OPTICAL ELEMENT FOR A EUV PROJECTION EXPOSURE SYSTEM

Abstract

In a method for producing an optical element for an EUV projection exposure apparatus, a shaping layer (22.sub.1) is applied onto a substrate (20) so as to have a surface roughness of at most 0.5 nm rms directly after the application of the shaping layer onto the substrate.

Claims

1. An optical element for an extreme ultraviolet (EUV) projection exposure apparatus, comprising: a substrate specifying a curved basic topography, at least two shaping layers, applied onto the substrate, each with a layer thickness (Di(s)) according to a specified layer thickness profile (Div(s)), a plurality of etch stop layers, an EUV radiation-reflecting layer, wherein at least one of the shaping layers is structured to form a grating structure with a bottom region, a front side, and a flank, wherein a transition from the bottom region to the flank has a radius of curvature of at most 5 μm, and wherein the EUV-radiation-reflecting layer is applied at least to the bottom region and to the front side of the grating structure.

2. The optical element as claimed in claim 1, wherein the etch stop layers and the shaping layers are applied to the substrate in an alternating sequence.

3. The optical element as claimed in claim 1, wherein the layer thickness (Di) in each case has a maximum deviation of at most 50 nm from a specified layer thickness (Dvi).

4. The optical element as claimed in claim 1, wherein the bottom region has a surface roughness of at most 0.5 nm rms.

5. The optical element as claimed in claim 1, wherein the bottom region has a surface roughness which is at most 20% greater than a surface roughness of the front side.

6. The optical element as claimed in claim 1, wherein at least one of the etch stop layers is arranged between the substrate and at least one of the shaping layers.

7. The optical element as claimed in claim 1, wherein a proportion of an area of the flank in a top view of the optical element is at most 2% of a total surface area in the top view of the optical element.

8. The optical element as claimed in claim 1, wherein at least one of the shaping layers comprises a nanolaminate.

9. The optical element as claimed in claim 1 and configured as a spectral filter.

10. A method for producing an optical element for an extreme ultraviolet (EUV) projection exposure apparatus, comprising: providing a substrate with a basic topography, applying a shaping layer onto the substrate with a layer thickness (Di(s)) according to a specified layer thickness profile (Div(s)), applying an etch stop layer, wherein the etch stop layer is applied under the shaping layer, wherein the shaping layer has a surface roughness of at most 0.5 nm rms directly after said applying onto the substrate, wherein all of the layers are applied in vacuum conditions, wherein the vacuum is maintained between said applying of the shaping and the etch stop layers.

11. The method as claimed in claim 10, wherein said applying of the shaping layer is performed such that a layer thickness (Di) of the shaping layer deviates by at most 1% from a layer thickness (Div) specified for the shaping element.

12. The method as claimed in claim 10, wherein said applying of the shaping layer comprises a roughness-preserving method or a smoothing method.

13. The method as claimed in claim 10, further comprising applying a further shaping layer onto the substrate such that the further shaping layer is separated from the applied shaping layer by a further etch stop layer.

14. The method as claimed in claim 10, wherein said applying steps comprise exclusively additive steps and selective structuring steps.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0103] Further details and advantages of the invention will become apparent from the description of exemplary embodiments with reference to the figures. The figures show in:

[0104] FIG. 1 schematically a meridional section through a projection exposure apparatus for EUV projection lithography,

[0105] FIGS. 2A to 2E schematic details from a cross section through an optical element with a two-step grating structure at different stages in the production process, specifically an intermediate product provided for a first structuring step (FIG. 2A), following a first structuring step (FIG. 2B), following a physical etching step (FIG. 2C), following a second structuring step (FIG. 2D), and following application of a radiation-reflecting layer (FIG. 2E).

[0106] FIG. 3 schematically a sequence of process steps from the process chain for producing the optical element, and

[0107] FIG. 4 schematically a detail through a cross section of an intermediate product for the production of a collector mirror.

DETAILED DESCRIPTION

[0108] Firstly, the general construction of a microlithographic projection exposure apparatus 1 will be described.

[0109] FIG. 1 schematically shows a microlithographic projection exposure apparatus 1 in a meridional section. An illumination system 2 of the projection exposure apparatus 1 has, besides a radiation source 3, an illumination optical unit 4 for the exposure of an object field 5 in an object plane 6. In this case, a reticle (not illustrated in the drawing) that is arranged in the object field 5 and is held by a reticle holder (likewise not illustrated) is exposed. A projection optical unit 7 serves for imaging the object field 5 into an image field 8 in an image plane 9. A structure on the reticle is imaged onto a light-sensitive layer of a wafer (likewise not illustrated in the drawing) that is arranged in the region of the image field 8 in the image plane 9 and is held by a wafer holder (likewise not illustrated).

[0110] The radiation source 3 is an EUV radiation source with emitted used radiation in the range of between 5 nm and 30 nm. This may be a plasma source, for example a GDPP (gas discharge-produced plasma) source or an LPP (laser-produced plasma) source. By way of example, tin can be excited to form a plasma using a carbon dioxide laser operating at a wavelength of 10.6 μm, that is to say in the infrared range. A radiation source based on a synchrotron can also be used for the radiation source 3. Information about such a radiation source can be found by a person skilled in the art for example in U.S. Pat. No. 6,859,515 B2. EUV radiation 10 emerging from the radiation source 3 is focused by a collector 11. A corresponding collector is known from EP 1 225 481 A. Downstream of the collector 11, the EUV radiation 10 propagates through an intermediate focal plane 12 before being incident on a field facet mirror 13 with a multiplicity of field facets 13a. The field facet mirror 13 is arranged in a plane of the illumination optical unit 4 which is optically conjugate with respect to the object plane 6.

[0111] The EUV radiation 10 is also referred to hereinafter as illumination light or as imaging light.

[0112] Downstream of the field facet mirror 13, the EUV radiation 10 is reflected by a pupil facet mirror 14 with a multiplicity of pupil facets 14a. The pupil facet mirror 14 is arranged in a pupil plane of the illumination optical unit 4, which is optically conjugate with respect to a pupil plane of the projection optical unit 7. With the aid of the pupil facet mirror 14 and an imaging optical assembly in the form of a transfer optical unit 15 with mirrors 16, 17 and 18 designated in the order of the beam path, individual field facets 19, which are also referred to as subfields or as individual mirror groups, of the field facet mirror 13 are imaged into the object field 5. The last mirror 18 of the transfer optical unit 15 is a grazing incidence mirror.

[0113] With the aid of the projection exposure apparatus 1, at least one part of the reticle in the object field 5 is imaged onto a region of a light-sensitive layer on the wafer in the image field 8 for the lithographic production of a microstructured or nanostructured component, in particular of a semiconductor component, for example of a microchip. Depending on the embodiment of the projection exposure apparatus 1 as a scanner or as a stepper, the reticle and the wafer are moved in a temporally synchronized manner in the y-direction continuously in scanner operation or step by step in stepper operation.

[0114] A method for producing an optical element of the projection exposure apparatus 1 and intermediate products in the production of this optical element are described below with reference to FIGS. 2A to 2E and FIG. 3.

[0115] The optical element can in particular be a mirror, in particular a mirror of the illumination optical unit 4 or the projection optical unit 7. In particular, it can be a mirror of the collector 11. It can also be a spectral filter, in particular a filter for suppressing infrared radiation (IR radiation). In particular, it is an EUV-reflecting mirror with an IR-suppressing effect. For further details of such an optical element, reference is made by way of example to PCT/EP 2019/082 407, which is incorporated herein by reference in its entirety.

[0116] First, a substrate 20 is provided in a provision step 19. The substrate 20 is used to specify a basic topography of the optical element. In particular, it can have a non-planar, i.e. a curved, surface, such as e.g., a convex or concave surface. The substrate can have an aspheric, in particular an ellipsoidal, or a paraboloidal basic topography.

[0117] In an application step 21, a sequence of etching layers 22.sub.i (i≥1) and etch stop layers 23.sub.i (i≥1) is applied to the substrate 20.

[0118] The etching layers 22.sub.i are applied in particular through a deposition method, in particular through a sputtering method, such as a magnetron sputtering method (MSD, magnetron sputter deposition).

[0119] The etch stop layers 23.sub.i are in particular grown.

[0120] The etching layers 22.sub.i are applied with a thickness D.sub.i. The layer thickness D.sub.i can vary over the surface of the substrate 20, D.sub.i=D.sub.i(s), where s denotes the position on the surface of the substrate 20. The etching layer 22.sub.i is applied onto the substrate 20 in particular with a layer thickness D.sub.i(s) according to a specified layer thickness profile D.sub.iv(s).

[0121] The layer thickness D.sub.i(s) deviates, in particular in the region of the entire surface of the substrate 20, by at most 1% from the specified layer thickness D.sub.iv(s).

[0122] The etching layers 22.sub.i have a smooth surface. Their surface roughness is in particular 0.15 nm rms. This specification refers in particular to the range of high spatial frequencies, in particular of at least 1/μm.

[0123] The etching layers 22.sub.i have in particular a thickness D.sub.i of a few μm. The thickness D.sub.i of the etching layers 22.sub.i can in particular lie in the range from 1 μm to 10 μm, in particular in the range from 3 μm to 7 μm.

[0124] The overall thickness of the coating of the substrate 20, in particular the sum of the thickness of all the etching layers 22.sub.i and etch stop layers 23.sub.i, is in particular at most 20 μm, in particular at most 10 μm. These details should not be understood as limiting.

[0125] The etching layers 22.sub.i can be made, for example, of amorphous silicon, SiO.sub.2 or Si.sub.3N.sub.4.

[0126] Their thickness D.sub.i is set directly during coating. The thickness D.sub.i can in particular be set with an accuracy of better than 1%, in particular better than 0.5%, in particular better than 0.3%, in particular better than 0.2%.

[0127] The etch stop layers 23.sub.i are made of a material with a selectivity for the intended etching process. The etch stop layers 23.sub.i can, for example, be made of ruthenium or aluminum oxide (Al.sub.2O.sub.3).

[0128] The etch stop layers 23.sub.i can be in particular grown, particularly grown smoothly. They have a thickness D in the range of a few nm, in particular in the range from 1 nm to 20 nm, in particular in the range from 3 nm to 10 nm. In particular, they have a maximum surface roughness which corresponds to the surface roughness of the etching layers 22.sub.i.

[0129] The shaping etching layers 22.sub.i and the etch stop layers 23.sub.i are applied in particular with a roughness-preserving, in particular a smoothing, process.

[0130] They are applied with great precision. The maximum thickness deviation over the optically used surface area of the optical component is in particular at most 2%, in particular at most 1%, in particular at most 0.5%, in particular at most 0.3%, in particular at most 0.2%. In the event of a layer thickness of the etching layer 22.sub.i in the range of a few micrometers, the maximum thickness deviation can be in particular at most 50 nm, in particular at most 30 nm, in particular at most 20 nm, in particular at most 10 nm. The etching layers 22.sub.i are therefore also referred to as shape-retaining or shaping layers.

[0131] The term shape-retaining layer is used in particular if the layer has a constant thickness. Layers of varying thickness are referred to as shaping layers.

[0132] After applying all the etching layers 22.sub.i and etch stop layers 23.sub.i onto the substrate 20, an intermediate product 24 for producing the optical element is present. FIG. 4 shows an example of an intermediate product 24 for producing a collector shell. In this case, the substrate 20 has a curved surface, in particular an ellipsoidal or a paraboloidal surface.

[0133] In a first structuring step 25, the uppermost etching layer 22.sub.1 is structured. A lithography step 26 and a subsequent etching step 27 are provided for this purpose. Since the etching depth is limited by the etch stop layer 23.sub.1, the demands relating to the etching process are significantly reduced. In particular, it is permissible to over-etch during the etching step 27 without running the risk in the process of removing too much material.

[0134] The intermediate product 24 is shown in FIG. 2B at the stage after the first structuring step 25.

[0135] For selective opening, i.e. for selective, region-by-region removal of the etch stop layer 23.sub.1, a physical etching step 28, in particular a dry etching process step, is provided.

[0136] In particular, reactive ion etching can serve as the etching step 28. It can include reactive (chemical) and sputtering (physical) components. The etching step 28 is, in particular, a directional, anisotropic process. In this way, it can be ensured that an overlying etch stop layer is not flushed from underneath in a second etching step.

[0137] Here, the etch stop layer 23.sub.1 is selectively removed in the bottom region 29 of the first trench structure 30 produced in the first structuring step 25. Here, sections of the bottom region 29 are left standing in order to subsequently form steps 31.

[0138] For further details of the structuring step, reference is made to DE 10 2018 220 629.5.

[0139] In FIG. 2C, the intermediate product 24 is shown as an example after the physical etching step 28 has been completed.

[0140] Then, in a second structuring step 32, the second etching layer 22.sub.2 is structured. The second structuring step 32 comprises a lithography step 33 and an etching step 34 corresponding to the first structuring step 25. For details of the second structuring step 32, reference is made to the description of the first structuring step 25. The structuring steps 25, 32 can be substantially identical. They can also differ in one or more details. This is provided in particular if the etching layers 22.sub.1, 22.sub.2 are not identical in design.

[0141] FIG. 2D shows by way of example the state of the intermediate product 24 after the second structuring step 32. The intermediate product 24 now has a two-step trench structure 35, i.e. a structure with three levels L.sub.1, L.sub.2, L.sub.3.

[0142] The uppermost or frontmost level L.sub.1 forms a front side 40 of the trench structure 35.

[0143] The trench structure 35 also has a bottom region 41.

[0144] Finally, the trench structure 35 has flanks 42.

[0145] In the example shown in FIG. 2D, the trench structure 35 has an intermediate step, namely the step 31. It is therefore a two-step grating structure.

[0146] This is to be understood to be an example. With the method described above, one-step or multi-step, in particular three-step or four-step grating structures can also correspondingly be produced.

[0147] The transition from the bottom region 41 to the flank 42 is sharp-edged. It has a radius of curvature rB of at most 5 μm, in particular at most 3 μm, in particular at most 2 μm, in particular at most 1 μm.

[0148] The transition from the front side 40 to the flank 42 is sharp-edged. In particular, it has a radius of curvature rV of at most 5 μm, in particular at most 3 μm, in particular at most 2 μm, in particular at most 1 μm.

[0149] The flanks 42 form a loss region. In particular, they do not contribute to the transmission of the EUV radiation in the direction specified by the front side 40 and the bottom region 41. It is therefore advantageous for the flanks 42 to be as steep as possible. The angle between one of the flanks 42 and a surface normal to the bottom region 41 and/or to the front side 40 is preferably at most 15°, in particular at most 10°, in particular at most 5°, in particular at most 3°, in particular at most 2°, in particular at most 1°.

[0150] In a subsequent application step 36, a radiation-reflecting layer 37 is applied. The radiation-reflecting layer 37 is applied in particular to all three levels L.sub.1, L.sub.2, L.sub.3.

[0151] The radiation-reflecting layer 37 is in particular an EUV radiation-reflecting layer. The radiation-reflecting layer 37 is in particular a layer stack made of molybdenum-silicon double layers.

[0152] Between the radiation-reflecting layer 37 and the shaping etching layer 22.sub.i there may be further possible layers. In particular, protective layers or other functional layers can be applied onto the etching layers 22.sub.i, in particular onto the uppermost of the etching layers 22.sub.i.

[0153] The radiation-reflecting layer 37 is applied directly onto the layers 22.sub.1, 23.sub.1 and 23.sub.2. Due to the low surface roughness of these layers, a preceding polishing step can be dispensed with.

[0154] In principle, the uppermost etching layer 22.sub.i can also be polished.

[0155] The intermediate product 24 with the radiation-reflecting layer 37 is shown schematically in FIG. 2E.

[0156] The method described above leads to advantages in particular with regard to integral parts of the collector 11, in particular collector shells. In particular, this makes it possible to produce a collector 11 with improved IR suppression. This is due to a reduction in the step depth error. At the same time, the method according to the invention leads to a considerable simplification of the process chain, in particular to a reduction in throughput time. This is due to the bypassing of polishing steps and of the ability to omit the etch depth determination.

[0157] In the following text, different aspects of the invention are described again in the form of keywords. These aspects lead in each case to advantages individually or in combination.

[0158] A shape-retaining or a shaping method is used to deposit the etching layers 22.sub.i. The etching layers 22.sub.i are therefore also referred to as shaping layers.

[0159] A deposition method, in particular a roughness-preserving, preferably a smoothing, deposition method is used in particular to apply the shaping layers. The layers thus have a specified layer thickness profile and a very low surface roughness immediately after their application.

[0160] Each coating step can comprise at least one of the following elementary processes: separation, removal and smoothing. These elementary processes can take place sequentially or simultaneously.

[0161] Each of these elementary processes can act globally, in particular on the entire optically used surface of the optical element, or locally, selectively.

[0162] Smoothing can take place before coating, during coating and/or after coating.

[0163] An ion beam method, in particular a reactive ion beam method, a plasma method, in particular a reactive plasma method, a plasma jet method, a remote plasma method, atomic layer etching, in particular spatial atomic layer etching, electron beam-assisted etching or another method can be used, in particular to remove and/or smooth them for selectively removing individual regions of the etching layers 22.sub.i and/or of the etch stop layers 23.sub.i. Spatial atomic layer processing or processing with a focused electron beam can also be provided.

[0164] The use of nanolaminates can be advantageous for a particularly low surface roughness. These layers of alternating material combinations, which are a few nanometers thick, can be smoothed using the methods mentioned above, although smoothing the pure volume material would actually not be possible due to their hardness. For example, tantalum carbide (TaC), which is a very hard material, can be smoothed in this way.