Optical assembly for a topography measurement system

Abstract

An optical assembly (30) for a topography measurement system, the optical assembly (30) including a plurality of optical channels each comprising: a first curved reflector (32) and a second curved reflector (34) arranged to image an object with a magnification magnitude of 1. In an embodiment, each channel comprises a first planar reflector (33) optically between the first curved reflector (32) and the second curved reflector (34); and a second planar reflector (31) located such that the first curved reflector (32) is optically between the first planar reflector (33) and the second planar reflector (31).

Description

OPTICAL ASSEMBLYCROSS-REFERENCE TO RELATED APPLICATION

[0001] The Application claims priority of US provisional application number 63 / 747,289 which was filed on 20 January, 2025 and which is 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 the lithographic apparatus, 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 plurality of optical channels each comprising: a first curved reflector and a second curved reflector arranged to image an object with a magnification magnitude of 1.

[0007] According to an embodiment, there is provided a method of making an optical assembly for a topography measurement system, the method comprising, for each of a plurality of optical channels: arranging a first curved reflector and a second curved reflector to image an object with a magnification magnitude of 1.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The accompanying drawings, which are incorporated in and constitute a part of thespecification, 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 a topography measurement system comprising an optical assembly.

[0013] Figure 5 schematically depicts part of the optical assembly.

[0014] Figure 6 is a perspective view of the topography measurement system shown in Figure 4.

[0015] Figure 7 schematically depicts fields of view of the topography measurement system shown in Figure 4.

[0016] Figure 8 schematically depicts fields of view of a topography measurement system.

[0017] Figure 9 schematically depicts the measurement system comprising a different optical assembly.

[0018] Figure 10 schematically depicts part of the optical assembly shown in Figure 9.

[0019] Figure 11 schematically depicts the optical assembly shown in Figure 9 and Figure 10.

[0020] Figure 12 schematically depicts a perspective view of the optical assembly shown in Figure 10.

[0021] Figure 13 schematically depicts a perspective view of the optical assembly shown in Figure 11.

[0022] Figure 14 schematically depicts construction details of the topography measurement system shown in Figure 9.

[0023] Figure 15 schematically depicts a top view of the topography measurement system according to some embodiments of the invention.DETAILED DESCRIPTION

[0024] 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 disclosurewill 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.

[0025] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV and / or a DUV radiation beam B and to supply the EUV and / or DUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT (e.g., a mask table, a reticle table, a reticle stage) configured to support a patterning device MA (e.g., a mask, a reticle), a projection system PS, and a substrate table WT configured to support a substrate W.

[0026] The illumination system IL is configured to condition the EUV and / or DUV radiation beam B before the EUV and / or DUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror or other optical device 10 and a faceted pupil mirror or other optical device 11. The device 10 and device 11 together provide the EUV and / or DUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the device 10 and device 11.

[0027] After being thus conditioned, the EUV and / or DUV radiation beam B interacts with the patterning device MA. This interaction may be reflective (as shown), which may be preferred for EUV radiation. This interaction may be transmissive, which may be preferred for DUV radiation. As a result of this interaction, a patterned EUV and / or DUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV and / or DUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors or other optical elements 13, 14 which are configured to project the patterned EUV and / or DUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV and / or DUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as havingonly two mirrors or other optical elements 13, 14 in FIG. 1, the projection system PS may include a different number of mirrors or other optical elements (e.g., six or eight mirrors or other optical elements).

[0028] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV and / or DUV radiation beam B’, with a pattern previously formed on the substrate W.

[0029] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus (or other apparatus such as a metrology or inspection apparatus to measure a parameter other than topography), 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, e.g., 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 or for measurement by a metrology or inspection apparatus to measure a parameter other than topography. 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.

[0030] 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 comprises a radiation source LSO providing a beam of radiation LSB which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or nonpolarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to redirect from a surface of a substrate.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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).

[0035] 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.

[0036] 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.

[0037] 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 its entirety by reference. In PCT Patent Application Publication No. WO2016102127, 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.

[0038] 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.

[0039] As shown in Figure 3, in an embodiment the topography measurement system comprises one or more optical assemblies 30. As shown in Figure 3, in an embodiment the projection unit LSP comprises a projection optical assembly 30. The projection optical assembly 30 is located downbeam of the projection grating PGR. The projection optical assembly 30 is located upbeam of the substrate W. The projection optical assembly 30 is configured to receive radiation from the projection grating PGR. The radiation may be patterned radiation. The projection optical assembly 30 is configured toproject the patterned beam of radiation, for example the beam of radiation BE1 towards the substrate W.

[0040] As shown in Figure 3, in an embodiment the detection unit LSD comprises a detection optical assembly 30. The detection optical assembly 30 is located downbeam of the substrate W. The detection optical assembly 30 is located upbeam of the detection grating DGR. In an embodiment the detection optical assembly 30 is configured to receive the redirected patterned beam of radiation BE2. The detection optical assembly 30 is configured to output radiation towards the detection grating DGR.

[0041] The projection optical assembly 30 and the detection optical assembly 30 are configured to operate on the radiation. The projection optical assembly 30 is configured to operate on the radiation used for measuring the substrate W. The detection optical assembly 30 is configured to operate on the redirected patterned beam of radiation BE2.

[0042] Figure 4 schematically depicts a topography measurement system, which may be a level sensor. As shown in Figure 4, the topography measurement system comprises a projection unit LSP and a detection unit LSD. The projection unit LSP comprises an optical assembly 30. As shown in Figure 4, in an embodiment the detection unit LSD comprises another instance of the optical assembly 30. Alternatively, different types of optical assembly may be used for the projection unit LSP and the detection unit LSD.

[0043] In an embodiment the projection unit LSP comprises the optical assembly 30 optically between the projection grating PGR and the substrate W. In an embodiment the detection unit LSD comprises the optical assembly 30 optically between the substrate W and the detection grating DGR.

[0044] As shown in Figure 4, in an embodiment the optical assembly comprises a first curved reflector 32 and a second curved reflector 34. As shown in Figure 4, in an embodiment the second curved reflector 34 is located optically between the first curved reflector 32 and the substrate W. In an embodiment the first curved reflector 32 is located optically between the second curved reflector 34 and the projection grating PGR or detection grating DGR.

[0045] In an embodiment the first curved reflector 32 and the second curved reflector 34 are arranged to image an object with a magnification magnitude of 1. For example, in the context of the projection unit LSP, the optical assembly 30 comprises the first curved reflector 32 and the second curved reflector 34 arranged to image the projection grating PGR onto the substrate W with a magnification magnitude of 1. For example, the magnification of the optical assembly 30 may be -1. In the context of the detection unit LSD, the optical assembly 30 may comprise the first curved reflector 32 and the second curved reflector 34 arranged to image the substrate W onto the detection grating DGR with a magnification magnitude of 1. For example, the magnification of the optical assembly 30 of the detection unit LSD may be -1.

[0046] Providing that the optical assembly 30 has a magnification magnitude of 1, aberrations may be decreased. For example, the aberrations may be decreased compared to an alternative designhaving a greater field of view on the substrate W. With a relatively small field of view on the substrate W, the aberrations of the optical assembly 30 comprising the first curved reflector 32 and the second curved reflector may be sufficiently low. In an embodiment the optical assembly 30 is substantially diffraction limited.

[0047] Figure 5 schematically depicts part of the optical assembly 30. Figure 5 schematically depicts the first curved reflector 32 and the second curved reflector 34. As shown in Figure 5, for example, in an embodiment the first curved reflector 32 and the second curved reflector 34 are concave.

[0048] In an embodiment the reflector 32 and the second curved reflector 34 are tilted. In Figure 5, in an embodiment the tilts of each surface are around the X axis and the Y axis that define the plane transverse to the optical axis ray impinging on the reflectors 32, 34. In Figure 5, the first curved reflector 32 may be considered to be tilted with respect to the X axis. In Figure 5, in an embodiment the second curved reflector 34 is tilted about the axis, i.e., the same axis about which the first curved reflector 32 is tilted. However, it is not essential for the first curved reflector 32 and the second curved reflector 34 to be tilted about the same axis.

[0049] In an alternative arrangement, the second curved reflector 34 may be tilted about an axis that is different from the axis about which the first curved reflector 32 is tilted. For example, the second curved reflector 34 may be tilted about an axis that is orthogonal to the axis about which the first curved reflector 32 is tilted. In the coordinate system shown in Figure 5, the first curved reflector 32 may be tilted about the X axis while the second curved reflector 34 may be tilted about the Y axis. In an alternative arrangement, the first curved reflector 32 may be tilted about the Y axis, with the second curved reflector 34 tilted about the X axis.

[0050] By tilting the first curved reflector 32 and the second curved reflector 34 about different axes, aberrations of the optical assembly 30 may be reduced. In particular, by providing that the first curved reflector 32 and the second curved reflector 34 are tilted about orthogonal axes, aberrations associated with the optical assembly 30 may be reduced.

[0051] By tilting the first curved reflector 32 and the second curved reflector 34, astigmatism of the optical assembly 30 may be reduced.

[0052] Figure 6 is a perspective view of the measurement system shown in Figure 4. Figure 6 shows further details of the optical assembly 30.

[0053] As shown in Figure 6, in an embodiment the optical assembly 30 comprises a plurality of optical channels. Each optical channel corresponds to an optical path for the projection unit LSP, each optical channel corresponds to an optical path between the projection grating PGR and the substrate W. For the detection unit LSD, each channel corresponds to an optical path between the substrate W and the detection grating DGR.

[0054] By providing a plurality of optical channels of the assembly 30, the physical size of each of the reflectors may be reduced. An embodiment of the invention is expected to reduce the cost ofmaking the image assembly 30 for the topography measurement system.

[0055] In Figure 6, in an embodiment each optical channel of the optical assembly 30 comprises a first curved reflector 32 and a second curved reflector 34. The first curved reflector 32 and the second curved reflector 34 are arranged to image an object with a magnification magnitude of 1. The optical assembly 30 comprises, for each optical channel, two curved reflectors 32, 34 generating a one-to-one image of a small field of view of a grating on the substrate.

[0056] In Figure 6, the optical beams corresponding to five of the optical channels are illustrated for the projection unit LSP. For each of the five optical channels, the first curved reflector 32a-e and the second curved reflector 34a-e is labelled. For example, a first optical channel of the optical assembly 30 of the projection unit LSP comprises the first curved reflector 32a and the second curved reflector 34a. The second optical channel comprises the first curved reflector 32b and the second curved reflector 34b. The third optical channel comprises the first curved reflector 32c and the second curved reflector 34c. The fourth optical channel comprises the first curved reflector 32d and the second curved reflector 34d. The fifth optical channel comprises the first curved reflector 32e and the second curved reflector 34e.

[0057] The number of optical channels of the optical assembly 30 is not particularly limited. By increasing the number of optical channels, the overall field of view of the optical assembly 30 may be increased. In an embodiment the optical assembly 30 comprises at least 5, optionally at least 10, optionally at least 20, optionally at least 50, optionally at least 100, optionally at least 200 or optionally at least 500 optical channels. As specific examples, the optical assembly 30 may comprise 150 optical channels or 300 optical channels.

[0058] By providing the optical assembly 30 with a plurality of optical channels, the overall field of view of the optical assembly 30 may be increased. By increasing the overall field of view, the number of scans of the substrate W by the topography measurement system comprising the optical assembly 30 may be reduced. An embodiment of the invention is expected to reduce the time required to scan the substrate. An embodiment of the invention is expected to reduce the time required to measure topography of the substrate W.

[0059] As shown in Figure 4, in an embodiment each channel of the optical assembly 30 comprises a first planar reflector 33. As shown in Figure 4, in an embodiment the first planar reflector 33 is optically between the first curved reflector 32 and the second curved reflector 34. The first planar reflector 33 may be a flat folding mirror. The first planar reflector 33 is configured to fold radiation beams between the first curved reflector 32 and the second curved reflector 34. By providing the first planar reflector 33, the optical assembly 30 may fit into a smaller overall volume.

[0060] As shown in Figure 4, in an embodiment each channel comprises a second planar reflector 31. As shown in Figure 4, in an embodiment the second planar reflector 31 is located such that the first curved reflector 32 is optically between the first planar reflector 33 and the second planar reflector 31. By providing the second planar reflector 31, the overall volume taken up by the opticalassembly 30 may be reduced. In an embodiment the second planar reflector 31 is a flat folding mirror. The second planar reflector 31 may be located optically between the projection grating PGR (or the detection grating DGR) and the first curved reflector 32. The second planar reflector 31 may be configured to fold radiation beams between the projection grating PGR (or detection grating DGR) and the first curved reflector 32.

[0061] As shown in Figure 6, in an embodiment each channel of the optical assembly 30 is provided with a corresponding first planar reflector 33 and a corresponding second planar reflector 31. In Figure 6, the first planar reflector 33a-e is labelled for each of the five optical channels for which the optical beams are shown.

[0062] In an embodiment the optical assembly 30 comprises only two curved reflectors 32, 34 for each optical channel. In an embodiment the optical assembly 30 comprises only two planar reflectors 33, 31 for each optical channel. In an embodiment the optical assembly 30 comprises an array of optical channels. An embodiment of the invention is expected to achieve a miniaturized version of the topography measurement system that has an extended field of view. In an embodiment the optical assembly 30 comprises an array of reflectors. For example, as shown in Figure 6, the optical assembly 30 may comprise an array of first curved reflectors 32a-e, an array of second curved reflectors 34a-e, an array of first planar reflectors 33a-e and an array of second planar reflectors 31.

[0063] In an embodiment, the first curved reflector 32, or the second curved reflector 34, or both the first curved reflector 32 and the second curved reflector 34 is a spherical reflector. For example, the first curved reflector 32 and the second curved reflector 34 may be spherical mirrors. However, it is not essential for the first curved reflector 32 or the second curved reflector 34 to be spherical mirrors. In an alternative embodiment, the first curved reflector 32, or the second curved reflector 34, or both the first curved reflector 32 and the second curved reflector 34 is selected from the group consisting of a parabolic reflector, an aspherical reflector and a freeform surface. By providing that the first curved reflector 32 and / or the second curved reflector 34 are spherical mirrors, the cost of manufacturing the optical assembly 30 may be reduced. An embodiment of the invention is expected to reduce the cost of manufacturing the topography measurement system.

[0064] Figure 7 schematically depicts fields of view 41 of individual optical channels of the optical assembly 30 on the substrate W. As shown in Figure 7, the optical assembly 30 has a plurality of individual fields of view 41 corresponding to respective optical channels of the optical assembly 30. As shown in Figure 7, in an embodiment a first plurality of the optical channels have fields of view 41 arranged in a first linear array 51.

[0065] As shown in Figure 7, in an embodiment the optical assembly 30 is arranged such that there is a gap 45 between adjacent fields of view 41 within the first linear array 51 of fields of view 41. However, it is not essential for the individual fields of view 41 to be spaced apart from each other. In an alternative arrangement, the fields of view 41 are contiguous with each other.

[0066] Although the arrangement shown in Figure 7 shows a first linear array 51 of fields of view41, this is not essential. In an alternative arrangement, the fields of view 41 are arranged in a nonlinear pattern, for example in a curve.

[0067] Figure 8 schematically depicts fields of view 41, 43 of optical channels of a topography measurement system comprising a different optical assembly 30 from that shown in Figure 6. As shown in Figure 8, in an embodiment a second plurality of the optical channels have fields of view 43 arranged in a second linear array 52. As shown in Figure 8, in an embodiment the second linear array 52 is offset from the first linear array 51. For example, as shown in Figure 8, the fields of view 43 of the second linear array 52 may be centered substantially equidistantly between the fields of view 41 of the first linear array 51. By providing the second linear array 52, the overall field of view of the optical assembly 30 may be increased. By providing the fields of view 43 of the second linear array 52, the fields of view 43 may at least partly compensate for the gaps between the fields of view 41 of the first linear array 51.

[0068] As shown in Figure 8, in an embodiment, smaller gaps may remain between adjacent fields of view 41, 43 of the two linear arrays 51, 52. Alternatively, the two linear arrays 51, 52 may be arranged such that fields of view 43 of the second linear array 52 partly overlap with the fields of view 41 of the first linear array 51. That is, there may be substantially no gaps between the fields of view 41, 43 when both the linear arrays 51, 52 are taken into account. Figure 8 corresponds to a staggered design of the optical assembly 30.

[0069] In an embodiment the second curved reflector 34 is located at most 2 mm, optionally at most 1 mm or optionally at most 0.6 mm from the plane of the substrate W. By decreasing the distance between the second curved reflector 34 and the plane of the substrate W, the angle of incidence of the patterned beam of radiation BE1 (or equivalently the reflected patterned beam of radiation BE2) may be increased.

[0070] In an embodiment the curved reflector 34 is at least 0.1 mm, optionally at least 0.2 mm, optionally at least 0.5 mm, or optionally at least 0.6 mm from the plane of the substrate W. By increasing the distance between the second curved reflector 34 and the plane of the substrate W, the intolerances of the optical assembly may be increased.

[0071] In an embodiment the optical assembly 30 is arranged such that the angle of incidence of the patterned beam of radiation BE1 (or the reflected patterned beam of radiation BE2) relative to the substrate W is at least 40°, optionally at least 70°, optionally at least 78°, optionally at least 80°, optionally at least 85°, optionally at least 86° or optionally at least 87°. By increasing the angle of incidence, the optical performance of the optical assembly 30 may be improved.

[0072] In an embodiment at least one of the first curved reflector 32, the second curved reflector 34, the first planar reflector 33 and the second planar reflector 31 has a dimension of at least 1 mm, optionally at least 2 mm or optionally at least 2.2 mm. For example, the dimension may be the diameter available for reflecting radiation beams.

[0073] In an embodiment the optical assembly 30 is arranged such that each optical channel has afield of view 41, 43 on the substrate W of a dimension of at least 0.2 mm, optionally at least 0.5 mm or optionally at least 1 mm. For example, each illumination spot size may be of the order of 1 mm x 1 mm.

[0074] In Figure 2, in an embodiment, the projection unit LSP comprises the radiation source LSO configured to provide the beam of radiation LSB. The projection unit LSP may further comprise the projection grating PGR configured to pattern the beam of radiation LSB so as to form the patterned beam of radiation BEL The projection unit LSP may comprise the optical assembly 30 located such that the projection grating PGR is at an object plane of the optical assembly 30. In an embodiment the projection unit LSP is located such that the substrate W is at the image plane of the optical assembly 30.

[0075] In the context of the detection unit LSD, the detection grating DGR may be located at the image plane of the optical assembly 30. The detection unit LSD may comprise the detector DET configured to produce a detector output signal indicative of radiation received. In an embodiment the detection unit LSD is located such that the substrate W is at the object plane of the optical assembly 30.

[0076] As shown in Figure 4, in an embodiment the first curved reflector 32 substantially faces towards the substrate W. The second curved reflector 34 substantially faces orthogonal to the surface of the substrate W. However, it is not essential for the optical assembly 30 to be arranged in this way.

[0077] Figure 9 schematically depicts a different optical assembly 30. Features of the optical assembly 30 shown in Figure 9 that are the same as described above with reference to Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8 are not repeated for brevity.

[0078] Figure 9 schematically shows an optical assembly 30 of the projection unit LSP outputting the patterned beam of radiation BE1, and a further optical assembly 30 configured to image the reflected patterned beam of radiation BE2.

[0079] As shown in Figure 9, each optical assembly 30 comprises, for each optical channel, the first curved reflector 32, the second curved reflector 34, the first planar reflector 33 and the second planar reflector 31. In the arrangement shown in Figure 9, the first curved reflector 32 and the second curved reflector 34 face substantially similar directions. This is different from the arrangement shown in Figure 4, in which the first curved reflector 32 and the second curved reflector 34 face substantially orthogonal directions from each other. In the arrangement shown in Figure 9, the first planar reflector 33 and the second planar reflector 31 substantially oppose the first curved reflector 32 and the second curved reflector 34. This is different from the arrangement shown in Figure 4, in which the first planar reflector 33 and the second planar reflector 31 are arranged roughly at 45° angles to both the first curved reflector 32 and the second curved reflector 34.

[0080] Figure 10 is a schematic view of part of the optical assembly 30 shown in Figure 9. Figure 10 shows further details of the physical construction of the optical assembly 30.

[0081] As shown in Figure 10, in an embodiment the optical assembly 30 may have a substantiallymonolithic construction. For example, as shown in Figure 10 the optical assembly 30 may comprise a first supporting member 61. The first supporting member 61 may be configmed to physically support the first curved reflector 32 and the second curved reflector 34. In an embodiment the first supporting member 61 is substantially monolithic. The first supporting member 61 may be a monolithic component. A method of making the optical assembly 30 may comprise arranging the first curved reflector 32 and the second curved reflector 34 on a monolithic component.

[0082] As shown in Figure 10, in an embodiment, the first supporting member 61 is further configmed to mechanically support the second planar reflector 31. In an embodiment the first supporting member 61 has an approximate L-shape.

[0083] Figure 11 schematically depicts further details of the physical construction of the optical assembly 30 shown in Figme 9 and Figure 10. As shown in Figme 11, in an embodiment the optical assembly 30 comprises a second supporting member 62. In an embodiment the second supporting member 62 is configured to mechanically support the first planar reflector 33. In an embodiment the second supporting member 62 is configured to be fixed to the first supporting member 61. In an embodiment the first supporting member 61 forms a bottom part and the second supporting member 62 forms a top part of the optical assembly 30.

[0084] Figure 12 schematically depicts a perspective view of the arrangement shown in Figure 10. Figure 12 more clearly shows the different optical channels of the optical assembly 30. As shown in Figure 12, in an embodiment the same first supporting member 61 mechanically supports the first curved reflector 32, the second curved reflector 34 and the second planar reflector of a plurality (optionally all) of the optical channels of the optical assembly 30.

[0085] Figure 13 schematically depicts a perspective view of the arrangement shown in Figure 11. As shown in Figme 13, in an embodiment the second supporting member 62 is configmed to mechanically support the first planar reflector 33 of a plurality (optionally all) of the optical channels of the optical assembly 30 receiving the redirected patterned beam of radiation BE2. As shown ,the first supporting member 61 forms a bottom part and the second supporting member 62 forms a top part of the optical assembly 30.

[0086] In the arrangement shown in Figure 6, the optical assembly 30 may comprise a first reflector member 42 configmed to support the first curved reflectors 32a-e of the optical assembly 30. The optical assembly 30 may comprise a second reflector member 44 configmed to support the second cmved reflectors 34a-e of the optical assembly 30.

[0087] Figure 14 schematically depicts the arrangement shown in Figure 13 applied to both the projection unit LSP and the detection unit LSD of a topography measurement system. As shown in Figure 14, in an embodiment the projection unit LSP comprises a first supporting member 61a and a second supporting member 62a. The detection unit LSD may comprise a first supporting member 61b and a second supporting member 62b.

[0088] As shown in Figme 14, in an embodiment the topography measurement system is arrangedto fit around another optical system 65. For example, the other optical system 65 may comprise an optical element such as an objective lens, or an exposure lens. The other optical system may comprise an alignment sensor, or part of an alignment sensor. The other optical system may be part of sensor system to measure a parameter other than topography. A plurality of optical elements of the other optical system 65 may be arranged in the X direction. The assemblies 30 of the projection unit LSP and the detection unit LSD are arranged to fit on either side of the other optical system 65 in the Y direction.

[0089] In an embodiment the object plane and the image plane are tilted relative to a plane perpendicular to an optical axis of the optical assembly 30 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, for example. However, it is not essential for the Scheimpflug condition to be satisfied.

[0090] In an embodiment the projection unit LSP is configured to illuminate one or more illumination spot on the substrate W. For example, the illumination spots may be formed in a line, which may be a straight line. In an embodiment the measurement location MLO comprises an array of illumination spots. The illumination spots may be arranged substantially symmetrically. In an embodiment the measurement location MLO comprises an odd number of illumination spots such that there is a single central illumination spot.

[0091] Figure 15 schematically depicts the measurement location MLO formed as a linear array. In an embodiment the measurement location MLO is formed as a plurality of arrays, for example multiple arrays of spots. It is not essential for the measurement location MLO to be formed as one or more linear arrays. In an alternative embodiment the measurement location MLO is formed as a curved array or a plurality of curved arrays, for example.

[0092] Figure 15 further shows a field of view 72 of the other optical systems 65. As shown in Figure 14, the other optical system 65 may be located directly above the substrate W. In an embodiment the field of view 72 of the other optical system 65 is directly below the other optical system 65.

[0093] Figure 15 schematically depicts a field of view 71 of the projection unit LSP. The field of view 71 may be the field of view of the typography measurement system.

[0094] As shown in Figure 15, in an embodiment the field of view 71 of the projection unit LSP at least partially overlaps with the field of view 72 of the other optical system 65. In the arrangement shown in Figure 15, the field of view 72 of the other optical system 65 is substantially fully within the field of view 71 of the projection unit LSP. Alternatively, there may be only a partial overlap between the field of view 72 of the other optical system 65 and the field of view 71 of the projection unit LSP.

[0095] As shown in Figure 15, in an embodiment the measurement location MLO of the projection unit LSP at least partially overlaps with the field of view 72 of the other optical system 65. Themeasurement location MLO may be selected from within the field of view 71 of the projection unit LSP.

[0096] As shown in Figure 14, in an embodiment the other optical system 65 is located between the optical assemblies 30 of the projection unit LSP and the detection unit LSD. As shown in Figure 15, in an embodiment the field of view 72 of the other optical system 65 is located between the optical assemblies 30 of the projection unit LSP and the detection unit LSD. As shown in Figure 15, in an embodiment the projection unit LSP comprises an optical element 73 that is the final optical element of the projection unit LSP upbeam of the field of view 71 of the projection unit LSP. In an embodiment the optical element 73 comprises a folding mirror or a multiple folding mirror. However, it is not essential for the optical element 73 to comprise a folding mirror. The optical element 73 may be different from a multiple folding mirror.

[0097] As shown in Figure 15, in an embodiment the detection unit LSD comprises an optical element 74 that is the first optical element of the detection unit LSD downbeam of the field of view 71 of the projection unit LSP. In an embodiment the optical element 74 comprises a folding mirror or a multiple folding mirror. However, it is not essential for the optical element 74 to comprise a folding mirror. The optical element 74 may be different from a multiple folding mirror.

[0098] In an embodiment the optical elements 73, 74 are located on opposite sides of the field of view 72 of the other optical system 65. In an embodiment the other optical system 65 is located between the optical elements 73, 74 when viewed in plan view. In an embodiment the optical elements 73, 74 are located on opposite sides of the substrate W.

[0099] It is not essential for the projection unit LSP and the detection unit LSD to comprises the optical elements 73, 74. In an alternative embodiment the other optical system 65 is located between the projection unit LSP and the detection unit LSD. For example, the projection unit LSP and the detection unit LSD may be located where the optical elements 73, 74 are shown in Figure 15.

[0100] An embodiment of the invention is expected to increase the freedom of choice of the location of the measurement location MLO. An embodiment of the invention is expected to reduce the possibility of components mechanically interfering with the beam of radiation BE1 and the redirected patterned beam of radiation BE2.

[0101] By locating the measure location MLO to partially coincide with the field of view 72 of the other optical system 65, downstream processing of the measurement results may be made easier. For example, when the other optical system 65 is an alignment sensor, then the alignment measurements and the level sensing measurements may be made from substantially the same locations on the substrate W.

[0102] In an embodiment the central illumination spot of the illumination spots that form the measurement location MLO coincides with the field of view 72 of the other optical system 65. As indicated in Figure 15, there may be other illumination spots of the measurement location MLO that extend beyond the field of view 72 of the other optical system 65.

[0103] In an embodiment a method of making the optical assembly 30 comprises making the first curved reflector 32 and / or the second curved reflector 34. For example, in an embodiment, the first curved reflector 32 and / or the second curved reflector 34 is made by at least one of lithographic manufacturing, precision molding and computer numerical control milling. An embodiment of the invention is expected to reduce the cost of making an optical assembly 30 for a topography measurement system.

[0104] Figure 8 schematically depicts two linear arrays 51, 52 of fields of view 41, 43 of the optical assembly 30. In an alternative embodiment, the optical assembly 30 is arranged to have three or more than three linear arrays of fields of view on the substrate W. For example, the optical assembly 30 may comprise two, three or more than three arrays of each of the first curved reflector 32, the second curved reflector 34, the first planar reflector 33 and the second planar reflector 31.

[0105] In an embodiment, each of the reflectors is larger than the field of view 41, 43. An embodiment of the invention is expected to increase telecentricity. As a result of providing that each reflector is larger than the image or object, the gap 45 may be present between adjacent fields of view 41.

[0106] 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.

[0107] 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.

[0108] 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 numberedclauses.1. An optical assembly for a topography measurement system, the optical assembly comprising a plurality of optical channels each comprising:a first curved reflector and a second curved reflector arranged to image an object with a magnification magnitude of 1.2. The optical assembly of clause 1, wherein each channel comprises:a first planar reflector optically between the first curved reflector and the second curved reflector; anda second planar reflector located such that the first curved reflector is optically between the first planar reflector and the second planar reflector.3. The optical assembly of clause 1 or 2, wherein at least one of the first curved reflector and the second curved reflector is a spherical reflector.4. The optical assembly of any preceding clause, wherein a first plurality of the optical channels have fields of view of an image plane or an object plane arranged in a first linear array.5. The optical assembly of clause 4, wherein a second plurality of the optical channels have fields of view of the image plane or the object plane arranged in a second linear array offset from the first linear array.6. The optical assembly of clause 5, wherein linear arrays of fields of view of the optical channels partly overlap each other in a direction parallel to the linear arrays.7. The optical assembly of any of clauses 4-6, wherein the object plane and the image plane are tilted relative to a plane perpendicular to an optical axis of the optical assembly so as to satisfy the Scheimpflug principle.8. The optical assembly of any preceding clause, wherein the first curved reflector is tilted about a first axis and the second curved reflector is tilted about a second axis orthogonal to the first axis. 9. The optical assembly of any of clauses 1-7, wherein the first curved reflector is tilted about a first axis and the second curved reflector is tilted about the first axis.10. The optical assembly of any preceding clause, wherein the first curved reflector and the second curved reflector are concave.11. 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; andthe optical assembly of any preceding clause located such that the projection grating is at an object plane of the optical assembly.12. The projection unit of clause 11, located such that a substrate is at an image plane of the optical assembly.13. 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. 14. The detection unit of clause 13, located such that a substrate is at an object plane of the optical assembly.15. A topography measurement system comprising at least one of the projection unit of any of clauses 11-12 and the detection unit of any of clauses 13-14.16. The topography measurement system of clause 15, arranged to fit around another optical system.17. The topography measurement system of clause 16, wherein a field of view of the projection unit on a substrate to be measured at least partially overlaps with a field of view of the other optical system on the substrate.18. The topography measurement system of clause 16 or 17, wherein the other optical system is located between the optical assemblies of the projection unit and the detection unit.19. A method of making an optical assembly for a topography measurement system, the method comprising, for each of a plurality of optical channels:arranging a first curved reflector and a second curved reflector to image an object with a magnification magnitude of 1.20. The method of clause 19, comprising making at least one of the first curved reflector and the second curved reflector by at least one of lithographic manufacturing, precision molding and computer numerical control milling.21. The method of clause 19 or 20, comprising arranging the first curved reflectors and the second curved reflectors on a monolithic component.

Claims

1. CLAIMS1. An optical assembly for a topography measurement system, the optical assembly comprising a plurality of optical channels, each channel comprising a first curved reflector and a second curved reflector arranged to image an object with a magnification magnitude of 1.

2. The optical assembly of claim 1, wherein each channel comprises:a first planar reflector optically between the first curved reflector and the second curved reflector; anda second planar reflector located such that the first curved reflector is optically between the first planar reflector and the second planar reflector.

3. The optical assembly of claim 1 or claim 2, wherein the first curved reflector and / or the second curved reflector is a spherical reflector.

4. The optical assembly of any preceding claim, wherein a first plurality of the optical channels have fields of view of an image plane or an object plane arranged in a first linear array.

5. The optical assembly of claim 4, wherein a second plurality of the optical channels have fields of view of the image plane or the object plane arranged in a second linear array offset from the first linear array.

6. The optical assembly of claim 5, wherein linear arrays of fields of view of the optical channels partly overlap each other in a direction parallel to the linear arrays.

7. The optical assembly of any preceding claim, wherein the first curved reflector is tilted about a first axis and the second curved reflector is tilted about a second axis orthogonal to the first axis.

8. The optical assembly of any of claims 1-6, wherein the first curved reflector is tilted about a first axis and the second curved reflector is tilted about the first axis.

9. The optical assembly of any preceding claim, wherein the first curved reflector and the second curved reflector are concave.

10. 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; andthe optical assembly of any preceding claim located such that the projection grating is at an object plane of the optical assembly.

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

12. A detection unit for a topography measurement system, the detection unit comprising:the optical assembly of any of claims 1-9;a detection grating located at an image plane of the optical assembly; anda detector configured to produce a detector output signal indicative of radiation received.

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

14. A topography measurement system comprising the projection unit of claim 10 or claim 11 and / or the detection unit of claim 12 or claim 13.

15. A method of making an optical assembly for a topography measurement system, the method comprising, for each of a plurality of optical channels, arranging a first curved reflector and a second curved reflector to image an object with a magnification magnitude of 1.

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