Diffraction grating

Abstract

An optical assembly for a topography measurement system, the optical assembly comprises: a first reflector; a second reflector; and a third reflector located such that the second reflector is located along an optical path between the first reflector and the third reflector; wherein a focal length of the first reflector is different from a focal length of the third reflector such that the optical assembly has a magnification dependent on a ratio of the focal length of the first reflector to the focal length of the third reflector.

Description

DIFFRACTION GRATINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24211599.6 which was filed on 07 November 2024 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The present disclosure relates generally to a diffraction grating, a projection unit, a detection unit, a topography measurement system and a method of making a diffraction grating.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern of a patterning device (e.g., a mask, a reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, deep ultraviolet (DUV) radiation with a wavelength of 157 nm or 193 nm or 248 nm.

[0005] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of the substrate. There is a general need to improve the diffraction grating of such a system, for example reducing manufacturing cost, making it easier to manufacture and / or enabling a wider variety of configurations of the topography measurement system.SUMMARY

[0006] According to an embodiment, there is provided a diffraction grating for a topography measurement system, wherein the diffraction grating comprises: first areas configured to output incoming radiation in a first direction, wherein the first areas are of a first type selected from the group consisting of reflective areas configured to reflect the incoming radiation and transmissive areas configured to transmit the incoming radiation; and second areas configured to output incoming radiation in a second direction different from the first direction, wherein the first areas alternate with the second areas along the diffraction grating, wherein the second areas are of a second type different from the first type and selected from the group consisting of transmissive areas configured to transmit the incoming radiation and open areas configured for passage of the incoming radiation.

[0007] According to an embodiment, there is provided a method of making a diffraction grating for a topography measurement system, the method comprising: forming first areas configured to output radiation in a first direction, wherein the first areas are of a first type selected from the group consisting of reflective areas configured to reflect the incoming radiation and transmissive areas configured to transmit the incoming radiation; and forming second areas configured to output radiation in a second direction different from the first direction, wherein the first areas alternate with the second areas along the diffraction grating, wherein the second areas are of a second type different from the first type and selected from the group consisting of transmissive areas configured to transmit the incoming radiation and open areas configured for passage of the incoming radiation.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0009] Figure 1 schematically depicts a lithographic system comprising a radiation source and a lithographic apparatus.

[0010] Figure 2 schematically depicts a level sensor, for example of the lithographic apparatus of Figure 1.

[0011] Figure 3 schematically depicts a detection grating, for example of the level sensor of Figure 2.

[0012] Figure 4 schematically depicts a comparative example of a detection grating.

[0013] Figure 5 schematically depicts an alternative detection grating, for example of the level sensor of Figure 2.

[0014] Figure 6 schematically depicts an alternative detection grating, for example of the level sensor of Figure 2.

[0015] Figure 7 schematically depicts an alternative detection grating, for example of the level sensor of Figure 2.

[0016] Figure 8 schematically depicts an alternative detection grating, for example of the level sensor of Figure 2.

[0017] Figure 9 schematically depicts an alternative detection grating, for example of the level sensor of Figure 2.

[0018] Figure 10 schematically depicts part of a detection unit, for example of the level sensor of Figure 2.

[0019] Figure 11 schematically depicts part of an alternative detection unit, for example of the level sensor of Figure 2.

[0020] Figure 12 schematically depicts part of an alternative detection unit, for example of the level sensor of Figure 2.DETAILED DESCRIPTION

[0021] Embodiments of the present disclosure are described in detail with reference to the drawings, which are provided as illustrative examples of the disclosure so as to enable those skilled in the art to practice the disclosure. Notably, the figures and examples below are not meant to limit the scope of the present disclosure to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present disclosure can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present disclosure will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the disclosure. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the disclosure is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present disclosure encompasses present and future known equivalents to the known components referred to herein by way of illustration. Although specific reference may be made in this text to the manufacture of ICs, it should be explicitly understood that the description herein has many other possible applications.

[0022] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV and / or a DUV radiation beam B and to supply the EUV and / or DUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT (e.g., a mask table, a reticle table, a reticle stage) configured to support a patterning device MA (e.g., a mask, a reticle), a projection system PS, and a substrate table WT configured to support a substrate W.

[0023] The illumination system IL is configured to condition the EUV and / or DUV radiation beam B before the EUV and / or DUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV and / or DUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0024] After being thus conditioned, the EUV and / or DUV radiation beam B interacts with the patterning device MA. This interaction may be reflective (as shown), which may be preferred for EUV radiation. This interaction may be transmissive, which may be preferred for DUV radiation. As a result of this interaction, a patterned EUV and / or DUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV and / or DUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV and / or DUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV and / or DUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in FIG. 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0025] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV and / or DUV radiation beam B’, with a pattern previously formed on the substrate W.

[0026] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0027] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or nonpolarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate.

[0028] The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0029] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0030] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0031] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR (not shown).

[0032] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0033] In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0034] Various height sensors of a general type are disclosed for example in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In W02016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating.

[0035] Figure 3 schematically depicts a diffraction grating. The diffraction grating is for a topography measurement system LS. The topography measurement system LS may be, for example, a level sensor or a height sensor. Features of the diffraction grating are described in the context of thediffraction grating being used as a detection grating DGR of the topography measurement system LS. It is not essential for the diffraction grating to be used as a detection grating DGR. Alternatively, or additionally, the diffraction grating may be used as a projection grating PGR, for example.

[0036] In an embodiment the detection grating DGR is configured to separate incoming radiation into two different signals of the topography measurement system LS. The two signals may be referred to as a left signal and a right signal (or as an up signal and a down signal, for example). The detection grating DGR may be configured to receive incoming radiation in one direction and to output that radiation in two different directions.

[0037] As shown in Figure 3, in an embodiment the detection grating DGR comprises first areas 11 configured to output the incoming radiation in a first direction and second areas 12 configured to output the incoming radiation in a second direction. The second direction is different from the first direction. Figure 3 shows a first beam 31 of radiation output by the first areas 11 and a second beam 32 of radiation output by the second areas 12.

[0038] As shown in Figure 3, in an embodiment the detector DET comprises a first detection element 21 and a second detection element 22. The first detection element 21 may be configured to detect one signal (for example the left signal) of the topography measurement system LS. The second detection element 22 may be configured to detect another signal (e.g. the right signal) of the topography measurement system LS. As shown in Figure 3, in an embodiment the first detection element 21 is configured to receive the first beam 31 of radiation output by the first areas 11. The second detection element 22 may be configured to receive the second beam 32 of radiation output by the second areas 12 of the detection grading DGR.

[0039] In an embodiment the first areas 11 and the second areas 12 are of different types. Different types of areas of a grating that may output radiation are reflective areas, transmissive areas and open areas. Reflective areas are configured to reflect the incoming radiation. For example, in the arrangement shown in Figure 3 the first areas 11 are reflective areas. Transmissive areas are configured to transmit the incoming radiation. The incoming radiation passes through a volume of material forming the transmissive areas. For example, in the arrangement shown in Figure 3 the second areas 12 are transmissive areas. Open areas are areas where the detection grating DGR is open. The open areas correspond to gaps or apertures in the detection grating DGR. The open areas allow passage of the incoming radiation.

[0040] Figure 4 schematically depicts a comparative example of a detection grating DGR. In the comparative example shown in Figure 4, the detection grating DGR is transmissive and includes prism shapes to separate the left and the right signals of the topography measurement system. The incoming radiation 30 is separated into the left signal 34 and the right signal 33 which are detected by the first detection element 21 and the second detection element 22, respectively.

[0041] As shown in Figure 4, it is necessary for the incoming radiation 30 to be incident on the detection grating DGR substantially normally, i.e. perpendicular to the direction across the detectiongrating DGR. If radiation is received at a significantly different angle, then the radiation may significantly reflect off the grating. In contrast, as shown in Figure 3 the detection grating DGR may allow the incoming radiation 30 to be received at an angle. For example the incoming radiation 30 may be angled relative to a direction perpendicular to the direction across the detection grating DGR. The term “angled” means at a non-zero angle, i.e. not parallel. For example, an angle defined between the incoming radiation 30 and a direction perpendicular to the direction across the detection grating DGR may be greater than zero and less than 90 degrees. An embodiment of the invention is expected to increase the different possible configurations of the topography measurement system LS.

[0042] For example, by allowing the incoming radiation 30 to be received at an angle, the detection grating DGR may be in a Scheimpflug condition. By satisfying the Scheimpflug principle, focus of the radiation on the substrate may be improved. For example, radiation may be emitted onto the substrate W at an angle (i.e. not perpendicularly to the surface of the substrate W). The reflected radiation may leave the surface of the substrate at an angle. The substrate W may be in the object plane or the image plane of the optical system comprising the diffraction grating. For example, when the diffraction grating is a detection grating DGR, then the substrate W may be at the object plane. By satisfying the Scheimpflug principle, such an angle between the direction of the incoming radiation 30 and the direction perpendicular to the direction across the detection grating DGR may be accommodated without unduly causing defocusing (e.g. blurring) of the radiation.

[0043] In an embodiment the optical system comprising the diffraction grating may have a magnification different from 1. By satisfying the Scheimpflug principle, magnification of the optical system of the topography measurement system may be increased. An embodiment of the invention is expected to increase the field of view of the topography measurement system LS. By increasing the field of view of the topography measurement system LS, the number of scans required to measure a substrate may be reduced. The time required to measure a substrate may be reduced. An embodiment of the invention is expected to reduce the time required for measuring the topography of a substrate.

[0044] As shown in Figure 4, the prisms of the detection grating DGR may only be able to separate the light path with a limited angle. This creates the need for an optical system downbeam of the detection grating DGR for separating the signals. The separating optics may need a long path length.

[0045] As shown in Figure 3, an embodiment of the invention is expected to increase the angle between the first beam 31 and the second beam 32 output by the first areas 11 and the second areas 12 of the detection grating DGR. An embodiment of the invention is expected to simplify the separation optics downbeam of the detection grating DGR.

[0046] In an embodiment an angle between the first direction and the second direction is at least as great as the numerical aperture, i.e. the cone of incident light on the detection grating DGR. In an embodiment an angle between the first direction and the second direction is at least 2 degrees, optionally at least 5 degrees, optionally at least 10 degrees, optionally at least 20 degrees, optionally atleast 30 degrees, optionally at least 45 degrees, optionally at least 50 degrees, optionally at least 60 degrees, optionally at least 70 degrees, and optionally at least 80 degrees.

[0047] As shown in Figure 3, in an embodiment the detection grating DGR comprises a transmissive substrate 14. The transmissive substrate 14 is configured to transmit the incoming radiation 30. For example, the transmissive substrate may comprise a glass.

[0048] As shown in Figure 3, in an embodiment the reflective areas 11 are at a surface 13 of the transmissive substrate 14. For example, in the arrangement shown in Figure 3, the reflective areas 11 are at the facing surface 13 of the transmissive substrate 14. The facing surface 13 faces the incoming radiation 30. The facing surface 13 is the surface of the detection grating DGR on which the incoming radiation 30 is incident.

[0049] The incoming radiation 30 that is incident on the reflective areas 11 is reflected as the first beam 31 towards the first detector element 21. The incoming radiation 30 that is incident on the second areas 12 is transmitted as the second beam 32 towards the second detector element 22. The second areas 12 are transmissive areas formed in regions between the first areas 11 (i.e. the reflective areas).

[0050] In an embodiment the reflective areas 11 are formed by a coating 15. For example, the coating 15 may be applied to the transmissive substrate 14. The coating 15 may be patterned so as to form the first areas 11. The coating 15 may be patterned lithographically so as to form the reflective areas. Transmissive areas may be formed in the gaps between the sections of coating 15. The coating 15 may be applied to the facing surface 13 of the transmissive substrate 14.

[0051] As shown in Figure 3, in an embodiment the first areas 11 and the second areas 12 are substantially coplanar. An embodiment of the invention is expected to make it easier to manufacture the detection grating DGR.

[0052] As shown in Figure 3, in an embodiment a surface of the detection grating DGR configured to receive the incoming radiation 30 is substantially planar. For example, in the arrangement shown in Figure 3 the facing surface 13 is substantially planar. It is possible that the facing surface 13 may not be completely flat. It is possible that the first areas 11 and the second areas 12 may not be perfectly coplanar. For example, the coating 15 that forms the first areas 11 may add to the overall thickness of the detection grating DGR compared to the second areas 12.

[0053] It is not essential for the first areas 11 and the second areas 12 to be exactly coplanar. For example, Figure 5 schematically depicts an alternative embodiment of the detection grating DGR. As shown in Figure 5, in an embodiment the transmissive areas, i.e. the second areas 12 are angled relative to the reflective areas (i.e. the first areas 11). The reflective areas are substantially flush with the facing surface 13 of the detection grating DGR. The first areas 11 are formed substantially parallel with the plane of the detection grating DGR. The first areas 11 are substantially parallel to a direction across the detection grating DGR. The second areas 12 have a topography relative to the plane of the detection grating DGR.

[0054] For example, as shown in Figure 5 in an embodiment the transmissive areas are formed at concave portions 17 of the detection grating DGR. In an embodiment each concave portion 17 is defined by a first surface 18 and a second surface 19. The first surface 18 may be substantially normal to the direction of the incoming radiation 30. The second surface 19 may be substantially perpendicular to the first surface 18. The second surface 19 may be substantially parallel to the direction of the incoming radiation 30. In an embodiment at least one of the concave portions 17 is replaced by a convex portion. In an embodiment each convex portion is defined by a first surface 18 and a second surface 19. The first surface 18 may be substantially normal to the direction of the incoming radiation 30. The second surface 19 may be substantially perpendicular to the first surface 18. The second surface 19 may be substantially parallel to the direction of the incoming radiation 30.

[0055] By providing that the transmissive areas are formed by the first surface 18 of the concave portion 17, undesirable reflection of the incoming radiation 30 may be reduced. An embodiment of the invention is expected to reduce spectral losses of the incoming radiation 30.

[0056] The embodiment of Figure 5 may additionally have convex portions 27 located on the radiation exit surface to ensure that internal reflection of radiation in the body of the detection grating is minimised. For example, in an embodiment as shown in Figure 6, each convex portion is defined by a first surface 28 and a second surface 29. The first surface 28 may be substantially normal to the direction of the incoming radiation 30, and accordingly substantially parallel to the first surface 18 of the concave portion 17. The second surface 29 may be substantially perpendicular to the first surface 28, and in substantially the same plane as the second surface 19 of the concave portion 17.

[0057] In an embodiment the method of manufacturing the detection grating DGR comprises etching the transmissive substrate 14 between the reflective areas so as to form concave portions 17 in which the transmissive areas are provided. For example, a coating 15 may be applied to the transmissive substrate 14. The coating 15 may be patterned, for example in a lithography step. The transmissive substrate 14 may then be etched so as to form the concave portions 17.

[0058] It is not essential for the reflective areas that form the first areas 11 to be provided at the facing surface 13 of the detection grating DGR. Figure 7 schematically depicts an alternative embodiment in which the reflective coating 15 is provided on the other side of the detection grating DGR. As shown in Figure 7, in an embodiment the reflective areas (i.e. the first areas 11) are at a surface of the transmissive substrate 14 facing away from the incoming radiation 30. The incoming radiation 30 is transmitted through at least a portion of the transmissive substrate 14 before being reflected at the reflective areas (i.e. the first areas 11). An embodiment of the invention is expected to increase flexibility of how the detection grating DGR is used.

[0059] In the arrangements shown in Figure 3 and Figure 7, for example, the first areas 11 are reflective areas and the second areas 12 are transmissive areas. However, it is not essential for the second areas 12 to be transmissive areas, for example by providing a transmissive substrate 14.

[0060] Figure 8 schematically depicts an alternative embodiment in which the second areas 12 correspond to open areas. In the arrangement shown in Figure 8, the first areas 11 are of the reflective area type and the second areas 12 are of the open area type. The second areas 12 are configured for passage of the incoming radiation 30. Part of the incoming radiation 30 passes through the open areas 12 so as to form the second beam 32 towards the second detector element 22.

[0061] As shown in Figure 8, in an embodiment the detection grating DGR comprises a substrate 16. The substrate 16 is not required to be a transmissive substrate. For example, the substrate 16 may be a silicon substrate. Alternatively, it is possible for the substrate 16 to be a transmissive substrate.

[0062] As shown in Figure 8, in an embodiment the substrate 16 comprises the reflective areas.For example, the substrate 16 may comprise a reflective material such that the facing surface 13 of the substrate 16 forms the first areas 11 as reflective areas. However, it is not essential for the substrate 16 to be made of a reflective material. As shown in Figure 8, in an embodiment the detection grating DGR comprises a coating 15 at the facing surface 13 of the substrate 16. The coating 15 may be considered to be part of the substrate 16. Alternatively, the coating 15 may be considered to be a separate component from the substrate 16. The coating 15 is attached to the substrate 16. The coating 15 may form the facing surface 13 of the detection grating DGR.

[0063] As shown in Figure 8, in an embodiment the open areas (i.e. the second areas 12) correspond to gaps 17 in the substrate 16. The gaps 17 may be apertures. The gaps 17 may be longitudinally shaped. The gaps 17 are formed between the first areas 11.

[0064] In an embodiment a method of manufacturing the detection grating DGR comprises providing the substrate 16. In an embodiment the method comprises applying the coating 15 to a surface of the substrate 16 for forming the reflective areas. Alternatively, the substrate 16 may be a reflective substrate.

[0065] In an embodiment the method comprises removing portions of the substrate 16 to form the gaps 17 that form the open areas. For example, the substrate 16 may be etched to form the gaps 17.In an embodiment the substrate 16 is etched along the grain direction of the substrate 16. The substrate 16 may be selected to have a specific crystal orientation so that the etching along the grain direction is in the desired direction of the gaps 17. For example, as shown in Figure 8, in an embodiment the gaps 17 are formed substantially in the same direction as the incoming radiation 30. The incoming radiation 30 may be angled relative to the facing surface 13 of the detection grating DGR. The gaps 17 may be formed by etching through the substrate 16 at an angle relative to the facing surface 13 of the substrate 16.

[0066] Figure 9 schematically depicts an alternative embodiment of the detection grating DGR. As shown in Figure 9, in an embodiment the first areas 11 are of the transmissive area type and the second areas 12 are of the open area type. It is not essential for either the first areas 11 or the second areas 12 to be formed as reflective areas. Different properties of the first areas 11 and the secondareas 12 of the detection grating DGR are used so as to separate the incoming radiation 30 into the first beam 31 and the second beam 32.

[0067] In the arrangement shown in Figure 9, some of the incoming radiation 30 is transmitted through the first areas 11 to form the first beam 31 of radiation. As shown in Figure 9, the incoming radiation 30 may be bent as it is transmitted through the first areas 11 to form the first beam 31. As shown in Figure 9, the detection grating DGR may be oriented such that the incoming radiation 30 is incident on the first areas 11 at an angle (i.e. not normal to the detection grating DGR). Some of the incoming radiation 30 passes through the gaps 17 that form the second areas 12 to form the second beam 32 of radiation.

[0068] As shown in Figure 9, in an embodiment the detection grating DGR comprises a transmissive substrate 14. The method of manufacturing the detection grating DGR may comprise providing the transmissive substrate 14. The transmissive substrate 14 is configured to transmit he incoming radiation 30.

[0069] In the arrangement shown in Figure 3 or Figure 7, for example, the method of manufacturing the detection grating DGR may comprise applying a coating 15 to a surface of the transmissive substrate 14 for forming the reflective areas. In the arrangement shown in Figure 9, it is not necessary to apply any coating to the transmissive substrate 14 because it is not necessary to form any such reflective areas.

[0070] Figure 10 schematically depicts part of a detection unit LSD, for example of the topography measurement system LS shown in Figure 2. As shown in Figure 10, in an embodiment the detection unit LSD is arranged such that the radiation detected by the first detection element 21 and the radiation detected by the second detection element 22 experience substantially the same spectral losses along their respective optical paths. Some of the incoming radiation 30 may be lost in the process of outputting the radiation by the detection grating DGR towards the first detection element 21 and the second detection element 22. For example, spectral losses may occur as the radiation is transmitted through a transmissive substrate 14. Spectral losses may occur as the radiation is reflected by a reflective area.

[0071] By arranging the detection unit LSD such that the spectral losses are substantially the same, the different branches corresponding to the different signals of the incoming radiation 30 may be matched. This may help to improve the accuracy of the measurements made by the topography measurement system LS. The radiation may comprise radiation of different wavelengths.Aberrations can occur when radiation of different wavelengths interact differently with the substrate W, for example. By arranging the detection unit LSD such that the spectral losses are substantially the same, chromatic aberration may be reduced.

[0072] In the example shown in Figure 10, the second areas 12 are formed as transmissive areas configured to transmit the incoming radiation 30. As shown in Figure 10, in an embodiment the detection unit LSD comprises a transmissive element 44 in the optical path of the radiation output bythe first areas 11. By providing the transmissive element 44, spectral losses experienced by the radiation output by the first areas 11 substantially match spectral losses experienced by the radiation output by the transmissive areas. Some spectral loss may be experienced by the transmission through the transmissive substrate 14 when forming the second beam 32 of radiation. In an embodiment the transmissive element 44 comprises substantially the same material as the transmissive substrate 14. The transmissive element 44 may have substantially the same thickness as the transmissive substrate 14. The angle between the transmissive element 44 and the second beam 31 may be substantially equal to the angle between the incoming radiation 30 and the transmissive substrate 14.

[0073] As shown in Figure 10, in an embodiment the first areas 11 are reflective areas. In an embodiment a reflective element 45 is provided in the optical path of the second beam 32. By providing the reflective element 45, the spectral losses experienced by the second beam 32 may substantially equal the spectral losses experienced by the first beam 31 due to reflection. For example, some radiation may be lost when the incoming radiation 30 is reflected at the coating 15 on the transmissive substrate 14. A similar (or the same) spectral loss may be experienced when the second beam 32 is reflected by the reflective element 45. In an embodiment the angle between the second beam 32 and the reflective element 45 is substantially equal to the angle between the incoming radiation 30 and the first areas 11. For example, the reflective element 45 may be oriented substantially parallel to the detection grating DGR.

[0074] As shown in Figure 10, in an embodiment the transmissive element 44 is located between the detection grating DGR and the detection DET (e.g. the first detection element 21). As shown in Figure 10, in an embodiment the reflective element 45 is located in the optical path between the detection grating DGR and the second detection element 22.

[0075] Figure 11 schematically depicts an alternative embodiment for branch matching. As shown in Figure 11, in an embodiment the reflective areas are embedded within the transmissive substrate 14. As shown in Figure 11, in an embodiment the transmissive substrate 14 comprises a first piece 51 and a second piece 52. The first piece 51 may be substantially parallel to the second piece 52. The first piece 51 may be secured, for example bonded, to the second piece 52 so as to form the transmissive substrate 14. Alternatively, the transmissive substrate 14 may be formed as an integral single piece.

[0076] In an embodiment the first areas 11 are formed at the junction between the first piece 51 and the second piece 52. For example, a coating 15 may be applied to a surface of one of the first piece 51 and the second piece 52. The other of the first piece 51 and the second piece 52 may be bonded to the piece that has the coating 15 applied. The coating may be patterned so as to form the first areas 11 as reflective areas before the first piece 51 and the second piece 52 are secured together.

[0077] In an embodiment the first piece 51 and the second piece 52 have substantially the same thickness. The incoming radiation 30 that is output as the first beam 31 (i.e. by being reflected by the first areas 11) is transmitted through the first piece 51 twice. The incoming radiation 30 that istransmitted through the transmissive substrate 14 at the second areas 12 passes through the first piece 51 and the second piece 52. In an embodiment both the first beam 31 and the second beam 32 are transmitted through substantially the same thickness of the material that forms the transmissive substrate 14. In an embodiment substantially the same spectral losses are experienced by the first beam 31 and the second beam 32.

[0078] As shown in Figure 11, in an embodiment the reflective element 45 is provided in the optical path of the second beam 32. The reflective element 45 helps to match the spectral losses due to reflection for the two branches (i.e. for the first beam 31 and the second beam 32). In an alternative embodiment the reflective element 45 is omitted. In an embodiment the spectral losses caused by reflection at the reflective areas is insignificant compared to the spectral losses due to transmission through the transmissive areas.

[0079] As mentioned above, in an embodiment the Scheimpflug principle is satisifed. In an embodiment the detection grating DGR is comprised in the detection unit LSD together with an optical assembly and the detector DET. In an embodiment the optical assembly and the detection grating DGR are arranged such that a direction of a beam of radiation from the optical assembly and incoming on the detection grating DGR is angled relative to a direction perpendicular to a direction along the detection grating DGR. The detection grating DGR may be located at the image plane of the optical assembly. The image plane may be tilted relative to a plane perpendicular to an optical axis of the optical assembly. In other words, the detection grating DGR may be tilted relative to the plane perpendicular to the optical axis of the optical assembly. In an embodiment the substrate that is to be measured by the topography measurement system LS is at the object plane of the optical assembly. The object plane and the image plane may be tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.

[0080] Additionally or alternatively, the diffraction grating that is described primarily as a detection grating DGR may be used as a projection grating PGR. The projection grating PGR may be comprised in a projection unit LSP for the topography measurement system LS. The projection unit may further comprise the radiation source LSO and an optical assembly. The optical assembly may be located such that the projection grating PGR is at an object plane of the optical assembly. In an embodiment the radiation source LSO and the projection grating PGR are arranged such that a direction of the beam of radiation incoming on the projection grating PGR is angled relative to a direction perpendicular to a direction along the projection grating PGR. The optical assembly is located such that the projection grating PGR is at the object plane of the optical assembly. In an embodiment the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly. In other words, the projection grating PGR may be tilted relative to a plane perpendicular to the optical axis of the optical assembly. In an embodiment the substrate that is to be measured by the topography measurement system LS is at the image plane of the optical assembly. In anembodiment the object plane and the image plane are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.

[0081] When the diffraction grating is used as the projection grating PGR, one of the first beam 31 and the second beam 32 may be output by the diffraction grating to be incident on the substrate W. The other of the first beam 31 and the second beam 32 may be blocked. For example, a radiation absorber may be provided to absorb the other of the first beam 31 and the second beam 32.

[0082] Figure 12 schematically depicts an alternative embodiment of part of a detection unit LSD. As shown in Figure 12, in an embodiment the detector DET is integrated with the detection grating DGR. For example, the first detection element 21 and / or the second detection element 22 may be secured to the detection grating DGR. Alternatively, the first detection element 21 and the second detection element 22 may be distanced from the detection grating DGR, for example as shown in Figure 3.

[0083] By integrating the detector DET with the detection grating DGR, the detection unit LSD may be more compact. An embodiment of the invention is expected to reduce the volume taken up by the topography measurement system LS.

[0084] In the embodiments described above, the gratings are configured to separate incoming radiation into two different signals. The gratings comprise first areas 11 configured to output the incoming radiation in a first direction and second areas 12 configured to output the incoming radiation in a second direction. The gratings may alternatively be configured to separate incoming radiation into more than two different signals, each in different directions. For example, the grating may be configured to separate incoming radiation into three different signals. In such a case, the grating comprises first areas configured to output the incoming radiation in a first direction, second areas configured to output the incoming radiation in a second direction different to the first direction, and third areas configured to output the incoming radiation in a third direction different to the first and second directions. The output radiation signals in the first, second and third directions may for example be referred to as a left signal, a right signal, and a center signal. Any combination of transmissive areas, for example in the form of shaped prisms, and / or reflective areas, and / or open areas may be used to achieve an embodiment configured to separate incoming radiation into more than two different signals. When such a grating is used in a detection unit, any suitable detector arrangement which may comprise one or more detectors configured to detect the more than two signals. Such arrangements advantageously provide an increased sensitivity and range of measurement, meaning that a larger height range may be detected without any change in illumination of the grating.

[0085] While the present invention has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

[0086] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below. Aspects of the invention are set out in the clauses below.1. A diffraction grating for a topography measurement system, wherein the diffraction grating comprises: first areas configured to output incoming radiation in a first direction, wherein the first areas are of a first type selected from the group consisting of reflective areas configured to reflect the incoming radiation and transmissive areas configured to transmit the incoming radiation; and second areas configured to output incoming radiation in a second direction different from the first direction, wherein the first areas alternate with the second areas along the diffraction grating, wherein the second areas are of a second type different from the first type and selected from the group consisting of transmissive areas configured to transmit the incoming radiation and open areas configured for passage of the incoming radiation.2. The diffraction grating of clause 1, wherein the first areas are reflective areas configured to reflect the incoming radiation and the second areas are transmissive areas configured to transmit the incoming radiation.3. The diffraction grating of clause 2, comprising: a transmissive substrate configured to transmit the incoming radiation.4. The diffraction grating of clause 3, wherein the reflective areas are at a surface of the transmissive substrate.5. The diffraction grating of clause 4, wherein the reflective areas are at a surface of the transmissive substrate facing the incoming radiation.6. The diffraction grating of clause 4, wherein the reflective areas are at a surface of the transmissive substrate facing away from the incoming radiation such that the incoming radiation is transmitted through at least a portion of the transmissive substrate before being reflected at the reflective areas.7. The diffraction grating of clause 4, wherein the reflective areas are embedded within the transmissive substrate.8. The diffraction grating of any of clauses 2-7, wherein the transmissive areas are angled relative to the reflective areas.9. The diffraction grating of clause 8, wherein the transmissive areas are formed at concave portions of the diffraction grating.10. The diffraction grating of clause 8, wherein the transmissive areas are formed at convex portions of the diffraction grating.11. The diffraction grating of clause 1, wherein the first areas are reflective areas configured to reflect the incoming radiation and the second areas are open areas configured for passage of the incoming radiation.12. The diffraction grating of clause 11, comprising: a substrate comprising the reflective areas, wherein the open areas correspond to gaps in the substrate.13. The diffraction grating of clause 1, wherein the first areas are transmissive areas configured to transmit the incoming radiation and the second areas are open areas configured for passage of the incoming radiation.14. The diffraction grating of any preceding clause, wherein the first areas and the second areas are substantially coplanar.15. The diffraction grating of any preceding clause, wherein a surface of the diffraction grating configured to receive the incoming radiation is substantially planar.16. The diffraction grating of any preceding clause, configured such that an angle between the first direction and the second direction is at least 10 degrees, optionally at least 20 degrees, and optionally at least 50 degrees.17. A projection unit for a topography measurement system, the projection unit comprising: a radiation source configured to provide a beam of radiation; the diffraction grating of any preceding clause configured to pattern the beam of radiation; and an optical assembly located such that the diffraction grating is at an object plane of the optical assembly.18. The projection unit of clause 17, wherein the radiation source and the diffraction grating are arranged such that a direction of the beam of radiation incoming on the diffraction grating is angled relative to a direction perpendicular to a direction along the diffraction grating.19. The projection unit of clause 17 or 18, wherein the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.20. The projection unit of clause 19, located such that a substrate is at an image plane of the optical assembly.21. The projection unit of clause 20, wherein the object plane and the image plane are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.22. A detection unit for a topography measurement system, the detection unit comprising: an optical assembly; the diffraction grating of any of clauses 1-16 located at an image plane of the optical assembly; and a detector configured to produce a detector output signal indicative of radiation received.23. The detection unit of clause 22, wherein the optical assembly and the diffraction grating are arranged such that a direction of a beam of radiation from the optical assembly and incoming on the diffraction grating is angled relative to a direction perpendicular to a direction along the diffraction grating.24. The detection unit of clause 22 or 23, wherein the detector comprises: a first detection element configured to detect radiation output by the diffraction grating in the first direction; and a second detection element configured to detect radiation output by the diffraction grating in the second direction.25. The detection unit of clause 24, arranged such that the radiation detected by the first detection element and the radiation detected by the second detection element experience substantially the same spectral losses along their respective optical paths.26. The detection unit of clause 25, wherein the first areas or the second areas are transmissive areas configured to transmit the incoming radiation, and the detection unit comprises: a transmissive element in the optical path of the radiation output by the other of the first areas and the second areas so as to substantially match spectral losses experienced by the radiation output by the transmissive areas due to transmission through the transmissive areas.27. The detection unit of clause 26, wherein the transmissive element is located between the diffraction grating and the detector.28. The detection unit of any of clauses 25-27, wherein the first areas are reflective areas configured to reflect the incoming radiation, and the detection unit comprises: a reflective element in the optical path of the radiation output by the second areas so as to substantially match spectral losses experienced by the radiation output by the reflective areas due to reflection at the reflective areas.29. The detection unit of clause 28, wherein the reflective element is located between the diffraction grating and the second detection element.30. The detection unit of any of clauses 22-29, wherein the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.31. The detection unit of clause 30, located such that a substrate is at an object plane of the optical assembly.32. The detection unit of clause 31, wherein the object plane and the image plane are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.33. The detection unit of any of clauses 22-32, wherein the detector is integrated with the diffraction grating.34. A topography measurement system comprising at least one of the projection unit of any of clauses 17-21 and the detection unit of any of clauses 22-33.35. A topography measurement system comprising the diffraction grating of any of clauses 1- 16.36. A method of making a diffraction grating for a topography measurement system, the method comprising: forming first areas configured to output radiation in a first direction, wherein the first areas are of a first type selected from the group consisting of reflective areas configured to reflect the incoming radiation and transmissive areas configured to transmit the incoming radiation; and forming second areas configured to output radiation in a second direction different from the first direction, wherein the first areas alternate with the second areas along the diffraction grating, wherein the second areas are of a second type different from the first type and selected from the group consisting of transmissive areas configured to transmit the incoming radiation and open areas configured for passage of the incoming radiation.37. The method of clause 36, comprising: providing a transmissive substrate configured to transmit the radiation.38. The method of clause 37, comprising: applying a coating to a surface of the transmissive substrate for forming the reflective areas.39. The method of clause 38, comprising: lithographically patterning the coating so as to form the reflective areas.40. The method of any of clauses 37-39, comprising: etching the transmissive substrate between the reflective areas so as to form concave portions in which the transmissive areas are provided.41. The method of any of clauses 37-39, comprising: etching the transmissive substrate between the reflective areas so as to form convex portions in which the transmissive areas are provided.42. The method of clause 36, comprising: providing a substrate comprising the reflective areas.43. The method of clause 42, comprising: applying a coating to a surface of the substrate for forming the reflective areas.44. The method of clause 42 or 43, comprising: etching the substrate to form the gaps that form the open areas.

Claims

CLAIMS1. A diffraction grating for a topography measurement system, wherein the diffraction grating comprises: first areas configured to output incoming radiation in a first direction, wherein the first areas are of a first type selected from the group consisting of reflective areas configured to reflect the incoming radiation and transmissive areas configured to transmit the incoming radiation; and second areas configured to output incoming radiation in a second direction different from the first direction, wherein the first areas alternate with the second areas along the diffraction grating, wherein the second areas are of a second type different from the first type and selected from the group consisting of transmissive areas configured to transmit the incoming radiation and open areas configured for passage of the incoming radiation.

2. The diffraction grating of claim 1, wherein the first areas are reflective areas configured to reflect the incoming radiation and the second areas are transmissive areas configured to transmit the incoming radiation.

3. The diffraction grating of claim 2, comprising: a transmissive substrate configured to transmit the incoming radiation, and wherein the reflective areas are at a surface of the transmissive substrate..

4. The diffraction grating of claim 3, wherein the reflective areas are at a surface of the transmissive substrate facing the incoming radiation, or wherein the reflective areas are at a surface of the transmissive substrate facing away from the incoming radiation such that the incoming radiation is transmitted through at least a portion of the transmissive substrate before being reflected at the reflective areas.

5. The diffraction grating of claim 4, wherein the reflective areas are embedded within the transmissive substrate.

6. The diffraction grating of any of claims 2-5, wherein the transmissive areas are angled relative to the reflective areas.

7. The diffraction grating of claim 6, wherein the transmissive areas are formed at concave portions of the diffraction grating or wherein the transmissive areas are formed at convex portions of the diffraction grating.

8. The diffraction grating of claim 1, wherein the first areas are reflective areas configured to reflect the incoming radiation and the second areas are open areas configured for passage of the incoming radiation, or wherein the first areas are transmissive areas configured to transmit the incoming radiation and the second areas are open areas configured for passage of the incoming radiation.

9. A projection unit for a topography measurement system, the projection unit comprising: a radiation source configured to provide a beam of radiation; the diffraction grating of any preceding claim configured to pattern the beam of radiation; and an optical assembly located such that the diffraction grating is at an object plane of the optical assembly.

10. A detection unit for a topography measurement system, the detection unit comprising: an optical assembly; the diffraction grating of any of claims 1-8 located at an image plane of the optical assembly; and a detector configured to produce a detector output signal indicative of radiation received.

11. The detection unit of claim 10, wherein the optical assembly and the diffraction grating are arranged such that a direction of a beam of radiation from the optical assembly and incoming on the diffraction grating is angled relative to a direction perpendicular to a direction along the diffraction grating.

12. The detection unit of claim 10 or 11, wherein the detector comprises: a first detection element configured to detect radiation output by the diffraction grating in the first direction; and a second detection element configured to detect radiation output by the diffraction grating in the second direction, and wherein the detection unit is arranged such that the radiation detected by the first detection element and the radiation detected by the second detection element experience substantially the same spectral losses along their respective optical paths.

13. The detection unit of claim 12, wherein the first areas or the second areas are transmissive areas configured to transmit the incoming radiation, and the detection unit comprises: a transmissive element in the optical path of the radiation output by the other of the first areas and the second areas so as to substantially match spectral losses experienced by the radiationoutput by the transmissive areas due to transmission through the transmissive areas, wherein the transmissive element is located between the diffraction grating and the detector.

14. The detection unit of any of claims 12 or 13, wherein the first areas are reflective areas configured to reflect the incoming radiation, and the detection unit comprises: a reflective element in the optical path of the radiation output by the second areas so as to substantially match spectral losses experienced by the radiation output by the reflective areas due to reflection at the reflective areas, wherein the reflective element is located between the diffraction grating and the second detection element.

15. A topography measurement system comprising at least one of the projection unit of claim 9 and the detection unit of any of claims 10-14.

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