Device for imaging micro- and / or nanostructures

Abstract

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

Description

[0001] The invention relates to a device for imaging micro- and / or nanostructures, in particular structures on microlithography masks.

[0002] Microlithography is used in the production of microstructured components, such as integrated circuits. The microlithography process is carried out in a so-called projection exposure system, which has an illumination device and a projection device. The image of a mask (also called a "reticle") illuminated by the illumination device is projected by the projection device onto a substrate, e.g., a silicon wafer, 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 systems have a reduction factor of, for example, 8:1, the structures of the masks must already be highly accurate due to the advancing miniaturization in the semiconductor sector and the transition in wavelength during exposure from DUV (e.g., 193 nm) to EUV (e.g., 13.5 nm). To ensure that a mask meets these accuracy requirements and that a microstructured component produced with it also exhibits the desired properties and functionality, a mask is checked using suitable methods in inspection and / or metrology systems before use in projection exposure systems.

[0004] In known inspection or metrology systems, the object to be inspected - for example, a mask for the production of microstructured components - is illuminated by a light source in such a way that either the radiation partially reflected by the object or the radiation partially transmitted by it falls on a sensor, whose sensor data can then be suitably evaluated.

[0005] The sensor can, for example, be an imaging sensor, from whose sensor data (usually) two-dimensional images of a specific measured variable, namely the radiation intensity, can be derived. An imaging sensor itself can be designed as a flat surface and record all measured variables for a specific image at once; however, it is also possible for an imaging sensor to be designed as a point or line sensor only, namely as a point or line sensor, and thus can only determine parts of the measured variables required for a desired image at once, while the other measured variables required for the image must be recorded in further steps, each after suitable realignment of the imaging sensor and / or the object, in order to ultimately obtain the desired image of the measured variables.The described realignment of the imaging sensor and / or the object can also be realized as a continuous relative movement of the two components, with the sensor then virtually continuously collecting data that can be combined into a two-dimensional image. This process is also referred to as "scanning."

[0006] Using images of objects obtained in this way, various checks can then be performed depending on the measured values ​​recorded or displayed therein, or on values ​​derived from the measured values. In the case of masks for semiconductor production, an image that visually reflects the actual micro- and / or nanostructure on the surface of the mask can, for example, be compared with an image of a target structure in order to detect possible defects in the mask. If the resulting image is largely distortion-free and / or the image scale is known, measurements of the mask structure can also be made based on the image in order to compare them with target values ​​or to compare different measured values ​​with one another.

[0007] Particularly when imaging micro- and / or nanostructures, the required components must be aligned with high precision to achieve high resolution in the measured values. In other words, the objective is to ensure that the measured value obtained for a specific point on the object reflects, as far as possible, the measured value at that point and is not influenced by the corresponding values ​​of the surrounding points. This allows for the most accurate image of the object's actual surface to be obtained.

[0008] Particularly if the sensor does not capture the entire surface of an object to be inspected with micro- and / or nanostructures at once, but rather captures sections or continuously, refocusing of the capture device is regularly required, even during the actual capture process, to ensure that the resulting image is as sharp as possible over the entire imaged area. If micro- and / or nanostructures are to be imaged, even changes in the arrangement, particularly of the object and sensor, can lead to defocusing, which is detrimental to the sharpness of the image. Such changes in the arrangement can occur, for example, as a result of thermal expansion of the components or the structure supporting the components. Deflection of the object, which can occur due to its storage, may also require refocusing during a capture process.

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

[0010] This object is achieved by a device according to the main claim. Advantageous further developments are the subject of the dependent claims.

[0011] Accordingly, the invention relates to a device for the image capture of micro- and / or nanostructures, in particular structures on microlithography masks, comprising an illumination device for illuminating an object with illumination radiation and an image capture unit for capturing the illumination radiation transformed by the object, wherein the device is designed to be focusable, wherein the image capture unit comprises at least two image capture sensors which are arranged such that at least one image capture sensor is defocused regardless of the focusing of the device.

[0012] First, some terms used in connection with the invention are explained:

[0013] In the context of the invention, “focusing” refers to the adjustment of the image capture unit and / or the distance between the image capture unit and the object (and thus generally the length of the object-side beam path) with which the image of the object is imaged as optimally as possible with regard to a predetermined criterion on a specific or all image capture sensors. “Optimal focusing” can, for example, refer to the state in which the sharpness on a given image capture sensor is maximum as the optimization criterion. In this context, “sharpness” can - if necessary - be the edge sharpness, which generally corresponds to the contrast of the image. Alternatively, the optimization criterion can be, for example,The square sum of the representative size for the individual images of all image acquisition sensors should also be used, which should then be as minimal or maximum as possible, depending on the selected characteristic size.

[0014] “Direction of focus” refers to the direction of the adjustment available for focusing, in which the image capture unit and / or the distance between the image capture unit and the object is to be changed, for example whether the said distance is to be increased or decreased.

[0015] If an image capture sensor achieves optimal focus (taking into account an appropriate tolerance), the image capture sensor is considered "in focus." With such optimal focus, the area of ​​the object captured by the image capture unit lies in the object focal plane, and the image capture sensor lies in the image focal plane.

[0016] An image sensor is "defocused" when its focus cannot be considered in focus, even within a reasonable tolerance range. The distance of the image sensor from the image focal plane and / or the distance of the object from the object focal plane, at which optimal focus would be achieved, is considered the defocus and can therefore be expressed numerically. With optimal focus, the defocus is zero. A difference in defocus between two image sensors is referred to as "relative defocus."

[0017] A radiation or electromagnetic wave is "transformed" within the meaning of the invention when it is at least partially reflected, absorbed, and / or transmitted by an object. In microlithography masks, depending on their design, reflection and absorption, or transmission and absorption, usually occur together, depending on whether they are reflective or transmissive masks, in which the structure is represented by absorbing regions. Phase-shifting masks are also known.

[0018] The invention has recognized that by capturing at least two images by an image capture unit with two different image capture sensors, which fundamentally have a different defocus due to their arrangement, rapid refocusing is possible, even immediately during image capture. If a variable characteristic of the focusing of the individual image capture sensors is determined for the two images or at least parts thereof from the images captured by the image capture sensors, rapid refocusing can take place depending on the selected optimization criterion. With fundamental knowledge of the relationship between the representative variable and the focusing or defocus, a possible optimization criterion can then be the reduction of the defocus of an individual, possibly predetermined image capture sensor to zero. Alternatively, it is possible to use, for example, as an optimization criterion.The sum of squares of the representative value for all image acquisition sensors should also be considered, which should then ideally form an optimum—i.e., a maximum or a minimum. Instead of the aforementioned sum of squares, other optimization criteria that take all image acquisition sensors into account can also be used.

[0019] The characteristic variable of the images captured by the individual image sensors used for optimization can often be used not only to determine the fundamental need for refocusing—for example, whether all image sensors are defocused, whether the square sum of the characteristic variable for all image sensors exceeds or falls below a specified limit, or whether it has changed compared to a previously determined analog sum—but also to determine in which direction the focus of the image acquisition unit needs to be adjusted so that the selected optimization criterion is truly optimal again. For this purpose, the difference between the characteristic variables determined for the images of the various image sensors can be used, for example.

[0020] To achieve this, the representative size must have sufficient resolution for focusing, resulting in fundamentally different defocus values ​​for each image sensor. This resolution depends on various factors, such as the resolution of the image sensors, the relative defocus between the image sensors, and the intensity of the object's illumination. However, a specialist is easily able to coordinate the relevant factors in such a way that the required resolution for the representative size is achieved.

[0021] The contrast of the captured image, for example, can be used as a representative parameter for focusing. It can be assumed that, with optimal focusing of the image sensor, the contrast generally has a single maximum, or at least a clearly identifiable main maximum. For example, the root mean square across all or some of the pixels of a captured image or the standard deviation from the mean (both determined across all or some of the pixels of a captured image) can be used as a value for contrast. This parameter also has a single optimum or a clearly identifiable main optimum at which optimal focusing is achieved.The procedure described below as an example and for reasons of clarity exclusively for the contrast or its inverse as a representative quantity for focusing can also be directly applied to this quantity, as well as to alternative quantities reflecting the contrast.

[0022] If the relationship between a representative quantity and the focus is not so clear, for example, and exhibits more than one optimum, the procedure described below can still be implemented in principle with sufficiently precise knowledge of the relationship between the characteristic quantity and the focus. However, this may require more than two or three image acquisition sensors to determine the direction in which any necessary refocusing should occur with sufficient certainty.

[0023] If an image capture unit with two image capture sensors requires refocusing, which can also be derived, for example, from a change in the representative quantity(s) used to determine the optimal focus, the focus can be changed toward the image capture sensor for which, for example, the higher contrast was determined. This change can be continued until the contrast of the image capture sensor in question no longer changes (or decreases again, whereupon the focus can then be corrected slightly in the opposite direction). Alternatively, the change can be continued until a practically identical contrast is determined for both image capture sensors or until the square sum of the reciprocals of the contrasts of both image capture sensors is minimal.

[0024] If the image capture unit comprises three image capture sensors, which is preferred, if refocusing is necessary, the refocusing can be performed directly by changing the focus toward the image capture sensor with, for example, the higher or highest contrast. The degree of refocusing required can also be determined from the representative values ​​determined for the individual image capture sensors. A similar approach applies to image capture units with more than three image capture sensors.

[0025] The determination of the direction in which focusing is to occur, as described above by way of example, already enables the refocusing of a device for imaging micro- and / or nanostructures even during the actual image acquisition. Because the device according to the invention immediately knows the direction in which focusing is to occur when the need for refocusing is determined, following this specification immediately improves the focusing; further defocusing and thus a deterioration, for example, of the sharpness of the image acquisition are ruled out. The device according to the invention also allows the current focusing to be monitored during refocusing and to determine when optimal focusing has been achieved during refocusing.

[0026] However, it is preferred that the device not only enables the described refocusing during the actual image acquisition, but also performs it automatically. For this purpose, the device preferably comprises a control device configured to determine a characteristic variable for the focusing for each individual image from the image acquisition sensors and to adjust the focusing of the device in a direction derived from the characteristic variables, so that the focusing of a, preferably predetermined, image acquisition sensor or across all image acquisition sensors of the image acquisition unit is optimal. In the former case, the defocus of one or the predetermined image acquisition sensor is preferably zero; in the latter case, the sum of squares of the characteristic variable can be optimal for all image acquisition sensors.Possible control strategies for the control device to achieve this are outlined in the above explanations.

[0027] It is preferred if the distance between the image capture unit and the object can be changed for focusing purposes. It is irrelevant whether the image capture unit, the object, or even both can be moved appropriately to adjust the focusing. The distance between the image capture unit and the object can preferably be adjusted so precisely, particularly during automatic refocusing via a control device, that the minimal defocus already determined by the adjustment via the characteristic value for the focusing can be compensated for and, in particular, no further defocusing due to "overshooting" of the optimal focusing due to insufficient precision in the adjustment of the distance between the image capture unit and the object.

[0028] The image acquisition sensors of the image acquisition unit can be arranged perpendicular to a predetermined image focal plane of the image acquisition unit, offset from one another by a predetermined distance, which essentially results in a relative defocus between the image acquisition sensors. By changing the distance between the image acquisition unit and the object as described, the defocus of all image acquisition sensors is changed by changing the position of the object relative to the object focal plane corresponding to the predetermined image focal plane. The different distances of the image acquisition sensors from the image focal plane ensure a different defocus of the individual image acquisition sensors.

[0029] However, it is also possible for the image acquisition 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 acquisition sensors to be specified by a suitable tilt angle of the image acquisition unit relative to the object or its surface. Such a tilt means that the surface of the object is not in the object focal plane, so that at least for some of the sections of the surface captured by the individual image acquisition sensors, defocus generally results directly from the tilt angle. If the image acquisition sensors are arranged in a common plane, a single image acquisition chip can be provided instead of separate image acquisition sensors, with the individual image acquisition sensors then being formed by separate sub-regions of the image acquisition chip.

[0030] It is also possible to combine the above measures to generate relative defocus between the image acquisition sensors, but it is important to ensure that the measures - offset arrangement of the image acquisition sensors and tilting - do not cancel each other out.

[0031] It is preferred if the tilt angle is variable. By changing the tilt angle, the difference between the defocus of the individual image acquisition sensors can be changed, whereby it is particularly ensured that the distances are fundamentally changed uniformly. In this way, even under potentially unfavorable boundary conditions, it can be ensured that the characteristic variable for focusing two adjacent image acquisition sensors is fundamentally sufficiently different to carry out refocusing based on the different characteristic variables, if necessary. The tilt angle can be changed before image acquisition begins for a specific object. However, it is also possible for the tilt angle to be changed during image acquisition, e.g. by the control device. The control strategy can be used, for example,On the one hand, an attempt should be made to ensure a sufficient difference in the characteristic focusing parameters between the individual image capture sensors to determine the direction of any necessary refocusing, while on the other hand, this difference should be minimized as much as possible. The smaller the said difference, the less the defocusing of at least one non-optimally focused image capture sensor will be. The image captured by this image capture sensor can, despite the actual defocusing, still exhibit sufficient sharpness to be used for further inspection of the captured micro- and / or nanostructures of the object.

[0032] To simplify this, among other things, it is preferable for the object-side beam path of the image acquisition unit to be telecentric. Consequently, the images acquired by the individual image acquisition sensors generally have the same size despite the different defocus. Consequently, characteristic focusing parameters derived from the acquired images can be directly compared. In this case, the images can also be more easily combined into a single image—if necessary and despite the defocus of at least one of the images.

[0033] It is even preferred if the device comprises an image processing unit with which the images from the individual image capture sensors are combined into a single image. Whether the combination is achieved by superimposition or, for example, using a known "focus stacking method" is fundamentally irrelevant. By combining multiple images of the same object, even if they are at least partially not optimally focused, it is possible to incorporate as much image information as possible into the resulting image, which can, for example, further improve the contrast of the resulting image or reduce image noise.

[0034] It is fundamentally possible for all image capture sensors of the image capture unit to simultaneously capture images of the same area on the surface of the object. However, this generally requires complex optics between the object and the image capture sensors. However, it is preferred if the image capture unit is designed so that its image capture sensors each capture individual adjacent areas in an object plane, and if an object stage is provided for moving the object in the object plane in at least one direction, with which an object arranged on the object stage can be moved such that the image capture sensors can each successively capture an image of the same area of ​​the surface. In other words, the device can be suitably designed for "scanning" as is already fundamentally known from the prior art.

[0035] The image acquisition sensors can then be line sensors, i.e. sensors that only capture a single line of an image at a time, and a two-dimensional image is created by capturing each line step by step. However, it is preferred if the image acquisition sensors are designed as "time delay integration" sensors (TDI sensors). Such sensors can be understood as several line sensors arranged directly next to one another, in which, however, an image line captured by a first line sensor is passed on to a neighboring line sensor synchronously with the change in the position of the object relative to the sensor, so that the neighboring line sensor recaptures the same area depicted by the previously captured image line and adds it to the already captured image line in order to obtain a high-contrast image after passing through all the line sensors.Compared to a simple line sensor, a TDI sensor can achieve a comparably bright image through multiple exposures.

[0036] Although the device according to the invention can in principle also be designed for transmissive objects, in which case radiation passing through the object is detected by the image capture unit, it is preferred if the device is designed for reflective objects. In both cases, the illumination radiation ultimately detected by the image capture unit or its image capture sensors is transformed by the object—e.g., by partial reflection, partial absorption—in such a way that the micro- and / or nanostructure on the object is imaged on the image capture sensors.

[0037] It is particularly preferred if the illumination device and the image capture unit are designed for illumination radiation in the EUV range, i.e., for radiation with a wavelength of 5 nm to 30 nm, in particular 13.5 nm. Since only reflective optical elements are known for a corresponding wavelength, not only should the device be designed for reflective objects, but also all optical elements of the device should be designed to be reflective.

[0038] The invention will now be described by way of example using advantageous embodiments with reference to the accompanying drawings. They show: Fig. 1: a schematic representation of a first embodiment of a device according to the invention; Fig. 2: a schematic representation of a second embodiment of a device according to the invention; and Fig. 3: a schematic representation of the signals transmitted by the control devices of the devices according to Fig. 1 and Fig. 2 used relation between characteristic size and focus.

[0039] In Fig. Figure 1 schematically shows a first embodiment of a device 1 according to the invention for imaging micro- and / or nanostructures on an object 20. The object 20 to be imaged is a reflective microlithography mask with corresponding structures on its surface. Radiation incident on the object is generally reflected, with the incident radiation being partially transformed by the micro- and / or nanostructures, for example, by absorbing portions of the radiation.

[0040] Particularly when it is a microlithography mask, the object 20 can have an aspect ratio between 1:1 and 1:3, preferably between 1:1 and 1:2, particularly preferably 1:1 or 1:2. The object 20 can be substantially rectangular and is preferably 5 to 7 inches (12.70 to 17.78 cm) long and wide, more preferably 6 inches (15.24 cm) long and wide. Alternatively, the object 20 can be 5 to 7 inches (12.70 to 17.78 cm) long and 10 to 14 inches (25.40 to 35.56 cm) wide, and is preferably 6 inches (15.24 cm) long and 12 inches (30.48 cm) wide.

[0041] The device 1 comprises an illumination device 10, with which the object 20 can be sufficiently illuminated. The illumination device 10 is adapted to the object 10 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, so that the image acquisition unit 40 can capture the illumination radiation transformed by the object 20 and the illumination radiation thus captured actually yields image information about the surface of the object 20. For this purpose, the wavelength of the illumination radiation must be sufficiently low to actually be able to image the micro- and / or nanostructures. If the object 20 is, for example, a microlithography mask for microlithography in the EUV range, the illumination radiation of the illumination device 10 must generally also be in the EUV range, i.e.between 5 nm and 30 nm, preferably 13.5 nm, since only at this wavelength can the micro- and / or nanostructures typically found on corresponding microlithography masks be effectively imaged. Furthermore, the reflective properties of corresponding microlithography masks are optimized for these wavelengths.

[0042] The object 20, which is illuminated by the lighting device 10, is arranged on an object stage 30, with which the object 20 can be moved, in particular, in the direction indicated by the arrow 31.

[0043] For the actual detection of the micro- and / or nanostructures on the surface of the object 20, an image detection unit 40 is provided, with which the illumination radiation of the illumination device 10 reflected and transformed by the object 20 is detected in such a way that a two-dimensional image of the micro- and / or nanostructures on the surface of the object 20 is ultimately obtained.

[0044] For this purpose, the image capture unit 40 has an optical system 41 that is telecentric on the object side. The optical system 41 comprises exclusively mirrors as optical elements, which are designed to reflect the illumination radiation used, e.g., EUV radiation. For illumination radiation in wavelength ranges for which transmissive optical elements, such as lenses, are also known, the optical system 41 can also comprise such optical elements.

[0045] The image capture unit 40 comprises three image capture sensors 45, 46, 47. The three image capture sensors 45, 46, 47 are line sensors, each of which captures individual regions of the object 30 located one behind the other in the direction of movement of the object 20 indicated by the arrow 31. As the object 20 moves in direction 31 with the aid of the object stage 30, a specific region of the object 30 successively passes through the detection regions of the three image capture sensors 47, 46, 45.

[0046] One of the image capture sensors 46 defines the image focal plane 42 of the image capture unit 40, from which the object focal plane 43 is directly derived via the optics 41. The image capture unit 40 as a whole can be moved in a direction parallel to the incident radiation into the optics 41 (indicated by the double arrow 44), so that the distance between the object 20 and the image capture unit 40 can be changed. By changing the distance between the object 20 and the image capture unit 40, the image capture sensor 46 can be focused, among other things.

[0047] The two other image acquisition sensors 45, 47 are arranged offset perpendicularly from the image focal plane 42 defined by the image acquisition sensor 46 by a predetermined distance ±Δz, whereby this distance ±Δz cannot be changed. If the distance between object 20 and image acquisition unit 40 is changed by moving the image acquisition unit 40, this not only changes the focus of the image acquisition sensor 46 but also directly changes the focus of the image acquisition sensors 45, 47. The different distances from the image focal plane 42 ensure that a different defocus is achieved for each of the individual image acquisition sensors 45, 46, 47. The defocus of one of the image acquisition sensors 45, 46, 47 can of course also be zero, but then the other two image acquisition sensors inevitably have a defocus of ±Δz or ±2Δz.

[0048] The device 1 comprises an image processing unit 60 with which those images from the image capture sensors 45, 46, 47, which each image the same line-shaped region of the object 20, are combined into a single line-shaped image and, from a plurality of line-shaped images, a two-dimensional image is generated which reflects the surface of the object 20. Although a specific line-shaped region of the object 20 is not captured simultaneously by the individual image capture sensors 45, 46, 47, the movement of the object 20 in direction 31 allows the region in question to be captured successively by all image capture sensors 45, 46, 47.

[0049] Depending on the degree of defocus of the individual line-shaped images relative to one another, which is largely determined by the distance ±Δz, the images from the individual image acquisition sensors 45, 46, 47 can be directly superimposed or combined using a known "focus stacking method." The individual line-shaped images can then be combined into a two-dimensional image, as is common and well-known in scanning processes. This image can be transmitted via an interface 61 to external devices and units, e.g., to check the image for defects.

[0050] The device 1 further comprises a control device 50, which is connected to both the image acquisition unit 40 and the object stage 30 and controls at least their respective movements. The control of the object stage 30 and, if applicable, also the acquisition by the image acquisition sensors 45, 46, 47 are carried out according to the known scanning principle, so the relevant functioning of the control device 50 need not be explained in detail.

[0051] The control device 50 also receives the images captured by the image capture sensors 45, 46, 47 of the image capture unit 40 in order to control the focusing by adjusting the distance between the object 20 and the image capture unit 40 in the direction 44. The control strategy used for this purpose will be described later with reference to Fig. 3 in more detail, also because it is analogous to the embodiment according to Fig. 2 applies.

[0052] In the Fig. 2 schematically illustrated embodiment, individual components are similar to those in Fig. 1 or are even identical thereto. Components of the device 1 according to the invention, which are not explained in detail below, Fig. 2, reference is therefore made to the above comments.

[0053] The Fig. The device 1 shown in Figure 2 for imaging micro- and / or nanostructures comprises an illumination device 10 for illuminating the object 20 arranged on an object stage 30, a reflective microlithography mask with micro- and / or nanostructures on the surface. The illumination device 10, the object stage 30, and the object 20 are essentially identical to the devices shown in Figure 2. Fig. 1. However, the object stage 30 not only enables movement of the object 20 parallel to the object surface 21 (see, among others, arrow 31), but also in a direction perpendicular thereto, which ultimately also enables movement in a direction perpendicular to the object focal plane 43 of the image acquisition unit 40 described below, which is indicated by the double arrow 44.

[0054] In the image acquisition unit 40, the image acquisition sensors 45, 46, 47, which in this case are designed as TDI sensors, are arranged in a common image focal plane 42, which then, together with the optics 41, also results in the object focal plane 43. The optics 41 is analogous to that of Fig. 1 and is particularly telecentric on the object side.

[0055] Comparable to the embodiment according to Fig. 1, the three image acquisition sensors 45, 46, 47 capture fundamentally different areas of the object 20. However, by appropriate movement of the object 20 through the object stage 30 in the direction 31, an area on the object 20 is captured successively by all image acquisition sensors 45, 46, 47. The images thus captured by the image acquisition sensors 45, 46, 47 can be Fig. 1 known image processing unit 60 to form a two-dimensional image of the surface of the object 20 and made available via the interface 61.

[0056] The image acquisition unit 40 is arranged tilted relative to the object 20 or its surface 21, wherein the angle 49 between the surface normals of the surface 21 of the object 20 and the object focal plane 43 can be assumed as the angle for the tilt. The image acquisition unit 40 is arranged to be movable in the direction of the double arrow 49 in order to be able to change the tilt angle 48 if necessary. Since the three image acquisition sensors 45, 46, 47 capture different areas of the object, a different defocus for the individual image acquisition sensors 45, 46, 47 results due to the tilt angle 48 on the object side, although they are arranged in the common image focal plane 42. While the Fig. 2, the defocus shown as zero for the image sensor 46 remains constant regardless of the tilt angle 48, the relative defocus for the other two image acquisition sensors 45, 47 is ±Δz, where Δz is directly dependent on the tilt angle 48: the larger the tilt angle 48, the larger ±Δz is.

[0057] By moving the image acquisition unit 40 in direction 49 and the associated change in the tilt angle 48, the relative defocus ±Δz between the individual image acquisition sensors 45, 46, 47 can ultimately be changed.

[0058] The control device 50 is basically related to Fig. 1 already explained and the following based on the Fig. 3. However, to adjust the distance, it is not the image acquisition unit 40 that is moved, but rather the object stage 30. In particular, the control device 40 is designed to simultaneously effect a movement in or against the direction 31 when the object stage 30 is moved in direction 33, so that the sections of the surface 21 of the object 20 detected by the individual image acquisition sensors 45, 46, 47 do not change.

[0059] The control device is additionally designed to adjust the tilt angle 48 by moving the image acquisition unit 40 in the direction 49 so that the distance ±Δz and thus the defocusing of the two image acquisition sensors 45, 47 are, on the one hand, as small as possible 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 sufficiently large to be able to now, in particular, based on the Fig. 3 to enable refocusing.

[0060] In all the above-described embodiments, the control unit 50 is designed to determine characteristic variables for the focusing of the images captured by the different image capture sensors 45, 46, 47. Characteristic variables are preferably determined for images of the same area on the object 20 in order to exclude any impairment of the characteristic variables by different recorded structures of the object 20. The corresponding images are recorded in a device 1 according to Fig. 1 or Fig. 2 are not detected simultaneously, but in quick succession due to the known scanning process carried out by the control device 50.

[0061] In this case, the contrast K of the captured images is determined as a characteristic value for the focusing, whereby the images in question are line images due to the image capture sensors 45, 46, 47 used. Since the focusing in the devices 1 according to Fig. 1 and Fig. 2 occurs solely by changing the distance z between the image acquisition unit or an image acquisition sensor 45, 46, 47 and the object 20, the relationship between contrast and focus can be sketched as a one-dimensional curve. Such a curve is shown as an example in Fig. 3 in a contrast-distance diagram. While the position of the curve in the contrast-distance diagram and the specific shape of the curve may vary depending on various influencing factors, such as the illumination intensity and the specific configuration of the object 20 in the captured area, the contrast-distance curve generally exhibits a clear maximum, which occurs when one of the image acquisition sensors 45, 46, 47 is optimally focused.

[0062] Since the device according to the invention provides several, in particular three, image capture sensors 45, 46, 47, which each have a relative defocus to one another, at least one, in the present embodiments two image capture sensors 47, 46, 45 are defocused, even if one of the image capture sensors 45, 46, 47 is optimally focused, which is why the images of a specific area of ​​the object 20 captured by these image capture sensors 47, 46, 45 have a lower contrast than the image of the same area captured by the optimally focused image capture sensor 45, 46, 47.

[0063] In Fig. 3 shows, by way of example, the contrasts determined by the control device 50 for the three image acquisition sensors 45, 46, 47 at three different distances between the image acquisition unit 40 and the object 20.

[0064] At the Fig. 3, the contrast determined for the image captured by the image acquisition sensor 46 corresponds to the maximum contrast value. Consequently, the contrast values ​​for the images from the image acquisition sensors 45, 47 with relative defocus ±Δz are both necessarily lower. If only the contrast values ​​determined by the control device are available, it can be assumed, with a corresponding ratio of the contrast values ​​to one another, that the internal image sensor 46 is optimally focused at least within a certain tolerance, which also depends, among other things, on the magnitude of the relative defocus ±Δz.

[0065] In cases where the image capture sensor 46 is not optimally focused, the contrast value for the image captured by one of the other two image capture sensors 45, 47 is necessarily greater than the corresponding value for the image of the same area captured by the image capture sensor 46. This is in Fig. 3 (cf. point groups away from the three points at or next to the maximum). From the ratio of the contrasts determined in this way, it is not only possible to determine that the image acquisition sensor 46 is defocused, but also in which direction the distance between the image acquisition unit 40 and the object 20 needs to be changed in order to optimally focus the image acquisition sensor 46 again. If the course of the contrast-distance curve is precisely known or can be approximated with sufficient accuracy, the determined contrasts can be used to determine not only the direction of the change in distance required for focusing, but also to estimate the size of the required change in the distance between the image acquisition unit 40 and the object 20.

[0066] As an alternative to the described procedure, in which one of the image capture sensors 46 - in particular the image capture sensor 46 which lies in the image focus plane 42 of the image capture unit 40 - is always optimally focused, it is also possible to form the square sum of the contrast or the inverse of the contrast for the images of the individual image capture sensors (45, 46, 47) of the image capture unit (40) and to regulate the distance between the image capture unit 40 and the object 20 such that the square sum in question is maximized or minimized.

[0067] For devices 1 according to Fig. 1 and Fig.2, the control unit 50 is designed to effect the distance between the image capture unit 40 and the object 20 by changing the position of the image capture unit 40 or the object stage 30 accordingly, depending on the contrasts of the images of the same area on the object 20 captured by the individual image capture sensors 45, 46, 47, which contrasts are determined and evaluated as described above.

Claims

[1] Device (1) for the image acquisition of micro- and / or nanostructures, in particular structures on microlithography masks, comprising an illumination device (10) for illuminating an object (20) with illumination radiation and an image acquisition unit (40) for acquiring the illumination radiation transformed by the object (20), wherein the device (10) is designed to be focusable, characterized by , that the image acquisition unit (40) comprises at least two image acquisition sensors (45, 46, 47) arranged such that, regardless of the focusing of the device (1), at least one image acquisition sensor (45, 46, 47) is defocused. [2] Device according to claim 1, characterized by, that the device (1) comprises a control device (50) which is configured to determine a characteristic quantity for focusing for each of the individual images of the image acquisition sensors (45, 46, 47) and to adjust the focusing of the device (1) in a direction derivable from the characteristic quantities, so that the focusing of an image acquisition sensor (45, 46, 47) of the image acquisition unit (40) is as optimal as possible. [3] Device according to claim 2, characterized by , that the image acquisition sensor (45, 46, 47) used to optimize focus is predetermined. [4] Device according to claim 1, characterized by, that the device (1) comprises a control device (50) which is configured to determine a characteristic quantity for focusing for each of the individual images of the image acquisition sensors (45, 46, 47) and to adjust the focusing of the device (1) in a direction derivable from the characteristic quantities, so that the focusing is as optimal as possible when viewed across all image acquisition sensors (45, 46, 47) of the image acquisition unit (40). [5] Device according to any one of claims 2 to 4, characterized by , that the determined characteristic quantity is the contrast (K) of a captured image or the inverse thereof. [6] Device according to any one of claims 2 to 5, characterized by , that the optimal focus is achieved at an optimum of the sum of squares of the characteristic size for each of the image acquisition sensors (45, 46, 47) of the image acquisition unit (40). [7] Device according to any one of the preceding claims, characterized by , that the image acquisition unit (40) comprises at least three image acquisition sensors (45, 46, 47) spaced apart from each other perpendicular to an image focus plane (42) of the image acquisition unit (40). [8] Device according to any one of the preceding claims, characterized by , that the distance between the image acquisition unit (40) and the object (20) can be changed for focusing. [9] Device according to any one of the preceding claims, characterized by , that the image acquisition sensors (45, 46, 47) are fixedly arranged in the image acquisition unit (40), preferably in a common plane (48), wherein the relative defocus of the image acquisition sensors (45, 46, 47) to each other is determined by a preferably variable tilting angle (49) of the image acquisition unit (40) relative to the object (20). [10] Device according to any one of the preceding claims, characterized by, that the object-side beam path of the image acquisition unit (40) is telecentric. [11] Device according to any one of the preceding claims, characterized by , that the device (1) comprises an image processing unit (60) with which the images from the individual image acquisition sensors (45, 46, 47) are combined into a single image. [12] Device according to any one of the preceding claims, characterized by , that the image acquisition unit (40) is designed such that its image acquisition sensors (45, 46, 47) each detect individual adjacent areas in an object plane (31), and an object stage (30) is provided for moving the object (20) in the object plane (31) in at least one direction (31), with which an object (20) arranged on the object stage (30) can be moved in such a way that the image acquisition sensors (45, 46, 47) can successively each detect an image of the same area of ​​the surface (21) of the object (20). [13] Device according to any one of the preceding claims, characterized by , that the device (1) is designed for reflective objects. [14] Device according to claim 13, characterized by , that the lighting device (20) and the image acquisition unit (40) are designed for illumination radiation in the EUV range. [15] Device according to any one of the preceding claims, characterized by , that the image acquisition sensors (45, 46, 47) are TDI sensors.

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