Optical elements and lithography systems

Abstract

The present invention relates to an optical element (1) that reflects radiation, in particular EUV radiation (2), comprising a substrate (3) that has a surface (4) that is provided with a reflective coating (5) and that has formed therein at least one channel (6) through which a coolant (7) can preferably flow, and that is made of quartz glass, in particular titanium-doped quartz glass, or a glass ceramic, in which the channel (6) has a length (L) below the surface (4) that is provided with the reflective coating (5) of at least 10 cm, preferably at least 20 cm, and in which the cross-sectional area (A) of the channel (6) varies by no more than + / - 20%, preferably no more than + / - 10%, particularly preferably no more than + / - 2%, over the length (L) of the channel (6).

Description

[Technical Field]

[0001] [Reference to Related Application] This application claims priority from German Patent Application No. 102019219179.7 filed on December 9, 2019, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to an optical element for reflecting radiation, in particular EUV radiation, comprising a substrate having a surface provided with a reflective coating and formed with at least one channel, preferably through which a coolant can flow (i.e. forming a cooling channel), and made of quartz glass, in particular titanium-doped quartz glass, or glass ceramic. The present invention further relates to a lithography system, in particular an EUV lithography system, comprising at least one such optical element. [Background technology]

[0003] In EUV lithography systems in the form of EUV lithography apparatus, reflective optical elements in the form of mirrors, in particular mirrors of the projection system, are subjected to a high radiation flux. With the increasing power of the EUV radiation source, the average power radiated on the mirror can amount to as much as 50 W, of which 1 / 3 to 1 / 2 is absorbed in the layer system of the reflective coating, leading to localized heating in the area of ​​the mirror or of the substrate. This heating leads to deformations of the surface of the mirror to which the reflective coating is applied, even when so-called zero-expansion materials are used (for example in the form of titanium-doped quartz glass, in particular ULE®, Asahi Zero, or in the form of glass ceramics, in particular Zerodur® or Clearceram®). These deformations are particularly related to the coefficient of (linear) thermal expansion (CTE) within the volume of the substrate or to the zero-crossing temperature (T ZC ) and that the coefficient of thermal expansion is significantly different from zero away from the zero cross temperature.

[0004] To reduce the temperature of mirrors in EUV lithography systems, it is known to introduce channels into the substrate through which a cooling fluid flows. In EUV lithography systems, such channels are used in particular for mirrors and support structures with substrates made of SiSiC (EUV collectors) or aluminum (grazing incidence mirrors). The channels are milled into the substrate during manufacturing, and a cover is welded or sintered onto it.

[0005] Since the mirror is mounted in a suspended manner, turbulence and vibrations (flow-induced vibrations, FIV) that may result from the flow of the (typically liquid) coolant should be avoided, as this has a generally adverse effect on image aberrations. However, the inside of the milled channels is generally rough, which is a disadvantage in terms of FIV.

[0006] Patent document 1 describes an optical element for reflecting radiation, which comprises a substrate having at least one tube through which a cooling fluid can flow. The substrate is produced by mechanical molding and sintering, in particular by hot isostatic pressing. In one example, both the substrate and the tube through which the cooling fluid can flow are made of glass, preferably titanium-doped quartz glass.

[0007] Selective laser-induced etching (SLE) can be used to create microchannels, shaped bores, etc. in transparent component parts made of, for example, quartz glass, borosilicate glass, sapphire, or ruby ​​(see, for example, Non-Patent Document 1). In selective laser-induced etching, light in the form of ultrashort pulsed laser radiation (ps or fs pulses) is focused into a volume of a transparent workpiece. The pulse energy is absorbed only within the focal volume as a result of multiphoton processes. In the focal volume, the transparent material changes its optical and chemical properties so that it can be selectively chemically etched without cracks or, in some cases, with microcracks. Depending on the laser parameters used, the material modification can be microcracks or other subsurface damage. By deflecting the focal point on the material, for example, with a microscanner system, successive regions are modified, which can then be removed by wet chemical etching. In wet chemical etching, the component is typically immersed in an etching solution for weeks or months, and the etching solution preferably (selectively) removes the modified material. By scanning or moving the laser radiation in the volume of the workpiece, any desired hollow structure, for example a channel, can be created.

[0008] A limitation of selective laser-induced etching of channels in quartz glass and titanium-doped quartz glass is the relatively low etching selectivity of about 1:500 to about 1:1500 compared to other transparent materials, such as sapphire, which has an etching selectivity of 1:10,000 (see Non-Patent Document 2). During etching, low etching selectivity results in channels that are wider at the edge of the component, where the etchant first strikes, than deeper within the component volume. Areas that are too wide can lead to uneven cooling of the component and may be mechanically persistent. In extreme cases, etching that is too aggressive can lead to short circuits between adjacent channels.

[0009] The channels in the substrate can also be used for cooling in other optical systems that primarily contain lens elements instead of mirrors, for example in lithography systems for the DUV wavelength range. However, the focus of the application in that case is generally not on the use of the channels for cooling, but on the integration of decoupling or actuators into each channel. In the context of this application, the EUV wavelength range is understood to be the wavelength range from about 5 nm to about 30 nm, and in the context of this application, the DUV wavelength range is understood to be the wavelength range from about 30 nm to about 370 nm. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] DE 10 2017 221 388 A1 [Non-patent literature]

[0011] [Non-Patent Document 1] www.ilt.fraunhofer.de / de / mediathek / prospekte / themenbroschuere-selektiveslaseraetzen.html [Non-patent document 2] "SLE with LightFab 3D Printer" by Lightfab GmbH ("www.lightfab.de"), retrievable from "www.lightfab.de / files / Downloads / SLE_3D_printed_glass.pdf" Summary of the Invention [Problem to be solved by the invention]

[0012] It is an object of the present invention to provide an optical element and a lithography system that can be cooled efficiently even in the case of high heat loads. [Means for solving the problem]

[0013] This object is achieved by an optical element of the aforementioned type, in which the channel has a length below the reflectively coated surface of at least 10 cm, preferably at least 20 cm, and in which the cross-sectional area of ​​the channel varies by no more than + / - 20%, preferably no more than + / - 10%, particularly preferably no more than + / - 2% over the length of the channel below the reflectively coated surface.

[0014] In the context of this application, a channel through which a coolant can flow is understood to mean, in the simplest case, a channel having two open ends, i.e., a channel forming a through channel in the substrate. If a plurality of channels interconnected by junctions is provided in the substrate, the (nominal) average channel cross section usually varies with each division into two or more channels or with each merging of two or more channels at each junction. In this case, a channel is understood to mean the channel section between two adjacent junctions, i.e., a channel with a constant (nominal) channel cross section.

[0015] In the following, the channels are also referred to as cooling channels, assuming that a coolant can flow through the channels, however, it is understood that the channels do not necessarily have to be used for cooling, but may be used, for example, for incorporating components into the substrate or for other applications.

[0016] In the optical element according to the invention, at least one channel of considerable length is used for cooling the optical element. Such a channel is particularly advantageous for efficient cooling of mirrors of the projection system of an EUV lithography apparatus, which have relatively large optically active mirror surfaces, for example with dimensions of 10 cm x 10 cm to 100 cm x 100 cm. As mentioned above, the large cross-sectional area of ​​a channel with a highly varying cross-section can lead to non-uniform cooling of the component and possibly mechanical damage to the component.

[0017] In the context of this application, a variation in the cross-sectional area of ​​a channel of + / - x% is defined as the average cross-sectional area A Mis understood to mean a deviation of + / - x% from the mean cross-sectional area A M is the maximum cross-sectional area A along the length of the channel MAX and minimum cross-sectional area A MIN and the average value (A M =(A MAX +A MIN ) / 2).

[0018] The surface provided with a reflective coating is understood to mean the surface or surface region of the substrate that is provided with the reflective coating. The surface or edge of the substrate may optionally extend laterally beyond this surface region. In this case, the length of the channel is understood to mean the length over which the channel extends in a (partial) volume region of the substrate that is located below the surface provided with the reflective coating in the thickness direction of the substrate. The part of the channel that protrudes laterally beyond this (partial) volume region does not have to fulfill the above-mentioned condition regarding the constancy of the cross-sectional area, even though this may be advantageous in principle. Preferably, the channel fulfills the above-mentioned condition regarding the constancy of the cross-sectional area over its entire length, i.e., also in a sub-volume region of the substrate that may be outside the sub-volume region of the substrate covered with the reflective coating.

[0019] The substrate is preferably monolithic. A monolithic substrate is understood to mean that the substrate is one piece and has no bonding surface. In contrast, a substrate with milled cooling channels has at least one bonding surface at the same height as the cooling channels. If all cooling channels are arranged substantially along a common curved or flat surface, such a bonding surface usually extends parallel to the surface containing the cooling channels. Such a bonding surface on an optical element or substrate can be identified, for example, by numerous small bubbles or refractive index jumps that occur between two bonded parts or sections of the substrate.

[0020] In principle, it is also possible for the substrate to have a joining surface in the region of the cooling channels or optionally elsewhere. In this case, two or more substrate parts are interconnected at the joining surface by a bonding method, generally omitting the use of a bonding agent. By way of example, the bonding method can be high-temperature bonding, direct bonding, silicate bonding, etc. If the joining surface extends into the region of the cooling channels or intersects with the cooling channels, the two substrate parts can be interconnected, particularly at the joining surface, before the cooling channels are formed in the substrate.

[0021] The production of monolithic or possibly multi-part substrates with at least one cooling channel having the properties already described above is usually carried out by the above-mentioned selective laser-induced etching, during which, as a result of multiphoton excitation, the chemical bond between silicon and oxygen in the case of a substrate made of quartz glass (SiO2) or between titanium and oxygen in the case of a substrate made of titanium-doped quartz glass can be broken.

[0022] For silica glass, such bonds can recombine or become saturated with hydrogen present in the glass. Therefore, it can be assumed that when hydrogen is present, the bonds between titanium and oxygen are saturated as well. Titanium-doped silica glass, when produced by direct deposition (e.g., ULE®), is 10 17 molecules / cm 3 Even when titanium-doped quartz glass is produced by the soot method, the hydrogen concentration is usually 10 15 molecules / cm 3 Higher.

[0023] Generally, the etch selectivity in conventional selective laser-induced etching is not sufficient to maintain the above requirement of a substantially constant cross section of the cooling channels over significant lengths of more than 10 cm or 20 cm for substrates made of quartz glass or titanium-doped quartz glass.

[0024] It is known that the bond saturation is metastable and can be broken again in quartz glass, for example, as a result of absorption of UV radiation. Therefore, to enhance etching selectivity, during the etching process, the etch front, i.e., the area where the etching solution is attacking the substrate material, can be irradiated not necessarily with ultrashort-pulse laser radiation, but with laser radiation used to modify substrate materials, e.g., with wavelengths of about 1 μm, or with radiation of other wavelengths, e.g., UV radiation. In particular, it is possible to perform actual damage or modification to the substrate material during selective laser-induced etching in the etching bath. While this increases the processing time in the exposure or scanner system, the acceleration of the etching rate can be such that it is still economical to equip each etching apparatus with an exposure or scanner system for selective laser-induced etching. Etching selectivity within the scope of selective laser-induced etching of (titanium-doped) quartz glass can be optionally enhanced by methods other than irradiating the etch front. The etching bath or etching solution can be a (weakly) acidic, substantially neutral, or alkaline etching solution. The advantage of a substantially neutral etching solution is that it minimizes roughening.

[0025] In one embodiment, the channel is 100 μm 2 ~25mm 2 , especially 1mm 2 ~25mm 2 Channels for cooling optical elements introduced into the material of the substrate using conventional manufacturing methods, such as milling, typically have a relatively large cross-sectional area, typically about 1 cm x 1 cm, and typically have a high degree of roughness, which may be due to the etching process subsequent to milling, particularly required to deal with the mechanical stresses generated during milling.

[0026] However, the above order, i.e., several hundred μm 2 or mm 2It has been found that the use of channels with smaller cross sections, on the order of 1 L / min, is more advantageous for cooling. To avoid FIV, the flow through the channels should remain as clearly laminar as possible, i.e., the Reynolds number should be significantly below 1000. Since the Reynolds number is smaller the smaller the diameter of the channel, it has been found that providing channels with small cross sections is advantageous in terms of FIV. Even with such small cross sections, it is possible to achieve a total volumetric flow rate of cooling fluid, summed over all parallel-connected channels, of the order of 1 L / min or more, which has been found to be particularly advantageous for cooling optical elements.

[0027] In yet another embodiment, the cross-sectional area of ​​the channel has a height to width ratio of less than 5:1. Generally, it is advantageous from a structural standpoint for the channel height to be greater than the width, as this reduces the risk of the channel being indented when polishing the surface. In this case, the channel height is measured in the thickness direction of the substrate, i.e., perpendicular to the bottom of the substrate. The width is measured in a direction perpendicular to the thickness direction of the substrate and aligned perpendicular to the longitudinal direction of the channel at each location within the substrate. It is understood that the channels can have rectangular or square cross sections, or circular or elliptical cross sections.

[0028] In yet another embodiment, the surface to which the reflective coating is applied has a maximum dimension in at least one direction perpendicular to the thickness direction of the substrate of 10 cm to 100 cm. If the surface of the optical element is rotationally symmetric, the surface has a diameter in the range of 10 cm to 100 cm. If the surface of the optical element is not rotationally symmetric, the maximum dimension of the substrate refers to the maximum distance between two points along the edge of the substrate or optical surface in each direction.

[0029] In yet another embodiment, at least one channel extends at a substantially constant distance from the surface provided with the reflective coating, said surface being particularly curved, and the distance is preferably 1 to 3 times, in particular 1.5 to 2.2 times, the distance between adjacent channels.

[0030] In the context of this application, a substantially constant distance of the channels from the surface is understood to mean that the distance of the center of the cross-sectional area of ​​the channels from the surface (measured in the thickness direction, i.e., in the Z direction) varies by no more than + / - 20%, preferably no more than + / - 10%, in particular no more than + / - 5% along the length of the channel. As in the case of the channel cross-section, a variation in the distance of the channels of + / - x% is understood to mean a deviation of + / - x% from the average distance of the channels from the surface. The average distance is defined as the average value of the maximum and minimum distances from the surface along the length of the channel. For cooling purposes, it has been found advantageous if the (constant) distance is 1 to 3 times, or preferably 1.5 to 2.2 times, the distance between adjacent channels. The distance between adjacent cooling channels is measured between the centerlines of two adjacent cooling channels.

[0031] If the optical surface of the substrate has a curvature in one or possibly two directions, the channels follow the curvature of the optical surface, i.e., the channels do not extend linearly in the thickness direction of the substrate. If the optical surface does not have a curvature in the direction of channel progression, or if the optical surface is flat, the channels may extend linearly in the thickness direction. The channels may also have a curvature in a plane perpendicular to the thickness direction. By way of example, the channels may have a spiral, serpentine, etc. For examples of (cooling) channels with different geometries, see DE 10 2009 039 400 A1, the contents of which are incorporated herein by reference in their entirety.

[0032] In yet another embodiment, the substrate has a plurality of channels, the (maximum) distance between adjacent channels being preferably less than or equal to the (usually constant) distance of the channels from the surface provided with the reflective coating. By way of example, the channels through the substrate may be aligned in parallel. In order to generate a temperature distribution at the substrate that is as uniform as possible, it has been found to be advantageous if adjacent (cooling) channels are not too far apart from each other. As already mentioned above, the distance between adjacent cooling channels is measured between the centerlines of two adjacent cooling channels.

[0033] In yet another embodiment, the substrate is made of titanium-doped quartz glass, and the variation in the zero-cross temperature of the titanium-doped quartz glass in the volumetric region of the substrate between the surface and at least one channel is 10 K or less peak-to-valley, preferably 3 K or less peak-to-valley, and / or the variation in the thermal expansion coefficient of the titanium-doped quartz glass in this volumetric region is less than 0.5 K / cm. The titanium-doped quartz glass has a so-called zero-cross temperature at which the temperature-dependent thermal expansion coefficient crosses zero. The thermal expansion coefficient of the titanium-doped quartz glass, and therefore the zero-cross temperature, is adjusted by the titanium content of the quartz glass. When manufacturing the substrate, care must be taken to ensure that the titanium content is as constant as possible, especially in the volumetric region between the surface and each channel, in order to achieve a zero-cross temperature that is as uniform as possible and to minimize the position-dependent gradient of the thermal expansion coefficient of the substrate material. In (further) volumetric regions of the substrate that are further from the optical surfaces than the channels, the requirements for the thermal properties of the substrate material are less stringent. For example, it may be sufficient if the zero cross temperature of the titanium-doped quartz glass varies peak-to-valley by no more than 20 K in said further volume region and / or if the thermal expansion coefficient of the titanium-doped quartz glass varies by less than 2 K / cm in said further volume region.

[0034] In yet another embodiment, the channel has an inner roughness R a is less than 5 μm rms, preferably less than 2 μm rms. a is understood to mean the root mean square roughness. The above roughness values ​​can be achieved when creating channels by selective laser-induced etching.

[0035] A further aspect of the invention relates to a lithography system, in particular an EUV lithography system, comprising at least one optical element as already described above. The EUV lithography system can be an EUV lithography apparatus for exposing wafers or any other optical apparatus using EUV radiation, for example an EUV inspection system for inspecting masks, wafers, etc. used in EUV lithography. However, the optical element can also be used in other optical systems, for example in lithography systems, in particular in lithography apparatus for the DUV wavelength range.

[0036] In particular in the latter case, at least one channel may serve to integrate an assembly such as an actuator, a decoupling, etc. into the substrate. It is therefore not essential that a coolant can flow through the channel, which may have only one end that is open to the periphery, i.e., in the form of a blind hole. In contrast, a channel through which a coolant can flow is understood to mean a channel which itself has two ends that are open to the periphery, i.e., forms a through channel, or, in the case of a substrate having several interconnected channels, a channel through which a coolant can flow via both ends that are open to the periphery.

[0037] In one embodiment, the lithography system comprises a cooling device for flowing a coolant, in particular a cooling liquid, through at least one channel in the substrate of the optical element. The cooling device may have corresponding ports and may also have lines for supplying and removing a cooling liquid, e.g., cooling water, to each channel. The cooling device may include a pump or the like for circulating the cooling liquid. An alternative option is for the cooling device to communicate with a cooling water source via a cooling liquid port, e.g., a cooling water port.

[0038] In one further development, the cooling device is designed to make the coolant flow through at least one channel in the substrate of the optical element with a total volumetric flow rate of 1 L / min or more. The total volumetric flow rate of the cooling fluid is understood to mean the sum of the volumetric flows of the cooling fluid flowing in parallel through all channels or through the channel sections of each branching level. A high volumetric flow rate of this order is required to dissipate a thermal power of the order of 10 W or more during operation of the optical element.

[0039] In yet another development, the cooling device is designed to cause the coolant to flow through at least one channel in the substrate of the optical element at a Reynolds number of less than 1000. To avoid FIV, it is advantageous for the coolant flow to remain in a clearly laminar flow region, i.e., a Reynolds number significantly below 1000. Since the Reynolds number is smaller the smaller the diameter of the channel, it has been found to be advantageous in terms of FIV to provide channels with small cross-sections. The above condition regarding the Reynolds number should be met in at least a partial volume of the substrate below the surface provided with the reflective coating. It is advantageous, but not essential, that the above condition be met over the entire length of the cooling channel within the substrate volume.

[0040] Generally, as is conventional, the Reynolds number is Re=v M d / ν where V M where ρ denotes the average flow velocity of the refrigerant through the (constant) average cross-sectional area of ​​the channel, d denotes the (average) diameter of the channel assuming it is circular, and ν denotes the viscosity of the cooling fluid. As is generally conventional, the average flow velocity denotes the average value of the flow velocity at each point in the cross-sectional area of ​​the channel. For channels with non-circular cross-sections, the hydraulic diameter is used to calculate the Reynolds number.

[0041] Further features and advantages of the invention are apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawings which show the details essential to the invention, and from the claims. The individual features can each be realized alone and separately or in any desired combination in a variant of the invention.

[0042] Exemplary embodiments are shown in the schematic drawings and explained in the following description. [Brief explanation of the drawings]

[0043] [Figure 1a] 1 shows a schematic diagram of an optical element in the form of an EUV mirror. [Figure 1b] 1 shows a schematic diagram of an optical element in the form of an EUV mirror. [Figure 1c] 1 shows a schematic diagram of an optical element in the form of an EUV mirror. [Figure 2] 1 shows a schematic diagram of an EUV lithography apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0044] In the following description of the drawings, the same reference numbers are used for identical or functionally identical components.

[0045] 1a to 1c show a schematic structure of an optical element 1 that reflects EUV radiation 2. The optical element 1 comprises a substrate 3 having a surface 4 provided with a reflective coating 5, which is designed to reflect EUV radiation 2. The reflective coating 5 comprises alternating individual layers of silicon and molybdenum to reflect EUV radiation 2 at an operating wavelength of approximately 13.5 nm. Depending on the operating wavelength used, combinations of other materials are possible as well, such as molybdenum and beryllium, ruthenium and beryllium, or lanthanum and B4C. The reflective multilayer coating 5 typically comprises further functional layers that will not be discussed in more detail here.

[0046] In the example shown in FIGS. 1a to 1c, the material of the substrate 3 is titanium-doped quartz glass (ULE®), which has a small coefficient of thermal expansion CTE at the operating temperature of the optical element 1 and a zero-cross temperature T ZCAlternatively, the substrate 3 may be made from a different material with a low coefficient of thermal expansion (CTE), for example titanium doped quartz glass in the form of Asahi Zero, or a form of glass ceramic, for example Zerodur® or Clearceram®.

[0047] Figure 1c shows a plan view of the surface 4 of the substrate 3 on which the optical element 1 or reflective coating 5 is applied, Figure 1a shows a cross section of the substrate 3 along the XZ plane extending through the centre of the surface 4, and Figure 1b shows a cross section of the substrate 3 along the YZ plane also extending through the centre of the substrate 3.

[0048] In the example shown in Figures 1a-1c, a plurality of channels 6, for example 15, are formed in the substrate 3, each forming a through-channel in the substrate 3 through which a coolant 7, for example in the form of cooling water, can flow. The channels 6 will therefore be referred to hereinafter as cooling channels 6. As can be seen from Figure 1c, the cooling channels 6 run parallel to one another through the substrate 3 along the Y direction, and each have a different length L. Figure 1b shows a cooling channel having a maximum length L of 25 cm. As can be seen from Figure 1c, most of the other cooling channels 6 similarly have somewhat longer lengths L of 20 cm or more.

[0049] The length of the cooling channel 6 shown in FIG. 1b is the maximum dimension E of the surface 4 of the substrate 3 in the Y direction. Y corresponds to the maximum dimension E of the substrate 3 in the x-direction, which in the illustrated example is approximately 35 cm. X The maximum size E of the surface 4 in the XY plane or in the projection onto the XY plane perpendicular to the thickness direction Z of the substrate 3 is smaller than X , E Y Typical values ​​for range from about 10 cm to about 100 cm.

[0050] As can be seen from FIG. 1a, the surface 4 provided with the reflective coating 5 has a curvature in the XZ plane or in the X direction. In contrast, the surface 4 of the substrate 3 is not curved along the Y direction (see FIG. 1b). In the example shown in FIGS. 1a-1c, the cooling channels 6 extend at a substantially constant distance D in the Z direction from the curved surface 4, which in the example shown is about 1 cm. In this case, the distance D is measured in the Z direction between the center of the cross-sectional area of ​​each cooling channel 6 in the Z direction and the surface 4. A substantially constant distance D of the cooling channels 6 from the surface is understood to mean that the distance D varies by no more than + / −20%, preferably no more than + / −10%, in particular no more than + / −5% from the average distance over the length of the cooling channels 6, the average distance being the average of the maximum and minimum distances of the cooling channels 6 from the surface 4 over the entire length L of the cooling channels 6.

[0051] In the example shown in Figures 1a-1c, not only is the distance D constant over the length L of a single cooling channel 6, but each individual cooling channel 6 is also positioned at the same distance D in the Z direction from the surface 4. As is clear from Figure 1a, this results in different cooling channels 6 being positioned at different heights in the Z direction within the substrate 3. For cooling purposes, it has been found to be advantageous if the (constant) distance D is 1 to 3 times, in particular 1.5 to 2.2 times, the distance d between adjacent channels 6. In this case, the distance d is measured between the centerlines of two adjacent cooling channels 6, as is clear from Figure 1c.

[0052] For cooling purposes, it has been found to be advantageous if the distance d between adjacent cooling channels 6 is less than or equal to the distance D between the cooling channels 6 and the surface 4 provided with the reflective coating 5 .

[0053] The cooling channels 6 are formed in the monolithic substrate 3 by selective laser-induced etching. During selective laser-induced etching, an ultrashort pulsed laser beam is focused through the surface 4 into a focal volume within the substrate 3, which is transparent to the laser beam. In this case, the pulse energy is absorbed only within the focal volume as a result of a multiphoton process. In the focal volume, the optical and chemical properties of the transparent material of the substrate 3 are altered to allow selective chemical etching. By deflecting the focal point of the material of the substrate 3, for example with a scanner, successive regions are modified and can be removed by wet chemical etching, resulting in the formation of the cooling channels 6 shown in Figures 1a-1c.

[0054] To increase selectivity during wet-chemical etching, the substrate 3 is irradiated with laser radiation, not necessarily in the form of ultrashort pulses, but with a wavelength of about 1 μm, or with radiation of other wavelengths, such as UV radiation, used to modify the material of the substrate 3 during the etching process. In particular, selective laser-induced etching can be performed directly in the etching bath. In this case, the ultrashort-pulse laser radiation is typically focused into a focal volume that simultaneously forms the etch front of the etching process. In this way, the modified material is already released from the substrate 3 during selective laser-induced etching, thereby increasing the etching selectivity in the case of titanium-doped quartz glass.

[0055] The cooling channel 6 shown by way of example in FIG. 1b has a cross-sectional area A (see FIG. 1a) that varies by about + / - 1.5% over the length L of the cooling channel 6. A variation in the cross-sectional area of ​​the cooling channel 6 of + / - x% is M is understood to mean a deviation of + / - x% from the mean cross-sectional area A M is the maximum cross-sectional area A along the length of the channel MAX and minimum cross-sectional area A MIN and the average value (A M =(A MAX +A MIN ) / 2) To make the representation of Figure 1b clearer, the maximum cross-sectional area A MAX and minimum cross-sectional area A MINIn principle, the variation in the cross-sectional area A of the cooling channel 6 can be larger than in the example shown in Figure 1b, e.g. it can vary by up to + / - 20% or even up to + / - 10% of the total length L of the cooling channel 6.

[0056] In principle, it would be advantageous if all cooling channels 6 had a length L of less than 20 cm or even 10 cm, and all cooling channels 6 met the above criteria for constancy of the cross-sectional area A. In contrast to what is shown in Figures 1a-1c, the substrate 3 may also comprise a network of several cooling channels 6 interconnected by junctions. In order to generate a constant flow rate, the (nominal) average channel cross section A M changes at each junction for each division into two or more cooling channels 6 or for each junction of two or more cooling channels 6 (at each branch level). In this case, the above criteria relate to the cooling channels 6 at each branch level between two junctions or between a junction and an opening at the edge of the substrate. The cooling channels at each branch level connected in parallel with respect to the through-flow have the same average cross-sectional area A M It is advantageous to have

[0057] To effectively cool the optical element 1, the cooling channel 6 is 100 μm 2 ~25mm 2 , especially 1mm 2 ~25mm 2 The average cross-sectional area A M It has been found to be advantageous to have a cross-sectional area A of the cooling channels 6. Although the cross-sectional area A of the cooling channels 6 is shown as a circle in FIG. 1a, it may have a different geometric shape. Ideally, the cross-sectional area A of each cooling channel 6 has a height h in the thickness direction of the substrate 3 (Z direction) that is very slightly larger than the width b of the cross-sectional area A of the cooling channel 6, which extends in the X direction in the example shown. The ratio of the height h to the width b of each cooling channel 6 holds true at each position along the length L of the cooling channel 6 (in the Y direction): h / b<5:1. By way of example, the ratio of the height h to the width b of the cooling channel 6 holds true: h / b>1.0 or >0.9.

[0058] The cooling channels 6 are produced by selective laser-induced etching, so that the cooling channels 6 have a roughness R a is less than 5 μm rms, in particular less than 2 μm rms, ie a significantly lower roughness than if the cooling channels 6 were made by machining, for example by milling.

[0059] 1a shows a dashed curve along which the centres of the cross-sectional areas A of all cooling channels 6 are located. The centres or centrelines of all channels 6 run through this (imaginary) line or through the (imaginary) boundary line between a volume region 3a of the substrate 3 formed between the surface 4 and the cooling channels 6 and a further volume region 3b located further away from the surface 4 of the substrate 3.

[0060] The zero-cross temperature T of the titanium-doped silica glass substrate 3 ZC is a peak-to-valley temperature T of the titanium-doped quartz glass that varies less than 10 K, especially less than 3 K, in the volume region 3a close to the surface. Furthermore, the coefficient of thermal expansion CTE of the titanium-doped quartz glass varies less than 0.5 K / cm in the volume region 3a close to the surface. The requirement for homogeneity of the titanium-doped quartz glass material is usually relaxed in the volume region 3b more remote from the surface 4. Here, the zero-cross temperature T ZC It is sufficient if the variation is peak-to-valley of 20 K or less. The spatial variation of the coefficient of thermal expansion CTE of the titanium-doped quartz glass is also larger than that of the volume region 3a close to the surface, and can be, for example, less than 2.0 K / cm.

[0061] The optical element 1 shown in FIGS. 1a to 1c can be used in different optical systems, for example in an EUV lithography apparatus 100, the schematic structure of which is explained below with reference to FIG.

[0062] The EUV lithography apparatus 100 shown in Fig. 2 includes a beam shaping and illumination system 102 and a projection system 104. The beam shaping and illumination system 102 and the projection system 104 are each provided in a vacuum housing shown in Fig. 2, and each vacuum housing is evacuated using an evacuation device (not shown). The vacuum housing is surrounded by a machine room (not shown) in which a driving device for mechanically moving or setting optical elements is provided. Furthermore, an electric controller and the like may also be provided in the machine room.

[0063] The EUV lithography apparatus 100 includes an EUV light source 106. The EUV light source 106 can be, for example, a plasma source (or a synchrotron) that emits radiation 108 in the EUV range, for example in the wavelength range of 5 nm to 20 nm. In the beam shaping and illumination system 102, the EUV radiation 108 is focused and a desired operating wavelength is filtered from the EUV radiation 108. Since the EUV radiation 108 generated by the EUV light source 106 has a relatively low transmittance in air, the beam guiding spaces in the beam shaping and illumination system 102 and the projection system 104 are evacuated.

[0064] 2 includes five mirrors 110, 112, 114, 116, and 118. After passing through beam shaping and illumination system 102, EUV radiation 108 is directed to a photomask (reticle) 120. Photomask 120 is also in the form of a reflective optical element and may be located outside systems 102, 104. Furthermore, EUV radiation 108 may be directed to photomask 120 by mirror 122. Photomask 120 has a structure that is imaged by projection system 104 at a reduced size, such as onto wafer 124.

[0065] The projection system 104 (also referred to as a projection lens) has six mirrors M1-M6 for imaging the photomask 120 onto the wafer 124. It should be noted that the number of mirrors in the EUV lithography apparatus 100 is not limited to the number shown; more or fewer mirrors may be provided. Furthermore, the mirrors are typically curved on their front sides for beam shaping, as already discussed above in relation to Figures 1a-1c. By way of example, the optical element described in relation to Figures 1a-1c may be one of the six mirrors M1-M6 of the projection system 104.

[0066] 2 shows a cooling device 126 for cooling the first mirror M1 of the projection system 104. The cooling device 126 is designed to supply a coolant 7, in the example shown, in the form of cooling water, and for this purpose has supply and removal lines, not shown, for supplying the coolant 7 to the channels 6 or removing the coolant 7 from the channels 6. In the example shown, the cooling device 126 is designed to cause the coolant 7 to flow through the channels 6 in the substrate 3 of the optical element M1 with a total volumetric flow rate of 1 L / min or more. The total volumetric flow rate of the cooling fluid 7 is understood to mean the sum of the volumetric flows of the coolant 7 flowing in parallel through all the channels 6. The cooling device 126 is designed to cause the coolant 7 to flow through each of the channels 6 formed in the substrate 3 with a Reynolds number of less than 1000. For this purpose, the cooling device 126 generates in each channel 6 a volumetric flow rate or average flow velocity of the refrigerant 7 that ensures laminar flow of the refrigerant 7 in the channel 6 under given geometric conditions, i.e. a given (hydraulic) diameter of the channel 6 and a given viscosity of the refrigerant 7.

[0067] Instead of a reflective coating for EUV radiation 2, the optical element 1 described above can also be provided with a reflective coating for radiation of a different wavelength range, for example for the DUV wavelength range. Generally, the requirements regarding the thermal expansion of the substrate 3 of such a reflective optical element 1 are less stringent, so that substrate materials other than those described above can be used, for example conventional quartz glass. In this case, in particular, the channels 6 formed in the substrate 3 may not necessarily be necessary for or suitable for the flow of a coolant. In this case, the channels 6 can serve, for example, to integrate component parts, such as actuators, into the substrate 3 or to form decouplings.

Claims

1. An optical element (1) for reflecting radiation, a substrate (3) made of quartz glass or glass ceramic, having a surface (4) with a reflective coating (5) and at least one channel (6) through which a coolant (7) can flow; An optical element (1) comprising: the channel (6) has a length (L) below the surface (4) on which the reflective coating (5) is applied of at least 10 cm, and the cross-sectional area (A) of the channel (6) varies by no more than + / - 20% over the length (L) of the channel (6); a total volumetric flow rate of the refrigerant (7) of at least 1 L / min is achieved over all the channels (6); The at least one channel (6) extends a substantially constant distance (D) from the surface (4) on which the reflective coating (5) is applied, the surface having a curvature, and the at least one channel follows the curvature of the surface.

2. 2. An optical element according to claim 1, wherein the substrate (3) is monolithic.

3. 3. An optical element according to claim 1 or 2, wherein the channel (6) has a diameter of 100 μm. ２ ~25mm ２ The average cross-sectional area (A Ｍ ) an optical element having

4. An optical element according to any one of claims 1 to 3, wherein the cross-sectional area (A) of the channel (6) has a height (h) to width (b) ratio of less than 5:

1.

5. In the optical element according to any one of claims 1 to 4, the surface (4) on which the reflective coating (5) is applied has a maximum dimension (E) of 10 cm to 100 cm in at least one direction (X, Y) perpendicular to the thickness direction (Z) of the substrate (3). Ｘ , E Ｙ ) an optical element having

6. 6. The optical element according to any one of claims 1 to 5, wherein the substantially constant distance (D) is between 1 and 3 times the distance (d) between adjacent channels (6).

7. 7. An optical element according to claim 1, comprising a plurality of channels (6), wherein the distance (d) between adjacent channels (6) is preferably equal to or less than the distance (D) of the channels (6) from the surface (4) on which the reflective coating (5) is applied.

8. 8. The optical element according to claim 1, wherein the substrate (3) is made of titanium-doped quartz glass, and the zero-cross temperature (T ＺＣ ) peak-to-valley fluctuation of 5K or less.

9. 9. The optical element according to claim 1, wherein the channel (6) has a roughness R ａ An optical element having a radii of less than 2 μm rms.

10. Lithography system, comprising at least one optical element (1, M1-M6) according to any one of claims 1 to 9.

11. 11. The lithography system of claim 10, further comprising a cooling device (126) for causing a coolant (7) to flow through at least one channel (6) in a substrate (3) of the optical element (M1).

12. 12. A lithography system according to claim 11, wherein the cooling device (126) is designed to flow the coolant (7) through the at least one channel (6) of the substrate (3) of the optical element (M1) at a total volumetric flow rate of 1 L / min or more.

13. 13. A lithography system according to claim 11 or 12, wherein the cooling device (126) is designed to cause the coolant (7) to flow through the at least one channel (6) in the substrate (3) of the optical element (M1) at a Reynolds number of less than 1000.

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