Cross-reference to Related Applications

This application claims priority to German Patent Application No. 10 2014 206 765.0 filed on April 8, 2014, the entire contents of which are hereby incorporated by reference in the disclosure of this application .

Background of the invention

The invention relates to a mirror arrangement, which has a mirror with a substrate and with a reflective coating for EUV radiation, a projection lens for EUV lithography and an EUV lithography apparatus, as well as a method for operating such an EUV lithography apparatus .

The use of wavefront manipulators for correcting aberrations in microlithographic projection lenses is known. Such manipulators typically generate the wavefront correction by mechanical manipulation, for example by a change in position and/or by generating a deformation of the element serving as a manipulator. However, mechanical manipulators can typically only correct low-order wavefront errors, while higher order wavefront errors, as may be caused by a high thermal load on the optical elements, are generally not compensated for sufficiently by mechanical manipulators. Therefore, thermal actuators are used for correcting higher order wavefront errors in order to change the optical properties of optical elements by targeted thermal influencing. By way of example, WO 2009/026970 Al discloses the provision of thermal actuators for influencing the temperature distribution in an optical correction device, which is produced by at least two sub-elements that differ in terms of their suitability for heat transport or in terms of their thermal conductivity, respectively. The optical correction device can be an optical element, for example a lens element or a mirror. The thermal actuators can be ohmic heating elements, which are arranged in a grid arrangement in order to supply local heat to the correction device in a targeted manner. For correcting an imaging property of a projection system in a projection exposure apparatus for the VUV wavelength range, US 8,111,378 B2 proposes the application of radiation in a wavelength range, which differs from a wavelength range of an exposure beam of the projection exposure apparatus, to at least part of an optical element, typically a lens element, by way of a spatial waveguide mechanism.

In lithography apparatuses for the EUV wavelength range, i.e. for wavelengths in the range between approximately 5 nm and approximately 35 nm, use is typically only made of reflecting optical elements (mirrors) which have a substrate which only has a very small coefficient of thermal expansion (CTE) in the region of the operating temperatures used therein as a result of the extremely high requirements on geometric tolerances and stability that are to be met by the mirror surfaces, in particular in the projection lenses used there. Such substrate materials typically have two constituents, the coefficients of thermal expansion of which have an opposing dependence on temperature such that the coefficients of thermal expansion almost completely compensate one another at the temperatures occurring at the mirrors during operation of the EUV lithography apparatus. The dependence of the thermal expansion (change in length) of such materials on the temperature is approximately parabolic in the relevant temperature range, i.e. there is an extremum of the thermal expansion at a specific temperature. The derivative with respect to temperature of the thermal expansion of zero-expansion materials (i.e. the coefficient of thermal expansion) has an approximately linear dependence on the temperature in this range and changes sign at the temperature at which the thermal expansion has an extremum, which is why this temperature is referred to as zero crossing temperature (ZCT) .

WO 2013/110518 discloses the practice of matching the operating temperature (or the mean temperature) and the zero crossing temperature of a mirror element of an optical arrangement to one another in such a way that wavefront errors are reduced or minimized. In the case of a spatially dependent temperature distribution at the optical surface of the mirror element which occurs during the operation of the optical arrangement and is dependent on a local irradiance, the optical element comprises a substrate, the zero crossing temperature of which is greater than the mean temperature at the optical surface of the mirror element in the case where the mean temperature is less than the mean value of the minimum temperature and the maximum temperature at the optical surface.

In order to generate a temperature profile at the surface of an EUV mirror which is as homogeneous as possible, WO 2012/013747 Al discloses the practice of controlling the spatially dependent temperature distribution in the substrate in two or three spatial directions with the aid of a temperature-control device. The temperature-control device can have ohmic heating elements, Peltier elements, etc., which can be arranged in a grid arrangement. Provision can also be made for at least one radiation source, which acts on the substrate by heating radiation (IR radiation) , in order to exert a thermal influence thereon. If a mirror for EUV lithography is intended to be used as a thermal manipulator, said mirror, in particular the optical surface thereof, should have high sensitivity to temperature variations. However, a mirror for EUV lithography has additional EUV radiation applied thereon during operation, which introduces a high thermal load into the mirror and possibly leads to strong parasitic errors in the wavefront if the sensitivity of the mirror to temperature variations is too large .

Object of the invention

It is an object of the invention to provide a mirror arrangement, a projection lens with such a mirror arrangement and an EUV lithography apparatus, as well as a method for operating an EUV lithography apparatus, in which the optical properties of a mirror used as thermal manipulator for correcting wavefront errors are improved.

Subject matter of the invention

According to a first aspect, this object is achieved by a mirror arrangement comprising: a mirror with a substrate and with a reflective coating for EUV radiation, wherein the substrate is formed from a material with a temperature-dependent coefficient of thermal expansion, which, in a first volume region, has a zero crossing at a first zero crossing temperature and which, in at least one second, surface-near volume region, has a zero crossing at a second zero crossing temperature that differs from the first. The mirror arrangement also comprises at least one thermal actuator for the thermal actuation of the second volume region of the substrate.

According to a second aspect, the object is achieved by a mirror arrangement comprising: a mirror with a substrate and with a reflective coating for EUV radiation, wherein at least one thermally actuatable layer made of a material with a coefficient of thermal expansion of more than 10xlO ^"6 /K, preferably of more than 50xlO ^_6 / , is applied between the substrate and the reflective coating, and also at least one thermal actuator for the thermal actuation of the thermally actuatable layer.

In a development of the mirror arrangement in accordance with the second aspect, the substrate is formed from a material with a temperature-dependent coefficient of thermal expansion which, in a first volume region, has a zero crossing at a first zero crossing temperature and which, in at least one second, surface-near volume region, has a zero crossing at a second zero crossing temperature that differs from the first. Provision is made for at least one thermal actuator for the thermal actuation of the second volume region of the substrate.

The substrate of the mirror can have one or more surface-near volume regions, which have a higher sensitivity to temperature variations than is the case in the first volume region which typically makes up the majority, i.e. more than approximately 95%, of the substrate volume. Alternatively or additionally, the mirror of the mirror arrangement can have a thermally actuatable layer, i.e. a layer which has a high coefficient of thermal expansion and which therefore has a high sensitivity to temperature variations or to targeted thermal influencing. Both the second volume region or regions and the thermally actuatable layer are comparatively thin or only have a small volume such that, when these are thermally influenced, deformations or changes in length occur which are comparatively small in comparison with the overall volume or the overall thickness of the substrate. Therefore, only comparatively small deformations can be generated on the mirror by means of the thermally actuatable second volume region or by means of the thermally actuatable layer, but these changes are sufficient for correcting wavefront errors.

In one embodiment of the mirror arrangement, the second zero crossing temperature differs by more than 2 K, preferably by more than 5 K, in particular by more than 10 K from the first zero crossing temperature. The first zero crossing temperature typically corresponds substantially to the expected (mean) operating temperature in the substrate during the operation of the mirror in an optical arrangement, in particular in an EUV lithography apparatus, which expected (mean) operating temperature is caused by the thermal load on the optical surface of the mirror from EUV radiation. In the case of small fluctuations of the operating temperature of the mirror about the zero crossing temperature, there are only small thermally caused deformations, and so the sensitivity of the first volume region to temperature variations is low.

The zero crossing temperature of the second volume region deviates from the zero crossing temperature in the first volume region by at least 2 K. If the temperature in the second volume region corresponds to the operating temperature, the thermal expansion of the second volume range is not extreme, i.e. the coefficient of thermal expansion at the operating temperature of the mirror is greater in the second volume region than in the first volume range. The second volume region therefore has a higher sensitivity than the first volume region for targeted thermal influencing, wherein a temperature deviating from the operating temperature is generated in the second volume region, and it is therefore suitable for targeted thermal manipulation or deformation of the optical surface of the mirror.

In a further embodiment of the mirror arrangement in accordance with the second aspect, the material of the thermally actuatable layer is selected from the group comprising: ZrMo ₂ 0 ₈ , ZrW ₂ 0 ₈ , Zr ₂ (Mo0 ) 3 , Zr ₂ ( 0 ₄ ) ₃ , Y ₂ W ₃ 0i ₂ , BiNi0 ₃ , Rh, Cu, Mo, Zr, Nb, Y, Si, Ge, Ni, NiSi, Ru, Ru0 ₂ and mixtures or compounds containing these substances. The materials mentioned here firstly have a high coefficient of thermal expansion and secondly have a low surface roughness after polishing, which simplifies the application of the reflective coating.

In a further embodiment, the surface-near, thermally actuatable volume region is formed on a surface of the substrate facing the reflecting coating or on a lateral surface of the substrate. The provision of the volume region, used for thermal actuation, in the vicinity of the reflecting coating is advantageous since the thermal profile generated by means of the thermal actuator or the temperature distribution generated by the thermal actuator has a direct effect on the optical surface, the surface geometry of which is intended to be manipulated. By contrast, if the surface-near volume region is applied on the surface of the substrate lying opposite to the reflecting coating, the heat or temperature distribution introduced by means of the thermal actuator must propagate through the substrate, wherein the thermal profile would flow apart due to thermal conduction, i.e. the resolution of the thermal profile generated by means of an actuator arranged at this location would be lower. The lateral provision of the second volume region on the substrate renders it possible to thermally influence an edge- side portion of the optical surface or of the substrate in a targeted manner. Here, the second volume region can, in particular, extend in a strip- shaped manner along the lateral surface in the direction of the thickness of the substrate and be restricted in the circumferential direction to a comparatively small angular range of e.g. 10° or less. In particular, a plurality of surface-near volume regions can be arranged along the lateral surface of the substrate, typically extending substantially in a (circular) cylindrical manner. The second volume regions can be arranged along the circumference of the substrate in such a way that specific wavefront errors can be corrected in a targeted manner. By way of example, an astigmatic deformation of the optical surface of the mirror can be generated in a targeted manner by the provision of second volume regions on diametrically opposing points on the lateral surface. It is understood that more complex deformations of the optical surface, and therefore more complex optical effects, can be achieved by suitable other geometric arrangements . In one embodiment, the surface-near volume region extends from the surface of the substrate to a maximum depth of 5 μπι. The surface -near volume region should have a sufficient thickness to generate the desired thermally caused deformation of the optical surface required for the wavefront correction. However, it is generally disadvantageous to extend the thermally actuatable second volume region too far into the volume of the substrate. The thermally actuatable layer also has a sufficient thickness to generate the desired deformation of the optical surface. Depending on the utilized material, the thickness of the thermally actuatable layer can for example be more than 50 nm. As a rule, the thermally actuatable layer has a layer thickness that is less than approximately 0.5 μτη.

In a further embodiment, the thermal actuator is embodied as a heating device. The heating device can be embodied to generate a heat flux into the mirror without a heating element coming into contact with the mirror; however, it is also possible for the heating device to generate a heating flux by contact heat. It is understood that a cooling device can also be used as a thermal actuator instead of a heating device, said cooling device having cooling elements e.g. in the form of Peltier elements. A thermal actuator, which has a combination of heating and cooling devices, is also possible .

In a development, the heating device has a plurality of ohmic heating elements, which convert electrical energy into heat energy when an electric current passes therethrough. The ohmic heating elements are preferably arranged in a grid arrangement or in a grid. The ohmic heating elements can in particular be (thin) heating wires which extend along lines and columns of a grid and the current supply of which can be set individually such that a thermal profile with a resolution which in the ideal case corresponds to the distance between the heating wires can be generated by the grid arrangement. The distances between the heating elements can remain constant over the grid; however, it is also possible to vary the distances between the heating elements in order to generate, in a targeted manner, a higher resolution in specific surface regions of the optical surface than in others .

In an advantageous development, the ohmic heating elements are arranged on a surface of the substrate facing the reflecting coating. The ohmic heating elements can be applied directly onto the upper side of the substrate, for example in the form of a circuit, as described in WO 2012/013747 Al, which was cited in the introduction. The ohmic heating elements can be covered by an insulating layer, i.e. an electrically non- conductive layer, which, in particular, can be embodied as the thermally actuatable layer. As an alternative or in addition to the thermal actuation of the layer, the ohmic heating elements can also serve to heat the adjoining surface-near volume region of the substrate.

In a further development, the heating device has at least one radiation source for generating heating radiation. The radiation source can be embodied to generate infrared radiation, for example in the form of an IR laser or as an IR laser diode. The radiation source is typically positioned or aligned in such a way that the heating radiation can, in a targeted manner, irradiate a surface region of the substrate, to which the thermally actuatable second volume region is applied, or irradiate the thermally actuatable layer. Provision can be made for a radiation guiding device, which guides the heating radiation to a desired position and aligns it on the mirror. The radiation guide device can be embodied in the form of e.g. fibre optics, waveguides, etc. A further aspect of the invention relates to a projection lens for EUV lithography, comprising: a plurality of mirrors, wherein one of the plurality of mirrors is a thermally actuatable mirror of a mirror arrangement as described above. In general, merely one of the mirrors of the projection lens is thermally actuatable; however, optionally, a plurality of mirror arrangements with a plurality of thermally actuatable mirrors can also be arranged in the projection lens, as described further below. The suitability of a mirror of the projection lens as the thermally actuatable mirror depends, inter alia, on the position of the mirror in the beam path of the projection lens, i.e. not all mirrors of the projection lens are suitable as thermally actuatable mirrors to the same extent. The maximum number of thermally actuatable mirrors or mirror arrangements is therefore typically at most half the total number of mirrors in the projection lens. In one embodiment, the thermally actuatable mirror is arranged at a position in the beam path of the projection lens at which a thermal load applied by EUV radiation on the mirror is at less than 50% of a thermal load applied by EUV radiation on a first mirror in the beam path of the projection lens. Typically, only a portion of at most approximately 70% of the incident EUV radiation is reflected at an EUV mirror, the remainder being absorbed by the mirror. The power of the EUV radiation and therefore also the thermal load incident on a further EUV mirror, which is arranged downstream in the beam path, are reduced by a corresponding portion.

In order in the case of the thermally actuatable mirror to, firstly, implement a sensitivity to temperature changes that is as high as possible and, secondly, avoid unwanted parasitic errors in the wavefront, which could be caused by a thermal sensitivity that is too high in the case of irradiation with EUV radiation, the thermally actuatable mirror should only absorb a thermal load that is as small as possible, i.e. it should be arranged at a location in the beam path of the EUV radiation at which the power of the incident EUV radiation is comparatively small.

In one embodiment, the thermally actuatable mirror is arranged at a position in the beam path of the projection lens at which a thermal load applied by EUV radiation on the mirror is less than the median of the thermal loads applied by EUV radiation on the (i.e. on all of the) mirrors of the projection lens. As illustrated above, such a position is typically situated in the rear part of the projection lens, i.e. in a region in the beam path of the EUV radiation at which the thermal load of a respective mirror is less than the median of the thermal loads of all mirrors of the projection lens.

In the case of an odd number n of mirrors in the projection lens which are sorted according to the strength of the thermal load incident thereon, the median is that thermal load applied to the (n+l)/2-th mirror of the projection lens. In the case of an even number n of mirrors in the projection lens which are sorted according to the strength of the thermal load incident thereon, the median value is the mean value of the thermal loads applied to the n/2-th and (n+l)/2-th mirror .

Preferably, the thermally actuatable mirror forms the ultimate or penultimate mirror in the beam path of the projection lens. As described further above, the thermal load on the penultimate and, in particular, on the ultimate mirror of the projection lens is lowest, and so these mirrors are particularly well suited to being thermally actuatable mirrors.

In a further embodiment, the thermally actuatable mirror has an optically used surface region that is greater than the median of the (i.e. of all of the) optically used surface regions of the mirrors of the projection lens. The optically used surface region is that area of the optical surface which is arranged within the beam path of the EUV radiation. In general, mirrors which have a large optically used surface region also have a large mirror diameter, and so a measure equivalent to the size of the optically used surface region is the diameter of the mirror or the area (or the area content thereof) provided with the reflective coating.

Reflective coatings for EUV radiation are only designed for a restricted temperature range. Therefore, a local temperature change that is too large is disadvantageous for a thermally manipulable mirror, i.e. the maximum heating and cooling power which can be introduced into the mirror is restricted. In the case of a given heating and cooling power, the maximum possible spatial resolution of a temperature profile generated by thermal actuation is restricted by the subaperture size or the mirror size. Therefore, the use of a large mirror (with a large subaperture or optically used surface region) as a thermally actuatable mirror is advantageous if small structures in the wavefront are intended to be corrected.

In one development, the thermally actuatable mirror is a pupil-near mirror of the projection lens. In the case of pupil-near mirrors, i.e. in the case of mirrors situated in the vicinity of a pupil plane of the projection lens, small structures to be corrected occur in the wavefront, particularly in the case of critical illumination settings of an illumination system arranged upstream of the projection lens. Therefore, a mirror arrangement with a thermally actuatable large mirror, which is arranged near the pupil, is particularly advantageous.

Within the meaning of this application, a mirror in which the ratio of chief ray height H to the marginal ray height R is less than 0.5, preferably less than 0.2, is understood to be a pupil-near mirror in a projection lens, which images an object field with a maximum object height onto an image field under a given aperture. The chief ray height H and the marginal ray height R are measured proceeding from an object point with a maximum object height, to be precise in a given plane which extends parallel to a pupil plane of the projection lens. A further aspect of the invention relates to an EUV lithography apparatus, which has a projection lens that is embodied as described further above. In addition to the projection lens, the EUV lithography apparatus has an illumination system which illuminates an object field to be imaged on an image field by the projection lens as homogeneously as possible. The EUV lithography apparatus also has an EUV radiation source for generating the EUV radiation. In one embodiment, the EUV lithography apparatus additionally comprises a temperature control device for open- loop or closed-loop control of the thermal actuator of the mirror arrangement . The thermal actuator can be actuated by the temperature control device depending on the structure of the mask or of the object to be imaged, the illumination settings of the illumination system, etc., in order to correct wavefront errors in the projection lens. To the extent that one or more sensors are provided for registering the temperature or the temperature distribution of the thermally actuatable mirror, in particular at the optical surface of the thermally actuatable mirror, in the EUV lithography apparatus, there can also be closed- loop control of the thermal actuator for generating a desired temperature profile at the optical surface. The temperature profile generated at the optical surface by means of the thermal actuator leads to a deformation of the optical surface which is selected depending on the optical properties of the projection lens in such a way that wavefront errors of the projection lens can be corrected.

The invention also relates to a method for operating such an EUV lithography apparatus, wherein the thermally actuatable mirror is subject to open- loop or closed- loop control by the temperature control device during the operation of the EUV lithography apparatus.

Further features and advantages of the invention emerge from the subsequent description of exemplary embodiments of the invention, on the basis of the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can, in a variant of the invention, each be realized individually on their own or together in any combination.

Drawing

Exemplary embodiments are depicted in the schematic drawing and are explained in the following description. In detail: Figure 1 shows a schematic illustration of a mirror arrangement comprising an EUV mirror with a thermally actuatable layer and with a thermal actuator,

Figure 2 shows a schematic illustration of a mirror arrangement analogous to Figure 1, in which the EUV mirror has a substrate with a thermally actuatable volume region,

Figure 3 shows a schematic illustration of a mirror arrangement, in which a plurality of thermally actuatable volume regions and a plurality of radiation sources for the thermal actuation thereof are provided,

Figure 4 shows a schematic illustration of an EUV lithography apparatus comprising an EUV light source for generating EUV radiation and a projection lens with a thermally actuatable mirror,

Figure 5 shows an illustration of the thermal load at six mirrors of the projection lens from Figure 4 ,

Figures 6 and b show illustrations of a temperature distribution, generated by means of a thermal actuator, at an optical surface of the ultimate mirror of the projection lens of

Figure 4 , and

Figures 7a and b show illustrations of a temperature distribution, generated by means of a thermal actuator, at a penultimate mirror of the projection lens of Figure 4. In the following description of the drawings, identical reference signs are used for equivalent or functionally equivalent components. Figure 1 schematically shows a mirror arrangement 1, which has an EUV mirror 2 with a substrate 3, a reflective coating 4 and a thermally actuatable layer 5, which is arranged between the substrate 3 and the reflective coating 4. The mirror arrangement 1 also comprises a thermal actuator 6, which comprises a plurality of ohmic heating elements 7 in the form of heating wires, which are applied in a grid arrangement, i.e. in a regular arrangement in lines and columns, to the surface 3a of the substrate 3 facing the reflective coating 4.

The ohmic heating elements 7 actuate at least 50%, preferably at least 90% of the surface 3a of the substrate 3 provided with the reflective coating 4. The thermal actuator 6 has a voltage source 8 which supplies the ohmic heating elements 7 with a current that is individually adjustable for each one of the heating elements 7 so as to generate a locally varying temperature profile at an optical surface 4a at the upper side of the reflecting coating 4, at which EUV radiation 9 impinging on the mirror 2 is reflected.

The thermally actuatable layer 5 consists of a material which has a coefficient of thermal expansion CTE of more than 10xl0 ^"e /K, preferably of more than 50xlO ^"6 /K. In the shown case, in which the heating wires 7 are applied to the substrate 3, it is necessary for the thermally actuatable layer 5 to consist of electrically insulating material in order to electrically insulate the reflecting coating 4 from the heating wires 7 of the thermal actuator 6. In the shown example, the thermally actuatable layer 5 consists of ZrMo ₂ 0 ₈ , but it can also be formed from different materials, for example from ZrW ₂ 0 ₈ , Zr ₂ (Mo0 ₄ ) ₃ , Zr ₂ (W0 ₄ ) _3/ Y ₂ W ₃ 0i ₂ , BiNi0 ₃ , Rh, Cu, Mo, Zr, Nb, Y, Si, Ge, Ni, NiSi, Ru, Ru0 ₂ and from mixtures or compounds containing these substances. It is understood that the thermally actuatable layer 5 can also be formed from an electrically conductive material, e.g. BiNi0 ₃ , in the case where the heating wires 7 do not come into contact with the thermally actuatable layer 5. In this case, the thermally actuatable layer 5 can be applied e.g. to an insulating layer, e.g. made of quartz, which completely covers and electrically insulates the heating wires 7. In the shown example, the thermally actuatable layer 5 has a layer thickness that lies at less than approximately 0.5 μτα and at more than 50 nra.

As can be recognized in an exemplary manner in Figure 1, the thermal actuator 6 heats the thermally actuatable layer 5 which expands to a comparatively great extent due to the high coefficient of thermal expansion, even in the case of a comparatively small heat influx. As can likewise be recognized in Figure 1, the local deformation of the thermally actuatable layer 5 is transferred to the reflective coating 4 or to the optical surface 4a.

The reflective coating 4 has - as indicated in Figure 1 - a plurality of individual layers (not denoted in any more detail) , which typically consist of layer pairs made of two materials with different refractive indices. If use is made of EUV radiation at a wavelength in the region of 13.5 nm, the individual layers usually consist of molybdenum and silicon. Depending on the wavelength used, other material combinations, such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and materials such as rhodium, palladium, silver, are also possible. In addition to the individual layers, the reflective coating 4 can also contain intermediate layers for preventing diffusion and a capping layer or a capping layer system for preventing oxidation and corrosion. One or else more functional layers can be provided between the thermally actuatable layer 5 and the reflecting coating 4, for example layers to protect the substrate 3 from the EUV radiation 9. The mirror 2 shown in Figure 1 has a substrate 3 made of quartz glass doped with titanium, with a silicate glass proportion of more than 80%. Such a commercially available silicate glass is distributed under the trade name ULE® (Ultra Low Expansion glass) . The zero crossing temperature T _zc of such a glass can be set to a certain extent by way of the Ti0 ₂ proportion of the quartz glass material and, in the shown example, is selected in such a way that the substrate 3 has a desired zero crossing temperature T _zc (which is typically as constant as possible for the whole substrate volume) . The use of a so-called zero-crossing material, i.e. a material in which the coefficient of thermal expansion has a zero crossing in the relevant temperature range between approximately 15 °C and, as a rule, at most 100°C, is necessary in order to meet the high demands on the geometric tolerances on the mirror 2.

As an alternative to using a doped quartz glass, in particular a Ti0 ₂ doped quartz glass, it is also possible to use glass ceramics as zero crossing material, where the ratio of the crystal phase to the glass phase is set in such a way that the coefficients of thermal expansion of the different phases virtually cancel one another out. By way of example, such glass ceramics are offered under the trade names Zerodur® and Clearceram® . The zero crossing temperature T _zc in the volume of the substrate 3 is typically selected in such a way that it corresponds to the (mean) operating temperature T _B of the mirror 2 during operation in an optical arrangement, e.g. a projection lens. The stationary operating temperature T _B , which sets in at the mirror 2 during operation, is dependent on the thermal load from the EUV radiation 9 incident on the mirror 2, the heat transport in the volume of the substrate 3 and heat sinks for dissipating the thermal load, arranged in the region of the substrate 2. The (mean) operating temperature T _B of the mirror 2 can be calculated by simulations or determined by experiment. Small, in particular local deviations of the operating temperature T _B from the zero crossing temperature T _zc , at which the thermal expansion of the substrate 3 has a minimum, only lead to small deformations of the substrate 3, i.e. the mirror 2 is not sensitive to small temperature fluctuations around the operating temperature T _B .

Figure 2 shows a mirror 2 with a substrate 3, which has a first volume region VI forming the main body of the substrate 2, and a second, significantly smaller volume region V2 arranged adjacent to the surface 3a of the substrate 3 facing the reflecting coating 4. The second volume region V2 has a second zero crossing temperature Tzc2 _/ which deviates from the first zero crossing temperature T _ZC i by a value of more than 2 K, optionally by more than 5 K or by more than 10 K, i.e. the difference between the first and the second zero crossing temperature is | T _ZC i - T _ZC2 1 > 2 K or | T _ZC i - T _ZC 2 I > 5 K, in particular | T _ZC i - T _ZC2 | > 10 K.

As a result of the deviation of the second zero crossing temperature T _2C 2 from the first zero crossing temperature T _ZC i , the sensitivity to temperature variations around the operating temperature T _B of the second volume region V2 is increased compared to the first volume region VI. Like in Figure 1, the zero crossing temperature T _ZC i of the first volume region VI substantially corresponds to the (mean) operating temperature T _B of the mirror 2 or of the substrate 3.

As depicted in Figure 2, a heat influx can be introduced into the second volume region V2 in a local or targeted manner with the aid of the thermal actuator 6, analogous to Figure 1, which heat influx leads to a local expansion of the second volume region V2 and therefore to a deformation of the reflecting coating 4 and the optical surface 4a. The thickness D of the second volume region V2 or the maximum distance of the second volume region V2 from the surface 3a of the substrate 3 lies at no more than 0.5 μπι in the example shown .

In the example shown in Figure 2, an insulating layer 10 made of quartz is applied to the heating wires 7 in order to electrically insulate the reflective coating 4 from the heating wires 7. It is understood that the thermally actuatable layer 5 shown in Figure 1 and the thermally actuatable second volume region V2 shown in Figure 2 can also be implemented together at a mirror 2. Moreover, a thermal actuator which is embodied as a cooling device or which forms a combination of heating and cooling device can be used as an alternative to the thermal actuator 6 embodied as a heating device. By way of example, the heating wires 7 can be wholly or partly replaced by cooling channels for holding a cooling medium. It is understood that other possibilities for implementing a thermal actuator 6 also exist, said other possibilities, for example in the form of Peltier elements, having a cooling effect on the thermally actuatable layer 5 or on the second volume region V2.

A further option for thermal actuation of the substrate 3 of the mirror 2 is depicted in Figure 3. The illustration of the reflecting coating was dispensed with in Figure 3, i.e. all that is shown is the substrate 3 and the optical surface 4a at the upper side of the reflecting coating, said upper side having an optically used surface region F in which the EUV radiation 9 is incident on the optical surface 4a. Along a circumferential lateral surface 3b, the substrate 3 has three second volume regions V2a, V2b, V2c, which, like the second volume region V2 in Figure 2, have a zero crossing temperature T _ZC2 deviating from that of the first volume region VI forming the main body of the substrate 3, said zero crossing temperature being identical for all three second volume regions V2a, V2b, V2c in the example shown. There is a radiation source 6a, 6b, 6c assigned to each one of the second volume regions V2a to V2c, said radiation source generating heating radiation in the IR wavelength range and possibly being embodied as e.g. an IR laser or IR diode. Each one of the radiation sources 6a to 6c is aligned onto one of the three second volume regions V2a to V2c in order to introduce heating radiation into the latter for thermal actuation. The second volume regions V2a to V2c applied laterally to the substrate 3 have a strip- shaped embodiment and respectively extend over an angular range of approximately 2° to 15° in the circumferential direction.

The three second volume regions V2a to V2c and a fourth second volume region (not depicted in the image) on the rear side of the circumferential surface 3b are respectively arranged at an angle of 90° from one another in the circumferential direction. One or more of the second volume regions V2a to V2c can selectively be impinged by heating radiation by the radiation sources 6a, 6b, 6c in order to generate a deformation in an edge region of the substrate 3, and therefore also at the optical surface 4a. By way of example, an astigmatic deformation of the optical surface 4a can be generated in a targeted manner by introducing, by means of the radiation sources 6a, 6c, an identical thermal load into the two second volume regions V2a, V2c lying diametrically opposite to one another. More complex deformations of the optical surface 4a and therefore also more complex optical effects can be achieved by other geometric arrangements of the second volume regions V2a to V2c or by the introduction of heating radiation with different power into the various volume regions V2a, V2b, V2c with the aid of the individually actuatable radiation sources 6a to 6c.

By way of example, the mirror arrangement depicted in Figure 1 to Figure 3 can be used in an EUV lithography apparatus 101, as is depicted in Figure 4. The EUV lithography apparatus 101 has an EUV light source 102 for generating EUV radiation, which has a high energy density in an EUV wavelength range below 50 nm, in particular between approximately 5 nm and approximately 15 nm. By way of example, the EUV light source 102 can be embodied in the form of a plasma light source for generating a laser- induced plasma or it can be embodied as a synchrotron radiation source. Particularly in the first case, use can be made of a collector mirror 103, as shown in Figure 1, to focus the EUV radiation from the EUV light source 102 so as to form an illumination beam 104 and thus further increase the energy density. The illumination beam 104 serves to illuminate a structured object M by means of an illumination system

110 which, in the present example, has five reflecting optical elements 112 to 116 (mirrors) . By way of example, the structured object M can be a reflective mask, which has reflecting and non- reflecting or at least less strongly reflecting regions for generating at least one structure at the object . Alternatively, the structured object can be a multiplicity of micromirrors which are arranged in a one-dimensional or multi-dimensional arrangement and which optionally are movable about at least one axis in order to set the angle of incidence of the EUV radiation 104 on the respective mirror.

The structured object M reflects part of the illumination beam 104 and shapes a projection beam path 105 which carries the information about the structure of the structured object M and which is radiated into a projection lens 120, which generates an image of the structured object M, or of a respective portion thereof, on a substrate W. The substrate W, for example a wafer, comprises a semiconductor material, e.g. silicon, and is arranged on a holder which is also referred to as wafer stage WS .

In the present example, the projection lens 120 has six reflective optical elements 121 to 126 (mirrors) in order to generate on the wafer an image of the structure present on the structured object M. The number of mirrors in a projection lens 120 is typically between four and eight, but optionally use can also be made of only two mirrors.

In order to achieve a high imaging quality when imaging a respective object point OP of the structured object M on a respective image point IP on the wafer W, very high demands should be met by the surface shape of the mirrors 121 to 126, and the position or the alignment of the mirrors 121 to 126 in relation to one another or relative to the object M and to the substrate W also requires precision in the nanometre range.

In particular, the generation of a diffraction- limited image, which enables the maximum possible resolution, is only possible if the wavefront aberrations of the projection lens 120 are sufficiently small. In order to achieve this, the surface form of the mirrors 121 to 126 must be set very precisely and the mirrors 121 to 126 must likewise be positioned very accurately.

During the operation of the projection lens 120, a portion of the radiation of the projection beam path 105, which may be up to 70%, is absorbed by a respective mirror 121 to 126. Figure 5 shows the thermal load l to W6 or the power of the EUV radiation 9 which is incident on a respective mirror 121 to 126. The power Wl, which is reflected by the mask M and which is incident on the first mirror 121 is 100%. The power or thermal load W2 incident on the second mirror

122 in the projection beam path 105 is approximately 70%, the thermal load W3 on the third mirror 123 is approximately 50%, the thermal load W4 on the fourth mirror 124 is approximately 35% etc.

It is advantageous for a thermally actuatable mirror if the latter is only exposed to a small thermal load W by the EUV radiation. Therefore, it is advantageous if the thermal load W applied to thermally actuatable mirror is less than 50% of the thermal load Wl applied to the first mirror 121. As can be seen from Figure 5, this is the case for the thermal loads W4 to W6 of the last three mirrors 124 to 126 of the projection lens 120. The fifth and sixth mirrors 125, 126 of the projection lens 120 are particularly well suited to being thermally actuatable mirrors as a result of the low thermal load. Moreover, the thermal load W4 to W6 applied by the EUV radiation at the last three mirrors 124 to 126 of the projection lens 120 is less than the median of the thermal loads l to W6 of all mirrors 121 to 126 of the projection lens 120. In the shown example, in which the projection lens 120 has an even number of mirrors 121 to 126, the median is the mean value of the thermal load W3 of the third mirror 123 and of the thermal load W4 of the fourth mirror 124, i.e. the median lies at approximately ½(50% + 35%) = 42.5 %.

In the projection lens 120 depicted in Figure 4, the sixth mirror 126 is embodied as a thermally actuatable mirror 2, which is part of a mirror arrangement 1 that can be embodied as depicted further above in conjunction with Figure 1 to Figure 3. The thermal actuator 6 of the mirror arrangement 1 is signal connected to a temperature control device 130 of the EUV lithography apparatus 101. The temperature control device 130 actuates the thermal actuator 6 in order to generate a predetermined temperature profile at the optical surface 4a (cf. Figure 1 to Figure 3) of the sixth mirror 126. If one or more sensors for registering the temperature of the mirror 126 or of the optical surface 4a and/or the temperature of the substrate 3 are arranged in the EUV lithography apparatus 101, the temperature control device 130 can subject the temperature distribution generated by the thermal actuator 6 or the temperature profile generated thereby to closed- loop control such that said temperature profile corresponds to a reference temperature profile which generates the desired correction of wavefront errors of the projection lens 120. Temperature sensors can be embedded outside of the substrate 3, but possibly also into the substrate 3 or into the volume of the substrate 3. Performing the thermal actuation on the last mirror 126 of the projection lens 120 is also advantageous for another reason in addition to the low occurring thermal load W6 : the last mirror 126 is a pupil-near mirror 126, which has a comparatively large optical surface 4a or a large diameter. As is possible to identify on the basis of the projection beam path 105 of Figure 4, the last mirror 126 is the mirror with the largest diameter or with the largest optically used surface region in the projection lens 120. The optically used surface region F (cf. Figure 3) is that region of the optical surface 4a which is exposed to EUV radiation 9 or which is arranged in the projection beam path 105 of the projection lens 120. The use of mirrors which have an optically used surface region F that is greater than the median of the optically used surface regions at the mirrors 121 to 126 of the projection lens 120 was found to be advantageous since on such mirrors small structures in the wavefront can be corrected, as is described below on the basis of Figures 6a and 6b and Figures 7a and 7b, which depict a plan view of the optical surface 4a with the grid structure of the ohmic heating wires 7 from Figure 1 and Figure 2, which are arranged at a constant distance L from one another.

Figure 6a shows a temperature distribution of the optical surface 4a of the last mirror 126, immediately after a heating zone HI was heated with the aid of the thermal actuator 6 to a temperature that is higher than that of the surroundings. The heating zone HI shown in Figure 6a comprises an area of 7x7 distances L between adjacent heating wires 7, i.e. the heating zone HI is comparatively large. Figure 6b shows the temperature distribution at a subsequent time, at which the temperature distribution is stationary and has expanded to a larger area than the original area of 7x7 distances L due to the thermal conduction.

Figures 7a and 7b show an analogous illustration of a temperature distribution, in which a second heating zone H2 , which comprises lxl distances L, was heated to the same temperature as in Figure 6a, directly after the activation of the second heating zone H2. Figure 7b shows the stationary temperature distribution which sets in in a stationary state when maintaining the heating power introduced into the second heating zone H2. It is clearly possible to recognize that the temperature profile in Figure 7b in the stationary state is dominated by thermal conduction, i.e. the heating zone H2 originally only measuring lxl distances L extends over an area of 5x5 distances L.

Since the peak heating and cooling power of the thermal actuator 6 is predetermined and cannot be increased arbitrarily so as not to cause negative effects on the reflective coating 4, the spatial resolution of the temperature profile generated with the aid of the thermal actuator 6 cannot be increased arbitrarily. If the area or the subaperture at a mirror available for the thermal actuation is large, it is also possible to correct small structures of the wavefront with a comparatively low spatial resolution since a relatively large number of heating elements 7 are available; this immediately follows from a comparison between Figures 6a and 6b and Figures 7a and 7b, which show the grid arrangement with 16x16 heating wires 7 at the ultimate, large mirror 126 of the projection lens 120 and the grid arrangement with 10x10 heating wires 7 at the penultimate, smaller mirror 125 of the projection lens 120. Small structures of the wavefront to be corrected occur, in particular, on pupil-near mirrors such that, in particular, large pupil-near mirrors are suitable as thermal manipulators. The ultimate mirror 126 in the projection beam path 105 of the projection lens 120 is such a pupil-near mirror, i.e. a mirror in which the ratio of chief ray height H to marginal ray height R is less than 0.5, even less than 0.2 in the shown case.