Abstract:
The invention concerns a method of and an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus. In an embodiment the mirror is an EUV mirror and a method according to the invention comprises the following steps: deflecting at least one input measuring beam on to the mirror; ascertaining at least one optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and determining the heating condition of the mirror on the basis of the parameter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of German Patent Application DE 10 2010 061 950.7 filed on Nov. 25, 2010, and U.S. 61/417,552, filed on Nov. 29, 2010. The content of these applications is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the invention 
         [0003]    The invention concerns a method and arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus. 
         [0004]    2. State of the art 
         [0005]    Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The micro lithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated by means of the illumination system is projected by means of the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate. 
         [0006]    Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. A problem which arises in practice is that the EUV mirrors experience a rise in temperature and linked thereto thermal expansion or deformation, as a consequence of absorption of the radiation emitted by the EUV light source but also because of cleaning in a hydrogen atmosphere, and such expansion or deformation in turn can result in worsening of the imaging properties of the optical system. To be able to assess those effects and possibly compensate for them there is a need to determine the extent of the mirror heating as accurately as possible. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide a method of and an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, which without substantially worsening operation of the optical system, permit reliable characterisation of the heating condition. 
         [0008]    That object is attained by the method in accordance with the features of independent claim  1  and the arrangement in accordance with the features of further independent claim  19 . 
         [0009]    A method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the mirror is an EUV mirror, comprises the following steps:
       deflecting at least one input measuring beam on to the mirror;   ascertaining at least one optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   determining the heating condition of the mirror on the basis of said optical parameter.       
 
         [0013]    The operation of determining the change in temperature, which is effected in contact-less fashion by analysis of an output measuring beam produced from the input measuring beam after interaction with the mirror can be effected directly in the EUV projection objective and also during operation thereof, wherein, depending on the respective actual structure involved, as is described in greater detail hereinafter, the measuring light can be supplied and also carried away by way of optical fibers. Consequently, no additional electronics are required within the typically evacuated housing of the projection objective, and no stray light is introduced into the optical system. The method can be carried out in particular with a measuring arrangement which is connected in the form of a separate module to the projection objective. 
         [0014]    In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining at least one value dependent on the refractive index of the mirror material. In that case it is possible in accordance with the invention to make use of the fact that the low coefficients of expansion which typically exist in the mirror material of EUV mirrors involve a comparatively pronounced temperature dependency in respect of the refractive index, which in turn makes it possible to infer a change in temperature of the mirror from measurement of the change in refractive index, with comparatively great accuracy (as described in fuller detail hereinafter). 
         [0015]    In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining an optical path length change. 
         [0016]    In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the reflectivity at a predetermined wavelength. 
         [0017]    In a further embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the beam deflection of the output measuring beam relative to the input measuring beam. 
         [0018]    In a further embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the wavelength of the output measuring beam. 
         [0019]    In an embodiment a plurality of input measuring beams is deflected on to the mirror. Those input measuring beams can be deflected on to the mirror in particular at different azimuth angles with respect to an optical axis of the optical system in order to obtain information about the temperature distribution of the mirror and possibly also any symmetry in respect of the heating effect in conjunction with certain illumination settings. 
         [0020]    In an embodiment the output measuring beam is produced from the input measuring beam by passing at least once through the mirror. 
         [0021]    The inventions also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a micro lithographic projection exposure apparatus, wherein the method comprises the following steps:
       deflecting at least one input measuring beam on to the mirror;   ascertaining at least one optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   determining the heating condition of the mirror on the basis of said optical parameter,   wherein the input measuring beam passes at least once through the region between the reflecting surface of the mirror and the mirror rear side.       
 
         [0026]    The mirror comprises a substrate and a reflecting surface. Accordingly, the input measuring beam passes at least once through the substrate (corresponding to the region between the reflecting surface of the mirror and the mirror rear side). In particular, the input measuring beam can pass at least once through the substrate without being reflected at the (EUV-)reflecting surface of the mirror. 
         [0027]    In particular the output measuring beam can be produced from the input measuring beam by passing through the mirror (or the mirror substrate, respectively) a plurality of times, whereby it is possible to achieve an increase in the length of the measuring section and thus an increase in measuring accuracy. 
         [0028]    In an embodiment the output measuring beam is produced from the input measuring beam by reflection at an optical element arranged at the mirror. 
         [0029]    The optical element can have at least one DBR (DBR=‘Distributed Bragg Reflector’). In addition the optical element can be a surface-emitting laser (VCSEL). 
         [0030]    The invention also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the method comprises the following steps:
       deflecting at least one input measuring beam on to the mirror;   ascertaining at least one optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   determining the heating condition of the mirror on the basis of said optical parameter,   wherein the output measuring beam is produced from the input measuring beam by reflection at an optical element arranged at the mirror, wherein the optical element has at least one DBR.       
 
         [0035]    In particular, the mirror comprises a substrate and a reflecting surface, wherein the at least one DBR can be provided on the substrate of the mirror. 
         [0036]    The invention also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the method comprises the following steps:
       deflecting at least one input measuring beam on to the mirror;   ascertaining at least one optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   determining the heating condition of the mirror on the basis of said optical parameter,   wherein the output measuring beam is produced from the input measuring beam by reflection at an optical element arranged at the mirror, wherein the optical element is a surface-emitting laser (VCSEL).       
 
         [0041]    In particular, the mirror comprises a substrate and a reflecting surface, wherein the VCSEL can be provided on the substrate of the mirror. 
         [0042]    The invention also comprises embodiments in which the input measuring beam (from the light source) passes through the mirror substrate in order to reach the DBR or the VCSEL and/or in order to arrive (from the DBR or the VCSEL) at a detector. 
         [0043]    In a further aspect the invention concerns an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the mirror is an EUV mirror, said arrangement comprising:
       a measuring light source unit for producing at least one input measuring beam;   a detector unit for ascertaining an optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   an evaluation unit for determining the heating condition of the mirror on the basis of said parameter.       
 
         [0047]    In an embodiment the measuring light source unit has a plurality of measuring light sources. In addition the detector unit can have a plurality of detectors. In particular a plurality of measuring light sources and/or a plurality of detectors can be arranged around the mirror at different azimuth angles with respect to an optical axis of the optical system. 
         [0048]    In an embodiment a plurality of measuring light sources and/or a plurality of detectors are arranged matrix-like. 
         [0049]    In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, said arrangement comprising:
       a measuring light source unit for producing at least one input measuring beam;   a detector unit for ascertaining an optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   an evaluation unit for determining the heating condition of the mirror on the basis of said parameter;   wherein the input measuring beam passes at least once through the region between the reflecting surface of the mirror and the mirror rear side.       
 
         [0054]    In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, said arrangement comprising:
       a measuring light source unit for producing at least one input measuring beam;   a detector unit for ascertaining an optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   an evaluation unit for determining the heating condition of the mirror on the basis of said parameter;   wherein the mirror comprises a substrate and a reflecting surface, and wherein at least one DBR is provided on the substrate.       
 
         [0059]    In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, said arrangement comprising:
       a measuring light source unit for producing at least one input measuring beam;   a detector unit for ascertaining an optical parameter of at least one output measuring beam produced from the input measuring beam after interaction with the mirror; and   an evaluation unit for determining the heating condition of the mirror on the basis of said parameter;   wherein the mirror comprises a substrate and a reflecting surface, and wherein at least one surface-emitting laser (VCSEL) is provided on the substrate.       
 
         [0064]    Further configurations of the invention are set forth in the description and the appendant claims. 
         [0065]    The invention is described in greater detail hereinafter by means of embodiments by way of example illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0066]    In the drawings: 
           [0067]      FIG. 1  shows a diagrammatic view to illustrate a measuring arrangement in accordance with an embodiment of the invention, 
           [0068]      FIG. 2  shows a diagrammatic view to illustrate a further embodiment of a measuring arrangement according to the invention, 
           [0069]      FIGS. 3   a - g  show diagrammatic views of possible embodiments of temperature adjustment or regulation effected in accordance with the invention in relation to a mirror, and 
           [0070]      FIGS. 4-7  show diagrammatic views to illustrate further embodiments of a measuring arrangement according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0071]    Hereinafter embodiments of the invention are described with reference to  FIGS. 1 through 3 , in which contact-less measurement of the temperature-induced change in refractive index is effected interferometrically, as relative and integral measurement. 
         [0072]    A measuring arrangement  100  by way of example for temperature measurement at a mirror  101  is shown in  FIG. 1 . 
         [0073]    Referring to  FIG. 1  a linearly polarised laser beam  105  produced by a laser light source (not shown) is coupled into the measuring arrangement  100  and is firstly incident on a polarisation-neutral or polarisation-independent beam splitter  110 . Without the invention being restricted thereto for example an infrared laser involving a wavelength of a few micrometers (μm) can be used as the measuring light source. The laser beam  105  is coupled into the measuring arrangement  100  in such a way that the polarisation direction of the light incident on the beam splitter  110  is at an angle of 45° relative to the plane defined by the incident beam, the transmitted beam and the reflected beam so that as the outcome the reflected and the transmitted beams coincide in respect of their proportion of the overall intensity of the coupled-in light. 
         [0074]    The light transmitted by the beam splitter  110  passes through the mirror  101  to be measured, is reflected at a deflection prism  120  and after again passing through the mirror  101  passes through a lambda/ 4  plate  125  which converts the light which is substantially linearly polarised (as a consequence of the negligible polarisation influence of the deflection prism  120 ) into circularly polarised light. Here and in the following embodiments (and as can be gathered from the drawings) the mirror  101  comprises a substrate and a reflecting surface. The substrate can e.g. be made from a material, i.e. can comprise a material or consist of a material, said material having a zero-cross temperature (zero-crossing temperature), i.e. also a temperature-dependent thermal expansion coefficient (=coefficient of thermal expansion), such as e.g. zerodur® or ULE®. 
         [0075]    As can be gathered from  FIG. 1 , the input measuring beam passes at least once through the mirror substrate without being reflected at the reflecting surface (which is the concave, upper surface of the mirror in  FIG. 1 ) of the mirror  101 . With other words, the input measuring beam passes at least once (in the specific embodiment, twice) through the region between the reflecting surface of the mirror  101  and the rear side (lower surface of the mirror in  FIG. 1 ) of the mirror  101 . 
         [0076]    Polarisation beam splitters  130  and  140  serve for measurement of the component which is then transmitted and reflected respectively by the beam splitter  110 . Downstream of the second polarisation beam splitter  140  there are angles in respect of the polarisation direction of 0° and 90° for the component transmitted by the beam splitter  110 , the associated light components being measured by way of photodiodes  150  and  170  respectively. Downstream of the first polarisation beam splitter  130  there are angles in respect of the polarisation direction of 180° and 270° for the component reflected in the beam splitter  110 , the associated light components being measured by way of photodiodes  180  and  160  respectively. 
         [0077]    Accordingly comparatively disturbance-insensitive evaluation of four push-pull signals is effected by means of the measuring arrangement of  FIG. 1 . In addition the arrangement adopted in  FIG. 1  provides for the measuring (laser) beam to be coupled in and out on the same side of the measuring arrangement  100  and thus provides that the light passes twice through the mirror material, leading to increased measuring accuracy. 
         [0078]    In practice after the light source of the optical system has been brought into operation or switched on the start temperature or switch-on temperature (for example T 0 =22° C.) is determined by means of an absolute temperature sensor and used as a reference temperature by a counter at that start time being set to zero and then the interference stripes which occur upon heating of the measured mirror  101  in interferometric measurement being counted. In that way, the change in temperature of the mirror can be ascertained by means of equation (1), for example with degrees of accuracy in the millikelvin (mK) range, a degree of resolution by way of example which can be achieved according to the invention being 0.4 mK. The relationship between the optical path length difference OPD which is interferometrically determined by means of the measuring arrangement and the change in temperature causing that path length difference OPD is given by: 
         [0000]      OPD(δ)= dn/dδ*Δδ*L    (1)
 
         [0000]    wherein δ denotes the temperature (in units of Kelvin, K), dn/dδ denotes the temperature-dependent change in refractive index (in units of 1/K) and L denotes the measuring distance. The value of dn/dδ is known for the respective mirror material used and for example in the case of glasses with a low degree of thermal expansion is dn/dδ&gt;10*10 −6  K −1 . 
         [0079]    Determining the change in temperature in accordance with the invention can be effected directly in the EUV projection objective and also during operation thereof, for which purpose for example the measuring light can be both supplied and also taken away by way of optical fibers whereby the measuring arrangement can be connected in the form of a separate module to the projection objective. That configuration has the advantage that no additional electronics are required within the typically evacuated housing of the projection objective and also no stray light is introduced into the optical system. 
         [0080]    It will be appreciated that the measuring arrangement described with reference to  FIG. 1  is only by way of example and it is also possible to use another suitable measuring arrangement. In principle the measuring arrangement according to the invention can operate both in accordance with the so-called homodyne method (that is to say with superimposition of signals of identical frequency, as described hereinbefore) and also in accordance with the so-called heterodyne method (that is to say with superimposition of signals of differing frequency). As an example of the last-mentioned principle, it is also possible to use two laser modes at different frequencies, insofar as Zeeman splitting is effected in a resonator by the application of an external magnetic field so that two different polarisation components (with right-handed and left-handed circular polarisation respectively) are produced, which in turn can be evaluated by means of a polarisation divider. 
         [0081]    Referring to  FIG. 2  measurement according to the invention can be effected in such a way that a plurality of input measuring beams are deflected on to the mirror  201 , wherein the measuring light is coupled in and out respectively at mutually different azimuth angles. In the  FIG. 2  embodiment four measuring arrangements as were described with reference to  FIG. 1  are arranged around a mirror  201  shown in plan view, in respective mutually displaced relationship through 90° in the peripheral direction, wherein, based on the structure of  FIG. 1  for each of those measuring arrangements, a respective pair of a photodiode  250 - 253  and a deflection prism  220 - 223  are indicated here. 
         [0082]    The number and azimuthal orientation of the measuring arrangements can be selected variably in each case in accordance with the specific requirements. For example, in a further embodiment, eight measuring arrangements of the structure described with reference to  FIG. 1  can also be provided in azimuthally displaced relationship in the peripheral direction around the mirror  201  to be measured. In that case, in the course of the above-described evaluation of the different azimuthal path length changes a corresponding change in temperature and also deformation involved with that change in temperature in the axial direction (with respect to the optical axis of the system) can be inferred. Accordingly it is possible in that way to ascertain what imaging errors are produced by the deformation of the mirror, that is involved with the change in temperature caused for example by a given illumination setting. 
         [0083]    As soon as the heating condition of the mirror  101  or  201  has been determined suitable correction measures can be initiated. Reference is made to  FIG. 3  hereinafter to show how that can be effected using an external heating or cooling device in order to compensate for or homogenise the imaging errors which occur. Conventionally that is effected for example by additionally introducing specifically deformed optical elements into the beam path. An alternative approach (in which it is possible to dispense with such additional deformed optical elements) is described hereinafter with reference to  FIG. 3 . 
         [0084]    That approach is based on the fact that the effect which dominates for heat transport in the atmosphere typically used (for example krypton, argon etc) is that of heat conduction. For that purpose the structure shown in  FIG. 3  includes a matrix-like arrangement  310  of Peltier elements  310   a,  . . . on a (for example water-)cooled base plate  320 . The base plate  320  serves to take account of the limited temperature range of the Peltier elements  310   a,  . . . , within which the Peltier elements  310   a,  . . . can set or maintain a given temperature. Typically that difference temperature range is about 60-80° C. A suitable offset can be predetermined by the cooled base plate  320  for that temperature range so as to permit a reduction in the temperature to for example 10° C. or 20° C. 
         [0085]    Associated with the Peltier elements  310   a,  . . . is a measuring sensor  330  which in the illustrated embodiment is disposed on a ceramic substrate  325  and directly measures the temperature with which the respective Peltier element  310   a,  . . . radiates into the mirror  301 . Suitable cooling or heating bores can be arranged on the rear side of the mirror, which prevent the Peltier elements  310   a,  . . . from touching each other and prevent an unwanted ‘cross-talk’ occurring between them. 
         [0086]    Temperature regulation can be effected by means of the structure shown in  FIG. 3  and for example the change in temperature of the mirror  301 , resulting from a given illumination setting, can be compensated (so that for example there is no need to introduce specifically deformed transmissive elements into the beam path). 
         [0087]    Although the structure in  FIG. 3  can advantageously be combined with the characterisation of the change in temperature of the mirror, described hereinbefore with reference to  FIGS. 1 and 2 , in order in that way for example to implement regulation of the temperature on the basis of the ascertained change in temperature, the invention is not restricted thereto. Thus in further embodiments the structure of  FIG. 3  can also be used independently of measurement of the change in temperature or as a stand-alone unit in order to effect specifically targeted influencing of the temperature of the mirror  301  for example in dependence on the respective illumination setting and as a result to achieve homogenisation of the variation in temperature in the mirror  301  or optimisation of the imaging characteristics of the mirror  301 . 
         [0088]    As shown in  FIG. 3   d  (in cross-section) and  FIG. 3   e  (in plan view), the arrangement and number of the cooling or heating bores or the Peltier elements  310   a,  . . . can be varied in accordance with the respective specific requirements involved to achieve a temperature distribution which is as homogenous as possible, while  FIG. 3   f  (in section) and  FIG. 3   g  (in plan view) shows for example a concentric arrangement of the cooling and heating bores or the Peltier elements on the rear side of the mirror. 
         [0089]    Embodiments of the invention are described hereinafter with reference to  FIGS. 4   ff,  in which a change in refractive index or a refractive index gradient which is continuously set over the mirror to be measured is utilised to determine the heating condition of the mirror. 
         [0090]      FIG. 4 , to illustrate the measuring principle, firstly shows a mirror  401  to be measured in the form of a concave mirror on which light  405  is incident from an EUV light source (not shown) so that the mirror  401  heats up. The mirror  401  can be cooled on its rear side. 
         [0091]    Accordingly, there is a refractive index gradient from the light entrance surface  401   a  (which is heated most strongly) to the mirror rear side  401   b.    
         [0092]    Now, as shown in  FIG. 4 , a light beam  411  from a laser diode  410  is deflected to a position detector  420  (for example in the form of a CCD camera or a CCD array) through the mirror  401  transversely to the direction of propagation of the EUV light  405 , that is in the z-direction in the co-ordinate system, that is to say in the x-direction. Instead of the laser diode  410  it is also possible to use any other suitable light source, wherein it is preferably monochromatic in order to avoid influence of the wavelength dependency of the refractive index, and the choice of wavelength is such that the material of the mirror  401  is sufficiently translucent. 
         [0093]    As can be gathered from  FIG. 4 , the input measuring beam passes at least once through the mirror substrate without being reflected at the reflecting surface (which is the concave, upper surface of the mirror in  FIG. 4 ) of the mirror  401 . With other words, the input measuring beam passes at least once through the region between the reflecting surface of the mirror  401  and the mirror rear side  401   b.    
         [0094]    The light beam  411  which passes through the mirror  401  perpendicularly to the refractive index gradient produced by virtue of the heating condition of the mirror  401  ‘sees’ different refractive indices as a consequence of the refractive index gradient over the beam cross-section. In particular expressed in graphic terms, the left-hand edge of the light beam  411  (whose diameter can be in the range of 1-5 mm just by way of example and without the invention being restricted thereto) experiences a higher or lower refractive index than the right-hand edge of the light beam  411 , which results in different speeds of propagation over the cross-section of the light beam  411  and thus deflection of the latter. Accordingly therefore the refractive index gradient which is continuously set over the mirror  401  to be measured is converted into a change in position of the light beam  411 , that can be measured by means of the position detector  420 . 
         [0095]    It will be appreciated that the invention is not limited to the geometry shown in  FIG. 4  in respect of the configuration of the light beam  411  through the mirror  410 . All that is required for functioning of the measurement principle described with reference to  FIG. 4  is that the refractive index gradient has a component perpendicularly to the direction of propagation of the light beam  411  so that as a result that provides for bending of the wave front and thus beam deflection of the light beam  411 . 
         [0096]    In further embodiments a plurality of light beams can also be deflected by the mirror  401  insofar as a corresponding arrangement (linear, in matrix form or any other) of laser diodes (for example at a spacing of a few millimeters from each other) is arranged along the mirror and the change in position is ascertained by way of a suitable position detector arrangement (for example in the form of a quadrant photodiode or another, in particular matrix-form detector arrangement). In further embodiments therefore a plurality of light beams can also be deflected by the mirror  401  at different beam angles, whereby in particular account can also be taken of the configuration (being concave in the example) of the curved optically effective surface of the mirror  401  and the variation involved therewith in the change in temperature of the mirror in the x- and y-directions respectively. 
         [0097]    A plurality of light sources or laser diodes  401  can also be arranged in azimuthally displaced relationship in the peripheral direction around the mirror  401  to provide information about the temperature distribution of the mirror  401  and any symmetry in heating in conjunction with given illumination settings. In that way it is possible to take account of or measure a non-homogenous heating effect of the mirror  401 , that is involved with the use of given illumination settings. The passage of a plurality of light beams through the mirror  401  at different beam angles can be effected both by means of an arrangement of a plurality of light sources or laser diodes  410  at different angles or also by a suitable configuration of reflecting mirror side surfaces on the mirror  401 . 
         [0098]    In further embodiments the measuring section can also be increased in length by the respective mirror side surfaces being of a reflecting nature by means of a suitable coating (for example of aluminum) so that this provides an increase in the resulting beam deflection and thus also an increase in the accuracy of measurement. When using only one measuring section or laser diode, it may be advantageous if, as shown in  FIG. 4 , it is arranged in the immediate proximity (for example at a spacing of less than 2 mm) of the light entrance surface  410   a  or tangentially in relation thereto as the refractive index gradient to be utilised can be particularly pronounced in that region, depending on the respective configuration of the mirror  401  (in particular if it is made from a material of high thermal conductivity). 
         [0099]    The operation of determining the change in refractive index, which is effected as shown in  FIG. 4 , can also be effected directly in the EUV projection objective and also directly during operation thereof, for which purpose for example the measuring light can be supplied and also carried away by way of optical fibers. That design configuration has the advantage that no additional electronics are required within the typically evacuated housing of the projection objective and also no additional stray light can be introduced into the optical system. 
         [0100]    A calibration operation can be performed prior to the above-described measurement procedure, in the course of which calibration the relationship between the change in position of the light beam  411  and the change in temperature of the mirror  401  is ascertained for predetermined and specifically set values of the change in temperature. 
         [0101]    Although the measurement principle described with reference to  FIG. 4  was described using the example of a concave mirror  401  the invention is not limited thereto. Thus the mirror for example can also be convexly curved or planar. In particular the refractive index gradient required for the measurement procedure is afforded for example when active cooling is effected on the rear side of the mirror, within all of the mirror material, so that the light beam  410  can also pass through at a greater spacing from the optically effective surface  401   a  and the curvature of that optically effective surface  401   a  no longer plays any significant part. However, even in the situation where the refractive index gradient is produced substantially over a distance of a few millimeters within the mirror material and thus only in the immediate proximity of the optically effective surface  401   a,  the configuration of the light beam  411  through the mirror  401 , as described hereinbefore, can be implemented at different beam angles and thus can be adapted to the curvature of the optically effective surface  401   a.    
         [0102]    In accordance with further embodiments, to suppress interfering measurement noise and to increase measuring accuracy by means of the measuring arrangement of  FIG. 4 , a lock-in procedure can be employed. In that way, it is possible to take account of the fact that, in operation of a projection exposure apparatus, illumination is typically effected in pulsed fashion, wherein a typical frequency in a projection exposure apparatus designed for EUV can be in the region of 10 kHz so that under some circumstances the temperature gradient can also be built up and reduced again with a corresponding frequency, in the mirror material. 
         [0103]    Further embodiments of the invention are described hereinafter with reference to  FIGS. 5 through 7 , which are based on the change in the refractive index of semiconductor layers with temperature. Those semiconductor layers can be in the form of so-called DBRs (=‘Distributed Bragg Reflectors’). DBRs have an alternate arrangement of high-refractive and low-refractive materials (for example AlGaAs and GaAs semiconductors), which provides that upon irradiation with light for example of a 1 μm wavelength, a proportion of the light is reflected back at each second interface. 
         [0104]    As shown in  FIG. 5 , such a DBR  610  is applied to a mirror  601  to be measured (comprising a substrate and a reflecting surface), this in practice being effected in the optically unused region of the mirror  601 . In particular, the DBR  610  can be applied to the mirror substrate of the mirror  601 . Contacting can be effected for example by way of a silver conductive adhesive or indium to achieve a resistance which is as low as possible at the transition between the mirror  601  and the DBR  610 . The DBR  610  is kept at the temperature of the mirror  601  as a consequence of the contact therewith. 
         [0105]    A light source  620  (for example a laser diode) whose wavelength is preferably in the infrared range (for example at λ=1 μm) deflects the measurement light on to the DBR  610  and a suitable detector  630  (for example a CCD camera) measures the light reflected at the DBR  610 . The invention also comprises embodiments in which the input measuring beam from the light source  620  passes through the mirror substrate in order to reach the DBR  610  and/or in order to arrive (from the DBR  610 ) at the detector  630 . The use of infrared light is particularly suitable for the reason that the influence of temperature on the reflectivity curve in the infrared range is substantially greater than for example with EUV light. The measurement arrangement  600  of  FIG. 5 , like the measuring arrangements of the above-described embodiments, has the advantage that measurement is effected in completely contact-less fashion. 
         [0106]    As a consequence of mounting the DBR  610  as shown in  FIG. 5  to the mirror  601  to be measured a change in temperature of the mirror  601  leads to a shift in the reflection spectrum as the refractive index, the band gap and (as a consequence of thermal expansion) also the layer thickness of the DBR  610  change as a consequence of the variation in temperature. The change in temperature of the mirror  601  can thus be inferred from the change in reflectivity of the DBR  610 , which is involved with a change in temperature of the mirror  601 . 
         [0107]    In accordance with a further measurement principle described with reference to  FIGS. 6 and 7 , an optical element  710  can also be mounted to a mirror  701 (comprising a substrate and a reflecting surface) to be measured, wherein two DBRs are combined in the element  710  with a laser-active zone to give a surface-emitting laser (VCSEL). In particular, the optical element  710  can be applied to the mirror substrate of the mirror  701 . Light has to be reflected to and from between the two DBRs to excite the laser-active zone. Upon a change in temperature the reflection capability of the DBRs now changes, while in addition thermal expansion of the laser-active zone therebetween also occurs, so that a change in temperature is involved with a change in the base emission mode of the surface-emitting laser (VCSEL), which in turn is determined as shown in  FIG. 6  with a spectrometer  730 . In other words the change in temperature of the mirror  701  ultimately leads to a shift in the wavelength of the light emitted by the surface-emitting laser (VCSEL), wherein that shift which is dependent on the temperature of the mirror  701  can typically be in the range of 0.2-0.3 nm/K. In that case a surface-emitting laser (VCSEL) which is for example suitable according to the invention is based on AlGaAs and GaAs semiconductors which can have an emission wavelength in the range of between 800 nm and 1100 nm. The invention also comprises embodiments in which the input measuring beam from the light source  720  passes through the mirror substrate in order to reach the optical element  710  and/or in order to arrive (from the optical element  710 ) at the spectrometer  730 . 
         [0108]    In further embodiments, instead of the VCSEL, it is also possible to use a quantum cascade laser which can be excited by way of microwaves (by using a microwave source instead of the light source  710 ). 
         [0109]    As the change in emission wavelength of the surface-emitting laser, that is utilised with the measurement principle described hereinbefore with reference to  FIG. 6 , also occurs with slight changes in temperature of the mirror (for example by only 0.01 K), the limiting factor for temperature resolution is ultimately the wavelength resolution of the spectrometer  730 . In that respect, temperature resolutions in the range of 0.3-0.5 K can already be achieved with spectrometers which are available as standard. Higher levels of temperature resolution for example up to 0.05-0.1 K are possible for example when using a Fourier-transforming spectrometer which permits levels of wavelength resolution in the region below 0.1 nm (typically between 0.035 mm and 1.5 μm). 
         [0110]    As regards the working wavelength of the measurement light source  620  or  720  respectively, used in the measuring arrangements of  FIG. 5  and  FIG. 6 , that is preferably approximately at the center of the reflection curve with the approach described with reference to  FIG. 6 , so that an effect in respect of the laser-active zone that is as pronounced as possible is achieved (in which the change in temperature leads in particular to a change in the band gap and thermal expansion in the laser-active zone). In contrast, with the approach described hereinbefore with reference to  FIG. 5 , a working wavelength in the region of the edge of the reflection spectrum R(λ) is preferably adopted so that the influence of the change in temperature on the reflectivity of the DBR  610  is as great as possible or at its maximum. 
         [0111]      FIG. 7  shows a specific measuring arrangement by way of example, in which an optical element  810  mounted to the mirror  801  to be measured, in the form of a surface-emitting laser, is optically pumped, whereby electric feed lines to the surface-emitting laser are avoided and contact-less measurement is made possible. For that purpose, as shown in  FIG. 7 , a commercially available laser diode  820  with a glass fiber connection serves as the pump source, wherein a wavelength different from the wavelength of the surface-emitting laser is selected as the wavelength of the laser diode  820 . In this case the pump light should lie in terms of energy above the laser emission and outside the stopband of the DBR. 
         [0112]    The light of the laser diode  820  is passed by way of a glass fiber  805  and a beam splitter  830  as well as an UHV-compatible glass fiber transmission means  840  into the vacuum chamber of the projection objective of the projection exposure apparatus. There the light can be further passed by way of UHV-compatible glass fibers to the respective measurement points at one or more mirrors  801  of the projection objective. A suitable collimator optical system (not shown) at the end of the glass fibers focuses the light on to the surface-emitting laser  810  on the mirror  801  and again couples the light emitted by the surface-emitting laser  810  into the glass fiber  805 . The collimator optical system can be of such a design configuration that the surface-emitting laser  810  or VCSEL chip is of a size of 0.5*0.5 mm 2 . The emitted light is passed back on to the beam splitter  830  by way of the glass fiber  805  and from there coupled into a detector  860  by way of an edge filter  850  which fades out the pump wavelength. 
         [0113]    Coupling the pump light into and out of the surface-emitting laser  810 , which is adopted in the  FIG. 7  structure, and also coupling the light emitted by the surface-emitting laser  810  into the same glass fiber  805 , has the advantage that additional glass fibers are unnecessary. On the other hand, the unwanted introduction of stray light into the optical system is also prevented in comparison for example with optical pumping (which in principle is also possible) in a direction sideways or parallel to the mirror surface. 
         [0114]    A thermal contact of the component which is respectively used (DBR or VCSEL) in relation to the mirror to be measured is necessary in each case in the embodiments described with reference to  FIGS. 5-7 , but in other respects ‘interrogation’ of the component in question (DBR or VCSEL) is effected in contact-less fashion. Similarly to the above-described embodiments it is also possible to use a plurality of the respective components employed (DBR or VCSEL), which in turn can be effected for example in a matrix-like arrangement. 
         [0115]    Even if the invention has been described by reference to specific embodiments numerous variations and alternative embodiments will be apparent to the man skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly it will be appreciated by the man skilled in the art that such variations and alternative embodiments are also embraced by the present invention and the scope of the invention is limited only in the sense of the accompanying claims and equivalents thereof.