Method for operating a mask inspection device, EUV camera, mask inspection device and computer program product

Abstract

The invention relates to a method for operating a mask inspection device, in which EUV radiation is guided onto a photomask (17) and in which EUV radiation reflected off the photomask (17) is guided via a projection lens (22) onto an image sensor (24) of an EUV camera (23) such that the photomask (17) is imaged on the image sensor (24) The photomask (17) is moved in a scan direction (32) while the image sensor (24) is exposed. Image data that have different resolutions in the scan direction (32) and a cross-scan direction (33) are obtained using the image sensor (24). A dimensionally compressed image (34) of the photomask (17) is generated from the image data. The invention also relates to a mask inspection device, an EUV camera and a computer program product.

Description

Method for operating a mask inspection device, EUV camera, mask inspection device and computer program product

[0001] The invention relates to a method for operating a mask inspection device, an EUV camera, a mask inspection device and a computer program product.

[0002] Photomasks are used in microlithographic projection exposure apparatuses, which are used to produce integrated circuits with particularly small structures. The photomask illuminated by very short-wave extreme ultraviolet radiation (EUV radiation) is imaged onto a lithography object in order to transfer the mask structure to the lithography object.

[0003] To ensure a high quality of the image created on the lithography object, it is necessary for the photomask to be true to size and not adversely affected by contaminations. It is known practice to subject photomasks to an inspection, either prior to the operation in a microlithographic projection exposure apparatus or during a break in operation. To this end, what is known as an aerial image of a portion of the photomask is created, the photomask in the process being imaged not on a lithography object but on an image sensor of an EUV camera. Using the imaging onto the image sensor as a basis, it is possible to make an assessment as to whether the photomask is without defects and contaminations.

[0004] The image sensor generates significant amounts of data during the operation of a mask inspection device. For example, the amount of data may be of the order of several 10 GB / sec- ond, and so data processing is accompanied by a significant outlay .

[0005] The problem addressed by the invention is that of presenting a method for operating a mask inspection device , an EUV camera, a mask inspection device and a computer program product , by means of which the aforementioned disadvantages are reduced . The problem is solved by the features of the independent claims . Advantageous embodiments are speci fied in the dependent claims .

[0006] In the method according to the invention for operating a mask inspection device , EUV radiation is guided onto a photomask . EUV radiation reflected of f the photomask is guided via a proj ection lens onto an image sensor of an EUV camera such that the photomask is imaged on the image sensor . The photomask is moved in a scan direction while the image sensor is exposed . Image data that have di f ferent resolutions in the scan direction and a cross-scan direction are obtained using the image sensor . A dimensionally compressed image of the photomask is generated from the image data .

[0007] The following considerations underpin the invention . There are microlithographic proj ection exposure apparatuses in which the photomask is imaged onto the lithography obj ect in anamorphic fashion, i . e . in which the imaging scale in the scan direction does not correspond to the imaging scale in the cross-scan direction . A square structure of the photomask with sides aligned parallel to the scan direction and the crossscan direction is imaged onto the lithography obj ect as a rectangle in that case . The scan direction and the cross-scan direction span a plane that corresponds to the plane of the photomask . The cross-scan direction is at right angles to the scan direction .

[0008] In relation to mask inspection, this background indicates that there is no advantage in analysing a photomask with the same resolution in both the scan direction and the cross-scan direction . A high resolution in the direction in which the microlithographic proj ection exposure apparatus has the smaller imaging scale would open up the possibility of correcting errors in this dimension that are no longer correctable in the other dimension . However, this would not add value because the quality of the structure on the lithography obj ect overall is limited by the larger of the two imaging scales of the microlithographic proj ection exposure apparatus .

[0009] This invention proposes to take account of this di f ference between the two dimensions of a photomask already when recording image data of the photomask, by virtue of obtaining the image data with di f ferent resolutions in the cross-scan direction and the scan direction . This opens up the possibility of avoiding image data that supply no added value in view of the subsequent use of the photomask in a microlithographic proj ection exposure apparatus before much outlay has arisen due to the processing of image data obtained . The outlay arising during the further data processing is reduced because the amount of image data to be processed is less .

[0010] The anamorphic imaging scales of the microlithographic proj ection exposure apparatus may be aligned such that a circular structure of the photomask is imaged onto the lithography obj ect as an oval , wherein the oval has a greater extent in the cross-scan direction than in the scan direction . A consequence of this is that a circular error on the photomask has a greater ef fect in the cross-scan direction than in the scan direction . In this case , the concept of the invention can be used by virtue of the image data obtained using the image sensor having a higher resolution in the cross-scan direction than in the scan direction . The situation would be reversed should the anamorphic imaging scales of the microlithographic proj ection exposure apparatus have an inverse relationship .

[0011] An image sensor is exposed when EUV radiation is incident on the image sensor, and the image sensor generates charge carriers , which may be read in order to obtain image data, on the basis of the incident EUV radiation . Within the meaning of the invention, an image is generated from image data when the image data are provided in a form on the basis of which conclusions may be drawn about structures that are present on the photomask . The generation of an image does not assume that the image is created physically or represented in a form perceivable to humans . An image is dimensionally compressed i f a structure that has the same extent in the scan direction and in the cross-scan direction on the photomask has di f ferent extents in the scan direction and the cross-scan direction in the image .

[0012] The image sensor may comprise a multiplicity of pixels . The pixels may span a pixel array that is aligned in the scan direction and cross-scan direction . The image sensor may comprise pixel lines that extend in the scan direction . The cross-scan direction may be spanned by a plurality of parallel pixel lines . It is also possible for the pixel lines to form an angle with the scan direction that di f fers from 0 ° . The angle may be smaller than 5 ° , preferably smaller than 2 ° and further preferably smaller than 1 ° . The image sensor may be designed to be read line by line .

[0013] The method may be performed in such a way that charge carriers created by incident EUV radiation are displaced from pixel to pixel within a pixel line in the image sensor before the number of charge carriers is read . The displacement speed of the charge carriers may be matched to the scanning speed at which the photomask is moved in the scan direction . In particular, the displacement speed and the scanning speed may be matched to one another in such a way that the speed at whichthe image of the photomask moves over the image sensor corresponds to the displacement speed . This opens up the possibility of summing up the charge carriers , which were created by the imaging, over the duration of a scanning procedure , and this has an advantageous ef fect on the accuracy and the sig- nal-to-noise ratio of the data obtained . The image sensor may be designed as a CCD ( charge-coupled device ) sensor or as a CMOS ( complementary metal-oxide-semiconductor ) . In an embodiment , the image sensor is designed as a TDI ( time delay and integration) sensor .

[0014] The term scan direction is used uni formly for the direction in which the photomask is moved relative to the proj ection lens and the direction in which the imaging of the photomask moves relative to the image sensor as a result . This use of the term scan direction serves illustrative purposes and does not express that the movement direction of the photomask and the movement direction of the imaging must have a speci fic spatial arrangement relative to one another . In particular, it is not necessary for the image sensor to have a spatial alignment that is parallel to the photomask .

[0015] The maximum resolution of an image sensor depends on the density of the pixels on the surface of the image sensor . The resolution of the image sensor in the cross-scan direction can be increased by virtue of reducing the distance between adj acent pixel lines . The resolution of the image sensor in the scan direction can be increased by virtue of reducing the distance between the pixels within a pixel line . Since image sensors are generally designed such that adj acent pixels are immediately adj acent to one another, an improvement in the resolution is usually accompanied by a reduction in the pixels .

[0016] Image information can be obtained by virtue of the number of charge carriers in a pixel line portion being ascertained, wherein the length of the pixel line portion is greater than the distance between two adj acent pixel lines .The number of charge carriers represents the amount of EUV radiation that was incident on the corresponding pixel line portion during the exposure procedure . The image information may be supplied to a control unit that is able to store the image information in a form allowing the generation of an image of the imaged photomask . To this end, the image information may be stored, for example together with information about the pixel line from where the image information came and together with information about the time at which the image information was read out . Image data according to the invention correspond to a set of image information from which an image of the imaged photomask may be generated .

[0017] In an embodiment , the image sensor is equipped with pixels whose length is longer than their width . The length of the pixels may extend parallel to a pixel line of the image sensor . The length of the pixels may extend parallel to the scan direction . The width of a pixel extends transversely thereto . In particular, the width of the pixel may extend in the cross-scan direction . Since the number of charge carriers within a pixel is ascertained during the readout of the image sensor, the resolution in relation to a direction reduces with the length of the pixel in this direction . The length of a pixel may be greater than the width of said pixel by at least a factor of 1 . 3 , preferably by at least a factor of 1 . 5 and further preferably by at least a factor of 1 . 8 . The length of the pixel may be twice the si ze of the width of the pixel . These statements may apply to at least 80% , preferably at least 90% and further preferably 100% of the pixels of the image sensor .

[0018] The ratio between the length of the pixel and the width of the pixel may be matched to the ratio between the imaging scales of the anamorphic imaging of the microlithographic projection exposure apparatus. The two imaging scales px in the cross-scan direction, py in the scan direction may be (px, py) = (+ / -0.25, + / -0.125) , for example. An imaging scale p of 0.25 corresponds here to a reduction with a ratio 4:1, while an imaging scale p of 0.125 results in a reduction with a ratio of 8:1. A positive sign in the case of the imaging scale p means imaging without image inversion; a negative sign means imaging with image inversion. In the case of an imaging scale which is twice as large in the cross-scan direction as in the scan direction, the length of the pixel may be twice the width of the pixel .

[0019] The anamorphic imaging scales px, pyof the microlithographic projection exposure apparatus are reflected in the photomask. Structures that should have the same size in the scan direction and in the cross-scan direction on the lithography object are found on the photomask 17 in a dimensionally compressed form. A photomask for which this holds true is referred to as an anamorphic photomask. The method according to the invention can be performed with an anamorphic photomask. The image of the photomask on the image sensor may be non-ana- morphic. In other words, the image of the photomask on the image sensor may have the same imaging scale in both the scan direction and the cross-scan direction. The photomask may have an aspect ratio of between 1:1 and 1:3, preferably between 1:1 and 1:2 and particularly preferably of 1:1 or 1:2. The photomask may be of substantially rectangular design. The photomask may be preferably 5 to 7 inches (12.7 cm to 17.8 cm) in length and width, particularly preferably 6 inches (15.2 cm) in length and width. In an alternative to that, the photomask may be 5 to 7 inches (12.7 cm to 17.8 cm) in length and 10 to 14inches ( 25 . 4 cm to 35 . 6 cm) in width, preferably 6 inches ( 15 . 2 cm) in length and 12 inches ( 30 . 5 cm) in width .

[0020] A reduced image resolution in one direction may alternatively also be achieved by virtue of the charge carriers of adj acent pixels of a pixel line being added when reading out the image sensor . In particular, the number of charge carriers from two adj acent pixels may be added in each case . The charge carriers may be summed up before the number of charge carriers is processed further as image information . In the method according to the invention, the number of charge carriers may initially be read out individually per pixel and subsequently added . It is also possible for the number of charge carriers of the plurality of pixels to be directly read out j ointly . This procedure may be applied in the case of an image sensor whose pixels have corresponding length and width . In particular, the image sensor may have square pixels .

[0021] The number of charge carriers from one or more pixels obtained thus corresponds to a respective piece of image information . To generate an image , suitable image data may be derived from a multiplicity of such pieces of image information by virtue of the image information being stored such that the image information may be spatially assigned . In particular, a spatial assignment may be rendered possible by virtue of the image information being assigned to a pixel line and a time .

[0022] The EUV camera may comprise a first operating mode and a second operating mode , wherein the charge carriers of more than one pixel are summed up in the first operating mode in order to generate image information and wherein image information is ascertained on the basis of the number of the charge carriers of the individual pixels in the second operating mode . The charge carriers of adj acent pixels , in particularthe charge carriers of two adj acent pixels , may be summed up in the first operating mode .

[0023] The invention also relates to a mask inspection device comprising an EUV camera, a positioning device for a photomask and a proj ection lens in order to image the photomask onto an image sensor of the EUV camera . The positioning device is designed to move the photomask in a scan direction while the image sensor is exposed . The EUV camera is designed to obtain image data that have di f ferent resolutions in the scan direction and a cross-scan direction . The invention encompasses EUV cameras that comprise more than one image sensor .

[0024] The invention also relates to an EUV camera for such a mask inspection device . The EUV camera comprises an image sensor having a pixel array that is spanned by a multiplicity of a pixel lines . The EUV camera is designed to obtain image information for an image of a photomask by virtue of the number of charge carriers in a pixel line portion being ascertained, wherein the length of the pixel line portion is greater than the distance between two adj acent pixel lines .

[0025] The invention also relates to a computer program product or a set of computer program products comprising program parts which, when loaded into a computer or networked computers connected to a mask inspection device according to the invention, are designed to perform the method according to the invention .

[0026] In a further step, the image data obtained according to the invention may be compared with dimensional speci fications for structures on the imaged portion of the photomask . I f a di f ference between the dimensions of structures on the portion of the photomask, as represented in the image data, and the dimensional speci fications for structures on the portion ofthe photomask is determined to be equal to or greater than a predetermined quality threshold value , then structures on the portion of the photomask can be modi fied in order to create modi fied structures on the portion of the photomask . The modification of the structures on the portion of the photomask may comprise a treatment of the photomask using an ion beam . One or more of the steps mentioned in this paragraph may be repeated until a di f ference between the dimensions of the structures on the portion of the photomask, as represented in the image data, and the dimensional speci fications for structures on the portion of the photomask is smaller than the predetermined quality threshold value , or until a termination criterion is satis fied .

[0027] The disclosure encompasses developments of the method with features that are described in the context of the mask inspection device according to the invention or in the context of the EUV camera according to the invention . The invention encompasses developments of the mask inspection device and developments of the EUV camera that are described in the context of the method according to the invention .

[0028] The invention is described by way of example below on the basis of advantageous embodiments with reference to the accompanying drawings , in which :Fig . 1 : shows a schematic illustration of a mask inspection device according to the invention;Fig . 2 : shows a schematic illustration of a photomask;Fig . 3 : shows a perspective view of an EUV camera according to the invention;Fig . 4 : shows a schematic illustration of aspects of theEUV camera from Fig . 3 ;Fig. 5: shows a schematic illustration of a photomask and of an image according to the invention of the photomask;Fig. 6: shows a schematic illustration of steps of the method according to the invention;Figs 7, 8: show views according to Fig. 6 in alternative embodiments of the invention.

[0029] Microlithographic photomasks 17 can be examined using a mask inspection apparatus as shown in Fig. 1.

[0030] In general, microlithographic photomasks 17 are provided for use in a microlithographic projection exposure apparatus (not depicted) . In the microlithographic projection exposure apparatus, the photomask 17 is illuminated with extreme ultraviolet radiation (EUV radiation) at a wavelength of for example 13.5 nm in order to image a structure formed on the photomask 17 onto the surface of a lithographic object in the form of a wafer. The wafer is coated with a photoresist that reacts in response to the EUV radiation. The mask inspection device is used to examine whether the photomask meets the specifications and is free from contaminations.

[0031] According to Fig. 1, the photomask 17 is arranged in the mask inspection device such that an EUV beam path 15 emanating from an EUV radiation source 14 is guided via an illumination system 16 onto the photomask 17. The illumination system 16 is used to shape the EUV radiation to form a beam used to illuminate, with uniform brightness, an examination field 20 on the surface of the photomask 17. The examination field 20, which is small in comparison with the area of the photomask 17, is depicted in Fig. 2 in an illustration that is not true to scale. For example, the illuminated region 20 mayhave dimensions of 0.5 mm x 0.8 mm. The edge lengths of the photomask 17 may be between 100 mm and 200 mm, for example. A field stop used to delimit the illuminated region to the examination field 20 on the surface of the photomask 17 is arranged in the illumination system 16. Using an XY-positioning mechanism 26, it is possible to move the photomask in the XY- plane in order to bring different examination fields 20 into the region of the EUV beam path.

[0032] The EUV beam path 15 reflected off the photomask 17 continues through a projection lens 22 to an EUV camera 23, which is equipped with an image sensor 24. The projection lens is used to image the examination field 20 of the photomask 17 onto the image sensor 24 of the EUV camera 23. The EUV radiation source 14, the illumination system 15, the photomask 17, the projection lens 22 and the EUV camera 23 are arranged in a vacuum housing 40, in which a negative pressure prevails during the operation of the mask inspection device.

[0033] The EUV radiation source 14 is a plasma radiation source, in which the EUV radiation is emitted from a plasma at a wavelength of 13.5 nm. Tin is a medium that may be used to generate a plasma suitable for emitting such EUV radiation. A laser beam may be made to impinge on a droplet of the medium for the purpose of generating the plasma.

[0034] The illumination system 16 and the projection lens 22 may comprise mirrors off which the EUV radiation is reflected. The mirrors may be designed as EUV mirrors which have a particularly high reflectivity for EUV radiation. The optical surface of the EUV mirrors may be formed by a highly reflective coating. A multilayer coating may be involved, in particular a multilayer coating having alternating layers of molybdenum and silicon. Using such a coating, it is possible to reflect approximately 70% of the incident EUV radiation.

[0035] The proj ection lens 22 has a magni fication factor of more than 100 . In order to be able to record the entirety of the image generated by the examination field 20 of the photomask 17 , the area of the image sensor 24 is greater than the area of the examination field 20 in accordance with the magnification factor . For example , the image sensor 24 may have dimensions of the order of 100 mm to 200 mm . The image sensor 24 comprises a multiplicity of pixels 31 , which span a pixel array 50 in a scan direction 32 and in a cross-scan direction 33 .

[0036] According to Fig . 4 , the EUV camera 23 comprises a control unit 30 which communicates with the image sensor 24 . The control unit 30 controls the image sensor 24 , inter alia in order to determine the times at which the image sensor 24 is exposed in order to create an image recording . The amount of incident EUV radiation is then registered in each pixel 31 and converted into a corresponding number of free charge carriers . Image data may be obtained by reading out the number of charge carriers for the individual pixels 31 .

[0037] The image sensor 24 comprises a multiplicity of pixel lines 36 , with each pixel line 36 extending over the entire length of the image sensor 24 in the scan direction 32 . The mutually parallel pixel lines 36 span the width of the image sensor 24 in the cross-scan direction 33 .

[0038] The image sensor 24 is read out by virtue of the charge carriers created by a pixel 31 being displaced from pixel to pixel in the scan direction 32 within each pixel line 36 . With each displacement step, the charge carriers from the last pixel in a pixel line 36 ( far left in Fig . 4 ) are displaced to a readout cell 37 . The number of charge carriers is ascertained in the readout cell 37 . Information about the number ofcharge carriers in a readout cell 37 is trans ferred as image information to the control unit 30 .

[0039] The image sensor 24 may be configured such that the charge carriers from all pixel lines 36 are simultaneously displaced by one pixel 31 in a readout step, and so charge carriers may be trans ferred to an associated readout cell 37 for each pixel line 36 . Image data from which an image spanned in the scan direction 32 and in the cross-scan direction 33 can be generated may be obtained by a multiplicity of readout steps .

[0040] According to the invention, the EUV camera 23 is designed to obtain image data from the photomask 17 that have a lower resolution in the scan direction 32 than in the crossscan direction 33 . Fig . 5 shows a portion 20 of the photomask 17 and an image taken from the portion 20 . The image was generated from image data 34 that were obtained using the image sensor 24 . The image data 34 have not been reproduced true to scale ; in fact , an image generated from the image data 34 is generally substantially larger than the imaged portion 20 of the photomask 17 . It is evident from Fig . 5 that the aspect ratio in the image generated from the image data 34 does not correspond to the imaged portion 20 . Since the image resolution in the scan direction 32 is lower than in the cross-scan direction 33 , the portion 20 and structures 38 present in the portion 20 are dimensionally compressed in the scan direction 32 .

[0041] The image sensor 24 is designed as a TDI ( time delay and integration) sensor, as explained in detail on the basis of Fig . 6 . Figure 6 (A) depicts a portion of a pixel line 36 adj acent to the readout cell 37 . A number of charge carriers that correspond to a structure 38 on the photomask 17 have accumulated in the two pixels 31 far right . The structure 38 isindicated schematically by a combination of a circle and a cross . During an image recording, the photomask 17 is displaced in a scan direction 32 parallel to the X-direction using the XY-positioning mechanism 26 . A consequence of the photomask 17 being displaced relative to the proj ection lens 22 is that the image of the photomask 17 that is created on the image sensor 24 is displaced relative to said image sensor 24 . The direction in which the image is displaced on the image sensor 24 when the photo mask 17 is moved in the scan direction 32 is also referred to as scan direction 32 even i f the two directions are not necessarily parallel to one another in space . In the cross-scan direction 33 , which is at right angles to the scan direction 32 in the plane of the image sensor 24 , the position of the image of the photomask 17 remains unchanged relative to the image sensor 24 .

[0042] The control unit 30 controls the image sensor 24 such that the speed at which the charge carriers are displaced from pixel to pixel corresponds to the speed at which the image of the photomask 17 moves relative to the image sensor 24 . Fig . 6 shows , in a plurality of steps (A) to ( G) , the displacement of the charge carriers from pixel to pixel . The exposure procedure continues during this time interval , and so the number of charge carriers becomes ever larger ; this is indicated by an ever-thickening line for the circle and the cross . The first trans fer into the readout cell 37 of charge carriers belonging to the structure 38 is implemented in step ( F) . The charge carriers are collected there for one step, and so the charge carriers from the next pixel 31 are additionally trans ferred into the readout cell 37 in step ( G) . Only the sum of the charge carriers of the two pixels 31 is transmitted to the control unit 30 . Hence image information is obtained by virtue of the number of charge carriers in a pixel line portion 49 being ascertained, the length of said pixel line portion being greater than the distance between two adj acent pixel lines 36 .

[0043] The control unit 30 obtains image data 34 on the basis of the transmitted number of charge carriers , by virtue of the number of charge carriers being stored together with further information . This further information includes a statement regarding the pixel line 36 in which the charge carriers were generated, and a statement regarding the time at which the readout cell 37 was read out . The image data 34 are stored in a memory module 35 of the control unit 30 . The amount of image data 34 has been halved in comparison with a conventional method in which image information is stored on an individual basis for each pixel .

[0044] Fig . 7 depicts a variant in which the charge carriers trans ferred into the readout cell 37 are read out in each step (A) to ( G) , and information about the number of charge carriers is transmitted to a digital module 39 . The number of charge carriers ascertained in two successive readout procedures are added in the digital module 39 . Only the ascertained sum is trans ferred to the control unit 30 . In this way, the amount of image data 34 to be processed can be reduced to the same extent as in the method in which the charge carriers from two pixels 31 are collected directly in the readout cell 37 .

[0045] The control unit 30 may be designed to switch the EUV camera 23 between a first operating mode and a second operating mode . In the first operating mode , a sum over two pixels 31 is formed in the digital module 39 in each case , as described, before data is trans ferred to the control unit 30 . The digital module 39 may be deactivated in the second operating mode , and so each number of charge carriers ascertained by a readout cell 37 is transmitted to the control unit 30 . This opens up the possibility, where necessary, of creating image data 34 whose resolution in the scan direction 32 equals the resolution in the cross-scan direction 33 . With the second operating mode , it is accepted that the amount of data to beprocessed has doubled in comparison with the first operating mode .

[0046] In the embodiment according to Fig. 8, the pixels 31 are not square but rectangular. The longer side of the rectangle is aligned parallel to the scan direction 32, and the shorter side of the rectangle parallel to the cross-scan direction 33. In the exemplary embodiment shown, the length 40 of a pixel 31 is twice the width 41 of the pixel.

[0047] Other aspect ratios between the length 40 and the width 41 of a pixel 31 are possible. In particular, the aspect ratio 31 of the pixels may be matched to the anamorphic imaging scales with which the photomask 17 is imaged onto a lithography object in an associated microlithographic projection exposure apparatus .

[0048] The projection exposure apparatus may have an imaging scale Py in the scan direction, which differs from the imaging scale pxin the cross-scan direction. In an embodiment, two imaging scales px, pyof the projection system 20 are (px, py) = (+ / -0.25, + / -0.125) . An imaging scale p of 0.25 corresponds here to a reduction with a ratio 4:1, while an imaging scale p of 0.125 results in a reduction with a ratio of 8:1. A positive sign in the case of the imaging scale p means imaging without image inversion; a negative sign means imaging with image inversion.

[0049] In the aforementioned example, the imaging scale in the cross-scan direction is twice the imaging scale in the scan direction. A pixel 31 of the image sensor 24 that is matched to the anamorphic imaging scales of the microlithographic projection exposure apparatus has a length 40 that is twice its width 41. In the case of a different ratio of the anamorphicimaging scales px, py, the ratio of length 40 to with 41 of a pixel 31 matched thereto is changed accordingly .

[0050] The anamorphic imaging scales px, pyof the microlitho- graphic proj ection exposure apparatus are also reflected in the photomask 17 . Structures that should have the same si ze in the scan direction and in the cross-scan direction on the lithography obj ect are found on the photomask 17 in a dimension- ally compressed form . A photomask 17 for which this holds true is referred to as an anamorphic photomask 17 . The method according to the invention can be performed with an anamorphic photomask 17 .

[0051] On account of the rectangular shape of the pixels 31 , the structure 38 of the photomask 17 , which was imaged on two pixels 31 in the preceding exemplary embodiments , fits on a single pixel 31 in Fig . 8 . The number of charge carriers belonging to this structure 38 is trans ferred collectively into the readout cell 37 in step (E ) . The amount of image data 34 to be processed is reduced by hal f in comparison with a method in which the structure 38 is imaged on two square pixels and the image data obtained by the pixels are stored individually .

Claims

Claims1. Method for operating a mask inspection device, in which EUV radiation is guided onto a photomask (17) and in which EUV radiation reflected off the photomask (17) is guided via a projection lens (22) onto an image sensor (24) of an EUV camera (23) such that the photomask (17) is imaged on the image sensor (24) , wherein the photomask (17) is moved in a scan direction (32) while the image sensor (24) is exposed, wherein image data that have different resolutions in the scan direction (32) and a cross-scan direction (33) are obtained using the image sensor (24) , and wherein a dimensionally compressed image (34) of the photomask (17) is generated from the image data.

2. Method according to Claim 1, wherein the image data obtained using the image sensor (24) have a higher resolution in the cross-scan direction (33) than in the scan direction(32) .

3. Method according to Claim 1 or 2, wherein charge carriers created by incident EUV radiation are displaced from pixel (31) to pixel (31) within a pixel line (36) in the image sensor (24) before the number of charge carriers is read.

4. Method according to Claim 3, wherein the pixel lines (36) of the image sensor (24) are aligned in the scan direction (32) .

5. Method according to Claim 3 or 4, wherein the displacement speed at which the charge carriers are displaced within the pixel line (36) is matched to the scanning speed at which the photomask (17) is moved in the scan direction (32) .

6. Method according to any of Claims 3 to 5, wherein image information is obtained by virtue of the number of chargecarriers in a pixel line portion (49) being ascertained, wherein the length of the pixel line portion (49) is greater than the distance between two adjacent pixel lines (36) .

7. Method according to any of Claims 1 to 6, wherein the image sensor (24) comprises a multiplicity of pixels (31) and wherein the length (40) of the pixels (31) is greater than the width (41) of the pixels (31) .

8. Method according to Claim 7, wherein the length (40) of the pixels (31) extends parallel to a pixel line (36) of the image sensor (24) .

9. Method according to Claim 7 or 8, wherein the length (40) of the pixels (31) is twice as long as the width (41) of the pixels ( 31 ) .

10. Method according to any of Claims 1 to 9, wherein the charge carriers of adjacent pixels (31) in a pixel line (36) are added when reading out the image sensor (24) .

11. Method according to any of Claims 1 to 9, wherein image information is generated by virtue of the charge carriers of more than one pixel (31) of the image sensor (24) being summed up in a first operating mode of the EUV camera (23) and wherein image information is ascertained on the basis of the number of the charge carriers of individual pixels (31) in a second operating mode.

12. Method according to any of Claims 1 to 11, wherein an ana- morphic photomask (17) is imaged onto the image sensor13. Method according to Claim 12, wherein the anamorphic photomask (17) is imaged onto the image sensor (24) in non-ana- morphic fashion.

14. Mask inspection device, comprising an EUV camera (23) , a positioning device (26) for a photomask and a projection lens (22) in order to image the photomask (17) onto an image sensor (24) of the EUV camera (23) , wherein the positioning device (26) is designed to move the photomask (17) in a scan direction (32) while the image sensor (24) is exposed, wherein the EUV camera (23) is designed to obtain image data that have different resolutions in the scan direction (32) and a cross-scan direction (33) .

15. Mask inspection device according to Claim 14, wherein the image data obtained using the image sensor (24) have a higher resolution in the cross-scan direction (33) than in the scan direction (32) .

16. Mask inspection device according to Claim 14 or 15, wherein the pixel lines (36) of the image sensor (24) are aligned in the scan direction (32) .

17. Mask inspection device according to any of Claims 14 to 16, wherein image information is obtained by virtue of the number of charge carriers in a pixel line portion (49) being ascertained, wherein the length of the pixel line portion (49) is greater than the distance between two adjacent pixel lines (36) .

18. Mask inspection device according to any of Claims 14 to 17, wherein the image sensor (24) comprises a multiplicity of pixels (31) and wherein the length (40) of the pixels (31) is greater than the width (41) of the pixels (31) .

19. Mask inspection device according to Claim 18, wherein the length (40) of the pixels (31) extends parallel to a pixel line (36) of the image sensor (24) .

20. Mask inspection device according to Claim 18 or 19, wherein the length (40) of the pixels (31) is twice as long as the width (41) of the pixels (31) .

21. Mask inspection device according to any of Claims 14 to 20, wherein the EUV camera (23) is designed to obtain image information by virtue of the charge carriers of adjacent pixels (31) in a pixel line (36) being added when reading out the image sensor (24) .

22. Mask inspection device according to any of Claims 14 to 20, wherein the EUV camera (23) has a first operating mode and a second operating mode, wherein image information is generated by virtue of the charge carriers of more than one pixel (31) of the image sensor (24) being summed up in the first operating mode and wherein image information is ascertained on the basis of the number of the charge carriers of individual pixels (31) in the second operating mode.

23. Mask inspection device according to any of Claims 14 to 22, further comprising a photomask (17) , wherein the photomask (17) is an anamorphic photomask (17) .

24. Mask inspection device according to Claim 23, wherein the projection lens (22) is designed to image the anamorphic photomask (17) onto the image sensor (24) in non-anamorphic fashion .

25. EUV camera for a mask inspection device according to any of Claims 14 to 24, comprising an image sensor (24) having a pixel array that is spanned by a multiplicity of pixellines (36) , wherein the EUV camera (23) is designed to obtain image information for an image of a photomask by virtue of the number of charge carriers in a pixel line portion (49) being ascertained, wherein the length of the pixel line portion (49) is greater than the distance between two adjacent pixel lines (36) .

26. Computer program product or set of computer program products, comprising program parts which, when loaded into a computer or networked computers connected to a mask inspec- tion device according to any of Claims 14 to 24, are designed to perform the method according to any of Claims 1 to 13.

Images