Mirror element, lithographic system and method of providing mirror element

Abstract

A mirror element (20) having a mirror surface (26) comprising an aspheric target region (22) and an extension region (28) proximate to an edge (24) of the target region (22) wherein the edge (24) is described by a closed curve (b) that is continuously differentiable at least twice; wherein the target area (22) has a respective edge curvature at each edge point (s) located on the curve (b); wherein, starting from the edge point (s) in a contour direction transverse to the edge (24), the extension region (28) has a curvature contour which has no more than one local extremum and the absolute value of the curvature of which is less than twice the absolute value of the edge curvature. Also disclosed is a lithographic system having a mirror element (20), and a method of providing a mirror element (20).

Description

[0001] This patent application claims the priority of German Patent Application DE 10 2022 211 866.9, filed on November 9, 2022, the content of which is hereby incorporated by reference in its entirety into this application. Field of the Invention

[0002] The present invention generally relates to a mirror element having a mirror surface that includes a target region and an extended region adjacent to an edge of the target region, to a lithography system including such a mirror element, and to a method for providing such a mirror element. Background Art

[0003] Microlithography systems are used to produce integrated circuits with particularly small structures. A mask (mask blank) irradiated by extreme ultraviolet or deep ultraviolet radiation (DUV or EUV radiation) is imaged onto a lithography object to transfer the mask structure onto the lithography object.

[0004] A lithography system includes a plurality of mirrors that reflect radiation. The mirrors have a precisely defined shape and are precisely positioned such that the imaging of the mask onto the lithography object has sufficient quality.

[0005] The mirror surface of such a mirror includes a region that reflects extreme ultraviolet radiation during operation of the lithography system. In this region, which is called the target region of the mirror surface, the shape of the mirror surface is specified based on the fact that the wavefront of the radiation has a specific shape in the object plane. The manufacture of the mirror, including a polishing step, requires the mirror surface to extend beyond the edge of the target region into an extended region. Only when the entire circumference of the polishing tool can be guided beyond the edge of the target region can sufficient surface quality be achieved at the edge of the target region.

[0006] A mathematical description of the surface profile of the surface can be used for the target region. This mathematical description can be extrapolated beyond the edge of the target region, thereby defining the shape of the extended region. It has been found that this process can damage the surface quality at the edge of the target region. This is especially true for surface profiles described by high-order polynomials, where the absolute value of the curvature or local astigmatism increases with the distance from the target region, making it more difficult to achieve precise polishing and provide a mirror surface with sufficient surface quality. Summary of the Invention

[0007] In view of the above background of the problem, an object of the present invention is therefore to provide an improved mirror element, in particular, the surface quality at the edge of the target region can be avoided from being damaged.

[0008] The solution according to the present invention lies in the features of the independent claims. The dependent claims relate to advantageous developments.

[0009] According to the present invention, a mirror element having a mirror surface is disclosed, the mirror surface comprising an aspherical target region and an extension region adjacent to the edge of the target region, wherein the edge can be described by at least two continuously differentiable closed curves, wherein the target region has respective edge curvatures at each edge point located on the curve, and wherein, starting from the edge point in a direction transverse to the contour of the edge, the extension region has a curvature profile that has no more than one local extremum and whose absolute value of curvature is less than twice the absolute value of the edge curvature.

[0010] For example, the mirror element according to the present invention is a mirror element having a circular mirror surface comprising a non-rotationally symmetric target region and is used in an illumination optical unit or a projection optical unit of an EUV lithography system.

[0011] First, some terms used throughout the present invention are explained below:

[0012] The "principal curvatures" of a point should be understood to represent the minimum and maximum values of the curvature of a planar curve that is generated by the intersection of a known surface with a plane determined by the surface normal vector and the tangent direction of the point. They are a measure of the degree to which the surface bends differently in different directions at that point. The associated tangent directions are called "principal curvature directions".

[0013] Starting from a point in the contour direction, the "curvature profile" should be understood to represent the profile of those curvatures that have an intersection curve corresponding to the intersection of the mirror surface with the plane spanned by the normal vector at the edge point and the contour direction.

[0014] The "target region" denotes the region of the mirror surface that is provided as the usage region. The surface profile of the target region can be described by a high-order polynomial, for example, a 20th-order polynomial. At each point on the edge, the target region has an edge curvature that, in terms of absolute value, corresponds to the maximum principal curvature at that point. These edge points are located on the curve that describes the edge.

[0015] The "extension region" is configured to be adjacent to the target region. At the edge point, the transition from the target region via the edge to the extension region can have a profile without sudden changes and without kinks. For example, the surface profile of the mirror surface is at least twice continuously differentiable.

[0016] The present invention is an advantageous configuration of a mirror element, which has a mirror surface including a target area and an extension area. In fact, along a contour direction transverse to the edge from an edge point, the extension area has a curvature profile that has no more than one local extremum, and the absolute value of its curvature is less than twice the absolute value of the edge curvature. This allows the surface in the extension area to be largely uniform, and allows the mean curvature, that is, the sum of the two principal curvatures, and the astigmatism, that is, the difference between the two principal curvatures, to have only long-wave variations. Thus, the present invention provides a mirror element for which the special configuration of the extension area can particularly effectively avoid a reduction in the quality of the mirror surface in the target area, especially due to manufacturing reasons - particularly within the required polishing range of the mirror surface.

[0017] According to one embodiment, the absolute value of the curvature of the curvature profile is less than or equal to the edge curvature.

[0018] This ensures that the absolute value of the curvature occurring along the curvature profile within the extension area does not exceed the absolute value of the edge curvature, making the surface configuration of the extension area more uniform. If the curvature profile has a local extremum, then in this case it is thus assumed that the latter is located at the edge point.

[0019] For example, the curvature profile has principal curvatures, and the absolute value of the principal curvatures is less than or equal to the edge curvature.

[0020] This limitation of the principal curvature within the curvature profile can result in the values of the mean curvature and the astigmatism in the extension area not exceeding the values existing at the respective edge points. Thus, the absolute value of the possible variation in curvature can be reduced.

[0021] In one embodiment, the absolute value of the curvature decreases along the curvature profile. Alternatively or additionally, the curvature profile can be a monotonic curvature profile.

[0022] In this way, during the manufacturing of the mirror element, bothersome curvature variations in the extension area can be limited or avoided in terms of their amplitude.

[0023] Alternatively, the curvature is constant along the curvature profile.

[0024] This allows the curvature profile along the contour direction in the extension area to be described as a part of the said one circular edge. Thus, in three dimensions, the surface profile of the extension body can be described as a part of a toroidal surface. This allows the surface profile of the extension area to be determined particularly simply.

[0025] Alternatively, the curvature profile along the contour direction in the extension area can be described as a part of a parabola. Thus, the surface profile of the extension area can generally be considered as a part of an elliptic paraboloid surface.

[0026] According to another embodiment, the profile direction is perpendicular to the edge. In this case, the profile direction corresponds to the surface normal of the tangential surface at the edge point from which the considered curvature profile emanates.

[0027] Alternatively, the profile direction may follow the principal curvature direction of the edge point from which the considered curvature profile emanates.

[0028] For example, the curvature profile has no discontinuities in the extension region. In this case, the curvature profile is continuous. This is especially true if the surface profile of the mirror surface is at least twice continuously differentiable.

[0029] Thus, the curvature profile along the curve without discontinuities and the surface profile of the extension region can be described along the surface without discontinuities of the three-dimensional object. This can reduce the cost for determining the surface profile of the extension region.

[0030] In a further embodiment, at least a part of the surface of the extension body in contact with the edge covers the extension region, wherein for a plurality of edge points, the extension body has a cross-sectional area perpendicular to the edge, which is spanned by a circular or parabolic contact region, and for an edge point, the contact region can be determined based on beam parameters, where the beam parameters describe the length of the beam emitted from the edge point along the beam direction extending transversely to the curve, more specifically the beam direction is perpendicular to the curve, and the beam completely covers the extension region in the beam direction.

[0031] This represents a possible description of the surface profile of the extension region.

[0032] Additionally, the contact region can be determined based on installation space conditions or based on the optimization of the curvature in the extension region. This can be implemented by adding supplementary terms to supplement the surface description of the contact region. For example, further optimization of the mirror surface in view of astigmatism reduction can also be implemented by adding supplementary terms.

[0033] Further supplementary terms can be provided, especially for minimizing a function of the following form:

[0034]

[0035] where is the parameterization of the surface in its sag z in the extension region De, are the two principal curvatures of the target region, is the surface integral metric, 、 、 The values of are specified by the edge of the target region and its attributes.

[0036] The curve describing the edge can be expanded in basis functions, such as Fourier components, for efficient optimization.

[0037] For example, the extension region extends at least 50 mm from the edge of the target region in a direction perpendicular to the edge. The size of the extension region should be such that the entire circumference of the polishing tool can be guided beyond the end of the target region in each part of the edge, and the polishing tool does not reach the outer peripheral edge of the extension region. In individual cases, this may result in the surface area of the extension region being larger than the surface area of the target region.

[0038] Furthermore, the present invention discloses a lithography system including a mirror element according to the present invention. In particular, this may relate to an EUV lithography system.

[0039] Therein, the object field in the object plane can be illuminated by means of an illumination system. This illumination system includes a plurality of optical elements that image the illumination radiation emitted by an exposure radiation source into the object field disposed in the object plane. In a lithography system, a projection system can be used to image the object field into the image plane via a plurality of optical elements. For example, a mask (also referred to as a mask blank) disposed in the object plane can be imaged onto the light-sensitive layer of a wafer disposed in the image plane. In particular, the projection system should be understood to refer to a system containing a plurality of optical elements that are continuously arranged in the light beam path to shape the radiation entering the projection system. The optical elements of the projection system, especially as a whole, can be in the form of mirrors. This is particularly helpful if the radiation source emits EUV radiation because EUV radiation typically undergoes high transmission losses. If the radiation is only reflected and not transmitted, transmission losses can be avoided. The mirror can be configured for grazing incidence in the light beam path.

[0040] It should be understood that the illumination system and the projection system may also particularly include a plurality of mirror elements according to the present invention.

[0041] Furthermore, the present invention discloses a method for providing a mirror element having a mirror surface with an aspherical target region and an extension region adjacent to the edge of the target region, the method comprising:

[0042] - determining at least two continuously differentiable closed curves describing the edge in the extension region;

[0043] - for each edge point located on the curve, determining a beam parameter that describes the length of a beam emitted from the edge point along a beam direction that extends transversely to the curve, more specifically perpendicular to the curve, and the beam completely covers the extension region in the beam direction;

[0044] - based on the beam parameter, determining a circular or parabolic contact region for each edge point;

[0045] - Determine an extension body that contacts the edge, and for each edge point, the extension body has a cross-sectional area spanned by the determined contact area;

[0046] - Manufacture a mirror element, wherein at least a part of the surface of the extension body covers the extension area.

[0047] In one embodiment, determining the contact area additionally depends on the installation space conditions or the optimization of the curvature in the extension area.

[0048] For a detailed explanation of further advantageous developments of the method, reference is made to the above developments of the device. Similarly, further features described throughout the method can be used to develop the device.

[0049] The above embodiments and configurations should be understood as being merely exemplary and not limiting the invention in any way. Description of the Drawings

[0050] Hereinafter, the present invention will be explained in detail in an exemplary manner based on advantageous embodiments with reference to the drawings, wherein:

[0051] Figure 1 A schematic diagram showing an exemplary embodiment of a lithography system;

[0052] Figure 2 A schematic diagram showing an exemplary embodiment of a mirror element;

[0053] Figure 3 A schematic diagram showing exemplary adaptation coordinates for providing a mirror element according to an exemplary embodiment;

[0054] Figure 4 A schematic diagram showing an exemplary contact area for providing a mirror element according to an exemplary embodiment;

[0055] Figure 5 A schematic diagram showing a part of an exemplary extension body for providing a mirror element according to an exemplary embodiment; and

[0056] Figure 6 A diagram showing the mirror surface characteristics of a mirror element according to an exemplary embodiment. Detailed Description of the Invention

[0057] As an exemplary embodiment of a lithography system, Figure 1 The EUV lithography system 1 is schematically shown. The EUV lithography system 1 includes an illumination optical unit 10 and a projection optical unit 11. The illumination optical unit 10 is used to illuminate the object field 13 in the object plane 12.

[0058] The illumination optical unit 10 includes an illumination radiation source 14 that emits electromagnetic radiation in the EUV range, that is, a wavelength especially between 5 nanometers (nm) and 100 nanometers. The illumination radiation emitted from the illumination radiation source 14 is first focused by a condenser 15 onto an intermediate focal plane 16.

[0059] The illumination optical unit 10 includes a deflection mirror 17 via which the illumination radiation emitted by the illumination radiation source 14 is deflected to a first faceted mirror 18. A second faceted mirror 19 is arranged downstream of the first faceted mirror 18. Each of the first faceted mirror and the second faceted mirror 19 includes a plurality of micromirrors that can be pivoted individually about two respective axes extending perpendicular to each other. The second faceted mirror 19 is used to image the individual facets of the first faceted mirror 18 onto the object field 13.

[0060] Using a projection optical unit 11, the object field 13 is imaged into an image plane 9 using a plurality of mirrors 8. Arranged in the object plane 12 is a mask (also referred to as a mask reticle) that is imaged onto a light-sensitive layer of a wafer arranged in the image plane 9. Each of the mirrors of the EUV lithography system 1 that reflects the illumination radiation is in the form of an EUV mirror. The EUV mirror has a highly reflective coating, for example, in the form of a multilayer coating, especially having alternating layers of molybdenum and silicon.

[0061] Figure 2 A mirror element 20 is schematically shown in a plan view. The mirror element is, for example, a mirror 8 of the projection optical unit 11 of the EUV lithography system 1. However, it is also conceivable that the mirror element 20 is a part of a mirror in the illumination optical unit 10. The mirror element 20 includes a mirror surface 26 that has a target region 22 and an extended region 28 adjacent to an edge 24 of the target region. The edge 24 can be described using a twice continuously differentiable closed curve b on which there are a plurality of edge points s. The target region 22 has a respective edge curvature at each edge point s. Starting from the edge point s along a contour direction transverse to the edge 24, the extended region 28 has a curvature profile whose absolute value of the curvature is less than or equal to the edge curvature. Similarly, the curvature profile has a principal curvature whose absolute value is less than or equal to the absolute value of the edge curvature. Thus, the mean curvature and astigmatism of the curvature profile are not greater than the mean curvature and astigmatism at the edge point(s).

[0062] The following refers to Figures 2 to 5 an exemplary setting of such a mirror element 20.

[0063] The closed curve b that describes the edge 24 should be determined first, for example, in a Cartesian coordinate system, as Figure 2As shown. If it is not possible to describe the actual edge of the target region 22 by the curve b, for example because it is not free of mutations or kinks, it is also conceivable that the actual edge of the target region 22, for example the convex hull of the actual edge, is approximated by the curve b in the extension region 28.

[0064] The curve b is parameterized according to the arc length, where the curve b is represented by a B-spline of a fifth-degree polynomial, taking into account an appropriate smoothing parameter. The smoothing parameter is defined based on the bending strength of the curve b (i.e., the curvature of the curve b). In one variant, the curve b can also be represented by a low-pass filtered Fourier expansion of the edge 24.

[0065] The determined representation of the curve b is transferred to an adapted coordinate system. For example, there can be a transformation from Cartesian coordinates to generalized polar coordinates, where the curve b is parameterized by the edge point s, and the second coordinate t represents the light beam for each edge point s, which is transverse to the edge 24 at each edge point s and covers the extension region 28. Figure 3 Shows the application to Figure 2 A diagram of the result of such a coordinate transformation for an example. The coordinate transformation is at least twice continuously differentiable, for example at least three times. In Figure 2 and Figure 3 In the example shown, the light beam t of the edge point s is perpendicular to the edge 24.

[0066] For the edge point s, a corresponding contact region 32 that contacts the edge 24 at the edge point s is formed along the light beam t. The contact region 32 can be circular or parabolic, as Figure 4 shown, or have a shape according to a higher-order polynomial. When using a parabolic contact region, the latter should be configured such that its maximum curvature is at the edge point s.

[0067] When interpolating the intermediate space between two contact regions 32 between two edge points s, an extension body 30 that contacts the edge 24 is generated, and a part of the surface of the extension body covers the extension region 28. Figure 5 A schematic diagram showing a part of an exemplary extension body 30 is shown. In the example shown, the shape of all contact regions 32 of the extension body 30 is circular. Therefore, the shape of the extension body is annular. The surface z in the extension region 28 e is described by the following formula:

[0068]

[0069] Here, the parameters and are determined by the twice continuously differentiable surface profile of the mirror surface.

[0070] Thus, along the contour direction transverse to the edge 24 starting from the edge point s, the extension region 28 has a curvature profile, the absolute value of whose maximum curvature is equal to the edge curvature of the edge point s. The curvature in the curvature profile is constant and thus also has the same sign. In the presented example, the edge curvature of the edge point s considered separately corresponds to the reciprocal of the radius of the circular contact area 32. In contrast, if a parabolic contact area is used, the curvature in the curvature profile will have the same sign but a reduced absolute value.

[0071] In this example, the surface z in the additional adjustment extension region 28 e is described to be able to meet the specified installation space conditions for the mirror element 20. The description of the surface z e is supplemented by adding the term where represents at least a twice continuously differentiable function, where the applicable condition is . The parameter is determined by optimization depending on the specified installation space conditions. The parameter is expanded in a Fourier series and can be set to 0 above the Fourier order M, corresponding to low-pass filtering. Therefore, since only N*M parameters need to be effectively considered, the additional consideration for supplementation can be reduced.

[0072] The characteristics of the mirror surface 26 of the mirror element 20 provided in the described manner are shown in Figure 6 based on two diagrams. The target region 22 and the extension region 28 adjacent to the edge 24 of the target region 22 are also shown. In these two diagrams, the x-axis and y-axis represent the positions of the mirror surface 26 in the x (right / left) direction and y (up / down) direction in millimeters (mm). The drawn streamline corresponds to the respective principal curvature direction.

[0073] The shading intensity in the left diagram reproduces the sum value of the two principal curvatures of the points of the mirror surface 26 . Obviously, the sum of the principal curvatures has a very high value, especially for multiple points at the lower edge 24 of the target region 22. However, in principle, when starting from multiple points on the edge 24, the value in the extension region 28 decreases in the direction transverse to the edge 24.

[0074] In the right diagram, the shading intensity reproduces the subtraction value of the two principal curvatures corresponding to astigmatism of multiple points of the mirror surface 26 . For multiple points on the edge 24 at the lower end of the target region 22, this value is also large. However, when starting from multiple points on the edge 24, in principle, it decreases again in the extension region 28 in the direction transverse to the edge 24.

[0075] When proceeding from points on the edge 24 respectively in a direction transverse to the edge 24, the fact that the values of both the addition and subtraction of the two principal curvatures decrease in the extension region 28 clearly indicates that the provided mirror element 20 has a mirror surface 26, where the extension region 28 has a maximum curvature in a direction perpendicular to the edge 24 starting from each considered point on the edge 24, and the absolute value of this maximum curvature is less than or equal to the edge curvature.

[0076] The embodiments of the present invention described in this specification and the alternative features and characteristics separately listed in this regard should also be understood to be disclosed in all combinations with each other. In particular, the description of the features included in the embodiments (assuming no contrary explicit interpretation) should not, in the current context, be construed as meaning that the feature is essential or necessary for the function of the embodiment.

Claims

1. A mirror element (20) having a mirror surface (26), the mirror surface comprising: An aspherical target region (22) and an extension region (28) adjacent to an edge (24) of the target region (22), wherein the edge (24) can be described by a closed curve (b) that is at least twice continuously differentiable; wherein the target region (22) has respective edge curvatures at each edge point (s) located on the curve (b); wherein, starting from the edge point (s) and in a direction transverse to the contour of the edge (24), the extension region (28) has a curvature profile that has no more than one local extremum and whose absolute value of the curvature is less than twice the absolute value of the edge curvature.

2. The mirror element (20) according to claim 1, wherein the absolute value of the curvature of the curvature profile is less than or equal to the edge curvature.

3. The mirror element (20) according to any one of the preceding claims, wherein the curvature profile has a principal curvature, and the absolute value of the principal curvature is less than or equal to the edge curvature.

4. The mirror element (20) according to any one of the preceding claims, wherein the absolute value of the curvature decreases along with the curvature profile.

5. The mirror element (20) according to any one of the preceding claims, wherein the curvature profile is a monotonic curvature profile.

6. The mirror element (20) according to any one of claims 1 to 3, wherein the curvature is constant along the curvature profile.

7. The mirror element (20) according to any one of the preceding claims, wherein the profile direction is perpendicular to the edge.

8. The mirror element (20) according to any one of the preceding claims, wherein there is no abrupt change in the curvature profile in the extension region (28).

9. The mirror element (20) according to any one of the preceding claims, wherein at least a part of the surface of the extension body (30) in contact with the edge (24) covers the extension region (28); for a plurality of edge points (s), the extension body (30) has a cross-sectional area perpendicular to the edge (24), and the cross-sectional area is spanned by a circular or parabolic contact region (32), and for the edge point (s), the circular or parabolic contact region can be determined based on the beam parameter (t); the beam parameter (t) describes the length of the beam emitted along the beam direction from the edge point (s), the beam direction extends transversely to the curve (b), more specifically, the beam direction is perpendicular to the curve (b), and the beam completely covers the extension region (28) in the beam direction.

10. The mirror element (20) according to claim 9, wherein the contact region (32) can be determined based on the installation space conditions or based on the optimization of the curvature in the extension region (28).

11. The mirror element (20) according to any one of the preceding claims, wherein the target region (22) is non-rotationally symmetric.

12. The mirror element (20) according to any one of the preceding claims, wherein the extension region (28) extends at least 50 mm along a direction perpendicular to the edge (24) from the edge (24) of the target region (22).

13. The mirror element (20) according to any one of the preceding claims, wherein the area of the extension region (28) is larger than the area of the target region (22).

14. A lithographic system (1) comprising a mirror element (20) according to any one of the preceding claims.

15. A method of providing a mirror element (20) having a mirror surface (26) comprising an aspherical target region (22) and an extension region (28) adjacent to an edge (24) of the target region (22), the method comprising: Determine a closed curve (b) that is at least twice continuously differentiable in the extension region (28) and that describes the edge (24); For each edge point (s) located on the curve (b), determine a beam parameter (t) that describes the length of a beam emitted from the edge point (s) in a beam direction that extends transversely to the curve (b), more specifically perpendicular to the curve (b), and that the beam completely covers the extension region (28) in the beam direction; Based on the beam parameter (t), determine a circular or parabolic contact region (32) for each edge point (s); Determine an extension body (30) that contacts the edge (24) and that, for each edge point (s), has a cross-sectional area perpendicular to the edge (24) that is spanned by the determined contact region; Manufacture the mirror element (20), wherein at least a portion of the surface of the extension body (30) covers the extension region (28).

16. The method according to claim 15, wherein determining the contact region (32) additionally depends on installation space conditions or optimization of the curvature in the extension region (28).

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