood that other angles γ deviating from 90° are also possible, which improve the fluid dynamics of the flowing fluid F. With the oblique orientation of the distributor channel 37 relative to the inlet channel 36, the respective convergence section 41, 41', ... generally has a geometry that is not rotationally symmetrical with respect to the longitudinal direction of the distributor channel 37, as shown in Fig. Figure 6c shows the linearly tapered funnel shape on the left and the radially tapered funnel shape on the right of the respective convergence section 41, 41'. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2021115643A1

[0012] WO 2022 / 008155 A1

[0014]

Claims

[1] Optical element (25) comprising: a preferably monolithic substrate (26) with an optical surface (27) to which a reflective coating (28) for reflecting radiation, in particular for reflecting EUV radiation (16), is applied, wherein in the substrate (26) below the optical surface (27) there runs a plurality of channels (29) through which a fluid (F) can flow, which channels are preferably aligned in parallel, characterized by , that a cross-sectional area (A) of at least one channel (29) varies in the longitudinal direction (x) of the channel (29) and / or that a lateral distance (p) between each two adjacent channels (29) varies. [2] Optical element according to claim 1, wherein at least one channel (29) has at least one divergence section (30) with increasing cross-sectional area (A) and / or at least one convergence section (31) with decreasing cross-sectional area (A), wherein the divergence section (30) and / or the convergence section (31) preferably run between channel sections (33a, 32; 32, 33b) with constant cross-sectional area (A). [3] Optical element according to claim 2, wherein the cross-sectional area (A) of the channel (29) in the divergence section (30) increases in both spatial directions (z, y) and / or wherein the cross-sectional area (A) of the channel (29) in the convergence section (31) decreases in both spatial directions (z, y). [4] Optical element according to claim 2, wherein the cross-sectional area (A) of the channel (29) in the divergence section (30) increases only in one spatial direction (z; y) and / or wherein the cross-sectional area (A) of the channel (29) in the convergence section (31) decreases only in one spatial direction (z; y). [5] Optical element according to claim 4, wherein the spatial direction (z; y) in which the cross-sectional area (A) increases or decreases is aligned parallel to the normal direction (z) of the optical surface (27) or perpendicular to the normal direction (z) of the optical surface (27). [6] Optical element according to one of the preceding claims, wherein in the divergence section (30) and / or in the convergence section (31) a distance (d) of the channel (29) from the optical surface (27) or a distance (d') of the channel (29) from a surface (27') of the substrate (26) facing away from the optical surface (27) is constant. [7] Optical element according to one of claims 2 to 6, wherein in the region of a divergence section (30) of a first channel (29) an adjacent second channel (29') has a convergence section (31'), and vice versa. [8] Optical element according to claim 7, wherein a divergence section (30) of the first channel (29) is connected to a convergence section (31') of the second channel (29') via a connecting section (34), and vice versa. [9] Optical element according to one of the preceding claims, wherein a distance (d) of at least one channel (29), in particular of all channels (29), from the optical surface (27) is 10 mm or less, preferably 5 mm or less. [10] Optical element according to the preamble of claim 1, in particular according to one of the preceding claims, in which the channels (29) are connected via distributor channels (37, 37', ...) to a fluid distributor which is designed as an inlet channel (36), wherein the distributor channels (37, 37', ...) branch off from the inlet channel (36) at a respective convergence section (41, 41', ...) with decreasing cross-sectional area (A), and in which the channels (29) are connected via collector channels (38) to a fluid collector which is designed as an outlet channel (39), wherein the collector channels (38) branch off from the outlet channel (39) at a respective convergence section with decreasing cross-sectional area. [11] Optical element according to claim 10, wherein the convergence section (41) has a constant aperture angle (α) or wherein the convergence section (41') is rounded. [12] Optical element according to one of claims 10 or 11, wherein the convergence section (41) has an edge (43) which is oriented perpendicular to the inlet channel (36) or perpendicular to the outlet channel (39). [13] Optical element according to one of claims 10 to 12, in which an angle (γ) between the inlet channel (36) and a respective distributor channel (37) is between 60° and 120°, preferably between 80° and 100° and / or in which an angle between the outlet channel (39) and the respective collector channel (38) is between 60° and 120°, preferably between 80° and 100°. [14] Optical element according to claim 13, wherein the convergence section (41, 41') has a non-rotationally symmetrical geometry with respect to a longitudinal direction of the distribution channel (37) or the collector channel (38). [15] Lithography system (1), in particular EUV lithography system, comprising: at least one optical element (25) according to one of the preceding claims.

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