Magnifying imaging optics for a metrology system for the examination of objects

Abstract

A magnifying imaging optic (20c) is part of a metrology system for examining objects. The magnifying imaging optic has a maximum of four mirrors (M1 to M4) that image an object field (4) in an object plane (17) into an image field (21) in an image plane (22). Reflecting surfaces of small-area mirrors (M2, M3), which are used to guide imaging light (3) along an imaging beam path, deviate from a spherical shape by a maximum of 10 µm. The result is a magnifying imaging optic that, for a given manufacturing effort, produces an imaging result that meets the strict requirements of a metrology system.

Description

[0001] The invention relates to a magnifying imaging optic for a metrology system for examining objects. Furthermore, the invention relates to an illumination optic for a metrology system adapted to such an imaging optic, an optical system comprising such an imaging optic and an illumination optic, and a metrology system comprising such an optical system.

[0002] A mask inspection system is known from US 8,842,284, from US 2013 / 0250428 A1, from WO 2016 / 012436 A1, from US 10,042,248 B2, DE 102 20 815 A1 and from WO 2012 / 101269 A1.

[0003] It is an object of the present invention to further develop a magnifying imaging optics in such a way that, with a given manufacturing effort, a good imaging result is obtained which meets the strict requirements of a metrology system.

[0004] According to a first aspect, this object is achieved according to the invention by a magnifying imaging optic according to claim 1.

[0005] According to the invention, it was recognized that it is possible to implement the magnifying imaging optics with small-area mirrors that deviate only slightly or not at all from a spherical shape without undesirable compromises in image quality. This facilitates the production of the small-area mirrors of the magnifying imaging optics and offers corresponding manufacturing advantages.

[0006] The deviation of the reflecting surfaces from a spherical shape is measured relative to a spherical shape that best fits the respective mirror reflecting surface. Such a best fit can be determined by minimizing the square error when comparing the respective reflecting surface to a spherical surface.

[0007] The deviation of the reflection surface of the respective small-area mirror from a spherical shape can be a maximum of 5 µm, or even a maximum of 1 µm. With such a small deviation, processing during mirror production is possible with a small number of processing cycles. The small-area mirror can be designed as a nanoasphere, in which the deviation of the reflection surface from a spherical shape is a maximum of ten times the useful wavelength. Such a nanoasphere can be measured using a measuring technology designed to measure a spherical reflection surface.

[0008] An intermediate image can be located between a first and a second mirror in the imaging beam path of the magnifying imaging optics. This can be used to improve the aberration correction effect. Furthermore, this can be used to create a particularly compact beam path in the area of ​​the mirrors arranged near the intermediate focus or near the field.

[0009] To characterize such a "near-field" property, a parameter P can be used, which is defined in WO 2009 / 024164 A1. A mirror is considered near-field if the parameter P is less than 0.5, and in particular less than 0.4, less than 0.3, less than 0.25, or even less than 0.2. This parameter P is usually greater than 0.05 for real mirrors.

[0010] A final mirror of the magnifying imaging optics in the imaging beam path can be designed close to the field.

[0011] A first and, if necessary, a second mirror in the imaging beam path of the magnifying imaging optics can be positioned near the pupil. The parameter P for these near-pupil mirrors is greater than 0.5, can be greater than 0.6, can be greater than 0.7, and can even be greater than 0.8. For a real near-pupil mirror, the parameter P is usually less than 0.95.

[0012] The magnifying imaging optics can be designed for use with EUV imaging light, in particular with a wavelength in the range between 5 nm and 30 nm, for example, 13.5 nm. The magnifying imaging optics can have highly reflective coatings designed, in particular, for corresponding EUV wavelengths.

[0013] The overall length of the magnifying imaging optics can be a maximum of 1250 mm, resulting in a compact optic.

[0014] The mirrors of the magnified imaging optics can be designed so that none of the mirrors has a diameter of a reflection surface used to guide the imaging light along the imaging beam path that is larger than 400 mm. This also results in a compact optics system.

[0015] In an embodiment according to claim 2, the reflection surfaces of all mirrors deviate only slightly, namely by a maximum of 25 µm, from a spherical shape, thus also including mirrors with a diameter of at least 50 mm, which are also referred to below as large-area mirrors. This results in corresponding manufacturing advantages for all mirrors of the magnifying imaging optics. The deviation of the reflection surface mirrors from the spherical shape can be at most 20 µm, at most 15 µm, or can also be as large for all mirrors as discussed above in connection with the at least one small-area mirror.

[0016] According to a further aspect, the object mentioned at the outset is achieved according to the invention by a magnifying imaging optic having the features specified in claim 3.

[0017] According to the invention, it was recognized that a non-elliptical entrance pupil with an aspect ratio other than 1 offers additional adaptation options for guiding the imaging beam path within the magnified imaging optics, in particular to a beam path of illumination light illuminating the object field, which can improve imaging quality and / or light throughput of the imaging optics. The boundary shape of the entrance pupil can be adapted to imaging requirements, for example, to different typical object structure sizes in two mutually perpendicular field dimensions. Alternatively or additionally, adaptation to a diffraction effect of the object structures and / or to an illumination pupil of an illumination optics of the metrology system, which in turn has a boundary shape with an aspect ratio other than 1, is possible.

[0018] The x:y aspect ratio of the entrance pupil of the magnifying imaging optics can range between 1.1:1 and 5:1, with the x-coordinate being perpendicular to a meridional plane of the magnifying imaging optics. The x:y aspect ratio can be 2:1, for example.

[0019] A smallest resulting object-side numerical aperture of the magnifying imaging optics can be greater than 0.1, can be greater than 0.12 and can be, for example, 0.125 or even 0.135.

[0020] A boundary shape of the entrance pupil according to claim 4 can be well adapted to structural requirements and, in particular, to the spatial requirements of the imaging beam path. A correspondingly high light throughput is the result.

[0021] The same applies to an entrance pupil with a recess section according to claim 5. The recess section can take into account an obscuration by at least one mirror of the magnifying imaging optics.

[0022] In an embodiment according to claim 6, the advantages of an entrance pupil with an aspect ratio other than 1 are particularly well utilized. The at least one mirror, whose edge corresponds to that of the entrance pupil, can be a near-pupil mirror. This can be the first and possibly also the second mirror of the magnifying imaging optics.

[0023] Spacing ratios according to claim 7 ensure that imperfections and / or contamination on the last mirror in the imaging beam path of the magnifying imaging optics have no undesirable effects on the imaging quality of the optics. This applies in particular when the last mirror is designed as a near-field mirror. The distance of the last mirror to the image plane can be greater than 65% of the distance between the object plane and the image plane. This distance is usually smaller than the distance between the object plane and the image plane.

[0024] The distance between the last and the third-to-last mirror in the imaging beam path of the magnifying imaging optics along a coordinate perpendicular to the image plane can, assuming a corresponding design of the magnifying imaging optics, be less than 15%, less than 12%, or even less than 10% of the distance between the object plane and the image plane. This distance between the last and the third-to-last mirror is usually greater than 1% of the distance between the object plane and the image plane.

[0025] A magnification ratio according to claim 8 has proven successful in practice. Such a magnification ratio can be adapted to the pixel sizes of a spatially resolving detection device of the metrology system that captures the image field.

[0026] The magnifying imaging optics can have an object field with an extension in the two object field dimensions in the range between 0.1 mm and 1 mm and can have an area of, for example, 0.1 mm 2 up to 0.5 mm 2 A typical object field size is 0.3 mm × 0.6 mm or 0.5 mm × 0.5 mm.

[0027] Angles of incidence according to claims 9 and 10 have proven successful in practice and lead to favorable reflection conditions and good imaging ratios. The angle of incidence on the mirrors can be a maximum of 13° or even smaller.

[0028] An RMS wavefront error according to claim 11 results in good imaging quality. A field-side Petzval radius of the magnifying imaging optics can be greater than 500 mm.

[0029] The advantages of an illumination optics according to claim 12 correspond to those already explained above in connection with the various aspects of the magnifying imaging optics.

[0030] A boundary shape of the illumination pupil can be at least approximately elliptical, can be at least approximately stadium-shaped and can also be at least approximately semicircular.

[0031] The advantages of an optical system according to claim 13 or 14 and of a metrology system according to claim 15 correspond to those already explained above with reference to the magnifying imaging optics and the illumination optics.

[0032] The light source of the metrology system can be an EUV light source.

[0033] The detection device may comprise at least one TDI camera.

[0034] The metrology system can be designed as a mask inspection system or as a wafer inspection system.

[0035] The inspection system may comprise an object holder for holding the object to be inspected, which is mechanically coupled to an object displacement drive so that a scanning displacement of the object is possible during illumination.

[0036] The inspection system can be a system for actinic mask or wafer inspection.

[0037] An embodiment of the invention is explained in more detail below with reference to the drawing, in which: 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, with imaging optics and with at least one EUV energy sensor device; Fig. 2 shows in a meridional section a schematic view of a further embodiment of an imaging optics which can be used instead of the imaging optics according to Fig. 1 can be used in the mask inspection system; Fig. 3 A schematic view of the imaging optics from viewing direction III in Fig. 2; Fig. 4 in a meridional section a further embodiment of an imaging optics, which instead of the imaging optics according to Fig. 1 can be used in the mask inspection system; Fig. 5 in pupil coordinates an entrance pupil of the imaging optics according to Fig. 4; Fig. 6 in a meridional section a design of an imaging optics, which instead of the imaging optics according to Fig. 1 can be used in the mask inspection system; and Fig. 7 in pupil coordinates an entrance pupil of the imaging optics according to Fig. 6.

[0038] An illumination optics 1 is a component 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 in the Fig. 1 illustrates the illumination optics 1 via marginal rays and a main ray. The illumination light 3 illuminates an illumination field 4 of the mask inspection system.

[0039] The illumination light 3 is generated by an EUV light source 5 into a source region 6. The light source 5 can generate useful EUV radiation in a wavelength range between 2 nm and 30 nm, for example in the range between 2.3 nm and 4.4 nm or in the range between 5 nm and 30 nm, for example at 13.5 nm.

[0040] Light source 5 is designed as a plasma light source. This can be, for example, a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). Such plasma sources are known in principle as light sources for EUV projection exposure systems. Alternatively, light source 5 can also be designed as a high-harmonic EUV source. A pulse frequency of light source 5 can be in the kHz range.

[0041] To facilitate positional relationships, a Cartesian xyz coordinate system is used below. The x-axis is perpendicular to the drawing plane of the Fig. 1 and runs into it. The y-axis runs in the Fig. 1 horizontally to the left and the z-axis runs in the Fig. 1 vertically upwards.

[0042] After emission by the light source 5, the illumination light 3 first passes through a useful light filter 8, which is 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 1. The useful light filter 8 can be one of a plurality of filters that are stored in the metrology system 2a, for example, in a filter magazine. A further useful light filter can be arranged in a waiting position outside the illumination light beam path of the illumination optics 1. The useful light filters 8 can have the same transmission characteristics, whereby a change between the useful light filters can then be made if a degradation of the filter effect of the operating useful light filter 8 is detected.Alternatively, the useful light filters can also have different filter characteristics and, for example, allow different useful light wavelength ranges to pass through into the subsequent illumination light beam path or be optimized to filter out different amounts of false light.

[0043] The useful light filters can be designed in such a way that they filter out pump light carried in the illumination light beam path, in particular, which was used in the generation of useful light in the source volume 6.

[0044] After the filter 8 and the mirror IL1, the illumination light 3 first passes through an aperture stop 9 that limits the edge of a beam of the illumination light 3. Subsequently, the illumination light beam 3 is transferred to a beam-homogenizing element 11 of the illumination optics 1. The mirror IL1 serves as the coupling optics 10 for coupling the illumination light 3 into the beam-homogenizing element 11.

[0045] Between the source volume 6 and the beam homogenizing element 11, usually after the first mirror IL1 of the illumination optics 1, the illumination light 3 passes through an opening in a wall of a vacuum chamber VK, which is in the Fig. 1 in the illumination light beam path between the mirror IL1 and the illumination light aperture diaphragm 9.

[0046] 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 in the range between 0.02 and 0.2, for example in the range between 0.07 and 0.15 or also in the range between 0.05 and 0.08. Alternatively or in addition to the aperture stop 9, an aperture-limiting stop can be arranged between the beam homogenizing element 11 and a subsequent optical component of the illumination optics 1, as shown in the Fig.1 at 9a. It is also possible to arrange such a further aperture stop in the beam path of the illumination light 3 after the beam homogenizing element 11 between two downstream optical components of the illumination optics 1.

[0047] 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 focal point of the ellipsoidal mirror IL1 is therefore in the source region 6, and a second focal point of the ellipsoidal mirror IL1 is in the entrance opening 12. The ellipsoidal mirror IL1 focuses 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 entry into the entrance opening 12 can be in the range from 0.02 to 0.2, for example in the range of 0.05.

[0048] The angle of incidence of a central principal ray of the illumination light beam 3 on the coupling mirror IL1 can be in the range between 10° and 20°. The ellipsoidal mirror IL1 can be a normal incidence (NI) mirror, but can also be designed as a grazing incidence (GI) mirror.

[0049] The inlet opening 12 and an outlet opening 14 of the beam homogenizing element 11 are each square or rectangular with typical dimensions in the range between 0.5 mm and 5 mm and, for example, between 0.5 mm and 2 mm or also between 0.5 mm and 1 mm.

[0050] An aspect ratio of the entrance opening 12 and an equally 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 the exit opening 14 of the beam homogenizing element 11 is, for example, 0.5 mm × 1.0 mm, 0.75 mm × 0.75 mm, 1.0 mm × 2.0 mm or 1.5 mm × 2.0 mm.

[0051] The beam homogenizing element 11 can be designed as a hollow waveguide.

[0052] 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, in the range between 50 mm and 500 mm, e.g. in the range between 50 mm and 150 mm, in particular in the range between 50 mm and 100 mm.

[0053] An angle between a normal to the entrance plane 13 of the beam homogenizing element 11 and the main beam of the illuminating light beam 3 incident into the entrance opening 12 can be 0° or alternatively can be different from 0° and, for example, in the range between 0° and 1.5°, for example between 0.25° and 0.75° and in particular in the range of 0.5°.

[0054] A ratio of the distance between the inlet plane 13 and the outlet plane 15, and a size or the typical diameter of the inlet opening or the outlet opening 12, 14, is in the range between 50 and 1000 and can, for example, be in the range between 50 and 200.

[0055] An imaging output mirror optics 16 arranged downstream of the beam homogenizing element 11, comprising two mirrors IL2, IL3, images the exit opening 14 of the beam homogenizing element 11, located in an exit plane 15, into the illumination field 4 in an object plane 17. An image-side numerical aperture of this image can be in the range from 0.05 to 0.2.

[0056] In the illustrated embodiment, the output mirror optics 16 has exactly two mirrors, namely mirrors IL2 and IL3. The aperture stop described above, which may be used after the beam homogenizing element 11, can be arranged between the beam homogenizing element 11 and the mirror IL2 or between the mirrors IL2 and IL3.

[0057] The output mirror optics 16 are designed in the manner of a Wolter telescope, namely a Type I Wolter optic. Such Wolter optics are described in J.D. Mangus and J.H. Underwood, "Optical Design of a Glancing Incidence X-ray Telescope," Applied Optics, Vol. 8, 1969, page 95, and the references cited therein. Instead of a paraboloid, a hyperboloid can also be used in such Wolter optics. Such a combination of an ellipsoidal mirror with a hyperboloidal mirror also represents a Type I Wolter optic.

[0058] An embodiment of the output coupling mirror optics 16 is described in US 10,042,248 B2.

[0059] An imaging factor β1 of the input-coupling mirror optics 10 can range between 0.1 and 50, thus it can reduce by a factor of 10 up to a factor of 50. An imaging factor β2 of the output-coupling mirror optics 16 can range between 0.02 and 10, thus it can in turn reduce by a factor of 50 up to a factor of 50. A product β1, β2 of the two imaging factors can range between 0.25 and 10 for the illumination optics 1.

[0060] A reticle 18 to be inspected is arranged in the object plane 17 as the object to be inspected or the mask to be inspected, which is held by a reticle holder 19. The reticle holder 19 is mechanically connected to a reticle displacement drive 20, via which the reticle 18 is displaced along an object displacement direction y during a mask inspection. This enables a scanning displacement of the reticle 18 in the object plane 17.

[0061] The illumination field 4 has a typical dimension in the object plane 17 of less than 1.5 mm. In the illustrated embodiment, the extension of the illumination field 4 is 1 mm in the x-direction and 0.5 mm in the y-direction.

[0062] The x / y aspect ratio of the illumination field 4 corresponds to the x / y aspect ratio of the exit opening 14.

[0063] The illumination field 4, or a portion of the illumination field 4, which then represents an object field, is imaged into an image field 21 in an image plane 22 using projection optics 20a. The size of the image field 21 can be in the range of 150 mm x 300 mm. The shorter image field extension runs along the scanning direction y.

[0064] The projection optics 20a has a magnification ratio of the image of the object or illumination field 4 into the image field 21 of 500. Depending on the design of the projection optics 20a, this magnification ratio can be in the range between 250 and 500.

[0065] The projection optics 20a has numbered mirrors M1, M2, and M3 in the imaging beam path of the projection optics 20a, thus comprising a total of three mirrors. Depending on the design of the projection optics 20a, the number of mirrors may be greater than three. Further designs that can be used instead of the projection optics 20a are described below with reference to Fig. 2 ff. will be explained further.

[0066] An aperture stop 9b is arranged in an entrance pupil plane EP of the projection optics 20a, which lies in the imaging beam path of the illumination or imaging light between the reflecting reticle 18 and the first mirror M1. This aperture stop 9b can also serve to specify any internal obscuration of the projection optics 20a.

[0067] The mirrors M1 and M2 of the projection optics 20a are designed as NI mirrors with an angle of incidence of the illumination and imaging light 3 of less than 45°. The maximum angle of incidence of individual rays of the illumination light 3 on the mirrors M1 and M2 is in the range of 14°.

[0068] The illumination or imaging light 3 strikes the image field 21 with an angle of incidence that is less than 5°.

[0069] The mirror M1 has an edge of a reflection surface which is used to guide the imaging or illumination light 3 along the imaging beam path, which corresponds to the edge of an entrance pupil EP which is defined by the aperture stop 9b.

[0070] At least one of the mirrors M1 and M2 can be designed as an aspherical mirror. The projection optics 20a can have one aspherical mirror or two aspherical mirrors.

[0071] The illumination optics 1 of the metrology system 2a, in turn, has an illumination pupil, defined by the aperture stop 9b, which is adapted to the entrance pupil EP. This illumination pupil of the illumination optics 1 can have a boundary shape other than an ellipse, whose aspect ratio is not equal to 1. The illumination pupil can have an approximately elliptical, an approximately stadium-shaped, or an approximately semicircular boundary shape with a corresponding aspect ratio other than 1. The aspect ratio of the illumination pupil can correspond to that of the entrance pupil EP of the imaging optics 20a.

[0072] Based on the Fig. 2 and Fig.3, a further embodiment of a projection optics 20b is described below, which can be used instead of the projection optics 20a in the mask inspection system 2a. Components and functions that correspond to those described above with reference to the Fig. 1 have already been described, in particular, bear the same reference numbers and will not be discussed in detail again.

[0073] The projection optics 20b has a total of four mirrors M1, M2, M3 and M4 in the imaging beam path between the object or illumination field 4 and the image field 21, which are in turn numbered in the order in which they are exposed to the illumination or imaging light 3. The imaging beam path of the projection optics 20b is in the Fig. 2 by edge rays emanating from two spaced object field points. In the view according to Fig.3 these are three spaced field points.

[0074] At least one of the mirrors M1 to M4 can be designed as an aspherical mirror. The projection optics 20b can have one aspherical mirror, two aspherical mirrors, or even three aspherical mirrors. All four mirrors of the projection optics 20b can also be designed aspherical.

[0075] Between the object field 4 and the mirror M1 there is an entrance pupil plane in which the aperture stop 9b is arranged.

[0076] A boundary shape of an entrance pupil EP of the projection optics 20b, which is determined by an inner boundary of the aperture stop 9b, is semicircular and corresponds to the shape of the entrance pupil, which will be described below in connection with the further design of the projection optics from the Fig. 4 and Fig. 5 is explained.

[0077] A diameter extension of this semicircular shape of the entrance pupil EP runs parallel to the x-coordinate. In the area of ​​a corresponding diameter boundary section of the entrance pupil EP running along the x-coordinate, the entrance pupil EP is bounded by an obscuration accentuated by the mirror M2.

[0078] The x:y aspect ratio of the entrance pupil EP of the projection optics 20b is 2:1. Depending on the design of the projection optics 20b, this aspect ratio can be in the range between 5:1 and 1.1:1. The object-side numerical aperture of the projection optics 20b lies in the yz-plane according to Fig. 2 at approximately 0.125 (NAy=0.125) and in the xz plane perpendicular thereto at approximately 0.25 (NAx=0.25). Depending on the design of the projection optics 20b, the NAx can be between 0.1 and 0.5 and the NAy between 0.05 and 0.25.

[0079] An intermediate image 24 lies between the mirrors M1 and M2 in the imaging beam path of the projection optics 20b.

[0080] On mirrors M1 and M2, individual rays belonging to different field points but with the same illumination angle are spaced at a comparatively small distance from one another, which amounts to at most one-quarter of the total used reflection surface diameter of the respective mirror. A parameter P, which characterizes the field or pupil proximity of the respective mirror and is defined in WO 2009 / 024164 A1, has a value of P>0.5 for mirrors M1 and M2. Mirrors M1 and M2 are therefore near the pupil. In particular, mirror M1 is a near-pupil mirror.

[0081] On mirrors M3 and M4, individual rays belonging to the same field points but with different illumination angles are again spaced apart by a distance of at most one-quarter of the total used reflection surface diameter of the respective mirror. The parameter P (again, see the definition from WO 2009 / 024164 A1) is less than 0.5 for each of mirrors M3 and M4. Therefore, mirrors M3 and M4 are arranged close to the field in the imaging beam path of projection optics 20b. In particular, mirror M4 is a close-to-field mirror.

[0082] The edge of a reflection surface of mirror M1, which is used to guide the imaging light 3 along the imaging beam path, corresponds to the edge of the entrance pupil EP. This approximately also applies to mirror M2. The shape of the edge of mirror M2 is mirrored around the xz plane compared to the edge of mirror M1 and also to the edge of the entrance pupil EP.

[0083] A border of a reflection surface of mirror M4, which is used to guide the imaging light 3 along the imaging beam path, corresponds to a border of the image field 21, which is usually rectangular or square. This also applies in a similar way to mirror M3.

[0084] The mirror M4, i.e. the last mirror in the imaging beam path, has a distance A to the image plane 22 which is greater than 60% of a distance B between the object plane 17 and the image plane 22.

[0085] A distance C between the mirrors M4 and M2, i.e. a distance between the last and the third-to-last mirror along a coordinate perpendicular to the image plane 22, is less than 15% of the distance B between the object plane 17 and the image plane 22.

[0086] A distance D between the penultimate mirror M3 in the imaging beam path of the projection optics 20b and the image plane 22 is greater than 20%, greater than 25%, greater than 30% and also greater than 35% of the distance B between the field planes 17, 22.

[0087] The projection optics 20b in turn has a magnification ratio when imaging the object field 4 into the image field 21 in the range between 250 and 500.

[0088] Within the imaging beam path of the projection optics 20b, individual rays have an angle of incidence on the mirrors M1 to M4 of a maximum of 13° each. The angle of incidence of the individual rays of the imaging light 3 on the image field 21 of the projection optics 20b is a maximum of 5°.

[0089] Based on the Fig. 4 and Fig.5, a further embodiment of a projection optics 20c is explained below, which can be used in the mask inspection system 2a instead of the projection optics described above. Components and functions that correspond to those described above with reference to the Fig. 1 to 3 have already been described, bear in particular the same reference numbers and will not be discussed again in detail.

[0090] An imaging beam path of the projection optics 20c basically corresponds to that of the projection optics 20b.

[0091] A distance A between the mirror M4 and the image plane 22 is approximately 69% of the distance B between the object plane 17 and the image plane 22. A distance C between the mirrors M4 and M2 is approximately 13% of the distance B.

[0092] In the projection optics 20c, the distance D between the penultimate mirror M3 in the imaging beam path and the image plane 22 is approximately 35% of the distance B between the field planes 17, 22.

[0093] The object-field numerical aperture NAx for the 20c projection optics is 0.27. The object-field numerical aperture NAy is 0.135. The object-field field size is 0.74 mm x 0.28 mm with a field offset of 0.06 mm in y.

[0094] The projection optics 20c has a magnification ratio of 435.

[0095] The distance A in the projection optics 20c is so large that an imperfection on the reflection surface of the mirror M4 with a typical size of 0.16 mm does not lead to the shading of a pixel dimension in the image field 21.

[0096] No single beam within the imaging beam path of the projection optics 20c has an angle of incidence on one of the mirrors M1 to M4 that is greater than 13°.

[0097] The imaging light 3 strikes the image field 21 at the projection optics 20c with an angle of incidence that is less than 5°.

[0098] An RMS wavefront error across the image field 21 for the projection optics 20c is at most 20 mλ, namely 10 mλ in the case of the exemplary embodiment of the projection optics 20c. The Petzval radius on the image field side of the projection optics 20c is greater than 500 mm. The distortion on the image field side is 1 nm.

[0099] The mirrors M1 and M2 each have a border of a reflection surface, which is used to guide the imaging or illumination light 3 along the imaging beam path, which corresponds to the border of the entrance pupil EP. The border shape of the mirror M2 is different from that in the Fig. 5 shown boundary shape of the entrance pupil EP is mirrored around the xz-plane.

[0100] The optical design data of the projection optics 20c are summarized below in Tables 1a / b.

[0101] The first column of Table 1a indicates the respective optical area, starting with object field 4.

[0102] The second column of Table 1a indicates a radius of curvature of the respective optical surface.

[0103] The following column of Table 1a gives a radius of curvature of the sphere fitted to the optical surface.

[0104] The fourth column of Table 1a indicates a z-distance to the previous surface.

[0105] The fifth column of Table 1a indicates the optical power of the surface, if any. For mirrors M1 to M4, this optical power is "REFL," meaning reflective.

[0106] The first column of Table 1b shows a maximum value of an incidence height (distance perpendicular to the optical axis) of the respective surface description of the optical surface in mm.

[0107] The second column of Table 1b indicates a maximum deviation of the respective aspheric optical surface from the best-fit sphere, again in mm.

[0108] In addition, coefficients K, C1, C2 and C3 are given for the mirror surfaces of the mirrors M1, M2 and M4 in Table 2 below, according to the following asphere surface formula: p(h)=[((1 / r)h2) / (1+SQRT(1−(1_K)(1 / r)2h2))]+C1⋅h4+C2⋅h6+C3⋅h8…

[0109] Where p is the sagittal height, h is the incidence height, r is the radius of curvature, K is the conic constant and C1, C2 and C3 are the first three even coefficients of the aspheric correction polynomial.

[0110] The deviation from the best-fitted sphere results from the difference in the heights of the arrows according to the area formula of the asphere and the best-fitted sphere. Table 1a for Fig. 4 / 5 Optical surface Radius [mm] Radius of best-fit sphere [mm] Thickness [mm] Effect Object field Infinite 0.000 1: Infinite 297.654 Aperture diaphragm Infinite 0.000 3: Infinite 391.216 M1 540,167 541.1 -470.928 reflective M2 51,130 51.226 598.100 reflective M3 -44,815 -44.815 -434.431 reflective M4 1174,354 1174,824 867.233 reflective Image field Infinite 0.000000 Table 1b for Fig. 4 / 5 Optical surface Max. incidence height [mm] Max. deviation asphere from sphere [mm] Object field 1: Aperture diaphragm 83.5 3: M1 185 0.0150 M2 11.5 0.0007 M3 7.7 0 M4 160 0.009 Image field Table 2 for Fig. 4 / 5 Mirror K C1 C2 C3 M1 -0.0572 0.000000E+00 0.135E-17 0.779E-23 M2 -0.1520 0.000000E+00 0.360E-10 0.202E-13 M4 -0.6361 0.000E+00 -0.576E-18 0.795E-22

[0111] The mirrors M1, M2, and M4 in the projection optics 20c are therefore designed as aspheres. The mirror M3 is a spherical mirror.

[0112] In the Fig. Figure 5 illustrates the obscuring effect of mirror M2 with respect to the entrance pupil EP in the OBS. The mirror M2 is designed so that the obscuring OBS does not overlap with the semicircular entrance pupil EP.

[0113] Based on the Fig. 6 and Fig.7, a further embodiment of a projection optics 20d is explained below, which can be used in the mask inspection system 2a instead of the projection optics described above. Components and functions that correspond to those described above with reference to the Fig. 1 to 5 have already been described, bear in particular the same reference numbers and will not be discussed again in detail.

[0114] The object-field numerical aperture NAx for the 20d projection optics is 0.25. The object-field numerical aperture NAy is 0.125. The object-field field size is 0.56 mm x 0.36 mm with a field offset of 0.06 mm in y.

[0115] The projection optics 20d also has four mirrors M1 to M4. A beam path in the projection optics 20d is shown mirrored around the xz plane compared to the beam path of the projection optics 20c. Otherwise, the beam path within the projection optics 20d essentially corresponds to that within the projection optics 20c.

[0116] The distance ratio A to B for the projection optics 20d is: A / B≃69%.

[0117] The other distance ratios are: C / B≃10% D / B≃20%.

[0118] Fig.7 shows an edge contour of the entrance pupil EP, which simultaneously corresponds to an inner edge contour of the aperture diaphragm 9b. The entrance pupil EP of the projection optics 20d has an edge shape that includes a semicircular edge section 25 and a diameter edge section 26. Along the diameter edge section 26, which together with the semicircular edge section 25 results in an entire edge of the entrance pupil EP, there is a recess section 27 in the edge of the entrance pupil EP. A central recess of this recess section 27 runs parallel to the diameter edge section 26, offset from it in the positive y-direction. This central recess of the recess section 27 merges into the diameter edge section 26 via two oblique edge sections 28, 29.

[0119] The y-distance of the central recess section 27 from the diameter boundary section 26 is less than 15% of the y-extension of the entire entrance pupil EP. The entire recess in the diameter boundary section thus has a negligible area compared to the semicircular envelope around the entrance pupil EP and also compared to the area of ​​the entrance pupil EP itself.

[0120] The recess section 27 is caused by an obscuration of the entrance pupil EP, which is caused by the mirror M2 of the projection optics 20d.

[0121] The magnification ratio of the projection optics is 20d 435.

[0122] A wavefront error RMS over the image field 21 for the projection optics 20d is 15 mλ.

[0123] The Petzval radius on the image field side of the 20d projection optics is in the range of 20,000 mm.

[0124] The following optical design data for the projection optics 20d are again summarized in two tables, which correspond in structure to the tables for the Fig. 4 and Fig. 5, i.e. corresponding to the projection optics 20c. Table 1a / b for Fig. 6 / 7 Optical surface Radius [mm] Radius of best-fit sphere [mm] Thickness [mm] Effect Object field Infinite 0.000 1: Infinite 196.000 Aperture diaphragm Infinite 0.000 3: Infinite 479.56 M1 527.767 528.588 -459.1 reflective M2 50.000 50.095 584.25 reflective M3 44.030 44.03 -489.63 reflective M4 1263.26 1264.587 688.9 reflective Image field Infinite 0.000000 Table 1b for Fig. 6 / 7 Optical surface Max. incidence height [mm] Max. deviation asphere from sphere [mm] Object field 1: Aperture diaphragm 3: M1 170 0.012 M2 10.5 0.0006 M3 7.6 0 M4 180 0.004 Image field Table 2 for Fig. 6 / 7 Mirror K C1 C2 C3 C4 M1 -0.0584 0.000E+00 0.17E-17 0.666E-23 0 M2 -0.161 0.000E+00 0.438E-10 0.5450E-14 0.642E-17 M4 -0.205 0.000E+00 -3.1 E-18 0.5 E-22 -1E-28

[0125] The aspherical mirrors of the projection optics 20a, as well as the aspherical mirrors M1, M2, and M4 of the projection optics 20c and 20d, have reflection surfaces that deviate from a spherical shape by a maximum of 25 µm. The small mirrors M2 and M3, which are also referred to as small-area mirrors with a reflection surface area of ​​less than 50 mm, deviate from a spherical shape by a maximum of 5 µm. In the projection optics 20c and 20d, the spherical mirror M3 does not deviate from a spherical shape at all.

[0126] An illumination light beam path of the illumination light 3 for illuminating the reticle 18 and an imaging light beam path of the projection optics 20a for imaging the object field 4 into the image field 21 intersect in an intersection region. This intersection region can be located in the area of ​​the entrance pupil plane EP of the projection optics 20a. Here, the imaging light beam path intersects with the illumination light beam path between the exit opening 14 and the mirror IL2 of the illumination optics 1, as well as between the mirrors IL2 and IL3 of the illumination optics 1.

[0127] The image field 21 is captured by a detection device 23, e.g., by a CCD camera or multiple CCD cameras. For details of the imaging in 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 can also be implemented as a TDI (time delay integration) detection device with a plurality of TDI detectors.

[0128] The detection device 23 is designed with spatial resolution. The detection device 23 can have sensor pixels with a typical pixel size of at most 20 µm × 20 µm. This pixel size can be smaller and can be, for example, 15 µm × 15 µm or even 10 µm × 10 µm. A pixel dimension along an image field coordinate x and / or y can be in the range between 1 µm and 20 µm.

[0129] With the mask inspection system 2a, an inspection of, for example, a structure on the reticle 18 is possible. QUOTES CONTAINED IN THE DESCRIPTION

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

[0000] US 8,842,284

[0002] US 2013 / 0250428 A1

[0002] WO 2016 / 012436 A1

[0002] US 10,042,248 B2 [0002, 0058, 0127] DE 102 20 815 A1

[0002] WO 2012 / 101269 A1

[0002] WO 2009 / 024164 A1 [0009, 0080, 0081] Cited non-patent literature

[0000] J. D. Mangus, J. H. Underwood „Optical Design of a Glancing Incidence X-ray Telescope“, Applied Optics, Vol. 8, 1969, Seite 95

[0057]

Claims

[1] Magnifying imaging optics (20a; 20b; 20c; 20d) for a metrology system (2a) for examining objects (18), - with a maximum of four mirrors (M1, M2, M3; M1, M2, M3, M4) which image an object field (4) in an object plane (17) into an image field (21) in an image plane (22), wherein at least one of the mirrors is a small-area mirror (M2, M3) with a diameter of the reflection surface which is smaller than 50 mm, wherein the reflection surface of the small-area mirror (M2, M3) deviates from a spherical shape by a maximum of 10 µm. [2] Magnifying imaging optics according to claim 1, characterized in that the reflection surfaces of all mirrors deviate from a spherical shape by at most 25 µm. [3] Magnifying imaging optics (20a; 20b; 20c; 20d) for a metrology system (2a) for examining objects (18), - with a maximum of 4 mirrors (M1, M2; M3; M1, M2, M3, M4) which image an object field (4) in an object plane (17) into an image field (21) in an image plane (22) along an imaging beam path, - with an entrance pupil (EP) with a boundary shape other than an ellipse, the aspect ratio of which is not equal to 1. [4] Magnifying imaging optics according to claim 3, characterized by that the entrance pupil (EP) has a boundary shape which has a semicircular boundary section (25). [5] Magnifying imaging optics according to claim 4, characterized by that along a diameter boundary section (26), which together with the semicircular boundary section (25) results in an entire boundary of the entrance pupil (EP), there is a recess section (27) in the boundary. [6] Magnifying imaging optics according to one of claims 3 to 5, characterized byat least one mirror (M1; M1, M2) with an edge on the reflection surface, which is used to guide imaging light (3) along the imaging beam path, which corresponds to the edge of the entrance pupil (EP). [7] Magnifying imaging optics according to one of claims 1 to 6, wherein a last mirror (M3; M4) in the imaging beam path has a distance from the image plane (22) which is greater than 60% of a distance between the object plane (17) and the image plane (22). [8] Magnifying imaging optics according to one of claims 1 to 7, characterized by a magnification ratio in the range between 250 and 500. [9] Magnifying imaging optics according to one of claims 1 to 8, characterized by that no single beam within the imaging beam path has an angle of incidence on one of the mirrors (M1, M2; M1 to M4) that is greater than 14°. [10] Magnifying imaging optics according to one of claims 1 to 9, characterized bythat the imaging beam path is designed such that the imaging light (3) strikes the image field (21) with an angle of incidence which is less than 5°. [11] Magnifying imaging optics according to one of claims 1 to 10, characterized by a maximum RMS wavefront error of 50 mλ. [12] Illumination optics (1) for a metrology system (2a) for examining objects (18), - with an illumination pupil adapted to an entrance pupil (EP) of an imaging optics according to one of claims 3 to 11, - wherein the illumination pupil has a boundary shape other than an ellipse, the aspect ratio of which is not equal to 1. [13] Optical system (2) with an imaging optics (20a; 20b; 20c; 20d) according to one of claims 1 to 11 and with an illumination optics (1) for illuminating the object field (4) with illumination light (3). [14] Optical system (13) with an illumination optics according to claim 12. [15] Metrology system (2a) - with an optical system according to claim 13 or 14, - with a light source (5), - with a spatially resolving detection device (22) which covers the image field (21).

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