WO2025201933A1

WO2025201933A1 - Optical measurement system

Info

Publication number: WO2025201933A1
Application number: PCT/EP2025/057156
Authority: WO
Inventors: Ahmet Burak CUNBUL; Stanislav Smirnov
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2024-03-29
Filing date: 2025-03-17
Publication date: 2025-10-02
Anticipated expiration: 2026-09-29

Abstract

Disclosed is an optical measurement system for measuring a topology of a surface of a substrate. The optical measurement system includes a light projector configured to project a light beam to the surface of the substrate and a light detector configured to receive the light beam from the light projector that has been reflected from the surface. It also includes a mirror relay comprising a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the light detector and increase a field of view of the optical measurement system at the surface.

Description

OPTICAL MEASUREMENT SYSTEM

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The application claims priority of US provisional application number 63/572,098 which was filed on 29 March, 2024 and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to an optical design of an optical measurement system with improved field of view (FOV) such as when used in conjunction with measuring surface features of a semiconductor substrate.

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 (also often referred to as "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as 'Moore's law'. To keep up with Moore's law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. 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 are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 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, radiation with a wavelength of 193 nm.

[0005] Products that may be produced by the above methods (e.g., wafers used with the production of computer chip components) often require measurements of surface features to quantify the quality of production or to find unacceptable errors or deviations. Optical scanning of the surface of such wafers to determine height can then be a part of the production process. However, some scanning systems can have limited fields of view which can require multiple scans and thereby slow production throughput. Accordingly, there is a need for scanning systems with improved fields of view such that overall scanning time can be reduced and throughput increased.

SUMMARY

[0006] In one aspect an optical measurement system for measuring a topology of a surface of a substrate is disclosed. The optical measurement system includes a light projector configured to project a light beam to the surface of the substrate and a light detector configured to receive the light beam from the light projector that has been reflected from the surface. The optical measurement system also includes a mirror relay comprising a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the light detector and increase a field of view of the optical measurement system at the surface.

[0007] In some variations, the multiple lateral reflections cause there to be lateral locations in the mirror relay with reduced aberrations. The concave mirror and/or the convex mirror can comprise multiple mirrors arranged in the lateral direction at the lateral locations, and the lateral reflections are between the convex mirror and the concave mirror. The multiple mirrors can have the same center of curvature.

[0008] In other variations, the light beam can have an elongate cross-section at the multiple mirrors and the multiple mirrors comprise elongate mirrors are configured to receive the light beam. Two or more of the multiple mirrors can have different aspheric shapes that reduce field curvature and distortion. The aspheric shapes in the multiple mirrors can be symmetric about a central axis of the mirror relay.

[0009] In yet other variations, the mirror relay can be rotated relative to a scan direction of the substrate, and wherein an aspect ratio of the mirror relay is modified relative to an unrotated configuration such that a reduction in a field of view of the mirror relay is at least partially compensated. Modifying the aspect ratio can include enlarging an X dimension of the mirror relay relative to a Y dimension of the mirror relay.

[0010] In some variations, the mirror relay can further comprise an aperture stop as a separate mirror with multiple concave mirrors, as a separate mirror with multiple convex mirrors, part of the concave mirror, or part of the convex mirror.

[0011] In other variations, the convex mirror can be a single convex mirror, the concave mirror can be single concave mirror.

[0012] In yet other variations, the optical measurement system can include a projection grating and or a detection grating or may be part of a lithographic apparatus. [0013] In an interrelated aspect, a level sensor can include a projection unit arranged to direct a beam of radiation received from a light projector to a surface of a substrate, comprising a projection grating having a period P, the projection grating configured to provide a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P; a detection unit arranged to receive the beam of radiation reflected at the surface of the substrate, one or more detectors; a processing unit configured to determine a position of the surface of the substrate based on the beam of radiation received by the one or more detectors; and wherein the projection unit and/or detection unit comprise a mirror relay having a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the one or more detectors and increase a field of view of the optical measurement system at the surface.

[0014] In a further interrelated aspect, a method of determining a height level of a substrate is disclosed. The method includes: projecting a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P onto the substrate; receiving a reflected patterned measurement beam after reflection on the substrate on to detector; determining the height level of the substrate based on one or more signals from the detector, wherein at least one of projecting the patterned measurement beam and receiving the reflected patterned measurement beam is by means of a mirror relay having a convex mirror and a concave mirror arranged to reflect the measurement beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the measurement beam and increase a field of view of the optical measurement system at the surface.

[0015] In some variations, the method includes moving the substrate and the patterned measurement beam relative to each other to make a scanning movement of the patterned measurement beam over the surface.

[0016] In some variations, the height level of the substrate can be determined based on a single stroke scanning-measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed implementations. In the drawings, [0018] Figure 1 depicts a schematic overview of a lithographic apparatus;

[0019] Figure 2 depicts a schematic overview of a level or height sensor; [0020] Figure 3A depicts a simplified side view of a mirror relay;

[0021] Figure 3B depicts a simplified side view of a mirror relay that includes additional lateral reflections that increase the field-of-view;

[0022] Figure 3C depicts a simplified side view of a mirror relay that includes mirror segments with different aspheric shapes that reduce aberrations;

[0023] Figure 3D depicts a side view of a mirror relay forming part of an optical measurement system; [0024] Figure 4 depicts a top view of a mirror relay;

[0025] Figure 5 depicts a bottom view of a mirror relay;

[0026] Figure 6 depicts a simplified representation of an aperture stop being part of a concave mirror;

[0027] Figure 7 depicts a simplified representation of an aperture stop being part of a convex mirror;

[0028] Figure 8 depicts a mirror relay rotated relative to a scan direction for imaging a wafer; and

[0029] Figures 9-11 show examples of mirror relays having different orientations in an optical measurement system.

DETAILED DESCRIPTION

[0030] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0031] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, electromagnetic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0032] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, and/or electromagnetic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0033] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fdl a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in U.S. 6,952,253, which is incorporated herein by reference.

[0034] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0035] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0036] In operation, the radiation beam B may be incident on the patterning device, e.g., mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which may focus the beam onto a target portion C of the substrate W or onto a sensor arranged at a stage. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C. Substrate alignment marks Pl, P2 may also be arranged in the target portion C area as in -die marks. These in-die marks may also be used as metrology marks, for example, for overlay measurements. [0037] To clarify the present disclosure, in some instances a Cartesian coordinate system may be used. The Cartesian coordinate system has three axis, i.e., an X-axis, a Y-axis and a Z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the Y-axis is referred to as an Ry-rotation. A rotation around the Z-axis is referred to as an Rz-rotation. The X-axis and the Y -axis define a horizontal plane, whereas the Z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the disclosure and is used for clarification only. Instead, another coordinate system, such as a cylindrical or spherical coordinate system, may be used. The orientation of the Cartesian coordinate system may be different, for example, such that the Z-axis has a component along the horizontal plane.

[0038] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0039] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS 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 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 W or from a layer at the substrate W.

[0040] The projection grating PGR is a grating comprising, for example, a periodic structure resulting in a beam of radiation BE1 having a periodically varying spatial intensity. The beam of radiation BE1 with the periodically varying spatial intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.

[0041] In order to determine the height level at the measurement location MLO, the level sensor LS further comprises a detection unit LSD comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0042] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having, for example, a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0043] 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.

[0044] 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.

[0045] 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 (e.g., measurements at several measurement locations) or spots covering a larger measurement range.

[0046] Various height sensors of a general type are disclosed for example in U.S. 7,265,364 and U.S. 7,646,471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. 2010/233600 Al, incorporated by reference. In WO 2016/102127 Al, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating. The combinations and sub-combinations of the elements disclosed herein constitute separate embodiments and are provided as examples only. Also, the descriptions are intended to be illustrative, not limiting.

[0047] Figure 3A depicts a simplified side view of a mirror relay 300a. Mirror relays of the present disclosure can be utilized to relay light from the measured surface to components of the optical measurement system (e.g., detector DET). The disclosed mirror relays, in addition to providing a 1: 1 source/image size ratio, can provide other benefits such as including increasing the FOV. As used herein, the term “field of view” can either be a two-dimensional area, or it can refer to the linear width of an area swept out by scanning the width in a direction orthogonal to the width (e.g., a scan direction of a wafer). In the context of measuring surface features, such as height, increasing the FOV can include increasing the number of measurement spots, so that an extended FOV can project the measurement spots onto the wafer and the mirror relay project the reflected light beams to a detector.

[0048] Figure 3A shows an example of mirror array 300a with concave mirror 301a and convex mirror 302a. In this example, light beam BE2 is shown having three reflections (two off of concave mirror 301a and one off of convex mirror 302a). To the right of the mirror diagram is a depiction of exemplary calculated root mean square (RMS) wavefront error (WFE), which can be due to aberrations (e.g., field curvature, astigmatism, etc.). From the center of the spherical mirror(s), there is one radial distance where the field curvature present in the reflected image will largely be cancelled by the astigmatism or, more generally, where aberrations cancel such that the RMS WFE is at or near a local minimum. This forms a narrow band 303a where the aberrations are substantially reduced. Generally, an inscribed rectangle 304a can be shown where the width generally corresponds to the width of the band 303a and therefore giving it a corresponding length where it is substantially within band 303a. By reducing the WFE over a larger region, more measurement points at the wafer can be included. Thus, this length (or area) can be considered related to the measurement system’s field-of-view and is described in Figure 3 A by field-of-view (mirror) FOVM1 (to be distinct from FOV at the wafer surface).

[0049] Figure 3B depicts a simplified side view of mirror relay 300b that includes additional lateral reflections that increase the field-of-view. With such additional lateral reflections, the calculated aberrations include multiple bands 303b with very low aberrations and region 305b between them with fairly low aberrations. This creates an expanded area shown by inscribed rectangle 304b having tolerable aberrations, and importantly, an expanded field-of-view FOVM2. In this way, mirror relays can be provided that include numerous areas with low aberrations.

[0050] Figure 3C depicts a simplified side view of mirror relay 300c that includes a plurality of mirror segments 306c with different aspheric shapes that reduce aberrations. As explained above, the areas of the mirrors outside the low-aberration bands (e.g., bands 303a and 303b) are generally not desirable for reflecting light. The simplified embodiment of Figure 3C shows how mirror segments 306c can be provided at the reflection locations rather than providing a continuous mirror. Mirror segments 306c can be understood as, for example, being rectangularly shaped portions of a spherical mirror. As explained in further detail herein, the shapes of different mirror segments 306c can be aspheric such that they correct different orders of aberrations. This can have the effect depicted by band 303c where, for example, the two bands 303b shown in Figure 3B have substantially merged to form a single wider band 303c of very low aberrations. In this way, embodiments of the present disclosure can, individually or in combination, provide improved fields of view and images with reduced aberrations.

[0051] Figure 3D depicts a side view of a mirror relay 300d forming part of an optical measurement system, for example the sensor as illustrated in Figure 2. Figure 4 depicts atop view of the mirror relay 300d. Figure 5 depicts a bottom view of the mirror relay 300d. Mirror relay 300 can be utilized with an optical measurement system (e.g., level sensor LS), for example, for measuring topology of a surface of a substrate (e.g., a wafer W) such as depicted in Figure 2. The optical measurement system can include a light projector LSO (e.g., radiation source LSO) configured to project light beam BE1 (e.g., beam of radiation BE1) to the surface of the substrate. The optical measurement system can also include a light detector (e.g., detector DET) configured to receive light beam BE2 from light projector LSO that has been reflected from the surface. Any of the optical measurement systems disclosed herein can also include additional mirrors 340 to direct the light beam to/from the surface, a mirror relay, light projector, and/or a light detector.

[0052] The depicted example of a mirror relay 300d is shown as including convex mirror 320 (or mirrors) and a concave mirror 310 (or mirrors). The convention used herein to describe the curvatures of the mirrors is relative to the light beam that reaches their mirror surfaces. With this convention, the larger radius mirror(s) has the light beam impinging on its concave surface and the smaller radius mirror(s) has light impinging on its convex surface.

[0053] Concave mirror 310 and convex mirror 320 can be arranged to reflect the light beam in a substantially radial direction RD and at least partially in a lateral direction LD between the reflective surfaces. This can cause multiple lateral reflections between the concave mirror 310 and the convex mirror 320 that extend a path length of the light beam between light projector LSO and light detector DET. As explained with reference to Figures 3A-C, these multiple lateral reflections cause there to be lateral locations in the mirror relay with reduced aberrations. Having a mirror (or mirrors) at these locations can increase the field-of-view of the optical measurement system.

[0054] In some embodiments, the convex and concave mirrors can have their centers of curvature CC be coaxial (e.g., along central axis CA about which the mirrors may be symmetrical) and optionally, but not necessarily, coincident (i.e., at substantially the same point in space). In some embodiments, the convex and concave mirrors can be substantially spherical, and having their centers of curvature at substantially the same location (i.e., coinciding at a single point). The concave mirror 310 and/or convex mirror 320 can have any required geometry, for example, spherical, aspherical, paraboloidal, ellipsoidal, etc. Also depicted is an example of aperture stop 330, see Figures 3D, 4, and 5, which is described in further detail with reference to Figure 5. [0055] Mirror relay 300d shown in Figure 3D depicts a partial system where, for example, light can be relayed from wafer W to detector DET, which can be referred as the detection unit LSD of Figure 2. However, the present disclosure also contemplates embodiments where mirror relay 300d can be on a projection side of the wafer (i.e., to relay light from light projector LSO to wafer W), as illustrated as the projection unit LSP in Figure 2. In some embodiments, there can be separate mirror relays 300d on both sides of wafer W, thus as part of the project unit LSP and the detection unit LSD of the sensor LS of Figure 2.

[0056] While the concave mirror 310 and the convex mirror 320 are shown in Figure 3D as having discrete segments (and a distinct aperture stop 330), the present disclosure contemplates that such mirrors can have any number of segments. As shown in Figure 3D, the concave mirror 310 and/or the convex mirror 320 can include multiple mirrors arranged in the lateral direction LD (e.g., at lateral locations as described above). As shown, the lateral reflections can be between the convex mirror and the concave mirror. Such multiple mirrors can have the same centers of curvature at substantially the same point (e.g., barring aspheric features or other minor deviations), and for example, arranged to have their radii located on central axis CA, etc. In other embodiments, the multiple mirrors may have some centers of curvature located off central axis CA. The example of Figure 3D shows an embodiment where concave mirror 310 has six segments, with three mirror segments on either side of central axis CA. Similarly, convex mirror 320 can have five segments, with the third segment located at central axis CA and the remaining mirror segments symmetrically distributed on either side. While the particular shape of the mirrors can vary with the application, in the depicted embodiment of FIG. 3, the light beam can have an elongate crosssection and the multiple mirrors can be elongate mirrors configured to receive the light beam.

[0057] The surfaces of any of the disclosed mirrors can also be aspherical, where the shape can be selected to reduce aberrations. The general form of a rotationally symmetric aspheric surface equation can be represented as follows:

[0058] In Equation (1), Z is the sagitta, or the distance from the vertex of the mirror to a point on its surface, along the particular mirror’s axis of symmetry; r is the radial distance from the axis of symmetry to a point on the surface; c is the curvature at the vertex (the reciprocal of the radius of curvature); k is the conic constant, which defines the primary shape of the mirror (e.g., parabolic, hyperbolic, elliptical); and A,B, C, . . . are the coefficients for the higher-order terms, which define the aspheric deviation at different powers of r. [0059] In some embodiments, two or more of the multiple mirrors can have different aspheric shapes that reduce field curvature and/or distortion, or other optical aberrations. For example, field curvature can be an aberration when the focal plane (e.g., at the surface of wafer W) of the system is curved rather than flat. Distortions can include effects such as warping or bending of images. Different aspheric shapes for the multiple mirrors can address such aberrations by having differences in one or more of the coefficients described above. In some embodiments, the aspheric shapes in the multiple mirrors can be symmetric about central axis CA of the mirror relay 300d. One advantage of having multiple mirror segments is that complementary mirror segments (e.g., on opposite sides of the central axis CA) can be constructed to have the same aspheric shape, thus avoiding having to construct a mirror whose aspheric shape (e.g., as set forth by Equation 1) would otherwise vary across its surface. In some embodiments, the base radius (e.g., r) of the concave mirror 310 or the convex mirror 320 can be the same, but the aspherical coefficients (A, B, C, etc.) may vary to correct aberrations and distortions. In some embodiments, the multiple mirrors can have aspheric coefficients that are between 4^th and 12^th order to reduce aberrations. For example, this can include extending Equation (1) to include two more coefficients D and E for the 10^th and 12^th order (in r) terms. In other embodiments, the higher-order terms and their coefficients can go up to 20^th order.

[0060] The surface of each mirror segment can be a free-form surface meaning its sagitta Z can be described by any function of x and y coordinates, i.e., Z = F(x,y) or equation F(x,y,z) = 0, e.g., a general 2D polynomial of x and y.

[0061] One proposed manufacturing method for the convex and/or concave mirrors’ 310, 320 aspherical segments can be to generate an optical blank on a glass, metal, or ceramic substrate. Molded aspherical glass or plastics can then be added. The aspherical segments can be coated with multilayer reflective thin films.

[0062] As a result of the improvements disclosed herein, embodiments of the present disclosure can have FOVs in the range of, for example, 100 to 450 mm, or 150 to 300 mm, and in particular embodiments approximately 300 mm. Exemplary embodiments of the present disclosure can have FOVs of approximately the substrate diameter. For example, the disclosed embodiments can create a wider measurement area at the wafer surface. As such, the measurement area covers a larger area on the substrate, which is perpendicular to the scan direction (relative movement). As such, a single measurement stroke can be sufficient to measure the complete surface.

[0063] While different number of mirrors can be utilized in mirror relay 300d, some embodiments can include single-segment mirrors where convex mirror 320 is a single convex mirror and/or concave mirror 310 is a single concave mirror. In such embodiments, an aperture stop (or aperture) may be disposed between convex mirror 320 or concave mirror 310, or anywhere along the light path. [0064] The botom view shown in Figure 5 also depicts an aperture stop 330 that may be used with embodiments of the disclosed mirror relay 300d. Figure 6 depicts a simplified representation of aperture stop 630 being part of the concave mirror 610. Figure 7 depicts a simplified representation of aperture stop 730 being part of the convex mirror 720. As shown in Figure 5, the mirror relay 300d can include aperture stop 330 that can be configured to provide a limitation on the light that is reflected between the mirror(s). Aperture stop 330 can also affect the numerical aperture (NA) of the optical measurement system. Physically, the aperture stop 330 can be a mirror with an optionally controllable aperture such as a shuter or diaphragm to limit the amount of light entering or exiting. Aperture stop 330 can be circular, as shown, but in other embodiments may be rectangular similar to other mirror segments. Also, aperture stop 330 can be a planar mirror, a curved mirror (e.g., spherical, parabolic, etc.), etc.

[0065] As shown in Figure 5, in some embodiments, mirror relay 300d can have aperture stop 330 as a separate mirror with the multiple convex mirrors 320. Figure 6 shows a similar embodiment to that of Figure 5, with mirror relay 600 having concave mirror 610, convex mirror 620, and aperture stop 630 as part of concave mirror 610 arrangement. Figure 7 shows an example where mirror relay 700 can include aperture stop 730 as a separate mirror with the multiple convex mirrors 720. Similar embodiments are possible for single-segment mirror configurations. In such embodiments, aperture stop 630, 730 can be part of the (single -segment) concave mirror 610 or part of the (single-segment) convex mirror 720. It is also contemplated that, in some embodiments, an aperture stop can be located outside the mirror “plane” of the curved mirrors such that it is between the mirrors, outside the concave mirror (e.g., at a larger radius), or inside the concave mirror (e.g., at a smaller radius).

[0066] Figure 8 depicts a mirror relay 810 rotated relative to a scan direction 820 used when measuring the surface of a wafer W. One way to make the disclosed mirror relays more compact can be to have the mirror relay rotated relative to a scan direction of the substrate. This is illustrated by wafer W and exemplary mirror relay 800 that is considered non-rotated and depicted (for illustrative purposes) as essentially square (e.g., a square shape surrounding the envelope of concave mirror 310 shown in Fig. 4). To retain the field-of-view of mirror relay 800, but with a rotated mirror relay 810, the aspect ratio of mirror relay 810 must change relative to mirror relay 800. This can further include moving the focal positions of the mirror relay (at the substrate and at a corresponding optical element) closer to the optical axis of the mirror relay. In the example shown, the mirror relay becomes more rectangular (e.g., longer in one dimension, and shorter in the other). Thus, the aspect ratio (e.g., ratio of a mirror segment’s X dimension vs. its Y dimension) of the mirror relay can be modified relative to an unrotated configuration such that a reduction in a field-of-view of the mirror relay is at least partially compensated. For example, modifying the aspect ratio can include enlarging an X dimension of the mirror relay relative to a Y dimension of the mirror relay. For the purposes of discussion, the X dimension may be in the generally lateral direction LD (but possibly on a curved surface for the depicted spherical mirrors) with the Y dimension generally orthogonal to the X dimension (but possibly on a similarly curved surface). Such modifications relative to non-rotated designs can, for example, be useful in decreasing a critical dimension for manufacturing or installation purposes.

[0067] Figures 9-11 show examples of mirror relays 900, 1000, 1100 having different orientations in an optical measurement system 910, 1010, 1110, for example, a sensor arranged to measure a topology of a surface. Based on space constraints, or other system needs, embodiments of the mirror relays can be sized and oriented as needed. Depending on the particular geometry of the optical measurement system, other optical elements (e.g., additional mirrors 940, 1040, 1140) can be utilized to direct light to/from a mirror relay as shown in the depicted examples. While this provides three examples of configurations for mirror relays and a general optical measurement system, the present disclosure contemplates numerous other configurations as appropriate for the particular optical measurement system utilized with the mirror relays.

[0068] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of items that may be optionally claimed in any combination: [0069] Item 1 : An optical measurement system for measuring a topology of a surface of a substrate, comprising: a light projector configured to project a light beam to the surface of the substrate; a light detector configured to receive the light beam from the light projector that has been reflected from the surface; and a mirror relay comprising a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the light detector and increase a field of view of the optical measurement system at the surface.

[0070] Item 2: The optical measurement system of Item 1, wherein the multiple lateral reflections cause there to be lateral locations in the mirror relay with reduced aberrations.

[0071] Item 3 : The optical measurement system of any one of the preceding Items, wherein the concave mirror and/or the convex mirror comprise multiple mirrors arranged in the lateral direction at the lateral locations, and the lateral reflections are between the convex mirror and the concave mirror.

[0072] Item 4: The optical measurement system of any one of the preceding Items, wherein the multiple mirrors have the same center of curvature.

[0073] Item 5 : The optical measurement system of any one of the preceding Items, wherein the light beam has an elongate cross-section at the multiple mirrors and the multiple mirrors comprise elongate mirrors are configured to receive the light beam. [0074] Item 6: The optical measurement system of any one of the preceding Items, wherein two or more of the multiple mirrors have different aspheric shapes that reduce field curvature and distortion.

[0075] Item 7 : The optical measurement system of any one of the preceding Items, wherein the aspheric shapes in the multiple mirrors are symmetric about a central axis of the mirror relay.

[0076] Item 8: The optical measurement system of any one of the preceding Items, wherein the mirror relay is rotated relative to a scan direction of the substrate, and wherein an aspect ratio of the mirror relay is modified relative to an unrotated configuration such that a reduction in a field of view of the mirror relay is at least partially compensated.

[0077] Item 9: The optical measurement system of any one of the preceding Items, wherein modifying the aspect ratio includes enlarging an X dimension of the mirror relay relative to a Y dimension of the mirror relay.

[0078] Item 10: The optical measurement system of any one of the preceding Items, the mirror relay further comprising an aperture stop as a separate mirror with multiple concave mirrors.

[0079] Item 11 : The optical measurement system of any one of the preceding Items, the mirror relay further comprising an aperture stop as a separate mirror with multiple convex mirrors.

[0080] Item 12: The optical measurement system of any one of the preceding Items, the mirror relay further comprising an aperture stop that is part of the concave mirror.

[0081] Item 13: The optical measurement system of any one of the preceding Items, the mirror relay further comprising an aperture stop that is part of the convex mirror.

[0082] Item 14: The optical measurement system of any one of the preceding Items, wherein the convex mirror is a single convex mirror.

[0083] Item 15: The optical measurement system of any one of the preceding Items, wherein the concave mirror is a single concave mirror.

[0084] Item 16: The optical measurement system of any one of the preceding Items, further comprising a projection grating and or a detection grating.

[0085] Item 17: A lithographic apparatus comprising the optical measurement system according to any one of the preceding Items.

[0086] Item 18: A level sensor comprising a projection unit arranged to direct a beam of radiation received from a light projector to a surface of a substrate, comprising a projection grating having a period P, the projection grating configured to provide a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P; a detection unit arranged to receive the beam of radiation reflected at the surface of the substrate, one or more detectors; a processing unit configured to determine a position of the surface of the substrate based on the beam of radiation received by the one or more detectors; and wherein the projection unit and/or detection unit comprise a mirror relay having a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the one or more detectors and increase a field of view of the optical measurement system at the surface.

[0087] Item 19: A method of determining a height level of a substrate, the method comprising: projecting a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P onto the substrate; receiving a reflected patterned measurement beam after reflection on the substrate on to detector; determining the height level of the substrate based on one or more signals from the detector, wherein at least one of projecting the patterned measurement beam and receiving the reflected patterned measurement beam is by means of a mirror relay having a convex mirror and a concave mirror arranged to reflect the measurement beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the measurement beam and increase a field of view of the optical measurement system at the surface.

[0088] Item 20: The method according to Item 19, further comprising: moving the substrate and the patterned measurement beam relative to each other to make a scanning movement of the patterned measurement beam over the surface.

[0089] Item 21: The method according to Item 19 or 20, wherein the height level of the substrate is determined based on a single stroke scanning-measurement.

[0090] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The lithographic apparatus may be a lithographic exposure apparatus. The lithographic apparatus may be a metrology apparatus configured to measure characteristics of an area at a substrate.

[0091] Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that, where the context allows, the subject matter of the present disclosure is not limited to optical lithography and may be used in other applications, for example imprint lithography, e-beam lithography, or directed self-assembly.

[0092] Where the context allows, embodiments of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present disclosure may also 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 actuators or other devices to interact with the physical world.

[0093] While specific embodiments of the present disclosure have been described above, it will be appreciated that various embodiments may be practiced otherwise than as described. 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 to various embodiments as described without departing from the scope of the claims set out below. Other aspects of the invention are set-out as in the following numbered clauses.

1. An optical measurement system for measuring a topology of a surface of a substrate, comprising: a light projector configured to project a light beam to the surface of the substrate; a light detector configured to receive the light beam from the light projector that has been reflected from the surface; and a mirror relay comprising a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the light detector and increase a field of view of the optical measurement system at the surface.

2. The optical measurement system of clause 1, wherein the multiple lateral reflections cause there to be lateral locations in the mirror relay with reduced aberrations.

3. The optical measurement system of clause 2, wherein the concave mirror and/or the convex mirror comprise multiple mirrors arranged in the lateral direction at the lateral locations, and the lateral reflections are between the convex mirror and the concave mirror.

4. The optical measurement system of clause 3, wherein the multiple mirrors have the same center of curvature.

5. The optical measurement system of clause 3, wherein the light beam has an elongate crosssection at the multiple mirrors and the multiple mirrors comprise elongate mirrors are configured to receive the light beam. 6. The optical measurement system of clause 3, wherein two or more of the multiple mirrors have different aspheric shapes that reduce field curvature and distortion.

7. The optical measurement system of clause 6, wherein the aspheric shapes in the multiple mirrors are symmetric about a central axis of the mirror relay.

8. The optical measurement system of clause 1, wherein the mirror relay is rotated relative to a scan direction of the substrate, and wherein an aspect ratio of the mirror relay is modified relative to an unrotated configuration such that a reduction in a field of view of the mirror relay is at least partially compensated.

9. The optical measurement system of clause 8, wherein modifying the aspect ratio includes enlarging an X dimension of the mirror relay relative to a Y dimension of the mirror relay.

10. The optical measurement system of clause 3, the mirror relay further comprising an aperture stop as a separate mirror with multiple concave mirrors.

11. The optical measurement system of clause 3, the mirror relay further comprising an aperture stop as a separate mirror with multiple convex mirrors.

12. The optical measurement system of clause 1, the mirror relay further comprising an aperture stop that is part of the concave mirror.

13. The optical measurement system of clause 1, the mirror relay further comprising an aperture stop that is part of the convex mirror.

14. The optical measurement system of clause 1, wherein the convex mirror is a single convex mirror.

15. The optical measurement system of clause 1, wherein the concave mirror is a single concave mirror.

16. The optical measurement system of clause 1, further comprising a projection grating and or a detection grating.

17. A lithographic apparatus comprising the optical measurement system according to any one of the preceding clauses.

18. A level sensor comprising : a projection unit arranged to direct a beam of radiation received from a light projector to a surface of a substrate, comprising a projection grating having a period P, the projection grating configured to provide a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P; a detection unit arranged to receive the beam of radiation reflected at the surface of the substrate, one or more detectors; a processing unit configured to determine a position of the surface of the substrate based on the beam of radiation received by the one or more detectors; and wherein the projection unit and/or detection unit comprise a mirror relay having a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the one or more detectors and increase a field of view of the optical measurement system at the surface.

19. A method of determining a height level of a substrate, the method comprising: projecting a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P onto the substrate; receiving a reflected patterned measurement beam after reflection on the substrate on to detector; determining the height level of the substrate based on one or more signals from the detector, wherein at least one of projecting the patterned measurement beam and receiving the reflected patterned measurement beam is by means of a mirror relay having a convex mirror and a concave mirror arranged to reflect the measurement beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the measurement beam and increase a field of view of the optical measurement system at the surface.

20. The method according to clause 14, further comprising: moving the substrate and the patterned measurement beam relative to each other to make a scanning movement of the patterned measurement beam over the surface.

Claims

2. The optical measurement system of claim 1, wherein the multiple lateral reflections cause there to be lateral locations in the mirror relay with reduced aberrations.

3. The optical measurement system of claim 2, wherein the concave mirror and/or the convex mirror comprise multiple mirrors arranged in the lateral direction at the lateral locations, and the lateral reflections are between the convex mirror and the concave mirror.

4. The optical measurement system of claim 3, wherein the multiple mirrors have the same center of curvature.

5. The optical measurement system of claim 3, wherein the light beam has an elongate cross-section at the multiple mirrors and the multiple mirrors comprise elongate mirrors are configured to receive the light beam.

6. The optical measurement system of claim 3, wherein two or more of the multiple mirrors have different aspheric shapes that reduce field curvature and distortion.

7. The optical measurement system of claim 6, wherein the aspheric shapes in the multiple mirrors are symmetric about a central axis of the mirror relay.

8. The optical measurement system of claim 1, wherein the mirror relay is rotated relative to a scan direction of the substrate, and wherein an aspect ratio of the mirror relay is modified relative to an unrotated configuration such that a reduction in a field of view of the mirror relay is at least partially compensated.

9. The optical measurement system of claim 8, wherein modifying the aspect ratio includes enlarging an X dimension of the mirror relay relative to a Y dimension of the mirror relay.

10. The optical measurement system of claim 3, the mirror relay further comprising an aperture stop as a separate mirror with multiple concave mirrors, or as a separate mirror with multiple convex mirrors.

11. The optical measurement system of claim 1, the mirror relay further comprising an aperture stop that is part of the concave mirror, or that is part of the convex mirror.

12. The optical measurement system of claim 1, further comprising a projection grating and or a detection grating.

13. A level sensor comprising : a projection unit arranged to direct a beam of radiation received from a light projector to a surface of a substrate, comprising a projection grating having a period P, the projection grating configured to provide a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P; a detection unit arranged to receive the beam of radiation reflected at the surface of the substrate, one or more detectors; a processing unit configured to determine a position of the surface of the substrate based on the beam of radiation received by the one or more detectors; and wherein the projection unit and/or detection unit comprise a mirror relay having a convex mirror and a concave mirror arranged to reflect the light beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the light beam between the light projector and the one or more detectors and increase a field of view of the optical measurement system at the surface.

14. A method of determining a height level of a substrate, the method comprising: projecting a patterned measurement beam having a periodically varying intensity distribution in a first direction having the period P onto the substrate; receiving a reflected patterned measurement beam after reflection on the substrate on to detector; determining the height level of the substrate based on one or more signals from the detector, wherein at least one of projecting the patterned measurement beam and receiving the reflected patterned measurement beam is by means of a mirror relay having a convex mirror and a concave mirror arranged to reflect the measurement beam in a substantially radial direction and at least partially in a lateral direction to cause multiple lateral reflections between the concave mirror and the convex mirror that extend a path length of the measurement beam and increase a field of view of the optical measurement system at the surface.

15. The method according to claim 14, further comprising: moving the substrate and the patterned measurement beam relative to each other to make a scanning movement of the patterned measurement beam over the surface.