JP7791939B2 - Optical elements and optical devices for reflecting VUV radiation - Google Patents

Description

[Reference to Related Applications]
This application is a continuation of German patent application DE 10 2018 221 191 filed on December 7, 2018.
This application claims priority from application Ser. No. 09/199,394, filed on Mar. 1, 2009, 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 having 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 short-wave ultraviolet wavelength range of about 100 nm to about 200 nm, also known as the vacuum ultraviolet wavelength range (VUV wavelength range), not only transmissive optical elements but also reflective optical elements are often used.
Optical devices for radiation in the UV wavelength range can be used, for example, to perform optical inspection of wafers or masks or in the manufacture of semiconductor components.

Optical elements that reflect VUV radiation typically have a reflective coating, which in certain applications, for example when inspecting wafers, should have a high reflectivity over a large spectral range within 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 more 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 native aluminum oxide (Al ₂ O ₃ ) layer with a thickness of about 2 nm to 3 nm. This Al ₂ O ₃ layer 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 aluminum layer from oxidation, it is known, for example from the article by [1], 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 those that may occur in lithography, especially during mask and wafer inspection, severe degradation of reflective optical elements occurs within hours or even days, accompanied by significant losses in reflectivity. Even the protective layers made of the metal fluorides mentioned above, which in fact offer very good environmental protection, are unable to prevent oxidation of the aluminum layer in the event of irradiation. Significant reflectivity losses have also been observed when the oxygen or moisture content of the optical element's environment is reduced to prevent oxidation.

For optical devices for the VUV wavelength range, an additional problem generally exists: contamination of optical surfaces due to the inability to completely suppress unwanted gas components in the environment of the optical elements. These gas components deposit on the optical surfaces and are "burned in" there during irradiation. This problem exists not only for optical surfaces of reflective optical elements, but also for 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

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical element for reflecting radiation in the VUV wavelength range and an optical device for use in the VUV wavelength range that can have an extended service life.

The object is to provide at least one hydrogen catalyst for the dissociation of (molecular) hydrogen.
This is achieved with optical elements of the type described above, in which a layer of aluminium is applied to the aluminium layer.

The inventors have recognized that certain materials are suitable for catalytic hydrogen cracking.
This leads to oxidation (or dissociation) of molecular hydrogen, which, if added in the form of a compound to the environment of the optical element, dissociates in the material of the hydrogen catalyst layer, forming active hydrogen, especially at high irradiation doses. Active hydrogen is understood to mean hydrogen radicals, hydrogen ions, and/or hydrogen in an excited electronic state. This active hydrogen can usually prevent or at least significantly slow down oxidation of the aluminum layer, even at high irradiation intensities.

In one embodiment, the material of the hydrogen catalyst layer is selected from the group consisting of Ru, Pt, Pd, Ni, and Rh. It has been shown that for these materials, especially Ru and Pt, oxidation can be virtually completely prevented by adding hydrogen to the environment, for example in a residual gas atmosphere. It has also been shown 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.
It is known from lithography in the EUV wavelength range.

In an advantageous embodiment, the hydrogen catalyst layer has a thickness of 0.1 nm to 3.0 nm, preferably 0.1
The layer thickness is between 1.0 nm and 1.0 nm. The hydrogen catalyst materials specified above typically 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 would reduce 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 layer thickness, the layer cannot usually be applied in a completely sealed manner. For example, for Ru as the layer material, this is the case for a layer thickness of about 1.0 nm or less. Therefore, below this (material-dependent) value, the hydrogen catalyst layer may not completely cover the aluminum layer, despite its application.
A portion of the surface of the aluminum layer is exposed, and native aluminum oxide can form in this portion. However, because a native aluminum oxide layer cannot form in the covered portion or portions of the aluminum layer, even a hydrogen catalyst layer that is not completely sealed can reduce the oxidation rate during irradiation. Furthermore, when hydrogen is added to the environment of the optical element, the catalytic effect of the material of the hydrogen catalyst layer can prevent oxidation from occurring even in the exposed surface region of the aluminum layer in some cases.

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 proportion of the surface of the hydrogen catalyst layer relative to the total surface of the aluminum layer (opposite to the substrate). Typically, the coverage with the hydrogen catalyst layer occurs in the form of island-like, spaced-apart accumulations of material of the hydrogen catalyst layer on the aluminum layer. This is advantageous because, 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 complete or partial peeling of the hydrogen catalyst layer. To prevent the formation of bubbles or degradation of the coating of optical elements 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 desorbing material that 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 includes a protective layer applied to the aluminum layer and the hydrogen catalyst layer. For reflection reasons, if the hydrogen catalyst layer is applied thinly enough that it does not completely cover the aluminum layer, part of the surface of the aluminum layer will be exposed. If a native oxide layer forms on the exposed surface area of the aluminum layer, this reaction generally cannot be reversed by adding hydrogen because the enthalpy of reaction of aluminum with oxygen is relatively high. The above also applies to active hydrogen formed on the hydrogen catalyst layer, although active hydrogen is significantly more reactive than molecular hydrogen. However, the situation is different if the oxidation reaction has not yet occurred. 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 use a protective layer to protect the aluminum layer from oxidation at least in the surface area(s) not covered by the hydrogen catalyst layer until a sufficient amount of hydrogen to prevent oxidation is present in the environment of the optical element, which is the case when a suitable hydrogen-containing environment is provided therein, for example during operation of the optical element in an optical device.

The protective layer preferably forms a sealing layer. As mentioned above, the protective layer must cover at least the exposed surface area(s) of the aluminum layer. This is typically achieved by applying the sealing protective layer to the aluminum layer and the hydrogen catalyst layer that partially covers it. It should be understood that, alternatively, the protective layer may be applied only to the exposed surface area(s) of the aluminum layer. However, due to the small thickness of the hydrogen catalyst layer, such a procedure is generally impractical.

The protective layer is made of a transparent, in particular fluoride material, for example a metal fluoride, for example A
Preferably, it is made of 1F _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 can be stripped by irradiation with radiation in the VUV wavelength range and/or contact with hydrogen. In this embodiment, the protective layer is applied reversibly. That is, the protective layer 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 the reflective coating, and therefore the protective layer, is anyway exposed to radiation in the VUV wavelength range. To make the protective layer strippable, the protective layer 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. The protective layer eliminates the need for a complex concept of handling and transporting the optical element in an inert gas/nitrogen or even a vacuum in order to transfer the optical element into the optical device in which it is to be used. When the optical device is serviced, the optical element is exposed to the atmosphere, 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 can be achieved, for example, by
This is the case for a thin protective layer containing carbon or carbohydrates that are reduced to carbon compound gases during operation of the optical element. By adding carbon or hydrocarbons to the environment of the reflective optical element of the optical device, it is possible to (re)deposit the protective layer onto the hydrogen catalyst layer. The hydrocarbons that form 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 to have a vacuum environment inside where the optical element is located, to which hydrogen is additionally supplied via the gas inlet. To supply hydrogen to the interior, the gas inlet typically has a gas reservoir used for storing molecular hydrogen.

A sufficient amount of molecular hydrogen is provided inside the optical element to form active hydrogen on the hydrogen catalyst layer and thus protect the aluminum layer from oxidation. It should be understood that an attempt should also be made 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 vacuum. As a result of these measures taken together, 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 over a sufficiently long irradiation time, thereby minimizing the need to replace the optical element or to renew the reflective coating.

A further aspect of the invention relates to an optical apparatus of the aforementioned type, which may, but not necessarily, be designed in accordance with the second aspect of the invention, the optical apparatus comprising a plasma generation device into which a plasma gas is supplied via a gas inlet in order to generate 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 a reflective coating thereof, or alternatively, the optical element may be a transmissive optical element through which radiation in the VUV wavelength range passes, in either case the optical surface being 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 they become contaminated over time due to the presence of gaseous components in the environment which deposit on the optical surfaces of said optical elements. In order to clean the optical surfaces, this aspect of the invention proposes to use atmospheric pressure plasma to remove unwanted deposits on the optical surface(s). Compared to plasmas generated under vacuum conditions, atmospheric pressure plasmas, i.e. above 100 mbar, preferably around 100 mbar, are advantageous, in particular because the optical device can be operated in a flushing gas atmosphere and no vacuum pumps are required.
It has been found advantageous to use a plasma at a pressure of 100 bar. In this case, compressed air or another type of flushing gas, such as nitrogen and/or a noble gas, such as argon, can be used as the plasma gas or as the main component of the plasma gas. To enhance the cleaning effect of the plasma, reactive gases or species, such as hydrogen, oxygen, or water, can optionally be added to the plasma gas.

In one embodiment, the plasma generation device is designed to generate 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 through a gas inlet. Alternatively, the plasma gas and hydrogen can be supplied to the interior through 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 to clean the aluminum layer of the reflective optical element, the exposed surface of which brings the hydrogen plasma into contact with the surface of the aluminum layer, and the native _Al2O3 layer already mentioned _above is completely or partially reduced to aluminum by the excited hydrogen ions of the plasma, resulting in an increase in the reflectivity of the reflective optical element and an increase in the transmittance of the optical device.

As already mentioned above, the reaction enthalpy of aluminum with oxygen is high, so that it is not possible to reduce aluminum oxide to aluminum by simply supplying hydrogen or hydrogen activated on a hydrogen catalyst layer.However, when cleaning with hydrogen plasma, such a reduction reaction is possible, and therefore the hydrogen catalyst layer already mentioned above for dissociating molecular hydrogen can be omitted in some cases.If the hydrogen plasma is not generated continuously but intermittently during interruptions in operation, for example at predetermined time intervals, or if the reflectivity of the optical element is significantly reduced, it is advantageous to combine the generation of hydrogen plasma with the hydrogen catalyst layer already mentioned above.

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 that 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 designed to direct a plasma gas flow of plasma gas toward the optical surface or toward at least one partial region of the optical surface. In this case, the gas inlet is a plasma nozzle, and its gas outlet is or can be directed toward 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 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 may be doped with, for example, hydrogen or another active gas, enters the interior of the plasma nozzle in a plasma state. An example of a plasma nozzle that allows the generation of an active gas jet that should be electrically neutral immediately after leaving the plasma nozzle is described in German Patent Application Publication No. 10145131, the entire contents of which are incorporated herein by reference.

Alternatively or additionally, the plasma generation device can 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) can be used to generate the plasma in a targeted manner at a desired internal location. In this case, for example, an aluminum layer or another metal layer of the reflective coating can serve as a counter electrode or to provide a ground potential for the plasma electrode. The electrode(s) can 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 the partial region of the optical surface.

In yet another embodiment, the plasma generation 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, with each electrode or plasma nozzle assigned to a partial area of the optical surface. Targeted activation of the individual plasma nozzles or electrodes allows the atmospheric pressure plasma on the optical surface to 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 be individually set, 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, etc. 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 about the oxidation or contamination of the optical surface.

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 pressure plasma only occurs during interruptions in the operation of the optical device.

The cleaning or reducing effect of atmospheric pressure plasma, as already mentioned above, can increase the reflectivity of optical elements, i.e., without the need to remove the optical elements from the optical device, thus eliminating the need to frequently remove each optical element from the optical device and replace it with a new, structurally identical optical element, or remove and then reapply the reflective coating.

Further features and advantages of the invention are apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawings which show the details essential to the invention, and from the claims. Each of the individual features can be implemented alone or in any combination of several in a 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 device for the VUV wavelength range in the form of a VUV lithography device. 1 shows a schematic diagram of an optical apparatus in the form of a wafer inspection system. 1 shows a schematic diagram of the wavelength-dependent reflectance of an unoxidized aluminum layer and an aluminum layer that has been oxidized upon irradiation. 1 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 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 reflective 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.

1 shows diagrammatically 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 carrying out an exposure process, which may be, for example, a 1
It may be an excimer laser emitting radiation 11 at a wavelength in the VUV wavelength range of 93 nm, 157 nm or 126 nm and may be an integral part of the VUV lithography apparatus 1 .

The radiation 11 emitted by the radiation source 10 is projected onto a mask 1, also called a reticle, by means of an illumination system 12.
1, the illumination system 12 includes both transmissive and reflective optical elements. For example, the transmissive optical element 12 focuses the radiation 11.
1, a reflecting optical element 121 that deflects radiation 11. As is known, various transmissive, reflective, or other optical elements can be combined with each other in illumination system 12 in any desired and more complex manner.

The mask 13 has a structure on its surface that is to be transferred onto an element 15, e.g., a wafer, that is exposed using a projection system 14 as part of the manufacture of a semiconductor component.
3 is embodied as a transmissive optical element. Mask 13 can also be embodied as a reflective optical element in an alternative embodiment. Projection system 14 has at least one transmissive optical element in the example shown. In the example shown, two transmissive optical elements 140, 141 are representatively shown, which serve, for example, to reduce the structures on mask 13 to a size desired for exposure of wafer 15. 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 should also be pointed out that optical arrangements without transmissive optical elements can also be used for VUV lithography.

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

The wafer inspection system 2 comprises a radiation source 20, whose radiation 21 is directed by an optical system 22 to a wafer 25. For this purpose, the radiation 21 is directed from a concave mirror 220 to a wafer 25.
In the case of mask inspection system 2, a mask to be inspected may be placed in place of wafer 25. Radiation reflected, diffracted, and/or refracted from wafer 25 is reflected by a further concave mirror 221, also associated with optical system 22, onto 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 between 100 nm and 200 nm, particularly preferably between 110 nm and 250 nm.
It is in the range of 0 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, the reflective optical surface 1 of each reflective optical element 121, 220, 221
Oxidation of the reflective optical elements 121, 220, and 221a may occur.
can have a metal mirror layer that reflects VUV radiation 11, and the aluminum layer
For example, it is known that the metal mirror layer has high reflectivity in a wide wavelength range of 100 nm to 200 nm.

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

The solid curve in Figure 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 wavelengths λ less than about 200 nm, which is relevant here, drops significantly, specifically to a value less than 0.1 for wavelengths λ less than about 140 nm. Using the reflectance R curve shown in Figure 3, it is quite clear that oxidation of an aluminum layer acting as a metallic mirror layer should be avoided when it is used to reflect radiation 11 in the VUV wavelength range.

4a and 4b are embodied to reflect radiation 11 in the VUV wavelength range, and are not shown in FIG.
4a and 4b show an optical element 4, which may form, for example, one of the reflective optical elements 121, 220, 221 of the present invention. The optical element 4 shown in FIGS. 4a and 4b is a mirror having a substrate 41, which in the illustrated example 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 comprising 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 FIGS. 4a and 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, although care should be taken to ensure sufficient adhesion to both the substrate 41 and the aluminum layer 43. Other functional layers, such as smoothing and/or polishing layers, may also be provided between the aluminum layer 43 and the substrate 41.

In the example shown in Figures 4a and 4b, a hydrogen catalyst layer 45 is applied to the aluminum layer 43, which serves to dissociate molecular hydrogen _H2 into active hydrogen or hydrogen radicals. The material of the hydrogen catalyst layer 45 is:
For example, it can be Ru, Pt, Pd, Ni, or Rh.

The above materials have a relatively low reflectivity for radiation 11 in the VUV wavelength range. Therefore, it is beneficial if the thickness D of the hydrogen catalyst layer 45 is as small as possible, which is less than 1.0 nm in the example shown in Figures 4a and 4b. A hydrogen catalyst layer 45 with 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 therefore the thickness D of the hydrogen catalyst layer 45 is not sufficient to completely cover the aluminum layer 43, i.e., in the form of a sealing layer.
4b, island-like spaced apart accumulations of material of hydrogen catalyst layer 45 are formed on aluminum layer 43.

The coverage of the aluminum layer 43 by the hydrogen catalyst layer 45 is between about 10% and about 90%, preferably between 30% and 70%, which allows for a hydrogen catalyst layer 45 with a small thickness D so that the reflectivity R of the optical element 5 is not reduced too much, and further allows for 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 _H2 in the environment of the reflective optical element 4. The molecular hydrogen H2 in the VUV lithography apparatus 1 shown in FIG.
The hydrogen _{H 2} is supplied via a gas inlet 123 formed in the housing 122 of the illumination system 12 to an interior 122a of the housing 122 in which the reflecting optical element 121 is arranged. For this purpose, the gas inlet 123 has a gas reservoir (not shown) for containing molecular hydrogen H _2. If a sufficient amount of molecular hydrogen H ₂ is present in the environment of the reflecting optical element 121, this will cause the hydrogen catalyst layer 4 to react with the hydrogen H 2.
5 can be converted to active hydrogen, thus protecting the aluminum layer 43 from oxidation.

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

Alternatively, the protective layer 46 can be applied reversibly to the hydrogen catalyst layer 45 and 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 released upon irradiation with radiation 11 in the VUV wavelength range and/or upon contact with (molecular) hydrogen _H2 .
The materials of the protective layer 46 that can be stripped due to irradiation include carbon or at least one hydrocarbon,
For example, it can be parylene.

The aluminum layer 43, the hydrogen catalyst layer 45 applied thereto, and the possible protective layer 46 applied thereto 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. In addition to 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 and 221 of the wafer inspection system 2 shown in FIG. 2 are also shown in FIGS. 4a and 4b.
The optical system 22 of the wafer inspection system 2 can be designed as described in connection with
has a housing 24, in an interior 24a of which are disposed two reflecting optical elements 220, 221. 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 which is 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 aluminium 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 apparatus 1, 2 in which the reflecting optical element 5 of Figures 5a, 5b is arranged comprises a plasma generation device 50 for generating 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 to 52f,
These are arranged in a uniform circumferential distribution around the optical surface 5a, which is circular in top view, specifically outside the beam path 53 of the VUV radiation 11, 21, shown in dashed lines in Figure 5b.

The plasma nozzles 52a to 52f emit plasma gas in the form of respective plasma gas streams 54a to 54f.
5a and 5b, each plasma gas flow 54a-54f is designed to supply an arc of the optical surface 5a, which is circular in top view.
As can be seen in Figure 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 example shown, the plasma generation device 50 is designed to generate a hydrogen plasma 51. For this purpose, plasma gas flows 54a-54f issuing from plasma nozzles 52a-52f are
4f has 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 the above gases. The addition of (molecular) hydrogen _H2 to the plasma gases 54a-54f improves the cleaning effect of the atmospheric pressure plasma 51 and reduces the amount of cleaning of the optical surface 5 during irradiation.
The thin aluminum oxide layer formed on a is reduced back to aluminum.

As discussed above in connection with Figure 3, the reflectivity R of the reflective optical element 5 can be significantly increased in this way. The use of plasma nozzles 54a-54f, which are typically located in each interior 122a, 24a in which the reflective optical element 5 is also located, makes it possible to eliminate 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 generation device 60 that is used to generate atmospheric pressure plasma 61 at the optical surface 6a of a reflective optical element 6 that is designed to reflect radiation 11, 21 in the VUV wavelength range and that may form, for example, one of the reflective optical elements 121, 220, 221 from Figures 1 or 2. The plasma generation device 60 of Figures 6a and 6b differs from the plasma generation device 50 shown in Figures 5a and 5b in that it has six pointed electrodes 62a to 62f for generating atmospheric pressure plasma 61 at the optical surface 6a of the reflective optical element 6. The six electrodes 62a to 62f are arranged outside the beam path 63 of the optical arrangement 1, 2 in which the optical element 6 is used.

The plasma generating device 60 generates a potential difference between the electrodes 62a to 62f and the optical surface 6a of the aluminum layer 54, which is kept at a predetermined potential and connected to ground potential in the illustrated example, and as a result, atmospheric pressure plasma 61 is generated between each of the electrodes 62a to 62f and the optical surface 6a, extending to the optical surface 6a. In the plasma generating device 60 shown in Figures 6a and 6b, each of the electrodes 62a
1 or 2. The plasma gas 64 is supplied to the optical surface 6a through the associated interiors 122a, 24a, 25a, 26a, 27a, 28a, 29a, 30a, 31a, 32a, 33a, 34a, 35a, 36a, 37a, 38a, 39a, 40a, 41a, 42a, 43a, 44a, 45a, 46a, 47a, 48a, 49a, 50a, 51a, 52a, 53a, 54a, 55a, 56a, 57a, 58a, 59a, 60a, 61a, 62a, 63a, 64a, 65a, 65a, 66a, 67
1 and 2, molecular hydrogen _H2 can also be supplied to the interiors 122a, 24a via gas inlets 123, 26, 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 Figures 5a and 5b and the plasma generating device 60 shown in Figures 6a and 6b, the atmospheric pressure plasmas 51, 61 on the optical surfaces 5a, 6a may vary depending on their locations.
For example, in the plasma generation device 50 shown in Figures 5a and 5b, the flow through each plasma nozzle 52a-52f and/or the plasma gas flow 54 can be individually controlled.
The time for which the hydrogen plasma 61 flows to each of the partial regions 55a to 55f of the optical surface 5a can be individually set. Correspondingly, the potential or potential difference between each of the electrodes 62a to 62f and the optical element 6 or the aluminum layer 43 can be changed to change the strength of the hydrogen plasma 61 generated in each of the partial regions 65a to 65f of the optical surface 6a.

In this way, each plasma generating device 50, 60 can be used to carry out targeted plasma cleaning of previously oxidized or contaminated partial areas 55a-55f, 65a-65f of the optical surfaces 5a, 6a.
Plasma nozzles 52a to 52f or electrodes 62a in locations not belonging to 55f, 65a to 65f
5a, 5b, and 6a, 6b, and 6c, the atmospheric pressure plasmas 51, 61 are generated by the plasma nozzles 52a to 52f or the electrodes 62a to 62f, so it is not necessarily necessary to activate all the plasma nozzles 52a to 52f or all the electrodes 62a to 62f to generate the atmospheric pressure plasmas 51, 61.
The number of six plasma nozzles 52a-52f or six electrodes 62a-62f shown in FIG. 6b is just an example, i.e. the plasma generation devices 50, 51 may have fewer or more plasma nozzles 52a-52f or electrodes 62a-62f.

Using the plasma generation devices 50 and 60 shown in FIGS. 5a, 5b and 6a, 6b,
Atmospheric pressure plasmas 51, 61 can be generated not only at the front reflective surfaces 5a, 6a of the reflective optical elements 5, 6, but also at the transmissive optical elements, for example the two transmissive optical elements 140, 141 of the projection system 14 shown in Figure 1. In this case, the atmospheric pressure plasmas 51, 61 can be used to remove contaminants from the respective transmissive optical surfaces 140a, 140b, 140c, 140d, 140e, 140f, 140g, 140h, 140h, 140i ...
To enhance the cleaning effect, reactive gas components, such as the above-mentioned hydrogen or possibly oxygen or water, can be removed from the atmospheric pressure plasma 51, 61.
This is possible because projection system 14 does not have any reflective optical elements with aluminum layers 43 or layers made of any other material that may be damaged by the oxidizing effects of oxygen.

The measures already mentioned above prevent the reflection of the reflective optical elements 121, 220,
The reduction in the reflectivity R of the aluminum layer 54 can be suppressed by generating hydrogen plasma 51, 72 at atmospheric pressure. The oxidation reaction of the aluminum layer 54 can be further reversed, i.e., the aluminum oxide already formed can be re-reduced to metallic aluminum. The action of the atmospheric pressure plasma 51, 61 reduces the reflectivity R of the optical surface 140 of the transmissive optical element 140, 141.
a, 140b, 141a, 141b can also be cleaned by removing contaminants.

Claims (14)

An optical element (4) in the form of a mirror for reflecting radiation (11, 21) in the VUV wavelength range,
An optical element (4) comprising a substrate (41) and a reflective coating (42) for reflecting radiation (11, 21) in the VUV wavelength range, the reflective coating (42) having at least one aluminum layer (43) applied to the substrate (41),
At least one hydrogen catalyst layer (45) for dissociation of molecular hydrogen (H ₂ ) is applied to said aluminum layer (43) ,
and at least one protective layer (46) applied to the aluminum layer (43) and the hydrogen catalyst layer (45).
The optical element , wherein the protective layer is made of a fluoride material . 2. The optical element of claim 1, wherein the hydrogen catalyst layer (45) completely covers the aluminum layer (43). 3. The optical element according to claim 1, wherein the material of the hydrogen catalyst layer (45) is selected from the group consisting of Ru, Pt, Pd, Ni, and Rh. The optical element according to claim 1 or 2, wherein the hydrogen catalyst layer (45) has a layer thickness (D) of 0.1 nm to 3.0 nm. The optical element according to any one of claims 1 to 4, wherein the hydrogen catalyst layer (45) is a continuous layer. 2. The optical element according to claim 1 , wherein said protective layer (46) forms a sealing layer. 7. The optical element according to claim 1 or 6 , wherein the protective layer (46) is made of a transparent material. 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;
and at least one gas inlet (123, 26) for supplying gas (H ₂ ) to the interior (122a, 24a),
Optical device, characterized in that the optical element (121, 220, 221, 4) is designed as claimed in any one of claims 1 to 7 and the gas inlet (123, 26) is designed to supply hydrogen (H ₂ ) to the interior (122a, 24a). 9. The optical device according to claim 8 ,
a plasma generating device (50, 60) for supplying plasma gas (54a to 54f, 64) to the interior (122a, 24a) via the gas inlet (123, 26) to generate atmospheric pressure plasma (51, 61) in at least one partial region (55a to 55f , 65a to 65f ) of the optical surface (5a, 6a) of the optical element (5, 6 );
An optical device comprising: 10. The optical arrangement according to claim 9 , wherein the plasma generation device (50, 60) is designed to generate a hydrogen plasma (51, 61) at the optical surface (5a, 6a) of the optical element (5, 6). 11. The optical device according to claim 9 or 10 , wherein the gas inlet is designed as a plasma nozzle (52a-52f) for supplying the plasma gas (54a-54f) to at least one partial region (55a-55f) of the optical surface (5a). 12. The optical apparatus according to claim 9 , wherein the plasma generating device (60) has at least one electrode (62a-62f) spaced apart from the optical surface (6a) to generate the atmospheric pressure plasma (61) in the at least one partial region (65a-65f) of the optical surface (6a). 13. The optical arrangement according to any one of claims 9 to 12 , wherein the plasma generating device (50, 60) is designed to generate a spatially variable atmospheric pressure plasma (51, 61) at the optical surface (5a, 6a). The optical device according to any one of claims 9 to 13 , configured as a wafer inspection system (2) or as a VUV lithography device (1).