Illumination optical unit for a mask inspection system using EUV illumination light

Abstract

The illumination optical unit (1) is part of a mask inspection system for use with EUV illumination light (3). The hollow waveguide (11) serves to guide the illumination light (3). In the case of the illumination light (3), the hollow waveguide (11) has an entrance aperture (12) at the entrance surface (13) and an exit aperture (14) at the exit surface (14). An input coupling mirror optical unit (10) is disposed 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 aperture (12) of the hollow waveguide (11). The output coupling mirror optical unit (16) serves to image the exit aperture (14) of the hollow waveguide (11) into the illumination field (4). Thereby, an illumination optical unit with optimized utilization efficiency of the EUV illumination light is obtained.

Description

Technical Field

[0001] The contents of German Patent Application No. 102022205767.8, German Patent Application No. 102023110173.0, and German Patent Application No. 102023110174.9 are incorporated herein by reference.

[0002] The present invention relates to an illumination optical unit for a mask inspection system for use with EUV illumination light. Furthermore, the present invention relates to 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 Patent No. 10,042,248, German Patent No. 10220815, and WO2012 / 101269.

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to develop an illumination optical unit for such an inspection system such that the utilization efficiency for EUV illumination light is optimized.

Means for Solving the Problems

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

[0006]

[0007] It has been recognized that an input coupling mirror optical unit for imaging an EUV light source with which the illumination optical unit interacts into the entrance aperture of a hollow waveguide provides a high input coupling efficiency. An optical unit known from US Patent No. 10,042,248 can be used as an output coupling mirror optical unit.The input coupling mirror optical unit can comprise at least one mirror for grazing incidence (GI) embodied for an angle of incidence of the illumination light of more than 45°. Alternatively or in addition, the input coupling mirror optical unit can comprise at least one mirror for normal incidence (NI) embodied for an angle of incidence of the illumination light of less than 45°, in particular less than 30°.

[0008] The input coupling mirror optical unit can comprise exactly one mirror or, if not, a plurality of mirrors, for example two mirrors.

[0009] The arrangement of the input coupling mirror optical unit can be such that the geometric centroid ray of the rays of the illumination light beam impinges on the incident surface of the incident aperture at an angle of less than 2°, less than 1.5°, less than 1°, and in some cases of about 0.5° with respect to the normal of this incident surface. In particular, the geometric centroid can impinge on the incident aperture of the hollow waveguide in a manner orthogonal to the incident surface. Alternatively, the angle conditions described above can be applied to the principal ray of the illumination light beam. 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 illuminated homogeneously and / or symmetrically.

[0010] Depending on the angle of incidence and / or the position of the incident principal ray of the illumination light beam or the geometric centroid ray of the peripheral rays of the illumination light beam on the reflective inner wall of the hollow waveguide of the incident surface, as a result, a monopolar, dipolar, or multipolar, for example quadrupolar, illumination angle distribution of the irradiation field can be obtained.

[0011] The mirrors of the illumination optical unit, that is, in particular the input coupling mirror optical unit and / or the output coupling mirror optical unit, can comprise free-form reflecting surfaces.

[0012] The embodiment of the input coupling mirror optical unit according to claim 2 as an elliptical mirror enables an input coupling mirror optical unit having exactly one reflection between the source region of the light source and the incident aperture of the hollow waveguide, and enables a high EUV throughput of the input coupling mirror optical unit.

[0013] The adjustability of the incident angle according to claim 3 enables an embodiment of an illumination optical unit that enables setting a defined illumination angle distribution for illuminating an illumination field. Depending on the selection of the incident angle of the illumination light beam with respect to the incident surface of the incident aperture, it is possible to obtain, for example, a monopolar, bipolar, or multipolar, for example quadrupolar, illumination angle distribution of the illumination field.

[0014] The incident angle according to claim 4 enables a high overall transmittance of the input coupling mirror optical unit. Alternatively or in addition, the incident angle may be less than 45°, for example 0° to 45°, for example 0° to 30°.

[0015] The input coupling mirror optical unit according to claim 5 enables accurately imaging the source region of the EUV light source within the incident aperture having an accurately specifiable imaging factor. The input coupling mirror optical unit can be embodied as a Wolter type mirror optical unit, particularly a Wolter type I mirror optical unit. A combination of a hyperbolic mirror and a parabolic mirror for the input coupling mirror optical unit is also possible. For example, it is possible to use the design principle as described in U.S. Patent No. 10,042,248. Alternatively or in addition, a mirror having a free-form reflecting surface may be used.

[0016] The rectangular entrance aperture according to claim 6 has been found to be particularly suitable for specifying defined illumination conditions and can be manufactured precisely. It can also have an embodiment with a square entrance aperture. The edge length of the boundary edge of the entrance aperture can be less than 2 mm, particularly less than 1 mm. The ratio of the length of the hollow waveguide to the general diameter of the entrance aperture, for example, the edge length of a rectangle, can be greater than 10, can be greater than 30, can be in the range of 40 to 80, can be greater than 100, can be greater than 200, and can be in the range of 200 to 300. Such a length / diameter ratio is usually less than 1000.

[0017] The rotatable embodiment of the illumination optical unit component according to claim 7 facilitates, in particular, the adjustment of the illumination optical unit, i.e., the illumination setting of the illumination optical unit, for the purpose of specifying the illumination angle distribution. By means of a properly embodied actuator system, at least one component of the illumination optical unit embodied to be rotatable about at least one axis of rotation can be displaced in relation to two or more rotational and / or translational degrees of freedom, for example, in relation to 2, 3, 4, 5, or 6 degrees of freedom.

[0018] The rotatable component of the illumination optical unit can be a hollow waveguide. The axis of rotation of the hollow waveguide can be located on the entrance surface of the entrance aperture. The rotational adjustment of the hollow waveguide can be used for the étendue optimization of the illumination optical unit.

[0019] For example, the term étendue is explained in the document "Non-imaging Optics" by Benitez, P. G., Minano, J. C., Winston, R., Narkis Shatz and John C. Bortz, W. c. b. (2005). The étendue optimization can be carried out such that the étendue when exiting the hollow waveguide is as low as possible, i.e., particularly as low as possible at the pupil coordinates, and the angular diameter of the illumination light beam exiting the hollow waveguide is kept as small as possible with respect to the exit normal.

[0020] The possible displacement possibilities of the hollow waveguide for multiple degrees of freedom can be used to set the translational and / or rotational or pivoting positions of the hollow waveguide within the illumination optical unit.

[0021] The number of reflections of all individual light rays of the illumination light within the hollow waveguide can be made less than a maximum upper limit. This upper limit of the number of reflections can be set to 50. Other numbers of reflections are also possible. The change between different illumination angle distributions due to a small change in the incident angle of the illumination light beam at the incident aperture of the hollow waveguide can be enabled by multiple reflections. Furthermore, such an embodiment can be implemented in an etendue-optimized form.

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

[0023] The pivotability of the light source according to claim 10 enables the specification of the illumination angle of the illumination light beam, particularly the incident illumination angle, at the incident aperture of the hollow waveguide. Thereby, firstly, it also becomes possible to specify the illumination angle distribution and / or to perform etendue optimization.

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

[0025] Correspondingly, a wafer inspection system can also be constructed. The inspection system includes an object holder, and the object holder mechanically couples to an object displacement drive so as to hold the object to be inspected and enable scan displacement of the object during illumination.

[0026] The inspection system can be a system for optical mask inspection.

[0027] Exemplary embodiments of the present invention will be described in more detail below with reference to the drawings.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

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Figure 14

DETAILED DESCRIPTION OF THE INVENTION

[0029] The illumination optical unit 1 is a component of the 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 indicated by the peripheral rays. The illumination field 4 of the mask inspection system is illuminated by the illumination light 3.

[0030] The illumination light 3 is generated by an EUV light source 5 in a source region or source volume. The light source 5 can generate EUV radiation used in the wavelength range of 2 nm to 30 nm, for example, in the range of 2.3 nm to 4.4 nm, or in the range of 5 nm to 30 nm, for example, at 13.5 nm.

[0031] The light source 5 can be embodied as a plasma light source (a harmonic EUV light source is also possible). By way of example, this can relate to a laser-produced plasma (LPP) or a discharge-produced plasma (DPP). In principle, such plasma light sources are light sources known for EUV projection exposure apparatuses.

[0032] For the sake of clarity of the positional relationship, a Cartesian xyz coordinate system will be used hereinafter. The x-axis is perpendicular to the plane of the figure in FIG. 1. The y-axis extends horizontally to the right in FIG. 1, and the z-axis extends vertically upward in FIG. 1.

[0033] The source region 6 has a generally elliptical shape and has a maximum extension, also called the main direction of extension, parallel to the y-axis. The main emission direction of the illumination light 3 from the source region 6 extends along this main direction of extension, i.e., when approximating the ellipse, it extends along the longest major axis of the elliptical source region 6. The swivel drive unit 7 enables the source region 6 of the light source 5 to be swiveled around a swivel axis 8 extending parallel to the z-axis. The swivel drive unit 7 can be embodied as a linear drive unit and / or a piezoelectric drive unit. The swivel drive unit 7 can comprise a hexapod actuator, and as a result, a displacement of the source region 6 with up to six degrees of freedom is possible. For this reason, the source region 6 can be displaced with up to three rotational degrees of freedom and / or up to three translational degrees of freedom with the assistance of the swivel drive unit 7. The general swivel angle of the source region 6 around the swivel axis 8 is in the range of + / - 15°, for example in the range of + / - 2°.

[0034] Subsequent to the emission by the light source 5, the illumination light 3 first passes through an aperture stop 9 that defines the edge of the beam of the illumination light 3.

[0035] The aperture stop 9 can be designed to be exchangeable. For example, a diaphragm wheel can be provided for this purpose, and the diaphragm wheel stores various aperture stop embodiments that can be used alternately within the beam path of the illumination light 3. With such an exchangeable aperture stop design, various input apertures of the illumination light 3 can be specified.

[0036] The aperture stop 9 can be embodied to be exchangeable and / or adjustable and / or adjustable with respect to the diaphragm edge. As a result, various diaphragm shapes of the aperture stop 9 can be realized and / or set. As an example, a specifiable diaphragm shape can be circular with a selectable diameter and / or elliptical with a selectable ellipse size and optionally a selectable semi-axis ratio of the ellipse. Such a semi-axis ratio of the ellipse specifiable by the aperture stop 9 can be 2:1.

[0037] Subsequent to the aperture stop 9, the illumination light beam 3 is transmitted from the input coupling mirror optical unit 10 to the hollow waveguide 11 of the illumination optical unit 1.

[0038] The aperture stop 9 restricts the numerical aperture of the illumination light beam 3 emitted by the source region 6 to a numerical aperture value that ranges from 0.02 to 0.3, for example, ranges from 0.02 to 0.1 or from 0.05 to 0.08. The numerical aperture specified by the aperture stop 9 that is greater than 0.1, i.e., ranges from 0.1 to 0.3, enables a higher light collection efficiency in the illumination light beam path between the source volume 6 and the irradiation field 4.

[0039] An incoherent illumination setting can be used.

[0040] The aperture stop 9 can be embodied to follow the movement of the hexapod actuator of the swivel drive unit 7. In particular, the aperture stop 9 can be coupled to the hexapod actuator. As an alternative to or in addition to the aperture stop 9, an aperture limiting stop can be arranged between the hollow waveguide 11 and the optical components downstream of the illumination optical unit 1. Such an additional aperture stop can also be arranged in the beam path of the illumination light 3 downstream of the hollow waveguide 11 between two downstream optical components of the illumination optical unit 1.

[0041] 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 aperture 12 within the entrance surface 13 of the hollow waveguide 11. Accordingly, the first focus of the elliptical mirror IL1 is located within the source region 6, and the second focus of the elliptical mirror IL1 is located at or within the region of the entrance aperture 12. The elliptical mirror IL1 is used to converge the illumination light beam 3 onto the entrance aperture 12 of the hollow waveguide 11. The numerical aperture on the incident side of the illumination light beam 3 upon incidence on the entrance aperture 12 can range from 0.02 to 0.2, for example, be about 0.15 or about 0.05 or 0.1.

[0042] The incident angle α of the central principal ray of the illumination light beam 3 on the input coupling mirror IL1 In falls within the range of 70° to 75°. In the embodiment of the illumination optical unit 1 according to FIG. 1, the elliptical mirror IL1 represents a mirror for grazing incidence (GI).

[0043] The incident aperture 12 and the exit aperture 14 are each square or rectangular, and the general dimensions range from 0.5 mm to 5 mm. The aspect ratio of the equally sized incident aperture 12 and the exit aperture 14 of the hollow waveguide 11 for the illumination light 3 on the exit surface 15 ranges from 0.25 to 4, for example from 0.5 to 2. The general dimensions of the incident aperture 12 and the exit aperture 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.

[0044] The inner wall of the waveguide cavity of the hollow waveguide 11 is provided with a highly reflective coating for the illumination light 3, for example a ruthenium coating. The waveguide cavity is a cuboid according to the rectangular incident and exit apertures 12, 14. The hollow waveguide 11 has a general length in the beam direction of the illumination light 3 that ranges from 10 to 500 mm, for example from 20 mm to 500 mm, from 20 mm to 300 mm, or otherwise from 20 mm to 80 mm.

[0045] The incident angle of the illumination light 3 with respect to the inner wall of the waveguide cavity of the hollow waveguide 11 is greater than 60°. The illumination light 3 strikes the inner wall at a grazing incidence.

[0046] The angle α between the longitudinal axis of the hollow waveguide 11 and the principal ray CR of the illumination light beam 3 incident on the incident aperture 12 CR can be 0°, or alternatively, it may be different from 0°, for example, it may range from 0° to 1.5°, for example from 0.25° to 0.75°, and in particular may be about 0.5°.

[0047] The ratio of the length of the hollow waveguide 11, i.e., the distance between the incident surface 13 and the exit surface 15, and the general diameter of the hollow waveguide 11, i.e., the general size or general diameter of the incident aperture or exit aperture 12, 14, ranges from 10 to 1000, for example, 10 to 500, 30 to 500, 30 to 300, or otherwise, 30 to 80 or 200 to 500.

[0048] The imaging output coupling mirror optical unit 16, which is arranged downstream of the hollow waveguide 11 and has two mirrors IL2, IL3, images (image) the exit aperture 14 located on the exit surface 15 of the hollow waveguide 11 into the irradiation field 4 on the object surface 17. This imaging can have an image-side numerical aperture ranging from 0.1 to 0.3.

[0049] The two mirrors IL2, IL3 of the output coupling mirror optical unit 16 are realized as mirrors for the oblique incidence of the illumination light 3. The average incident angle α1 of the mirror 14 and the average incident angle α2 of the mirror 15 are each greater than 60° in each case. In the case of the illumination optical unit 1, the sum α = α1 + α2 of these two average incident angles is approximately 150°.

[0050] In the illustrated embodiment, the output coupling mirror optical unit 16 has exactly two mirrors for oblique incidence, i.e., the mirrors IL2 and IL3. The optionally used aperture stop downstream of the hollow waveguide 11, as described above, can be arranged between the hollow waveguide 11 and the mirror IL2, or otherwise, between the mirrors IL2 and IL3.

[0051] The output coupling mirror optical unit 16 is embodied in the form of a Wolter telescope, i.e., in the form of a type I Wolter optical unit. Such Wolter optical 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 optical units, hyperboloids can also be used instead of paraboloids. Such a combination of an elliptical mirror and a hyperboloidal mirror also represents a type I Wolter optical unit.

[0052] An exemplary embodiment of the output coupling mirror optical unit 16 is described in U.S. Patent No. 10,042,248. Alternatively, the mirrors of the output coupling mirror optical unit 16 may also include reflective surfaces in the form of freeform surfaces.

[0053] The reticle 18 to be inspected, held by the reticle holder 19, is disposed on the object plane 17. The reticle holder 19 is mechanically operably connected to the reticle displacement drive unit 20, whereby the reticle 18 is displaced in the object displacement direction y during mask inspection. In this way, the scan displacement of the reticle 18 on the object plane 17 is enabled.

[0054] The irradiation field 4 on the object plane 17 has typical dimensions of less than 1 mm and may be less than 0.5 mm. In the illustrated embodiment, the extension of the irradiation field 4 is 0.5 mm in the x direction and 0.5 mm in the y direction.

[0055] The x / y aspect ratio of the irradiation field 4 can correspond to the x / y aspect ratio of the exit aperture 14.

[0056] Using a projection optical unit not shown in FIG. 1, the irradiation field 4 is imaged (image) within the image field in the image plane.

[0057] The image field is detected by a detection device, for example, one or more CCD cameras. For details of imaging on the image field, reference is made to U.S. Patent No. 10,042,248, as well as the references specified in this specification and U.S. Patent No. 10,042,248.

[0058] For example, inspection of the structure on the reticle 18 is possible by a mask inspection system.

[0059] The imaging coefficient β of the input coupling mirror optical unit 10 1 can range from 0.1 to 50, that is, its effect can vary from one-tenth to 50 times. The imaging coefficient β2 of the output coupling mirror optical unit 16 can range from 0.02 to 10, that is, its effect can then vary from one-fiftieth to 10 times. In the case of the illumination optical unit 1, the product of the two imaging coefficients β 1 β 2 can range from 0.25 to 10.

[0060] Figure 2 shows a plan view of the optical system 2 having the illumination optical unit 1. The incident surface 13 is emphasized in Figure 2.

[0061] The chief ray incident angle α of the illumination light beam 3 at the incident aperture 12 CR , that is, the angle of the chief ray CR of the illumination light beam 3 with respect to the longitudinal axis of the hollow waveguide 11, is varied by causing the source region 6 of the light source 5 to rotate around the rotation axis 8 with the assistance of the rotation drive unit 7. The influence of this rotation is shown by comparing Figures 1 and 2 representing the situation before rotation with Figures 3 and 4 representing the situation after rotation. The influence of the corresponding inclination of the chief ray direction passing through the angle α in the xy plane compared to the original chief ray direction according to Figure 2 can be particularly collected from the plan view according to Figure 4. As a result of the imaging effect of the mirror IL1, this inclination is converted into a corresponding change in the chief ray angle at the incidence of the chief ray of the illumination light beam 3 on the incident aperture 12 at the incident surface 13.

[0062] FIG. 5 schematically shows the effect of the non-zero principal ray angle α of the principal ray CR of the illumination light beam 3 incident on the incident aperture 12 of the hollow waveguide 11 with respect to the longitudinal axis L of the hollow waveguide 11. CR The angle α CR causes both the principal ray CR and the other individual rays of the illumination light beam 3 to be reflected at least once on the inner wall of the waveguide cavity of the hollow waveguide 11. As a result, the angular distribution within the illumination light beam after exiting from the exit aperture 14 is affected. This exit angle distribution is schematically shown in FIG. 5 based on a number of individual rays 21 of the illumination light beam 3.

[0063] To specify a unipolar illumination angle distribution, the incident illumination light 3 shines with the principal ray extending along the longitudinal axis L (α CR = 0). In this case, a deformed form in which the illumination angle distribution of the incident illumination light 3 is symmetric around the longitudinal axis L is preferable. At this time, the illumination light beam 3 emitted from the hollow waveguide 11 has a corresponding illumination angle distribution centered on the longitudinal axis L, and this illumination angle distribution corresponds to the angle distribution of the incident illumination light beam 3 from the viewpoint of angle variation. Due to reflection on the inner wall of the hollow waveguide 11, the illumination angle of the emitted illumination light beam 3 is redistributed within the illumination angle variation of the incident illumination light beam 3, but no new illumination angle is generated. This redistribution can lead to homogenization of the intensity distribution within the illumination angle of the illumination light beam 3.

[0064] The following FIGS. 6 and 7 show the reflection conditions in the hollow waveguide 11 for the case of a single incident illumination angle. In actual relevant cases, the incident light 3 has not only one incident illumination angle but many incident illumination angles.

[0065] FIG. 6 shows a variant of the reflection situation of "odd number of reflections" in the case of an idealized version with exactly one non-zero principal ray angle α CR where there is exactly one exit angle α of the illumination light beam 3 exiting from the exit aperture 14 following the exit from the exit aperture 14. In this case, the incident angle α of the principal ray at the incident aperture 12 out exists. CRlies only in the yz plane, i.e., in the plane of the figure of Fig. 6. In the projection onto the xy plane of Fig. 6, the incident principal ray of the illumination light beam 3 extends parallel to the longitudinal axis L of the hollow waveguide 11. In Fig. 6, as well as in the corresponding Figs. 7 and 12 described later, the hollow waveguide 11 is not drawn to scale, and as a result, the angle of incidence α CR is also shown in an exaggerated form in each case. The illumination angle distribution according to Fig. 6 is realized by odd-numbered reflections of the illumination light beam 3 on the inner wall of the waveguide cavity of the hollow waveguide 11. In this case, all individual rays of the illumination light beam 3 undergo odd-numbered reflections. This number of reflections is usually greater than 1, and as a result, Fig. 6 should be understood schematically from this perspective. In reality, the odd-numbered reflections can be much larger, for example, about 50 times, or even of a larger odd order.

[0066] Fig. 7 shows a more idealized angle of the incident radiation and reflection configuration, where the illumination light beam 3 is radiated at a different angle of incidence α CR than, for example, at a larger angle compared to the configuration according to Fig. 6. Following the first reflection of the entire illumination light beam 3 on the inner wall of the hollow waveguide 11, half of the entire cross-section of the illumination light beam 3 is reflected a second time on the opposite inner wall of the hollow waveguide 11 and exits from the exit aperture 14 as the illumination light component beam 3 1 . The remaining part of the incident illumination light beam 3 is not reflected again after the first reflection and exits from the exit aperture 14 as the illumination light component beam 3 2 . For this reason, on the inner wall of the waveguide cavity of the hollow waveguide 11, there are some odd-numbered reflections of the illumination light beam and some even-numbered reflections of the illumination light beam, thus resulting in a dipole-type illumination angle distribution. According to what was described above in relation to Fig. 6, the actual number of reflections is usually larger than the number of reflections in the schematic diagram according to Fig. 7.

[0067] Fig. 12 shows a further idealized incident and reflection configuration, where the angle of incidence α of the principal ray CR of the illumination light beam 3 with respect to the longitudinal axis L of the hollow waveguide 11 CRAs compared with FIG. 7, it is increased again in the yz plane. And the overall illumination light beam 3 here undergoes exactly two reflections on the opposing inner walls of the hollow waveguide 11, and as a result, following the emission from the emission aperture 14 in the idealized illustrated case of FIG. 12, there is exactly one illumination emission angle α of the overall illumination light beam 3 out exists.

[0068] Non-zero chief ray incident angle α CR As long as there are a plurality or a large number of further illumination angles of the incident illumination light beam 3 around it, there is an overlap of the reflection configurations according to FIGS. 6, 7, and 12, and using this, the dipole and multipole illumination angle distributions of the emitted illumination light beam 3 can be generated.

[0069] FIG. 8 shows the pupil representation (the pupil coordinates σ corresponding to the spatial coordinates x and z) of the illumination angle distribution at the incidence of the illumination light beam 3 on the incident aperture 12 x , σ z ). At this incidence, there is a bounded continuum of illumination angles. This illumination angle continuum is shifted from the center with respect to the pupil coordinates, and as a result, α CR ≠0 applies. If the continuum of incident illumination angles shown in FIG. 8 is centered at σ x = 0, σ z = 0, there will be a case of generating a monopole-like illumination angle distribution as described above. In the actually shown deformed form, the case α CR ≠0 exists.

[0070] FIG. 9 shows the situation according to FIG. 7 at the emission of the illumination light beam 3 from the emission aperture 14. Here, there is a bounded, dipole-like continuum of illumination angles, that is, a dipole-like illumination angle distribution. As long as there are an even number of reflections, the emitted illumination light beam has an illumination angle distribution corresponding to the illumination light component beam 3 1 . In the case of an odd number of reflections, there is an illumination angle distribution corresponding to the illumination light component beam 3 2 . Therefore, overall, as shown in FIG. 9, as a result, the illumination angle distribution of the emitted illumination light 3 becomes a dipole form.

[0071] Figures 10 and 11 show the configuration of the incident radiation and reflection angles, where, in both the projection in the yz plane and the projection in the xy plane, the non-zero incident radiation angle α of the principal ray CR of the illumination light beam 3 with respect to the longitudinal axis L of the hollow waveguide 11 CR exists. As a result, as shown in, for example, FIG. 7, splitting of the cross-sectional components of the incident illumination light beam 3 occurs in both the yz plane and the xy plane orthogonal thereto.

[0072] FIG. 10 shows this incident angle situation in the pupil representation corresponding to that of FIG. 8. In the case of FIG. 10, the principal ray 3 incident on the incident aperture 12 is incident on the incident aperture 12 at a non-zero angle with respect to the longitudinal axis L of the hollow waveguide 11 in both the yz plane and the xy plane. In each case, in a state where splitting into two illumination light component beams occurs in both the yz and xy planes, that is, a total of four illumination light component beams 3 1 、3 2 、3 3 and 3 4 have occurred, following the emission from the emission aperture 14, a quadrupole-like illumination angle distribution of the illumination light beam 3 appears in the pupil representation according to FIG. 11.

[0073] Therefore, as a result of the source region 6 tilting around the pivot axis 8 and a further pivot axis, particularly a further pivot axis arranged orthogonally to the pivot axis 8, starting from the monopole-like illumination angle distribution at α CR =0, it is possible to generate both the dipole-like illumination angle distribution according to FIGS. 6, 7, 12 and 9 and the quadrupole-like illumination angle distribution according to FIG. 11, and then each of them can be used to illuminate the irradiation field 4.

[0074] An alternative input coupling mirror optical unit 22 that can be used instead of the input coupling mirror optical unit 10 will be described hereinafter with reference to FIG. 13. The components and functions already described with reference to FIGS. 1 to 12 have the same reference numerals and will not be discussed in detail again.

[0075] As already explained above in connection with the output coupling mirror optical unit 16, the input coupling mirror optical unit 22 according to FIG. 13 comprises exactly two mirrors IL1a, IL1b which together form a type-I Wolter optical unit. The two mirrors IL1a, IL1b are embodied, for example, as an elliptical mirror and a hyperbolic mirror.

[0076] What applies to all mirrors IL1a, IL1b and mirrors IL2, IL3 in the case of the illumination optical unit 1 according to FIG. 13 is that the incident angle of the illumination light beam is significantly larger than 60° in each case.

[0077] FIG. 14 shows an alternative illumination optical unit 1 of a mask inspection system which can be used instead of the illumination optical unit described above. The components and functions already explained with reference to FIGS. 1 to 13 have the same reference numerals and will not be discussed in detail again. In FIG. 14, the beam path of the illumination light 3 is shown starting from the intermediate focus IF, to which the source region 6 is transmitted with the aid of a suitable condenser optical unit.

[0078] The input coupling mirror optical unit 23 of the illumination optical unit 1 according to FIG. 14 is designed as a NI mirror and has exactly one mirror IL1 against which the illumination light 3 impinges at an incident angle of less than 30°. The mirror IL1 has a coating made of an alternating molybdenum / silicon double layer which is highly reflective with respect to the illumination light 3.

[0079] The output coupling mirror optical unit 24 of the illumination optical unit 1 according to FIG. 14, which can be used instead of the output coupling mirror optical unit 16, comprises, as a GI mirror, two mirrors IL2 and IL3 against which the illumination light 3 impinges obliquely in succession. The incident angle of the central principal ray of the illumination light 3 on the object plane 17 of the object field 4 is slightly larger in the case of the output coupling mirror optical unit 24 than in the case of the output coupling mirror optical unit 16. In the case of the output coupling mirror optical unit 24, this incident angle is also less than 10°.

[0080] The two mirrors IL2 and IL3 of the output coupling mirror optical unit 24 also have a ruthenium coating embodied as a highly reflective coating for the illumination light 3.

[0081] In particular, the NI mirror of the illumination optical unit as a component of the input coupling mirror optical unit such as the mirror IL1 is carried together with the illumination light 3, but enables significant suppression of wavelength components different from the optical wavelength used for the illumination light. The coating on the NI mirror that highly reflects the used optical wavelength can, as a result, serve as a band-pass filter for the used optical wavelength and can block by reflection other wavelengths, for example, the pump optical wavelength for generating the source plasma in the source volume 6.

[0082] The corresponding reflective coating can be realized by alternating double layers made of molybdenum and silicon in the form of a multilayer coating. Such a coating can pass the used optical wavelength within a range of 5 nm to 30 nm with a bandwidth of, for example, 2 nm and a maximum reflectivity of, for example, 60%. Around the specified used optical wavelength range, the suppression of such a multilayer coating can be better than 1×10 -3 and may be better than 1×10 -4 and may be better than 1×10 -5 and may be even better than 1×10

[0083] The two mirrors IL2 and IL3 of the output coupling mirror optical unit 24 can have a reflective surface that can be described as a free-form surface. For example, such a free-form surface can be parameterized as follows.

Number

[0084] Using an optimization algorithm, starting from the raw aspheric shape, the polynomial coefficients aik, the radius R, the conic constant k, and the basic position of the mirrors IL2 and IL3, in particular, their distances from the upstream and downstream components of the output coupling mirror optical unit 24, are optimized such that the residual aberration is minimized during the adaptation of the illumination intensity distribution and / or the illumination angle distribution of the illumination light 3 over the illumination field 4 to the requirements of the downstream imaging optical unit for imaging the object field 4 into the image field of the mask inspection system.

[0085] According to the rotatability of the light source 5, the hollow waveguide 11 can also be embodied to be rotatable about at least one axis of rotation with the assistance of a corresponding rotation actuator. This axis of rotation of the hollow waveguide can be arranged on the incident surface 13 of the incident aperture 12. It is possible to use the rotation drive unit design described with reference to the rotation drive unit 7 of the light source 5.

Claims

1. An illumination optical unit (1) for a mask inspection system for use with EUV illumination light (3), - comprising a hollow waveguide (11) that serves to guide the illumination light (3), having an entrance aperture (12) for the illumination light (3) that defines an entrance surface (13) of the hollow waveguide (11), and an exit aperture (14) for the illumination light (3) that defines an exit surface (15) of the hollow waveguide (11), - comprising an input coupling mirror optical unit (10; 22) disposed upstream of the hollow waveguide (11) in the beam path of the illumination light (3) and having at least one mirror (IL1; IL1a, IL1b) for imaging a source region (6) of an EUV light source (5) into the entrance aperture (12) of the hollow waveguide (11), - an illumination optical unit comprising an output coupling mirror optical unit (16) for imaging the exit aperture (14) of the hollow waveguide (12) into an illumination field (4).

2. The illumination optical unit according to claim 1, characterized in that the input coupling mirror optical unit (10) is embodied as an elliptical mirror (IL1).

3. The angle (α CR ) between the normal to the incident surface (13) of the incident aperture (12) and the incident principal ray of the beam of the illumination light (3) is in the range of 0° to 5°, and the illumination optical unit according to claim 1 or 2, characterized by such a configuration.

4. The angle of incidence (α) of the principal ray of the beam of the illumination light (3) with respect to the at least one mirror of the input coupling mirror optical unit (10; 22). In ), characterized in that it takes a range of 70° to 89.9°, the illumination optical unit according to any one of claims 1 to 3.

5. The illumination optical unit according to claim 1, 3 or 4, characterized in that the input coupling mirror optical unit (22) is embodied 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 aperture (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 to be rotatable about at least one axis of rotation.

8. The illumination optical unit according to claim 7, characterized in that the rotatable 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 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 to be rotatable about at least one axis (8) passing through the source region (6) of the light source (5).

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

Images