OPTICAL ELEMENT FOR REFLECTING EUV RADIATION, EUV LITHOGRAPHY SYSTEM AND METHOD FOR SEALING A GAP

Abstract

An optical element (1) for reflecting EUV radiation (4) includes: a substrate (2); a coating (3) applied to the substrate (2), which coating reflects the EUV radiation (4); a top layer (5) protecting the reflective coating (3), which top layer is applied to the reflective coating (3); and an intermediate layer (6) having at least one reactive material (7) which, together with an activating gas (O2) penetrating through a gap (5a) in the top layer 95), forms at least one reaction product (8) sealing the gap (5a). A related EUV lithography system has at least one such reflective optical element (1), and a related method for sealing a gap (5a) in the top layer (5) of such an optical element (1) are also disclosed.

Claims

1. An optical element for reflecting extreme ultraviolet (EUV) radiation, comprising: a substrate, a reflective coating applied to the substrate and configured to reflect the EUV radiation, a capping layer applied to the reflective coating and configured to protect the reflective coating, and an intermediate layer arranged between the reflective coating and the capping layer, wherein the intermediate layer comprises at least one reactive material which, together with an activating gas penetrating through a gap in the capping layer, forms at least one reaction product sealing the gap, and wherein the intermediate layer has at least one ply composed of a glass material.

2. The optical element as claimed in claim 1, wherein the reactive material is selected from the group consisting essentially of: borides, silicides and carbides.

3. The optical element as claimed in claim 1, wherein the activating gas is selected from the group consisting essentially of: oxygen (O.sub.2), nitrogen, hydrogen and combinations thereof.

4. The optical element as claimed in claim 3, wherein the activating gas is water.

5. The optical element as claimed in claim 1, wherein the ply is formed from an aluminosilicate glass or from a borosilicate glass.

6. The optical arrangement as claimed in claim 1, wherein the ply contains at least one material selected from the group consisting essentially of: Al, Ti, Si, Ba, V, B, O, N, Zr, Sc, Mn, Ge, Pd, Cr.

7. The optical element as claimed in claim 1, wherein the reactive material is introduced into the glass material.

8. The optical element as claimed in claim 7, wherein the reactive material is introduced into the glass material as nanoparticles.

9. The optical element as claimed in claim 1, wherein the reactive material is introduced into at least one further ply of the intermediate layer.

10. The optical element as claimed in claim 1, wherein the intermediate layer has a thickness of between 0.2 nm and 10 nm.

11. The optical element as claimed in claim 1, wherein the intermediate layer and/or the capping layer are/is applied by a method selected from the group consisting essentially of: laser beam evaporation, atomic layer deposition, magnetron sputtering and electron beam evaporation.

12. The optical element as claimed in claim 1, wherein the capping layer comprises at least one metallic material, an oxide or a nitride.

13. The optical element as claimed in claim 1, wherein the material of the capping layer is selected from the group consisting essentially of: Ru, Rh, Pd, Ir, Ta, AlO.sub.x, HfO.sub.x, ZrO.sub.x, TaO.sub.x, TiO.sub.x, NbO.sub.x, WO.sub.x, CrO.sub.x, TiN, SiN, ZrN, YO.sub.x, LaO.sub.x, CeO.sub.x and combinations thereof.

14. The optical element as claimed in claim 1, wherein the capping layer has a thickness of between 0.5 nm and 10 nm.

15. The optical element as claimed in claim 1, wherein the reflective coating forms a multilayer coating for reflecting EUV radiation incident on the reflective optical element with normal incidence, wherein the multilayer coating has alternating plies composed of a first material and a second material having different refractive indices.

16. The optical element as claimed in claim 1, wherein the reflective coating is configured for reflecting EUV radiation incident on the reflective optical element with grazing incidence.

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

18. A method for sealing a gap in a capping layer of an optical element as claimed in claim 1, comprising: forming the reaction product with the activating gas penetrating through the gap in the capping layer, and sealing the gap with the formed reaction product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

[0035] FIGS. 1A-1C show schematic illustrations of a conventional EUV mirror, comprising a reflective multilayer coating and a capping layer, both without (FIG. 1A) and with (FIG. 1B) a crack that exposes the multilayer coating to an oxidizing gas (FIG. 1C),

[0036] FIGS. 2A and 2B show schematic illustrations analogous to FIGS. 1A-C, in which a self-healing intermediate layer sealing the crack is arranged between the capping layer and the multilayer coating, before (FIG. 2A) and after (FIG. 2B) conversion into a reaction product,

[0037] FIGS. 3A and 3B show schematic illustrations analogous to FIGS. 2A and 2B respectively with a reflective coating in the form of a single ply, and

[0038] FIG. 4 shows a schematic illustration of an EUV lithography apparatus.

DETAILED DESCRIPTION

[0039] In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

[0040] FIGS. 1A-C schematically show the construction of an optical element 1 comprising a substrate 2 and a reflective multilayer coating 3 for reflecting EUV radiation 4, said multilayer coating being applied to the substrate 2. A capping layer 5 forming an interface with the environment of the optical element 1 is applied to the reflective multilayer coating 3. In the example shown, the capping layer 5 is formed from Ru. An intermediate layer 6 is arranged between the capping layer 5 and the reflective multilayer coating 3, which intermediate layer, in the example shown, consists of C and serves as a barrier layer for preventing the Ru material from penetrating into the reflective multilayer coating 3.

[0041] 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 α of incidence of typically less than approximately 45° with respect to the surface normal. In this case, the reflective coating 3 is embodied as a multilayer coating and has a plurality of, e.g. more than fifty, alternating plies 3a, 3b formed from materials having different refractive indices.

[0042] In the example shown, in which the EUV radiation 4 has a used wavelength of 13.5 nm, the materials are silicon and molybdenum (see FIG. 1A). Depending on the used wavelength employed, other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium, or lanthanum and B.sub.4C are likewise possible. The substrate 2 is generally formed from a so-called zero expansion material having a very low coefficient of thermal expansion, for example composed of Zerodur® or composed of titanium-doped quartz glass (ULE®).

[0043] During operation of the optical element 1 in an EUV lithography apparatus, damage to the capping layer 5 can occur for various reasons, said damage resulting in the occurrence of a gap 5a in the capping layer 5. As can be discerned in FIG. 1B, the gap 5a extends over the entire thickness D of the capping layer 5 as far as the intermediate layer 6. The gap 5a illustrated in FIG. 1B can be a crack or a hole, for example. Through the gap 5a gases, for example oxygen O.sub.2, from the environment can pass through the capping layer 5 to the intermediate layer 6 and diffuse through the latter into the reflective coating 3. In the reflective coating 3 the oxygen can oxidize the materials of the alternating plies 3a, 3b. In the example shown, the Si of the first plies 3a is at least partly oxidized to Si.sub.Ox and the Mo of the second plies 3b is at least partly oxidized to MoO.sub.x, as is illustrated in FIG. 1C. The oxidation of the materials of the plies 3a, 3b alters the optical properties thereof, in particular the refractive index thereof, which has the effect that the reflectivity of the optical element 1 for the EUV radiation 4 decreases significantly.

[0044] In order to prevent such damage of the reflective coating 3 as a result of oxidation or to counteract the latter, the optical element 1 shown in FIGS. 2A,B has a self-healing intermediate layer 6, which seals the gap 5a or the crack, such that the oxygen O.sub.2 penetrating through the capping layer 5 cannot diffuse as far as the reflective coating 3. In order to achieve this, in the example shown in FIGS. 2A,B, the intermediate layer 6 has a first, upper ply 6a composed of a glass material or a composite glass, composed of, in particular, aluminosilicate glass, and a second, lower ply 6b composed of vanadium boride (VB).

[0045] As has been described further above in association with FIGS. 1A-C, the oxygen O.sub.2 in the form of a plasma passes through the gap 5a in the capping layer 5 and firstly impinges on the upper ply 6a of the intermediate layer 6. The upper ply 6a is formed from an aluminosilicate glass that has a zeolite structure and is porous. Such a ply 6a composed of aluminosilicate glass can be produced for example in the manner described in the article by S. Shaikhutdinov and H-J. Freund cited in the introduction. The layer thickness of the upper ply 6a is typically very small and can be for example less than approximately 10 nm or optionally 1 nm or less. In particular, the upper ply 6a can be formed only by one monolayer or optionally by a few monolayers of the aluminosilicate glass. Instead of an aluminosilicate, the upper ply 6a can also be formed from a silicate glass material in which Al is replaced by another metallic material, for example by Ti, Zr, etc.

[0046] The oxygen O.sub.2 that has passed through the upper ply 6a and is present in the form of an O.sub.2 plasma impinges on the lower ply 6b or diffuses into the latter. The O.sub.2 plasma serves as activating gas for the vanadium boride material of the lower ply 6b, which constitutes a chemically reactive material 7 and is oxidized to VO.sub.x and BO.sub.x by the O.sub.2 plasma at a comparatively low temperature of less than approximately 100° C. VO.sub.x and BO.sub.x are liquid or volatile reaction products 8 which penetrate from the lower ply 6b into the upper ply 6a and possibly partly further into the gap 5a and seal or close the latter. In this case, the reaction products 8 additionally react with the glass matrix of the upper ply 6a, such that the latter loses its porous structure and seals the gap 5a in the manner of a plug.

[0047] As has been described further above, it is possible to deposit or apply the upper ply 6a with a very small thickness. The same applies to the lower ply 6b composed of vanadium boride. The intermediate layer 6 can therefore have overall a very small thickness d that is between approximately 0.2 nm and approximately 10 nm. In this way it is ensured that the reflectivity of the optical element 1 is only slightly reduced by the presence of the intermediate layer 6.

[0048] The capping layer 5, too, has a thickness D that is between 0.5 nm and 10 nm in the example shown, in order to prevent the reflectivity of the optical element 1 from being excessively reduced by the presence of the capping layer 5. Besides the thickness D of the capping layer 5, the decrease in reflectivity is also dependent on the material of the capping layer 5. The capping layer 5 can comprise a metallic material, an oxide or a nitride, for example. In addition or as an alternative to the Ru described above, the material of the capping layer 5 can be selected from the group comprising: Rh, Pd, Ir, Ta, AlO.sub.x, HfO.sub.x, ZrO.sub.x, TaO.sub.x, TiO.sub.x, NbO.sub.x, WO.sub.x, CrO.sub.x, TiN, SiN, ZrN, YO.sub.x, LaO.sub.x, CeO.sub.x and combinations thereof. In a departure from the illustration in FIGS. 1A-C and in FIGS. 2A,B, the capping layer 5 can comprise two or more plies.

[0049] Instead of an optical element 1 having a self-healing intermediate layer 6 comprising two plies 6a, 6b, it is also possible to use a self-healing intermediate layer 6 which comprises only a single ply or which consists of the single ply, as is described below with reference to FIGS. 3A,B. The intermediate layer 6 shown in FIGS. 3A,B consists of a glass material in the form of a borosilicate glass or a silicate glass containing boron particles. The boron particles have a diameter of typically less than approximately 10 nm and are embedded into the glass matrix. Besides SiO.sub.2, the glass material comprises further constituents, specifically Al.sub.2O.sub.3, CaO and BaO. The glass material of the intermediate layer 6 can correspond in particular to the composition described in the article in J. Am. Ceram. Soc. 99, 849-855 (2016) cited in the introduction. The glass material can additionally or alternatively also comprise other materials, for example Ti, N, Zr and/or V, B (see below).

[0050] The boron particles 7 form a reactive material which reacts with oxygen O.sub.2 as activating gas (cf. FIG. 3A) and in this case forms liquid boron oxide (B.sub.2O.sub.3) as reaction product 8, said boron oxide sealing the gap 5a in the capping layer 5 (cf. FIG. 3B) by formation of bridges in the glass material and partly in the gap 5a, which limit or prevent the diffusion of oxygen O.sub.2 into the reflective coating 3.

[0051] In contrast to the optical element 1 illustrated in FIGS. 2A,B, the optical element 1 illustrated in FIGS. 3A,B is designed for reflecting EUV radiation 4 incident with grazing incidence, i.e. for EUV radiation 4 which impinges on the optical element 1 at angles α of incidence of more than approximately 60° with respect to the surface normal. For this purpose, the reflective coating 3 comprises a single ply composed of ruthenium. In contrast to the illustration in FIGS. 3A,B, the reflective coating 3 can comprise two or more plies. Instead of ruthenium, the ply (plies) of the reflective coating 3 can also contain other materials or consist of other materials, e.g. composed of Mo or Nb. The substrate 2 of the optical element 1 illustrated in FIGS. 3A,B is formed from a ceramic material, for example composed of aluminum oxide (Al.sub.2O.sub.3) or composed of silicon carbide (SiC).

[0052] As an alternative to the examples described further above, the activating gas can be hydrogen or nitrogen or combinations thereof which, together with a suitable reactive material, form a reaction product which seals the gap 5a in the capping layer 5 and in this way prevents as completely as possible the diffusion of the active gas into the underlying reflective coating 3. The reactive material 7 can in principle be borides, silicides and carbides, for example the vanadium boride described further above. Boron or boron particles, vanadium or vanadium particles and optionally other types of particles can also serve as reactive material 7.

[0053] In the examples described further above, both the intermediate layer 6 and the capping layer 5 were applied by laser beam evaporation. However, it is also possible for the capping layer 5 and in particular the intermediate layer 6 to be applied to the substrate 2 or to the respective underlying ply or layer by some other coating method, for example by atomic layer deposition, magnetron sputtering or electron beam evaporation. Besides laser beam evaporation, atomic layer deposition, in particular, makes it possible to deposit very thin plies with a thickness of a few monolayers.

[0054] The optical elements 1 illustrated in FIGS. 2A,B and in FIGS. 3A,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. 4.

[0055] 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. 4 is designed for an operating wavelength of the EUV radiation of 13.5 nm, for which the optical elements 1 illustrated in FIGS. 2A,B and in FIGS. 3A,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.

[0056] The EUV lithography apparatus 101 furthermore comprises a collector mirror 103 in order to focus the EUV radiation of the EUV light source 102 to form an illumination beam 104 and to increase the energy density further in this way. The illumination beam 104 serves for the illumination of a structured object M with an illumination system 110, which in the present example has five reflective optical elements 112 to 116 (mirrors).

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 and water is formed in the vacuum environment 127.

[0061] The optical element 1 illustrated in FIGS. 2A,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. The optical element 1 shown in FIGS. 3A,B and designed for grazing incidence of the EUV radiation 4 can be the last optical element 116 of the illumination system 110. In contrast to the illustration in FIG. 4, further reflective optical elements 103, 112 to 115 of the illumination system 110 and/or reflective optical elements 121 to 126 of the projection system 120 can be configured for EUV radiation 4 incident with grazing incidence.