METHOD FOR MANUFACTURING A MULTI-COMPONENT MIRROR OF A PROJECTION EXPOSURE APPARATUS FOR MICROLITHOGRAPHY - Patent application

Abstract

The invention relates to a method for manufacturing a mirror (26) of a projection exposure apparatus for microlithography, comprising providing at least one material blank (34) made of a material with a very low coefficient of thermal expansion and having a failure zone (33), in which at least one material parameter deviates from a specified value by more than a minimum deviation. A first mirror part (27) having a first connecting surface (29) is manufactured from the material blank (34). A second mirror part (28) having a second connecting surface (30) is manufactured from said material blank (34) or from a different material blank (34). The first mirror part (27) and the second mirror part (28) are permanently connected to each other in the region of the first connecting surface (29) of the first mirror part (27) and the second connecting surface (30) of the second mirror part (28). The volumetric area of ​​the material blank (34) in which the first mirror part (27) is to be manufactured and / or the volumetric area of ​​the material blank (34) or the further material blank (34) in which the second mirror part (28) is to be manufactured is determined based on the spatial formation of a defect zone (33) in the material blank (34) or in the material blank (34) or in the further material blank (34).

Description

[Technical field]

[0001] The invention relates to a method for manufacturing a multi-component mirror of a projection exposure apparatus for microlithography The invention also relates to a multi-component mirror of a projection exposure apparatus for microlithography, an illumination optical unit, a projection optical unit and a projection exposure apparatus for microlithography. [Background technology]

[0002] Projection exposure apparatus for microlithography are used in particular in the manufacture of semiconductors and generally comprise an illumination optical unit and a projection optical unit. The illumination optical unit generates from the light of a light source a light distribution desired for the illumination of a reticle, often also called a mask. Light is to be understood in this case in the general sense of electromagnetic radiation, i.e. without being limited to a specific wavelength. The terms "light" and "radiation" are therefore used synonymously in the following, i.e. the light source can also be called a radiation source and the light distribution can also be called a radiation distribution and vice versa. By means of the projection optical unit, the reticle is imaged onto a photosensitive material, which is applied in particular to a wafer of semiconductor material or to some other substrate. In this way, the photosensitive material is structured and exposed to a predefined pattern by the reticle. Since the reticle has tiny structural elements that are intended to be transferred to the substrate with high precision, it is necessary that the illumination optical unit generates the desired light distribution accurately and reproducibly and that the imaging by the projection optical unit is performed accurately and reproducibly.

[0003] In addition to further optical elements, the illumination optical unit and the projection optical unit may have at least one mirror in the light path, which deflects the light in a predetermined way by reflection at its optical surface. The specific way in which the light is deflected depends on the shape of the optical surface. The optical surface may be formed, for example, as a metal layer or as a series of layers with alternating refractive indices.

[0004] It may be advantageous or even necessary to form the mirror from two or more pieces, for example if the mirror is to have cooling channels or is to be larger than the blank of material available for its manufacture.

[0005] A multi-component mirror with cooling channels is known from the non-prior document US Pat. No. 5,399,633.

[0006] Multi-component implementations of mirrors can adversely affect the optical properties of the mirror, especially if the mirror components do not have exactly the same material properties.

[0007] If the mirror is a component of an illumination optical unit, the light distribution produced by the illumination optical unit may deviate from specifications, and if the mirror is a component of a projection optical unit, imaging aberrations may occur during imaging with the projection optical unit.

[0008] As miniaturization in semiconductor manufacturing continues, illumination and imaging of the reticle must be performed with ever greater precision, which places increasingly stringent demands on the optical properties of optical elements, e.g. mirrors, used in the illumination or projection optical units. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] German Patent Application No. 102020208648.6 Summary of the Invention [Problem to be solved by the invention]

[0010] The invention is based on the object of developing a multi-component mirror for a projection exposure apparatus for microlithography such that its optical properties meet very strict requirements: in particular, the mirror should have good optical properties and be reliably usable even under thermal load. [Means for solving the problem]

[0011] This object is achieved by the combination of features of claim 1.

[0012] In a first variant of the method according to the invention for manufacturing a mirror of a projection exposure apparatus for microlithography, at least one material blank is provided, which consists of a material with a very low coefficient of thermal expansion and has a failure zone, in which at least one material parameter deviates from a specified value by more than a minimum deviation. A first mirror part with a first connecting surface is manufactured from the material blank. A second mirror part with a second connecting surface is manufactured from the material blank or from a further material blank. The first and second mirror parts are permanently connected to each other in the region of the first connecting surface of the first mirror part and the second connecting surface of the second mirror part. The volume area of ​​the material blank in which the first mirror part is manufactured and / or the volume area of ​​the material blank or the further material blank in which the second mirror part is manufactured is determined on the basis of the spatial formation of the failure zone in the material blank or the material blank or the further material blank. In this case, the volume areas of the material blank or the first and second mirror parts in the material blank or the material blank or the further material blank are selected in such a way that after the connection of the first and second mirror parts, the failure zone continues from the first mirror part to the second mirror part.

[0013] The method according to the invention is advantageous because it allows the manufacture of multi-component mirrors of projection exposure devices for microlithography, whose optical properties meet very strict requirements. By taking into account the spatial formation of the defect zones in the material blank, it is possible to manufacture mirrors that exhibit very good thermal behavior and have good optical properties even under thermal load.

[0014] The continuation of the failure zones from the first mirror part to the second mirror part can be defined such that the failure zones continue from the first mirror part to the second mirror part without lateral offset or only with a small lateral offset. In this case, a small lateral offset of the failure zones can be defined such that the lateral offset of the failure zones of 50% or more, preferably 80% or more, is less than 50%, preferably less than 30%, particularly preferably less than 10% of the dimension of each failure zone in the direction of the offset. In this case, the direction of the offset can be considered for each failure zone individually as the direction in which the lateral offset between the first mirror part and the second mirror part is maximum for each failure zone.

[0015] In particular, the volumetric region of the second mirror part of the material blank can be selected to be displaced along the failure zone relative to the volumetric region of the first mirror part of the material blank, in this way properties associated with the continuation of the failure zone from the first mirror part to the second mirror part, e.g. low mechanical stresses in the region of the connecting surface, can be targeted.

[0016] By way of example, the specified value can be an average value of the material parameter generated by averaging over the entire volume or a part of the volume of each material blank, which also applies to the further variants of the method according to the invention described below.

[0017] In yet another variant of the method according to the invention for producing a mirror of a projection exposure apparatus for microlithography, at least one material blank is provided, which consists of a material with a very low coefficient of thermal expansion and has a failure zone, in which at least one material parameter deviates from a specified value by more than a minimum deviation. A first mirror part with a first connecting surface is produced from the material blank. Furthermore, a second mirror part with a second connecting surface is produced from the material blank. The first mirror part and the second mirror part are permanently connected to each other in the region of the first connecting surface of the first mirror part and the second connecting surface of the second mirror part. The first mirror part and the second mirror part are produced from volumetric regions of the material blank that are spaced apart from each other by the sum of the additional material amount for the production of the first mirror part and the additional material amount for the production of the second mirror part. In this case, the additional material amount for the production of the two mirror parts may correspondingly include an additional material amount for the processing of each mirror part, for example by grinding and polishing, as well as an additional material amount for the separation of the mirror part from the material blank, for example by sawing or cutting. As a result, the individual material additions correspond respectively to the material ablation due to each processing step, i.e. for example cutting losses, grinding losses or polishing losses. In particular, the distance should be kept as small as possible so that only the necessary material additions are barely observable.

[0018] This further variant of the method according to the invention also allows the production of a multi-component mirror of a projection exposure apparatus for microlithography, which has very good optical properties and very good thermal behavior.A further advantage of this further variant is that it requires relatively little expenditure.It is not essential to know the spatial formation of the defect zone.However, this is useful in order to be able to estimate the properties of the produced mirror.

[0019] The advantage of both variants is that they reduce the lateral offset of possible failure zones at the transitions between the mirror parts, which in turn can have a positive effect on the durability of the mirror and its thermal properties.

[0020] Within the scope of the method according to the invention, in particular at least one material blank can be provided from quartz glass, titanium-doped quartz glass or glass ceramic. These materials are available in very good quality and have excellent suitability for use in projection exposure apparatus for microlithography. Material blanks made of quartz glass or titanium-doped quartz glass can be produced, for example, by the direct deposition method or the soot method. With these processes, high-quality material blanks with precisely defined specifications can be produced.

[0021] The first and second mirror parts can be manufactured from the same material blank or from two separate material blanks. Manufacturing from one material blank has the advantage that the material parameters generally vary only slightly, so that both mirror parts are made of practically exactly the same material. The use of two separate material blanks has the advantage that there is more freedom for the arrangement of the failure zones of the mirror parts. For the two material blanks, the spatial formation of the failure zones should be similar. By way of example, this can be done by manufacturing the material blanks with the same manufacturing equipment in uninterrupted time and with the same process parameters. Similar formed failure zones allow a continuous transition between the two mirror parts, reducing the risk of the connected mirror parts being detached from each other.

[0022] The material parameter can be a specification on the material composition, such as titanium content or OH content, or a specification on the material properties, such as zero cross temperature or slope of the thermal expansion coefficient.

[0023] In an embodiment of the method according to the invention, the volumetric regions of the first and second mirror parts of the material blank, in which the image aberrations during operation of the projection exposure apparatus are minimized, can be simulated on the basis of the spatial formation of the defect zones. The simulation can be based on a specific usage scenario of the projection exposure apparatus. In this way, particularly good optical properties can be obtained and the risk that the manufactured mirror does not meet the requirements can be reduced. Furthermore, an optimal use of the available material blanks is made possible.

[0024] The first and second mirror parts can be separated from the material blank along a curved separation surface at least in certain regions. The curved separation surface can extend between a volume region of the first mirror part and a volume region of the second mirror part. In particular, the curved separation surface can extend between parts of the material blank provided for the formation of the first connecting surface of the first mirror part and the second connecting surface of the second mirror part. Furthermore, the curvature of the separation surface can substantially correspond to the curvature of the first connecting surface of the first mirror part and the second connecting surface of the second mirror part. These measures have the advantage that the additional amount of material required for the production of the first mirror part in the region of the first connecting surface and the additional amount of material required for the production of the second mirror part in the region of the second connecting surface can be kept small. Furthermore, this allows the volume regions of the first and second mirror parts to be arranged at a very short distance from each other in the material blank. It is therefore quite possible that the failure zones of the mirror parts produced in this way are formed very similarly at the location of the first connecting surface and at the location of the second connecting surface and do not have a significant lateral offset from each other, i.e. the lateral offset can be further reduced. Separation along a curved separation surface can be achieved by separation ball grinding.

[0025] The relative orientation in which the first and second mirror parts are connected to each other can be determined based on the spatial formation of failure zones in the material blank or in the material blank or in the further material blank. By way of example, the relative orientation during connection is selected such that the failure zones run from the first mirror part to the second mirror part without significant lateral offset. It is also possible to select the relative orientation during connection such that the failure zones run from the first mirror part to the second mirror part with significant lateral offset. In particular, it is possible to select a maximum lateral offset. As a result of both procedures having different effects on the formation of local stresses in the region of the connecting surfaces of the mirror parts and on the thermal behavior of the mirror, it is possible to have a targeted influence in this respect. For example, in the case of mirrors located near the pupil of a projection exposure apparatus, i.e. in the case of mirrors arranged near the pupil plane or a plane of the projection exposure apparatus conjugate thereto, a large lateral offset can be advantageous. In particular, in the case of periodically formed failure zones, a large lateral offset can be advantageous.

[0026] In particular, the first and second mirror parts can be manufactured from laterally offset volume regions of a material blank. In this case, the lateral offset of the volume regions can be specified based on the spatial formation of the failure zone in the material blank. In this way, for example, the profile of the failure zone can be reproduced by the offset of the volume regions, as a result of which it is possible to prevent material losses during processing of the mirror parts from resulting in a lateral offset of the failure zone between the mirror parts in the finished mirror.

[0027] It is also possible to manufacture the first and second mirror parts from a volume area of ​​the material blank that is inclined with respect to an outer surface or axis of the material blank. In this case, the volume area of ​​the material blank can be inclined with respect to all outer surfaces of the material blank. In particular, the volume area can be inclined such that the defect zone in the material blank extends approximately parallel to the part of the material blank that is provided for the formation of the outer surface of the second mirror part on which the optical surface is formed. As a result, only a small defect zone intersects with this outer surface and does not promote the formation of a wavy surface that adversely affects the optical surface during the processing of the second mirror part as a result of its slightly different material hardness.

[0028] The first and second mirror parts can be connected to each other in the same relative orientation as they are in the material blank. In this way, an offset of the defect zone between the first and second mirror parts, for example due to twisting of the mirror parts, can be avoided. At least one auxiliary frame can be used or at least one marking can be attached to the material blank to mark the orientation of the material blank. As a result, the connection in the same relative orientation can be ensured using relatively simple means.

[0029] The volumetric areas of the material blank or the first and second mirror parts of the material blank or the further material blank can be selected such that the occurrence of a failure zone at the location of the first connecting surface of the first mirror part and at the location of the second connecting surface of the second mirror part, respectively, is below a threshold value. As a result, regardless of the orientation of the failure zone, the possible negative effects of the failure zone can be limited by the area of ​​the connecting surfaces. In particular, the occurrence of a failure zone is below a threshold value whenever the proportion of the area of ​​the failure zone at each connecting surface is below a threshold value. In other words, the area of ​​each connecting surface occupied by the entire failure zone divided by the total area of ​​each connecting surface must be below a threshold value.

[0030] The threshold value can be defined as the arithmetic mean of the maximum and minimum values ​​of the area percentages of the defective zones on each connecting surface when the volumetric regions of the first and second mirror parts in the material blank or in the material blank or in the further material blank are arranged differently. In this case, it is also possible to determine the minimum and maximum values ​​for the area percentages of the first and second mirror parts separately and set the threshold values ​​for the first and second connecting surfaces accordingly.

[0031] Furthermore, it is possible to require that the interval between the minimum value of the percentage area of ​​the defective zones and the threshold value be, for example, more than 30%, in particular more than 60%, below the threshold value.

[0032] The regions can be selected for each material blank such that the spatial formation of the failure zone is similar, and the first mirror part can be produced from a volume area in one of the regions and the second mirror part can be produced from a volume area in another region. In particular, the volume areas of the first and second mirror parts are arranged so that they are directly adjacent to each other when the volume area of ​​the second mirror part is transferred to the region containing the volume area of ​​the first mirror part while maintaining its position and orientation with respect to the failure zone. An arrangement of the volume areas in which the transfer results in the first and second connecting surfaces being produced from directly adjacent parts of the volume areas is particularly advantageous. These measures have the advantage that, despite material losses during cutting and processing of the mirror parts, the lateral offset of the failure zone between the mirror parts can be kept small.

[0033] The invention also relates to a mirror of a projection exposure apparatus for microlithography. The mirror according to the invention comprises a first mirror part and a second mirror part made of a material with a very low coefficient of thermal expansion. The first mirror part and the second mirror part are permanently connected to each other in the region of a first connecting surface of the first mirror part and a second connecting surface of the second mirror part. The first mirror part and the second mirror part have failure zones in which at least one material parameter deviates from a specified value by more than a minimum deviation. The failure zones of the first mirror part adjoin the failure zones of the second mirror part over more than 50% of the total area they occupy at the location of the first connecting surfaces.

[0034] The mirror according to the invention has the advantage that it exhibits very good thermal behavior and that its optical properties meet very high requirements, especially under thermal load. Problems that may arise due to abrupt changes in material parameters during the transition from the first mirror part to the second mirror part can be minimized. This is particularly important for mirrors of the projection exposure apparatus that are particularly susceptible to such abrupt changes, for example mirrors in the vicinity of the field, i.e. mirrors that are arranged for example in the vicinity of the object plane or a plane of the projection exposure apparatus conjugate thereto.

[0035] By way of example, the specified value may be an average value of the material parameter produced by averaging over the entire volume or a portion of the volume of each mirror part.

[0036] In particular, the failure zones of the first mirror part may adjoin the failure zones of the second mirror part over 75% or more, particularly preferably over 90% of the total area that they occupy at the location of the first connecting surfaces.

[0037] The second mirror component may have an optical surface.

[0038] It is likewise possible for the failure zones of the second mirror part to extend substantially parallel to the optical surface. In particular, the surface of the second mirror part on which the optical surface is formed may have no more than 10 failure zones, preferably no more than 5, particularly preferably no more than 2 failure zones.

[0039] This makes it possible to limit undesirable undulations of the optical surface that may result from local variations in the surface hardness of the second mirror part.

[0040] The first and / or second mirror parts may contain titanium and / or OH. In particular, the first and / or second mirror parts may be made of quartz glass, titanium-doped quartz glass, or glass ceramic. In terms of mass, the first and / or second mirror parts may have an OH content of less than 400 ppm. In terms of mass, it is also possible for the first and / or second mirror parts to have an OH content of more than 600 ppm. The mirror may have cooling channels in the area of ​​the first and / or second mirror parts.

[0041] The material parameter defining the failure zone may be, for example, titanium content, OH content, zero cross temperature, or gradient of the thermal expansion coefficient.

[0042] When averaged over a lateral area of ​​the optical surface, the second mirror part may have a zero cross temperature that is offset from the zero cross temperature of the first mirror part by -0.5 K to 3 K, preferably -0.5 K to +1.5 K. In particular, the second mirror part may have a higher zero cross temperature, when averaged in this way, than the first mirror part. The second mirror part tends to have a higher temperature than the first mirror part during operation of the projection exposure apparatus, and therefore a lower thermal expansion at a higher zero cross temperature as a result. The second mirror part may have a more uniform distribution of the zero cross temperature over the volume than the first mirror part. Since the second mirror part has an optical surface, the material properties of the second mirror part are more important for the optical properties of the mirror than the material properties of the first mirror part.

[0043] The first and second mirror parts may have a similar titanium content and / or a similar OH content in the first volume area extending from the first connecting surface to a distance of 10 mm in the first mirror part and in the second volume area extending from the second connecting surface to a distance of 10 mm in the second mirror part. In particular, the titanium dioxide content of the second volume area of ​​the second mirror part may deviate from the titanium dioxide content of the first volume area by less than 0.04% by weight, preferably less than 0.02% by weight, particularly preferably less than 0.1% by weight. Furthermore, the OH content of the second volume area of ​​the second mirror part may deviate from the OH content of the first volume area by less than 5% by weight, preferably less than 2% by weight, particularly preferably less than 1% by weight. In the case of the OH content, the percentage specification for this deviation therefore relates to the OH content as a relative value. The average values ​​in the respective volume areas can be used as values ​​for the titanium dioxide content and the OH content. If the second mirror part is less than 10 mm thick, the entire second mirror part can be used as the second volume area.

[0044] The failure zones can be continued from the first mirror part to the second mirror part. In particular, the failure zones can be continued from the first mirror part to the second mirror part without lateral offset or only with a small lateral offset. By way of example, the lateral offset of the failure zones of 50% or more, preferably 80% or more, can be less than 50%, preferably less than 30%, particularly preferably less than 10% of the dimension of each failure zone in the direction of the offset. In this case, the direction of the offset can be considered for each failure zone individually as the direction in which the lateral offset between the first mirror part and the second mirror part is maximum for each failure zone.

[0045] Furthermore, the invention relates to a mirror of a microlithographic projection exposure apparatus, comprising a first mirror part and a second mirror part made from a material with a very low coefficient of thermal expansion, the first mirror part and the second mirror part being permanently connected to each other in the region of a first connecting surface of the first mirror part and a second connecting surface of the second mirror part. The first mirror part has a first average value of the zero cross temperature in a first volumetric region extending to a distance of 10 mm from the first connecting surface of the first mirror part. The second mirror part has a second average value of the zero cross temperature in a second volumetric region extending to a distance of 10 mm from the second connecting surface of the second mirror part. The first average value of the zero cross temperature deviates from the second average value of the zero cross temperature by less than 1 K.

[0046] The mirror according to the invention has the advantage that it exhibits very good thermal behavior and in particular meets very high requirements in terms of optical properties even under thermal load, in particular as a result of the small difference in the average values ​​of the zero cross temperatures in the region of the connecting surfaces, the risk of mechanical stresses forming during changes in the mirror temperature, which can lead to deformations of the optical surfaces, is reduced.

[0047] In particular, the first average value of the zero cross temperatures may deviate from the second average value of the zero cross temperatures in particular by less than 0.5 K, particularly preferably by less than 0.1 K.

[0048] If the second mirror part has a thickness of less than 10 mm, the entire second mirror part can be used as the second volume region.

[0049] Outside the first volumetric region, the first mirror component may have a third average value of zero cross temperatures that deviates more from the second average value of zero cross temperatures than the first average value of zero cross temperatures.

[0050] In particular, the third average value of the zero cross temperatures may deviate from the second average value of the zero cross temperatures by 0.1 K or more, preferably by 1 K or more, particularly preferably by 3 K or more, from the first average value of the zero cross temperatures.

[0051] The invention also relates to an illumination optical unit comprising a mirror according to the invention.

[0052] Furthermore, the invention relates to a projection optical unit comprising a mirror according to the invention.

[0053] Finally, the invention relates to a projection exposure apparatus for microlithography, which comprises an illumination optical unit according to the invention and / or a projection optical unit according to the invention.

[0054] The invention is explained in more detail below on the basis of exemplary embodiments shown in the attached drawings. [Brief description of the drawings]

[0055] [Figure 1] 1 illustrates an exemplary embodiment of a projection exposure apparatus for EUV projection lithography in schematic meridian section; [Diagram 2] 1 shows a schematic diagram of an exemplary embodiment of a projection exposure apparatus for DUV projection lithography; [Diagram 3] 1 shows a schematic cross-sectional view of an exemplary embodiment of a mirror according to the invention; [Figure 4] 1 shows a schematic cross-section of a material blank for the manufacture of a mirror according to a first configuration of the mirror according to the invention; [Diagram 5] 4 shows a schematic cross-section of a material blank for the manufacture of a mirror according to yet another configuration of the mirror according to the invention; [Figure 6] 4 shows a schematic cross-section of a material blank for the manufacture of a mirror according to yet another configuration of the mirror according to the invention; [Figure 7] 4 shows a schematic cross-section of a material blank for the manufacture of a mirror according to yet another configuration of the mirror according to the invention; [Figure 8] 4 shows a schematic cross-section of a material blank for the manufacture of a mirror according to yet another configuration of the mirror according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] FIG. 1 shows an exemplary embodiment of a projection exposure apparatus for EUV projection lithography in schematic meridian section.

[0057] In the following, the essential components of a microlithography projection exposure apparatus 1 will be described by way of example, initially with reference to Figure 1. The description here of the basic construction of the projection exposure apparatus 1 and its components should not be regarded as limiting.

[0058] An embodiment of the illumination system 2 of the projection exposure apparatus 1 comprises, in addition to the light source or radiation source 3, an illumination optical unit 4 for illumination of an object field 4 in an object plane 6. In an alternative embodiment, the radiation source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not comprise the radiation source 3.

[0059] A reticle 7, also called a mask, arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable, in particular in the scanning direction, by a reticle displacement drive 9.

[0060] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x direction extends perpendicular to the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. The scanning direction extends along the y direction in Fig. 1. The z direction extends perpendicular to the object plane 6.

[0061] The projection exposure apparatus 1 comprises a projection optical unit 10. The projection optical unit 10 serves to image the object field 5 into an image field 11 in an image plane 12. The image plane 12 extends parallel to the object plane 6. Alternatively, angles other than 0° between the object plane 6 and the image plane 12 are also possible.

[0062] Structures on the reticle 7 are imaged onto a photosensitive layer of a wafer 13 or some other substrate, which is arranged in the region of the image field 11 of the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable, in particular in the y-direction, by a wafer displacement drive 15. The displacement of the reticle 7, firstly by the reticle displacement drive 9 and secondly by the wafer displacement drive 15, can be performed synchronously with respect to one another.

[0063] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits illumination radiation 16, also referred to in the following as used radiation or illumination light. In the illustrated exemplary embodiment, the illumination radiation 16 has a wavelength in the EUV range between 5 nm and 30 nm. The radiation source 3 can be a plasma source, for example an LPP (Laser Produced Plasma) source or a GDPP (Gas Discharge Plasma) source. It can also be a synchrotron-based radiation source. Likewise, the radiation source 3 can be a Free Electron Laser (FEL).

[0064] The illumination radiation 16 coming from the radiation source 3 is focused by a collector 17. The collector 17 may be a collector with one or more ellipsoidal and / or hyperbolic reflecting surfaces. The illumination radiation 16 may be incident on at least one reflecting surface of the collector 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45°, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. The collector 17 may be structured and / or coated, firstly to optimize the reflectivity for the illumination radiation 16 and secondly to suppress extraneous light.

[0065] Downstream of the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may represent a separation between the source module comprising the radiation source 3 and the collector 17 and the illumination optical unit 4.

[0066] The illumination optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 arranged downstream thereof in the beam path. The deflection mirror 19 may be a plane deflection mirror or a mirror with a beam-influencing effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 may be embodied in the form of a spectral filter that separates the used optical wavelength of the illumination radiation 16 from extraneous light of wavelengths deviating therefrom. If the first facet mirror 20 is arranged in a plane of the illumination optical unit 4 that is optically conjugate with the object plane 6 as the field plane, it is also referred to as a field facet mirror. The first facet mirror 20 comprises a plurality of individual first facets 21, also referred to as field facets in the following. Only some of these first facets 21 are shown by way of example in FIG. 1 .

[0067] The first facet 21 can be embodied as a macroscopic facet, in particular as a rectangular facet, or as a facet having an arcuate or part-circular peripheral contour. The first facet 21 can be embodied as a planar facet or as a convexly or concavely curved facet.

[0068] As is known, for example, from DE 10 2008 009 600 A1, the first facet 21 itself can also consist of a number of individual mirrors, in particular a number of micromirrors. The first facet mirror 20 can in particular be in the form of a microelectromechanical system (MEMS system). For further details, see DE 10 2008 009 600 A1.

[0069] Illumination radiation 16 travels horizontally, ie in the y direction, between collector 17 and deflection mirror 19 .

[0070] In the beam path of the illumination optical unit 4, a second facet mirror 22 is arranged downstream of the first facet mirror 20. If the second facet mirror 22 is arranged in a pupil plane of the illumination optical unit 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged away from the pupil plane of the illumination optical unit 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US 2006 / 0132747 A1, EP 1 614 008 A1 and US 6,573,978 B1.

[0071] The second facet mirror 22 includes a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

[0072] The second facet 23 may likewise be a macroscopic facet, which may for example have a circular, rectangular or hexagonal boundary, or it may be a facet made up of a micromirror - in this respect reference is also made to DE 10 2008 009 600 A1.

[0073] The second facet 23 may have a planar reflective surface or a convexly or concavely curved reflective surface.

[0074] The illumination optical unit 4 results in a double-faceted system. This basic principle is also called a fly's eye condenser (fly's eye integrator).

[0075] It may be advantageous not to arrange the second facet mirror 22 exactly in a plane that is optically conjugate with the pupil plane of the projection optical unit 10. In particular, the second facet mirror 22 may be arranged at an angle with respect to the pupil plane of the projection optical unit 10, as described, for example, in DE 10 2017 220 586 A1.

[0076] The individual first facets 21 are imaged into the object field 5 by means of a second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

[0077] In yet another embodiment (not shown) of the illumination optical unit 4, a transfer optical unit, which notably contributes to the imaging of the first facet 21 into the object field 5, can be arranged in the beam path between the second facet mirror 22 and the object field 5. The transfer optical unit can comprise exactly one mirror or two or more mirrors arranged in succession in the beam path of the illumination optical unit 4. In particular, the transfer optical unit can comprise one or two normal incidence mirrors (NI mirrors) and / or one or two grazing incidence mirrors (GI mirrors).

[0078] In the embodiment shown in FIG. 1, the illumination optical unit 4 comprises exactly three mirrors downstream of the collector 17 , in particular a deflection mirror 19 , a first facet mirror 20 and a second facet mirror 22 .

[0079] In yet another embodiment of the illumination optical unit 4, the deflection mirror 19 is not required, so that the illumination optical unit 4 can then have exactly two mirrors downstream of the collector 17, in particular a first facet mirror 20 and a second facet mirror 22.

[0080] The imaging of the first facet 21 onto the object plane 6 by the second facet 23 or by means of the second facet 23 and the transfer optical unit is usually only an approximate imaging.

[0081] The projection optical unit 10 comprises a number of mirrors Mi, which are numbered according to their location in the beam path of the projection exposure apparatus 1 .

[0082] In the example shown in Fig. 1, the projection optical unit 10 comprises six mirrors M1-M6. A substitution of 4, 8, 10, 12 or any other number of mirrors Mi is equally possible. The projection optical unit 10 is a double-shielded optical unit. The penultimate mirror M5 and the final mirror M6 each have a through aperture through which radiation contributing to the exposure passes on its way from the reticle 7 to the wafer 13 during exposure of the wafer 13. The projection optical unit 10 has an image-side numerical aperture which is greater than 0.5, may be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0083] The reflective surface of the mirror Mi can be in the form of a freeform surface without an axis of rotational symmetry. Alternatively, the reflective surface of the mirror Mi can be designed as an aspheric surface with exactly one axis of rotational symmetry of the reflective surface shape. Like the mirrors of the illumination optical unit 4, the mirror Mi can have a coating that is highly reflective with respect to the illumination radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0084] The projection optical unit 10 has a large object-image offset in the y direction between the y coordinate of the center of the object field 5 and the y coordinate of the center of the image field 11. Said object-image offset in the y direction may be approximately the same size as the z distance between the object plane 6 and the image plane 12.

[0085] The projection optical unit 10 can have, in particular, an anamorphic form. In particular, it has different imaging scales βx, βy in the x-direction and the y-direction. The two imaging scales βx, βy of the projection optical unit 10 are preferably (βx, βy)=(+ / -0.25, + / -0.125). A positive imaging scale β means imaging without image inversion. A negative sign of the imaging scale β means imaging with image inversion.

[0086] The projection optical unit 10 results in a size reduction in the x-direction, ie perpendicular to the scanning direction, by a ratio of 4:1.

[0087] The projection optical unit 10 provides a size reduction of 8:1 in the y-direction, ie the scan direction.

[0088] Other imaging scales are possible as well. Imaging scales of the same sign and the same absolute value in the x and y directions, for example 0.125 or 0.25, are also possible.

[0089] The number of intermediate image planes in the x- and y-directions in the beam path between the object field 5 and the image field 11 may be the same or different depending on the embodiment of the projection optical unit 10. Examples of projection optical units 10 with different numbers of such intermediate images in the x- and y-directions are known from US 2018 / 0074303 A1.

[0090] Each of the second facets 23 is assigned to exactly one of the first facets 21 to respectively form an illumination channel for illuminating the object field 5. This makes it possible to obtain an illumination in particular according to the Köhler principle. The far field is decomposed by means of the first facets 21 into a number of object fields 5. The first facets 21 generate a number of images of intermediate foci on the respectively assigned second facets 23.

[0091] By means of the assigned second facets 23, the first facets 21 are respectively imaged onto the reticle 7 in an overlapping manner in order to illuminate the object field 5. The illumination of the object field 5 is in particular as uniform as possible. Its uniformity error is preferably less than 2%. By overlapping the different illumination channels, field uniformity can be obtained.

[0092] The illumination of the entrance pupil of the projection optical unit 10 can be geometrically defined by the arrangement of the second facets 23. By selecting the illumination channels that guide light, in particular a subset of the second facets 23, the intensity distribution in the entrance pupil of the projection optical unit 10 can be set. This intensity distribution is also called the illumination setting or illumination pupil filling.

[0093] An equally favorable pupil uniformity in the region of a defined illumination portion of the illumination pupil of the illumination optical unit 4 can be achieved by redistribution of the illumination channels.

[0094] Further aspects and details of the illumination of the object field 5, in particular the entrance pupil of the projection optical unit 10, are explained below.

[0095] The projection optical unit 10 may in particular have a concentric entrance pupil. It may be accessible. It may also be inaccessible.

[0096] The entrance pupil of the projection optical unit 10 cannot often be illuminated exactly by the second facet mirror 22. When the projection optical unit 10 images the center of the second facet mirror 22 telecentrically onto the wafer 13, the aperture rays often do not intersect at a single point. However, it is possible to find a surface for which the determined separation of the pair of aperture rays is minimal. This surface represents the entrance pupil or a real space surface conjugate thereto. In particular, this surface has a finite curvature.

[0097] The projection optical unit 10 may have different entrance pupil positions for the tangential and sagittal beam paths. In this case, an imaging element, in particular an optical component of the transfer optical unit, should be arranged between the second facet mirror 22 and the reticle 7. This optical element can be used to take into account the different positions of the tangential and sagittal entrance pupils.

[0098] In the arrangement of the components of the illumination optical unit 4 shown in Fig. 1, the second facet mirror 22 is arranged in a plane conjugate with the entrance pupil of the projection optical unit 10. The first facet mirror 20 is arranged so as to be tilted with respect to the object plane 6. The first facet mirror 20 is arranged so as to be tilted with respect to the arrangement plane defined by the deflection mirror 19.

[0099] The first facet mirror 20 is disposed so as to be inclined with respect to a placement plane defined by the second facet mirror 22 .

[0100] 2 shows in a schematic diagram an exemplary embodiment of a projection exposure apparatus 1 for DUV projection lithography, where DUV stands for "deep ultraviolet". In particular, the projection exposure apparatus can be designed for operation at a wavelength of 193 nm.

[0101] The projection exposure apparatus 1 comprises an illumination optical unit 4 and a projection optical unit 10. The internal structure of the illumination optical unit 4 and of the projection optical unit 10, in each case including for example optical components, sensors, manipulators etc., are not shown in detail. In the case of the projection optical unit 10, a mirror M is shown as representative of the optical components. The mirror M can be cooled by means of a cooling medium provided by a cooling device 24. The cooling medium is a fluid, for example water. Additionally or alternatively, the illumination optical unit 4 can comprise one cooled mirror M and one associated cooling device 24. The projection optical unit 10 and / or the illumination optical unit 4 can also comprise several cooled mirrors M and cooling devices 24. In the case of the illumination optical unit 4 and in the case of the projection optical unit 10, lenses and further mirrors, which may or may not be cooled, may be present as further optical components.

[0102] Likewise, the exemplary embodiment of the projection exposure apparatus 1 shown in FIG. 1 may also be provided with at least one cooling device 24, which may for example be connected to the mirror M3.

[0103] The radiation necessary for the operation of the projection exposure apparatus 1 is produced by a radiation source 3. The radiation source 3 can in particular be an excimer laser, for example an argon fluoride laser, which produces illumination radiation 16 of a wavelength of 193 nm.

[0104] Between the illumination optical unit 2 and the projection optical unit 10 a reticle holder 8 is arranged, on which a reticle 7 is fixed. The reticle holder has a reticle displacement drive 9. Downstream of the projection optical unit 10 in the radial direction a wafer holder 14 is arranged, which supports a wafer 13 or some other substrate and has a wafer displacement drive 15.

[0105] Figure 2 further shows a control device 25, which is connected to the illumination optical unit 4, the projection optical unit 10, the cooling device 24, the radiation source 3, the reticle holder 8 or reticle displacement drive 9, and the wafer holder 14 or wafer displacement drive 15. Similarly, the projection exposure apparatus of Figure 1 also comprises a control device 25, which can be connected to corresponding components.

[0106] The projection exposure apparatus 1 serves the purpose of imaging a reticle 7 with high precision onto a wafer 13. For this purpose, the reticle 7 is illuminated by means of an illumination optical unit 4 and the illuminated reticle 7 is imaged onto the wafer 13 by means of a projection optical unit 10. In particular, the following procedure is adopted:

[0107] The illumination optical unit 4 converts the illumination radiation 16 generated by the radiation source 3 by means of its optical components in a precisely defined manner and directs it to the reticle 7. Depending on the embodiment, the illumination optical unit 4 can be formed to illuminate the entire reticle 7 or only partial areas of the reticle 7. The illumination optical unit 4 is able to illuminate the reticle 7 in such a way that the illumination conditions are substantially identical at each illumination point of the reticle 7. In particular, the intensity and the angular distribution of the incident illumination radiation 16 are substantially identical for each illumination point of the reticle 7.

[0108] The illumination optical unit 4 can illuminate the reticle 7 with illumination radiation 16 possibly in several different angular distributions. These angular distributions of illumination radiation are also called illumination settings. The desired illumination settings are generally selected depending on the structure elements formed on the reticle 7. For example, dipole or quadrupole illumination settings are more often used, in which case the illumination radiation 16 is incident on each illumination point of the reticle 7 from two or four different directions, respectively. Depending on the form of the illumination optical unit 4, different illumination settings can be produced, for example, by different diffractive optical elements in combination with a zoom axicon optical unit or by a mirror array, which each has several small mirrors arranged next to one another and whose angular position can be set individually.

[0109] The reticle 7 can for example be formed as a glass plate, which is transparent to the illumination radiation 16 provided by the illumination optical unit 4 and which is provided with an opaque structure, for example in the form of a chrome coating.

[0110] The projection exposure apparatus 1 can be made such that the entire reticle 7 is illuminated simultaneously by the illumination optical unit 4 and completely imaged onto the wafer 13 by the projection optical unit 10 in a single exposure step.

[0111] Alternatively, the projection exposure apparatus 1 can also be configured in such a way that only partial areas of the reticle 7 are illuminated simultaneously by the illumination optical unit 4 and that, by control of the reticle displacement drive 9 by the control device 25, the reticle 7 is moved relative to the illumination optical unit 4 during exposure of the wafer 13, so that the illuminated partial areas are moved over the reticle 7 as a result. By appropriately adapted control of the wafer displacement drive 15, which also takes into account the imaging properties of the projection optical unit 10, the wafer 13 is moved synchronously so that the respectively illuminated partial areas of the reticle 7 are imaged onto the corresponding partial areas of the wafer 13. This movement of the reticle and the wafer 13 is also referred to as scanning.

[0112] In order to be able to transfer a latent image produced by exposure of the wafer 13 in both embodiments of the projection exposure apparatus 1 into a physical structure, a photosensitive layer is applied to the wafer 13. The image of the reticle 7 is formed in this photosensitive layer by exposure and a permanent structure can be produced therefrom in the wafer 13 using subsequent chemical processes.

[0113] The reticle 7 is generally not imaged once but several times side by side on the wafer 13. For this purpose, for each imaging of the reticle 7 on the wafer 13, the wafer holder 14 is displaced laterally, which corresponds to the size of the image of the reticle 7 on the wafer 13. The imaging of the reticle 7 can here be performed in each case as a whole or sequentially by scanning. Chemical treatment of the wafer 13 only begins after the desired number of imaging of the reticle 7 on the wafer 13 has been performed.

[0114] FIG. 3 shows a schematic cross-sectional view of an exemplary embodiment of a mirror 26 according to the present invention.

[0115] The mirror 26 can be used in one of the projection exposure apparatus 1 shown in Figures 1 and 2 and has a lower part 27 and an upper part 28. The choice of the terms "lower part" and "upper part" is due to the fact that the lower part 27 is generally made much thicker than the upper part 28 and therefore, so to speak, supports the upper part 28. However, these terms have no relation to the orientation of the mirror 26 with respect to the direction of gravity in its mounted state. During operation of the projection exposure apparatus 1, the upper part 28 may be arranged above or below or to the side of the lower part 27 with respect to the direction of gravity or may take any other relative position with respect to this. The lower part 27 is also referred to as the first mirror part. The upper part 28 is also referred to as the second mirror part.

[0116] The lower part 27 and the upper part 28 are rigidly connected to each other in the region of the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28. In the illustrated exemplary embodiment, the connecting surface 29 of the lower part 27 is formed concavely. Its curvature can be formed according to a spherical, aspherical or free-form surface. The connecting surface 30 of the upper part 28 has a convex curvature that is complementarily curved with the connecting surface 29 of the lower part 27 and can therefore be formed according to a spherical, aspherical or free-form surface. The connecting surface 30 of the upper part 28 and the connecting surface 29 of the lower part 27 can therefore be in close contact with each other. Instead of being formed curved, the connecting surface 30 of the upper part 28 can also be formed planar.

[0117] The lower part 27 is formed planarly on the side opposite its connecting surface 29, shown in the lower part of Fig. 3. The upper part 28 is formed concavely on the side opposite its connecting surface 30, shown in the upper part of Fig. 3, and has a reflecting optical surface 31 with the same curvature, which can be formed according to a spherical, aspherical or freeform surface and in particular corresponds to and runs parallel to the curvature of the connecting surface 30 of the upper part 28. As an alternative to this, the optical surface 31 can also be formed planar. This applies in particular if the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28 are formed planar.

[0118] The optical surface 31 is embodied in particular as a coating applied to the upper part 28. The formation of the coating depends on the wavelengths for which the optical surface 31 is intended to provide its reflective effect. If reflection in the DUV range is desired, i.e. in the case of the mirror M of FIG. 2, the coating is generally formed as an aluminum layer, which is dielectrically reinforced and protected from oxidation by the coating. On the other hand, if reflection in the EUV range is intended, such as in the case of the mirror M3 of FIG. 1, the coating may in particular be formed from alternating successive layers of silicon and molybdenum, and possibly one or more further layers of different composition, which act for example as protective layers.

[0119] The lower part 27 has a number of elongated cooling channels 32 which run parallel to one another and to the connection surface 29 of the lower part 27 and extend laterally in the area of ​​the optical surface 31 and possibly somewhat beyond it. The cooling channels 32 in the case of the illustrated exemplary embodiment are therefore formed in a curved shape. The cooling channels 32 are formed so as to open towards the connection surface 29 of the lower part 27. The transverse dimension of the cooling channels 32 may be approximately 0.2 mm to 10 mm, the depth of the cooling channels 32, i.e. the dimension approximately perpendicular to the connection surface 30 of the upper part 28 in the exemplary embodiment of FIG. 3, being approximately the same size as the width, i.e. the dimension approximately parallel to the connection surface 30 of the upper part 28. However, it is also possible that the depth of the cooling channels 32 is substantially greater than its width. For example, the depth of the cooling channels 32 may be more than twice the width of the cooling channels 32.

[0120] By way of example, the cooling channel 32 may be connected to the cooling device 24 shown in FIG. 2 by means of a further channel, not visible in FIG. 3, which can be used to generate a flow of fluid through the cooling channel 32 and thus remove heat from the mirror 26. The heat input to the mirror 26 can be realized, for example, by radiation reflected by the optical surface 31 during operation of the projection exposure apparatus 1. Since the optical surface 31 does not completely reflect the incident radiation, part of the radiation is absorbed by the optical surface 31 and, depending on the configuration of the optical surface 31, is also absorbed by the upper part 28 and converted into heat. Since the optical surface 31 and the upper part 28 have a certain thermal conductivity, part of this heat can be led to the cooling channel 32 where it can be taken up by the fluid and carried away. In this way, the temperature rise of the mirror 26 caused by the radiation can be limited and deformations of the optical surface 31 caused by thermal expansion effects can be reduced compared to an uncooled mirror 26. As a result, the image aberrations caused by the deformations are also reduced.

[0121] In order to keep the adverse effects of temperature fluctuations, which occur despite the cooling measures, as small as possible, materials with a very low coefficient of thermal expansion are used to manufacture the lower part 27 and the upper part 28. Suitable materials are, for example, quartz glass, titanium-doped quartz glass or special glass ceramics.

[0122] By way of example, the material may have, averaged over its volume, a zero cross temperature of 22°C to 25°C, where the thermal expansion of the material is minimal and in the ideal case equal to zero. Here, the average zero cross temperature is adjusted to match the desired temperature during operation of the projection exposure apparatus 1, taking into account the cooling of the mirror 26. Depending on the method of manufacturing the material, the increase in the thermal expansion coefficient at 22°C can be around 1.35 ppb / K. 2 ~1.8ppb / K 2 Preferably, the zero cross temperature uniformity is better than ±5 K in the lateral region of the optical surface 31, i.e. the zero cross temperature varies spatially in this region by less than ±5 K. It is also possible to make the average zero cross temperature significantly higher, especially if local heating of the mirror is provided in addition to cooling.

[0123] The lower part 27 and the upper part 28 may have a similar titanium content and / or a similar OH content in a first volume region extending in the lower part 27 to a distance of 10 mm from the first connecting surface 29 and in a second volume region extending in the upper part 28 to a distance of 10 mm from the second connecting surface 30. In particular, the titanium dioxide content of the second volume region of the upper part 28 may deviate from the titanium dioxide content of the first volume region by less than 0.04% by weight, preferably less than 0.02% by weight, particularly preferably less than 0.01% by weight. Furthermore, the OH content of the second volume region of the upper part 28 may deviate from the OH content of the first volume region by less than 5% by weight, preferably less than 2% by weight, particularly preferably less than 1% by weight of this OH content of the first volume region. In the case of the OH content, the percentage specifications for the deviation relate to the OH content as a relative value. The average values ​​in the respective volume regions can be used as values ​​for the titanium dioxide content and the OH content. If the upper part 28 is less than 10 mm thick, the entire upper part 28 can be used as the second volume.

[0124] The lower part 27 has a first average value of the zero cross temperature in a first volumetric region, and the upper part 28 has a second average value of the zero cross temperature in a second volumetric region, the first average value of the zero cross temperature may deviate from the second average value of the zero cross temperature by less than 1 K, in particular by less than 0.5 K, particularly preferably by less than 0.1 K.

[0125] If the upper part 28 is less than 10 mm thick, the entire upper part 28 can be used as the second volume.

[0126] Outside the first volumetric region, the lower part 27 may have a third average value of zero cross temperatures that deviates more from the second average value of zero cross temperatures than the first average value of zero cross temperatures, in particular the third average value of zero cross temperatures may deviate from the second average value of zero cross temperatures by more than 0.1 K, preferably by more than 1 K, particularly preferably by more than 3 K from the first average value of zero cross temperatures.

[0127] It is also possible that the upper part 28 has a deviation of -0.5 K to +3 K from the zero cross temperature of the lower part 27, averaged over the lateral region of the optical surface 31. In the case of a mirror 26 which is relatively close to the light source or in the case of an actively heated mirror 26, this deviation can be up to +5 K. The deviation is preferably between -0.5 K and +1.5 K. The asymmetric tolerance deviation with respect to the cold and hot side is due to the fact that the upper part 28 generally has a higher temperature than the lower part 27 during operation of the projection exposure apparatus 1 and that the thermal expansion is minimal around the zero cross temperature.

[0128] The mirror 26 has failure zones 33 both in the area of ​​the lower part 27 and in the area of ​​the upper part 28, in which at least one material parameter deviates from a specified value by more than a minimum deviation. By way of example, the specified value can be an average value of the material parameter, generated by averaging over the entire volume or a part of the volume of the lower part 27 and the upper part 28, respectively. The material parameter can be a specification on the material composition, for example the titanium content or the OH content, or a specification on the material properties, for example the zero cross temperature or the gradient of the thermal expansion coefficient. The failure zones 33 can appear in different degrees, i.e. the magnitude of the deviation from the specified value can differ.

[0129] The failure zones 33 have a strip-like form in the exemplary embodiment of FIG. 3, i.e. extend with a substantially constant cross section towards the plane of the drawing. The failure zones 33 extend substantially parallel to one another and are slightly inclined to the vertical in the illustration of FIG. 3, and therefore do not extend perpendicularly to the side opposite the connecting surface 29 of the lower part 27. The illustrated exemplary embodiment is further characterized in that the failure zones 33 continue from the lower part 27 to the upper part 28 without a lateral offset. There may also be a lateral offset of the failure zones 33 at the transition from the lower part 27 to the upper part 28. In this case, the lateral offset of the failure zones 33 of 50% or more, preferably 80% or more, may be less than 50%, preferably less than 30%, particularly preferably less than 10% of the dimension of each failure zone 33 in the direction of the offset. In this case, the direction of the offset can be considered for each failure zone 33 individually as the direction in which the lateral offset between the lower part 27 and the upper part 28 is maximum for each failure zone 33.

[0130] The failure zones 33 may have different spatial formations. In general, however, it is advantageous if the failure zones 33 of the lower part 27 adjoin the failure zones 33 of the upper part 28 over more than 50% of the total area that they occupy at the location of the connection surface 29. Alternatively, there is the option of providing only a small overlap of the failure zones 33 in the region of the connection surface 29 of the lower part 27 and / or the connection surface 30 of the upper part 28, thus providing a large lateral offset at the transition from the lower part 27 to the upper part 28. In this way, for example, it is possible to try to compensate to some extent for the influence of the failure zones 33.

[0131] To manufacture the mirror 26, the lower part 27 and the upper part 28 are manufactured as separate parts and then connected to each other, for example by a thermal bonding process. The optical surface 31 is generally formed only after the connection of the lower part 27 and the upper part 28, in order to achieve the greatest possible precision, especially with regard to its shape, and to avoid damage to the optical surface 31 during the connection process.

[0132] Several aspects of the fabrication of lower and upper pieces 27, 28 and their combination to form mirror 26 are described below.

[0133] 4 shows a schematic cross-section of a material blank 34 for the manufacture of a mirror 26 according to a first configuration of the method according to the invention. The contours of the lower part 27 and the upper part 28 are also shown in FIG.

[0134] For example, the material blank 34 may consist of titanium-doped quartz glass, in particular quartz glass to which titanium dioxide is added. Likewise, the material blank 34 may also contain ternary compounds containing further metals in addition to silicon and titanium. Furthermore, doping with fluorine is possible. Material blanks 34 made of titanium-doped quartz glass may differ from batch to batch, for example in terms of titanium content, OH content, zero cross temperature, etc. Furthermore, there may also be corresponding variations at different locations within the same material blank 34, and thus non-negligible deviations from the specified values ​​at this point. By way of example, the specified values ​​may be average values ​​of the respective material parameters, which are generated by averaging over the entire volume or a part of the volume of the material blank 34. By way of example, failure zones 33 may be formed in the material blank 34, in which the deviations from the specified values ​​exceed a minimum value.

[0135] To limit the adverse effects of batch variations, the lower part 27 and the upper part 28 of the mirror 26 may be separated from the same material blank 34. By taking the failure zone 33 into account during the manufacture of the mirror 26 from the material blank 34, the adverse effects of the failure zone 33 may be limited.

[0136] Depending, for example, on the manufacturing method and geometric shape of the material blank 34, several steps occur for the separation of the lower part 27 and the upper part 28 from the material blank 34 and the assembly of the mirror 26 from the lower part 27 and the upper part 28, which are described in detail below.

[0137] The material blank 34 shown in FIG. 4 was produced by the direct deposition method. With respect to its mass, the material blank 34 has an OH content of more than 600 ppm. The failure zone 33, which is caused by a local deviation of the concentrations of titanium and OH from the specified values, can be formed as a three-dimensional band pattern consisting of a plurality of bands running parallel to one another, as already described with respect to FIG. 3, with corresponding deviations of the gradients of the thermal expansion coefficient and the zero cross temperature from the corresponding specified values. As in FIG. 3, the longitudinal extent of the band in FIG. 4 runs approximately perpendicular to the plane of the figure, so that FIG. 4 shows a cross section of the band-like failure zone 33 running across its longitudinal extent. As in FIG. 3, the band-like failure zone 33 is also inclined with respect to the vertical line in FIG. 4 and therefore does not run parallel or perpendicular to the outer surface of the material blank 34.

[0138] With the material blank 34 thus embodied, one possible procedure consists in separating the lower part 27 and the upper part 28 from the material blank 34 with the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28 facing each other, minimizing the distance between them. A possible separation surface 35 in this respect is shown in Figure 4. The separation surface 35 has a planar form and extends in a straight line in the region of the shown cutting surface. This makes it possible to use a separating tool of relatively simple design.

[0139] After separation, the lower part 27 and the upper part 28 are brought to the desired shape, for example by milling, grinding and polishing. For this reason, when cutting out the lower part 27 and the upper part 28, in addition to the additional material amount for the separation procedure, an additional additional material amount for the subsequent processing must be provided. For the production of the lower part 27, therefore, a corresponding additional material amount for the separation procedure and an additional material amount for the processing of the lower part 27 must be provided as additional material amounts in the region of the connection surface 29. For the production of the upper part 28, a corresponding additional material amount for the separation procedure and an additional material amount for the processing of the upper part 28 must be provided as additional material amounts in the region of the connection surface 30. The minimum distance between the volumetric areas of the material blank 34 from which the lower part 27 and the upper part 28 are manufactured therefore corresponds to the sum of the additional amount of material for the manufacture of the lower part 27 and the additional amount of material for the manufacture of the upper part 28, or in other words the sum of the additional amount of material for the separation of the lower part 27 and the upper part 28, the additional amount of material for the subsequent processing of the lower part 27 and the additional amount of material for the subsequent processing of the upper part 28.

[0140] The connecting surface 29 of the lower part 27 may in particular be shaped to generally correspond to the intended shape of the optical surface 31 that will later be formed on the upper part 28 .

[0141] The cutting out at a minimum distance results in the failure zone 33 in the region of the connecting surface 29 of the lower part 27 and in the region of the connecting surface 30 of the upper part 28 having a very similar morphology. Furthermore, the lateral offset of the failure zone 33 between the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28 caused by the inclination of the failure zone 33 with respect to the vertical associated with the material ablation accompanying the processing of the lower part 27 and the upper part 28 is relatively small. As a result, it is possible to interconnect the lower part 27 and the upper part 28 in the region of the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28, such that the failure zone 33 continues practically unchanged in the mirror 26 from the lower part 27 to the upper part 28.

[0142] To achieve this, the lower part 27 and the upper part 28 are connected to each other in a substantially correct positional orientation, i.e. in substantially the same relative position and orientation as in the material blank 34. By way of example, the connection can be made by thermal gluing. In this case, the connection surfaces 29, 30 can be directly interconnected in the blank state or the connection surfaces 29, 30 can be coated with additives. However, additives that remain permanently on the connection surfaces 29, 30 are avoided if possible, since they may have different thermal expansion coefficients. In some cases, the lower part 27 and the upper part 28 can be interconnected by optical contact bonding before gluing.

[0143] To allow for separation and subsequent material ablation surface processing along the planar separation surface 35, the lower and upper parts 27, 28 contact each other in the finished mirror 26 and are aligned relative to each other in the material blank 34, resulting in a slight misalignment of their relative positions in the material blank 34. As mentioned above, this misalignment leads to a slight lateral offset of the failure zone 33 at the transition from the lower part 27 to the upper part 28, as well as a tilt of the failure zone 33 shown in FIG.

[0144] The relative orientation of the lower part 27 and the upper part 28 in the material blank 34 with respect to one another can be maintained very accurately when the lower part 27 and the upper part 28 are connected. By way of example, this can be ensured by the use of an auxiliary frame or by the formation of one or more markings on the material blank 34. The markings can be formed in the areas which are separated from the material blank 34 for the production of the lower part 27 and / or in the areas which are separated from the material blank 34 for the production of the upper part 28. By way of example, the markings can be formed as perforations, notches or grooves.

[0145] By means of the markings, the lower part 27 and the upper part 28 can also be positioned with high precision relative to one another during connection, i.e. not only the angular coordinates but also the spatial coordinates can be maintained. However, limitations arise as to the reproduction of the relative positioning in the material blank 34, on the one hand as a result of the lower part 27 and the upper part 28 being in contact in the area of ​​the contact surfaces 29, 30 and being interconnected without a gap, and on the other hand as a result of the material being ablated in the area of ​​the contact surfaces 29, 30 during its processing, i.e. there must be a gap between the connection surfaces 29, 30 in the material blank 34. With the configuration of the method according to the invention, which will be described in more detail below, it is possible to reduce the material losses and thus the necessary gaps in the material blank 34 and the associated limitations.

[0146] Deviating from the illustration in FIG. 4, the material blank 34 can also be made substantially larger than is necessary for the production of the lower part 27 and the upper part 28. In particular, the material blank 34 can be so large that several lower parts 27 and / or several upper parts 28 can be produced therefrom. In that case, more parting surfaces 35 are needed to cut out the individual lower parts 27 and / or upper parts 28 accordingly. Even in the case of a material blank 34 used for the production of only one lower part 27 and one upper part 28, it is possible to provide further parting surfaces 35 for separating off excess material, in particular if the material blank 34 is significantly larger than is necessary for the production of the lower part 27 and the upper part 28.

[0147] Figure 5 shows a schematic cross-section of a material blank 34 for the manufacture of a mirror 26 according to yet another configuration of the method according to the invention. As with Figure 4, in the configuration shown in Figure 5 the lower part 27 and the upper part 28 are separated from the material blank 34 along a separation surface 35 and are subsequently brought to the desired shape by several processing steps. The remaining explanations for Figure 4 apply analogously to Figure 5. However, there are differences with regard to the procedure in cutting out the lower part 27 and the upper part 28 from the material blank 34 and with regard to the formation of the separation surface 35.

[0148] In FIG. 5, the separation surface 35 has a curved rather than planar configuration in the region between the connecting surface 29 of the lower part 27 and the connecting surface 30 of the upper part 28 .

[0149] In particular, the separation surface 35 may have a spherical form in this region. The curvature of the separation surface 35 corresponds approximately to the curvature of the connection surfaces 29, 30 of the lower part 27 and the upper part 28. In this way, the additional amount of material required for processing both the connection surface 29 of the lower part 27 and the connection surface 30 of the upper part 28 can be obtained, even if the distance between the lower part 27 and the upper part 28 in the material blank 34 is smaller than in the configuration of FIG. 4. In other words, by separating the lower part 27 and the upper part 28 from the material blank 34 along the curved separation surface 35, it is possible to reduce the material loss between the connection surfaces 29, 30 and therefore the required distance between the lower part 27 and the upper part 28 in the material blank 34. Thus, the misalignment during the connection of the lower part 27 and the upper part 28 is also reduced compared to the situation in the material blank 34. This results in a much smaller change in the continuation of the failure zone 33 from the lower part 27 to the upper part 28 in the finished mirror 26 than in the configuration shown in FIG. 4. With the canted arrangement of the failure zone 33, this also results in a smaller lateral offset of the failure zone 33 at the transition from the lower component 27 to the upper component 28 than in the configuration shown in FIG.

[0150] The separation of the lower part 27 and the upper part 28 from the material blank 34 along the curved separation surface 35 can be carried out by means of a process also called separating ball grinding. During this process, a rotary grinding tool embodied as a curved rotating surface is gradually inserted into the material blank 34 rotating about a vertical axis in FIG. 5 and finally cuts through it. The lower part 27 and the upper part 28 thus separated from the material blank 34 each have a curved surface in the area of ​​processing with the rotary grinding tool. Separating ball grinding is described in detail in DE 102 333 777 A1.

[0151] The two above-mentioned configurations of the method according to the invention are primarily based on the separation of the lower part 27 and the upper part 28 from the material blank 34 in volumetric regions that are as close as possible. Yet another possible procedure consists in separating the lower part 27 and the upper part 28 from regions of the material blank 34 that are subject to similar conditions and in particular have a similar spatial distribution of the defect zones 33. This is explained in more detail below on the basis of FIG.

[0152] Figure 6 shows a schematic cross-section of a material blank 34 for the manufacture of a mirror 26 according to yet another configuration of the method according to the invention. As in Figure 4, in the configuration shown in Figure 6, the lower part 27 and the upper part 28 are separated from the material blank 34 along a separation plane 35, shown using a dashed line, and are subsequently brought to the desired shape by several processing steps. The remaining explanations regarding Figure 4 apply analogously to Figure 6. However, there are differences regarding the procedure during cutting of the lower part 27 and the upper part 28 from the material blank 34 and the subsequent processing of the upper part 28.

[0153] In the configuration of Fig. 6, the contour of the upper part 28 is laterally offset relative to the contour of the lower part 27, as a result of which the connection surface 30 of the upper part 28 is not vertically coplanar with the connection surface 29 of the lower part 27. To illustrate this, a vertical auxiliary line is shown in Fig. 6 (using a dotted line). Furthermore, the direction of the lateral offset is indicated by an arrow. The absolute value of the lateral offset of the upper part 28 relative to the lower part 27 corresponds approximately to the absolute value of the lateral offset of the failure zone 33 between the connection surface 29 of the lower part 27 and the connection surface 30 of the upper part 28, which is caused by the inclination of the failure zone 33.

[0154] This makes it possible to connect the upper part 28 to the lower part 27 without or only with a small lateral offset of the failure zone 33, despite the additional amount of material required for processing. For this purpose, the lower part 27 and the upper part 28 are separated from the material blank 34 along a separation plane 35 shown in FIG. 6. The side surface of the upper part 28, i.e. the surface which delimits the upper part 28 to the left and right in the representation in FIG. 6, is then milled and / or ground in such a way that if the lower part 27 and the upper part 28 are connected in such a way that they are in the same plane with respect to the connection surfaces 29, 30, there will be no or only a small offset of the failure zone 33 between the lower part 27 and the upper part 28 in the region of the connection surfaces 29, 30. For this purpose, different amounts of material ablation are performed at different points of the side surface of the upper part 28 in order to obtain a lateral displacement of the failure zone 33 relative to the outer contour of the upper part 28. Since the inclination of the failure zone 33 is generally of uniaxial nature, the optimum lateral displacement of the failure zone 33 can be determined from the angle of this inclination and the sum of the amount of material ablation on the connecting surfaces 29, 30, which corresponds to the distance between the connecting surfaces 29 and 30 in the material blank 34. If there is a significant material loss during the separation of the lower part 27 and the upper part 28 from the material blank 34 along the separation surface 35, this material loss can be proportionally added to the amount of material ablation on the connecting surfaces 29 and 30, respectively, in order to slightly increase the accuracy in determining the lateral displacement.

[0155] Alternatively, there is the possibility of determining the lateral distribution of the failure zones 33 in the area of ​​the connecting surfaces 29, 30 and determining the optimum lateral displacement by autocorrelation.

[0156] Likewise, the spatial formation of the failure zone 33 in the material blank 34 can be used to simulate the absolute value and direction of the lateral displacement, which results in the minimum image aberration during operation of the projection exposure apparatus 1 under given operating conditions, and the upper part 28 can be processed accordingly. However, the displacement determined in this way does not necessarily result in the minimum lateral offset of the failure zone 33 between the connecting surfaces 29, 30. In the simulation, the twisting of the upper part 28 relative to the lower part 27 around an axis vertical in the illustration in FIG. 6 can additionally be used as a further variable. In some cases, when determining the appropriate lateral displacement, it is also possible to include not only the image aberration, but also local stresses resulting from the lateral offset of the failure zone 33 in the region of the connecting surfaces 29, 30, since local stresses can have a negative effect on the durability of the connection between the lower part 27 and the upper part 28.

[0157] After processing of the side surface of the upper part 28, further processing of the lower part 27 and the upper part 28 and the connection of the lower part 27 and the upper part 28 can be carried out in a similar manner to the configuration of the method according to the invention already described above.

[0158] The lateral offset of the failure zone 33 can also be at least partially compensated for by milling or grinding the side surface of the upper part 28 in the configuration shown in FIG. 5, in which the lower part 27 and the upper part 28 are separated from the material blank 34 by separate ball grinding.

[0159] In addition to or as an alternative to the failure zones 33 described so far, the material blank 34 may have further failure zones 33 which must be taken into account when separating the lower part 27 and the upper part 28 from the material blank 34. This is explained with the aid of FIG.

[0160] FIG. 7 shows a schematic cross-sectional view of a material blank 34 for the manufacture of a mirror 26 according to yet another configuration of the method according to the invention. The material blank 34 was manufactured by direct deposition similarly to the configuration shown in FIG. 4. As a result of the layer-by-layer deposition of material during the manufacturing process, there may be periodic variations in the titanium content and the OH content in the direction extending vertically in FIG. 7. As a result, layer-like failure zones 33 of about 2 layers / mm to 30 layers / mm arise. These failure zones 33 may arise in addition to the failure zones 33 shown in FIG. 4. The following description focuses on the layer-like failure zones 33.

[0161] For the sake of clarity, only some of the failure zones 33 are shown in Fig. 7. It is clear from Fig. 7 that the failure zones 33 are not formed flat but as curved layers. If the lower part 27 and the upper part 28 are separated from the material blank 34 parallel to a horizontal plane, many of the failure zones 33 will intersect with the area of ​​the upper part 28 where the optical surface 31 is formed. Firstly, this area is processed by grinding and polishing, and secondly, the failure zones 33 have a slightly different material composition and therefore also a different hardness, so that an uneven distribution corresponding to the layer pattern is formed in this area as a result of the ablation rate varying with hardness.

[0162] To avoid this, the lower part 27 and the upper part 28 can be produced from a volumetric area of ​​the material blank 34 that has an orientation adapted to the formation of the failure zone 33, i.e. the contours of the lower part 27 and the upper part 28 shown in FIG. 7 are inclined with respect to the outer surface or axis of the material blank 34. The separation of the lower part 27 and the upper part 28 from the material blank 34 can therefore also be carried out along a similarly inclined separation surface 35. As shown in FIG. 7, this can be achieved, for example, by inclining the separation surface 35 with respect to the horizontal according to the local profile of the failure zone 33, so that the separation surface 35 runs approximately parallel to the failure zone 33. Alternatively, the profile of the failure zone 33 can also only be taken into account when separating the upper part 28 from the material blank 34, by having a wedge-shaped cutting gap between the lower part 27 and the upper part 28. In the illustration of FIG. 7, this means that the orientation of the upper part 28 remains unchanged, while the lower part 27 has a horizontal orientation. In any case, the upper part 28 is further processed in such a way that the face of the upper part 28 on which the optical surface 31 is subsequently formed runs approximately parallel to the defect zones 33 and intersects with this face with as few defect zones 33 as possible. In particular, the face of the upper part 28 on which the optical surface 33 is formed may have no more than 10, preferably no more than 5, particularly preferably no more than 2 defect zones 33. Otherwise, the processing of the lower part 27 and the upper part 28 can be carried out in the same way as already described above.

[0163] The lower part 27 and the upper part 28 can also be separated from a material blank 34 produced by another method other than direct deposition. By way of example, the material blank 34 can be produced by a soot process, in which a cylindrically shaped rod is rotated above an array of burners, and material from the gas phase is continuously deposited on the cylinder. As a result of slightly different deposition rates of material in the area of ​​the individual burners, defective zones 33 can be formed. Each defective zone 33 can extend over the entire cross section of the cylinder, run perpendicular to the cylinder axis, and several defective zones 33 can be continuous along the cylinder axis. The cylinders produced in this way can be reshaped into material blanks 34 by pouring the cylinders, heated until softened, into a mould. Two material blanks 34 produced in this way are shown in FIG. 8.

[0164] 8 shows a schematic cross-section of two material blanks 34 for the manufacture of a mirror 26 according to yet another configuration of the method according to the invention. In terms of mass, the material blanks 34 have an OH content of up to 400 ppm. The material blanks 34 were each manufactured by the soot process and reshaped by pouring into a mould. As a result of the reshaping process, curved failure zones 33 emerge from the planar failure zones 33 oriented perpendicular to the cylindrical axis, such that each material blank 34 has a number of continuous curved failure zones 33.

[0165] By way of example, if the lower part 27 and the upper part 28 are cut out along the same parting plane 35 from a material blank 34 shown on the left side of Figure 8, when the lower part 27 and the upper part 28 are assembled after processing to form the mirror 26, gaps will appear in the curved failure zone 33 due to material loss due to this subsequent processing of the lower part 27 and the upper part 28. Such gaps can be avoided or at least reduced by various measures.

[0166] One measure consists in separating the lower part 27 and the upper part 28 from different material blanks 34 with a similar embodiment in terms of the spatial distribution of the failure zones 33. In particular, similar forms of the failure zones 33 can be achieved by producing the material blanks 34 using the same production equipment and by producing the material blanks 34 in succession in time using the same process parameters. The two material blanks 34 shown in FIG. 8 are material blanks 34 of similar embodiment produced in this way. The two material blanks 34 therefore have similar or identically formed failure zones 33, the material blank 34 shown on the left side of FIG. 8 being used for producing the lower part 27 and the material blank 34 shown on the right side of FIG. 8 being used for producing the upper part 28. In particular, the following procedure is adopted:

[0167] The lower part 27 is separated from the material blank 34 shown on the left side of Fig. 8 along a separation plane 35. The lower part 27 is then processed as already described above until it has the contour shown on the left side of Fig. 8.

[0168] In the material blank 34 shown on the right side of FIG. 8, the contour of the lower part 27 is shown by means of a dotted line in a position that corresponds to the position of the contour of the lower part 27 in the material blank 34 shown on the left side of FIG. 8 with respect to the relative arrangement with respect to the failure zone 33. The orientation of the contour of the lower part 27 shown in the material blank 34 shown on the right side also corresponds to the orientation of the contour of the lower part 27 in the material blank 34 shown on the left side. The contour of the upper part 28 is likewise shown in the material blank 34 shown on the right side and is directly adjacent to the contour of the lower part 27 such that the connection surface 29 of the lower part 27 and the connection surface 30 of the upper part 28 are directly adjacent to each other. However, the lower part 27 is not separated from the material blank 34 shown on the right side. Instead, only the upper part 28 is separated from the material blank 34 shown on the right side, with the separation surface 35 offset towards the lower part 27 compared to the material blank 34 shown on the left side. The offset between the parting plane 35 for cutting out the lower part 27 and the parting plane 35 for cutting out the upper part 28 corresponds to the sum of the material additions of the lower part 27 and the upper part 28 in the region of the connecting surfaces 29, 30. Like the lower part 27, the upper part 28 is also processed after separation as already described above.

[0169] The lower part 27 and the upper part 28 are then assembled to form the mirror 26, which has no or only a small gap in the region of the connection surfaces 29, 30 in the defective zone 33. This is based on the fact that the offset separation of the lower part 27 and the upper part 28 from two similar material blanks 34 makes it possible to eliminate the material losses that occur during separation and processing, so that in the end the same result is obtained as if the lower part 27 and the upper part 28 had been produced without material losses from directly adjacent volumetric areas of the same material blank 34. In other words, if the volumetric area of ​​the lower part 27, shown in FIG. 8 by the outline of the lower part 27, is transferred from the material blank 34 shown on the left to the material blank 34 of a similar embodiment shown on the right, while maintaining its positional orientation with respect to the defective zone 33, this volumetric area will be directly adjacent to the volumetric area of ​​the upper part 28, shown by the outline of the upper part 28. As a result, the connection surface 29 of the fully processed lower part 27 and the connection surface 30 of the fully processed upper part 28 are positioned identically with respect to the spatial distribution of the failure zone 33. The failure zone 33 therefore continues seamlessly at the transition from the lower part 27 to the upper part 28 in the finished mirror 26, and there is no abrupt change between the connection surface 29 of the lower part 27 and the connection surface 30 of the upper part 28 with respect to the material parameters on which the failure zone 33 is based.

[0170] Therefore, the size of the gap in the failure zone 33 that occurs when joining the lower part 27 and the upper part 28 varies depending on the volume area of ​​the material blank 34 shown on the left side of Figure 8 from which the lower part 27 is manufactured and the volume area of ​​the material blank 34 shown on the right side of Figure 8 from which the upper part 28 is manufactured.

[0171] The procedure described with reference to FIG. 8 can also be applied to a single material blank 34, provided that the material blank 34 has sufficiently large spatial dimensions.

[0172] A further measure consists in reducing the material loss during cutting and machining of the lower part 27 and the upper part 28 to such an extent that comparatively small gaps arise. For this purpose, the lower part 27 and the upper part 28 can be separated from the material blank 34 along a curved separation surface 35, for example by the already mentioned separating ball grinding. In order to keep the material loss as small as possible, the curvature of the separation surface 35 can be adapted to the curvature of the connecting surfaces 29, 30.

[0173] Separation along the curved separation surface 35 involves an improvement over separation along a planar separation surface 35, however, upon assembly of the lower part 27 and the upper part 28 to form the mirror 26, gaps arise in the region of the connecting surfaces 29, 30 in defect zones 33 that were not present in the material blank 34.

[0174] As a further measure, it is also possible to characterize the material blank 34 and to arrange a parting surface 35 for cutting out the lower part 27 and the upper part 28, respectively, in the area of ​​the material blank 34 that is free of significant failure zones 33. This measure is also applicable in the case of a single material blank 34, if the material blank 34 has a sufficient size. Furthermore, this procedure can also be used with different forms of failure zones 33 and / or material blanks 34 of different manufacture. In particular, the volumetric areas of the lower part 27 and the upper part 28 of the material blank or blanks 34 can be selected such that the occurrence of failure zones 33 at the location of the first connecting surface 29 of the lower part 27 and at the location of the second connecting surface of the upper part 28, respectively, is below a limit value. In particular, the occurrence of failure zones is below a limit value whenever the proportion of the area of ​​the failure zones at the connecting surface 29 or the connecting surface 30 is below a threshold value. The threshold value can be defined as the arithmetic mean of the maximum and minimum values ​​of the area percentage of the failure zone 33 on the connecting surface 29 or the connecting surface 30 for different arrangements of the volumetric areas of the lower part 27 and the upper part 28 on one or more material blanks 34. In this case, it is also possible to determine the respective minimum and maximum values ​​for the area percentages of the lower part 27 and the upper part 28 separately and to set the respective threshold values ​​of the connecting surface 29 or the connecting surface 30 accordingly. Furthermore, it is possible to require that the interval between the minimum value of the area percentage of the failure zone 33 and the threshold value be below the threshold value by, for example, more than 30%, in particular more than 60%.

[0175] As a further measure, one may attempt to homogenize each material blank 34. The options and procedures in this regard vary depending on the material and manufacturing process.

[0176] In all of the described configurations of the method according to the invention, instead of maintaining the profile of the failure zone 33 of the material blank 34, it is also possible to intentionally deviate from the original profile of the failure zone 33 when connecting the lower part 27 and the upper part 28. By way of example, the position and orientation of the lower part 27 and the upper part 28 relative to one another during connection can be such that the lateral offset of the failure zone 33 at the transition from the lower part 27 to the upper part 28 is as large as possible. As a result, the failure zones 33 in the region of the connection surface 29 of the lower part 27 and the connection surface 30 of the upper part 28 can be arranged in a certain sense in antiphase with one another, and deviations in the material composition or material properties associated with the failure zone 33 can be partially compensated in the region of the connection surfaces 29, 30.

[0177] Instead of using the upper part 28 separated from the material blank 34, it is also possible to produce the upper part 28 by pouring onto molten metal (float glass), by pouring over long breakaway edges, by pressure sintering of glass soot or indirectly by grey body. With these methods it is possible to produce slabs with good homogeneity several millimeters thick. [Explanation of symbols]

[0178] 1. Projection exposure equipment 2. Lighting system 3 Radiation source 4. Illumination optical unit 5 Object field of view 6 Object plane 7 Reticle 8 Reticle Holder 9 Reticle Displacement Drive 10 Projection optical unit 11 Image field 12 Image plane 13 Wafer 14 Wafer holder 15 Wafer Displacement Drive 16 Lighting radiation 17 Collector 18 Intermediate focal plane 19 Deflection Mirror 20 1st Facet Mirror 21 First Facet 22 Second Facet Mirror 23 Second Facet 24 Cooling device 25 Control device 26. Mirror 27 Lower parts 28 Upper part 29 Connection Surface 30 Connection Surface 31 Optical surface 32 Cooling Channels 33 Bad Zone 34 Material Blanks 35 Separation plane M Mirror M1 Mirror M2 Mirror M3 Mirror M4 Mirror M5 Mirror M6 Mirror

Claims

1. A mirror of a projection exposure apparatus (1) for microlithography, comprising: The mirror (26) comprises a first mirror part (27) and a second mirror part (28) made from a material having a very low coefficient of thermal expansion; the first mirror part (27) and the second mirror part (28) are permanently connected to one another in the region of a first connecting surface (29) of the first mirror part (27) and a second connecting surface (30) of the second mirror part (28), the first mirror part (27) has a first average value of zero cross temperatures in a first volumetric region extending from the first connecting surface (29) to a distance of 10 mm in the first mirror part (27); the second mirror part (28) has a second average value of zero cross temperatures in a second volumetric region extending from the second connecting surface (30) to a distance of 10 mm on the second mirror part (28); The first average value of the zero cross temperature deviates from the second average value of the zero cross temperature by less than 1 K.

2. A mirror of a projection exposure apparatus (1) for microlithography, comprising: The mirror (26) comprises a first mirror part (27) and a second mirror part (28) made from a material having a very low coefficient of thermal expansion; the first mirror part (27) and the second mirror part (28) are permanently connected to one another in the region of a first connecting surface (29) of the first mirror part (27) and a second connecting surface (30) of the second mirror part (28), the first mirror part (27) and the second mirror part (28) have a failure zone (33) in which at least one material parameter deviates from a specified value by more than a minimum deviation, A mirror in which the failure zones (33) of the first mirror part (27) adjoin the failure zones (33) of the second mirror part (28) by more than 50% of the total area that they occupy at the location of the first connecting surface (29).

3. 3. The mirror of claim 2, wherein the failure zone (33) continues from the first mirror part (27) to the second mirror part (28).

4. An illumination optical unit comprising a mirror (26) according to any one of claims 1 to 3.

5. A projection optical unit comprising a mirror (26) according to any one of claims 1 to 3.

6. A microlithography projection exposure apparatus comprising an illumination optical unit (4) according to claim 4 and / or a projection optical unit (10) according to claim 5.

7. A method for manufacturing a mirror (26) of a projection exposure apparatus (1) for microlithography, comprising the steps of: Providing at least one material blank (34) made of a material with a very low coefficient of thermal expansion and having a failure zone (33) in which at least one material parameter deviates from a specified value by more than a minimum deviation; A first mirror part (27) having a first connecting surface (29) is produced from said material blank (34), A second mirror part (28) having a second connecting surface (30) is manufactured from another material blank (34); said first mirror part (27) and said second mirror part (28) being permanently connected to one another in the region of said first connecting surface (29) of said first mirror part (27) and said second connecting surface (30) of said second mirror part (28); determining a volumetric area of ​​the material blank (34) for producing the first mirror part (27) and / or a volumetric area of ​​the further material blank (34) for producing the second mirror part (28) based on a spatial formation of the defect zone (33) in the material blank (34) or in the further material blank (34), A method for selecting volumetric regions of the first mirror part (27) and the second mirror part (28) in the material blank (34) or in the further material blank (34) such that after connection of the first mirror part (27) and the second mirror part (28), the defect zone (33) continues from the first mirror part (27) to the second mirror part (28).

8. A method for manufacturing a mirror (26) of a projection exposure apparatus (1) for microlithography, comprising the steps of: Providing at least one material blank made of a material with a very low coefficient of thermal expansion and having a failure zone (33) in which at least one material parameter deviates from a specified value by more than a minimum deviation; A first mirror part (27) having a first connecting surface (29) is produced from said material blank (34), producing a second mirror part (28) having a second connecting surface (30) from said material blank (34); said first mirror part (27) and said second mirror part (28) being permanently connected to one another in the region of said first connecting surface (29) of said first mirror part (27) and said second connecting surface (30) of said second mirror part (28); A method for manufacturing the first mirror part (27) and the second mirror part (28) from volumetric regions of the material blank (34) spaced apart from each other by an additional amount of material for the manufacture of the first mirror part (27) plus an additional amount of material for the manufacture of the second mirror part (28).

9. 9. The method according to claim 7 or 8, wherein the first mirror part (27) and the second mirror part (28) are separated from the material blank (34) along a separation surface (35) that is curved at least in certain regions.

10. The method according to any one of claims 7 to 9, wherein the relative orientation in which the first mirror part (27) and the second mirror part (28) are connected to each other is determined based on the spatial formation of the defect zone (33) in the material blank (34) or in the material blank (34) or in the further material blank (34).

11. The method according to any one of claims 7 to 10, wherein the first mirror part (27) and the second mirror part (28) are manufactured from laterally offset volumetric regions of the material blank (34).

12. The method according to any one of claims 7 to 11, wherein the first mirror part (27) and the second mirror part (28) are manufactured from a volumetric region of the material blank (34) inclined with respect to an outer surface or axis of the material blank (34).

13. The method according to any one of claims 7 to 12, wherein the first mirror part (27) and the second mirror part (28) are connected to each other in the same relative orientation as they are in the material blank (34).

14. The method according to any one of claims 7 to 13, comprising selecting the material blank (34) or the volumetric regions of the first mirror part (27) and the second mirror part (28) of the material blank (34) or of the further material blank (34) such that the occurrence of the failure zone (33) at the location of the first connecting surface (29) of the first mirror part (27) and at the location of the second connecting surface (30) of the second mirror part (28), respectively, is below a limit value.

15. The method according to any one of claims 7 to 14, wherein each zone is selected for a material blank (34) such that the spatial formation of the defect zones (33) is similar, and the first mirror part (27) is manufactured from a volume area in one of the zones and the second mirror part (28) is manufactured from a volume area in another zone.

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