Device for capturing images of structures on photolithographic masks

Abstract

The invention relates to a device (1) for capturing images of structures on microlithographic masks, in particular structures on microlithographic masks In the case of the device (1), a region of a photolithographic mask (20) arranged on an object stage (30) is in each case captured with a time offset by a plurality of image capturing sensors (42) of an image capturing unit (40) during a scanning operation, wherein an adjustment to the distance between the photolithographic mask (20) and the image capturing unit (40) during the scanning operation ensures that the captured images of the same region on the photolithographic mask (20) are focused differently. The variables that are characteristic for the defocus arising from the images are then used to ascertain whether a refocusing of the device (1) is necessary, in particular whether the distance between the image capturing unit (40) and the object stage (30) must be adapted beyond the variation required for the capture of differently focused images.

Description

CZVS112PW024.10.2025 / BRDevice for capturing images of structures on photolithographic masks

[0001] The invention relates to a device for capturing images of structures on photolithographic masks.

[0002] Photolithography is used for producing microstructured components, such as for example integrated circuits. The photolithography process is carried out in what is known as a projection exposure apparatus, which comprises an illumination device and a projection device. The image of a mask (also called "reticle") illuminated by means of the illumination device is projected in this case by means of the projection device onto a substrate, for example a silicon wafer, that is coated with a light-sensitive layer (so-called "photoresist") and arranged in the image plane of the projection device in order to transfer the mask structure to the light-sensitive coating of the substrate. In subsequent production steps, the transferred structure is implemented in the substrate, e.g. by etching .

[0003] Even if the projection devices of projection exposure apparatuses have a reduction factor of e.g. 4:1, the structures of the masks already need to have a high accuracy owing to the advancing miniaturization in the semiconductor field and the transition in the wavelength during exposure from DUV (e.g. 193 nm) to the EUV (e.g. 13.5 nm) . In order to ensure that a mask satisfies these quality requirements and a microstructured component produced thereby also has the desired properties and manner of functioning, a mask is checked by means of suitable methods in inspection and / or metrology apparatuses before use in a projection exposure apparatus.

[0004] In the case of known inspection and / or metrology apparatuses, the object to be checked - i.e. for example a maskfor producing microstructured components - is illuminated by an illumination source in such a way that either the radiation partially reflected back of f the obj ect or the partially transmitted radiation is incident on a sensor, the sensor data of which can then be suitably evaluated .

[0005] The sensor can be e . g . an imaging sensor, from the sensor data of which ( in general ) two-dimensional image representations of a speci fic measurement variable , namely of the radiation intensity, can be derived . In this case , an imaging sensor itsel f can be configured in an areal fashion and capture all the measurement variables for a speci fic image representation all at once ; however, it is also possible that an imaging sensor is configured only in a point-type or linear fashion, namely as a point or line sensor, and thus can only determine portions of the measurement variables required for a desired image representation all at once , while the further measurement variables required for the image representation have to be captured in further steps in each case after suitable realignment of the imaging sensor and / or of the obj ect in order to obtain the desired image representation of the measurement variables as a result . In this case , the described realignment of the imaging sensor and / or of the obj ect can also be reali zed as a continuous relative movement of the two components with respect to one another, wherein the sensor then captures practically continuous data which can be combined to form a two-dimensional image representation . A corresponding procedure is also referred to as " scanning" .

[0006] On the basis of image representations of obj ects obtained in this way, it is then possible - depending on the measurement variable captured therefor or represented therein or depending on the variables derived from measurement variables - to carry out various checks . In the case of masks for semiconductor fabrication, an image representationconstituting an image-based reflection of the actual structure on the surface of the mask can be compared e . g . with an image representation of a target structure in order thereby to be able to recogni ze possible defects in the mask . I f the resulting image representation is free of distortion to the greatest possible extent and / or the imaging scale is known, the image representation can be taken as a basis also for performing measurements of variables of the structure of the mask in order to compare same with target values or in order to compare di f ferent measured variables among one another .

[0007] Particularly when capturing images of structures on photolithographic masks , the requisite components have to be arranged highly accurately with respect to one another in order to obtain a high sharpness in the captured measurement variables . In other words , it ought to be ensured that the measurement variable obtained for a speci fic point on the obj ect reflects the measured variable at precisely this point as far as possible exclusively and is not influenced somewhat by the corresponding variables of the surrounding points . An image representation of the actual surface of the obj ect that is as exact as possible can be obtained as a result .

[0008] Particularly i f the sensor does not capture all at once the entire area of a photolithographic mask that is to be checked, rather portion-by-portion or continuous capture takes place , refocusing of the capture device is regularly required even during the actual capture process , in order to ensure that the resulting image representation is as sharp as possible over the entire imaged region . This is because i f structures of photolithographic masks are intended to be imaged, changes in the arrangement of obj ect and sensor, in particular, may already result in defocusing, which is detrimental to the sharpness of the image representation . Corresponding changes in the arrangement may occur e . g . on account ofthermal expansions of the components or the structure carrying the components . Moreover, bending of the obj ect , which may occur on account of its mounting, may require refocusing during a capture process .

[0009] The problem addressed by the present invention is that of providing a device for capturing images of structures on photolithographic masks which enables refocusing during a scanning process .

[0010] This problem is solved by a device according to the main claim . The dependent claims relate to advantageous developments .

[0011] Accordingly, the invention relates to a device for capturing images of structures on photolithographic masks , comprising an illumination device for illuminating an extensive obj ect arranged on an obj ect stage with illumination radiation, an image capturing unit that uses at least two image capturing sensors arranged in a j oint plane to perform scanning capture of illumination radiation trans formed by means of the photolithographic mask, and a control device for controlling a relative movement between the obj ect stage and image capturing unit in a scanning direction, wherein the device can be focused by adj usting the distance between image capturing unit and obj ect stage , wherein the control device is designed to adj ust the distance between the image capturing unit and obj ect stage during the relative movement between the obj ect stage and image capturing unit in the scanning direction starting from an initial distance , in such a way that images of the same region of the obj ect captured by at least two di fferent image capturing sensors of the image capturing unit are successively focused di f ferently and a respective variable that is characteristic for the defocus is ascertained for the images by di f ferent image capturing sensors of the same regionof the object, and to adapt the initial distance between image capturing unit and object stage in a direction that is derivable from the characteristic variables should a non-optimal initial distance be established on the basis of the ascertained variables that are characteristic for the defocus.

[0012] Firstly, some terms used in the context of the invention are explained:

[0013] Radiation is "transformed" within the meaning of the invention if it is at least partially reflected, absorbed and / or transmitted by an object. In the case of photolithographic masks, depending on the configuration as a reflective or transmissive mask, in which the respective structures are represented by absorbent regions, in general reflection and absorption or transmission and absorption occur jointly.

[0014] A device "can be focused" if the image capture plane, onto which an object is actually imaged and which for example is formed by an image capturing sensor, can be brought into correspondence with the image plane, in which the object is imaged with optimum sharpness and which for example arises from the known lens equation, in the optical representation obtained as a result of modifications to the device. Apart from the adjustment of the distance between the lens and image capture plane, for example as known from photography, focusing may also be brought about by adjusting the distance between object and rigid lens-image capture plane system. The distance to be adjusted may also be an optical path length.

[0015] A device or an image captured by this device is "optimally focused" if the image capture plane of the image capturing sensor in said device is located in the image plane of the device. The captured image then is "sharp". By contrast, a device or an image captured by this device is "defocused" if theimage capture plane of the image capturing sensor is spaced apart from the image plane of the device.

[0016] In this context, the distance of the image capture plane from the image plane is referred to as "defocus" and may be expressed numerically as a signed distance. From the sign of the defocus, it is possible to read off the direction in which the device must be adjusted so that it is "optimally focused" again. In the case of devices having an image capturing sensor that can be displaced vis-a-vis the lens, the image capturing sensor that forms the image capturing plane may be moved into the image plane of the lens; in the case of devices having a rigid arrangement of the lens and image capturing sensor, whereby the distance between the lens and image capturing plane is fixed, the distance between the lens and image plane may be adapted in such a way by adjusting the distance between the lens and object that the image plane and image capturing plane coincide. Should a device be optimally focused, the image plane is located in the image capturing plane, whereby the defocus is zero.

[0017] Two images are "differently focused "if they each have a different focus, wherein one of the two defocuses may also be zero.

[0018] For a device for capturing images of structures on photolithography masks, in which the image is captured using the scanning method and at least individual regions of the object to be captured are captured by different image capturing sensors at different times during the scanning process, the invention has recognized that adjusting the distance between the image capturing unit and the object during the scanning process allows differently focused images of the same region of the object to be captured.

[0019] If at least two differently focused images of one region of the object are available, variables that are characteristic for the defocus of the images, which may be ascertained from the images themselves, allow an estimation as to the location of the distance between the image capturing unit and object stage or an object arranged thereon for optimal focusing or a defocus of zero, relative to the respective distances at which the respective images were recorded. Should the reference or initial distance used as a starting point for the described variation in the distance between the image capturing unit and the object stage or object for the purpose of capturing differently focused images be established as being non-optimal as a result thereof, said distance may be adapted accordingly, wherein it is possible in principle to also use the characteristic variables ascertained for the individual images to derive the direction in which the initial distance should be adjusted in order to attain the desired optimum, i.e. whether said initial distance should be reduced or increased .

[0020] The direction in which the initial distance should be adapted can be ascertained particularly easily if more than two differently focused images of the same region of the object, for which respective variables that are characteristic for the defocus of the images can be ascertained, are available. Moreover, it is regularly also possible to estimate at least an approximate order of magnitude of the required adaptation of the initial distance.

[0021] For example, the desired optimum for the initial distance may be achieved at the distance at which a defocus of zero is present. However, a variable offset from this distance, said offset being predetermined or dependent on other parameters, may also be specified as an optimum. In this case,the initial distance may be adapted during the scanning process , whereby the desired refocusing may be achieved .

[0022] So that at least individual regions of the obj ect to be captured are captured by di f ferent image capturing sensors at di f ferent times during the scanning process , provision is made for the image capturing unit to comprise at least two image capturing sensors , the respective image capturing regions of which are preferably arranged at a distance from one another in the direction of the image representation of the scanning direction . Further preferably, the image capturing regions should at the same time at least partially overlap in a proj ection onto a plane perpendicular to the direction of the image representation of the scanning direction . In this case , "direction of the image representation of the scanning direction" means the direction in which the image representation of the photolithographic mask moves in the image capturing unit when the obj ect stage and image capturing unit are moved relative to each other in the scanning direction . Such an arrangement ensures that , during the scanning process , at least one portion of the region of the obj ect captured by a first image capturing sensor at a first time is captured by another image capturing sensor at a second time on account of the relative movement between the obj ect stage and image capturing unit . Should more than two image capturing sensors be provided, individual regions of the obj ect may also be captured by multiple image capturing sensors , in particular by three or four image capturing sensors , during the scanning process .

[0023] Such an arrangement of further image capturing sensors in a direction perpendicular to the direction of the image representation of the scanning direction may increase the width of the region that can be captured by the image capturing unit during a movement of the obj ect in the scanning direction . In this case , multiple image capturing sensors , theimage capturing regions of which are spaced apart from one another in the direction of the image representation of the scanning direction, may also be arranged over the entire width in each case . Not only does this increase the number of the di f ferently focused images that are usable for the adaptation of the initial distance or the overall si ze of the region on the surface of the obj ect captured multiple times , but this also provides more image data for the preferred fusion of the images captured by the image capturing sensors to form a single image . Even i f at least some of the images have a defocus , albeit regularly only a small defocus , even higher contrast images may be obtained by fusion, even in the event of a low radiation intensity of the trans formed illumination radiation incident on the image capturing sensors . The controller may be designed to combine the images of the images captured by the various image capturing sensors in a single image .

[0024] In order to obtain di f ferently focused images , the invention provides for the distance between the image capturing unit and the obj ect stage or the obj ect arranged thereon to be altered during the scanning process . In this case , it is particularly preferable for the control device to be embodied to adj ust the distance between the image capturing unit and the obj ect stage around the initial distance in regularly repeating fashion, starting from the initial distance . In other words , the distance should vary around an initial value , for example the initial distance , in temporally repeating fashion, wherein the variation is preferably periodic, undamped and / or continuously di f ferentiable . In particular and particularly preferably, the regular repetition may lead to a sinusoidal temporal adj ustment of the distance around the initial distance . Abrupt accelerations of the image capturing unit and / or of the obj ect stage , which should be moved to adj ust the distance in question, are avoided by a corresponding sinusoidal change in distance .

[0025] In this context , it is preferable for the period of the regularly repeating adj ustment of the distance between the image capturing unit and the obj ect stage to be matched to a constant relative movement between the obj ect stage and the image capturing unit in the scanning direction and the arrangement of the image capturing regions of the individual image sensors , in such a way that the distance in at least two , preferably at least three images of the same region of the obj ect captured by di f ferent image capturing sensors is di f ferent . Adapting the period of the regularly repeating adj ustment to the distance between the image capturing unit and the obj ect stage accordingly ensures that at least two or even at least three di f ferently focused images are always available for each region during the scanning process , from at least two or at least three image capturing sensors as a matter of principle .

[0026] To ensure that the check as to whether a non-optimal initial distance is present is possible , the variables that are characteristic for the defocus should di f fer from one another for di f ferently focused images as a matter of principle . At the same time , the defocus in the individual images should be as small as possible in each case so that images that image the same region of the obj ect may also be combined to form a single image with as little loss of quality as possible . The required variation in the distance between the image capturing sensor and obj ect may be chosen and defined - in particular in a manner dependent on the surface structure to be captured, which regularly has an ef fect on the characteristic variable or the change thereof in the case of a defocus . In the case of a regularly repeating adj ustment to the distance between the image capturing unit and the obj ect stage or obj ect , it is preferable for the control device to be embodied such that the amplitude of the regularly repeating adj ustment to the distance between the image capturing unit and the obj ect stage isadaptable so that the variables that are characteristic for the focusing are different for at least two, preferably at least three images of the same region of the object that are captured by different image capturing sensors. The amplitude may be suitably chosen in ob ect-dependent fashion, i.e. depending on the structure on a photolithographic mask in particular, in order to obtain a defocus that is as small as possible but at the same time is sufficient for the focusing according to the invention. For example, an amplitude of ±10 nm to ±100 nm may be chosen for the inspection of masks for the EUV range, while a range of ±20 nm to ±300 nm is chosen for masks for the DUV range.

[0027] It is also possible that the amplitude is adjusted during the scanning process in order to fulfil the specified boundary conditions - specifically the capturable large change in the variable that is characteristic for the focus and smallest possible adjustment to the distance - even in the event of different surface structures. The control unit may be embodied to ascertain and perform an optionally required adaptation to the amplitude on the basis of the ascertained variables that are characteristic for the defocus in images of the same region of the object.

[0028] To adjust the distance between the image capturing unit and the object stage or an object arranged thereon, it is preferable for the object stage to be movable in a direction perpendicular to the scanning direction - and hence regularly also perpendicular to the object or to the photolithographic mask. In principle, the image capturing unit may then have a stationary arrangement; this is advantageous in view of the vibrating masses, especially in the event of a regularly repeating adjustment of the aforementioned distance. In this case, the distance between the image capturing unit and object can be adapted in the course of the automatic refocusing byway of the control device preferably in a precise manner so that , by means of the adaptation, it is already possible to compensate for defocusing established by way of the variable that is characteristic for the defocus , but in particular no renewed defocusing occurs as a result of "overshooting" on account of excessively low precision in the adaptation of the distance between the image capturing unit and obj ect stage or ob ect .

[0029] The device preferably comprises at least three , preferably four image capturing sensors that are arranged perpendicular to the direction of the image representation of the scanning direction and at least partially overlap in the direction of the image representation of the scanning direction - more precisely, in a proj ection onto a plane perpendicular to the direction of the imaging of the scanning direction . A corresponding number of image capturing sensors allows at least three di f ferently focused images of the same region on the obj ect to be captured . Should four image capturing sensors that are arranged perpendicular to the direction of the image representation of the scanning direction and at least partially overlap in the direction of the image representation of the scanning direction be provided, an of fset arrangement of the image capturing sensors and the provision of multiple image capturing sensors in the direction perpendicular to the direction of the image representation of the scanning device ensures that even in regions in which two image capturing sensors that are adj acent in the direction perpendicular to the direction of the image representation of the scanning direction collide and hence do not allow direct image capture , three other image capturing sensors that are adj acent in the direction of the image representation of the scanning direction are available for capturing three di f ferently focused im- ages .

[0030] A variable derived from the contrast of the captured image may be used as variable characteri zing the defocus of an image . This may also directly be the contrast of the captured image or its reciprocal . For the contrast , the assumption may be made that it regularly has only a single maximum ( a minimum for the reciprocal of the contrast ) in a certain region around the optimal focus . For example , the root mean square of the absolute value of the gradient over all or some of the image points of a captured image or the standard deviation vis-a-vis the mean value (both ascertained over all or some of the image points of a captured image ) may be used as a value for the contrast . This variable also has only a single optimum, at which optimal focusing is attained, in a certain region around the optimal focus . The procedure described below by way of example and, for reasons of clarity, exclusively for the contrast or the reciprocal thereof as a representative variable for the focusing can also be applied directly to this variable , and also to alternative variables reflecting the contrast .

[0031] I f a relationship between a characteri zing variable and the defocus has more than one optimum, the procedure described below can nevertheless be carried out , in principle , given suf ficiently accurate knowledge of the relationship between the characteristic variable and the defocus . However, this may then optionally necessitate more than two or three di f ferently focused images of the same region in order to be able to determine the direction in which a possibly required adaptation of the initial distance has to take place with suf ficient certainty .

[0032] Should the necessity for an adj ustment of the initial distance be established on the basis of two di f ferently focused images , for example because the two characteristic variables ascertained for these images deviate beyond apredetermined measure from the characteristic variables ascertained in advance during the scanning process or because the di f ference between the two characteristic variables exceeds a predetermined measure , the initial distance may be adj usted in the direction of that distance in which the image with the greater deviation of the characteristic variable from a value ascertained in advance or in which the image with the larger characteristic variable was recorded . For example , the adj ustment of the initial distance may be continued until practically identical values that are characteristic for the defocus are ascertained for two di f ferently focused captured images or until the sum of the squares of the reciprocals of corresponding values is minimal .

[0033] I f three di f ferently focused images of the same region are available , the necessity of adj usting the initial distance may be established in comparable fashion . Depending on the chosen variable that is characteristic for the defocus , an optimal initial distance may be present i f the characteristic variable of the image at the mean distance of the three distances , at which the three images were captured, is greater than ( or less than) the characteristic variables of the two other images .

[0034] Independently of how many di f ferent focused images of a region are available , it is preferable for the initial distance to be adj usted until the sum of the squares of the characteristic variables for the images of the same region of the obj ect from di f ferent image capturing sensors is optimal . Whether a minimum or a maximum of the sum of the squares represents the optimum in this respect depends on the characteristic variable that is used to form the sum of squares .

[0035] The very act of ascertaining - as described by way of example above - the direction in which focusing has to takeplace makes it possible to refocus a device for capturing images of structures on photolithographic masks even during the actual image capture . Speci fically, in the case of the device according to the invention, upon the need for adapting the initial distance between the image capturing unit and the obj ect stage or a photolithographic mask arranged thereon being established, by virtue of the fact that the direction in which this adj ustment has to take place is known directly, an improvement of the focusing is achieved directly upon this speci fication being complied with; by contrast , more extensive defocusing and thus a deterioration in e . g . the sharpness of the image capture are precluded . The device according to the invention furthermore makes it possible to monitor the instantaneous focusing during the refocusing and to establish when the optimal distance between the image capturing unit and the obj ect stage or photolithographic mask was attained in the course of refocusing . In this case , it is irrelevant that the scanning process is continued during the refocusing, i . e . the repeated ascertainment and checking of the variables that are characteristic for the defocusing is not always implemented for the same region of the photolithographic mask as a matter of principle but instead implemented on the basis of the respective current regions located in the image capturing region of the individual image capturing sensors .

[0036] The image capturing sensors can then be line sensors , i . e . sensors that each capture only a single line of an image , and a two-dimensional image arises as a result of step-by- step, line-by-line capture . It is preferred, however, i f the image capturing sensors are designed as " time delay integration" sensors ( TDI sensors ) . Corresponding sensors can be regarded as a plurality of line sensors which are arranged directly next to one another but in which an image line captured by a first line sensor is trans ferred synchronously with the change in the position of the obj ect vis-a-vis the sensor toan adjacent line sensor, such that the adjacent line sensor again captures the same region imaged by the previously captured image line and additively appends it to the already captured image line, in order thus to obtain a high-contrast image after passing through all the line sensors. In comparison with a simple line sensor, a comparatively strongly exposed image can be attained by multiple exposure using a TDI sensor, wherein the object, however, need remain at a specific position for a shorter time in each case such that the use of TDI sensors can achieve a reduction in the time required for the scanning of an object.

[0037] It is preferable for the object-side beam path of the image capturing unit to be telecentric. As a consequence, the images captured by the individual image capturing sensors have the same size or the same imaging scale, in principle, despite the different focusing. As a consequence, characteristic variables for the focusing that are derived from the captured images can be directly compared with one another. Moreover, in this case - if necessary and despite the defocusing for at least one of the images - the images can more easily be combined to form a single image.

[0038] In principle, the device according to the invention may be designed for transmissive objects, in which case radiation passing through the object is captured by the image capturing unit. The device may also be designed for reflective objects. In both cases, the illumination radiation ultimately captured by the image capturing unit or the image capturing sensors thereof is transformed by the object in such a way - e.g. by partial reflection, partial absorption - that the structure situated on the object is imaged on the image capturing sen- sors .

[0039] It is particularly preferred i f the illumination device and the image capturing unit are designed for illumination radiation in the EUV range , i . e . for radiation having a wavelength of 5 nm to 30 nm, in particular of 13 . 5 nm . Since exclusively reflective optical elements are known for a corresponding wavelength, it is necessary not only to design the device for reflective obj ects , but also to configure any optical elements of the device in a reflective fashion .

[0040] The above-described possibility of refocusing during the scanning process is based on the ascertainment of variables that are characteristic for the defocus on the basis of recorded images . Especially i f images are captured from a region of the obj ect in which the obj ect has no structure , for example the edge region of photolithographic masks which is referred to as the "black border" , it is not possible to ascertain corresponding variables such as the contrast or determine any di f ferences in the characteristic variables for di fferently focused images . In order to nevertheless enable focusing of the device in such situations , provision can be made for a distance sensor, by means of which the distance between the image capturing unit and the obj ect stage or the obj ect is ascertained in the region in which the image capturing regions of the image capturing sensors of the image capturing unit are arranged . The distance between the image capturing unit and the obj ect stage or obj ect may then be adj usted in such a way that the measured distance corresponds to a predetermined distance , which may for example be derived from the lens equation . Focusing on the basis of the distance sensor is frequently suf ficient in order to assume that a focus with optimal contrast that can be found in the vicinity of this focusing is the optimal focus since there is only a single optimum for the contrast in this region, speci fically at the optimal focus .

[0041] The provision of more than one distance sensor, the respective ascertained distances of which may be combined to form a distance value with increased accuracy following a suitable plausibility check, is also possible. Should more than one distance sensor be provided, the use of at least two different types of distance sensors is preferable. A distance sensor may be a confocal sensor, a white light interferometer, a laser interferometer and / or a capacitive sensor.

[0042] The photolithographic mask whose image should be captured by the device 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 photolithographic mask may be configured in substantially rectangular fashion. The photomask can preferably have a length and a width of 5 to 7 inches, particularly preferably a length and a width of 6 inches. As an alternative thereto, the photomask can have a length of 5 to 7 inches and a width of 10 to 14 inches, preferably a length of 6 inches and a width of 12 inches.

[0043] The invention will now be described by way of example on the basis of advantageous embodiments with reference to the accompanying drawings, in which:Figure 1: shows a schematic illustration of an exemplary embodiment of a device according to the invention;Figure 2: shows a schematic illustration of the arrangement of the image capturing sensors of the device from Figure 1; andFigure 3: shows a schematic illustration of the dependence of the contrast of captured images on thedistance between image capturing unit and obj ect stage in the device from Figure 1 .

[0044] Figure 1 schematically illustrates an exemplary embodiment of a device 1 according to the invention for capturing images of structures on a photolithographic mask 20 . The photolithographic mask 20 to be captured as an image is a reflective photolithographic mask 20 , having corresponding structures on the surface . Radiation incident on the obj ect is reflected, in principle , wherein the incident radiation is partially trans formed by the structures on the photolithographic mask 20 , e . g . by some of the radiation being absorbed . The device 1 comprises an illumination device 10 , by which the photolithographic mask 20 can be suf ficiently illuminated . In this case , the illumination device 10 is adapted to the obj ect 10 in such a way that the illumination radiation is suf ficiently reflected of f the photolithographic mask 20 or at least parts of the structures thereon, such that the image capturing unit 20 can capture the illumination radiation trans formed by the photolithographic mask 20 and image-pertaining information concerning the surface of the photolithographic mask 20 actually arises from the illumination radiation thus captured . For this purpose , the wavelength of the illumination radiation must be short enough that the structures on the photolithographic mask 20 can actually be imaged . I f the photolithographic mask is for photolithography in the EUV range , the illumination radiation of the illumination device 10 should regularly likewise be in the EUV range , i . e . between 5 nm and 30 nm, preferably at 13 . 5 nm, since it is only at such a wavelength that the structures typically situated on corresponding photolithographic masks 20 can actually be imaged . Moreover, the reflective properties of corresponding photolithographic masks are optimi zed for corresponding wavelengths .

[0045] The photolithographic mask 20 illuminated by the illumination device 20 is arranged on an object stage 30, by which the photolithographic mask 20 can in particular also be moved in the scanning direction indicated by the arrow 31. Moreover, the object stage 30 can be moved in the direction indicated by the double-headed arrow 32 that is perpendicular to the scanning direction 31.

[0046] For actually capturing the structures on the surface of the photolithographic mask 20, an image capturing unit 40 is provided, by which the illumination radiation of the illumination device 10 that has been reflected and transformed by the photolithographic mask 20 is captured in such a way as ultimately to give rise to a two-dimensional image representation of the structures on the surface of the photolithographic mask 20.

[0047] For this purpose, the image capturing unit 40 has an optical unit 41, which is telecentric on the object side. The optical unit 41 comprises exclusively mirrors as optical elements, which are suitably configured for reflecting the used illumination radiation, e.g. EUV radiation. Shown here by way of example is an optical unit consisting of 2 mirrors. However, any other desired number of mirrors is also conceivable, in particular 3, 4 or 6 mirrors. In the case of illumination radiation in wavelength ranges for which transmissive optical elements, such as e.g. lens elements, are also known, the optical unit 41 can also comprise such optical elements.

[0048] The image capturing unit 40 comprises multiple image capturing sensors 42, which are embodied as TDI sensors. As evident from the schematic plan view of the image capturing sensors 42 in Figure 2, the image capturing sensors are offset in four rows 43 that extend perpendicular to the direction of the image representation of the scanning direction 31,indicated in Figure 2 , and are arranged with an overlap in a proj ection onto a plane 45 that is perpendicular to the direction of the image representation of the scanning direction 31 , in such a way that a region of the photolithographic mask 20 is successively captured by three or four image capturing sensors 42 in each case . The region in which at least three images of the obj ect are captured in succession in the direction of the image representation of the scanning direction 31 is indicated as overall capturing region 44 of the image capturing unit 40 . The portions in which only three images can be captured are indicated by curly brackets 45 ' . Four images may be captured in the regions not marked by the curly brackets 45 ’ .

[0049] As sketched out in Figure 1 , the arrangement of the image capturing sensors 42 in rows 43 yields that the rows 43 of image capturing sensors 42 in each case capture individual regions of the obj ect 30 that are located one behind the other in the movement direction of the photolithographic mask 20 that is indicated by the arrow 31 . During movement of the photolithographic mask 20 with the aid of the obj ect stage 30 in direction 31 , a speci fic region of the obj ect 30 successively passes through the capture regions of the rows 43 of image capturing sensors 42 on the photolithographic mask 20 .

[0050] The image capturing unit 40 furthermore comprises a distance sensor 46 for ascertaining the distance between the image capturing unit 40 and the obj ect stage 30 or the photolithographic mask 20 arranged thereon, in the region in which the image capturing regions of the image capturing sensors 42 of the image capturing unit 40 are located . The distance sensor 46 is a laser interferometer . In situations , in which the option, yet to be described below, of refocusing the device 1 is not available , for example because the images captured by the image capturing sensors 42 have no contrast since they forexample image the "black border" of the photolithographic mask 20 , the distance ascertained by the distance sensor 46 can be used to displace the obj ect stage 30 in the direction 32 until the measured distance corresponds to a predetermined value . By preference , the predetermined value corresponds to an initial distance , in the vicinity of which the optimal distance , at which the defocus is zero , is situated or assumed .

[0051] This control and the control described below are performed by the control device 50 in the device 1 . To this end, the control device 50 is connected both to the image capturing unit 40 and to the obj ect stage 30 and may in particular control the movement of the obj ect stage 30 both in the scanning direction 31 and in the direction 32 . The control of the obj ect stage 30 in the scanning direction 31 and optionally also the capture by the image capturing sensors 42 take place according to the known principle of scanning, and so the manner of functioning of the control device 50 in this regard need not be explained in greater detail .

[0052] In this case , the control device 50 is directly designed to combine the images from the individual image capturing sensors 42 from the entire scanning process of the photolithography mask 20 to form a single two-dimensional image representation of the surface of the photolithographic mask 20 , as is conventional in a scanning method . This image can be communicated via an interface 51 to external devices and units , e . g . for checking the image for defects .

[0053] For the individual images captured by the image capturing sensors 42 , the control device 50 is furthermore embodied to ascertain a respective variable that is characteristic for the defocus of the respective image . In the present case , the contrast of the respective recorded image is ascertained for this purpose as characteristic variable , from which it isknown that - provided it can be ascertained (see above) it is at a maximum in the case of a defocus of zero.

[0054] During the scanning process comprising a movement of the object stage 30 in the scanning direction 31 at constant speed, the control device 50 is embodied to adjust the position of the object stage 30 in the direction 32 perpendicular to the scanning direction 31, starting from an initial value z0, in regularly repeated fashion in the form of a sinusoidal oscillation with the amplitude Az, as a result of which - since the image capturing unit 40 is stationary - there is a correspondingly regularly repeating adjustment in the distance between the image capturing unit 40 and the object stage 30 or the photolithographic mask 20 arranged thereon. As a consequence, this distance, too, is likewise adjusted sinusoidally around an initial distance.

[0055] As a result of adjusting the Z-position of the object stage 30, and hence adjusting the distance between the image capturing unit 40 and the object stage 30 or the photolithographic mask 20, there is also a change in the position of the image plane 47 in the image capturing unit 40 (depicted in Figure 1 by planes 47' ) . On account of the stationary arrangement of the image capturing sensors 42 in the image capturing unit 40, the image plane 47 is intermittently in the image capture plane formed by the image capturing sensors 42 (defocus zero) , or this yields a signed distance between the image plane 47 and said image capture plane, i.e. a defocus.

[0056] The frequency of the adjustment of the distance between the image capturing unit 40 and the object stage 30, which repeats regularly and starts from the initial distance, is matched to the speed of the movement in the scanning direction 31 such that at least two images of the same region of the photolithographic mask 20, which are captured with a timeoffset by different image capturing sensors, are focused differently, i.e. have different defocuses present during the capture thereof. In this case, the frequency may be chosen such that at least three differently focused images are always available for the regions that are scanned by four image capturing sensors (cf . Figure 2) . In that case, this also automatically ensures that at least two differently focused images are present for each of the regions of the photolithographic mask 20 which are scanned by only three image capturing units 42 during the scanning process (cf . Figure 2) .

[0057] The differently focused images of a region of the photolithographic mask 20 may be used to refocus the device 1 by adapting the described initial distance. Thus, whether the initial distance is optimally chosen can be read off from the contrast which was ascertained for the individual images as the variable that is characteristic for the defocus. The contrast is at a maximum for a defocus of zero and drops off with increasing defocus. For example, in the event of three differently focused images, the initial distance has not been chosen optimally should the contrast for the image that was captured when the distance between the image capturing unit 40 and the object stage 30 or photolithographic mask 20 is between the corresponding distances when the other two images were captured not be greater than the contrasts of the other two images .

[0058] Figure 3 depicts by way of example how the contrast of captured images of an (unchanging) region changes about an initial distance given a regularly repeating adjustment of the distance between the image capturing unit 40 and the object stage 30 or photolithographic mask 20. Together with its change over time, which is plotted along the x-axis, the distance in question is represented by the line 90.

[0059] In the middle diagram of Figure 3, in which the initial distance is chosen optimally, it is hardly possible to identify changes in the contrast, which is represented as line 91.

[0060] By contrast, if the initial distance is chosen to be too small, the image reproduced at the top of Figure 3 arises, where the contrast (line 91) behaves in the opposite sense to the regularly repeatedly changing distance (line 90) .

[0061] Should the initial distance be too large, the image reproduced at the bottom of Figure 3 arises. The contrast (line 91) also changes in a manner dependent on the regularly re- peatingly changing distance (line 90) in this case, albeit in the same sense in this case.

[0062] On account of the regularly repeating change in the z- position of the object stage 30 and the resultant adjustment to the distance between the image capturing unit 40 and the photolithographic mask 20, three or four images arise as a matter of principle for a region of the surface of the photolithographic mask 20 on account of the plurality of image capturing sensors 42, at least two or at least three of said images being focused differently. In this case, the differently focused images can be represented as specific points on the x- axis of the diagrams from Figure 3. From the relative positions of the contrasts for the individual images with respect to one another and in conjunction with the distance between the image capturing unit 40 and the object stage 30 at the respective time of capture, it is possible to derive whether and, if so, in which direction the initial distance needs to be adjusted, i.e. reduced or increased.

[0063] The defocus of the individual images of the same region of the object should be sufficiently large so that a distance in the contrast can be established. At the same time, thedefocus should be as small as possible so that a contrast that is as high as possible is obtained in the envisaged combination of the individual images to form a j oint image . The control device 50 may be embodied to adapt the amplitude ±Az in such a way during the scanning process that the conditions above are satis fied . However, it is also possible to fixedly prescribe the amplitude ±Az . For example , an amplitude of ± 10 nm to ± 100 nm may be appropriate for photolithographic masks for the EUV range , while a range of ±20 nm to ±300 nm is appropriate for masks for the DUV range .

Claims

Claims1. Device (1) for capturing images of structures on photolithographic masks, comprising an illumination device (10) for illuminating a photolithographic mask (20) arranged on an object stage (30) with illumination radiation, an image capturing unit (40) that uses at least two image capturing sensors (42) arranged in a joint plane to perform scanning capture of illumination radiation transformed by means of the photolithographic mask (20) , and a control device (50) for controlling a relative movement between the object stage (30) and image capturing unit (40) in a scanning direction (31) , wherein the device (1) can be focused by adjusting the distance between image capturing unit (40) and object stage (30) , wherein the control device (50) is designed to adjust the distance between the image capturing unit (40) and object stage (30) during the relative movement between the object stage (30) and image capturing unit (40) in the scanning direction (31) starting from an initial distance, in such a way that images of the same region of the photolithographic mask (20) captured by at least two different image capturing sensors (42) of the image capturing unit (40) are successively focused differently and a respective variable that is characteristic for the defocus is ascertained for the images by different image capturing sensors (42) of the same region of the photolithographic mask (20) , and to adapt the initial distance between image capturing unit (40) and object stage (30) in a direction that is derivable from the characteristic variables should a non-optimal initial distance be established on the basis of the ascertained variables that are characteristic for the defocus.

2. Device according to Claim 1, characterized in thatthe control device (50) is embodied to adjust the distance between the image capturing unit (40) and the object stage (30) around the initial distance in regularly repeating fashion, starting from the initial distance.

3. Device according to Claim 2, characterized in that the period of the regularly repeating adjustment of the distance between the image capturing unit (40) and the object stage (30) is matched to a constant relative movement between the object stage (30) and the image capturing unit (40) in the scanning direction (31) and the arrangement of the image capturing regions of the individual image sensors (42) , in such a way that the distance in at least two images of the same region of the photolithographic mask (20) captured by different image capturing sensors (42) is different .

4. Device according to Claim 3, characterized in that the distance in at least three images of the same region of the photolithographic mask (20) captured by different image capturing sensors (42) is different.

5. Device according to Claim 2 to 4, characterized in that the control device (50) can adapt the amplitude of the regularly repeating adjustment to the distance between the image capturing unit (40) and the object stage (30) so that the variables that are characteristic for the focusing are different for at least two, preferably at least three images of the same region of the photolithographic mask (20) that are captured by different image capturing sensors (42) .

6. Device according to any of the preceding claims, characterized in that the object stage (30) is movable in a direction (32) perpendicular to the scanning direction (31) for adjusting the distance between the image capturing unit (40) and object stage ( 30 ) .

7. Device according to any of the preceding claims, characterized in that the device (1) comprises at least three image capturing sensors (42) arranged perpendicularly to a direction of the image representation of the scanning direction (31) .

8. Device according to any of the preceding claims, characterized in that the device (1) comprises four image capturing sensors (42) arranged perpendicularly to a direction of the image representation of the scanning direction (31) .

9. Device according to any of the preceding claims, characterized in that the image capturing regions of the image capturing sensors (42) are arranged at a distance from one another in the direction of the image representation of the scanning direction ( 31 ) .

10. Device according to Claim 9, characterized in that the image capturing regions of the image capturing sensors (42) are arranged at least partially overlapping in a projection onto a plane perpendicular to the image representation of the scanning direction (31) .

11. Device according to any of the preceding claims, characterized in thatthe determined characteristic variable is the contrast (K) of a captured image K.

12. Device according to any of the preceding claims, characterized in that the control device (50) is designed to adapt the initial distance in such a way that the sum of the squares of the characteristic variables for the images of the same region of the photolithographic mask (20) from different image capturing sensors (42) is optimal.

13. Device according to any of the preceding claims, characterized in that the device comprises a distance sensor (46) for ascertaining the distance between the image capturing unit (40) and the object stage (30) or a photolithographic mask (20) arranged on the object stage (30) , in the region in which the image capturing regions of the image capturing sensors (42) of the image capturing unit (40) are arranged.

14. Device according to any of the preceding claims, characterized in that the image capturing sensors (42) are TDI sensors.

15. Device according to any of Claims 1 to 14, characterized in that the image capturing sensors (42) are line sensors.

16. Device according to any of the preceding claims, characterized in that the object-side beam path of the image capturing unit (40) is telecentric.

17. Device according to any of the preceding claims, characterized in thatthe device (1) is designed for reflective photolithographic masks (20) .

18. Device according to any of the preceding claims, characterized in that the illumination device (10) and the image capturing unit(40) are designed for illumination radiation in the EUV range .

19. Device according to any of the preceding claims, characterized in that the control device (50) is designed to combine the images captured by the various image capturing sensors (42) in a single image.

Images