EP3323022A1

EP3323022A1 - Mirror, in particular for a microlithographic projection exposure apparatus

Info

Publication number: EP3323022A1
Application number: EP16741264.2A
Authority: EP
Inventors: Oliver Dier; Kerstin HILD; Hartmut Enkisch; Matus Kalisky
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2015-07-15
Filing date: 2016-07-07
Publication date: 2018-05-23
Also published as: US10331048B2; JP2018522281A; JP6814195B2; US20180217507A1; DE102015213253A1; WO2017009185A1

Abstract

The invention relates to a mirror, in particular for a microlithographic projection exposure apparatus, wherein the mirror has an optical effective area, having a mirror substrate (205, 305), a reflection layer (220, 320), which is designed such that the mirror has a reflectivity of at least 50% for electromagnetic radiation of a specified operating wavelength, which impinges on the optical effective area (200a, 300a) at an angle of incidence of at least 65° relative to the respective surface normal, and a substrate protection layer (210, 310), which is arranged between the mirror substrate (205, 305) and the reflection layer (220, 320), wherein the substrate protection layer (210, 310) has a transmission of less than 0.1% for EUV radiation.

Description

Mirror, in particular for a microlithographic

Projection exposure system

The present application claims the priority of German Patent Application DE 10 2015 213 253.6 filed on 15 July 2015. The content of this DE application is incorporated by reference into the present application text by "incorporation by reference".

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a mirror, in particular for a microlithographic projection exposure apparatus.

State of the art

Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection objective. The image of a mask (= reticle) illuminated by means of the illumination device is hereby projected onto a substrate (eg a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective in order to apply the mask structure to the photosensitive coating of the Transfer substrate. In projection lenses designed for the EUV sector, for example at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process because of the lack of availability of suitable light-transmissive refractive materials.

It is u.a. the operation of mirrors under grazing incidence known. Such mirrors operated under grazing incidence are understood here and below to mean mirrors for which the reflection angles occurring in the reflection of the EUV radiation and which refer to the respective surface normal are at least 65 °. Such mirrors are sometimes referred to as gl-mirror ("grazing incidence"). Basically, the use of such Gl levels u.a. in view of the comparatively high achievable reflectivities (of, for example, 80% and more).

In contrast, for example, to mirrors operated under normal incidence (also referred to as Nl mirror, "normal incidence" = "perpendicular incidence"), such GI mirrors do not require a multilayer system in the form of an alternating sequence of numerous individual layers to achieve the respective reflectivities as a reflection layer from at least two different layer materials, but only a single layer, which consist for example of ruthenum (Ru) and, for example may have a typical layer thickness in the range of 40 nm.

However, a problem which also occurs in practice when using GI mirrors, in particular in the projection objective, is that electromagnetic radiation, for example in the form of transmitted light or scattered light, can pass through the respective reflection layer to the mirror substrate, which is favored, for example, by the fact that the object in question electromagnetic radiation differs in the angular and / or wavelength distribution from the actual useful light. The relevant electromagnetic radiation reaching the mirror substrate can then be damaged in the mirror substrate material. or changes, for example in the form of radiation-induced local density changes (compaction), which in turn leads to undesired changes in the wavefront during operation of the optical system and thus ultimately to impairment of the performance of the optical system or of the projection exposure apparatus.

For the state of the art, reference is merely made by way of example to DE 10 2009 032 779 A1, DE 10 2009 054 653 A1, US 2013/0038929 A1, DE 10 2009 049 640 A1 and DE 10 2012 202 675 A1.

SUMMARY OF THE INVENTION

Against the above background, it is an object of the present invention to provide a mirror, in particular for a microlithographic projection exposure apparatus, which enables a grazing incidence application while at least largely avoiding the problems described above. This object is solved by the features of independent claim 1.

According to one aspect of the invention, a mirror, in particular for a microlithographic projection exposure apparatus, has an optical active surface, as well as a mirror substrate, a reflection layer, which is designed such that the electromagnetic radiation mirror of a predetermined operating wavelength, which is incident on the optical active surface the respective surface normal incident angle of at least 65 ° impinges, has a reflectivity of at least 50%, and a substrate protective layer disposed between the mirror substrate and the reflective layer, the EUV radiation substrate protective layer having a transmission of less than 0.1%.

The invention is based in particular on the concept of reducing or eliminating undesirable wavefront changes associated with electromagnetic radiation in a mirror designed for operation in grazing incidence by electromagnetic radiation and thus eliminating undesired wavefront changes between the reflection layer of the optical system Mirror and the mirror substrate, a substrate protective layer is disposed, which by sufficient absorption effect, the relevant electromagnetic radiation keeps away from the mirror substrate material. According to one embodiment, the substrate protection layer for EUV radiation has a transmission of less than 0.01%, in particular less than 0.001%. Preferably, the condition according to which the transmission for EUV radiation is less than 0.1%, in particular less than 0.01%, more particularly less than 0.001%, also for angles of incidence other than the "operating angles", in particular also for EUV impinging perpendicularly on the substrate protective layer Furthermore, the condition according to which the transmission of the substrate protective layer is less than 0.1%, in particular less than 0.01%, more particularly less than 0.001%, is preferably also satisfied for wavelengths other than the operating wavelength.

In embodiments of the invention, the substrate protective layer according to the invention is designed in such a way that, in addition to the above-described protection of the mirror substrate against electromagnetic radiation, a layer voltage compensation is also achieved in the entire layer structure of the mirror. According to one embodiment, the substrate protective layer for this purpose has at least one layer voltage compensation layer made of a second material. Such a layer stress (which may be in the form of compressive stress or tensile stress depending on the concrete embodiment of the reflection layer of the GI-mirror) can lead to deformation of the mirror substrate and thus likewise to undesired changes of the wavefront during operation of the respective optical system without further measures.

For at least partial compensation of said layer stress, the substrate protection layer according to the invention may also have as a multilayer system an alternating succession of absorber layers (the substrate protection against electromagnetic radiation) of a first material and layer stress compensation layers of a second material (said layer voltage at least partially compensating).

According to the invention, the circumstance that the existing requirements for a layer voltage compensation in the case of a mirror designed for operation under grazing incidence (GI mirrors) as a rule are substantially lower than in the case of a vertical operation, can advantageously also be exploited Incident designed mirror (Nl mirror). The layer stresses which occur (which can typically be of the order of magnitude of 200 MPa-600 MPa for the reflection layer stack of an NI mirror) are in absolute terms greater at a Gl level (with a typical layer voltage of 800 MPa-1200 MPa) , but due to the lower total thickness of the reflective layer to effectively lower deformation.

Furthermore, in the case of a GI-mirror, the wavefront modifications possibly caused by said layer stresses are less pronounced, since these wavefront modifications are ultimately confused with the overall layer modification. Thick scale, which in turn at a Gl-mirror (eg, with a reflective layer of ruthenium with about 40 nm thickness) are substantially lower than in a Nl mirror whose reflective layer stack, for example in the form of an alternating sequence of molybdenum and silicon layers typically has a total layer thickness in the range of 350 nm.

The lower requirements, as described above, for a layer voltage compensation in a GI mirror can now be utilized by the fact that in the structure of the substrate protective layer according to the invention the said stress compensation layers can be reduced in their proportion or the respective layer thickness, with the result that that the entire thickness of the layer structure in the GI-mirror can be limited or reduced accordingly.

This, in turn, results in effects that are scaling with the layer thickness, such as e.g. The scattered light components resulting from the layer structure as well as wavefront effects of any layer thickness errors are ultimately less pronounced.

As a result, an effective protection of the mirror substrate against electromagnetic radiation and a reduction or minimization of undesired wavefront modifications during operation of an optical system having the GI mirror according to the invention can thus be achieved according to the invention.

In embodiments of the invention, the stress compensation layers may in particular have a maximum thickness of 5 nm. Depending on the specific design of the reflection layer in the GI mirror according to the invention, the requirements for a layer stress compensation can even be reduced to almost zero, with the result that it is ultimately sufficient if the substrate protective layer according to the invention exclusively from the described absorber layer - waiving any voltage compensation layers - exists. In this case, it is also possible to use the further circumstance, which is advantageous in comparison with NI mirrors, that "roughening effects", which may be relatively pronounced in NI mirrors and therefore possibly also require so-called "interrupter layers", are less problematical at a Gl level , so that under this aspect, the realization of the substrate protective layer according to the invention as a single layer (from only one absorber layer) comes into consideration.

According to one embodiment, the first material is selected from the group consisting of iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), silver (Ag), gold (Au), platinum (Pt), germanium ( Ge), tungsten (Wo), chromium (Cr), tin (Sn), zinc (Zn), indium (In), tellurium (Te), iridium (Ir), palladium (Pd), osmium (Os), tantalum ( Ta) and alloys thereof.

In one embodiment, the second material is selected from the group consisting of molybdenum (Mo), ruthenium (Ru), boron (B), boron carbide (B ₄ C), carbon (C), and silicon (Si).

According to a further aspect of the invention discussed below, the embodiment of the substrate protective layer according to the invention and, in particular, also of the reflective layer preferably takes place in such a way that the period length of the multilayer system forming the substrate protective layer - and ultimately also the lateral thickness profile of the substrate protection layer

Substrate protective layer - metrologically accessible in the simplest possible way. In this way, according to the invention, the further problem can be taken into account that a respectively desired, typically constant thickness profile of the substrate protective layer can basically only be realized with finite precision, ie in practice a certain lateral thickness variation of the substrate protective layer is unavoidable. In order to have such an undesired variation in thickness either already in the relevant mirror itself (by optimizing the layer thickness profile) or elsewhere in the relevant To be able to compensate for the optical system, as exact a knowledge as possible of the respective course of the layer thickness is required.

In the advantageous embodiment of the period length of the multi-layer system forming the substrate protective layer on the one hand and optionally also the thickness of the reflective layer on the other hand, the invention is based on the consideration that in principle the relationship between the period length of the multilayer system forming the substrate protective layer on the one hand and the spectral peak position in the for the relevant mirror by reflectometry determined dependence of the reflectivity of the wavelength of the incident electromagnetic radiation on the other hand for determining said period length and thus, when measured at a plurality of locations on the mirror, the lateral thickness profile can be used, wherein in this reflectometry measurement the electromagnetic radiation substantially perpendicular (preferably with an incident on the respective surface normal angle of incidence of a maximum of 10 °) on de n mirror is directed.

In this case, although the reflectivities obtained for the mirror (designed for grazing incidence operation) are comparatively low (relative to a NI mirror designed for vertical incidence), they are comparatively low (for example, by a factor of 100), however, for said period length of the substrate protective layer characteristic multilayer system, as will be explained in more detail below. With suitable specification of the period length of the multilayer system, it can now be achieved that the spectral peak position in the wavelength-dependent reflectivity profiles obtained for the relevant mirror can be accessed with a reflectometer which is already available or available for EUV lithography and can thus be easily determined metrologically.

According to one embodiment, the substrate protective layer has a multiple-layer system of a plurality of individual layers. According to one embodiment, these individual layers have a maximum thickness of less than 100 nm, in particular of less than 50 nm, more particularly of less than 25 nm. According to one embodiment, the multilayer system comprises an alternating sequence of first layers of a first material and second layers of a second material different from the first material. According to one embodiment, this multi-layer system has a period length in the range from 6 nm to 8 nm, in particular in the range from 6.5 nm to 7.5 nm.

According to one embodiment, the total layer thickness which the mirror has above the maximum of a standing wave field generated by the multilayer system corresponds to an integral multiple of the period length of the multilayer system, up to a maximum deviation of ± 10%. By this choice of layer thickness or its matching to the period length of the substrate protective layer forming multilayer system can - as also explained in more detail below - a comparatively favorable in terms of the evaluation, namely substantially symmetrical course of each peak in the wavelength dependence of the reflectivity can be achieved. This concept according to the invention is the further one

Consideration that the above-mentioned total layer thickness (including the thickness of the reflective layer) affects the spectral response of the reflectivity of the relevant mirror, as by the multi-layer system forming the substrate protective layer, a standing wave field is generated, which extends through the reflection layer up to the Mirror adjoining vacuum continues. The choice of the above-mentioned total layer thickness as an integer multiple of the period length in this context now, as explained in more detail below, leads to a particularly pronounced peak or a comparatively strong oscillation of the electric field strength at the interface between the reflection layer and the vacuum can be achieved, with the result that the spectral peak position which is ultimately sought is also particularly easy to determine metrologically.

The above-described and hereinafter also explained in more detail reflectometric determination of the lateral course of the period length of the substrate protective layer forming multilayer system can alternatively be performed either before the application of the reflective layer or after its application (i.e., with the electromagnetic radiation through said reflective layer). In other words, the invention comprises on the one hand embodiments in which a reflectometric characterization is first carried out for the substrate protective layer itself (even without a reflection layer thereon) and optionally only subsequently or sequentially for the reflection layer then applied. On the other hand, however, the invention also encompasses embodiments in which, while reducing the metrological effort or the number of required process steps, the spectral peak position of the reflectivity of the multiple-layer system forming the substrate protective layer is determined even after the reflection layer has been applied.

According to one embodiment, the working wavelength is less than 30 nm, wherein it may be in particular in the range of 10 nm to 15 nm. The invention further relates to an optical system of a microlithographic

Projection exposure apparatus, which has at least one mirror with the features described above.

According to one embodiment, this mirror is arranged in the optical system in such a way that the reflection angles occurring in the operation of the optical system upon reflection of electromagnetic radiation at the mirror and related to the respective surface normal are at least 50 °, in particular at least 65 °. The invention further relates to a microlithographic projection exposure apparatus having a lighting device and a projection lens, wherein the illumination device illuminates a mask located in an object plane of the projection lens during operation of the projection exposure apparatus and the projection objective structures on this mask onto a photosensitive layer located in an image plane of the projection objective wherein the projection exposure apparatus has an optical system with the features described above.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

Figure 1 is a schematic representation of a designed for operation in the EUV projection exposure system; Figure 2-3 are schematic representations to explain the possible

Structure of a mirror in exemplary embodiments of the invention; and

Figure 4-6 diagrams for explaining further possible embodiments of the invention. DETAILED DESCRIPTION PREFERRED

EMBODIMENTS

1 shows a schematic representation of an exemplary projection exposure apparatus 100 designed for operation in the EUV, in which the present invention can be implemented.

According to FIG. 1, a lighting device of the projection exposure apparatus 100 has a field facet mirror 103 and a pupil facet mirror 104. On the field facet mirror 103, the light of a light source unit comprising a plasma light source 101 and a collector mirror 102 is directed. In the light path after the pupil facet mirror 104, a first telescope mirror 105 and a second telescope mirror 106 are arranged. A deflecting mirror 107, which operates under a grazing incidence, is arranged downstream of the light path, which deflects the radiation impinging on it onto an object field in the object plane of a projection objective merely indicated in FIG. At the location of the object field, a reflective structure-carrying mask 121 is arranged on a mask table 120, which is imaged by means of a projection objective 150 into an image plane in which a photosensitive layer (photoresist) coated substrate 161 is located on a wafer table 160.

The structure according to the invention of a mirror operating under a grazing incidence can be realized, for example, in one or more mirrors of the projection objective 150 or also in the deflection mirror 107 provided within the illumination device.

Hereinafter, possible embodiments of an under-light mirror according to the present invention will be described with reference to the schematic figures of FIGS. 2-3.

2, a mirror 200 designed for operation under grazing incidence may, in a first embodiment, be incorporated in a manner known per se Mirror substrate 205 (which may be made of any suitable substrate material, such as under the designation ULE ^® by the company Corning Inc. marketed Titanium-silicate glass) and having an optical effective surface 200a forming reflection layer 220, which approximately, for example in the execution of ruthenium (Ru) is prepared and may have a typical exemplary thickness in the range of 20 nm to 200 nm. In further embodiments, layer systems which are known per se for GI levels with more than one single (eg ruthenium (Ru)) layer can likewise be used. With regard, for example, to possible layer designs of GI mirrors, reference is made, for example, to DE 10 201 1075 579 A1.

In order to protect the mirror substrate 205 from electromagnetic radiation incident on the optical active surface 200a during operation of the mirror 200 and penetrating the reflective layer 220 as a result of transmission, e.g. Through the ruthenium (Ru) layer, the mirror 200 has a substrate protection layer 210 between the reflection layer 220 and the mirror substrate 205.

In the exemplary embodiment of FIG. 2, the substrate protective layer 210 has an alternating succession of absorber layers 212 made of a first material (with a correspondingly high absorption fraction in the refractive index, eg silver (Ag), platinum (Pt), tin (Sn), zinc (Zn) , Copper (Cu), tungsten (W), nickel (Ni) or cobalt (Co)), and layer stress compensation layers 21 1 of a second material (eg, molybdenum (Mo) or ruthenium (Ru)). While the layer voltage compensation layers 21 1 serve to compensate for an undesirable mechanical layer voltage that may be present in the entire layer structure of the mirror 200, the absorber layers 212 provide EUV radiation (in particular of less than 0.1%, in particular less than 0.01%) owing to their low transmission %, more particularly less than 0.001%) for keeping electromagnetic radiation in the form of stray light away from the mirror substrate 205 where this electromagnetic radiation could cause compaction or otherwise damage to the mirror substrate material. In particular, the total number of cycles of the alternating sequence within the substrate protective layer 210 may be in the range of 10 to 40 (each period corresponding to a partial stack of respectively one absorber layer 212 and one layer voltage compensation layer 21 1). Furthermore, the thickness of the film voltage compensation layers 21 1 may be in the range of 2 nm to 4 nm by way of example only. In particular, the thickness of the layer stress compensation layers 21 1 can be significantly lower than in a (Nl) mirror designed for operation under normal incidence, wherein advantage is taken of the fact that the layer voltage-dependent deflections, which may have to be compensated for, are affected in the under-incidence mirror 200 according to FIG 2 are much lower than at said NI level. 3 shows a schematic representation for explaining a further exemplary embodiment of a mirror according to the invention, in which analogous or essentially functionally identical components with reference numbers increased by "100" are designated in comparison to FIG the substrate protective layer 310 exclusively of an absorber layer with a total thickness of typically 50 nm or 100 nm required for absorbing the electromagnetic radiation penetrating the reflective layer 320, wherein here additional layer stress compensating layers or a corresponding alternating layer structure in the substrate protective layer 310 is completely dispensed with and the substrate protective layer thus consists only of a single absorber layer analogous to the absorber layer 212. Such a configuration is particularly suitable if a mech Anic layer voltage is negligibly small.

In further embodiments, the substrate protective layer present according to the invention between the mirror substrate and the reflection layer can also be made of 2 (ie absorption of electromagnetic radiation penetrating the reflective layer such as transmitted light or scattered light) as well as the function of the layer voltage compensation layers 21 1 (ie compensation of an overall layer structure of the Mirror existing mechanical layer stress). For example, alloys of the abovementioned absorber materials silver (Ag), platinum (Pt), tin (Sn), zinc (Zn), copper (Cu), tungsten (W), nickel (Ni), cobalt can be used for this purpose (Co) with, for example, silicon (Si), vanadium (V), carbon (C), boron (B) or molybdenum (Mo). Depending on the absorption behavior, the layer thicknesses can be between 50 and 300 nm. Likewise, by suitably selecting the process parameters for both purely elemental absorber layers and for alloys, the layer stress can be adjusted over a wide range (eg from +1000 MPa to -1000 MPa).

The invention can be implemented particularly advantageously in a projection objective which has at least one mirror for grazing incidence of the illumination light (with angles of incidence greater than 65 °), for example in a projection objective as shown in DE 10 2012 202 675 A1. In further embodiments, the invention can also be implemented in projection lenses with a different structure or in other optical systems.

In the following, embodiments of the invention are described with reference to FIGS. 4-6, in which an advantageous embodiment of the multilayer system forming the substrate protective layer and possibly also of the reflective layer is realized insofar as this embodiment is particularly favorable for a metrological determination of the lateral thickness profile of the substrate protective layer , Such a metrological determination of the lateral thickness profile is in turn desirable, as already described above, in order to be able to compensate for undesired deviations of this thickness profile from a desired, eg typically constant, thickness profile (the said compensation being achieved either in the layer thickness range). course of the mirror in question itself or else elsewhere in the optical system can take place).

4a-4d respectively show wavelength dependencies of the reflectivity of a mirror according to the invention, wherein for different values of the period length of the multilayer system forming the substrate protective layer (from d = 6nm to d = 8nm) and different thicknesses of the reflective layer (which in Fig. 4a is 25nm, in Fig. 4b is 28nm, in Fig. 4c is 30nm and in Fig. 4d is 35nm) the individual reflectivity gradients are plotted. It can be seen that for the selected period lengths the spectral peak position is between 12.5nm and 14.5nm, which is within the range that is already accessible in any case for reflectors commonly used and available in the EUV field. 5 shows a diagram for illustrating a standing wave field generated by the multilayer system forming the substrate protective layer in the individual regions of a mirror according to the invention and in particular at the interfaces located between substrate protective layer or multilayer system, reflection layer and vacuum. In this case, in the exemplary embodiment (but without the invention being limited thereto), the multilayer system forming the substrate protective layer is formed as an alternating sequence of boron carbide (B C) and chromium (Cr) layers, whereas the reflection layer also consists, for example, of ruthenium (Ru). 6a-c show the respective reflectivities in their wavelength profile as a function of the thickness of the reflection layer.

The individual curves in FIG. 5 correspond to different thicknesses of the reflection layer, the respective boundary surfaces between reflection layer and vacuum being indicated for the different samples by short vertical lines. As can be seen from FIG. 5, the oscillation of the electric field strength in a vacuum is particularly great when the relevant interface between reflective layer and vacuum is in the vicinity of a maximum of the standing wave field (which in the example shown for thicknesses of the reflective layer of 27nm and 34nm is given). If, on the other hand, the relevant interface lies near a minimum of the standing wave field (as in the example for a thickness of the reflection layer of 31 nm), the relevant oscillation of the field strength is particularly small. As a result, it is found that a pronounced peak in the wavelength dependence of the reflectivity is achieved when the total thickness of the layers above the maximum of the standing wave field in the multilayer system is chosen to be approximately an integer multiple of the period length of the multilayer system.

While the invention has been described in terms of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and / or exchange of features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be embraced by the present invention, and the scope of the invention is limited only in terms of the appended claims and their equivalents.

Claims

claims

Mirror, in particular for a microlithographic projection exposure apparatus, wherein the mirror has an optical active surface, with

A mirror substrate (205, 305);

• a reflection layer (220, 320), which is designed such that the mirror for electromagnetic radiation of a predetermined operating wavelength which is incident on the optical active surface (200a, 300a) at an incident angle of at least 65 ° relative to the respective surface normal, a reflectivity has at least 50%; and

A substrate protective layer (210, 310) which is arranged between the mirror substrate (205, 305) and the reflection layer (220, 320), wherein the substrate protective layer (210, 310) has a transmission of less than 0.1% for EUV radiation.

2. Mirror according to claim 1, characterized in that the substrate protective layer (210, 310) for EUV radiation has a transmission of less than 0.01%, in particular less than 0.001%.

3. Mirror according to claim 1 or 2, characterized in that the substrate protective layer (210, 310) has at least one absorber layer of a first material.

A mirror according to claim 3, characterized in that the first material is selected from the group consisting of iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), silver (Ag), gold (Au) , Platinum (Pt), germanium (Ge), tungsten (Wo), chromium (Cr), tin (Sn), zinc (Zn), indium (In), tellurium (Te), iridium (Ir), palladium (Pd) , Osmium (Os), tantalum (Ta) and alloys thereof.

5. Mirror according to one of the preceding claims, characterized in that the substrate protective layer (210) further comprises at least one layer voltage compensation layer (21 1) made of a second material.

6. Mirror according to claim 5, characterized in that the second material is selected from the group consisting of molybdenum (Mo), ruthenium (Ru), boron (B), boron carbide (B ₄ C), carbon (C) and silicon ( Si) contains.

7. Mirror according to one of the preceding claims, characterized in that the substrate protective layer (210) comprises a multi-layer system of a plurality of individual layers.

8. Mirror according to claim 7, characterized in that these individual layers have a maximum thickness of less than 100 nm, in particular of less than 50 nm, more particularly of less than 25 nm.

A mirror according to claim 7 or 8, characterized in that the multilayer system comprises an alternating succession of first layers of a first material and second layers of a second material different from the first material.

10. Mirror according to claim 9, characterized in that the multi-layer system has a period length in the range of 6 nm to 8 nm, in particular in the range of 6.5 nm to 7.5 nm.

1 1. Mirror according to claim 9 or 10, characterized in that the total layer thickness, which has the mirror above the maximum of a standing wave field generated by the multi-layer system, an integer multiple of the period length of the multi-layer system corresponds to a maximum deviation of ± 10%.

12. Mirror according to one of claims 9 to 1 1, characterized in that the first layers absorber layers (212) and the second layers layer voltage compensation layers (21 1).

13. Mirror according to one of claims 1 to 4, characterized in that between the mirror substrate (305) and the reflection layer (320) exclusively an absorber layer is arranged as a substrate protective layer (310).

14. Mirror according to one of the preceding claims, characterized in that the working wavelength is less than 30 nm, in particular in the range of 10 nm to 15 nm.

15. An optical system of a microlithographic projection exposure apparatus, characterized in that it comprises at least one mirror according to one of the preceding claims.

16. Optical system according to claim 15, that this mirror is arranged in the optical system such that the reflection angle occurring during operation of the optical system upon reflection of electromagnetic radiation at the mirror, related to the respective surface normal reflection angle at least 50 °, in particular at least 65 °, be.

17. A microlithographic projection exposure apparatus with an illumination device and a projection objective, wherein the illumination device illuminates a mask located in an object plane of the projection objective during operation of the projection exposure apparatus and the projection objective images structures on this mask onto a photosensitive layer located in an image plane of the projection objective, the projection exposure apparatus at least a mirror according to any one of claims 1 to 14.