JP7520837B2 - Optical elements and optical devices for reflecting VUV radiation - Patents.com - Google Patents

Description

[Reference to Related Applications]
This application claims priority from German Patent Application No. DE 10 2018 221 191.4 filed on December 7, 2018, the entire disclosure of which is incorporated herein by reference.

The present invention relates to an optical element for reflecting radiation in the VUV wavelength range, the optical element comprising a substrate and a reflective coating applied to the substrate and comprising at least one aluminum layer. The present invention relates to an optical device for the VUV wavelength range, in particular a wafer inspection system or a VUV lithography device, the optical device comprising an interior in which at least one optical element is arranged and at least one gas inlet for supplying gas to the interior.

In particular, in the shortwave ultraviolet wavelength range from about 100 nm to about 200 nm, also referred to as the vacuum ultraviolet wavelength range (VUV wavelength range), not only transmissive but also reflective optical elements are often used. Optical devices for radiation in the VUV wavelength range can be used, for example, to perform optical inspection of wafers or masks or to manufacture semiconductor components.

Optical elements that reflect VUV radiation usually have a reflective coating, which in certain applications, e.g. when inspecting wafers, should have a high reflectivity over a large spectral range in the VUV wavelength range. Since aluminum has a high reflectivity of about 0.9 or 90% in the VUV wavelength range, it has been found to be advantageous if such a reflective coating has one or possibly several aluminum layers as an underlayer(s).

A common problem with aluminum layers in the VUV wavelength range is that when the aluminum layer comes into contact with air or the atmosphere surrounding the reflective optical element, it immediately forms a natural aluminum oxide (Al ₂ O ₃ ) layer having a layer thickness of about ₂ nm to ₃ nm in magnitude, which is highly absorbing in the VUV wavelength range, making the aluminum layer unattractive as a reflective layer for use in the VUV wavelength range unless additional measures are taken to protect it from oxidation.

In order to protect the aluminium layer against oxidation, it is known, for example from the article by G. K. et al., J. Appl. Phys. 1999, 141:1311-1321, to provide it with a protective layer or coating in the form of a metal fluoride, for example MgF ₂ , AlF ₃ or LiF, or in the form of a triple-layer protective coating made of these materials.

However, it has been observed that at high radiation intensities, such as may occur in lithography, especially during mask and wafer inspection, severe degradation of the reflecting optical elements occurs within hours or days, accompanied by significant reflectance losses. Even the protective layers made of the above-mentioned metal fluorides, which in fact show a very good protective effect against the environment, are unable to prevent the oxidation of the aluminum layer in the case of irradiation. A significant decrease in reflectance has also been observed when the oxygen or moisture content in the environment of the optical elements is reduced in order to prevent oxidation.

In the case of optical devices for the VUV wavelength range, there is generally an additional problem of contamination of the optical surfaces, since unwanted gas components in the environment of the optical elements cannot be completely suppressed. These gas components deposit on the optical surfaces and are "burned in" there during irradiation. This problem exists not only for the optical surfaces of reflective optical elements, but also for the optical surfaces of transmissive optical elements.

"Protected and enhanced aluminum mirrors for the VUV" by S. Wilbrandt et al., Applied Optics, Vol. 53, No. 4, February 2014

The object of the present invention is to provide an optical element that reflects radiation in the VUV wavelength range and an optical device for use in the VUV wavelength range that can extend the useful life.

This object is achieved by an optical element of the aforementioned type, in which at least one hydrogen-catalytic layer for the dissociation of (molecular) hydrogen is applied to the aluminum layer.

The inventors have realized that certain materials cause catalytic hydrogen splitting or dissociation of molecular hydrogen, which, when added to the environment of the optical element, possibly in the form of a compound, dissociates in the material of the hydrogen catalyst layer to form active hydrogen, especially at high doses of irradiation. Active hydrogen is understood to mean hydrogen radicals, hydrogen ions, and/or hydrogen in an excited electronic state. This active hydrogen is usually capable of preventing or at least significantly slowing down the oxidation of the aluminum layer, even at high radiation intensities.

In one embodiment, in the case of the material of the hydrogen catalyst layer, this is selected from the group comprising Ru, Pt, Pd, Ni, Rh. It has been shown that for these materials, and in particular for Ru and Pt, oxidation can be virtually completely prevented by adding hydrogen to the environment, for example in a residual gas atmosphere. It is known from lithography in the EUV wavelength range that ruthenium oxide formed in a vacuum environment can be reduced to ruthenium by subsequent addition of hydrogen, i.e. the oxidation reaction can be reversed.

In an advantageous embodiment, the hydrogen catalyst layer has a layer thickness of 0.1 nm to 3.0 nm, preferably 0.1 nm to 1.0 nm. The hydrogen catalyst materials specified above usually have too low a reflectance or too high an absorption in the VUV wavelength range to allow for large layer thicknesses. For example, Ru has a reflectance well below 0.6 in the VUV wavelength range. Correspondingly, a ruthenium layer having a thickness of only 3 nm reduces the overall reflectance of the optical element by about 0.2, which is generally unacceptable.

In yet another embodiment, the hydrogen catalyst layer does not completely cover the aluminum layer. Below a certain value, the layer cannot usually be applied in a completely sealed form. For example, for Ru as the layer material, this is the case for layer thicknesses of about 1.0 nm or less. Thus, below this (material-dependent) value, parts of the surface of the aluminum layer are exposed, and native aluminum oxide can form on said parts, despite the application of the hydrogen catalyst layer. However, even a hydrogen catalyst layer that is not completely sealed can reduce the oxidation rate during irradiation, since a native aluminum oxide layer cannot form on the covered partial area(s) of the aluminum layer. Furthermore, if hydrogen is added to the environment of the optical element, oxidation cannot possibly occur even on the exposed surface areas of the aluminum layer due to the catalytic effect of the material of the hydrogen catalyst layer.

In a further development, the hydrogen catalyst layer covers the aluminum layer with a coverage of approximately 10% to 90%, preferably 30% to 70%. Coverage is understood to mean the percentage of the surface of the hydrogen catalyst layer relative to the entire surface of the aluminum layer (opposite the substrate). Usually, the coverage with the hydrogen catalyst layer takes place in the form of accumulations of island-like spaced apart material of the hydrogen catalyst layer on the aluminum layer in this case. This is advantageous, since, as with a sealed hydrogen catalyst layer, (molecular) hydrogen cannot accumulate between the aluminum and the hydrogen catalyst layer, leading to the formation of bubbles, which would normally lead to a complete or partial peeling of the hydrogen catalyst layer. In order to prevent the formation of bubbles or a deterioration of the coating of the optical element due to the action of hydrogen, it is known from DE 10 2017 222 690 A1 that the upper layer of the coating contains a hydrogen desorption material, which converts hydrogen atoms adsorbed on the surface of the upper layer into molecular hydrogen and desorbs it from the surface.

In yet another embodiment, the reflective coating comprises a protective layer applied to the aluminum layer and the hydrogen catalyst layer. If, for reflection reasons, the hydrogen catalyst layer is applied thin enough not to completely cover the aluminum layer, part of the surface of the aluminum layer is exposed. If a native oxide layer forms on the exposed surface area of the aluminum layer, this reaction generally cannot be reversed by the addition of hydrogen since the reaction enthalpy of aluminum with oxygen is relatively high. The above also applies to active hydrogen formed on the hydrogen catalyst layer, although active hydrogen is much more reactive than molecular hydrogen. However, the situation is different if the oxidation reaction has not yet taken place. The oxygen or hydroxide molecules are much more loosely bound to the surface before the reaction and are therefore more susceptible to reduction by hydrogen radicals, i.e. the oxidation rate of aluminum is reduced in this case.

Therefore, it is usually advantageous to protect the aluminum layer from oxidation at least in the surface area or areas not covered by the hydrogen catalyst layer with a protective layer until a sufficient amount of hydrogen is present in the environment of the optical element to prevent oxidation. This is the case, for example, when a suitable hydrogen-containing environment is provided therein during operation of the optical element in an optical device.

The protective layer preferably forms a sealing layer. As mentioned above, the protective layer should cover at least the exposed surface area(s) of the aluminum layer. This is typically achieved by applying a sealing protective layer to the aluminum layer and the hydrogen catalyst layer partially covering it. It will be appreciated that, alternatively, the protective layer may be applied only to the exposed surface area of the aluminum layer. However, due to the small thickness of the hydrogen catalyst layer, such a procedure is generally not practical.

The protective layer is preferably made of a transparent, in particular fluoride material, for example formed from a metal fluoride, for example AlF _3. In this case, the protective layer is usually irreversible, i.e. it is permanently applied to the hydrogen catalyst layer.

In one embodiment, the protective layer is formed from a material that is strippable by irradiation with radiation in the VUV wavelength range and/or by contact with hydrogen. In this embodiment, the protective layer is applied reversibly, i.e., it can be easily removed during operation of the optical element in the optical device, since hydrogen is anyway introduced to protect the aluminum layer from oxidation during irradiation, or since the reflective coating, and thus the protective layer, is anyway exposed to radiation in the VUV wavelength range. To make the protective layer strippable, it is usually a top layer of the reflective coating.

The peelable protective layer is generally applied during or after the manufacture of the optical element before it is exposed to an air environment. In order to transfer the optical element into the optical device for use, the protective layer eliminates the need for complex concepts of handling and transporting the optical element in inert gas/nitrogen or even vacuum. If the optical device is to be serviced, the optical element is then exposed to the atmosphere and so there is no longer a sufficient amount of hydrogen to prevent oxidation of the aluminum layer, but if necessary the protective layer can be applied again, for example by adding a suitable substance to the environment of the optical element, which may be for example a vacuum environment or an environment with flushing gas.

In a further development, the protective layer is formed from carbon or at least one hydrocarbon. As mentioned above, it is advantageous if the protective layer can be easily peeled off and reapplied. This is the case, for example, for thin protective layers that contain carbon or carbohydrates and are reduced to carbon compound gases during operation of the optical element. By adding carbon or hydrocarbons to the environment of the reflecting optical element of the optical device, it becomes possible to (re)deposit the protective layer on the hydrogen catalyst layer. The hydrocarbons forming the protective layer can be, for example, volatile alkanes or alkenes, which subsequently adhere to the surface to form a polymer layer.

A second aspect of the invention relates to an optical device of the aforementioned type, in particular a wafer inspection system or a VUV lithography device, in which the optical element is designed as already described above and the gas inlet is designed or configured to supply (molecular) hydrogen to the interior. A flushing gas can be flowed into the interior via the gas inlet, to which hydrogen is additionally supplied, but it is also possible that the interior in which the optical element is located is a vacuum environment, to which hydrogen is additionally supplied via the gas inlet. For the supply of hydrogen to the interior, the gas inlet usually has a gas reservoir used for storing molecular hydrogen.

A sufficient amount of molecular hydrogen is provided inside where the optical element is located in order to form active hydrogen on the hydrogen catalyst layer and thus protect the aluminum layer from oxidation. It should be understood that one should also try to keep the amount of oxidizing agents such as oxygen or water as low as possible in the environment of the optical element, i.e. in the surrounding flushing gas or in vacuum. As a result of all these measures, the oxidation rate of the aluminum in the aluminum layer can usually be reduced to such an extent that a high reflectivity of the optical element is ensured for a sufficiently long irradiation time, so that the optical element needs to be replaced as little as possible or the reflective coating needs to be renewed as little as possible.

Yet another aspect of the invention relates to an optical device of the aforementioned type, which may, but not necessarily, be designed according to the second aspect of the invention. The optical device has a plasma generation device which supplies a plasma gas to the interior via a gas inlet in order to generate an atmospheric pressure plasma in at least one partial region of the optical surface of the optical element.

The optical element may be a reflective optical element for radiation in the VUV wavelength range, with the optical surface formed on the reflective coating. The optical element may alternatively be a transmissive optical element through which radiation in the VUV wavelength range passes. In either case, the optical surface is at least partially disposed in the beam path of the optical device.

As already mentioned above, a problem with both reflective and transmissive optical elements used in optical devices for radiation in the VUV wavelength range is that said optical elements become contaminated over time due to the presence in the environment of gas components that deposit on the optical surfaces of said optical elements. In order to clean the optical surfaces, in this aspect of the invention it is proposed to remove unwanted deposits on the optical surface(s) using atmospheric pressure plasma. It has been found that the use of atmospheric pressure plasma, i.e. plasma at a pressure of more than 100 mbar, preferably about 1 bar, is advantageous compared to plasmas occurring under vacuum conditions, in particular since the optical device can be operated in a flushing gas atmosphere and no vacuum pumps need to be provided. In this case, compressed air or another type of flushing gas, for example nitrogen and/or noble gases, for example argon, can be used as plasma gas or as the main component of the plasma gas. In order to enhance the cleaning effect of the plasma, reactive gases or species, for example hydrogen, oxygen or water, can optionally be added to the plasma gas.

In one embodiment, the plasma generation device is designed to generate a hydrogen plasma on the optical surface of the optical element. In this embodiment, hydrogen can be added to the plasma gas, which is supplied to the interior via a gas inlet. Alternatively, the plasma gas and hydrogen can be supplied to the interior via two or more separate gas inlets. It is important that the hydrogen plasma is formed in the environment of the optical surface.

Hydrogen plasma can be used particularly advantageously for cleaning the aluminum layer of the reflective optical element, the hydrogen plasma coming into contact with the surface of the aluminum layer due to the exposed surface of the reflective optical element, the native _{Al2O3 -} layer already mentioned above being completely or partially reduced to aluminum by the excited hydrogen ions of the plasma, as a result of which the reflectivity of the reflective optical element is increased and the transmittance of the optical device is increased.

As already mentioned above, due to the high enthalpy of reaction of aluminum with oxygen, it is not possible to reduce aluminum oxide to aluminum by merely supplying hydrogen or hydrogen activated on a hydrogen catalyst layer. However, during cleaning with hydrogen plasma, such a reduction reaction is possible, so that the already mentioned hydrogen catalyst layer for dissociation of molecular hydrogen can possibly be dispensed with. If the hydrogen plasma is not generated continuously but intermittently during interruptions in operation, for example at certain time intervals, or if the reflectivity of the optical element is significantly reduced, the combination of hydrogen plasma generation with the already mentioned hydrogen catalyst layer is advantageous.

The plasma gas, or more precisely the plasma gas ions produced during plasma formation, can be delivered to the optical surface in a variety of ways.

In one embodiment, the gas inlet is designed as a plasma nozzle, which supplies plasma gas to at least one partial region of the optical surface. In this case, the gas inlet can have at least one outlet opening, which is designed to direct a plasma gas flow of plasma gas to the optical surface or to at least one partial region of the optical surface. In this case, the gas inlet is a plasma nozzle, the gas outlet of which is or can be directed to the optical surface, if the plasma nozzle or its outlet opening can be moved, for example tilted and/or displaced, relative to the optical surface. In the case of a plasma nozzle, the plasma gas, for example in the form of oil-free compressed air or in the form of a flushing gas, for example nitrogen or another inert gas, typically flows through a discharge section, where it is excited and converted into a plasma state. The plasma gas, which can for example be doped with hydrogen or another active gas, enters the interior from the plasma nozzle in a plasma state. An example of a plasma nozzle that allows the generation of an active gas jet, which should be electrically neutral immediately after leaving the plasma nozzle, is described in DE-A-101 45 131, which is hereby incorporated by reference in its entirety.

Alternatively or additionally, the plasma generating device may have at least one electrode spaced apart from the optical surface for generating atmospheric pressure plasma in at least one partial region of the optical surface. The electrode(s) may be used to target the plasma at a desired internal location. In this case, for example, an aluminum layer or another metal layer of the reflective coating may serve as a counter electrode or to provide a ground potential for the plasma electrode. The electrode(s) may be tapered, for example, to ionize the plasma gas in the region of the electrode tip or to generate a plasma there that extends to a partial region of the optical surface.

In yet another embodiment, the plasma generating device is designed to generate a location-variable atmospheric pressure plasma on the optical surface. It has been found to be advantageous if partial areas of the optical surface are only exposed to the plasma to the extent that they were previously contaminated or oxidized. This can be achieved by a suitable arrangement of the plasma nozzle(s) and/or electrode(s). For example, the electrodes and/or plasma nozzles can be arranged in a ring around the optical element, such that each electrode or plasma nozzle is respectively assigned to a partial area of the optical surface. By targeted activation of the individual plasma nozzles or electrodes, the atmospheric pressure plasma on the optical surface can be location-variable. In particular, the time during which each plasma nozzle or each electrode is activated for a cleaning operation or reduction reaction can also be set individually, in particular depending on the degree of contamination or oxidation of each partial area of the optical surface.

To determine the degree of contamination or oxidation of each partial area, the optical apparatus may have an inspection device, for example in the form of a camera. If a plasma is generated during interruptions in operation, a camera or another inspection device can be introduced inside during interruptions in operation to obtain location-dependent information on the oxidation or contamination of the optical surfaces.

In the examples already mentioned above, it was assumed that the plasma nozzle(s) or electrode(s) are arranged outside the beam path of the optical device. However, it is also possible to introduce the plasma nozzle(s) or electrode(s) into the beam path of the optical device and remove them again using suitable actuators. This can be particularly useful if the generation of atmospheric plasma only occurs during interruptions in the operation of the optical device.

The cleaning operation or reduction effect of atmospheric plasma already mentioned above allows the reflectivity of the optical elements to be increased, i.e. without the need to remove the optical elements from the optical device. In this way, it is possible to frequently remove each optical element from the optical device and replace it with a structurally identical new optical element, or to remove the reflective coating so that it does not have to be reapplied after removal.

Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention with reference to the figures of the drawings showing the details essential to the invention, and from the claims. Each of the individual features can be implemented alone or in any combination of multiple in one variant of the invention.

An exemplary embodiment is shown in the schematic diagram and described in detail below.

1 shows a schematic diagram of an optical apparatus for the VUV wavelength range in the form of a VUV lithography apparatus. 1 shows a schematic diagram of an optical apparatus in the form of a wafer inspection system. FIG. 2 shows a schematic diagram of the wavelength-dependent reflectance of a non-oxidized aluminum layer and an aluminum layer that has been oxidized upon irradiation. FIG. 2 shows a schematic diagram of an optical element with a reflective coating having an aluminum layer and a hydrogen catalyst layer applied to the aluminum layer. FIG. 2 shows a schematic diagram of an optical element with a reflective coating having an aluminum layer and a hydrogen catalyst layer applied to the aluminum layer. 1 shows a plasma generation device with six plasma nozzles for generating atmospheric pressure plasma at the optical surface of a reflecting optical element. 1 shows a plasma generation device with six plasma nozzles for generating atmospheric pressure plasma at the optical surface of a reflective optical element. 1 shows a plasma generation device with six electrodes for generating atmospheric pressure plasma at the optical surface of a reflective optical element. 1 shows a plasma generation device with six electrodes for generating atmospheric pressure plasma at the optical surface of a reflective optical element.

In the following description of the drawings, the same reference numbers are used for identical or functionally identical components.

Figure 1 shows a schematic representation of an optical arrangement 1 in the form of a VUV lithography apparatus, in particular for wavelengths in the range of 100 nm to 200 nm or 190 nm. The VUV lithography apparatus 1 has as essential components two optical systems in the form of an illumination system 12 and a projection system 14. The VUV lithography apparatus 1 has a radiation source 10 for performing an exposure process, which may be an excimer laser emitting radiation 11 at a wavelength in the VUV wavelength range of, for example, 193 nm, 157 nm or 126 nm, and which may be an integral part of the VUV lithography apparatus 1.

The radiation 11 emitted by the radiation source 10 is conditioned by means of an illumination system 12 so as to completely illuminate a mask 13, also called a reticle. In the example shown in FIG. 1, the illumination system 12 comprises both transmissive and reflective optical elements. For example, a transmissive optical element 120 that focuses the radiation 11 and a reflective optical element 121 that deflects the radiation 11 are representatively shown in FIG. 1. As is known, various transmissive, reflective, or other optical elements can be combined with each other in the illumination system 12 in any desired and more complex manner.

The mask 13 has a structure on its surface that is to be transferred to an element 15, e.g. a wafer, that is to be exposed as part of the manufacture of a semiconductor component using the projection system 14. In the illustrated example, the mask 13 is embodied as a transmissive optical element. The mask 13 can also be embodied as a reflective optical element in alternative embodiments. The projection system 14 has at least one transmissive optical element in the illustrated example. In the illustrated example, two transmissive optical elements 140, 141 are representatively shown, which serve, for example, to reduce the structure on the mask 13 to a size desired for the exposure of the wafer 15. The projection system 14 can also be provided with, in particular, reflective optical elements, and any optical elements can be combined with one another in any known manner. It is also to be noted that optical arrangements without transmissive optical elements can also be used for VUV lithography.

Figure 2 shows a schematic representation of an exemplary embodiment of an optical device in the form of a wafer inspection system 2. The description given below applies equally to an inspection system for mask inspection.

The wafer inspection system 2 comprises a radiation source 20, the radiation 21 of which is directed by an optical system 22 to the wafer 25. For this purpose, the radiation 21 is reflected from a concave mirror 220 to the wafer 25. In the case of a mask inspection system 2, a mask to be inspected may be arranged instead of the wafer 25. The radiation reflected, diffracted and/or refracted at the wafer 25 is directed by a further concave mirror 221, also associated with the optical system 22, to a detector 23 for further evaluation. By way of example, the radiation source 20 may be only one radiation source or a combination of several individual radiation sources to provide a substantially continuous radiation spectrum. In a variant, one or several narrowband radiation sources 20 may also be used. Preferably, the wavelength or wavelength band of the radiation 21 generated by the radiation source 20 is in the range of 100 nm to 200 nm, particularly preferably 110 nm to 190 nm.

During operation of an optical apparatus, such as the VUV lithography apparatus 1 from FIG. 1 or the wafer or mask inspection system 2 from FIG. 2, oxidation of the reflective optical surfaces 121a, 221a, 222a of each reflective optical element 121, 220, 221 can occur. The reflective optical elements 121, 220, 221 can have a metal mirror layer that reflects VUV radiation 11, and it has been found that an aluminum layer is a metal mirror layer that has a high reflectivity in a wide wavelength range, for example from 100 nm to 200 nm.

As an example, the reflectance R of such an aluminum layer is shown in FIG. 3 as a function of wavelength λ for the wavelength range from about 120 nm to about 280 nm, i.e., covering almost the entire VUV wavelength range from about 100 nm to 200 nm. The dashed curve in FIG. 3 in this case corresponds to a reflectance R of a non-oxidized aluminum layer that is greater than 0.9 over the entire wavelength range shown.

The solid curve in FIG. 3 shows the reflectance R of an aluminum layer oxidized by irradiation with radiation 11, 21 in the VUV wavelength range. It can be clearly seen that the reflectance R for the relevant wavelengths λ of less than about 200 nm drops significantly, in particular to values of less than 0.1 for wavelengths λ of less than about 140 nm. Using the curve of reflectance R shown in FIG. 3, it is quite clear that oxidation of an aluminum layer acting as a metallic mirror layer should be avoided if it is used to reflect radiation 11 in the VUV wavelength range.

4a, 4b show an optical element 4 embodied to reflect radiation 11 in the VUV wavelength range and which may for example form one of the reflecting optical elements 121, 220, 221 of FIG. 1 or FIG. 2. The optical element 4 shown in FIG. 4a, 4b is a mirror having a substrate 41, which in the example shown may be quartz (glass), in particular titanium-doped quartz glass, ceramic or glass ceramic. A reflective coating 42 is applied to the substrate 41, said reflective coating having a continuous aluminum layer 43 acting as a metallic mirror layer. The aluminum layer 43 may be applied directly to the substrate 41. In the example shown in FIG. 4a, 4b, a functional layer in the form of an adhesion-promoting layer 44 is applied between the aluminum layer 43 and the substrate 41. The material of the adhesion-promoting layer 44 can be chosen from a number of materials, but care should be taken that there is sufficient adhesion to both the substrate 41 and the aluminum layer 43. Other functional layers, for example smoothing and/or polishing layers, may also be provided between the aluminum layer 43 and the substrate 41.

4a, 4b, a hydrogen catalyst layer 45, which serves to dissociate molecular hydrogen _H2 into active hydrogen or hydrogen radicals, is applied to the aluminum layer 43. The material of the hydrogen catalyst layer 45 can be, for example, Ru, Pt, Pd, Ni, or Rh.

The above materials have a relatively low reflectivity for radiation 11 in the VUV wavelength range. It is therefore beneficial if the thickness D of the hydrogen catalyst layer 45, which in the example shown in Figures 4a and 4b is less than 1.0 nm, is as small as possible. A hydrogen catalyst layer 45 having such a small thickness D cannot be applied to the aluminum layer 43 in a completely covering manner, i.e. in the form of a sealing layer, and island-like spaced apart material accumulations of the hydrogen catalyst layer 45 form on the aluminum layer 43, as shown in Figures 4a and 4b.

The coverage of the aluminum layer 43 by the hydrogen catalyst layer 45 is about 10% to about 90%, preferably 30% to 70%, which allows the realization of a hydrogen catalyst layer 45 with a small thickness D so that the reflectance R of the optical element 5 is not too low, and further allows a dissociation effect of the hydrogen catalyst layer 45 sufficient to prevent oxidation of the aluminum layer 43 during irradiation with VUV radiation 11, 21.

A necessary condition for the protective effect of the hydrogen catalyst layer 45 is the availability of molecular hydrogen H ₂ in the environment of the reflecting optical element 4. Molecular hydrogen H ₂ in the VUV lithography apparatus 1 shown in FIG. 1 is supplied to the interior 122a of the housing 122 in which the reflecting optical element 121 is arranged, via a gas inlet 123 formed in the housing 122 of the illumination system 12. For this purpose, the gas inlet 123 has a gas reservoir (not shown) that contains molecular hydrogen H _2. If a sufficient amount of molecular hydrogen H ₂ is present in the environment of the reflecting optical element 121, it can be converted into active hydrogen in the hydrogen catalyst layer 45, thus protecting the aluminum layer 43 from oxidation.

When placed in the interior 122a of the housing 122 of the illumination system 12, the reflective optical element 121 is generally exposed to the atmosphere, which may lead to a possibly irreversible oxidation of the hydrogen catalyst layer 45 or the aluminum layer 43. It is therefore beneficial to apply a protective layer 46 to the hydrogen catalyst layer 45, as shown by way of example in Fig. 4b. The protective layer 46 should form a sealing layer covering both the aluminum layer 43 and the hydrogen catalyst layer 45. The protective layer 46 may be permanently applied to the hydrogen catalyst layer 45. In this case, the material of the protective layer 46 should be a transparent material that provides a good protective effect for the aluminum layer 43. In this case, the material of the protective layer 46 may be, for example, a metal fluoride, for example in the form of a fluoride material, for example _AlF3 .

Alternatively, the protective layer 46 can be applied reversibly to the hydrogen catalyst layer 45 and to the aluminum layer 43. In this case, the protective layer 46 can be removed again after the reflective optical element 121 has been introduced into the illumination system 122. For this purpose, a material is used for the protective layer 46 which is peeled off upon irradiation with radiation 11 in the VUV wavelength range and/or upon contact with (molecular) hydrogen _H2 . The material of the protective layer 46 which can be peeled off due to irradiation can be carbon or at least one hydrocarbon, for example parylene.

The aluminum layer 43, the hydrogen catalyst layer 45 applied thereto and the possibly applied protective layer 46 are preferably deposited by atomic layer deposition. Atomic layer deposition allows the deposition of particularly thin and smooth layers and thus allows the reduction of reflectivity losses and scattering due to absorption. Besides atomic layer deposition, other coating processes are also suitable, such as magnetron sputtering, ion-assisted deposition, plasma-enhanced deposition, thermal evaporation, etc.

The reflective optical elements 220, 221 of the wafer inspection system 2 shown in Figure 2 can also be designed as described in connection with Figures 4a, 4b. The optical system 22 of the wafer inspection system 2 has a housing 24 in an interior 24a of which two reflective optical elements 220, 221 are arranged. A gas inlet 26 is formed in the housing 24, through which molecular hydrogen _H2 can be supplied to the interior 24a of the housing 24.

Figures 5a and 5b show a reflective optical element 5 designed to reflect radiation 11, 21 in the VUV wavelength range and which may for example form one of the reflective optical elements 121, 220, 221 of Figures 1 or 2. The reflective optical element 5 of Figures 5a and 5b is designed substantially like the reflective optical element 4 shown in Figures 4a and 4b, but does not have the hydrogen catalyst layer 45 or the protective layer 36, i.e. the aluminum layer 43 is directly exposed to the environment and its upper part forms the reflective optical surface 5a.

In order to protect the aluminum layer 43 of the reflecting optical element 5 from oxidation, each optical device 1, 2 in which the reflecting optical element 5 of Fig. 5a, Fig. 5b is arranged has a plasma generation device 50 which generates an atmospheric pressure plasma 51 on the reflecting optical surface 5a of the reflecting optical element 5. For this purpose, the plasma generation device 50 has six gas inlets in the form of plasma nozzles 52a-52f, which are arranged with a uniform distribution circumferentially around the optical surface 5a, which is circular in top view, specifically outside the beam path 53 of the VUV radiation 11, 21, shown by the dashed line in Fig. 5b.

The plasma nozzles 52a-52f are designed to supply the optical surface 5a with plasma gas in the form of respective plasma gas flows 54a-54f. In the example shown in Fig. 5a, 5b, each plasma gas flow 54a-54f is supplied to one of six partial regions 5a-5f of the optical surface 5a that form an arc of the optical surface 5a, which is circular in top view. As can be seen in Fig. 5b, which shows a cross section of the reflecting optical element 5, each plasma nozzle 52a-52f, and therefore also the plasma gas flow 54a-54f, is directed obliquely with respect to the optical surface 5a.

In the illustrated example, the plasma generation device 50 is designed to generate a hydrogen plasma 51. For this purpose, the plasma gas flows 54a-54f exiting the plasma nozzles 52a-52f have a relatively low proportion of hydrogen in addition to a flushing gas, for example in the form of nitrogen, a noble gas or a mixture of said gases. By adding (molecular) hydrogen _H2 to the plasma gases 54a-54f, the cleaning effect of the atmospheric plasma 51 is improved, and the thin aluminium oxide layer formed on the optical surface 5a during irradiation is re-reduced to aluminium.

As mentioned above in connection with Figure 3, the reflectivity R of the reflecting optical element 5 can thus be significantly increased. The use of plasma nozzles 54a-54f, typically arranged in each interior 122a, 24a in which the reflecting optical element 5 is also located, makes it possible to dispense with the gas inlets 123, 26 shown in Figures 1 and 2 for supplying molecular hydrogen _H2 to each interior 122a, 24a.

Figures 6a and 6b show a plasma generating device 60 that is designed to reflect radiation 11, 21 in the VUV wavelength range and is used to generate an atmospheric pressure plasma 61 at the optical surface 6a of a reflecting optical element 6, which may for example form one of the reflecting optical elements 121, 220, 221 from Figures 1 or 2. The plasma generating device 60 of Figures 6a and 6b differs from the plasma generating device 50 shown in Figures 5a and 5b in that it has six pointed electrodes 62a-62f for generating an atmospheric pressure plasma 61 at the optical surface 6a of the reflecting optical element 6. The six electrodes 62a-62f are arranged outside the beam path 63 of the optical device 1, 2 in which the optical element 6 is used.

The plasma generating device 60 generates a potential difference between the electrodes 62a-62f and the optical surface 6a of the aluminum layer 54, which is held at a defined potential and connected to ground potential in the illustrated example, resulting in an atmospheric pressure plasma 61 extending up to the optical surface 6a between each electrode 62a-62f and the optical surface 6a. In the plasma generating device 60 shown in Fig. 6a, 6b, each electrode 62a-62f is assigned to a partial area 65a-65f of the optical surface 6a forming a circular arc, respectively. A plasma gas 64 is supplied to the environment of the optical surface 6a via a respective gas inlet 123, 26 to the associated interior 122a, 24a from Fig. 1 or 2 in this example. Here, molecular hydrogen _H2 can additionally be supplied to the interior 122a, 24a via the gas inlet 123, 26 as shown in Fig. 1 and 2, but this is generally added to the plasma gas 64 in the form of a small admixture.

In both the plasma generating device 50 shown in Figs. 5a and 5b and the plasma generating device 60 shown in Figs. 6a and 6b, the atmospheric plasma 51, 61 on the optical surface 5a, 6a can vary depending on the location. For this purpose, the plasma nozzles 52a-52f or the electrodes 62a-62f can be individually controlled. For example, in the plasma generating device 50 shown in Figs. 5a and 5b, the flow through each plasma nozzle 52a-52f and/or the time for the plasma gas flow 54a-54f to each partial area 55a-55f of the optical surface 5a can be individually set. Correspondingly, the potential or potential difference between each electrode 62a-62f and the optical element 6 or the aluminum layer 43 can also be changed in order to vary the strength of the hydrogen plasma 61 generated in each partial area 65a-65f of the optical surface 6a.

In this way, it is possible to carry out targeted plasma cleaning of previously oxidized or contaminated partial regions 55a-55f, 65a-65f of the optical surfaces 5a, 6a using each plasma generating device 50, 60. Since (less intense) atmospheric pressure plasma 51, 61 is also generated by the plasma nozzles 52a-52f or electrodes 62a-62f at locations of the optical surfaces 5a, 6a that do not belong to the respective relevant partial regions 55a-55f, 65a-65f, it is not necessarily necessary to activate all the plasma nozzles 52a-52f or all the electrodes 62a-62f to generate the atmospheric pressure plasma 51, 61. The number of six plasma nozzles 52a-52f or six electrodes 62a-62f shown in Figures 5a, 5b and 6a, 6b is merely an example, i.e., the plasma generating devices 50, 51 may have fewer or more plasma nozzles 52a-52f or electrodes 62a-62f.

The plasma generation devices 50, 60 shown in Figs. 5a, 5b and 6a, 6b can be used to generate atmospheric pressure plasmas 51, 61 not only at the front reflecting surfaces 5a, 6a of the reflecting optical elements 5, 6 but also at the transmitting optical elements, for example the two transmitting optical elements 140, 141 of the projection system 14 shown in Fig. 1. In this case, the atmospheric pressure plasmas 51, 61 can be used to remove contaminants from each transmitting optical surface 140a, 140b, 141a, 141b. To enhance the cleaning effect, reactive gas components, for example hydrogen as mentioned above or possibly oxygen or water, can be added to the plasma gas used to generate the atmospheric pressure plasmas 51, 61. This is possible because no reflecting optical elements with an aluminum layer 43 or a layer made of any other material that could be damaged by the oxidizing effect of oxygen are arranged in the projection system 14.

The measures already mentioned above make it possible to prevent a decrease in the reflectivity R of the reflective optical elements 121, 220, 221 due to oxidation of the aluminum layer 43. By generating a hydrogen plasma 51, 72 at atmospheric pressure, the oxidation reaction of the aluminum layer 54 can be further reversed, i.e. the already formed aluminum oxide can be re-reduced to metallic aluminum. By the action of the atmospheric pressure plasma 51, 61, the optical surfaces 140a, 140b, 141a, 141b of the transmissive optical elements 140, 141 can also be cleaned by removing contaminants.

Claims (16)

An optical element (4) for reflecting radiation (11, 21) in the VUV wavelength range,
An optical element (4) comprising a substrate (41) and a reflective coating (42) applied to the substrate (41) and having at least one aluminum layer (43),
At least one hydrogen catalyst layer (45) for dissociation of molecular hydrogen (H ₂ ) is applied to said aluminum layer (43) ,
The optical element is characterized in that the hydrogen catalyst layer (45) is not completely closed and does not completely cover the aluminum layer (43), so that a part of the surface of the aluminum layer (43) is exposed to the environment . The optical element according to claim 1, wherein the hydrogen catalyst layer (45) is selected from the group including Ru, Pt, Pd, Ni, and Rh. 3. The optical element according to claim 1, wherein the hydrogen catalyst layer (45) has a layer thickness (D) of 0.1 nm to 1.0 nm. 4. The optical element according to claim 1 , wherein the hydrogen catalyst layer (45) covers the aluminum layer (43) with a coverage of 10% to 90%. An optical element according to any one of the preceding claims, further comprising at least one protective layer (46) applied to said aluminium layer (43) and to said hydrogen catalyst layer (45). 6. The optical element according to claim 5 , wherein said protective layer (46) forms a sealing layer. 7. The optical element according to claim 5 or 6 , wherein the protective layer (46) is made of a transparent material . 8. The optical element according to claim 5 , wherein the protective layer (46) is made of a material that can be stripped by irradiation with radiation (11, 21) in the VUV wavelength range and/or by contact with hydrogen ( _H2 ). 9. The optical element of claim 8 , wherein the protective layer (46) is formed from carbon or at least one hydrocarbon. 1. An optical device for use in the VUV wavelength range, comprising:
an interior (122a, 24a) in which at least one optical element (121, 220, 221; 140, 141; 4, 5, 6) is arranged;
An optical device comprising at least one gas inlet (123, 26, 52a-52f) for supplying a gas (H ₂ ) to the interior (122a, 24a), characterized in that the optical elements (121, 220, 221, 4) are designed as described in any one of claims 1 to 9 , and the gas inlet (123, 26) is designed to supply hydrogen (H ₂ ) to the interior (122a, 24a). 11. The optical device according to claim 10, wherein the optical device for VUV wavelength range is a wafer inspection system (2) or a VUV lithography device (1). 12. The optical device according to claim 10 ,
a plasma generation device (50, 60) for supplying a plasma gas (54a-54f) to said interior (122a, 24a) via said gas inlets (123, 26, 52a-52f) in order to generate an atmospheric pressure plasma (51, 61) in at least one partial region (55a-55f) of the optical surface (5a, 6a) of said optical element (5, 6);
An optical device comprising: The optical device according to claim 12, wherein the plasma generating device (50, 60) is designed to generate a hydrogen plasma (51, 61) at the optical surface (5a, 6a) of the optical element (5, 6). The optical device according to claim 12 or 13, wherein the gas inlet is designed as a plasma nozzle (52a-52f) that supplies the plasma gas (54a-54f) to at least one partial region (55a-55f) of the optical surface (5a). The optical device according to any one of claims 12 to 14, wherein the plasma generating device (60) has at least one electrode (62a to 62f) spaced apart from the optical surface (6a) for generating the atmospheric pressure plasma (61) in at least one partial region (65a to 65f) of the optical surface (6a). The optical device according to any one of claims 12 to 15, wherein the plasma generating device (50, 60) is designed to generate a location-variable atmospheric pressure plasma (51, 61) at the optical surface (5a, 6a).

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