Method and apparatus for producing at least one hollow structure, mirror, EUV lithography system, fluid supply device and method for supplying a fluid - Patents.com

Abstract

The invention relates to a method for producing a hollow structure (28) in a workpiece (25) in the form of a substrate for a mirror, in particular an EUV mirror (M4), by a material removal process with pulsed laser radiation (35), comprising the steps of irradiating the pulsed laser radiation (35) from a radiation inlet side (27) onto the workpiece (25) made of a material transparent to the pulsed laser radiation (35), focusing the pulsed laser radiation (35) in a focal region (39) and moving the focal region (39) along a movement pattern (41) to focus the workpiece (25). The method comprises the steps of forming a removal front (46) for areal removal of material of the workpiece (25) and manufacturing a hollow structure (28) by moving the removal front (46) in the workpiece (25), wherein the removal front (46) which is not aligned perpendicularly to the incident radiation direction (Z) of the pulsed laser radiation (35) at the radiation inlet side (27) of the workpiece (25) is formed at least intermittently during the manufacturing of the hollow structure (28), the hollow structure being manufactured in the form of a channel through which a fluid can flow. The invention also relates to a method for manufacturing a flow channel (28) in a workpiece (25) in the form of a substrate (25) for a mirror (M4), the flow channel (28) being manufactured by a material removal process by pulsed laser radiation (35), and wherein a fluid supply (50) is introduced at least partially into the flow channel (28) during the manufacturing of the flow channel (28). The invention also comprises an apparatus for carrying out the method, and a mirror, in particular an EUV mirror, an EUV lithography apparatus, a fluid supply apparatus and a fluid supply method.

Description

[Technical field]

[0001] The invention relates to a method for forming at least one hollow structure in a workpiece, preferably in the form of a substrate for a mirror, in particular for an EUV mirror, by a material removal process with pulsed laser radiation, comprising the steps of emitting pulsed laser radiation from the radiation incidence side to a workpiece made of a material transparent to the pulsed laser radiation, focusing the pulsed laser radiation in a focal area, moving the focal area along a movement pattern to create a removal front for area-like removal of material of the workpiece, and moving the removal front in the workpiece to create a hollow structure. The invention also relates to a method for creating a hollow structure in the form of a channel in a workpiece, preferably in the form of a substrate for a mirror, in particular for an EUV mirror. The invention also relates to a mirror, in particular an EUV mirror, and to an EUV lithography system having at least one such EUV mirror.

[0002] The invention further relates to an apparatus for forming at least one hollow structure, in particular at least one flow channel, in a workpiece, preferably in the form of a substrate for a mirror, in particular for an EUV mirror, comprising a laser source for generating pulsed laser radiation, a holder for accommodating the workpiece, a focusing device for focusing the laser radiation in a focal region, and a scanner optical unit designed to emit pulsed laser radiation onto the radiation entrance side of the workpiece accommodated in the holder and to move the focal region.

[0003] The invention also relates to a fluid supply device for supplying a fluid to at least one removal front during removal of material by laser ablation, in particular by multiphoton laser ablation, from a workpiece, in particular a monolithic substrate, preferably for an EUV mirror.The invention also relates to a method for supplying a fluid to at least one removal front. [Background technology]

[0004] In the present application, an EUV lithography system is understood to mean an optical system that can be used in the field of EUV lithography. In addition to EUV lithographic projection exposure apparatuses that are used for producing semiconductor components, a lithography system can for example be an inspection system for the inspection of photomasks that are used in such projection exposure apparatuses, hereinafter also referred to as reticles, an inspection system for the inspection of semiconductor substrates to be structured, hereinafter also referred to as wafers, or a metrology system that is used for the measurement of EUV lithographic projection exposure apparatuses or parts thereof, for example for the measurement of a projection optical unit.

[0005] In order to make the structure widths of the semiconductor components formed as small as possible, state-of-the-art projection exposure apparatus, also known as EUV lithography apparatus, are designed for operating wavelengths in the extreme ultraviolet wavelength range, also known as the EUV wavelength range, i.e. in the range of about 5 nm to about 30 nm. Due to the short wavelength radiation, coated mirrors, also known as EUV mirrors, are used for beam steering and focusing, said mirrors comprising a substrate made of a material with a very low thermal expansion coefficient. By way of example, the substrate material can be titanium-doped quartz glass, which has a very low thermal expansion coefficient. The productivity during the formation of the exposed wafers depends heavily on the power of the EUV light source used to generate the EUV radiation. However, a high power of radiation incident on the EUV mirror leads to an increased thermal load on said EUV mirror. Despite the very low thermal expansion coefficient, the thermal power introduced into the substrate leads to shape deviations of the mirror surface with high precision. To meet the demands for increased productivity and, therefore, for more powerful EUV light sources, active cooling of the EUV mirror can be performed.

[0006] An efficient method is provided by volumetric cooling in the form of internal channels through which a liquid, for example water, flows to cool the substrate, for this reason these channels are also referred to below as cooling channels. The channels through which the liquid can flow form elongated cavities closed in the circumferential direction, which do not have branching points and extend between a first and a second end of the channel. At one or both ends the channel may merge with a further hollow structure located in the volume of the substrate. It is also possible that one or both ends of the channel open to the outside of the substrate. The challenge in this situation is to realize hollow structures in the form of cooling channels in the volume of the substrate, which have a relatively large diameter, generally more than about 1 mm, and which extend a small distance, usually less than about 10 mm deep, from the surface of the substrate that is applied with a reflective coating for reflecting EUV radiation.

[0007] To provide and remove the cooling fluid, it is usually necessary to provide a cooling channel oriented substantially parallel to the surface of the mirror that is irradiated with EUV radiation, with curved forward and return channels that may be connected, for example, to the backside of the substrate.

[0008] One approach to creating hollow structures consists in material processing using laser radiation, where the material to be processed is damaged by moderately high pulse intensities. Conventional glasses such as quartz glass, borosilicate glass, or titanium-doped quartz glass are transparent to laser radiation with wavelengths in the visible to near-infrared range. Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide a method and an apparatus which allows the formation of hollow structures having complex shapes, in particular curved and / or bent shapes, in a workpiece, preferably a substrate for a mirror, in particular for an EUV mirror. It is yet another object of the present invention to provide a mirror, in particular an EUV mirror, and an EUV lithography apparatus comprising such a hollow structure. It is yet another object of the present invention to provide a mirror, in particular an EUV mirror, in which flow-induced vibrations occurring when a fluid flows through the hollow structure are reduced. It is yet another object of the present invention to provide a fluid supply apparatus and a method which allow the supply of fluid to at least one removal front even when forming complex hollow structures. [Means for solving the problem]

[0010] In a first aspect of the invention, this object is achieved by a method of the aforementioned type, at least intermittently forming an ablation front not oriented perpendicularly to the incident radiation direction of the pulsed laser radiation on the radiation incident side of the workpiece to form hollow structures, preferably made in the form of channels through which a fluid, in particular a cooling fluid, can flow. The fluid can be a temperature control medium used for heating or cooling. Typically the fluid is a liquid, but it can also be a gas.

[0011] In this application, the term "ablation front" is used synonymously with the term "removal front." Material of the workpiece is removed at the removal front. The removal front forms an interface between the workpiece material and the formed portion of the hollow structure. The sides of the formed portion of the hollow structure are adjacent to the removal front, more precisely the edge contours of the removal front.

[0012] As recognized by the inventors, the advantage of forming a hollow structure having an undercut, e.g. a curved or bent shape, is that the removal front is, at least intermittently, not oriented perpendicular to the incident radiation direction of the pulsed laser radiation on the radiation incident side of the workpiece, but rather the removal front is inclined and oriented at an angle other than 90° to the incident radiation direction, i.e. an angle between 0° and 89°.

[0013] Generally, the inclined ablation front extends in a plane inclined with respect to the incident radiation direction, but in principle it is also possible for the ablation front to be non-planar and deviate from a planar shape. In this case, the angle at which the ablation front is oriented with respect to the incident radiation direction of the pulsed laser radiation is understood to be the angle with respect to the equivalent plane of the ablation front. The equivalent plane is a plane that has the smallest possible distance from all the points of the ablation front. The equivalent plane is determined by conventional ordinary least squares (OLS) methods. In particular, the ablation front can deviate only slightly from a planar or planar shape and can approximate a planar shape, for example in the form of small steps.

[0014] The incident radiation direction is understood to mean the direction in which the pulsed laser radiation is incident on the radiation incidence side. The pulsed laser radiation focused at the respective focal position is usually a laser beam whose propagation direction corresponds to the incident radiation direction. If the pulsed laser radiation is not emitted perpendicularly to the radiation incidence side of the workpiece, the propagation direction of the pulsed laser radiation in the material of the workpiece differs from the incident radiation direction at the workpiece due to refraction.

[0015] The incident radiation direction of the pulsed laser radiation on the radiation incidence side of the workpiece is approximately constant during the formation of the ablation front. If the incident radiation direction of the pulsed laser changes during the formation of the ablation front, the incident radiation direction is understood to mean the arithmetic mean of the incident radiation directions on the radiation incidence side at all focus positions along the trajectory of the movement pattern.

[0016] Generally, the thickness direction of the workpiece is substantially parallel to the incident radiation direction of the pulsed laser radiation at the workpiece. The thickness direction corresponds to the normal direction of the radiation incident side at the location where the pulsed laser radiation passes through the radiation incident side. The radiation incident side, on which the pulsed laser radiation enters the workpiece, can be approximately planar, oriented substantially perpendicularly to the incident radiation direction. However, it is also possible for the radiation incident side to form a curved surface, for example a spherically curved surface, an aspherically curved surface or a free-form surface. However, for example, the planar radiation incident side can be mechanically or otherwise processed, even after the hollow structure or the formation of the hollow structure, in order to form a curved mirror surface. It is also possible for the pulsed laser radiation to be incident non-perpendicularly on the radiation incident side.

[0017] If it is intended to form a hollow structure with an undercut, the removal front may be oriented at an angle other than 90°, i.e. between 0° and 89°, at least during the creation of the undercut, relative to the incident radiation direction. By way of example, such an orientation is advantageous if it is intended to form a part of a hollow structure whose longitudinal axis is substantially parallel to the radiation incident side. Of course, the removal front may also be oriented at an angle other than 90° relative to the incident radiation direction, this angle being constant or possibly varying depending on the position of the removal front in the workpiece during the creation of the entire hollow structure. The angle of the removal front should be changed continuously and slowly, i.e. the difference angle between two removal fronts formed in direct succession should be small.

[0018] The removal of material from the workpiece is usually caused by multiphoton absorption of the applied laser radiation, so the removal process is also called multiphoton laser ablation. Multiphoton absorption can occur in the workpiece material and / or in a fluid, usually a liquid, that is in contact with the removal front during material removal, as will be described in more detail below. Other chemical and / or physical processes can also result in material removal. The parameters of the applied laser radiation can be adapted or optimized for each process that is to result in material removal.

[0019] The hollow structure is preferably produced in the form of curved channels through which a fluid, in particular a cooling fluid, can flow. As further described above, in particular hollow structures provided with undercuts, for example hollow structures in the form of curved channels through which a fluid can flow, can be formed by the method further described above.

[0020] In a variant, the pulsed laser radiation first exits the workpiece material in the region of the ablation front, where the pulsed laser radiation propagates in the workpiece material between the radiation incidence side of the workpiece and the ablation front, i.e. there is no cavity between the radiation incidence side and the ablation front.

[0021] In a development of this variant, the pulsed laser radiation is re-imposed on the workpiece material after it has left it, as is the case when it is intended to form hollow structure sections that do not normally extend parallel to the incident radiation direction. In this case, the workpiece material on which the part of the pulsed laser radiation that was not completely absorbed by the workpiece material in the respective focal zone is incident is located on the side of the removal front that is remote from the radiation incidence surface in the incident radiation direction of the pulsed laser radiation.

[0022] In yet another variant, the removal front in the workpiece is at least intermittently moved in a direction towards the radiation incidence side of the workpiece. If parts of the hollow structure are formed that run parallel to the direction of the incident radiation, the movement of the removal front in the direction towards the radiation incidence side of the workpiece can be parallel to the direction of the incident radiation. However, it is also possible that the movement of the removal front in the direction towards the radiation incidence side is superimposed with a movement perpendicular thereto, for example if bends or curves of the hollow structure are formed.

[0023] In yet another variant, during movement of the removal front within the workpiece, material of the workpiece at least intermittently abuts the side of the edge of the removal front remote from the radiation incident side of the workpiece, and as further mentioned above, this is the case when it is intended to form parts of hollow structures that do not normally extend parallel to the incident radiation direction.

[0024] In yet another variant, the removal front is moved starting from the side of the workpiece opposite to the radiation incidence side. The side of the workpiece opposite to the radiation incidence side on which the removal front starts can be the back side of the workpiece, which is generally oriented substantially parallel to the beam incidence side. However, the side of the workpiece on which the removal front starts can also be a surface located within the volume of the workpiece. By way of example, this can be the case when a hollow structure is already present in the workpiece and the hollow structure formed by the method results from an already formed hollow structure. The hollow structure already present in the volume of the workpiece can have been formed using the method described herein, but in principle it can also have been formed using a different material removal method, such as for example a mechanical grinding method.

[0025] In one variant, a removal front is at least intermittently formed which is oriented at an angle of 0° to 89°, preferably 0° to 80°, particularly preferably 20° to 70°, in particular 30° to 60° to the incident radiation direction. As further mentioned above, such an orientation of the removal front prevents or reduces the introduction of material modifications in the volumetric regions of the workpiece located below the removal front and therefore significantly reduces the stresses introduced into the workpiece material when forming hollow structures.

[0026] In yet another variant, the focal regions are moved along mutually offset trajectories of the movement patterns. In the method described herein, the trajectories of the movement patterns preferably extend linearly with a mutually parallel orientation. By way of example, the distance between adjacent trajectories may be about 0.01 mm to 0.5 mm. However, such an orientation is not mandatory, i.e. the trajectories of the movement patterns may optionally be arranged in the form of concentric contours, for example in the form of concentric circles, or in any other manner. The cross-sectional shape of the movement pattern or its projection onto a plane perpendicular to the movement direction of the removal front corresponds to the cross-sectional shape of the hollow structure. In principle, the cross-sectional shape of the hollow structure may be as desired and may be, for example, circular, elliptical, polygonal, etc. By modifying the spatial extent of the movement pattern depending on the position of the removal front in the workpiece, it is also possible to modify the diameter of the cross section of the hollow structure during the movement of the removal front within a certain range.

[0027] In one variant, the trajectories of the movement patterns are offset from one another by offsetting the focal regions in or along the positive or negative incident radiation direction in order to form ablation fronts that are not oriented perpendicular to the incident radiation direction. In this variant, the trajectories of the movement patterns usually extend in straight lines and are oriented parallel to one another. In the variant described here, the individual trajectories of the movement patterns are offset from one another along the incident radiation direction in order to generate ablation fronts that are oblique to the incident radiation direction. A movement in a certain direction, for example the Z direction, is understood to mean a movement along or parallel to that direction, without taking into account the sign of the direction (+Z, -Z).

[0028] In the simplest case of this variant, the focal range is offset in the direction of the incident radiation by a constant absolute value starting from a trajectory at the outer edge of the movement pattern and proceeding to an adjacent trajectory. As an example, a dynamic zoom lens can be used to offset the focal range in the direction of the incident radiation.

[0029] In this variant, the inclined removal front is formed by fast scanning of a movement pattern in combination with an offset of the focal range in the direction of the incident radiation by a scanner optical unit with a dynamic zoom lens. To move the removal front in the workpiece, a movement of the workpiece, which is usually slower compared to the scanning movement, is superimposed on the scanning movement. Alternatively, in the case of a fixed workpiece, the removal front can be moved by changing the location of the movement pattern in the scanning field of the scanner optical unit. It is possible to modify the position of the removal front by a movement of the workpiece in order to form a first part of the hollow structure and to move the removal front in the workpiece by modifying the position of the movement pattern in the scanning field in order to form a second part of the hollow structure.

[0030] In yet another variant, the pulse energy of the pulsed laser radiation in the mutually offset trajectories of the movement pattern is changed to form an ablation front that is not oriented perpendicular to the incident radiation direction, and the focal area is preferably moved in a plane perpendicular to the incident radiation direction. As an alternative or in addition to creating an offset of the focal area in the incident radiation direction, the pulse energy of the pulsed laser radiation in the mutually offset trajectories can also be changed. The pulse energy in this case is usually continuously increased or decreased starting from a trajectory of a first outer edge of the movement pattern to a trajectory of a second opposite edge of the movement pattern. By way of example, a fast adjustment of the pulse energy can be achieved using an acousto-optical or electro-optical modulator acting on a laser source for generating pulsed laser radiation and having a response time of the order of less than 1 μs.

[0031] Generally, the variants of the method described herein are not combined with offsets of the focal area in the direction of incident radiation, so that it is possible to dispense with a device for dynamically adapting the focal position. Thus, the pulsed laser radiation of the focal area is focused on a focal plane and moved in the focal plane during the generation of the ablation pattern. During the scanning of the trajectory with the scanner optical unit, the focusing of the pulsed laser radiation on the focal plane can be realized by using an Fθ lens or a telecentric lens. At the beginning of the ablation process, the pulsed laser radiation can be focused on a focal plane that is located, for example, on the back side of the workpiece in this case.

[0032] Even when the pulsed laser radiation is focused in a focal plane perpendicular to the incident radiation direction, an oblique removal front can be formed by changing the pulse energy between the individual trajectories, since there was a change in the area of ​​influence of the laser pulses. Depending on the wavelength and the pulse time of the laser pulse, a certain energy density threshold, or alternatively a certain threshold intensity, is necessary for the occurrence of removal of the processed material. The higher the pulse energy, the larger the focal range or the area in the direction of the incident radiation, in which removal of the material occurs starting from the focal plane. Thus, a fast adjustment of the pulse energy allows the formation of a removal front starting from the side of the workpiece far from the radiation entrance side, the removal front being directed at an angle other than 0° to the focal plane, through which the focal area moves. In this case too, the removal front can be moved within the workpiece by the movement of the workpiece, in order to form hollow structures. In this way, an oblique removal front can be generated without the need for additional moving elements for this purpose, for example actuators of a zoom optical unit.

[0033] In one variant, in order to form a portion of the hollow structure which preferably extends substantially parallel to the radiation incidence side, the removal front is at least intermittently moved in a movement direction transverse to the incident radiation direction within the workpiece during the creation of the hollow structure, the side of the removal front closer to the radiation incidence side being preferably oriented at an angle of less than 90°, preferably less than 70°, to the movement direction during the transverse movement relative to the incident radiation direction. Within the meaning of the present application, the phrase "substantially parallel" is understood to mean a parallel orientation or an orientation at an angle of ±10° to the parallel orientation.

[0034] Moreover, the above-mentioned method can be used in particular to form "horizontal" parts of the hollow structure, i.e. parts that run substantially parallel to the radiation entrance side of the workpiece. In the case of a workpiece in the form of a substrate for an EUV mirror, such parts are generally located at a slight distance from the radiation entrance side, which in this case corresponds to the optical surface that will be applied with a reflective coating after the formation of the hollow structure. The "horizontal" parts make it possible to achieve an efficient temperature control of the optical surface of the EUV mirror.

[0035] In yet another variant, the removal front is moved at least intermittently substantially parallel to the incident radiation direction during the creation of the hollow structure in order to form a portion of the hollow structure extending substantially parallel to the incident radiation direction starting from the side of the workpiece opposite to the radiation incidence side, and preferably to form a further portion of the hollow structure extending substantially parallel to the incident radiation direction. After the formation of the hollow structure, in order to form a continuous hollow structure, for example in the form of a through channel, through which a fluid can flow for temperature control, in particular cooling, of the workpiece or substrate, it is generally necessary to connect said "horizontal" part of the hollow structure to the side of the workpiece remote from the radiation incidence side or to any other side of the workpiece. For this purpose, a "vertical" channel part of the hollow structure can be formed starting from the side of the workpiece remote from the radiation incidence side, said "vertical" channel part merging into the "horizontal" channel part of the hollow structure.

[0036] When forming a continuous hollow structure, it is possible to form a first "vertical" portion of the hollow structure and only a part of the "horizontal" portion in a first process step, i.e. a hollow structure is formed that ends with a removal front in the "horizontal" portion. To form a continuous hollow structure, a second "vertical" channel portion of the hollow structure is formed in a second process step, said second "vertical" channel portion starting at a different position on the side of the workpiece that is offset from the position where the already formed first "vertical" portion of the hollow structure starts by the length of the "horizontal" portion, away from the radiation incident side of the workpiece.

[0037] The removal front during the second process step corresponds to the removal front during the first process step mirrored in the incident radiation direction. The second "vertical" part transitions into a further part of the "horizontal" part of the hollow structure, which extends to the same height in the incident radiation direction as the formed part of the "horizontal" part. The two ends of the "horizontal" part of the hollow structure are interconnected in an overlap region by continuous processing. In this way, the hollow structure forms a continuous flow channel which is open and can be supplied with a cooling fluid via a fluid inlet at the first end, said cooling fluid being able to be removed again from the hollow structure at a fluid outlet at the other end. In the overlap region, a seam region is formed. Within the seam region, the nature of the flow channel, in particular the nature of the walls of the flow channel, is different from the nature of the flow channel, in particular the nature of the walls of the flow channel, outside the seam region, as will be described in more detail below.

[0038] In yet another variant, a first part of a hollow structure and an adjacent second part of a hollow structure are produced whose longitudinal directions are oriented at an angle of 70° to 100° relative to each other, preferably at an angle of 90°. As further mentioned above, the formation of hollow structures with undercuts is possible in the case of the method described herein. By way of example, hollow structures in the form of curved cooling channels can be formed using the method described herein. In particular, it is possible to form hollow structures with a 90° curvature, i.e. with a change of direction of about 90° between adjacent parts. By way of example, this is beneficial for the formation of the above-mentioned transition between a "vertical" part of a hollow structure and a "horizontal" part of a hollow structure. However, it is understood that the transition in the manner of a curvature of about 90° described herein does not necessarily have to occur between a "horizontal" part of a hollow structure and a "vertical" part of a hollow structure.

[0039] In one variant, a rounded portion is formed during the creation of the hollow structure, the first and second portions meeting each other at said rounded portion. It has been found to be advantageous that, when the curvature between two adjacent portions is about 90°, a discontinuous transition in the form of a kink does not occur, but the transition is realized continuously along the rounded portion of the hollow structure. Generally, the rounded portion has a constant radius of curvature, but this is not essential and the radius of curvature may vary. The rounded portion simplifies the tracking by a flexible tube for supplying a flux to the removal front or to the hollow structure, which flux effectively removes removal products from the area of ​​the removal front, as will be explained in more detail below.

[0040] In yet another variant, the removal front is brought into contact with a fluid during the creation of the hollow structure, which fluid tracks the removal front during its movement by a fluid supply that is introduced at least partially into the hollow structure. For the removal of ablation products and for cooling the workpiece, it is usually necessary to wash the focal area with a liquid that is transparent, in particular to the pulsed laser radiation. The liquid can be directed to the back side of the workpiece in the form of a free liquid jet by means of a nozzle, or the workpiece can be partially immersed in a liquid bath. However, instead of a liquid, a gas, for example compressed air, can be brought into contact with the removal front to remove the ablation products.

[0041] In the case of a long length of the hollow structure, for example in the form of a channel, a fluid supply must be introduced at least partially into the hollow structure in order to allow localized cleaning in the region of the removal front. For this purpose, a fluid supply or a part of a fluid supply, for example a nozzle, can be introduced at least partially into the hollow structure. By way of example, the fluid supply can be formed as or comprise a rigid tube, at whose free end a nozzle is attached, the tube or nozzle being introduced at least partially into the hollow structure. However, it is also possible that the fluid supply does not have a nozzle. The fluid supply, for example in the form of a tube, has an outer diameter that is slightly smaller than the diameter of the hollow structure, so that the fluid leaving the nozzle can be taken up by a gap between the tube and the wall of the hollow structure.

[0042] In a development of this variant, the fluid supply includes at least one flexible tube or the fluid supply forms at least one flexible tube and the removal front is tracked by the fluid using the at least one flexible tube, in particular starting from the side of the workpiece remote from the radiation incidence side.

[0043] When forming curved hollow structures, optionally with 90° bends or 90° bends, a flexible tube must be introduced into the formed part of the hollow structure for effective removal of the removal products. The free end of the tube, to which the nozzle is usually attached, is in this case positioned at a relatively short distance from the removal front. The tube has a diameter slightly smaller than that of the hollow structure to ensure that the fluid leaving the nozzle can be removed through the gap between the tube and the wall of the hollow structure, usually in a direction towards the back side of the workpiece. The free end or another flexible element with an end piece for the evacuation of the fluid may follow the removal front.

[0044] In yet another variant, the workpiece is moved, in particular displaced, in order to move the removal front. By way of example, the movement of the workpiece can be carried out in the form of a displacement in one, two or three spatial directions. In particular, if there is a transition between two parts of a hollow structure oriented at approximately 90° to one another, it is advantageous to displace or move the workpiece in two spatial directions in superimposed movements. In the case of the formation of hollow structures whose size does not exceed the processing area of ​​the scanner optical unit, the movement of the removal front in the workpiece can also be realized by a movement of the scanning pattern in the processing area without moving the workpiece. In this case too, the orientation of the removal front can be changed with respect to the incident radiation direction during the formation of the hollow structure.

[0045] In yet another variant, the focal area is moved along mutually offset trajectories of a movement pattern by the scanner optical unit to form the ablation front. The movement of the focal area to form the ablation front is in this case performed by fast deflection of the pulsed laser radiation by means of the scanner optical unit, which for this purpose may comprise one or more scanner mirrors, for example in the form of a galvanometer mirror. To form hollow structures, the scanning movement to form the ablation front is usually superimposed with a relatively slow movement of the workpiece. To focus the focal area in the focal plane during the scanning movement, the pulsed laser radiation usually needs to pass through an F-theta lens or a telecentric lens. When generating an offset of the focal area in the incident radiation direction to form an oblique ablation front, such lenses are generally used in combination with dynamic zoom lenses as well. Generally, F-theta lenses or telecentric lenses are also used when generating an oblique ablation front by changing the pulse energy of the pulsed laser radiation.

[0046] In yet another variant, the hollow structure has a circular cross section with a diameter of 1 mm to 20 mm, preferably 1 mm to 5 mm. As further mentioned above, the flow passages can also have a cross section that deviates from a circular cross section. In this case, the diameter of the flow passage is understood to be the so-called equivalent diameter, i.e. the diameter of a circle with an area corresponding to the flow passage cross section of the flow passage, which in this case is not circular. Flow passages with diameters in the above-mentioned value ranges have been found to be advantageous for the flow of fluids.

[0047] In yet another variant, the hollow structure has a length of 10 cm or more, preferably 15 cm or more, particularly preferably 20 cm or more, in particular 70 cm or more. As further mentioned above, hollow structures, in particular in the form of curved channels having a significant length, can be formed using the methods described herein.

[0048] The invention also relates to a method of the aforementioned type, which may also include one or more variants of the method. In this method, the hollow structure is defined by a side surface other than the removal front, and the removal front during the creation of the hollow structure is oriented at a removal front angle relative to an area of ​​the side surface of the hollow structure adjacent to the removal front, the removal front angle being at least intermittently greater than the removal front minimum angle of 1° and at least intermittently smaller than the removal front maximum angle of 89°. The removal front minimum angle is preferably 5°, 10°, 20° or 30°. The removal front maximum angle is preferably 85°, 80°, 70° or 60°. The removal front angle during the creation of the hollow structure may be permanently greater than the removal front minimum angle and / or permanently smaller than the removal front maximum angle, but this is not required. The side surface may be, for example, a cylindrical side surface, but this is not required. By way of example, the hollow structure may be a channel, preferably through which a fluid can flow.

[0049] The invention also relates to a method for producing in a workpiece in the form of a substrate for a mirror, in particular for an EUV mirror, channels through which a fluid can flow, the channels being formed by a material removal process by pulsed laser radiation and a fluid supply being introduced at least partially into the channels during the production of the channels. Furthermore, as mentioned above, the formation of channels having a relatively long length requires the use of a fluid supply which is introduced at least partially into the channels.

[0050] In one variant of the method, a fluid supply is used to supply fluid to the area where the material removal process is performed, in particular to the removal front formed during the material removal process, and the removal front is preferably tracked by the fluid supply during its movement in the workpiece, in particular automatically tracked. Within the scope of material removal processes by pulsed laser radiation, which can for example be carried out by the method further described above, the formation of long channels requires tracking of the removal front by the fluid supply for cooling of the removal front and removal of ablation products. By way of example, the fluid supply can be a tube or the like, the free end of which, to which the nozzle may be attached, is usually arranged at a small distance from the removal front. Tracking of the removal front with the fluid is also necessary when straight channels of considerable length are formed. In this case, it may be possible to optionally dispense with the oblique orientation of the removal front described above. By way of example, automatic tracking can be carried out by means of a tracking device as described below.

[0051] In yet another variant, the fluid supply comprises a flexible element, preferably introduced at least partially into the curved flow path, the flexible element preferably forming a flexible tube. As further mentioned above, the introduction of a flexible element in the form of a flexible fluid line, for example in the form of a flexible tube, allows tracking of the removal front by the fluid even when the flow path is curved and has undercuts, for example in the form of 90° bends.

[0052] In yet another variant, the channel is formed with a length of 10 cm or more, preferably 15 cm or more, particularly preferably 20 cm or more, in particular 70 cm or more. As mentioned above, tracking is necessary when channels with a relatively long length are formed. As mentioned above, the channel is formed by a material removal process with pulsed laser radiation. Parts formed in a different way, for example by machining, for example by drilling, can adjoin the channel. The length of these parts is not taken into account when determining the length of the channel.

[0053] In yet another variant, the substrate is monolithic. In order to avoid as much as possible stresses in the substrate material that occur when joining two or more parts to form the substrate, it is advantageous if the substrate of the mirror, for example of the EUV mirror, is monolithic, i.e. formed in one piece. Furthermore, using the above-mentioned method, it is possible to form hollow structures having virtually any desired shape in such a monolithic substrate. The hollow structures can in particular be cooling channels, i.e. hollow structures that allow the passage of a cooling liquid, for example water. The hollow structures can be continuous cooling channels, but it is also possible for the hollow structures to have one or more branching points. The coolant inlets and coolant outlets of the hollow structures can be arranged on the rear side of the substrate, but this is not necessary, especially if other material removal methods are additionally used to form the entire hollow structure or a number of hollow structures.

[0054] In one variant, the substrate consists of titanium-doped quartz glass or glass ceramic.As further mentioned above, substrates for EUV mirrors usually consist of so-called zero expansion materials, which have a very small coefficient of thermal expansion.

[0055] In principle, hollow structures can also be introduced with the above-mentioned method into workpiece materials that are not suitable for use as substrates for EUV mirrors. Workpiece materials suitable for this method are glasses, crystals and semiconductors. The prerequisite is that the respective material is transparent to the incident laser radiation. For example, in the case of silicon, this applies to wavelengths in the near infrared wavelength range above about 1060 nm.

[0056] In yet another variant, the material of the substrate has a zero cross temperature between 0 ° C. and 100 ° C., preferably between 19 ° C. and 40 ° C., particularly preferably between 19 ° C. and 32 ° C. Zero expansion materials, for example in the form of doped quartz glass, in particular titanium-doped quartz glass, or in the form of certain glass ceramics, have components or phases with a positive thermal expansion coefficient opposed to components or phases with a negative thermal expansion coefficient. This results in a virtually non-linear relationship between thermal expansion and temperature, such that there is only one temperature value at which there is no thermal expansion or which is least susceptible to temperature changes. To be precise, this is the so-called zero cross temperature, also called ZCT.

[0057] In one variant, the material of the substrate has a spatial variation of zero-cross temperature that is less than 3 K, preferably less than 2 K, particularly preferably less than 1 K, in particular less than 0.1 K. It is advantageous if the zero-cross temperature is as constant as possible throughout the volume of the substrate, i.e. it shows as little variation as possible. Spatial variation is understood to mean the difference between the maximum zero-cross temperature and the minimum zero-cross temperature in the volume of the substrate. The spatial variation of zero-cross temperature refers to the substrate after the formation of the hollow structure in the substrate, i.e. the material ablated during the creation of the hollow structure is not taken into account when determining the spatial variation of zero-cross temperature.

[0058] The invention also relates to a mirror, in particular an EUV mirror, comprising a substrate and a coating applied to the substrate and serving to reflect radiation, in particular EUV radiation, in which the substrate comprises at least one hollow structure, preferably in the form of a channel, particularly preferably in the form of a channel through which a fluid can flow, in particular in the form of a cooling channel through which a cooling fluid can flow, formed using the method described above. Generally, the hollow structure is used for temperature control of the EUV mirror, a fluid being passed through it for this purpose. However, the hollow structure can also be introduced in the substrate for any other purpose, for example to integrate one or more components in the substrate, for example in the form of sensors, actuators, etc. In order to avoid interactions occurring between the pulsed laser radiation and the material of the reflective coating, the substrate usually does not yet have a reflective coating at the time of the formation of the hollow structure. The reflective coating is therefore usually only applied to the substrate after the formation of the hollow structure.

[0059] The invention also relates to a mirror, in particular an EUV mirror, comprising a substrate with at least one channel, preferably through which a fluid can flow, the channel being formed by a material removal process by pulsed laser radiation, having a curved morphology and a diameter of 1 mm to 20 mm, preferably 1 mm to 5 mm, and / or a length of 10 cm or more, preferably 15 cm or more, in particular 20 cm or more. The channel may be formed by a removal front moving through the substrate, formed by pulsed laser radiation, in particular during a material removal process by pulsed laser radiation as described above, i.e. by the method described above. In particular, the removal front in this case may not be oriented perpendicularly to the incident radiation direction of the pulsed laser radiation at the radiation incident side of the workpiece, at least intermittently. The curved channel may in particular be formed for the passage of a fluid, for example a cooling fluid.

[0060] In one embodiment, the substrate is monolithic. As discussed above, stresses in the substrate material that occur when two or more sections are joined to form a multi-component substrate may be avoided in a monolithic substrate.

[0061] In yet another embodiment, the flow channel has a first portion and an adjacent second portion whose longitudinal axes are oriented at an angle between 70° and 100°, preferably 90°, relative to one another. A relatively large angle of deflection, of about 90°, can be advantageous in directing fluid through the flow channel to provide effective temperature control of the substrate.

[0062] In one embodiment, the first and second portions merge into one another at a rounded portion. A continuous transition between the two portions along a rounded portion has been found to be advantageous over a transition in the form of a kink, as will be described in more detail below.

[0063] Yet another aspect of the invention relates to an optical element for reflecting radiation in the form of a mirror, in particular an EUV mirror, comprising a preferably monolithic substrate, a reflective coating for reflecting radiation, in particular EUV radiation, the coating being preferably applied to the surface of the monolithic substrate, and at least one hollow structure, preferably extending within the monolithic substrate and designed for the flow of a fluid, the hollow structure having a first part and an adjacent second part oriented at an angle between 60° and 120°, preferably between 80° and 100°, in particular at an angle of 90° relative to each other, and a rounded part where the first part and the second part join each other. The two parts are usually flow passage parts extending substantially linearly immediately adjacent to the rounded part. Immediately adjacent to the rounded part, the two parts have their longitudinal axes oriented at said angle relative to each other. In particular, the first part and the second part may be oriented at an angle of more than 90° relative to each other, for example more than 100°.

[0064] In particular, at the transition between the two parts of the hollow structure of the substrate in the form of a corner or a sharp edge, flow separations can occur at the wall of the hollow structure, which leads to turbulence and causes flow-induced vibrations, if the two parts are oriented approximately perpendicular to each other, i.e. at an angle of 60° to 120° relative to each other. For this reason, it is proposed that the two parts of the hollow structure merge into each other at a rounded section with a profile as streamlined as possible.

[0065] A rounded section is understood to mean a section without corners. As a result, the first section merges continuously into the second section at the rounded section. The cross section or diameter of the hollow structure is usually constant within the rounded section, but may also vary arbitrarily. Generally, the cross section or diameter of the rounded section corresponds to the cross sections of the two sections, but this is not necessary if the rounded section is located at the bifurcation point.

[0066] The substrate is preferably monolithic, that is, formed in one piece and has no interface where two or more sections of the substrate are interconnected.

[0067] The rounded portion can be produced by a material removal process by pulsed laser radiation, as described above in the context of the method for producing a hollow structure or the method for producing a channel, in which a removal front is produced that is oriented obliquely with respect to the incident radiation direction. As described above, hollow structures that deviate from a straight shape and in particular do not run parallel to the thickness direction of the substrate can be produced by an oblique orientation of the processing surface or removal front. By way of example, the rounded portion of a hollow structure can be produced by a simultaneous displacement of an oblique removal front in two mutually perpendicular directions.

[0068] In one embodiment, the R / D ratio between the radius of curvature R of the rounded portion and the diameter D of the rounded portion is between 2 and 6, preferably between 2.5 and 5, in particular between 2.5 and 3.5. If the R / D ratio is more than 2, a significant improvement in terms of flow-induced vibrations can already be achieved, for example more than 50%. Ideally, the R / D ratio is about 2.5 to 3.5, for example 3.0, since the maximum improvement in terms of flow-induced vibrations is usually achieved at this ratio. The R / D ratio should not exceed 6. In this embodiment, the rounded portion has a constant radius of curvature.

[0069] The flow cross section of the rounded portion is usually circular, but can also deviate from the circular shape and have, for example, an elliptical shape, in which case, as already mentioned above, the diameter of the rounded portion is understood to be the so-called equivalent diameter, i.e. the diameter of a circle having an area corresponding to the flow cross section of the rounded portion, which in this case is not circular.

[0070] It was found that the ratio of the radius of curvature of the rounded section to the diameter of the rounded section represents a crucial parameter for turbulence-free, streamlined flow guidance and thus the avoidance of flow-induced vibrations.

[0071] In yet another embodiment, the diameter D of the rounded portion is between 2 mm and 20 mm, preferably between 2 mm and 12 mm. A diameter of the rounded portion or of the channel structure of the hollow structure of the specified order allows the generation of a volume flow rate sufficient for efficient temperature control of the optical element for a given boundary condition. Typically, the flow velocity of the fluid in the hollow structure is of the order of meters per second.

[0072] In one embodiment, the hollow structure comprises a plurality of temperature control channels, in particular in the form of cooling channels, extending below the surface to which the reflective coating is applied, the hollow structure comprising a fluid distribution section connected to the temperature control channels, in particular the cooling channels, via distribution channels, and a fluid return section connected to the temperature control channels, in particular the cooling channels, via return channels. The temperature control channels, which are also referred to below as cooling channels, as they usually serve to cool the substrate, generally extend below the surface in a near-surface region. Near-surface region is understood to mean a distance of 10 mm or less from the surface of the substrate. The distance from the surface is generally measured in the thickness direction of the substrate, oriented perpendicularly to the substantially planar underside of the substrate. As a result of the small distance of the cooling channels from the surface, an effective cooling of the surface of the mirror can be obtained. This distance is understood to mean the minimum distance between each temperature control channel and the surface to which the reflective coating is applied.

[0073] Typically, the fluid distribution and fluid return sections each have a larger cross section than the individual cooling channels. This allows advantageous flow conditions to be set. The fluid distribution and / or fluid return sections are preferably arranged at a greater distance from the surface to be provided with the reflective coating than the cooling channels. This arrangement allows the deformation of the surface due to fluid pressure in the fluid distribution and / or fluid return sections, which generally have cavities with a larger surface area than the cooling channels, to be limited to acceptable limits. The fluid distribution sections are usually connected to a fluid inlet and the fluid return sections are usually connected to a fluid outlet. Each cooling channel may be connected to only one distribution channel and only one return channel, but in principle it is also possible for groups of two or optionally more cooling channels to be connected to a common distribution channel and a common return channel.

[0074] In yet another embodiment, the first portion forms an end portion of the temperature control channel, particularly a cooling channel, adjacent to the distribution channel and the second portion forms a distribution channel portion adjacent to the end portion, and / or the first portion forms an end portion of the temperature control channel, particularly a cooling channel, adjacent to the return channel and the second portion forms a return channel portion adjacent to the end portion.

[0075] The cooling channels typically run substantially parallel to the surface on which the reflective coating is applied. Due to limited installation space within the substrate, the distribution or return channels connected to each cooling channel typically run away from the surface carrying the reflective coating at approximately a right angle, i.e. the return or distribution channel portion and the adjacent end portion of the cooling channel typically run at approximately a right angle to each other, i.e. there is an approximately 90° deflection of the fluid flowing through the hollow structure.

[0076] The above-mentioned rounded portions can avoid or at least substantially reduce flow-induced vibrations, particularly by selecting an appropriate ratio of radius of curvature to diameter.

[0077] In principle, the fluid distribution parts and the fluid connectors can have different designs. For example, the flow cross sections of the fluid distribution parts and the flow cross sections of the fluid return parts can taper, for example in the manner of a funnel, starting from the distribution channels and return channels, respectively, so that the cavities formed by the fluid distribution parts and the fluid return parts in the substrate are not unnecessarily large.

[0078] In yet another embodiment, the fluid distribution section forms an inlet channel from which the distribution channel branches off, and / or the fluid return section forms an outlet channel from which the return channel branches off. In this embodiment, the fluid return section and the fluid distribution section generally extend substantially transversely to the longitudinal direction of the distribution channel and substantially transversely to the longitudinal direction of the return channel. Generally, the distribution channel and the return channel branch off substantially at right angles from the inlet channel and the outlet channel, respectively. By way of example, the fluid distribution section and the fluid return section may be formed as cylindrical channels in this case, which cylindrical channels extend into the substrate starting from the inlet and outlet openings, respectively, outside the substrate. In this case, the inlet and outlet channels may be formed, for example, as perforations, but they can also be formed by the above-mentioned ablation method.

[0079] In a development of this embodiment, the first portion forms a confluence portion of the distribution flow path adjacent to the inlet flow path and the second portion forms a branched portion of the inlet flow path adjacent to the confluence portion, and / or the first portion forms a confluence portion of the return flow path adjacent to the outlet flow path and the second portion forms a branched portion of the outlet flow path adjacent to the confluence portion of the return flow path.

[0080] Furthermore, as mentioned above, the longitudinal direction of the inlet and outlet channels runs substantially perpendicular to the longitudinal direction of the respective return and distribution channels. At each branching point of the distribution or return channels, a streamlined shape is also advantageous, which can be obtained by providing a rounded portion at the branching point of the inlet or outlet channel. In this way, steps can be avoided and edges can be rounded, so that the shape of the hollow structure can be designed to be more streamlined, and separation of the fluid in the inlet and outlet channels can be avoided or at least significantly reduced.

[0081] The ratio of the diameter to the radius of the rounded portion is preferably within the above-mentioned value range. However, the rounded portion may not have a constant radius of curvature at the branching point. The flow diameter of the rounded portion at the branching point does not necessarily have to be constant either. As an example, the cross section of the rounded portion may taper starting from the inlet flow path or from the outlet flow path.

[0082] In yet another embodiment, the angle between the diverging portion of the inlet channel and the merging portion of the distribution channel is greater than 90°, preferably greater than 100°, and / or the angle between the diverging portion of the outlet channel and the merging portion of the return channel is greater than 90°, preferably greater than 100°. It has been found to be beneficial for flow guidance if the diverging portions of the inlet and outlet channels and the diverging portions of the distribution and return channels, respectively, are oriented at obtuse angles with respect to each other.

[0083] In yet another embodiment, the material of the substrate is selected from the group comprising quartz glass, in particular titanium-doped quartz glass, and glass ceramics. To avoid deformation of the surface to which the reflective coating is applied, which may result from non-uniform heating of the substrate material, the substrate of the mirror for EUV lithography is usually made using so-called zero-expansion materials, which have a very small thermal expansion coefficient. As mentioned above, these materials are hard and brittle, which makes mechanical processing difficult. However, using the above-mentioned laser ablation method, hollow structures of virtually any shape can be made with such materials.

[0084] In yet another embodiment, the material of the substrate has a zero cross temperature between 0° C. and 100° C., preferably between 19° C. and 40° C., particularly preferably between 19° C. and 32° C. As mentioned above, the zero cross temperature depends inter alia on the average incident radiant flux during operation of the EUV mirror.

[0085] In one embodiment, the material of the substrate has a spatial variation of the zero cross temperature that is less than 3 K, preferably less than 2 K, particularly preferably less than 1 K, in particular less than 0.1 K. As mentioned above, a high spatial uniformity of the zero cross temperature is usually necessary for efficient operation of the mirror.

[0086] In yet another embodiment, the hollow structure, preferably in particular the channel through which a fluid can flow, has a seam area. As mentioned above, the seam area is usually formed during the creation of the hollow structure in the form of a channel, in which two parts of the channel formed by laser ablation are joined to form a continuous channel. In the seam area, the nature of the channel, in particular the wall of the channel, differs in at least one characteristic from the nature of the channel, in particular the wall of the channel, outside the seam area. By way of example, the surface or surface structure of the wall of the channel in the seam area may differ from the surface or surface structure of the wall of the channel outside the seam area.

[0087] In a development of this embodiment, the hollow structure, preferably in particular the flow channel through which a fluid can flow, has an edge contour of the removal front, at least one bulge, a lateral offset or another structural modification in the seam region.

[0088] In the seam area, the contour or edge contour of the removal front, or optionally of the two removal fronts, where the laser ablation of each part of the channel has ended, may be discernible in the surface structure of the hollow structure in the form of the channel or may be inscribed therein. The edge contour of the removal front discernible in the surface structure of the channel may in particular be at an angle of, for example, 45° to the region of the side of the channel adjacent to the removal front. This angle is usually due to the removal front not being oriented perpendicularly to the incident radiation direction during the creation of the channel. The edge contour of the removal front may be discernible over the entire circumference or possibly only in a portion of the surface structure of the channel.

[0089] The walls of the channel in the seam region may have one or more bulges that respectively form an expansion or contraction of the cross section of the channel defined locally in the longitudinal direction. Furthermore, the walls of the channel may have a slight lateral offset in the seam region in the form of a step, which occurs during the joining of the two parts of the channel and / or because the two parts of the channel have slightly different cross sections. It is understood that the nature of the channel or the walls of the channel in the seam region may also have other structural modifications that distinguish the surface structure of the channel in the seam region from the surface structure of the channel outside the seam region.

[0090] Yet another aspect of the invention relates to a mirror, in particular an EUV mirror, comprising a substrate including a particularly curved channel through which a fluid can preferably flow, said channel having a seam region. As mentioned above, the seam region occurs when two parts of a channel formed by laser ablation are joined to form a continuous channel. The seam region is usually spaced apart from both ends of the channel. The seam region may have approximately the same distance from both ends of the channel, but this is not required. As mentioned above, the properties of the channel, in particular the walls of the channel, within the seam region differ in at least one characteristic from the properties of the channel, in particular the walls of the channel, outside the seam region.

[0091] In one embodiment, the flow channel has an edge contour of the removal front, at least one bulge, lateral offset, or another structural modification in the seam region.

[0092] The mirror according to this aspect of the invention may in particular have the features of the mirror according to the above-mentioned aspect of the invention. The substrate material may in particular be titanium-doped quartz glass or glass ceramic. The substrate may have a monolithic form, although this is not essential.

[0093] Yet another aspect of the invention relates to an EUV lithography system comprising at least one EUV mirror designed as described above and a temperature control device, in particular a cooling device, designed to flow a temperature control fluid, in particular a cooling fluid, through at least one hollow structure, in particular in the form of a channel. The EUV lithography system can be an EUV lithography device for exposing a wafer, or it can be any other optical device using EUV radiation, for example an EUV inspection system for inspection of masks, wafers, etc. used in EUV lithography.

[0094] The temperature control device can act as a cooling device, for example being formed to pass a coolant in the form of a cooling fluid, for example a coolant in the form of a cooling liquid, for example a cooling liquid in the form of cooling water, through the hollow structure in the form of a channel in particular. For this purpose, the temperature control device or the cooling device can optionally have a pump and also suitable supply and removal lines. The temperature control device can also act as a heating device for heating the substrate. In this case, a temperature control fluid in the form of a heating fluid, which is generally also liquid, is supplied to the hollow structure in the form of a channel. It is also possible that the temperature control device is designed to both heat and cool the mirror. In both the cooling and heating cases, water is preferably used as the temperature control fluid passed through the hollow structure in the form of a channel.

[0095] The hollow structure in the substrate has an inlet opening through which fluid enters and an outlet opening through which fluid exits. The inlet and outlet openings can be connected to ports of fluid supply and removal lines, respectively, to connect the flow paths to a temperature controller. When multiple fluidly isolated hollow structures or flow paths extend into the substrate, they are connected to the temperature controller by separate inlet and outlet openings.

[0096] Yet another aspect of the invention relates to an apparatus of the aforementioned type, comprising a fluid supply that can be introduced at least partially into the flow channel, the apparatus being designed to carry out the above-mentioned method of creating the flow channel, within which the material removal process is carried out and within which the fluid supply is introduced at least partially into the flow channel. As mentioned above, the relatively long flow channels require the removal of the ablated processed material with an appropriate fluid supply and cooling of the removal front.

[0097] In one embodiment, the fluid supply is designed to supply fluid to the area where the material removal process is performed, in particular to the removal front formed during the material removal process, with the fluid supply being preferably capable of tracking the removal front during its movement in the workpiece. For this purpose, the fluid supply can remain fixed and the workpiece can move relative to the fluid supply, but there is also the possibility of moving the fluid supply itself, for example by means of a suitable actuator. In this case, the fluid supply typically comprises an element that can be introduced into the flow path, for example in the form of a tube or the like.

[0098] In yet another embodiment, the fluid supply is designed to introduce a flexible element, in particular a flexible tube, at least partially into the flow path. In order to remove the ablated material from the removal front as effectively as possible, the fluid needs to be supplied in the vicinity of the removal front, such as by means of a flexible tube, in particular when this concerns curved flow paths. At the free end, the flexible tube may have a nozzle for issuing a fluid, which may be, for example, water or compressed air. In the case of non-bent cavities or relatively short cavity lengths, for example less than 20 mm, where there is usually a distance between the removal front and the discharge of the fluid from the tube or nozzle, the flexible tube can be arranged in a fixed state and introduced automatically into the cavity by the movement of the workpiece, without any external action on the tube for this purpose. The device described here may comprise a fluid supply device and, optionally, a tracking device for automatic tracking of the removal front by the flexible tube, as will be explained in the context of the fluid supply device described below.

[0099] In yet another embodiment, the scanner optical unit is designed to move the focal region along a movement pattern to form an ablation front for area-like ablation of the workpiece material, and the device is designed to form an ablation front that is not oriented perpendicular to the incident radiation direction of the pulsed laser radiation at the workpiece accommodated by the holder. Usually, the incident radiation direction at the workpiece, more precisely at the radiation incident side of the workpiece, corresponds to the gravity direction. When placed in the holder, the workpiece is aligned such that its thickness direction, which is usually perpendicular to the radiation incident side, corresponds to the gravity direction. The laser source is designed to generate pulsed laser radiation in the form of ultrashort laser pulses, which can generate multiphoton absorption for ablation of the workpiece material.

[0100] There are many options for designing a device that aligns the ablation front in a direction or plane other than perpendicular to the incident radiation direction.

[0101] In one embodiment, the apparatus further comprises a focus offset device for offsetting a focal region of the pulsed laser radiation in or along an incident radiation direction of the pulsed laser radiation, and a controller designed or programmed to control the focus offset device to offset the trajectories of the movement patterns relative to each other in or along the incident radiation direction to form an ablation front that is not oriented perpendicular to the incident radiation direction.

[0102] In this embodiment, the apparatus comprises a focus offset device for dynamically offsetting the focal region along the incident radiation direction of the pulsed laser radiation. By way of example, the focus offset device can be formed as a dynamic zoom lens. The control device can be implemented in suitable hardware and / or software form.

[0103] In yet another embodiment, the device comprises a control device designed or programmed to control the laser source to change the pulse energy of the pulsed laser radiation of the mutually offset trajectories of the moving pattern to form an ablation front that is not oriented perpendicularly to the incident radiation direction. As described above in the context of the method, the pulsed laser radiation is focused by the focusing optical unit without a focus offset device, usually in a plane oriented perpendicularly to the incident radiation direction. In order to focus the pulsed laser radiation on the same focal plane at all positions within the scanning field of the scanner optical unit, the device may comprise an Fθ lens or a telecentric lens. In order to orient the ablation front obliquely with respect to the focal plane of the moving focal region, the pulse energy of the pulsed laser radiation is changed between the mutually offset trajectories in this case.

[0104] To vary the pulse energy, the laser source may include one or more acousto-optic or electro-optic modulators acted upon by a controller. As described above in the context of the method, the pulse energy generally increases or decreases from one edge of the ablation pattern to the opposite edge of the ablation pattern, resulting in the formation of an ablation front that is obliquely oriented relative to the focal plane.

[0105] In yet another embodiment, the device comprises a positioning device for moving the removal front in the workpiece, preferably starting from the side opposite to the radiation incidence side of the workpiece, for the purpose of forming a flow path, the positioning device being designed to displace the workpiece in or along the direction of the incident radiation, preferably in at least one direction transverse to the direction of the incident radiation. For this purpose, the positioning device usually acts on a holder for the workpiece. The positioning device may include one or more drives, for example in the form of linear motors, for realizing a superimposed movement or displacement of the workpiece, in particular in two or three different spatial directions. In principle, the positioning device may also be designed to rotate the non-workpiece.

[0106] Yet another aspect of the invention relates to a fluid supply device of the type mentioned at the beginning, comprising at least one flexible fluid line, preferably a plurality of flexible fluid lines, for supplying fluid to at least one removal front, preferably a plurality of removal fronts, and an insertion component for insertion into a cavity of the workpiece, the insertion component having at least one guide channel for guiding or being able to guide the at least one flexible fluid line, for supplying fluid to the at least one removal front.

[0107] For the supply of fluid to at least one removal front moving in the material of the workpiece during the formation of the hollow structure, the fluid supply device according to this aspect of the invention comprises at least one flexible fluid line capable of tracking the removal front during its movement through the workpiece. In the case of the fluid supply device according to the invention, in order to position the flexible fluid line, more precisely its free end from which the fluid exits, at a specified position in the substrate, the flexible fluid line is guided in a guide channel of an insertion component inserted in the cavity of the workpiece.

[0108] This is particularly advantageous when one or more structures formed by laser ablation, particularly multiphoton laser ablation, branch off from the wall of the cavity, because in this case, an insertion component or guide channel can be used to position each end of a fluid line at the location on the wall of the cavity where the structure emerges, allowing the removal front to be tracked as the structure is formed.

[0109] Prior to the insertion of the insert component into the cavity, a short portion of each structure branching off from the cavity may already have been created by laser ablation. To supply fluid to the removal front that is then formed, the workpiece may be at least partially immersed in a liquid bath. Immersion in the liquid bath is generally not sufficient, since as soon as the removal front is more than a distance, typically about 20-40 mm, from the wall of the cavity, it is no longer possible to remove sufficient ablated material and the ablation process gradually stops. Therefore, to create structures branching off from the cavity and having a length greater than about 20-40 mm, flexible fluid lines are used to fluidically track the movement of the removal front.

[0110] If the hollow structure created by laser ablation does not branch or have other overly complicated shapes, the flexible fluid line can also track the removal front without the insertion component. However, as described above, the insertion component can be used to place the flexible fluid line at the location where the structure should start or branch off from the cavity. Thus, even in the case of a hollow structure with a branching point, the insertion component can be used to ensure the supply of fluid to the removal front without having to manually thread the flexible fluid line from the cavity to the structure where it branches off.

[0111] Laser ablation can be carried out in particular in the form of multiphoton laser ablation. In order to form hollow structures in the case of multiphoton laser ablation, pulsed laser radiation, typically ultrashort pulsed laser radiation, is emitted through the material of the substrate to a point on the back side of the workpiece or on a surface within the workpiece, for example on the wall of the cavity, from which the structure to be formed begins. In the case of multiphoton laser ablation, a removal front is formed, starting from which it moves through the material of the substrate to form the hollow structure. For more information on material removal by multiphoton laser ablation, please refer to the above-mentioned method for creating hollow structures by material removal processing with pulsed laser radiation. As described there, in order to create hollow structures of complex shape, the removal front or processing surface can be aligned not perpendicular to the incident radiation direction of the pulsed laser radiation, but inclined with respect to a plane perpendicular to the incident radiation direction. In this way, it is also possible to create hollow structures by laser ablation that have undercuts that are not linear.

[0112] The fluid is usually a liquid, e.g., water, which exits the flexible fluid line at a relatively high pressure. Instead of a liquid, a gas, e.g., compressed air, can be brought into contact with the ablation front to remove the ablation products. A nozzle for exiting the fluid can be attached to the free end of the flexible fluid line or tube, but this is not required.

[0113] The workpiece is preferably a monolithic substrate, in particular for an EUV mirror. A monolithic substrate is formed in one piece and does not have a joining surface where two or more sub-bodies of the substrate are interconnected. As mentioned above, the hollow structure of such a monolithic substrate cannot be easily produced by machining, for example drilling or grinding, in hard and brittle glass materials, which can be, for example, titanium-doped quartz glass or glass ceramics. The above-mentioned cavities can be produced by machining, for example the cavities can be holes milled in the substrate. However, even if multiphoton laser ablation is time-consuming when producing cavities with large diameters, the cavities can also be produced by this method.

[0114] In one embodiment, the insert component has a number of guide channels, each of which guides a flexible fluid line. Therefore, in the case of a hollow structure, optionally having a significant number of channels or other structures emanating from the cavity, it is advantageous to simultaneously create a number of channels or other structures. For this purpose, a number of pulsed laser beams can be simultaneously emitted through the volume of the workpiece in order to simultaneously form a number of removal fronts along which the material of the workpiece is ablated, so that a number of structures or channels branching out from the cavity can be simultaneously formed.

[0115] To simultaneously form multiple removal fronts, fluids must be simultaneously fed to the removal fronts with a corresponding number of guide channels or flexible fluid lines that track each removal front. Ideally, all structures branching off from the cavity can be created simultaneously. If there are too many branching structures, these structures can be divided into groups and each can be processed simultaneously. Different forms of insertion components can be used to form each group of structures.

[0116] In yet another embodiment, a gap, in particular a ring gap, through which the fluid can flow is formed between the fluid line and the channel wall of the guide channel in order to return the fluid from the removal front. The flexible fluid line has a diameter selected so that the fluid supplied to the removal front in the fluid line can be removed again through the passable gap. In general, the flow cross-sectional area of ​​the gap should at least correspond to the flow cross-sectional area of ​​the fluid in the flexible fluid line.

[0117] In yet another embodiment, the guide channel has at least one rounded section for changing the direction of the flexible fluid line. Generally, the structures branching from the cavity of the workpiece do not run parallel to the direction in which the insert component is inserted or introduced into the workpiece. Therefore, when each guide channel starts from an end face of the insert component, it is generally necessary to change the direction of the flexible fluid line in the insert component. Such a change in direction is ideally performed by guiding the fluid line along a rounded or curved section of the guide channel. In the rounded section, the change in direction of the flexible fluid line is preferably performed at an oblique angle, i.e., at an angle greater than 90°.

[0118] In yet another embodiment, the insert component is rod-shaped and at least one guide channel extends from the end face of the insert component to the side face of the insert component. In this case, the cavity of the workpiece is a generally straight channel, preferably having a constant diameter, which starts from an opening in the side face of the workpiece and extends into the volume of the workpiece. In this case, the insert component is inserted into the cavity by pushing the insert component through the opening of the workpiece into the cavity. In this case, the end face of the insert component is accessible from the outside through the opening of the workpiece, so that the flexible fluid lines move away from the workpiece at the end face of the insert component and can be connected to a fluid supply device, including a pump or the like. The above-mentioned rounded portion of the guide channel can be used to guide each flexible fluid line from the end face of the insert component to the side face of the insert component.

[0119] In a development of this embodiment, the guide channels merge into openings at the sides of the insert component, which openings are preferably arranged next to each other in the longitudinal direction of the insert component, in particular arranged equidistantly from each other in the longitudinal direction of the insert component. Arrangement of adjacent openings in the longitudinal direction of the insert component is understood to mean that the openings extend along a common straight line or line extending in the longitudinal direction of the insert component. In other words, the openings are not offset from each other in the circumferential direction of the insert component. This is advantageous if it is intended to form a number of structures branching off from the cavity along a common line by multiphoton laser ablation. If the structures to be formed are arranged equidistantly from each other, the openings are also arranged equidistantly, i.e. at the same distance from each other in the longitudinal direction of the insert component.

[0120] As a development of this embodiment, the rod-shaped insert component has a cylindrical form and preferably has a diameter of 5 mm to 10 mm. The shape of the insert component is adapted to match the shape of the cavity, which in this case also has a cylindrical form. The diameter of the cavity is slightly larger than the diameter of the insert component. A cylindrical cavity with a relatively large diameter can be formed by machining, for example by grinding, and can be, for example, in the form of a blind hole. In this case, the insert component is usually pressed into the cylindrical cavity until it abuts against the end face of the cavity. In this way, the position of the opening in the longitudinal direction of the insert component is defined. The insert component is further aligned or rotated in such a way that the opening in the side of the insert component is positioned in the circumferential direction corresponding to the point at which the structure branching off from the cavity begins. Furthermore, the end face of the insert component can have a protruding part, i.e. a tongue, which hooks into a notch or groove in the workpiece in order to simplify the axial positioning. Before the insertion of the insert component, a spacer, for example in the form of a solid cylinder, can be introduced into the cavity and the insert component can be brought into contact with the end face of said spacer. In this way, the length of the insert component can be reduced.

[0121] In yet another embodiment, the at least one guide channel has an inner diameter of between 1 mm and 4 mm. The structure exiting the cavity generally has a significantly smaller diameter than the insert component. This can be advantageous as it allows the insert component to accommodate multiple guide channels extending into the insert component.

[0122] In yet another embodiment, at least one fluid line has an outer diameter of 1 mm or less. As mentioned above, a gap for fluid return must generally remain between the fluid line and the wall of each guide channel. Thus, the outer diameter of the fluid line is correspondingly smaller than the inner diameter of the guide channel.

[0123] The insert component can be formed in various ways. For example, the guide channel can be formed as a tube, e.g., a stainless steel tube or a plastic tube, that is bent or curved to form a rounded section. A guide channel in the form of a tube, e.g., a stainless steel tube or a plastic tube, can be put together and molded in a suitable material to create an insert component with the desired shape, e.g., in the manner of a cylinder.

[0124] Alternatively, the insert component can be produced by additive manufacturing, where it consists of a body usually produced by means of 3D printing, with the guide channels in the form of hollow structures being formed during additive manufacturing. The production of the insert component can be made of metal, plastic or glass-like materials typical for 3D printing.

[0125] In yet another embodiment, the fluid supply device comprises a fluid make-up device for supplying fluid to at least one flexible fluid line. The fluid make-up device may include a fluid reservoir for making up the fluid. As mentioned above, it is usually advantageous to have the fluid leave the fluid line at a relatively high pressure. It is therefore advantageous to supply the fluid to the flexible fluid line by means of a pump that is part of the fluid make-up device and generates a corresponding high pressure.

[0126] In yet another embodiment, the fluid supply device comprises at least one tracking device for automatic tracking of the movement of the removal front in the workpiece material by means of at least one flexible fluid line. In the case of multiphoton laser ablation, the removal front usually moves in the volume of the workpiece at a constant processing speed. Automatic tracking by the flexible fluid line is likewise performed at the processing speed. For tracking, the flexible fluid line advances, for example by unwinding at a constant speed from a coil. For tracking by a flexible fluid line, the material of the fluid line usually needs to have sufficient shear stiffness, which is usually the case for materials used for the flexible fluid line. If the removal front moves in the workpiece material at different processing speeds, the tracking device can be designed so that the fluid line can be tracked at an individually adapted speed.

[0127] Yet another aspect of the invention relates to a method for supplying fluid to at least one removal front by a fluid supply device designed as described above during removal of material from a workpiece by multiphoton laser ablation, preferably from a monolithic substrate for an EUV mirror, comprising the steps of inserting an insert component into a cavity of the workpiece and supplying fluid to at least one removal front through at least one flexible fluid line guided in at least one guide channel of the insert component. As described above, the fluid supply device designed as described above allows the flexible fluid line to automatically track the removal front during the movement of the removal front in the workpiece.

[0128] In one variant, the cavity is filled with fluid before the insertion of the insert component, and starting from the cavity filled with fluid, a number of channel sections adjacent to the cavity are formed by multiphoton laser ablation. For the formation of relatively short channel sections or other structures starting from or branching off from the cavity, localized supply of fluid to the removal front by means of flexible fluid lines is not usually necessary. If the length of the channel sections as a whole is less than or equal to about 20 mm to 40 mm, it is sufficient to fill the entire cavity and thus also the channel sections formed during multiphoton absorption with fluid. Usually, for this purpose, the workpiece is partially immersed in a liquid or a liquid bath with a fluid, usually in a water bath.

[0129] In an expanded version of this variant, to form multiple flow paths, multiple removal fronts are formed starting from the end faces of the flow path portions after insertion of the insert component into the cavity and moved within the workpiece material, and multiple flexible fluid lines track the movement of the removal fronts within the workpiece material.

[0130] If the workpiece is a substrate for an EUV mirror, it may for example have two cavities that act as a fluid distribution section and a fluid return section, into which an insert component is inserted, respectively. The two cavities are fluidically interconnected by a number of channels that branch off from the first cavity and open into the second cavity. In this case, it is possible to form a first or second channel section, respectively, from each of the two cavities by multiphoton laser ablation, corresponding to approximately half the length of each channel. Approximately in the middle of the channel length, the removal fronts of the two channel sections being created overlap, resulting in a continuous channel that connects the fluid distribution section and the fluid return section.

[0131] When the channels are cooling channels or structures for an EUV mirror, they have distribution channels that typically do not run in a straight line between a fluid distribution section and a fluid return section, but are angled, and typically have a distribution channel that transports the fluid near the surface starting from the fluid distribution section that is generally at a relatively large distance from the optical surface of the EUV mirror. The fluid, typically in the form of cooling water, is guided along the surface in the channel sections that form the cooling channels before being removed from the surface in the return channel and supplied to the fluid connector.

[0132] Further advantages and aspects of the invention will become apparent from the following description of an embodiment of the invention, with reference to the figures of the drawing which show the essential details of the invention, and from the claims. The individual features can be implemented either alone or in any combination in one variant of the invention.

[0133] Exemplary embodiments are shown in the schematic drawings and explained in the following description. [Brief description of the drawings]

[0134] [Figure 1] 1 shows a schematic illustration of a meridian section of a projection exposure apparatus for EUV projection lithography; [Figure 2a] FIG. 2 shows a schematic diagram of an EUV mirror having a substrate into which hollow structures in the form of cooling channels are introduced. [Figure 2b] FIG. 2 shows a schematic diagram of an EUV mirror having a substrate into which hollow structures in the form of cooling channels are introduced. [Diagram 3] 2b shows a schematic diagram of an apparatus for forming the hollow structure of FIG. 2a by material removal processing of a substrate by pulsed laser radiation while forming an undercut. [Figure 4a] 1 shows a schematic diagram of a plan view of a movement pattern having mutually offset trajectories in the incident radiation direction of pulsed laser radiation at a substrate. [Figure 4b] 1 shows a schematic diagram of a side view of a movement pattern having mutually offset trajectories in the incident radiation direction of pulsed laser radiation at a substrate. [Figure 4c] FIG. 4 is a schematic diagram illustrating the formation of an obliquely oriented ablation front using the apparatus of FIG. 3. [Figure 5a] 1 shows a schematic diagram of a plan view of a movement pattern with mutually offset trajectories that arises upon focusing of pulsed laser radiation with different pulse energies. [Figure 5b] FIG. 5a shows a schematic cross-sectional view of a substrate with an oblique removal front resulting upon focusing of pulsed laser radiation onto a focal plane with ablation patterns at different pulse energies shown. [Figure 6a] 4 is a schematic diagram similar to FIG. 3 including a fluid supply including a nozzle or flexible tube at one of three different stages in the formation of a hollow structure. [Figure 6b] 4 is a schematic diagram similar to FIG. 3 including a fluid supply including a nozzle or flexible tube at one of three different stages in the formation of a hollow structure. [Figure 6c] 4 is a schematic diagram similar to FIG. 3 including a fluid supply including a nozzle or flexible tube at one of three different stages in the formation of a hollow structure. [Figure 7a] 2 shows two process steps during the creation of a hollow structure in the form of a continuous cooling channel. [Figure 7b] 2 shows two process steps during the creation of a hollow structure in the form of a continuous cooling channel. [Figure 7c] The seam area formed at this time is shown. [Figure 8a] 2 shows a schematic cross-sectional view of a mirror of the projection exposure apparatus of FIG. 1, comprising a hollow structure with a plurality of temperature control channels in the form of cooling channels whose end portions merge via rounded sections into distribution or recovery channels. [Figure 8b] 2 shows a schematic cross-sectional view of a mirror of the projection exposure apparatus of FIG. 1, comprising a hollow structure with a plurality of temperature control channels in the form of cooling channels whose end portions merge via rounded sections into distribution or recovery channels. [Figure 9a] Schematic diagrams of the rounded section between the distribution channel and the end of the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 9b] Schematic diagrams of the rounded section between the distribution channel and the end of the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 9c] Schematic diagrams of the rounded section between the distribution channel and the end of the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 9d] Schematic diagrams of the rounded section between the distribution channel and the end of the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 10a]Schematic diagrams of the rounded section between the end of the return channel and the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 10b] Schematic diagrams of the rounded section between the end of the return channel and the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 10c] Schematic diagrams of the rounded section between the end of the return channel and the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 10d] Schematic diagrams of the rounded section between the end of the return channel and the cooling channel for one of four different radii of curvature with the same channel diameter are shown. [Figure 11a] FIG. 8 shows a perspective view of a substrate for an EUV mirror having a hollow structure similar to that of FIGS. 8a and 8b, in which the end portions of the cooling channels are arranged at an obtuse angle to the distribution or recovery channels. [Figure 11b] 1 shows a schematic of a radius at the transition between the end portion of the cooling channel and the distribution channel. [Figure 12a] 8a and 8b show diagrams of a substrate for an EUV mirror having a hollow structure similar to that of FIG. 8a and FIG. 8b, in which the distribution and recovery channels are arranged at obtuse angles to the inlet and outlet channels, respectively, and merge into the inlet or outlet channels at rounded portions. [Figure 12b] 8a and 8b show diagrams of a substrate for an EUV mirror having a hollow structure similar to that of FIG. 8a and FIG. 8b, in which the distribution and recovery channels are arranged at obtuse angles to the inlet and outlet channels, respectively, and merge into the inlet or outlet channels at rounded portions. [Figure 12c] 8a and 8b show diagrams of a substrate for an EUV mirror having a hollow structure similar to that of FIG. 8a and FIG. 8b, in which the distribution and recovery channels are arranged at obtuse angles to the inlet and outlet channels, respectively, and merge into the inlet or outlet channels at rounded portions. [Figure 12d] 8a and 8b show diagrams of a substrate for an EUV mirror having a hollow structure similar to that of FIG. 8a and FIG. 8b, in which the distribution and recovery channels are arranged at obtuse angles to the inlet and outlet channels, respectively, and merge into the inlet or outlet channels at rounded portions. [Figure 13a]2 shows a schematic cross-sectional view of a mirror of the projection exposure apparatus of FIG. 1 including a hollow structure with a number of cooling channels; [Figure 13b] 2 shows a schematic cross-sectional view of a mirror of the projection exposure apparatus of FIG. 1 including a hollow structure with a number of cooling channels; [Figure 13c] FIG. 13 shows a schematic diagram of the mirror of FIGS. 13a, 13b during the creation of a hollow structure using a fluid supply device that includes two different insert components. [Figure 14a] 1 shows a schematic diagram of a first embodiment of a fluid delivery device including an insert component having a plurality of guide channels in the form of bent tubes for guiding flexible fluid lines. [Figure 14b] 1 shows a schematic diagram of a first embodiment of a fluid delivery device including an insert component having a plurality of guide channels in the form of bent tubes for guiding flexible fluid lines. [Figure 14c] 1 shows a schematic diagram of a first embodiment of a fluid delivery device including an insert component having a plurality of guide channels in the form of bent tubes for guiding flexible fluid lines. [Figure 15a] FIG. 13 shows a schematic diagram of a second embodiment of a fluid supply device in which an insert component is formed by additive manufacturing; [Figure 15b] FIG. 13 shows a schematic diagram of a second embodiment of a fluid supply device in which an insert component is formed by additive manufacturing;

[0135] In the following description of the drawings, the same reference numbers are used for identical or functionally identical components.

[0136] The essential components of an optical apparatus for EUV lithography in the form of a microlithography projection exposure apparatus 1 are described below by way of example with reference to Fig. 1. The description of the basic arrangement of the projection exposure apparatus 1 and its components should not be understood as limiting in this case.

[0137] An embodiment of the illumination system 2 of the projection exposure apparatus 1 comprises, in addition to the light source or radiation source 3, an illumination optical unit 4, which illuminates 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.

[0138] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable in particular in the scanning direction by a reticle displacement drive 9.

[0139] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x direction extends perpendicular to the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. In Fig. 1, the scanning direction extends in the y direction. The z direction extends perpendicular to the object plane 6.

[0140] The projection exposure apparatus 1 comprises a projection optical unit 10, which serves to image the object field 5 into an image field 11 in an image plane 12. A structure on a reticle 7 is imaged onto a photosensitive layer of a wafer 13, which is arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14, which is displaceable in particular along the y direction by a wafer displacement drive 15. The displacement of the reticle 7 firstly by the reticle displacement drive 9 and the displacement of the wafer 13 secondly by the wafer displacement drive 15 can be performed synchronously with respect to one another.

[0141] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, also referred to below as working radiation, illumination radiation or illumination light. In particular, the working radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 can be a plasma source, for example an LPP source, i.e. a laser-produced plasma source, or a GDPP source, i.e. a gas discharge plasma source. It can also be a synchrotron-based radiation source. The radiation source 3 can be a free electron laser, also referred to by the acronym FEL.

[0142] The illumination radiation 16 coming from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector with one or more ellipsoidal and / or hyperbolic reflecting surfaces. On at least one reflecting surface of the collector mirror 17 the illumination radiation 16 may be incident at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. The collector mirror 17 may be structured and / or coated, firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0143] The illumination radiation 16 propagates downstream of the collector 17 mirror at an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may represent a separation between the source module comprising the radiation source 3 and the collector mirror 17 and the illumination optics unit 4.

[0144] The illumination optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 arranged downstream thereof in the beam path. The deflection mirror 19 can be a plane deflection mirror or a mirror with a beam-influencing effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be in the form of a spectral filter which separates the used light wavelengths of the illumination radiation 16 from extraneous light of different wavelengths. The first facet mirror 20 comprises a number of individual first facets 21, also referred to as field facets in the following. FIG. 1 shows only some of said facets 21 by way of example. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optical unit 4. The second facet mirror 22 comprises a number of second facets 23.

[0145] The illumination optical unit 4 therefore forms a double-facet system. This basic principle is also called a fly's eye condenser (fly's eye integrator). By means of a 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-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

[0146] The projection system 10 comprises a number of mirrors Mi, which are consecutively numbered according to their position in the beam path of the projection exposure apparatus 1 .

[0147] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1-M6. Four, eight, ten, twelve or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have a passing aperture for the illumination radiation 16. The projection system 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture which may be greater than 0.4 or 0.5, may be greater than 0.6, and may for example be 0.7 or 0.75.

[0148] Like the mirrors of the illumination optical unit 4 , the mirror Mi may have a highly reflective coating for the illumination radiation 16 .

[0149] 2a, 2b show an exemplary mirror M4 of the projection system 10, which comprises a substrate 25 with a surface 27 that is applied with a reflective coating 26. In the example shown, the material of the substrate 25 is titanium-doped quartz glass, which has a very low coefficient of thermal expansion. The substrate 25 can also be made of another material, for example a glass ceramic, with an as low as possible coefficient of thermal expansion. These materials are zero-expansion materials, in which components or phases with a positive coefficient of thermal expansion are opposed to components or phases with a negative coefficient of thermal expansion. These materials have only one temperature value at which thermal expansion disappears or is least susceptible to temperature changes. Precisely, this is the so-called zero-cross temperature T ZC In the example described here, the material of the substrate 25, titanium-doped quartz glass, which has a very small expansion coefficient, has a zero cross temperature T ZC The zero cross temperature T ZC is substantially constant throughout 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, and the spatial variation is less than the zero cross temperature T ZC This indicates the difference between the maximum and minimum values.

[0150] In the illustrated example, the substrate 25 has a monolithic form. In the illustrated example, the reflective coating 26 has a number of layer pairs of materials with different real parts of the refractive index, which may be formed, for example, of Si and Mo for a wavelength of 13.5 nm of the EUV radiation 16. The surface 27 of the substrate 25 is represented as a flat region in Figures 2a,b, but may also have a curvature.

[0151] In the example shown in Fig. 2a, 2b, the substrate 25 has a continuous hollow structure 28 in the form of a channel through which a coolant can flow, in the form of a temperature control fluid, in this case a cooling fluid 32a, indicated by arrows, which in the example described here is water. The hollow structure 28 may therefore be referred to below as a cooling channel. It is understood that a heating fluid can also flow through the hollow structure 28 for the purpose of heating the substrate 25. The cooling channel 28 has a first portion 28a that extends in a vertical direction, i.e. in the Z direction of the XYZ coordinate system, starting from a coolant inlet 30 formed in the back side 29 of the substrate 25. The vertical direction Z corresponds to the thickness direction Z of the substrate 25. In the example described here, the upper side 27 and the lower side 29 of the substrate 25 are each oriented perpendicular to the thickness direction Z.

[0152] The first vertical portion 28a of the cooling channel 28 is adjacent to a portion 28b extending in a horizontal direction, i.e. in the X-direction, through which the coolant 33 flows into a third portion 28c of the cooling channel 28 which extends vertically and opens into a coolant outlet 31 on the backside 29 of the substrate 25. With the exception of two transitions between each vertical portion 28a, 28c and the horizontal portion 28b, the hollow structure 28 in the form of a cooling channel shown in FIG. 2a has a circular cross section with a constant diameter, which in the example shown is of the order of about 1 mm to 5 mm. The continuous cooling channel 28 shown in FIG. 2b differs from the continuous cooling channel 28 shown in FIG. 2a in that the portions of each of the vertical portions 28a, 28c starting from the coolant inlet 30 and the coolant outlet 31, respectively, have a larger diameter than the short portions of the vertical portions 28a, 28c located at the horizontal portion 28b and at the transitions to the horizontal portion 28c of the cooling channel 28, respectively.

[0153] For supplying the coolant 32a to the coolant inlet 30 and for removing the coolant 32a from the coolant outlet 31, the projection exposure apparatus 1 comprises a cooling device 32, which is represented diagrammatically in FIG. 1. In the illustrated example, the cooling device 32 serves to supply the coolant 32a in the form of cooling water to the cooling channels 28 or to the mirror M4, and for this purpose comprises a supply line, not shown here, which is fluid-tightly connected to the coolant inlet 30. The cooling device 32 also comprises a removal line, not shown here, for removing the cooling water 32a from the coolant outlet 31. The other mirrors M1-M3, M5, M6 of the projection system 10 can also have a hollow structure 28, which for cooling purposes is connected to the cooling device 32 or is optionally connected to further cooling devices provided for this purpose. It is understood that in principle any mirror can have a hollow structure 28 through which a coolant can flow. By way of example, this may concern mirrors designed to reflect radiation in the DUV / VUV, visible and / or infrared wavelength ranges. Instead of the cooling device 32, the projection exposure apparatus 1 can also be provided with a temperature control device, i.e. a device used for cooling and / or heating the mirrors M1-M6. A suitable temperature control fluid 32a can be used for heating, e.g. water which is heated to the desired temperature before being fed into the hollow structure 28.

[0154] The device 33, which is represented diagrammatically in Fig. 3, serves to form the cooling channels 28 shown in Fig. 2a. The device 33 comprises a laser source 34 which generates a pulsed laser radiation 35, represented in Fig. 3 in the form of dashes. The laser source 34 is an ultrashort pulsed laser source designed to generate laser pulses with a pulse duration of the order of picoseconds, for example less than 10 ps, ​​and has a peak pulse power in the MW range. The laser source 34 is designed to generate a pulsed laser radiation 35 having a wavelength in the near infrared wavelength range, more precisely at 1030 nm. However, the laser source 34 could also be designed to generate a pulsed laser radiation 35 at a wavelength in the visible wavelength range or at another wavelength in the near infrared wavelength range for which the material of the substrate 25 is transparent.

[0155] The device 33 also comprises a scanner optical unit 36 ​​and a focusing device 37. In the illustrated example, the focusing device 37 is designed as an Fθ lens and serves to focus the pulsed laser radiation in the form of a pulsed laser beam onto a focal region 39 in the substrate 25. The scanner optical unit 36 ​​serves to emit the pulsed laser radiation 35 onto the beam entrance side 27 of the substrate 25 in an incident radiation direction Z oriented perpendicularly to the beam entrance side 27 and to move the focal region 39 in the substrate 25, said scanner optical unit comprising for this purpose a galvanometer mirror 40 tiltable in two directions. Instead of one galvanometer mirror 40, it is also possible that two galvanometer mirrors, each tiltable in one direction, are arranged in the scanner optical unit 36. Instead of a galvanometer mirror, it is also possible to use other types of mirrors, for example a piezoelectric mirror.

[0156] As mentioned above, the focal region 39 can be moved within the substrate 25 by tilting the mirror 40. As a result of the F-theta lens 37, which corrects for field curvature in the case of different alignments of the galvanometer mirror 40, the focal region 39 moves in the XY plane perpendicular to the incident radiation direction Z as the mirror 40 is tilted. Thus, the incident radiation direction Z of the pulsed laser radiation 35 at the substrate 25 substantially corresponds to the thickness direction Z of the substrate 25.

[0157] Using the scanner optical unit 36, the focal region 39 is moved along an ablation pattern 41, which is shown in plan view in Fig. 4a. The ablation pattern 41 has a number of linear tracks 42 that are oriented in parallel, extend in the Y direction and are arranged equidistantly from one another in the X direction in the example shown. It is not essential that the tracks 42 are arranged at the same distance from one another, and the distance between adjacent tracks 42 can also vary within the ablation pattern 41 in a process-dependent manner.

[0158] To create the vertical portion 28a of the cooling channel shown in Fig. 3, an ablation pattern 41 shown in Fig. 4a is generated in the XY-plane on the back side 29 of the uncoated substrate 25 by means of the scanner optical unit 36. While the scanner optical unit 36 ​​generates the ablation pattern 41 shown in Fig. 5a a number of times, the substrate 25 is displaced downward in the incident radiation direction Z of the pulsed laser radiation 35 at the beam incident side 27 of the substrate 25 by means of a positioning device 43, represented very diagrammatically in Fig. 3 and acting on a holder 44 for the substrate 25. In this way, starting from the back side 29 of the substrate 25, the vertical portion 28a of the cooling channel 28 shown in Fig. 3 is formed.

[0159] 3 shows a step of making the hollow structure 28 when forming a transition in the form of an undercut with an undercut angle of 90° between the vertical portion 28a of the cooling channel 28 and the horizontal portion 28b of the cooling channel 28. In this case, when pulsed laser radiation 35 is directed laterally along the top edge of the vertical portion 28a in order to remove material to form the horizontal portion, as shown in FIG. 3, this leads to an interaction of the pulsed laser radiation 35 with the material of the substrate 25 thereunder, which induces deformations and stresses in the material of the substrate 25, as shown in FIG. 3 by the solid vertical line 45.

[0160] This problem is rectified by generating a removal front 46 that is not oriented perpendicular to the incident radiation direction Z. In the example shown in Fig. 4c, the removal front 46 has an angle α of 45° with respect to the incident radiation direction Z. However, the angle α with respect to the incident radiation direction Z may be larger or smaller, for example oriented in a range of values ​​between 0° and 89°, between 10° and 80°, between 20° and 70° or between 30° and 60° with respect to the incident radiation direction Z.

[0161] In Fig. 4c, in order to create the horizontal portion 28b of the cooling channel 28, the removal front 46 is tilted in a direction towards the radiation incidence side 27 of the substrate 25, which is shown in dashed lines in Fig. 4c before creation. To create the horizontal portion 28b of the cooling channel 28, the substrate 25 is displaced in the X direction by the positioning device 43 in order to continuously ablate material along the removal front 46, as indicated by the arrow in Fig. 4c. As is also evident from Fig. 4c, during the movement of the removal front 46 in the horizontal X direction, the upper edge 46a of said removal front closer to the radiation incidence side 27 of the substrate 25 is oriented at an angle β of 45° with respect to the direction of movement of the removal front 46 in the substrate 25, which corresponds to the negative X direction. The lower edge 46b of the removal front 46 remote from the radiation incidence side 27 protrudes in the direction of movement more than the upper edge 46a of the removal front 46 during the formation of the horizontal portion 28b of the hollow structure 28. In this way, interaction of the pulsed laser radiation 35 with the material of the substrate 25 during production of the horizontal portion 28b can be minimized, since only the lower edge 46b of the removal front 46 adjoins the material of the substrate 25. As is also evident from Fig. 4c, it is in the region of the removal front 46 that the pulsed laser radiation 35 first exits the material of the workpiece 25 after incidence at the radiation incidence side 27.

[0162] In the example shown in FIG. 4c, the inclined ablation front 46 is generated by offsetting the focal area 39 between two adjacent tracks 42 in the incident radiation direction Z by a value Δz, which is constant in the illustrated example, as shown in FIG. 4b. The distance A between adjacent tracks 42 along the ablation front 46 inclined at 45° to the incident radiation direction Z is in the illustrated example about 0.01 mm to 0.5 mm, for example about 0.03 mm. In order to be able to rapidly offset the focal area 39 in the incident radiation direction Z between the traverses of adjacent tracks 42, the device 33 includes a focus offset device 47 in the form of a dynamic zoom lens. The focus offset device 47 is controlled by a control device 48 for generating an offset in the incident radiation direction Z. The control device 48 also serves to control the laser source 34 and the scanner device 36 in order to synchronize the offset in the incident radiation direction Z with the movement of the focal area 39 along each track 42. The diameter of the circular cross section of the cooling channel 28 in the illustrated example is about 2 mm.

[0163] 5a, b show a further option for forming an oblique removal front 46, also oriented at an angle α of 45° to the incident radiation direction Z. As is evident from FIG. 5b, the device 33 for producing the hollow structure 28 does not have a focus offset device. In the device 33 shown in FIG. 5b, the pulsed laser radiation 35 is focused in a focal plane FE oriented perpendicularly to the incident radiation direction Z. As is evident from FIG. 5a, the pulse energy E of the pulsed laser radiation 35 P is gradually increased between adjacent trajectories 42 or corresponding scan lines in the focal plane FE to form an ablation front 46, as indicated by the increasing line thickness of the trajectories 42 in FIG. 5a.

[0164] The control device 48 controls the pulse energy E P The laser source 34 acts to increase the pulse energy E P The ablation pattern 41 includes a device for adjusting the pulse energy E between two adjacent tracks 42 of the ablation pattern 41, which may for example be in the form of an acousto-optical or electro-optical modulator. Such a modulator has a response time of the order of microseconds or less and adjusts the pulse energy E between two adjacent tracks 42 of the ablation pattern 41, respectively. PAs is evident from FIG. 5b, the extent to which the pulsed laser radiation 35 has an effect on the material of the substrate 25 depends, in addition to other parameters, on the pulse energy E P Depending on the specified parameters, such as the wavelength of the pulsed laser radiation 35 and the pulse duration of the pulsed laser radiation 35, a certain threshold energy density or intensity is required to ablate the material of the substrate 25. The pulse energy E P The larger , the larger the size of the area starting from the focal plane FE where material removal can occur.

[0165] FIG. 5b shows the contour range of energy density within the substrate 25, along which there is a boundary 49 where ablation can still occur in the incident radiation direction Z. As shown by the arrows in FIG. 5a, the pulse energy E P As a result of the gradual increase in the thickness of the substrate 20, it is possible to displace the boundary 49 in the incident radiation direction Z to form a removal front 49 shown in Fig. 5b which is oriented at an angle of 45° to the incident radiation direction Z. As a result, an oblique removal front 46 can be formed as explained in conjunction with Figs. 5a, 5b, without the device 33 having additional moving elements for this purpose, such as a focus offset device 47. As explained in connection with Fig. 3, the vertical portion 28a of the channel 28 can also be formed by a downward movement in the Z direction of the substrate 25, in this case as indicated by the arrow in Fig. 5b.

[0166] 6a-6c show three stages of the formation of a curved hollow structure 28 with a vertical portion 28a and a horizontal portion 28b that merge into one another at a rounded portion 28d or a curved portion. In this case, the device 33 for producing the hollow structure 28 is designed as in Fig. 4a-4c, i.e. includes a focus offset device 47 for forming an oblique removal front 46. As is evident from Fig. 6a, the vertical channel portion 28a is formed in a first stage by displacing the substrate 25 downwards, as a result of which the removal front 46 moves within the substrate 25 and material of the substrate 25 is successively ablated, while the scanner optical unit 36 ​​remains fixed. Since it is the relative movement between the removal front 46 and the substrate 25 that is of primary importance for material removal, the scanner optical unit 36 ​​may alternatively be moved upwards, while the substrate 25 remains fixed in the incident radiation direction Z, in order to move the removal front 46 within the substrate 25. In principle, overlapping movements in the Z direction of the substrate 25 and the scanner optical unit 36 ​​are also possible.

[0167] The removed material is removed from the removal front 46 by means of a fluid supply 50. In the illustrated example, the fluid supply 50 comprises a fixed nozzle 51 from which a liquid, in the illustrated example water 32b, emerges and flows vertically and is supplied to the removal front 46. The vertical upward orientation of the nozzle 51 allows targeted removal of the removal particles from the removal front 46, which are removed by gravity through the gap between the nozzle 51 and the wall of the vertical part 28a of the hollow structure 28. In this way, the removal front 46 is substantially free of deposits and removal can be performed without interruption. At the same time, the supply of the liquid 32b allows active cooling of the removal front 46 or the processing zone, which results in a reduction in the residual heat in the substrate 25. Instead of supplying the liquid 32b, the fluid supply 50 can also be used to supply a gas, for example compressed air, to the removal front 46.

[0168] 6b shows a stage during the fabrication of the hollow structure 28 where a rounded portion 28d of the hollow structure 28 is formed, forming a 90° transition between the vertical portion 28a and the horizontal portion 28b of the hollow structure 28. During the formation of the rounded portion 28d, as indicated by the arrows, the substrate 25 is displaced in the X direction in addition to the Z direction, but the 45° inclined removal front 46 is still formed in the manner described above.

[0169] The rounded portion 28d allows the introduction of a flexible tube 52 into the hollow structure 28, which tracks the removal front 46 during the creation of the horizontal portion 28b of the hollow structure 28, as shown in FIG. 6c. In this way, removal products can be effectively removed from the removal front 46 even during the creation of the horizontal portion 28b. As a result of the continuous tracking by the tube 52, the achievable length of the hollow structure 28 is limited only by the size of the substrate 25 and the length of the tube 52.

[0170] The above-mentioned tilting of the removal front 46 does not necessarily have to be performed when a hollow structure, for example in the form of a straight channel, is produced by the material removal process with the pulsed laser radiation 35. Even if it is intended to produce a hollow structure in the form of a channel 28 with a relatively long linear length, the fluid supply 50 or a part of the fluid supply 50 must be introduced at least partially into the channel 28 in order to supply the rinsing liquid 32b to the removal front 46. For this purpose, the fluid supply 50 may comprise, for example, a rigid tube or the like which is introduced at least partially into the channel 28. In particular, when a curved channel 28 is to be produced, the fluid supply 50 may comprise a flexible element, for example in the form of a flexible tube 52, which is introduced at least partially into the channel 28 in order to follow the removal front 46 with its free end. A nozzle may be attached to the free end of the tube 52, but this is not essential.

[0171] As is also evident from Fig. 6c, the material of the substrate 25 during the creation of the horizontal portion 28b of the hollow structure 28 is adjacent to the side 46c of the edge 46b of the removal front 46 that is remote from the radiation incidence side 27 of the substrate 25, i.e. in Fig. 6c the material of the substrate 25 is located below the lower edge 46b of the removal front 46. Furthermore, during the creation of the horizontal portion 28b of the hollow structure 28, part of the laser radiation 35 that leaves the area of ​​the removal front 46 or is emitted from the removal front 46 into the hollow structure 28 re-enters the material of the substrate 25, precisely at the lower edge of the side of the hollow structure in the form of the channel 28 in Fig. 6c.

[0172] As is also evident from Figure 6c, the removal front 46 includes an angle β' with the side 57 of the channel 28, which in the illustrated example is cylindrical, also referred to below as the removal front angle. During creation of the channel 28, the removal front angle β' is typically at least intermittently greater than a removal front minimum angle of 1°, 5°, 10°, 20°, or 30° and at least intermittently less than a removal front maximum angle of 89°, 85°, 80°, 70°, or 60°. The removal front angle β' during creation of the channel 28 may be permanently greater than the removal front minimum angle and / or permanently less than the removal front maximum angle, but this is not required.

[0173] Figures 7a, 7b show two further stages or steps during the production of the hollow structure 28, following the stage described in conjunction with Figures 6a-6c. As is evident from Figure 7a, starting from the rear side 29 of the substrate 25, a further vertical portion 28c of the hollow structure 28 is produced, followed by a further rounded portion 28e, in a manner corresponding to the stage shown in Figure 6a. In the stage of formation of the hollow structure 28 shown in Figure 7a, the orientation of the removal front 46 is mirrored compared to Figure 6a and the substrate is displaced in the negative X direction in order to form a horizontal portion 28b. As is evident from Figure 7b, the mirrored removal front 46' is displaced horizontally by continuous processing until it reaches the processed portion of the horizontal portion 28b of the hollow structure 28, so that a continuous horizontal portion 28b is produced and the hollow structure 28 in the form of a channel is opened.

[0174] A seam region 53 occurs upon opening of the channel 28, said seam region extending to two adjacent dashed regions in Fig. 7b corresponding to the last removal fronts 46, 46' respectively formed during the creation of each portion of the channel 28. The properties of the channel 28 in the seam region 53, more precisely the properties of the walls of the channel 28, differ in at least one property from the properties of the walls of the channel 28 outside the seam region, or the seam region 53 has at least one structural modification with respect to the remaining channel 28.

[0175] Figure 7c shows three examples of such structural modifications. In the example shown in Figure 7c, firstly, the surface structure of the walls of the channel 28 in the seam region 53 differs from the surface structure of the walls of the channel 28 outside the seam region 53 in that an edge contour 54 of the removal front 46 is observable on the surface structure of the seam region 53. The edge contour of the other removal front 46' is also partially written into the surface structure of the channel 28 in the seam region 53, but this is not shown in Figure 7c. The edge contour 54 of the removal front 46 extends in the illustrated example in an elliptical shape and is oriented at an angle of about 45° to the side of the channel 28, i.e. at the same angle as the removal front 46 itself (see Figure 6c).

[0176] In the example shown in FIG. 7c, four bulges 55 are further formed in the wall of the channel 28 in the seam region 53, each of which locally increases the cross section of the channel 28. Bulges that reduce the cross section of the channel 28 are likewise possible. Furthermore, the wall of the channel 28 has a slight lateral offset 56 in the manner of a step in the seam region 53, which may result from a slightly different cross-sectional area of ​​the two parts of the channel 28. The lateral offset may also result from a slightly offset positioning of the two parts of the channel 28 when forming the continuous opening. It is understood that both the bulges 55 and the lateral offset 56 are exaggerated for the sake of clarity.

[0177] As a result of the material removal process by the pulsed laser radiation 35, hollow structures in the form of curved channels 28 can be produced in the substrate 25, which have a diameter D of 1 mm to 20 mm, in particular 1 mm to 5 mm, and / or a length L of 10 cm or more, 15 cm or more, 20 cm or more, or 70 cm or more.C The flow channel 28 shown in FIG. C and has a diameter D of 5 mm. The zero cross temperature T ZC is within the range of values ​​specified above and is virtually constant over the volume of the monolithic substrate 25. That is, the variation in zero cross temperature ΔT ZC , ie, the difference between the maximum and minimum zero cross temperatures in the volume of substrate 25, is also within the value range specified above.

[0178] As explained in conjunction with Fig. 2a, 2b, a hollow structure 28 that can be used for the passage of a cooling liquid is formed as described above. Differently from what has been described above, the backside removal can also start not from the backside 29 of the substrate 25, but also from the side of the substrate 25 that is opposite to the beam incidence side 27 and that is arranged within the substrate 25. By way of example, this can be the upper end of a vertical hole shown in Fig. 2b, which is the starting point for the creation of the two vertical parts 28a, 28c of the hollow structure 28. In this case, the hollow structure 28 is produced using a hybrid production method that combines machining of the substrate 25 with a material removal process by pulsed laser radiation 35. It is understood that instead of a vertical hole, a differently oriented hole or cavity can serve as the starting point for the above-mentioned formation of the hollow structure 28 using the pulsed laser radiation 35.

[0179] In summary, high aspect ratio vertical and horizontal macroscopic hollow structures can be introduced in the substrate 25 of the EUV mirror as described above. It is understood that hollow structures or parts of hollow structures that deviate from a horizontal or vertical orientation can also be produced in this way. It is understood that not only one of the mirrors M1-M6 of the projection system 10 can be processed as described above to form a hollow structure, but also any other mirror, in particular the EUV mirror. Hollow structures more complex than a single continuous cooling channel 28, for example hollow structures with Y-junctions or T-junctions, can also be produced by the above described method. Hollow structures with junctions can also be produced by the above described method without damaging or stressing the material of the substrate.

[0180] 8a, 8b show yet another example of an embodiment of a mirror M4 with a monolithic substrate 125 that is part of the projection system 10. In the example shown, the material of the substrate 125 is titanium-doped quartz glass, which has a very low coefficient of thermal expansion. The substrate 125 may also be made of another material, for example glass ceramic, with an as low coefficient of thermal expansion as possible. The zero cross temperature T ZC is within the range of values ​​specified above and is virtually constant over the volume of the monolithic substrate 125. That is, the variation in zero cross temperature ΔT ZC , ie, the difference between the maximum and minimum zero cross temperatures in the volume of substrate 125, is also within the value range specified above.

[0181] A reflective coating 126 that reflects EUV radiation 16 is applied to a surface 125a of the substrate 125. A part of the surface 125a located within the reflective coating 126 is struck by the EUV radiation 116 of the projection system 10 and forms an optically used part of the reflective coating 126, not shown here. To reflect the EUV radiation 16, the reflective coating 126 can for example comprise a number of layer pairs of materials with different real parts of the refractive index, the layers could for example be made of Si and Mo for a wavelength of EUV radiation 16 of 13.5 nm.

[0182] The substrate 125 has a hollow structure 127 through which a fluid 128 can flow, which in the illustrated example is water. The fluid 128, indicated by arrows in Fig. 8a, enters the substrate 125 via a lateral inlet opening 129 so as to flow through a number of cooling channels 131 forming part of the hollow structure 127, thus cooling in particular the surface 125a of the substrate 125 which is applied with the reflective coating 126.

[0183] In order to supply the fluid 128 to the inlet opening 129 and to remove the fluid 128 from an outlet opening, not shown in Figures 8a, 8b, the projection exposure apparatus 1 comprises the above-mentioned temperature control device 32 in the form of a cooling device. In the example shown, the cooling device 32 serves to supply the fluid 128 in the form of cooling water to the hollow structure 127 or to the mirror M4 and for this purpose comprises a supply line, not shown here, which is fluid-tightly connected to the inlet opening 129. The cooling device 132 also comprises a removal line, not shown here, for removing the cooling water via an outlet opening in the substrate 125 or from the hollow structure 127.

[0184] As is evident from Fig. 8a, the fluid 128 enters the hollow structure 127 through an inlet opening 129 into an inlet channel 133 which forms a fluid distribution section from which a number of distribution channels 134 branch off, each connected to one of a number of temperature control channels, hereafter referred to as cooling channels 131. The cooling channels 131 are arranged at a distance A' of about 5 mm from the surface 125a of the substrate 125, which in the illustrated example is planar, and run parallel to the surface 125a, i.e. parallel to the XY plane of the XYZ coordinate system. The cooling channels 131 run linearly, are oriented parallel and extend in a longitudinal direction corresponding to the Y direction over substantially the entire part of the surface 125a of the substrate 125 covered by the coating 126, see Fig. 8b. From the cooling channels 131, the fluid 128 flows via a number of return channels 136 to a fluid return section, which in the example shown in Fig. 8b is formed as an outlet channel 135. The outlet channel 135 has the above-mentioned outlet opening, not shown in FIGS. 8 a , 8 b , through which the fluid 128 leaves the hollow structure 127 of the substrate 125 .

[0185] As is clear from Fig. 8b, the hollow structure 127 has a first rounded portion 137a where each distribution channel 134 merges into the cooling channel 131. The hollow structure 127 therefore also has a second rounded portion 137b where each cooling channel 131 merges into the return channel 136. In the illustrated example, the cooling channels 131 extend linearly in a horizontal direction corresponding to the Y direction, while the distribution channels 134 and the return channel 136 extend linearly in a vertical direction corresponding to the Z direction. The longitudinal axis of the cooling channel 131 is therefore oriented at an angle γ of 90° to the distribution channels 134 and the return channel 136. The rounded portions 137a, 137b serve to generate a flow guidance that is as streamlined as possible in order to avoid or at least significantly reduce the occurrence of turbulence, as would occur in the case of a non-rounded "angular" 90° bend. The reduction in turbulence results in a reduction in flow-induced vibrations of the reflecting optical element M4.

[0186] As shown in Figures 9a-9d and 10a-10d, for optimized flow guidance in a 90° bend it is advantageous if the rounded sections 137a, 137b have a constant radius of curvature R. A parameter essential for optimal flow guidance is represented by the ratio of the radius of curvature R of the rounded sections 137a, 137b to the flow passage diameter D.

[0187] 9a-9d show a first rounded portion 137a where the end portion 131a of each cooling channel 131 and the distribution channel portion 134a adjacent to the end portion 131a are adjacent to each other for four different ratios of the radius of curvature R of the rounded portion 137a to the diameter D of the rounded portion 137A. In this case, the radius of curvature R is measured at the center of the rounded portion 137a as shown in FIG. 9a-9d. The diameter D of the rounded portion 137a is 5 mm in all four examples shown. The diameter D of the rounded portion 137a in this case corresponds to the diameter D of the distribution channel 134 and the diameter D of the cooling channel 131. The length L is about 50 mm in the illustrations of FIG. 9a-9d. As can be seen from FIG. 9a-9d, the R / D ratios are R / D=2, R / D=3, R / D=4, and R / D=5 in the four examples shown, respectively.

[0188] Similar to Figs. 9a-9d, Figs. 10a-10b show a second rounded portion 137b where the end portion 131b of each cooling channel 131 and the return channel portion 136a adjacent to the end portion 131b join each other. The diameter D of the rounded portion 137b is 10 mm in Figs. 10a-10d. In the illustrations of Figs. 10a-10d, the length L is about 60 mm. In the illustrations of Figs. 10a-10d, the R / D ratios are R / D=2, R / D=3, R / D=4, and R / D=5, respectively, in the four examples shown. The diameter D of each rounded portion 137a, 137b is typically 2 mm to 20 mm, and ideally 2 mm to 12 mm.

[0189] As mentioned above, there is an optimum ratio between the radius of curvature R and the diameter D of each rounded portion 137a, 137b, such that the centrifugal force acts such that the pressure of the flowing fluid 128 outside the rounded portion 137a, 137b increases only slightly compared to the inside of the rounded portion 137a, 137b, thereby achieving a reduction in boundary layer separation upstream and downstream of the rounded portion 137a, b. Figures 9a-9d and 10a-10d show the contours of the regions where the turbulent kinetic energy of the flowing fluid 128 exceeds a specified value. In this case, it was assumed that the fluid 128 enters each end portion 131a or 131b of the cooling channel 131 from the distribution channel portion 134a or from the return channel portion 136a.

[0190] For this purpose, a ratio of the radius of curvature R of the rounded portions 137a, 137b to the diameter D of the rounded portions 137a, 137b of 2 to 6 or less, more preferably 2.5 to 5, ideally 2.5 to 3.5, has proven to be particularly advantageous. If the R / D ratio is less than 2, a significant reduction of the boundary layer separation is usually not possible. The optimum value of the R / D ratio is typically 2.5 to 3.5, although the optimum value may in some cases be outside this value range. If the R / D ratio is more than 6.0, the flow behavior usually deteriorates.

[0191] In reality, the rounded portions 137a, 137b cannot be produced by conventional processing methods in the monolithic substrate 125, as described above. In the illustrated example, only the inlet channel 133 and the outlet channel 135 are produced by conventional processing methods, precisely by introducing respective holes into the substrate 125. In contrast, the distribution channel 134, the cooling channel 131 and the recovery channel 136 are produced by laser ablation of the material of the substrate 25, as will be described below.

[0192] To create the distribution channels 134, the cooling channels 131, and the recovery channels 136 of the hollow structure 127, a pulsed laser beam is emitted from the surface 125a of the substrate 125 through the material of the substrate 125 and focused thereon above the inlet channels 133, before the application of the reflective coating 126, to generate a movement pattern with multiple parallel ablation paths that form a removal front 130a oriented at an angle of 45° with respect to the thickness direction Z of the substrate 25. From this position, the removal front 130a is displaced multiple times in the thickness direction corresponding to the Z direction with respect to the substrate 125 for ablation of the material of the substrate 125 and formation of the distribution channels 34. During the displacement, the removal front 130a may remain fixed, and the substrate 125 is displaced upward in the Z direction until the removal front 130a is located directly below the first rounded portion 137a.

[0193] To create the first rounded portion 137a, the removal front 130a or the substrate 125 is displaced with overlapping movements in both the Z and Y directions. After formation of the first rounded portion 137a, in order to form the cooling channel 131 or an end portion 131a of the cooling channel 131 adjacent to the distribution channel 134, the removal front 130a oriented at 45° to the thickness direction corresponding to the Z direction is only displaced in the longitudinal direction of the cooling channel 131 corresponding to the Y direction until the removal front is located approximately in the center of the longitudinal direction of the cooling channel 131.

[0194] The creation of the recovery channel 136, the second rounded portion 137b and the end portion 131b of the cooling channel 131 adjacent to the second rounded portion 137b is similarly performed by laser ablation starting from the outlet channel 135, on the upper side of which the pulsed laser radiation is first focused through the substrate 125. The further removal front 30b formed in this way is also oriented at 45° to the thickness direction or XY plane of the substrate 25, but is mirror-imaged in the XZ plane with respect to the removal front 130a described above. The orientation of the removal fronts 130a, 130b at an angle to the thickness direction or incident radiation direction Z of the pulsed laser beam or pulsed laser radiation is typically performed as described above in connection with Figures 4a-4c and 5a, 5b. A fluid is supplied to each removal front 130a, 130b for removing the ablated material from each removal front 130a, 130b or for cooling purposes. The supply of fluid is typically achieved by a fluid supply, ie by at least partial introduction of the fluid supply into the hollow structure 127, as described in connection with Figures 6a to 6c.

[0195] In the hollow structure 127 described above, only two portions 137a, 137b are rounded, while the distribution channels 134, the return channels 136 and the cooling channels 131 run in straight lines. However, more complex hollow structures 127 can also be produced using the laser ablation method described above. Figures 11a, 11b show an example of such a hollow structure 127 in a substrate 125, which substantially corresponds to the hollow structure 127 shown in Figures 8a, 8b. The hollow structure 127 differs from the hollow structure 127 in Figures 8a, 8b in that the cooling channels 131 have a slight curvature that follows the curvature of the surface 125a, which in the example shown is convexly curved. In the example shown in Figure 11b, the end portion 131a of each cooling channel 131 adjacent to the distribution channel 134 is oriented at an angle γ of about 115°. Although the cooling channel 131 has a curvature extending in the ZX plane, it is possible to define a longitudinal axis that defines an angle γ with respect to the end 131a adjacent to the rounded portion 131a. It is understood that the second rounded portion 137b, not shown in Figures 11a and 11b, is embodied in a manner that corresponds to the first rounded portion 137a. The R / D ratio of the radius of curvature R and the straightness D of each rounded portion 137a, 137b is typically within the above-mentioned value range.

[0196] In the case of the substrate 125 shown in Figures 12a-12d, the hollow structure 127 is designed substantially like the hollow structure 127 shown in Figures 8a, 8b, but differs from the latter in that the distribution channels 134 and the return channels 136 do not extend vertically, but are oriented at an angle of about 25° to the thickness direction Z of the substrate 125. Like the hollow structure 127 shown in Figures 8a, 8b, the hollow structure 127 shown in Figures 12a-12d has two rounded portions 137a, 137b, not shown here, between each distribution channel 134 or return channel 136 and the cooling channel 131. The angle γ between the distribution channel 134 or return channel 136 and the cooling channel 131 is again 90°, but extends in a plane inclined by about 25° to the thickness direction Z, as is evident from Figure 12d, which shows that the angle γ' between the longitudinal axis of the inlet channel 133 and each distribution channel 134 is about 115°.

[0197] The hollow structure 127 shown in Figures 12a-12d has a rounded section 138 where the junction section 134b of each distribution channel 134 transitions into the inlet channel 134, more precisely into the branching section 134a of the inlet channel 134, or where the junction section 136b of each return channel 136 transitions into the branching section 135a of the outlet channel 135. In the illustrated example, each rounded section 138 does not have a constant diameter or flow section, instead the flow section becomes smaller starting from the branching section 134a. The rounded section 138 does not have a constant radius of curvature R, as is the case for the two curved sections 137a, 137b extending between each distribution channel 134 or return channel 136 and each cooling channel 131. Therefore, an optimized ratio of the radius of curvature R to the diameter D cannot be specified. The rounded section 138 can also be produced using the laser ablation method described above.

[0198] It will be appreciated that the hollow structure 127 having at least one rounded portion 137a, 137b, 138 is not limited to the above example, and in principle other more complex hollow structures 127 having one or more such portions may also extend through the substrate 125. Furthermore, not only the cooling channels 131 having a curvature as described in connection with Figures 11a, 11b, but also the distribution channels 134 and the return channels 136 may extend in a curved manner.

[0199] 13a-b show yet another example of a mirror M4 which is part of the projection system 10, in the example shown having a monolithic substrate 225. In the example shown, the material of the substrate 225 is titanium doped quartz glass, which has a very low coefficient of thermal expansion. The substrate 225 may also be made from a different material, for example glass ceramic, with an as low coefficient of thermal expansion.

[0200] The reflective coating 226 is applied to a surface 225a of the monolithic substrate 225. A part of the surface 225a located within the reflective coating 226 is struck by the EUV radiation 16 of the projection system 10 and forms an optically useful part of the reflective coating 226, not shown here. To reflect the EUV radiation 16, the reflective coating 226 can for example comprise a number of layer pairs of materials with different real parts of the refractive index, the layers could for example be made of Si and Mo for a wavelength of the EUV radiation 16 of 13.5 nm.

[0201] The substrate 225 has a hollow structure 227 through which a fluid 228 can flow, which in the illustrated example is water. The fluid 228, indicated by arrows in Fig. 13a, enters the substrate 225 via a lateral inlet opening 229 so as to flow through a number of cooling channels 231 forming part of the hollow structure 227, thus cooling in particular the surface 225a of the substrate 225 which is applied with a reflective coating 226.

[0202] For supplying the fluid 228 to the inlet opening 229 and for removing the fluid 228 from an outlet opening, not shown in FIG. 13c, the projection exposure apparatus 1 comprises the above-mentioned temperature control device in the form of a cooling device 32, shown very diagrammatically in FIG. 1. In the example shown, the cooling device 32 serves to supply the fluid 228 in the form of cooling water to the hollow structure 227 or to the mirror M4 and for this purpose comprises a supply line, not shown here, which is fluid-tightly connected to the inlet opening 229. The cooling device 32 also comprises a removal line, not shown here, for removing the cooling water via an outlet opening in the substrate 225 or from the hollow structure 227. The other mirrors M1-M3, M5, M6 of the projection system 10 and the mirrors of the illumination system 2 are also connected, for cooling purposes, to the cooling device 32 or optionally to further temperature control devices or cooling devices provided for this purpose.

[0203] As is clear from FIG. 13a, the fluid 228 enters the inlet channel 233 of the hollow structure 227 via the inlet opening 229, which inlet channel forms a fluid distribution section from which a number of distribution channels 234 branch off, each connected to one of the cooling channels 231. The cooling channels 231 are arranged at a distance A' of about 2 mm to about 5 mm from the surface 225a of the substrate 225, which in the illustrated example is planar, and extend parallel to the surface 225a, i.e. parallel to the XY plane of the XYZ coordinate system. The cooling channels 231 extend in straight lines, are oriented parallel and extend longitudinally, i.e. longitudinally, over substantially the entire part of the surface 225a of the substrate 225 covered by the coating 226. See FIG. 13b. From the cooling channels 231, the fluid 228 flows via a number of return channels 236 to a fluid return section, which in the illustrated example is formed as a cylindrical second cavity 235. The fluid 228 leaves the hollow structure 227 of the substrate 225 through the outlet opening 230. In the illustrated example, the cooling channels 231 extend linearly in a horizontal direction corresponding to the X direction, and the distribution channels 234 and the return channels 236 extend linearly in a vertical direction corresponding to the Z direction. The longitudinal axis of the cooling channels 231 is therefore oriented at an angle of 90° to the distribution channels 234 and the return channels 236. However, such an orientation is not required.

[0204] To produce the hollow structure 227 shown in Figures 13a-c, the following procedure is carried out: First, the two cylindrical cavities 233, 235 forming the fluid distribution section and the fluid connector are introduced into the material of the substrate 225 by machining, for example by grinding or ultrasonic grinding. The substrate 225 is then immersed in a liquid bath, more precisely in a water bath, as a result of which a fluid 228 in the form of water enters the cavity 233 of the fluid distribution section through the inlet opening 228 and enters the cavity 235 of the fluid recovery section through the outlet opening 230, filling these cavities. Starting from each cavity 233, 235 filled with fluid 238, a number of short channel sections 237 adjacent to the cavity 233, 235 are produced by multiphoton laser ablation, as is evident from Figure 13c.

[0205] To create the flow channel portion 237, multiple pulsed laser beams are emitted and focused on the surface of the cavity 233 forming the fluid distribution portion starting from the surface 225a of the substrate 225 through the material of the substrate 225. The upper side of the cavity 233 thus forms the side of the substrate 225 opposite to the radiation incident side in the form of the surface 225a. Each irradiating pulsed laser beam moves the focal area in a movement pattern having multiple parallel ablation paths that are mutually offset in the Z direction and form ablation fronts 230a oriented at an angle of about 45° with respect to the thickness direction Z of the substrate 225. The focal position of the irradiating laser beam is changed in the Z direction resulting in an offset of the ablation paths in the Z direction.

[0206] Starting from a point on the surface of the cavity 233 where the channel portion 237 starts, the removal front 230a is displaced multiple times in the thickness direction, i.e. in the Z direction, relative to the substrate 225 for ablation of the material of the substrate 225 and formation of the channel portion 237. During the displacement, the removal front 230a may remain stationary, and the substrate 225 is displaced downwards in the Z direction until the removal front 230a has a distance of about 20-40 mm from the upper side of the cavity 233. In order to form the channel portion 237 with a relatively long length by multiphoton laser ablation, the removal front 230a must be locally washed with a fluid 228, as will be explained in more detail below. The creation of the channel portion 237 starting from the cavity 235 forming the fluid recovery is carried out in a corresponding manner by multiphoton laser ablation. A further removal front 230b formed in the process is also oriented at about 45° relative to the thickness direction of the substrate 225 or the XY plane, but is mirror-imaged in the XZ plane relative to the removal front 230a.

[0207] To form the hollow structure 227 shown in Figures 13a, 13b, the substrate 225 is removed from the liquid bath and the liquid 228 is removed from the cavities 233, 235. To form the remaining hollow structure 227, a fluid supply 238 is used to supply fluid 228 in the form of water to each removal front 230a, 230b, which is shown very diagrammatically in Figure 13c. The fluid supply 238 comprises two insertion components 239, 240 and a number of flexible fluid lines 24. In the illustrated example, the fluid supply 238 comprises seven fluid lines 241. The fluid lines 241 are connected to a fluid supply 243 of the fluid supply 238, which comprises a pump that pumps the fluid 228 in the flexible fluid lines 241 at a pressure of typically a few bars. The fluid supply device 243 also includes a tracker 244 that serves to track, or more precisely push, the flexible fluid lines 241 as the removal fronts 230a, 230b are moved within the substrate 225 to form the hollow structure 227. By way of example, the tracker 244 may include a reel or the like that winds up a portion of each flexible fluid line 241. For tracking purposes, the reel may be rotated at a constant angular velocity that corresponds to the speed of movement of the removal front. It is understood that the tracker 244 may be formed in different forms.

[0208] To form the hollow structure 277, the rod-shaped, cylindrical first insert component 239 is introduced through the inlet opening 229 into the first cavity 233 until the end face of the insert component abuts against the end of the first cavity 233 forming a blind hole. The insert component 239 is circumferentially aligned so that the openings 246 formed in the side surface 245 of the insert component 239 are located at positions on the wall of the cavity 233 where the number of flow passage portions 237 corresponding to the number of openings 246 start or branch off. The circumferential alignment of the insert component 239 can also be achieved by providing the end surface 239a of the rod-shaped insert component 239 with a lateral protrusion (see FIG. 13c), which acts as a tongue and engages in a correspondingly shaped groove in the side surface of the base plate 225, as shown by dashed lines in FIG. 13b.

[0209] To introduce the flexible fluid lines 241 into the flow channel portion 237, the insert component 239 includes seven guide channels 247 in the example shown, see FIG. 14. It is understood that the insert component 239 can also have a greater or lesser number of guide channels 247. As is evident from FIG. 14a, which shows the insert component 239 in a plan view of its end face 239a shown in FIG. 13c, six of the guide channels 247 are arranged around a central guide channel 247. In the example shown in FIGS. 14a-c, the guide channels 247 are stainless steel tubes embedded or cast in a solid material 248 that defines a diameter D' of the insert component 39, in the example shown, about 9 mm. The diameter H of the cylindrical cavity 233, more precisely of its inner wall 233a, is slightly larger, about 10 mm. The inner diameter d of each guide channel 247 is about 2 mm. The inner diameter F of the flexible fluid line 241 is about 1 mm.

[0210] As can be seen in Fig. 14a, each fluid line 241 extends centrally through a respective guide channel 247. A ring gap 249 is located between the fluid line 241 and an inner wall 247a of the respective guide channel 247. The ring gap 249 serves to return the fluid 228 that is supplied to each removal front 230a, 230b through the flexible fluid line 241. The returned fluid 228 exits the end face 239a of the insert component 239 for removal therefrom.

[0211] 14b, each guide channel 247 of the insert component 239 has a rounded portion 250 for changing the alignment of the flexible fluid lines 241 from a longitudinal direction corresponding to the Y direction of the insert component 239 to a direction perpendicular thereto so that the flexible fluid lines 241 can exit from each opening 246 on the side surface 245 of the rod-like insert component 239. The rounded portion 250 is made by bending each stainless steel tube forming the guide channel 247.

[0212] As is evident from Figures 13c and 14c, the openings 246 in the side surface 245 of the insert component 239 are arranged along a common line corresponding to the longitudinal direction of the insert component 239, i.e. the Y direction. This is usually necessary, since the channel portions 237 likewise extend along a common line corresponding to the longitudinal direction of the cavity 233, i.e. the Y direction. The distance A'' between adjacent openings 246 in the side surface 245 corresponds to the distance between two adjacent channel portions 237 of the substrate 225. It is not essential that the channel portions 237 and the openings 246 in the side surface of the insert component 239 extend along a common line, and deviations from such an arrangement along a common line are possible.

[0213] In order to arrange the openings 246 in the side surfaces 245 next to each other in the longitudinal direction, it is usually necessary to design the guide channels 247 with different lengths and to twist the curved portions 250 relative to each other, which is very clear from Figures 15a, 15b, which show an insert component 239 which differs from the insert component 239 shown in Figures 14a-c in that it has been produced by additive manufacturing. The insert component 239 shown in Figures 15a, 15b consists of a cylindrical body in which the guide channels 247 are formed during additive manufacturing.

[0214] The free ends of the fluid lines 241 shown in FIG. 15b, which protrude beyond the side surface 245 of the insert component 239, protrude into the channel portion 237 during operation of the fluid supply device 238. To form the portion of the hollow structure 227 shown in FIG. 13c using dashed lines, removal fronts 230a, 230b are created at the end faces of the already formed channel portion 237 by irradiation with pulsed laser radiation. Starting from the end face of the already formed channel portion 237 which forms the side opposite to the radiation incidence side, the removal fronts 230a, 230b are moved relative to the substrate 225 by displacing the substrate 225 downwards in the Z direction until the removal front 230a is at the same height as the horizontally extending cooling channels 231. Alternatively, for example, the focal position of the irradiated pulsed laser radiation can be offset upwards by the same value at all points along the removal front 230a in order to move the removal front 230a within the substrate 225 while the substrate 225 itself remains stationary.

[0215] To form the horizontal cooling channel 231, the removal front 230a, which is oriented at approximately 45° with respect to the thickness direction of the substrate 225, i.e. the Z direction, or with respect to the incident radiation direction of the pulsed laser radiation, is displaced in the longitudinal direction of the cooling channel 231, i.e. the X direction, until it is located approximately in the center of the cooling channel 231. In this case, the substrate 225 is normally stationary, and an optical unit for emitting a laser beam on the substrate 225 is displaced and moved in an appropriate manner to displace the removal front 230a in the horizontal direction.

[0216] Using further insert components of a further fluid supply device, not shown here, a plurality of recovery channels 235, usually seven, are simultaneously created by multiphoton laser ablation starting from the second cavity 235 by displacing the substrate 225 downwards or by displacing the focal position of the irradiated pulsed laser radiation upwards by a certain value at each point of the removal front 230b while the substrate 225 remains stationary. The removal front 230b is then moved in the Y direction while the substrate 225 remains stationary. The two removal fronts 230a, 230b overlap approximately in the center of each cooling channel 231, which may have a length of, for example, about 400 mm, so that a continuous cooling channel 231 is formed, connected by a distribution channel 234 to the first cavity 233 acting as a fluid distributor and by a recovery channel 236 to the second cavity 235 acting as a fluid collector.

[0217] In this manner, the seven distribution channels 234, cooling channels 231, and return channels 236 can be fabricated simultaneously, which significantly reduces the time required to fabricate the hollow structure 227. To fabricate the remaining seven distribution channels 234, cooling channels 231, and return channels 236 of the hollow structure 227 shown in FIG. 13c, an insert component 240 shown in FIG. 13c can be utilized, in which the opening 246' for the fluid line 241 to exit is offset relative to the opening 246 of the first insert component 239. By inserting the second insert component 240 into the first cavity 233, the fabrication of the second group of seven distribution channels 234, cooling channels 231, and return channels 236 is performed in a similar manner to the formation of the first group.

[0218] It is understood that, unlike that shown in FIG. 13c, the second insert component 240 may be shorter in length than the first insert component 239. In this case, the second insert component 240 may be introduced after a spacer, for example in the form of a solid cylinder, is introduced into the cavity 233, so that the insert component 240 abuts against an end face of said spacer when pressed in. Alternatively, one or more protruding portions on the end face of the insert component 240 may act as a stopper or as an abutment surface to limit the movement of the second insert component 240 when introduced into the cavity 233. The stopper attached to the end face of the second insert component 240 may also act as a tongue to engage a corresponding groove on the outside of the substrate 225 in order to properly circumferentially align the second insert component 240 as described above.

[0219] Within the scope of the creation of a complex hollow structure 227 shown in Fig. 13c, multiple removal fronts 230a, 230b created simultaneously can be automatically tracked by flexible fluid lines 241 using the fluid supply device 238 described above. In this way, multiple structures, e.g. cooling channels 231, can be created simultaneously, resulting in significant time savings or productivity increases during the creation of complex hollow structures 227.

[0220] The fluid supply 50 described above in connection with Figures 6a-6c may for example be designed as the fluid supply device 238 described in connection with Figures 13c, 14a-14c and 15a-15b. It is however also possible that the fluid supply 50 only comprises some of the components of the fluid supply device 238, such as the fluid supply device 243 and optionally the tracking device 244. The latter may for example be used for automatic tracking by at least one flexible fluid line 241 in the form of a flexible tube 52 as described in connection with Figures 6a-6c.

[0221] The present invention also includes aspects described in the following paragraphs which form part of this specification but are not claimed.

[0222] 1. A method for producing a hollow structure (28) in a workpiece (25), in particular a substrate for an EUV mirror (M4), by material removal processing using pulsed laser radiation (35), comprising: irradiating a workpiece (25) made of a material transparent to the pulsed laser radiation (35) from a radiation incident side (27); focusing pulsed laser radiation (35) at a focal region (39); creating a removal front (46) for area-wise removal of material of the workpiece (25) by moving the focal region (39) along mutually offset trajectories (42) of a movement pattern (41); creating a hollow structure (28) by moving the removal front (46) within the workpiece (25) starting from the side (29) of the workpiece (25) opposite the radiation incident side (27); In a method comprising: A method characterized in that during the production of a hollow structure (28), a removal front (46) is formed at least intermittently on the radiation incident side (27) of the workpiece (25) that is not oriented perpendicularly to the incident radiation direction (Z) of the pulsed laser radiation (35).

[0223] 2. A method according to claim 1, in which a removal front (46) is formed at least intermittently, oriented at an angle (α) of 20° to 70°, preferably 30° to 60°, relative to the incident radiation direction (Z) of the pulsed laser radiation (35) on the radiation incident side (27) of the workpiece (25).

[0224] 3. A method according to paragraph 1 or paragraph 2, in which the trajectories (42) of the movement patterns (41) are offset from one another in the incident radiation direction (Z) by offsetting the focal region (39) in the incident radiation direction (Z) to form an ablation front (46) that is not oriented perpendicular to the incident radiation direction (Z).

[0225] 4. In the method according to any one of claims 1 to 3, the pulse energy (E P ) to form an ablation front (46) that is not oriented perpendicular to the incident radiation direction (Z), and the focal region (39) is moved preferably in a plane (FE) perpendicular to the incident radiation direction (Z).

[0226] 5. A method according to any one of paragraphs 1 to 4, wherein during creation of the hollow structure (28), the removal front (46) is at least intermittently moved in a movement direction (-X) transverse to the incident radiation direction (Z) within the workpiece in order to form a portion (28b) of the hollow structure (28) that preferably extends substantially parallel to the radiation incident side (27), and an end (46a) of the removal front (46) closer to the radiation incident side (27) is oriented at an angle (β) of less than 90°, preferably less than 70°, with respect to the movement direction (-X) during the lateral movement relative to the incident radiation direction (Z).

[0227] 6. A method according to any one of paragraphs 1 to 5, wherein the removal front (46) is moved at least intermittently substantially parallel to the incident radiation direction (Z) during the creation of the hollow structure (28) in order to create a portion (28a) of the hollow structure (28) extending substantially parallel to the incident radiation direction (Z) starting from the side (29) opposite the radiation incident side (27) of the workpiece (25) and preferably in order to create a further portion (28c) of the hollow structure (28) extending substantially parallel to the incident radiation direction (Z).

[0228] 7. A method according to any one of paragraphs 1 to 6, comprising producing a first portion (28a, 28c) of a hollow structure (28) and an adjacent second portion (28b) of the hollow structure (28), in which the longitudinal directions (X, Z) are oriented at an angle (γ) of 70° to 100°, preferably an angle (γ) of 90°, relative to each other.

[0229] 8. The method according to paragraph 7, wherein rounded portions (28d, 28e) are formed during the production of the hollow structure (28), and the first portion (28a, 28c) and the second portion (28b) merge with each other at the rounded portions.

[0230] 9. A method according to any one of paragraphs 1 to 8, wherein the removal front (46) is brought into contact with the fluid (32b) during creation of the hollow structure (28), and the fluid (32b) tracks the removal front (46), preferably by means of a flexible tube (52), during its movement starting from the side (29) of the workpiece (25) remote from the radiation incidence side (27).

[0231] 10. The method according to any one of claims 1 to 9, wherein the workpiece is displaced to move the removal front (46).

[0232] 11. A method according to any one of claims 1 to 10, wherein the focal region (39) is moved along mutually offset trajectories (42) of a movement pattern (41) by a scanner optical unit (36) to form an ablation front (46).

[0233] 12. The method according to any one of claims 1 to 11, wherein the substrate (25) is monolithic and made of titanium-doped quartz glass or glass ceramic.

[0234] 13. An EUV mirror (M4), A substrate (25); a coating (26) applied to the substrate (25) that serves to reflect EUV radiation (16); wherein the substrate (25) includes at least one hollow structure (28) fabricated using the method according to any one of paragraphs 1 to 12.

[0235] 14. An EUV lithography system (1) comprising at least one EUV mirror (M4) as described in paragraph 13 and a cooling device (32) designed to flow a cooling fluid (32a) through at least one hollow structure (28).

[0236] 15. An apparatus (33) for producing at least one hollow structure (28) in a workpiece (25), in particular in a substrate for an EUV mirror (M4), comprising: a laser source (34) generating pulsed laser radiation (35); a focusing device (37) for focusing the laser radiation (35) at a focal region (39); a holder (44) for accommodating a workpiece (25); a scanner optical unit (36) designed to emit pulsed laser radiation (35) onto a radiation entrance side (27) of a workpiece (25) accommodated in a holder (44) and to move a focal region (39) along mutually offset trajectories (42) of a movement pattern (41) in order to form a removal front (46) for area-like removal of material of the workpiece (25); a positioning device (43) for moving a removal front (46) within the workpiece (25) starting from a side (29) of the workpiece (25) opposite to the radiation incidence side (27) in order to create a hollow structure (28); Equipped with The apparatus (33) is characterized in that it is designed to form a removal front (46) in a workpiece (25) accommodated by a holder (44) that is not oriented perpendicular to the incident radiation direction (Z) of the pulsed laser radiation (35).

[0237] 16. The device according to claim 15, further comprising a focus offset device (47) for offsetting the focal region (39) of the pulsed laser radiation (35) in the incident radiation direction (Z), a control device (48) designed to control the focus offset device (47) to offset the trajectories (42) of the movement patterns (41) relative to one another in the incident radiation direction (Z) to form an ablation front (46) that is not oriented perpendicular to the incident radiation direction (Z); The apparatus further comprises:

[0238] 17. The device according to claim 15 or 16, The pulse energies (E) of the pulsed laser radiation (35) of the mutually offset trajectories (42) of the moving pattern (41) are adjusted to form an ablation front (46) that is not oriented perpendicular to the incident radiation direction (Z). P a control device (48) designed to control the laser source (34) to vary the The apparatus further comprises:

[0239] 18. An apparatus according to any one of paragraphs 15 to 17, wherein the positioning device (43) is designed to displace the workpiece (25) in the direction of incident radiation (Z), preferably in at least one direction (X, Y) transverse to the direction of incident radiation (Z).

[0240] 19. The device according to any one of items 15 to 18, a fluid supply (50) designed to supply a fluid (32b) to the removal front (46), preferably having a flexible tube (52) that is tracked by the fluid (32b) during movement of the removal front (46) starting from the side (29) opposite the radiation incidence side (27) of the workpiece (25); The apparatus further comprises:

[0241] 20. An optical element (M4) for reflecting radiation, in particular EUV radiation (16), A monolithic substrate (25); a reflective coating (26) applied to a surface (25a) of the monolithic substrate (25); at least one hollow structure (27) extending within the monolithic substrate (25) and designed to carry a fluid (28), the hollow structure (27) having a first portion (31a, 31b; 34b, 36b) and a second adjacent portion (34a, 36a; 33a, 35a) oriented at an angle (γ, γ') between 60° and 120°, preferably at an angle (γ, γ') between 80° and 100°, in particular at an angle (γ, γ') of 90° relative to one another; Equipped with An optical element, wherein the hollow structure (27) has rounded portions (37a, 37b, 38) where the first portions (31a, 31b; 34b, 36b) and second portions (34a, 36a; 33a, 35a) join each other.

[0242] 21. The optical element according to item 20, wherein the R / D ratio of the radius of curvature R of the rounded portions (37a, 37b) to the diameter D of the rounded portions (37a, 37b) is 2 to 6, preferably 2.5 to 5, particularly 2.5 to 3.5.

[0243] 22. The optical element according to item 20 or 21, wherein the diameter D of the rounded portions (37a, 37b) is 2 mm to 20 mm, preferably 2 mm to 12 mm.

[0244] 23. An optical element described in any one of items 20 to 22, wherein the hollow structure (27) includes a plurality of cooling flow paths (31) extending below a surface (25a) on which a reflective coating (26) is applied, and the hollow structure (27) includes a fluid distribution section (33) connected to the cooling flow paths (31) via a distribution flow path (34), and a fluid recovery section (35) connected to the cooling flow path (31) via a recovery flow path (36).

[0245] 24. An optical element as described in paragraph 23, wherein the first portion forms an end portion (31a) of the cooling flow path (31) adjacent to the distribution flow path (34), and the second portion forms a distribution flow path portion (34a) adjacent to the end portion (31a), and / or the first portion forms an end portion (31b) of the cooling flow path (31) adjacent to the recovery flow path (36), and the second portion forms a recovery flow path portion (36a) adjacent to the end portion (31b).

[0246] 25. An optical element as described in item 23 or 24, wherein the fluid distribution section forms an inlet flow path (33) from which the distribution flow path (34) branches, and / or the fluid recovery section forms an outlet flow path (35) from which the recovery flow path (36) branches.

[0247] 26. An optical element as described in paragraph 25, wherein the first portion forms the confluence portion (34b) of the distribution flow path (34) adjacent to the inlet flow path (33), and the second portion forms the branch portion (33a) of the inlet flow path (33) adjacent to the confluence portion (34b), and / or the first portion forms the confluence portion (36b) of the recovery flow path (36) adjacent to the outlet flow path (35), and the second portion forms the branch portion (35a) of the outlet flow path (35) adjacent to the confluence portion (36b) of the recovery flow path (36).

[0248] 27. An optical element as described in paragraph 26, wherein the angle (γ') between the branching portion (33a) of the inlet flow path (33) and the joining portion (34b) of the distribution flow path (34) is greater than 90°, preferably greater than 100°, and / or the angle (γ') between the branching portion of the outlet flow path (35) and the joining portion (36b) of the recovery flow path (35) is greater than 90°, preferably greater than 100°.

[0249] 28. The optical element according to any one of items 20 to 27, wherein the material of the substrate (25) is selected from the group consisting of silica glass, in particular titanium-doped silica glass, and glass ceramics.

[0250] 29. An optical apparatus, in particular an EUV lithography system (1), comprising: At least one optical element (M4) according to any one of clauses 20 to 28, a temperature control device, in particular a cooling device (32) designed to direct a fluid (28) through at least one hollow structure (27); An optical device comprising:

[0251] 30. A fluid supply device (38) for supplying a fluid (28) to at least one ablation front (30a, 30b) during removal of material by multiphoton laser ablation from a workpiece, preferably from a monolithic substrate (25) for an EUV mirror (M4), comprising: at least one flexible fluid line (41), preferably a plurality of flexible fluid lines (41), for supplying fluid (28) to at least one ablation front (30a, 30b); at least one insertion component (39, 40) for inserting the workpiece (25) into the cavity (33, 35), the insertion component (39, 40) having at least one guide channel (47) for guiding at least one flexible fluid line (41) to supply fluid (28) to at least one ablation front (30a, 30b); A fluid supply device comprising:

[0252] 31. The fluid supply device according to claim 30, wherein the insertion components (39, 40) each have a plurality of guide channels (47) for guiding a flexible fluid line (41).

[0253] 32. A fluid supply device as described in paragraph 30 or paragraph 31, in which a gap, in particular a ring gap (49), through which the fluid can flow is formed between the fluid line (41) and the flow path wall (47a) of the guide flow path (47) to return the fluid (28) from the ablation front (30a, 30b).

[0254] 33. The fluid supplying device according to any one of items 30 to 32, wherein the guide flow path (47) has at least one rounded portion (50) for changing the direction of the flexible fluid line (41).

[0255] 34. In the fluid supply device according to any one of items 30 to 33, the insert component (39, 40) is rod-shaped, and the guide flow path (47) extends from an end face (48) of the insert component (39) to a side face (45) of the insert component (39). Fluid supply device.

[0256] 35. A fluid supply device as described in paragraph 34, wherein the guide passage (41) merges with an opening (46) on a side surface (45) of the insert component (39), and the openings are preferably arranged adjacent to each other in the longitudinal direction (Y) of the insert component (39), in particular at an equal distance (A) from each other in the longitudinal direction (Y) of the insert component (39).

[0257] 36. A fluid supplying device according to item 34 or 35, wherein the rod-shaped insertion component (39) has a cylindrical shape and preferably has a diameter (D) of 5 mm to 10 mm.

[0258] 37. The fluid supplying device according to any one of items 30 to 36, wherein at least one guide channel (47) has a diameter (d) of 1 mm to 4 mm.

[0259] 38. The fluid supplying device according to any one of items 30 to 37, wherein at least one fluid line (41) has an outer diameter (F) of 1 mm or less.

[0260] 39. The fluid supply device according to any one of paragraphs 30 to 38, further comprising a fluid refill device (43) that supplies fluid (28) to at least one flexible fluid line (41).

[0261] 40. A fluid supply device according to any one of paragraphs 30 to 39, further comprising at least one tracking device (44) for automatically tracking the movement of the ablation front (30a, 30b) within the material of the workpiece (25) via at least one flexible fluid line (41).

[0262] 41. A method for the removal of material from a workpiece by multiphoton laser ablation, in particular from a preferably monolithic substrate (25) for an EUV mirror (M4), comprising the steps of: supplying a fluid (28) to at least one ablation front (30a, 30b) by a fluid supply device (38) according to any one of clauses 30 to 40, Inserting an insert component (39, 40) into a cavity (33, 35) of a workpiece (25); supplying fluid (28) to at least one ablation front (30a, 30b) through at least one flexible fluid line (41) guided in at least one guide channel (47) of the insertion component (39, 40); The method includes:

[0263] 42. A method according to claim 41, further comprising filling the cavities (33, 35) with a fluid (28) prior to insertion of the insertion components (39, 40), and forming a plurality of flow channel portions (37) adjacent to the cavities (33, 35) by multiphoton laser ablation, starting from the cavities (33, 35) filled with the fluid (28).

[0264] 43. A method as described in paragraph 42, wherein a plurality of ablation fronts (30a, 30b) are formed starting from a flow path portion (37) after insertion into the cavity (33, 35) of the insertion component (39, 40) and moved within the material of the workpiece (25) to form a plurality of flow paths (31, 34, 35), and a plurality of flexible fluid lines (41) track the movement of the ablation fronts (30a, 30b) within the material of the workpiece (25).

Claims

1. 1. A method for producing hollow structures (28) in a workpiece (25) in the form of a substrate for a mirror, in particular for an EUV mirror (M4), by material removal processing with pulsed laser radiation (35), comprising: irradiating the pulsed laser radiation (35) from a radiation incident side (27) onto the workpiece (25) made of a material transparent to the pulsed laser radiation (35); focusing the pulsed laser radiation (35) into a focal region (39); moving the focal region (39) along a movement pattern (41) to form a removal front (46) for area-wise removal of material of the workpiece (25); creating the hollow structure (28) by moving the removal front (46) within the workpiece (25); In a method comprising: A method characterized in that during the creation of a hollow structure (28), an ablation front (46) is at least intermittently formed on the radiation incident side (27) of the workpiece (25) that is not oriented perpendicular to the incident radiation direction (Z) of the pulsed laser radiation (35), and the hollow structure (28) is created in the form of a flow channel (28) through which a fluid (32a) can flow.

2. 2. The method of claim 1, wherein the hollow structure is made in the form of a curved channel (28) through which a fluid (32a) can flow.

3. 2. The method of claim 1, wherein the pulsed laser radiation (35) first exits the material of the workpiece (25) in the region of the removal front (46).

4. 4. The method of claim 3, wherein the pulsed laser radiation (35) re-enters the workpiece (25) material after exiting the workpiece (25).

5. 2. The method of claim 1, wherein the removal front (46) within the workpiece (25) is moved at least intermittently in a direction toward the radiation incident side (27) of the workpiece (25).

6. 2. The method of claim 1, wherein, during movement of the removal front (46) within the workpiece (25), material of the workpiece (25) is at least intermittently adjacent to a side (46c) of an edge (46b) of the removal front (46) that is remote from the radiation entrance side (27) of the workpiece (25), and the edge (46b) is remote from the radiation entrance side (27) of the workpiece (25).

7. 2. The method of claim 1, wherein the removal front (46) is moved starting from a side (29) of the workpiece (25) opposite the radiation incident side (27).

8. 2. The method according to claim 1, wherein the removal front (46) is oriented at an angle (α) of between 20° and 70°, preferably between 30° and 60°, relative to the incident radiation direction (Z). ) at least intermittently.

9. 2. The method of claim 1, wherein the focal regions (39) are moved along mutually offset trajectories (42) of the movement pattern (41).

10. 10. The method of claim 9, wherein the trajectories (42) of the movement patterns (41) are offset from one another along the incident radiation direction (Z) by offsetting the focal region (39) along the incident radiation direction (Z) to form the removal front (46) that is not oriented perpendicular to the incident radiation direction (Z).

11. 10. The method according to claim 9, wherein the pulse energy (EP) of the pulsed laser radiation (35) of the mutually offset trajectories (42) of the movement pattern (41) is changed to form the ablation front (46) not oriented perpendicular to the incident radiation direction (Z), and the focal region (39) is moved preferably in a plane (FE) perpendicular to the incident radiation direction (Z).

12. 2. The method of claim 1, wherein during creation of the hollow structure (28), the removal front (46) is at least intermittently moved within the workpiece in a movement direction (-X) transverse to the incident radiation direction (Z) to form a portion (28b) of the hollow structure (28) that preferably extends substantially parallel to the radiation incident side (27), and an end (46a) of the removal front (46) close to the radiation incident side (27) is oriented at an angle (β) of less than 90°, preferably less than 70°, with respect to the movement direction (-X) during the transverse movement relative to the incident radiation direction (Z).

13. 2. The method of claim 1, wherein the removal front (46) is moved at least intermittently substantially parallel to the incident radiation direction (Z) during creation of the hollow structure (28) in order to create a portion (28a) of the hollow structure (28) extending substantially parallel to the incident radiation direction (Z) starting from a side (29) of the workpiece (25) opposite the radiation incident side (27) and preferably to create a further portion (28c) of the hollow structure (28) extending substantially parallel to the incident radiation direction (Z).

14. 2. The method according to claim 1, wherein the longitudinal directions (X, Z) are oriented at an angle (γ) between 70° and 100°, preferably 90°, relative to one another. A method for making a first portion (28a, 28c) of a hollow structure (28) and an adjacent second portion (28b) of said hollow structure (28).

15. 15. The method of claim 14, wherein a rounded portion (28d, 28c) is formed during the creation of the hollow structure (28), and the first portion (28a, 28c) and the second portion (28b) merge into each other at the rounded portion.

16. 2. The method of claim 1, wherein the removal front (46) is contacted with a fluid (32b) during creation of the hollow structure (28), the fluid (32b) tracking the removal front (46) as the removal front (46) moves by a fluid supply (50) that is introduced at least partially into the hollow structure (28).

17. 17. The method according to claim 16, wherein the fluid supply (50) comprises at least one flexible tube (52), and the removal front (46) is tracked by the fluid (32b) using the at least one flexible tube (52), in particular starting from the side (29) of the workpiece (25) remote from the radiation incident side (27).

18. 2. The method of claim 1, wherein the workpiece is displaced to move the removal front (46).

19. 2. The method of claim 1, wherein the focal region (39) is moved by a scanner optical unit (36) along mutually offset trajectories (42) of the movement pattern (41) to form the removal front (46).

20. 2. The method according to claim 1, wherein the hollow structure has a circular cross section with a diameter (D) of 1 mm to 20 mm, preferably 1 mm to 5 mm.

21. 2. The method according to claim 1, wherein the hollow structure has a length (LC) of 10 cm or more, preferably 15 cm or more, particularly preferably 20 cm or more, in particular 70 cm or more.

22. In a method according to the preamble of claim 1, in particular according to any one of claims 1 to 21, The hollow structure (28) is defined by a side surface (57) other than the removal front (46), and the removal front (46) during fabrication of the hollow structure (28) is oriented at a removal front angle (β') relative to an area of ​​the side surface (57) of the hollow structure (28) adjacent to the removal front (46), and the removal front angle (β') is such that the removal front is at least intermittently and at least intermittently less than a removal front maximum angle of 89°.

23. 1. A method for producing a channel (28) in a workpiece (25) in the form of a substrate (25) for a mirror, in particular for an EUV mirror (M4), wherein the channel (28) is produced by material removal processing using pulsed laser radiation (35), and a fluid supply (50) is introduced at least partially into the channel (28) during the production of the channel (28).

24. 24. The method of claim 23, wherein the fluid supply (50) is used to supply fluid (32b) to the area where the material removal process is being performed, in particular to a removal front (46) formed during the material removal process, and the removal front (46) is preferably tracked, in particular automatically, by the fluid supply (50) as it moves within the workpiece (25).

25. 24. The method of claim 23, wherein the fluid supply (50) comprises a flexible element preferably introduced at least partially into the curved flow path (28), the flexible element preferably forming a flexible tube (52).

26. 24. The method according to claim 23, wherein the channel (28) is made with a length (LC) of 10 cm or more, preferably 15 cm or more, particularly preferably 20 cm or more, in particular 70 cm or more.

27. 2. The method of claim 1, wherein the substrate (25) is monolithic.

28. 2. The method of claim 1, wherein the substrate (25) is made of titanium-doped quartz glass or glass ceramic.

29. 2. The method according to claim 1, wherein the material of the substrate (25) is heated to a temperature between 0°C and 100°C, preferably between 19°C and 40°C, particularly preferably between 19°C and 32°C. The method has a loss temperature (TZC).

30. 2. The method according to claim 1, wherein the material of the substrate (25) has a spatial variation of the zero cross temperature (ΔTZC) of less than 3 K, preferably less than 2 K, particularly preferably less than 1 K, in particular less than 0.1 K.

31. A mirror, in particular an EUV mirror (M4), A substrate (25); a coating (26) applied to said substrate (25) that reflects radiation, in particular EUV radiation (16); 10. A mirror comprising: a substrate (25) including at least one channel (28) fabricated using the method of claim 1.

32. A mirror, in particular an EUV mirror (M4), a substrate (25) including at least one channel (28) through which a fluid (32b) can preferably flow; wherein the flow passage (28) is formed by material removal processing using pulsed laser radiation (35); the flow channel (28) has a curved configuration; and The channel (28) has a diameter (D) of 1 mm to 20 mm, preferably 1 mm to 5 mm, and / or a length (LC) of 10 cm or more, preferably 15 cm or more, in particular 20 cm or more.

33. Mirror according to claim 32, wherein the substrate (25) is monolithic.

34. Mirror according to claim 32, wherein the channels (28) are oriented such that their longitudinal directions (X, Z) are mutually oriented at an angle (γ) between 70° and 100°, preferably at an angle (γ) of 90°. , a mirror having a first portion (28a, 28c) and an adjacent second portion (28b).

35. 35. A mirror according to claim 34, wherein the first portion (28a, 28c) and second portion (28b) meet each other at a rounded portion (28d, 28e).

36. A mirror, in particular an EUV mirror (M4), a preferably monolithic substrate (125); a reflective coating (126) that reflects radiation, in particular EUV radiation (16), the reflective coating (126) being applied to a surface (125a) of said preferably monolithic substrate (125); At least one hollow structure (127) extending in said preferably monolithic substrate (125) and designed to carry a fluid (128), is arranged at an angle (γ, γ') between 60° and 120° relative to one another, preferably at an angle (γ, γ') between 80° and 100°, in particular at an angle (γ, γ') of 90°. γ, γ′) and the adjacent first portions (131a, 131b; 134b, 136b). and a hollow structure (127) having a second portion (134a, 136a; 133a, 135a), the hollow structure (127) having rounded portions (137a, 137b, 138) where the first portion (131a, 131b; 134b, 136b) and the second portion (134a, 136a; 133a, 135a) meet each other.

37. 37. The mirror according to claim 36, wherein the R / D ratio of the radius of curvature R of the rounded portions (137a, 137b) to the diameter D of the rounded portions (137a, 137b) is between 2 and 6, preferably between 2.5 and 5, in particular between 2.5 and 3.

5.

38. Mirror according to claim 36, wherein the diameter D of the rounded portions (137a, 137b) is between 2 mm and 20 mm, preferably between 2 mm and 12 mm.

39. 37. A mirror according to claim 36, wherein the hollow structure (127) comprises a plurality of temperature control channels (131), in particular in the form of cooling channels, extending below the surface (125a) on which the reflective coating (126) is applied, and wherein the hollow structure (127) comprises a fluid distribution section (133) connected to the temperature control channels (131) via distribution channels (134) and a fluid recovery section (135) connected to the temperature control channels (131) via recovery channels (136).

40. 40. A mirror as described in claim 39, wherein the first portion forms an end portion (131a) of the temperature control flow path (131) adjacent to a distribution flow path (134), and the second portion forms a distribution flow path portion (134a) adjacent to the end portion (131a), and / or the first portion forms the end portion (131b) of the temperature control flow path (131) adjacent to a recovery flow path (136), and the second portion forms a recovery flow path portion (136a) adjacent to the end portion (131b).

41. 40. A mirror as claimed in claim 39, wherein the fluid distribution section forms an inlet flow path (133) from which a distribution flow path (134) branches, and / or the fluid recovery section forms an outlet flow path (135) from which a recovery flow path (136) branches.

42. 42. A mirror according to claim 41, wherein the first portion forms a converging portion (134b) of the distribution flow path (134) adjacent to the inlet flow path (133), and the second portion forms a branched portion (133a) of the inlet flow path (133) adjacent to the converging portion (134b), and / or the first portion forms a converging portion (136b) of the recovery flow path (136) adjacent to the outlet flow path (135), and the second portion forms a branched portion (135a) of the outlet flow path (135) adjacent to the converging portion (136b) of the recovery flow path (136).

43. A mirror as described in claim 42, wherein the angle (γ') between the branching portion (133a) of the inlet flow path (133) and the confluence portion (134b) of the distribution flow path (134) is greater than 90°, preferably greater than 100°, and / or the angle (γ') between the branching portion of the outlet flow path (135) and the confluence portion (136b) of the recovery flow path (135) is greater than 90°, preferably greater than 100°.

44. Mirror according to claim 31, wherein the material of the substrate (125) is selected from the group comprising quartz glass, in particular titanium-doped quartz glass, and glass ceramic.

45. Mirror according to claim 31, wherein the material of the substrate (25, 125) has a temperature range of 0°C to 100°C, preferably 19°C to 40°C, particularly preferably 19°C to 32°C. A mirror having a zero cross temperature (TZC) of

46. Mirror according to claim 31, wherein the material of the substrate (25, 125) has a spatial variation of the zero cross temperature (ΔTZC) of less than 3K, preferably less than 2K, particularly preferably less than 1K, in particular less than 0.1K.

47. Mirror according to claim 31, wherein the hollow structure, in particular in the form of the channel (28), has a seam region (53).

48. Mirror according to claim 47, wherein the hollow structure, in particular in the form of the channel (28), has an edge contour (54) of the removal front (46), at least one bulge (55), a lateral offset (56) or another structural modification in the seam region (53).

49. A mirror, in particular an EUV mirror (M4), A substrate (25) including a particularly curved channel (28) through which a fluid (32b) can flow, said channel having a seam region (53). Mirror with.

50. 50. The mirror of claim 49, wherein the flow path (28) has an edge contour (54) of the removal front (46), at least one bulge (55), a lateral offset (56), or another structural modification in the seam region (53).

51. 51. An EUV lithography system (1) comprising at least one EUV mirror (M4) according to any one of claims 31 to 50 and a temperature control device, in particular a cooling device (32), designed to flow a temperature control fluid, in particular a cooling fluid (32a, 128), through at least one hollow structure (127), in particular in the form of a channel (28).

52. 1. An apparatus (33) for producing at least one channel (28) in a workpiece (25) in the form of a substrate (25) for a mirror, in particular for an EUV mirror (M4), comprising: a laser source (34) for generating pulsed laser radiation (35); a focusing device (37) for focusing the laser radiation (35) into a focal region (39); a holder (44) for accommodating the workpiece (25); a scanner optical unit (36) designed to emit the pulsed laser radiation (35) onto the radiation incident side (27) of the workpiece (25) accommodated in the holder (44) and to move the focal region (39); In an apparatus (33) comprising: The device (33) comprises a fluid supply (50) at least partially introducible into said flow path (28).

53. 53. The apparatus of claim 52, wherein the fluid supply (50) is designed to supply fluid (32b) to the area where the material removal process is performed, in particular to a removal front (46) formed during the material removal process, and the fluid supply (50) is preferably capable of tracking the removal front (46) as it moves within the workpiece (25).

54. 53. Device according to claim 52, wherein the fluid supply (50) is designed to at least partially introduce a flexible element, in particular a flexible tube (52), into the flow path (28).

55. 53. The apparatus of claim 52, wherein the scanner optical unit (36) is designed to move the focal region (39) along a movement pattern (41) to form a removal front (46) for area-like removal of material from the workpiece (25), and the apparatus (33) is designed to form a removal front (46) that is not oriented perpendicular to the incident radiation direction (Z) of the pulsed laser radiation (35) on the workpiece (25) accommodated by the holder (44).

56. 56. The apparatus of claim 55, further comprising: a focus offset device (47) for offsetting the focal region (39) of the pulsed laser radiation (35) along the incident radiation direction (Z); a control device (48) designed to control the focus offset device (47) to offset the trajectories (42) of the movement patterns (41) relative to one another along the incident radiation direction (Z) in order to form the ablation front (46) not oriented perpendicular to the incident radiation direction (Z); The apparatus further comprises:

57. 56. The apparatus of claim 55, a control device (48) designed to control the laser source (34) to vary the pulse energy (EP) of the pulsed laser radiation (35) of the mutually offset trajectories (42) of the movement pattern (41) in order to form the ablation front (46) not oriented perpendicular to the incident radiation direction (Z). The apparatus further comprises:

58. 53. The apparatus according to claim 52, further comprising a positioning device (43) for moving the removal front (46) within the workpiece (25), preferably starting from the side (29) of the workpiece (25) opposite the radiation incident side (27) in order to create the flow path (28), the positioning device (43) being designed to displace the workpiece (25) along the incident radiation direction (Z), preferably along at least one direction (X, Y) transverse to the incident radiation direction (Z).

59. a fluid supply device (238) for supplying a fluid (228) to at least one removal front (230a, 230b) during removal of material by laser ablation from a workpiece, preferably from a monolithic substrate (225) for an EUV mirror (M4), in particular, at least one flexible fluid line (241), preferably a plurality of flexible fluid lines (241), for supplying said fluid (228) to said at least one removal front (230a, 230b); at least one insertion component (239, 240) for inserting the workpiece (225) into the cavity (233, 235), the insertion component (239, 240) having at least one guide channel (247) for guiding the at least one flexible fluid line (241) to supply the fluid (228) to the at least one removal front (230a, 230b); A fluid supply device comprising:

60. 60. The fluid supply device of claim 59, wherein the insert component (239, 240) has a plurality of guide channels (247) for respectively guiding the flexible fluid lines (241).

61. 60. A fluid supply device as described in claim 59, wherein a gap, in particular a ring gap (249), through which fluid can flow is formed between the fluid line (241) and the flow path wall (247a) of the guide flow path (247) to return the fluid (228) from the removal front (230a, 230b).

62. 51. The fluid supply device of claim 50, wherein the guide channel (247) has at least one rounded portion (250) for redirecting the flexible fluid line (241).

63. 60. The fluid supply device of claim 59, wherein the insert component (239, 240) is rod-shaped and the guide channel (247) extends from an end face (248) of the insert component (239) to a side face (245) of the insert component (239).

64. A fluid supply device as described in claim 63, wherein the guide channels (241) merge into openings (246) on the side surfaces (245) of the insert component (239), the openings preferably being arranged adjacent to each other in the longitudinal direction (Y) of the insert component (239), in particular being arranged at equal distances (A'') from each other in the longitudinal direction (Y) of the insert component (239).

65. Fluid supply device according to claim 63, wherein said rod-shaped insert component (239) has a cylindrical form and preferably has a diameter (D') between 5 mm and 10 mm.

66. A fluid supply device according to claim 60, wherein the guide channel (247) has a diameter (d) between 1 mm and 4 mm.

67. 60. A fluid supply device according to claim 59, wherein said at least one fluid line (241) has an outer diameter (F) of 1 mm or less.

68. 60. The fluid supply apparatus of claim 59, further comprising a fluid supply (243) for supplying said fluid (228) to said at least one flexible fluid line (241).

69. 60. The fluid supply apparatus of claim 59, further comprising at least one tracking device (244) for automatically tracking the movement of the removal front (230a, 230b) within the material of the workpiece (225) by means of the at least one flexible fluid line (241).

70. 55. A method for supplying a fluid (228) to at least one removal front (230a, 230b) during removal of material by laser ablation from a workpiece, in particular from a preferably monolithic substrate (225) for an EUV mirror (M4), by means of a fluid supply device (238) according to claim 54, comprising: inserting insert components (239, 240) into cavities (233, 235) of said workpiece (225); supplying said fluid (228) to at least one removal front (230a, 230b) through at least one flexible fluid line (241) guided in at least one guide channel (247) of said insertion component (239, 240); A method comprising:

71. 71. The method of claim 70, wherein the cavity (233, 235) is filled with the fluid (228) before inserting the insert component (239, 240), and starting from the cavity (233, 235) filled with the fluid (228), a plurality of flow path portions (237) adjacent to the cavity (233, 235) are formed by laser ablation.

72. 72. The method of claim 71, wherein a plurality of removal fronts (230a, 230b) are formed starting from the flow path portion (237) after insertion of the insert component (239, 240) into the cavity (233, 235) and moved within the material of the workpiece (225) to form a plurality of flow paths (231, 234, 235), and the plurality of flexible fluid lines (241) track the movement of the removal fronts (230a, 230b) within the material of the workpiece (225).

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