OPTICAL ELEMENT, EUV LITHOGRAPHY SYSTEM, AND METHOD FOR FORMING NANOPARTICLES

Abstract

An optical element (1)includes: a substrate (2), applied to the substrate (2), a multilayer system (3) which reflects EUV radiation (4), and applied to the multilayer system (3), a protective layer system (5) having an uppermost layer (5a). Nanoparticles (7) are embedded into the material of the uppermost layer (5a) of the protective layer system (5) which nanoparticles contain at least one metallic material. An EUV lithography system which includes at least one such optical element (1) designed as indicated above, and a method of forming nanoparticles (7) in the uppermost layer (5a) of the protective layer system (5) are also disclosed.

Claims

1. An optical element, comprising: a substrate, applied to the substrate, a multilayer system which reflects extreme ultraviolet (EUV) radiation, and applied to the multilayer system, a protective layer system having an uppermost layer, wherein nanoparticles containing at least one metallic material are embedded into the uppermost layer of the protective layer system, and wherein the embedded nanoparticles reduce a reflectivity (Rvuv) of the optical element for radiation at wavelengths greater than wavelengths of the EUV radiation.

2. The optical element as claimed in claim 1, wherein the nanoparticles contain at least one material that does not correspond to a material of the uppermost layer that surrounds the nanoparticles.

3. The optical element as claimed in claim 1, wherein the nanoparticles contain at least one material present in a material of the uppermost layer that surrounds the nanoparticles.

4. The optical element as claimed in claim 1, wherein the nanoparticles have average particle sizes between 0.5 nm and 2 nm.

5. The optical element as claimed in claim 1, wherein the embedded nanoparticles reduce a reflectivity (Rvuv) of the optical element for radiation in at least one of very ultraviolet (VUV) wavelength regions and deep ultraviolet (DUV) wavelength regions.

6. The optical element as claimed in claim 1, wherein the material of the nanoparticles is selected from the group consisting of at least one of: Ru, Pd, Pt, Rh, Ir, Au, Ag, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La, W.

7. The optical element as claimed in claim 1, wherein the uppermost layer has a thickness between 1.0 nm and 5.0 nm.

8. The optical element as claimed in claim 1, wherein the protective layer system comprises at least one further layer disposed between the uppermost layer and the multilayer system.

9. The optical element as claimed in claim 8, wherein the at least one further layer has a thickness between 0.1 nm and 5.0 nm.

10. The optical element as claimed in claim 1, wherein a material of the uppermost layer is formed of a stoichiometric or nonstoichiometric oxide or of a stoichiometric or nonstoichiometric mixed oxide.

11. The optical element as claimed in claim 8, wherein a material of the at least one further layer is formed of a stoichiometric or nonstoichiometric oxide or of a stoichiometric or nonstoichiometric mixed oxide.

12. The optical element as claimed in claim 10, wherein the oxide or mixed oxide comprises at least one chemical element selected from the group consisting of at least one of: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr.

13. The optical element as claimed in claim 8, wherein at least one of the further layers is formed from at least one metal.

14. The optical element as claimed in claim 13, wherein at least one of the further layers comprises or consists of a metal selected from the group consisting of at least one of: Ru, Pd, Pt, Rh, Ir, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La and mixtures thereof.

15. The optical element as claimed in claim 8, wherein the material of at least one of the further layers is selected from the group consisting of at least one of: C, B4C, BN, Si.

16. The optical element as claimed in claim 1, wherein the protective layer system has a thickness of less than 10 nm.

17. The optical element as claimed in claim 16, wherein the protective layer system has a thickness of less than 7 nm.

18. The optical element as claimed in claim 16, configured as a collector mirror.

19. An EUV lithography system comprising: an illumination beam source comprising a first optical element and outputting an illumination beam, an illumination system comprising further optical elements adapting the illumination beam, and a projection lens comprising additional optical elements projecting the adapted illumination beam as a projected beam into a projected image, wherein at least one of the first, further and/or additional optical elements is an optical element as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Working examples are shown in the schematic drawing and are detailed in the description that follows. The figures show:

[0045] FIG. 1A and FIG. 1B schematic diagrams of an optical element in the form of an EUV mirror having a reflective multilayer system and a protective layer system with an uppermost layer into which nanoparticles are being (FIG. 1A) / have been (FIG. 1B) embedded, and

[0046] FIG. 2 a schematic representation of an EUV lithography apparatus.

DETAILED DESCRIPTION

[0047] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0048] FIG. 1A and FIG. 1B show a schematic of the construction of an optical element 1 which comprises a substrate 2 composed of a material having a low coefficient of thermal expansion, for example of Zerodur®, ULE® or Clearceram®. The optical element 1 shown in FIGS. 1A, 1B is configured for reflection of EUV radiation 4 incident on the optical element 1 with normal incidence, i.e. at angles α of incidence of typically less than about 45° with respect to the surface normal. For the reflection of EUV radiation 4, a reflective multilayer system 3 is applied to the substrate 2. The multilayer system 3 comprises alternately applied layers of a material having a comparatively high real part of the refractive index at the operating wavelength (also called “spacer” 3b) and of a material having a comparatively low real part of the refractive index at the operating wavelength (also called “absorber” 3a), where an absorber-spacer pair forms a stack. As a result of this construction of the multilayer system 3, a crystal is simulated, to a certain degree, with lattice planes corresponding to the absorber layers at which Bragg reflection takes place. In order to ensure sufficient reflectivity, the multilayer system 3 comprises a number of generally more than fifty alternating layers 3a, 3b.

[0049] The thicknesses of the individual layers 3a, 3b and also of the repeating stacks can be constant over the entire multilayer system 3 or else vary, depending on what spectral or angle-dependent reflection profile is intended to be achieved. The reflection profile can also be influenced in a targeted manner by the basic structure composed of absorber 3a and spacer 3b being supplemented by additional more and less absorbing materials in order to increase the possible maximum reflectivity at the respective operating wavelength. To that end, in some stacks absorber and/or spacer materials can be mutually interchanged, or the stacks can be constructed from more than one absorber and/or spacer material. The absorber and spacer materials can have constant or varying thicknesses over all the stacks in order to optimize the reflectivity. Furthermore, it is also possible to provide additional layers for example as diffusion barriers between spacer and absorber layers 3a, 3b.

[0050] In the present example, in which the optical element 1 has been optimized for an operating wavelength of 13.5 nm, in other words for an optical element 1 which exhibits maximum reflectivity at a wavelength of 13.5 nm under substantially normal incidence of EUV radiation 4, the stacks of the multilayer system 3 comprise alternating silicon layers 3a and molybdenum layers 3b. In this system, the silicon layers 3b correspond to the layers having a comparatively high real part of the refractive index at 13.5 nm and the molybdenum layers 3a correspond to the layers having a comparatively low real part of the refractive index at 13.5 nm. Depending on the exact value of the operating wavelength, other material combinations, such as e.g. molybdenum and beryllium, ruthenium and beryllium, or lanthanum and B.sub.4C, are likewise possible.

[0051] In order 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 FIG. 1A, B, the protective layer system consists of a number n of layers 5a, ..., 5n, where n typically assumes a value from 1 to 10. The first layer 5a is the furthest removed from the multilayer coating 3 and forms an uppermost layer 5a of the protective layer system 5. A surface 6 formed on the uppermost layer 5a forms an exposed interface with the environment. The further layers 5b, ..., 5n, i.e. the second layer 5b to the nth layer 5n of the protective layer system 5, are arranged closer than the uppermost layer 5a in relation to the multilayer system 3. It is not absolutely necessary for the protective layer system 5 to have the further layers 5b, ..., 5n; instead, the protective layer system 5 may consist solely of a single (uppermost) layer 5a.

[0052] The uppermost layer 5a has a first thickness d.sub.1 between 1.0 nm and 5.0 nm. The second layer 5b to the nth layer 5n each have a thickness d.sub.2, ..., d.sub.n between 0.1 nm and 5.0 nm. The protective layer system 5 has a total thickness D (here: D = d.sub.1 + d.sub.2 + ... + d.sub.n) which is less than 10 nm, optionally less than 7 nm.

[0053] In the example shown, the material 8 of the uppermost layer 5a is a (stoichiometric or nonstoichiometric) oxide or a (stoichiometric or nonstoichiometric) mixed oxide which comprises at least one chemical element selected from the group comprising: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr.

[0054] The material of at least one of the second layer 5b to the nth layer 5n may likewise be a (stoichiometric or nonstoichiometric) oxide and/or a (stoichiometric or nonstoichiometric) mixed oxide containing at least one chemical element selected from the above-specified group comprising: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, W, Cr.

[0055] Alternatively to an oxide or mixed oxide, the material of at least one of the second to nth layers 5b, ..., 5n may comprise (at least) one metal. The metal may be selected, for example, from the group comprising: Ru, Pd, Pt, Rh, Ir, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La and mixtures thereof.

[0056] The material of at least one of the further layers 5b, ..., 5n may alternatively be selected from the group comprising: C, B4C, BN, Si. These materials have been found to be advantageous as diffusion barriers.

[0057] The choice of suitable materials for the second to nth layers 5b, ..., 5n depends upon factors including the arrangement thereof in the protective layer system 5. For example, it may be favorable to produce the nth layer 5n directly adjoining the reflective multilayer system 3 from a material that forms a diffusion barrier, i.e., for example, from C, B.sub.4C, Bn or optionally from Si.

[0058] The protective effect of the protective layer system 5 is dependent not only on the materials which are selected for the layers 5a, ..., 5n but also on whether these materials are a good fit in terms of their properties - for example, with regard to their lattice constants, their coefficients of thermal expansion, their free surface energies, etc.

[0059] There follows a description of an example of a protective layer system 5 having three layers 5a, 5b, 5c harmonized with one another in terms of their properties. The first layer 5a is formed of TiO.sub.x and has a thickness d.sub.1 of 1.5 nm, the second layer 5b is formed of Ru and has a thickness d.sub.2 of 2 nm, and the third layer 5c is formed of AlO.sub.x and likewise has a thickness d.sub.3 of 2 nm. It will be appreciated that as well as the example described here, other combinations of materials are also possible, and also the thicknesses of the three (or optionally more or fewer) layers 5a-c of the protective layer system 5 may differ from the values indicated above.

[0060] In the examples shown in FIG. 1A, B, nanoparticles 7 are embedded into the material 8 of the uppermost layer 5a of the protective layer system 5. The nanoparticles 7 in the example shown are metallic nanoparticles. The metal from which the nanoparticles are formed may, for example, be Ru, Pd, Pt, Rh, Ir, Au, Ag, Al, Ta, Cr, Mo, Zr, Y, Sc, Ti, V, Nb, La or W.

[0061] As described above, the material 8 of the uppermost layer 5a into which the nanoparticles 7 are embedded is a stoichiometric or nonstoichiometric oxide or a stoichiometric or nonstoichiometric mixed oxide. The nanoparticles 7 embedded into the oxide or mixed oxide 8 increase the stability of the uppermost layer 5a with respect to damage factors, for example EUV radiation 4, elevated temperatures, plasma, and oxidation and reduction processes.

[0062] The formation of the embedded nanoparticles 7 is induced by ion implantation, meaning that, for the embedding of the nanoparticles 7, the surface 6 of the uppermost layer 5a of the protective layer system 5 is irradiated with ions 9 in the production of the optical element 1, as shown in FIG. 1B.

[0063] In principle, the material of the embedded nanoparticles 7 may correspond to the material of the ions 9 which is used in the ion irradiation of the optical element 1. In this case, the material of the embedded nanoparticles 7 generally comprises foreign atoms, i.e. chemical elements that do not correspond to the material of the uppermost layer 5a surrounding the nanoparticles 8.

[0064] In the example shown, the ions 9 that are used for the irradiation are a metallic material, for example a precious metal, in particular gold (Au) or silver (Ag). In the example shown, the material of the uppermost layer 5a surrounding the embedded nanoparticles 7 is titanium dioxide TiO2 or a mixed titanium oxide (TiO.sub.x). In this case, the embedded nanoparticles 7 enable not only the stabilization of the uppermost layer 5a against external damage factors but also an increase in the absorption of the uppermost layer 5a for radiation at wavelengths outside the EUV wavelength range, and in this way a reduction in the reflectivity R.sub.DUV of the optical element 1 with respect to an optical element 1 of identical construction for this wavelength range, for example 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 uppermost layer 5a of the protective layer system 5. The reflectivity R.sub.EUV of the optical element 1 for EUV radiation 4, by contrast, is reduced only extremely slightly, if at all, by the embedding of the nanoparticles 7.

[0065] Alternatively to the embedding of nanoparticles 7 in the form of foreign atoms into the surrounding material 8 of the uppermost layer 5a as described above, the nanoparticles 7 may contain at least one material present in the surrounding material 8 of the uppermost layer 5a. The nanoparticles 7 here may additionally contain the material of the ions 9 which are used in the irradiation, but it is also possible that the irradiation with the ions 9 leads to formation of nanoparticles 7 formed exclusively from the chemical elements present in the material 8 of the uppermost layer 5a before or without the irradiation with the ions 9.

[0066] In particular, the irradiation with the ions 9 can lead to structure formation in which nanoparticles 7 are formed in the material of the uppermost layer 5a in that the oxide or mixed oxide in the uppermost layer 5a is chemically reduced. As described in the article by E. M. Hunt cited above, it is possible, for example, to use ions 9 in the form of Y.sup.+, Ca.sup.+, Mg.sup.+ or Zr.sup.+ for irradiation, in order to reduce monocrystalline aluminum oxide (AI.sub.2O.sub.3) to Al. The Al formed in the reduction can subsequently form clusters and react with other elements in order to form the embedded Al nanoparticles 7. The implantation of Mg.sup.+ into AI2O.sub.3 forms nanoparticles 7 in the form of MgA1.sub.2O.sub.4 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 uppermost layer 5a (i.e. AI.sub.2O.sub.3) prior to the irradiation. If the material of the uppermost layer 5a is quartz glass (SiO.sub.2), irradiation with Zr.sup.+ ions 9 can form ZrSi.sub.2 nanoparticles 7 in the uppermost layer 5a.

[0067] The ion dose required for the above-described formation of nanoparticles 7 is typically in the order of magnitude between about 10.sup.15 ions/cm.sup.2 or about 10.sup.16 ions/cm.sup.2 and about 10.sup.17 ions/cm.sup.2. Typical energies of the ions 9 in the implantation or in the irradiation are in the order of magnitude of about 100-200 keV.

[0068] In the cases described above, it has been found to be favorable when the nanoparticles 7 have average particle sizes p between about 0.5 nm and about 2 nm. The average particle size p of the nanoparticles 7 may be adjusted - within certain limits - by suitable selection of the parameters in the irradiation with the ions 9. The average particle size p affects the absorption of the uppermost layer 5a for radiation outside the EUV wavelength range and may be chosen such that particularly strong absorption and hence a reduction in the reflectivity R.sub.DUV of the optical element 1 is established within a wavelength range of interest.

[0069] It will be appreciated that the uppermost layer 5a, alternatively to the materials described above, may also be formed from different materials, especially in the form of oxides or mixed oxides, into which nanoparticles 7 are embedded in the manner described above.

[0070] The optical elements 1 illustrated in FIG. 1A, B can be used in an EUV lithography system in the form of an EUV lithography apparatus 101, as is illustrated schematically below in the form of a so-called wafer scanner in FIG. 2.

[0071] The EUV lithography apparatus 101 comprises 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 about 5 nanometers and about 15 nanometers. The EUV light source 102 can be embodied, for example, in the form of a plasma light source for generating a laser-induced plasma. The EUV lithography apparatus 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 illustrated in FIG. 1A, B are also designed. However, it is also possible for the EUV lithography apparatus 101 to be configured for a different operating wavelength in the EUV wavelength range, such as 6.8 nm, for example.

[0072] The EUV lithography apparatus 101 further comprises a collector mirror 103 in order to focus the EUV radiation of the EUV light source 102 to form a bundled illumination beam 104 and to increase the energy density further in this way. The illumination beam 104 serves to illuminate a structured object M with an illumination system 110, which in the present example has five reflective optical elements 112 to 116 (mirrors).

[0073] The structured object M can be for example a reflective photomask, which has reflective and non-reflective, or at least less reflective, regions for producing at least one structure on the object M. Alternatively, the structured object M can be a plurality of micro-mirrors, which are arranged in a one-dimensional or multi-dimensional arrangement and which are optionally movable about at least one axis, in order to set the angle of incidence of the EUV radiation on the respective mirror.

[0074] The structured object M reflects part of the illumination beam 104 and shapes a projection beam 105, which carries the information about the structure of the structured object M and is radiated into a projection lens 120, which generates a projected image of the structured object M or of a respective partial region thereof on a substrate W. The substrate W, for example a wafer, comprises a semiconductor material, for example silicon, and is disposed on a mounting, which is also referred to as a wafer stage WS.

[0075] In the present example, the projection lens 120 has six reflective optical elements 121 to 126 (mirrors) in order to generate an image of the structure that is present at the structured object M on the wafer W. The number of mirrors in a projection lens 120 typically lies between four and eight; however, as few as two mirrors can also be used, if appropriate.

[0076] The reflective optical elements 103, 112 to 116 of the illumination system 110 and the reflective optical elements 121 to 126 of the projection lens 120 are arranged in a vacuum environment 127 during the operation of the EUV lithography apparatus 101. A residual gas atmosphere containing, inter alia, oxygen, hydrogen and nitrogen is formed in the vacuum environment 127.

[0077] The optical element 1 illustrated in FIG. 1A, B can be one of the optical elements 103, 112 to 115 of the illumination system 110 or one of the reflective optical elements 121 to 126 of the projection lens 120 which are designed for normal incidence of the EUV radiation 4. In particular, the optical element 1 of FIG. 1A, B may be the collector mirror 103, which in the operation of the EUV lithography apparatus 101 is exposed not only to reactive hydrogen but also to Sn contaminations. The protective layer system 5 described in connection with FIG. 1A, B enables the lifetime of the collector mirror 103 to be significantly extended, and in particular this mirror can be used again after cleaning, for example.