JP2022510701A - Optical elements and devices that reflect VUV radiation - Google Patents

Abstract

The present invention is an optical element (4) that reflects radiation in the VUV wavelength range, a reflective coating (42) that is applied to the substrate (41) and has at least one aluminum layer (43). The present invention relates to an optical element (4) provided with and. At least one hydrogen catalyst layer (45) for dissociation of molecular hydrogen (H2) is applied to the aluminum layer (43). The present invention also relates to an optical device for the VUV wavelength range, the optical device including an interior in which at least one optical element is arranged and at least one gas inlet for supplying gas to the interior. In one aspect of the invention, the optical element (4) is designed as described above and hydrogen is supplied internally using a gas inlet. In yet another aspect of the invention, the optical device comprises a plasma generating device that supplies plasma gas internally through a gas inlet to generate atmospheric pressure plasma in at least one partial region of the optical surface of the optical element. ..

Description

[Refer to related applications]
This application claims the priority of German Patent Application No. DE10 2018 221 191.4 of December 7, 2018, which is incorporated herein by reference in its entirety.

The present invention relates to an optical element that reflects radiation in the VUV wavelength range and includes a substrate and a reflective coating applied to the substrate and having at least one aluminum layer. The present invention is an optical device for the VUV wavelength range, particularly a wafer inspection system or a VUV lithography device, which has an interior in which at least one optical element is arranged and at least one gas inlet for supplying gas to the interior. Regarding optical equipment.

In particular, in the short-wave ultraviolet wavelength range of about 100 nm to about 200 nm, which is also called the vacuum ultraviolet wavelength range (VUV wavelength range), not only the transmission optical element but also the reflection optical element is often used. Optical devices for radiation in the VUV wavelength range can be used, for example, to perform optical inspections of wafers or masks or to manufacture semiconductor components.

Optical elements that reflect VUV radiation usually have a reflective coating, which should have high reflectance over a large spectral range within the VUV wavelength range for certain applications, for example when inspecting wafers. be. Aluminum has a high reflectance of about 0.9 or 90% in the VUV wavelength range, so if such a reflective coating has one or, in some cases, multiple aluminum layers as the underlying layer (s). It turns out to be advantageous.

When using an aluminum layer in the VUV wavelength range, when the aluminum layer comes into contact with the atmosphere or the atmosphere around the catoptric element, it immediately forms a natural aluminum oxide (Al ₂ O ₃ ) layer with a layer thickness of about 2 nm to 3 nm. There is generally the problem of forming aluminum. Since this Al ₂ O ₃ layer has strong absorption in the VUV wavelength range, the aluminum layer is not attractive as a reflective layer used in the VUV wavelength range unless further protective measures from oxidation are taken.

In order to protect the aluminum layer from oxidation, for example, from the paper of Non-Patent Document 1, the aluminum layer is made of a metal fluoride form, for example, MgF ₂ , AlF ₃ , or LiF form, or three layers made of these materials. It is known to apply a protective layer or protective coating in the form of a protective coating.

However, it has been confirmed by lithography that severe deterioration of the catoptrics occurs within hours or days and is accompanied by large reflectance loss, especially at high radiant intensities that can occur during inspection of masks and wafers. In fact, even the above-mentioned protective layer made of metal fluoride, which shows a very good protective effect on the environment, cannot suppress the oxidation of the aluminum layer in the case of irradiation. A significant decrease in reflectance was also observed when the oxygen or moisture content of the optics in the environment was reduced to prevent oxidation.

In the case of an optical device for the VUV wavelength range, the problem that the optical surface is contaminated is generally added because the unnecessary gas component in the environment of the optical element cannot be completely suppressed. These gas components deposit on the optical surface and are "burned in" there during irradiation. This problem exists not only with respect to the optical surface of the reflective optical element but also with respect to the optical surface of the transmissive optical element.

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

An object of the present invention is to provide an optical element that reflects radiation in the VUV wavelength range and an optical device for the VUV wavelength range, which can extend the service life.

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

We recognize that certain materials cause catalytic hydrogen splitting or dissociation of molecular hydrogen. When molecular hydrogen is added to the environment of an optical element in the form of a compound in some cases, it separates in the material of the hydrogen catalyst layer at a particularly high irradiation amount to form active hydrogen. Active hydrogen is understood to mean hydrogen radicals, hydrogen ions, and / or hydrogen in excited electronic states. This active hydrogen can usually prevent or at least significantly slow down the oxidation of the aluminum layer even at high radiant intensities.

In one embodiment, for the material of the hydrogen catalyst layer, it is selected from the group comprising Ru, Pt, Pd, Ni, Rh. For these materials, especially Ru and Pt, it has been shown that oxidation can be virtually completely prevented, for example, by adding hydrogen to the environment 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 hydrogenation, that is, 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 material specified above usually has too low reflectance or too much absorption in the VUV wavelength range, so that a large layer thickness cannot be applied. For example, Ru has a reflectance well below 0.6 in the VUV wavelength range. Correspondingly, the ruthenium layer having a thickness of only 3 nm reduces the overall reflectance of the optics by about 0.2, which is generally unacceptable.

In yet another embodiment, the hydrogen catalyst layer does not completely cover the aluminum layer. If the layer thickness is below a certain value, the layer will usually not be able to be applied in a completely sealed form. For example, when Ru is used as a layer material, this is the case when the layer thickness is about 1.0 nm or less. Therefore, when the thickness is lower than this (depending on the material), a part of the surface of the aluminum layer is exposed and natural aluminum oxide is formed in the above part even though the hydrogen catalyst layer is applied. obtain. However, since the natural aluminum oxide layer cannot be formed in the covered partial region (s) of the aluminum layer, even a hydrogen catalyst layer that is not completely sealed can reduce the oxidation rate during irradiation. can. Further, when hydrogen is added to the environment of the optical element, oxidation cannot occur even in the exposed surface region of the aluminum layer in some cases due to the catalytic effect of the material of the hydrogen catalyst layer.

In the developed form, the hydrogen catalyst layer covers the aluminum layer with a coverage of about 10% to 90%, preferably 30% to 70%. Coverage is understood to mean the ratio of the surface of the hydrogen catalyst layer to the entire surface of the aluminum layer (opposite the substrate). Usually, the coating with the hydrogen catalyst layer occurs in this case in the form of an island-like isolated material deposit of the hydrogen catalyst layer on the aluminum layer. Like a closed hydrogen catalyst layer, (molecular) hydrogen accumulates between the aluminum and the hydrogen catalyst layer, leading to the formation of bubbles that would normally lead to complete or partial exfoliation of the hydrogen catalyst layer. This is advantageous because there is no such thing. In order to prevent the formation of bubbles or deterioration of the coating of the optical element due to the action of hydrogen, the hydrogen atom adsorbed on the surface of the upper layer of the coating is converted into molecular hydrogen and desorbed from the surface. Is known to contain a hydrogen desorbing material from German Patent Application Publication No. 1020172222690.

In yet another embodiment, the reflective coating comprises a protective layer applied to the aluminum layer and the hydrogen catalyst layer. For reflection reasons, if the hydrogen catalyst layer is applied so thin that it does not completely cover the aluminum layer, a portion of the surface of the aluminum layer will be exposed. When the natural oxide layer is formed in the exposed surface region of the aluminum layer, this reaction is generally irreversible by the addition of hydrogen due to the relatively high enthalpy of reaction of aluminum with oxygen. Active hydrogen is considerably more reactive than molecular hydrogen, but the above also applies to active hydrogen formed on the hydrogen catalyst layer. However, the situation is different if the oxidation reaction has not yet occurred. Oxygen or hydroxide molecules are even more loosely bonded to the surface prior to the reaction and are therefore more likely to be reduced by hydrogen radicals, i.e. the oxidation rate of aluminum is reduced in this case.

Therefore, the protective layer is used to oxidize the aluminum layer at least in the surface region (s) not covered by the hydrogen catalyst layer until sufficient hydrogen is present in the environment of the optic to prevent oxidation. It is usually advantageous to protect from. This is the case, for example, when an appropriate hydrogen-containing environment is provided there during operation of the optical element in an optical device.

The protective layer preferably forms a closed layer. As mentioned above, the protective layer needs to cover at least the exposed surface area (s) of the aluminum layer. This is usually achieved by providing a sealed protective layer between the aluminum layer and the hydrogen catalyst layer that partially covers it. It should be understood that as an alternative, 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 impractical.

The protective layer is preferably made of a transparent material, particularly a fluoride material, such as AlF ₃ , which is formed from, for example, metal fluoride. In this case, the protective layer is usually irreversible, i.e. permanently applied to the hydrogen catalyst layer.

In one embodiment, the protective layer is formed from a material that can be exfoliated by irradiation with radiation in the VUV wavelength range and / or contact with hydrogen. In this embodiment, the protective layer is reversibly applied. That is, because hydrogen is introduced anyway to protect the aluminum layer from oxidation during irradiation, or because the reflective coating, and thus the protective layer, is exposed to radiation in the VUV wavelength range anyway, in optics. The protective layer can be easily removed when the optical element operates. To allow the protective layer to be peeled off, the protective layer is usually an upper layer of the reflective coating.

The peelable protective layer is generally applied during or after the manufacture of the optic before exposure to the air environment. Since there is a protective layer to move the optics into the optics to be used, the complex concept of handling and transporting the optics in an inert gas / nitrogen or possibly vacuum is not required. When the optics are installed, the optics are exposed to the atmosphere so that there is not enough hydrogen to prevent oxidation of the aluminum layer, but if necessary, for example in a vacuum environment or flushing gas. The protective layer can be reapplied, for example, by adding, for example, a substance suitable for the environment of the optical element, which may be a flowing environment.

In the developed form, the protective layer is formed from carbon or at least one hydrocarbon. As described above, it is advantageous if the protective layer can be easily peeled off and reapplied. This is the case, for example, in the case of a thin protective layer containing carbon or carbohydrates and reduced to a carbon compound gas during operation of the optical element. By adding carbon or hydrocarbons to the environment of the catoptric elements of the optics, the protective layer can be (re) deposited 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 is the aforementioned type of optics, particularly wafer inspection systems, in which the optics are designed as described above and the gas inlet is designed or configured to supply (molecular) hydrogen internally. Or related to VUV lithography equipment. Flushing gas can flow inside through the gas inlet, and hydrogen is additionally supplied to the gas inlet, but it is also possible to create a vacuum environment inside where the optical element is placed, and this vacuum environment. Hydrogen is additionally supplied through the gas inlet. For the supply of hydrogen to the interior, the gas inlet usually has a gas reservoir used to store molecular hydrogen.

In order to form active hydrogen on the hydrogen catalyst layer and thus protect the aluminum layer from oxidation, a sufficient amount of molecular hydrogen is supplied to the inside where the optical element is arranged. It should be understood that attempts should also be made to keep the amount of oxidants such as oxygen or water as low as possible in the environment of the optics, i.e. in the surrounding flushing gas or in vacuum. As a result of these measures combined, the oxidation rate of the aluminum in the aluminum layer can usually be reduced to the extent that the high reflectance of the optical element is ensured over a sufficiently long irradiation time, resulting in optics. The need to replace the element is as low as possible, or the reflective coating needs to be renewed as little as possible.

Yet another aspect of the invention relates to the aforementioned type of optics which can be designed according to the second aspect of the invention but is not necessarily so. The optical device has a plasma generating device that supplies plasma gas internally through a gas inlet to generate atmospheric pressure plasma in at least one partial region of the optical surface of the optical element.

The optical element can be a reflective optical element for radiation in the VUV wavelength range, and an optical surface is formed on the reflective coating thereof. As an alternative, the optical element can be a transmitted optical element through which radiation in the VUV wavelength range passes through the optical surface. In either case, the optical surface is at least partially located in the beam path of the optical device.

As already mentioned above, the problem with both catoptric and transmissive optics used in optical devices for radiation in the VUV wavelength range is that the gas component deposited on the optical surface of the optical element is in the environment. The optical element is contaminated over time. To clean the optical surface, it is proposed in this aspect of the invention to use atmospheric pressure plasma to remove unwanted deposits on the optical surface (s). In particular, since the optical device can operate in a flushing gas atmosphere and does not require a vacuum pump, the atmospheric pressure plasma, that is, more than 100 mbar, preferably about 1 bar, is compared with the plasma generated under vacuum conditions. The use of pressure plasma has proved to be advantageous. In this case, compressed air or another type of flushing gas, such as nitrogen and / or noble gas, such as argon, can be used as the plasma gas or as the main component of the plasma gas. Reactive gas or reactive species such as hydrogen, oxygen, or water can optionally be added to the plasma gas to enhance the cleaning effect of the plasma.

In one embodiment, the plasma generating 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 internally via the gas inlet. Alternatively, plasma gas and hydrogen can be supplied internally 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 particularly advantageously used for cleaning the aluminum layer of the catoptric element, and the exposed surface of the catoptric element causes the hydrogen plasma to come into contact with the surface of the aluminum layer. The natural Al ₂ O ₃ layer already described above is completely or partially reduced to aluminum by the excited hydrogen ions of the plasma, and as a result, the reflectance of the reflecting optical element is improved and the transmittance of the optical device is improved. ..

As already described above, since the reaction enthalpy of aluminum with oxygen is high, it is impossible to reduce aluminum oxide to aluminum only by supplying hydrogen or activated hydrogen on the hydrogen catalyst layer. However, during cleaning with hydrogen plasma, such a reduction reaction is possible and therefore the already described hydrogen catalyst layer for dissociation of molecular hydrogen can be eliminated in some cases. When hydrogen plasma is generated intermittently during interruption of operation rather than continuously, for example at predetermined time intervals, or when the reflectance of the optical element is significantly reduced, the generation of hydrogen plasma and the hydrogen catalyst layer already described above The combination with is advantageous.

Plasma gas, more precisely plasma gas ions generated during plasma formation, can be supplied to the optical surface in various 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, which is designed to direct the plasma gas flow of the 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 and the gas outlet is directed or directed to the optical surface if the plasma nozzle or its outlet opening can be moved, eg tilted and / or displaced with respect to the optical surface. Can be In the case of a plasma nozzle, plasma gas, for example in the form of oil-free compressed air or flushing gas, such as nitrogen or another inert gas, usually flows through the discharge section, where it is excited and converted into a plasma state. .. For example, a plasma gas to which hydrogen or another active gas can be added enters the interior through a 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 as soon as it exits the plasma nozzle is described in German Patent Application Publication No. 10145131, see in its entirety. Will be part of this application.

Alternatively or additionally, the plasma generating device may have at least one electrode that is separated from the optical surface to generate atmospheric pressure plasma in at least one partial region of the optical surface. Electrodes (s) can be used to generate plasma as intended at desired locations within. In this case, for example, a reflective coated aluminum layer or another metal layer can serve as a counter electrode or to provide a ground potential for the plasma electrode. The electrodes (s) can be tapered, for example, to ionize the plasma gas in the region of the electrode tip or to generate plasma there that extends to a partial region of the optical surface.

In yet another embodiment, the plasma generating device is designed to generate locationally variable atmospheric pressure plasma on the optical surface. It has been found to be advantageous if a partial region of the optical surface is exposed to plasma only to the extent that it was previously contaminated or oxidized. This can be achieved with the appropriate placement of plasma nozzles (s) and / or electrodes (s). For example, electrodes and / or plasma nozzles can be arranged in a ring around the optics so that each electrode or plasma nozzle is assigned to a partial region of the optical surface, respectively. By activating the individual plasma nozzles or electrodes as intended, the atmospheric pressure plasma on the optical surface can be altered depending on the location. In particular, the time at which each plasma nozzle or each electrode is activated for a cleaning operation or reduction reaction can be set individually according to the degree of contamination or the degree of oxidation of each partial region of the optical surface.

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

In the example already described above, it is assumed that the plasma nozzle (s) or electrodes (s) or electrodes (s) are arranged outside the beam path of the optical device. However, it is also possible to introduce plasma nozzles (s) or electrodes (s) or electrodes (s) into the beam path of the optics and remove them again using suitable actuators. This can be particularly useful if the generation of atmospheric pressure plasma occurs only during the interruption of the operation of the optics.

The reflectance of the optical element can be increased by the above-mentioned cleaning operation or reduction effect of the atmospheric pressure plasma, that is, it is not necessary to remove the optical element from the optical device. In this way, each optical element can be frequently removed from the optical device and replaced with a new structurally identical optical element, or the reflective coating can be removed so that it does not need to be reapplied after removal.

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

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

A schematic diagram of an optical device for the VUV wavelength range in the form of a VUV lithography device is shown. A schematic diagram of an optical device in the form of a wafer inspection system is shown. The schematic diagram of the wavelength-dependent reflectance of the non-aluminum oxide layer and the aluminum layer oxidized by irradiation is shown. FIG. 6 shows a schematic view of an optical element provided with a reflective coating having an aluminum layer and a hydrogen catalyst layer applied to the aluminum layer. FIG. 6 shows a schematic view of an optical element provided with a reflective coating having an aluminum layer and a hydrogen catalyst layer applied to the aluminum layer. A plasma generation device equipped with six plasma nozzles for generating atmospheric pressure plasma on the optical surface of a catoptric element is shown. A plasma generation device equipped with six plasma nozzles for generating atmospheric pressure plasma on the optical surface of a catoptric element is shown. A plasma generation device provided with six electrodes for generating atmospheric pressure plasma on the optical surface of a catoptric element is shown. A plasma generation device provided with six electrodes for generating atmospheric pressure plasma on the optical surface of a catoptric element is shown.

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

FIG. 1 schematically illustrates an optical device 1 in the form of a VUV lithography device, especially for wavelengths in the range of 100 nm to 200 nm or 190 nm. The VUV lithography apparatus 1 has two optical systems in the form of an illumination system 12 and a projection system 14 as essential components. The VUV lithography apparatus 1 has a radiation source 10 for carrying out an exposure process, which can be an excimer laser emitting radiation 11 having a wavelength in the VUV wavelength range of, for example, 193 nm, 157 nm, or 126 nm. It can be an integral part of the lithography device 1.

The radiation 11 emitted by the radiation source 10 is adjusted by using the illumination system 12 so that the mask 13 also referred to as a reticle can be completely illuminated. In the example shown in FIG. 1, the illumination system 12 has both a transmission optical element and a reflection optical element. For example, FIG. 1 typically shows a transmission optical element 120 that focuses the radiation 11 and a reflection optical element 121 that deflects the radiation 11. As is known, various transmissive optics, catoptrics, or other optics can be combined with each other in the illumination system 12 in any desired more complex way.

The mask 13 has a structure on its surface that is transferred to an element 15, such as a wafer, that is exposed as part of the manufacture of semiconductor components 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 catoptric element in an alternative embodiment. 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 typically shown, which serve, for example, to reduce the structure on the mask 13 to the size desired for exposure of the wafer 15. The projection system 14 may also be provided with a particularly reflective optical element, and any optical element may be combined with each other by any known method. It should also be pointed out that optical devices without transmissive optics can also be used for VUV lithography.

FIG. 2 schematically illustrates an exemplary embodiment of an optical device of the form of a wafer inspection system 2. The instructions given below also apply to inspection systems for mask inspection.

The wafer inspection system 2 has a radiation source 20, and the radiation 21 thereof is directed to the wafer 25 by the optical system 22. For this purpose, the radiation 21 is reflected from the concave mirror 220 to the wafer 25. In the case of the mask inspection system 2, the mask to be inspected may be arranged instead of the wafer 25. Radiation reflected, diffracted, and / or refracted by the wafer 25 is directed to the detector 23 for further evaluation by yet another concave mirror 221 also associated with the optical system 22. As an example, the source 20 can be a single source alone or a combination of multiple individual sources to provide a substantially continuous emission spectrum. In the modified form, one or more 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.

For example, during the operation of an optical device such as the VUV lithography device 1 from FIG. 1 or the wafer or mask inspection system 2 from FIG. 2, oxidation of the reflective optical surfaces 121a, 221a, and 222a of the reflective optical elements 121, 220, and 221 occurs. obtain. The catoptric elements 121, 220, and 221 can have a metal mirror layer that reflects VUV radiation 11, and the aluminum layer may be a metal mirror layer having a high reflectance in a wide wavelength range of, for example, 100 nm to 200 nm. I know.

As an example, FIG. 3 shows the reflectance R of such an aluminum layer as a function of the wavelength λ in the wavelength range of about 120 nm to about 280 nm, that is, the wavelength range including almost the entire VUV wavelength range of about 100 nm to 200 nm. show. In this case, the curve shown by the broken line in FIG. 3 corresponds to the reflectance R of the non-aluminum oxide layer exceeding 0.9 in the entire wavelength range shown in the figure.

The curve shown by the solid line in FIG. 3 shows the reflectance R of the aluminum layer oxidized by irradiation with radiations 11 and 21 in the VUV wavelength range. Here, it is clearly seen that the reflectance R with respect to the corresponding wavelength λ of less than about 200 nm is remarkably reduced to a value of less than 0.1 at the wavelength λ of less than about 140 nm. Using the curve of reflectance R shown in FIG. 3, it is quite clear that the aluminum layer acting as the metal mirror layer should avoid its oxidation when it is used to reflect the radiation 11 in the VUV wavelength range.

4a and 4b show an optical element 4 that is embodied to reflect radiation 11 in the VUV wavelength range and can form, for example, one of the catoptric elements 121, 220, 221 of FIG. 1 or 2. .. The optical element 4 shown in FIGS. 4a and 4b is a mirror having a substrate 41, and the substrate 41 may be quartz (glass), particularly titanium-doped quartz glass, ceramic, or glass-ceramic in the illustrated example. A reflective coating 42 is applied to the substrate 41, which has a continuous aluminum layer 43 that acts as a metal mirror layer. The aluminum layer 43 can be applied directly to the substrate 41. In the example shown in FIGS. 4a and 4b, the functional layer in the form of the 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 selected from a large number of materials, but care should be taken to ensure sufficient adhesion to both the substrate 41 and the aluminum layer 43. Other functional layers, such as a smoothing layer and / or a polishing layer, may be provided between the aluminum layer 43 and the substrate 41.

_In the example shown in FIGS. 4a and 4b, a hydrogen catalyst layer 45 useful for dissociation of molecular hydrogen H2 in 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 material has a relatively low reflectance for radiation 11 in the VUV wavelength range. Therefore, it is beneficial if the thickness D of the hydrogen catalyst layer 45, which is less than 1.0 nm in the examples shown in FIGS. 4a and 4b, is as thin as possible. The hydrogen catalyst layer 45 having such a small thickness D cannot be applied to the aluminum layer 43 so as to completely cover it, that is, in the form of a closed layer, and as shown in FIGS. 4a and 4b, the hydrogen catalyst layer 45 cannot be applied to the aluminum layer 43. An island-like isolated material deposit of layer 45 is formed on the aluminum layer 43.

The coverage of the aluminum layer 43 by the hydrogen catalyst layer 45 is about 10% to about 90%, preferably 30% to 70%, and since it enables the realization of the hydrogen catalyst layer 45 having a small thickness D, it is an optical element. The reflectance R of 5 does not decrease too much, and the dissociation effect of the hydrogen catalyst layer 45 sufficient to prevent the oxidation of the aluminum layer 43 during irradiation with the VUV radiations 11 and 21 becomes possible.

A necessary condition for the protective effect of the hydrogen catalyst layer 45 is that the molecular hydrogen H2 can be used in the environment of the reflection optical element ₄ . The molecular hydrogen H ₂ in the VUV lithography apparatus 1 shown in FIG. 1 is supplied to the inside 122 a of the housing 122 in which the reflecting optical element 121 is arranged via the gas inlet 123 formed in the housing 122 of the lighting system 12. .. For this purpose, the gas inlet 123 has a gas reservoir ₍ not shown) containing the molecular hydrogen H2. If a sufficient amount of molecular hydrogen H ₂ is present in the environment of the catoptric element 121, it can be converted to active hydrogen at the hydrogen catalyst layer 45, thus protecting the aluminum layer 43 from oxidation.

When installed inside 122a of the housing 122 of the illumination system 12, the catoptric element 121 is generally exposed to the atmosphere, which can lead to potentially irreversible oxidation of the hydrogen catalyst layer 45 or the aluminum layer 43. Therefore, it is beneficial to provide the protective layer 46 on the hydrogen catalyst layer 45, as shown as an example in FIG. 4b. The protective layer 46 should form a closed layer that covers both the aluminum layer 43 and the hydrogen catalyst layer 45. The protective layer 46 can 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 gives a good protective effect to the aluminum layer 43. In this case, the material of the protective layer 46 may be, for example, a fluoride material, for example, a metal fluoride in the form of AlF ₃ .

Alternatively, the protective layer 46 can be reversibly applied 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 is introduced into the illumination system 122. For this purpose, a material that is stripped off upon irradiation with radiation 11 in the VUV wavelength range and / or on contact with ₍ molecular) hydrogen H2 is used for the protective layer 46. The material of the protective layer 46 that can be stripped due to irradiation can be carbon or at least one hydrocarbon, such as parylene.

The aluminum layer 43, the hydrogen catalyst layer 45 applied thereto, and the protective layer 46 that may be applied to the aluminum layer 43 are preferably deposited by atomic layer deposition. Atomic layer deposition allows for the deposition of particularly thin and smooth layers, and thus reduces reflectance loss and scattering due to absorption. In addition to atomic layer deposition, other coating processes such as magnetron sputtering, ion assisted deposition, plasma enhanced deposition, thermal deposition and the like are also suitable.

The catoptric elements 220 and 221 of the wafer inspection system 2 shown in FIG. 2 can also be designed as described in relation to FIGS. 4a and 4b. The optical system 22 of the wafer inspection system 2 has a housing 24, and two reflection optical elements 220 and 221 are arranged inside the housing 24 a. A gas inlet 26 capable of supplying the molecular hydrogen H ₂ to the inside 24a of the housing 24 is formed in the housing 24.

5a and 5b are catoptric elements designed to reflect radiation 11, 21 in the VUV wavelength range and capable of forming, for example, one of the catoptric elements 121, 220, 221 of FIG. 1 or 2. 5 is shown. 5a The catoptric element 5 of FIG. 5b is designed substantially like the catoptric element 4 shown in FIGS. 4a and 4b, but has neither a hydrogen catalyst layer 45 nor a protective layer 36, that is, The aluminum layer 43 is directly exposed to the environment, the upper part of which forms a catoptric surface 5a.

In order to protect the aluminum layer 43 of the reflective optical element 5 from oxidation, each of the optical devices 1 and 2 in which the reflective optical element 5 of FIGS. 5a and 5b is arranged is large on the reflective optical surface 5a of the reflective optical element 5. It has a plasma generation device 50 that generates atmospheric pressure plasma 51. For this purpose, the plasma generating device 50 has six gas inlets in the form of plasma nozzles 52a-52f, which are concretely distributed circumferentially around an optical surface 5a which is circular in the top view. It is arranged outside the beam path 53 of the VUV radiations 11 and 21 shown by the broken line in FIG. 5b.

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

In the illustrated example, the plasma generation device 50 is designed to generate hydrogen plasma 51. For this purpose, the plasma gas streams 54a-54f exiting the plasma nozzles 52a-52f have a relatively low proportion of hydrogen in addition to, for example, a flushing gas in the form of nitrogen, a noble gas, or a mixture of the above gases. By adding (molecular) hydrogen H ₂ to the plasma gases 54a to 54f, the cleaning effect of the atmospheric pressure plasma 51 is improved, and the thin aluminum oxide layer formed on the optical surface 5a during irradiation is re-reduced to aluminum. Will be done.

As described above in relation to FIG. 3, the reflectance R of the reflective optical element 5 can be significantly increased in this way. FIGS. 1 and ₂ show that the molecular hydrogen H2 is supplied to the internal 122a and 24a by using the plasma nozzles 54a to 54f which are normally arranged in the internal 122a and 24a where the reflective optical element 5 is also positioned. The indicated gas inlets 123 and 26 can be omitted.

6a and 6b are reflection optics designed to reflect radiation 11, 21 in the VUV wavelength range and capable of forming, for example, one of the reflection optics 121, 220, 221 from FIG. 1 or 2. 6 shows a plasma generating device 60 used to generate atmospheric pressure plasma 61 on the optical surface 6a of 6. The plasma generation device 60 of FIGS. 6a and 6b differs from the plasma generation device 50 shown in FIGS. 5a and 5b in that six sharp electrodes (6 sharp electrodes) for generating atmospheric pressure plasma 61 on the optical surface 6a of the reflection optical element 6 ( pointed electrodes) It is a point having 62a to 62f. The six electrodes 62a to 62f are arranged outside the beam paths 63 of the optical devices 1 and 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 connected to the ground potential in the illustrated example, and as a result, each of the electrodes 62a to 62a. An atmospheric pressure plasma 61 extending to the optical surface 6a is formed between the 62f and the optical surface 6a. In the plasma generating device 60 shown in FIGS. 6a and 6b, the electrodes 62a to 62f are assigned to the partial regions 65a to 65f of the optical surface 6a forming the arc, respectively. The plasma gas 64 is supplied to the environment of the optical surface 6a in this example via the respective gas inlets 123, 26 to the relevant internal 122a, 24a from FIG. 1 or 2. Here, as shown in FIGS. 1 and ₂ , the molecular hydrogen H2 can be further supplied to the internal 122a, 24a via the gas inlets 123, 26, which is generally in the form of a small admixture. Is added to the plasma gas 64.

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 pressure plasmas 51 and 61 on the optical surfaces 5a and 6a may change depending on the location. For this purpose, the plasma nozzles 52a to 52f or the electrodes 62a to 62f can be individually controlled. For example, in the plasma generation device 50 shown in FIGS. 5a and 5b, the flow through each of the plasma nozzles 52a to 52f and / or the time during which the plasma gas flows 54a to 54f flow to the respective partial regions 55a to 55f of the optical surface 5a are individually. Can be set. Correspondingly, in order to change the strength of the hydrogen plasma 61 generated in each partial region 65a to 65f of the optical surface 6a, the potential or potential difference between each electrode 62a to 62f and the optical element 6 or the aluminum layer 43 is set. You can change it.

In this way, the plasma generation devices 50 and 60 can be used to perform plasma cleaning as intended in the previously oxidized or contaminated partial regions 55a to 55f and 65a to 65f of the optical surfaces 5a and 6a. .. Plasma nozzles 52a to 52f or electrodes 62a to 62f in locations of the optical surfaces 5a and 6a that do not belong to the relevant partial regions 55a to 55f and 65a to 65f also generate (not very strong) atmospheric pressure plasmas 51 and 61. Therefore, it is not always necessary to activate all the plasma nozzles 52a to 52f or all the electrodes 62a to 62f to generate the atmospheric pressure plasmas 51 and 61. The number of the six plasma nozzles 52a to 52f or the six electrodes 62a to 62f shown in FIGS. 5a, 5b and 6a, 6b is only an example, that is, the plasma generating devices 50 and 51 are smaller or larger. The plasma nozzles 52a to 52f or the electrodes 62a to 62f of the above can also be provided.

Using the plasma generating devices 50 and 60 shown in FIGS. 5a, 5b and 6a and 6b, it is possible to generate atmospheric pressure plasmas 51 and 61 on the reflecting surfaces 5a and 6a on the front side of the reflecting optical elements 5 and 6. Instead, transmission optical elements, for example, the two transmission optical elements 140 and 141 of the projection system 14 shown in FIG. 1 can also generate atmospheric pressure plasmas 51 and 61. In this case, the atmospheric pressure plasmas 51 and 61 can be used to remove contaminants from the transmitted optical surfaces 140a, 140b, 141a and 141b. In order to enhance the cleaning effect, reactive gas components such as the hydrogen described above or perhaps oxygen or water can be added to the plasma gas used to generate the atmospheric pressure plasmas 51, 61. This is possible because no catoptric element with an aluminum layer 43 or a layer made of any other material that can be damaged by the oxidative effects of oxygen is located in the projection system 14.

The measures already described above can suppress the decrease in the reflectance R of the reflective optical elements 121, 220, and 221 due to the oxidation of the aluminum layer 43. By generating the hydrogen plasmas 51 and 72 at atmospheric pressure, the oxidation reaction of the aluminum layer 54 can be further reversed, that is, the already formed aluminum oxide can be re-reduced to metallic aluminum. By the action of the atmospheric pressure plasmas 51 and 61, the optical surfaces 140a, 140b, 141a and 141b of the transmission optical elements 140 and 141 can also be cleaned by removing contaminants.

Claims (16)

An optical element (4) that reflects radiation (11, 21) in the VUV wavelength range.
In 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).
An optical element, characterized in that at least one hydrogen catalyst layer (45) for dissociation of molecular hydrogen (H ₂ ) is applied to the aluminum layer (43). In the optical element according to claim 1, the hydrogen catalyst layer (45) is an optical element selected from the group containing 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, preferably 0.1 nm to 1.0 nm. The optical element according to any one of claims 1 to 3, wherein the hydrogen catalyst layer (45) does not completely cover the aluminum layer (43). In the optical element according to claim 4, the hydrogen catalyst layer (45) covers the aluminum layer (43) with a coverage of 10% to 90%. The optical element according to any one of claims 1 to 5, further comprising at least one protective layer (46) applied to the aluminum layer (43) and the hydrogen catalyst layer (45). In the optical element according to claim 6, the protective layer (46) is an optical element that forms a closed layer. In the optical element according to claim 6 or 7, the protective layer (46) is an optical element formed of a transparent material, particularly a fluoride material. In the optical element according to any one of claims 6 to 8, the protective layer (46) is irradiated with radiation (11, 21) in the VUV wavelength range and / or by contact with hydrogen (H ₂ ). An optical element formed from a peelable material. In the optical element according to claim 9, the protective layer (46) is an optical element formed of carbon or at least one hydrocarbon. An optical device for the VUV wavelength range, particularly a wafer inspection system (2) or a VUV lithography device (1).
Inside (122a, 24a) where at least one optical element (121, 220, 221; 140, 141; 4, 5, 6) is located,
In an optical device provided with at least one gas inlet (123, 26, 52a to 52f) for supplying gas (H ₂ ) to the inside (122a, 24a), the optical element (121, 220, 221, 4). Is designed as described in any one of claims 1-10, and the gas inlets (123, 26) are designed to supply hydrogen (H ₂ ) to the interior (122a, 24a). An optical device characterized by that. The optical device according to claim 11, especially in the optical apparatus according to claim 11.
The gas inlets (123, 26, 52a) to generate atmospheric pressure plasma (51, 61) in at least one partial region (55a-55f) of the optical surface (5a, 6a) of the optical element (5, 6). Plasma generating devices (50, 60) that supply plasma gas (54a to 54f) to the inside (122a, 24a) via (52f).
An optical device characterized by. In the optical device according to claim 12, the plasma generating device (50, 60) generates hydrogen plasma (51, 61) on the optical surface (5a, 6a) of the optical element (5, 6). An optical device designed. In the optical device according to claim 12 or 13, the gas inlet supplies the plasma gas (54a to 54f) to at least one partial region (55a to 55f) of the optical surface (5a). An optical device designed as ~ 52f). In the optical apparatus according to any one of claims 12 to 14, the plasma generating device (60) is separated from the optical surface (6a), and the at least one partial region of the optical surface (6a) is separated from the optical surface (6a). An optical device having at least one electrode (62a to 62f) that generates the atmospheric pressure plasma (61) at (65a to 65f). In the optical device according to any one of claims 12 to 15, the plasma generating device (50, 60) is an atmospheric pressure plasma (51, 61) that is variable depending on the location on the optical surface (5a, 6a). ) Is an optical device designed to generate.

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