OPTICAL ARRANGEMENT FOR THE FUV / VUV WAVELENGTH RANGE

Description

Reference to related application

[0001] This application claims priority from German patent application DE 10 2021 202 848.9 of March 24, 2021. Background of the invention

[0002] The invention relates to an optical arrangement for the FUV / VUV wavelength range, in particular a wafer inspection system or an FUV / VUV lithography system, which has an interior space, wherein at least one transmitting optical element for transmitting radiation in the FUV / VUV wavelength range is arranged in the interior space, which preferably has a crystalline, in particular ionic substrate, and / or wherein at least one reflective optical element for reflecting radiation in the FUV / VUV wavelength range is arranged in the interior space, which has a substrate and a reflective coating, a radiation source for irradiating the at least one optical element with radiation in the FUV / VUV wavelength range and at least one gas inlet for supplying at least one gas into the interior space.For the purposes of this application, the VUV wavelength range is understood to be a wavelength range between 100 nm and 200 nm (VUV wavelength range according to DIN 5031 Part 7). For the purposes of this application, the FUV wavelength range is understood to be a wavelength range between 200 nm and approximately 280 nm. The optical arrangement described here can be designed or configured for operation in the VUV wavelength range, for operation in the FUV wavelength range, or for operation in both wavelength ranges.

[0003] For the inspection of the smallest structures on a wafer using a wafer inspection system, it is advantageous to reduce the wavelengths previously used for inspection in the FUV wavelength range to smaller wavelengths in the VUV wavelength range, see, for example, the article "Extending Optical Inspection to the VUV", Keith Wells et al., Int. Conf. on Frontiers of Characterization and Metrology for Nanoelectronics (FCMN), 2017, pages 92-101 or US 2016 / 0258878 A1, which deals with a wafer inspection system for the VUV wavelength range.

[0004] As described in the above-cited article, operating a wafer inspection system at wavelengths in the VUV wavelength range presents significant challenges. For example, highly reflective mirrors, traditionally made of aluminum at VUV wavelengths, exhibit a significant decrease in reflectivity at high irradiances, even when the aluminum surface is protected with dielectric materials, e.g., fluoride materials such as MgF 2 , as described, for example, in US 2017 / 0031067 A1 or US 2004 / 0006249 A1. DE102019219177.0 proposes using a Mangin mirror for the VUV wavelength range to solve this problem.

[0005] DE 10 2018 211 498 A1 describes an optical element comprising a reflective surface having a protective layer of fluorides. The optical element can be designed for the VUV wavelength range. The reflective surface can be formed as a coating on a substrate and have a metal layer, which can in particular be a layer of aluminum or an aluminum alloy.

[0006] In addition to reflective optical elements, transmissive optical elements are also used in optical arrangements for the FUV / VUV wavelength range. Optical elements designed to transmit FUV / VUV radiation often have a material or substrate in the form of an ionic (single) crystal, for example a fluoride (single) crystal. Irradiation of such optical elements with high irradiances in the FUV / VUV wavelength range can lead to defects in the form of color centers, e.g. in the form of F centers, see for example the article "The radiation damage process in MgF2", MR Buckton and D. Pooley, J. Phys. C: Solid State Phys. 5 (1972), pages 1553-1563 or the article "Swelling and creation of color centers in MgF2 single crystals irradiated with energetic heavy ions", AS El-Said et al., Nucl. Instr. Meth. B 245 (2006), pages 250-254.The formation of the color centers leads to an increased absorption of FUV / VUV radiation in the material of the ionic crystal, e.g. in MgF 2 .

[0007] Irradiation of a crystalline material, particularly an ionic crystal, with high-energy radiation in the FUV or VUV wavelength range can also result in a significant energy input into the surface of the crystalline material. This energy input can lead to an undesirable change in the geometry of the surface (degradation). A surface has a less favorable energetic state than the bulk of a solid. In addition, the (finishing) processing during surface production (grinding, polishing, etc.) can introduce defects into the surface, which represent particular weak points. This problem occurs particularly when an imperfect surface is irradiated with high-energy radiation, since such a surface exhibits increased absorption, meaning that a significantly greater energy input occurs there than in the bulk of the substrate of the optical element.

[0008] US 6,495,202 B2 proposes replacing the fluorine atoms in an optical element that has fluorine at least in a near-surface region with OH groups and / or with O atoms. US 6,269,661 B1 proposes increasing the transmission of an optical component by heat treatment and etching treatment on a surface where the component was cut from a block material, e.g., in the form of CaF 2 . WO 2019 / 007927 A1 discloses at least partially removing a contamination layer from an optical surface of an optical element by an atomic layer etching process.

[0009] WO 2020 / 115112 A2 describes an optical arrangement in the form of a wafer inspection system of the type mentioned above. The wafer inspection system has a gas inlet designed or configured to supply an adsorbate, in particular water, into the interior at least during irradiation of the surface. Supplying the adsorbate to the surface of an optical element is intended to mitigate surface degradation: The adsorbate can absorb a portion of the thermal energy generated by absorption of radiation at the surface and dissipate it through desorption, generating evaporative cooling. This results in less energy being available at the surface for rearrangement effects and other surface-damaging effects.

[0010] PCT / EP2019 / 083632 describes that, when the substrate is an ionic crystal, the addition of a polar adsorbate, i.e., an adsorbate exhibiting a high dipole moment, has proven beneficial. Since the surfaces of ionic crystals, which exhibit increased surface energy, typically have a dipole character, the dipoles on these surfaces can be saturated by the attachment of molecules exhibiting a dipole moment. PCT / EP2019 / 083632 states that water is a suitable adsorbate due to its large dipole moment. To achieve the desired effect, a minimum water concentration of more than 1 ppm or more than 10 ppm (relative to the total pressure) is required in the interior space.

[0011] It has been observed that at the above-mentioned water concentrations in the interior, functional optical layers or components can be damaged at high irradiances in the FUV and especially in the VUV wavelength range. For example, it has been shown that a reflective optical element arranged in the interior, in the form of a dielectrically protected aluminum mirror, exhibits a significant decrease in reflectivity. Object of the invention

[0012] The object of the invention is to provide an optical arrangement which counteracts the degradation of transmitting and / or reflecting optical elements during irradiation with radiation in the DUV / VUV wavelength range. Subject of the invention

[0013] This object is achieved by an optical arrangement of the type mentioned above, in which the gas inlet is designed to supply ammonia (NH 3 ) into the interior at least during the irradiation of the surface.

[0014] Like water (H2O), ammonia is a permanent dipole with a comparably large dipole moment (dipole moments: H2O = 1.85 D, NH3 = 1.47 D). By adding ammonia to the interior space, the same effect on the surface of the substrate or the ionic crystal of a transmitting optical element can be produced as by adding water. As described below, however, the negative effects of water on functional optical layers or components can be avoided if ammonia is added to the interior space instead of water. The addition of a polar adsorbate can also have advantageous effects on reflective optical elements, for example when these are designed as rear-surface mirrors in which, for example, an aluminum layer is applied to the back of a transparent carrier material in the form of a fluoride, e.g. in the form of MgF2.

[0015] Water is photochemically split very efficiently (efficiency almost 1) when irradiated with UV radiation, especially with VUV radiation at wavelengths between 115 nm and 160 nm, producing highly reactive oxygen and / or hydroxide radicals and / or ions.

[0016] Two of these photochemical reactions are given below: H 2 O + hv → H* + OH* H 2 O + hv → H2 + O*

[0017] These oxidic / hydroxidic reaction products can subsequently oxidize functional optical layers and / or components, for example Al mirrors, forming Al 2 O 3 , Al(OH) 3 .

[0018] In contrast, the photochemical reaction products of NH 3 induced by VUV radiation are significantly less reactive.

[0019] Two of the photochemical reactions of NH 3 are given below: NH 3 + hv → NH 2 * + H NH 3 + hv→ NH* + H 2

[0020] The photochemical reaction products listed above under FUV / VUV irradiation are oxygen-free and unreactive with materials such as fluorides or metals such as aluminum. A direct chemical reaction between metals such as aluminum and these reaction products is therefore unlikely if the residual gas atmosphere in the interior is sufficiently dry.

[0021] In one embodiment, the optical arrangement comprises an adjustment device for adjusting the ammonia content in the interior space. To achieve the desired effect of the ammonia, namely preventing degradation of the surface of the transmitting optical element, a minimum ammonia concentration is typically required in the interior space. However, the ammonia concentration should not be chosen too high to prevent the radiation within the gas volume of the interior space from being partially absorbed by the ammonia, thus reducing the transmission of the optical arrangement.

[0022] The adjustment device serves to keep the concentration or content of ammonia in the interior space within a predetermined value range, at least during irradiation. The supply of the adsorbate to the interior space is typically only necessary while the surface is being loaded, i.e., irradiated. The adjustment device can therefore prevent the supply of ammonia during breaks in operation. The adjustment device can be connected to a dosing device that allows the amount of ammonia supplied to the interior space via the gas inlet to be adjusted. The dosing device can, for example, be a controllable valve that allows the ammonia content in the interior space to be dosed in the ppm range.

[0023] According to the invention, the gas inlet or gas inlet system for supplying the ammonia into the interior also has a reservoir containing the ammonia. The deposition of the ammonia on the surface is a reversible process.

[0024] In a further development, the optical arrangement comprises a measuring device (a sensor) for measuring the concentration of ammonia in the interior. The measuring device measures the actual concentration of ammonia in the interior, particularly near the surface. In this case, the adjustment device is typically designed to regulate the concentration of ammonia in the interior. With the aid of a control device or a controller provided in the adjustment device, the actual value of the ammonia concentration measured by the measuring device can be regulated to a target value.

[0025] The concentration or content of ammonia can be measured absolutely, for example, in the form of partial pressure, or relatively, i.e., relative to the total pressure or to the total volume / total number of particles in the interior. The ratio of the volume of ammonia to the total volume (volume mixing ratio, unit: ppmV) is used below to define the ammonia content in the interior.

[0026] The in-situ measuring device should be designed to measure the ammonia content in the interior with an accuracy of the order of 1 ppmV, ideally with an accuracy of 100 ppbV. A residual gas analyzer with a mass spectrometer can serve as an in-situ measuring device, for example. An electrochemical sensor or a tunable diode laser for photoacoustic analysis or spectroscopy can also be used as an in-situ measuring device.

[0027] In a further embodiment, ammonia is introduced into the interior at a concentration of more than 0.1 ppmV, preferably more than 1 ppmV. A minimum ammonia concentration greater than the specified values ​​is generally sufficient for substrates in the form of ionic crystals to prevent surface degradation due to rearrangement. As a rule, a maximum ammonia concentration in the interior of more than approximately 10 ppmV should not be exceeded to prevent excessive radiation absorption.

[0028] In a further embodiment, the optical arrangement is designed to supply at least one inert gas into the interior space, which in particular takes place via the same gas inlet as the supply of the ammonia, but which can optionally also take place via a different gas inlet. The interior space can be formed, for example, in a housing of the optical arrangement, e.g. a wafer inspection system or a lithography system. In this embodiment, the interior space generally has a comparatively high total pressure, typically close to atmospheric pressure, which is essentially determined by the partial pressure of the inert gas. The inert gas can be, for example, a noble gas, e.g. He, Ne, Ar, Kr, Xe or nitrogen (N 2 ).

[0029] In a further development, the gas inlet is designed to introduce the ammonia and inert gas into the interior as a gas mixture. This is advantageous because ammonia is a toxic gas. By using a gas mixture of NH3 and the inert gas, e.g., Ar, it is possible to not exceed the MAK value (maximum workplace concentration) for the concentration of ammonia at the workplace of 20 ppmV, which is prescribed for occupational safety. Due to the toxicity of ammonia, the installation of an ammonia monitoring system may be required in the (laboratory) area surrounding the optical arrangement.

[0030] In a further development, the optical arrangement has a drying device for drying the inert gas before it is fed into the interior. Drying the supplied inert gas, e.g., in the form of a noble gas, is advantageous for generating the lowest possible water partial pressure and the lowest possible oxygen partial pressure in the interior. A commercially available drying or purification device ("gas purifier"), such as that used for purifying or drying inert gases, e.g., for drying argon, can be used for drying.

[0031] In one embodiment, water with a content of less than 1 ppmV, preferably less than 0.1 ppmV, is introduced into the interior space. As described above, it is advantageous if the water content or partial pressure in the interior space is as low as possible so that as few reactive species as possible are formed during photochemical reactions with radiation in the FUV / VUV wavelength range. This is particularly advantageous or necessary if optical elements are present in the interior space that have a high affinity for oxide formation, for example, aluminum.

[0032] In one embodiment, the reflective coating of the at least one reflective optical element comprises at least one aluminum layer. Optical elements for reflecting FUV / VUV radiation typically have a reflective coating which, for certain applications, for example, in the inspection of wafers, should have high reflectivity over a large spectral range within the FUV / VUV wavelength range. Since aluminum has a high reflectivity of approximately 0.9 or 90% in the FUV / VUV wavelength range, it has proven advantageous if such a reflective coating has one or more aluminum layers as base layer(s). The reflective optical element can also be designed as a rear surface mirror. In this case, the aluminum layer is applied to the back of a substrate that is transparent to the FUV / VUV radiation, e.g., a fluoride substrate.

[0033] A general problem with the use of aluminum layers is that upon contact with the atmosphere surrounding the reflective optical element, the aluminum layer almost immediately forms a native aluminum oxide (Al 2 O 3 ) layer with a layer thickness on the order of approximately 2-3 nm. This Al 2 O 3 layer absorbs so strongly in the VUV wavelength range that an aluminum layer is not attractive as a reflective layer for use in the VUV wavelength range without the provision of additional measures to protect it from oxidation.

[0034] In a further development, at least one protective layer made of a fluoride material is applied to the reflective coating. To protect the aluminum layer from oxidation, it is known, for example, from the article "Protected and enhanced aluminum mirrors for the VUV" by S. Wilbrandt et al., Applied Optics, Vol. 53, No. 4, February 2014, to apply protective layers or coatings in the form of metal fluorides to the aluminum layer, e.g., in the form of MgF 2 , AlF 3 , or LiF.

[0035] However, it has been observed that at high radiation intensities, such as those encountered in lithography and especially during the inspection of masks and wafers, severe degradation of the reflective optical elements occurs within just a few hours or days, despite the protective layers described above, resulting in a significant loss of reflectivity if the oxygen or water content in the interior or surrounding area is too high. Therefore, both the water content and the oxygen content in the interior should be kept as low as possible.

[0036] In a further embodiment, the optical arrangement has at least one vacuum pump for evacuating the interior. In this embodiment, a total pressure of less than approximately 10 -3 mbar is generally generated in the interior. In this example, an inert gas can also be supplied to the interior, but this is not mandatory.

[0037] In a further development of this embodiment, the partial pressure of ammonia in the interior space is more than 10 -4 mbar, preferably more than 10 -3 mbar. At a partial pressure of ammonia that is above the specified values, degradation of the surface of an ionic crystal can typically be suppressed, thus increasing the service life of the transmitting optical element and thus of the entire optical arrangement.

[0038] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention. drawing

[0039] Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows Fig. 1 is a schematic representation of an optical arrangement for the VUV wavelength range in the form of a VUV lithography system, and Fig. 2 is a schematic representation of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system.

[0040] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0041] In Fig. 1 is schematically an optical arrangement 1 in the form of a VUV lithography system, in particular for wavelengths in the range between 100 nm and 200 nm or 190 nm. The VUV lithography system 1 has as essential components two optical systems in the form of an illumination system 2 and a projection system 3 To carry out an exposure process, the VUV lithography system 1 has a radiation source 4 which can be, for example, an excimer laser that emits radiation 5 emitted at a wavelength in the VUV wavelength range of, for example, 193 nm, 157 nm or 126 nm and which can be an integral part of the VUV lithography system 1.

[0042] The radiation 5 emitted by the radiation source 4 is processed by the illumination system 2 in such a way that a mask 6, also called reticle, can be illuminated. In the Fig. 1 In the example shown, the illumination system 2 has both transmitting and reflecting optical elements. Fig. 1 a transmitting optical element 7, which focuses the radiation 5, as well as a reflective optical element 8 which, for example, deflects the radiation 5. In a known manner, various transmitting, reflecting, or other optical elements can be combined in the illumination system 2 in any desired, even more complex, manner.

[0043] The mask 6 has a structure on its surface which is directed towards an optical element to be exposed 9,For example, a wafer, as part of the production of semiconductor components, is transferred using the projection system 3. In the example shown, the mask 6 is designed as a transmitting optical element. In alternative embodiments, the mask 6 can also be designed as a reflective optical element. In the example shown, the projection system 2 has at least one transmitting optical element. In the example shown, two transmitting optical elements are representative. 10, 11, which serve, for example, to reduce the structures on the mask 6 to the size desired for exposing the wafer 9. Reflective optical elements can also be provided in the projection system 3, and any optical elements can be combined with one another in a known manner. It should be noted that optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0044] In Fig. 2 is a schematic illustration of an exemplary embodiment of an optical arrangement in the form of a wafer inspection system 21 The following explanations also apply analogously to inspection systems for inspecting masks.

[0045] The wafer inspection system 21 comprises an optical system 22 with a radiation source 24 whose radiation 25 by means of the optical system 22 onto a wafer 29For this purpose, the radiation 25 is deflected by a concave mirror 26 reflected onto the wafer 29. In a mask inspection system 2, a mask to be examined could be arranged instead of the wafer 29. The radiation reflected, diffracted and / or refracted by the wafer 29 is reflected by a further concave mirror also belonging to the optical system 22. 28 via a transmitting optical element 27 on a detector 30for further evaluation. The radiation source 24 can, for example, be precisely one radiation source or a combination of several individual radiation sources in order to provide a substantially continuous radiation spectrum. In modifications, one or more narrowband radiation sources 24 can also be used. The wavelength or wavelength band of the radiation 25 generated by the radiation source 24 is preferably in the range between 100 nm and 200 nm, particularly preferably between 110 nm and 190 nm.

[0046] In the Fig. 1 In the example shown, the lighting system 2 has a housing 12 in which an interior 13 is formed in which the transmissive optical element 7 and the reflective optical element 8 are arranged in the form of a mirror. Accordingly, the optical system 22 of the wafer inspection system 2 of Fig. 2a housing 32 in which an interior 33 is formed in which the two mirrors 26, 28 and the transmissive optical element 27 are arranged.

[0047] When irradiating the respective transmitting optical element 7, 27 with the radiation 5, 25 of the radiation source 4, 24, which typically has a high intensity, it can happen, in particular in the case that the transmitting optical element 7, 27 is a substrate 7a, 27ain the form of an ionic crystal, e.g. in the form of MgF 2, the energy input can lead to a rearrangement of the surfaces of the substrate 7a, 27a that are exposed to the interior 13, 33. This rearrangement can result in degradation of the surfaces, in particular in the form of roughening of the surfaces. Such roughening during operation of the optical arrangement 1, 21 is undesirable, since the roughening results in an enlargement of the respective surface, which promotes the accumulation of contaminating substances on the surface of the substrate 7a, 27a of a respective transmitting optical element 7, 27. The roughening of the surfaces of the substrate 7a, 27a can also lead to the formation of scattered light within the respective optical arrangement 1, 21.

[0048] In order to prevent a rearrangement of the surfaces of the crystalline substrate 7a of the transmitting optical element 7, the lithography system 1 has a gas inlet 14 which is used to supply an adsorbate in the form of ammonia NH3 into the interior 13 of the housing 12 of the illumination system 2. Accordingly, the wafer inspection system 21 also has a gas inlet 34 which serves to supply an adsorbate in the form of ammonia NH 3 into the interior 33 of the housing 32 of the optical system 22. In the Fig. 1 and Fig. 2 In the examples shown, in addition to the adsorbate in the form of ammonia NH 3, an inert gas in the form of a noble gas, more precisely in the form of argon, is added to the respective interior space 13, 33. Ar, The gas inlet 14, 34 or the gas inlet system for supplying the ammonia NH 3 into the respective interior space 13, 33 has a reservoir in which the ammonia NH 3 is contained.

[0049] The concentration or content c NH3 of the ammonia NH 3 in the respective interior space 13, 33 is more than 0.1 ppmV, possibly more than 1 ppmV, in order to achieve the greatest possible coverage of the surfaces of the respective transmitting optical elements 7, 27 with ammonia NH 3. The content of ammonia NH 3 in the respective interior space 13, 33 should generally be no more than approximately 10 ppmV. Alternatively or in addition to argon Ar, another inert gas, for example another noble gas, or nitrogen N 2 can be introduced into the respective interior space 13, 33.

[0050] Since ammonia NH3 is a toxic gas, the gas inlet 14, 34 is designed to introduce the ammonia NH3 and the inert gas Ar together as a gas mixture into the respective interior space 13, 33. For this purpose, the gas inlet 14, 34 can have a mixing valve (not shown). In this way, it can be ensured that the MAK value for the ammonia concentration of 20 ml / m3< is not exceeded when supplying the ammonia NH3.

[0051] In order to adjust the concentration c NH3 of the ammonia NH 3 in the respective interior space 13, 33, the lithography system 1 of Fig. 1 and the wafer inspection system 21 from Fig. 2 one adjustment device each 15, 35 which acts on a controllable valve to adjust or regulate the flow of ammonia NH 3 through the gas inlet 14, 34. In the example shown, a measuring device 16, 36arranged in the respective interior space 13, 33 in order to measure the actual concentration c NH3 of ammonia NH 3 in the respective interior space 13, 33. The measuring device 16, 36 can, for example, be a residual gas analyzer which has a mass spectrometer for determining the actual concentration c NH3 of the ammonia NH 3. The measuring device 16, 36 can also be an electrochemical sensor or a tunable diode laser for performing photoacoustic spectroscopy. Alternatively, the measuring device 16, 36 can be arranged outside the interior space 13, 33, but connected to it via a pipeline or the like. The adjusting device 15, 35 has a controller for regulating the measured value of the ammonia concentration c NH3 to a target value or to a target interval.

[0052] The addition of a polar adsorbate in the form of ammonia NH 3 instead of water H 2 O is advantageous because photoinduced chemical reactions of ammonia NH 3 with radiation 5, 25 in the VUV wavelength range result in comparatively few reactive reaction products. In contrast, photoinduced chemical reactions of radiation 5, 25 in the VUV wavelength range with water H 2 O result in the formation of highly reactive reaction products, e.g. in the form of oxygen radicals, hydrogen radicals, etc. The addition of water H 2 O can therefore lead to undesirable effects such as the oxidation of functional optical layers or components arranged in the interior space 13, 33.

[0053] During the operation of optical arrangements such as the VUV lithography system 1 of Fig. 1 or the wafer or mask inspection system 2 of Fig. 2Due to these highly reactive reaction products, the presence of a comparatively high concentration of water H 2 O in the respective interior space 13, 33 can lead in particular to oxidation of the surfaces of the reflective optical elements 8, 26, 27, the structure of which is described in more detail below.

[0054] The reflective optical elements 8, 26, 28 have a substrate 8a, 26a, 28a e.g. made of a fluoride material or silicon, on which a reflective coating 8b, 26b, 28b to reflect the VUV radiation 5, 35. The reflective optical elements 8, 26, 28 each have a metal coating in the form of an aluminum layer as a reflective coating 8b, 26b, 28b. To protect the reflective coating 8b, 26b, 28b from oxidation, Fig. 1 and Fig. 2 shown examples a protective layer 8c, 26c, 28cmade of a fluoride material, for example MgF 2 , AlF 3 , LiF, LaF 3 , GdF 3 , BaF 2 or other transparent fluorides.

[0055] However, the protection of the reflective coating 8b, 26b, 28b in the form of the aluminum layer by the protective layer 8c, 26c, 28c is only effective if the water content and also the oxygen content in the respective interior space 13, 33 are extremely low. In particular, the water content c H2O in the respective interior space 13, 33 should be less than 1 ppmV, especially less than 0.1 ppmV.

[0056] In order to ensure the lowest possible water concentration and oxygen concentration in the respective interior space 13, 33, the respective optical arrangement 1, 21 of Fig. 1 or from Fig. 2 a drying device 17, 37for drying the inert gas Ar before it is fed into the interior space 13, 33. A conventional gas purifier can be used as the drying device 17, 37, which filters out not only water but also other undesirable gas components, e.g., oxygen.

[0057] The (total) pressure p in the respective interior space 13, 33 is in the Fig. 1 and Fig. 2 In the examples shown, the pressure is approximately at atmospheric pressure. In another embodiment of the wafer inspection system 2 (or the lithography system 1), a Fig. 2 vacuum pump shown in dashed lines 38 communicate with the interior space 33 in order to evacuate it. With the aid of the vacuum pump 38, a total pressure p can be generated in the interior space 33 that is approximately 10 -3< mbar or less.

[0058] In order to produce a sufficient coverage of the surfaces of the substrate 7a of the transmitting optical element 7 in this case, a partial pressure p NH3 of ammonia NH 3 in the interior space 33 of typically more than 10 -4< mbar, preferably more than 10 -3< mbar, is required. It is understood that the interior space 13 of the illumination system 2 of the Fig. 1 shown lithography system 1 can be evacuated by means of a vacuum pump. Also, the particles in the interior of the projection system 3 can be Fig. 1 arranged transmitting optical elements 10, 11 are protected from degradation in the manner described above in connection with the interior 13 of the lighting system 2.

[0059] In the optical arrangements 1, 2 described above, the lifetime of transmitting optical elements 7, 27 can be increased due to the addition of ammonia NH 3, without reducing the lifetime of the reflective optical elements 8, 26, 28, as would be the case with the addition of water H 2 O into the respective interior space 13, 33. In addition, the provision of a dosing device for water can be dispensed with. The lifetime of the reflective optical elements 8, 26, 28 can also be increased in this way, in particular if they are used differently than in Fig. 1 and in Fig. 2shown are designed as rear surface mirrors. The supply of ammonia NH 3 can also be advantageously used in optical arrangements 1, 2 that are designed for operation in the FUV wavelength range between approximately 200 nm and approximately 280 nm. It is understood that the addition of ammonia described above can also be carried out in optical arrangements 1, 2 in which only transmitting optical elements 7, 27 or in which only reflecting optical elements 8, 26, 28 are arranged in the respective interior space 13, 33.

Claims

1. Optical arrangement for the FUV / VUV wavelength range, especially FUV / VUV lithography system (1) or wafer inspection system (2), comprising: an interior (13, 33), in which interior (13, 33) at least one transmitting optical element (7, 27) for transmission of radiation (5, 25) in the FUV / VUV wavelength range is disposed, which preferably has a crystalline, especially ionic, substrate (7a, 27a), and / or in which interior (13, 33) at least one reflective optical element (8, 26, 28) for reflection of radiation (5, 25) in the FUV / VUV wavelength range is disposed, having a substrate (8a, 26a, 28a) and a reflective coating (8b, 26b, 28b), a radiation source (4, 24) for irradiating the optical element (7, 8, 26, 27, 28) with radiation (5, 25) in the FUV / VUV wavelength range, and at least one gas inlet (14, 34) for supplying at least one gas to the interior (13, 33), characterized in that the gas inlet (14, 34) is designed to supply ammonia (NH3) to the interior (13, 33) at least during the irradiation of the optical element (7, 8, 26, 27, 28) and in that the gas inlet (14, 34) has a reservoir containing ammonia (NH3).

2. Optical arrangement according to any of the preceding claims, further comprising: a setting device (15, 35) for setting a content (cNH3) of the ammonia (NH3) in the interior (13, 33).

3. Optical arrangement according to Claim 1 or 2, comprising a measurement device (16, 36) for measuring a content (cNH3) of the ammonia (NH3) in the interior (13, 33).

4. Optical arrangement according to any of the preceding claims, in which ammonia (NH3) has been introduced into the interior (13, 33) with a content (cNH3) of more than 0.1 ppmv, preferably of more than 1 ppmv.

5. Optical arrangement according to any of the preceding claims, designed to supply at least one inert gas to the interior (13, 33), especially via the gas inlet (14, 34).

6. Optical arrangement according to Claim 5, in which the gas inlet (14, 34) is designed to introduce the ammonia (NH3) and the inert gas (Ar) into the interior (13, 33) as a gas mixture.

7. Optical arrangement according to Claim 5 or 6, having a drying device (17, 37) for drying the inert gas (Ar) prior to supply to the interior (13, 33).

8. Optical arrangement according to any of the preceding claims, in which water (H2O) has been introduced into the interior (13, 33) with a content (cH2O) of less than 1 ppmv, preferably of less than 0.1 ppmv.

9. Optical arrangement according to any of the preceding claims, in which the reflective coating (8b, 26b, 28b) of the reflective optical element (8, 26, 28) comprises at least one aluminium layer.

10. Optical arrangement according to Claim 9, in which at least one protective layer (8c, 26c, 28c) composed of a fluoridic material has been applied to the reflective coating (8b, 26b, 28b).

11. Optical arrangement according to any of the preceding claims, having at least one vacuum pump (38) for evacuating the interior (33).

12. Optical arrangement according to Claim 11, in which a partial pressure (pNH3) of ammonia (NH3) in the interior (33) is more than 10-4 mbar, preferably more than 10-3 mbar.