Focus measurement and control in metrology and related wedge configurations

Abstract

A wedge configuration is disclosed that includes a plurality of wedge elements disposed around an optical axis. The plurality of wedge elements includes at least a first wedge element, a second wedge element, and a third wedge element. The first wedge element includes a first optical surface and a second optical surface, the first optical surface being planar and not perpendicular to the optical axis, and the second optical surface being non-planar such that the first wedge element has a non-linear thickness variation. The second wedge element includes a third optical surface and a fourth optical surface, each of the third optical surface and the fourth optical surface being planar and not parallel. The third wedge element includes a fifth optical surface and a sixth optical surface, the fifth optical surface being planar and not perpendicular to the optical axis. Also disclosed is a metrology device that uses such a wedge configuration to determine the focus of measurement radiation during measurement.

Description

Technical Field

[0001] Cross - reference to related applications

[0001] This application claims the priority of European Patent Application No. 22181325.6 filed on June 27, 2022, which is hereby incorporated by reference in its entirety into this specification.

[0002]

[0002] The present invention relates to a metrology apparatus and method that can be used, for example, to perform metrology in the manufacture of devices by lithography technology. The present invention further relates to a method for monitoring the focus of measurement radiation in such a metrology apparatus.

Background Art

[0003]

[0003] A lithography apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that case, a patterning device, alternatively called a mask or reticle, may be used to generate the circuit patterns formed on the individual layers of the IC. This pattern can be transferred onto a target portion (e.g., including a part of one or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is generally by imaging onto a layer of radiation - sensitive material (resist) provided on the substrate. Generally, a single substrate includes a network of adjacent target portions that are successively patterned. In a lithography process, it is often desirable to frequently measure the created structures, for example, for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes, which are often used to measure critical dimensions (CDs), and dedicated tools for measuring the accuracy of alignment of two layers within a device, i.e., overlay.

[0004]

[0004] In recent years, various forms of scatterometers have been developed for use in the lithography field. These devices direct a radiation beam onto a target and measure one or more characteristics of the scattered light (e.g., the intensity of a single reflection angle as a function of wavelength, the intensity of one or more wavelengths as a function of reflection angle, or polarization as a function of reflection angle), thereby obtaining a diffraction "spectrum" from which the characteristics of interest of the target can be determined.

[0005]

[0005] Examples of known scatterometers include angle-resolved scatterometers of the type described in U.S. Patent Application No. 2006033921A1 and U.S. Patent Application No. 2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm × 40 μm, and the grating and measurement beam produce a spot smaller than the grating (i.e., the grating is underfilled). Examples of dark-field imaging metrology can be found in U.S. Patent Application No. 20100328655A1 and U.S. Patent Application No. 2011069292A1, which are hereby incorporated by reference in their entirety. Further developments of this technology are described in published patent publications, U.S. Patent No. 20110027704A, U.S. Patent No. 20110043791A, U.S. Patent Application No. 2011102753A1, U.S. Patent No. 20120044470A, U.S. Patent No. 20120123581A, U.S. Patent No. 20130258310A, U.S. Patent No. 20130271740A, and International Publication No. 2013178422A1. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. Composite grating targets can be used to measure multiple gratings within a single image. The contents of all these applications are also hereby incorporated by reference.

[0006]

[0006] In many typical metrology tools, separate focus monitoring branches are used to measure or monitor the focus of the radiation for measurement of a target of measurement symmetry or a structure. However, this requires a significant amount of additional hardware (e.g., in particular, detectors, lenses, beam splitters, and other optical elements). Such hardware adds a great deal of cost, complexity, and bulk to the metrology tool.

[0007]

[0007] It is desirable to improve such focus monitoring techniques in metrology.

Summary of the Invention

[0008]

[0008] In a first aspect, the present invention provides a wedge configuration including a plurality of wedge elements arranged around an optical axis, the plurality of wedge elements including at least a first wedge element, a second wedge element, and a third wedge element, the first wedge element including a first optical surface and a second optical surface, the first optical surface being planar and not perpendicular to the optical axis, the second optical surface being non-planar such that the first wedge element has a non-linear thickness variation, the second wedge element including a third optical surface and a fourth optical surface, each being planar and not parallel, the third wedge element including a fifth optical surface and a sixth optical surface, the fifth optical surface being planar and not perpendicular to the optical axis.

[0009]

[0009] In a second aspect, the present invention provides a metrology device including a detection branch, the detection branch including the wedge configuration of the first aspect.

[0010]

[0010] In a third aspect, the present invention provides a metrology device, the metrology device including a processor, an irradiation branch for irradiating a sample with measurement radiation, and a detection branch for detecting scattered radiation, the scattered radiation including the above-mentioned measurement radiation scattered by the sample, the detection branch including at least one detector for detecting the above-mentioned scattered radiation, the detection branch being in a wedge configuration, separating at least non-zero order diffracted radiation in the above-mentioned scattered radiation from zero order radiation in the scattered radiation, such that the non-zero order diffracted radiation and the zero order scattered radiation are imaged on different regions of the above-mentioned at least one detector, and separating a first portion of the above-mentioned zero order radiation from a second portion of the above-mentioned zero order radiation, such that the above-mentioned first portion of the above-mentioned zero order radiation and the above-mentioned second portion of the above-mentioned zero order radiation are imaged on different regions of the above-mentioned at least one detector so as to form a first zero order image and a second zero order image respectively, and being operable to impose defocus on at least one of the above-mentioned first portion of the above-mentioned zero order radiation and the above-mentioned second portion of the above-mentioned zero order radiation, the above-mentioned processor being operable to determine a focus value of the above-mentioned measurement radiation on the sample from the above-mentioned first zero order image and the second zero order image.

[0011]

[0011] In a fourth aspect, the present invention provides a method for determining a focus value of measurement radiation on a sample during measurement of the sample, the method including separating scattered radiation from the sample, which is scattered radiation scattered from the sample after irradiating the sample with the above-mentioned 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 the non-zero order diffracted radiation, a first zero order image from the first portion of the zero order radiation, and a second zero order image from the second portion of the zero order radiation in separate regions of a detection plane, imposing defocus on at least one of the above-mentioned first portion of the above-mentioned zero order radiation and the above-mentioned second portion of the above-mentioned zero order radiation, and determining a focus value of the above-mentioned measurement radiation on the sample from the above-mentioned first zero order image and the second zero order image.

[0012]

[0012] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are merely presented for illustrative purposes herein. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein.

Brief Description of the Drawings

[0013]

[0013] Here, embodiments of the present invention will be described by way of example only with reference to the accompanying schematic drawings. In the drawings, like reference numerals indicate like parts.

[0014]

Figure 1

Figure 2

Figure 3

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Figure 8

DETAILED DESCRIPTION OF THE INVENTION

[0015]

[0014] Before describing embodiments of the present invention in detail, it is beneficial to present an example of an environment in which embodiments of the present invention can be implemented.

[0016]

[0015] FIG. 1 schematically depicts a lithographic apparatus LA. The apparatus includes 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., 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 according to certain parameters, two substrate tables (e.g., wafer tables) WTa and WTb each constructed to hold a substrate (e.g., a 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., a refractive projection lens system) PS configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF serves as a reference for connecting the various components and setting and measuring the positions of the patterning device and the substrate and the positions of the features thereon.

[0017]

[0016] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for guiding, shaping, or controlling radiation.

[0018]

[0017] 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 is in a desired position, for example, with respect to the projection system.

[0019]

[0018] As used herein, the term "patterning device" is to be broadly interpreted as referring to any device that can be used to impart a radiation beam having a pattern in its cross-section in order to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern of the target portion of the substrate, for example, when the pattern includes phase shift features or so-called assist features. In general, the pattern imparted to the radiation beam corresponds to a particular functional layer within the device being created in the target portion, such as an integrated circuit.

[0020] As shown herein, the apparatus is transmissive (e.g., employing a transmissive patterning device). Alternatively, the apparatus may be reflective (e.g., employing a programmable mirror array of the type mentioned above or a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. The use of the terms "reticle" or "mask" in this specification can be considered synonymous with the more general term "patterning device". The term "patterning device" can also be interpreted to refer to a device that stores pattern information in digital form for use in controlling such programmable patterning devices.

[0021] As used herein, the term "projection system" should be broadly interpreted to encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, that is appropriate for the exposure radiation being used or for other elements such as the use of a liquid immersion or the use of a vacuum. The use of the term "projection lens" in this specification can be considered synonymous with the more general term "projection system".

[0022] The lithographic apparatus may be of a type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system and the substrate. The liquid immersion may also be provided in other spaces of the lithographic apparatus, for example, between the mask and the projection system. Liquid immersion techniques for increasing the numerical aperture of the projection system are well known in the art.

[0023]

[0022] During operation, the illuminator IL receives a radiation beam from the radiation source SO. The radiation source and the lithographic apparatus may be separate entities, for example when the radiation source is an excimer laser. In such cases, the radiation source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the radiation source SO to the illuminator IL using a beam delivery system BD including, for example, suitable guiding mirrors and / or a beam expander. In other cases, the radiation source may be an integrated part of the lithographic apparatus, for example when the radiation source is a mercury lamp. The radiation source SO and the illuminator IL may, together with the beam delivery system BD if required, be referred to as a radiation system.

[0024]

[0023] The illuminator IL may include, for example, an adjuster AD for adjusting the angular intensity distribution of the radiation beam, the integrator IN and the condenser CO. The illuminator may be used to adjust the radiation beam so that it has a desired uniformity and intensity distribution across its cross-section.

[0025]

[0024] The radiation beam B is incident on a patterning device MA held on a patterning device support MT and is patterned by the patterning device. After passing through the patterning device (e.g. a mask) MA, the radiation beam B passes through a projection system PS that focuses the beam onto a target portion C of the substrate W. A second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder, a 2D encoder or a capacitance sensor) can be used to accurately move the substrate table WTa or WTb, for example to position different target portions C within the path of the radiation beam B. Similarly, a first positioner PM and another position sensor (not explicitly depicted in FIG. 1) can be used to accurately position the patterning device (e.g. a reticle / mask) MA with respect to the path of the radiation beam B, for example after a mechanical search of a mask library or during a scan.

[0026]

[0025] The patterning device (e.g., reticle / mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Substrate alignment marks as shown occupy dedicated target portions, although these may be located in the space between the target portions (these are known as scribe line alignment marks). Similarly, in a situation where two or more dies are provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be incorporated within the die, even within the device features, in which case it is desirable that the marker is as small as possible and requires different imaging or process conditions from adjacent features. An alignment system for detecting the alignment markers is described further below.

[0027]

[0026] The illustrated apparatus can be used in various modes. In the scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously, during which the pattern imparted to the radiation beam is projected onto the target portion C (i.e., single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the magnification (reduction) ratio and image inversion characteristics of the projection system PS. In the scan mode, the width of the target portion in a single dynamic exposure (in the non-scan direction) is limited by the maximum size of the exposure field, while the height of the target portion (in the scan direction) is determined by the length of the scan operation. As is well known in the art, other types of lithographic apparatus and operating modes are also possible. For example, the step mode is known. In so-called “maskless” lithography, although the programmable patterning device is held stationary, the pattern changes and the substrate table WT is moved or scanned.

[0028]

[0027] The combinations and / or variations of the usage modes described above, or completely different usage modes, can also be used.

[0029]

[0028] The lithographic apparatus LA is of the so-called dual-stage type, which has two substrate tables WTa, WTb and two stations (exposure station EXP and measurement station MEA), and the substrate tables can be exchanged between these stations. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparation steps can be carried out. Thereby, the throughput of the apparatus can be significantly increased. The preparation steps may include mapping the contour of the surface height of the substrate using the level sensor LS and measuring the position of the alignment markers on the substrate using the alignment sensor AS. When the position sensor IF cannot measure the position of the substrate table while the substrate table is at the measurement station and also while it is at the exposure station, a second position sensor may be provided to enable tracking of the position of the substrate table with respect to the reference frame RF at both stations. Other configurations are known and can be used instead of the illustrated dual-stage configuration. For example, other lithographic apparatuses with a substrate table and a measurement table provided are known. These are docked to each other when performing the preparation measurement and are undocked while the substrate table is being exposed.

[0030]

[0029] As shown in FIG. 2, the lithography apparatus LA sometimes forms part of a lithographic cell LC, also sometimes called a litho cell or cluster, which includes apparatus for performing pre- and post-exposure processing on a substrate. Conventionally, these have included a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, and a bake plate BK. A substrate handler or robot RO picks up substrates from input / output ports I / O1, I / O2 and moves them between different process apparatuses, and then delivers them to the loading bay LB of the lithography apparatus. These devices, generally referred to as a track, are under the control of a track control unit TCU which is itself controlled by a supervisory control system SCS (which also controls the lithography apparatus by means of a lithography control unit LACU). Thus, by operating the different apparatuses, throughput and processing efficiency can be maximized.

[0031]

[0030] In order to ensure that substrates exposed by the lithography apparatus are consistently and correctly exposed, it is desirable to inspect the exposed substrates and measure target characteristics or parameters such as the resulting overlay between layers, line width, critical dimension (CD), etc. Thus, the manufacturing facility in which the litho cell LC is located also includes a metrology system MET which receives some or all of the substrates W processed within the litho cell. The metrology results are provided directly or indirectly to the supervisory control system SCS. If an error is detected, adjustments may be made to the exposure of subsequent substrates, particularly if the inspection can be carried out quickly enough so as to still allow other substrates in the same batch to be exposed. Also, substrates that have already been exposed may be stripped and reprocessed or discarded in order to improve yield, thereby avoiding further processing of substrates known to be defective. If only a part of a target portion of a substrate has a defect, further exposure may be carried out only on the good target portion.

[0032]

[0031] In a metrology system MET, an inspection device is used to determine the characteristics of a substrate, in particular to check how the characteristics of different substrates or different layers of the same substrate vary from layer to layer. The inspection device may be integrated with a lithography apparatus LA or a lithocell LC, or may be a stand-alone device. In order to enable the most rapid measurements, it is desirable for the inspection device to measure the characteristics of 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 difference in refractive index between the resist portion exposed to radiation and the unexposed resist portion), and not all inspection devices have sufficient sensitivity to perform an effective measurement of the latent image. Therefore, the measurement may be performed after the post-exposure bake step (PEB), which is the step customarily first performed on the exposed substrate and which enhances the contrast between the exposed and unexposed portions of the resist. At this stage, the image in the resist may be referred to as a semi-latent image. It is also possible to perform the measurement of the developed resist image at the time when the exposed or unexposed portion of the resist has been removed, or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking a defective substrate, but may still provide useful information.

[0033]

[0032] A metrology apparatus suitable for use in an embodiment of the present invention is shown in FIG. 3(a). Note that this is only an example of a suitable metrology apparatus. Alternative suitable metrology apparatuses may use EUV radiation, for example, as disclosed in WO 2017 / 186483 A1. A number of other types of metrology apparatuses are known and can equally be used to implement the concepts disclosed herein. The target structure T and the diffracted rays of the measurement radiation used to illuminate the target structure are shown in more detail in FIG. 3(b). The illustrated metrology apparatus is of a type known as a dark-field metrology apparatus. The metrology apparatus may be a stand-alone device or may be incorporated, for example, in a lithography apparatus LA at a measurement station or in a lithographic cell LC. An optical axis having several branches throughout the apparatus is represented by the dotted line O. In this apparatus, light emitted by a light source 11 (for example, a xenon lamp) is guided onto a substrate W via a beam splitter 15 by an optical system including lenses 12, 14, and an objective lens 16. These lenses are arranged in a double sequence in a 4F configuration. Different lens arrangements can be used if they still provide an image of the substrate on the detector and at the same time allow access to the intermediate pupil plane for spatial frequency filtering. Thus, the angular range at which the radiation is incident on the substrate can be selected by defining the in-plane spatial intensity distribution that represents the spatial spectrum of the substrate plane, herein called the (conjugate) pupil plane. Specifically, this can be done by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in the plane that is the rear-projected image of the objective lens pupil plane. In the example shown, the aperture plate 13 has different forms (designated 13N and 13S) and allows different illumination modes to be selected. The illumination system of this example forms an off-axis illumination mode. In the first illumination mode, the aperture plate 13N provides off-axis illumination from a direction designated "north" for illustrative purposes only. In the second illumination mode, similar illumination is provided from the opposite direction designated "south" using the aperture plate 13S. Other illumination modes are possible using different apertures.A particular alternative, aperture plate 13Q, has a four-segmented illumination aperture, with illumination being inserted through two diagonally opposed quadrants and the other two quadrants being blocked (the illuminated and blocked quadrants may be interchanged from those shown). Such a configuration may be used for simultaneous imaging at diffraction orders +1 and -1, and is further described in U.S. Patent Application Publication No. 2010201963A1, supra. The remainder of the pupil plane is desirably darkened since unwanted light outside the desired illumination mode interferes with the desired measurement signal.

[0034]

[0033] As shown in FIG. 3(b), target structure T is arranged such that substrate W is perpendicular to the optical axis O of objective lens 16. Substrate W may be supported by a support (not shown). Measurement radiation beam I impinging on target structure T from an off-axis angle generates a zero-order beam (solid line 0) and two first-order beams (dash-dotted line +1 and dotted line -1), which are hereinafter referred to as a pair of complementary diffraction orders. Note that the pair of complementary diffraction orders may be any higher-order pair, such as a +2, -2 pair, etc., and is not limited to the first-order complementary pair. Note that in the case of an overfilled small target structure, these beams are just one of many parallel beams covering the area of the substrate including the metrology target structure T and other features. Since the aperture of plate 13 has a finite-width beam (necessary to accept a useful amount), incident beam I actually occupies a certain angular range, and diffracted beams 0 and +1 / -1 are somewhat spread. According to the point spread function of the small target, each of orders +1 and -1 further spreads over a certain angular range (not a single ideal beam as shown). Note that the grating pitch of the target structure and the illumination angle can be designed or adjusted such that the first-order beam entering the objective lens is precisely aligned with the central optical axis. The beams shown in FIGS. 3(a) and 3(b) are shown slightly off-axis merely to be able to distinguish them more easily in the figures.

[0035]

[0034] 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 guided to return through the beam splitter 15. Returning to Fig. 3(a), both the first and second illumination modes are illustrated by specifying the diametrically opposite apertures labeled North (N) and South (S). When the incident light ray I of the measurement radiation is from the north side of the optical axis, i.e., when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray (labeled +1(N)) enters the objective lens 16. In contrast, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (labeled -1(S)) is the diffracted ray that enters the lens 16.

[0036]

[0035] The second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 forms a diffraction spectrum (pupil plane image) of the target structure on the first sensor 19 (e.g., a CCD or CMOS sensor) using the 0th and 1st order diffracted beams. Each diffraction order hits a different point on the sensor so that the image processing can compare and contrast the orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the 1st order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0037]

[0036] In the second measurement branch, the optical systems 20, 22 form an image of the target structure T on a sensor 23 (for example, a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21a is provided in a plane 21 conjugate to the pupil plane. The aperture stop 21a functions to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed only from the -1 or +1 order beams. Instead of a simple aperture stop 21a, the metrology tool may use a wedge configuration 21b that separates the first order and the zero order, so that they can be imaged separately (apart) on the detector 23. Such a configuration is described in the above-mentioned US Patent Application Publication No. 2011 / 02753A1 and can be used to simultaneously acquire the +1 and -1 diffraction orders in one or both directions of the substrate plane (for example, in combination with a four-segment illumination aperture 13Q).

[0038]

[0037] The images captured by the sensors 19 and 23 are output to a processor PU that processes the images (the function of which depends on the specific type of measurement being performed). Here, the term "image" is used in a broad sense. Therefore, when only one of the -1 and +1 orders exists, an image of the grating lines is not formed.

[0039]

[0038] Position errors may occur due to overlay errors (often referred to as "overlay"). Overlay is the error in placing the first feature during the first exposure with respect to the second feature during the second exposure. The lithographic apparatus minimizes overlay by accurately aligning each substrate with respect to a reference before patterning, for example, by measuring the position of alignment marks on the substrate using alignment sensors, and by using feedback correction in the exposure process, for example, by using an appropriate metrology tool such as that shown in Figure 3(a) to measure the overlay of the metrology target after exposure.

[0040]

[0039] One of the known metrology methods that can be implemented using a metrology tool such as that shown in FIG. 3(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 detector 23) and are based on the intensity asymmetry or intensity difference between a first diffraction order and a second diffraction order of a pair of complementary diffraction order pairs (usually a complementary pair of first diffraction orders, i.e., as shown in FIG. 3(b), the first diffraction order may include +1 order and the second diffraction order may include -1 order) to determine the asymmetry within the structure. In such cases, in this context, the terms “first” and “second” are not used to refer to the number of diffraction orders, but are simply used to distinguish the two diffraction orders of the complementary pair. Note that the first and second diffraction orders may be +2 and -2 diffraction orders, or a pair of higher-order complementary diffraction orders. The zero order (specular radiation) is usually blocked or diverted elsewhere (e.g., to another part of the detector for monitoring purposes). The zero order is not used in μDBO metrology. The main “image” used for inferring the parameter of interest is formed only from the higher (e.g., first) diffraction order. The asymmetry of the structure can be used to estimate parameters of interest such as overlay or focus according to the design of the target.

[0041]

[0040] The overlay is just one of the parameters of interest measurable using such techniques. Another parameter of interest can be the exposure focus. This exposure focus may be measured using diffraction-based focus (DBF) or micro-diffraction-based focus (μDBF) metrology. The measurement principles of μDBO and μDBF are essentially the same (measurement of target asymmetry through intensity asymmetry at complementary diffraction orders). The main difference between these two techniques lies in the target. That is, the target of μDBO has an overlay-dependent asymmetry, and the target of μDBF has an asymmetry based on the exposure focus. Note that the exposure focus and DBF / μDBF metrology in this context are different from the focus of the focus monitoring method disclosed in this specification. DBF / μDBF estimates the actual focus of the exposure radiation on the substrate from the exposed target (scanner focus), that is, the scanner focus when the target (and other structures) are exposed on the wafer. The concepts disclosed in this specification relate to the metrology tool focus or the focus of the measurement radiation on the wafer during measurement. All references to μDBO or DBO below should be understood to include measurements of any parameter of interest using any diffraction-based imaging metrology method based on single diffraction order imaging, including DBF / μDBF.

[0042]

[0041] Figure 4 shows a known configuration that uses a wedge configuration (e.g., the wedge configuration 21b described above) to simultaneously image both diffraction orders and the zero order of a complementary diffraction order pair of +1 / -1 in separate regions of a detector (or multiple separate detectors). The detection pupil plane 400 (or the conjugate pupil plane with respect to the objective detection pupil plane) may be divided into four wedge elements or quadrants (e.g., by using a four-segmented illumination aperture within an illumination branch such as the illumination aperture 13Q described above), where two quadrants 405 correspond to the zero order 0, and two quadrants 410 correspond to the desired diffraction orders +1X, -1X, -1Y, +1Y received from targets in the X and Y directions, respectively (where X and Y are two perpendicular directions in the substrate plane). The wedge configuration 415 is disposed within this (conjugate) detection pupil plane and may include four (similar) quadrants Q1, Q2, Q3, Q4, each quadrant having respective optical wedges oriented to separate the angle of the zero order from the desired diffraction order such that the desired diffraction orders are disposed at four different positions within the μDBO image plane. Each wedge is oriented in a similar direction, and a cross-section is shown at the bottom of this figure. The resulting image 420 includes an image 425 of +1, an image 430 of -1, and two (similar) images 435 of the zero order. Note that it should be noted that the "image" referred to here is not the image of a conventional dark-field microscope, as each of the images 425 and 430 of +1 and -1 contains only half of the first-order diffracted radiation. Individual grating lines are not dissipated. Each grating is simply represented by an area of a specific intensity level, and the asymmetry of the target, and thus the parameter of interest, can be estimated using the difference in intensity levels (for each direction) between image 425 and image 430.

[0043]

[0042] FIG. 5 shows a known focus monitoring configuration that may be used in a metrology device such as that shown in FIG. 3(a). Such a focus monitoring configuration may include a focus branch 500 as shown in FIG. 5(a), which employs the confocal focus principle. The hardware shown in FIG. 5(a) is a very simplified schematic and shows a wafer plane WP that can be moved as indicated by the arrows for focusing. Scattered radiation 505 from the wafer is collected by an objective lens configuration 510 (e.g., via a beam splitter not shown) and may be split using a beam splitter 515. The first split portion 520 of the scattered radiation is focused at a first focus plane FP1 in front of a first detector 525, and the second split portion 530 of the scattered radiation is focused at a second focus plane FP2 behind a second detector 535. Respective pinholes or apertures 540, 545 are disposed in front of each detector 525, 535.

[0044]

[0043] FIG. 5(b) shows how focus is estimated from the detection signals captured by the first detector 525 and the second detector 535. This figure shows a plot of signal strength (e.g., amplitude or intensity) SS versus defocus f (centered about zero defocus). The dotted lines include a first signal 550 from the first detector 525 and a second signal 555 from the second detector 535. The differential signal 560 between these signals 550, 555 has a linear dependence on defocus in the vicinity of the zero defocus region. This signal can be obtained and used to detect the focus of the illumination for measurement on the wafer with good sensitivity.

[0045]

[0044] The problem with such a focusing configuration is that additional hardware branching is required, and this branching becomes much more complex than that shown. It is desirable to reduce the cost, bulk, and complexity of such a focus branch. Further, the current implementation of such a focus branch uses only two wavelengths to perform focusing, which does not properly represent all the wavelengths used for measurements taking chromatic aberration into account. Further, these two wavelengths cannot be used in the imaging branch of the μDBO measurement.

[0046]

[0045] In known alternative methods of focus monitoring in metrology tools, fringe contrast may be used. Usually, in μDBO imaging, since only the zero or first diffraction order is allowed in any quadrant, there are no fringes at all. For example, by using a low wavelength / target pitch ratio, if a diffraction order higher than the first exists within one quadrant, the target grid lines will be imaged within that quadrant with visible fringes. The contrast of the line fringes depends on the defocus, i.e., the highest fringe contrast is achieved at the best focus.

[0047]

[0046] Figure 6 is a plot of the first-order fringe contrast FRCO versus defocus f, showing this dependence. This dependence enables focus detection / calibration, but it cannot be used in real time during μDBO measurements because the μDBO image should not contain diffraction orders higher than the first. Further, it can be seen from the plot that the sensitivity is lowest in the region closest to the best focus, and that this method can only measure the magnitude of the defocus and not its direction. Also, a large target is required for fringe contrast analysis, which is not desirable for μDBO.

[0048]

[0047] To address these problems, a method and metrology configuration are proposed that use a focus-dependent metrics-based differential detection method. The focus-dependent metrics may include, for example, spatial frequency or fringe contrast. Such a method may use the wedge configuration of the new design at the detection pupil plane or its conjugate. This concept enables real-time focus detection in the imaging branch instead of using a separate focus monitoring branch (e.g., when implementing μDBO / μDBF metrology or similar dark field metrology techniques).

[0049]

[0048] The effect of defocus is to blur the image and thus reduce the higher spatial frequency components. This also applies to μDBO / μDBF images or similar images. Therefore, the focus can be determined by obtaining the spatial frequency from such an image.

[0050]

[0049] FIG. 7 shows the basic principle underlying (at least a part of) the concepts disclosed herein. Images 700a, 700b, 700c are shown, each corresponding to a respective different metrology focus value of +1, 0, -1 (units are arbitrary), where 0 corresponds to the best focus. Each of these images (or regions thereof) can be Fourier transformed (e.g., a 2D transform such as a 2D fast Fourier transform FFT) to obtain respective spatial frequencies 710a, 710b, 710c. In the illustrated example, the region of the transformed image may include one or both of the zero-order images, or a region including one or both of the zero-order images. This enables focus monitoring in the detected radiation that is not used for parameter estimation. A plot 730 of the spatial frequency metric SF versus focus f is shown. The spatial frequency metric 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 here specifically, it is the k x direction component in the pupil space corresponding to the X direction. It can be seen that the spatial frequency metric SF has a (nonlinear) dependence on the defocus f. It is proposed to use this dependence to measure the metrology focus.

[0051]

[0050] The spatial frequency metric used in the method described herein may relate to a single frequency component of each image, or a single frequency component for each direction (e.g., in the k x direction and the k y direction), or two or more spatial frequencies (e.g., for each direction or otherwise). For example, a number of frequencies (e.g., any frequency other than the lowest frequency at the center of each transformed image 710a, 710b, 710c) can be used. To obtain the spatial frequency metric, signals from different spatial frequencies can be averaged or combined in another way.

[0052]

[0051] In certain use cases, some spatial frequencies may be more sensitive to defocus than other spatial frequencies. Accordingly, the proposed method may include an optional spatial frequency selection step to optimize the defocus control performance.

[0053]

[0052] In particular, the spatial frequency metric used in the method described herein may include a differential spatial frequency metric that includes the difference between a first spatial frequency metric from a first zero-order image and a second spatial frequency metric from a second zero-order image, where the first and second zero-order images receive different foci (e.g., at the detection pupil plane or its conjugate).

[0054]

[0053] In a practical implementation of this principle, a novel wedge configuration is used to obtain a substantially linear or at least monotonic dependence between the (differential) spatial frequency metric and the focus over a range of focus of interest (e.g., the focus range around the best focus, including the expected focus error range of a metrology tool). The proposed wedge configuration may be similar to the wedge configuration 415 described above, but at least one of the wedge elements (e.g., quadrants) corresponding to the zero-order region within the detection pupil plane has a surface curvature so as to induce a defocused wavefront for each zero-order. For example, this curvature may be on the surface opposite the angled planar surface, which shifts the zero-order at the pupil plane and separates its zero-order image from the first-order image and the complementary zero-order image at the image plane. This enables determination of a differential signal between spatial frequency metrics respectively derived from two zero-order images that have a monotonic and / or substantially linear relationship with focus.

[0055]

[0054] Accordingly, the wedge configuration includes a plurality of wedge elements arranged around the optical axis, and the plurality of wedge elements include at least a first wedge element, a second wedge element, and a third wedge element. The first wedge element includes a first optical surface and a second optical surface. The first optical surface is planar and not perpendicular to the optical axis. The second optical surface is non-planar such that the first wedge element has a non-linear thickness variation. The second wedge element includes a third optical surface and a fourth optical surface, each of which is planar and not parallel. The third wedge element includes a fifth optical surface and a sixth optical surface. The fifth optical surface is planar and not perpendicular to the optical axis. The wedge configuration may also include a fourth wedge element that includes a seventh optical surface and an eighth optical surface, each of which is planar and not parallel.

[0056]

[0055] In a convenient configuration, the first wedge element may correspond to a first zero-order pupil region, the second wedge element may correspond to a first diffractive radiation pupil region, the third wedge element may correspond to a second zero-order pupil region, and the fourth wedge element may correspond to a second diffractive radiation pupil region. The terms "first diffractive radiation pupil region" and "second diffractive radiation pupil region" are for distinguishing the so-called pupil regions from each other, and the terms "first" and "second" do not refer to the number of diffraction orders. As will be understood, the first diffractive radiation pupil region and the second diffractive radiation pupil region usually correspond to the respective diffraction orders of a complementary pair of diffraction orders, for example, such that the first diffractive radiation pupil region corresponds to a +1 diffraction order and the second diffractive radiation pupil region corresponds to a -1 diffraction order.

[0057]

[0056] In a convenient configuration, the first wedge element and the third wedge element may each include a curved surface with opposite curvatures so as to impart defocus in opposite directions. In such a configuration, optionally, the curvatures (and thus the defocus) imparted by the first wedge element and the third wedge element may be equal in magnitude.

[0058]

[0057] Each of the first wedge element and the third wedge element may direct the zero-order radiation to different regions of the image plane (detection plane) so as to obtain the first zero-order image and the second zero-order image. For each of the first zero-order image and the second zero-order image, respective first and second spatial frequency metrics can be determined, and the difference between these spatial frequency metrics can be used to determine the focus.

[0059]

[0058] FIG. 8 is a schematic diagram showing such a method and configuration. The upper part of this figure corresponds to FIG. 4, and for similar elements, no further explanation will be given. The main difference is the wedge configuration 815, where the surface curvatures of the Q1' and Q3' quadrants (wedge elements) each have a curved surface that bends in the opposite direction. For example, quadrant Q1' has a convex surface that induces a negative defocus wavefront, and quadrant Q3' has a concave surface that induces a positive defocus wavefront. The magnitude of the applied defocus may be the same for both quadrants Q1' and Q3'.

[0060]

[0059] Image 420 includes a first zero-order image 835a and a second zero-order image 835b, each corresponding to a different imposed defocus. Each of these images can be Fourier-transformed (e.g., 2D FFT) to obtain respective transformed images or spatial frequency representations 840a, 840b. From each of these spatial frequency representations 840a, 840b, respective spatial frequency metrics can be determined (e.g., from one or more spatial frequency components as described above). The spatial frequency metric used for focus estimation may at this time include the difference between these spatial frequency metrics.

[0061]

[0060] Focus estimation may be achieved by referring to a previously calibrated relationship 845 that represents the variation of the spatial frequency metric with focus. This relationship may be obtained from the difference between two calibration relationships 850a, 850b that represent the variation of the spatial frequency metric of each defocused zero-order image with focus. Note that the peaks of each of the calibration relationships 850a, 850b are shifted in opposite directions with respect to the best focus.

[0062]

[0061] The method described herein may include an initial calibration stage for determining the calibrated relationship 845. Calibration is to obtain a first set of zero-order images and a second set of zero-order images, each set being imaged during the measurement of the target at a corresponding different focus setting (e.g., through focus), and determining the relationship of the spatial frequency metric through focus (e.g., via Fourier transform) for each set of those images, and determining the calibrated relationship 845 as the difference between these two relationships. Calibration may be performed for each application, e.g., for each stack and / or for each type of target, and inferences may be speculated using an appropriate calibrated relationship.

[0063]

[0062] The differential zero-order wedge configuration disclosed herein may be used in alternative focus estimation methods based on fringe contrast measurement. Such methods may include generating fringes in the zero-order pupil region. This may be achieved by capturing diffraction orders greater than the first diffraction order in the zero-order quadrants, for example, by using the ratio of the small measurement wavelength to the target pitch. Each of the first and second fringe contrast metrics (e.g., a measure of fringe contrast) can be determined from the resulting first zero-order image and second zero-order image that are imaged on the detector. A differential fringe contrast metric is determined that includes the difference between the first fringe contrast metric and the second fringe contrast metric, and this can be used to estimate focus based on a calibrated relationship between the fringe contrast metric (differential fringe contrast metric) and focus. This relationship can be determined in a calibration step similar to that already described, but is determined for fringe contrast rather than spatial frequency. This calibrated relationship exhibits a monotonic and / or linear dependence on focus over the focus range of interest and provides a much higher focus sensitivity than the known fringe contrast methods described above. However, such embodiments cannot be used during μDBO or μDBF measurements because additional diffraction orders (second order and higher) also exist in the first-order quadrants.

[0064]

[0063] In all embodiments, the metrology device can control focus (e.g., move the wafer relative to the measurement beam) using the measured focus value so as to minimize defocus. All determination steps, processing steps, and mathematical steps may be performed by a processor of the metrology device.

[0065]

[0064] According to the concepts disclosed herein, focus detection becomes possible in a metrology tool, more specifically in the imaging branch of a metrology tool that detects an image of a focused target on a detector. This means that a separate focus monitoring branch is not required, reducing cost, bulk, and system complexity. Further, the proposed method enables detection of focus at all wavelengths used (i.e., all wavelengths of the radiation for measurement), which is an improvement over current implementations of focus monitoring branches that use only two wavelengths for focus monitoring. Also, frequency difference detection can be used in real time during measurement (e.g., μDBO or μDBF measurement) and does not require imaging fringes in the uDBO image, and thus can be used during uDBO detection, and there are no requirements for the wavelength / pitch ratio or the target size.

[0066]

[0065] The fringe contrast difference detection method is advantageous in that, compared to current fringe contrast-based methods, the difference detection provides a linear region near zero focus and thus much higher focus detection sensitivity and the direction of defocus. This method has been described from the perspective of measuring a structure or target on a substrate. However, this method may be applied to the measurement of any sample, for example, it may include measuring focus during measurement on a flat area (e.g., a thin film sample).

[0067]

[0066] Further embodiments are disclosed in the list of numbered clauses that follow. 1. A wedge configuration including a plurality of wedge elements arranged around an optical axis, the plurality of wedge elements including at least a first wedge element, a second wedge element, and a third wedge element, the first wedge element of the above includes a first optical surface and a second optical surface, the first optical surface is planar and not perpendicular to the optical axis, and the second optical surface is non-planar such that the first wedge element of the above has a non-linear thickness change. The above-described second wedge element includes a third optical surface and a fourth optical surface, which are each planar and not parallel, The above-described third wedge element includes a fifth optical surface and a sixth optical surface, and the fifth optical surface is planar and not perpendicular to the optical axis, in a wedge configuration. 2. The sixth optical surface is non-planar such that the above-described third wedge element has a non-linear thickness variation, according to the wedge configuration of clause 1. 3. The above-described second optical surface is convex and the above-described sixth optical surface is concave, according to the wedge configuration of clause 2. 4. The magnitude of defocus imposed by the above-described first wedge element and the magnitude of defocus imposed by the above-described second wedge element are substantially the same, according to the wedge configuration of clause 3. 5. The plurality of above-described wedge elements are arranged substantially in the same plane around the above-described optical axis, according to the wedge configuration of any one of the preceding clauses. 6. Further including a fourth wedge element including a seventh optical surface and an eighth optical surface, each being planar and not parallel, according to the wedge configuration of any one of the preceding clauses. 7. Each wedge element includes a quartered configuration, and the plurality of wedge elements are arranged in a circular configuration within the above-described wedge configuration, according to the wedge configuration of clause 6. 8. A metrology device including a detection branch, wherein the above-described detection branch includes the wedge configuration of any one of the preceding clauses and at least one detector, a metrology device. 9. The above-described at least one detector is operable to detect at least a first zero-order image from the zero-order radiation of the scattered radiation displaced by the above-described first wedge element and a second zero-order image from the zero-order radiation of the scattered radiation displaced by the above-described third wedge element, according to the metrology device of clause 8. 10. The metrology device is a processor, determining a first focus-dependent metric from the above-described first zero-order image and determining a second focus-dependent metric from the above-described second zero-order image, Determine a differential focus-dependent metric from the difference between the above-described first focus-dependent metric and the above-described second focus-dependent metric, Determine a focus value of the radiation for measurement on a sample to be measured from the above-described differential focus-dependent metric, The metrology device according to clause 9, further comprising a processor operable to operate as described above. 11. The metrology device according to clause 10, wherein each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a spatial frequency metric related to the spatial frequency of the first zero-order image and the second zero-order image. 12. The metrology device according to clause 11, wherein the processor is operable to perform a Fourier transform on each of the above-described first zero-order image and second zero-order image and determine the above-described first focus-dependent metric and the above-described second focus-dependent metric. 13. The metrology device according to clause 10, wherein each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a fringe contrast metric related to the fringe contrast of the first zero-order image and the second zero-order image. 14. The metrology device according to any one of clauses 10 to 13, wherein the above-described processor is operable to determine the above-described focus value by referring to a calibrated relationship between the differential focus-dependent metric and the focus. 15. The above-described at least one detector Detect non-zero order diffracted radiation on the above-described detector, Determine a parameter of interest from the above-described non-zero order diffracted radiation, The metrology device according to any one of clauses 10 to 14, operable to operate as described above. 16. The metrology device according to clause 15, including determining the above-described parameter of interest and the above-described focus value from a single image acquisition on the above-described at least one detector. 17. The metrology device according to clause 15 or 16, wherein the above-mentioned first zero-order image, the above-mentioned second zero-order image, the first diffracted-order image of the first diffracted order of the complementary diffracted-order pair, and the second diffracted-order image of the second diffracted order of the complementary diffracted-order pair are detected in separate regions on the above-mentioned at least one detector. 18. The metrology device according to clause 17, wherein the above-mentioned first diffracted order is the +1 diffracted order and the above-mentioned second diffracted order is the -1 diffracted order. 19. The metrology device according to any one of clauses 10 to 18, which is operable to control focus using the above-mentioned focus value and minimize defocus of the above-mentioned measurement radiation on the sample. 20. The metrology device according to any one of clauses 8 to 19, wherein the above-mentioned wedge configuration is included within the pupil plane of the above-mentioned detection branch or its conjugate. 21. A metrology device, a processor, an illumination branch for irradiating a sample with measurement radiation, a detection branch for detecting scattered radiation, the scattered radiation including the above-mentioned measurement radiation scattered by the sample, the detection branch including at least one detector for detecting the above-mentioned scattered radiation, the detection branch is a wedge configuration, separating at least non-zero-order diffracted radiation within the above-mentioned scattered radiation from zero-order radiation within the above-mentioned scattered radiation such that non-zero-order diffracted radiation and zero-order scattered radiation are imaged on different regions of the above-mentioned at least one detector, separating a first portion of the above-mentioned zero-order radiation from a second portion of the above-mentioned zero-order radiation such that the first portion of the above-mentioned zero-order radiation and the second portion of the above-mentioned zero-order radiation are imaged on different regions of the above-mentioned at least one detector so as to form a first zero-order image and a second zero-order image respectively, further including a wedge configuration operable to impose defocus on at least one of the first portion of the above-mentioned zero-order radiation and the second portion of the above-mentioned zero-order radiation. The above-mentioned processor is a metrology device operable to determine a focus value of the above-mentioned measurement radiation on a sample from the above-mentioned first zero-order image and the second zero-order image. 22. The processor determines a first focus-dependent metric from the above-mentioned first zero-order image and determines a second focus-dependent metric from the above-mentioned second zero-order image, determines a differential focus-dependent metric from a difference between the above-mentioned first focus-dependent metric and the above-mentioned second focus-dependent metric, and is operable to determine a focus value of the above-mentioned measurement radiation on a sample from the above-mentioned differential focus-dependent metric, the metrology device according to clause 21. 23. Each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a spatial frequency metric related to the spatial frequencies of the first zero-order image and the second zero-order image, the metrology device according to clause 22. 24. The processor is operable to perform a Fourier transform on each of the above-mentioned first zero-order image and the second zero-order image and determine the above-mentioned first focus-dependent metric and the above-mentioned second focus-dependent metric, the metrology device according to clause 23. 25. Each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a fringe contrast metric related to the fringe contrast of the first zero-order image and the second zero-order image, the metrology device according to clause 22. 26. The above-mentioned processor is operable to determine the above-mentioned focus value by referring to a calibrated relationship between the differential focus-dependent metric and the focus, the metrology device according to any one of clauses 22 to 25. 27. The wedge configuration described above includes a plurality of wedge elements arranged around the optical axis of the detection branch. The plurality of wedge elements include at least a first wedge element configured to deflect the first portion of the above-described zero-order radiation to a first region on the at least one detector, a second wedge element configured to deflect at least a portion of the above-described non-zero-order diffracted radiation to a second region on the at least one detector, and a third wedge element configured to deflect the second portion of the above-described zero-order radiation to a third region on the at least one detector. The metrology device according to any one of clauses 21 to 26. 28. The above-described first wedge element includes a first optical surface and a second optical surface. The first optical surface is planar and not perpendicular to the optical axis. The second optical surface is non-planar and has a non-linear thickness variation such that the above-described first wedge element defocuses the first portion of the above-described zero-order radiation. The above-described second wedge element includes a third optical surface and a fourth optical surface, which are each planar and not parallel. The above-described third wedge element includes a fifth optical surface and a sixth optical surface. The fifth optical surface is planar and not perpendicular to the optical axis. The metrology device according to clause 26. 29. The sixth optical surface is non-planar and has a non-linear thickness variation such that the above-described third wedge element defocuses the second portion of the above-described zero-order radiation. The metrology device according to clause 28. 30. The above-described second optical surface is convex, and the above-described sixth optical surface is concave. The metrology device according to clause 29. 31. The magnitude of the defocus imposed by the above-described first wedge element and the magnitude of the defocus imposed by the above-described second wedge element are substantially the same. The metrology device according to clause 30. 32. The above-described plurality of wedge elements are substantially arranged around the above-described optical axis, within the common pupil plane of the detection branch, or within its conjugate. The metrology device according to any one of clauses 27 to 31. 33. The wedge configuration further includes a fourth wedge element including a seventh optical surface and an eighth optical surface, each being planar and non-parallel, of the metrology device according to any one of clauses 27 to 32. 34. Each wedge element includes a quartering configuration, and a plurality of wedge elements are arranged in a circular configuration within the above-described wedge configuration, of the metrology device according to clause 33. 35. The second wedge element described above is configured to deflect the first diffraction order of a complementary pair of diffraction orders to the second region on the at least one detector described above, and the fourth wedge element described above is configured to deflect the second diffraction order of a complementary pair of diffraction orders to the fourth region on the at least one detector, of the metrology device according to clause 33 or 34. 36. The first diffraction order described above is the +1 diffraction order, and the second diffraction order described above is the -1 diffraction order, of the metrology device according to clause 35. 37. The processor is operable to determine a parameter of interest from the detected first diffraction order and the detected second diffraction order, of the metrology device according to clause 35 or 36. 38. The processor is operable to determine the parameter of interest and the focus value described above from a single image acquisition at the at least one detector described above, of the metrology device according to clause 37. 39. The metrology device according to any one of clauses 21 to 38, operable to control focus using the focus value described above and minimize defocus of the measurement radiation on the sample. 40. A method for determining a focus value of measurement radiation on a sample during measurement of the sample, separating scattered radiation from the sample, scattered from the sample after irradiating the sample with the measurement radiation described above, into at least non-zero order diffracted radiation, a first portion of the zero order radiation, and a second portion of the zero order radiation; detecting at least one image of non-zero order diffracted radiation, a first zero order image from the first portion of the zero order radiation, and a second zero order image of the second portion of the zero order radiation in separate regions of a detection plane; Imposing defocus on at least one of the above-mentioned first part of the zero-order radiation and the above-mentioned second part of the zero-order radiation, Determining the focus value of the above-mentioned measurement radiation on the sample from the above-mentioned first zero-order image and the second zero-order image, a method comprising. 41. The above-mentioned step of determining the focus value Determining a first focus-dependent metric from the above-mentioned first zero-order image and determining a second focus-dependent metric from the above-mentioned second zero-order image, Determining a differential focus-dependent metric from the difference between the above-mentioned first focus-dependent metric and the above-mentioned second focus-dependent metric, Determining the focus value of the above-mentioned measurement radiation on the sample from the above-mentioned differential focus-dependent metric, the method according to clause 40. 42. Each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a spatial frequency metric related to the spatial frequencies of the first zero-order image and the second zero-order image, the method according to clause 41. 43. The method according to clause 42, including Fourier-transforming each of the above-mentioned first zero-order image and the second zero-order image to determine the above-mentioned first focus-dependent metric and the above-mentioned second focus-dependent metric. 44. Each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a fringe contrast metric related to the fringe contrast of the first zero-order image and the second zero-order image, the method according to clause 41. 45. The method according to any one of clauses 41 to 44, including determining the above-mentioned focus value by referring to a calibrated relationship between the differential focus-dependent metric and the focus. 46. The method according to any one of clauses 40 to 45, including determining a parameter of interest from the above-mentioned non-zero-order diffracted radiation. 47. The method according to clause 46, comprising determining the above-mentioned interest parameters and the above-mentioned focus value from a single image acquisition. 48. The method according to any one of clauses 41 to 47, comprising separating the above-mentioned non-zero order diffracted radiation into a first diffraction order of a complementary diffraction order pair and a second diffraction order of the complementary diffraction order pair, and detecting the first diffraction order and the second diffraction order in separate regions on the above-mentioned at least one detector. 49. The method according to clause 48, wherein the above-mentioned first diffraction order is the +1 diffraction order and the above-mentioned second diffraction order is the -1 diffraction order. 50. The method according to any one of clauses 40 to 49, comprising controlling the focus using the above-mentioned focus value and minimizing the defocus of the above-mentioned measurement radiation on the sample.

[0068]

[0067] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 355, 248, 193, 157, or 126 nm or near thereto) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 to 20 nm), as well as particle beams such as ion beams or electron beams.

[0069]

[0068] The term "lens" may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components, where the context permits.

[0070]

[0069] The term "target" should not be construed to mean only a dedicated target formed for a specific purpose of metrology. The term "target" should be understood to encompass other structures, including product structures, having characteristics suitable for metrology applications.

[0071] From the foregoing description of the specific embodiments, it will be evident to others skilled in the art that various modifications and / or adaptations can be made to such specific embodiments in various applications without undue experimentation and without departing from the general concept of the invention. Thus, such adaptations and modifications are intended to be within the meaning and scope of equivalents of the disclosed embodiments based on the teachings and guidance presented herein. Since the terminology or expressions herein are for the purpose of illustration by way of example and not for the purpose of limitation, the terminology or expressions herein are to be construed by those skilled in the art in light of this teaching and guidance.

[0072]

[0071] The breadth and scope of the present invention is not limited by any of the above exemplary embodiments, but is defined only by the following claims and their equivalents.

Claims

1. A wedge configuration including a plurality of wedge elements arranged around an optical axis, wherein the plurality of wedge elements includes at least a first wedge element, a second wedge element, and a third wedge element, the first wedge element includes a first optical surface and a second optical surface, the first optical surface is planar and not perpendicular to the optical axis, and the second optical surface is non-planar such that the first wedge element has a non-linear thickness variation, the second wedge element includes a third optical surface and a fourth optical surface, which are each planar and not parallel, the third wedge element includes a fifth optical surface and a sixth optical surface, the fifth optical surface is planar and not perpendicular to the optical axis, wedge configuration.

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

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

4. The magnitude of defocus imposed by the first wedge element and the magnitude of defocus imposed by the second wedge element are substantially the same, the wedge configuration according to claim 3.

5. The plurality of wedge elements are arranged in substantially the same plane around the optical axis, the wedge configuration according to any one of claims 1 to 4.

6. Further including a fourth wedge element including a seventh optical surface and an eighth optical surface, each of which is planar and not parallel, the wedge configuration according to any one of claims 1 to 5.

7. Each wedge element includes a quartered configuration, and the plurality of wedge elements are arranged in a circular configuration within the wedge configuration, the wedge configuration according to claim 6.

8. A metrology device including a detection branch, the detection branch including the wedge configuration according to any one of claims 1 to 7 and at least one detector, metrology device.

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

10. The metrology device is a processor, determining a first focus-dependent metric from the first zero-order image and a second focus-dependent metric from the second zero-order image, determining a differential focus-dependent metric from a difference between the first focus-dependent metric and the second focus-dependent metric, determining a focus value of the measurement radiation on the sample to be measured from the differential focus-dependent metric, The metrology device according to claim 9, further comprising a processor operable to:

11. Each of the first focus-dependent metric, the second focus-dependent metric, and the differential focus-dependent metric includes a spatial frequency metric related to the spatial frequency of the first zero-order image and the second zero-order image. The metrology device according to claim 10.

12. The at least one detector is detecting non-zero order diffracted radiation on the detector, determining a parameter of interest from the non-zero order diffracted radiation, The metrology device according to claim 10 or 11, operable to:

13. The first zero-order image, the second zero-order image, the first diffracted order image of the first diffracted order of a complementary diffracted order pair, and the second diffracted order image of the second diffracted order of the complementary diffracted order pair are detected in separate regions on the at least one detector. The metrology device according to claim 12.

14. The metrology device according to claim 13, wherein the first diffracted order is the +1 diffracted order and the second diffracted order is the -1 diffracted order.

15. A method for determining a focus value of measurement radiation on a sample during measurement of the sample, comprising: separating scattered radiation from the sample, scattered by the sample with the measurement radiation, into at least non-zero order diffracted radiation, a first portion of the zero-order radiation, and a second portion of the zero-order radiation; detecting at least one image of the non-zero order diffracted radiation, a first zero-order image from the first portion of the zero-order radiation, and a second zero-order image of the second portion of the zero-order radiation in separate regions of a detection plane; imposing defocus on at least one of the first portion of the zero-order radiation and the second portion of the zero-order radiation; A method comprising determining the focus value of the measurement radiation on the sample from the first zero-order image and the second zero-order image.

Images