Mask metrological measuring device and method for examining a photomask

Abstract

Mask metrological measuring device comprising a radiation source (14), an illumination system (16), an imaging system (19) and an image sensor (20), wherein the illumination system (16) is configured to illuminate a first illumination field (22) on a photomask (17) with radiation emitted by the radiation source (14) in a first measurement state, and wherein the imaging system (19) is configured to generate an image on the image sensor (20) from the first illumination field (22), wherein the measuring device is switchable between the first measurement state and a second measurement state, wherein in the second measurement state a second illumination field (23) on the photomask (17) is illuminated, wherein the first illumination field (22) is arranged within the second illumination field (23), wherein in the second measurement state the first illumination field (22) is not illuminated, and wherein stray light generated in the second measurement state is recorded with the image sensor (20).

Description

[0001] The invention relates to a mask metrological measuring device and a method for examining a photomask.

[0002] Photomasks are used in microlithographic projection exposure systems for the fabrication of integrated circuits with extremely small structures. The photomask (= reticulum), illuminated with very short-wavelength, deep ultraviolet or extreme ultraviolet radiation (DUV or EUV radiation), is projected onto a lithography object to transfer the mask structure.

[0003] For high-quality images produced on lithographic objects, the dimensional accuracy of the structures on the photomask is crucial. The production of a photomask typically involves verifying that the structure conforms to specifications after its creation. Any areas of the photomask where this is not the case are corrected. A measuring device for verifying the dimensional accuracy of a photomask is described, for example, in DE 10 2013 212 613 A1.

[0004] For correction, a laser beam can be applied to the photomask substrate to create local scattering centers, known as pixels. These pixels, corresponding to changes in the photomask's material structure, can be created in selected areas of the photomask, which may extend over several millimeters, for example. Incident radiation is scattered at the pixels, reducing the intensity of the radiation passing through the photomask in these areas. This local reduction in radiation intensity improves the image quality on the lithographed object.

[0005] After the pixels are written, a check is performed to determine whether the intended correction effect has occurred. A known method for this purpose is to generate a so-called aerial image of the photomask, in which the photomask is projected not onto a lithographic object, but onto an image sensor. Based on the image on the sensor, an assessment can be made as to whether the photomask correction was successful.

[0006] In this check, the entire photomask is not mapped onto the image sensor, but rather a local section of the photomask is illuminated and mapped, in which the pixels have been added.

[0007] Local illumination of a section of the photomask is insufficient to fully determine the effects of the inscribed pixels on the lithographic object. If the photomask is illuminated over a larger area, pixels located some distance from the local section can also influence the image. This is because the radiation is scattered in various directions by the pixels, and some of this scattered light can reach the relevant area of ​​the image. This scattered light component is not taken into account when only a local section of the photomask is illuminated and imaged onto the image sensor.

[0008] An additional check is required to determine the effect on the image when not just a local section, but a larger area of ​​the photomask is illuminated. Currently, this check is performed using a wafer print, in which a wafer is exposed in a microlithographic projection exposure system. This type of check is time-consuming and expensive.

[0009] The invention is based on the objective of presenting a mask metrological measuring device and a method for examining a photomask, with which these disadvantages are avoided. This objective is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0010] The mask metrological measuring device according to the invention comprises a radiation source, an illumination system, an imaging system, and an image sensor. The illumination system is designed to illuminate a first illumination field on a photomask with radiation emitted by the radiation source in a first measurement state. The imaging system is designed to generate an image on the image sensor from the first illumination field. The measuring device is switchable between the first measurement state and a second measurement state. In the second measurement state, a second illumination field on the photomask is illuminated, wherein the first illumination field is arranged within the second illumination field and the first illumination field is not illuminated. Stray light generated in the second measurement state is recorded by the image sensor.

[0011] The first measurement state corresponds to the conventional procedure, in which a local section of the photomask is illuminated with the first illumination field to verify the mapping of the first illumination field after the pixels have been introduced. The second measurement state examines the area surrounding the first illumination field while the first illumination field itself is not illuminated. Since there is no direct radiation incident on the image sensor due to the first illumination field being blocked, the image sensor only measures the scattered light component that reaches it from the area surrounding the first illumination field. Because the pixels are not arranged in the same plane as the photomask structure examined in the first measurement state, no mapping of the pixels onto the image sensor occurs. The sole purpose is to determine the amount of scattered light that is dispersed by the pixels towards the image sensor.

[0012] The measuring device can be configured such that, in the first measurement state, the entire first illumination field is not imaged onto the image sensor, but rather a section of the first illumination field is imaged onto the image sensor. The imaged section can be a central portion of the first illumination field, completely surrounded by the first illumination field. The section can extend over at least 40%, preferably at least 60%, of the edge length of the first illumination field. This can apply to both dimensions of the first illumination field. Stray light components arising in the edge regions of the first illumination field can also fall onto the image sensor.

[0013] Assuming that areas of the photomask outside the second illumination field are so far from the first illumination field that there is no significant stray light incident from there, the sum of the radiation in the first measurement state and in the second measurement state is equal to the total radiation received by the image sensor when the photomask is illuminated over a large area. The total radiation can be calculated from an image taken in the first measurement state and an image taken in the second measurement state using appropriate calculation steps.

[0014] In the second measurement state, an inverted aperture can be positioned in the illumination beam path, blocking the first illumination field. This means that the inverted aperture is positioned in the beam path of the illumination system such that the radiation emitted by the light source does not reach the first illumination field. The inverted aperture should be removed from the illumination beam path in the first measurement state.

[0015] The first illumination field can correspond to a small section of the photomask. For example, if the photomask has a square shape and an edge length between 100 mm and 200 mm, the edge length of the also square first illumination field can be between 5 µm and 100 µm, preferably between 10 µm and 50 µm. The area of ​​the first illumination field can be increased by at least a factor of 10. 5 , preferably by at least a factor of 10 7be smaller than the area of ​​the photomask.

[0016] The second illumination field completely surrounds the first illumination field, with the area of ​​the second illumination field being larger than the area of ​​the first illumination field. For example, if the second illumination field is square, it can have an edge length between 1 mm and 10 mm, preferably between 2 mm and 5 mm. The area of ​​the second illumination field can be increased by at least a factor of 10. 3 , preferably by at least a factor of 10 4 must be larger than the area of ​​the first illumination field. The area of ​​the second illumination field can be at least 10 times larger. 2 , preferably by at least a factor of 10 3 be smaller than the area of ​​the photomask. For the purposes of this information, the area of ​​the first illumination field is not included in the area of ​​the second illumination field.

[0017] Unlike a measuring device that only examines the first illumination field, the second measurement state places higher demands on the amount of radiation transmitted by the illumination system, since the radiation is distributed over a larger area in the second illumination field. The illumination system must provide a greater amount of radiation in the second measurement state to ensure a clear signal is displayed on the image sensor. Against this background, the invention proposes a number of measures to ensure that the second illumination field is supplied with a sufficient amount of radiation.

[0018] A pupil-shaping mirror element or a mirror array can be arranged in the pupil plane of the illumination system. The mirror array can comprise a frame structure and a plurality of mirror elements movably mounted on the frame structure. Because the mirror elements are arranged in the pupil plane, the radiation reflected by each individual mirror element is distributed across the entire area of ​​the illumination field in the plane of the photomask. The angle of incidence at which the radiation strikes the illumination field varies from mirror element to mirror element. The angular distribution of the incident radiation is called the illumination setting.

[0019] The illumination setting can be influenced in a desired way by adjusting the mirror elements of the mirror array. The current approach involves adjusting individual mirror elements of the array so that only a portion of the radiation striking the mirror element is directed onto the illumination field. The remaining portion of the radiation is lost and does not reach the image sensor.

[0020] The illumination setting is generally determined by the photomask under investigation, in that the photomask should be examined using the same illumination setting that is applied when using the photomask in the microlithographic projection exposure system. Dividing the radiant power incident on the illumination field by the number of mirror elements in the mirror array yields the average contribution that each individual mirror element makes to the radiation on the illumination field. In any given illumination setting, there are above-average mirror elements that make an above-average contribution, and below-average mirror elements that make a below-average contribution.

[0021] The measuring device according to the invention can be configured such that the radiation incident on the mirror array has an intensity distribution adapted to the illumination setting. In other words, there is at least one above-average mirror element onto which a higher radiant power is directed than onto the average of the mirror elements. In particular, at least 20%, preferably at least 50%, and more preferably at least 70% of the above-average mirror elements can be directed with a higher radiant power than onto the average of the mirror elements. There can be at least one below-average mirror element onto which a lower radiant power is directed than onto the average of the mirror elements.In particular, a lower radiant power can be directed onto at least 20%, preferably at least 50%, and further preferably at least 70% of the below-average mirror elements than onto the average of the mirror elements.

[0022] In the beam path of the illumination system, a first optical element can be arranged between the radiation source and the mirror array. This element shapes the beam path to have an intensity distribution adapted to the illumination setting. It can be an optical element through which the illumination beam passes. In one embodiment, the first optical element is a diffractive optical element (DOE). The DOE can comprise a glass substrate equipped with an optical grating. The optical grating can be designed such that, starting from an input beam with a predetermined intensity distribution, an output beam is produced whose intensity distribution is adapted to the illumination setting. Embodiments are also possible in which the first optical element is designed as a lens or lens array.

[0023] To enable the use of the measuring device with different illumination settings, the illumination system can include a switching device arranged between the radiation source and the mirror array. In a first state of the switching device, the radiation incident on the mirror array has an intensity distribution adapted to a first illumination setting, and in a second state of the switching device, the radiation incident on the mirror array has an intensity distribution adapted to a second illumination setting. In the first state of the switching device, a first variant of the first optical element can be arranged in the illumination beam path, and in the second state of the switching device, a second variant of the first optical element can be arranged.The switching device can be designed to switch more than two variants of the first optical element into the illumination beam path, in particular at least three, preferably at least five variants. With each variant of the first optical element, the intensity distribution of the radiation incident on the mirror array can be adapted to a different illumination setting.

[0024] The switching device can be actuated to adjust the intensity distribution, in order to switch the measuring device between a first type of photomask and a second type of photomask. It is also possible for the switching device to be actuated when the measuring device is switched between the first and second measurement states.

[0025] In existing measuring devices for examining microlithographic photomasks, the illumination system is typically designed such that the beam path strikes a mirror array with a plurality of mirror elements multiple times. The invention proposes an alternative embodiment of the illumination system in which the beam path is reflected only once by such a mirror array. This contributes to reducing radiation losses in the illumination system.

[0026] To switch the measuring device between the first and second measuring states, a mechanical change can be made to optical components arranged in the illumination beam path. In particular, optical components can be inserted into or removed from the beam path. The measuring device can include one or more actuators to move the components. A control unit can be provided that sends control signals to actuate the actuators. The control unit can be designed to switch the measuring device between the first and second measuring states.

[0027] In the first measurement state, a first field stop can be positioned in the illumination beam path, so that the radiation hits the first illumination field and areas of the photomask lying outside the first illumination field are not illuminated. In the second measurement state, the first field stop can be removed from the illumination beam path.

[0028] In the first measurement state, a diffractive optical element in the form of a first field-DOE can be arranged in a pupil plane of the illumination system located between the mirror array and the condenser optics for field generation.

[0029] In the second measurement state, a second field diaphragm can be arranged in the illumination beam path, so that the radiation does not strike the area of ​​the photomask lying outside the second illumination field. In the first measurement state, the second field diaphragm can be removed from the illumination beam path. The inverse diaphragm, which blocks out the first illumination field, can be arranged in the same plane of the illumination beam path as the second field diaphragm. In one embodiment, the inverse diaphragm is a component of the second field diaphragm.

[0030] In the second measurement state, a diffractive optical element in the form of a second field DOE can be positioned in a pupil plane of the illumination system located between the mirror array and the condenser optics. The second field DOE can be positioned in the same pupil plane as the first field DOE or in a different pupil plane. In the first measurement state, the second field DOE can be located away from the illumination beam path.

[0031] The state of the imaging system, the state of a condenser optic of the illumination system, and / or the position of the image sensor can be identical in the first and second measurement states. As a result, a large portion of the radiation directed onto the second illumination field does not reach the image sensor, since the first illumination field, which is imaged onto the image sensor, is blocked. Therefore, in the second measurement state, only radiation scattered towards the image sensor by pixels located outside the first illumination field reaches the image sensor.

[0032] The invention encompasses the concept of removing optical components, which are necessary in the first measurement state to achieve sufficient image quality and which are arranged in the beam path of the illumination system in the first measurement state, from the beam path of the illumination system in the second measurement state. This can include a polarizing filter, a polarizing adjuster, and / or a beam attenuator. An adjustable beam attenuator is also possible; this attenuates the radiation emitted by the radiation source in the first measurement state and is switched to transmit mode in the second measurement state.

[0033] A further contribution to reducing radiation losses in the lighting system can be made by a prism arrangement with the following features. The prism arrangement preferably comprises two prisms positioned between the radiation source and the first optical element, which adapts the intensity distribution to the lighting setting. The prism arrangement can be configured to deflect the illumination beam by 90°. Each of the preferably two prisms can have a prism angle between 55° and 65°. It can be advantageous to select identical prism angles for the preferably two prisms. Additionally, the angle of incidence of the radiation can be the same for each of the prisms. Adjusting the prism arrangement can be simplified if the exit face of the first prism is oriented perpendicular to the beam entering the first prism.Such a prism arrangement can help to stretch an incoming beam laterally in one dimension. This can be helpful if the incoming beam, viewed in cross-section, has a greater extent in one dimension than in another. Furthermore, such a prism arrangement can help to increase the divergence of the incoming radiation in one dimension if it contains multiple wavelengths, by making the direction of propagation of the radiation wavelength-dependent after passing through the prism arrangement. This can be advantageous if the incoming beam has a greater divergence in one dimension than in another.

[0034] Another way to obtain sufficient signal strength on the image sensor is to adjust the sensor's exposure time. A control unit of the measuring device can be configured so that the exposure time in the second measurement state is longer than in the first. In particular, the exposure time in the second measurement state can be at least twice as long, and preferably at least five times longer, than the exposure time in the first measurement state. The exposure time in the first measurement state can, for example, be between 0.1 s and 0.5 s.

[0035] It is further advantageous if the number of mirror surfaces at which the radiation is reflected within the lighting system is small. The invention proposes to design the lighting beam path such that the radiation is reflected at no more than five mirror surfaces, preferably no more than four.

[0036] A beam splitter can be arranged in the illumination beam path to direct a portion of the radiation to an energy monitor. The readings from the energy monitor can serve as a reference for the total amount of radiation directed onto the illumination field. Based on these readings, the image data acquired by the image sensor can be subjected to energy normalization. The beam splitter can be positioned between the mirror array and a condenser optic within the illumination beam path.

[0037] The measuring device according to the invention can include an XY positioner that carries the photomask. The XY positioner allows the photomask to be moved perpendicular to the direction of the illumination beam path in order to position the photomask so that the illumination beam path strikes a different area of ​​the photomask. This opens up the possibility of examining different areas of the photomask.

[0038] The radiation source can be a laser. The emitted illumination radiation preferably has the same wavelength as that used in the microlithographic projection exposure system in which the photomask is used. It can be DUV radiation in the deep ultraviolet spectral range with a wavelength between 100 nm and 300 nm. In one embodiment, the wavelength is 193 nm.

[0039] In an alternative embodiment of the invention, in the second measurement state, the second illumination field is not subjected to a uniform exposure process, but instead scanned with a laser beam. For this purpose, the illumination system can be designed such that a laser beam with a small beam diameter is directed onto a scanning optic, and the scanning optic is controlled so that the laser beam scans the second illumination field in a temporal sequence. The scanning optic can be controlled such that the first illumination field is excluded from the scanning process. In this way, the scanning can be limited to the second illumination field without the need for an aperture in the beam path of the illumination system that would block out the first illumination field. The scanning optic can comprise two scan mirrors, each of which is pivotably mounted about an axis.

[0040] The axes can be orthogonal to each other. A single scanning mirror, pivoted around two axes, is also possible. Alternatively, a design is possible in which the scanning optics comprise a scanning mirror and a rotating wedge.

[0041] The invention also relates to a method for examining a photomask, in which, in a first measurement pass, a first illumination field on a photomask is illuminated with radiation emitted from a radiation source, and in which an image is generated on the image sensor from the first illumination field. In a second measurement pass, a second illumination field on the photomask is illuminated, wherein the first illumination field is arranged within the second illumination field, and wherein, in the second measurement state, the first illumination field is not illuminated. Stray light generated in the second measurement state is recorded by the image sensor.

[0042] The disclosure includes further developments of the method with features described in connection with the measuring device according to the invention.

[0043] The invention is described below by way of example with reference to the accompanying drawings and advantageous embodiments. The drawings show: Fig. 1: a schematic representation of a measuring device according to the invention; Fig. 2: A top view of a photomask with a second lighting field; Fig. 3: the second lighting field off Fig. 2 in an enlarged view; Fig. 4: a schematic representation of ray paths in a photomask; Fig. 5: a schematic representation of the first measurement state; Fig. 6: a schematic representation of the second measurement state; Fig. 7: the lighting system of a measuring device according to the invention in the first measuring state; Fig. 8: the lighting system of a measuring device according to the invention in the second measuring state; Fig. 9: a schematic representation of a 4f optic; Fig. 10: the prism arrangement of the lighting system made of Fig. 8, Fig. 9; Fig. 11: a detail of an alternative embodiment of a measuring device according to the invention; Fig. 12: An exemplary intensity distribution in a lighting beam path adapted to the lighting setting.

[0044] A mask metrological measuring device according to the invention serves to examine the structure of a photomask 17. The photomask 17 is intended for use in a microlithographic projection exposure system (not shown). In the microlithographic projection exposure system, the photomask 17 is illuminated with deep ultraviolet radiation (DUV radiation) with a wavelength of, for example, 193 nm in order to image a structure formed on the photomask 17 onto the surface of a lithographic object in the form of a wafer. The wafer is coated with a photoresist that reacts to the DUV radiation. The measuring device is used to examine whether the structure on the photomask 17 corresponds to the dimensional specifications.

[0045] The measuring device is designed according to Fig. 1. The photomask 17 is arranged such that a beam path emanating from a laser radiation source 14 passes through the photomask 17 and is directed to an image sensor 20. The radiation has a wavelength of 193 nm, which corresponds to the DUV radiation used in the microlithographic projection exposure system. An illumination system 16 is arranged between the laser radiation source 14 and the photomask 17. This system shapes the laser beam emitted by the laser radiation source 14 so that it uniformly illuminates an illumination field within the area of ​​the photomask 17. An imaging system 19 images the structure of the photomask 17 onto an image sensor 20. The section of the beam path between the laser light source 14 and the photomask 17 is referred to as the illumination beam path 15. The section of the beam path between the photomask 17 and the image sensor 20 is referred to as the imaging beam path 21.

[0046] According to Fig. 2 The illumination field lit by the lighting system 16 is small compared to the area of ​​the photomask 17. In Fig. Figure 2 shows the second lighting field 23 in a representation not to scale; the first lighting field 22 is shown according to Fig. 3 is significantly smaller again. In an exemplary embodiment, the photomask 17 is square with an edge length of approximately 150 mm. The second illumination field can also be square and have an edge length of, for example, 4 mm. The first illumination field 22, which is also square, can have an edge length of, for example, 30 µm.

[0047] In the measuring device, the photomask 17 is arranged on an XY positioner 18, which is in Fig. Figure 1 is shown schematically. By moving the photomask 17 in the XY plane, the illumination beam path 15 can be directed onto different areas of the photomask 17.

[0048] Conventional measuring devices are designed to generate a high-resolution image of the first (small) illumination field 22 on the image sensor. In Fig. Figure 4, shown in the right part of the diagram, illustrates the case where the illumination beam path 15 only strikes the first illumination field 22. A portion of the radiation is scattered at pixels 24 generated in the substrate of the photomask 17. The scattered light components 25 do not reach the lens 26 of the imaging system 19, and therefore do not contribute to the signal measured by the image sensor 20.

[0049] This is different in the left part of the illustration in Fig. 4, where the illumination beam path 15 illuminates a larger area of ​​the photomask 17. Furthermore, only the part of the illumination beam path 15 that strikes the first (small) illumination field 22 enters the lens 26 of the imaging system 19 directly. However, stray light components 25 are generated from areas of the photomask 17 located outside the first illumination field 22, which enter the lens 16 of the imaging system 19 and thus contribute to the signal that the image sensor 20 measures.

[0050] The invention proposes equipping the measuring device with a first measuring state in which the illumination beam path 15 illuminates only the first illumination field 22, so that an image of the photomask structure in the area of ​​the first illumination field 22 is formed on the image sensor 20. In a second measuring state, the illumination beam path 15 is directed onto the second (larger) illumination field 23, while the first illumination field 22 is blocked. In the second measuring state, only the scattered light components 25 enter the lens 26 of the imaging system 19 and are recorded by the image sensor 20. By suitable calculation steps, a complete image can be calculated from an image recorded in the first measuring state and the scattered light recorded in the second measuring state. This complete image corresponds to the radiation that strikes the relevant section of the wafer in the microlithographic projection exposure system.

[0051] In Fig. Figure 5 schematically depicts a section of the illumination beam path 15 in the first measurement state. A first field stop 28 is arranged in the illumination beam path 15, so that, in conjunction with a condenser optic 31, the first illumination field 22 is illuminated with uniform intensity. The first illumination field 22 is located on the underside of the photomask 17, where the structure of the photomask 17 is applied in the form of a chrome coating. The substrate of the photomask 17 is made of glass.

[0052] In the second measurement state, according to Fig. 6. Instead of the first field diaphragm 28, a second field diaphragm 29 is arranged in the illumination beam path 15. The second field diaphragm 29 includes an inverted diaphragm 30, which blocks the first illumination field 22. In other words, the inverted diaphragm 30 prevents the illumination radiation 15 from reaching the first illumination field 22. The outer edge of the inverted diaphragm 30 defines the inner edge of the second illumination field 23. The inner edge of the second field diaphragm 29 corresponds to the outer edge of the second illumination field 23. The second illumination field 23 is illuminated uniformly via the condenser optics 31.

[0053] In Fig. Figure 7 schematically depicts the illumination beam path 15 between the laser radiation source 14 and the condenser optics 31 for the first measurement state of the measuring device. A laser beam emitted from the laser radiation source 14 is first guided through a beam attenuator 32. The beam attenuator 32 is adjusted so that the intensity of the laser beam is matched to the sensitivity of the image sensor 20. With a prism arrangement 33, which is in Fig. As shown in Figure 10, the illumination beam path 15 is deflected by 90° and the beam cross-section is simultaneously stretched laterally. The prism arrangement 33 comprises two prisms 42, 43, which have the same prism angles and are arranged such that the angle of incidence is the same for both prisms 42, 43. The exit surface of the first prism 42 is oriented perpendicular to the direction of the incoming beam, which facilitates the adjustment of the prism arrangement 33.

[0054] Following the prism arrangement 33, the illumination beam path 15 encounters a first optical assembly 34. The optical assembly 34 comprises a 4f optic with a first lens 44 and a second lens 46. The first lens 44 is arranged at a distance f1 from the output of the laser light source 14, which corresponds to the focal length of the first lens 44, see Fig. 9. At the same distance f1 on the other side of the first lens 44, a first DOE 45 is arranged, with which the intensity distribution in the cross-section of the illumination beam path 15 is adapted to the illumination setting. The first DOE 45 corresponds to a first optical element within the meaning of the invention.

[0055] In Fig. Figure 12 shows the cross-section of the illumination beam path 15 on the horizontal axis and the intensity of the radiation on the vertical axis. In this example, the illumination beam path 15 has a ring-shaped intensity distribution 58 with a low intensity in the center of the illumination beam path 15 and a high intensity extending in a ring around the center.

[0056] Other lighting settings require different intensity distributions in the cross-section of the lighting beam path 15. The first DOE 45 is therefore supported by a changer device 47, which is designed to position different DOE at this point in the lighting beam path 15, depending on the desired lighting setting.

[0057] According to Fig. A second lens 46 of the 4f optics is arranged at a distance f2 from the first DOE 45. The distance f2, which is smaller than the distance f1, corresponds to the focal length of the second lens 46. On the other side of the second lens 46, at a distance f2, a pupil-shaping mirror element or a mirror array 35 is arranged, comprising a plurality of mirror elements that are movably suspended from a frame structure and whose orientation relative to the frame structure can be individually adjusted. The near field of the laser light source 14, shaped by the first DOE 45, is imaged onto the mirror array 35.

[0058] The mirror array 35 is arranged in a pupil plane 57 of the illumination system 16, such that radiation reflected from the mirror array 35 is distributed with uniform intensity over the first illumination field 22. By adjusting the mirror elements of the mirror array 35, the illumination setting can be varied, i.e., the angular distribution at which the radiation strikes the first illumination field 22. The adjustment of the intensity distribution to the illumination setting by the first DOE 45 is configured such that mirror elements that contribute disproportionately to the radiation incident on the first illumination field 22 are illuminated disproportionately, and vice versa.

[0059] A second optical assembly 36, which also includes a 4f optic and serves for spatial filtering, directs the illumination radiation to a third optical assembly 37. The third optical assembly 37 comprises a first field DOE 48, which generates the field, and a first field stop 49. Via a tube lens 38 and a polarizing filter / polarizing controller 39, the illumination beam path 15 is directed to a beam splitter 41, through which a first portion of the radiation is directed to the condenser optic 31 and a second portion to an energy monitor 40. The energy monitor 40 serves as a reference for the amount of radiation directed onto the first illumination field 22. During subsequent image processing, the measurement data recorded by the energy monitor 40 can be used to perform energy normalization of the image data acquired by the image sensor 20.

[0060] In the second measurement state, which in Fig. As shown schematically in Figure 8, a second field DOE 51 is arranged between the beam splitter 41 and the condenser optics 31. The field DOE 51 generates the field for the second (large) illumination field 23. A fourth optical assembly 52 adapts the beam path to the condenser optics 31. The fourth optical assembly 52 comprises another 4f optic. The second field stop 29, 30 is arranged between the two lenses of the 4f optic, limiting the second illumination field 23 and blocking out the first illumination field 22.

[0061] Some components that were arranged in the illumination beam path 15 in the first measurement state are removed in the second measurement state. These include the first field stop 49, which limits the field in the first measurement state. Also included are the polarization filter / polarization controller 39 and the beam attenuator 32.

[0062] The measuring device comprises in Fig. The device includes eight schematically indicated actuators 60, which can be used to insert components into or remove them from the illumination beam path 15. The measuring device comprises a control unit 59, which is designed to control the actuators 60 with control signals. In particular, the control unit 59 is designed to switch the measuring device between the first and second measuring states. The control unit 59 can also be designed to control the exposure process. The exposure process can be longer in the second measuring state than in the first measuring state, for example, 0.2 s in the first measuring state and 2 s in the second measuring state.

[0063] In an alternative embodiment of a measuring device according to the invention, which is located in Fig. As shown in Figure 11, the image sensor 20 is not exposed in a single exposure process; instead, the second illumination field 23 is scanned with a laser beam. The illumination system 16 is configured so that the radiation, in the form of a concentrated laser beam, strikes the beam splitter 41. The orientation of the beam splitter 41 can be varied about two axes, so that the beam splitter 41 forms a scanning optic with which the second illumination field 23 can be scanned. An actuator 56 of the scanning optic is located in Fig. 11 indicated.

Claims

[1] Mask metrological measuring device comprising a radiation source (14), an illumination system (16), an imaging system (19) and an image sensor (20), wherein the illumination system (16) is configured to illuminate a first illumination field (22) on a photomask (17) with radiation emitted by the radiation source (14) in a first measurement state, and wherein the imaging system (19) is configured to generate an image on the image sensor (20) from the first illumination field (22), wherein the measuring device is switchable between the first measurement state and a second measurement state, wherein in the second measurement state a second illumination field (23) on the photomask (17) is illuminated, wherein the first illumination field (22) is arranged within the second illumination field (23), wherein in the second measurement state the first illumination field (22) is not illuminated,and wherein scattered light generated in the second measurement state is recorded with the image sensor (20). [2] Measuring device according to claim 1, wherein in the second measuring state an inverted aperture (30) is arranged in the illumination beam path and wherein the inverted aperture (30) blocks out the first illumination field (22). [3] Measuring device according to claim 1 or 2, wherein the area of ​​the second illumination field (23) is increased by at least a factor of 10 5 , preferably by at least a factor of 10 7 smaller than the area of ​​the photomask (17). [4] Measuring device according to one of claims 1 to 3, wherein the area of ​​the first illumination field (22) is increased by at least a factor of 10 3 , preferably by at least a factor of 10 4 smaller than the area of ​​the second illumination field (23). [5] Measuring device according to one of claims 1 to 4, wherein a pupil-shaping mirror element or a mirror array (35) is arranged in a pupil plane (57) of the illumination system (16), comprising a frame structure and a plurality of mirror elements movably mounted on the frame structure, and wherein the illumination system (16) is arranged such that the radiation incident on the mirror array (35) has an intensity distribution (58) adapted to the illumination setting. [6] Measuring device according to claim 5, wherein the illumination system (16) has a switching device (47) arranged between the radiation source (14) and the mirror array (35), such that in a first state of the switching device (47) the radiation incident on the mirror array (35) has a first intensity distribution (58) adapted to a first illumination setting and that in a second state of the switching device (47) the radiation incident on the mirror array has a second intensity distribution adapted to a second illumination setting. [7] Measuring device according to claim 5 or 6, wherein the lighting system is arranged such that the beam path is reflected exactly once at the mirror array (35). [8] Measuring device according to one of claims 1 to 7, comprising one or more actuators (60) for moving optical components arranged in the illumination beam path (15), and with a control unit (59) designed to control the actuators (60) in order to switch the measuring device between the first measuring state and the second measuring state. [9] Measuring device according to one of claims 1 to 8, wherein the exposure time of the image sensor (20) in the second measuring state is longer than in the first measuring state. [10] Measuring device according to one of claims 5 to 9, wherein a beam splitter (41) directs part of the radiation to an energy monitor (40) and wherein the beam splitter (41) is arranged in the illumination beam path (15) between the mirror array (35) and a condenser optic (31) of the illumination system (16). [11] Measuring device according to claim 1, wherein in the second measuring state the illumination beam path (15) is directed onto a scanning optic (41, 56) and wherein the second illumination field (23) is scanned with a laser beam. [12] Measuring device according to one of claims 1 to 11, wherein the radiation in the lighting system (16) is reflected on no more than five mirror surfaces, preferably on no more than four mirror surfaces. [13] Method for examining a photomask, wherein in a first measurement pass a first illumination field (22) on a photomask (17) is illuminated with radiation emitted from a radiation source (14), wherein an image is generated from the first illumination field (22) on an image sensor (20), wherein in a second measurement pass a second illumination field (23) on the photomask (17) is illuminated, wherein the first illumination field (22) is arranged within the second illumination field (23) and wherein in the second measurement state the first illumination field (22) is not illuminated, and wherein stray light generated in the second measurement state is recorded with the image sensor (20).

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