Optical element having a coating, optical arrangement and method of producing an optical element

Abstract

The invention relates to an optical element (1) comprising: a substrate (2), and a coating (3) which is partly reflective or antireflective to radiation (4), preferably to radiation in the UV or VIS wavelength region, wherein the coating (3) has a plurality of first layers (6) and second layers (7), wherein the first layers (6) include a first, preferably fluoridic or oxidic material and the second layers (7) include a second, preferably fluoridic or oxidic material, the refractive index (n2) of which for the for radiation (4) differs from the refractive index (n1) of the first material for the radiation (4). The coating (3) has at least one group of layers (8) for reflectivity matching, which comprises a third layer (9) of a third material which does not correspond to the first material and the second material. The invention also relates to an optical arrangement, preferably a laser or a semiconductor technology system, comprising: at least one optical element (1) as described further up, and to a method of producing an optical element (1).

Description

[0001] Stuttgart, 30.06.2025 SZ00388PCT Rp / pt

[0002] Optical element having a coating, optical arrangement and method of producing an optical element

[0003] Cross-Reference to related application

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

[0005] 102024208564.2, filed September 9, 2024, the entire disclosure of which is considered part of and is incorporated by reference in the disclosure of this application.

[0006] Background of the invention

[0007] The invention relates to an optical element comprising: a substrate, and a coating which is partly reflective or antireflective to radiation, preferably to radiation in the UV or VIS wavelength region, in particular to radiation in the VUV wavelength region, wherein the coating has a plurality of first layers and second layers, wherein the first layers include a first, preferably fluoridic or oxidic material and the second layers include a second, preferably fluoridic or oxidic material, the refractive index of which for the radiation differs from the refractive index of the first material for the radiation. 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. The invention further relates to a method of producing an optical element, comprising: applying a coating which is partly reflective or antireflective to radiation, preferably to radiation in the UV or VIS wavelength region, to a substrate of the optical element, where the coating has a plurality of first layers and second layers, where the first layers include a first, preferably fluoridic or oxidic material and the second layers include a second, preferably fluoridic or oxidic material, the refractive index of which for

[0008] 2023P00550WO 30.06.25 SZ00388PCT the for radiation differs from the refractive index of the first material for the radiation.

[0009] Optical elements having a partly reflective or antireflective coating to radiation in the UV wavelength region, for example in the form of beam splitters or transmissive optical elements, 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.

[0010] In the context of this application, a partly reflective coating means a coating which has a maximum reflectivity of not more than 95%, typically not more than 80%, 70% or 60%, for the incident radiation and transmits the remaining incident radiation (without taking account of losses). A partly reflective coating is therefore simultaneously also a partly transmitting coating. While a partly reflective coating has a maximum reflectivity for the incident radiation which is greater than the "natural" reflectivity of the material of the substrate, which is about 4% in each case, for example, for quartz glass in the VIS wavelength region and for CaF2 at 193 nm, reflectivity for the incident radiation in the case of an antireflective coating is less than the "natural" reflectivity of the substrate and is ideally 0%.

[0011] In order to ensure the functionality of optical elements, it is typically necessary to provide them with a suitable partly reflective or antireflective coating. The aim of this coating is, for example, an only slightly fluctuating (variable) reflectivity for a certain range of angles of incidence (with simultaneously constant wavelength) or for a particular wavelength range (with simultaneously constant angle of incidence).

[0012] 2023P00550WO 30.06.25 SZ00388PCT The requirements regarding the progression of reflectivity as a function of wavelength or angle of incidence and possibly stability over the lifetime can generally be met by choice of suitable layer materials and combination thereof with appropriate layer thicknesses and the number of individual layers used.

[0013] In order to meet the requirement with regard to suitability for production, care must be taken to ensure that the layer design is as "stable" as possible: Small single-thickness defects that can occur on application or deposition of the coating must have only a minimal effect on the optical performance of the coating. The stability of optical designs of coatings is generally inversely proportional to the number of individual layers they contain and the variation in the layer thicknesses of a material from layer to layer, and proportional to the closeness of the design to the "A / 4-layer design", meaning minimum deviation of the optical thickness of the individual layers from A / 4, where A denotes the used wavelength or centre of the spectral width of the reflectivity curve.

[0014] However, the degree to which requirements with regard to design stability and hence also suitability for manufacturing are met is inversely proportional to the spectral or angular width of the coating. It is also frequently necessary to deviate from a "A / 4-layer design" in order to create a (spectral or angular) plateau with a defined value of reflectivity.

[0015] Object of the invention

[0016] It is an object of the invention to provide an optical element, a semiconductor technology system comprising such an optical element and a method of producing an optical element, which have a coating that enables fine tuning of reflectivity without significantly affecting the stability of the optical design.

[0017] Subject-matter of the invention

[0018] 2023P00550WO 30.06.25 SZ00388PCT This object is achieved by an optical element of the type specified at the outset, in which the coating has at least one group of layers for reflectivity matching, which comprises a third layer of a third material that does not correspond to the first and the second material.

[0019] The inventors have recognized that it is favourable for meeting of the requirements on the coating, in particular for provision of a layer design of maximum stability with respect to single layer thickness defects, when reflectivity is matched not by exclusively matching the optical thicknesses of the layers of the first material and the second material, but when the coating includes a third material which is distinctly different in terms of optical properties, in particular with regard to refractive index, from the materials of the first and second layers. In this way, the absolute value of the desired broadband reflectivity curve of the partly reflective or antireflective coating can typically be established, meaning that fine tuning of reflectivity, in particular maximum reflectivity, is possible without requiring excessively significant deviation from the stable "A / 4-layer design" and hence an efficiently producible coating. The group(s) of layers can also be used to achieve alterations or matching of reflectivity of less than 0.5%, which is not readily possible by a change in the number of pairs of layers I periods of the layers of the first and second material, especially in the case of coatings having a small number of layers (see below).

[0020] In one embodiment, the coating has not more than three, preferably not more than two, groups of layers for reflectivity matching. As described further up, the aim should be minimum deviation from a stable layer design of the coating. Increasing the number of groups of layers for reflectivity matching leads to a more complex manufacturing process, since more individual layers have to be produced. The effect of the reflectivity matching of a second, third or further group of layers is also greatly reduced compared to the effect of the first group of layers. For that reason, generally more than three, typically more than two, groups of layers for reflectivity matching are not technically feasible.

[0021] 2023P00550WO 30.06.25 SZ00388PCT It should be pointed out here that the third material of the respective group of layers is not a barrier layer intended to prevent mixing of the neighbouring layers: barrier layers would have to be used at all equivalent interfaces; moreover, these should not exceed an optical thickness of about 0.1 to 0.2 x A / 4. The third layer, on the other hand, may have a greater optical thickness.

[0022] In a further embodiment, the first layers, the second layers and preferably the group of layers for reflectivity matching have the same optical thickness, which is typically A / 4, where A denotes the used wavelength of the incident radiation. As described further up, it is possible in this way to create a design of the coating that has high robustness to relatively small layer thickness defects. In this case, the optical thickness of the group of layers for reflectivity matching corresponds to the optical thickness of one of the first layers or one of the second layers.

[0023] In a further embodiment, the group of layers for reflectivity matching forms a pair of layers comprising the third layer and either a layer of the first material or a layer of the second material. The pair of layers typically has an optical thickness of A / 4. The coating generally has a sequence of alternating first and second layers, and the pair of layers for reflectivity matching replaces one of the layers of the sequence of alternating first and second layers. The effect of the group of layers or pair of layers on the reflectivity of the coating depends upon factors including the position of the pair of layers in the overall structure of the coating. An optical thickness of the third layer is established here depending on the desired reflectivity matching; the effect of the third layer on the reflectivity of the coating increases with the optical thickness of the third layer. The effect of the third layer on the reflectivity of the coating is an increase or decrease in the reflectivity of the coating, with only a slight change in the spectral or angledependent progression of the reflectivity of the coating. By sharing the optical thickness of a first or a second layer between a pair of layers, it is possible to

[0024] 2023P00550WO 30.06.25 SZ00388PCT continuously adjust the reflectivity of the coating within a certain range, depending on the ratio of the distribution. The sequence of the third layer and the layer of the first or second material in the pair of layers does affect the reflectivity or the reflectivity curve, but does not reverse the direction of the effect, i.e. whether the effect is a decrease or an increase in reflectivity is not dependent on the sequence of the layers in the pair of layers. It is possible that the optical thickness of the third layer and the layer of the first or second material are each about half the optical thickness of the pair of layers, but this is not absolutely necessary.

[0025] In a further embodiment, the coating has at least one phase shift layer, where the phase shift layer preferably has an optical thickness other than an optical thickness of the first layers and of the second layers, and in particular corresponds to twice the optical thickness of the first layers and the second layers, where the phase shift layer is preferably formed from the first material or from the second material.

[0026] The phase shift layer typically serves, proceeding from a layer design having a sequence of first and second layers having the same optical thickness (typically A / 4) in which the coating has a marked reflectivity maximum in the region of the used wavelength, to reduce reflectivity in the region of the reflectivity maximum and / or to create a plateau of the spectral and / or angle-dependent reflectivity curve with only little variable reflectivity around the reflectivity maximum. The creation of a comparatively broad plateau in the reflectivity curve has been found to be favourable especially in the case of a partly reflective coating. In the case of an antireflective coating, the phase shift layer is generally used to create an antireflective effect, i.e. to reduce reflectivity in the region of the reflectivity maximum.

[0027] The spectral width of the plateau in which reflectivity is only slightly variable, i.e. the region of the reflectivity curve in which maximum reflectivity and minimum

[0028] 2023P00550WO 30.06.25 SZ00388PCT reflectivity do not vary from one another by more than 5%, preferably by more than 3%, depends on various factors, for example on the used wavelength, the desired maximum reflectivity, the angle of incidence, the material combination of the layers of the coating, and also the basic design (e.g. one or more phase shift layers, etc.). The maximum width of the plateau can be estimated from the number of periods I double layers or the maximum reflectivity of a coating with a A / 4 design that does not have a phase shift layer. It is fundamentally the case that the maximum spectral width of the plateau decreases as the number of periods I double layers increases or as the maximum reflectivity of the coating increases.

[0029] It has been found to be favourable when the phase shift layer is formed from the material of the first layers or from the material of the second layers. It is also favourable for the stability of the coating when the optical thickness of the phase shift layer is a whole multiple of the optical thickness of the first layers or of the second layers, for example corresponds to twice the optical thickness of the first or second layers.

[0030] In one development of this embodiment, the group of layers for reflectivity matching is disposed between the phase shift layer and the clear surface of the coating. The clear surface of the coating is the interface of the coating to the environment. As described further up, the effect of the group of layers for reflectivity matching depends on the position within the reflective coating. It may be found to be favourable when the group of layers is disposed between the phase shift layer and the clear surface of the coating for reflectivity matching, but this is not absolutely necessary. The position of the group of layers for reflectivity matching is fundamentally dependent on the optical design of the coating and should be determined individually.

[0031] In a further embodiment, the coating comprises at least one compensation layer for at least partial compensation of a change in a spectral and / or angle-

[0032] 2023P00550WO 30.06.25 SZ00388PCT dependent progression in the reflectivity of the coating by the at least one group of layers for reflectivity matching and / or by the at least one phase shift layer, where the compensation layer is preferably formed from the first material or from the second material and has an optical thickness that deviates from the optical thickness of the first layers and from the optical thickness of the second layers, where the deviation in particular is not greater than 20% of the optical thickness of the first layers and not greater than 20% of the optical thickness of the second layers. In this embodiment, the optical thickness of the group of layers for reflectivity matching typically corresponds to the optical thickness of the respective first and second layers.

[0033] The deviation of the thickness of the compensation layer is intended to compensate for or at least reduce undesirable effects of the group of layers for reflectivity matching on the spectral and / or angle-dependent progression of the reflectivity of the coating. This may be, for example, an adjustment of the inclination of a plateau of the spectral or angle-dependent reflectivity curve. Overall, a layer design that combines the group of layers or the pair of layers for reflectivity matching with the compensation layer has much better producibility compared to other layer designs that generate comparable layer performance, since only few layers deviate from A / 4 thickness. In particular, it is favourable when the deviation of the optical thickness of the compensation layer from the A / 4 thickness is not too great, typically not greater than 20% of the A / 4 thickness.

[0034] In a further embodiment, the first material, the second material and / or the third material is selected from the group comprising: AIF3, MgF2, LiFs, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3, SiO2, AI2O3, TiO2, ZrO, HfO2, Ta2O5, SisN4, GaN, AIN, InN and mixtures thereof. The fluoridic materials listed here and possibly others, and also the oxidic materials SiO2 and AI2O3, have a comparatively low absorption coefficient in the VUV wavelength region, especially at a wavelength of about 193 nm, and can therefore be used as

[0035] 2023P00550WO 30.06.25 SZ00388PCT materials for a coating for the VUV wavelength region. The first material, the second material and / or the third material need not necessarily be an oxidic or fluoridic material; instead, it is also possible to use nitridic materials or metallic materials, especially if the coating is designed not for radiation in the UV wavelength region but, for example, in the VIS wavelength region. Examples of mixtures of the materials specified further up that may be used include AIGaN or AllnGaN with variable proportions of Al, Ga and In.

[0036] The choice of layer materials is typically subject to restrictions relating to the requirement for coating stability. For example, the first material with the high refractive index may be LaFs and the second material with the smaller refractive index may be MgF2, but of course other material combinations are also possible. The third material may also be a fluoridic material, but this is not absolutely necessary.

[0037] The number of first and second layers of the coating arises from the fact that the maximum reflectivity of a coating formed exclusively from these layers with a respective optical thickness of A / 4 should have distinctly greater reflectivity than required for the partly reflective or antireflective coating. Typically, the number of first and second layers is chosen such that the maximum reflectivity of the coating formed exclusively from layers with an optical thickness of A / 4 is about 1 / 3 greater than the maximum reflectivity of the partly reflective coating.

[0038] In principle, it is advantageous when, in the case of a coating for the UV wavelength region, specifically for the VUV wavelength region, the three materials are fluoridic materials since these have a low absorption coefficient in this wavelength region.

[0039] In a further embodiment, the first material and the second material are fluoridic materials and the third material is an oxidic material which is preferably selected from the group comprising: SiO2 and AI2O3. If the coating is used for the UV

[0040] 2023P00550WO 30.06.25 SZ00388PCT wavelength region and the material used for the third layer is an oxidic material, this should have an absorption coefficient as low as possible. SiCh has been found to be a favourable oxidic material for the third layer since it has a very low absorption coefficient which is only slightly greater than the absorption coefficient of most fluoridic materials that are typically used for production of the coating if it is intended for radiation in the UV or VUV wavelength region. Although AI2O3 has a greater absorption coefficient, AI2O3 can also be used as material for the third layer.

[0041] Assuming a used wavelength of 193 nm, the refractive index for LaFs as the first material is about 1 .71 and the refractive index for MgF2 as the second material is about 1.43. The refractive index of SiO2 at a used wavelength of 193 nm is about 1.56, i.e. between the refractive index of the first material and the refractive index of the second material. The refractive index of a layer of AI2O3 at a used wavelength of 193 nm is about 1 .84 and is thus greater than the refractive index of the first material and the second material.

[0042] In a further embodiment, the coating has a number of first layers and / or of second layers which is 15 or less, preferably 10 or less, in particular 5 or less. As described further up, the group of layers for reflectivity matching is used for fine tuning of the reflectivity of the optical element. For such fine tuning, it is especially advantageous when the coating has a comparatively small number of pairs of first and second layers, as explained below:

[0043] The maximum achievable reflectivity of a coating is achieved with a periodic A / 4 design, i.e. with a design having first and second layers with an optical thickness of A / 4 each. The maximum achievable reflectivity value depends on the difference between the refractive indices of the two materials used and on the number of layers or pairs of layers. Proceeding from the maximum achievable reflectivity, the reflectivity of the coating can be altered, more specifically reduced, by omitting pairs of layers or double layers. Especially if

[0044] 2023P00550WO 30.06.25 SZ00388PCT the coating has a large number of layers with an optical thickness of A / 4 or a corresponding pair of layers, the loss of reflectivity or gain in reflectivity resulting from the omitting or the adding of a particular pair of layers is so small that, in general, no fine tuning by a group of layers is required for reflectivity matching. The number of layers below which fine tuning by the group of layers for reflectivity matching is viable can be defined, for example, as that number of layers for which the change in reflectivity AR in the event of a change An in the number of periods or of double layers by one period or double layer exceeds a threshold of 0.5%. If the coating has a greater number of layers / periods than that number of layers, the omitting or the adding of a period can create a change in reflectivity of less than 0.5%, which may be sufficient for fine tuning of reflectivity. The number of layers of the coating for which the above-specified threshold is attained depends upon factors including the layer materials used.

[0045] In a further embodiment, the coating is antireflective to the radiation and the optical element has reflectivity at at least one wavelength of less than 1 .0%, preferably of less than 0.5%, in particular of less than 0.2%. With the aid of the at least one group of layers for reflectivity matching, it is possible to undertake fine tuning of the reflectivity of an antireflective coating in order to reduce its reflectivity virtually to zero for a wavelength that typically corresponds to the used wavelength. In the case of an antireflective coating having only two layer materials, such a reduction is impossible without compromising the stability of the layer design.

[0046] A further aspect of the invention relates to an optical arrangement, preferably a laser or 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

[0047] 2023P00550WO 30.06.25 SZ00388PCT 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 element may, for example, be a beam splitter having a partly reflective coating, or a transmitting optical element having an antireflective coating. It will be apparent that the optical element can also be used in other optical systems. 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.

[0048] There are fundamentally no restrictions with regard to the wavelength of the radiation that hits the partly reflective or antireflective coating. However, if layers of materials with a high absorption coefficient are used, a nominally reflectivityenhancing design, for example, may also have a contrary effect. This may be the case, for example, if the used wavelength in the EUV wavelength region is between about 5 nm and about 30 nm, since most materials have a high absorption coefficient for radiation in this wavelength region.

[0049] A further aspect of the invention relates to a method of the type specified at the outset for producing an optical element, in which, on application of the partly reflective or antireflective coating, at least one group of layers for reflectivity matching is applied, comprising a third layer of a third material which does not correspond to the first material and the second material. The application of the partly reflective coating is preferably preceded by determination of an optical thickness of the third layer depending on a desired reflectivity match. Alternatively or additionally, the group of layers for reflectivity matching preferably forms a pair of layers including the third layer and a further layer either of the first material or of the second material.

[0050] As described further up, the extent of the reflectivity matching, i.e. the change in the reflectivity of the optical element, depends on the optical thickness of the

[0051] 2023P00550WO 30.06.25 SZ00388PCT third layer of the group of layers for reflectivity matching. The change in relativity typically increases with the optical thickness of the third layer. When a pair of layers consisting of the third layer and the further layer of the first or second material is used, the optical thickness of the further layer decreases with increasing optical thickness of the third layer. The thickness of the third layer may be selected, for example, such that the reflectivity plateau described further up has a desired value or in such a way that, in the case of an anti reflective coating, a reflectivity of virtually zero is achieved.

[0052] A variant of the method comprises: defining a starting design of the partly reflective coating, having a preferably alternating sequence of the first layers and the second layers, selecting at least one of the first layers or the second layers of the starting design for reflectivity matching of the coating, replacing the selected first or second layer of the starting design with the group of layers for reflectivity matching, and applying the coating with the starting design modified by the at least one group of layers for reflectivity matching.

[0053] In a further variant, on application of the coating, at least one compensation layer for at least partial compensation of a change in a spectral and / or angledependent progression in the reflectivity of the coating by the at least one group of layers for reflectivity matching and / or by at least one phase shift layer is applied, where the compensation layer preferably has an optical thickness that deviates from the optical thickness of the first layers and the second layers, where the deviation in particular is not greater than 20% of the optical thickness of the first layers and the second layers, and where the compensation layer is preferably formed from the first material or from the second material.

[0054] The coating applied to the substrate with the modified starting design may be formed in particular as in the case of the above-described optical element. The coating with the modified starting design may, for example, have a phase shift

[0055] 2023P00550WO 30.06.25 SZ00388PCT layer which reduces the maximum reflectivity of the optical element and generates a reflectivity curve with a plateau of reflectivity.

[0056] 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 together in any combination in a variant of the invention.

[0057] Drawing

[0058] Working examples are shown in the schematic drawing and are elucidated in the description that follows. The figures show:

[0059] Fig. 1 a a schematic diagram of an optical element in the form of a mirror having a coating having a sequence of first and second layers having identical optical thickness,

[0060] Fig. 1 b a schematic diagram of a beam splitter in which one of the second layers is replaced by a phase shift layer,

[0061] Fig. 2 a schematic diagram of the reflectivity of the mirror of Fig. 1a and of the beam splitter of Fig. 1 b as a function of wavelength,

[0062] Fig. 3a, b schematic diagrams of the beam splitter of Fig. 1 b or of a beam splitter having multiple layers that have different optical thicknesses for reflectivity matching,

[0063] Fig. 4 a schematic diagram of the reflectivity of the beam splitter of Fig. 3a, b as a function of wavelength,

[0064] 2023P00550WO 30.06.25 SZ00388PCT Fig. 5 a schematic diagram of a beam splitter having a pair of layers with a third layer of a third material for reflectivity matching,

[0065] Fig. 6a, b schematic diagrams of the reflectivity of the beam splitter of Fig. 5 as a function of the wavelength at different optical thicknesses of the third layer,

[0066] Fig. 7 a schematic diagram of a transmissive optical element with an antireflective coating,

[0067] Fig. 8a, b schematic diagrams of the reflectivity of the transmissive optical element of Fig. 7 as a function of wavelength at different optical thicknesses of a third layer of a pair of layers for reflectivity matching,

[0068] Fig. 9 a schematic diagram of the reflectivity of a mirror as a function of the number of periods and the change in reflectivity on addition of a period,

[0069] Fig. 10a,b schematic diagrams of the maximum width of a reflectivity plateau in the optical element of Fig. 5 or of Fig. 7,

[0070] Fig. 11 a schematic diagram of a semiconductor technology system in the form of a VUV lithography system,

[0071] Fig. 12 a schematic diagram of a semiconductor technology system in the form of a wafer inspection system, and

[0072] Fig. 13 a schematic diagram of a laser chamber window.

[0073] 2023P00550WO 30.06.25 SZ00388PCT In the description of the drawings that follows, identical reference symbols are used for identical or functionally identical components.

[0074] Fig. 1a shows, in highly schematic form, an optical element in the form of a mirror 1 having a substrate 2 and a coating 3. In the example shown, 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 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 clear surface 5 of the coating 3 as shown in Fig. 1 a, but penetrates into the coating 3.

[0075] In the mirror 1 shown in Fig. 1 a, the coating 3 has a plurality of first fluoridic layers 6 and second fluoridic layers 7 in an alternating arrangement. In the example shown, the first layers 6 have a first fluoridic material having a first refractive index ni which is greater for the used wavelength A of the radiation to be reflected 4 than a second refractive index n2 of the second fluoridic material of the second layers 7. In the example shown, the layers 6, 7 of the coating 3 have the same optical thickness, which is A / 4. The layer design shown in Fig. 1 a is also referred to hereinafter as a A / 4-layer design.

[0076] In the example shown in Fig. 1 a, 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 fluoridic materials AIF3, Li F3, CaF2, YF3, YbFs, GdFs, CeFs, SmFs, NdFs. A respective pair of coating materials for the coating 3 is selected on the basis of the refractive index ni , n2, more specifically the difference ni - n2 of the refractive indices, of the two fluoridic materials of the layers 6, 7 and on the basis of the absorption coefficient of the respective material in order to generate maximum reflectivity of the mirror 1.

[0077] 2023P00550WO 30.06.25 SZ00388PCT Fig. 1b shows an optical element 1 in the form of a beam splitter, which, in contrast to Fig. 1a, reflects a first portion 4a of the radiation 4 incident on the clear surface 5 and transmits a second portion 4b of the incident radiation 4. For this purpose, the beam splitter 1 has a substrate 2 which is transparent to radiation 4 at the used wavelength A and at wavelengths around the used wavelength A and is formed, for example, from CaF2. In contrast to the beam splitter 1 of Fig. 1 b, in the case of the mirror 1 of Fig. 1a, the nonreflected portion 4b of the radiation 4 is essentially lost. The substrate 2 may also be formed from a different material, for example quartz glass, especially when the optical element 1 is designed for radiation 4 in the VIS wavelength region.

[0078] A requirement for the optical element 1 to function as a beam splitter is adjustment of the maximum reflectivity of the coating 3. It is also favourable when the coating 3 has a plateau of the spectral reflectivity curve in the region of the used wavelength A. This is achieved in that one of the second layers 7 of the A / 4 layer structure of Fig. 1a is replaced by a phase shift layer 7a, the optical thickness of which corresponds to twice the optical thickness of the first and second layers 6, 7, i.e. the phase shift layer 7a has an optical thickness of A / 2. The coating 3 of the beam splitter 1 of Fig. 1b also has a compensation layer 6', the optical thickness of which differs slightly from the optical thickness of the other layers 6, 7, which is A / 4, and which serves for fine adjustment of the curve shape of the reflectivity curve of the plateau produced by the phase shift layer 7a, more specifically to avoid a tilt of the plateau.

[0079] Table 1 below shows the sequence of the layers of the coating 3 of Fig. 1 a,b, with the layer numbered 1 disposed adjacent to the surface 2a of the substrate 2, and where the layer numbered 11 denotes the layer adjacent to the clear surface 5.

[0080] 2023P00550WO 30.06.25 SZ00388PCT

[0081] Table 1

[0082] Fig. 2 shows the reflectivity curve of the mirror of Fig. 1a as a function of wavelength with a dashed line and the reflectivity curve of the beam splitter 1 of Fig. 1 b with a solid line. It is clearly apparent that the phase shift layer 7a distinctly reduces the maximum reflectivity of the beam splitter 1 , namely by more than about 15%. Simultaneously with the reduction in maximum reflectivity, the reflectivity curve flattens out and a reflectivity plateau is formed in the area of the maximum reflectivity of the A / 4-layer design in which the reflectivity of the beam splitter 1 is essentially constant.

[0083] As can be seen in Fig. 2, the reflectivity of the beam splitter 1 in the plateau is about 52%, but this does not correspond to the desired value in the example shown. It is therefore necessary to adjust the reflectivity of the beam splitter 1 , more specifically to reduce it, in order to achieve a reflectivity closer to 50%. In order to enable this, in addition to the optical thickness of the compensation layer 6’, the optical thicknesses of three further layers 6”, 7’, 7” can be adjusted, as shown in table 2 below:

[0084] 2023P00550WO 30.06.25 SZ00388PCT

[0085] Table 2

[0086] Fig. 3a once again shows the beam splitter 1 with the layer design of the coating 3 of Fig. 1 b; Fig. 3b shows the beam splitter 1 with the matched layer design.

[0087] Fig. 4 shows the reflectivity of the beam splitter 1 of Fig. 3a with the layer thicknesses given on the left in Table 2 with a solid line, and the reflectivity of the beam splitter of Fig. 3b with the matched layer design shown on the right in Table 2 with a dotted line, each as a function of wavelength. As can be seen in Fig. 4, as well as the lowering of reflectivity, there is also a change in the spectral dependence of reflectivity, but this can be corrected by the matching of the optical thicknesses of further layers of the coating 3. In this way, the layer design of the coating 3, however, becomes further removed from the A / 4 layer design which is stable from a manufacturing point of view and hence from a readily manufacturable coating 3.

[0088] 2023P00550WO 30.06.25 SZ00388PCT This problem is additionally increased in that the sensitivities of the individual layers of the coating 3, i.e. the effect of a change in layer thickness of the spectral dependence of reflectivity, change depending on the layer design. With each additional adjustment of the optical thicknesses, this problem is aggravated and becomes appreciably less controllable. The problems described here for the partly reflective coating 3 of the beam splitter 1 also occur analogously in antireflective coatings or in (partly) transmitting coatings.

[0089] Fig. 5 shows an optical element in the form of a beam splitter 1 in which the coating 3, compared to the layer design described in association with Fig. 1 b or with Fig. 3a, has been modified in that one of the two layers 6, 7 of the coating 3 is replaced by a group of layers in the form of a pair of layers 8 which, for reflectivity matching, has a third layer 9 composed of a third material which does not correspond to the materials of the first layers 6 and the second layers 7. In the example shown, the third material is S iC>2, and the pair of layers 8 has a further layer 10 of the first material.

[0090] The pair of layers 8 has an optical thickness of A / 4, shared between the optical thickness x of the third layer 9 and the optical thickness 1-x of the further layer 10 of the pair of layers 8. It will be apparent that the pair of layers 8 may alternatively consist of the third layer 9 and a further layer of the second material.

[0091] Whether the third layer 9 reduces or increases the reflectivity of the beam splitter 1 depends upon factors including whether the refractive index ns of the third material of the third layer 9 is or is not between the refractive index ni of the first layers 6 and the refractive index n2 of the second layers 7. The issue of whether the third layer 9 reduces or increases the reflectivity of the beam splitter 1 also depends on the arrangement of the third layer 9 within the pair of layers 8 or within the coating 3, and on the material of the further layer 10 of the pair of layers 8.

[0092] 2023P00550WO 30.06.25 SZ00388PCT In the example shown in Fig. 5, the third material of the third layer 9 is Si O2, the refractive index ns of which is between the refractive index ni of LaFs as material of the first layers 6 and the refractive index n2 of MgF2 as material of the second layers 7. In the example shown, reflectivity of the coating 3 decreases by virtue of the pair of layers 8.

[0093] Table 3 below shows the layer design of the beam splitter 1 of Fig. 5 on the right-hand side. The pair of layers 8 or the double layer here replaces the seventh layer of the layer structure of Fig. 1 b, or of Fig. 3a shown on the left in Table 3. As can be seen in Table 3, the ninth layer of the layer design of the beam splitter 1 of Fig. 5 has an optical thickness that likewise differs from A / 4. As described further up, this layer serves as compensation layer 6' for at least partial compensation of the lateral inclination of the reflectivity plateau which is caused by the different dispersion of SiO2 and of LaFs. The change 5 in the optical thickness of the compensation layer 6' compared to the value described in Table 1 , where the effect of the phase shift layer 7a on the lateral inclination of the reflectivity plateau is compensated for, is in the order of magnitude of about 0.1 x A / 4.

[0094] 2023P00550WO 30.06.25 SZ00388PCT

[0095] Table 3

[0096] Fig. 6a, b show the reflectivity of the beam splitter 1 of Fig. 5 with the layer design, the layer thicknesses of which are shown in Table 3 on the right. As shown in Fig. 6a, reflectivity decreases with increasing optical thickness x of the third layer 9 of SiCh and correspondingly decreasing optical thickness 1-x of the first layer 6a of the pair of layers 8: The reflectivity curve has the greatest values when x = 0 and decreases with increasing optical thickness of the Si O2 layer 9. The form of dependence of reflectivity on wavelength changes only slightly; it is essentially a scaling, as also apparent from Fig. 6b, in which the reflectivity curve is shown over a greater wavelength range. As can be seen from Fig. 6b, the loss of spectral width of reflectivity with increasing optical thickness x of the third layer 9 is negligible.

[0097] It is possible in principle that the coating 3 of the optical element 1 of Fig. 5 has more than one pair of layers 8 for reflectivity matching. In general, the coating 3, however, should have not more than three, typically not more than two, such pairs of layers 8 since the optical effect of the second, third and further pairs of layers 8 is less than the optical effect of the first pair of layers 8, and robustness to layer thickness defects of the coating 3 decreases as the number of pairs of layers 8 for reflectivity matching increases.

[0098] 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. The coating 3 may also be designed as an antireflective coating having minimum reflectivity for the wavelength of the used radiation A. In this case, the

[0099] 2023P00550WO 30.06.25 SZ00388PCT optical element is not a beam splitter but a transmissive optical element, for example a lens or a window.

[0100] Fig. 7 shows such a transmissive optical element 1 with an antireflective coating 3 which is designed for radiation 4 at a used wavelength A of 600 nm in the VIS wavelength region. The material of the substrate 2 in the example shown is quartz glass (SiCh). The material of the first layers 6 is AI2O3; the material of the second layers 7 is SiCh. The first and second layers 6, 7 each have an optical thickness of A / 4. The coating 3 has a phase shift layer 7a of SiO2 with a thickness of A / 2 in order to generate the antireflective effect. Like the coating 3 of Fig. 5, the coating 3 of Fig. 7 has a pair of layers 8 for reflectivity matching, which consists of a third layer 9 of TiCh and of a further layer 10 of the second material, i.e. of SiC>2, and replaces the eighth layer of the layer structure.

[0101] The layer design of the coating 3 at different thicknesses x of the third layer 9 of the pair of layers 8 for reflectivity matching is shown in Table 4 below for five optical thicknesses x = 0.0 x A, 0.1 x A, 0.2 x A, 0.3 x A and 0.4 x A:

[0102] Table 4

[0103] 2023P00550WO 30.06.25 SZ00388PCT As is the case for the coating 3 of Fig. 5, the coating 3 of Fig. 7 also has a compensation layer 6' in order to compensate for an unwanted spectral shift in the reflectivity curve by the insertion of the layer 9 of the third material. As can be seen in Table 4, the compensation layer 6' has a deviation 5 from an optical thickness of A / 4, which, in the example shown, has a negative sign and does not exceed about 20% of the optical thickness of A / 4.

[0104] The reflectivity curve of the coating 3 or of the transmissive optical element 1 of Fig. 7 as a function of wavelength is shown in Fig. 8a, b. As can be seen in Fig. 8b in particular, the transmissive optical element without the third layer 9, i.e. when x = 0, has a minimum reflectivity at the used wavelength A of 600 nm of about 1 .0%. As can be seen in Fig. 8b, the increasing of the optical thickness x of the third layer 9 can lower the reflectivity to less than 1 .0% until, for a value of x = 0.4, a minimal reflectivity of the transmissive optical element 1 is attained that is virtually zero. In the case of optical thicknesses x of the third layer 9 that are greater than x = 0.4, there is again a rise in the minimal reflectivity of the transmissive optical element 1.

[0105] The reflectivity range in per cent in which the use of a pair of layers 3 having a third layer 9 of a third material for reflectivity matching is viable is limited at the lower end by 0% reflectivity, as apparent from Fig. 8a, b. In the case of coatings having a large number of layers or of pairs of layers / periods composed of first layers 6 and second layers 7, fine adjustment of reflectivity can also be effected by varying the number of periods. Fig. 9 shows, in a black curve, the maximum reflectivity of a coating 3 of the structure as shown in Fig. 1a and a periodic sequence of first layers 6 of LaFs and second layers 7 of MgFs, each of which has an optical thickness of A / 4 and which have been applied to a substrate 2 of SiO2. As apparent in Fig. 9, there is at first a significant rise in reflectivity, but the rise begins to distinctly flatten out at a number of about 10 periods. The adding of many further periods can theoretically lead to a maximum reflectivity

[0106] 2023P00550WO 30.06.25 SZ00388PCT of 98.15%. A grey curve in Fig. 9 shows the change in reflectivity on addition of the nth period, which decreases as the number of periods of the coating 3 increases and which is less than 0.1 % over and above a number of 20 periods.

[0107] The number of layers below which fine tuning by the group of layers 8 for reflectivity matching is useful can be defined, for example, as that number of periods for which the change in reflectivity AR in the event of a change An in the number of periods or of double layers by one period / double layer exceeds a threshold of 0.5%. The number of layers in the coating 3 for which the abovespecified threshold is attained depends on factors including the layer materials used, and in the example shown in Fig. 9 is 15 layers or periods. From the 15th period, the gain in reflectivity for each additional period AR I An is less than 0.5%, meaning that, in this case, fine adjustment of reflectivity can be generated by the change in the number of periods. The reflectivity of the coating 3 or mirror 1 in this case is about 97%. In general, the initial rise in reflectivity in the case of a few periods and the maximum achievable reflectivity of a mirror 1 depends on the combination of materials. There is also corresponding variation in the number of periods for which: AR / An = 0.5%. The upper limit in the reflectivity region for which the use of the pair of layers 8 for reflectivity matching is still useful is typically in the order of magnitude of about 95%.

[0108] Fig. 10a, b show an estimate of the maximum width of the reflectivity plateau which is created by the phase shift layer 7a (cf. also Fig. 2) as a function of reflectivity. The width of the plateau depends on factors including the combination of materials in the layers of the coating 3 and of the substrate 2, on the design of the coating in detail, and on the reflectivity value in the region of the plateau. The estimate was based on the assumption that the width of the plateau corresponds roughly to the width of the reflectivity curve of the coating without the plateau, i.e. the reflectivity curve of the A / 4 design. Fig. 10a shows the width of the plateau as a function of maximum reflectivity for a coating 3 which, as in Fig. 5, has LaFs and MgF2 as layer materials, and Fig. 10b for a

[0109] 2023P00550WO 30.06.25 SZ00388PCT coating 3 which, as in Fig. 7, has AI2O3 and SiCh as layer materials on an SiCh substrate for a used wavelength of 600 nm. As described further up, the reflectivity of the coating 3 depends on the number of pairs of layers in the coating 3 and increases with the number of pairs of layers or periods.

[0110] The optical element 1 described further up with the at least one pair of layers 8 for reflectivity matching can be used in different optical systems, for example in semiconductor technology systems.

[0111] Fig. 11 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.

[0112] 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.

[0113] 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.

[0114] 2023P00550WO 30.06.25 SZ00388PCT 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.

[0115] Both in the illumination system 22 and in the projection system 23, a wide variety of transmissive, partly reflective, 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.

[0116] Fig. 12 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.

[0117] At least one of the optical elements 27, 30, 31 of the VUV lithography system 21 shown in Fig. 7 and at least one optical element 47 of the wafer inspection

[0118] 2023P00550WO 30.06.25 SZ00388PCT system 41 shown in Fig. 8, like the optical element 1 of Fig. 5 and like a corresponding optical element, are formed with an antireflective coating.

[0119] Fig. 13 shows an optical element for transmitting radiation in the VUV wavelength region in the form of a laser chamber window 60 of a laser chamber 61 of an excimer laser 62. The laser beam emitted by the excimer laser 62 exits to the outside through the laser chamber window 60. The inside of the laser chamber window 60 has an antireflective coating 3 which prevents laser radiation from the excimer laser 62 from being reflected at the laser chamber window 60. The excimer laser 62 may, for example, form the radiation source 24 of the VUV lithography apparatus 21 of Fig. 7.

[0120] 2023P00550WO 30.06.25 SZ00388PCT

Claims

Claims1 . Optical element (1 ) comprising: a substrate (2), and a coating (3) which is partly reflective or antireflective to radiation (4), preferably to radiation in the UV or VIS wavelength region, where the coating (3) has a plurality of first layers (6) and second layers (7), where the first layers (6) include a first, preferably fluoridic or oxidic material and the second layers (7) include a second, preferably fluoridic or oxidic material, the refractive index (n2) of which for the radiation (4) differs from the refractive index (ni) of the first material for the radiation (4), characterized in that the coating (3) includes at least one group of layers (8) for reflectivity matching, comprising a third layer (9) of a third material which does not correspond to the first material and the second material, wherein the third material is selected from the group comprising: AIF3, MgF2, LiFs, CaF2, YF3, YbFs, GdFs, LaFs, CeFs, SmFs, NdFs, SiO2, AI2O3, TiO2, ZrO, HfO2, Ta20s, SisN4, GaN, AIN, InN and mixtures thereof.

2. Optical element according to Claim 1 , in which the coating (3) has not more than three, preferably not more than two, groups of layers (8) for reflectivity matching.

3. Optical element according to Claim 1 or 2, in which the first layers (6), the second layers (7) and preferably the group of layers (8) for reflectivity matching have the same optical thickness (A / 4).

4. Optical element according to any of the preceding claims, in which the group of layers for reflectivity matching forms a pair of layers (8) comprising the third layer (9) and a further layer (10) of the first material or of the second material.2023P00550WO 30.06.25 SZ00388PCT5. Optical element according to any of the preceding claims, in which the coating (3) has at least one phase shift layer (7a), where the phase shift layer (7a) preferably has an optical thickness (A / 2) other than an optical thickness (A / 4) of the first layers (6) and of the second layers (7), and in particular corresponds to twice the optical thickness (A / 2) of the first layers(6) and the second layers (7), where the phase shift layer (7a) is preferably formed from the first material or from the second material.

6. Optical element according to Claim 5, wherein the group of layers (8) for reflectivity matching is disposed between the phase shift layer (7a) and a clear surface (5) of the coating (3).

7. Optical element according to any of the preceding claims, in which the coating (3) comprises at least one compensation layer (6') for at least partial compensation of a change in a spectral and / or angle-dependent progression in the reflectivity of the coating (3) by the at least one group of layers (8) for reflectivity matching and / or by the at least one phase shift layer (7a), where the compensation layer (6') preferably has an optical thickness (A / 4 + / - 5) that deviates from the optical thickness (A / 4) of the first layers (6) and the second layers (7), where the deviation (5) in particular is not greater than 20% of the optical thickness (A / 4) of the first layers (6) and the second layers(7), and where the compensation layer (6') is preferably formed from the first material or the second material.

8. Optical element according to any of the preceding claims, in which the first material and / or the second material are selected from the group comprising: AIF3, MgF2, LiF3, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3, SiO2, AI2O3, TiO2, ZrO, HfO2, Ta2Os, Si3N4, GaN, AIN, InN and mixtures thereof.2023P00550WO 30.06.25 SZ00388PCT9. Optical element according to any of the preceding claims, in which the first material and the second material are fluoridic materials and the third material is an oxidic material which is preferably selected from the group comprising: SiO2 and AI2O3.

10. Optical element according to any of the preceding claims, in which the coating (3) has a number of first layers (6) and / or of second layers (7) which is 15 or less, preferably 10 or less, in particular 5 or less.11 . Optical element according to any of the preceding claims, in which the coating (3) is antireflective to the radiation (4) and the optical element (1 ) has reflectivity at at least one wavelength (A) of less than 1 .0%, preferably of less than 0.5%, in particular of less than 0.2%.

12. 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 , 28, 46, 48, 60) according to any of the preceding claims.

13. Method of producing an optical element (1 ), especially an optical element according to any of Claims 1 to 11 , comprising: applying a coating (3) which is partly reflective or antireflective to radiation (4), preferably to radiation in the UV or VIS wavelength region, to a substrate (2) of the optical element (1 ), where the coating (3) has a plurality of first layers (6) and second layers (7), where the first layers (6) include a first, preferably fluoridic or oxidic material and the second layers (7) include a second, preferably fluoridic or oxidic material, the refractive index (n2) of which for the radiation (4) differs from the refractive index (ni) of the first material for the radiation (4), characterized in that2023P00550WQ 30.06.25 SZ00388PCTon application of the coating (3), at least one group of layers (8) for reflectivity matching is applied, comprising a third layer (9) of a third material which does not correspond to the first material and the second material, where the application of the coating (3) is preferably preceded by determination of an optical thickness (x) of the third layer (9) depending on a desired reflectivity match and / or where the group of layers for reflectivity matching preferably forms a pair of layers (8) including the third layer (9) and a further layer (10) either of the first material or of the second material.

14. Method according to Claim 13, further comprising: defining a starting design of the coating (3), having a preferably alternating sequence of the first layers (6) and the second layers (7), selecting at least one of the first layers (6) or the second layers (7) of the starting design for reflectivity matching of the coating (3), replacing the selected first or second layer (6, 7) of the starting design with the group of layers (8) for reflectivity matching, and applying the coating (3) with the starting design modified by the at least one group of layers (8) for reflectivity matching.

15. Method according to Claim 13 or 14, in which, on application of the coating (3), at least one compensation layer (6') for at least partial compensation of a change in a spectral and / or angle-dependent progression in the reflectivity of the coating (3) by the at least one group of layers (8) for reflectivity matching and / or by at least one phase shift layer (7a) is applied, where the compensation layer (6') preferably has an optical thickness (A / 4 + / - 5) that deviates from the optical thickness (A / 4) of the first layers (6) and the second layers (7), where the deviation (5) in particular is not greater than 20% of the optical thickness (A / 4) of the first layers (6) and the second layers (7), and where the compensation layer (6') is preferably formed from the first material or from the second material.2023P00550WQ 30.06.25 SZ00388PCT

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