OPTICAL ELEMENT AND EUV LITHOGRAPHIC SYSTEM

Abstract

An optical element (1) includes: a substrate (2); applied to the substrate (2), a multilayer system (3) which reflects EUV radiation (4); and also applied to the multilayer system (3), a protective layer system (5) which comprises a first layer (5a), a second layer (5b) and a third, in particular topmost layer (5c), where the first layer (5a) is disposed closer to the multilayer system (3) than the second layer (5b), and where the second layer (5b) is disposed closer to the multilayer system (3) than the third layer (5c). The second layer (5b) and the third layer (5c) and also preferably the first layer (5a) each have a thickness (d.sub.2, d.sub.3, d.sub.1) of between 0.5 nm and 5.0 nm. A related EUV lithography system having at least one such optical element is also described.

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 which comprises a first layer, a second layer and a third layer, where the first layer is disposed closer to the multilayer system than is the second layer, and where the second layer is disposed closer to the multilayer system than is the third layer, wherein the second layer and the third layer both have a thickness of between 0.5 nm and 5.0 nm, wherein the first layer is formed of a stoichiometric or nonstoichiometric oxide or of a stoichiometric or nonstoichiometric mixed oxide, and wherein the oxide or the mixed oxide of the first layer comprises at least one chemical element selected from the group consisting essentially of: Al, Zr, Y.

2. The optical element as claimed in claim 1, wherein the third layer is a topmost layer of the protective layer system, and wherein the first layer also has a thickness between 0.5 nm and 5.0 nm.

3. The optical element as claimed in claim 1, wherein the second layer and/or the third layer are/is formed of a stoichiometric or nonstoichiometric oxide or of a stoichiometric or nonstoichiometric mixed oxide.

4. The optical element as claimed in claim 3, wherein the oxide or mixed oxide of the third layer comprises at least one chemical element selected from the group consisting essentially of: Zr, Ti, Nb, Y, Hf, Ce, La, Ta, Al, Er, W, Cr.

5. The optical element as claimed in claim 3, wherein the oxide or mixed oxide of the second layer comprises at least one chemical element selected from the group consisting essentially of: Al, Zr, Y, La.

6. An optical element comprising: a substrate; applied to the substrate, a multilayer system which reflects EUV radiation; and applied to the multilayer system, a protective layer system which comprises a first layer, a second layer and a third layer, where the first layer is disposed closer to the multilayer system than is the second layer, and where the second layer is disposed closer to the multilayer system than is the third layer, wherein the second layer and the third layer both have a thickness of between 0.5 nm and 5.0 nm, and wherein the first layer comprises or consists of a metal selected from the group consisting essentially of: Al, Mo, Ta, Cr.

7. The optical element as claimed in claim 6, wherein the third layer is a topmost layer of the protective layer system, and wherein the first layer also has a thickness between 0.5 nm and 5.0 nm.

8. The optical element as claimed in claim 6, wherein the second layer is formed of at least one metal.

9. The optical element as claimed in claim 8, wherein the second layer comprises or consists of a metal selected from the group consisting essentially of: Al, Zr, Y, Sc, Ti, V, Nb, La and noble metals.

10. The optical element as claimed in claim 9, wherein the second layer comprises or consists of Ru, Pd, Pt, Rh, or Ir.

11. An optical element comprising: a substrate; applied to the substrate, a multilayer system which reflects EUV radiation; and applied to the multilayer system, a protective layer system which comprises a first layer, a second layer and a third, topmost layer, where the first layer is disposed closer to the multilayer system than is the second layer, and where the second layer is disposed closer to the multilayer system than is the third layer, wherein the first layer directly adjoins the second layer, and the second layer directly adjoins the third layer, wherein the second layer and the third layer both have a thickness of between 0.5 nm and 5.0 nm, and wherein the material of the first layer is selected from the group consisting essentially of: C, B.sub.4C, BN.

12. The optical element as claimed in claim 11, wherein the first layer also has a thickness between 0.5 nm and 5.0 nm.

13. The optical element as claimed in claim 1, wherein ions and/or metallic particles are implanted into the first layer, into the second layer and/or into the third layer, and/or wherein metallic particles are applied to the first layer, to the second layer and/or to the third layer.

14. The optical element as claimed in claim 13, wherein the metallic particles are selected from the group consisting of: Pd, Pt, Rh, Ir.

15. The optical element as claimed in claim 1, wherein the protective layer system comprises at least one further layer having a thickness of 0.5 nm or less and comprising at least one metal.

16. The optical element as claimed in claim 15, wherein the at least one further layer is a sub-monolayer layer and wherein the at least one metal is selected from the group consisting essentially of: Pd, Pt, Rh, Ir.

17. The optical element as claimed in claim 1, wherein the multilayer system comprises a topmost layer having a thickness of more than 0.5 nm.

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

19. The optical element as claimed in claim 1, which is configured as a collector mirror.

20. An EUV lithography system comprising: at least one optical element as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

[0040] FIG. 1A shows a schematic illustration of an optical element in the form of an EUV mirror, which comprises a reflective multilayer system and also a protective layer system having three layers;

[0041] FIG. 1B shows a schematic illustration of the optical element of FIG. 1A, wherein ions and also metallic (nano)particles are implanted into the second layer of the protective layer system and deposited on the top side of the third layer,

[0042] FIG. 1C shows a schematic illustration of the optical element of FIGS. 1A and 1B, wherein the protective layer system comprises a fourth layer, composed of a noble metal; and

[0043] FIG. 2 shows a schematic illustration of an EUV lithography apparatus.

DETAILED DESCRIPTION

[0044] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0045] FIGS. 1A-C schematically show the construction of an optical element 1 which comprises a substrate 2 consisting of a material having a low coefficient of thermal expansion, e.g., of Zerodur®, ULE® or Clearceram®. The optical element 1 shown in FIGS. 1A-C is configured for reflecting EUV radiation 4 which is incident on the optical element 1 with normal incidence, i.e. at angles of incidence a of typically less than approximately 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 higher real part of the refractive index at the operating wavelength (also called “spacer” 3b) and of a material having a comparatively lower 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, in a certain way, 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.

[0046] 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.

[0047] In the present example, wherein 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 relatively higher real part of the refractive index at 13.5 nm and molybdenum layers 3a correspond to layers having a comparatively lower 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.

[0048] 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, the protective layer system consists of a first layer 5a, a second layer 5b and a third layer 5c. In this arrangement, the first layer 5a is disposed closer to the multilayer system 3 than the second layer 5b. The second layer 5b is disposed closer to the multilayer system 3 than the third layer 5c, which forms the topmost layer of the protective layer system 5, on whose exposed surface the interface with the surrounding environment is formed.

[0049] The first layer 5a has a first thickness d.sub.1, the second layer 5b has a second thickness d.sub.2, and the third layer 5c has a third thickness d.sub.3, each of these thicknesses being situated in a range between 0.5 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.3) which is less than 10 nm, optionally less than 7 nm.

[0050] In the example shown, the material of the third, topmost layer 5c 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, Er, W, Cr.

[0051] The material of the second layer 5b may likewise be a (stoichiometric or nonstoichiometric) oxide and/or a (stoichiometric or nonstoichiometric) mixed oxide which is selected from the group comprising: Al, Zr, Y, La. Alternatively to an oxide or mixed oxide, the material of the second layer 5b may comprise (at least) one metal. The metal may be selected, for example, from the group comprising: Al, Zr, Y, Sc, Ti, V, Nb, La and noble metals, preferably platinum metals, in particular Ru, Pd, Pt, Rh, Ir.

[0052] The material of the first layer 5a may likewise be a (stoichiometric or nonstoichiometric) oxide or a (stoichiometric or nonstoichiometric) mixed oxide. The oxide or the mixed oxide typically comprises at least one optical element selected from the group comprising: Al, Zr, Y. Alternatively the first layer 5a may comprise or consist of (at least) one metal. The metal may in particular be selected from the group comprising: Al, Mo, Ta, Cr. The material of the first layer 5a may alternatively be selected from the group comprising: C, B.sub.4C, BN. These materials have been found to be advantageous as diffusion barriers.

[0053] The protective effect of the protective layer system 5 is dependent not only on the materials which are selected for the three layers 5a-5c but also on whether the 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.

[0054] Described below are two examples of a protective layer system 3, in which the materials have been harmonized with one another in terms of their properties. In the first example, the third layer 5c is formed of TiO.sub.x and has a thickness d.sub.3 of 1.5 nm, the second layer 5b is formed of Ru and has a thickness d.sub.2 of 2 nm, and the first layer 5a is formed of AlO.sub.x and likewise has a thickness d.sub.1 of 2 nm. In the second example, the third layer 5c is formed of YO.sub.x and has a thickness d.sub.3 of 2 nm, the second layer 5b is formed of Rh and has a thickness d.sub.2 of 1.5 nm, and the first layer 5a is formed of Mo and has a thickness d.sub.1 of 3 nm. The total layer thickness D of the protective layer system 5 is 5.5 nm in the first example and 6.5 nm in the second example. It will be appreciated that as well as the examples described here, other combinations of materials are also possible, and that the thicknesses of the three layers 5a-c of the protective layer system 5 may differ from the values indicated above.

[0055] FIG. 1B shows an optical element 1 wherein small amounts of ions 6 have been implanted into the second layer 5b in order to counteract the implantation of Sn ions possibly present in the environment of the optical element 1. The ions 6 may be, for example, noble gas ions, e.g., Ar ions, Kr ions or Xe ions. The implanted ions may also be noble metal ions, as for example Pd ions, Pt ions, Rd ions, or possibly Ir ions. The noble metal ions serve as hydrogen and/or oxygen blockers.

[0056] Additionally or alternatively to the implantation of ions, it is also possible for metallic particles to be implanted into the second layer 5b, by, for example, doping the second layer 5b with metallic (nano)particles 7, in particular with particles and/or with (foreign) atoms of a noble metal, e.g., of Pd, Pt, Rh, Ir. It will be appreciated that the implantation of ions 6 and of metallic particles 7 may also take place for the first layer 5a and for the third layer 5c.

[0057] In the example shown in FIG. 1B, metallic (nano)particles 7, more specifically noble metal particles and/or noble metal atoms, have been applied to the third, topmost layer 5c. The application of (nano)particles 7, in particular in the form of Pd, Pt, Rh, Ir, may take place in individualized form, in particular in the form of individual atoms, or else in clusters (e.g. in groups of not more than 25 atoms).

[0058] In the example shown in FIG. 1B, the multilayer system 3 of the optical element 1 comprises a topmost layer 3b′ of silicon with a thickness d of more than 0.5 nm. The thickness d of the topmost layer 3b′ is selected such that the reflection of the multilayer system 3 is at its maximum. It will be appreciated that, alternatively, the topmost layer 3b′ of the multilayer system 3 may be embodied as in FIG. 1A, i.e. may have a thickness of less than 0.5 nm.

[0059] FIG. 1C shows a protective layer system 3 which between the first layer 5a and the second layer 5b comprises a further, fourth layer 5d, which has a thickness d.sub.4 of not more than 0.5 nm. The fourth layer 5d comprises a metal, more specifically a noble metal, for example Pd, Pt, Rh and/or Ir. The fourth (thin) layer 5d forms a sub-monolayer layer and contributes to the minimization of defects, hence being able to serve as a barrier to the penetration of hydrogen and/or oxygen into the underlying first layer 5a. It will be appreciated that the fourth layer 5d for the purpose of minimizing defects may also be formed between the second layer 5b and the third layer 5c or optionally on the third layer 5c, which in that case does not form the topmost layer of the protective layer system 5. The protective layer system 5 may also optionally comprise a fifth, sixth, . . . layer, in order to minimize the number of defects and/or in order to form a barrier for hydrogen and/or for oxygen.

[0060] The optical elements 1 illustrated in FIGS. 1A-C 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.

[0061] 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 approximately 5 nanometers and approximately 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 FIGS. 1A-C 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.

[0062] 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 is arranged 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).

[0063] 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.

[0064] The structured object M reflects part of the illumination beam 104 and shapes a projection beam path 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.

[0065] 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, only two mirrors can also be used, if appropriate.

[0066] 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.

[0067] The optical element 1 illustrated in FIGS. 1A-C 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 FIGS. 1A-C 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 FIGS. 1A-C 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.