OPTICAL ELEMENT, IN PARTICULAR FOR REFLECTING EUV RADIATION, OPTICAL ARRANGEMENT, AND METHOD FOR MANUFACTURING AN OPTICAL ELEMENT

Abstract

A reflective optical element (17), in particular for reflecting EUV radiation (16), includes: a substrate (25), and a reflective coating (26) applied to the substrate (25). In one disclosed aspect, the substrate (25) is doped within its volume (V) with at least one precious metal (27). In a further disclosed aspect, the reflective coating (26) and/or a structured layer (28) that is formed between the substrate (25) and the reflective coating (26) is doped with at least one precious metal (27). Also disclosed are an optical arrangement, preferably a projection exposure apparatus for microlithography, in particular for EUV lithography, which includes at least one such reflective optical element (17), and a method of producing such a reflective optical element (17).

Claims

1. A reflective optical element, comprising: a substrate defining a volume, and a reflective coating applied to the substrate, wherein the substrate is doped within the volume with at least one precious metal in a volume region that extends from a surface of the substrate to a distance from the surface of the substrate of more than 1 mm.

2. (canceled)

3. The reflective optical element as claimed in claim 1, wherein the substrate is formed from glass or from a composite material.

4. The reflective optical element as claimed in claim 3, wherein the glass is titanium-doped quartz glass, a glass ceramic, a ceramic, a silicon carbide ceramic, a silicon carbonitride ceramic, a magnesium aluminum silicate ceramic, or a cordierite ceramic.

5. The reflective optical element as claimed in claim 4, wherein the ceramic is a silicon nitride ceramic, a silicon carbide ceramic, a silicon carbonitride ceramic, a magnesium aluminum silicate ceramic, or wherein the composite material is silicon-infiltrated silicon carbide composite, SiSiC.

6. The reflective optical element as claimed in claim 1, wherein the substrate is formed from silicon.

7. The reflective optical element as claimed in claim 6, wherein the substrate is formed from monocrystalline, quasi-monocrystalline or polycrystalline silicon.

8. The reflective optical element as claimed in claim 1, wherein the volume region in which the substrate is doped with the at least one precious metal extends from a surface of the substrate to a distance from the surface of the substrate of more than 2 mm.

9. The reflective optical element as claimed in claim 1, wherein the volume region in which the substrate is doped with the at least one precious metal extends from the surface of the substrate to a distance from the surface of the substrate of more than 5 mm.

10. The reflective optical element as claimed in claim 1, wherein the substrate is doped with the at least one precious metal throughout the volume.

11. A reflective optical element, comprising: a substrate defining a volume, a reflective coating applied to the substrate, and a structured layer formed between the substrate and the reflective coating, wherein the structured layer is doped with at least one precious metal.

12. The reflective optical element according to claim 11, wherein the structured layer forms a grating structure.

13. The reflective optical element according to claim 1, wherein the reflective coating is doped with the at least one precious metal.

14. The reflective optical element as claimed in claim 11, wherein the structured layer and/or the reflective coating contains silicon that is doped with the precious metal.

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

16. The reflective optical element as claimed in claim 1, wherein the at least one precious metal is selected from the group consisting of: Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au.

17. The reflective optical element as claimed in claim 1, wherein a dopant concentration of the at least one precious metal is between 10.sup.10 cm.sup.−3 and 10.sup.20 cm.sup.−3.

18. The reflective optical element as claimed in claim 1 and configured as a collector mirror for an illumination optical system of a projection exposure apparatus.

19. An optical arrangement configured as at least one of a projection exposure apparatus for microlithography or a lithography apparatus for EUV radiation, comprising: at least one reflective optical element as claimed in claim 1.

20. A method of producing a reflective optical element, comprising: providing a substrate, and applying a reflective coating to the substrate, wherein the reflective coating is applied by sputtering deposition, and wherein the sputtering deposition comprises using a sputtering target doped with a precious metal.

21. A method of producing a reflective optical element, comprising: providing a substrate, applying a structurable layer to the substrate, and applying a reflective coating to the substrate, wherein the reflective coating and/or the structurable layer are/is applied by sputtering deposition, and wherein the sputtering deposition comprises using a sputtering target doped with a precious metal.

22. The method according to claim 21, wherein said applying of the reflective coating is preceded by said applying of the structuable layer, and wherein said applying of the reflective coating precedes a structuring of the structuable layer.

23. The method as claimed in claim 20, wherein the substrate defines a volume and is doped within the volume with at least one precious metal.

24. The method as claimed in claim 20, wherein the sputtering target comprises silicon.

25. A reflective optical element, comprising: a substrate defining a volume, and a reflective coating applied to the substrate, wherein the reflective coating is doped with at least one precious metal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Working examples are shown in the schematic drawing and are detailed in the description which follows. The figures show:

[0046] FIG. 1 a schematic in meridional section of a projection exposure apparatus for EUV lithography,

[0047] FIG. 2 a schematic diagram of a reflective optical element of the projection exposure apparatus of FIG. 1, with a substrate doped with a precious metal,

[0048] FIG. 3 a schematic diagram analogous to FIG. 2, in which the reflective optical element has a structured layer doped with a precious metal, and

[0049] FIG. 4 a schematic diagram of a sputtering deposition system with a sputtering target doped with a precious metal.

DETAILED DESCRIPTION

[0050] In the description of the drawings that follows, identical reference signs are used for components that are analogous or the same or have an analogous or the same function.

[0051] The predominant constituents of a projection exposure apparatus 1 for 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.

[0052] 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.

[0053] For purposes of explanation, 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 they direction in FIG. 1. The z direction runs perpendicularly to the object plane 6.

[0054] 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 configured to displace by way of a wafer displacement drive 15, especially 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.

[0055] 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).

[0056] 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 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.

[0057] 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.

[0058] 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.

[0059] 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 also the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5.

[0060] 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.

[0061] In the example shown 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 similarly 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 can also be greater than 0.6 and, for example, can be 0.7 or 0.75.

[0062] Just like the mirrors of the illumination optical unit 4, the mirrors Mi can have a highly reflective coating for the illumination radiation 16.

[0063] FIG. 2 shows the deflecting mirror 19 of the illumination optical system 4, having a substrate 25 of monocrystalline silicon to which a reflective coating 26 for reflection of the illumination radiation 16 is applied. The deflecting mirror 19 is exposed to reactive hydrogen species in the form of hydrogen ions (H+) and hydrogen radicals (H*). The reactive hydrogen species H+, H* may react with silicon material of the substrate 25 at exposed, for example lateral, surfaces 25a of the substrate 25, and form volatile hydrides, for example in the form of silanes. The volatile hydrides may in turn be deposited on optical surfaces, which leads to degradation thereof.

[0064] In order to counteract the formation of volatile hydrides, the substrate 25 of the deflecting mirror 19, in the example shown in FIG. 2, is doped throughout its volume V with a precious metal 27, more specifically with gold. The precious metal 27 in the form of gold atoms implanted into the silicon substrate 25 serves as hydrogen recombination material, and has the effect that the reactive hydrogen species H+, H* react to give molecular hydrogen, and therefore counteracts the formation of volatile hydrides.

[0065] The doping of the substrate 25 with the gold atoms has been effected in the production of the monocrystalline silicon substrate 25. The monocrystalline silicon substrate 25 has been pulled from the melt in the production thereof (Czochralski method). The material of the melt from which the substrate 25 has been pulled was doped here with the precious metal 27. It is likewise possible to produce the monocrystalline silicon substrate 25 doped with a precious metal 27 in other ways, for example with the aid of the Bridgeman-Stockbarger method. It is also possible to produce a quasi-monocrystalline silicon substrate 25 or a polycrystalline silicon substrate doped with a precious metal, for example with gold.

[0066] It is likewise possible to dope the silicon substrate 25 with other precious metals, for example with Ru, Rh, Pd, Ag, Os, Ir, Pt and combinations or alloys thereof. The doping of the silicon substrate 25 with Au or with Pt has been found to be favorable, since materials of this kind are already commercially available. However, it is of course likewise possible to dope the silicon substrate 25 with at least one precious metal other than Au or Pt.

[0067] The doping of the substrate 25 with a precious metal as described above can also be undertaken in the case of other substrate materials that are suitable for the production of reflective optical elements for EUV lithography. These substrate materials are, for example, quartz glass, glasses or glass ceramics having very low thermal expansion, for example ULE®, Zerodur®, Clearceram® etc., ceramics, e.g. silicon nitride, silicon carbide, in particular silicon-infiltrated silicon carbide composite (SiSiC), magnesium aluminum silicate ceramics such as cordierite ceramics, etc. It will be apparent that the doping of the substrate 25 can also be undertaken with two or more different precious metals 27.

[0068] FIG. 3 shows, by way of example, the collector mirror 17 of the illumination optical unit 2 of the projection exposure apparatus 1 of FIG. 1. The collector mirror 17 differs from the deflecting mirror 19 shown in FIG. 2 in that a structured layer 28 is formed between the substrate 25 and the reflective coating 26. The structured layer 28 has a structured surface in the form of a grating structure 29 and is formed from amorphous silicon. The grating structure 29 serves as spectral filter for suppression of extraneous light, i.e. of radiation at wavelengths outside the EUV wavelength range, for example in the infrared wavelength range. The reflective coating 26 is applied to the structured layer 28 or to the grating structure 29.

[0069] In principle, the structured layer 28 is protected from the reactive hydrogen species H+, H* by the reflective coating 26 applied. In the example shown in FIG. 3, however, the (maximum) edge steepness of the grating structure 29 is high and is about 90°. The applying of the reflective coating 26 in the form of a continuous layer that fully covers the structured layer 28 is possible even in the case of such a great edge steepness when the applying is effected by an isotropic coating method, for example by atomic layer deposition. However, the applying of the reflective coating 26, which, in the example shown, forms a multilayer coating with a number of about 50 twin layers of Si/Mo with the aid of an isotropic coating method is very complex. In addition, the collector mirror 17 is not planar, as shown in FIG. 3, but typically has an ellipsoidal or hyperboloid curvature, which additionally makes it difficult to achieve coating by atomic layer deposition.

[0070] In the example shown, the reflective coating 26 is applied to the structured layer 28 by a non-isotropic coating method, more specifically by sputtering deposition. The structured layer 28 is doped with a precious metal 27 for protection from reactive hydrogen species H+, H*. The same applies to the reflective coating 26 applied to the structured layer 28, since this, or more specifically the silicon-containing layers thereof, is likewise exposed to reactive hydrogen species H+, H*, in particular along the steep flanks of the grating structure 29. It is possible to apply a protective layer system (not shown in the figure) to the reflective coating 26, in which a precious metal may likewise be implanted in one or more layers.

[0071] For efficient performance of the doping of the structured layer 28, of the reflective coating 26 and optionally of one or more layers of the protective layer system, sputtering deposition is conducted, in which a sputtering target 37 doped with a precious metal 27 is used, as described hereinafter with reference to FIG. 4.

[0072] FIG. 4 shows, in highly simplified form, a sputtering deposition system 30 having a process chamber 31 in which there is a high vacuum. The process chamber 31 is supplied with a noble gas 32 in the form of argon via a gas inlet. The noble gas 32 enters the process chamber 31 in an interspace between a cathode 33 in plate form and an anode 34 in plate form, in which an electrical field constant over time is generated. For the generation of the electrical field, a voltage which is constant over time is applied between the cathode 33 and the anode 34. Magnets 35 that are disposed on a side of the cathode 33 remote from the interspace generate a magnetic field 36 in the interspace in addition to the electrical field.

[0073] The noble gas 32 is ionized in the interspace between the cathode 33 and the anode 34, and forms noble gas ions 32a that are accelerated to the cathode 33 and strike negatively charged particles 38 out of a sputtering target 37 mounted there, which are accelerated in the direction of the anode 34 and are deposited on a substrate 25 of the reflective optical element 17 that is mounted there.

[0074] The sputtering target 37 in the example shown is formed from monocrystalline or quasi-monocrystalline silicon doped with a precious metal 27. The effect of the doping is that a structurable layer 28′ deposited on the substrate 25 in the sputtering deposition likewise has doping with the precious metal 27. In a corresponding manner, it is also possible to deposit the reflective coating 26, or more specifically the silicon layers of the reflective coating 26, with the aid of a silicon sputtering target 37 doped with a precious metal 27.

[0075] Before the reflective coating 26 is applied, the structurable layer 28′ is structured in order to form the structured layer 28 with the grating structure 29. The structuring can be effected, for example, with the aid of a dry- or wet-chemical etching process on the structurable layer 28′ using a structuring layer. The structuring layer that serves as sacrificial layer may be structured, for example, with the aid of a lithographic exposure or in some other way.

[0076] A dopant concentration of the precious metal 27 in the volume V of the substrate 25, in the structured layer 28 and in the reflective coating 26 is typically in an order of magnitude between 10.sup.10 cm.sup.−3, and 10.sup.20 cm.sup.−3, in particular between 10.sup.12 cm.sup.−3 and 10.sup.16 cm.sup.−3. In the case of such a dopant concentration, there is no expectation of a significant increase in the absorption of the doped silicon in the reflective coating 26 or in the structured layer 28 or the substrate 25, and so the doping described here can achieve, at low cost and inconvenience, improved HIO resistance and radiation resistance of the reflective optical element 17.

[0077] It will be apparent that the doping with the precious metal 27 need not necessarily be effected both in the substrate 25 and in the structured layer 28, and in the reflective coating 26. For example, it is possible to dispense with doping of the substrate 25 if it is protected from the reactive hydrogen species in some other way. Such protection can be achieved, for example, with a shield as described in WO2019025162A1, cited at the outset, which is incorporated into this application in its entirety by reference. It may also be the case that doping of the structured layer 28 is not required if it is covered completely by the reflective coating 26.