This application claims priority to German Patent Application 10 2019 205 265.7 filed April 11 , 2019, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

Background of the invention

The invention relates to two methods for producing a glass body which has at least one cooling channel, preferably a plurality of cooling channels. The invention also relates to a reflective optical element, in particular for reflecting EUV radiation, which has a substrate having at least one cooling channel, and also to an optical arrangement, in particular an EUV lithography system, which comprises at least one such reflective optical element.

In an EUV lithography apparatus, reflective optical elements in the form of mirrors, especially in the form of mirrors of a projection system, are subjected to high radiant power, causing warming of the mirrors or the mirror substrate. The warming of the mirror substrate causes deformations of the mirror surface, impairing the imaging quality of the projection system. In order to counter this problem, substrates used are made typically of materials which have a low coefficient of thermal expansion.

For reducing the temperature of the mirrors it is known practice to introduce, into the mirror substrate, cooling channels through which a cooling fluid flows. The production of the cooling channels necessitates the incorporation or introduction, into the substrate, of tubes and/or of passage channels having a geometry which is dependent on the particular mirror. When the substrate is worked, stresses may occur in a glass material, and may likewise have adverse consequences, since during the shaping of the mirror, as for example when milling the mirror surface, they can lead to unanticipated changes in shape.

DE 102017221388 A1 describes a method for producing a component, through which a cooling fluid can flow, for a lithography system, said method comprising the steps of: surrounding a tube section disposed within a capsule, it being possible for the cooling fluid to flow through this tube section, with a pulverulent filling material, with at least one end of the tube section projecting from the capsule, and producing the component by hot isostatic pressing of the pulverulent filling material arranged in the capsule.

WO 2012/126830 A1 describes an optical element for a projection exposure apparatus in semiconductor lithography, said apparatus having an optically active surface and at least one cooling component for cooling the optical element. The cooling component is connected to at least two separate cooling circuits and is designed in such a way that the optically active surface can be cooled to a greater extent in at least one sub-region than in another sub-region.

US 7,591 ,561 B2 disclosed a liquid-cooled mirror for EUV lithography, where at least one microchannel is arranged beneath an optical surface. In one example, a base body is provided, in which a microchannel has been formed. The base body, at a surface at which the microchannel is formed, is connected or coupled with a cap in order to seal the microchannel along its periphery.

Object of the invention

It is an object of the invention to specify methods for producing a glass body wherein the glass material is minimally influenced by the introduction of a cooling channel, in particular using a placeholder which is introduced into the cooling channel. Subject matter of the invention

This object is achieved, according to a first aspect, by a method of the type stated at the outset, comprising: providing a first partial body and a second partial body of the glass body, forming at least one cooling channel by working, in particular mechanically working, the glass material at a surface of the first partial body and/or of the second partial body, and producing the glass body by joining the first partial body to the second partial body at the worked surface by high-temperature bonding.

In this aspect of the invention, it is proposed to form at least one, generally a plurality of, cooling channel(s) in at least one of the two partial bodies which are subsequently assembled to form the glass body, by means of mechanical working, typically by milling. When the two partial bodies are connected along the worked surface, the cooling channels are closed along their periphery. In the case of joining by high-temperature bonding (direct bonding), the two partial bodies are typically heated to a temperature of more than around 1000°C, in general between around 1300°C-2000°C, so that the partial bodies or the respective surface undergo partial melting and the two partial bodies are joined to one another along the worked surface, without the use of a joining agent. The cooling channels may be formed only in the first of the two partial bodies, this being the lower partial body, though it is also possible for the cooling

channels - or a part of the cross section of the cooling channels - to be formed in the second, upper partial body or in both partial bodies. In the case of joining by high-temperature bonding, the second partial body is placed onto the first partial body generally with a surface whose geometry corresponds to that of the worked surface.

In the variants described below, it is assumed that the cooling channels, on production of the glass body by joining of the two partial bodies by high- temperature bonding, are free from placeholders, from tubular bodies, etc. In one variant, during the working cooling channels are formed which are separated from one another by webs, where a web width A in lateral direction is at least 10 times, preferably at least 20 times, more preferably at least 50 times, in particular at least 100 times the width B of a respective cooling channel. The web width A corresponds typically to the lateral or side spacing A between adjacent cooling channels in a direction transverse to a main direction of the cooling channels, along which the cooling channels extend through the glass material. The cooling channels are introduced into the glass material generally with constant spacing A, i.e. with constant web width A, though this is not mandatory. The ratio between the web width A and the width B of the cooling channels ought to be comparatively large, in order to prevent the webs formed between the cooling channels in the glass material widening or flipping under the effect of gravity during the high-temperature bonding. The width B of the cooling channels ought preferably to be between around 0.5 mm and around 3 mm.

In another variant, during the working, the cooling channels are formed with a depth C which is between 2 times and 10 times the width B of a respective cooling channel. This is in particular favourable if the ratio between the web width A and the width B of the cooling channels is comparatively large, in order to ensure sufficient flow of a cooling liquid through the cooling channels.

In a further variant, during the working, at least two channel-free regions are formed on the surface that have a width D in lateral direction which corresponds to at least three times the depth of the cooling channels (D > 3 C) and/or to at least five times the web width A of the webs (D > 5 A). In particular in the event that the channel fraction is comparatively great, i.e. the ratio between the web width A and the width B of a respective cooling channel is comparatively small (A/B < 20) and/or the cooling channels are very deep, so that the ratio between the width B and the depth C of the cooling channels is comparatively small (B/C < 5), it has proved to be favourable to form at least two channel-free regions which have the width indicated above. The channel-free regions may in particular be formed adjacent to the lateral edges of the surface of the respective partial body, though this is not mandatory.

In a further variant, the providing comprises the dividing of the glass body into the first partial body and into the second partial body along a surface, in particular a curved surface. The glass body may be divided by sawing along a desired, optionally curved, contour. A curved surface can be generated when dividing the glass body by means, for example, of separative ball grinding. This is especially favourable if the glass body is used for producing a mirror substrate whose mirror surface is itself curved. In the event that the curvature of the surface produced on dividing corresponds substantially to the curvature of the mirror surface, the cooling channels, which are made preferably in the vicinity of the mirror surface, may be formed in the glass body in an

approximately constant spacing from the mirror surface.

In another variant, before the two partial bodies are joined, a placeholder made of a temperature-resistant material is introduced into a respective cooling channel. The use of placeholders is favourable in order to prevent the webs formed between the cooling channels from broadening or possibly flipping (see above) during high-temperature bonding. With the use of placeholders there is generally no need to observe the conditions described earlier on above with regard to the web width and/or the ratio between the web width and the width of the cooling channels, etc., although to observe them is possible in principle. The placeholder or placeholders may be removed from the cooling channels after high-temperature bonding or - in the case of a tubular configuration - may be intended to remain in the glass body or in the respective cooling channel.

A further aspect of the invention relates to a method for producing a glass body having at least one cooling channel, preferably having a plurality of cooling channels, comprising: embedding at least one placeholder made of a temperature-resistant material into the glass material of the glass body during the production of the glass body, to form the at least one cooling channel in the glass body. In this aspect of the invention, it is proposed to embed the placeholders into the glass material during production of the glass body. A prerequisite for this is that the placeholders have a certain robustness. This is generally ensured if the placeholders consist of a metal or of a largely densely sintered ceramic.

In one variant, the method comprises: providing a first partial body and a second partial body of the glass body, and producing the glass body by joining the first partial body to the second partial body by high-temperature bonding, with embedding of the at least one placeholder between the two partial bodies. As in the case of the first aspect, the first and second partial bodies may be provided by dividing the glass body. In contrast to the first aspect, however, in the present case the partial body or bodies are not worked mechanically in order to introduce cooling channels by milling or otherwise; instead, before the high-temperature bonding, the placeholders are introduced or inserted between the two partial bodies. In this case, during the high-temperature bonding itself, the placeholders are embedded into the glass material, although there may possibly be increased blistering in the region of the former parting surface between the partial bodies.

In an alternative variant, the method comprises: depositing glass material on a carrier, embedding the placeholder into the deposited glass material, and cooling the glass material to produce the glass body. In the case of direct deposition of the glass material, as described for example in US 5,970,751 for an Si02-Ti02 glass material, the titanium-doped glass material is generally deposited in a container and/or on a carrier, in the form of a turntable, for example, of a refractory oven. In this case the deposited glass material solidifies with the exception of a few millimetres directly below the hot surface on which the material is deposited. The placeholders may optionally be introduced directly into the container of the refractory oven, if the deposition process is halted after a certain running time. In this case, a network of placeholders, which is stabilized mechanically by cross-connectors, for example, may be laid onto the already solidified glass material, and the deposition process can be continued thereafter, so that the placeholders are embedded into the glass material. Alternatively, the placeholder may optionally be introduced before or during the filling of liquid glass material into a container, especially if the glass material comprises fused silica or a glass-ceramic, for example Zerodur®. The placeholders may optionally be introduced into the container even before the deposition of the glass material, and fixed there in a suitable way.

In one variant, the placeholder is tubular and remains in the cooling channel after the production of the glass body. In this case, the cooling liquid is not passed directly through the cooling channel formed in the glass material, but instead through the cavity formed in the tubular placeholder.

In an alternative variant, the placeholder is removed from the glass body after the production of the glass body, to form the cooling channel. In this case the placeholder does not remain in the cooling channel. The cooling liquid is passed directly through the cooling channel and in this case is in contact with the glass material. There are various options for the removal of the placeholder, of which a number are described in detail later on below.

The majority of temperature-resistant materials or solid bodies have a higher thermal expansion than the glass material, and so on cooling the material of a respective placeholder contracts to a greater extent than the glass material. Typically, therefore, on cooling, the placeholders detach from the glass material uncontrolledly over large parts of their periphery. This is generally favourable to the facilitated removal of the placeholders. Even before the detachment of the placeholders, however, local tensile stresses may be exerted by the

placeholders on the glass material, and may possibly in part be no longer able to relax before complete solidification, and are frozen in. Mechanical stresses of this kind are unfavourable, since in the case of a change in the mirror shape, by milling of the mirror surface, for example, they can lead to unanticipated changes in shape. Likewise, because of the uncontrolled detachment of the placeholders from the glass material, the heat transfer and thus the cooling rate are uneven, and this may lead to poorly controlled changes in the homogeneity of the coefficient of thermal expansion, because the coefficient of thermal expansion is dependent on the cooling rate in particular in the temperature range between around 1300°C and 800°C. Described below are a number of options for how the introduction of mechanical stresses into the glass material on detachment of the placeholders can be avoided or at least reduced. In the event that the cooling channels are introduced into the glass material by milling, the placeholders may optionally be designed to be slightly smaller than the cross section of a respective cooling channel. Suitable dimensions of the cross section of the placeholders may be calculated, for example, using finite-element methods.

In one variant, the placeholder has a base body, in particular a solid base body, on whose periphery there are lamellae formed which extend in a longitudinal direction and are separated from one another by gaps. In this case, typically, millings extending in a longitudinal direction are introduced into a solid placeholder, and lead to the formation of, for example, L-shaped or T-shaped lamellae. With the exertion of a small tensile force on a large part of their surface, the lamellae can remain joined to the glass material, and detach only via their web or via their web like attachment to the solid base body. The width of a respective gap between the lamellae may be for example between around 0.5 mm and around 3 mm. In one variant, the placeholder is tubular and either has slots spaced apart from one another or is configured as a spiral tube. In the former case, the slots may be designed, for example, as slots which extend in a longitudinal direction but which typically do not extend continuously along virtually the entire length of the tubular placeholder, but instead are arranged at an offset to one another and have only a comparatively low length, so that the tubular placeholder retains a maximum tangential cohesion. In order to avoid the development of stresses along the tubular placeholder, the slots may also have slight coiling. The use of a spiral tube as tubular placeholder is also possible in principle, though generally leads to intensified radial stresses when the placeholder detaches from the glass material.

In a further variant, the placeholder has a base body, in particular a solid or very largely solid base body, which is embedded into a layer which is more yielding or which, on detachment, generates a lower mechanical stress than the base body, so that the layer reduces the introduction of mechanical stresses on cooling of the glass material. The layer in which the base body is embedded may be formed, for example, of granules/sand and/or of a solid foam.

In a further variant, the temperature-resistant material of the placeholder has a melting point of more than 1500°C, preferably of more than 2000°C, in particular of more than 3000°C, and is formed preferably of a metallic material or a semi-metal. As described earlier on above, the high-temperature bonding is carried out in general at temperatures of more than 1000°C, generally between around 1300°C or 1500°C and 2000°C. The production of the glass body by direct deposition is also carried out at correspondingly high temperatures. It is therefore necessary for the temperature-resistant material of the placeholder to withstand temperatures of around 1300°C to 2000°C and to be able either to serve itself as a tube, in which case it remains in the cooling channel, or to be removed from the glass body without damage to the glass material. Moreover, in the course of cooling, the placeholder ought to introduce only very minimal stresses (< 2 MPa) into the glass material.

In a further variant, the temperature-resistant material of the placeholder is selected from the group comprising: Ti, Pa, Pt, Zr, Cr, V, Rh, Hf, Tc, Ru, B, Ir, Nb, Mo, Ta, Os, Re, W, C. All of these materials have a melting point of more than 1500°C: titanium (Ti) 1660°C, protactinium: 1750°C, platinum: 1772 °C, zirconium 1852°C, chromium 1857°C, vanadium: 1890°C, rhodium: 1966°C, hafnium: 2150°C, technetium: 2200°C, ruthenium: 2250°C, boron: 2300°C, iridium: 2410°C, niobium: 2468°C, molybdenum: 2617°C, tantalum: 2996°C, osmium 3045°C, rhenium: 3180°C, tungsten: 3410°C, carbon: 3500°C. A feature common to the majority of the materials stated above is that at elevated temperatures, as occur during high-temperature bonding, for example, they are oxidized or combust.

In a further variant, the production of the glass body, i.e. the high-temperature bonding, or the deposition of the glass material, is carried out under inert gas, such as in a nitrogen atmosphere, for example. In general these operations are not carried out in an inert gas atmosphere. It is, however, necessary to carry out these operations under inert gas if there is a risk of the material of the

placeholder burning during the high-temperature bonding or during the deposition of the glass material.

In a further variant, the placeholder comprises a non-combustible material, preferably an oxide, in particular an oxide ceramic, or a nitride, or the

placeholder is formed of the non-combustible material, preferably an oxide, in particular an oxide ceramic, or a nitride. The majority of oxide ceramics and nitrides, respectively, are very stable chemically. Examples suitable for the present application include boron nitride or titanium nitride. Examples of appropriate oxides are MgO or CaO, with the latter in particular also possessing a high temperature resistance (melting point 2580°C). Where a placeholder is used which is composed of a material which is non-combustible even at temperatures of more than around 1300°C, it is possible generally to do without the implementation of the bonding process or the deposition process, respectively, in an inert gas atmosphere.

In another variant, a coating which is inert to oxidation is applied to the

(generally combustible) placeholder. The application of a thin surface coating of a material inert to oxidation, e.g. of platinum or of osmium, to a tubular or substantially solid placeholder composed of a metal other than those stated, or of graphite, allows the bonding or deposition process to be carried out without inert gas. The coating is generally limited to the periphery of the placeholder, but may also, optionally, be applied additionally to the end faces on the free ends of the placeholder, and also - in the case of a tubular placeholder - to the inside thereof.

In a further variant, a coating which is inert to reaction with the glass material of the glass body is applied to the placeholder or to the wall(s) of the cooling channel. A (thin) surface coating of this kind is especially practical in the event that the tubular or substantially solid placeholder is formed of a material which reacts with the glass material of the glass body - for example, an oxide ceramic, e.g. CaO or MgO. Particularly in the case of magnesium oxide ceramics or calcium oxide ceramics which are readily removable from the cooling channel with water, there is a risk that, without such a coating, these ceramics will be bound into the glass material, since CaO and MgO are constituents of the conventional soda-lime glasses. As materials for the coating inert to reaction with the glass material of the glass body it is possible, for example, to use osmium or platinum. Platinum wets the glass surface and prevents further reactions, and even at elevated temperatures does not react with atmospheric oxygen, and is therefore also suitable for preventing oxidation of the material of the placeholder. Where a tubular placeholder is used, it is sufficient if the coating inert to reaction with the glass material is applied only along the outer periphery of the placeholder, since the placeholder comes into contact with the glass material only along its external periphery. In the event that the cooling channels are made in the glass material by mechanical working, for example by milling, the inert coating may also be applied to the wall(s) of the cooling channel.

In a further variant, the placeholder is a pressed or sintered body or a body formed from a mixture joined with a binder. The placeholder used may be a dimensionally stable, porous and/or pressed or (in general roughly or

incompletely) sintered body. Also possible is the use as placeholder of a body formed from sand or from granules and from a binder. On cooling, such a body will rapidly rupture along a roughly tangential line, and is therefore unable to exert any substantial tensile stresses on the glass material in the course of cooling. The removal of such a body is also facilitated by the circumferential crack. The bodies described earlier on above preferably comprise an oxide ceramic and/or a nitride. For example, the body may be formed of a mixture of a sand composed of aluminium oxide or of boron nitride with a little CaO and/or MgO as binder.

In another variant, the placeholder is formed by granules or a sand of

temperature-resistant material, the sand being optionally graded in terms of its particle size, and the material being, for example, an oxide ceramic or a nitride, which is pressed into the milled-out cooling channels. A placeholder of this kind composed of granules and/or sand which has not been too tightly pressed is generally not dimensionally stable and can be removed from the cooling channel in a simple way after the cooling of the glass material, with the use, for example, of a waterjet nozzle and/or a mechanical tool, e.g. a wire.

The placeholders described earlier on above as well, in the form of pressed or roughly sintered bodies or in the form of a mixture joined by a binder, can typically be removed from the cooling channels by washing with a waterjet or the like, by ultrasound and/or by using a suitable mechanical tool. Where CaO is used as temperature-resistant material, however, it should be borne in mind that the reaction of CaO with water is highly exothermic, and so the supply of water when removing the CaO material ought to be restricted. In this regard, it is favourable to use CaO as binder in a mixture with a sand, composed of aluminium oxide and/or boron nitride, for example. With a mixture of this kind, the CaO fraction is greatly reduced, and so the evolution of heat can be managed and the remaining sand can be rinsed out easily. In contrast to CaO, MgO undergoes virtually no reaction with water, meaning that the shelf life of MgO is greater. There is also a reduction in the risk of the placeholder reacting with the glass material during bonding. The comparatively weak reaction of MgO with water is nevertheless typically sufficient to lower the strength of a body made from a lightly bound mixture to a sufficient extent to allow it to be rinsed out of the cooling channel with the aid of a waterjet nozzle.

In the case of placeholders where the temperature-resistant material is not granular but instead solid, there are likewise various possibilities for removing them from the cooling channels:

In one variant, the placeholder is removed by being burnt off with the supply of an oxidizing gas, in particular oxygen. Burning off is possible with the majority of the placeholders described earlier on above and made from metallic or semi- metallic materials, and is appropriate in particular where the temperature- resistant material is carbon. During the burning-off of the placeholder, the glass material ought not to be heated to temperatures of more than around 600°C. This can be achieved by restricting the supply of oxygen and, where

appropriate, by local supply of oxygen via a probe.

In a further variant, the removal of the preferably metallic placeholder is accomplished by electrochemical ablation and/or by an etching treatment using an etchant. With electrochemical ablation, the generally metallic placeholder is polarised as the anode, and the tool as the cathode. In the case of the etching treatment, the etchant is chosen in dependence on the nature of the

temperature-resistant material. For example, tungsten can be dissolved in a mixture, heated to around 400°C-600°C, of sodium nitrate and sodium

carbonate or similar alkali metal salts. Titanium, for example, can be dissolved in concentrated sulfuric acid, which does not attack the glass material.

In a further variant, the glass material of the glass body is formed from fused silica, preferably from titanium-doped fused silica, or from a glass-ceramic, for example from Zerodur®. Titanium-doped fused silica, sold for example under the tradename ULE® by Corning, and also certain glass-ceramics, e.g.

Zerodur®, have a particularly low coefficient of thermal expansion and are therefore especially suitable for the production of substrates for EUV mirrors. With the so-called direct deposition of titanium-doped fused silica (Si02-Ti02 glass), the glass material is deposited in general on a carrier of a refractory oven, as is described in US 5,970,751 , for example. Where the cooling channels are to be embedded during the deposition of the glass material, the deposition process can be halted and the spacers can be placed where appropriate directly onto the already solidified glass material. The deposition process is subsequently continued, so as to embed the placeholders into the glass material. Alternatively, the glass-ceramic or the fused silica material produced by direct deposition or in a soot process can be divided into two or possibly more partial bodies, in order to form the cooling channels in or between the partial bodies, which are subsequently joined to one another or

(re)assembled by means of high-temperature bonding.

In a further variant, the method additionally comprises: producing a substrate for an optical element by working, in particular mechanically working, the glass body. In general the glass body is subjected to (final) mechanical working in order to produce the desired shape of the substrate. For example, the substrate can be cut from the glass body by removing a marginal region of the glass body, which is cut off or milled off, for example. The region in which a reflective coating is to be applied to the substrate is generally polished, prior to the application of the coating, in order to generate low surface roughness.

A further aspect of the invention relates to a reflective optical element, in particular for reflecting EUV radiation, comprising: a substrate which has at least one cooling channel and which is produced by the method described earlier on above, and also a reflective coating applied to the substrate. The reflective coating may be designed in particular for reflecting EUV radiation with a wavelength of between around 5 nm and around 30 nm. The specific configuration of such a reflective coating is familiar to the skilled person, and so will not be described more closely in the present application. With an optical element of this kind where the cooling channel has been produced using a placeholder, there are typically slight residues of the spacer and/or an impression of the spacer left in the respective cooling channel, providing information about the production of the optical element and/or the placeholder used in each case. The same is true of the cooling channels made by mechanical working, for example by milling, and also for bonding planes, which are likewise detectable on the substrate or on the optical element.

A further aspect of the invention relates to an optical arrangement, in particular an EUV lithography system, comprising: at least one reflective optical element, which is designed as described earlier on above, and also a cooling device, which is designed for the flowing of a cooling liquid through the at least one cooling channel. The EUV lithography system may be an EUV lithography apparatus for exposing a wafer, or may be some other optical arrangement that uses EUV radiation, for example an EUV inspection system, for example for inspecting masks, wafers or the like that are used in EUV lithography. The reflective optical element may in particular be a mirror of a projection system of an EUV lithography apparatus. The cooling device may be designed, for example, to cause a cooling liquid in the form of cooling water or the like to flow through the channel passage. For this purpose, the cooling device may optionally have a pump and also suitable feed and offtake lines.

It will be appreciated that the glass body may in principle also be used for purposes other than for the production of a (reflective) optical element. In that case as well, the cooling channel or channels may be used for cooling the glass body, if the latter is traversed by a flow of a cooling fluid, for example cooling water.

Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized in each case individually by themselves or as a plurality in any desired combination in a variant of the invention.

Drawing

Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

Figs 1 a,b show schematic representations of a first partial body of a glass body, into which cooling channels have been milled, and of a second partial body of the glass body,

Figs 2a, b show schematic representations in analogy to Figs 1 a,b, wherein three different kinds of placeholders are installed in the cooling channels, Figs 3a, b show representations in analogy to Figs 1 a,b, in which the placeholders are embedded between the two partial bodies during high-temperature bonding,

Figs 4a, b show two representations of a carrier on which a glass material is deposited, during the introduction of placeholders and also during the embedding of the placeholders into the glass material,

Figs 5a-c show representations of a placeholder with a solid base body, with lamellae formed on its periphery and, respectively, with the placeholder embedded in a layer of granules, and

Figs 6a-d show representations of a tubular placeholder which has slots spaced apart from one another formed on its periphery or which is designed as a spiral tube.

In the following description of the drawings, for identical or

functionally identical components, the same reference signs are used.

Figs 1 a,b show an example of a method for producing a glass body 1 which has a plurality of cooling channels 2. The method provides a first, lower partial body 3a and a second, upper partial body 3b of a substantially cylindrical glass body 1 which is shown in Fig. 1 b. In the example shown, the lower partial body 3a of the glass body 1 , or more precisely of its glass material 4, is mechanically worked at a surface 5a, by the milling of a plurality of cooling channels 2 into the glass material 4.

The two partial bodies 3a, b are joined to one another at the worked surface 5a by mounting the second partial body 3b with an unworked surface 5b onto the first partial body 3a and subjecting both partial bodies 3a, b jointly to a high- temperature bonding process. During the high-temperature bonding, the two partial bodies 3a, b are heated to temperatures of more than 1000°C, generally between around 1300°C or 1500°C and 2000°C, and the two surfaces 5a, b that are in contact with one another are generally fused to one another, so forming a permanent bond between the two partial bodies 3a, b.

The two partial bodies 3a, b are provided by sawing the solid glass body 1 (without cooling channels) by means of separated ball grinding along a parting face, which is curved in the example shown, to form the likewise curved surfaces 5a, b. The two surfaces 5a, b at which the partial bodies 3a, b are joined to one another are therefore matched to one another with accurate fit. The sawing of the glass body 1 along a curved parting face is advantageous in order to generate a constant spacing between the cooling channels 2 and a likewise curved optical surface or mirror face 6, which is shown with dashed lines in Fig. 1 a. The mirror face 6 is produced by mechanical working, e.g. by milling or the like, wherein a mirror substrate having a desired geometry is generated from the glass body 1. In the course of the mechanical working to produce the mirror substrate, the glass body 1 is in general additionally trimmed at the margins.

To produce a reflective optical element, a reflective coating is applied to the mirror face 6 of the substrate or of the mechanically worked glass body 1. In the example shown, in which the glass body 1 is used for producing an EUV mirror, the reflective coating is designed to reflect EUV radiation at an operational wavelength in the EUV wavelength range between around 5 nm and around 30 nm, and for that purpose has a plurality of alternating layers of a material of high refractive index and a material of low refractive index.

In order to ensure sufficient dimensional stability of the glass body 1 and to prevent unwanted deviations in the geometry of the mirror substrate produced from the glass body 1 , it is necessary in particular for the lower partial body 3a, provided with the cooling channels 2, to remain dimensionally stable during the high-temperature bonding. In order to achieve this without introducing placeholders into the cooling channels 2 for the high-temperature bonding, it is advantageous or necessary to observe predefined geometry parameters, as described below with reference to Fig. 1 a.

In the example shown in Fig. 1 a, the cooling channels 2 are introduced parallel to one another along an X-direction of an XYZ coordinate system, which forms the main direction or the longitudinal direction of the cooling channels 2. In the example shown, the cooling channels 2 each have a width B in the range between around 0.5 mm and 3.0 mm. As is likewise apparent from Fig. 1 a, in the course of the mechanical working, webs 7 are formed in the glass material 4 of the lower partial body 3a between each pair of adjacent cooling channels 2. The web width A of the webs 7 in Y-direction, which corresponds to the spacing A between two adjacent cooling channels 2 in Y-direction, is more than 10 times, optionally more than 20 times, more than 50 times or even more than 100 times, the width B of a respective cooling channel 2, i.e.: A/B > 10 or else > 20, > 50 or > 100. It should be pointed out that, in order to simplify the representation, Fig. 1 a is not to scale.

The relatively low fraction of the width B of the cooling channels 2 as a proportion of the overall width of the glass body 2 in Y-direction ensures that the webs 7 remain stable during bonding and do not bend or flip under the effect of gravity. In order to ensure a sufficient flow of cooling liquid through the cooling channels 2 in spite of the large A/B ratio, it is favourable if the depth C of the cooling channels 2 in Z-direction is selected to be comparatively large. For the depth C of the cooling channels 2, it ought to be the case that: 20 B > C > 2 B.

In particular in the event that the channel fraction is comparatively large, i.e. in the case that A/B is < 20, or in the case that the cooling channels are

comparatively high (B/C < 5), it may be advantageous to form at least two channel-free regions in the lower partial body 3a, the width D thereof obeying the following condition: D > 3 C and/or D > 5 A. Fig. 1 a, by way of example, shows two channel-free regions 8a, b, which are formed at the two side edges of the lower partial body 3a.

Represented in Fig. 2a, b is an example of the production of a glass body 1 , analogous to the example shown in Figs 1 a,b, in which case a placeholder 9a-c is introduced into a respective cooling channel 2 in each case, before the two partial bodies 3a, b are joined by high-temperature bonding. In Figs 2a, b, to simplify the representation, three examples of different placeholders 9a-c are shown, which are arranged in each case in a cooling channel 2. In general, in the production of a glass body 1 , only one single kind of placeholder 9a-c is used.

On the left in Fig. 2a, a placeholder 9a is formed from granules or from a granular material which is not dimensionally stable. The granules comprise a sand, optionally graded in its particle size, composed of a temperature-resistant material, e.g. an oxide ceramic or a nitride, which has been pressed into the milled-out cooling channel 2. Introduced into the cooling channel 2 represented in the centre in Fig. 2a is a solid, wire-like placeholder 9b, while a tubular placeholder 9c is introduced into the cooling channel 2 shown on the right in Fig. 2a.

In order that the respective placeholder 9a-c retains its dimensional stability during the high-temperature bonding, it is necessary for the temperature- resistant material of the placeholder 9a-c to have a comparatively high melting point of more than around 1500°C, of more than around 2000°C, ideally of more than around 3000°C. The temperature-resistant material may be formed of different materials, for example of metals or semi-metals, of carbon, etc.

Materials which have a melting point of more than around 1500°C and which are therefore suitable as temperature-resistant materials for the placeholder are, for example, Ti, Pa, Pt, Zr, Cr, V, Rh, Hf, Tc, Ru, B, Ir, Nb, Mo, Ta, Os, Re, W, C.

As can be seen in Fig. 2b, the placeholders 9a, b shown on the left and in the middle in Fig. 2a are removed from the respective cooling channel 2 after the joining of the two partial bodies 3a, b, while the tubular placeholder 9c shown on the right remains in the cooling channel 2 after the production of the glass body 1. In the case of the cooling channel 2 shown on the right in Fig. 2b, therefore, the cooling liquid is guided through the cavity in the tubular placeholder 9c. The placeholder composed of the granules and shown on the left in Fig. 2a is removed from the cooling channel 2 after the cooling of the glass body 1 , by means of a waterjet nozzle and with use, optionally, of a wire. The solid placeholder 9b shown in the middle in Fig. 2a is also removed from the cooling channel 2. The solid placeholder 9b is formed of a temperature-resistant but combustible material, more precisely of carbon, and can therefore be removed from the cooling channel 2 by being burnt off, with the accompanying supply of an oxidizing gas in the form of oxygen. In order to avoid the glass material 4 in this case undergoing local heating to more than 600°C, the supply of oxygen in the cooling channel 2 is restricted by the introduction of a probe 11 , which is indicated with dashed lines. Besides carbon, the majority of the materials recited earlier on above are also combustible and may be removed from the respective cooling channel, where appropriate, by controlled burning-off. In order to prevent uncontrolled burning-off of the non-refractory placeholder 9b during high-temperature bonding, the high-temperature bonding is carried out under inert gas or in an inert gas atmosphere.

Figs 3a, b show two partial bodies 3a, b of glass material 4, which are joined to one another as in Figs 1 a,b by high-temperature bonding. For this purpose, as in Figs 1 a,b, the upper partial body 3b is placed onto the lower partial body 3a. In contrast to Figs 1 a,b, however, the cooling channels 2 are not produced by working one or both partial bodies 3a, b by milling. Instead, a plurality of placeholders 9d are placed on the top side 5a of the lower partial body 3a, and are embedded into the glass material 4 of the glass body 1 during high- temperature bonding. The approach shown in Figs 3a, b is favourable in particular in the case that the placeholders 9b have a certain robustness, since they are formed, for example, of a metal. As described earlier on above, a problem with the majority of metallic materials is that they burn at high temperatures in contact with oxygen from the environment.

In order not to have to carry out high-temperature bonding under an inert gas atmosphere, an inert, refractory coating 12 of platinum is applied to the placeholder 9 which is shown in Figs 3a, b and which is made of a solid, combustible material, e.g. of tungsten or of graphite. Other non-combustible materials as well, osmium for example, are suitable materials for a coating which is inert to oxidation (refractory).

In the event that, as in Figs 3a, b, a solid placeholder 9d is used which is not tubular, it must be removed from the glass material 4 of the glass body 1 so as to form the cooling channels. The metallic material or the graphite can be removed by burning off as described earlier on above. If the placeholder 9d used is made of a metallic material, an alternative option is to subject it to electrochemical ablation. Chemical ablation of the material through an etching treatment is possible as well, in which case the nature of the etchant is dependent on the nature of metallic material. Tungsten, for example, can be dissolved in a mixture, heated to a temperature of around 400°C-600°C, of sodium nitrate and sodium carbonate or similar alkali metal salts. A placeholder 9d of titanium can be dissolved, for example, in concentrated sulfuric acid, which does not attack the glass material 4.

Figs 4a, b show a further possibility for embedding placeholders 9e into the glass material 4 of the glass body 1 , where the embedding takes place during the deposition of glass material 4 onto a carrier, which in the example shown is designed in the form of a container, but which may also be formed as a turntable or the like. In the example shown, the glass material 4 comprises titanium-doped fused silica (ULE®), which is produced by direct deposition, as is described in US 5,970,751 , for example. The glass material is deposited on the carrier 13 until this material has reached a predefined height. In a

subsequent step, the placeholders 9e are placed onto the solidified glass material 4 already deposited, and are optionally stabilized by means of a network of cross-connectors, which are not shown in the image. Next, the deposition of the quartz glass material 4 is continued, until the desired height of the glass body 1 is reached. When the glass body 1 has cooled throughout its volume to below its solidification temperature, it is typically taken from the carrier 13 or from the container, for further working. Instead of the method described here, in which the deposition process has to be interrupted in order for the placeholders 9e to be inserted, an alternative option is to fix the placeholders 9e suitably in the container even before the start of the deposition process. In this way, the process of depositing the glass material 4 can be carried out without interruption.

In the example shown in Figs 4a, b, the placeholder 9e is a body 14 made of a mixture which comprises sand held together with a binder. Alternatively the placeholder 9e may be a porous or granular body 14 made of a roughly sintered or pressed material. Particularly appropriate for the production of such a body 14 are non-combustible materials, e.g. oxides, in particular oxide ceramics, or nitrides, e.g. boron nitride or titanium nitride. With materials of these kinds there is no need to apply a refractory coating.

Where the placeholder 9e used is a body 14 made of a CaO or MgO ceramic, a problem is that these materials may react with the glass material 4 of the glass body 1 , and so undesirably are bound into the glass material 4. To prevent this, with the example shown in Figs 4a, b, a coating 15 which is inert to reaction with the glass material 4 of the glass body 1 is applied to the body 14. The coating 15 in the example shown is formed of platinum, which wets the glass surface and prevents further reactions. As described earlier on above, platinum does not react with atmospheric oxygen even at high temperatures, and is therefore also suitable as refractory coating 12.

Where nitrides, for example boron nitride or titanium nitride, are used for producing the placeholder 9e or the body 14, there is generally no need to apply a coating inert to reaction with the glass material 4, because these materials are already chemically inert to such a reaction. In the event that, as shown in Figs 1 a,b, the cooling channels 2 are introduced into the glass material 4 by mechanical working, the coating 15 inert to reaction with the glass material 4 may be applied to the wall of the cooling channel 2.

The porous or pressed bodies 14 described earlier on above can in general be broken up by a waterjet or by an ultrasound treatment and so removed from the cooling channels 2. CaO as oxide ceramic has the advantage of a high melting temperature of around 2580°C, but the reaction of CaO with water is strongly exothermic. It is therefore favourable not to select too high a fraction of CaO in the body 14. This is possible, for example, if the body 14, as described earlier on above, is formed of a mixture of sand, for example composed of aluminium oxide or of boron nitride, with CaO as binder. Such a body 14 may be removed from the glass body 1 by the addition of water or by a waterjet, since the evolution of heat is manageable. For the production of such a body 14 it is also possible to use another kind of binder, e.g. MgO. The reaction of MgO with water is comparatively weak. This reaction, however, may be enough to lower the strength of a comparatively weakly bound mixture to an extent which allows it to be removed easily from the glass body 1.

The majority of the temperature-resistant materials described earlier on above have a higher thermal expansion than the glass material 4 of the glass body 1. On cooling of the glass body 1 after the bonding process described in connection with Figs 1 a,b and Figs 3a, b or, respectively, after the glass deposition process described in connection with Figs 4a, b, therefore, the placeholders 9a-e typically contract to a greater extent than the glass material 1 , and, in so doing, undergo uncontrolled detachment from the glass material 1 over large parts of their periphery. In principle, while this does simplify the removal of the placeholders 9a-e after cooling, it is nevertheless the case that, before the detachment, the placeholders 9a-e exert tensile stresses on the glass material 4, which in part can no longer be relaxed before solidification, and are frozen in. Described below are a number of examples of placeholders 9b, 9c, which allow a reduction in the tensile stresses during the cooling of the glass body.

The placeholders 9b shown in Figs 5a-c each have a solid base body 16, which in the example shown is formed of a metallic material. In the case of the placeholder 9b shown in Fig. 5a, a number of four T-shaped lamellae 17 are distributed along the periphery 16a of the placeholder 9b. The lamellae 17 are each separated from one another by gaps 18 which have a gap width s in the periphery direction of between around 0.5 mm and 3.0 mm. While exerting a low tensile force over a major part of their area, the lamellae 17 remain joined to the glass material 4 and detach only by way of their web-like attachment to the solid base body 16. Fig. 5b shows a placeholder 9b having a solid base body 16 in analogy to Fig. 5a, formed along the periphery 16a of which are a number of eight L-shaped lamellae 17. The lamellae 17 are produced by introducing millings having the desired geometry into the base body 16.

With the example shown in Fig. 5c, the solid base body 16 of the placeholder 9b is embedded in a layer 19 which, on contraction of the placeholder 9b in the course of cooling, exerts a lower tensile stress on the glass material 4 than does the solid base body 16. The layer 19 in which the base body 16 is embedded may be formed, for example, of granules/sand, optionally joined with a binder, or of a solid foam. Figs 6a-d show examples of tubular placeholders 9c, for which likewise it is possible to reduce the tensile stress exerted on the glass material 4 during cooling. Introduced in the tubular placeholder 9c shown in Figs 6a, b are slots 20 which extend in the longitudinal direction and which are arranged at an offset to one another in the peripheral direction. As can be seen in Fig. 6a, the slots 20 are preferably arranged with a uniform distribution in the peripheral direction along the placeholder 9c. In order to prevent stresses which build up in the longitudinal direction of the placeholder 9c, the slots 20, in the example shown in Fig. 6c, are slightly coiled. Fig. 6d shows a placeholder 9c, which is designed in the form of a spiral tube 21.

When placeholders 9a-c are introduced into the cooling channels 2 of Figs 2a, b, formed by mechanical working, a problem is that during the high-temperature bonding, the placeholders 9a-c generally expand to a greater extent than the glass material 4 of the glass body 1. In order to prevent stresses being introduced into the glass material 4 during heating, the placeholder 9b shown in the middle in Figs 2a-b may optionally be given smaller sizing than the corresponding cooling channel 2.

As described earlier on above, a final mechanical working is carried out on the glass body 1 provided with cooling channels 2, in order to produce or to shape a substrate for a reflective optical element. In the mechanical working for producing the substrate, typically a circumferential margin of the glass body 1 is cut off, and the optical surface 6 shown with dashed lines in Fig. 1 a is formed. Additionally, in general, the surface 6 (with concave curvature in Fig. 1 a), to which a reflective coating is applied, is polished in order to reduce the surface roughness. The reflective coating may be designed, for example, to reflect radiation at an operational wavelength in the EUV wavelength range between around 5 nm and around 30 nm. A reflective optical element of this kind or a mirror of this kind for the reflection of EUV radiation may be disposed in an EUV lithography system, for example in an EUV lithography apparatus. In that case an EUV lithography system of this kind has a cooling device which enables a cooling fluid, in particular cooling water, to be passed as a flow through the at least one cooling channel 2 in the substrate. The cooling device may have corresponding ports and also lines for the supply and removal, respectively, of the cooling fluid into and from the respective cooling channels 2. The cooling device may have a pump or the like in order to circulate the cooling fluid. An alternative option is for the cooling device to be in communication with a cooling water supply, via a cooling water port. It will be appreciated that the use of the glass body 1 with the cooling channels 2 is not limited to EUV lithography, but instead that the glass body 1 and/or a substrate formed thereof may also be used in other optical

arrangements.