REFLECTIVE OPTICAL ELEMENT, ILLUMINATION OPTICAL UNIT, PROJECTION EXPOSURE APPARATUS, AND METHOD FOR PRODUCING A PROTECTIVE LAYER

Abstract

A reflective optical element (17), in particular for an illumination optical unit of a projection exposure apparatus includes: a structured surface (25a) that preferably forms a grating structure (29), and a reflective coating (36) that is applied to the structured surface (25a). The reflective coating (36) covers the structured surface (25a) discontinuously, and the reflective optical element (17) has at least one protective layer (37) that covers the structured surface (25a) continuously. Also disclosed are an illumination optical unit (4) for a projection exposure apparatus (1) including at least one reflective optical element (17) of this type, to a projection exposure apparatus (1) including an illumination optical unit (4) of this type, and to a method for producing a protective layer (37) on a reflective optical element (17) of this type.

Claims

1. A reflective optical element configured as a collector mirror for an illumination optical unit of a projection exposure apparatus, comprising: a structured surface, and a reflective coating applied to the structured surface, wherein the reflective coating covers the structured surface discontinuously, and the reflective optical element has at least one protective layer that covers the structured surface continuously.

2. The reflective optical element as claimed in claim 1, wherein the structured surface forms a grating structure.

3. The reflective optical element as claimed in claim 1, wherein the structured surface has a maximum flank steepness (α) of more than 60°.

4. The reflective optical element as claimed in claim 3, wherein the grating structure has a maximum flank steepness (α) of more than 90°.

5. The reflective optical element as claimed in claim 1, wherein the protective layer has a thickness (d) of 100 nm or less.

6. The reflective optical element as claimed in claim 5, wherein the protective layer has a thickness (d) of 5 nm or less.

7. The reflective optical element as claimed in claim 1, wherein the reflective coating forms a multilayer coating for reflection of extreme ultraviolet radiation.

8. The reflective optical element as claimed in claim 1, wherein the protective layer is formed between the structured surface and the reflective coating.

9. The reflective optical element as claimed in claim 1, further comprising a cap layer applied to the reflective coating.

10. The reflective optical element as claimed in claim 9, wherein the protective layer forms the cap layer that fully covers the structured surface.

11. The reflective optical element as claimed in claim 1, wherein the structured surface is formed in a functional layer applied to a substrate and/or in the substrate.

12. The reflective optical element as claimed in claim 11, wherein the functional layer and/or the substrate includes at least one material selected from the group consisting essentially of: amorphous silicon (aSi), silicon (Si), nickel-phosphorus (Ni:P), metals, alloys thereof, oxides, and combinations thereof.

13. The reflective optical element as claimed in claim 12, wherein the functional layer and/or the substrate includes at least one material selected from the group consisting essentially of: titanium (Ti), platinum (Pt), gold (Au), aluminum (Al), nickel (Ni), copper (Cu), silver (Ag), tantalum (Ta), tungsten (W), silicon dioxide (SiO.sub.2), aluminum oxide (AlO.sub.x), titanium oxide (TiO.sub.x), tantalum oxide (TaO.sub.x), niobium oxide (NbO.sub.X), zirconium oxide (ZrO.sub.x), mixed oxides, ceramics, glasses, glass ceramics, and composite materials.

14. The reflective optical element as claimed in claim 1, wherein the protective layer includes at least one material selected from the group consisting essentially of: metals and alloys thereof, oxides, carbides, carbonitrides, borides, nitrides, silicides and combinations thereof.

15. The reflective optical element as claimed in claim 14, wherein the protective layer includes at least one material selected from the group consisting essentially of: copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), gold (Au), tungsten (W), alloys thereof, oxides, carbides, carbonitrides, borides, nitrides, silicides and combinations thereof.

16. The reflective optical element as claimed in claim 15, wherein the oxides are selected from the group consisting essentially of: aluminum oxide (AlO.sub.x), zirconium oxide (ZrO.sub.x), titanium oxide (TiO.sub.x), niobium oxide (NbO.sub.x), tantalum oxide (TaO.sub.x), hafnium oxide (HfO.sub.x), and chromium oxide (CrO.sub.x), mixed oxides, ceramics, glasses, glass ceramics, and composite materials.

17. An illumination optical unit for a projection exposure apparatus, comprising: at least one reflective optical element as claimed in claim 1.

18. A projection exposure apparatus for microlithography, comprising: an illumination optical unit as claimed in claim 17 and configured to transfer illumination radiation from a radiation source onto a reticle comprising structures to be imaged, and a projection optical unit configured to image the structures of the reticle onto a wafer.

19. A method, comprising: providing a reflective optical element as claimed in claim 1, and forming the protective layer on the reflective optical element by applying the protective layer to the structured surface or to the reflective coating with an isotropic coating method.

20. The method as claimed in claim 19, wherein the isotropic coating method is selected from the group consisting essentially of: chemical vapor deposition and physical vapor deposition.

21. The method as claimed in claim 20, wherein the isotropic coating method is selected from the group consisting essentially of: atomic layer deposition and physical vapor deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Working examples are shown in the schematic drawing and are detailed in the description which follows. The figures show:

[0036] FIG. 1 a schematic in meridional section of a projection exposure apparatus for EUV lithography;

[0037] FIG. 2 a schematic diagram of a process sequence in the production of a structured surface that forms a grating structure,

[0038] FIGS. 3A and 3B schematic diagrams of a detail of the structured surface of FIG. 2 with grating flanks having different flank steepnesses (90° and >90°, respectively),

[0039] FIGS. 4A and 4B schematic diagrams analogously to FIGS. 3A and 3B, respectively, with a reflective coating applied to the structured surface that covers the structured surface in a noncontinuous manner,

[0040] FIGS. 5A and 5B schematic diagrams analogously to FIGS. 3A and 3B, respectively, with a protective layer which is formed between the grating structure and the reflective coating and covers the grating structure in a continuous manner,

[0041] FIG. 6 a schematic diagram analogously to FIG. 5A, in which the protective layer is applied to the reflective coating and covers the grating structure in a continuous manner, and

[0042] FIG. 7 a schematic diagram analogously to FIG. 5A, in which a cap layer that covers the grating structure in a non-continuous manner is applied to the reflective coating.

DETAILED DESCRIPTION

[0043] In the description of the drawings that follows, identical reference signs are used for components that are the same or have the same function, or are analogous or have analogous function.

[0044] The salient constituents of a projection exposure apparatus 1 of microlithography are described hereinafter by way of example with reference to FIG. 1. The description of the basic setup of the projection exposure apparatus 1 and constituents thereof should not be considered here to be restrictive.

[0045] An illumination system 2 of the projection exposure apparatus 1, as well as a radiation source 3, has an illumination optical unit 4 for illumination of an object field 5 in an object plane 6. What is exposed here is a reticle 7 disposed in the object field 5. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9, in particular in a scanning direction.

[0046] For purposes of description, a Cartesian xyz coordinate system is shown in FIG. 1. The x direction runs perpendicularly to the plane of the drawing. The y direction runs horizontally, and the z direction runs vertically. The scanning direction runs in the y direction in FIG. 1. The z direction runs perpendicularly to the object plane 6.

[0047] The projection exposure apparatus 1 comprises a projection optical unit 10. The projection optical unit 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by way of a wafer displacement drive 15, in particular in the y direction. The displacement of the reticle 7 on the one hand by way of the reticle displacement drive 9 and of the wafer 13 on the other hand by way of the wafer displacement drive 15 may be synchronized with one another.

[0048] The radiation source 3 is an EUV radiation source. The radiation source 3 emits EUV radiation 16 in particular, which is also referred to below as used radiation or illumination radiation. In particular, the used radiation has a wavelength in the range of between 5 nm and 30 nm. The radiation source 3 may be a plasma source, for example an LPP (“laser produced plasma”) source or a GDPP (“gas discharged produced plasma”) source. It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).

[0049] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 may be a collector mirror with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The illumination radiation 16 may be incident on the at least one reflection surface of the collector mirror 17 with grazing incidence (GI), i.e. at angles of incidence of greater than 45°, or with normal incidence (NI), i.e. at angles of incidence of less than 45°. The collector mirror 17 may be structured and/or coated, firstly to optimize its reflectivity for the used radiation and secondly to suppress extraneous light.

[0050] The illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18 downstream of the collector mirror 17. The intermediate focal plane 18 may constitute a separation between a radiation source module, having the radiation source 3 and the collector mirror 17, and the illumination optical unit 4.

[0051] The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream thereof in the beam path, a first facet mirror 20. The first facet mirror 20 comprises a multiplicity of individual first facets 21, which are also referred to as field facets below. FIG. 1 depicts only some of these facets 21 by way of example. In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. The second facet mirror 22 comprises a plurality of second facets 23.

[0052] The illumination optical unit 4 consequently forms a doubly faceted system. This basic principle is also referred to as fly’s eye integrator. With the aid of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-shaping mirror or actually the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5.

[0053] The projection optical unit 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1.

[0054] In the example illustrated in FIG. 1, the projection optical unit 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are likewise possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection optical unit 10 is a double-obscured optical unit. The projection optical unit 10 has an image-side numerical aperture which is greater than 0.5 and which may also be greater than 0.6 and, for example, can be 0.7 or 0.75.

[0055] Just like the mirrors of the illumination optical unit 4, the mirrors Mi may have highly reflective coatings for the illumination radiation 16. The collector mirror 17 of the illumination optical unit 4, in order to suppress extraneous light, has a structured surface 25a that forms a grating structure 29. The grating structure 29 serves as a spectral filter in order to filter extraneous light in a predefined wavelength range, for example in the IR wavelength range.

[0056] FIG. 2 shows a schematic of an example of a process procedure in the production of the structured surface 25a, formed in a functional layer 25 of the collector mirror 17 that can be structured by etching. For the structuring, a structuring layer 26 in the form of a photoresist is first applied over the area of the functional layer 25. The structuring layer 26 is selectively exposed in a subsequent step with the aid of a laser 27. In a further step, the exposed part of the structuring layer 26 is removed. The structuring layer 26 can also be structured in a different way than by irradiation with a laser 27. For example, the structuring layer 26 may be exposed in a lithography process.

[0057] In a subsequent etching step, the functional layer 25 is selectively etched using the structuring layer 26 as etching mask, with formation of the structured surface 25a on the functional layer 25. The structured surface 25a forms the grating structure 29, the geometry of which is chosen such that it serves as spectral filter and suppresses extraneous light at wavelengths in a respectively defined wavelength range.

[0058] The functional layer 25 can be structured with the aid of a dry etching method or with the aid of a wet-chemical etching method. For details of this etching method, reference is made to DE 10 2018 220 629 A1, which is mentioned in the introduction and which is incorporated into this application in its entirety by reference.

[0059] FIG. 3A shows a detail, represented by a dotted line in FIG. 2, of the structured surface 25a after the removal of the structuring layer 26. The structured surface 25a or the grating structure 29 has a multitude of grating webs 31, each comprising a top face 32 and flanks 33. Grooves 34 having a base 35 are formed in each case between the grating webs 31. The structured functional layer 25 with the grating structure 29 is formed on a substrate 30 of the collector mirror 17.

[0060] By contrast with what is shown in FIG. 2 and FIG. 3A, the structured surface 25a may also be formed in the substrate 30, or the structured surface 25 may be formed partly in the functional layer 25 and partly in the substrate 30, as described in DE 10 2018 220 629 A1.

[0061] The functional layer 25 or substrate 30 includes at least one material that has good processibility or good structurability by etching. The material of the functional layer 25 or substrate 30 may, for example, be amorphous silicon (aSi), silicon (Si), nickel-phosphorus, metals, in particular from the group of titanium (Ti), platinum (Pt), gold (Au), aluminum (Al), nickel (Ni), copper (Cu), silver (Ag), tantalum (Ta), tungsten (W) and alloys thereof; oxides, in particular from the group of silicon dioxide (SiO2), aluminum oxide (AlO.sub.x), titanium oxide (TiO.sub.x), tantalum oxide (TaO.sub.x), niobium oxide (NbO.sub.x), zirconium oxide (ZrO.sub.x) and combinations thereof (for example mixed oxides, ceramics, glasses, glass ceramics, composite materials).

[0062] The grating webs 31 shown in FIG. 3A each have flanks 33 having a flank steepness α of 90°. The flank steepness α is measured in relation to a local tangential plane corresponding to the top face of the substrate 30. In an equivalent manner thereto, the flank steepness α of the flank 33 may also be measured against a tangential plane corresponding to the base 35 of the groove 34 adjacent to the flank 33. The flank steepness α is measured against a local tangential plane since the surface of the substrate 30 or of the collector mirror 17 is not planar, but rather has a generally ellipsoidal and/or hyperboloid geometry, as described above.

[0063] FIG. 3B shows a case in which the flank steepness α is more than 90°, i.e. forms an undercut in the flank 33. Such a flank 33 with an undercut may be produced, for example, in (directed) wet-chemical etching.

[0064] In the case of the steep flanks 33 shown in FIGS. 3A and 3B, or in the case of a (maximum) flank steepness α typically more than about 60°, in the case of application of a reflective coating 36 to the structured surface 25a by a conventional coating method, for example by magnetron sputtering, it is not possible to achieve continuous coverage of the structured surface 25a. Instead, there will be gaps in the coverage as a result of the reflective coating 36 in the region of the flanks 33. Subregions of the structured surface 25a are thus exposed and, as described above, are subject to an etching attack by molecular hydrogen, by free hydrogen radicals and by hydrogen ions and/or to the above-described degradations, for example oxidation. If the material of the functional layer 25 includes silicon, such an etching attack forms silanes (e.g. SiH3, SiH4), which are deposited on optically utilized surfaces, which can lead to a loss of reflectivity or to degradation of the layer materials used there, possibly to the extent of under-etching and large-area defects.

[0065] In order to protect the functional layer 25 from the etching attack by reactive hydrogen, in the examples shown in FIGS. 5A and 5B, a protective layer 37 is formed between the structured surface 25a and the reflective coating 36. By contrast with the reflective coating 36, the protective layer 37 covers the structured surface 25a continuously, i.e. over the full area and without gaps, in the region of the flanks 33.

[0066] In order to achieve continuous coverage of the structured surface 25a by the protective layer 37, the protective layer 37 is applied or deposited by an isotropic coating method. The isotropic coating method in the example shown is atomic layer deposition, but it is also possible to use another isotropic CVD coating method or a PVD coating method with a suitably selected directed geometry for this purpose, which reduces the anisotropy that typically exists in the PVD method. It is possible, for example, to combine multiple material sources which may, for example, be in a tilted arrangement. In this case, the coating rates can be controlled by shadowing, which suppresses particular angles of incidence. In addition, a tilt or pivoting motion of the optical element 17 to be coated is possible.

[0067] The thickness d of the protective layer 37 in the example shown is about 5 nm and generally does not exceed a thickness of 100 nm. On account of the comparatively low thickness of the protective layer 37 and on account of the somewhat lower demands on control of layer thickness compared to the reflective coating 36, the isotropic coating method may be conducted with a comparatively manageable degree of complexity.

[0068] The reflective coating 36 is a multilayer coating for reflection of the EUV radiation 16, having a multitude of twin layers formed from molybdenum and silicon. The number of twin layers may be in the order of magnitude of, for example, 30 to 80, but may also be greater or smaller. Correspondingly, the reflective multilayer coating 36 has a considerable thickness. The applying of the reflective multilayer coating 35 with an isotropic coating method, for example by atomic layer deposition, would therefore be very complex.

[0069] The protective layer 37 may have a single layer, but it is also possible that the protective layer 37, like the reflective coating 36, has multiple layers of different materials. In principle, the material(s) of the protective layer 37 should as far as possible prevent the diffusion of molecular hydrogen, of hydrogen ions and hydrogen radicals through the protective layer 37, and also not enter into any chemical reaction with these hydrogen species and with other contaminating substances, for example tin. For this purpose, the protective layer 37 typically includes at least one material selected from the group comprising: metals, in particular from the group of copper (Cu), cobalt (Co), platinum (Pt), iridium (Ir), palladium (Pd), ruthenium (Ru), gold (Au), tungsten (W), and alloys thereof, oxides, in particular from the group of aluminum oxide (AlOx), zirconium oxide (ZrOx), titanium oxide (TiOx), niobium oxide (NbOx), tantalum oxide (TaOx), hafnium oxide (HfOx), chromium oxide (CrOx), carbides, borides, nitrides, silicides and combinations thereof (for example mixed oxides, ceramics, glasses, glass ceramics, composite materials).

[0070] In particular if the protective layer 37 is formed between the structured surface 25a and the reflective coating 36, it is favorable when the protective layer 37 has low roughness.

[0071] FIG. 6 shows a collector mirror 17 in which the protective layer 37 forms a cap layer applied to the reflective multilayer coating 36 with the aid of an isotropic coating method. The protective layer 37 in this case fully and continuously covers both the reflective coating 36 and a subregion of the structured surface 25a which is not covered by the reflective coating 36. In the example shown in FIG. 6, the protective layer 37 likewise has a low thickness d of less than about 10 nm and is formed from a material or a combination of materials having a comparatively low absorption for EUV radiation 16. In this way, it is possible to ensure that the protective layer 37 does not excessively reduce the reflectivity of the collector mirror 17.

[0072] FIG. 7 shows an example of a collector mirror 17 in which the protective layer 37, as in FIGS. 5A and 5B, is formed between the structured surface 25a and the reflective coating 36. In addition, a cap layer 38 is applied to the reflective coating 36. By contrast with FIG. 6, the cap layer shown in FIG. 7 does not form a complete protective layer since it does not cover the structured surface 25a continuously, but rather has gaps, as is also the case for the reflective coating 36. The cap layer 38 may be formed, for example, from one of the materials described above in connection with the protective layer 37. In principle, in the examples shown in FIGS. 5A and 5B as well, a cap layer 38 may be applied to the reflective coating 36. The cap layer 38 may be applied by an isotropic coating method, as shown in FIG. 6, or by an anisotropic coating method, as shown in FIG. 7.

[0073] The protective layer 37 may be applied not only in the case of the collector mirror 17 but also in the case of other structured reflective optical elements of the projection exposure apparatus 1, in order to protect these from the etching attack of a hydrogen plasma. It is also possible to use the protective layer in reflective optical elements designed to reflect radiation at different wavelengths than the EUV wavelength range. The protective layer 37 may also serve to protect the structured surface 25a from an etching attack by chemical elements other than hydrogen. In this case, the materials from which the protective layer 37 is formed should be matched to the chemical elements from which the structured surface 25a is to be protected.