Reflective optical element, illumination optical unit, projection exposure apparatus, and method for producing a protective layer

Abstract

The invention relates in particular to a reflective optical element (17) of an illumination optical unit of a projection exposure apparatus, which comprises a structured surface (25 a), which preferably forms a grating structure (29), and a reflective coating (36) applied to the structured surface (25 a). The reflective coating (36) discontinuously covers the structured surface (25 a), and the reflective optical element (17) comprises at least one protective layer (37) continuously covering the structured surface (25 a). The invention also relates to an illumination optical unit (4) of a projection exposure apparatus (1) comprising at least one reflective optical element (17) of the above type, a projection exposure apparatus comprising an illumination optical unit of this type, and a method for producing a protective layer (37) on a reflective optical element (17) of the above type.

Description

[Technical Field]

[0001] This application claims priority from German Patent Application No. 102020210553.7 filed on August 20, 2020, the entire disclosure of which is incorporated herein by reference.

[0002] The invention relates to a reflective optical element, in particular for an illumination optical unit of a projection exposure apparatus, which comprises a structured surface, preferably forming a grating structure, and a reflective coating applied to the structured surface. The invention also relates to an illumination optical unit of a projection exposure apparatus comprising at least one such reflective optical element, a projection exposure apparatus comprising such an illumination optical unit, and a method for forming a protective layer on an optical element. [Background technology]

[0003] Since virtually all known materials have low transmittance for radiation in the EUV wavelength range, i.e., at wavelengths from about 5 nm to about 30 nm, the illumination and projection optical units of projection exposure apparatuses for EUV lithography usually contain only reflective optical elements, particularly in the form of mirrors. A mirror as used herein has a substrate to which a reflective coating is applied in order to reflect EUV radiation. The reflective coating can be configured as a multilayer coating that serves as an interference layer system for the operating wavelength. For an operating wavelength of about 13.5 nm, the reflective multilayer coating can 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 to arrange one or more functional layers between the reflective coating and the substrate material that serve to protect the substrate, for example, as a polishing layer or as an adhesion promoter.

[0004] Residual gases in the vicinity of reflective optical elements absorb EUV radiation and therefore reduce its transmittance as it passes through the projection exposure apparatus. For this reason, projection exposure apparatuses for EUV lithography are usually operated under vacuum conditions. Residual gases in the vacuum environment may contain small amounts of hydrogen and / or other reactive gases, such as oxygen, which have a protective or cleaning effect, particularly with respect to reflective optical elements, and only slightly absorb EUV radiation.

[0005] During operation of the projection exposure apparatus, a hydrogen plasma is typically formed in such a vacuum environment under the influence of EUV radiation, i.e. activated hydrogen is formed in the form of hydrogen ions or free hydrogen radicals, which cause etching erosion of exposed surfaces of components arranged in the vacuum environment.

[0006] The exposed surface may be, for example, an exposed surface area of ​​a reflective optical element, which may contain, for example, silicon, on the surface subjected to etching erosion. Etching erosion forms volatile substances, such as SiH3 and SiH4 (silane), on the surface, which may result in the removal of the exposed surface and the deposition of volatile substances on the optically utilized surface. This leads to a decrease or deterioration of the reflectivity of the layer materials used therein, and thus to under-etching and widespread defects. For example, penetration of hydrogen atoms into the material of the (partially) exposed surface of a mirror substrate can further generate stress, which can lead to delamination. Hydrogen atoms further penetrate the exposed material and collect in the form of hydrogen molecules at defects or interfaces, where they cannot escape, which can also lead to delamination. A reflective coating, particularly in the form of a Mo / Si bilayer, can act as a protective layer against hydrogen penetration in the surface area covered by the reflective coating, if the reflective coating is continuous and not altered, for example, by oxidation.

[0007] Reflective optical elements, for example in the form of mirrors of an illumination optical unit of a projection exposure apparatus, can have a structured surface in the form of a grating structure. The grating structure can act as a spectral filter to filter out radiation in an unwanted wavelength range, which can be in the infrared or ultraviolet wavelength range, for example. If a reflective coating is applied to such a structured surface, the flanks formed thereon have a very high flank steepness, which can cause problems in that the reflective coating does not completely cover the structured surface when coated, resulting in gaps in the reflective coating that can cause etching erosion in the exposed surface areas of the structured surface.

[0008] Patent document 1 discloses a mirror for an illumination optical unit of a projection exposure apparatus, which has a spectral filter in the form of a grating structure. The grating structure has a maximum flank steepness in the range of 15° to 60°. The grating structure can be completely covered by a continuous protective layer having a plurality of double Si-Mo layers and forming a reflective coating. The low flank steepness of the grating structure improves the coverage of the grating structure or other structured surface with the protective layer, thus increasing the hydrogen stability of the reflective optical element.

[0009] In the solution described in WO 2007 / 024994, the problem is that the maximum flank angle of the structured surface or grating structure is limited, which leads to a loss of reflectivity of the optical element, which occurs, for example, due to scattered light losses in this flank area, because the radiation that should be reflected is not reflected in the direction determined by the optical unit.

[0010] Patent Document 2 discloses a reflective optical element for reflecting EUV radiation, which includes a substrate on at least one side of which is formed a multilayer stack having multiple alternating material layers that reflect EUV radiation. A spectral filter in the form of a three-dimensional profile is formed on the surface, on a scale much larger than the wavelength of EUV radiation. The multilayer stack includes a surface-conforming coating stack formed on the substrate after the three-dimensional profile is formed. Patent Document 2 states that magnetron sputtering, which is typically used to apply multilayer stacks, results in high surface roughness of the applied layers in each case. Therefore, the coating process proposed for applying multilayer stacks is a conformal or isotropic coating process in the form of atomic layer deposition, which achieves an essentially constant layer thickness along the three-dimensional profile. However, applying a reflective multilayer coating, which may have more than 50 double Mo-Si layers, by atomic layer deposition is very complicated. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] DE 10 2018 220 629 A1 [Patent Document 2] International Publication No. 2013 / 113537 Summary of the Invention [Problem to be solved by the invention]

[0012] It is an object of the present invention to specify a reflective optical element, an illumination optical unit, a projection exposure apparatus and a method for producing a protective layer, which allow efficient protection of structured surfaces from etching erosion. [Means for solving the problem]

[0013] This object is achieved by an optical element of the aforementioned type, in which the reflective coating discontinuously covers the structured surface and the reflective optical element has at least one (additional) protective layer continuously covering the structured surface.

[0014] According to the present invention, instead of a protective layer in the form of a reflective coating which acts as a continuous protective layer and requires a low flank steepness of the structured surface or a complex and surface-accurate coating method, it is proposed to apply an additional protective layer which continuously covers the structured surface.

[0015] In one embodiment, the structured surface, in particular the grating structure, has a maximum flank steepness of greater than 60°, preferably greater than 80°, in particular greater than 90°, where the (maximum) flank steepness is measured relative to a tangent to the (local) surface of the reflective optical element (e.g. the surface of the substrate) or, in the case of a grating structure, in the region between two grating webs.

[0016] As shown in Patent Document 1, reflective coatings generally cannot be applied faithfully to surfaces in the form of multilayer coatings in the case of such high flank steepness. Nevertheless, additional protective layer(s) continuously covering the structured surface can effectively protect the structured surface from etching erosion. It is also possible to cover flanks with flank steepness greater than 90°, i.e., under-etched flanks, such as can be formed by wet chemical etching, for example.

[0017] In yet another embodiment, the protective layer has a thickness of, for example, 100 nm or less, preferably 10 nm or less, more preferably 5 nm or less. To create a continuous protective layer with the above-mentioned flank steepness, it is usually necessary to apply the protective layer by a relatively complicated isotropic coating method. However, to protect the structured surface from etching erosion, only a relatively small thickness of the protective layer is required. In contrast, reflective coatings, which generally have much larger thicknesses, can be applied by anisotropic coating methods. Such coating methods are less complicated, but the reflective coating does not continuously cover the structured surface.

[0018] In yet another embodiment, the coating forms a multi-layer coating for reflecting EUV radiation. Such a multi-layer coating typically has multiple alternating layers of materials with high refractive index at the operating wavelength and low refractive index at the operating wavelength. The materials can be, for example, silicon and molybdenum, although other material combinations are possible depending on the operating wavelength.

[0019] In yet another embodiment, a protective layer is formed between the structured surface and the reflective coating.When applying a protective layer under the reflective coating, this is advantageous because the protective layer does not cause the loss of reflectivity.In some cases, the protective layer itself can also be structured to create a sub-lattice structure or further structure of the lattice structure under the protective layer.

[0020] In yet another embodiment, a cap layer is applied to the reflective coating. The cap layer serves to protect the underlying layer of the reflective coating from further environmental influences, for example from oxidation or from tin contamination caused by an EUV radiation source. It is also possible to select a material for the protective layer such that contamination, particularly in the form of tin contamination, can potentially be removed from the surface of the cap layer. The cap layer generally does not cover the structured surface continuously like the reflective coating, and is usually applied by an anisotropic coating method.

[0021] In yet another embodiment, the cap layer forms a protective layer that completely covers the structured surface. In this embodiment, the cap layer is typically applied by an isotropic coating method and forms an overcoat that continuously covers the (possibly steep) flanks of the structured surface or grating structure. A protective layer in the form of a cap layer can optionally be combined with a further protective layer, as described above, arranged between the reflective coating and the structured surface. As is customary for cap layers, the material of the protective or cap layer absorbs part of the EUV radiation and thus reduces the reflectivity of the reflective optical element. The cap layer can be a single layer, but it is also possible for the cap layer itself to form a multilayer coating with two or more layers of different materials.

[0022] In yet another embodiment, the structured surface is formed in a functional layer applied to the substrate and / or in the substrate. The functional layer is usually easy to process (e.g., by material removal, polishing, structuring by etching, etc.). It will be apparent that in addition to the functional layer processed to form the structured surface, other functional layers may be applied to the substrate. For example, this may include one or more layers that allow processing into the shape of the surface features of the reflective optical element, for example, by material removal, polishing, etc. The functional layer may also be an adhesion-promoting layer or the like. The substrate of the reflective optical element, which has a much greater thickness than the functional layer(s), may also have a structured surface. It is also possible that the structured surface is formed partly in the functional layer and partly in the substrate.

[0023] In yet another embodiment, the substrate and / or the functional layer is made of a material selected from the group consisting of amorphous silicon (aSi), silicon (Si), nickel phosphorus (Ni:P), in particular titanium (Ti), platinum (Pt), gold (Au), aluminum (Al), nickel (Ni), copper (Cu), silver (Ag), tantalum (Ta), tungsten (W), and alloys thereof, in particular silicon dioxide (SiO), aluminum oxide (AlO x ), titanium oxide (TiO x ), tantalum oxide (TaO x ), niobium oxide (NbO x ), zirconium oxide (ZrOx ), and combinations thereof (e.g., mixed oxides, ceramics, glasses, glass-ceramics, composite materials). As described in the above-mentioned patent application WO 2007 / 024994, these materials have proven to be particularly useful for components of EUV projection exposure apparatus.

[0024] In yet another embodiment, the protective layer is made of a metal, in particular copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), gold (Au), tungsten (W), etc., and alloys and oxides thereof, in particular aluminum oxide (AlO x ), zirconium oxide (ZrO x ), titanium oxide (TiO x ), niobium oxide (NbO x ), tantalum oxide (TaO x ), hafnium oxide (HfO x ), chromium oxide (CrO x ), carbides, borides, nitrides, silicides, and combinations thereof (e.g., mixed oxides, ceramics, glasses, glass-ceramics, composites). The protective layer may be formed from a single layer, but it is also possible for the protective layer itself to form a multi-layer coating having two or more layers of different materials.

[0025] The material(s) of the protective layer must meet several requirements. First, the protective layer must be able to block hydrogen molecules (H2), hydrogen ions (H2), and hydrogen ions (H2) passing through the (thin) protective layer. + ), and free hydrogen radicals (H * Second, the protective layer or its material should be able to substantially prevent the diffusion of H2, H + , H * The protective layer shall also have high thermal stability, high reduction and oxidation stability, high EUV resistance, and high cleaning process resistance, especially for removal of deposits (e.g., tin) as a result of system operation.

[0026]

[0004] In particular, if the protective layer is applied under a reflective coating, it should not cause a deterioration in the roughness of the substrate or functional layer to which it is applied. It would also be advantageous if the protective layer could be relatively simply removed in a process known as a refurbishment process, in which the old coating is removed and replaced with a new one, or if it would not be eroded or undesirably roughened by such a process. The above-mentioned materials meet most of the above requirements.

[0027] In yet another embodiment, the reflective optical element takes the form of a collector mirror of an illumination optical unit of a projection exposure apparatus. Such a collector mirror may have, for example, one or more ellipsoidal and / or hyperbolic reflective surfaces corresponding to the surfaces with a reflective coating. The illumination radiation may be incident on the reflective surface of the collector mirror at grazing incidence (GI), i.e., at an angle of incidence greater than 45°, or at normal incidence (NI), i.e., at an angle of incidence smaller than 45°.

[0028] The collector mirror typically has a structured surface in the form of a grating structure that acts as a spectral filter to suppress extraneous light, i.e. radiation at wavelengths outside the EUV wavelength range, for example in the infrared or ultraviolet wavelength range. It will be clear that the reflective optical element does not necessarily have to take the form of a collector mirror, but may be any other reflective optical element having a structured surface.

[0029] An aspect of the invention relates to an illumination optical unit of a projection exposure apparatus, comprising at least one reflective optical element as described above, which may be, for example, the collector mirror as described above.

[0030] Yet another aspect of the invention relates to a projection exposure apparatus for microlithography, in particular for EUV lithography, comprising an illumination optical unit as described above for transmitting illumination radiation from a radiation source to a reticle containing a structure to be imaged, and a projection optical unit for imaging the structure of the reticle onto a wafer.

[0031] In yet another aspect, the present invention also relates to a method of forming a protective layer on a reflective optical element as described above, comprising applying the protective layer to the structured surface or reflective coating by an isotropic coating method.

[0032] As mentioned above, it is possible to apply a continuous protective layer even to structured surfaces with high flank steepness using an isotropic coating method. In contrast, a reflective coating can be applied using an anisotropic coating method, for example by PVD coating methods such as magnetron sputtering. The use of an anisotropic coating method for applying a reflective coating is advantageous because the reflective coating generally has a (much) greater thickness than the protective layer.

[0033] In one variant, the isotropic coating method is selected from the group comprising chemical vapor deposition (CVD), in particular atomic layer deposition (ALD), or physical vapor deposition (PVD). When the coating is applied by PVD, a specific directional geometry is usually required, which reduces the actual anisotropy of these methods. It is now possible to combine multiple material sources, which may be inclined, for example. In this case, the coating rate can be controlled by shadowing, which suppresses certain angles of incidence. Furthermore, tilting or pivoting of the mirror to be coated is possible.

[0034] The isotropic coating method used to apply the protective layer can be performed at a relatively low temperature, thereby enabling the protection of the substrate. Furthermore, the isotropic coating method allows the application of a protective layer with uniform coverage, i.e., essentially constant thickness, even when the flank steepness of the structured surface is high, as described above. The isotropic coating method also allows the deposition of a protective layer with a relatively low roughness. This is particularly advantageous when the protective layer is formed between the structured surface and the reflective coating.

[0035] Further features and advantages of the invention will become apparent from the following description of an embodiment of the invention, with reference to the figures of the drawing which show the details essential to the invention, and from the claims, each of the individual features can be implemented alone or together in any combination in one variant of the invention.

[0036] Examples are shown in the schematic drawings and explained in the following description. [Brief explanation of the drawings]

[0037] [Figure 1] 1 is a schematic meridional section of a projection exposure apparatus for EUV lithography; [Figure 2] FIG. 1 is a schematic diagram of a process for creating a structured surface that forms a lattice structure. [Figure 3a] 3b is a schematic diagram of a detail of the structured surface of FIG. 2 in which the flank steepness of the grating flanks differs from that of FIG. 3b. [Figure 3b] 3b is a schematic diagram of a detail of the structured surface of FIG. 2 in which the flank steepness of the grating flanks differs from that of FIG. 3a. [Figure 4a] 3b is a schematic view similar to FIG. 3a, but with a reflective coating applied to the structured surface that discontinuously covers the structured surface. [Figure 4b] 3b is a schematic view similar to FIG. 3b, but with a reflective coating applied to the structured surface that discontinuously covers the structured surface. [Figure 5a] FIG. 3b is a schematic view similar to FIG. 3a, with a protective layer formed between the grating structure and the reflective coating, continuously covering the grating structure. [Figure 5b] FIG. 3c is a schematic view similar to FIG. 3b, with a protective layer formed between the grating structure and the reflective coating, continuously covering the grating structure. [Figure 6] FIG. 5B is a schematic diagram similar to FIG. 5a, b, in which a protective layer is applied to the reflective coating to continuously cover the grating structure. [Figure 7] FIG. 5B is a schematic diagram similar to FIG. 5a, but with a cap layer applied to the reflective coating that discontinuously covers the grating structure. DETAILED DESCRIPTION OF THE INVENTION

[0038] In the following description of the drawings, the same reference numerals are used for components that are the same or have the same function.

[0039] The essential components of a microlithographic projection exposure apparatus 1 are described below by way of example with reference to Figure 1. The description here of the basic construction of the projection exposure apparatus 1 and its components should not be considered limiting.

[0040] The illumination system 2 and the radiation source 3 of the projection exposure apparatus 1 comprise an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. What is exposed here is a reticle 7 arranged in the object field 5. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable, in particular in the scanning direction, by a reticle displacement drive 9.

[0041] For purposes of illustration, a Cartesian xyz coordinate system is shown in Figure 1. The x direction extends perpendicular to the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. The scanning direction extends in the y direction in Figure 1. The z direction extends perpendicular to the object plane 6.

[0042] The projection exposure apparatus 1 comprises a projection optical unit 10, which serves to image the object field 5 into an image field 11 in an image plane 12. Structures on a reticle 7 are imaged onto a photosensitive layer of a wafer 13, which is arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14, which is displaceable, in particular in the y-direction, by a wafer displacement drive 15. The displacement of the reticle 7 by the reticle displacement drive 9, on the one hand, and the displacement of the wafer 13 by the wafer displacement drive 15, on the other hand, can be synchronized with one another.

[0043] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, which is also referred to below as working radiation or illumination radiation. In particular, the working radiation has a wavelength in the range of 5 nm to 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).

[0044] The illumination radiation 16 emerging 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 hyperbolic reflecting surfaces. The illumination radiation 16 may be incident on at least one reflecting surface of the collector mirror 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. The collector mirror 17 may be structured and / or coated firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0045] The illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18 downstream of the collector mirror 17. The intermediate focal plane 18 may separate between the radiation source module comprising the radiation source 3 and the collector mirror 17 and the illumination optics unit 4.

[0046] The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20. The first facet mirror 20 comprises a plurality of individual first facets 21, also referred to as field facets in the following. Figure 1 shows only some of said facets 21 by way of example. A second facet mirror 22 is arranged in the beam path of the illumination optical unit 4 downstream from the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

[0047] The illumination optical unit 4 consequently forms a double-facet system. This basic principle is also called a fly's eye integrator. By means of a second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

[0048] The projection optical unit 10 comprises a number of mirrors Mi, which are numbered consecutively according to their location in the beam path of the projection exposure apparatus 1 .

[0049] 1, the projection optical unit 10 includes six mirrors M1 to M6. 4, 8, 10, 12 or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have a passage aperture for the illumination radiation 16. The projection optical unit 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture that is greater than 0.5, may be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0050] Like the mirrors of the illumination optical unit 4, the mirror Mi may have a highly reflective coating for the illumination radiation 16. The collector mirror 17 of the illumination optical unit 4 has a structured surface 25a that forms a grating structure 29 in order to suppress extraneous light. The grating structure 29 acts as a spectral filter to filter extraneous light in a predetermined wavelength range, for example in the IR wavelength range.

[0051] 2 shows a schematic diagram of an example of a processing procedure for the creation of a structured surface 25a formed in the functional layer 25 of the collector mirror 17, which can be structured by etching. For structuring, a structured layer 26 in the form of a photoresist is first applied to areas of the functional layer 25. In a subsequent step, the structured layer 26 is selectively exposed with a laser 27. In a further step, the exposed parts of the structured layer 26 are removed. The structured layer 26 can also be structured in a manner other than irradiation with a laser 27. For example, the structured layer 26 can be exposed in a lithographic process.

[0052] In a subsequent etching step, the functional layer 25 is selectively etched using the structured layer 26 as an etching mask, resulting in the formation of a structured surface 25a in the functional layer 25. The structured surface 25a forms a grating structure 29, the geometry of which is selected to act as a spectral filter and suppress extraneous light of wavelengths within a respective defined wavelength range.

[0053] The functional layer 25 can be structured using dry or wet chemical etching methods, for details of which see the aforementioned US Pat. No. 6,213,999, which is incorporated herein by reference in its entirety.

[0054] 2. Figure 3a shows the detail represented by the dotted lines in Figure 2 of the structured surface 25a after removal of the structured layer 26. The structured surface 25a or grating structure 29 has a plurality of grating webs 31, each having a top surface 32 and a flank 33. Grooves 34, each having a bottom surface 35, are formed between the grating webs 31. The structured functional layer 25 with the grating structure 29 is formed on the substrate 30 of the collector mirror 17.

[0055] In contrast to what is shown in Figures 2 and 3a, the structured surface 25a may be formed on the substrate 30, or as described in Patent Document 1, the structured surface 25 may be formed partly on the functional layer 25 and partly on the substrate 30.

[0056] The functional layer 25 or the substrate 30 includes at least one material that is highly processable or easily etched to form a structure, such as amorphous silicon (aSi), silicon (Si), nickel phosphorus, particularly titanium (Ti), platinum (Pt), gold (Au), aluminum (Al), nickel (Ni), copper (Cu), silver (Ag), tantalum (Ta), tungsten (W), and metals from the group of alloys thereof, particularly silicon dioxide (SiO), aluminum oxide (AlO). x ), titanium oxide (TiO x ), tantalum oxide (TaO x ), niobium oxide (NbO x ), zirconium oxide (ZrO x ), and combinations thereof (e.g., mixed oxides, ceramics, glasses, glass-ceramics, composites).

[0057] 3a shows grating webs 31 each having flanks 33 with a flank steepness α of 90°. The flank steepness α is measured relative to a local tangent plane corresponding to the top surface of the substrate 30. Equivalently, the flank steepness α of a flank 33 may be measured relative to a tangent plane corresponding to the bottom surface 35 of the groove 34 adjacent to the flank 33. As mentioned above, the flank steepness α is measured relative to a local tangent plane because the surface of the substrate 30 or the collector mirror 17 has an approximately ellipsoidal and / or hyperbolic geometry rather than a plane.

[0058] Figure 3b shows the case where the flank steepness α is greater than 90°, i.e. forming an undercut in the flank 33. Such a flank 33 with an undercut can be produced, for example, by (directional) wet chemical etching.

[0059] 3a and 3b, or in the case of a (maximum) flank steepness α typically greater than approximately 60°, applying a reflective coating 36 to the structured surface 25a by conventional coating methods, e.g., magnetron sputtering, does not allow for continuous coverage of the structured surface 25a; instead, gaps in the coverage result in the reflective coating 36 in the region of the flank 33. Consequently, partial regions of the structured surface 25a are exposed and, as described above, are subject to etching erosion and / or degradation, e.g., oxidation, by hydrogen molecules, free hydrogen radicals, and hydrogen ions. If the material of the functional layer 25 contains silicon, such etching erosion can form silanes (e.g., SiH3, SiH4), which can deposit on the optically useful surface, potentially leading to a reduction in the reflectivity or degradation of the layer materials used therein, even to the extent of underetching and widespread defects.

[0060] 5a, b a protective layer 37 is formed between the structured surface 25a and the reflective coating 36. In contrast to the reflective coating 36, the protective layer 37 covers the structured surface 25a continuously, i.e. without gaps, over the entire area in the region of the flanks 33.

[0061] To achieve continuous coverage of the structured surface 25a with the protective layer 37, the protective layer 37 is applied or deposited by an isotropic coating method. The isotropic coating method in the illustrated example is atomic layer deposition, but other isotropic CVD coating methods or PVD coating methods with appropriately selected directional geometries, thereby reducing the anisotropy typically present in PVD methods, can also be used for this purpose. For example, it is possible to combine several material sources, which can be in a tilted arrangement. In this case, the coating rate can be controlled by shadowing, which suppresses certain angles of incidence. Furthermore, tilting or pivoting of the optical element 17 to be coated is possible.

[0062] The thickness d of the protective layer 37 in the illustrated example is about 5 nm and generally does not exceed 100 nm in thickness. Due to the relatively small thickness of the protective layer 37 and the somewhat less demanding layer thickness control compared to the reflective coating 36, the isotropic coating method can be implemented with a relatively manageable level of complexity.

[0063] The reflective coating 36 is a multilayer coating for reflecting EUV radiation 16, having a plurality of bilayers formed from molybdenum and silicon. The number of bilayers may be, for example, about 30 to 80, but may be more or less. Correspondingly, the reflective multilayer coating 36 has a considerable thickness. Therefore, applying the reflective multilayer coating 35 by isotropic coating, for example by atomic layer deposition, is very complicated.

[0064] The protective layer 37 may comprise a single layer, but it is also possible that the protective layer 37 comprises several layers of different materials, like the reflective coating 36. In principle, the material(s) of the protective layer 37 should prevent as much as possible the diffusion of hydrogen molecules, hydrogen ions and hydrogen radicals through the protective layer 37, and should not undergo chemical reactions with these hydrogen species and with other contaminants, such as tin. For this purpose, the protective layer 37 is made of a metal, in particular from the group of copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), gold (Au), tungsten (W) and their alloys, in particular aluminum oxide (AlO x ), zirconium oxide (ZrO x ), titanium oxide (TiO x ), niobium oxide (NbO x ), tantalum oxide (TaO x ), hafnium oxide (HfO x ), chromium oxide (CrO x ), oxides, carbides, borides, nitrides, silicides, and combinations thereof (e.g., mixed oxides, ceramics, glasses, glass-ceramics, composites).

[0065] In particular, if the protective layer 37 is formed between the structured surface 25a and the reflective coating 36, it is advantageous if the protective layer 37 has a low roughness.

[0066] 6 shows a collector mirror 17 in which a protective layer 37 forms a cap layer that is applied to the reflective multilayer coating 36 by means of an isotropic coating method. The protective layer 37 in this case completely and continuously covers both the reflective coating 36 and the partial areas of the structured surface 25a that are not covered by the reflective coating 36. In the example shown in FIG. 6, the protective layer 37 likewise has a thickness d of less than about 10 nm and is formed from a material or combination of materials that has a relatively low absorption for EUV radiation 16. In this way, it is possible to ensure that the protective layer 37 does not reduce the reflectivity of the collector mirror 17 too much.

[0067] FIG. 7 shows an example of a collector mirror 17 in which a protective layer 37 is formed between the structured surface 25a and the reflective coating 36, as in FIGS. 5a and 5b. In addition, a capping layer 38 is applied to the reflective coating 36. In contrast to FIG. 6, the capping layer shown in FIG. 7 does not form a complete protective layer, since it does not continuously cover the structured surface 25a but has gaps, as is the case with the reflective coating 36. The capping layer 38 may, for example, be made of one of the materials mentioned above in connection with the protective layer 37. In principle, the capping layer 38 could also be applied to the reflective coating 36 in the example shown in FIGS. 5a and 5b. The capping layer 38 can be applied by an isotropic coating method, as shown in FIG. 6, or by an anisotropic coating method, as shown in FIG. 7.

[0068] It will be clear that the protective layer 37 can be applied not only to the collector mirror 17 but also to other structured reflective optical elements of the projection exposure apparatus 1 in order to protect them from etching attack by hydrogen plasma. It is also possible to use a protective layer on reflective optical elements designed to reflect radiation of wavelengths other than the EUV wavelength range. The protective layer 37 can also serve to protect the structured surface 25 a from etching attack by chemical elements other than hydrogen. In this case, the material from which the protective layer 37 is made should be adapted to the chemical elements from which the structured surface 25 a is to be protected.

Claims

1. A reflective optical element (17) of an illumination optical unit (4) of a projection exposure apparatus (1), a structured surface (25a) forming a lattice structure (29); a reflective coating (36) deposited on said structured surface (25a); A reflective optical element (17) comprising: the reflective coating (36) discontinuously covers the structured surface (25a); and The reflective optical element (17) has at least one protective layer (37) continuously covering said structured surface (25a). A reflective optical element characterized by:

2. 2. A reflective optical element according to claim 1, wherein the structured surface (25a) has a maximum flank steepness (α) of greater than 80°.

3. 3. A reflective optical element according to claim 1, wherein said protective layer (37) has a thickness (d) of 100 nm or less.

4. A reflective optical element according to any one of claims 1 to 3, wherein said reflective coating (36) forms a multi-layer coating for reflection of EUV radiation (16).

5. A reflective optical element according to any one of claims 1 to 4, wherein the protective layer (37) is formed between the structured surface (25a) and the reflective coating (36).

6. A reflective optical element according to any one of claims 1 to 5, wherein a cap layer (38) is applied to said reflective coating (36).

7. 7. A reflective optical element according to claim 6, wherein the protective layer (37) forms the cap layer completely covering the structured surface (25a).

8. A reflective optical element according to any one of claims 1 to 7, wherein the structured surface (25a) is a functional layer (25) applied to a substrate (30) and / or formed on the substrate (30).

9. In the reflective optical element according to claim 8, the functional layer (25) and / or the substrate (30) are made of a metal from the group of amorphous silicon (aSi), silicon (Si), nickel phosphorus (Ni:P), in particular titanium (Ti), platinum (Pt), gold (Au), aluminum (Al), nickel (Ni), copper (Cu), silver (Ag), tantalum (Ta), tungsten (W) and alloys thereof, in particular silicon dioxide (SiO ２ ), aluminum oxide (AlO ｘ ), titanium oxide (TiO ｘ ), tantalum oxide (TaO ｘ ), niobium oxide (NbO ｘ ), zirconium oxide (ZrO ｘ ), and combinations thereof, in particular mixed oxides, ceramics, glasses, glass-ceramics, composite materials.

10. A reflecting optical element according to any one of claims 1 to 9, wherein the protective layer (37) is made of a metal, in particular from the group of copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), gold (Au), tungsten (W) and alloys thereof, in particular aluminum oxide (AlO ｘ ), zirconium oxide (ZrO ｘ ), titanium oxide (TiO ｘ ), niobium oxide (NbO ｘ ), tantalum oxide (TaO ｘ ), hafnium oxide (HfO ｘ ), chromium oxide (CrO ｘ ) oxides, carbides, carbonitrides, borides, nitrides, silicides, and combinations thereof, in particular mixed oxides, ceramics, glasses, glass-ceramics, composite materials.

11. An illumination optical unit (4) of a projection exposure apparatus (1), comprising at least one reflective optical element (17) according to any one of claims 1 to 10.

12. 12. A projection exposure apparatus (1) for microlithography, comprising: an illumination optical unit (4) according to claim 11 for transmitting illumination radiation (16) from a radiation source (3) to a reticle (7) containing a structure to be imaged; and a projection optical unit (10) for imaging the structure of the reticle (7) onto a wafer (13).

13. A method for forming a protective layer (37) on a reflective optical element (17) according to any one of claims 1 to 10, comprising the steps of: applying said protective layer (37) to said structured surface (25a) or to said reflective coating (36) by an isotropic coating method; A method comprising:

14. 14. The method according to claim 13, wherein the isotropic coating method is selected from the group comprising chemical vapor deposition, in particular atomic layer deposition, or physical vapor deposition.

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