Layer forming method, optical element, and optical system

Abstract

The invention relates to a method for forming at least one layer (3) on a substrate (2) made of a fluoride material, comprising the steps of depositing at least one coating material (9) on the substrate (2) to form a layer (3) and generating a plasma (12) which aids in the deposition of the coating material (9). The plasma (12) is formed from a gas mixture (14) containing a first gas (G) and a second gas (H), the second gas (H) having an ionization energy smaller than that of the first gas (G), the first gas (G) being a noble gas and the second gas (H) being a reactive gas in the form of another noble gas or a fluorine-containing gas. The invention also relates to an optical element comprising a substrate (2) made of a fluoride material, in particular a metal fluoride, which has a coating (18) comprising at least one layer (3) formed by the aforementioned method. The invention also relates to an optical system, in particular for the DUV wavelength range, which comprises at least one of the optical elements described above.

Description

[Technical field]

[0001] [Reference to Related Applications] This application claims priority from German Patent Application No. 10 2021 200 747.3, filed on January 28, 2021, the entire disclosure content of which is incorporated herein by reference.

[0002] The invention relates to a method for forming at least one layer on a substrate, comprising the steps of depositing at least one coating material on the substrate to form a layer and generating a plasma which assists the deposition of the coating material. The invention also relates to an optical element comprising a substrate, preferably made of a fluoride material, in particular a metal fluoride, and a coating with at least one layer formed by the above-mentioned method. The invention also relates to an optical system, in particular for the EUV wavelength range (wavelengths below 250 nm), comprising at least one of the above-mentioned optical elements. [Background technology]

[0003] In this application, the expression "depositing on a substrate" does not necessarily mean depositing a coating material directly on the substrate. Rather, "depositing on a substrate" can also mean depositing a coating material on one or more layers already applied to the substrate.

[0004] For optical elements subject to high stress in the DUV / VUV wavelength range below 250 nm, especially for microlithography and inspection systems in the semiconductor industry, fluorides are generally used as substrate materials, especially fluorite (CaF 2 ) and magnesium fluoride (MgF 2 Under high-intensity irradiation, about 10 6 Even after the pulse, CaF 2 The first damage appears at the surface of the material (see non-patent document 1). As a result of interaction with DUV / VUV radiation, localized fluorine vacancies occur within the volume of such optical elements, leading to the formation of Ca metal colloids, which themselves become nuclei for larger degradation. Fluorine vacancies occur even faster at the surface, and the released fluorine atoms can be emitted into the environment.

[0005] For example, CaF 2 20 mJ / cm outside the laser chamber window of an excimer laser consisting of 2 Degradation in the form of a white powdery coating was observed at laser energy densities above 1000 nm. In contrast, no damage was observed on the inside of the laser chamber window in contact with the fluorine-containing laser gas, suggesting that fluorine is an important agent in the deposition of the powdery coating and thus in avoiding degradation. To avoid fluorine loss, a very low fluorine concentration in the laser gas mixture is sufficient, which can be, for example, on the order of 0.1% to 0.2% by volume.

[0006] Sealing with dense metal fluoride layers, either sputtered (Patent Document 1, Patent Document 2), reactively evaporated (Patent Document 3), or post-treated (Patent Document 4, Patent Document 5, Patent Document 6) in a fluorine-containing atmosphere, is a promising option to increase the radiation resistance of fluoride optical components.

[0007] However, especially in the case of metal fluoride coatings, the use of equipment components resistant to fluorine gas is inconvenient and expensive, so that the sealing success is only low while the technical complexity is high. Moreover, the layers produced in this way suffer from high optical losses caused by the formation of color centers in the fluoride material by interaction with the plasma in the case of plasma ion-assisted deposition (PIAD) or by contamination with reaction products of fluorine with the components and walls of the coating equipment (Non-Patent Document 2).

[0008] Another method to improve radiation resistance is the use of Al oxide. 2 O 3 Or SiO 2 or fluorinated SiO 2for sealing optical elements (e.g. outside the laser chamber window) in (Patent Document 7, Patent Document 8, Patent Document 9, Patent Document 10, Patent Document 11, Patent Document 12, Patent Document 13, Patent Document 14), but the fluoride in this case is not densified. To avoid damage of the fluoride by high energy particles, Patent Document 15 proposes a non-densified oxide intermediate layer.

[0009] For the functioning of optical components in optical systems, the surfaces generally have to be modified (e.g. they have to be provided with a reflective or anti-reflective coating). Anti-reflective or reflective coatings are in principle carried out by application of interference layers, in which case two materials with a very high and a very low refractive index are required. Only a few oxide materials are suitable for high-power applications in the wavelength range <200 nm due to their high absorption, e.g. SiO 2 Or Al 2 O 3 Therefore, it is preferable to use alternating combinations of fluoride and oxide layers (Patent Document 16, Patent Document 17). The aim is to achieve maximum densification in both the oxide and fluoride layers to avoid spectral shifts caused by moisture absorption. This can lead to increased light losses due to damage during the deposition of the layers, which should be avoided if possible.

[0010] The packing density (hereinafter also referred to as compactness) of the coating material of an optical coating is also related to the lifetime of the optical coating. A higher compactness or a lower porosity of the coating generally leads to better chemical, mechanical, environmental and laser stability. Furthermore, the refractive index of the deposited coating material varies depending on the compactness. The compactness can be increased if the substrate is heated during coating, for example to a temperature above about 200° C. However, it is not always possible to carry out coating at high temperatures, or the effect is often insufficient.

[0011] Known layer densification methods are ion or plasma ion assisted (PIAD). See, for example, US Pat. No. 5,399,633, in which the term "activation energy per molecule (EPM)" is introduced. This term also refers to the energy input of an ion beam effective in the layer growth, normalized to a given coating rate. The energy per molecule EPM is proportional to the number of ions per unit time N i and the ion's energy E i The product of this and the number of incident coating molecules per unit time N M It is defined as the quotient of EPM=(N i E i ) / N M (1) It is.

[0012] The ion energy E is set to minimize damage caused by high-energy ions. i Therefore, the density should be increased by increasing the EPM (see Non-Patent Document 3). Therefore, the ion current density or the number of ions per unit time N i (see Non-Patent Document 4) and / or the coating speed, i.e., the number of incident coating molecules per unit time N M Yet another aspect that must be considered is that the density must be constant over a wide range of deposition angles in order for the layer to grow uniformly.

[0013] As described in US Pat. No. 5,999,136, in most plasma sources, the ion energy and the ion current density are directly linked, so that a reduction in the ion energy leads to a reduction in the ion current density, and thus a significant reduction in the effect of increasing the density. Therefore, US Pat. No. 5,999,136 proposes the use of a high-frequency plasma source described in US Pat. No. 5,999,136 for PIAD. In the high-frequency plasma source described in the above document, which has a high-frequency matching network with a primary circuit and a secondary circuit, the active ion current density and the active ion energy are independently adjustable, and the optimal combination of active ion energy and active energy per molecule can be set. However, the plasma source described in US Pat. No. 5,999,136 is complicated in terms of its manufacture. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] US Patent Application Publication No. 2005 / 0023131 [Patent Document 2] JP 2003-193231 A [Patent Document 3] Japanese Patent Application Publication No. 11-172421 [Patent Document 4] US Patent Application Publication No. 2004 / 0006249 [Patent Document 5] Japanese Patent Application Publication No. 11-140617 [Patent Document 6] JP 2004-347860 A [Patent Document 7] German Patent No. 103 50 114 [Patent Document 8] DE 10 2006 004 835 A1 [Patent Document 9] European Patent No. 1614199 [Patent Document 10] US Patent Application Publication No. 2003 / 0021015 [Patent Document 11] U.S. Patent Application Publication No. 2004 / 0202225 [Patent Document 12] U.S. Patent No. 6,466,365 [Patent Document 13] U.S. Patent No. 6,833,949 [Patent Document 14] U.S. Patent No. 6,872,479 [Patent Document 15] European Patent No. 3111257 [Patent Document 16] U.S. Patent No. 9,933,711 [Patent Document 17] U.S. Patent No. 10,642,167 [Patent Document 18] German Patent Application Publication No. 10 2005 017 742 [Patent Document 19] International Publication No. 01 / 163981 [Non-Patent Document]

[0015] [Non-Patent Document 1] U. Natura, S. Rix, M. Letz, L. Parthier, "Study of haze in 193nm high dose irradiated CaF2 crystals", Proc. SPIE 7504, Laser-Induced Damage in Optical Materials: 2009, 75041P (2009). [Non-Patent Document 2] M. Bischoff, O. Stenzel, K. Friedrich, S. Wilbrandt, D. Gabler, S. Mewes, and N. Kaiser, “Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers”, Appl.. Opt. 50 (2011) C232-C238. [Non-Patent Document 3] R. Kaneriya, RR Willey, K. Patel, “Improved Magnesium Fluoride Process by Ion-Assisted Deposition”, Proc. Soc. Vac. Coat. 53 (2010) 313-319. [Non-Patent Document 4] J. Targove et al., "Ion-assisted deposition of lanthanum fluoride thin films", Appl. Opt. 26 (1987) 3733-3737. [Non-Patent Document 5] M.J. Shaw, "Penning ionization", Contemporary Physics, 15 (1974), 445-464. [Non-Patent Document 6] J. Targove and HA Macleod, "Verification of momentum transfer as the dominant densifying mechanism in ion-assisted deposition", Appl. Opt. 27 (1988) 3779-3781. [Non-Patent Document 7] Lias, SG & Liebman, JF Ion Energetics Data. NIST Chemistry WebBook, NIST Standard Reference Database Number 69. 165-220 (2009) [Non-Patent Document 8] internet <URL:https: / / www.cup.uni-muenchen.de / ph / aks / wanner / newhome / uploads / Main / Ionisierungsenergien_Tabelle.pdf> [Non-Patent Document 9] J. Harhausen, RP Brinkmann, R. Foest, M. Hannemann, A. Ohl and B. Schroder, "On plasma ion beam formation in the Advanced Plasma Source", Plasma Sources Sci. Technol. 21 (2012) 035012. Summary of the Invention [Problem to be solved by the invention]

[0016] It is an object of the present invention to provide a simple method for forming at least one layer of an (optically active) coating, which allows the coating material to be deposited with high density, low absorption and low contamination.It is yet another object of the present invention to provide an optical element provided with a coating having at least one such layer, and an optical system provided with at least one such optical element. [Means for solving the problem]

[0017] This object is achieved in a first aspect by a method of the above type, in which a plasma is formed from a gas mixture containing a first gas and a second gas, the second gas having an ionization energy smaller than that of the first gas. The term "ionization energy" in this application is understood to mean the first ionization energy of each chemical element of each compound.

[0018] The inventors have realized that when Penning ionization or the Penning effect is utilized to form a plasma, the charge carrier density and thus the ion current density can be increased for the same ion energy (see Non-Patent Document 5). Penning ionization is a specific form of chemical ionization, i.e. the transfer of excitation energy upon particle collision. If excited atoms of one particle type G occur in a gas mixture, with excitation energy greater than the (first) ionization energy of a second particle type M, it is possible to transfer the excitation energy of G to M, such that M is ionized upon collision. G * +M→MG*→M + +e - +G (2)

[0019] Here, the intermediate does not necessarily have to dissociate, i.e. the following types of ionization are also possible: G * +M→MG*→MG + +e - (3)

[0020] Therefore, by Penning ionization or by using a suitable gas mixture, the activation ion energy E i The activation energy per molecule EPM is increased without increasing the . In this way, coating materials can be applied with high packing density or compactness without the risk of damaging the coating with high energy ions.

[0021] In one variant of the present invention, the first gas of the mixed gas is a noble gas. Noble gases generally have a relatively high ionization energy. It is therefore advantageous if the first gas in the mixed gas is a noble gas. Furthermore, generally, the plasma is formed in the plasma source using an inert gas, for example a noble gas.

[0022] In one development, the second gas of the gas mixture is a second noble gas. The noble gases He, Ne, Ar, Kr, and Xe each have a different (first) ionization energy, the ionization energy being greater for the higher atomic number, i.e. He has the highest ionization energy and Xe the lowest.

[0023] In one development of this variant, the noble gas is Ar and another noble gas is selected from the group including Kr and Xe. Plasma sources serving to generate plasma for plasma ion assist often operate with Ar as plasma gas. Therefore, depending on the type of plasma source, it may be advantageous if the noble gas is Ar, since the plasma source may be designed or optimized for operation with Ar.

[0024] In an alternative development of this variant, the noble gas is Kr and the other noble gas is Xe. According to Non-Patent Document 6, the densification of the deposition of coating materials is mainly influenced by the transfer of momentum and not by the ionization energy. Thus, the use of heavier noble gases provides a higher effect in terms of densification than the use of lighter noble gas ions (Kr>Ar>Ne). Therefore, for densification, it may be advantageous if the mixed gas contains heavy noble gases or noble gas ions.

[0025] In yet another alternative development, the noble gas is Ne and the other noble gas is selected from the group including Ar, Kr and Xe. The noble gas neon has a relatively large first ionization energy compared to Ar, Kr and Xr, which is favorable for Penning ionization since a relatively large energy transfer is possible. By mixing with a heavy noble gas such as Kr or Xe, a relatively large momentum transfer to the coating material and thus a high densification of the coating material can be achieved even with the relatively light noble gas neon.

[0026] In yet another variant, the second gas is a reactive gas. A mixture of a noble gas and a reactive gas having a smaller ionization energy than the noble gas may also be advantageous for supporting the deposition of the coating material. The reactive gas may be, for example, a fluorine-containing gas, but other reactive gases, such as oxygen or ozone, may also be used.

[0027] The mixed gas is, for example, krypton (ionization energy 14 eV) and NF 3 (ionization energy 13 eV) or xenon (ionization energy 12.1 eV) and tetrafluoroethylene C 2 F 4 (ionization energy 10.1 eV). Other gas mixtures containing a noble gas as the first gas and a reactive gas as the second gas are in principle possible, provided that the ionization energy of the reactive gas is smaller than that of the noble gas. The ionization energies of many chemical elements or compounds can be found, for example, in Non-Patent Document 7 or Non-Patent Document 8.

[0028] In a gas mixture consisting of a first gas and a second gas, the second gas is typically present in a proportion of less than 10% by volume, less than 1% by volume, or less than 0.1% by volume. It will be apparent that in principle there may be more than two gases present in the gas mixture, in particular more than two noble gases.

[0029] In yet another variation, the gas mixture includes or a third gas is added to the gas mixture. The third gas may be O 2 , N2 The third gas, e.g., a fluorine-containing gas, may serve to deposit the coating material in a fluoride atmosphere. The fluorine-containing gas may, for example, be F. 2 , C.F. 4 ,SCIENCE FICTION 6 , xenon fluorides, e.g. XeF 2 , XeF 4 , XeF 6 , N.F. 3 , H.F., B.F. 3 , C.H. 3 F, C 2 F 4 The gas may be selected from the group including:

[0030] In one development, the third gas is added to the gas mixture in a proportion of less than 2% by volume, preferably less than 1% by volume, more preferably less than 0.01% by volume, even more preferably less than 0.001% by volume, in particular less than 0.001% by volume. Generally, it is advantageous if only a small proportion of the third gas is added to the gas mixture.

[0031] The plasma is generated in a plasma source. Typically, only one gas, for example a noble gas, is fed into a conventional plasma source to generate the plasma. An ionization volume is usually formed in the plasma source, in which the fed gas is ionized to form the plasma. There are various methods for generating the deposition-supporting plasma from the aforementioned gas mixture.

[0032] In one variant, the first gas and the second gas and / or the mixed gas are introduced into the plasma source for generating plasma via at least one gas inlet. After forming the mixed gas, it can be fed into the plasma source or into the ionization space via the gas inlet. If the plasma source has two or more gas inlets, the first gas is fed into the first gas inlet and the second gas into the second gas inlet. In that case, the mixed gas is formed only in the plasma source. A plasma source with two gas inlets for gas feeding is described, for example, in Non-Patent Document 9.

[0033] In an alternative variation, the mixed gas is formed by introducing a first gas via a gas inlet into a plasma source that generates the plasma and a second gas into a vacuum chamber in which the substrate is located, or vice versa, i.e., by introducing a second gas via a gas inlet into the plasma source and introducing the first gas into the vacuum chamber.

[0034] In that case, the mixed gas is formed only outside the plasma source in the vacuum chamber where the coating with the coating material takes place. It is advantageous if the first / second gas is supplied to the vacuum chamber close to the plasma source, more particularly close to the outlet opening of the plasma source. This purpose can be achieved by a gas shower arranged directly adjacent to the outlet opening of the plasma source, as described in [9]. This supply of the second gas outside the plasma source is advantageous, especially when the second gas is a reactive gas, since the inflow or supply of the second gas to the plasma source can lead to deterioration of the cathode or other components of the plasma source.

[0035] In yet another variation, the coating rate or deposition rate of the deposition of the coating material is 10 -10 m / s. As previously discussed in relation to equation (1), the coating speed or the number of incident coating molecules per unit time N M It is possible to increase the energy per molecule EPM by decreasing the deposition angle. However, the coating speed should not be selected too low, since this would introduce residual gases present in the vacuum chamber into the deposited coating material. A low coating speed can also produce an essentially constant compactness during deposition, i.e. a compactness that depends only slightly, if at all, on the deposition angle.

[0036] In yet another embodiment, the active ion energy of the ions present in the plasma is less than 100 eV, preferably between 45 eV and 100 eV. For a definition of the active ion energy, see Patent Document 18, which is incorporated herein by reference in its entirety. It has been found that active ion energies within the above order are favorable for the deposition of dense layers, as is evident, for example, from FIG. 1 of Non-Patent Document 3. When ion energies above 100 eV are used for deposition, there may generally be an increase in damage of the coating by high-energy ions. If the active ion energy is too small, the density decreases.

[0037] In yet another variant, the substrate is selected from the group comprising fluoride materials, preferably metal fluorides, in particular alkaline earth metal fluorides. As mentioned above, for applications in the DUV wavelength range of 250 nm to 200 nm wavelengths, substrates used for transmissive optical elements are often made of metal fluorides, in particular alkaline earth metal fluorides, such as fluorite (CaF 2 ) or magnesium fluoride (MgF 2 ). The above method can in principle also be applied to other substrates, e.g. quartz glass (SiO 2 It will be apparent that the present invention can also be applied to substrates such as

[0038] The coating material can be deposited, for example, using a PVD (physical vapor deposition) method. For example, the coating material can be deposited by thermal evaporation, in particular by electron beam evaporation. Other methods of depositing the coating material can also be used, for example sputtering. Any deposition method in which plasma ion assistance is possible or feasible can be used in the process described here. However, the temperature selected for deposition should not be too high, generally not exceeding about 200°C.

[0039] The coating material may be, for example, an oxide material, such as SiO 2 , or fluoride materials, such as MgF 2It will be apparent that other materials may be used to form the layers of the coating.

[0040] Yet another aspect of the invention relates to an optical element comprising a substrate, preferably made of a fluoride material, in particular a metal fluoride, carrying a coating comprising at least one layer formed by the method described above. The material of the substrate is, for example, quartz glass (SiO 2 When the substrate is made of a fluoride material, the material may be, for example, CaF 2 or MgF 2 The substrate may take the form of a planar sheet, but may also be a curved substrate, and correspondingly, the surface of the substrate onto which the layer or coating is applied may also be planar or curved.

[0041] The coating may include one or more layers, which may contain, for example, oxide or fluoride materials. It is not necessary that all layers of the coating be applied to the substrate using plasma ion assistance according to the aforementioned method. Instead, as described in the above-mentioned US Pat. No. 5,999,363, individual layers may not be densified. However, in general, to achieve maximum densification, it is advantageous if all layers of the coating are applied using the aforementioned method.

[0042] The coating may take the form of, for example, a stack of interference layers and may include two layers with a very high refractive index and a very low refractive index. The coating may serve, for example, to increase or decrease the reflection of the optical element. Here, it is preferred to use alternating combinations of fluoride and oxide layers, as described, for example, in US Pat. No. 5,399,623 or US Pat. No. 5,499,623. In addition to oxide or fluoride materials, there are other useful coating materials, for example oxyfluorides.

[0043] Yet another aspect of the invention relates to an optical system, in particular for the DUV wavelength range, i.e. for wavelengths below 250 nm, comprising at least one optical element formed as described above. The optical system can be, for example, a DUV lithography system or an inspection system, for example for inspection of masks or wafers, but can also be a laser, for example an excimer laser.

[0044] 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 show the essential details of the invention, and from the claims. Each of the individual features can be implemented alone or in any combination of several in a variant of the invention.

[0045] Examples are shown in the schematic drawings and explained in the following description. [Brief description of the drawings]

[0046] [Figure 1] FIG. 1 is a schematic diagram of a coating apparatus for coating a substrate. [Diagram 2] FIG. 1 is a schematic diagram of first ionization energies of several chemical elements as a function of atomic number. [Diagram 3] FIG. 1 is a schematic diagram of compactness as a function of ion energy, coating speed, and deposition angle. [Figure 4] 2 is a schematic diagram of an example optical element having a coating applied with the coating apparatus of FIG. 1. [Diagram 5] FIG. 5 is a schematic diagram of an example of a DUV lithography system including the optical element of FIG. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] In the following description of the drawings, the same reference numbers are used for components that are the same or have the same function.

[0048] FIG. 1 shows a schematic view of a coating apparatus 1 for forming or depositing a layer, in the illustrated example layer 3, on a substrate 2. The coating apparatus 1 comprises a high vacuum airtight vessel 4 which encloses a vacuum chamber 5. A turbomolecular pump 6 serves to evacuate the vacuum chamber 5. Inside the vacuum chamber 5 a holding device 7 is arranged on which a number of substrates can be mounted, of which only one substrate 2 is shown in FIG. 1. The holding device 7 is formed like a planetary system and has a main axis which rotates the substrates during coating, as indicated by an arrow in FIG. 1. During coating, a particular substrate 2 further rotates around its axis of symmetry, also indicated by an arrow in FIG. 1. The substrate 2 in the illustrated example is a plane-parallel plate to which an anti-reflective coating is applied. The substrate 2 in the illustrated example is made of calcium fluoride (CaF 2 ), but may also be made of a different material. The material of the substrate 2 may for example be another fluoride material, for example a metal fluoride, in particular an alkaline earth metal fluoride.

[0049] Arranged within the vacuum chamber 5 is an electron beam evaporator 8, into which a coating material 9 is introduced. During deposition, the electron beam evaporator 8 generates an evaporation cone 10, within which the substrate 2 to be coated is placed, so that the coating material 9 can be deposited on the surface of the substrate 2 to be coated facing the electron beam evaporator 8. The coating material 9 in the illustrated example is SiO 2 However, it could also be another material, for example an oxide or fluoride material or an oxyfluoride.

[0050] Also arranged in the vacuum chamber 5 is a plasma source 11 serving to generate a plasma 12, which is directed in the form of a plasma jet towards the surface of the substrate 2 to be coated. As described in detail in [9], the plasma source 11 is a DC plasma source. It has a rod-shaped cathode 13a surrounded by a ring-shaped anode 13b. Between the cathode 13a and the anode 13b, an electric field for the generation of the plasma 12 is generated in order to ionize a gas, more particularly a mixed gas 14, in the ionization space of the plasma source 11 and thus generate the plasma 12. A coil (not shown) surrounds the outside of the anode 13b and generates an axial magnetic field in the ionization space.

[0051] 1, the gas mixture 14 is supplied to the plasma source 11 via a gas inlet 15, which in the illustrated example is located at the bottom of the ionization volume of the plasma source 11. It will be apparent that the gas inlet 15 may be positioned elsewhere in the plasma source 11 to supply the gas mixture 14 to the plasma source 11.

[0052] As can also be seen in Fig. 1, the gas mixture 14 is formed in a mixing unit 16, which is also arranged outside the vacuum chamber 5. The mixing unit 16 has a valve arrangement for mixing a first gas G in a first gas reservoir with a second gas H in a second gas reservoir. Using the valve arrangement, it is possible in some cases to adjust the mixing ratio of the two gases G, H, but this is not necessary. For the applications described here, it is generally sufficient if the second gas H is present in the gas mixture 14 in a proportion of less than 10% by volume, less than 1% by volume, or less than 0.1% by volume.

[0053] The gas mixture 14 is fed to the plasma source 11 in order to generate a chemical ionization, i.e. a transfer of excitation energy upon particle collision between molecules or atoms of a first gas G and molecules or atoms of a second gas H. In the following discussion, the second gas H of the gas mixture 14 has a (first) ionization energy that is smaller than the (first) ionization energy of the first gas G. In this case, for example Penning ionization can be performed according to the above formulas (2) and (3), i.e. upon collision the excitation energy of the first gas G can be transferred to the second gas M such that the second gas M is ionized, or the two gases G, M can be ionized, for example GM + These may together form ionized species of the form:

[0054] In the example shown in Fig. 1, a third gas K is added to the mixed gas 14 containing two gases G and H. The third gas K can be added in the mixing unit 16, but it is also possible to supply the third gas K to the mixed gas 14 via a separate gas inlet 15a of the plasma source 11, as shown in Fig. 1. The separate gas inlet 15a is a so-called gas shower, which brings the third gas K close to the exit opening of the plasma source 11 before it is discharged. Thus, the mixed gas 14 of the two gases G and H is formed at the exit of the plasma source 11 or at the exit of the ionization space.

[0055] The third gas in the illustrated example is a fluoride gas, but may be another type of gas, e.g., O 2 Or N 2 In this case, the proportion of the third gas K in the mixed gas 14 is less than 0.001% by volume. The proportion of the third gas K may alternatively be greater, for example less than 1% by volume or less than 2% by volume.

[0056] Fluoride gases include, for example, F 2 , C.F. 4 ,SCIENCE FICTION 6 , xenon fluorides, e.g. XeF 2 , XeF 4 , XeF 6 , N.F. 3 , H.F., B.F. 3 , C.H. 3 F, C 2 F4 The third fluoride gas K in this case serves to generate a fluorine-containing atmosphere in the vacuum chamber 5. Fluorinated SiO 2 For deposition of the coating material 9, it is also possible to fluorinate the coating material 9 using a fluoride gas K for deposition on the substrate 2. It will be apparent that a third fluoride gas K can also serve for the fluorination of other coating materials 9.

[0057] As an alternative to supplying the mixed gas 14 via the (first) gas inlet 15, it is possible to supply the first gas G via the first gas inlet 15 and the second gas H via a second gas inlet (not shown) to the plasma source 11 or to the ionization space. In that case, the mixed gas 14 is only formed in the plasma source 11, more specifically in the ionization space.

[0058] Alternatively, the first gas G can be introduced into the plasma source 11 or into the ionization space, for example via the first gas inlet 15, and the second gas H is introduced into the vacuum chamber 5 outside the plasma source 11. In that case, for example, the second gas H can be supplied near the exit opening of the plasma source 11 via another gas inlet 15a forming a gas shower, as described above in connection with the third gas K. In principle, alternatively, the second gas H may be introduced elsewhere in the vacuum chamber 5 to form the gas mixture 14. The roles of the first gas G and the second gas H can also be exchanged.

[0059] The supply of the second gas H via a separate gas inlet 15a is particularly advantageous when the first gas G is a noble gas and the second gas H is a reactive gas, e.g. oxygen, ozone or a fluorine-containing gas. The supply of a reactive gas to the ionization space of the plasma source 11 may result in damage to components arranged therein. The supply of the second gas H via a separate gas inlet 15a, which may for example be in the form of a ring and have radial inlet openings, makes it possible to avoid such damage. The mixed gas 14 may for example be krypton (ionization energy 14 eV) and NF 3(ionization energy 13 eV) or xenon (ionization energy 12.1 eV) and tetrafluoroethylene C 2 F 4 (ionization energy 10.1 eV), but other types of mixed gases 14 can also be used.

[0060] The first gas G and the second gas N may be a noble gas, and FIG. 2 shows its (first) ionization energy E + (eV) as a function of atomic number N. + The order for is (from highest to lowest ionization energy): He, Ne, Ar, Kr, Xe.

[0061] There are various options for the two noble gases G, H of the gas mixture 14. For example, the first gas G can be Ar and the second gas H can be Kr or Xe. Since the plasma source 11 shown in Fig. 1 is designed for operation with Ar, it is advantageous to use a gas mixture 14 containing Ar. In particular, a gas mixture of Ar and Kr has been found to be advantageous.

[0062] Alternatively, the first gas G of the gas mixture 14 can be Ne and the second gas H of the gas mixture 14 can be selected from the group including Ar, Kr, and Xe. When the second gas H is Ar, the result is a typical example of Penning ionization (E + (Ne)=16.5 eV, E + (Ar)=15.8 eV, i.e., E + (Ne)>E + (Ar)): Ne * +Ar → Ar + +e - +Ne (4)

[0063] Alternatively, the gas mixture 14 may contain krypton as the first noble gas G and xenon as the second noble gas H. The density of the layer 3 depends on the ion H present in the plasma 12, rather than on the ion energy. +This is advantageous because the densification is primarily affected by momentum transfer of the gas mixture 14. Thus, in general, heavier noble gases in the gas mixture 14 will result in greater densification than lighter noble gases.

[0064] As is evident from FIG. 3, the compactness D (arbitrary units) also varies as a function of other parameters, such as the ion energy E, the deposition or coating speed R, and the deposition angle β (see FIG. 1). The four diagrams shown in FIG. 3 have different ionization energies E, which increase from left to right (E1 to E4). The diagrams shown in FIG. 3 respectively show the compactness at four coating speeds R1 to R4, which increase from left to right. At each coating speed R1 to R4, it is shown that the compactness depends on the deposition angle β. As is evident from FIG. 3, the compactness increases with the minimum deposition angle β in most diagrams. min From the maximum deposition angle β max increases to.

[0065] The coating rate R of the deposition of the coating material 9 is 10 -10 It has been found to be advantageous if the coating speed R is less than m / s. This is advantageous, firstly, because, as is evident from equation (1), the smaller the coating speed R, the higher the energy per molecule EPM. This is likewise advantageous, since at low coating speeds R the compactness is essentially constant or independent of the deposition angle. However, the selected coating speed R should not be too small, in order to avoid introducing residual gases present in the vacuum chamber 5 into the deposited coating material 9.

[0066] For example, as is clear from FIG. 1 of Non-Patent Document 3, it is favorable for the density if the active ion energy E is less than about 100 eV. Ions H present in the plasma 12 + A convenient value for the activation ion energy E is between about 60 eV and about 100 eV.

[0067] Figure 4 shows an optical element 17 in which all layers 3 of coating 18 have been applied as previously described in the coating apparatus 1 of Figure 1. The design of the example optical element 17 shown in Figure 4 is as described in US Patent No. 5,399,623 or US Patent No. 5,399,623, cited above. However, it will be apparent that the design of the optical element 17 or coating 18 may be different.

[0068] The optical element 17 is CaF 2 The coating 18 further comprises a substrate 2 consisting of a fluorinated compound having a low refractive index. A first layer 3a consisting of a fluorinated compound having a low refractive index is applied to the substrate 2. The first layer 3a can be applied directly to the substrate 2, but it is also possible that an adhesion-promoting layer or another type of functional layer is applied between the substrate 2 and the first layer 3a. The coating 18 further comprises a layer system 19 arranged between the first layer 3a and a final layer 3d of the coating 18. The final layer 3d in the illustrated example is an oxidic compound.

[0069] The example layer system 19 shown in Fig. 4 comprises two pairs of alternating layers 3b, 3c, each of which comprises a fluoride compound and an oxide compound. The layer 3b adjacent to the first layer 3a of the layer system 19 comprises an oxide compound. The layer 3c applied to the layer 3b comprises a fluoride compound. The entire coating 18 therefore consists of an alternating sequence of three fluoride and oxide layers 3a-3d. The material of the oxide layers 3b, 3b is e.g. SiO 2 The material of the fluoride layers 3a and 3c can be, for example, AlF 3 , MgF 2 4, coating 18 may also have an odd number of layers, in which case the first and last layers may be, for example, oxide layers, thereby maintaining the alternating sequence of oxide and fluoride layers in coating 18.

[0070] The coating 18 shown in Figure 4 acts as an anti-reflective coating to avoid reflection of DUV radiation at the surface of the substrate 2 to which the coating 18 is applied. It will be apparent that the substrate 2 may also have a corresponding coating 18 on the opposite side. The substrate 2 may be made of materials other than fluoride materials, such as quartz glass (SiO 2 ) can also be formed.

[0071] 5 shows a schematic diagram of an optical system 21 in the form of a DUV lithography apparatus for wavelengths below 250 nm, in particular in the wavelength range of 100 nm to 200 nm or 190 nm. The DUV lithography apparatus 21 has as essential components two optical systems in the form of an illumination system 22 and a projection system 23. For the implementation of an exposure process, the DUV lithography apparatus 21 has a radiation source 24, which may be for example an excimer laser emitting radiation 25 at a wavelength in the DUV wavelength range, for example 193 nm, 157 nm or 126 nm, and which may be an integral part of the DUV lithography apparatus 21.

[0072] The radiation 25 emitted by the radiation source 24 is conditioned by means of an illumination system 22 so that it can illuminate a mask 26, also called a reticle. In the example shown in Figure 5, the exposure system 22 comprises both transmissive and reflective optical elements. Typically, Figure 5 shows, for example, a transmissive optical element 27 that focuses the radiation 25 and a reflective optical element 28 that deflects the radiation 25. As is known, a wide variety of transmissive, reflective or other optical elements can be combined with each other in any and even more complex ways in the illumination system 22.

[0073] The mask 26 has on its surface a structure that is to be transferred by means of the projection system 23 onto an optical element 29 to be exposed, for example a wafer, in the context of the production of semiconductor components. In the illustrated example, the mask 26 is designed as a transmissive optical element. In an alternative implementation, the mask 26 can also be designed as a reflective optical element. The projection system 22 has at least one transmissive optical element in the illustrated example. The illustrated example shows two transmissive optical elements 30, 31 which typically serve, for example, to reduce the structures on the mask 26 to a size desired for the exposure of the wafer 29. The projection system 23 can also in particular be provided with reflective optical elements, any optical elements being able to be combined with one another in a known manner as desired. It should be noted that optical arrangements without transmissive optical elements can also be used for DUV lithography.

[0074] The optical element 17 shown in FIG. 4 can be, for example, a transmissive optical element 27 of an exposure system 22 of a DUV lithography system 21, or a mask 26, or a transmissive optical element 30, 31 of a projection system 23.

[0075] As an alternative to the optical system 21 shown in Fig. 5 in the form of a DUV lithography system, another optical system, particularly for the DUV wavelength range, can also include at least one optical element 17 shown in Fig. 4. The optical system 21 can be, for example, a wafer inspection system or a mask inspection system. The excimer laser 24 can also include an optical element 17, which includes a substrate 2 with a coating 18 applied thereto as described above. The optical element 17 can be, for example, an exit window of a laser chamber.

[0076] In summary, as described above, layers 3, 3a-3d or coatings 18 can be deposited on the substrate 2 of the optical element 17, which ensures high compactness combined with a long lifetime or high radiation resistance of the optical element 17 even in the case of irradiation with high radiation intensities.

Claims

1. A method for forming at least one layer (3) on a substrate (2) made of a fluoride material, comprising the steps of: Depositing at least one coating material (9) on said substrate (2) to form said layer (3); generating a plasma (12) supporting the deposition of the coating material (9), the plasma (12) being directed in the form of a plasma jet towards the surface of the substrate (2) to be coated, the plasma (12) being formed from a gas mixture (14) containing a first gas (G) and a second gas (H), the second gas (H) having an ionization energy smaller than the ionization energy of the first gas (G), the first gas (G) being an inert gas and the second gas (H) being another inert gas; The method includes:

2. 2. The method of claim 1, wherein the second gas (H) of the mixed gas consisting of the first gas (G) and the second gas (H) is present in a proportion of less than 10% by volume.

3. 3. The method of claim 1, wherein the noble gas is Ar and the other noble gas is selected from the group consisting of Kr and Xe.

4. 3. The method of claim 1, wherein the noble gas is Kr and the other noble gas is Xe.

5. 3. The method of claim 1, wherein the noble gas is Ne and the other noble gas is selected from the group consisting of Ar, Kr, and Xe.

6. The method according to any one of claims 1 to 5, wherein the mixed gas consists of the first gas (G) and the second gas (H).

7. The method according to any one of the preceding claims, wherein a third gas (K) is added to the gas mixture (14).

8. 8. The method of claim 7, wherein the third gas (K) is O ２ , N ２ , O ３ , N ２ O, H ２ O ２ and a fluorine-containing gas.

9. 9. The method according to claim 7 or 8, wherein the third gas (K) is added to the gas mixture (14) in a proportion of less than 2% by volume.

10. The method according to any one of claims 1 to 9, wherein the first gas (G) and the second gas (H) and / or the mixed gas (14) are introduced via at least one gas inlet (15) into a plasma source (11) which generates the plasma (12).

11. The method according to any one of claims 1 to 10, wherein the mixed gas (14) is formed by introducing the first gas (G) via a gas inlet (15) into a plasma source (11) which generates the plasma (12) and introducing the second gas (H) into a vacuum chamber (5) in which the substrate (2) is placed, or the mixed gas (14) is formed by introducing the second gas (H) via a gas inlet (15) into a plasma source (11) which generates the plasma (12) and introducing the first gas (G) into a vacuum chamber (5) in which the substrate (2) is placed.

12. The method according to any one of claims 1 to 11, wherein the coating rate (R) during deposition of the coating material (9) is 10 －１０ The method is less than m / s.

13. The method according to any one of claims 1 to 12, characterized in that ions (H ＋ ) of the active ion energy (E ｉ ) is less than 100 eV.

14. The method according to any one of the preceding claims, wherein the substrate (2) is a metal fluoride.

15. The method according to claim 14, wherein the substrate (2) is an alkaline earth metal fluoride.

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