Optical element and EUV lithography system

Abstract

The invention relates to an optical element (25) for reflecting EUV radiation (16), comprising: a substrate (26), a reflective coating (27) for reflecting the EUV radiation (16), and an etchable material (29) covered by the reflective coating (27), which is abraded upon contact with an etching medium (31). In a first aspect, the reflective coating (27) is subjected to a mechanical prestress to cause the reflective coating (27) to curve towards the substrate (26) when the etchable material (29) is abraded by the etching medium (31) upon damage to the reflective coating (27).In a second aspect, the etchable material (29) is doped with a dopant that is not removed upon contact with the etching medium (31) and that preferably accumulates at an etching surface of the etchable material when the etchable material (29) is removed by the etching medium (31) due to damage to the reflective coating (27). The invention also relates to an EUV lithography system comprising at least one such optical element (25) that is exposed to an etching medium (31) during operation of the EUV lithography system.

Description

Background of the invention

[0001] The invention relates to an optical element for reflecting EUV radiation, comprising: a substrate, a reflective coating for reflecting the EUV radiation, and an etchable material covered by the reflective coating, which is ablated upon contact with an etching gas or can be ablated upon contact with an etching gas. The invention also relates to an EUV lithography system comprising at least one such optical element.

[0002] For the purposes of this application, an EUV lithography system is understood to mean an optical system or an optical arrangement for EUV lithography, i.e., an optical system that can be used in the field of EUV lithography. In addition to an EUV lithography system used to manufacture semiconductor components, the optical system can be, for example, an inspection system for inspecting a photomask (hereinafter also referred to as a reticle) used in an EUV lithography system, for inspecting a semiconductor substrate to be structured (hereinafter also referred to as a wafer), or a metrology system used to measure an EUV lithography system or parts thereof, for example, for measuring a projection system.

[0003] EUV radiation refers to radiation in a wavelength range between approximately 5 nm and approximately 30 nm, for example, at 13.5 nm. Since EUV radiation is strongly absorbed by most known materials, it is typically guided through the EUV lithography system using reflective optical elements.

[0004] Optical elements for reflecting EUV radiation, also referred to as EUV mirrors, are exposed to harsh conditions during operation in an EUV lithography system, particularly in an EUV lithography system. For example, the reflective coating is hit by EUV radiation with a high radiant power. Residual gases in a vacuum environment in which EUV mirrors are typically operated (e.g., oxygen, nitrogen, hydrogen, water, and other residual gases commonly found in ultra-high vacuums) can be converted into reactive species such as ions or radicals by the action of the EUV radiation, for example, into a hydrogen-containing plasma.

[0005] An etchable material may be located beneath the reflective coating, which is etched away or removed upon contact with an etching medium, for example in the form of an etching gas, e.g. in the form of hydrogen radicals. This is generally not a problem if the reflective coating completely covers the etchable material. However, if the reflective coating is damaged and has local defects, e.g. in the form of holes, the etching medium can penetrate to the etchable material beneath the reflective coating and remove it. In this way, during operation of the optical element, the reflective coating is under-etched at the location of the respective defect and in the vicinity of the defect. Under-etching can create very large damaged areas on the optical surface of the reflective optical element.The same applies if the surface of the reflective coating is cleaned using plasma or wet-chemical methods, e.g., using a corrosive cleaning agent, e.g., in the form of an acid. In this case, too, the corrosive cleaning agent may penetrate through holes or other damage in the reflective coating and partially etch away the etchable material.

[0006] DE102019212736A1 describes an optical element for reflecting EUV radiation, comprising an EUV radiation-reflecting coating to which a cover layer is applied. An intermediate layer is arranged between the reflective coating and the cover layer. The intermediate layer comprises at least one reactive material that forms at least one reaction product that seals the gap with an activating gas penetrating through a gap in the cover layer.

[0007] DE102015225510A1 describes a mirror element comprising a substrate and a layer stack comprising at least one reflective layer system. A curvature of the mirror element is generated based on a desired target curvature for a given operating temperature by a non-zero bending force exerted by the layer stack. The generated curvature varies by no more than 10% over a temperature interval of at least 10K.

[0008] DE102018204364A1 describes an optical arrangement for EUV lithography, comprising at least one component having a base body with at least one surface region exposed to activated hydrogen during operation of the optical arrangement. The base body contains at least one material that forms at least one volatile hydride upon contact of the surface region with the activated hydrogen. Noble metal ions are implanted into the base body at the surface region. Noble metals, especially in the form of noble metal ions, generally exhibit a strong catalytic effect for the recombination of activated hydrogen into molecular hydrogen and are intended to prevent the formation of the volatile hydride.

[0009] DE102020213639A1 describes a reflective optical element for reflecting EUV radiation, comprising a substrate and a reflective coating applied to the substrate. The reflective coating, a structured layer formed between the substrate and the reflective coating, and / or the substrate are doped with at least one noble metal. Object of the invention

[0010] The object of the invention is to provide an optical element and an EUV lithography system in which material removal of the etchable material is spatially limited in the event of damage to the reflective coating. Subject of the invention

[0011] This object is achieved according to a first aspect by an optical element of the type mentioned at the outset, in which the reflective coating is subjected to a mechanical prestress in order to cause a curvature of the reflective coating in the direction of the substrate when the etchable material is removed by the etching medium in the event of damage to the reflective coating.

[0012] In this aspect of the invention, it is proposed to subject the reflective coating to a mechanical prestress in order to generate a bending force or a bending moment. The bending force is so small that it has only a minor influence on the substrate or its surface and does not bend it or only bends it negligibly. However, the bending force is sufficient to bend the reflective coating towards the substrate when the etchable material located beneath the reflective coating is removed. The prestress of the reflective coating, possibly in combination with another layer arranged between the reflective coating and the substrate (see below), generates a force or a bending moment towards the substrate.

[0013] As described above, the removal of the etchable material occurs when the reflective coating has a defect, for example in the form of a hole or the like, wherein in this case the etchable material is removed not only directly beneath the defect but also in the area surrounding the defect, i.e. the reflective coating is under-etched in the area surrounding the defect. Due to the bending force, the under-etched part of the reflective coating bends in this case towards the substrate, more precisely towards a cavity formed by the etching of the material, thus preventing the etching medium passing through the defect, e.g. in the form of a plasma, from accessing an etching front on the removed material. If the etching medium, e.g.In the form of one or more etching gases, for example, a hydrogen plasma, the lateral extent of the ablated area is limited. The etchable material can, in particular, be a material that forms volatile hydrides with hydrogen.

[0014] In a further development of this embodiment, the reflective coating is subjected to compressive stress at least in a coating section facing away from the substrate. The coating section facing away from the substrate typically extends from a surface of the reflective coating facing away from the substrate over a predetermined thickness or depth into the reflective coating. The coating section facing away from the substrate can have one or more layers subjected to compressive stress. The compressive stress of the coating section facing away from the substrate can be combined with a tensile stress (see below) in order to bend the reflective coating toward the substrate in the manner of a bimetallic strip when the underlying etchable material is removed.In contrast to a bimetallic strip, the bending force is not generated by differential thermal expansion, but rather the bending force or bending moment typically results from compressive stress in combination with tensile stress or from different compressive stresses of two or more layers, with the layer(s) further away from the substrate having a greater compressive stress than the layer(s) closer to the substrate.

[0015] In a further development of this embodiment, the reflective coating is under tensile stress at least in a coating section facing the substrate. The coating section facing the substrate typically extends from a surface facing the substrate over a predetermined thickness into the reflective coating. The reflective coating can have one or more layers under tensile stress in the coating section facing the substrate. In this embodiment, the coating can consist of the coating section facing away from the substrate and under compressive stress and the coating section facing the substrate. In this case, the reflective coating bends towards the substrate due to the mechanical prestress or bending force when the underlying etchable material is removed.In principle, it is also possible that between the coating section under compressive stress and the coating section under tensile stress there is another coating section that is not under tensile stress or under compressive stress.

[0016] As described above, the reflective coating may alternatively be subjected to a compressive stress, at least in the coating portion facing the substrate, that is less than the compressive stress in the coating portion facing away from the substrate. In this case, too, the bias of the reflective coating may cause the reflective coating to bend toward the substrate as the etchable material is removed.

[0017] Whether compressive or tensile stress occurs in the first and / or second coating section, and the intensity of the compressive or tensile stress, can be influenced by the respective manufacturing process or coating method used to apply the respective layer(s), as well as by the layer material used. In general, there are materials that are more prone to the formation of tensile stresses and materials that are more prone to the formation of compressive stresses, whereby the formation of tensile or compressive stress can be influenced by the manufacturing process.

[0018] The manufacturing processes for the reflective coating typically involve physical vapor deposition (PVD) coating processes such as thermal evaporation or e-beam / electron beam evaporation, ion beam & magnetron sputtering or pulsed laser deposition, or coating processes based on chemical vapor deposition (CVD). Other coating processes, such as atomic layer deposition (ALD), are also possible.

[0019] The material(s) of the coating section facing away from the substrate can, for example, be applied using a process that typically generates compressive stress. For example, the coating section facing away from the substrate can be applied by sputtering, since this coating process typically generates compressive stress. The coating section facing the substrate can, for example, be produced using electron beam evaporation, which generally generates tensile stress rather than compressive stress.

[0020] In an alternative development, the entire reflective coating is under compressive stress and at least one layer under tensile stress is arranged between the reflective coating and the etchable material.

[0021] As described above, there are basically materials that are more prone to the development of tensile stresses and materials that are more prone to the development of compressive stresses. The materials used for the layers of the reflective coating have an optical function and therefore cannot be chosen arbitrarily. This does not apply to the material of the layer subject to tensile stress, as this generally does not fulfill an optical function. The material of the layer subject to tensile stress can be, for example, Mo, MoSi, Mo-Si multilayer, MgF2, Ni, Cr or Cr alloys. Ideally, the layer subject to tensile stress is directly adjacent to the reflective coating under compressive stress in order to generate the greatest possible bending force, but this is not absolutely necessary.

[0022] As an alternative to the layer subjected to tensile stress, at least one layer subjected to compressive stress can be arranged between the reflective coating and the etchable material, whereby the compressive stress of this layer is lower than the compressive stress of the reflective coating. In this case, too, a bending moment and thus a curvature of the reflective coating toward the substrate can be generated. However, the smaller the difference between the two compressive stresses, the smaller the bending moment and thus the curvature of the reflective coating. It is therefore generally more advantageous to use a layer subjected to tensile stress than a layer subjected to compressive stress.

[0023] A second aspect of the invention relates to an optical element of the type mentioned above, in which the etchable material is doped with a dopant that is not ablated or cannot be ablated upon contact with the etching medium, wherein the dopant preferably accumulates at an etching point of the etchable material when the etchable material is ablated by the etching medium upon damage to the reflective coating. Since the etching medium is usually atomic hydrogen or a hydrogen plasma, in principle all materials that do not form volatile hydrides are suitable as dopant. Preference is given to using materials as dopant that interfere as little as possible with subsequent process steps (cleaning, polishing, measuring, structuring, plasmating, coating, etc.) and that are also stable with respect to EUV radiation and thermal stress over their lifetime.The inventors have found that when the etchable material is removed, the dopant typically accumulates at the etching front and forms a net-like structure there, which after a short time leads to the formation of a passivation layer on the etched material, which prevents further material removal, ie the material removal by etching comes to a standstill.

[0024] In a further development of this embodiment, the dopant is selected from the group comprising: metals, oxides, preferably SiO x , GeO x , BO x , AlO x , TiO x , TaO x, nitrides, carbides, borides. The oxides, nitrides, carbides, or borides can be stoichiometric or non-stoichiometric oxides, nitrides, carbides, or borides. It is possible for the non-etchable dopant to form an oxide, nitride, carbide, or boride of an etchable material, i.e., a material that is removed upon contact with the etching medium, as is the case with Si, for example. The oxide in this case can be SiO x where the nitride is SiN x , where carbide is SiC X and the boride is SiB xThe metal can be a noble or a base metal. The oxides, nitrides, carbides, or borides can be metal oxides, metal carbides, metal nitrides, or metal borides, but they can also be oxides, nitrides, carbides, or borides of non-metallic materials. It is understood that a combination of the aforementioned materials can also be used as a dopant.

[0025] In a further development of this embodiment, the metal is selected from the group comprising: Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides.

[0026] The dopant should be as insensitive as possible to the etching medium so that it can accumulate sufficiently on the surface of the etch front and is not removed when the etchable material is etched away. As described above, the dopant should also be resistant to irradiation with EUV radiation and interfere with subsequent process steps as little as possible.

[0027] In a further embodiment, the dopant concentration is between 0.01 at% and 30 at%, preferably between 0.1 at% and 20 at%. Dopant concentrations in the specified range have proven to be favorable.

[0028] Depending on the material used as a dopant, higher or lower doping concentrations are also possible. Typically, the etchable material can be doped with the dopant with a comparatively high doping concentration, since the etchable material is located beneath the reflective coating, and therefore the effect of the doping on the absorption of the useful radiation does not need to be considered.

[0029] In one embodiment, the etchable material is selected from the group comprising: amorphous silicon (aSi), silicon (Si), SiO2, Si-containing glasses, Ge, GeO, B, BO, materials containing silicon, as well as other materials that form volatile hydrides with hydrogen, are generally etchable by an etching medium in the form of reactive hydrogen or in the form of a hydrogen plasma.

[0030] In a further embodiment, the reflective coating has at least one damaged area where the etchable material is exposed to the environment. As described above, the etching removal of the etchable material begins in the area of ​​the damaged area or below the damaged area in the reflective coating and continues in a lateral direction, partially under-etching the reflective coating. To prevent under-etching, contact between the etching medium and the etching front is prevented, as described above.

[0031] In a further embodiment, the etchable material is contained in an intermediate layer between the reflective coating and the substrate. The intermediate layer is a functional layer that typically has no optical function. The intermediate layer can be, for example, an adhesion promoter layer or a structurable layer, which can be used to create, for example, a grating structure or the like.

[0032] In another embodiment, the etchable material is contained in the substrate. In this embodiment, the substrate itself contains the etchable material, which is protected from the etching medium or from etching removal by means of the reflective coating. In this case, the reflective coating is typically applied directly to the substrate. However, it is also possible in principle for both the intermediate layer and the substrate to contain an etchable material.

[0033] In a further embodiment, the reflective coating forms a multilayer coating for reflecting EUV radiation incident on the reflective optical element under normal incidence, wherein the multilayer coating comprises alternating layers of a first material and a second material with different refractive indices.

[0034] Normal incidence of EUV radiation typically means incidence of EUV radiation at an angle of incidence of typically less than approximately 45° to the surface normal of the reflective optical element. The reflective multilayer coating is typically optimized for the reflection of EUV radiation at a given wavelength, which usually corresponds to the useful wavelength of the EUV lithography system in which the optical element is used. For this purpose, the multilayer coating typically comprises a plurality of alternating layers made of a material with a high real part of the refractive index at the useful wavelength and a material with a low real part of the refractive index at the useful wavelength. The materials can be, for example, silicon and molybdenum, but other material combinations are also possible depending on the useful wavelength.

[0035] In a further embodiment, the reflective coating is designed to reflect EUV radiation impinging on the reflective optical element at grazing incidence. Grazing incidence of EUV radiation is typically understood to mean incidence of EUV radiation at an angle of incidence of more than approximately 60° to the surface normal of the surface of the reflective optical element. A reflective coating designed for grazing incidence typically has a maximum reflectivity at at least one angle of incidence that is greater than 60°. Such a reflective coating is typically formed from at least one material that has a low refractive index and low absorption for the EUV radiation impinging at grazing incidence. In this case, the reflective coating can contain a metallic material orbe made of a metallic material, for example Mo, Ru or Nb.

[0036] A further aspect of the invention relates to an EUV lithography system, comprising: at least one optical element, as described above, which is exposed to an etching medium during operation of the EUV lithography system. The EUV lithography system can be an EUV lithography system for exposing a wafer or another optical arrangement that uses EUV radiation, for example an EUV inspection system, e.g., for inspecting masks, wafers, or the like used in EUV lithography. The optical element can be, for example, an EUV mirror of a projection system or an illumination system, for example, a collector mirror.The optical element, and thus also the reflective coating, is exposed to an etching medium, at least during operation of the EUV lithography system. This etching medium typically consists of one or more etching gases present in the environment of the optical element. The etching medium can be, for example, activated hydrogen or a hydrogen plasma, i.e., excited hydrogen and / or hydrogen radicals.

[0037] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention. drawing

[0038] Examples of embodiments are shown in the schematic drawing and are explained in the following description. Fig. 1 schematically shows in meridional section a projection exposure system for EUV projection lithography, Fig. 2a,b schematic representations of an EUV mirror of the projection exposure system of Fig. 1 with a substrate and with a reflective coating which is undercut in the area of ​​a damaged area, Fig. 3a,b schematic representations of an EUV mirror analogous to Fig. 2b, in which the reflective coating is curved towards the substrate in the vicinity of the damage site, and Fig. 4a-c schematic representations of an EUV mirror analogous to Fig. 2b, in which an etchable material located beneath the reflective coating is doped with a non-etchable dopant.

[0039] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0040] In the following, with reference to Fig. Figure 1 describes, by way of example, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography. The description of the basic structure of the projection exposure system 1 and its components is not intended to be limiting.

[0041] One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0042] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.

[0043] In Fig. 1 shows a Cartesian xyz coordinate system for explanation. The x-direction is perpendicular to the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. The scanning direction is in the Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0044] The projection exposure system 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0045] The radiation source 3 is an EUV radiation source. The radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example, an LPP source (laser produced plasma) or a DPP source (gas discharged produced plasma). It can also be a synchrotron-based radiation source. The radiation source 3 can be a free-electron laser (FEL).

[0046] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (GI), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0047] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0048] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Of these facets 21, Fig. 1 shows only a few examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

[0049] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or actually the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0050] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0051] In the Fig. In the example shown in Figure 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical system. The projection optical system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0052] The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

[0053] Fig. 2a,b show a highly simplified detail of a reflective optical element in the form of an EUV mirror 25 of the projection exposure apparatus 1 of Fig. 1, which can be, for example, one of the mirrors Mi of the projection optics 10, the collector mirror 17, or one of the mirrors 19, 20, 22 of the illumination optics 4. The mirror 25 has a substrate 26 and a highly reflective coating 27 for reflecting the EUV radiation 16. Between the reflective coating 27 and the substrate 25 is an intermediate layer 28, which is covered by the reflective coating 27.

[0054] The intermediate layer 28 can serve as a structurable layer, for example, to create a lattice structure not shown, or to fulfill another function. The intermediate layer 28 is formed from a material that can be easily processed by etching. The material of the intermediate layer 28 can be, for example, amorphous silicon, silicon, SiO2, Si-containing glasses, Ge, GeO, B, B0, etc.

[0055] As shown in the top view of the reflective coating 27 in Fig. 2a, the reflective coating 27 has a locally limited damaged area 30 in the form of a hole that extends over the entire thickness of the reflective coating 27. An etching medium 31 in the form of an etching gas, more precisely in the form of activated hydrogen, present in the vicinity of the optical element 25 comes into contact with the material 29 of the intermediate layer 28 via the damaged area 30. The material 29 of the intermediate layer 28 is removed upon contact with the etching medium 31, i.e. it is an etchable material that reacts with the etching medium 31 and in the process forms a volatile material 32. In the event that the etchable material 32 contains silicon, the volatile material 32 can be, for example, a volatile hydride in the form of an SiH compound.

[0056] As in Fig. 2b, the removal of the etchable material 29 is not limited to a volume region of the intermediate layer 28 lying directly beneath the damaged area 30, but rather the etchable material 29 is also removed in an area surrounding the damaged area 30 whose lateral extent is significantly greater than the lateral extent of the damaged area 30. In this way, an under-etched area in the form of a cavity 33 is created beneath the damaged area 30 and in the area surrounding the damaged area 30 of the reflective coating 27. In the example shown, the substrate 26 is made of a material that is not removed upon contact with the etching medium 31. The material of the substrate 26 can be, for example, one or more metals, e.g. Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides or one or more oxides, e.g. SiO x , GeO x , BO x , AlO x , TiO x, TaO x , one or more carbides, nitrides or borides.

[0057] In order to reduce the lateral extent of the under-etched region 33, it is necessary to prevent or reduce the contact between the etching medium 31 and the etchable material 29.

[0058] In the Fig. 3a,b, the lateral extent of the under-etched region 33 is reduced by the reflective coating 27 having a mechanical prestress, which causes a curvature of the reflective coating 27 in the direction of the substrate 26 when the etchable material 29 is removed by the etching medium 31 upon damage to the reflective coating 27. As shown in Fig. 3a, the mechanical prestressing of the reflective coating 27 exerts a bending force or a bending moment on the part of the reflective coating 27 located above the under-etched region 33. The bending force or the bending moment results in the reflective coating 27 being bent in the vicinity of the damaged area 30 towards the substrate 26 and protruding into the volume of the cavity 33 or the under-etched region. As shown in Fig. 3a,b, the access of the etching medium 31 to the etchable material 29 is limited or closed in this way.

[0059] The mechanical preload is determined by the Fig. 3a, the reflective coating 27 comprises a first coating section 27a facing away from the substrate 26, which is subject to compressive stress, and a second coating section 27b facing the substrate 26, which is subject to tensile stress and which, in the thickness direction, directly borders the first coating section 27a. The tensile stress and the compressive stress are shown in Fig. 3a,b indicated by arrows.

[0060] In the example shown, the reflective coating 27 is a multilayer coating for reflecting EUV radiation 16 under normal incidence, which coating comprises alternating layers 34a, 34b made of silicon and molybdenum. For example, both the first coating section 27a and the second coating section 27b each comprise a pair of alternating layers 34a, 34b made of silicon and molybdenum; however, the number of layer pairs is generally significantly larger. The reflective coating 27 can also comprise further layers, for example a cover layer, which can be formed, for example, from Ru, or functional layers, for example, barrier layers, to prevent interdiffusion between a respective layer 34a made of Si and an adjacent layer 34b made of Mo.

[0061] In the Fig. In the example shown in Figure 3a, the first coating section 27a, more precisely the layers 34a, 34b of the first coating section 27a, is applied by a coating process that generates a compressive stress, for example, by magnetron sputtering. The second coating section 27b, more precisely its layers 34a, 34b, are applied by a coating process that generates a tensile stress, specifically by electron beam evaporation in the example shown. Alternatively, the second coating section 27b can be subjected to a compressive stress that is lower than the compressive stress of the first coating section 27a.

[0062] The Fig. The optical element 25 shown in Figure 3b differs from that shown in Fig. 3a in that the etchable material 29 is not contained in an intermediate layer, but in the substrate 26 itself. In the case of the Fig. The example shown in Figure 3b refers to the Fig. 2a,b and Fig. 3a, an undercut area 33 is formed. In the case of Fig. 3b, the reflective coating 27 is designed to reflect EUV radiation 16 under grazing incidence and has only a single layer made of a metallic material, more precisely of Ru. The reflective coating 27 in the form of the Ru layer is under compressive stress as a whole. Beneath the reflective coating 27 there is a layer 35 subject to tensile stress, which can be formed, for example, from Mo, MoSi, Mo-Si multilayer, MgF2, Ni, Cr or Cr alloys. The compressive stress of the reflective coating 27, together with the tensile stress of the layer 35 subject to tensile stress, causes the reflective coating 27 to bend in the direction of the substrate 26 or towards the cavity 33 when the etchable material 29 in the vicinity of the damaged area 30 is removed. Alternatively, instead of the layer 35 subject to tensile stress, aa layer under compressive stress may be used whose compressive stress is lower than the compressive stress of the reflective coating 27.

[0063] Fig. 4a-c show an optical element 25 which differs from the one shown in Fig. 3a,b in that the intermediate layer 28, which comprises the etchable material 29, is doped with a dopant 36 that is not removed upon contact with the etching medium 31. In the example shown, the dopant 36 accumulates at an etching point 37 of the etchable material 29 when the etchable material 29 is removed by the etching medium 31 upon damage to the reflective coating 27. As shown in Fig. 4b, the thickness of a passivation layer 38 made of the dopant 36 at or below the etching front 37 increases with increasing etching removal until the etching removal stops completely, as shown in Fig. 4c is the case.

[0064] The dopant 36 may, for example, be one or more metals, in particular Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides, one or more oxides, in particular SiO x , GeO x , BO x , AlO x , TiO x , TaO x , or one or more nitrides, carbides, or borides. A concentration of the dopant 36 in the etchable material 29 is typically between 0.01 at% and 30 at%, e.g., between 0.1 at% and 20 at%.

[0065] It is understood that the substrate 26 can also be doped with a non-etchable dopant 36 if the substrate 26 has an etchable material 29. In this case too, the etching removal on the etchable material 29 of the substrate 26 can be based on the methods described in connection with Fig. 4a-c can be brought to a standstill. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 102019212736A1

[0006] DE 102015225510A1

[0007] DE 102018204364A1

[0008] DE 102020213639A1

[0009]

Claims

[1] Optical element (25) for reflecting EUV radiation (16), comprising: a substrate (26), a reflective coating (27) for reflecting the EUV radiation (16), an etchable material (29) covered by the reflective coating (27) which is removed upon contact with an etching medium (31), characterized by , that the reflective coating (27) is under a mechanical prestress in order to cause a curvature of the reflective coating (27) in the direction of the substrate (26) when the etchable material (29) is removed by the etching medium (31) in the event of damage to the reflective coating (27). [2] Optical element according to claim 1, wherein the reflective coating (27) is under compressive stress at least in a coating section (27a) facing away from the substrate (26). [3] Optical element according to claim 2, wherein the reflective coating (27) is under tensile stress at least in a coating section (27a) facing the substrate (26). [4] Optical element according to claim 2, wherein the reflective coating (27) is under compressive stress and wherein at least one layer (35) under tensile stress is arranged between the reflective coating (27) and the etchable material (29). [5] Optical element according to the preamble of claim 1, characterized by in that the etchable material (29) is doped with a dopant (36) which is not removed upon contact with the etching medium (31), wherein the dopant (36) preferably accumulates at an etching point (37) of the etchable material when the etchable material (29) is removed by the etching medium (31) upon damage to the reflective coating (27). [6] Optical element according to claim 5, wherein the dopant (36) is selected from the group comprising: metals, oxides, preferably SiO x , GeO x , BO x , AlO x , TiO x , TaO x , nitrides, carbides, borides. [7] Optical element according to claim 6, wherein the metal is selected from the group comprising: Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides. [8] Optical element according to one of claims 5 to 7, wherein a concentration of the dopant (36) in the etchable material (29) is between 0.01 at% and 30 at%, preferably between 0.1 at% and 20 at%. [9] Optical element according to one of the preceding claims, wherein the etchable material (29) is selected from the group comprising: amorphous silicon, silicon, SiO2, Si-containing glasses, Ge, GeO, B, BO. [10] Optical element according to one of the preceding claims, in which the reflective coating (27) has at least one damaged area (30) at which the etchable material (29) is exposed to the environment. [11] Optical element according to one of the preceding claims, wherein the etchable material (29) is contained in an intermediate layer (28) between the reflective coating (27) and the substrate (26). [12] Optical element according to one of the preceding claims, wherein the etchable material (29) is contained in the substrate (26). [13] Optical element according to one of the preceding claims, wherein the reflective coating (27) forms a multi-layer coating for reflecting EUV radiation (16) incident on the reflective optical element (25) under normal incidence, the multi-layer coating comprising alternating layers (34a, 34b) of a first material and a second material having different refractive indices. [14] Optical element according to one of claims 1 to 13, wherein the reflective coating (27) is designed to reflect EUV radiation (16) incident on the reflective optical element (25) under grazing incidence. [15] EUV lithography system (1), comprising: at least one optical element (25) according to one of the preceding claims, which is exposed to an etching medium (31) during operation of the EUV lithography system (1).

Images