Optical module in ultraviolet wavelength range

Abstract

The invention relates to an optical module (300) comprising an optical element (301) for an operating wavelength range in the ultraviolet wavelength range and a holder (303), in which the optical element (301) is bonded by radiation (309) to the holder (303) with an adhesive (305) curable in a curing wavelength range in the ultraviolet wavelength range, and in which the module (300) has a protective adhesive coating (307), wherein the protective adhesive coating (307) is multi-layered and is highly reflective and slightly absorptive in the operating wavelength range. The invention further relates to an optical module, in which the adhesive protective coating that absorbs at the operating wavelength has an anti-reflective coating, and to an optical module, which has a diffractive structure in the region of the adhesive.

Description

Technical Field

[0001] The present invention relates to an optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by an adhesive that is curable by radiation at a curing wavelength in the ultraviolet wavelength range, and wherein the module has an adhesive protective coating. Furthermore, the present invention relates to an optical system and an apparatus comprising one of the aforementioned optical modules. This application claims priority from German patent application 10 2023 201 742.3 filed February 27, 2023, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0002] DE 10 2011 080 369 A1 discloses an assembly for a DUV microlithography projection exposure apparatus, specifically consisting of a holder and a component adhesively bonded together by an adhesive and transmitting radiation in the ultraviolet spectral range. The adhesive is curable by UV light. Within the adhesive region, a layer is present that transmits UV light from the spectral range suitable for curing the adhesive and absorbs UV light from the spectral range used. Particularly suitable is an absorption layer composed of tantalum pentoxide, which achieves high transmittance at a curing wavelength of 365 nm and very low transmittance at an operating wavelength of 248 nm or less, while exhibiting high absorption and low reflection in both spectral ranges. Summary of the Invention

[0003] The object of the present invention is to further develop the known assembly.

[0004] This object is achieved by an optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by an adhesive that can be cured by radiation at a curing wavelength in the ultraviolet wavelength range, and wherein the module has an adhesive protective coating, wherein the adhesive protective coating has a multilayer design and is highly reflective and slightly absorptive at the operating wavelength.

[0005] It has been discovered that providing a reflective multilayer adhesive protective coating makes it possible to tailor the adhesive protective coating to any desired combination of operating and curing wavelengths. This further makes it possible to avoid undesirable changes in the optical properties of the optical element due to heating from absorbed radiation, such as the resulting inhomogeneous changes in the refractive index in the lens, which in turn leads to undesirable wavefront deformation that affects the imaging properties of the lens. The choice of layer materials, layer thicknesses, and number of layers allows for flexible influence on the reflection, transmission, and absorption of specific wavelengths or wavelength ranges. In general, a wavelength is referenced for wavelength ranges with a width of ±1%. For wider wavelength ranges, the central wavelength can be considered instead. In particular, not only can the reflection at the operating wavelength be set as high as possible and the absorption, particularly at the operating wavelength, be set as low as possible, but the transmission, particularly at the curing wavelength, can also be set high enough to facilitate the adhesive curing process. Advantageously, the absorption at both the operating and curing wavelengths should be low enough to prevent undesirable heating of the optical element, which would result in changes in optical properties beyond manufacturing tolerances. In this context, any residual absorption present can be used to further reduce the radiation load on the adhesive at the operating wavelength above and above the reflection at the adhesive protective layer. Highly reflective is understood here to mean a reflection of 70% or more, and slightly absorptive to mean an absorption of 30% or less.

[0006] Advantageously, the adhesive protective coating comprises at least three layers, so that, in particular, the reflection at the operating wavelength can be selectively influenced and increased by the choice of the layer materials and layer thicknesses.

[0007] In a preferred embodiment, the adhesive protective coating comprises a layer consisting of a material having a higher refractive index at the operating wavelength and a layer consisting of a material having a lower refractive index at the operating wavelength, wherein the layers consisting of the material having the higher refractive index and the layers consisting of the material having the lower refractive index are arranged alternately in each case. Reflection can occur at each layer boundary. The corresponding selection of the layer thickness as a function of the incident wavelength makes it possible to increase the total reflection at the adhesive protective coating and reduce absorption as a whole. In this case, it is particularly advantageous to select a layer thickness on the order of one quarter of the incident wavelength. In order to obtain the highest possible reflection at a certain wavelength, preferably, an optical thickness of the order of half a wavelength is adopted for the layer pair consisting of the layer consisting of the material having the higher refractive index and the layer consisting of the material having the lower refractive index. The refractive index of each layer material affects the optical thickness.

[0008] Advantageously, the adhesive protective coating comprises at least two layers of a material having a higher refractive index at the operating wavelength and at least one layer of a material having a lower refractive index at the operating wavelength, so as to achieve an increase in reflection and a simultaneous reduction in absorption, particularly at the operating wavelength. Furthermore, in this case, the transmittance at the curing wavelength remains sufficiently high to cure the adhesive.

[0009] For these purposes, the adhesive protective coating preferably has between three and sixteen layers of a material having a higher refractive index at the operating wavelength and between three and sixteen layers of a material having a lower refractive index at the operating wavelength. These numbers of layers allow the reflection to be set effectively without incurring excessive coating costs.

[0010] Advantageously, the adhesive protective coating has a layer consisting of a material with a higher refractive index at the operating wavelength as the outermost layer on the adhesive side. Equally advantageously, the adhesive protective coating has a layer consisting of a material with a higher refractive index at the operating wavelength as the outermost layer on its side away from the adhesive. These two measures can contribute to increasing the reflection at the operating wavelength individually or in combination. In the latter case, preferably, the thickness of the outermost layer away from the adhesive can be a thickness different from the thickness of other layers consisting of a material with a higher refractive index at the operating wavelength. In the case of a multilayer adhesive protective coating optimized for particularly high reflection within the scope of the operating wavelength, the transmission within the scope of the curing wavelength can be improved by means of this measure.

[0011] Preferably, all layers of materials with a higher or lower refractive index at the operating wavelength have the same thickness. The resulting (due to its periodicity) construction of the adhesive protective coating allows for a significant increase in reflection, particularly for narrow operating wavelength ranges with widths in the nanometer or subnanometer range, or a significant reduction in absorption. Consequently, such modules are particularly suitable for use, for example, with lasers as radiation sources.

[0012] In a preferred embodiment, the operating wavelength is not equal to the curing wavelength. In this case, the adhesive protective coating has a reflection of greater than 70%, preferably greater than 80%, and particularly preferably greater than 90% at the operating wavelength, and a transmission of greater than 90% at the curing wavelength. Thus, after unimpeded curing, impairment of the optical properties due to undesired heating of the optical element can be particularly effectively avoided. Very particularly preferably, the optical module is designed for an operating wavelength of approximately 193 nm or 248 nm and a curing wavelength of approximately 365 nm.

[0013] In another preferred embodiment, the operating wavelength is equal to the curing wavelength. In this case, the adhesive protective coating has a reflection of greater than 70%, preferably greater than 80%, and particularly preferably greater than 90% at the operating wavelength, and a transmission of greater than 2%, preferably 6%, at the curing wavelength. In this way, any impairment of the optical properties due to undesired heating of the optical element can be particularly effectively avoided without hindering the curing of the adhesive. Very particularly preferably, the optical module is designed for an operating and curing wavelength of approximately 365 nm.

[0014] Preferably, the adhesive protective coating comprises one material, preferably two materials, selected from the group consisting of silicon dioxide, aluminum oxide, hafnium dioxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, pyroxene, and magnesium fluoride. Particularly preferably, the adhesive protective coating comprises a material selected from the group consisting of hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide, and zinc sulfide as the material having the higher refractive index at the operating wavelength, and a material selected from the group consisting of silicon dioxide, aluminum fluoride, magnesium fluoride, cryolite, and pyroxene as the material having the lower refractive index at the operating wavelength. These materials, particularly for the ultraviolet wavelength range between 150 nm and 400 nm, allow the production of multilayer adhesive protective coatings with large differences in refractive index at the operating wavelength, which can result in correspondingly high reflections. Particularly advantageous are combinations of lower refractive index silicon dioxide with higher refractive index aluminum oxide, in particular for operating wavelengths of approximately 193 nm and 248 nm, or with higher refractive index hafnium dioxide, in particular for an operating wavelength of 248 nm, and with higher refractive index tantalum pentoxide, for an operating wavelength of approximately 365 nm.

[0015] On the other hand, the object is achieved by an optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by an adhesive, and wherein the module has an adhesive protective coating that is absorptive at the operating wavelength, and the adhesive protective coating has an antireflection coating.

[0016] The optical module offers the following advantages: the adhesive protective coating with an anti-reflective coating not only protects the adhesive from radiation damage but also suppresses residual reflections that could cause disturbing radiation. The module is particularly suitable for combining with optical components that, when installed in the beam path of an optical system, do not significantly affect the imaged wavefront when heated.

[0017] In a preferred embodiment, the antireflection coating comprises layers composed of a material with a higher refractive index at the operating wavelength and layers composed of a material with a lower refractive index within the operating wavelength range, wherein the layers composed of the material with the higher refractive index and the layers composed of the material with the lower refractive index are arranged alternately. The choice of layer materials, layer thicknesses, and number of layers allows for flexible influence on the reflection, transmission, and absorption of specific wavelengths or wavelength ranges. In general, for wavelength ranges with a width of ±1%, reference is made to a wavelength. For wider wavelength ranges, the central wavelength can be considered instead. To achieve the lowest possible reflection at a wavelength, an optical thickness of the order of one quarter of the wavelength is employed for layer pairs composed of a layer composed of a material with a higher refractive index and a layer composed of a material with a lower refractive index, but significantly greater or lesser thicknesses are also possible. The refractive index of the individual layer materials influences the optical thickness. Preferably, the antireflective coating of the adhesive protective coating, such as the above-mentioned multi-layer adhesive protective coating, comprises one material, preferably two materials, selected from the group consisting of silicon dioxide, aluminum oxide, hafnium dioxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, pyroxene, and magnesium fluoride. Particularly preferably, the adhesive protective coating comprises a material selected from the group consisting of hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide, and zinc sulfide as the material having a higher refractive index at the operating wavelength, and a material selected from the group consisting of silicon dioxide, aluminum fluoride, magnesium fluoride, cryolite, and pyroxene as the material having a lower refractive index at the operating wavelength.

[0018] In a further preferred embodiment, the antireflection coating is implemented as a coating with a refractive index gradient. This allows the antireflection effect to be effective over a wider wavelength range. Particularly advantageously, the refractive index changes from the refractive index of the optical element material to the refractive index of the adhesive protective coating material. A most continuous variation of the refractive index can achieve a particularly good antireflection effect.

[0019] In another aspect, the object is achieved by an optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by means of an adhesive, and wherein the module has a diffractive structure in the region of the adhesive.

[0020] The provision of a diffractive structure ensures that radiation at the operating wavelength does not reach the adhesive and damage it in the process, that it does not reflect at the edges of the optical element and reduce the contrast of the image, and that it does not be absorbed and contribute to heating of the optical element, which could have a disruptive effect on the wavefront and therefore on the image, particularly in the case of uneven or only localized heating. To this end, the diffractive structure is preferably structured so that the radiation incident thereon is diffracted from the beam path. Particularly preferably, a diffractive structure embodied as a periodic diffraction grating is provided. The provision of the diffractive structure can also be combined with a highly reflective or, in particular, anti-reflective adhesive protective coating to remove interfering radiation of different wavelengths or different angles of incidence from the beam path.

[0021] Preferably, the diffractive structure is tilted relative to the surface normal of the optical element, in particular if the diffractive structure is implemented as a periodic diffraction grating. This has the significant advantage that, compared to a diffractive structure without a tilt, not only the first and second orders are diffracted from the beam path, but also the zeroth order, which is also the order with the highest intensity. With diffractive structures having varying heights and tilts, an antireflection effect can also be achieved for a wider range of wavelengths or angles of incidence.

[0022] Advantageously, the optical module has a radiation absorber which can absorb the diffracted radiation and thus effectively remove it from the beam path.

[0023] Preferably, in all the modules mentioned above, the optical element is designed as a lens. Therefore, the optical module is particularly suitable for UV lithography devices and inspection systems for inspecting masks exposed by means of lithography or inspecting wafers before or after exposure.

[0024] Furthermore, the object is achieved by an optical system comprising one of the optical modules described above. Furthermore, the object is achieved by an apparatus comprising one of the optical modules described above or comprising the just-mentioned optical system, wherein the apparatus is designed as a UV lithography apparatus or an inspection apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be explained in more detail with reference to preferred exemplary embodiments. In this respect:

[0026] Figure 1 shows a basic schematic diagram of a UV lithography apparatus;

[0027] Figure 2 shows a basic schematic diagram of the inspection system;

[0028] Figure 3 shows a basic schematic diagram of an optical module;

[0029] Figure 4 A basic schematic diagram showing an adhesive protective coating for an optical module;

[0030] Figure 5 shows the reflectance as a function of wavelength for a first exemplary embodiment and a second exemplary embodiment of an adhesive protective coating;

[0031] Figure 6 shows the transmission as a function of wavelength for a first exemplary embodiment and a second exemplary embodiment of an adhesive protective coating;

[0032] Figure 7 shows the reflectance as a function of wavelength for a third exemplary embodiment and a fourth exemplary embodiment of an adhesive protective coating;

[0033] Figure 8 shows the transmission as a function of wavelength for a third and fourth exemplary embodiments of an adhesive protective coating;

[0034] Figure 9 shows the reflectance as a function of wavelength for a fifth and sixth exemplary embodiments of an adhesive protective coating;

[0035] Figure 10 shows the transmission as a function of wavelength for a fifth and sixth exemplary embodiments of an adhesive protective coating;

[0036] Figure 11 shows a basic schematic diagram of the second optical module;

[0037] Figure 12 shows the ratio of oxygen concentration as a function of distance from the lens surface for a first embodiment of an adhesive protective coating with an antireflective coating;

[0038] Figure 13 shows the reflection as a function of wavelength for a first embodiment of an adhesive protective coating with an antireflective coating;

[0039] Figure 14 shows the reflection as a function of the angle of incidence for a first embodiment of an adhesive protective coating with an antireflective coating;

[0040] Figure 15 shows the reflection as a function of wavelength for a second embodiment of an adhesive protective coating with an antireflective coating;

[0041] Figure 16 shows the transmission as a function of wavelength for a second embodiment of an adhesive protective coating with an anti-reflective coating;

[0042] Figure 17shows a basic schematic diagram of a third optical module; and

[0043] Figure 18 A basic schematic diagram of an embodiment of a third optical module is shown. DETAILED DESCRIPTION

[0044] Figure 1 A basic schematic diagram of an apparatus 1 for UV lithography, particularly for wavelengths in the 190 nm to 400 nm range, is shown. The UV lithography apparatus 1 comprises two optical systems 12 and 14, an illumination system 12, and a projection system 14 as essential components. Lithography requires a radiation source 10, preferably an excimer laser emitting at, for example, 193 nm or 248 nm, which can be an integral part of the UV lithography apparatus. Alternatively, a mercury lamp can be used, whose i-line emission around 368 nm can be used as an operating wavelength and / or as a curing wavelength for adhesive bonding processes performed outside the corresponding optical system. Radiation 11 emitted by radiation source 10 is conditioned by means of illumination system 12 so that a mask 13 (also called a reticle) can be illuminated. In the example shown here, illumination system 12 includes transmissive and reflective optical elements. Transmissive optical element 120, for example, for focusing radiation 11, and reflective optical element 121, for example, for deflecting radiation, are shown as representative elements. All optical elements can be part of the optical module presented here. In a known manner, various transmissive, reflective and other optical elements can be combined with one another in any desired, even more complex, manner in the illumination system 12. It should be noted that the mask 13 can also be part of the optical module proposed here.

[0045] Mask 13 has structures on its surface that are transferred to element 15 to be exposed, such as a wafer in the context of semiconductor component manufacturing, with the aid of projection system 14. In this example, mask 13 is embodied as a transmissive optical element. In other embodiments, it can also be configured as a reflective optical element. In the example shown here, projection system 14 includes at least one transmissive optical element. In the example shown here, two transmissive optical elements 140 and 141 are shown as representative elements, which are used, for example, in particular to reduce the structures on mask 13 to the dimensions required for exposure of wafer 15. Projection system 14 can also include, in particular, reflective optical elements, and a variety of optical elements can be combined with one another in any manner known in the art. It should be noted that an optical system without transmissive optical elements can also be provided, particularly in the case of optical systems optimized for wavelengths below 200 nm. All optical elements of projection system 14 can also be part of the optical module presented here.

[0046] An optical module comprising an optical element and a holder for an operating wavelength in the ultraviolet wavelength range can also be used in a wafer or mask inspection system, wherein the optical element is adhesively bonded to the holder by an adhesive, the adhesive being curable by radiation in the curing wavelength range in the ultraviolet wavelength range, and wherein the module has an adhesive protective coating, wherein the adhesive protective coating has a multi-layer design and is highly reflective and slightly absorptive at the operating wavelength. Figure 2 An exemplary embodiment of a wafer inspection system 2 is schematically illustrated in FIG. These explanations apply equally to a mask inspection system.

[0047] Wafer inspection system 2 includes a radiation source 20, whose radiation is directed onto a wafer 25 by means of an optical system 22. To this end, the radiation is reflected onto wafer 25 by a concave reflector 220. In the case of a mask inspection system, a mask to be inspected may be provided instead of wafer 25. Radiation reflected, diffracted, and / or refracted by wafer 25 is directed onto a detector 23 for further evaluation by a concave reflector 221, which is also associated with optical system 22. Radiation source 20 may be, for example, a single radiation source or a combination of multiple individual radiation sources to provide a substantially continuous radiation spectrum. In modifications, one or more narrowband radiation sources may also be used. The inspection system shown here by way of example is designed for an operating wavelength range between 190 nm and 300 nm, particularly preferably between 190 nm and 200 nm. In addition to and / or as an alternative to the two concave reflectors 220, 221, a lens may also be provided in the wafer or mask inspection system. In the case of an inspection system designed for an operating wavelength in the range of 200 nm to 400 nm, in particular 300 nm to 400 nm, all optical elements can be embodied as lenses.

[0048] Figure 3An exemplary embodiment of an optical module 300 is shown. In this example, optical element 301 is a concave lens that is transparent for operating wavelengths in the ultraviolet range. Lens 301 is mounted in a holder 303. To this end, the lens is fixedly adhesively bonded to holder 303 using a UV-curable adhesive 305, preferably outside the area used optically during operation. For example, an epoxy-based adhesive that cures with UV radiation from the i-line of a mercury lamp can be used as the mounting adhesive. As a result of exposure to the lens during operation, particularly through stray light or multiple reflections, the adhesive can become damaged by radiation and lose its adhesion or deform over time, causing lens 301 to no longer maintain its originally adjusted position in holder 303 and potentially leading to imaging aberrations. Therefore, an adhesive protective coating 307 is provided between lens 301 and adhesive 305. It has a multilayer design and is highly reflective and slightly absorptive at the operating wavelength. Like, for example, dielectric multilayer antireflection coatings commonly applied to lenses, the adhesive protective coating can be applied using conventional thin-film coating methods, such as thermal electron beam evaporation or magnetron sputtering. To increase the mechanical strength of the optical module 300, an ion-assisted coating method is preferably selected. To simplify processing, an adhesive protective coating 307 can be locally applied to the lens 301 before the lens 301 is adhesively bonded to the holder 303. The adhesive can then be cured by irradiation (indicated by dashed arrow 309) at a curing wavelength for 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, making the absorption at the adhesive bond joint sufficiently low to avoid localized heating of the lens 301, which could result in imaging aberrations caused, for example, by wavefront aberrations. Absorption, which may be present at a sufficiently low level to avoid undesirable heating, can further help protect the adhesive 305 from radiation damage.

[0049] exist Figure 4 The region of the adhesive protective coating of the optical module is shown in greater detail in FIG. Adhesive protective coating 407 is disposed between optical element 401 and adhesive 405. Adhesive protective coating 407 is constructed in a multilayer manner, preferably comprising at least three layers. The number of layers, layer thicknesses, and layer materials are selected such that, for any desired combination of operating wavelength and curing wavelength, reflection, transmission, and absorption can be established at the relevant wavelengths. The reflection is as high as possible and the absorption is as low as possible to avoid undesirable heating of optical element 401 and radiation damage to adhesive 405, while the transmission at the curing wavelength remains sufficiently high to enable curing of adhesive 405.

[0050] It has proven particularly advantageous if the adhesive protective coating 407 comprises layers 471 of a material having a higher refractive index at the operating wavelength and layers 473 of a material having a lower refractive index at the operating wavelength, wherein the layers 471 of the material having a higher refractive index and the layers 473 of the material having a lower refractive index are arranged alternately. This allows for a particularly effective design of the adhesive protective coating 407, which meets the requirements for high reflection, low absorption, and sufficient transmission for freely selectable operating and curing wavelengths in the UV wavelength range. Preferably, the adhesive protective coating 407 comprises at least two layers 471 of a material having a higher refractive index at the operating wavelength and at least one layer 473 of a material having a lower refractive index at the operating wavelength. Particularly preferably, the adhesive protective coating 407 comprises three to sixteen layers 471 of a material having a higher refractive index at the operating wavelength and three to sixteen layers 473 of a material having a lower refractive index at the operating wavelength.

[0051] exist Figure 4In the example shown, four layers 471 of a material having a higher refractive index at the operating wavelength and three layers 473 of a material having a lower refractive index at the operating wavelength are provided. Layers 471', 471 composed of a material having a higher refractive index are provided as the respective outermost layers on the adhesive side and facing away from the adhesive. In the example shown here, all layers 471 of a material having a higher refractive index and all layers 473 of a material having a lower refractive index each have the same thickness, while the thickness of the outermost layer 471' on the adhesive side is different from the thickness of the other layers 471 composed of a material having a higher refractive index. Thus, there is essentially a periodic layer sequence characterized by particularly high reflection at the wavelength, particularly if there is a unit composed of two adjacent layers 471, 473 composed of materials having a higher and a lower refractive index at approximately one-quarter of the wavelength. If desired, the periodicity can be disrupted by the outermost layer on the adhesive side to slightly increase the transmittance at the curing wavelength to allow for faster curing. As an alternative, the bottom layer 471 on the optical substrate 401, which comprises a material with a higher refractive index, can also have a thickness that is different from one-quarter of the operating wavelength, in order to thereby achieve the highest possible transmission at the curing wavelength in a targeted manner, while the reflection at the operating wavelength remains high. Silicon dioxide, aluminum oxide, hafnium oxide, tantalum pentoxide, titanium dioxide, zinc sulfide, aluminum fluoride, cryolite, pyroxene cryolite, and magnesium fluoride have proven to be suitable materials for the adhesive protective coating 407 in combination with one or more of these materials or one or more additional materials. The combination of hafnium oxide, aluminum oxide, tantalum pentoxide, titanium dioxide, or zinc sulfide as materials with a higher refractive index and silicon dioxide, aluminum fluoride, magnesium fluoride, cryolite, and pyroxene cryolite as materials with a lower refractive index has proven to be particularly suitable.

[0052] Reflection and transmission curves for various embodiments of adhesive protective coatings are shown in the following figures. Figure 5 Reflection in Figure 6The transmission in FIG is plotted as a function of wavelength for two adhesive protective coatings, A and B, both optimized for an operating wavelength λ1 of 193 nm and a curing wavelength λ2 of 365 nm. Adhesive protective coating A has fourteen 27 nm thick layers composed of aluminum oxide as the material with the higher refractive index, and thirteen 31 nm thick layers composed of silicon dioxide as the material with the lower refractive index, arranged alternately. The reflection and transmission of adhesive protective coating A are plotted as dashed lines, respectively. Adhesive protective coating B has only ten 27 nm thick layers composed of aluminum oxide as the material with the higher refractive index, and nine 31 nm thick layers composed of silicon dioxide as the material with the lower refractive index, arranged alternately. The reflection and transmission of adhesive protective coating B are plotted as solid lines, respectively. Adhesive protective coating B has a reflection of almost 80% and a transmission of almost 20% at the operating wavelength λ1, and virtually 0% reflection and virtually 100% transmission at the curing wavelength λ2. This means that virtually no absorption of the corresponding radiation occurs within the operating wavelength λ1 and within the curing wavelength λ2 range, preventing undesirable heating of the corresponding optical element. However, the adhesive can cure as if the adhesive protective coating were not present, and the cured adhesive remains protected from radiation damage. Increasing the number of layers in adhesive protective coating A even leads to a reflection of more than 90% and a transmission of less than 10% at the operating wavelength λ1, thereby providing even better protection from radiation damage and a correspondingly longer lifetime. The residual absorption of less than 1% in the adhesive protective coating only results in lens heating, which is so low that the wavefront and imaging properties do not change beyond the necessary requirements. Rather, it serves to additionally protect the adhesive from radiation damage by absorbing the residual radiation energy.

[0053] exist Figure 7 Reflection in Figure 8The transmission in FIG is plotted as a function of wavelength for two adhesive protective coatings, C and D, both optimized for an operating wavelength λ1 of 248 nm and a curing wavelength λ2 of 365 nm. Adhesive protective coating C has seven alternating layers of hafnium oxide (the material with a higher refractive index) with a thickness of 30 nm and seven alternating layers of silicon dioxide (the material with a lower refractive index). Finally, another 15 nm thick layer of hafnium oxide follows on the adhesive side as the topmost layer. The reflection and transmission of adhesive protective coating C are plotted as dashed lines, respectively. Adhesive protective coating D has only five alternating layers of hafnium oxide (the material with a higher refractive index) with a thickness of 30 nm and five alternating layers of silicon dioxide (the material with a lower refractive index). The reflection and transmission of adhesive protective coating D are plotted as solid lines, respectively. Adhesive protective coating D has a reflection of almost 95% and a transmission of just over 5% at the operating wavelength λ1, and virtually 0% reflection and virtually 100% transmission at the curing wavelength λ2. This means that virtually no absorption of the corresponding radiation occurs within the operating wavelength λ1 and the curing wavelength λ2, preventing undesirable heating of the corresponding optical element. However, the adhesive can cure as if the adhesive protective coating were not present, and the cured adhesive remains protected from radiation damage. Increasing the number of layers in adhesive protective coating C even leads to a reflection of almost 100% and a transmission of almost 0% at the operating wavelength λ1, thereby providing even better protection from radiation damage and a correspondingly longer lifetime. At the curing wavelength λ2, the reflection is approximately 2% and the transmission is approximately 98%, yet this still allows the adhesive to cure completely unimpeded. The residual absorption of less than 1% in the adhesive protective coating only results in lens heating, which is so low that the wavefront and imaging properties do not change beyond the necessary requirements. Rather, it serves to additionally protect the adhesive from radiation damage by absorbing the residual radiation energy.

[0054] exist Figure 9 Reflection in Figure 10 The transmission in is plotted as a function of wavelength for two adhesive protective coatings E and F, both for an operating wavelength of 365 nm and a curing wavelength λ 1 / 2was optimized. The adhesive protective coating E has seven layers with a thickness of 42 nm composed of tantalum pentoxide as a material with a higher refractive index and six layers with a thickness of 60 nm composed of silicon dioxide as a material with a lower refractive index, which are arranged alternately. The reflection and transmission of the adhesive protective coating E are plotted as dashed lines. The adhesive protective coating F has only five layers with a thickness of 42 nm composed of tantalum pentoxide as a material with a higher refractive index and four layers with a thickness of 60 nm composed of silicon dioxide as a material with a lower refractive index, which are arranged alternately. The reflection and transmission of the adhesive protective coating F are plotted as solid lines. The adhesive protective coating E at the operating wavelength and the curing wavelength λ 1 / 2 At λ, the reflectance is about 97% and the transmission is slightly above 3%, so that in this wavelength range practically no absorption of the corresponding radiation occurs, so that no undesired heating of the corresponding optical element occurs. The relatively low transmittance can be compensated by a longer irradiation duration for adhesive curing. Reducing the number of layers in the adhesive protective coating F still results in a difference between the operating wavelength and the curing wavelength λ. 1 / 2 With a reflection of about 93% and a transmission of about 7%, the adhesive cures to about twice as fast as in the case of the first approximate adhesive protective coating E.

[0055] In order to estimate the transmission required for curing the adhesive, for example in the case of the 365 nm i-line of a mercury lamp as the curing wavelength, one can resort to the required irradiation dose D (in J / cm2) of the mercury lamp for curing on the optical element at the curing wavelength in each case. 2 The maximum permissible exposure duration t (in s) and power density P (in W / cm 2 The required transmission (percent) at the curing wavelength is then calculated as

[0056] .

[0057] The minimum allowable reflection at the operating wavelength can be calculated accordingly from the guaranteed lifetime t LT (in s), the maximum radiation dose D that still maintains the adhesive function degradation (in J / cm 2 as units), as well as stray light and residual reflection P stray (in W / cm 2 The power density (in J / cm2) occurring at the adhesive joint 2 Units) to estimate, and then calculate as

[0058] .

[0059] These estimates are particularly important if the curing wavelength is equal to the operating wavelength. If necessary, first, the reflection can be increased to the desired value by increasing the number of layers, and second, the transmission at the curing wavelength can be increased by appropriately detuning the outermost layer on the adhesive side composed of the material with the higher refractive index by selecting a different layer thickness than the other layers composed of the material with the higher refractive index.

[0060] Figure 11 An exemplary embodiment of a second optical module 1100 is shown. In this example, optical element 1101 is a convex lens transparent to operating wavelengths in the ultraviolet range. Lens 1101 is mounted in a holder 1103. To this end, the lens is fixedly adhered to holder 1103 using a UV-curable adhesive 1105, preferably outside the area of ​​optical use during operation. For example, an epoxy-based adhesive that cures with UV radiation from the i-line of a mercury lamp can be used as the mounting adhesive. As a result of exposure to the lens during operation, particularly through stray light or multiple reflections, the adhesive can sustain radiation damage and lose its adhesion or deform over time, causing lens 1101 to no longer maintain its originally adjusted position in holder 1103 and potentially leading to imaging aberrations. Therefore, an adhesive protective coating 1111, comprising an absorption layer 1110 and an antireflection coating 1112, is provided between lens 1101 and adhesive 1105. The antireflection coating 1112 can be applied using conventional thin-film coating methods, such as thermal electron beam evaporation or magnetron sputtering. In order to increase the mechanical strength of the optical module 1100 , an ion-assisted coating method is preferably selected for applying the adhesive protective coating 1111 , in particular the anti-reflective coating 1112 thereof.

[0061] In one embodiment, the antireflective coating of the adhesive protective coating is embodied as a coating having a refractive index gradient. A significant antireflective effect is possible if the coating between the material of the optical element and the adhesive protective coating has a refractive index gradient that continuously changes from the refractive index of the material of the optical element to the refractive index of the material of the adhesive protective coating.

[0062] In this example, an optical module is considered, in which the optical element is designed as a lens made of quartz glass, and the material of the absorption layer of the adhesive protective coating is amorphous silicon, which strongly absorbs radiation with a wavelength of less than 400nm. Between the lens and the amorphous silicon layer, for the anti-reflection effect, a SiOx gradient layer is applied, the oxygen content of which decreases continuously from the lens surface (where x=2) to x=0 at the transition to the silicon layer, as shown in FIG. Figure 12 For this purpose, for example, silicon can first be reactively sputtered from a target in an oxygen-saturated atmosphere, with the result that a SiO2 layer is first formed. The oxygen content in the coating apparatus is then continuously reduced, with the result that the oxygen content in the SiOx layer also decreases and x decreases from 2 to 0. Finally, at less than 10-5 Silicon is sputtered in a high vacuum at a pressure of 100 mbar, ie in the absence of oxygen. Figure 13 The reflection of the optical module is shown as a function of wavelength for a given quasi-normal incidence. The vertical variable dashed lines represent wavelengths 193 nm, 248 nm, and 365 nm. The solid reflection profile shows the reflection of the optical module including an anti-reflective adhesive protective coating (i.e., including amorphous silicon), which is used to pass through a device having a structure according to Figure 12 The refractive index gradient coating is used to protect the adhesive and anti-reflection coating. The dashed reflection profile shows the reflection of a corresponding optical module without anti-reflection coating (i.e., only amorphous silicon on quartz glass) for comparison. Figure 14 The reflection at a wavelength of 248 nm is shown as a function of the angle of incidence. The solid reflection profile shows the reflection with an antireflection coating on an adhesive protective coating composed of amorphous silicon, and the dashed reflection profile shows the reflection without an antireflection coating on an adhesive protective coating composed of amorphous silicon for comparison. The antireflection coating of the adhesive protective coating with the coating having a refractive index gradient results in very low reflections of less than 1% over a very wide wavelength range of 180 nm to 390 nm and a very wide angle of incidence range of up to 50°, and thus this optical module can be used very flexibly.

[0063] In a further embodiment, the antireflection coating can have layers of a material having a higher refractive index at the operating wavelength and layers of a material having a lower refractive index in the operating wavelength range, wherein the layers of the material having the higher refractive index and the layers of the material having the lower refractive index are arranged alternately in each case. The resulting dielectric multilayer antireflection coating can be similar to Figure 3 and Figure 4The following describes the high-reflection coating of the first optical module in FIG. The fundamental difference lies in the different layer thicknesses used. However, for an antireflection effect, the optical thickness of the layer pairs composed of materials with higher and lower refractive indices is generally on the order of approximately one-quarter of the operating wavelength of the corresponding optical element; for particularly high reflections, it is on the order of approximately half the operating wavelength. For optical thickness, the geometric thickness is weighted by the refractive index of the corresponding layer materials. The specific thicknesses of the higher and lower refractive index layers are optimized to achieve the highest possible antireflection effect. Suitable materials for the higher refractive index layer include, in particular, aluminum oxide for wavelengths of 193 nm or greater, hafnium oxide for wavelengths of 248 nm or greater, and tantalum pentoxide for wavelengths of 365 nm or greater. These materials, cited by way of example, have little absorption above the respective wavelengths indicated. A suitable material for the lower refractive index layer is silicon dioxide, particularly for wavelengths of 193 nm or greater. Applying the higher and lower refractive index layers using an ion-assisted coating process ensures high mechanical stability and adequate layer adhesion.

[0064] For optical modules having a dielectric multilayer antireflection layer on their adhesive protective coating, Figure 15 The reflection as a function of wavelength is shown in Figure 16 Figure 2 shows transmission as a function of wavelength. In this case, the dashed vertical 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 an optical element in the form of a lens made of quartz glass and an adhesive protective coating in the form of a layer of tantalum pentoxide with a thickness of 180 nm. The antireflection coating applied to it is a coating composed of 22 nm aluminum oxide, 33 nm silicon dioxide, and 22 nm aluminum oxide. The dashed reflection and transmission profiles show the reflection and transmission with the antireflection coating, while the solid line reflection and transmission profiles show the reflection and transmission without the antireflection coating for comparison. The antireflection coating, which is a multilayer dielectric coating, produces a targeted antireflection effect around the operating wavelength. In this case, combined with negligible transmission, the entire incident radiation is absorbed in the adhesive protective coating. In contrast, at the curing wavelength, the transmission is very high, with both the presence and absence of the antireflection coating, with low reflection and absorption, allowing the adhesive to cure very well through the quartz glass lens and the adhesive protective coating of tantalum pentoxide.

[0065] In general, any material with a relatively high refractive index that is also suitable as a layer material for a multilayer antireflection coating or a multilayer highly reflective adhesive protective coating can be used as an absorbing material for the adhesive protective coating in a wavelength-dependent manner. For absorbing UV radiation with a relatively long wavelength (e.g., 365 nm), metals are suitable, more particularly chromium, which also has good adhesion properties on conventional lens materials. Suitable materials for shorter wavelengths (e.g., in the range of 248 nm or 193 nm) are, for example, fluoride crystals (e.g., calcium fluoride) or quartz glass, while suitable materials for longer wavelengths (e.g., approximately 365 nm) are, for example, leaded glass or borosilicate glass. Depending on the choice of materials for the adhesive protective coating and for the optical element, an adhesion promoter may also be used.

[0066] Figure 17 and Figure 18 A further optical module 1700, 1800 is schematically shown, comprising an optical element 1701, 1801 for an operating wavelength in the ultraviolet wavelength range and a holder 1703, 1803, wherein the optical element 1701, 1801 is adhesively bonded to the holder 1703, 1803 by means of an adhesive 1705, 1805. The optical module 1700, 1800 shown here has a diffractive structure 1713, 1813 in the region of the adhesive 1705, 1805. In this example, the optical element 1701, 1801 is embodied as a lens.

[0067] Because lenses 1701, 1801 are specifically designed in their edge regions (where the corresponding holders 1703, 1803 are arranged) so that radiation 1721, 1723, 1821 incident thereon is diffracted from the beam path, it is possible to ensure that radiation 1721, 1723, 1821 does not reach adhesive 1705, 1805 and damage it in the process, nor does radiation 1721, 1723, 1821 reflect at the lens edges, thereby reducing contrast. Furthermore, radiation 1721, 1723, 1821 is not absorbed and contributes to lens heating, which has a disruptive effect on the wavefront and, therefore, on the imaging. The diffraction structure can be selected, for example, in the form of a periodic diffraction grating 1713, without tilting the diffraction structure relative to the lens surface. Thus, first- and higher-order diffraction is deflected from disruptive reflection paths, although, depending on the diffraction efficiency, it is necessary to ensure that the diffraction path leads to relatively non-critical regions.

[0068] exist Figure 17In the example shown, a radiation absorber 1715 is provided on the optical module 1700. The radiation absorber has a low thermal coupling to the lens 1701 and substantially absorbs the radiation 1722, 1724 incident on it and diffracted from the beam path by the diffraction grating 1713, and thus it ultimately removes said radiation from the optical module 1700.

[0069] However, it is very advantageous if the diffraction structure is designed as a tilted structure 1813, in particular a grating, such as a blazed grating or an echelle grating, for example, in which the zeroth order is already diffracted from the beam path (see also Figure 18 In this example, the tilt angle of the diffraction structure 1813 is designed so that the diffracted radiation 1822 is directed to a radiation absorber which causes absorption as completely as possible, which is not shown in FIG. Figure 18 In this example, the depth of structure 1813 is 1 / 4 of the operating wavelength to be diffracted, i.e., 193 nm / 4 (i.e., approximately 48 nm), 248 nm / 4 (i.e., 62 nm), or 365 nm / 4 (i.e., approximately 91 nm). If structure 1813 is filled with adhesive 1805, as shown in FIG. Figure 18 In the example shown, the depth should also be divided by the refractive index of the binder. During the fabrication of the diffractive structure, an etch stop layer can be used, where the material of the etch stop layer should be as transparent as possible at the operating wavelength. As an example, the diffractive structure can be produced by etching a silicon dioxide layer having a thickness corresponding to the depth of the structure to be etched, and on an aluminum layer approximately 5 to 10 nm thick that is inert to reactive etching processes.

[0070] Reference Signs List

[0071] 1VUV lithography equipment

[0072] 2Wafer inspection system

[0073] 3Reflective optical elements

[0074] 4 reflective optical elements

[0075] 5Reflective optical elements

[0076] 6 reflective optical elements

[0077] 10 Radiation Source

[0078] 11. Radiation

[0079] 12 Lighting System

[0080] 13 Mask

[0081] 14 Projection System

[0082] 15 Components to be exposed

[0083] 20 Radiation Source

[0084] 21 Radiation

[0085] 22 Optical System

[0086] 23 Detector

[0087] 25 chips

[0088] 50 optical elements

[0089] 51 substrate

[0090] 120 lens

[0091] 121 reflector

[0092] 140 lens

[0093] 141 lens

[0094] 220 reflector

[0095] 221 reflector

[0096] 300 optical modules

[0097] 301 optical components

[0098] 303 retainer

[0099] 305 adhesive

[0100] 307 adhesive protective coating

[0101] 309UV radiation

[0102] 401 optical components

[0103] 405 adhesive

[0104] 407 adhesive protective coating

[0105] 471,471'Higher refractive index layer

[0106] 473 lower refractive index layer

[0107] 1100 optical module

[0108] 1101 Optical Components

[0109] 1103 retainer

[0110] 1105 adhesive

[0111] 1109UV radiation

[0112] 1110 absorption layer

[0113] 1111 adhesive protective coating

[0114] 1112 anti-reflective coating

[0115] 1210 reflective surface

[0116] 1700 optical modules

[0117] 1701 Optical Components

[0118] 1703 retainer

[0119] 1705 adhesive

[0120] 1713 diffraction structure

[0121] 1721UV radiation

[0122] 1722UV radiation

[0123] 1723UV radiation

[0124] 1724UV radiation

[0125] 1800 optical modules

[0126] 1801 Optical Components

[0127] 1803 retainer

[0128] 1805 adhesive

[0129] 1813 diffraction structure

[0130] 1821UV radiation

[0131] 1822UV radiation

Claims

1. An optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by an adhesive that can be cured by radiation at a curing wavelength in the ultraviolet wavelength range, and wherein the module has an adhesive protective coating, characterized in that The adhesive protective coating (307, 407) has a multi-layer design and is highly reflective and slightly absorptive 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, characterized in that The adhesive protective coating (307, 407) has a layer (471) consisting of a material with a higher refractive index at the operating wavelength and a layer (473) consisting of a material with a lower refractive index in the operating wavelength range, wherein the layers (471) consisting of the material with the higher refractive index and the layers (473) consisting of the material with the lower refractive index are arranged alternately in each case.

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

5. The optical module according to any one of claims 1 to 4, characterized in that The adhesive protective coating (307, 407) has three to sixteen layers (471) of a material having a higher refractive index in the operating wavelength range and three to sixteen layers (473) of a material having a lower refractive index in the operating wavelength range.

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

7. The optical module according to claim 6, wherein: The adhesive protective coating (307, 407) has, on its side facing away from the adhesive, a layer (471) as the outermost layer, which layer (471) consists of a material having a relatively high refractive index at the operating wavelength.

8. The optical module according to claim 7, wherein: The outermost layer (471') facing away from the adhesive or facing the adhesive has a thickness that is different from the thickness of the other layers (471) made of a material having a higher refractive index at the operating wavelength.

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

10. The optical module according to any one of claims 1 to 9, wherein the operating wavelength is not equal to the curing wavelength, characterized in that: The adhesive protective coating (307, 407) has a reflection of more than 70%, preferably more than 80%, particularly preferably more than 90% at the working wavelength, and a transmission of more than 90% at the curing wavelength.

11. The optical module according to claim 10, wherein: The optical module is designed for an operating wavelength of approximately 193 nm or 248 nm and a curing wavelength of approximately 365 nm.

12. The optical module according to any one of claims 1 to 9, wherein the operating wavelength is equal to the curing wavelength, characterized in that: The adhesive protective coating (307, 407) has a reflection of greater than 70%, preferably greater than 80%, particularly preferably greater than 90% at the working wavelength, and a transmission of greater than 2%, preferably greater than 6% at the curing wavelength.

13. The optical module according to claim 12, wherein: The optical module is designed for an operating and curing wavelength of approximately 365 nm.

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

15. The optical module according to claim 14, wherein: The adhesive protective coating (307, 407) has a material selected from the group formed by hafnium dioxide, aluminum oxide, tantalum pentoxide, titanium dioxide and zinc sulfide as the material having a higher refractive index at the operating wavelength, and has a material selected from the group formed by silicon dioxide, aluminum fluoride, magnesium fluoride, cryolite and pyroxene cryolite as the material having a lower refractive index at the operating wavelength.

16. An optical module comprising an optical element for an operating wavelength in the ultraviolet wavelength range and a holder, wherein the optical element is adhesively bonded to the holder by an adhesive, and wherein the module has an adhesive protective coating that is absorptive 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 antireflection coating (1112) has a layer consisting of a material with a higher refractive index at the operating wavelength and a layer consisting of a material with a lower refractive index in the operating wavelength range, wherein the layers consisting of the material with the higher refractive index and the layers consisting of the material with the lower refractive index are arranged alternately in each case.

18. The optical module according to claim 16, wherein: The anti-reflection coating (1112) is implemented as a coating with a refractive index gradient.

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

20. The optical module according to claim 19, wherein The diffractive structure (1813) is inclined at an angle relative to the normal to the surface of the optical element (1801).

21. The optical module according to claim 19 or 20, characterized in that The optical module has a radiation absorber (1715).

22. The 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 the optical module according to any one of claims 1 to 22.

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

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