DE102022210514A1 - Method and apparatus for producing a fluoride protective coating for a reflective optical element - Google Patents

Abstract

The invention relates to a method for producing a fluoridic protective coating (11) for protecting a metallic reflection layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising the step of: irradiating a native oxide layer (16) of the metallic reflection layer (12) with UV/VUV radiation (17) in the presence of a fluorination agent (FW) to convert the native oxide layer (16) into a fluoridic layer (18) of the fluoridic protective coating (11). The invention also relates to a reflective optical element (13) with a fluoridic protective coating (11) that was produced by means of this method, and to an optical arrangement with at least one such reflective optical element (13).

Description

Background of the invention

The invention relates to a method and a device for producing a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range. The invention also relates to a reflective optical element for use in the VUV wavelength range with a fluoridic protective coating that was produced by means of this method, and to an optical arrangement for the VUV wavelength range that has at least one such reflective optical element.

In this application, the VUV wavelength range is understood to mean the wavelength range of electromagnetic radiation between 115 nm and 190 nm. The VUV wavelength range is particularly important for microlithography. Radiation in the VUV wavelength range is used, for example, in projection exposure systems and wafer or mask inspection systems.

Reflective optical elements are often used in such systems, which have a metallic reflection layer and a fluoride protective coating. In order to achieve a high degree of reflection, metallic reflection layers made of aluminum are used in particular, which are typically produced by physical vapor deposition, in particular by electron beam evaporation. The aluminum is typically deposited on an unheated substrate, since higher temperatures have a detrimental effect on the reflectivity.

Fluoride protective coatings are made at least partially, and in particular completely, from fluorides. In contrast to oxides, fluorides have the advantage of being sufficiently transparent in the VUV wavelength range. The fluoride protective coating can be used in particular to protect the underlying metallic reflective layer from oxidation. For example, a fluoride protective coating can be used to counteract the formation of a native oxide layer on a metallic reflective layer made of aluminum, which leads to considerable losses in reflectivity due to the high extinction of aluminum oxide in the VUV wavelength range.

For example, in the WO 2006/053705 A1 For the protection against degradation of a reflective metal layer made of aluminum, which serves to reflect wavelengths between 120 nm and 260 nm, the use of a protective layer made of chiolite is described. Furthermore, the EN 10 2018 211 499 A1 a reflective optical element comprising a substrate, a metal layer, a metal fluoride layer applied to the metal layer and an oxide layer applied to the latter, as well as a method for its production.

Fluoride layers are typically deposited using physical vapor deposition (PVD). These processes include thermal evaporation, electron beam evaporation (ESV) and ion beam sputtering (IBS). The deposition is typically carried out at elevated temperatures; in particular, the substrate can be heated in order to obtain a protective coating that is as dense as possible with as little absorption as possible. However, heating an unprotected aluminum layer leads to severe losses in reflectivity, which are caused in particular by the formation of a native aluminum oxide layer that is strongly absorbing in the VUV wavelength range and possibly contamination.

A process for producing MgF ₂ protected Al mirrors that partially solves this problem is described in US 2017/0031067 A1 and the article “Enhanced MgF ₂ and LiF Over-Coated Al Mirrors for FUV Space Astronomy” by MA Quijada et al., Proc. SPIE 8450, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 84502H (2012). The process described therein consists of three steps: First, a substrate is coated with aluminum in the intended thickness at room temperature. As quickly as possible, the aluminum layer is then coated with a 4 nm to 5 nm thin MgF ₂ layer at room temperature to protect the aluminum from oxidation and contamination. Finally, the substrate is heated and additional MgF ₂ is deposited on the thin MgF ₂ layer up to the intended thickness.

However, even with this procedure, the formation of an oxide layer cannot be completely avoided, as can be seen from the following example: Commercial PVD coating chambers without a lock (typically referred to as a "loadlock") are generally operated in a high vacuum. The residual gas pressure before coating is between 10 ^-3 mbar and 10 ^-8 mbar, typically between approx. 10 ^-6 mbar and 10 ^-7 mbar. According to Langmuir's adsorption model, the time period in which a monolayer of adsorbates forms on the layer is in the single-digit second range at a residual gas pressure in the range of 10 ^-6 mbar to 10 ^-7 mbar. Even at these low pressures, a monolayer of adsorbates forms very quickly. This Adsorbates (H ₂ O and O ₂ ) oxidize the aluminum layer, which leads to a significant loss of reflectivity despite the cold protective layer.

Furthermore, a number of other processes for producing fluoride layers or optical elements with fluoride layers are known from the prior art.

For example, the EN 10 2005 017 742 A1 a method for coating a substrate by plasma-assisted deposition of a coating material, for example a fluoride material. The plasma contains ions whose effective ion energy is relatively small, while the effective energy per molecule is relatively large, which should lead to low absorption and contamination of a deposited layer with a high packing density at the same time.

The EN 10 2020 208 044 A1 discloses a method for producing an optical element, for example a mirror, window or beam splitter, for the VUV wavelength range with a coating with a fluorine scavenger layer, which can be applied to a fluoride layer. The purpose of the fluorine scavenger layer is to prevent the degradation of the fluoride layer, which is associated with a longer service life of the optical element. The underlying mechanism is a significant reduction in the mobility of interstitial fluorine atoms by so-called fluorine scavengers in the fluorine scavenger layer.

Further variants of the physical vapor deposition of fluoride layers are also available in the US2013/0122252 A1 , the JP11172421A and the JP2003193231A2 described.

In addition to methods for producing fluoride layers or optical elements with fluoride layers, the post-treatment of fluoride layers is also proposed in the prior art. Post-treatment means that the treatment takes place after the deposition of the fluoride layers has been completed. The post-treatment typically serves to reduce the absorption of the fluoride layers and their degradation.

For example, the US 2004/0006249 A1 a process for the post-fluorination of fluoride layers. The fluoride layer is exposed to gaseous fluorine at a temperature between 10°C and 150°C and a fluorine concentration between 1000 ppm and 100%. As a result of the post-treatment, the composition of the fluoride layer approaches the stoichiometric ratio.

Also the EN 10 2021 200 490 A1 describes a post-treatment of a metal fluoride layer by means of irradiation, whereby electromagnetic radiation with at least one wavelength of less than 300 nm is used. The metal fluoride layer is applied to a metal layer of a reflective optical element for use in the VUV wavelength range. The irradiation results in a passivation of the metal fluoride layer, which counteracts degradation of the metal layer. However, the passivating protective layer is typically an oxide layer, which leads to the disadvantages described above.

Finally, the prior art proposes the use of UV light to support the deposition of layers, for nanostructuring or for irradiating optical elements during the operation of a microlithography system.

This is how the US 7 798 096 B2 the use of UV light to support the deposition of high-k dielectrics with good layer quality using chemical vapor deposition or atomic layer deposition is described. The UV light is used to excite or ionize the process gas and thereby initiate or intensify surface reactions during deposition.

The EN 10 2018 221 190 A1 further discloses the nanostructuring of a substrate for the transmission of radiation in the FUV/VUV wavelength range by introducing an energy input, e.g. by irradiation with UV/VUV radiation. The substrate is crystalline, for example the substrate is a MgF ₂ single crystal. The irradiation can reorganize the surface of the MgF ₂ single crystal in such a way that an anti-reflective effect occurs.

From the EN 10 2021 201 477 A1 Furthermore, a method for operating an optical arrangement for microlithography is known, which has an optical element with a fluoride coating or made of a fluoride substrate. In order to heal defects in the fluoride during operation, the optical element is irradiated with UV light with wavelengths that are longer than the wavelength of the working light of the optical arrangement, which is less than or equal to 300 nm.

Object of the invention

In contrast, it is the object of the invention to provide a method for producing a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range, which leads to the highest possible reflection degree of the reflective optical element in the VUV wavelength range.

Subject of the invention

This object is achieved according to a first aspect by a method for producing a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range, comprising the step of: irradiating a native oxide layer of the metallic reflection layer with UV/VUV radiation in the presence of a fluorinating agent to convert the native oxide layer into a fluoride layer of the fluoridic protective coating.

For the purposes of this application, UV/VUV radiation refers to electromagnetic radiation in the wavelength range between 115 nm and 350 nm. The native oxide layer is preferably irradiated in the presence of the fluorinating agent with VUV radiation in the wavelength range between 115 nm and 190 nm (see above).

The fluorination agent is a preferably gaseous substance which, when irradiated with UV/VUV radiation, forms molecular and/or atomic, in particular ionized and/or excited, fluorine (hereinafter referred to collectively as fluorine species). The fluorine species formed in this way substitute oxygen molecules in the native oxide layer and thus lead to the conversion of the native oxide layer into the fluoride layer. The formation of the native oxide layer is therefore not minimized as in the process described in the article by A. Quijada et al. cited above, but the native oxide layer is converted into the fluoride layer.

In addition to the photodissociation of the fluorination agent, UV/VUV radiation can also be used to provide activation energy for the fluorination process, i.e. the fluorination of the native oxide layer. The spectrum of UV/VUV radiation is preferably adapted to the activation energy of the fluorination process.

The method according to the invention leads in particular to a significantly increased degree of reflection of the reflective optical element in the VUV wavelength range. This is due in particular to the fact that the native oxide layer, which has a high absorption in the VUV wavelength range, is converted into the fluoride layer, which absorbs significantly less strongly.

The increased reflectance is accompanied by a significantly increased system transmission of optical arrangements for the VUV wavelength range with at least one reflective optical element with such a fluoridic protective coating, which leads to a higher throughput, for example in the case of VUV microlithography systems.

Furthermore, the lower absorption significantly reduces the thermal load, i.e. heating of the optical elements during operation (also known as “lens heating”). This is associated with a reduction in the imaging errors caused by lens heating. Furthermore, the reduced lens heating is accompanied by an extension of the service life of the reflective optical elements, since thermally activated processes which drive the degradation of the optical elements take place more slowly. These include in particular diffusion processes with Arrhenius-type activation, i.e. D ∝ exp(-E _D /k _B T), where D is the diffusion constant, E _{D is} the activation energy for diffusion, k _{B is} the Boltzmann constant and T is the temperature. Such diffusion processes can, for example, increase the roughening of the metallic reflective layer, which leads to higher stray light losses. The reflective optical elements can therefore be replaced at a reduced frequency compared to the prior art.

A further advantage of the method according to the invention is that the fluoride layer has a higher density compared to a fluoride layer deposited at room temperature, in particular compared to the MgF ₂ layer deposited at room temperature described in the article by A. Quijada et al. cited at the beginning. This can be a consequence in particular of the increase in the mobility of the atoms due to the UV/VUV radiation.

In a variant of this method, the UV/VUV radiation for photodissociation of the fluorinating agent has a first spectral range that includes at least one wavelength whose energy, E _ph , is at least as great as the dissociation energy, E _diss , of the fluorinating agent. The dissociation energy, E _diss , refers to the energy that is required to break the chemical bond of the fluorinating material by means of electromagnetic radiation (light).

Preferably, the first spectral range comprises mostly, particularly preferably exclusively, wavelengths whose energy, E _ph , is at least as great as the dissociation energy, E _diss , of the fluorinating agent. In this case, the relation applies to all photons of the first spectral range $$E_{\text{diss}}\le E_{\text{ph}}.$$

It may also be advantageous to restrict the first spectral range to higher energies in such a way that potentially negative and/or competing effects are suppressed or reduced in their rate. It is therefore preferable to choose E _UP smaller than a threshold energy above which negative and/or competing effects are intensified or occur for the first time, where E _UP is the highest energy of the first spectral range, i.e. E _ph ≤ E _UP . Examples of such negative and/or competing effects are the absorption of light in the solid state and the photodissociation of potentially oxidizing species (e.g. O ₂ and H ₂ O) in the gas phase.

In a further development of this variant, the highest energy, E _UP , of the first spectral range is at most 100%, preferably at most 50%, greater than the dissociation energy, E _diss , of the fluorinating agent.

In a further variant of this process, the highest energy, E _UP , of the first spectral range is at most as large as the band gap energy, E _G , of the fluoride layer being formed, preferably at most as large as 75% of the band gap energy, E _G , of the fluoride layer being formed. This reduces the photoabsorption in the fluoride layer and thus potentially the formation of point defects (eg F centers).

To reduce the photodissociation of potentially oxidizing species (e.g. O ₂ and H ₂ O) in the gas phase, it is further advantageous to restrict the first spectral range towards smaller and larger energies in such a way that the effective rate for the formation of potentially fluorinating species r _fluorinating , such as F, F ₂ , F-, F* or HF is greater than the rate of potentially oxidizing species r _oxidizing such as O, O ₂ , O*, O ^- , OH* and OH ^- : $$r_{\text{fluorinated}}>r_{\text{oxidizing}}.$$

In a further variant of this method, the UV/VUV radiation has a second spectral range for mobilizing atoms on the surface and/or in the volume (i.e. at the grain boundaries and/or in the grain volume) of the fluoride layer being formed, wherein the second spectral range lies in an energy range between 75% and 100%, preferably between 80% and 95% of a band gap energy of the fluoride layer.

By UV/VUV radiation close to the band edge of the corresponding fluoride, surface atoms or atoms in the volume can be mobilized without being desorbed, as was the case, for example, in the above-cited EN 10 2018 221 190 A1 This increased mobility can help to create a dense and smooth fluoride layer.

In a further variant, the UV/VUV radiation or further electromagnetic radiation with which the fluoride layer being formed is additionally irradiated has a spectral range for healing at least one crystal defect in the fluoride layer being formed, which at least partially overlaps with an absorption range of the at least one crystal defect, wherein the spectral range preferably comprises an absorption energy of the crystal defect, wherein particularly preferably an average energy of the spectral range deviates from the absorption energy of the crystal defect by no more than 0.5 eV, in particular by no more than 0.25 eV. The irradiation for healing the at least one crystal defect can take place during the conversion of the native oxide layer into the fluoride layer or after the conversion.

A potential problem with irradiation with VUV radiation is that it can cause crystal defects, particularly F/H center defect pairs, in the fluoride via one-photon processes. These crystal defects can be healed by irradiation in a spectral range that at least partially overlaps with the absorption range.

The absorption energy of the crystal defect is the energy or wavelength at which the absorption coefficient of the crystal defect has a maximum. The absorption range of the crystal defect is a range in which the absorption coefficient is greater than one hundredth of the value at the maximum of the absorption coefficient. The absorption energies of crystal defects of several fluorides relevant for the present applications are given below as examples: MgF ₂ : 260 nm (4.77 eV), AlF ₃ : 190 nm (6.53 eV), 170 nm (7.29 eV), LaF ₃ : 459 nm (2.7 eV), 564 nm (2.2 eV), 729 nm (1.7 eV).

In another variant of this process, the metallic reflection layer is an aluminum layer. In this case, the fluoride layer is made of AlF ₃ in particular. AlF ₃ has the advantage that it typically forms very smooth layers, as it generally has an X-ray amorphous structure. In addition, the aluminum layer has a high reflectivity in the VUV wavelength range.

In a further variant of this process, the irradiation of the native oxide layer is carried out in a protective gas atmosphere, preferably in a vacuum. The protective gas is preferably transparent for electromagnetic radiation in the UV/VUV Wavelength range. Furthermore, protective gases are preferred which are less reactive towards optically relevant oxides and fluorides. Inert gases in the form of the light noble gases helium, neon and argon are particularly suitable as protective gases, the latter being particularly suitable. Mixtures of noble gases, in particular the noble gases mentioned, can also be used as protective gases. The irradiation of the native oxide layer is typically carried out in a process chamber in the interior of which the protective gas atmosphere prevails. The protective gas atmosphere can be created by introducing (dosing) the fluorinating agent in a carrier gas in the form of a protective gas, typically in the form of an inert gas, into the evacuated interior of the process chamber. Alternatively, the irradiation can take place in a process chamber in the interior of which there is a protective gas atmosphere at higher pressure, e.g. at atmospheric pressure. In this case, the fluorinating agent is additionally introduced into the interior of the process chamber at the existing atmospheric pressure.

In a further variant of this process, the fluorinating agent comprises at least one substance from the group: F ₂ , HF, XeF ₂ , NF ₃ , CF ₄ , SF ₆ .

In a further variant of this process, the partial pressure of the fluorinating agent during irradiation of the native oxide layer is between 10 ^-8 mbar and 10 mbar, preferably between 10 ^-6 mbar and 10 ^-1 mbar.

In a further variant of this process, the partial pressure of the fluorinating agent is regulated to a desired value during irradiation of the native oxide layer.

In a further variant, the method additionally comprises the step of depositing a further fluoride layer of the fluoridic protective coating on the fluoride layer. The deposition of the further fluoride layer takes place after the conversion of the native oxide layer into the fluoride layer has been completed. The deposition of the further fluoride layer can take place in particular by means of physical vapor deposition.

As stated above, the fluoride layer resulting from irradiating the native oxide layer with UV/VUV radiation in the presence of the fluorination agent is particularly dense and smooth. Both aspects, the increased density and the low roughness, promote the growth of the additional fluoride layer. In particular, the additional fluoride layer is also particularly dense and smooth.

A further aspect of the invention relates to a reflective optical element for use in the VUV wavelength range, comprising a fluoridic protective coating produced by the method described above.

A further aspect of the invention relates to an optical arrangement for the VUV wavelength range, in particular a VUV lithography system or a wafer inspection system, comprising at least one reflective optical element as described above. The optical arrangement can be, for example, a (VUV) lithography system, a wafer or mask inspection system, a laser system, etc.

Finally, a further aspect of the invention relates to a device for producing a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range, comprising: a process chamber, a supply device for supplying inert gas and a fluorinating agent into the process chamber, the inside of the process chamber being resistant to the fluorinating agent and its subsequent products, and a UV/VUV radiation source for irradiating a native oxide layer of the metallic reflection layer with UV/VUV radiation in the presence of the fluorinating agent to convert the native oxide layer into a fluoridic layer of the fluoridic protective coating. With regard to the advantages achieved with the device, reference is made to the above statements in relation to the method according to the invention and its variants.

The UV/VUV radiation emitted by the UV/VUV radiation source is partially absorbed by the fluorination agent. The fluorine species subsequently formed by photodissociation of the fluorination agent cause a conversion of the native oxide layer into the fluoride layer. In addition, the UV/VUV radiation source can also be used to emit UV/VUV radiation to provide activation energy for the fluorination process and/or for UV/VUV radiation-driven mobilization of atoms on the surface of the fluoride layer being formed. However, the purpose of the UV/VUV radiation source does not necessarily have to go beyond the photodissociation of the fluorination agent. One or more additional UV/VUV radiation sources can also be used to provide the activation energy and mobilize atoms. In addition to the UV/VUV radiation source, the device preferably has one or two further UV/VUV radiation sources and/or further radiation sources for generating further electromagnetic radiation, e.g. for healing crystal defects or to provide the activation energy of the fluorination process.

The process chamber can be sealed gas-tight. The byproducts of the fluorination agent are the fluorine species and the chemical compounds formed from them (e.g. HF). The resistance is to be understood in particular in the sense that a passivating layer forms on the inside of the process chamber. In particular, no volatile fluorine compounds may form that could precipitate on the reflective optical element. Accordingly, the metals used should be free of Cr and Ti. The process chamber can in particular be made of Monel steel.

Alternatively, the inside of the process chamber can have a fluorine-resistant coating to prevent corrosion. Such a coating is preferably applied using a galvanic process. NiP, Pt or Ru/Rh mixtures are particularly suitable materials.

The feed device is preferably designed to introduce the fluoridation agent diluted in the inert gas into the process chamber. Furthermore, the feed device can comprise a suitable metering valve, in particular a needle valve or a mass flow controller (MFC), in order to set the partial pressure of the fluoridation agent in the process chamber to a desired value.

Preferably, the device also comprises a fluorine gas sensor (e.g. a residual gas analyzer) and/or a dedicated sensor for the fluoridation agent. The device can further comprise a control device for controlling the partial pressure of the fluoridation agent, wherein the control preferably takes place by means of the measured values of the fluorine gas sensor and/or the dedicated sensor for the fluoridation agent.

The partial pressure of the fluorinating agent can either be adjusted indirectly by adjusting the flow or it can be actively controlled via a sensor and a control device.

In one embodiment, the device additionally comprises: at least one coating source for depositing the metallic reflection layer and/or a further fluoride layer of the fluoridic protective coating. It is advantageous if the production of the fluoridic protective coating and the deposition of the metallic reflection layer are carried out in one and the same process chamber, since in this case no transport of the substrate on which the deposition takes place is required.

Further features and advantages of the invention emerge from the following description of embodiments of the invention, based on the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in groups in any combination in a variant of the invention.

drawing

• 1 eine schematische Darstellung der Herstellung einer Schutzbeschichtung aus MgF₂ für einen Aluminium-Spiegel mittels eines aus dem Stand der Technik bekannten Verfahrens,
• 2 den berechneten Reflexionsgrad von fluoridisch geschützten Aluminium-Spiegeln mit und ohne nativer Aluminiumoxidschicht in Abhängigkeit der Wellenlänge,
• 3 eine schematische Darstellung der Herstellung einer fluoridischen Schutzbeschichtung zum Schutz einer metallischen Reflexionsschicht eines reflektiven optischen Elements für den VUV-Wellenlängenbereich mittels des erfindungsgemäßen Verfahrens, umfassend die Bestrahlung einer nativen Oxidschicht der metallischen Reflexionsschicht mit VUV-Strahlung,
• 4 eine schematische Illustration der für die Bestrahlung der nativen Oxidschicht beim Einsatz des erfindungsgemäßen Verfahrens besonders relevanten Absorptions- und Spektralbereiche,
• 5 eine schematische Darstellung einer erfindungsgemäßen Vorrichtung zur Herstellung einer fluoridischen Schutzbeschichtung zum Schutz einer metallischen Reflexionsschicht eines reflektiven optischen Elements für den VUV-Wellenlängenbereich,
• 6 eine schematische Darstellung einer optischen Anordnung für den VUV-Wellenlängenbereich in Form einer VUV-Lithographieanlage, sowie
• 7 eine schematische Darstellung einer optischen Anordnung für den VUV-Wellenlängenbereich in Form eines Wafer-Inspektionssystems.

Examples of embodiments are shown in the schematic drawing and are explained in the following description. It shows

• 1 a schematic representation of the production of a protective coating of MgF ₂ for an aluminium mirror by means of a process known from the prior art,
• 2 the calculated reflectance of fluoride-protected aluminium mirrors with and without native aluminium oxide layer as a function of wavelength,
• 3 a schematic representation of the production of a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range by means of the method according to the invention, comprising the irradiation of a native oxide layer of the metallic reflection layer with VUV radiation,
• 4 a schematic illustration of the absorption and spectral ranges particularly relevant for the irradiation of the native oxide layer when using the method according to the invention,
• 5 a schematic representation of an apparatus according to the invention for producing a fluoridic protective coating for protecting a metallic reflection layer of a reflective optical element for the VUV wavelength range,
• 6 a schematic representation of an optical arrangement for the VUV wavelength range in the form of a VUV lithography system, as well as
• 7 a schematic representation of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system.

The 1 shows a schematic representation of the production of a protective coating 1 made of MgF ₂ for an aluminium mirror 2 by means of the in the above-quoted US 2017/0031067 A1 and in the article by MA Quijada et al. also cited at the beginning. The protective coating 1 disclosed therein comprises a first MgF ₂ layer 3 and a second MgF ₂ layer 4. In 1 The aluminum mirror 2, which comprises an aluminum layer 6 applied to a substrate 5, is shown in four snapshots MS1, MS2, MS3, MS4, wherein only a small section in the vicinity of the surface 7 of the aluminum layer 6 is shown in each case.

The first snapshot MS1 shows the aluminum mirror 2 after the deposition of the aluminum layer 6 on the substrate 5 and before the formation of a native oxide layer 8.

The second snapshot MS2 shows the deposition of the first MgF ₂ layer 3 of the protective coating 1, which is carried out as quickly as possible after the deposition of the aluminum layer 6 at room temperature. However, the formation of the native oxide layer 8 cannot be completely avoided in this way. In addition, the deposited first MgF ₂ layer 3 is typically a porous layer.

The third snapshot MS3 shows the aluminum mirror 2 after completion of the deposition of the first MgF ₂ layer 3. The first MgF ₂ layer 3 has a thickness of 4 nm to 5 nm in the example shown.

The fourth snapshot MS4 shows the aluminum mirror 2 after completion of the deposition of the second MgF ₂ layer 4. The substrate 5 is heated for the deposition of this second MgF ₂ layer 4. The higher temperature leads to a higher density of the second MgF ₂ layer 4.

The fact that the formation of the native oxide layer 8 cannot be completely avoided is due in particular to the fact that the native oxide layer 8 forms within a few seconds even at the relatively low residual gas pressures in PVD coating chambers. The formation of the native oxide layer 8 leads to a loss of reflectivity of the aluminum mirror 2, as will be shown below using 2 described.

In the 2 the calculated reflectance 9 of a fluoride-protected aluminum mirror with a native oxide layer is compared with the calculated reflectance 10 of a fluoride-protected aluminum mirror without a native oxide layer. The dependence of the reflectances 9 and 10 on the wavelength is shown. The native oxide layer is located at the aluminum-fluoride transition and has a thickness of approximately 1 nm.

The results presented show that even a native oxide layer with a relatively small thickness of 1 nm leads to a reflectivity loss of up to 10 % in the VUV wavelength range.

The 3 shows a schematic representation of the production of a fluoridic protective coating 11 for protecting a metallic reflection layer 12 of a reflective optical element 13 for the VUV wavelength range by means of a method which avoids the reflection loss described above, which is attributable to the native oxide layer 8. Four snapshots M1, M2, M3, M4 are shown, with only a small section in the vicinity of the surface 14 of the metallic reflection layer 12 being shown in each case.

The first snapshot M1 shows the reflective optical element 13 with the metallic reflection layer 12 and a substrate 15 before the production of the fluoridic protective coating 11. A native oxide layer 16 has not yet formed in this exemplary representation. The metallic reflection layer 12 is, but not necessarily, an aluminum layer in the example shown.

If the coating methods for depositing the metallic reflective layer 12 and the fluoridic protective coating 11 are not compatible, the reflective optical element 13 can be transferred to a process chamber (not shown here) to produce the fluoridic protective coating 11. If the coating methods are compatible, the reflective optical element 13 typically remains in the coating chamber in which the metallic reflective layer 12 is deposited. In both cases, as shown in the second snapshot M2, the native oxide layer 16 of the metallic reflective layer 12 is irradiated with UV/VUV radiation 17 in the presence of a fluorinating agent FW. As a result of the irradiation, the fluorinating agent FW dissociates and forms fluorine species F,F ₂ ,F*. The fluorine species F,F ₂ ,F* react with the native oxide layer 16 and a fluoride layer 18 is formed there, which is part of the fluoridic protective coating 11.

For example, the fluorination agent FW is NF ₃ , but it can also be another substance that can provide the fluorine species F,F ₂ ,F* via photodissociation, in particular at least one substance from the group comprising: F ₂ , HF, XeF ₂ , NF ₃ , CF ₄ and SF ₆ .

The irradiation with UV/VUV radiation 17 also leads, for example but not necessarily, to a mobilization of atoms on the surface 18' and in the volume 18" of the fluoride layer 18 that is being formed.

The third snapshot M3 shows the reflective optical element 13 after irradiation of the native oxide layer 16. By irradiation with UV/VUV radiation 17 in the presence of the fluorination agent FW, the native oxide layer 16 was converted into a fluoride layer 18. Since the metallic reflection layer 12 in the example shown is an aluminum layer, the fluoride layer 18 is here, for example, predominantly made of AlF _3. The fluoride layer 18 is also particularly dense and smooth.

In the example shown, but not necessarily, in a subsequent step, a further fluoride layer 19 of the fluoridic protective coating 11 is deposited on the fluoride layer 18.

The fourth snapshot M4 shows the reflective optical element 13 after the deposition of the further fluoride layer 19 of the fluoridic protective coating 11. The further fluoride layer 19 is formed here by way of example, but not necessarily, from AlF ₃ and was deposited by means of physical vapor deposition, wherein the substrate 15 was heated.

The 4 illustrates the absorption and spectral ranges that are particularly relevant for the irradiation of the native oxide layer 16 with the UV/VUV radiation 17 in the presence of the fluorination agent FW. The energy is plotted on the abscissa axis and the absorption cross section on the ordinate axis. Schematically shown are the dissociation energy E _diss of the fluorination agent FW, the absorption cross section 61 of the fluoride layer 18 that is being formed, including an Urbach tail 61', and the absorption cross section 65 of a crystal defect in the fluoride layer.

The UV/VUV radiation 17 with which the native oxide layer 16 is irradiated has a first spectral range 62 for the photodissociation of the fluorination agent FW. The first spectral range 62 comprises, for example, at least one wavelength whose energy E _ph is at least as great as the dissociation energy E _diss of the fluorination agent FW.

Furthermore, the largest energy E _UP of the first spectral range 62 is, by way of example but not necessarily, less than 50% greater than the dissociation energy E _diss of the fluorinating agent FW (ie less than 1.5 x E _diss , cf. 4 ). This suppresses potentially negative and/or competing effects. The greatest energy E _UP of the first spectral range 62 can also be at most as large as the band gap energy E _G of the fluoride layer 18, preferably at most as large as 75% of the band gap energy E _G of the fluoride layer 18.

Furthermore, the UV/VUV radiation 17 has a second spectral range 63 for mobilizing atoms on the surface 18` and/or in the volume 18" of the fluoride layer 18 being formed. This second spectral range 63 lies here in an energy range between 75% and 100% of the band gap energy E _G of the fluoride layer 18. Preferably, the second spectral range 63 can also lie in an energy range between 80% and 95% of the band gap energy E _G of the fluoride layer 18.

Furthermore, for example, the fluoride layer 18 to be formed is irradiated with further electromagnetic radiation 20 in order to heal at least one crystal defect 10 of the fluoride layer 18. The further electromagnetic radiation 20 has a spectral range 64 that overlaps with an absorption range 66 of the at least one crystal defect. In the example shown, the spectral range 64 of the further electromagnetic radiation 20 lies within the absorption range 66 of the crystal defect, which is an F center, but this is not absolutely necessary. Alternatively, the UV/VUV radiation 17 can also have a corresponding spectral range.

In the example shown, the spectral range 64 of the further electromagnetic radiation 20 includes the absorption energy E _A of the crystal defect at which the absorption cross section is maximum. However, this is not necessarily the case. The absorption range 66 of the crystal defect is defined by a drop to one hundredth of the maximum value of the absorption cross section (FWHM) at the absorption energy E _A of the crystal defect. Furthermore, it is advantageous if an average energy E _m of the spectral range 64 deviates from the absorption energy E _A of the crystal defect by no more than 0.5 eV, in particular by no more than 0.25 eV.

The 5 shows a device 70 for producing a fluoridic protective coating 11 for protecting a metallic reflection layer 12 of a reflective optical element 13 for the VUV wavelength range by means of the method described above in connection with 3 The device comprises a process chamber 71, a feeding device 72, and a UV/VUV radiation source 73.

The reflective optical element 13 with the metallic reflection layer 12 and the native oxide layer 16 is mounted within the process chamber 71 on a substrate holder 74 which is rotatable about a rotation axis 75. However, in contrast to the example shown here, the device 70 does not have to comprise a rotatable substrate holder 74.

The feed device 72 serves to feed inert gas IG and the fluorination agent FW into the process chamber 71, wherein the feed device 72 comprises a first valve 76 for the controlled feed of the inert gas IG and a second valve 77 for the controlled feed of the fluorination agent FW. The second valve 77 is also a controllable metering valve. The device 70 also comprises a vacuum pump 78 for evacuating the process chamber 71. The fluorination agent FW is metered into the evacuated interior of the process chamber 71 in the inert gas IG, which serves as a carrier gas. Alternatively, the irradiation of the native oxide layer 16 in the presence of the fluorination agent FW can be carried out at higher pressures, e.g. at atmospheric pressure, in a protective gas atmosphere. In this case, the process chamber 71 is flooded with inert gas and the fluorination agent FW is additionally added.

The UV/VUV radiation source 73 is used to irradiate the native oxide layer 16 with UV/VUV radiation 17 in the process chamber 71 in the presence of the fluorination agent FW. This serves to convert the native oxide layer 16 into the fluoride layer 18 (not shown here). For example, the UV/VUV radiation 17 enters the process chamber 71 through an MgF ₂ window 79.

Furthermore, the device 70 comprises, by way of example but not necessarily, a second UV/VUV radiation source 80 for irradiating the native oxide layer 16 with UV/VUV radiation 17 for mobilizing atoms on the surface 18` or in the volume 18" of the fluoride layer 18 being formed. For the second UV/VUV radiation source 80, a further MgF ₂ window 81 is provided here by way of example.

Instead of at least one of the MgF ₂ windows 79, 81, windows made of other materials, for example CaF ₂ , SrF ₂ and/or BaF ₂ , can in principle also be used, whereby sufficient transparency at the wavelengths used is crucial.

Alternatively, the device 70 can also have a different number of UV/VUV radiation sources. In particular, a single UV/VUV radiation source can serve both for converting the native oxide layer 16 into the fluoride layer 18 and for mobilizing atoms on the surface 18` or in the volume 18" of the fluoride layer 18 being formed. The device 70 can also have a further radiation source (not shown in the image) for irradiating the fluoride layer 18 formed during the conversion with further electromagnetic radiation 20 in the manner described above in connection with 4 described spectral range 64 for healing at least one crystal defect of the fluoride layer 18. The further radiation source for healing the crystal defect can be designed, for example, as a further UV/VUV radiation source.

The process chamber 71 can be sealed gas-tight. The inner side 82 of the process chamber 71 is also resistant to the fluorinating agent FW and its byproducts.

In the example shown, but not necessarily, the device 70 also comprises a sensor 83 for measuring the partial pressure p _FW of the fluoridation agent FW in the process chamber 71. The partial pressure p _FW of the fluoridation agent FW is preferably between 10 ^-8 mbar and 10 mbar, particularly preferably between 10 ^-6 mbar and 10 ^-1 mbar, during the irradiation of the native oxide layer 16.

The device 70 further comprises a control device 84 for controlling the partial pressure p _FW of the fluorinating agent FW in the process chamber 71 to a target value, wherein the control is carried out by means of the actual measured value M of the sensor 83 for measuring the partial pressure p _FW of the fluorinating agent FW in the process chamber 71 and by means of controlling the second valve 77. The sensor 83 can only be designed to measure the partial pressure p _FW of the fluorinating agent FW, but it can also be a residual gas analyzer which can also determine the partial pressures of other gases contained in the process chamber 71.

In the example shown, the device 70 also includes, but does not necessarily include, a coating source 85 which serves to deposit the metallic reflection layer 12 and the further fluoride layer 19. For example, the coating source 85 here is a device for electron beam evaporation, but it can also be another coating source, in particular a sputter source or a thermal evaporator.

Alternatively, the device 70 may also have no or more than one coating source In particular, the device 70 for depositing the metallic reflection layer 12 and the further fluoride layer 19 can each have a dedicated coating source.

The device 70 also has a plasma source 86 to assist in the activation of the fluoridation agent FW. Alternatively, the device 70 may also have another ion source or no plasma source. During or after the conversion of the oxide layer 16 into the fluoride layer 18, the fluoridation agent FW may also be activated by increasing the temperature of the substrate 15.

6 shows an optical arrangement for the VUV wavelength range in the form of a VUV lithography system 21. The VUV lithography system 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography system 21 also has a radiation source 24, which can be an excimer laser, for example.

The radiation 25 emitted by the radiation source 24 is processed with the aid of the illumination system 22 in such a way that a mask 26, also called a reticle, is illuminated. In the example shown, the illumination system 22 has a housing 32 in which both transmitting and reflecting optical elements are arranged. A transmitting optical element 27, which bundles the radiation 25, and a reflecting optical element 28, which deflects the radiation, are shown as representatives.

The mask 26 has a structure on its surface which is transferred to an optical element 29 to be exposed, for example a wafer, for producing semiconductor components, using the projection system 23. In the example shown, the mask 26 is designed as a transmitting optical element. In alternative embodiments, the mask 26 can also be designed as a reflective optical element.

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

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

7 shows an optical arrangement 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 system 42 with a radiation source 54, the radiation 55 of which is directed onto a wafer 49 by means of the optical system 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 another concave mirror 48, which also belongs to the optical system 42, via a transmitting optical element 47 to a detector 50 for further evaluation. 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 radiation source 54 can, for example, be exactly one radiation source or a combination of several individual radiation sources in order to provide an essentially continuous radiation spectrum. In modifications, one or more narrow-band radiation sources 54 can also be used.

At least one of the reflective optical elements 28 of the 6 shown VUV lithography system 21 and at least one of the reflective optical elements 46, 48 of the in 7 The wafer inspection system 41 shown in FIG. 1 are designed as described above. The at least one of the reflective optical elements 28, 46, 48 thus has at least one fluoride layer, which is applied by means of the method described above in connection with 3 described procedure.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

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Cited non-patent literature

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Claims (16)

Method for producing a fluoridic protective coating (11) for protecting a metallic reflection layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising the step: - irradiating a native oxide layer (16) of the metallic reflection layer (12) with UV/VUV radiation (17) in the presence of a fluorinating agent (FW) to convert the native oxide layer (16) into a fluoride layer (18) of the fluoridic protective coating (11). Procedure according to Claim 1 , characterized in that the UV/VUV radiation (17) for photodissociation of the fluorinating agent (FW) has a first spectral range (62) which comprises at least one wavelength whose energy (E _ph ) is at least as great as the dissociation energy (E _diss ) of the fluorinating agent (FW). Procedure according to Claim 2 , characterized in that the greatest energy (E _UP ) of the first spectral range (62) is at most 100%, preferably at most 50%, greater than the dissociation energy (E _diss ) of the fluorinating agent (FW). Procedure according to Claim 2 or 3 , characterized in that the greatest energy (E _UP ) of the first spectral range (62) is at most as large as the band gap energy (E _G ) of the fluoride layer (18) being formed, preferably at most as large as 75% of the band gap energy (E _G ) of the fluoride layer (18). Method according to one of the preceding claims, characterized in that the UV/VUV radiation (17) has a second spectral range (63) for mobilizing atoms on the surface (18`) and/or in the volume (18") of the fluoride layer (18) being formed, wherein the second spectral range (63) lies in an energy range between 75% and 100%, preferably between 80% and 95% of a band gap energy (E _G ) of the fluoride layer (18). Method according to one of the preceding claims, characterized in that the UV/VUV radiation (17) or further electromagnetic radiation (20) with which the forming fluoride layer (18) is additionally irradiated has a spectral range (64) for healing at least one crystal defect of the fluoride layer (18), which at least partially overlaps with an absorption range (65) of the at least one crystal defect, wherein the spectral range (64) preferably comprises an absorption energy (E _A ) of the crystal defect, wherein particularly preferably an average energy (E _m ) of the spectral range (64) deviates from the absorption energy (E _A ) of the crystal defect by no more than 0.5 eV, in particular by no more than 0.25 eV. Method according to one of the preceding claims, characterized in that the metallic reflection layer (12) is an aluminum layer. Method according to one of the preceding claims, characterized in that the irradiation of the native oxide layer (16) is carried out in a protective gas atmosphere, preferably in a vacuum. Process according to one of the preceding claims, characterized in that the fluorinating agent (FW) comprises at least one substance from the group: F ₂ , HF, XeF ₂ , NF ₃ , CF ₄ , SF ₆ . Method according to one of the preceding claims, characterized in that the partial pressure (p _FW ) of the fluorinating agent (FW) during the irradiation of the native oxide layer (16) is between 10 ^-8 mbar and 10 mbar, preferably between 10 ^-6 mbar and 10 ^-1 mbar. Method according to one of the preceding claims, characterized in that the partial pressure (p _FW ) of the fluorination agent (FW) is regulated to a desired value during the irradiation of the native oxide layer (16). Method according to one of the preceding claims, characterized in that the method additionally comprises the step: - depositing a further fluoride layer (19) of the fluoridic protective coating (11) on the fluoride layer (18). Reflective optical element (13) for use in the VUV wavelength range, comprising a fluoridic protective coating (11) produced by the method according to one of the preceding claims. Optical arrangement for the VUV wavelength range, in particular VUV lithography system (21) or wafer inspection system (41), comprising at least one reflective optical element (28, 46, 48) according to Claim 13 . Device (70) for producing a fluoridic protective coating (11) for protecting a metallic reflection layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising - a process chamber (71), - a supply device (72) for supplying inert gas (IG) and a fluorinating agent (FW) into the process chamber (71), the inside (82) of the process chamber (71) being resistant to the fluorinating agent (FW) and its subsequent products, and - a UV/VUV radiation source (73) for irradiating a native oxide layer (16) of the metallic reflection layer (12) with UV/VUV radiation (17) in the presence of the fluorinating agent (FW) to convert the native oxide layer (16) into a fluoride layer (18) of the fluoridic protective coating (11). Device (70) according to Claim 15 , characterized in that the device (70) additionally comprises: - at least one coating source (85) for depositing the metallic reflection layer (12) and/or a further fluoride layer (19) of the fluoridic protective coating (11).

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