Optical assembly

Abstract

An optical assembly for a topography measurement system, the optical assembly including: 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 ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.

Description

OPTICAL ASSEMBLYCROSS-REFERENCE TO RELATED APPLICATION

[0001] The Application claims priority of US provisional application number 63 / 701,003 which was filed on 30 September, 2024 and of US provisional application number 63 / 747,776 which was filed on 21 January, 2025, and which are incorporated herein its entirety by reference.TECHNICAL FIELD

[0002] The present disclosure relates generally to an optical assembly, a projection unit, a detection unit, a topography measurement system and a method of measuring topography.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 a lithographic apparatus or a metrology apparatus arranged to measure a parameter other topography, is arranged to measure a topography of a top surface of the substrate. There is a general need to reduce the measurement time.

[0006] According to an embodiment, there is provided an optical assembly for a topography measurement system, the optical assembly comprising: a first reflector; a second reflector; and a third reflector located such that the second reflector is located along an optical pathbetween 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 a ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.

[0007] According to an embodiment, there is provided a method of measuring topography, the method comprising: reflecting a beam of radiation with a first reflector of an optical assembly; reflecting the beam of radiation with a second reflector of the optical assembly; and reflecting the beam of radiation with a third reflector of the optical assembly, the 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 a ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.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 an overview of a level sensor.

[0012] Figure 4 schematically depicts an optical assembly, for example of the level sensor of Figure 2 or Figure 3.

[0013] Figure 5 is a diagram showing a Scheimpflug condition applied to the optical assembly of Figure 4.

[0014] Figure 6 schematically depicts an alternative view of the optical assembly of Figure 4.

[0015] Figure 7 is a close-up view of the optical assembly of Figure 4 in the vicinity of the substrate.

[0016] Figure 8 schematically depicts a close-up view of the optical assembly in the vicinity of the substrate when the Scheimpflug condition is not applied.DETAILED DESCRIPTION

[0017] 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.

[0018] 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 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.

[0019] The illumination system IL is configured to condition the beam B before the beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include afaceted 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 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.

[0020] After being thus conditioned, the 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 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 beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned 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).

[0021] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned beam B’, with a pattern previously formed on the substrate W.

[0022] 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 for or during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focused 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.

[0023] 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 anoptical 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 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 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.

[0024] 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 redirected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0025] 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.

[0026] By means of triangulation or other 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.

[0027] 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).

[0028] 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.

[0029] 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.

[0030] Various height sensors of a general type are disclosed for example in U.S. 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 U.S. 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.

[0031] Figure 3 is a schematic overview of part of the level sensor module shown in Figure 2. Figure 3 schematically shows features of the level sensor that are not shown in Figure 2.

[0032] As shown in Figure 3, in an embodiment the topography measurement system comprises one or more optical assemblies 11, 12. As shown in Figure 3, in an embodiment the projection unit LSP comprises a projection optical assembly 11. The projection optical assembly 11 is located downbeam of the projection grating PGR. The projection optical assembly 11 is located upbeam of the substrate W. The projection optical assembly 11 is configured to receive radiation from the projection grating PGR. The radiation may be patterned radiation. The projection optical assembly 11 is configured to project the patterned beam of radiation, for example the beam of radiation BE1 towards the substrate W.

[0033] As shown in Figure 3, in an embodiment the detection unit LSD comprises a detection optical assembly 12. The detection optical assembly 12 is located downbeam of the substrate W. The detection optical assembly 12 is located upbeam of the detection grating DGR. In an embodiment the detection optical assembly 12 is configured to receive the redirected beam of radiation BE2. The detection optical assembly 12 is configured to output radiation towards the detection grating DGR.

[0034] The projection optical assembly 11 and the detection optical assembly 12 are configured to operate on the radiation. The projection optical assembly 11 is configured tooperate on the radiation used for measuring the substrate W. The detection optical assembly 12 is configured to operate on the redirected beam of radiation BE2.

[0035] Figure 4 schematically depicts an optical assembly 20. The optical assembly 20 is for a topography measurement system. For example, the optical assembly 20 may form the projection optical assembly 11 of the projection unit LSP shown in Figure 3. Additionally or alternatively, the optical assembly 20 may form the detection optical assembly 12 of the detection unit LSD shown in Figure 3.

[0036] As shown in Figure 4, in an embodiment the optical assembly 20 comprises a first reflector 21. The first reflector 21 comprises a reflective surface configured to reflect radiation. The first reflector 21 may comprise a mirror, for example.

[0037] As shown in Figure 4, in an embodiment the first reflector 21 is configured to reflect radiation from the radiation source LSO. The first reflector 21 may be configured to receive the beam of radiation LSB provided by the radiation source LSO. The first reflector 21 is configured to reflect the beam of radiation LSB.

[0038] As shown in Figure 4, in an embodiment one or more optical elements are provided upbeam of the first reflector 21. Figure 4 shows an arrangement in which a reflective element 25 is located downbeam of the radiation source LSO and upbeam of the first reflector 21. The reflective element 25 is configured to reflect the beam of radiation LSB provided by the radiation source LSO towards the first reflector 21. Reflective element 25 may be a field mirror with a freeform, toroidal, aspherical or cylindrical surface. The use of one or more field mirrors helps ensure improved aberration and performance requirements, such as telecentricity and distortion. In an alternative arrangement, the reflective element 25 may be omitted.

[0039] As shown in Figure 4, in an embodiment the optical assembly 20 is located above the substrate W. That is, the surface of the substrate W that is measured by the topography measurement system faces towards the optical assembly 20. In an alternative arrangement, the optical assembly 20 may be located elsewhere, with radiation beams reflected onto the substrate W from the optical assembly 20.

[0040] As shown in Figure 4, in an embodiment, the optical assembly 20 comprises a second reflector 22. The second reflector 22 comprises a reflective surface configured to reflect radiation. The second reflector 22 is configured to receive the beam of radiation LSB from the first reflector 21 and to reflect the beam of radiation LSB. In an embodiment the second reflector 22 is configured to reflect the beam of radiation LSB towards a third reflector 23. In an embodiment the second reflector 22 comprises a mirror.

[0041] As shown in Figure 4, in an embodiment the first reflector 21 has a reflective surface that faces generally towards the substrate W. The first reflector 21 may face downwards. The second reflector 22 may have a reflective surface that faces generally upwards. The second reflector 22 may face generally away from the substrate W.

[0042] As shown in Figure 4, in an embodiment the optical assembly 20 comprises a third reflector 23. The third reflector 23 comprises a reflective surface configured to reflect radiation. In an embodiment the third reflector 23 comprises a mirror.

[0043] In an embodiment the third reflector 23 is configured to receive radiation reflected by the second reflector 22. The third reflector 23 is configured to reflect the radiation downbeam. For example, the third reflector 23 may be configured to reflect the radiation generally towards the substrate W.

[0044] As shown in Figure 4, in an embodiment the third reflector 23 is located such that the second reflector 22 is located along an optical path between the first reflector 21 and the third reflector 23. The second reflector 22 is downbeam of the first reflector 21. The third reflector 23 is downbeam of the second reflector 22.

[0045] As shown in Figure 4, in an embodiment the third reflector 23 is different from the first reflector 21. The third reflector 23 may have a location that is fixed relative to the first reflector 21. However, the reflective surface of the third reflector 23 is discontinuous with the reflective surface of the first reflector 21. In an embodiment the third reflector 23 is physically separate from the first reflector 21.

[0046] As shown in Figure 4, in an embodiment, one or more optical elements are provided downbeam of the third reflector 23. The one or more further optical elements may be located upbeam of the substrate W. For example, the arrangement shown in Figure 4 comprises one or more (e.g., a plurality ol) reflective elements 24 configured to reflect the radiation from the third reflector 23 on to the substrate W. Reflective element 24 may be a field mirror with a freeform, toroidal, aspherical or cylindrical surface. The use of one or more field mirrors helps ensure improved aberration and performance requirements, such as telecentricity and distortion. In an alternative arrangement, such one or more reflective elements 24 may be omitted.

[0047] In an embodiment a focal length of the first reflector 21 is different from a focal length of the third reflector 23. The radius of curvature of the first reflector 21 may be different from the radius of curvature of the third reflector 23. In the arrangement shown in Figure 4, the focal length of the first reflector 21 is less than the focal length of the third reflector 23. The reflective surface of the first reflector 21 is shaped differently from thereflective surface of the third reflector 23. In an embodiment the focal length of the first reflector 21 is different from the focal length of the third reflector 23, such that the ratio of the focal length of the first reflector 21 to the focal length of the third reflector 23 contributes to a magnification of the optical assembly. In an embodiment the optical assembly 20 has a magnification. In an embodiment the magnitude of the magnification is greater than 1. The optical assembly 20 may be configured such that the dimension of the patterned beam of radiation BE1 on the substrate W is greater than the dimension of the beam of radiation LSB received by the optical assembly 20.

[0048] In an embodiment, the optical assembly 20 is configured to increase the field of view of a topography measurement system comprising the optical assembly 20. In an embodiment, the optical assembly 20 has a magnification with an absolute value of at least 2, and optionally at least 3. In an embodiment the magnification is negative.

[0049] An embodiment of the invention 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 of the invention is expected to reduce the time required to measure the substrate W with a topography measurement system.

[0050] For example, when the projection optical assembly 11 of the topography measurement system has a magnitude of magnification of one, then the topography measurement system may require, for example, about six strokes over a substrate W in order to measure a height map of the substrate W. By increasing the field of the topography measurement system, the focus scanning can be done in fewer strokes. For example, by providing that the topography measurement system has a magnitude of magnification of at least three, the number of strokes required to measure the substrate W may be reduced to two.

[0051] In an embodiment, the topography measurement system comprising the optical assembly 20 as the projection optical assembly 11 has a magnitude of magnification of at least four, optionally at least five and optionally at least six. When the magnitude of magnification is six, the substrate W may be measured in a single stroke of the topography measurement system. An embodiment of the invention is expected to increase throughput of substrate measurements.

[0052] In an embodiment, the optical assembly 20 consists of mirrors. For example, the optical assembly 20 may not have any lenses as optical elements. By using a mirror system, chromatic aberrations may be reduced compared to a lens system, for example.

[0053] In an embodiment, the second reflector 22 is located substantially at a focal point of the first reflector 21. In an embodiment, the second reflector 22 is located substantially at a focal point of the third reflector 23. The optical assembly 20 may be arranged such that the focal point of the first reflector 21 coincides with the focal point of the third reflector 23. The second reflector 22 may be located at that focal point. By providing that the second reflector 22 is located at the focal point, telecentricity of the optical assembly 20 may be improved. An embodiment of the invention is expected to improve telecentricity of a topography measurement system.

[0054] In an embodiment, the first reflector 21 is concave, the second reflector 22 is convex and the third reflector 23 is concave. In an embodiment, the second reflector 22 has an optical power of the opposite sign of that of the first reflector 21 and the third reflector 23. For example, the first reflector 21 and the third reflector 23 may have a positive optical power. The second reflector 22 may have a negative optical power.

[0055] In an embodiment, a sum of an optical power of the first reflector 21, an optical power of the second reflector 22 and an optical power of the third reflector 23 is substantially zero. By providing that the sum of optical powers is substantially zero, any undesirable field curvature aberration associated with the optical assembly 20 may be reduced. In an embodiment, the Petzval sum of the optical assembly 20 is substantially zero. An embodiment of the invention is expected to reduce field curvature aberration associated with the topography measurement system.

[0056] In an embodiment, the focal length of the third reflector 23 is greater than the focal length of the first reflector 21 such that the optical assembly 20 has a magnification with an absolute value greater than one. For example, the focal length of the third reflector 23 may be selected so as to provide a target magnification of the optical assembly 20.

[0057] By providing that the third reflector 23 is different and separate from the first reflector 21, parameters associated with the third reflector 23 may be controlled so as to correct for an undesirable aberration.

[0058] In an embodiment, the first reflector 21 has a spherical reflective surface. Alternatively, the first reflector 21 may have an aspherical reflective surface. As a further alternative, the first reflector 21 may have a freeform reflective surface.

[0059] In an embodiment, the second reflector 22 has a spherical reflective surface. Alternatively, the second reflector 22 may have an aspherical reflective surface. As a further alternative, the second reflector 22 may have a freeform reflective surface.

[0060] In an embodiment, the third reflector 23 has a spherical reflective surface.Alternatively, the third reflector 23 may have an aspherical reflective surface. As a further alternative, the third reflector 23 may have a freeform reflective surface.

[0061] The surface type (e.g., spherical, aspherical and freeform) of the reflectors may be selected so as to optimize performance of the optical assembly 20.

[0062] In the arrangement shown in Figure 4, the optical assembly 20 is located upbeam of the substrate W. The optical assembly 20 may be part of the projection unit LSP of a topography measurement system. The optical assembly 20 may form the projection optical assembly 11 shown in Figure 3, for example.

[0063] In an arrangement, the optical assembly 20 may be located downbeam of the substrate W. The optical assembly 20 may be part of the detection unit LSD of a topography measurement system. For example, the optical assembly 20 may form the detection optical assembly 12 shown in Figure 3.

[0064] Figure 5 is a schematic diagram illustrating magnification of radiation beams when the optical assembly 20 is used for the projection optical assembly 11 upbeam of the substrate W and another of the optical assembly 20 is used for the detection optical assembly 12 downbeam of the substrate W. Figure 5 schematically represents locations of optical elements along the optical path, without showing their relative physical positions.

[0065] As shown in Figure 5, in an embodiment the focal length of the third reflector 23 (see the detection optical assembly 12 in the right hand side of Figure 5) is less than the focal length of the first reflector 21 such that the optical assembly 20 (e.g., the detection optical assembly 12) has a magnification with an absolute value less than 1. By providing a magnification with an absolute value less than 1, the dimension of the overall reflected patterned beam of radiation BE2 on the substrate W may be reduced to fit onto the detection grating DGR, for example.

[0066] In an embodiment, the projection optical assembly 11 upbeam of the substrate W may have a magnification with an absolute value greater than 1, while the detection optical assembly 12 downbeam of the substrate W may have a magnification with an absolute value less than 1. In an embodiment, the product of the absolute values of the magnifications of the projection optical assembly 11 and the detection optical assembly 12 is substantially 1. For example, when the projection optical assembly 11 has a magnification with an absolute value of 3, then the detection optical assembly 12 may have a magnification with an absolute value of 1 / 3.

[0067] In an embodiment, the optical arrangement of the projection optical assembly 11 is substantially symmetrical to the optical arrangement of the detection optical assembly 12.

[0068] An embodiment of the invention is expected to increase the dimension of the patterned beam of radiation BE1 on the substrate W without unduly increasing the size of the optical assemblies of the topography measurement system. For example, the projection grating PGR and the detection grating DGR may remain of the same size, while reducing the number of scans required to measure the substrate W.

[0069] As indicated in Figure 5, for example, in an embodiment, the optical assembly 20 comprises an aperture stop 26. The aperture stop 26 may be configured to shape the beam of radiation, for example the patterned beam of radiation BE1 directed towards the substrate W. In an embodiment, the aperture stop 26 is located substantially at a focal point of the first reflector 21. In this embodiment, the ratio of the focal length of the first reflector 21 to the focal length of the second reflector 22 and third reflector 23 contributes to the magnification of the optical assembly 20. In an embodiment, the aperture stop 26 is located substantially at a focal point of the third reflector 23. In this embodiment, the ratio of the focal length of the first reflector 2 land the second reflector 22 to the focal length of the third reflector 23 contributes to the magnification of the optical assembly 20.

[0070] For example, in an embodiment, the aperture stop 26 is located substantially at the second reflector 22. In the embodiment where the aperture stop 26 is located substantially at the second reflector in the arrangement as illustrated in Figure 5, the magnification of the optical assembly is equal to the focal length ratio of the third reflector 23 over the first reflector 21. In an embodiment, the aperture stop 26 is formed by providing a radiation absorbing area surrounding a radiation reflecting area at the reflective surface of the second reflector 22. For example, a black coating may be provided outside of a selected shape so as to absorb radiation incident on the second reflector 22 beyond that shape. As such, the aperture stop 26 may not physically have an aperture. In an alternative embodiment, the aperture stop may be distanced from the second reflector 22. In this embodiment, where the aperture stop is distanced from the second reflector 22, the focal length ratio of the first reflector 21 to the focal length ratio of the third reflector 23 contributes to the magnification of the optical assembly 20. The optical power of the second reflector 22 also contributes to the magnification. The aperture stop may be formed by a plate with an aperture defined in it for the passage of the radiation.

[0071] By providing that the aperture stop 26 is at the focal point of the first reflector 21 and the focal point of the third reflector 23, for example at the second reflector 22, thetel ecentri city of the optical assembly 20 may be improved. In the arrangement shown in Figure 5, it may be particularly advantageous for the projection optical assembly 11 to be substantially telecentric. It may also be advantageous for the detection optical assembly 12 to be substantially telecentric.

[0072] The embodiments described in relation to the illustrations of Figure 4 and Figure 5 have a first reflector 21, a second reflector 22 and a third reflector 23. In an embodiment, the first reflector 21 may be a first plurality of reflectors located before (i.e., upbeam of) the aperture stop 26 and the third reflector 23 may be a second plurality of reflectors located after (i.e., downbeam of) the aperture stop 26. Such an embodiment provides an optical assembly comprising more than three reflectors, for example four reflectors, five reflectors, six reflectors or more. In such an embodiment, the ratio of the focal length of the optics (e.g., reflectors and reflective elements) preceding the aperture stop 26 to the focal length of the optics (e.g., reflectors and reflective elements) succeeding the aperture stop 26 contributes to the magnification of the optical assembly 20. In other words, the ratio of the focal length of the first plurality of reflectors to the focal length of the second plurality of reflectors contributes to the magnification of the optical assembly 20.

[0073] As shown in Figure 2, in an embodiment a projection unit LSP for a topography measurement system comprises a radiation source LSO, a projection grating PGR and the optical assembly 20 as a projection optical assembly 11. The radiation source LSO is configured to provide a beam of radiation LSB. The projection grating PGR is configured to pattern the beam of radiation LSB to form a patterned beam of radiation BE1. The projection optical assembly 11 is located such that the projection grating PGR is at an object plane of the projection optical assembly 11. In an embodiment the substrate W is at the image plane of the projection optical assembly 11. The projection unit LSP is configured to image the projection grating PGR onto the substrate W.

[0074] As can be seen from Figure 5, in an embodiment the object plane is tilted relative to a plane 61 perpendicular to an optical axis 63 of the projection optical assembly 11. The projection grating PGR may be angled relative to the plane 61. Figure 5 shows an angle 62 between the plane 61 and the projection grating PGR at the object plane.

[0075] As can be seen in Figure 5, in an embodiment the image plane is tilted relative to a plane 61 perpendicular to the optical axis 63 of the projection optical assembly 11. Figure 5 shows the substrate W angled relative to the plane 61. For example, an angle 64 may be provided between the substrate W and the plane 61. The angle 64 may be acute.

[0076] In an embodiment, the object plane and the image plane are tilted relative to a plane 61 perpendicular to the optical axis 63 of the projection optical assembly 11 so as to satisfy the Scheimpflug principle. By satisfying the Scheimpflug principle, the absolute value of the magnification of the projection optical assembly 11 may be increased.

[0077] It is not essential for the Scheimpflug condition to be satisfied. In an alternative embodiment, the projection grating PGR is substantially parallel with the plane 61 perpendicular to the optical axis 63. Even when the Scheimpflug principle is not satisfied, the mirror system formed by the projection optical assembly 11 has a magnification with an absolute value of greater than 1. By satisfying the Scheimpflug principle, the magnification may be further increased. An embodiment of the invention is expected to increase the field of view of the topography measurement system, without unduly increasing the size of the topography measurement system.

[0078] As shown in Figure 2, in an embodiment the detection unit LSD comprises the optical assembly 20 as a detection optical assembly 12, a detection grating DGR and a detector DET. The detection grating DGR may be located at the image plane of the detection optical assembly 12. The detector DET may be configured to produce a detector output signal indicative of radiation received.

[0079] As can be seen in Figure 5, in an embodiment the image plane (i. e. , where the detection grating DGR may be located) is tilted relative to a plane 71 perpendicular to an optical axis of the detection optical assembly 12. For example, Figure 5 shows the detection grating DGR angled relative to the plane 71. There may be an angle 72 between the detection grating DGR and the plane 71 that is perpendicular to the optical axis of the detection optical assembly 12.

[0080] In an embodiment the detection unit LSD is located such that the substrate W is at the object plane of the detection optical assembly 12. As can be seen in Figure 5, in an embodiment the object plane (e.g., where the substrate W is located) is tilted relative to the plane 71 perpendicular to the optical axis of the detection optical assembly 12. The substrate W may be angled relative to the plane 71. As shown in Figure 5, an angle 74 may be formed between the substrate W and the plane 71 that is perpendicular to the optical axis of the detection optical assembly 12. The angle 74 may be acute.

[0081] In an embodiment the object plane and the image plane are tilted relative to the plane 71 perpendicular to the optical axis of the detection optical assembly 12 so as to satisfy the Scheimpflug principle. By satisfying the Scheimpflug principle, the absolute value of the magnification of the detection optical assembly 12 may be further from one. By satisfying theScheimpflug principle, the magnification of the detection optical assembly 12 may be enhanced. However, it is not essential for the Scheimpflug principle to be satisfied.

[0082] Figure 6 schematically depicts an alternative view of the optical assembly 20 shown in Figure 4. As shown in Figure 6, in an embodiment the optical assembly 20 is configured such that the beam of radiation BE1 exiting the optical assembly 20 has a slit shape. The slit shape has a long dimension that is longer than a short dimension. For example, when the optical assembly 20 is used as the projection optical assembly 11 of a projection unit LSP, the projection unit LSP is configured such that the beam of radiation BE1 exiting the projection unit LSP has a slit shape. The slit shape may have a longitudinal direction angled relative to the image plane. In the arrangement shown in Figure 6, the substrate W may be located at the image plane of the optical assembly 20. The beam of radiation BE1 that exits the optical assembly 20 may be oriented substantially vertically.

[0083] Figure 7 schematically depicts a close up view of the arrangement shown in Figure 6. The orientation of the radiation beam is indicated by the arrow 51 between the optical assembly 20 and the substrate W.

[0084] As shown in Figure 7, in an embodiment the projection unit LSP is arranged such that the orientation of the radiation slit on the substrate W is substantially horizontal. The orientation of the radiation slit on the substrate W is indicated in Figure 7 by the arrow 52. In an embodiment the radiation beam that exits the projection unit LSP is angled relative to the radiation slit on the substrate W. For example, the radiation beam may be substantially perpendicular to the radiation slit on the substrate W. This may be a consequence of the Scheimpflug principle being satisfied.

[0085] Figure 8 schematically depicts an alternative arrangement in which the Scheimpflug principle is not satisfied. As shown in Figure 8, in an alternative arrangement, the radiation beam output by the optical assembly 20 is oriented substantially horizontally (i.e. , parallel with the plane of the substrate W). The radiation slit on the substrate W as indicated by the arrow 52 may be substantially horizontal. The radiation beam as indicated by the arrow 51 may be substantially parallel to the radiation slit on the substrate W indicated by the arrow 52.

[0086] In an embodiment, the scanning direction for scanning the radiation beam across the surface of the substrate W may be varied depending on whether or not the Scheimpflug principle is satisfied.

[0087] Embodiments are provided as follows:1. An optical assembly for a topography measurement system, the optical assemblycomprising: 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 ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.2. The optical assembly of clause 1, wherein the second reflector is located substantially at a focal point of first reflector and a focal point of the third reflector.3. The optical assembly of clause 1 or 2, wherein the first reflector is concave, the second reflector is convex and the third reflector is concave.4. The optical assembly of any preceding clause, wherein a sum of an optical power of the first reflector, an optical power of the second reflector and an optical power of the third reflector is substantially zero.5. The optical assembly of any preceding clause, wherein the focal length of the third reflector is greater than the focal length of the first reflector such that the optical assembly has a magnification with an absolute value greater than 1.6. The optical assembly of any of clauses 1-4, wherein the focal length of the third reflector is less than the focal length of the first reflector such that the optical assembly has a magnification with an absolute value less than 1.7. The optical assembly of any preceding clause, comprising an aperture stop.8. The optical assembly of clause 7, wherein the aperture stop is located substantially at a focal point of first reflector and a focal point of the third reflector.9. The optical assembly of clause 7 or 8, wherein the aperture stop is located at the second reflector.10. The optical assembly of clause 9, wherein the magnification of the optical assembly is dependent on a ratio of the focal length of the first reflector to the focal length of the third reflector.11. The optical assembly of any preceding clause, wherein the third reflector is downbeam of the first reflector.12. The optical assembly of any preceding clause, further comprising a half-field mirror.13. The optical assembly of any preceding clause, wherein the first reflector is a first plurality of reflectors and wherein the third reflector is a second plurality of reflectors.14. A projection unit for a topography measurement system, the projection unit comprising: a radiation source configured to provide a beam of radiation; a projection grating configured to pattern the beam of radiation; and the optical assembly of any preceding clause located such that the projection grating is at an object plane of the optical assembly.15. The projection unit of clause 14, wherein the object plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.16. The projection unit of clause 15, located such that a substrate is at an image plane of the optical assembly.17. The projection unit of clause 16, 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.18. The projection unit of any of clauses 14-17, configured such that the beam of radiation exiting the projection unit has a longitudinal shape with its longitudinal direction angled relative to the image plane.19. A detection unit for a topography measurement system, the detection unit comprising: the optical assembly of any preceding clause; a detection grating located at an image plane of the optical assembly; and a detector configured to produce a detector output signal indicative of radiation received.20. The detection unit of clause 19, wherein the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.21. The detection unit of clause 20, located such that a substrate is at an object plane of the optical assembly.22. The detection unit of clause 21, 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.23. A topography measurement system comprising at least one of the projection unit of any of clauses 14-18 and the detection unit of any of clauses 19-22.24. A method of measuring topography, the method comprising: reflecting a beam of radiation with a first reflector of an optical assembly; reflecting the beam of radiation with a second reflector of the optical assembly; and reflecting the beam of radiation with a third reflector of the optical assembly, the third reflector located such that the second reflector is located along an optical path between thefirst 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 ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.

[0088] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. For example, one or more steps of a method described herein may be caused by hardware, firmware, software, or any combination thereof. Embodiments of the invention may be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause one or more devices to interact with the physical world.

[0089] 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 being indicated by the following claims.

[0090] 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.

Claims

CLAIMS1. An optical assembly for a topography measurement system, the optical assembly comprising: 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 ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.

2. The optical assembly of claim 1, wherein the second reflector is located substantially at a focal point of first reflector and a focal point of the third reflector.

3. The optical assembly of claim 1 or claim 2, wherein the first reflector is concave, the second reflector is convex and the third reflector is concave.

4. The optical assembly of any preceding claim, wherein a sum of an optical power of the first reflector, an optical power of the second reflector and an optical power of the third reflector is substantially zero.

5. The optical assembly of any preceding claim, wherein the focal length of the third reflector is greater than the focal length of the first reflector such that the optical assembly has a magnification with an absolute value greater than 1.

6. The optical assembly of any of claims 1-4, wherein the focal length of the third reflector is less than the focal length of the first reflector such that the optical assembly has a magnification with an absolute value less than 1.

7. The optical assembly of any preceding claim, comprising an aperture stop, and wherein the aperture stop is located at least one of; substantially at a focal point of first reflector and a focal point of the third reflector, and at the second reflector, and wherein the magnification of the optical assembly is dependent ona ratio of the focal length of the first reflector to the focal length of the third reflector.

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

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

10. The projection unit of claim 9, located such that a substrate is at an image plane of the optical assembly, 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.

11. A detection unit for a topography measurement system, the detection unit comprising: the optical assembly of any of claims 1-7; a detection grating located at an image plane of the optical assembly; and a detector configured to produce a detector output signal indicative of radiation received.

12. The detection unit of claim 11, wherein the image plane is tilted relative to a plane perpendicular to an optical axis of the optical assembly.

13. The detection unit of claim 12, located such that a substrate is at an object plane of the optical assembly, and 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.

14. A topography measurement system comprising the projection unit of any of claims 8- 10 and / or the detection unit of any of claims 11-13.

15. A method of measuring topography, the method comprising:reflecting a beam of radiation with a first reflector of an optical assembly; reflecting the beam of radiation with a second reflector of the optical assembly; and reflecting the beam of radiation with a third reflector of the optical assembly, the 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 ratio of the focal length of the first reflector to the focal length of the third reflector contributes to a magnification of the optical assembly.

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