Optical system

Abstract

An optical system for a topography measurement system, the optical system configured to project radiation onto a surface and direct radiation from the surface, the optical system including: an optical assembly having: at least one concave reflector; and at least one convex reflector, wherein the optical system is arranged such that the radiation projected onto the surface and the radiation directed from the surface are reflected by the at least one concave reflector and the at least one convex reflector.

Description

2025P00005WG 1OPTICAL SYSTEMCROSS-REFERENCE TO RELATED APPLICATION

[0001] The Application claims priority of US provisional application number 63 / 756,642 which was filed on 10 February, 2025 and which is incorporated herein its entirety by reference.TECHNICAL FIELD

[0002] The present disclosure relates generally to an optical system, a topography measurement system, an apparatus and a method of projecting and directing radiation for topography measurement.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.SUMMARY

[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 increase the design freedom for the configuration of the topography measurement system.

[0006] According to an embodiment, there is provided an optical system for a topography measurement system, the optical system configured to project radiation onto a surface and direct radiation from the surface, the optical system comprising:an optical assembly comprising:at least one concave reflector; andat least one convex reflector,wherein the optical system is arranged such that the radiation projected onto the surface and the radiation directed from the surface are reflected by the at least one concave reflector and the at least one convex reflector.

[0007] According to an embodiment, there is provided a method of projecting radiation onto a2025P00005WG 2surface and directing radiation from the surface for topography measurement, the method comprising:projecting radiation onto the surface by reflecting radiation by at least one concave reflector and at least one convex reflector; anddirecting radiation from the surface by reflecting radiation by the at least one concave reflector and the at least one convex reflector.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 is a schematic view of a level sensor, for example of the lithographic apparatus of Figure 1.

[0012] Figure 4 is a perspective view of the level sensor shown in Figure 3.

[0013] Figure 5 schematically depicts an overview of a comparative level sensor.

[0014] Figure 6 is a schematic view of an alternative level sensor, for example of the lithographic apparatus of Figure 1.DETAILED DESCRIPTION

[0015] 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 other2025P00005WG 3embodiments 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.

[0016] 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 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.

[0017] The illumination system IL is configured to condition the radiation beam B before the 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 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.

[0018] After being thus conditioned, the 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 radiation beam B’ is generated. The projection system PS is configured to project the patterned 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 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 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).

[0019] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned radiation beam B’, with a pattern previously formed on the substrate W.

[0020] 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 a2025P00005WG 4substrate (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, for example, 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.

[0021] An example of a level or height sensor LS 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 receives radiation from 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 non-polarized, 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 may be visible light. 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 be redirected, e.g., reflected, from a surface of a substrate. The radiation may be DUV and / or EUV radiation.

[0022] 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 beam of radiation BE1 is redirected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0023] 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 radiation received, for example indicative of the intensity of the radiation 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.

[0024] By means of triangulation or other techniques, the height level at the measurement location2025P00005WG 5MLO 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.

[0025] 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 beam of radiation between the projection grating PGR and the detection grating DGR (not shown).

[0026] 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.

[0027] 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.

[0028] Various height sensors of a general type are disclosed for example in United States patent nos. US7265364 and US7646471, both incorporated herein in their entireties by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in United States patent application publication no. US2010233600, incorporated herein in its entirety by reference. In PCT patent application publication no. W02016102127, incorporated herein in its entirety 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.

[0029] Figure 3 schematically depicts an optical system 30. The optical system 30 is for a topography measurement system such as a level sensor LS. The level sensor LS may be part of a lithographic apparatus, for example as shown in Figure 1.

[0030] In an embodiment the optical system 30 is configured to project radiation onto a surface. For example, as shown in Figure 3, in an embodiment the optical system 30 is configured to project the beam of radiation BE1 onto the substrate W. The substrate W may comprise the surface onto which the optical system 30 is configured to project radiation. In an embodiment the optical system 30 is configured to project the radiation onto the surface (e.g. of the substrate W) for topography measurement.

[0031] As shown in Figure 3, in an embodiment the optical system 30 is configured to direct radiation from the surface. For example, as shown in Figure 3 the optical system 30 may be configured to direct the redirected beam of radiation BE2 from the substrate W. The substrate W may comprise the surface from which the optical system 30 is configured to direct radiation. In an embodiment the optical system 30 is configured to direct radiation from the surface (e.g. of the substrate W) for topography measurement. The directed radiation may be detected so as to provide topography measurement signals.

[0032] As shown in Figure 3, in an embodiment the optical assembly 30 comprises at least one2025P00005WG 6concave reflector 31. In the arrangement shown in Figure 3, the optical system 30 comprises two concave reflectors 31, 33. Each concave reflector 31, 33 is configured to reflect radiation incident on it. In an embodiment the at least one concave reflector 31 comprises a curved reflecting surface.

[0033] As shown in Figure 3, in an embodiment the optical assembly 30 comprises at least one convex reflector 32. The convex reflector 32 is configured to reflect radiation incident on it. In an embodiment the convex reflector 32 comprises a curved reflective surface.

[0034] As shown in Figure 3, in an embodiment the optical system 30 is arranged such that the radiation projected onto the surface and the radiation directed from the surface are reflected by the at least one concave reflector 31 and the at least one convex reflector 32. The same optical system functions as the objective for both the projected radiation (projected onto the surface) and the directed radiation (directed from the surface). The optical system 30 is configured such that the radiation passes two times through the single optical system 30.

[0035] By providing that the optical system 30 is arranged such that the radiation projected onto the surface and the radiation directed on the surface are reflected by the at least one concave reflector 31 and the least one convex reflector 32, the number of optical systems can be reduced. For example, in the context of a topography measurement system, the number of objectives for the topography measurement system may be reduced from two to one. An embodiment is expected to increase the lifetime of a topography measurement system. By reducing the number of optical systems used, the number of components of the topography measurement system may be reduced. By reducing the number of components, the possibility of replacing a component may be reduced. By having the same optical system 30 for radiation projected / directed in both directions, the complexity of a topography measurement system may be reduced. An embodiment is expected to make it easier to manufacture a topography measurement system such as a level sensor LS.

[0036] In an embodiment the level sensor LS is configured to determine a topography map of the substrate W by triangulation. In an embodiment the level sensor LS is configured to project an image on the substrate W and to re-image that image. As shown in Figure 3, in an embodiment the level sensor LS comprises the projection unit LSP. The projection unit LSP comprises the optical system 30 and the projection grating PGR. The projection grating PGR is configured to pattern the beam of radiation BE1 such that an image is projected on the substrate W.

[0037] As shown in Figure 3, in an embodiment the level sensor LS comprises the detection unit LSD. The detection unit LSD comprises the detection grating DGR. In addition, the detection unit LSD comprises the optical system 30. That is, the optical system 30 is comprised in both the projection unit LSP and the detection unit LSD. In an embodiment the optical system 30 is configured to function as both the projection objective and the detection objective. The optical system 30 is configured to project the image onto the substrate W. In an embodiment the image is slit-shaped. In an embodiment the optical system 30 is configured to function as the detection objective. In an embodiment the optical system 30 is configured to re-image the redirected beam of radiation BE22025P00005WG 7onto a detector DET, for example.

[0038] By providing that the optical system 30 functions as both the projection objective and the detection objective, it is ensured that both objectives are identical. An embodiment is expected to reduce aberrations.

[0039] As shown in Figure 3, in an embodiment the optical system 30 is arranged such that the radiation projected onto the surface follows an optical path between the at least one concave reflector 31, 33 and the at least one convex reflector 32. In Figure 3, the optical path is shown by the solid long arrows. The optical path for the projected radiation extends from the first concave reflector 31 to the convex reflector 32 and then to the second concave reflector 33.

[0040] As shown in Figure 3, in an embodiment the projection optical path employs the reflectors 31-33 in an arrangement opposite to that of the detection optical path. The optical path followed by the radiation directed from the surface of the substrate W is shown in dashed long arrows in Figure 3. As shown in Figure 3, in an embodiment the optical path for the radiation directed from the surface of the substrate W extends from the second concave reflector 33 to the convex reflector 32 and then to the first concave reflector 31.

[0041] In an embodiment the optical path of the projected radiation and the optical path of the reflected radiation partially overlap each other. The footprints of the projected radiation beams BE1 and redirected radiation beams BE2 are adjacent to each other. The projected radiation beams and the redirected radiation beams are reflected by the same optical surfaces.

[0042] In the arrangement shown in Figure 3, the optical system 30 comprises the first concave reflector 31 and the second concave reflector 33. As shown in Figure 3, in an embodiment the convex reflector 32 is located optically between the first concave reflector 31 and the second concave reflector 33. This means that the convex reflector 32 is on the optical path between the first concave reflector 31 and the second concave reflector 33. For radiation projected onto the surface of the substrate W, the convex reflector 32 is downbeam of the first concave reflector 31 and upbeam of the second concave reflector 33. For the radiation directed away from the surface of the substrate W, the convex reflector 32 is downbeam of the second concave reflector 33 and upbeam of the first concave reflector 31.

[0043] In an embodiment the optical system 30 has a magnification. In an embodiment the magnitude of the magnification is greater than one. The optical system 30 may be configured such that the dimension of the beam of radiation BE1 on the substrate W is greater than the dimension of the beam of radiation LSB received by the optical system 30.

[0044] In an embodiment the focal length of the first concave reflector 31 is different from the focal length of the second concave reflector 33. For example, the focal lengths of the first concave reflector 31 and the second concave reflector 33 may be selected such that the ratio of the focal length of the first concave reflector 31 to the focal length of the second concave reflector 33 contributes to a magnification of the optical assembly.2025P00005WG 8

[0045] In an embodiment the optical assembly comprises an aperture stop. The aperture stop may be configured to shape the beam of radiation, for example the beam of radiation BE1 directed towards the substrate W. In an embodiment the aperture stop of the optical assembly is on the convex reflector 32. When the aperture stop of the optical assembly is on the convex reflector 32, the optical system 30 may have a magnification dependent on a ratio of the focal length of the first concave reflector 31 to the focal length of the second concave reflector 33.

[0046] However, it is not essential for the aperture stop of the optical assembly to be on the convex reflector 32. When the aperture stop is not be located on the convex reflector 32, the optical system 30 may have a magnification dependent on or contributed to by (a) a ratio of the focal length of the first concave reflector 31 to the focal length of the second concave reflector 33 and (b) the optical power of the second concave reflector 33.

[0047] In an embodiment the optical system 30 is configured to increase the field of view of a topography measurement system comprising the optical system 30. In an embodiment the optical system 30 has a magnification with an absolute value of at least two, and optionally at least three. In an embodiment the magnification is negative.

[0048] An embodiment is expected to increase the field of view of a topography measurement system. By increasing the field of view of the topography measurement system, the topography measurement system may be configured to measure the topography of the substrate W in a smaller number of strokes over the substrate W. An embodiment is expected to reduce the time required to measure the substrate W with a topography measurement system.

[0049] As shown in Figure 3, in an embodiment the convex reflector 32 is located substantially at a focal point of the at least one concave reflector 31 and / or 33. For example, the convex reflector 32 may be located substantially at the focal point of the first concave reflector 31. In an embodiment the convex reflector 32 is located substantially at the focal point of the second concave reflector 33. The optical system 30 may be arranged such that the focal point of the first concave reflector 31 substantially coincides with the focal point of the second concave reflector 33. By providing that the convex reflector 32 is located at the focal point, telecentricity of the optical system 30 may be improved. An embodiment is expected to improve telecentricity of a topography measurement system.

[0050] In an embodiment the first concave reflector 31 and second concave reflector 33 have a positive optical power. The convex reflector 32 may have a negative optical power.

[0051] In an embodiment a sum of an optical power of the at least one concave reflector 31 and / or 33 and an optical power of the at least one convex reflector 32 is substantially zero. By providing that the sum of optical powers is substantially zero, any undesirable field curvature aberration associated with the optical system 30 may be reduced. An embodiment is expected to reduce field curvature aberration associated with the topography measurement system.

[0052] As shown in Figure 3, in an embodiment the optical system 30 comprises at least one2025P00005WG 9projection planar reflector 34, 35. The at least one projection planar reflector 34, 35 is configured to reflect radiation from the optical assembly 30 onto the surface. For example, Figure 3 shows that the level sensor LS may comprise two projection planar reflectors 34, 35. The projection planar reflectors 34, 35 may be configured to reflect the beam of radiation BE1 from the optical system 30 onto the surface (e.g. of the substrate W). In an embodiment the projection planar reflectors 34, 35 comprise folding mirrors.

[0053] As shown in Figure 3, in an embodiment one of the projection planar reflectors 35 is located such that the substrate W is between the projection grating PGR and the projection planar reflector 34 when viewed in plan view. In an embodiment two projection planar reflectors 34, 35 are located such that the substrate W is between the two projection planar reflectors 34, 35 when viewed in plan view.

[0054] As shown in Figure 3, in an embodiment the optical system comprises at least one detection planar reflector 36, 37. In an embodiment the at least one detection planar reflector 36, 37 is / are different from the at least one projection planar reflector 34, 35. In an embodiment the at least one detection planar reflector 36, 37 is configured to reflect radiation from the surface to the optical assembly 30. For example, in the arrangement shown in Figure 3, the optical system comprises two detection planar reflectors 36, 37. The detection planar reflectors 36, 37 are configured to reflect the redirected beam of radiation BE2 from the surface of the substrate W to the optical assembly 30.

[0055] As shown in Figure 3, in an embodiment one of the detection planar reflectors 36 is located such that the substrate W is between the detection grating DGR and the detection planar reflector 37 when viewed in plan view. In an embodiment two detection planar reflectors 36, 37 are located such that the substrate W is between the two detection planar reflectors 36, 37 when viewed in plan view.

[0056] As embodiment of the invention is expected to reduce the volume required for the level sensor LS. By providing the planar reflectors 34-37, the level sensor LS can fit into a smaller volume while projecting radiation onto the substrate W and directing redirected radiation from the substrate W.

[0057] As shown in Figure 3, in an embodiment the optical system comprises a planar reflector 38 configured to reflect radiation from the projection grating PGR to the optical assembly 30. As shown in Figure 3, in an embodiment the planar reflector 38 is different from the detection planar reflector 36 and / or 37 that reflects the redirected beam of radiation from the substrate W to the optical assembly 30.

[0058] As shown in Figure 3, in an embodiment the level sensor LS comprises a planar reflector 39 configured to reflect the redirected beam of radiation BE2 from the optical assembly 30 to the detection grating DGR. As shown in Figure 3, in an embodiment the planar reflector 39 is different from the projection planar reflector 34 and / or 35 that reflects the beam of radiation BE1 from the optical assembly 30 onto the surface of the substrate W.

[0059] As shown in Figure 3, in an embodiment the level sensor LS comprises six planar reflectors2025P00005WG 10

[0060] As shown in Figure 3, in an embodiment the projection grating PGR is at an object plane of the optical assembly 30. That is, when the optical assembly 30 is considered as the projection objective, the projection grating PGR is at the object plane of the optical assembly 30.

[0061] In an embodiment the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly 30. In an embodiment the optical system is configured such that the radiation exiting the projection unit LSP forms a beam that has a longitudinal shape with its longitudinal direction angled relative to the surface.

[0062] In an embodiment the optical system is arranged such that a substrate W is at an image plane of the optical assembly 30. The substrate W comprises the surface onto which radiation is projected by the optical assembly 30. When the optical assembly 30 is considered as the projection objective, the substrate W is at the image plane of the optical assembly 30. In an embodiment the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly 30.

[0063] In an embodiment the optical system is arranged such that the substrate W is at an object plane of the optical assembly 30. When the optical assembly 30 is considered as the detection objective, the substrate W is at the object plane of the optical assembly 30.

[0064] In an embodiment the object plane and the image plane of the optical assembly 30 are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle. By satisfying the Scheimpflug principle, the absolute value of the magnification of the optical assembly 30 may be increased.

[0065] Figure 4 schematically depicts a perspective view of the level sensor LS shown in Figure 3. As shown in Figure 4, the beam of radiation BE1 and the redirected beam of radiation BE2 straddle the substrate W. This is possible without requiring that separate projection and detection objectives straddle the substrate W. The same optical assembly 30 functions as both the projection objective and the detection objective. As shown in Figure 4, in an embodiment a projection planar reflector 35 and a detection planar reflector 36 are arranged on opposite sides of the surface of the substrate W.

[0066] An embodiment is expected to increase the design freedom for the configuration of the level sensor LS. It is not necessary to provide separate projection and detection objectives on either side of the substrate W. This enables a Scheimpflug optical layout, which may otherwise be prohibited as a result of a 90° rotation of the measurement slit of the level sensor LS when separate projection and detection objectives are provided on either side of the substrate W.

[0067] As shown in Figure 4, in an embodiment an apparatus comprises a frame 40. The apparatus may be a lithographic apparatus, for example as shown in Figure 1. The apparatus may be a metrology apparatus, for example for performing measurements on a substrate W.

[0068] As shown in Figure 4, in an embodiment the optical system is adjustably mounted to the frame 40. For example, in an embodiment the apparatus comprises a mount 41 configured to mount the optical system to the frame 40.

[0069] In an embodiment one or more components of the optical system can have its position2025P00005WG 11relative to the frame 40 adjusted via the mount 41. An embodiment of the invention is expected to make it easier to maintain position stability for the level sensor LS. For example, in an embodiment the projection grating PGR may have its position adjusted to obtain the correct image and focus plane location on the substrate W. In an embodiment the position of the detection grating DGR may be adjusted relative to the frame 40.

[0070] In an embodiment the mount 41 forms a frame interface for the optical system. In an embodiment the optical assembly 30 can be located so as to be accommodated by an existing frame interface for a projection objective or a detection objective. For example, an apparatus that comprises separate projection and detection objectives may be modified so as to accommodate the optical assembly 30 into the mounting position of one or both of the projection objective and detection objective of the apparatus. An embodiment is expected to allow for an apparatus to be retrofitted with a level sensor LS having a combined projection and detection objective while minimising undesirable effects on other components of the apparatus.

[0071] Figure 5 schematically depicts a comparative level sensor LS. In the comparative level sensor LS shown in Figure 5, projection objective 30A and the detection objective 30B are separately provided. The topography measurement system comprises two optical assemblies 30a, 30b. The projection unit LSP comprises a projection optical assembly 30a. The projection optical assembly 30a is located downbeam of the projection grating PGR. The projection optical assembly 30a is located upbeam of the substrate W. The projection optical assembly 30a is configured to receive radiation from the projection grating PGR. The radiation may be patterned radiation. The projection optical assembly 30a is configured to project the patterned beam of radiation, for example the beam of radiation BE1 towards the substrate W.

[0072] The detection unit LSD comprises a detection optical assembly 30b. The detection optical assembly 30b is located downbeam of the substrate W. The detection optical assembly 30b is located upbeam of the detection grating DGR. In an embodiment the detection optical assembly 30b is configured to receive the redirected beam of radiation BE2. The detection optical assembly 30b is configured to output radiation towards the detection grating DGR.

[0073] The projection optical assembly 30a and the detection optical assembly 30b are configured to operate on the radiation. The projection optical assembly 30a is configured to operate on the radiation used for measuring the substrate W. The detection optical assembly 30b is configured to operate on the redirected beam of radiation BE2.

[0074] In the arrangement shown in Figure 3, the optical assembly 30 comprises the first concave reflector 31 and the second concave reflector 33. However, it is not essential for the optical assembly 30 to comprise two concave reflectors 31, 33.

[0075] For example, Figure 6 schematically depicts a level sensor LS comprising an alternative optical assembly 30. As shown in Figure 6, in an embodiment the optical assembly comprises only one concave reflector 31. The concave reflector 31 is arranged to reflect onto the at least one convex2025P00005WG 12reflector 32 to be projected onto the surface. The concave reflector 31 is further arranged to receive from the at least one concave reflector 32 the radiation directed from the surface. In Figure 6, the projection beam path is shown by solid long arrows. The detection beam path is shown by long dashed arrows.

[0076] Features of the level sensor LS shown in Figure 6 that are substantially the same as those of the level sensor LS shown in Figure 3 are not described in detail. As shown in Figure 6, the number of concave reflectors 31 may be one.

[0077] In an embodiment the optical assembly 30 forms an Offner relay.

[0078] In an embodiment the projection grating PGR is integrated with the optical assembly 30. Additionally or alternatively, the detection grating DGR may be integrated with the optical assembly 30.

[0079] By providing that the projection and / or detection gratings PGR, DGR are integrated into the body of the optical assembly 30 (i.e. the combined level sensor objective), substantially all of the mechanical hardware that is relevant for the performance of the level sensor LS is obtained in a single structure. In particular, the optical assembly 30, the projection grating PGR and the detection grating DGR may be contained in a single structure. By providing such a single structure, the structure may be individually shipped. In an embodiment, the structure comprising the optical assembly 30 with the integrated projection grating PGR and detection grating DGR can be adjusted outside the apparatus, for example outside the frame 40. An embodiment is expected to reduce down time of the apparatus, for example down time of a scanner or a metrology apparatus. An embodiment is expected to increase throughput.

[0080] An embodiment is expected to reduce position tolerances. In an embodiment the number of degrees of freedom by which the positions of the detection grating DGR and / or the projection grating PGR are required to be adjusted may be reduced. An embodiment is expected to simplify adjustment mechanisms and procedures for the level sensor LS.

[0081] An embodiment is expected to increase imaging stability. The number of mechanical interfaces between the level sensor LS and the frame 40 may be reduced. In an embodiment the distance between the projection grating PGR and the detection grating DGR may be reduced. By reducing the distance between the projection grating PGR and the detection grating DGR, it may be easier to obtain desired position stability.

[0082] However, it is not essential for the projection grating PGR or the detection grating DGR to be integrated with the optical assembly 30. In an alternative arrangement, the projection grating PGR and / or the detection grating DGR is provided separately from the optical assembly 30.

[0083] 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 and examples be considered as exemplary only, with a true scope and spirit of the invention being2025P00005WG 13indicated by the following claims.

[0084] 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. Other aspects of the invention are set-out as in the following numbered clauses.1. An optical system for a topography measurement system, the optical system configured to project radiation onto a surface and direct radiation from the surface, the optical system comprising:an optical assembly comprising:at least one concave reflector; andat least one convex reflector,wherein the optical system is arranged such that the radiation projected onto the surface and the radiation directed from the surface are reflected by the at least one concave reflector and the at least one convex reflector.2. The optical system of clause 1, arranged such that the radiation projected onto the surface follows a first optical path between the at least one concave reflector and the at least one convex reflector, and the radiation directed from the surface follows a second optical path that partially overlaps the first optical path.3. The optical system of clause 1 or clause 2, wherein the optical assembly comprises a first concave reflector and a second concave reflector, wherein the at least one convex reflector is located optically between the first concave reflector and the second concave reflector.4. The optical system of clause 3, wherein a focal length of the first concave reflector is different from a focal length of the second concave reflector such that the ratio of the focal length of the first concave reflector to the focal length of the second concave reflector contributes to a magnification of the optical assembly.5. The optical system of any preceding clause, wherein the optical assembly has a magnification with an absolute value greater than 1.6. The optical system of clause 1 or clause 2, wherein the optical assembly comprises only one concave reflector arranged to reflect onto the at least one convex reflector and to receive from the concave reflector the radiation to be projected onto the surface and to receive the radiation directed from the surface.7. The optical system of clause 6, wherein the optical assembly forms an Offner relay.8. The optical system of any preceding clause, wherein the at least one convex reflector is located substantially at a focal point of the at least one concave reflector.9. The optical system of any preceding clause, wherein a sum of an optical power of the at least one concave reflector and an optical power of the at least one convex reflector is substantially zero.10. The optical system of any preceding clause comprising:2025P00005WG 14at least one projection planar reflector configured to reflect radiation from the optical assembly onto the surface; andat least one detection planar reflector different from the at least one projection planar reflector and configured to reflect radiation from the surface to the optical assembly.11. The optical system of any preceding clause comprising a projection unit, wherein the optical assembly is located optically between the projection unit and the surface.12. The optical system of clause 11, wherein the projection unit comprises a projection grating configured to pattern the radiation to be projected onto the surface.13. The optical system of clause 12, wherein the projection grating is integrated with the optical assembly.14. The optical system of clause 12 or clause 13, wherein the projection grating is at an object plane of the optical assembly.15. The optical system of clause 14, wherein the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.16. The optical system of any of clauses 11-15, configured such that the radiation exiting the projection unit forms a beam that has a longitudinal shape with its longitudinal direction angled relative to the surface.17. The optical system of any of clauses 11-16, wherein the projection unit comprises a radiation source configured to provide the radiation to be projected onto the surface.18. The optical system of any preceding clause, further comprising a detection unit, wherein the optical assembly is located optically between the surface and the detection unit.19. The optical system of clause 18, wherein the detection unit comprises a detector configured to produce a detector output signal indicative of radiation received.20. The optical system of clause 18 or clause 19, wherein the detection unit comprises a detection grating located at an image plane of the optical assembly.21. The optical system of clause 20, wherein the detection grating is integrated with the optical assembly.22. The optical system of any preceding clause, arranged such that a substrate is at an image plane of the optical assembly, the substrate comprising the surface.23. The optical system of clause 22, wherein the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.24. The optical system of clause 22 or clause 23, arranged such that the substrate is at an object plane of the optical assembly.25. The optical system of clause 24, wherein the object plane and the image plane of the optical assembly are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.26. A topography measurement system comprising the optical system of any preceding clause2025P00005WG 15and configured to measure a topology of the surface.27. An apparatus comprising:a frame; andthe optical system of any of clauses 1-25 adjustably mounted to the frame.28. A method of projecting radiation onto a surface and directing radiation from the surface for topography measurement, the method comprising:projecting radiation onto the surface by reflecting radiation by at least one concave reflector and at least one convex reflector; anddirecting radiation from the surface by reflecting radiation by the at least one concave reflector and the at least one convex reflector.

Claims

2025P00005WG 16CLAIMS1. An optical system for a topography measurement system, the optical system configured to project radiation onto a surface and direct radiation from the surface, the optical system comprising:an optical assembly comprising:at least one concave reflector; andat least one convex reflector,wherein the optical system is arranged such that the radiation projected onto the surface and the radiation directed from the surface are reflected by the at least one concave reflector and the at least one convex reflector.

2. The optical system of claim 1, arranged such that the radiation projected onto the surface follows a first optical path between the at least one concave reflector and the at least one convex reflector, and the radiation directed from the surface follows a second optical path that partially overlaps the first optical path.

3. The optical system of claim 1 or claim 2, wherein the optical assembly comprises a first concave reflector and a second concave reflector, wherein the at least one convex reflector is located optically between the first concave reflector and the second concave reflector.

4. The optical system of claim 3, wherein a focal length of the first concave reflector is different from a focal length of the second concave reflector such that the ratio of the focal length of the first concave reflector to the focal length of the second concave reflector contributes to a magnification of the optical assembly.

5. The optical system of claim 1 or claim 2, wherein the optical assembly comprises only one concave reflector arranged to reflect onto the at least one convex reflector and to receive from the concave reflector the radiation to be projected onto the surface and to receive the radiation directed from the surface.

6. The optical system of any preceding claim, wherein the at least one convex reflector is located substantially at a focal point of the at least one concave reflector.

7. The optical system of any preceding claim comprising:at least one projection planar reflector configured to reflect radiation from the optical assembly onto the surface; andat least one detection planar reflector different from the at least one projection planar2025P00005WG 17reflector and configured to reflect radiation from the surface to the optical assembly.

8. The optical system of any preceding claim comprising a projection unit that comprises a radiation source configured to provide the radiation to be projected onto the surface, and a projection grating configured to pattern the radiation to be projected onto the surface, wherein the optical assembly is located optically between the projection unit and the surface.

9. The optical system of claim 8, wherein the projection grating is at an object plane of the optical assembly, wherein the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.

10. The optical system of any of claims 7-9, configured such that the radiation exiting the projection unit forms a beam that has a longitudinal shape with its longitudinal direction angled relative to the surface.

11. The optical system of any preceding claim, further comprising a detection unit that comprises a detector configured to produce a detector output signal indicative of radiation received, and a detection grating located at an image plane of the optical assembly, wherein the optical assembly is located optically between the surface and the detection unit.

12. The optical system of any preceding claim, arranged such that a substrate is at, at least one of an image plane or an object plane of the optical assembly, the substrate comprising the surface, and wherein the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.

13. The optical system of claim 12, wherein the object plane and the image plane of the optical assembly are tilted relative to a plane perpendicular to the optical axis of the optical assembly so as to satisfy the Scheimpflug principle.

14. A topography measurement system comprising the optical system of any preceding claim and configured to measure a topology of the surface.

15. A method of projecting radiation onto a surface and directing radiation from the surface for topography measurement, the method comprising:projecting radiation onto the surface by reflecting radiation by at least one concave reflector and at least one convex reflector; and2025P00005WG 18directing radiation from the surface by reflecting radiation by the at least one concave reflector and the at least one convex reflector.

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