Optical element including at least one smoothing layer and optical arrangement

Abstract

The invention relates to an optical element (1) comprising: a substrate (2), and an optical coating (3) for reflection of radiation (4) in the UV or VIS wavelength region, having a plurality of layers (6, 7) that are successive from the direction of the substrate (2) in the direction of a free surface (5) of the optical coating (3) and have increasing layer roughness (12) with increasing distance from the substrate (2). At least one smoothing layer (9) which is disposed within the coating (3) in addition to the plurality of layers (6, 7) has lower layer roughness (12) than one of the layers (6, 7). The invention also relates to an optical arrangement comprising: at least one optical element (1) as described further up.

Description

[0001] Optical element including at least one smoothing layer and optical arrangement

[0002] Reference to related

[0003] This application claims the priority of the German patent application DE102024206143.3 of July 1 , 2024, the entire disclosure of which is incorporated into this application by reference.

[0004] Background of the invention

[0005] The invention relates to an optical element, in particular a mirror, comprising: a substrate, and an optical coating, preferably for reflection of radiation, in particular for reflection of radiation in the UV or VIS wavelength region, having a plurality of layers that are successive from the direction of the substrate in the direction of a free surface of the optical coating and have increasing layer roughness with increasing distance from the substrate. The invention also relates to an optical arrangement, preferably a laser or a semiconductor technology system, in particular a UV lithography system or a wafer inspection system, that includes at least one such optical element.

[0006] Optical coatings of optical elements can be used for different purposes. For example, the optical coating of an optical element can serve as an antireflection coating that serves to prevent reflections of the incident radiation. The optical coating can also be used as an optical filter, for example as an edge filter, as used for example in spectacle lenses, or as a partly reflective coating. It is assumed hereinafter that the optical coating serves to reflect incident radiation, particularly in the UV or VIS wavelength region, but this is not absolutely necessary. Optical elements for the reflection of radiation in the UV wavelength region, in particular in the form of mirrors, are used, for example, in semiconductor technology. The UV wavelength region encompasses wavelengths between 100 nm and 380 nm, one region of which is the VUV wavelength region with wavelengths between 100 nm and 200 nm (VUV wavelength region according to DIN 5031 Part 7). The VIS wavelength region encompasses wavelengths between 380 nm and 780 nm. The coating of the optical element described herein may be designed in particular for reflection of radiation in the VUV wavelength region.

[0007] Typically, the coating takes the form of a highly reflective coating for reflection of radiation at a used wavelength in the UV or VUV or in the VIS wavelength region. In this case, the optical element is typically a mirror having a reflectivity of more than 80%, generally more than 90%, for radiation at a used wavelength in the UV wavelength region, especially in the VUV wavelength region, or in the VIS wavelength region.

[0008] The layers of such coatings can be applied or deposited on the substrate in different ways. If the layers are deposited by means of coating techniques that work with low-energy thermal material particles, for example with the aid of thermal evaporation or electron beam evaporation, the deposited layers generally have elevated layer roughness compared to other coatings or to the substrate surface, and this leads to a non-negligible loss of reflection. This problem occurs especially in the case of thick single layers and especially in the case of coatings that have a multitude of thick single layers or of pairs of layers. Possible consequences that have been observed are, for example, losses in maximum achievable reflectivity owing to elevated scattered light and a reduction in lifetime owing to porous layer growth. In a very simple growth model, layer roughness increases in a linear manner with the number of periods of pairs of layers or with the number of pairs of layers that are present between the substrate and the surface of the respective layer. Layer roughness thus depends not only on the layer material and the mode of deposition but also on the position of the respective layer in the coating. The loss of reflectivity of the coating owing to scattered light, on the other hand, increases with the square of layer roughness.

[0009] In order to avoid problems with layer roughness, it is also possible in principle to use other coating techniques that work with higher-energy material particles, for example with particles at energies of more than about 5 eV, for example magnetron sputtering or ion beam sputtering. However, fluoridic materials, i.e. materials containing fluorine, cannot currently be deposited by sputtering without simultaneously accepting severe losses in the quality of the deposited layers. Although some approaches in this regard exist for the material MgF2 (see, for example, the article "Effect of sputtering power on MgF2 thin films deposited by sputtering technique under fluorine trapping", R. De et al., AIP Conference Proceedings 1731 , 080078 (2016) or the article "Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al", J. Larruquert et al., Optics Communications, Vol. 215 Issues 1-3, pp. 93-99 (2003)), the coating technique described therein is not yet mature.

[0010] In principle, layers can also be deposited using epitaxial growth, but this typically takes place at temperatures just below the decomposition temperature of the material to be grown. This melting or sublimation temperature is more than about 1000°C for most materials that are suitable for use in coating and is thus greater than generally allowed by the coating systems in current use. A further problem with the use of such high temperatures is the dimensional stability of the substrate, in particular the dimensional stability of the surface on which the coating is applied. In the case of typical substrate materials, for example CaF2, at such high temperatures, there can be possibly severe deformations of the shape or finish of the substrate surface. In the worst case, the substrate can become unusable for the application. Object of the invention

[0011] It is an object of the invention to reduce the roughness of a coating of an optical element and to provide an optical arrangement comprising such an optical element.

[0012] Subject-matter of the invention

[0013] This object is achieved by an optical element of the type specified at the outset, in which at least one smoothing layer disposed within the coating in addition to the plurality of layers has lower layer roughness than one of the layers. The expression "within the coating" means that the smoothing layer is not at the clear surface, i.e. at the interface of the coating to the environment, or at the interface of the coating to the substrate. Instead, the smoothing layer is between two layers of the plurality of layers that have greater layer roughness. The smoothing layer is typically formed from a different material from the plurality of layers that have increasing layer roughness with increasing distance from the substrate. The introducing of the smoothing layer composed of the different material generally does not result in any (appreciable) loss of reflectivity as a result of the design. The smoothing layer thus helps to reduce the loss of reflectivity that arises through the roughening without itself causing a loss of reflectivity that would additionally have to be recovered.

[0014] In one embodiment, the smoothing layer is applied by sputtering and / or at a higher temperature than the plurality of layers. Sputtering, in particular ion beam sputtering or magnetron sputtering, can result in deposition of layers having comparatively low roughness. Another option may be to deposit the smoothing layer by an epitaxial deposition process, for example MOCVD (organometallic chemical vapour deposition). It is generally impossible or impracticable to reprocess the smoothing layer by polishing to reduce the layer roughness of the smoothing layer and the subsequent layers. It will be apparent that other coating processes that use higher-energy material particles for the deposition and therefore enable non-roughening deposition can also be used for deposition of the smoothing layer. Alternatively or additionally to applying of the smoothing layer by sputtering, the smoothing layer can be deposited at a higher temperature or at higher temperatures than the plurality of layers. Deposition at the higher temperature can result in deposition of the smoothing layer with lower roughness compared to the plurality of layers. The material of the smoothing layer here may match the material of the plurality of layers or of one of the plurality of layers.

[0015] By contrast, the plurality of layers which have increasing surface roughness with increasing distance from the substrate and which increase the layer roughness of the coating are generally deposited by a coating method that uses low- energy material particles. The coating method may be, for example, thermal evaporation or electron beam evaporation. The plurality of layers having increasing layer roughness with increasing distance from the substrate may be formed, for example, from fluoridic materials, since fluoridic materials generally cannot be deposited by sputtering or by comparable processes without considerable losses in quality.

[0016] It is possible that the material of the at least one smoothing layer is a fluoridic material which is applied, for example, at a higher temperature than the plurality of the other fluoridic layers and therefore has a lower layer roughness than the plurality of the layers. In this case too, the fluoridic material may match the material of one of the plurality of layers having a layer roughness that increases with increasing distance from the substrate.

[0017] Alternatively, the fluoridic material of the smoothing layer differs from the materials of the plurality of layers that are generally likewise, but need not necessarily be, fluoridic materials. The fluoridic material of the smoothing layer may, for example, be AIF3 or YbFs. It has been found that AIF3 can be deposited with low roughness compared to other fluoridic materials. The same is true of YbFs, the latter having higher absorption at a used wavelength in the VUV wavelength range than AIF3.

[0018] In a further embodiment, the plurality of layers comprises first layers and second layers, where the first layers comprise a first material and the second layers comprise a second material having a refractive index for the radiation that is smaller than the refractive index of the first material for the radiation. In particular, highly reflective coatings for optical elements for reflection of radiation generally have multiple repetitions of double layers or pairs of layers of a material with a very high refractive index and a partner made of a material with a small refractive index, where the refractive index means the real part of the refractive index. Such a coating with pairs of layers of materials with higher and lower refractive index enables reflection of radiation based on interference effects. The difference in the refractive index and the number of periods of the pairs of layers determine the final reflectivity of the coating, with occurrence of saturation of maximum reflectivity over and above a certain number of periods of the pairs of layers. An antireflective coating generally also has multiple repetitions of pairs of layers composed of two different materials.

[0019] In a further embodiment, the first material and / or the second material is / are a fluoridic material which is preferably selected from the group comprising: AIF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3. The fluoridic materials listed here, and others that form a compound of fluorine with at least one chemical element selected from the group comprising: Al, Ba, Ca, Dy, Er, Eu, Gd, La, Li, Mg, Na, Nd, Sm, Y, Yb, have a comparatively low absorption coefficient in the VUV wavelength region, in particular at a wavelength of about 193 nm or 170 nm, and can therefore be used as materials for a reflective, partly reflective, transmitting or partly transmitting coating in the VUV wavelength region.

[0020] In general, the first fluorid ic material and the second fluoridic material selected are two materials that have a maximum difference in refractive index. The first material with the high refractive index is preferably LaFs, and the second material with the smaller refractive index MgF2, but of course other material combinations are also possible. In particular, the first material may be LaFs, the second material MgF2, and the material of the smoothing layer AIF3.

[0021] In a further embodiment, the material of the at least one smoothing layer is an oxidic material which is preferably selected from the group comprising: SiC>2, AI2O3, TiO2, ZrO, HfO2, Ta2Os. SiO2 in particular has been found to be a favourable material for the smoothing layer since it has a very low absorption coefficient in the VUV wavelength region which is only slightly greater than the absorption coefficient of most fluoridic materials that are typically used for production of the coating. Although AI2O3 has a greater absorption coefficient, AI2O3 can also be used as oxidic material in the smoothing layer. The other oxidic materials, i.e. TiCh, ZrO, HfCh and Ta2Os, can also be used as material for the smoothing layer.

[0022] The inventors have recognized that not only fluoridic materials are suitable for the reflection of radiation in the UV wavelength region, especially in the VUV wavelength region, but it is also possible to use particular oxidic materials that have a suitable refractive index and a comparatively small absorption coefficient. Accordingly, in the case of reflective coatings that must necessarily contain fluoridic materials in order to ensure their optical functionality, the additional single oxidic layer(s) may be coated in a non-roughening way or this / these may reduce the existing roughness of the coating, such that the formation of layer roughness recommences from the smoothing layer with a lower layer roughness. In coatings in which solely fluoridic materials are used for optical functionality, there are no such oxidic layers that can be deposited with a low roughness. If there are no relevant opposing reasons in respect of the lifetime of the optical element, single oxidic layers in the form of smoothing layers can also be introduced in the case of such coatings. In this case, the oxidic smoothing layer(s) should have little influence, ideally no influence, on the optical functionality of the coating. Disadvantages that theoretically occur as a result of the additional oxidic smoothing layer, for example a slight reduction in reflectivity, can be more than compensated for or surpassed by the improvement in the layer roughness of the coating.

[0023] In an alternative embodiment, the material of the at least one smoothing layer is a nitridic material which is preferably selected from the group comprising: SisN4, GaN, AIN. Nitridic materials can also be deposited with a low layer roughness, for example by sputtering. Owing to their comparatively high absorption, smoothing layers made of nitridic materials generally cannot however be used in reflective coatings for the UV or VUV wavelength region. But smoothing layers made of nitridic materials can be used in the case of reflective coatings, for example for the VIS wavelength region, that have a plurality of layers having layer roughness that increases with increasing distance from the substrate. These layers may be fluoridic or, if necessary, oxidic materials, provided that the latter are deposited not by sputtering but, for example, by thermal evaporation.

[0024] In a further embodiment, the smoothing layer has a physical thickness of more than 2 nm, preferably of more than 10 nm and / or of not more than 100 nm. For sufficient smoothing of the coating by the smoothing layer, a minimum thickness of the smoothing layer is typically required, depending on the type of material used and the roughness of the layer to be smoothed. The same applies to the maximum thickness of the smoothing layer. If a large number of smoothing layers are used, the respective smoothing layers typically are of comparatively low thickness.

[0025] In a further embodiment, the coating comprises a sequence of groups of layers or periods that preferably each have the same optical thickness. The optical thickness of the groups of layers is typically an integer multiple of half the used wavelength of the radiation that is to be reflected by the coating or for which the coating has a maximum reflectivity. The coating with a sequence of groups of layers having the same optical thickness is typically a reflective coating. It will be apparent that the coating does not necessarily have to have a periodic sequence of layers. This is generally not the case, for example, when the coating is designed as a partly reflective coating.

[0026] The identical optical thickness of the groups of layers is usually half the used wavelength A, i.e. the thickness of the groups of layers is typically A / 2. The groups of layers having the same optical thickness are generally adjacent to each other, such that the sequence of the groups of layers is periodic. This is advantageous in order to create a maximum reflectivity of the coating. The expression "same optical thickness" means that the groups of layers ideally have exactly the same optical thickness. It will be apparent that, for production- related reasons, there may unintentionally be slight variances between the optical thicknesses of the groups of layers.

[0027] In one development of this embodiment the sequence includes groups of layers that have no smoothing layer, where the groups of layers that have no smoothing layer preferably take the form of pairs of layers each consisting of one of the first layers and one of the second layers, where the first layer and the second layer of a respective pair of layers in particular have the same optical thickness. By virtue of the periodic arrangement or sequence of pairs of layers or double layers, the high reflectivity of the coating can be generated for the used wavelength, which is typically more than 90%, 95% or more than 97% for the used wavelength. For this purpose, a number of pairs of layers is usually required that is in the order of about 10 pairs of layers, of about 20 pairs of layers or more. As described further up, the reflectivity of the optical element cannot be increased over and above a particular number of pairs of layers, which is typically in the order of about 40-50 pairs of layers. The two layers of a respective pair of layers generally have the same optical thickness, which is typically A / 4, but this is not absolutely necessary, i.e. the optical thicknesses of the two layers may differ - in particular slightly - from one another without impairing the maximum reflectivity.

[0028] In another development, the sequence of groups of layers includes at least one group of layers including the smoothing layer. The at least one group of layers including the smoothing layer is arranged within the sequence of pairs of layers and enables smoothing, which reduces the layer roughness for all groups of layers that follow on from or are deposited on the group of layers including the smoothing layer, for example in the form of the pairs of layers described further up, and also for the interfaces between these layers. Because of the identical thickness of the group of layers that includes the smoothing layer and of the other groups of layers that do not include a smoothing layer, the periodicity of the sequence of groups of layers is maintained.

[0029] As described further up, the smoothing layer should contribute only slightly, if at all, to the optical functionality of the coating. It is therefore a feature of the coating described here that the coating, when the smoothing layer is omitted, or more specifically when the group of layers including the smoothing layer is omitted - apart from the consequences of increased roughness - essentially has the same optical performance. In one development of this embodiment, there is at least a number of two groups of layers, preferably at least a number of five groups of layers, more preferably at least a number of ten groups of layers, in particular at least a number of fifteen groups of layers, that do not include a smoothing layer between the substrate and the at least one group of layers including the smoothing layer and / or between a clear surface of the coating and the at least one group of layers.

[0030] As described further up, assuming a simple growth model for layer growth, roughness increases in a linear manner with the number of groups of layers or pairs of layers, and loss of reflectivity increases with the square of the number of pairs of layers or groups of layers when there is no smoothing layer. Depending on the extent of the increase in roughness with the number of pairs of layers, it is therefore favourable when a group of layers including a smoothing layer is applied after a certain number of, for example 5, 10 or optionally 15, groups of layers, in particular in the form of pairs of layers, that are applied to the substrate. Accordingly, it is favourable when a number of, for example, 5, 10, 15 or more, groups of layers or pairs of layers that do not include a smoothing layer are arranged between the group of layers including the smoothing layer and the clear surface of the coating toward the environment.

[0031] In one development of this embodiment, the sequence of groups of layers comprises exactly one group of layers including a smoothing layer, where a number of groups of layers that do not include a smoothing layer and are disposed between the group of layers including the smoothing layer and the substrate differs by less than four groups of layers, preferably by less than two groups of layers, from a number of groups of layers that do not include a smoothing layer and are disposed between the group of layers including the smoothing layer and the clear surface. In particular, the number of groups of layers that do not include a smoothing layer and are arranged between the group of layers including the smoothing layer and the substrate may correspond to the number of groups of layers that do not include a smoothing layer and are arranged between the group of layers including the smoothing layer and the clear surface.

[0032] If the coating has only a single group of layers with a smoothing layer, it is generally favourable when this group of layers is disposed roughly in the middle of the sequence of groups of layers. Assuming that the smoothing layer restores the original roughness, in this case, the part-stacks of the sequence that are arranged between the substrate and the group of layers including the smoothing layer and between the group of layers including the smoothing layer and the clear surface or the interface to the environment have not more than half of the roughness that otherwise occurs in each case. The loss of reflectivity owing to the roughness is therefore reduced to about a quarter of the loss of reflectivity that occurs without the smoothing layer.

[0033] In a further development, the sequence of groups of layers comprises at least two groups of layers including a smoothing layer, where there is at least a number of two groups of layers, preferably at least a number of five groups of layers, more preferably at least a number of ten groups of layers, in particular at least a number of fifteen groups of layers, that do not include a smoothing layer between two groups of layers that are adjacent in the sequence and include the smoothing layer. The use of two or more additional groups of layers with a smoothing layer is advisable, for example, if the increase in roughness is too fast, such that a single smoothing layer, typically disposed essentially in the middle of the sequence of groups of layers (see above), does not allow a sufficient reduction in roughness, or when the reduction in roughness is not possible in one step, i.e. with the aid of a single smoothing layer.

[0034] In one embodiment, the group of layers including the smoothing layer forms a further pair of layers which, in addition to the smoothing layer, comprises either a first layer or a second layer, where the smoothing layer and the first layer or the second layer preferably have the same optical thickness. In this case, typically in addition to the pairs of layers that generate the reflectivity of the coating, at least one additional pair of layers that includes the smoothing layer is added. In the further pair of layers, either the first layer or the second layer is replaced by the smoothing layer, while the respective other layer of the pair is retained. The thickness of the smoothing layer typically corresponds here to the thickness of the replaced first layer or the replaced second layer. Typically, the smoothing layer has the same thickness as the layer that is replaced by the smoothing layer. If one of the two layers is replaced by an oxidic material with a small absorption coefficient, the maximum reflectivity to be achieved is virtually unchanged and, depending on the oxidic material used, only the spectral width of the coating is somewhat altered.

[0035] In an alternative development, the or a respective group of layers including the smoothing layer consists of the smoothing layer, i.e. the group of layers is formed as a single layer in this case. It is also possible that one or more groups of layers or one or more periods in the sequence of groups of layers take the form of a smoothing layer. It has been found that, in this case too, the maximum reflectivity of the coating remains virtually unchanged in the case of an oxidic material in the smoothing layer that has a small absorption coefficient, for example in the case of S iC>2, and only the spectral width of the reflectivity curve is somewhat reduced.

[0036] If an oxidic material with a greater absorption coefficient is used as material for the smoothing layer, it is favourable, when the latter or the group of layers including the smoothing layer is disposed not too far from the substrate in the sequence, since the majority of the reflection of the incident radiation is effected in the pairs of layers disposed adjacent to the clear surface of the coating that forms the interface to the environment. Accordingly, the electrical field of the stationary wave formed on reflection of the radiation in the coating is greater there than in pairs of layers near the substrate. It is therefore favourable when, in this case, the group of layers including the smoothing layer is in the middle of the sequence of the groups of layers or between the middle of the sequence of the groups of layers and the substrate.

[0037] In a further development, the at least one group of layers including the smoothing layer comprises the smoothing layer and one of the first layers and one of the second layers, where the smoothing layer is disposed between the first layer and the second layer, where the first layer is closer to the substrate than the second layer, and where the smoothing layer preferably has an optical thickness corresponding to less than 25%, in particular less than 15%, of the optical thickness of the group of layers including the smoothing layer.

[0038] In particular, if the oxidic material of the smoothing layer has a comparatively large absorption coefficient, it is favourable when the smoothing layer(s) in the coating is / are in positions where the higher absorption has the least possible effect, which is the case in positions where the electrical field of the reflected radiation has a minimum.

[0039] The inventors have recognized that, in the coating described here, the minima of the electrical field at the interfaces occur at the transition from the first layer with the higher refractive index to the second layer with the lower refractive index when the first layer is closer to the substrate than the second layer. The maxima of the electrical field, by contrast, occur at the interfaces between the second layer and the first layer when the second layer is closer to the substrate than the first layer. It is therefore favourable when the smoothing layer is disposed between the first layer and the second layer of a respective group of layers, the optical layer thicknesses of which are adjusted accordingly in order to assure a uniform thickness of the groups of layers. In order to reduce absorption, the smoothing layer in the respective group of layers has a comparatively small optical thickness, which is typically in the order of magnitude of less than about 25% or less than about 15% of the optical thickness of the group of layers.

[0040] It is possible that all groups of layers in the sequence of groups of layers have the structure described here including the smoothing layer, and also the first and second layer. In order to reduce the loss of reflectivity resulting from the oxidic smoothing layer, it is alternatively possible for only some groups of layers to have the structure described here including the smoothing layer and including the first and second layer, while other groups of layers may take the form, for example, of pairs of layers that include only the first and second layer or consist solely of the first and second layer. For example, it may be the case that only every Nth group of layers of the sequence has a smoothing layer, where N denotes an integer other than one, meaning that: N= 2, 3, 4, 5, ... .

[0041] Alternatively or additionally, the adjustment of the optical thicknesses of the two layers of a respective group of layers may be unsymmetric, i.e. is possible to deviate from an identical thickness of the first and the second layers in the respective group of layers including the smoothing layer. For example, only the optical thickness of one of the two layers can be reduced by the optical thickness of the smoothing layer, while the optical thickness of the other layer is unchanged. In this way, in addition to a - correctable - spectral shift, it is possible to achieve the effect of no significant change in the maximum achievable reflectivity or the spectral width of the coating by the smoothing layer.

[0042] In principle, thin barrier layers may optionally be disposed between the first and second layers of the coating, which are intended to serve to prevent diffusion of the materials at the interface between the two layers. Such barrier layers generally exist in each period or group of layers, but they do not generally contribute to smoothing, i.e. they do not reduce the layer roughness of the coating. In a further embodiment, an electrical field of a standing wave in the region of at least the one smoothing layer that forms on reflection of radiation at a used wavelength in the UV wavelength region has a minimum. The used wavelength is typically that wavelength at which the coating has its maximum reflectivity, or the used wavelength differs only slightly from that wavelength.

[0043] Irrespective of the design of the coating, it is generally favourable for reduction of absorption when the oxidic smoothing layer is at a position within the coating where the electrical field of the standing wave that forms has a minimum, as described, for example, in DE102018211499A1 . This describes a reflective optical element for the VUV wavelength region, which comprises a substrate and a metal layer. The optical element has at least one metal fluoride layer on the side of the metal layer that faces away from the substrate, and at least one oxide layer on the side thereof that faces away from the substrate. The electrical field of a stationary wave that forms on reflection should have a minimum in the region of the at least one oxide layer.

[0044] A further aspect of the invention relates to an optical arrangement, preferably a semiconductor lithography system, more preferably a UV lithography system, in particular a VUV lithography system or a wafer inspection system, comprising at least one optical element as described further up. The semiconductor lithography system may be a UV lithography system or a VUV lithography system, i.e. a projection exposure system for exposing a wafer, or part of such a projection exposure system, for example an illumination system or a projection lens. The semiconductor lithography system may also be an inspection system, for example for inspection of masks, wafers or the like that are used in semiconductor lithography. The optical arrangement may also be a laser, for example an excimer laser for a wavelength of 193 nm or 248 nm, or another optical arrangement which is possibly not used in semiconductor technology. Further features and advantages of the invention will be apparent from the description of working examples of the invention that follows, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be implemented individually or collectively in any combination in a variant of the invention.

[0045] Drawing

[0046] Working examples are shown in the schematic drawing and are elucidated in the description that follows. The figures show:

[0047] Fig. 1 a a schematic diagram of an optical element in the form of a mirror having a sequence of groups of layers in the form of pairs of layers with identical optical thickness, each consisting of two fluorid ic layers,

[0048] Fig. 1 b,c schematic diagrams of the mirror from Fig. 1 a that has an additional pair of layers including an oxidic smoothing layer and including one of the two fluorid ic layers from Fig. 1 a,

[0049] Fig. 2 a schematic diagram of the refractive indices and the absorption coefficients of various fluoridic and oxidic layer materials for a wavelength of 193 nm,

[0050] Fig. 3a, b schematic diagrams of the reflectivity of the mirror of Fig. 1 a-c as a function of wavelength,

[0051] Fig. 4a, b schematic diagrams of the reflectivity of a mirror analogously to Fig. 1a-c, which has pairs of layers of oxidic layers and a nitridic smoothing layer, Fig. 5a, b schematic diagrams of the reflectivity of a mirror analogously to Fig. 1a-c, which has pairs of layers of fluoridic layers and a fluoridic smoothing layer,

[0052] Fig. 6a, b a schematic diagram of the mirror from Fig. 1a and a mirror having an additional group of layers in the form of an oxidic smoothing layer,

[0053] Fig. 7a, b schematic diagrams of the reflectivity of the mirror of Fig. 5a, b as a function of wavelength with SiCh as smoothing layer material,

[0054] Fig. 8a, b schematic diagrams analogously to Fig. 7a, b with AI2O3 as smoothing layer material,

[0055] Fig. 9a, b schematic diagrams analogously to Fig. 7a, b with AIF3 as smoothing layer material,

[0056] Fig. 10a, b schematic diagrams analogously to Fig. 7a, b with YbFs as smoothing layer material,

[0057] Fig. 11a,b a schematic diagram of the mirror from Fig. 1a and a mirror having two additional groups of layers in the form of oxidic or fluoridic smoothing layers,

[0058] Fig. 12a,b schematic diagrams of the reflectivity of the mirror of Fig. 11a,b as a function of wavelength with SiCh or with AI2O3 as the smoothing layer material,

[0059] Fig. 13a,b schematic diagrams of the reflectivity of the mirror of Fig. 11a,b as a function of wavelength with AIF3 or with YbFs as the smoothing layer material, Fig. 14a,b a schematic diagram of a mirror having a coating, which, like the mirror of Fig. 11 b, has two additional groups of layers in the form of smoothing layers, and the layer roughness of the layers of the coating,

[0060] Fig. 15 a schematic diagram of the electrical field of a standing wave of the radiation reflected at the coating as a function of the layer thickness of the coating,

[0061] Fig. 16a,b schematic diagrams of the reflectivity of a mirror, the coating of which has a sequence of groups of layers each having two fluoridic layers of the same thickness and an oxidic smoothing layer, as a function of wavelength,

[0062] Fig. 17a,b schematic diagrams analogously to Fig. 16a,b, in which the fluoridic layers have a different thickness,

[0063] Fig. 18 a schematic diagram of a semiconductor technology system in the form of a VUV lithography system,

[0064] Fig. 19 a schematic diagram of a semiconductor technology system in the form of a wafer inspection system, and

[0065] Fig. 20 a schematic diagram of a detail of a laser with an end mirror.

[0066] In the description of the drawings that follows, identical reference symbols are used for identical or functionally identical components.

[0067] Fig. 1a-c show, in highly schematic form, an optical element in the form of a mirror 1 having a substrate 2 and a coating 3. The coating 3 is designed for the reflection of radiation 4 in the VUV wavelength region, more specifically at a used wavelength A of 193 nm. For all the considerations that follow, it is assumed that the radiation 4, by contrast to what is shown in Fig. 1 a-c, is incident on the coating 3 at an angle of 0° to the surface normal, more specifically on the clear surface 5 of the coating 4 that forms the interface to the surroundings of the mirror 1 . It will be apparent that the radiation 4 is not reflected at the surface 5 of the coating 3 as shown in Fig. 1 a-c, but penetrates into the coating 3, where it forms a standing wave, as described in detail further down.

[0068] In the mirror 1 shown in Fig. 1a, the coating 3 has a plurality of first fluoridic layers 6 and second fluoridic layers 7 that collectively form a group of layers in the form of a pair of layers 8. In the example shown, the first layers 6 have a first fluoridic material having a first refractive index which is greater for the used wavelength A of the radiation to be reflected 4 than a second refractive index of the second fluoridic material of the second layers 7. The sequence of the two layers 6, 7 in the respective pair of layers 8 may also be reversed, i.e. in place of the first layer 6, the second layer 7 can be positioned closer to the substrate 2. In the example shown, the pairs of layers 8 of the coating 3 have the same optical thickness, which is A / 2. The first layer 6 and the second layer 7 of a respective pair of layers 8 have the same thickness in the example shown, which is A / 4 in the example shown.

[0069] In the examples shown in Fig. 1 a-c, the first fluoridic material of the first layers 6 is LaFs and the second fluoridic material of the second layers 7 is MgF2. It will be apparent that the fluoridic materials of the two layers 6, 7 may also be other materials, for example the further fluoridic materials AIF3, LiF, CaF2, YF3, YbFs, GdFs, CeFs, SmFs, NdFs that are shown in Fig. 2. A respective pair of coating materials for the coating 3 is selected on the basis of the refractive index, more specifically the difference between the refractive indices, of the two fluoridic materials of the layers 6, 7 and on the basis of the absorption coefficient k of the respective material in order to generate maximum reflectivity of the mirror 1 .

[0070] In the mirror 1 shown in Fig. 1a, the fluoridic layers 6, 7 were deposited by electron beam evaporation or by thermal evaporation, as a result of which the coating 3 has high roughness. The roughness of the applied fluoridic layers 6, 7 increases in a linear manner with increasing layer thickness of the coating 3, on the basis of a simple growth model. The reduction in reflectivity owing to scattered light which is caused by the roughness of the layers 6, 7 increases with the square of the roughness.

[0071] Fig. 1 b,c show a mirror 1 in which roughness is reduced by positioning of a smoothing layer 9 within the coating 3 that consists of an oxidic material. In the example shown, the oxidic material is SiC>2, which has a comparatively low absorption coefficient k (see Fig. 2). The smoothing layer 9 of SiCh is applied in the example shown by sputtering, more specifically by magnetron sputtering, and has comparatively low roughness that ideally corresponds to the original roughness of the substrate 2, more specifically to that of the surface 2a of the substrate 2 to which the coating 3 has been applied, or is less than the original roughness of the substrate 2.

[0072] In the example shown, the physical thickness d of the smoothing layer 9 results from the condition that the optical thickness of the smoothing layer 9 is A / 4. The physical thickness dPhySis determined from the optical thickness dopt, taking account of the refraction index n of SiCh of 1.56 at the used wavelength A of 193 nm, and is at a vertical incidence of the radiation 4 at about 31 nm (dPhys= dopt I n). It will be apparent that the physical thickness d of the smoothing layer 9 can also assume other values that are typically in the order of magnitude between about 2 nm or 10 nm and about 100 nm. The smoothing layer 9 should have as little influence as possible on the optical functionality of the mirror 1. This is achieved in the example shown in Fig. 1 b,c in that the smoothing layer 9 is included in a further group of layers 10 with an optical thickness of A / 2, such that the periodicity of the coating 3, more specifically the sequence of groups of layers 8, 10, is not disrupted by the further group of layers 10.

[0073] The further group of layers 10 including the smoothing layer 9, in the examples shown in Fig. 1 b,c, is disposed in the middle of the coating 3 or the sequence of pairs of layers 8, meaning that the same number of pairs of layers 8 are disposed between the substrate 2 and the group of layers 10 including the smoothing layer 9 as are disposed between the group of layers 10 including the smoothing layer 9 and the clear surface 5 that forms the interface to the environment. As a result of the positioning of the group of layers 10 including the smoothing layer 9 in the middle of the coating 3, the latter has two partstacks of pairs of layers 8 that each correspond to half the layer thickness of the coating 3 of Fig. 1 a. The coating 3 from Fig. 1 b,c therefore has only a maximum of half of the roughness of Fig. 1 a when the smoothing layer 9 restores the original roughness. The loss of reflectivity owing to the roughness of the layers 6, 7 of the coating 3 from Fig. 1 b, c is therefore reduced compared to the coating 3 from Fig. 1 a to about a quarter.

[0074] In the example shown in Fig. 1 b, the further group of layers 10, in addition to the smoothing layer 9, has a second fluoridic layer 7 of MgF2. The smoothing layer 9 of SiO2 replaces the first fluoridic layer 6 of the pairs of layers 8 that do not include a smoothing layer 9, and has an optical thickness of A / 4.

[0075] In the example shown in Fig. 1 c, the further group of layers 10, in addition to the smoothing layer 9, has a first fluoridic layer 6 of LaFs. In this case, the smoothing layer 9 of SiO2 replaces the second fluoridic layer 7 of the pairs of layers 8 that do not include a smoothing layer 9, and likewise has an optical thickness of A / 4.

[0076] Fig. 3a, b show the reflectivity of the coating 3 for the three cases described in Fig. 1a-c as a function of the wavelength. The reflectivity curves shown in Fig. 3a, b and the reflectivity curves shown in the further figures are based on a numerical simulation in which additional losses owing to absorption and transmission were neglected. The simulation is based on a sequence of twenty- four pairs of layers 8, of which only sixteen pairs of layers 8 are shown in Fig. 1 a-c for reasons of clarity. The loss of reflectivity owing to the roughness of layers 6, 7 and the reduction thereof by the smoothing layer 9 were not taken into account in the numerical simulation. The numerical simulation assumed idealized smooth layers, i.e. the reflectivity curves shown correspond to the maximum achievable reflectivity without loss of reflectivity resulting from the layer roughness.

[0077] As apparent in Fig. 3b, the addition of the additional group of layers 10 including the smoothing layer 9 does not cause any loss of reflectivity compared to the coating 3 shown in Fig. 1a, the reflectivity curve of which is shown in Fig. 3a, b by a solid line. The reflectivity curve of the coating 3 shown in Fig. 1 b is represented by a narrow-dashed line; the reflectivity curve of the coating 3 shown in Fig. 1c is represented by a broad-dashed line. As apparent in Fig. 3a, on replacement of MgF2 with SiC>2, only the spectral width is somewhat reduced, since there is lower optical contrast to LaFs.

[0078] Fig. 4a, b show the reflectivity of a mirror 1 which is formed as shown in Fig. 1a- c, but which is designed for reflection of radiation 4 not in the VUV wavelength region but in the VIS wavelength region at a used wavelength A of 500 nm. In this mirror 1 , the first layers 6 consist of an oxidic material, more specifically TiO2, and the second layers 7 likewise consist of an oxidic material, more specifically HfCh. In the example shown in Fig. 1a-c, the smoothing layer 9 is formed not from an oxidic material but from a n itridic material, namely from SisN4. As apparent in Fig. 4a, b, the replacing of a layer 6 of TiCh or of a layer 7 of HfO2 by the smoothing layer 9 of SisN4 results not in a decrease in reflectivity of the mirror 1 but a slight increase. The spectral bandwidth of the mirror 1 changes only slightly.

[0079] Fig. 5a, b show the reflectivity of a mirror 1 which is formed as shown in Fig. 1a- c, i.e. the mirror 1 has first layers 6 of LaFs and second layers 7 of MgF2. The mirror 1 is designed for reflection of radiation 4 in the VUV wavelength region at a used wavelength A of 170 nm. In the example shown here, the smoothing layer 9 is formed from a fluoridic material, namely from AIF3. The substrate 3 used is CaF2.

[0080] As apparent in Fig. 5a, b, the replacing of a layer 6 of LaFs or of a layer 7 of MgF2 by the smoothing layer 9 of AIF3 results in virtually no decrease in reflectivity of the mirror 1. The spectral bandwidth of the mirror 1 decreases slightly when the first layer 6 of LaFs is replaced by the smoothing layer 9 of AIF3. When the second layer 7 of MgF2 is replaced by the smoothing layer 9, the width of the reflectivity curve, by contrast, remains virtually constant.

[0081] The layer structure of the mirror 1 in the case of replacement of one of the second layers 7 of MgF2 by AIF3 in the example of Fig. 5a, b is as follows: (LaF3 / MgF2)A12 / LaF3 / AIF3 / (LaF3 / MgF2)A12 / LaF3 / CaF2substrate. The layer structure of the mirror 1 on replacement of one of the first layers 6 of LaF2 by AIF3 in the example of Fig. 5a, b is as follows: (LaF3 / MgF2)A13 1 AIF31 MgF21 (LaFs I MgF2)A11 I LaFs I CaF2 substrate. The optical thicknesses of the respective layers are A / 4.

[0082] Fig. 6a once again shows the mirror 1 with the coating 3 from Fig. 1 a; Fig. 6b shows the mirror 1 with a coating 3 in which an additional group of layers 10 has been inserted, consisting of a single smoothing layer 9. The smoothing layer 9, like the sequence of pairs of layers 8, has an optical thickness of A / 2. As apparent in Fig. 7a, b, the maximum reflectivity is virtually unchanged by the additional smoothing layer 9; there is merely a slight reduction in the spectral width of the reflectivity curve.

[0083] The fact that the smoothing layer 9 has virtually no effect on the reflectivity of the mirror 1 is attributable to the fact that this oxidic material has a very low absorption coefficient k (see Fig. 2), and so the smoothing layer 9 is virtually invisible. If AI2O3 is used as oxidic material in place of SiC>2, this results in the reflectivity curves shown in Fig. 8a, b for the example shown in Fig. 6a, b. In addition to a further reduction in spectral width, a reduction in reflectivity is also apparent at least to some extent. This reduction is only so small because there is only a single smoothing layer 9 of AI2O3 in the coating 3 and this is positioned in the middle of the coating 3. Since the majority of the reflection is generated at the pairs of layers 8 disposed close to the clear surface 5 that forms the interface to the environment, the effect on the reflectivity increases with the closeness of the smoothing layer 9 of AI2O3 to the clear surface 5 of the coating 3.

[0084] Fig. 9a, b show a diagram of a mirror 1 analogous to Fig. 6a, b in which the smoothing layer 9 has an optical thickness of A / 2 and is formed of AIF3. As apparent in Fig. 9a, b, this slightly reduces the spectral width of the reflectivity curve, while maximum reflectivity remains virtually constant. The layer structure of the mirror 1 in the example of Fig. 9a, b is as follows: (LaFs / MgF2)A12 I LaFs I AIF3A2 / MgF2 / (LaF3 / MgF2)A11 / LaF3 / CaF2substrate.

[0085] Fig. 10a, b show a diagram of a mirror 1 analogous to Fig. 6a, b, in which the smoothing layer 9 has an optical thickness of A / 2 and is formed from YbFs. As apparent in Fig. 10a, b, this likewise only slightly reduces the spectral width of the reflectivity curve, while maximum reflectivity remains virtually constant. The higher absorption of YbFs compared to AIF3 has only a minor effect on reflectivity because the smoothing layer 9 is in the middle of the coating 3. The layer structure of the mirror 1 in the example of Fig. 10a, b is as follows: (LaF3 / MgF2)A12 / LaF3 / YbF3A2 / MgF2 / (LaF3 / MgF2)A11 / LaF3 / CaF2substrate. The optical thicknesses of the respective layers are A / 4.

[0086] Fig. 11a, b show a diagram of a mirror 1 analogously to Fig. 6a, b, in which, in addition to the smoothing layer 9 in the middle of the coating 3, a further smoothing layer 9a is disposed within the coating 3, which is in the region of about 3 / 4 of the total layer thickness of the coating 3 proceeding from the substrate 2. The second smoothing layer 9a likewise has an optical thickness of A / 2. Fig. 12a,b show the reflectivity curve for the two cases that the smoothing layers 9, 9a are formed from SiO2or from AI2O3. As is apparent, the second smoothing layer 9a of SiO2is still virtually invisible in terms of maximum reflectivity, while two smoothing layers 9, 9a of AI2O3lead to a distinct drop in reflectivity in addition to the reduction in spectral bandwidth. If smoothing layers 9, 9a of SiO2are used in the coating 3, the arrangement thereof in the coating 3 plays a minor role with regard to the decrease in reflectivity.

[0087] Fig. 13a, b show a diagram of the mirror 1 analogously to Fig. 11 a,b, in which the two smoothing layers 9, 9a are formed from AIF3or from YbF3. As is apparent, the second smoothing layer 9a of AIF3is still virtually invisible in terms of maximum reflectivity, while two smoothing layers 9, 9a of YbF3lead to a distinct drop in reflectivity in addition to the reduction in spectral bandwidth. If YbF3is used as material for the smoothing layer 9, 9a, this should be positioned in the lower half of the coating 3. The positioning of the smoothing layer 9, 9a of AIF3in the coating 3, on the other hand, plays a minor role with regard to the decrease in reflectivity.

[0088] The layer structure of the mirror 1 in the example of Fig. 13a,b when AIF3is used for the smoothing layers 9, 9a is as follows: (LaF3 / MgF2)A6 1 LaF31 AIF3A2 I MgF2 / (LaF3ZMgF2)A5 / LaF3 / AIF3A2 / MgF2 / (LaF3 / MgF2)A11 / LaF3 / CaF2 substrate. The layer structure of the mirror 1 in the example of Fig. 13a,b when YbF3is used for the smoothing layers 9, 9a is as follows: (LaF3 / MgF2)A6 1 LaF3I YbF3A2 / MgF2 / (LaF3 / MgF2)A5 / LaF3 / YbF3A2 / MgF2 / (LaF3 / MgF2)A11 / LaF31 CaF2substrate. The optical thicknesses of the respective layers are A / 4.

[0089] If only one smoothing layer 9 is used in the coating 3, it is favourable when this is positioned roughly in the middle of the coating 3, since there is a reduction in roughness in this way for half of the total number of the pairs of layers 8 in each case. It may be advantageous when at least a number of two, five, ten or fifteen pairs of layers 8 is disposed between the substrate 3 and the smoothing layer 9. The distance between the substrate 3 and the smoothing layer 9 depends on the extent of the smoothing effect of the smoothing layer 9. The number of pairs of layers 8 that should be disposed between the smoothing layer 9 and the substrate 3 also depends on the extent of the increase in roughness as a function of the layer thickness of the coating 3. If two or more smoothing layers 9, 9a, ... are disposed within the coating 3, it may be favourable when at least a number of two, five, ten or fifteen groups of layers 8 that do not include a smoothing layer 9, 9a, ... are disposed between two adjacent groups of layers 9, 9a in the sequence.

[0090] Because of the problems described further up, it is favourable particularly in the case of a smoothing layer 9 of an oxidic material with a comparatively large absorption coefficient when the latter is not too close to the clear surface 5 that forms the interface to the environment. In particular, there should be a number of at least two, five, ten or fifteen pairs of layers 8 that do not include a smoothing layer 9 between the smoothing layer 9 and the clear surface 5.

[0091] Fig. 14a,b show the layer roughness 12 of layers 6, 7, 9, 9a of a coating 3 of a mirror 1 , which, like the mirror 1 shown in Fig. 11 b, has two smoothing layers 9, 9a. In contrast to the mirror 1 of Fig. 11 b, the thickness of the first layers 6 of LaF3of a respective pair of layers 8 differs from the thickness of the second layers 7 of MgF2 of a respective pair of layers 8, where the respective pair of layers 8 has a total optical thickness of A / 2. The two smoothing layers 9, 9a replace, as in the mirror of Fig. 1c, the second layer 7 of a respective pair of layers 8.

[0092] The diagram of Fig. 14a shows the coating 3 with the physical thicknesses of the layers 6, 7, 9, 9a. Fig. 14a shows, on the right, alongside layers 6, 7, 9, 9a of the coating 3, the respective layer roughness 12, i.e. the roughness on the respective layer 6, 7, 9, 9a. Fig. 14b shows the layer roughness depending on the number of the respective layer in the layer sequence proceeding from the substrate 2 in arbitrary units. The layer roughness of Fig. 10b can be measured, for example, in the form of the square roughness Rq(rms value) or in other ways.

[0093] As apparent in Fig. 14a,b, layer roughness 12, proceeding from the substrate 2, increases in an essentially linear manner with the number of layers 6, 7 up to the first smoothing layer 9. The first smoothing layer 9 reduces layer roughness 12 virtually to the roughness value at the surface 2a of the substrate 2. After the first smoothing layer 9, layer roughness 12 increases with the number of layers 6, 7, again in an essentially linear manner, up to the second smoothing layer 9a which again reduces the layer roughness 12. The layer roughness 12 then increases again in an essentially linear manner proceeding from the second smoothing layer 9a up to the interface surface 5 to the environment. The layer roughness 12 of the coating 3 at the interface 5 to the environment is therefore significantly lower in the case of the mirror 1 shown in Fig. 14a than would be the case with a coating 3 without smoothing layers 9, 9a.

[0094] If the oxidic material of the smoothing layer 9 has a comparatively large absorption coefficient, the absorption is less disruptive when an electrical field, more specifically an electrical field strength, of a standing wave 11 formed on reflection of radiation 4 at the used wavelength A, as shown in Fig. 15, has a minimum in the region of the smoothing layer 9. The minimum of the electrical field strength at which it is equal to zero is in the region of the interface between the first fluoridic layer 6 and the second fluoridic layer 7 of the respective pair of layers 10, shown by dashed lines in Fig. 11 . Fig. 15 shows each of the two fluoridic layers 6, 7 with a layer thickness of A / 4, with the substrate 2 being on the left and the surface 5 of the coating 3 being on the right in Fig. 15. The oxidic smoothing layer 9, which is indicated in Fig. 15 by an arrow, is disposed between the first layer 6 and the second layer 7 of a respective group of layers 10, with a reduction in the optical thickness of the two layers 6, 7 by the corresponding amount.

[0095] Table 1 below shows the layer thicknesses of the fluoridic layers 6, 7 of LaFs and of MgF2 and the oxidic smoothing layer 9 of AI2O3 for three versions of the coating 3 which in each case has twenty-four groups of layers 10 having the first fluoridic layer 6, the second fluoridic layer 7, and the oxidic smoothing layer

[0096] 9 of AI2O3 in between. The smoothing layer 9 has an optical thickness of less than 25% for version 2 and 3, and less than 15% for version 1 , of the optical thickness A / 2 of the group of layers 10 including the smoothing layer 9.

[0097]

[0098] Table 1

[0099] Fig. 16a,b show the reflectivity curve in the case that the coating 3 is formed according to version 1 or version 2; "standard" in Fig. 14a,b refers to the case that the groups of layers 10 have only the first fluoridic layer 6 and the second fluoridic layer 7 each with an optical layer thickness of A / 4. As apparent from the reflectivity curves shown, the spectral width of reflectivity is reduced only minimally by the smoothing layer 9, but the decrease in reflectivity depends on the layer thickness used in the smoothing layer 9. In order to reduce the loss of reflectivity, the number of smoothing layers 9 can be reduced; for example, only every Nth group of layers 10 may be provided with a smoothing layer 9, while the other groups of layers 8 have only the two fluoridic layers 6, 7, but no smoothing layer 9.

[0100] Alternatively or additionally, the adjustment of the optical thicknesses of the two fluoridic layers 6, 7 may be made unsymmetric, as is the case for version 3 in Table 1 above. As apparent from the reflectivity curves shown in Fig. 17a,b, in this case, aside from a correctable spectral shift, no significant change in the maximum achievable reflectivity or spectral width of the reflectivity curve of the mirror 1 is apparent. Similar results are obtained when smoothing layers 9 of AI2O3 are replaced by fluoridic smoothing layers 9 of AIF3. It will be apparent that the options described further up for reducing reflectivity loss can also be combined in one and the same coating 3 in order to obtain an optical element 1 or a mirror having maximum reflectivity, ideally of greater than 95% or 97%. It will be apparent that the coating 3 or the optical element 1 can also be designed for used wavelengths other than 193 nm in that the optical thicknesses and optionally the layer materials are adjusted correspondingly, as described in connection with Fig. 4a, b. The optical element 1 described further up that includes the at least one pair of layers 9 can be used in different optical systems, for example in semiconductor technology systems.

[0101] Fig. 18 shows a semiconductor technology system for the VUV wavelength region in the form of a VUV lithography apparatus 21. The VUV lithography apparatus 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 additionally has a radiation source 24, which may be an excimer laser for example.

[0102] The radiation 25 emitted by the radiation source 24 is conditioned with the aid of the illumination system 22 such that a mask 26, also called a reticle, is illuminated thereby. In the example shown, the illumination system 22 has a housing 32, in which there are disposed both transmissive and reflective optical elements. In a representative manner, the illustration shows a transmissive optical element 27, which focuses the radiation 25, and a reflective optical element 28, which deflects the radiation.

[0103] The mask 26 has, on its surface, a structure which is transferred to an optical element 29 to be exposed, for example a wafer, with the aid of the projection system 23 for the purpose of producing semiconductor components. In the example shown, the mask 26 is designed as transmissive optical element. In alternative embodiments, the mask 26 may also be designed as reflective optical element. The projection system 22 comprises at least one transmissive optical element in the example illustrated. The example shown illustrates, in a representative manner, two transmissive optical elements 30, 31 , which serve, for example, to reduce the structures on the mask 26 to the size desired for the exposure of the wafer 29.

[0104] Both in the illumination system 22 and in the projection system 23, a wide variety of transmissive, reflective or other optical elements can be combined with one another as desired, including in a more complex manner. Optical assemblies without transmissive optical elements can also be used for VUV lithography.

[0105] Fig. 19 shows a semiconductor technology system for the VUV wavelength region in the form of a wafer inspection system 41 , but this may also be a mask inspection system. The wafer inspection system 41 has an optical system 42 with a radiation source 54, from which radiation 55 is directed onto a wafer 49 by means of the optical system 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In the case of a mask inspection system, it would be possible to replace the wafer 49 with a mask to be examined. The radiation reflected, diffracted and / or refracted by the wafer 49 is directed onto a detector 50 for further evaluation by a further concave mirror 48, which is likewise associated with the optical system 42, via a transmissive optical element 47. The wafer inspection system 41 additionally has a housing 52, in which there are disposed the two mirrors 46, 48 and the transmissive optical element 47. The radiation source 54 may, for example, be exactly one radiation source or a combination of a plurality of individual radiation sources in order to provide a substantially continuous radiation spectrum. In modifications, it is also possible to use one or more narrowband radiation sources 54.

[0106] At least the optical element 28 of the VUV lithography apparatus 21 shown in Fig. 14 and at least one of the optical elements 46, 48 of the wafer inspection system 41 shown in Fig. 15 are designed here like the optical element 1 of Fig.

[0107] 1 b,c of Fig. 6b, of Fig. 11 b or as in Fig. 15 and have low layer roughness and high reflectivity. Fig. 20 shows an optical element for reflection of radiation in the VUV wavelength region in the form of an end mirror 60 of a laser chamber 61 of a resonator of an excimer laser 62. The laser light emitted by the excimer laser 62 is reflected at the end mirror 60, more specifically at a reflective coating 3 of the end mirror 60, which is applied to a substrate 2. The excimer laser 62 may, for example, form the radiation source 24 of the VUV lithography apparatus 21 of Fig. 18.

Claims

Claims1 . Optical element (1 ) comprising: a substrate (2), and an optical coating (3) for reflection of radiation (4) in the UV or VIS wavelength region, having a plurality of layers (6, 7) that are successive from the direction of the substrate (2) in the direction of a clear surface (5) of the optical coating (3) and have increasing layer roughness (12) with increasing distance from the substrate (2), characterized in that at least one smoothing layer (9, 9a) disposed within the coating (3) in addition to the plurality of layers (6, 7) has lower layer roughness (12) than one of the layers (6, 7), where the material of the at least one smoothing layer (9, 9a) is a fluoridic material.

2. Optical element according to Claim 1 , wherein the at least one smoothing layer (9, 9a) is applied by sputtering and / or at a higher temperature than the plurality of layers (6, 7).

3. Optical element according to Claim 1 or 2, in which the fluoridic material of the smoothing layer (9, 9a) is selected from the group comprising: AIF3, YbF3.

4. Optical element according to any of the preceding claims, in which the plurality of layers (6, 7) comprises first layers (6) and second layers (7), where the first layers (6) comprise a first material and the second layers (7) comprise a second material having a refractive index for the radiation (4) that is smaller than the refractive index of the first material for the radiation (4).

5. Optical element according to Claim 4, in which the first material and / or the second material is / are a fluoridic material.

6. Optical element according to Claim 5, in which the first material and / or the second material is selected from the group comprising: AIF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3.

7. Optical element according to Claim 6, in which the first material is LaF3, in which the second material is MgF2and in which the fluoridic material of the smoothing layer (9, 9a) is AIF3.

8. Optical element according to any of the preceding claims, in which the smoothing layer (9) has a physical thickness (d) of more than 2 nm, preferably of more than 10 nm, and / or of not more than 100 nm.

9. Optical element according to any of the preceding claims, in which the coating (3) comprises a sequence of groups of layers (8, 10, 10a) that preferably each have the same optical thickness (A / 2).

10. Optical element according to Claim 9, in which the sequence includes groups of layers (8) that have no smoothing layer (9, 9a), where the groups of layers preferably take the form of pairs of layers (8) each consisting of one of the first layers (6) and one of the second layers (7), where the first layer (6) and the second layer (7) of a respective pair of layers (8) in particular have the same optical thickness (A / 4).11 . Optical element according to Claim 9 or 10, in which the sequence of groups of layers (8, 10, 10a) includes at least one group of layers (10, 10a) with a smoothing layer (9, 9a).

12. Optical element according to Claim 11 , in which there is at least a number of two groups of layers (8), preferably at least a number of five groups of layers (8), more preferably at least a number of ten groups of layers (8), in particular at least a number of fifteen groups of layers (8), that do not include a smoothing layer (9, 9a) between the substrate (3) and the at least one group of layers (10, 10a) including the smoothing layer (3, 9a) and / or between a clear surface (5) of the coating (9) and the at least one group of layers (9, 10a) including the smoothing layer (9, 10a) .

13. Optical element according to Claim 11 or 12, in which the sequence of groups of layers (8, 10) comprises exactly one group of layers (10) including a smoothing layer (9), where a number of groups of layers (8) that do not include a smoothing layer (3) and are disposed between the group of layers (8) including the smoothing layer (9) and the substrate (9) differs by less than four groups of layers (8), preferably by less than two groups of layers (8), from a number of groups of layers (8) that do not include a smoothing layer (9) and are disposed between the group of layers (8) including the smoothing layer (9) and the clear surface (5).

14. Optical element according to Claim 11 or 12, in which the sequence of groups of layers (8, 10, 10a) comprises at least two groups of layers (10, 10a) including a smoothing layer (8, 9a), where there is at least a number of two groups of layers (9), preferably at least a number of five groups of layers (8), more preferably at least a number of ten groups of layers (8), in particular at least a number of fifteen groups of layers (8), that do not include a smoothing layer (9, 9a) between two groups of layers (10, 10a) that are adjacent in the sequence and include the smoothing layer (9, 9a).

15. Optical element according to any of Claims 11 to 14, in which the group of layers including the smoothing layer (9) forms a pair of layers (10) which, in addition to the smoothing layer (9), comprises either a first layer (6) or asecond layer (7), where the smoothing layer (9) and the first layer (6) or the second layer (7) preferably have the same optical thickness (A / 4).

16. Optical element according to any of Claims 11 to 15, in which the group of layers (10, 10a) including the smoothing layer (9, 9a) consists of the smoothing layer (9, 9a).

17. Optical element according to any of Claims 11 to 16, in which the at least one group of layers (10) including the smoothing layer (9) comprises the smoothing layer (9) and one of the first layers (6) and one of the second layers (7), where the smoothing layer (9) is disposed between the first layer (6) and the second layer (7), where the first layer (6) is closer to the substrate (7) than the second layer (7), and where the smoothing layer (9) preferably has an optical thickness corresponding to less than 25%, in particular less than 15%, of the optical thickness (A / 2) of the group of layers (10) including the smoothing layer (9).

18. Optical element according to any of the preceding claims, in which an electrical field of a standing wave (11 ) in the region of at least the one smoothing layer (9) that forms on reflection of radiation (4) at a used wavelength (A) in the UV wavelength region has a minimum.

19. Optical arrangement, preferably laser (62) or semiconductor technology system, in particular UV lithography system (21 ) or wafer inspection system (41 ), comprising: at least one optical element (1 , 60, 46, 48, 28) according to any of the preceding claims.

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