Device for the image capture of structures on photolithography masks

Abstract

The invention relates to a device (1) for the image capture of structures on photolithography masks. In the device (1), during a scanning process, several image acquisition sensors (42) of an image acquisition unit (40) capture a region of a photolithography mask (20) arranged on an object stage (30) at staggered intervals. By changing the distance between the photolithography mask (20) and the image acquisition unit (40) during the scanning process, it is ensured that the captured images of the same region of the photolithography mask (20) are in different focus. Based on the defocus characteristics derived from the images, it is then determined whether refocusing of the device (1) is necessary, in particular whether the distance between the image acquisition unit (40) and the object stage (30) must be adjusted beyond the variation required to capture images with different focus.

Description

[0001] The invention relates to a device for the image capture of structures on photolithography masks.

[0002] Photolithography is used in the fabrication of microstructured components, such as integrated circuits. The photolithography process is carried out in a projection exposure system, which includes an illumination unit and a projection unit. The image of a mask (also called a "reticule") illuminated by the illumination unit is projected by the projection unit onto a substrate, such as a silicon wafer, coated with a photosensitive layer (called a "photoresist") and positioned in the image plane of the projection unit. This transfers the mask structure onto the photosensitive coating of the substrate. In subsequent manufacturing steps, the transferred structure is incorporated into the substrate, for example, by etching.

[0003] Even if the projection devices of a projection exposure system have a reduction factor of, for example, 4:1, the mask structures themselves must exhibit high accuracy due to the ongoing miniaturization in the semiconductor industry and the transition of the 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 possesses the desired properties and functionality, a mask is inspected using suitable methods in inspection and / or metrology facilities before being used in a projection exposure system.DE 10 2019 201 916 A1 discloses a method for inspecting nano- or microstructures of an object by comparing a plurality of optical images of the nano- or microstructures under investigation with a plurality of reference images of reference structures that are ideally identical to the nano- or microstructures under investigation. JP 2024 - 137 016 A discloses a sample inspection method and a sample inspection device.

[0004] In known inspection and 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 falls onto a sensor, whose sensor data can then be appropriately evaluated.

[0005] The sensor can be, for example, an imaging sensor, from whose sensor data (usually) two-dimensional images of a specific measurement quantity, namely radiation intensity, can be derived. An imaging sensor itself can be designed as a flat area and acquire all the measurements required for a specific image at once; however, it is also possible that an imaging sensor is designed only as a point or line sensor, i.e., as a point or line sensor, and thus can only determine parts of the measurements required for a desired image at once, while the remaining measurements required for the image must be acquired in subsequent steps, each after appropriate reorientation of the imaging sensor and / or the object, in order to ultimately obtain the desired image of the measurements.The described realignment of the imaging sensor and / or the object can also be implemented as a continuous relative movement of the two components to each other, whereby the sensor then practically continuously acquires data that can be combined to form a two-dimensional image. This process is also known as "scanning".

[0006] Based on the resulting images of objects, various checks can then be carried out – depending on the measured quantity recorded or depicted, or quantities derived from measured quantities. In the case of masks for semiconductor manufacturing, for example, an image that visually reflects the actual structure on the surface of the mask can 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's structure can also be taken from the image to compare them with target values ​​or to compare different measured quantities with each other.

[0007] Especially when capturing structures on photolithography masks, the necessary components must be arranged with high precision to obtain high sharpness in the measured values. In other words, it must be ensured that the measured value obtained for a specific point on the object reflects, as far as possible, only the measured value at that point and is not influenced by the corresponding values ​​of the surrounding points. This allows for the most accurate possible representation of the object's actual surface.

[0008] Especially when the sensor does not capture the entire surface of a photolithography mask being inspected at once, but rather captures it section by section or continuously, refocusing of the scanning device is regularly necessary, even during the actual capture process, to ensure that the resulting image is as sharp as possible across the entire captured area. If the structures of photolithography masks are to be imaged, even changes in the arrangement, particularly of the object and sensor, can lead to defocusing, which negatively impacts image sharpness. Such changes in the arrangement can occur, for example, due to thermal expansion of the components or the supporting structure. Deflection of the object, which can occur due to its mounting, can also necessitate refocusing during a capture process.

[0009] The object of the present invention is to create a device for the image capture of structures on photolithography masks, which enables refocusing during a scanning process.

[0010] This problem is solved 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 structures on photolithography masks, comprising a lighting device for illuminating a planar object arranged on an object stage with illumination radiation, an image capture unit for scanning illumination radiation transformed by means of the photolithography mask with at least two image capture sensors arranged in a common plane, and a control device for controlling a relative movement between the object stage and the image capture unit in a scan direction, wherein the device can be focused by changing the distance between the image capture unit and the object stage, wherein the control device is configured to change the distance between the image capture unit and the object stage from an initial distance during the relative movement in the scan direction between the object stage and the image capture unit in such a way as tothat images of the same area of ​​the object, successively captured by at least two different image capture sensors of the image capture unit, are focused differently; for the images of the same area of ​​the object from different image capture sensors, a quantity characteristic of the defocus is determined; and if a non-optimal initial distance is detected, the initial distance between the image capture unit and the object stage is adjusted in a direction derivable from the characteristic quantities based on the determined quantities characteristic of the defocus.

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

[0013] Radiation is "transformed" within the meaning of the invention when it is at least partially reflected, absorbed, and / or transmitted by an object. In photolithography masks, depending on whether they are reflective or transparent, with the structure represented by absorbing areas, reflection and absorption or transmission and absorption typically occur together.

[0014] A device is "focusable" if, by modifying the device, the image plane onto which an object is actually projected (e.g., by an image capture sensor) can be aligned with the image plane in which the object is projected with optimal sharpness (e.g., as determined by the lens equation). Aside from adjusting the distance between the lens and the image plane, as is common in photography, focusing can also be achieved by changing the distance between the object and the fixed lens-image plane system. This distance can also be an optical path length.

[0015] A device, or an image captured by that device, is "optimally focused" when its image capture sensor lies within the image plane of the device. The captured image is then "sharp." Conversely, a device, or an image captured by that device, is "defocused" when the image capture plane of the image sensor is located away from the image plane of the device.

[0016] The distance between the image capture plane and the image plane is called "defocus" and can be expressed numerically as a signed distance. The sign of the defocus indicates in which direction the device must be adjusted to achieve optimal focus. In devices with a movable image capture sensor relative to the lens, the sensor, which defines the image capture plane, can be moved into the image plane of the lens. In devices with a fixed arrangement of lens and image capture sensor, where the distance between the lens and the image capture plane is fixed, the distance between the lens and the object can be adjusted so that the image plane and the image capture plane coincide. When a device is optimally focused, the image plane lies in the image capture plane, and the defocus is zero.

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

[0018] The invention has recognized that in a device for the image capture of structures on photolithography masks, in which the image capture is carried out in the scanning process and at least individual areas of the object to be captured are captured by different image capture sensors at different times during the scanning process, differently focused images of the same area of ​​the object can be captured by changing the distance of the image capture unit and the object during the scanning process.

[0019] If at least two differently focused images of an area of ​​the object are available, the distance between the image acquisition unit and the object stage, or an object positioned on it, can be estimated based on parameters characteristic of the image defocus, which can be determined from the images themselves. This estimate determines the distance between the image acquisition unit and the object stage, or between the object stage and the object stage, relative to the respective distances at which the images are taken, for optimal focus or zero defocus. If this establishes the reference or initial distance from which the described variation of the distance between the image acquisition unit and the object stage is derived, the next step is to determine the optimal focus and the distance between the image acquisition unit and the object stage.If the object used to capture images with different focus is not optimal, it can be adjusted accordingly, whereby the characteristic sizes determined for the individual images can also be used to derive in which direction the initial distance should be changed to achieve the desired optimum, i.e., whether it should be decreased or increased.

[0020] If more than two differently focused images of the same area of ​​the object are available, for which characteristic values ​​for the defocus of the images can be determined, the direction in which an adjustment of the initial distance must be made can be determined particularly easily. Furthermore, at least an approximate order of magnitude of the required adjustment of the initial distance can usually be estimated.

[0021] The desired optimum for the initial distance can be achieved, for example, at the distance where there is zero defocus. Alternatively, a predefined or variable offset from this distance, dependent on other parameters, can be specified as the optimum. The initial distance can be adjusted during the scanning process, thus achieving the desired refocusing.

[0022] To ensure that at least individual areas of the object to be scanned are captured by different image capture sensors at different times during the scanning process, the image capture unit is provided to have at least two image capture sensors, whose respective image capture areas are preferably spaced apart from each other in the direction of the scan direction projection. Furthermore, the image capture areas should preferably overlap at least partially in a projection onto a plane perpendicular to the direction of the scan direction projection. "Direction of the scan direction projection" refers to the direction in which the image of the photolithography mask moves within the image capture unit when the object stage and the image capture unit are moved relative to each other in the scan direction.Such an arrangement ensures that, during the scanning process, at least part of the area of ​​the object captured by a first image acquisition sensor at a first point in time is captured by another image acquisition sensor at a second point in time due to the relative movement between the object stage and the image acquisition unit in the scanning direction. If more than two image acquisition sensors are provided, individual areas of the object can also be captured by more, in particular three or four, image acquisition sensors during the scanning process.

[0023] By arranging additional image acquisition sensors in a direction perpendicular to the scanning direction, the width of the area that the image acquisition unit can capture when the object moves in the scanning direction can be increased. Multiple image acquisition sensors, whose image acquisition areas are spaced apart from each other in the scanning direction, can also be arranged across the entire width. This not only increases the number of differently focused images available for adjusting the initial distance, or the overall size of the multiply captured area of ​​the object's surface, but also provides more image data for the preferential merging of the images captured by the image acquisition sensors into a single image.Even if at least some of the images exhibit a slight defocus, high-contrast images can still be achieved by combining them, even with low intensity of the transformed illumination radiation striking the image acquisition sensors. The control unit can be configured to combine the images captured by the various image acquisition sensors into a single image.

[0024] To obtain images with different focuses, the invention provides for changing the distance between the image acquisition unit and the object stage, or the object arranged thereon, during the scanning process. It is particularly preferred if the control device is configured to change the distance between the image acquisition unit and the object stage regularly and repeatedly by the initial distance. In other words, the distance should fluctuate repeatedly over time by an initial value, such as the initial distance, wherein the fluctuation is preferably periodic, undamped, and / or continuously differentiable. In particular, and especially preferably, the regular repetition can lead to a sinusoidal change in the distance over time by the initial distance.By using a corresponding sinusoidal change in distance, abrupt accelerations of the image acquisition unit and / or the object stage, which would have to be moved to change the distance in question, are avoided.

[0025] It is preferred that the period of the regularly repeating change in the distance between the image acquisition unit and the object stage is coordinated with a constant relative movement between the object stage and the image acquisition unit in the scan direction and with the arrangement of the image acquisition areas of the individual image sensors, such that the distance differs for at least two, preferably at least three, images of the same area of ​​the object captured by different image acquisition sensors. If the period of the regularly repeating change in the distance between the image acquisition unit and the object stage is adjusted accordingly, it is ensured that at least two or even at least three differently focused images are always available for each area captured by at least two or at least three image acquisition sensors during the scanning process.

[0026] To enable verification of suboptimal initial distances, the defocus characteristics should fundamentally differ between images taken with different focus points. Simultaneously, the defocus should be as small as possible for each individual image, allowing images depicting the same area of ​​the object to be combined into a single image with minimal loss of quality. The required variation in the distance between the image acquisition sensor and the object can be selected and defined, particularly depending on the surface structure being captured, which regularly influences the characteristic value and its changes during defocus. In the case of a regularly recurring change in the distance between the image acquisition unit and the object stage, or...It is preferred that the control device is designed in such a way that the amplitude of the regularly repeating change in the distance between the image acquisition unit and the object stage is adjustable so that the quantities characteristic for focusing are different for at least two, preferably at least three, images of the same area of ​​the object captured by different image acquisition sensors.

[0027] The amplitude can be selected to suit the object, i.e., in particular the structure on a photolithography mask, in order to obtain the smallest possible defocus that is simultaneously sufficient for the focusing according to the invention. For example, when inspecting masks for the EUV range, an amplitude of ±10 nm to ±100 nm can be selected, and for masks for the DUV range, ±20 nm to ±300 nm.

[0028] It is also possible that the amplitude is changed during the scanning process in order to best meet the aforementioned boundary conditions—namely, a detectable large change in the size characteristic of the defocus and the smallest possible change in distance—even with different surface structures. The control unit can be configured to determine and implement any necessary amplitude adjustments based on the measured sizes characteristic of the defocus in images of the same area of ​​the object.

[0029] To change the distance between the image acquisition unit and the object stage, or an object mounted on it, it is preferred if the object stage is movable in a direction perpendicular to the scan direction—and thus, generally, also perpendicular to the object or the photolithography mask. The image acquisition unit can then be fixed in place, which is advantageous with regard to the masses that vibrate, especially when the aforementioned distance is changed regularly.The distance between the image acquisition unit and the object can preferably be adjusted so precisely via the control device during automatic refocusing that the defocus already detected by the adjustment above the characteristic size for the defocus can be compensated for, but in particular no further defocusing occurs due to "overshooting" due to insufficient precision in adjusting the distance between the image acquisition unit and the object stage or object.

[0030] It is preferred that the device comprises at least three, preferably four, image acquisition sensors arranged perpendicular to the direction of the scan direction and at least partially overlapping in the direction of the scan direction – more precisely, in projection onto a plane perpendicular to the direction of the scan direction. With a corresponding number of image acquisition sensors, at least three differently focused images of the same area on the object can be acquired.If four image acquisition sensors arranged perpendicular to the direction of the scan direction and at least partially overlapping in the direction of the scan direction are provided, then, with an offset arrangement of the image acquisition sensors and the provision of several image acquisition sensors in a direction perpendicular to the direction of the scan direction, it is ensured that even in areas where two adjacent image acquisition sensors meet in a direction perpendicular to the direction of the scan direction and therefore no direct image acquisition is possible, three other image acquisition sensors adjacent in the direction of the scan direction are available to acquire three differently focused images.

[0031] A quantity derived from the contrast of a captured image can be used to characterize the defocus of that image. This can be either the contrast of the captured image itself or its inverse. The contrast can be assumed to exhibit only a single maximum (or a minimum, if its inverse is the contrast) within a certain range around the optimal focus. For example, the root mean square of the gradient across all or some pixels of a captured image, or the standard deviation from the mean (both calculated across all or some pixels of a captured image), can be used as a value for the contrast. This quantity also exhibits only a single optimum within a certain range around the optimal focus, at which optimal focus is achieved.The procedure described below, using contrast or its inverse as a representative quantity for focusing, as an example and for the sake of clarity, can be directly applied to this quantity as well as to alternative quantities reflecting contrast.

[0032] If the relationship between a characteristic parameter and the defocus has more than one optimum, the procedure described below can still be carried out in principle, provided the relationship between the characteristic parameter and the defocus is known with sufficient accuracy. However, this may require more than two or three differently focused images of the same area in order to determine with sufficient certainty the direction in which any necessary adjustment of the initial distance should be made.

[0033] If, based on two differently focused images, it is determined that the initial distance needs to be adjusted—for example, because both characteristic values ​​determined for these images deviate by a predetermined amount from the characteristic values ​​previously determined during the scanning process, or because the difference between the two characteristic values ​​exceeds a predetermined amount—the initial distance can be adjusted towards the distance at which the image with the greater deviation of its characteristic value from a previously determined value, or at which the image with the larger characteristic value was captured. This adjustment of the initial distance can be continued, for example, until practically identical defocus characteristic values ​​are determined for the two captured differently focused images, or until the sum of the squares of the reciprocals of these values ​​is minimal.

[0034] If three differently focused images of the same area are available, the need to adjust the initial distance can be determined in a comparable way. Depending on the chosen characteristic value for the defocus, an optimal initial distance may exist if the characteristic size of the image at the average distance of the three distances at which the three images were captured is larger (or smaller) than the characteristic sizes of the other two images.

[0035] Regardless of how many differently focused images of an area are available, it is preferable to vary the initial distance until the sum of the squares of the characteristic values ​​for the images of the same area of ​​the object from different image acquisition sensors is optimal. Whether a minimum or a maximum of the sum of squares represents the optimum depends on the characteristic value used to calculate the sum of squares.

[0036] The determination of the direction in which focusing must occur, as described above, enables the refocusing of a device for the image capture of structures on photolithography masks even during the actual image capture process. In the device according to the invention, when the need for an adjustment of the initial distance between the image capture unit and the object stage or a photolithography mask arranged thereon is identified, the direction in which this change must occur is immediately known. Following this guideline results in an immediate improvement in focusing; further defocusing and thus a deterioration, for example, in the sharpness of the image capture, are, however, prevented.The device according to the invention further allows the current focus to be monitored during refocusing and enables the determination of when the optimal distance between the image acquisition unit and the object stage or photolithography mask has been reached during refocusing. It is irrelevant that the scanning process continues during refocusing; that is, the repeated determination and verification of the parameters characteristic of defocusing is not always performed for the same area of ​​the photolithography mask, but rather based on the areas currently located within the image acquisition range of the individual image acquisition sensors.

[0037] The image acquisition sensors can be line sensors, meaning sensors that capture only a single line of an image at a time, and a two-dimensional image is created through stepwise, line-by-line acquisition. However, it is preferred if the image acquisition sensors are designed as "Time Delay Integration" (TDI) sensors. Such sensors can be understood as several line sensors arranged directly next to each other, where an image line captured by a first line sensor is passed to an adjacent line sensor synchronously with the change in the object's position relative to the sensor. This allows the adjacent line sensor to capture the same area represented by the previously captured image line again and add it additively to the already captured image line, thus obtaining a high-contrast image after all line sensors have passed through.In comparison to a simple line sensor, a TDI sensor allows for a comparably bright image to be achieved through multiple exposures, but the object only needs to remain in a specific position for a shorter time each time, so that the time required to scan an object can be reduced by using TDI sensors.

[0038] It is preferred if the object-side beam path of the image acquisition unit is telecentric. Consequently, the images captured by the individual image acquisition sensors, despite different focus points, generally have the same size or image scale. As a result, characteristic focus parameters derived from the captured images can be directly compared. Furthermore, in this case, the images can be more easily combined into a single image, if necessary and despite defocusing in at least one of them.

[0039] The device according to the invention can, in principle, be designed for transmissive objects, in which case radiation passing through the object is detected by the image acquisition unit. The device can also be designed for reflective objects. In both cases, the illumination radiation ultimately detected by the image acquisition unit or its image acquisition sensors is transformed by the object – for example, by partial reflection and partial absorption – such that the structure on the object is imaged onto the image acquisition sensors.

[0040] It is particularly preferred if the illumination device and the image acquisition unit are designed for illumination radiation in the EUV range, i.e., for radiation with a wavelength of 5 nm to 30 nm, especially 13.5 nm. Since only reflective optical elements are known for a corresponding wavelength, not only must the device be designed for reflective objects, but also all optical elements of the device must be designed to be reflective.

[0041] The refocusing method described above, which occurs during the scanning process, is based on determining parameters characteristic of defocus from captured images. However, particularly when images are captured of an area of ​​the object where the object has no structure, such as the "black border" of masks used in photolithography, corresponding parameters, such as contrast, cannot be determined, or no differences in these characteristic parameters can be observed between differently focused images. To enable focusing of the device in such situations, a distance sensor can be provided. This sensor determines the distance between the image acquisition unit and the object stage or the object itself in the area where the image acquisition sensors of the image acquisition unit are located.The distance between the image capture unit and the object stage or object can then be adjusted so that the measured distance corresponds to a predetermined distance, which can be derived, for example, from the lens equation. Focusing based on the distance sensor is often sufficient to assume that a focus point with optimal contrast found near this focus point is the optimal focus, since there is only one optimum for contrast in this range, namely at the optimal focus.

[0042] It is also possible to provide more than one distance sensor, the distances of which can be combined into a single distance value with increased accuracy after suitable plausibility checks. If more than one distance sensor is provided, it is preferred to use at least two different types of distance sensors. A distance sensor can be a confocal sensor, a white light interferometer, a laser interferometer, and / or a capacitive sensor.

[0043] The photolithography mask to be captured by the device can have an aspect ratio between 1:1 and 1:3, preferably between 1:1 and 1:2, and particularly preferably 1:1 or 1:2. The photolithography mask can be substantially rectangular. The photomask can preferably be 5 to 7 inches long and wide, and particularly preferably 6 inches long and wide. Alternatively, the photomask can be 5 to 7 inches long and 10 to 14 inches wide, and preferably 6 inches long and 12 inches wide.

[0044] The invention will now be described by way of example with reference to advantageous embodiments and the accompanying drawings. These show: Fig. 1: a schematic representation of an embodiment of a device according to the invention; Fig. 2: a schematic representation of the arrangement of the image acquisition sensors of the device Fig. 1; and Fig. 3: a schematic representation of the dependence of the contrast of captured images on the distance between the image capture unit and the object stage of the device Fig. 1.

[0045] In Fig. Figure 1 schematically illustrates an embodiment of a device 1 according to the invention for the image capture of structures on a photolithography mask 20. The photolithography mask 20 to be imaged is a reflective photolithography mask 20 with corresponding structures on its surface. Radiation incident on the object is generally reflected, with the incident radiation being partially transformed by the structures on the photolithography mask 20, for example, by absorbing parts of the radiation. The device 1 comprises a lighting device 10 with which the photolithography mask 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 photolithography mask 20 or at least parts of the structures on it, so that the image acquisition unit 20 can detect the illumination radiation transformed by the photolithography mask 20 and actually obtain image information about the surface of the photolithography mask 20 from the illumination radiation thus detected. For this to occur, the wavelength of the illumination radiation must be sufficiently short to actually image the structures on the photolithography mask 20. If the photolithography mask is in the EUV range for photolithography, the illumination radiation of the illumination device 10 should also regularly be in the EUV range, i.e.,The wavelengths should be between 5 nm and 30 nm, preferably at 13.5 nm, since only at such a wavelength can the structures typically found on corresponding photolithography masks 20 actually be imaged. Furthermore, the reflective properties of the corresponding photolithography masks are optimized for these wavelengths.

[0046] The photolithography mask 20, which is illuminated by the lighting device 10, is arranged on an object stage 30, which allows the photolithography mask 20 to be moved, in particular in the scanning direction indicated by the arrow 31. Furthermore, the object stage 30 can be moved in the direction indicated by the double arrow 32, perpendicular to the scanning direction 31.

[0047] For the actual capture of the structures on the surface of the photolithography mask 20, an image capture unit 40 is provided, with which the illumination radiation of the illumination device 10 reflected and transformed by the photolithography mask 20 is captured in such a way that a two-dimensional image of the structures on the surface of the photolithography mask 20 is ultimately obtained.

[0048] The image acquisition unit 40 includes 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. An example optical system consisting of two mirrors is shown here. However, any other number of mirrors is also conceivable, in particular three, four, or six mirrors. For illumination radiation in wavelength ranges for which transmissive optical elements, such as lenses, are known, the optical system 41 can also include such optical elements.

[0049] The image acquisition unit 40 comprises several image acquisition sensors 42, which are configured as TDI sensors. As shown in the schematic top view of the image acquisition sensors 42 in Fig. As can be seen in section 2, the image acquisition sensors are arranged in four perpendicular directions. Fig. The rows 43 running in the direction indicated by the scan direction 31 are offset and arranged overlapping in a projection onto a plane 45 perpendicular to the scan direction 31, such that an area of ​​the photolithography mask 20 is successively captured by three or four image capture sensors 42. The area in which at least three images of the object are successively captured in the direction of the scan direction 31 is indicated as the total capture area 44 of the image capture unit 40. The sub-areas in which only three images can be captured are indicated by the brackets 45'. In the areas not marked by the brackets 45', four images can be captured.

[0050] The arrangement of the image acquisition sensors 42 in rows 43 results, as shown in Fig. Figure 1 illustrates that the rows 43 of image acquisition sensors 42 each capture individual areas of the object 30 in the direction of movement of the photolithography mask 20 indicated by the arrow 31. As the photolithography mask 20 is moved by the object stage 30 in the direction 31, a specific area of ​​the object 30 successively passes through the capture areas of the rows 43 of the image acquisition sensors 42 on the photolithography mask 20.

[0051] The image acquisition unit 40 further comprises a distance sensor 46 for determining the distance between the image acquisition unit 40 and the object stage 30 or the photolithography mask 20 arranged thereon in the area where the image acquisition areas of the image acquisition sensors 42 of the image acquisition unit 40 are located. The distance sensor 46 is a laser interferometer. In situations where the refocusing option of the device 1 described below is not available, e.g., because the images captured by the image acquisition sensors 42 have no contrast, as they depict, for example, the "black border" of the photolithography mask 20, the distance determined by the distance sensor 46 can be used to move the object stage 30 in the direction 32 until the measured distance corresponds to a predetermined value.The specified value preferably corresponds to an initial distance near which the optimal distance, at which the defocus is zero, is located or is presumed to be.

[0052] This and the following regulation is carried out in device 1 by the control device 50. The control device 50 is connected to both the image acquisition unit 40 and the object platform 30 and can, in particular, control the movement of the object platform 30 in both the scan direction 31 and the direction 32. The control of the object platform 30 in the scan direction 31, as well as, if applicable, the acquisition by the image acquisition sensors 42, is carried out according to the known principle of scanning, so that the corresponding operation of the control device 50 does not need to be explained in more detail.

[0053] The control device 50 is specifically designed to combine the images from the individual image acquisition sensors 42, which are acquired during the entire scanning process of the photolithography mask 20, into a single two-dimensional image of the surface of the photolithography mask 20, as is typical in scanning processes. This image can be transmitted via an interface 51 to external devices and units, e.g., for checking the image for defects.

[0054] The control device 50 is further configured to determine a characteristic value for the defocus of each image captured by the image acquisition sensors 42. In this case, the contrast of each captured image is determined as the characteristic value, which is known to be at its maximum when the defocus is zero, provided it can be determined (see above).

[0055] The control device 50 is designed to change the position of the object stage 30 in direction 32 perpendicular to the scan direction 31 during the scanning process by regularly repeating a movement of the object stage 30 in the scan direction 31 at a constant speed, starting from an initial value z0, in the form of a sinusoidal oscillation with amplitude Δz. Since the image acquisition unit 40 is stationary, this results in a correspondingly regularly repeating change in the distance between the image acquisition unit 40 and the object stage 30, or rather, the photolithography mask 20 arranged on it. This distance also subsequently changes sinusoidally around an initial distance.

[0056] By changing the Z-position of the object stage 30 and thus the distance between image acquisition unit 40 and object stage 30 or photolithography mask 20, the position of the image plane 47 in the image acquisition unit 40 also changes (in Fig. 1 represented by planes 47'). Due to the fixed arrangement of the image acquisition sensors 42 in the image acquisition unit 40, the image plane 47 is temporarily located in the image capture plane formed by the image acquisition sensors 42 (zero defocus) or a signed distance results between the image plane 47 and the said image capture plane, i.e. a defocus.

[0057] The frequency of the regularly repeating change in the distance between the image acquisition unit 40 and the object stage 30, starting from the initial distance, is synchronized with the speed of movement in the scan direction 31 such that at least two images of the same area of ​​the photolithography mask 20, which are captured at different times by different image acquisition sensors, are in different focus, i.e., they exhibit different defocus at the time of capture. The frequency can be selected such that of the areas covered by four image acquisition sensors (see...). Fig. 2) always at least three differently focused images are available. This also automatically ensures that the areas of the photolithography mask 20, which are only covered by three image capture sensors 42 during the scanning process (see Fig. 2) at least two differently focused images are available.

[0058] The differently focused images of an area of ​​the photolithography mask 20 can be used to refocus the device 1 by adjusting the described initial distance. The contrast, determined for each image as a characteristic of the defocus, allows the user to determine whether the initial distance is optimally chosen. The contrast is maximum at zero defocus and decreases with increasing defocus. For example, if, with three differently focused images, the contrast for the image where the distance between image acquisition unit 40 and object stage 30 (or photolithography mask 20) ​​is the same as the distance between the other two images, is not greater than the contrast of the other two images, then the initial distance is not optimally chosen.

[0059] In Fig. Figure 3 illustrates how the contrast of captured images of a (constant) area changes when the distance between image capture unit 40 and object stage 30 or photolithography mask 20 is regularly varied by an initial distance. The distance in question, along with its change over time, plotted along the x-axis, is represented by line 90.

[0060] In the middle diagram in Fig. In step 3, where the initial distance is optimally chosen, hardly any changes in the contrast, which is represented as line 91, can be seen.

[0061] If, however, the initial distance is chosen too small, the following results: Fig. 3. Image shown above, where the contrast (line 91) is inversely proportional to the regularly repeating and changing distance (line 90).

[0062] If the initial distance is too large, this results in Fig. 3. Image shown below. In this case, too, the contrast (line 91) changes depending on the regularly repeating distance (line 90), but in this case, the changes are constant.

[0063] Due to the regularly repeating change in the z-position of the object stage 30 and the resulting change in the distance between the image acquisition unit 40 and the photolithography mask 20, the multiple image acquisition sensors 42 generally produce three or four images for a given area of ​​the surface of the photolithography mask 20, of which at least two or at least three are focused differently. The differently focused images can be represented as specific points on the x-axis of the diagrams. Fig.3. From the relative positions of the contrasts for the individual images to each other in connection with the distance between image acquisition unit 40 and object stage 30 at the respective time of acquisition, it can be deduced whether and, if so, in which direction the initial distance must be changed, i.e., reduced or increased.

[0064] The defocus of the individual images of the same area of ​​the object should be sufficiently large to allow a difference in contrast to be detected. At the same time, the defocus should be as small as possible to achieve the highest possible contrast when the individual images are combined into a single image. The control device 50 can be configured to adjust the amplitude ±Δz during the scanning process to meet the aforementioned conditions. Alternatively, the amplitude ±Δz can be fixed. For example, photolithography masks for the EUV range can have an amplitude of ±10 nm to ±100 nm, while masks for the DUV range can have an amplitude of ±20 nm to ±300 nm.

Claims

[1] Device (1) for the image capture of structures on photolithography masks, comprising a lighting device (10) for illuminating a photolithography mask (20) arranged on an object stage (30) with illumination radiation, an image capture unit (40) for scanning illumination radiation transformed by means of the photolithography mask (20) with at least two image capture sensors (42) arranged in a common plane, and a control device (50) for controlling a relative movement between the object stage (30) and the image capture unit (40) in a scan direction (31), wherein the device (1) is focusable by changing the distance between the image capture unit (40) and the object stage (30), wherein the control device (50) is configured toStarting from an initial distance, the distance between the image acquisition unit (40) and the object stage (30) is to be changed during the relative movement in the scan direction (31) between the object stage (30) and the image acquisition unit (40) such that images of the same area of ​​the photolithography mask (20) captured successively by at least two different image acquisition sensors (42) of the image acquisition unit (40) are focused differently, a quantity characteristic of the defocus is determined for each of the images of the same area of ​​the photolithography mask (20) from different image acquisition sensors (42), and if a non-optimal initial distance is detected, the initial distance between the image acquisition unit (40) and the object stage (30) is adjusted in a direction derivable from the characteristic quantities based on the determined quantities characteristic of the defocus. [2] Device according to claim 1, characterized by, that the control device (50) is designed to change the distance between the image acquisition unit (40) and the object stage (30) regularly and repeatedly by the initial distance. [3] Device according to claim 2, characterized by , that the period of the regularly repeating change of the distance between the image acquisition unit (40) and object stage (30) is such that the distance is coordinated with a constant relative movement between the object stage (30) and the image acquisition unit (40) in the scan direction (31) and the arrangement of the image acquisition areas of the individual image sensors (42) is such that the distance is different for at least two images of the same area of ​​the photolithography mask (20) captured by different image acquisition sensors (42). [4] Device according to claim 3, characterized by, that the distance is different for at least three images of the same area of ​​the photolithography mask (20) captured by different image acquisition sensors (42). [5] Device according to claims 2 to 4, characterized by , that the amplitude of the regularly repeating change of the distance between the image acquisition unit (40) and object stage (30) can be adjusted by the control device (50) such that the quantities characteristic for focusing are different for at least two, preferably at least three images of the same area of ​​the photolithography mask (20) captured by different image acquisition sensors (42). [6] Device according to any one of the preceding claims, characterized by , that the object stage (30) can be moved in a direction (32) perpendicular to the scan direction (31) to change the distance between the image acquisition unit (40) and the object stage (30). [7] Device according to any one of the preceding claims, characterized by , that the device (1) comprises at least three image acquisition sensors (42) arranged perpendicular to a direction of the imaging of the scan direction (31). [8] Device according to any one of the preceding claims, characterized by , that the device (1) comprises four image acquisition sensors (42) arranged perpendicular to a direction of the imaging of the scan direction (31). [9] Device according to any one of the preceding claims, characterized by , that the image acquisition areas of the image acquisition sensors (42) are spaced apart from each other in the direction of the imaging of the scan direction (31). [10] Device according to claim 9, characterized by , that the image acquisition areas of the image acquisition sensors (42) are arranged at least partially overlapping in a projection onto a plane perpendicular to the image of the scan direction (31). [11] Device according to any one of the preceding claims, characterized by , that the determined characteristic quantity is a quantity derived from the contrast (K) of a captured image. [12] Device according to any one of the preceding claims, characterized by , that the control device (50) is designed to adjust the output distance such that the sum of the squares of the characteristic quantities for the images of the same area of ​​the photolithography mask (20) from different image acquisition sensors (42) is optimal. [13] Device according to any one of the preceding claims, characterized by , that the device includes a distance sensor (46) for determining the distance between the image acquisition unit (40) and the object stage (30) or a photolithography mask (20) arranged on the object stage (30) in the area in which the image acquisition areas of the image acquisition sensors (42) of the image acquisition unit (40) are arranged. [14] Device according to any one of the preceding claims, characterized by , that the image acquisition sensors (42) are TDI sensors. [15] Device according to any one of claims 1 to 14, characterized by , that the image acquisition sensors (42) are line sensors. [16] 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. [17] Device according to any one of the preceding claims, characterized by that the device (1) is designed for reflective photolithography masks (20). [18] Device according to any one of the preceding claims, characterized by , that the lighting device (10) and the image acquisition unit (40) are designed for illumination radiation in the EUV range. [19] Device according to any one of the preceding claims, characterized by, that the control device (50) is designed to combine the images captured by the various image acquisition sensors (42) into a single image.

Images