Abstract:
An arrangement for and a method of characterizing the polarization properties of an optical system, in particular an optical system of a microlithographic projection exposure apparatus. The arrangement includes at least one polarization state generator ( 130, 230, 330 ) which sets a defined polarization state of radiation incident on the optical system, and a polarization state detector ( 140, 240, 340 ) adapted to measure the exit polarization state of radiation issuing from the optical system, wherein the optical system is designed for a working wavelength of less than 15 nm, and wherein the polarization state generator and/or the polarization state detector are so designed that their polarization-optical action on an incident light beam is substantially constant over an angle spectrum of the light beam of at least 10°.

Description:
The present application is a Continuation of International Application No. PCT/EP2010/068713, filed on Dec. 2, 2010, which claims priority of German Patent Application 10 2010 001 336.6, filed on Jan. 28, 2010. The contents of these applications are hereby incorporated by reference in their respective entireties. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The invention concerns an arrangement for and a method of characterising the polarisation properties of an optical system. 
     Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated by the illumination system is projected by 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. 
     It is known that, in high-resolution imaging systems such as for example the above-described microlithographic projection exposure apparatus, particularly with high numerical apertures, the influence of the imaging system on the polarisation state of the radiation which in operation passes through the imaging system can no longer be disregarded. That is to be attributed to changes in the imaging contrast due to polarisation-influencing effects (for example stress birefringence induced by holder components in the material of the optical components such as for example lenses or mirrors, polarisation-influencing effects of dielectric layers and so forth). 
     It is thus desirable to determine the polarisation properties of such imaging systems, in particular of high aperture, as reliably as possible, in order on the one hand to draw appropriate conclusions concerning the polarisation-dependent imaging quality, and on the other hand to be able to take possibly suitable measures for manipulation of the polarisation properties. 
     U.S. Pat. No. 7,286,245 B2 discloses inter alia a method of and an apparatus for determining the influence of the polarisation state of optical radiation by an optical imaging system, wherein a defined entrance polarisation state is afforded in an object plane of the imaging system and wherein the exit polarisation state of radiation issuing from the imaging system is measured in pupil-resolved relationship within a predeterminable pupil region of the imaging system. The imaging system can be for example a projection objective of a projection exposure apparatus designed for the wavelength range around 248 nm or 193 nm. 
     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. In that case the problem which arises in connection with the above-mentioned characterisation of the polarisation properties is that the implementation of a measurement structure like that described hereinbefore using reflective optical components can lead to considerable problems in regard to the required structural space, going as far as lack of viability. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an arrangement for and a method of characterising the polarisation properties of an optical system, which permit reliable characterisation of the polarisation properties even at working wavelengths in the EUV, and preferably with a compact structure. 
     An arrangement for characterising the polarisation properties of an optical system, in particular an optical system of a microlithographic projection exposure apparatus, comprises:
         at least one polarisation state generator which sets a defined polarisation state of radiation incident on the optical system; and   a polarisation state detector adapted to measure the exit polarisation state of radiation issuing from the optical system;   wherein the optical system is designed for a working wavelength of less than 15 nm; and   wherein the polarisation state generator and/or the polarisation state detector are so designed that their polarisation-optical action on an incident light beam is substantially constant over an angle spectrum of said light beam of at least 10°.       

     The criterion whereby ‘the polarisation-optical action’ of the polarisation state generator and/or the polarisation state detector on an incident light beam is constant signifies that the polarisation state generator or polarisation state detector produce the same polarisation state over said angle spectrum, which in turn can be defined in that, for the light issuing from the polarisation state generator and/or polarisation state detector, the so-called IPS value (=‘intensity in preferred state’) is constant over said angle spectrum if the IPS value is constant for the light incident in the polarisation state generator or the polarisation state detector. In that respect the expression ‘substantially constant’ is also intended to embrace fluctuations in the IPS value of up to 10%, in particular up to 5%. In addition the desired polarisation state is not necessarily linear polarisation but can also be any other polarisation state, for example elliptical or circular polarisation. 
     With the arrangement according to the invention, the invention pursues in particular the concept of designing the components used for characterising the polarisation state, the polarisation state generator and the polarisation state detector, respectively for operation in the transmission mode at working wavelengths in the EUV range. In accordance with another approach the polarisation state generator and/or the polarisation state detector have at least one optical element which is operated in the transmission mode. In that way once again particularly in optical systems with a working wavelength of about 13.5 nm, a markedly more compact structure is achieved as reflecting polarisation-optical components with comparatively complicated beam paths and more complex and expensive actuators are avoided. 
     In that respect it is to be noted in particular that the option which exists in the DUV wavelength range, that is to say at wavelengths of for example about 248 nm or about 193 nm, of being able to operate the polarisation-optical components in a parallel beam geometry (that is to say with mutually parallel beam portions of the beam which passes through the respective component) is no longer afforded in the EUV range embraced in accordance with the invention as none of the suitable optical components used in the DUV range, in the form of lens systems, are now available to transform the divergent or convergent beam path into a parallel beam path. 
     A further advantage of the configuration according to the invention is that the components operated in the transmission mode can be relatively easily incorporated at the respectively appropriate positions in the beam path without a significant change in the beam path being required for that purpose. 
     Furthermore, in accordance with the invention, in spite of the above-mentioned divergent or convergent beam paths, it is possible to implement simultaneous or parallel measurement of the exit polarisation state of radiation issuing from the optical system. That means that the exit pupil of the system is measured ‘all at once’, that is to say beams are simultaneously passed with larger aperture angles through the respective polarisation-optical component or are detected by a detector element at the end of the arrangement such as for example a CCD camera. That is advantageous insofar as scanning of the exit pupil—which is usual in conventional measurement operations for the polarisation properties in EUV—and in which a comparatively narrow parallel beam has to be ‘displaced’ successively over the entire angular range to perform successive individual measurements is dispensable. Besides the time advantages resulting from simultaneous measurement that affords further advantages in terms of avoiding positioning problems and drift errors. 
     Although an advantageous use of the invention is measurement of the polarisation properties of a projection objective the invention is not limited thereto. Rather, the term ‘optical system’, the polarisation properties of which are characterised in accordance with the invention, also embraces any other optical systems and in particular also individual optical elements such as for example mirrors. 
     In an embodiment the polarisation state generator and/or the polarisation state detector are so designed that their polarisation-optical action on an incident light beam is substantially constant over an angle spectrum of that light beam of at least 15°, in particular at least 20°, and further particularly over the entire angle spectrum of said light beam. 
     In an embodiment the polarisation state generator is adapted for pupil-resolved measurement of the exit polarisation state. In that respect the values implemented for pupil resolution can be for example at least 30 pixels, in particular at least 40 pixels, further particularly at least 50 pixels, over the pupil radius. 
     In an embodiment the polarisation state generator and/or the polarisation state detector has a rotatable polariser. In addition the polarisation state generator and/or the polarisation state detector can have a rotatable retarder. 
     In an embodiment the polariser and/or the retarder has at least one multi-layer system having a plurality of individual layers. Preferably in that respect the multi-layer system has a light entrance surface which is at least region-wise curved, i.e., curved at least in regions along the surface. In accordance with the invention that can be implemented in particular by the multi-layer system being of a varying thickness. In a further embodiment the plurality of individual layers can also be disposed on a substrate which is at least region-wise curved. 
     In an embodiment the plurality of individual layers is arranged either in free-standing or substrate-less relationship or on a substrate of a thickness of a maximum of 400 nm, preferably a maximum of 100 nm, and further preferably a maximum of 50 nm to achieve a sufficiently large proportion of the transmitted light. Suitable substrate materials to be considered are in particular those of comparatively low transmission, for example silicon (Si), quartz glass (=fused silica, SiO 2 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), polymers, quartz (SiO 2 ,), zirconium (Zr), diamond, niobium (Nb) and molybdenum (Mo). 
     In an embodiment the polariser and/or the retarder can also have a plurality of substrates each provided with a multi-layer system. In that respect at least two of those substrates (in particular also all of those substrates) can be arranged tilted relative to each other to provide the arrangement consisting of the substrates respectively provided with the multi-layer system with the desired homogeneity of the polarisation-influencing action in the angle spectrum. 
     In an embodiment the respective polariser produces a linear exit polarisation degree of radiation issuing from the polariser of at least 95%, in particular at least 97%, further particularly at least 99%. In that respect the expression ‘linear exit polarisation degree’ is used to denote the ratio of the intensity of the linearly polarised light component to the total intensity of the light, which ratio applies for the light issuing from the respective polariser. 
     In an embodiment the arrangement further has a wavelength filter which again is preferably adapted to filter a predetermined wavelength band out of the wavelength spectrum of the light source in the (measurement) arrangement. That is advantageous in regard to the fact that, in a projection exposure apparatus designed for EUV, the transmission bandwidth of the projection objective is comparatively great at about 13.5±0.3 nm and is also exceeded by the bandwidth of the plasma light source used as the light source for the illumination system. 
     Measurement involving integration over those relatively wideband wavelength ranges could not be readily transferred to the actual operating characteristics of the projection objective if the spectra of the plasma light source used in the measurement operation and the plasma light source used in the actual lithography procedure are not the same. In contrast, by using the wavelength filter (which is sufficiently narrowband, for example involving a bandwidth of at most 1/20 of the transmission bandwidth of the projection objective), it is possible to provide for a wavelength-resolved measurement in order to be able to make a prediction of the polarisation properties of the projection objective for the respective entrance spectrum. 
     In an embodiment the optical system is designed for a numerical aperture of at least 0.3, in particular at least 0.5, further particularly at least 0.7. With increasing apertures the invention is particularly advantageous as then overcoming the problems involved with the larger aperture angles which occur in the exit pupil is particularly useful. 
     The invention further concerns an apparatus for EUV lithography comprising a projection objective designed for operation in the EUV and an arrangement for characterising the polarisation properties of said projection objective, wherein that arrangement is designed like the above-described arrangement for characterising the polarisation properties of an optical system. 
     Furthermore the invention concerns a method of characterising the polarisation properties of an optical system, in particular an optical system of a microlithographic projection exposure apparatus, wherein the method comprises the following steps:
         setting a defined polarisation state of radiation incident on the optical system with a polarisation state generator; and   measuring the exit polarisation state of radiation issuing from the optical system with a polarisation state detector;   wherein the optical system is designed for a working wavelength of less than 15 nm; and   wherein the polarisation state generator and/or the polarisation state detector have at least one optical element operated in the transmission mode.       

     Further configurations of the invention are found in the description and the appendant claims. The invention is described in greater detail hereinafter by means of preferred embodiments by way of example with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a diagrammatic view to illustrate the structure of an arrangement according to the invention for characterising the polarisation properties in a first embodiment, 
         FIGS. 2-3  show diagrammatic views to illustrate further embodiments of the invention which are simplified in relation to  FIG. 1 , 
         FIGS. 4   a - c  show diagrammatic views to illustrate embodiments by way of example of multi-layer systems used in an arrangement as shown in  FIGS. 1-3 , 
         FIGS. 5-6  show diagrammatic views to illustrate further embodiments of multi-layer systems according to the invention, and 
         FIGS. 7-11  show diagrams for characterising various embodiments of multi-layer systems used according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  firstly shows a diagrammatic view of a possible measurement structure using an arrangement according to the invention. The arrangement serves for characterising the polarisation properties of a projection objective  120  which is designed for operation in the EUV range and which with an illumination system  110  also designed for EUV forms a microlithographic projection exposure apparatus. 
     The method implemented using the measurement structure shown in  FIG. 1  is known as such from U.S. Pat. No. 7,286,245 B2 for wavelengths in the DUV range (for example about 193 nm or about 248 nm). The subject-matter of the present application is however the use of that method in the EUV range, that is to say at wavelengths below 15 nm, wherein account is taken of the problems arising in that use as explained hereinafter by a suitable design configuration of the polarisation-optical components. More precisely the method according to the invention is distinguished by the use of wideband polarisation-optical components both in respect of the wavelength spectrum and also in respect of the angle spectrum. 
     In that respect the invention pursues in particular the concept of using the polarisation-optical components in the transmission mode in spite of the use in the EUV range and thus avoiding reflecting polarisation-optical components with comparatively complicated beam paths and also more complex and expensive actuators. 
     As shown in  FIG. 1  the arrangement according to the invention includes, in the light propagation direction, downstream of the illumination system  110 , a polarisation state generator  130  which has in succession in the light propagation direction a rotatable polariser  131  and a rotatable retarder  132 . The configuration of the polariser  131  and the retarder  132  is described in greater detail hereinafter with reference to  FIGS. 4-6 . 
     The polariser  131  is preferably so designed that it produces a polarisation state which is linear at least to a good level of approximation, wherein the preferred polarisation direction is variable by rotation of the polariser  131 . The production of light which is polarised linearly as completely as possible by the polariser  131  also provides for maximising the light contribution to the operation of determining the polarisation state, which takes place in the further procedure in the polarisation state detector  140  (whereas a non-polarised light component which remains on issuing from the polariser  131  would not contribute to the measurement procedure according to the invention). 
     The retarder  132  preferably has an effective retardation of a quarter of the working wavelength λ of the system (that is to say for example (13.5/4) nm), in which case the polarisation state generator  130  makes it possible to set any (including circular) polarisation states. The polarisation state generator  130  accordingly thus makes it possible to set different elliptical polarisation states in the entrance pupil of the projection objective  120 . 
     Disposed downstream of the polarisation state generator  130  in the light propagation direction is an apertured mask  150  (=pinhole) which approximately forms a point light source in the (entrance) field plane of the projection objective  120 . 
     The light from the pinhole  150  then passes through the projection objective  120 , wherein the angles of the beam portions coming from the point light source formed by the pinhole  150  correspond to positional co-ordinates in the pupil plane (not shown) of the projection objective  120 , which in turn are imaged at an angle in the exit pupil of the projection objective  120 . Polarisation distribution over that exit pupil, that is to say the exit polarisation state of the light issuing from the projection objective  120 , is ascertained in pupil-resolved relationship with a polarisation state detector  140 . That operation of determining the exit polarisation state is effected in a manner which is basically also known, simultaneously over the exit pupil, that is to say the exit pupil of the system is measured ‘all at once’, insofar as beams are also simultaneously passed at relatively large aperture angles through the respective polarisation-optical component, or are detected by a detector element at the end of the arrangement such as for example a CCD camera. 
     For that purpose the polarisation state detector  140  in turn has a rotatable retarder  141  and downstream thereof in the light propagation direction a polariser  142  which is preferably also rotatable as well as a detector element  143  for example in the form of a CCD camera. Accordingly the CCD camera measures the distorted projection image of the exit pupil of the projection objective  120 . 
     The retarder  141  preferably also has a retardation corresponding to a quarter of the working wavelength (=lambda/4). That affords an optimum signal-to-noise ratio in the measurement operation as then the variation in intensity which occurs on the detector element  143  or the CCD camera during the measurement operation becomes a maximum in dependence on the rotary position of the retarder  141  and the polarisation properties of the projection objective  120 . 
     An important property of the illustrated arrangement is now that the light is incident on the polarisation-optical components in the polarisation state detector  140  and also in the polarisation state generator  130 , at different angles of incidence, since, as already explained in the opening part of this specification, the lens systems usual in the DUV range are not available for converting a divergent beam path into a parallel beam path. In accordance with the invention the problems arising out of the inclined passage of light both on the part of the polarisation state generator  130  and also on the part of the polarisation state detector  140  are resolved by a suitable design in respect of the polarisation-optical components, as is described in greater detail hereinafter with reference to  FIGS. 4-6 . 
     The measurement structure described hereinbefore with reference to  FIG. 1  makes it possible to determine the polarisation properties of any optical system. Simplified measurement structures for characterising optical systems with more specific polarisation properties are described hereinafter with reference to  FIG. 2  and  FIG. 3 . 
       FIG. 2  shows an embodiment which is an alternative to the measurement structure in  FIG. 1 , wherein mutually corresponding components or components of substantially the same function are denoted by references increased by ‘100’. 
     The measurement structure in  FIG. 2  differs from that in  FIG. 1  in that the polarisation state generator  230  or the polarisation state detector  240  respectively each has only a rotatable polariser  231  and  241  respectively, but no retarder. Such a structure which is simplified in comparison with  FIG. 1  is suitable for determining the polarisation properties of a projection objective  230 , whose pupil Jones matrices in a good approximation have exclusively linearly polarised eigen polarisations. 
     The polarisers  231  and  241  can either be oriented in mutually crossed relationship (to produce a dark-field polarimeter) or in mutually parallel relationship (to produce a light-field polarimeter) and rotate synchronously with each other about the optical axis of the projection objective  230 . During that rotation measurement is made by means of the detector element  143  or the CCD camera of the variation in intensity, from which in turn polarisation properties, namely retardation (that is to say the phase difference between two orthogonal polarisation states) and diattenuation (that is to say the ratio of the amplitudes between two orthogonal polarisation states) of the projection objective are calculated. 
       FIG. 3  shows a further simplified measurement structure, wherein once again components which are similar to  FIG. 2  or which involve essentially the same function are identified by references increased by ‘100’. 
     The measurement structure in  FIG. 3  differs from that in  FIG. 2  in that a rotatable polariser  331  is provided only within the polarisation state generator  330 , and therefore the polariser  331  in the polarisation state generator  330  represents the sole polarisation-optical element of the measurement structure in  FIG. 3 . In contrast the polarisation state detector  340  includes only a measurement head with CCD camera which makes it possible to record the distribution of intensity in the exit pupil and measurement of the exit wave front (for example by interferometric means). 
     In operation of the arrangement shown in  FIG. 3  the polariser  331  is rotated about the optical axis or light propagation direction of the projection objective  320 . In that case, in dependence on the rotary position of the polariser  331  (that is to say in dependence on the direction of the linear entrance polarisation) the resulting variation in the intensity in the exit pupil and the wave front is ascertained. The retardation and also diattenuation (in each case both in respect of magnitude and also axis directions) can be ascertained in per se known manner from the amplitude and the phase of that variation in intensity. 
     Reference will now be made to  FIGS. 4   a  and  4   b  to describe embodiments by way of example of multi-layer systems used in the arrangements shown in  FIGS. 1-3  for embodying the polarisation-optical components used there. 
     In these embodiments the multi-layer system used in the polarisation-optical elements (polariser and/or retarder) used according to the invention is respectively so designed that account is taken of the fact that, in contrast to a projection exposure apparatus operated at wavelengths in the DUV range (for example of about 193 nm or about 248 nm), in the present case, that is to say in the EUV range, no refractive optical elements suitable for affording a structural space that is still acceptable are available for producing a beam path parallel to the optical axis. 
     In accordance with the invention both at the entrance side and also at the exit side, in relation to the projection objective  320 , that problem is resolved in that, in spite of the divergent beam path shown in each of  FIGS. 4   a - b , homogeneity of the polarisation-optical effect produced by the multi-layer system is achieved in the angle spectrum both in the entrance pupil and also in the exit pupil. 
     What is common to the embodiments shown in  FIGS. 4   a - b  is that in each case a multi-layer system  460  and  470  respectively is so designed that it has a light entrance surface which is at least region-wise curved. 
     As shown in  FIG. 4   a  that curved light entrance surface is embodied in that the application of a plurality of individual layers, identified by reference  462 , to a curved substrate  461 , is effected in such a way that each of the beam portions S 1 , S 2  and S 3  is incident on the multi-layer system  460 , having regard to the aperture of the system, at almost the same angle of incidence γ. 
     Multiple reflection of the incident radiation occurs in the multi-layer system, wherein the proportion which is finally transmitted through the multi-layer system and which accordingly is to be as large as possible is considered. For that purpose the substrate  461  is preferably of a relatively small thickness of for example not more than 400 nm. 
     Suitable substrate materials are for example silicon (S), silicon nitride (Si 3 N 4 ) or silicon carbide (SiC). The multi-layer system  460  and  470  respectively alternately comprises comparatively high-refractive and low-refractive layers (for example a succession of molybdenum (Mo) and silicon (Si)). 
       FIG. 4   b  shows an alternative configuration of a multi-layer system  470 , in which case the plurality of individual layers, identified by reference  472 , for producing the multi-layer system  470 , is provided on a flat substrate  471  with a varying layer thickness (shown on an exaggerated scale in  FIG. 4   b ). The multi-layer system  470  is made up of optically anisotropic layer materials, in which respect advantage is taken of the fact that, as a consequence of the anisotropy and the variable layer thickness, once again all beam portions S 1 , S 2  and S 3  experience the same polarisation action or the same retardation having regard to the aperture of the system. 
       FIG. 4   c  shows a multi-layer system  480  which is similar to the embodiment of  FIG. 4   b  with a varying thickness profile, having a plurality of individual layers identified by reference  482 , being arranged in tilted relationship with the beam path or the optical axis of the system on a flat substrate  481  to achieve the desired homogeneity of the polarisation-influencing action in the angle spectrum, having regard to the aperture of the system. 
       FIGS. 5   a  and  5   b  show further embodiments for embodying the above-described polarisation-optical elements. In these embodiments it is assumed that the respectively desired polarisation-optical action cannot be achieved over a sufficiently large range of angles of incidence, even with the curved light entrance surfaces, in accordance with the configurations shown in  FIGS. 4   a - c.    
     For that case it is advantageous, as shown in  FIG. 5   a , to provide a plurality of substrates which in the example are in the form of plane-parallel plates and of which only five substrates  561  through  565  are shown by way of example in  FIG. 5   a , with a respective multi-layer system, and to arrange same in suitably tilted relationship at different positions in the beam path. The substrates  561  through  565  also preferably involve a relatively small thickness of for example a maximum of 400 nm, preferably a maximum of 100 nm and further preferably a maximum of 50 nm. 
     While the arrangement  560  shown in  FIG. 5   a  comprising the plurality of substrates  561  through  565  for use in a parallel-beam geometry has the same respective angle for the individual substrates  561  through  565  relative to the light propagation direction, in accordance with the non-parallel-beam geometry shown in  FIG. 5   b  the individual substrates  571  through  575  of the arrangement  570  are tilted at different angles to the optical axis. Accordingly, in both embodiments in  FIGS. 5   a  and  5   b , the desired homogeneity of the polarisation-influencing action in the angle spectrum is again achieved, having regard to the aperture of the system. 
     In a further embodiment diagrammatically shown in  FIG. 6 , to implement a wavelength filter, a wedge-shaped coating  682  with a material with a refractive index different from one (for example molybdenum (Mo) or ruthenium (Ru)) is also applied to a substrate  681 , for example using displaceable orifice plate members, so that the effect ultimately achieved corresponds to that of a plurality of individual prisms and is ‘tunable’ by way of the tilt. In that case advantage is taken of the wavelength dependency of the deflection angles in prisms by virtue of the dispersion of the refractive index. 
     Reference is now made to  FIGS. 7-11  and Tables 1-4 to describe embodiments by way of example of multi-layer systems suitable for use in polarisation-optical components used according to the invention. 
     Table 1 firstly shows a design of a multi-layer system suitable for embodying a polariser, for example in the polarisation state generator  130 ,  230  or  330  or in the polarisation state detector  140 ,  240  or  340 . The layer design of Table 1 uses exclusively molybdenum (Mo) and silicon (Si) as layer materials. That layer design is optimised in the transmission mode for an angle of 43° in the proximity of the pseudo-Brewster angle (near 45°). In regard to the layer design advantage is taken of the fact that s-polarised light exhibits markedly greater Fresnel reflection at the Mo—Si interfaces than p-polarised light. The layer design is comparable to that of a mirror designed for the corresponding angle of incidence, typical individual layer thicknesses being in the region of a quarter of the working wavelength. In general terms, in the configuration of the layer design both for the polariser and also for the wavelength filter, attention is to be paid to the fact that the total thickness does not become too great as otherwise the transmitted proportion of light becomes excessively small. 
     As can be seen from the transmission characteristics shown in  FIG. 7   a  in dependence on the angle of incidence for s- and p-polarised light respectively, s-polarised light is greatly reflected in the relevant angle range around about 43° while predominantly p-polarised light is transmitted. 
       FIG. 8  shows, for the Table 1 layer design, the dependency of transmission for s- or p-polarised light respectively on the angle of incidence for different wavelengths ( FIG. 8   a ) and for different layer thicknesses ( FIG. 8   b ). It will be seen from  FIG. 8   a  that the multi-layer system is in each case only suitable in a restricted wavelength and angle range with the desired action so that, in the case of a variation in the wavelength, over a greater range, the multi-layer system is to be arranged in correspondingly tilted relationship.  FIG. 8   b  further shows that the angle of incidence at which the multi-layer system exhibits the desired action can be varied by altering the total layer thickness (wherein all individual thicknesses are altered by the same factor). 
     Table 2 and  FIGS. 9   a - 9   b  show views similar to the embodiment of Table 1 and  FIGS. 7   a - 7   b  of a further embodiment of a multi-layer system in which, in addition to molybdenum (Mo) and silicon (Si), ruthenium (Ru) is also used as a further layer material. The use of ruthenium (Ru) as a further layer material is in this case only by way of example and it is also possible to employ other suitable materials with different or deviating refractive index n as well as comparatively slight attenuation such as for example silicon (Si), potassium (K), silicon carbide (SiC), yttrium (Y), zirconium (Zr), boron carbide (B 4 C), boron (B), carbon (C), silicon nitride (Si 3 N 4 ), boron nitride (BN), niobium (Nb), molybdenum carbide (MoC), molybdenum (Mo) or rhodium (Rh). 
     Table 3 and  FIGS. 10   a - b  describe an embodiment for a retarder layer, for example for use in the polarisation state generator  130 ,  230  or  330  or the polarisation state detector  140 ,  240  or  340 . The layer system is optimised for an angle of incidence of about 55°, wherein—insofar as similar to the embodiment in Table 1—once again exclusively molybdenum (Mo) and silicon (Si) are used as layer materials and the individual layer thicknesses are again in the region of a quarter of the working wavelength. 
     Table 4 and  FIG. 11  shows an embodiment for a layer design which is suitable for embodying a wavelength filter or a ‘monochromator layer’ in the above-described measurement arrangements. Although in this embodiment molybdenum (Mo), silicon (Si) and ruthenium (Ru) are used as layer materials, that is not absolutely necessary, but similarly to the above-described embodiments it is also possible to implement a layer design using only two different layer materials (for example molybdenum and silicon). 
     The property of the layer system of Table 4 whereby only a given wavelength of the incident light is transmitted is ‘tunable’ as shown in  FIG. 11  by way of the angle of incidence (specified in the legend), that is to say the wavelength to be transmitted can be predetermined or selected by way of the tilt angle of the substrate. In particular operation is also possible with almost perpendicular light incidence. 
     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. 
     The above description of the embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Layer design for FIGS. 7-8 
               
             
          
           
               
                 Layer No. 
                 Thickness[nm] 
                 Material 
               
               
                   
               
             
          
           
               
                   
                   
                 Substrate 
               
               
                 1 
                 4.854 
                 Mo 
               
               
                 2 
                 5.17 
                 Si 
               
               
                 3 
                 4.912 
                 Mo 
               
               
                 4 
                 5.185 
                 Si 
               
               
                 5 
                 4.795 
                 Mo 
               
               
                 6 
                 5.196 
                 Si 
               
               
                 7 
                 4.776 
                 Mo 
               
               
                 8 
                 5.203 
                 Si 
               
               
                 9 
                 4.763 
                 Mo 
               
               
                 10 
                 5.208 
                 Si 
               
               
                 11 
                 4.753 
                 Mo 
               
               
                 12 
                 5.212 
                 Si 
               
               
                 13 
                 4.747 
                 Mo 
               
               
                 14 
                 5.214 
                 Si 
               
               
                 15 
                 4.743 
                 Mo 
               
               
                 16 
                 5.216 
                 Si 
               
               
                 17 
                 4.74 
                 Mo 
               
               
                 18 
                 5.217 
                 Si 
               
               
                 19 
                 4.738 
                 Mo 
               
               
                 20 
                 5.217 
                 Si 
               
               
                 21 
                 4.738 
                 Mo 
               
               
                 22 
                 5.217 
                 Si 
               
               
                 23 
                 4.738 
                 Mo 
               
               
                 24 
                 5.217 
                 Si 
               
               
                 25 
                 4.74 
                 Mo 
               
               
                 26 
                 5.216 
                 Si 
               
               
                 27 
                 4.743 
                 Mo 
               
               
                 28 
                 5.214 
                 Si 
               
               
                 29 
                 4.747 
                 Mo 
               
               
                 30 
                 5.212 
                 Si 
               
               
                 31 
                 4.753 
                 Mo 
               
               
                 32 
                 5.208 
                 Si 
               
               
                 33 
                 4.762 
                 Mo 
               
               
                 34 
                 5.203 
                 Si 
               
               
                 35 
                 4.776 
                 Mo 
               
               
                 36 
                 5.196 
                 Si 
               
               
                 37 
                 4.795 
                 Mo 
               
               
                 38 
                 5.185 
                 Si 
               
               
                 39 
                 4.912 
                 Mo 
               
               
                 40 
                 5.17 
                 Si 
               
               
                 41 
                 4.991 
                 Mo 
               
               
                   
                   
                 Incidence medium 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Layer design for FIG. 9 
               
             
          
           
               
                 Layer No. 
                 Thickness[nm] 
                 Material 
               
               
                   
               
             
          
           
               
                   
                   
                 Substrate 
               
               
                 1 
                 0.97 
                 Mo 
               
               
                 2 
                 2.24 
                 Ru 
               
               
                 3 
                 1.773 
                 Mo 
               
               
                 4 
                 5.125 
                 Si 
               
               
                 5 
                 1.428 
                 Mo 
               
               
                 6 
                 1.792 
                 Ru 
               
               
                 7 
                 1.823 
                 Mo 
               
               
                 8 
                 5.14 
                 Si 
               
               
                 9 
                 1.609 
                 Mo 
               
               
                 10 
                 1.5 
                 Ru 
               
               
                 11 
                 1.869 
                 Mo 
               
               
                 12 
                 5.15 
                 Si 
               
               
                 13 
                 4.828 
                 Mo 
               
               
                 14 
                 5.157 
                 Si 
               
               
                 15 
                 4.816 
                 Mo 
               
               
                 16 
                 5.162 
                 Si 
               
               
                 17 
                 4.808 
                 Mo 
               
               
                 18 
                 5.165 
                 Si 
               
               
                 19 
                 4.802 
                 Mo 
               
               
                 20 
                 5.168 
                 Si 
               
               
                 21 
                 4.798 
                 Mo 
               
               
                 22 
                 5.169 
                 Si 
               
               
                 23 
                 4.795 
                 Mo 
               
               
                 24 
                 5.17 
                 Si 
               
               
                 25 
                 4.794 
                 Mo 
               
               
                 26 
                 5.171 
                 Si 
               
               
                 27 
                 4.793 
                 Mo 
               
               
                 28 
                 5.171 
                 Si 
               
               
                 29 
                 4.794 
                 Mo 
               
               
                 30 
                 5.17 
                 Si 
               
               
                 31 
                 4.795 
                 Mo 
               
               
                 32 
                 5.169 
                 Si 
               
               
                 33 
                 4.798 
                 Mo 
               
               
                 34 
                 5.168 
                 Si 
               
               
                 35 
                 4.802 
                 Mo 
               
               
                 36 
                 5.165 
                 Si 
               
               
                 37 
                 4.808 
                 Mo 
               
               
                 38 
                 5.162 
                 Si 
               
               
                 39 
                 4.816 
                 Mo 
               
               
                 40 
                 5.157 
                 Si 
               
               
                 41 
                 4.828 
                 Mo 
               
               
                 42 
                 5.15 
                 Si 
               
               
                 43 
                 1.869 
                 Mo 
               
               
                 44 
                 1.498 
                 Ru 
               
               
                 45 
                 1.61 
                 Mo 
               
               
                 46 
                 5.14 
                 Si 
               
               
                 47 
                 1.823 
                 Mo 
               
               
                 48 
                 1.79 
                 Ru 
               
               
                 49 
                 1.43 
                 Mo 
               
               
                 50 
                 5.125 
                 Si 
               
               
                 51 
                 1.773 
                 Mo 
               
               
                 52 
                 2.238 
                 Ru 
               
               
                 53 
                 1.11 
                 Mo 
               
               
                   
                   
                 Incidence medium 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Layer design for FIG. 10 
               
             
          
           
               
                 Layer No. 
                 Thickness[nm] 
                 Material 
               
               
                   
               
             
          
           
               
                   
                   
                 Substrate 
               
               
                 1 
                 1.661 
                 Mo 
               
               
                 2 
                 6.127 
                 Si 
               
               
                 3 
                 3.633 
                 Mo 
               
               
                 4 
                 6.444 
                 Si 
               
               
                 5 
                 3.775 
                 Mo 
               
               
                 6 
                 6.456 
                 Si 
               
               
                 7 
                 3.839 
                 Mo 
               
               
                 8 
                 6.571 
                 Si 
               
               
                 9 
                 3.971 
                 Mo 
               
               
                 10 
                 6.662 
                 Si 
               
               
                 11 
                 3.956 
                 Mo 
               
               
                 12 
                 6.562 
                 Si 
               
               
                 13 
                 3.771 
                 Mo 
               
               
                 14 
                 6.384 
                 Si 
               
               
                 15 
                 3.719 
                 Mo 
               
               
                 16 
                 6.447 
                 Si 
               
               
                 17 
                 3.919 
                 Mo 
               
               
                 18 
                 6.619 
                 Si 
               
               
                 19 
                 4.028 
                 Mo 
               
               
                 20 
                 6.619 
                 Si 
               
               
                 21 
                 3.928 
                 Mo 
               
               
                 22 
                 6.504 
                 Si 
               
               
                 23 
                 3.852 
                 Mo 
               
               
                 24 
                 6.54 
                 Si 
               
               
                 25 
                 3.957 
                 Mo 
               
               
                 26 
                 6.674 
                 Si 
               
               
                 27 
                 3.973 
                 Mo 
               
               
                 28 
                 6.601 
                 Si 
               
               
                 29 
                 3.689 
                 Mo 
               
               
                 30 
                 6.279 
                 Si 
               
               
                 31 
                 3.501 
                 Mo 
               
               
                 32 
                 6.358 
                 Si 
               
               
                 33 
                 3.931 
                 Mo 
               
               
                 34 
                 6.789 
                 Si 
               
               
                 35 
                 4.334 
                 Mo 
               
               
                 36 
                 6.982 
                 Si 
               
               
                 37 
                 4.341 
                 Mo 
               
               
                 38 
                 6.84 
                 Si 
               
               
                 39 
                 3.843 
                 Mo 
               
               
                 40 
                 6.256 
                 Si 
               
               
                 41 
                 3.144 
                 Mo 
               
               
                 42 
                 6.076 
                 Si 
               
               
                 43 
                 3.583 
                 Mo 
               
               
                 44 
                 6.716 
                 Si 
               
               
                 45 
                 4.197 
                 Mo 
               
               
                 46 
                 6.932 
                 Si 
               
               
                 47 
                 4.183 
                 Mo 
               
               
                 48 
                 6.671 
                 Si 
               
               
                 49 
                 2.614 
                 Mo 
               
               
                   
                   
                 Incidence medium 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Layer design for FIG. 11 
               
             
          
           
               
                 Layer No. 
                 Thickness[nm] 
                 Material 
               
               
                   
               
             
          
           
               
                   
                   
                 Substrate 
               
               
                 1 
                 2.593 
                 Mo 
               
               
                 2 
                 4.264 
                 Si 
               
               
                 3 
                 3.19 
                 Mo 
               
               
                 4 
                 4.265 
                 Si 
               
               
                 5 
                 3.259 
                 Mo 
               
               
                 6 
                 4.447 
                 Si 
               
               
                 7 
                 3.219 
                 Mo 
               
               
                 8 
                 4.726 
                 Si 
               
               
                 9 
                 3.147 
                 Mo 
               
               
                 10 
                 4.785 
                 Si 
               
               
                 11 
                 3.142 
                 Mo 
               
               
                 12 
                 4.604 
                 Si 
               
               
                 13 
                 3.159 
                 Mo 
               
               
                 14 
                 4.448 
                 Si 
               
               
                 15 
                 3.067 
                 Mo 
               
               
                 16 
                 4.345 
                 Si 
               
               
                 17 
                 2.694 
                 Mo 
               
               
                 18 
                 4.154 
                 Si 
               
               
                 19 
                 2.33 
                 Mo 
               
               
                 20 
                 4.132 
                 Si 
               
               
                 21 
                 2.861 
                 Mo 
               
               
                 22 
                 4.225 
                 Si 
               
               
                 23 
                 3.219 
                 Mo 
               
               
                 24 
                 4.268 
                 Si 
               
               
                 25 
                 3.305 
                 Mo 
               
               
                 26 
                 4.415 
                 Si 
               
               
                 27 
                 3.258 
                 Mo 
               
               
                 28 
                 4.919 
                 Si 
               
               
                 29 
                 3.217 
                 Mo 
               
               
                 30 
                 5.408 
                 Si 
               
               
                 31 
                 1.693 
                 Mo 
               
               
                 32 
                 1.817 
                 Ru 
               
               
                 33 
                 4.58 
                 Si 
               
               
                 34 
                 0.52 
                 Mo 
               
               
                 35 
                 2.414 
                 Ru 
               
               
                 36 
                 0.628 
                 Mo 
               
               
                 37 
                 4.531 
                 Si 
               
               
                 38 
                 1.706 
                 Ru 
               
               
                 39 
                 1.799 
                 Mo 
               
               
                 40 
                 4.99 
                 Si 
               
               
                 41 
                 3.193 
                 Mo 
               
               
                 42 
                 4.734 
                 Si 
               
               
                 43 
                 3.079 
                 Mo 
               
               
                 44 
                 4.41 
                 Si 
               
               
                 45 
                 2.999 
                 Mo 
               
               
                 46 
                 4.367 
                 Si 
               
               
                 47 
                 2.864 
                 Mo 
               
               
                 48 
                 4.622 
                 Si 
               
               
                 49 
                 2.707 
                 Mo 
               
               
                 50 
                 5.103 
                 Si 
               
               
                 51 
                 2.771 
                 Mo 
               
               
                 52 
                 4.781 
                 Si 
               
               
                 53 
                 3.018 
                 Mo 
               
               
                 54 
                 4.254 
                 Si 
               
               
                 55 
                 3.075 
                 Mo 
               
               
                 56 
                 4.033 
                 Si 
               
               
                 57 
                 2.896 
                 Mo 
               
               
                 58 
                 3.895 
                 Si 
               
               
                 59 
                 2.528 
                 Mo 
               
               
                 60 
                 3.841 
                 Si 
               
               
                 61 
                 2.632 
                 Mo 
               
               
                 62 
                 4.04 
                 Si 
               
               
                 63 
                 0.149 
                 Mo 
               
               
                 64 
                 0.636 
                 Ru 
               
               
                 65 
                 2.142 
                 Mo 
               
               
                 66 
                 4.284 
                 Si 
               
               
                 67 
                 0.512 
                 Ru 
               
               
                 68 
                 2.447 
                 Mo 
               
               
                 69 
                 4.953 
                 Si 
               
               
                 70 
                 2.614 
                 Mo 
               
               
                 71 
                 5.335 
                 Si 
               
               
                 72 
                 2.528 
                 Mo 
               
               
                 73 
                 4.788 
                 Si 
               
               
                 74 
                 2.59 
                 Mo 
               
               
                 75 
                 4.56 
                 Si 
               
               
                 76 
                 2.457 
                 Mo 
               
               
                 77 
                 4.844 
                 Si 
               
               
                 78 
                 1.796 
                 Mo 
               
               
                 79 
                 14.561 
                 Si 
               
               
                 80 
                 0.891 
                 Mo 
               
               
                 81 
                 1.661 
                 Ru 
               
               
                 82 
                 4.301 
                 Si 
               
               
                 83 
                 0.698 
                 Mo 
               
               
                 84 
                 1.953 
                 Ru 
               
               
                 85 
                 0.229 
                 Mo 
               
               
                 86 
                 4.004 
                 Si 
               
               
                 87 
                 0.55 
                 Mo 
               
               
                 88 
                 1.961 
                 Ru 
               
               
                 89 
                 0.369 
                 Mo 
               
               
                 90 
                 3.845 
                 Si 
               
               
                 91 
                 0.5 
                 Mo 
               
               
                 92 
                 1.957 
                 Ru 
               
               
                 93 
                 0.395 
                 Mo 
               
               
                 94 
                 3.777 
                 Si 
               
               
                 95 
                 0.492 
                 Mo 
               
               
                 96 
                 2.049 
                 Ru 
               
               
                 97 
                 0.394 
                 Mo 
               
               
                 98 
                 3.763 
                 Si 
               
               
                 99 
                 0.451 
                 Mo 
               
               
                 100 
                 2.137 
                 Ru 
               
               
                 101 
                 0.443 
                 Mo 
               
               
                 102 
                 3.903 
                 Si 
               
               
                 103 
                 2.356 
                 Ru 
               
               
                 104 
                 0.534 
                 Mo 
               
               
                 105 
                 3.747 
                 Si 
               
               
                 106 
                 0.311 
                 Mo 
               
               
                 107 
                 2.06 
                 Ru 
               
               
                 108 
                 0.606 
                 Mo 
               
               
                 109 
                 3.859 
                 Si 
               
               
                 110 
                 2.156 
                 Ru 
               
               
                 111 
                 0.616 
                 Mo 
               
               
                 112 
                 3.883 
                 Si 
               
               
                 113 
                 2.139 
                 Ru 
               
               
                 114 
                 0.565 
                 Mo 
               
               
                 115 
                 3.776 
                 Si 
               
               
                 116 
                 0.342 
                 Mo 
               
               
                 117 
                 2.005 
                 Ru 
               
               
                 118 
                 0.492 
                 Mo 
               
               
                 119 
                 3.959 
                 Si 
               
               
                 120 
                 2.262 
                 Ru 
               
               
                 121 
                 0.448 
                 Mo 
               
               
                 122 
                 3.803 
                 Si 
               
               
                 123 
                 0.399 
                 Mo 
               
               
                 124 
                 2.09 
                 Ru 
               
               
                 125 
                 0.444 
                 Mo 
               
               
                 126 
                 3.82 
                 Si 
               
               
                 127 
                 0.378 
                 Mo 
               
               
                 128 
                 2.097 
                 Ru 
               
               
                 129 
                 0.477 
                 Mo 
               
               
                 130 
                 3.85 
                 Si 
               
               
                 131 
                 0.337 
                 Mo 
               
               
                 132 
                 2.047 
                 Ru 
               
               
                 133 
                 0.516 
                 Mo 
               
               
                 134 
                 3.893 
                 Si 
               
               
                 135 
                 0.32 
                 Mo 
               
               
                 136 
                 1.93 
                 Ru 
               
               
                 137 
                 0.487 
                 Mo 
               
               
                 138 
                 3.929 
                 Si 
               
               
                 139 
                 0.401 
                 Mo 
               
               
                 140 
                 1.795 
                 Ru 
               
               
                 141 
                 0.317 
                 Mo 
               
               
                 142 
                 3.956 
                 Si 
               
               
                 143 
                 0.61 
                 Mo 
               
               
                 144 
                 1.752 
                 Ru 
               
               
                 145 
                 0.117 
                 Mo 
               
               
                 146 
                 3.996 
                 Si 
               
               
                 147 
                 0.793 
                 Mo 
               
               
                 148 
                 1.802 
                 Ru 
               
               
                 149 
                 4.004 
                 Si 
               
               
                 150 
                 0.922 
                 Mo 
               
               
                 151 
                 1.697 
                 Ru 
               
               
                 152 
                 3.871 
                 Si 
               
               
                 153 
                 1.248 
                 Mo 
               
               
                 154 
                 1.288 
                 Ru 
               
               
                 155 
                 3.715 
                 Si 
               
               
                 156 
                 1.832 
                 Mo 
               
               
                 157 
                 0.87 
                 Ru 
               
               
                 158 
                 3.822 
                 Si 
               
               
                 159 
                 1.972 
                 Mo 
               
               
                 160 
                 0.667 
                 Ru 
               
               
                 161 
                 0.371 
                 Mo 
               
               
                 162 
                 3.889 
                 Si 
               
               
                 163 
                 3.08 
                 Mo 
               
               
                 164 
                 3.481 
                 Si