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

Abstract

An energy detection assembly (78) is suitable for an illumination system of a mask inspection system for use with EUV illumination light (3). The assembly (78) has at least one EUV energy sensor device (77) that is embodied such that it captures energy of the illumination light (3) guided along an illumination light beam path. The EUV energy sensor device (77) has at least one energy value sensor and at least one energy variation sensor (79). The energy value sensor serves to capture an illumination energy of the illumination light (3) over an exposure period. The energy variation sensor (79) serves to capture a temporal development of the illumination energy of the illumination light (3) over the exposure period. This results in an energy detection assembly, by means of which an inspection accuracy of the 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 present patent application claims the priority of German patent application DE 10 2024 203 353.7, the content of which is incorporated by reference herein.

[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 2020 207 566 Al discloses an apparatus and a method for characterizing a microlithographic mask. US 2019 / 0277772 Al discloses an inspection device and an inspection method. US 2015 / 0146182 Al discloses a radiation source for EUV radiation.

[0005] The problem addressed by the present invention is that of helping to improve the inspection accuracy of a mask inspection system.

[0006] According to the invention, this problem is solved by an energy detection assembly for an illumination system of a mask inspection system for use with EUV illumination light, having the features specified in Claim 1.

[0007] According to the invention, it was recognized that in addition to knowledge about an illumination energy over an exposure period for a mask to be inspected, it may be important to have knowledge about a temporal development of the exposure energy over the exposure period, in particular in detection-dependent fashion. The EUV energy sensor device according to the invention allows high-precision capture of both parameters, "illumination energy" on the one hand and "temporal development of the illumination energy" on the other hand, by way of the energy value sensor on the one hand and by way of the energy variation sensor on the other hand. In particular, this allows compensation or correction of detection falsification effects that may arise on account of pulsed illumination light incident on the mask to be inspected during the exposure period. The energy detection assembly may comprise a plurality of EUV energy sensor devices, each with at least one energy value sensor and at least one energy variation sensor, increasing a measurement accuracy of the energy detection assembly. An energy sensor of the EUV energy sensor device may be embodied as EUV photodiode.

[0008] A signal of the energy variation sensor can be used to computationally correct the fact that different sections of an object to be inspected were exposed to light pulses that differ in number and / or in intensity and originate from a light source which operates in pulsed fashion in that case and which is used in the mask inspection system.

[0009] The EUV energy sensor device according to Claim 1 is able to capture the illumination light that is guided along the illumination light beam path outside of the entrance opening of the beam-homogenizing element. In this case, the EUV energy sensor device 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 marginal boundary of the entrance opening of the beam-homogenizing element may regularly be smaller than 50% of a mean diameter of said entrance opening. This distance might be even smaller, for example smaller than 40%, smaller than 30%, smaller than 25%, smaller than 20%, smaller than 15%, smaller than 10% or else smaller than 5% of this mean diameter of the entrance opening. This 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, the 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 and within predetermined marginal aperture boundaries of optical components that guide the illumination light.

[0011] The energy detection assembly may comprise or be signal-connected to an open- loop / closed-loop control device. The respective energy value sensor may be signal-connected to the open-loop / closed-loop control device.

[0012] The respective energy variation sensor may be signal-connected to the open-loop / closed- loop control device.

[0013] This allows data regarding the illumination energy or the temporal development of the illumination energy of the illumination light, as captured by the at least one energy value sensor and the at least one energy variation sensor, respectively, to be used by way of the open- loop / closed-loop control device for open-loop / closed-loop control of further components of the assembly or of an overarching mask inspection system. In particular, this may be used to compensate or correct for unwanted effects that may arise on account of, in particular, illumination light incident in pulsed fashion on a mask to be inspected during the exposure period.

[0014] A temporal resolution of the energy variation sensor according to Claim 2 is well adapted to typical pulse frequencies of a light source, which may be in the kHz-range, for example in the range between 2 and 50 kHz and in particular between 2.5 and 25 kHz. The temporal resolution of the energy variation sensor may be better than 1 ps and in particular may be better than 0.2 ps.

[0015] A energy variation sensor according to Claim 3 may be embodied highly precisely and, in particular, with a very large temporal bandwidth, i.e. a very high time resolution. The detection light may be extraneous light which is simultaneously co-created during a creation of the illumination light. A wavelength of the detection light may be located within the UV range and / or within the VIS range and / or within the NIR range.

[0016] In practice, a conversion medium according to Claim 4 has proven its worth, especially in the context of detecting EUV light. The conversion medium might be a fluorescence medium and / or a scintillation medium. The advantages of a mask inspection system according to Claim 5 correspond to those which have already been explained above with reference to the energy detection assembly.

[0017] A wafer inspection system may also be constructed accordingly.

[0018] 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.

[0019] The inspection system may be a system for actinic mask inspection.

[0020] The mask inspection system may comprise an open-loop / closed-loop control device or be signal-connected to such an open-loop / closed-loop control device. Depending on the embodiment of the mask inspection system, the open-loop / closed-loop control device may be a part of the energy detection assembly and / or a part of the mask inspection system.

[0021] The light source of the mask inspection system may be signal-connected to the open- loop / closed-loop control device.

[0022] The spatially resolving detection device of the mask inspection system may be signal-connected to the open-loop / closed-loop control device.

[0023] Influencing the light source and / or the spatially resolving detection device by way of openloop or closed-loop control allows a compensation or correction of unwanted effects to be achieved, said unwanted effects being detected in particular by way of an interplay between the at least one energy value sensor and the at least one energy variation sensor.

[0024] Should at least one TDI detector according to Claim 6 be used, a temporal resolution of the energy variation sensor allows inference in respect of the amount of illumination light energy that arrives at a respective section of a mask to be inspected during an object displacement, through the object field of the mask inspection system, of the mask to be inspected. Detection falsifications that may be based thereon are then rendered correctable or avoidable. Such detection falsifications may be corrected, for example by virtue of the intensity data as measured by an image capturing device, in particular the TDI detector, being corrected by computation in a data processing apparatus.

[0025] The advantages of the energy detection assembly are particularly pronounced when a pulsed light source according to Claim 7 is used. The energy variation sensor is able to capture a temporal position of an illumination pulse with great accuracy, in particular allowing a synchronization of the detection with a readout rate or a charge shift of at least one TDI detector in the detection device. During the operation of the mask inspection system, it is possible to determine both the quantity of light pulses and the specific light pulses of the light source that were used to image on the detection device, which may be embodied as a camera, specific parts of the mask to be inspected or of the object to be inspected. The intensity data measured by the detection device may then be corrected with the aid of an algorithm. In particular, a corresponding correction may be implemented by virtue of intensity information obtained by the spatially resolving detection device being combined with time information obtained by way of the energy variation sensor of the EUV energy sensor device.

[0026] In particular, the energy variation sensor can be used to determine, in absolute temporal terms, precisely when a light pulse from the light source is incident on the object to be inspected or on the mask to be inspected. In particular, this plays a role in those pulsed light sources in which a light pulse duration is very much shorter than a time interval between two light pulses. A light pulse duration may be of the order of 1 ps or even shorter. Depending on a light pulse frequency of the pulsed light source, which may for example range between 2 kHz and 50 kHz, a time interval between two temporally adjacent light pulses of the pulsed light source may range between 100 ps and 500 ps.

[0027] The energy of the respective light pulse can be measured using the energy value sensor. The energy variation sensor is able to measure the absolute temporal development of the light pulse, in particular an exact time of incidence of the light pulse on the object to be inspected or on the mask to be inspected. The energy variation sensor may in that case also be understood to be a sensor for capturing an absolute temporal development of a light pulse of the light source operating in pulsed fashion in that case, i.e. a pulse-time sensor.

[0028] An exemplary embodiment of the invention is explained in greater detail below with reference to the drawing, in which:

[0029] 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;

[0030] 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;

[0031] Fig. 2 schematically shows a perspective, entrance-side view of an embodiment of the beam-homogenizing element;

[0032] 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 surroundings 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;

[0033] 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; 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 beam-homogenizing 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;

[0034] 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;

[0035] 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;

[0036] 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;

[0037] 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;

[0038] Fig. 10 shows a plan view of an illumination pupil of the illumination optics 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 optics unit; 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;

[0039] 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;

[0040] 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;

[0041] 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;

[0042] 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;

[0043] 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;

[0044] Fig. 17 shows an embodiment of a light source of the mask inspection system with a energy variation 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;

[0045] 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 energy variation sensor arranged in a source chamber of the light source;

[0046] 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 beam-homogenizing element in a meridional section, with components of an input coupling sensor device of the illumination system being depicted;

[0047] Fig. 20 shows, in a perspective view, an input coupling location sensor unit of the input coupling 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;

[0048] 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.

[0049] An illumination optics unit l 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 optics unit 1 in Fig.

[0050] 1. An illumination field 4 of the mask inspection system is illuminated by the illumination light 3.

[0051] The illumination light 3 is created by an EUV light source 5 in a source region 6. The light source 5 can created 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. The light source 5 is embodied as a plasma light source. For example, this 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 embodied as a high-harmonic EUV source. A pulse frequency of the light source 5 may be in the kHz range.

[0052] 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.

[0053] 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 optics 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 optics 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.

[0054] 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.

[0055] 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 optics unit 1. In this case, the mirror IL1 serves as an input coupling optics unit 10 for input coupling the illumination light 3 into the beam-homogenizing element 11.

[0056] Between the source volume 6 and the beam- homogenizing element 11, generally downstream of the first mirror IL1 of the illumination optics 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.

[0057] The aperture stop 9 limits a numerical aperture of the illumination light beam 3 emitted by the source region 6 to 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 optics unit 1 as indicated at 9a in Fig. 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 optics unit 1 is also possible.

[0058] 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.

[0059] 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 embodied as a grazing incidence (GI) mirror. 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.

[0060] Fig. 1A elucidates details of the beam-homogenizing element 11.

[0061] A pair of successive micromirror arrays 15a, 15b is arranged in the beam path of the beamhomogenizing element 11 between the entrance opening 12 and the exit opening 14. Fig. 1 A illustrates three 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 embodied 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.

[0062] 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. Alternatively, the beam-homogenizing element 11 may be embodied as a hollow waveguide.

[0063] 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.

[0064] 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°.

[0065] 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.

[0066] An imaging output coupling mirror optics 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.

[0067] In the illustrated embodiment, the output coupling mirror optics 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.

[0068] 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 optics unit. Such Wolter optics 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 optics 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 optics unit.

[0069] An exemplary embodiment of the output coupling mirror optics unit 16 is described in US 10,042,248 B2.

[0070] An imaging factor Pi of the input coupling mirror optics 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 optics 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. In the case of the illumination optics unit 1, a product Pi, P2 of the two imaging factors can range between 0.25 and 10.

[0071] 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 rendered possible.

[0072] 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.

[0073] The x / y aspect ratio of the illumination field 4 corresponds to the x / y aspect ratio of the exit opening 14.

[0074] 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. 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.

[0075] The mirrors Ml and M2 of the projection optical unit PO are embodied as NI mirrors with an angle of incidence of the illumination and imaging light 3 of less than 45°.

[0076] 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 cross one another 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 optics unit 1 and also between the mirrors IL2 and IL3 of the illumination optics unit 1.

[0077] 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 embodied as a TDI (time delay integration) detection device comprising a plurality of TDI detectors. A corresponding embodiment will be described in detail below.

[0078] An inspection of a structure on the reticle 18, for example, is possible by means of the mask inspection system 2a.

[0079] 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. In a manner enlarged in comparison with Fig. 2, Fig. 3 shows a portion 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 optics unit 1. The energy detection assembly 26 also includes the beam-homogenizing element 11.

[0080] The four EUV deflection mirrors 251 to 254 are arranged outside of the entrance opening 12 and 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.

[0081] 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.

[0082] 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 region 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. 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 251 is regularly in the range of less than 1 x 10'4of the entire energy of the illumination light 3.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] An energy value capture by way of the energy value sensor 27 might be more accurate than 3%, might be more accurate than 1%, might be more accurate than 0.5% and might be more accurate than 0.2%.

[0087] 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.

[0088] 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.

[0089] Further energy sensors 27i and 273 not visible in Fig. 4 are associated accordingly with the deflection mirrors 251 and 25s. 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.

[0090] 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.

[0091] 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.

[0092] 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 to 4, bear the same reference signs and will not be discussed again in detail.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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 princi- pie, in the case of the squarely extended entrance opening 12Q, a deflection mirror arrangement with deflection mirrors 251 to 254 might 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.

[0097] 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 beam-homogenizing element of the energy detection assembly 26 according to Fig. 3.

[0098] 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 at least 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.

[0099] Accordingly, the EUV energy sensor device 29 is designed such that it captures the sensor component 3s 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 but beyond this used exit opening region, specifically in the exit opening sensor component 14s. 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.

[0100] 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.

[0101] 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.

[0102] 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 relate to a deflection mirror of the type of deflection mirror 25i or an energy sensor of the type of energy sensors 27i.

[0103] A plurality of such EUV energy sensor devices 31 may also be present in the energy detection assembly 30. 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?.

[0104] 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.

[0105] The detection light 34 is light at a wavelength that is reflected out by the used light filter 8, and this wavelength may differ from the wavelength of the illumination light 3. In particular, the wavelength of the detection light 34 may be located within the UV range or visible range, but also within 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.

[0106] The detection light 34 is light that is not passed along the subsequent illumination light beam path by the used light filter 8.

[0107] 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. At least one of the used light filters 8 housed in the filter wheel 7 may have a metallic support structure.

[0108] An alternative 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.

[0109] 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.

[0110] In the energy detection assembly 41 according to Fig. 9, one of the mirrors of the illumination optics unit 1, specifically the mirror IL2, is embodied 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.

[0111] 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.

[0112] Fig. 9 depicts the beam path of the illumination light 3 from the exit plane 15 of the beamhomogenizing 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. 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.

[0113] 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.

[0114] In a plan view, Figure 10 depicts an embodiment of the aperture stop 9 of the illumination optics 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 parts 51, which are indicated in Fig. 10 and which may have a very thin design.

[0115] 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 optics unit 1 covered by the inner obscuration stop section 50. Once again, a part 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.

[0116] As illustrated in Fig. 10, the EUV energy sensor device 52 may comprise further EUV deflection mirrors 54i, 542, 54s and 544, which may in turn be constituent parts of further EUV energy sensor devices of the type of EUV energy sensor device 52. 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.

[0117] An EUV sensor device 56 of the energy detection assembly 55 is embodied 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 optics unit 1 is once again captured by way of the energy sensor 27 of the energy detection assembly 55.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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 embodied 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.

[0122] 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.

[0123] A part 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.

[0124] The beam path of the illumination light 3 is guided through the ionization space 66.

[0125] 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. 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.

[0126] The electrodes 71b, 72b are embodied as grid electrodes.

[0127] 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.

[0128] 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.

[0129] Fig. 15 illustrates an operating scheme for one of the TDI sensors 32i of the detection device 23 of the mask inspection system 2a.

[0130] The reticle 18 with a circular edge in the example of Fig. 15 is displaced in 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 optics unit 1 are also shown schematically to the left of Fig. 15.

[0131] 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.

[0132] The sensor columns I to IV are read out in synchronized fashion with the object displacement of the reticle 18 relative to the TDI sensor 32i, the readout also being implemented in 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.

[0133] 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 the 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.

[0134] 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 in 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.

[0135] 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. This is illustrated in Fig. 16, which illustrates a temporal sequence of a pulsed exposure of the reticle 18 that is displaced in 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; said illumination field also migrates in 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.

[0136] 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 respective light pulse, is located in the overlap region 76i, which is also emphasized by dashed lines in Fig. 16.

[0137] The overlap region 762 is illuminated by exactly one light pulse since it is located exclusively in the illumination field 4s.

[0138] 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.

[0139] 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.

[0140] 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 energy variation 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.

[0141] 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. 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.

[0142] 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.

[0143] 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.

[0144] 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 energy variation sensor 79 of the EUV energy sensor device 77. The energy variation 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 (NIR) light.

[0145] 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 energy variation 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 energy variation sensors 79 and 88.

[0146] The energy variation sensors 79 and 88 have a temporal resolution better than 10 ps. The temporal resolution may be better than 1 ps and in particular may be even better still.

[0147] 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.

[0148] 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.

[0149] The input coupling location sensor unit 92 is arranged in the region of the entrance opening 12 of the beam-homogenizing element 11.

[0150] 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.

[0151] 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, by way of example, 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.

[0152] 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 27s 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.

[0153] The deflection mirrors 94i may be embodied 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 else, alternatively, a convex embodiment.

[0154] In an alternative to a coating, the deflection mirrors 94i may be applied to this entrance-side end wall, for example adhesively bonded thereto. In a further alternative, 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.

[0155] 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:

[0156] 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.

[0157] Ii and I3 are the sensor signals from the energy sensors 27i and 27s. r|xis a scaling parameter that can be obtained within the scope of a calibration.

[0158] Accordingly, a control signal Ay for the y-coordinates can be obtained by way of the following relationship:

[0159] I2 and I4 are the sensor signals from the energy sensors 272 and 274. r|yonce again is a scaling factor, which may be obtained by means of a preceding calibration step.

[0160] 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.

[0161] 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 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.

[0162] 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.

[0163] 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.

[0164] 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, the energy sensors 27i arranged thus represent sections of the aperture stop 9.

[0165] 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 the generation of control signals Ax, Ay and, thereby, direction control of the beam of illumination light 3.

[0166] 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 96s 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 embodied 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.

[0167] In an alternative to the aforementioned embodiments of the input coupling sensor device 91 or in addition to that, in particular in an alternative or in addition to the input coupling location sensor unit 92, use can also be made of variants which were already explained 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 the at least one corresponding energy sensor in marginal fashion.

[0168] In yet a further 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.

[0169] In yet a further 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.

[0170] 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.

[0171] 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.

[0172] 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. Such effective components 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.

[0173] 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.

[0174] Both a respective alternating control of input coupling location and input coupling direction and also a combined control are possible.

[0175] 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-direction as well. For example, the beam- homogenizing element 11 may be displaced in the beam direction, i.e. in the z-direction perpendicular to the entrance plane 13, in order to open up this degree of freedom of adjustment.

[0176] The above-explained embodiments of the energy value sensors or of the energy variation 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.

[0177] The respective energy value sensors or energy variation 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 energy variation sensor, respectively, of the above-described embodiments of the energy detection assemblies. A corresponding highly accurate image generation of the structures of the reticle 18 is the consequence.

[0178] The open-loop / closed-loop control device 27a is signal-connected, in particular, to the input coupling mirror optics unit 10 for the purpose of performing the above-explained open- loop / closed-loop control processes.

Claims

Claims1. Energy detection assembly (78; 86) for an illumination system of a mask inspection system (2a) for use with EUV illumination light (3), having at least one EUV energy sensor device (77; 87) that is embodied such that it captures energy of the illumination light (3) guided along an illumination light beam path, wherein the EUV energy sensor device (77; 87) comprises:— at least one energy value sensor (27; 35; 45; 71, 72) for capturing an illumination energy of the illumination light (3) over an exposure period;— at least one energy variation sensor (79; 88) for capturing a temporal development of the illumination energy of the illumination light (3) over the exposure period.

2. Energy detection assembly according to Claim 1, characterized in that the energy variation sensor (79; 88) has a temporal resolution better than 10 ps.

3. Energy detection assembly according to Claim 1 or 2, characterized in that the energy variation sensor (79; 88) comprises a photodiode for time-resolved capture of detection light (82) at a longer wavelength than the EUV illumination light (3).

4. Energy detection assembly according to any of Claims 1 to 3, characterized in that the energy value sensor (27) and / or the energy variation sensor comprises a conversion medium for converting the EUV illumination light (3) into longer-wavelength detection light.

5. Mask inspection system (2a) for use with EUV illumination light (3), having an illumination system with a light source (5) for creating illumination light (3) and with an illumination optics unit (1) for guiding the illumination light (3) to an object field (4) in which a mask (18) to be inspected is arrangeable, having an imaging optics 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), having at least one energy detection assembly (78; 86) according to any of Claims 1 to 4.

6. A mask inspection system according to Claim 5, characterized in that the detection device comprises at least one TDI detector.

7. Mask inspection system according to Claim 5 or 6, characterized in that the light source (5) is embodied as pulsed light source.

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