Method and apparatus for characterizing microlithographic masks

Abstract

The present invention relates to a method and an apparatus for characterizing microlithographic masks. In the method according to the invention, a plurality of individual imaging processes are carried out with a pixel resolution specified by a detector unit, wherein the individual imaging processes differ from one another with respect to the position of at least one polarization optical element located in the imaging optical unit, wherein the image data recorded by the detector unit are evaluated in an evaluation unit, wherein polarization-dependent effects due to the polarization dependence of the interference of electromagnetic radiation occurring in the wafer plane during operation of the microlithographic projection exposure apparatus are simulated, wherein a transformation of the image data obtained in the individual imaging processes is carried out, which is in each case based on at least one calibration image obtained by imaging a structureless area of the mask onto the detector unit, wherein the calibration image used in each case is selected differently depending on the position of the at least one polarization optical element.

Description

[0001] This application claims the benefit of German patent application DE 10 2020 123 615.8, filed on September 10, 2020. The contents of this application are incorporated herein by reference. Background of the Invention Technical Field

[0003] The present invention relates to a method and an apparatus for characterizing microlithographic masks. The invention is applicable both for finding and characterizing defects and for characterizing structures on the mask, for example in position determination or for determining the line width (CD = “critical dimension”) of the structures and for determining the process window (for example by determining the line width as a function of dose and defocus), among other applications. Background Art

[0004] For example, microlithography is used to manufacture microstructured components such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which comprises an illumination device and a projection lens. In this case, an image of a mask (reticle) illuminated by the illumination device is projected via the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens. This transfers the mask structure to the substrate's photosensitive coating.

[0005] In photolithography, unwanted defects on masks have a particularly negative impact, as they can recur with every exposure step. Therefore, it is desirable to analyze the imaging effects of possible defect locations directly in order to minimize mask defects and achieve successful mask repair. Therefore, there is a need for a fast and simple way to measure or qualify masks as accurately as possible under the same conditions as those actually encountered in projection exposure systems.

[0006] To this end, it is known to record and evaluate aerial images of sections of the mask in a mask inspection apparatus. To record the aerial image, the structures to be measured on the mask are illuminated by an illumination optical unit, wherein the light from the mask is projected onto a detector unit via an imaging optical unit and detected. To enable measurements of the mask under conditions similar to those in a projection exposure apparatus, the mask is typically illuminated in the mask inspection apparatus in a manner consistent with that in the projection exposure apparatus, wherein in the mask inspection apparatus, in particular, the same wavelength (e.g., approximately 248 nm, approximately 193 nm, or approximately 13.5 nm), the same numerical aperture, and, if appropriate, the same (polarization) illumination settings are provided.

[0007] In practice, however, a problem arises from the fact that, in the imaging optics of a mask inspection system, the imaging of the mask onto the detector unit differs from the imaging onto the wafer in a projection exposure system and does not occur in a reduced-size manner, but rather in a significantly magnified manner. Consequently, the significantly different numerical apertures present in the respective projection or imaging optics (which are practically zero in the imaging optics of a mask inspection system) lead to significantly different polarization-dependent vector effects occurring in the imaging onto the wafer during the lithography process and in the imaging onto the detector unit in the mask inspection system. In this context, "vector effects" are understood to mean the polarization-dependence of the interference of the electromagnetic radiation occurring in the respective image plane.

[0008] In order to take the above-mentioned problems into account and in order to determine the vector effects occurring in microlithography projection exposure equipment and in order to take them into account in the generation of aerial images, it is particularly known to carry out a plurality of individual imaging processes with a mask inspection device, during which different polarization optical components are placed at different positions in the illumination and / or imaging optical unit and the correspondingly generated images are combined with one another and subjected to calculations.

[0009] However, in practice, the introduction of one or more polarization optical elements into the beam path leads to further problems which therefore make a reliable simulation of the conditions occurring in the projection exposure apparatus during mask characterization by calculating the correct combination of the individual images with one another more difficult: Firstly, the generally unavoidable contamination and / or inhomogeneities present on the polarization optical elements lead to artifacts in the obtained measurement images, which is further complicated by the fact that the effect of said contamination or inhomogeneity behaves differently depending on the position of the polarization optical element or elements.

[0010] Furthermore, the introduction of polarization optics into the imaging beam path of the mask inspection apparatus leads to image shifts and, moreover, to changes in the imaging scale due to unavoidable manufacturing errors.

[0011] Within the scope of ever-increasing demands on the accuracy of mask characterization, achieving correct vector effect calculations for simulating polarization-dependent effects during mask characterization therefore represents a significant challenge.

[0012] With regard to the prior art, reference is made merely by way of example to DE 10 2007 045 891 A1, DE 10 2017 115 262 A1, DE 10 2004 033 603 A1, DE 10 2004 033 602 A1, DE 10 2005 062 237 A1, DE 102007 009 661 A1 and EP 1 615 062 B1. Summary of the Invention

[0013] The object of the present invention is to provide an apparatus and a method for characterizing microlithographic masks which enable a more accurate characterization taking into account the conditions given in the lithographic process including polarization-dependent effects occurring therein, while at least partially avoiding the above-mentioned problems.

[0014] This object is achieved by a device and a method according to the features of the alternative independent claims.

[0015] The invention also relates in particular to a method for characterizing a microlithographic mask,

[0016] - wherein the illumination optical unit illuminates a structure of a mask intended for use in a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit images the mask onto a detector unit, wherein the detector unit has a plurality of pixels;

[0017] wherein a plurality of individual imaging processes are carried out with a pixel resolution specified by the detector unit, wherein the individual imaging processes differ from one another with regard to the position of at least one polarization optical element located in the imaging optical unit;

[0018] - wherein the image data recorded by the detector unit are evaluated in an evaluation unit, wherein polarization-dependent effects due to polarization dependence of interference of electromagnetic radiation occurring in the wafer plane during operation of the microlithographic projection exposure apparatus are simulated during the evaluation;

[0019] - wherein the evaluation comprises a conversion of image data obtained in a separate imaging process, which are in each case based on at least one calibration image obtained by imaging a structure-free region of the mask onto the detector unit;

[0020] wherein the calibration image respectively used during the conversion is selected differently depending on the position of the at least one polarization optical element during the respective individual imaging process.

[0021] According to one embodiment, a calibration image obtained for the respective same position of the at least one polarization optical element in the imaging optical unit is used in each case during the conversion of the image data obtained in the individual imaging processes.

[0022] In particular, the invention is based on the concept of carrying out a transformation of the image data obtained when characterizing a mask by performing a plurality of individual imagings, based on calibration images obtained by imaging structure-free areas of the mask, in each case with a different position of one or more polarization optical elements in the imaging optical unit for the purpose of simulating polarization-dependent effects, the transformation being carried out in such a way that not only the illumination intensity and transmission properties of the optical system, inhomogeneities of the detector unit are taken into account, but also undesirable effects of the polarization optical elements (in particular with regard to inhomogeneities and current contamination of the polarization optical elements) are “computationally removed” from the simultaneous measurement results.

[0023] In particular, the invention comprises the following principle: when converting the image data obtained during the individual imaging processes for the purpose of computationally removing the above-mentioned effects, not one and the same calibration image is used to convert all measurement images, but rather a respective dedicated calibration image is recorded for each position of the polarization optical element in the imaging optical unit, so that the measurement images recorded at different positions of the polarization optical element in the imaging optical unit by computation are then combined in each case with the “fitted” calibration image.

[0024] According to the present invention, the increased measurement complexity required to record multiple calibration images of different positions of one or more polarization optical elements in the imaging optical unit is deliberately accepted in order to achieve, in return, increased accuracy in the simulation of polarization-dependent effects during mask characterization, while at least partially avoiding the problems described in the introduction. In particular, the additional measurement complexity during operation according to the present invention prevents contamination and inhomogeneities on the polarization optical elements from occurring, which could lead to incorrect calculations of the vector effects and, therefore, ultimately, to faulty mask characterizations.

[0025] According to one embodiment, before the conversion, at least some of the calibration images are subjected to a pre-processing, during which the brightness of the calibration images are matched to one another.

[0026] This preprocessing takes into account the fact that the different brightness levels of the calibration images recorded according to the invention for different positions of the polarization optical element, which are always present in the case of polarized illumination, will ultimately lead to a loss of desired information relating to the different brightness levels of the images during their combination by calculation with the corresponding associated measurement images (since in this case the "corrected" measurement images ultimately resulting from the conversion will all have the same intensity).

[0027] According to one embodiment, if the illumination setting set in the illumination optics unit is a non-polarized illumination setting, the above-mentioned pre-processing (i.e., matching the brightness level of the calibration image) is omitted. This takes into account the fact that, in the case of a deliberately non-polarized illumination setting, the "computational removal" of any small residual polarization that still exists is desirable, and thus the method is obtainable by omitting the above-mentioned pre-processing step.

[0028] According to one embodiment, if the average intensity in a calibration image obtained for a given position of the at least one polarization optical element falls below a specified threshold, the image data obtained during a separate imaging process carried out at this position of the at least one polarization optical element is converted based on the calibration image instead, which was recorded without the polarization optical element in the imaging optical unit. This takes into account the fact that a conversion based on such a particularly dark calibration image would result in significant background noise in the ultimately obtained image data, and therefore no meaningful image information would be obtained.

[0029] According to one embodiment, the image data recorded by the detector unit during the individual imaging process are subjected to low-pass filtering before evaluation. The invention is based on the idea that, since the spatial resolution of the detector unit is generally always substantially higher than that of the imaging optics unit, relatively high spatial frequencies in the image data recorded by the detector unit are generally noise components. Proceeding from this idea, the invention now includes the further concept of applying a low-pass filter to the image data obtained in each case during the individual imaging process (this application being essential in this respect), so that this image data, from which the noise components have been removed, forms the basis for the actual vector effect calculation mentioned above. In this regard, reference should be made to DE 10 2015 213 163 A1.

[0030] The above-mentioned application of a low-pass filter to the image data obtained during a separate imaging process is also advantageous, independently of the use of the above-mentioned different calibration images. According to further aspects, the present invention therefore also relates to a method for characterizing a microlithography mask,

[0031] - wherein the illumination optical unit illuminates a structure of a mask intended for use in a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit images the mask onto a detector unit, wherein the detector unit has a plurality of pixels;

[0032] wherein a plurality of individual imaging processes are carried out with a pixel resolution specified by the detector unit, wherein the individual imaging processes differ from one another with regard to the position of at least one polarization optical element located in the imaging optical unit;

[0033] - wherein the image data recorded by the detector unit are evaluated in an evaluation unit, wherein polarization-dependent effects due to polarization dependence of interference of electromagnetic radiation occurring in the wafer plane during operation of the microlithographic projection exposure apparatus are simulated during the evaluation; and

[0034] - wherein the image data recorded by the detector unit during the individual imaging processes are subjected to low-pass filtering before evaluation.

[0035] According to one embodiment, during the evaluation of the image data recorded by the detector unit, changes in the imaging scale of the imaging optical unit that are dependent on the position of the polarization optical element during the relevant individual imaging process and image shifts that are dependent on the position of one or more polarization optical elements during the relevant individual imaging process are at least partially corrected.

[0036] According to one embodiment, the correction is achieved by displacing the stretch center of the central stretch applied to the image recorded by the detector unit relative to the center of the camera field of the detector unit by a value that depends on the position of the at least one polarization optical element.

[0037] According to one embodiment, a focus stack is recorded for each pixel of the detector unit in the form of a plurality of individual image representations that differ from one another with respect to the respective distances between the mask and the imaging optical unit, wherein the relative focus position is corrected pixel by pixel by individually fitting the image data correspondingly obtained for each pixel when recording the focus stack. This allows a focus stack to be obtained whose focus accuracy can be more accurate than the mechanical reproducibility of the focus adjustment mechanism. The number of focal planes of the interpolated focus stack can be greater or smaller than the number of measured focal planes. The focus increments between the focal planes of the interpolated focus stack can also be greater or smaller than the focus increments between the measured focal planes. The focus increments between the focal planes of the measured and / or interpolated focus stacks can also be non-equidistant. The result can be a focus stack or only a single image, for example, in an ideal optimal focus plane. To achieve even more accurate focus accuracy, the interpolation for focus correction can be applied multiple times (e.g., twice) in succession.

[0038] The above-mentioned pixel-by-pixel correction of the relative focus position is also advantageous by performing a separate fitting of the image data corresponding to each pixel obtained when recording the focus stack, which is independent of the use of the above-mentioned different calibration images. According to further aspects, the present invention therefore also relates to a method for characterizing a microlithography mask,

[0039] - wherein the illumination optical unit illuminates a structure of a mask intended for use in a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit images the mask onto a detector unit, wherein the detector unit has a plurality of pixels;

[0040] - wherein the image data recorded by the detector unit are evaluated in an evaluation unit; and

[0041] - wherein for each pixel of the detector unit a focus stack is recorded in the form of a plurality of individual image representations which differ from one another with respect to the respective distances between the mask and the imaging optical unit, wherein a correction of the relative focus position is carried out pixel by pixel by individual fitting of the image data respectively obtained for each pixel when recording the focus stack.

[0042] According to one embodiment, the mask is designed for an operating wavelength of less than 250 nm, in particular for an operating wavelength of less than 200 nm, more in particular for an operating wavelength of less than 15 nm.

[0043] Furthermore, the present invention relates to an apparatus for characterizing a microlithography mask, comprising: an illumination optical unit for illuminating the structure of a mask intended for use in a lithography process in a microlithography projection exposure apparatus; a detector unit; an imaging optical unit for imaging the mask onto the detector unit; and an evaluation unit for evaluating data recorded by the detector unit, wherein the apparatus is designed to carry out a method having the above-mentioned features.

[0044] With regard to further advantages and preferred configurations of the method, reference is made to the above explanations in connection with the device according to the invention.

[0045] Further configurations of the invention can be derived from the description and the dependent claims.

[0046] The invention is explained in more detail below based on the exemplary embodiments shown in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In the attached figure:

[0048] Figure 1-3 A schematic diagram illustrating the effect of the method according to the invention is shown;

[0049] Figure 4 shows a schematic diagram illustrating an exemplary configuration of a mask inspection apparatus; and

[0050] Figure 5 A schematic diagram is shown for explaining focus correction implemented according to an aspect of the present invention. DETAILED DESCRIPTION

[0051] Figure 4 First, a schematic diagram is shown to explain a possible configuration of a mask inspection apparatus 400 that can be implemented by the present invention.

[0052] according to Figure 4In a mask inspection apparatus 400, a mask 421 is mounted on a mask holder 420. Illumination light generated by a light source 401 illuminates the structure to be measured on the mask 421 via an illumination optical unit 410. The light from the mask 421 is imaged and detected by a detector unit 440 by an imaging optical unit 430. The image data recorded by the detector unit 440 is evaluated in an evaluation unit 450 to determine the position of the structure.

[0053] By means of the pupil filter present in pupil plane PP in the illumination optical unit 410 and by means of the polarizer 415 in the illumination optical unit 410, an illumination setting can be set which corresponds with respect to the intensity distribution and the polarization distribution to an illumination setting predefined in the microlithographic projection exposure apparatus. In this case, the polarized illumination setting predefined for the lithographic process can be, by way of example only, a quasi-tangential illumination setting or a quadrupole setting with xy polarization.

[0054] In order to now also determine the vector effects that occur in the microlithography projection exposure device for the purpose of simulating the conditions prevailing during an actual microlithography process in the best possible way, at least one polarization optical element is introduced into the imaging beam path of the imaging optical unit 430 in a manner that is in principle known per se, wherein a plurality of individual imaging processes are carried out for different positions of the at least one polarization optical element 435.

[0055] The image data obtained during the individual imaging processes are transformed based on calibration images, each of which is obtained by imaging a structureless area of mask 421 onto detector element 440. Using the appropriately transformed image data, the above-described vector effect calculation is performed in a manner known per se to a person skilled in the art. For this purpose, reference is made, for example, to EP 1 615 062 B1.

[0056] According to the invention, instead of just a single calibration image being recorded for the aforementioned conversion of the image data obtained during a single imaging process and forming the basis for said conversion, a plurality of calibration images are recorded instead, and these form the basis for said conversion depending on the different positions of the polarization optical element or elements in the imaging optical unit 430. Thus, not only are the inhomogeneities of the detector unit, the illumination intensity and the transmission properties of the optical system taken into account during the calibration process, but also the inhomogeneities and unavoidable contamination of the polarization optical element or elements are removed from the measurement results by calculation.

[0057] Refer to the following Figure 1 a-1c, Figure 2 a-2c and Figure 3The figures shown in Figures 3a-3c illustrate the advantageous effects of the method according to the present invention. For the sake of simplicity, each of these figures shows the intensity curve (of a measurement or calibration image, as described below) only along the x-coordinate, with the intensity curve along the y-coordinate omitted for easier explanation. Furthermore, to simplify the explanation of the effects achieved according to the present invention, reference is made in each case to a measurement image recorded when imaging a region of mask 421 in which mask 421 does not have a mask structure.

[0058] Figure 1 a first shows the intensity curve of such a measurement image along the x-coordinate, without the presence of polarization optics in the imaging optical unit 430. The inhomogeneity of the intensity profile is illustrated in an exaggerated manner. Figure 1 b shows the intensity curve along the x-coordinate obtained in the corresponding calibration image when there is no polarization optical element in the imaging optical unit 430. Since the calibration image appears by definition from the image representation of the structureless area of the mask 421, Figure 1 The intensity curve of b corresponds to Figure 1 The intensity curve of a. Figure 1 The intensity ratio of the measured and calibration images plotted in c therefore has a constant value of 1 along the x-coordinate.

[0059] Similar to Figure 1 In the manner of a, if the polarization optical element is located at a certain position of the imaging optical unit 430, then Figure 2 The schematic diagram of a shows the intensity curve along the x-coordinate obtained in the measured image (again at a location where the mask 421 has no structure). The presence of one or more polarization optical elements firstly causes an image shift and, in addition, local intensity drops caused by contamination and / or inhomogeneities, so that according to Figure 2 The intensity curve of a has been compared to Figure 1 The intensity curve of a is significantly modified. Because, according to the invention, the image data obtained during the associated individual imaging process are now converted using a calibration image, which was obtained for the same position of the polarization optical element in the imaging optical unit 430 and therefore has Figure 2 b shows the intensity profile along the x-coordinate, according to Figure 2 c The intensity value ratio of the measured image and the calibration image plotted along the x-coordinate again supplies the correctly expected constant result for the considered “simple” case of imaging a structureless area of the mask 421 .

[0060] In contrast, if only one calibration image is used to transform all measurement images in the conventional way, Figure 3 The scenario shown in a-3c. In this case, Figure 3 The intensity curve of the measured image shown in a (with Figure 2The intensity curve of a is consistent with that of the image) will be combined by calculation with a calibration image determined in the absence of polarization optics in the imaging optical unit 430, i.e. with Figure 3 The intensity curve shown in b is Figure 1 The intensity curves in b are consistent. Figure 3 Drawn in c Figure 3 a and Figure 3 The ratio of the two intensity profiles of b therefore deviates from the correct, desired constant value along the x-coordinate, since the image shift caused by the polarization optical element or elements and the effects of contamination and inhomogeneities present on said polarization optical elements were not taken into account during calibration. This would therefore lead to an erroneous calculation of the vector effects, which is precisely avoided according to the present invention.

[0061] References below Figure 5 Other aspects of the present invention are explained.

[0062] The present invention proceeds from a method known per se for recording a focus stack for each pixel of a detector unit in the form of a plurality of individual image representations that differ from one another in terms of the respective distances between the mask and the imaging optical unit, in order to characterize the mask not only at optimal focus but also "when defocused," thereby determining the permissible process window (with respect to dose and focus). According to the present invention, the relative focus position can now be corrected pixel by pixel by individually fitting the image data correspondingly obtained for each pixel when recording the focus stack. In this way, a focus stack can be obtained whose focus accuracy can be more accurate than the mechanical reproducibility of the focus adjustment mechanism.

[0063] according to Figure 5 Schematic diagram showing that when determining the contrast in the focal plane, the actual optimal relative focus position (“optimal focus position”) for achieving the maximum contrast slightly deviates from the position that forms the basis for recording the focus stack. According to the invention, this deviation is now taken into account by calculating an image corresponding to the shifted position from the measured image (for example by spline interpolation of the intensity of the focal position of each camera pixel itself) and outputting said image accordingly.

[0064] Here, the number of focal planes of the interpolated focus stack can be greater or less than the number of focal planes measured (where Figure 5 The number of focal planes is also shown to increase from 5 to 7 while decreasing the focus increment as an example only. Likewise, the focus increments between the focal planes of the interpolated focus stack may be larger or smaller than the focus increments between the measured focus planes. The focus increments between the focal planes of the measured and / or interpolated focus stacks may also be non-equidistant. Figure 5The result can be a focus stack or just a single image, for example in an ideal best focus plane. To obtain a more accurate focus accuracy, the interpolation for focus correction can be applied multiple times in succession (for example twice).

[0065] Although the present invention has been described based on the specified embodiments, many variations and alternative embodiments will be apparent to those skilled in the art, for example by combining and / or exchanging features of the individual embodiments. Therefore, it goes without saying that the present invention encompasses such variations and alternative embodiments, and the scope of the present invention is limited only by the meaning of the appended claims and their equivalents.

Claims

1. A method for characterizing a microlithography mask, - wherein the illumination optical unit (410) illuminates a structure of a mask (421) intended for a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit (430) images the mask (421) onto a detector unit (440), wherein the detector unit (440) has a plurality of pixels; - wherein a plurality of individual imaging processes are carried out at a pixel resolution specified by the detector unit (440), wherein the individual imaging processes differ from one another with respect to the position of at least one polarization optical element located in the imaging optical unit (430); - wherein the image data recorded by the detector unit are evaluated in an evaluation unit (450), wherein polarization-dependent effects due to the polarization dependence of the interference of electromagnetic radiation occurring in the wafer plane during operation of the microlithographic projection exposure apparatus are simulated during the evaluation; - wherein the evaluation comprises a conversion of the image data obtained in the individual imaging processes, which is in each case based on at least one calibration image obtained by imaging a structure-free region of the mask (421) onto the detector unit (440); - wherein the calibration image respectively used during the conversion is selected differently depending on the position of the at least one polarization optical element during the relevant individual imaging process.

2. The method according to claim 1, characterized in that During this conversion, a calibration image is used in each case which is obtained for the correspondingly identical position of the at least one polarization optical element in the imaging optical unit (430).

3. The method according to claim 1 or 2, characterized in that Prior to this conversion, at least one of the calibration images is subjected to a preprocessing, during which the brightness of the calibration images is matched to one another.

4. The method according to claim 3, characterized in that If the illumination setting set in the illumination optical unit (410) is a non-polarized illumination setting, this pre-processing is omitted.

5. The method according to claim 1 or 2, characterized in that In the event that the average intensity in the calibration image obtained for a given position of the at least one polarization optical element falls below a specified threshold, the conversion of the image data obtained during the individual imaging process carried out at this position of the at least one polarization optical element is instead carried out based on the calibration image, which was recorded without the at least one polarization optical element being present in the imaging optical unit (430).

6. The method according to claim 1 or 2, characterized in that Prior to the evaluation, the image data recorded by the detector unit (440) during the individual imaging process are subjected to low-pass filtering.

7. The method according to claim 1 or 2, characterized in that During the evaluation of the image data recorded by the detector unit (440), changes in the imaging scale of the imaging optical unit (430) that are dependent on the position of the at least one polarization optical element during the respective individual imaging process and image shifts that are dependent on the position of the at least one polarization optical element during the respective individual imaging process are at least partially corrected.

8. The method according to claim 7, characterized in that The correction is achieved by displacing the center of the stretching of the central stretching applied to the image recorded by the detector unit (440) relative to the center of the camera field of the detector unit (440) by a value that depends on the position of the at least one polarization optical element.

9. A method for characterizing a microlithography mask, - wherein the illumination optical unit (410) illuminates a structure of a mask intended for a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit (430) images the mask (421) onto a detector unit (440), wherein the detector unit (440) has a plurality of pixels; - wherein a plurality of individual imaging processes are carried out at a pixel resolution specified by the detector unit (440), wherein the individual imaging processes differ from one another with respect to the position of at least one polarization optical element located in the imaging optical unit (430); - wherein the image data recorded by the detector unit are evaluated in an evaluation unit (450), wherein polarization-dependent effects due to the polarization dependence of the interference of electromagnetic radiation occurring in the wafer plane during operation of the microlithographic projection exposure apparatus are simulated during the evaluation; - wherein the image data recorded by the detector unit (440) during the individual imaging process are subjected to low-pass filtering before evaluation; in, During the evaluation of the image data recorded by the detector unit (440), a change in the imaging scale of the imaging optical unit (430) which is dependent on the position of the at least one polarization optical element during the respective individual imaging process and an image shift which is dependent on the position of the at least one polarization optical element during the respective individual imaging process are at least partially corrected; as well as The correction is achieved by displacing the center of the stretching of the central stretching applied to the image recorded by the detector unit (440) by a value relative to the center of the camera field of the detector unit (440), which value depends on the position of the at least one polarization optical element.

10. The method according to claim 1 or 9, characterized in that For each pixel of the detector unit (440), a focus stack is recorded in the form of a plurality of individual image representations, which differ from one another with respect to the respective distances between the mask (421) and the imaging optical unit (430), wherein a correction of the relative focus position is performed pixel by pixel by individual fitting of the image data respectively obtained for each pixel when recording the focus stack.

11. A method for characterizing a microlithography mask, - wherein the illumination optical unit (410) illuminates a structure of a mask (421) intended for a lithographic process in a microlithographic projection exposure apparatus, and wherein the imaging optical unit (430) images the mask (421) onto a detector unit (440), wherein the detector unit (440) has a plurality of pixels; - wherein the image data recorded by the detector unit (440) are evaluated in an evaluation unit (450); wherein for each pixel of the detector unit (440), a focus stack is recorded in the form of a plurality of individual image representations, said plurality of individual image representations differing from one another with respect to the respective distances between the mask (421) and the imaging optical unit (430), wherein the correction of the relative focus position is performed pixel by pixel by performing an individual fitting of the image data respectively obtained for each pixel when recording the focus stack; - wherein during the evaluation of the image data recorded by the detector unit (440), changes in the imaging scale of the imaging optical unit (430) that are dependent on the position of the at least one polarization optical element during the respective individual imaging process and image shifts that are dependent on the position of the at least one polarization optical element during the respective individual imaging process are at least partially corrected; as well as - wherein the correction is achieved by means of a displacement of the center of the stretching of the central stretching applied to the image recorded by the detector unit (440) relative to the center of the camera field of the detector unit (440) by a value that depends on the position of the at least one polarization optical element.

12. The method according to claim 1, 9 or 11, characterized in that: The mask (421) is designed for an operating wavelength less than 250 nm.

13. The method according to claim 12, characterized in that The operating wavelength is less than 200 nm.

14. The method according to claim 12, characterized in that The operating wavelength is less than 15 nm.

15. An apparatus for characterizing a microlithography mask, comprising: an illumination optical unit (410) for illuminating a structure of a mask (421) intended for use in a lithographic process in a microlithographic projection exposure apparatus, a detector unit (440); an imaging optical unit (430) for imaging the mask (421) onto the detector unit (440); and an evaluation unit (450) for evaluating the data recorded by the detector unit (440); It is characterized by: The device is designed to carry out the method according to any of the preceding claims.

16. The device according to claim 15, characterized in that The mask (421) is designed for an operating wavelength less than 250 nm.

17. The device according to claim 16, characterized in that The operating wavelength is less than 200 nm.

18. The device according to claim 16, characterized in that The operating wavelength is less than 15 nm.

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