EP4150644A1

EP4150644A1 - Optical element, euv lithography system, and method for forming nanoparticles

Info

Publication number: EP4150644A1
Application number: EP21722416.1A
Authority: EP
Inventors: Anastasia Gonchar
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2020-05-14
Filing date: 2021-04-27
Publication date: 2023-03-22
Also published as: WO2021228545A1; DE102020206117A1; US20230076667A1

Abstract

The invention relates to an optical element (1) comprising: a substrate (2); a multi-layer system (3) which is applied to the substrate (2) and reflects EUV radiation (4); and a protective layer system (5) which is applied to the multi-layer system (3) and comprises an uppermost layer (5a). Nanoparticles (7) are embedded in the material of the uppermost layer (5a) of the protective layer system (5), which nanoparticles preferably contain at least one metal material. The invention also relates to an EUV lithography installation which comprises at least one optical element (1) designed as described above, and to a method for forming nanoparticles (7) in the uppermost layer (5a) of the protective layer system (5).

Description

Optical element, EUV lithography system and method for forming nanoparticles

Reference to related application

This application claims the priority of German patent application DE 102020206117.3 of May 14, 2020, the entire disclosure content of which is incorporated into the content of this application by reference.

Background of the invention

The invention relates to an optical element, comprising: a substrate, a multilayer system which is applied to the substrate and reflects E UV radiation, and a protective layer system which has an uppermost layer and which is applied to the multilayer system. The invention also relates to an EUV lithography system which has at least one such optical element. The invention also relates to a method for forming nanoparticles which are embedded in the uppermost layer of the protective layer system of the optical element.

For the purposes of this application, an EUV lithography system is understood to mean an optical system or an optical arrangement for EUV lithography, ie an optical system which can be used in the field of EUV lithography. In addition to an EUV lithography system, which is used to manufacture semiconductor components, the optical system can be, for example, an inspection system for inspecting a photomask (also called reticle in the following) used in an EUV lithography system, for inspecting a semiconductor substrate to be structured (also called wafer in the following) or a metrology system which is used to measure an EUV lithography system or parts thereof, for example to measure a projection system.

EUV radiation is understood to mean radiation in a wavelength range between approx. 5 nm and approx. 30 nm, for example at 13.5 nm. Since EUV radiation is strongly absorbed by most known materials, the EUV radiation is typically guided through the EUV lithography system with the aid of reflective optical elements.

The layers of a reflective multilayer system in the form of a coating of a reflective optical element (EUV mirror) are exposed to harsh conditions during operation in an EUV lithography system, in particular in an EUV lithography system has high radiation output. The EUV radiation also causes some of the EUV mirrors to heat up to high temperatures of possibly several 100 ° C. The residual gases in a vacuum environment, in which the EUV mirrors are usually operated, can also affect the layers of the reflective multilayer system in the form of the coating, especially if these gases are converted into reactive species such as ions or by the action of EUV radiation Radicals are converted. Ventilation of the vacuum environment during a break in operation and unintentional leaks can also damage the layers of the reflective multilayer system. In addition, the layers of the reflective multilayer system can be contaminated or damaged by hydrocarbons produced during operation, by volatile hydrides, by tin drops or tin ions, by cleaning media, etc.

To protect the layers of the reflective multilayer system of the optical element, a protective layer system which is applied to the multilayer system and which itself can have one or more layers is used. the Layers of the protective layer system can fulfill different functions in order to avoid typical damage patterns, for example the formation of bubbles or the detachment of layers (delamination), in particular due to a plasma present in the residual gas atmosphere, which in addition to reactive hydrogen also contains other gas components, e.g. reactive oxygen, water, Contains nitrogen, noble gases and hydrocarbons. The protective layer system can also protect the multilayer system from the effects of EUV radiation or from thermal influences. Surface processes, for example oxidation / reduction cycles, take place on the surface of the top layer of the protective layer system and not on the multilayer system.

WO 2014/139694 A1 describes an optical element in which the protective layer system has at least a first and a second layer, the first layer being arranged closer to the multilayer system than the second layer. The first layer can have a lower solubility for hydrogen than the second layer. The protective layer system can have a third, uppermost layer which is formed from a material which has a high recombination rate for hydrogen. The first layer, the second layer and / or the third layer can be formed from a metal or from a metal oxide. The material of the third, uppermost layer can be selected from the group comprising: Mo, Ru, Cu, Ni, Fe, Pd, V, Nb and their oxides.

EP 1 065568 B1 describes a lithographic projection device which has a reflector with a multilayer reflective coating and with a cover layer. The cover layer can have a thickness between 0.5 nm and 10 nm. The cover layer can have two or three layers made of different materials. The top layer can consist of Ru or Rh, the second layer of B4C, BN, diamond-like carbon, S13N4 or SiC. The material of the third layer corresponds to the material of a layer of the multilayer reflective coating, for example it can be Mo. A reflective optical element with a protective layer system that comprises two layers is known from EP 1 402542 B1. The protective layer system described there has a top layer made of a material which resists oxidation and corrosion, for example Ru, Zr, Rh, Pd. The second layer serves as a barrier layer, which consists of B4C or Mo and which is intended to prevent the material of the top layer of the protective layer system from diffusing into the top layer of the multilayer system that reflects the EUV radiation.

From EP 1 364231 B1 and US Pat. No. 6,664,554 B2 it is known to provide a self-cleaning optical element in an EUV lithography system which has a catalytic cover layer made of Ru or Rh, Pd, Ir, Pt, Au to protect a reflective coating from oxidation having. A metallic layer made of Cr, Mo or Ti can be introduced between the cover layer and the surface of the mirror.

A method and a device have become known from EP 1 522895 B1 in which at least one mirror is provided with a dynamic protective layer in order to protect the mirror from being etched with ions. The method includes supplying a gaseous substance (as needed) into a chamber containing the at least one mirror. The gas is typically a gaseous hydrocarbon (CXHY). The protective effect of the carbon layer deposited in this way is limited, however, and the supply and monitoring of the mirror are complex.

Further protective layer systems, which are or can be formed from several layers, are described in JP2006080478 A and JP4352977 B2. An optical element which is designed as described above is also known from WO 2013/124224 A1. The optical element has a protective layer system with an uppermost layer and with at least one further layer below the uppermost layer, the thickness of which is greater than the thickness of the uppermost layer. The material of the top layer is selected from the group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides.

DE 102019212910.2 describes an optical element which has a protective layer system with a first layer, a second layer and a third, in particular topmost, layer. Metallic particles and / or ions can be implanted into at least one layer of the protective layer system. The ions can be metal ions, e.g. noble metal ions, in particular platinum metal ions, or noble gas ions. The implanted ions are intended to prevent Sn ions from being implanted in the material of the respective layer, which arise when the EUV radiation is generated in an EUV radiation source. The noble metal ions can also serve as hydrogen and / or oxygen blockers. At least one layer of the protective layer system can be doped with metallic (nano) particles, e.g. with (foreign) atoms in the form of noble metal particles.

Object of the invention

The object of the invention is to provide an optical element and an EUV lithography system in which damage to the protective layer system is prevented or at least slowed down so that the service life of the optical element is increased.

Subject of the invention This object is achieved by an optical element of the type mentioned at the outset, in which nanoparticles are embedded in the material of the top layer of the protective layer system, which nanoparticles preferably contain at least one metallic material. The formation of the embedded nanoparticles is typically induced by ion implantation. The material of the embedded nanoparticles does not necessarily match the material of the ions with which the top layer is irradiated to form the nanoparticles. The nanoparticles are also not particles introduced into the material of the top layer by doping. It has been shown that the top layer of the protective layer system or the entire protective layer system can be stabilized with respect to the damage factors described above through the embedded nanoparticles.

In one embodiment, the nanoparticles contain at least one material that does not match the material of the top layer surrounding the nanoparticles. In this case, the embedded nanoparticles consist of the material of the ions used in the ion implantation or the nanoparticles contain the material from which the ions used in the irradiation are formed. This type of formation of embedded nanoparticles in the form of gold particles or nanoclusters embedded in yttrium-doped zirconium dioxide is described in the article “X-Ray Photoelectron Spectroscopy of Stabilized Zirconia Films with Embedded Au Nanoparticles Formed under Irradiation with Gold Ions ", S. Yu. Zubkov et al. , Physics of the Solid State 2018, Vol. 60, No. 3, pp. 598-602.

In a further embodiment, the nanoparticles contain at least one material that is contained in the material of the top layer surrounding the nanoparticles. In this case, the ion implantation can induce, for example, a reduction in the (basic) material of the top layer or a typically metallic component of the material of the top layer. Examples of embedded nanoparticles generated in this way, are for example in the article Jon Implantation-induced Nanoscale Particle Formation in AI2O3 and S1O2 via Reduction “, EM Hunt et al. , Acta mater., Vol. 47, no. 5, pages 1497-1511, 1999.

In the article by EM Hunt it is stated, for example, that by implantation of ions selected according to thermodynamic laws, e.g. Y ⁺ , Ca ⁺ , Mg ⁺ or Zr ⁺ , monocrystalline aluminum oxide (Al2O3) to Al or quartz glass (S1O2) Si can be reduced. The Al or Si formed during the reduction can subsequently form clusters and react with other elements to form nano-dimensional deposits. When Y ⁺ or Ca ^{+ is} implanted in Al2O3, Al nanoparticles with mean diameters of 12.5 nm and 8.0 nm are formed. When Mg ^{+ is} implanted in Al2O3, MgAl ₂ _{O 4} platelets are formed. The implantation of Zr ⁺ in quartz glass leads to the formation of ZrSh particles with a size between approx. 1 nm and approx. 17 nm.

In a further embodiment, the nanoparticles have mean particle sizes between 0.5 nm and 2 nm. The mean particle size of the nanoparticles can, for example, be based on the values described in the above-cited article by S.

Yu. Zubkov et al. described manner, i.e. by recording photoelectron spectra, see section 3.2 "Determination of the Average Diameter of Gold Clusters in the YSZ Matrix". It goes without saying that the mean particle size must not be greater than the thickness of the top layer of the protective layer system. The mean particle size and the material of the nanoparticles can optionally be determined as a function of a function of the nanoparticles that goes beyond the stabilization of the first layer, as will be described below.

In a further embodiment, the nanoparticles reduce the reflectivity of the top layer for radiation at longer wavelengths than EUV radiation, in particular for radiation in the VUV wavelength range or in the DUV wavelength range. The embedded nanoparticles can reduce the reflectivity of the optical element for radiation with wavelengths outside the EUV wavelength range compared to an identically constructed optical element without nanoparticles embedded in the top layer. In the case of radiation outside the EUV wavelength range, it can in particular be radiation in the VUV wavelength range, i.e. at wavelengths between 100 nm and 200 nm (VUV wavelength range according to DIN 5031 Part 7) or in the DUV wavelength range in an interval between 100 nm and act 300 nm. The absorption of radiation, especially in the DUV / VUV wavelength range, is favorable, since radiation in this wavelength range is usually generated by the EUV radiation source in addition to the EUV radiation and its propagation through the EUV lithography system is undesirable.

The reflectivity of the optical element for the radiation outside the EUV wavelength range is typically generated by an increased absorption of the first layer for radiation in this wavelength range. The absorption of the top layer or the embedded nanoparticles for radiation outside the EUV wavelength range depends not only on the material of the nanoparticles, but also on other parameters, for example on the (mean) particle size of the nanoparticles.

As in the article "Enhanced light absorption of T1O2 in the near-ultraviolet band by Au nanoparticles", Shu-Ya Du et al. , Optics Leiters, Vol. 35, No. October 20, 2010, is described, the arrangement of Au nanoparticles next to Ti0 ₂ nanoparticles (in rutile phase), the absorption of the Ti0 ₂ nanoparticles for radiation in the near UV wavelength range can be increased. Correspondingly, the implantation of Ag ions in T1O2 can also increase the absorption at wavelengths in the UVA / is wavelength range, as described, for example, in the article "Applications of Ion Implantation for Modification of T1O2: A review", AL Stepanov, Rev. Adv. Mater. May be. 30 (2012), 150-165. All four articles cited above are incorporated by reference in their entirety into the content of this application.

In a further embodiment, the material of the nanoparticles is selected from the group comprising: Ru, Pd, Pt, Rh, Ir, Au, Ag, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La , W. As described above, the absorption effect of the embedded nanoparticles for radiation outside the EUV wavelength range depends on the material of the nanoparticles. The choice of a suitable material for the nanoparticles depends not only on the reinforcing effect on the absorption of the “out-of-band” radiation, but also on its effect on the stability of the material of the top layer during operation in the EUV lithography system. The choice of the material for the nanoparticles typically also depends on the (base) material of the top layer in which the nanoparticles are embedded.

In a further embodiment, the top layer has a thickness between 1.0 nm and 5.0 nm. For the embedding of the nanoparticles, a minimum thickness of the top layer is required, which is typically 1.0 nm. With a suitable choice of the materials of the individual layer (s) (see below) or with a suitable design of the protective layer system, a sufficient protective effect and thus a long service life of the optical element can be guaranteed even with a comparatively small thickness of the individual layer (s). The comparatively small thickness of the layer (s) of the protective layer system generally leads to a reduction in the absorption of the EUV radiation which passes through the protective layer system, so that the reflectivity of the reflective optical element is increased.

In a further embodiment, the protective layer system has at least one further layer which is arranged between the top layer and the multilayer system. As described above, can The protective layer system only consists of the top layer, but it is also possible that further layers are arranged under the top layer, for example to block the passage of hydrogen / oxygen ions to the multilayer system or as a barrier to prevent the material from mixing the top layer of the protective layer system with the material of the top layer of the multilayer system (e.g. Si) can serve.

In a further development, the (or each) further layer has a thickness between 0.1 nm and 5.0 nm. As a rule, no nanoparticles are embedded in the further layer (s), so that these layer (s) can have a very small thickness, which contributes to a reduction in the absorption effect of this layer (s).

In a further embodiment, the material of the top layer in which the nanoparticles are embedded and / or the material of at least one further layer is / are formed from a stoichiometric or non-stoichiometric oxide or from a stoichiometric or non-stoichiometric mixed oxide. The oxide or mixed oxide can be a stoichiometric oxide or mixed oxide or a non-stoichiometric oxide or mixed oxide. Mixed oxides are composed of several oxides, i.e. their crystal lattice is composed of oxygen ions and the cations of several chemical elements. The use of oxides in the layers of the protective layer system has proven to be beneficial, since these have a high level of absorption for DUV radiation, which in the case of the top layer can be additionally reinforced by the embedded nanoparticles.

In a further development, the oxide or the mixed oxide contains at least one chemical element selected from the group comprising: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr. In order to prevent the degradation of the layers of the multilayer system or to counteract a reduction in reflectivity, the material of the top layer and - if present - the other layer (s) should be resistant to cleaning media (aqueous, acidic, basic, organic solvents and surfactants ), as well as against reactive hydrogen (H ^* ), ie hydrogen ions and / or hydrogen radicals, which are used when cleaning the surface of the protective layer system or the top layer.

In the event that the optical element is arranged in the vicinity of the EUV radiation source, the material of the top layer should be resistant to Sn or not mix with Sn. In particular, Sn contamination deposited on the top layer should be able to be removed from the surface of the third layer with reactive hydrogen (FT). The material of the top layer should also be resistant to redox reactions, i.e. neither oxidize nor - e.g. in contact with hydrogen - be reduced. The top layer should also not contain any substances that are volatile in an atmosphere containing oxygen and / or hydrogen. The oxides or mixed oxides of the metals described above meet these conditions or most of these conditions.

In a further embodiment, the further layer or at least one of the further layers is formed from at least one metal (or from a mixture of metals or an alloy). In contrast to the topmost layer, which is preferably formed from an oxide or from a mixed oxide, the further layer (s) can be formed from (at least) one metal. The requirements for resistance to cleaning media etc. are lower for the other layer (s) than for the top layer.

In a further development of this embodiment, the or at least one further layer contains a metal or consists of a metal that is selected from the group comprising: Ru, Pd, Pt, Rh, Ir, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La and mixtures thereof. These metallic materials also meet the requirements described above for the material of the top layer well.

In a further embodiment, the material of the further layer is selected from the group comprising: C, B ₄ C, BN, Si. These materials have proven to be advantageous, in particular with regard to their properties as diffusion barrier layers, in order to prevent the material of the top layer of the multilayer system from diffusing into the protective layer system.

In a further embodiment, the protective layer system has a thickness of less than 10 nm, preferably less than 7 nm. As described above, with a suitable choice of the materials of the individual layer (s) or with a suitable design of the protective layer system, even with a comparatively small thickness of the individual layer (s), an adequate protective effect and thus a long service life of the optical element can be achieved guaranteed. The comparatively small thickness of the layers of the layer system also generally leads to a reduction in the absorption of the EUV radiation that passes through the protective layer system, so that the reflectivity of the reflective optical element is increased. It goes without saying that materials should be selected for the layers of the protective layer system which do not have too great an absorption for EUV radiation.

When selecting suitable materials for the top layer and for the other layer (s), coordination with regard to their properties is necessary, in particular the lattice constants, the coefficient of thermal expansion (CTE) and the free surface energies of the materials of the layers are matched to one another will. Not every combination of the materials described above is therefore equally suitable for producing the protective layer system.

The layers of the protective layer system and the layers of the reflective multilayer system can in particular be applied by a PVD (“physical vapor deposition”) coating process or by a CVD (“chemical vapor deposition”) coating process. The PVD coating process can be, for example, electron beam evaporation, magnetron sputtering or laser beam evaporation (“pulsed laser deposition”, PLD). The CVD coating process can be, for example, a plasma-assisted CVD process (PE-CVD) or an atomic layer deposition (ALD) process. In particular, atomic layer deposition enables very thin layers to be deposited.

In a further embodiment, the optical element is designed as a collector mirror. In EUV lithography, collector mirrors are typically used as the first mirror behind the EUV radiation source, for example behind a plasma radiation source, in order to collect the radiation emitted in different directions by the radiation source and to reflect it in a bundled manner to the next mirror. Because of the high radiation intensity in the vicinity of the radiation source, there is a particularly high probability that molecular hydrogen present in the residual gas atmosphere can be converted into reactive (atomic or ionic) hydrogen with high kinetic energy, so that collector mirrors are particularly at risk due to the penetration of reactive hydrogen To show signs of separation on the layers of the protective layer system or on the upper layers of your multilayer system. Another aspect of the invention relates to an EUV lithography system, comprising: at least one optical element, as described above. The EUV lithography system can be an EUV lithography system for exposing a wafer or another optical arrangement that uses EUV radiation, for example an EUV inspection system, e.g. for inspecting masks used in EUV lithography, Wafers or the like.

Another aspect of the invention relates to a method for forming nanoparticles that are embedded in a top layer of a protective layer system of an optical element which is designed as described above, the method comprising: forming the embedded nanoparticles by irradiating the top layer of the protective layer system with ions . As described above, the nanoparticles are formed in the top layer by irradiation with ions. The nanoparticles can be the implanted ions. However, it is also possible that the nanoparticles contain a material or consist of a material that is contained in the topmost layer and that does not match the material of the ions used in the irradiation.

The ion dose required for the irradiation is typically of the order of magnitude between approx. 10 ¹⁵ ions / cm ² or approx. 10 ¹⁶ ions / cm ² and approx. 10 ¹⁷ ions / cm ² . Typical ion energies during implantation are in the order of magnitude of approx. 100-200 keV.

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

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

1a, b are schematic representations of an optical element in the form of an EUV mirror, which has a reflective multilayer system and a protective layer system with an uppermost layer in which nanoparticles are or will be embedded, and

2 shows a schematic representation of an EUV lithography system.

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

Figs. 1a, b show schematically the structure of an optical element 1 which has a substrate 2 made of a material with a low coefficient of thermal expansion, for example Zerodur®, ULE® or Clearceram®. The optical element 1 shown in FIGS. 1 a, b is designed to reflect EUV radiation 4 which strikes the optical element 1 at normal incidence, ie at angles of incidence a of typically less than approximately 45 ° to the surface normal. For the reflection of the EUV radiation 4, a reflective multilayer system 3 is applied to the substrate 2. The multilayer system 3 has alternately applied layers of a material with a higher real part of the refractive index at the working wavelength (also called spacer 3b) and a material with a lower real part of the refractive index at the working wavelength (also called absorber 3a), with an absorber-spacer pair Stack forms. As a result of this structure of the multilayer system 3, a crystal is simulated in a certain way, the lattice planes of which correspond to the absorber layers at which Bragg reflection takes place. In order to ensure sufficient reflectivity, the multilayer system 3 has a number of generally more than fifty alternating layers 3a, 3b.

The thicknesses of the individual layers 3a, 3b as well as the repeating stacks can be constant or also vary over the entire multilayer system 3, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be influenced in a targeted manner by adding more and less absorbing materials to the basic structure of absorber 3a and spacer 3b in order to increase the possible maximum reflectivity at the respective working wavelength. For this purpose, absorbers and / or spacer materials can be exchanged for one another in some stacks or the stacks can be built up from more than one absorber and / or spacer material. The absorber and spacer materials can have constant or also varying thicknesses over all stacks in order to optimize the reflectivity. Furthermore, additional layers can also be provided, for example as diffusion barriers between spacer and absorber layers 3a, 3b.

In the present example, in which the optical element 1 has been optimized for an operating wavelength of 13.5 nm, ie with an optical element 1 which has the maximum reflectivity at essentially normal incidence of EUV radiation 4 at a wavelength of 13.5 nm has, the stacks of the multilayer system 3 have alternating silicon layers 3a and molybdenum layers 3b. The silicon layers 3b correspond to the layers with a higher real part of the refractive index at 13.5 nm and the molybdenum layers 3a correspond to the layers with a lower real part of the refractive index at 13.5 nm Molybdenum and beryllium, ruthenium and beryllium or lanthanum and B4C are also possible. To protect the multilayer system 3 from degradation, a protective layer system 5 is applied to the multilayer system 3. In the example shown in FIGS. 1a, b, the protective layer system consists of a number n of layers 5a, 5n, where n typically assumes a value between 1 and 10. The first layer 5a is furthest away from the multilayer coating 3 and forms an uppermost layer 5a of the protective layer system 5. On the uppermost layer 5a, a surface 6 is formed which forms an exposed interface with the environment. The further layers 5b,. It is not absolutely necessary for the protective layer system 5 to have the further layers 5b,..., 5n, rather the protective layer system 5 can only consist of a single (topmost) layer 5a.

The top layer 5a has a first thickness di which is between 1.0 nm and 5.0 nm. The second layer 5b to the n-th layer 5n each have a thickness d2,..., D _n which is between 0.1 nm and 5.0 nm. The protective layer system 5 has a total thickness D (here: D = di + d2 +... + D _n ) which is less than 10 nm, possibly less than 7 nm.

In the example shown, the material 8 of the uppermost layer 5a is a (stoichiometric or non-stoichiometric) oxide or a (stoichiometric or non-stoichiometric) mixed oxide which contains at least one chemical element selected from the group comprising: Zr , Ti, Nb, Y, Hf, Ce, La, Ta, AI, W, Cr.

The material of at least one of the second layers 5b to n-th layers 5n can also be a (stoichiometric or non-stoichiometric) oxide or a (stoichiometric or non-stoichiometric) mixed oxide that contains at least one chemical element that is selected is from the group given above, comprising: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr.

As an alternative to an oxide or mixed oxide, the material of at least one of the second to n-th layers 5b,..., 5n can be (at least) one metal. For example, the metal can be selected from the group comprising: Ru, Pd, Pt, Rh, Ir, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La and mixtures thereof.

The material of at least one of the further layers 5b, ..., 5n can alternatively be selected from the group comprising: C, B4C, BN, Si. These materials have proven to be beneficial as diffusion barriers.

The choice of suitable materials for the second to n-th layers 5a, ..., 5n depends, among other things, on their arrangement in the protective layer system 5. For example, it can be advantageous to manufacture the nth layer 5n, which directly adjoins the reflective multilayer system 3, from a material that forms a diffusion barrier, i.e. for example from C, B4C, Bn or possibly from Si.

The protective effect of the protective layer system 5 depends not only on the materials that are selected for the layers 5a, ..., 5n, but also on whether these materials match well in terms of their properties, for example what their lattice constants, their thermal expansion coefficients, their surface free energies, etc.

An example of a protective layer system 5 is described below which has three layers 5a, 5b, 5c which are adapted to one another with regard to their properties. The first layer 5a is formed from TiO _x and has a thickness di of 1.5 nm, the second layer 5b is formed from Ru and has a thickness d2 of 2 nm and the third layer 5c is formed from AlO _x and also has one Thickness ch of 2 nm. It goes without saying that, in addition to the example described here, other material combinations are also possible and the thicknesses of the three (or possibly more or less) layers 5a-c of the protective layer system 5 can also deviate from the values given above.

In the examples shown in FIGS. 1 a, b, 5 nanoparticles 7 are embedded in the material 8 of the top layer 5 a of the protective layer system. In the example shown, the nanoparticles 7 are metallic nanoparticles. The metal from which the nanoparticles are formed can be, for example, Ru, Pd, Pt, Rh, Ir, Au, Ag, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, Act la or w.

As has been described above, the material 8 of the uppermost layer 5a, in which the nanoparticles 7 are embedded, is a stoichiometric or non-stoichiometric oxide or a stoichiometric or non-stoichiometric mixed oxide. As a result of the nanoparticles 7 embedded in the oxide or in the mixed oxide 8, the stability of the uppermost layer 5a with respect to damage factors such as EUV radiation 4, increased temperatures, plasma and oxidation or reduction processes is increased.

The formation of the embedded nanoparticles 7 is induced by ion implantation, i.e. for the embedding of the nanoparticles 7, the surface 6 of the top layer 5a of the protective layer system 5 is irradiated with ions 9 during the manufacture of the optical element 1, as shown in Fig. 1b.

In principle, the material of the embedded nanoparticles 7 can match the material of the ions 9 that is used for the ion irradiation of the optical element 1. In this case, the material of the embedded nanoparticles 7 is usually foreign atoms, ie about chemical elements that do not match the material of the top layer 5a surrounding the nanoparticles 8.

The ions 9 which are used for the irradiation are, in the example shown, a metallic material, for example a noble metal, in particular gold (Au) or silver (Ag). The material of the uppermost layer 5a, which surrounds the embedded nanoparticles 7, is titanium oxide T1O2 or a titanium mixed oxide (TiO _x ) in the example shown. The embedded nanoparticles 7 make it possible in this case not only to stabilize the top layer 5a against external damage factors, but also to increase the absorption of the top layer 5a for radiation at wavelengths that are outside the EUV wavelength range and in this way to increase the reflectivity RDUV of the optical Element 1 compared to an identically constructed optical element 1 for this wavelength range, e.g. the DUV wavelength range between 100 nm and 300 nm, compared to an optical element 1 in which no nanoparticles 7 are embedded in the top layer 5a of the protective layer system 5. By contrast, the reflectivity REUV of the optical element 1 for the EUV radiation 4 is not or only extremely slightly reduced by the embedding of the nanoparticles 7.

As an alternative to the above-described embedding of nanoparticles 7 in the form of foreign atoms in the surrounding material 8 of the uppermost layer 5a, the nanoparticles 7 can contain at least one material that is contained in the surrounding material 8 of the uppermost layer 5a. The nanoparticles 7 can additionally contain the material of the ions 9 that are used in the irradiation, but it is also possible that the irradiation with the ions 9 leads to the formation of nanoparticles 7 that are formed exclusively from the chemical elements that are contained in the material 8 of the uppermost layer 5a before or without the irradiation with the ions 9. In particular, the irradiation with the ions 9 can lead to a structure formation in which nanoparticles 7 are formed in the material of the uppermost layer 5a by chemically reducing the oxide or the mixed oxide of the uppermost layer 5a. As described in the article by EM Hunt cited above, for example ions 9 in the form of Y ⁺ , Ca ⁺ , Mg ⁺ or Zr ⁺ can be used for irradiation in order to reduce monocrystalline aluminum oxide (Al2O3) to Al. The Al formed during the reduction can subsequently form clusters and react with other elements in order to form the embedded Al nanoparticles 7. When Mg ^{+ is} implanted in Al2O3, nanoparticles 7 are formed in the form of MgA O ^ platelets. In this case, the nanoparticles 7 contain both the material of the ions 9 used in the irradiation and the constituents or chemical elements of the material of the top layer 5a (ie Al2O3) before the irradiation. In the event that the material of the uppermost layer 5a is quartz glass (S1O2), ZrSh nanoparticles 7 can be formed in the uppermost layer 5a ^{by irradiation with Zr + ions 9.}

The ion dose required for the formation of nanoparticles 7 described above is typically of the order of magnitude between approx. 10 ¹⁵ ions / cm ² or approx. 10 ¹⁶ ions / cm ² and approx. 10 ¹⁷ ions / cm ² . Typical energies of the ions 9 during implantation or during irradiation are in the order of magnitude of approx. 100-200 keV.

In the cases described above, it has proven to be advantageous if the nanoparticles 7 have mean particle sizes p between approx. 0.5 nm and approx. 2 nm. The mean particle size p of the nanoparticles 7 can be set - within certain limits - by a suitable choice of the parameters during the irradiation with the ions 9. The mean particle size p influences the absorption of the top layer 5a for radiation outside the EUV wavelength range and can be selected so that there is particularly strong absorption in a wavelength range of interest so that a reduction in the reflectivity RDUV of the optical element 1 is set for this wavelength range.

It goes without saying that, as an alternative to the materials described above, the top layer 5a can also be formed from other materials, in particular in the form of oxides or mixed oxides, in which nanoparticles 7 are embedded in the manner described above.

The optical elements 1 shown in FIGS. 1 a, b can be used in an EUV lithography system in the form of an EUV lithography system 101, as is shown schematically below in the form of a so-called wafer scanner in FIG.

The EUV lithography system 101 has an EUV light source 102 for generating EUV radiation which has a high energy density in the EUV wavelength range below 50 nanometers, in particular between approx. 5 nanometers and approx. 15 nanometers. The EUV light source 102 can be designed, for example, in the form of a plasma light source for generating a laser-induced plasma. The EUV lithography system 101 shown in FIG. 2 is designed for an operating wavelength of the EUV radiation of 13.5 nm, for which the optical elements 1 shown in FIGS. 1a, b are also designed. However, it is also possible for the EUV lithography system 101 to be configured for a different working wavelength of the EUV wavelength range, such as, for example, 6.8 nm.

The EUV lithography system 101 also has a collector mirror 103 in order to bundle the EUV radiation from the EUV light source 102 to form an illumination beam 104 and in this way to further increase the energy density. The illumination beam 104 is used to illuminate a structured object M by means of an illumination system 110, which in the present example has five reflective optical elements 112 to 116 (mirrors). The structured object M can be, for example, a reflective photomask that has reflective and non-reflective or at least less strongly reflective areas for generating at least one structure on the object M. Alternatively, the structured object M can be a plurality of micromirrors which are arranged in a one-dimensional or multi-dimensional arrangement and which can optionally be moved about at least one axis in order to set the angle of incidence of the EUV radiation on the respective mirror.

The structured object M reflects part of the illuminating beam 104 and forms a projection beam 105, which carries the information about the structure of the structured object M and which is irradiated into a projection objective 120, which images the structured object M or a respective sub-area thereof a substrate W generated. The substrate W, for example a wafer, has a semiconductor material, for example silicon, and is arranged on a holder, which is also referred to as a wafer stage WS.

In the present example, the projection objective 120 has six reflective optical elements 121 to 126 (mirrors) in order to generate an image of the structure present on the structured object M on the wafer W. The number of mirrors in a projection objective 120 is typically between four and eight, but only two mirrors can optionally be used.

The reflective optical elements 103, 112 to 116 of the lighting system 110 and the reflective optical elements 121 to 126 of the projection objective 120 are arranged in a vacuum environment 127 during operation of the EUV lithography system 101. In the In the vacuum environment 127, a residual gas atmosphere is formed in which, among other things, oxygen, hydrogen and nitrogen are present.

The optical element 1 shown in Fig. 1a, b can be one of the optical elements 103, 112 to 115 of the lighting system 110 or one of the reflective optical elements 121 to 126 of the projection lens 120, which for normal incidence of the EUV Radiation 4 are designed. In particular, the optical element 1 from FIGS.

Exposed to contamination. The protective layer system 5 described in connection with FIGS. 1a, b can significantly increase the service life of the collector mirror 103, in particular it can be reused, for example, after cleaning.

Claims

1. Optical element (1), comprising: a substrate (2), a multilayer system (3) which is applied to the substrate (2) and reflecting EUV radiation (4), and a protective layer system (5) which is applied to the multilayer system (3) , which has an uppermost layer (5a), characterized in that nanoparticles (7) which contain at least one metallic material are embedded in the material of the uppermost layer (5a) of the protective layer system (5).

2. Optical element according to claim 1, in which the nanoparticles (7) contain at least one material that does not match the material (8) of the top layer (5a) surrounding the nanoparticles (7).

3. Optical element according to claim 1 or 2, in which the nanoparticles (7) contain at least one material which is contained in the material (8) of the top layer (5a) surrounding the nanoparticles (7).

4. Optical element according to one of the preceding claims, in which the nanoparticles (7) have mean particle sizes (p) between 0.5 nm and 2 nm.

5. Optical element according to one of the preceding claims, in which the embedded nanoparticles (7) have the reflectivity (Rvuv) of the optical element (1) for radiation at longer wavelengths than EUV radiation (4), in particular in the VUV wavelength range or in the DUV - Reduce the wavelength range.

6. Optical element according to one of the preceding claims, in which the material of the nanoparticles (7) is selected from the group comprising: Ru, Pd, Pt, Rh, Ir, Au, Ag, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La, W.

7. Optical element according to one of the preceding claims, in which the top layer (5a) has a thickness (di) between 1.0 nm and 5.0 nm.

8. Optical element according to one of the preceding claims, in which the protective layer system (5) has at least one further layer (5b, ..., 5n) which is arranged between the top layer (5a) and the multilayer system (3).

9. Optical element according to claim 8, in which the at least one further layer (5b, ... 5n) has a thickness (d2; ..., d _n ) between 0.1 nm and 5.0 nm.

10. Optical element according to one of the preceding claims, in which the material of the top layer (5a) and / or of the at least one further layer (5b, ..., 5n) is made of a stoichiometric or non-stoichiometric oxide or of a stoichiometric or not stoichiometric mixed oxide is / are formed.

11. The optical element according to claim 10, wherein the oxide or the mixed oxide contains at least one chemical element selected from the group comprising: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr .

12. Optical element according to one of claims 8 to 11, in which at least one of the further layers (5b, ..., 5n) is formed from at least one metal.

13. The optical element according to claim 12, wherein at least one of the further layers (5b, ..., 5n) contains a metal or consists of a metal selected from the group comprising: Ru, Pd, Pt, Rh, Ir , Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La and mixtures thereof.

14. Optical element according to one of claims 8 to 13, in which the material of at least one of the further layers (5b, ..., 5n) is selected from the group comprising: C, B ₄ C, BN, Si.

15. Optical element according to one of the preceding claims, in which the protective layer system (5) has a thickness (D) of less than 10 nm, preferably of less than 7 nm.

16. Optical element according to one of the preceding claims, which is designed as a collector mirror (103).

17. EUV lithography system (101), comprising: at least one optical element (1, 103, 112 to 115, 121 to 126) according to one of the preceding claims.

18. A method for forming nanoparticles (7) which are embedded in an uppermost layer (5a) of a protective layer system (5) of an optical element (1) according to one of claims 1 to 16, comprising:

Irradiating the top layer (5a) of the protective layer system (5) with ions (9) to form the nanoparticles (7) embedded in the top layer (5a).