Energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light

Abstract

An energy detection assembly (26) for an illumination system of a mask inspection system for use with EUV illumination light (3) has a beam-homogenizing element (11) for guiding the illumination light (3). The beam-homogenizing element (11) has an entrance opening (12) and an exit opening (14) for the illumination light (3). The assembly (26) has at least one EUV energy sensor device (24i), which captures illumination light (3) which is guided outside the entrance opening (12) of the beam-homogenizing element (11) along an illumination light beam path. In further embodiments of corresponding assemblies, the EUV energy sensor device captures illumination light which is guided within the exit opening or else illumination light which is guided remote from a detection region of a spatially resolving detection device. In a further embodiment of the assembly, an EUV energy sensor device captures detection light which is not allowed through into a downstream illumination light beam path by a used light filter. A further embodiment of the assembly comprises an EUV energy sensor device, which measures a photocurrent. A further embodiment of the assembly comprises an energy sensor device arranged in the beam path of fluorescence detection light, generated in a fluorescence layer of an EUV mirror. A further embodiment of the assembly comprises an EUV energy sensor device, which captures illumination light which is guided in a central region of an illumination pupil or of an illumination light beam path of the illumination optical unit. A further embodiment of the assembly comprises an ion detector or an electron detector. This results in an energy detection assembly, by means of which an inspection accuracy of a mask inspection system can be improved.

Description

[0001] Energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light

[0002] The content of German patent application DE 10 2024 203 350.2 is incorporated herein by reference.

[0003] The invention relates to an energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light. Furthermore, the invention relates to a mask inspection system having such an energy detection assembly.

[0004] Such a mask inspection system is known from US 10,042,248 B2, DE 102 20 815 Al and WO 2012 / 101269 Al. DE 10 2021 213 327 B3 discloses a metrology system for examining objects with EUV measurement light. DE 10 2020 207 566 Al discloses an apparatus and a method for characterizing a microlithographic mask. DE 10 2012 219 169 Al discloses a beam closed-loop control apparatus for an illumination beam and a metrology system having an optical system comprising such a beam closed-loop control apparatus. DE 10 2016 225 563 Al discloses a hollow waveguide for guiding EUV light having a used wavelength. US 6,456,362 Bl discloses an integrating waveguide for use in a lithographic projection exposure apparatus.

[0005] It is an object of the present invention to help improve the inspection accuracy of a mask inspection system.

[0006] According to the invention, this object is achieved by an energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light, comprising the features specified in Claims 1 and 7.

[0007] It has been found according to the invention that an energy detection assembly of this type offers the possibility of providing highly precise mask inspection even when using a light source of the mask inspection system whose energy or intensity fluctuates over time. In particular, this may be the case in pulsed EUV light sources. Using the EUV energy sensor device, energy fluctuations of the light source may be detected, so that these can be removed from the calculation during post-processing by means of sensor data of the EUV energy sensor device, in particular during image post-processing from image data of a detection device, of the mask inspection system. For each part of the image data, it is possible to sum or average over the energy of the light pulses which have contributed to the measurement of this part of the image data. This can be effected in particular if a scanning measurement is performed by means of the EUV energy sensor device, which can then be designed as a TDI camera. Corresponding summation or averaging results can then in turn be input variables for the closed-loop control of an intensity of the light source. An energy value measured by means of the EUV energy sensor device can be used to correct measured image or camera data in particular by calculation. Alternatively or in addition, the sensor data of the EUV energy sensor device can be used as a closed-loop signal for controlling the intensity of the light source in a closed loop. An input-coupling location and / or an input-coupling direction of a coupling of the illumination light into the beam-homogenizing element can be variables to be controlled in a closed loop here too. The energy detection assembly can comprise a plurality of EUV energy sensor devices which can capture or monitor in particular different partial beams of the illumination light. An energy sensor of the at least one EUV energy sensor device may be designed as EUV photodiode. EUV photodiodes of this type are known from the market.

[0008] A measurement accuracy of energy measurement taken by the EUV energy sensor device can be better than 3%, can be better than 1%, can be better than 0.5% and can in particular be better than 0.2%. A correspondingly precise measurement of specific energy-dependent inspection parameters is then possible using a mask inspection system which is equipped with the energy detection assembly.

[0009] The EUV energy sensor device according to Claim 1 captures the illumination light that is guided along the illumination light beam path outside of the entrance opening of the beam-homogenizing element. The EUV energy sensor device thus captures the illumination light that does not pass through the entrance opening of the beam-homogenizing element. A distance between illumination light capture regions of the EUV energy sensor device and a peripheral boundary of the entrance opening of the beam-homogenizing element is regularly smaller than 50% of a mean diameter of the entrance opening. This distance can also be even smaller, for example smaller than 40%, smaller than 30%, smaller than 25%, smaller than 20%, smaller than 15%, smaller than 10% or smaller than 5% of this mean diameter of the entrance opening. The distance is regularly greater than 0.01% of the mean diameter.

[0010] The illumination light captured by the EUV energy sensor device is guided along an illumination light beam path when said illumination light, captured by the EUV energy sensor device, is guided between a source region of an EUV light source that creates the EUV illumination light and the beam-homogenizing element within predetermined marginal aperture boundaries of optical components that guide the illumination light.

[0011] Since the EUV energy sensor device according to Claim 1 captures illumination light that is guided along the illumination light beam path outside of the entrance opening of the beam-homogenizing element, there is the possibility for an energy measurement without any disturbing illumination light loss. The energy detection assembly avoids light loss because illumination light is used for the energy detection which is not available for illumination of an object to be inspected at any rate because it is not guided by the beam-homogenizing element.

[0012] The beam- homogenizing element produces a desired intensity distribution and / or illumination angle distribution of the illumination light over the exit opening of the beam-homogenizing element. This effect of the beam-homogenizing element is then used for the illumination of an object field of the mask inspection system which is specified accordingly with respect to a field intensity distribution and / or with respect to a field illumination angle distribution.

[0013] The beam- homogenizing element can comprise at least one micromirror array, which is used to split the illumination light into a plurality of illumination channels which are guided such that they are superposed on one another in the exit opening. The beam-homogenizing element can comprise a plurality of such micromirror arrays arranged one after another in the beam path of the illumination light. The beam-homogenizing element can comprise exactly two such micromirror arrays. Micromirrors of the at least one micromirror array can be designed as concave micromirrors. If at least two such micromirror arrays are used, radii of curvature of the micromirrors can be dimensioned such that the illumination light is guided in parallelized fashion along the illumination channels between at least two successive micromirror arrays of the beamhomogenizing element. Alternatively, the beam-homogenizing element may be designed as a hollow waveguide.

[0014] The energy detection assembly may comprise a spatially resolving detection device.

[0015] The energy detection assembly may comprise a light filter.

[0016] The energy detection assembly may comprise at least one EUV energy sensor device that is designed such that it captures illumination light which is guided at least through the beam-homogenizing element.

[0017] The energy detection assembly may comprise at least one EUV energy sensor device that is designed such that it captures illumination light which is guided through the beam-homogenizing element and reflected by at least one additional mirror behind the beam-homogenizing element in the illumination light beam path.

[0018] The energy detection assembly comprises at least one EUV energy sensor device that is designed such that it captures illumination light which is guided through the beam-homogenizing element and reflected by at least two additional mirrors behind the beam-homogenizing element in the illumination light beam path.

[0019] The energy detection assembly comprises at least one EUV energy sensor device that is designed such that it captures illumination light which is guided through the beam-homogenizing element and reflected by at least three additional mirrors behind the beam-homogenizing element in the illumination light beam path.

[0020] The energy detection assembly being designed such that it captures illumination light which is guided at least through the beam-homogenizing element may comprise at least one EUV deflection mirror.

[0021] The EUV deflection mirror may be carried by an aperture stop section. The EUV deflection mirror may be held by a stop.

[0022] The EUV energy sensor device may be designed such that it captures illumination light which is guided outside an object field. The object field may be arranged in an object plane. In the object field a mask to be inspected is arrangeable.

[0023] An EUV energy sensor device according to Claim 2 does not result in a disturbing illumination light loss, wherein the assembly adjacent to the beam- homogenizing element can be of a compact design, in particular due to a correspondingly small structural size of the EUV deflection mirror, which enables energy detection even with tight spatial conditions in the vicinity of the beam-homogenizing element.

[0024] An energy sensor device according to Claim 3, which can additionally also be used for measuring a directional stability of the illumination light in the manner of, for example, a quadrant detector, has corresponding advantages. The four EUV energy sensor devices can be arranged with an equal distribution around the entrance opening of the beam-homogenizing element. The entrance opening of the beam-homogenizing element can be rectangular. The EUV energy sensor devices can then be arranged close to the four rectangle sides outside of the entrance opening. Regarding the distances of the regions distributed around the entrance opening from the entrance opening, what was already said above in relation to the capturing of the illumination light outside of the entrance opening may apply. In the circumferential direction, the regions which are distributed around the entrance opening and in which the EUV energy sensor devices capture the illumination light can cover, for example, between 5% and 50% of a total circumference around a centre of the entrance opening, for example in the range between 10% and 40%, in particular in the region of 25%. A thus covered partial circumference around the entrance opening represents a good compromise between a complexity of the sensor system on the one hand, and an information gain by way of the capture of the illumination light in the capture regions distributed around the entrance opening on the other hand.

[0025] An input-coupling sensor device according to Claim 4 makes an energy- efficient coupling of the illumination light into the entrance opening possible. If the input-coupling location and the input- coupling direction are monitored at the same time, in particular an adjustment optimization with respect to both dimensions is made possible.

[0026] The input-coupling location can be monitored along the two spatial coordinates which define an entrance plane in which the entrance opening of the beam-homogenizing element is located, and also along a plane perpendicular thereto. In this way it is possible to monitor the position in particular of an input-coupling focus of the illumination light relative to the position of the entrance opening in all three spatial directions. In particular a defocus in the input-coupling of the illumination light into the entrance opening can then be detected.

[0027] The input-coupling sensor device can be connected for the transmission of signals to an open- loop / closed-loop control device of the energy detection assembly to enable open-loop or closed- loop control of the input-coupling location and / or the input-coupling direction.

[0028] The input-coupling sensor device can at the same time represent the EUV energy sensor device, that is to say also assume the function thereof. An embodiment of the input-coupling sensor device can correspond to the embodiments of the EUV energy sensor device which has already been discussed above.

[0029] An input-coupling sensor device according to Claim 5 has proven useful in practice. The inputcoupling location and the input-coupling direction are monitored separately using the respective sensor units, wherein the corresponding monitoring results can be fed to a joint open- loop / closed-loop control device for enabling input-coupling location and / or input-coupling direction control.

[0030] Using such closed-loop control, a control loop for an input-coupling location and / or for an inputcoupling direction of a beam of the illumination light into the entrance opening of the beam-homogenizing element can be realized.

[0031] Such a control loop can have a control bandwidth which leads to a shorter control reaction time than temporal changes in an actual input-coupling location to be adjusted and / or an actual inputcoupling direction to be adjusted. An embodiment of the input-coupling direction sensor unit according to Claim 6 is fit for practice.

[0032] An energy detection assembly according to Claim 7 in fact detects illumination light which is guided inside the exit opening of the beam-homogenizing element along the illumination light beam path, that is to say within a beam path of the beam-homogenizing element, and could therefore in principle be used for object field illumination. This capturing of the illumination light which is guided within the exit opening consequently leads to great monitoring reliability.

[0033] In a configuration of the EUV energy sensor device according to Claim 8, the EUV deflection mirror arranged within the exit opening of the beam-homogenizing element can be very small, which can lead to a minimization of illumination light loss.

[0034] A configuration according to Claim 9 avoids illumination light loss, that is to say a loss of illumination light which can be used effectively for object field illumination. The used exit opening region is that region of the exit opening of the beam-homogenizing element within which illumination light is guided that is used for illuminating an object to be inspected.

[0035] The input-coupling location sensor unit and / or the input-coupling direction sensor unit can comprise a plurality of EUV energy sensors. The latter can in turn capture light, which is coupled out of an incident beam of the illumination light using deflection mirrors of the energy detection assembly, in particular of the input-coupling sensor device, in particular of the input-coupling location sensor unit and / or the input-coupling direction sensor unit. The embodiment of such an input-coupling direction sensor unit can again correspond to those which has already been discussed above in relation to the input-coupling location sensor unit or the EUV energy sensor device.

[0036] The plurality of deflection mirrors may comprise at least one EUV deflection mirror. Such deflection mirror may be designed according to at least one of the embodiments given in this specification. The plurality of deflection mirrors may comprise at least two pairs of EUV deflection mirrors.

[0037] The plurality of deflection mirrors may be arranged at least partially around an entrance opening of the beam-homogenizing element. In particular, such deflection mirrors arranged at least partially around the entrance opening of the beam-homogenizing element may delimit contour sections of an outer contour of the entrance opening at different circumferential positions.

[0038] The deflection mirrors of the energy detection assembly, in particular of the input-coupling location sensor unit, can be arranged in exactly one arrangement plane. Alternatively, the deflection mirrors can be arranged in a plurality of arrangement planes located one after the other with respect to an illumination light beam path, for example in exactly two arrangement planes. These arrangement variants offer good combinations of structural compactness and production requirements for the deflection mirrors.

[0039] The plurality of arrangement planes may comprise at least one first arrangement plane and at least one second arrangement plane being substantially parallel to each other. At least two of the plurality of deflection mirrors may be arranged in the first arrangement plane. At least two of the plurality of deflection mirrors may be arranged in the second arrangement plane. The at least two deflection mirrors arranged on the first arrangement plane may be tilted to a first plane being substantially perpendicular to the first arrangement plane. The at least two deflection mirrors arranged on the second arrangement plane may be tilted to a second plane being substantially perpendicular to the second arrangement plane. The first plane and the second plane may be substantially parallel to each other or may be substantially perpendicular to each other. In that respect, two planes are substantially parallel in case these planes have an angle of at most 20°, of at most 15°, of at most 10°, of at most 7°, of at most 5°, of at most 3°, of at most 2°, of at most 1° to each other or have an angle of 0° to each other. Further, in that respect, two planes are substantially perpendicular in case they have an angle of at least 70°, of at least 75°, of at least 80°, of at least 83°, of at least 85°, of at least 87°, of at least 88°, of at least 89° or an angle of 90° to each other. The at least two deflection mirrors being tilted to the first plane may be tilted symmetrically to the first plane. The at least two deflection mirrors being tilted to the second plane may be tilted symmetrically to the second plane.

[0040] The deflection mirrors can be arranged in pairs and can delimit opposite sides of a through opening for the illumination light which has not been deflected. This through opening can coincide with the entrance opening of the beam-homogenizing element.

[0041] The at least one of the pairs of the deflection mirrors may comprise two deflection mirrors being arranged symmetrically to a symmetry plane. The two mirrors of this pair of mirrors may both be tilted symmetrically towards the symmetry plane.

[0042] The through opening can have an opening area in the range of between 0.1 mm2and 10 mm2, in particular in the region of 1 mm2.

[0043] At the location where the respective deflection mirror delimits a through opening for the illumination light which is not deflected, the deflection mirror can be trapezoidal, in particular symmetrically trapezoidal, or designed with a taper in the shape of a wedge. This simplifies an alignment of the deflection mirrors delimiting the through opening for the illumination light relative to one another.

[0044] The deflection mirror can have a mirror thickness of less than 10 pm in the region of a delimitation edge of the deflection mirror where the illumination light to be deflected is separated from the illumination light which is not deflected. This mirror thickness can be in particular in the range between 1 pm and 10 pm.

[0045] An edge roughness of the deflection mirror in the region of the delimitation edge may be smaller than 50 pm and can be, for example, at most 25 pm.

[0046] Partial beams deflected by the deflection mirrors or sensor components of the illumination light can have a typical beam diameter in the range of 0.1 mm to 1 mm, for example in the region of 0.5 mm. The deflection mirrors can have a coating which is highly reflective for the illumination light in particular in the region of the delimitation edge.

[0047] The object mentioned in the introductory part is additionally achieved according to the invention by an energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light, having the features mentioned in the following aspects 10, 12, 14 to 17, 18 and 21. Advantageous further configurations can be found in the following aspects 11 and 13 and in the aspects 19, 20 and 22 to 30.

[0048] 10. Energy detection assembly (30) for an illumination system of a mask inspection system (2a) for use with EUV illumination light, having a spatially resolving detection device (23) for capturing the illumination light (3), in an image field (21), wherein the illumination light (3) is guided into the image field (21) via an object field (4), wherein a mask (18) to be inspected is arranged in the image field (21), having at least one EUV energy sensor device (31), which is designed such that it captures illumination light (3) which is guided along an illumination light beam path, remote from a detection region (32i) of the detection device (23).

[0049] An energy detection assembly according to this aspect 10 avoids undesired illumination or imaging light loss. The detection region of the detection device may be defined by detection surfaces of CCD and / or TDI sensors of the detection device, which determine respective detection sections of the detection region. With regard to configurations of TDI sensors that are known per se, reference is made, merely by way of example, to DE 197 14 221 Al.

[0050] 11. Energy detection assembly according to aspect 10, characterized in that the EUV energy sensor device (31) is designed such that it captures illumination light (3) which is guided between two detection sections (32i, 32j) of the detection region (32) of the detection device (23) along the illumination light beam path.

[0051] A design of the EUV energy sensor device according to this aspect 11 can be easily realized. A plurality of such EUV energy sensor devices can be arranged between the detection sections of the detection region of the detection device, in particular between adjacent CCD and / or TDI sensors.

[0052] 12. Energy detection assembly (33) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having a used light filter (8i), which is arrangeable in a beam path of the illumination light (3) between a light source (5) and an object field (4), in which a mask (18) to be inspected is arrangeable, and having at least one EUV energy sensor device (36), which is designed such that it captures detection light (34) which is not allowed through into the downstream illumination light beam path by the used light filter (8i).

[0053] An energy detection assembly according to this aspect 12 avoids illumination light loss. It is not the illumination light but detection light which is energetically correlated to the former that is captured, from which conclusions can be drawn regarding the energy of the illumination light. The EUV energy sensor device can capture in particular detection light reflected by the used light filter.

[0054] 13. Energy detection assembly according to aspect 12, characterized by a bandpass filter (37), which is arrangeable in a beam path between the used light filter (8i) and an energy sensor (35) of the EUV energy sensor device (36).

[0055] A bandpass filter according to this aspect 13 enables a reduction of out-of-band (oob) light, which could falsify the detection.

[0056] 14. Energy detection assembly (39) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having a used light filter (8i), which is arrangeable in a beam path of the illumination light (3) between a light source (5) and an object field (4), in which a mask (18) to be inspected is arrangeable, and having at least one EUV energy sensor device (40), which is designed such that it measures a photocurrent which is produced in the used light filter or in a carrying structure (38) of the used light filter (8i) by way of the illumination light (3).

[0057] An energy detection assembly according to this aspect 14 enables a refined energy measurement which does not take place in an optical way, but is based on a charge or photocurrent measurement.

[0058] 15. Energy detection assembly (41) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having an illumination optical unit (1) for guiding the illumination light (3) towards an object field (4), in which a mask (18) to be inspected is arrangeable, wherein the illumination optical unit (1) comprises at least one EUV mirror (IL3), which has a fluorescence layer for converting a fraction of the incident illumination light (3) into fluorescence detection light (43) having a wavelength which differs from a used light wavelength, and having an energy sensor device (46), which is arranged in the beam path of the fluorescence detection light (43) and has at least one energy sensor (45) for capturing an energy of the fluorescence detection light (43).

[0059] An energy detection assembly according to this aspect 15 enables the use of sensitive energy sensors which are sensitive for the fluorescence detection light having a larger wavelength than the illumination light wavelength. Such larger wavelengths, within which such sensitive energy sensors can be sensitive, are wavelengths in the DUV range, in the UV range, in the VIS range, in the NIR range or else in the IR range. The mirror with the fluorescence layer can be a last mirror of the illumination optical unit in the illumination light beam path upstream of the object field.

[0060] 16. Energy detection assembly (48; 55; 57; 60) for an illumination system of a mask inspection system (2a) with EUV illumination light (3), having an illumination optical unit (1) for guiding the illumination light (3) towards an object field (4), in which a mask (18) to be inspected is arrangeable, and having at least one EUV energy sensor device (52; 56; 58; 62), which is designed such that it captures illumination light (3) which is guided in a central region of an illumination pupil and / or in the central region of an illumination light beam path of the illumination optical unit (1).

[0061] An energy detection assembly according to this aspect 16 can take advantage of the fact that a mask inspection system often simulates an optical production system having a centre obscuration. In this case, a central region of an illumination pupil of the illumination optical unit and / or a central region of an illumination light beam path is effectively not used for the object illumination or for the object imaging, which is why illumination light which is guided via such a central region can also be used for energy monitoring. The EUV energy sensor device can comprise an EUV deflection mirror, which is arranged in the central region of the illumination pupil or of the illumination light beam path, and an energy sensor which is arranged in a manner such that EUV light which is guided along the illumination light beam path and is incident on the EUV deflection mirror is guided towards the energy sensor. Such an EUV deflection mirror may be small, which reduces in particular obscuration light loss.

[0062] 17. Energy detection assembly (64) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having a gas source (67) for guiding ionization gas (68) in an ionization space (66), through which an illumination light beam path of the illumination system is guided, and having an ion detector (71) and / or having an electron detector (72) for detecting a number of ions (71a) and / or electrons (72a), which are produced in the ionization space (66) by the illumination light (3) by way of the ionization of the ionization gas (68).

[0063] An energy detection assembly according to this aspect 17 in turn utilizes a different measurement principle for the energy measurement, to be precise an ionization of an ionization gas by way of the illumination light. In this way, low-loss energy monitoring can be made possible.

[0064] 18. Energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light, having at least one EUV energy sensor device, which is designed such that it captures illumination light which is guided outside an object field, which is arranged in an object plane and in which a mask to be inspected is arrangeable.

[0065] In an energy detection assembly according to this aspect 18, utilization is made of the fact that an object field to be imaged is regularly smaller than an illumination field illuminated in the object plane. EUV illumination light which can be used for detection purposes and in particular by use within an energy detection assembly is therefore regularly available remote from the object field to be imaged.

[0066] 19. Energy detection assembly according to aspect 18, characterized by at least one energy sensor, which is arranged in the region of the object plane.

[0067] An arrangement of an EUV energy sensor according to this aspect 19 can be made to be particularly compact. The energy sensor can be carried by an object holder.

[0068] 20. Energy detection assembly according to aspect 18 or 19, characterized by at least one energy sensor and at least one EUV deflection mirror, which is arranged in the region of the object plane in a manner such that it guides the illumination light captured outside the object field towards the energy sensor.

[0069] An arrangement according to this aspect 20 may be realized with a structurally very small EUV deflection mirror, the effective mirror surface of which may be smaller than 5 mm2, than 3 mm2or even than 1 mm2. The energy sensor in this aspect can then have a greater volume, without this causing any undesirable conflicts in terms of installation space. The EUV deflection mirror may be carried by an object holder.

[0070] 21. Illumination optical unit for guiding EUV illumination light from an EUV light source towards an object field, in which a mask to be inspected is arrangeable, within an illumination system of a mask inspection system, having at least one stop in the beam path of the EUV illumination light between the EUV light source and the object field, having an energy detection assembly for use with the EUV illumination light, comprising at least one EUV energy sensor device, which is designed such that it captures illumination light which is guided remote from an illumination light beam which is determined by the stop.

[0071] An illumination optical unit according to this aspect 21 utilizes the fact that an illumination light beam is guided illumination-optically regularly through an aperture stop or an aperture stop section, which delimits at least one circumferential section of a beam of the illumination light, for specifying a defined illumination angle distribution. In the region of this stop or of this stop section, the illumination light can then be used for being guided towards the energy sensor device, that is to say be fed to an energy monitoring operation.

[0072] 22. Illumination optical unit according to aspect 21, characterized by at least one energy sensor, which is arranged in the region of the stop.

[0073] An arrangement according to this aspect 22 can again be made to be compact. Exactly one energy sensor can be used. A plurality of, for example two, three, four, five, eight, ten or even more energy sensors may also be used. The number of energy sensors used is regularly less than 25. The energy sensor can be carried by the stop.

[0074] 23. Illumination optical unit according to aspect 21 or 22, characterized by at least one energy sensor and by at least one EUV deflection mirror, which is arranged in the region of the stop in a manner such that it guides the illumination light captured remote from the stop towards the energy sensor.

[0075] The advantages of a mirror sensor arrangement according to this aspect 23 in principle correspond to those that have already been discussed above for example in relation to aspect 20. The EUV deflection mirror can be carried by the stop.

[0076] 24. Illumination optical unit according to any of the aspects 21 to 23, characterized in that the stop is arranged as an aperture stop in the region of a pupil plane of the illumination optical unit. An embodiment of the stop according to this aspect 24 has proven its worth in practice.

[0077] The stop and in particular the aperture stop can be formed in one or even more pieces.

[0078] At least one of the stop sections can be arranged in the region of a pupil plane of the illumination optical unit. Such a stop section, a plurality of such stop sections and in particular all stop sections can be equipped with an energy sensor and / or with an EUV deflection mirror according to the above aspects 22 and 23.

[0079] 25. Illumination optical unit according to any of the aspects 21 to 24, characterized by a beamhomogenizing element for guiding the illumination light, which has an entrance opening for the illumination light and an exit opening for the illumination light.

[0080] The advantages of a beam-homogenizing element according to this aspect 25 have already been discussed above in relation to the aforementioned embodiments of the energy detection assembly.

[0081] 26. Illumination optical unit according to aspect 25, characterized in that the stop is arranged in the illumination light beam path between the beam- homogenizing element and the object field.

[0082] 27. Illumination optical unit according to any of the aspects 21 to 26, characterized in that the stop is arranged in the illumination light beam path between the light source and the beamhomogenizing element.

[0083] Stop arrangements according to these aspects 26 and 27 are particularly advantageous depending on the illumination-optical design and depending on the installation space requirements of the mask inspection system. 28. Energy detection assembly according to any of Claims 1 to 17, or according to any of the aspects 18 to 20, or illumination optical unit according to any of the aspects 21 to 27, characterized in that the energy sensor has a wavelength-dependent sensitivity for the EUV illumination light.

[0084] An energy sensor according to this aspect 28 can be adapted to a wavelength dependence of an imaging optical unit of the mask inspection system for imaging the object field into a detectionside image field and / or to a wavelength sensitivity of the spatially resolving detection device.

[0085] 29. Energy detection assembly or illumination optical unit according to aspect 28, with the use of an EUV deflection mirror for the EUV illumination light, characterized in that the EUV deflection mirror has a wavelength-dependent reflectivity for the EUV illumination light.

[0086] The advantages of this aspect 29 correspond to those that have already been discussed above in relation to the aspect 28.

[0087] 30. Energy detection assembly or illumination optical unit according to aspect 28 or 29, characterized in that the energy sensor and / or the EUV deflection mirror carry a wavelength-dependent coating.

[0088] A wavelength-dependent coating according to this aspect 30 has proven particularly suitable for the defined specification of a wavelength-dependent sensitivity or a wavelength-dependent reflectivity. The coating can be an interference coating. The coating can be a multi-layer coating. Coating materials which can be used are in particular molybdenum, silicon or ruthenium.

[0089] The energy detection assemblies claimed or discussed above or individual components or functions thereof can also be used in combination with one another within a mask inspection system.

[0090] The advantages of a mask inspection system according to Claim 21 correspond to those which have already been explained above with reference to the respective energy detection assembly. Such a mask inspection system can alternatively or additionally also be designed with an energy detection assembly or with an illumination optical unit according to any of the previously explained aspects.

[0091] A wafer inspection system may also be constructed accordingly.

[0092] The inspection system may comprise an object holder that serves to hold the object to be inspected and is mechanically coupled to an object displacement drive, with the result that a scanning displacement of the object is possible during the illumination.

[0093] The inspection system can be a system for actinic mask or wafer inspection.

[0094] The features of all the claims and aspects mentioned above may be combined individually resulting in further feature combinations which may be sub feature combinations of claims and / or aspects.

[0095] An exemplary embodiment of the invention is explained in greater detail below with reference to the drawing, in which:

[0096] Fig. 1 schematically shows, in a meridional section, a mask inspection system for lithography masks for use with EUV illumination light with an illumination system, comprising an energy detection assembly with a beam-homogenizing element and with at least one EUV energy sensor device;

[0097] Fig. 1 A shows the beam-homogenizing element of the mask inspection system, in a manner that is enlarged in comparison with Fig. 1 and that shows internal details;

[0098] Fig. 2 schematically shows a perspective, entrance-side view of an embodiment of the beam-homogenizing element;

[0099] Fig. 3 shows a region around an entrance opening of the beam-homogenizing element for the illumination light, in a manner that is enlarged in comparison with Fig. 2, with four deflection mirrors that each belong to an EUV energy sensor device of the energy detection assembly being additionally depicted in the vicinity of the entrance opening, an input-coupling intensity distribution of the illumination light in an entrance plane of the beam-homogenizing element being additionally illustrated;

[0100] Fig. 4 schematically shows a section according to the line IV-IV in Fig. 3 with an illustration of an illumination and detection light path between two deflection mirrors of the energy detection assembly and associated energy sensors;

[0101] Fig. 5 shows, in an illustration similar to Fig. 4, a further embodiment of an energy detection assembly having an EUV energy sensor device for capturing illumination light guided along an illumination beam path within an exit opening of the beamhomogenizing element, with a detection light beam path in turn being illustrated between a deflection mirror and an energy sensor of the EUV energy sensor device of the energy detection assembly;

[0102] Fig. 6 shows, in an illustration similar to Fig. 3, a plan view of the entrance opening of the beam-homogenizing element, suitable for use with an energy detection assembly according to Fig. 5;

[0103] Fig. 7 shows, in an illustration similar to Fig. 6, a plan view of an exit opening of the beam-homogenizing element according to Fig. 6;

[0104] Fig. 8 shows a plan view of a further embodiment of an energy detection assembly for the mask inspection system according to Fig. 1, having a spatially resolving detection device and an EUV energy sensor device embodied to capture illumination light that is guided along an illumination light beam path beyond a detection region of the detection device, specifically between two detection sections of the detection region;

[0105] Fig. 9 shows, in an illustration similar to Fig. 1, an illumination light beam path downstream of the exit opening of the beam-homogenizing element, with moreover a further embodiment of an energy detection assembly having an EUV mirror with a fluorescence layer and an energy sensor device arranged in the beam path of fluorescence detection light being depicted;

[0106] Fig. 10 shows a plan view of an illumination pupil of the illumination optical unit of an embodiment of the mask inspection system having an embodiment of an energy detection assembly with an EUV energy sensor device for capturing illumination light which, inter alia, is guided in a central region of the illumination pupil of the illumination optical unit;

[0107] Fig. 11 shows, in a meridional section, a section of an illumination light beam path downstream of an exit from an embodiment of the beam-homogenizing element with an energy sensor of a further embodiment of an energy detection assembly arranged centrally in an exit illumination light beam;

[0108] Fig. 12 shows, in an illustration similar to Fig. 11, a further embodiment of an energy detection assembly, in which an output-coupling mirror of a further embodiment of an energy detection assembly is arranged centrally in the exit illumination light beam, and a detection light beam path is illustrated towards an energy sensor of the energy detection assembly;

[0109] Fig. 13 shows, in an illustration similar to Figures 11 and 12, a further embodiment of an energy detection assembly having an output-coupling mirror and a downstream deflection mirror for detection light, towards an energy sensor of the energy detection assembly;

[0110] Fig. 14 shows a cross section through an ionization chamber of a further embodiment of an energy detection assembly with an ion / electron detection for capturing charged particles which are created by the illumination light by ionizing an ionization gas in the ionization chamber; Fig. 15 schematically shows a function of a time delay integration (TDI) sensor, which may be used for the spatially resolving detection device of the mask inspection system for capturing an image of the mask to be inspected;

[0111] Fig. 16 shows, in an illustration corresponding to Fig. 15, a temporal progression of an illumination of the lithography mask to be inspected using illumination light from a pulsed light source of the mask inspection system;

[0112] Fig. 17 shows an embodiment of a light source of the mask inspection system with a change-in-energy sensor of an embodiment of the energy detection assembly, otherwise not depicted in Fig. 17, for capturing a temporal development of an illumination energy of the illumination light over an exposure period;

[0113] Fig. 18 shows, in an illustration similar to Fig. 17, the light source of the mask inspection system with a further embodiment of an energy detection assembly with a changein-energy sensor arranged in a source chamber of the light source;

[0114] Fig. 19 shows, not true to scale in comparison with Fig. 1, a portion of the illumination system in the region of an aperture stop and an entrance-side section of the beamhomogenizing element in a meridional section, with components of an input-coupling sensor device of the illumination system being depicted;

[0115] Fig. 20 shows, in a perspective view, an input-coupling location sensor unit of the inputcoupling sensor device, comprising four deflection mirrors arranged around an entrance opening of the beam-homogenizing element and four energy sensors associated with these deflection mirrors;

[0116] Fig. 21 shows, likewise in a perspective illustration, an embodiment of an input-coupling direction sensor unit of the input-coupling sensor device, arranged in the region of the aperture stop; Fig. 22 shows, in an illustration similar to Fig. 20, a further embodiment of an input-coupling location sensor unit of the input-coupling sensor device, again comprising four deflection mirrors arranged around an entrance opening of the beam-homogenizing element and four energy sensors associated with these deflection mirrors;

[0117] Fig. 23 shows, in a viewing direction comparable to that of Fig. 3, a top view of an arrangement of the four deflection mirrors of the input-coupling location sensor unit according to Fig. 22;

[0118] Fig. 24 shows a perspective top view of a section of an object in the form of a lithography mask to be inspected with an object field that is illuminated by means of the EUV illumination light thereon and an energy sensor arranged in the region of an object plane as part of a further embodiment of an energy detection assembly having an EUV energy sensor device;

[0119] Fig. 25 shows, in an illustration similar to Fig. 24, a further embodiment of an energy detection assembly having an EUV energy sensor device having an energy sensor and having an EUV deflection mirror, which is arranged in the region of the object plane in a manner such that it guides EUV illumination light captured outside the object field towards the energy sensor;

[0120] Fig. 26 shows, in an illustration similar to Fig. 1, a section of an illumination optical unit of a further embodiment of a mask inspection system having an aperture stop in the region of a pupil plane and energy sensors, which are arranged in the region of the aperture stop in a manner such that they capture EUV illumination light which is present outside the aperture stop;

[0121] Fig. 27 shows, in an illustration similar to Fig. 26, a further embodiment of the energy detection assembly comprising an energy sensor and an EUV deflection mirror, which is arranged in the region of an aperture stop of the illumination optical unit in a manner such that it guides EUV illumination light captured remote from the stop towards the energy sensor; Fig. 28 shows, again in an illustration similar to Fig. 26, a portion of the illumination optical unit comprising a multi-part aperture stop, wherein an EUV deflection mirror is arranged in the region of a stop section between a last mirror of the illumination optical unit upstream of the object field and the object field in such a way that it guides EUV illumination light captured remote from the stop section towards an energy sensor of an EUV energy sensor device of the energy detection assembly; and

[0122] Fig. 29 shows, in an illustration similar to Fig. 1, a further embodiment of the illumination optical unit including a beam-homogenizing element and a configuration with an EUV deflection mirror and an energy sensor corresponding to Fig. 28, wherein an aperture stop of the illumination optical unit is formed from multiple parts and has a stop section upstream of, and a stop section downstream of, the beam-homogenizing element.

[0123] An illumination optical unit 1 is a constituent part of an optical system 2 of a mask inspection system 2a for use with EUV illumination light 3. A beam path of the illumination light 3 is illustrated by way of marginal rays and a chief ray for the illumination optical unit 1 in Fig. 1. An illumination field 4 of the mask inspection system is illuminated by the illumination light 3.

[0124] The illumination light 3 is created by an EUV light source 5 in a source region 6. The light source 5 can generate EUV used radiation in a wavelength range of between 2 nm and 30 nm, for example in the range of between 2.3 nm and 4.4 nm or in the range of between 5 nm and 30 nm, for example at 13.5 nm.

[0125] The light source 5 is designed as a plasma light source. For example, it may be a laser plasma source (LPP; laser produced plasma) or else a discharge source (DPP; discharge produced plasma). In principle, such plasma sources are known as light sources for EUV projection exposure apparatuses. Alternatively, the light source 5 may also be designed as a high-harmonic EUV source. A pulse frequency of the light source 5 may be in the kHz range. In order to facilitate positional relationships, a Cartesian xyz-coordinate system will be used hereinafter. The x-axis is perpendicular to the plane of the drawing in Figure 1 and runs into the latter. The y-axis runs horizontally towards the left in Figure 1, and the z-axis runs vertically upwards in Figure 1.

[0126] After emission by the light source 5, the illumination light 3 firstly passes through a used light filter 8 arranged in an operating position in the beam path of the illumination light 3 between the source volume 6 and a first ellipsoidal mirror IL1 of the illumination optical unit 1. The used light filter 8 can be a filter from a plurality of filters kept available for example in a filter magazine in the metrology system 2a. A further used light filter can be arranged in a waiting position outside the illumination light beam path of the illumination optical unit 1. The used light filters 8 may have the same transmission characteristic, in which case a changeover between the used light filters can be effected if a degradation of a filter effect of the operating used light filter 8 is ascertained. Alternatively, the used light filters may also have different filter characteristics and for example transmit different used light wavelength ranges into the downstream illumination light beam path or be optimized for filtering out different extraneous light components.

[0127] The used light filters may be designed such that they filter out in particular pump light which is concomitantly guided in the illumination light beam path and which was used during the generation of used light in the source volume 6.

[0128] Downstream of the filter 8 and the mirror IL1, the illumination light 3 firstly passes through an aperture stop 9 which delimits the edge of a beam of illumination light 3. After that, the illumination light beam 3 is transferred towards a beam-homogenizing element 11 of the illumination optical unit 1. In this case, the mirror IL1 serves as an input-coupling optical unit 10 for input-coupling the illumination light 3 into the beam-homogenizing element 11.

[0129] Between the source volume 6 and the beam- homogenizing element 11, generally downstream of the first mirror IL1 of the illumination optical unit 1, the illumination light 3 passes through an opening in a wall of a vacuum chamber VK, which is indicated between the mirror IL1 and the illumination light aperture stop 9 in the illumination light beam path in Fig. 1. The aperture stop 9 delimits a numerical aperture of the illumination light beam 3 emitted by the source region 6 and a value of the numerical aperture ranging between 0.02 and 0.2, for example ranging between 0.07 and 0.15 or else ranging between 0.05 and 0.08. As an alternative or in addition to the aperture stop 9, an aperture-limiting stop may be arranged between the beam-homogenizing element 11 and a downstream optical component of the illumination optical unit 1. An arrangement of such a further aperture stop in the beam path of the illumination light 3 downstream of the beam-homogenizing element 11 between two downstream optical components of the illumination optical unit 1 is also possible.

[0130] The ellipsoidal mirror IL1 serves to image the source region 6 of the EUV light source 5 into an entrance opening 12 in an entrance plane 13 of the beam-homogenizing element 11. A first focus of the ellipsoidal mirror IL1 is therefore located in the source region 6 and a second focus of the ellipsoidal mirror IL1 is located in the entrance opening 12. The ellipsoidal mirror IL1 is used to focus the illumination light beam 3 into the entrance opening 12 in the entrance plane 13 of the beam-homogenizing element 11. An entrance-side numerical aperture of the illumination light beam 3 upon entrance into the entrance opening 12 can range between 0.02 and 0.2, for example be of the order of 0.05.

[0131] An angle of incidence of a central chief ray of the illumination light beam 3 on the input-coupling mirror IL1 can range between 10° and 20°. The ellipsoidal mirror IL1 may be a normal incidence (NI) mirror but may also be designed as a grazing incidence (GI) mirror.

[0132] The entrance opening 12 and an exit opening 14 of the beam-homogenizing element 11 are each square or rectangular with typical dimensions ranging between 0.5 mm and 5 mm and for example between 0.5 mm and 2 mm or else between 0.5 mm and 1 mm. An aspect ratio of the entrance opening 12 and of an identically sized exit opening 14 of the beam-homogenizing element 11 for the illumination light 3 in an exit plane 15 is between 0.5 and 2. A typical size of the entrance opening 12 and of the exit opening 14 of the beam-homogenizing element 11 is e.g. 0.5 mm x 1.0 mm, 0.75 mm x 0.75 mm, 1.0 mm x 2.0 mm or 1.5 mm x 2.0 mm.

[0133] Fig. 1A elucidates details of the beam-homogenizing element 11. A pair of micromirror arrays 15a, 15b is arranged successively in the beam path of the beam-homogenizing element 11 between the entrance opening 12 and the exit opening 14. Fig. 1 A illustrates exemplary component beam paths of illumination channels 3i (i = 1 to 4), which are each guided by a micromirror 15c of the first micromirror array 15a in the beam path and by a micromirror 15d of the second micromirror array 15b that is disposed downstream in the beam path. An actual number of illumination channels 3i is greater in practice and may for example be of the order of several hundred illumination channels, which are guided over corresponding pairs of micromirrors 15c, 15d of the two micromirror arrays 15a, 15b. The micromirrors 15c and 15d are each designed as concave mirrors and have a radius of curvature such that the illumination light 3 is in each case parallelized along the illumination channels 3i in the beam path between the micromirrors 15c and 15d. The micromirror arrays 15a and 15b are arranged and dimensioned such that the component beams of the illumination light 3 along the illumination channels 3i are superimposed on one another in the exit opening 14, and so there is a beam homogenization of the illumination light 3 between the entrance opening 12 and the exit opening 14 in the beam-homogenizing element 11.

[0134] The two micromirror arrays 15a and 15b are housed in a housing 15e of the beam-homogenizing element 11. The entrance opening 12 on the one hand and the exit opening 14 on the other hand are formed in this housing 15e.

[0135] Alternatively, the beam-homogenizing element 11 may be designed as a hollow waveguide.

[0136] The beam- homogenizing element 11 has a typical length perpendicular to the planes 13 and 15, i.e. along a main beam direction of the illumination light 3, ranging between 50 mm and 500 mm, e.g. ranging between 50 mm and 150 mm, in particular ranging between 50 mm and 100 mm.

[0137] An angle between a normal to the entrance plane 13 of the beam- homogenizing element 11 and the chief ray CR of the illumination light beam 3 incident into the entrance opening 12 can be 0° or can alternatively also differ from 0° and for example range between 0° and 1.5°, for example between 0.25° and 0.75°, and in particular be of the order of 0.5°. A ratio of the distance between the entrance plane 13 and the exit plane 15 and a size or the typical diameter of the entrance opening and respectively the exit opening 12, 14 ranges between 50 and 1000 and can for example range between 50 and 200.

[0138] An imaging output-coupling mirror optical unit 16 situated downstream of the beam- homogenizing element 11 and having two mirrors IL2, IL3 images the exit opening 14, located in an exit plane 15, of the beam-homogenizing element 11 into the illumination field 4 in an object plane 17. This imaging may have an image-side numerical aperture ranging between 0.05 and 0.2.

[0139] In the illustrated embodiment, the output-coupling mirror optical unit 16 has exactly two mirrors, namely the mirrors IL2 and IL3. The above-described, optionally used aperture stop downstream of the beam-homogenizing element 11 may be arranged between the beam-homogenizing element 11 and the mirror IL2, or else between the mirrors IL2 and IL3.

[0140] The output-coupling mirror optical unit 16 may be embodied in the style of a Wolter telescope, specifically in the style of a Type I Wolter optical unit. Such Wolter optical units are described in J. D. Mangus, J. H. Underwood "Optical Design of a Glancing Incidence X-ray Telescope", Applied Optics, Vol. 8, 1969, page 95, and the references cited therein. In such Wolter optical units, a hyperboloid may also be used in place of a paraboloid. Such a combination of an ellipsoidal mirror with a hyperboloid mirror also constitutes a Type I Wolter optical unit.

[0141] An exemplary embodiment of the output-coupling mirror optical unit 16 is described in US 10,042,248 B2.

[0142] An imaging factor Pi of the input-coupling mirror optical unit 10 may range between 0.1 and 50, i.e. its action may vary from a reduction by a factor of 10 to a magnification of a factor of 50. An imaging factor P2 of the output-coupling mirror optical unit 16 may range between 0.02 and 10, i.e. its action in turn may vary from a reduction by a factor of 50 to a magnification of a factor.

[0143] In the case of the illumination optical unit 1, a product Pi, P2 of the two imaging factors can range between 0.25 and 10. A reticle 18 to be inspected, which is held by a reticle holder 19, is arranged as object to be inspected or mask to be inspected in the object plane 17. The reticle holder 19 is mechanically operatively connected to a reticle displacement drive 20, by means of which the reticle 18 is displaced along an object displacement direction y during a mask inspection. In this way, a scanning displacement of the reticle 18 in the object plane 17 is possible.

[0144] The illumination field 4 has a typical dimension in the object plane 17 which is less than 0.5 mm. In the embodiment illustrated, the extent of the illumination field 4 is 0.5 mm in the x-direction and 0.5 mm in the y-direction.

[0145] The x / y aspect ratio of the illumination field 4 corresponds to the x / y aspect ratio of the exit opening 14.

[0146] The illumination field 4 or a part of the illumination field 4, this part then constituting an object field, is imaged into an image field 21 in an image plane 22 by a projection optical unit PO. A size of the image field 21 can be in the range of 150 mm x 250 mm. The shorter image field extent runs along the scanning direction y.

[0147] The projection optical unit PO has mirrors Ml, M2 consecutively numbered in the imaging beam path of the projection optical unit PO, i.e. comprises a total of two mirrors. Depending on the embodiment of the projection optical unit PO, the number of mirrors may also be greater than two. An aperture stop 9b is arranged in an entrance pupil plane EP of the projection optical unit PO, which is located between the reflective reticle 18 and the first mirror Ml in the imaging beam path of the illumination or imaging light. Said aperture stop 9b may also serve for predefining an internal obscuration of the projection optical unit PO that may be present.

[0148] The mirrors Ml and M2 of the projection optical unit PO are designed as NI mirrors with an angle of incidence of the illumination and imaging light 3 of less than 45°.

[0149] An illumination light beam path of the illumination light 3 for illuminating the reticle 18 and an imaging light beam path of the projection optical unit PO for imaging the object field 4 into the image field 21 intersect in a crossing region K. This crossing region K lies in the region of the entrance pupil plane EP of the projection optical unit PO. The imaging light beam path here crosses the illumination light beam path between the exit opening 14 and the mirror IL2 of the illumination optical unit 1 and also between the mirrors IL2 and IL3 of the illumination optical unit 1.

[0150] The image field 21 is captured by a detection device 23, for example one CCD camera or a plurality of CCD cameras. For details of the imaging into the image field, reference is made to US 10,042,248 B2 and the references cited in US 10,042,248 B2. The detection device 23 may also be designed as a TDI (time delay integration) detection device comprising a plurality of TDI detectors. A corresponding embodiment will be described in detail below.

[0151] An inspection of a structure on the reticle 18, for example, is possible by means of the mask inspection system 2a.

[0152] Fig. 2 shows a perspective and schematic view of an embodiment of the beam-homogenizing element 11, the viewing direction being towards the entrance opening 12.

[0153] In a manner enlarged in comparison with Fig. 2, Fig. 3 shows a detail of an entrance end face of the beam- homogenizing element 11, arranged on the entrance plane 13, with the entrance opening 12. EUV energy sensor devices 24i (i = 1 to 4), of which four EUV deflection mirrors 25i (i = 1 to 4) are depicted in Fig. 3, are part of an embodiment of an energy detection assembly 26 for an illumination system of the mask inspection system 2a, which also includes the light source 5 in addition to the illumination optical unit 1. The energy detection assembly 26 also includes the beam-homogenizing element 11.

[0154] The four EUV deflection mirrors 251 to 254 are arranged outside of the entrance opening 12 around the latter. In this case, each of the EUV deflection mirrors 25i is in close vicinity of one of the four sides of the entrance opening 12 and arranged centrally in relation to the respective side. Fig. 3 moreover shows isolines of an energy distribution of the illumination light 3 in the entrance plane 13. The vast majority of the illumination light energy is input coupled into the entrance opening 12.

[0155] A measure for the respective illumination light energy present in the entrance plane is illustrated in Fig. 3 using different types of hatching, a measurement chart being supplied to the right of Fig. 3 as a legend in this respect. The input coupled illumination light energy is greatest at the centre of the entrance opening 12 in the region of the chief ray CR and drops radially, approximately in rotationally symmetric fashion, with the distance from the chief ray CR. Since the entrance opening 12 is rectangular, slightly more illumination light energy is cut off in the region of the long sides of the entrance opening 12 than in the region of the short sides of the entrance opening 12. The two deflection mirrors 251 and 25s arranged at the long sides therefore capture more illumination light 3 not input coupled into the entrance opening 12 than the two deflection mirrors 252 and 254 arranged at the short sides of the entrance opening 12.

[0156] The percentage of energy of the illumination light 3 not input coupled is less than 30% and regularly less than 5%. The proportion of the illumination light 3 captured by the four deflection mirrors 25i is regularly in the range of less than 1 x 10'4of the entire energy of the illumination light 3.

[0157] In a longitudinal section illustration, Fig. 4 elucidates, in particular, a further detection beam path of the two energy sensor devices 244 with the deflection mirror 254 and of the energy sensor device 242 with the deflection mirror 252.

[0158] The illumination light 3 incident on the deflection mirror 252 is steered towards an energy sensor 272. This may be a photodiode that is sensitive to EUV light. An EUV sensitivity may be achieved by way of appropriate fluorescence or scintillation layers.

[0159] The energy sensor 27 represents an energy value sensor for capturing an illumination energy of the illumination light 3 over an exposure period of the reticle 18. An energy value capture by way of the energy value sensor 27 may be more accurate than 3%, may be more accurate than 1%, may be more accurate than 0.5% and may be more accurate than 0.2%.

[0160] An energy value capture by way of the energy value sensor 27 may be implemented with pulse resolution, i.e. separately for each light pulse from the light source 5.

[0161] Accordingly, the further deflection mirror 254 visible in Fig. 4 steers the proportion of illumination light 3 incident thereon to an associated energy sensor 274 of the energy sensor device 244.

[0162] Further energy sensors 27i and 273 not visible in Fig. 4 are associated accordingly with the deflection mirrors 251 and 25s.

[0163] Thus, overall, the energy detection assembly 26 comprises four deflection mirrors 25i to 254 and four associated energy sensors 27i to 274, i.e. a total of four EUV energy sensor devices 24i to 244.

[0164] The energy sensors 27 are signal-connected to a central open-loop / closed-loop control device 27a (cf. Fig. 1) of the mask inspection system 2a.

[0165] What can also be taken from the illumination light beam path of Fig. 4, visible in the meridional section, is that the deflection mirrors 252 and 254 only deflect illumination light 3 that does not enter the entrance opening 12 of the beam-homogenizing element 11.

[0166] A further embodiment of an energy detection assembly 28 is described hereinafter on the basis of Figures 5 to 7; said energy detection assembly can be used as an alternative to the energy detection assembly 26 according to Figures 3 and 4 or in addition to that. Components and functions corresponding to those which have already been explained above with reference to Figures 1 to 4, and in particular with reference to Figures 3 and 4, bear the same reference signs and will not be discussed again in detail. The energy detection assembly 28 has an EUV energy sensor device 29 that is embodied such that it captures illumination light 3 which is guided within the exit opening 14 of the beam-homogenizing element 11 along an illumination light beam path illustrated in Fig. 5.

[0167] A corresponding exit-side sensor component 3s of the illumination light 3 is created without a meaningful loss of light by virtue of a beam-homogenizing element 11 with a square entrance opening 12Q being used instead of a beam-homogenizing element with a rectangular entrance opening 12 like in the energy detection assembly 26 according to Figures 3 and 4.

[0168] Fig. 6 elucidates this difference. The rectangular entrance opening in the variant from the energy detection assembly 26 according to Figures 3 and 4 is depicted using a solid line in Fig. 6, this time in "standing" form. The squarely extended entrance opening 12Q from the energy detection assembly 28 according to Fig. 5 is depicted in Fig. 6 using dashed lines. In principle, in the case of the squarely extended entrance opening 12Q, a deflection mirror arrangement with deflection mirrors 25i to 254 may also be present outside of this square entrance opening 12Q, as explained above on the basis of the energy detection assembly 26, in particular according to Fig. 4.

[0169] In the case of the energy detection assembly 28, the sensor component 3 s, as a result of the square expansion, is available in an exit opening region 14s (cf. Fig. 7, left) of the entire exit opening 14Q on the exit side downstream of the exit opening 14Q that is likewise square in that case. The rest of the exit opening, i.e. the cross section of the square exit opening 14Q reduced by the sensor component 14s can then be used to illuminate the illumination field 4. Alternatively, a further sensor component 14S2 of the exit opening 14Q on the opposite side, the right side in Fig. 7, may also be used for energy sensor purposes or other purposes, and so a rectangular exit opening 14 effectively remains on the side of the exit opening 14Q; it corresponds to that of the beamhomogenizing element of the energy detection assembly 26 according to Fig. 3.

[0170] Thus, the exit opening 14Q of the beam-homogenizing element 11 is designed to be larger than a used exit opening region which is in the form of the effectively rectangular exit opening 14 and required for the subsequent illumination of the object field in which the mask to be inspected or the reticle 18 is arranged. As a result, some of the illumination light 3 output coupled from the beam-homogenizing element 11 is no longer available for illuminating the object field 4. This effective reduction in the used illumination light for illuminating the object field 4 is compensated for, in any case in part, by virtue of the fact that the entrance opening 12Q of the beam-homogenizing element 11 is also larger, and hence more illumination light 3 can be input coupled into the beam-homogenizing element 11.

[0171] Accordingly, the EUV energy sensor device 29 is designed such that it captures the sensor component 3 s of the illumination light 3 which is guided along the illumination light beam path within the exit opening 14Q of the beam-homogenizing element 11 beyond this used exit opening region, specifically in the exit opening sensor component 14s.

[0172] In the case of the EUV energy sensor device 29, the sensor component 3 s of the illumination light 3 is guided via a first deflection mirror 29a, which is arranged in the exit-side region of the sensor component 14s of the exit opening 14Q. Subsequently, the sensor component 3s is guided to an energy sensor 27 via a further deflection mirror 29b with an imaging embodiment.

[0173] The EUV detection assembly 28 may comprise a plurality of EUV energy sensor devices 29i of the type of EUV energy sensor device 29 depicted in Fig. 5, wherein for example a respective deflection mirror of the type of deflection mirror 29amay be arranged on both sides of the rectangular used exit opening 14, with each deflection mirror then being able to be followed in the beam path of the then further sensor component of the illumination light by components of the type of deflection mirror 29b and of energy sensor 27.

[0174] A further embodiment of an energy detection assembly 30 is described hereinafter on the basis of Fig. 8; said energy detection assembly can be used as an alternative to the energy detection assemblies explained above or in addition to them.

[0175] Part of the energy detection assembly 30 is formed by an EUV energy sensor device 31, which captures the illumination light 3 that is guided along the beam path of the illumination light 3 beyond a detection region of the detection device 23. This detection region of the detection device 23 is predetermined by detection surfaces of TDI or CCD sensors 32i to 3212, which are arranged in the style of a 4 x 3 array in the image plane 22 of the detection device 23. There is a y-spacing between the lines of this sensor array of the sensors 32i that is sufficiently large to ensure that a component of the EUV energy sensor device 31 may be arranged in the region of this line spacing. This may be a deflection mirror of the type of deflection mirrors 25i or an energy sensor of the type of energy sensors 27i.

[0176] A plurality of such EUV energy sensor devices 31 may also be present in the energy detection assembly 30.

[0177] Thus, the energy sensor device 31 is embodied such that it captures a component of the illumination light 3 which is guided along the illumination light beam path between two detection sections of the detection region of the detection device 23, specifically between the detection sections of the sensors 324 and 32?.

[0178] In a further embodiment of an energy detection assembly 33 (cf. Fig. 1) which may be used instead of the above-discussed energy detection assemblies, the respectively utilized used light filter 8 simultaneously serves as deflection mirror for detection light 34, which is reflected out of the beam path of the illumination light 3 by the respective used light filter 8. In Fig. 1, this is depicted at the used light filter 8, which reflects detection light 34 towards an energy sensor 35. The used light filter 8 used to reflect the detection light 34 in each case and the energy sensor 35 represent an EUV sensor device 36 of the energy detection assembly 33.

[0179] The detection light 34 is light at a wavelength that is reflected out by the used light filter 8 and may differ from the wavelength of the illumination light 3. In particular, the wavelength of the detection light 34 may be located in the UV range or in the visible range, but also in the NIR range. Accordingly, the energy sensor 35 is sensitive to the wavelength of the detection light 34 reflected in each case. Thus, the energy sensor 35 may be a photodiode that is sensitive to UV light or else to VIS light or NIR light.

[0180] The detection light 34 is light that is not passed along the subsequent illumination light beam path by the used light filter 8.

[0181] The energy detection assembly 33 may additionally include a bandpass filter 37, which is arranged in the beam path of the detection light 34 between the used light filter 8 and the energy sensor 35 of the EUV energy sensor device 36. The bandpass filter 37 may have filter properties that filter out wavelength ranges that would falsify an inference between the measurement result of the energy sensor 35 and the energy content of the illumination light 3. Thus, the bandpass filter 37 ensures a reduction in what is known as out-of-band light.

[0182] At least one of the used light filters 8 housed in the filter wheel 7 may have a metallic support structure.

[0183] An alternatively and additionally usable further variant of an energy detection assembly 39 may comprise an EUV energy sensor device 40, which is schematically illustrated in Fig. 1 and embodied in such a way that it measures a photocurrent that is created by way of the illumination light 3 in the metallic support structure of the used light filter 8 or in the used light filter 8 itself. The strength of the measured photocurrent then is a measure for the energy content of the illumination light 3.

[0184] A further embodiment of an energy detection assembly 41 is described hereinafter on the basis of Fig. 9. Components and functions which have already been explained above with reference to Figures 1 to 8 bear the same reference signs and will not be discussed again in detail.

[0185] In the energy detection assembly 41 according to Fig. 9, one of the mirrors of the illumination optical unit 1, specifically the mirror IL2, is designed as a fluorescent mirror. To this end, the mirror IL2 has a fluorescence layer 42 for converting a fraction of the incident illumination light 3 into fluorescence detection light 43 at a wavelength that differs from a used light wavelength of the illumination light 3.

[0186] A wavelength of the detection light 43 is longer than that of the EUV illumination light 3. The wavelength of the fluorescence detection light 43 may be located in the UV range, may be located in the visible (VIS) range or may else be located in the near infrared (NIR) range.

[0187] Fig. 9 depicts the beam path of the illumination light 3 from the exit plane 15 of the beam-homogenizing element 11 to downstream of the mirror IL2, and a beam path of the fluorescence detection light 43. The latter extends, starting from the fluorescence layer 42, beyond a lens element 44, which focuses the detection light 43 towards an energy sensor 45. The lens element 44 and the energy sensor 45 are constituent parts of an energy sensor device 46 of the energy detection assembly 41 arranged in the beam path of the fluorescence detection light 43.

[0188] In the detection light beam path between the lens element 44 and the energy sensor 45, the detection light 43 passes through a window 47 in the wall of the vacuum chamber VK.

[0189] A further embodiment of an energy detection assembly 48 is described hereinafter on the basis of Fig. 10; said energy detection assembly can be used as an alternative to the above-described embodiments of the energy detection assembly or in addition to them.

[0190] In a plan view, Fig. 10 depicts an embodiment of the aperture stop 9 of the illumination optical unit 1. It has an outer aperture stop section 49, which delimits the numerical aperture of the illumination light 1 in the beam path upstream of the entrance opening 12 of the beam- homogenizing element 11 and which is depicted using broken lines to the outside in Fig. 10. Furthermore, the aperture stop 9 has an inner obscuration stop section 50, which serves to predetermine a central obscuration of the illumination light 3 in a pupil centre. The two stop sections 49, 50 are connected to one another by way of three connecting pieces 51, which are indicated in Fig. 10 and which may have a very thin design.

[0191] The energy detection assembly 48 has an EUV energy sensor device 52, which is embodied such that it captures that portion of the illumination light 3 incident on the aperture stop 9 which is guided in the central region of an illumination pupil of the illumination optical unit 1 covered by the inner obscuration stop section 50. Once again, a piece of the EUV energy sensor device 52 is formed by an EUV deflection mirror 53 and an energy sensor of the type of energy sensor 27, which is arranged in the beam path of the deflected illumination light 3 but not depicted in Fig. 10. The structure of the EUV energy sensor device 52 may be in the manner as described above, for example in the context of one of the EUV energy sensor devices 24i according to Fig. 3 and Fig. 4. As illustrated in Fig. 10, the EUV energy sensor device 52 may comprise further EUV deflection mirrors 54i, 542, 54s and 544, which may be a constituent part in turn of further EUV energy sensor devices of the type of EUV energy sensor device 52.

[0192] A further embodiment of an energy detection assembly 55 is described hereinafter on the basis of Fig. 11; said energy detection assembly can be used as an alternative to the above-described variants of the energy detection assemblies or in addition to them. Components and functions which have already been explained above with reference to Figures 1 to 10 bear the same reference signs and will not be discussed again in detail.

[0193] An EUV sensor device 56 of the energy detection assembly 55 is designed as an energy sensor of the type of energy sensors 27, for example. In the energy detection assembly 55, the energy sensor 27 is arranged centrally in the illumination light beam in the beam path of the illumination light 3 downstream of the exit plane 15 of the beam-homogenizing element 11, and so a component of the illumination light 3 guided in a central region of an illumination pupil of the illumination optical unit 1 is once again captured by way of the energy sensor 27 of the energy detection assembly 55.

[0194] A further embodiment of an energy detection assembly 57 is described hereinafter on the basis of Fig. 12; said energy detection assembly can be used as an alternative to the above-described variants of the energy detection assemblies or in addition to them. Components and functions which have already been explained above with reference to Figures 1 to 11 bear the same reference signs and will not be discussed again in detail.

[0195] A structure of the energy detection assembly 57 according to Fig. 12 in principle corresponds to that of the energy detection assembly 55 according to Fig. 11. An EUV energy sensor device 58 of the energy detection assembly 57 is constructed in a manner comparable to that according to Fig. 4, for example, and has a focusing deflection mirror 59 which, in a manner comparable to the energy sensor 27 of the energy detection assembly 55 according to Fig. 11, in turn is arranged in a central region of the illumination light beam path downstream of the exit plane 15 and which focuses the component of the illumination light 3 extending there onto an energy sensor 27 of the EUV energy sensor device 58. A further embodiment of an energy detection assembly 60 is described hereinafter on the basis of Fig. 13; said energy detection assembly can be used as an alternative to the above-described variants of the energy detection assemblies or in addition to them. Components and functions which have already been explained above with reference to Figures 1 to 12 bear the same reference signs and will not be discussed again in detail.

[0196] In contrast to the energy detection assembly 57, a deflection mirror 61 of an EUV energy sensor device 62 of the energy detection assembly 60 according to Fig. 13 is designed as a plane deflection mirror. The deflection mirror 61 of the energy detection assembly 60 is arranged at the location of the deflection mirror 59 of the energy detection assembly 57, i.e., once again, centrally in an illumination light beam path downstream of the exit plane 15. The plane deflection mirror 61, which can be embodied to be small, steers the component of the illumination light 3 incident thereon onward to a further deflection mirror 63, which in turn focuses this component of the illumination light 3 on an energy sensor 27.

[0197] A further embodiment of an energy detection assembly 64 is described hereinafter on the basis of Fig. 14; said energy detection assembly can be used as an alternative to the above-described variants of the energy detection assemblies or in addition to them. Components and functions which have already been explained above with reference to Figures 1 to 13 bear the same reference signs and will not be discussed again in detail.

[0198] A piece of the energy detection assembly 64 is formed by an ionization chamber 65, which may be arranged in the beam path of the illumination light 3, for example upstream of the vacuum chamber VK (cf. also Fig. 1). The ionization chamber 65 specifies an ionization space 66. A gas source 67 serves to guide ionization gas 68 into the ionization chamber 65, i.e. into the ionization space 66, via a control valve 69.

[0199] The beam path of the illumination light 3 is guided through the ionization space 66. An EUV energy sensor device 70 of the energy detection assembly 64 has an ion detector 71 for detecting a total number of ions 71a which are generated in the ionization space 66 by the illumination light 3 by way of the ionization of the ionization gas 68. Furthermore, the EUV energy sensor device 70 comprises an electron detector 72 for detecting a total number of electrons 72a which are generated in the ionization space 66 by the illumination light 3 by way of the ionization of the ionization gas 68.

[0200] The detectors 71, 72 each have an acceleration electrode 71b, 72b, which may for example be at a potential of -50 V in the case of the ion detector 71 and at a potential of 20 kV in the case of the electron detector 72.

[0201] The electrodes 71b, 72b are designed as grid electrodes.

[0202] A metallic ion trap 71c of the ion detector 71 and a metallic electron trap 72c of the electron detector 72 serve to capture the accelerated ions 71a and the accelerated electrons 72a, respectively. These traps 71c, 72c are at 0 V reference potential and may have a comb-like structure, as indicated in Fig. 14.

[0203] The two detectors 71, 72 each have a charge signal receiver 7 Id, 72d, which records a charge signal captured by the traps 71c, 72c and generates an energy signal therefrom. The charge receivers 7 Id, 72d thus represent energy sensors of the EUV energy sensor device 70 of the energy detector assembly 64.

[0204] Fig. 15 illustrates an operating scheme for one of the TDI sensors 32i of the detection device 23 of the mask inspection system 2a.

[0205] The reticle 18 with a circular edge in the example of Fig. 15 is displaced along the object displacement direction y relative to the object field 4 during a mask inspection step, as indicated in a side view to the left of Fig. 15. In this schematic view according to Fig. 15, the reticle 18 is shown as a transmissive object in order to simplify the illustration. The light source 5 and the illumination optical unit 1 are also shown schematically to the left of Fig. 15. Fig. 15, top centre, illustrates how on account of the object displacement during the mask inspection step, an image of the reticle 18 migrates over a detection surface of the TDI sensor 32i, said detection surface being depicted in columns in Fig. 15 and having sensor columns I to IV. This is illustrated on the basis of an image 75 of the circular edge of the reticle 18 on the detection surface, which migrates from left to right over the detection surface in the instantaneous recording situations A to D shown in Fig. 15, top centre.

[0206] In synchronization with the object displacement of the reticle 18 relative to the TDI sensor 32i, the sensor columns I to IV are read, again along the object displacement direction y. A charge accumulation q, building up in the process, over the spatial coordinate y on the detection surface of the TDI sensor 32i is depicted at the bottom of Fig. 15, once again for the instantaneous situations A to D.

[0207] Once the image of the reticle 18 has been displaced by one column width of the sensor columns I to IV in the object displacement direction y, there is a charge shift on the TDI sensor 32i from one column to the next column in the y-direction at the same time. This leads to a growth of a charge q in the columns which capture image structures, for example a boundary section of the image 75, at each displacement / charge shift step, as illustrated by the charge distribution curves at the bottom centre in Fig. 15 for the instantaneous situations A to D. Situation D shows the moment at which a leading section of the image 75 has migrated in full over all columns I to IV of the TDI sensor 32i, and the image charge corresponding to this image section has accumulated to a maximum extent at the sensor column IV furthest right as a result of the synchronized charge shift.

[0208] On the right, Fig. 15 shows the resultant overall image 75 of the imaged structure on the reticle 18 after this entire structure was displaced through the object field 4 along the y-direction. This yields a more contrast-rich image on account of the synchronized displacement / charge shift than would be the case in the event of a non-time-shift-integrated recording by means of a conventional CCD sensor.

[0209] Additionally, a pulsed design of the light source 5 must be taken into account when imaging the reticle 18 using the mask inspection system 2a. Different sections of the reticle 18 see different numbers of light pulses from the light source 5 during the object displacement of the reticle 18 through the object field 4.

[0210] This is illustrated in Fig. 16, which illustrates a temporal sequence of a pulsed exposure of the reticle 18 that is displaced along the object displacement direction y. What is shown in each case is the illumination field 4i (i = 1 to 5) instantaneously exposed on the reticle 18 during a light pulse from the light source 5 and also migrating along the object displacement direction y on account of the displacement of the reticle 18 over same. The arrows emphasize certain illumination field overlap regions 76i.

[0211] A section of the reticle, which is exposed by a total of three light pulses since it is located in the three illumination fields 4i, 42 and 4s each assigned to a light pulse, is located in the overlap region 76i, which is also emphasized by dashed lines in Fig. 16.

[0212] The overlap region 762 is illuminated by exactly one light pulse since it is located exclusively in the illumination field 4s.

[0213] Overlap regions 76s and 764 are each exposed by exactly two light pulses from the light source 5 since they are located firstly (overlap region 76s) in the illumination fields 4s and 44 and secondly (overlap region 764) in the illumination fields 44 and 4s.

[0214] Thus, it is not true that every object section of the reticle 18 is exposed by the same number of light pulses from the light source 5. Thus, the reticle 18 is impinged unevenly by the illumination light 3 in this sense.

[0215] To compensate for this exposure unevenness, an EUV energy sensor device 77 of an energy detector assembly 78, which is depicted in Fig. 17, also has at least one change-in-energy sensor 79 for capturing a temporal development of an illumination energy of the illumination light 3 over an exposure period of the reticle 18, in particular for capturing a pulsed time-energy curve of the light source 5, in addition to an energy value sensor of the type of energy sensors, for example energy sensors 27, of the exemplary embodiments described above. Functions and components already explained above with reference to Figures 1 to 16 bear the same reference signs and will not be discussed in detail again.

[0216] In turn, Fig. 17 schematically shows components of the light source 5, specifically the source volume 6 that is housed in a source chamber 80 and an exit opening 81 in the source chamber 80 that specifies an exit aperture of a beam of illumination light 3.

[0217] On account of the plasma generation method of the light source 5, stray light 82 emanates from the source volume 6 and some of said stray light is used as a detection light and runs along a detection beam path illustrated in Fig. 17 by way of example. Along this detection beam path, the detection light 82 is reflected off an inner wall 83 of the source chamber 80 and passes through the exit opening 81 of the source chamber 80 at an angle to a chief ray of the illumination light 3, i.e. it crosses the beam of illumination light 3 in the exit opening 81.

[0218] In comparison with the illumination light 3, the detection light 82 has a significantly longer wavelength, for example in the visible wavelength range. The detection light 82 is extraneous light which is simultaneously co-created during the creation of the illumination light 3. The detection light 82 is also emitted by the plasma in the source volume 6, in a manner correlated in time with the emission of the illumination light 3.

[0219] Following the passage through the exit opening 81, the detection light 82 passes through a window 84, for example made of glass, and a lens element 85, which focuses the detection light 82 on the change-in-energy sensor 79 of the EUV energy sensor device 77. The change-in-energy sensor 79 is a fast photodiode with a bandwidth of several MHz, which, depending on the wavelength of the detection light 82, may be sensitive to UV light, to visible light (VIS) or else to near infrared light (NIR).

[0220] Fig. 18 shows a further variant of an energy detection assembly 86, which may be used as an alternative or in addition to the energy detection assembly 78 according to Fig. 17. The energy detection assembly 86 has an EUV energy sensor device 87, which comprises a change-in-energy sensor 88 that is housed in the source chamber 80 and directly captures as detection light stray light 82 emitted by the source volume 6. A temporal development of an energy of the detection light 82 correlates directly with the temporal development of an energy of the illumination light 3, and so a temporal development of the illumination energy of the illumination light 3 can be captured by way of the change-in-energy sensors 79 and 88.

[0221] The change-in-energy sensors 79 and 88 have a temporal resolution better than 10 ps. The temporal resolution may be better than 1 ps and may be even better still.

[0222] Fig. 19 shows a meridional section of a portion of the illumination system of the mask inspection system 2a in the region between the aperture stop 9 and an entrance-side section of the beam-homogenizing element 11 in the region of the entrance opening 12.

[0223] Fig. 19 illustrates an input-coupling sensor device 91 for monitoring an input-coupling location and an input-coupling direction of a beam of illumination light 3 into the entrance opening 12 of the beam- homogenizing element 11. The input-coupling sensor device 91 has an input-coupling location sensor unit 92 and an input-coupling direction sensor unit 93.

[0224] The input-coupling location sensor unit 92 is arranged in the region of the entrance opening 12 of the beam-homogenizing element 11.

[0225] Fig. 20 shows an embodiment of the input-coupling location sensor unit 92 in a perspective illustration. For illustrative purposes, individual beams 3i of the entire beam of illumination light 3 are plotted in the region of the beam path upstream of the entrance opening 12 of the beam-homogenizing element 11. Apart from the entrance opening 12, the beam-homogenizing element 11 has been omitted from Fig. 20.

[0226] The input-coupling location sensor unit 92 has a total of four deflection mirrors 94i, 942, 94s and 944 that are arranged around the entrance opening 12 in such a way that the entrance opening 12 is completely delimited by the deflection mirrors 94i. Thus, illumination light 3 not passing through the entrance opening 12 during the input-coupling process is reflected off one of the deflection mirrors 94i to 944. Moreover, Fig. 20 shows exemplary individual ray beam paths of light components not input- coupled into the entrance opening 12, i.e. sensor components 3s of the illumination light 3 that is reflected off a respective one of the deflection mirrors 94i. Energy sensors 27i, 272, 27s and 274 of the input-coupling location sensor unit 92 are arranged in the beam direction of these sensor components 3 s of the illumination light 3 following reflection at a respective one of the deflection mirrors 94i to 944.

[0227] The deflection mirrors 94i are each oriented such that the respective sensor component 3s is reflected to the associated energy sensor 27i in a manner crossing the beam of illumination light 3 incident in the direction of the entrance opening 12. This is also made clear by Fig. 19 which, in section, depicts the deflection mirrors 94i and 94s and the energy sensors 27i and 273 associated therewith. In Fig. 19, the deflection mirror 94i is arranged above the entrance opening 12, i.e. above the incident beam of illumination light 3, and the associated energy sensor 27i is arranged below the incident beam of illumination light 3 in Fig. 19. Accordingly, the deflection mirror 94s is depicted below and the associated energy sensor 27s is depicted above this incident beam of illumination light 3 in Fig. 19.

[0228] The deflection mirrors 94i may be designed as a coating on an entrance-side end wall of the housing 15e of the beam-homogenizing element 11. In the deflection mirror arrangement according to Fig. 19, this entrance-side end wall may have a concave embodiment or, alternatively, also a convex embodiment.

[0229] In an alternative to a coating, the deflection mirrors 94i may be applied to this entrance-side end wall, for example adhesively bonded thereto. Once again, alternatively, the deflection mirrors 94i may be components that are separate from the beam-homogenizing element 11 and in particular adjustable in a manner separately therefrom.

[0230] The energy sensors 27i may be selected in the style of a quadrant detector for closed-loop control of the input-coupling location of the beam of illumination light 3 into the entrance opening 12. Adjustment control pulses can be processed with the aid of energy or intensity values generated by the energy sensors 27i, as set forth below: A control signal Ax in one of the dimensions of the entrance opening 12 may be generated in accordance with the following relationship:

[0231] Here, Ax is a control signal for an adjustment component for displacing the beam of illumination light 3 in the x-direction. To simplify matters, the assumption is made here that the entrance opening 12 is located in the xy-plane.

[0232] Ii and I3 are the sensor signals from the energy sensors 27i and 27s. r|xis a scaling parameter that can be obtained during a calibration.

[0233] Accordingly, a control signal Ay for the y-coordinates can be obtained by way of the following relationship:

[0234] I2 and I4 are the sensor signals from the energy sensors 272 and 274. r|yis again a scaling factor which can be obtained by means of a preceding calibration step.

[0235] The control signals Ax, Ay generated thus may be used as input signals for a location control of the beam of illumination light 3 relative to the entrance opening 12. In this way, there is location control of the input-coupling location of the beam of illumination light 3 into the entrance opening 12 of the beam-homogenizing element 11.

[0236] In principle, the input-coupling location sensor unit 92 may be constructed like the energy detection assembly 26, which was explained above on the basis of Fig. 4. Accordingly, the input-coupling location sensor unit 92 may also concomitantly adopt the function of the energy detection assembly 26, which was explained above. Fig. 21 shows an embodiment of the input-coupling direction sensor unit 93. Once again, the beam of illumination light 3 is illustrated by a multiplicity of individual rays 3i.

[0237] In the ideal case, the beam of illumination light 3 passes through the aperture stop 9 without significant components of the illumination light 3 being cut off by the aperture stop 9.

[0238] Once again, four energy sensors 27i, 272, 27s and 274 are arranged in the beam path of the illumination light 3, directly upstream or else directly downstream of the aperture stop 9. These energy sensors 27i of the input-coupling direction sensor unit 93 are held by way of a sensor frame 95 that also surrounds the beam of illumination light 3.

[0239] The energy sensors 27i are so closely adjacent to the beam of illumination light 3 that, should a sensor component 3 s of the illumination light 3 be incident on one of the energy sensors 27i of the input-coupling direction sensor unit 93, this sensor component 3s would also be cut off by the aperture stop 9. To the extent that the energy sensors 27i are arranged in the beam path downstream of the aperture stop 9, these energy sensors 27i arranged thus represent sections of the aperture stop 9.

[0240] The input-coupling direction of the beam of illumination light 3 into the entrance opening 12 of the beam-homogenizing element 11 is monitored with the aid of the energy sensors 27i to 274 of the input-coupling direction sensor unit 93. Moreover, appropriate evaluation of the energy sensors 27i, once again in the style of a quadrant sensor as explained above in the context of the input-coupling location sensor unit 92, allows generation of control signals Ax, Ay and, thereby, direction control of the beam of illumination light 3.

[0241] In an alternative to the embodiment according to Fig. 21, the input-coupling direction sensor unit 93, as illustrated in Fig. 19, may also comprise deflection mirrors 96i, of which the mirrors 96i and 963 are depicted in Fig. 19, and associated energy sensors 27i and 27s. The deflection mirrors 96i are then arranged around the aperture of the aperture stop 9, corresponding to the arrangement of the energy sensors 27i in the arrangement according to Fig. 21. Corresponding to what was explained above in the context of the deflection mirrors 94i, the deflection mirrors 96i may be designed as coatings of the aperture stop 9, as deflection mirrors applied to the aperture stop 9 or else as deflection mirrors that are separate from the aperture stop 9 and, in particular, adjustable separately therefrom. The deflection mirrors 96i are designed such that they reflect sensor components 3 s of the illumination light that has not passed through the aperture in the aperture stop 9 to the outside, to the respectively assigned energy sensor 27i.

[0242] A further embodiment of an input-coupling location sensor unit 97, which can be used instead of the input-coupling location sensor unit 92 according to Figures 19 and 20, is explained in the following text on the basis of Figures 22 and 23. Components and functions corresponding to those which have already been explained above with reference to Figures 1 to 21, and in particular with reference to the input-coupling sensor device 91 according to Figures 19 and 20, bear the same reference signs and will not be discussed in detail again.

[0243] While the four deflection mirrors 94i to 944 are arranged in the input-coupling location sensor unit 92 in a single arrangement plane, which is perpendicular to a chief ray of an input-coupling beam path of the illumination light 3, deflection mirrors 981 to 984 of the input-coupling location sensor unit 97, which otherwise in principle correspond to the deflection mirrors 94i to 944 in terms of their function, are arranged in two arrangement planes which are arranged at a distance from one another and one after the other with respect to the input-coupling beam path of the illumination light 3.

[0244] The two deflection mirrors 981 and 98s on the one hand and the two deflection mirrors 982 and 984 on the other hand form in each case a deflection mirror pair 99 and 100, respectively.

[0245] The deflection mirrors 98i (i = 1 to 4) again delimit the entrance opening 12 of the beam- homogenizing element 11. The deflection mirror pair 99 is arranged here to guide the illumination light 3 in the beam direction of the input-coupling beam path, that is to say in an arrangement plane which is located further away from the beam-homogenizing element 11 than the arrangement plane of the further deflection mirror pair 100 having the deflection mirrors 982 and 984. At the point where the deflection mirror pair 100 delimits the entrance opening 12, the deflection mirrors 982 and 984 of the deflection mirror pair 100 are thus located between the deflection mirrors 98i, 983 of the deflection mirror pair 99 and the beam-homogenizing element 11. The deflection mirrors 98i delimit overall a through opening for the illumination light 3 not deflected by the deflection mirrors 98i which coincides with the entrance opening 12 of the beamhomogenizing element 11.

[0246] As is illustrated in particular by the top view according to Fig. 23, the deflection mirrors 98i each have the same basic shape, and so it will suffice to describe this shape with reference to the deflection mirror 98i . At the point where the deflection mirror 98i delimits the through opening 12, the deflection mirror 98i is symmetrically trapezoidal and has two wedge edge regions 101, 102, which taper in the form of a wedge, and a boundary edge region 103, which in fact delimits the through opening 12. A length of the boundary edge region 103 approximately corresponds to a third of an overall width B of the deflection mirror 981. A thickness of the boundary edge region 103, that is to say a thickness of the deflection mirror 98i in the region of a delimitation edge for the through opening 12, is smaller than 10 pm and can be in the range from 1 pm to 10 pm.

[0247] An edge roughness of the deflection mirror 98i in the region of the wedge edge region is smaller than 25 pm.

[0248] The area of the through opening 12 corresponds to that of the entrance opening 12. The through opening 12 has an opening area for example in the range of between 0.1 mm2and 10 mm2. Partial beams of the illumination light 3 which are deflected by the deflection mirrors 98i have a typical beam diameter in the range of between 0.1 mm and 1 mm.

[0249] The deflection mirrors 98i deflect sensor components 3Sof the illumination light 3 towards the energy sensors 27i, in accordance with what has been discussed above in particular in relation to Fig. 19 and 20.

[0250] In an alternative or additionally to the aforementioned embodiments of the input-coupling sensor device 91, in particular to the input-coupling location sensor unit 92 or 97, use can also be made of variants which were already discussed above in the context of the various embodiments of the energy detection assembly. In particular, part of a variant of the input-coupling location sensor unit may be made up by a deflection mirror which reflects at least a plurality of sensor components 3 s, and preferably a plurality of sensor components in the circumferential direction, of the beam of illumination light 3 guided to the entrance opening 12 to at least one corresponding energy sensor in marginal fashion.

[0251] Once again, in an alternative to that or in addition, an input-coupling sensor device with a corresponding control function may be realized by scanning corresponding relative movement components for the purpose of displacing location and direction of the incident beam of illumination light 3 to the beam-homogenizing element 11 and in particular to the entrance opening of the beam-homogenizing element 11. Depending on the respective scan position, it is then possible to measure the amount of illumination light 3 that is input coupled into the entrance opening 12, and it is then possible to control this to an input-coupling maximum.

[0252] Once again, in an alternative to that or in addition, input-coupling of the illumination light 3 into the entrance opening 12 can be sensor-monitored by means of a corresponding input-coupling location sensor unit by arranging an EUV fluorescent screen in the entrance plane 13, wherein the screen is in particular attached around the entrance opening 12, in a manner comparable to the deflection mirrors 94i. Fluorescence created by offshoots of the beam of illumination light 3 on this fluorescent screen during the input-coupling into the entrance opening 12 may then be monitored by way of a corresponding camera, and the input-coupling of the beam of illumination light 3 into the entrance opening 12 can be adjusted in respect of minimizing this captured fluorescence or rendering it symmetric.

[0253] In principle, the above-described sensor units of the input-coupling sensor device 91 may be arranged in the beam path upstream of an opening of the respective sensor unit, i.e. upstream of the aperture stop 9 and / or upstream of the entrance opening 12, or be arranged downstream of a corresponding opening in an alternative.

[0254] Arrangement upstream of the opening provides the option of obtaining an evaluable sensor signal even in the case of a centred, optimally input-coupled state provided the energy sensors are still located in the beam path of the illumination light 3 in that case and accordingly receive a sensor component 3s. Arrangement downstream of the respective opening enables a measurement with high signal dynamics because the energy sensors used in that case are able to capture a strong drop in intensity in the respective input-coupling peripheral region.

[0255] Adjustment control of input-coupling location and input-coupling direction may be implemented with identical control steps. Setting of input-coupling direction and input-coupling location may be brought about by moving optically effective components of the illumination system in one or more degrees of freedom of translation and / or rotation. Components thus optically effective for setting the input-coupling direction and the input-coupling location may be components of the light source and / or else further beam guiding components.

[0256] Input-coupling location on the one hand and input-coupling direction on the other hand may be iteratively adjusted in alternation and thus be optimized.

[0257] Both a respective alternating control of input-coupling location and input-coupling direction or else a combined control are possible.

[0258] When monitoring the input-coupling location, the input-coupling sensor device 91 can carry out not only monitoring in the two spatial dimensions x and y in the entrance plane 13 of the entrance opening 12 but also monitoring along the component z perpendicular thereto. This can be implemented by linking this monitoring with a focus adjustment option for an input-coupling focus of the illumination light 3 relative to the position of the entrance opening 12 in this z-direc- tion as well. For example, the beam-homogenizing element 11 may be displaced along the beam direction, i.e. along the z-direction perpendicular to the entrance plane 13, in order to open up this degree of freedom of adjustment.

[0259] The above-explained embodiments of the energy value sensors or of the change-in-energy sensors may each comprise a conversion medium for converting light incident on the respective sensor into a detection light at a longer wavelength, for example a fluorescence medium and / or a scintillation medium. The respective energy value sensors or change-in-energy sensors are signal-connected to the open-loop / closed-loop control device 27a, which in turn is signal-connected to the light source 5 and to the detection device 23. In this way, it is possible to correlate a detection result with the performance of the light source 5, in particular with the respective energy of a light pulse of the light source 5 and a respective temporal development of this energy; this may be captured by way of the at least one energy value sensor and the at least one change-in-energy sensor, respectively, of the above-described embodiments of the energy detection assemblies. A correspondingly highly accurate image generation of the structures of the reticle 18 is the consequence.

[0260] The open-loop / closed-loop control device 27a is signal-connected, in particular, to the input-coupling mirror optical unit 10 for the purpose of performing the above-explained open-loop / closed- loop control processes.

[0261] With reference to Figures 24 ff, further embodiments of energy detection assemblies and further embodiments of illumination optical units will be described below, which can be used in a correspondingly adapted illumination system of the mask inspection system 2a. Components, functions and coordinates corresponding to those which have already been explained above with reference to Figures 1 to 23 are identically labelled, bear the same reference signs and will not be discussed in detail again.

[0262] Fig. 24 perspectively shows a top view of a section of the reticle 18 as a lithography mask to be inspected. The reticle 18 is arranged in the object plane 17 in a manner such that a section of the reticle 18 to be inspected is located in the object field 4a, which in turn lies within the illumination field 4. The illumination field 4 is extended further in the x-direction and also in the y-direc- tion than the object field 4a.

[0263] An energy sensor 105 of a further embodiment of an EUV energy sensor device according to Fig. 24 is arranged in the region of the object plane 17, that is to say either in the object plane 17 or in the z-direction adjacent to the object plane 17. A distance between the energy sensor 105 and the object plane 17 is regularly smaller than 5 mm. The energy sensor 105 is arranged above the object plane 17, that is to say on the side of the object plane 17 facing the last mirror IL3 of the illumination optical unit 1. The energy sensor 105 is arranged such that it captures the EUV illumination light 3 which is guided outside the object field 4a but within the illumination field 4. Owing to this arrangement of the energy sensor 105, no EUV light loss thus occurs in the object field 4 and therefore no throughput loss. The function of the energy sensor 105 corresponds to that which has already been discussed above in relation to the energy sensors of the embodiments of the energy detection assemblies according to Figures 1 to 23. The energy sensor 105 can be carried by the object holder 19.

[0264] Fig. 25 shows a variant of such an energy detection assembly. An EUV deflection mirror 106, which guides the illumination light 3, which is incident on the object plane 17 outside the object field 4a but within the illumination field 4, towards an energy sensor 107, is arranged there in the region of the object plane 17. The energy sensor 107 can in this embodiment according to Fig. 25 be located further away from the object plane 17, with the result that there is no conflict in terms of installation space between the energy sensor 107 and components of the mask inspection system which are arranged close to the object plane 17.

[0265] The EUV deflection mirror 106 is arranged in the region of the object plane 17 in a manner such that it guides EUV illumination light 3 captured outside the object field 4a towards the energy sensor 107. The EUV deflection mirror 106 can be carried by the object holder 19.

[0266] The function of the energy sensor 107 corresponds to that of the energy sensor 105 according to Fig. 24.

[0267] A further embodiment of an energy detection assembly is described hereinafter on the basis of Fig. 26. Components and functions corresponding to those which have already been described above with reference to Figures 1 to 25 bear the same reference signs and will not be discussed in detail again.

[0268] Fig. 26 illustrates a beam path of the illumination light 3 between an output plane 108 and the illumination field 4. The output plane 108 can be a source plane, that is to say an arrangement plane of the source volume 6 of the light source 5. The output plane 108 can also be an intermediate focus plane, in particular a plane into which the source volume 6 of the light source 5 has been imaged. The output plane 108 can also be the exit plane 15 of the beam-homogenizing element 11 according to Figures 1, 1A.

[0269] A stop 109 is arranged in the illumination light beam path between the mirrors IL2 and IL3 of the illumination optical unit according to Fig. 26. The stop 109 is an aperture stop of the illumination optical unit 1, which is arranged in the region of a pupil plane 110 of the illumination optical unit 1. A total beam of the illumination light 3 is fully accessible in the region of the pupil plane 110.

[0270] An EUV energy sensor device of the energy detection assembly according to Fig. 26 is designed such that it captures a portion of the EUV illumination light 3 which is guided remote from an illumination light beam which is specified by the stop 109. For this purpose, the energy sensor device has EUV energy sensors 27i, 272, which are arranged in the region of the stop 109. The energy sensors 27i are carried directly by the stop 109 in the embodiment according to Fig. 26. These energy sensors 27i are arranged on an entrance side of the stop 109 for the illumination light 3. The energy sensors 27i are facing the illumination optical unit mirror IL2.

[0271] The EUV energy sensor device according to Fig. 26 can comprise an energy sensor 27, as illustrated in Fig. 26, two energy sensors 27i, 272 or else a larger number of energy sensors 27i, for example three, four, five, six, eight, ten or even more energy sensors 27i.

[0272] Fig. 27 shows a further variant of an energy detection assembly. Components and functions corresponding to those which have already been explained above with reference to Figures 1 to 26, and in particular with reference to Figures 1 and 26, bear the same reference signs and will not be discussed again in detail.

[0273] An EUV energy sensor device of the energy detection assembly according to Fig. 27 has, instead of the energy sensors 27i of the embodiment according to Fig. 26, EUV deflection mirrors 25i, of which exactly one deflection mirror 25 is illustrated in Fig. 27.

[0274] The EUV deflection mirror 25 is arranged, in the energy detection assembly according to Fig. 27, in the region of the stop 109 in a manner such that it guides a portion of illumination light 3 captured remote from the stop 109 towards the energy sensor 27. The deflection mirror 25 is held by the stop 109.

[0275] The energy sensor 27 can be arranged, in the embodiment according to Fig. 27, in such a way that no conflicts in terms of installation space with further components of the mask inspection system 2a occur.

[0276] Fig. 28 shows a further variant of an energy detection assembly. Components and functions corresponding to those which have already been discussed above with reference to Figures 1 to 27, and in particular with reference to Figures 1, 26 and 27, bear the same reference signs and will not be discussed again in detail.

[0277] In Fig. 28, an aperture stop I l l is formed from multiple parts and has, as is illustrated by way of example in Fig. 28, two stop sections 1111, 11 U. The aperture stop section H U is arranged in the beam path of the illumination light 3 between the mirrors IL2 and IL3 and delimits a partial circumference of a total beam of the illumination light 3 between these two mirrors IL2 and IL3. The further aperture stop section 11 U is arranged in the downstream illumination light beam path between the mirror IL3, that is to say the last mirror of the illumination optical unit 1 upstream of the illumination field 4, and the illumination field 4 and again delimits a partial circumference of the illumination light total beam. The arrangement of the two aperture stop sections 1111 and 11 h can be such that the illumination light total beam is delimited from two opposite sides of the beam by these aperture stop sections I l li and 11 U. A fully circumferential aperture delimitation of the total beam of the illumination light 3 is achieved in this way with the multi-part aperture stop 111 with aperture stop sections I l li.

[0278] An EUV deflection mirror 25 of the EUV energy sensor device of the energy detection assembly according to Fig. 28 is carried by the aperture stop section 11 U. This EUV deflection mirror 25 is arranged in the region of the aperture stop section 11 U in a manner such that it guides the illumination light 3 captured remote from the aperture stop section 11 U towards an energy sensor 27 of the EUV energy sensor device according to Fig. 28. In the embodiment according to Fig. 28 as well, a plurality of EUV deflection mirrors 25i may be provided, which are carried by the respective aperture stop section I l li, which guide respective partial beams 3i of the illumination light 3 towards associated EUV energy sensors 27i.

[0279] Instead of the deflection mirror 25, in the case of the aperture stop section 11 U, an energy sensor 25 may also be arranged directly, corresponding to the embodiment which was discussed above in relation to Fig. 26.

[0280] Fig. 29 shows a further variant of an energy detection assembly. Components and functions corresponding to those which have already been explained above with reference to Figures 1 to 28, and in particular with reference to Figures 1 and 26 to 28, bear the same reference signs and will not be discussed again in detail.

[0281] Fig. 29 illustrates an embodiment of the illumination optical unit 1 between an output plane 112 and the illumination field 4. The output plane 112 can be the arrangement plane of the source volume 6 of the light source 5 or an intermediate focus plane into which this source volume 6 is imaged.

[0282] An aperture stop 111 in the illumination optical unit 1 according to Fig. 29 is again formed from multiple parts. Fig. 29 shows two aperture stop sections 1111 and 11 U of this aperture stop 111. The aperture stop section 1111 is arranged in the beam path of the illumination light 3 between the illumination optical unit mirror IL1 and the beam-homogenizing element 11 and corresponds to the aperture stop 9 of the embodiment according to Fig. 1 in terms of its arrangement position. The further aperture stop section 11 U is arranged in the illumination beam path between the mirror IL3 and the illumination field 4 and corresponds to the aperture stop section 11 h according to Fig. 28 in terms of its arrangement.

[0283] The aperture stop section 11 U in the embodiment according to Fig. 29 again carries an EUV deflection mirror 25, which, as in the embodiment according to Fig. 28, guides illumination light 3 captured remote from the aperture stop section 11 U towards an energy sensor 27 of the EUV energy sensor device according to Fig. 29. Such an EUV deflection mirror, which again guides illumination light captured remote from the aperture stop section H U towards an EUV energy sensor associated with this EUV deflection mirror, can also be arranged in the region of the aperture stop section 1111. One variant in which an energy sensor is arranged directly in the region of the aperture stop section 1111 is also possible, comparable with the embodiment according to Fig. 26.

[0284] In order to capture energy fluctuations of the light source 5 by means of the above-described variants of the energy detection assemblies as precisely as possible for post-processing the sensor data from the detection device 23, the EUV energy sensors can be designed such that their spectral capture characteristic is matched firstly with a spectral transmission characteristic of the projection optical unit PO (cf. Fig. 1) and / or secondly with a spectral sensitivity of the detection device 23. The respective EUV energy sensor 27 can be optimized in such a way that its spectral sensitivity corresponds to a transmission curve T (X), for which:

[0285] T (X) = T Optik ( ) T Kamera (X)

[0286] Here, T Optik ( ) is a spectral transmission of the projection optical unit PO and T Kamera (X) is a spectral sensitivity of the detection device 23.

[0287] In order to simulate this spectral characteristic T ( ) as a sensitivity characteristic of the EUV energy sensor 27, the respective EUV energy sensor 27 can have a specific coating, which is in particular wavelength-dependent in the EUV. Alternatively or additionally, the associated EUV deflection mirror 25 can have such a wavelength-dependent coating.

[0288] The coating can be an interference coating. The coating can be a multi-layer coating. The coating can be designed as a layer sequence of coating materials, for example molybdenum, silicon, ruthenium. Exactly two of these materials can be used.

Claims

Patent Claims1. Energy detection assembly (26) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having a beam-homogenizing element (11) for guiding the illumination light (3), which has an entrance opening (12) for the illumination light (3) and an exit opening (14) for the illumination light (3), having at least one EUV energy sensor device (24i) that is designed such that it captures illumination light (3) which is guided outside of the entrance opening (12) of the beamhomogenizing element (11) along an illumination light beam path.

2. Energy detection assembly according to Claim 1, characterized in that the EUV energy sensor device (24) has an EUV deflection mirror (25i) arranged outside the entrance opening (12) of the beam-homogenizing element (11) and an energy sensor (27i), which is arranged in a manner such that illumination light (3), which is guided along the illumination light beam path and is incident on the EUV deflection mirror (25i), is guided towards the energy sensor (27i).

3. Energy detection assembly according to Claim 1 or 2, characterized by a plurality of EUV energy sensor devices (24i to 244), which are designed such that they capture illumination light (3) which is guided to a plurality of regions outside the entrance opening (12) of the beam-homogenizing element (11), which are distributed around the entrance opening (12), along the illumination light beam path.

4. Energy detection assembly according to any of Claims 1 to 3, characterized by an inputcoupling sensor device (91) for monitoring an input-coupling location and / or an input-coupling direction of a beam of the illumination light (3) into the entrance opening (12) of the beam-homogenizing element (11).

5. Energy detection assembly according to Claim 4, characterized in that the input-coupling sensor device (91) comprises:an input-coupling location sensor unit (92; 97) for monitoring an input-coupling location of the beam of the illumination light (3) into the entrance opening (12) of the beamhomogenizing element (11), and / or separately herefrom, an input coupling direction sensor unit (93) for monitoring an input-coupling direction of the beam of the illumination light (3) into the entrance opening (12) of the beam-homogenizing element (11).

6. Energy detection assembly according to Claim 4 or 5, characterized in that the input-coupling sensor device (91) comprises at least one EUV energy sensor (27i), which captures illumination light (3) which is guided outside an aperture of an aperture stop (9) of the illumination system.

7. Energy detection assembly (28) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having a beam-homogenizing element (11) for guiding the illumination light (3), having an entrance opening (12) for the illumination light (3) and having an exit opening (14) for the illumination light (3), and having at least one EUV energy sensor device (29), which is designed such that it captures illumination light (3) which is guided within the exit opening (14) of the beam-homogenizing element (11) along an illumination light beam path.

8. Energy detection assembly according to Claim 7, characterized in that the EUV energy sensor device (29) comprises an EUV deflection mirror (29a) arranged within the exit opening (14) of the beam-homogenizing element (11), and an energy sensor (27), which is arranged such that illumination light (3) which is guided along the illumination light beam path and is incident on the EUV deflection mirror (29a) is guided towards the energy sensor (27).

9. Energy detection assembly according to Claim 7 or 8, characterized in that the exit opening (14Q) of the beam-homogenizing element (11) is larger than a used exit opening region (14), which is required for subsequently illuminating an object field (4) in which a mask (18) to be inspected is arrangeable, wherein the EUV energy sensor device (29) is designedsuch that it captures illumination light (3) which is guided within the exit opening (14Q) of the beam-homogenizing element (11) remote from the used exit opening region (14) along an illumination light beam path.

10. Energy detection assembly according to one of Claims 1 to 9, comprising a plurality of deflection mirrors which are arranged to couple out an incident beam of the illumination light (3) to the at least one EUV energy sensor device (24i), in particular to the at least one EUV energy sensor (27i) or to a plurality of EUV energy sensors (27i).

11. Energy detection assembly according to Claim 10, wherein the deflection mirrors are arranged in exactly one arrangement plane.

12. Energy detection assembly according to Claim 10, wherein the deflection mirrors are arranged in a plurality of arrangement planes located one after the other with respect to an illumination light beam path.

13. Energy detection assembly according to one of Claims 10 to 12, wherein the deflection mirrors are arranged in pairs.

14. Energy detection assembly according to Claim 13, wherein the deflection mirrors of one of the pairs of deflection mirrors delimit opposite sides of a through-opening for the illumination light (3) which has not been deflected.

15. Energy detection assembly according to Claim 14, wherein the through-opening coincides with the entrance opening (12) of the beam-homogenizing element (11).

16. Energy detection assembly according to one of Claims 10 to 15, wherein at least one of the deflection mirrors is, at a location where the respective deflection mirror delimits a through- opening for the illumination light (3) which is not deflected, trapezoidal or is designed with a taper in the shape of a wedge.

17. Energy detection assembly according to one of Claims 10 to 16, wherein the at least one of the plurality of deflection mirrors has a mirror thickness of less than 10 pm in a region of a delimitation edge of the deflection mirror where the illumination light (3) to be deflected is separated from the illumination light (3) which is not deflected.

18. Energy detection assembly according to one of Claims 10 to 17, wherein an edge roughness of the deflection mirror in the region of a delimitation edge is smaller than 50 pm.

19. Energy detection assembly according to one of Claims 10 to 18, wherein the at least one deflection mirror is arranged such that a partial beam of the illumination light (3) deflected by the deflection mirror has a typical beam diameter in the range of 0,1 mm to 1 mm.

20. Energy detection assembly according to one of Claims 10 to 19, wherein the at least one deflection mirror has a coating which is highly reflective for the illumination light (3).

21. Mask inspection system (2a) for use with EUV illumination light (3), having an illumination system with a light source (5) for generating illumination light (3) and with an illumination optical unit (1) for guiding the illumination light (3) towards an object field (4) in which a mask (18) to be inspected is arrangeable, having an imaging optical unit (PO) for imaging the object field (4) into an image field (21), having a spatially resolving detection device (23) for capturing the illumination light (3) guided into the image field (21), and having at least one energy detection assembly (26; 28; 30; 33; 39; 41; 48; 55; 57; 60; 64; 78; 86) according to any of Claims 1 to 20.

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