Device for capturing images of structures on photolithographic masks

Abstract

The invention relates to a device (1) for capturing images of structures on photolithographic masks (20), comprising an illumination device (10) for illuminating a photolithographic mask (20) arranged on an object stage (30) with illumination radiation, an image capture unit (40) that uses at least one image capture sensor (42) arranged in an image capture plane to perform scanning capture of illumination radiation transformed by means of the photolithographic mask (20), and a control device (50) for controlling a relative movement between the object stage (30) and the image capture unit (40) in a scanning direction (31), wherein the device (20) can be focused by changing the distance between the image capture unit (40) and the object stage (30), wherein the image capture unit (40) comprises an optical unit (41) for imaging at least one imaging region (91) and at least one focusing region (90) onto the at least one image capture sensor (42), wherein the optical unit (41) is designed such that the image plane (47) onto which the at least one imaging region (91) is imaged is spaced apart from the image plane (47') onto which the at least one focusing region (90) is imaged, and the control device (50) is designed to determine in each case a variable that is characteristic of the defocus for imaging at least one imaging region (91) and at least one focusing region (90) and, if a non-optimum distance is determined on the basis of the determined variables that are characteristic of the defocus, to adapt the distance between the image capture unit (40) and the object stage (30) in a direction that can be derived from the characteristic variables.

Description

24.10.2025 / BRDevice for capturing images of structures on photolithographic masks

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

[0002] Photolithography is used for producing microstructured component parts, 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 EUV (e.g. 13.5 nm) . In order to ensure that a mask satisfies these accuracy requirements and a microstructured component part produced thereby also has the desired properties and manner of functioning, a mask is checked by means of suitable methods in inspection and / or metrology apparatuses before use in a projection exposure apparatus.

[0004] In the case of known inspection and / or metrology apparatuses, the object to be checked - i.e. for example a mask for producing microstructured component parts - is illuminatedby 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 practically continuously captures data which can be combined to form a two-dimensional image representation . A corresponding process is also referred to as " scanning" .

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

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

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

[0009] The obj ect of the present invention is to provide a device for capturing images of structures on photolithographic masks which enables refocusing during a scanning process .

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

[0011] Accordingly, the invention relates to a device for capturing images of structures on photolithographic masks , comprising an illumination device for illuminating a photolithographic mask arranged on an obj ect stage with illumination radiation, an image capture unit that uses at least one image capture sensor arranged in an image capture plane to perform scanning capture of illumination radiation trans formed by means of the photolithographic mask, and a control device for controlling a relative movement between the obj ect stage and the image capture unit in a scanning direction, wherein the device can be focused by changing the distance between the image capture unit and the obj ect stage , wherein the image capture unit comprises an optical unit for imaging at least one imaging region and at least one focusing region onto the at least one image capture sensor, wherein the optical unit is designed such that the image plane onto which the at least one imaging region is imaged is spaced apart from the image plane onto which the at least one focusing region is imaged, and the control device is designed to determine in each case a variable that is characteristic of the defocus for imaging at least one imaging region and at least one focusing region and, i f a non-optimum distance is determined on the basis of thedetermined variables that are characteristic of the defocus, to adapt the distance between the image capture unit and the object stage in a direction that can be derived from the characteristic variables.

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

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

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

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

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

[0017] In the device according to the invention, it is provided that , in addition to an imaging region in which a region of a photolithographic mask is actually imaged, on the basis of which possible defects of the photolithographic mask can be determined, images of at least one further region of the mask - namely the focusing region - are also captured . The imaging region may be at least a first part of the photolithographic mask . The focusing region may be at least a second part of the photolithographic mask . The first part of the photolithographic mask and the second part of the photolithographic mask can be designed separately from each other . Alternatively, the first part of the photolithographic mask and the second part of the photolithographic mask may at least partially overlap .

[0018] Appropriate design of the optical unit ensures that the image plane for imaging the imaging region ( s ) and the image plane ( s ) for the focusing region ( s ) are spaced from each other . I f the device is optimally focused with regard to the imaging of the at least one imaging region - i . e . , the image plane for imaging the at least one imaging region coincides with the image capture plane of the at least one image capture sensor - the respective imaging of the at least one focusing region inevitably has a defocus , since the image plane for the focusing regions is spaced apart from the image plane for imaging the at least one imaging region and is thus also spaced apart from the image capture plane during the aforementioned focusing .

[0019] On the other hand, i f the device is not optimally focused, there is a distance between the image plane for imaging the at least one imaging region and the image capture plane . The respective distance between an image plane for the focusing regions and the image capture plane is also changed compared to the optimal focusing state and can even be zero for one of the image planes .

[0020] By providing, according to the invention, for a variable that is characteristic of the defocus to be determined in each case for the image representations of the at least one imaging region and the at least one focusing region that are captured by the image capture unit , a deviation from the optimal focusing can be determined from the absolute values for the defocus and / or the relative ratio of the defocus values to each other, and it is also possible to determine in particular the direction in which the focusing must be adapted in order to achieve optimal focusing for the at least imaging region .In the device according to the invention, it can therefore be determined in particular whether the distance between the image capture unit and the obj ect stage must be increased orreduced . I f there are , for example , variables that are characteristic of the defocus for an imaging region and a focusing region, on the basis of which it is determined that the defocus for the focusing region is lower than the defocus for the imaging region, the distance between the image capture unit and the obj ect stage must be changed such that the image plane for the imaging region moves in the direction of the image plane for the focusing region .

[0021] The determination of the characteristic variables for the defocus both in the at least one imaging region and in the at least one focusing region can be carried out during the scanning process and can also be repeated regularly . As a result , the device can also be focused during a process of scanning a photolithographic mask, namely by adapting the distance between the obj ect stage and the image capture unit .

[0022] I f more than one focusing region is provided, in which case the image planes for at least two focusing regions are spaced apart from each other, thus resulting in a di f ferent defocus for each of the two focusing regions , the direction in which the distance needs to be adapted is particularly easy to determine . Moreover, it is regularly also possible to estimate at least an approximate order of magnitude of the required adaptation of the distance . It is preferred i f the optical unit is designed such that the image plane onto which the at least one imaging region is imaged is arranged between at least two image planes onto which at least one focusing region is imaged in each case .

[0023] In particular, i f the structure on a photolithographic mask has a broad regularity, it is possible to directly compare the characteristic values for the defocus in a imaging region and in a focusing region . However, it is preferred i f at least one imaging region and at least one focusing regionare arranged in such a way that they capture the same region on the photolithographic mask at a short time interval during the scanning process . Preferably, the at least one focusing region may be arranged with an of fset with respect to an imaging region in the direction of the image representation of the scanning direction . Further, it is possible that the at least one focus region is then overlapping the imaging region in a proj ection onto a plane perpendicular to the scanning direction . In this case , "direction of the image representation of the scanning direction" is used to mean that direction in which the image representation of the photolithographic mask moves in the image capture unit when the obj ect stage and the image capture unit are moved relative to each other in the scanning direction . Subsequently, a region of the photolithographic mask, which initially lies in the imaging region, is moved directly into the focusing region by the movement of the photolithographic mask, which takes place for scanning . It goes without saying that it is possible that a region on the photolithographic mask first passes through a focusing region and only then passes through an imaging region . I f the imaging region is larger than a focusing region, the determination of the variable characteristic of the defocus can be limited to that part of the imaging region that is also in a focusing region - with a time delay .

[0024] In order to achieve an image plane for a focusing region that is spaced apart from the image plane for the imaging region, provision may be made for the beam path in the optical unit for imaging a focusing region to be lengthened or shortened relative to the beam path for imaging an imaging region, preferably by suitably arranging additional optical elements .

[0025] The image capture unit may have at least two image capture sensors , the respective image capture regions of which are spaced apart from each other in a direction of the imagerepresentation of the scanning direction of the device , but at the same time overlap at least partially in a proj ection onto a plane perpendicular to the direction of the image representation of the scanning direction . Such an arrangement ensures that a region on the photolithographic mask is captured by a plurality of image capture sensors during the scanning process . The image representations obtained in this way can then be combined, for example by the control unit , to form a single image representation . Capturing regions on the photolithographic mask multiple times by means of di f ferent image capture sensors also makes it possible to achieve , even when the radiation intensity of the trans formed illumination radiation incident on the image capture sensors is low, a combined image representation with greater contrast or a signal-to-noise ratio , in which imaging aberrations of individual image capture sensors can also be at least partially compensated . I f more than two image capture sensors are provided, individual regions of the obj ect may also be captured by multiple image capture sensors , in particular by three or four image capture sensors , during the scanning process .

[0026] Arranging further image capture sensors in a direction perpendicular to the direction of the image representation of the scanning direction makes it possible to increase the width of the region that can be captured by the image capture unit during a movement of the obj ect in the scanning direction . In this case , a plurality of image capture sensors , the image capture regions of which are spaced apart from one another in the direction of the image representation of the scanning direction, may also be arranged over the entire width in each case . I f more than one row of image capture sensors is provided, the image capture sensors in the individual rows are preferably arranged in such a way that some of the image capture sensors in each case overlap at least partially in aproj ection onto a plane perpendicular to the direction of the image representation of the scanning direction .

[0027] A variable derived from the contrast of a captured image may be used as a variable characteri zing the defocus of an image . This may also directly be the contrast of a captured image or its reciprocal . For the contrast , the assumption may be made that it regularly has only a single maximum ( a minimum for the reciprocal of the contrast ) in a certain region around the optimal focus . For example , the root mean square of the absolute value of the gradient over all or some of the image points of a captured image or the standard deviation vis-a-vis the mean value (both ascertained over all or some of the image points of a captured image ) may be used as a value for the contrast . This variable also has only a single optimum, at which optimal focusing is attained, in a certain region around the optimal focus .

[0028] I f a relationship between a characteri zing variable and the defocus has more than one optimum, the procedure described below can nevertheless be carried out , in principle , given suf ficiently accurate knowledge of the relationship between the characteristic variable and the defocus . However, this may then optionally necessitate more than two or three di f ferently focused images of the same region in order to be able to determine the direction in which a possibly required adaptation of the distance needs to be carried out with suf ficient certainty .

[0029] I f the need to change the distance between the image capture unit and the obj ect stage is determined on the basis of at least two variables that are characteristic of the defocus and are determined for an imaging region and / or a focusing region, e . g . because at least one of the characteristic variables di f fers from the characteristic variables previouslydetermined during the scanning process by more than a predefined amount , or the di f ference between the two characteristic variables exceeds a predefined amount , the distance in question can be adapted in that direction in which a reduction in the defocus for the imaging region can be assumed ( see above ) . The change in the distance can be continued, for example , until the desired defocus for the imaging region is achieved .

[0030] The very act of ascertaining the direction in which focusing needs to take place makes it possible to refocus a device for capturing images of structures on photolithographic masks even during the actual image capture . Speci fically, in the case of the device according to the invention, upon establishing the need to adapt the distance between the image capture unit and the obj ect stage or the photolithographic mask arranged thereon, by virtue of the fact that the direction in which this change needs to take place is known directly, an improvement in the focusing is achieved directly upon complying with this speci fication; by contrast , more extensive defocusing and thus a deterioration in e . g . the sharpness of the image capture are precluded . The device according to the invention furthermore makes it possible to monitor the instantaneous focusing during the refocusing and to establish when the optimal distance between the image capture unit and the obj ect stage or obj ect was attained in the course of refocusing . In this case , it is irrelevant that the scanning process is continued during the refocusing, i . e . the repeated ascertainment and checking of the variables that are characteristic of the defocusing are not always carried out for the same region of the obj ect as a matter of principle but instead carried out on the basis of the respective regions currently located in the image capture region of the individual image capture sensors .

[0031] The image capture sensors can then be line sensors , i . e . sensors that each capture only a single line of an image ,and a two-dimensional image arises as a result of step-by- step, line-by-line capture . It is preferred, however, i f the image capture sensors are designed as "time delay integration" sensors ( TDI sensors ) . Corresponding sensors can be regarded as a plurality of line sensors which are arranged directly next to one another but in which an image line captured by a first line sensor is trans ferred synchronously with the change in the position of the obj ect vis-a-vis the sensor to an adj acent line sensor, such that the adj acent line sensor again captures the same region imaged by the previously captured image line and additively appends it to the already captured image line , in order thus to obtain a high-contrast image after passing through all the line sensors . In comparison with a simple line sensor, a comparatively strongly exposed image can be attained by multiple exposure using a TDI sensor, wherein the obj ect , however, need remain at a speci fic position for a shorter time in each case such that the use of TDI sensors can achieve a reduction in the time required for the scanning of an obj ect .

[0032] It is preferred i f the obj ect-side beam path of the image capture unit is telecentric . As a result , the images captured by the individual image capture sensors , despite di f ferent focusing, generally have the same si ze or the same imaging scale , and so image representations of the same region of the photolithographic mask that are captured with di f ferent defocus due to refocusing can also be more easily combined to form a single image representation .

[0033] To change the distance between the image capture unit and the obj ect stage or an obj ect arranged thereon, it is preferred i f the obj ect stage is movable in a direction perpendicular to the scanning direction - and hence usually also perpendicular to the obj ect or to the photolithographic mask .

[0034] In principle, the device according to the invention may be designed for transmissive objects, in which case radiation passing through the object is then captured by the image capture unit. The device may also be designed for reflective objects. In both cases, the illumination radiation ultimately captured by the image capture unit or the image capture sensors thereof is transformed by the object - e.g. by partial reflection, partial absorption - in such a way that the structure situated on the object is imaged onto the image capture sensors .

[0035] It is particularly preferred if the illumination device and the image capture unit are designed for illumination radiation in the EUV range, i.e. for radiation having a wavelength of 5 nm to 30 nm, in particular of 13.5 nm. Since exclusively reflective optical elements are known for a corresponding wavelength, it is necessary not only to design the device for reflective objects, but also to configure any optical elements of the device in a reflective fashion.

[0036] The above-described possibility of refocusing during the scanning process is based on the ascertainment of variables that are characteristic of the defocus on the basis of recorded image representations. Especially if image representations are captured from a region of the object in which the object has no structure, for example the edge region of photolithographic masks which is referred to as the "black border", it is not possible to ascertain corresponding variables such as the contrast or determine any differences in the characteristic variables for differently focused images. In order to nevertheless make it possible to focus the device in such situations, it is possible to provide a distance sensor that is used to ascertain the distance between the image capture unit and the object stage or the photolithographic mask arranged thereon in the region in which the image capture regions ofthe image capture sensors of the image capture unit are arranged. The distance between the image capture unit and the object stage or object may then be changed in such a way that the measured distance corresponds to a predefined distance which may be derived for example from the lens equation. Focusing on the basis of the distance sensor is frequently sufficient in order to assume that a focus with optimal contrast that can be found in the vicinity of this focusing is the optimal focus since there is only a single optimum for the contrast in this region, specifically at the optimal focus.

[0037] It is also possible to provide more than one distance sensor, the respective ascertained distances of which may be combined to form a distance value with increased accuracy following a suitable plausibility check. If more than one distance sensor is provided, it is preferred to use at least two different types of distance sensors. A distance sensor may be a confocal sensor, a white light interferometer, a laser interferometer and / or a capacitive sensor.

[0038] The photolithographic mask whose image should be captured by the device may have an aspect ratio of between 1:1 and 1:3, preferably between 1:1 and 1:2 and particularly preferably of 1:1 or 1:2. The photolithographic mask may be configured in substantially rectangular fashion. The photomask can preferably have a length and a width of 5 to 7 inches, particularly preferably a length and a width of 6 inches. As an alternative thereto, the photomask can have a length of 5 to 7 inches and a width of 10 to 14 inches, preferably a length of 6 inches and a width of 12 inches.

[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 an exemplary embodiment of a device according to the invention;Figure 2 : shows a schematic illustration of the arrangement of the image capture sensors of the device from Figure 1 ;Figure 3 : shows a schematic illustration of the optical unit of the device from Figure 1 ; andFigure 4 : shows a schematic illustration of the dependence of the contrast of captured images on the distance between the image capture unit and the obj ect stage in the device from Figure 1 .

[0040] Figure 1 schematically illustrates an exemplary embodiment of a device 1 according to the invention for capturing images of structures on a photolithographic mask 20 . This is a reflective photolithographic mask 20 having structures on the surface , as known from the prior art . Radiation incident on the obj ect is reflected, in principle , wherein the incident radiation is partially trans formed by the structures on the photolithographic mask 20 , e . g . by some of the radiation being absorbed .

[0041] The device 1 comprises an illumination device 10 that can be used to suf ficiently illuminate the photolithographic mask 20 . In this case , the illumination device 10 is adapted to the obj ect 20 in such a way that the illumination radiation is suf ficiently reflected of f the photolithographic mask 20 or at least parts of the structures thereon, such that the image capture unit 40 can capture the illumination radiation transformed by the photolithographic mask 20 and image-pertaining information concerning the surface of the photolithographicmask 20 actually arises from the illumination radiation thus captured . For this purpose , the wavelength of the illumination radiation must be short enough to actually be able to image the structures on the photolithographic mask 20 . I f the photolithographic mask is for photolithography in the EUV range , the illumination radiation of the illumination device 10 should usually likewise be in the EUV range , i . e . between 5 nm and 30 nm, preferably at 13 . 5 nm, since it is only at such a wavelength that the structures typically situated on corresponding photolithographic masks 20 can actually be imaged . Moreover, the reflective properties of corresponding photolithographic masks are optimi zed for corresponding wavelengths .

[0042] The photolithographic mask 20 illuminated by the illumination device 10 is arranged on an obj ect stage 30 that can be used to also move the photolithographic mask 20 in particular in the scanning direction indicated by the arrow 31 . Moreover, the obj ect stage 30 can be moved in the direction indicated by the double-headed arrow 32 that is perpendicular to the scanning direction 31 .

[0043] For actually capturing the structures on the surface of the photolithographic mask 20 , an image capture unit 40 is provided and is used to capture the illumination radiation of the illumination device 10 that has been reflected and transformed by the photolithographic mask 20 in such a way as to ultimately give rise to a two-dimensional image representation of the structures on the surface of the photolithographic mask 20 .

[0044] For this purpose , the image capture unit 40 has an optical unit 41 which is telecentric on the obj ect side and basically images onto the image plane 47 . The optical unit 41 comprises exclusively mirrors as optical elements , which aresuitably configured for reflecting the illumination radiation used, e . g . EUV radiation . Shown here by way of example is an optical unit consisting of 2 mirrors . However, any other desired number of mirrors is also conceivable , in particular 3 , 4 or 6 mirrors . In the case of illumination radiation in wavelength ranges for which transmissive optical elements , e . g . lens elements , are also known, the optical unit 41 can also comprise such optical elements .

[0045] The image capture unit 40 comprises a plurality of image capture sensors 42 which are embodied as TDI sensors . As evident from the schematic plan view of the image capture sensors 42 in Figure 2 , the image capture sensors are of fset in four rows 43 that extend perpendicular to the direction of the image representation of the scanning direction 31 , indicated in Figure 2 , and are arranged with an overlap in a proj ection onto a plane 45 that is perpendicular to the direction of the image representation of the scanning direction 31 , in such a way that a region of the photolithographic mask 20 is successively captured by three or four image capture sensors 42 in each case . The region in which at least three images of the obj ect are captured in succession in the direction of the image representation of the scanning direction 31 is indicated as the overall capture region 44 of the image capture unit 40 . The portions in which even four images can be captured are indicated by the curly brackets 45 ' . Only three images may be captured in the regions not marked by the curly brackets 45 ' .

[0046] As sketched out in Figure 1 , the arrangement of the image capture sensors 42 in rows 43 results in the rows 43 of image capture sensors 42 in each case capturing individual regions of the obj ect 20 that are located one behind the other in the movement direction of the photolithographic mask 20 that is indicated by the arrow 31 . During movement of the photolithographic mask 20 with the aid of the obj ect stage 30 indirection 31 , a speci fic region of the obj ect 20 successively passes through the capture regions of the rows 43 of image capture sensors 42 on the photolithographic mask 20 .

[0047] As a result of the Z-position of the obj ect stage 30 and hence the distance between the image capture unit 40 and the obj ect stage 30 or the photolithographic mask 20 being changed, there is also a change in the position of the image plane 47 in the image capture unit 40 . Due to the stationary arrangement of the image capture sensors 42 in the image capture unit 40 , the only possibility for focusing the device 1 is also present herein . The image plane 47 is ideally located in the image capturing plane formed by the image capture sensors 42 ( zero defocus ) . However, even during the scanning process , a signed distance between the image plane 47 and the said image capturing plane , i . e . a defocus , can result .

[0048] The image capture unit 40 furthermore comprises a distance sensor 46 for ascertaining the distance between the image capture unit 40 and the obj ect stage 30 , or the photolithographic mask 20 arranged thereon, in the region in which the image capture regions of the image capture sensors 42 of the image capture unit 40 are located . The distance sensor 46 is a laser interferometer . In situations in which the option, also described below, of refocusing the device 1 is not available , for example because the images captured by the image capture sensors 42 have no contrast since they image for example the "black border" of the photolithographic mask 20 , the distance ascertained by the distance sensor 46 can be used to displace the obj ect stage 30 in the direction 32 until the measured distance corresponds to a predefined value . The predefined value preferably corresponds to a distance , in the vicinity of which the optimal distance , at which the defocus is zero , is situated or at least assumed .

[0049] This control and the control described below are performed by the control device 50 in the device 1 . To this end, the control device 50 is connected both to the image capture unit 40 and to the obj ect stage 30 and may in particular control the movement of the obj ect stage 30 both in the scanning direction 31 and in direction 32 . The control of the obj ect stage 30 in the scanning direction 31 and optionally also the capture by the image capture sensors 42 take place according to the known principle of scanning, and so the manner of functioning of the control device 50 in this regard need not be explained in greater detail .

[0050] In this case , the control device 50 is directly designed to combine the images from the individual image capture sensors 42 , during the entire process of scanning the photolithographic mask 20 , to form a single two-dimensional image representation of the surface of the photolithographic mask 20 , as is conventional in scanning methods . This image can be communicated via an interface 51 to external devices and units , e . g . for checking the image for defects .

[0051] As also shown in Figure 3 , the optical unit 41 comprises additional optical elements 48 that are used to lengthen or shorten the beam path between the surface of the photolithographic mask 20 and the image capture sensors 42 in certain regions . For example , the beam path can be shortened by providing a single additional mirror as an optical element 48 ; the beam path can be lengthened as shown with two mirrors as optical elements 48 . By virtue of the fact that the beam path is shortened or lengthened in certain regions , the image transmitted by the optical unit 41 is imaged, in the region of the beam path shortened or lengthened by the additional optical elements 48 , onto image planes 47 ' other than the image plane 47 remote from the shortened or lengthened beam paths . As a result , the image representations of thephotolithographic mask 20 in the region of a shortened or lengthened beam path, which are imaged onto the image capture plane formed by the image capture sensors 42 , have a fundamentally di f ferent defocus than in the regions in which imaging onto the image plane 47 is carried out .

[0052] It goes without saying that other configurations of the optical unit 41 which deviate from the illustrated exemplary embodiment and in which the imaging is carried out in the image plane 47 ' deviating from the actual image plane 47 are also possible .

[0053] In Figure 2 , the regions in which the imaging is carried out onto the image capture sensors 42 via the beam paths shortened or lengthened by the additional optical elements 48 are illustrated using hatching and are referred to as focusing regions 90 . The regions in which the imaging onto the image capture sensors 42 is not af fected by the additional optical elements 48 are referred to as imaging regions 91 . As is clear directly from Figure 2 , the focusing regions 90 are selected in such a way that , when scanning in the scanning direction 31 , the region of the photolithographic mask 20 , which is also imaged in a focusing region 90 , is also captured in each case by three other image capture sensors 42 in the imaging region 91 ( cf . the portions 92 of the imaging regions 91 identi fied by the dash-dotted line ) . As a result , suf ficient image representations with a defocus identical to the defocus of the remaining imaging regions 91 are also available for the af fected regions on the photolithographic mask 20 in order to achieve a two-dimensional image representation of the surface of the photolithographic mask 20 .

[0054] The di f ferently focused image representations of a region of the photolithographic mask 20 in the focusing regions and in the portions 92 of the imaging regions 91 can be usedto refocus the device 1 by adapting the distance between the photolithographic mask 20 and the image capture unit 40 . Thus , whether the current distance is optimally chosen can be read of f from the contrast which was ascertained for the individual image representations in the said regions as the variable that is characteristic of the defocus . The contrast is at a maximum for a defocus of zero and drops of f with increasing defocus . Due to the additional optical elements 48 and the resulting image planes 47 , 47 ' , it is possible to determine not only a defocus of the image plane 47 for the actual imaging regions 91 , but also the direction in which the distance between the photolithographic mask 20 and the image capture unit 40 must be changed so that the defocus is reduced, preferably to zero , for this very image plane 47 . For example , i f it can be assumed, based on the distance sensor 46 , that the device is in the region of optimal focusing, and i f the contrast ascertained for one of the focusing regions 90 is higher for a certain region of the photolithographic mask 20 than the contrast for the same region when it is captured in one of the portions 92 of the imaging region 91 , it can be concluded that the defocus for the image plane 47 ' in the corresponding focusing region 90 is lower than for the image plane 47 . The distance between the photolithographic mask 20 and the image capture unit 40 can then be changed - even during the scanning process - in such a way that the image plane 47 is shi fted in the direction of the image plane 47 ' with the higher contrast so as to reduce the defocus relevant to the actual imaging of the photolithographic mask 20 in the imaging regions 91 . The adaptation of the distance can be continuously monitored by capturing image representations in the focusing regions 90 and the imaging regions 92 .

[0055] Figure 4 illustrates by way of example how the contrast of a ( constant ) region of the photolithographic mask behaves in the imaging region 91 , in particular the imaging regions 92and the focusing regions 90 . In the case of centrally illustrated optimal focusing, the image plane 47 of the imaging regions 92 is in the image plane defined by the image capture sensors 42 , whereby the defocus is zero and the contrast characteristic of the defocus is at a maximum . I f the defocus for the image plane 47 is zero , the defocus for the image planes 47 ' of the focusing regions 90 is not equal to zero and consequently the contrast is slightly lower .

[0056] I f the image plane 47 is not in the image capture plane , there is a signed defocus ( cf . left-hand and right-hand illustrations in Figure 4 ) . Due to the distances between the image plane 47 for the imaging regions 92 and the image planes 47 ' for the focusing regions 90 , however, the image representation in one of the focusing regions 90 has a lower defocus and therefore a higher contrast than the image representation in the other of the two focusing regions 90 and the image representation in the imaging region 92 . From the di f ference in the contrast values of the image representations in the focusing and imaging regions 90 , 92 , it is then possible to determine the direction in which the distance between the image capture unit 40 and the obj ect stage 30 or photolithographic mask 20 should be changed in order to reduce the defocus for the image plane 47 .

Claims

24Patent claimsDevice (1) for capturing images of structures on photolithographic masks (20) , comprising an illumination device (10) for illuminating a photolithographic mask (20) arranged on an object stage (30) with illumination radiation, an image capture unit (40) that uses at least one image capture sensor (42) arranged in an image capture plane to perform scanning capture of illumination radiation transformed by means of the photolithographic mask (20) , and a control device (50) for controlling a relative movement between the object stage (30) and the image capture unit (40) in a scanning direction (31) , wherein the device (20) can be focused by changing the distance between the image capture unit (40) and the object stage (30) , wherein the image capture unit (40) comprises an optical unit (41) for imaging at least one imaging region (91) and at least one focusing region (90) onto the at least one image capture sensor (42) , wherein the optical unit (41) is designed such that the image plane (47) onto which the at least one imaging region (91) is imaged is spaced apart from the image plane (47' ) onto which the at least one focusing region (90) is imaged, and the control device (50) is designed to determine in each case a variable that is characteristic of the defocus for imaging at least one imaging region (91) and at least one focusing region (90) and, if a non-optimum distance is determined on the basis of the determined variables that are characteristic of the defocus, to adapt the distance between the image capture unit (40) and the object stage (30) in a direction that can be derived from the characteristic variables.

2. Device according to Claim 1, characterized in thatat least two focusing regions (90) are provided and the optical unit (41) is designed such that the image plane (47) onto which the at least one imaging region (91) is imaged is arranged between at least two image planes (47' ) onto which at least one focusing region (90) is imaged in each case.

3. Device according to one of the preceding claims, characterized in that the beam path in the optical unit (41) for imaging a focusing region (90) is lengthened or shortened relative to the beam path for imaging an imaging region (91) .

4. Device according to Claim 3, characterized in that the lengthening or shortening of the beam path in the optical unit (41) is achieved by suitably arranging additional optical elements (48) .

5. Device according to one of the preceding claims, characterized in that the image capture unit (40) comprises at least two image capture sensors (42) arranged in a common plane.

6. Device according to one of the preceding claims, characterized in that the device (1) comprises at least three image capture sensors ( 42 ) .

7. Device according to one of the preceding claims, characterized in that the device (1) comprises four image capture sensors (42) .

8. Device according to one of the preceding claims, characterized in that at least one focusing region (90) is arranged with anoffset with respect to an imaging region (91) in the scanning direction (31) .

9. Device according to Claim 7, characterized in that the at least one focusing region (90) is arranged in a manner overlapping the imaging region (91) in a projection onto a plane perpendicular to the scanning direction (31) .

10. Device according to one of the preceding claims, characterized in that the characteristic variable determined is a variable derived from the contrast (K) of a captured image representation .

11. Device according to one of the preceding claims, characterized in that the image capture sensors (42) are TDI sensors.

12. Device according to one of Claims 1 to 10, characterized in that the image capture sensors (42) are line sensors.

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

14. Device according to one of the preceding claims, characterized in that the object stage (30) is movable in a direction (32) perpendicular to the scanning direction (31) for changing the distance between the image capture unit (40) and the ob- j ect stage ( 30 ) .

15. Device according to one of the preceding claims, characterized in that the device (1) is designed for reflective photolithographic masks (20) .

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

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