Optical system for a metrology system and metrology system with such an optical system

Abstract

An optical system (8) for a metrology system (1) is used to measure an object (2). An optical focusing component (10) is arranged in the beam path of illuminating light (4) between a light source (5) and an object field (12) and is used to generate an illuminating focus (14). A detection device (6) is used to detect the illuminating light (4) in the beam path downstream of the object field (12). The optical focusing component (10) is designed as a zone plate with at least two zones. Portions of the illuminating light (4) that strike the zones interact with one another through diffraction.Depending on the design of the zone plate, the zones of the zone plate are arranged such that the zone plate is chromatically corrected for the illumination light (4), such that the zone plate is aspherically corrected for the illumination light (4), such that the zones of the zone plate are reflective for the illumination light (4), and / or such that the zones of the zone plate are arranged such that at least one diffraction order of the illumination light (4) is preferred as the default diffraction order compared to at least one other diffraction order with regard to the useful illumination light intensity guided by the zone plate. The result is an optical system in which the possibilities of a zone plate design of the optical focusing component are optimized to improve application possibilities of the metrology system.

Description

[0001] The invention relates to an optical system for a metrology system for measuring an object. Furthermore, the invention relates to a metrology system for measuring an object using such an optical system.

[0002] A metrology system of the type mentioned above is known, for example, from US 2012 / 0 008 123 A1. Other systems for measuring lithography masks are known from the specialist articles by Na J. et al. "Application of actinic mask review system for the preparation of HVM EUV lithography with defect-free mask", Proc. of SPIE Vol. 10145, 101450M-1, by Goldberg K. et al. "Actinic mask imaging: recent results and future directions from the SHARP EUV microscope", Proc. of SPIE Vol. 9049, 90480Y-1 and by Naulleau et al. “Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source”, Optics Express, Vol. 22, 20144, 2014. Another metrology system is known from US 9,904,060.

[0003] It is an object of the present invention to further develop an optical system for a metrology system in such a way that possibilities of a zone plate design of the optical focusing component are optimized to improve application possibilities of the metrology system.

[0004] This object is achieved according to the invention by an optical system having the features specified in claim 1.

[0005] According to the invention, it was recognized that zone plates offer design possibilities that lead to optical properties that can be used to improve the application possibilities of the metrology system.

[0006] A chromatic correction of the zone plate according to a first aspect avoids undesirable color errors.

[0007] An aspherical correction of the zone plate according to a further aspect avoids undesired aberrations and / or creates the possibility of compensating or correcting aberrations that are generated via other components of the metrology system and / or creates the possibility of deliberately introducing aberrations into the metrology system.

[0008] A reflective design of the zones of the zone plate according to another aspect leads to the possibility of increasing the illumination light throughput of the metrology system. This can improve measurement accuracy and / or reduce the requirements for a light source of the metrology system.

[0009] A zone plate design that is selective with respect to at least one predefined diffraction order according to a further aspect also leads to an increase in throughput and can also prevent the occurrence of unwanted stray light or unwanted channel crosstalk in a detection device that is then implemented with spatial resolution. The zone plate can be designed, in particular, for chromatic correction of diffraction of the illumination light.

[0010] The focal length of the optical focusing component, i.e., the zone plate, can be a maximum of 10 mm. The focal length can be, for example, 5 mm or even 0.5 mm.

[0011] A number of zone rings according to claim 2 results in an advantageous diffraction effect of the zone plate. The number of zone rings can be greater than 150, greater than 200, and even greater than 250. The number of zone rings is usually less than 1000.

[0012] An embodiment of the zone plate as a transmitting phase plate according to claim 3 has proven successful in practice.

[0013] An embodiment of the zone plate as a photon sieve according to claim 4 enables, in particular, chromatic correction of the zone plate. A diameter of pinholes of the photon sieve can be on the order of magnitude of a radial extension of a width of the respective zone ring. Instead of pinholes, correspondingly extended reflective elements can also be used, which are also referred to below as pinholes. The pinholes can be randomly distributed across the respective zone ring. Within a specific zone ring, the pinholes can have a binary distribution, i.e., a cumulative hole circumference of all pinholes can correspond to a cumulative gap circumference of gaps between adjacent pinholes in the circumferential direction of the respective zone ring.

[0014] An embodiment of the photon sieve according to claim 5 avoids unwanted stray light. The preferred diffraction order can be a plus / minus first diffraction order. The photon sieve can be designed such that a zeroth diffraction order is suppressed.

[0015] An aspherical correction embodiment according to claim 6 represents one possibility for the design of an aspherically corrected zone plate. An aspherical correction can have a radial distribution of the ring widths of the zone rings that corresponds to that of one or the sum of several rotationally symmetric Zernike polynomials, for example, the Zernike polynomial Z36. An aspherical correction can also have a distribution of the ring widths of the zone rings that deviates from rotational symmetry and corresponds to that of one or the sum of several arbitrary Zernike polynomials, for example, the Zernike polynomial Z8.

[0016] A design according to claim 7 allows the zone plate to be designed as a reflective zone plate. The reflective layer can be designed as a multilayer layer.

[0017] A design according to claim 8, for example, allows the zone plate to be designed as a chromatically corrected zone plate. The zone layer can, in particular, comprise multiple absorber layers, with different absorber materials being used for each absorber layer. The optionally different absorber layers can then be assigned to zones optimized for different wavelengths.

[0018] A design according to claim 9 enables a highly reflective design of the zone plate.

[0019] If the zone layer has a multilayer portion of the reflection layer, a chromatically corrected zone plate can again be realized, wherein one zone layer portion is designed as an absorber layer and another zone layer portion is designed as a reflective multilayer layer.

[0020] An embodiment according to claim 10 provides diverse design options for the zones of the zone plate. The zones can have a sawtooth profile, which enables a blazed design of the zone plate. The zones can be designed as a stepped profile with at least two steps of different step heights. This can, for example, approximate a continuous cross-sectional profile of the zones, for example, as close as possible to a sinusoidal or blazed profile. A rectangular envelope of a respective zone ring can be divided into several radially spaced profile sections of the same step height. This can be used to specify certain preferred diffraction orders of the illumination light guided over the zone plate.

[0021] The advantages of a metrology system according to claim 11 correspond to those already explained above with reference to the optical system. A spectral width Δλ / λ (FWHM, full width half max) of the illumination light generated by the light source can be at least 5 × 10 -4 , at least 1 × 10 -3 , at least 3 × 10 -3 , at least 5 × 10 -3 , at least 1 × 10 -2 , at least 1 × 10 -1 and can, for example, be in the range between 1 / 250 and 1 / 300.

[0022] The optical system can be part of a scanning EUV microscope. A scanning EUV microscope is otherwise described in the article by Naulleau et al., "Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source," Optics Express, Vol. 22, 2014, 2014.

[0023] An EUV light source according to claim 12 enables actinic measurement, in particular of an EUV lithography mask as an object. The EUV light source can be a plasma light source, in particular pumped by a solid-state laser, or a high harmonic generation (HHG) light source.

[0024] Embodiments of the invention are explained in more detail below with reference to the drawings, in which: Fig. 1 schematically shows in a meridional section a metrology system for measuring an object comprising a zone plate in an illuminating light beam path in front of an object field, designed as a transmissive zone plate; Fig. 2 in one to Fig. 1 similar representation of another embodiment of a metrology system with a reflective zone plate in the illumination light beam path in front of the object field; Fig. 3 a perspective and also schematic view, not to scale, of a zone plate to illustrate the parameters relevant for zone plate design; Fig. 4 a flow diagram to illustrate an embodiment of the zone plate as a chromatically corrected photon sieve; Fig. 5 in an enlarged, not to scale meridional section, an illustration of a diffractive effect of the photon sieve or a zone lens, with beam paths of different diffraction orders highlighted; Fig. 6 in one to Fig. 4 similar representation a scheme for the design of another version of a zone plate, in this case aspherically corrected; Fig. 7 shows in cross section an embodiment of a reflective zone plate with a zone layer which, like a reflective layer of the zone plate, is designed as a multilayer layer; Fig. 8 in one to Fig. 7 shows an embodiment of a reflective zone plate with a zone layer comprising a multilayer portion of the reflective layer and an absorber portion arranged thereabove; Fig. 9 again in a Fig. 7 shows an embodiment of a reflective zone plate in which a zone layer is designed as an absorber layer made of an absorber material; Fig. 10 again in a Fig. 7 shows an embodiment of a reflective zone plate in which the zone layer is designed as an absorber layer in two layers, each of the two layers being made of a different absorber material; Fig. 11 shows a section through some adjacent zone rings of an embodiment of the zone plate in which exactly one diffraction order is preferred as the default diffraction order compared to other diffraction orders of the illumination light guided by the zone plate, the zone rings being designed as a sawtooth profile in cross section; Fig. 12 in one to Fig. 11 similar representation in comparison to this, a diffractive effect of zone rings with a binary rectangular cross-section, where the diffractive effect is shown for different diffraction orders; Fig. 13 again in a Fig. 11 shows a diffractive effect of zone rings, which are designed in cross-section as two step profiles of different step heights, whereby the diffractive effect for different diffraction orders is also shown here, according to the representation in Fig. 12; Fig. 14 again in a Fig. 11 similar representation of a diffractive effect of an embodiment of the zone rings as a subdivision of a rectangular envelope into several radially spaced profile sections with the same step height, wherein the diffractive effect is shown for different diffraction orders; Fig. 15 in one to Fig. 11 similar representation a design of the zone plate with sinusoidal zone rings in cross-section; and Fig. 16 again in a Fig. 11 similar representation a version of the zone plate with zone rings, designed as step profiles with a total of three steps and two step heights each.

[0025] Fig. Figure 1 schematically shows a metrology system 1 for measuring an object 2. An example of the object 2 to be measured is a lithography mask for projection lithography for the production of micro- or nanostructured semiconductor components. The figure shows a beam path of an edge-limited beam 3 of illumination light 4 between a light source 5 and a detection device 6 of the metrology system 1.

[0026] The light source 5 is an EUV light source for generating the EUV illumination light 4 with a central useful wavelength in the range between 5 nm and 30 nm, in particular 13.5 nm. A spectral width Δλ / λ (FWHM, full width half max) of the EUV illumination light 4, which is used for illuminating the object 2, is at least 1 × 10 -4and can, for example, be in the range between 1 / 250 and 1 / 300. The light source 5 can be a plasma light source or an HHG light source. In the schematically illustrated variant of the light source 5, it has a pump laser 5a in the form of a Ti:sapphire laser, whose pump light 5b is focused by means of a mirror focusing optics into a gas cell 5c to generate the illumination light 4. A pinhole 5d of the light source 5 serves to separate the illumination light 4 from the pump light 5b.

[0027] The pinhole 5d can also be used to separate the useful illumination light 4 from, in particular, unwanted debris entrained therein. A stray light filter 7 is arranged downstream of the light source 5 in the beam path of the illumination light 4 to separate the useful illumination light 4 from unwanted wavelength components entrained in the beam path.

[0028] After the light source 5, the illumination light 4 is guided by an optical system 8 of the metrology system 1. The optical system 8 comprises an EUV mirror 9 and an optical focusing component in the form of a zone plate 10, for which various exemplary embodiments are explained below. The optical focusing component 10 is arranged in the beam path of the illumination light 4 between the light source 5 and an object field 12 in an object plane 13 of the optical system 8. The optical focusing component 10 serves to generate an illumination focus 14 in the beam path of the illumination light 4 after the optical focusing component 10.

[0029] A focal length of the optical focusing component 10 is at most 10 mm and is typically 0.5 mm.

[0030] The zone plate 10 is designed to be transmissive to the illumination light 4, thus allowing at least a portion of it to pass through. The zone plate 10 is designed as a transmissive phase plate, with zones of the zone plate 10 differently influencing the phase of the illumination light 4 incident on the zone plate 10.

[0031] To clarify the positional relationships between components of the metrology system, Fig. 1 a Cartesian xyz coordinate system is drawn. The x-direction runs in the Fig. 1 to the right. The y-direction runs in the Fig. 1 perpendicular to the plane of the drawing into it. The z-direction runs in the Fig. 1 up.

[0032] An object holder 15 of the optical system is used to hold the object 2 in the object plane 13 so that a section of the object 2 lies in the object field 12. The object holder 15 can be displaced perpendicular to the object plane 13 via an actuator 16, as shown in the Fig. 1 by a displacement double arrow Δz. Via the actuator 16a, the object holder 15 with the object 2 can also be displaced parallel to the object plane 13, as shown in the Fig. 1 is indicated by a double arrow Δx / y.

[0033] A main ray angle α with which the illuminating light 4 enters the object field 12 can be smaller than 6°.

[0034] An object-side numerical aperture of the illumination light beam path can be in the range between 0.075 and 0.2, in particular in the range of 0.1.

[0035] The object 2 is designed as a reflective object. Illumination light 4 reflected by the object 2 is guided as detection light by the optical system 8 to the detection device 6.

[0036] In the beam path of the illuminating light 4 between the object field 12 and a spatially resolving detector 16 of the detection device 6, an aperture stop 17 is arranged, which in the Fig. 1 is shown in a top view for illustration purposes. The aperture diaphragm 17 can additionally have an inner obscuration diaphragm section. The aperture diaphragm 17 can be designed to emulate an exit pupil of a projection optics system of a projection exposure system, the imaging effect of which can then be simulated.

[0037] Depending on the design, the detection device 6 can have two, three, five, ten, or even more sensor elements. The detection device 6 can be designed as a sensor line or as a two-dimensional sensor array, for example in the form of a CCD or CMOS array. The detector 16 of the detection device 6 is arranged in an arrangement or detection plane 6a.

[0038] The detector 16 detects a diffraction field of an object point in the object field 12. The light transmitted through the aperture diaphragm 17 is integrated by the detector 16 and defines an intensity determined for the respective object point being measured. By scanning across the object 2, an image of the object 2 is created from the pixels assigned to the individual object points.

[0039] The zone plate 10 serves to specify an illumination angle distribution and / or an illumination intensity distribution of the illumination light 4 across the object field 12, in particular a point illumination of the object, which is adapted to an illumination setting of a projection exposure system for EUV lithography. The main ray angle α is also specified via the zone plate 10.

[0040] Fig. Figure 2 shows a further embodiment of the metrology system 18. Components and functions corresponding to those described above with reference to the Fig. 1 have already been explained, bear the same reference numbers and will not be discussed in detail again.

[0041] In the metrology system 18, a zone plate 19 is provided, which basically performs the function of the zone plate 10 according to Fig. 1 has been carried out reflectively.

[0042] Fig. Figure 3 shows a schematic distribution of zones Z1 to Z N of the zone plates 10 and 19 respectively. Along a radius coordinate r emanating from a center of the zone plate 10, 19, each of the zones Z i in a zone ring ZR i with a ring radius r i (i=1, ...N).

[0043] The number N of zone rings ZR i is in the Fig. 3 is clearly understated. In fact, this number N is in the range between 50 and 500, for example, in the range between 200 and 300, for example, around 250.

[0044] The zone plate 10 or 19 has an entrance pupil diameter D, which has a diameter of the outermost zone Z N The diameter D can be in the range between 10 µm and 250 µm, for example in the range between 50 µm and 150 µm, for example in the range of 80 µm.

[0045] Furthermore, the zone plate 10, 19 has a focal length f. The focal length f can be in the range between 0.25 mm and 10 mm, for example 0.5 mm.

[0046] For a radius extension Δr1 of an innermost zone ring ZR1, 20 nm ≤ Δr1 ≤ 200 nm applies. This radius extension Δr1 can, for example, be in the range of 80 nm. This radius extension Δr i usually increases towards the radially outer zone rings ZR i in a defined manner, so that a desired diffraction effect of the zone plate 10, 19 for the illumination light 4 is provided.

[0047] Based on the Fig. 4 and Fig. 5, an embodiment of a zone plate 20 is described below, which can be used instead of the zone plates 10 or 19. Components and functions corresponding to those described above with reference to the Fig. 1 to 3 have already been explained, have the same reference numbers and will not be discussed in detail again.

[0048] Fig. Figure 4 shows a flow diagram for the design of a zone plate 20, which can be used instead of zone plates 10 or 19. Zone plate 20 is designed as a photon sieve. Zone plate 20 is chromatically corrected for the illumination light 4.

[0049] This photon sieve version of the zone plate 20 includes a variety of on the different zone rings ZR1 to ZR N arranged hole or reflector structures 21a, which are referred to below as pinholes.

[0050] The flow chart according to Fig. 4 illustrates how the photon sieve 20 is designed.

[0051] The starting point is a zone plate with closed zone rings ZR i which in the Fig. 4 shown on the left. Instead of the closed zone rings ZR i are now, as in the Fig. 4 right, randomly over a circumference of the respective zone ring ZR i distributed pinholes PH i j set.

[0052] An innermost of the zone rings, ZR1, has, when executed according to Fig. 4 total of six pinholes PH1 1 to PH1 6 , which in the Fig. 4 on the right are numbered counterclockwise. In the radially outwardly adjacent zone ring ZR2 there are 14 pinholes (PH2 1 up to PH2 14 ). In the example in the Fig. 4 radially outermost zone ring ZR N There are about 60 pinholes (PH N 1 , ...PH N 60 ).

[0053] A diameter of the respective pinholes PH i j corresponds to a radial width, i.e. a radial extension, of the respective zone ring ZRi .

[0054] A pinhole diameter can also be larger than the radial extent of the respective zone ring and can be 1.5 times, 3.5 times, or even 5.5 times. The ratio between the pinhole diameter and the radial extent of the respective zone ring can therefore range between 0.5 and 10. The diameter of a focus for point illumination of the object 2, or the focus quality provided by the zone plate 10, can be improved by adjusting the ratio between the pinhole diameter and the radial extent of the respective zone ring. In this regard, reference is made to R. Menon's doctoral thesis: Diffractive optics for maskless lithography and imaging, Thesis (Ph. D.) - Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.

[0055] About the radial distribution of the zone rings ZR i, the dimensioning of the pinholes PH i j and the number of pinholes PH i j in the respective zone ring ZR i a diffraction effect of the photon sieve of the zone plate 20 can be defined.

[0056] Experts can find details on the design in the article “Huang, KL (2017). Huang, K., Liu, H., Si, G., Wang, Q., Lin, J., Teng, J., Laser & Photonics Reviews 2017, 11, 1700025. Laser & Photonics Reviews, p. 1700025”

[0057] The Pinholes PH i j of a specific zone ring ZR j can be used within this zone ring ZR j be binary distributed. A cumulative pinhole circumference, i.e. a pinhole diameter sum of all pinholes PH i j within this zone ring ZR j then corresponds to a cumulative gap circumference of the gaps between the adjacent pinholes PH i jin the circumferential direction within this zone ring ZR j . For the innermost zone ring ZR1, the cumulative hole circumference is six times the diameter of a single pinhole PH i 1 The cumulative gap circumference within this zone ring ZR1 is then the same size as the cumulative hole circumference.

[0058] Fig. 5 illustrates a diffractive effect of the photon sieve zone plate 20, shown in this case in meridional section. The effect of a transmissive photon sieve is shown, in which the pinholes PH i j are designed as pinholes.

[0059] A beam 3 of the illumination light 4 incident from the left, schematically indicated by an arrow, is diffracted in a +1st diffraction order 21 towards the illumination focus 14 in the object plane 13.

[0060] A zeroth diffraction order 22 passes through the photon sieve zone plate 20 undiffracted, whereby an area-related intensity in the range of this zeroth diffraction order is orders of magnitude lower than the intensity of the +1st diffraction order in the illumination focus 14. Depending on the arrangement of the pinholes PH i j the zeroth diffraction order can be suppressed even more than in the Fig. 5 indicated.

[0061] Shown in the Fig. 5 also shows the course of a -1st diffraction order 23 with a significantly lower surface intensity compared to the +1st diffraction order 21, which is focused into the illumination focus 14.

[0062] Higher diffraction orders are due to the arrangement of the pinholes PH i j the photon sieve zone plate 20 is effectively suppressed.

[0063] The photon sieve zone plate 20 can be designed to prevent longitudinal chromatic aberration. Different diffracted wavelengths of the illumination light 4 are thus focused in the first diffraction order at the same z-focal position of the illumination focus 14. The photon sieve zone plate 20 is thus chromatically corrected.

[0064] The photon sieve 20 is designed such that the illumination focus 14 of the diffraction order +1 is preferred compared to illumination foci of other diffraction orders, for example +2, +3,..., with regard to the useful illumination intensity guided by the photon sieve zone plate 20.

[0065] Based on the Fig. 6, an embodiment of a zone plate 24 is described below, which can be used instead of the zone plates 10 or 19. Components and functions corresponding to those described above with reference to the Fig. 1 to 5 have already been explained, have the same reference numbers and will not be discussed in detail again.

[0066] Fig. Figure 6 shows the design of the zone plate 24 as a result of a design flow diagram. The zone plate 24 has zones Z arranged as follows: i or zone rings ZR i that the zone plate 24 is aspherically corrected for the illumination light 4. The zone plate 24 thus has the effect of an aspheric lens or an aspheric mirror. The zone plate 24 can thus be used to influence the imaging parameters of the optical system 8.

[0067] The design of the aspherical zone plate 24 is based on a raw zone plate 25 with spherical wavefront effect of the zone rings ZR i , which in the Fig. 6 is shown on the left.

[0068] An aspherical correction function 26 is impressed on the zone distribution of this raw zone plate 25, as shown in the Fig. 6, middle. This correction function can, for example, include significant contributions from the radially symmetric Zernike polynomial Z36.

[0069] Due to this aspherical correction, a discontinuous distribution of ring widths Δr results i or radius extensions of the zone rings ZR i , as in the Fig. 6. This ring width Δr i decreases starting from the center of the asphere zone plate 24 and then increases again in a middle radius range, decreases again, then increases again radially outwards before it reaches the very outside in the area of ​​the outermost zone rings ZR N-1 , ZR N This corresponds to the course of the Zernike polynomial Z36. Fig. 7 to 10, further designs of zone plates are described below, each of which is designed to reflect the illumination light 4, i.e. instead of the zone plate 19 according to Fig. 2. Components and functions corresponding to those described above with reference to the Fig. 1 to 6 have already been explained, have the same reference numbers and will not be discussed in detail again.

[0070] Fig. 7 shows an embodiment of a reflecting zone plate 27 in transverse or meridional section.

[0071] The reflective zone plate 27 has a zone layer 28 which contains the zones Z i , arranged on the zone rings ZR i , has. The zones Z i the zone layer 28 of the zone plate 27 each have a rectangular profile or an approximately rectangular profile.

[0072] This zone layer 28 is applied to a base reflection layer 29 of the zone plate 27. At the same time, the zone layer 28 is part of an overall reflection layer 28, 29, which is constructed as a multilayer layer with alternating layers L1, L2, or L1, L2, L3, made of materials with different refractive indices.

[0073] The zone layer 28 itself represents a multilayer portion of the total reflection layer 28, 29. The zones Z i of the zone layer 28 are in turn stacks of the individual layers L i the multilayer layer.

[0074] Fig. Figure 8 shows a variant of a reflective zone plate 30 in which the zone layer 28 has zones Z i which deviate from a rectangular profile in the meridional section.

[0075] The zones Z iare designed as a stepped profile with steps S1 and S2 in zone plate 30. A lower step profile of step S1 projects radially on both sides beyond the upper step profile of step S2, resulting in step projections of the lower step profile S1 on both sides of the upper step profile S2.

[0076] The upper step profiles S2 of zones Z i are made of absorber material and the lower step profiles S1 are multilayer layers with individual layers L i as described above in connection with the Fig. 7 already explained.

[0077] The zone layer 28 of the reflective zone plate 30 is in turn supported by a basic reflection layer 29 according to the embodiment according to Fig. 7.

[0078] The absorber material is at least partially or completely absorbent for the illumination light 4.

[0079] Fig. Figure 9 shows another variant of a reflective zone plate 31, which can be used instead of the zone plates 27 or 30. A zone layer 28 is made up of zones Z in the zone plate 31. i formed entirely from the absorber material. A profiling of the zones Z i the zone layer 28 of the zone plate 31 corresponds to the rectangular profiling of the zone plate 27 according to Fig. 7.

[0080] This zone layer with the zones Z i from the absorber material is carried by the reflection layer 29 in the zone plate 31.

[0081] Fig. 10 shows another embodiment of a reflective zone plate 32 which can be used instead of the zone plates 27, 30 and 31.

[0082] Zones Z i a zone layer 28 of the reflective zone plate 32 are stepped in the manner of the zones Z i the zone plate 30 to Fig. 8 formed. The lower step profile S ı is made of a first absorber material A1, and the overlying step profile S2 is made of a second absorber material A2. The absorber materials A1 and A2 of the step profiles S1 and S2 differ from each other in terms of their absorbing effect on the illumination light 4.

[0083] Material examples for the individual layers L i The multilayer layers are molybdenum (Mo), silicon (Si), or beryllium (Be). Examples of absorber materials are TiN, Cr, TaN, TaBN, and TaBO.

[0084] The zones Z i the zone layer 28 are also carried by the multilayer reflection layer 29 in the zone plate 32.

[0085] The wavelength bandwidth of a zone plate can be increased using zone plates such as those shown in Figures 30, 31, and 32. The resulting zone plate can be understood as a superposition of several zone plates, for example, a superposition of two or three zone plates, each of which is optimized for a different target wavelength.

[0086] One of these wavelengths can be the wavelength of the useful illumination light 4. The other wavelengths can be neighboring or more distant wavelengths.

[0087] This can be used to design zone plates similar to zone plates 30, 31, 32, which are chromatically corrected with respect to the useful wavelength of the illumination light 4. Details on the design of corresponding chromatically corrected zone plates can be found in the technical article "Cai. H. et al. (2019), Ultrathin transmissive metasurfaces for multi-wavelength optics in the visible. Appl. Phys. Lett., S071106."

[0088] Based on the Fig. 11 to 16, further zone plate designs with zones Z i which deviate from a rectangular profile in cross-section or meridional section. Components and functions corresponding to those described above with reference to the Fig. 1 to 10 have already been explained, have the same reference numbers and will not be discussed in detail again.

[0089] Fig. 11 shows a zone plate 33 with zones Z i , Zi+1 , Z i+2 , which have a sawtooth profile in cross-section or meridional section. Zone plate 33 represents a transmitting phase plate.

[0090] An inclined surface 34 of the respective zone Z i can be adapted with respect to its surface angle s to a plate plane 35 of the zone plate 33, which runs parallel to the xy plane, in such a way that a blazed zone plate 33 results. The angle s is then a blaze angle, which in the case of the zone plate 33 can be adapted to the useful wavelength of the illumination light 4 in such a way that only a +1st diffraction order of the illumination light 4 is structurally guided by the zone plate 33.

[0091] This is in the Fig. 11 schematically shows a transmissive variant of the zone plate 33, where the illumination light 4 is radiated from below and a +1st diffraction order is guided, deflected by a diffraction angle b. Other diffraction orders of the incident illumination light 4, while in this sawtooth profile design of the zones Z i the zone plate 33 is largely or completely suppressed.

[0092] In the case of the zone plate 33, the +1st diffraction order therefore represents a default diffraction order which is preferred over at least one further diffraction order with regard to a useful illumination light intensity guided by the zone plate 33.

[0093] Fig. 12 shows, in comparison to the blaze effect of the zone plate 33, an embodiment of a zone plate 35 in which the zones Z i are designed as rectangular profiles.

[0094] A diffraction effect of such a rectangular profile design of the zones Z i the zone plate 35 is in the Fig. 12 analogous to the representation according to Fig. 11. The diffracted light 4, again incident from below, is split into a plurality of diffraction orders, namely a -3rd, a -2nd, a -1st, a 0th, a +1st, a +2nd, and a +3rd diffraction order. Higher diffraction orders are also possible, but have a negligible diffraction intensity compared to the diffraction orders shown. Compared to the zone plate 33, only a comparatively small proportion of the total incident illumination light energy is diffracted into the +1st diffraction order.

[0095] Fig. 13 shows another variant of Zones Z i whose profile deviates from a rectangular profile. In this case, the zone plate 36 again has stepped zones Z iwith step profiles S1, S2. The Fig. 13 The lower step profile S1 has a larger radial extension than the upper step profile S2, for example, twice the radial extension. The lower step profile S1 only projects beyond the upper step profile S2 in one radial direction, for example, along the positive r-coordinate. Overall, the step-shaped zones Z i of the zone plate 36 as an approximation to the sawtooth profiling of the zones Z i the zone plate 33 to Fig. 11 can be understood.

[0096] Regarding the Fig. 13 also analogous to the Fig. 11 and Fig. 12, there is a preference for the +1st diffraction order also in the step profile according to Fig. 13 according to the Blaze variant Fig. 11, where the zone plate 36 is Fig. 13 the other diffraction orders -3, -2, -1, 0, +2 and +3 are not completely suppressed.

[0097] Fig. 14 shows a variant of a zone plate 37 with the zone plate 36 according Fig. 3 comparable diffraction effect. In the zone plate 37, the individual zones Z i in the form of a subdivision of a rectangular envelope 38 of the respective zone ring ZR i Such a rectangular envelope 38 is shown in the Fig. 14 dashed at the zone ring ZR i+2 indicated.

[0098] In the zone plate 37, this subdivision of the rectangular envelope 38 takes place into several radially spaced profile sections A1, A2, A3, and A4 of the same step height. Radial extensions of the profile sections A i and the radial distances r i between the adjacent profile sections A i , A i+1 and a step height ΔS are coordinated so that the Fig. 14 results in a diffraction effect in which the +1st diffraction order of the illumination light 4 is again preferred.

[0099] Fig. 15 shows a further embodiment of a zone plate 39 with zones Z deviating from a rectangular profile i . Similar to the Fig. 11 to 14 are in the Fig. 15 again three radially adjacent zones Z i , Z i+1 and Z i+2A cross-sectional or meridional profile of these zones of the zone plate 39 is a sinusoidal profile or a profile approximating a sinusoidal profile. The zone plate 39 has a diffractive effect for incident diffracted light 4, provided a corresponding adjustment of the period and amplitude of the sinusoidal profile, in reflection or transmission, in which the illumination light 4 is guided exclusively into a +1st and a -1st diffraction order, with the other diffraction orders being at least largely or even completely suppressed.

[0100] In the zone plate 39, the + / -1st diffraction order represents default diffraction orders which are preferred over the other diffraction orders with regard to the useful illumination light intensity guided by the zone plate.

[0101] Fig. 16 shows a further embodiment of a zone plate 40 with a profile of the zones Z that deviates from a rectangular profile. i in cross-section or meridional section. The zones Z i are designed as a step profile on the zone plate 40, which approximates the sinusoidal profile of the zone plate 39.

[0102] The stepped profiles have, comparable to the profiling according to the Fig. 8 and Fig. 10, again has a lower step profile S1 and an upper step profile S2, with the lower step profile S1 projecting equally far beyond the upper step profile S2 on both sides for sinusoidal approximation. A distance between adjacent lower step profiles S1 corresponds to a radial extension of the upper step profile S2.

[0103] The step profiling of the zone plate 40 approximates the sinusoidal profile Fig. 15 over a total of three levels. Alternatively, an approximation using a different number of levels is also possible, for example, four, five, six, or even more. The higher the number of levels, the better the approximation to a sinusoidal profile can be achieved.

[0104] Accordingly, the diffraction effect for the zone plate 40 is Fig. 16 or a corresponding zone plate with a sinusoidal profile approximated over a higher number of step levels, a diffraction effect in which the +1st and -1st diffraction orders of the illumination light 4 are preferred.

[0105] To measure the structure of object 2, an image of the object structure in the object field 12 is created by the detection device 6 by sequentially scanning the individual points of the object structure in the x / y direction. Depending on the measurement method, either a single image or an image stack (aerial image) is acquired in multiple z-positions. In this case, the object 2 is moved to corresponding z-positions using the object holder 15 and the actuator 16. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2012 / 0 008 123 A1

[0002] US 9,904,060

[0002] Zitierte Nicht-Patentliteratur

[0000] Na J. et al. „Application of actinic mask review system for the preparation of HVM EUV lithography with defect free mask“, Proc. of SPIE Vol. 10145, 101450M-1

[0002] Goldberg K. et al. „Actinic mask imaging: recent results and future directions from the SHARP EUV microscope“, Proc. of SPIE Vol. 9049, 90480Y-1

[0002] Naulleau et al. „Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source“, Optics Express, Vol. 22, 20144, 2014 [0002, 0022] R. Menon: Diffractive optics for maskless lithography and imaging, Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003

[0054] Huang, K. L. (2017). Huang, K., Liu, H., Si, G., Wang, Q., Lin, J., Teng, J., Laser & Photonics Reviews 2017, 11, 1700025. Laser & Photonics Reviews, S. 1700025

[0056] Cai. H. et al. (2019), Ultrathin transmissive metasurfaces for multi-wavelength optics in the visible. Appl. Phys. Lett., S071106

[0087]

Claims

[1] Optical system (8) for a metrology system (1) for measuring an object (2), - with an object holder (15) for holding an object (2) in an object plane (13), - with an optical focusing component (10; 19; 20; 24; 27; 30; 31; 32; 33; 35; 36; 37; 39; 40) arranged in the beam path of illuminating light (4) between a light source (5) of the metrology system (1) and an object field (12) in the object plane (13), for generating an illumination focus (14) in the beam path of the illuminating light (4) after the optical focusing component (10; 19; 20; 24; 27; 30; 31; 32; 33; 35; 36; 37; 39; 40), - with a detection device (6) for detecting the illuminating light (4) in the beam path after the object field (12), - wherein the optical focusing component (10; 19; 20; 24; 27; 30; 31; 32; 33; 35; 36; 37; 39; 40) is designed as a zone plate with at least two zones (Z i , Z i+1), wherein a portion of the illumination light (4) directed onto a first (Z i ) of the zones, with a further portion of the illuminating light (4) directed to another (Z i+1 ) of the zones, interacts by diffraction, - where -- the zones (Z i ) of the zone plate (20; 30; 32) are arranged such that the zone plate (20; 30; 32) is chromatically corrected for the illuminating light (4), and / or -- the zones (Z i ) of the zone plate (24) are arranged such that the zone plate (24) is aspherically corrected for the illuminating light (4), and / or -- the zones (Z i ) of the zone plate (19; 20; 24; 27; 30; 31; 32; 33; 35; 36; 37; 39; 40) are designed to reflect the illuminating light (4) and / or -- the zones (Z i) of the zone plate (10; 19; 20; 24; 27; 30; 31; 32; 33; 36; 37; 39; 40) are arranged such that at least one diffraction order (+1; + / -1) of the illumination light (4) is preferred as the default diffraction order in comparison to at least one further diffraction order with regard to the useful illumination light intensity guided by the zone plate (10; 19; 20; 24; 27; 30; 31; 32; 33; 36; 37; 39; 40). [2] Optical system according to claim 1, characterized by that the zones (Z i ) are arranged in a ring shape, wherein a number of zone rings (ZR i ) of the zone plate (10; 19; 20; 24; 27; 30; 31; 32; 33; 35; 36; 37; 39; 40) is greater than 100. [3] Optical system according to claim 1 or 2, characterized by that the zone plate (10; 20; 24; 33; 35; 36; 37; 39; 40) is designed as a transmitting phase plate, wherein the zones (Z i) influence the phase of the illuminating light (4) incident on the zone plate (10; 20; 24; 33; 35; 36; 37; 39; 40) differently. [4] Optical system according to one of claims 1 to 3, characterized by that the zone plate (20) is designed as a photon sieve. [5] Optical system according to claim 4, characterized by that the photon sieve (20) is designed such that an illumination light focus (14) of a specific diffraction order (+1) is preferred over an illumination light focus of at least one further diffraction order with regard to the useful illumination light intensity guided by the zone plate. [6] Optical system according to one of claims 1 to 5, characterized by that a discontinuous radial distribution of ring widths (Δr i ) of the zone rings (ZR i ) of the zone plate or pinholes of the photon sieve (20) an aspherical correction is impressed. [7] Optical system according to one of claims 1 to 6, characterized by that a zone layer (28) which contains the zones (Z i ) is applied to a reflection layer (29) of the zone plate (27; 30; 31; 32). [8] Optical system according to claim 7, characterized by that the zone layer (28) has at least one absorber layer made of an absorber material. [9] Optical system according to claim 7 or 8, characterized by that the reflection layer (28, 29) is designed as a multilayer layer, wherein the zone layer (28) has at least a multilayer portion of the reflection layer (28, 29). [10] Optical system according to one of claims 1 to 9, characterized by that at least some of the zones (ZR i ) in the meridional section of the zone plate (33; 36; 37; 39) deviate from a rectangular profile. [11] Metrology system (1) for measuring an object (2), - with an optical system according to one of claims 1 to 9, - with a light source (5) for generating illuminating light (4). [12] Metrology system according to claim 11, characterized by that the light source (5) is an EUV light source.

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