Optical element, semiconductor technology system and method of producing an optical element

Abstract

The invention relates to an optical element (30) comprising: a substrate (31), preferably composed of a ceramic or metallic material, a reflective coating (34) for reflection of radiation, in particular for reflection of EUV radiation (16), and a structuring layer (33, 33') having a structured surface (33a, 33a') that preferably forms a grating structure, where the reflective coating (34) is applied to the structured surface (33a, 33a'). In the optical element (30), the structuring layer (33) is applied to a base layer (32) in the form of a carbide layer and / or the structuring layer (33') is in the form of a carbide layer. The invention also relates to a semiconductor lithography system, in particular an EUV lithography system, including at least one such optical element (30). The invention also relates to a method of producing such an optical element (30).

Description

[0001] Stuttgart, 13.01.2026 SZ00395PCT Rp / pt

[0002] Optical element, semiconductor technology system and method of producing an optical element

[0003] Cross-Reference to related application

[0004] This application claims priority to German Patent Application No.

[0005] 102025102083.3, filed January 21, 2025, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

[0006] Background of the invention

[0007] The invention relates to an optical element comprising: a substrate, preferably composed of a ceramic or metallic material, a reflective coating for reflection of radiation, in particular for reflection of EUV radiation, and a structuring layer having a structured surface that preferably forms a grating structure, where the reflective coating is applied to the structured surface. The invention also relates to a semiconductor technology system including at least one such optical element, and to a method of producing such an optical element, comprising: providing the substrate, depositing the structuring layer, structuring the structuring layer to form the structured surface, in particular the grating structure, and applying the reflective coating to the structured surface.

[0008] Owing to the low transmittance of virtually all known materials to radiation in the EUV wavelength range, i.e. at wavelengths between about 5 nm and about 30 nm, both the illumination optical unit and the projection optical unit in a semiconductor technology system, in particular in a projection exposure

[0009] 2023P00659WO 13.01.26 SZ00395PCTapparatus for EUV lithography, typically contain exclusively reflective optical elements, especially in the form of mirrors.

[0010] The mirrors used therein have a substrate to which a reflective coating is applied in order to reflect the EUV radiation. The reflective coating may be configured as a multilayer coating which acts as an interference layer system for the operating wavelength. If the operating wavelength is about 13.5 nm, the reflective multilayer coating may have, for example, alternating layers of molybdenum and silicon. The reflective coating can be applied directly to the substrate material, but it is also possible for one or more functional layers that serve, for example, to protect the substrate, as a polishing layer or as adhesion promoter to be disposed between the reflective coating and the substrate material.

[0011] Reflective optical elements, for example mirrors for illumination optical units of projection exposure apparatuses, may also have a structured surface, for example in the form of a grating structure. The grating structure may serve as a spectral filter in order to filter out radiation within a wavelength range which is unwanted and which may, for example, be in the infrared or ultraviolet wavelength region. The structured surface need not necessarily be a grating structure; it may instead also be a different type of structuring. The structuring of the surface is typically used to alter the reflective and / or scattering properties of the optical element. The structured surface is typically formed on a functional layer intended for the purpose in the form of a structuring layer. The structuring layer must enable structuring or the providing of a grating structure. It is also necessary that the structuring layer be processible such that the structured surface has sufficient HSFR ("high spatial frequency roughness") smoothness in order to apply the reflective coating. The structuring layer should also grow homogeneously, with a minimum level of defects and so as to be amenable to structuring processes on the substrate.

[0012] 2023P00659WO 13.01.26 SZ00395PCTDuring operation of the projection exposure apparatus, a hydrogen plasma typically forms in a vacuum environment under the influence of the EUV radiation, meaning that activated hydrogen in the form of hydrogen ions and free hydrogen radicals is formed. The hydrogen ions or free hydrogen radicals cause an etch attack on exposed surfaces of components disposed in the vacuum environment. It is therefore advantageous when the material of the structuring layer is stable to such an etch attack since the absorption or intercalation of hydrogen can lead to stresses in the structuring layer and to the formation of small bubbles that can lead to partial or complete layer detachment.

[0013] WO2022 / 037846A1 describes a reflective optical element, in particular for an illumination optical unit of a projection exposure apparatus, comprising: a structured surface that preferably forms a grating structure, and a reflective coating applied to the structured surface. The structured surface may be formed in a functional layer applied to a substrate and / or in the substrate. The functional layer typically includes a material of good processibil ity . The material of the functional layer and / or the material of the substrate may be selected from the group comprising: amorphous silicon, silicon, nickel-phosphorus, metals and alloys thereof; oxides, in particular from the group of silicon dioxide, aluminium oxide, titanium oxide, tantalum oxide, niobium oxide, zirconium oxide and combinations thereof, in particular mixed oxides, ceramics, glass, glass ceramics, composites. WO2022 / 037846A1 proposes protection of the structured surface against an etch attack using a protective layer which covers the structured surface in a closed manner.

[0014] As described above, there may be an arrangement of further functional layers between the reflective coating and the structuring layer, which form a base layer system. A tie layer may also be provided between the substrate and the structuring layer, which creates adhesion to the substrate in the order of more than 10-20 MPa, for example. It is also possible to apply a functional layer to

[0015] 2023P00659WO 13.01.26 SZ00395PCTthe tie layer or to an interlayer, which provides a defect-free surface for the applying of the structuring layer, where the surface can be mechanically preprocessed by material-removing processes, in order that the surface corresponds to the geometric requirements as exactly as possible.

[0016] Tie layers and interlayers are frequently composed of metallic materials, the presence of which is typically undesirable in a semiconductor technology system. Economic production of numerous metallic layers of higher layer thickness is also possible only with high raw material expenditure in the form of metal salts, which are obtained using hazardous substances and with creation of large volumes of hazardous wastewater and are subsequently used for layer deposition. The tie layers and interlayers can also introduce high thermally induced residual stresses into the optical element, which are caused by differences in the coefficients of thermal expansion of the layers and limit the thermal resistance of the optical element, for example a mirror.

[0017] The functional layers of such a base system are frequently applied galvanically - in the case of metallic layers - or wet-chemically, which makes the production thereof complex, time-consuming and costly, and also labour-intensive owing to assembly and disassembly operations, and additionally entails elevated expenditure for the procurement and care of requisites necessary for galvanization. This type of application also requires tight control of numerous coating parameters and complex quality assurance of the layers produced.

[0018] As described further up, any adverse effect on layer growth and layer adhesion of the structuring layer and any underlying functional layers resulting from the particle size distribution of the typically ceramic or metallic substrate should be avoided. The occurrence of epitaxial growth, especially in the case of a metallic substrate, and the requirement of freedom from defects and layer adhesion therefore result in restrictions in the permissible grain size distributions of the substrate.

[0019] 2023P00659WO 13.01.26 SZ00395PCTObject of the invention

[0020] It is an object of the invention to provide an optical element, a semiconductor technology system and a method of producing an optical element, which improve the properties, in particular the service life, of the optical element and simplify its production.

[0021] Subject-matter of the invention

[0022] This object is achieved by an optical element of the type specified at the outset, in which the structuring layer is applied to a base layer in the form of a carbide layer and / or in which the structuring layer is in the form of a carbide layer. The base layer may be applied to a surface of the substrate. Preferably, the base layer is applied directly to the surface of the substrate.

[0023] The inventors have recognized that it is possible to apply, rather than a galvanochemically deposited base layer system with multiple tie layers or interlayers, a uniform, low-defect and firmly adhering base layer of a ceramic in the form of a carbide layer to the substrate, which has or combines the favourable properties of the layers of a multilayer base layer system. A carbide layer means a layer containing at least one carbide. The carbide layer may consist of exactly one carbide, but it is also possible that the carbide layer includes multiple carbides or that the carbide layer is in the form of a solid solution, where one of the constituents of the solid solution is a carbide.

[0024] A conventional structuring layer and a conventional reflective coating may be applied to the base layer in the form of the carbide layer, where the structured surface is generated before the reflective coating is applied to the structuring layer. The base layer in the form of the carbide layer, the structuring layer and the reflective coating can be applied in each case by a gas phase deposition.

[0025] 2023P00659WO 13.01.26 SZ00395PCTHowever, it is also possible that the carbide layer itself serves as a structuring layer, i.e. that the structured surface is created on the carbide layer by structuring it in a suitable manner (see below), and the reflective coating is applied to the structuring layer in the form of the carbide layer. In principle, it is also possible to combine a base layer in the form of a carbide layer with a structuring layer in the form of a further carbide layer (composed of another carbide).

[0026] A base layer and also a structuring layer in the form of a carbide layer can be deposited by flame spraying or cold gas spraying or with conventional vacuum coating processes (see below), which simplifies their production, in particular reduces costs, one reason being that no complex tools, for example in the form of galvanic tooling, are required, which additionally require care. The throughput times in the manufacture of the optical element can also be reduced, because the work involved in the complex set-up and for associated visual inspections can be reduced. In addition, it is possible in this way to reduce any defects in the base layer and hence also in the layers applied thereto, which reduces rework rates and reduces lifetime risks for the optical element.

[0027] A further benefit is the simplification of possible reworking processes in which the reflective coating and the structuring layer are to be removed by wetchemical means. This is because it is known that carbides, compared to metals, have improved chemical stability to the acids and alkalis used for removal, and hence enable clean removal of the reflective coating and the structuring layer without endangering the base layer. The same is true if the structuring layer is in the form of a carbide layer.

[0028] The base layer or the structuring layer in the form of the carbide layer also has the advantage that, in the event that the layers applied above are damaged, such that they are partly exposed to the vacuum environment in the

[0029] 2023P00659WO 13.01.26 SZ00395PCTsemiconductor technology system, there is generally no damage to the base layer or the structuring layer.

[0030] In one embodiment, the base layer and / or the structuring layer in the form of a carbide layer includes at least one carbide selected from the group comprising: boron carbide (B4C), silicon carbide (SiC), metallic carbides, in particular titanium carbide (TiC), combinations and compositions thereof. The abovementioned carbides may be deposited by vapour deposition, especially by CVD. The carbide layer may also be in the form of a carbide-containing solid solution, for example in the form of titanium carbonitride, Ti(C,N), which is a solid solution of titanium carbide and titanium nitride.

[0031] In one embodiment, the structuring layer and / or the base layer is / are deposited by vapour deposition, in particular by chemical vapour deposition or by sputtering. Chemical vapour deposition (CVD) has been found to be favourable for the deposition of particular carbide materials, for example of SiC. In sputtering, material in the form of particles, typically in the form of ions, is knocked out of a bombarded target and deposited on the substrate. Reactive sputtering using a reactive gas, for example tetrafluoromethane, can also result in deposition of carbides by sputtering on a substrate.

[0032] In a further embodiment, the base layer is applied directly to the surface of the substrate and preferably the structuring layer is applied directly to the base layer. It is possible that, when a base layer is used in the form of a carbide layer, no further layers are required both between the base layer and the surface of the substrate and between the base layer and the structuring layer. This is advantageous since the small number of layers can reduce the layer stresses caused by different coefficients of thermal expansion of the layer materials and hence increase the thermal durability of the optical element. The complexity involved in the manufacture of the optical element is also reduced in this way.

[0033] 2023P00659WO 13.01.26 SZ00395PCTIn a further embodiment, the structuring layer is preferably applied directly to the base layer and contains at least one material selected from the group comprising: silicon-containing materials, metals and alloys thereof. If the optical element has a base layer in the form of a carbide layer, a conventional structuring layer can be used, i.e. a structuring layer composed of a material which has good processibility to create the structured surface and which makes it possible to provide the low roughness required for the application of the reflective coating. The silicon-containing material may especially be silicon or a modification of silicon.

[0034] It is favourable when the structured surface of the structuring layer in the form of a carbide layer, which is preferably applied directly to a surface of the substrate, has minimum roughness. As described further up, the applying of the reflective coating requires a low roughness of the structured surface. In this case, the intrinsic roughness of the structuring layer in the form of a carbide layer should support the high final roughness requirements. It is possible to rework the structuring layer in the form of a carbide layer in order to meet the roughness requirements, as described in greater detail further down.

[0035] In a further embodiment, the structuring layer in the form of a carbide layer and / or the base layer has a thickness of at least 10 pm, preferably of at least 100 pm, more preferably of at least 200 pm. When a structuring layer and / or a base layer in the form of a carbide layer is used, the carbide material is generally not attacked or damaged owing to its high chemical stability in further processing operations in which the reflective coating or its layers and the structuring layer are removed by a wet-chemical etching process. The carbide material can nevertheless be applied or deposited like the conventionally used layer materials with a comparatively large thickness of, for example, 10 pm, 100 pm or more than 100 pm, if partial removal should be necessary. Partial removal of the structuring layer or of the base layer is possible by further

[0036] 2023P00659WO 13.01.26 SZ00395PCTprocessing, for example by ultrashort-pulse laser ablation, by grinding, polishing, or by dry etching processes (see below), if such partial removal should be required in a further processing operation.

[0037] In a further embodiment, the ceramic material of the substrate includes or consists of a carbide, wherein the ceramic material of the substrate is preferably selected from the group comprising: silicon carbide, silicon-infiltrated silicon carbide (SiSiC) or sintered silicon carbide (SSiC). A substrate composed of one of the materials mentioned, more specifically a surface of the substrate, may be ground in order to produce - at least roughly - the approximate surface shape or the fit that the optical element or the optical surface thereof is to have.

[0038] It is possible to process the ceramic substrate of the optical element with a laser, in particular with an ultrashort-pulse laser, in order to produce the desired surface shape; see also "https: / / www.pulsar-photonics.de / en / application-areas / ceramic-processing / ”. Depending on the type of substrate material, it is also possible to roughen the surface of the substrate by a wet-chemical process in order to improve the adhesion of the base layer or the structuring layer to the surface of the substrate.

[0039] It is not absolutely necessary for the substrate material to be a ceramic material. The substrate material may also be a different material, for example a metallic material.

[0040] In a further embodiment, the reflective coating forms a multilayer coating for reflection of EUV radiation. Such a multilayer coating typically has a multitude of alternating layers of a material having a high refractive index at the operating wavelength and a material having a low refractive index at the operating wavelength. The materials may, for example, be silicon and molybdenum, but other material combinations are possible depending on the operating wavelength. In particular, if the reflective coating is used for reflection of EUV

[0041] 2023P00659WO 13.01.26 SZ00395PCTradiation at grazing incidence, the reflective coating is generally not designed as a multilayer coating for reflection by interference effects. In that case, the reflective coating may have a single layer or, if necessary, multiple layers consisting of a suitable material, for example of Ru, Nb or Mo.

[0042] A further aspect of the invention relates to a semiconductor technology system, in particular a EUV lithography system, comprising: at least one optical element, as described further up, and preferably a device for measuring the conductivity of the base layer or the structuring layer in the form of a carbide layer.

[0043] For the purposes of this application, a semiconductor technology system means an optical system or optical arrangement for lithography, i.e. an optical system that can be used in the field of lithography. Apart from a lithography apparatus which serves for the production of semiconductor components, the apparatus may, for example, be an inspection system for inspection of a photomask used in a lithography apparatus (also referred to hereinafter as reticle), for inspection of a semiconductor substrate to be structured (also referred to hereinafter as wafer) or a metrology system which is used for surveying a lithography apparatus or parts thereof, for example for surveying a projection system.

[0044] The semiconductor technology system may include a device for measuring the conductivity of the base layer or the structuring layer in the form of a carbide layer. Carbide materials, especially SiC, become metallic under the influence of hydrogen. If there is a rise in hydrogen content in the base layer or in the structuring layer formed from the carbide, this indicates a leak in the reflective coating. Monitoring of the conductivity of the base layer or of the structuring layer formed from the carbide can therefore be used to conclude that the reflective coating is impermeable or damaged. The device for conductivity measurement may, for example, have one or more electronic contacts or the like that are mounted laterally on the structuring layer or on the base layer in order to contact them electrically. Conductivity is typically measured by

[0045] 2023P00659WO 13.01.26 SZ00395PCTmeasuring the electrical or ohmic resistance of the structuring layer or the base layer.

[0046] A further aspect of the invention relates to a method of the type specified at the outset, in which the base layer in the form of a carbide layer, to which the structuring layer is applied, and / or the structuring layer in the form of a carbide layer is applied by gas phase deposition, in particular by chemical vapour deposition or by sputtering. As described further up, the structuring layer in the form of a carbide layer or the base layer can be deposited by a vacuum process, i.e. without requiring a galvanic or a wet-chemical deposition process for the purpose. This increases environmental sustainability by avoiding resource- and wastewater-intensive galvanic processes. The reflective coating and the conventional structuring layer which is not in the form of a carbide layer are typically likewise deposited by vacuum processes, i.e. not galvanically or by wet-chemical processes. In this way, the entire coating process chain is free of wet-chemical processes. All that are therefore required in the coating process chain are vacuum processes, which simplifies the production environment.

[0047] It is possible that the base layer and the conventional structuring layer which is not in the form of a carbide layer are deposited in one and the same coating system. In particular, the base layer and the conventional structuring layer may be deposited one after another in the coating system without having to remove the substrate from the coating system.

[0048] In one variant, the base layer and / or the structuring layer in the form of a carbide layer is / are subjected to further processing by laser ablation, preferably by ultrashort pulse laser ablation, or by a dry etching process, preferably by ion beam processing. Owing to the hardness of the carbide material, further processing of the structuring layer or the base layer, for example by materialremoving processes, is generally unviable. However, the carbide material of the base layer or the structuring layer can be further processed, for example, by

[0049] 2023P00659WO 13.01.26 SZ00395PCTultrashort-pulse laser ablation, in order to achieve fine adjustment of the surface shape or the fit of its surface. Alternatively or additionally, the base or structuring layer, more specifically the surface thereof, can be further processed by a dry etching process, in particular by ion beam processing or by plasma treatment.

[0050] In a further variant, surface defects of the substrate are corrected on deposition of the base layer and / or the structuring layer in the form of a carbide layer. Surface defects that exist after the surface of the substrate has been processed can be corrected when the structuring layer and / or the base layer is deposited, i.e. the surface defects on the surface of the substrate are not transferred to the surface of the structuring layer or the base layer. In order to enable this, surface or fit defects of the substrate can be compensated for by controlled adjustment of the deposition parameters of the carbide material, possibly with the aid of other technical systems, for example, stops, dwell time control or the like, during the coating operation. In this way, it is also possible to correct non-rotationally symmetric surface defects or surface defects on free-form surfaces.

[0051] In a further variant, the depositing of the base layer and / or of the structuring layer in the form of a carbide layer on a surface of the substrate is preceded by processing of the surface of the substrate, preferably mechanically or by means of a laser, in particular by means of an ultrashort pulse laser. The mechanical processing operation may, for example, be a grinding processing operation. Processing with a laser is also possible, in particular with an ultrashort-pulse laser, in order to create the rough target surface shape or fit of the mirror on the surface. If surface defects occur in the processing of the surface of the substrate, these can be corrected, for example, in the depositing of the base layer or of the structuring layer in the form of a carbide layer. Alternatively or additionally, further processing of the respective layer is also possible, as described further up.

[0052] 2023P00659WO 13.01.26 SZ00395PCTFurther features and advantages of the invention will be apparent from the description of working examples of the invention that follows, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features may each be implemented individually on their own or as a plurality in any desired combination in one variant of the invention.

[0053] Drawing

[0054] Working examples are illustrated in the schematic drawing and are elucidated in the description that follows. The figures show:

[0055] Fig. 1 a schematic meridional section through a projection exposure apparatus for EUV projection lithography,

[0056] Fig. 2a a schematic diagram of a mirror of the projection exposure system of Fig. 1 , which has a base layer in the form of a carbide layer and a structuring layer, and

[0057] Fig. 2b a schematic diagram analogous to Fig. 2a, in which the mirror has a structuring layer in the form of a carbide layer with a structured surface.

[0058] In the description of the drawings that follows, identical reference signs are used for identical or functionally identical components.

[0059] There follows an illustrative description, with reference to Fig. 1, of the essential constituents of an optical arrangement for EUV lithography in the form of a projection exposure apparatus 1 for microlithography or of an EUV lithography apparatus. The description of the basic structure of the projection exposure

[0060] 2023P00659WO 13.01.26 SZ00395PCTapparatus 1 and of the components thereof should not be regarded here as having any limiting effect.

[0061] An embodiment of an illumination system 2 of the projection exposure apparatus 1 has, in addition to a light or radiation source 3, an illumination optics unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided in the form of a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.

[0062] 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 a scanning direction by way of a reticle displacement drive 9.

[0063] By way of elucidation, Fig. 1 shows a Cartesian xyz coordinate system, x direction runs perpendicularly to the plane of the drawing, y direction runs horizontally, and z direction runs vertically. Scanning direction runs in y direction in Fig. 1. z direction runs perpendicularly to the object plane 6.

[0064] The projection exposure apparatus 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable, in particular in y direction, by way of a wafer displacement drive 15. The displacement, firstly, of the reticle 7 by way of the reticle displacement drive 9 and, secondly, of the wafer 13 by way of the wafer displacement drive 15 can be synchronized with one another.

[0065] The radiation source 3 is an EUV radiation source. The radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as used

[0066] 2023P00659WO 13.01.26 SZ00395PCTradiation, illumination radiation or illumination light. The used radiation has in particular a wavelength in the range of between 5 nm and 30 nm. The radiation source 3 may be a plasma source, for example an LPP (Laser Produced Plasma) source or a GDPP (Gas Discharge Produced Plasma) source. It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).

[0067] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The illumination radiation 16 may be incident on the at least one reflection surface of the collector mirror 17 with grazing incidence (Gl), i.e. at angles of incidence of greater than 45°, or with normal incidence (Nl), i.e. at angles of incidence of less than 45°. The collector mirror 17 may be structured and / or coated, firstly to optimize its reflectivity for the used radiation and secondly to suppress extraneous light.

[0068] Downstream of the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may constitute a separation between a radiation source module, having the radiation source 3 and the collector mirror 17, and the illumination optics unit 4.

[0069] The illumination optics unit 4 comprises a deflection mirror 19 and, disposed downstream thereof in the beam path, a first facet mirror 20. The deflection mirror 19 may be a planar deflection mirror or alternatively a mirror with a beaminfluencing effect that goes beyond the pure deflection effect. Alternatively or additionally, the deflection mirror 19 may be in the form of a spectral filter, which separates a used light wavelength of the illumination radiation 16 from extraneous light of a different wavelength. The first facet mirror 20 comprises a multiplicity of individual first facets 21 , which are also referred to below as field

[0070] 2023P00659WO 13.01.26 SZ00395PCTfacets. Fig. 1 illustrates only some of these facets 21 by way of example. In the beam path of the illumination optics unit 4, a second facet mirror 22 is disposed downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

[0071] The illumination optical unit 4 thus forms a doubly faceted system. This basic principle is also referred to as a fly's eye condenser (fly's eye integrator). The second facet mirror 22 is used to image the individual first facets 21 into the object field 5. The second facet mirror 22 is the last beam-shaping mirror or else actually the last mirror for the illuminating radiation 16 in the beam path upstream of the object field 5.

[0072] The projection system 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.

[0073] In the example illustrated in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are likewise possible. The second-last mirror M5 and the last mirror M6 each have a through opening for the illuminating radiation 16. The projection system 10 is a doubly obscured optical unit. The projection optics unit 10 has an image-side numerical aperture which is greater than 0.4 or 0.5 and which may also be greater than 0.6 and which may be for example 0.7 or 0.75.

[0074] Just like the mirrors of the illumination optics unit 4, the mirrors Mi may have a highly reflective coating for the illumination radiation 16.

[0075] Fig. 2a shows an optical element in the form of a mirror 30 for reflection of EUV radiation 16, which may be one of the mirrors 17, 19, 20, 22 of the illumination optics unit 4 or optionally one of the mirrors Mi of the projection optics unit 10. In the example shown, the mirror 30 has a substrate 31 composed of a ceramic

[0076] 2023P00659WO 13.01.26 SZ00395PCTmaterial, which may be, for example, a carbide. For example, the ceramic material may be selected from the group comprising: silicon carbide, silicon-infiltrated silicon carbide (SiSiC) or sintered silicon carbide (SSiC). Alternatively, the substrate 31 may also be formed from another material, in particular from a metallic material.

[0077] In the mirror 30 shown in Fig. 2a, a base layer 32 in the form of a carbide layer is applied directly to a surface 31 a of the substrate 31. In the example shown, the carbide of the carbide layer is silicon carbide (SiC), but it may also be a different type of carbide, for example boron carbide (B4C) or a metallic carbide, e.g. titanium carbide (TiC), combinations of these carbides or compositions thereof, for example, titanium carbonitride Ti(C,N), i.e. a solid solution of titanium carbide and titanium nitride.

[0078] In the example shown, the base layer 32 is applied directly to the surface 31a of the substrate 31. On a surface 32a of the base layer 32, in the example shown, a structuring layer 33 is applied directly. In the example shown, the structuring layer 33 contains a material selected from the group comprising: silicon-containing materials, e.g. silicon, in particular modifications of silicon, metals and alloys thereof. The structuring layer 33 has a structured surface 33a that forms a grating structure in the example shown. The grating structure or the structured surface 33a serves as spectral filter in order to filter extraneous light in a defined wavelength range, for example in the IR wavelength region.

[0079] A reflective coating 34 is applied directly to the structured surface 33a of the structuring layer 33. The reflective coating 34 is a multilayer coating having alternating layers of Si and Mo in order to reflect the EUV radiation 16 incident on the mirror 30 by interference effects. Alternatively, the reflective coating 34 may be designed for reflection of EUV radiation 16 under grazing incidence. In this case, the reflective coating 34 may be formed, for example, from a layer of ruthenium or of another suitable material.

[0080] 2023P00659WO 13.01.26 SZ00395PCTIn the example shown in Fig. 2a, both the base layer 32 and the structuring layer 33 and the reflective coating 34 have been deposited by gas phase deposition on the surface 31 a of the substrate 31. The base layer 32 has been deposited on the substrate 31 by chemical vapour deposition. The structuring layer 33 and the reflective coating 34 have been deposited in the example shown by sputtering, e.g. by magnetron sputtering, but these can also be deposited by a different type of gas phase deposition. The base layer 32 in the form of the carbide layer can optionally also be deposited by sputtering, in particular by reactive sputtering, but deposition by chemical vapour deposition (CVD) is also possible.

[0081] For structuring of the structuring layer 33 for formation of the structured surface 33a, a lithography process is conducted, in which a photoresist is applied to the structuring layer 33 as an etching mask. The photoresist is selectively exposed by a laser and subsequently removed selectively. Subsequently, the material of the structuring layer 33 is selectively or locally etched and structured at the same time, with the aid of a dry etching process or with the aid of a wet chemical etching process. For details of these etching methods, reference is made to DE 102018220629 A1, which is incorporated into this application in its entirety by reference.

[0082] In the example shown, the base layer 32 has a thickness D of at least 10 pm, of at least 100 pm or of at least 200 pm. The depositing of the base layer 32 with a comparatively large thickness may be found to be favourable when, in a further processing operation in which the reflective coating 34 and also the structuring layer 33 are removed by a chemical etching process, some of the thickness of the base layer 32 is optionally also to be removed. This may be necessary in particular when contamination applied by the chemical etching process that can no longer be cleaned off has to be removed.

[0083] 2023P00659WO 13.01.26 SZ00395PCTFig. 2b shows a mirror 30, which, in contrast to the mirror 30 shown in Fig. 2a, has no base layer 32 but only a structuring layer 33' in the form of a carbide layer. The structuring layer 33' in the form of the carbide layer has a structured surface 33a' to which the reflective coating 34 is applied directly. In the example shown in Fig. 2b, therefore, no additional base layer is required. The structuring layer 33' in the form of a carbide layer has a thickness D of at least 10 pm, of at least 100 pm or of at least 200 pm, in order that the structuring layer 33' can be partly removed repeatedly if required in further processing operations.

[0084] Both the base layer 32 from Fig. 2a and the structuring layer 33' in the form of a carbide layer from Fig. 2b can be processed further in order to create a desired surface shape or fit of the respective surface 32a, 33a'. For this purpose, the carbide layer can be further processed by laser ablation, in particular by ultrashort-pulse laser ablation, or by a dry etching process, in particular by ion beam processing. As described further up, the further processing can be used to finely adjust the surface shape or the fit of the respective surface 32a, 33a' by implementing a controlled local removal of material on the respective surface 32a, 33a'. In particular, the structuring of the surface 33a' of the structuring layer 33' in the form of a carbide layer can also be effected by laser ablation or by a dry etching process.

[0085] Further processing is favourable or may be required in order to create a low roughness in the case of the structuring layer 33' in the form of a carbide layer, but also in the base layer 32 in the form of the carbide layer, at the respective surface 33a', 32a, as required for the applying of the reflective coating 34 or for the applying of the structuring layer 33 to the base layer 32 in order to achieve the desired reflectivity of the mirror 30.

[0086] Before the base layer 32 or the structuring layer 33' in the form of a carbide layer is applied, the surface 31a of the substrate 31 is typically processed mechanically to create the desired surface shape, in particular by grinding, or by

[0087] 2023P00659WO 13.01.26 SZ00395PCTlaser processing, in particular by means of an ultrashort-pulse laser, in order to achieve the closest possible approximation of the target surface shape of the mirror 30.

[0088] It is likewise possible to correct surface defects on the surface 31a of the substrate 31 when depositing the structuring layer 33' in the form of a carbide layer and / or the base layer 32. Fig. 2a shows such a local surface defect 35 in the form of a depression or hollow on the surface 31a of the substrate 31 , which in the example shown should have a flat surface shape as the target surface shape. As can be seen in Fig. 2a, the surface defect 35 was corrected when applying the base layer 32 in the form of the carbide layer in that the deposition parameters in the deposition of the carbide layer were suitably adjusted with the aid of stops in order to vary the local thickness in the deposition of the base layer 32 at the site of the depression and in that way to correct the local surface defect 35 on the surface 31 a of the substrate 31.

[0089] Although the mirrors 30 shown in Fig. 2a, b have a flat target surface shape, it is also possible in the manner described further up to coat mirrors 30 having a surface shape other than a flat target surface shape, for example with a spherical surface shape or with a surface shape in the form of a free-form surface.

[0090] Since the base layer 32 or the structuring layer 33' in the form of a carbide layer can be deposited by gas phase deposition and no galvanic or wet-chemical deposition process is required for the purpose, the production of the mirror 30 is significantly simplified. In particular, the throughput times and the costs in the manufacture of the mirror 30 can be significantly reduced.

[0091] A further benefit in the case of use of a base layer 32 or a structuring layer 33' in the form of a carbide layer, especially a SiC layer, is that it changes its specific electrical conductivity under the influence of hydrogen. Therefore, a

[0092] 2023P00659WO 13.01.26 SZ00395PCTconductivity measurement on the mirror 30, more specifically on the carbide layer 32 and 33', allows monitoring of the long-term behaviour of the mirror 30 with regard to damage that allows the penetration of hydrogen. In particular, it is possible to recognize leaks in the reflective coating 24 that lead to penetration of hydrogen into the carbide layer 32 or 33'.

[0093] For measurement of the conductivity of the mirror 30, it is possible to exploit the fact that the ceramic substrate 31 , at least if it is formed from SiSiC or SSiC, is also electrically conductive, such that the electrical conductivity measurement on the substrate 31 of the mirror 30 can be conducted, for example, in the axial direction. However, it is also possible that, for the measurement of electrical conductivity, the carbide layer 32 or 33' is electrically contacted on at least one side, typically on two opposite sides, for example via electrical wires that are part of a device 36 symbolically indicated by a square in Fig. 2a, b for measurement of conductivity of the base layer 32 in the form of a carbide layer or of the structuring layer 33' in the form of a carbide layer.

[0094] 2023P00659WO 13.01.26 SZ00395PCT

Claims

Claims1. Optical element (30) comprising:a substrate (31 ), preferably composed of a ceramic or metallic material, a reflective coating (34) for reflection of radiation, in particular for reflection of EUV radiation (16),a structuring layer (33, 33') having a structured surface (33a, 33a') that preferably forms a grating structure, where the reflective coating (34) is applied to the structured surface (33a, 33a'),characterized in thatthe structuring layer (33) is applied to a base layer (32) in the form of a carbide layer, and the base layer (32) is applied to a surface (31a) of the substrate (31).

2. Optical element according to Claim 1 , in which the structuring layer (33') takes the form of a carbide layer.

3. Optical element according to Claim 1 or 2, in which the base layer (32) and / or the structuring layer (33') includes at least one carbide selected from the group comprising: boron carbide, B4C, silicon carbide, SiC, metallic carbides, in particular titanium carbide, TiC, combinations and compositions thereof.

4. Optical element according to any of the preceding claims, in which the base layer (32) and / or the structuring layer (33, 33') is / are deposited by gas phase deposition, in particular by chemical vapour deposition or by sputtering.

5. Optical element according to any of the preceding claims, in which the base layer (32) is applied directly to the surface (31a) of the substrate (31) and in which the structuring layer (33) is applied directly to the base layer (32).2023P00659WO 13.01.26 SZ00395PCT6. Optical element according to any of the preceding claims, in which the structuring layer (33) is preferably applied directly to the base layer (32) and contains a material selected from the group comprising: silicon-containing materials, metals and alloys thereof.

7. Optical element according to any of the preceding claims, in which the structuring layer (33') in the form of a carbide layer and / or the base layer (32) has a thickness of at least 10 pm, preferably of at least 100 pm, more preferably of at least 200 pm.

8. Optical element according to any of the preceding claims, in which the ceramic material of the substrate (31 ) includes or consists of a carbide, wherein the ceramic material of the substrate (31) is preferably selected from the group comprising: silicon carbide, silicon-infiltrated silicon carbide (SiSiC) or sintered silicon carbide (SSiC).

9. Optical element according to any of the preceding claims, in which the reflective coating (34) forms a multilayer coating for reflection of EUV radiation (16).

10. Semiconductor technology system, in particular EUV lithography system (1), comprising:at least one reflective optical element (30) according to any of the preceding claims, and preferably a device (36) for measuring the conductivity of the base layer (32) or of the structuring layer (33') in the form of a carbide layer.

11. Method of producing an optical element (30) according to any of Claims 1 to 9, comprising the steps of:providing the substrate (31),depositing the structuring layer (33, 33'),2023P00659WO 13.01.26 SZ00395PCTstructuring the structuring layer (33, 33') to form the structured surface (33a, 33a'), in particular the grating structure, andapplying the reflective coating (34) to the structured surface (33a, 33a'), characterized in thatthe base layer (32) in the form of a carbide layer, to which the structuring layer (33) is applied, and / or the structuring layer (33') in the form of a carbide layer is applied by gas phase deposition, in particular by chemical vapour deposition or by sputtering.

12. Method according to Claim 11 , in which the base layer (32) and / or the structuring layer (33') in the form of a carbide layer is / are subjected to further processing by laser ablation, preferably by ultrashort-pulse laser ablation, or by a dry etching process, preferably by ion beam processing.

13. Method according to Claim 11 or 12, in which the depositing of the base layer (32) and / or of the structuring layer (33') in the form of a carbide layer corrects surface defects (35) of the substrate (31).

14. Method according to any of Claims 11 to 13, in which the depositing of the base layer (32) and / or of the structuring layer (33') in the form of a carbide layer on a surface (31 a) of the substrate (31 ) is preceded by processing of the surface (31a) of the substrate (33), preferably mechanically or by means of a laser, in particular by means of an ultrashort-pulse laser.2023P00659WO 13.01.26 SZ00395PCT

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