Optical module for the ultraviolet wavelength range

Abstract

The present invention relates to an optical module (300) comprising an optical element (301) and a holder (303) for use in the ultraviolet wavelength range, wherein the optical element (301) is bonded to the holder (303) by irradiation (309) with an adhesive (305) that can be cured in the ultraviolet wavelength range, and the module (300) has an adhesive protective coating (307), the adhesive protective coating (307) is multilayer and has high reflectivity and low absorption in the operating wavelength range. Furthermore, the invention proposes an optical module having an anti-reflective adhesive protective coating that is absorbent at the operating wavelength, and an optical module having a diffractive structure in the adhesive region.

Description

[Technical Field]

[0001] The present invention relates to an optical module comprising an optical element and a holder for operating wavelengths in the ultraviolet wavelength range, wherein the optical element is bonded to the holder by an adhesive curable by radiation at a curing wavelength in the ultraviolet wavelength range, and the module has an adhesive protective coating. The present invention further relates to an optical system and an apparatus comprising one of the optical modules mentioned. This application claims priority to German Patent Application No. 10 2023 201 742.3 of 27 February 2023, the entire disclosure of which is incorporated herein by reference. [Background technology]

[0002] Patent Document 1 discloses an assembly of a DUV microlithography projection exposure apparatus, comprising a holder and a component bonded by an adhesive and transmitting radiation in the ultraviolet spectral range. The adhesive is curable by UV light. The adhesive region contains a layer that transmits UV light from a spectral range suitable for curing the adhesive and absorbs UV light from the spectral range of use. Particularly suitable is an absorption layer made of tantalum pentoxide, which exhibits high transmittance at a curing wavelength of 365 nm and very low transmittance at a wavelength of use below 248 nm, while simultaneously exhibiting high absorptivity and low reflectivity across both spectral ranges. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] German Patent Application No. 10 2011 080 369 Specification [Overview of the project] [Problems that the invention aims to solve]

[0004] The objective of this invention is to further develop known assemblies. [Means for solving the problem]

[0005] This objective is achieved by an optical module comprising an optical element and a holder for operating wavelengths in the ultraviolet wavelength range, wherein the optical element is bonded to the holder with an adhesive curable by radiation at a curing wavelength in the ultraviolet wavelength range, and the module has an adhesive protective coating, wherein the adhesive protective coating has a multilayer design and is highly reflective and low-absorbent at the operating wavelength.

[0006] By providing a reflective multilayer adhesive protective coating, it becomes possible to adapt the adhesive protective coating to suit any desired combination of operating and curing wavelengths. This further helps to avoid undesirable changes in the optical properties of optical elements caused by heating due to absorbed radiation, such as non-uniform changes in refractive index within the lens leading to undesirable wavefront deformation affecting the imaging characteristics of the lens. The reflectance, transmittance, and absorptance for specific wavelengths or wavelength ranges can be flexibly influenced by the selection of layer materials, layer thicknesses, and number of layers. Overall, a single wavelength is referred to when the wavelength range width is ±1%. For broader wavelength ranges, the central wavelength can be considered as an alternative. In particular, it is possible to set the reflectance at the operating wavelength as high as possible, the absorptance at the operating wavelength as low as possible, and the transmittance at the curing wavelength to be sufficiently high for the adhesive curing process. An advantage is that since the absorptance at both the operating and curing wavelengths should be very low, undesirable heating of the optical element, which would result in changes in optical properties exceeding manufacturing tolerances, does not occur. In this regard, the residual absorption present can be used to further reduce the radiation load on the adhesive at the operating wavelength, in addition to the reflection in the adhesive protective layer. Here, high reflectivity means a reflectance of 70% or more, and low absorption means an absorptive rate of 30% or less.

[0007] An advantage is that the adhesive protective coating has at least three layers, allowing for the intentional influence and enhancement of reflectivity, particularly at the operating wavelength, through the selection of layer materials and layer thicknesses.

[0008] In a preferred embodiment, the adhesive protective coating comprises a layer made of a material having a high refractive index at the operating wavelength and a layer made of a material having a low refractive index at the operating wavelength, with the layers alternating. Reflection can occur at each layer boundary. By selecting the layer thickness in correspondence with the incident wavelength, it is possible to increase the total reflectivity and reduce absorption in the adhesive protective coating as a whole. In this case, it is particularly advantageous to select the layer thickness to be approximately 1 / 4 of the incident wavelength. To obtain the maximum reflectivity at a certain wavelength, it is preferable to use an optical thickness of approximately half the wavelength for each layer pair made of the high-refractive-index and low-refractive-index materials. The refractive index of each layer material affects the optical thickness.

[0009] An advantage is that the adhesive protective coating has at least two layers of material with a high refractive index at the operating wavelength and at least one layer of material with a low refractive index at the operating wavelength, in order to increase reflectivity at the operating wavelength while simultaneously reducing absorptivity. In this case, the transmittance at the curing wavelength can also be kept high enough to allow the adhesive to cure.

[0010] For this purpose, the adhesive protective coating preferably has 3 to 16 layers of material with a high refractive index at the operating wavelength and 3 to 16 layers of material with a low refractive index at the operating wavelength. This number of layers allows for efficient setting of reflectivity while avoiding excessively high coating costs.

[0011] An advantage is that the adhesive protective coating has a layer made of a material with a high refractive index at the operating wavelength as the outermost layer on the adhesive side. It is equally advantageous if the adhesive protective coating has a layer made of a material with a high refractive index at the operating wavelength as the outermost layer on the opposite side of the adhesive. Both measures, individually or in combination, can contribute to increasing reflectivity at the operating wavelength. In the latter case, preferably, the thickness of the outermost layer on the opposite side of the adhesive may be different from the thickness of the other layers made of the material with a high refractive index at the operating wavelength. In the case of multilayer adhesive protective coatings optimized for particularly high reflectivity in the operating wavelength range, this measure can increase transmittance in the curing wavelength range.

[0012] Preferably, all layers of the material having a high refractive index or low refractive index at the operating wavelength have the same thickness. This results in a so-called periodic adhesive protective coating configuration that significantly increases reflectivity or significantly reduces absorptivity, particularly in the narrow operating wavelength range of nanometers or sub-nanometers. Therefore, such modules are particularly suitable for use, for example, with lasers as radiation sources.

[0013] In one preferred embodiment, the operating wavelength is not equal to the curing wavelength, in which case the adhesive protective coating has a reflectance of more than 70%, preferably more than 80%, and particularly preferably more than 90% at the operating wavelength, and a transmittance of more than 90% at the curing wavelength. Thus, degradation of optical properties due to unwanted heating of the optical element can be particularly effectively avoided after uninterrupted curing. Very preferably, the optical module is designed for an operating wavelength of about 193 nm or 248 nm and a curing wavelength of about 365 nm.

[0014] In a more preferred embodiment, the operating wavelength is equal to the curing wavelength, in which case the adhesive protective coating has a reflectance of more than 70%, preferably more than 80%, and particularly preferably more than 90% at the operating wavelength, and a transmittance of more than 2%, preferably more than 6% at the curing wavelength. In this way, degradation of optical properties due to unwanted heating of the optical element can be avoided particularly effectively without interfering with the curing of the adhesive. Very preferably, the optical module is designed for operating and curing wavelengths of about 365 nm.

[0015] Preferably, the adhesive protective coating comprises one material, preferably two, from the group consisting of silicon dioxide, aluminum oxide, hafnium dioxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, thiolite, and magnesium fluoride. Particularly preferably, the adhesive protective coating comprises one material from the group consisting of hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide, and zinc sulfide as a material having a high refractive index at the operating wavelength, and one material from the group consisting of silicon dioxide, aluminum fluoride, magnesium fluoride, thiolite, and cryolite as a material having a low refractive index at the operating wavelength. In particular, in the ultraviolet wavelength range of 150 nm to 400 nm, these materials can be used to produce a multilayer adhesive protective coating with a large difference in refractive index at the operating wavelength, and correspondingly, a high reflectivity can be obtained. Particularly advantageous are combinations of low refractive index silicon dioxide with aluminum oxide, which has a high refractive index especially for operating wavelengths of approximately 193 nm and 248 nm; or combinations with hafnium dioxide, which has a high refractive index for an operating wavelength of approximately 248 nm; and combinations with tantalum pentoxide, which has a high refractive index for an operating wavelength of approximately 365 nm.

[0016] In yet another embodiment, this objective is achieved by an optical module comprising an optical element and a holder for an operating wavelength in the ultraviolet wavelength range, wherein the optical element is bonded to the holder by an adhesive, the module has an adhesive protective coating that is absorbent at the operating wavelength, and the adhesive protective coating has an anti-reflective coating.

[0017] This optical module features an adhesive protective coating with an anti-reflective coating, which not only protects the adhesive from radiation damage but also suppresses residual reflections that could lead to ambient light. The module is particularly suitable for use with optical elements that do not significantly affect the image wavefront even when heated, when mounted in the beam path of an optical system.

[0018] In one preferred embodiment, the anti-reflective coating comprises a layer made of a material having a high refractive index at the operating wavelength and a layer made of a material having a low refractive index in the operating wavelength range, with the layers alternating. The choice of layer material, layer thickness, and number of layers allows for flexible influence on the reflectance, transmittance, and absorptance for a specific wavelength or wavelength range. Overall, a wavelength is referred to as one wavelength when the width of the wavelength range is ±1%. For a broad wavelength range, the midpoint wavelength can be considered as an alternative. To obtain minimum reflectance at a given wavelength, the optical thickness of the layer pair consisting of the high refractive index layer and the low refractive index layer is approximately 1 / 4 of the wavelength, and also significantly larger or smaller. The refractive index of each layer material affects the optical thickness. Preferably, the anti-reflective coating of the adhesive protective coating, such as the multilayer adhesive protective coating described above, comprises one material, preferably two materials, from the group consisting of silicon dioxide, aluminum oxide, hafnium dioxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, thiolite, and magnesium fluoride. Particularly preferably, the adhesive protective coating comprises one material from the group consisting of hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide, and zinc sulfide as a material having a high refractive index at the operating wavelength, and one material from the group consisting of silicon dioxide, aluminum fluoride, magnesium fluoride, thiolite, and cryolite as a material having a low refractive index at the operating wavelength.

[0019] In yet another preferred embodiment, the antireflection coating is embodied as a coating having a refractive index gradient. As a result, the antireflection effect appears in a wider wavelength range. Particularly advantageous is that the refractive index changes from the refractive index of the material of the optical element to the refractive index of the material of the adhesive protective coating. By means of a refractive index change that is as continuous as possible, a particularly good antireflection effect can be obtained.

[0020] In yet another aspect, the above object is achieved by an optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, the optical element being adhered to the holder by an adhesive, and the module having a diffraction structure in the region of the adhesive.

[0021] By providing a diffraction structure, it is possible to prevent the radiation of the operating wavelength from reaching the adhesive and causing damage, or from being reflected at the edge of the optical element and reducing the contrast of the image formed by the optical element, or from being absorbed and contributing to the heating of the optical element, especially when the optical element is heated only non-uniformly or locally, which would otherwise have an adverse effect on the wavefront and thus on the image formation. For this purpose, the diffraction structure is preferably configured such that the incident useful radiation is diffracted out of the beam path. Particularly preferably, a diffraction structure embodied as a periodic diffraction grating is provided. In order to remove interfering radiation of different wavelengths or different angles of incidence from the beam path, the installation of the diffraction structure can also be combined with a highly reflective or particularly antireflective adhesive protective coating.

[0022] Preferably, especially when the diffraction structure is embodied as a periodic diffraction grating, the diffraction grating has an inclination angle with respect to the normal to the surface of the optical element. This has the great advantage that, unlike a diffraction structure without an inclination angle, not only the first and second order diffractions but also the zero order diffraction, which is the order of maximum intensity at the same time, is diffracted out of the beam path. In the case of diffraction structures having various heights and inclination angles, it is also possible to achieve an antireflection effect in a larger wavelength or angle of incidence range.

[0023] An advantage is that the optical module has a radiation absorber. The radiation absorber absorbs diffracted radiation and can therefore efficiently remove it from the beam path.

[0024] Preferably, in all of the above modules, the optical elements are designed as lenses. Therefore, the optical modules are particularly suitable for use in UV lithography apparatus and inspection systems for inspecting masks exposed by lithography or wafers before or after exposure.

[0025] Furthermore, the above objective is achieved by an optical system equipped with one of the optical modules described above. Moreover, this objective is achieved by an apparatus designed as an EUV lithography apparatus or inspection apparatus, equipped with one of the optical modules described above or the optical system described above.

[0026] The present invention will be described in more detail with reference to preferred exemplary embodiments.

[0027] [Brief explanation of the drawing]

[0028] [Figure 1] This shows a basic schematic diagram of a UV lithography system. [Figure 2] A basic schematic diagram of the inspection system is shown. [Figure 3] A basic schematic diagram of the optical module is shown. [Figure 4] This shows a basic schematic diagram of the adhesive protective coating for optical modules. [Figure 5] The reflectance as a function of wavelength in exemplary first and second embodiments of adhesive protective coatings is shown. [Figure 6] The transmittance as a function of wavelength in the first and second exemplary embodiments of the adhesive protective coating is shown. [Figure 7] The reflectance as a function of wavelength in exemplary third and fourth embodiments of the adhesive protective coating is shown. [Figure 8]The transmittance as a function of wavelength in exemplary third and fourth embodiments of the adhesive protective coating is shown. [Figure 9] The reflectance as a function of wavelength in exemplary fifth and sixth embodiments of the adhesive protective coating is shown. [Figure 10] The transmittance as a function of wavelength in exemplary fifth and sixth embodiments of the adhesive protective coating is shown. [Figure 11] A basic schematic diagram of the second optical module is shown. [Figure 12] This shows the ratio of oxygen concentration as a function of distance from the lens surface in a first embodiment of an adhesive protective coating having an anti-reflective coating. [Figure 13] The reflectance as a function of wavelength in a first embodiment of an adhesive protective coating having an anti-reflective coating is shown. [Figure 14] The reflectance as a function of the angle of incidence in a first embodiment of an adhesive protective coating having an anti-reflective coating is shown. [Figure 15] The reflectance as a function of wavelength in a second embodiment of an adhesive protective coating having an anti-reflective coating is shown. [Figure 16] The transmittance as a function of wavelength in a second embodiment of an adhesive protective coating having an anti-reflective coating is shown. [Figure 17] A basic schematic diagram of the third optical module is shown. [Figure 18] A basic schematic diagram of one embodiment of the third optical module is shown. [Modes for carrying out the invention]

[0029] Figure 1 shows a basic schematic diagram of apparatus 1 for UV lithography, particularly for wavelengths in the range of 190 nm to 400 nm. The UV lithography apparatus 1 comprises, as essential components, two optical systems 12 and 14, an illumination system 12, and a projection system 14. Lithography requires a radiation source 10, particularly preferably an excimer laser, which emits light at, for example, 193 nm or 248 nm and may be an integral part of the UV lithography apparatus. A mercury lamp may also be used, with the emitted i-line of approximately 368 nm being used as the operating wavelength and / or as the curing wavelength in bonding processes performed outside each optical system. The radiation 11 emitted by the radiation source 10 can be regulated using the illumination system 12 to illuminate a mask 12, also called a reticle. In this example, the illumination system 12 includes a transmission optical element and a reflection optical element. For example, a transmission optical element 120 for focusing the radiation 11 and a reflection optical element 121 for deflecting the radiation are shown here as representative examples. All optical elements can be part of an optical module as proposed here. As is known, the illumination system 12 can combine various transmissive optical elements, reflective optical elements, and other optical elements in any more complex way. Note that the mask 13 can also be part of an optical module as proposed here.

[0030] The mask 13 has a structure on its surface, which is transferred to the exposure element 15, for example, a wafer in the manufacturing of a semiconductor component, using a projection system 14. In this example, the mask 13 is embodied as a transmission optical element. In other embodiments, it can also be configured as a reflective optical element. The projection system 14 includes at least one transmission optical element in the example shown herein. In this example, two transmission optical elements 140 and 141 are shown representatively, which, for example, reduce the structure on the mask 13 to a size desirable for exposure of the wafer 15. The projection system 14 can also include a reflective optical element, and various optical elements can be arbitrarily combined with each other in known ways. Note that, especially in the case of an optical system optimized for wavelengths less than 200 nm, an optical system without a transmission optical element can also be provided. All optical elements of the projection system 14 can also be part of the optical module proposed herein.

[0031] An optical module comprising an optical element and a holder for operating wavelengths in the ultraviolet wavelength range, wherein the optical element is bonded to the holder with an adhesive that can be cured by irradiation at a curing wavelength in the ultraviolet wavelength range, and the module has an adhesive protective coating, which is multilayered and has high reflectivity and low absorption at the operating wavelength, can also be used in wafer or mask inspection systems. An exemplary embodiment of wafer inspection system 2 is schematically shown in Figure 2. The description applies similarly to a mask inspection system.

[0032] The wafer inspection system 2 includes a radiation source 20, the radiation of which is directed to the wafer 25 by an optical system 22. For this purpose, the radiation is reflected to the wafer 25 from a concave mirror 220. In the case of a mask inspection system, the mask under inspection may be placed in place of the wafer 25. The radiation reflected, diffracted, and / or refracted by the wafer 25 is directed to a detector 23 for further evaluation by a concave mirror 221, which is also associated with the optical system 22. The radiation source 20 can be, for example, just one radiation source or a combination of multiple individual radiation sources to provide a substantially continuous radiation spectrum. In modified forms, one or more narrowband radiation sources may also be used. The inspection system shown here as an example is designed for operating wavelengths in the range of 190 nm to 300 nm, particularly preferably 190 nm to 200 nm. Lenses may also be provided in addition to and / or as replacements for the two concave mirrors 220, 221 in the wafer or mask inspection system. In the case of inspection systems designed for operating wavelengths in the 200nm to 400nm range, particularly in the 300nm to 400nm range, all optical elements can be realized as lenses.

[0033] Figure 3 shows an exemplary embodiment of the optical module 300. In this example, the optical element 301 is a concave lens transparent to the operating wavelength in the ultraviolet wavelength range. The lens 301 is mounted to a holder 303. For this purpose, the lens is bonded to the holder 303 by a UV-curable adhesive 305, particularly preferably outside the area optically used during operation. As an example, an epoxy adhesive that can be cured using the i-line UV radiation of a mercury lamp is used as the mounting adhesive. In particular, as a result of irradiation of the lens during operation due to stray light or multiple reflections, the adhesive may be damaged by radiation over time, impairing adhesion or deformation, which can cause the lens 301 to no longer be held in its original adjusted position within the holder 303, resulting in imaging aberrations. Therefore, an adhesive protective coating 307 is provided between the lens 301 and the adhesive 305. This is a multilayer design and is highly reflective and low-absorbent at the operating wavelength. For example, adhesive protective coatings, such as dielectric multilayer anti-reflective coatings commonly found on lenses, can be applied by conventional coating methods using thin-film techniques, such as thermal electron beam deposition or magnetron sputtering. To enhance the mechanical strength of the optical module 300, an ion-assisted coating method is preferred. For simplification of handling, the adhesive protective coating 307 can be applied locally to the lens 301 before bonding it to the holder 303. The adhesive can then be cured by irradiation at a curing wavelength (indicated by the dashed arrow 309), to which the adhesive protective coating 307 is sufficiently transparent. At the operating wavelength, most of the incident radiation 309 is reflected out of the beam path, so the absorption at the adhesive joint is low enough to avoid localized heating of the lens 301, which could lead to imaging aberrations, such as those caused by wavefront aberration. Absorption that can be low enough to avoid undesirable heating may further contribute to protecting the adhesive 305 from radiation damage.

[0034] Figure 4 shows in detail the area of ​​the adhesive protective coating of the optical module. The adhesive protective coating 407 is placed between the optical element 401 and the adhesive 405. The adhesive protective coating 407 is multilayered, preferably consisting of at least three layers. By selecting the number of layers, layer thickness, and layer material, the reflectance, transmittance, and absorptance at the corresponding wavelengths can be set for any desired combination of operating wavelength and curing wavelength, making it possible to ensure sufficient transmittance at the curing wavelength to enable curing of the adhesive 405 while keeping the reflectance as high as possible and the absorptance as low as possible to avoid undesirable heating of the optical element 401 and radiation damage to the adhesive 405.

[0035] It has been found that the adhesive protective coating 407 is particularly useful if it has a layer 471 made of a material having a high refractive index at the operating wavelength and a layer 473 made of a material having a low refractive index at the operating wavelength, and in either case the layer 471 made of the high refractive index material and the layer 473 made of the low refractive index material are arranged alternately. In this way, it is possible to design an adhesive protective coating 407 that satisfies the requirements of high reflectivity, low absorption rate, and sufficient transmittance for the operating wavelength and curing wavelength in the freely selected ultraviolet wavelength range. Preferably, the adhesive protective coating 407 has at least two layers 471 made of a material having a high refractive index at the operating wavelength and at least one layer 473 made of a material having a low refractive index at the operating wavelength. Particularly preferably, the adhesive protective coating 407 has 3 to 16 layers 471 made of a material having a low refractive index at the operating wavelength and 3 to 16 layers 473 made of a material having a low refractive index at the operating wavelength.

[0036] In the example shown in Figure 4, four layers 471 of a material with a high refractive index at the operating wavelength and three layers 471 of a material with a low refractive index at the operating wavelength are provided. Layers 471' and 471 made of the high refractive index material are provided as the outermost layers on both the adhesive side and the opposite side of the adhesive. In this example, all of the layers 471 of the high refractive index material and all of the layers 473 of the low refractive index material have the same thickness, however the thickness of the outermost layer 471' on the adhesive side is different from the thickness of the other layers 471 made of the high refractive index material. Therefore, a periodic layer arrangement characterized by particularly high reflectivity at a certain wavelength is essentially obtained when there is a unit consisting of two adjacent layers 471 and 473 made of materials with high and low refractive index materials, and its thickness is approximately 1 / 4 of a certain wavelength. If necessary, curing can be accelerated by breaking the periodicity with the outermost layer on the adhesive side to increase the transmittance at the curing wavelength somewhat. As an alternative, the bottom layer 471 on the optical substrate 401, which includes a material with a high refractive index, can also have a thickness different from 1 / 4 of the operating wavelength, thereby intentionally achieving maximum transmittance at the curing wavelength while maintaining a high reflectance at the operating wavelength. Silicon dioxide, aluminum oxide, hafnium oxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, thiolite, and magnesium fluoride, when combined with other or more of these materials, or one or more further materials, have been found to be useful as materials for the adhesive protective coating 407. Combinations of hafnium oxide, aluminum oxide, tantalum pentoxide, titanium dioxide, or zinc sulfide as materials with a high refractive index, and silicon dioxide, aluminum fluoride, magnesium fluoride, thiolite, and cryolite as materials with a low refractive index, have been found to be particularly useful.

[0037] The reflectance and transmittance curves for various embodiments of adhesive protective coatings are shown in the following figures. Figure 5 shows the reflectance and Figure 6 shows the transmittance, both expressed as functions of wavelength for two adhesive protective coatings A and B optimized for an operating wavelength λ1 of 193 nm and a curing wavelength λ2 of 365 nm. Adhesive protective coating A has 14 layers of aluminum oxide with a thickness of 27 nm as a high refractive index material, and 13 layers of silicon dioxide with a thickness of 31 nm as a low refractive index material, which are arranged alternately. The reflectance and equalization rate of adhesive protective coating A are shown as dashed lines. Adhesive protective coating B has only 10 layers of aluminum oxide with a thickness of 27 nm as a high refractive index material, and 9 layers of silicon dioxide with a thickness of 31 nm as a low refractive index material, which are arranged alternately. The reflectance and transmittance of adhesive protective coating B are shown as solid lines. Adhesive protective coating B has a reflectivity of approximately 80% and transmittance of approximately 20% at the operating wavelength λ1, and a reflectivity of virtually 0% and transmittance of virtually 100% at the curing wavelength λ2. Therefore, virtually no absorption of radiation occurs in both the operating wavelength λ1 and curing wavelength λ2 ranges, and no undesirable heating of the optical elements occurs. The adhesive can be cured as if the adhesive protective coating were not present, and nevertheless, the cured adhesive is protected from radiation damage. Increasing the number of layers of adhesive protective coating A results in a reflectivity exceeding 90% and transmittance of less than 10% at the operating wavelength λ1, thus better protecting the adhesive from radiation damage and correspondingly extending its lifespan. The residual absorption of the adhesive protective coating here is less than 1%, resulting in minimal lens heating, and the wavefront and imaging characteristics do not deviate from the required requirements. Rather, here, this works to further protect the adhesive from radiation damage by absorbing residual radiant energy.

[0038] Figure 7 shows the reflectance, and Figure 8 shows the transmittance, both as functions of wavelength for two adhesive protective coatings C and D, optimized for an operating wavelength λ1 of 248 nm and a curing wavelength λ2 of 365 nm. Adhesive protective coating C consists of seven 30 nm thick layers of hafnium oxide, a material with a high refractive index, and seven 40 nm thick layers of silicon dioxide, a material with a low refractive index, arranged alternately. Finally, as the top layer, another 15 nm thick layer of hafnium dioxide follows on the adhesive layer side. The reflectance and transmittance of adhesive protective coating C are shown as dashed lines. Adhesive protective coating D consists of only five 30 nm thick layers of hafnium oxide, a material with a high refractive index, and five 40 nm thick layers of silicon dioxide, a material with a low refractive index, arranged alternately. The reflectance and transmittance of adhesive protective coating D are shown as solid lines. Adhesive protective coating D has a reflectivity of nearly 95% and a transmittance of slightly over 20% at the operating wavelength λ1, and a reflectivity of virtually 0% and a transmittance of virtually 100% at the curing wavelength λ2. Therefore, virtually no absorption of radiation occurs in both the operating wavelength λ1 and curing wavelength λ2 ranges, and no undesirable heating of the optical elements occurs. The adhesive can be cured as if the adhesive protective coating were not present, and nevertheless, the cured adhesive is protected from radiation damage. Increasing the number of layers of adhesive protective coating C results in a reflectivity of nearly 100% and a transmittance of nearly 0% at the operating wavelength λ1, so the adhesive is better protected from radiation damage and, correspondingly, its lifespan is extended. At the curing wavelength λ2, the reflectivity is about 2% and the transmittance is about 98%, which still allows for completely unhindered curing of the adhesive. The residual absorption of the adhesive protective coating here is less than 1%, so lens heating is minimal, and the wavefront and imaging characteristics do not deviate from the required requirements. Rather, in this case, it works to further protect the adhesive from radiation damage by absorbing residual radiant energy.

[0039] Figure 9 shows the reflectance, and Figure 10 shows the transmittance, both at the operating wavelength of 365 nm and the curing wavelength λ. 1 / 2It is expressed as a function of the wavelengths of two adhesive protective coatings E and F optimized for. Adhesive protective coating E has 7 layers with a thickness of 42 nm made of tantalum pentoxide as a material with a high refractive index, and 6 layers with a thickness of 60 nm made of silicon dioxide as a material with a low refractive index, and these are arranged alternately. The reflectance and transmittance of adhesive protective coating E are represented as dashed lines, respectively. Adhesive protective coating F has only 5 layers with a thickness of 42 nm made of tantalum pentoxide as a material with a high refractive index, and 4 layers with a thickness of 60 nm made of silicon dioxide as a material with a low refractive index, and these are arranged alternately. The reflectance and transmittance of adhesive protective coating F are represented as solid lines, respectively. Adhesive protective coating E has a reflectance of about 95% and a transmittance slightly exceeding 3% at the operating wavelength and the curing wavelength λ 1 / 2 so that in this wavelength range, absorption of each radiation hardly occurs and undesirable heating of each optical element does not occur. This considerably low transmittance can be compensated by lengthening the irradiation period for curing the adhesive. Reducing the number of layers of adhesive protective coating F results in a reflectance of about 93% and a transmittance of about 7% at the operating wavelength and the curing wavelength λ 1 / 2 so that the adhesive cures at approximately twice the speed of the case of adhesive protective coating E in a first approximation.

[0040] To estimate the transmittance required for curing the adhesive, for example, in the case of the 365 nm i-line of a mercury lamp as the curing wavelength, the required irradiation dose D (J / cm 2 ), the maximum allowable irradiation period t (seconds), and the power density P (W / cm 2 ) of the mercury lamp used for curing on the optical element can each be utilized. The required transmittance at the curing wavelength in percent is T = 100% × D / (P × t) and is calculated as such. Correspondingly, the minimum allowable reflectance at the operating wavelength is the guaranteed life t LT (seconds), the maximum irradiation dose D degradation (J / cm 2 ) for which the function of the adhesive is maintained, and the power density P of stray light and residual reflection occurring at the adhesive joint stray(W / cm 2 ) can be estimated from, R = 100% × (1-D) degradation / P stray ×t LT ) These are calculated as follows. These estimates are especially important when the curing wavelength is equal to the operating wavelength. If necessary, firstly, the reflectance can be increased to the desired value by increasing the number of layers, and secondly, the transmittance at the curing wavelength can be increased by appropriately adjusting the outermost layer on the adhesive side, which is made of a high refractive index material, by selecting a different layer thickness from the other layers made of a high refractive index material.

[0041] Figure 11 shows an exemplary embodiment of the second optical module 1100. In this example, the optical element 1101 is a convex lens transparent to the operating wavelength in the ultraviolet wavelength range. The lens 1101 is mounted on a holder 1103. For this purpose, the lens is bonded to the holder 1103 by a UV-curable adhesive 1105, particularly preferably outside the area optically used during operation. As an example, an epoxy adhesive that can be cured using the i-line UV radiation of a mercury lamp is used as the mounting adhesive. In particular, as a result of irradiation of the lens during operation due to stray light or multiple reflections, the adhesive may be radiation-damaged over time, impairing adhesion or deformation, which may cause the lens 1101 to no longer be held in its original adjusted position within the holder 1103, resulting in imaging aberrations. Therefore, an adhesive protective coating 1111, including an absorption layer 1110 and an anti-reflective coating 1112, is provided between the lens 1101 and the adhesive 1105. The anti-reflective coating 1112 can be applied by conventional coating methods using thin-film technology, such as thermal electron beam deposition or magnetron sputtering. To increase the mechanical strength of the optical module 1100, it is preferable to select an ion-assisted coating method for applying the adhesive protective coating 1111, and especially its anti-reflective coating 1112.

[0042] In one embodiment, the anti-reflective coating of the adhesive protective coating is embodied as a coating having a refractive index gradient. A substantial anti-reflective effect is possible when the coating between the optical element material and the adhesive protective coating has a refractive index gradient that changes continuously from the refractive index of the optical element material to the refractive index of the adhesive protective coating material.

[0043] In the optical module considered in this example, the optical elements are designed as lenses made of quartz glass, and the material of the absorption layer of the adhesive protective coating is amorphous silicon, which strongly absorbs radiation with wavelengths less than 400 nm. An SiOx gradient layer is applied between the lens and the amorphous silicon layer for an anti-reflective effect, and its oxygen content decreases continuously from the lens surface at x=2 to x=0 at the transition point to the silicon layer, as shown in Figure 12. For this purpose, for example, firstly, an SiO2 layer is formed by reactive sputtering silicon from a target in a saturated oxygen atmosphere. Subsequently, as the oxygen content of the coating apparatus continues to decrease, the oxygen content of the SiOx layer also decreases, and x drops from 2 to 0. Finally, at a pressure of 10 -5Silicon is sputtered in a high vacuum of mbar, i.e., in the absence of oxygen. Figure 13 shows the reflectance of the thus obtained optical module as a function of wavelength for quasi-normal incidence. The various vertical dashed lines represent wavelengths of 193 nm, 248 nm, and 365 nm. The solid reflectance profile shows the reflectance of the optical module including an anti-reflective adhesive protective coating, i.e., an anti-reflective coating with amorphous silicon to protect the adhesive and a coating with the refractive index gradient shown in Figure 12. For comparison, the dashed reflectance profile shows the reflectance of the corresponding optical module without the anti-reflective coating, i.e., only amorphous silicon on quartz glass. Correspondingly, Figure 14 shows the reflectance at a wavelength of 248 nm as a function of incidence angle, with the solid reflectance profile showing the reflectance with the anti-reflective coating, and for comparison, the dashed reflectance profile showing the reflectance without the anti-reflective coating on the adhesive protective coating made of amorphous silicon. The anti-reflective coating of the adhesive protective coating with a refractive index gradient provides extremely low reflectivity of less than 1% over a very wide wavelength range of 180nm to 390nm and a very wide incident angle range of up to 50°, making this type of optical module extremely flexible to use.

[0044] In yet another embodiment, the anti-reflective coating may have a layer made of a material having a high refractive index at the operating wavelength and a layer made of a material having a low refractive index in the operating wavelength range, with the layers of the high refractive index material and the layers of the low refractive index material being arranged alternately. The resulting dielectric multilayer anti-reflective coating can be described in the same way as the high-reflectance coating of the first optical module shown in Figures 3 and 4. The fundamental difference is the use of different layer thicknesses. For anti-reflective effect, the optical thickness of the layer pair made of high-reflectance and low-reflectance materials is about 1 / 4 of the operating wavelength of each optical element, whereas for particularly high reflectivity, this optical thickness is about half the operating wavelength. The optical thickness is the geometric thickness weighted by the refractive index of each layer material. The specific thicknesses of the high-reflectance and low-reflectance layers are optimized for the maximum anti-reflective effect. Suitable materials for the high-reflectance layer include aluminum oxide at wavelengths above 193 nm, hafnium oxide at wavelengths above 248 nm, and tantalum pentoxide at wavelengths above 365 nm. These materials, shown as examples, have almost no absorption beyond the indicated wavelengths. A suitable material for the low refractive index layer is silicon dioxide, especially at wavelengths above 193 nm. By applying both the high and low refractive index layers using an ion-assisted coating process, high mechanical stability and sufficiently good layer adhesion are ensured.

[0045] Figure 15 shows the reflectance and Figure 16 shows the transmittance as functions of wavelength for an optical module having a dielectric multilayer anti-reflective layer on an adhesive protective coating. In this case, the vertical dashed line represents the operating wavelength of 193 nm, and the dotted line represents the wavelength of 365 nm at which the adhesive cures. The optical module has optical elements in the form of lenses made of quartz glass and an adhesive protective coating in the form of a layer made of tantalum pentoxide with a thickness of 180 nm. The anti-reflective coating applied thereto is in the form of a coating made of 22 nm aluminum oxide, 33 nm silicon dioxide, and 22 nm aluminum oxide. The dashed reflectance and transmittance profiles show the reflectance and transmittance with the anti-reflective coating, while the solid reflectance and transmittance profiles show the reflectance and transmittance without the anti-reflective coating for comparison. The anti-reflective coating by the multilayer dielectric coating intentionally produces an anti-reflective effect near the operating wavelength, in which case there is negligible transmission and the entire incident radiation is absorbed by the adhesive protective coating. In contrast, at the curing wavelength, regardless of the presence or absence of an anti-reflective coating, the transmittance is very high while the reflectance and absorptiveness are low, so the adhesive can be cured very well through the adhesive protective coating made of quartz glass lens and tantalum pentoxide.

[0046] Overall, as an absorbing material for adhesive protective coatings, any material can be used that, depending on the wavelength, would also be suitable as a layer material with a high refractive index for multilayer anti-reflective coatings or multilayer high-reflectance adhesive protective coatings. For example, for absorbing UV radiation with long wavelengths such as 365 nm, metals, more specifically chromium, are suitable, and this also has good adhesion characteristics compared to conventional lens materials. For short wavelengths such as 248 nm or 193 nm, fluoride crystals such as calcium fluoride or quartz glass are suitable, and for long wavelengths such as approximately 365 nm, lead-containing glass or borosilicate glass are suitable. Depending on the selection of materials for the adhesive protective coating and optical elements, adhesion promoters can also be used in addition.

[0047] Figures 17 and 18 schematically show yet another optical module 1700, 1800 having optical elements 1701, 1801 and holders 1703, 1803 for operating wavelengths in the ultraviolet wavelength range, where the optical elements 1701, 1801 are bonded to the holders 1703, 1803 by adhesives 1705, 1805. The optical module 1700, 1800 shown here has a diffraction structure 1713, 1813 in the area of ​​the adhesives 1705, 1805. In this example, the optical elements 1701, 1801 are embodied as lenses.

[0048] Lenses 1701 and 1801 are intentionally structured to diffract the incident radiation 1721, 1723, and 1821 away from the beam path in the edge regions where each holder 1703, 1803 is positioned. This prevents radiation 1721, 1723, and 1821 from reaching and damaging the adhesive 1705, 1805, as well as reducing contrast as a result of radiation 1721, 1723, and 1821 being reflected at the lens edges, and preventing radiation 1721, 1723, and 1821 from being absorbed and contributing to lens heating, thereby adversely affecting the wavefront and, consequently, image formation. For example, a diffraction structure can be selected in the form of a periodic diffraction grating 1713 in which the diffraction structure is not tilted with respect to the lens surface. This ensures that first-order and higher diffracted light is deflected away from obstructive reflection paths, but that the diffraction paths are directed towards relatively less important regions, depending on the diffraction efficiency.

[0049] In the example shown in Figure 17, a radiation absorber 1715 is provided on the optical module 1700. The radiation absorber has little thermal coupling with the lens 1701 and substantially absorbs the incident radiation 1722 and 1724 that is diffracted from the beam path by the diffraction grating 1713, thus ultimately removing this radiation from the optical module 1700.

[0050] However, it is highly advantageous for the diffraction structure to be designed as a gradient structure 1813, particularly as a lattice such as a blazed or echelet lattice that already diffracts the zero-order diffracted light from the beam path (see also Figure 18). In this example, the gradient angle of the diffraction structure 1813 is designed so that the diffracted radiation 1822 is directed towards a radiation absorber that provides as complete absorption as possible, although this radiation absorber is not shown in Figure 18 for clarity. In this example, the depth of the structure 1813 is 1 / 4 of the operating wavelength to be diffracted, i.e., 193 nm / 4, i.e., about 48 nm, 248 nm / 4, i.e., 62 nm, or 365 nm / 4, i.e., about 91 nm. If the structure 1813 is filled with adhesive 1805 as in the example shown in Figure 18, the depth should also be divided by the refractive index of the adhesive. An etch stop layer can be used during the fabrication of the diffraction structure, and the material of the etch stop layer should be as transparent as possible at the operating wavelength. For example, diffracted structures can be fabricated on an aluminum layer approximately 5 nm to 10 mm thick, which is inert to reactive etching processes, by etching from a silicon dioxide layer having a thickness corresponding to the structural depth to be etched. [Explanation of Symbols]

[0051] 1 VUV lithography system 2. Wafer inspection system 3 Reflective optical elements 4 Reflective optical elements 5 Reflective optical elements 6 Reflective optical elements 10 Radiation source 11. Radiation 12 Lighting Systems 13 masks 14 Projection system 15 Exposure-Targeted Element 20 Radiation source 21 Radiation 22 Optical system 23 Detectors 25 wafers 50 Optical elements 51 circuit boards 120 lenses 121 Mirror 140 lenses 141 Lens 220 Mirror 221 Miller 300 Optical Modules 301 Optical elements 303 Holder 305 Adhesive 307 Adhesive protective coating 309 UV radiation 401 Optical elements 405 Adhesive 407 Adhesive protective coating 471, 471' High refractive index layer 473 Low refractive index layer 1100 Optical Module 1101 Optical elements 1103 Holder 1105 Adhesive 1109 UV radiation 1110 Absorption layer 1111 Adhesive protective coating 1112 Anti-reflective coating 1210 Reflective surface 1700 Optical Module 1701 Optical elements 1703 Holder 1705 Adhesive 1713 Diffraction structure 1721 UV radiation 1722 UV radiation 1723 UV radiation 1724 UV radiation 1800 Optical Module 1801 Optical element 1803 Holder 1805 Adhesive 1813 Diffraction structure 1821 UV radiation 1822 UV radiation

Claims

1. An optical module comprising an optical element and a holder for operating wavelengths in the ultraviolet wavelength range, wherein the optical element is bonded to the holder with an adhesive that can be cured by irradiation at a curing wavelength in the ultraviolet wavelength range, and the module has an adhesive protective coating, wherein the adhesive protective coating (307, 407) is multilayer in design and has high reflectivity and low absorption in the operating wavelength range.

2. The optical module according to claim 1, wherein the adhesive protective coating (307, 407) has at least three layers (471, 473).

3. The optical module according to claim 1 or 2, wherein the adhesive protective coating (307, 407) comprises a layer (471) made of a material having a high refractive index at the operating wavelength and a layer (473) made of a material having a low refractive index at the operating wavelength, and the layer (471) made of the material having a high refractive index and the layer (473) made of the material having a low refractive index are arranged alternately.

4. An optical module according to any one of claims 1 to 3, characterized in that the adhesive protective coating (307, 407) comprises at least two layers (471) of a material having a high refractive index at the operating wavelength and at least one layer (473) of a material having a low refractive index at the operating wavelength.

5. An optical module according to any one of claims 1 to 4, characterized in that the adhesive protective coating (307, 407) comprises 3 to 16 layers of a material having a high refractive index at the operating wavelength (471) and 3 to 16 layers of a material having a low refractive index at the operating wavelength (473).

6. An optical module according to any one of claims 1 to 5, characterized in that the adhesive protective coating (307, 407) has a layer (471) made of a material having a high refractive index at the operating wavelength as the outermost layer on the adhesive side.

7. The optical module according to claim 6, characterized in that the adhesive protective coating (307, 407) has a layer (471) made of a material having a high refractive index at the operating wavelength as the outermost layer opposite to the adhesive.

8. The optical module according to claim 7, characterized in that the thickness of the outermost layer (471') on the side opposite to the adhesive or on the adhesive side is different from the thickness of the other layers (471) made of a material having a high refractive index at the operating wavelength.

9. An optical module according to any one of claims 3 to 7, characterized in that all of the layers (471, 473) of the material have the same thickness.

10. An optical module according to any one of claims 1 to 9, wherein the operating wavelength is not equal to the curing wavelength, and the adhesive protective coating (307, 407) has a reflectance of more than 70%, preferably more than 80%, and particularly preferably more than 90% at the operating wavelength, and a transmittance of more than 90% at the curing wavelength.

11. An optical module according to claim 10, characterized in that it is designed for an operating wavelength of about 193 nm or 248 nm and a curing wavelength of about 365 nm.

12. An optical module according to any one of claims 1 to 9, wherein the operating wavelength is equal to the curing wavelength, and the adhesive protective coating (307, 407) has a reflectance of more than 70%, preferably more than 80%, and particularly preferably more than 90% at the operating wavelength, and a transmittance of more than 2%, preferably more than 6% at the curing wavelength.

13. An optical module according to claim 12, characterized in that it is designed for an operating wavelength and a curing wavelength of approximately 365 nm.

14. An optical module according to any one of claims 1 to 13, characterized in that the adhesive protective coating (307, 407) comprises one material, preferably two materials, from the group consisting of silicon dioxide, aluminum oxide, hafnium dioxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, thiolite, and magnesium fluoride.

15. The optical module according to claim 14, wherein the adhesive protective coating (307, 407) comprises one material having a high refractive index at the operating wavelength, selected from the group consisting of hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide, and zinc sulfide, and one material having a low refractive index at the operating wavelength, selected from the group consisting of silicon dioxide, aluminum fluoride, magnesium fluoride, thiolite, and cryolite.

16. An optical module comprising an optical element and a holder for an operating wavelength in the ultraviolet wavelength range, wherein the optical element is bonded to the holder with an adhesive, and the module has an adhesive protective coating that is absorbent at the operating wavelength, characterized in that the adhesive protective coating (1111) has an anti-reflective coating (1112).

17. The optical module according to claim 16, wherein the anti-reflective coating (1112) comprises a layer made of a material having a high refractive index at the operating wavelength and a layer made of a material having a low refractive index in the operating wavelength range, and the layer made of the material having a high refractive index and the layer made of the material having a low refractive index are arranged alternately.

18. The optical module according to claim 16, characterized in that the anti-reflective coating (1112) is embodied as a coating having a refractive index gradient.

19. An optical module comprising an optical element and a holder for operating wavelengths in the ultraviolet wavelength range, wherein the optical element is bonded to the holder by an adhesive, characterized in that the module (1700) has a diffraction structure (1713) in the region of the adhesive (1705).

20. An optical module according to claim 19, characterized in that the diffraction structure (1813) has an inclination angle with respect to a perpendicular to the surface of the optical element (1801).

21. An optical module according to claim 19 or 20, characterized in that it has a radiation absorber (1815).

22. An optical module according to any one of claims 1 to 21, characterized in that the optical elements (301, 1101, 1701, 1801) are designed as lenses (120, 140, 141).

23. An optical system (2, 12, 14) comprising an optical module according to any one of claims 1 to 22.

24. An apparatus comprising an optical module according to any one of claims 1 to 22 or an optical system according to claim 23, the apparatus being designed as a UV lithography apparatus (1) or an inspection apparatus (2).

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