Device for capturing images of micro- and / or nanostructures

Abstract

The invention relates to a device (1) for capturing images of micro- and / or nanostructures, in particular structures on microlithographic masks The device (1) comprises an illumination device (10) for illuminating an object (20) with illumination radiation and an image capturing unit (40) for capturing the illumination radiation transformed by the object (20), wherein the device (10) is configured in a focusable fashion, wherein the image capturing unit (40) comprises at least two image capturing sensors (45, 46, 47) arranged in such a way that at least one image capturing sensor (45, 46, 47) is defocused independently of the focusing of the device (1).

Description

Device for capturing images of micro- and / or nanostructures

[0001] The invention relates to a device for capturing images of micro- and / or nanostructures, in particular structures on microlithographic masks.

[0002] Microlithography is used for producing microstructured components, such as for example integrated circuits. The microlithography process is carried out in what is known as a projection exposure apparatus, which has 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. 8:1, 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 obj ect to be checked - i . e . e . g . a mask for producing microstructured components - is illuminated by an illumination source in such a way that either the radiation partially reflected back from 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 measurementvariables - to carry out various checks . In the case of masks for semiconductor fabrication, an image representation constituting an image-based reflection of the actual micro- and / or nanostructure 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 o f 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 micro- and / or nanostructures , 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 o f 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 an obj ect having micro- and / or nanostructures that is to be checked, rather portion-by- portion or continuous capture takes place , refocusing of the capturing device is regularly required even during the actual capturing process , in order to ensure that the resulting image representation i s as sharp as possible over the entire imaged region . This is because i f micro- and / or nanostructures are intended to be imaged, changes in the arrangement of obj ectand 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 of thermal expansions of the components or the structure carrying the components. Moreover, bending of the object, which may occur on account of its mounting, may require refocusing during a capturing process.

[0009] It is an object of the present invention to provide a device for capturing images of micro- and / or nanostructures which enables refocusing during a capturing process.

[0010] This object is achieved 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 micro- and / or nanostructures, in particular structures on microlithographic masks, comprising an illumination device for illuminating an object with illumination radiation and an image capturing unit for capturing the illumination radiation transformed by the object, wherein the device is configured in a focusable fashion, wherein the image capturing unit comprises at least two image capturing sensors arranged in such a way that at least one image capturing sensor is defocused independently of the focusing of the device.

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

[0013] In the context of the invention, "focusing" denotes the adaptation of the image capturing unit and / or of the distance between image capturing unit and object (and thus in general the length of the object-side beam path) , by which the imagerepresentation of the object becomes as optimal as possible with regard to a predefined criterion on a specific one or all of the image capturing sensors. In this respect, "optimal focusing" may denote e.g. the state in which, as optimization criterion, the sharpness on a predefined image capturing sensor is maximal. In this context - if necessary - "sharpness" may be taken to be the edge sharpness, which in general corresponds to the contrast of the image.Alternatively, as optimization criterion, it is also possible to use e.g. the sum of the squares of the representative variable for the individual images from all the image capturing sensors, which is then intended to be as minimal or as maximal as possible - depending on the characteristic variable chosen.

[0014] What is denoted by "direction of focusing" is that direction of the adaptation available for focusing in which the image capturing unit and / or the distance between image capturing unit and object ought to be changed, i.e. e.g. whether said distance ought to be increased or decreased.

[0015] If optimal focusing (taking into account an appropriate tolerance) is present for an image capturing sensor, an image capturing sensor is deemed to be "focused". In the case of such optimal focusing, the region of the object that is captured by the image capturing unit lies in the object focal plane and the image capturing sensor lies in the image focal plane .

[0016] An image capturing sensor is "defocused" if its focusing cannot be deemed to be focused, even taking into account an appropriate tolerance range. The distance between the image capturing sensor and the image focal plane, and / or the distance between the object and the object focal plane, in which the optimal focusing would be attained, are / is regardedhere as defocus and can thus be expressed numerically. In the case of optimal focusing, the defocus is then zero. A difference in defocus between two image capturing sensors is referred to as "relative defocus".

[0017] Radiation or an electromagnetic wave 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 microlithographic 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. Phase-shifting masks are also known .

[0018] The invention has recognized that by capturing at least two images by way of an image capturing unit having two different image capturing sensors, which in principle have a different defocus on account of their arrangement, rapid refocusing is possible even directly during image capture. If, for the two images or at least portions thereof, a variable that is characteristic of the focusing of the individual image capturing sensors is determined from the images captured by the image capturing sensors, rapid refocusing can take place depending on the optimization criterion chosen. Given fundamental knowledge of the relationship between the representative variable and the focusing or the defocus, a possible optimization criterion may then be the reduction of the defocus of an individual, optionally predefined, image capturing sensor to zero. Alternatively, it is possible for the optimization criterion used also to be e.g. the sum of the squares of the representative variable for all the image capturing sensors, which then ought to form as far as possible an optimum - i.e. a maximum or a minimum. Instead of the aforementioned sum of the squares, other optimization criteriathat take into account all the image capturing sensors may also be used.

[0019] In this case, from the characteristic variable of the images captured by the individual image capturing sensors that is used for optimization purposes, often it is possible to establish not only the fundamental requirement for refocusing, namely e.g. whether possibly all the image capturing sensors are defocused or the sum of the squares of the characteristic variable for all the image capturing sensors exceeds or falls below a predefined limit value or changes in relation to a sum of the squares determined analogously beforehand, but also in what direction the focusing of the image capturing unit needs to be changed in order that the optimization criterion chosen is actually optimal again. For this purpose, it is possible to use e.g. the difference between the respective characteristic variables determined for the images from the different image capturing sensors.

[0020] In order to make this possible, the representative variable for the focusing must have a sufficient resolution, such that in principle different values for the respective defocus arise for the image capturing sensors. This resolution is dependent on various factors, such as e.g. the resolving power of the image capturing sensors, the relative defocus between the image capturing sensors, but also the intensity of the illumination of the object. However, a person skilled in the art is readily able to coordinate the relevant factors among one another in such a way as to attain the required resolution for the representative variable.

[0021] By way of example, the contrast of the recorded image can be used as a representative variable for the focusing. The contrast can be assumed to have, given optimal focusing of the image capturing sensor, in general a single maximum, but atleast an unambiguously identifiable primary maximum. By way of example, the root mean square over all or some of the image points of a recorded image or the standard deviation vis-a-vis the mean value (both determined over all or some of the image points of a recorded image) can be used as a value for the contrast. This variable, too, has a single optimum or an unambiguously identifiable primary optimum in conjunction with which optimal focusing is attained. 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.

[0022] If a relationship between a representative variable and the focusing is not unambiguous in this way and has e.g. more than one optimum, the procedure described below can nevertheless be carried out, in principle, given sufficiently accurate knowledge of the relationship between the characteristic variable and the focusing. However, this may then necessitate more than two or three image capturing sensors in order to be able to ascertain the direction in which renewed focusing that may be required has to take place with sufficient certainty.

[0023] If, in the case of an image capturing unit having two image capturing sensors, there is a need for refocusing, which e.g. is also derivable from a change in the representative variable (s) used for determining the optimum focusing, it is possible to change the focusing in the direction of that image capturing sensor for which e.g. the higher contrast was established. This change can be continued until the contrast of the image capturing sensor in question no longer changes (or decreases again, whereupon the focusing can then be corrected again somewhat in the opposite direction) .Alternatively, the change can be continued until a practically identical contrast is determined for both image capturing sensors or until the sum of the squares of the reciprocals of the contrasts o f both image capturing sensors is minimal .

[0024] I f the image capturing unit comprises three image capturing sensors , which is preferred, upon a need for refocusing, the refocusing can comparably be ef fected directly by changing the focusing in the direction of that image capturing sensor with e . g . the higher or highest contrast . The measure of the required refocusing can optionally also be determined from the representative variables determined for the individual image capturing sensors . The same comparably applies to image capturing units having more than three image capturing sensors .

[0025] The very act of determining - as described by way of example above - the direction in which focusing has to take place makes it possible to refocus a device for capturing images of micro- and / or nanostructures even during the actual image capturing . Speci fically, in the case of the device according to the invention, upon the need for refocusing being established, by virtue of the fact that the direction in which the focusing 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 capturing are precluded . The device according to the invention furthermore makes it possible to monitor the instantaneous focusing during the refocusing and to establish when optimal focusing was attained in the course of refocusing .

[0026] What is preferred, however, is for the device not j ust to make possible the described refocusing during the actualimage capturing, but rather for this to be carried out in an automated manner as well . For this purpose , the device preferably comprises a control device designed to determine a variable that is characteristic of the focusing for each of the individual images from the image capturing sensors and to adapt the focusing of the device in a direction derivable from the characteristic variables , such that the focusing of a, preferably prede fined, image capturing sensor, or as viewed over all the image capturing sensors of the image capturing unit , becomes optimal . In the first-mentioned case , preferably, the defocus of a or the predefined image capturing sensor becomes zero ; in the last-mentioned case , the sum of the squares over the characteristic variable for all the image capturing sensors can be optimal . Possible regulation strategies for the control device in order to achieve this are evident from the explanations above .

[0027] It is preferred i f the distance between image capturing unit and obj ect is variable for focusing purposes . In this case , it is unimportant whether the image capturing unit , the obj ect or even both can be suitably moved in order to adapt the focusing . In this case , the distance between image capturing unit and obj ect can be adapted particularly in the course of the automatic refocusing by way of a control device preferably in a precise manner so that , by means of the adaptation, it is already possible to compensate for minimal defocusing established by way of the characteristic variable for the focusing, and in particular no renewed defocusing as a result of "overshooting" of the optimal focusing on account of excessively low precision in the adaptation of the distance between image capturing unit and obj ect occurs .

[0028] The image capturing sensors of the image capturing unit can be arranged of fset with respect to one another in the image capturing unit perpendicularly to a predefined imagefocal plane of the image capturing unit, thus resulting, in principle, in a relative defocus between the image capturing sensors. By virtue of the described changing of the distance between image capturing unit and object, the defocus of all the image capturing sensors is changed by the position of the object being changed relative to the object focal plane matching the predefined image focal plane, wherein the different distances between the image capturing sensors and the image focal plane ensure a different defocus of the individual image capturing sensors.

[0029] However, it is also possible for the image capturing sensors to be arranged next to one another in a common plane or image focal plane, and for the different defocus at the individual image capturing sensors to be predefined by a suitable tilt angle of the image capturing unit vis-a-vis the object or the surface thereof. By virtue of such a tilt, the surface of the object does not lie in the object focal plane, such that defocuses resulting directly from the tilt angle arise, in principle, at least for some of the portions of the surface that are captured by the individual image capturing sensors. If the image capturing sensors are arranged in a common plane, a single image capturing chip can also be provided instead of separate image capturing sensors, wherein the individual image capturing sensors are then formed by separate sub-regions of the image capturing chip.

[0030] It is also possible to combine the above measures for producing relative defocuses between the image capturing sensors; care should merely be taken to ensure that the measures - offset arrangement of the image capturing sensors and tilting - do not mutually cancel one another out.

[0031] It is preferred if the tilt angle is variable. Varying the tilt angle makes it possible to vary the differencebetween the defocus of the individual image capturing sensors, in particular ensuring that the distances are varied uniformly, in principle. It can thus be ensured, even under possibly unfavourable constraints, that the characteristic variable for the focusing of two adjacent image capturing sensors is sufficiently different, in principle, to carry out refocusing as necessary on the basis of the different characteristic variables. In this case, the variation of the tilt angle can be performed before the beginning of the image capturing for a specific object. However, it is also possible for the tilt angle to be varied during the image capturing, e.g. by means of the control device. An attempted regulation strategy may involve e.g. on the one hand ensuring a difference in the characteristic variables for the focusing between the individual image capturing sensors, said difference being sufficient for determining the direction of refocusing that is possibly necessary, but on the other hand minimizing said difference as far as possible. This is because the defocusing of the at least one image capturing sensor that is not optimally focused turns out to be all the smaller, the smaller said difference. The image captured by this image capturing sensor, despite the actual defocusing, can in particular then still have enough sharpness that it can be used for the further checking of the captured micro- and / or nanostructures of the object.

[0032] In order to simplify this, inter alia, it is preferred if the object-side beam path of the image capturing unit is telecentric. As a consequence, the images captured by the individual image capturing sensors have the same size, in principle, despite the different defocus. 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 thedefocusing for at least one of the images - the images can more easily be combined to form a single image.

[0033] It is even preferred if the device comprises an image processing unit, by which the images from the individual image capturing sensors are combined to form a single image. Whether the combination takes place by superimposition or e.g. using a known "focus stacking method" is unimportant, in principle. What can be achieved by the combination of a plurality of images of the same object, even if these images, at least in part, are not optimally focused, is that as much image information as possible is taken into account for the resulting image, whereby e.g. the contrast of the resulting image can be improved even further or image noise can be reduced .

[0034] It is possible, in principle, for all the image capturing sensors of the image capturing unit simultaneously to capture images of the same region on the surface of the object. However, this generally requires a complex optical unit between the object and the image capturing sensors. However, it is preferred if the image capturing unit is designed to the effect that the image capturing sensors thereof each capture individual adjacent regions in an object plane, and provision is made of an object stage for moving the object in the object plane in at least one direction, which allows an object arranged on the object stage to move in such a way that the image capturing sensors can successively each capture an image of the same region of the surface. In other words, the device can be suitably designed for the "scanning" that is already known in principle from the prior art.

[0035] 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, if 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 transferred synchronously with the change in the position of the object vis-a-vis the sensor to an 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 TDI sensor makes it possible to attain a comparably highly exposed image by multiple exposure.

[0036] Even if the device according to the invention can in principle also be designed for, in principle, transmissive objects, in which case radiation passing through the object is then captured by the image capturing unit, it is preferred if the device is 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 micro- and / or nanostructure situated on the object is imaged on the image capturing sensors.

[0037] It is particularly preferred if 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 designthe device for reflective objects, but also to configure any optical elements of the device in a reflective fashion.

[0038] 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 a first exemplary embodiment of a device according to the invention;Figure 2: shows a schematic illustration of a second exemplary embodiment of a device according to the invention; andFigure 3: shows a schematic illustration of the relation between characteristic variable and focusing that is used by the control devices of the devices in accordance with Figures 1 and 2.

[0039] Figure 1 schematically illustrates a first exemplary embodiment of a device 1 according to the invention for capturing images of micro- and / or nanostructures on an object 20. The object 20 to be captured as an image is a reflective microlithographic mask, having corresponding structures on the surface. Radiation incident on the object is reflected, in principle, wherein the incident radiation is partially transformed by the micro- and / or nanostructures, e.g. by parts of the radiation being absorbed.

[0040] The device 1 comprises an illumination device 10, by which the object 20 can be sufficiently illuminated. In this case, the illumination device 10 is adapted to the object 20 in such a way that the illumination radiation is sufficiently reflected by the object 20 or at least parts of the micro- and / or nanostructures, such that the image capturing unit 40can capture the illumination radiation transformed by the object 20 and image-pertaining information concerning the surface of the object 20 actually arises from the illumination radiation thus captured. For this purpose, the wavelength of the illumination radiation must be low enough that the micro- and / or nanostructures can actually be imaged. If the object 20 is e.g. a microlithographic mask for microlithography in the EUV range, the illumination radiation of the illumination device 10 must 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 micro- and / or nanostructures typically situated on corresponding microlithographic masks can actually be imaged. Moreover, the reflective properties of corresponding microlithographic masks are optimized towards corresponding wavelengths.

[0041] The object 20 illuminated by the illumination device 10 is arranged on an object stage 30, by which the object 20 can in particular also be moved in the direction indicated by the arrow 31.

[0042] For actually capturing the micro- and / or nanostructures on the surface of the object 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 object 20 is captured in such a way as ultimately to give rise to a two-dimensional image representation of the micro- and / or nanostructures on the surface of the object 20.

[0043] 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. In the case of illumination radiation in wavelength ranges for whichtransmissive optical elements, such as e.g. lens elements, are also known, the optical unit 41 can also comprise such optical elements .

[0044] The image capturing unit 40 comprises three image capturing sensors 45, 46, 47. The three image capturing sensors 45, 46, 47 are line sensors that each capture individual regions of the object 20 lying one behind the other in the direction of movement of the object 20 as indicated by the arrow 31. During movement of the object 20 with the aid of the object stage 30 in direction 31 a specific region of the object 20 successively passes through the capture regions of the three image capturing sensors 47, 46, 45.

[0045] One of the image capturing sensors 46 defines the image focal plane 42 of the image capturing unit 40, from which the object focal plane 43 also directly arises by way of the optical unit 41. The image capturing unit 40 as a whole is displaceable in a direction parallel to the incident radiation into the optical unit 41 (indicated by the double-headed arrow 44) , such that the distance between object 20 and image capturing unit 40 can be changed. Changing the distance between object 20 and image capturing unit 40 makes it possible to focus the image capturing sensor 46, inter alia.

[0046] The other two image capturing sensors 45, 47 are arranged offset vis-a-vis the image focal plane 42 - defined by the image capturing sensor 46 - by a predefined distance ±Az perpendicularly thereto, wherein this distance ±Az cannot change. If the distance between object 20 and image capturing unit 40 is changed by displacement of the image capturing unit 40, this changes not only the focusing of the image capturing sensor 46 but directly also the focusing of the image capturing sensors 45, 47, wherein the different distances from the image focal plane 42 ensure that a different defocusarises for each of the individual image capturing sensors 45, 46, 47. In this case, the defocus of one of the image capturing sensors 45, 46, 47 can of course also be zero, which then means, however, that the other two image capturing sensors necessarily have a defocus by ±Az and ±2Az, respectively .

[0047] The device 1 comprises an image processing unit 60, by which those images from the image capturing sensors 45, 46, 47 which each image the same linear region of the object 20 are merged to form an individual linear image and a two- dimensional image reflecting the surface of the object 20 is generated from a multiplicity of linear images. In this case, although a specific linear region of the object 20 is not captured simultaneously by the individual image capturing sensors 45, 46, 47, the movement of the object 20 in direction 31 enables the region in question to be captured successively by all the image capturing sensors 45, 46, 47.

[0048] Depending on the degree of defocusing of the individual linear images with respect to one another, which crucially results from the distance ±Az, the images from the individual image capturing sensors 45, 46, 47, can be directly superimposed or they are combined with the aid of a known "focus stacking method". The individual linear images can then be merged to form a two-dimensional image, as is customary and known in scanning methods. This image can be communicated via an interface 61 to external devices and units, e.g. for checking the image for defects.

[0049] The device 1 furthermore comprises a control device 50, which is connected both to the image capturing unit 40 and to the object stage 30 and controls at least the respective movement thereof. The control of the object stage 30 and optionally also the capture by the image capturing sensors 45,46, 47 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 .

[0050] The control device 50 furthermore obtains the images captured by the image capturing sensors 45, 46, 47 of the image capturing unit 40 in order on this basis to control the focusing by adapting the distance between object 20 and image capturing unit 40 in direction 44. The regulation strategy employed for this purpose will be explained in greater detail later with reference to Figure 3, also because it is analogously applicable to the exemplary embodiment in accordance with Figure 2.

[0051] In the exemplary embodiment illustrated schematically in Figure 2, individual components are similar to those from Figure 1 or are even identical thereto. Regarding components of the device 1 according to the invention in accordance with Figure 2 that are not explained comprehensively below, reference is therefore supplementarily made to the explanations above.

[0052] The device 1 for capturing images of micro- and / or nanostructures as illustrated in Figure 2 comprises an illumination device 10 for illuminating the object 20 arranged on an object stage 30, said object being a reflective microlithographic mask having micro- and / or nanostructures on the surface. In this case, illumination device 10, object stage 30 and object 20 are identical, in principle, to the components shown in Figure 1. However, the object stage 30 makes possible movements of the object 20 not only parallel to the object surface 21 (cf. arrow 31, inter alia) but also in a direction perpendicular thereto, as a result of which a movement in a direction perpendicular to the object focalplane 43 of the image capturing unit 40 described below is also made possible, which is indicated by the double-headed arrow 44.

[0053] In the case of the image capturing unit 40, the image capturing sensors 45, 46, 47, configured as TDI sensors in this case, are arranged in a common image focal plane 42, which together with the optical unit 41 then also gives rise to the object focal plane 43. The optical unit 41 is constructed analogously to that from Figure 1 and is telecentric on the object side, in particular.

[0054] In a comparable manner to the exemplary embodiment in accordance with Figure 1, the three image capturing sensors 45, 46, 47 capture different regions of the object 20, in principle. However, suitable movement of the object 20 by means of the object stage 30 in direction 31 allows a region on the object 20 to be captured successively by all the image capturing sensors 45, 46, 47. The images thus captured by the image capturing sensors 45, 46, 47 can be combined by the image processing unit 60 - already known from Figure 1 - to form a two-dimensional image representation of the surface of the object 20 and can be made available via the interface 61.

[0055] The image capturing unit 40 is arranged in a manner tilted vis-a-vis the object 20 or the surface 21 thereof, wherein the angle 49 between the surface normal of the surface 21 of the object 20 and the object focal plane 43 can be assumed as angle for the tilting. The image capturing unit 40 is arranged so as to be movable in the direction of the double-headed arrow 49 in order to be able to vary the tilt angle 48 as necessary. Since the three image capturing sensors 45, 46, 47 capture different regions of the object, the result, even though they are arranged in the common image focal plane 42, is a different defocus for the individualimage capturing sensors 45 , 46 , 47 on account of the tilt angle 48 on the obj ect side . While the defocus represented as zero in Figure 2 for the image sensor 46 remains constant independently of the tilt angle 48 , the relative defocus with respect thereto for the other two image capturing sensors 45 , 47 is ±Az , where Az is directly dependent on the tilt angle 48 : The greater the tilt angle 48 , the greater ±Az is as well .

[0056] Movement of the image capturing unit 40 in direction 49 and the attendant changing of the tilt angle 48 thus ultimately make it possible to vary the relative defocus ±Az with respect to one another between the individual image capturing sensors 45 , 46 , 47 .

[0057] The control device 50 is designed in principle for the functions already explained in association with Figure 1 and also for those yet to be explained below with reference to Figure 3 . For adj usting the distance , however, the obj ect stage 30 is moved, rather than the image capturing unit 40 . In particular, the control device 50 is designed, upon movement of the obj ect stage 30 in direction 33 , simultaneously to bring about a movement in or counter to the direction 31 , such that the portions of the surface 21 of the obj ect 20 that are captured by the individual image capturing sensors 45 , 46 , 47 do not change .

[0058] The control device is additionally designed to adj ust the tilt angle 48 by moving the image capturing unit 40 in direction 49 such that the distance ±Az and thus the defocusing of the two image capturing sensors 45 , 47 are on the one hand possibly small in order to increase the quality of the combination of the various image data by the image processing unit 60 , and on the other hand are large enough to enable the refocusing that will now be explained in particular with reference to Figure 3 .

[0059] In all the exemplary embodiments described above, the control unit 50 is designed to determine variables that are characteristic of the focusing for each of the images captured by the different image capturing sensors 45, 46, 47. In this case, characteristic variables are preferably determined for images of the same region on the object 20 in order to preclude any impairment of the characteristic variables by different recorded structures of the object 20. In the case of a device 1 in accordance with Figure 1 or Figure 2, the corresponding images are not captured simultaneously, but in close succession on account of the known scanning process carried out by the control device 50.

[0060] In the present case, the contrast K of the captured images is determined as a characteristic variable for the focusing, wherein the images in question are line images on account of the image capturing sensors 45, 46, 47 used. Furthermore, since the focusing for the devices 1 in accordance with Figures 1 and 2 is effected in each case solely by the variation of the distance z between the image capturing unit or an image capturing sensor 45, 46, 47 and the object 20, the relation between contrast and focusing can be depicted schematically as a one-dimensional curve. Such a curve is shown by way of example in a contrast-distance diagram in Figure 3. Although the position of the curve in the contrast-distance diagram and the specific shaping of the curve may vary depending on various influencing factors, such as the illumination intensity, but also the specific configuration of the object 20 in the captured region, the contrast-distance curve, in principle, has an unambiguous maximum which occurs in conjunction with optimal focusing of one of the image capturing sensors 45, 46, 47.

[0061] Since, in the case of the device according to the invention, a plurality of, in particular three, imagecapturing sensors 45, 46, 47 are provided, which each have a relative defocus with respect to one another, there is - even if one of the image capturing sensors 45, 46, 47 is optimally focused - defocusing of at least one, and in the case of the present exemplary embodiments two image capturing sensors 47, 46, 45, for which reason the images of a specific region of the object 20 that are captured by these image capturing sensors 47, 46, 45 have a lower contrast than the image of the same region that is captured by the optimally focused image capturing sensor 45, 46, 47.

[0062] Figure 3 illustrates by way of example the contrasts determined by the control device 50 for the three image capturing sensors 45, 46, 47 in the case of three different distances between image capturing unit 40 and object 20.

[0063] In the case of the distance illustrated in the middle of Figure 3, the contrast determined for the image captured by the image capturing sensor 46 corresponds to the maximum contrast value. As a consequence, the contrast values for the images from the image capturing sensors 45, 47 with a relative defocus ±Az are both necessarily lower. If exclusively the contrast values determined by the control device are present, given a corresponding relation between the contrast values it can be assumed that the inner image sensor 46 is optimally focused at least within a certain tolerance, which inter alia also depends on the magnitude of the relative defocus ±Az.

[0064] In cases in which the image capturing sensor 46 is not optimally focused, the contrast value for the image captured by one of the other two image capturing sensors 45, 47 is necessarily greater than the corresponding value for the image of the same region captured by the image capturing sensor 46. This is likewise depicted schematically in Figure 3 (cf. groups of points away from the three points at or next to themaximum) . From the relation between the contrasts thus determined, however, it is possible to deduce not only that defocusing of the image capturing sensor 46 is present , but also the direction in which the distance between image capturing unit 40 and obj ect 20 should be changed in order to optimally focus the image capturing sensor 46 again . I f the profile of the contrast-distance curve is accurately known or can be approximated suf ficiently exactly, it is possible from the determined contrasts , i f appropriate , not only to determine the direction of the change in distance as needed for the focusing, but also to estimate the magnitude of the required change in the distance between image capturing unit 40 and obj ect 20 .

[0065] As an alternative to the described procedure in which one of the image capturing sensors 46 - in particular the image capturing sensor 46 lying in the image focal plane 42 of the image capturing unit 40 - is optimally focused, in principle , it is also possible to form the sum of the squares of the contrast or of the reciprocal of the contrast for the images from the individual image capturing sensors 45 , 46 , 47 of the image capturing unit 40 and to regulate the distance between image capturing unit 40 and obj ect 20 such that the sum of squares in question becomes maximal or minimal .

[0066] In the case of the devices 1 in accordance with Figures 1 and 2 , the control unit 50 is in each case designed, depending on the - as described above - determined and evaluated contrasts of the images of the same region on the obj ect 20 that are captured by the individual image capturing sensors 45 , 46 , 47 , to bring about the distance between image capturing unit 40 and obj ect 20 by correspondingly changing the position of image capturing unit 40 or obj ect stage 30 .

Claims

Patent Claims1. Device (1) for capturing images of micro- and / or nanostructures, in particular structures on microlithographic masks, comprising an illumination device (10) for illuminating an object (20) with illumination radiation and an image capturing unit (40) for capturing the illumination radiation transformed by the object (20) , wherein the device (10) is configured in a focusable fashion, characterized in that the image capturing unit (40) comprises at least two image capturing sensors (45, 46, 47) arranged in such a way that at least one image capturing sensor (45, 46, 47) is defocused independently of the focusing of the device (1) .

2. Device according to Claim 1, characterized in that the device (1) comprises a control device (50) designed to determine a variable that is characteristic of the focusing for each of the individual images from the image capturing sensors (45, 46, 47) and to adapt the focusing of the device (1) in a direction derivable from the characteristic variables, such that the focusing of a, preferably predefined, image capturing sensor (45, 46, 47) of the image capturing unit (40) , or as viewed over all the image capturing sensors (45, 46, 47) of the image capturing unit (40) , becomes as optimal as possible.

3. Device according to Claim 2, characterized in that the determined characteristic variable is the contrast (K) of a captured image or the reciprocal thereof.

4. Device according to Claim 2 or 3, characterized in that the optimal focusing is attained in conjunction with an optimum of the sum of the squares of the characteristic variable for each of the image capturing sensors (45, 46, 47) of the image capturing unit (40) .

5. Device according to any of the preceding claims, characterized in that the image capturing unit (40) comprises at least three image capturing sensors (45, 46, 47) arranged at a distance from one another perpendicularly to an image focal plane (42) of the image capturing unit (40) .

6. Device according to any of the preceding claims, characterized in that the distance between image capturing unit (40) and object (20) is variable for focusing purposes.

7. Device according to any of the preceding claims, characterized in that the image capturing sensors (45, 46, 47) are arranged fixedly, preferably in a common plane (48) , in the image capturing unit (40) , wherein the relative defocus of the image capturing sensors (45, 46, 47) with respect to one another is predefined by a preferably variable tilt angle (49) of the image capturing unit (40) vis-a-vis the object (20) .

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

9. Device according to any of the preceding claims, characterized in that the device (1) comprises an image processing unit (60) , by which the images from the individual image capturing sensors (45, 46, 47) are combined to form a single image.

10. Device according to any of the preceding claims, characterized in that the image capturing unit (40) is designed in such a way that the image capturing sensors (45, 46, 47) thereof each capture individual adjacent regions in an object plane (31) , and provision is made of an object stage (30) for moving the object (20) in the object plane (31) in at least one direction (31) , which allows an object (20) arranged on the object stage (30) to move in such a way that the image capturing sensors (45, 46, 47) can successively each capture an image of the same region of the surface (21) of the ob j ect (20) .

11. Device according to any of the preceding claims, characterized in that the device (1) is designed for reflective objects.

12. Device according to Claim 11, characterized in that the illumination device (20) and the image capturing unit (40) are designed for illumination radiation in the EUV range .

13. Device according to any of the preceding claims, characterized in that the image capturing sensors (45, 46, 47) are TDI sensors.

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