Magnifying imaging optical unit for metrology system for inspecting object

Abstract

A magnifying imaging optical unit (20c) is disclosed as part of a metrology system for inspecting an object. The magnifying imaging optical unit has at most four mirrors (M1 to M4), which image an object field (4) in an object plane (17) into an image field (21) in an imaging plane (22). According to one aspect, an entrance pupil of the magnification imaging optical unit has a boundary shape deviating from an ellipse, and an aspect ratio thereof is not equal to 1. According to another aspect, the reflective surfaces of the small area mirrors (M2, M3) for guiding the imaging light (3) along the imaging beam path deviate from the spherical shape by at most 10 [mu] m. This results in a magnifying imaging optical unit, in which the generated imaging results can meet the strict requirements of a metrology system for specific manufacturing expenses.

Description

[0001] The content of German Patent Application No. DE 10 2023 213 267.2 is hereby incorporated by reference into this document for reference. Technical Field

[0002] The present invention relates to a magnifying imaging optical unit for a metrology system for inspecting an object. Furthermore, the present invention relates to an illumination optical unit for a metrology system, which is adapted to this imaging optical unit, an optical system comprising this imaging optical unit and the illumination optical unit, and a metrology system comprising this optical system. Background Art

[0003] A mask inspection system is known from US 8,842,284, US2013 / 0250428 A1, WO 2016 / 012426 A1, US

[0004] 10,042,248B2, DE 102 20 815A1 and WO 2012 / 101269 A1. DE 10 2010 029 050 A1 and DE 10 2011 084 255 A1 each disclose a magnifying imaging optical unit and a metrology system comprising this imaging optical unit. DE 10 2010 029 049A1 discloses an illumination system for a metrology system and a metrology system comprising this illumination optical unit. Summary of the Invention

[0005] The object of the present invention is to develop a magnifying imaging optical unit such that for a given manufacturing cost, the resulting good imaging result meets the strict requirements of a metrology system.

[0006] According to a first aspect, this object is achieved according to the present invention by a magnifying imaging optical unit as described below.

[0007] The present invention provides a magnifying imaging optical unit for a metrology system for inspecting an object, comprising: at most four mirrors, which image an object field in an object plane along an imaging beam path to an image field in an image plane; an entrance pupil, the boundary shape of which deviates from an ellipse and the aspect ratio of which is not equal to 1.

[0008] According to the present invention, it has been found that a non-elliptical entrance pupil with an aspect ratio different from 1 provides additional possibilities for guiding the imaging beam path within the magnifying imaging optical unit, in particular for adapting it to the beam path of the illumination light for illuminating the object field, and this can improve the imaging quality and / or the light flux of the imaging optical unit. The boundary shape of the entrance pupil can be adapted to the imaging requirements, for example, to different typical object structure sizes in two mutually perpendicular field dimensions. Alternatively or additionally, it is possible to adapt the illumination pupil of the illumination optical unit to the diffraction effects of the object structure and / or the metrology system, which itself has a boundary shape with an aspect ratio different from 1.

[0009] The x:y aspect ratio range of the entrance pupil of the magnifying imaging optical unit can be in the range between 1.1:1 and 5:1, where the x coordinate can be perpendicular to the meridian plane of the magnifying imaging optical unit. For example, the x:y aspect ratio can be 2:1.

[0010] The minimum composite-side numerical aperture of the magnifying imaging optical unit can be greater than 0.1, can be greater than 0.12, and can be, for example, 0.125 or 0.135.

[0011] In one embodiment, the entrance pupil includes a boundary shape having a semi-circular boundary portion. The boundary shape of the entrance pupil as described above can be perfectly adapted to the structural requirements and in particular to the spatial requirements of the imaging beam path. This results in a relatively high light flux.

[0012] This also applies relatively to the entrance pupil with a cutout portion as described below. The cutout portion can take into account the shielding of at least one mirror of the magnifying imaging optical unit. In one embodiment, along the diameter boundary portion that forms the total boundary of the entrance pupil together with the semi-circular boundary portion, there is a cutout portion in this boundary.

[0013] In one embodiment, at least one mirror includes a boundary at the reflective surface for guiding the imaging light along the imaging beam path, and this boundary corresponds to the boundary of the entrance pupil. In the case of the construction as described above, the advantages of an entrance pupil with an aspect ratio different from 1 are particularly evident. At least one mirror (whose boundary corresponds to the boundary of the entrance pupil) can be a near-pupil mirror. This can relate to the first mirror of the magnifying imaging optical unit and optionally also to the second mirror.

[0014] According to another aspect, the object mentioned in the introduction is achieved according to the present invention by a magnifying imaging optical unit having the features described below.

[0015] The present invention provides a magnifying imaging optical unit for a metrology system for inspecting an object, comprising: at most four mirrors that image an object field in an object plane to an image field in an image plane, wherein at least one of the mirrors is a small-area mirror having a reflection surface diameter of less than 50 mm, and wherein the reflection surface of the small-area mirror deviates from a spherical shape by at most 10 μm.

[0016] According to the present invention, it has been found that small-area mirrors that deviate only slightly or not at all from a spherical shape can be used to implement the magnifying imaging optical unit without causing an unnecessary reduction in the imaging quality. This enables the production of small-area mirrors for the magnifying imaging optical unit and provides corresponding manufacturing advantages.

[0017] The deviation of the reflection surface from a spherical shape is measured relative to the spherical shape that best fits the reflection surface of the corresponding mirror. This best fit is determined by minimizing the squared error when comparing the corresponding reflection surface with a sphere.

[0018] The deviation of the reflection surface of the corresponding small-area mirror from a spherical shape can be at most 5 μm, or can also be at most 1 μm. This small deviation enables processing with fewer processing cycles during the production of the mirror. The small-area mirror can be implemented as a nanosphere, wherein the deviation of the reflection surface from a spherical shape reaches at most ten times the wavelength used. Such a nanosphere can be measured by a measurement technique designed for measuring a spherical reflection surface.

[0019] An intermediate image can be located between a first mirror and a second mirror in the imaging beam path of the magnifying imaging optical unit. The intermediate image can be used to improve the imaging aberration correction effect. Furthermore, this can be used to establish a particularly compact beam path in the region of the mirror disposed near the intermediate focus or near field.

[0020] The parameter P defined in WO 2009 / 024164 A1 can be used to characterize this "near field" property. 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 less than 0.2, the mirror is considered to be a near field. For a real mirror, this parameter P is typically greater than 0.05.

[0021] The last mirror in the imaging beam path of the magnifying imaging optical unit can be specifically implemented as a near field.

[0022] The first mirror and optionally the second mirror in the imaging beam path of the magnifying imaging optical unit can be implemented as near pupils. For these near pupil mirrors, the parameter P is greater than 0.5, can be greater than 0.6, can be greater than 0.7, or can also be greater than 0.8. For a real near pupil mirror, the parameter P is typically less than 0.95.

[0023] The magnifying imaging optical unit can be designed to be used in conjunction with EUV imaging light, the wavelength range of the EUV imaging light being especially between 5 nm and 30 nm and the wavelength being, for example, 13.5 nm. In particular, the magnifying imaging optical unit can have a high-reflection coating implemented for the corresponding EUV wavelength.

[0024] The structural length of the magnifying imaging optical unit can be at most 1250 mm at maximum, resulting in a compact optical unit.

[0025] The mirrors of the magnifying imaging optical unit can be implemented such that no mirror has a reflective surface diameter for guiding the imaging light along the imaging beam path that is greater than 400 mm. This also results in a compact optical unit.

[0026] In one embodiment, the reflective surfaces of all the mirrors deviate from a spherical shape by at most 25 μm. In the case of the embodiments described above, the reflective surfaces of all the mirrors deviate only slightly from a spherical shape, that is, at most 25 μm, that is, even for 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 the mirrors of the magnifying imaging optical unit. The deviation of the reflective surface of the mirror from a spherical shape can be at most 20 μm, at most 15 μm, or for all the mirrors, the size can also be the same as the size discussed above in connection with at least one small-area mirror.

[0027] The features of the magnifying imaging optical unit according to the two aspects described above can also be presented in combination with each other.

[0028] In one embodiment, the distance between the last mirror in the imaging beam path and the image plane is greater than 60% of the distance between the object plane and the image plane. The distance ratio as described above has the result that defects and / or contamination on the last mirror in the imaging beam path of the magnifying imaging optical unit do not have any undesirable effect on the imaging quality of the optical unit. This is especially the case if the last mirror is implemented as a near-field mirror. The distance between the last mirror and the image plane can be greater than 65% of the distance between the object plane and the image plane. This distance is generally less than the distance between the object plane and the image plane.

[0029] Assuming corresponding embodiments of the magnifying imaging optical unit, the distance between the last mirror and the penultimate mirror in the imaging beam path of the magnifying imaging optical unit along the coordinate perpendicular to the image plane can be less than 15%, less than 12% or less than 10% of the distance between the object plane and the image plane. The distance between the last mirror and the penultimate mirror is generally greater than 1% of the distance between the object plane and the image plane.

[0030] In one embodiment, the magnification is between 250 and 500. The magnification as described above has proven to be valuable in practice. Such a magnification can be adapted to the pixel size of the spatial resolution detection device of the metrology system for capturing the image field.

[0031] The magnifying imaging optical unit can have an object field, the range of which in two object field dimensions is respectively between 0.1 mm and 1 mm, and its area can be, for example, 0.1 mm 2 to 0.5 mm 2 . Typical object field ranges are 0.3 mm × 0.6 mm or 0.5 mm × 0.5 mm.

[0032] In one embodiment, no individual ray in the imaging beam path has an incident angle greater than 14° on one of the mirrors. In one embodiment, the imaging beam path is implemented such that the imaging light is incident on the image field at an incident angle less than 5°. The incident angles as described above have proven to be valuable in practice and result in favorable reflection conditions or good imaging conditions. In each case, the incident angle on the mirror can be at most 13° or even smaller.

[0033] In one embodiment, the maximum RMS wavefront aberration is 50 mλ. The RMS wavefront aberration as described above results in good imaging quality. The Petzval radius on the image field side of the magnifying imaging optical unit can be greater than 500 mm.

[0034] In one embodiment, the reflective surface of at least one of the mirrors deviates from a spherical shape by at least twice the wavelength used. The minimum value of the deviation of the reflective surface of the mirror from a spherical shape as described above has proven to be valuable in practice. When the wavelength used is 13.5 nm, this corresponds to a minimum deviation of 25 nm, i.e., the lower limit of the deviation. According to an embodiment of the magnifying imaging optical unit, a plurality of mirrors, for example two or three mirrors, can meet this criterion of the lower limit of the deviation, or exactly one of the plurality of mirrors. In another embodiment, the lower limit of the deviation of all the mirrors of the magnifying imaging optical unit meets twice the wavelength used.

[0035] The present invention provides an illumination optical unit for a metrology system for inspecting an object, comprising an illumination pupil, which is adapted to the entrance pupil (EP) of the imaging optical unit as described above, wherein the boundary shape of the illumination pupil deviates from an ellipse and its aspect ratio is not equal to 1. The advantages of the illumination optical unit as described above correspond to those that have been explained above in connection with various aspects of the magnifying imaging optical unit.

[0036] The 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 semi-circular.

[0037] The present invention provides an optical system, comprising an imaging optical unit as described above, and comprising an illumination optical unit for illuminating the object field with illumination light. The present invention provides an optical system, comprising an illumination optical unit as described above. The present invention provides a metrology system, comprising: an optical system as described above; a light source; and a spatially resolved detection device for capturing the image field. The advantages of the optical system as described above and the metrology system as described above correspond to those explained above with reference to the magnification imaging optical unit and the illumination optical unit.

[0038] The light source of the metrology system can be an EUV light source.

[0039] The detection device can have at least one TDI camera.

[0040] The metrology system can be embodied as a mask inspection system or a wafer inspection system.

[0041] The inspection system can comprise an object carrier for holding the object to be inspected and mechanically coupled to an object displacement drive such that a scanning displacement of the object during illumination is also possible.

[0042] The inspection system can be a system for photochemical mask or wafer inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] A exemplary embodiment of the present invention will be explained in more detail with reference to the following drawings, in which:

[0044] Figure 1 A mask inspection system for a lithography mask for use with EUV illumination light is schematically shown in a meridional section, which comprises an illumination system having an energy detection assembly comprising a beam homogenizing element, an imaging optical unit and at least one EUV energy sensor device;

[0045] Figure 2 A schematic view of an alternative embodiment of the imaging optical unit is shown in a meridional section, which can be used in the mask inspection system instead of the imaging optical unit according to Figure 1 ;

[0046] Figure 3 Shows Figure 2 A schematic view of the imaging optical unit in the viewing direction III of

[0047] Figure 4 Another embodiment of the imaging optical unit is shown in a meridional section, which can be used in the mask inspection system instead of the imaging optical unit according to Figure 1 ;

[0048] Figure 5 Shows the imaging optical unit in pupil coordinates according toFigure 4 the entrance pupil of the imaging optical unit;

[0049] Figure 6 shows an embodiment of the imaging optical unit in the meridional section, which can replace the imaging optical unit according to Figure 1 and be used in a mask inspection system; and

[0050] Figure 7 shows the entrance pupil of the imaging optical unit according to Figure 6 in pupil coordinates. Detailed Description

[0051] The illumination optical unit 1 is a component of the optical system 2 of a mask inspection system 2a that uses EUV illumination light 3. Figure 1 The beam path of the illumination light 3 is shown by the marginal rays and chief rays passing through the illumination optical unit 1. The illumination field 4 of the mask inspection system is illuminated by the illumination light 3.

[0052] The illumination light 3 is generated by an EUV light source 5 and enters the source region 6. The light source 5 can generate radiation used for EUV, with a wavelength range between 2 nm and 30 nm, for example, between 2.3 nm and 4.4 nm or between 5 nm and 30 nm, and the wavelength is, for example, 13.5 nm.

[0053] The light source 5 is implemented as a plasma light source. For example, this can be a laser-produced plasma (LPP) or a discharge-produced plasma (DPP). In principle, such plasma light sources are known as light sources for EUV projection exposure equipment. Alternatively, the light source 5 can also be implemented as a high-harmonic EUV source. The pulse frequency of the light source 5 can be in the kHz range.

[0054] To facilitate the positioning relationship, the Cartesian xyz coordinate system will be used hereinafter. The x-axis is perpendicular to Figure 1 the drawing plane in Figure 1 and enters the latter. The y-axis extends horizontally to the left in Figure 1 and the z-axis extends vertically upward in

[0055] After being emitted by the light source 5, the illumination light 3 first passes through the filter 8 used, which is arranged in the beam path of the illumination light 3 at an operating position between the source volume 6 and the first ellipsoidal mirror IL1 of the illumination optical unit 1. The filter 8 used can be, for example, a filter from among a plurality of filters kept available in a filter cassette at the metrology system 2a. Another filter used can be arranged in a waiting position outside the illumination beam path of the illumination optical unit 1. The filters 8 used can have the same transmission characteristics, in which case, if it is determined that the filtering effect of the filter 8 used during operation has decreased, a conversion between the filters used can be carried out. Alternatively, the filters used can also have different filter characteristics, for example, different used light wavelength ranges can be transmitted into the downstream illumination beam path, or they can be optimized to filter out different foreign light components.

[0056] The filter used is embodied such that it filters out a specific pump light, which is guided along in the illumination beam path and was used during the generation of the used light in the source volume 6.

[0057] Downstream of the filter 8 and the mirror IL1, the illumination light 3 first passes through the aperture stop 9, which delimits the beam edge of the illumination light 3. Thereafter, the illumination light beam 3 is transmitted towards the beam homogenizing element 11 of the illumination optical unit 1. In this case, the mirror IL1 serves as an input coupling optical unit 10 for coupling the illumination light 3 into the beam homogenizing element 11.

[0058] Between the source volume 6 and the beam homogenizing element 11, typically downstream of the first mirror IL1 of the illumination optical unit 1, the illumination light 3 passes through an opening in the wall of the vacuum chamber VK, which is indicated as being between Figure 1 the mirror IL1 and the illumination light aperture stop 9 in the illumination beam path shown.

[0059] The aperture stop 9 limits the numerical aperture of the illumination light beam 3 emitted from the source region 6 to a numerical aperture value in the range between 0.02 and 0.2, for example in the range between 0.07 and 0.15 or in the range between 0.05 and 0.08. As an alternative or addition to the aperture stop 9, an aperture limiting stop can be arranged between the beam homogenizing element 11 and the downstream optical components of the illumination optical unit 1, as shown at 9a in Figure 1 It is also possible to arrange this additional aperture stop in the beam path of the illumination light 3 downstream of the beam homogenizing element 11, between two downstream optical components of the illumination optical unit 1.

[0060] The ellipsoidal mirror IL1 is used to image the source region 6 of the EUV light source 5 into the incident opening 12 in the incident plane 13 of the beam homogenizing element 11. Therefore, the first focus of the ellipsoidal mirror IL1 is located in the source region 6, and the second focus of the ellipsoidal mirror IL1 is located in the incident opening 12. The ellipsoidal mirror IL1 is used to focus the illumination beam 3 into the incident opening 12 in the incident plane 13 of the beam homogenizing element 11. The range of the incident-side numerical aperture of the illumination beam 3 when entering the incident opening 12 can be between 0.02 and 0.2, for example, it can be on the order of 0.05.

[0061] The range of the incident angle of the central principal ray of the illumination beam 3 on the incident coupling mirror IL1 can be between 10° and 20°. The ellipsoidal mirror IL1 can be a normal incidence (NI) mirror, but can also be implemented as a grazing incidence (GI) mirror.

[0062] The incident opening 12 and the exit opening 14 of the beam homogenizing element 11 are each square or rectangular, and their typical dimensions range between 0.5 mm and 5 mm, for example, between 0.5 mm and 2 mm or between 0.5 mm and 1 mm. The aspect ratio of the incident opening 12 and the exit opening 14 of the same size of the beam homogenizing element 11 for the illumination light 3 in the exit plane 15 is between 0.5 and 2. The typical dimensions of the incident opening 12 and the exit opening 14 of the beam homogenizing element 11 are, 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.

[0063] The beam homogenizing element 11 can be specifically implemented as a hollow waveguide.

[0064] The beam homogenizing element 11 has a typical length perpendicular to the planes 13 and 15, that is, along the main beam direction of the illumination light 3, and the range is between 50 mm and 500 mm. For example, the range is between 50 mm and 150 mm, especially the range is between 50 mm and 100 mm.

[0065] The angle between the normal of the incident plane 13 of the beam homogenizing element 11 and the main ray of the illumination beam 3 incident on the incident opening 12 can be 0°, or alternatively, it can also be different from 0°, and for example, the range is between 0° and 1.5°, for example, the range is between 0.25° and 0.75°, and especially it has an order of magnitude of 0.5°.

[0066] The distance ratio between the incident plane 13 and the exit plane 15, and the range of the dimensions or typical diameters of the incident opening and the corresponding exit openings 12, 14 is between 50 and 1000, and for example, the range can be between 50 and 200.

[0067] The imaging output coupling mirror optical unit 16 is located downstream of the beam homogenizing element 11 and has two mirrors IL2, IL3, which image the exit opening 14 of the beam homogenizing element 11 located in the exit plane 15 to the illumination field 4 in the object plane 17. The range of the numerical aperture on the image side of this imaging can be between 0.05 and 0.2.

[0068] In the illustrated embodiment, the imaging output coupling mirror optical unit 16 exactly has two mirrors, namely mirror IL2 and mirror IL3. The aforementioned selectively used aperture stop downstream of the beam homogenizing element 11 can be arranged between the beam homogenizing element 11 and mirror IL2, or between mirror IL2 and IL3.

[0069] The imaging output coupling mirror optical unit 16 is implemented in the form of a Wolter telescope, that is, in the form of a type-I Wolter optical unit. Such Wolter optical units are described in "Optical Design of a Glancing Incidence X-ray Telescope" by J.D. Mangus, J.H. Underwood, etc. in Applied Optics, Vol. 8, 1969, p. 95 and the references cited therein. In such Wolter optical units, hyperboloids can also be used to replace paraboloids. This combination of an ellipsoidal mirror and a hyperboloidal mirror also constitutes a type-I Wolter optical unit.

[0070] An exemplary embodiment of the output coupling mirror optical unit 16 is described in US10,042,248B2.

[0071] The imaging factor β1 of the input coupling mirror optical unit 10 can range between 0.1 and 50, that is to say, its function can vary from reducing by a factor of 10 to magnifying by a factor of 50. The imaging factor β2 of the output coupling mirror optical unit 16 can range between 0.02 and 10, that is to say, its function can vary from reducing by a factor of 50 to magnifying by a factor of 1. In the case of the illumination optical unit 1, the product β1, β2 of the two imaging factors can be in the range between 0.25 and 10.

[0072] The mask 18 to be inspected held by the mask carrier 19 is configured as the object to be inspected or the mask to be inspected in the object plane 17. The mask carrier 19 is mechanically operatively connected to the mask displacement drive 20, through which the mask 18 is displaced along the object displacement direction y during mask inspection. Thus, the displaced mask 18 can be scanned in the object plane 17.

[0073] The typical size of the illumination field 4 in the object plane 17 is less than 1.5 mm. In the illustrated embodiment, the illumination field 4 ranges 1 mm in the x direction and 0.5 mm in the y direction.

[0074] The x / y aspect ratio of the illumination field 4 corresponds to the x / y aspect ratio of the exit aperture 14.

[0075] The illumination field 4 or a part of the illumination field 4 (which then constitutes the object field) is imaged by the projection optical unit 20a into the image field 21 in the image plane 22. The size of the image field 21 can range within 150 mm × 300 mm. The shorter image field extent extends along the scanning direction y.

[0076] The projection optical unit 20a has a magnification of 500 for imaging the object field or the illumination field 4 into the image field 21. According to an embodiment of the projection optical unit 20a, the range of the magnification can be between 250 and 500.

[0077] The projection optical unit 20a has mirrors M1, M2, M3 successively numbered in the imaging beam path of the projection optical unit 20a, i.e., a total of three mirrors are included. According to an embodiment of the projection optical unit 20a, the number of mirrors can also be greater than three. The following will also refer to Figure 2 and the following diagrams to explain further embodiments that can be used to replace the projection optical unit 20a.

[0078] The aperture stop 9b is arranged in the entrance pupil plane EP of the projection optical unit 20a, which is located between the reflection mask 18 and the first mirror M1 in the imaging beam path of the illumination or imaging light. The aperture stop 9b can also be used to predefine the possible internal shielding of the projection optical unit 20a.

[0079] The mirrors M1 and M2 of the projection optical unit 20a are implemented as NI mirrors with an incident angle of the illumination light and the imaging light 3 less than 45°. The maximum incident angle of the individual rays of the imaging light 3 on the mirrors M1 and M2 is approximately 14°.

[0080] The illumination or imaging light 3 is incident on the image field 21 at an incident angle less than 5°.

[0081] The mirror M1 has a boundary of the reflective surface for guiding the imaging or illumination light 3 along the imaging beam path, and the boundary corresponds to the boundary of the entrance pupil EP predefined by the aperture stop 9b.

[0082] At least one of the mirrors M1 and M2 can be implemented as an aspherical mirror. The projection optical unit 20a can have one aspherical mirror or can have two aspherical mirrors.

[0083] As such, the illumination optical unit 1 of the metrology system 2a has an illumination pupil predefined by the aperture stop 9b, which is adapted to the entrance pupil EP. The boundary shape of this illumination pupil of the illumination optical unit 1 can deviate from an ellipse and its aspect ratio is not equal to 1. The boundary shape of the illumination pupil can be approximately elliptical, approximately stadium-shaped, or approximately semi-circular, and its corresponding aspect ratio deviates from 1. The aspect ratio of the illumination pupil can correspond to the aspect ratio of the entrance pupil EP of the imaging optical unit 20a.

[0084] The following refers to Figure 2 and Figure 3 to describe another embodiment of the projection optical unit 20b, which can be used to replace the projection optical unit 20a in the mask inspection system 2a. The components and functions corresponding to those already referred to above Figure 1 are specifically provided with the same reference numerals and will not be discussed in detail again.

[0085] The projection optical unit 20b has a total of 4 mirrors M1, M2, M3, and M4 in the imaging beam path between the object field or illumination field 4 and the image field 21, and these mirrors are again consecutively numbered in the order in which the illumination or imaging light 3 impinges thereon. In Figure 2 the marginal rays emitted from two object field points spaced apart from each other illustrate the imaging beam path of the projection optical unit 20b. In accordance with Figure 3 the view, the number of field points spaced apart from each other is three.

[0086] At least one of the mirrors M1 to M4 can be implemented as an aspherical mirror. The projection optical unit 20b can have one aspherical mirror, can have two aspherical mirrors, or can have three aspherical mirrors. It is also possible that all four mirrors of the projection optical unit 20b are implemented as aspherical.

[0087] The entrance pupil plane (where the aperture stop 9b is arranged) is located between the object field 4 and the mirror M1.

[0088] The boundary shape of the entrance pupil EP of the projection optical unit 20b (which is predefined by the inner boundary of the aperture stop 9b) is semi-circular and corresponds to the shape of the entrance pupil, which will also be explained below in connection with Figure 4 and Figure 5 further embodiments of the projection optical unit.

[0089] The diameter range of the semi-circular shape of the entrance pupil EP is parallel to the x coordinate. In the region of the corresponding diameter boundary portion extending along the x coordinate through the entrance pupil EP, the entrance pupil EP is bounded by the occlusion caused by the mirror M2.

[0090] In the case of the projection optical unit 20b, the x:y aspect ratio of the entrance pupil EP is 2:1. According to an embodiment of the projection optical unit 20b, the aspect ratio range can be between 5:1 and 1.1:1. The object-side numerical aperture of the projection optical unit 20b is about 0.125 (NAy = 0.125) in the yz plane according to Figure 2 and about 0.25 (NAx = 0.25) in the xz plane perpendicular thereto. Depending on the embodiment of the projection optical unit 20b, NAx can be between 0.1 and 0.5 and NAy can be between 0.05 and 0.25.

[0091] The intermediate image 24 is located between the mirrors M1 and M2 in the imaging beam path of the projection optical unit 20b.

[0092] On the mirrors M1 and M2, the distance between individual rays for different field points but the same illumination angle is relatively small, which is at most a quarter of the total used reflective surface diameter of the corresponding mirror. The parameter P characterizing the field or pupil proximity of the corresponding mirror and defined in WO 2009 / 024164 A1 has a value of P > 0.5 for each of the mirrors M1 and M2. Thus, the mirrors M1 and M2 are near-pupil. Specifically, the mirror M1 is a near-pupil mirror.

[0093] On the mirrors M3 and M4, the individual rays for the same field point but different illumination angles are again separated from each other by a distance that is at most a quarter of the total used reflective surface diameter of the corresponding mirror; for each of the mirrors M3 and M4, the parameter P (again see the definition in WO

[0094] 2009 / 024164 A1) is less than 0.5. Thus, the mirrors M3 and M4 are mirrors in a near-field configuration in the imaging beam path of the projection optical unit 20b. Specifically, the mirror M4 is a near-field mirror.

[0095] The boundary of the reflective surface of the mirror M1 for guiding the imaging light 3 along the imaging beam path corresponds to the boundary of the entrance pupil EP. This also generally applies to the mirror M2. In the case of the mirror M2, the boundary shape is mirror-reflected about the xz plane compared to the boundary of the mirror M1 and compared to the boundary of the entrance pupil EP.

[0096] The boundary of the reflective surface of the mirror M4 for guiding the imaging light 3 along the imaging beam path corresponds to the boundary of the image field 21, and the boundary of this image field is generally implemented as a rectangle or a square. In a similar form, this also applies to the mirror M3.

[0097] The distance A between the mirror M4 (which is the last mirror in the imaging beam path) and the image plane 22 is greater than 60% of the distance B between the object plane 17 and the image plane 22.

[0098] The distance C between the mirrors M4 and M2, that is, the distance between the last mirror and the penultimate mirror along the 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.

[0099] The distance D between the penultimate mirror M3 in the imaging beam path of the projection optical unit 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.

[0100] The projection optical unit 20b again has a magnification for imaging the object field 4 into the image field 21, which ranges between 250 and 500.

[0101] Within the imaging beam path in the case of the projection optical unit 20b, the individual rays have an incident angle of at most 13° on the mirrors M1 to M4 in each case. In the case of the projection optical unit 20b, the incident angle of the individual rays of the imaging light 3 on the image field 21 is at most 5°.

[0102] The following reference Figure 4 and Figure 5 explains another embodiment of the projection optical unit 20c, which can be used to replace the aforementioned projection optical unit in the mask inspection system 2a. The components and functions corresponding to those already referred to Figures 1 to 3 above are specifically provided with the same reference numerals and will not be discussed in detail again.

[0103] The imaging beam path of the projection optical unit 20c corresponds in principle to the imaging beam path of the projection optical unit 20b.

[0104] The 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. The distance C between the mirrors M4 and M2 is approximately 13% of the distance B.

[0105] In the case of the projection optical unit 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.

[0106] In the case of the projection optical unit 20c, the numerical aperture NAx on the object field side is 0.27. The numerical aperture NAy on the object field side is 0.135. The object-side field size is 0.74 mm × 0.28 mm, and the field offset in the y direction is 0.06 mm.

[0107] The magnification of the projection optical unit 20c is 435.

[0108] In the case of the projection optical unit 20c, the size of the distance A is such that a defect on the reflective surface of the mirror M4 with a typical size of 0.16 mm does not cause occlusion of the pixel size in the image field 21.

[0109] The incident angle of individual rays within the imaging beam path of the projection optical unit 20c on one of the mirrors M1 to M4 is not greater than 13°.

[0110] In the case of the projection optical unit 20c, the imaging light 3 is incident on the image field 21 at an incident angle of less than 5°.

[0111] In the case of the projection optical unit 20c, the wavefront aberration RMS on the image field 21 is at most 20 mλ, specifically 10 mλ in the case of an exemplary embodiment of the projection optical unit 20c. In the case of the projection optical unit 20c, the Petzval radius on the image field side is greater than 500 mm. The distortion on the image field side is 1 nm.

[0112] The mirrors M1 and M2 each have a boundary of the reflective surface for guiding the imaging or illumination light 3 along the imaging beam path, and this boundary corresponds to the boundary of the entrance pupil EP. In the case of the mirror M2, compared to Figure 5 the boundary shape of the shown entrance pupil EP, the boundary shape is mirror-symmetric with respect to the xz plane.

[0113] The optical design data of the projection optical unit 20c are summarized in Table 1a / b below.

[0114] The first column of Table 1a indicates the corresponding optical surface starting from the object field 4.

[0115] The second column of Table 1a indicates the radius of curvature of the corresponding optical surface.

[0116] The subsequent columns of Table 1a indicate the radius of curvature of the sphere fitted to the optical surface.

[0117] The fourth column of Table 1a indicates the z-distance relative to the corresponding previous surface.

[0118] The fifth column of Table 1a indicates the optical effect of the surface (if there is such an optical effect). In the case of the mirrors M1 to M4, this optical effect is "REFL", that is, reflection.

[0119] The first column of Table 1b indicates the maximum value of the incident height (distance perpendicular to the optical axis) of the corresponding surface description of the optical surface, in millimeters.

[0120] The second column of Table 1b indicates the maximum deviation of the corresponding aspherical optical surface from the best-fitting sphere, also in millimeters.

[0121] In addition, for the mirror surfaces of mirrors M1, M2, and M4, Table 2 below also indicates the coefficients K, C1, C2, and C3 according to the following aspherical surface formula:

[0122] p(h) = [((1 / r)h 2 ) / (1 + SQRT(1 – (1 + K)(1 / r) 2 h 2 ))] + C1·h 4 + C2·h 6 + C3·

[0123] h 8 ....

[0124] In this case, p is the sagittal height, h is the incident 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 aspherical correction polynomial.

[0125] The deviation from the best-fitting sphere is caused by the difference between the sagittal heights according to the surface formulas of the aspherical and the best-fitting sphere.

[0126]

[0127] Figure 4 Table 1a of / 5

[0128]

[0129]

[0130] Figure 4 Table 1b of / 5

[0131] Mirror K <![CDATA[C1]]> <![CDATA[C2]]> <![CDATA[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

[0132] Figure 4 Table 1c of / 5

[0133] Therefore, in the case of the projection optical unit 20c, mirrors M1, M2, and M4 are implemented as aspherical. Mirror M3 is a spherical mirror.

[0134] In Figure 5 the shading effect of mirror M2 with respect to the entrance pupil EP is also shown at OBS. Mirror M2 is implemented such that the shading OBS does not overlap with the semi-circular entrance pupil EP.

[0135] The following references Figure 6 and Figure 7Explain another embodiment of the projection optical unit 20d, which can be used to replace the projection optical unit in the above-mentioned mask inspection system 2a. Corresponding components and functions to those already referred to above Figures 1 to 5 are particularly given the same reference numerals and will not be discussed in detail again.

[0136] In the case of the projection optical unit 20d, the object field side numerical aperture NAx is 0.25. The object field side numerical aperture NAy is 0.125. The object side field size is 0.56 mm × 0.36 mm, and the field offset in the y direction is 0.06 mm.

[0137] The projection optical unit 20d also has four mirrors M1 to M4. In the case of the projection optical unit 20d, in the figure, compared with the beam path of the projection optical unit 20c, the beam path is mirror-symmetric with respect to the xz plane. Otherwise, the beam path in the projection optical unit 20d corresponds in principle to the beam path in the projection optical unit 20c.

[0138] In the case of the projection optical unit 20d, the following holds for the distance ratio A to B:

[0139]

[0140] Another distance ratio is:

[0141]

[0142]

[0143] Figure 7 Show the edge profile of the entrance pupil EP, which simultaneously corresponds to the inner boundary profile of the aperture stop 9b. The entrance pupil EP of the projection optical unit 20d has a boundary shape including a semi-circular boundary portion 25 and a diameter boundary portion 26. Along the diameter boundary portion 26 that together with the semi-circular boundary portion 25 forms the total boundary of the entrance pupil EP, a cutout portion 27 exists in the boundary of the entrance pupil EP. The central cut of this cutout portion 27 extends parallel to the diameter boundary portion 26 in a manner offset in the positive y direction with respect to the diameter boundary portion 26. The central cut of the cutout portion 27 merges into the diameter boundary portion 26 via two inclined boundary portions 28, 29.

[0144] The y distance between the central cutout portion 27 and the diameter boundary portion 26 is less than 15% of the y-direction range of the total entrance pupil EP. Therefore, compared with the semi-circular envelope around the entrance pupil EP and compared with the area of the entrance pupil EP itself, the total cutout in the diameter boundary portion has a negligible area.

[0145] The cutout portion 27 is attributable to the shielding of the entrance pupil EP caused by the mirror M2 of the projection optical unit 20d.

[0146] In the case of the projection optical unit 20d, the magnification is 435.

[0147] In the case of the projection optical unit 20d, the RMS of the wavefront aberration above the image field 21 is 15 mλ.

[0148] In the case of the projection optical unit 20d, the Petzval radius on the image field side is on the order of about 20.000 mm.

[0149] The optical design data regarding the projection optical unit 20d are again summarized in the following two tables, whose structures correspond to Figure 4 and Figure 5 (that is, the table structure of the projection optical unit 20c).

[0150]

[0151]

[0152] Figure 6 Table 1a / b of / 7

[0153]

[0154] Figure 6 Table 1b of / 7

[0155] Mirror K <![CDATA[C1]]> <![CDATA[C2]]> <![CDATA[C3]]> <![CDATA[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 -3.1E-18 0.5E-22 -1E-28

[0156] Figure 6 Table 2 of / 7

[0157] The aspherical mirrors of the projection optical unit 20a and the aspherical mirrors M1, M2, and M4 of the projection optical units 20c and 20d have reflection surfaces that deviate from the spherical shape by at most 25 μm. Regarding the extension of the reflection surfaces of the small mirrors M2 and M3, these mirrors are also called small-area mirrors with a reflection surface diameter of less than 50 mm, and they deviate from the spherical shape by at most 5 μm. In the case of the projection optical units 20c and 20d, the corresponding spherical mirror M3 does not deviate from the spherical shape at all.

[0158] The illumination beam path of the illumination light 3 for illuminating the reticle 18 and the imaging beam path of the projection optical unit 20a for imaging the object field 4 into the image field 21 cross each other in the crossing region. This crossing region can be located in the region of the entrance pupil plane EP of the projection optical unit 20a. Here, the imaging beam path crosses the illumination beam path between the exit opening 14 and the mirror IL2 of the illumination optical unit 1 and between the multiple mirrors IL2 and IL3 of the illumination optical unit 1.

[0159] The image field 21 is captured by a detection device 23, such as a CCD camera or multiple CCD cameras. For details regarding imaging onto the image field, reference is made to US10,042,248B2 and the references cited therein. The detection device 23 may also be implemented as a time delay integration (TDI) detection device containing multiple TDI detectors.

[0160] The detection device 23 is implemented in a spatially resolved manner. The detection device 23 may include sensor pixels having a typical pixel size of at most 20 μm × 20 μm. This pixel size may be smaller and may be, for example, 15 μm × 15 μm or 10 μm × 10 μm. The pixel size along the image field coordinates x and / or y may range between 1 μm and 20 μm.

[0161] For example, the mask 18 may be inspected using a mask inspection system 2a.

Claims

1. A magnifying imaging optical unit (20a, 20b, 20c, 20d) of a metrology system (2a) for inspecting an object (18), comprising: at most four mirrors (M1, M2; M3; M1, M2, M3, M4) which image an object field (4) in an object plane (17) along an imaging beam path to an image field (21) in an image plane (22); The entrance pupil (EP) has a boundary shape that deviates from an ellipse and whose aspect ratio is not equal to 1.

2. The magnifying imaging optical unit according to claim 1, characterized in that: The entrance pupil (EP) comprises a boundary shape having a semicircular boundary portion (25).

3. The magnifying imaging optical unit according to claim 2, characterized in that: Along a diameter boundary portion (26) which together with the semicircular boundary portion (25) forms the overall boundary of the entrance pupil (EP), there is a cutout portion (27) in the boundary.

4. The magnifying imaging optical unit according to any one of claims 1 to 3, characterized in that: At least one mirror (M1; M1, M2) comprises a boundary at a reflective surface for directing imaging light (3) along the imaging beam path, the boundary corresponding to a boundary of the entrance pupil (EP).

5. A magnifying imaging optical unit (20a, 20b, 20c, 20d) of a metrology system (2a) for inspecting an object (18), comprising: At most four reflectors (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 reflectors is a small-area reflector (M2, M3) having a reflective surface diameter of less than 50 mm, wherein the reflective surface of the small-area reflector (M2, M3) deviates from a spherical shape by at most 10 μm.

6. The magnifying imaging optical unit according to claim 5, characterized in that: The reflective surfaces of all mirrors deviate from a spherical shape by a maximum of 25 μm.

7. The magnifying imaging optical unit according to any one of claims 1 to 6, wherein: The distance between the last mirror (M3; M4) in the imaging beam path and the image plane (22) is greater than 60% of the distance between the object plane (17) and the image plane (22).

8. The magnifying imaging optical unit according to any one of claims 1 to 7, characterized in that: The magnification is between 250 and 500.

9. The magnifying imaging optical unit according to any one of claims 1 to 8, characterized in that: No individual ray in the imaging beam path has an angle of incidence greater than 14° on one of the mirrors (M1, M2; M1 to M4).

10. The magnifying imaging optical unit according to any one of claims 1 to 9, characterized in that: The imaging beam path is implemented such that the imaging light (3) is incident on the image field (21) at an incident angle of less than 5 degrees.

11. The magnifying imaging optical unit according to any one of claims 1 to 10, characterized in that: The maximum RMS wavefront aberration is 50mλ.

12. The magnifying imaging optical unit according to any one of claims 1 to 11, characterized in that: The reflective surface of at least one of the mirrors deviates from a spherical shape by at least two times the wavelength used.

13. An illumination optical unit (1) for a metrology system (2a) for inspecting an object (18), comprising an illumination pupil adapted to an entrance pupil (EP) of an imaging optical unit according to any one of claims 1 to 12, in, The boundary shape of the illumination pupil deviates from an ellipse and its aspect ratio is not equal to 1.

14. An optical system (2) comprising an imaging optical unit (20a, 20b, 20c, 20d) as claimed in any one of claims 1 to 12 and comprising an illumination optical unit (1) for illuminating the object field (4) using illumination light (3).

15. An optical system (13) comprising the illumination optical unit according to claim 13.

16. A metering system (2a), comprising: An optical system as claimed in claim 14 or 15; a light source (5); and A spatial resolution detection device (23) is used to capture the image field (21).

Images