WO2012101090A1

WO2012101090A1 - Process for producing a substrate for a reflective optical element for euv lithography

Info

Publication number: WO2012101090A1
Application number: PCT/EP2012/050968
Authority: WO
Inventors: Holger Maltor; Claudia Ekstein
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2011-01-25
Filing date: 2012-01-23
Publication date: 2012-08-02
Anticipated expiration: 2013-07-25
Also published as: DE102011003077A1

Abstract

In order to counteract subsequent deformations, a process for producing a substrate for a reflective optical element for EUV lithography is proposed comprising the following steps: a block of material is provided; a surface of an area of at least 100 cm² of the block of material is machined by stress-free material removal on the surface with a removal rate of at least 0.5 μm/h; the block of material is machined further. In preferred embodiments, before the stress-free machining step, the surface of the block of material is machined by stress-inducing material removal on the surface.

Description

Process for producing a substrate for a reflective optical element for EUV lithography

The present invention relates to a process for producing a substrate for a reflective optical element for EUV lithography, to a substrate for a reflective optical element for EUV lithography which is produced thereby and also to a reflective optical element for EUV lithography having such a substrate.

The entire contents of the priority application DE 10 201 1 003 077.8 of January 25, 201 1 is incorporated into the present application by reference.

In order to make it possible to create ever finer structures using lithographic methods during the production of semiconductor components, for example, use is made of light having an ever shorter wavelength. If light in the extreme ultraviolet (EUV) wavelength range is used, for instance at wavelengths of between about 5 nm and 20 nm, it is no longer possible to use lens-like elements in transmission, but instead illumination and projection objectives are built up from mirror elements with highly reflective coatings which are adapted to the respective operating wavelength. In contrast to mirrors in the visible and ultraviolet wavelength ranges, it is also the case in theory that maximum reflectivities only of less than 80% can be achieved per mirror. Since EUV projective devices generally have a plurality of mirrors, it is necessary for each of these to have the highest possible reflectivity in order to ensure sufficiently high overall reflectivity.

In order both to keep losses in intensity as a result of stray radiation as low as possible and to avoid imaging aberrations, mirror substrates or mirrors which are produced by applying a highly reflective layer to the mirror substrate should have the lowest possible microroughness. The root mean squared (RMS) roughness is calculated from the mean value of the squares of the deviation of the measured points over the surface with respect to a central area, which is laid through the surface such that the sum of the deviations with respect to the central area is minimal. Particularly for optical elements for EUV lithography, the roughness in a spatial frequency range of 0.1 μηη to 200 μηη is particularly important for avoiding negative influences on the imaging quality of the optical elements.

In addition to the microroughness, it is also particularly important that the surface course of the optically used surface, which has to be worked very precisely in order to observe the optical specifications necessary for EUV lithography, remains as constant as possible over time. With respect to a substrate in which the surface course of the surface which is optically used in the finished reflective optical element has already been laid out as far as possible, it is particularly important that the surface of the substrate, as the latter is being produced, is machined such that the surface course remains as constant as possible over time.

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

This object is achieved by a process for producing a substrate for a reflective optical element for EUV lithography, comprising the following steps:

- a block of material is provided;

- a surface of an area of at least 100 cm² of the block of material is machined by stress-free material removal on the surface with a removal rate of at least 0.5 μηι/|-ι;

- the block of material is machined further. It has been found that the known surface machining methods can be separated into methods in which stresses are induced in the machined material, in particular on the surface thereof, and other methods in which no stresses are induced or such small stresses are induced that they are negligible compared with the stresses induced by the stress-inducing methods, and which are referred to here as stress-free methods. For the specific use as a substrate for reflective optical elements for EUV lithography, the stresses induced by the surface machining can have the effect that the substrate or the reflective optical element can subsequently become distorted, which can lead to an altered course of the optically used surface and therefore altered optical or imaging properties. Thus, the invention proposes the provision of at least one stress-free machining step for machining a block of material to form a substrate for a reflective optical element for EUV lithography. As a result, it is possible to reduce the degree to which stresses are present in the substrate produced, and it is thereby easier to ensure the necessary imaging properties of a reflective optical element produced as a result. It will be noted that the substrates with a reduced degree of stresses can be produced in reasonable amounts of time due to the high removal rate of at least 0.5 μηι/|-ι, preferably 1 μηΊ/h over areas of 100 cm² or more, preferably 250 cm² or more, most preferably 500 cm² or more.

In particularly preferred embodiments, before the stress-free machining step, the surface of the block of material is machined by stress-inducing material removal on the surface.

Advantageously, at least one stress-inducing surface machining step is combined with at least one surface machining step which does not induce any additional stresses, but instead removes surface in which stresses have been induced by previous machining. In this way, it is possible to precisely incorporate a surface course and at the same time to reduce the degree of induced stresses which might subsequently lead to deformations of the surface. In particular, it is possible to remove more material in a relatively short time in many stress- inducing machining methods, and therefore the combination of stress-inducing and stress-free machining steps firstly makes it possible to produce substrates for reflective optical elements for EUV lithography as quickly and economically as possible, and at the same time makes it possible to reduce the stresses present in the finished substrate to the smallest possible level.

It is preferable for the surface of the block of material to be subjected to stress-inducing material removal by milling, turning, honing, polishing, in particular coarse polishing, electric discharge machining or lapping. These types of machining make it possible to already provide the substrate and the surface which is later optically used in the reflective optical element with a coarse form with relatively little outlay and in a relatively short time. The surface course, in particular, is predefined by turning, honing, polishing, in particular coarse polishing, electric discharge machining or lapping. By milling, it is possible, for example, to expose individual milling webs which can later be singulated to form individual substrates or reflective optical elements.

In preferred embodiments, the surface of the block of material is subjected to stress-free material removal by etching, plasma etching, electropolishing or plasma polishing. For areas of at least 100 cm², preferably 100 cm² to 1000 cm², particulary 300 cm² to 600 cm², removal rates of ca. 350 μηΊ/h to 800 μηΊ/h can be achieved by etching, of up to ca. 1 μηΊ/h can be achieved by plasma etching, of ca. 3 μηΊ/h to 900 μηΊ/h can be achieved by electropolishing and of ca. 1 μηΊ/h to 75 μηΊ/h can be achieved by plasma polishing. The material removal can be varied in a targeted manner areally over the surface and these types of surface machining are particularly suitable for finely machining the respective surface within reasonable times. In particular variants of the proposed production process, the production of the substrate may also involve only stress-free machining steps, in which case different stress-free machining steps can be combined with one another. Under certain circumstances, it is also possible to reduce residual stresses present in the block of material in the surface region by one or more stress-free machining steps.

Particularly in the case of surfaces which have grain boundaries in the material thereof, it has proved to be advantageous if the material removed during the stress-free surface machining corresponds at least to the average grain size. It is assumed that, during stress-inducing surface machining methods, the stresses induced end predominantly at the grain boundaries of the grains which adjoin the surface, where they accumulate. Since these grains are removed substantially by stress-free surface machining, the stress induced in the surface can be reduced significantly.

Depending on the surface material and surface machining, it may be advantageous if the block of material is additionally subjected to a stress-reducing heat treatment before and/or after the stress-free surface machining. Particularly if stresses can be induced very quickly in the material or the material already has high residual stresses, or if surface machining steps via which particularly high stresses are induced are proceeded with, the stress in the block of material and also at the surface thereof can additionally be reduced as a whole by a heat treatment.

In preferred embodiments, a polishing layer is applied to the block of material, it also being possible for said polishing layer to be so thick that it not only serves to polish the surface to the lowest possible microroughness, but also a significant part of the surface course is

incorporated therein. Depending on the desired surface course and thickness of the polishing layer, the latter can be applied before or after stress-inducing and stress-free surface machining steps or else therebetween. The provision of a polishing layer makes it possible to optimize the base material of the block of material to properties such as a low coefficient of thermal expansion or good coolability, for example, while the polishing layer is optimized for good polishability. Examples of suitable materials for a polishing layer are metals and metal alloys or metal composites, silicon or silicon oxide. If necessary, an adhesion-promoter layer can be provided between the polishing layer and the underlying material. In the case of metallic polishing layers, particular preference is given to polishing layers applied by electroplating on account of their usually very low residual stresses. It is preferable for the block of material to be machined by laser ablation, ion beam figuring (IBF, ion milling), milling webs, polishing and/or singulation, in order 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 explained process. Such substrates are distinguished by a very precisely worked and thus resistant surface course, and therefore they are particularly suitable for use as a substrate for a reflective optical element for EUV lithography.

In particularly preferred embodiments, the block of material and/or the surface thereof is made of a copper or copper alloy composite or of a copper alloy. In terms of machinability, strength and dimensional stability, in particular under the action of heat, these materials are particularly suitable for substrates for reflective optical elements for EUV lithography. As a polishing layer, they can be applied inter alia by electroplating. By way of example, particular preference is given to high-strength copper alloys such as, inter alia, copper alloys comprising 0.8% by weight chromium and 0.08% by weight zirconium (commercially available for example under the trade name Elmedur X^® from Thyssen Durometall), or oxides, borides and/or carbides of dispersion-strengthened copper (commercially available for example under the trade name Glidcop^® or Discup^®).

Furthermore, the object is achieved by a reflective optical element for EUV lithography, which has a substrate as mentioned with an optically active coating on the basis of a multilayer system of alternating layers of materials having a differing real part of the refractive index at a wavelength in the EUV wavelength range. Such multilayer systems are regularly used in reflective optical elements which are to be used in the case of substantially normal incidence in the EUV wavelength range. In this case, the materials and also the number, the thicknesses and the specific sequence of the layers are chosen in a known manner primarily in view of the highest possible reflectivity at the wavelength at which the lithography is to be carried out. By way of example, multilayer systems for wavelengths of 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 in the form of a facet mirror for an EUV lithography apparatus. Facet mirrors in particular have particularly small dimensions, and therefore it is particularly highly probable that stresses induced in the surface could subsequently lead to disruptive deformations, but these are lowered using the substrates produced in the manner described. The features mentioned above and further features are apparent not only from the claims but also from the description and the drawings, wherein the individual features can in each case be realized by themselves or as a plurality in the form of subcombinations in an embodiment of the invention and in other fields and can constitute advantageous and inherently protectable embodiments.

The present invention will be explained in more detail with reference to a preferred exemplary embodiment. In this respect,

Figure 1 schematically shows an EUV lithography apparatus;

Figure 2 schematically shows an illumination system of an EUV lithography

apparatus; Figures 3a, b schematically show the influence of a stress-free surface treatment on the stress in a block of material; Figures 4a, b schematically show the influence of a stress-free heat treatment on the stress in a block of material; and

Figures 5a-c schematically show individual machining states during the production of reflective optical elements for EUV lithography.

Figure 1 shows an outline view of an EUV lithography apparatus 100 for producing

microelectronic components, for example, which is operated in a scanning mode along a scanning direction 126 at an operating wavelength in the EUV range and which can have one or more optical elements with an additional coating. The EUV lithography apparatus 100 shown in Figure 1 has a punctiform plasma radiation source. The radiation from the laser source 102 is directed via a condenser lens 104 onto suitable material, which is introduced via the supply 108 and is excited to form a plasma 106. The radiation emitted by the plasma 106 is imaged onto the intermediate focus Z by the collector mirror 1 10. Appropriate apertures 1 1 1 at the intermediate focus Z ensure that no undesirable stray radiation impinges on the following mirrors 1 12, 1 14, 1 16, 1 18, 120 of the illumination system of the projection exposure apparatus 100. The plane mirror 122 serves for folding the system, in order to provide installation spaces for mechanical and electronic components in the object plane in which the mount for the reticle 124 is arranged. In the illumination system, the mirror 1 12 is followed by a field facet mirror 1 14 and a pupil facet mirror 1 16 in the present example. The field facet mirror 1 14 serves to project a multiplicity of images of the radiation source of the EUV lithography apparatus into a pupil plane in which there is arranged a second facet mirror, which serves as a pupil facet mirror 1 16 and superimposes the images of the facets of the field facet mirror 1 14 in the object plane, in order to make the most homogeneous illumination possible possible. The mirrors 1 18 and 120, which follow the facet mirrors 1 14, 1 16, serve substantially to form the field in the object plane. A structured reticle 124 is arranged in the object plane and the structure thereof is imaged onto the object 130 to be exposed, for instance a wafer, by means of a projection objective 128 which, in the present example, has six mirrors. In the EUV lithography apparatus 100, which here is designed as a scanning system, the reticle 124 can be moved in the indicated direction 126, and is illuminated successively in certain portions in order to appropriately project the respective structures of the reticle 124 onto a wafer 130, for example, using the projection objective. Figure 2 shows a radiation source in conjunction with an illumination system 1 1 which is part of an EUV lithography apparatus. A collector 1 is arranged around a light source which is formed by a plasma droplet 2 which is excited by an infrared laser 3. In order to obtain wavelengths in the region around 13.5 nm, for example, in the EUV wavelength range, it is possible to use a carbon dioxide laser which operates at a wavelength of 10.6 μηη to excite tin, for example, to form a plasma. Instead of a carbon dioxide laser, it is also possible to use solid-state lasers, for example. A field facet mirror 16 having individual facets 18 and a pupil facet mirror 17 having individual facets 19 follow the collector 1 after the aperture 5 at the intermediate focus 4.

Before the rays impinge on the reticle 13 to be scanned in the y direction having the structure to be projected onto a wafer, they are further deflected by a folding mirror 12. The folding mirror 12 has a lesser optical function, but instead serves to optimize the space required for the illumination system 1 1.

It should be pointed out that a very wide variety of radiation sources can be used in EUV lithography, including plasma sources, which can be based for example on laser excitation (LPP sources) or gas discharge (DPP sources), synchrotron radiation sources or free electron lasers (FEL). Furthermore, the collectors can be in any desired form, including even in the form of a Wolter collector or in the form of an ellipsoid collector, preferably adapted to the radiation source respectively used. In addition, the illumination system of an EUV lithography apparatus can also be in any desired form and, in addition or else instead of the facet mirrors, can have, inter alia, fly's eye condensers, specular reflectors, movable optical components or the like.

The facet mirrors described in conjunction with Figures 1 and 2, but also other reflective optical elements for use in EUV lithography, can be produced on the basis of substrates which are produced, for example, by

- providing a block of material;

- machining a surface of the block of material by stress-inducing material removal on the surface;

- subsequently a surface of an area of at least 100 cm² of the block of material is machined by stress-free material removal on the surface with a removal rate of at least 0.5 μηι/|-ι;

- machining the block of material further.

It will be noted that the substrates with a reduced degree of stresses can be produced in reasonable amounts of time due to the high removal rate of at least 0.5 μηι/|-ι, preferably 1 μηΊ/h over areas of several hundred square centimeters.

In particular, the surface of the block of material is subjected to stress-free material removal by etching, plasma etching, electropolishing or plasma polishing. For areas of at least 100 cm², preferably 100 cm² to 1000 cm², in particular 300 cm² to 600 cm², removal rates of ca. 350 μηη/η to 800 μηη/η can be achieved by etching, of up to ca. 1 μηΊ/h can be achieved by plasma etching, of ca. 3 μηΊ/h to 900 μηΊ/h can be achieved by electropolishing and of ca. 1 μηΊ/h to 75 μηΊ/h can be achieved by plasma polishing.

The material of the block of material is preferably selected on the basis that it has a good dimensional stability and/or a low thermal expansion. Suitable materials include metals, intermetallic compounds, copper, copper alloys, copper composites, aluminum alloys, aluminum composites, doped glasses, glass-ceramics. If the block of material is prestructured by milling, turning, honing, polishing, in particular coarse polishing, electric discharge machining or lapping and/or the surface for later optical use is provided with a desired course, which can be planar, curved or even aspherical, stresses are thereby induced in particular in the region of the machined surface in addition to residual stresses present in the block of material.

This is shown schematically in Figures 3a, b using the example of a cylindrical block of material 30 having a height I. The stress σ, which has a value of greater than zero in the block of material 30, increases greatly in the region of the machined surface at the height I. A high stress gradient is present. If a height ΔΙ is then removed by stress-free surface machining, for example by etching, electropolishing or plasma polishing, such that the end height of the block of material 30 is then only Γ, it is thereby possible to reduce the stress from σ to σ'. Particularly in the case of materials which have grain boundaries, for example in the case of crystalline materials, it has proved to be advantageous for reducing the stress if the material removed during the stress-free surface machining corresponds at least to the average grain size.

If, as shown in Figures 4a, b, a heat treatment is carried out on a block of material 40 which has been subjected to stress-inducing machining on its surface, the stress can be reduced as a whole from, for example, σ to σ' at the surface at the height I. Substantially no influence is exerted on the stress profile. By a combination of the stress-free surface machining and the heat treatment, it is possible to achieve particularly low stresses on the surface, in order to make it possible to ensure a particularly high constancy of the surface course.

Once a stress-free surface has been created, the block of material can be machined further. Thus, the surface course can be adjusted by ion beam figuring or laser ablation. Additionally or alternatively, the surface can be polished and/or provided with a polishing layer which is likewise polished, for example, and in the process can be given the microroughness which is necessarily low for reflective optical elements for EUV lithography. The block of material can thus serve as a substrate for EUV mirrors, for example. To produce the EUV mirror, the polished surface is provided with a highly reflective coating, for example on the basis of a multilayer system of alternating layers of materials having a differing real part of the refractive index at a wavelength in the EUV wavelength range.

A block of material 50 having a polishing layer 52 on its machined surface 51 with reduced stresses is shown schematically in Figure 5a, by way of example. The material for the polishing layer is chosen in particular in view of the smallest possible achievable roughness. Examples of suitable materials are amorphous silicon, nickel-phosphorus (in particular with high phosphorus concentrations), silicon dioxide or else copper. Polishing layers on a metal basis are preferably applied by electroplating or without external current or chemically. In the case of copper polishing layers, it is preferable for electroplating to be used, since layers applied by electroplating have a particularly low residual stress and therefore can also have relatively large thicknesses. Typical thicknesses d are in the region of 10 μηη, for example. Particularly if a plurality of substrates having relatively small dimensions, for example for use for facet mirrors, are to be obtained from a large block of material, the latter can first of all be machined in such a way that individual webs 53 are milled out or diamond cut (see Figures 5b, c). The surfaces 54 thereof can firstly be smoothed by lapping, for example, and then the uppermost region of the machined surface can be subjected to stress-free material removal, in order to reduce the stresses in the surface region as much as possible, before the surface 54 is polished.

A polishing layer 52 is applied to the polished surface 54 with reduced stresses. If necessary, an adhesion-promoter layer is firstly applied to the polished surface 54 of the original block of material 56. The surface 55 of the polishing layer 52 is then polished in such a way that the microroughness thereof is reduced adequately for EUV mirror substrates. The webs 53 thus prepared can then be singulated along the lines 57, for example, and can later be provided with a highly reflective coating on the surface 55 thereof, so that they can be used as facet mirrors.

The following examples are intended to show various variants of the substrate production by way of example. Here, it is assumed, without restricting the generality, that the block of material is made of a copper alloy or a copper particulate composite in which ceramic particles are dispersed in a copper or copper alloy matrix.

Example 1 : A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface. The surface for later optical use was then subjected to stress-free etching using nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrlZr (material no. CW 106-C), by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness (frequency domain: (100-1000) μηη and (10-100) μηη in each case less than 5 nm) and fit (radius of the workpiece). In this case, the maximum material removal was 20 μηη.

For etching copper microstructures, in particular, it is possible to use, by way of example, nitric acid, dilute nitric acid with silver nitrate, ammonium sulfate solution with hydrochloric acid or iron chloride or copper chloride solution or dilute ethanol with iron nitrate, the concentration being adapted to the desired material removal.

Example 2:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface, and the optical surfaces were exposed in the form of milling webs by milling. The surface for later optical use was then subjected to stress-free etching using nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrl Zr, by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness

(frequency domain: (100-1000) μηη and (10-100) μηη in each case less than 5 nm) and fit (radius of the workpiece). In this case, the maximum material removal was 20 μηη.

Example 3:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface, and the optical surfaces were exposed in the form of milling webs by milling. Then, the block of material was subjected to stress-relief annealing at about 400°C for up to 5 h, and the surface for later optical use was subjected to stress-free etching using nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrl Zr, by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness

In modifications of Example 3, the annealing can also be carried out after the etching or both before and after the etching.

Example 4:

Following the procedure outlined in Example 3, the surfaces which were polished with alumina were covered with an acid-resistant coating, and the milling flanks were subjected to stress- free etching. Then, the coating was removed again. The surfaces can also be polished by a chemical mechanical process. Mixtures of concentrated acetic acid, phosphoric acid, nitric acid and optionally hydrochloric acid can be used, by way of example, as the chemical polishing agent. Before the coating and/or after the coating has been removed, it is possible to carry out stress-relief annealing at about 400°C for at most 5 h.

Example 5:

A cylindrical block of material was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface, and the optical surfaces were exposed in the form of milling webs by milling. The optical surfaces were covered with an acid-resistant coating, and the milling flanks were subjected to stress-free etching. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrlZr, by way of example, the material removal was max. 300 μηη. Then, the coating was removed again. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness (frequency domain: (100- 1000) μηη and (10-100) μηη in each case less than 5 nm) and fit (radius of the workpiece). In this case, the maximum material removal was 20 μηη. In modifications of Example 5, the block of material can be subjected to stress-relief annealing at about 400°C for up to 5 h before the etching, after the etching or both before and after the etching. Example 6:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface. The surface for later optical use was then subjected to stress-free electropolishing. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrl Zr, by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness

In modifications of Example 6, plasma polishing can also be carried out instead of

electropolishing or both types of polishing can be combined. In the case of the copper alloy CuCrlZr, by way of example, the material removal over the entire end face due to plasma polishing was max. 300 μηη, which took approximately 5 h.

In further modifications of Example 6, plasma etching can also be carried out instead of electropolishing. In the case of the copper alloy CuCrl Zr, by way of example, the material removal over the entire end face due to plasma etching was max. 50 μηη, which took approximately 55 h.

Example 7:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface. The surface for later optical use was then subjected to stress-free etching using nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrlZr, by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. Then, the surface was subjected to further targeted material removal by means of ion beam figuring (IBF, ion milling). This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired microroughness (frequency domain: (100-1000) μηη and (10-100) μηη in each case less than 5 nm) and fit (radius of the workpiece). In this case, the maximum material removal was 20 μηη.

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

Example 8:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface. The surface for later optical use was then subjected to stress-free etching using nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrlZr, by way of example, the material removal over the entire end face was max. 300 μηη, which took approximately 30 min. Then, the surface was subjected to further targeted material removal by means of laser ablation. This was followed by fine polishing using a polishing agent based on oxidic alumina, as is generally used for polishing metal or in optics, in order to establish the desired

microroughness (frequency domain: (100-1000) μηη and (10-100) μηη in each case less than 5 nm) and fit (radius of the workpiece). In this case, the maximum material removal was 20 μηι.

In modifications of Example 8, the laser ablation can also be carried out after the fine polishing or both before and after the fine polishing.

Example 9:

A cylindrical block of material with a diameter of 240 mm was pre-turned in order to establish planarity and parallelism of the end faces. Then, the surface of the block of material was very finely turned with a mean material removal of 200 μηη, with the aim of reducing the stress gradient toward the surface. The surface for later optical use was then subjected to stress-free material removal using dilute nitric acid. The material removal depends on the mean grain size within the material microstructure. In the case of the copper alloy CuCrl Zr, by way of example, the material removal over the entire end face was at most 50 μηη, which took approximately 9 min. This was followed by chemical mechanical polishing in order to establish the desired microroughness (frequency domain: (100-1000) μηη and (10-100) μηη in each case less than 5 „„

14 nm) and fit (radius of the workpiece). In this case, the overall material removal was at most 300 μηη.

It will be noticed that the various possibilities of stress-free can be combined with each other in any way as well as with stress reducing heat treatments and various further machining steps.

Claims

Patent Claims

1. A process for producing a substrate for a reflective optical element for EUV lithography, comprising the following steps:

- a block of material is provided;

- the block of material is machined further.

2. The process according to claim 1 , characterized in that, before the stress-free machining step, the surface of the block of material is machined by stress-inducing material removal on the surface.

3. The process according to claim 2, characterized in that the surface of the block of material is subjected to stress-inducing material removal by milling, turning, honing, polishing, electric discharge machining or lapping.

4. The process according to one of claims 1 to 3, characterized in that the surface of the block of material is subjected to stress-free material removal by etching, plasma etching, polishing or plasma polishing.

5. The process according to one of claims 1 to 4, wherein grain boundaries are formed in the material of the surface of the block of material to be machined, characterized in that the material removed during the stress-free surface machining corresponds at least to the average grain size.

6. The process according to one of claims 1 to 5, characterized in that the block of material is additionally subjected to a stress-reducing heat treatment before and/or after the stress-free surface machining.

7. The process according to one of claims 1 to 6, characterized in that a polishing layer is applied to the block of material.

8. The process according to one of claims 1 to 7, characterized in that the block of material is machined by ion beam figuring, laser ablation, milling webs, polishing and/or singulation.

9. A substrate for a reflective optical element for EUV lithography, produced by the process according to one of claims 1 to 8.

10. The substrate according to claim 9, characterized in that the block of material and/or the surface thereof is made of a copper or copper alloy composite or of a copper alloy.

1 1 . A reflective optical element for EUV lithography having a substrate according to claim 9 or 10 with an optically active coating on the basis of a multilayer system of alternating layers of materials having a differing real part of the refractive index at a wavelength in the EUV wavelength range.

12. The reflective optical element according to claim 1 1 , characterized in that it is in the form of a facet mirror for an EUV lithography apparatus.