Optical hollow waveguide component

Abstract

An optical hollow waveguide component (25), in particular for the homogenization of EUV light, comprises two support elements (26) and two insertion elements (28), in particular two plate-shaped insertion elements. The insertion elements (28) are inserted between the support elements (26) in such a way that a hollow waveguide (11) is formed between the insertion elements (28). The hollow waveguide component (25) is suitable in particular for use in an illumination optics unit for a mask inspection system (1) for use with EUV illumination light. An optical system having such an illumination optics unit and a mask inspection system having the optical system are also described.

Description

[0001] Optical hollow waveguide component

[0002] The content of the German patent application DE 10 2024 205 382.1 is incorporated by reference herein.

[0003] The invention relates to an optical hollow waveguide component, in particular for the homogenization of illumination light, for example EUV illumination light. The invention also relates to an illumination optics unit for a mask inspection system for use with EUV illumination light, having such a hollow waveguide component. The invention moreover relates to an optical system having such an illumination optics unit and to a mask inspection system.

[0004] A mask inspection system is known from DE 10 2022 205 767 Al, for example. EUV light used for the illumination is homogenized with the aid of a hollow waveguide. The use of such hollow waveguides with EUV light is complicated by the requirements on the manufacturing precision of the hollow waveguide.

[0005] Hence, the problem addressed by the present invention is that of improving an optical hollow waveguide, in particular with regard to its possible application for the homogenization of EUV light, in particular that of specifying an optical hollow waveguide, which is simple and precise to manufacture.

[0006] According to the invention, this problem is solved by an optical hollow waveguide component, in particular for the homogenization of EUV light, having the features specified in Claim 1.

[0007] The inventors have recognized that the insertion of insertion elements between support elements allows more precise positioning of the insertion elements. The hollow waveguide, in particular its cross section, is precisely defined. In particular, the arrangement between the support elements allows extensive contacting of the support elements with the insertion elements. This increases angular accuracy in relative positioning. In addition, holding forces are introduced by the support elements substantially in the direction perpendicular to the support element surfaces in contact with the insertion elements, and so transverse forces are reduced. The hollow waveguide component has a particularly high dimensional accuracy and is particularly suitable for use with EUV light, for example as a beam-homogenizing element.

[0008] In the present case, the hollow waveguide is formed as a cavity or intermediate space between the insertion elements, in particular between the insertion elements and the support elements. The hollow waveguide comprises an entrance opening arranged in an entrance plane and an exit opening arranged in an exit plane. The entrance plane and the exit plane may be formed, for example, in the region of the respective end faces of the insertion elements. In particular, they are parallel planes. The hollow waveguide extends along its longitudinal axis between the entrance opening and the exit opening. The longitudinal axis of the hollow waveguide is in particular parallel to the propagation direction of the beam of the illumination light to be homogenized. In particular, the longitudinal axis extends perpendicular to the entrance plane and the exit plane.

[0009] Here and in the following, a cross section of the hollow waveguide is understood to be the cross section perpendicular to the longitudinal axis of the hollow waveguide. The cross section is defined by the insertion elements, in particular by the insertion elements and the support elements. Here and in the following, surfaces of the insertion elements that face the hollow waveguide are also referred to as bordering surfaces. The bordering surfaces of the insertion elements delimit the hollow waveguide, in particular its cross section.

[0010] The insertion elements are arranged between the support elements. The insertion elements should therefore be understood to be components inserted between the support elements. The support elements are therefore spaced apart by the insertion elements. A distance between the support elements is precisely defined, in particular by the insertion elements.

[0011] Here and in the following, surfaces of the support elements that face the insertion elements and hence the other support element are also referred to as support element surfaces facing one another or support element surfaces. The support element surfaces facing one another can delimit the cross section of the hollow waveguide in regions. The support element surfaces facing one another are parallel to one another in particular. The insertion elements are arranged between the support element surfaces facing one another.

[0012] The insertion elements contact the support element surfaces extensively in particular.

[0013] Delimiting the hollow waveguide between the bordering surfaces of the insertion elements allows a particularly simple and precise determination of the hollow waveguide geometry, in particular of its cross section. An extent of the cross section in the direction perpendicular to the support element surfaces facing one another is defined in particular by a thickness of the insertion elements in the region of the end surfaces. An extent of the cross section parallel to the support element surfaces is given by the spacing of the end surfaces of the insertion elements in particular. In particular, the cross section can be rectangular.

[0014] A contour of the bordering surfaces of the insertion elements corresponds to at least a part of a cross-sectional contour of the hollow waveguide. For example, the bordering surfaces of the insertion elements may form bordering surfaces of the hollow waveguide. The bordering surfaces delimiting the hollow waveguide may extend in linear or other fashion along the respective cross-sectional contour regions and may be angled and / or bent in particular.

[0015] In particular, the hollow waveguide may be delimited by the bordering surfaces of the insertion elements and at least one of the support element surfaces. It is also possible that the hollow waveguide is delimited by the bordering surfaces of the insertion elements and the two support element surfaces facing one another of the support elements.

[0016] As a result of the hollow waveguide being delimited by the bordering surfaces of the insertion elements and optionally by at least one of the support element surfaces, it is advantageously possible to realize any desired cross-sectional shape for the hollow waveguide. In particular, the hollow waveguide is not restricted to rectangular cross sections. For example, the cross section of the hollow waveguide may be triangular or polygonal, in particular triangular, quadrilateral, pentagonal or hexagonal. Curved cross-sectional contours are also possible.

[0017] For example, the bordering surfaces of the insertion elements may be parallel to one another. Together with the support element surfaces facing one another, the parallel bordering surfaces may delimit a hollow waveguide with a quadrilateral cross section. For example, the bordering surfaces may be perpendicular to the support element surfaces in order to form a rectangular hollow waveguide cross section. The bordering surfaces may also be at an angle of less than 90° to the support element surfaces, and so a hollow waveguide with a parallelogram-shaped cross section is formed.

[0018] It is also possible that the bordering surfaces are at an angle to one another in a plane perpendicular to a longitudinal axis of the hollow waveguide. This enables the formation of triangular or pentagonal cross sections in particular. For example, a triangular cross section of the hollow waveguide may be formed by converging bordering surfaces and one of the support element surfaces.

[0019] It is also possible that the bordering surfaces along the cross-sectional contour themselves have one or more kinks or bends. For example, the bordering surfaces may have grooves, in particular V-shaped grooves. In this way, hexagonal or polygonal cross sections can be easily realized.

[0020] On account of the extensive placement of the insertion elements against the support element surfaces, transition regions between the bordering surfaces and the support element surfaces can be manufactured particularly precisely and stably. In particular, an edge accuracy in the region of the edges of the bordering surfaces in the submicrometre range is possible. The resultant transition regions do not affect transmission. This is advantageous for the homogenization of EUV light in particular.

[0021] The extensive contact of the insertion elements with the support element surfaces has the further advantage of distributing the holding forces over a large area. This avoids a punctiform application of holding forces, especially in the region of edges of the bordering surfaces, and hence damage to these edges.

[0022] Plate-shaped insertion elements according to Claim 2 have the advantage of particularly simple and precise positioning. Transverse forces that could affect the distance between the bordering surfaces are avoided. By preference, the insertion elements are plane-parallel plates. The support element surfaces facing one another are parallel to one another. Here and in the following, the plate-shaped insertion elements are also referred to as plate elements.

[0023] In particular, plate-shaped should be understood to mean that the insertion elements have a much smaller extent in one direction than in the two directions perpendicular thereto. Here and in the following, the smallest extent is referred to as the thickness of the plate elements. The thickness can be the same, especially over the plate element. Surfaces spaced apart by the thickness are parallel to one another in particular. Plate elements having two parallel surfaces spaced apart by a constant thickness are also referred to as plane-parallel plates.

[0024] By preference, the plate-shaped insertion elements are arranged between the support elements in such a way that the support elements are spaced apart from one another by a thickness of the plate-shaped insertion elements. This allows particularly good extensive contact between the insertion elements and the support elements.

[0025] A thickness of the plate elements corresponds in particular to an extent of the hollow waveguide cross section in the direction perpendicular to the support element surfaces.

[0026] The ratio of the width B of the insertion elements to their thickness d according to Claim 3 has proven to be particularly advantageous. The relatively high width ensures particularly precise positioning with a large contact surface on the support element surfaces. Holding forces are distributed over a large area.

[0027] The main extent of the hollow waveguide cross section according to Claim 4 has proven to be particularly advantageous, especially with regard to the surface processing of the bordering surfaces. In applications of the hollow waveguide relevant in practice, especially in illumination optics units for mask inspection systems, the cross section of the hollow waveguide is rectangular with different side lengths. The main extent corresponds to the longer side length of the rectangle. By selecting the main extent to correspond to the thickness of the insertion elements, in particular the thickness of plate-shaped insertion elements, these have a maximum possible thickness in the region of the side surface for the desired hollow waveguide cross section. This improves and simplifies surface processing in the region of the bordering surfaces. For example, a coating of the bordering surfaces with an EUV-reflective material, in particular ruthenium, is improved. The surfaces delimiting the hollow waveguide, in particular edges in the region of the bordering surfaces of the insertion elements, can be manufactured particularly precisely. The hollow waveguide component is particularly suitable for use with EUV light.

[0028] The positioning elements according to Claim 5 ensure particularly precise positioning of the insertion elements to one another and in particular to the support elements as well.

[0029] The positioning elements according to Claim 6 ensure good accessibility and simple and precise alignment of the insertion elements and / or support elements to one another without adversely affecting the placement of the insertion elements against the support element surfaces. In particular, the positioning elements may be arranged on the outer surfaces of the hollow waveguide component, in particular of the insertion elements and / or support elements. For example, positioning grooves may be introduced into the outer surfaces.

[0030] In an alternative to external positioning elements or in addition, positioning elements may also be introduced into the hollow waveguide to be formed. For example, positioning balls may be used, which are removed after positioning.

[0031] The arrangement of the positioning elements according to Claim 7 on the end faces does not adversely affect the placement of the insertion elements against the support element surfaces. The positioning elements are easily accessible. In this respect, the end faces should be understood to mean, in particular, the outer sides of the hollow waveguide component which are perpendicular to the longitudinal direction of the hollow waveguide.

[0032] Positioning grooves according to Claim 8 are simple and precise to manufacture. The positioning grooves may have any desired groove cross section perpendicular to a direction of extent of the groove. By preference, the groove cross section tapers, in particular continuously, from the opening of the groove to its base. V-shaped positioning grooves were found to be particularly advantageous. V-shaped positioning grooves have a uniform taper of the groove cross section, enabling particularly precise positioning. For example, positioning elements with a cross section corresponding to the positioning grooves or with a round cross section may be introduced into the positioning grooves in order to position the support elements and the insertion elements relative to one another.

[0033] The positioning grooves according to Claim 9 are easily accessible. An introduction of positioning elements, in particular a temporary introduction, may be implemented easily and precisely. In particular, the positioning grooves are introduced in outer surfaces, in particular in end faces, of the support elements and / or insertion elements such that these result in continuous grooves in the assembled state.

[0034] The support elements and / or the insertion elements may be made of glass, quartz, ceramic and / or metal in particular. On their surface facing the hollow waveguide, the support elements and / or insertion elements, in particular elements made of glass, quartz and / or metal, may have a coating of EUV-reflective material, in particular ruthenium, zirconium and / or molybdenum.

[0035] Producing the support elements and / or insertion elements from metal according to Claim 10 increases the stability of the respective elements. In particular, the support elements and / or insertion elements are made entirely of metal, in particular ruthenium. Holding forces can be absorbed better on account of metal being ductile, especially in comparison with glass-like materials. Damage, especially in the region of the edges of the bordering surfaces, is avoided.

[0036] The use of metals also enables particularly high manufacturing accuracies, in particular a precise production of the transition regions between the bordering surfaces and the support element surfaces. In particular, a high edge accuracy is possible.

[0037] The support elements and insertion elements may be made of the same or different materials. In particular, different materials may be flexibly combined. For example, the insertion elements may be made of metal, in particular ruthenium. The support elements may be made of another metal, for example copper, or glass-like materials, for example glass. The support element surfaces facing one another may be coated with a suitable material, in particular ruthenium. The choice of the same materials for the support elements and the insertion elements has the advantage in particular of a corresponding thermal expansion behaviour. Ruthenium was found to be particularly suitable for the reflection of UV light, especially EUV light. Further metals particularly suitable for this purpose are zirconium and / or molybdenum. For example, ruthenium, zirconium and / or molybdenum may be used for the production of the support elements and / or insertion elements and / or for a surface coating of these elements.

[0038] In particular, the use of metal for the support elements and insertion elements is an independent aspect of the present invention, in particular independent of the arrangement of the insertion elements between support element surfaces facing one another. In particular, differently arranged and / or formed support elements and / or insertion elements may be used for the hollow waveguide component.

[0039] Insertion elements according to Claim 11 have proven to be particularly advantageous for use with EUV light. In particular, plate-shaped insertion elements can be made from ruthenium with comparatively low material costs. Production costs are reduced, especially in comparison with complex coatings of surfaces with ruthenium and the surface processing required to this end.

[0040] A contact plate according to Claim 12 may be formed in correspondence with plate-shaped insertion elements in particular. In particular, the contact plate may consist of the same material as the insertion elements. For example, the contact plate may have the same thickness as the insertion elements. For example, the support elements may be embodied as contact plate. In an alternative, the support elements may also have a substrate in addition to the contact plate. Contact plates of low thickness in particular can be stabilized with the aid of the substrate.

[0041] Support elements according to Claim 13 have proven to be particularly suitable for EUV light. Particularly advantageously, the support elements, in particular contact plates of the support elements, and the insertion elements are made of ruthenium. The support elements may have a single-part or multiple-part embodiment. A single-part embodiment of the support elements has the advantage of a simple and stable arrangement.

[0042] The insertion elements may have a single-part or multiple-part embodiment. A single-part configuration, in particular as single-part plate elements, allows a simple arrangement between the support elements. The insertion elements are particularly stable.

[0043] Insertion elements according to Claim 14 allow a particularly flexible design of the hollow waveguide, in particular of its cross section. For example, the insertion elements may comprise a plurality of insertion element parts, which in particular in the form of insertion element plates are arranged on top of one another in the direction of a surface normal of the support element surface. Complex bordering surface contours may be created easily on the basis of different inclinations of the bordering surfaces of the individual insertion element parts, in particular insertion element plates. For example, the bordering surfaces of different insertion element parts may be inclined differently, and so a corresponding polygonal profile of the bordering surfaces emerges.

[0044] The advantages of the illumination optics unit according to Claim 15 correspond to those which have already been explained above with reference to the hollow waveguide component.

[0045] The input coupling mirror optics unit may comprise at least one mirror for grazing incidence (GI), embodied in particular for an angle of incidence of the illumination light of greater than 45°. The input coupling mirror optics unit may comprise exactly one mirror or else comprise a plurality of mirrors, e.g. two mirrors.

[0046] An optics unit as known from US 10,042,248 B2, for example, may be used as an output coupling mirror optics unit.

[0047] The advantages of an optical system according to Claim 16 and of a mask inspection system according to Claim 17 correspond to those which were already explained above with reference to the hollow waveguide component. A wafer inspection system may also be constructed accordingly. The inspection system may comprise an object holder that serves to hold the object to be inspected and is mechanically coupled to an object displacement drive, with the result that a scanning displacement of the object is possible during the illumination.

[0048] The inspection system may be a system for actinic mask inspection.

[0049] Further features, details and advantages of the invention will become apparent from the following description of exemplary embodiments based on the attached drawings. In these drawings:

[0050] Fig. 1 schematically shows, in a meridional section, a mask inspection system for lithography masks for use with EUV illumination light with an illumination optics unit, comprising a beam-homogenizing element in the form of a hollow waveguide;

[0051] Fig. 2 shows schematic angle relationships of an illumination light beam upon entrance into an entrance opening of the hollow waveguide and upon exit from an exit opening of the hollow waveguide;

[0052] Fig. 3 shows a schematic, not true-to-scale perspective of a hollow waveguide component for realizing the hollow waveguide of the illumination optics unit, wherein the hollow waveguide component is depicted in partially transparent fashion;

[0053] Fig. 4 shows a schematic, not true-to-scale cross section through the hollow waveguide component;

[0054] Fig. 5 shows a schematic, not true-to-scale cross section through a further exemplary embodiment of a hollow waveguide component for realizing the hollow waveguide of the illumination optics unit; and Figs 6 to 9show schematic, not true-to-scale cross sections through further exemplary embodiments of a hollow waveguide component with different cross-sectional shapes of the hollow waveguide.

[0055] An illumination optics unit BO is a constituent part of an optical system 2 of a mask inspection system 1 for use with EUV illumination light 3. A beam path of the illumination light 3 is illustrated by way of marginal rays and a chief ray for the illumination optics unit BO in Fig. 1. An illumination field 4 of the mask inspection system 1 is illuminated by the illumination light 3.

[0056] The illumination light 3 is created by an EUV light source 5 in a source region 6. The light source 5 can create EUV used radiation in a wavelength range of between 2 nm and 30 nm, for example in the range of between 2.3 nm and 4.4 nm or in the range of between 5 nm and 30 nm, for example at 13.5 nm.

[0057] The light source 5 is designed as a plasma light source. For example, it may be a laser plasma source (LPP; laser produced plasma) or else a discharge source (DPP; discharge produced plasma). In principle, such plasma sources are known as light sources for EUV projection exposure apparatuses. Alternatively, the light source 5 may also be designed as a high- harmonic EUV source. A pulse frequency of the light source 5 may be in the kHz range.

[0058] In order to facilitate positional relationships, a Cartesian xyz-coordinate system will be used hereinafter. The x-axis is perpendicular to the plane of the drawing in Figure 1 and runs into the latter. The y-axis runs horizontally towards the left in Figure 1, and the z-axis runs vertically upwards in Figure 1.

[0059] After emission by the light source 5, the illumination light 3 firstly passes through a used light filter 8 arranged in an operating position in the beam path of the illumination light 3 between the source volume 6 and an ellipsoidal mirror 10 of the illumination optics unit BO. The used light filter 8 may be a filter or a plurality of filters kept available for example in a filter magazine in the mask inspection system 1. A further used light filter 8 may be arranged in a waiting position outside the illumination light beam path of the illumination optics unit BO. The used light filters 8 may have the same transmission characteristic, in which case a changeover between the used light filters 8 can be effected if a degradation of a filter effect of the used light filter 8 is determined. Alternatively, the used light filters 8 may also have different filter characteristics and for example transmit different used light wavelength ranges into the downstream illumination light beam path or be optimized for filtering out different extraneous light components.

[0060] The used light filter 8 may be designed such that it filters out in particular pump light which is concomitantly guided in the illumination light beam path and which was used during the creation of used light in the source volume 6.

[0061] Downstream of the used light filter 8 and the ellipsoidal mirror 10, the illumination light 3 firstly passes through an aperture stop 9 which delimits the edge of a beam of illumination light 3. After that, the illumination light beam 3 is transferred towards a beam-homogenizing element 11 in the form of a hollow waveguide of the illumination optics unit BO. In this case, the mirror 10 serves as an input-coupling optics unit for input-coupling the illumination light 3 into the beam-homogenizing element 11.

[0062] Between the source volume 6 and the beam- homogenizing element 11, generally downstream of the ellipsoidal mirror 10 of the illumination optics unit BO, the illumination light 3 passes through an opening in a wall of a vacuum chamber VK, which is indicated between the ellipsoidal mirror 10 and the aperture stop 9 in the illumination light beam path in Fig. 1.

[0063] The aperture stop 9 restricts 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 else in the range between 0.05 and 0.08. In an alternative to the aperture stop 9 or in addition, an aperture-limiting stop may be arranged between the beam-homogenizing element 11 and a downstream optical component of the illumination optics unit BO. An arrangement of such a further aperture stop in the beam path of the illumination light 3 downstream of the beam-homogenizing element 11 between two downstream optical components of the illumination optics unit BO is also possible. The ellipsoidal mirror 10 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 10 is therefore located in the source region 6 and a second focal point of the ellipsoidal mirror 10 is located in the entrance opening 12. The ellipsoidal mirror 10 is used to focus the illumination light beam 3 into the entrance opening 12 in the entrance plane 13 of the beam-homogenizing element 11. An entrance-side numerical aperture of the illumination light beam 3 upon entrance into the entrance opening 12 may range between 0.02 and 0.2, for example be of the order of 0.05.

[0064] An angle of incidence of a central chief ray of the illumination light beam 3 on the inputcoupling mirror 10 may range between 10° and 20°. The ellipsoidal mirror 10 may be a normal incidence (NI) mirror but may also be designed as a grazing incidence (GI) mirror.

[0065] The entrance opening 12 and an exit 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 else between 0.5 mm and 1 mm. An aspect ratio of the entrance opening 12 and of an identically sized exit opening 14 of the beamhomogenizing element 11 for the illumination light 3 in an exit plane 15 is between 0.5 and 2. A typical size of the entrance opening 12 and of the exit opening 14 of the beamhomogenizing element 11 is e.g. 0.5 mm x 1.0 mm, 0.75 mm x 0.75 mm, 1.0 mm x 2.0 mm or 1.5 mm x 2.0 mm.

[0066] The beam- homogenizing element 11 is designed as a hollow waveguide and is also referred to here and in the following as hollow waveguide 11.

[0067] The beam- homogenizing element 11 has a typical length perpendicular to the planes 13 and 15, i.e. along its longitudinal axis L 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 ranging between 50 mm and 100 mm.

[0068] An angle between a normal to the entrance plane 13 of the beam- homogenizing element 11 and the chief ray CR of the illumination light beam 3 incident into the entrance opening 12 can be 0° or can alternatively also differ from 0° and for example range between 0° and 1.5°, for example between 0.25° and 0.75°, and in particular be of the order of 0.5°.

[0069] A ratio of the distance between the entrance plane 13 and the exit plane 15 and a size or the typical diameter of the entrance opening and respectively the exit opening 12, 14 ranges between 50 and 1000 and can for example range between 50 and 200.

[0070] An imaging output-coupling mirror optics unit 16 situated downstream of the beamhomogenizing element 11 images the exit opening 14, located in an exit plane 15, of the beam-homogenizing element 11 into the illumination field 4 in an object plane 17. This imaging may have an image-side numerical aperture in the range between 0.05 and 0.2.

[0071] The output-coupling mirror optics unit 16 may be designed in the style of a Wolter telescope, specifically in the style of a Type I Wolter optics unit. Such Wolter optics units are described in J. D. Mangus, J. H. Underwood "Optical Design of a Glancing Incidence X-ray Telescope", Applied Optics, Vol. 8, 1969, page 95, and the references cited therein. In such Wolter optics units, a hyperboloid may also be used in place of a paraboloid. Such a combination of an ellipsoidal mirror with a hyperboloid mirror also constitutes a Type I Wolter optics unit.

[0072] An exemplary embodiment of the output-coupling mirror optics unit 16 is described in US 10,042,248 B2.

[0073] An imaging factor Pi of the input-coupling mirror optics unit 10 may range between 0.1 and 50, i.e. its action may vary from a reduction by a factor of 10 to a magnification of a factor of 50. An imaging factor P2 of the output-coupling mirror optics unit 16 may range between 0.02 and 10, i.e. its action in turn may vary from a reduction by a factor of 50 to a magnification of a factor. In the case of the illumination optics unit BO, a product Pi, P2 of the two imaging factors may range between 0.25 and 10.

[0074] A reticle 18 to be inspected, which is held by a reticle holder 19, is arranged as object to be inspected or mask, in particular photomask, to be inspected in the object plane 17. The reticle holder 19 is mechanically operatively connected to a reticle displacement drive 20, by means of which the reticle 18 is displaced along an object displacement direction y during a mask inspection. In this way, a scanning displacement of the reticle 18 in the object plane 17 is possible.

[0075] The mask to be inspected may have an aspect ratio of between 1 : 1 and 1 :3, preferably between 1 : 1 and 1 :2 and particularly preferably of 1 : 1 or 1 :2. The mask may be designed in a substantially rectangular fashion. The mask may preferably have a length and width of 5 to 7 inches, particularly preferably a length and width of 6 inches. In an alternative thereto, the photomask may be 5 inches to 7 inches long and 10 inches to 14 inches wide, preferably 6 inches long and 12 inches wide.

[0076] The illumination field 4 has a typical dimension in the object plane 17 of less than 0.5 mm. In the embodiment illustrated, the extent of the illumination field 4 is 0.5 mm in the x-direction and 0.5 mm in the y-direction.

[0077] The x / y aspect ratio of the illumination field 4 corresponds to the x / y aspect ratio of the exit opening 14.

[0078] The illumination field 4 or a part of the illumination field 4, this part then constituting an object field, is imaged into an image field 21 in an image plane 22 by a projection optics unit PO. A size of the image field 21 can be in the range of 150 mm x 250 mm. The shorter image field extent runs along the scanning direction y.

[0079] The image field 21 is captured by a detection device 23, e.g. one CCD camera or a plurality of CCD cameras. For details of the imaging into the image field, reference is made to US 10,042,248 B2 and the references cited in US 10,042,248 B2. The detection device 23 may also be designed as a TDI (time delay integration) detection device having a plurality of TDI detectors.

[0080] An inspection of a structure on the reticle 18, for example, is possible by means of the mask inspection system 1. Figure 2 schematically shows the effect of a non-zero chief ray angle acR of a chief ray CR of the illumination light beam 3 which enters the entrance opening 12 of the beam-homogenizing element 11 in the form of a hollow waveguide, with respect to the longitudinal axis L of the hollow waveguide 11. On account of the angle acR, both the chief ray CR and the other individual rays of the illumination light beam 3 are reflected at least once off the inner wall of the waveguide cavity of the hollow waveguide 11. This leads to an angle distribution within the illumination light beam 3 being influenced post exit from the exit opening 14. This exit angle distribution is indicated schematically in Figure 2 on the basis of a multiplicity of individual rays 24 of the illumination light beam 3. This redistribution may be used for a homogenization of an intensity distribution within the illumination angles of the illumination light beam 3.

[0081] For example, the illumination light 3 may be radiated-in with a chief ray extending along the longitudinal axis L (acR = 0), wherein an illumination angle distribution of the incident illumination light 3 preferably extends symmetrically about the longitudinal axis L. The illumination light beam 3 emerging from the hollow waveguide 11 then has in turn a corresponding illumination angle distribution which is centred about the longitudinal axis L and which corresponds in terms of its angle variation to the angle distribution of the incident illumination light beam 3. On account of the reflections off the inner wall of the hollow waveguide 11, the illumination angles of the emerging illumination light beam 3 are redistributed within the illumination angle variation of the incident illumination light beam 3, with no new illumination angles occurring however.

[0082] A monopole-like illumination angle distribution arises for a chief ray angle acR = 0. For chief ray angles UCR 0, dipole and multipole illumination angle distributions and mixtures thereof may be created, depending on the chief ray angle.

[0083] Figs 3 and 4 show an exemplary embodiment of a hollow waveguide component 25 for realizing the hollow waveguide 11. The hollow waveguide component 25 comprises two support elements 26, which are substantially cuboid in the exemplary embodiment shown. Two insertion elements 28 are arranged between two opposing support element surfaces 27. The insertion elements 28 are designed as plane-parallel plates. With their respective plate surfaces, the insertion elements 28 are placed against the support element surfaces 27. The hollow waveguide 11 is formed between the insertion elements 28 and the support element surfaces 27. A cross section of the hollow waveguide 11 is delimited by bordering surfaces 29 of the insertion elements 28 and the support element surfaces 27 facing one another. The hollow waveguide 11 has a rectangular cross section, which corresponds in terms of its aspect ratio to the aspect ratio of the illumination field 4.

[0084] The insertion elements 28 and the support elements 26 have a length along the longitudinal axis L of the hollow waveguide 11 that corresponds to the length of the hollow waveguide 11. A width b of the hollow waveguide 11 that extends parallel to the support element surfaces 27 and perpendicular to the longitudinal axis L is determined by the spacing of the insertion elements 28, in particular of their bordering surfaces 29 from one another. A height of the cross section of the hollow waveguide 11 perpendicular to the width b is determined by the thickness d of the insertion elements 28. The width b and the thickness d correspond to the aforementioned dimensions of the entrance opening 12 and the exit opening 14 in particular. By preference, the main extent of the cross section is perpendicular to the support element surfaces 27, and so the main extent of the cross section corresponds to the thickness d of the insertion elements 28. Although this increases the thickness d and hence the material requirement for the insertion elements 28, the increased thickness d also facilitates the surface processing of the bordering surfaces 29.

[0085] The insertion elements 28 are designed as thin plates. A width B of the insertion elements 28 perpendicular to the longitudinal axis L exceeds the thickness d. The following applies to the ratio of width B to thickness d: B / d > 5, in particular B / d > 10, in particular B / d > 15; for example, the ratio B / d is approximately 20. A large width B causes a large contact surface, and so forces that act on edges in the region of the bordering surfaces 29 are reduced. In addition, large contact surfaces allow precise positioning and insensitivity to angular errors. A thickness D of the support elements 26 may be chosen flexibly. In particular, the stability of the hollow waveguide component 25 can be influenced by the thickness D. An exemplary thickness D may be approximately 15 mm.

[0086] A width T of the support elements 26 perpendicular to the longitudinal axis L preferably corresponds to the two widths B of the insertion elements 28 plus the width b of the hollow waveguide 11. An exemplary width T may be approximately 40 mm.

[0087] For relative positioning of the insertion elements 28, corresponding positioning elements in the form of positioning grooves 30 are arranged on the end faces of the support elements 26 and insertion elements 28.

[0088] In the exemplary embodiment shown, a respective positioning groove 30 is present for each insertion element 28 and for each end face, i.e. on the side of the entrance opening 12 and on the side of the exit opening 14. The positioning grooves 30 are V-shaped. During assembly, positioning elements, for example round-bar-shaped positioning elements, may engage in the positioning grooves 30 in order to position the support elements 26 and the insertion elements 28 precisely with respect to one another in the width direction.

[0089] The insertion elements 28 are held between the support elements 26. In this context, the support elements 26 exert a holding force F, which is shown schematically in Fig. 4, on the insertion elements 28.

[0090] There are various options for the connection between the support elements 26 and the insertion elements 28. Firstly, the elements may be adhesively bonded to one another, as shown schematically in Fig. 3. Thus, pockets which have an adhesive projection 31 and a surrounding degassing channel 32 are formed in the region of the support element surfaces 27. The adhesive projections 31 are slightly recessed vis-a-vis the support element surface 27 so that an adhesive can be applied. The adhesive may degas via the degassing channels 32.

[0091] In addition to adhesive bonding or in an alternative, the support elements 26 may be clamped with the insertion elements 28. For example, to this end clamping fixtures 33 may be formed in the support elements 26 and optionally in the insertion elements 28 for the purpose of receiving clamping elements.

[0092] The support elements 26 and the insertion elements 28 may be made of the same or different materials. In particular, material combinations are possible.

[0093] By preference, the insertion elements 28 and the support elements 26 are made of a metal. The metals have increased ductility, especially vis-a-vis glasses, which simplifies the application of force. In particular, damage in the region of the edges of the bordering surfaces 29 is avoided. In a preferred exemplary embodiment, the insertion elements 28 are made of ruthenium. Ruthenium was found to be particularly suitable for the total-internal reflection of EUV light. The use of plate-shaped insertion elements 28 reduces the material requirements of ruthenium. This reduces manufacturing costs, especially also in comparison with components, for example made of glass, coated with ruthenium.

[0094] The support elements 26 may be made of metal or glass. For example, the support elements 26 may also be made of ruthenium. It is also possible to make the support elements 26 from a further, ductile material, for example a metal, in particular copper. A coating, in particular of ruthenium, may be present in the region of the support element surfaces 27.

[0095] A further exemplary embodiment of a hollow waveguide component 125 is described on the basis of Fig. 5. Components which have already been described in relation to the exemplary embodiment in Figs 3 and 4 bear the same reference signs and are not explained in detail again.

[0096] The hollow waveguide component 125 only differs from the above-described hollow waveguide component 25 in terms of the design of the support elements 126. The support elements 126 comprise contact plates 35 that form the support element surfaces 27 facing one another. The contact plates 35 contact the insertion elements 28 extensively. For example, the contact plates 35 may be designed as plane-parallel plates. For example, the contact plates 35 have the same thickness d as the insertion elements 28. By preference, the contact plates 35 are made of ruthenium. This is possible with relatively low material costs on account of the small thickness of the contact plates 35. The contact plates 35 are particularly preferably made of the same material, for example ruthenium, as the insertion elements 28. The contact plates 35 and the insertion elements 28 have the same thermal expansion behaviour.

[0097] The support elements 126 optionally comprise substrates which are arranged on the side of the contact plates 35 facing away from the insertion elements 28. The substrates 36 may serve for stabilizing the contact plates 35, especially if these have a small thickness d.

[0098] A particular advantage of the use of insertion elements consists in the fact that a cross- sectional shape of the hollow waveguide can be easily and flexibly adapted, as is described in exemplary fashion with reference to Figures 6 to 9. Components which have already been described in relation to the exemplary embodiment in Figures 3 and 4 bear the same reference signs and are not explained in detail again.

[0099] The hollow waveguide component 225 according to Fig. 6 comprises insertion elements 228, the bordering surfaces 229 of which are inclined to one another. This results in a hollow waveguide 211 with a pentagonal cross section.

[0100] In the hollow waveguide component 325 according to Fig. 7, the mutually inclined bordering surfaces 329 of the insertion elements 328 contact one another, and so a triangular cross section of the hollow waveguide 311 arises. The hollow waveguide 311 is delimited by the side surfaces 329 and the support element surface 27 of one of the support elements, the lower support element in the present case.

[0101] The hollow waveguide component 425 according to Fig. 8 comprises insertion elements 428, the bordering surfaces 429 of which extend parallel to one another but obliquely at an angle of less than 90° with respect to a surface normal of the support element surfaces 27. This results in a hollow waveguide 411 with a parallelogram-like cross section. In the hollow waveguide component 525 according to Fig. 9, the insertion elements 528 each consist of a plurality of insertion element parts 540, two in the present case. The insertion element parts 540 are formed as insertion element plates which are arranged one above the other in the direction of a surface normal of the support element surfaces 27. A V-shaped bordering surface 529 of the insertion elements 528 arises on account of the bordering surfaces of the insertion element parts 540 in each case extending at an angle to one another. Overall, this results in a hollow waveguide 511 with a hexagonal cross section.

[0102] In the variant of Fig. 9, the insertion elements 528 may also be formed in one piece. In that case, a V-shaped groove may be introduced into the region of the bordering surfaces 529 in order to realize the hexagonal cross section of the hollow waveguide 511. Other contours of the bordering surfaces are also possible.

Claims

Claims1. Optical hollow waveguide component (25; 125; 225; 325; 425; 525), in particular for the homogenization of EUV light, comprising two support elements (26; 126) and two insertion elements (28; 228; 328; 428; 528), which are inserted between the support elements (26; 126) in such a way that a hollow waveguide (11) is formed between the insertion elements (28; 228; 328; 428; 528).

2. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to Claim 1, characterized in that the insertion elements (28; 228; 328; 428; 528) are plate-shaped, in particular plane-parallel plates.

3. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to Claim 1 or 2, characterized in that the insertion elements (28; 228; 328; 428; 528) have a thickness d, which corresponds to a minimum spacing of the support elements (26; 126), and a width B, which extends perpendicular to the longitudinal axis (L) of the hollow waveguide (11) and perpendicular to thickness d, wherein the following applies to the ratio of width B to thickness d: B / d > 5, in particular B / d > 10, in particular B / d > 15.

4. Hollow waveguide component (25; 125; 228; 328; 428; 528) according to any of the preceding claims, characterized in that a main extent of a cross section of the hollow waveguide (11) perpendicular to the longitudinal axis (L) of the hollow waveguide (11) corresponds to a thickness d of the insertion elements (28; 228; 328; 428; 528).

5. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of the preceding claims, characterized by positioning elements (30) on the support elements (26) and / or the insertion elements (28; 228; 328; 428; 528) for the relative positioning of the insertion elements (28; 228; 328; 428; 528) and / or support elements (26) to one another.

6. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to Claim 5, characterized in that the positioning elements (30) are arranged in the region of the outer sides, in particular on outer surfaces, of the hollow waveguide component (25; 125; 225; 325; 425; 525).

7. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to Claim 5 or 6, characterized in that the positioning elements are arranged on the end faces of the support elements (26) and / or the insertion elements (28; 228; 328; 428; 528).

8. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of Claims 5 to 7, characterized in that the positioning elements (30) are formed as positioning grooves (30), in particular V-shaped positioning grooves (30).

9. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of Claims 5 to 8, characterized in that the positioning grooves (30) are introduced in the outer surfaces, in particular the end-face surfaces, of the support elements (26) and / or the insertion elements (28; 228; 328; 428; 528).

10. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of the preceding claims, characterized in that the support elements (26; 126) and / or the insertion elements (28; 228; 328; 428; 528) are made of metal.

11. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to Claim 10, characterized in that the insertion elements (28) are made of ruthenium.

12. Hollow waveguide component (125) according to any of the preceding claims, characterized in that the support elements (126) each have a contact plate (35), which contacts the insertion elements.

13. Hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of the preceding claims, characterized in that the support elements (26; 126), in particular the contact plates (35) of the support elements (126), are made of ruthenium.

14. Hollow waveguide component (525) according to any of the preceding claims, characterized in that the insertion elements (528) are formed from multiple parts, in particular consist of a plurality of overlaid insertion element plates (540).

15. Illumination optics unit (BO) for a mask inspection system (1) for use with EUV illumination light (3), having a hollow waveguide component (25; 125; 225; 325; 425; 525) according to any of the preceding claims, having an input coupling mirror optics unit which is disposed upstream of the hollow waveguide component (25; 125; 225; 325; 425; 525) in the beam path of the illumination light (3) and has at least one mirror (10) for imaging a source region (6) of an EUV light source (5) into the entrance opening (12) of the hollow waveguide (11) and having an output coupling mirror optics unit (16) for imaging an exit opening (14) of the hollow waveguide (11) into an illumination field.

16. Optical system (2) having an illumination optics unit (BO) according to Claim 15 and having an EUV light source (5) for the illumination light (3).

17. Mask inspection system (1) having an optical system (2) according to Claim 16, having a projection optics unit (PO) for imaging the illumination field (4) into an image field (21) and having a detection device (23) for detecting illumination light (3) incident on the image field (21).

Images