Illumination optical unit of mask inspection system for use with EUV illumination light

Abstract

The illumination optical unit (1) is part of a mask inspection system used with EUV illumination light (3). The hollow waveguide (11) is used to guide the illumination light (3). For the illumination light (3), the hollow waveguide (11) has an entry opening (12) in an entry plane (13) and an exit opening (14) in an exit plane (14). An in-coupling mirror optical unit (10) is arranged upstream of the hollow waveguide (11) in the beam path of the illumination light (3) and has at least one mirror (IL1) for imaging the source region (6) of the EUV light source (5) into the entrance opening (12) of the hollow waveguide (11). The outcoupling mirror optical unit (16) is used to image the exit opening (14) of the hollow waveguide (11) into the illumination field (4). As a result, an illumination optical unit which is optimized for the utilization efficiency of the EUV illumination light can be obtained.

Description

[0001] The contents of German patent applications DE 10 2022 205 767.8, DE 10 2023 110 173.0 and DE 102023110 174.9 are incorporated herein by reference. Technical Field

[0002] The present invention relates to an illumination optical unit of a mask inspection system used in conjunction with EUV illumination light, an optical system having such an illumination optical unit, and a mask inspection system having such an illumination optical unit. Background Art

[0003] Such mask inspection systems are known from US Pat. No. 10,042,248 B2, DE 102 20 815 A1 and WO 2012 / 101269 A1. Summary of the invention

[0004] The object of the present invention is to develop an illumination optical unit for such an inspection system in such a way that its use of EUV illumination light is optimized.

[0005] According to the invention, this object is achieved by an illumination optical unit comprising the features specified in claim 1 .

[0006] According to the invention, an input coupling mirror optical unit interacting with an illumination optical unit is realized for imaging the EUV light source into the entrance opening of the hollow waveguide to provide a high input coupling efficiency. The optical unit known from US 10,042,248 B2 can be used as an output coupling mirror optical unit.

[0007] The input coupling mirror optical unit may include at least one mirror for grazing incidence (GI), which is implemented for an angle of incidence of the illumination light greater than 45°. Alternatively or additionally, the input coupling mirror optical unit may include at least one mirror for normal incidence (NI), which is implemented for an angle of incidence of the illumination light less than 45° and in particular less than 30°.

[0008] The input coupling mirror optical unit may comprise only one mirror or a plurality of mirrors, for example two mirrors.

[0009] The arrangement of the input coupling reflector optical unit may be such that a geometric centroid ray among the rays of the illumination light enters the plane of incidence of the entrance opening at an angle of less than 2°, less than 1.5°, less than 1° and possibly of the order of 0.5° to the normal of said plane of incidence. In particular, the geometric centroid ray may enter the entrance opening of the hollow waveguide perpendicular to the plane of incidence. Alternatively, the above-mentioned angular conditions may apply to the principal ray of the illumination light. The geometric centroid ray on the one hand and the principal ray on the other hand may not coincide, in particular if the pupil of the illumination optical unit is not uniformly illuminated and / or not symmetrically illuminated.

[0010] Depending on the incident angle and / or position of the incident plane of the incident principal ray of the illumination light beam relative to the reflective inner wall of the hollow waveguide or the incident geometric centroid ray of the edge ray of the illumination light beam, this can result in a monopole-like, dipole-like or multipole-like, for example a quadrupole-like, illumination angle distribution of the illumination field.

[0011] The mirrors of the illumination optics, ie in particular the incoupling mirror optics and / or the outcoupling mirror optics, can contain free-form reflective surfaces.

[0012] According to an embodiment of the input coupling reflector optical unit according to claim 2, as an elliptical reflector, the input coupling reflector optical unit can accurately perform one reflection between the source region of the light source and the incident opening of the hollow waveguide, allowing the input coupling reflector optical unit to have a high EUV flux.

[0013] The adjustability of the angle of incidence according to claim 3 enables embodiments of the illumination optical unit to allow setting a defined illumination angle distribution for illuminating the illumination field. Depending on the choice of the angle of incidence of the illumination light beam relative to the plane of incidence of the entrance opening, the illumination angle distribution of the illumination field can, for example, be monopole-like, dipole-like or multipole-like (e.g. quadrupole-like).

[0014] An angle of incidence according to claim 4 enables a high total transmittance of the incoupling mirror optical unit. Alternatively or additionally, the angle of incidence can also be less than 45° and for example in the range between 0° and 45°, for example in the range between 0° and 30°.

[0015] The input coupling reflector optical unit according to claim 5 enables a precise imaging of the source region of the EUV light source into the entrance opening with a precisely specifiable imaging factor. The input coupling reflector optical unit can be embodied as a Wolter-type reflector optical unit, in particular as a Wolter Type I reflector optical unit. A combination of a hyperbolic reflector and a parabolic reflector for the input coupling reflector optical unit is also possible. For example, a design principle as described in US 10,042,248 B2 can be used. Alternatively or additionally, a reflector with a free-form reflective surface can be used.

[0016] The rectangular entrance opening according to claim 6 has been found to be particularly suitable for specifying defined lighting conditions and can be manufactured precisely. The entrance opening can also be embodied in a square shape. The edge length of the boundary edge of the entrance opening can be less than 2 mm and in particular less than 1 mm. The ratio of the length of the hollow waveguide to the typical diameter of the entrance opening (e.g. rectangular edge length) can be greater than 10, can be greater than 30, can be in the range between 40 and 80, can be greater than 100, can be greater than 200, can be in the range between 200 and 300. Such a length / diameter ratio is typically less than 1000.

[0017] The pivotable embodiment of the illumination optical unit components according to claim 7 facilitates the adjustment of the illumination optical unit, in particular for the purpose of specifying the illumination angle distribution, that is to say the illumination setting of the illumination optical unit. By means of a suitably implemented actuator system, at least one component of the illumination optical unit is embodied so as to be pivotable about at least one pivot axis and can be displaced with respect to more than one rotational and / or translational degree of freedom, for example with respect to 2, 3, 4, 5 or 6 degrees of freedom.

[0018] The pivotable illumination optical unit component may be a hollow waveguide. The pivot axis of the hollow waveguide may be located in the incident plane of the incident opening. The pivot adjustment of the hollow waveguide may be used to optimize the etendue of the illumination optical unit.

[0019] For example, the term "etendue" is explained in the book "Non-imaging Optics" by Benitez, PG, Minano, JC, Winston, R., Narkis Shatz and John C. Bortz, WCB (2005). The etendue can be optimized in such a way that the etendue at exit from the hollow waveguide is reduced as much as possible, that is, the angular diameter of the illumination light emerging from the hollow waveguide relative to the exit normal is kept as small as possible, especially in pupil coordinates.

[0020] By means of the actuator system, the hollow waveguide is involved in possible displacements with multiple degrees of freedom, which can be used to set the translational and / or rotational or pivoting position of the hollow waveguide within the illumination optical unit.

[0021] The number of reflections of all individual rays of illumination light within the hollow waveguide may be less than a maximum upper limit. The upper limit of the number of reflections may be 50, or other numbers of reflections. By utilizing small changes in the angle of incidence of the illumination beam at the entrance opening of the hollow waveguide, changes between different illumination angle distributions may be achieved through the number of reflections. In addition, such an embodiment may be implemented in a manner that optimizes the optical etendue.

[0022] The advantages of the optical system according to claim 9 correspond to those already explained above with reference to the illumination optical unit.

[0023] The pivotability of the light source according to claim 10 enables specification of the illumination angle, in particular the incident illumination angle of the illumination beam at the entrance opening of the hollow waveguide. This also allows, firstly, specification of the illumination angle distribution and / or etendue optimization.

[0024] The advantages of the mask inspection system according to claim 11 correspond to the advantages already explained above with reference to the illumination optical unit and the optical system.

[0025] The wafer inspection system may also be constructed accordingly.The inspection system may comprise an object holder for supporting the object to be inspected and mechanically coupled to the object displacement drive such that a scanning displacement of the object can be performed during illumination.

[0026] The inspection system may be a system for actinic mask inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings, in which:

[0028] Figure 1 schematically illustrates an optical system having an illumination optical unit for a mask inspection system for use with EUV illumination light;

[0029] Figure 2 is from Figure 1 A top view of the optical system as seen from the observation direction II;

[0030] Figure 3 Show according to Figure 1 The optical system, and Figure 1 In contrast, the source region of the EUV light source of the optical system has been pivoted about a pivot axis;

[0031] Figure 4 Shown from Figure 3 A view of the optical system as seen from the viewing direction IV in FIG.

[0032] Figure 5 Schematically showing the angle relationship between the illumination light beam when entering the incident opening of the hollow waveguide of the illumination optical unit and when emitting from the exit opening of the hollow waveguide;

[0033] Figure 6 shows schematically and in an idealized manner the reflection conditions in a hollow waveguide in the case where illumination light incident at exactly one incident angle undergoes an odd number of internal reflections in the hollow waveguide;

[0034] Figure 7 shows in a schematic and idealized manner the reflection conditions in a hollow waveguide in the case of a mixture of odd and even reflections associated with internal reflections in the hollow waveguide of an illumination light beam;

[0035] Figure 8 The pupil shows the illumination beam entering the Figure 6 and Figure 7 The incident angle at the incident opening;

[0036] Fig. 9 Similar to Figure 8 The illumination beam is shown from Figure 6 and Figure 7 The exit angle of the exit opening when the light is emitted;

[0037] Fig.10 Similar to Figure 8 Shown in the corresponding Figure 6 and 7 The incident angle of the illumination beam when entering in the configuration is used to produce quadrupole illumination of the illumination field;

[0038] Fig.11 Shown in accordance with Fig.10 The angular distribution of the outgoing illumination light under the condition of the incident angle, that is, the quadrupole pupil;

[0039] Fig.12 and Figure 6 , Figure 7 Similarly, the reflection conditions of the illumination light in the hollow waveguide when the number of reflections is even are shown;

[0040] Fig.13 Similar to Figure 1 Another embodiment of an optical system having an illumination optics unit for a mask inspection system for use with EUV illumination light is shown; and

[0041] Fig.14 Similar to Figure 1 Another embodiment of an optical system having an illumination optics unit for a mask inspection system for use with EUV illumination light is shown. DETAILED DESCRIPTION

[0042] The illumination optical unit 1 is a component of an optical system 2 of a mask inspection system for use with EUV illumination light 3. In the figure, the beam path of the illumination light 3 is shown by marginal rays. The illumination light 3 illuminates an illumination field 4 of the mask inspection system.

[0043] The illumination light 3 is generated by an EUV light source 5 in a source region or source volume. The EUV radiation generated by the light source 5 may have 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, for example at 13.5 nm.

[0044] The light source 5 can be embodied as a plasma light source (high harmonic EUV sources are also possible). This can be, for example, a laser plasma source (LPP; laser produced plasma) or a discharge source (DPP; discharge produced plasma). In principle, such plasma sources are known light sources for EUV projection exposure apparatuses.

[0045] In order to clarify the positional relationship, the Cartesian xyz coordinate system will be used below. Figure 1 drawing plane. Figure 1 In the figure, the y-axis extends horizontally to the right and the z-axis is Figure 1 Extends vertically upward.

[0046] The source region 6 has an approximately elliptical shape and has a maximum extension direction parallel to the y-axis, which is also referred to as the main extension direction. The main emission direction of the illumination light 3 from the source region 6 travels along the main extension direction, that is, along the longest main axis of the elliptical source region 6 in the case of elliptical approximation. The pivot drive 7 enables the source region 6 of the light source 5 to pivot around a pivot axis 8 extending parallel to the z-axis. The pivot drive 7 can be implemented as a linear drive and / or a piezoelectric drive. The pivot drive 7 may include a hexapod actuator, resulting in the source region 6 being displaceable in up to six degrees of freedom. Therefore, the source region 6 can be displaced in up to three rotational degrees of freedom and / or up to three translational degrees of freedom by means of the pivot drive 7. The typical pivot angle range of the source region 6 around the pivot axis 8 is within + / -15°, for example within + / -2°.

[0047] After the light source 5 emits the illumination light 3 , the illumination light 3 initially passes through the aperture stop 9 , which defines a beam edge of the illumination light 3 .

[0048] The aperture stop 9 can be designed to be interchangeable. For example, an aperture wheel can be provided for this purpose, which stores various aperture stop embodiments that can be used alternately in the light path of the illumination light 3. The design of such an interchangeable aperture stop can be used to specify different input apertures of the illumination light 3.

[0049] The aperture stop 9 can be designed to be interchangeable and / or adjustable and / or adjustable relative to its aperture edge. As a result, different aperture geometries of the aperture stop 9 can be realized and / or set. For example, the aperture geometry that can be specified can be a circle with a selectable diameter and / or an ellipse with a selectable ellipse size and optionally a selectable ellipse semi-axis ratio. The semi-axis ratio of such an ellipse that can be specified with the aperture stop 9 can be 2:1.

[0050] Along the aperture stop 9 the illumination light beam 3 is transmitted from the in-coupling mirror optical unit 10 to the hollow waveguide 11 of the illumination optical unit 1 .

[0051] The aperture stop 9 limits the numerical aperture of the illumination beam 3 emitted by the source volume 6 to a numerical aperture range between 0.02 and 0.3, for example, between 0.02 and 0.1 or between 0.05 and 0.08. A numerical aperture greater than 0.1, that is to say in the range between 0.1 and 0.3, specified by the aperture stop 9, enables a greater light output in the illumination beam path between the source volume 6 and the illumination field 4.

[0052] An incoherent lighting setup may be used.

[0053] The aperture stop 9 can be implemented so that it follows the movement of the hexapod actuator of the pivot drive 7. In particular, the aperture stop 9 can be coupled to the hexapod actuator. As an alternative or in addition to the aperture stop 9, an aperture-limiting stop can be arranged between the hollow waveguide 11 and the downstream optical components of the illumination optical unit 1. Such another aperture stop can also be arranged in the optical path of the illumination light 3 downstream of the hollow waveguide 11 between two downstream optical components of the illumination optical unit 1.

[0054] The input coupling mirror optical unit 10 is embodied as exactly one elliptical mirror IL1 and serves to image the source region 6 of the EUV light source 5 into the entrance opening 12 in the entrance plane 13 of the hollow waveguide 11. Therefore, a first focus of the elliptical mirror IL1 is located in the source region 6 and a second focus of the elliptical mirror IL1 is located in or in the region of the entrance opening 12. The elliptical mirror IL1 serves to focus the illumination light beam 3 into the entrance opening 12 in the entrance plane 13 of the hollow waveguide 11. The incident-side numerical aperture of the illumination light beam 3 when entering the entrance opening 12 may be in the range between 0.02 and 0.2, for example in the order of 0.15 or in the order of 0.05 or 0.1.

[0055] The incident angle α of the central principal ray of the illumination light 3 at the input coupling reflector IL1 In The range is between 70° and 75°. Figure 1 In the embodiment of the illumination optical unit 1 , the elliptical reflector IL1 represents a reflector for grazing incidence (GI).

[0056] The entrance opening 12 and the exit opening 14 are both square or rectangular, with typical dimensions ranging between 0.5 mm and 5 mm. The entrance opening 12 and the exit opening 14 (same dimensions) of the hollow waveguide 11 for the illumination light 3 have an aspect ratio in the exit plane 15 ranging between 0.25 and 4, for example between 0.5 and 2. Typical dimensions of the entrance opening 12 and the exit opening 14 of the hollow waveguide 11 are 0.75 mm×0.75 mm, 1.0 mm×2.0 mm or 1.5 mm×2.0 mm.

[0057] The inner wall of the waveguide cavity of the hollow waveguide 11 is provided with a coating, which is highly reflective for the illumination light 3, such as a ruthenium coating. The waveguide cavity is a rectangular parallelepiped, consistent with the rectangular entrance opening 12 and the exit opening 14. The typical length of the hollow waveguide 11 in the beam direction of the illumination light 3 ranges between 10 mm and 500 mm, such as between 20 mm and 500 mm, between 20 mm and 300 mm, or between 20 mm and 80 mm.

[0058] The incident angle of the illumination light 3 on the inner wall of the waveguide cavity of the hollow waveguide 11 is greater than 60°. The illumination light 3 is irradiated onto the inner wall in a grazing incidence manner.

[0059] The angle α between the longitudinal axis of the hollow waveguide 11 and the chief ray CR of the illumination light 3 incident into the incident opening 12 is CR It may or may not be 0°, for example, in the range between 0° and 1.5°, for example, between 0.25° and 0.75°, and in particular in the order of 0.5°.

[0060] The ratio of the length of the hollow waveguide 11 (i.e. the distance between the incident plane 13 and the exit plane 15) to the typical diameter of the hollow waveguide 11 (i.e. the typical size or typical diameter of the incident opening 12 or the exit opening 14) ranges between 10 and 1000, and can be, for example, between 10 and 500, between 30 and 500, between 30 and 300, or between 30 and 80, or between 200 and 500.

[0061] An imaging output coupling mirror optical unit 16 is located downstream of the hollow waveguide 11 and has two mirrors IL2, IL3, which images the exit opening 14 of the hollow waveguide 11 located on the exit plane 15 into the illumination field 4 located on the object plane 17. The image side numerical aperture of the imaging can range between 0.1 and 0.3.

[0062] The two mirrors IL2, IL3 of the outcoupling mirror optical unit 16 are designed as mirrors for grazing incidence of the illumination light 3. The average angle of incidence α1 of the mirror 14 and the average angle of incidence α2 of the mirror 15 are each greater than 60°. In the case of the illumination optical unit 1, the sum of the two average angles of incidence α=α1+α2 is approximately 150°.

[0063] In the embodiment shown, the output coupling mirror optical unit 16 has exactly two mirrors for grazing incidence, namely mirrors IL2 and IL3. The aperture stop downstream of the hollow waveguide 11 optionally used above can be arranged between the hollow waveguide 11 and the mirror IL2 or between the mirrors IL2 and IL3.

[0064] The output coupling reflector optical unit 16 is implemented in the form of a Walter telescope, that is, in the form of a Walter I optical unit. Such a Walter optical unit is described in JD Mangus, JH Underwood, "Optical Design of Grazing X-ray Telescopes Applied Optics, Vol. 8, 1969, p. 95" and in the references cited therein. In such a Walter optical unit, a hyperboloid can also be used instead of a parabola. Such a combination of an elliptical reflector and a hyperboloid reflector also represents a Walter I optical unit.

[0065] An exemplary embodiment of the output coupling reflector optical unit 16 is disclosed in US Pat. No. 10,042,248 B2. Alternatively, the reflector of the output coupling reflector optical unit 16 may also include a reflective surface in the form of a free-form surface.

[0066] A reticle 18 to be inspected, supported by a reticle holder 19, is arranged in an 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 in the object displacement direction y during mask inspection. In this way, the reticle 18 can be displaced in a scanning manner in the object plane 17.

[0067] The illumination field 4 in the object plane 17 has a typical size of less than 1 mm and may be less than 0.5 mm. In the embodiment shown, the extent of the illumination field 4 is 0.5 mm in the x-direction and 0.5 mm in the y-direction.

[0068] The x / y aspect ratio of the illumination field 4 can correspond to the x / y aspect ratio of the exit opening 14 .

[0069] use Figure 1 The projection optical unit (not shown) images the illuminated field 4 into an image field in an image plane.

[0070] The image field is detected by a detection device, such as a CCD camera or a plurality of CCD cameras. For details on imaging into the image field, reference is made to US 10,042,248 B2 and the references specified therein and in US 10,042,248 B2.

[0071] For example, the structures on the reticle 18 may be inspected with the aid of a mask inspection system.

[0072] The imaging factor β1 of the input coupling mirror optical unit 10 may range between 0.1 and 50, i.e. its effect may vary from a reduction of 10 times to an enlargement of 50 times. The imaging factor β2 of the output coupling mirror optical unit 16 may range between 0.02 and 10, i.e. its effect may vary from a reduction of 50 times to an enlargement of 10 times. In the case of the illumination optical unit 1, the product of the two imaging factors β1, β2 may range between 0.25 and 10.

[0073] Figure 2 1 shows a plan view of an optical system 2 having an illumination optical unit 1. Figure 2 The incident plane 13 is clearly shown.

[0074] In order to change the incident angle α of the principal ray of the illumination light 3 at the incident opening 12 CR , that is, the angle of the chief ray CR of the illumination light 3 relative to the longitudinal axis of the hollow waveguide 11, the source region 6 of the light source 5 is pivoted about the pivot axis 8 by means of the pivot drive 7. By Figure 1 and Figure 2 and represents the situation after the pivot Figure 3 and Figure 4 Comparison is made to show the effect of this pivot. Figure 2 Compared with the original main ray direction, it can be specifically obtained from Figure 4 Due to the imaging effect of the reflector IL1, when the chief ray of the illumination beam 3 enters the incident opening 12 in the incident plane 13, this tilt is converted into a corresponding change in the chief ray angle.

[0075] Figure 5 Schematically showing a non-zero chief ray angle α of the chief ray CR of the illumination light beam 3 entering the entrance opening 12 of the hollow waveguide 11 relative to the longitudinal axis L of the hollow waveguide 11 CR Due to the angle α CR The main light ray CR and other individual light rays of the illumination light beam 3 are reflected at least once at the inner wall of the waveguide cavity of the hollow waveguide 11. This causes the angular distribution of the illumination light beam emitted from the emission opening 14 to be affected. Figure 5The emission angle distribution is schematically shown in FIG. 1 based on a plurality of individual light rays 21 of the illumination beam 3 .

[0076] In order to define a monopole type illumination angle distribution, the incident illumination light 3 is incident along the longitudinal axis L (α CR =0). In this case, a variant in which the illumination angle distribution of the incident illumination light 3 is symmetrical about the longitudinal axis L is preferred. The illumination beam 3 emitted from the hollow waveguide 11 then has a corresponding illumination angle distribution, which is centered on the longitudinal axis L and corresponds to the angular distribution of the incident illumination beam 3 in terms of its angle variation. Due to reflection at the inner wall of the hollow waveguide 11, the illumination angle of the outgoing illumination beam 3 is redistributed within the illumination angle variation of the incident illumination beam 3, but no new illumination angle occurs. This redistribution can homogenize the intensity distribution within the illumination angle of the illumination beam 3.

[0077] the following Figure 6 and Figure 7 The reflection conditions in the hollow waveguide 11 are shown for a single incident illumination angle. In a practically relevant case, the incident light 3 has not only one incident illumination angle, but a plurality of incident illumination angles.

[0078] Figure 6 Show that at exactly one non-zero chief ray angle α CR In the idealized version of the case of the "odd-number reflection" reflection case, the illumination light beam 3 emitted from the exit opening 14 has exactly an exit angle α after emitting from the exit opening 14. out In this case, the incident angle α of the principal ray at the entrance opening 12 is CR Only in the yz plane, that is, in Figure 6 In the drawing plane. Figure 6 In the projection on the xy plane, the incident principal ray of the illumination beam 3 travels parallel to the longitudinal axis L of the hollow waveguide 11. Figure 6 and the corresponding Figure 7 and Fig.12 In the figure, the hollow waveguide 11 is not drawn to scale, so the angle of incidence α in each case is CR The reflection of the illumination beam 3 at the inner wall of the waveguide cavity of the hollow waveguide 11 is achieved by an odd number of reflections of the illumination beam 3 at the inner wall of the waveguide cavity of the hollow waveguide 11. Figure 6 In this case, all individual rays of the illumination beam 3 undergo an odd number of reflections. This number of reflections is usually greater than 1, so that in this respect it should be understood schematically. Figure 6 In practice, the number of odd-order reflections may be much higher, such as on the order of fifty, or even on the order of a greater odd number.

[0079] Figure 7shows a further idealized incident radiation angle and reflection configuration, where, according to Figure 6 Compared with the configuration of CR For example, at a greater angle of incidence α CR After the entire illumination light beam 3 is reflected for the first time at the inner wall of the hollow waveguide 11, half of the entire cross-section of the illumination light beam 3 is reflected for the second time at the opposite inner wall of the hollow waveguide 11, and is emitted from the exit opening 14 as an illumination light component beam 31. The remaining portion of the incident illumination light beam 3 is no longer reflected after the first reflection, and leaves the exit opening 14 as an illumination light component beam 32. In this way, the illumination light beam is partially reflected for an odd number of times and partially reflected for an even number of times at the inner wall of the waveguide cavity of the hollow waveguide 11, thereby forming a dipole illumination angle distribution. According to the above description of Figure 6 The actual number of reflections is usually greater than that explained in the context of Figure 7 The number of reflections in the schematic diagram.

[0080] Fig.12 shows a further idealized incident and reflected configuration, where Figure 7 In comparison, the incident angle α of the main ray CR of the illumination beam 3 relative to the longitudinal axis L of the hollow waveguide 11 is CR In the yz plane, the entire illumination beam 3 now undergoes exactly two reflections at the opposite inner walls of the hollow waveguide 11, with the result that Fig.12 In the ideal case shown, after emitting from the emission opening 14, the entire illumination light beam 3 has exactly one illumination emission angle α out .

[0081] If a plurality or several other illumination angles of the incident illumination beam 3 exist at non-zero chief ray incidence angle α CR Around, there is a basis Figure 6 , 7 and 12 , and this can be used to generate dipole and multipole illumination angle distributions of the outgoing illumination beam 3 .

[0082] Figure 8 The pupil representation (pupil coordinates σ x , σ z Corresponding to spatial coordinates x and z). There is a bounded continuum of illumination angles at the entrance opening. This illumination angle continuum is offset from the center relative to the pupil coordinates, resulting in α CR ≠0 applies. If Figure 8 If the incident illumination angle continuum depicted in is arranged at the center with σx = 0 and σz = 0, then a situation will occur where a quasi-monopole illumination angle distribution is produced as described above. The variant actually described has α CR≠0.

[0083] Fig. 9 The illumination light beam 3 is shown as it is emitted from the emission opening 14 according to Figure 7 There is now a bounded, dipole-like continuum of illumination angles, that is, a dipole-like illumination angle distribution. In the case of an even number of reflections, the outgoing illumination beam has an illumination angle distribution corresponding to illumination light component beam 31. In the case of an odd number of reflections, there is an illumination angle distribution corresponding to illumination light component beam 32. Thus, overall Fig. 9 As shown, this results in an illumination angle distribution of the outgoing illumination light 3 in the form of a dipole.

[0084] Fig.10 and 11 The incident radiation and reflection angle configuration is shown, wherein the chief ray CR of the illumination beam 3 has a non-zero incident radiation angle α relative to the longitudinal axis L of the hollow waveguide 11 in both the projection on the yz plane and the projection on the xy plane. CR This results in the cross-sectional components of the incident illumination beam 3 in the yz plane (e.g. Figure 7 as shown) and split in the xy plane perpendicular to the yz plane.

[0085] Fig.10 Shown with Figure 8 The corresponding pupil representation shows this incident angle; Fig.10 In the case of , the main light ray 3 incident into the entrance opening 12 enters the entrance opening 12 at a non-zero angle relative to the longitudinal axis L of the hollow waveguide 11, both relative to the yz plane and relative to the xy plane. After exiting from the exit opening 14, it is split into two illumination light component beams in each case in the yz and xy planes, that is, with the splitting into a total of four illumination light component beams 31, 32, 33 and 34, a quadrupole-like illumination angle distribution of the illumination light beam 3 occurs, as shown in Fig.11 The pupil representation is shown.

[0086] Since the source region 6 is tilted about the pivot axis 8 and about another pivot axis, in particular another pivot axis arranged perpendicular thereto, the source region 6 is tilted from α CR = 0, according to the single-pole illumination angle distribution Figure 6 , 7 , 12 and 9 generate dipole-like illumination angle distributions and according to Fig.11 Quadrupole-like illumination angle distributions are generated, each of which can then be used to illuminate the illumination field 4 .

[0087] The following is based on Fig.13 An alternative input coupling mirror optical unit 22 which can be used instead of the input coupling mirror optical unit 10 is explained. Figures 1 to 12Components and functions explained with the same reference numerals will not be discussed again in detail.

[0088] according to Fig.13 The incoupling mirror optical unit 22 comprises precisely two mirrors IL1a, IL1b, which together form a Walter I-type optical unit, as explained above in the description of the outcoupling mirror optical unit 16. The two mirrors IL1a, IL1b are designed, for example, as elliptical mirrors and hyperbolic mirrors.

[0089] In accordance with Fig.13 In the case of an illumination optical unit 1 of FIG. 1 , it is suitable for all reflectors IL1a , IL1b as well as for reflectors IL2 , IL3 that the angle of incidence of the illumination light beam is in each case significantly greater than 60°.

[0090] Fig.14 An alternative illumination optical unit 1 for a mask inspection system is shown, which can be used instead of the illumination optical unit described above. Figures 1 to 13 Components and functions explained with the same reference numerals will not be discussed again in detail. Fig.14 The beam path of the illumination light 3 is shown starting from the intermediate focus IF, into which the source region 6 is transmitted by means of suitable focusing optics.

[0091] according to Fig.14 The incoupling mirror optical unit 23 of the illumination optical unit 1 has exactly one mirror IL1 which is designed as an NI mirror and on which the illumination light 3 impinges at an angle of incidence of less than 30°. The mirror IL1 has a coating made of an alternating molybdenum / silicon double layer which is highly reflective for the illumination light 3.

[0092] according to Fig.14 The output coupling mirror optical unit 24 of the illumination optical unit 1, which can be used to replace the output coupling mirror optical unit 16, comprises two mirrors IL2 and IL3 as GI mirrors, on which the illumination light 3 is then irradiated at grazing incidence. In the case of the output coupling mirror optical unit 24, the incident angle of the central principal ray of the illumination light 3 at the object plane 17 of the object field 4 is slightly greater than in the case of the output coupling mirror optical unit 16. In the case of the output coupling mirror optical unit 24, the incident angle is also less than 10°.

[0093] The two mirrors IL2 , IL3 of the outcoupling mirror optical unit 24 also have a ruthenium coating, which is a highly reflective coating for the illumination light 3 .

[0094] The NI mirrors of the illumination optical unit, in particular as a component of the input coupling mirror optical unit, for example the mirror IL1, enable a significant suppression of wavelength components which are carried with the illumination light 3 but which differ from the used light wavelength of the illumination light. The coating on the NI mirror can reflect the used light wavelength to a large extent and can therefore act as a bandpass filter for the used light wavelength and can reflectively block other wavelengths, for example the pump light wavelength used to generate the source plasma in the source volume 6.

[0095] The corresponding reflectivity coating can be in the form of a multilayer coating, which is realized by alternating double layers made of molybdenum and silicon. The wavelength range of light used is between 5nm and 30nm and can pass through such a coating, with a bandwidth of, for example, 2nm and a maximum reflectivity of, for example, 60%. In the surrounding environment of the specific wavelength range of light used, the suppression of such a multilayer coating can be better than 1x10 -3 , can be better than 1x10 -4 , and can be better than 1x10 -5 .

[0096] The two mirrors IL2, IL3 of the output coupling mirror optical unit 24 may have a reflective surface, which may be described as a free-form surface. Such a free-form surface may be parameterized as follows:

[0097]

[0098] Here, z is the corresponding sag of the reflective surface to be described, x and y are the Cartesian coordinates of the respective surface reference coordinate system used, R is the radius of curvature corresponding to the usual aspheric equation, and k is the conic constant corresponding to the usual aspheric equation. The free-form surface equation is supplemented by polynomial expansions of powers of x and y. Each exponent of the x and y power expansion has a specified coefficient a for i, k ik .

[0099] Using the optimization algorithm, the following optimization can be performed starting from the original aspheric shape: The polynomial coefficient a ik , radius R, cone constant k and the basic positions of the mirrors IL2 and IL3, in particular their distances from upstream and downstream components of the output coupling mirror optical unit 24, in such a way that residual aberrations are minimized during an adaptation of the illumination intensity distribution and / or the illumination angle distribution of the illumination light 3 on the illumination field 4 to the requirements of a downstream imaging optical unit for imaging the object field 4 into the image field of the mask inspection system.

[0100] Depending on the pivotability of the light source 5, the hollow waveguide 11 can also be implemented to be pivotable around at least one pivot axis by a corresponding pivot actuator. The hollow waveguide pivot axis can be located in the incident plane 13 of the incident opening 12. The pivot drive explained above with reference to the pivot drive 7 of the light source 5 can be used for design.

Claims

1. An illumination optical unit (1) of a mask inspection system for use with EUV illumination light (3), - comprising a hollow waveguide (11) for guiding the illumination light (3) and having an entrance opening (12) for the illumination light (3), the entrance opening designating an entrance plane (13) of the hollow waveguide (11), and having an exit opening (14) for the illumination light (3), The exit opening specifies an exit plane (15) of the hollow waveguide (11), - comprising an input coupling mirror optical unit (10; 22) which is arranged upstream of the hollow waveguide (11) in the optical path of the illumination light (3) and has at least one mirror (IL1; IL1a, IL1b) for imaging a source region (6) of an EUV light source (5) into the entrance opening (12) of the hollow waveguide (11), - comprises an output coupling mirror optical unit (16) which images the exit opening (14) of the hollow waveguide (12) into the illumination field (4).

2. The illumination optical unit according to claim 1, wherein: The incoupling mirror optical unit (10) is implemented as an elliptical mirror (IL1).

3. The illumination optical unit according to claim 1 or 2, characterized in that: The angle (α) between the normal line of the incident plane (13) of the incident opening (12) and the incident principal ray of the light beam of the illumination light (3) is CR ) is arranged between 0° and 5°.

4. The illumination optical unit according to any one of claims 1 to 3, characterized in that: The incident angle (α) of the main ray of the light beam of the illumination light (3) on at least one reflector of the input coupling reflector optical unit (10; 22) is In ) ranges from 70° to 89.9°.

5. The illumination optical unit according to claim 1, 3 or 4, characterized in that: The incoupling mirror optical unit (22) is implemented as a combination of an elliptical mirror and a hyperbolic mirror.

6. The illumination optical unit according to any one of claims 1 to 5, characterized in that: The entrance opening (12) of the hollow waveguide (11) has a rectangular embodiment.

7. The illumination optical unit according to any one of claims 1 to 6, characterized in that: At least one of the components of the illumination optical unit (1) is embodied so as to be pivotable about at least one pivot axis.

8. The illumination optical unit according to claim 7, wherein: The pivotable component of the illumination optical unit (1) is the hollow waveguide (11).

9. An optical system (2) comprising the illumination optical unit (1) according to any one of claims 1 to 8, and comprising an EUV light source (5) for the illumination light (3).

10. The optical system according to claim 9, characterized in that The light source (5) is embodied so as to be pivotable about at least one axis (8) which passes through a source region (6) of the light source (5).

11. A mask inspection system - comprising an optical system according to claim 9 or 10, - comprises a projection optical unit for imaging the illumination field (4) into an image field, and - comprising detection means for detecting illumination light (3) incident on the image field.

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