Workpiece with a hollow structure, method for at least partially forming a hollow structure, mirror, and lithography system

Abstract

The invention relates to a workpiece comprising: at least one hollow structure (27) which extends in the workpiece (25) and is designed to be flowed through by a fluid (28). The hollow structure (27) has a first portion and a second, adjacent portion which are oriented at an angle (γ) of between 60° and 120° to one another. The hollow structure (27) has a rounded portion (37a, 37b) at which the first portion and the second portion transition into one another. A surface of a wall of the hollow structure (27) in the first portion, in the second portion and / or in the rounded portion (37a, 37b) has a roughness Ra of 25 µm or less, preferably of 10 µm or less, particularly preferably of 5 µm or less, in particular of 2 µm or less. The invention also relates to a method for at least partially forming a hollow structure (27) in a workpiece (25) by selective laser etching. The invention also relates to a mirror, in particular an EUV mirror (24), comprising: a workpiece in the form of a substrate (25), which is formed as described above, and a reflective coating (26) for reflecting radiation, in particular for reflecting EUV radiation (16), which is applied to a surface (25a) of the substrate (25). The invention also relates to a lithography system, in particular an EUV lithography system, having at least one such workpiece and / or having at least one such mirror (M4).

Description

[0001] Workpiece with a hollow structure, method for at least partially forming a hollow structure, mirror and lithography system

[0002] Background of the invention

[0003] The invention relates to a workpiece, preferably a substrate for a mirror, in particular a substrate for an EUV mirror, comprising: at least one hollow structure which runs in the substrate and which is designed for a fluid to flow through. The invention also relates to a method for at least partially forming a hollow structure in a workpiece. The invention also relates to a mirror, in particular an EUV mirror, which comprises a workpiece in the form of a substrate which is designed as described above. The invention also relates to a lithography system, in particular an EUV lithography system, comprising: at least one mirror which is designed as described above, and a temperature control device, in particular a cooling device which is designed for a temperature control fluid, in particular a cooling fluid, to flow through the at least one hollow structure.

[0004] The lithography system can be a lithography system for exposing a wafer or another optical arrangement used for lithography, for example an inspection system, e.g. an arrangement for measuring or inspecting masks, wafers, or the like used in lithography. The lithography system can, for example, be designed for operation with radiation in the EUV wavelength range. For the purposes of this application, the EUV wavelength range is understood to mean a wavelength range between approximately 5 nm and approximately 30 nm. In an EUV lithography system in the form of an EUV lithography system, optical elements for reflecting radiation in the form of mirrors, in particular in the form of mirrors of a projection system, are exposed to a high radiation power.As the power of the EUV radiation source increases, the average power radiated onto the mirrors reaches up to 50 W, of which one third to one half is absorbed in the layer system of the reflective coating and leads to flat and local heating of the mirror or the substrate. This heating leads to changes in the shape of the surface of the mirror to which the reflective coating is applied, even when using so-called zero-expansion material, e.g. in the form of titanium-doped quartz glass or in the form of a glass ceramic. These changes in shape are due, among other things, to inhomogeneities in the (linear) coefficient of thermal expansion (CTE) or the zero-crossing temperature (ZCT).This is due to the low temperature (Zero-Crossing Temperature, Tzc) within the volume of the substrate and to the fact that the thermal expansion coefficient is significantly different from zero away from the zero-crossing temperature.

[0005] To reduce the temperature of the mirrors in EUV lithography systems, it is known to introduce hollow structures in the form of cooling channels into the mirror substrate and to flow a cooling fluid through these hollow structures. Such channels can be milled into the substrate during production and sealed with a lid.

[0006] Due to the floating suspension of the mirrors and the generally unfavorable effect on image aberrations, turbulent flows and vibrations attributable to the flow of the (usually liquid) cooling medium (“flow-induced vibrations” (FIV)) must be avoided. However, the insides of the milled channels are generally angular and rough, which is unfavorable with regard to FIV. Selective laser etching (SLE) can be used to produce microchannels, shaped bores, etc. in transparent components made of, for example, quartz glass, borosilicate glass, sapphire or ruby. In selective laser etching, light in the form of ultrashort pulsed laser radiation (ps or fs pulses) is focused in the volume of a transparent workpiece (focal volume). The pulse energy is absorbed only within the focal volume by multi-photon processes. In the focal volume, the transparent material is etched crack-free or, if necessary, completely.with microcracks, its optical and chemical properties are altered in such a way that it becomes selectively chemically etchable. Depending on the materials used.

[0007] Depending on the laser parameters, the modification of the material can involve microcracks or other deep damage. By deflecting the focus in the material, e.g., using a microscanner system, contiguous areas (contiguous irradiation volumes) are modified, which can then be removed using wet-chemical etching. In wet-chemical etching, the component is typically immersed in an etching solution for several weeks or months, which preferentially (selectively) releases the modified material. By scanning or moving the laser radiation within the volume of the workpiece, any hollow structures, e.g., in the form of channels, can be created.

[0008] A limiting factor for the selective laser etching of channels in quartz glass and other materials, e.g. in titanium-doped quartz glass, is the comparatively low etch selectivity of approximately 1:500 to approximately 1:1500 compared to other transparent materials, e.g. sapphire, which has an etch selectivity of 1:10,000. This low etch selectivity results in the channel being wider in an area at the edge of the component, where the etching fluid first attacks, than further inside the component. Areas that are too wide can lead to inhomogeneous cooling of the component and possibly mechanically push through it: in extreme cases, excessive etching can lead to short circuits between adjacent channels. In general, the conical channel cross-section that results from insufficient etch selectivity is also undesirable.

[0009] WQ2021 / 115643 A1 describes an optical element for reflecting EUV radiation, in which at least one channel is formed in the substrate, through which a cooling medium can preferably flow. The substrate is made of quartz glass, in particular titanium-doped quartz glass, or of a glass ceramic. The channel has a length of at least 10 cm, and a cross-sectional area of ​​the channel varies by no more than + / - 20% over the length of the channel. WQ2021 / 115643 A1 describes that the production of a channel with such properties is typically carried out by selective laser etching, whereby the etch selectivity should be increased to produce such a channel. The inside of such a channel can have a low roughness.

[0010] DE102019200750A1 describes a manufacturing method for components of a projection exposure system for semiconductor lithography, in which a hollow structure is created in the component, e.g., in a mirror body, by selective laser etching. The hollow structure can be designed as a temperature control channel that runs in a temperature control plane between two openings aligned perpendicular to the temperature control plane and serving as an inlet and outlet for the temperature control fluid. The temperature control channel is connected to the openings via two angular 90° bends.

[0011] Object of the invention

[0012] The object of the invention is to provide a workpiece, a mirror, and a lithography system in which flow-induced vibrations are reduced when a fluid flows through the hollow structure.

[0013] This object is achieved by a workpiece of the type mentioned at the outset, in which the hollow structure has a first section and a second, adjacent section, which are aligned at an angle between 60° and 120°, preferably at an angle between 80° and 100°, in particular at an angle of 90°, to one another, wherein the hollow structure has a rounded section at which the first section and the second section merge into one another, and wherein a surface of a wall of the hollow structure in the first section, in the second section and / or in the rounded section has a roughness R a of 25 pm or less, preferably of 10 pm or less, particularly preferably of 5 pm or less, in particular of 2 pm or less. The roughness R a, also called mean roughness, indicates the average distance of a measuring point on the surface from the surface's centerline. The mean roughness thus corresponds to the arithmetic mean of the absolute deviation from the centerline.

[0014] The inventors have recognized that at a transition between two sections of the hollow structure of the workpiece in the form of a corner or a sharp edge, in particular when the two sections are aligned approximately perpendicular to one another (i.e. at an angle of between 60° and 120°), at the typical flow velocities at which the fluid flows through the hollow structure, flow separation occurs at the wall of the hollow structure. This leads to turbulence and causes flow-induced vibrations. For this reason, it is proposed that the two sections of the hollow structure merge into one another at a rounded section which has a course that is as streamlined as possible. A rounded section is understood to be a section which has no corners. At the rounded section, the first section thus merges continuously into the second section. The cross-section orThe diameter of the hollow structure within the rounded section is typically constant, but can also vary. The cross-section or diameter of the rounded section usually corresponds to the cross-section of both sections, but this is not necessarily the case if the rounded section is located at a junction (see below).

[0015] The hollow structure with the rounded section can be manufactured entirely or partially by selective laser etching. The hollow structure has a wall that forms the interface between the interior of the hollow structure and the workpiece material. In particular, when the hollow structure is manufactured by selective laser etching, a low surface roughness of the wall of the hollow structure can be achieved, which counteracts the generation of flow-induced vibrations. This is particularly advantageous in the rounded section described above. Typically, both the rounded section and the first section and the second section meet the roughness requirement specified above.

[0016] For the production of the workpiece described above, it is particularly advantageous if the etching selectivity is increased during selective laser etching. The increase in etching selectivity can be achieved in various ways, for example by guiding a flexible hose along the etching front. The rounded section facilitates the tracking of such a flexible hose. Since the hose can get caught on the wall, particularly on the rounded section, if the microscopic roughness of the surface of the wall of the hollow structure is too great, the low roughness of the surface of the wall of the hollow structure also facilitates the tracking of such a hose. In one embodiment, the surface of the wall of the hollow structure has recesses.In this case, the surface of the wall facing the interior of the hollow structure has a surface structure with recesses, which are also referred to below as depressions or depressions and which typically have a concave shape.

[0017] In a further development of this embodiment, the recesses are crater-shaped. For the purposes of this application, a crater-shaped recess is understood to be a depression or recess whose bottom is enclosed by a raised, annular wall, which is also referred to below as the crater rim. The crater-shaped recess can have a substantially circular geometry in plan view; however, it is also possible for the recess to have a geometry in plan view that deviates from a circular geometry, for example, a polygonal geometry, an angular geometry, etc.

[0018] In a further development, adjacent recesses on the wall of the hollow structure merge into one another. The number of recesses on the surface and their lateral extent is typically so large that adjacent recesses merge into one another. In the case where the recesses are crater-shaped, a respective crater rim enclosing a recess forms, on its side facing away from the recess, a section of a crater rim of an adjacent recess or merges into the base of an adjacent recess. In this development, adjacent recesses not only border one another but at least partially overlap one another.

[0019] In a further development, the recesses on the surface of the wall of the hollow structure form a honeycomb-like surface structure. As described above, adjacent recesses typically do not just border one another, as is the case with honeycombs, but rather merge into one another. Unlike with honeycombs, the recesses are generally not arranged in a regular grid. The shape and size of the recesses forming the honeycomb-like surface structure also vary. The edges between two adjacent recesses, which may in particular be designed in the form of crater edges, generally form a net-like surface structure complementary to the honeycomb structure.

[0020] In a further development, the recesses have a maximum lateral extent of no more than 500 pm, preferably no more than 450 pm, in particular no more than 400 pm. The maximum lateral extent is understood to be the maximum lateral distance between two points along the edge of the recess in a plan view of the recess. In a crater-shaped recess, the maximum lateral extent refers to the maximum lateral distance between two points on the respective crater edge.

[0021] In a further development, the recesses have a maximum depth of no more than 20 pm, preferably no more than 15 pm, in particular no more than 10 pm. The maximum depth is understood to be the distance in the vertical direction measured between the bottom of the recess and the highest point at the edge of the recess or at the crater rim.

[0022] The surface of the wall of the hollow structure can be flat, for example, if the hollow structure has a channel with a rectangular or square cross-section. In the event that the surface of the wall of the hollow structure has a curvature, for example, because the hollow structure forms a channel with a circular cross-section, it is assumed for the determination of the lateral distance and the depth of the recesses that the surface of the hollow structure is approximately flat in the partial area used for the measurement. If necessary, the development of the cylindrical surface, for example, which forms the lateral surface of the channel, can be used to determine the lateral distance and the depth of the recesses.

[0023] In a further embodiment, the first section, the second, adjacent section, and the rounded section each form a channel section of a channel through which a fluid can flow. A channel through which a fluid or liquid can flow forms an elongated, circumferentially closed cavity that has no branches and extends between a first end of the channel and a second end of the channel. The channel can open at one or both ends into further hollow structures that lie within the volume of the workpiece. It is also possible for one or both ends of the channel to open on the outside of the workpiece. In this case, the cross-section or diameter of the rounded section generally essentially corresponds to the cross-section of both sections of the channel. In this case, the wall of the hollow structure forms the lateral surface of the channel.

[0024] In a further development of this embodiment, the channel has a diameter of between 1 mm and 20 mm, preferably between 1 mm and 5 mm, and / or a length of at least 10 cm, preferably of at least 15 cm, in particular of at least 20 cm. The channel can have a round cross-section, but the channel can also have a cross-section that deviates from a round geometry. In this case, the diameter of the channel is understood to be the so-called equivalent diameter, i.e. the diameter of a circle whose area corresponds to the cross-section of the channel, which in this case is non-circular. Channels with a diameter within the value range specified above have proven to be advantageous for the flow of a fluid. In particular, in the event that the workpiece has a large volume, it is advantageous if the channel has a comparatively great length. The diameter of the channel can, if necessary.in the longitudinal direction of the channel, but as a rule it is advantageous if the diameter of the channel varies as little as possible in the longitudinal direction.

[0025] In one embodiment, the cross-sectional area of ​​the channel varies over the length of the channel by no more than + / - 20%, preferably by no more than + / - 10%, in particular by no more than + / - 2%. In this embodiment, the channel typically has a length of at least 10 cm, preferably of at least 15 cm, in particular of at least 20 cm. A variation in the cross-sectional area of ​​the channel of + / - x% is understood, for the purposes of this application, to mean a deviation of + / - x% from an average cross-sectional area AM of the channel. The average cross-sectional area AM is defined as the mean value of the maximum cross-sectional area AMAX and the minimum cross-sectional area AMIN along the length of the channel (AM = (AMAX + AMIN) / 2), as described in WQ2021 / 115643 A1 cited at the outset, which is incorporated into this application in its entirety by reference.

[0026] In one embodiment, a radius of curvature R of the rounded section and a diameter D of the rounded section have a ratio R / D that is between 2 and 6, preferably between 2.5 and 5, in particular between 2.5 and 3.5. With an R / D ratio of more than 2, significant improvements in flow-induced vibrations, e.g. > 50%, can already be achieved. Ideally, the R / D ratio is between approximately 2.5 and 3.5, e.g. 3.0, since this is typically where the greatest improvements in flow-induced vibrations are typically achieved. A value of the R / D ratio of more than 6 should not be exceeded. In this embodiment, the rounded section has a constant radius of curvature. The flow cross-section of the rounded section is typically circular, but can deviate from a circular geometry and, for example, have an elliptical geometry.In this case, the diameter of the rounded section is understood to be the so-called equivalent diameter, which was defined above.

[0027] It has been found that the ratio between the diameter of the rounded section and the radius of curvature of the rounded section is an essential parameter for a streamlined flow pattern without turbulence and thus for the avoidance of flow-induced vibrations.

[0028] In a further embodiment, the diameter D of the rounded section is between 2 mm and 20 mm, preferably between 2 mm and 12 mm. A diameter of the rounded section or of the channel structures of the hollow structure within the specified range allows for the generation of a sufficient volume flow for efficient temperature control of the optical element under the given boundary conditions. The flow velocity of the fluid in the hollow structure is generally in the order of meters per second.

[0029] In one embodiment, the hollow structure has a plurality of temperature control channels that run beneath a surface of the workpiece. The hollow structure has a fluid distributor connected to the temperature control channels via distribution channels, as well as a fluid collector connected to the temperature control channels via collector channels. The temperature control channels typically serve to cool the workpiece and are therefore also referred to below as cooling channels. The temperature control channels generally run in a near-surface region beneath a surface, which is usually a surface to be temperature-controlled. A near-surface region is understood to be a distance of 10 mm or less from the surface of the workpiece to be temperature-controlled.The distance from the surface to be tempered is measured in the thickness direction of the workpiece, which is perpendicular to the surface to be tempered, beneath which the cooling channels run. The short distance between the cooling channels and the surface to be tempered allows for effective cooling of the workpiece surface to be tempered. The distance is defined as the minimum distance between the respective cooling channel and the surface of the workpiece to be tempered.

[0030] The fluid distributor and the fluid collector generally each have a larger flow cross-section than a single cooling channel. This enables favorable flow conditions to be set. The fluid distributor and / or the fluid collector are preferably arranged at a greater distance from the surface to be tempered than the cooling channels. This arrangement makes it possible to keep the deformation of the surface due to the fluid pressure in the fluid distributor and / or the fluid collector, which generally have larger cavities than the cooling channels, within acceptable limits. The fluid distributor is typically connected to a fluid inlet, and the fluid collector is typically connected to a fluid outlet. Each cooling channel can be connected to exactly one distributor channel and exactly one collector channel, but it is also fundamentally possible for a group of two or, if necessary, three to be connected.more than two cooling channels are connected to a common distribution channel and a common collector channel.

[0031] In a further embodiment, the first section forms an end section of the tempering channel adjacent to a distribution channel and the second section forms a distribution channel section adjacent to the end section and / or the first section forms an end section of the tempering channel adjacent to a collector channel and the second section forms a collector channel section adjacent to the end section.

[0032] The cooling channels typically run essentially parallel to the surface to be temperature controlled, to which a reflective coating is applied in the case of the workpiece being a substrate for a mirror. Since the installation space within the substrate is limited, a distribution channel or a collector channel that is connected to a respective cooling channel is generally led away from the surface with the reflective coating at an approximately right angle, i.e. the collector or distribution channel section and an adjacent end section of the cooling channel generally run at approximately a right angle to one another, i.e. there is approximately a 90° deflection of the fluid flowing through the hollow structure.

[0033] By using the rounded section described above, especially by choosing a suitable ratio of radius of curvature to diameter, flow-induced vibrations can be avoided or at least significantly reduced in this area.

[0034] In principle, the fluid distributor and the fluid collector can be designed in different ways. For example, the flow cross-section of the fluid distributor or of the fluid collector can taper starting from the distributor channels or from the collector channels, e.g. in the manner of a funnel, so that the cavities formed by the fluid distributor and the fluid collector in the workpiece are not unnecessarily large. In a further embodiment, the fluid distributor forms an inlet channel from which the distributor channels branch off and / or the fluid collector forms an outlet channel from which the collector channels branch off. In this embodiment, the fluid collector and the fluid distributor generally run essentially transversely to the longitudinal direction of the distributor channels or transversely to the longitudinal direction of the collector channels. The distributor channels or the collector channels generally branch off from the inlet channel or the outlet channel at essentially right angles.In this case, the fluid distributor and the fluid collector can be designed, for example, in the form of cylindrical channels that extend from an inlet opening or an outlet opening on an outer side of the workpiece into the workpiece. The inlet channel and the outlet channel can be designed, for example, in the form of bores, but it is also possible for them to be produced by the selective laser etching described above.

[0035] In a further development of this embodiment, the first section forms an opening section of the distributor channel adjacent to the inlet channel and the second section forms a branching section of the inlet channel adjacent to the opening section and / or the first section forms an opening section of the collector channel adjacent to the outlet channel and the second section forms a branching section of the outlet channel adjacent to the opening section of the collector channel.

[0036] As described above, the longitudinal direction of the inlet channel or outlet channel runs essentially perpendicular to the longitudinal direction of a respective collector channel or distributor channel. A streamlined geometry is also advantageous at a respective branch of a distributor or collector channel, which can be achieved by providing a rounded section at a branch of the inlet channel or outlet channel. In this way, steps can be avoided and edges rounded, making the geometry of the hollow structure more streamlined and preventing or at least significantly reducing fluid separation in the inlet channel and outlet channel.

[0037] The ratio of diameter to radius of the rounded section preferably lies within the range of values ​​described above. However, it is also possible for the rounded section at the junction to not have a constant radius of curvature. The flow diameter of the rounded section at the junction is also not necessarily constant. For example, the cross-section of the rounded section can taper from the inlet channel or from the outlet channel.

[0038] In a further embodiment, the angle between the branching section of the inlet channel and the mouth section of the distributor channel is greater than 90°, preferably greater than 100°, and / or the angle between the branching section of the outlet channel and the mouth section of the collector channel is greater than 90°, preferably greater than 100°. It has been found that it is advantageous for flow guidance if the branching section of the inlet channel or the outlet channel and the mouth section of the distributor channel or the collector channel are aligned at an obtuse angle to one another.

[0039] A further aspect of the invention relates to a workpiece of the type mentioned above, in which a surface of a wall of the hollow structure has recesses. In this case, the hollow structure is typically produced by selective laser etching. The wall of the hollow structure has a characteristic surface structure with recesses, which is formed as described above, as will be described in more detail below: In one embodiment, the recesses are crater-shaped.

[0040] In a further embodiment, adjacent crater-shaped recesses merge into one another.

[0041] In a further embodiment, the recesses on the surface of the wall of the hollow structure form a honeycomb-like structure.

[0042] In a further embodiment, the recesses each have a maximum lateral extent of not more than 500 pm, preferably not more than 450 pm, in particular not more than 400 pm.

[0043] In one embodiment, the recesses have a maximum depth of not more than 20 pm, preferably not more than 15 pm, in particular not more than 10 pm.

[0044] In one embodiment, the surface of the wall of the hollow structure has a roughness R a of 25 pm or less, preferably of 10 pm or less, particularly preferably of 5 pm or less, in particular of 2 pm or less.

[0045] In one embodiment, the hollow structure is designed in the form of a preferably curved channel through which a fluid can flow.

[0046] In a further development of this embodiment, the channel has a diameter between 1 mm and 20 mm, preferably between 1 mm and 5 mm, and / or a length of at least 10 cm, preferably of at least 15 cm, in particular of at least 20 cm.

[0047] In a further development of this embodiment, a cross-sectional area of ​​the channel varies over the length of the channel by no more than + / - 20%, preferably by no more than + / - 10%, in particular by no more than + / - 2%.

[0048] In both aspects described above, the material of the workpiece is preferably selected from the group comprising: quartz glass, in particular titanium-doped quartz glass, and glass ceramic. In this case, the workpiece is typically a substrate for a mirror, more precisely a substrate for an EUV mirror. In order to avoid deformations of the surface, to which a reflective coating is applied in this case, which may be due to inhomogeneous heating of the workpiece material, the substrates of mirrors for EUV lithography are typically made of so-called zero-expansion material, which has a very low thermal expansion coefficient. As described above, these materials are hard and brittle and are therefore difficult to machine.However, the selective laser etching process described above can also be used to produce hollow structures of virtually any shape in such materials.

[0049] In a further embodiment, the material of the workpiece has a zero-crossing temperature that lies between 0°C and 100°C, preferably between 19°C and 40°C, particularly preferably between 19°C and 32°C. The zero-crossing temperature is determined, among other things, as a function of the average incident radiation power during operation of an EUV mirror.

[0050] In one embodiment, the material of the workpiece has a spatial variation of the zero crossing temperature which is less than 3 K, preferably less than 2 K, particularly preferably less than 1 K, in particular less than 0.1 K. Typically, a high spatial homogeneity of the zero crossing temperature is required in order to be able to operate an EUV mirror efficiently. In a further embodiment, the workpiece is monolithic, i.e. it is formed in one piece and has no joining surface at which two or more partial bodies of the workpiece are connected to one another. A rounded section in a monolithic workpiece cannot easily be produced by mechanical processing, e.g. by drilling or milling, in the hard and brittle glass material. It is also possible in principle for the workpiece to be composed of two or more partial bodies.In this case, the joining surface typically does not pass through the rounded section, i.e. the joining surface does not intersect the rounded section.

[0051] A further aspect of the invention relates to a method for at least partially forming a hollow structure in a workpiece, in particular in a workpiece which is designed as described above, by selective laser etching, the method comprising: focusing pulsed laser radiation into a typically contiguous irradiation volume in the workpiece, and at least partially forming the hollow structure by selectively etching the workpiece in the irradiation volume.

[0052] As described above, the hollow structure can be formed entirely or only partially, in particular in sections, by selective laser etching. In particular, the first section described above, the second section, and / or the rounded section of the hollow structure, e.g., in the form of channel sections, can be formed by selective laser etching. For the sake of simplicity, it is assumed below that the hollow structure formed by the method is a channel.

[0053] As described above, the etching process starts from the edge or surface of the workpiece and enters the irradiation volume of the workpiece. The etching creates a channel section which extends within the irradiation volume from an entrance to the channel section on the surface of the workpiece to an end face of the channel section where an etching front is formed. At the etching front or end face, further material from the workpiece is gradually removed along the irradiation volume until the channel is completely formed, i.e. until the channel extends over the entire irradiation volume. During the etching process, the length of the already etched channel section is gradually increased, similar to drilling a tunnel.

[0054] As also described above, depending on the material of the workpiece in which the channel is formed, the etching selectivity in the irradiated volume compared to the non-irradiated volume of the workpiece is comparatively low and may be as low as 1:500. The channel is therefore wider at the edge of the workpiece, where the etching liquid first attacks, than further inside the volume of the workpiece, since the period in which the channel wall is exposed to the etching medium in the volume of the workpiece is significantly shorter than at the edge of the workpiece.

[0055] This problem can be counteracted by etching an end face of a channel section formed in the irradiation volume during selective laser etching at a higher etching rate than a (circumferential, already etched) channel wall of the channel section.

[0056] Preferably, the ratio of the etching rate at the channel wall of the channel section to the etching rate at the end face of the channel section, or the etching selectivity, is at least 1:1500, particularly preferably at least 1:2000. For the purposes of this application, an increase in etching selectivity means that the ratio described above decreases, i.e., that the etching rate at the end face of the channel section increases relative to the etching rate at the channel wall. Except for the case described below in which the channel wall is sealed against etching, increasing the etching rate at the end face of the channel relative to the etching rate at the channel wall leads to an overall higher etching rate.

[0057] In this way, a channel with a possibly considerable length can be formed in the workpiece, wherein the cross-sectional area of ​​the channel is essentially constant over the length of the channel or varies only slightly, namely by no more than + / - 20%, possibly by no more than + / - 10% or + / - 2%. This also applies if the workpiece is a substrate, e.g. for a reflective optical element, which is made of quartz glass, in particular titanium-doped quartz glass, or of a glass ceramic, as described in WQ2021 / 115643 A1. The workpiece, which may, for example, be a substrate for a reflective optical element, can in particular be monolithic (see above).

[0058] To increase the etching rate at the front side compared to the etching rate at the channel wall, which is advantageous or necessary to produce a cross-section that is essentially constant over the length of the channel, various measures can be carried out individually or in combination.

[0059] One such measure consists in increasing the etching rate by generating a temperature at the front side of the channel section that is at least 20 K, preferably at least 40 K, in particular at least 60 K higher than a temperature at the channel wall of the channel section. In this case, the etching selectivity is increased by keeping the workpiece and the etching solution or etching liquid at the lowest possible temperature, which is just above or, if necessary, just below the freezing point of the etching solution. In contrast, the etching front at the front side of the channel is kept at the highest possible temperature, which is at least 20 K, preferably at least 40 K, ideally at least 60 K higher than the temperature of the etching solution and the (remaining) workpiece and thus also the temperature at the (circumferential) channel wall of the channel section.

[0060] The front side of the channel section can be heated by at least one heating device, which is preferably moved along with the front side of the channel section during channel formation. As described above, the position of the front side of the channel or the etching front changes during channel formation, i.e., it moves along the entire length of the irradiation area. To generate the higher temperature at the etching front / front side compared to the surrounding material of the workpiece, it is therefore advantageous to move the heating device along with the etching front.

[0061] The heating device can be located outside the channel, e.g., on or near the surface of the workpiece that is closest to the channel. In this case, the heating device can be carried along the surface parallel to the channel or to the etching front in the channel. In this case, the heating device can be, for example, a resistance heater that is in contact with the surface in order to transfer contact heat to the material of the workpiece. However, the heating device can also be a heating light source, e.g., an infrared light source, or a laser that is focused on the etching front along the direction of the channel or the channel section, or, if necessary, through the material of the workpiece onto the front side of the channel section. It is also possible to place the heating device (e.g., in the form of a resistance heater or a light source) through the

[0062] channel section and ideally to keep the heating device at a constant distance from the etching front. In this case, the heating device can be attached to a suitable support element that has a smaller dimension than the channel diameter. Such a support element is referred to below as a probe. In a further measure, to increase the etching rate, the front side of the channel section is exposed to a higher throughput of an etching solution than the channel walls. The throughput of the etching solution at the front side of the channel section can be increased, for example, by swirling the etching solution. For this purpose, a probe can be inserted into the channel section permanently or (periodically) intermittently. The probe can be a swirling device, e.g.in the form of a propeller, a turbine or the like in order to swirl the etching solution and thus increase the throughput of the etching solution at the etching front.

[0063] In a further measure, the front side of the channel section is mechanically freed of etched particles in order to increase the etching rate at the front side of the channel section. In this case, a nozzle can be used to increase the etching rate at the front side of the channel section. This nozzle is arranged permanently or intermittently in front of the entrance to the channel section or is introduced permanently, progressively or intermittently into the channel section with the aid of a probe or a fluid supply device, e.g. in the form of a hose. Due to the fact that the surrounding, non-irradiated material of the workpiece is etched at the channel wall, the flow of etching solution generated by the nozzle removes more etched particles from the front side of the channel section than from the channel wall. Alternatively, or in addition to the nozzle, a probe can also have a mechanical stirrer, a broom or the like, which is placed near the etching front oris carried along with it to remove etched particles.

[0064] In a further measure, in order to reduce the ratio of the etching rate at the channel wall of the channel section to the etching rate at the front of the channel section, i.e. to increase the etching selectivity, the front of the channel section is exposed to ultrasonic waves. The effect of the ultrasound or ultrasonic waves can be to loosen etched particles, to circulate the etching solution and / or to heat the etching front. In order to expose the front of the channel section to ultrasound, an ultrasonic generator can be used which is arranged outside the workpiece and which radiates the ultrasonic waves through a surface of the workpiece adjacent to the channel section onto the front of the channel section. Alternatively or additionally, an ultrasonic generator on a probe can be inserted into the channel section in order to expose the front of the channel section to the ultrasonic waves.

[0065] Another measure to increase the etching rate is to seal the channel wall of the channel section to prevent etching. A protective lacquer is preferably applied to the channel wall during the sealing process. In this variant, the already etched channel section is sealed against etching with the etching solution along the entire channel wall - but not on the front side. A lacquer, such as a polymer lacquer, can be used for sealing. This lacquer has a protective effect against etching and is not or only slightly attacked by the etching solution. For sealing, it is advisable to periodically, for example daily, remove the workpiece from the etching bath or etching solution, rinse it, and dry it in order to seal a channel section that has been newly etched during the day.Alternatively, the entire previous seal of the channel section can be removed by using an organic solvent, for example, and a new seal can be applied that extends up to just before the etching front or just before the front side of the channel.

[0066] The seal can be applied by dipping the workpiece into a protective lacquer. In doing so, it is necessary to leave the front side of the channel section, which forms the future etching front, free. This can be done by inserting a probe and mechanical cleaning or irradiation with (laser) light to remove the lacquer from the front side. A UV-curing lacquer can also be used. In this case, a probe can be inserted into the channel section, which radiates to the side, i.e. towards the circumferential channel wall, but not in the direction of the channel, i.e. not towards the front side. Finally, the uncured lacquer is rinsed out of the already etched channel section. Alternatively, a sponge or felt body soaked in lacquer can be inserted into the channel section on a probe. This is then prevented from wetting the front surface of the channel, i.e. the future etching front, for example by using a spacer dome or the like.

[0067] As described above, the workpiece can be made of quartz glass, particularly titanium-doped quartz glass, or of a glass-ceramic. As described above, the etching selectivity of quartz glass is comparatively low compared to other materials such as sapphire, so increasing the etching selectivity of this material is desirable. However, quartz glass, particularly titanium-doped quartz glass, is frequently used for the production of substrates for reflective optical elements. The process described above can also be advantageously applied to glass-ceramic materials.

[0068] In selective laser etching, the laser radiation focused into the volume of the workpiece typically has a wavelength of approximately 1 pm or—when using frequency-doubled light—on the order of approximately 500 nm. Neither of these wavelengths allows coupling into the IR absorption bands of quartz glass or titanium-doped quartz glass, nor excitation into the conduction band of quartz glass in a two-photon process. The use of laser radiation with a wavelength on the order of approximately 1 pm is therefore highly inefficient for these materials, as the light is not absorbed linearly but only in a multi-photon process.

[0069] During selective laser etching for at least partial formation of the hollow structure, it is therefore advantageous—regardless of whether the increase in etch selectivity described above is realized or not—if the pulsed laser radiation is focused into the irradiation volume at at least one wavelength that is absorbed in an absorption band of the workpiece material in a wavelength range between 2500 nm and 3120 nm, between 2150 nm and 2230 nm, or between 1380 nm and 1400 nm. The workpiece material can be, in particular, quartz glass or titanium-doped quartz glass.

[0070] For both materials, i.e., quartz glass and titanium-doped quartz glass, it is advisable to use laser radiation that can be absorbed in a two-photon process at approximately 2500 nm, 2230 nm, or 1380 nm, i.e., one that has, for example, twice the wavelength of one of these absorption bands or wavelengths. Alternatively, laser radiation with different wavelengths can also be used, provided the combined photon energy corresponds to that of one of the absorption bands described above.

[0071] The absorption bands of hydroxyl groups in quartz glass result from the transmission curve of quartz glass as a function of wavelength, which can be found, for example, under

[0072] "https: / / www.heraeus.com / media / media / hqs / doc_hqs / products_and_solutions_8 / optics / Data_and_Properties_Quarzglas_fuer_die_Optik_DE.pdf". The absorption bands of titanium-doped quartz glass, or rather the transmission as a function of wavelength, can be found, for example, at "www.pgo-online.com / de / kurven / ule_tkurve.html". It is clear that a higher intensity must be used for a low-OH glass than for a high-OH glass.

[0073] In the case of a workpiece made of quartz glass, in order to at least partially form the hollow structure, the pulsed laser radiation can be focused into the irradiation volume at at least one wavelength that is 351 nm or less, preferably 308 nm or less, particularly preferably 275 nm or less, in particular 266 nm or less. For absorption into the conduction band, it is suitable to use light or laser radiation from an excimer laser at wavelengths of 351 nm, 308 nm, 248 nm, or 193 nm, or frequency-multiplied light from a solid-state laser with a wavelength of 266 nm. A laser emitting at a wavelength of approximately 275 nm can also be used for this purpose.

[0074] For a workpiece made of titanium-doped quartz glass, the pulsed laser radiation can be focused into the irradiation volume at at least one wavelength between 260 nm and 520 nm during at least partial formation of the hollow structure. For absorption into the conduction band, it has proven advantageous to use laser radiation between 260 nm and 520 nm for focusing into the irradiation volume with titanium-doped quartz glass.

[0075] During the at least partial formation of the hollow structure by selective laser etching described above, pulsed (spatially and temporally) coherent laser radiation, in particular pulsed coherent excimer laser radiation, can be focused into the irradiation volume. It has been shown that when quartz glass or titanium-doped quartz glass is irradiated with coherent laser radiation, typically at wavelengths in the UV wavelength range, first optical fiber structures and then small channels (microchannels) form, while the formation occurs significantly delayed after the breaking of the spatial coherence of the laser light. It is to be expected that the optical fiber structures already exhibit a high density of broken bonds and are therefore easily etchable. In any case, the (micro)channels offer the etching solution an increased surface area for the etching process.

[0076] Therefore, in the case described here, instead of IR radiation with pulse durations in the ps or fs range, it is proposed to use excimer laser radiation or frequency-multiplied solid-state laser radiation in the UV wavelength range at wavelengths of approximately 351 nm or less, for example, at 275 nm or less, for selective laser etching. The pulse durations of the pulsed laser radiation are generally in the low ns or high ps range. In this case, the collimated and unfiltered laser beam generated by a laser source is preferably focused directly into the irradiation volume, or more precisely, into a focal volume within the irradiation volume, to prevent the laser beam from losing its spatial and temporal coherence.

[0077] During at least partial formation of the hollow structure, the selective etching in the irradiation volume can be carried out using a reactive plasma, wherein the reactive plasma is preferably supplied to an end face of a channel section formed in the irradiation volume during the selective etching. In this case, too, the end face of the channel section formed in the irradiation volume can be etched at a higher etching rate than a channel wall of the channel section.

[0078] The reactive plasma or reactive plasma species can be, for example, reactive oxygen species, e.g., oxygen radicals, but also other reactive species, e.g., reactive hydrogen species. For etching with the reactive plasma, a comparatively small plasma source on a probe or the like can be guided along the pre-irradiated channels, or more precisely, along an already etched channel section, up to the end face of the channel section, in order to locally expose the end face of the channel section to the reactive plasma.

[0079] The plasma source can be periodically inserted into the channel section and periodically withdrawn, and the debris rinsed out. Alternatively, continuous or quasi-continuous flushing can be used. Flushing without a probe inserted into the channel section is preferably carried out with a liquid solution or a liquid jet, while flushing with a probe inserted is carried out with a gas jet. Since the smallest plasma sources currently have a diameter of approximately 10 mm, the creation of channels with significantly smaller diameters is not possible by inserting a probe into a respective channel. Instead, in this case it is necessary to use a more powerful plasma source that remains close to the entrance of the channel (outside the workpiece). In this case, the plasma can be guided along the etching front or the cooling channel by feeding the plasma via a feeder close to the front of the cooling channel oris guided close to the front side of the already etched channel section. A tube or hose, for example, can be used to feed the plasma or the plasma species, the free end of which is located near the etching front on the front side of the channel. The hose can have ring-shaped or spiral-shaped reinforcing elements to stabilize its cross-section while maintaining good flexibility. Since this results in losses of reactive atoms or species, it is necessary to design such an external plasma source with correspondingly higher performance. In this case, the feed, e.g. in the form of a tube or the like, must be replaced regularly if necessary or can be made of an etch-resistant material or provided with an etch-resistant inner coating or lining.

[0080] As described in WQ2021 / 115643 A1 cited above, it is possible to increase etch selectivity during the etching process by irradiating the etching front, i.e., the area in which the etching solution is currently attacking the substrate material, with the laser radiation used to modify the material or with laser radiation at other wavelengths. In this case, the actual damage or modification of the material during selective laser etching can only occur in the etching bath. In most of the other options described above, the formation of the irradiation volume by focusing pulsed laser radiation and the selective etching of the workpiece in the irradiation volume can also be carried out not only sequentially but, if necessary, in parallel.In the latter case, the etching system must be equipped with an exposure system that enables the focusing of the pulsed laser radiation in the irradiation volume when the workpiece is placed in the etching bath or etching solution. The etching bath or etching solution can be a (slightly) acidic, a substantially neutral, or a basic etching solution. The advantage of a substantially neutral etching solution is that it minimizes roughening. Neutral or slightly acidic, and in particular demineralized or distilled water, can also be used as an etching solution; see also the article "Water-assisted femtosecond laser ablation for fabricating three-dimensional microfluidic chips," Yan Li, Shiliang Qu, Current Applied Physics, Vol. 13, Issue 7, 2013, pages 1292-1295.

[0081] A further aspect of the invention relates to a mirror, in particular an EUV mirror, comprising: a workpiece in the form of a substrate configured as described above, and a reflective coating for reflecting radiation, in particular for reflecting EUV radiation, which is applied to a surface of the substrate. The reflective coating can have a plurality of layer pairs made of materials, each with a different real part of the refractive index, for reflecting radiation.

[0082] A further aspect of the invention relates to a lithography system, in particular an EUV lithography system, comprising: at least one workpiece, as described above, and / or at least one mirror, in particular an EUV mirror, as described above, and a temperature control device, in particular a cooling device, which is designed for a temperature control fluid, in particular a cooling fluid, to flow through the at least one hollow structure. The workpiece can be an optical component or a non-optical, for example mechanical, component, for example a wafer chuck, a wafer table or a structural component of the lithography system, e.g.in the form of a holder, in particular in the form of a frame for holding optical elements, a frame for holding sensors or in the form of a support frame, as used in EUV lithography systems, especially in EUV lithography systems.

[0083] The temperature control device can serve as a cooling device and can, for example, be designed to allow a cooling medium in the form of a cooling fluid, for example a cooling liquid, e.g. in the form of cooling water, to flow through the hollow structure. For this purpose, the temperature control or cooling device can optionally have a pump as well as suitable supply and discharge lines. The temperature control device can also serve as a heating device for heating the workpiece or the substrate. In this case, a temperature control fluid in the form of a heating fluid, which is usually also a liquid, is supplied to the hollow structure in the form of the channel. It is also possible for the temperature control device to be designed both for heating and for cooling the mirror. Water is preferably used as the temperature control fluid for flowing through the hollow structure in the form of the channel - both for cooling and heating.

[0084] The hollow structure of the workpiece or substrate has an inlet opening for the entry of the fluid and an outlet opening for the exit of the fluid. The inlet opening and the outlet opening can be connected to a connection of a fluid supply line or a fluid discharge line in order to connect the hollow structure to the temperature control device. In the event that several fluidically separated hollow structures or channels run in the workpiece or substrate, these are connected to the temperature control device via separate inlet and outlet openings. Further features and advantages of the invention emerge from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in groups in any combination in a variant of the invention.

[0085] drawing

[0086] Examples of embodiments are shown in the schematic drawing and are explained in the following description.

[0087] Fig. 1 shows a meridional section of a projection exposure system for EUV projection lithography,

[0088] Fig. 2a, b schematic sectional views of a mirror of the projection exposure apparatus of Fig. 1 with a hollow structure having a plurality of temperature control channels in the form of cooling channels, the end sections of which merge into distribution channels or collector channels via rounded sections,

[0089] Fig. 3a-d schematic representations of a rounded section between a distribution channel and an end section of a cooling channel with identical flow diameters at four different curvature radii,

[0090] Fig. 4a-d schematic representations of a rounded section between a collector channel and an end section of a cooling channel with identical flow diameters at four different curvature radii,

[0091] Fig. 5a is a perspective view of a substrate for an EUV mirror with a hollow structure analogous to Fig. 2a, b, in which the end sections of the cooling channels are aligned at an obtuse angle to the distribution channels or to the collector channels, Fig. 5b is a schematic view of a rounded section at the transition between an end section of a cooling channel and a distribution channel, and

[0092] Fig. 6a-d representations of a substrate for an EUV mirror with a hollow structure analogous to Fig. 2a, b, in which the distribution channels or the collector channels are aligned at an obtuse angle to an inlet channel or an outlet channel and open into the inlet channel or the outlet channel at a rounded section,

[0093] Fig. 6e,f representations of a multi-part substrate for an EUV mirror with a hollow structure similar to Fig. 6a-d,

[0094] Fig. 7a, b schematic representations of two steps of a method for selective laser etching of a channel into a substrate,

[0095] Fig. 8 is a schematic representation of a channel section formed during selective laser etching with an end face having a higher temperature than the rest of the substrate,

[0096] Fig. 9 is a schematic representation analogous to Fig. 8, in which the front side of the channel section is exposed to an increased throughput of an etching solution in order to increase the etching rate,

[0097] Fig. 10 is a schematic representation analogous to Fig. 8, in which the front side of the channel section is mechanically freed from etched particles to increase the etching rate, Fig. 11 is a schematic representation analogous to Fig. 8, in which the front side of the channel section is exposed to ultrasonic waves to increase the etching selectivity,

[0098] Fig. 12 is a schematic representation analogous to Fig. 8, in which a channel wall of the channel section is sealed with a protective varnish,

[0099] Fig. 13 is a schematic representation analogous to Fig. 8, in which the etching is carried out by means of a reactive plasma which is supplied to the end face of the channel section,

[0100] Fig. 14a-c schematic representations of a channel section formed during selective laser etching with an end face to which a rinsing fluid is supplied via a flexible hose, as well as a nozzle attached to the outlet end of the hose,

[0101] Fig. 15a-c are schematic representations of a flexible hose being introduced into a channel section using a rigid guide element and a tracking device comprising a drive roller and a guide roller,

[0102] Fig. 16a-d schematic representations of the insertion and tracking of the flexible hose by means of a tracking device with two chucks,

[0103] Fig. 17a, b are schematic representations of a surface of a wall of a channel produced in the manner described above in connection with Fig. 7a, b, Fig. 18a, b are schematic representations analogous to Fig. 17a, b on a different scale, and

[0104] Fig. 19 is a further schematic representation of a surface of a wall of a channel manufactured in the manner described above in connection with Fig. 7a, b.

[0105] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0106] The following describes, by way of example, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography with reference to Fig. 1. The description of the basic structure of the projection exposure system 1 and its components is not intended to be limiting.

[0107] One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0108] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.

[0109] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction in Fig. 1 runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0110] The projection exposure system 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0111] The radiation source 3 is an EUV radiation source. The radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example, an LPP source (laser produced plasma, plasma generated using a laser) or a DPP source (gas discharged produced plasma, plasma generated by gas discharge). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser.

[0112] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0113] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0114] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Only a few of these facets 21 are shown by way of example in Fig. 1. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

[0115] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or indeed the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0116] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0117] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical system. The projection optical system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0118] The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

[0119] Fig. 2a, b show an example of a configuration of the mirror M4 of the projection system 10, which in the example shown comprises a monolithic workpiece in the form of a substrate 25. In the example shown, the material of the substrate 25 is titanium-doped quartz glass with a very low coefficient of thermal expansion. The substrate 25 can also be formed from another material that has the lowest possible coefficient of thermal expansion, for example, a glass ceramic. The zero-crossing temperature Tzc of the substrate 25 is between 0°C and 100°C, typically between 19°C and 40°C, in particular between 19°C and 32°C.The zero crossing temperature Tzc is substantially constant in the volume of the substrate 25 and has a spatial variation of less than 3 K, less than 2 K, less than 1 K, or less than 0.1 K, wherein the spatial variation denotes the difference between the maximum and minimum zero crossing temperatures Tzc.

[0120] A reflective coating 26 for reflecting EUV radiation 16 is applied to a surface 25a of the substrate 25, which is shown in Fig. 1. A partial area of ​​the surface 25a, which is located within the reflective coating 26, is struck by the EUV radiation 16 of the projection system 10 and forms an optically used partial area of ​​the reflective coating 26 (not shown). For reflecting the EUV radiation 16, the reflective coating 26 can, for example, have a plurality of layer pairs made of materials, each with a different real part of the refractive index, which can be formed, for example, from Si and Mo at a wavelength of the EUV radiation 16 of 13.5 nm.

[0121] The substrate 25 has a hollow structure 27 through which a fluid 28 can flow, which in the example shown is water. The fluid 28, indicated by an arrow in Fig. 2a, enters the substrate 25 via an inlet opening 29 on a side surface to flow through a plurality of temperature control channels in the form of cooling channels 31, which form part of the hollow structure 27, in order to thereby cool, in particular, the surface 25a of the substrate 25 to be temperature-controlled, to which the reflective coating 26 is applied.

[0122] For supplying the fluid 28 to the inlet opening 29 and for discharging the fluid 28 from an outlet opening not shown in Fig. 2a, b, the projection exposure system 1 has a temperature control device 32 which is designed in the form of a cooling device, which is shown schematically in Fig. 1. In the example shown, the cooling device 32 serves to supply a coolant in the form of cooling water 28 to the mirror M4 and for this purpose has a supply line (not shown) which is fluid-tightly connected to the coolant inlet 29. The cooling device 32 also has a discharge line (not shown) for discharging the cooling water 28 from the coolant outlet (not shown). The other mirrors M1-M3, M5, M6 of the projection system 10 can also have a hollow structure 27 which is connected to the cooling device 32 or, if appropriate,connected to additional cooling devices provided for this purpose. Instead of a cooling device 32, a temperature control device can also be provided in the projection exposure system 1, i.e., a device used for cooling and / or heating the mirrors M1-M6. A suitable temperature control fluid 28 can be used for heating, for example, water, which is heated to a desired temperature before being supplied to the hollow structure 27.

[0123] As can be seen in Fig. 2a, the fluid 28 enters an inlet channel 33 of the hollow structure 27 via the inlet opening 29, which forms a fluid distributor and from which a plurality of distributor channels 34 branch off, each of which is connected to one of the plurality of temperature control channels, which are referred to below as cooling channels 31. The cooling channels 31 are arranged at a distance A of approximately 5 mm from the flat surface 25a of the substrate 25 in the example shown and extend parallel to the surface 25a, i.e. parallel to an XY plane of an XYZ coordinate system. The cooling channels 31 run in a straight line, are aligned parallel and extend in the longitudinal direction, which corresponds to the Y direction, over approximately the entire portion of the surface 25a of the substrate 25 covered by the coating 26, cf. Fig. 2b. From the cooling channels 31, the fluid 28 flows via a plurality of collector channels 36 to a fluid collector, which in the case of the device shown in Fig.2b, is designed as an outlet channel 35. The outlet channel 35 has the outlet opening described above, not illustrated in Fig. 2a, b, through which the fluid 28 exits the hollow structure 27 of the substrate 25.

[0124] As can be seen in Fig. 2b, the hollow structure 27 has a first rounded section 37a, at which a respective distribution channel 34 merges into a cooling channel 31. Correspondingly, the hollow structure 27 also has a second rounded section 37b, at which a respective cooling channel 31 merges into a collector channel 36. In the example shown, the cooling channels 31 run straight in the horizontal direction, which corresponds to the Y direction, and the distribution channels 34 and the collector channels 36 run straight in the vertical direction, which corresponds to the Z direction. Accordingly, the longitudinal axes of the cooling channels 31 are aligned at an angle y of 90° to the distribution channels 34 and the collector channels 36, respectively.The rounded section 37a, b serves to create the most streamlined flow path possible, thus avoiding or at least significantly reducing the occurrence of turbulence, which would occur with a non-rounded, "angular" 90° bend. The reduction in turbulence results in a reduction in the flow-induced vibrations of the reflective optical element M4.

[0125] The distribution channel 34 shown in Fig. 2a, b, the cooling channel 31 adjacent thereto, and the collector channel 36 adjacent to the cooling channel 31 form three sections of a continuous, curved channel 31, 34, 36, the length Lc of which is approximately 10 cm in the example shown. However, the length Lc of the channel 31, 34, 36 k can also be greater and be approximately 15 cm or more, approximately 20 cm, or more than approximately 20 cm. The diameter of the channel 31, 34, 36 can, for example, be between approximately 1 mm and approximately 20 mm, in particular between approximately 1 mm and approximately 5 mm. In principle, the cross-sectional area A q of the channel 31, 34, 36 over the length Lc of the channel 31, 34, 36 may not vary by more than + / - 20% or by more than + / - 10%. In the example shown, the channel 31, 34, 36 has a cross-sectional area A q which varies by no more than + / - 2% over the length Lc of the channel 31, 34, 36.

[0126] For optimized flow guidance at the 90° bend, it is advantageous if the rounded section 37a, b has a constant radius of curvature R, as shown in Fig. 3a-d and Fig. 4a-d, respectively. A key parameter for optimal flow guidance is the ratio between the radius of curvature R of the rounded section 37a, 37b and the flow diameter D.

[0127] Fig. 3a-d show the first rounded section 37a, at which an end section 31a of a respective cooling channel 31 and a distribution channel section 34a adjacent to the end section 31a adjoin one another, with four different ratios between the radius of curvature R of the rounded section 37a and the diameter D of the rounded section 37a. The radius of curvature R is measured in the center of the rounded section 37a, as shown in Fig. 3a-d. In all four examples shown, the diameter D of the rounded section 37a is 5 mm. The diameter D of the rounded section 37a corresponds to the diameter D of the distribution channel 34 and the diameter D of the cooling channel 31. The length L in the illustrations in Fig. 3a-d is approximately 50 mm. As can be seen in Fig. 3a-d, the ratio R / D in the four examples shown is R / D = 2, R / D = 3, R / D = 4 and R / D = 5, respectively.

[0128] Fig. 4a-d show, analogously to Fig. 3a-d, the second rounded section 37b, at which an end section 31b of a respective cooling channel 31 and a collector channel section 36a adjacent to the end section 31b merge into one another. The diameter D of the rounded section 37b is 10 mm in Fig. 4a-d. The length L is approximately 60 mm in the illustrations in Fig. 4a-d. In the illustration in Fig. 4a-d, the ratio R / D in the four examples shown is also R / D = 2, R / D = 3, R / D = 4 or R / D = 5. The diameter D of a respective rounded section 37a, 37b is typically between 2 mm and 20 mm, ideally between 2 mm and 12 mm.

[0129] As described above, there is an optimal relationship between the radius of curvature R and the diameter D of the respective rounded section 37a, 37b, at which the centrifugal force acts such that the pressure of the flowing fluid 28 on the outside of the rounded section 37a, 37b increases only minimally compared to the inside of the rounded section 37a, b, and in this way a reduction in boundary layer separation before and after the rounded section 37a, b can be achieved. Fig. 3a-d and Fig. 4a-d show the outlines of regions in which the turbulent kinetic energy of the flowing fluid 28 exceeds a predetermined value. It was assumed here that the fluid 28 flows from the distributor channel section 34a or from the collector channel section 36a into the respective end section 31a or 31b of the cooling channel 31.

[0130] For this purpose, a ratio between the radius of curvature R of the rounded section 37a, 37b and the diameter D of the rounded section 37a, 37b has proven particularly advantageous, which lies between 2 and 6, better between 2.5 and 5, ideally between 2.5 and 3.5. With a ratio R / D less than 2, typically no significant reduction in boundary layer separation can be achieved. An optimal value for the ratio R / D is typically between 2.5 and 3.5, but the optimal value can also lie outside this range. With a ratio R / D of more than 6.0, the flow behavior typically deteriorates. As described above, the rounded section 37a, b cannot be produced in a monolithic substrate 25 in practice using conventional machining methods.In the example shown, only the inlet channel 33 and the outlet channel 35 are manufactured using a conventional machining process, namely by drilling a respective hole into the substrate 25. The distribution channels 34, the cooling channels 31, and the collector channels 36, however, were manufactured by selective laser etching of the material of the substrate 25, as described in more detail below. In principle, the inlet channel 33 and the outlet channel 35 can also be manufactured by selective laser etching.

[0131] In the hollow structure 27 described above, only the two sections 37a, 37b are rounded, while the distribution channels 34, the collector channels 36, and the cooling channels 31 run in a straight line. However, more complex hollow structures 27 can also be produced using the selective laser etching described below. Fig. 5a, b show an example of such a hollow structure 27 in a substrate 25, which essentially corresponds to the hollow structure 27 shown in Fig. 2a, b. The hollow structure 27 differs from the hollow structure 27 of Fig. 2a, b in that the cooling channels 31 have a slight curvature that follows the curvature of the concavely curved surface 25a in the example shown. An end section 31a of a respective cooling channel 31 adjacent to the distribution channel 34 is oriented at an angle y of approximately 115° in the example shown in Fig. 5b.Despite the fact that the cooling channel 31 has a curvature that runs in the ZX plane, a longitudinal axis that defines the angle y can be defined for the end section 31a adjacent to the rounded section 31a. It is understood that the second rounded section 37b, not shown in Fig. 5a, b, is formed correspondingly to the first section 37a. The ratio R / D between the radius of curvature R and the diameter D of the respective rounded sections 37a, b typically lies within the range of values ​​described above.

[0132] In the substrate 25 shown in Fig. 6a-f, the hollow structure 27 is formed essentially like the hollow structure 27 shown in Fig. 2a, b, but differs from this in that the distribution channels 34 and the collector channels 36 do not run in the vertical direction, but are aligned at an angle of approximately 25° to the thickness direction Z of the substrate 25. The hollow structure 27 shown in Fig. 6a-d, like the hollow structure 27 shown in Fig. 2a, b, has two rounded sections 37a, 37b (not shown) between the respective distribution channels 34 or collector channels 36 and the cooling channels 31. The angle y between the distribution channels 34 or the collector channels 36 and the cooling channels 31 is also 90° in this case, but this runs in a plane which is inclined by approximately 25° to the thickness direction Z, as can be seen in Fig. 6d, which has an angle y' of approximately115° between the longitudinal axis of the inlet channel 33 and a respective distribution channel 34.

[0133] The hollow structure 27 shown in Fig. 6a-d has rounded sections 38, at which an opening section 34b of a respective distribution channel 34 merges into the inlet channel 33, more precisely into a branching section 33a of the inlet channel 33, or at which an opening section 36b of a respective collector channel 36 merges into a branching section 35a of the outlet channel 35. In the example shown, the respective rounded section 38 does not have a constant diameter or flow cross-section; rather, the flow cross-section decreases starting from the branching section 33a. The rounded section 38 also does not have a constant radius of curvature R, as is the case with the two curved sections 37a, b, which run between the respective distribution channels 34 or collector channels 36 and a respective cooling channel 31. Accordingly, no optimized ratio of curvature radius R to diameter D can be specified.The rounded section 38 can also be produced using the selective laser etching process described below.

[0134] The hollow structure 27 shown in Fig. 6e,f differs from the hollow structure 27 shown in Fig. 6a-d in that the substrate is not monolithic, but is composed of three substrate parts 39a, 39b, 39c made of titanium-doped quartz glass. The second substrate part 39b and the third substrate part 39d are attached to the underside of the first substrate part 39a, more precisely, they are permanently connected to the underside of the first substrate part 39a. In the example shown, the permanent connection is established by thermal bonding, in which the flat underside of the first substrate part 39a is connected to the flat top side of the second substrate part 39a and to the flat top side of the third substrate part 39b in the bare state. Optionally, the two substrate parts 39a, 39b; 39a, 39c can each be connected to one another by wringing before bonding.

[0135] The cooling channels 31, the respective rounded sections 37a, 37b and the distributor channels 34 and collector channels 36 adjoining them, with the exception of the respective mouth sections 34b, 36b, run in the first substrate part 39a. The mouth sections 34b of the distributor channels 34 run in the second substrate part 39b, and the mouth sections 36b of the collector channels 36 run in the third substrate part 39c. The inlet channel 33 of the hollow structure 27 also runs in the second substrate part 39b, and the outlet channel 35 of the hollow structure 27 runs in the third substrate part 39c. Both the inlet channel 33 and the outlet channel 35 are introduced into the second and third substrate parts 39a, 39b, respectively, by drilling. The mouth sections 34b of the distribution channels 34 are drilled into the second substrate part 39b starting from its upper side before it is connected to the underside of the first substrate part 39a.Accordingly, the opening sections 36b of the collector channels 36 are drilled into the third substrate part 39c, starting from the upper side thereof. As can be seen in Fig. 6f, no rounded section is formed at the respective opening sections 34b, 36b.

[0136] As can also be seen in Fig. 6f, the respective distribution channels 34, which are formed in the first substrate part 39a, directly adjoin the respective mouth section 34b. The distribution channels 34 and the collector channels 36 are also largely formed by creating a bore starting from the underside of the first substrate part 39a. Only the rounded sections 37a, b and the cooling channels 31 are produced in the first substrate part 39a by selective laser etching. For this purpose, the first substrate part 39a, with the respective pre-drilled distribution channels 34 and collector channels 36, is immersed in an etching solution, as described in more detail below. The bonding of the second and third substrate parts 39b, c is typically carried out after the selective laser etching of the first substrate part 39a. In principle, however, it is also possible to carry out the selective laser etching only after the permanent connection orBonding of the three substrate parts 39a-c.

[0137] It is understood that the hollow structure 27, which has at least one rounded section 37a, b, 38, is not limited to the examples described above, but that, in principle, other, more complex hollow structures 27 can also extend in the substrate 25, which have one or more such sections. Furthermore, not only can the cooling channels 31 have a curvature, as described in connection with Fig. 5a, b, but also the distribution channels 34 and the collector channels 36 can be curved.

[0138] Fig. 7a, b shows two process steps of a selective laser etching process for

[0139] Forming a channel 31 in a substrate 25 for one of the six mirrors Mi of the projection optics 10 of Fig. 1. After the channel 31 has been formed, or more precisely after a plurality of channels 31 have been formed, the highly reflective coating 26 mentioned above is applied to the substrate 25. In the example shown, the channel 31 is a through-channel which can be used as a cooling channel for the flow of a cooling medium (not shown), typically a cooling liquid, e.g. in the form of water, through the substrate 25. To form the channel 31 in the substrate 25, pulsed laser radiation, more precisely a pulsed laser beam 40, is focused into a coherent irradiation volume 41 in the substrate 25 in the method step shown in Fig. 7a.The laser beam 40 is generated by a laser source 42 and impinges in free beam propagation on a focusing optics 43, which in the simplest case can be a focusing lens.

[0140] The substrate 25 is transparent to the wavelength AL of the laser beam 40 and thus enables the focusing of the laser beam 40 onto a focus volume V around a focus position of the laser beam 40. The pulse energy of a respective pulse of the pulsed laser beam 40 is typically only absorbed within the focus volume V by multiphoton processes. In the focus volume V, the transparent material of the substrate 25 is modified in its optical and chemical properties, either without cracks or possibly with microcracks, such that it becomes selectively chemically etchable. By deflecting or moving the focus volume V in the substrate 25, for example by means of a microscanner system (not shown), contiguous regions in the substrate 25 can be modified, forming a contiguous irradiation volume 41. In the embodiment shown in Fig.In the example shown in Fig. 7a, the focus volume V is moved along the Y-axis of an XYZ coordinate system to form a rectilinear irradiation volume 41. Depending on the laser parameters used, the modification of the material of the substrate 25 can be microcracks or other deep damage. The irradiation volume 41 modified by the pulsed laser beam 40 is subsequently removed by wet-chemical etching, as shown in Fig. 7b. During wet-chemical etching, the substrate 25 is typically immersed in an etching solution 44 for several weeks or months, which preferentially (i.e., selectively) dissolves the modified material in the irradiation volume 41 from the substrate 25 until this material is completely removed, so that the channel 31 forms in the substrate 25.

[0141] During selective laser etching, the laser beam 40 is typically focused into the focal volume V in the form of ultrashort pulsed laser radiation (ps or fs pulses). The laser wavelength AL can, for example, be in the IR wavelength range at approximately 1 pm. In the present example, the material of the substrate 25 is titanium-doped quartz glass. With this material, it has proven advantageous if the pulsed laser beam 40 is focused into the irradiation volume 41 at at least one wavelength AL, which lies between 260 nm and 520 nm, in order to absorb the laser radiation in the conduction band of the titanium-doped quartz glass. In the event that the material of the substrate 25 is (undoped) quartz glass, it is favorable for absorption into the conduction band if the wavelength AL of the laser beam 40 is in the UV wavelength range, typically less than 266 nm, e.g., 248 nm or 193 nm.In the latter case, the laser source 42 can be designed, for example, as an excimer laser or as a frequency-multiplied solid-state laser.

[0142] Especially when the material of the substrate 25 is quartz glass or titanium-doped quartz glass, it has proven to be advantageous if the laser beam 40 in the focus volume V satisfies the coherence condition, ie if a coherent laser beam 40 is irradiated into the irradiation volume 41, for the following reason: It has been shown that when quartz glass is irradiated with coherent laser radiation, first light guide structures and then microchannels are formed, whereas when non-coherent laser radiation is used, the formation of such structures and thus a corresponding material modification only occurs significantly later.The use of a coherent laser beam 40 with wavelengths in the UV wavelength range, for example at wavelengths of 351 nm or less, 308 nm or less, 275 nm or less or 266 nm or less, as generated, for example, by an excimer laser or a frequency-multiplied solid-state laser, in combination with comparatively long pulse durations, e.g. in the order of magnitude of approximately 100 picoseconds to 100 nanoseconds, as generated by Q-switched solid-state lasers or gas discharge lasers / excimer lasers, has therefore proven advantageous for the selective laser etching of quartz glass or titanium-doped quartz glass.

[0143] Alternatively, or possibly in addition to focusing laser radiation at wavelengths in the UV wavelength range, particularly in the case of the two materials mentioned above, i.e. quartz glass or titanium-doped quartz glass, the irradiation volume 41 in the substrate 25 can also be irradiated with a laser beam 40 which has (at least) one wavelength AL which is absorbed in an IR absorption band of the material of the substrate 25 in a wavelength range between 2500 nm and 3120 nm, between 2150 nm and 2230 nm or between 1380 nm and 1400 nm. For this purpose, the laser beam 40 can, for example, have twice the wavelength AL of the above-mentioned wavelength ranges in order to be absorbed in a two-photon process in the corresponding absorption band. Alternatively, the laser source 42 can also generate laser radiation at different wavelengths AL whose added photon energy corresponds to one of the above-mentioned absorption bands.For this purpose, the laser source 42 can optionally have two or more lasers. It is understood that in Fig. 7a, b a single irradiation volume 41 is shown merely to simplify the illustration, but that in principle two or more irradiation volumes 41 can be formed in the substrate 25 in order to form two or more channels 31 in the substrate 25. It is further understood that a network of interconnected (cooling) channels can also be formed in the substrate 25. In principle, curved channels 31 can also be formed by selective laser etching, i.e. a respective channel 31 does not necessarily have to be straight, as is shown in Fig. 7a, b. By scanning or moving the laser beam 40 in the volume of the substrate 25, in principle any hollow structures 27 can be produced.

[0144] The channel 31 can be designed, in particular, as described in connection with Fig. 2a, b or Fig. 5a, b and can adjoin a distributor channel 34 or a collector channel 36. In particular, a respective end section 31a, 31b of the channel 31 can merge at a rounded section 37a, b into a respective distributor channel section 34a of the distributor channel 34 or into a respective collector channel section 36a of the collector channel 36.

[0145] The selective laser etching process described above is not limited to the substrate 25 of a mirror for an EUV lithography system 1, but can also be used, for example, to form channels in a substrate 25 of a reflective or transmissive optical element for a DUV lithography system. The selective laser etching process can also be used to form channels in other workpieces or components of a lithography system, for example, in workpieces that serve as holders for optical elements and into which components, e.g., in the form of actuators, sensors, etc., are to be integrated, or for workpieces in the form of a wafer chuck or a wafer table, which are to be tempered using the hollow structure or using channels 31.

[0146] As can be seen in Fig. 8, in the wet-chemical etching step illustrated in Fig. 7b, the etching process begins at the edge or from a lateral surface 25b of the substrate 25 into the irradiation volume 41. The etching forms a channel section 45 that extends within the irradiation volume 41 from a channel inlet on the lateral surface 25b of the substrate 25 to an end face 47 of the channel section, where an etching front is formed. The etching front or the end face 47 of the already etched channel section 45 adjoins a not yet etched volume region 41a of the irradiation volume 41. As the duration of the etching process increases, further material is gradually removed along the irradiation volume 41, ie the not yet etched volume region 41a of the irradiation volume 41 decreases until the channel 31 extends over the entire irradiation volume 41.

[0147] When etching materials such as quartz glass, titanium-doped quartz glass, certain glass ceramics, etc., the etching selectivity in the irradiation volume 41 is comparatively low compared to the surrounding, non-irradiated volume of the substrate 25 and may only be in the order of 1:500. This may result in the channel 31 formed during etching having a significantly larger cross-sectional area near the respective channel inlet on the surface 25a of the substrate 25 than further inside the volume of the substrate 25. However, a cross-sectional area that is not constant over the length of the channel 31 is typically unfavorable, for example with regard to vibrations that are attributable to the flow of the (usually liquid) cooling medium.

[0148] In order to produce a cross-sectional area of ​​the channel 31 that is essentially constant over the length of a respective channel 31, it is advantageous if the end face 47 of the channel section 45 is etched at a higher etching rate As than the respective (circumferential) channel wall 46 of the channel section 45 is etched (etching rate AR < As). In this way, the etching selectivity can be increased, i.e. more material is removed in the direction of the remaining irradiation volume 41a or the later channel 31 than in the direction of the channel wall 46, i.e. transversely to the direction of the channel 31, which in the example shown corresponds to the Y direction of the XYZ coordinate system shown in Fig. 7a, b.

[0149] The etching rate As at the front side of the channel section 45 is typically in the order of magnitude of approximately 100 pm / h up to several mm / h. For increasing the etching rate As at the front side 47 of the channel section 45 or the etching selectivity, i.e., reducing the ratio AR / As between the etching rate AR at the channel wall 46 of the channel section 45 and the etching rate As at the front side 47 of the channel section 45, in the material of the substrate 25, there are various possibilities, several of which are described in more detail below. These possibilities can be used individually or in combination to increase the etching rate As or to reduce the ratio AR / As. In this way, an etching selectivity or a ratio AR / As of more than 1:1500 or possibly more than 1:2000 can be achieved. In order to reduce the overall processing time, the substrate 25 can also be irradiated with several laser sources 42 simultaneously in order to create several channels 31 orto generate several etching fronts simultaneously.

[0150] In the example shown in Fig. 8, in order to increase the etching rate As, a temperature Ts is generated at the end face 47 of the channel section 45 that is at least 20 K, at least 40 K, or ideally at least 60 K higher than a temperature TR at the channel wall 46 of the channel section 45. The temperature TR at the channel wall 46 typically corresponds to the temperature of the etching solution 44 or the substrate 25, with the exception of the end face 47 of the channel section or the etching front. The temperature TR of the etching solution 44 should be as low as possible, i.e., ideally just above or, if appropriate, just below the freezing point of the etching solution 44. In contrast, the etching front at the end face 47 of the channel section 45 is kept at the highest possible temperature Ts, which is ideally at least 60 K higher than the temperature TR at the channel wall 46.

[0151] In order to maintain the temperature Ts at the end face 47 of the channel section 4 during the progress of the etching process, the end face 47 of the channel section 45 is heated by means of a heating device 48, which is moved along with the end face 47 of the channel section 45 during the formation of the channel 31. In this way, the heating device 48 is kept at a constant distance from the end face 47 of the channel section 45 and makes it possible to keep the temperature Ts at the end face 47 of the channel section 45 approximately constant.

[0152] In the example shown in Fig. 8, the heating device 48 is located outside the channel section 45 and rests on the top side 25a of the substrate 25, which is the surface of the substrate 25 that is closest to the channel 31. After the selective laser etching, the reflective coating is applied to the top side 25a in order to form the mirror Mi. In the example shown in Fig. 8, the heating device 48 is guided along the top side 25a of the substrate 25 parallel to the channel 31 or to the etching front on the end face 47 in the Y direction, for which purpose a suitable mechanical movement device can be provided in the etching system shown in Fig. 7b.

[0153] In the example shown in Fig. 8, the heating device 48 is a resistance heater that is in direct contact with the surface 25a in order to transfer contact heat to the material of the substrate 25. However, the heating device 48 can also be a heating light source, for example an infrared light source, or a laser that focuses radiation onto the end face 47 of the channel section 45 and is carried along during the formation of the channel 31. Alternatively, it is also possible to guide or thread the heating device 48 (e.g. in the form of a resistance heater or a light source) through the channel section 45 and to ideally keep the heating device 48 at a constant distance from the etching front or from the end face 47 of the channel section 45. In this case, the heating device 48 can be attached to a suitable support element that has a smaller dimension than the channel diameter.

[0154] Such a support element (probe) 49, which is inserted into the channel section 45 to increase the etching rate As at the end face 47 of the channel section 45, is shown in Fig. 9. In the example shown, the probe 49 is inserted into the channel section 45 and, during the formation of the channel 31, is carried along with the end face 47 of the already formed channel section 45. The probe 49 can, for example, carry a heating device 48, e.g., in the form of a resistance heater.

[0155] In the example shown in Fig. 9, however, the probe 49 serves to carry a swirling device 50 in the form of a propeller in order to increase the throughput of etching solution 44 at the end face 47 of the channel section 45 and thus increase the etching rate As at the end face 47 of the channel section 45. It is understood that instead of a propeller, another type of swirling device, for example a turbine or the like, can be attached to the probe 49. It is also not absolutely necessary for the probe 49 to continuously follow the end face 47 of the channel section 45; rather, the probe 49 can be (periodically) intermittently inserted into the channel section 45 and removed again from it. In Fig.10, to increase the etching rate As, the end face 47 of the channel section 45 is mechanically freed of etched particles 52 by permanently or intermittently arranging a nozzle 51 in front of the channel inlet of the channel section 45. Alternatively, the nozzle 51 can be continuously, progressively or intermittently introduced into the channel section 45 with the aid of a probe 49, as described in connection with Fig. 9. Alternatively, or in addition to the nozzle shown in Fig. 10, a probe 49 can also comprise a mechanical stirrer, a broom, or the like, which is placed near the end face 47 of the channel section 45 or carried along with it in order to remove loosely etched particles 52 from the end face 47 of the already formed channel section 45.

[0156] In the example shown in Fig. 11, to increase the etching selectivity, the end face 47 of the channel section 45 is exposed to ultrasonic waves 53. The ultrasonic waves 53 are generated by an ultrasonic exciter 54 which, as in Fig. 9, is inserted into the channel section 45 on a probe 49 and positioned near the end face 47 of the channel section 45. The effect of the ultrasonic waves can be to loosen etched particles 52, to circulate the etching solution 44 and / or to heat the etching front or the end face 47 of the channel section 45. To generate the ultrasonic waves 53, the ultrasonic exciter 54 can alternatively be arranged outside the substrate 25, e.g., on its upper side 25a, similar to Fig. 8, and can be carried along with the etching front.

[0157] Fig. 12 shows a further possibility for increasing the etching rate As at the end face 47 of the channel 31, in which the channel wall 46 of the channel section 45 is sealed against etching or against attack by the etching solution 44. For this purpose, in the example shown in Fig. 11, a protective lacquer 55 is applied to the channel wall 46. The protective lacquer 55 can, for example, be a polymer lacquer that withstands attack by the etching solution. To apply the protective lacquer 55, it is necessary to remove the substrate 25 from the etching solution 44, rinse it, and dry it. The removal and application of the protective lacquer 55 can take place periodically at predetermined time intervals, for example once a day, in order to seal a channel section newly etched during the day or the entire already etched channel section 45 along the channel wall 46.In the latter case, the entire previous seal of the channel section 45 is removed by using, for example, an organic solvent and a new seal is applied which extends up to just before the etching front or just before the end face 47 of the channel section 45.

[0158] For sealing, the entire substrate 25 is typically immersed in a suitable protective resist 55. It is necessary to leave the end face 47 of the channel section 45 exposed, which forms the subsequent etching front. This can be done by inserting a probe 49 and mechanically cleaning it with a scraper 56, as shown highly schematically in Fig. 12. Alternatively, irradiation with (laser) light can also be used to remove the protective resist 55 from the end face 47 of the channel section 45.

[0159] A UV-curing protective lacquer 55 can also be used for sealing. In this case, a probe 49 can be inserted into the channel section 45, which radiates to the side, i.e., towards the circumferential channel wall 46, but not in the channel direction (Y direction), i.e., not towards the end face 47. Finally, the uncured protective lacquer 55 is rinsed out of the already etched channel section 45. Alternatively, a sponge or felt body impregnated with the protective lacquer 55 can be inserted into the channel section 45 on a probe 49. In this case, for example, the use of a spacer dome or the like can prevent the protective lacquer 55 from wetting the end face 47 of the channel 31, i.e., the future etching front. In the example shown in Fig. 13, instead of the wet-chemical etching shown in Fig. 7b, the channel 31 in the substrate 25 is etched by means of a reactive plasma 57, ieIn this case, no liquid etching solution is used.

[0160] 44 is required. In this example, a plasma source 58 is used for etching, which generates the reactive plasma 57 in the form of reactive plasma species, in the example shown in the form of oxygen radicals.

[0161] In the example shown in Fig. 13, the etching rate As is also increased at the end face 47 of the channel section 45 by supplying the reactive plasma 57 to the end face 47 of the channel section 45. Since the channel 31 in Fig. 13 has a small diameter d of approximately 5 mm, but the plasma source 58 has a diameter of approximately 10 mm, it cannot be introduced into the channel section 45 via a probe 49. The plasma source 58 is therefore arranged near the entrance of the channel section 45 and remains outside the substrate 25.

[0162] In order to guide the reactive plasma 57 to the end face 47 of the channel section 45, a feed 59 in the form of a rigid tube is used in Fig. 13, the outlet end of which is directed close to the end face 47 of the channel section

[0163] 45. A hose or the like can also be used as the feed for the plasma or the plasma species 57, the free, outlet-side end of which is located near the end face 47 of the channel 31. The hose can have annular or spiral-shaped reinforcing elements to enable stabilization of its cross-section while maintaining good flexibility. In the example shown in Fig. 13, the feed 59, e.g., in the form of a small tube, pipe, or the like, must be replaced regularly if necessary, be made of an etch-resistant material, or be provided with an etch-resistant inner coating or lining.

[0164] In the event that the channel 31 or the channel section 45 has a larger diameter, the plasma source 58 can be introduced into the channel section 45 on a probe 49 or the like and guided up to the end face 47 of the channel section 45 in order to locally expose the end face 47 to the reactive plasma 57. Generating the reactive plasma 57 in the immediate vicinity of the end face 47 of the channel section 45 is advantageous because losses of reactive species occur during transport by means of the feed device 59, so that the external plasma source shown in Fig. 13 must be designed to be correspondingly more powerful.

[0165] The feed device 59 or, if applicable, the plasma source 58 can be periodically inserted into the channel section 45 and periodically withdrawn during the etching process, and the debris can be flushed out. Alternatively, continuous or quasi-continuous flushing can be used. Flushing without the feed device 59 or probe 49 inserted into the channel section 45 is preferably carried out with a liquid solution or a liquid jet; flushing with the feed device 59 or probe 49 inserted is carried out with a gas jet.

[0166] Fig. 14a-c describe a further possibility for flushing the end face 47 of the channel section 45 and the channel wall 46 adjacent to the end face 47 of the channel section 45 and thereby freeing them of etched particles. In the example shown in Fig. 14a, a fluid supply in the form of a flexible hose 60 is introduced into the channel section 45, which is curved in the example shown, for this purpose. At an outlet end 60a of the flexible hose 60, a fluid stream 61 emerges, which is directed towards the end face 47 of the channel section 45. The flushing fluid of the fluid stream 61 can be water, for example.

[0167] To ensure efficient flushing, the outlet-side end 60a of the hose 60 should be arranged at an average distance A' of at least approximately 5 mm from the end face 47 of the channel section 45, as this enables suitable swirling of the fluid flow 61 in the region of the end face 47 of the channel section 45. In addition, this achieves flushing of edge regions of the channel wall 46 of the channel section 45 in order to detach particles that have not yet completely separated from the channel wall 46. Such particles can, for example, be lamellar, have a considerable length of, for example, approximately 3 mm, and adhere in particular to the upper side of the channel wall 46. Such particles and bubbles can block the narrow gap between the upper side of the channel wall 46 and the hose 60, thereby blocking the return flow of the flushing fluid, which is why such particles should be removed by means of the fluid flow 61.

[0168] The outlet-side end 60a of the flexible hose 60 should—depending on the outflow volume and the outflow pressure of the fluid stream 61—be arranged at an average distance A' of no more than approximately 15 mm from the end face 47 of the channel section 45, since otherwise the backflow effect of the particle-fluid mixture can no longer be maintained. To enhance the swirling and backflow effect, the hose 60, when guided along the end face 47 of the channel section 45 during the production of the channel 31, can be moved cyclically back and forth, with the distance A' of the outlet-side end 60a of the hose 60 ideally varying within the optimal range of the distance A' between 5 mm and 10 mm.

[0169] As shown in Fig. 14b, for optimal flow onto the end face 47 of the channel section 45, the outlet end 60a of the hose 60 can have a nozzle 51 or an attachment, as described above in connection with Fig. 10. The nozzle 51 serves to atomize the escaping fluid stream 61 into a significantly larger solid angle range than is the case in the example shown in Fig. 14a, in which no nozzle is attached to the outlet end 60a of the hose 60. In this way, a fluid movement is caused that is independent of the orientation of the hose 60, and the end face 47 of the channel section 45 is completely flowed onto. An equivalent or average angle of incidence of the fluid flow 61 at the end face 47 of the channel section 45 is thus independent of the exact orientation of the outlet-side end 60a of the hose 60.

[0170] The nozzle 51 or the hose attachment mounted at the outlet end 60a can, for example, consist of a thin-walled, convexly curved film with a thickness between approximately 10 μm and approximately 50 μm made of a flexible, ductile, and tear-resistant material, e.g., stainless steel, brass, a carbon composite, etc., which is provided with micro-bores. Fig. 14c shows a plan view of such a nozzle 51 with a plurality of micro-bores. The area ratio of the bores to the area of ​​the nozzle 51 shown in Fig. 14c should be greater than 50% to ensure a sufficient flow rate. Alternatively, the nozzle 51 can also consist of a close-meshed net, a fine-pored plug, or a membrane with a comparable atomizing effect.In order to increase the atomization effect and to achieve an optimized backward movement of the rinsing fluid, the hose 60 can be additionally perforated on its circumferential surface in the region of the outlet-side end 60a, as indicated by the circular holes shown in Fig. 14b.

[0171] In order to increase the impact or erosion effect of the rinsing fluid on the end face 47 of the channel section 45 and on the channel wall 46, the fluid flow 61 can be repetitively switched on and off, i.e., intermittent rinsing can occur. In this way, smaller particles close to the end face 47 of the channel section 45 as well as larger, thin-walled and lamellar particles, e.g., on the upper side of the channel wall 46, which detach from a critical length of approximately 4 mm, can be effectively removed. For efficient rinsing of the end face 47 of the channel section 45 and the channel wall 46 adjacent to it, it is necessary to continuously guide the flexible hose 60 described in Fig. 14a-c along the etching front or the end face 47 of the channel section 45 as it migrates within the volume of the substrate 25. The flexible hose 60 may also have to be inserted into areas of the substrate 25 that are difficult to access.To ensure that the hose 60 is inserted into the already formed channel section 45 without kinking, it is necessary that the force point for guiding the hose 60 is positioned as close as possible to a rigid wall defining the channel section 45, and that the force direction is aligned parallel to the initial course of the channel section 45. Fig. 15a illustrates this situation using the transition between the inlet channel 33 drilled into the substrate 25 and the mouth section 34b of the distribution channel 34 of the hollow structure 27 shown in Fig. 5a, b.

[0172] The force point indicated by a horizontal line in Fig. 15a when threading the hose 60 into the mouth section 34b of the distribution channel 34 should be as close as possible to the wall or the material edge of the inlet channel 33 at the transition to the mouth section 34b, and the force direction indicated by an arrow in Fig. 15a when guiding the hose 60 should be aligned as parallel as possible to the longitudinal direction of the mouth section 34b of the distribution channel 34. In the event that the force direction and the force point deviate significantly from the position or alignment shown in Fig. 15a, this inevitably leads to kinking of the flexible hose 60 when inserted into the mouth section 34b of the distribution channel 34, which may prevent further insertion and / or damage the hose 60.

[0173] For the kink-free insertion and feeding of the hose 60 into the opening section 34b of the distribution channel 34, which is oriented at a 90° angle to the inlet channel 33, it is advantageous if the force point and the force direction for the hose feed are shifted outward from the substrate 25. For this purpose, it is advantageous if a stable and rigid connection is established between the starting point of the opening section 34b of the distribution channel 34 and the hose 60.

[0174] Such a connection can be made with the aid of a rigid guide element 62 shown in Fig. 15b. The guide element 62 has a rod-shaped section whose outer diameter is slightly smaller than the diameter of the cylindrical inlet channel 33. The rigid guide element 62 also has an internal channel 63 whose diameter is slightly larger than the diameter of the hose 60 to be guided. The guide element 62 is introduced into the inlet channel 33 with the rod-shaped section. An opening of the internal channel 63 formed on the outer surface of the rod-shaped section is positioned opposite the opening of the mouth section 34b of the distribution channel 34, whereby a virtually positive connection is formed. With the aid of the rigid guide element 62, the point of force and the direction of force for the supply and discharge of the hose 60 are determined.the guide of the tube 60 from the substrate 25 is shifted outwards to the front side of the rigid guide element 62 and enables a kink-free insertion of the tube 60.

[0175] The precise course of the channel 63 within the rigid guide element 62, the exit position and exit angle of the internal channel 63 on the lateral surface of the rod-shaped section, as well as the shape or geometry of the guide element 62 can be adapted or determined depending on the desired course of the hollow structure 27. In the example shown, the internal channel 63 runs along a longitudinal axis in the center of the guide element 62 and is led out of the rod-shaped section of the guide element 62 in an arc shape with a desired exit angle tangentially therefrom at the free end of the rod-shaped section of the guide element 62. The guide element 62 can be designed essentially rotationally symmetrical to the longitudinal axis, but this is not absolutely necessary.

[0176] Adjacent to the rod-shaped portion, the guide element 62 has a portion that projects laterally beyond the substrate 25 and has a slightly larger diameter than the rod-shaped portion that is inserted into the inlet channel 33. This is advantageous because the shoulder formed between the rod-shaped portion and the projecting portion can serve as a stop surface during the insertion of the rigid guide element 62.

[0177] In order to specifically divert the backflow of the rinsing fluid between the hose 60 and the wall of the internal channel 63, in the example shown in Fig. 15b, a backflow adapter 64 is attached in a fluid-tight or watertight manner to the end face of the section of the guide element 62 that laterally projects beyond the substrate 25. The backflow adapter 64 serves to divert the backflowing rinsing fluid laterally at a slotted part of the internal channel 63 via a radial channel indicated in Fig. 15b to the outer surface of the backflow adapter 64 and to supply it to a fluid line (not shown).

[0178] The rigid guide element 62 described above can be manufactured monolithically, for example, by additive manufacturing, e.g., using a 3D printing process. An Al-Si alloy, for example, can be used as the material for the rigid guide element 62, which meets the requirements for complexity and watertightness. As described above, the design of the guide element 62 manufactured using the 3D printing process can be adapted to the geometry of the hollow structure 27. For example, hose guides can be realized with different exit angles of the hose 60 from the outer surface of the rigid guide component 62, adapted to the respective channel geometry. For example, an exit angle of 90° to the longitudinal axis of the rigid guide element 62 can be realized. The return adapter 64 can also be manufactured using the 3D printing process. Unlike in Fig.15b, the return adapter 64 may not be a separate component, but may be integrated into the guide component 62.

[0179] It is possible for the guide element 62 to have a plurality of separate internal channels 63 to enable simultaneous feeding of multiple hoses into multiple channel sections that are processed in parallel. The exit positions on the outer surface of the rod-shaped section of the guide element 62 can be selected differently in the longitudinal direction and / or in the circumferential direction or in the radial direction.

[0180] For automated tracking of the hose 60, a tracking device 65 can be used, as shown in Fig. 15b and Fig. 15c. In the example shown, the tracking device 65 has a guide roller 66a and a directly driven drive roller 66b. As can be seen in Fig. 15c, the guide roller 66a and the drive roller 66b have a throaty cross-sectional shape, and the casing cross-section is also adapted to the cross-section of the hose 60 in order to exert effective contact pressure on the hose 60. In this way, sufficient feed force can be applied to guide the hose 60 and critical indentation of the hose 60 can be prevented. For improved adhesion, the casing surface of the guide roller 66a and the drive roller 66b can also be roughened or knurled.

[0181] As can be seen in Fig. 15b, for tracking the hose 60, the guide roller 66a and the drive roller 66b are arranged directly adjacent to the guide element 62 and the return adapter 64, respectively, to prevent kinking and to guide the hose 60 through the inner channel 63 of the rigid guide element 62 into the mouth section 34b of the distribution channel 34. Alternatively, the tracking device 65 can also be positioned near the entrance of a respective channel of the hollow structure 27 without using the guide element 62.

[0182] The tracking device 65 shown in Fig. 15b,c with the guide roller 66a and the drive roller 66b is generally sufficient for tracking the tube 60—similar to endoscopic tools—if the channel 31 to be formed or an already formed channel section 45 is essentially straight or slightly curved and has a radius of curvature of more than 10 mm. With the help of the tracking device 65, the already inserted tube 60 can also be quickly removed from the respective channel section 45.

[0183] However, in the case of channel walls 46 with smaller curvature radii of less than 10 mm or channel paths with numerous changes of direction, difficulties arise during advancement because the hose 60—pre-bent by the previous path of the channel section 45—is pressed against the outer side of the channel wall 46 in a rounded section, and therefore a greater force must be applied for advancement. Furthermore, the front edge of the hose can become caught on the microscopically roughened channel wall 46 of the channel section 45. Due to the limited contact pressure of the tracking device 65 described in Fig. 15b, c, this can lead to slippage or kinking of the hose 60, which prevents further advancement of the hose 60.

[0184] Fig. 16a-d show a tracking device with which an active rotation of the hose 60 can be realized parallel to the feed movement. The tracking device has an automated, e.g. pneumatically, electrically, ... clampable chuck 67 which can be driven directly via a rotation axis. In addition, the chuck 67 is mounted on a linear axis indicated by a rectangle in Fig. 16a-d. In order to guide the hose 60 along the end face 47 of the channel section 45, the chuck 67, when released from the hose 60, is moved away from the rigid guide element 62 by a corresponding distance, which is typically a few millimeters, without moving the hose 60, as shown in Fig. 16a. The hose 60 is then automatically clamped in the chuck 67 and moved to the guide element 62 with the aid of the linear axis, as shown in Fig. 16b.In this way, the hose 60 is inserted into the channel section 45 by a programmable push length. The chuck 67 can then be automatically released again, and the process repeated to guide the hose 60 step by step through the channel section 45.

[0185] By rotating the entire chuck 67, the hose 60 can be rotated in addition to the linear feed movement when its outlet end is guided along a sharply curved channel section, as indicated in Fig. 16c. This rotation enables the hose 60 to be screwed into the sharply curved channel section with as little friction as possible. To prevent the hose 60 from turning back or from being accidentally pulled out of the channel section 45 when the chuck 67 is repositioned during retraction, the tracking device has an additional chuck 68 that is fixedly mounted. The additional chuck 68 is automatically clamped before the chuck 67 is released for retraction, as indicated in Fig. 16d. In this way, the position of the hose 60 is not changed during repositioning.As soon as the chuck 67 has reached its starting position during the return movement and clamps the hose 60 again, the additional, stationary chuck 68 is released again. Automated, continuous tracking of the hose 60 can also be achieved in this way. It is fundamentally possible to combine the chucks 67, 68 shown in Fig. 16a-d with the guide roller 66a and the drive roller 66b shown in Fig. 15b,c in one and the same tracking device. The tracking device described above makes it possible to automatically track a non-self-moving hose 60 along the end face 47 of an already formed channel section 45, even in complex hollow structures 27 or channels 31 with a large aspect ratio of length to diameter, which can be more than 10:1.

[0186] It should be noted that, to simplify the illustration, only one channel section 45 was shown on the substrate 25 in Fig. 8 to Fig. 16a-d, but that in a through-channel 31, as shown in Fig. 7a, b, a respective channel section 45 with a channel wall 46 and with an etching front on its end face 47 is formed on both sides of the substrate 25. The etching step described further above, in particular with the increase in the etching rate, e.g. by feeding a tube 60, is typically carried out simultaneously on both channel sections of the through-channel 31.

[0187] Fig. 17a shows a microscope image of a partial area of ​​a surface 46a of the wall 46 of a channel 31, which was produced in the manner described in connection with Fig. 7a, b, i.e. by selective laser etching. The channel 31 is not a through-channel as shown in Fig. 7a, b, but rather the temperature control channel 31 shown in Fig. 2a, b, which runs beneath the surface 25a of the substrate 25, to which the coating 26 is applied. To produce the temperature control channel 31, which merges into the distribution channel 34 and the collector channel 36 at the two rounded sections 37a, 37b, the flushing described above in connection with Fig. 14a-c to Fig. 16a-d was carried out using the flexible hose 60. As shown in Fig.17a, the surface 46a has a honeycomb-shaped surface structure with a plurality of substantially circular recesses 70, wherein adjacent recesses 70 merge into one another. As can be seen from Fig. 17a and from Fig. 17b, which shows a profile section of the surface 46a of the wall 46 of the channel 31 along the horizontal line shown in dashed lines in Fig. 17a, the recesses 70 are crater-shaped, ie they each form a depression with a bottom enclosed by an annular raised wall, which is also referred to as a crater rim. As can be seen in the profile section of Fig. 17b, the crater rim of a respective crater-shaped recess 70 is generally not the same height at every point in the circumferential direction, but varies depending on the position in the circumferential direction, which is particularly due to the intertwining or overlapping between the recesses 70.The crater edges of the respective recesses 70 form a net-like surface structure.

[0188] The surface 46a shown in Fig. 17a,b with the crater-shaped recesses 70 has a roughness R a of less than 25 pm. The roughness R a The surface 46a of the wall 46 of the channel 31 is typically 20 pm or less, 10 pm or less, 5 pm or less, and may in particular be 2 pm or less. Both the tempering channel 31 and the distribution channels 34 and the collector channels 36 have a roughness R a in the value range specified above and have a surface structure with the crater-shaped recesses 70 described above.

[0189] While Fig. 17a,b shows a comparatively small section of the surface 46a with a lateral extent of approximately 250 pm by 190 pm, Fig. 18a shows a larger region of the surface 46a with a lateral extent of approximately 2000 pm by 2000 pm. Fig. 18b shows a profile section of the surface 46a of Fig. 18a along the horizontal line shown in Fig. 18a. Fig. 18b shows a particularly large and deep crater-shaped recess 70 whose lateral extent L, which in the example shown corresponds to the diameter of the crater-shaped recess 70 which is circular in plan view, is approximately 300 pm. The depth T of the crater-shaped recess 70 is approximately 2 pm. In general, the crater-shaped recesses 70 usually have a maximum lateral extent L which is not greater than 500 pm, not greater than 450 pm or not greater than 400 pm.The maximum depth T of the crater-shaped recesses 70 is typically not more than 20 pm, not more than 15 pm or not more than 10 pm.

[0190] Fig. 19 shows the surface 46a of the wall 46 of the channel 31, on which an etching treatment was also performed, whereby different laser parameters were used during the preceding irradiation. As can be seen in Fig. 19, the surface 46a also has crater-shaped recesses 70, which have a polygonal basic shape and form a honeycomb-like surface structure. The recesses 70 of the surface 46a shown in Fig. 19 also have the properties described above in connection with Figs. 17a,b and 18a,b with regard to the maximum lateral extent E and the depth T. The surface 46a shown in Fig. 19 also has the values ​​for the roughness R specified above. a on.

Claims

Patent claims 1 . Workpiece, preferably a substrate (25) for a mirror, in particular a substrate (25) for an EUV mirror (M4), comprising: at least one hollow structure (27) which runs in the workpiece (25) and which is designed for the flow of a fluid (28), characterized in that the hollow structure (27) has a first section (31a, 31b; 34b, 36b) and a second, adjacent section (34a, 36a; 33a, 35a), which are aligned with one another at an angle (y, y') between 60° and 120°, preferably at an angle (y, y') between 80° and 100°, in particular at an angle (y, y') of 90°, wherein the hollow structure (27) has a rounded section (37a, 37b, 38) on which the first section (31a, 31b; 34b, 36b) and the second section (34a; 36a; 33a, 35a) merge into one another, and wherein a surface (46a) of a wall (46) of the hollow structure (27) in the first section (31a, 31b; 34b, 36b), in the second section (34a; 36a;33a, 35a) and / or in the rounded section (37a, 37b, 38) a roughness R; a of 25 pm or less, preferably of 10 pm or less, particularly preferably of 5 pm or less, in particular of 2 pm or less.

2. Workpiece according to claim 1, wherein the surface (46a) of the wall (46) of the hollow structure (27) has recesses (70).

3. Workpiece according to claim 2, wherein the recesses (70) are crater-shaped.

4. Workpiece according to claim 2 or 3, in which adjacent recesses (70) overlap one another.

5. Workpiece according to one of claims 2 to 4, wherein the recesses (70) on the surface (46a) of the wall (46) of the hollow structure (27) form a honeycomb-like surface structure.

6. Workpiece according to one of claims 2 to 5, in which the recesses (70) have a maximum lateral extent (E) of not more than 500 pm, preferably not more than 450 pm, in particular not more than 400 pm.

7. Workpiece according to one of claims 2 to 6, wherein the recesses (60) have a maximum depth (T) of not more than 20 pm, preferably not more than 15 pm, in particular not more than 10 pm.

8. Workpiece according to one of the preceding claims, in which the first section (31a, 31b), the second, adjacent section (34a; 36a) and the rounded section (37a, 37b) form channel sections of a channel (31, 34, 36) through which a fluid (28) can flow.

9. Workpiece according to claim 8, wherein the channel (31, 34, 36) has a diameter (D) between 1 mm and 20 mm, preferably between 1 mm and 5 mm, and / or a length (Lc) of at least 10 cm, preferably of at least 15 cm, in particular of at least 20 cm.

10. Workpiece according to claim 8 or 9, wherein a cross-sectional area (A q ) of the channel (31, 34, 36) over the length (L c ) of the channel (31, 34, 36) varies by no more than + / - 20%, preferably by no more than + / - 10%, in particular by no more than + / - 2%.

11. Workpiece according to one of the preceding claims, wherein a radius of curvature R of the rounded portion (37a, 37b) and a diameter D of the rounded portion (37a, 37b) have a ratio R / D which is between 2 and 6, preferably between 2.5 and 5, in particular between 2.5 and 3.

5.

12. Workpiece according to one of the preceding claims, wherein the diameter D of the rounded portion (37a, 37b) is between 2 mm and 20 mm, preferably between 2 mm and 12 mm.

13. Workpiece according to one of the preceding claims, in which the hollow structure (27) has a plurality of tempering channels (31) which run below a surface (25a) of the workpiece (25), and in which the hollow structure (27) has a fluid distributor (33) connected to the tempering channels (31) via distributor channels (34) and a fluid collector (35) connected to the tempering channels (31) via collector channels (36).

14. Workpiece according to claim 13, in which the first section forms an end section (31 a) of the tempering channel (31 ) adjacent to a distribution channel (34) and the second section forms a distribution channel section (34a) adjacent to the end section (31 a) and / or in which the first section forms an end section (31 b) of the tempering channel (31 ) adjacent to a collector channel (36) and in which the second section forms a collector channel section (36a) adjacent to the end section (31 b).

15. Workpiece according to claim 13 or 14, wherein the fluid distributor (33) forms an inlet channel (33) from which the distributor channels (34) branch off and / or wherein the fluid collector (35) forms an outlet channel (35) from which the collector channels (36) branch off.

16. Workpiece according to claim 15, wherein the first portion has a Inlet channel (33) adjacent mouth section (34b) of the distributor channel (34) and in which the second section forms a branching section (33a) of the inlet channel (33) adjacent to the mouth section (34b) and / or in which the first section forms a mouth section (36b) of the collector channel (36) adjacent to the outlet channel (35) and in which the second section forms a branching section (35a) of the outlet channel (35) adjacent to the mouth section (36b) of the collector channel (36).

17. Workpiece according to claim 16, wherein the angle (y') between the branching section (33a) of the inlet channel (33) and the mouth section (34b) of the distributor channel (34) is greater than 90°, preferably greater than 100° and / or wherein the angle (y') between the branching section of the outlet channel (35) and the mouth section (36b) of the collector channel (35) is greater than 90°, preferably greater than 100°.

18. Workpiece, preferably substrate (25) for a mirror, in particular substrate (25) for an EUV mirror (M4), comprising: at least one hollow structure (27) which runs in the workpiece (25) and which is designed for a fluid (28) to flow through, characterized in that a surface (46a) of a wall (46) of the hollow structure (27) has recesses (70).

19. Workpiece according to claim 18, wherein the recesses (70) are crater-shaped.

20. Workpiece according to one of claims 18 or 19, in which adjacent recesses (70) overlap one another. 21 . Workpiece according to one of claims 18 to 20, wherein the recesses (70) on the surface (46a) of the wall (46) of the hollow structure (27) form a honeycomb-like structure.

22. Workpiece according to one of claims 18 to 21, in which the recesses (70) each have a maximum lateral extent (E) of not more than 500 pm, preferably not more than 450 pm, in particular not more than 400 pm.

23. Workpiece according to one of claims 18 to 22, wherein the recesses (70) have a maximum depth (T) of not more than 20 pm, preferably not more than 15 pm, in particular not more than 10 pm.

24. Workpiece according to claim 18 or 23, wherein the surface (46a) of the wall (46) of the hollow structure (27) has a roughness R a of 25 pm or less, preferably of 10 pm or less, particularly preferably of 5 pm or less, in particular of 2 pm or less.

25. Workpiece according to one of claims 17 to 24, wherein the hollow structure (27) is designed in the form of a preferably curved channel (31, 34, 36) through which a fluid (28) can flow.

26. Workpiece according to claim 25, wherein the channel (31, 34, 36) has a diameter (D) between 1 mm and 20 mm, preferably between 1 mm and 5 mm, and / or a length (Lc) of at least 10 cm, preferably of at least 15 cm, in particular of at least 20 cm.

27. Workpiece according to claim 25 or 26, wherein a cross-sectional area (A q ) of the channel (31, 34, 36) over the length (L c ) of the channel (31 , 34, 36) in order not to more than + / - 20%, preferably by no more than + / - 10%, in particular by no more than + / - 2%.

28. Workpiece according to one of the preceding claims, the material of which is selected from the group comprising: quartz glass, in particular titanium-doped quartz glass, and glass ceramic.

29. Workpiece according to one of the preceding claims, the material of which has a zero-crossing temperature (Tzc) which is between 0°C and 100°C, preferably between 19°C and 40°C, particularly preferably between 19°C and 32°C.

30. Workpiece according to one of the preceding claims, the material of which has a spatial variation of the zero crossing temperature (ATzc) which is less than 3 K, preferably less than 2 K, particularly preferably less than 1 K, in particular less than 0.1 K. 31 .Workpiece according to one of the preceding claims, which is monolithic.

32. A method for at least partially forming a hollow structure (27) in a workpiece (25), in particular in a workpiece (25) according to one of the preceding claims, by selective laser etching, comprising: focusing pulsed laser radiation (40) into an irradiation volume (41) in the workpiece (25), and at least partially forming the hollow structure (27) by selectively etching the workpiece (25) in the irradiation volume (41).

33. Mirror, in particular EUV mirror (24), comprising: a workpiece in the form of a substrate (25) according to one of the preceding claims, and a reflective coating (26) for reflecting radiation, in particular for reflecting EUV radiation (16), which is applied to a surface (25a) of the substrate (25).

34. Lithography system, in particular EUV lithography system (1), comprising: at least one workpiece according to one of claims 1 to 31 and / or at least one mirror (M4) according to claim 33, and a tempering device, in particular a cooling device (32), which is designed for a tempering fluid, in particular a cooling fluid (28), to flow through the at least one hollow structure (27).

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