Method for operating an optical system and optical system

Abstract

The invention relates to a method for operating an optical system, in particular for the FUV / VUV wavelength range, wherein the optical system comprises at least one reflective optical element (1) having a metallic surface (3a) to which a metal fluoride layer (4) is applied, wherein, during operation of the optical system, the optical element (1) is irradiated with radiation (6), in particular in the FUV / VUV wavelength range. In the method, the optical system is operated during a first time interval under oxidizing ambient conditions (OB) of the optical element (1) and during a second time interval under fluorinating ambient conditions (FB) of the optical element (1), wherein the first time interval follows the second time interval or the second time interval follows the first time interval.Preferably, the optical system (10) is operated alternately under oxidizing ambient conditions (OB) of the optical element (1) and under fluorinating ambient conditions (FB) of the optical element (1). The invention also relates to an optical system (10).

Description

Background of the invention

[0001] The invention relates to a method for operating an optical system, in particular for the FUV / VUV wavelength range, wherein the optical system comprises at least one reflective optical element having a metallic surface to which a metal fluoride layer is applied, wherein, during operation of the optical system, the optical element is irradiated with radiation, in particular with radiation in the FUV / VUV wavelength range. The invention also relates to an optical system, preferably for the FUV / VUV wavelength range, in particular an FUV / VUV lithography system or a wafer inspection system, comprising: at least one reflective optical element having a metallic surface to which a metal fluoride layer is applied, and a light source for irradiating the reflective optical element with radiation, in particular in the FUV / VUV wavelength range.

[0002] 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 280 nm. The optical system 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] Particularly in the VUV wavelength range, it is often not possible to work exclusively with transmissive optical elements; rather, it is usually necessary to also use reflective optical elements. Reflective optical elements with a metallic surface, often made of or containing aluminum, have proven to be effective in this case, as this material has high reflectivity for the VUV wavelength range. It is possible for the metallic surface to be formed on a substrate of the optical element made of a metallic material. Typically, the metallic surface is formed on a metal layer applied to a substrate of the optical element.An overlying protective layer can be applied to protect the metal layer from oxidation, which is usually a metal fluoride layer because metal fluorides have a large band gap.

[0004] Despite the application of a protective layer in the form of a metal fluoride, it has been shown that in an optical system operated with high radiation intensities, such as those encountered in lithography and, in particular, during the inspection of masks and wafers, a gradual degradation of the reflective optical element occurs, accompanied by a significant loss of reflectivity, which shortens the lifetime of the reflective optical element. This is due to the fact that in such an optical system, the atmosphere surrounding the reflective optical element, which can be a vacuum or a purge gas, inevitably contains a residual concentration of oxygen-containing species, e.g., water or oxygen, which, in combination with the high-energy photons, oxidize the surface of the protective metal fluoride layer.It is fundamentally possible to significantly slow down the oxidation or to passivate the metal fluoride layer by appropriately selecting the parameters in the environment of the reflective optical element (see below). However, significant losses in reflectivity still occur over a longer period of operation (e.g. 20% reflectivity loss for a layer of approx. 3 nm MgO on MgF2, 15% for a layer of approx. 3 nm Al2O3 on AlF3), which typically cannot be accepted in view of the overall transmission of the optical system.

[0005] DE 10 2018 211 499 A1 describes a reflective optical element designed as described above, wherein an oxide layer is additionally applied to the metal fluoride layer or layer in order to protect the underlying layers and thus increase the service life of the reflective optical element. When applying an additional protective layer in the form of an oxide layer, the problem described above arises: most oxides have high absorption at wavelengths of less than 160 nm and can therefore result in large losses in reflectivity. DE 10 2018 211 499 A1 proposes reducing the loss in reflectivity by minimizing a standing wave of the electric field that forms during reflection in the region of the oxide layer.

[0006] DE 10 2021 200 490 A1 describes a method for forming a protective layer on a reflective optical element configured as described above. To form the protective layer, the metal fluoride layer is irradiated with electromagnetic radiation having at least one wavelength of less than 300 nm. The irradiation results in passivation of the metal fluoride layer, which counteracts degradation of the metal layer. The passivating protective layer is typically an oxide layer, which leads to the disadvantages described above.

[0007] US 11,262,664 B2 describes a system and method for protecting an optical element from damage during irradiation with VUV light. The system comprises a light source for generating VUV light and a chamber containing a fluorine-based compound with a defined partial pressure. The optical element is arranged in the chamber and exposed to the fluorine-based compound. The VUV light generated by the light source has sufficient energy to convert the fluorine-based compound in the chamber into a primary product, which may contain atomic fluorine. This is intended to prevent fluorine loss and the associated oxidation of the optical element, which may consist of a metal fluoride, for example.

[0008] However, in the process described in US 11,262,664 B2, excess fluorine atoms adsorbed on the surface of the metal fluoride layer can reach the interface to the underlying metal layer by diffusion and fluorinate it, which can also lead to a decreasing reflectivity of the optical element. Theoretically, an equilibrium between oxidizing and fluorinating species could be found in the environment of the optical element. In practice, however, this is not feasible or can only be achieved with considerable effort, since each operating mode of the optical system (different wavelength bands, different intensities of VUV radiation, etc.) would require a different concentration of fluorine-containing gases, which would have to be known and precisely adjusted. As a consequence, the optical system described in US 11,262,664 B2 might have to be modified.operated at an excessively high concentration of fluorine-containing gases, which would accelerate the degradation of the optical element and thus significantly reduce the transmission of the optical system. Object of the invention

[0009] The object of the invention is to provide a method for operating an optical system and an optical system whose transmission decreases only slightly even during a long period of operation. Subject of the invention

[0010] According to one aspect, this object is achieved by a method of the type mentioned at the outset, in which the optical system is operated during a first time interval under oxidizing ambient conditions of the optical element and during a second time interval under fluorinating ambient conditions of the optical element, wherein the first time interval follows the second time interval or the second time interval follows the first time interval.

[0011] According to the invention, it is proposed to operate the optical element in the optical system during a first time interval under oxidizing ambient conditions and during a second time interval under fluorinating ambient conditions. The inventors have discovered that when operating the optical element under static ambient conditions, as described for oxidizing ambient conditions in DE102021200490A1 and for fluoridating ambient conditions in US Pat. No. 11,262,664 B2, harmful interactions during operation of the optical system progress continuously with increasing operating time of the optical system – under oxidizing ambient conditions through superficial oxidation of the metal fluoride layer, and under fluoridating ambient conditions through fluorination at the metallic surface or the interface to the metallic layer.

[0012] Preferably, the optical system is operated alternately under oxidizing ambient conditions of the optical element and under fluorinating ambient conditions of the optical element. In this case, the reversible surface oxidation of the metal fluoride layer can be limited by refluorination, while simultaneously preventing excessive addition of fluorine and thus irreversible fluorination of the interface between the metal fluoride layer and the metal layer or the metallic surface. This exploits the fact that the oxidation and refluorination of the surface of the metal fluoride layer is reversible, in contrast to the fluorination of the metallic surface, which typically forms the interface between the metal layer and the metallic fluoride layer.

[0013] In one variant, an oxide layer is formed on a side of the metal fluoride layer facing away from the metallic surface, or an oxide layer is formed under oxidizing ambient conditions of the optical element, the thickness of the oxide layer increasing with increasing duration of the oxidizing ambient conditions. The oxide layer can be formed upon irradiation of the surface of the metal fluoride layer, typically with radiation in the FUV / VUV wavelength range, under oxidizing ambient conditions, i.e. in the presence of an oxygen-containing gas in the environment of the reflective optical element. During the formation of the oxide layer, a volume region of the metal fluoride layer near the surface is converted into the oxide layer. The oxide layer formed upon irradiation under oxidizing ambient conditions therefore does not extend over the entire thickness of the metal fluoride layer.For details of the ambient atmosphere parameters and the irradiation parameters that cause or promote the formation of the oxide layer, reference is made to DE102021200490A1 cited above, which is incorporated by reference in its entirety into this application. Alternatively, the thin oxide layer may have been applied to the metal fluoride layer during production, or the oxide layer may have been formed under previous oxidizing ambient conditions and not completely removed under fluorinating ambient conditions (see below).

[0014] In all the cases described above, the thickness of the oxide layer on the metal fluoride layer increases with increasing duration of the oxidizing ambient conditions. The thin oxide layer acts as a diffusion barrier and prevents fluorine atoms present in the environment from reaching the metallic surface or the interface between the metal layer and the metal fluoride layer. With increasing duration of the fluorinating ambient conditions, the thickness of the thin oxide layer on the surface of the metal fluoride layer decreases, as it is completely or partially converted back into a fluoride or refluorinated under fluorinating ambient conditions, as described in more detail below.

[0015] In an advantageous development, the oxide layer remains on the side of the metal fluoride layer facing away from the metallic surface under the fluorinating ambient conditions of the optical element. In this case, the fluorinating ambient conditions are switched to the oxide ambient conditions before the oxide layer is completely removed. In this way, the fluorination of the metallic surface or the interface between the metal fluoride layer and the metallic layer can be stopped or at least significantly slowed down, which increases the service life of the reflective optical element compared to the method described in US Pat. No. 11,262,664 B2.By operating the optical system under alternating environmental conditions of the reflective optical element, harmful but reversible reactions are limited to the surface of the protective metal fluoride layer and irreparable deep damage caused by fluorination of the metallic surface or the interface between the metal fluoride layer and the metal layer is avoided.

[0016] In a further development, the thickness of the metal fluoride layer is selected such that the electric field of a standing wave that forms when the reflective optical element is irradiated with radiation at a useful wavelength of the optical system is at a minimum in the region of the oxide layer. The thickness of the metal fluoride layer as well as the thickness(es) of any other layers that may be present and applied to the metallic surface are selected in this variant such that the electric field or its amplitude in the oxide layer is minimized as much as possible, so that the absorption of the useful radiation in the oxide layer is as low as possible. For details of the design or the suitable selection of the thickness of the metal fluoride layer and any other layers that may be applied to the metallic surface, e.g.in the form of adhesion promoter layers, reference is made to DE102018211499A1 cited at the beginning, which is incorporated into this application in its entirety by reference.

[0017] In a further variant, to switch between oxidizing ambient conditions and fluorinating ambient conditions, a concentration of at least one fluorine-containing gas and / or at least one oxygen-containing gas in the environment of the reflective optical element is changed. Preferably, at least one fluorine-containing gas is supplied to the environment of the reflective optical element under fluorinating ambient conditions and no fluorine-containing gas is supplied under oxidizing ambient conditions. To switch between oxidizing ambient conditions and fluorinating ambient conditions, a change in the concentration or dosage of at least one fluorine-containing gas in the environment of the reflective optical element is typically sufficient.To change the concentration, for example, at least one fluorine-containing gas can be supplied to the environment of the reflective optical element under fluorinating ambient conditions and no fluorine-containing gas can be supplied under oxidizing ambient conditions. As a rule, it is not necessary to supply an oxidizing gas to the environment of the optical element under oxidizing ambient conditions, since the residual concentrations of oxygen and / or water in the purge gas are sufficient to create the oxidizing ambient conditions. The concentration of the oxidizing gas in the environment of the optical element can therefore be kept constant. However, it is also fundamentally possible to change the concentration of an oxygen-containing gas in the environment of the reflective optical element alternatively or additionally.The concentration of the fluorinating and / or oxidizing gas in the environment of the reflective optical element is selected such that the surface of the optical element, which forms the interface with the environment, is oxidized under oxidizing ambient conditions and refluorinated under fluorinating ambient conditions. Under oxidizing ambient conditions, a thin oxide layer typically forms, or the thickness of an existing oxide layer is increased, whereas under fluorinating ambient conditions, the thin oxide layer is converted back into a fluoride, or its thickness is reduced. Even under fluorinating ambient conditions, a residual thickness of the oxide layer may remain on the metal fluoride layer (see above).

[0018] In a further development, the concentration of the at least one fluorine-containing gas and / or the at least one oxygen-containing gas in the environment of the optical element is controlled as a function of at least one control parameter that represents a measure of the reflectivity of the reflective optical element. The concentration of the at least one fluorine-containing gas and / or the at least one oxygen-containing gas, and thus the time of a change between the fluorinating and the oxidizing ambient conditions, is preferably controlled as a function of at least one control parameter that is based on a measured variable that represents a measure of the reflectivity of the optical element. The control allows for automated responses to different operating modes of the optical system, e.g., a variation in the electromagnetic spectrum of the useful radiation, the intensity of the useful radiation, etc.In the method described in US 11,262,664 B2, however, either the ideal concentration of the fluorine-containing gas must be known depending on the operating mode of the optical system or too much fluorine-containing gas is added, which results in a reduction in the lifetime of the reflective optical element.

[0019] In a further development, the concentration of the at least one fluorine-containing gas and / or the at least one oxygen-containing gas is kept constant if the control parameter lies between a first, lower threshold value and a second, upper threshold value. As described above, the control parameter or a variable derived therefrom forms a measure of the reflectivity of the optical element. In the event that the optical system has a total transmission or the optical element has a reflectivity that lies between an upper and a lower threshold value, the concentration of the fluorine-containing gas and / or the oxygen-containing gas can be kept constant, i.e. no alternation between fluorinating and oxidizing ambient conditions is necessary. The lower threshold value can, for example, be on the order of magnitude of approximately 95% of the transmission of the optical system orthe reflectivity of the optical element, with the upper threshold being approximately 99%. Of course, other values ​​are also possible depending on the optical system or control parameters.

[0020] In a further development, the concentration of the fluorine-containing gas is increased and / or the concentration of the oxygen-containing gas is reduced if the control parameter falls below the first threshold value, and / or the concentration of the fluorine-containing gas is reduced and / or the concentration of the oxygen-containing gas is increased if the control parameter exceeds the second threshold value. If the first, lower threshold value is undershot, the thickness of the oxide layer is too great, which results in a reduction in the transmission of the optical system. By reducing the concentration of the oxygen-containing gas and / or increasing the concentration of the fluorine-containing gas, the transmission of the optical system can be increased. Accordingly, when the second threshold value is exceeded, the thickness of the oxide layer is so small that degradation of the metallic surface orat the interface between the metal layer and the metal fluoride layer. This can be counteracted by reducing the concentration or partial pressure of the fluorine-containing gas and / or by increasing the concentration or partial pressure of the oxygen-containing gas.

[0021] Preferably, after changing the concentration of the at least one fluorine-containing gas and / or the at least one oxygen-containing gas, the optical system is operated at constant ambient conditions for at least a predetermined holding time. It is advantageous not to run the control loop continuously, but rather to set or specify a holding time during which the optical system is operated with constant ambient conditions after a change or modification of the ambient conditions has occurred. The holding time is typically on the order of hours or days. It is understood that even after the holding time has elapsed, a change in the ambient conditions does not occur automatically, but is performed depending on the value of the control parameter.The holding time can be adjusted, particularly depending on the concentration of the fluoride gas, so that a thin oxide layer always remains as a diffusion barrier for the fluorine atoms.

[0022] In one development, the control parameter is selected from the group comprising: total transmittance of the optical system, reflectivity of the reflective optical element, chemical composition of the surface of the optical element, temperature of the optical element. The at least one control parameter is measured using a suitable measuring system. To measure the total transmittance of the optical system, the measuring system can be arranged as an additional component outside a chamber into which the reflective optical element is introduced during operation of the optical system. To measure the reflectivity of the optical element, the latter can optionally be irradiated with light from a measuring light source, e.g. in the VUV wavelength range, and the light reflected by the optical element can be detected by a detector. This takes advantage of the fact that oxides are strongly absorbent in the VUV wavelength range, while fluorides are transparent.A temperature sensor can be used to measure the temperature, for example, in the form of a thermocouple integrated into a mirror holder, a pyrometer, or the like. The temperature is a measure of the absorption of the reflective optical element and thus of the degree of oxidation of the reflective optical element, or its reflectivity.

[0023] Alternatively or additionally, a chemical composition of a surface or a volume region near the surface of the optical element and thus the chemical composition or stoichiometry of the reflective optical element or the metal fluoride layer can be measured. For this purpose, measurement methods known to the person skilled in the art, e.g. XPS, i.e. X-ray photoelectron spectroscopy, XRF, i.e. X-ray fluorescence spectroscopy, etc. can be used. In this case, the chemical composition or a measure of the chemical composition, e.g. a concentration of fluorine and / or oxygen at the surface which forms the interface to the environment of the optical element, can be used as a control parameter. It is understood that the control parameters described here and other control parameters can also be combined when controlling the optical system.

[0024] A further aspect of the invention relates to an optical system of the type mentioned above, which is designed for operation under oxidizing ambient conditions of the optical element during a first time interval and under fluorinating ambient conditions of the optical element during a second time interval, wherein the first time interval follows the second time interval or the second time interval follows the first time interval. Preferably, the optical system is designed for operation under alternating oxidizing ambient conditions of the optical element and fluorinating ambient conditions of the optical element. The optical system has the advantages described above in connection with the method.

[0025] In one embodiment, the reflective optical element is arranged in a chamber and the optical system has a supply device for supplying at least one fluorine-containing gas and / or at least one oxygen-containing gas into the chamber, wherein the supply device is designed to adjust a concentration of the fluorine-containing gas and / or a concentration of the oxygen-containing gas in the chamber.

[0026] The following fluorinating gases can be used as fluorinating agents or fluorinating gases: F2, HF, XeF2, NF3, CF4, SF6. When operating under fluorinating ambient conditions, the partial pressure of the fluorinating agent is typically between 10 -9 mbar and 10 -1 mbar or between 1pptV and 100 ppmV in the purge gas, in particular between 10 -6 mbar and 10 -3mbar or between 1 ppbV and 1 ppmV in the purge gas. The fluorine-containing gas can be adjusted to the desired partial pressure, for example using a needle valve, and measured or regulated using a dedicated fluorine gas sensor (e.g. a residual gas analyzer). As described above, the concentration of the at least one oxygen-containing gas can be kept constant or changed in a manner corresponding to the adjustment of the concentration of the fluorine-containing gas. For this purpose, the optical system can have a dosing unit for the oxygen-containing gas or the oxidizing species such as H2O or O2. A corresponding sensor for determining the partial pressure or concentration of the oxidizing species or the oxygen-containing gas can also be integrated into the optical system.

[0027] The light source for illuminating the reflective optical element is typically the optical system's useful light source, whose wavelength or wavelength range is typically in the FUV / VUV wavelength range. To adjust the fluorinated ambient conditions, the light source typically has to fulfill two tasks: The light source is intended to cause photodissociation of the at least one fluorine-containing gas to provide fluorine atoms for the fluorination of the oxide layer. The wavelength range of the light source can be adapted to the absorption cross-section of the fluorine-containing gas in order to provide fluorine atoms as efficiently as possible. For this purpose, the light source used can, for example, provide light in the wavelength range between 115 nm and 1000 nm, preferably between 120 nm and 170 nm, particularly preferably between 140 nm and 170 nm.

[0028] The light source must also provide the activation energy for the fluorination process. For this purpose, the wavelength of the light provided by the light source can be appropriately adjusted or specified. For this purpose, the light source used should generate light in the wavelength range from 115 nm to 1000 nm, preferably between 120 nm and 200 nm, and particularly preferably between 140 nm and 200 nm.

[0029] Alternatively or additionally, the photodissociation of the fluorine-containing gas or the activation energy can be achieved by increasing the temperature or by other means, for example, by using a plasma. For example, it is known to perform plasma-induced cleaning of PECVD coating systems with NF3.

[0030] In a further embodiment, the optical system comprises a control device for controlling the concentration of the at least one fluorine-containing gas and / or the at least one oxygen-containing gas in the environment of the optical element as a function of at least one control parameter that forms a measure of the reflectivity of the reflective optical element, as well as at least one measuring device for measuring the at least one control parameter. As described above in connection with the method, it is advantageous if the concentration of the fluorine-containing or oxygen-containing gas or the switching time between oxidizing ambient conditions and fluoridizing ambient conditions is controlled.For this purpose, a dosing unit for the fluorine-containing gas is preferably coupled to the control parameter measuring device via a feedback loop, so that the required concentration of the fluorine-containing gas and / or the oxygen-containing gas is automatically adjusted. For this purpose, the control device can, for example, comprise a mass flow controller.

[0031] In a further development, the at least one control parameter is selected from the group comprising: total transmittance of the optical system, reflectivity of the reflective optical element, chemical composition of the surface of the optical element, temperature of the optical element. As described above in connection with the method, the control parameter can be measured using a suitable measuring device or using a suitable sensor. The sensor or measuring device can be integrated into the chamber in which the reflective optical element is arranged. However, it is also possible for the measuring device to be arranged outside the chamber or the housing, for example if the total transmittance of the optical system is measured as the control parameter.

[0032] The housing or chamber in which the reflective optical element is arranged, as well as all other components that come into contact with the fluorine-containing gas (e.g. NF3, XeF2, SF6, CF4, HF, F2), its photodissociated species (e.g. F, F2, F*) and / or its derivatives (e.g. HF), must be resistant to these species. Resistant means that, for example, a passivating layer forms on the inside of the chamber. The following materials and their alloys can be used: Ni, Fe, Cu, Co, Sc, Y and Hf. An example of such a material is Monel metal, which is an alloy of Ni, Cu and Fe. In particular, no volatile fluorine compounds must be formed that could precipitate on the reflective optical element or other optics. Accordingly, the metals used must be free of Cr or Ti. Alternatively, the inside of the chamber orThe chamber wall can be coated with a fluorine-resistant coating to prevent corrosion. Such a coating can be applied, for example, in a galvanic process. Possible coating materials include NiP, Pt, or Ru / Rh mixtures.

[0033] In a further embodiment, the chamber has an entrance window for the entry of light from the light source into the chamber and / or an exit window for the exit of light from the light source from the chamber. The entrance or exit window can be used to separate the environment of the reflective optical element with the fluorine-containing and / or oxygen-containing gases from the rest of the optical system. In principle, the chamber can also contain the entire optical system, more specifically, all optical components of the optical system, if separation of the environmental conditions is not required or if the components are resistant to fluorine or fluorine species.

[0034] The metal fluoride layer of the reflective optical element may comprise at least one material selected from the group comprising: magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, chiolite, cryolite, erbium fluoride, neodymium fluoride, gadolinium fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, hafnium fluoride, lanthanum fluoride, europium fluoride, lutetium fluoride, cerium fluoride, barium fluoride, yttrium fluoride.

[0035] The metallic surface of the reflective optical element may be formed on a metal layer comprising at least one material selected from the group comprising: aluminum, rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, germanium or a combination thereof.

[0036] The optical system described above, which operates under alternating oxidizing and fluorinating ambient conditions, allows the reflective optical element, which is typically a mirror, to be replaced with reduced frequency, thereby increasing the service life of the optical system while maintaining low costs.

[0037] 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

[0038] Examples of embodiments are shown in the schematic drawing and are explained in the following description. Fig. 1a is a schematic representation of the irradiation of a reflective optical element under oxidizing ambient conditions, Fig. 1b is a schematic representation of the irradiation of a reflective optical element under fluorinated ambient conditions, Fig. 1c shows a schematic representation of the irradiation of a reflective optical element under alternating oxidizing and fluorinating ambient conditions, Fig. 2a a schematic representation of a control circuit for regulating alternating oxidizing or fluorinating ambient conditions, Fig. 2b a schematic representation of the total transmission of the optical system as a function of time, Fig. 3 is a schematic representation of an optical system with a reflective optical element designed to control alternating oxidizing and fluorinating conditions in the environment of the reflective optical element, Fig. 4 a schematic representation of an optical system for the VUV wavelength range in the form of a VUV lithography system, and Fig. 5 a schematic representation of an optical system for the VUV wavelength range in the form of a wafer inspection system.

[0039] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0040] Fig. 1a-c show a highly schematic detail of a reflective optical element 1 in the form of a mirror. The reflective optical element 1 has a substrate 2 on which a metal layer 3 is formed with a metallic surface 3a located on a side facing away from the substrate 2. In the example shown, the substrate 2 is made of silicon; however, it is understood that the substrate 2 can also be made of another metallic or non-metallic material, e.g., a glass ceramic, a ceramic, etc.

[0041] The metal fluoride layer 4 was applied using a conventional coating process (by evaporation). The metal fluoride layer 4 serves to protect the underlying metal layer 3 from oxidation. In the example shown, the material of the metal layer 3 is aluminum, and the material of the metal fluoride layer 4 is magnesium fluoride (MgF2), but other materials can also be used.

[0042] It has been shown that the presence of the metal fluoride layer 4 alone is not sufficient to protect the optical element 1 from degradation when the reflective optical element 1 is used in the operation of a Fig. 1a-c is irradiated with high powers or irradiances: Despite the presence of the metal fluoride layer 4, the Al material of the metal layer 3 is oxidized to Al2O3 within a comparatively short period of several hours or days, resulting in a significant reduction in the reflectivity of the reflective optical element 1. If the loss of reflectivity is too great, replacement of the reflective optical element 1 is necessary.

[0043] In order to increase the lifetime of the reflective optical element 1, a thin passivating oxide layer 5 can be applied to the reflective optical element 1, as shown in Fig. 1a. For this purpose, the reflective optical element 1 is irradiated with radiation 6 at at least one wavelength λ of less than 300 nm, typically less than 200 nm. The irradiation can in particular take place at one or more wavelengths λ lying between 115 nm and 200 nm. The irradiation takes place under oxidizing ambient conditions OB, i.e., in an environment in which at least one oxygen-containing gas is present at a sufficient concentration.

[0044] During the formation of the oxide layer 5, a volume region of the metal fluoride layer 4 near the surface is converted into the oxide layer 5. The oxide layer 5 formed during irradiation under oxidizing ambient conditions OB therefore does not extend over the entire thickness of the metal fluoride layer 4. In the example shown, in which the material of the metal fluoride layer is MgF2, the oxide layer 5 consists of MgO. For details of the oxidizing ambient conditions OB, which are favorable for the formation of the thin oxide layer 5, reference is made to DE102021200490A1. As in Fig. 1a, the thickness d of the oxide layer 5 increases with increasing time during which the reflective optical element 1 is exposed to the oxide ambient conditions OB.

[0045] Fig. 1b shows the optical element 1 of Fig. 1a under fluorinating ambient conditions FB. The fluorinating ambient conditions FB are intended to prevent the formation of an oxide layer on the metal fluoride layer 4, which contributes to reducing the reflectivity of the reflective optical element 1 (see above). The fluorinating ambient conditions can be implemented, for example, in the manner described in US Pat. No. 11,262,664 B2.

[0046] When the reflective optical element 1 is operated under fluorinating ambient conditions FB, fluorine atoms 8 can diffuse to the metallic surface 3a, resulting in a thin intermediate layer of AlF3 forming between the metal layer 3 and the metal fluoride layer 4, which becomes thicker with increasing duration of the fluorinating ambient conditions FB, which also leads to a decrease in the reflectivity of the reflective optical element 1.

[0047] Fig. Figure 1c shows the reflective optical element 1, which is operated alternately in oxidizing ambient conditions OB and in fluorinating ambient conditions FB. As in Fig. 1c, the thickness d of the oxide layer 5 increases under oxidizing ambient conditions OB and decreases under fluorinating ambient conditions FB. With a suitable choice of the times of switching between oxidizing ambient conditions OB and fluorinating ambient conditions FB, a thin oxide layer 5 can permanently remain on the metal fluoride layer 4, i.e. the oxide layer 5 that is or was formed under oxidizing ambient conditions OB of the reflective optical element 1 or that was already applied to the metal fluoride layer 4 during the manufacture of the reflective optical element 1, also remains under the fluorinating ambient conditions FB on the side of the metal fluoride layer 4 facing away from the metallic surface 3a. Since the oxide ambient conditions OB do not persist permanently, an increase in the oxide layer 5 to an excessively large thickness d can be prevented, i.e.the thickness d of the oxide layer 5 can be limited in order to prevent or limit a decrease in the reflectivity of the optical element 1.

[0048] To minimize the decrease in the reflectivity of the optical element, it is advantageous if the thickness D of the metal fluoride layer 4 is selected such that the electric field of a standing wave formed upon irradiation of the reflective optical element 1 with radiation 6 at a useful wavelength λ is minimized in the region of the oxide layer 5. This can be achieved, for example, in the manner described in DE102018211499A1.

[0049] It is advantageous if a respective switching point between the oxidizing ambient conditions OB and the fluorinating ambient conditions FB is controlled. Such a control or such a control process can, for example, be applied to the following in connection with Fig. 2a,b described manner on an optical system 10 which is in Fig. 3 and which is described in more detail below.

[0050] The Fig. The optical system 10 shown in Figure 3 comprises a chamber 11 in which the reflective optical element 1 of Fig. 1c, a feed device 12, and an FUV / VUV radiation source 13. The reflective optical element 1 is arranged in an interior of the chamber 11, which forms an environment 14 of the reflective optical element 1. The reflective optical element 1 is attached to an optics holder, in which a measuring device in the form of a temperature sensor 15 is embedded, which serves to measure the temperature T of the reflective optical element 1.

[0051] In the example shown, the supply device 12 serves to supply protective gas in the form of inert gas IG, to supply at least one oxygen-containing reactive gas OG, and to supply at least one fluorine-containing reactive gas FG into the chamber 11. The supply device 12 comprises a first valve 16a for the controlled supply of the inert gas IG, a second valve 16b for the controlled supply of the at least one oxygen-containing gas OG, and a third valve 16c for the controlled supply of the at least one fluorine-containing gas FG. The second valve 16b and the third valve 16c are each controllable metering valves. The optical system 10 also comprises a gas inlet 17a into the chamber 11 in the region of the supply device 12 and a gas outlet 17b.

[0052] The IR inert gas in the example shown is argon, but other IR inert gases can also be used, for example, other light noble gases such as helium or neon. Mixtures of noble gases, especially those mentioned above, can also be used as IR inert gases. Oxygen-containing gases can be water (H2O) or molecular oxygen (O2). Fluorine-containing gases can include, for example, F2, HF, XeF2, NF3, CF4, and SF6.

[0053] The FUV / VUV radiation source 13 serves to irradiate the reflective optical element 1, more precisely its surface 1a, with FUV / VUV radiation 6. The radiation 6 thus has a useful wavelength λ in the FUV / VUV wavelength range. For example, in the illustrated example, the FUV / VUV radiation 6 enters the chamber 11 through an entrance window 20a made of MgF2 and exits the chamber 11 through an exit window 20b made of MgF2. The chamber 11 is sealed gas-tight by the two windows 20a and b.

[0054] An inner side 11a of the chamber 11 is resistant to the fluorine-containing gas FG and its byproducts. For this purpose, the chamber 11 in the example shown is formed, at least on its inner side 11a, from a metal in the form of Monel steel, which forms a passivating layer to prevent corrosion. In principle, the chamber 11 can also be formed from other corrosion-resistant metals, provided they are free of Cr and Ti. Alternatively, a corrosion-resistant coating, e.g., made of NiP, Pt, or Ru / Rh mixtures, can be applied to the inner side 11a of the chamber 11. The corrosion-resistant coating can be applied to the inner side 11a of the chamber 11, for example, by means of an electroplating process. The components arranged in the chamber 11 that come into contact with the fluorine-containing gas FG are also resistant to the fluorine-containing gas FG and its byproducts.

[0055] Furthermore, the optical system 10 comprises, for example, a first sensor 18a for measuring the concentration c OG of the oxygen-containing gas OG in the chamber 11 and a second sensor 18b for measuring the concentration c FG of the fluorine-containing gas FG in the chamber 61. It is possible that the respective sensor 18a, 18b is designed to measure the concentration of different oxygen-containing or fluorine-containing gases in the chamber 11. For example, the first sensor 18a can be designed to measure the concentration c OG or the partial pressure of water H2O and oxygen O2 in the chamber 11.

[0056] The optical system 10 also has a control device 19 for controlling the partial pressure or the concentration c FG of the fluorine-containing gas FG in the chamber 11 to a desired value, wherein the control is carried out by means of an actual measured value of the second sensor 18b for measuring the concentration c FGof the fluorine-containing gas FG in the chamber 11 and by controlling the third valve 16c. The second sensor 18b can only measure the partial pressure c FW of the fluorine-containing gas FG, but it can also be a residual gas analyzer that can also determine the partial pressures or concentrations of other gases contained in the chamber 11. It is possible that such a residual gas analyzer can replace the function of the two Fig. 3. In the event that the third valve 16c is a metering valve, for example a mass flow controller, the use of the second sensor 18b for measuring the concentration c FG of the fluorine-containing gas FG in chamber 11.

[0057] The control device 19 also serves to change or switch between the functions associated with Fig. 1c and the fluorinating ambient conditions FB in the chamber 11 or in the environment 14 of the reflective optical element 1, which takes place as a function of at least one control parameter which is a measure of the reflectivity R of the optical element 1.

[0058] The control parameter can be, for example, the total transmission T(t) of the optical system 10 or a control variable derived from it. Fig. The total transmission T(t) of the optical system 10 at a time t shown in Figure 2b is defined as the quotient of the light intensity I measured at the output of the optical system 10 transmittiert (t) to the light intensity I generated by the light source 13 at time t Lichtquelle (t): T(t) = I transmittiert (t) / I Lichtquelle(t). The respective light intensities and thus the total transmission T(t) can be determined using measuring devices not shown, e.g., optical sensors.

[0059] The control parameter P used for controlling the operation of the optical system 10 is the quotient of the measured total transmittance T(t) at time t and a total transmittance T0(t) which is attributable to long-term changes in the optical system 10 that are not attributable to changes in the reflectivity R of the reflective optical element 1. The prerequisite for using the control parameter P is that comparatively short-term changes or fluctuations in the total transmittance T(t) of the optical system 10 on an hourly or daily basis are attributable to changes in the reflectivity R of the reflective optical element 1, while long-term changes which are caused, for example, by other optical components are known or can be factored out for the control. As a consequence, the total transmittance T(t) or T0(t) which decreases over time isthe control parameter P is a direct measure of the oxidation of the surface 1a or of the reflectivity R of the reflective optical element 1.

[0060] For the regulation, three cases are distinguished in the example shown, as in Fig. 2a: In the first case, the total transmission or the control parameter P is small and lies below a lower threshold P1, which can be, for example, approximately 95%. In this case, the concentration c FG of the fluorine-containing gas FG is increased in order to reverse the surface oxidation of the reflective optical element 1 or to re-fluorinate it and to increase the total transmission of the optical system 10 or the control parameter P. In principle, the concentration c OG of the oxidizing gas(es) OG in chamber 11, but this is not absolutely necessary.

[0061] In the second case, where the control parameter P lies between the first, lower threshold P1 and a second, upper threshold P2, which can be approximately 99%, the concentration c FG of the fluorine-containing gas FG is kept constant.

[0062] In the third case, where the control parameter P exceeds the second threshold P2, the oxide layer 5 is almost completely fluorinated. Therefore, the dosage or concentration c FG of the fluorine-containing gas FG is regulated down, so that oxidizing conditions are restored. In this case in particular, the concentration c OG of oxidizing gases OG in chamber 11 to accelerate the oxidation process.

[0063] As in Fig. 2a, the control circuit in the control device 19 does not run permanently, but rather after changing the concentration c FG , c OGof the at least one fluorine-containing gas FG and / or the at least one oxygen-containing gas OG when the respective threshold value P1, P2 is undershot or exceeded, the optical system 10 at least for a predetermined holding time t H operated at constant ambient conditions before rechecking whether a change between the oxidizing ambient conditions OB and the fluorinating ambient conditions FB is necessary. The holding time t H is typically on the order of several hours or days.

[0064] Alternatively or in addition to the control via the total transmittance or the control parameter P, the control of the switching time between the oxidizing ambient conditions OB and the fluorinating ambient conditions FB can be carried out by means of the control device 19 with the aid of other control parameters determined using suitable measuring devices. For example, the reflectivity R of the reflective optical element 1 can be measured using a measuring device 15a, 15b, which has a VUV measuring light source 15a for irradiating the reflective optical element 1 with measuring light and a detector 15b for detecting the measuring light reflected by the optical element 1. In addition or alternatively to the reflectivity R of the optical element 1, the absorption of the optical element 1 can also be determined by measuring the temperature T of the optical element 1 using the temperature sensor 15 described above.The chemical composition of the surface 1a of the reflective optical element 1 can also be determined using suitable measuring devices or measuring methods, e.g., using XPS or XRF. To control the ambient conditions OB, FB of the optical element 1, one of the control parameters P, R, T, ... can be measured; however, it is also possible to perform the control based on two or more of the control parameters P, R, T, ... The control device 19 can be embodied in the form of suitable hardware and / or software and is connected to the respective measuring devices 15, 15a,b via electrical lines.

[0065] The concentration c FGof the fluorine-containing gas FG in the chamber 11, in which fluorinating ambient conditions FB exist in the environment 14 of the optical element 1, depends on the type of fluorine-containing gas FG. When operating the chamber 11 in a vacuum, ie without an inert gas IG, the partial pressure or concentration c FG of the fluorine-containing gas FG typically between 10 -9 mbar and 10 -1 mbar, especially between 10 -6 mbar and 10 -3 mbar. When operating chamber 11 using the purge or inert gas IG, the concentration c FG typically between 1pptV and 100 ppmV, in particular between 1 ppbV and 1 ppmV, in the purge or inert gas IG.

[0066] The light source 13 generates light in the FUV / VUV wavelength range, which causes both the photodissociation of the fluorine-containing gas FG to fluorine species, e.g. in the form of atomic fluorine F or fluorine radicals, and provides the activation energy required for the conversion of the oxide of the oxide layer 5 into a fluoride.

[0067] Under oxidizing ambient conditions OG the concentration c FG of the fluorine-containing gas FG below the values ​​specified above, typically at zero, ie no fluorine-containing gas FG is supplied to the environment 14 or the chamber 11 under oxidizing ambient conditions OG. The concentration c OG of oxygen-containing gas in the form of oxygen O2 is usually between approximately 1 pptV and 100 ppmV. The concentration c OGThe presence of water as the oxygen-containing gas OG in chamber 11 should be low and typically not exceed a value of approximately 100 ppbV. The residual gas concentrations of oxygen O2 and water in chamber 11 may be sufficient, in the presence of a purge gas, to create the oxidizing ambient conditions OG. In this case, it is not necessary to additionally supply an oxygen-containing gas to chamber 11 to create the oxidizing ambient conditions OG.

[0068] In the optical system 10 of Fig. 3, the chamber 11 is separated from the remaining components of the optical system 10 by the windows 20a,b, but this is not mandatory. The chamber 11 can also contain all optical components of the optical system 10, provided they are resistant to the fluorine-containing gas FG or their reaction products. The optical system 10 of Fig. 3 can be designed in different ways. Two examples of such an optical system 10 are described below.

[0069] Fig. 4 shows an optical system for the VUV wavelength range in the form of a VUV lithography system 21. The VUV lithography system 21 comprises two optical arrangements, namely an illumination system 22 and a projection system 23. The VUV lithography system 21 also has a light source 24, which can be, for example, an excimer laser.

[0070] The radiation 25 emitted by the light source 24 is processed by the illumination system 22 to illuminate a mask 26, also called a reticle. In the example shown, the illumination system 22 has a housing 32 in which both transmitting and reflecting optical elements are arranged. Representative elements are a transmitting optical element 27, which focuses the radiation 25, and a reflecting optical element 28, which deflects the radiation.

[0071] The mask 26 has a structure on its surface that is transferred to an optical element 29 to be exposed, for example, a wafer for the production of semiconductor components, using the projection system 23. In the example shown, the mask 26 is designed as a transmissive optical element. In alternative embodiments, the mask 26 can also be designed as a reflective optical element.

[0072] In the example shown, the projection system 22 has at least one transmitting optical element. Two transmitting optical elements 30, 31 are shown as representative elements, which serve, for example, to reduce the structures on the mask 26 to the desired size for exposing the wafer 29.

[0073] In both the illumination system 22 and the projection system 23, a wide variety of transmitting, reflecting, or other optical elements can be combined in any desired, even more complex, way. Optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0074] Fig. 5 shows an optical system for the VUV wavelength range in the form of a wafer inspection system 41, but it can also be a mask inspection system. The wafer inspection system 41 has an optical arrangement 42 with a light source 54, whose radiation 55 is directed onto a wafer 49 by means of the optical arrangement 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In a mask inspection system, a mask to be examined could be arranged instead of the wafer 49. The radiation reflected, diffracted, and / or refracted by the wafer 49 is guided by a further concave mirror 48, also belonging to the optical arrangement 42, via a transmitting optical element 47 to a detector 50 for further analysis. The wafer inspection system 41 also has a housing 52 in which the two mirrors 46, 48 and the transmissive optical element 47 are arranged.The light source 54 can, for example, be a single light source or a combination of several individual light sources to provide a substantially continuous radiation spectrum. In modifications, one or more narrowband light sources 54 can also be used.

[0075] The Fig. 4 shown VUV lithography system 21 and the one in Fig. The wafer inspection system 41 shown in Figure 5 is designed to operate under alternating oxidizing ambient conditions OB and fluorinating ambient conditions FB of the respective mirrors 28 and 46, 48, respectively. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2018 211 499 A1 [0005, 0016, 0048] DE 10 2021 200 490 A1 [0006, 0011, 0013, 0044] US 11, 262,664 B2 [0007, 0008, 0011, 0015, 0018, 0045]

Claims

[1] Method for operating an optical system (10), in particular for the FUV / VUV wavelength range, wherein the optical system (10) comprises at least one reflective optical element (1) having a metallic surface (3a) to which a metal fluoride layer (4) is applied, wherein during operation of the optical system (10) the optical element (1) is irradiated with radiation (6), in particular in the FUV / VUV wavelength range, characterized by that the optical system (10) is operated during a first time interval under oxidizing ambient conditions (OB) of the optical element (1) and during a second time interval under fluorinating ambient conditions (FB) of the optical element (1), wherein the first time interval follows the second time interval or the second time interval follows the first time interval. [2] Method according to claim 1, wherein the optical system (10) is operated alternately under oxidizing ambient conditions (OB) of the optical element (1) and under fluorinating ambient conditions (FB) of the optical element (1). [3] Method according to claim 1 or 2, wherein an oxide layer (5) is formed on a side of the metal fluoride layer (4) facing away from the metallic surface (3a) or is formed under oxidizing ambient conditions (OB) of the optical element (1), wherein a thickness (d) of the oxide layer (5) increases with increasing duration of the oxidizing ambient conditions (OB). [4] Method according to claim 3, wherein, under the fluorinating ambient conditions (FB) of the optical element (1), the oxide layer (5) remains on the side of the metal fluoride layer (4) facing away from the metallic surface (3a). [5] Method according to claim 3 or 4, wherein the thickness (D) of the metal fluoride layer (4) is selected such that the electric field of a standing wave formed when the reflecting optical element (1) is irradiated with radiation (6) at a useful wavelength (λ) of the optical system (10) has a minimum in the region of the oxide layer (5). [6] Method according to one of the preceding claims, in which, for switching between oxidising ambient conditions (OB) and fluorinating ambient conditions (FB), a concentration (c FG , c OG) at least one fluorine-containing gas (FG) and / or at least one oxygen-containing gas (OG) in the environment (14) of the reflective optical element (1) is changed, wherein preferably at least one fluorine-containing gas (FG) is supplied to the environment (14) of the reflective optical element (1) under fluorinating ambient conditions (FB) and no fluorine-containing gas (FG) is supplied under oxidizing ambient conditions (OB). [7] Method according to claim 6, wherein the concentration (c FG , c OG ) of the at least one fluorine-containing gas (FG) and / or the at least one oxygen-containing gas (OG) in the environment (14) of the optical element (1) is controlled as a function of at least one control parameter (P, R, T) which forms a measure of the reflectivity (R) of the reflective optical element (1). [8] A method according to claim 7, wherein the concentration (c FG) of the at least one fluorine-containing gas (FG) and / or of the at least one oxygen-containing gas (c OG ) is kept constant when the control parameter (P) lies between a first, lower threshold value (P1) and a second, upper threshold value (P2). [9] A method according to claim 8, wherein the concentration (c FG ) of the fluorine-containing gas (FG) is increased and / or the concentration (c OG ) of the oxygen-containing gas (OG) is reduced when the control parameter (P, R, T) falls below the first threshold value (P1) and / or when the concentration (c FG ) of the fluorine-containing gas (FG) is reduced and / or the concentration (c OG ) of the oxygen-containing gas (OG) is increased when the control parameter (P, R, T) exceeds the second threshold value (P2). [10] Method according to one of claims 6 to 9, wherein the optical system (1) after changing the concentration (c FG , c OG) of the at least one fluorine-containing gas (FG) and / or the at least one oxygen-containing gas (OG) at least for a predetermined holding time (t H ) is operated under constant ambient conditions. [11] Method according to one of claims 6 to 10, wherein the control parameter is selected from the group comprising: total transmission (P) of the optical system (1), reflectivity (R) of the reflective optical element (1), chemical composition of the surface (1a) of the optical element (1), temperature (T) of the optical element (1). [12] Optical system, preferably for the FUV / VUV wavelength range, in particular FUV / VUV lithography system (21) or wafer inspection system (41), comprising: at least one reflective optical element (1) having a metallic surface (3a) to which a metal fluoride layer (4) is applied, a light source (13) for irradiating the reflective optical element (1) with radiation, in particular in the FUV / VUV wavelength range, characterized by , that the optical system (1) is designed to operate in oxidizing Ambient conditions (OB) of the optical element (1) during a first time interval and under fluorinating ambient conditions (FB) of the optical element (1) during a second time interval, wherein the first time interval follows the second time interval or the second time interval follows the first time interval. [13] Optical system according to claim 12, wherein the optical system (10) is designed to operate under alternating oxidizing ambient conditions (OB) of the optical element (1) and fluorinating ambient conditions (FB) of the optical element (1). [14] Optical system according to claim 12 or 13, wherein the reflective optical element (1) is arranged in a chamber (11) and the optical system (1) has a supply device (12) for supplying at least one fluorine-containing gas (FG) and / or at least one oxygen-containing gas (OG) into the chamber (11), wherein the supply device is configured to adjust a concentration (c FG ) of the fluorine-containing gas (FG) and / or a concentration (c OG ) of the oxygen-containing gas (OG) in the chamber (11). [15] Optical system according to one of claims 12 to 14, further comprising: a control device (19) for controlling the concentration (c FG , c OG) of the at least one fluorine-containing gas (FG) and / or the at least one oxygen-containing gas (OG) in the environment (14) of the optical element (1) as a function of at least one control parameter (P, R, T) which forms a measure of the reflectivity (R) of the reflective optical element (1), and at least one measuring device (15, 15a,b) for measuring the at least one control parameter (P, R, T). [16] Optical system according to claim 15, wherein the at least one control parameter is selected from the group comprising: total transmission (P) of the optical system (1), reflectivity (R) of the reflective optical element (1), chemical composition of the surface (1a) of the optical element (1), temperature (T) of the optical element (1). [17] Optical system according to one of claims 12 to 16, wherein the chamber (11) has an entrance window (20a) for the entry of light (6) of the light source (13) into the chamber (11) and / or an exit window (20b) for the exit of light (6) of the light source (13) from the chamber (11).

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