WO2013014182A1

WO2013014182A1 - Mirror, optical system comprising mirror and method for producing a mirror

Info

Publication number: WO2013014182A1
Application number: PCT/EP2012/064594
Authority: WO
Inventors: Oliver Dier
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2011-07-28
Filing date: 2012-07-25
Publication date: 2013-01-31
Also published as: DE102011080052A1

Abstract

A mirror has a substrate (112), which consists of a substrate material having a specific thermal conductivity of less than 10W/(m*K), and a reflection coating (116) applied on the substrate, said reflection coating having a reflective effect for vacuum ultraviolet radiation or radiation from the extreme ultraviolet range. A heat distribution intermediate layer (120, 220) is arranged between the substrate (112) and the reflection coating (116), said heat distribution intermediate layer consisting of a layer material having a specific thermal conductivity that is at least 10 times the magnitude of the specific thermal conductivity of the substrate material. The heat distribution intermediate layer has a layer thickness of at least 1 µm.

Description

Mirror, optical system comprising mirror and method for producing a mirror The following disclosure is based on German Patent Application No. 10 201 1 080 052.2 filed on July 28, 201 1 , which is incorporated into this application by reference.

BACKGROUND

Technical field

The invention relates to a mirror in accordance with the preamble of claim 1 , to an optical system comprising a mirror in accordance with the preamble of claim 15, and to a method for producing a mirror in accordance with the preamble of claim 18. One preferred field of application is microlithography at operating wavelengths from the range of vacuum ultraviolet radiation (VUV) radiation or extreme ultraviolet radiation (EUV).

Description of the prior art

Nowadays predominantly microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components. In this case, use is made of masks (reticles) or other patterning devices which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object sur- face of the projection lens and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as projection radiation through the projection lens, which images - - the pattern onto the substrate to be exposed, which is coated with a radiation-sensitive layer.

The pattern is illuminated with the aid of an illumination system, which, from the radiation of a primary radiation source, forms an illumination radiation which is directed onto the pattern and which is characterized by specific illumination parameters and impinges on the pattern within an illumination field of defined form and size. Within the illumination field, a predefined local intensity distribution should be present, which is nor- mally intended to be as uniform as possible.

In general, depending on the type of structures to be imaged, different illumination modes (so-called illumination settings) are used, which can be characterized by different local intensity distributions of the illumina- tion radiation in a pupil surface of the illumination system. It is thereby possible to predefine in the illumination field a specific illumination angle distribution or a specific distribution of the impinging intensity in the angle space. In order to be able to produce ever finer structures, various approaches are pursued. By way of example, the resolution capability of a projection lens can be increased by enlarging the image-side numerical aperture (NA) of the projection lens. Another approach consists in employing shorter wavelengths of the electromagnetic radiation.

For applications in the field of microlithography, optical systems which operate with vacuum ultraviolet radiation (VUV radiation), in particular at an operating wavelength of approximately 193 nm, or with radiation from the extreme ultraviolet range (EUV) are often being used in the mean- time. . .

If it is attempted to improve the resolution by increasing the numerical aperture, then problems can arise by virtue of the fact that as the numerical aperture increases, the depth of focus (DOF) that can be achieved decreases. This is disadvantageous because a depth of focus of the order of magnitude of at least 0.1 μητι is desirable for example for reasons of the achievable flatness of the substrates to be structured and mechanical tolerances.

For this reason, inter alia, optical systems have been developed which operate with moderate numerical apertures and achieve the increase in the resolution capability substantially by means of the short wavelength of the used electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nm and 30 nm. In the case of EUV lithography having operating wavelengths of around 13.5 nm, for example given image-side numerical apertures of NA=0.3 it is theoretically possible to achieve a resolution of the order of magnitude of 0.03 μητι in conjunction with typical depths of focus of the order of magnitude of approximately 0.15 μητι. Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. A mirror (EUV mirror) having a reflective effect for radiation from the EUV range typically has a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range (EUV) and having a large number of layer pairs comprising alternately low refractive index and high refractive index layer material. Layer pairs for EUV mirrors are often constructed with the layer material combinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si). - -

EUV mirrors having multilayer reflection coatings typically achieve maximum reflectances of around 70%. A portion of the non-reflective radiation intensity of the incident radiation is absorbed in the layer materials with generation of heat. The absorption of radiation results in local heating in the irradiated regions. As a consequence of the heating, deformations of the mirror surface can occur on account of thermal expansion of the reflection coating and possibly of the substrate regions adjoining the latter. These thermally induced deformations can become apparent as imaging aberrations when the EUV mirrors are used in an imaging system. Said imaging aberrations generally cannot be completely compensated for, or can be completely compensated for only with high outlay in respect of apparatus. Depending on the spatial distribution of the irradiation intensity on the mirror surface, very different imaging aberrations can be induced. The thermally induced deformations are generally reversible, such that the mirror surface reverts to its initial form after the illumination has been switched off.

A local temperature increase at a mirror surface can possibly also lead to irreversible aberrations in the region of the mirror surface, e.g. to in- creased layer compaction in the reflection coating. The optical performance of the optical system can therefore be detrimentally affected over the lifetime of the optical system or depending on the accumulated irradiation dose. Numerous proposals have already been made for avoiding or reducing thermally induced aberrations in optical systems comprising EUV mirrors.

One approach for limiting thermally induced disturbances in optical sys- terns consists in using materials having extremely low coefficients of thermal expansion in the production of the optical elements, such that disturbances caused by thermal expansion are reduced from the outset . .

to an amount that can be afforded tolerance. It is known, for example, to use specific glass ceramics as substrate material for mirrors. One glass ceramic suitable for producing mirror substrates for microlithography systems is sold under the trade designation ZERODUR^® (Schott AG). Coefficients of thermal expansion of 0 + 0.10^*10^"6K^"1 are specified for these glass ceramics for the temperature range between 0°C and 50°C. Another suitable glass ceramic having substantially identical coefficients of thermal expansion is sold under the trade designation CLEARCERAM^® (Ohara, Inc.). Even lower coefficients of thermal ex- pansion are achieved in the case of specific titanium silicate glasses, also known as "Ultra Low Expansion Glass". For one such titanium silicate glass sold under the trade designation ULE^® by Corning, Inc., an average coefficient of thermal expansion of 0 + 30^*10^"9 K^"1 is specified for the temperature range between 5°C and 35°C.

The European patent application EP 0 955 565 A2 describes EUV mirrors for use in an illumination system or a projection lens of a microlithography projection exposure apparatus, in which mirrors the substrate of the EUV mirror consists of a metallic substrate material, for example of aluminum, copper, beryllium, silver, gold, or an alloy comprising one or more of these elements. A thin film composed of an amorphous material is applied to the front side of the substrate, and the front side of said film facing away from the substrate is smoothed by polishing and can thus serve as a support surface for the reflection coating applied thereon. What is intended to be achieved by using a metallic substrate material having a high thermal conductivity is that the heat evolved upon irradiation of the mirror surface in the region of the reflection coating can be dissipated rapidly and efficiently to the cooled rear side of the mirror substrate. Aberrations on account of thermal deformations in the region of the mirror surface are intended thereby to be avoided. . .

The patent application US 2002/00741 15 A1 (corresponds to DE 100 50 125) discloses an apparatus for temperature compensation for thermally loaded bodies composed of materials having a low specific thermal conductivity, wherein a thermally loaded body can be, for exam- pie, a mirror carrier for an optical system. In the apparatus, a heat distribution device comprising one or more heat distribution bodies is adapted to surfaces of the thermally loaded body in such a way that a gap remains between the thermally loaded body and a heat distribution body, said gap being filled with a fluid for the purpose of thermally coupling the thermally loaded body and the heat distribution body with at the same time mechanical decoupling. The intention is thereby to achieve a separation between a mechanical coupling and a thermal coupling relative to the thermally loaded body. By way of example, solid bodies of copper, aluminum or silver are proposed as heat distribution bodies, and, by way of example, water or mercury or metal alloys that are liquid at room temperature is/are proposed as coupling fluid.

The international patent application WO 2010/020337 A1 discloses a reflective reticle for EUV lithography which is intended to reduce or avoid thermally induced pattern distortions. The reflective reticle has an optical layer serving as a carrier layer for the pattern, said optical layer being composed of a material having an extremely low coefficient of thermal expansion. In one embodiment, an Ultra Low Expansion Glass (ULE glass) is used. By contrast, the substrate of the reticle consists of a sub- strate material whose thermal conductivity is significantly greater than the thermal conductivity of the optical layer. In one exemplary embodiment, the substrate consists of cordierite, a ceramic material which, at the temperatures typical of the use of EUV mirrors, has a practically vanishing coefficient of thermal expansion, but a thermal conductivity (ap- proximately 3.0 W/(m*K) at 25°C) about twice as high in comparison with other conventional materials having a very low coefficient of thermal expansion. A conductive layer is arranged between the substrate and the - - optical layer, said conductive layer being in contact with the substrate surface and/or a facing surface of the optical layer and consisting of aluminum in one exemplary embodiment. In order to reduce thermally induced aberrations in lens elements or mirrors, it has also already been proposed to compensate for inhomogene- ous heating of the optical component on account of inhomogeneous irradiation by heating less highly irradiated regions and/or cooling more highly heated regions, with the result that the temperature distribution is made more uniform (e.g. EP 0 678 768 B1 ).

PROBLEM AND SOLUTION

One problem addressed by the invention is to provide a mirror which can be used e.g. in an optical system of a microlithography projection exposure apparatus and affords a high stability of the reflection properties under different operating conditions and over the entire lifetime of the projection exposure apparatus. A further problem is to provide a mirror which is relatively insensitive to adverse influences of inhomogeneous irradiation loading.

In order to solve these problems, the invention provides a mirror comprising the features of claim 1. Furthermore, an optical system comprising the features of claim 15 is provided. Furthermore, a method for pro- ducing a mirror comprising the features of claim 18 is provided.

Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference in the content of the description.

In the case of a mirror having such a construction, a predominant proportion of the heat that arises during the irradiation in the reflection coat- . .

ing is not dissipated by the substrate material, furnished with a relatively low specific thermal conductivity, but rather via the heat distribution intermediate layer, which is capable of thermal conduction significantly better than the substrate. Therefore, a predominant proportion of the heat dissipation is not effected perpendicularly to the reflection coating through the substrate in the direction of the rear side thereof, but rather parallel to the reflection coating through the heat distribution intermediate layer in a lateral direction in a region between the reflection coating and the substrate. In this case, a considerable proportion of the heat flow follows the lateral temperature gradient within the heat distribution intermediate layer, such that heat is redistributed from locally heated regions laterally into regions which are cooler relative thereto and which can readily heat up, if appropriate, on account of the thermal energy supplied.

The lateral redistribution of thermal energy has an effective action toward the goal of increasing the uniformity of, or homogenizing, the thermal loading of the mirror surface in the case of locally concentrated irradiation. The heat distribution intermediate layer thus acts as a homoge- nizing layer. In this case, it is not necessary for a completely homogeneous (uniform) temperature distribution actually to be achieved. A decrease or reduction of lateral temperature gradients or a reduction of the temperature inhomogeneity is primarily important. What is achieved by the lateral redistribution of thermal energy within the heat distribution intermediate layer is that an inhomogeneous irradiation intensity distribution - often desired during the operation of the projection exposure apparatus - at the mirror surface does not lead to the same or a corresponding inhomogeneous temperature distribution in the sub- strate material and the associated thermally induced deformations of the mirror surface. Rather, a partial decoupling of the heat input into the substrate material from the local distribution of the heat generation in the . .

reflection coating is achieved by the lateral redistribution of thermal energy. The heat distribution intermediate layer distributes the heat input by radiation in the region between the reflection coating and the mirror substrate and thereby increases the uniformity of, or homogenizes, both the temperature distribution in the substrate and the temperature distribution in the reflection coating.

The reflection coating is situated on that side of the heat distribution intermediate layer which faces away from the substrate, and thus on the light entrance side of the mirror. The reflection coating, which is preferably constructed from a plurality of individual layers, is preferably designed such that practically no intensity or only a vanishingly small proportion of the incident radiation reaches the substrate. The incident radiation is therefore predominantly (e.g. to the extent of more than 40% or more than 60%) reflected and possibly partly absorbed by the reflection coating. The transmittance of the reflection coating at the operating wavelength is typically significantly lower than the reflectance. For the operating wavelength range of the mirror, the reflection coating has a relatively high reflectance, which is normally more than 40% or more than 60%. In the case of reflection coatings designed for vacuum ultraviolet radiation, reflectances of more than 90% or even more than 95% are typically achieved. In the case of reflection coatings having a reflective effect for radiation from the extreme ultraviolet range, the maximum reflectances that can be achieved are generally lower and are typically at most 70%.

The mirror has a substrate composed of a substrate material having poor thermal conductivity. A "poor thermal conductivity" within the meaning of this application is afforded, in particular, when the substrate mate- rial has a specific thermal conductivity of less than 10 W/(m^*K). In the case of many substrate materials which may be attractive on account of low thermal expansion and/or for other reasons, the thermal conductivity - - is significantly lower still. In particular, the specific thermal conductivity of the substrate material can be approximately 3 W/(m^*K) or less.

In order to concentrate a heat redistribution largely on the region of the heat distribution intermediate layer, it can therefore be expedient to use substrate materials having a very low thermal conductivity. In some embodiments, the substrate material has a specific thermal conductivity of less than 2 W(m*K) or less than 1.5 W(m*K). In particular, the titanium silicate glasses (e.g. ULE^® with approximately 1 .31 W/(m^*K) at 25°C) or glass ceramics (e.g. ZERODUR^® (with approximately 1.46 W/(m^*K) or CLEARCERAM^® with approximately 1 .51 W/(m^*K)) mentioned in the introduction or comparable substrate materials can be used.

In general, it is advantageous if the thermal conduction ratio TCR be- tween the specific thermal conductivity of the layer material and the specific thermal conductivity of the substrate material is as high as possible. In some embodiments, TCR > 50 or TCR > 70 or even TCR > 100 holds true. This fosters a strong homogenizing effect. A prerequisite for an effective lateral redistribution of thermal energy by the heat distribution intermediate layer is a highest possible specific thermal conductivity of the layer material of the heat distribution intermediate layer, said layer material also being designated as intermediate layer material in this application. In general, it is advantageous if the in- termediate layer material has a specific thermal conductivity of more than 50 W/(m^*K) or more than 100 W/(m^*K) or more than 200 W/(m^*K) or more than 500 W/(m^*K).

The intermediate layer material can be, for example, a metal or a metal alloy. On account of their high specific thermal conductivity, for example aluminum or copper or silver or alloys comprising one of these materials as main constituent are appropriate, possibly also nickel or other metals. - -

In some embodiments, the intermediate layer material is a nonmetallic material. By way of example, it is possible to use crystalline semiconductor materials as intermediate layer material. Thus, the heat distribution intermediate layer can consist, for example, exclusively or predominantly of crystalline silicon or crystalline germanium.

Crystalline semiconductor materials are regarded as advantageous here for the following reasons, inter alia. In metals, heat is transferred primarily by electrons. In the Wiedemann-Franz law, the electrical conductivity and thermal conductivity in metals are linked via the Lorenz number. In crystalline insulators or semiconductors, by contrast, the transfer of heat is dominated by lattice vibrations (phonons). The greater the extent to which the lattice can vibrate without disturbance (that is to say no defects, impurities, alloys or the like), the better the thermal conduction. Completely or predominantly crystalline structures can thus be advantageous compared with partly or completely amorphous structures that are likewise possible.

In some embodiments, the intermediate layer material, i.e. the layer ma- terial of the heat distribution intermediate layer, substantially consists of carbon. Depending on the hybridization ratios, the material properties of this layer material undergo transition fluidly from those of graphite to those of diamond. Diamond-like carbon (DLC) is preferably used as intermediate layer material. A carbon layer can have a thermal conductiv- ity of 500 W/(m*K) or more, or even 800 W/(m*K) or more or even 1000 W/(m*K) or more, such that a very efficient heat redistribution is possible in principle.

The structure of the intermediate layer material at the atomic level is of importance here, too. Specific lattice forms favor the transfer of heat on account of their symmetry: thus, in the case of diamond, on account of the purely cubic crystal geometry and the tetrahedral bonds of equal - - length (sp³), the phonon propagation is undisturbed over very wide regions; this results in the good thermal conductivity. The more amorphous the layer, the lower the thermal conductivity becomes. Therefore, the thermal conductivity of DLC is variable and fluctuates depending on the production method. The diamond-like properties of DLC are determined in the percentage of sp³ hybridizations that make up the diamond crystal structure. A high proportion of sp³ hybridizations is regarded as expedient. A layer which substantially consists of diamond-like carbon (DLC) is also designated in a simplified manner as "DLC layer" in this application.

In order to enable a sufficiently strong heat flow within the heat distribution intermediate layer under all operating conditions, the heat distribu- tion intermediate layer should generally have a layer thickness of at least 1 μητι. In the case of layer thicknesses distinctly less than that, an effective heat distribution is generally possible only imperfectly. Layer thicknesses of 10 μητι or more are often advantageous. The ability of the heat distribution intermediate layer to carry out effective redistribution of thermal energy can also be specified by the thermal conduction product TCP, which for the purposes of this application is defined as the product of the specific thermal conductivity of the intermediate layer material (in [W/(m^*K)]) and the layer thickness of the heat distribution intermediate layer (in [m]). Preferably, the thermal conduction product TCP of the heat distribution intermediate layer is at least 1 *10 ³ W/K; in particular, the thermal conduction product can be greater than 1 *10^~2 W/K or even greater than 1 *10^~1 W/K. While with regard to maximizing the heat distribution efficiency it may be desirable to have the largest possible layer thicknesses of the heat distribution intermediate layer, on the other hand it should also be taken - -

into consideration that disadvantageous effects owing to thermal expansion of the heat distribution intermediate layer can increase as the layer thickness increases. Therefore, it is generally expedient if the heat distribution intermediate layer has a layer thickness of at most 500 μητι, wherein the layer thickness is preferably less than 200 μητι. If these upper limits of the layer thickness are significantly exceeded, then this generally results in hardly any advantages with regard to the heat distribution, while instead possible problems owing to thermal expansion can increase.

In preferred embodiments, the layer thickness of the heat distribution intermediate layer is between approximately 20 μητι and approximately 100 μητι. A good compromise between efficient heat distribution and low absolute thermal expansion can be obtained as a result.

Since reflection coatings in the EUV range or VUV range often have total layer thicknesses of the order of magnitude of up to a maximum of 1 μητι, a layer thickness ratio between the layer thickness of the heat distribution intermediate layer and the total layer thickness of the reflection coat- ing applied directly or with an intermediate layer thereon in the case of EUV mirrors and VUV mirrors is generally at least 10 or at least 50.

The heat distribution intermediate layer can be directly in contact with the reflection coating applied thereon, as a result of which a particularly good thermal conduction contact is possible. A first layer (closest to the substrate) of the reflection coating can therefore have been or be applied directly to the heat distribution intermediate layer.

If the free surface of the heat distribution intermediate layer, after the production thereof, should not be directly coated, for instance owing to a surface roughness exceeding the specification, the surface of the heat distribution intermediate layer, before a subsequent layer is applied, can - -

be processed by polishing or some other smoothing material processing process in order to produce a sufficiently low surface roughness. It is also possible to reduce the surface roughness of the heat distribution intermediate layer by means of a smoothing coating process by a smoothing layer being produced between heat distribution intermediate layer and reflection coating.

In other embodiments, before the reflection coating is applied, a polishing layer is also applied to the heat distribution intermediate layer, this polishing layer consisting of a polishing layer material which can be provided with an optically smooth surface by means of polishing. In some embodiments, for this purpose a layer composed of amorphous silicon is applied, which can be processed by polishing using established processing processes with high surface quality, in particular with low surface roughness. The reflection coating can subsequently be applied.

The heat distribution intermediate layer can be applied directly to a correspondingly processed front surface of the substrate, such that the intermediate layer material is directly in contact with the substrate mate- rial. In other embodiments, between the heat distribution intermediate layer and the substrate there is arranged a (at least one) further intermediate layer, the layer properties of which can be designed, for example, so as to result in an improved adhesive strength between substrate and heat distribution intermediate layer. Depending on the substrate ma- terial, the layer material of the further intermediate layer can also be chosen so as to result in an adaptation of the coefficients of thermal expansion between heat distribution intermediate layer and substrate. In some embodiments, the further intermediate layer is an adhesion promoting intermediate layer, wherein for example hexamethyldisiloxane (HDMSO), titanium or chromium can be provided as material for the further intermediate layer. - -

In some embodiments, a mirror has only a single heat distribution intermediate layer between substrate and reflection coating. That can be expedient for reasons of simple production, inter alia. However, it is also possible to provide two or more heat distribution intermediate layers, be- tween which further layers composed of layer materials having poorer or better thermal conductivity can be situated. As a result, a profile with varying thermal conductivity can be provided possibly perpendicularly to the layer extent. Such layer structures can e.g. also be expedient in order to keep down layer stresses in the heat distribution intermediate layer.

In accordance with another formulation, the invention relates to a mirror comprising a substrate, which consists of a substrate material, and a reflection coating applied on the substrate, said reflection coating having a reflective effect for vacuum ultraviolet radiation or radiation from the extreme ultraviolet range, wherein the mirror is characterized by a heat distribution intermediate layer arranged between the substrate and the reflection coating, said heat distribution intermediate layer having a layer thickness and consisting of a layer material having a specific thermal conductivity, wherein the layer thickness is in a range from 1 μητι to 100 μητι, a thermal conduction product of the heat distribution intermediate layer is defined as the product of the specific thermal conductivity of the layer material and the layer thickness of the heat distribution intermediate layer, wherein the thermal conduction product of the heat distribution intermediate layer is at least 1 ^*10^"3 W/K.

The thermal conduction product can even be greater than 1 *10^"2 W/K, in particular greater than 1 *10^"1 W/K. Particularly in conjunction with substrates having poor thermal conductivity (having a specific thermal conductivity of less than 10 W/(m*K)), considerable practical advantages can arise. A substantial heat redistri- - -

bution is also possible in the case of substrate materials having a higher thermal conductivity, the specific thermal conductivity of which is greater than or equal to 10 W/(m^*K). The invention also relates to an optical system for a microlithography projection exposure apparatus comprising at least one mirror of the type described in this application. The optical system generally has one or a plurality of further optical elements. In systems which operate at operating wavelengths from the VUV range, the further optical elements can be lens elements and/or mirrors. In systems for EUV lithography, normally one or a plurality of further mirrors and no lens elements are provided. The optical system can be, for example, a projection lens of a micro- lithography projection exposure apparatus. By using at least one mirror in accordance with this disclosure, thermally induced imaging aberra- tions attributed e.g. to wavefront deformations can be considerably reduced in comparison with conventional systems. Alternatively or additionally, at least one mirror of the type described in this application can be used in an illumination system of a microlithography projection exposure apparatus.

The invention also relates to a method for producing a mirror. In this case, use is made of a substrate composed of a substrate material which has a specific thermal conductivity of less than 10 W/(m*K). A front surface of the substrate is processed in order to produce a surface having a predefined surface form. In this case, the term "surface form" refers both to the macroform (e.g. concave, convex, plane, non- rotationally symmetrical (freeform surface), undulation, etc.) and to the microform (e.g. characterized by values for the surface roughness, etc.). Furthermore, a reflection coating is produced, wherein the reflection coating has a reflective effect for vacuum ultraviolet radiation or radiation from the extreme ultraviolet range. In the finished mirror, the reflection coating is carried by the substrate. The method is characterized in that - - after processing the front surface of the substrate and before producing the reflection coating, a heat distribution intermediate layer is produced, which consists of a layer material having a specific thermal conductivity that is at least 10 times the magnitude of the specific thermal conductiv- ity of the substrate material, and which has a layer thickness of at least 1 μητι.

These and further features emerge not only from the claims but also from the description and the drawings, wherein the individual features can be realized in each case 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. Exemplary embodiments are illustrated in the drawings and are explained in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows, in schematic sectional illustration, an embodiment of a mirror according to the invention, this mirror having a reflective effect for EUV radiation;

Figure 2 shows a schematic detail illustration of the illuminated region A from Fig. 1 ; Figure 3 shows a schematic lateral temperature profile in the region of the illuminated region A from Figure 2;

Figures 4 to 8 show simplified simulations concerning the spatial temperature distribution in a substrate cross section along a sectional axis which begins in the center of an illuminated region and ends at the edge of the substrate; - -

Figure 9 shows a comparative diagram illustrating the temperatures at selected points of the mirror as a function of the layer thickness of the heat distribution intermediate layer; Figure 10 shows an exemplary embodiment of an EUV microlithogra- phy projection exposure apparatus comprising a projection lens containing a mirror in accordance with an embodiment of the invention; and

Figure 1 1 shows, in schematic sectional illustration, an embodiment of a mirror according to the invention, this mirror having a reflective effect for VUV radiation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows an embodiment of an EUV mirror 100 in schematic illustration. In this context, the term "EUV mirror" means that the mirror is designed to have a highest possible reflectance in the wavelength range of extreme ultraviolet radiation (EUV radiation). Figure 2 shows a schematic detail illustration of the illuminated region A from Figure 1.

The EUV mirror 100, which is also designated simply as mirror 100 hereinafter, is designed as a front surface mirror and has a substrate 1 12, the concavely curved front surface 1 14 of which is embodied as an optical surface processed with optical quality. The mirror 100 is provided for incorporation into an optical system of an EUV microlithography projection exposure apparatus and has at its front surface 1 14 a reflection coating 1 16 in the form of a multilayer arrangement having a high reflectance for EUV radiation having a used wavelength in the range of 5 to 15 nm. The reflection coating can be constructed with alternate molyb- denum/silicon layers, for example. It can have uniform or locally varying layer thicknesses. - -

In general, a reflection coating that is reflective in the EUV range has a multilayer arrangement comprising a multiplicity of layer pairs (bilayers) each having alternately applied layers of a layer material having a higher real part of the refractive index (also called "spacer") and of a layer ma- terial having relative thereto a lower real part of the refractive index (also called "absorber"). In the case of the example, relatively thin layers comprising molybdenum (Mo) as absorber material are applied alternately with relative thereto thicker layers comprising silicon (Si) as spacer material. A layer pair can also contain at least one further layer, in particular an interposed barrier layer, which e.g. can consist of C, B₄C, Si_xN_y, SiC or a composition comprising one of these materials and is intended to prevent interdiffusion at the interface. As a result, it is possible to ensure permanently sharply defined interfaces even under radiation loading. At that surface of the layer stack which is remote from the substrate, it is also possible to apply a cap layer for protecting the underlying layers. The cap layer can e.g. consist of ruthenium, rhodium, gold, palladium, silicon, titanium or molybdenum and possibly the oxides, nitrides or carbides thereof (e.g. S1O2, Si_xN_y, SiC) or contain one of these materials. The free surface of the cap layer forms the ray entrance surface. Non- periodic reflection coatings are also possible.

Examples of suitable EUV reflection coatings can be found e.g. in the patent US 6,01 1 ,646, the disclosure content of which is incorporated by reference in the content of this description.

During operation, the EUV radiation 130 is incident on the reflection coating from the side facing away from the substrate 1 12 and is reflected in this case at the coated front surface. The multilayer reflection coating acts as a Bragg reflector in this case.

The mirror 100 is embodied, for example, as a concave mirror having positive refractive power. However, it can also be embodied as a convex . -

mirror having negative refractive power or as a plane mirror without refractive power. The optical surface can be fashioned as a spherical or aspherical rotationally symmetrical surface, but possibly also as a non- rotationally symmetrical (rotationally asymmetrical) freeform surface. The diameter of the mirror is adapted to the respective application. In a projection lens for a microlithography projection exposure apparatus, the diameter is typically between 100 mm and 500 mm; larger or smaller diameters are also possible. The substrate 1 12 consists of a substrate material having an extremely low coefficient of thermal expansion, which is less than 0.1 *10^~6 K^"1 in the range around room temperature (at 20°C), wherein the coefficient of thermal expansion is preferably at least half an order of magnitude lower still, e.g. a maximum of 5^*10^"8 K^"1. Materials of this type are occasionally also designated as ultra low expansion materials.

In the exemplary embodiment, the substrate 1 12 substantially consists of a titanium silicate glass ((Si02-Ti02) glass) having a high proportion of at least 90% by weight of S1O2 and a remainder (e.g. approximately 7% by weight) consisting predominantly of titanium oxide (T1O2). Suitable glass materials are available e.g. under the trade name ULE^® glass (Corning, Inc). For the temperature range of between 5°C and 35°C, an average coefficient of thermal expansion of 0 + 30^*10^"9 K^"1 is specified. The specific thermal conductivity of the substrate materials, with a value of approximately 1 .31 W/(m^*K), at 25°C is also very low.

In an embodiment which is not illustrated in the figures, the substrate material of the substrate is a glass ceramic containing crystalline phase components distributed in a glass matrix. By combining the thermal characteristics of the different phases, it is possible to obtain extremely low coefficients of thermal expansion, which can even become zero or slightly negative in some temperature ranges. - -

An individual heat distribution intermediate layer 120 is arranged between the substrate 1 1 2 and the multilayer arrangement forming the reflection coating 1 16, said heat distribution intermediate layer consisting of a layer material (intermediate layer material) whose specific thermal conductivity is greater than the specific thermal conductivity of the substrate material by at least one order of magnitude, i .e. at least by the factor 10).

In the case of the example, the heat distribution intermediate layer is ap- plied directly to the front surface 1 14 of the substrate 1 12 without interposition of a further intermediate layer, such that there is a large-area touching contact between the first interface 122 - near the substrate - of the heat distribution intermediate layer and the front surface 1 14. Furthermore, the second interface 124 - remote from the substrate - of the heat distribution intermediate layer is directly in touching contact with that individual layer of the reflection coating 1 16 which is closest to the substrate, which is thus applied directly to the heat distribution intermediate layer without interposition of a further layer. The heat distribution intermediate layer substantially consists of diamond-like carbon (DLC), for which reason it can also be designated simply as "DLC layer". In this material , the carbon atoms are present substantially without long range order. Depending on the sp² : sp³ hybridization ratio, the material properties lie between those of graphite and those of diamond . Both the thermal expansion and the thermal conductivity of DLC layers are greatly dependent on the ratio between the sp³ (diamond) and sp² (graphite) hybridization. Since the coefficient of thermal expansion of diamond is approximately 1 .2*10^"6 1 /K and that of graphite, depending on orientation, is between approximately 0.5 and 6*10^"6 1 /K, the coefficient of thermal expansion of the DLC layer to a first approximation is of the order of magnitude of 1 *1 0^"6 11K. - -

A special feature of this layer material is the extremely high specific thermal conductivity, which is significantly greater than that of highly conductive metals and which, in the case of the example, can be of the order of magnitude of up to approximately 1000 W/(m^*K), possibly even higher than that. The specific thermal conductivity of the heat distribution intermediate layer therefore exceeds the specific thermal conductivity of the substrate material (approximately 1.31 W/(m^*K)) and also that of the reflection coating 1 16 (approximately 0.1 W/(m^*K) to approximately 2.5 W/(m^*K) depending on the basis of the estimation) by several orders of magnitude.

The layer thickness of the heat distribution intermediate layer measured perpendicularly to the surface normal of the substrate front surface 1 14 (in the z-direction) is dimensioned such that the heat distribution inter- mediate layer is able to transport relatively large amounts of heat in a lateral direction, i.e. in all directions perpendicular to the layer normal. In the case of the example of EUV mirrors, layer thicknesses in the range of between approximately 20 m and approximately 100 m have proved to be suitable for this purpose. The optimum layer thickness is primarily dependent on the thermal conductivity which is obtainable in the diamond-like intermediate layer, said thermal conductivity being dependent in turn on the layer deposition process.

The heat distribution intermediate layer is accordingly characterized by a thermal conduction product TCP of the order of magnitude of 0.02 to 0.1 W/K.

The interposition of a heat distribution intermediate layer having very good thermal conductivity between the reflection coating 1 16 having relatively poor thermal conductivity and the substrate material likewise having very poor thermal conductivity makes it possible to decouple the substrate material to a considerable extent from inhomogeneous tern- . -

perature distributions that occur in an operation-dependent manner in the region of the reflection coating 1 16. What can thereby be achieved is that a spatially inhomogeneous irradiation which may indeed be desired depending on the illumination setting of the optical system does not re- suit, or results only to an extent that can be afforded tolerance, in inhomogeneous heating of the substrate and associated thermally induced deformations of the mirror in proximity to the mirror surface. This will be explained in greater detail with reference to Figures 2 to 9. The lifetime of the reflection coating can also be lengthened, if appropriate, by the reduction of disadvantageous thermally induced layer alterations, such as e.g. compaction.

In the case of the example from Figure 1 , the projection exposure apparatus containing the mirror 100 is operated with a dipole illumination set- ting. The mirror is arranged in proximity to a pupil surface of the projection lens. A considerable proportion of the radiation passing through the optical system is thereby concentrated substantially on two regions A and B situated diametrically opposite with respect to the optical axis 1 13 and situated at a distance outside the optical axis. The illumination in- tensity is particularly high there in comparison with surrounding regions.

The locally concentrated irradiation of the reflection coating 1 16 in the region A results, on account of absorption processes within the layer materials of the reflection coating, in local heating of the region of the local irradiation in comparison with the adjacent regions that are not irradiated or are irradiated less intensively. If the x-coordinate x = 0 is chosen for the center of the irradiated region A, then this gives rise schematically to the lateral temperature profile shown in Figure 3 with a local maximum in the center of the region A. Such a local temperature in- crease, in the absence of a heat distribution intermediate layer, would result in local heating of the substrate material below the irradiated region, which would in turn cause deformation to a greater or lesser extent . -

depending on the coefficient of thermal expansion of the substrate material. Even if, as in this case, a substrate material having a very low coefficient of thermal expansion is chosen, the thermally induced deformations can have a disturbing effect.

The interposition of a heat distribution intermediate layer having very good thermal conductivity makes it possible for the substrate material largely to be decoupled from the spatially inhomogeneous heat generation in the region of the reflection coating. Figure 2 schematically indi- cates in this respect, with the aid of heat flow arrows, that a large part of the generated heat is dissipated through the heat distribution intermediate layer parallel to the layer extent or perpendicular to the z-direction laterally outward (heat flow arrows W1 , W2) and only a significantly smaller proportion (heat flow arrows W3, W4) can still pass into the sub- strate. Accordingly, the substrate material is largely decoupled thermally from the reflection coating.

The thermally homogenizing effect of the heat distribution intermediate layer is primarily dependent on the specific thermal conductivity of the intermediate layer material and the layer thickness thereof. Simulations were carried out for quantitatively demonstrating the homogenizing effect, the results of which simulations are illustrated in Figures 4 to 8 and summarized once again in Figure 9. Figures 4 to 8 in each case show the simplified simulations for the temperature distribution in a substrate cross section along a sectional axis which begins in the center of an illuminated region (for example the illuminated region A) (x = 0 mm) and ends at the edge of the substrate (x = 50 mm). The depth coordinate z [mm], which runs in the direction of the normal to the mirror surface or the layer extent, is plotted on the ordinate. The interface between the heat distribution intermediate layer 120 and the reflection coating 1 16, that is to say the interface of the reflection - - coating 1 16 facing the substrate, is situated at z = 0 mm. For the simulations, the reflection coating has a thickness of 1 μητι and is homogeneously subjected to heat between x = 0 mm and x = 1 0 mm. Heat distribution intermediate layers of varying layer thickness (0 μητι, 1 μητι, 1 0 μητι, 50 μητι and 1 00 μητι) were simulated between the reflection coating and the substrate. On account of the small layer thicknesses of the two layers (heat distribution intermediate layer and reflection coating), in comparison with the substrate, they are not discernible as sepa- rate elements in the figures. The geometry of the arrangement and the power densities of the incident illumination and the heat generation were taken from typical scenarios of thermal mirror loading in an EUV projection lens. Since the negative x-axis exhibits the same temperature profile, these redundant points have not been illustrated.

For reasons appertaining to simulation, a planar heat sink having a constant temperature T = 300 K was assumed at z = -20 mm. In all of the simulations, the coldest location T₄ was situated in proximity to the heat sink at the outer edge of the region under consideration, that is to say at a location at the greatest distance with respect to the region of the heat input. Directly in the center of the irradiation spot, the point x = 0 mm having the highest temperature Ti is situated on the free surface. The temperature T₂ in each case prevails at the edge of the illuminated region at x=1 0 mm, while the temperature T₃ prevails at the edge of the illustrated substrate region (x = 50 mm) at the surface. The lines within the diagrams in each case represent isothermal lines (isotherms), that is to say lines of identical temperature T relative to the temperature T₄ at the coldest location. Even a first qualitative comparison of the figures shows that the heat is distributed spatially further on the mirror and in the substrate as the layer thickness of the heat distribution intermediate layer increases, and , - - therefore, both absolutely and relatively, the temperatures in the illuminated region decrease.

While temperature boosts in comparison with the coldest location of up to approximately 55 K arise for example without a heat distribution intermediate layer (Figure 4) in the center of the illuminated region, the corresponding boost is only approximately 35 K given a layer thickness of 10 μητι (Figure 6) and only a little above 20 K given a layer thickness of 100 μητι (Figure 8). It can be discerned directly from the spatial density of the isothermal lines that the temperature gradients within the substrate become smaller and smaller as the layer thickness of the heat distribution intermediate layer increases.

Figure 9 shows a comparative diagram illustrating the temperatures T (in [K]) at the selected points with temperatures Ti to T₄ as a function of the layer thickness d [μητι] of the heat distribution intermediate layer (x-axis). As mentioned, the point x = 0 mm having the highest temperature Ti is situated directly in the center of the irradiation spot. The decrease in temperature as the layer thickness of the heat distribution intermediate layer increases is greatest at this location. At the edge of the illuminated region (point x = 10 mm, temperature T₂), a decrease in temperature can likewise also be discerned, both the extent of the absolute temperatures and the extent of the decrease being somewhat smaller than in the center of the illumination spot. At the edge of the illustrated region (x = 50 mm, T₃), the temperature rises slightly from 324 K to approximately 326.5 K on account of the homogenizing effect as a result of the heat distribution intermediate layer. Especially this slight rise in temperature shows that the heat distribution intermediate layer redistributes heat from the particularly highly heated region to less highly heated regions, thus resulting in a homogenizing effect. The coldest location T₄ in proximity to the heat sink maintains its temperature of 324 K in a constant fashion. . -

The results illustrated by way of example can be applied qualitatively to other power densities. In order to take account of the influence of an increased power density at the location of heat generation, by way of example, in each case the difference between the hottest and coldest points is formed and this is expressed as a relationship for a simulation with a 1 00 μητι and without a heat distribution intermediate layer. The difference between Ti and T₄ without the heat distribution intermediate layer is approximately 9.4 K, while with the presence of a 1 00 μητι thick heat distribution intermediate layer the difference is only 4.0 K and thus only just under 43% of the original temperature.

If the power density is then increased, these relative temperature swings scale largely linearly. In the case where the power density is increased by the factor of 5, the temperature difference will rise to 47 K, for exam- pie, in the absence of a heat distribution intermediate layer, while with a 100 μητι DLC layer the difference would likewise attain the 43% value of 20 K.

In contrast to various heating scenarios in which more and more heating power has to be introduced in order to homogenize the temperature distribution and the absolute temperatures thus rise further and further, when a homogenizing layer or heat distribution intermediate layer is used the relative temperature distribution for a given thickness of the homogenizing layer remains the same. As a result, the rise in the abso- lute temperatures is very much more weakly pronounced than in heating scenarios.

In the case where a heat distribution intermediate layer is used , therefore, it is possible to considerably reduce the relative thermal expansion of the substrate material in comparison with conventional systems without a heat distribution intermediate layer. Furthermore, it is also possible to increase the lifetime of the reflection coating since the local thermal - - loading of the reflection coating is almost halved in the case of the example.

In a method for producing an EUV mirror with a heat distribution inter- mediate layer, the front surface 1 14 of the substrate 1 12 is firstly polished to optical quality, wherein, for example, a peak-to-valley height of less than approximately 10 nm is produced. Afterward, the heat distribution intermediate layer is applied directly to the polished front surface 1 14 by means of a suitable coating method. In the exemplary embodi- ments, the heat distribution intermediate layer is applied by means of plasma-enhanced chemical vapor deposition (PECVD). Good layer qualities can be achieved e.g. at growth rates of between approximately 1 μιη/h and 2 μητι/h. In the case of the layer thicknesses usually sought for the DLC layer in the range of between 10 μητι and 100 μητι or more, generally the free surface of the DLC layer, after the conclusion of the coating, is not directly suitable for applying the reflection coating, since generally stringent requirements made of the deformation and the figure of the surface to be coated have to be fulfilled here. Therefore, in preferred embodiments, after the deposition of the heat distribution intermediate layer, a surface is produced which is suitable as a support for the reflection coating and which fulfils the form and figure tolerances. For this purpose, the free surface of the DLC layer can be directly processed by means of a polish- ing step in order to reduce the surface roughness and to produce the form. Since polishing the very hard layer material involves a high expenditure, in some embodiments provision is made for also applying a polishing layer composed of a layer material that can be polished, for example amorphous silicon, before the reflection coating is deposited. The surface of said layer is then polished before the first layer of the reflection coating 1 16 is applied. - -

All conventional production methods can be used for the production of the multilayer arrangement serving as a reflection coating.

Although the heat distribution intermediate layer, as in the case of the example, can be applied directly to the front surface 1 14 of the substrate, said front surface having been processed with optical quality, in other embodiments provision is made for also introducing a further intermediate layer between the substrate surface and the heat distribution intermediate layer. This can involve an adhesion promoting intermediate layer, for example, which improves the adhesion between substrate and heat distribution intermediate layer. The further intermediate layer can also be designed such that it can serve as a stress compensation layer.

Depending on the material combinations, however, it is also possible in many cases that functionalities such as stress compensation or protection of the substrate against EUV radiation can also be realized by the heat distribution intermediate layer. It is thereby possible to produce such an EUV mirror in a relatively small number of work steps. Moreover, the number of necessary interfaces within the layer construction is kept small, which can be advantageous overall for the lifetime of the entire layer system.

An EUV mirror with a heat distribution intermediate layer affords particular advantages if there is the risk that the performance of the entire layer arrangement or of the optical system equipped with EUV mirrors could be detrimentally affected by high local power input and poor thermal conductivity of the substrate.

Figure 10 shows optical components of an EUV microlithography projec- tion exposure apparatus 1000 for exposing a radiation-sensitive substrate arranged in the region of an image plane 1060 of a projection lens 1030 with at least one image of a pattern of a reflective patterning device - - or mask arranged in the region of an object plane 1020 of the projection lens.

The apparatus is operated with radiation from a primary radiation source 1014. An illumination system 1010 serves for receiving the radiation from the primary radiation source and for shaping illumination radiation directed onto the pattern. The projection lens 1030 serves for imaging the structure of the pattern onto a light-sensitive substrate. The primary radiation source 1014 can be, inter alia, a laser plasma source or a gas discharge source or a synchrotron-based radiation source. Such radiation sources generate a radiation 1020 in the EUV range, in particular having wavelengths of between 5 nm and 15 nm. In order that the illumination system and the projection lens can operate in this wavelength range, they are constructed with components that are reflective to EUV radiation.

The radiation 1020 emerging from the radiation source 1014 is collected by means of a collector 1015 and directed into the illumination system 1010. The illumination system comprises a mixing unit 1012, a telescope optical unit 1016 and a field shaping mirror 1018. The illumination system shapes the radiation and thus illuminates an illumination field situated in the object plane 1020 of the projection lens 1030 or in proximity thereto. In this case, the form and size of the illumination field determine the form and size of the effectively used object field in the object plane 1020.

A reflective reticle or some other reflective patterning device is arranged in the object plane 1020 during operation of the apparatus. The projec- tion lens 1030 in this case has six mirrors M1 to M6 and images the pattern of the patterning device into the image plane, in which a substrate to be exposed, e.g. a semiconductor wafer, is arranged. - -

The mixing unit 1012 substantially consists of two facet mirrors 1070, 1080. The first facet mirror 1070 is arranged in a plane of the illumination system that is optically conjugate with respect to the object plane 1020. It is therefore also designated as a field facet mirror. The second facet mirror 1080 is arranged in a pupil plane of the illumination system that is optically conjugate with respect to the pupil plane of the projection lens. It is therefore also designated as a pupil facet mirror. With the aid of the pupil facet mirror 1080 and the imaging optical assembly which is displaced downstream in the beam path and which comprises the telescope optical unit 1016 and the grazing incidence field shaping mirror 1018, the individual mirroring facets (individual mirrors) of the first facet mirror 1070 are imaged into the object field.

The spatial (local) illumination intensity distribution at the field facet mirror 1070 determines the local illumination intensity distribution in the object field. The spatial (local) illumination intensity distribution at the pupil facet mirror 1080 determines the illumination angle intensity distribution in the object field.

EUV projection exposure apparatuses having a similar basic construction are known e.g. from WO 2009/100856 A1 or WO 2010/049020 A1 , the disclosure of which is incorporated by reference in the content of this description.

Individual mirrors of the microlithography projection exposure apparatus, in particular at least one mirror of the illumination system 1010 and/or at least one mirror of the projection lens 1030, are embodied as thermally self-compensating EUV mirrors in accordance with one embodiment of this disclosure. . .

Thermally critical components are, in particular, the mirrors of the projection lens 1030, which, on account of the heating by the EUV radiation, change their form admittedly only to a small extent, but the imaging of the object into the image plane 1060 can thereby be severely disturbed. In order to minimize heat-induced deformations of the mirrors, all of the mirrors M1 to M6 have a mirror substrate composed of a titanium silicate glass having a coefficient of thermal expansion of a few 10^~9 K^"1 (i.e. a few parts per billion per kelvin). In particular, it is possible to use a material for which an average coefficient of thermal expansion of 0 + 30^* 10^"9 K^"1 is specified for the temperature range of between 5°C and 35°C.

The heating of the mirrors of the projection lens 1030 is dependent firstly on their order in the beam path. Thus, the integral power of the impinging EUV radiation decreases from mirror to mirror on account of the ab- sorption in the multilayer coatings. Secondly, however, the heating is also dependent on the diameter of the mirror. If a small mirror is involved, then the integral light power impinges on a smaller area than in the case of a large mirror, with the result that smaller mirrors are heated to a greater extent. This is the case particularly for the third mirror M3 and fifth mirror M5 in the ray passage direction.

Furthermore, the mirrors are not always heated homogeneously over the mirror surface. Thus, in particular the second mirror M2 of the projection lens 1030 in the ray passage direction, said second mirror being ar- ranged in the pupil plane, will generally have an inhomogeneous illumination depending on the so-called illumination setting. The illumination setting defines the angular spectrum with which an object to be imaged within the object field is illuminated by the illumination system 1010. To put it another way: the illumination setting describes the local intensity distribution of the illumination of the entrance pupil of the projection lens. This can involve, for example, a conventional illumination setting (corresponding to an axially centered circular illumination region in the en- - - trance pupil), an annular, a dipole, a quadrupole or some other multipole illumination setting.

In the case of a dipole illumination setting, the illumination of a pupil plane is characterized by two diametrically opposite intensity maxima situated outside the reference axis of the optical system. The mirror M2 arranged in the pupil plane is thus heated by absorption in the multilayer reflection coating principally in two diametrically opposite regions (cf. Figure 1 ). This can give rise to an astigmatic deformation of the mirror surface, wherein the orientation of the connecting line of the particularly hot zones is dependent on the orientation of the dipole. The heat distribution can therefore have substantially a 2-fold rotational symmetry.

In order to minimize the effects of the non-uniform heating from the im- aging quality, it is possible to use an EUV mirror in accordance with one embodiment of this disclosure. By way of example, the second mirror M2, between the substrate 1 12 and the multilayer reflection coating 1 16, can have a heat distribution intermediate layer 120 composed of diamond-like carbon (DLC) and having a very good thermal conductivity, this heat distribution intermediate layer automatically bringing about a lateral heat redistribution.

This automatically decreases or reduces temperature gradients in the region of the mirror surface in the direction of temperature homogeniza- tion, with the result that the lateral redistribution of thermal energy leads to a reduction of thermally dictated aberrations.

If a different or differently oriented inhomogeneous thermal loading of the mirror surface is established after a change of the illumination set- ting, then the EUV mirror automatically adapts itself to the new intensity distribution and compensates for a considerable part of the inhomogene- ity by lateral redistribution of the thermal energy. - -

Figure 1 1 shows a schematic section through one embodiment of a VUV mirror 200. In this context, the term "VUV mirror" means that the mirror is designed to have a highest possible reflectance in the wavelength range of vacuum ultraviolet radiation (VUV radiation).

The VUV mirror 200 is designed as a dielectrically amplified front surface mirror and has a substrate 212 composed of a glass material or a glass ceramic. The front surface 214 of the substrate is embodied as an optical surface processed with optical quality. The mirror 200 is provided for incorporation into an optical system of a VUV microlithography projection exposure apparatus (e.g. projection lens), and has at its front surface 214 a reflection coating 216 in the form of a multilayer arrangement, which has a high reflectance of more than 95% for VUV radiation having a used wavelength in the region of 193 nm.

The reflection coating 216 substantially consists of an optically dense aluminum layer 217 having a layer thickness of approximately 150 nm and - applied thereon - a dielectric interference layer system 219 having three layers composed of alternately low refractive index and high re- fractive index dielectric material, e.g. fluoride materials and/or oxide materials. The optical layer thicknesses thereof can be in the range of around 20% to 30% of the operating wavelength.

Examples of suitable VUV reflection coatings can be found e.g. in the patent US 6,809,871 B2, the disclosure content of which in this respect is incorporated by reference in the content of this description.

An individual heat distribution intermediate layer 220 is arranged between the substrate 212 and the the reflection coating 216, said heat distribution intermediate layer consisting of a layer material (intermediate layer material) whose specific thermal conductivity is greater than the - - specific thermal conductivity of the substrate material by at least one order of magnitude, i.e. at least by the factor 10).

In the case of the example, the heat distribution intermediate layer is ap- plied directly to the front surface 214 of the substrate 212 without interposition of a further intermediate layer, such that there is a large-area touching contact between the first interface 222 - near the substrate - of the heat distribution intermediate layer and the front surface 1 14. Furthermore, the second interface 224 - remote from the substrate - of the heat distribution intermediate layer is directly in touching contact with that individual layer closest to the substrate of the reflection coating 1 16, which is formed here by the aluminum layer 217.

The heat distribution intermediate layer has a layer thickness that is at least 10 times the magnitude of the layer thickness of the aluminum layer 217 belonging to the reflection coating and substantially consists of diamond-like carbon (DLC), the specific thermal conductivity of which is multiply greater than that of aluminum.

Claims

Patent claims

1. Mirror (100, 200) comprising:

a substrate (1 12, 212), which consists of a substrate material having a specific thermal conductivity of less than 10 W/(m^*K), and a reflection coating (1 16, 216) applied on the substrate, said reflection coating having a reflective effect for vacuum ultraviolet radiation or radiation from the extreme ultraviolet range, characterized by

a heat distribution intermediate layer (120, 220) arranged between the substrate (1 12, 212) and the reflection coating (1 16, 216), wherein said heat distribution intermediate layer is composed of a layer material having a specific thermal conductivity that is at least 10 times the magnitude of the specific thermal conductivity of the substrate material and has a layer thickness of at least 1 μητι.

2. Mirror according to claim 1 , wherein the heat distribution intermediate layer (120, 220) has a layer thickness which is at least 10 μητι.

3. Mirror according to claim 1 or 2, wherein the heat distribution intermediate layer (120) has a layer thickness of less than 500 μητι, in particular of less than 200 μητι.

4. Mirror according to any of the preceding claims, wherein the heat distribution intermediate layer (120, 220) has a layer thickness of between 10 μητι and 100 μητι.

5. Mirror according to any of the preceding claims, wherein a thermal conduction ratio TCR between the specific thermal conductivity of the layer material and the specific thermal conductivity of the substrate material is greater than 50.

6. Mirror according to any of the preceding claims, wherein the layer material of the heat distribution intermediate layer (120, 220) has a specific thermal conductivity of more than 50 W/(m*K), wherein the specific thermal conductivity of the layer material is preferably greater than 500 W/(m^*K).

7. Mirror according to any of the preceding claims, wherein the layer material of the heat distribution intermediate layer (120, 220) consists substantially of carbon, in particular substantially of diamondlike carbon.

8. Mirror according to any of the preceding claims, wherein a thermal conduction product of the heat distribution intermediate layer (120, 220) is defined as the product of the specific thermal conductivity of the layer material and the layer thickness of the heat distribution intermediate layer, wherein the thermal conduction product of the heat distribution intermediate layer is at least 1 *10^~3 W/K, wherein the thermal conduction product is in particular greater than 1 *10^" ² W/K.

9. Mirror according to any of the preceding claims, wherein a polishing layer is arranged between the heat distribution intermediate layer and the reflection coating, said polishing layer consisting of a polishing layer material that can be provided with an optically smooth surface by means of polishing.

10. Mirror according to any of the preceding claims, wherein a further intermediate layer is arranged between the heat distribution intermediate layer and the substrate.

1 1. Mirror according to any of the preceding claims, wherein the reflection coating (1 16) has a multilayer arrangement having a re- flective effect for radiation from the extreme ultraviolet range (EUV), said multilayer arrangement comprising a multiplicity of layer pairs having alternate layers composed of a high refractive index layer material and a low refractive index layer material.

12. Mirror (100, 200) comprising:

a substrate (1 12, 212), which consists of a substrate material, and a reflection coating (1 16, 216) applied on the substrate, said reflection coating having a reflective effect for vacuum ultraviolet ra- diation or radiation from the extreme ultraviolet range, characterized by

a heat distribution intermediate layer (120, 220) arranged between the substrate (1 12, 212) and the reflection coating (1 16, 216), said heat distribution intermediate layer having a layer thickness and consisting of a layer material having a specific thermal conductivity, wherein

the layer thickness is in a range from 1 μητι to 100 μητι, and a thermal conduction product of the heat distribution intermediate layer (120, 220) is defined as the product of the specific thermal conductivity of the layer material and the layer thickness of the heat distribution intermediate layer, wherein the thermal conduction product of the heat distribution intermediate layer is at least 1 ^*10^"3 W/K.

13. Mirror according to claim 12, wherein the thermal conduction product is greater than 1 *10^"2 W/K, in particular greater than 1 *10^" ¹ W/K.

14. Mirror according to claim 12 or 13, characterized by the features of the characterizing part of at least one of claims 5 to 1 1 .

15. Optical system of a microlithography projection exposure apparatus comprising at least one mirror according to any of claims 1 to 14.

16. Optical system according to claim 15, wherein the optical system is a projection lens (1030) of the microlithography projection exposure apparatus.

17. Optical system according to claim 16, wherein the mirror (M2) is arranged in the region of a pupil plane of the projection lens (1030).

18. Method for producing a mirror comprising the following steps:

providing a substrate, which consists of a substrate material having a specific thermal conductivity of less than 10 W/(m*K);

processing a front surface of the substrate in order to produce a surface having a predefined surface form; and

producing a reflection coating, wherein the reflection coating has a reflective effect for vacuum ultraviolet radiation or radiation from the extreme ultraviolet range,

characterized in that

after processing the front surface of the substrate and before producing the reflection coating, a heat distribution intermediate layer is produced, which consists of a layer material having a specific thermal conductivity that is at least 10 times the magnitude of the specific thermal conductivity of the substrate material and has a layer thickness of at least 1 μητι.

19. Method according to claim 18, wherein the heat distribution intermediate layer is applied directly to the front surface of the substrate.