Method for manufacturing a reflective optical element for the EUV wavelength range and a reflective optical element for the EUV wavelength range

Abstract

In particular, for producing a reflective optical element for the EUV wavelength range that has a grating structure or that can serve for phase shifting, a method is proposed that includes the following steps: applying a structurable layer to a substrate, applying a reflective coating to the substrate provided with the structurable layer and locally irradiating the structurable layer beneath the reflective coating, or locally irradiating the structurable layer and applying a reflective coating to the structurable layer, wherein at least two layers of different materials are applied as the structurable layers, and the materials exothermically mix and / or exothermically react with each other under the influence of irradiation. A reflective optical element (50) for the EUV wavelength range, in particular a reflective optical element produced in this way, comprises a substrate (59) and a reflective coating (54), with a structurable layer (60) arranged between the substrate (59) and the reflective coating (54).

Description

[Technical Field]

[0001] The present invention provides a method for manufacturing a reflective optical element for use in the EUV wavelength range, comprising the steps of: applying a structurable coating to a substrate; applying a reflective coating to the substrate provided with the structurable coating, the structurable coating comprising at least two layers of different materials; providing localized irradiation to the structurable coating beneath the reflective coating; and a method for manufacturing a reflective optical element for the EUV wavelength range, comprising: applying a structurable coating to a substrate; subjecting the structurable coating to local irradiation, followed by applying a reflective coating to the structurable coating, the structurable coating comprising at least two layers of different materials; The invention further relates to a reflective optical element for use in the EUV wavelength range produced by this method, a reflective optical element for use in the EUV wavelength range having a substrate and a reflective coating, with a structurable coating disposed between the substrate and the reflective coating and comprising at least two layers of different materials, and a reflective optical element for use in the EUV wavelength range having a substrate and a reflective coating, with the structurable coating disposed between the substrate and the reflective coating. [Background technology]

[0002] Reflective optical elements for the EUV wavelength range (wavelengths in the range of 5 nm to 20 nm) can have structures, for example, to enable their use as phase-shift masks or to filter or deflect radiation of unwanted wavelengths. A known method for producing reflective optical elements with lateral structures is, for example, the use of lithography, in which a radiation-sensitive layer, also called a resist, is effectively exposed to photons, ions, or electrons, transferring the desired pattern to the radiation-sensitive layer, which is then structured, for example, by etching or selective deposition. This structure can then be transferred to the reflective optical element. In this way, high-resolution structures in the nanometer range can be produced. This method requires intensive cleaning processes, especially to control particle contamination. This method is very demanding, as the requirements for cleanliness are particularly high for optical elements for the EUV wavelength range.

[0003] Patent document 1 discloses the irradiation of microstructured or nanostructured components made of glass, glass ceramic, or ceramic with photons or electrons. This leads to material compression at the irradiated site. A reflective coating can then be applied to the structured component, e.g., as a substrate for a reflective optical element.

[0004] According to US Patent No. 5,619,239, surface topography correction of an EUV mirror already provided with a reflective coating is performed by irradiating the EUV mirror with electrons of an energy that results in a penetration depth large enough to introduce a laterally varying compression into the substrate, especially if it is made of glass, glass ceramic or ceramic.

[0005] In contrast to this, Patent Document 3 proposes to locally influence the optical properties of a reflective coating in the form of a multilayer system by locally influencing its period or total thickness by inputting energy through irradiation with electrons, photons or ions, preferably by inputting thermal energy, in order to locally change the density. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] DE 10 2012 212 199 A1 [Patent Document 2] DE 10 2011 084 117 A1 [Patent Document 3] U.S. Patent No. 6,844,272 Summary of the Invention [Problem to be solved by the invention]

[0007] It is an object of the present invention to indicate further means for structuring reflective optical elements for the EUV wavelength range and to provide further structured reflective optical elements for the EUV wavelength range. [Means for solving the problem]

[0008] This object is to provide, in a first aspect, a method for manufacturing a reflective optical element for the EUV wavelength range, comprising the steps of: applying a structurable coating to a substrate; applying a reflective coating to the substrate provided with the structurable coating, the structurable coating comprising at least two layers of different materials; providing localized irradiation to the structurable coating beneath the reflective coating; wherein the materials exothermically mix and / or react with each other under the influence of irradiation, Also, a method for manufacturing a reflective optical element for the EUV wavelength range, comprising: applying a structurable coating to a substrate; subjecting the structurable coating to local irradiation, followed by applying a reflective coating to the structurable coating, the structurable coating comprising at least two layers of different materials; wherein the materials exothermically mix and / or react with each other under the influence of irradiation.

[0009] The inventors have recognized the advantages of applying the structurable coating as at least two layers of different materials, which are structured by localized irradiation and are an integral part of the resulting reflective optical element. This includes applying materials that exothermically mix and / or exothermically react with each other under the influence of localized irradiation, particularly at the site of localized irradiation of the structured layer. This has the advantage that the structurable coating is metastable before irradiation. Only when activation energy is introduced by irradiation does the mixing or reaction of the materials form a stable state. In this way, in the production of reflective optical elements by localized irradiation, it is possible to introduce permanent structures into the structurable coating, both before and after the application of a reflective coating. Since structuring can be carried out without a resist, no stringent cleaning steps are necessary. If necessary, it is also possible to introduce structures by localized irradiation before or after the application of a reflective coating. In particular, controlling the irradiation of the structurable coating, for example by appropriately selecting the nature, energy, etc. of the irradiation, makes it possible to avoid adverse effects on the substrate and / or the reflective coating during the structuring process.

[0010] It is more advantageous to irradiate the structurable coating with electrons locally. Generally, since the electron beam can be focused more or less as needed and its energy can be adjusted without significant complexity, it is possible to ensure, by commonly known means and low-cost equipment, that neither the reflective coating nor the substrate is exposed to a large energy input during irradiation of the structurable coating, especially if a reflective coating is already applied, and thus retain their respective properties unchanged. It has been found particularly advantageous to use electrons with energies ranging from 5 keV to 80 keV, preferably from 5 keV to 40 keV, and more preferably from 10 keV to 25 keV, for irradiation. It has also been found advantageous to use electron beams with diameters ranging from 5 nm to 1000 μm for irradiation. Diameters approaching 1000 μm are suitable, for example, for introducing binary gratings to deflect imprecise infrared light from the beam path. Diameters approaching 5 nm make it possible to introduce high-resolution structures, such as those used for phase-shift masks. When irradiated with electrons, at least one of the materials of the layers of the structurable coating preferably has a large absorption or small penetration depth for electrons so that electron energy can be converted very efficiently into activation energy for inducing reaction or mixing, and so that the minimum total thickness of the structurable coating can be sufficient to achieve the desired thickness change by local irradiation.

[0011] In particular if a reflective coating is applied before local irradiation of the structurable coating, the layer materials are preferably selected such that upon mixing or reaction under the influence of irradiation they have a Gibbs free energy in the range of -10 kJ / mol to -900 kJ / mol, thereby ensuring that upon irradiation-induced exothermic mixing or reaction of the layer materials of the structurable coating there is no release of excessive amounts of heat which could damage the substrate material or in particular the reflective coating.

[0012] Preferably, polishing is performed on at least one of the at least two layers of the structurable coating. This has been found to be advantageous, particularly for structurable coatings with relatively thick layers, so as to reduce the surface roughness of the finished reflective optical element, which may adversely affect reflectivity. Polishing can be performed before, during, or after the deposition of at least one layer to reduce the roughening effect. Regardless of when polishing is performed, it can be performed using, for example, ion-assisted polishing (U.S. Pat. No. 6,441,963; see also A. Kloidt et al. (1993), "Smoothing of interfaces in ultrathin Mo / Si multilayers by ion bombardment", Thin Solid Films 228 (1-2), 154 to 157; E. Chason et al. (1993), "Kinetics of Surface Roughening and Smoothing During Ion Sputtering", MRS Proceedings, 317, 91), plasma-assisted polishing (see also German Patent Application No. 10 2015 119 325), reactive ion-assisted polishing (Ping, Study of chemically assisted ion beam etching of GaN using HCl gas, Appl. Phys. Lett. 67 (9) 1995). 1250), reactive plasma-assisted polishing (see also U.S. Patent No. 6,858,537), plasma immersion polishing (see also U.S. Patent No. 9,190,239), bias-plasma-assisted polishing (see also S. Gerke et al. (2015), "Bias-plasma Assisted RF Magnetron Sputter Deposition of Hydrogen-less Amorphous Silicon," Energy Procedia 84, 105 to 109), and polishing by magnetron sputtering with pulsed direct current (Y.Any method can be used, including atomic layer polishing (see also Pei (2009), "Growth of nanocomposite films: From dynamic roughening to dynamic smoothening," Acta Materialia, 57, 5156-5164), atomic layer polishing (U.S. Patent No. 8,846,146; see also Keren J. Kanarik, Samantha Tan, and Richard A. Gottscho, Atomic Layer Etching: Rethinking the Art of Etch, The Journal of Physical Chemistry Letters 2018 9 (16), 4814-4821, DOI: 10.1021 / acs.jpclett.8b00997).

[0013] In a preferred embodiment, the applied structurable coating comprises at least two layers of different materials, which mix and / or react with each other under the influence of local irradiation, resulting in a thickness change of the structurable coating, particularly at the irradiated site(s), with the layer thicknesses selected such that the thickness does not change further after the desired thickness change of the structurable coating is achieved. With regard to the process, a major advantage of this approach is that the procedure for structuring the structurable coating by irradiation is self-terminating. This is because the thicknesses of the individual layers of the structurable coating are selected such that after a certain dose, the individual layers completely mix or react with each other, so that the structuring process cannot continue even if the irradiation is continued further. In this way, it is possible to achieve a precision of the structuring, particularly of the resulting layer thickness changes, and thus of the surface profile of the manufactured reflective optical element, that exceeds the control of the irradiation itself.

[0014] In yet another aspect, the above object is achieved by a reflective optical element produced as described above, or a reflective optical element for use in the EUV wavelength range having a substrate and a reflective coating, wherein a structurable coating is disposed between the substrate and the reflective coating, and wherein the structurable coating comprises at least two layers of different materials, the materials of the layers being materials that can exothermically react or exothermically mix with each other, and a reflective optical element for use in the EUV wavelength range having a substrate and a reflective coating, wherein the structurable coating is disposed between the substrate and the reflective coating, and wherein the structurable coating comprises at least two materials that have very low compatibility at room temperature but high compatibility at temperatures of 300°C or above.

[0015] The inventors have realised that there are advantages to providing a structurable coating with the specifically mentioned material properties, which is structured by irradiation and is an integral part of the resulting reflective optical element. In particular, the provision of a dedicated structured coating allowed the introduction of structure into the reflective optical element during the structuring process without any significant adverse effects on the substrate and / or the reflective coating.

[0016] Advantageously, the structurable coating has lateral thickness variations. Thickness variations may be caused by local irradiation of the structurable coating, which may lead to local changes in the thickness of the structurable coating and thus to structuring of this layer. These thickness variations may be introduced to impart to the reflective optical element the effect of a phase shift mask or a spectral filter, for example in the form of a diffraction grating. Thickness variations may in particular be correlated to structural and / or stoichiometric differences between the materials of the regions of different densities.

[0017] In a preferred embodiment, the structurable coating has a coating weight of 12 g / cm 3 or more, preferably 15 g / cm 3 More preferably, 18 g / cm 3It is known that the penetration depth into a material upon irradiation with photons, ions, and especially electrons is inversely proportional to the density of the material. Providing the structurable coating with a material of the mentioned minimum density makes it possible to prevent localized irradiation from penetrating through the structurable coating into the substrate of the reflective optical element, which could lead to, for example, undesired compression of the substrate material, while keeping the structurable coating very thin in order to reduce adverse effects such as high layer stresses or excessive roughening.

[0018] Preferably, the structurable coating comprises at least two layers of different materials. More preferably, the structurable coating has multiple layers of at least two materials arranged alternately. This configuration of the structurable coating allows for the introduction of structure into the structurable coating, since activation energy is supplied to the at least two materials by irradiation of the layers to induce mixing or reaction of the at least two materials at their adjacent surfaces. The provision of multiple layers increases the number of interfaces at which these processes can occur. Most preferably, the layer materials are materials that can exothermically react or exothermically mix with each other. This has the advantage that the structurable coating is metastable. Only upon introduction of activation energy, for example, by irradiation, is there mixing or reaction of the materials that forms a stable state. In this way, it is possible to introduce permanent structure into a reflective optical element by locally introducing activation energy into the structurable coating, for example, for phase shifting or wavelength filtering. Advantageously, the layer materials are selected so that upon mixing or reaction upon introduction of activation energy into the structurable coating, for example, by irradiation, the Gibbs free energy is in the range of -10 kJ / mol to -900 kJ / mol. This makes it possible to ensure that there is no excessive heat release which may damage the substrate material or the reflective coating in particular.

[0019] Advantageously, the structurable coating comprises one or more of the following materials: tungsten, rhenium, osmium, iridium, tantalum, hafnium, ruthenium, platinum, gold, alloys thereof, oxides thereof, carbides thereof, nitrides thereof, borides thereof. This firstly allows the thickness of the structurable coating to be minimized in order to avoid further layer stresses as much as possible, and secondly makes it possible to avoid that EUV radiation is highly absorbed by the mentioned materials and thus penetrates to the substrate during operation of the reflective optical element, which could lead to damage to the substrate. In particular, the structurable coating can comprise, for example, metal and ceramic materials.

[0020] The structurable coating in the above-mentioned variants preferably comprises at least one further material from the group consisting of carbon, boron, silicon, boron carbide and boron nitride, which, after the supply of activation energy by irradiation, reacts efficiently with materials, in particular from the group consisting of tungsten, rhenium, osmium, iridium, tantalum, hafnium, ruthenium, platinum and gold, to form compounds with densities that are completely different from the respective starting materials, so that structures with different thicknesses can be introduced into the structured layer by local irradiation, or such structures may already be introduced.

[0021] In yet another preferred variant, the structurable coating comprises at least two materials that have very low compatibility at room temperature but high compatibility at temperatures above 300°C. More preferably, these two materials are applied alternately in the form of multiple layers. A structurable coating consisting of at least two materials with such different compatibility is metastable at room temperature. When locally heated to a sufficiently high temperature by energy input through irradiation, these materials can intermix, which can lead to density changes and therefore structurization. More preferably, in these variants, the structurable coating comprises a first material from the group consisting of tungsten, tantalum, and indium, and a further material from the group consisting of vanadium, titanium, rhodium, platinum, and chromium.

[0022] The present invention will now be described in detail with reference to preferred embodiments. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 10 is a diagram of a first alternative embodiment of a reflective optical element having a structurable coating. [Figure 2] FIG. 10 is a diagram of a second alternative embodiment of a reflective optical element having a structurable coating. [Figure 3] FIG. 3 is a diagram of a third variant embodiment of a reflective optical element having a structurable coating in a first state. [Figure 4] FIG. 10 is a diagram of a third alternative embodiment of a reflective optical element having a structurable coating in a second state. [Figure 5] 1 is a schematic flow diagram of a first method for manufacturing a reflective optical element. [Figure 6] FIG. 10 is a schematic flow diagram of a second method for manufacturing a reflective optical element. DETAILED DESCRIPTION OF THE INVENTION

[0024] 1 shows a schematic diagram of a configuration of a reflective optical element 50 having a structurable coating 60 on a substrate 59 and a reflective coating 54 thereon, which in this example comprises layers of a material having a relatively large real part of its refractive index at the operating wavelength, e.g., at which lithographic exposure occurs (also referred to as spacer 56), and layers of a material having a relatively small real part of its refractive index at the operating wavelength (also referred to as absorber 57), applied alternately to substrate 51, with the absorber-spacer pairs forming stack 55. In a sense, this mimics a crystal with lattice planes corresponding to the absorber layers where Bragg reflection occurs. Typically, reflective optical elements for EUV lithography tools or optical systems are designed so that each wavelength of maximum reflectivity substantially matches the wavelength of use for the lithography process or other application of the optical system.

[0025] The thicknesses of the individual layers 56, 57 and the repeat stack 55 can be constant throughout the multilayer system 54 or can vary across its area or thickness, depending on the spectral or angle-dependent reflectance profile or maximum reflectance at the operating wavelength to be achieved. When the layer thicknesses throughout the multilayer system 54 are essentially constant, it is referred to as a period 55 rather than a stack 55. To increase the maximum reflectance at each operating wavelength, the basic structure consisting of the absorber 57 and spacer 56 can be supplemented with other more and less absorbing materials to specifically influence the reflectance profile. To this end, the absorber and / or spacer materials may be interchanged in some stacks, or the stack may be composed of two or more absorber and / or spacer materials. Furthermore, additional layers can be provided between the spacer and absorber layers 56, 57 as diffusion barriers. For example, a typical material combination for an operating wavelength of 13.4 nm is molybdenum as the absorber material and silicon as the spacer material. The period 55 here often has a thickness of approximately 6.7 nm, and the spacer layer 56 is typically thicker than the absorber layer 57. Further typical material combinations are silicon-ruthenium or molybdenum-beryllium, among others.Furthermore, a protective layer 53, which may also be of multilayer design, can be provided in the multilayer system .

[0026] Typical substrate materials for reflective optical elements for EUV lithography are silicon, silicon carbide, silicon-impregnated silicon carbide, fused silica, titanium-doped fused silica, glass, and glass-ceramic. For such substrate materials in particular, a layer of a material highly absorbing for radiation in the EUV wavelength range used in the operation of reflective optical element 50 can be provided between reactive coating 54 and substrate 59 to protect substrate 59 from radiation damage, e.g., unwanted compression. Furthermore, the substrate can also be made of copper, aluminum, a copper alloy, an aluminum alloy, or a copper-aluminum alloy.

[0027] The structurable coating may include at least two layers of different materials. The structurable coating may include multiple layers, preferably at least two materials, arranged alternately. In the example shown in FIG. 1 , the structurable coating 60 includes, without limitation to generality, multiple layers 63, 64 of two different materials arranged alternately. More preferably, the materials are capable of exothermic reaction or exothermic mixing with each other. Such a combination of materials initially forms the structurable coating 60 in a metastable state. When a certain amount of energy sufficient to act as activation energy is introduced into the structurable coating, these two materials can react with each other to form one or more other materials, or they can mix or dissolve together. The structurable coating 60 then becomes more stable at the locations where the activation energy was introduced. Because the changes in the structurable coating 60 caused by the activation energy are more or less related to density changes, permanent structures, such as binary gratings or phase shifters, can be introduced into the reflective optical element 50 in this manner. It is advantageous for the structurable coating 60 not to be too thick to avoid excessive layer stress.

[0028] The more layers 63, 64 are provided, the more interfaces there are at which reaction or intermixing can occur. Advantageously, at least one of the selected materials is highly absorbing for the radiation used to introduce the activation energy, firstly to ensure sufficient conversion of the radiation energy into activation energy and secondly to protect the substrate 59 from damage due to the structuring radiation, and / or is highly absorbing for the EUV radiation used in the operation of the reflective optical element 50, in order to protect the substrate 59 from corresponding radiation damage. It is also possible to provide additional layers between the structurable coating 60 and the substrate 59 to protect the substrate 59 from the structuring radiation and possibly also from EUV radiation during operation. If the structuring is performed by electron radiation, it would be possible, for example, to provide a layer containing a metal with high electron absorption. By way of example only, it would be possible to provide the structurable coating 60 with tungsten layers 63 and 64 and a total thickness of about 300 nm for electrons with an energy of 10 keV, and about 600 nm for electrons with an energy of about 20 keV.

[0029] In some cases, a dedicated polishable layer can be provided between the structurable coating 60 and the reflective coating 54 so that the roughening of the structurable coating 60 does not continue through to the reflective coating 54 and reduce the reflectivity of the reflective optical element 50. In a further variation, the structurable coating can be formed from two or more sections, each consisting of at least one layer, with a polishable layer disposed between each section. Any desired polishing method can be used, including, for example, ion-assisted polishing, plasma-assisted polishing, reactive ion-assisted polishing, reactive plasma-assisted polishing, plasma immersion polishing, bias plasma-assisted polishing, polishing by magnetron sputtering with pulsed direct current, and atomic layer polishing.

[0030] Alternatively or additionally, polishing can be performed on at least one or more, or possibly all, of the layers 63, 64 of the structurable coating. This has been found to be advantageous, particularly for structurable coatings having relatively thick layers, so as to reduce the surface roughness of the finished reflective optical element, which may adversely affect reflectivity. Polishing can be performed before, during, or after the deposition of at least one layer to reduce the roughening effect. Regardless of when polishing is performed, any desired polishing method can be used, including, for example, ion-assisted polishing, plasma-assisted polishing, reactive ion-assisted polishing, reactive plasma-assisted polishing, plasma immersion polishing, bias plasma-assisted polishing, polishing by magnetron sputtering with pulsed direct current, and atomic layer polishing.

[0031] In the embodiment of the reflective optical element 51 shown in FIG. 2 , the structurable coating 61 disposed between the substrate 59 and the reflective coating 54, which may be configured as already described in connection with FIG. 1 , comprises a plurality of layers 65, 66, 67 of three different materials arranged in a repeating stack. The third material may likewise be a material that exothermically reacts or mixes with the other two materials. Alternatively, it is possible to provide materials that reduce or even completely compensate for the stresses caused by the reflective coating 54 and the structurable coating 61. In particular, the material, number, and thickness of at least one of the layers 65, 66, 67 may be optimized with respect to compensating for the stresses caused by the reflective coating 54. Furthermore, it is also possible to take all other known measures to compensate or reduce stresses, such as providing a symmetrical coating on the back side of the substrate, an additional layer between the substrate and the structurable coating or between the structurable coating and the reflective coating, which may also have a multilayer structure. Alternatively, the materials of the structurable coating, or of any layers forming the structurable coating, can be selected so that the overall layer stress both before and after structuring by local irradiation is minimized, and so that the layer stress of the structurable coating counters the layer stress caused by the reflective coating. The layer stress can also be influenced by the coating parameters. A reduction in the overall layer stress can also reduce the risk of layer delamination. It has also proven useful to select at least one material for the layer of the structurable coating that can plastically adapt in the event of deformation caused by structuring and / or layer stress. Countermeasures against delamination of the structurable coating and the substrate can include providing an adhesion-promoting layer therebetween.

[0032] 3 and 4 show a third embodiment of a reflective optical element 52, 52' for the EUV wavelength range at the beginning of localized irradiation (FIG. 3) and after the end of localized irradiation (FIG. 4). In the reflective optical element 52, 52', a structurable coating 62, 62' is arranged between a substrate 59 and a reflective coating 54, similar to the example shown in FIG. 1. In the structurable coating 62, 62', in the example shown here, multiple layers 68, 69 of two different materials, each alternately arranged, are applied, similar to the example from FIG. 1, which exothermically mix and / or react with each other under the influence of activation energy, e.g., by irradiation. It has proven advantageous to select the layer materials so that their Gibbs free energy during mixing or reaction under the influence of irradiation is in the range of -10 kJ / mol to -900 kJ / mol. On the one hand, the heat generation is sufficiently low so as not to damage either the substrate or the existing reflective coating. On the other hand, the state of the structurable coating after reaction or mixing is significantly more stable than its original state.

[0033] To introduce the activation energy required to induce a reaction or intermixing into the structurable coating 62, the structurable coating 62 is irradiated with electrons (represented by wavy arrows) in the example shown. This has the advantage that both the electrical energy and the diameter of the electron beam can be adjusted very precisely over a wide range. For example, irradiation with electrons in the energy range of 5 keV to 80 keV, preferably 5 keV to 40 keV, and more preferably 10 keV to 25 keV, has proven useful, firstly to penetrate the reflective coating and minimize attenuation, and secondly to avoid damaging the substrate. Regarding the electron beam diameter, it is possible to operate with two or more electron beams, either sequentially or in parallel, but it is preferred to operate with a diameter in the range of 5 nm to 1000 μm. Diameters approaching 1000 μm are suitable, for example, for introducing binary gratings to bend imprecise infrared light out of the beam path. Diameters approaching 5 nm allow for the introduction of high-resolution structures, such as those used for phase-shift masks.

[0034] Local irradiation of the structurable coating can result in local variations in the thickness of the structurable coating, and thus in structuring of this layer. Thickness variations can be particularly correlated to structural and / or stoichiometric differences between materials at different density regions. In the example shown in FIG. 4, electron irradiation has a compressive effect such that depressions are formed under the reflective coating 54 at the irradiated regions. Depending on the magnitude of the optical path difference of the EUV light reflected when using the reflective optical element 52′ at the depression regions compared to the non-depressed regions, for example, for relatively small values, there can be a phase shift. In the case of larger structures, for example, inaccurate radiation at higher wavelengths present in the EUV beam can be bent out of the beam path.

[0035] In the example shown, the structurable coating is applied at a dose of 12 g / cm to limit the penetration depth of the radiation into the structurable coating as much as possible and to prevent adverse effects on the substrate material. 3 or more, preferably 15 g / cm 3 More preferably, 18 g / cm 3 The method has at least one layer of material having a density of 1000 .mu.m.sup.-1000

[0036] In a first preferred variant, the materials of layers 68, 69 are selected so that they have very low compatibility at room temperature and high compatibility at temperatures above 300°C. In this variant, the structurable coating is metastable at room temperature. When locally heated to a sufficiently high temperature by energy input through irradiation, mixing of these materials can occur, which can lead to density changes and therefore structuring. More preferably, in these variants, the structurable coating comprises a first material from the group consisting of tungsten, tantalum, and indium, and a further material from the group consisting of vanadium, titanium, rhodium, platinum, and chromium.

[0037] In a further preferred variant, the structurable coating 62, 62′ comprises one or more materials from the group consisting of tungsten, rhenium, osmium, iridium, tantalum, hafnium, ruthenium, platinum, gold, alloys thereof, oxides thereof, carbides thereof, nitrides thereof, and borides thereof. These materials have the advantage of protecting the substrate from radiation damage during operation of the reflective optical element with EUV radiation. These materials also have a high absorption for electrons, allowing for particularly efficient conversion of electron energy into activation energy. Due to these properties, the total thickness of the structurable coating 62, 62′ can be kept smaller than in the case of materials with low absorption for electrons and EUV radiation, thereby more simply minimizing the resulting layer stress. In particular, it has been found that the structurable coating 62, 62′ is useful for exothermic reactions when it comprises at least one additional material from the group consisting of carbon, boron, silicon, boron carbide, and boron nitride. Boron carbide and boron nitride are used in the B x C y or B x N z Preferably, the carbon layer may be applied as an amorphous layer or as a diamond-like layer.

[0038] It should be noted that tantalum, platinum, and titanium layers in particular have a plastic adaptability to deformation. If the structurable coating terminates on the substrate side with a layer of chromium, tantalum, niobium, molybdenum, titanium, or one of their alloys or compounds, and the substrate consists of silicon, silicon carbide, silicon-impregnated silicon carbide, quartz glass, titanium-doped quartz glass, glass, and glass ceramic, an adhesion-promoting layer between the structurable coating and the substrate can have a particularly good adhesive effect.

[0039] The following is a non-exhaustive list of exemplary possible material combinations.

[0040] Starting material Final material(s) Gibbs free energy W+C WC -38.3kJ / mol W+2 Si WSi2 -90.9kJ / mol Re+2 Si ReSi2 -90.3kJ / mol Hf+2 B HfB2 -332kJ / mol Hf+C HfC -249kJ / mol 3 Hf+B4C 2 HfB2+HfC -851kJ / mol TaB2+Hf HfB2+Ta -126kJ / mol WC+Ta TaC+W -104kJ / mol RuO2+Hf Ru+HfO2 -836kJ / mol RuO2+Re ReO2+Ru -138kJ / mol SiO2+Hf HfO2+Si -232kJ / mol TaB2+HfC TaC+HfB2 -19.7kJ / mol

[0041] The estimation of radiation-induced thickness changes is illustrated by the following example with reference to a structurable coating consisting of multiple tungsten and silicon layers. Using the densities and molar masses of the tungsten and silicon starting materials, the respective molar volumes can be calculated. The density of tungsten is 19.25 g / cm. 3 and molar mass 183.84 and density of silicon 2.336 g / cm 3 and molar mass 2.09 g / mol, the molar volume of tungsten is 9.47 cm 3 / mol, 12.06 cm for silicon 3 / mol. For tungsten silicide formed by radiation-induced reactions, the density is 9.3 g / cm 3and a molar mass of 24.01 g / mol, resulting in a molar volume of 25.81 g / mol. Considering that the molar ratio of tungsten to silicon in the structurable coating should be 1:2, the shrinkage of the structurable coating as a result of irradiation is approximately 23% for full conversion to tungsten silicide at the irradiated sites. This means that the total thickness of the structurable coating should be 4.2 nm for a 1 nm reduction. This approach can be adapted to any desired material combination.

[0042] In order to achieve particularly good control of the layer thickness variations in the structurable coating, it has proven to be particularly advantageous if the layer thicknesses are selected such that the thickness does not change further after the desired thickness variation of the structurable coating has been achieved, in other words, the thicknesses of the individual layers of the structurable coating should be selected such that after a certain dose the individual layers have completely mixed or reacted with each other, so that the structuring process cannot continue even if the irradiation is continued further, i.e., the structuring process is self-terminating.

[0043] 5 and 6 show schematically two basic alternative sequences by which the reflective optical element for the EUV wavelength range can be manufactured as proposed here. The sequence shown in FIG. 5 corresponds to the procedure described in connection with the preceding example. In a first step 501, a structurable coating is first applied to a substrate, then in step 503, a reflective coating is applied to the substrate provided with the structurable coating, such that the structurable coating is located between the substrate and the reflective coating. Irradiation is then carried out in step 505 in order to structure the structurable coating located below the reflective coating.

[0044] 6, in contrast, after applying a structurable coating to the substrate in step 601, this structurable coating is irradiated for structuring in a further step 603, before applying a reflective coating to the structurable and in fact structured layer in step 605. As a further option not shown here, a first sub-structuring is performed by irradiation of the structurable coating before applying the reflective coating, and a further sub-structuring is performed by further irradiation after applying the reflective coating.

[0045] In both the variants shown in Figure 5 and 6, the applied structurable coating can be at least two layers of different materials. The layer materials can be materials that can exothermically react or exothermically mix with each other. The applied structurable coating can include at least two materials that have very little compatibility at room temperature but high compatibility at temperatures above 300°C. These materials can be advantageously applied as at least two layers of different materials to form the structurable coating.

[0046] In variations of these techniques, structuring may be performed by local irradiation in two or more sub-steps, for example using different irradiation parameters such as energy, dose, irradiation mode and / or corresponding design of the structurable coating by division into two or more sub-stacks and changing layer materials and layer thicknesses, to first structure areas of the structurable coating farther from the substrate and then in subsequent irradiation step(s) structure closer to the substrate by acting at different penetration depths.

[0047] The above explanations relating to the procedure for manufacturing the proposed reflective optical element, and in particular the procedure for irradiating the structurable coating for structuring, are equally applicable to the two latter options.

[0048] It should be noted that in the manufacture of reflective optical elements with a structurable coating that has at least two layers, each made of a different material, the reflectivity can be increased by polishing one, preferably two or more or all layers by ion irradiation during the coating operation or after applying each layer and before applying the next layer. Otherwise, too high a surface roughness can lead to a lower reflectivity than would be expected based on the structure of the reflective coating. A particularly favorable effect has been observed in the case of silicon layers.

[0049] The proposed reflective optical elements for the EUV wavelength range can in particular be used as EUV mirrors, for example in EUV lithography apparatus or mask or wafer inspection systems, where they can also be used as masks.

[0050] If necessary, repair of these reflective optical elements can be performed by measuring the surface profile of the reflective coating and comparing it to a target profile, and irradiating the substrate and / or the structurable coating at one or more locations where the surface profile differs from the target profile, if any. Local irradiation of the substrate and the structurable coating can introduce thickness variations at those locations, particularly due to density variations, with the effect of reducing the difference in the actual surface profile from the target profile at those locations. Advantageously, electron irradiation can also be used for repair, using electrons with higher energy than for the previously performed structuring, in order to achieve a greater penetration depth and therefore local density variations in deeper areas. [Explanation of symbols]

[0051] 50 Reflective optical elements 51 Reflective optical elements 52, 52' Reflective optical element 53 Protective layer 54, 54' reflective coating 55 stacks 56 Absorber 57 Spacer 59 Circuit Board 60 structurable coatings 61 structurable coatings 62, 62' structurable coating 63 First Layer 64 Second Layer 65 First Layer 66 Second Layer 67 Third Layer 68 First Layer 69 Second Layer 501 method steps 503 method steps 505 method steps 601 method steps 603 method steps 605 method steps

Claims

1. 1. A method for manufacturing a reflective optical element for EUV wavelength range, comprising: applying a structurable coating to a substrate; applying a reflective coating to the substrate provided with the structurable coating, the structurable coating comprising at least two layers of different materials; applying localized irradiation to the structurable coating beneath the reflective coating; wherein the materials of the structurable coating exothermically mix and / or exothermically react with each other under the influence of the irradiation, thereby introducing a permanent structure due to the localized irradiation, and the materials are selected such that upon mixing or reacting under the influence of the localized irradiation, they have a Gibbs free energy in the range of -10 kJ / mol to -900 kJ / mol.

2. 1. A method for manufacturing a reflective optical element for EUV wavelength range, comprising: applying a structurable coating to a substrate; subjecting the structurable coating to localized irradiation, followed by applying a reflective coating to the structurable coating, the structurable coating comprising at least two layers of different materials; wherein the materials of the structurable coating exothermically mix and / or exothermically react with each other under the influence of the irradiation, thereby introducing a permanent structure due to the localized irradiation, and the materials are selected such that upon mixing or reacting under the influence of the localized irradiation, they have a Gibbs free energy in the range of -10 kJ / mol to -900 kJ / mol.

3. 3. The method according to claim 1 or 2, characterized in that the local irradiation is performed by electrons.

4. 4. The method according to any one of claims 1 to 3, characterized in that polishing is performed on at least one of the at least two layers of the structurable coating.

5. 5. The method according to claim 1, wherein the materials mix and / or react with each other under the influence of the localized irradiation, thereby causing a thickness change in the structurable coating, and wherein the layer thickness is selected such that there is no further change in thickness after a desired thickness change in the structurable coating has been achieved.

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