WO2013164091A1

WO2013164091A1 - Optical element having a coating and method for checking the optical element

Info

Publication number: WO2013164091A1
Application number: PCT/EP2013/001297
Authority: WO
Inventors: Oli T. EHRLER; Holger Kierey; Johannes Kraus
Original assignee: Carl Zeiss Laser Optics GmbH
Current assignee: Carl Zeiss Laser Optics GmbH
Priority date: 2012-05-03
Filing date: 2013-05-02
Publication date: 2013-11-07
Anticipated expiration: 2014-11-03
Also published as: DE102012207369A1

Description

Optical element having a coating and method for checking the optical element The invention relates to an optical element having a coating, and to a method for checking the optical element. Furthermore, the invention relates to a method for repairing an optical element. Lithography methods are used for producing microelectronic components or other micro- or nanostructured elements. The associated projection exposure apparatuses are increasingly being operated at short wavelengths in order that a high resolution is ensured. By way of example, a radiation source can be provided which can generate radiation in the extreme ultraviolet wavelength range (EUV) having a wavelength of 13 nm. Furthermore, the projection exposure apparatuses have optical units having a multiplicity of mirrors, including a collector which is arranged in proximity to the radiation source and focuses and passes on the radiation from the EUV radiation source.

Optical elements used in EUV projection exposure apparatuses have to be able to withstand extreme conditions. Alongside high thermal loading and irradiation by the EUV radiation, they are often also subjected to loading resulting from impinging particles from the radiation source, whereby damage and contamination of the optically active layers of the optical elements can occur. If a plasma-based radiation sources is used in the EUV projection exposure apparatus, particulate or film- like deposits of the plasma material on the EUV-reflective layers of the optical elements can occur, which lead to losses in reflectivity and ultimately require replacement of the optical elements.

C©f il¾MATION COPY In order to increase the effective lifetime of the optical elements, a final protective layer can be applied on the EUV-reflective layers of the optical elements, said protective layer protecting the optical layer against defects resulting from fast particles and ionizing radiation from the EUV radiation source. Deposits of the plasma material can be removed ex situ or in situ by means of plasma-based or wet-chemical etching processes. The fast particles formed in the radiation source and the ionizing radiation also lead, however, to damage to the protective layer, such that the latter is slowly eroded and/or locally damaged during operation. This has the consequence that the EUV-reflective layers are finally damaged after erosion of the protective layer, such that the optical element becomes unusable.

Accordingly, one object of the present invention is to provide an optical element which makes it possible, in a simple way, to detect damage in particular to the layers of the optical element which are essential for the optical effect. A further object of the invention is to specify a method by which damage to the layers of the optical element which are essential for the optical effect can easily be ascertained.

The object is achieved by means of an optical element comprising the features of Claim 1. The optical element comprises a first coating and a second coating, wherein the second coating is arranged between the first coating and a surface of the optical element. The second coating has a physical property that differs from the physical properties of the first coating, such that a conclusion about a thickness of the second coating and/or a presence of the second coating is made possible by a measurement of the physical property. By determining the thickness and/or the existence of the second coating, it is possible to draw conclusions about the state of the underlying first coating. If the second coating has a certain minimum thickness and/or is existent at all, it can be assumed that the underlying first coating is at least largely intact.

In one development of the invention, the second coating joins the first coating. Therefore, from the determination of the thickness and/or the existence of the second coating, the state of the underlying first coating can be deduced directly.

In one development of the invention, the first coating comprises layer plies composed of molybdenum and silicon. The first coating thus has a high reflectivity to radiation in the EUV wavelength range. The tiniest of damage to the first coating already has great effects on the reflectivity of the optical element, and so when these optical elements are used in an EUV projection exposure apparatus, a check should regularly be made to determine whether the optical element still fulfils the quality requirements. Alongside the layer plies composed of molybdenum and silicon, the first coating can also contain further layers of non-metals or other metals or other semiconductors, which are arranged between the layer plies composed of molybdenum and silicon and the thickness of which is less than the thickness of the layer plies composed of molybdenum or silicon. One function of the further layers consists in separating the layer plies composed of silicon and molybdenum.

In one development of the invention, the second coating comprises a material which emits radiation or particles. In particular, the second coating comprises a material which spontaneously emits radiation or particles, for example a weakly radioactive element. The second coating can also be doped with an element that emits rays or particles . By measuring the radiation or particles, it is possible directly to draw conclusions about a presence and/or a thickness of the second coating. What is essential here is that at least the layers of the optical element which adjoin the second coating do not comprise the material of the second coating or are not doped with the same element.

In one development of the invention, the second coating comprises a material which emits radiation or particles as a response to an external excitation. The second coating can also be doped with a corresponding material or element. The external excitation is effected by an external energy source, for example a light emitter or a heat emitter. By measuring the emitted radiation or particles, it is possible directly to draw conclusions about a presence and/or a thickness of the second coating. Here, too, it is essential that at least the layers of the optical element which adjoin the second coating do not comprise the material of the second coating or are not doped with the same element.

In one development of the invention, the second coating comprises a luminescent material, for example a fluorescent or phosphorescent material . A particularly simple detection of the second coating is made possible in this way.

In one development of the invention, the second coating comprises an electrical conductor or at least one electrically conductive material. After an external temporally modulated electric field has been applied, a response of the electrical conductor or of the electrically conductive material in the second coating is easily detectable, from which in turn it is possible to draw conclusions about the presence and/or the thickness of the second coating. In one development of the invention, the second coating is colored in a color that differs from the color of the first coating. The existence of the second coating can thus be easily visually detected.

In one development of the invention, the second coating is embodied as a protective layer. In this case, a protective layer should be understood to mean a layer composed of a material which is at least largely impenetrable to fast particles and ionizing radiation. The configuration of the second coating as a protective layer makes it possible directly to deduce the state of the protective layer by a measurement of the physical properties .

In one development of the invention, the second coating comprises a metal, a metal oxide, a semiconductor oxide, a semiconductor nitride or a combination thereof. Thus, materials are provided which are simple to process and which manifest a good protective effect against fast particles and ionizing radiation.

In one development of the invention, a third coating is arranged between the second coating and the surface of the optical element. An optical element constructed in this way affords the advantage that during a repair of the third coating by measuring the physical property it is possible to ensure that the second coating is still present, which means that, in particular, the first coating arranged below the second coating is also still undamaged and can fully manifest its optical effect.

In one development of the invention, the third coating comprises a metal or a metal oxide or a semiconductor oxide or a semiconductor nitride or a combination of the materials mentioned. These materials afford good protection against fast particles and ionizing radiation, such that an optical element configured in this way is particularly suitable for use in the EUV wavelength range.

The object is furthermore achieved by means of a method for checking an optical element comprising the features of Claim 13. The optical element comprises a first coating and a second coating, wherein the second coating is arranged between the first coating and a surface of the optical element, and wherein the second coating has a physical property that differs from the physical properties of the first coating. In the method according to the invention, the physical property is measured and a characteristic number for a layer thickness and/or an existence of the second coating are/is determined from the measured value of the physical property. By determining the thickness and/or the existence of the second coating, it is possible to draw conclusions about the state of the underlying first coating. If the second coating has a certain minimum thickness and/or is existent at all, it can be assumed that the underlying first coating is at least largely intact.

In one development of the method, the physical property is measured at a plurality of locations over the surface of the optical element. It is thus possible to determine the presence and/or the thickness of the second coating in a spatially resolved manner distributed over the surface of the optical element.

In one development of the method, the second coating comprises a material which emits radiation or particles, and the emission of the radiation or particles from the second coating is determined as the physical property. In particular, the second coating comprises a material which spontaneously emits radiation or particles, for example a weakly radioactive element. The second coating can also be doped with an element that emits radiation or particles. By measuring the radiation or particles, it is possible directly to draw conclusions about a presence and/or a thickness of the second coating.

In one development of the method, the second coating comprises a material which emits radiation or particles as a response to an excitation by means of an energy source from outside, and the emission of radiation or particles from the second coating on account of the excitation is determined as the physical property. The external excitation is preferably effected by an external energy source, for example a light emitter or a heat emitter. By measuring the emitted radiation or particles, it is possible directly to draw conclusions about a presence and/or a thickness of the second coating.

In one development of the method, the second coating comprises a luminescent material and the optical element is irradiated with a first electromagnetic radiation with an excitation wavelength and an emission of a second electromagnetic radiation on account of the irradiation of the luminescent material with the first electromagnetic radiation is measured as the physical property. A particularly simple detection of the second coating is made possible in this way.

In one development of the method, the second coating comprises an electrical conductor or at least one electrically conductive material, and the optical element is excited with an electromagnetic or electric field and an inductance is determined as the physical property. This affords an alternative possibility for drawing conclusions about the presence and/or the thickness of the second coating with little outlay. In one development of the method, an absorption spectrum which arises upon excitation of the optical element by an electric or electromagnetic field is determined as the physical property. For this purpose, the second coating is preferably embodied from a material or doped with a material which, upon irradiation with an electromagnetic radiation, has characteristic absorption lines or absorption bands which differ from the absorption lines or absorption bands of the adjoining layers of the optical element distinctly and in an easily detectable manner.

In one development of the invention, a surface of the optical element is wetted with a liquid, and a wetting behavior is determined over the surface of the optical element. From the wetting behavior, it is possible directly to draw conclusions about the material of the wetted surface. What is essential in this case is that, for the first coating and the second coating, materials are chosen whose wetting behaviors differ with respect to the chosen liquid, to be precise preferably in such a way that the different wetting behavior upon the wetting of the first and second coatings with the same liquid can be ascertained visually.

In a method for repairing an optical element having a first coating, a second coating and a third coating, wherein the second coating is arranged between the first coating and a surface of the optical element and the third coating is arranged between the second coating and the surface of the optical element, firstly the optical element is checked by one of the methods mentioned above. On the basis of the checking results it is ascertained whether the second coating is formed continuously along the surface of the optical element and/or whether the second coating has a sufficient thickness. In this case, the surface of the optical element should be understood to mean that area of the optical element to which radiation from the optical beam path is applied during the operation of the optical element. If a continuous second coating and/or a sufficient thickness of the second coating of the optical element are/is ascertained, subsequently the third coating is completely or partly stripped away by a suitable method and a new third coating is applied.

Further advantages, characteristics and features of the present invention will become clear in the course of the following detailed description of exemplary embodiments with reference to the accompanying drawings . In this case, specifically in the figures: gure 1 shows an illustration of an EUV projection exposure apparatus m the present invention can be used;

Figure 2 shows a general illustration of an optical element according to the invention for elucidating the principle of the invention;

Figure 3 shows a schematic illustration of a measurement of the physical property;

Figure 4 shows a further exemplary embodiment an optical element according to invention and

Figure 5 shows an exemplary embodiment of an optical element according to the invention comprising an additional third coating . Figure 1 shows an EUV projection exposure apparatus in a purely schematic illustration. Such a projection exposure apparatus comprises a radiation source 1 for generating a radiation in the extreme ultraviolet (EUV) range and a collector 2 for focusing and passing on the electromagnetic radiation emitted by the radiation source 1. An illumination system 3 comprises a plurality of optical elements in the form of mirrors. By means of the mirrors 4 to 9, the EUV radiation 16 can be deflected onto a reticle 17, which has a structure to be imaged onto a wafer 18. The imaging is effected by means of a projection optical unit, which in turn comprises a plurality of optical elements in the form of mirrors 10 to 15. The mirrors 4 to 15 and the collector 2 have first coatings in the form of reflection coatings which are constructed from a multiplicity of thin layers and form a Bragg reflector.

In particular the collector 2, arranged in direct proximity to the radiation source 1, is subjected to high thermal loading and also, alongside the radiation loading, to possible bombardment of particles from the radiation source 1, such that the coatings arranged on the surface of the collector can incur damage.

Damage to the reflection coating impairs the reflection behavior of the optical elements and leads to a deterioration in the efficiency of the EUV projection exposure apparatus .. For this reason, it is expedient to regularly check the optical elements in order to ensure that a reflection coating and/or a protective layer for the reflection coating are/is present with sufficient quality. For this purpose, the optical element is equipped with a second coating in the form of a detection layer. Figure 2 illustrates a section through an optical element according to the invention. The optical element is a mirror or a collector of an EUV projection exposure apparatus. Arranged on a substrate 20 is a first coating in the form of an EUV-reflective layer 21 formed from alternately deposited plies of molybdenum 22 and silicon 23. This construction has a particularly high reflectivity for EUV radiation. The detection layer 40 is formed on the EUV-reflective layer. What is essential to the detection layer 40 is that it is formed from a material or contains a material or is doped with a material or element which has a physical property that differs at least from the physical properties of the adjoining first coating, preferably also from the physical properties of all other materials used in the optical element. In this way, a presence and/or a thickness of the detection layer 40 can be deduced by a measurement of the physical property. In this case, physical properties should be considered to be, in particular, those properties which make it possible to examine the optical active area of the optical element with a sufficient spatial resolution. Examples of corresponding physical properties are presented below.

The principle of the measurement of the physical property is illustrated in Figure 3. The optical element shown in Figure 3 has been subjected to EUV irradiation for a period of time. In particular the fast particles and ionizing radiation emitted by the EUV radiation source have led to damage to the topmost coating, in this case the detection layer 40. In particular, the detection layer 40 is completely eroded in a partial region 45. This optical element would be usable only to a limited extent in an EUV projection exposure apparatus, since the EUV-reflective layer 21 would be damaged during further operation, such that the reflectivity of the optical element would be impaired. In order to ascertain whether there is the risk of damage to the EUV-reflective layer 21, the optical element is measured in situ or ex situ. For this purpose, the physical property that the second coating has in contrast to the adjoining coatings and/or to all other materials or elements of the optical element is determined with the aid of measuring probes 41. A corresponding measurement result is illustrated schematically in the upper part of Figure 3. In this case, the physical property that allows conclusions about a presence of the detection layer 40 was measured. Location-dependent characteristic numbers were determined from the measurements, and are illustrated in the graph 42. It is clearly evident that the characteristic numbers assume a different value in the partial region 45, in which the detection layer 40 is completely eroded, compared with the adjoining regions, from which the state of the detection layer 40 in this partial region can be deduced.

In an alternative configuration (not illustrated) of the invention, a thickness of the detection layer or a characteristic value for a thickness of the detection layer is determined by means of a measurement of a corresponding physical property of the detection layer. If a predefined limit value for the thickness is undershot, the optical element is taken out of service. Alternatively, the optical element can also be restored by means of a suitable method or a protective layer can be applied to the detection layer.

The detection layer 40 is provided with a material or embodied from a material which makes it possible, with a sufficient spatial resolution, to examine the entire optically active area of the optical element or at least a significant partial region of the entire optically active area. In this case, an optically active area should be understood to mean that area of the optical element which is penetrated by an optical beam path of the device into which the optical element is incorporated. A physical property of the detection layer 40 within the meaning of the present invention can for example generally be an emission of radiation or particles as spontaneous behavior. In a first exemplary embodiment, the detection coating 40 for this purpose is doped with a weakly radioactive material or element or produced from a radioactive material .

A further possible physical property of the detection layer 40 consists in the emission of radiation or particles as a response to an excitation by means of an external energy source . In a second exemplary embodiment, the detection layer 40 is doped with a fluorescent or phosphorescent element or an organic fluorescent marker is deposited on or in the detection layer. In an alternative exemplary embodiment, a detectable property of the detection layer 40 consists in a local or locally detectable change in material properties of the detection layer 40 by account of an external excitation.

In a third exemplary embodiment, the detection layer 40 is produced from a material or it comprises a material which, upon irradiation with electromagnetic radiation, in particular in the visible or x-ray range, has characteristic absorption lines or absorption bands that are optically easy to detect. In this case, the measurement of the physical property consists of a measurement of an absorption spectrum and an analysis of the absorption spectrum for the existence of the absorption lines or absorption bands characteristic of the detection layer 40.

In particular, the detection layer 40 can be produced from a material or can comprise a material, whose electrons are being excited when exposed to electromagnetic radiation of a short wavelength (for example x-ray) or a flux of charged particles (electrons and/or ions) . This results in complex physical processes which can be used for various methods of surface analysis, for example x-ray photoelectron spectroscopy XPS, Auger electron spectroscopy AES, secondary ion mass spectroscopy SIMS, energy dispersive x-ray analysis EDX, near-edge x-ray absorption fine structure NEXAFS or x-ray absorption near-edge structure spectroscopy XANES, electron energy loss spectroscopy EELS, secondary neutral mass spectrometry SNMS or glow discharge optiocal (emission) spectroscopy GDO(E)S. In x-ray photoelectron spectroscopy XPS the surface of the optical element is irradiated with monochromatic x- ray radiation resulting in photoelectrons being emitted from the surface. The energy of these photoelectrons is measured in a spectrometer. The measured energy spectrum comprises lines which are characteristic for a specific chemical element. These lines represent a "finger print" of the material of the detection layer. By analyzing the energy spectrum conclusions can be drawn about the chemical composition of the surface and/or the physical properties or existence of the detection layer.

If Auger electron spectroscopy AES is used for analysis of the detection layer 40, the detection layer of the optical element must contain a material which, upon irradiated with a photon or an electron beam focused on the surface, releases an electron of a lower energy level, leaving behind a hole in the corresponding electron shell. As this is an unstable state, the hole is filled by an outer shell electron, whereby the electron moving to the lower energy level loses an amount of energy equal to the difference in orbital energies . The transition energy is coupled to a second outer shell electron, which will be emitted from the atom if the transferred energy is greater than the orbital binding energy. Since the difference in energy between the shells is characteristic for the element it is possible to detect or even determine the material of the detection layer by measuring the energy of the emitted electron. The peak intensity depends on the concentration of the electron emitting material in the optical element, so that quantitative analysis is also possible.

In energy dispersive x-ray analysis EDX the optical element is irradiated with an electron beam. The electrons penetrate the optical element and interact with the electrons of the atoms, whereby x-ray radiation is emitted. The energy of the x-ray radiation is characteristic for the atom emitting the x-ray radiation. By measuring the energy of the x-ray radiation with an energy spectrometer it is possible to draw conclusions about the material of the probe or more specifically about a presence of the material of the detection layer. Measuring the intensity at the respective energies allows conclusions about the concentration of the material emitting x-ray radiation in the optical element.

In electron energy loss spectroscopy the optical element is irradiated with a focused electron beam. Due to interaction between the electron beam and the optical element part of the energy of the electron beam is transferred to the electrons of the material of the detection layer. Some electrons of the detection layer material receive sufficient energy to leave the electron shell and the energy of the irradiating beam is diminished. By analyzing the energy distribution of the electron beam conclusions can be drawn about the binding energy and the presence and/or composition of the detection layer material. The analysis methods mentioned in this alternative embodiment are typically limited to analysis of a few atomic layers at the surface of the probe. By using one or more of the analysis methods it is possible to determine the presence and/or thickness of the detection layer with high precision.

In a fourth exemplary embodiment, the detection layer 40 is colored, such that the existence of the detection layer can be checked visually.

In a fifth exemplary embodiment, the detection layer 40 is produced from a material having a different wetting behavior in combination with a wetting liquid (for example water or an oil-containing solution) compared with the material of a layer adjoining the detection layer 40, in particular of the EUV-reflective layer 21 adjoining the detection layer. By wetting the optical element with the wetting liquid and by checking the surface of the optical element for variations in the wetting behavior of the wetting liquid, it is possible to ascertain in a simple manner whether the detection layer 40 is fully formed.

Figure 4 illustrates a sixth exemplary embodiment of an optical element according to the invention. In this case, an electrical conductor in the form of a conductor loop 46 is arranged in the detection layer 40. If an electric field is then applied externally, an inductance generated by the conductor loop can be measured as the physical property. In the case of damage to the detection layer 40, the conductor loop 46 or the electrical conductor is likewise damaged, such that a changed inductance is established upon an excitation with the electric field. Conclusions about the state of the detection layer 40 can be drawn from this . In an alternative exemplary embodiment (not illustrated) , the detection layer 40 is doped with a conductive material.

In all the exemplary embodiments presented previously, the detection layer 40 itself can also be configured as a protective layer. In this case, a protective layer should be understood to mean a layer which protects the EUV-reflective layer 21 against defects as a result of fast particles and ionizing radiation from the EUV radiation source. A detection layer 40 configured as a protective layer preferably comprises a metal, a metal oxide, a semiconductor oxide, a semiconductor nitride or a combination thereof, since these materials afford particularly effective protection. The measurement of the physical property thus makes it possible directly to deduce the state of the protective layer and thus the remaining protective effect of the protective and/or detection layer 40. A detection layer 40 configured as a protective layer can be obtained for example by introducing small amounts of fluorescent elements into an otherwise transparent detection layer, or by a doping of the detection layer by means of isotopes or impurity elements.

In a seventh exemplary embodiment in accordance with Figure 5, a third coating in the form of a separate protective layer 24 is arranged on the detection layer 40. In this exemplary embodiment, the detection layer can be reduced to a very thin layer since sufficient protection of the optical element against particles and ionizing radiation is provided by the separate protective layer 24. Figure 5 furthermore illustrates tin deposits 27 on the protective layer 24, which can arise in a plasma-based EUV radiation source and deposit on the optical element. During a use of the optical element in an EUV projection exposure apparatus, the detection layer 40 experiences no modification as long as the protective layer 24 arranged thereabove has a finite residual thickness. It is only if the protective layer 24 is eroded on account of the conditions prevailing in the EUV projection exposure apparatus that the detection layer 40, now at least locally uncovered, is eroded, as a result of which the physical properties of the detection layer change. The change in the physical properties can in turn be determined by means of one of the methods described above. It is thus possible to ascertain the presence of a residual portion of the protective layer 24. The protective layer 24 ensures that the EUV-reflective layer 21 has not experienced any damage as a result of the use of the optical element in the EUV projection exposure apparatus.

In a further exemplary embodiment, an etching stop layer is arranged between the protective layer 24 and the detection layer 40 and/or between the detection layer 40 and the EUV-reflective layer 21. An etching stop layer should be understood to mean a layer composed of a material that has a good transmissivity for EUV radiation and at the same time in combination with a chemical solution, for example an aqueous acid, has a significantly lower etching rate than in combination with the protective layer 24 and/or the detection layer 40. It is thereby possible to remove the protective layer 24 and, if appropriate, the detection layer 40 for repairing the optical element by means of a chemical solution, damage to the underlying EUV-reflective layer being at least largely precluded by the etching stop layer. Subsequently, a new detection layer and/or a new protective layer can be applied to the optical element. It should be taken into account that all the exemplary- embodiments mentioned can be combined with one another. In particular, it is also possible to realize optical elements whose second coating has two, three or more of the physical properties described.

In a method for repairing the optical element, the protective layer 24 and/or the detection layer 40 are/is removed as soon as damage to the detection layer 40 is ascertained by a measurement of the physical property. Subsequently, a new detection layer and/or protective layer are/is applied by suitable methods.

Claims

Patent Claims :

1. Optical element having a first coating (21) and a second coating (40) , wherein the second coating (40) is arranged between the first coating (21) and a surface of the optical element, and wherein the second coating

(40) has a physical property which differs from the physical properties of the first coating (21) , such that a conclusion about a thickness of the second coating (40) and/or a presence of the second coating

(40) is made possible by a measurement of the physical property.

2. Optical element according to Claim 1,

characterized in that

the second coating (40) adjoins the first coating (21) .

3. Optical element according to either of Claims 1 and 2 ,

characterized in that

the first coating (21) comprises layer plies composed of molybdenum (22) and silicon (23) .

4. Optical element according to any of Claims 1 to 3 , characterized in that

the second coating (40) comprises a material which emits radiation or particles.

5. Optical element according to any of Claims 1 to 4 , characterized in that

the second coating (40) comprises a material which emits radiation or particles as a response to an external excitation.

6. Optical element according to Claim 5,

characterized in that the second coating (40) comprises a luminescent material .

7. Optical element according to any of Claims 1 to 6, characterized in that

the second coating (40) comprises an electrical conductor (46) .

8. Optical element according to any of Claims 1 to 6 , characterized in that

the second coating (40) is colored in a color that differs from the color of the first coating (21) .

9. Optical element according to any of Claims 1 to 8 , characterized in that

the second coating (40) is embodied as a protective layer .

10. Optical element according to Claim 9,

characterized in that

the second coating (40) comprises a metal, a metal oxide, a semiconductor oxide, a semiconductor nitride or a combination thereof .

11. Optical element according to any of Claims 1 to 10,

characterized in that

a third coating (24) is arranged between the second coating (40) and the surface of the optical element.

12. Optical element according to Claim 11,

characterized in that

the third coating (24) comprises a metal or a metal oxide or a semiconductor oxide or a semiconductor nitride or a combination of the materials mentioned.

13. Method for checking an optical element having a first coating (21) and a second coating (40) , wherein the second coating (40) is arranged between the first coating (21) and a surface of the optical element, and wherein the second coating (40) has a physical property that differs from the physical properties of the first coating (21), comprising the following method steps: measuring the physical property and

determining a characteristic number (42) for a layer thickness and/or an existence of the second coating (40) from the measured value of the physical property.

14. Method according to Claim 13,

characterized in that

the physical property is measured at a plurality of locations over the surface of the optical element.

15. Method according to either of Claims 13 and 14, characterized in that

the second coating (40) comprises a material which emits radiation or particles and in that the emission of the radiation or particles from the second coating (40) is determined as the physical property.

16. Method according to any of Claims 13 to 15, characterized in that

the second coating (40) comprises a material which emits radiation or particles as a response to an excitation by means of an energy source from outside, and in that the emission of radiation or particles from the second coating (40) on account of the excitation is determined as the physical property.

17. Method according to Claim 16,

characterized in that

the second coating (40) comprises a luminescent material, and in that the optical element is irradiated with a first electromagnetic radiation with an excitation wavelength, and in that an emission of a second electromagnetic radiation on account of the irradiation of the luminescent material with the first electromagnetic radiation is measured as the physical property.

18. Method according to any of Claims 13 to 17, characterized in that

the second coating (40) comprises an electrical conductor (46) , and in that the optical element is excited with an electromagnetic field, and in that an inductance is determined as the physical property.

19. Method according to any of Claims 13 to 18, characterized in that

an absorption spectrum which arises upon excitation of the optical element by an electric or electromagnetic field is determined as the physical property.

20. Method according to any of Claims 13 to 19, characterized in that

a surface of the optical element is wetted with a liquid, and in that a wetting behavior over the surface of the optical element is determined.

21. Method for repairing an optical element having a first coating (21) , a second coating (40) and a third coating (24) , wherein the second coating (40) is arranged between the first coating (21) and a surface of the optical element and the third coating (24) is arranged between the second coating (40) and the surface of the optical element, comprising the following method steps:

checking the optical element by means of a method according to any of Claims 13 to 19,

- ascertaining on the basis of the checking results whether the second coating (40) is formed continuously over a surface of the optical element, if a continuous coating of the optical element with the second coating (40) is ascertained: completely or partly stripping away the third coating (24) and applying a new third coating.