Method and device for the inspection of semiconductor substrates

Abstract

The invention relates to a method for the inspection of semiconductor substrates, and to a device (1) designed for carrying out this method. The method for the inspection of semiconductor substrates using a device (1) for the image-based capture of structures on semiconductor substrates, the device for the image-based capture being designed for capturing at least one region of a semiconductor substrate with a different focus position, comprises the following steps: a) capturing a region of a semiconductor substrate with a first focus position; b) capturing the region of the semiconductor substrate with a second focus position deviating from the first focus position; and c) comparison of the two captured images with different focus positions for the purpose of determining defects in the captured region of the semiconductor substrate. The device (1) is designed for carrying out the method.

Description

28.10.2025 / BRMethod and device for the inspection of semiconductor substrates

[0001] The invention relates to a method for the inspection of semiconductor substrates, and to a device designed for carrying out this method.

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

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

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

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

[0006] On the basis of image representations of obj ects obtained in this way, it is then possible - depending on the measurement variable captured therefor or represented therein or depending on the variables derived from measurementvariables - to carry out various checks . In the case of masks for semiconductor fabrication, an image representation constituting an image-based reflection of the actual structure on the surface of the mask can be compared e . g . with an image representation of a target structure in order thereby to be able to recogni ze possible defects in the mask .

[0007] Defects determined in this way are regularly so-called amplitude defects , i . e . defects that locally change the light intensity captured on the image representation, which can then result in deviations in relation to the image representation of the target structure . For example , a foreign body on a mi- crolithographic mask may be imaged during capture with a signi ficantly higher or else a signi ficantly lower light intensity than the actual structure of the microlithographic masks at the location of the foreign body, which becomes noticeable in a comparison with the image representation of the target structure , where the light intensity at the location in question actually corresponds to that of the actual structure of the microlithographic masks .

[0008] In addition to amplitude defects , phase defects are also of signi ficance in photolithography during the production of microstructured components . Corresponding phase defects , in which the phase inter alia of the exposure radiation emanating from an illuminated microlithographic mask is inadvertently changed locally, may lead to imaging aberrations e . g . on account of interferences that occur only as a result of these defects . Such phase defects may be caused e . g . by virtue of the fact that in the case of reflective masks , a reflective surface is a few nanometres higher or lower than intended . In the case of transmissive masks , a local deviation of the refractive index from the target value may lead to a phase defect .

[0009] Even though in practice almost all real defects on a microlithographic mask each have both an amplitude defect portion and a phase defect portion, the methods known from the prior art for the inspection of microlithographic masks primarily serve for recogni zing amplitude defects .

[0010] Microstructured components produced with the aid of photolithography can likewise be examined in the course of an inspection for possible defects using the methods explained above for microlithographic masks . Here as well , however, the known methods primarily serve for recogni zing amplitude defects .

[0011] It is an obj ect of the present invention to provide a method for the inspection of semiconductor substrates in which phase defects can also be recogni zed, and a device suitably designed for this purpose .

[0012] This obj ect is achieved by a method according to Claim 1 and a device according to Claim 11 . The dependent claims relate to advantageous developments .

[0013] Accordingly, the invention relates to a method for the inspection of semiconductor substrates using a device for the image-based capture of structures on semiconductor substrates , the device for the image-based capture being designed for capturing at least one region of a semiconductor substrate with a di f ferent focus position, the method comprising the following steps : a ) capturing a region of a semiconductor substrate with a first focus position; b ) capturing the region of the semiconductor substrate with a second focus position deviating from the first focus position; andc) comparison of the two captured images with different focus positions for the purpose of determining defects in the captured region of the semiconductor substrate.

[0014] Furthermore, the invention relates to a device for the image-based capture of structures on semiconductor substrates, comprising an illumination device for illuminating an object with illumination radiation, an image capturing unit for capturing the illumination radiation transformed by the object (20) , and a control device, the device being configured in a focusable fashion, the image capturing unit (40) comprising at least two image capturing sensors (45, 46, 47) arranged in such a way that they have a different focus position independently of the focusing of the device (1) , and a control device being designed to suitably control the device (1) for carrying out a method according to the invention and carry out the comparison of the images captured by the image capturing sensors

[0015] Firstly, some terms used in connection with the invention are explained.

[0016] "Semiconductor substrate" denotes in principle that substrate which can be examined with the aid of the method according to the invention. The term encompasses in particular microlithographic masks and / or microstructured substrates or components produced with the aid of photolithography, in particular .

[0017] Radiation is "transformed" within the meaning of the invention if it is at least partially reflected, absorbed and / or transmitted by an object. In the case of photolithographic masks, depending on the configuration as a reflective or transmissive mask, in which the respective structures arerepresented by absorbent regions, in general reflection and absorption or transmission and absorption occur jointly.

[0018] A device for the image-based capture of structures on semiconductor substrates is suitably designed for capturing images of semiconductor substrates, for which purpose the device must necessarily have a sufficient resolution to render the structures of the semiconductor substrates in a predefined degree of detail.

[0019] In the context of the invention, "focusing" denotes the adaptation of the image capturing unit and / or of the distance between image capturing unit and object (and thus in general the length of the object-side beam path) , by which the image representation of the object becomes as optimal as possible with regard to a predefined criterion on a specific one or all of the image capturing sensors. In this respect, "optimal focusing" may denote e.g. the state in which, as optimization criterion, the sharpness on a predefined image capturing sensor is maximal. In this context - if necessary - the edge sharpness may be used as "sharpness".

[0020] "Focus position" denotes the distance between the image plane and the image capture plane of an image capturing sensor of the device for the image-based capture of structures on semiconductor substrates during corresponding image-based capture. Depending on the configuration of the device, the focus position can be influenced by varying the distance between the device and the semiconductor substrates and / or by settings on the device itself.

[0021] In the event of the distance in question and hence the focus position being equal to zero, the image plane and the image capture plane of the image capturing sensor coincide. The device is then deemed to be optimally focused with regardto the image capturing sensor in question as well. In the case of a focus position deviating from zero, a captured image is "defocused". In the not equal to zero case, a focus position or the distance between the image plane and the image capture plane of an image capturing sensor is also referred to as "defocus". The defocus can be expressed in a signed fashion, the sign indicating the direction in which the focus position or the distance exists, proceeding from the image plane or the image capturing plane.

[0022] The invention has recognized that during the imagebased capture of the same region of a semiconductor substrate with a different focus position, defects on the semiconductor substrate can be determined from the differences between The two captured images that are ascertainable during a comparison .

[0023] Different types of defects can be determined depending on the configuration of the comparison. Since phase defects, however, can be determined particularly well from the imagebased capture with different focus positions, the comparison is preferably designed for determining precisely such phase defects .

[0024] For this purpose, the comparison can comprise difference formation, in particular, which involves forming in particular the individual difference between the optical values determined for each image point, such as e.g. the light intensity captured for this image point, such that the differences in question yield image points of a difference map. Depending on the focus position of the two captured images, it is thus possible to determine locations on the semiconductor substrate at which different difference values are present, even though identical difference values should actually be expected onaccount of the predefined structure of the semiconductor substrate .

[0025] It is particularly preferred if the first focus position comprises a defocus from the optimal focusing and the second focus setting comprises a defocus from the optimal focusing that is equal to the defocus of the first focus position in terms of absolute value. Since the two focus positions are furthermore intended to be different, what directly becomes apparent is that The two focus positions lie on different sides of the optimal focusing. Since phase defects are mirrored in captured images in a signed fashion depending on the sign of a defocus, while defect-free regions are independent of the sign of the defocus, difference formation as described above makes it possible to create a difference map that ultimately records only the potentially defective regions in the captured region of the semiconductor substrate; at the regions which are free of phase defects, the result is a difference of zero.

[0026] It has been found that in particular in the case of defocuses that are identical in terms of absolute value, but have different signs, for the first and second focus positions, the respective defocus preferably has a length amounting to a proportion of approximately 1 / 8 to approximately 1, preferably of approximately 1 / 4 to approximately 1 / 2, of the Rayleigh length of the radiation used for the exposure of the semiconductor substrate that is used for capturing the images.

[0027] As evident from the explanations above, potential phase defects can already be identified directly from the comparison of The two captured images, without further data being needed. In principle, however, it is also possible, of course, to compare the result of a comparison of the two captured images with target value specifications which e.g. also arise fromdesign data of the semiconductor substrate and / or simulations ( "Die-to-Database" procedure ) .

[0028] Especially i f no target values , limit values or the like are available for the evaluation of the result of a comparison of two captured images of the same region with a di fferent focus position, it is possible , and optionally even preferred, i f in a further region comparable to the region with regard to the structures of the semiconductor substrate , the semiconductor substrate is subj ected to image-based capture with two di f ferent focus positions , and the comparison comprises di f ference formation applied to comparisons carried out for each of The two regions from the respectively captured images with di f ferent focus positions . In other words , for each of The two regions in each case , as described above , a comparison of two images of these regions captured with a di fferent focus position is intended to be determined, and then the di f ference is formed from the results of these two comparisons . I f a di f ference is actually present , this indicates - especially i f it lies above a predefined tolerance threshold - a phase defect in one of The two captured regions . The procedure is thus similar to the known "Die-to-Die" mask inspection method for determining amplitude defects .

[0029] Especially i f one of the captured images has no defocus or only a small defocus , in addition to the described comparison it is also possible to carry out a further comparison of at least one captured image with a reference image of the captured region of the semiconductor substrate . In this regard, without an image additionally having to be captured for this purpose , an examination in respect of possible amplitude defects can directly be carried out as well . Especially i f the defocus is not considerable for all captured images of a region, it is possible for the captured images of a region to beadded for the comparison with a reference image. The addition results in an overall image having higher contrast.

[0030] The semiconductor substrate examined with the aid of the method according to the invention is preferably a micro- lithographic mask. The corresponding inspection of microlitho- graphic masks helps to establish that microstructured components produced with the aid of a microlithographic mask also actually conform to specifications, which inter alia also necessitates ensuring that no relevant phase defects are present in the microlithographic mask.

[0031] In order to be able to capture two images of a region with different focus positions in a simplified manner, a device that is particularly suitable for carrying out the method is proposed. In this case, the device for the image-based capture of structures on semiconductor substrates comprises an illumination device for illuminating an object, i.e. preferably a semiconductor substrate, with illumination radiation and an image capturing unit for capturing the illumination radiation transformed by the object (20) , the device (1) being configured in a focusable fashion, and the image capturing unit (40) comprising at least two image capturing sensors (45, 46, 47) arranged in such a way that they have a different focus position independently of the focusing of the device. Furthermore, the device comprises a control unit designed to suitably control the device for carrying out a method according to the invention and in particular also carry out the comparison of the images captured by the device. By virtue of the device comprising at least two image capturing sensors, which at any rate capture images with at least two different focus positions, it is possible to dispense with an active focus adjustment for capturing two images with a different focus position.

[0032] The at least two image capturing sensors of the image capturing unit can be arranged in the image capturing unit offset with respect to one another by a predefined distance perpendicular to their respective image planes, which in principle results in different focus positions for the at least two image capturing sensors. While the distance between the image capturing sensors remains constant, focusing of the device as a whole enables joint adaptation of the focus positions for the different image capturing sensors.

[0033] However, it is also possible for the at least two image capturing sensors to be arranged next to one another in a common plane, and for the different focused positions at the individual image capturing sensors to be predefined by a suitable tilt angle of the image capturing unit vis-a-vis the object or the surface thereof. By virtue of such a tilt, the surface of the object does not lie in the object focal plane, such that focus positions resulting directly from the tilt angle arise, in principle, at least for some of the portions of the surface that are captured by the individual image capturing sensors. If the image capturing sensors are arranged in a common plane, a single image capturing chip can also be provided instead of separate image capturing sensors, wherein the individual image capturing sensors are then formed by separate sub-regions of the image capturing chip.

[0034] It is preferred if the distance between image capturing unit and object is variable for focusing purposes. In this case, it is unimportant whether the image capturing unit, the object or even both can be suitably moved in order to adapt the focusing.

[0035] It is possible, in principle, for all the image capturing sensors of the image capturing unit simultaneously to capture images of the same region on the surface of the object.However, this generally requires a complex optical unit between the object and the image capturing sensors. However, it is preferred if the image capturing unit is designed to the effect that the image capturing sensors thereof each capture individual adjacent regions in an object plane, and provision is made of an object stage for moving the object in the object plane in at least one direction, which allows an object arranged on the object stage to move in such a way that the image capturing sensors can successively each capture an image of the same region of the surface. In other words, the device can be suitably designed for the "scanning" that is already known in principle from the prior art.

[0036] In principle, the device according to the invention can be designed for transmissive objects, in principle, in which case radiation passing through the object is captured by the image capturing unit. It is also possible for the device to be designed for reflective objects. In both cases, the illumination radiation ultimately captured by the image capturing unit or the image capturing sensors thereof is transformed by the object in such a way - e.g. by partial reflection, partial absorption - that the micro- and / or nanostructure situated on the object is imaged on the image capturing sensors.

[0037] It is particularly preferred if the illumination device and the image capturing unit are designed for illumination radiation in the EUV range, i.e. for radiation having a wavelength of 5 nm to 30 nm, in particular of 13.5 nm. Since exclusively reflective optical elements are known for such a wavelength, it is necessary not only to design the device for reflective objects, but also to configure any optical elements of the device in a reflective fashion.

[0038] A device as described above is in principle also described in the German patent application DE 10 2024 110 447.3,to which reference is made for further details concerning the device . In the case of the invention explained in the cited application, the images captured by at least two image capturing sensors with a di f ferent focus position are evaluated in order to attain as optimal focusing of the device as possible . It is possible for the images captured by a device with a di fferent focus position to be used in parallel both for focusing as described in the application DE 10 2024 110 447 . 3 , and for determining defects in the manner according to the invention .

[0039] The invention will now be described by way of example on the basis of advantageous embodiments with reference to the accompanying drawings , in which :Figure 1 : shows a schematic illustration of a first exemplary embodiment of a device according to the invention;Figure 2 : shows a schematic illustration of a second exemplary embodiment of a device according to the invention; andFigure 3 : shows a schematic illustration of the identi fication of a phase defect .

[0040] Figure 1 schematically illustrates a first exemplary embodiment of a device 1 according to the invention for the image-based capture of structures on an obj ect 20 . The obj ect 20 to be subj ected to image-based capture is a reflective mi- crolithographic mask as a semiconductor substrate to be examined, having corresponding structures on the surface . Radiation incident on the obj ect is reflected, in principle , wherein the incident radiation is partially trans formed by the micro- and / or nanostructures , e . g . by parts of the radiation being absorbed .

[0041] Especially if a microlithographic mask is involved, the object 20 can have an aspect ratio of between 1:1 and 1:3, preferably between 1:1 and 1:2 and particularly preferably of 1:1 or 1:2. The object 20 can in this case be substantially rectangular and preferably has a length and width of 5 to 7 inches (12.70 to 17.78 cm) , with further preference a length and width of 6 inches (15.24 cm) . As an alternative thereto, the object 20 can have a length of 5 to 7 inches (12.70 cm to 17.78 cm) and a width of 10 to 14 inches (25.40 cm to 35.56 cm) , preferably a length of 6 inches (15.24 cm) and a width of 11.2 inches (28.45 cm) .

[0042] The device 1 comprises an illumination device 10, by which the object 20 can be sufficiently illuminated. In this case, the illumination device 10 is adapted to the object 20 in such a way that the illumination radiation is sufficiently reflected by the object 20 or at least parts of the micro- and / or nanostructures, such that the image capturing unit 40 can capture the illumination radiation transformed by the object 20 and image-pertaining information concerning the surface of the object 20 actually arises from the illumination radiation thus captured. For this purpose, the wavelength of the illumination radiation must be low enough that the micro- and / or nanostructures can actually be imaged. If the object 20 is e.g. a microlithographic mask for microlithography in the EUV range, it is advantageous if the illumination radiation of the illumination device 10 is regularly likewise be in the EUV range, i.e. between 5 nm and 30 nm, preferably at 13.5 nm, since it is only at such a wavelength that the micro- and / or nanostructures typically situated on corresponding microlithographic masks can actually be imaged. Moreover, the reflective properties of corresponding microlithographic masks are optimized towards corresponding wavelengths.

[0043] The object 20 illuminated by the illumination device 10 is arranged on an object stage 30, by which the object 20 can in particular also be moved in the direction indicated by the arrow 31.

[0044] For actually capturing the micro- and / or nanostructures on the surface of the object 20, an image capturing unit 40 is provided, by which the illumination radiation of the illumination device 10 that has been reflected and transformed by the object 20 is captured in such a way as ultimately to give rise to a two-dimensional image representation of the micro- and / or nanostructures on the surface of the object 20.

[0045] For this purpose, the image capturing unit 40 has an optical unit 41, which is telecentric on the object side. The optical unit 41 comprises exclusively mirrors as optical elements, which are suitably configured for reflecting the used illumination radiation, e.g. EUV radiation. In the case of illumination radiation in wavelength ranges for which transmissive optical elements, such as e.g. lens elements, are also known, the optical unit 41 can also comprise such optical elements .

[0046] The image capturing unit 40 comprises three image capturing sensors 45, 46, 47. The three image capturing sensors45, 46, 47 are "time delay integration" sensors (TDI sensors) sensors that each capture individual regions of the object 20 lying one behind the other in the direction of movement of the object 20 as indicated by the arrow 31. During movement of the object 20 with the aid of the object stage 30 in direction 31 a specific region of the object 20 successively passes through the capture regions of the three image capturing sensors 47,46, 45.

[0047] One of the image capturing sensors 46 defines the so- called image focal plane 42 of the image capturing unit 40, from which the object focal plane 43 also directly arises by way of the optical unit 41. The image capturing unit 40 as a whole is displaceable in a direction parallel to the incident radiation into the optical unit 41 (indicated by the doubleheaded arrow 44) , such that the distance between object 20 and image capturing unit 40 can be changed. By changing the distance between object 20 and image capturing unit 40, it is possible in particular for the image capturing sensor 46 to be focused such that it captures an image with optimal focusing.

[0048] The other two image capturing sensors 45, 47 are arranged offset vis-a-vis the image focal plane 42 - defined by the image capturing sensor 46 - by a predefined distance ±Az perpendicularly thereto, wherein this distance ±Az cannot change. If the distance between object 20 and image capturing unit 40 is changed by displacement of the image capturing unit 40, this changes not only the focusing of the image capturing sensor 46 but directly also the focusing of the image capturing sensors 45, 47, wherein the different distances from the image focal plane 42 ensure that a different focus position arises for each of the individual image capturing sensors 45, 46, 47. If the image capturing sensor 46 is optimally focused, then a defocus that is identical in terms of absolute value, but different in terms of sign, specifically of ±Az, arises for each of the image capturing sensors 45 and 47.

[0049] The device 1 furthermore comprises a control device 50, which is connected both to the image capturing unit 40 and to the object stage 30 and controls at least the respective movement thereof. The control of the object stage 30 and optionally also the capture by the image capturing sensors 45, 46, 47 take place according to the known principle of scanning,and so the manner of functioning of the control device 50 in this regard need not be explained in greater detail.

[0050] The control device 50 is designed to carry out the procedure for automatic focusing described in the patent application DE 10 2024 110 447.3. For this purpose, the control device 50 obtains the images captured by the image capturing sensors 45, 46, 47 of the image capturing unit 40 in order on this basis to control the focusing by adapting the distance between object 20 and image capturing unit 40 in direction 44. The closed-loop control strategy employed for this purpose is described thoroughly in the cited patent application. The latter also describes an image processing unit integrated into the control device 50, which enables the images from the image capturing sensors 45, 46, 47 to be merged to form a common image. Such a common image is suitable e.g. for recognizing amplitude defects.

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

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

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

[0054] In a comparable manner to the exemplary embodiment in accordance with Figure 1, the three image capturing sensors 45, 46, 47 capture different regions of the object 20, in principle. However, suitable movement of the object 20 by means of the object stage 30 in direction 31 allows a region on the object 20 to be captured successively by all the image capturing sensors 45, 46, 47. The images thus captured by the image capturing sensors 45, 46, 47 can be combined by the control unit 50 or the image processing unit integrated therein to form a two-dimensional image representation of the surface of the object 20 and can be used e.g. for the determination of amplitude defects.

[0055] The image capturing unit 40 is arranged in a manner tilted vis-a-vis the object 20 or the surface 21 thereof, wherein the angle 49 between the surface normal of the surface 21 of the object 20 and the object focal plane 43 can be assumed as angle for the tilting. The image capturing unit 40 is arranged so as to be movable in the direction of the doubleheaded arrow 44 in order to be able to vary the tilt angle 49 as necessary. Since the three image capturing sensors 45, 46,47 capture different regions of the object, the result, even though they are arranged in the common image focal plane 42, is a different focus position for the individual image capturing sensors 45, 46, 47 on account of the tilt angle 49 on the object side. While the focus position represented as zero in Figure 2 for the image sensor 46 remains constant independently of the tilt angle 49, the defocus for the other two image capturing sensors 45, 47 amounts to ±Az, where Az is directly dependent on the tilt angle 49: the greater the tilt angle 49, the greater the absolute value of Az is as well.

[0056] Movement of the image capturing unit 40 in direction 44 and the attendant changing of the tilt angle 49 thus ultimately make it possible to vary the relative focus position with respect to one another of the individual image capturing sensors 45, 46, 47.

[0057] The control device 50 is designed in principle for the functions already explained in association with Figure 1. For setting the distance, however, the object stage 30 is moved, rather than the image capturing unit 40. In particular, the control device 50 is designed, upon movement of the object stage 30 in direction 33, simultaneously to bring about a movement in or counter to the direction 32, such that the portions of the surface 21 of the object 20 that are captured by the individual image capturing sensors 45, 46, 47 do not change .

[0058] The control device is additionally designed to set the tilt angle 49 by movement of the image capturing unit 40 in direction 44 in order to make possible firstly the automatic focusing described in the patent application DE 10 2024 110 447.3, and secondly the method described below.

[0059] Even though in the present exemplary embodiment the tilt angle 49 is set solely by movement of the image capturing unit 40 in direction 44, it is alternatively or additionally possible to suitably move the object stage 30, in particular rotate it about an axis perpendicular to the depiction direction, in order to set the tilt angle 49.

[0060] In all the exemplary embodiments described above, the control unit 50 is (additionally) designed to optimally focus the image capturing sensor 46 and to determine potential phase defects from the images captured by the image capturing sensors 45, 47. The explanations below assume that the image capturing sensor 46 is optimally focused and remains so - e.g. with the aid of the focusing from the patent application DE 10 2024 110 447.3.

[0061] Albeit not simultaneously, but rather with a slight temporal offset owing to the scanning, the image capturing sensors 45, 47 in principle capture the same region of the object 20, but with different focus positions. One image capturing sensor 45 captures an image with a defocus of -Az, while the image from the other image capturing sensor 47 captures a defocus +Az that is equal in terms of absolute value but different in terms of sign.

[0062] From the two images thus captured, a difference is then formed, which gives rise to a two-dimensional difference map for the captured region of the microlithographic mask 20, which may already exhibit potential phase defects.

[0063] If neither reference maps for a further comparison nor analysis methods for directly identifying phase defects on the difference map are available, the method described above is carried out for a further region of the microlithographic mask 20, in which further region the structure of themicrolithographic mask 20 should in principle be identical to the region analysed previously. If the structure in both regions is actually identical, the respective difference maps are also identical. If this is not the case, the locations at which the structures in The two regions differ from one another become evident upon difference formation over The two difference maps. These locations can then be examined more closely, as necessary.

[0064] Figure 3 schematically shows how a phase defect can be identified e.g. using one of the devices in accordance with Figures 1 and 2.

[0065] Figure 3a illustrates the actual situation on an object 20, which is once again a reflective microlithographic mask as a semiconductor substrate to be examined. In the illustrated region of the object 20, elevated structures 22 are provided on the object surface 21, and valleys 23 having a flat base in principle form between said elevated structures. In one of these valleys 23, however, the base is not completely flat, but rather has a location 24 with a height deviation. As illustrated below, this height deviation can be identified as a phase defect by the method according to the invention, if carried out e.g. by a device 1 in accordance with Figure 1 or Figure 2.

[0066] Figure 3b illustrates by way of example the appearance of the images of the region illustrated in Figure 3a that are captured by the individual image capturing sensors 45, 46, 47 of one of the devices 1 in accordance with Figures 1 and 2. It is assumed here that the image capturing sensor 46 is optimally focused, as a result of which the images from the other two image capturing sensors 45, 47 have a defocus of -Az and +Az, respectively.

[0067] Whereas in the image captured by the image capturing sensor 46 the location 24 on the object surface 21 is not discernible or - as illustrated - is only scarcely discernible and in particular does not permit a reliable conclusion to be drawn about the presence of a defect at this location, precisely this location 24 in the images from the image capturing sensors 45, 47 in each case causes a clearly altered image representation .

[0068] If the difference is then formed from the images cap- tured by the two image capturing sensors 45, 47, this results in the difference image illustrated in Figure 3c. The difference image has a phase defect 25 at precisely the location 24 on the surface 21 of the object 20 where a height deviation is present (cf . Figure 3a) . By contrast, the other regions in the difference image clearly have no phase defect.

Claims

23Patent Claims1 . Method for the inspection of semiconductor substrates using a device ( 1 ) for the image-based capture of structures on semiconductor substrates , the device for the imagebased capture being designed for capturing at least one region of a semiconductor substrate with a di f ferent focus position, the method comprising the following steps : a ) capturing a region of a semiconductor substrate with a first focus position; b ) capturing the region of the semiconductor substrate with a second focus position deviating from the first focus position; and c ) comparison of the two captured images with di f ferent focus positions for the purpose of determining defects in the captured region of the semiconductor substrate .2 . Method according to Claim 1 , characteri zed in that the comparison of the two captured images is designed for determining phase defects .3 . Method according to either of the preceding claims , characteri zed in that the comparison of the two captured images comprises di fference formation .4 . Method according to any of the preceding claims , characteri zed in that the first focus position comprises a defocus from the optimal focusing and the second focus setting comprises a defocus from the optimal focusing that is equal to thedefocus of the first focus position in terms of absolute value .

5. Method according to Claim 4, characterized in that the defocus has a length amounting to a proportion of approximately 1 / 8 to approximately 1 of the Rayleigh length of the radiation used for the exposure of the semiconductor substrate that is used for capturing the images.

6. Method according to Claim 5, characterized in that the defocus has a length amounting to a proportion of approximately 1 / 4 to approximately 1 / 2 of the Rayleigh length of the radiation used for the exposure of the semiconductor substrate that is used for capturing the images.

7. Method according to any of the preceding claims, characterized in that in a further region comparable to the region with regard to the structures of the semiconductor substrate, the semiconductor substrate is subjected to image-based capture with two different focus positions, and the comparison comprises difference formation applied to comparisons carried out for each of The two regions from the respectively captured images with different focus positions.

8. Method according to any of the preceding claims, characterized in that a comparison of at least one captured image with a reference image of the captured region of the semiconductor substrate is additionally carried out.

9. Method according to Claim 8, characterized in thatthe captured images of a region are added for the comparison with a reference image.

10. Method according to any of the preceding claims, characterized in that the semiconductor substrate is a microlithographic mask.

11. Device (1) for the image-based capture of structures on semiconductor substrates, comprising an illumination device (10) for illuminating an object (20) with illumination radiation and an image capturing unit (40) for capturing the illumination radiation transformed by the object (20) , the device (1) being configured in a focusable fashion, and the image capturing unit (40) comprising at least two image capturing sensors (45, 46, 47) arranged in such a way that they have a different focus position independently of the focusing of the device (1) , characterized in that the device (1) comprises a control device (50) designed to suitably control the device (1) for carrying out a method according to any of the preceding claims and carry out the comparison of the images captured by the image capturing sensors (45, 46, 47) .

12. Device according to Claim 11, characterized in that the image capturing unit (40) comprises at least two image capturing sensors (45, 46, 47) arranged at a distance from one another perpendicular to their respective image planes .

13. Device according to Claim 11 or 12, characterized in that the image capturing sensors (45, 46, 47) are arranged fixedly in the image capturing unit (40) , the relative focus26 position of the image capturing sensors (45, 46, 47) with respect to one another being predefined by a tilt angle (49) of the image capturing unit (40) in relation to the ob ect (20) .

14. Device according to Claim 13, characterized in that the image capturing sensors (45, 46, 47) are arranged in a common plane (48) .

15. Device according to Claim 13 or 14, characterized in that the tilt angle (49) is variable.

16. Device according to any of Claims 11 to 15, characterized in that the distance between image capturing unit (40) and object(20) is variable for focusing purposes.

17. Device according to any of Claims 11 to 16, characterized in that the image capturing unit (40) is designed in such a way that the image capturing sensors (45, 46, 47) thereof each capture individual adjacent regions in an object plane (31) , and provision is made of an object stage (30) for moving the object (20) in the object plane (31) in at least one direction (31) , which allows an object (20) arranged on the object stage (30) to move in such a way that the image capturing sensors (45, 46, 47) can successively each capture an image of the same region of the surface(21) of the object (20) .

18. Device according to any of Claims 11 to 17, characterized in that the device (1) is designed for reflective objects.2719. Device according to Claim 18, characterized in that the illumination device (20) and the image capturing unit(40) are designed for illumination radiation in the EUV range .

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