Focusing measurement and control in metrology, and associated optical wedge arrangement

Abstract

Disclosed is an optical wedge arrangement comprising a plurality of optical wedge elements arranged around an optical axis, the plurality of optical wedge elements comprising at least a first optical wedge element, a second optical wedge element, and a third optical wedge element; wherein the first optical wedge element comprises a first optical surface and a second optical surface, the first optical surface is planar and is not perpendicular to the optical axis, and the second optical surface is non-planar, so that the first optical wedge element has a nonlinear thickness change; the second optical wedge element comprises a third optical surface and a fourth optical surface which are planar and are not parallel; and the third optical wedge element comprises a fifth optical surface and a sixth optical surface, and the fifth optical surface is planar and is not perpendicular to the optical axis. Also disclosed is a metrology device that uses such an optical wedge arrangement in determining the focus of the measurement radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22181325.6 filed on June 27, 2022, and the entire contents of this European application are incorporated herein by reference. Technical Field

[0003] The present invention relates to a metrology apparatus and a method which can be used for performing metrology, for example in device manufacturing by lithography. The invention also relates to such a method for monitoring the focusing of a measuring radiation in such a metrology apparatus. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate, usually to a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that case, a pattern forming device, alternatively referred to as a mask or reticle, can be used to produce a circuit pattern to be formed on a single layer of the IC. This pattern can be transferred to a target portion (e.g., a portion including or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will include a network of adjacent target portions that are patterned continuously.

[0005] During the lithography process, it is desirable to frequently make measurements of the structures produced, for example, for process control and verification. Various tools for making these measurements are well known, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools for measuring overlay (i.e., the alignment accuracy of two layers in a device). Recently, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a radiation beam onto a target and measure one or more properties of the scattered radiation, for example, intensity at a single reflection angle as a function of wavelength, intensity at one or more wavelengths as a function of reflection angle, or polarization as a function of reflection angle, to obtain a diffraction "spectrum" from which properties of interest of the target can be determined.

[0006] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The target used by such scatterometers is a relatively large (e.g., 40 μm×40 μm) grating, and the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled). Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1, the documents of which are hereby incorporated by reference in their entirety. Further developments of the technology have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. Composite grating targets may be used to measure multiple gratings in one image. The contents of all these applications are also incorporated herein by reference.

[0007] In many typical metrology tools, a separate focus monitoring branch is used to measure or monitor the focus of the measurement radiation on the target or structure being measured. However, this requires significant additional hardware (e.g., detectors, lens beam splitters and other optical elements, among others). Such hardware adds considerable cost, complexity and volume to the metrology tool.

[0008] It would be desirable to improve such focus monitoring methods in metrology. Summary of the invention

[0009] The present invention provides in a first aspect an optical wedge arrangement, the optical wedge arrangement comprising a plurality of optical wedge elements arranged around an optical axis, the plurality of optical wedge elements comprising at least a first optical wedge element, a second optical wedge element and a third optical wedge element; wherein: the first optical wedge element comprises a first optical surface and a second optical surface, wherein the first optical surface is planar and not perpendicular to the optical axis, and the second optical surface is non-planar, so that the first optical wedge element has a non-linear thickness change; the second optical wedge element comprises a third optical surface and a fourth optical surface, both of which are planar and non-parallel; and the third optical wedge element comprises a fifth optical surface and a sixth optical surface, wherein the fifth optical surface is planar and not perpendicular to the optical axis.

[0010] In a second aspect, the present invention provides a measurement device comprising a detection branch comprising the optical wedge arrangement of the first aspect.

[0011] The present invention provides in a third aspect a measurement device, comprising: a processor; an illumination branch, the illumination branch being used to illuminate a sample with measurement radiation; and a detection branch for detecting scattered radiation, the scattered radiation comprising the measurement radiation that has been scattered by the sample, the detection branch comprising at least one detector for detecting the scattered radiation; wherein the detection branch further comprises an optical wedge arrangement, the optical wedge arrangement being operable to: separate at least non-zero-order diffracted radiation within the scattered radiation from zero-order radiation within the scattered radiation, so that the non-zero-order diffracted radiation and zero-order scattered radiation are imaged on different regions of the at least one detector; separate a first portion of the zero-order radiation from a second portion of the zero-order radiation, so that the first portion of the zero-order radiation and the second portion of the zero-order radiation are imaged on different regions of the at least one detector to form a first zero-order image and a second zero-order image, respectively; and apply defocus to at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation; wherein the processor is operable to determine a focus value of the measurement radiation on the sample from the first zero-order image and the second zero-order image.

[0012] The present invention provides in a fourth aspect a method for determining a focusing value of measurement radiation on a sample during measurement of the sample, the method comprising: separating scattered radiation from the sample that has been scattered from the sample after irradiating the sample with the measurement radiation into at least non-zero-order diffraction radiation, a first portion of zero-order radiation, and a second portion of zero-order radiation; detecting at least one image of the non-zero-order diffraction radiation, a first zero-order image of the first portion of the zero-order radiation, and a second zero-order image of the second portion of the zero-order radiation at independent regions of a detection plane; applying defocus to at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation; and determining the focusing value of the measurement radiation on the sample from the first zero-order image and the second zero-order image.

[0013] The following describes in detail other features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. These embodiments are presented herein only for illustrative purposes. Based on the teachings included in the present invention, those skilled in the relevant art will understand additional embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference characters indicate corresponding parts, and in which:

[0015] Figure 1 Describe the lithographic equipment;

[0016] Figure 2 depicting a lithographic cell or cluster in which an inspection device according to the invention can be used; and

[0017] Figure 3 schematically illustrates an inspection apparatus adapted to perform angle-resolved scatterometry and dark-field imaging inspection methods;

[0018] Figure 4 Diagram for using such Figure 3 The metrology tool illustrated in FIG. 1 images four diffraction orders (pairs of first order diffraction orders in each of two substrate plane directions) in a known arrangement;

[0019] Figure 5 Graphics are used for Figure 3 , and showing (a) a schematic illustration of a focus monitoring branch and (b) a known signal processing method for inferring focus using such a focus monitoring branch;

[0020] Figure 6 is a graph of fringe contrast versus focus illustrating the basis of a known fringe contrast based focus monitoring method;

[0021] Figure 7 is a flow chart and graph illustrating spatial frequency metrics versus focus underlying a spatial frequency based focus monitoring method according to the concepts disclosed herein; and

[0022] Figure 8 is a flow chart and graph illustrating spatial frequency metrics versus focus for novel optical wedge arrangements and methods according to embodiments. DETAILED DESCRIPTION

[0023] Before describing embodiments of the present invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0024] Figure 1A lithographic apparatus LA is schematically depicted. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation), a patterning device support or support structure (e.g. mask table) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters, two substrate tables (e.g. wafer stages) WTa and WTb each configured to hold a substrate (e.g. resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted by the patterning device MA to the radiation beam B onto a target portion C (e.g. comprising one or more dies) of the substrate W. A reference frame RF connects the various components and serves as a reference for setting and measuring the position of the patterning device and substrate and the positions of features on the patterning device and substrate.

[0025] The illumination optics system may include various types of optical components for directing, shaping, or controlling radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof.

[0026] The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions (such as whether the patterning device is held in a vacuum environment). The patterning device support can take many forms; the patterning device support can ensure that the patterning device (for example) is in a desired position relative to the projection system.

[0027] The term "patterning device" as used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to produce a pattern in a target portion of the substrate. It should be noted that, for example, if the pattern imparted to the radiation beam includes phase-shifting features or so called assist features, the pattern may not exactly correspond to the desired pattern in the target portion of the substrate. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device (such as an integrated circuit) produced in the target portion.

[0028] As depicted herein, the apparatus is of the transmissive type (e.g., employing a transmissive patterning device). Alternatively, the apparatus may be of the reflective type (e.g., using a programmable mirror array of the type mentioned above, or using a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the term "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device." The term "patterning device" may also be interpreted as referring to a device that stores pattern information in digital form for controlling such a programmable patterning device.

[0029] The term "projection system" as used herein should be broadly interpreted as covering any type of projection system suitable for the exposure radiation used or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system".

[0030] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, such as the space between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of a projection system.

[0031] In operation, the illuminator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In these cases, the source is not considered to form part of the lithographic apparatus, and the radiation beam is delivered from the source SO to the illuminator IL by means of a beam delivery system BD comprising, for example, suitable directional mirrors and / or a beam expander. In other cases, for example, when the source is a mercury lamp, the source may be an integral part of the lithographic apparatus. The source SO and the illuminator IL together with the beam delivery system BD may be referred to as a radiation system when necessary.

[0032] The illuminator IL may, for example, comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam to have a desired uniformity and intensity distribution in a cross-section of the radiation beam.

[0033] The radiation beam B is incident on a patterning device MA which is held on a patterning device support MT and is patterned by the patterning device. Having traversed the patterning device (e.g. a mask) MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor IF (e.g. an interferometer, a linear encoder, a 2D encoder or a capacitive sensor), the substrate table WTa or WTb can be accurately moved, for example in order to position a different target portion C in the path of the radiation beam B. Similarly, the first positioner PM and the further position sensor (which is not in the Figure 1 ) can be used to accurately position the patterning device (eg, reticle / mask) MA relative to the path of the radiation beam B, for example, after mechanical retrieval from a mask library or during scanning.

[0034] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the pattern forming device (e.g., reticle / mask) MA with the substrate W. Although the illustrated substrate alignment marks occupy dedicated target portions, the marks can be located in the spaces between target portions (these marks are called scribe alignment marks). Similarly, where more than one die is provided on the pattern forming device (e.g., mask) MA, the mask alignment marks can be located between the dies. Small alignment marks can also be included in the die among device features, in which case it is desirable to make the marks as small as possible and without requiring any different imaging or process conditions than adjacent features. An alignment system for detecting alignment marks is further described below.

[0035] The depicted apparatus may be used in a number of modes. In scanning mode, the pattern forming device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the pattern forming device support (e.g., mask table) MT may be determined by the (or less) magnification and image reversal characteristics of the projection system PS. In scanning mode, the maximum size of the exposure field limits the width of the target portion in a single dynamic exposure (in the non-scanning direction), while the length of the scanning movement determines the height of the target portion (in the scanning direction). Other types of lithographic apparatus and operating modes are possible, as is known in the art. For example, stepping modes are known. In so-called "maskless" lithography, the programmable pattern forming device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0036] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be used.

[0037] The lithographic apparatus LA is of the so-called dual-platform type, having two substrate tables WTa, WTb and two stations (exposure station EXP and measurement station MEA), between which the substrate table can be exchanged. While exposing one substrate on one substrate table at the exposure station, another substrate can be loaded onto another substrate table at the measurement station and various preparatory steps can be performed. This situation achieves a significant increase in the throughput of the apparatus. The preparatory steps may include using a horizontal sensor LS to map the surface height profile of the substrate, and using an alignment sensor AS to measure the position of the alignment mark on the substrate. If the position sensor IF is unable to measure the position of the substrate table when the substrate table is at the measurement station and when the substrate table is at the exposure station, a second position sensor may be provided to enable tracking of the position of the substrate table relative to the reference frame RF at the two stations. Instead of the dual-platform arrangement shown, other arrangements are known and available. For example, other lithographic apparatuses providing a substrate table and a measurement table are well known. These substrate tables and measurement tables are docked together when performing preparatory measurements, and then disconnected when the substrate table is exposed.

[0038] like Figure 2 As shown, the lithography equipment LA forms part of the lithography unit LC (sometimes also called a lithography cell or lithography cluster), which also includes equipment for performing pre-exposure processes and post-exposure processes on the substrate. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a baking plate BK. The substrate conveyor or robot RO picks up the substrate from the input / output ports I / O1, I / O2, moves the substrate between different process equipment, and then transfers the substrate to the feed station LB of the lithography equipment. These devices, which are often collectively referred to as coating and developing systems, are under the control of the coating and developing system control unit TCU, which itself is controlled by the management control system SCS, which also controls the lithography equipment via the lithography control unit LACU. Therefore, different equipment can be operated to maximize production volume and processing efficiency.

[0039] In order to correctly and consistently expose a substrate exposed by the lithography apparatus LA, it is desirable to inspect the exposed substrate to measure properties or parameters of interest, such as overlap between subsequent layers, line thickness, critical dimension (CD), etc. Therefore, the manufacturing facility in which the lithography unit LC is located also includes a measurement system MET, which receives some or all of the substrates W that have been processed in the lithography unit. The measurement results are provided directly or indirectly to the management control system SCS. If an error is detected, the exposure of subsequent substrates can be adjusted, especially if the inspection can be performed quickly and quickly enough that other substrates of the same batch are still to be exposed. In addition, substrates that have already been exposed can be stripped and reworked to improve yield or discarded, thereby avoiding further processing of substrates that are known to be defective. In the case where only some target portions of the substrate are defective, further exposure can be performed only on those target portions that are good.

[0040] In the metrology system MET, an inspection device is used to determine the properties of the substrate, and in particular to determine how the properties of different substrates or different layers of the same substrate vary between different layers. The inspection device can be integrated into the lithographic apparatus LA or the lithography unit LC, or can be a separate device. In order to achieve the fastest measurement, it is desirable to have the inspection device measure the properties in the exposed resist layer immediately after exposure. However, the latent image in the resist has a very low contrast (there is only a very small refractive index difference between the part of the resist that has been exposed to the radiation and the part of the resist that has not been exposed to the radiation) and not all inspection devices have sufficient sensitivity to perform useful measurements of the latent image. Therefore, the measurement can be performed after the post-exposure baking step (PEB), which is usually the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed parts of the resist. At such a stage, the image in the resist can be called a semi-latent image. Measurements can also be made on a developed resist image (at which point the exposed or unexposed portions of the resist have been removed) or on a developed resist image after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking a defective substrate, but can still provide useful information.

[0041] exist Figure 3 A suitable metrology device for use in embodiments of the present invention is shown in (a). It should be noted that this metrology device is only one example of a suitable metrology device. Alternative suitable metrology devices may use EUV radiation, such as disclosed in WO2017 / 186483A1. Many other types of metrology devices are known and may also be used to implement the concepts disclosed herein. Figure 3(b) of FIG. 1 illustrates in more detail the target structure T and the diffracted rays of the measuring radiation used to illuminate the target structure. The illustrated metrology apparatus is of a type known as a dark field metrology apparatus. The metrology apparatus may be a separate device or incorporated in a lithography apparatus LA or a lithography unit LC at, for example, a measurement station. An optical axis having several branches running through the apparatus is represented by a dash-dotted line O. In such an apparatus, light emitted by a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in a double sequence of a 4F arrangement. Different lens arrangements may be used, provided that the lens arrangement still provides an image of the substrate onto the detector and simultaneously allows access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range of radiation incident on the substrate may be selected by defining the spatial intensity distribution in a plane presenting the spatial spectrum of the substrate plane (here referred to as the (conjugate) pupil plane). In particular, this selection can be made by inserting an aperture plate 13 of suitable form between lenses 12 and 14 in the plane of the back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms (labeled 13N and 13S), allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis from a direction designated as "North" for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but provides illumination from the opposite direction labeled "South". Other illumination modes are possible by using different holes. A specific alternative aperture plate 13Q includes a four-part illumination aperture, wherein illumination is received via two diagonally opposed quadrants, and the other two quadrants are blocked (the illuminated and blocked quadrants can be swapped with those shown). Such an arrangement can be used for simultaneous imaging of the +1 and -1 diffraction orders and is further described in the aforementioned US2010201963A1.The remainder of the pupil plane is desirably dark, since any unwanted light outside the desired illumination pattern will interfere with the desired measurement signal.

[0042] As in Figure 3As shown in (b) of FIG. 1 , the target T is placed in such a way that the substrate W is perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A measurement radiation ray I impinging on the target structure T from an angle offset from the axis O produces a zero-order ray (solid line 0) and two order rays (dash-dotted line +1 and double dash-dotted line -1) referred to hereinafter as a pair of complementary diffraction orders. It should be noted that the pair of complementary diffraction orders may be any higher order pair; for example, a +2, -2 pair, etc., and is not limited to a first order complementary pair. It should be remembered that in the case of an overfilled small target structure, these rays are only one of many parallel rays covering the area of ​​the substrate including the measurement target structure T and other features. Because the hole in the plate 13 has a finite width (necessary to receive a useful amount of light), the incident ray I will actually occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. According to the point spread function of the small target, each order +1 and -1 will be further spread out over the angular range, rather than a single ideal ray as shown. It should be noted that the grating pitch and illumination angle of the target structure can be designed or adjusted so that the first-order rays entering the objective lens are closely aligned with the central optical axis. Figure 3 (a) and Figure 3 The rays illustrated in (b) are shown slightly off-axis simply to enable the rays to be more easily distinguished in the figure.

[0043] At least the 0th and +1st orders diffracted by the target structure T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Figure 3 (a) illustrates both the first illumination mode and the second illumination mode by indicating diametrically opposite apertures labeled as north (N) and south (S). When the incident ray I of the measurement radiation is from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray labeled as +1 (N) enters the objective lens 16. On the contrary, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (labeled as 1 (S)) is the diffracted ray that enters the lens 16.

[0044] The second beam splitter 17 divides the diffraction beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order diffraction beam and the first-order diffraction beam to form a diffraction spectrum (pupil plane image) of the target structure on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast several orders. The pupil plane image captured by the sensor 19 can be used to focus the measurement device and / or for normalizing the intensity measurement results of the first-order beam. The pupil plane image can also be used for various measurement purposes such as reconstruction.

[0045] In the second measurement branch, the optical system 20, 22 forms an image of the target structure T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21a is arranged in a plane 21 conjugate to the pupil plane. The aperture stop 21a serves to block the zeroth order diffraction beam so that the image of the target formed on the sensor 23 is formed by only -1 or +1 order beams. Instead of a simple aperture stop 21a, the metrology tool instead uses a wedge arrangement 21b that separates the first and zeroth orders so that the first and zeroth orders can be imaged separately (separated) on the detector 23. Such an arrangement is described in the aforementioned US2011102753A1 and can be used (e.g., in combination with a quartered illumination aperture 13Q) to simultaneously acquire the +1 and -1 diffraction orders in one or two directions in the substrate plane.

[0046] The images captured by sensors 19 and 23 are output to a processor PU which processes the image, the function of which will depend on the specific type of measurement being performed. It should be noted that the term "image" is used in a broad sense here. Thus, if only one of the -1 order and the +1 order is present, no image of the raster lines will be formed.

[0047] Position errors may occur due to overlay errors (often referred to as "overlay"). Overlay is the error in the placement of a first feature during a first exposure relative to the placement of a second feature during a second exposure. Lithographic equipment minimizes overlay by accurately aligning each substrate to a reference prior to patterning, for example by measuring the position of alignment marks on the substrate using alignment sensors; and using feedback corrections to the exposure process; for example based on the use of sensors such as in Figure 3 A suitable metrology tool is illustrated in (a) to measure the overlay on a metrology target after exposure.

[0048] You can use, for example, Figure 3 One known metrology method performed by the metrology tool illustrated in (a) is known as diffraction-based overlay (DBO) or micro-diffraction-based overlay (μDBO). Such μDBO techniques use the imaging branch of the metrology tool (the branch through the detector 23) and are based on a pair of complementary pairs of diffraction orders (usually a pair of complementary first-order diffraction orders, i.e., the first diffraction order may include the +1 order and the second diffraction order may include the -1 order, as in Figure 3The intensity asymmetry or intensity difference between the first and second diffraction orders in (b) of FIG. 4 determines the asymmetry of the structure. Therefore, in this context, the terms "first" and "second" do not refer to the diffraction order and are simply used to distinguish a pair of complementary two diffraction orders, note that the first and second diffraction orders can be +2 and -2 diffraction orders, or a pair of complementary higher diffraction orders. The zeroth order (mirror radiation) is usually blocked or shunted elsewhere (e.g., to another part of the detector for monitoring purposes); the zeroth order is not used in μDBO measurements. The main "image" used for the inference of the parameter of interest is formed only by the higher (e.g., first) diffraction orders. The asymmetry of the structure can be used to infer the parameters of interest, such as overlap or focus, depending on the target design.

[0049] Overlap is only one parameter of interest that can be measured using such a technique. Another parameter of interest may be the exposure focus. Such exposure focus may be measured using diffraction-based focusing (DBF) or micro-diffraction-based focusing (μDBF) metrology. The measurement principles of μDBO and μDBF are essentially the same (target asymmetry is measured via intensity asymmetry in complementary diffraction orders). The main difference in the two techniques is the target: the μDBO target has an asymmetry that depends on overlap, while the μDBF target has an asymmetry based on the exposure focus. It should be noted that the exposure focus and DBF / μDBF measurements in this context are different from the focusing of the focus monitoring method disclosed in this article. When the target (and other structures) are exposed on the wafer, DBF / μDBF infers the actual focus of the exposure radiation on the substrate (scanner focus), i.e., the scanner focus, from the exposed target. The concepts disclosed in this article relate to the focus of the metrology tool, or the focus of the measurement radiation on the wafer during measurement. All references to μDBO or DBO below should be understood as measuring any parameter of interest using any diffraction-based imaging metrology method based on imaging a single diffraction order, including DBF / μDBF.

[0050] Figure 4The illustration shows the use of a known arrangement of a wedge arrangement (e.g., the wedge arrangement 21b described above) to image two diffraction orders of a +1 / -1 complementary pair of diffraction orders and the zeroth order simultaneously onto multiple zones of a detector (or different detectors). The detection pupil plane 400 (or a pupil plane conjugated to the target detection pupil plane) can be divided into four wedge elements or quadrants (e.g., by using a quartered illumination aperture in the illumination branch, such as the illumination aperture 13Q described above), where two quadrants 405 correspond to the zeroth order 0, and two quadrants 410 correspond to the desired diffraction orders +1X, -1X, -1Y, +1Y received from an X-oriented target and a Y-oriented target, respectively (where X and Y are two perpendicular directions to the substrate plane). The wedge arrangement 415 may be located in the (conjugate) detection pupil plane and comprises four (similar) quadrants Q1, Q2, Q3, Q4, each quadrant comprising a corresponding wedge oriented to separate the angle of the zero order from the desired diffraction order, such that the zero order and the desired diffraction order are located at four different positions in the μDB0 image plane. Each wedge is shown in a cross section at the bottom of the image, similarly oriented. The resulting image 420 comprises a +1 image 425, a -1 image 430 and two (similar) zero order images 435. It should be noted that by including only half of the first order diffracted radiation in each of the +1 and -1 images 425, 430, the "images" referred to herein are not conventional dark field microscopy images. Individual grating lines will not be resolved. Each grating will simply be represented by an area of ​​a certain intensity level, and the difference between the intensity levels of the images 425 and 430 (per direction) can be used to infer target asymmetry and hence the parameters of interest.

[0051] Figure 5 The diagram shows a known focus monitoring arrangement, which can be used in Figure 3 The focus monitoring arrangement is used on the measurement device shown in (a) of FIG. Such a focus monitoring arrangement may include a focus branch 500, such as Figure 5 As shown in (a), the focusing branch 500 uses the confocal focusing principle. Figure 5 The hardware shown in (a) of FIG. 5 is highly simplified schematically, showing a wafer plane WP that can be moved as indicated by the arrows for focusing. Scattered radiation 505 from the wafer can be collected by an objective arrangement 510 (e.g., via a beam splitter not shown), and the scattered radiation 505 is split using a beam splitter 515. A first split portion 520 of the scattered radiation is focused at a first focal plane FP1 in front of a first detector 525 and a second split portion 530 of the scattered radiation is focused at a second focal plane FP2 behind a second detector 535. A corresponding pinhole or aperture 540, 545 is positioned in front of each detector 525, 535.

[0052] Figure 5 (b) illustrates how focus can be inferred from the detection signals captured by the first detector 525 and the second detector 535. The figure shows a graph of signal strength (e.g., amplitude or intensity) SS versus defocus f (centered at zero defocus). The dashed line includes a first signal 550 from the first detector 525 and a second signal 555 from the second detector 535. The difference signal 560 between these signals 550, 555 has a linear dependence on defocus near the zero defocus condition. Such a signal can be determined and used to detect the focus of the illumination on the wafer with good sensitivity.

[0053] The problem with such a focusing arrangement is that it requires an additional hardware branch which is significantly more complex than the illustrated case. It is desirable to reduce the cost, volume and complexity of such a focusing branch. In addition, current implementations of such a focusing branch use only two wavelengths to perform focusing, which cannot accurately represent all wavelengths for measurements taking into account chromatic aberrations. In addition, these two wavelengths cannot be used in the imaging branch for μDBO measurements.

[0054] An alternative known method for focus monitoring in metrology tools may use fringe contrast. Normally, in μDBO imaging, there are no fringes, since only the zeroth or first order diffraction orders are allowed in any quadrant. When diffraction orders higher than the first order are present in one quadrant, for example by using a low wavelength / target pitch ratio, the grating lines of the target will be imaged as visible fringes in that quadrant. The contrast of the line fringes is dependent on the amount of defocus, i.e., the highest fringe contrast is achieved at optimal focus.

[0055] Figure 6 is a graph of the first order fringe contrast FRCO versus the amount of defocus f showing this dependence. This dependence allows focus detection / calibration; however, it cannot be used in real time during μDBO measurements, since μDBO images should not include diffraction orders higher than the first order. Furthermore, it can be seen from the graph that the lowest sensitivity is at close to optimal focus conditions, and this method can only measure the amount of defocus, not the direction of defocus. Furthermore, large targets are required for fringe contrast analysis, which is not desirable for μDBO.

[0056] To address these issues, a method and a metrology arrangement are proposed that use a differential detection method based on a focus-dependent metric. For example, the focus-dependent metric may include spatial frequency or fringe contrast. This method may use a new design of a wedge arrangement in the detection pupil plane or in its conjugate plane. This concept enables real-time focus detection in the imaging branch (e.g. when performing μDBO / μDBF metrology or similar dark field metrology techniques) instead of using a separate focus monitoring branch.

[0057] The defocus effect blurs the image and thus reduces the higher spatial frequency components of the image. This is also true for μDBO / μDBF images or similar images. Therefore, by determining the spatial frequency from such an image, the focus can be determined.

[0058] Figure 7 The basic principles underlying (at least some of) the concepts disclosed herein are illustrated. Images 700a, 700b, 700c are shown, each corresponding to a different measured focus value +1, 0, -1 (in arbitrary units), where 0 corresponds to optimal focus. Each of these images (or a region thereof) can be Fourier transformed (e.g., a 2D transform, such as a 2D Fast Fourier Transform FFT) to obtain a corresponding spatial frequency 710a, 710b, 710c. In the example shown, the transformed image region can include one zero-order image or two zero-order images in the zero-order image, or a region that can include one zero-order image or two zero-order images in the zero-order image. This enables focus monitoring of detected radiation that is not used in parameter inference. A graphical representation 730 of the spatial frequency indicator SF relative to the focus value f is shown. The spatial frequency index may include, for example, the intensity or amplitude of the corresponding frequency components 720a, 720b, 720c from each spatial frequency representation 710a, 710b, 710c, or a combination of the corresponding frequency components from each spatial frequency representation 710a, 710b, 710c. The specific frequency components shown here are arbitrary, and specifically here are k corresponding to the X direction in pupil space. x Directional component. It can be seen that the spatial frequency indicator SF has a (non-linear) dependence on the defocus amount f. It is proposed to use this dependence for measuring the metrological focus.

[0059] The spatial frequency index used in the methods described herein may relate to a single frequency component, a single frequency component per direction (e.g., in k x and k yThe spatial frequency metric may be used to determine the spatial frequency of the image 710a, 710b, 710c. The spatial frequency metric may be used to determine the spatial frequency of the image 710a, 710b, 710c. The spatial frequency metric may be used to determine the spatial frequency of the image 710a, 710b, 710c. The spatial frequency metric may be used to determine the spatial frequency of the image 710a, 710b, 710c.

[0060] It is possible that, in certain use cases, some spatial frequencies are more sensitive to defocus than others. Therefore, the proposed method may include an optional spatial frequency selection step to optimize the defocus control performance.

[0061] Specifically, the spatial frequency indicator used in the method described herein may include a difference spatial frequency indicator, which includes the difference between a first spatial frequency indicator from a first zero-order image and a second spatial frequency indicator from a second zero-order image, the first zero-order image and the second zero-order image (for example, in a detection pupil plane or a conjugate plane thereof) being subjected to different focusing.

[0062] A practical implementation of this principle may use a novel wedge arrangement to obtain a substantially linear or at least monotonic dependence of a (difference) spatial frequency indicator on focus within a focus range of interest (e.g., a focus range about optimal focus and including an expected focus error range for a metrology tool). The proposed wedge arrangement may be similar to the wedge arrangement 415 described above, but wherein at least one of the wedge elements (e.g., quadrants) corresponds to a zero-order region in a detection pupil plane having a surface curvature or curved portion to induce a defocused wavefront associated with the zero-order corresponding thereto. For example, the curvature or curved portion may be on a surface opposite to an inclined planar surface for shifting the zero-order in the pupil plane so as to separate an image of the zero-order from a first-order image and a complementary zero-order image in the image plane. This enables determination of a differential signal between spatial frequency indicators derived from two zero-order images, respectively, which differential signal has a monotonic and / or substantially linear relationship to focus.

[0063] Thus, the optical wedge arrangement may include a plurality of optical wedge elements arranged around an optical axis, the plurality of optical wedge elements including at least a first optical wedge element, a second optical wedge element, and a third optical wedge element; wherein: the first optical wedge element includes a first optical surface and a second optical surface, wherein the first optical surface is planar and not perpendicular to the optical axis, and the second optical surface is non-planar, so that the first optical wedge element has a non-linear thickness change; the second optical wedge element includes a third optical surface and a fourth optical surface, both of which are planar and not parallel; and the third optical wedge element includes a fifth optical surface and a sixth optical surface, wherein the fifth optical surface is planar and not perpendicular to the optical axis. The optical wedge arrangement may also include a fourth optical wedge element, the fourth optical wedge element including a seventh optical surface and an eighth optical surface, both of which are planar and not parallel.

[0064] In a convenient arrangement, the first wedge element may correspond to a first zeroth order pupil zone, the second wedge element may correspond to a first diffracted radiation pupil zone, the third wedge element may correspond to a second zeroth order pupil zone, and the fourth wedge element may correspond to a second diffracted radiation pupil zone. The terms "first diffracted radiation pupil zone" and "second diffracted radiation pupil zone" are so referred to in order to distinguish the pupil zones from one another, and the terms "first" and "second" do not refer to diffraction order numbers. As will be appreciated, the first diffracted radiation pupil zone and the second diffracted radiation pupil zone will typically correspond to corresponding diffraction orders of a pair of complementary diffraction orders, for example, such that the first diffracted radiation pupil zone corresponds to the +1 diffraction order and the second diffracted radiation pupil zone corresponds to the -1 diffraction order.

[0065] In a convenient arrangement, the first optical wedge element and the third optical wedge element may each include a curved surface having opposite curvatures so as to impart defocus in opposite directions. In such an arrangement, optionally, the curvatures (and therefore the defocus) imparted by the first optical wedge element and the third optical wedge element may be equal in magnitude.

[0066] Each of the first and third optical wedge elements may direct zero-order radiation to different regions of an image plane (detector plane) to obtain a first and second zero-order image. A corresponding first and second spatial frequency index for each of the first and second zero-order images may be determined and the difference between these spatial frequency indexes is used to determine focus.

[0067] Figure 8 is a schematic diagram illustrating this method and arrangement. The top portion of the figure corresponds to Figure 4, and similar elements will not be described again herein. The main difference is the wedge arrangement 815 for which the surface curvatures of the Q1' and Q3' quadrants (wedge elements) each have curved surfaces with opposite curves, e.g., quadrant Q1' has a convex surface that induces a negative defocused wavefront, and quadrant Q3' has a concave surface that induces a positive defocused wavefront. The magnitude of the applied defocus is the same for both quadrants Q1' and Q3'.

[0068] The image 420 will now include a first zero-order image 835a and a second zero-order image 835b, each corresponding to a different applied defocus amount. Each of these images can be Fourier transformed (e.g., 2D FFT) to obtain a corresponding transformed image or spatial frequency representation 840a, 840b. From each of these spatial frequency representations 840a, 840b, a corresponding spatial frequency indicator can be determined (e.g., as described, from one or more spatial frequency components). The spatial frequency indicator used for focus inference can then include the difference between these spatial indicators.

[0069] Focus inference can be achieved by referring to a previously calibrated relationship 845 that describes the change in spatial frequency index with focus. This relationship can be obtained from the difference between the two calibration relationships 850a, 850b, thereby describing the change in spatial frequency index of each defocused zero-order image with focus. Note that the corresponding peaks in the calibration relationships 850a, 850b are shifted in opposite directions with respect to optimal focus.

[0070] The methods described herein may include an initial calibration phase for determining a calibrated relationship 845. The calibration may include obtaining a set of first zero-order images and a set of second zero-order images, each set of images being imaged at a corresponding different focus setting (e.g., by focusing) during measurement of the target; determining, for each set of images, a relationship of the spatial frequency index by focusing (e.g., via Fourier transform); and determining the calibrated relationship 845 as the difference between the two relationships. The calibration may be performed for each application, e.g., for each stack and / or target type, and the appropriate calibration relationship is used to infer the inferred result.

[0071] The differential zero-order wedge arrangement disclosed herein may be used in an alternative focus inference method based on fringe contrast measurements. Such a method may include generating fringes in a zero-order pupil region. This may be achieved by capturing diffraction orders greater than the first diffraction order in a zero-order quadrant, for example by using a small measurement wavelength to target pitch ratio. Corresponding first and second fringe contrast indicators (e.g., measures of fringe contrast) may be determined from the resulting first and second zero-order images (as imaged on a detector). A differential fringe contrast indicator comprising the difference between the first and second fringe contrast indicators may be determined and used to infer focus based on a calibrated relationship between the fringe contrast indicator (differential fringe contrast indicator) and focus. This relationship may be determined in a calibration phase similar to the calibration phase previously described, but for fringe contrast rather than spatial frequency. This calibrated relationship will exhibit a monotonic and / or linear dependence on focus within the focus range of interest and provide much higher focus sensitivity than the known fringe contrast methods previously described. However, such an embodiment cannot be used during μDBO or μDBF measurements, since additional diffraction orders (≥ 2nd order) will also be present in the 1st order tetrad.

[0072] In all embodiments, the measured focus values ​​may be used by the metrology device to control the focus (eg, move the wafer relative to the measurement beam) in order to minimize the amount of defocus. All determination steps, processing steps and mathematical steps may be performed by a processor of the metrology device.

[0073] The concepts disclosed herein allow for the detection of focus in a measurement tool and, more in detail, for the detection of focus in an imaging branch of a measurement tool that detects a focused image of a target on a detector. This situation means that no separate focus monitoring branch is required, thereby reducing cost, volume and system complexity. In addition, the proposed method enables the detection of focus at all used wavelengths (i.e., all wavelengths of the measuring radiation), which is an improvement over current implementations of the focus monitoring branch that use only two wavelengths for focus monitoring. Furthermore, frequency differential detection can be used in real time during measurements (e.g., μDBO or μDBF measurements), there is no requirement to have imaging fringes in the uDBO image, and therefore frequency differential detection can be used during uDBO detection, and there are no requirements for wavelength / pitch ratio or target size.

[0074] The advantage of the fringe contrast differential detection method over current fringe based contrast methods is that the differential detection provides a linear regime around the zero focus value and therefore provides a much higher focus detection sensitivity and provides the direction of out-of-focus. The method has been described in terms of measuring structures or targets on a substrate. However, the method can be applied to measurements on any sample and can, for example, include measuring focus during measurements on flat areas (e.g. thin film samples).

[0075] Further embodiments are disclosed in the subsequent lists of numbered aspects:

[0076] 1. An optical wedge arrangement, comprising a plurality of optical wedge elements arranged around an optical axis, the plurality of optical wedge elements comprising at least a first optical wedge element, a second optical wedge element and a third optical wedge element; wherein:

[0077] The first optical wedge element includes a first optical surface and a second optical surface, wherein the first optical surface is planar and non-perpendicular to the optical axis, and the second optical surface is non-planar, so that the first optical wedge element has a non-linear thickness variation;

[0078] The second optical wedge element includes a third optical surface and a fourth optical surface, the third optical surface and the fourth optical surface are both planar and non-parallel; and

[0079] The third optical wedge element includes a fifth optical surface and a sixth optical surface, wherein the fifth optical surface is planar and non-perpendicular to the optical axis.

[0080] 2. The optical wedge arrangement of clause 1, wherein the sixth optical surface is non-planar such that the third optical wedge element has a non-linear thickness variation.

[0081] 3. The optical wedge arrangement of clause 2, wherein the second optical surface is convex and the sixth optical surface is concave.

[0082] 4. An optical wedge arrangement according to clause 3, wherein the magnitude of the defocus applied by the first optical wedge element is substantially the same as the magnitude of the defocus applied by the second optical wedge element.

[0083] 5. An optical wedge arrangement according to any preceding aspect, wherein the plurality of optical wedge elements are arranged in substantially the same plane around the optical axis.

[0084] 6. The optical wedge arrangement according to any preceding aspect, further comprising a fourth optical wedge element, the fourth optical wedge element comprising seventh and eighth optical surfaces which are both planar and non-parallel.

[0085] 7. The optical wedge arrangement of clause 6, wherein each optical wedge element comprises a quadrant configuration, and the plurality of optical wedge elements are arranged in a circular configuration in the optical wedge arrangement.

[0086] 8. A metrology device comprising a detection branch comprising an optical wedge arrangement according to any preceding aspect and at least one detector.

[0087] 9. A metrology device according to aspect 8, wherein the at least one detector is operable to: detect at least a first zeroth order image from zeroth order radiation of the scattered radiation shifted by the first optical wedge element, and to detect a second zeroth order image from zeroth order radiation of the scattered radiation shifted by the third optical wedge element.

[0088] 10. The measurement device according to aspect 9, wherein the measurement device further comprises a processor, wherein the processor is operable to:

[0089] determining a first indicator that depends on focus from the first zero-order image and determining a second indicator that depends on focus from the second zero-order image;

[0090] determining a focus-dependent difference indicator from a difference between the first focus-dependent indicator and the second focus-dependent indicator; and

[0091] A focus value of the measurement radiation on the sample being measured is determined from the focus-dependent difference indicator.

[0092] 11. A measurement device according to aspect 10, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a spatial frequency indicator related to the spatial frequency of the first zero-order image and the second zero-order image.

[0093] 12. The metrology apparatus of clause 11, wherein the processor is operable to Fourier transform each of the first and second zero-order images and determine the first and second focus-dependent indicators.

[0094] 13. A measurement device according to aspect 10, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a fringe contrast indicator related to the fringe contrast of the first zero-order image and the second zero-order image.

[0095] 14. A metrology apparatus according to any of clauses 10 to 13, wherein the processor is operable to determine the focus value by reference to a calibrated relationship between the focus dependent difference indicator and focus.

[0096] 15. A metrology apparatus according to any of clauses 10 to 14, wherein the at least one detector is operable to: detect non-zero order diffracted radiation on the detector; and

[0097] A parameter of interest is determined from the non-zero order diffracted radiation.

[0098] 16. The metrology apparatus of clause 15, comprising determining the parameter of interest and the focus value from a single image acquisition on the at least one detector.

[0099] 17. A measuring device according to aspect 15 or 16, wherein the first zero-order image, the second zero-order image, a first diffraction order image of a first diffraction order in a pair of complementary diffraction orders, and a second diffraction order image of a second diffraction order in the pair of complementary diffraction orders are detected on independent areas on the at least one detector.

[0100] 18. The measurement device according to clause 17, wherein the first diffraction order is a +1 diffraction order and the second diffraction order is a -1 diffraction order.

[0101] 19. A metrology device according to any of clauses 10 to 18, the metrology device being operable to use the focus value to control focus and minimise the amount of defocus of the measurement radiation on the sample.

[0102] 20. A metrology device according to any one of clauses 8 to 19, wherein the wedge arrangement is comprised in a pupil plane of the detection branch or a conjugate plane thereof.

[0103] 21. A measuring device, comprising:

[0104] processor;

[0105] an illumination branch for illuminating the sample with measurement radiation; and

[0106] a detection branch for detecting scattered radiation, said scattered radiation comprising said measurement radiation which has been scattered by said sample, said detection branch comprising at least one detector for detecting said scattered radiation;

[0107] Wherein the detection branch further comprises an optical wedge arrangement, the optical wedge arrangement being operable to:

[0108] separating at least non-zero order diffracted radiation within the scattered radiation from zero order radiation within the scattered radiation such that the non-zero order diffracted radiation and the zero order scattered radiation are imaged onto different regions of the at least one detector,

[0109] separating a first portion of the zero-order radiation from a second portion of the zero-order radiation such that the first portion of the zero-order radiation and the second portion of the zero-order radiation are imaged onto different regions of the at least one detector to form a first zero-order image and a second zero-order image, respectively; and

[0110] applying defocusing to at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation;

[0111] Wherein the processor is operable to determine a focus value of the measurement radiation on the sample from the first and second zero order images.

[0112] 22. A metrology apparatus according to clause 21, wherein the processor is operable to: determine a first indicator dependent on focus from the first zero-order image, and determine a second indicator dependent on focus from the second zero-order image;

[0113] determining a focus-dependent difference indicator from a difference between the first focus-dependent indicator and the second focus-dependent indicator; and

[0114] The focus value of the measurement radiation on the sample is determined from the focus-dependent difference indicator.

[0115] 23. A measurement device according to aspect 22, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a spatial frequency indicator related to the spatial frequency of the first zero-order image and the second zero-order image.

[0116] 24. The metrology apparatus of clause 23, wherein the processor is operable to Fourier transform each of the first zero-order image and the second zero-order image and determine the first focus-dependent indicator and the second focus-dependent indicator.

[0117] 25. A measurement device according to aspect 22, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a fringe contrast indicator related to the fringe contrast of the first zero-order image and the second zero-order image.

[0118] 26. A metrology apparatus according to any of clauses 22 to 25, wherein the processor is operable to determine the focus value by reference to a calibrated relationship between the focus dependent difference indicator and focus.

[0119] 27. A measurement device according to any one of aspects 21 to 26, wherein the optical wedge arrangement includes a plurality of optical wedge elements arranged around the optical axis of the detection branch, the plurality of optical wedge elements including at least a first optical wedge element arranged to divert the first portion of the zero-order radiation to a first area on the at least one detector, a second optical wedge element arranged to divert at least a portion of the non-zero-order diffracted radiation to a second area on the at least one detector, and a third optical wedge element arranged to divert the second portion of the zero-order radiation to a third area on the at least one detector.

[0120] 28. The measurement device according to aspect 26, wherein:

[0121] The first optical wedge element comprises a first optical surface and a second optical surface, wherein the first optical surface is planar and non-perpendicular to the optical axis and the second optical surface is non-planar, such that the first optical wedge element has a non-linear thickness variation to impart a defocus to the first portion of the zero-order radiation;

[0122] The second optical wedge element includes a third optical surface and a fourth optical surface, the third optical surface and the fourth optical surface are both planar and non-parallel; and

[0123] The third optical wedge element includes a fifth optical surface and a sixth optical surface, wherein the fifth optical surface is planar and non-perpendicular to the optical axis.

[0124] 29. The metrology apparatus of clause 28, wherein the sixth optical surface is non-planar such that the third optical wedge element has a non-linear thickness variation in order to impart a defocus to the second portion of the zero-order radiation.

[0125] 30. The metrology device of clause 29, wherein the second optical surface is convex and the sixth optical surface is concave.

[0126] 31. The metrology apparatus of clause 30, wherein the magnitude of the defocus applied by the first optical wedge element and the magnitude of the defocus applied by the second optical wedge element are substantially the same.

[0127] 32. The measurement device according to any one of aspects 27 to 31, wherein the plurality of optical wedge elements are arranged approximately in a common pupil plane of the detection branches or a conjugate plane thereof around the optical axis.

[0128] 33. The metrology device according to any one of clauses 27 to 32, wherein the optical wedge arrangement further comprises a fourth optical wedge element, the fourth optical wedge element comprising a seventh optical surface and an eighth optical surface which are both planar and non-parallel.

[0129] 34. The metrology apparatus of clause 33, wherein each optical wedge element comprises a quadrant configuration, and the plurality of optical wedge elements are arranged in a circular configuration in the optical wedge arrangement.

[0130] 35. A measurement device according to aspect 33 or 34, wherein the second optical wedge element is arranged to steer a first diffraction order of a pair of complementary diffraction orders to the second region on the at least one detector, and the fourth optical wedge element is arranged to steer a second diffraction order of the pair of complementary diffraction orders to a fourth region on the at least one detector.

[0131] 36. The metrology apparatus of clause 35, wherein the first diffraction order is a +1 diffraction order and the second diffraction order is a -1 diffraction order.

[0132] 37. A metrology apparatus according to clause 35 or 36, wherein the processor is operable to determine a parameter of interest from the detected first diffraction order and the detected second diffraction order.

[0133] 38. A metrology device according to clause 37, wherein the processor is operable to determine the parameter of interest and the focus value from a single image acquisition on the at least one detector.

[0134] 39. A metrology device according to any of clauses 21 to 38, the metrology device being operable to use the focus value to control focus and minimise the amount of defocus of the measurement radiation on the sample.

[0135] 40. A method of determining a focus value of measurement radiation on a sample during measurement of the sample, the method comprising:

[0136] separating scattered radiation from the sample that has been scattered from the sample after irradiation of the sample with the measurement radiation into at least non-zero order diffracted radiation, a first portion of zero order radiation and a second portion of zero order radiation;

[0137] detecting at least one image of said non-zero order diffracted radiation, a first zeroth order image from said first portion of zeroth order radiation, and a second zeroth order image of said second portion of zeroth order radiation at separate regions of a detection plane;

[0138] applying defocusing to at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation; and

[0139] The focus value of the measurement radiation on the sample is determined from the first and second zero-order images.

[0140] 41. A method according to clause 40, wherein the step of determining the focus value comprises:

[0141] determining a first indicator that depends on focus from the first zero-order image and determining a second indicator that depends on focus from the second zero-order image;

[0142] determining a focus-dependent difference indicator from a difference between the first focus-dependent indicator and the second focus-dependent indicator; and

[0143] A focus value of the measurement radiation on the sample is determined from the focus-dependent difference indicator.

[0144] 42. A method according to aspect 41, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a spatial frequency indicator related to the spatial frequency of the first zero-order image and the second zero-order image.

[0145] 43. A method according to clause 42, comprising Fourier transforming each of the first zero order image and the second zero order image and determining the first focus dependent indicator and the second focus dependent indicator.

[0146] 44. A method according to aspect 41, wherein each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator includes a fringe contrast indicator related to the fringe contrast of the first zero-order image and the second zero-order image.

[0147] 45. A method according to any of clauses 41 to 44, comprising determining the focus value by reference to a calibrated relationship between the focus dependent difference indicator and focus.

[0148] 46. ​​A method according to any one of clauses 40 to 45, comprising determining a parameter of interest from the non-zero order diffracted radiation.

[0149] 47. A method according to clause 46, comprising determining the parameter of interest and the focus value from a single image acquisition.

[0150] 48. The method according to any one of aspects 41 to 47 comprises: separating the non-zero order diffraction radiation into a first diffraction order of a pair of complementary diffraction orders and a second diffraction order of the pair of complementary diffraction orders; and detecting the first diffraction order and the second diffraction order on independent areas on the at least one detector.

[0151] 49. The method of clause 48, wherein the first diffraction order is a +1 diffraction order and the second diffraction order is a -1 diffraction order.

[0152] 50. A method according to any of clauses 40 to 49, comprising using the focus value to control focus and minimise the amount of defocus of the measurement radiation on the sample.

[0153] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm to 20 nm); and particle beams (such as ion beams or electron beams).

[0154] The term "lens" may refer, where the context permits, to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0155] The term target should not be interpreted to mean a dedicated target formed only for the specific purpose of metrology. The term target should be understood to encompass other structures, including product structures, having properties suitable for metrology applications.

[0156] The foregoing description of specific embodiments will therefore fully reveal the general nature of the invention, so that others can easily modify and / or adapt these specific embodiments for various applications by applying the knowledge understood by those skilled in the art without departing from the general concept of the invention, without undue experimentation. Therefore, based on the teachings and guidance presented herein, it is hoped that these adaptations and modifications are within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the words or terms herein are for description by example and are non-restrictive, so that the terms or words of this specification will be interpreted by those skilled in the art in view of the teachings and the guidance.

[0157] The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. An optical wedge arrangement, comprising a plurality of optical wedge elements arranged around an optical axis, the plurality of optical wedge elements comprising at least a first optical wedge element, a second optical wedge element and a third optical wedge element; wherein: The first optical wedge element includes a first optical surface and a second optical surface, wherein the first optical surface is planar and non-perpendicular to the optical axis, and the second optical surface is non-planar, so that the first optical wedge element has a non-linear thickness variation; The second optical wedge element includes a third optical surface and a fourth optical surface, the third optical surface and the fourth optical surface are both planar and non-parallel; and The third optical wedge element includes a fifth optical surface and a sixth optical surface, wherein the fifth optical surface is planar and non-perpendicular to the optical axis.

2. The optical wedge arrangement according to claim 1, wherein: The sixth optical surface is non-planar such that the third optical wedge element has a non-linear thickness variation.

3. The optical wedge arrangement according to claim 2, wherein: The second optical surface is convex and the sixth optical surface is concave.

4. The optical wedge arrangement according to claim 3, wherein: The magnitude of the defocus applied by the first optical wedge element is substantially the same as the magnitude of the defocus applied by the second optical wedge element.

5. An optical wedge arrangement according to any preceding claim, wherein: The plurality of optical wedge elements are arranged in substantially the same plane around the optical axis.

6. An optical wedge arrangement according to any preceding claim, further comprising a fourth optical wedge element comprising seventh and eighth optical surfaces which are both planar and non-parallel.

7. The optical wedge arrangement of claim 6, wherein: Each optical wedge element comprises a quadrant configuration, and the plurality of optical wedge elements are arranged in a circular configuration in the optical wedge arrangement.

8. A metrology device comprising a detection branch comprising an optical wedge arrangement according to any preceding claim, and at least one detector.

9. The measuring device according to claim 8, wherein: The at least one detector is operable to detect at least a first zeroth order image from zeroth order radiation of the scattered radiation displaced by the first optical wedge element and a second zeroth order image from zeroth order radiation of the scattered radiation displaced by the third optical wedge element.

10. The measuring device according to claim 9, wherein: The measurement device also includes a processor, the processor being operable to: determining a first indicator that depends on focus from the first zero-order image and determining a second indicator that depends on focus from the second zero-order image; determining a focus-dependent difference indicator from a difference between the first focus-dependent indicator and the second focus-dependent indicator; as well as A focus value of the measurement radiation on the sample being measured is determined from the focus-dependent difference indicator.

11. The measuring device according to claim 10, wherein: Each of the first focus-dependent indicator, the second focus-dependent indicator and the focus-dependent difference indicator comprises a spatial frequency indicator related to a spatial frequency of the first zero-order image and the second zero-order image.

12. The measuring device according to any one of claims 10 to 11, wherein: The at least one detector is operable to detect non-zero order diffracted radiation on the detector; and A parameter of interest is determined from the non-zero order diffracted radiation.

13. The measuring device according to claim 12, wherein: The first zeroth order image, the second zeroth order image, a first diffraction order image of a first diffraction order in a pair of complementary diffraction orders, and a second diffraction order image of a second diffraction order in the pair of complementary diffraction orders are detected at separate areas on the at least one detector.

14. The measuring device according to claim 13, wherein: The first diffraction order is a +1 diffraction order, and the second diffraction order is a -1 diffraction order.

15. A method of determining a focus value of measurement radiation on a sample during measurement of the sample, the method comprising: separating scattered radiation from the sample that has been scattered from the sample after irradiation of the sample with the measurement radiation into at least non-zero order diffracted radiation, a first portion of zero order radiation and a second portion of zero order radiation; detecting at least one image of said non-zero order diffracted radiation, a first zeroth order image from said first portion of zeroth order radiation, and a second zeroth order image of said second portion of zeroth order radiation at separate regions of a detection plane; applying defocusing to at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation; as well as The focus value of the measurement radiation on the sample is determined from the first and second zero-order images.

Images