METHOD FOR MEASURING AN EFFECT OF A WAVELENGTH-DEPENDENT MEASURING LIGHT REFLECTIVITY AND AN EFFECT OF A POLARIZATION OF MEASURING LIGHT ON A MEASURING LIGHT IMPINGEMENT ON A LITHOGRAPHY MASK

Abstract

To measure an effect of a wavelength-dependent measuring light reflectivity R.sub.Ret of a lithography mask, a measuring light beam is caused to impinge on said lithography mask within a field of view of a measuring apparatus. The measuring light has a wavelength bandwidth between a wavelength lower limit and a wavelength upper limit differing therefrom. The reflected measuring light emanating from an impinged section of the lithography mask is captured by a detector. A filter with a wavelength-dependent transmission within the wavelength bandwidth is introduced into a beam path of the measuring light beam between the measuring light source and the detector. The measuring light reflected by the lithography mask is captured again by the detector once the filter has been introduced. The wavelength-dependent reflectivity R.sub.Ret or an effect of the wavelength-dependent reflectivity R.sub.Ret is determined on the basis of the capture results. In comparison with the prior art, this yields an improved method for measuring an effect of a measuring light reflectivity on a lithography mask. Additionally, a method for measuring an effect of a polarization of measuring light on a measuring light impingement on a lithography mask is specified, wherein as a result of this the effect of the lithography mask on measuring light is made accessible in respect of further optical parameters of a measurement.

Claims

1. A method for measuring an effect of a polarization of measuring light on a measuring light impingement on a lithography mask, including the following steps: providing the lithography mask, providing a polarization measuring apparatus, having a measuring light source for producing measuring light with a raw polarization state, having a mask holder for holding the lithography mask, having a detector for capturing the measuring light in a measuring light beam path downstream of the mask holder, causing a measuring light beam to impinge on the lithography mask within a field of view of the measuring apparatus, capturing the measuring light emanating from an impinged section of the lithography mask using the detector, introducing a polarization filter with a polarization-dependent effect on the measuring light into the beam path of the measuring light beam between the measuring light source and the detector for the purposes of producing measuring light with a filter polarization state that differs from the raw polarization state, the polarization filter having such a polarization-dependent effect on the measuring light that said polarization filter attenuates a certain polarization component of the incident measuring light in relation to another polarization component of the incident measuring light, capturing the measuring light emanating from the impinged section of the lithography mask again using the detector once the polarization filter has been introduced, determining the effect of the polarization of the measuring light on the measuring light impingement on the lithography mask on the basis of the capture results.

2. The method of to claim 1, wherein the steps of introducing the filter and capturing the measuring light once the polarization filter has been introduced are repeated for a plurality of polarization filters which differ in terms of their polarization effect on the measuring light.

3. The method of claim 1, wherein the polarization measuring apparatus comprises a projection optical unit for imaging an object field arranged in the field of view into an image field, a change in a structure resolution of the image representation as a result of the introduction of the polarization filter being determined as effect.

4. A method for measuring an effect of a wavelength-dependent measuring light reflectivity R.sub.Ret on a lithography mask, including the following steps: providing the lithography mask, providing a reflectivity measuring apparatus, having a measuring light source for producing measuring light with a wavelength bandwidth between a wavelength lower limit and a wavelength upper limit differing therefrom, having a mask holder for holding the lithography mask, having a detector for capturing measuring light that was reflected by the lithography mask in the mask holder, causing a measuring light beam to impinge on the lithography mask within a field of view of the measuring apparatus, capturing the reflected measuring light emanating from an impinged section of the lithography mask using the detector, introducing a filter with a wavelength-dependent transmission within the wavelength bandwidth into a beam path of the measuring light beam between the measuring light source and the detector, capturing the reflected measuring light emanating from the impinged section of the lithography mask again using the detector once the filter has been introduced, determining the wavelength-dependent reflectivity R.sub.Ret or an effect of the wavelength-dependent reflectivity R.sub.Ret on the basis of the capture results.

5. The method of claim 4, wherein the steps of introducing a filter and capturing the reflected measuring light once the filter has been introduced are repeated for a plurality of filters which differ in terms of their wavelength-dependent transmission.

6. The method of claim 4, wherein the reflectivity measuring apparatus comprises a projection optical unit for imaging an object field arranged in the field of view into an image field, a change in a structure resolution of the image representation as a result of the introduction of the filter being determined as effect.

7. The method of claim 4, wherein the wavelength-dependent reflectivity R.sub.Ret is determined on the basis of the capture results in a wavelength range around a target wavelength (λ.sub.0) within the wavelength bandwidth.

8. The method of claim 7, wherein an influence of structures on the lithography mask on the wavelength-dependent reflectivity R.sub.Ret is determined when determining the wavelength-dependent reflectivity R.sub.Ret.

9. The method of claim 4, wherein the filter has a non-rotationally symmetric filter effect in a filter arrangement plane and is introduced into the beam path of the measuring light beam with a defined orientation relative to an axis of rotation perpendicular to the filter arrangement plane.

10. A measuring apparatus for measuring an effect of a lithography mask on measuring light, comprising: a measuring light source for producing a measuring light, a mask holder for holding the lithography mask such that a section of the lithography mask is arranged within a field of view on which the measuring light can impinge, a detector for capturing the measuring light that is reflected by the lithography mask in the mask holder, at least one filter holder with at least one filter, arranged in a beam path of the measuring light beam between the measuring light source and the detector, wherein the filter holder is designed as an interchangeable filter holder, and a signal acquisition and evaluation device that is signal connected to the detector.

11. The measuring apparatus of claim 10, wherein the filter holder is arranged in a pupil plane of the measuring apparatus.

12. The measuring apparatus of claim 10, wherein the filter holder is arranged in the beam path of the measuring light beam between the measuring light source and the mask holder and/or in the beam path of the measuring light beam between the mask holder and the detector.

13. The measuring apparatus of claim 10, comprising a projection optical unit for imaging an object field arranged in the field of view into an image plane, with the detector being arranged in the image plane.

14. The measuring apparatus of claim 10, wherein the filter holder is oriented such that a filter arrangement plane is tilted in relation to a normal plane to the measuring light beam path.

15. The measuring apparatus of claim 10, wherein the filter has a plurality of filter sections that are tilted in relation to a normal plane (xy) to a beam direction (z) of the measuring light.

16. The method of claim 2, wherein the polarization measuring apparatus comprises a projection optical unit for imaging an object field arranged in the field of view into an image field, a change in a structure resolution of the image representation as a result of the introduction of the polarization filter being determined as effect.

17. The method of claim 5, wherein the reflectivity measuring apparatus comprises a projection optical unit for imaging an object field arranged in the field of view into an image field, a change in a structure resolution of the image representation as a result of the introduction of the filter being determined as effect.

18. The method of claim 5, wherein the wavelength-dependent reflectivity R.sub.Ret is determined on the basis of the capture results in a wavelength range around a target wavelength (λ.sub.0) within the wavelength bandwidth.

19. The method of claim 5, wherein the filter has a non-rotationally symmetric filter effect in a filter arrangement plane and is introduced into the beam path of the measuring light beam with a defined orientation relative to an axis of rotation perpendicular to the filter arrangement plane.

20. The measuring apparatus of claim 11, wherein the filter holder is arranged in the beam path of the measuring light beam between the measuring light source and the mask holder and/or in the beam path of the measuring light beam between the mask holder and the detector.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0039] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings, in which:

[0040] FIG. 1 schematically shows a measuring apparatus for measuring an effect of a lithography mask on measuring light, comprising an illumination system, an imaging optical unit and a spatially resolving detection device;

[0041] FIG. 2 shows, in an illustration similar to FIG. 1, a further embodiment of the measuring apparatus;

[0042] FIG. 3 shows a further embodiment of the measuring apparatus in a representation which exhibits a measuring light beam path between an illumination-side pupil plane and the detection device;

[0043] FIG. 4 shows a plan view of a filter for measuring a wavelength-dependent measuring light reflectivity of a lithography mask as a constituent part of the measuring apparatus;

[0044] FIG. 5 shows the filter according to FIG. 4 with an orientation rotated 90° clockwise in comparison with FIG. 4;

[0045] FIG. 6 shows, in an illustration similar to FIG. 4, a further embodiment of a filter for measuring a wavelength-dependent reflectivity;

[0046] FIG. 7 shows, in an illustration similar to FIG. 5, the filter according to FIG. 6;

[0047] FIG. 8 shows, in an illustration similar to FIG. 4, a further embodiment of a filter for measuring a wavelength-dependent reflectivity;

[0048] FIG. 9 shows, in an illustration similar to FIG. 5, the filter according to FIG. 8;

[0049] FIG. 10 shows an effect of a polarization filter as a constituent part of an embodiment of the measuring apparatus on incident measuring light in a perspective illustration;

[0050] FIG. 11 in turn shows a meridional section of a further embodiment of a polarization filter with a plurality of filter sections that are tilted in relation to a normal plane to a measuring light beam path; and

[0051] FIG. 12 shows a plan view of a lithography mask to be measured by the measuring apparatus.

DETAILED DESCRIPTION

[0052] FIG. 1 shows, in a sectional view corresponding to a meridional section, a beam path of EUV illumination light or EUV imaging light 1 in a metrology system 2, which is embodied in the form of a reflectivity and polarization measuring apparatus. The illumination light 1 is produced by an EUV light source 3. The illumination light 1 is also referred to as measuring light.

[0053] In order to facilitate the representation of positional relationships, a Cartesian xyz-coordinate system is used hereinafter. The x-axis in FIG. 1 extends perpendicular to the plane of the drawing and out of the latter. The y-axis in FIG. 1 extends toward the right. The z-axis in FIG. 1 extends upwardly. A local Cartesian xy- or xyz-coordinate system is also used in the subsequent figures. In this local coordinate system, the x-axis in each case extends parallel to the x-axis of the global coordinate system according to FIG. 1. With the x-axis of the local coordinate system, the y-axis spans an optical surface of the respectively depicted optical component. Accordingly, the y- and z-axes of the local coordinate systems are optionally tilted in relation to the y- and z-axes of the global coordinate system according to FIG. 1.

[0054] The light source 3 can be a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). In principle, a synchrotron-based light source can also be used, for example a free electron laser (FEL). A used wavelength of the illumination light 1 can lie in the range of between 5 nm and 30 nm. In principle, in the case of a variant of the metrology system 2, it is also possible to use a light source for another used light wavelength, for example for a used wavelength of 193 nm. The measuring light 1 of the light source 3 can be collected by a collector. In this case, the collector can be an ellipsoidal collector or a nested collector, for example.

[0055] FIG. 1 shows a beam path of the measuring light 1 by way of a chief ray CR, drawn as a solid line, of a central field point and by way of two marginal rays, shown in dashes. Said marginal rays in each case delimit a pupil in a pupil plane of the metrology system 2.

[0056] The illumination light 1 is conditioned in a schematically illustrated illumination optical unit 4 of an illumination system 5 of the metrology system 2, of which the light source 3 is also a part, in a manner such that a specific illumination setting of the illumination is provided, that is to say a specific illumination angle distribution. A specific intensity distribution of the illumination light 1 in a pupil of the metrology system 2 corresponds to said illumination setting. In this case, it is possible to specify for example a multi-pole illumination setting, in particular a quadrupole illumination setting. Other illumination settings can also be specified within the metrology system 2, for example a conventional illumination setting in which practically all illumination angles are used for object illumination, in particular with the exception of illumination angles close to perpendicular or average incidence on the object to be illuminated, an annular illumination setting with small illumination angles overall, that is to say illumination angles close to perpendicular or average incidence, which itself can in turn be omitted, or dipole illumination settings, wherein the individual poles can each have a “leaflet” contour in a pupil plane, that is to say an edge contour that corresponds approximately to the section through a biconvex lens element.

[0057] The illumination optical unit 4 has a total of three EUV mirrors, denoted in FIG. 1 in the order of impingement thereof by IL1, IL2 and IL3. A filter plane 6 is located between the light source 3 and the first mirror IL1 of the illumination optical unit 4, in which filter plane for example a debris filter can be arranged that protects optical components downstream in the beam path of the measuring light 1 against radiation components and/or particle components that come from the light source 3 and are undesirably guided along.

[0058] An intermediate focal plane 7 of the illumination system 5 is located between the mirrors IL1 and IL2. An intermediate focus IF of the measuring light 1 is arranged here. A pinhole stop that can have a function corresponding to that of the debris filter can be arranged in the intermediate focal plane 7.

[0059] A pupil plane 8 of the illumination optical unit 4 is located between the intermediate focal plane 7 and the mirror IL2 of the illumination optical unit 4. A setting stop for specifying the illumination setting can be arranged in said pupil plane 8.

[0060] Together with an imaging optical unit or projection optical unit 9, the illumination system 5 constitutes an optical measuring system 10 of the metrology system 2. The projection optical unit 9 has a total of three EUV mirrors, denoted in FIG. 1 in the order of impingement thereof by M1, M2 and M3.

[0061] With the illumination setting that is respectively set, the illumination light 1 illuminates an object field or field of view 11 of an object plane 12 of the metrology system 2. A lithography mask 13 is arranged in the object plane 12. As an alternative to such a structured object, an unstructured object can also be measured, for example a mask blank.

[0062] The object plane 12 extends parallel to the xy-plane. The object field 11 simultaneously constitutes a field of view of the projection optical unit 9 or of the metrology system 2.

[0063] The lithography mask or the object 13 is carried by an object holder 14 which is depicted schematically in FIG. 1 and which is also referred to as a mask holder. The object 13 can be displaced in the object plane 12 in at least two translational degrees of freedom with the aid of an object displacement drive 14a, which cooperates with the object holder 14. A further degree of freedom of displacement provided by the object displacement drive 14a lies perpendicular to the object plane 12, that is to say along the is z-direction.

[0064] The illumination light 1 is reflected by the lithography mask 13, as depicted schematically in FIG. 1, and enters an entrance pupil of the imaging optical unit 9 in a pupil plane 15. The pupil plane 15 is indicated schematically within the projection optical unit 8 in FIG. 1. The pupil used of the imaging optical unit 9 can have a circular or elliptic boundary.

[0065] The imaging optical unit 9 images the object field 11 in an operating situation of the metrology system 2 into an image field or measurement field 16 in an image plane or measurement plane 17 of the metrology system 2. A magnifying imaging scale during the imaging by the projection optical unit 9 is greater than 500. Depending on the embodiment of the projection optical unit 9, the magnifying imaging scale can be greater than 100, can be greater than 200, can be greater than 250, can be greater than 300, can be greater than 400 and can also be significantly greater than 500. The imaging scale of the projection optical unit 9 is regularly less than 2000.

[0066] The projection optical unit 9 serves for imaging a section of the object 13, which section is arranged in the object field 11, into the image plane 17.

[0067] A spatially resolving detection device or a detector 18 of the metrology system 2 is arranged in the image plane or measurement plane 17. This detection device can be a CCD (charge-coupled device) camera. Accordingly, the image field 16 will also be referred to as measurement field hereinafter. The detection device 18, which is also referred to as a detector, serves for capturing measuring light 1 reflected by the object 13.

[0068] The field of view 11 has an extent in the xy-plane that is smaller than 50 μm×50 μm, for example 10 μm×10 μm.

[0069] The detector 18 can have for example in the measurement field 16 a resolution of 1000×1000 pixels.

[0070] A spatial resolution of the detector 18 within the field of view 11 can be better than 100 nm and be for example in the range between 1 nm and 10 nm.

[0071] Depending on the embodiment, the detector 18 can be used firstly to implement a spatially resolved capture of the measuring light 1 over the measuring field 16 and secondly also implement an illumination angle-resolved capture such that an illumination angle distribution of the measuring light 1 at a field point can also be determined for each field point with the aid of the detector 18.

[0072] The measuring apparatus or the metrology system 2 serves to measure an effect of a wavelength-dependent measuring light reflectivity on the lithography mask 13. Alternatively or in addition, the measuring apparatus 2 serves to measure an effect of a polarization of the measuring light 1 on a measuring light impingement of the lithography mask 13. A wavelength-dependent reflectivity or a polarization effect of a component of the measuring apparatus 2 can also be measured therewith.

[0073] The measuring apparatus 2 provides various filter holders for the purposes of fulfilling these measurement tasks, specifically a filter holder 19 at the location of the filter plane 6 also used by the debris filter, a filter holder 20 at the location of the intermediate focal plane 7, a filter holder 21 at the location of the illumination optical unit pupil plane 8 and a filter holder 22 at the location of the projection optical unit pupil plane 15.

[0074] These filter holders 19 to 22 are all arranged in the beam path of the measuring light beam 1 between the measuring light source 3 and the detector 18.

[0075] A signal acquisition and evaluation device 23 of the metrology system 2 is signal connected to the detector 18 (not shown here). The signal acquisition and evaluation device 23 can additionally be signal connected to further components of the measuring apparatus 2, for example to drives (not shown here in more detail) of the filter holders 19, 20, 21, 22, in particular for the driven filter displacement and/or filter changes. In addition, the signal acquisition and evaluation device 23 can be signal connected to the object displacement drive 14a for the purposes of controlling the latter.

[0076] The measuring light 1 has a wavelength bandwidth between a wavelength lower limit and a wavelength upper limit differing therefrom. An example of such a wavelength bandwidth is specified in the specialist article “Application of a high-brightness electrodeless Z-pinch EUV source for metrology, inspection, and resist development,” S. F. Horne et al., SPIE Proceedings of the 31st International Symposium on Microlithography, 2006. By way of example, the wavelength lower limit λ.sub.UG can be at 13.0 nm and the wavelength upper limit λ.sub.OG can be at 14.0 nm. A central wavelength λ.sub.0 of the measuring light 1 can be at 13.5 nm.

[0077] One of the filters that can be inserted into one of the filter holders 19 to 22 of the metrology system 2 can be a filter with a wavelength-dependent transmission within this wavelength bandwidth.

[0078] One of the filters can be embodied as a low-pass filter with a transmission function 24 that is depicted in an insert in FIG. 1 as a solid line. For relatively short wavelengths shorter than λ.sub.0, the transmission function 24 is such that a high transmission T is present there. The transmission T of the low-pass filter 24 is low for wavelengths longer than the central wavelength λ.sub.0. A corresponding transmission function of a high-pass filter 25 is plotted in the insert of FIG. 1 using a dashed line and a transmission function of a bandpass filter 26 is plotted using a dash-dotted line. In the case of the high-pass filter 25, wavelengths longer than λ.sub.0 are transmitted on account of a correspondingly high transmission of the high-pass filter 25 and wavelengths shorter than λ.sub.0 are absorbed to a greater extent. In the case of the bandpass filter 26, wavelengths around the central wavelength λ.sub.0 are transmitted, and wavelengths below a respective wavelength band around λ.sub.0 and above a wavelength band blocked in each case

[0079] A transmission function 27 of a notch filter is also depicted in the insert of FIG. 1 using a dotted line, said notch filter acting like an inverse bandpass filter with a very small bandwidth, that is to say it blocks a very tight wavelength range around the central wavelength λ.sub.0. A further variant of a filter that is usable in the metrology system or the measuring apparatus 2 is an inverted notch filter which transmits the measuring light 1 in a tight wavelength range around the central wavelength λ.sub.0 and otherwise blocks said measuring light.

[0080] As indicated in the case of the filter holder 20 in the intermediate focal plane 7 by way of a dashed line, the filter holder 20 can be oriented in such a way that a filter arrangement plane 28 (cf. FIG. 1) is tilted in relation to a normal plane to the measuring light beam path 1, which coincides with the intermediate focal plane 7.

[0081] There will be further details regarding the filters that can be arranged in the filter holders 19 to 22 in the further course of the description.

[0082] FIG. 2 shows a further embodiment of the measuring apparatus 29, which can be used instead of the one according to FIG. 1. Components and functions corresponding to those which have already been explained above with reference to FIG. 1 bear the same reference signs and will not be discussed in detail again.

[0083] In the case of the measuring apparatus 29, there are a plurality of energy sensors 30, for example three energy sensors 30, in the region of the illumination optical unit pupil plane 8, said energy sensors for example capturing marginal individual rays of the measuring light beam 1 in the region of the illumination optical unit pupil plane 8. In this way, it is possible to obtain normalization data by means of which variations of measuring light quantities between the measuring light source 3 and the illumination optical unit pupil plane 8 can be separated from the effects on the measuring light 1 of subsequent components in the measuring light beam path. Moreover, a quality of a setting of an illumination setting can also be monitored by way of the energy sensors 30.

[0084] Moreover, a pellicle 31, that is to say a protective film for the lithography mask 13, is indicated in FIG. 2 at a distance from the lithography mask 13 and spaced apart from the object plane 12 in the negative z-direction. Otherwise, the structure of the measuring apparatus 29 corresponds to that of the measuring apparatus 2.

[0085] A further embodiment of a measuring light beam path in a further embodiment of a measuring apparatus 32 is described below on the basis of FIG. 3. Functions and components corresponding to those which have already been explained above with reference to FIGS. 1 and 2 bear the same reference signs and will not be discussed in detail again.

[0086] The measuring apparatus 32 is depicted in FIG. 3 for the measuring light beam path between the illumination optical unit pupil plane 8 and the detector 18. The illumination system 5 of the measuring apparatus 32 has no optical components that influence the measuring light beam 1 between the illumination optical unit pupil plane 8 and the object field 11. Thus, the illumination optical unit mirrors IL2 and IL3 are lacking in the measuring apparatus 32 in comparison with the measuring apparatuses 2 and 29. Otherwise, the measuring apparatus 32 corresponds to the measuring apparatuses 2 and 29.

[0087] An example of a filter with a wavelength-dependent transmission function is described below on the basis of FIGS. 4 and 5. FIGS. 4 and 5 show the wavelength filter 33 in two different orientations, in each case in a plan view, the orientation according to FIG. 5 being rotated 90° clockwise in relation to that of FIG. 4 around a central filter axis 34, which is perpendicular to the plane of the drawing of FIGS. 4 and 5.

[0088] The wavelength filter 33 has a circular edge 35 and is divided in two halves in terms of its transmission function, said halves being separated from one another by way-of a separation line 36 that is perpendicular in FIG. 4. One half of the wavelength filter 33, to the left in FIG. 4, is embodied as a high-pass filter section 37 with the transmission function 25. The measuring light 1 that radiates through the high-pass filter section 37 is suppressed at short wavelengths that are shorter than the central wavelength λ.sub.0.

[0089] The half of the wavelength filter 33 to the right in FIG. 4 is embodied as a low-pass filter section 38, which has a transmission function corresponding to the transmission function 24 in the insert of FIG. 1. In the low-pass filter section 38, the wavelengths of the measuring light 1 that are shorter than the central wavelength λ.sub.0 are transmitted and the wavelengths longer than the central wavelength λ.sub.0 are suppressed.

[0090] By way of example, when carrying out a method for measuring an effect of a wavelength-dependent measuring light reflectivity of the lithography mask 13, the wavelength filter 33 is used as follows:

[0091] First, the lithography mask 13 and the measuring apparatus 2, 29 or 32 are provided, the latter then operating as a reflectivity measuring apparatus. Then, the measuring light beam 1 is caused to impinge on the lithography mask 13 within the field of view 11. In this case, no further filter is arranged in the filter holders 19 to 22 of the measuring apparatus 2, 29 or 32 with the exception of the debris filter in the filter plane 6. Then, the reflected measuring light 1 emanating from the section of the lithography mask 13 impinged by the measuring light beam 1 is captured by the detector 18.

[0092] Subsequently, the wavelength filter 33 is introduced into the filter holder 21 in the illumination optical unit pupil plane 8 or alternatively into the filter holder 22 in the projection optical unit pupil plane 15, for example with the orientation depicted in FIG. 4. The rotational orientation of the wavelength filter 33 in the filter holder 21 or 22 around the filter axis 34 can be adapted to an orientation of structures on the lithography mask 13.

[0093] Following introduction of the wavelength filter 33, the reflected measuring light emanating from the impinged section of the lithography mask 13 is captured again by the detector 18. Then, either the wavelength-dependent reflectivity of the lithography mask around the central wavelength λ.sub.0 is determined directly or an effect of the wavelength-dependent reflectivity around the central wavelength λ.sub.0 is determined on the basis of the two capture results of “without filter/with an introduced filter.”

[0094] The effect of the wavelength-dependent reflectivity can be a change in a structure resolution of the image representation of structures of the lithography mask 13 as a result of the introduction of the filter 33. By way of example, a critical dimension (CD) could be determined firstly on the basis of measuring light 1 captured without an introduced filter 33 and secondly on the basis of measuring light 1 captured with an introduced wavelength filter 33.

[0095] Instead of the subdivided wavelength filter 33, it is also possible to introduce a wavelength filter which has a transmission function, for example in the style of the transmission functions 24 to 27 explained above, over its entire surface.

[0096] In particular, whether a minimal achievable CD changes on account of the introduction of the wavelength filter can be determined in this way. In this way, it is possible to deduce a wavelength-dependent reflectivity of the lithography mask 13 around the central wavelength λ.sub.0.

[0097] The following formalism can be used to directly determine the wavelength-dependent reflectivity around the target wavelength λ.sub.0 on the basis of the capture results “without filter/without filter.”

[0098] Using pupil coordinates (k.sub.x, k.sub.y) in the respective pupil plane, an intensity S.sub.σ(λ) of the measuring light 1 passing through a pupil plane of the measuring apparatus 2, 29, 32 can be written as:

S.sub.σ(λ)=∫.sub.ΣP.sub.σ(k.sub.x,k.sub.y)I(k.sub.x,k.sub.y,λ)dk.sub.xdk.sub.y

P.sub.σ is a pupil function which reproduces where measuring light 1 is present in the utilized pupil of the respective pupil plane 8 or 5. In the case of a conventional illumination setting, P.sub.σ is for example one for all pupil coordinates k.sub.x, k.sub.y within the pupil of the measuring apparatus 2, 29 or 32 and zero everywhere else. In principle, the values of the pupil function P.sub.σ can also adopt “greyscale” values of between zero and one.

[0099] I(k.sub.x, k.sub.y, λ) is the luminous intensity of the measuring light 1 from one pupil coordinate, that is to say from exactly one illumination direction, for the wavelength λ.

[0100] A luminous intensity S.sub.ES detected by the energy sensors 30 can be written as:

[00001]$S_{\mathrm{ES}}=\underset{{\lambda }_{\min }}{\stackrel{{\lambda }_{\max }}{\int }}{\int }_{\mathrm{ES}}{P}_{\sigma }\left(k_x,k_y\right)I\left(k_x,k_y,\lambda \right){\mathrm{dk}}_x{\mathrm{dk}}_{y}d\lambda$

The energy sensors 30 integrate over all wavelengths.

[0101] A luminous intensity S.sub.CCD(x,y) detected by a CCD detector of the detection device 18 with spatial resolution in the x- and y-coordinates of the measuring field 16 can, as a function of S.sub.σ(λ), be written as

[00002]$\begin{array}{cc}{S}_{\mathrm{CCD}}\left(x,y\right)=\underset{{\lambda }_{\min }}{\stackrel{{\lambda }_{\max }}{\int }}{S}_{\sigma }\left(\lambda \right)T_{\mathrm{IL}2}\left(\lambda \right)T_{\mathrm{IL}3}\left(\lambda \right)R_{\mathrm{Re}t}\left(x,y,\theta ,\lambda \right)T_{\mathrm{NA}}\left(\lambda \right)T_{M1}\left(\lambda \right)T_{M2}\left(\lambda \right)T_{M3}\left(\lambda \right)A_{\mathrm{CCD}}\left(\lambda \right)d\lambda & \left(1\right)\end{array}$

[0102] In this case:

[0103] T.sub.IL2(λ), T.sub.IL3(λ) are the wavelength-dependent reflectivities of the mirrors IL2 and IL3 of the illumination optical unit 4. A wavelength-dependent reflectivity of the mirror IL1 can also be considered accordingly.

[0104] R.sub.Ret(x, y, θ, λ) is the reflectivity of the lithography mask 13 as a function of the field location x, y depending on the illumination angle θ of the measuring light 1 at the object plane 12 of the respective location of the field of view 11.

[0105] T.sub.NA is the wavelength-dependent transmission of the filter, in particular of the filter 33, in the pupil plane 8 or 15, that is to say for example one of the transmission functions 24 to 27 of the insert of FIG. 1, where T.sub.NA can also depend on the pupil coordinates should a subdivided wavelength filter 33 be used, for example in the style of the wavelength filter 33 according to FIG. 4.

[0106] T.sub.M1(λ), T.sub.M2(λ), T.sub.M3(λ) are the wavelength-dependent reflectivities of the three mirrors M1, M2 and M3 of the projection optical unit 9.

[0107] A.sub.CCD(λ) is a wavelength-dependent absorption coefficient or a wavelength-dependent sensitivity of the CCD sensor of the detector 18.

[0108] Below, the assumption is made that, firstly, the reflectivity of the lithography mask 13 around the central wavelength λ.sub.0 and, secondly, the transmission function of the filter 33 around the central wavelength λ.sub.0 do not change abruptly. Then, as conventional in perturbation theory, both R.sub.Ret(x, y, θ, λ) and T.sub.NA(λ) can be written as an expansion according to powers of a deviation of the wavelength from the central wavelength λ.sub.0.

[0109] A product T.sub.NA(λ)R.sub.Ret(x, y, θ, λ) can now be inserted into Formula (1) above, with additionally a normalized intensity .Math.(x, y)=S.sub.CCD(x, y)/S.sub.ES also being taken into account. This yields:

[00003]$\begin{array}{cc}t\left(x,y\right)=T_{\mathrm{NA}}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(0\right)}+\left(\frac{d}{d\lambda }{T}_{\mathrm{NA}}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}\left({\lambda }_0\right)\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)\right){\Sigma }^{\left(1\right)}+\frac{1}{2}\left(\frac{d^2}{d{\lambda }^2}{T}_{\mathrm{NA}}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+2\frac{d}{d\lambda }{T}_{\mathrm{NA}}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}\left({\lambda }_0\right)\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)\right){\Sigma }^{\left(2\right)}& \left(2\right)\end{array}$

[0110] Here, the terms Σ.sup.(0), Σ.sup.(1) and Σ.sup.(2) represent

[00004]${\Sigma }^{\left(0\right)}=\underset{{\lambda }_{\min }}{\stackrel{{\lambda }_{\max }}{\int }}\frac{S_{\sigma }\left(\lambda \right)}{S_{\mathrm{ES}}}{T}_{\mathrm{IL}2}\left(\lambda \right)T_{\mathrm{IL}3}\left(\lambda \right)T_{M1}\left(\lambda \right)T_{M2}\left(\lambda \right)T_{M3}\left(\lambda \right)A_{\mathrm{CCD}}\left(\lambda \right)d\lambda$$\left(0\mathrm{th}\mathrm{order}\right)$${\Sigma }^{\left(1\right)}=\stackrel{{\lambda }_{\max }}{\underset{{\lambda }_{\min }}{\int }}\left(\lambda -{\lambda }_0\right)\frac{S_{\sigma }\left(\lambda \right)}{S_{\mathrm{ES}}}{T}_{\mathrm{IL}2}\left(\lambda \right)T_{\mathrm{IL}3}\left(\lambda \right)T_{M1}\left(\lambda \right)T_{M2}\left(\lambda \right)T_{M3}\left(\lambda \right)A_{\mathrm{CCD}}\left(\lambda \right)d\lambda$$\left(1\mathrm{st}\mathrm{order}\right)$${\Sigma }^{\left(2\right)}=\stackrel{{\lambda }_{\max }}{\underset{{\lambda }_{\min }}{\int }}{\left(\lambda -{\lambda }_0\right)}^2\frac{S_{\sigma }\left(\lambda \right)}{S_{\mathrm{ES}}}{T}_{\mathrm{IL}2}\left(\lambda \right)T_{\mathrm{IL}3}\left(\lambda \right)T_{M1}\left(\lambda \right)T_{M2}\left(\lambda \right)T_{M3}\left(\lambda \right)A_{\mathrm{CCD}}\left(\lambda \right)d\lambda$$\left(2\mathrm{nd}\mathrm{order}\right)$

[0111] To a first approximation, the quantities Σ.sup.(0), Σ.sup.(1) and Σ.sup.(2) do not depend on intensity fluctuations of the light source 3.

[0112] To determine the following quantities of interest: [0113] R.sub.Ret(x, y, λ.sub.0), i.e., the reflectivity of the lithography mask 13 at the central wavelength λ.sub.0,

[00005]$\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right),$

i.e., the first derivative of the reflectivity of the lithography mask 13 at the target wavelength λ.sub.0 and

[00006]$\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right),$

i.e., the curvature of the wavelength-dependent reflectivity at the central wavelength λ.sub.0 two transmission functions T.sup.(1).sub.NA(λ.sub.0)T.sup.(2).sub.NA(λ.sub.0) are required first, said transmission functions differing firstly in the form of the transmission function around the central wavelength λ.sub.0 and also in terms of their first and second derivatives

[00007]$\frac{d}{d\lambda }{T}_{\mathrm{NA}}^{\left(1/2\right)}\left({\lambda }_0\right)$$\mathrm{and}$$\frac{d^2}{d{\lambda }^2}{T}_{\mathrm{NA}}^{\left(1/2\right)}\left({\lambda }_0\right).$

A third transmission function T.sup.(3).sub.NA arises trivially by a measurement without a wavelength field, and so the following applies T.sup.(3).sub.NA(λ.sub.0)=1 and has values for the first derivative and the curvature of 0 in each case.

[0114] The following system of equations arises for the three measurements with the first filter function T.sup.(1).sub.NA with the first wavelength filter, for example a filter with the transmission function 24 according to FIG. 1, with the second filter with the filter function T.sup.(2).sub.NA, for example with the transmission function 25 according to FIG. 1, and without a filter with the transmission function T.sup.(3).sub.NA:

[00008]$\begin{array}{cc}{l}_1\left(x,y\right)=T_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(0\right)}+(\frac{d}{d\lambda }{T}_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(1\right)}+\frac{1}{2}\left(\frac{d^2}{d{\lambda }^2}{T}_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+2\frac{d}{d\lambda }{T}_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}^{\left(1\right)}\left({\lambda }_0\right)\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)\right){\Sigma }^{\left(2\right)}& \left(3\right)\end{array}$$\begin{array}{cc}{l}_2\left(x,y\right)=T_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(0\right)}+(\frac{d}{d\lambda }{T}_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(1\right)}+\frac{1}{2}\left(\frac{d^2}{d{\lambda }^2}{T}_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+2\frac{d}{d\lambda }{T}_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)+T_{\mathrm{NA}}^{\left(2\right)}\left({\lambda }_0\right)\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)\right){\Sigma }^{\left(2\right)}& \left(4\right)\end{array}$$\begin{array}{cc}{l}_3\left(x,y\right)=R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(0\right)}(\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(1\right)}+\frac{1}{2}\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right){\Sigma }^{\left(2\right)}& \left(5\right)\end{array}$

Then, the desired values for

[00009]$R_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right),\frac{d}{d\lambda }{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)\mathrm{and}\frac{d^2}{d{\lambda }^2}{R}_{\mathrm{Re}t}\left(x,y,{\lambda }_0\right)$

can be determined from this linear system of equations. To the extent that use is made of wavelength filters in the style of the wavelength filter 33 according to FIG. 4 with transmission function that varies over the filter surface, these values for the reflectivities, the gradients thereof and the curvatures thereof at the central wavelength λ.sub.0 can still be determined as a function of the illumination angle. In this way, structure influences, in particular, can also be acquired.

[0115] Within the scope of carrying out the measuring method, the wavelength filter 33 according to FIG. 4 can be introduced into the respective filter holder, for example into the filter holders 21 and 22, in a plurality of orientations.

[0116] FIG. 5 shows such a further orientation, in the case of which the wavelength filter 33 is oriented rotated clockwise through 90° about the drawing axis 34 in comparison with the orientation according to FIG. 4. Accordingly, the separation line 36 is horizontal in the case of the orientation of the wavelength filter 33 according to FIG. 5, the high-pass filter section 37 is located at the top and the low-pass filter section 38 is located at the bottom.

[0117] FIGS. 6 and 7 show a further wavelength filter 39, which can be used as an alternative or in addition to the wavelength filter 33 when carrying out the measuring method. Instead of the low-pass filter section 38, the wavelength filter 39 has a fully transmissive filter section 40, for which the following applies: T=1. The wavelength filter 39 also has the high-pass filter section 37 like the wavelength filter 33.

[0118] FIG. 6 shows the wavelength filter 39 in an orientation with a perpendicular separation line 36, corresponding to the orientation of the wavelength filter 33 according to FIG. 4.

[0119] Similar to FIG. 5, FIG. 7 shows the wavelength filter 39 rotated 90° clockwise.

[0120] FIGS. 8 and 9 show a further embodiment of a wavelength filter 39a, which can be used as an alternative or in addition to the wavelength filters 33 and 39 or to the other filters, already explained above, with the transmission functions according to FIG. 1. The structure of the wavelength filter 39a corresponds to that of the wavelength filter 39 according to FIGS. 6 and 7, Instead of the high-pass filter section 37, the wavelength filter 39a has a low-pass filter section 38 which corresponds to that already explained above in relation to the wavelength filter 33.

[0121] FIGS. 8 and 9 show the wavelength filter 39a in two orientations, which correspond to those of the wavelength filter 39 in FIGS. 6 and 7.

[0122] FIG. 10 shows a polarization filter 41 and its effect on a polarization state of a measuring light beam 1 passing through the polarization filter 41.

[0123] The polarization filter 41 can be used in a method for measuring an effect of a polarization of the measuring light 1 on a measuring light impingement on the lithography mask 13 using the measuring apparatus 2, 29 or 32, as an alternative or in addition to the above-described method for measuring the effect of the wavelength-dependent measuring light reflectivity on the lithography mask 13.

[0124] The polarization filter 41 is arranged tilted about an axis parallel to the y.sub.P-axis in relation to the plane of the drawing of FIG. 10, which is spanned by polarization coordinates x.sub.P, y.sub.P. The coordinates x.sub.F, y.sub.F which span a filter arrangement plane 28 of the polarization filter 41 are likewise plotted in FIG. 10. The y.sub.F-coordinate extends parallel to the y.sub.P-coordinate. The x.sub.F-coordinate extends at an angle to the x.sub.P-coordinate. This angle corresponds to a tilt angle of the filter arrangement plane x.sub.Fy.sub.F in relation to a normal plane xy to the measuring light beam path 1, which in turn extends parallel to the polarization plane x.sub.Py.sub.P.

[0125] A tilt angle of the filter arrangement plane x.sub.Fy.sub.F with respect to the polarization plane x.sub.Py.sub.P is such that the Brewster condition is satisfied for an x-polarization component 42 of the measuring light 1. Consequently, the x-polarization component 42 is not attenuated during the passage through the polarization filter 41. The x-polarization component 42 is the p-polarization component in relation to a plane of incidence of the measuring light 1 on the filter arrangement plane x.sub.Fy.sub.F. This plane of incidence is parallel to the xz-plane of the polarization coordinates of the measuring light beam 1.

[0126] A y-polarization component 43 of the measuring light 1 is attenuated during the passage through the polarization filter 41 since this y-polarization component 43 represents an s-polarization component in relation to the plane of incidence xz.

[0127] Thus, the polarization filter 41 changes the polarization state of the measuring light 1. Following the passage through the polarization filter 41, the measuring light 1 is present in a filter polarization state that differs from a raw polarization state of the measuring light 1 prior to impingement on the polarization filter 41.

[0128] Should the x/y-polarization components 42, 43 have been present with an intensity ratio of for example 1/1 prior to entry in the polarization filter 41, this ratio has changed following the passage through the polarization filter 41 to a ratio of 1/2 or 0.5/1.

[0129] To measure the effect of the polarization of the measuring light 1 on a measuring light impingement on the lithography mask 13, the lithography mask 13 and the measuring apparatuses of one of the embodiments of the measuring apparatuses 2, 29 or 32 are once again initially provided. The measuring light source 3 produces the measuring light 1 in the raw polarization state. Following a first round of causing the measuring light beam 1 to impinge on the lithography mask 13 within the field of view 11 of the measuring apparatus 2 and of capturing the measuring light 1 emanating from the impinged section of the lithography mask 13 with the detector 13 and with a still unutilized polarization filter, the polarization filter 41 is subsequently inserted into one of the filter holders of the measuring apparatus 2, 29 or 32, for example into the filter holder 19 and/or into the filter holder 20.

[0130] Subsequently, the measuring light 1 emanating from the impinged section of the lithography mask 13 is captured again using the detector 18 once the polarization filter 41 has been introduced.

[0131] The effect of the polarization of the measuring light 1 on the measuring light impingement of the lithography mask 13 is then determined on the basis of the results of the two capture steps “without/with polarization filter.”

[0132] This polarization effect can be determined for the two polarization directions x.sub.P, y.sub.P with the aid of the formalism that was explained above in conjunction with the determination of the reflectivity at the wavelength λ.sub.0. To this end, the steps of “introducing the polarization filter” and “capturing the measuring light once the polarization filter has been introduced” are repeated for a plurality of polarization filters which differ in terms of their polarization effect on the measuring light 1. Alternatively or in addition, it is also possible for one and the same polarization filter 41 to be introduced into the beam path of the measuring light 1 with different orientations. Such orientations, which differ from the orientation according to FIG. 10, can differ in terms of the tilt angle about the y-axis and/or in terms of the orientation of the filter arrangement plane x.sub.Fy.sub.F relative to the coordinates xyz of the measuring light beam 1. Thus, for example, the polarization filter 41 can also be oriented in such a way that the Brewster condition is satisfied for the y-polarization component 43.

[0133] In a manner analogous to determining the change in structure resolution on the basis of the reflectivity of the lithography mask 13, as described above in the context of the reflectivity measuring method, the polarization measuring method can be used to determine, as an effect, a change of a structure resolution of the image representation of the structures on the lithography mask 13 by the introduction of the respective polarization filter 41. To this end, the CD can be determined in turn. This change in the structure resolution can be determined depending on, in particular, an orientation of structures on the lithography mask 13.

[0134] FIG. 11 shows a further embodiment of a polarization filter 44, which can be used instead of the polarization filter 41. The polarization filter 44 is depicted held in a filter holder, which could be one of the filter holders 19 and 20, for example. FIG. 11 shows the filter holder 20 in the intermediate focal plane 7 in exemplary fashion. A plane of the drawing of FIG. 11 is perpendicular to this intermediate focal plane 7.

[0135] The polarization filter 44 has a plurality of filter sections, a total of four filter sections 45, 46, 47, 48 in the depicted embodiment, the filter arrangement planes 28.sub.1, 28.sub.2, 28.sub.3 and 28.sub.4 of said filter sections being located parallel to one another and each being tilted in relation to a normal plane xy to the beam path of the measuring light 1, which strikes the polarization filter 44 from the top in FIG. 11, that is to say in the negative z-direction. Despite the tilt of the filter arrangement planes 28.sub.i (=x.sub.Fy.sub.F) in relation to the normal planes xy located parallel to the intermediate focal plane 7, an advantageously small installation space of the projection filter 44 arises in the measuring light beam direction, that is to say in the z-direction of FIG. 11, on account of the subdivision of the polarization filter 44 into the filter sections 45 to 48.

[0136] FIG. 12 shows by way of example an embodiment of a lithography mask 13 that is to be measured. The latter is divided into a central zone 13.sub.1, in which a structuring is provided that is imaged onto a wafer to be structured during the use of the lithography mask 13 during the projection exposure. This central zone 13.sub.1 is approximately square.

[0137] The central zone 13.sub.1 is surrounded by a transition zone 13.sub.2 with a maximum reflectivity for the illumination light 1 that is reduced compared to the central zone 13.sub.1. Reflection sections 50 can be arranged at the location of the transition zone 13.sub.2 for determining a signal strength of the measuring light 1 impinging on the object 13. In addition to the measuring light beam 1 impinging on the field of view 11, the illumination system 5 can be designed such that at least one of the reflection sections 50 on the object 13 is impinged upon with the aid of at least one further measuring light beam. Then, the respective further measuring light beam is directed by the reflection sections 50 to at least one energy sensor 51 corresponding to the above-described energy sensors 30 for determining the measuring light signal strength

[0138] Such energy sensors 51, which are depicted schematically in FIG. 12, can be arranged on a spatially fixed component of the measuring apparatus 2, which is not displaced when the object 13 is displaced.

[0139] The transition zone 13.sub.2 is designed to extend around an outer periphery of the central zone 13.sub.1. The transition zone 13.sub.2 in turn is surrounded by a peripheral zone 13.sub.3 of the object 13. The latter serves for cooperating with the object holder 14 and is generally designed to be non-reflective, but can likewise carry reflection sections 50, as indicated in FIG. 12.

[0140] A maximum reflection of the central zone 13.sub.1 can lie in the range between 60% and 70% of the incident measuring light 1. A maximum reflection of the transition zone 13.sub.2 can lie in the range between 55% and 65% of the incident measuring light 1.

[0141] The above-described filters can be realized as thin films. In particular, these filters can be constructed as a multilayer system. The influence of such multilayer systems on a wavelength-dependent transmission and/or on a polarization of transmitted measuring light can be calculated with the aid of algorithms described in the technical literature. Refractive indices of appropriate materials which are used in the construction of such a multilayer system are tabulated and, for example, available from the Internet site http://henke.lbl.gov/optical_constants/.

[0142] The polarization filter 41 can attenuate a certain polarization component of the incident measuring light in relation to another polarization component, for example s-polarized measuring light in relation to p-polarized measuring light. Such a polarization filter can be designed by specifying and optimizing the multilayer system. An angle of incidence of the measuring light on the polarization filter 41 can be specified and/or optimized together with the multilayer design.

[0143] The multilayer system of the filter can be chosen to be periodic or aperiodic.

[0144] A reflectivity and a transmission for the two polarization components (s-polarized component, p-polarized component or tangential polarization component and sagittal polarization component) of the polarization filter 41 can be calculated with the aid of a transfer matrix formalism, as known from the technical literature.

[0145] While this specification contains many implementation details, these should not be construed as limitations on the scope of the invention or of what can be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. The separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a subcombination or variation of a subcombination.

[0146] Similarly, while operations are described in a particular order, this should not be understood as requiring that such operations be performed in the particular order, or that all operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments.

[0147] Thus, particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. In addition, the actions recited in the claims can be performed in a different order and still achieve desirable results.