DE102011003077A1 - Process for the preparation of a substrate for a reflective optical element for EUV lithography - Google Patents

Abstract

In order to counteract subsequent deformations, a method for producing a substrate for a reflective optical element for EUV lithography is proposed with the following steps: providing a material block; Processing a surface of the block of material by removing material on the surface in a stress-free manner; - further processing of the material block. In preferred embodiments, before the stress-free machining step, the surface of the material block is machined by removing material from the surface in a stress-inducing manner.

Description

The present invention relates to a method for producing a substrate for a reflective optical element for EUV lithography, to a thus prepared substrate for a reflective optical element for EUV lithography and to a reflective optical element for EUV lithography with such a substrate.

In order to be able to produce ever finer structures in the production of semiconductor devices, for example, by means of lithographic methods, work is always carried out with shorter-wavelength light. If one works in the extreme ultraviolet (EUV) wavelength range, for example at wavelengths between approx. 5 nm and 20 nm, it is no longer possible to work with lenticular elements in transmission, but illumination and projection objectives are constructed from mirror elements with highly reflective coatings adapted to the respective working wavelength , In contrast to mirrors in the visible and ultraviolet wavelength range, theoretically only maximum reflectivities of less than 80% per mirror can be achieved. Since EUV projectives usually have multiple mirrors, they must each have the highest possible reflectivity in order to ensure a sufficiently high overall reflectivity.

In order to minimize intensity losses due to scattered radiation as well as to avoid aberrations, mirror substrates or mirrors, which are produced by applying a highly reflective layer on the mirror substrate, should have the smallest possible microroughness. The quadratic roughness, also called root mean squared roughness (RMS) roughness, is calculated from the mean of the squares of the deviation of the measurement points across the surface to a mean surface which is thus laid through the surface, which is the sum of the deviations is minimal relative to the mean surface. In particular, for optical elements for EUV lithography, the roughness in a spatial frequency range of 0.1 .mu.m to 200 .mu.m is of particular importance in order to avoid negative effects on the imaging quality of the optical elements.

In addition to microroughness, it is also particularly important that the surface profile of the optically used surface, which must be worked very precisely in order to comply with the optical specifications required for EUV lithography, remain as constant as possible over time. With regard to a substrate in which the surface profile of the surface optically used in the finished reflective optical element has already been applied as far as possible, it is of particular importance that the surface of the substrate is processed during substrate production in such a way that the surface course is possible over time remains constant.

An object of the present invention is to propose a process for producing substrates which are suitable as substrates for reflective optical elements which are used at wavelengths in the EUV wavelength range, in particular in EUV lithography.

• – Bereitstellen eines Materialblocks;
• – Bearbeiten einer Oberfläche des Materialblocks, indem auf spannungsfreie Weise Material an der Oberfläche abgetragen wird;
• – weiteres Bearbeiten des Materialblocks.

This object is achieved by a method for producing a substrate for a reflective optical element for EUV lithography with the following steps:

• - Providing a block of material;
• - processing a surface of the block of material by removing material on the surface in a stress-free manner;
• - further editing of the material block.

It has been found that the known surface processing methods can be divided into methods in which stresses are induced in the machined material, in particular on its surface, and other methods in which no or so low voltages are induced to be compared with those by the stress-inducing methods are negligible, and these are referred to as stress-free methods. For the special application as a substrate for reflective optical elements for EUV lithography, the stresses induced by the surface processing can lead to the substrate or the reflective optical element being subsequently distorted, resulting in a changed course of the optically used surface and thus may result in altered optical or imaging properties. According to the invention, it is now proposed to provide at least one stress-free processing step when processing a material block to a substrate for a reflective optical element for EUV lithography. As a result, the level of existing stresses in the substrate produced can be reduced and thus the required imaging properties of a reflective optical element produced with it can be ensured more easily.

In particularly preferred embodiments, before the tension-free processing step, the surface of the material block is processed by removing material on the surface in a stress-inducing manner. Advantageously, at least one stress-inducing surface treatment step is combined with at least one surface treatment step which does not induce additional stresses but removes surface in which stress has been induced by previous processing. In this way, a surface course can be precise while reducing the level of induced stresses that could subsequently lead to surface deformation. In particular, more material can be removed in a shorter time in many stress-inducing machining methods, so that substrates for reflective optical elements for EUV lithography can be produced as quickly and economically as possible by combining stress-inducing and stress-free processing steps and at the same time the stresses present in the finished substrate be reduced to the lowest possible level.

Preferably, the surface of the block of material is removed stress-inducing by milling, turning, honing, polishing, in particular coarse polishing, erosion or cloth. As a result of these types of processing, the substrate and the surface optically used later on the reflective optical element can already be given a coarse shape with relatively little effort and in a relatively short time. About turning, honing, polishing, especially rough polishing, erosion or lobes in particular the surface profile is specified. By means of milling, for example, individual milling bridges can be exposed, which can later be separated into individual substrates or reflective optical elements.

In preferred embodiments, the surface of the block of material is prepared by etching, electropolishing, plasma polishing, ion beam assisted shaping, laser ablation or radiation, for example by a fluid and / or chemically assisted and using a variety of particles, i.a. a. Sand particles or glass beads, removed stress-free. These types of surface treatment have a relatively low removal per time, which can also be selectively varied spatially over the surface, so that they are particularly suitable for fine machining of the respective surface. In particular variants of the proposed manufacturing method, the production of the substrate can also be based only on stress-free processing steps, wherein different stress-free processing steps can be combined. Under certain circumstances, one or more stress-free processing steps can also reduce internal stresses in the surface area in the material block.

Especially with surfaces that have grain boundaries in their material, it has proven to be advantageous if at the stress-free surface machining of the removal corresponds to at least the average grain size. It is assumed that with stress-inducing surface treatment methods, the induced stresses end predominantly at the grain boundaries of the grains adjacent to the surface and accumulate there. By removing these grains essentially by stress-free surface treatment, the stress induced in the surface can be significantly reduced.

Depending on the surface material and surface treatment, it may be advantageous if the material block is additionally subjected to a stress-reducing heat treatment before and / or after the tension-free surface treatment. In particular, if stresses can be induced into the material quite quickly or if it already has high residual stresses, or if surface processing steps via which particularly high voltages are induced, a heat treatment can additionally reduce the total stress in the material block and also on its surface.

In preferred embodiments, a polishing layer is applied to the block of material, which may also be so thick that it serves not only to polish the surface to the smallest possible micro-roughness, but a substantial portion of the surface course is incorporated into it. Depending on the desired surface course and thickness of the polishing layer, it can be applied before or after stress-inducing and tension-free surface processing steps or even therebetween. The provision of a polishing layer allows the base material of the material block to be optimized, for example, for properties such as low thermal expansion coefficient or good coolability, while the polishing layer is optimized for good polishability. Suitable materials for a polishing layer are, for example, metals or metal alloys or metal composites, silicon or silicon oxide. If necessary, one can provide an adhesive layer between the polishing layer and the underlying material. In the case of metallic polishing layers, electroplated polishing layers are particularly preferred because of their usually very low residual stresses.

Preferably, the material block is machined by milling of webs, polishing and / or singulation to produce a finished substrate for reflective optical elements for EUV lithography.

The object is also achieved by a substrate for a reflective optical element for EUV lithography, which is produced by the method explained. Such substrates are characterized by a very precisely worked and thereby stable surface course, so that they are particularly well suited for use as a substrate for a reflective optical element for EUV lithography.

In particularly preferred embodiments, the material block and / or its surface is made of a copper composite material or a copper alloy. These materials are particularly suitable for substrates for reflective optical elements for EUV lithography in terms of machinability, strength and dimensional stability, especially when exposed to heat. As a polishing layer they can be applied galvanically. Particularly preferred are, for example, high-strength copper alloys such as copper alloys with 0.8 wt .-% chromium and 0.08 wt .-% zirconium (for example, commercially available under the trade name Elmedur X ^® Thyssen Durometall), or oxides, borides and / or carbides dispersion strengthened copper (for example, under the trade name Glidcop ^® or DISCUP ^® commercially available).

Furthermore, the object is achieved by a reflective optical element for EUV lithography, which is a substrate as explained with an optically active. Coating based on a multilayer system of alternating layers of materials with different real part of the refractive index at a wavelength in the EUV wavelength range. Such multi-layer systems are regularly used in reflective optical elements which are to be used in the case of a substantially normal incidence in the EUV wavelength range. In this case, in a known manner, the materials, as well as the number, the thicknesses and the specific sequence of the layers, are selected primarily with regard to the highest possible reflectivity at the wavelength at which the lithography is to be carried out. For example, multilayer systems for wavelengths between 12 nm and 15 nm are often based on several tens of alternating layers of silicon and molybdenum.

In preferred embodiments, the reflective optical element for EUV lithography is formed as a facet mirror for an EUV lithography device. Straight facet mirrors have particularly small dimensions, so that stresses induced in the surface with a particularly high probability could subsequently lead to disturbing deformations, but which is lowered with the substrates produced as described.

The above and other features are apparent from the claims and from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in an embodiment of the invention and in other fields be realized and advantageous as well protectable versions.

The present invention will be explained in more detail with reference to a preferred embodiment. Show this

1 schematically an EUV lithography device;

2 schematically an illumination system of an EUV lithography device;

3a Fig. 10 schematically shows the influence of a stress-free surface treatment on the stress in a block of material;

4a b schematically illustrates the influence of a stress-free heat treatment on the stress in a block of material; and

5a C schematically shows individual processing states in the production of reflective optical elements for EUV lithography.

In 1 is a principle view of an EUV lithography device 100 for the fabrication of, for example, microelectronic devices operating in a scan mode along a scan direction 126 operated at a working wavelength in the EUV range and may have one or more optical elements with additional coating. In the 1 shown EUV lithography device 100 has a punctiform plasma radiation source. The radiation of the laser source 102 is via a condenser lens 104 directed to suitable material, via the feed 108 is initiated and to a plasma 106 is stimulated. The plasma 106 emitted radiation is from the collector mirror 110 imaged on the intermediate focus Z. By appropriate screens 111 At the intermediate focus Z it is ensured that no unwanted scattered radiation on the following mirrors 112 . 114 . 116 . 118 . 120 of the illumination system of the projection exposure apparatus 100 meets. The plane mirror 122 is used to fold the system to build space for mechanical and electronic components in the object plane in which the reticle holder 124 is arranged to provide. In the lighting system follow in the present example on the mirror 112 a field facet mirror 114 and a pupil facet mirror 116 , The field facet mirror 114 serves to project a plurality of images of the radiation source of the EUV lithography device in a pupil plane in which a second facet mirror is arranged, which is a pupil facet mirror 116 serves and the images of the facets of the field facet mirror 114 superimposed in the object plane to allow the most homogeneous possible illumination.

The subsequent to the facet mirror 114 . 116 arranged mirrors 118 and 120 serve in the Essentially, to shape the field in the object plane. In the object plane is a structured reticle 124 arranged, its structure by means of a projection lens 128 with in the present example six mirrors on the object to be exposed 130 , about a wafer is imaged. The reticle 124 is in the here designed as a scanning system EUV lithography device 100 in the direction shown 126 movable and is successively illuminated in sections to the respective structures of the reticle 124 with the projection lens according to, for example, a wafer 130 to project.

In 2 is a radiation source in conjunction with a lighting system 11 which is part of an EUV lithography apparatus. A collector 1 is arranged around a light source, that of a plasma droplet 2 is formed by an infrared laser 3 is stimulated. In order to obtain wavelengths in the range of, for example, 13.5 nm in the EUV wavelength range, z. B. tin are excited by means of a working at a wavelength of 10.6 microns carbon dioxide laser to a plasma. Instead of a carbon dioxide laser, for example, solid state lasers can be used. On the collector 1 follow after the aperture 5 at the intermediate focus 4 a field facet mirror 16 with individual facets 18 and a pupil facet mirror 17 with individual facets 19 , in particular the facets 18 . 19 an additional coating to compensate for deformations due to the functional coating of the facets 18 . 19 exhibit. Before the rays on the reticle to be scanned in the y-direction 13 With the structure to be projected onto a wafer, it still becomes a folding mirror 12 diverted. The folding mirror 12 has less visual function, but rather serves the space requirements of the lighting system 11 to optimize.

It should be noted that in EUV lithography a variety of radiation sources can be used, u. a. Plasma sources based on laser excitation (LPP sources) or gas discharge (DPP sources), synchrotron radiation sources or free electron lasers (FEL). Furthermore, the collectors can be configured as desired, u. a. also as a Wolter collector or as an ellipsoidal collector, preferably adapted to the particular radiation source used. In addition, the illumination system of an EUV lithography device can be configured as desired and u. a. in addition or also in place of the facet mirrors, honeycomb condensers, specular reflectors, movable optical components or the like exhibit.

• – ein Materialblock bereitgestellt wird;
• – eine Oberfläche des Materialblocks bearbeitet wird, indem auf spannungsinduzierende Weise Material an der Oberfläche abgetragen wird;
• – die Oberfläche des Materialblocks anschließend bearbeitet wird, indem auf spannungsfreie Weise Material an der Oberfläche abgetragen wird;
• – der Materialblock weiterbearbeitet wird.

The in conjunction with the 1 and 2 described facet mirror, but also other reflective optical elements for use in EUV lithography can be prepared on the basis of substrates, for their preparation, for example

• A block of material is provided;
• A surface of the block of material is machined by removing material on the surface in a stress-inducing manner;
• - The surface of the block of material is subsequently processed by removing material on the surface in a stress-free manner;
• - the material block is processed further.

The material of the material block is preferably selected according to that it has a good dimensional stability and / or a low thermal expansion. Suitable materials are u. a. Metals, intermetallics, copper, copper alloys, copper composites, aluminum alloys, aluminum composites, doped glasses, glass ceramics. If the block of material is pre-structured by milling, turning, honing, polishing, in particular coarse polishing, erosion or lapping and / or the later optically usable surface with a desired course, which may be flat, curved or even aspherical provided thereby in addition to existing residual stresses in the material block, in particular in the area of the machined surface induces stresses.

This is in the 3a , b on the example of a cylindrical block of material 30 shown schematically with a height l. The stress σ in the material block 30 has a value greater than zero, increases sharply in the area of the machined surface at the height l. There is a high voltage gradient. If a height Δl is now applied by a stress-free surface treatment, for example by etching, electropolishing, plasma polishing, ion beam-assisted shaping, laser ablation or radiation, for example by a fluid and / or chemically assisted and using a wide variety of particles, including sand particles or glass beads that the final height of the block of material 30 only l ', this can reduce the stress from σ to σ'. In particular, in the case of materials which have grain boundaries, as for example in the case of crystalline materials, it has proved advantageous for stress reduction if the removal corresponds at least to the average grain size in the stress-free surface treatment.

It leads, as in the 4a , b, a heat treatment on a stress-inducing processed on its surface block of material 40 through, the total stress can be reduced from, for example, σ to σ 'at the surface at the height. There is essentially no influence on the voltage curve. By combining stress-free surface treatment and heat treatment It is possible to achieve particularly low surface tensions in order to guarantee a particularly high consistency of the surface course.

After a stress-free surface has been created, the block of material can be further processed. For example, the surface can be polished and / or provided with a polishing layer, which is also polished, and thereby brought to the required microroughness for reflective optical elements for EUV lithography. Thus, the block of material can serve as a substrate for example EUV mirror. To produce the EUV mirror, the polished surface is provided with a highly reflective coating, for example based on a multilayer system of alternating layers of materials with different real part of the refractive index at a wavelength in the EUV wavelength range.

A block of material 50 on his machined and tension reduced surface 51 a polishing layer 52 is, for example, schematically in FIG 5a shown. The material for the polishing layer is selected in particular with regard to the lowest possible achievable roughness. Suitable materials are for example amorphous silicon, nickel-phosphorus (especially at high phosphorus concentrations), silicon dioxide or copper. Polishing layers based on metal are preferably applied galvanically or without external current or chemically. In the case of copper polishing layers is preferably galvanically coated, since galvanically applied layers have a particularly low residual stress and therefore may have larger thicknesses. Typical thicknesses d are for example in the range of 10 microns. In particular, if a plurality of substrates with smaller dimensions, for example, for use for faceted mirror, to be obtained from a large block of material, this can first be processed so that individual webs 53 be cut out or diamond cut (see 5b , c). Their surfaces 54 can first be smoothed by, for example, rags and then the topmost area of the machined surface are removed stress-free, in order to reduce the stresses in the surface area as possible, before the surface 54 is polished.

On the polished and reduced voltage surface 54 becomes a polishing layer 52 applied. If necessary, first on the polished surface 54 of the original material block 56 applied a primer layer. The surface 55 the polishing layer 52 is now being polished so that its micro-roughness is sufficiently reduced for EUV mirror substrates. The so prepared webs 53 You can then, for example, along the lines 57 be isolated and later on their surface 55 be provided with a highly reflective coating to be used as a facet mirror can.

In the following examples, different variants of the substrate production will be shown by way of example. It is assumed without loss of generality that the material block is made of a copper alloy or a copper particle composite in which ceramic particles are dispersed in a copper or copper alloy matrix.

Example 1:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 μm, with the aim of reducing the stress gradient towards the surface. Thereafter, the later optically used surface was etched stress-free with nitric acid. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr (material No. CW 106-C), for example, the removal was max. 300 μm. Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

For example, nitric acid, dilute nitric acid with silver nitrate, ammonium sulfate solution with hydrochloric acid or ferric chloride or copper chloride solution or dilute ethanol with iron nitrate may be used to etch, in particular, copper composites, the concentration being adjusted to the desired erosion.

Example 2:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 microns, with the aim of reducing the voltage gradient towards the surface, and exposed by milling the optical surfaces in the form of Frässtegen. Thereafter, the later optically used surface was etched stress-free with nitric acid. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. This was followed by a fine polish with a polish based oxide cladding, as commonly used in polishing metal or optics, to achieve the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm each less than 5 nm) and pore shape (radius of the workpiece) manufacture. The maximum removal was 20 μm.

Example 3:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 microns, with the aim of reducing the voltage gradient towards the surface, and exposed by milling the optical surfaces in the form of Frässtegen. Thereafter, the material block was stress-relieved annealed at about 400 ° C for up to 5 h and the later optically used surface was stress-free etched with nitric acid. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

In variations of Example 3, annealing may also be performed after etching or both before and after etching.

Example 4:

Following the procedure described in Example 3, the surfaces polished with alumina were covered with acid-resistant lacquer and the milling flanks were etched stress-free. Then the paint was removed. Incidentally, the surfaces can also be polished chemically-mechanically. As a chemical polishing agent, for example, mixtures of concentrated acetic acid, phosphoric acid, nitric acid and optionally hydrochloric acid can be used. Before painting and / or after removing the paint, a low-stress annealing can be carried out at approx. 400 ° C for a maximum of 5 hours.

Example 5:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 microns, with the aim of reducing the voltage gradient towards the surface, and exposed by milling the optical surfaces in the form of Frässtegen. The optical surfaces were covered with acid-resistant paint and the milling edges were etched stress-free. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. Then the paint was removed. Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

In modifications of Example 3, the material block can be stress relieved annealed at about 400 ° C for up to 5 hours before etching, after etching or both before and after etching.

Example 6:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 μm, with the aim of reducing the stress gradient towards the surface. Thereafter, the surface to be optically used later was electro-polished stress-free. The removal depends on the mean grain size within the material structure. Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

In modifications of Example 6, instead of electropolishing, plasma polishing may be performed or both types of polishes may be combined.

Example 7:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 μm, with the aim of reducing the stress gradient towards the surface. Thereafter, the later optically used surface was etched stress-free with nitric acid. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. Subsequently, the surface was further selectively removed by means of ion beam etching (also called IBF, ion beam figuring, ion milling). Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

In modifications of Example 7, the ion beam etching may also be carried out after the fine polishing or both before and after the fine polishing.

Example 8:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 μm, with the aim of reducing the stress gradient towards the surface. Thereafter, the later optically used surface was etched stress-free with nitric acid. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. Subsequently, the surface was further selectively ablated by means of laser ablation. Thereupon, a fine polishing was carried out with an oxide-based abrasive as commonly used in polishing of metal or optics to obtain the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, respectively, smaller than 5 nm) and Passeform (radius of the workpiece). The maximum removal was 20 μm.

In modifications of Example 7, the laser ablation can also be performed after the fine polishing or both before and after the fine polishing.

Example 9:

A block of material in cylindrical form was pre-turned to produce planarity and parallelism of the faces. Subsequently, the surface of the block of material was microfinished with a mean removal of 200 μm, with the aim of reducing the stress gradient towards the surface. Thereafter, the surface to be used optically later was removed particle-controlled stress-free, for example by fine sandblasting. The removal depends on the mean grain size within the material structure. For copper alloy CuCr1Zr, for example, the removal was max. 300 μm. Subsequently, a chemical-mechanical polishing was carried out to produce the desired micro-roughness (frequency space: (100-1000) μm and (10-100) μm, each less than 5 nm) and Passeform (radius of the workpiece). The total removal was a maximum of 300 microns.

In a modification of Example 9, prior to polishing, the optical surface can be removed stress-free with dilute nitric acid with a maximum removal of 50 μm.

Claims (12)

Method for producing a substrate for a reflective optical element for EUV lithography, comprising the steps: - Providing a block of material; - processing a surface of the block of material by removing material on the surface in a stress-free manner; - further editing of the material block. A method according to claim 1, characterized in that prior to the stress-free processing step, the surface of the material block is processed by material is removed on the surface in a stress-inducing manner. A method according to claim 2, characterized in that the surface of the block of material is removed stress-inducing by milling, turning, honing, polishing, eroding or cloth. Method according to one of claims 1 to 3, characterized in that the surface of the block of material by means of etching, electropolishing, plasma polishing, ion beam assisted shaping, laser ablation or radiation is removed stress-free. Method according to one of claims 1 to 4, wherein in the material of the material block surface to be machined grain boundaries are formed, characterized in that in the stress-free surface treatment of the removal corresponds to at least the average grain size. Method according to one of claims 1 to 5, characterized in that the material block is additionally subjected before and / or after the stress-free surface treatment of a stress-reducing heat treatment. Method according to one of claims 1 to 6, characterized in that a polishing layer is applied to the block of material. Method according to one of claims 1 to 7, characterized in that the block of material is processed by milling webs, polishing and / or singulating. Substrate for a reflective optical element for EUV lithography, produced by the method according to one of claims 1 to 8. Substrate according to claim 9, characterized in that the material block and / or its surface is made of a copper or copper alloy composite material or a copper alloy. Reflective optical element for EUV lithography comprising a substrate according to claim 9 or 10 comprising an optical active coating based on a multilayer system of alternating layers of materials with different real part of the refractive index at a wavelength in the EUV wavelength range. Reflective optical element according to claim 11, characterized in that it is designed as a facet mirror for an EUV lithography device.

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