OPTICAL ELEMENT AND OPTICAL ASSEMBLY COMPRISING SAME

Abstract

An optical element (14), in particular for EUV lithography, includes a substrate (15), a reflective coating (16) arranged on the substrate (15), and an electrically conductive coating (19) extending between the substrate and the reflective coating, and having at least one first layer (22a) under tensile stress and at least one second layer (22b) under compressive stress. The electrically conductive coating has at least one section (20) that extends on the substrate laterally beyond the reflective coating. Also disclosed is an optical assembly, in particular an EUV lithography system, provided with at least one optical element of this type.

Claims

1. An optical element, comprising: a substrate, a reflective coating arranged on the substrate, and an electrically conductive coating extending between the substrate and the reflective coating, wherein the electrically conductive coating has at least one first layer under tensile stress and at least one second layer under compressive stress, and wherein the electrically conductive coating has at least one section that extends on the substrate laterally beyond the reflective coating.

2. The optical element as claimed in claim 1 and configured to reflect extreme ultraviolet (EUV) light.

3. The optical element as claimed in claim 1, wherein the first and/or the second layer are/is formed from a metallic material or a metallic alloy.

4. The optical element as claimed in claim 1, wherein a material of the first layer and/or a material of the second layer are/is selected from the group comprising: silver, copper, gold, aluminum, rhodium, iridium, tungsten, molybdenum, cobalt, nickel, ruthenium, indium, osmium, iron, platinum, palladium, chromium, tantalum, titanium, Zr, Re and alloys thereof.

5. The optical element as claimed in claim 1, wherein the electrically conductive coating comprises at least one barrier layer that is arranged between the first layer and the second layer.

6. The optical element as claimed in claim 1, further comprising at least one barrier layer arranged between the reflective coating and the electrically conductive coating.

7. The optical element as claimed in claim 5, wherein a material of the barrier layer is selected from the group comprising: W, Ta, Y, Mo, Zr, Ti, Hf, Sc, alloys and/or compounds thereof.

8. The optical element as claimed in claim 7, wherein the material of the barrier layer is selected from the group comprising: carbides, nitrides, borides, silicides, C and B.sub.4C.

9. The optical element as claimed in claim 6, wherein a material of the barrier layer is selected from the group comprising: W, Ta, Y, Mo, Zr, Ti, Hf, Sc, alloys and/or compounds thereof.

10. The optical element as claimed in claim 9, wherein the material of the barrier layer is selected from the group comprising: carbides, nitrides, borides, silicides, C and B.sub.4C.

11. The optical element as claimed in claim 1, wherein the electrically conductive coating has a thickness of between 50 nm and 1000 nm.

12. The optical element as claimed in claim 1, wherein the first layer has a thickness that is greater than a thickness of the second layer, and wherein a material of the first layer has an absorption for EUV radiation that is greater than an absorption for the EUV radiation of a material of the second layer.

13. The optical element as claimed in claim 1, wherein the second layer has a thickness that is greater than the thickness of the first layer, and wherein a material of the second layer has an absorption for EUV radiation that is greater than an absorption for the EUV radiation of a material of the first layer.

14. The optical element as claimed in claim 1, wherein the reflective coating comprises a plurality of alternating individual layers composed of materials having mutually different refractive indices.

15. The optical element as claimed in claim 1, further comprising at least one protective layer configured to protect the substrate against EUV radiation and arranged between the electrically conductive coating and the substrate.

16. An optical assembly, comprising: at least one optical element as claimed in claim 1.

17. The optical assembly as claimed in claim 16, configured as an EUV lithography system and comprising: a beam shaping system, an illumination system, and a projection system comprising the at least one optical element.

18. The optical assembly as claimed in claim 16, further comprising: an electrical line configured to contact the at least one section of the electrically conductive coating that extends on the substrate laterally beyond the reflective coating.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0032] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the drawing:

[0033] FIG. 1 shows a schematic illustration of an EUV lithography apparatus,

[0034] FIG. 2 shows a schematic illustration of a reflective optical element comprising an electrically conductive coating consisting of a first layer under a tensile stress,

[0035] FIG. 3 shows a schematic illustration analogously to FIG. 2, wherein the electrically conductive coating has a second layer under a compressive stress, which compensates for the tensile stress of the first layer, and

[0036] FIG. 4 shows a schematic illustration analogously to FIG. 3, wherein the electrically conductive coating extends on the top side of the substrate laterally beyond the reflective coating.

DETAILED DESCRIPTION

[0037] In the following description of the drawings, identical reference signs are used for identical, functionally identical or equivalent components.

[0038] FIG. 1 schematically shows an EUV lithography system in the form of a projection exposure apparatus 1 for EUV lithography. The projection exposure apparatus 1 comprises a beam shaping system 2, an illumination system 3 and a projection system 4, which are accommodated in separate vacuum housings and arranged successively in a beam path 6 emanating from an EUV light source 5 of the beam shaping system 2. A plasma source or a synchrotron can serve for example as the EUV light source 5. The radiation emerging from the light source 5 in the wavelength range between about 5 nm and about 20 nm is first focused in a collimator 7. With the aid of a downstream monochromator 8, the desired operating wavelength X,B, which in the present example is about 13.5 nm, is filtered out by variation of the angle of incidence, as indicated by a double-headed arrow. The collimator 7 and the monochromator 8 are configured as reflective optical elements.

[0039] The radiation treated with regard to wavelength and spatial distribution in the beam shaping system 2 is introduced into the illumination system 3, which has a first and a second reflective optical element 9, 10. The two reflective optical elements 9, 10 guide the EUV radiation onto a photomask 11 as further reflective optical element, which has a structure that is imaged onto a wafer 12 on a reduced scale with the projection system 4. For this purpose, a third and a fourth reflective optical element 13, 14 are provided in the projection system 4.

[0040] The design of the fourth optical element 14 is described in greater detail by way of example below with reference to FIG. 2 to FIG. 4; the first to third optical elements 11, 12, 13 have a corresponding design. The (fourth) optical element 14 comprises a substrate 15 composed of a material having a low coefficient of thermal expansion, which is typically less than 100 ppb/K at 22 C. or over a temperature range of approximately 5 C. to approximately 35 C. One material which has these properties is silicate or quartz glass doped with titanium dioxide and typically having a silicate glass proportion of more than 90%. Such a commercially available silicate glass is sold by Corning Inc. under the trade name ULE (Ultra Low Expansion glass). A further material group having a very low coefficient of thermal expansion is that of glass ceramics, in which the ratio of the crystal phase to the glass phase is set such that the coefficients of thermal expansion of the different phases nearly cancel one another out. Such glass ceramics are offered for example under the trade name Zerodur by Schott AG or under the trade name Clearceram by Ohara Inc.

[0041] A reflective coating 16 having a plurality of individual layers 17a, 17b consisting of different materials is applied on the substrate 15. In the present case, the individual layers alternately consist of materials having different refractive indices. If the operating wavelength kB is approximately 13.5 nm, as in the present case, then the individual layers usually consist of molybdenum and silicon. Other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and B.sub.4C are likewise possible. In addition to the individual layers described, the reflective coating 16 can also comprise intermediate layers and/or barrier layers for preventing diffusion. The illustration of such auxiliary layers has been omitted in the figures.

[0042] The reflective coating 16 also has a capping layer 18 in order to protect the underlying individual layers 17a, 17b and to prevent for example the oxidation thereof. The capping layer 18 consists of ruthenium in the present example. Other materials, in particular metallic materials such as rhodium, palladium, platinum, iridium, niobium, vanadium, chromium, zinc or tin, can also be used as capping layer materials. The capping layer 18 is transmissive to the EUV radiation 6.

[0043] As an alternative to the reflective coating 16 shown in FIG. 2, which is configured for normal-incidence EUV radiation 6, the reflective coating 16 can be configured or optimized for grazing-incidence EUV radiation 6. In this case, the reflective coating 16 can comprise, if appropriate, just a single layer, which can be formed for example from a metallic material, in particular from Mo, Ru or Nb.

[0044] In the case of the example shown in FIG. 2, an electrically conductive coating 19 consisting just of a single layer composed of a noble metal, composed of gold in the example shown, is arranged between the reflective coating 16 and the substrate 15. In the case of the example shown in FIG. 2, the electrically conductive coating 19 has a section that extends on a side surface of the substrate 15 in order to be able to electrically contact the electrically conductive coating 19. In the case of the example shown in FIG. 2, the electrical contacting is effected with an electrical line 21, which can be connected to ground potential, for example, in order to ground the optical element 14. The line 21 can also serve to conduct a photocurrent, generated in the reflective coating 16 as a result of the irradiation with the EUV radiation 6, away from the optical element 14 and to measure the photocurrent in a charge amplifier, for example.

[0045] The electrically conductive coating 19 is produced with the aid of conventional coating methods, specifically typically by Physical Vapor Deposition (PVD) methods such as e.g. thermal evaporation or electron beam evaporation, ion beam or magnetron sputtering or pulsed laser deposition, or coating methods based on Chemical Vapor Deposition (CVD). When the electrically conductive coating 19 is applied, a layer stress is produced which, in the case of the example shown in FIG. 2, wherein gold is used as material for the electrically conductive coating 19, leads to a tensile stress in the electrically conductive coating, as is indicated by two lateral arrows at the electrically conductive coating 19 in FIG. 2.

[0046] In the case of the thickness D of typically between approximately 50 nm and approximately 1000 nm, approximately 300 nm in the example shown, that is typically used for the electrically conductive coating 19, the tensile stress has the consequence that the substrate 15 warps, such that the latter forms a concave curvature at its side facing the reflective coating 16, said concave curvature being illustrated in a greatly exaggerated manner for elucidation purposes in FIG. 2. The electrically conductive layer 19 thus leads to a deviation of the surface shape of the optical element 14 from a desired surface shape (desired figure), which is a plane surface shape in the case of the example shown in FIG. 2. The deviation from the desired surface shape leads to aberrations of the optical element 14 during operation in the EUV lithography apparatus 1.

[0047] In order to avoid the undesired deformation of the substrate 15 and thus of the surface shape of the optical element 14, in the case of the example illustrated in FIG. 3, the electrically conductive coating 19 has a first layer 22a under a tensile stress and a second layer 22b under a compressive stress. The first layer 22a under the tensile stress and/or the second layer 22b under compressive stress can be formed for example from a material selected from the group comprising: silver, copper, gold, aluminum, rhodium, iridium, tungsten, molybdenum, cobalt, nickel, ruthenium, indium, osmium, iron, platinum, palladium, chromium, tantalum, titanium, Zr, Re. Both the first and the second layer 22a, 22b of the electrically conductive coating 19 can e.g. also be formed from an (electrically conductive) alloy instead of a metallic material. The fact of whether the first and/or second layer 22a, 22b are/is under a tensile stress or a compressive stress, depends not only on the material used but also on the coating method by which the respective material is applied. In the example shown, the first layer 22a, which consists of gold, is applied by electron beam evaporation, as a result of which a tensile stress is formed in the first layer 22a. In the example shown, the second layer 22b consists of ruthenium and was produced by sputtering, as a result of which a compressive stress is formed in the ruthenium material of the second layer 22b.

[0048] In the case of the example shown in FIG. 3, the tensile stress of the first layer 22a and the compressive stress of the second layer 22b are chosen such that the resulting layer stress of the electrically conductive coating 19 substantially disappears, with the result that the plane desired surface shape of the substrate 15 and thus also of the reflective surface of the optical element 14 is produced.

[0049] This makes use of the fact that the second layer 22b under compressive stress produces a warping of the substrate that is directed oppositely to the warping as a result of the first layer 22a under tensile stress, since the second layer 22b under compressive stress brings about a convex curvature of the substrate 15. In order to produce the compensation of the layer stresses of the electrically conductive coating 19, the layer materials and the thicknesses of the first and second layers 22a, 22b are suitably adapted to one another.

[0050] As can likewise be discerned in FIG. 3, a barrier layer 23 is arranged between the first layer 22a under tensile stress and the second layer 22b under compressive stress, said barrier layer being intended to substantially prevent a diffusion of the material of the first layer 22a into the second layer 22b (and vice-versa). The barrier layer 23 typically has such a small thickness (e.g. of less than 1 nm) that its layer stress on the electrically conductive coating 19 is practically negligible. However, if necessary, the influence of the tensile and/or the compressive stress of the barrier layer 23 on the resulting layer stress of the electrically conductive coating 19 must be taken into account. The electrically conductive coating 19 can have, if appropriate, more than one layer 22a under a tensile stress and more than one layer 22b under a compressive stress, and, if appropriate, further barrier layers. The material of the barrier layer 23 can be selected for example from the group comprising: W, Ta, Y, Mo, Zr, Ti, Hf, Sc, alloys and/or compounds thereof, in particular carbides, nitrides, borides, silicides, C and B.sub.4C.

[0051] Instead of a substantially complete compensation of the layer stresses of the layers 22a, 22b under tensile and respectively under compressive stress, it is also possible, with the aid of the electrically conductive coating 19, to produce a predefined resulting layer stress that can be used to wholly or partly compensate for layer stresses that occur in the reflective coating 16 as a result of the coating process. In order to be able to compensate for possibly locally varying layer stresses in the reflective coating 16, the electrically conductive coating 19 can exhibit a location-dependent variation of the thickness of the individual layers 22a, 22b under tensile and respectively under compressive stress. Such a possibly location-dependent variation of the layer thicknesses and/or of the layer materials used and/or of the composition of the layers 22a, 22b can, if appropriate, also be carried out for other reasons, for example in order to achieve protection against the influence of the EUV radiation 6 on the substrate 15.

[0052] In order to protect the substrate 15 against the EUV radiation 6, those layers 22a, 22b of the electrically conductive coating 19 which have a high absorption for the EUV radiation 6 should ideally have a larger thickness than those layers 22a, 22b which have a low absorption for the EUV radiation 6. In the case of the example shown in FIG. 2, the first layer 22a, which consists of gold, can have for example a thickness D1 that is greater than the thickness D2 of the second layer 22b, which consists of ruthenium, since gold has a greater absorption for the EUV radiation 6 than ruthenium. Materials that should be applied with a greater thickness in order to protect the substrate 15 can be selected for example from the group comprising: Fe, Ni, Co, Cu, Ag, Au, Pt, Wo, Cr, Zn, Ir, In, Sn and alloys and/or compounds thereof.

[0053] In the case of the example shown in FIG. 3, the electrically conductive coating 19, as in the case of the example shown in FIG. 2, has a section 20 that extends on the substrate 15 laterally beyond the reflective coating 16, specifically as in FIG. 2 along a side surface of the substrate 15. As in FIG. 2, the projecting section 20 serves for electrically contacting the electrically conductive coating 19. Instead of the section 20 of the electrically conductive coating 19, in the case of the example shown in FIG. 3, it is also possible for just a portion, for example a single one, of the layers 22a, 22b to project on the side surface of the substrate 15 beyond the reflective coating 16, since in this region the influence of the tensile and/or the compressive stress on the surface shape of the optical element 14 is typically less than on the top side of the substrate 15. In general, however, it is more advantageous for the entire (stress-compensated) electrically conductive coating 19 to form the projecting section 20, as is illustrated in FIG. 3.

[0054] FIG. 4 shows a further example of an optical element 14, which differs from the optical element 14 shown in FIG. 3 in that the electrically conductive coating 19 has a section 20 which projects laterally beyond the reflective coating 16 and which is not formed on the side surface of the substrate 15, but rather on the top side thereof. As can be discerned in FIG. 4, in this case the electrically conductive coating 19 on the top side of the substrate 15 can be contacted with an electrical line 21 in order to ground the optical element 14 and, if appropriate, to conduct a photocurrent away from the latter.

[0055] In the case of the example shown in FIG. 4, a barrier layer 23 is additionally formed between the reflective coating 16 and the electrically conductive coating 19 in order to prevent the diffusion of material from the reflective coating 16 into the electrically conductive coating 19 (and vice-versa). In the example shown, the barrier layer 23 consists of carbon, but can also be formed from some other material, for example from W, Ta, B.sub.4C or Mo.

[0056] In the case of the example shown in FIG. 4, a protective layer 24 is formed between the electrically conductive coating 19 and the substrate 15, said protective layer serving for protecting the substrate 15 against the EUV radiation 6. This may be necessary if the electrically conductive coating 19 does not sufficiently absorb the EUV radiation 6, such that the latter can pass to the substrate 15. The protective layer 24 in turn typically consists of a conductive material, for example of: iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), silver (Ag), gold (Au), platinum (Pt), tungsten (Wo), chromium (Cr), zinc (Zn), iridium (Ir), indium (In), tin (Sn) and alloys and/or compounds thereof. The layer stress of the protective layer 24 can be disregarded, if appropriate, but can also be taken into account, if appropriate, in the compensation of the layer stresses as described further above.

[0057] With the electrically conductive coating 19 described further above, the optical element 14 can be grounded without its optical properties being impaired. Moreover, the electrically conductive coating 19 can have a defined bias voltage applied to it or be used for conducting away a photocurrent. The electrically conductive coating 19 can in particular also serve for protecting the substrate 15 against the EUV radiation 6.