Measuring system for optical inspection of an adjustment state, method for optical inspection of an adjustment state, adjustment procedure

Abstract

Measuring system for optically checking the alignment of an illumination optic (16) relative to a projection optic (22). The illumination optic (16) is designed to illuminate an object (17) located in an object plane (25) of the projection optic (22), and the projection optic (22) is designed to image a reflection generated at the object onto an image plane of the projection optic (22). The measuring system comprises: a holder for positioning the illumination optic (16) and the projection optic (22); a partially transparent test object (29) arranged in the object plane (25); a measuring illumination unit (20) for generating a measuring beam path (27) that illuminates a test field (19) of the test object (29) from behind and is oriented such that the test field (19) is imaged onto a first sensor plane by the illumination optic (16) and onto a second sensor plane by the projection optic (22).a first sensor (31a) for detecting an image generated in the first sensor plane; a second sensor (31b) for detecting an image generated in the second sensor plane; an aperture device (30) arranged in the measuring beam path (27) between an illumination source (26) and the object plane (25), comprising a plurality of pixels (32) that are each switchable between a first transmission state and a second transmission state; and a control unit (34) for switching the plurality of pixels (32) between the first transmission state and the second transmission state. The switchable pixels of the aperture device allow the measuring beam path to be flexibly adapted to the measurement task without manipulation of mechanical elements.

Description

[0001] The invention relates to a measuring system for optically testing the alignment state of an illumination optic relative to a projection optic, a method for optically testing an alignment state, and a corresponding alignment method. The illumination optic and the projection optic can, in particular, be part of a mask inspection system for checking photomasks for semiconductor lithography.

[0002] Semiconductor lithography is used in the fabrication of microstructured components, such as integrated circuits. The microlithography process is carried out in a projection exposure system, which comprises an illumination system and a projection system. The image of a photomask (also called a "reticule") illuminated by the illumination system is projected by the projection system onto a substrate, such as a silicon wafer, coated with a photosensitive layer (called a "photoresist") and positioned in the image plane of the projection system. This transfers the mask structure onto the coated substrate. In subsequent fabrication steps, the transferred structure is implemented in the substrate, for example, by etching or material deposition.

[0003] For high-quality images produced on the coated substrate, the mask must be dimensionally accurate and free from contaminants. It is known to inspect photomasks before use in a microlithographic projection exposure system or during downtime. For this purpose, a so-called aerial image of the mask, or a section thereof, is generated using a mask inspection system. In this image, the photomask is projected not onto the coated substrate, but onto a photosensor. Based on this image, it can be assessed whether the photomask is free of defects and contaminants. To inspect the photomask with the highest accuracy, the illumination and projection optics of the mask inspection system must be precisely aligned.

[0004] The object of the present invention is to provide a measuring system for optically testing the alignment state of an illumination optic relative to a projection optic, a corresponding method for optically testing the alignment state, and an alignment method with which increased accuracy can be achieved. This object is achieved by the features of the independent claims. Advantageous embodiments are described in the dependent claims.

[0005] Accordingly, the invention relates to a measuring system for optically checking the alignment state of an illumination optic relative to a projection optic. The illumination optic is designed to illuminate an object located in an object plane of the projection optic. Furthermore, the projection optic is designed to image a reflection generated at the object onto an image plane of the projection optic. The measuring system comprises: - a bracket for positioning the lighting optics and the projection optics; - a partially transparent test object that is arranged in the object plane; - a measuring illumination unit for generating a measuring beam path that illuminates a test field of the test object from behind and is aligned in such a way that the test field is imaged onto a first sensor plane by the illumination optics and onto a second sensor plane by the projection optics; - a first sensor for capturing an image of the test field generated in the first sensor plane; - a second sensor for capturing an image of the test field generated in the second sensor plane; - an aperture device arranged in the measuring beam path between an illumination source and the object plane, comprising a plurality of pixels, each of which can be switched between a first transmission state and a second transmission state different from the first; and - a control unit for switching the majority of pixels between the first transmission state and the second transmission state.

[0006] The measuring beam path strikes the test object from the rear, i.e., on the side opposite the illumination and projection optics. It illuminates the test area of ​​the object, which is then projected onto the first sensor via a first partial beam from the measuring beam path through the illumination optics and onto the second sensor via a second partial beam from the measuring beam path through the projection optics. The test object is suitable for adjusting the illumination optics relative to the projection optics and may, for example, have suitable markings that can be detected by the sensors. These sensors can be, for example, CCD or CMOS sensors.

[0007] The illumination optics and the projection optics can be integrated into the measuring system for measurement. For this purpose, the projection optics and the illumination optics can be fixed to a support structure that can be inserted into the measuring system. The support structure can, for example, have a mounting surface on which it is positioned. By positioning the illumination optics and the projection optics, they assume a defined state relative to the measurement beam path generated by the measurement illumination unit. The illumination optics and the projection optics can then be adjusted relative to the measurement beam path and relative to each other. For example, the image of the test field captured by the sensors can be modified by suitable adjustment measures until a desired result is achieved on both sensors.Through adjustment measures, the exit pupil of the illumination optics can be aligned relative to the entrance pupil of the projection optics, whereby, in particular, the aperture cross-sections of the pupils and / or their relative orientations can be adjusted. The projection optics and the illumination optics can have suitable calibration devices for this purpose. After the adjustment has been carried out, the illumination optics and the projection optics, in their adjusted state, can be removed from the measuring system and installed in an optical system, especially a mask inspection system, and used there.

[0008] By imaging the illuminated test field onto separate sensors via both the illumination optics and the projection optics, the measurement beam path splits into a first and a second partial beam after passing through the test object. When a single measurement beam path is split into two partial beams that image the illuminated test field of the test object onto the two sensors, the problem can arise that, during the measurement of one of the partial beams, unwanted reflections from the test object are generated by the other partial beam. These reflections can also reach the sensor and thus distort the measurement.It was therefore known in the prior art to generate a spatially limited measuring beam with which the illuminated test field is projected onto only one of the sensors via only one of the optics at a time, wherein the measuring illumination unit is designed to pivot about a central fixed point located in the object plane for measuring the other optic. Since the measuring beam is more narrowly limited in this case and directed precisely onto the respective sensor used, unwanted reflections can be reduced. However, it was recognized within the scope of the invention that relocating the measuring illumination unit introduces mechanical vibrations into the measuring system, which can lead to misalignment of the components of the measuring system, thereby reducing the measuring accuracy.

[0009] With the aid of the aperture device according to the invention, a portion of the measuring beam path can be easily blocked without requiring any mechanical relocation of components of the measuring system. This makes it possible to perform a measurement using one of the partial beams sequentially, while the other partial beam is blocked by the aperture device, thus avoiding unwanted reflections. Furthermore, the control unit can selectively allow a beam corresponding to the respective pixel of the measuring beam path to pass through or at least partially block it. This allows a desired measuring beam profile to be generated, enabling the measuring beam path to be flexibly adapted to the specific measurement task.

[0010] In one embodiment, the control unit is configured to selectively switch a first pixel into one of two transmission states, independently of the switching state of a second pixel that differs from the first. In particular, it can also be provided that each of the plurality of pixels can be selectively switched into one of the different transmission states, independently of the switching state of the other pixels. Furthermore, the plurality of pixels can also be switched into several different transmission states. In this case, the different transmission states are distinguished by the transmission rate of the respective pixel. The independent switching of the pixels into the different transmission states also allows for the setting of more complex measurement beam profiles.This makes it possible, for example, to compensate for aberrations present in the optics being adjusted, thereby further improving the accuracy of the adjustment.

[0011] In one embodiment, the aperture device includes a pixel unit designed as a liquid crystal display. In this case, the pixels can be formed from liquid crystal segments in a generally known manner, and these segments can independently change their transparency, for example, by applying an electrical voltage to alter the orientation of the liquid crystals.

[0012] The aperture device can further include a pixel unit incorporating an absorption modulation (AMM) material. AMM materials have an absorption or transmission coefficient that can be modified by irradiation with light of a wavelength specific to the material. US Patent 2014 / 0300877 A1 proposes using such an AMM material to create a photolithographic mask, enabling the mask structures to be adjusted as desired. In the present embodiment, individual pixels of the pixel unit can consist of segments to which a desired transmission coefficient can be imprinted by irradiation with a suitable wavelength. Thus, in this embodiment as well, the pixels can be switched between different transmission states by irradiation with a suitable wavelength.In this case, the measuring system can have an irradiation source that can be controlled by the control unit and is designed to irradiate individual pixels with a wavelength suitable for changing the transmission state.

[0013] The aperture device can also include a pixel unit formed by a microelectromechanical system (MEMS). Using MEMS, it is also possible, in a generally known manner, to switch individual pixels between different transmission states without introducing (macroscopic) mechanical vibrations into the system.

[0014] In one embodiment, the aperture device comprises two pixel units whose pixels are aligned with each other in such a way as to increase the achievable contrast ratio between the first and second transmission states. The pixel units can be positioned parallel to each other and, in particular, perpendicular to the measuring beam path. A pixel of one of the pixel units is assigned to a corresponding pixel of the other pixel unit and can be switched simultaneously. If a portion of the measuring beam is to be blocked, both corresponding pixels are switched to a transmission state with low transmittance, so that the light beam loses intensity as it passes through each of the two corresponding pixels. In this way, the contrast can be significantly increased.It is also possible to combine more than two, for example three to five, mutually aligned pixel units into an aperture device.

[0015] In one embodiment, the aperture device has at least one of the following features: - a pixel size is between 100 µm and 400 µm; - the number of pixels is between 100 and 1000; and - The contrast ratio of the aperture device between the first transmission state and the second transmission state is between 1:10 6 and 1:3x10 7 .

[0016] It has been shown that these features can achieve an accuracy sufficient for the adjustment and inspection of a mask inspection system.

[0017] It can be provided that a pixel of the aperture device can be switched by the control unit into an intermediate state in which the pixel's transmittance is greater than in the first transmittance state and less than in the second transmittance state. In particular, a pixel can be switched by the control unit into a plurality of intermediate states in which the pixel exhibits different transmittances between those in the first transmittance state and those in the second transmittance state. By enabling the pixels to be switched into one or more different intermediate states, it is possible to adjust the measuring beam profile even more flexibly. In particular, this makes it possible to compensate for aberrations expected within the illumination optics or the projection optics by means of a suitable measuring beam profile.

[0018] The invention further relates to a method for optically testing the alignment state of an illumination optic relative to a projection optic. The illumination optic is configured to illuminate an object located in an object plane of the projection optic. Furthermore, the projection optic is configured to image a reflection generated at the object onto an image plane of the projection optic. The method comprises the following steps: - Arranging a partially transparent test object in the object plane; - Generating a measuring beam path that illuminates a test field of the test object from behind and is aligned in such a way that the test field is imaged onto a first sensor plane by the illumination optics and onto a second sensor plane by the projection optics; - Positioning an aperture device in the measuring beam path between the illumination source and the object plane, wherein the aperture device has a plurality of pixels, each of which can be switched between a first transmission state and a second transmission state; - Addressing the majority of pixels to generate a predefined measurement beam profile; - Capturing an image of the test field generated by the illumination optics and / or an image of the test field generated by the projection optics.

[0019] The disclosure includes further embodiments of the method, which can be further developed by features already described above in connection with the measuring system according to the invention. Furthermore, the disclosure includes further embodiments of the measuring system, which can be further developed by features described in connection with the method.

[0020] In one embodiment, the image of the test field generated by the illumination optics is captured while the second measuring beam is at least partially blocked, and the image of the test field generated by the projection optics is captured while the first measuring beam is at least partially blocked.

[0021] It may be possible to estimate the expected optical aberration of the illumination optics or the projection optics. Furthermore, the specified measurement beam profile can be generated in such a way that the expected optical aberration of the illumination optics or the projection optics is at least partially compensated.

[0022] The expected aberration can be caused, for example, by manufacturing or positioning inaccuracies of the optical elements, or by systematic inaccuracies in the measurement technology. Optical aberration can also be due to apodization of the illumination optics and / or the projection optics. Compensation creates an image of the measurement field with the most homogeneous intensity possible in the sensor plane. This improves the alignment accuracy. The expected optical aberration can be estimated in generally known ways, for example, using empirical data or mathematical calculations.

[0023] In one embodiment, the optical aberration of the illumination optics and / or the projection optics is estimated based on the image of the test field generated by the illumination optics and / or the projection optics. The predetermined measuring beam profile is generated such that the estimated optical aberration of the illumination optics and / or the projection optics is at least partially compensated. In this embodiment, a measurement is first performed to estimate any existing optical aberrations of the illumination optics or the projection optics based on the measurement result. Only in a subsequent measurement is the measuring beam profile adjusted accordingly to compensate for the measured optical aberration.

[0024] The invention further relates to a method for adjusting a lighting optic relative to a projection optic, in which an inventive method for optically testing an adjustment state of a lighting optic relative to a projection optic is carried out, wherein the lighting optic is adjusted relative to the projection optic based on the image of the test field produced by the lighting optic and the image of the test field produced by the projection optic.

[0025] Advantageous embodiments are explained below by way of example with reference to the accompanying drawings. These show: Fig. 1: a schematic representation of a mask inspection system; Fig. 2: a schematic representation of a measuring system according to the invention; Fig. 3: a schematic representation of the aperture device of the measuring system of the Fig. 2 in a first state; Fig. 4: a schematic representation of the aperture device of the measuring system of the Fig. 2 in a second state; Fig. 5: a schematic representation of the aperture device of the measuring system of the Fig. 2 in a third state; Fig. 6: a schematic representation of the measuring system of the Fig. 2 in a second state; Fig. 7: a schematic representation of the measuring system of the Fig. 2 in a third state; Fig. 8: a schematic representation of the aperture device of the measuring system of the Fig. 2 in a fourth state; Fig. 9: An illustration of a method for compensating for apodization using the aperture device of the measuring system of the Fig. 2; Fig. 10: An illustration of a method for compensating optical aberration using the aperture device of the measuring system of the Fig. 2.

[0026] Fig. Figure 1 shows a schematic representation of a mask inspection system for examining microlithographic photomasks 17. Microlithographic photomasks 17 are generally intended for use in a microlithographic projection exposure system (not shown). In the microlithographic projection exposure system, the photomask 17 is illuminated with extreme ultraviolet (EUV) radiation with a wavelength of, for example, 13.5 nm 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 EUV radiation. The measuring device is used to examine whether it meets the specifications and is free of contaminants.

[0027] The measuring device is designed according to Fig. 1. The photomask 17 is arranged such that an EUV beam path 15 emanating from an EUV radiation source 14 is directed onto the photomask 17 via an illumination system 16. The illumination system 16 shapes the EUV radiation into a beam that illuminates an inspection field on the surface of the photomask 17 with uniform brightness. The illuminated area can, for example, have dimensions of 0.5 mm x 0.8 mm. The edge lengths of the photomask 17 can, for example, be between 100 mm and 200 mm. A field stop is arranged in the illumination system 16, which limits the illuminated area to the inspection field on the surface of the photomask 17. An XY positioner 37 can be used to move the photomask in the XY plane to bring different inspection fields into the area of ​​the EUV beam path.

[0028] The EUV beam path 15, reflected at the photomask 17, continues via a projection lens 22 to an EUV camera 23, which is equipped with an image sensor 24. The projection lens maps the examination field 20 of the photomask 17 onto the image sensor 24 of the EUV camera 23. The EUV radiation source 14, the illumination system 15, the photomask 17, the projection lens 22, and the EUV camera 23 are arranged in a vacuum housing 40, in which a negative pressure is maintained during operation of the measuring device.

[0029] The EUV radiation source 14 is a plasma radiation source in which EUV radiation with a wavelength of 13.5 nm is emitted from a plasma. Tin is a medium suitable for generating a plasma for emitting such EUV radiation. To generate the plasma, a droplet of the medium can be exposed to a laser beam.

[0030] The mirrors of the illumination system 16 and the mirrors of the projection lens 22 are designed as EUV mirrors, exhibiting particularly high reflectivity for EUV radiation. The optical surface of the EUV mirrors can be formed by a highly reflective coating. This can be a multilayer coating, in particular a multilayer coating with alternating layers of molybdenum and silicon. With such a coating, approximately 70% of the incident EUV radiation can be reflected.

[0031] The projection lens 22 has a magnification factor of more than 100. In order to fully capture the image generated by the examination field 20 of the photomask 17, the area of ​​the image sensor 24 is larger than the area of ​​the examination field 20, corresponding to the magnification factor. The image sensor 24 can, for example, have dimensions on the order of 100 mm to 200 mm.

[0032] To enable highly accurate measurement of the photomask 17, the illumination system 16 must be very precisely adjusted relative to the projection system 22. In particular, it must be ensured that the beam path generated by the illumination system 16 strikes the photomask 17 at the correct angle and enters the projection optics at the correct angle, and that an exit pupil of the illumination system 16 is precisely aligned with an entrance pupil of the projection system. Adjustment is performed using a measuring system according to the invention. Such a measuring system is described below in conjunction with Fig. 2 described. The adjustment can be carried out, in particular, before the initial commissioning of the mask inspection system or also for maintenance purposes during an operational break of the mask inspection system. For this purpose, the lighting system 16 and the projection system 22 can be removed from the mask inspection system and installed in the measuring system described below.

[0033] Fig. Figure 2 shows a schematic representation of a measuring system according to the invention for optically checking the alignment state of an illumination optic 16 relative to a projection optic 22. The illumination optic 16 and the projection optic 22 are mounted on a (in Fig. The carrier device 41 (not shown) is attached and can be inserted into the measuring system in this state so that the adjustment status can be checked and the actual adjustment process can be carried out within the measuring system. After the adjustment has been carried out, the carrier device 41, together with the illumination optics 16 and the projection optics 22, can be removed from the measuring system again.

[0034] The measuring system comprises a measuring illumination unit 20 for generating a measuring beam path 27, a test object 29, a first sensor 31a and a second sensor 31b. In addition, the measuring system includes a holder 42 on which the support device 41 can be positioned in a predetermined orientation.

[0035] The measuring illumination unit 20 has a light source 26, the light from which is shaped into a parallel beam by means of a collimator 21. In the beam path behind the collimator 21 is an aperture device 30, which is configured, as explained in more detail below, to give the measuring beam path 27 a desired measuring beam profile. After passing through the aperture device 30, the measuring beam path reaches a condenser 28, which focuses the beam path onto a test field 19 of the test object 29. From the test field 19, the measuring beam path can enter either the illumination optics 16 or the projection optics 22.

[0036] The test object 29 is positioned relative to the holder 42 such that it lies in an object plane 25 of the projection optics 22. Since the optical path along which light enters the illumination optics 16 is in the measuring system of the Fig. 2 Conversely, the optical path within the mask inspection system is Fig. 1, the object level 25 can be present (unlike in the mask inspection system of the Fig. 1) can simultaneously be considered as object plane 25 of the illumination optics 16. Sensor 31a is located in an image plane of the illumination optics 16 and sensor 31b is located in an image plane of the projection optics 22.

[0037] The test field 19 of the test object 29 is back-illuminated by the measuring beam path 27. The test object 29 is at least partially transparent, so that the measuring beam path can pass at least partially through the test object 29. The test object 29 also includes markings in the area of ​​the test field 19, which simplify the adjustment process. After the measuring beam path 27 has passed through the test object 29, the following occurs in the state of Fig. 2. A first part 27a of the measuring beam path passes into the illumination optics 16. The test field 19 is thus imaged by the illumination optics 16 onto a sensor surface of the sensor 31a. In an analogous manner, in the state of Fig. 2 a second part 47b of the measuring beam path into the projection optics 22. The test field is thus imaged by the projection optics 22 onto a sensor surface of the sensor 31b.

[0038] In the Fig. In the condition shown in Figure 2, the problem can occur that the image of the test field captured by sensor 31a is disturbed by unwanted reflections caused by the second part 27b of the measuring beam 27. To avoid or at least reduce these unwanted reflections, the aperture device 30 has a plurality of switchable pixels 32, which allow flexible adjustment of a desired measuring beam profile. The aperture device 30 is connected to a control unit 43 for this purpose. To adjust a desired measuring beam profile, the control unit 43 sends corresponding control signals to the aperture device 30.

[0039] In the Fig. 3, Fig. 4 to Fig. Figure 5 shows the aperture device 30 in various states that can be achieved by the control unit outputting corresponding control signals. The aperture device 30 comprises a square pixel unit consisting of a total of 16 by 16 pixels 32, which is configured as a liquid crystal display. A pixel 32 of the liquid crystal display is formed in a generally known manner by a liquid crystal segment whose transmittance can be changed by applying a suitable voltage. The output of control signals by the control unit 43 can be accompanied by the application of a suitable voltage. The design of corresponding liquid crystal displays is generally known and therefore does not need to be explained in detail here.

[0040] In the state of Fig. 3 is a first group of 32'' pixels switched into a first transmission state with a high transmittance, in which they are essentially transparent and allow light to pass through unimpeded. These pixels are in Fig. 3 is illustrated by a white color. Another group of pixels 32' are switched to a low transmittance state, in which they almost completely block out the light. Low transmittance pixels 32' are in Fig. 3 are illustrated by a dark gray color and are subsequently also referred to as opaque pixels. The transparent pixels 32'' form an approximately circular opening. Through the in Fig. The circuit shown in section 3 can generate a circular measuring beam profile.

[0041] In the state of Fig. The transparent pixels 32'' form a semicircle that creates an opening located on the left side of the aperture device 30. All other pixels 32'' have a low transmittance and block the light.

[0042] In the state of Fig. The transparent pixels 32'' form a semicircle that creates an opening located on the right side of the aperture device 30. All other pixels 32'' have a low transmission rate.

[0043] Through the in the Fig. 3 and Fig. In the 4 states shown, the right or left half of the circular measuring beam can be hidden, thus creating a semicircular measuring beam profile.

[0044] Fig. Figure 6 shows the measuring system of the Fig. 2, after the aperture device 30 is in the Fig. The state shown in Figure 5 is switched in which only a semicircular beam profile on the right side of the aperture device 30 is transmitted, while the entire left half of the measuring beam 27 is blocked. In this state, the test field 19 is illuminated only by the second part 27b of the measuring beam 27, and only the second part 27b of the measuring beam 27 reaches the sensor 31b via the projection optics 22. By blocking the first part 27a of the measuring beam, interfering reflections are avoided or at least reduced, so that the test field 19 can be detected with high accuracy by the sensor 31b.

[0045] Fig. 7 shows the measuring system of the Fig. 2, after the aperture device 30 is in the Fig. The state shown in Figure 4 is such that only a semicircular beam profile on the left side of the aperture device 30 is transmitted, while the entire right half of the measuring beam 27 is blocked. In this state, the test field 19 is illuminated only by the first part 27a of the measuring beam 27, and only this first part 27a of the measuring beam 27 reaches the sensor 31a via the illumination optics 22. By blocking the second part 27b of the measuring beam, interfering reflections are avoided or at least reduced, so that the test field 19 can be detected with high accuracy by the sensor 31a.

[0046] Fig. Figure 8 shows the aperture device 30 in a further switching state, which starts from the state of the Fig. 3 is achieved by switching a third group of pixels 32''' into a state in which they have an average transmittance that lies between the transmittance of the transparent pixels 32'' and the transmittance of the opaque pixels 32'. These pixels 32''' are subsequently also referred to as semi-transparent pixels and in Fig. 8 is represented by a gray value that lies between that of the transparent pixel 32'' and the opaque pixel 32'. The semi-transparent pixel 32''' is located within a circle in the center of the opening, which is in Fig. 3 is formed by the transparent pixels 32'', so that only an outer ring of transparent pixels 32'' remains. Using the in Fig. In the switching state of the aperture device 30 shown in Figure 8, it is possible, for example, to compensate for an optical aberration of the illumination optics 16 or the projection optics 22. This will be shown below using the Fig. 9 explained in more detail.

[0047] Fig. Figure 9(a) shows a homogeneous pupil illumination generated by the measuring illumination unit 20 in the area of ​​the test field 19, which is obtained by irradiating the test field 19 with a correspondingly homogeneous measuring beam profile. Such a measuring beam profile can be achieved, for example, by the [missing information] Fig. The switching state shown in point 3 is achieved. High illumination is achieved in Fig. 9 is illustrated by a darker color and lower illumination by a lighter color.

[0048] The homogeneously illuminated test field 19 is projected onto the sensor 31a via the lighting system 16 and generates the following there Fig. Figure 9(c) shows inhomogeneous illumination, which exhibits lower intensity at the edges. The measurement indicates that a so-called apodization occurs within the illumination optics 16, resulting in intensity losses at the edges of the measurement beam path. The apodization of the illumination optics and its effect on the measurement beam path are described in Figure 9(c). Fig. 9(b) illustrates the intensity distribution of the Fig. 9(c) is obtained by convolution of the intensity distributions of Fig. 9(a) and Fig. 9(b), which is indicated in the figures by the mathematical convolution operator “x”.

[0049] Based on the in Fig. 9(c) The measurement shown allows the extent of the apodization of the illumination optics 16 to be estimated, and a measurement beam profile to be generated by the measurement illumination unit 20 can be derived from this to compensate for the apodization. In this case, the measurement beam profile must be suitable for illuminating a peripheral area of ​​the test field 19 more intensely in order to compensate for the intensity losses caused by the apodization in the peripheral area. A correspondingly illuminated test field 19 is shown in the Fig. Figure 9(d) illustrates the pupil illumination shown there, for example, with an aperture device 20 which, in the switching state of the Fig. 8. Due to the transparent pixels 32''' located in the edge region of the aperture, the outer area 33 of the test field 19 is illuminated more strongly than the central circular area of ​​the aperture, in which the semi-transparent pixels 32''' are located. Due to the apodization within the illumination optics 16, which in Fig. As shown again in 9(e), the test field 19 is mapped onto the sensor 31a with a homogeneous intensity, which in Fig. Figure 9(f) shows that the homogeneous illumination of sensor 31a improves measurement accuracy.

[0050] Fig. Figure 10 illustrates another example of the compensation of an optical aberration. The optical aberration, which in this case is attributable to the projection optics 22 and manifests itself as a decrease in intensity in the peripheral regions 35 of the pupil, is shown here in Fig. 10(b) shown. Fig. Figure 10(a) shows an illumination of the test field 19, which is required to detect the in Fig. to compensate for the intensity distribution shown in Figure 10(b). The stronger illumination of the edge regions 36, in combination with the weaker illumination of the edge regions 35, results in a homogeneous illumination of the sensor 31b (see Figure 10(b)). Fig. 10(c)). QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 2014 / 0300877 A1

[0012]

Claims

Measuring system for optically checking the alignment state of an illumination optic (16) relative to a projection optic (22), wherein the illumination optic (16) is designed to illuminate an object (17) located in an object plane (25) of the projection optic (22) and the projection optic (22) is designed to image a reflection generated on the object onto an image plane of the projection optic (22), wherein the measuring system comprises: - a holder for positioning the illumination optic (16) and the projection optic (22); - a partially transparent test object (29) arranged in the object plane (25); - a measuring illumination unit (20) for generating a measuring beam path (27) which illuminates a test field (19) of the test object (29) from behind and is oriented such that the test field (19) is imaged onto a first sensor plane by the illumination optic (16) and onto a second sensor plane by the projection optic (22);- a first sensor (31a) for detecting an image produced in the first sensor plane; - a second sensor (31b) for detecting an image produced in the second sensor plane; - an aperture device (30) arranged in the measuring beam path (27) between an illumination source (26) and the object plane (25) and comprising a plurality of pixels (32) each switchable between a first transmission state and a second transmission state; and - a control unit (34) for switching the plurality of pixels (32) between the first transmission state and the second transmission state. Measuring system according to claim 1, wherein the control unit is configured to selectively switch a first pixel (32) into the first transmission state or into the second transmission state independently of a switching state of a second pixel (32) different from the first. Measuring system according to claim 1 or 2, wherein the aperture device (30) has a pixel unit designed as a liquid crystal display. Measuring system according to one of claims 1 to 3, wherein the aperture device (30) has a pixel unit comprising an absorption modulation material. Measuring system according to one of claims 1 to 4, wherein the aperture device (30) has a pixel unit formed by a microelectromechanical system. Measuring system according to one of claims 3 to 5, wherein the aperture device (30) has two pixel units whose pixels are aligned to each other in such a way as to increase the achievable contrast ratio between the first transmission state and the second transmission state. Measuring system according to one of claims 1 to 6, wherein the aperture device (30) has at least one of the following features: - a pixel size is greater than 10 µm and is preferably between 100 µm and 400 µm; - a number of pixels (32) is more than 100, preferably more than 1000, and is further preferably between 100 and 1000000; - a contrast ratio of the aperture device (30) between the first transmission state and the second transmission state is more than 1:103 and preferably more than 1:104, further preferably more than 1:

106. Measuring system according to one of claims 1 to 7, in which a pixel (32) of the aperture device (30) can be switched by the control unit (34) into an intermediate state in which a transmittance of the pixel (32) lies between the transmittances of the first transmission state and the second transmission state. Measuring system according to claim 8, wherein a pixel (32) can be switched by the control unit (34) into a plurality of intermediate states in which the pixel (32) has different transmittances which lie between the transmittance in the first transmittance state and the transmittance in the second transmittance state. A method for optically testing the alignment state of an illumination optic (16) relative to a projection optic (22), wherein the illumination optic (16) is configured to illuminate an object (17) located in an object plane (25) of the projection optic (22) and the projection optic (22) is configured to image a reflection generated on the object into an image plane of the projection optic (22), comprising the following steps: - Arranging a partially transparent test object (29) in the object plane (25); - Generating a measuring beam path (27) that back-illuminates a test field (19) of the test object (29) and is oriented such that the test field (19) is imaged by the illumination optic (16) onto a first sensor plane and by the projection optic (22) onto a second sensor plane;- Positioning an aperture device (30) in the measuring beam path (27) between the illumination source (26) and the object plane (25), wherein the aperture device (30) has a plurality of pixels (32) which can each be switched between a first transmission state and a second transmission state in order to selectively allow a beam of light corresponding to the respective pixel (32) of the measuring beam path (27) to pass through or at least partially block it; - Controlling the plurality of pixels (32) to generate a predetermined measuring beam profile; - Capturing an image of the test field (19) generated by the illumination optics (16) and an image of the test field (19) generated by the projection optics (22).; Method according to claim 10, wherein the image of the test field (19) produced by the illumination optics (16) is captured while the second measuring beam (27b) is at least partially blocked, wherein the image of the test field (19) produced by the projection optics (22) is captured while the first measuring beam (27a) is at least partially blocked. Method according to claim 10 or 11, wherein an expected optical aberration of the illumination optics (16) or an expected optical aberration of the projection optics (22) is estimated, wherein the predetermined measuring beam profile is generated such that the expected optical aberration of the illumination optics (16) or the projection optics (22) is at least partially compensated. Method according to one of claims 10 to 12, wherein an optical aberration of the illumination optics (16) or the projection optics (22) is estimated on the basis of the image of the test field (19) generated by the illumination optics (16) or by the projection optics (22), wherein the predetermined measuring beam profile is generated in such a way that the estimated optical aberration of the illumination optics (16) or the projection optics (22) is at least partially compensated. Method for adjusting an illumination optic (16) relative to a projection optic (22), in which a method according to one of claims 10 to 13 is carried out, wherein the illumination optic (16) is adjusted relative to the projection optic (22) on the basis of the image of the test field (19) produced by the illumination optic (16) and the image of the test field (19) produced by the projection optic (22).

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